1. business and industrial
2. chemicals industry
3. plastics and polymers

INVESTIGATIONS ON THE STRUCTURAL, OPTICAL
AND MAGNETIC PROPERTIES OF NANOSTRUCTURED
CERIUM OXIDE IN PURE AND DOPED FORMS AND ITS
POLYMER NANOCOMPOSITES
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INVESTIGATIONS ON TIIE STRUCTURAL, OPTICAL AND MAGNETIC PROPERTIES OF
NANOSTRUCTURED CERIUM OXIDE IN PURE AND DOPED FORMS AND ITS POlYMER
NAN OCOMPOSITES
Ph.D thesis in the field of Material Science
Authon
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Dhannia.T
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Division for Research in Advanced Materials
Department of Physics
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Cochin University for Science 86 Technology
Kochi- 682022, Kerala, India.
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E mail:dhanniat@ gmailcorn
Supervisor:
Dr. S.]aya.lel<sl1n1i,
Professor,
Division for Research in AdvanceclMatc1ia.ls
Department of Physics
Cochin University for Science 86 Technology
Kocl1i- 682022, Kerala, India.
Email:jaya.lel<[email protected]
January 2012
Dr. S. Jayalekshmi
Professor
Division for Research in Advanced Materials
Department of Physics
Cochin University for Science 8| Technology
Kochi- 682022, Kerala, India.
[email protected]
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I hereby declare that the present work entitled “INVESTIGATIONS
ON THE STRUCTURAL, OPTICAL AND MAGNETIC PROPERTIES
OF NANOSTRUCTURED CERIUM OXIDE IN PURE AND DOPED
FORMS AND ITS POLYMER NANOCOMPOSITES” is based on the
original work done by me under the guidance of Dr. S. J ayalekshmi, Professor,
Department of Physics, Cochin University of Science And Technology, and has
not been included in any other thesis submitted previously for the award of any
other degree.
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Dhannia.T
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30/01/2012
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Acknowledgement
I would like to use this opportunity to express my sincere thanks and
gratitude to _Dr. S Jayalekshmi, Professor, Department of Physics, Cochin
University of Science and T echnologfy, for her excellent guidance and
encouragement throughout the course of this work.
I am thankful to Prof M. R. Anantharaman, the present Head of the
Department of Physics and also to the former Heads of the Department of
Physics Prof K. Babu Joseph , Prof M. Sabir, Prof K. P. Rajappan Nair,
Prof Elizabeth Mathai, Prof K.P Vijayakumar, Dr. “V. C. Kuriakose,
Dr. Ramesh Babu T, Dr. Godfrey Louis for providing all facilities. I express my
gratitude to my teachers Prof B. Pradeep, Prof C. Sudha Kartha,
Prof T.M Abdhul Rasheed, Mr. M. M Kuttappan, Mr. P. K. Sarangadharan of the
Department of Physics, Cochin University of Science and Technology.
I express my sincere thanks to Prof T. Balasubramanian and
Prof D. Sastilcumar Heads of the Department ofPhysics, National Institute of
Technology, Thiruchirappalli for permitting me to do part of my research work
in NIT Trichy.
I am grateful to Dr. A. Chandrabose, Dr. J. Hemalatha and
Dr. T. Prasada Rao, Department of Physics, National Institute of Technology,
T hiruchirappalli, for their help to initiate the work in Nanotechnology during
my stay in NI TT. I am also thankful to Dr. R. Justin Josephyus and his students
Mr. K. Prakash and Mr. T. Arun of Department of Physics, NIT T for their
valuable help in VSM and TGA measurements. I acknowledge with thanks the
help and encouragement of all faculty members and research scholars of NIT T.
I thank all the non-teaching stafl of Department of the Physics, C USAT,
for their help and co-operation. The help extended by the technical staff of ST 1C
and USIC, C USAT, is also acknowledged.
I remember late Dr. K. Ravindranath of Acquinas College, Dr. Sajeev U S,
Dr. Santhosh D shenoy, Dr. Sindhu S, Dr. Veena Gopalan, Dr. Vijutha Sunny,
Dr. Sagar and Ms. _Geetha with deep gratitude and aflection. I would always
remember Mr. Vimal, Dr. Pramitha, Saju sir and Anila teacher for the support
given to me all through my research carrier.
I acknowledge the co—operative and helpful attitude of all my fellow
research scholars in the Dream Lab in the Department of Physics.
Words are insujficient to express my acknowledgements for my
colleagues in the school of Engineering, C USAT, without whose advices, help
and motivations, it would have not been possible to complete the research work.
And at last but not the least I have no words to express my thanks to my
parents and husband, for they have been the inspiring force behind me.
Dhannia. T
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Page N0
Preface
list of Publications
Cfzapterr 1 Introduction .................................................... ..
O O I O O Q O O O O I O I O O O O O O O O O O O O O O Q OOI
..l
l.l. Nanoscience and Nanotechnology ................................ .. .................................. ..l
1.2. (erium oxide — structure, properties and applications ..................................... ..8
l.3. A Brief Review .......................................................... .. ...................................... ..l3
l.4. Objectives of the present work ................................. .. ...................................... ..l5
References ....................................................................... .. ...................................... ..l6
Qfiapte/12 Experimental Techniques .................................................................. ..27
2.l. introduction .............................................................. .. ...................................... ..27
2.2. Synthesis methods .................................................... ..
o Q n a o n n Q n u a o Q n n o n n a o n I Q n n Q o Q I o Q I u o 0I0
...28
2.2.l. fhemical precipitation ............................................ .. .......................................... ..28
2.2.2. Spray pyrolysis ....................................................... .. .......................................... ..29
2.2.3. Synthesis of polymer/(e02 nanocomposites ........ .. ........................................... ..35
2.3. Structural characterization..........-................................
2.3.l. X-ray diffraction.............................-...
.....37
2.3.2- Transmission Electron Microscapy.........
2.3.3. Scanning Electron Microscopy ......................................................................... .. ....39
2.3.4. Energy Dispersive X-ray Spectroscopy ............................................... ..
....40
2.3.5. Fourier Transform Infra-—Red Spectroscopy .................. .. . . - - . . . . - . - . - . . - - - Q Q - - Q - a A ¢ A ¢ - a04 ....4l
2.4. Optical characterization ............................................ ..
0 Q u Q a n Q ¢ n u o o I Q o o I I o Q I Q Q I u Q I I Q 0 I u Q I IOI
42
2.4.1. UV-Vis Absorption Spectroscopy ........................................................... ..
43
2.4.2. Diffuse Reflectance Spectroscopy
....44
2.4.3. Phatoluminescence Spectroscopy ......................... .. ............................................ ..48
2.5. Thermal characterization ..................................................................... .. ........... .. 50
2.5.1. Thermogravimetric Analysis ................................. .. ............................................ ..5U
2.6. Magnetic characterization ........................................... .. .................................... ..5l
2.6. l . Vibrating Sample Magnetometer
References ............................................................................................................... ..53
Qliaptm 3 Synthesis, structural and optical characterization of cerium oxide
nanocrystals ............................................................................................. ..55
3.1. Introduction ...................................................................................................... .. 55
3.2. Experimental techniques ................................................................................... .. 56
3.3. Results and discussion ...................................................................................... .. 57
3.4. (onclusion ......................................................................................................... .. 68
References ............................................................................................................... ..69
Qfiapte/c 4 Synthesis, structural and optical characterization of doped cerium
oxide nanocrystals ........................................................................... ..7l
4.I. Introduction ...................................................................................................... .. 7|
4.2. Experimental techniques ................................................................................... .. 72
4.3. Results and discussion ...................................................................................... .. 74
4.4 Eonclusion .......................................................................................................... ..95
References ............................................................................................................... ..95
Qfiapte/c 5 Thermal and magnetic properties of ceria and doped ceria nanacrystals ....... .. 97
5.l. Introduction ...................................................................................................... .. 97
5.2. Experimental techniques ................................................................................... .. 99
5.3. Results and discussion ...................................................................................... .. 99
5.4. fionclusion ......................................................................................................... .- I I2
References ............................................................................................................... .. I I3
(ffiaptev. 6 Preparation of Ceria Thin Films by Spray Pyrolysis and their
Characterizations ............................................................................. ..t I 5
6.l. Introduction ...................................................................................................... .. I I5
6.2. Experimental techniques ................................................................................... .. I I6
6.3. Results and discussion ...................................................................................... -. I I7
6.4. Conclusion ......................................................................................................... .. I 29
References ............................................................................................................... -. I30
Qliaptm 7 Synthesis, structural and optical properties of polymer/ceria nanocomposites
7.l. Introduction ................................................................................................. ..
7.2. Experimental techniques ............................................................................... ..
7.3. Results and discussion .................................................................................. ..
7.4. Conclusion .................................................................... ..
References ................................................... ..
Q Q Q Q n ¢ Q n ¢ ¢ n n Q Q n Q Q Q n n Q - - Q Q Q u Q Q Q Q Qas
Q Q Q Q Q o 0 Q 0 0 0 Q 0 0 Q Q o v v Q 0 Q Q I I Q o I u o I u o I u u U I u o I I v o v v v o 0 o 0 Q 0 000 0
ten 8 Summary and Conclusion ................................................................. ..
8.1. General Conclusion ...................................................... ..
8.1. Future scope of the present work ................................................................... ..
References .................................... ..
, - - , , - - , , q , , - - ¢ - - - 0 o . . . Q o A - - - - . . Q . - ¢ Q - Q Q - - Q Q Q Q Q n Q Q Q Q Q Q o n Q Q Q Q Q Q ¢ ¢ ¢ Q » A Q oin
PREFACE
In recent years, nanoscience and nanotechnology has emerged as one of
the most important and exciting frontier areas of research interest in almost all
fields of science and technology. This technology provides the path of many
breakthrough changes in the near future in many areas of advanced technological
applications. Nanotechnology is an interdisciplinary area of research and
development. The advent of nanotechnology in the modem times and the
beginning of its systematic study can be thought of to have begun with a lecture by
the famous physicist Richard Feynman. In 1960 he presented a visionary and
prophetic lecture at the meeting of the American Physical Society entitled “there zs
plenty qfmom at the bottom” where he speculated on the possibility and potential of
nanosized materials.
Synthesis of nanomaterials and nanostructures are the essential aspects of
nanotechnology. Studies on new physical properties and applications of
nanomaterials are possible only when materials are made available with desired
size, morphology, crystal structure and chemical composition. Cerium oxide (ceria)
is one of the important functional materials with high mechanical strength, thermal
stability, excellent optical properties, appreciable oxygen ion conductivity and
oxygen storage capacity. Ceria finds a variety of applications in mechanical
polishing of microelectronic devices, as catalysts for three-way automatic exhaust
systems and as additives in ceramics and phosphors. The doped ceria usually has
enhanced catalytic and electrical properties, which depend on a series of factors
such as the particle size, the structural characteristics, morphology etc. Ceria based
solid solutions have been widely identified as promising electrolytes for
intermediate temperature solid oxide fuel cells (SOFC). The success of many
promising device technologies depends on the suitable powder synthesis
techniques. The challenge for introducing new nanopowder synthesis techniques is
to preserve high material quality while attaining the desired composition. The
method adopted should give reproducible powder properties, high yield and must
be time and energy effective. The use of a variety of new materials in many
technological applications has been realized through the use of thin filmsof these
materials. Thus the development of any new material will have good application
potential if it can be deposited in thin film form with the same properties. The
advantageous properties of thin films include the possibility of tailoring the
properties according to film thickness, small mass of the materials involved and
high surface to volume ratio. The synthesis of polymer nanocomposites is an
integral aspect of polymer nanotechnology. By inserting the nanometric inorganic
compounds, the properties of polymers can be improved and this has a lot of
applications depending upon the inorganic filler material present in the polymer.
An important field of research is the development of new synthetic
processes to produce ultrafine CeO2 particles with nanocrystalline structure that
heighten the perfonnance of the material. So one of the important objectives of
the present work is to synthesize nanostructured cerium oxide in pristine and
doped forms using surfactant free, hydrolysis assisted chemical precipitation
method which is a simple and cost effective technique. This method using cerium
chloride and ammonia as precursors is a novel method for synthesizing
nanostmctured ceria, since this technique has not been pursued previously.
Another important objective of the present work is to search for Room
Temperature Ferromagnetism (RTFM) in nanostructured ceria (both in pristine
and doped fonns) and cerium oxide thin films deposited by spray pyrolysis
technique. Detailed investigations on the optical and photoluminescence
properties of hydrolysis assisted chemically synthesized nanostructured ceria both
in the pristine and doped fomis is another objective of the present work.
polymer/ceria nanocomposites constitute an interesting field of research area
which has not been subjected to extensive investigations. So in the present work
emphasis is also given to the synthesis of polymer/ceria nanocomposite using
different polymers and their characterizations for possible practical applications.
In the present work nanocrystalline CeO2 powder samples have been
prepared by hydrolysis assisted chemical precipitation method employing cerium
chloride and ammonia as precursors. Extensive investigations have been carried
out on the structural and optical properties of nanostructured ceria and iron,
aluminium and cobalt doped ceria. One of the highlights of these studies is the
observation that both pure and doped ceria offer the prospects of applications as
cost effective and non toxic inorganic material for efficient UV filtering in
sunscreen cosmetics. RTFM has been observed for the first time in pure ceria
nanocrystals synthesized by chemical precipitation technique and also in iron and
cobalt doped ceria nanocrystals. RTFM is also reported for the first time in high
quality cerium oxide thin films deposited by spray pyrolysis technique. The origin
of RTFM has been ascribed to the presence of oxygen vacancies on the surface of
ceria nanoparticles. Based on the experimental data good correlation between the
magnetic, structural and optical properties of these samples has also been
established. Based on the dependence of PL intensity on annealing temperature a
self trapped exciton (STE) mediated PL mechanism has been proposed for the
observed PL in ceria nanocrystals. The polymer/ceria nanocomposites prepared
using a variety of polymers such as PVDF, PVA, PMMA and PS possess good UV
absorption window regions of approximately 250 nm width. Hence these
nanocomposites offer prospects of potential applications in the development of
efficient UV filters.
The present thesis consists of eight chapters. The significance of
nanomaterials and their applications in different areas of science and technology
are briefly introduced in chapter 1. It gives some general ideas about
nanomaterials and a brief review on ceria and related compounds. The objectives
of the present investigations are also detailed in this chapter.
The second chapter describes the various experimental techniques
employed for the preparation and characterization of the materials relevant to the
present work. These include XRD, TEM, EDX and FTIR for structural
characterization, Photoluminescence, Diffuse Reflectanceand UV-Vis absorption
spectroscopic techniques for optical characterization, TGA for thermal studies and
VSM for magnetic studies.
In the third chapter, the preparation of cerium oxide nanoparticles by
hydrolysis assisted chemical precipitation method using cerium chloride and
ammonia precursors is addressed. This technique is a simple and cost effective
one, without the need for any surfactants or high temperature and pressure
conditions. The observation of STE mediated photoluminescence in pure cerium
oxide forms the highlight of this chapter.
The fourth chapter includes the synthesis and characterization of
aluminium, iron and cobalt doped cerium oxide nanoparticles prepared by the
same simple chemical precipitation technique similar to the one used for
synthesizing pure ceria. The structural and optical properties of these samples have
been investigated in detail. The presence of ot - Fe2O, phase in iron doped ceria
has been used to explain some of the observed features of the room temperature
ferromagnetic behaviour of this sample, which fonns a significant part of the next
chapter.
The fifth chapter gives detailed investigations on the magnetic and thermal
properties of pure ceria nanocrystals and transition metals (Fe and Co) doped ceria
nanocrystals. RTFM has been observed first time in cerium oxide, both in pure
and doped fomrs, synthesised by chemical precipitation technique. A detailed
theoretical explanation of the observed RTFM has also been attempted.
The sixth chapter describes the preparation and characterization of ceria
thin films using spray pyrolysis method. Spray pyrolysed thin films offer high film
quality and low processing costs compared to conventional thin films, prepared
using techniques such as pulsed laser deposition and chemical or physical vapour
deposition. Room temperature ferromagnetism has been observed in pure ceria
thin films of the present study, for the first time. Though the magnetization value
of ceria thin film is small compared to the bulk oxide, the observed signature of
FM in ceria thin films is significant from the view point of applications in the
development of spintronic devices.
In the seventh chapter a detailed discussion of the synthesis and various
characterization of CeO2 nanocomposites using different polymers such as PVDF,
is included. Optical absorption and photoluminescence characteristics of these
polymer nanocomposites have been investigated in detail.
Chapter eight is the concluding chapter of the thesis. General conclusions
and inferences anived at, based on the present investigations are summarized in
this chapter. Suggestions for the improvements in the synthesis conditions are also
discussed and the scope for further investigations are also high- lighted.
List of Publications
Journal Papers ~
[1] Effect of aluminium doping on structural and optical properties of
cerium oxide nanoparticles, T Dhannia, S J ayalekshmi, M C Santhosh
Kumar, T Prasada Rao and A Chandra Bose, J.Phys & Chem of solids,
70 (2009) 1443-1447.
[2] Effect of iron doping on structural and optical properties of cerium
oxide nanoparticles, T Dhannia, S J ayalekshmi M C Santhosh
Kumar, Prasada Rao and A Chandra Bose, J .Phys & Chem of solids
71 (2010) 1020-1025
[3] Self Trapped Excitons in chemically precipitated cerium oxide
nanocrystals, T Dhannia, S Jayalekshmi, M C Santhosh Kumar (under
review)
[4] Magnetic study of chemically prepared Fe-doped ceria, T Dhannia,
S J ayalekshmi M C Santhosh Kumar (communicated)
[5] Synthesis, structural and optical properties of Polymer/Ceria
nanocomposites, T Dhannia and S J ayalekshmi (communicated)
[6] Structural and optical properties of ceria thin film, T Dhannia,
T Prasada Rao‘ M C Santhosh Kumar, S J ayalekshmi (communicated)
Conference Papers
[1] T Dhannia, S Jayalekshmi‘ M C Santhosh Kumar, T Prasada Rao and
A Chandra Bose, Structural and optical properties of chemically
prepared cerium oxide nanocrystals, National Conference on New
Horizons in theoretical and experimental Physics (NI-ITEP-2007),
Department of Physics, Cochin University of Science & Technology.
[2] T Dhannia, S Jayalekshmi, M C Santhosh Kumar, T Prasada Rao and
A Chandra Bose, Lattice constant dependence of particle size of doped
cerium oxide nanoparticles on annealing, National conference on
Emerging materials -and technologies for India-2020, Department of
Metallurgical and Materials Engineering, NITT, TamilNadu, 2008.
[3] T Dhannia, S Jayalekshmi, M C Santhosh Kumar and T Prasada Rao
The effect of annealing on the structural and optical properties of
aluminium doped cerium oxide nanocrystals, Proceedings of 53'“ DAE
Solid State Physics Symposium, 2008.
[4] T Dhannia, S Jayalekshmi M C Santhosh Kumar, Aluminium doped
nanocrystalline cerium oxide—a new material for SOFC, Cochin nano­
2009, Department of Physics, Cochin University of Science &
Technology
(Z Q 0 p f ep
I
INTRODUCTION
l.l. Nonoscience and Nanotechnology
l.2. Cerium Oxide: structure, properties and upplicotions
l.3. A Brief Review
l.4 Objectives of the present work
_ References lg
1.1. Nanoscience and Nanotechnology
Research in nanoscience and nanotechnology has gained considerable
momentum in the last 15 years, owing to its present and future applications in
various walks of life like consumer electronics, sensors, health care,
nanomedicine, food technology etc. Nanoscience is the study of phenomena and
manipulation of materials at atomic, molecular and macromolecular scales,
where properties differ significantly from those at a larger scale.
Nanotechnology is a new field or a new scientific domain. Similar to quantum
mechanics, on nanometer scale, materials or structures may possess new
physical properties or exhibit new physical phenomena. Some of these
properties are already known. There may be many more unique physical
properties not known to us yet. These new physical properties or phenomena
may not satisfy everlasting human curiosity, but offer the prospects of new
advancements in technology. Nanotechnology also promises the possibility of
creating nanostructures of metastable phase with non-conventional properties
including superconductivity and magnetism. Yet another very important aspect
C/iapter-1
of nanotechnology is the miniaturization of current and new instruments,
sensors and machines that will greatly impact the new world we live in.
Nanotechnology has an extremely broad range of potential applications from
nanoscale. electronics and optics to nanobiological systems and nanomedicine,
to new materials and therefore it requires formation of and contribution from
multidisciplinary teams of physicists, chemists, material scientists, engineers
and molecular biologists. Such a multidisciplinary team of workers has to work
together on (l) synthesis and processing of nanomaterials and nanostructures,
(2) understanding the physical properties related to the nanometer scale, (3)
design and fabrication of nanodevices or devices with nanomaterials as building
blocks, and (4) design and construction of novel tools for characterization of
nanostructures and nanomaterials [1].
The applications of nanomaterials utilize not only chemical composition
but also the size, shape and surface dependent properties. These properties of
nanoparticles in novel applications lead to remarkable performance
characteristics. Particles with size smaller than the wavelength of visible light
have an important role in a broad range of applications in material science.
Synthesis of nanomaterials with well-controlled size, morphology and chemical
composition may open new opportunities in exploring new and enhanced
physical properties. The recent emergence of tools and techniques capable of
constructing structures with dimensions ranging from 0.1 nm to 50 nm has
opened up numerous possibilities for investigating new devices in this size
domain, which was inaccessible to experimental researchers till now.
Decreasing dimension in microelectronic and optoelectronic devices requires
knowledge of material properties below a critical size [2].
There are two approaches for the preparation of nanostructures. They are
top-down and bottom up approaches. In bottom-up approach, the atoms and
E , t . q ?Z?§i'/iiriéti/11¢/Iiiiiffllkr/‘Iii(1154?
Introduction
molecules are collected, consolidated and fastened together into the structure.
This is carried out by a sequence of chemical reactions controlled by catalysts.
In top-down method, a large scale object or pattern gradually gets reduced in
dimensions to form nanostructures. This can be accomplished by a technique
called lithography in which radiation through a template is given to a surface
coated with a radiation sensitive resist; the resist is then removed and the
surface is chemically treated to produce the nanostructures [2]. When the size or
dimension of a material is continuously reduced from a large or macroscopic
size to a very small size, the properties remain the same at first, and then small
changes begin to occur, until finally when the size drops below 100 mn,
dramatic changes in properties occur. If one dimension is reduced to the narrow
range while the other two dimensions remain large, then we obtain a structure
known as quantum well. If two dimensions are so reduced and one remains
large, the resulting structure is referred to as a quantum wire. The extreme case
of this process of size reduction in which all the three dimensions reach low
nanometer range is called quantum dot.
Nanoparticles can be produced by a variety of methods. These include
combustion synthesis, plasma synthesis, wet-phase processing, chemical
precipitation, sol-gel processing, microwave synthesis, mechanical processing,
mechanochemical synthesis, high-energy ball-milling, chemical vapour
deposition, laser ablation etc. In the sol-gel process, alkoxide or organo-metallic
compounds are usually used as precursors. However, the expensive precursor
materials and more complicated reaction mechanism often restrict the potential
use of sol-gel process. The reactions for hydrothermal synthesis and forced
hydrolysis are often carried out under several different conditions, such as
higher temperature, higher pressure and longer reaction time [3]. In microwave
synthesis, electromagnetic radiation with frequency range of 0.3-300GHz and
flepar/mentoff/rysirs, (1/we
Cliapter-I
corresponding wavelengths from lmm to lm is employed. In the microwave
irradiation region, the frequency of applied irradiation is low enough so that the
dipoles have time to respond to the alternating electric field and therefore
rotation. However, the frequency is not high enough for the rotation to precisely
follow the field, which causes energy to be lost from the dipole by molecular
fiiction and collision, giving rise to the dielectric heating [4]. In micro­
emulsion method, it is necessary to mix micro-emulsion containing ions to be
precipitated with another kind of micro emulsion, solid or gas containing
precipitants. It is a very efficient method for preparing highly monodispersed
nanoparticles, but is hard to scale up commercially. The oxide particles
obtained by ceramic processes are rather large and non unifonn in size. These
non unifonn particles result in scratching or rough surfaces in polishing [5].
