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Cite this: RSC Adv., 2014, 4, 5666
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Synthesis of Bi2S3–Bi2O3 composites and their
enhanced photosensitive properties†
Fangyuan Lu,a Renxiong Li,a Nengjie Huo,a Juehan Yang,a Chao Fan,a
Xiaozhou Wang,b Shengxue Yang*a and Jingbo Li*a
Bi2S3–Bi2O3 composites were synthesized by a facile hydrothermal method with surfactant as template.
The structure and morphology of the as-synthesized products were characterized in detail. The
photosensitive behavior of the Bi2S3–Bi2O3 composites was investigated carefully. Under the illumination
of a 650 nm laser in air or vacuum, the device displayed enhanced photosensitive performance
Received 31st October 2013
Accepted 17th December 2013
compared to the pure Bi2S3, pure Bi2O3 and mechanical mixture of Bi2S3 and Bi2O3 based devices. The
photoswitch ratio (Iphoto/Idark) was as high as 30 in a vacuum with fast photoresponse speed, which
DOI: 10.1039/c3ra46283h
indicated their potential applications in manufacturing photodetectors and optoelectronic devices. The
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possible mechanism of enhanced photosensitivity was also proposed.
1
Introduction
Photosensitive materials have been paid attention due to their
potential applications in photodetectors,1–4 eld-effect transistors,5 photoswitch microdevices,6 lasers,7 photoanodes8 and
light-emitting diode.9 Over the past few decades, various kinds
of photosensitive materials sensitive to ultraviolet, visible or
infrared light have been synthesized and applied in optoelectronic devices, such as ZnO,10 CdS.11 In order to develop more
new photosensitive materials and take full advantage of their
photosensitivity, many composites have been designed and
investigated. For example, S. X. Yang12 et al. reported the
fabrication of surface-functionalized metal oxide composites,
which showed much better photoresponse property than pure
metal oxides. B. Zhang11 et al. synthesized CdS/Mesoporous
Carbon composites with enhanced photosensitivity. Those
studies prove that the improvement in the photosensitivity is by
virtue of a coupling effect between the two components in the
composites that is benecial to the electron transfer and can
inhibit the recombination of the photo-generated carriers.
Bi2S3 and Bi2O3 are two important semiconductor materials
and have been extensively researched. Bi2S3 is a direct band gap
semiconductor with bandgap (Eg) of 1.3 eV,13 which shows great
promise in numerous applications, such as photovoltaic
converter, electrochemical hydrogen storage, photocatalyst, photoresponse, thermoelectric, X-ray computed tomography, etc.14–17
a
State Key Laboratory for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, P. R.
China. E-mail: [email protected]; [email protected]
b
Research Center for Light Emitting Diodes Zhejiang Normal University, Jinhua
321004, China
† Electronic supplementary information (ESI) available: Detailed synthesis and
characterization of the pure Bi2S3 and Bi2O3. See DOI: 10.1039/c3ra46283h
5666 | RSC Adv., 2014, 4, 5666–5670
In the few years, due to unique shape- and size-dependent
physical and chemical properties, different morphologies of Bi2S3
including nanoribbons, nanoowers, nanowires, nanorods and
mircrospheres,3,16,18,19 have been synthesized by microwave irradiation, sonochemical method, hydrothermal, solvothermal,
chemical vapor deposition, and so on.18–22 As one of the earliest
known photoconduction materials, the photoconductivity of
Bi2S3 was rst reported by Case in 1917, based on the studies on
mineral samples of bismuthinite or bismuth glance,23 and the
photosensitive properties of Bi2S3 with different morphologies
and structures in recent years have been studied in details. For
example, X. L. Yu18 et al. prepared the nanoowers of Bi2S3 and
investigated their photoreponse and eld-emission properties.
