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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
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Solvothermal synthesis ZnSeIn2S3eAg2S solid solution
coupled with TiO2LxSx nanotubes film for photocatalytic
hydrogen production
Fangzhou Jia a, Zhongping Yao a,b, Zhaohua Jiang a,*
a
School of Chemical Engineering and Technology, State Key Lab Urban Water Resource & Environment, Harbin Institute of Technology,
No. 92 Xidazhijie Street, Nangang, Harbin 150001, PR China
b
Materials Research Lab, The Pennsylvania State University, University Park, PA 16802, USA
article info
abstract
Article history:
ZnSeIn2S3eAg2S solid solution coupled with TiO2-xSx nanotubes film catalyst has been
Received 15 August 2011
successfully prepared by a two-step process of anodization and solvothermal methods for
Received in revised form
the first time. The as-prepared photo-catalysts are characterized by scanning electron
28 October 2011
microscopy (SEM), X-ray diffraction (XRD), UVeVisible diffuse reflectance spectra (UVeVis
Accepted 4 November 2011
DRS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The
Available online 14 December 2011
results show that the ZnSeIn2S3eAg2S solid solution are deposited on the surface of
Keywords:
into the lattice of TiO2 through substituting the sites of oxygen atoms. Such
TiO2NTs nanotubes under the solvothermal conditions, by which S atoms are incorporated
TiO2-xSx nanotubes
ZnSeIn2S3eAg2S@TiO2-xSx nanotubes composite presents the enhanced absorption in
Sulfur doping
visible region and the efficient transfer of photoelectron between the solid solution and
ZnSeIn2S3eAg2S
TiO2-xSx nanotubes, which determines the excellent photocatalytic activity for the photo-
Solid solution
catalytic hydrogen evolution from aqueous solutions containing the sacrificial reagents of
H2 production
Na2S and Na2SO3 under 500 W Xe lamp irradiation.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1.
Introduction
The photocatalytic splitting of water into H2 using solid solution photo-catalysts would be an environmentally friendly
way of producing clean and renewable hydrogen on a large
scale. Since the Honda-Fujishima effect was first reported [1],
all kinds of photo-catalysts for H2 production have been
studied. Over the last few years, considerable efforts have
been made to improve visible light response of photocatalysts. The recently prepared solid solution photocatalysts such as (AgIn)xZn2(1-x)S2 [2], ZnS-CuInS2-AgInS2 [3],
ZnSeIn2SeAg2S[4], Cd1-xZnxS[5e8], (CuIn)xZn2(1-x)S2 [9],
ZnSeIn2SeCuS[10], ZnmIn2S3þm [11], Sr-doped CdS-ZnS[12]
and ZnSeCuSeCdS[13,14]have shown excellent performance
for photocatalytic hydrogen production under visible light
irradiation because of their controllable band structures and
high quantum yield. Besides, it is reported that
ZnSeIn2S3eAg2S solid solution shows a high apparent yield
(19.8% at 420 nm) of hydrogen production from water containing sacrificial reagents of Na2S and Na2SO3 without
a cocatalyst [4].
Due to the problems of the separation and the recycle of
the powder particles, the immobilization of solid solution has
attracted increasing attentions [15,16]. In recent years, self-
* Corresponding author. Tel.: þ86 451 86402805; fax: þ86 451 86403379.
E-mail address: [email protected] (Z. Jiang).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.11.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
organized, well-ordered TiO2 nanotubes arrays (TiO2NTs) on
titanium plate fabricated in situ by electrochemical anodization present the large surface-to-volume ratio, high orientation and excellent electron percolation pathways for charge
transfer between interfaces [17e20]. The high cationexchange character and the particular open mesoporous
morphology as well as more free space make nano-particles
easily bond on the surface of TiO2NTs [21e30]. Especially,
when a photocatalyst with a higher conduction band level is
coupled with TiO2NTs, efficient electron transfer from the
sensitized semiconductor to a host titania matrix can reduce
recombination of photo-induced electrons and holes and
improve the photocatalytic activity of photocatalyst [31e35].
