Zinc Oxide–Titania Heterojunction-based Solid Nanospheres as Photoanodes for Electron-Trapping in Dye-Sensitized Solar Cells

DOI: 10.1002/ente.201600357
Zinc Oxide–Titania Heterojunction-based Solid
Nanospheres as Photoanodes for Electron-Trapping in
Dye-Sensitized Solar Cells
Kiran P. Shejale, Devika Laishram, Ritu Gupta,* and Rakesh K. Sharma*[a]
Introduction
Titania and zinc oxide are the most-explored metal oxide
semiconductors and are extensively used in a variety of ap-
plications such as environmental detoxification, sensing, solar
cells, and photocatalysis.[1–3]
Over the past few decades, com-
posites,[4]
layered structures,[5]
doping,[6]
and various nano-
morphologies[7–9]
of TiO2/ZnO have been extensively studied.
Despite the wide range of uses of TiO2/ZnO composites,
drawbacks such as charge recombination, low surface areas,
and poor electron transport make their application diffi-
cult.[10,11]
Generating a highly porous surface for the electron
trapping by preparing heterojunctions of ZnO and TiO2 sem-
iconductors is an efficient way to solve these issues, but limit-
ed efforts have been made to enhance poor electron trans-
port owing to a nonfunctional interface at the heterojunc-
tion.[12–16]
A good heterojunction should have matching band
levels for high electron mobility and a conductive interface.[1]
A heterojunction of TiO2 and ZnO with similar band struc-
ture usually forms a II-type heterojunction with high mobili-
ty, due to improved charge isolation compared to the pure
oxides, and reduces recombination during charge separation.
Titanium-doped ZnO has been reported to show high con-
ductivity and high surface area, which are important for elec-
tron trapping.[17]
ZnO has a higher electron mobility (205–
300 cm2
Vs@1
) than TiO2 (10@5
cm2
Vs@1
)[18,19]
and thus ZnO–
TiO2 heterojunction with appropriate architectures and opti-
mal doping can facilitate electron trapping.
Various architectures of both oxides, in the form of nano-
tubes, nanowires, clusters, hollow spheres, core–shell struc-
tures, and 3D hybrid arrays, have been explored for en-
hanced charge transport.[20–23]
Unfortunately, detrapping due
to random electrical paths, surface states, and in some cases
defects hinders their application in dye-sensitized solar cells
(DSSCs). Compared to other architectures the solid nano-
sphere structure reported for the first time in this study is ex-
hibits a higher internal surface area, enhanced light scatter-
ing ability, and more sites for dye anchoring. The highly
porous, micrometer-sized volume spheres significantly en-
hance the total surface area of the film.[24]
To understand the
role of the heterojunction, DSSCs are employed as a simple
and quick method. Attempts have been made towards ex-
ploring combinations of TiO2 and ZnO as photoanode mate-
rials in DSSCs where explicit structural and morphology de-
termines the photoconversion efficiency of the cell (literature
on ZnO–TiO2 architectures used in DSSC is summarized in
detail in Supporting Information, Table S2).[25,26]
.
Herein, we report the synthesis of a novel ZnO–TiO2 het-
erojunction of solid nanospheres decorated with different
percentages of ZnO. The solid nanospheres are synthesized
by an environmentally friendly and facile route. These meso-
porous structures are comprised of polydisperse aggregates
of both oxides, and possess type-II heterojunctions. The
ZnO–TiO2 heterojunction architecture can separate electrons
and holes and provide more electron-trapping sites. The
high-specific-surface-area solid nanospheres used as photoa-
node material in the DSSC can trap more light by facilitating
a higher number of sites for dye attachment. In addition,
these heterojunctions have potential to show increment in
the electron trapping, as a result, charge recombination is
Agile nanostructure architectures and smart combinations of
semiconducting metal oxide materials are key features of
high-performing dye-sensitized solar cells (DSSCs). Herein,
we synthesize mesoporous solid nanospheres of ZnO–TiO2
with type-II heterojunction and use these as an efficient pho-
toanode material for excellent photoconversion. These poly-
disperse aggregates doped with 1%, 5%, and 10% of ZnO
exhibit improved solar cell performance with respect to pris-
tine TiO2 under AM 1.5 G. The 1% ZnO doped TiO2 nano-
sphere possess high specific surface area (84.23 m2
g@1
) as
a photoanode and shows high photoconversion efficiency of
about 8.07% with ca. 18% improvement in the photocurrent
density (Jsc) compare to TiO2 nanosphere. The improved
solar cell performance (Dh=40%) of ZnO decorated TiO2
nanospheres is ascribed to type-II heterojunction of ZnO–
TiO2, that reduces the electron recombination and synergisti-
cally enhances the electron mobility and charge collection
capability.
