The incorporation of cadmium sulfide (CdS) into TiO2 nanoparticle thin films was investigated. The nanoparticle TiO2 thin film onto an indium doped–tin oxide (ITO) substrate was deposited by Electron Beam Deposition (EBD) combined with thermal process. Then a CdS thin film was vacuum-deposited onto the pre-deposited TiO2 film by a thermal evaporation technique. The obtained TiO2/CdS was characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM). The TiO2/CdS nanocomposite film was used in a photo-electrochemical (PEC) cell as a working electrode and a platinum electrode as a counter electrode. The electrolyte solution contains 1 M KCl and 0.1M Na2S. The results show that the cell with TiO2/CdS composite film electrode has significantly improved photoelectric capability in comparison with that of the pure TiO2 thin films

Chi, Le Ha

Chien, Dang Tran

Hoi, Pham Van

Long, Pham Duy

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 21, No. 1 (2011), pp. 57-62
NANOCOMPOSITE THIN FILM TiO2/CdS ELECTRODES
PREPARED BY THERMAL EVAPORATION PROCESS FOR
PHOTOVOLTAIC APPLICATIONS
DANG TRAN CHIEN
College of Technology, VNU
and
Hanoi College of Natural Resources and Environment
PHAM DUY LONG AND PHAM VAN HOI
Institute of Materials Science, VAST
LE HA CHI
College of Technology, VNU
and
Institute of Materials Science, VAST
Abstract. The incorporation of cadmium sulfide (CdS) into TiO2 nanoparticle thin films was
investigated. The nanoparticle TiO2 thin film onto an indium dopedtin oxide (ITO) substrate
was deposited by Electron Beam Deposition (EBD) combined with thermal process. Then a CdS
thin film was vacuum-deposited onto the pre-deposited TiO2 film by a thermal evaporation tech-
nique. The obtained TiO2/CdS was characterized by X-ray diffraction (XRD), field emission
scanning electron microscope (FE-SEM). The TiO2/CdS nanocomposite film was used in a photo-
electrochemical (PEC) cell as a working electrode and a platinum electrode as a counter electrode.
The electrolyte solution contains 1 M KCl and 0.1M Na2S. The results show that the cell with
TiO2/CdS composite film electrode has significantly improved photoelectric capability in compari-
son with that of the pure TiO2 thin films.
I. INTRODUCTION
In recent years, an increasing interest has been devoted to the study of interpar-
ticle electron transfer between dissimilar semiconductor particles. Combining two semi-
conductor particles offers an opportunity to sensitize a semiconductor material having
a large bandgap and energetically low-lying conduction band by another one having a
small bandgap and energetically high-lying conduction band [1]. Charge injection from
one semiconductor into another can lead to efficient and longer charge separation, which
is anticipated to have potential applications in photocatalysis and solar energy conversion
[2]. Nanoscale titania (TiO2) is of great interest mainly due to its unique properties, such
as high photoactivity, chemical stability and size dependent optical properties [3]. How-
ever, it is well known that the TiO2 acts as a catalyst under ultraviolet excitation [4] due
to its wide band gap (around 3.2 eV). To overcome this problem, various materials, like
58 DANG TRAN CHIEN, PHAM DUY LONG, PHAM VAN HOI, AND LE HA CHI
dyes and metallic nanoparticles, have been used as sensitizer to increase the photoactivity
of TiO2 in the visible range.
Among the various semiconductors CdSe, CdTe, CuInS2, InP etc., used as a sen-
sitizer [5-8], CdS has shown much prospect as an effective sensitizer [9,10]. CdS belongs
to the IIVI group, and it is typically sulfur deficient [11]. It is the most widely studied
nano-crystalline semiconductor as a photoanode in PEC cells because of its suitable band
gap, long lifetimes, excellent stability and easy fabrication [1214, 15]. In this study, CdS
was used to sensitize TiO2 nano-structures. In TiO2/CdS nanocomposite, CdS acts as a
visible sensitizer and TiO2, being a wide band semiconductor, is responsible for charge
separation which suppresses the recombination process. Hence, the prepared TiO2/CdS
nanocomposite thin films can effectively capture the visible light and quickly transfer the
photogenerated electrons into the TiO2 conduction band, and finally, the sensitization of
