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Structural, photoluminescence and photoelectrochemicalproperties of electrosynthesized ZnSe spheres
G. M. Lohar • J. V. Thombare • S. K. Shinde •
S. H. Han • V. J. Fulari
Received: 6 December 2013 / Accepted: 20 January 2014 / Published online: 16 February 2014
� Springer Science+Business Media New York 2014
Abstract Zinc selenide (ZnSe) spheres have been suc-
cessfully synthesized by a simple galvanostatic mode of
electrodeposition method for the photoelectrochemical
properties. These spheres are characterized for their
structural and morphological features using X-ray diffrac-
tion studies, scanning electron microscopy techniques. The
growth mechanism of the ZnSe spheres have been dis-
cussed in detail, which can be addressed as the 2H? gas
trap and cation exchange reaction. UV–vis absorption
spectra revealed that an absorbance peak at 380 nm and its
band gap energy was found to be *2.57 eV. Photolumi-
nescence measurement spectra revealed that, the spheres
exhibit strong emission at 675.94 nm. The photoelectro-
chemical measurement of ZnSe spheres was studied and
the calculated efficiency was found to be *0.05 %. In the
electrodeposition galvanostatic mode is good technique
applicable for controlling the film thickness. Hence, these
galvanostatically electrosynthesized ZnSe spheres could be
used as a buffer layer in the photoelectrochemical solar
cells.
1 Introduction
Recently, with the increasing development of science and
technology, the inorganic materials with solid spheres or
hollow spheres like morphology have shown a promising
perspective in many fields, such as catalysts, dyes or inks,
coatings, filters, and micro reactors for their low densities,
high surface areas, and unique optical, electrical, and sur-
face properties [1]. Chalcogenide materials are one of the
important classes of semiconductors. In particular, Zinc
selenide (ZnSe) based materials have been widely inves-
tigated in recent years for their potential optoelectronic
applications in high-density optical storage, full color dis-
plays [2], photovoltaic [3], thin film transistors [4], photo
sensors [5], laser screens [6], dielectric mirrors [7], light
emitting devices [8], photoelectrochemical cells [9], etc.
ZnSe thin film have been used as n-type window layer
for thin film heterojunction solar cells as well as a buffer/
window layer in chalcogenide-based thin film solar cells
[10]. Zinc selenide is well known n–type of II–IV semi-
conducting material having wide band gap energy of
2.7 eV. In recent years, many researchers are interested in
the development of various morphologies of ZnSe, such as
quantum dots [11], nanodots, nanoflowers, and nanotubes
[12, 13], nanobelts [14, 15], nanowires [16], nanorings [17]
and hollow spheres [1], Xiaolei Ren et al. [18] using
solvothermal method, shown that ZnSe microspheres are
about 5–6 lm in diameter. Guozhen Shen et al. [19]
studied the synthesis of cubic phase ZnSe spheres via rapid
polyol process and reported that longer the reaction time,
bigger the size of the final products. But after some time
spheres become rough and many nanoparticles get attached
with the surface, due to the more reaction time. Gong et al.
[20] prepared ZnSe microspheres, by hydrothermal syn-
thesis and they said that the ratio of Zn/Se has great effect
on the nucleation and growth of ZnSe sphere. High ratio of
Zn/Se is significant to prepare ZnSe microspheres.
Recently, Liu et al. [21] reported ZnSe Hollow Nano-
spheres from an Ionic Liquid Precursor using hydrothermal
G. M. Lohar � J. V. Thombare � S. K. Shinde � V. J. Fulari (&)
Thin Film Physics and Holography Laboratory, Department of
Physics, Shivaji University, Kolhapur 416 004, Maharashtra,
India
e-mail: [email protected]
S. H. Han
Department of Chemistry, Hanyang University, Seoul, South
Korea
123
J Mater Sci: Mater Electron (2014) 25:1597–1604
DOI 10.1007/s10854-014-1750-4
method. Yang et al. [22] reported ZnSe nanospheres by hot
injection method and reported its luminescent properties.