L. S. Li1 et al. studies the photoresponsive properties of the Bi2S3
core–shell microspheres and nanorod bundles. Y. P. Li3 et al.
studied the photoresponsive characteristics of the Bi2S3 nanorods. All these indicate that Bi2S3 is a good candidate for optoelectronic devices and photodetectors. Bi2O3 is a semiconductor
oxide of narrow band gap of 2.8 eV with suitable band edge
potentials (CB: 0.33 eV, VB: 3.13 eV).24 And it has been widely used
in superconductor ceramic glass manufacturing, photocatalyst,
electrolytes, and sensor optical coatings due to its special properties including high refractive index, dielectric permittivity
marked photoconductivity and photoluminescence.25–29 Because
of its low generation efficiency in photo-generated carries, the
photoresponse of the pure Bi2O3 is not apparent.30 However, in
order to explore the property of heterostructure between Bi2S3
between Bi2O3, some researches integrate these two materials
together to form new composites. The synthesis of Bi2S3–Bi2O3
composites has been reported before. P. M. Sirimanne31 et al.
applied scattered Bi2S3 microcrystals depositing on a compressed
and sintered Bi2O3 pellet to obtain the Bi2S3–Bi2O3 composites,
but they did not investigate the photosensitive activity of the
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Bi2S3–Bi2O3 composites. F. A. Liu30 et al. obtained the Bi2O3–Bi2S3
core–shell nanoparticles assembled thin lms by utilizing
successive oxidation and surface sulfurization of Bi nanoparticles
and investigated the photosensitive activity, but photosensitive
property was not very high.
In this letter, we applied a facile hydrothermal method for
the preparation of the Bi2S3–Bi2O3 composites with surfactant
PVP as the template. The photosensitive property of the asprepared Bi2S3–Bi2O3 composites was studied and they displayed enhancing photosensitivity compared with the pure
Bi2S3, pure Bi2O3 and mechanical mixture of Bi2S3 and Bi2O3
based devices. The possible mechanism of enhanced photosensitivity was also discussed.
2 Experimental section
All chemical reagents in our experiments were analytical and
used as received without further purication. The Bi2S3–Bi2O3
composites were prepared through a facile hydrothermal
method. In the typical experiment, 0.3 g of surfactant polyvinylpyrrolidone (PVP) and 0.75 g Bi(NO3)3$5H2O were rstly
dissolved in 30 ml distill water, and then 0.24 g of Na2S$9H2O
was added slowly to the solution. The solution was stirred about
10 min and then lled a certain amount of NH3$H2O to adjust
the pH value to 11. Aerwards, the whole mixture was transferred into a 50 ml Teon-lined autoclave, sealed and heated at
150 C for 24 h. The nal precipitates were collected and washed
with deionized water and pure ethanol several times and dried
in air at 80 C. The details of the synthesis of pure Bi2S3 and
pure Bi2O3 were similar to the above method and presented in
the ESI.†
The as-prepared samples were re-dispersed in ethanol by
ultrasonication and spun onto the clean SiO2/Si substrates.
Then a pair of Au electrodes was evaporated onto the glass
covered with samples with a gap of 30 um by using a gold wire as
shadow mask. A schematic of the photosensitive devices containing as-prepared products is shown in Fig. 2a.
The crystalline structures were analyzed by XRD, using Cu Ka
(l ¼ 0.15406 nm) radiation at 50 kV and 50 mA in a q range of 10
to 70 at room temperature. The sizes and morphologies of the
as-synthesis products were detected with SEM by using an
accelerating voltage of 15 kV. The photosensitivity was
measured on a CHI 660D electrochemical workstation using a
diode laser (red light, central wavelength of 650 nm) as light
source.
RSC Advances
Fig. 1 (a) XRD patterns of the Bi2S3–Bi2O3 composites. (b) and (c) SEM
images of the Bi2S3–Bi2O3 composites.
planes. The characteristic peaks of Bi2O3 are indexed to the
diffraction from the (210), (201), (211), (220), (400) planes. No
other peaks are observed in the XRD pattern, which indicates
that the as-prepared samples are only made of Bi2S3 and Bi2O3.
The XRD patterns of pure Bi2S3 and Bi2O3 are shown in the
Fig. S1,† respectively (see the ESI†). The SEM images of the assynthesized Bi2S3–Bi2O3 composites are displayed in Fig. 1. The
low-magnication SEM image (Fig. 1b) shows that the assynthesized composites are composed of microsheets and
nanowires which are inset in these microsheets. The highmagnication SEM image (Fig. 1c) shows that the general sizes
of nanowires in microsheets are about 1 um in lengths and
40 nm in diameters. Compared with the morphologies of the assynthesized Bi2O3 and Bi2S3 samples which are shown in Fig. S2
(in ESI†), we deduce that microsheets are Bi2O3 and nanowires
are Bi2S3 in the composites product.