These properties make TiO2NTs suitable to be used as
a promising support material for loading solid solution.
However, to our best knowledge, ZnSeIn2S3eAg2S solid
solution coupled with TiO2NTs for photocatalytic hydrogen
production has not been reported yet. Moreover, sulfur doping
TiO2 exhibited higher photocatalytic activity than TiO2
[36e40], especially when introducing S at the O sites which
could significantly modify the electronic structures of TiO2
because S has a large ionic radius. Therefore, in this work,
ZnSeIn2S3eAg2S is selected as a coupling material and
meantime big atomic size sulfur is expected to be incorporated into the crystal lattice of TiO2 to prepare
ZnSeIn2S3eAg2S solid solutions coupled with TiO2-xSx NTs.
2.
Experimental sections
2.1.
Synthesis of samples
absolute ethanol and finally air-dried. For comparison, ZnSIn2S3@TiO2NTs sample was similarly prepared under the
same conditions, except that AgNO3 pyridine solution was not
used in the experiment.
2.2.
Characterization of samples
Morphology of as-prepared samples was observed using
a scanning electron microscope (SEM; JSM-6480A, Japan). The
mass of ZnSeIn2S3eAg2S and ZnS-In2S3 solid solution on the
surface of TiO2 nanotubes can be obtained quantitatively by
the weight growth of TiO2NTs after the solvothermal
synthesis, which was measured by electronic analytical
balance with the accuracy of the 105 g (CP 225D, Sartorius,
German). The X-ray diffraction (XRD) patterns of as-prepared
samples were obtained on an X-ray diffractometer (D/max˚ , 45 kV, and 40 mA), X-ray
rB, Ricoh, Japan, Cu Ka, l ¼ 1.5418 A
photoelectron spectroscopy (XPS) analysis was conducted on
Phi5700 spectroscopy (ESCA system, U.S.A.) using a monochromated Al Ka X-ray source (1486.6 eV) operating at 15 kV.
The amounts of metal ions were determined by inductively
coupled plasma emission spectrometry (ICP; Perkin Elmer,
Optima 5300DV). Raman spectra were recorded using a Raman
spectrometer (Jobin-Yvon Labram HR 800) to study the fine
structure of the specimens. The photoabsorption property
was recorded with a diffuse reflectance UVeVis diffuse
reflectance spectrophotometer (UV-2400; Shimadzu, Japan).
BaSO4 was used as the reflectance standard.
2.3.
Highly ordered TiO2NTs were prepared by electrochemical
anodic oxidation. Prior to anodization, the pure titanium sheet
(purity 99.5%) of 0.5 mm thickness was pretreated by
mechanically polishing and acid bright pickling, the detailed
operating methods was presented in our previous paper [29].
Then, titanium sheet (reaction area 25 mm 40 mm) was
anodized in 0.14 M NaF and 1 wt. % H3PO4 aqueous solution at
20 V for 90 min. After anodization, the obtained TiO2NTs were
rinsed with the deionized water and subsequently calcined at
400 C for 1 h.
A solvothermal method was employed to prepare the
ZnSeIn2S3eAg2S solid solution coupled with TiO2NTs
(ZnSeIn2S3eAg2S@TiO2NTs) [4]. The mixture of Zn(Ac)2∙2H2O
(2.1 mmol), In(NO3)3∙4H2O (0.3 mmol) and thioacetamide
(TAA) (10.8 mmol) were dissolved in 23.5 ml pyridine together.