The solvothermal method, polyol process and hydrother-
mal synthesis, all these are prepared in non-aqueous solu-
tion as well as at high temperature. But in this work we
reported ZnSe nanospheres using simple galvanostatic
electrodeposition technique in aqueous solution at low
temperature and we studied its structural, morphological,
optical, photoluminescence and photoelectrochemical
properties. We claim that such type of work is not previ-
ously reported elsewhere.
Many experimental techniques have been used to deposit
ZnSe thin films, but electrodeposition is an attractive
approach for growing semiconductor compounds. Electro-
deposition is one of the most widely accepted techniques for
the inexpensive and well organized growth of the films in
aqueous medium, and numerous reports are available on the
deposition of various thin films by this technique. Synthesis
of ZnSe thin films by electrodeposition method is difficult
task because of the wide difference in the reduction potential
of Zn and Se ions and only few reports are available on the
potentiostatically electrodeposited ZnSe thin films [23–26].
We have successfully synthesized ZnSe spheres by simple
galvanostatic electrodeposition method.
In the present work, we are going to report synthesis of
galvanostatically electrosynthesized ZnSe spheres on tin
doped indium oxide (ITO) and stainless steel substrates and
its structural studies such as X-ray diffraction (XRD),
scanning electron microscopy (SEM), and fourier transform
Raman (FT-Raman) spectroscopy. Optical studies such as
UV–vis absorption spectroscopy, photoluminescence and
photoelectrochemical behavior (J–V measurement) are
reported. The galvanostatically electrosynthesized ZnSe
spheres and its PEC performance is most important part of
this work. Also we have improved the PEC performance of
ZnSe thin film by galvanostatic mode of electrodeposition.
2 Experimental
All the reagents used in this experiment were of analytical
reagent grade and used as received without further purifi-
cation. Zinc sulphate GR (heptahydrate) ZnSO4�7H2O,
selenium dioxide (SeO2), sulphuric acid 98 % (H2SO4),
acetone, constant current source, tin doped indium oxide
(ITO) substrate, stainless steel substrate.
The zinc selenide (ZnSe) thin films were grown by elec-
trodeposition technique in the electrolytic bath containing
0.25 M ZnSO4, 0.001 M SeO2 in an aqueous solution. In order
to remove contamination, the stainless steel was cleaned first
by using zero polish paper then by water, up to formation of a
good uniform film of water on substrate. When it happens,
substrate was again cleaned by double distilled water. The
substrates were air dried and finally dried with acetone before
deposition. The pH was adjusted between 2 and 2.5 by adding
sulphuric acid. The films were deposited on stainless steel
as well as ITO coated glass substrate by galvanostatic mode
of electrodeposition by applying constant current density
70 lA/cm2 for various deposition times such as 30, 60, 90 and
120 min. A stainless steel substrate was used as a working
electrode. The bath temperature was maintained at 65 �C by
using thermostat. Absorbance, reflectance, FT-Raman, pho-
toluminescence, photoelectrochemical properties were stud-
ied on the ITO substrate and for X-ray diffraction study,
stainless steel substrate was used.
The structural characterization of electrodeposited ZnSe
hallow spheres were carried out, by analyzing the X-ray
diffraction pattern obtained under Cu–Ka radiation from a
Bruker D2 phaser tabel top model. Surface morphology
was studied using JEOL JSM-6360 Japan, scanning elec-
tron microscope (SEM). Absorption spectra were recorded
at room temperature and near to normal incidence using a
UV-1800 Shimadzu, Japan, UV–vis spectrophotometer
using ITO as reference. The optical reflectance was
recorded using a Steller Net. Inc. USA Reflectometer
having UV–vis light source with CCD detector. Raman
scattering experiments were performed in air at room
temperature with Raman system from Bruker AXE Ana-
lytical Instruments PVT, Germany. The Raman spectra
were excited with Nd-yag Laser source at wavelength
1,064 nm from 200 to 1,000 cm-1 and Ge detector was
used. Photoluminescence study was carried out using Jasco
Spectrofluorometer FP-8300 using excitation wavelength
541 nm and Keithley 4200-SCS were used for study of the
photoelectrochemical cell property.