[a] K. P. Shejale, D. Laishram, Dr. R. Gupta, Dr. R. K. Sharma
Department of Chemistry
Indian Institute of Technology Jodhpur
Jodhpur, Rajasthan 342011 (India)
E-mail: ritu@iitj.ac.in
rks@iitj.ac.in
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/ente.201600357.
Energy Technol. 2017, 5, 489 – 494 T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 489

suppressed and the performance of the DSSC is greatly en-
hanced.
Results and Discussion
The 0, 1, 5, and 10% ZnO doped TiO2 solid nanospheres
were synthesized by a solvothermal crystallization reaction.
Firstly, TiO2 precursor beads were synthesized by hydrolysis
of titanium tetra-isopropoxide followed by addition of differ-
ent amount of zinc acetate dihydrate-precursor during the
solvothermal reaction, resulting in a hybrid nanostructure of
ZnO–TiO2 solid nanospheres. In X-ray diffraction (XRD)
analysis (shown in Figure 1), all the diffraction peaks corre-
late with anatase-phase TiO2 (Joint Committee on Powder
Diffraction Standards (JCPDS) file 21-1272). However, the
crystallinity of the TiO2 solid nanospheres improves with the
percentage of ZnO, and resulted in a relatively higher peak
intensity.[27]
The average crystalline size for TiO2 solid nano-
spheres was calculated, using the Debye–Scherrer equa-
tion,[28]
as ca. 12 nm, which increased to ca. 22 nm for 10%
ZnO doping as shown in the inset of Figure 1, left.
X-ray photoelectron spectroscopy (XPS) was carried out
to analyze the chemical composition and oxide species of the
ZnO-doped TiO2 solid nanospheres. The full-range spectra
show the presence of Ti2p, O1s, and Zn2p as shown in
Figure 2a and respective binding energy values are summar-
ized in Table 1. The spin–orbit components of Zn2p3/2 and
Zn2p1/2 shows binding energy values at around 1021 and
1044 eV respectively, and the splitting of 23 eV indicates that
zinc ions exist mainly as Zn2+
due to the formation of ZnO
(Figure 2b).[29,30]
The intensity of the Zn2p peak increases up
on increasing ZnO %, however, the peak position of Zn2p
exhibits a notable shift by ca. 0.5 eV towards higher binding
energy (BE) value upon introduction of ZnO. This clearly in-
dicates that Zn2+
persists more in the solid nanosphere lat-
tice rather than undergoing phase separation. The Ti2p spec-
tra are similar to typical Ti4+
in the form of TiO2.[31]
The
effect of ZnO on the Ti2p spectrum is visible by the shifting
of the Ti2p peak by 0.2 eV to lower BE values, as shown in
Figure 2c. The cause of this BE difference may be the inter-
face between ZnO and TiO2 nanoparticles. The broadness of
the O1s spectra (Figure 2d) indicates the presence of several
types of oxygen species, with O2@
appearing at 530 eV and
OH@
species at 531 eV. In all samples, the Zn/Ti atomic ratio
was less than 1 (Table 1), which suggests a random distribu-
tion of ZnO all over the TiO2 solid nanospheres.[32]
The O1s
peak of ZnO–TiO2 solid nanosphere shifts due to Zn@O@Ti
complex bond formation at the interface.