After Zn(Ac)2, In(NO3)3 and TAA dissolved completely, 1.5 ml
of 0.05 M AgNO3 pyridine solution was then added dropwise
into the above mixture solution under constant stirring. The
TiO2NTs was perpendicularly mounted into the Teflon-lined
stainless steel autoclave of 50 ml capacity and 25 mm in
diameter containing the prepared solution, which was maintained at 180 C for 18 h. Before the solvothermal reaction, the
solution was purged with Ar for 3 min in order to evacuate the
air in the solution. When the solvothermal reaction ended, the
TiO2NTs was taken out from the autoclave and washed with
ethanol and dried in air. In addition, the powders product
(ZnSeIn2S3eAg2S solid solution) in the autoclave was
collected by centrifugation, washed several times with
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Evaluation of photocatalytic properties of samples
Photocatalytic activities of the as-prepared film catalysts were
conducted in a closed gas circulation system by using a 500 W
high-pressure ball-shaped Xe lamp (XHA500 W, Shanghai
Ruizi Co. China) supplying the wavelength illumination from
300 to 800 nm. The film catalysts (the effective area for the film
catalysts is 10 cm2) were immersed in 40 ml aqueous solution
containing Na2S (0.1 M) and Na2SO3 (0.02 M) in a sideirradiation quartz reaction cell which was placed 10 cm
away from the Xe lamp. The solution was continuously stirred
with a magnetic stirrer. Nitrogen was purged through the cell
for 10 min before irradiation to remove oxygen in the solution.
The amount of H2 was determined using thermal conductivity
detector (TCD) gas chromatography (SP2100A, Beifen instrument, China).
3.
Results and discussion
3.1.
Composition and structure of samples
Fig. 1a shows the top view and cross sectional view (inset part)
of as-prepared TiO2NTs, it can be seen that the highly ordered
and porous TiO2 nanotubes with tube length of w600 nm and
pore diameter w 100 nm are formed on the Ti substrate. Fig. 1b
is morphology of the powders of ZnSeIn2S3eAg2S solid solution, a large number of nanorods with needle-like structure
appear and aggregate randomly. Fig. 1c is the SEM image of
ZnSeIn2S3eAg2S@TiO2NTs, it can be observed that TiO2
nanotubes still kept their tube-like structures after
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
Fig. 1 e SEM of (a) TiO2NTs (b) ZnSeIn2S3eAg2S (c) ZnSeIn2S3eAg2S@TiO2NTs
sovolthermal treatment, and the needle-like ZnSeIn2S3eAg2S
nanorods have been deposited on the most part of the surface
of TiO2NTs. The mass of ZnSeIn2S3eAg2S and ZnS-In2S3 solid
solution grown on the surface of TiO2 nanotubes are almost
the same (about 0.0016 g), which illustrates that such samples
are comparable in the following structural characterization
and performance tests. According to the analysis results of
inductively coupled plasma (ICP), the atomic ratios of Zn, In
and Ag are close to 1: 0.242: 0.056. Therefore, the composition
of the ZnSeIn2S3eAg2S sample can be expressed as ZnIn0.242Ag0.056S1.391. The XRD patterns of the samples are shown
in Fig. 2. For solid solution samples, it can be seen that the
crystal structures of the solid solutions depend on the
composition. The diffraction peaks of ZnS-In2S3 at 28.45 (111),
47.56 (200), and 56.36 (311) matched with the cubic zinc
blende structure. With the Ag doping, the hexagonal (wurtzite-type) structure was obtained and the peak position
slightly shifts to left, indicating the formation of the
ZnSeIn2S3eAg2S solid solution, which is in agreement with
the literature [4]. For the TiO2NTs, the pattern exhibited both
the anatase at 25.24 (101) and rutile TiO2 at 27.42 (110), 35.95
(101), 48.04 (200), 54.21 (211), and anatase is the major
constituent. The diffraction peaks at 35.09 , 38.28 , 40.06 and
52.9 corresponding to Ti are also detected. There is a significant structural change observed after the solvothermal
treatment: the diminishing of anatase and the intensifying of
the rutile, which indicates the partial crystal transformation
from antase to rutile at high temperature and high pressure
[41]. In addition, the diffraction peak of cubic ZnS at 28.5
appeared for the film samples, proving the existence of solid
solution in the films.