3 Results and discussion
3.1 X-ray diffraction (XRD) study
Figure 1 shows the X-ray diffraction pattern of electrosyn-
thesized ZnSe spheres. Phase identification using X-ray
diffraction depends mainly on the positions of the peaks in a
diffraction profile and to some extent on the relative inten-
sities of these peaks. XRD pattern were obtained with the
help of copper target having wavelength Ka = 1.5418 A�.
The ZnSe thin films were slowly scanned between 20� and
80�. All the diffraction peaks in XRD pattern can be indexed
to hexagonal (Wurtzite) structure, which is in very good
agreement with the JCPDS card no. 01-089-2940 for ZnSe
(a = b = 3.996 A, c = 6.6260 A). The several peaks of
hexagonal phase of ZnSe have been obtained due to dif-
fraction from (1 0 0), (1 0 1), (1 1 0), (1 0 3), (1 1 4), (2 0 4)
& (3 0 2) planes of ZnSe. The lattice constants were eval-
uated using the relation,
1598 J Mater Sci: Mater Electron (2014) 25:1597–1604
123
d ¼ affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðh2 þ k2 þ l2Þp
" #
ð1Þ
where, d is the inter-planar spacing of the atomic plane. The
crystallite size D was calculated from the Debye Scherer’s
formula from the full width at half maxima (FWHM),
D ¼ 0:9kb cos h
ð2Þ
where, b is the FWHM and the average crystallite size was
calculated as 60 nm.
The micro strain e was calculated using the relation;
b cos hk¼ 1
Dþ e sin h
kð3Þ
where, k is wavelength and h is Bragg’s angle. From
relation (3) it was found that the micro strain was
0.505 9 10-3 e (lin-2 m-4). The dislocation density d,
defined as the length of dislocation lines per unit volume of
the crystal, was evaluated from the relation [27, 28];
d ¼ 15eaD
ð4Þ
where ‘a’ is lattice constant.
According to the relation (4) the dislocation density was
found to be 2.015 9 1014 lines/m2. The texture coefficient
was calculated using following Eq. (5),
pðhikiliÞ ¼IðhikiliÞI0ðhikiliÞ
�
1
n
X IðhikiliÞI0ðhikiliÞ
� �
ð5Þ
where, I0 represents the standard intensity, I is the observed
intensity of (hi, ki, li) plane, and ‘n’ is the reflection number [29].
The crystallite shape of the ZnSe thin film is strongly
related to the texture of ZnSe thin films. The predominant
plane (101) orientation of the film has high texture
coefficient value. The calculated texture coefficient was
found to be 1.283. In this study we used the only film
deposited for 90 min because maximum thickness of the
thin film was observed at 90 min while for films deposited
with less than 90 min time thickness was very less. This less
thickness reflected in peaks only for stainless steel in the
XRD pattern. The syntheses of ZnSe nanoparticles at low-
temperature usually observed cubic sphalerite structure even
as hexagonal wurtzite phase are only produced at high
temperature but, Senthilkumar et al. [30] showed that the
hexagonal ZnSe phase at room temperature is possible. Also
this result is in good agreement with Senthilkumar et al.
3.2 Scanning electron microscopy (SEM)
Figure 2 shows the SEM images of electrosynthesized
ZnSe thin films with different deposition times as 30, 60,
90, 120 min, with different magnifications, which show
that the products are nano-spheres. At the 30 min deposi-
tion time well dispersed spheres of ZnSe are observed with
grain size 300 nm which is shown in Fig. 2 A1& A2). As
deposition time increased for to 60 and 90 min, grain size
is observed to be increased to 400 and 500 nm respectively.
It is because of increase in deposition time which is shown
in Fig. 2 B1, B2 and C1, C2. While for 90 min film some
overgrowth is observed. It is also observed that for 120 min
the surface of the spheres are uneven with lots of particles
attached, shown in Fig. 2 D1 and D2 also all details of
SEM mentioned in Table 1. The formation of the ZnSe
sphere may be demonstrated by the following reaction.