CdS on ITO/TiO2 strongly ameliorates the photoelectric performance of the TiO2/CdS
nanocomposite thin films.
There are numerous reports describing the fabrication of titania thin films using
techniques such as EBD [16], doctor blading [17], magnetron sputtering [18], solgel pro-
cessing [19], surfactant template self-assembly [20], pulsed laser deposition [21], and spray
pyrolysis, etc. In this work, TiO2 nanocrystalline thin films were obtained by evaporated
Ti films by EBD method and then thermal treated. The CdS thin films have been made
by a thermal evaporation technique. As compared with various techniques used for the
deposition of CdS thin films in [22-26], EBD and thermal evaporation are relatively simple
techniques for the large scale uniform coating to produce clean, dense and strong adhesion
to substrate thin films.
II. EXPERIMENTAL
The optically transparent and electrically conductive indium tin oxide (ITO) coated
glass substrate with a sheet resistance of 30 W/square was ultrasonically cleaned in a series
of organic solvents (ethanol, methanol and acetone) and deionized water. A 300nm thick
layer of Ti was deposited by EBD technique. In order to obtain TiO2 nanoparticles, the
Ti coated on the ITO substrate was annealed at 400˚ C in air for 8 h. Thin film of CdS
with thickness of 30 nm was vacuum-deposited by thermal evaporation at pressure of 10−2
torr onto the pre-deposited TiO2 films. The deposition rate was 0.2 nm/s and the thin
film thickness was measured during deposition using conventional quartz crystal monitor.
To obtain good crystallinity, the thin CdS films were annealed at 300˚ C in air for 1 hour.
The surface morphology of the samples was investigated by using a “Hitachi” Field Emis-
sion Scanning Electron Microscopy (FE-SEM). The Ultravioletvisible (UVvis) absorption
spectra were performed by using a Jasco UV-VIS-NIR V570 spectrometer, X-ray diffrac-
tograms were recorded on a Siemens D-5000 diffractometer using CuKα radiation with
the wavelength 1.5406A˚. In photoelectronic studies, a two-electrode Photoelectrochemical
(PEC) cell was used. The PEC cell is composed of ITO/TiO2 or ITO/TiO2/CdS used
as working electrode (working area is 0.6 cm2) and a platinum electrode separated by an
electrolyte containing 1 M KCl and 0.1M Na2S. The photocurrent was measured on an
Auto-Lab Potentiostat PGS-30. A halogen lamp was used as a light source.
NANOCOMPOSITE THIN FILM TiO2/CdS ELECTRODES... 59
III. RESULTS AND DISCUSSIONS
III.1. Morphology and structural characterizations of the TiO2 thin film and
the TiO2/CdS nanocomposite film
The FE-SEM images of the surface of nc-TiO2 film are presented in Fig. 1a. The
surface of CdS film on the ITO/TiO2 substrate is shown in Fig. 1b. From Fig. 1a it is
apparent that the average TiO2 particles size was found to be less than 20nm and a porous
structure could be seen. As shown in Fig. 1b, a homogeneous CdS ?lm with good quality
deposited onto ITO/TiO2 substrate and good ?lm-to-substrate adhesion were observed.
The vacuum deposition by thermal evaporation technique can effectively improve the
distribution of CdS nanoparticles around TiO2 nanoparticles and the porosity of the thin
film. The XRD pattern of ITO/TiO2/CdS thin film gives the enhancement in intensity
of the peak at 2θ position of 26.80 corresponding to the (002) plane indicates preferential
orientation in the (002) direction (Fig. 2). This result is in good agreement with literature
[27-29].
Fig. 1. FE-SEM images of the nc-TiO2 film (a), TiO2/CdS film (b)
III.2. Absorption spectra of the TiO2 and TiO2 /CdS nanocomposite thin films
Fig. 3 shows the UV-vis absorption spectra of ITO/TiO2 thin film (a) and CdS
thin film on the ITO/ TiO2 substrate (b). As it can be seen from Fig. 3, the TiO2 thin
film absorbs light mainly in the wavelength range from 380 nm to 420nm. The absorption
intensity of the TiO2/CdS nanocomposite film significantly increases. The main absorption
edge for the CdS film on ITO/TiO2 substrate is estimated to be about 550nm, which shifts
toward visible region compared with the pure TiO2 thin film. This is in good agreement
with Hong-gang Fu et al. [9]. Therefore, the modification of CdS thin film for ITO/TiO2
would effectively absorb the visible light of TiO2/CdS nanocomposite thin films.