Figure 3 shows the influence of the amount of ZnO (wt%)
on the structure and surface morphology of TiO2 solid nano-
spheres. The field-emission scanning electron microscopy
(FESEM) images in Figure 3a–d illustrate the porous nature
of these nanostructures, formed by the aggregation of the
nanocrystallites of TiO2 and ZnO. The porosity is greatly in-
fluenced by ZnO doping. With increasing amounts of ZnO,
the TiO2 solid nanospheres begin to disintegrate, with a de-
crease in average sphere size from 600:20 to 200:30 nm.
The detailed structure of the solid nanospheres was investi-
gated by transmission electron microscopy (TEM) images as
shown in Figure 3e–h. Interestingly, the solid spheres are no
longer packed with monodisperse nanocrystallites, however,
these consist of polydisperse aggregates. Moreover, there is
a gradual increase in crystallite size of TiO2 with increasing
Figure 1. XRD patterns of the TiO2 doped with ZnO of varying weight percent-
age (0%, 1%, 5% and 10%). The inset on right side shows the respective
(101) peak used for fitting and particle size calculation.
Figure 2. a) Full range XPS survey spectra, high resolution spectra of
b) Zn2p, c) Ti2p and d) O1s of TiO2 solid nanospheres with different ZnO
doping.
Table 1. The surface analysis parameters and atomic ratios calculated
from XPS spectra.
Amount of ZnO
in TiO2 [%]
Binding energy [eV] at%
Zn Zn Ti Ti O1s O1s Zn/Ti
2p3/2 2p1/2 2p3/2 2p1/2 (O2@
) (OH@
)
0 – – 458.6 464.4 530 532.2 –
1 1022.1 1045.2 458.6 464.4 530 530 0.04
5 1021.6 1044.8 458.4 464.2 529.7 531.8 0.14
10 1021.5 1044.7 458.4 464.2 529.6 531.7 0.2

ZnO resulting in smaller pores at the surface of the solid
sphere.
A typical TEM image of 5% ZnO in TiO2 sample was
chosen for detailed TEM studies as shown in Figure 4a. The
HRTEM image (Figure 4b) is taken from a selected area
marked in Figure 4a which shows lattice planes with two dif-
ferent d-spacings. The d-spacing of 0.35 nm corresponds to
(101) anatase TiO2 while the other value of 0.26 nm matches
well with the (002) wurtzite structure of ZnO.
No transitional layer was found at the interface between
the ZnO and TiO2 and the distinct boundary of nanocrystals,
which indicates that these solid nanospheres are a mix of
both nanostructures. In the electron diffraction (ED) pattern,
only anatase TiO2 is visible (Figure 4c) and ZnO is absent
due to the lower weight percentage of ZnO. Figure 4d–f con-
firms the compositional homogeneity of the solid nano-
spheres by the random distribution of elements Ti, Zn, and
O. Electron dispersive X-ray (EDX) spectra are shown in
Figure S1 (Supporting Information) and the elemental distri-
bution is summarized by bargraph in Figure 4g. The atomic
percentage of zinc gradually increases from 0 to 3.09%, with
increasing amounts of ZnO doping. The mismatch between
doping amount and atomic percentage controls the growth
behavior of ZnO/TiO2 solid nanosphere.[33]
The zinc ions
have a tendency to segregate into anatase TiO2 crystal lattice
only if their doping concentration (atomic) is <0.1%. This is
a result of a significant difference in charge and atomic prop-
erties between titanium and zinc ions which apparently hin-
ders the incorporation of Zn ions into TiO2 lattice. This leads
to a mismatch between the actual weight percentage and pre-
cursor doping percentage resulting in the heterojunction
crystallite formation of ZnO–TiO2 solid nanospheres.[34]
The adsorption–desorption isotherms of the ZnO-doped
TiO2 solid nanosphere are shown in Figure 5a. The shape of
the curve corresponds to a type-IV isotherm which is charac-
teristic of the mesoporous structure and the observed hyste-
resis suggests the presence of small pores, as observed loops
were not shifted to a high relative pressure.[35]
The specific
surface area obtained from Brunauer–Emmett–Teller (BET)
analysis shows a gradual but linear decrease from
95.23 m2
g@1
to 63 m2
g@1