Zn2þ þ H2Se ¼ ZnSeþ 2Hþ ð6Þ
According to Eq. (6), Zn2? and H2Se react with each other
to form ZnSe clusters accompanied by the release of 2H? gas
[25]. The final product was found to be a shell instead of solid
spheres. The shell cavity increases with increase in deposition
time. Mostly hollow morphologies are observed in the metal
chalcogenide due to the cation exchange reaction. The sche-
matic presentation of formation of ZnSe spheres are shown in
Fig. 3. Here we predict that Se at ZnSe core shell spherical
particle by partially reacting amorphous Se (a-Se) particles
with Zn atoms were formed and then transformed the shell into
Zinc selenide to produce Zinc selenide core shell particles
which is shown in Fig. 3. Selective removal of core Se gen-
erated zinc selenide hollow particles. Because the changes in
volume during the transformation were small, all the products
maintained the spherical shape with a similar diameter and
shell thickness [31].
3.3 UV–vis absorption spectroscopy
Figure 4 shows the typical UV–vis absorption spectrum of
an electrosynthesized ZnSe spheres deposited at various
20 40 60 80 100
Inte
nsit
y (A
.U)
2θ (degree )
(101
)
(110
)
(103
)
(114
)
(204
)
(302
)
(100
)
Δ
Δ − ZnSeO3
δ − Steelδ
δ
Fig. 1 A typical X-ray diffraction pattern of the electrosynthesized
ZnSe sphere
J Mater Sci: Mater Electron (2014) 25:1597–1604 1599
123
deposition times. Figure 4 provides an accessible evaluation
of absorption of a photon, leading to excitation of an electron
from the valence band to the conduction band, which is
connected with the band gap energy (Eg) instead of Fig. 4
shown the band gap energy plot. The band gap energy and
Bohr exciton radius of the bulk ZnSe is 2.69 eV (461 nm)
and is 4.5 nm respectively [32]. The first excitonic transition
380 nm was observed in the UV–visible spectra which is
clearly red-shifted with increase in deposition time. The
sphere deposited at 90 min shows the better absorbance than
the other. The variation of (ahm)2 versus hm is linear at the
absorption edge, which confirmed direct band gap transition
in ZnSe sphere. Extrapolating the straight-line portion of the
plot (ahm)2 versus hm for zero absorption coefficient value
gives direct band gap Eg as shown in inset of Fig. 4 [33].
ZnSe sphere has band gap energy of *2.57 eV. Band gap
shows the ‘Red-shifts’ of 0.12 eV from the standard bulk
band gap (Eg = 2.69 eV) because of increase in thickness
Fig. 2 Scanning electron
microscopic image of
electrosynthesized ZnSe sphere
with different magnification and
different deposition time, (1) A1and A2 at 30 min 915,000 and
920,000. (2) B1 and B2 at
60 min 915,000 and 920,000.
(3) C1 and C2 at 90 min
915,000 and 920,000, D1 (4)
D2 at 120 min 915,000 and
920,000
1600 J Mater Sci: Mater Electron (2014) 25:1597–1604
123
and deposition temperature (65 �C) which is higher than the
room temperature and may be due to the excess of selenium
in the ZnSe sphere.
3.4 Reflectance spectroscopy
The reflectance spectra of the electrosynthesized ZnSe
sphere were recorded in the spectral range 300–850 nm
which is shown in Fig. 5. As far as solar cell performance
is concerned the absorbance and reflection properties of
ZnSe sphere play an important role. The diffused reflec-
tance spectra will increase some light scattering effect. The
increase in light scattering effect can thus increase the light
travelling path and hence the interaction between thin film
and electrolyte, Then resulting in higher light harvesting
efficiency and corresponding higher photocurrent [34]. For
90 min film we observed good diffused reflectance spectra
than the other deposition time. The electrosynthesized
ZnSe spheres show the reflectivity near about 40 % at
90 min which is shown in Fig. 5. Okereke et al. [35]
showed that ZnSe thin film act as a good material for solar
cell application. This result demonstrates that electrosyn-
thesized ZnSe spheres are useful for buffer layer in the
photoelectrochemical solar cell.