Solvothermal synthesis utilizes a solvent under pressure and temperatures
above its critical point to increase the solubility of a solid and to speed up
reactions between solids [6]. Most of these techniques are complex, energy
consuming and expensive. Compared to these methods, precipitation is more
attractive because cheap salt precursors are needed and the operation is simple
and quite suitable for mass production. Moreover, the reaction conditions of
precipitation are mild and adjustable for satisfying a variety of purposes [3]. In
homogeneous precipitation process, the precipitants can be tmiformly generated
on-site by their precursors. Homogeneous precipitation method, which is based
on acid-base reaction, is convenient for preparing rare earth oxides from a view
point of industrial application as well as a lab scale experiments [7].
The optical properties of nanoparticles are markedly different from those
of bulk. In the case of metals, as the size of the particle decreases we start
observing oscillations of electron gas on the surface of nanoparticles. These
oscillations are called surface plasmons. So, if the nanoparticles are exposed to
Introfuction
an electromagnetic wave having a wavelength comparable to or greater than the
size of the nanoparticles and the light has a frequency close to that of the
surface plasmon then the surface plasmon would absorb energy. Thus
nanoparticles start exhibiting different colors as their size changes and the
frequency of the surface plasmon changes with it. This frequency of the surface
plasmon absorption is a function of the dielectric constant of the material, size
of the particles and also the specific geometrical shape that the particle has. In
the case of semiconducting naoparticles the pI'Ope1"[i6S change in a different
fashion. One of the important properties that changes as the size of the
nanoparticles changes is the absorption spectrum of the material. The strength
of the absorption depends on the material and wavelength passed. For a given
material in its bulk state, the absorption spectrum is unique. But when the
material is in the form of nanostructures then the absorption spectrum changes
and undergoes a blue shift. The nanostructures have a different density of states
compared with the bulk state. This change in density of states has implications
on the electrical properties of nanoparticles.
Physical properties of a semiconductor are related to its band gap, which
can be substantially modified (increased or decreased), by doping. The
magnitude of the shift is determined by two competing mechanisms. There is a
band gap narrowing (BGN) which is a consequence of many body effects on the
conduction and valence bands. The shrinkage is counteracted by the Burstein­
Moss effect, which gives a band gap widening (BGW) as a result of the
blocking of lowest states in the conduction band [8].
Magnetic nanoparticles show a variety of unusual magnetic behavior
when compared with the bulk materials. This is mostly due to surface/interface
effects, including symmetry breaking, electronic environment/charge transfer
and magnetic interactions. The diverse applications of magnets require the
Depar/meat bf (U541
Cfiapter-1
magnetization curve to have different properties. Nanostructuring of magnetic
materials can be used to design the magnetization curve of the material at hand.
The dynamics of magnetization and demagnetization of magnetic materials in
any device are govemed by the presence of domain walls and regions having
magnetization in different directions. These phenomena change the hysteresis
loop of magnetic nanoparticles as compared to bulk material. Due to the
formation of nanoparticles the atoms which are on the surface are now facing
different potentials in different directions. The resulting surface stress in
nanoparticles modifies its mechanical and structural properties.
Spray pyrolysis has been developed as a powerful tool for the deposition
of various kinds of thin films such as metal oxides and nanophase materials. In
comparison with other techniques, it has several advantages such as high purity
and excellent control of chemical uniformity in multi component system. It can
be adapted easily for production of large area films. Spray pyrolysis method is a
one step synthesis process in which a precursor solution is sprayed by means of
a compressed carrier gas. During spray-pyrolysis deposition, a precursor
solution of metal salts and solvents is sprayed as fine droplets onto a heated
substrate. When the droplets reach the heated substrate, they spread out and
undergo pyrolytic decomposition. Newly deposited flat droplets, with thickness
in the 10-20 mn range, pile up on the previously deposited ones and undergo
pyrolytic decomposition as well. This process continues until a film thickness
of 100-500 nm is reached. The degree of decomposition is determined by the
relation between substrate temperature, the boiling point of the solvents and the
melting point of the salts used for the precursor [9]. It is a simple and cost
effective method for the deposition of variety of thin films. Only limited data is
available on the preparation of ceria thin films using spray pyrolysis technique.
CeO2 thin films are attractive for various electronic and optical applications,
r . 1 i . iii”‘ *Y§' < w~* ~ ~ ~ ~it Peflflifmrflfvfffirr/rs fl/W
Introduction
such as silicon-on-insulator structures, miniaturized stable capacitors, oxygen
sensors, optical coatings etc.
Synthesis and characterization of nanosized metal polymer composites
have been particularly gaining importance due to their potential applications in
biomedical, electronic and optical devices and also as heterogeneous green
catalysts. Generally such composites are synthesized in two to three steps
wherein the polymer and metal particles are prepared separately followed by
mechanical mixing or metal/ metal oxide particles are generated inside the
already prepared polymer by vapor deposition or by reduction/oxidation etc.
Nanocomposites can be obtained by two different approaches viz, in-situ
and ex-situ techniques. In the in-situ methods, nanoparticles are generated
inside a polymer matrix by decomposition or chemical reduction of a metallic
precursor dissolved into the polymer. In the ex-situ approach, nanoparticles are
first produced by soft-chemistry routes and then dispersed into polymer
matrices. General methods for processing nanocomposites are mechanical
alloying, sol-gel synthesis and thennal spray synthesis. Mechanical alloying
occurs as a result of repeated breaking up and joining of the component
particles. This method can prepare highly metastable structures such as
amorphous alloys and nanocomposite structures with high flexibility. In sol-gel
method, metal or main group element compounds undergo hydrolysis and
condensation reactions giving gel materials with extended three-dimensional
structures. These methods are commonly used to prepare nanocomposite
materials because the method occurs readily with a wide variety of precursors
and can be conducted at or near room temperature. Thermal spray coating is
very effective because agglomerated nanocrystalline powders are melted,
accelerated against a substrate and quenched very rapidly in a single step.
. . . . I I I II I
Usually the preparative scheme allows obtaining nanoparticles whose surface
Cfiapter-1 H
can be passivated by monolayers of suitable agents like n- alkanethiol
molecules. Surface passivation has a fundamental role since it avoids
aggregation and surface oxidation or contamination phenomenon. General
methods for processing nanocomposites are mechanical alloying, sol-gel
synthesis, and thermal spray synthesis. Nowadays polymer based
nanocomposites are of considerable interest in research areas because of their
ability to combine the advantages of both polymers and filler components.
There are several applications of polymeric nanocomposites based on their
optical, electrical, mechanical and magnetic properties. The use of inorganic
nanoparticles into the polymer matrix can provide high performance novel
materials that find applications in many industrial fields. With this respect,
frequently considered features are optical properties such as light absorption
(UV and color), the extent of light scattering, photoluminescence and magnetic
properties such as super—paramagnetism, electromagnetic wave absorption etc
[l0].
1.2. Cerium Oxide: structure, properties and applications
The nanoscience and nanotechnology have brought about new chances
for new applications of some traditional materials, such as ceria-based
materials, which are of great interest due to their wide applications, in
particular, as redox or oxygen storage promoters in the three-way catalysts,
catalysts for H2 production from fuels, solid state conductors for fuel cells etc.
Cerium is the most abundant element in lanthanides or rare earth elements in
the periodic table in which the inner 4f electron shell is being filled. The most
stable oxide of cerium is cerium dioxide, CeO3, also called ceria or ceric oxide.
Cerium (atomic number 58, atomic weight 140.12) can be chemically present in
two stable valence states, Ce4+ (ceric) and Ce3+ (cerous), and this property
triggers several technological uses. The ground state of all neutral Ln atoms is
Q Deparfment, off/iys/'rs, (1/$47
I ntrozfuction
probably either [Xe] 4t“5d'6s2 or [Xe] 4t"+'6s2 where the increase in n from 0 to
14 corresponds to the change from La to Lu. Cerium is the second and most
reactive member of the Ln series. It is highly electro-positive and is
predominantly ionic due to the low ionization potential for the removal of the
three most weakly bound electrons. The energetics is such that for all
lanthanides, the most stable state is a trivalent one (Ln3+) with [Xe] 4f"
configuration; i.e. for Ce“, it is [Xe] 4f‘. The 4f electrons have well-shielded
inner orbital which are not influenced by the external enviromnent and hence
the chemical behavior of all Ln3+ ions, including Ce“, is very similar. At the
start of the Ln series, the 5d orbitals are not much higher in energy than the 4f
shell. However, in the case of cerium a potential 4f—5d charge transition
accounts for the absorption by Ce (III) compounds in the UV region just outside
the visible region. The relative increased stability of empty 4f°, half-full 4f7, and
completely full 4fl4 shells for certain elements can cause oxidation states other
than three also to be reasonably stable, in particular Ce4+ with a [Xe] 4fO
configuration [1 1].
'%~a\,_§“\_*‘“~_.
///,­
'“"“"‘~o~
Figure 1.1: The crystal structure of Ce0z
Ceria has the fluorite (CaF2) structure, space group Fm3m (a = 0.541134
nm, J CPDS 34-394) with 8-coordinate cations and 4-coordinate anions. In other
Cliapter-I 7 _ g g A 7 7
words, each cerium cation is coordinated to eight equivalents nearest oxygen
anions at the comer of a cube, each anion being tetrahedrally coordinated by
four cations. The structure is illustrated in figure 1.1. Extending this structure
by drawing cubes of oxygen ions at each comer reveals the eightfold cubic co­
ordination of each cerium, which altemately occupies the centre of the cube.
This clearly shows that there are large vacant octahedral holes in the structure, a
feature, which will be significant when movement of ions through the defect
structure is considered. Ceria has only one crystallographic form throughout the
range of temperatures. lt has a strong tendency to remain in the fluorite­
structured lattice even after losing considerable amount of oxygen, thus
stabilizing a structure with an elevated number of oxygen vacancies [1 ll].
Powdered ceria is slightly hygroscopic and will absorb a small amount
of carbon dioxide from the atmosphere. Cerium also forms cerium (111) oxide
but C602 is the most stable phase at room temperature and under atmospheric
conditions. Oxygen atoms in CeO; units are very mobile and easily leave the
ceria lattice, giving rise to a large variety of non-stoichiometric oxides with two
limiting cases being CeO2 and Ce2O3.
Oxides with the cubic fluorite structure like ceria (CeO2) are known to
be good solid electrolytes when they are doped with cations of lower valency
than the host cations. The high ionic conductivity of doped ceria makes it an
attractive electrolyte for solid oxide fuel cells, whose prospects as an
environmentally friendly power sources are very promising. In these
electrolytes, the current is carried by oxygen ions that are transported by oxygen
vacancies, present to compensate for the lower charge of the dopant cations.
Doped ceria with trivalent ions results in a lowering of the energy barrier for
oxygen migration. A clear physical picture of the connection between the
'7: ".1 I i 1: 09flUf/m§'fl/
I ntrozfuction
choice of a dopant and the improvement of ionic conductivity in ceria is still
lacking [12].
It has been proved that the lattice of CeO2 can be modified by
incorporating metal ions with smaller radius and lower valence such as Fe, Cu,
Mn or larger lanthanide elements with similar properties to Ce. The key factor
in the design of modified ceria is the choice of dopant elements, as well as their
amount introduced. In addition, the preparation method of the powder has also
very strong influence on the homogeneity and stability of the solid solutions.
Nanosized doped ceria with oxides of di or ‘trivalent metals are shown to be a
promising solid electrolyte with good ionic conductivity at low temperatures.
The partial replacement of Ce4+ ions with di or trivalent ions produces large
density of oxygen vacancies in ceria lattice enhancing the conductivity of these
materials. Because of its lower valence state than ceria and low price, a good
candidate for doping can be iron. No detailed analysis has been reported on the
properties of ceria prepared by chemical precipitation method and doped with
dopants such as iron, aluminium, cobalt etc.
The search for room temperature ferromagnetism (RTFM) in non
magnetic oxides including cerium oxide has been the subject of extensive
research during the past few years. Though there are a few reports related to the
observation of RTFM in nanostructured ceria both pristine and doped forms, in
all these reports the synthesis of cerium oxide involves special synthesis
conditions such as the presence of surfactants, high temperature and pressure
conditions etc. There are no reports on the observation of RTFM in cerium
oxide synthesized by hydrolysis assisted chemical precipitation technique. This
technique is simple and cost effective technique [13] and is the one employed
for the synthesis of cerium oxide in the present study.
I 1/J_’/II ' E
Cfiapter-1
The value of the magnetic moment is the measure of the strength of the
magnetism that is present in the material. Atoms in various transition series of
the periodic table have unfilled inner energy levels in which the spins of the
electrons are unpaired, giving the atom a net magnetic moment. Several recent
studies have pointed out that oxygen vacancies, F-centers and defects in diluted
magnetic semiconductor (DMS) systems play a major role in the magnetic
exchange mechanism. Ferromagnetic (F M) ordering through F -centre exchange
mechanism is mediated by oxygen vacancies and strongly depends on the
valence state of Fe dopant [14].
The major drawback of the ceria based electrolytes is that, Ce4+ can be
reduced to Ce“ under reducing conditions at high temperatures above 700 0C,
resulting in high electronic conductivity, which is detrimental to the functioning
of electrolyte in the SOFCs. Also in polycrystalline ceria based electrolytes, the
impurities such as Si and Ca segregate at grain boundaries and form thin blocking
layers within the grain boundary network, which affect the grain boundary
conductivity. One way of reducing the larger contribution of segregated
impurities is to reduce the grain size, so that the grain boundary per unit volume
is increased and the total amount of impurities can be spread over a large
interfacial area. Also, it has been speculated that, in the nanostructured materials,
grain boundary may provide fast diffusion pathway for ionic defects resulting in
enhanced ionic conductivity in the finely grained materials. So the nanocrystalline
materials with improved electrochemical properties are expected to overcome
some of the draw backs associated with microcrystalline ceria based electrolyte,
opening wide range of applications in the intermediate temperature range.
Therefore, in recent years, the nanostwctured ceria based electrolyte materials
doped with different dopants and dopant concentrations have attracted a great
interest for their development as electrolyte materials for SOFCs. Currently
179,51?///?7l9fl3'?fi"?lY3??3)in/547
Introfuction
enormous effects are being made to understand the electrolyte properties of
nanoscale ceria based materials for application purpose [12].
1.3. A Brief Review
Synthesis forms a vital aspect of the science of nanomaterials. Chemical
methods have proved to be more effective and versatile than physical methods
and have therefore, been employed widely to synthesize a variety of
nanomaterials including nanocrystals, nanowires, nanotubes etc.
1.3.1. Pure and doped ceria
The synthesis of nanomaterials with controlled size and composition has
become one of the important topics of colloid and materials chemistry because of
their size or shape dependent properties. Recently, several methods have been
developed to prepare pure and doped CeO; powder, including wet chemical
synthesis, thennal hydrolysis, flux method, hydrothermal synthesis, gas
condensation method, microwave technique, sonochemical, reverse micellar
synthesis, solvothermal method, mechano-chemical, sol-gel method, composite­
hydroxide-method, precipitation, thermal decomposition, polymer method
combustion synthesis, SPRT method, microemulsion method, electrochemical
method, low temperature decomposition method, precursor-growth- calcination
process, citrate method, solid state synthesis, polyol method, ball milling
method, freeze drying etc. [15 - 93]. Among these methods, precipitation is more
attractive because it is a simple technique which requires easily available and cost
effective precursors. There is also a possibility for the mass production quite
easily. Although CeO2 in doped and pristine form have already been studied,
there are no reports on systematic investigations related to the synthesis and
properties of nanostructured cerium oxide (both in pristine and doped forms)
C/iapter-1 \
pp q
using the cost effective technique of hydrolysis assisted chemical precipitation
technique.
1.3.2. Ceria thin films
Different thin film deposition methods have been employed for ceria.
The various techniques can be divided into three main classes: 1) chemical
processes, MO-CVD, spray pyrolysis, sol-gel, flame—assisted vapor deposition
etc. 2) Physical methods-sputtering, laser ablation etc. 3) ceramic powder
processes-tape casting, screen printing, atomic layer deposition etc. [94-125].
Spray pyrolysis is an attractive thin film preparation method because it
is inexpensive and can be used to deposit large area films. Spray-pyrolysed thin
films offer high film quality and low processing costs compared to conventional
thin film preparation techniques such as pulsed laser deposition and chemical or
physical vapor deposition. There are a few reports on the preparation of
nanostructured cerium oxide thin films using spray pyrolysis technique. Hence
in the present work, attempts have been made to prepare high quality cerium
oxide thin films using spray pyrolysis technique and investigate the structural,
optical and magnetic properties of these films.
1.3.3. Polymer/Ceria nanocomposites
Even though many polymer/metal nanocomposite systems have been
extensively studied, polymer/ceria nanocomposite systems have not been given
much attention [126-128]. Hence, the part of the present investigation is
devoted to the synthesis and characterization of polymer/ceria nanocomposite
systems using different polymers such as PVDF, PMMA, PS and PVA.
I 1 warm»:r»//next!/1/11
In t-rot:/‘act ion
1.4. Objectives of the present work
Most of techniques employed for the preparation of nanoparticles and
thin films are complex, energy consuming, and expensive. Therefore, simple
and cost effective routes to synthesize nanoparticles by utilizing cheap,
nontoxic and environmentally benign precursors have to be identified and
implemented. The work present in the thesis is centered around the synthesis of
nanostuctured ceria in pristine and doped forms, the preparation of ceria thin
films using spray pyrolysed technique, the synthesis of polymer/ceria
nanocomposites and detailed investigations on the various properties of these
systems. An important field of research is the development of new synthetic
processes to produce ultrafine C€O2 particles with nanocrystalline structure that
heighten the performance of the material.
The objectives of the present work can be summarized as follows.
0 In the present work emphasis is given to the synthesis of nanostructured
cerium oxide in pristine and doped fonns using surfactant free,
hydrolysis assisted chemical precipitation method which is a simple and
cost effective technique. This method, using cerium chloride and
ammonia as precursors is a new approach for synthesizing
nanostructured ceria, since this technique has not been pursued
previously.
0 Room temperature ferromagnetism (RTFM) has been reported in
nanostructured ceria synthesized using variety of techniques, however,
there are no reports related to RTFM in ceria synthesized by the
hydrolysis assisted precipitation method. One of the important objective
§0,é)Jb}t1i:e)7tafPby$i(st,i(1/SAT t
Cliapter-I _ 7 H A _ g 7 g _
of the present work is to search for RTFM in nanostructured ceria (both
in pristine and doped forms) and finds a correlation between the
observed magnetic, structural and optical properties.
0 Detailed investigations on the optical and photoluminescence properties
of hydrolysis assisted chemically synthesized nanostmctured ceria both
in the pristine and doped forms is the another objective of the present
work.
0 Attempts will be making for RTFM in cerium oxide thin films deposited
by spray pyrolysis deposition technique.
0 Polymer/ceria nanocomposites constitute an interesting field of research
area which has not been subjected to extensive investigations
0 In the present work emphasis is also given to the synthesis of
polymer/ceria nanocompoiste using different polymers and their
characterizations for possible practical applications.
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J A Punnoose, Phys. Rev. B 76 (2007) 165206
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A Verma, A K Bakhshi, S A Agnihotry, Solar Energy Mat. Solar
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j'jfj.f§f§j§_._ "'ffff__ r;;;;;* ‘S15ii:i?i.§~=fi§flepar(inéh(of/’/rys/?s,_ (1/IN
Introduction
[109] C Chaiitidis, P Patsalas, S Logothetidis, J. Phy: Conf. series 10
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[110] Siokou, S Ntais, V Copoulos, S Papaefthimiou, G Leftheriotis, P
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Appl. Surf. Sci. 254 (2007) 153
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[118] V Fernandes, R J O Mossanek, P Schio, J J Klein, A J A de Oliveira,
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Mosca, Phys. Rev. B 80 (2009) 035202
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ifleparrmebia/P/iyjairs; 1695473 ; 1 1 i i i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 A
Cfmpter-1 _ __ __ ___(_
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.-...-.... ...........
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2
rxprmmrmrnt TECHNIQUES
2.1. Introduction
2.2. Synthesis methods
2.3. Structuralthurotterizution
2.4. Optitulchuruclerizution
2.5. Thermulchuracterizution
2.6. Mognetitchuructerizaiion
References
2.1. Introduction
This chapter focuses mainly on the fundamentals and basic principles of
the preparation techniques of nanomaterials and thin films and the
characterization tools, which are used in the present investigation. The
characterization techniques include structural, optical, thermal and magnetic
characterization methods used to characterize the nanomatenals and
nanostructures. The structural characterizations include X-ray Diffraction
(XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray
Spectroscopy (EDX) and Transmission Electron Microscopy (TEM)
techniques. Optical characterizations include Diffuse Reflectance Spectroscopy
(DRS), Fourier Transfonn IR Spectroscopy (FTIR) and Photoluminescence
Spectroscopy (PL) methods. Vibrating Sample Mgnetometer (VSM) is used to
investigate the magnetic properties.
(jfiapter-2 g J
2.2. Synthesis methods
Synthesis forms an essential component of nanoscience and
nanotechnology. While nanomaterials have been generated by physical methods
such as laser ablation, arc-discharge and evaporation, chemical methods have
been proved to be more effective, as they provide better control on the size,
shape and functionalization of the nanomaterials synthesized [1].
2.2.1. Chemical precipitation
Precipitation technique of rare earth compounds from aqueous solutions
using suitable reagents is useful for the development of nanoparticles of oxides
for various applications. Homogeneous precipitation method, which is based on
acid-base reaction, is conventional for preparing functional rare earth oxides
fiom a view point of industrial applications. Chemical precipitation of various
salts (nitrates, chlorides etc.) under a fine control of pH using NaOH or NH4OH
solutions is used for synthesizing the corresponding oxide nanoparticles.
Particle size of the as-precipitated material is strongly dependent on the pH of
the precipitation medium and morlarity of the starting precursors.
Consequently, control over the particle size can be easily achieved. The reaction
and transport rates are affected by the concentration of reactants, temperature,
pH of the solutions and the order in which the reagents are added to the solution
and mixing. The stmcture and crystallinity of the particles can be influenced by
the reaction rates and impurities. Particle morphology is influenced by factors
such as supersaturation, nucleation and growth rates.
In the present investigation a modified version of chemical precipitation
is used. The modified technique is called hydrolysis assisted precipitation
method. Here the required quantity of precursor salts are dissolved in deionized
water and taken in round bottom flak fitted with a Liebig’s condenser. The
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solution is heated to about 120 OC using a mantle heater. The solution thus
vapourized is condensed in the Liebig’s condenser and fall back to the round
bottom flask. This hydrolysis is carried out for many hours (72 hours) to get the
required size distribution. After the completion of the hydrolysis process
ammonia solution is added to the solution to get precipitate. The precipitate is
washed many times with deionized water and then centrifuged. Then it is heated
to 100 OC to remove water and ground using agate mortar and pestle to get fine
nanoparticles [2].
2.2.2. Spray pyrolysis
Spray pyrolysis is one of the relatively simple and cheap methods, which
can easily be adopted for mass production of large — area coatings for industrial
applications. This method has been widely used for the preparation of
transparent conducting oxide films. In the present investigation, we have used
the spray pyrolysis technique for the preparation of CeO2 thin films.
Spray pyrolysis involves a thermally stimulated chemical reaction
between fine droplets of different chemical species. In this technique of film
preparation, a solution (usually aqueous) containing soluble salts of the number
of constituents atoms of the compound is sprayed onto a preheated substrate in
the form of fine droplets by a nozzle sprayer with the help of a carrier gas.
Upon reaching the hot surface, these droplets undergo pyrolytic decomposition
to be formed as a thin film on the substrate surface. The hot substrate provides
the thermal energy for the decomposition and subsequent chemical reaction.
The carrier gas may or may not play an active role in the pyrolytic reaction
process.
According to the previous studies, the decomposition process is the net
result of
a. Spreading of a drop into a disc
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Cliapter-2
b. Pyrolytic reaction between the decomposed reactants
c. Evaporation of the solvent
d. Repetition of the proceeding process with succeeding droplets
Consequently, the film contains overlapping discsj each corresponds to a
single droplet. A schematic diagram of a typical spray pyrolysis set up is
shown in figure 2.1.
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ii.
Figure 2.1: Schematic diagram of spray pyrolysis experimental setup.
The spray pyrolysis experimental setup consists of the following components.
