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Delivered by Publishing Technology to: Nanyang Technological University
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Copyright American Scientific Publishers
RESEARCH ARTICLE
Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 787–792, 2013
CdSe Nanocrystal Sensitized Anatase TiO2(001)
Tetragonal Nanosheet-Array Films for
Photovoltaic Application
Shuanglong Feng1, Junyou Yang1∗, Ming Liu1, and Yong Liu2
1State Key Laboratory of Material Processing and Die and Mould Technology, Huazhong University of
Science and Technology, Wuhan 430074, P. R. China
2Hubei Institute of Measurement and Testing Technology, Wuhan 430223, P. R. China
CdSe nanocrystal sensitized TiO2nanosheet array heterostructure films were fabricated by a two-
step method. Firstly, a single crystalline anatase TiO2tetragonal nanosheet-array film on a transpar-
ent conductive fluorine-doped tin oxide (FTO) substrate was successfully prepared by hydrothermal
method. Then, CdSe nanocrystalline sensitizers were deposited on the TiO2nanosheet array by
CBD method. The products were characterized with XRD, SEM, TEM and UV-vis absorption spec-
troscopy. The effect of the CdSe nanocrystal deposition time and the length of the TiO2sheet
on the photovoltaic performance of the resulting CdSe/TiO2nanosheet array electrodes were also
investigated. In comparison with the non-sensitized TiO2nanosheet array, the photocurrent of CdSe
sensitized TiO2nanosheet has a great enhancement, which gives some insight to the fundamental
mechanism of the performance improvement.
Keywords: TiO2, CdSe, Nanosheet, Solar Cell, Hydrothermal.
1. INTRODUCTION
Titanium dioxide (TiO2, which plays an important role in
many applications, such as photocatalysis1and solar cell,23
has been paid much attention to by many researchers
in recent years. TiO2films with various nanostructures,
such as nanotubes,4nanowires5and nanoparticles,6have
been synthesized and sensitized with specific inorganic
compounds to widen the absorption edge to the visi-
ble spectral range and improve visible light photoelectron
activity.7–10 Among these TiO2nanostructures, nanosheet
array with highly reactive {001} facets are of both theo-
retical and practical significance due to their many intrin-
sic shape-dependent properties.11 However, well-defined
anatase TiO2single crystals with {001} facets are very dif-
ficult to grow, and influence of the physical chemical and
electronic properties on the electrochemical and photoelec-
trochemical response of this films, has less been known yet.
In our previous work,12 an anatase single crystal TiO2
nanosheet with {001} facets array film was prepared on
FTO surface through a facile hydrothermal method and
its growth mechanism was discussed in detail. As is well
known, TiO2is an important wide band gap semiconduc-
tor, but it can’t absorb and utilize the visible region of the
∗Author to whom correspondence should be addressed.
solar spectrum. In order to investigate the photoelectro-
chemical response of TiO2nanosheet films, narrow band
gap sensitizers should be composited with them as visible-
light harvester. CdSe (Eg=22 eV) as an important chalco-
genide semiconductor is most widely studied because it
can absorb a light with wavelength more than 720 nm and
has superior ability to inhabit the charge recombination
at electrode/electrolyte interface.13 Niitsoo et al. reported
that CdSe sensitized nanocrystalline TiO2photoelectrode
and achieved better conversion efficiency under one sun
illumination.14 Lee et al. grew CdSe QDs between meso-
porous TiO2and CdS to reduce the pre-self-assembly of
CdS QDs and enhanced the conversion efficiency.15
In this work, TiO2nanosheets array film was fabri-
cated by a hydrothermal method reported by us before,12
and chemical bath deposition (CBD) was employed to
prepare the CdSe/TiO2nanosheet composites. The mor-
phology, structure, and light absorption properties were
characterized for the CdSe/TiO2nanosheet heterostructure
arrays and the photoelectrochemical performance was also
investigated.
2. EXPERIMENTAL DETAILS
TiO2tetragonal nanosheet-array films were synthesized by
a hydrothermal method. In a typical synthesis, 30 mL of
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 2 1533-4880/2013/13/787/006 doi:10.1166/jnn.2013.6026 787
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RESEARCH ARTICLE
CdSe Nanocrystal Sensitized Anatase TiO2(001) Tetragonal Nanosheet-Array Films for Photovoltaic Application Feng et al.
deionized water was mixed with 30 mL of concentrated
hydrochloric acid (36.5%–38% by weight) to reach a total
volume of 60 mL in a Teflon-lined stainless steel autoclave
(100 mL), 1 mL titanium butoxide and 0.5 g ammonium
hexafluorotitanate ((NH42TiF6were added into this solu-
tion. Then, two pieces of clean FTO substrates (F:SnO2,
Tec 15, 10 /), were placed at the bottom of the Teflon-
liner with the conductive side facing up. The hydrothermal
synthesis was conducted at 150 C for 1–24 h in an elec-
tric oven. After cooling down, the FTO substrates were
taken out and rinsed with deionized water thoroughly.