XPS has been used to analyze composition and chemical
states of the components in the samples. Fig. 3 is the XPS
Fig. 2 e XRD patterns of TiO2NTs, ZnS-In2S3,
ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs and
ZnSeIn2S3eAg2S@TiO2NTs
Fig. 3 e (a) XPS full spectra of TiO2NTs, ZnS-In2S3, ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs and ZnSeIn2S3eAg2S@TiO2NTs; (b)
The Zn2p3/2 high-resolution XPS spectra of ZnS-In2S3, ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs and
ZnSeIn2S3eAg2S@TiO2NTs; (c) The In3d high-resolution XPS spectra of ZnS-In2S3, ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs
and ZnSeIn2S3eAg2S@TiO2NTs; (d) The Ag3d high-resolution spectra of ZnSeIn2S3eAg2S and ZnSeIn2S3eAg2S@TiO2NTs;
(e) The S2p high-resolution spectra of ZnS-In2S3, ZnSeIn2S3eAg2S, ZnSeIn2S3@TiO2NTs and ZnSeIn2S3eAg2S@TiO2NTs; (f)
The Ti2p high-resolution spectra of TiO2NTs, ZnS-In2S3@TiO2NTs and ZnSeIn2S3eAg2S@TiO2NTs; (g) The O1 s highresolution spectra of TiO2NTs, ZnS-In2S3@TiO2NTs and ZnSeIn2S3eAg2S@TiO2NTs.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
analysis of the samples. The binding energies were calibrated
by taking the C 1 s peak at 284.6 eV as reference. It can be seen
from full XPS spectra (Fig. 3a), the signals of Zn, In, Ag and S
appear in the ZnSeIn2S3eAg2S@TiO2NTs sample, suggesting
the presence of ZnSeIn2S3eAg2S on the surface of TiO2NTs.
For ZnS-In2S3 and ZnSeIn2S3eAg2S samples, a weak peak of O
1 s at w532 eV is due to a small amount of hydroxyl groups
covering the surface.
The high-resolution XPS spectra of Zn 2p, In 3d for the
samples were investigated with the results shown in Fig. 3b
and c, respectively. For ZnS-In2S3 sample, the Zn 2p3/2 and In
3d5/2 XPS peaks are at binding energies of 1023.1 and
445.44 eV, respectively, which are higher than the binding
energy of Zn 2p3/2 (1021.5 eV) for pure ZnS and In 3d5/2
(444.3 eV) for In2S3 [42,43]. The high binding energies of Zn
2p3/2 and In 3d5/2 of ZnS-In2S3 sample can be attributed to
the In 5s5p orbital mixed with Zn 4s4p[2]. For
ZnSeIn2S3eAg2S sample, the peaks of Zn 2p3/2 and In 3d5/2
shift to the lower binding energies of 1022.5 and 444.87 eV,
respectively, compared with the ZnS-In2S3 sample, which
could result from the charge density difference between
metal ions[44]. The ion radii of Zn2þ, In3þ, and Ag are 74, 80,
and 115 pm, respectively. The higher charge density ions of
Zn and In can withdraw the out-shell electron clouds form
Ag, so the screening effect of electrons would increase for In
and Zn, which lead to the shift to the lower binding energies
for Zn 2p3/2 and In 3d5/2.
For ZnS-In2S3@TiO2NTs sample, the Zn 2p3/2 and In 3d5/2 can
be deconvoluted into two peaks. The low binding energies
around 1022.71 eV (Zn 2p3/2) and 444.82 eV (In 3d5/2) correspond
to the ZnS-In2S3. The high binding energies around 1024.23 eV
(Zn 2p3/2) and 446.37 eV (In 3d5/2) correspond to the oxidation
states of Zn and In, which can be associated with OeZneS and
OeIneS. This means that the oxidation of Zn and In likely
occurs on the interface between sulfides and TiO2NTs in spite
of the fact that the synthesis process used here took place
entirely under inert atmosphere (Ar). For the ZnSeIn2
S3eAg2S@TiO2NTs sample, the binding energies of Zn 2p3/2 and
In 3d5/2 are close to that of ZnSeIn2S3eAg2S sample, which
indicates that the doping of Ag efficiently suppress the oxidation phenomena of ZnSeIn2S3eAg2S solid solution on the
surface of TiO2NTs due to a more stable structure for
ZnSeIn2S3eAg2S than ZnS-In2S3. The high-resolution XPS
spectra of Ag 3d for ZnSeIn2S3eAg2S and ZnSeIn2
S3eAg2S@TiO2NTs samples is shown in Fig. 3d, the binding
energies of Ag 3d3/2 and Ag 3d5/2 are 373.72 and 367.72 eV,
respectively. The observed values for the binding energies are
close to the reported value of Ag2S [45].
