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Fabrication of large area nanorod like structured CdSphotoanode for solar H2 generation using spray pyrolysistechnique

Alka Pareek, Rekha Dom, Pramod H. Borse*

Solar H2 PEC Lab., International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO, Hyderabad, AP 500 005,

India

a r t i c l e i n f o

Article history:

Received 5 June 2012

Received in revised form

25 September 2012

Accepted 19 October 2012

Available online 22 November 2012

Keywords:

Nanostructure

Photoelectrochemical hydrogen

Large area

Solar-to-hydrogen

Solar energy

Spray pyrolysis film deposition

* Corresponding author. Tel.: þ91 4024452426E-mail address: phborse@arci.res.in (P.H.

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.10.0

a b s t r a c t

Large area nanorod like structured CdS films (9 � 9 cm2) were deposited on the FTO glass

substrate using simple and economic spray pyrolysis deposition technique for photo-

electrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the

deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen

generation capability. High photocurrent of 5 mA has been achieved from the PEC active

area of 37.5 cm2. Its unit area (1 cm2) counterpart yielded Solar-to-Hydrogen (STH)

conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under

solar simulator (AM1.5) with 80 mW/cm2 irradiance. The 500 nm thick film exhibiting

uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as

hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated

by the higher IPCE values (5e9%) in the wavelength range of 450e550 nm. It exhibited

superior optical properties (Eg w2.4 eV), and formation of high crystallinity hexagonal CdS

phase with space group P63MC. The superior performance of the photoanode is attributed

to the nanostructured morphology acquired under optimized spray pyrolysis conditions.

Large area photoanodes showed unaltered photo-activity indicating the homogeneity in

the film properties even in scaled-up version.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction are known as good photoanode candidates, but suffer by their

The solar radiation induced photoelectrochemical hydrogen

production is a green technology route to generate renewable

energy from eco-friendly entities. The hydrogen thus gener-

ated from water and solar light over a photocatalytic system,

is thus highly important formankind today till an efficient and

stable photocatalyst system is identified. There have been

exploratory studies over semiconducting oxides, chalcogen-

ides and ferrites systems, to identify an efficient photoanode

[1e5]. The potential metal oxides of TiO2, SrTiO3 etc. though

; fax: þ91 4024442699.Borse).2012, Hydrogen Energy P57

large band gap (Eg� 3.2 eV), which limits [6] their PEC activity

to UV light. On the other hand ferrites, an n-type Fe2O3

semiconductor, do exhibit a low band gap (Eg w2.0e2.2 eV),

but show a poor photo-oxidation efficiency due to its low

absorption coefficient and high electron-hole (e-h) recombi-

nation rates [7]. Chalcogenides have been known as the most

potent photocatalyst candidates working under visible light.

They exhibit desirable low band gap, and well suited valence/

conduction band edges with respect to water redox potential

levels. Thus till today, they are the most extensively sought

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Schematic of spray pyrolysis set up for large area

film deposition.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 4 37

semiconductors for the role of visible light absorbing photo-

anode. Cadmium sulphide, a well known low band gap, n-type

semiconductor (Eg w2.4 eV), is the most promising candidate

not only for PEC hydrogen production applications but also for

various other applications; viz. in opto-electronic devices,

such as laser material, transducers, photo-conducting cells,

photo-sensor, optical waveguides and non-linear integrated

optical devices [8]. Consequently it has been chosen in the

present study. Additionally, advent of modern material

development techniques needs to be exploited for the mate-

rial nanostructuring. Such attempts not only pave path to

achieve better material properties but also lead to demon-

strate the industrial processability of the methodology.

In past CdS has been deposited by several techniques viz.

chemical vapor deposition (CVD) [9,10], chemical bath depo-

sition (CBD) [11e14], vacuum evaporation [15], sputtering [16],

electro deposition [17], pulsed laser deposition [18], spray

pyrolysis deposition (SPD) [8,19e25], & SILAR [26] etc. SPD is

one of the most simple, inexpensive and versatile film depo-

sition techniques which displays high potential for its indus-

trial applicability. It can yield desired metal-oxide,

semiconductor film over a large substrate area. This technique

involves a controlled droplet-spraying of the molecularly

mixed liquid precursor over the thermally heated substrate.