1) Air Compressor
2) Pressure Controller
3) Temperature Controlle:
4) Nozzle for Air
5) Nozzle for Solution
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6) Solution Container
7) Stepper Motor
8) Microprocessor Controller
9) Temperature sensor
10) Heater setup
1 1) Computer
Spray pyrolysis is a simple technique that has become popular owing to
the number of advantages it offers over -the other thin film deposition
techniques. The quality of the spray deposited films depends on the deposition
parameters such as the spray rate, spray interval, spray time, substrate
temperature and ratio of the various constituents in the solutions. Compared to
other thin film deposition techniques, pyrolytic technique has been most widely
used for the deposition of semiconductor films such as zinc oxide, indium oxide
and tin oxide. In comparison with other deposition techniques, this method is
rather easy to use and most economical in terms of equipment cost. In a
properly designed reactor with suitable adjustment of process parameters, the
film deposited is highly reproducible. The spray pyrolysis technique has the
following advantages [3-5]:
0 It is a cost effective technique and the apparatus can be implemented in
the laboratory with greater ease.
0 The film production cost is quite low in comparison to the other thin
film deposition techniques.
0 It does not require high quality targets and vacuum at any stage.
0 Large area coating and high reproducibility are possible.
ffléjibrirnérkfaff/I/31?s,_(1/IA!‘ "
C/iapter~2
I It is possible to change the useful deposition parameters such as flow
rate of carrier gas, solution concentration level of the host and dopant
atoms, substrate temperature, and substrate-nozzle distance, spray angle,
spray rate, spray time and spray interval independently.
0 The thickness of the films and rate of deposition can be easily controlled
over a wide range, simply by controlling the spray parameters.
In the present investigation, the automated spray pyrolysis unit supplied
by Holmarc Opto-mechatronics Ltd, Kochi, India has employed for the
II
preparation of CeO3 thin films. Figure 2.2 depicts the photograph of the spray
pyrolysis unit. The important components of the system are discussed below.
Heater and Temperature Controller:
Resistive heating is achieved with the help of a heater that has heating
coils. The heating coils are wound on a ceramic kettle. The heater is provided
with proper thermal insulation (by means of glass wool) to avoid the thermal
loss in the form of radiation. A stainless steel plate kept on top of the heater is
used as the base for holding the substrates. A digital DTC503 temperature
controller is used for monitoring the substrate temperature with the resolution
of l °C. A Cromel-Alumel thermocouple that is kept on the surface of hot plate
is used as thermal sensor. The thermocouple is placed at the optimized position
in order to make sure that the measured temperature is approximately at par
with that on the substrates surface.
Air Compressor:
An air compressor with the container capacity of 45 litres has been
employed in the present study to supply the carrier gas. A pressure regulator
that is fixed at the outlet of the container is used to control the pressure of
carrier gas, which is a useful deposition parameter of spray pyrolysis technique.
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The outlet of this regulator is connected to air filter to remove the oil content or
the other impurities mixed with the carrier gas. The outlet of the air filter is
connected with an open/close valve, with which the spraying action can be
started or stopped.
Solution Container:
A teflon cylinder with a capacity of 250 ml is used as a solution
container. A tube (ianatics, pneumatic, 10 bar, D25j) is connected to the
cylinder and the other end is connected to the spray nozzle. The solution is
sprayed by the movement of a piston connected to a stepper motor and
controlled by micro-controller. The spray rate is adjusted through software and
can be stored for the future deposition.
Chamber:
A chamber has been designed and fabricated to cover the entire
experimental setup. One side of the chamber is covered with glass doors, which
can be used to operate the setup and as a see through port. All the other sides
are covered with iron sheets.
Deposition process
In this deposition technique, a starting solution, containing metal
precursors, is sprayed by means of a nozzle, assisted by a carrier gas, over a hot
substrate. When the fine droplets arrive at the hot substrate, the compounds in
the precursor decompose to become a new chemical compound in solid state,
which is then deposited on the substrate surface. All other byproducts are in the
gaseous form, and are taken away by the carrier gas. In the present
investigation, CeO2 thin films of various thicknesses were deposited on soda
lime glass substrates using the spray pyrolysis technique at a substrate
temperature of 623 K.
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The selection of the substrate and methods adopted for cleaning the
substrate surface are very important in the formation of thin films with
reproducible properties. Selection of substrates determines the surface
properties and these are -directly or indirectly related to the film formation
stages of adatom, nucleation, interface formation and film growth. The
material, nature, size and surface roughness and cleanliness of the substrate
surface play an important role on the thin film properties such as adhesion, pin
hole density, porosity, film microstnlcture, morphology and mechanical
properties. A well cleaned substrate surface is a pre-requisite for the
preparation of films with reproducible properties. In this studyiamicro glass
slides were chosen as substrates since a film deposited on the glass substrate
shows less dependence of interfacial energy on the film orientation and hence
smooth nucleation for the measurement of optical and electrical properties.
ln general, the contaminants present on the substrate surface can be
broadly classified under two headings such as organic and inorganic. Organic
contaminants can be easily removed by emulsifying the substrate surface with
washing solutions. However, a direct mechanical approach is required for the
removal of inorganic contaminants particularly when they are in particle form.
A number of procedures [4] are available for the substrate cleaning such as
immersing in solvents, ultrasonic cleaning, cleaning with solvents, cleaning by
heating and irradiation, spray cleaning, electrical discharge etc. Amongst the
different procedures, ultrasonic cleaning is one of the best methods. In this
method, ultrasonic waves with high accelerating force separate the
contaminating particles that are stuck to the substrate surface. The systematic
procedures adopted in this study for cleaning the substrates are outlined in the
following steps.
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lnitially, the substrates were wiped with acetone and cotton to
Step l
remove the visible contamination such as air dust.
Step 2
Cleaned thoroughly with industrial detergent in running water.
Step 3
Later, the substrates were subjected to ultrasonic agitation for 30
minutes followed by washing with double distilled water.
Then the substrates were dried in a hot air oven before mounting
Step 4
them on the substrate holder.
2.2.3. Synthesis of polymer/CeO2 nanocomposites
CeO2 nanoparticles are prepared using hydrolysis assisted chemical
precipitation method as described in 2.2.1. Polymer/ CeO2 nanocomposites are
prepared by adding 10 wt % of CeO2 powder into four different polymer
solutions (polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA),
polymethyl methaacrylate (PMMA) and polystyrene (PS) ) in respective
solvents (10 % w/v). The mixtures are then stirred for 2 hours then sonicated
for l0 min [6].
2.3. Structural Characterization
Characterization of nanomaterials and nanostructures has been largely
based on the surface analysis techniques and conventional characterization
methods developed for bulk materials. These include: XRD, TEM, SEM, EDS
and FT-IR. To understand a nanomaterial we must, first, leam about its
structure, i.e., determine the types of atoms that constitute its building blocks
and how these atoms are arranged relative to each other.
2.3.1. X-ray Diffraction
X-ray diffractometry (XRD) is a powerful technique for the
investigation of crystalline matter. Information conceming phase composition,
Depot/menliof/’/iysirs, (1/IA)’ . . . _ _
Cfiapter—2 7 7 7 A _
grain size, intemal lattice strain, preferred crystal orientation etc. can be
obtained. In this work, x-ray diffraction has been used as a method for grain
a '// I
size calculation and strain determination.
9
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5
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Figure 2.2: X-ray diffraction from crystal planes.
As the sample is exposed to a beam of x-rays each atom is the source of
a coherently scattered wave that will interfere in a constructive or destructive
way with the waves from other surrounding atoms. This is shown schematically
in figure 2.2. The Bragg law describes the path difference. The condition for
constructive reflection of the incident radiation is [7]
2d(h;d)SiI1 8 : 71/1
where drhkl) is the distance between atom planes, 9 is the angle of
rt/at
incidence, it is the wavelength of the x-ray, and n is the order of the reflection.
The Bragg law is a consequence of the periodicity of the space lattice. This law
does not refer to the arrangement or basis of atoms associated with each lattice
point. The composition of the basis determines the relative intensity of the
various orders “n” of diffraction from given set of parallel planes. Bragg
reflection can occur only for wavelengths X S 2d, since (n7t/2d) S l. Further,
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SchenTer’s formula is commonly used for determining the mean grain size, D,
in a crystalline material. This quantity is given by
D : 0.92. (2)
B0056
where D is the average dimension of the crystallites normal to the
reflecting planes, B is the crystallite-size breadth defined by |32=B2-b2 in which
B is the FWHM of observed peak, b is the instrumental factor. The Scherrer
method may be used when the effect of internal strain can be neglected. The
lattice constant ‘a’ is calculated using the following equation:
d(2m<1) az (3)
1 _[/12 +18 +18]
2.3.2. Transmission Electron Microscopy (TEM)
TEM is an imaging technique whereby a beam of electrons is focused onto a
specimen causing an enlarged version to appear on a fluorescent screen or layer
of photographic film or to be detected by a CCD camera. Electrons are
generated by a process known as thermionic discharge in the same manner as
the cathode ray tube, or by field emission; they are then accelerated by an
electric field and focused by electric and magnetic fields on to the sample. The
electrons can be focused onto the sample providing a resolution far better than
possible with light microscopes, and with improved depth of vision. Details of
the sample can be enhanced with light microscopy by the use of stains.
Similarly, with electron microscopy, compounds of heavy metals such as
osmium, lead or uranium can be used to selectively deposit in the sample to
enhance structural details, the electrons that remain in the beam can be detected
using a photographic film, or fluorescent screen. Figure 2.3 shows schematic
diagram of TEM.
Dcpdrlment or/1/1y;/(5, (1/54;
Cfiapter-2 _
High voltage
if
Electron gun
t Firstcondenserlens
Condenser aperture
Second condenser lens
Mfiondenser a pertu re
Specimen holder and air-lock
Objective lenses and aperture
Electron beam
Fluorescent screen and camera
Figure 2.3: Schematic diagram of TEM
A particle of mass m moving with a large velocity v behaves like a wave
of wavelength 7t = h/mv. This wave nature of the particle is used in the
construction of an electron microscope. An electron microscope is similar to
that of an optical microscope. In an electron microscope, a beam of electrons is
used in the place of ordinary light and focusing is done by electric and magnetic
fields.
Electron beams are not only capable of providing crystallographic
information about nanoparticle surfaces but also can be used to produce images
of the surface, and they play this role in electron microscopes. In a transmission
electron microscope, the electrons from a source such as an electron gun enter
the sample, are scattered as they pass through it, are focused by an objective
lens, are amplified by a magnifying lens, and finally produce the desired image.
A TEM can form images by the use of selected area electron diffraction
(SAED) aperture located between the objective and projector lenses. The main
i
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part of the electron beam transmitted by the sample consists of electrons that
have not undergone any scattering. The beam also contains electrons that have
lost energy through inelastic scattering with no deviation of their path, and
electrons that have been reflected by various (hkl) crystallographic planes. To
produce what is called a bright-field image, the aperture is inserted so that it
allows only the main undedicated transmitted electron beam to pass. The bright
­
field image is observed at the detector or viewing screen. If the aperture is
positioned to select only one of the beams reflected from a particular (hkl)
plane, it generates a dark —field image at the viewing screen [8].
2.3.3. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is often used for surface analysis
and imaging. The sample is hit by a beam of electrons, from a field emission or
thermionic cathode, accelerated by a voltage of 1 to 30 kV. When the beam of
electrons interacts with the surface of the sample, secondary electrons, back
scattered electrons, and Auger electrons can be collected using different
detectors; as shown in figure 2.4.
Electron beam
X-rays
. Backscattered electrons
Auger electrons
Secondary electrons
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Figure.2.4: Schematic diagram of electron beam interaction with material
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Cfiapter-2 7 __ H _ H L
Scanning electron microscopy can provide information on surface
morphology. Using additional equipment, for example an energy dispersive
spectrometer (EDS-detector), one can obtain elemental infonnation and produce
elemental maps of the sample surface. Upon impinging on the specimen, there
is an interaction between the atoms and electrons in the sample. This interaction
causes various signals to be generated and the most commonly used signals are
those from secondary and elastic back-scattered electrons. The secondary
electrons are electrons of very low energy and thus contain information of only
a few angstroms deep on the surface of the layer. These electrons are then
detected by a detector consisting of a scintillator-photomultiplier combination,
which in tum through the system electronics drive the cathode ray tube. These
images are the ones commonly used in SEM to interpret the morphology of a
sample [9].
2.3.4. Energy-Dispersive X-ray spectroscopy (EDX)
The chemical composition of the grown layers plays an important role in
deciding the electrical and optical properties. A small change in composition
from the desired concentration of elements can lead to drastic variations in the
electro-optical properties of thin films. In the present investigation, Energy
dispersive analysis of X-rays technique is employed to analyze the composition
of nanoparticles and films.
In EDX, high energy (>200 kV) electrons transmitted through sample
sections allow atomic resolution imaging as well as nanometer scale elemental
identification via characteristic X-ray generation. Its characterization capability
is mainly related to the fundamental principle that each element of the periodic
table has a unique atomic structure allowing X-rays that are characteristic of an
element's atomic structure to be uniquely distinguished from each other.
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The basic principle involved in EDX is that X-rays are produced as a
result of the ionization of an atom by high-energy radiation wherein an inner
shell electron is removed. When the ionized atom retums to its ground state, an
electron from a higher energy outer shell fills the vacant inner shell and, in the
process, an amount of energy equal to the potential energy difference between
the two shells is released. This excess energy, which is unique for every atomic
transition, will be emitted by the atom either as an X-ray photon or will be self
absorbed and emitted as an Auger electron. For example, if the K-shell is
ionizedand the ejected K-shell electron is replaced by an electron from the L­
shell, the emitted X-ray is labeled as a characteristic Kat; line. The hole that
exists in the L-shell will be filled by an electron from a higher shell, say the M­
shell, if one exists. This M-L transition may result in the emission of another
X-ray, labeled according to one of the many M-L transitions possible. The
cascade of transitions will continue until the last shell is reached. Thus, in an
atom with many shells, many emissions can result from a single primary
ionization.
The qualitative and quantitative analysis for all elements from carbon
onwards can be done and the detection limits typically 0.1 — 100 wt% for most
elements. Further the multi- element quantitative analysis of materials and
features Z 2 um are realizable using modem EDX. Based on the practical
observation the energy dispersive X-ray spectrometry in the SEM has limits of
approximately 0.1 wt% [9].
2.3.5. Fourier Transform Infra-red Spectroscopy (FTIR)
Molecules and crystals can be thought of as systems of balls (atoms or
ions) connected by springs (chemical bonds). These systems can be set into
vibration, and vibrate with frequencies determined by the mass of the balls
flezzairriiea/TL051?/wig(1/:4.z,4i;§§21¢; ssssssss at 5]]
Cfiapter-2 V
(atomic weight) and by the stiffness of the spring (bond-strength). The
mechanical molecular and crystal vibrations are at very high frequencies, which
is in the infra red regions of the electromagnetic spectrum.
Vibrational spectroscopy involves photons that induce transition
between vibrational states in molecules and solids, typically in the (IR)
frequency range. The energy gaps of many semiconductors are in this same
frequency region and can be studied by IR techniques.
If the impinging IR energy is in resonance with the energy of the
chemical bond in the sample the intensity of the beam is measured before and
after it interacts with the sample. The incident radiation can be detected in
transmission or reflection experiments. The intensity is then plotted as a
function of frequency in the IR spectrum. From the characteristic peaks,
different filnctional groups present in the compound can be identified [10].
2.4. Optical characterization
Optical spectroscopy has been widely used for the characterization on
nanomaterials, and the techniques can be generally categorized into two groups:
absorption and emission spectroscopy and vibrational spectroscopy. The former
determines the electronic structures of atoms, ions, molecules or crystals
through exciting electrons from the ground to excited states (absorption) and
relaxing from the excited to ground states (emission). The vibrational
techniques may be summarized as involving the interactions of photons with
species in a sample via vibrational excitation or de excitation. The vibrational
frequencies provide the infonnation of chemical bonds in the detecting samples.
Semiconductors are distinguished by the nature and mechanisms of the
processes that bring about the absorption or emission of light. Incident light
with photon energies less than the band gap energy Eg passes through the
sample without absorption, and higher-energy photons can raise electrons from
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the valence band to the conduction band, leaving behind holes in the valence
band. The technique of photoluminescence excitation has become a standard
one for obtaining information on the nature of nanostructures such as quantum
dots. One advantage of these techniques is that the optical spectroscopy systems
are inexpensive and have fast data acquisition.
2.4.1. UV-VIS absorption Spectroscopy
Optical absorption studies are necessary for the determination of the
band structure of semiconductors and these measurements are non-destructive.
This technique involves the interaction of light with matter in which the photo­
induced electronic transitions can occur between different bands by the
absorption or emission of radiation at a characteristic frequency. The basic
principle of optical spectroscopy is that electromagnetic radiations from a
source are dispersed and this dispersed light is passed through the sample under
study. A part of the transmitted radiation is studied as a function of incident
wavelength with a suitable detecting system. The optical spectrometer consists
of three main components, a light source with monochromator, sample and
reference port and the detecting system. There are various types of sources,
monochromators, and sample containers and detecting units, the choice of
which depends on the materials under study [1 1]
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Figure.2.5: Double beam UV-visible spectrometer
Cfiapter-((2 pp H pp p
UV-Vis-NIR radiation is a form of energy and a fruitful part of
electromagnetic radiation spectrum ranging from 300 to llO0 nm. When a
beam of UV-Vis-NIR radiation strikes any object it can be absorbed (A),
transmitted (T), scattered (S) and reflected (R). Since some of the energy of the
incident photon is retained in the molecule (or is lost by a non — radiative
process such as collision with another molecule) the emitted photon has less
energy and hence a longer wavelength than the absorbed photon. Surface
roughness, micro cracks, incorporated particulates and impurities act as sources
of scattering. Since the scattering effect is negligibly small, it is generally
omitted. Then we can emphasize the familiar relation as R+T+A=l00%.
Absorption of light by different materials can induce various types of
transitions such as band to band, between sub-bands, between impurity levels
and bands, interactions with free carriers in with a band, resonance due to
vibrational state of lattice and impurities. These lead to the appearance of bands
or absorption peaks in the absorption spectra.
A schematic diagram of a spectrophotometer is shown in figure 2.5. The
transmissions of the thin films were recorded using Shimadzu UV-1700
Spectrophotometer in the wavelength range 300 to 1100 nm in the present
studies.
2.4.2. Diffuse Reflectance Spectroscopy (DRS)
This technique is readily applicable for the characterization of
nanostructures and nanomaterials. The characteristic lines observed in the
reflection spectra of nearly isolated atoms and ions due to transitions between
quantum levels are extremely sharp. As a result, their wavelengths or photon
energies can be determined with great accuracy. The lines are characteristic of a
particular atom or ion and can be used for identification purposes. In solids, the
large degeneracy of the atomic levels is split by interactions into quasi­
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continuous bands (valence and conduction bands), and makes their optical
spectra rather broad. The energy difference between the highest lying valence
(the highest occupied molecular orbital, HOMO) and the lowest lying
conduction (the lowest unoccupied molecular orbital, LUMO) bands is
designated as the fundamental gap.
Diffuse
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Figure 2.6: Schematic diagram showing the diffuse reflection.
Figure 2.6 shows a schematic diagram of processes taking place when a
beam of radiation incident on a solid sample. In a diffuse reflectance spectrum,
the ratio of the light scattered from a thick layer of sample and an ideal non­
absorbing reference sample is measured as a function of wavelength. The
relation between the diffuse reflectance of the sample (Ra), absorption (K) and
scattering (S) coefficients are related by the Schuster-Kubelka-Munk (SKM)
remission function.
The absorption coefficients of the samples were calculated from Munl<­
Kubleka relation, which is useful for compounds that are difficult to analyze in
transmission mode [12].
S ZRDO
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1?-éiorrrrmezrrarr/smr, (mt J}, a a a i i i i i __
Cfiapter-2
R00 is the diffuse reflectivity from an infinitely thick layer of powder, k
the absorption coefficient and s the scattering factor independent of wavelength
for particle sizes larger than the wavelength of light.
The relation(ahv)=A(hv—Eg )” remains valid if the coefficient a is
replaced by k/s. Where Eg is the band gap, v is the frequency, A is a constant
and n can have values l/2, 3/2, 2 and 3 depending up on the mode of inter band
transition i.c. direct allowed, direct forbidden, indirect allowed and indirect
forbidden transition respectively. The direct optical band gap Eg is determined
by extrapolating the curve (hvk/s)2 vs. (hv) to zero absorption (Tauc plot).
Similarly plotting (hvk/s)"'2 as a function of photon energy (hv), and
extrapolating the linear portion of the curve to absorption equal to zero gives
the value of indirect band gap energy.
The study of the optical properties of solid is a powerful tool in
understanding the electronic and atomic structure of solids. Measurement of
optical constants, the refractive index (n), the extinction coefficient (k) and the
absorption coefficient (01) has crucial importance from the point of view of basic
and applied research. This is because, these constants are determined by the
structure and bonding of atoms in the solid. Hence the measurement of these
quantities as a function of wavelength can give valuable information regarding
the structure and bonding. Moreover any application of the materials in optics
or opto-electronics, n & k are the most important quantities.
Commonly used methods for measuring the optical constants are
spectroscopic ellipsometry, simultaneous measurement of transmission and
reflection, and measurement of reflection/transmission only. The spectroscopic
ellipsometry offers a precise determination of optical constants, particularly at
photon energies well above the fi.1l'ld21l’I1€l1t£1l edge. But the method is applicable
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only in the region of wavelength where polarizers and analyzers are available.
Moreover elaborate mathematical calculation is needed. One advantage of this
method is that the material need not be transparent to the radiation used.
Simultaneous measurement of transmission and reflection is the most used
method for the measurement of the optical constants. This method is applicable
in any region of the spectrum if suitable light source and detectors are available
and also if the material is fairly transparent. In the highly absorbing region of
the spectrum of the material, reflectivity measurements are the only available
method.
Intrinsic optical absorption of a single photon across the band gap is the
dominant optical absorption process in a semiconductor. When the energy of
the incident photon (hv) is larger than the band gap energy the excitation of
electrons from the valence band to the empty states of the conduction band
occurs. The light passing through the material is then absorbed and the number
of electron hole pairs generated depends on the number of incident photons
S@(v) (per unit area, unit time and unit energy). The frequency v is related to the
wavelength - by the relation, X: c/v, where c is the velocity of light. The photon
flux S(x,v) decreases exponentially inside the crystal according to the relation
S(x.v) = Soexp (-0111) (5)
where, the absorption coefficient Ot, (ot(v) = 4rtkv/c) is determined by the
absorption process in semiconductors and k is the extinction coefficient. For the
parabolic band structure, the relation between the absorption coefficient ((1) and
the band gap of the material is given by
ahv = A(hv — E9)" (6)
where, n = 1/2 for allowed direct transitions, n = 2 for allowed indirect transitions,
tr:/:11 1 T i
n = 3 for forbidden indirect transitions and n = 3/2 for forbidden direct transitions.
Cliapter-2 jjjjj jg
A is the parameter which depends on the transition probability. The absorption
coefficient can be deduced from the absorption or transmission spectra using the
1 : 106*“ (7)
relation
where, I is the transmitted intensity and I0 is the incident intensity of the light
and t is the thickness of the film. The absorption coefficient on is calculated
from the relation [5]
T I (1 — R)exp(—at) (8)
where T is the transmittance, R is the reflectance and t is the film thickness. In
the case of direct transition, from equation, (OLi'lV)2 will show a linear
dependence on the photon energy (hv). A plot of (othv)2 against hv will be a
straight line and the intercept on energy axis at (othv)2 equal to Zero will give
the band gap energy.
2.4.3. Photoluminescence Spectroscopy (PL)
Luminescence refers to the emission of light by a material through any
process other than black body radiation. The emission of light can result from a
variety of stimulations. In PL, one measures physical and chemical properties
of materials by using photons to induce excited electronic states in the material
system and analyzing the optical emission as these states relax. Typically light
is directed onto the sample for excitation, and the emitted luminescence is
collected by a lens and passed through an optical spectrometer onto a photon
detector. The spectral distribution and time dependence of the emission are
related to electronic transition probabilities within the sample and can be used
to provide qualitative and sometimes, quantitative information about chemical
composition, structure, impurities, kinetic process and energy transfer.