Fig. 3e shows the high-resolution XPS spectra of S 2p. For
ZnSeIn2S3eAg2S sample, the binding energies of S 2p (2p3/2,
2p1/2) doublets at 161.51 and 162.69 eV should be assigned to
S2 of ZnSeIn2S3eAg2S, which are close to the binding energy
of S 2p3/2 (161.5 eV) for pure ZnS and S 2p3/2 (161.6 eV) for In2S3
[42,43]. For the ZnSeIn2S3eAg2S@TiO2NTs sample, besides the
pair of S 2p3/2 and S 2p1/2 peaks at 161.51 and 162.69 eV,
another pair of spin-orbit component of S 2p3/2, S 2p1/2
appears at binding energies of 163.78 and 164.96 eV, respectively, which is consistent with OeTieS bonding[40], suggesting that S substituted for O in the TiO2 lattice. For ZnSIn2S3 sample, due to lack of Ag-S chemical bond, the S 2p
(2p3/2, 2p1/2) doublets in the chemical state of S2 are shifted to
higher binding energies of 162.0 and 163.18 eV, in comparison
with the ZnSeIn2S3eAg2S. For ZnS-In2S3@TiO2NTs sample,
the S 2p (2p3/2, 2p1/2) doublets at the binding energies of 162.88
and 164.06 eV can be attributed to the chemical state of
SeZneO and SeIneO, which is consistent with the results of
high-resolution Zn 2p and In 3d spectra.
Fig. 3f is the high-resolution Ti 2p spectra of the samples,
which exhibits two peaks corresponding to binding energies
of w 458.8 eV (Ti 2p3/2) and w 464.5 eV (Ti 2p1/2), respectively.
For ZnS-In2S3@TiO2NTs sample, the slight shift of Ti 2p3/2 to
the lower value of binding energy with respect to that of the
TiO2NTs sample indicates the interaction between ZnS-In2S3
and TiO2 nanotubes, in which electron transfer from ZnSIn2S3 to titania takes place [24]. In contrast, a shift to the
higher value of the Ti 2p3/2 for ZnSeIn2S3eAg2S@TiO2NTs
sample occurs as compared to that of the TiO2NTs sample,
which can be attributed to OeTieS species [40]. The result is
consistent with that of high-resolution S 2p spectra. This
means that ZnSeIn2S3eAg2S has stronger interaction with
TiO2 nanotubes than ZnS-In2S3 due to the existence of Ag in
the solid solution.
Fig. 3g shows the high-resolution XPS spectra of O 1 s. The
binding energy of O 1 s for the samples can be deconvoluted
into two peaks, the peak at w 530.3 eV, resulting from the
crystal lattice oxygen of TiO2 (OTi-O) and the peak at w 532 eV,
corresponding to the hydroxyl oxygen (OO-H). While the
ZnSeIn2S3eAg2S@TiO2NTs sample shows a higher ratio of OOH/OTi-O than that of TiO2NTs and ZnS-In2S3@TiO2NTs samples.