These droplets undergo thermal decomposition over the

substrate by making use of available energy. It is controllably

possible to modulate the nucleation-and-growth mediated

film properties by tuning the deposition conditions e flow

rate, deposition temperature, deposition time etc. Unlike CBD,

it offers the advantage of yielding high film crystallinity, and

film uniformity, without the loss of chemical precursor. The

ability to deposit large area nanostructure thin film is an

added asset of SPD [23]. Such capability of SPD is expected to

be beneficial in the field of PEC and solar-cell applications.

Thus we utilize this depositionmethod in the present study to

deposit large area thin films. Study of large area photoanodes

for H2 production is most desirable with respect to commer-

cialization and scale-up efforts for the hydrogen fuel utiliza-

tion. Though CdS is a very efficient PEC system, still the study

of its large area PEC performance is not reported in past.

Hence such study would be a great step toward the efforts to

evaluate scale-up approach to generate the green energy.

In present work we have focused on the large area (more

than 40 cm2) film deposition and its photoanode performance

for H2 generation. A rigorous optimization of the deposition

parameters with respect to flow rate, deposition temperature,

deposition time, precursor etc. has been done to achieve an

efficient photoanode. The main work related to the photo-

anode application for hydrogen generation is further elabo-

rated. It was expected that by tailoring the structural, optical

and morphological film properties, one is able to achieve an

improved nanostructured photoanode from such films. The

study presents the work in two major sections one being unit

area (1� 1 cm2) photoanode and other as large area (9� 9 cm2)

photoanode. Accordingly, in the first section the deposition

and PEC optimization have been described for unit area film.

The final section demonstrates large area film deposition. As

a step further, the study clearly demonstrates the feasibility of

deposition of large area films and their utilization as an effi-

cient solar-electrode, particularly for the PEC application.

2. Experimental

2.1. Large area film deposition and photo-electrodefabrication

Fig. 1 shows the schematic diagram of indigenously designed

spray pyrolysis equipment that was used for large area

(9 � 9 cm2) thin film deposition. The automated computer

interface of this SPD system has ability to control the inert gas

pressure, precursor flow rate, substrate temperature and the

film thickness. In order to deposit the desired film, equal

amounts of CdCl2 (0.1 M) & (NH2)2CS (0.1 M) chemicals were

dissolved in double distilled water to make the film precursor

solution. Unless otherwise stated, throughout the work,

certain fixed substrate area was chosen to achieve PEC active

area of 1.5 cm2. Fluorine doped tin oxide; FTO (Pilkington TCO-

15) with resistivity of 12e15 U/sq was used as a bottom elec-

trode. Films were deposited at various temperatures of

300e500 �C and characterized (Figs. SI.1 and 2). The film

thickness was varied by increasing the number of deposition

cycles in the range of 3e20 cycles. The nozzle-to-substrate

distance was varied from 15 to 25 cm, while the spray-rate

was maintained between 2 and 10 ml/min. Parameter opti-

mization was done on the basis of PEC performance (Figs. SI.3

and 4). In order to deposit large area film on FTO, optimized

parameters were used. Mainly the results of four different

substrate areas 1.5 cm2, 12 cm2, 37.5 cm2 & 81 cm2 are

described in the present work.

For the fabrication of electrode, the proposed active area

was located over the film. The electrical contact was made

over the bottom electrode, to apply the desired voltage biasing

from external circuit. Rest of the part was electrically insu-

lated using an epoxy. Thus, photoelectrode active area of

1 cm2 was achieved. The large area electrodes were also

fabricated in similar manner. The photo-electrode thus

Fig. 2 e Schematic of photoelectrochemical reactor set up

used for hydrogen generation.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 438

obtained was used for the photoelectrochemical hydrogen

generation.