(Figure 5b), however, the overall
surface area is still higher in comparison to P25
(51 m2
g@1
).[36]
The average pore size was calculated by Bar-
rett–Joyner–Halenda (BJH) analysis for different doping per-
centages and their distribution is shown in the inset of Fig-
ure 5a. Most interestingly, the average pore volume increases
for 1% doping of ZnO in TiO2 while it decremented step-
wise at higher doping levels. This deviation from a decreasing
trend is observed because the size of the pores formed in the
TiO2 solid nanospheres are about the size of the ZnO aggre-
gates.[37]
The average pore size decreases on increasing ZnO
content beyond 1%. This may be due to the bigger grain size
for higher ZnO percentages, as shown by the TEM images in
Figure 3. The UV-vis diffuse reflectance spectra in the wide
spectral range of 250–700 nm for TiO2 solid nanospheres
with different ZnO% are shown in Figure S2 (Supporting In-
formation). The spectra show a decrease in reflectance upon
increasing ZnO%. The decrease in reflectance is slightly
higher (DR~2%) for 1% doped ZnO as compared to 5%
and 10% doped ZnO-TiO2 samples, resulting in enhance-
ment in light scattering for 1% ZnO. The 1% ZnO/TiO2
solid nanospheres scatter visible light more efficiently as
compared to the TiO2 nanospheres alone because the aver-
Figure 3. a–d) FESEM images and e–h) TEM images of TiO2 solid nanosphere doped with ZnO of 0%, 1%, 5% and 10% by weight respectively.
Figure 4. a) TEM image, b) HRTEM image and c) ED pattern of TiO2 solid
nanosphere with 5% ZnO doping. d–f) EDS maps of Ti K (red), Zn K
(yellow) and O K (green). g) Bar graph displaying atomic percentages for dif-
ferent ZnO doping levels.

age size is of the order of the wavelength of light. The dense-
ly packed solid nanospheres of both oxides can result in mul-
tiple scattering of light owing to their random distribu-
tion.[38,39]
The ZnO-doped TiO2 solid nanosphere films were used as
photoanode layer in DSSCs fabricated as detailed in the Ex-
perimental Section.[40]
The ZnO-doped TiO2 solid nano-
sphere material was screen printed in the form of a paste
and annealed resulting in a photoanode with thickness of ca.
15 mm. The current density versus voltage (J–V) curves for
DSSC devices fabricated using TiO2 doped with ZnO of
varying weight percentages are shown in Figure 6a. The per-
formance parameters along with the dye loading amount on
various photoanodes are summarized in Table 2. The effect
of ZnO on the photovoltaic properties of TiO2 can be quanti-
tatively shown by the bar graph in Figure 6b. The 1% ZnO
cell shows the highest Jsc up to 18.96 mAcm@2
compared to
14.88, 15.63, and 15.22 mAcm@2
for 0%, 5% and 10%
doped cells. As the 1% showed maximum efficiency, we con-
sidered it important to examine the efficiencies of solar cells
with ZnO between 1% and 5%. The 2% and 3% ZnO-
doped TiO2-based DSSC exhibited JSC of 17.73, and
16.22 mAcm@2
respectively (see Supporting Information, Fig-
ure S5) which is expected and in accordance with the trend
as shown in Figure 6. The enhancement in photocurrent den-
sity for 1% ZnO–TiO2-based DSSCs increases the overall ef-
ficiency. Usually, a high photocurrent is an evidence of better
dye loading. This predominatly increases the light-harvesting
capability of the ZnO–TiO2-based photoanode material.[41]
The dye loading capability of the photoanode was deter-
mined by the dye desorption using 10 mM NaOH solution,
and the amount was calculated from the absorbance spectra
shown in Figure S3 (Supporting Information). On ZnO
doping, the increase in dye loading was lower (~21%) than
the expected value based on efficiency (Dh~40%), however,
the light harvesting capability is improved, which improves
the overall efficiency.
Figure 5. a) N2 adsorption-desorption BET isotherm of TiO2 solid nano-
spheres with different ZnO doping. b) Effect of varying ZnO doping level on
surface area and average pore size.