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Sensitivity is one of the strengths of the PL technique, allowing very small
quantities or low concentrations of material to be analyzed.
In PL, a material gains energy by absorbing photon at some wavelength
by promoting an electron from a low to a higher energy level. This may be
described as making a transition from the ground state to an excited state of an
atom or molecule, or from the valence band to conduction band of a
semiconductor crystal. The system then undergoes a non-radiative intemal
relaxation involving interaction with crystalline or molecular vibrational and
rotational modes, and the excited electrons move to a more stable excited level,
such as bottom of the conduction band or lowest vibrational molecular state.
After a characteristic lifetime in the excited state, electron will return to the
ground state. In the luminescent materials some or all of the energy released
during this final transition is in the form of light. The wavelength of the emitted
light is longer than that of the incident light [13].
Photoluminescence study is one of the techniques widely exploited by
the researchers to probe the native defect states in the photoluminescent
materials. The PL is a complementary technique to the absorption spectra. The
emission spectrum contains "fingerprint"- type peaks related to the energy of
each excited level and can be used as a sensitive probe to find impurities and
other defects in semiconductors.
Uses:
> To determine the band gap energy and/or the wavelength of maximum
gain
‘P To determine the composition of ternary or quaternary layers
Y» To determine impurity levels
> To investigate recombination mechanisms
p;e;iakim.é};t.r1ifi?/zygifrs, (mr
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Limitations:
‘P Identification of impurity and defect states depends on their optical
property
> Although PL is a very sensitive probe of radiative levels, one must rely
on secondary evidence to study the states that couple weakly with light
'2‘-> Materials with poor radiative efficiency, such as low quality indirect band
gap semiconductors are difficult to be studied through ordinary PL.
2.5. Thermal characterization
2.5.1. Thermo Gravimetric Analysis (TGA)
TGA is a technique widely used for determining the organic or inorganic
content of various materials. Its basic rule of function is the high precision
measurement of weight gain/loss with increasing temperature under inert or
reactive atmospheres. Each weight change corresponds to physical
(crystallization, phase transformation) or chemical (oxidation, reduction,
reaction) processes that take place by increasing the temperature. The sample is
placed into platinum or alumina pan and along with an empty or standard pan.
Both the pans are placed onto two high precision balances inside a high
temperature oven.
The substance is subjected to a programmed heating and cooling, it
nomially undergoes physical and chemical or mechanical changes. The gain or
loss in weight of a sample as a function of temperature is measured by
thennogravimetry (TG). It is very useful technique for the study of solid-gas
systems [14].
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2.6. Magnetic Characterization
2.6.1. Vibrating Sample Magnetometer (VSM)
The vibrating sample magnetometer (VSM) has become a widely used
instrument for determining magnetic properties of a large variety of materials:
diamagnetic, paramagnetic, ferromagnetic and anti-ferromagnetic. This experimental
technique was invented in 1956 by Simon Foner, a scientist of the MIT. A vibrating
sample magnetometer (VSM) operates on F araday’s Law of induction, which tells us
that a changing magnetic field will produce an electric field. This electric field can be
measured and provides information about the changing magnetic field. A VSM is
used to measure the magnetic behavior of magnetic materials. Using VSM the
hysteresis loop parameters namely saturation magnetization (Ms), coercive field
(Hc), remanence magnetization (Mr) and squareness ratio (Mr/Ms) can be derived.
In a VSM, the sample to be studied is placed in a constant magnetic field. If the
sample is magnetic, this constant magnetic field will magnetize the sample by
aligning the magnetic domains or the individual magnetic spins, with the field. The
stronger the constant field, the larger the magnetization. The magnetic dipole
moment of the sample will create a magnetic field around the sample, sometimes
called the magnetic stray field. As the sample is moved up and down, this magnetic
stray field change as a function of time and can be sensed by as set of pickup coils.
A transducer converts a sinusoidal ac drive signal provided by a circuit located in
the console into a sinusoidal vertical vibration of the sample rod and the sample is
thus made to undergo a sinusoidal motion in a uniform magnetic field. Coils
mounted on the pole pieces of the magnet pick up the signal resulting from the
sample motion. The altemating magnetic field will cause an electric field in the
pickup coil as according to Faradays law of induction, the current will be
proportional to the magnetization of the sample. The greater the magnetization the
greater is the induced current. The induction current is amplified by a trans­
:l_D9ir_iar5l1'rie)1'/0//’/rys/2:, ( l/SA 1']
Cfiapter-2
impedance amplifier and a lock—in amplifier. The various components are
interfaced via a computer.
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Figure.2.7: Block diagram of vibrating sample magnetometer.
Figure 2.7 shows the schematic diagram of a VSM set up. The sample is
fixed to a small sample holder located at the end of a sample rod mounted in a
electromechanical transducer. The transducer is driven by a power amplifier
which itself is driven by an oscillator at a frequency of 90 Hertz. So, the sample
vibrates along the Z axis perpendicular to the magnetizing field. The latter
induced a signal in the pick-up coil system that is fed to a differential amplifier.
The output of the differential amplifier is subsequently fed into a tuned
amplifier and an intemal lock-in amplifier that receives a reference signal
supplied by the oscillator. The output of this lock-in amplifier, or the output of
the magnetometer itself, is a DC signal proportional to the magnetic moment of
the sample being studied. The electromechanical transducer can move along X,
Y and Z directions in order to find the saddle point (which Calibration of the
vibrating sample magnetometer is done by measuring the signal of a pure Ni
standard of known the saturation magnetic moment placed in the saddle point.
g M g _ Eiqreflmentafqécfiniqzws
Using a vibrating sample magnetometer one can measure the DC
magnetic moment as a function of temperature, magnetic field, angle and time.
So, it allows performing susceptibility and magnetization studies. Some of the
most common measurements done are: hysteresis loops, susceptibility or
saturation magnetization as a function of temperature (thermomagnetic
analysis), magnetization curves as a function of angle (anisotropy), and
magnetization as a function of time [15].
In the present studies, Lakeshore (USA) model 7404 vibrating sample
magnetometer is used to determine the ferromagnetism induced in the sample
and coercivity of the sample at different concentration and annealing
temperature.
References
[I] C N R Rao, S R C Vivekchand, K Biswas, A Govindaraj, Synthesis of
inorganic nanomaterials, Dalton Trans. (2007) 3728
[2] A Chandra Bose, P Balaya, P Thangadurai, S Ramasamy, J. Phy.
Chem. Solids 64 (2003) 659
[3] H L I-Iartnagel, A L Dawar, A K Jain, C Jagadish, Semiconducting
Transparent Thin Films, Institute of Physics Publishing Ltd (1995)
[4] L I Maissel, M H Francombe, An Introduction to Thin Films, Gordon
and Breach, New York (1973)
[5] K L Chopra, S R Das, Thin Film Solar Cells, Plenum Press, New York
(1983)
[6] P P Jeeju, A M Sajimol, V G Sreevalsa, S J Varma, S Jayalekshmi, Polym.
Int. 60 (2011) 1263
[7] B D Cullity, Elements of X-ray Diffraction, Addition-Wesley,
Reading, MA (1978)
aepamnenr0/P/iy;/r;,;(0s4r
Cfiapter—2 7 My N
[8] P E J Flewit, R K Wild, Physical methods for material
characterization, Second edition, IOP Publishing, London (2003)
[9] D K Schroder, Semiconductor material and device characterization,
second edition, John Wiley & Sons, New York (1998)
[10] C N R Rao, UV—Visible Spectroscopy Chemical applications, Second
edition, Butterworth Co Publishers, London (1967)
[ll] H H Willard, L L Merret Jr., J D Dean, Instrumental Methods of
Analysis, Affiliated East-West Press, New Delhi (1965)
[I2] L Djellala, A Bougueliab, l-I M Kadi, M Trarib, Sol. Ener. Mater. Solar
Cells 92 (2008) 594
[13] K Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds: part-A; Theory and Application In Inorganic chemistry
Sm edn., John Wiley and sons, New York (l997)
[14] R K Krishnan & A Banerjee, Rev. Sci. Instrum. 70 (1995) 85
ww
055“!-"'00
3
SYNTHESIS. STRUCTURAL AND OPTICAL
CHARACTERIZATION OF CERIUM OXIDE NAIIIOOIIYSTALS
3.1. Introduction
3.2. Experimental techniques
3.3. Results and discussion
3.4. Conclusion
References A
3.1. Introduction
Cen'a is a rare earth oxide that has been attracting a goat deal of interest
due to its unique properties and applications in various fields including
catalysis, semiconductor industry, solid oxide fuel cells etc. [1]. It has a fluorite
cubic structure with a lattice constant of 0.5411 mn at room temperature,
matching closely to silicon (05431 run). It has very high chemical and thermal
stability even at high temperature, which is essential for the applications like
SOI (silicon on insulator) structure. The dielectric constant of bulk CeO2 is
around 26 which makes it a promising material for applications in high density
capacitor devices with small dimensions [2]. Preparation of cerium oxide
powders is by no means a new research subject; however, practical methods are
still needed for synthesizing high quality ultrafine powders with required
characteristics in temis of their size, uniformity, morphology and crystallinity.
Recently, several methods have been developed to synthesize nanoceria
powder, including thermal hydrolysis, co-precipitation process, combustion
method, micro emulsion technique etc. In all these, some special reacting
Cfiapter-3 1 g
conditions, such as high temperature, high pressure, capping agents, expensive
or toxic solvents etc. have been involved. The large scale production of
powders needs to be economically feasible and should not be more complex. So
compared to these methods, precipitation is more attractive due to cheap salt
precursors and simple operation [3]. The solution based routes have been
developed as the most important strategies, with advantages of low
temperatures, simple apparatus and easy control. The hydrolysis behavior of
cerium gives homogeneous compositions, tunable shapes and uniform sizes.
The preparation based on liquid-phase synthesis in aqueous and non aqueous
media gives direct precipitation of hydrated ceria from cerium (Ill) and cerium
(IV) salts in strongly basic solutions. Most of the advanced performances of
CeO;; generally appear at small size in particular less than l0 nm, since in that
regime the distinct properties are strongly size-dependent and would show
significant quantum size effect. Therefore, the priority in tailoring electronic
structures for performance improvements is to find the methodologies that can
be convenient in getting monodispersed CeO; nanoparticles with tunable sizes.
CeO2 nanoparticle growth dynamics is governed by the reaction time, reaction
temperature and molar ratio of reactants. In this chapter the realization of
synthesis of an environmentally friendly and economically efficient way for the
described. "
controlled ceria nanoparticles through aqueous-phase synthesis method is
3.2. Experimental techniques
3.2.1. Synthesis of cerium oxide
Cerium oxide nanociystalline powder samples were prepared by
hydrolysis assisted precipitation method. 0.1M solution of cerium chloride
(CeCl3.7H;O) was taken in a flask fitted with a Liebig’s condenser. The
-*w:i@i:i1_ s ii ifleparlirie/itaffbys/ks, (Z/SA!
Syrttfzesir, _Structura[am{Optica[Cfiaractenlzatiort Cerimn Orirflz Wanocrystalr
solution was heated at 120 OC in an electric oven and evaporated solution was
condensed back to the flask by the reflux condenser. This process of hydrolysis
was carried out for 72 hours. An appropriate amount of ammonium hydroxide
was added to the hydrolyzed solution and the obtained precipitate was washed
many times with doubly distilled water to remove the remaining ammonia
solution and then dried at 100 OC for 1 h [4, 5]. The process can be expressed by
the following chemical equations:
CeCl3+2H2O —> Ce (OH);Cl+2HCl
Ce (OH);_Cl +NH4OH (aq) —> CeO2+H2O+ NH4Cl
3.2.2. Characterizations
XRD investigations were carried out for the as-prepared and annealed
samples prepared under the optimum conditions, using a Rigaku Ultima-III X­
Ray Diffractometer using Cu Kot; radiation in the 29 range from 200 to 800 in
steps of 0.030. JEOL JEM-2010 was used to record the transmission electron
micrograms (TEM). FT-IR spectra of the samples were recorded using Perkin
Elmer FT-IR spectrometer. UV-Vis Diffuse Reflectance Spectra were recorded
using JASCO V570 UV-VIS-NIR Spectrometer. Photoluminescence (PL)
measurements were carried out using a luminescence spectrometer (J obin Yvon
Flourimeter) at room temperature.
a/ut
3.3. Results and discussion
Hydrolysis refers to those reactions of metallic ions with water that
liberate protons and produce hydroxide or oxide solids. The rate of cerium
oxide precipitation will critically depend on the reaction temperature, reaction
time and concentration of precursors. The effect of reaction temperature on size
distribution and morphology of precipitants can be ascribed to the effect of
Cfiapter-3 p pp __ A _
reaction temperature on the precipitation kinetics including nucleation and
growth rate. Both nucleation and growth rate are slow at low temperature, but
nucleation rate is higher than the growth rate, so it facilitate to form smaller
particles. However, growth rate is higher than nucleation rate at high
temperature, thereby the particle becomes larger. The size and size distribution
of particles vary significantly with the type of reagents used in the synthesis.
The optimum conditions for the preparation of cerium oxide nanocrystals are as
follows.
Precursor concentration - 0.1M/L
Hydrolysis time - 72 hours
Hydrolysis temperature - 120 OC
The molarity, hydrolysis duration and hydrolysis temperature are
optimized to the above values after many number of trials. When the molar
concentration was higher than 0.1M the solution is highly viscous and therefore
difficult to boil, while if it is lower, the quantity of powder produced is small.
When the hydrolysis temperature is below I20 OC, it takes long time for boiling
and if it is higher, the quantity of the solution decreases because of escape of
water vapour without condensation in the condenser.
The XRD spectra of the as-prepared and annealed cerium oxide samples
are shown in figure 3.1. The observed broad peaks in the as-prepared samples
indicate the formation of cubic nanocrystalline structure of cerium oxide
sample. The peaks become gradually sharper with increasing annealing
temperature, indicating that the crystallinity of the as-prepared CeO2
nanoparticles is improved by the annealing process [6, 7]. Grain size of the as—
prepared and annealed samples is calculated from XRD data using Scherrer’s
formula [8]
n r t as Fllezidirfive/?tvfP/irt/"H, fl/W
Syntfiesir, Structural and Optical Cfiaractenlzation qf Cerium Oagicfe _7\/anocrystalfs
Average grain
size rm, = , (1)
flcosél
where K is the wavelength of the incident X-rays (l.5406 A) ,8 is Full
Width Half Maximum and 9 the diffraction angle. Table 3.1 shows the variation
of the grain size and lattice parameter with annealing temperature. The particle
size is found to increase with increase in annealing temperature. The lattice
parameter for CeO2 fluorite structure is 5.4113 A (J CPDS 34-0394). The lattice
parameter calculated from XRD data is found to decrease with increase in
annealing temperature. The increase in lattice parameter of the nanoparticles
with decrease in particle size is of great importance and would strongly affect
the properties of CeO2 nanoparticles. As-prepared ceria tends to exhibit higher
lattice parameters due to the presence of defects and impurities. Such defects
are introduced during the sample preparation and vary according to method and
reagents used [9]. Deshpande et al. [10] have studied the relationship between
the crystallite size and lattice parameter of the synthesized ceria nanoparticles.
They have pointed out that the variation in the lattice parameter is attributed to
the lattice strain induced by the introduction of Ce3+ ions due to the formation
of oxygen vacancies.
~
N
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Y‘
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3
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ax
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’+~.3I AA2Q._
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é as-prepared E
\
20 30 40 so so 70 so
26 (degree)
Figure 3.1: XRD patterns of the as-prepared and annealed cerium oxide.
Department of P/rys/'0', (1/5,4!
C/iapter-3
_ I-:;J"’~ J
25 nu
>~p~»~
ii 25 nm
a) as-prepared b) annealed at 600 "C c) annealed at 900 "C
Figure 3.2: TEM images of cerium oxide nanoparticles
The morphology and structure of the ceria nanocrystals are investigated by
TEM. Figure 3.2 shows TEM images of cerium oxide in the as-prepared and
annealed states. Agglomeration of nanoparticles is a very common phenomenon
that occurs because the nanoparticles tend to decrease the exposed surface in order
to lower the surface energy and the smaller particle size results in stronger
agglomeration [1 1]. The increase of particle size with increase in annealing
temperature is also evident from the TEM images. Loose agglomeration of CeO2
nanoparticles is evident in annealed samples, which suggests that particles could be
separated fi'om each other. The inset picture shows the selected area electron
diffraction (SAED) pattem and Debye-Scherrer rings of the nanoparticles, which
can be indexed as those of cerium oxide with the cubic fluorite structure. The ring
pattems obtained in the SAED pattems confinn the polycrystalline nature and the
fluorite structure [12]. The as-prepared sample consists of particles of smaller size
compared with the annealed samples. The as-prepared particles are more or less
spherical in shape, but in the annealed samples the particles appear in different
shapes. These TEM results confirm that grains are nanometer in size and show
good agreement with the XRD results.
m Department off/rys/‘:5, (I/IN
Synt/iesir, StructurafamfOptica[C/iaracterrlzation of Cerium Oagife Wanocrystalk
Table 3.1: Variation of the grain size and lattice parameter with annealing temperature
sample
' TEM XRD TEM
,1
7 , XBD
3 is 4 3 i Grain size (nm) lattice parameter (A) l
_ As-prepared 9.42 10.5 5.428 p 5.395
At 600 "C 12.92 14.1 5.411 5.390
At 900 “C 25.84 27.0 5.410 5.350
Figure 3.3 shows EDX image of cerium oxide in the as-prepared state.
EDX shows the presence of cerium and oxygen. There is no other impurity
peaks in this spectrum.
mo°'
900 -­
800 —­
700 —-*
600 -*
%
O $00 —~
O
400 —­
300 ——
200­
N
°W
r am| ramrsmlcwrrmramIoml mm
om rm am
100 -*
149V
Figure 3.3: EDX of cerium oxide nanoparticles
The FT-IR spectrum in the transmission mode of the as-prepared cerium
oxide sample is presented in figure 3.4. The peaks appearing in that figure are
characteristic of the material structure. The broad transmission band in the first
region (around 3400 cm") is due to the presence of surface water. In ambient
atmosphere the carbonation of ceria is unavoidable. This effect can be identified
in the region below 1800 cm" with several peaks [13, 14]. In the range of
850 cm" to 450 cm" the prominent stretching vibration of Ce-O has been
observed, which confirms the formation of CeO;; [13-1 5].
Department of /’/rys/‘rs, (ll!/if
Cfiapter-3
70
l
,
T
4)ll
4* 1|
>
I00 (l
‘H
90 ­
‘N _
Kl)
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IS­
I0­
i
4il()(l.ll 3(»()ll 3100 Zllllll 2-tilt) Zllllll cm-I
I800 I600 I400 I100 llillil X00 600 -H0 0
Figure 3.4: FT-IR spectrum of cerium oxide nanoparticles
Figure 3.5 shows UV —Vis diffuse reflectance spectrum of cerium oxide.
Cerium oxide shows a strong absorption below 400 mn. The intensity of
absorption peak is more or less the same for the as-prepared as well as the
annealed samples. Due to the increase of particle size with increase in the
armealing temperature, the absorption edge slightly shifis towards longer
wavelength side resulting in a slight decrease in band gap energy [16, 17].
100 ,
_f as-prepared
so -‘
annealed 600 °C
-0-0 3
<<
._..
Q
annealed 900 °C
20-0
­
9200'1400
* r600* 800
r ' 1000
I * I1200
* |1400
* r1600
'I
Wavelength (nm)
Figure 3.5: Diffuse Reflectance spectra of cerium oxide nanoparticles
Department 0//’/zys/‘rs, (054!
Syntliesis, Structurafand'Optica[C/iaractenkation of Cerium OJ(1?[e Wanocrystab
The absorption coefficients of the samples were calculated from Munk­
Kubleka relation, which is usefiol for compounds that are difficult to analyze in
transmission mode [18].
S ZROO
k : (1-Rm)’
R00 is the diffuse reflectivity from an infinitely thick layer of powder, k
the absorption coefficient and s the scattering factor independent of wavelength
for particle sizes larger than the wavelength of light.
Z5
298
/-\
2.0 —
,­|
\-/
4-I
r ey
<­
0.0
-1 +3001 400
* 1500
' I 600
' I '700
| - 800
-1
200
Wavelength (nm)
Figure 3.6: Absorption coefficient of cerium oxide nanoparticles
Figure 3.6. shows the plot of absorption coefficient with wavelength of
cerium oxide nanoparticles. The absorption curve is composed of one large band,
and the maximum absorption is centered on 298 nm. The absorption edge is
located around 390 mn. For CeO2, the fundamental absorption is due to a charge
transfer between the filled 2p (O) orbital and the empty 4f (Ce) orbital, which
corresponds to an experimental band-gap value of 3.16 eV in the present case for
the unannealed sample.
The relation (ahv) = A(hv—Eg )n remains valid if the coefficient or is
replaced by k/s. Where Eg is the band gap, v is the frequency, A is a constant
llepar/mam‘ of P/Iysirs, (1/£4 T
Cliapter-3
and n can have values 1/2, 3/2, 2 and 3 depending up on the mode of inter band
transition i.e. direct allowed, direct forbidden, indirect allowed and indirect
forbidden transition respectively. The direct optical band gap Eg is detennined by
extrapolating the curve (hvk/s)2 vs. (hv) to zero absorption (Tauc plot) as shown in
figure 3.7 (a), which gives the value of the direct band gap Eg = 3.16 eV. Similarly
plotting (hvk/s)'/2 as a function of photon energy (hv), and extrapolating the
linear portion of the curve to absorption equal to zero as shown in figure 3.7
(b), which gives the value of indirect band gap energy Eg = 2.85 eV [20]. Table
3.2 shows the variation of the direct and indirect band gap energy with
annealing temperature. At the outermost CeO2 nanocrystal surface, Ce4+ ions
coexist with Ce3+ iones. When the particle size decreases, the concentration of
Ce“ increases, thereby introducing defect states due to oxygen vacancies,
which results in the decrease in the band gap [17, 21]. However in the present
case the increase in particle size is brought about by thermal annealing in air
and the annealing process converts majority of Ce3+ to Ce4+, which removes the
.._' ‘0.5?
H
K‘
' ' 00 i I 1
defect states and tends to increase the band gap. Therefore in the present
experiment although the particle size changes in the range 10-25 mn due to
annealing, the change in direct and indirect band gap energy is very small.
">\
__
“" ' M
_ '21-54
“
0.075
w~
u
- ~*‘WW“
Tann..|°d6o°.c
"*Ir';;r;';“pT;~"'&
' -~~""
*
.
~ -~- smeared 900 ‘C 2.5 -— __‘__ wooded no we /‘A
<­
:1 o.025— _ /f ‘F
\-I
0'
*-'1.0­
Ir
%
'11.0 +
+3.54.4.0 4.51 5.0+ &Z| 0‘12.5‘ 3.0 3 5
2.5 3.0
(a) Direct band gap (b) Indirect band gap
Figure 3.7: Tauc plot for cerium oxide nanoparticles
. t Department of/’/rys/‘rs, (1/IAI
Syntlieszk, StructurafandOptica[Cfiaractenlzati0n qfCerium Oxide ill/anocrystalr
Table 3.2: Comparison of direct and indirect band gap energies of the cerium oxide
nanoparticles
Samples
H
(av)
(av)
As-prepared 3.16 2.85
Direct-band gap energy l Indirect-band gap energy I
Annealed at 600 “C 2.92 2.30
Annealed at 900 “C 2.85 2.22
The ultra violet radiation reaching the earth’s atmosphere consists of UV
type B sub range (UVB, 290-320 nm) and the UV type A sub range (UVA,
320-400 mn). Theses wavelengths are the major cause of skin cancers. For the
filtration of UVB, nanostructured TiO2 is being used in sunscreen cosmetic
products. But for the filtration of UVA, the availability of suitable inorganic
material is limited. In the field of cosmetic products [19], the need for new
inorganic nontoxic materials, capable of filtering UVA radiations has increased
substantially. In the present investigations with a band gap of 3.16 eV, good
transparency in visible region, and no known toxicity, and strong absorption in
the 300 mn-400 nm region nanostructured ceria can be a promising inorganic
material for UVA filtering in sunscreen cosmetics. Thus from the present
investigations it is proved that ceria-based nanomaterials are suitable candidates
for application in the new generation of inorganic UV filters.