The photoresponsive curves of the Bi2S3–Bi2O3 composites
based device in air and vacuum with the red light (650 nm)
turning on and off periodically were measured and recorded by
CHI660D workstation under the bias voltage of 1 V. The
experimental schematic diagram of the Bi2S3–Bi2O3 composites
based device is shown in Fig. 2a.The I–V curves of the Bi2S3–
Bi2O3 composites based device measured in dark or under light
illumination are shown in Fig. 2b. In Fig. 2b, the I–V curves of
the device are all nearly linear, which indicates a very good
Ohmic contact between Au electrodes and the Bi2S3–Bi2O3
composites. Furthermore, compared to the current of the Bi2S3–
Bi2O3 composites in dark, the current under light illumination
is increased immensely, especially in vacuum, which presents
high photosensitivity. Fig. 2c shows the UV-Vis spectrum of the
as-synthesized Bi2S3–Bi2O3 composites, their bandgap is estimated about 1.31 eV, similar to the bandgap of Bi2S3,13 which
indicates that light energy is mainly absorbed by Bi2S3 in the
3 Results and discussion
The XRD pattern was usually used to characterize the phase
structures and purity of the as-prepared samples. The XRD
pattern of the obtained Bi2S3–Bi2O3 composites is just showed
in Fig. 1a. The sharp reection characteristic peaks of the XRD
patterns indicate that the as-synthesized products are
composed of the orthorhombic phase Bi2S3 (JCPDS 17-0320)
and orthorhombic phase Bi2O3 (JCPDS 27-0050). The characteristic peaks of Bi2S3 are indexed to the diffraction from the
(110), (120), (220), (101), (210), (311), (510), (061), (551), (152)
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Fig. 2 (a) Schematic diagram of the Bi2S3–Bi2O3 composites device.
(b) I–V curves of the Bi2S3–Bi2O3 composites device measured under
dark and light illumination in air and vacuum. (c) UV-Vis spectrum of
the Bi2S3–Bi2O3 composites.
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composites. Fig. 3 shows the photoresponse of the Bi2S3–Bi2O3
composites, pure Bi2S3, pure Bi2O3 and the mechanical mixture
of Bi2S3 and Bi2O3 based devices as a function of time in air and
vacuum. We observe that no matter when in air or vacuum, the
photocurrent of the Bi2S3–Bi2O3 composites, pure Bi2S3 and the
mechanical mixture of Bi2S3 and Bi2O3 based devices increases
rapidly and quickly reaches saturation and stable with the red
light (650 nm) on, but the photocurrent of pure Bi2O3 based
device does not increase apparently due to the wider band gap
of Bi2O3. Here we dene the photoswitch ratio as Iphoto/Idark,
where, Iphoto is the current under light illumination and Idark is
the current without light illumination. Although the photocurrent of the Bi2S3–Bi2O3 composites based device is lower than
that of the Bi2S3 based device at the same bias voltage due to the
Bi2O3 existing in the composites, the photosensitive activity of
the Bi2S3–Bi2O3 composites based device is highly enhanced in
air or vacuum and the photoswitch ratio is even as high as 30 in
vacuum (Fig. 3a). As shown in Fig. 3b and c, the photosensitive
property of both pure Bi2S3 and mechanical mixture of Bi2S3
and Bi2O3 based devices are very low and that of the Bi2O3 based
device is not apparent under the red light.
As an optical switch device, photoresponse speed is another
key parameter to show its performance besides photoswitch
ratio. Fast photoresponse speed indicates high sensitivity for
light. Photoreponse speed includes response time which is
dened as the time required for photocurrent to increase from
10% to 90% of Ipeak and recovery time which is opposite to the
response time.32 The photoresponse time of the Bi2S3–Bi2O3
composites based device in vacuum is shown in Fig. 4a and b. It
is observed that the response time and recovery time are about
100 ms and 530 ms, respectively. Furthermore, the photoresponse speed of pure Bi2S3 and mechanical mixture of Bi2S3
and Bi2O3 based devices were also measured in vacuum, as
shown in Fig. S3.† Compared to the photoresponse speed of
pure Bi2S3 and mechanical mixture of Bi2S3 and Bi2O3 based
devices, the fast photoresponse speed of the Bi2S3–Bi2O3
composites based device in vacuum displays its superiority in
practical application potentials. The photocurrent of the Bi2S3–
Bi2O3 composites based device for repetitive switching
The time functional on/off photocurrent responses of the
devices: (a) Bi2S3–Bi2O3 composites based device, (b) pure Bi2S3 based
device and Bi2O3 based device and (c) mechanical mixture of Bi2S3 and
Bi2O3 based device.