CdSe was deposited on TiO2substrates by the CBD
method with potassium nitrilotriacetate (N(CH2COOK)3=
NTA) as the complexing agent and NaSeSO3as the Se
source. An aqueous NaSeSO3solution was prepared by
refluxing 0.02 mol Se powder with 0.05 M Na2SO3at
70 C for seven hours (the PH of this precursor solution
wasn’t adjusted). The prepared TiO2nanosheet array sub-
strate was immersed into the aqueous solution consisting
of 2 mM CdSO4, 10 mM NaSeSO3, and 10 mM NTA, and
the bath temperature was kept at 0 C. The deposition time
for CdSe nanocrystal varied from 0 to 60 minutes. After
deposition for different times, the films were rinsed with
deionized water. The annealing process of the composite
films was performed at 350 CinN
2atmosphere.
Fig. 1. (a) The SEM image of TiO2film. (b) XRD pattern of the TiO2nanosheet. (c) The SEM image of CdSe sensitized TiO2nanosheet. (d) TEM
bright-field image of CdSe/TiO2, and the inset is SAED pattern of CdSe/TiO2composite structure.
XRD patterns of the as-prepared films were recorded in
a Philip X’pert X-ray diffractometer (Cu Kirradiation,
=015418 nm).2Morphology and structure information
of the products were examined with a Sirion 200 field
emission scanning electron microscope (FE-SEM) and
a FEI Tecnai G230 TEM respectively. A Perkin Elmer
Lambda35 spectrometer UV-vis system was used to obtain
the absorption spectra of the samples over a range of
380–700 nm.
Solar cells with different TiO2photoanodes were assem-
bled and the photovoltage characteristics were investigated
using the polysulfide electrolyte solution. The polysulfide
electrolyte solution consists of Na2S (0.5 M), S (0.125 M),
and KCl (0.2 M). The active area of the cells was 0.16 cm2.
The current–voltage (I–V) characteristics were recorded
using a potentiostat system (CHI604D), an Oriel 91192
AM 1.5 solar simulator was used as the light source.
3. RESULTS AND DISCUSSION
Figure 1 shows the typical SEM images of TiO2nano-
sheet films without and with CdSe nanocrystals. From
Figure 1(a), it can be clearly seen that the TiO2film
was composed of smooth, highly oriented nanosheet array.
XRD patten in Figure 1(b) shows that the nanosheet
788 J. Nanosci. Nanotechnol. 13, 787–792,2013
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RESEARCH ARTICLE
Feng et al. CdSe Nanocrystal Sensitized Anatase TiO2(001) Tetragonal Nanosheet-Array Films for Photovoltaic Application
Fig. 2. The EDX of CdSe/TiO2film.
TiO2is composed of anatase TiO2(JCPDS No. 71-1167).
The SEM image in Figure 1(c) shows that there are
many tiny CdSe particles coated on the surface of TiO2
nanosheets. The TEM image and SAED of a scraped
CdSe/TiO2film were shown in Figure 1(d). The diffraction
rings can be indexed as the polycrystalline cubic CdSe,16
while the spots in the SAED pattern comes from the sin-
gle crystal anatase TiO2with (001) facets. Furthermore,
EDX quantitative analysis of the as-prepared CdSe/TiO2
Fig. 3. (a) Optical absorption of TiO2nanosheets deposited with CdSe for 0, 30, 45, and 60 min. (b)–(d) The SEM images of CdSe deposited on
TiO2surface for 30, 45 and 60 min, respectively.
nanosheets, as shown in Figure 2, gave an approximately
1:1 stoichiometric ratio of Cd to Se (Cd, 4.42%; Se,
3.96%), which is consistent with the formula of CdSe
compound. These results suggested that CdSe nanocrystals
could be successfully deposited on TiO2nanosheet arrays
surface to form CdSe/TiO2composite structure by a facile
hydrothermal method.
The optical absorption spectra of the TiO2nanosheet
films with deposition of CdSe nanocrystals for differ-
ent times (0304560 min) were shown in Figure 3.
Figure 3(a) shows that a significant red-shifted absorp-
tion occurs in visible region from 400 nm to 600 nm
after sensitization with CdSe. Additionally, the absorbance
of the spectrum increases with prolonging the deposition
time, indicating an increased adsorption amount and size
of CdSe, SEM images of CdSe for different deposition
time in Figures 3(b)–(d) are in agreement with the results
in Figure 3(b). It means that CdSe nanocrystals should be
mainly responsible for electron generation and electronic
transition under visible light illumination.
Figure 4 shows the cross-sectional SEM images of TiO2
nanosheet arrays with different length for different growth
time. After growing for 12 h, the length of TiO2nanosheets
are about 500 nm (Fig. 4(a)). Prolonging growth time
from 12 h to 18 h, an ordered TiO2nanosheet array film
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RESEARCH ARTICLE
CdSe Nanocrystal Sensitized Anatase TiO2(001) Tetragonal Nanosheet-Array Films for Photovoltaic Application Feng et al.