The room temperature photoluminescence (PL) spectra of CeO2 powder
sample are recorded under the excitation wavelength of 325 nrn and are shown in
figure 3.8. The samples are annealed at 300 OC, 600 OC, 700 OC and 900 OC and PL
spectra are recorded under the same excitation wavelength and are also shown in
figure 3.8. In the PL spectra emission peaks located at 371, 442, 452, 469, 483 and
494 can be clearly observed. The emission peak at 371 mn can be related to the
hopping fi'om the localized Ce 4f state to the O 2p valence band. The emission
peaks ranging from 400 nm to 500 mn can be related to the hopping from different
Department of F/rys/rs, [l/I/if
Cliapter-3
defect levels to the O2p level [22]. These weak blue green emissions are possible
due to the surface defects in the CeO2 nanoparticles. A low intense green emission
is also observed at around 530 mn, possibly due to low density of oxygen
vacancies incorporated during the preparation of the Ce()2 sample [21]. The most
intense peak observed at 469 mn can be related to the presence of abundant defect
states, which helps in fast oxygen transportation.
From the figure 3.8, PL intensity is found to increase with increase in
annealing temperature up to 600 OC and thereafter decreases. This can be
understood from self trapping of excitons (STE) in nanocrystalline CeO2. In
cerium oxide, the strong coupling between lattice and electronic part of excitons
would result in the STE. That is, electrons in the valence band are excited to
some localized levels by absorbing incident photons and form small polarons,
and holes in the valence band interact with the polarons to form STEs. On the
basis of the size effect of excitons, it is assumed that the binding energy and the
‘442
469
452
1 483
‘f
0
radiative life time of STEs are strongly related to grain size. The spatial overlap
can
_- .
A 1 annealed at 600 C 494
oi
—
— -i 371 nnealed at 300 °C
T as-prepared
an
r as = ‘ ’1\_z\v A annealed at 700 OC
/ 7 annealed
at ~
900l°C
KT‘:
22 l
F’/T
I
l
*
|
F
359 400 450 500 550
waveIength(nm)
Figure 3.8: PL spectra of the as-prepared and annealed Ceflz nanoparticles
m Department of /’/lys/‘rs, [1/5.47
Syntfzesis, Stmctum[anJOptica[CIiaracterrZatron qfCen'umOJg'zfe Wanocrystais
between the envelop functions of electrons and holes increases with decreasing
grain size, leading to the enhancement of the STE binding energy. The
enhanced STE binding energy may prolong the STE radiative life time. The
dielectric confinement effect also enhances the binding energy because of the
penetration of electric force lines between electrons and holes into the
surrounding medium. The dielectric confinement effect would become stronger
in small sized CeO2, and in the present work the synthesized nanostructured
C€Og has a high dielectric constant value around 76 for l KHZ frequency at
room temperature. The small sized particles can introduce more opportunity for
the electric force lines to penetrate into the surrounding medium like air with
relatively much lower dielectric constant with respect to the CeO2. When CeO;
nanoparticles with high dielectric constant are surrounded by air with very low
dielectric constant, the dielectric confinement effect is certainly remarkable
[23]. Above 600 OC, however PL intensity starts to drop, and almost disappears
for the sample annealed at 900 OC. It is seen from the table 3.1 that the grain
size increases slightly from 9 nm to 12 nm up to 600 OC. Hence the influence of
the grain size on the concentration of STEs is not very significant in the
temperature range from room temperature to 600 OC. However, the dielectric
constant of CeO;; annealed at 600 OC has the highest value which is around 84.
It is observed that PL intensity is maximum for samples annealed at 600 OC and
it decreases for further increase in annealing temperatures. The reason for
highest PL intensity for the sample annealed at 600 OC can be related to the
highest dielectric constant value and the comparatively lower grain size. For the
samples annealed at 900 QC the dielectric constant drastically decreases and
grain size increases to around 26 nm. The dielectric constant sharply decreases
to 31 for the samples annealed at 900 OC. Hence the increase of concentration of
Department of/’/rysirs, (£154 T
Cliapter-3 g A ,,qm,__* g
STE is mainly due to the dielectric confinement effect and, it is only slightly
influence by the change in grain size. This STE phenomenon is observed in
SrTiO3 in which case maximum PL emission is observed for the samples annealed
at 700 OC. The present observation of STE phenomenon in nanostrucutred CeO2,
based on the dependence of PL emission on the annealing temperature is novel
and has not been reported earlier. Figure 3.8 also shows that the spectral lines
remain unchanged and the PL peak positions have no detectable shifts for
various annealing temperatures.
The nanostructured CeO2 having intense PL emission at around 469 nm
hence, can be used as excellent oxygen ion conductor. These nanociystals have
prospects of applications in developing efficient solid oxide fuel cells.
3.4. Conclusion
Nanocrystalline CeO2 powder samples have been prepared by hydrolysis
assisted chemical precipitation method employing cerium chloride and
ammonia as precursors. The prepared samples are air annealed at different
temperatures up to 900 OC. XRD and TEM studies confinn the cubic phase
CeO; nanocrystalline particle formation. Particle size increases with increase in
amiealing temperature and lattice parameter decreases. Strong PL emission has
been observed around 469 nm which is mainly due to the presence of oxygen
vacancies. The dependence of PL emission intensity on the annealing
temperatures is explained on the basis of the self trapped exciton phenomenon.
Though STE mechanism has already been reported in other oxide systems such
as SrTiO3, this is the first time that STE mediated PL emission has been reported
for nanostructured ceria samples synthesized by chemical precipitation techniques.
Ln the present work, the dependence of PL intensity on the annealing temperatures,
Syntfiesis, Struetura[am{Optica[Cfiaracterization 0f(jer1'um Oagicflr Wanocrystals
gives a clear indication of the effect of the dielectric confinement on the formation
of self trapped excitons.
References
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[11
Z Yang, K Zhou, X Liu, Q Tian, D Lu, S Yang, Nanotechnology 18
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[21
M Y Li, Z L Wang, S S Fan, Q T Zhao, G C Xiang, Nuclear
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it E-1
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[13]
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Bueno, J A Varela, E Longo, Mater. Lett. 62 (2008) 4509
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H Chen, H chang, Ceram. Int. 31 (2005) 795
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D Zhang, X Ni, H Zheng, X Zhang, J Song, Solid State Sci. 8
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[18]
L Djellala, A Bougueliab, H M Kadi, M Tralib, Sol. Ener. Mater.
Solar Cells 92 (2008) 594
[19]
L Tmffault, M Ta, T Devers, K Konstantinov, V Harel, C
Simmonard, C Andreazza, I P Nevirkovets, A Pineau, O Veron, J
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[20]
S Sathyamurthy, K J Leonard, R T Dabestani, M Parans
Paranthaman, Nanotechnology 16 (2005) 1960
[21]
S Phoka, P Laokul, E Swatsitang, V Promarak, S Seraphin, S
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.................................. ..
”7I“( ,_§;g;v*p¢mm@aw@n%wna;a%nr
Gaaptér
1!
SYNTHESIS, STRUCTURAL AND
OPTICAL CHARACTERIZATION OF DOPED
CERIUM OXIDE NANOCRYSTALS
4.l. lntroduttion
4.2. Experimental techniques
4.3. Results and distussion
References _
4.4 Conclusion
4.1. Introduction
Cerium oxide is a fluorite-structured material and it does -not show any
crystallographic structural change from room temperature up to its melting point
(2700 QC). Ceria based solid solutions have been widely accepted as promising
electrolytes for intermediate temperature solid oxide fuel cells (SOF C). Synthesis
of ceria-based solid solutions with controllable concentrations of oxygen
vacancies is of great fundamental significance because properties and
applications of ceria-based solid solutions are determined by defect chemistry
related to these concentrations, as well as effects from dopant size and valence.
Ceria shows much improved properties under doping. The fluorite lattice is also
capable of oxygen storage via electron flow between Ce“ ¢>Ce3+ [1]. The key
factor in the design of modified ceria is the choice of doping elements, as well as
their amount introduced. In addition, the preparation method of the powder has
also very strong influence on the homogeneity and stability of the solid solutions
[2]. Ionic conductivity in ceria is closely related to oxygen vacancy formation and
Chapter-4 My __
migration properties. A clear physical picture of the connection between the
choice of a dopant and the improvement of ionic conductivity in ceria is still
lacking [3]. It has been observed that the conductivity of ceria solid solutions
tends to be dependent upon the dopant concentration, and a separate maximum is
exhibited for each dopant. Attempts have been made in the present work to
synthesize solid solutions of metal doped ceria using iron, aluminium and cobalt
as dopants and investigate their structural, optical and magnetic properties. These
details are given in this chapter. However, due to time constraints and the lack of
facilities the dependence of ionic conductivity on dopant concentration couldnot
be investigated in detail.
Nano-crystalline fluorite~like structures of iron, aluminium and cobalt
doped cerium oxide compounds were prepared by chemical precipitation
method. The synthesized sampleswere characterized by XRD, EDX, TEM,
DRS, FT-IR and PL. The effect of thermal annealing on the particle size, lattice
parameter and band gap energy was also investigated.
4.2. Experimental techniques
4.2.1. Synthesis of doped ceria
a. Iron doped ceria
Aqueous solutions of cerium chloride (C€Cl3.7H2O) and iron chloride
(FeCl3) were taken in a flask fitted with a Liebig’s condenser. The mixed
solution was heated at 120 OC in an electric oven and the evaporated solution
was condensed back to the flask by the Liebig’s condenser. This process of
hydrolysis was carried out for 72 hours [4]. An appropriate amount of
ammonium hydroxide was added to the hydrolyzed solution and the obtained
Ii if § § f f flepar/mehto/Phys/Z;"(I/5.4!
_W W Syntliestlt, StructumfanzfOpticafCfiaracterzLzatz'0n of ®0peJ Cerium Oagide Wanocrystafs
precipitate was washed many times with doubly distilled water to remove the
remaining ammonia solution and then dried at 100 0C for 1 h.
b. Aluminium doped ceria
Aluminium doped ceria samples were similarly prepared by mixing 0.1 M
aqueous solution of aluminum chloride with that of cerium chloride. Exactly
similar process as above was adapted and the obtained product was thoroughly
washed and dried [5].
c. Cobalt doped ceria
Cobalt doped ceria samples were prepared adopting an exactly similar
procedure as above using 0.1 M aqueous solutions of cerium chloride and cobalt
chloride. As before an appropriate amount of ammonia solution was added till
precipitation and the obtained precipitate was washed and dried.
4.2.2. Characterizations
X-ray diffraction studies were carried out for the as-prepared and
annealed samples using a Rigaku Ultima-III X-ray diffractometer using Cu Kotl
radiation in the 29 range from 200 to 800 at 30 kV, 20 mA at a scanning rate of
30/min. 11201. JEM-2010 was used to record the TEM images. EDX spectra
were recorded using Jeon JSM- 6390 LA instrument. FT-IR spectra of the
samples were obtained using Perkin Elmer FT-IR spectrometer. UV-Vis
Diffuse Reflectance Spectra were recorded using JASCO V570 UV-VIS-NIR
rrrr
Spectrometer. The photoluminescence emission spectra were taken using J obin
Yvon Flourimeter using Xe lamp as excitation source under an excitation
wavelength of 325 nm.
C/iapter-4 g H H_
4.3. Results and discussion
The XRD patterns of the three different concentrations (5, 10, 15 mol %)
of iron, aluminum and cobalt doped cerium oxide samples in different
amrealing conditions are shown in figure 4.1 (a-i). The broad peaks obtained in
the samples indicate the nanocrystalline structure. It is observed that with
thermal annealing, broadening of the peak decreases; at the same time, the peak
intensity increases, suggesting larger crystallite sizes [4,5]. The characteristic
peaks are confirmed to be corresponding to the cubic fluorite structured CeO;;
(Fm3m, JCPDS file 81-0792).
Figure 4.1 (a - c) show the XRD pattems of iron doped ceria samples in
the as-prepared and annealed conditions. it is seen that all the XRD patterns
correspond to single—phased CeO2 phase which has a fluorite type cubic
structure. They display reflections only from cubic ceria (fluorite structure)
which means that homogeneous cubic ceria can be obtained through doped
ceria also with low concentration of the dopant Fe, in air. When the solid
solution is formed, its structure is corresponding to that of the cubic ceria in
which some of the Ce4+ cations have been substituted by Fe3+ cations. The solid
solution can be formulated as Ce|-,,Fe,,O;-;;, indicating that electrical neutrality is
achieved by the O-vacancy formation mechanism [6]. However, in the XRD
patterns of 15 % iron doped ceria (Figure 4.1 (0)), there is an additional peak
observed at ~35.70 which corresponds to Fe2O3 cubic phase in addition to
CeO2 cubic phase._ It is fonned from unsubstituted iron in the doped sample,
which is quite possible SiI1C€ ionic radii of Ce4+ (0.92/11°) and Fe3+ (O.64A° ) sic
quite different [4].
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Figure 4.1: XRD patterns of iron, aluminum and cobalt doped ceria
The XRD patterns of aluminium and cobalt doped samples (figure 4.l(d-i))
display reflections only from cubic ceria. As described above, in aluminum
doped ceria, some of the Ce“ cations have been substituted by A13 J’ cations and
in cobalt doped ceria some of the Ce“ cations have been substituted by Co2+
cations. So the solid solutions can be formulated as Ce1-xAlxO2-5 and
Ce1-xCoxO2-5, indicating that electrical neutrality is achieved by the O-vacancy
formation mechanism [6].
Grain size of the as-prepared and annealed samples is calculated from
XRD data using Scherrer’s formula [7]
Average grain
size tm, = , (1)
,6 cos6
Department of/’/rys/‘rs, (I/IA)‘
Syntfieszls, StructurafanzfOptica[CfiaractenLzation qf (Dopezf Cerium om 9\/anocrystafr
where 7» is the wavelength of the incident X-rays (l.5406 A) ,6 is Full Width
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Table 4.1: Variation of lattice parameter and grain size with annealing temperature
;.; I lattice parameter (A°)5.42
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Table 4.1 shows the variations of lattice parameter and grain size with
concentrations of iron, aluminium and cobalt and annealing temperature. In all
these samples, the lattice parameter increases with increase in doping
concentrations. The change in the lattice parameter in doped samples is directly
related to the ionic size of the dopants. The ionic sizes of the Ce“, Fe“, A13”
and Cozl are given below:
ca“ (0.92A°)
Fe“ (0.64A0)
A1“ (0.53A°)
Co2+ -----(0.70 A0)
Because of the smaller size of the dopants, there should be considerable
decrease in the lattice parameter of the CeO;; lattice. However, as the
concentration of dopants increases, there is a simultaneous increase in the oxide
ion vacancy and this leads to an increase in the lattice parameter [8]. The
increase of the lattice parameter with doping indicates that the volume of the
cell has increased due to the effective doping.
The grain size decreases with increase in dopant concentrations for all
the three dopants. Basically, the addition of a dopant into a crystalline structure
affects the crystalline growth kinetics. Before inserting itself into the CeO;
structure, the dopants are first located between the grain boundaries and thus
disturb the normal growth of C603 ciystallites [9].
Lattice parameter is found to be decreasing with increase in annealing
temperature because annealing removes the defect states in the samples. Grain
size is found to be increasing with increase in annealing temperature due to the
enhancement of crystallinity. With the increase of annealing temperature, the
growth rate of particles increases more rapidly than the nucleation rate does,
. tflerrr/ni éniafPbysi::..fl/W
Syntfiesir, Structurafam{Optica[Cfiaracterizati0n of (Doped Cerium Oxide Wanocrystalr
and the aggregation trend of particles becomes stronger. Therefore, the average
size of CeO2 particles increases with increase in annealing temperature [10].
The morphology of the doped cerium oxide nanopowder is investigated
by TEM. TEM images of the iron, aluminium and cobalt doped ceria
nanoparticles are shown in figure 4.2 (a-i).[Enlarged TEM images are given in
page no. 145-147]. The'images show that the fine particles are more or less
spherical in shape in iron and aluminum doped ceria and each particle is found
to be an aggregate of nanocrystallites. Basically, nanoparticles have a natural
tendency to agglomerate for two »_.main reasons. The agglomeration is a more
stable configuration from an energetic point of view. Thus, nanoparticles tend
to agglomerate to allow smooth crystallite growth [ll]. The number of these
agglomerates is observed to increase with increase in iron concentration. These
TEM results confirm that grains are nanometer in size and show good
agreement with the XRD results.
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15% cobalt doped ceria shows a rod like character. Actually the
morphology of the products will be changed a lot by adjusting pH value of the
solution. In acid solution, the product becomes spherical shape and the product
can be spherical and rod-like when the solution is neutral, but if the solution is
alkaline, the shape tums rod-like with a ratio of diameter and length about 1:10.
According to Shim'yo’s theory, the formation of rod-like and spherical
crystallites depends on the pH value of the solution in the course of Ce3+/Ce4+
conversion and the reaction mechanism in acid and alkaline solution obeys the
decomposition precipitation mechanism [12]. So the observation of both
spherical and rod like cerium oxide particles in the 15% cobalt doped sample
can be explained using the above theory.
Energy—dispersive x-ray (EDX) images of the doped cerium oxide
nanoaprticles are shown in figure 4.3(a-c). They clearly show the presence of
Ce, O and Fe in iron doped ceria, Ce, O and Al in aluminium doped ceria, Ce,
O and Co in cobalt doped ceria. The synthesized materials are hence pure
without contaminations.
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Figure 4.3: EDX of doped ceria
The FT-IR spectra in the transmission mode of iron, aluminium and
cobalt doped ceria in the as-prepared conditions and in different dopant
concentrations are presented in figure 4.4 (a-c). All the spectra show a strong
peak around 3500 cm" due to O-H bond of water molecules. The carbonation
of ceria is unavoidable. These effects can be clearly seen from bands in between
2000 cm" -1000 cm". In figure 4.4 (a) the peaks corresponding to 492 cm",
504 cm", 506 cm" represent Ce-O vibration mode, 761 cm" represents Fe­
OH, 880 cm" represents Fe-O mode. In figure 4.4(b), the peak corresponding to
499 cm" represents Ce-O mode and 715 cm" represents Al-O mode[l3, 14].
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Deparf/izenfdafl’/1;/sing (I/.I'/If
Cfiapter-4
Figures 4.5 (a-i) show the diffuse reflectance spectra of iron, aluminium
and cobalt doped ceria corresponding to different amiealing temperatures and
doping concentration. The dips in the iron doped ceria suggest the presence of
significant amount of midgap defect states in the system. Earlier studies about
defect states in ceria have shown that it is quite prone to oxygen vacancies,
while it still retains its fluorite structure over a wide range of non stoichiometic
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Figure 4.5: DRS spectra of iron, aluminum and cobalt doped ceria
The absorption coefficients of the samples were calculated from Munk­
Kubleka relation, which is useful for compounds that are difficult to analyze III
transmission mode [16].
Department of/’/rys/2'5, (1/I4 I
Cliapter-4
s 2Rw (2)
k (1 - 12,, )2
R00 is the diffuse reflectivity from an infinitely thick layer of powder, k
the absorption coefficient and s the scattering factor independent of wavelength
for particle sizes larger than the wavelength of light. Figures 4.6 (a-i) show the
plot of absorption coefficient with wavelength of doped ceria corresponding to
various doping concentrations and annealing conditions. The absorption curve
is composed of one large band, whose maximum is located at around 340 nm.
The low intensity band at 270 nm can be ascribed to the intrinsic character of
CeO2. The intensity of the absorption peak decreases with the increase in
particle size. For CeO2, the fundamental absorption is due to a charge transfer
between the filled 2p (O) orbital and the empty 4f (Ce) orbital, which
corresponds to an experimental band-gap value of 3.19 eV for the bulk [9]. Iron
doped ceria absorbs more UV radiation between 200 to 400 mn than pure ceria
sample. The absorption band around 560 nm is due to the presence of (1 Fe2O3.
This absorption feature is a characteristic of the d-d transitions of octahedrally
coordinated Fe3+ ions [17]. Intensity of this band increases with increasing the
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doped ceria with wavelength
Department off/rys/‘rs, (I/I/If
Cfiaprer-4 _ p
concentration of Fe3+ ions. Aluminium and cobalt doped samples also absorb in
the 200 to 400 nm wavelength range. In the case of aluminium and cobalt
doped ceria, intensity of absorbance is more or less same with doping
concentration. With increasing the dopant ion content, the absorption edge
gradually shifts towards higher wavelength.
The relation (ahr/)=A(hv—E, remains valid if the coefficient or is
replaced by k/s. ~Where Eg is the band gap, v is the frequency, A is a constant
and n can have values 1/2, 3/2, 2 and 3 depending up on the mode of inter band
transition i.e. direct allowed, direct forbidden, indirect allowed and indirect
forbidden transition respectively. The direct optical band gap Eg is determined
by extrapolating the curve (hvk/s)2 vs. (hv) to zero absorption (Tauc plot) as
shown in figure 4.7 (a-i). Similarly plotting (hvk/s)‘/2 as a function of photon
energy (hv), and extrapolating the linear portion of the curve to absorption
equal to zero as shown in figure 4.8 (a-i), gives the value of the indirect allowed
transition [18].
Table 4.2 shows a comparison of direct and indirect band gap energies
of the as-prepared nano-crystalline samples with different doping concentration
and annealing temperature. The heavy doping affects the density of states,
canier mobility, absorption, luminescence properties and hence device
properties. Also, heavy doping produces some changes in band structure of
semiconductors. One of the changes is band gap narrowing (BGN) or band gap
shrinkage due to the formation of density of states, and the tails are resulted
from inhomogeneous impurity distribution [19]. Here both direct and indirect
band gap energies are found to decrease with increase in the concentration of
dopants. This is due to the band tailing effect. The impurity distribution in a
i i i i . . . . . . . . . . T i i i4 4 Q9p0r(fl)enf0fP/)y5lZ$- (1/IN
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heavily doped semiconductor is not uniform. The fluctuation of potential
energy causes a spatial dependence of the local density of states.
Macroscopically, it results in a band tail of both conduction band and valence
band [20]. The decreasing trend also indicates that the dopants are incorporated
into CeO2 matrix.
As the annealing temperature increases, the particle size increases and
both direct and indirect band gap energies decrease. So the band gap has an
inverse dependence on the grain size. Electrons are highly localized in
nanoparticles and the interaction between these localized states give rise to the
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cobalt doped ceria nanocrystals.
Department of P/zys/2.9 (l/54/'
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Syntfiesis, .Structura[anz{Optica[Cliaractenkation qf (Doped Cerium Oaqidk Wanocrystals
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cobalt doped ceria nanocrystals
Table 4.2: Variation of direct and indirect band gap energies with annealing temperature
k r1i_sarnpleTaaif A Direct Band gap(eV) Indirect band gapleV)
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Department 0/P/iys/rs, (I151!
Syntfiesis, Structurafam{Optica[Cfiaracten;zationqf{I)0pfr1;Qeriurrr Wanocrystafi
The ultra violet radiation reaching the earth’s atmosphere consists of
UV type B sub range (UVB, 290-320 nm) and the UV type A sub range (UVA,
320-400 nm). Theses radiations are the major cause of skin cancers. For the
filtration of UVB, nanostructured TiO2 is being used in sunscreen cosmetic
products. But for the filtration of UVA, there are no suitable materials. The
need for new materials capable of filtering the UVA radiation has enhanced
extensive research in this direction [9]. With a band gap of 3.16 eV, good
transparency in visible region, and no known toxicity, nanostructurc ceria can
hence be a promising inorganic material for UV filtering applications in
sunscreen cosmetics. Present studies show that nanostructured doped ceria has
peak UV absorption at 320 nm and hence can be used as an efficient UV filter
for the UV A radiations.
The room temperature photoluminescence (PL) spectra of as-prepared
iron, aluminium and cobalt doped CeO2 nanoparticle samples are recorded
under the excitation wavelength of 325 nm and are shown in figure 4.9 (a-c).
The addition of trivalent ions Fe3+ and Al3+ and divalent ion Co2+ introduces
oxygen vacancies in the CeO2 nanoparticles. The majority of luminescence
peaks produced in these doped samples are due to the presence of the oxygen
V2lC2lI1Cl€S.