Raman spectrum is also used to investigate the structure of
these samples as shown in Fig. 4. For ZnS-In2S3 and
ZnSeIn2S3eAg2S samples, two broad bands around 559.8 cm1
and 1112.4 cm1 are observed, this again approved that ZnSIn2S3 and ZnSeIn2S3eAg2S have similar crystal structure, as
shown by XRD results. The Raman spectrum of TiO2NTs
shows five characteristic modes at approximately 143, 393.7,
445.3, 521.6 and 631.1 cm1. The mode at 146 cm1 is strong
and assigned to Eg phonon of the anatase structure and B1g
phonon of the rutile structure, the modes at 393.7, 521.6 and
631.1 cm1 are moderate and assigned as B1g, A1g, and Eg
modes of the anatase phase, respectively, the mode at
445.3 cm1 is weak and corresponded to Eg modes of the rutile
structure. Comparison with Raman spectra of TiO2NTs film,
the other two Raman characteristic rutile modes at approximately 237.8 and 608 cm1 (A1g) appear in the Raman spectra
and
ZnS-In2S3@TiO2NTs
of
ZnSeIn2S3eAg2S@TiO2NTs
samples. In the meantime, the anatase to rutile ratio decrease,
this is consistent with the results of XRD. In addition, two
Raman bands corresponding to carbon at approximately
1447.3 and 1567.3 cm1 are also detected, which are possibly
due to a small amount of carbon resulted from the carbonization of pyridine under sovolthermal conditions. Comparison with the ZnS-In2S3@TiO2NTs sample, it is to be noted that
a peak at w337 cm1 appears in the ZnSeIn2S3e
Ag2S@TiO2NTs sample, which belongs to A1g mode of Ti-S
vibration [46]. This indicates the existence of a type of Ti-S
chemical bond. In this way, it can be considered that Ag in
the solid solution plays an important role in the formation of
OeTieS species, which means the formation of Ti-S bonds in
TiO2NTs is closely related to the formation of ZnSeIn2S3eAg2S
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
3053
Fig. 5 e UVevis absorption spectra of TiO2NTs, ZnS-In2S3,
ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs and
ZnSeIn2S3eAg2S@TiO2NTs.
Fig. 4 e Raman spectra of TiO2NTs, ZnS-In2S3,
ZnSeIn2S3eAg2S, ZnS-In2S3@TiO2NTs and
ZnSeIn2S3eAg2S@TiO2NTs.
and the interaction between solid solution and TiO2NTs.
However, the detailed insight into S replacing O to form TiO2xSx nanotubes is unknown yet.
3.2.
Optical properties of samples
The optical absorption is represented by the Kubelka-Munk
function calculated from the DRS spectra. Optical absorption
spectra of the samples are shown in Fig. 5. It is found that the
TiO2NTs sample exhibits the fundamental absorption band at
419 nm, which corresponds to the TiO2 with mixed crystal
structure (anatase and rutile). The ZnS-In2S3 sample is white
and has a steep absorption band edge at 378 nm. The
ZnSeIn2S3eAg2S sample is yellow and its absorption edge
shifts monotonically to longer wavelengths due to the Ag
doping, indicating that the powders obtained are solid solutions rather than simple mixtures of ZnS, In2S3 and Ag2S.
Considering that solid solution has been deposited on the
most part of the TiO2NTs surface, composites mainly exhibit
the optical properties of solid solution. For the ZnSIn2S3@TiO2NTs sample, the absorption band is closed to that
of the ZnS-In2S3 sample and slightly red shifted. The red shift
of the spectra to some extent can be attributed to the interface
coupling effect between two semiconductors [31]. A noticeable
shift of the optical absorption edge up to the 435 nm in the
visible region is observed in the ZnSeIn2S3eAg2S@TiO2NTs
sample, which indicates the narrowed band gap of TiO2 by S
doping [38].
3.3.
ZnSeIn2S3eAg2S@TiO2NTs samples demonstrate the higher
photocatalytic activity as compared to TiO2NTs sample.
ZnSeIn2S3eAg2S@TiO2NTs shows the highest photoactivity
with corresponding k value of 25.02 mmol h1, while ZnSIn2S3@TiO2NTs has k ¼ 22.92 mmol h1.