Figure 6. a) J–V characteristics of DSSC with photoanode based on TiO2 solid nanospheres with different ZnO doping. b) Bar graph displaying photovoltaic pa-
rameters obtained for different DSSC devices fabricated by varying the ZnO doping levels. c) Nyquist plots and equivalent circuit (inset) of ZnO-TiO2 solid
nanosphere with various percentages of ZnO as photoanode in DSSC.
Table 2. Photovoltaic properties of dye-sensitized solar cells based on
ZnO-doped TiO2 solid nanospheres.[a]
Amount of ZnO
in TiO2 [%]
Average pore
size [nm]
N719 dye loading
[nMcm@2
]
JSC
[mAcm@2
]
VOC
[V]
FF
[%]
h
[%]
0 0.55 2.52 14.88 0.68 56 5.78
1 0.57 3.2 18.96 0.70 60 8.07
5 0.46 2.65 15.63 0.71 64 7.11
10 0.38 2.64 15.22 0.74 60 6.68
[a] Jsc, Voc, FF, and h are the short-circuit photocurrent density, open-circuit
potential, fill factor, and photoconversion efficiency, respectively. The solar
cell measurements were carried out under 1 sun illumination.

The electron transport in ZnO-doped TiO2 seems to be fa-
vorable, as seen from the lower electrode resistance (Fig-
ure 6c) and increase in the electron carrier density up to
47% with increasing ZnO% (summarized in the Supporting
Information, Table S1). The charge resistance at their inter-
face (Rct) increases with increasing ZnO, however the value
for 1% ZnO is comparatively minimal (~13.88 W) as derived
from the second semicircle and by fitting a Nyquist plot with
equivalent circuit as shown in Figure 6c. Because the con-
duction band of ZnO formed by the s orbital provides a sig-
nificantly smaller me (effective electron mass) compared to
the higher me corresponding to the d orbital of TiO2 conduc-
tion band, ZnO-doped TiO2 has more electron mobility than
TiO2.[20–23,42]
The enhancement in electron transport (ca.
32%) as shown in Table S1 (Supporting Information) im-
proves the photocurrent density, along with photovoltage
and fill factor, resulting in an increase in the photoconversion
efficiency of the DSSC device. For ZnO% <1 the photocur-
rent density is relatively lower, which can be attributed to
the difference in crystallite size up to 10 nm and lower pore
size of the nanosphere as shown in Table 2. This also reveals
the reduced anchoring sites for N719 dye molecules, further
inhibiting the photocurrent density in 5% and 10% nano-
spheres. Although both photocurrent density and efficiency
deteriorated at higher doping percentage, the open-circuit
voltage clearly shows enhancement up to 0.74 V (shown in
Figure 6b). The higher open circuit voltage is the result of
a lower recombination rate at electrolyte interfaces. Both
ZnO and TiO2 exhibit similar band structures, as shown in
the Supporting Information, Figure S4. The incorporation of
ZnO in TiO2 makes energy at conduction band (Ecb) of ZnO
more prominent, which easily transfers electrons to the Ecb
of TiO2 rather than the electrolyte species.[42]
The path length
is lowered at an optimized doping of 1% ZnO that further
reduces the recombination rate at the electrolyte.
Conclusions
A polydisperse aggregate structure of ZnO–TiO2 nano-
spheres is demonstrated to be an excellent photoanode mate-
rial. The anatase TiO2 solid nanospheres decorated with dif-
ferent percentages of ZnO are synthesized by a hydrated-
salt-assisted hydrothermal method. The aggregate structure
leads to good light-scattering properties along with a surface
area appropriate for electron trapping. The heterojunction
spheres exhibit higher photocurrent density, up to
18.96 mAcm@2
, which is due to spherical architecture increas-
ing the number anchoring sites for dye molecules with en-
hanced scattering effect. With 1% ZnO, solid nanospheres
the maximum photoconversion efficiency was about 8.07%.
An enhancement of open-circuit voltage was observed for
the ZnO decoration, also. These improvements are attributed
to enhanced electron transport efficiency and lower recombi-
nation with a better light-harvesting structure.