In these doped ceria emission peaks located at 421, 440, 452, 470, 483,
494 and 533 can be clearly observed. The emission peak at 371 nm can be
related to the hopping from the localized Ce 4f state to the O 2p valence band.
The emission peaks ranging from 400 nm to 500 nm can be related to the
hopping from different defect levels to the O2p level [21]. These weak blue
green emissions are possible due to the surface defects in the C€O2
nanoparticles. A low intense green emission is also observed at around 533 nm,
possibly due to low density of oxygen vacancies incorporated in the sample
ilte;%airr:»en1v/PlirsirsiF1114!i
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[22]. The most intense peak observed at 470 mn can be related to the presence
of abundant defect states, which helps in fast oxygen transportation.
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Figure 4.9: PL spectra of doped ceria
In the aluminium doped ceria, the most intense peak is slightly shifted to
higher wavelength region. In cobalt doped ceria, the peaks are found to be
broadened. A large number of defect peaks can be observed in iron doped ceria
compared to aluminium and cobalt doped PL spectra. The intensity of PL
peaks decreases with concentration of iron due to quenching effect of iron
whereas, it increases with concentration of aluminum and cobalt.
Department 0/P/Iys/"rs, (Z/541'
H Syntfiesis, _Structum[anrfOptz'ca[Cfiaracterzzation qf(I)0per{Cerz'urrr Oagzkfé Wanocrystafs
4.3. Conclusion
Nanocrystalline iron, aluminum and cobalt doped cerium oxide
nanopafticles are prepared by chemical precipitation method using optimum
conditions. The prepared samples are air annealed at 600 DC and 900 OC. XRD
and TEM studies confirm the cubic phase Ce1-,.Fe,.Og-.>;, Ce1-,,Al,.O2-5 and
Ce1_xCo,.O2-,5 nanocrystalline particle formation. Particle size increases with
increase in annealing temperature and lattice parameter decreases. FTIR studies
show the existence of Ce, Fe, A1 and O bonds very clearly. The optical
absorption studies of the doped ceria samples show that the samples can be
used as efficient UV filters for the UV A radiations. One of the highlights of the
present investigation is the identification of doped ceria as a promising non
toxic inorganic material for UV filtering applications in sunscreen cosmetics.
Due to the size effect, both direct and indirect band gap energies are found to
decrease with increase in particle size. Due to the band tailing effect, both direct
and indirect band gap energies are found to decrease with increase in doping
concentrations. PL spectra show the presence of the abundant defect states due
to the addition of di or tri valent cation in ceria lattice.
References
[1] G Li, R L Smith, 11 lnomata, .1. Am. Chem. Soc. 123 (2001) 11091
[2] B Matovic, Z Dohcevic-Mitrovic, M Radovic, Z Brankovic, G Brankovic,
S Boskovic, Z V Popovic, J. Power Sources 193 (2009) 146
[3] D A Anderson, S I Simak, N V Skorodumova, I A Abrikosov, B
Johansson, PNAS 83 (1996) 1
[4] T Dhannia, S J ayalekshmi, M C Santhosh Kumar, T Prasada Rao, A
Chandra Bose, J. Phys. Chem. solids, 71 (2010) 1020
[5] T Dhannia, S J ayalekshmi, M C Santhosh Kumar, T Prasada Rao, A
Chandra Bose, J. Phys. Chem. solids, 70 (2009) 1443
[6] l-I Lv, H Tu, B Zhao, Y Wu, K Hu, Solid State Ionics 177 (2007) 3467
ifiéflarfméfti 1?":I/.’/I)?!/‘II, (1/54 T A
C/iapter-4 pf A f
[7] B D Cullity, S R Stock, in: Elements of X-RAY Diffraction-third ed.,
Prentice Hall, New Jersey (2001)
[8] A Gayen, K R Priolkar, A K Shukla, N Ravishankar, M S Hegde,
Mater. Res. Bull. 40 (2005) 421
[9] L Truffault, M Ta, T Devers, K Konstantinov, V Harel, C Simmonard,
C Andreazza, I P Nevirkovets, A Pineau, O Veron, J Blondeau, Mat.
Res. Bull. 45 (2010) 527
:10] Y 1-1e, B Yang, G Cheng, Mat. Lett. 57 (2003)1880
1 1] C Hu, Z Zhang, H Liu, P Gao, Z L Wang, Nanotechnology 17 (2006) 5983
'12] M Yan, W Wei, N Zuoren, J. Rare earths 25 (2007) 53
ch
_13] K Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds: part-A; Theory and Application in
lnorganichemistry, 5m edn, John Wiley & Sons, New York (1997)
[141 M G Sujana, K 1< Chattopadyay, s Anand, Appl. Surfi Sci. 254 (2008)
7405
[15] A Tiwari, V M Bhosle, S Ramachandran, N Sudhakar, J Narayanan, S
Budak, A Gupta, Appl. Phys. Lett. 88 (2006) 142511
[16] L Djellala, A Bougueliab, I-I M Kadi, M Trarib, Sol. Energy Mat. Solar
cells 92 (2008) 594
[17] C Liu, L Luo, X Lu, Kinetics and Catalysis 49 (2008) 676
[18] S Sathyamurthy, K J Leonard, R T Dabestani, M Parans Paranthaman,
Nanotechnology 16 (2005) 1960
[19] M K Hudait, P Modak, S B Krupanidhi, Mater. Sci. Engg. B 56 (1999)1
[20] Y Pan, M Kleefstra, Semicond. Sci. Tech. 5 (1990)312
[21] S Phoka, P Laokul, E Swatsitang, V Promarak, S Seraphin, S Maensiri,
Mater. Chem. Phys. 115 (2009) 423
[22] P Patsalas, S Logothetidis, Phys. Rev. B 68 (2003) 035104
Gaaflfér
5
THERMAI. AND MAGNETIC PROPERTIES OF
CERIA AND DUPED CERIA NANDCRYSTALS
5.1. lntroduttion
5.2. Experimental techniques
5.3. Results and discussion
5.4. Conclusion
References
1*
5.1. Introduction
Diluted magnetic semiconductors (DMS) are currently being explored with
a strong drive to find room temperature ferromagnetic materials (RTFM) for
possible technological applications. DMS are experimentally obtained by doping
the oxide matrix with a small amount of magnetic transition metal ions such as
Fe3+, Co2+, Mn?” etc. This procedure may introduce ferromagnetism in otherwise
non magnetic materials and open up feasibility of applications in spintronics and
magneto optical devices [l]. Spintronics has vast potential in the storage,
processing, transmission of digital information and optical and magnetic sensors.
Many promising compounds such as: Transition Metals ( TM) doped ZnO,'TiO2,
CeO2 and Hit); have been observed to show RT-FM, but intrinsic origin of the FM
is still under debate. Therefore, these controversial results give an indication that
FM in DMS is very sensitive to preparation methods and preparation conditions.
High cutie temperature (Tc) FM observed in TM doped insulating oxide points
towards a mechanism of oxygen vacancy mediated FM so called F-Centre
exchange coupling (FCE) instead of the carrier induced FM. Since insulating
Cfiapter-5 g W M W H 7
materials have no free carriers, high temperature FM is attributed to the oxygen
vacancies. In FCE mechanism an electron is trapped in oxygen vacancy which acts
as a coupling centre, via which doped magnetic ions align in ferromagnetic order.
However, in most of the magnetic semiconductor doped magnetic ions exhibit very
low solubility in the host matrix and the origin of FM in some of the compounds
has been attributed to the magnetic impurities. Therefore, there is a great need of
the host matrix that has high solubility of magnetic ions to form
thennodynamically stable magnetic semiconductor [2].
Most of the room temperature ferromagnetics -DMS materials discovered
so far have noncubic crystal symmetry. The RTFM in cubic systems will facilitate
the integration of spintronic devices with advanced silicon based microelectronic
devices. Among the materials with cubic symmetry, cerium oxide is an interesting
candidate for advanced multifunctional devices. The recent discovery of RTFM in
pure and/or doped CeO2 nanoparticles and thin films give realization of future
spintronic devices [3].
CeO2 is one of the most important transparent rare earth oxides which are
endowed with high dielectric constant. CeO2 is used in variety of applications such
as a buffer layer for silicon on insulator devices and as a high k- dielectric material
in capacitors. These applications make CeO2 a good matrix to develop FM.
Therefore, the origin of RT-FM in -CeO; makes it a potential candidate for use in
novel multifunctional devices [3]. In the present investigation vibrating sample
magnetometer (VSM) measurements were carried out on ceria and doped ceria
nanocrystals to understand the magnetic properties.
Thermo-gravimetric analysis (TGA) is a type of testing performed on
samples that determines the changes in weight in relation to change in
temperature. TGA is the act of heating a mixture to a high enough temperature
so that some of the components decompose into a gas, which dissociates into
the air. It is a process that utilizes heat and stoichiometry ratios to detemrine the
percent by mass ratio of a solute. TGA is commonly employed in research and
ar,a¢ma;0/1»/iymrr/rir
"1" 5%-S
’I7iemza[amf9\4agnetic properties ofceria anzfdbpecfceria nanocrystali
testing to determine characteristics of materials to determine degradation
temperatures, absorbed moisture content of materials, the level of inorganic and
organic components in materials, decomposition points of explosives, and
solvent residues.
This chapter reports the investigations on the thermal and magnetic
properties of as-prepared cerium oxide and doped cerium oxide nanoparticles with
the variation in doping concentration and annealing conditions. Thermal properties
are investigated using TGA. Magnetic properties are explained based on the room
temperature VSM measurements.
5.2. Experimental techniques
5.2.1. Synthesis
The synthesis of cerium oxide and doped cerium oxide (iron and cobalt)
nanoparticles was carried out by hydrolysis assisted precipitation technique,
which are described in chapter 3 and 4 in detail.
5.2.2. Characterizations
Thermo-gravimetric Analysis (TGA) was done on a SITNT Exstar 6200
TGA/DTA instrument. Samples were heated to 1000 QC at a scan rate of 10 OC
per minute in nitrogen atmosphere. Magnetic measurements were carried using a
Lakeshore 7404 vibrating sample magnetometer (VSM).
5. 3. Results and discussion
Figures 5.1 to 5.3 show the TGA curves of cerium oxide and doped cerium
oxide nanocrystals. The thermal analysis has been carried out from room temperature
to 1000 OC with a heating rate of 10 OC/minute in nitrogen atmosphere. The TGA
curve exhibits very less weight loss upto 250 OC, due to losses of moisture and
trapped cos [4]. Almost no weight loss could be observed from 250 °c to 1000 °c,
suggesting the fomiation of ciystalline CeO2 as a decomposition product, as
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Department of P/rys/‘rs, (1/I/H
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Department of /’//ys/rs, (1/I/if
‘17ierma[ anzf Magnetic properties qfceria amfdbpecfceria nanocrystais
The magnetic properties of CeO2 nanocrystals are evaluated by
measuring the field dependence of magnetization at room temperature. Figure
5.4(a) shows the room temperature magnetization curves of Ce()2 in the as­
prepared and amiealed conditions. In this figure, CeO2 appears to be
diamagnetic. However, on close examination (Figure 5.4 (b)) room temperature
ferromagnetism can be observed in these figures which are characterized by a
closed hysteresis loop. The curves are composed of two parts: at lower field
(Hc< 3000 Oe), the curve exhibits magnetic hysteresis with the remanent
magnetization (Mr) of 0.010 emu/g anda coercivity of 198 Oe, whereas at
higher field (Hc> 3000 Oe), the curve shows diamagnetic behavior. These
results indicate the presence of a ferromagnetic component that is overlapped
with a diamagnetic one for the CeO2 nanocrystals at room temperature. Here,
the preparation method involves only cerium chloride and ammonia, so it does
not involve any magnetic element and therefore the possibility of contamination
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Department of P/))'5/(S, (I/I4!
Cliapter-5
The mechanism of the observed ferromagnetism and its explanations are
very much controversial for undoped cerium oxide system. The FM in these
insulating materials challenges the carrier induced FM mechanism and strongly
support the F center mediated exchange mechanism for diluted ferromagnetism [5].
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Figure 5.4: (h) Hysteresis loops for C002 nanoparticles
The origin of ferromagnetism in CeO2 is assumed to be resulting from
the oxygen vacancies at the surface of the nanoparticles [6]. Ideal CeO2 is Ce3+
free and therefore is diamagnetic because of the absence of magnetic moments
for Xe-and Ne-like closed-shell electronic configuration of Ce“ and O2"
respectively. According to the traditional ferromagnetic theory, the appearance
of ferromagnetism might be correlated to the interaction between the 3d and/or
4f open-shell electrons. So the oxygen vacancies and defects at the
surface/interface dominate the mechanism for room-temperature
ferromagnetism. For very small nanociystals defects may be expelled from the
intemal bulk lattice to the surfaces by a self purification process. These
defective surfaces always show high surface energies and as a result, almost all
Department of P/Iys/cs, (Z/I/If
"Hiernza[am{9Vlagnetz'c properties ofceria arzddbpedceria zzarzocrystafi
nanocrystals have a strong capacity of adsorption species on the surface to
reduce the surface energies. These adsorption species would act as the donors of
electrons or holes. And these oxygen vacancies and defects at the surfaces of
nanoparticles can easily trap the electrons or holes than the bulk vacancies. The
surface/interface atoms may have the feature of low coordination number,
which could also effectively reduce the distance of defects to the effective
magnetic exchange interaction scale. So this magnetic exchange interaction
between the high-spin state vacancies gives rise to the ferromagnetism in
cerium oxide nanocrystals [Z]. The ferromagnetism has been reported as an
intrinsic property in a number of undoped and nonmagnetic insulating oxides.
Actually a deeper insight into complex defect physics is necessary to
understand the ferromagnetic behavior in these systems. From the figure 5.4 it
is seen that the 2lI1I16£1i€d cerium oxide samples show a considerable reduction in
ferromagnetic behavior, which almost become diamagnetic at higher annealing
temperature (900 QC). This can be understood from the fact that the filling up of
the oxygen vacancies due to air annealing, which destroy the ferromagnetic
ordering and hence magnetic exchange interaction in the nanocrystalline
samples [8].
Figure 5.5 (a and b) shows the magnetization curves of 5% and 15% iron
doped CeO2 in the as-prepared and annealed conditions. The hysteresis loops
indicate that all the samples show clear RTFM. In the sample with lower iron
concentration (5%) there is also a linear paramagnetic component in the
unannealed sample, suggesting that not all of the dopant atoms participate in the
FM ordering. The Fe ions enter substitutionally in ceria lattice but certain
amount of Fe ions remain in the interstitial sites too. These isolated Fe ions are
not magnetically ordered, which can bring about paramagnetic behavior [8].
When doping CeO2 with divalent or trivalent ions, an oxygen vacancy is
veprrrmaa/r»rr»rr rt/wt a a a a . .
Cfiapter-5 Y W W _ W
formed to ensure charge neutrality. So the ions that enter substitutionally in
place of Ce4+ in lattice produce oxygen vacancies. These oxygen vacancies are
responsible for the FM behavior in these samples [9].
The key issue is to understand the role of oxygen vacancies in‘ the FM in
doped ceria. The exchange mechanisms that could establish the ferromagnetism
is Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions and/or bound
magnetic polarons (BMP). The former requires a metallic system, whereas the
latter can be observed in semiconducting/insulating materials. There is a sub
category of BMP theory named F-centre exchange (F CE). This mechanism has
been applied to explain the room temperature F M in magnetic insulators [IO].
The FCE mechanism for FM due to the presence of oxygen vacancies
can be summarized as follows. There are three possible charge states of an
oxygen vacancy. These are (a) 132+ center with no trapped electrons, (b) F+
centre with one trapped electron and (e) F0 centre with two trapped electrons. F0
centre charge states are in a singlet (S=0) state and form a shallow donor level
or lie above the conduction band edge. This can only mediate weak anti
ferromagnetic (AFM) exchange between magnetic dopants. In some eases, the
impurity band, which is formed due to the F0 centre states can overlap with the
conduction band of host oxide (4s band). This favors the FM, but its strength is
very weak. In contrast to this, singly occupied vacancies (F+ centre) lie deep in
the gap and favor a strong FM [10].
In the sample with lower iron concentration (5%), the presence of a
diamagnetic component in addition to the ferromagnetic one is observed in the
annealed samples in the magnetization curve. The magnetization value of the
5% iron doped ceria is observed to be less than pure ceria. The reduction in the
magnetization is possibly due to the presence of nearby interstitial Fe ions
which, in the absence of oxygen vacancy, can interact antiferromagnetically by
‘Ifierma[amf9\4agnetic properties Qfce-ria arzJJ0peJcer1'a nanocrystafir
the super exchange interaction. Another possible source of AFM interaction can
be the existence of oxygen vacancies with two trapped electrons. These
vacancies are in a singlet state can mediate weak AFM interaction between
nearby magnetic dopants. From the optical absorption studies it is observed that
there is a peak around 560 nm in all the iron doped C602 samples which
corresponds to the presence of antiferromagnetic phase orFe2O3. The intensity
of this peak is found to increase with increase in doping concentration of iron.
Although the magnetization curves of iron doped ceria samples show clear
RTFM behavior, the curves don-’ t saturate even for a magnetization field of
16000 Oe. The presence of the antiferromagnetic otFe2O3 phase in the iron
doped ceria samples could be the prime reason for the observed unsaturation of
magnetization curves. The magnetic behavior of Fe doped ceria presents a
subtle interplay of the competing magnetic interactions: ferromagnetic FCE
coupling strongly dependent on the valence state of Fe dopant, AFM interaction
and paramagnetism due to isolated Fe ions randomly distributed in host matrix
[8]­
In these samples, every Fe 3+ ions in low spin state (3d5) has one
unpaired electron spin which can participate in ferromagnetic ordering.
Therefore when Fe3+ ions enter substitutionally in ceria lattice besides an
increase in vacancy concentration one can expect more ferromagnetic
Fe3+-V0-Fe“ groups where V0 stands for oxygen vacancy. Thus in the 15%
iron doped ceria the higher concentrations of Fe“-Vo-Fe“ and Ce 3+- Vo- Ce“
complexes are responsible for ferromagnetic coupling and the obseived higher
magnetization, where V‘, stands for oxygen vacancy In this sample, both for the
unannealed and annealed cases (600 and 900 OC) the presence of diamagnetic
component is not observed. The unannealed 15 % iron doped sample shows the
highest magnetization which is around 4 X 10 '2 emu/g. Both the lack of
»@p@rr»»a/raft/irsrrsi (1/W
Cliapter-5
magnetic saturation (increase of slope at high field) and high coercivity (Hc
=ll64 Oe) indicate high magnetic anisotropy in the 15% Fe-doped cerium
oxide nanocrystals [1 1].
0.020
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Figure 5.5: (h) Hysteresis loops for CBo.asFBo.1s02-5
From figure 5.5(b) it is obvious that annealing degrades FM properties.
With annealing the particles grow and surface to volume ratio decreases
followed by decrease in Vo concentration. Hence oxygen vacancy induced FM
Department of P/lys/'(s, (1/Mi
‘Ifierma[and'9‘vlagnetz'c properties QfC8?"!-(1 am{r{oped'ceria nanocrystafs
should be weakened [8]. The remanence magnetization (Mr) changes from
0.014 emu/g to 0.012 emu/g. The coercivity increases with annealing
temperature from l 164 Oe to 1589 Oe. The increase of coercivity by increasing
the calcination temperature can be mainly attributed to the enhancement of
crystallite size within the single domain region.
Figure 5.6 (a and b) shows the magnetization curves of 5% and 15%
cobalt doped CeO;; in the as-prepared and annealed conditions. The hysteresis
loops indicate that all the nanoparticles have clear RTFM. The RTFM in Co
doped samples also can be explained on the basis of FCE mechanism. The Cobalt
ions that entered substitutionally in place of Ce4+ in the lattice produce oxygen
vacancies and FM behaviour. Each oxygen vacancy site easily traps an electron
from adsorption species forming an F centre. The exchange interaction between
the Co2+ ions and singly charged vacancy enables indirect FM coupling known as
F-centre exchange coupling (FCE). Magnetization is not saturated due to the
presence of antiferomagnetic or paramagnetic components [8].
When a divalent or trivalent Co ion is substituted in CeO;, an oxygen
vacancy is naturally formed to ensure charge neutrality. An oxygen vacancy in
CeO2 traps an electron to fomr F-centers. This F-centre with two Co ions
constitutes a Co2+ - Vo- Co2+ group, where V0 denotes the oxygen vacancy. The
electron trapped in the oxygen vacancy occupies an orbital which overlaps the d
shells of both Co ion neighbours. The radius of the electron orbital which
overlaps the d shells of both Co ion neighbours. Based on Hund’s mle and
Pauli’s exclusion principle, spin orientations of the trapped electrons and the
two neighbouring Co ions should be parallel in the same direction, thus
ferromagnetic ordering is achieved [3,l2].
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Co ions are randomly distributed in the CeO2 host matrix and can be
approximately divided into three subsets. One subset of Co is far apart from
other Co ions, thus the spins of these isolated Co ions are completely free and
the magnetization follows a simple PM behavior. In second set of ions, Co ions
have a smaller separation from each other and they are affected by FCE
interactions, leading to a typical FM behavior. Meanwhile, there must also be
some Co ions that have nearest neighbor Co ions (paired Co). The paired Co
ions mediated by one oxygen ion are expected to have super exchange
interaction with nearest neighbor Co ions thus giving rise to an
antifcrromagnetic behavior. When the Co content is small, the first and second
subsets of Co ions dominantly coexist, thus a mixture of PM and FM is
observed. As Co content increases, more Co ions are mediated by FCE
interaction due to smaller distance between Co ions and the increase of
concentration of oxygen vacancies. Therefore an increase of magnetization with
Co content is reasonably observed in the CeO2. With further increase of Co
content, the number of the third subset of Co ions increases and the
antiferromagnetic interactions quickly reduce the FM of Co doped CeO2
powder. Clearly a competition between the magnetic properties resulting from
. r
three subsets of Co ions underlies the complex magnetic behavior in Co doped
ceria nanoparticles [12]. Here magnetization value of Co doped ceria is less
than pure CeO2. This decrease of magnetization value with Co is ascribed by
the name “paired Co”. The paired Co ions via one oxygen ion give rise to an
antiferromagnetic behavior by super exchange interaction. Therefore, there
must be a critical density of both Vo defect and magnetic ions to establish the
percolation threshold for long range FM order [5].
Tliermaf a mf 9vlagnetz'c properties grceria ancfzfopez{cerz'a nanocrystafi
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‘*1
0000
*_.-A
-0008ELt L“"L-‘_‘.‘
‘="":*- ;
‘l rfi - r" - t ' ~ | re 1 - “ | rrfi
-8000 -6000 4000 -2000 0 2000 4000 6000 6000
-0.010 ‘
Magnetic field (Oe}
Figure 5.6: (bl Hysteresis loops for C9o.a5[§D0.15U2-5
The M-H curve of the 900 0C annealed sample shows a well defined
hysteresis loop with a coercive field of e200 Oe and magnetisation of
Ms-0.07 emu/ g. On annealing FM increases and saturation magnetization is
observed in Co doped ceria due to high crystallinity and lower surfacedisorder.
Annealing provides thermal energy and crystal lattice reconfinmation happens
to form high quality nanocrystals. So FM increases because of lowering of
surface spin disorder of larger magnetic nanoparticles [13, 14].
Zzlepor‘//nemf 'ofP/lyrics; (1,/5.4!?