III.3. Photoelectronic performance of the TiO2 /CdS nanocomposite film
Fig.4 shows the photocurrent-potential behaviors of the photo-electrochemical cell
with ITO/TiO2 and ITO/TiO2/CdS in turn under dark and illumination. Table 1 shows
the open-circuit photovoltage (Voc) and the short-circuit photocurrent (Jsc) of the ITO/TiO2
60 DANG TRAN CHIEN, PHAM DUY LONG, PHAM VAN HOI, AND LE HA CHI
10 20 30 40 50 60
0
500
1000
1500
2000
TiO
2
CdS
(103)(110)
(102)
(101)(100)
(002)
ITOITO
ITO
ITO
ITO
ITO
ITO
2-Theta-Scale
L
in
(C
p
s)
Fig. 2. XRD pattern of TiO2/ CdS nanocomposite thin film on ITO substrate
350 400 450 500 550 600 650
b
a
A
b
s
or
b
a
n
ce
[a
.u
]
Wavelength [nm]
a-TiO
2
b-TiO
2
/CdS
-0.2 0.0 0.2
1E-8
1E-7
1E-6
1E-5
1E-4
c
a
b
dJ[
A
c
m
-
2 ]
E[V]
a-TiO
2
dark b-TiO
2
light
c-T iO
2
/CdS dark d-TiO
2
/CdS light
Fig. 3. UV-vis absorption spectra of TiO2
film and TiO2 / CdS nano composite film.
Fig. 4. Photocurrent-potential behaviors of
ITO/TiO2 and the ITO/TiO2 /CdS electrodes.
and ITO/TiO2/CdS. It can be clearly seen that the photo-electrochemical cell made from
films consisting only of TiO2 has very low values of Voc and Jsc, whereas for the cell
with coating a thin film of CdS, both Voc and Jsc increase dramatically. It is clear that
the coupling CdS with TiO2 ameliorates the photoelectric performance of the TiO2/CdS
nanocomposite films.
Table 1. The Voc and Jsc of all samples used as working electrodes in the PEC cell
Sample Voc (mV) Jsc (µAcm−2)
TiO2 32 0.5
TiO2/CdS 143 11.5
NANOCOMPOSITE THIN FILM TiO2/CdS ELECTRODES... 61
It can be explained as the following: The electron affinity of the CdS is higher than
that of the TiO2. Therefore, according to Andersons model, a type-II heterojunction is
formed between CdS and TiO2 [30]. The light from the halogen lamp generates electron-
hole pairs in CdS. The electrons from the conduction band of CdS are quickly transferred
to the conduction band of TiO2, as shown in Fig.5. Once the electrons diffuse into the
conduction band of TiO2, the probability of its decay is small, because there can be
no free holes in TiO2 under visible excitation. As a result, the electrons accumulate in
the conduction band of the TiO2 and the holes accumulate in the valance band of the
CdS. In this way, the charge separation is achieved. The redox couple in the electrolyte,
after reducing the oxidized dye, can be renewed in the counter-electrode, making the
photoelectrochemical cell regenerative [17, 31]. It can be clearly seen that the coupling
of TiO2 with CdS can significantly enhance the separate of the electron and hole pairs.
These results are promising for improving the efficiency of photovoltaic devices.
Fig. 5. Schematic diagram showing the charge transfer at TiO2-CdS hetero-
junction interface
IV. CONCLUSIONS
TiO2/CdS nanocomposite thin films have been successfully prepared by a thermal
evaporation technique. X-Ray diffraction studies indicated that the CdS has preferential
orientation in the (002) direction. The prepared TiO2/CdS nanocomposite thin film has
been possessed more excellent photoelectrochemical performance than that of ITO/TiO2
film. It is believed that CdS thin films sensitizing TiO2 microporus structures can be used
in fabricating high efficiency photovoltaic devices. The differences in CdS layer thickness
on the ITO/TiO2 substrates will be investigated in our next study to find out a suitable
one that has the best of values of VOC and JSC.
ACKNOWLEDGMENTS
This work is supported in part by the National Program for Basic Researches in
Natural Science of Vietnam. A part of the work was done with the help of the Key
Laboratory in Electronic Materials and Devices, Institute of Materials Science, Vietnam
Academy of Science and Technology. We would like to thank the colleagues for their
contribution to the work.
62 DANG TRAN CHIEN, PHAM DUY LONG, PHAM VAN HOI, AND LE HA CHI
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Nanocomposite Thin Film TiO2/CdS  Electrodes Prepared  by Thermal Evaporation Process for Photovoltaic Applications

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Nanocomposite Thin Film TiO2/CdS Electrodes Prepared by Thermal Evaporation Process for Photovoltaic Applications

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