Fig. 4. SEM images of TiO2nanosheet arrays grown for different times: (a) 12 h, (b) 18 h, (c) 24 h, (d) 30 h.
about 1 m in thickness was obtained (Fig. 4(b)). After
growing for 24 h, the nanosheets became much more
densely and the thickness of the film increasing to 2 m
(Fig. 4(c)). But some new nanosheets appeared in the
interspaces of nanosheets were obtained after growing for
30 h (Fig. 4(d)). In order to study the effect of the length
of TiO2nanosheets on the photovoltaic performance of the
solar cells, the above TiO2samples were further sensitized
with CdSe nanocrystals for an hour without annealing pro-
cess. Figure 5 shows the variation of I–Vcurve with the
length of TiO2nanosheet. It can be observed that the short
circuit current density initially increased with increase of
TiO2nanosheet length, and then decreased when the length
reached 2.88 m. The highest conversion efficiency was
achieved at the length of 2 m. The optimum working cell
gave a conversion efficiency () of 0.18%, an open-circuit
photovoltage (Voc) of 0.32 V, and a short-circuit photocur-
rent density (Jsc) of 1.25 mA/cm2respectively. It is reason-
ablely that the longer and more ordered nanosheets could
provide the photo injected electrons with a direct elec-
trical pathway to the photoanode and lead to fast charge
transport17 and a larger surface area for a greater CdSe
loading to enhance the light-harvesting efficiency. How-
ever, the over-long TiO2nanosheets resulted in the forma-
tion of a large intercross of nanosheets with a low surface
area for CdSe loading on the bottom of the film, which
accounts for the diminution of electrons from CdSe to
TiO2nanosheets.18 Thus, nanosheets with an appropriate
surface area available for CdSe adsorption are very impor-
tant for the conversion efficiency.
Figure 6 shows the I–Vcurves of the solar cells based
on the CdSe/TiO2film before and after annealing. After
annealing, a significant enhancement of the photovoltaic
performance of the solar cell can be observed, the current
Fig. 5. I–Vcurves plotted as a function of the length of TiO2
nanosheets: (a) 500 nm, (b) 1 m, (c) 2 m, (d) 2.88 m.
790 J. Nanosci. Nanotechnol. 13, 787–792,2013
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RESEARCH ARTICLE
Feng et al. CdSe Nanocrystal Sensitized Anatase TiO2(001) Tetragonal Nanosheet-Array Films for Photovoltaic Application
Fig. 6. I–Vcurves of CdSe/TiO2films before (a) and after
(b) annealing.
density (Jscincreased from 1.25 to 2.75 mA/cm2.Itis
because that the crystallinity was improved and surface
defects were reduced after annealing,1920 and the electrical
contact was also improved after annealing, thus provided
an effective pathway for carriers.21
As shown in Figure 7, TiO2nanosheet array film
deposited with CdSe for 1 h, is superior to the others in
efficiency. In the CdSe/TiO2nanosheet array photoanode,
electrons are injected across the CdSe/nanosheet interface
into the TiO2, this process is facilitated by the overlap
between the electronic states in the CdSe and TiO2con-
duction band. Short CdSe deposition time means less load-
ing of CdSe, therefore it results in a less light harvest.
However, an over-long deposition time (80 min) resulted
in an excess deposition of CdSe nanoparticles on the
TiO2nanosheets, which reduced the collection probability
because it could result in a longer transport path for the
photo-generated electron–hole pairs in the sensitizer before
Fig. 7. I–Vcurves of CdSe/TiO2films prepared with different CdSe
deposition times: (a) 0 min, (b) 30 min, (c) 45 min, (d) 60 min,
(e) 80 min.
separated and collected, and thus acted as a potential bar-
rier for charge transfer and weakened the photocurrent.
4. CONCLUSIONS
In summary, CdSe/TiO2nanosheet array films as solar
cells photoanodes were prepared by a two-step method
and their photovoltaic performance were investigated.
The nanostructure of CdSe/TiO2was characterized. Also,
the effects of the thickness of TiO2film, CdSe nano-
crystal phase and CdSe deposition time on the photovoltaic
of the solar cell were investigated. The annealed 2 m
CdSe/TiO2nanosheets film showed the best photovoltaic
property. A power conversion efficiency of 0.3%, a pho-
tocurrent density of 2.75 mA/cm2and an open circuit
potential of 0.43 V were achieved under optimum para-
meters. Although the power-conversion efficiency is not
enough high in this initial work, but this study gives some
insights into the fundamental mechanisms that improve the
performance.
Acknowledgment: This work is co-financed by
National Natural Science Foundation of China (Grant Nos.
50827204 and 51072062), Research Fund for the Doctoral
Program of Higher Education (No. 20100142110016), the
Fundamental Research Funds for the Central Universi-
ties (2010ZD014), and the Cultivation Fund of the Key
Scientific and Technical Innovation Project, Ministry of
Education of China (No. 707044). The technical assistance
from the Analytical and Testing Center of HUST is also
gratefully acknowledged.
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CdSe Nanocrystal Sensitized Anatase TiO2(001) Tetragonal Nanosheet-Array Films for Photovoltaic Application Feng et al.
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Received: 12 September 2011. Accepted: 30 November 2011.
792 J. Nanosci. Nanotechnol. 13, 787–792,2013