Cfiapter-5 i g g j
5.4. Conclusion
Room temperature ferromagnetism (RTFM) has been obsen/ed in
nanocrystalline pure CeO2‘ iron doped CeO2 and cobalt doped CeO2 powders,
which are synthesised using a surfactant free, simple hydrolysis assisted
chemical precipitation technique with cerium chloride and ammonia as
precursors. The shape of the hysteresis loops indicates the presence of a
ferromagnetic component that is overlapped with a diamagnetic one for the
CeO2 nanocrystals at room temperature. The origin of room temperature
ferromagnetism in CeO2 can be ascribed to the exchange interactions between
localized electron spin moments resulting from the electrons trapped at the
oxygen vacancies at the surface of the nanoparticles (FCE). The width of the
hysteresis loops and coercivity decrease with increase in annealing temperature
due to the decrease of oxygen vacancies upon annealing. The hysteresis loops in
pure ceria show saturation behavior for both the as-prepared and annealed
samples. The sample with lower iron concentration (5%) shows the presence of
a diamagnetic component in addition to the ferromagnetic one in the
magnetization curve. The reduction of magnetization in the 5% iron doped
sample compared to pure ceria can be a consequence of the interplay of the
competing magnetic interactions which are (1) the ferromagnetic FCE coupling
which strongly depends on the valence state of Fe ions (2) the AF M interactions
due to the isolated Fe ions randomly distributed in the host cerium oxide matrix
and (3) paramagnetism due to the presence of ot Fe2O3. Annealing the samples
at 600 0C and 900 OC degrades the FM behavior in both the iron doped samples
because of the decrease of oxygen vacancy concentration. In the 15% iron doped
samples, the higher concentrations of Fe3+-Vo-Fe3+ and Ce3+-Vo- Ce3+
complexes are responsible for ferromagnetic coupling and the observed higher
magnetization, where V0 stands for oxygen vacancy, In the Co doped samples
- --P?l??4m??Pnf‘iiiliiiiifl‘-(1/541
‘I7ierma[and'9l/la(gnetz'c properties ofceria and'r[oper{cen'a nanocrystair
(both for 5% and 15%) the presence of a diamagnetic component in addition to
the ferromagnetic one is observed in the magnetization curve of the unannealed
samples. The magnetisation value of cobalt doped ceria is observed to be less
than pure ceria due to the presence of “paired Co”~. The paired Co ions mediated
by one oxygen ion give rise to an antiferromagnetic behaviour by super
exchange interaction. In the 15% cobalt doped ceria the magnetization is less
than that in the 5% Co doped ceria due to higher concentrations of paired cobalt
ions and in both cases magnetization is less than pure ceria. In these samples,
however magnetization increases with increase in annealing temperature due to
lower surface spin disorder arising from better crystallinity of larger particles
formed as a result of annealing. Thennal properties of pure ceria and transition
metals (Fe and Co) doped ceria nanocrystals show negligibly small mass loss
up to 1000 OC which establishes the excellent thermal stability of both the pure
and doped ceria.
References
[1] T Fukumura, Y Yamada, H Toyosaki, T Hasegawa, H Koinuma, M
Kawasaki, Appl. Surf. Sci. 223 (2004) 62
[2] S Kumar, Y J Kim, B H Koo, H Choi, C G.Lee, IEEE Transactions on
Magnetic 45 (2009) 2439
[3] A Tiwari, V M Bhosle, S Ramachandran, N Sudhakar, J Narayanan, S
Budak, A Gupta, Appl. Phys. Lett. 88 (2006) 14251l
[4] S Phoka, P Laokul, E Swatsitang, V Promarak, S Seraphin, S Maensiri,
Mater. Chem. Phys. 115 (2009) 423
[5] Q Wen, H Zhang, Y Song, Q Yang, H Zhu, IEEE Transactions on
Magnetic 44 (2008) 2704
fiepar/mentof/’/lyrics, (I/£47 4 3&1
Cfiapter-5 Q A N Q Q
[6] A Sudaresan, R Bhargavi, N Rangarajan, U Siddesh, C N R Rao, Phys.
Rev. B 74 (2006) 161306 (R)
[7] X Chen, G Li, Y Su, X Qiu, L Li, Z Zou, Nanotechnology 20 (2009)
' 115606
[8] Z D Dohcevic-Mitrovic, N Paunovic, M Radovic, Z V Popovic, B
Matovic, B Cekic, V Ivaanovski, Appl. Phys. Lett. 96 (2010) 203104
[9] P C A Brito, D A A Santos, J G S Duque, M A Macedo, Physica B 405
(2010) 1821
[10] L R Shah, B Ali, H Zhu, W G Wang, Y Q Song, H W Zhang, S I Shah,
J Q Xiao, J. Phys. Cond. Matt. 21 (2009) 486004
[11] D Peddis, N Yaacoub, M Fenetti, A Martinelli, G Piccaluga, A
Musinu, C Cannas, G Navarra, J M Greneche, D Fiorani, J.Phys:
Condens. Matter 23 (2001) 426004
[12] Q Wen, H Zhang, Y Song, Q Yang, H Zhu, J Q Xiao, J. Phys: Cond.
Matt. 19 (2007) 246205
[13] D Gao, Z Zhang, J Fu, Y Xu, J Qi, D Xue, J. Appl. Phy. 105
(2009)l 13928
[14] M P Morales, S Veintemillas, M I Montero, C J Serna, Chem.Mater 11
(1999) 3058
.................................. ..
atiaptep
6
PREPARATION or cram THIN FILMS BY SPRAY
_ (((( PYBOLYSIS mun mun CHABACTERIZATIDNS
1 6.1. lintroiliirtion
6.2. Experimental techniques
6.3. Results and distussion
6.4. fionclusion
References
6.1. Introduction
Thin film science has received tremendous attention in recent years
because of numerous applications of films in diverse fields such as electronic
industries, space science, solar energy utilization, optoelectronics,
superconductivity, sensors and microelectronics. The use of materials in many
technological applications has been realized through the use of thin films of
these materials. Thus the development of any new material will have good
application potential if it can be deposited in thin film form with the same
properties. The advantageous properties of thin films include the possibility of
tailoring the properties according to film thickness, small mass of the materials
involved and high surface to volume ratio. Ceria films have been used for
applications in optoelectronics, electrochromics, higher storage capacitor
devices, UV blocking filters, silicon on insulator structures for microelectronics
etc. Celia films are very promising as buffer layers for high temperature
superconductors because of their close match and similar thermal coefficients.
The aim of the present work is to investigate the possibility of production of
Cfiapter-6 g p W
high quality ceria films on glass substrates by spray pyrolysis method at lower
substrate temperature.
Spray pyrolysis is an attractive thin film preparation method because it is
inexpensive, easy when doping is required and can under optimal conditions,
produce good quality films [1]. The optical properties of oxide films, such as
transmittance and band gap energy, are critically dependent on the film
thickness, its corresponding microstructure and surface morphology.
This chapter deals with the deposition of CeO2 nanocrystalline thin films
on soda lime glass substrates using spray pyrolysis method. The effect of film
thickness on the microstructure, surface morphology, optical properties and
magnetic properties has been investigated in detail. The search for RTFM in
spray pyrolysed CeO; thin films also forms an integral part of the present
investigations.
6.2. Experimental techniques
6.2.1. Preparation method
The spray pyrolysis is a cost effective and simple method for the deposition
of thin films of metallic oxides. In this deposition technique, a starting solution,
containing metal precursors, is sprayed by means of a nozzle, assisted by a carrier
gas, over a hot substrate. When the fine droplets arrive at the hot substrate, the
compounds in the precursor decompose to become a new chemical compound in
solid state, which is then deposited on the substrate surface. All other byproducts
are in the gaseous form, and are taken away by the carrier gas. In the present
investigation, CeO;; thin films of various thicknesses were deposited on soda lime
glass substrates using the spray pyrolysis technique at a substrate temperature of
623 K. The substrates were cleaned in running water with an industrial detergent
solution followed by acetone and deionized water. The substrates were finally
cleaned with an ultrasonic bath for 30 minutes and dried in hot air oven. A solution
of 0.1 M CeCl;.7H;O was used as a precursor, prepared by dissolving the salt in
. . . . . s sis H/W
Q>reparatz'on 0fCen'a ‘Iliin Tifms Fry Spray G§1r0@s1's azzJ‘17iez'r (,‘fiaracten'zat1'0ns
deionized water. The nozzle was at a distance of 20 cm from the substrate during
deposition. The solution flow rate was held constant at 3 ml/min. Air was used as
the carrier gas, at the pressure of 2 bar. When aerosol droplets arrive closer to the
heated substrates, a pyrolytic process occurs and highly adherent CeO2 thin films
were deposited. Samples with different thickness were deposited under the same
preparation conditions, only varying the deposition time. The films were deposited
using Holmarc (India) automated spray pyrolysis unit.
6.2.2. Characterization techniques
The XRD pattems were recorded using Rigaku D/Max X-ray
diffractometer. The optical measurements of the C603 thin films were carried
out at room temperature using Shimadzu UV-1700 Spectrophotometer in the
wavelength range 300 to 1100 nm. The thickness was measured using Stylus
profile meter. Photoluminescence measurements were canied out using a
luminescence spectrometer (Jobin Yvon Flouzimeter) at room temperature.
Magnetic properties of films were investigated using Lakeshore 7404 vibrating
sample magnetometer (VSM)
6.3. Results and discussion
In the present work CeO2 thin films were deposited at various substrate
temperatures and conditions to optimize the deposition conditions. All
temperature below 573 K yielded powdery non-adherent films. When the
substrate temperature was at 573 K adherent films could be deposited. But these
films were found to be amorphous in nature. At a substrate temperature of 623
K (350 OC) highly crystalline thin films could be deposited by optimizing other
deposition conditions like carrier gas pressure and flow rate. Figure 6.1 shows
the XRD pattem of the CeO2 thin films deposited at 573 K and 623 K. The
films deposited at 573 K shows almost an amorphous nature from the XRD
pattem. These films were whitish-yellow in color with very poor optical
transmittance. The samples deposited at 623 K show the characteristic peaks at 26
= 28.48, 32.96, 48.26 and 56.38, which are in good agreement with JCPDS file
Department 0/flbyrzrs, _ (ls/5.4iI's 4 f
Cfzapter-6
No.81-0792. The X-Ray Diffraction pattems of the as prepared cerium oxide thin
films with different thickness are shown in figure 6.2.
I
1%
gr
Q-I
.—
tip .
digit!
WWvMI‘IW"-iWI'~M*rr~w.-p
‘.4
1
!- . . I _ 57: K I
‘ ""9 ..
} I ¥w*@“N%dw#wMfi%”h%»d*hhh‘§$Qhfiww
go ' 3'0 ' -do ' —s'o ' “sic: ' flridw —a'd—f_eo
20 (degree)
I;
Figure 5.1: XRD patterns of the as-prepared cerium oxide thin films at 573 K and 623 K.
J (200) 1251 nm
T (111)fi
"I J"" ti
*5 '1 (220) (311) 1
}*i"‘I""“" rrf M (400) 3
\‘l‘I"‘N%-:1 \7t"n|lIlr1I*)|\'t"‘II\I'F*\»Iiwa‘nI\4q,,n».,,,;;,i.
-1 I I I I“ 1 I I * I
r "“"‘ ' ' ' '"*' “"""""I"""""' """' 1" ' ' ' 7' ' ' ' ' ' "
i
1
1589 nm 1
2
um"
.._,,tp\rJt;;r’I't
I ' I ' I ' I ' I ' J-1
Y
i
_ Fm Hi 672 nrn
r I501801701B0I
20*30I40
\N"“""i"’I'*’*‘»~t»I*i"“-r\*1\\rtt~+~v#i»~»t~.r.,.r
20 {degree}
Figure 6.2: XRD patterns of the as-prepared cerium oxide thin films with
different thicknesses at 623 K substrate temperature.
mi": iiiiiiiiiii if
as I 3?”/1)’!/?$: (wt
A _Q’reparfatz'on 0fCerz'a "Min Tilirzs Spray Q§1r0()mLs am{‘Uieir Cfiaracterizations
.-_l
To describe the preferred orientation, the texture coefficient TC (h k l),
was calculated using the following expression
1 (mu-
)
L W1
TC (hk1)[ ["";’l ] ()
“i '1 2 I r(Hd A
where I(hkl) is the measured relative intensity of a plane (hkl), I0(hkl) is
the standard intensity of a plane (hkl) taken from the JCPDS data and n is the
number of diffraction peaks considered in the calculation [2]. It is clear from
the definition that the deviation of texture coefficient from unity implies the
film growth in preferred orientation. Texture coefficients calculated for (111),
(200) and (220) planes are shown in the table 6.1. The higher value of TC
indicates the preferred orientation of the films along that diffraction plane. This
means that the increase in preferred orientation is associated with the increased
number of grains along that plane [2]. The high value of TC along (200) plane
indicates the maximum preferred orientation of the films along the (200)
diffraction plane.
Table 6.1: Texture coefficient calculated for thin films of various thicknesses
572
-- lm
i 1.53i0.46
11757
,158900.42
.
0.54 2.03
I 1.91
0.54
” 0.53 C
.
. ._ l , .
:
According Baeur [3], there are two possible mechanisms; orientation due
to nucleation and final growth orientation. Both of these result from the
nucleation at the film/substrate interface. The initial orientation is favoured on
smooth surface with the tendency of nuclei to develop a minimum free energy
Cliqpter-6 up p _ p
configuration. The final growth orientation results from survival of nuclei
having an energetically unstable plane parallel to the substrate surface among
randomly oriented nuclei because of their different growth rates. This means,
that, the growth orientation is developed into one crystallographic direction
which is of the lowest surface energy. Then, the grains became larger as the
film grows with lower surface energy density [3].
The XRD pattern of lower thickness film shows a random crystal
orientation comparable to that of standard powder diffraction pattem of bulk
material. lt can be observed that the lower thickness film exhibits poor crystallinity.
As the thickness increases, the peak intensity increases and become narrower
indicating a better crystallinity. It is not the result of structural changes in the films,
but the result of increase in the film thickness [4].
Grain sizes are calculated from XRD data using Scherrer’s formula as
given below [5].
. . . /1
Average grain size rm, = —9~2—— (2)
' ,6 cos 6
where 7t is the wavelength of the incident X-rays (0.15406 nm), [3 the FWHM
and 6 the diffraction angle. The calculated particle sizes and lattice parameters
of the as-prepared ceria films from XRD spectra are shown in table 6.2. The
grain size is found to change dramatically with film thickness. The average
grain size increases with film thickness. As deposition time increases, the
amount of solute reaching the surface of the substrate increases to form film and
therefore the electrostatic interaction between solute particles becomes larger
thereby increasing the probability of more solute particles to be gathered
together forming a grain. Thus as the thickness increases, grain size also
increases [4]. In order to understand the composition and presents of impurities
i i i 111" s if s s A o o o , , r r , s i at-pmepr off/rys/‘rs. (aw
s
S
L
t'"j
a
2
E
1
2s 55
Qreparation offe-ria ‘Train ‘Fr'[ms 6) $pray Q§yr0{ys1's and'77iez'r Cfiaracterizations
in the sample EDX spectra were recorded. Figure 6.3 shows the EDX spectra of
cena thin films. The EDX spectra show the presence of cerium and oxygen,
confirming the purity of the films formed.
:l__
2
an
1
­
F
'19-‘
W4
é
2
2
w*~+
-5
no.6
g
.
‘
Ar
.
2m--=1
~<-s
=
?
1
M5 .' <4;-*. 1
Table 6.2: Variation of grain size and lattice parameter of Cell: thin film
672
21
5.37
1589
32
5.37
! 1757 . 45 5.37 i
_ _._ ..... ........_................
. "53
M
_‘ ~~~ ~~~~ ' ' ' " 7 -7 - 7 7 7_ 7 7 7_;~_~>_~-;~;>;;-3;pg-Q'_;¢¥~y-;¢_~+q_ Q - I I - -Ii
i»~=~;
-‘E
1
é
;41
_
:
\
'
T3
V
' 2.=21I;z
"“ r-I -4»:
=i Iw 5rH-@@
H i=1
53
:
At: ~u
5
i-7 li|'
i
'-5' ht-'
~'>
A
3
Q‘ .4
.€3
-9-:I11
, ,-___,-,-s...-.-....}..=,-e~,-,..1...._.'.
...
I_ _Q
$3
I I!"N tr all 11!! H‘!
-1
' II
JI -I"-‘
v )-*~*};.7.:,:,-1-;*:,~
an
30- ‘mi on as in =1» *1» H“ PM
4“. Q “C~ ~ ‘ *5 _ Y . —.“. . -;.;1_;. . _. -:Iq>-\.----w—-f~---- - - i~ ----- - ------- 4
la) 672 nm lb) 1589 nm
19¢ \ -7---,~;~_>-----= e 7 ~~ ~ -- <7? H .
I
.1!
M)
fl
iIv- L. - f
Q
.. _““ ' 7 ~_~: '.;~_~--.--.--r-Q---.-g.-~
$
bl?
(c) 1757 nm
Figure 6.3: EDX of ceria thin films
swims»iii»/rymzz/:4r i 1 . . _ - ­
Cliapter-6
Scanning electron micrograms (SEM) were recorded for the CeO2 thin
films with different thicknesses. The micrographs (figure 6.4) show uniform grain
distribution when the film thickness is 672 mn. These micrographs show that the
obtained films have good adherence to the substrates. As the film thickness
increases, more growth occurs on the surface of the films and some of these grains
grow as cubic crystallites on the surface of the films. The SEM results are in good
agreement with XRD results and the change in crystalline size with film thickness
is well in agreement with that found in other oxides [4].
a) 672 nm b) 1589 nm
c) 1757 nm
Figure 6.4: SEM images of ceria thin films
G’reparation qfCeria ’I7iin Tifms 5y Spray Qfyr0[ysis anr{TIieir Cliaracterizations
Accurate knowledge of the absorption coefficient, optical band gap and
refractive index of semiconductors is indispensable for the design and analysis of
various optical and optoelectronic devices. It is possible to determine direct and
indirect transitions occurring in the materials by optical absorption spectra. The
transmittance data can be analyzed to d€lZ€ITI1lI16 optical constants such as refractive
index, extinction coeflicient and dielectric constant [6]. The optical properties of
CeO2 thin films are determined from transmission and reflection measurements
conducted in the wavelength range of 300 nm to 1100 mn at room temperature.
The variation of transmittance and reflectance of cerium oxide thin films of
different thickness with wavelength is shown in figure 6.5. The sharp absorption
edge for these films is around 350 mn in ultra violet region. CeO2 films of
thickness 672 mn exhibits a high transparency (above 80 %) in the visible region.
The high transmittance is probably due to the existence of an interfacial layer with
low refractive index between CeO2 and glass [7]. The transmittance found to
decrease with the increase of film thickness. This is attributed to the increase in
film thickness, which subsequently increases the absorption.
100
_ 672 nmd “WM ________
158§_nn1’_ _____ _ '
it /’ ,/ 1151 nm_ M ..__
;'. ,1
/
]1 '
I’
4
1
‘ _7 ;
l .1 1‘;
L" ,9
25 -I gr
,.»-Y; 612 nm
. . /"
4" J‘; "--_. ___~—‘_f-------~--—-—j-'_ T__-~-___
J ‘\_ *:‘:_-“TT."‘f
1 issss11s1sr+e+; 0 (R)
0200-1
| I |0000 1000
| 1 |1200
||
400| 600
Wavelength (nm)
Figure 6.5: Variation of transmission and reflection of ceria thin film with wavelength
flepar/mam‘ of/’llys/'rs, (1/I/If
Cliapter-6
The transmission decreases sharply near the ultraviolet region due to the
band gap absorption. Additionally, a shifl of the absorption edge, proportional
to the thickness values, towards lower energies is observed fi'om the spectra.
Absorption coefficient ot is calculated from the transmission and reflection data
using the following relation
a=%ln(!_TR) (3)
where “t” is the film thickness and “T” is the transmittance and “R” is the
°_l
.
'7 - s Wm“
reflectance of the film [3]. The variation of absorption coefficient with
wavelength is displayed in figure 6.6. Absorption coefficient is found to
decrease with increase in wavelength.
1.8x10 _ ‘ ___6.,2nm
- 1sa9 i
A1"6"1°J" ‘* -I-1157:2115
%
.4x1o"°­
I-\
‘.4 1.2x1o'° ­
¢-I
A
­ .ox1o" - ‘
-_
01:10‘ — ‘
_. 9A
A‘
A
-.= e.ox1o‘ - '
_..
"
0-0
1 - 400
t - I450
- 1500I
300 350
.ox1o‘ ­
1
2.0110‘
A‘.
L“‘ IA ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ l Q A AA4
_ ‘UFO!!! I I'.|ll
I O I Q 0 0It
Wavelength (nm)
Figure 6.6: Variation of absorption coefficient (oz) with wavelength for ceria thin film
In the high absorption region Tauc [8] and Davis and Mott [9] have shown
that the absorption coefficient (on) and photon energy (hv) are related by the equation.
ahv=A(hv—Eg)” (4)
where A is a constant and Eg is the band gap of the material and n has values
l/2, 3/2, 2 and 3 depending on the optical absorption process. Here n = 1/2 is
0eparfmentYafl’lrySit.$;@(I/Ill’
G?reparati0n qfCeria ‘Iliin Tifms 6_y Spray Q§1r0(ysrlr anJ‘17ieir (,‘/iaracterrlzations
used for obtaining the direct band gap and n = 2 for indirect band gap. The
optical gap Eg is determined by extrapolating the curve (o1hu)2 vs. ho (Tauc
plots) to zero absorption, which gives the value of the direct band gap. Plotting
(orhu)'/2 as a function of photon energy (hu), and extrapolating the linear portion
of the curve to absorption equal to zero gives the value of the indirect band
gap. The Tauc plots are shown in figures 6.7. The variation of direct and
indirect bang gap values with film thickness is shown in table 6.3. Both the
direct and indirect band gap energies are found to decrease with film thickness.
The reduction in the band gap of the as deposited samples is due to the variation
of film thickness. The variation in band gap can be explained using size effect
in thin films. The variation of band gap with thickness [10] is given by
AE=§;.T,2' <5)
h
_
>
/
>
PI
A
'
I 1,,/ /
mo 3.0110’
mo"
1a=1o'
.
O
-| ~I—672nm
572 ""1
6flo,,_L
I £89 nrn I:0,'3"z' 1589
nmr
_ ‘ Q 1 2:10’
-—
'5 9 0:10’ - /
where m* is the effective mass of electron in the material, t the thickness of the film. »
5
1o“‘
u EI/
"A 3110‘ —
O
Q
__ _-_5',_"I“_ f ,I1.5110 ~ ~ 1757 nm;
­
2=1o"- :2 6.011110‘-— / % /1
026I 28
‘ 110
' ‘ 32
l ' 3A
l I I36001
F/{<4//‘
1 1321 34
1 136133
1
38 1
40112
34 25 231 10
hv (BV) hv (BV)
(a) Direct (bl Indirect
Figure 6.7: Variation of band gap with photon energy
The extinction coefficient (k) was calculated using the relation
or = 41ck/7» and refractive index n by the relation [6]
R (n+l)2 +k2 (6)
(n—D2+k2
Department of P/rys/‘rs, (Z/I/if
C/iapter-6
The variation of refractive index with wavelength is shown in figure 6.8.
It increases up to a certain wavelength in UV region and then it decreases
exponentially and finally becomes constant in the visible region.
2.5
.| __. -,
|"‘\}_
2 4 -1 ,4,
n
1* f ‘X
\
.
' 1757 nm
»-s
\-/an"| \\
.2 3' t"
2.3 - ~ I‘. '.
~.
u
­
1589 nm
- 1 73672 nm if
us
as 2.0 ­
1.8­
1'|'li|'1'I
un­
1.7­
200 400 600 800 1000 1200
wavelegth (nm)
Figure 6.8: Variation of refractive index with wavelength
The variation of refractive index with film thickness at 550 mn is given
in table 6.3. The variation of refractive index is in agreement with literature
reports in the visible region. The dielectric constant of the as-prepared ceria thin
films is expressed as; 5, =n2 —k2 where 8| is the real part the dielectric
constant. The dependence of dielectric constant on photon energy with different
thickness is shown in figure 6.9. The variations of refractive index with film
thickness at 550 mn are given in table 6.3.
Table 6.3: Variations of direct band gap, indirect band gap and refractive index
with film thickness.
(nm)
leV) (eV)
672
3.36
2.79
2.065
1589
3.35
2.75
2.090
1757 3.24 2.65 2.133
Film thickness Direct band Gap Indirect band gap Refiactiveindex
Department of P/1)’!/(S, (I/SAT
Treparation qfCeria qiiin Tilms 6y Spray Qfyr0[ysis anr[‘Iiieir (jliaracterizations
9'
672nm
5'5"
—l589nm
' "*‘ l757nm
6.0
/§
% 5.0­
id
i
­
1
‘E.
3.5 ­
<­
30­
2-51
I I800I 1000
II
200 400I600
Wavelength (nm)
Figure 6.9: Variation of dielectric constant with wavelength.