Experimental Section
Synthesis of photoanode material
All chemicals used in this synthesis were procured commercially
and used without further purification. In a typical synthesis of
the ZnO-TiO2 solid nanosphere, 1.97 g of hexadecylamine
(HDA) was added to 200 mL absolute ethanol, 0.8 mL of 0.1 M
KCl, and then the solution was vigorously stirred to dissolve
HDA completely. Next, 4 mL of titanium tetra-isopropoxide
(TTIP) was added dropwise. For doping, 1%, 5%, and 10 wt%
of ZnOTTIP was mixed followed by sonication for 10 min. The
mixture was kept static overnight and then filtered. The filtrate
was washed with distilled water and absolute ethanol several
times followed by drying in air at room temperature to obtain
a powder. A portion of the above powder (0.8 g) was dispersed
in 10 mL absolute ethanol having 0.45 M liquid ammonia and
transferred to a Teflon-lined autoclave (20 mL). It was kept in an
oven at 1808C for 16 h for complete reaction. The filtrate was
collected by washing with distilled water and ethanol and air an-
nealed at 5008C for 2 h.
Fabrication of dye-sensitized solar cells (DSSCs)
All solar cells were fabricated according to a typical procedure
as mentioned in our previous study.[34]
Briefly, ZnO–TiO2 solid
nanospheres and TiO2 (P25) powders were mixed in 4:6 ratio in
the form of a paste and uniformly grinded with ethyl cellulose in
a-terpinol and ethanol (wt% ratio: 2.7:1:3.38). The paste
(25 mg) was screen printed onto the FTO substrates and dried at
1208C for 6 min and repeatedly printed 5 times to increase the
layer thickness. The electrodes were sintered at 4508C for 30 min
in the air resulting in film of ~15 mm thickness. The platinum sol
was deposited onto the FTO counter electrode and then calcined
at 4508C for 30 min. The photoanodes were soaked in 0.5 mM
N719 dye solution for 18 hrs. Finally, the soaked photoanodes
along with platinum counter electrode were assembled using
a Surlyn spacer and Iodolyte Z 50 (Solaronix) as the electrolyte,
resulting in a sandwich structure.
Characterization
Powder X-ray diffraction (Bruker D8 Advance diffractometer
equipped with 1.54 c wavelength Cu Ka radiation) was used for
the structure and phase identification. The morphology was ex-
amined by field emission scanning electron microscopy
(FESEM) and high-resolution transmission electron microscopy
(HRTEM) equipped (Technai G2 T20 ST) with energy-disper-
sive X-ray spectroscopy (Burker X-100). Surface composition
and chemical states were measured with an Omicron nanotech-
nology (Oxford instruments) X-ray photoelectron spectroscopy
(XPS) instrument equipped with monochromatic Al Ka radia-
tion. Specific surface area was analyzed by N2 adsorption–de-
sorption isotherms (Quntachrome autosorb iQ3). The diffuse re-
flectance spectra were investigated using a UV-visible spectro-
photometer (Varian Cary 4000) over a wavelength range of 200–
800 nm. N719 dye loading amount on the photoanode was calcu-
lated by dye desorption from the photoanode which was washed
using 6 mL of 10 mM NaOH in distilled water. The photovoltaic
performance and electrochemical impedance spectroscopy (EIS)
were measured using electrochemical workstation CHI660E-CH
Instruments Inc. PET Photo Emission Tech SS50AAA solar sim-
ulator was used as a light source to get one sun irradiation. Elec-

trochemical impedance of the cells was studies in the range of
1 Hz to 0.1 MHz.
Acknowledgements
The authors acknowledge financial support from the Depart-
ment of Science and Technology (SR/FT/CS-144/2011), Govt.
of India, and the Indo-Portuguese Program of Corporation in
Science and Technology (INT/Portugal/PO2/2013). We also
acknowledge the Material Research Centre, MNIT Jaipur,
India for providing TEM Characterization facility.
Keywords: dye-seinsitized solar cells · metal oxides ·
heterojunctions · titania · zinc oxide
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