Paper
The enlarged view of photocurrent vs. time in the Bi2S3–Bi2O3
composites based device under vacuum condition: (a) response time
and (b) recovery time.
Fig. 4
characteristic under the red light illumination with the light
turning on or off in vacuum was measured and shown in Fig. 5.
The photocurrent of the device increases and decreases in
accordance with the light source on/off due to its fast photoresponse speed and even aer scores of the on/off sensing
cycles, the photocurrent of the device does not appear any
detectable degradation, which further demonstrates reversibility and stability of the device. These above characteristics
also suggest the potential applications of the as-synthesized
Bi2S3–Bi2O3 composites in high sensitivity photodetectors and
photoelectronic switches in the near future.
The photosensitive mechanism may involve a series of
process, which includes the electron–hole generation, trapping,
and recombination in a device. 19 So, we discussed the possible
mechanism for the Bi2S3–Bi2O3 composites based device with
enhanced photosensitive activity. Compared to the photosensitivity of pure Bi2S3, pure Bi2O3 and mechanical mixture of
Bi2S3 and Bi2O3 based devices, we conclude that the better
photosensitivity of the Bi2S3–Bi2O3 composites device under the
light illumination is probably ascribed to the formation of
heterostructure between Bi2S3 and Bi2O3 in the embedded
structure shown in SEM images. Formation of heterostructure
promotes the separation of electrons and holes and therefore
reduces the recombination between electrons and holes. The
Fig. 3
5668 | RSC Adv., 2014, 4, 5666–5670
Fig. 5 I–t curves of the Bi2S3–Bi2O3 composites based device
measured by periodically turning on and off red light under vacuum.
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potential in photodetectors and optoelectronic devices in the
future. This enhanced photoresponse in a vacuum was suggested to be due to the formation of heterostructure between
Bi2S3 and Bi2O3 and a few atmosphere molecules absorbed by
the defects in the composites acting as a medium to promote
electron–hole recombination, which led to the effective separation and decrease of the recombination of electrons and holes
in Bi2S3. Our studies prove that the properly designed composites
can display better properties than only one component and this
work might be of great value in developing new composite
materials for outstanding applications in the future.
Acknowledgements
Fig. 6 Band alignment of the Bi2S3 and Bi2O3 versus normal hydrogen
electrode (NHE) and possible charges separation process.
possible charges separation process in the Bi2S3–Bi2O3
composites is shown in Fig. 6. The electron–hole pairs are easily
generated in Bi2S3 with the red light illumination due to the
direct and narrow band gap. Because of the formation of heterostructure and the lower conduction band edge of Bi2O3,
electrons located in the conduction band of Bi2S3 tend to
transfer to the conduction band of Bi2O3, which leads to separation of electron–hole pairs effectively and prevents the electron–hole recombination. Therefore, the concentration of
carriers increases in circuit, which leads to enhancing the
conductivity. By comparing the photocurrent of the devices in
air and in vacuum, defects may be another reason for the
different photosensitivity of the Bi2S3–Bi2O3 composites based
device that the photosensitivity is much better in vacuum than
in air. Defects in the device can form the traps, which will
absorb the molecules in air, such as oxygen, nitrogen and water
molecules. The absorbed molecules act as mediums and
promote the recombination between electrons and holes. But
due to few atmosphere molecules in vacuum, this kind of
recombination can be avoided and the photoconductivity is
enhanced.
4 Conclusion
In summary, we have successfully synthesized Bi2S3–Bi2O3
composites through a facile hydrothermal method with
surfactant templates. The SEM images showed that the
morphology was an embedded structure with the Bi2S3 nanowires inset in Bi2O3 microsheets. The photoresponse of assynthesized Bi2S3–Bi2O3 composites was investigated in detail,
which showed enhanced and stable photosensitive properties
both in air and vacuum compared with the pure Bi2S3, pure
Bi2O3 and mechanical mixture of Bi2S3 and Bi2O3 based devices
and the photoswitch ratio in a vacuum could be as high as 30
with fast photoresponse speed, which showed huge application
This journal is © The Royal Society of Chemistry 2014
J. Li gratefully acknowledges nancial support from National
Science Found for Distinguished Young Scholar (Grant no.
60925016) and the National Basic Research Program of China
(Grant no. 2011CB921901). S. Yang acknowledges nancial
support from China Postdoctoral Science Foundation (no.
2013M540127).
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