The room temperature photoluminescence specra of CeO2 thin films at an
excitation wavelength of 325 mn is shown in figure 6.10. The higher thickness
films show PL spectra similar to those of nanostructured ceria, as explained in
chapter 3. Low thickness film shows PL emission centered around 403 mn. With
increasing film thickness most of the characteristic peaks appear in the spectrum.
453457A 1757 nm
419
43? 1":
_ _ / \- , 1.‘! 2
~"*-~\.I
’. ~.4
1.
$3‘ 492
I.‘
IIII
415 437 1589 nm
i
4&
a
1
.-»/
IIII
403 672 nm
/
\\
'4
Wavelength (nm)
Figure 6.10: PL spectra of ceria thin films with different thicknesses
Deparimem‘ 0//'/7}’!/(S, (1/ur
Cliapter-6
The PL spectra exhibit a broad-band character from 350 mn to 500 nm.
This could be the results of defects including oxygen vacancies in the crystal with
electronic energy levels below 4f band and is well explained in chapter 3.3.
The spray pyrolysed ceria thin films with high transmittance in the
visible region, high absorbance characteristic in the UV region, are promising
for optoelectronic applications. With a band gap of 3.36 eV, good transparency
in visible region, nanostructure ceria thin film prepared by spray pyrolysis
method can be used as a promising inorganic material for transparent UV
filtering films.
4 ~ 1589
3.0x10
1 672 nm
nm
4 0110 1
, 1757 nm
<­
i
/-\
\-r
­
Q-I
2.0x1o* —
i.
-1
" " nun.l ::'0o I I
.ox1o‘
-'-- ....
__ ' . ~
J90.
...lI|
0.0 -..|-u-n:.:::::-Q!-I-ate“-i{,,Q,:;,_'..-...-_..-..,,‘"
"llr|lI' ._m .'."II[ ."r|1
_ox1o‘- ' if . '°'i¥I:ZIZ:
-a.ox1o‘
-»
-4-0X10‘ -| ~ | ' | ' l ' l ' l * 4 1
- 0x10‘ —
-9000 -6000 -3000 0 3000 6000 9000
Magnetic field (Oe)
Figure 6.11: Hysteresis loops for cerium oxide thin films with different thickness
The magnetic properties of CeO2 nanocrystals are characterized by
measuring the field dependence of magnetization at room temperature. Figure 6.11
shows the RT magnetization curve of CeO2 with different thickness in the as­
prepared conditions. In this figure CeO2 appears to be diamagnetic. However, on
close examination room temperature ferromagnetism can be observed in these
figures which are characterized by a closed hysteresis loop. The origin of
ferromagnetism in CeO2 is assumed to be resulting from the oxygen vacancies at
the surface of the nanoparticles and is well explained in the chapter 5.3. With
Department of /’/rys/"rs, (1/IAT
(Preparat1'0-np ofCen'a ‘Uiin €l-"ifms 6y Spray qifyrofisis anrf’I7iez'r Cfiamctenlzations
increasing thickness in cerium oxide films diamagnetic property decreases whereas
ferromagnetism increases. Loops are not saturating in lower thickness films due to
the presence of diamagnetic component. But it tries to saturate with increase in
thickness. Remanence magnetization and coercivity of the ceria thin films are low
compared to cerium oxide nanoparticles. Though the remanence magnetization and
coercivity are low (Mr-8.86 x l0'6 emu/ g, Hc-153 Oe) compared to nanostructured
powder sample, this is the first time that RTFM has been observed in spray
pyrolysed ceiia thin films synthesized from CeCl3. The present study is significant
since it offers ample scope for optimizing the spray deposition conditions to obtain
CeO2 films with appreciable RTFM.
6.4. Conclusion
Detailed investigations have not been carried out on CeO2 thin films
deposited by spray pyrolysis technique. There are a few reports describing the
spray deposition of cerium oxide thin films. However in all these reports the
substrate temperature used is quite high which is around 850 K. The highlight of
the present work is the fact that good quality cerium oxide thin films could be
successfully deposited on glass substrate by spray pyrolysis technique using cost
effective precursors such as CeCl3 at comparatively much lower substrate
temperatures around 623 K. In the present work the effect of film thickness on the
structural, optical and magnetic properties of CeO2 thin films has been investigated
in detail. The most important outcome of the present study is the observation of
RTFM in cerium oxide thin films deposited by spray pyrolysis technique, which
has not been reported earlier. Although the area of hysteresis loop and the
magnetization value are low compared to those of nanostuctured CeO2 powder
sample, the present work offers much scope for further investigations on RTFM in
CeO2 films. By optimizing the deposition conditions it may be possible to improve
the ferromagnetic behavior of spray deposited CeO; films. This is quite
flézidrtrrié-i1t?1fP/Ir:/rs, fl/W? T
C/iapter-6 _p _ p _
significance from the view point of the prospecting applications of these cost
effective CeO2 films in the development of spintronic devices.
References
[1] B Elidrissi, M Addou, M Regragui, C Monty, A Bougrine, A
Kachouane, Thin Solid Films 379 (2000) 23
[2] T Prasada Rao, M C Santhosh Kumar, Appl. Sur. Sci., 255 (2009)
7212
[3] E Bauer, M H Francombe, H Sato (Eds.), Single Crystal Films,
Pergamon, London (1964)
[4] T Prasada Rao, M C Santhosh Kumar, Appl. Sur. Sci., 255 (2009)
4579
[5] B D Cullity, s R Stock, Elements of X-ray Diffraction, 3'“ Edn,
Prentice Hall, New Jersey (2001)
[6] F Yakuphanoglu, S llican, M Caglar, Y Caglar, J. Optoelectronics &
Advanced Materials 9 (2007) 2180
[7] S Debnath, M R Islam, M S R Khan, Bull. Mat. Sci. 30 (2007) 315
[8] J Tauc, in F Abels (Ed.), Optical Properties of Solids, North Hlland,
Amsterdam (1970)
[9] E A Davis, N F Mott, Phil. Mag. 22 (1970) 903
[10] K L Chopra, Thin film phenomena, McGraw-Hill book Company,
New York (1969)
55 55
altaple,
7
SYNTHESIS. STRUCTURAL AND OPTICAL PROPERTIES
OF POLYMERICEBIA NANUCOMPUSITES
7. I. Introduction
7.2. Experimental techniques
7.3. Results and discussion
7.4. Conclusion
References
7. 1. Introduction
The synthesis of polymeric matrixes embedded with nanoparticles has
attracted much interest in the field of nanomatefials. Polymers are considered as
good choice as host materials, because they can be designed to yield a variety of
bulk physical properties, and they nonnally exhibit long term stability and posses
flexible reprocessability. This new class of inorganic—polymer composites may
afford potential applications in molecular electronics, optics, photo electrochemical
cells, solvent-free coatings etc. Synthesis of polymer/nanoparticles composite
materials is very important in advanced material science. These materials combine
both the unique properties of nanoparticles and polymers and possess new
properties which are not specific to the original components. The polymer matrix
also prevents extremely active nanoparticles from aggregation. The addition of
inorganic spherical nanoparticles to polymers allows the modification of polymers
physical properties as well as the implementation of new features of inorganic
nanoparticles.
Cfiapter-7
Polymer nanocomposite structures provide a new method to improve the
processability and stability of materials with interesting optical properties.
Inorganic /polymer nanocomposites benefit from physical flexibility and ease
of processing. which are typical features of polymers. Further, nanocrystals
dispersed in suitable solid hosts can be stabilized for long period of time [1]. In
the present work, four different polymers are used as the matrix for nanocerium
oxide in the nanocomposite. In this study, CeO2 nanocrystals have been
prepared by hydrolysis assisted chemical method which could be suitably
embedded in four polymer matrices (polyvinylidene fluoride (PVDF), poly
vinyl alcohol (PVA), polymethyl methacrylate (PMMA) and polystyrene (PS).
PVDF is a special plastic material in the fluoropolymer family; it is used
generally in applications requiring the highest purity, strength, and resistance to
solvents, acids, bases and heat. Compared to other fluoropolymers, it has an
easier melt process because of its relatively low melting point of around 177 °C.
lt has a low density (1.78 g/cc) and low cost compared to the other
fluoropolymers. Polyvinyl alcohol (PVA) has excellent film forming,
emulsifying and adhesive properties. It has high tensile strength and flexibility.
PVA is fully degradable and dissolves quickly. PVA has a melting point of
230 °C. It decomposes rapidly above 200°C as it can undergo pyrolysis at high
temperatures. Polymethyl methacrylate (PMMA) is a transparent thermoplastic,
sometimes called acrylic glass. Chemically, it is the synthetic polymer of
methyl methacrylate. Polystyrene (PS) is an amorphous, optically clear
thermoplastic material, which is often chosen as a host matrix because of its
ideal properties for investigating optical properties. It is one of the most
extensively used plastic materials.
oepvmnror P/1y$i?3} §(l!5;4 I
g g_ Syntfiesis, Structurafam{Optz'ca[Q°r0pert1'es of Qofimer/Ceria 9V'an0co:rzposz'tes
7.2. Experimental techniques
7.2.1. Synthesis
CeO3 nanopaiticles were prepared by hydrolysis assisted precipitation
method as explained in chapter 3. PVDF/ CeO2 nanocompsite solutions were
prepared by adding 10 wt % of CeO2 powder sample into PVDF solution (l0%
w/v) in NMP. The mixtures were then stirred for 2h and then sonicated for 10
min [2]. Using the same procedure PVA (dissolved in water)/ CeO;_ PMMA
(dissolved in Tetrahydrofuran)/CeO2 and PS (dissolved in tetrahydrofuran)/
CeO_-; nanocomposites were also prepared.
7.2.2. Characterizations
XRD studies were carried out for the polymer/CeO2 nanocomposites
using a Rigaku Ultima-III X-Ray Diffractometer using Cu Kot; radiation in the
20 range from 10 0 to 70 O. Optical absorption studies were carried out using
JASCO V 570 Spectrophotometer at room temperature. Photoluminescence
spectra of composite samples were obtained using Flouromax-3
Spectrophotometer at room temperature under an excitation of 325 nm.
7.3. Results and discussion
XRD pattems of various polymer/ceria nanocomposites are shown in
figure 7.1. These indicate broad non crystalline peaks of polymer and sharp
peaks of cerium oxide. The diffraction peaks corresponding to (111), (200),
(220) and (311) planes indicate cubic structure of CeO2 (J CPDS 34-0394)_ The
broadening of XRD peaks indicates formation of nanosized particles in the as­
prepared sample. The presence of CeO2 produces neither new peaks nor peaks
shift with respect to polymer showing that nano CeO2 filled polymer composites
consist of two phase structures.
Department of?/zys/'0‘, (I/{Hg - ; I
Cfiapter-7
"ll" PSI Ceria
5’ (200)
,»@
»»./
XAi (311)
;\ (220)
fi I
i
|‘I|I|I|IIl1
‘J \\"*-0~o-4~a-~_rJ ‘tn,/\“.“__'________",-_
PMMAlCeria_
-A A
MW
~|~|'|=|'|'|
r\\\¢.,,.,M~»i\¢\_-mH~w/ 1
PVA/Ceria
\ PVDFI Ceria '
1.
'|'|'|'|'I'I
/\M~1~4"}\-1A\,. l\ Jr,
I p, ­
Ceriai
l
I ' |30
' F401 50
I *60
t *70t
10' 20
.,\~’;_'___\__~dVw‘/J‘ \--\vvw RH \&‘-J_/: W/er
2o (degree)
Figure 7.1: XRD patterns of the polymer! Ce0z nanocomposites
UV-Vis absorption spectra of the polymer/Ce(); nanocomposites are
shown in figure 7.2. Pure ceria shows a sharp absorption peak around 298 nm
and an absorption edge at 387 mn which is explained in chapter 3.3. In PMMA,
PVA and PVDF/ceria nanocomposites the absorption edge shifts to longer
wavelength side. But in PS/ceria nanocomposite absorption edge is almost at
300 mn. In all the composite samples there is a UV absorption window. The
width of the window is from 250 nm to 350 mn for all polymer/ceria
nanocomposites except for PS/ceria nanocomposite for which it is from 250 nm
to 300 mn.
Department of P/zys/rs, (0547
Syntliesir, StructurafamfOptica[G’r0perties qfq’0[ymer/Ceria Wanocomposites
g__ PS / CeO2
\
- ‘”
~.
_: I
1 I S00 ' I I ' I ' I I I
' \PMMA
r / CeO2
O -A-‘
\
____ L...‘-\_»-__;
._,- \__
-- ---- . ... _ g
V
t
~.
%
*~ -‘Q _ _
I .._.__
I ' I 4,“ I ' I I I ' I
' PVA
/ CeO2
. -3- g.-,5‘ . xxru‘
‘\
-I I I I\“50 I I I I ' “>1
' PVDF / CeO2
-» “*
_,-_r~
\
I
\\
1II'I'IIIII
510
zoo zoo 400 soo soo 100 aoo
Wavelength (nm)
Figure 7.2: UV-Vis absorption spectra of the polymer! Ceflz nanocomposites
‘2-‘IIJ,-_3­Q//’//
Pure polymers don’t show appreciable UV absorption. But polymer/ceria
nanocomposites are covering UV A and UV B regions and part of UV C region,
thus showing prospects of acting as efficient UV filters. The mechanism of UV
absorption in these materials involves the use of photon energy to excite electrons
from the valence band to conduction band.
BU)
*Iuewvce’
Pv~Fc!?:am
IIIV‘' PMMNQ;
;PS/C803
A PMMA.'Cena
3 "T
‘ __
A_ pslc g
'lP'VDF"C0riG§
600 ,_,: - e - 015 r V“ U13 ;
'5
'>
0.10­
21D--1 I V °_Q5_
“I hvIE”
(av) hv (av)
­,-\
*1
iU"2 1
r~
U
0I “r
1 09°
2 3I A6 ~5 517I O
1 5 12 IDI rr
2 5n-LU
3 0 3r ~5 I4+Qor4' 5I *5rOr
Ia) Direct band gap (b) Indirect band gap
Figure 7.3: Band gap energy of the polymer! Ceflz nanocomposites
Department of Phys/2.9 (0!/If I I
Cliapter-7
Direct and indirect band gap energies of polymer/CeO2 nanocomposites
are shown in figure 7.3 (a & b). Table 7.1. shows direct and indirect band gap
energies of polymer/CeO2 nanocomposites.
Table 7.1: Direct and indirect band gap energies of p0lymerICe0z nanocomposites
Sample 4 Direct band gap (eV)
Indirect band gap (eV)
PVDF
ICeria
2.43
2.05
PVAICeria
2.75
2.14
PMMA
ICeria
2.87
2.26
PS ICeria 4.13 3.99
The present findings reveal an enhanced band gap in the range of 4.13
eV for PS/ceria nanocomposite. So it could be used as a better photocatalyst.
Light below this wavelength has sufficient energy to excite electrons and hence
absorbed by CeO2. Light having a wavelength longer than the band gap energy
(towards the visible light) will not be absorbed. Therefore PS/CeO2
nanocomposite is an excellent UV absorber [3].
. 572
PSI Cena 555»
522 "Ln r‘
50;-E M597
319 _/ /ii 1 \~
'I'l'Illl
/_.
_. ._J"~.,~I—~//‘*’—'v""' fl
_ 466
1%:' \‘
5°? J‘=.
535
._.
'“
in/ .1I‘~,_
PMMA! Cena 1. -"\\
/ii \\
Ax ‘~\_ \ H
J,V“
\/.Tb
I' I 1 t | I I__*
+K_____‘
I|+i
VA! Ceria. 522
1
‘/\¢\547 61 5
319 / K.’
““~---.
/\
‘'l'1I1'I
/ _\\
1
381
\ \\< 1
ll //’1\..
ii
~\.
l
r=vor=/ Ceria fl
J:
’/ ""'*' ---_
I "T" i I | t i + t I |
r1
sso 400 450 soo sso soc sso
Wavelength (nm)
Figure 7.4: Pl. spectra of the polymer! Ceflz nanocomposites
7
Department of P/rys/'0', (I/IA)‘
Syntliesrlt, StmcturafarzrfOptica[@r0pertz'es qfQ’0[ymer/Ceria Wanocompositgs
PL spectra of the as-prepared polymer/CeO2 nanocomposites are shown
in figure 7.4. PL studies of polymer/ceria nanocomposites are done to
investigate the effects of the polymer matrix on the PL characteristics of ceria.
Pure ceria shows a series of high intensity PL emission peaks in the blue-green
region and also lower intensity peaks in the UV region which are well
explained in chapter 3.3. Polymer matrix provides surface passivation of the
ceria nanoparticles and hence modifies the PL spectrum. Passivation effect is
highly pronounced in PMMA/Ceria nanocomposite. The PMMA/Ceria
nanocomposite has the most intense PL emission peak at 466 nm compared to
pure ceria and other polymer composites.
lt is observed that free standing films of PS/ceria, PMMA/ceria and
PVDF/ceria can be obtained with thickness around 1p.m using solution casting.
These nanocomposite films also show high dielectric constant value around 20.
Due to time constraint, detailed investigations on the various properties of these
films couldnot be carried out.
7.4. Conclusion
Polymer/ceria nanocomposites using four different polymers have been
successfully synthesized. These nanocomposites show good UV absorption
window regions. Polymer matrix provides surface passivation of the ceria
nanoparticles and hence modifies the PL spectra. Passivation effect is highly
pronounced in PMMA/Ceria nanocomposite.
It is observed that free standing polymer nanocomposite films of ceria
can be prepared using PS, PMMA and PVDF which show considerably high
dielectric constant values (-20). These free standing high dielectric constant
nanocomposite films can be of profound applications as gate electrodes for
metal oxide semiconductor devices.
_C/Zapter-7 _ W
References
[1] L L Beecroft, C K Ober, Chem. Mater., 9 (1997) 1302
[2] P P Jeeju, A M Sajimol, V G Sreevalsa, S J Vanna, S Jayalekshmi,
Polym. 1111., 60 (2011) 1263
[3] P K Khanna, N Singh, S Charan, Mat. Lett. 61 (2007) 4725
°Q.'°€."@
Depart/hen! afiff/7}/“.1/159,1 (UM I
(Z It G P f 21'
3
SUMMARY AND CUlllCll.l$ll]lll
8.1. General conclusion
8.2. Future scope of the present work
References
8.1. General conclusion
Synthesis and processing of nanomatelials and nanostmctures are the
essential aspects of nanotechnology. Studies on new physical properties and
applications of nanomaterials and nanostructures are possible only when
nanostructured materials are made available with desired size, morphology,
crystal structure and chemical composition. Among the topics of current
interest in material science, the identification of rare earth compounds with
desirable properties has attracted much attention owing to their wide
applications. Among them, cerium oxide (ceria) is one of the important
functional materials with high mechanical strength, thermal stability, excellent
optical properties, appreciable oxygen ion conductivity and oxygen storage
capacity. Ceria finds a variety of applications in mechanical polishing of
microelectronic devices, as catalysts for three-way automatic exhaust systems
and as additives in ceramics and phosphors[l].
Recently, several methods have been developed to prepare pure and doped
CeO2 powder, including wet chemical synthesis, thermal hydrolysis, flux method,
hydrothermal synthesis, gas condensation method, microwave technique etc. In
all these, some special reaction conditions, such as high temperature, high
pressure, capping agents, expensive or toxic solvents etc. have been involved.
Cfiapter-8
The large scale production of powders needs to be economically feasible and
should not be more complex. So compared to these methods, chemical
precipitation is more attractive due to the use of cheap salt precursors and
simple operation[l]. Although CeO; in doped and pristine forms have already
been studied, there are no reports on the systematic investigations related to the
synthesis and properties of nanostructured cerium oxide (both in pristine and
doped forms) using the cost effective technique of hydrolysis assisted chemical
precipitation method.
In the present work nanocrystalline CeO2 powder samples have been
prepared by hydrolysis assisted chemical precipitation method employing
cerium chloride and ammonia as precursors. XRD and TEM studies confirm
the formation of the fluorite CeO; nanopartilces of average size around l0 nm.
Extensive investigations have been carried out on the structural and optical
properties of nanostructured ceria and iron, aluminum and cobalt doped ceria.
One of the high-lights of these studies is the observation that both pure and
doped ceria offer the prospects of applications as cost effective and non toxic
inorganic material for efficient UV filtering in sunscreen cosmetics.
One of the objectives of the present investigation is to look for room
temperature ferromagnetism in nanostructured "ceria and doped ceria (iron and
cobalt) synthesized using the hydrolysis assisted chemical precipitation technique.
RTFM has been observed for the first time in pure ceria nanocrystals synthesized
by chemical precipitation technique and also in iron and cobalt doped ceria
nanocrystals. The origin of RTFM has been ascribed to the presence of oxygen
vacancies on the surface of ceria nanoparti]ces[2]. Based on the experimental data
good correlation between the magnetic, structural and optical properties of these
samples has also been established.
I i s s r r s s T s s vemrmeaz arr»;/ms. (1/W
i A Summary amfC0nc[u.sion
Photoluminescence has already been reported in cerium oxide
synthesized by variety of techniques. In the present work detailed investigation­
has been carried out on the PL characteristics of cerium oxide nanocrystals.
Based on the dependence of PL intensity on annealing temperature a self
trapped exciton (STE) mediated PL mechanism has been proposed for the
observed photoluminescence in ceria nanocrystals. Though STE mechanism has
been reported in some oxide systems such as LigO3, SrTiO3 etc. [3]. it is the first
time that such a mechanism is being proposed for PL emission in cerium oxide
nanocrystals.
Another hi gh-li ght of the present work is room temperature ferromagnetism
in cerium oxdie thin films deposited by spray pyrolysis technique. High quality
CeO2 thin films could be deposited by the cost effective spray pyrolysis technique
at comparatively much lower substrate temperatures around 623 K using simple
precursors, CeCl3_ There are no reports on RTFM in spray deposited ceria films.
Though the magnetization value is much lower compared to that of
cerium oxide powder, the present work offers much scope for detailed
investigations on RTFM in cerium oxide films. It should be possible to
optimize the spray deposition conditions to improve the F M properties of C€Og
films with the prospects of applications in developing cost effective spintronic
devices.
The field of polymer/ceria nanocomposite with inorganic filler
nanomaterials has gained worldwide research attention owing to versatile
applications of these nanocomposites in many branches of science and
technology. In the present work attempts has been made to synthesize
ceria/polymer nanocomposites using a variety of polymers such as PVDF,
PVA, PMMA and PS. It is observed that these polymer/ceria nanocomposites
possess good UV absorption window regions of approximately 250 nm width.
Cfiapter-8 p___ g g g 7
Hence these nanocomposites having practical applications in developing UV
filters.
8.2. Future scope of the present work
The observation of RTFM in CeO;_ nanocrystals and iron and cobalt
doped CeO2 synthesized using surfactant free hydrolysis assisted chemical
precipitation is one of the highlights of the present investigations. Another
important result is the observation of RTFM in CeO2 thin films deposited by
spray pyrolysis technique. The present investigations offer ample scope for
further studies related to RTFM in CeO2 thin films which has not been pursued
widely. It should be possible to optimize the deposition conditions of CeO2
films in order to improve the FM behavior of CeO2 films.
The observation of self trapped exciton mediated PL in ceria
nanocrystals is another important outcome of the present study. STE mediated
mechanism has been proposed for CeO2 nanocrystals based on the dependence
of PL intensity on the annealing temperature. It would be interesting to extent
these investigations to the doped forms of cerium oxide and cerium oxide thin
films to get deeper insight into STE mechanism.
Due to time constraints detailed investigations could not be canied out
on the preparation and properties of free standing films of polymer/ceria
nanocomposites. It has been observed that good quality free standing films of
PVDF/ceria, PS/C61‘l8, PMMA/ceria can be obtained using solution casting
technique. These polymer nanocomposite films show high dielectric constant
around 20 and offer prospects of applications as gate electrodes in metal-oxide
semiconductor devices.
Summary anJCOnc[u.$z'0n
References
[1] H Chen, H Chang, Colloids and Surfaces A: Physicochc-:m.Eng. Aspects
242 (2004) 61
[2] A Sudaresan, R Bhargavi, N Rangarajan, U Siddesh, C N R Rao, Phys.
Rev. B 74 (2006) 161306 (R)
[3] W F Zhang, Z Yin, M S Zhang, Z L Du, W C Chen, J. Phys: Condens.
Matter 11 (1999) 5655
-.......-..... ...........
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