In comparison with the TiO2NTs, the enhancement of
photoactivity of ZnS-In2S3@TiO2NTs can be attributed to
interaction between solid solution and TiO2NTs. The conduction bands edge of ZnS-In2S3 solid solution are deemed to be
higher than that of TiO2 due to the high potential level of the
conduction band consisting primarily of Zn 4s4p. When solid
solution coupled with TiO2NTs, under 500 W Xe lamp irradiation, the excited photoelectrons from conduction band of solid
solution are easily injected into the conduction band of TiO2,
which suppresses the charge recombination of photo-induced
electrons and holes and improves the rate of electron transfer
to Hþ absorbed on the TiO2NTs surface. Consequently, the
photo-generated electron can effectively reduce Hþ to H2.
The higher H2 production activity of ZnSeIn2
S3eAg2S@TiO2NTs than ZnS-In2S3@TiO2NTs sample can be
attributed to the enhanced absorption in visible region. On the
Photocatalytic activity for H2 evolution
Based on the above analysis results, it is expected that
ZnSeIn2S3eAg2S@TiO2NTs sample can exhibit an excellent
photocatalytic activity. It can be clearly observed from Fig. 6
that the TiO2NTs sample shows the lowest photocatalytic
activity with a corresponding production rate (k) of
18.87 mmol h1. However, the ZnS-In2S3@TiO2NTs and
Fig. 6 e Amount of H2 evolution in Na2S D Na2SO3 solution
by photocatalytic process of film catalysts.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 0 4 8 e3 0 5 5
activity for the photocatalytic hydrogen evolution from
aqueous solutions containing the sacrificial reagents of Na2S
and Na2SO3 under 500 W Xe lamp irradiation.
Acknowledgment
This work was financially supported by the National High
Technology Research and Development Program of China
(National High Technology Research and Development
Program of China) (No. 2007AA03Z337).
Fig. 7 e The band structures and the mechanism of
electron transport for ZnSeIn2S3eAg2S solid solution
coupled with TiO2-xSx nanotubes film catalyst.
one hand, with the doping of Agþ, the valence band level of
ZnSeIn2S3eAg2S solid solution is shifted due to the hybrid
orbital of S 3p and Ag 4d, which results in a decrease in the
band gap energy and increasing visible light absorbtion [2]. On
the other hand, sulfur doping (S substituted for O in the TiO2
lattice) could create intra-band-states close to the valence
band edges due to the mixing the S 3p with the valence band
of TiO2, narrowing the band gap of TiO2 and inducing visible
light absorption at the sub-band-gap energies [38]. In addition,
S atoms doping could induce the position of flat band potential
shifted to a higher level than that of undoped TiO2, which
increases the driving force of electron injected from solid
solution and accelerates the reductive process of interfacial
electron transfer. The band structures and the mechanism of
electron transport for ZnSeIn2S3eAg2S solid solution coupled
with TiO2-xSx nanotubes film catalyst are showed in Fig. 7,
which is similar with the mechanism in the N doping TiO2
reported in Ref[47].
4.
Conclusion
ZnSeIn2S3eAg2S solid solution coupled with TiO2-xSx nanotubes film has been successfully prepared by a two-step
process of anodization and solvothermal methods for the
first time. The results showed some solid solution powders
were deposited on the TiO2NTs film through a certain interaction between TiO2NTs and the solid solution. The doping of
Ag can efficiently suppress the oxidation phenomena of
ZnSeIn2S3eAg2S solid solution on the surface of TiO2NTs.
Moreover, when ZnSeIn2S3eAg2S solid solution grows in situ
on the surface of TiO2NTs, the oxygen atoms in the TiO2NTs
are partly substituted by S to form TiO2-xSx nanotubes under
the solvothermal conditions. The large surface contact and
strong interaction between solid solution and TiO2NTs accelerate electron transfer on the interface, while the substitution
of sulfur for oxygen narrows the band gap of TiO2 and induces
the visible light absorption. These make ZnSeIn2S3e
Ag2S@TiO2NTs sample show an excellent photocatalytic
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