3.5 Raman spectroscopy
Raman Spectroscopy used to study the structures of the
ZnSe sphere. Figure 6 shows a typical room temperature
Raman spectra for the ZnSe spheres at 90 min. The strong
peak at 239 cm-1 is the characteristic vibrations of ZnSe
Table 1 SEM images of ZnSe Sphere with different magnification
Sr. no Time (min) Name Magnification
1. 30 A1 915,000
A2 920,000
2. 60 B1 915,000
B2 920,000
3. 90 C1 915,000
C2 920,000
4. 120 D1 915,000
D2 920,000
Glass substrate
ZnSe Nanoparticles Agglomeration of ZnSe nanoparticles
400 nm at 60 min
ITO Coating
ZnSe spheres at 30 min, grain size 300nm
I II III
IVV
500 nm at 90 min Over growth at 120 min αα-Se Zn2+ ZnSe
VI
Fig. 3 Schematic
representation of the Formation
of electrosynthesized ZnSe
spheres and inset formation of
single Sphere by cation
replacement reaction
300 400 500 600 700 800 900 1000
0.4
0.8
1.2
1.6
2.0
2.4
2.8
(a) - 30 min
(b) - 60 min
(c) - 90 min
(d) - 120 min
Abs
orba
nce
Wavelength (nm)
(a)
(b)
(d)
(c)
1.5 2.0 2.5 3.00
3
6
9
12
( αhυ
)2x1
010 (
eV/c
m2)
hυ (eV)
Fig. 4 UV–vis absorption and band gap spectra of ZnSe sphere
absorption spectra of ZnSe sphere with different deposition time and
inset energy plot ZnSe hallow sphere deposited at 90 min
J Mater Sci: Mater Electron (2014) 25:1597–1604 1601
123
spheres. They can be attributed to the longitudinal optic
(LO) phonon modes of ZnSe crystal, sharp and symmet-
rical Raman peaks disclose that the ZnSe spheres are all of
high crystalline quality and pure phase, which is consistent
with the above XRD results. Mostly ZnSe shows charac-
teristic peak at 251 and 202 cm-1 [36]. According to
Pejova et al. [37] the LO mode is red shifted compared
with the bulk value and it appears in the spectral region
from 239 to 234 cm-1, depending on the actual nano-
crystal size. Another proof for the red shift is explained in
the same, also the phonon frequency of about 177 cm-1,
which shows both the LO and the TO mode frequencies of
ZnSe. The peak at 199 cm-1 indicate the TO mode fre-
quency of ZnSe thin film.
3.6 Photoluminescence (PL)
Photoluminescence spectra for electrosynthesized ZnSe
spheres are shown in Fig. 7. The photoluminescence
spectrum was recorded at room temperature in the range of
600–900 nm using an excitation wavelength of 541 nm.
The excitation wavelength was found to be greater with
reference to earlier literature [38], i.e. less excitonic energy
is required, which corresponds to relaxation of quantum
confinement. This may be due to some excess selenium in
the ZnSe. This type of results is always observed in case of
electrodeposition of Zinc selenide thin film because of
difference in deposition potential of Zinc and selenium
[39]. The spectrum exhibits peaks at 675.95, 732.72,
824.24 and 868.10 nm. The PL emissions from ZnSe have
been recognized to the presence of native defects like zinc
and selenium vacancies or interstitial, which are likely to
be introduced during the growth process. The emission
observed at 675.94 nm corresponds to Stokes shift [40].
Our results are good agreement with Pol et al. [41] They
explained ZnSe emission peaks at 550–760 nm because of
ZnSe with a high density of structural defects. In this case
strong red emission is due to the selenium and oxygen
defects in the ZnSe sphere.
3.7 Photoelectrochemical study (J–V measurement)
The J–V characteristics of the films were recorded using a
SCS-4200 Keithley, Germany, with a two-electrode con-
figuration. The following cell configuration was used to
record J–V plots: Glass/ITO/ZnSe/NaOH–KI–I2/G. The
Fig. 8 shows the typical J–V characteristic of electrosyn-
thesized ZnSe spheres. The cell consists of electrosynthe-
sized ZnSe sphere as a working electrode, poly-iodide as
electrolyte and graphite as counter electrode. The power
conversion efficiency g of solar cells were calculated using
Eq. (7).
400 500 600 700 8000
10
20
30
40
50(a) - 30 min
(b) - 60 min
(c) - 90 min
(d) - 120 min
Ref
lect
ance
%
Wavelenght (nm)
(a)
(b)
(c)
(d)
Fig. 5 Reflectance Spectra of ZnSe sphere with different deposition
time
200 400 600 800 1000
Ram
an in
tens
ity
Raman shift (cm-1)
239 cm-1
177.9 cm-1
199 cm-1
Fig. 6 Raman spectroscopic study of ZnSe sphere deposited at
90 min
650 700 750 800 850 900
Inte
nsit
y (A
.U)
Wavelength (nm)
732.72
824.24
675.95
868.10
Fig. 7 Photoluminescence spectra of ZnSe sphere deposited at
90 min and inset schematic representation of emission of ZnSe sphere
1602 J Mater Sci: Mater Electron (2014) 25:1597–1604
123
g ¼ JSC � Voc
Pin
� FF ð7Þ
where, Voc is the open circuit voltage, Jsc is the short circuit
current density, Pin the power density of the incident light
and FF is the fill factor, calculated using Eq. (8) [42].
FF ¼ Imax � Vmax
Isc � Voc
ð8Þ
Zhou et al. [43] have Studied the I–V characteristics of
ZnSe, at the forward bias voltage of 10 V, the present single
ZnSe showed current of *470 nA. Here is drastic
improvement of the entire cell parameters such as open cir-
cuit voltage (Voc), short circuit current (Isc), fill factor (FF)
and cell conversion efficiency (g). We observed that open
circuit voltage (Voc) is 0.09 V, short circuit current (Isc) is 40
lA and calculated value of Fill Factor (FF) as well as effi-
ciency was found to be 41 and 0.05 %, respectively. For
90 min film we observed the thickness 200 nm which is
more than the other, so more efficient ZnSe sphere are
observed at 90 min. Galvanostatically electrosynthesized
ZnSe spheres have very low efficiency, but this is very good
material as buffer layer in the heterojunction solar cell.
4 Conclusion
In summary, ZnSe spheres were successfully synthesized
by a galvanostatic electro deposition method at compara-
tively low temperature. The constant current density is very
good parameter for controlling film thickness which is
helpful in the formation of buffer layer in solar cell. The
ZnSe thin films have shown hexagonal (Wurtzite) struc-
ture. Spheres are formed because of evolution of 2H? gas
and cation exchange reaction, the diameter of ZnSe sphere
was found to be near about 300–500 nm. ZnSe sphere
shows the band gap *2.57 eV and its absorption at
380 nm. Reflection spectra ZnSe sphere shows good
reflectivity and it is useful as buffer layer for photoelect-
rochemical solar cell. The strong Raman peak at 239 cm-1
is the characteristic vibration of ZnSe spheres. PL spectrum
exhibits peaks at 675.95, 732.72, 824.24 and 868.10 nm.
The peak at 675.95 nm, which is most intense peak, shows
the stocks shift. The photoelectrochemical performance
shows the efficiency *0.05 % which is good improvement
in PEC performance. Also these results explain that elec-
trosynthesized ZnSe spheres are useful material for buffer
layer in photoelectrochemical solar cells and photovoltaic
solar cells.
Acknowledgements The authors thank Prof. P. S. Patil for pro-
viding Keithleymeter facility, Dr. G. B. Kolekar for providing Pho-
toluminescence, Dr. K. Y. Rajpure for providing the Reflectance. The
authors wish to express their gratitude to the UGC India, for the
financial support received through the scheme No. F.4-1/2006 (BSR)/
7-167/2007 (BSR).
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Fig. 8 Photoelectrochemical cell performance of ZnSe sphere,
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