2.2. Film characterization

X-ray diffraction pattern was recorded by using Bruker AX D8

XRD equipment having CuKa X-ray source. The scan was

taken in the range 0e100� with an increase of 0.01� at durationof 2 s/step. The transmission spectra were recorded using

UVeVis spectrometer (Perkin Elmer, lambda 650). The detailed

analysis was carried out using Tauc relation [24]:

a ¼ K�hn� Eg

�n2

hn(1)

where K is constant, Eg is optical band gap, and n ¼ 1 for direct

band gapmaterial such as CdS. The band gap Eg is determined

from the E vs (ahn)2 graph. The intercept on the energy axiswas

obtainedby extrapolating the tangent to the curve,whichgives

the value Eg. Surfacemorphology of CdS thin filmswas studied

by field emission scanning electron microscopy, FESEM (Hita-

chi model S4300SE/N) operated at 20 kV. Energy dispersive

spectroscopy (EDS) was carried out for all the films during SEM

study. The film cross-sectional studies were carried out to

estimate the film thickness. For this the films were cracked

carefully to expose the cross-section and then were used for

the further study.Thusdeposited thinfilmswerenamedon the

basis of their deposition parameters as shown in Table 1.

2.3. Photo-electro-chemical characterization

Photoelectrochemical measurements were carried out using

two-electrode cell (or 3-electrode cell, wherever mentioned)

with the photoanode as a working electrode and graphite

(/platinum) as a counter electrode in a quartz PEC reactor. In

case of 3-electrode cell Ag/AgCl was used as reference elec-

trode. The electrolyte was made using 0.01 M Na2S & 0.02 M

Na2SO3 so as to minimize the photo-corrosion. The solar

simulator (Oriel Model 91160) equipped with AM0, AM1.0 and

AM1.5 Global (Newport) filters was utilized for the experi-

mentation. The irradiance of 80 mW/cm2 was used in present

case. The PEC measurements were done using two different

PEC cell reactors, one for unit area electrode and another being

for large area. A schematic of typical set is shown in Fig. 2. The

reactor cell is interfaced with gas chromatography (GC)

equipment and with an electrochemical set up (PARSTAT

2273). As shown, the specially designed and fabricated PEC

reactor consists of a quartz window on the top of the reactor.

Table 1 e Films deposited at 350 �C were named on thebasis of the deposition cycles and observed filmthickness as characterized from cross-sectionmeasurements.

Film name No. of deposition cycles Thicknessa (nm)

A 5 200

B 10 500

C 15 1000

a FESEM cross-section studies.

This enables one to vertically photo-illuminate the electrode

using solar simulator. The air tight reactor was utilized to

monitor the generated H2 evolution. The evolved gas was then

extracted and characterized by GC. During the PEC measure-

ments, the distance between working electrode and counter

electrode was maintained around 1 cm. The electrode

connections from the PEC reactor were directly connected to

an electrochemical system for photocurrent-potential and

chronoamperometric measurements. In order to carry out

Incident-Photon-Current-Conversion Efficiency (IPCE)

measurements, an Oriel monochromator (Oriel model 74125)

fitted with a 300 W Xenon lamp, that was capable of gener-

ating wavelengths in the range of 200e900 nm was used. The

quantitative estimation of photo-electrochemically generated

H2 was done by GC spectrometer, Model GC-2010 Plus (Shi-

madzu) that was equipped with TCD (Thermal conductivity

detector) detector.

3. Results and discussions

3.1. Physico-chemical characterization of thin films

Fig. 3 shows the XRD spectra for the films (A, B and C)

deposited under optimized temperature (Figs. SI.1 and 2) of

350 �C. All the films exhibited hexagonal crystal structure of

CdS lattice belonging to P63MC (SG no. 186) space group. There

was no trace of any impurity phase like CdO. The crystallinity

was found to improve fromA to C, with the increase in the net

deposited CdS content over the substrate. The FTO substrate

peak (200) was found in all cases. The inset of the figure

displays the variation in the CdS (002) peak intensity with the

film thickness from A to C, indicating the effect of film

thickness on film crystallinity. On the contrary, the intensity

of the FTO (200) peak was found to decrease due to increase in

the film thickness.

Further in order to estimate the crystallite size, Debye

Scherer’s formula [27] was used:

Fig. 3 e XRD patterns of CdS film deposited with different

thicknesses viz., A (200 nm), B (500 nm) and C (1000 nm).

Substrate (FTO) peaks are indicated by solid circle. Inset

shows the change in (002) peak intensity with the increase

in the film thickness from A to C.

Fig. 4 e Transmission spectra of CdS films deposited for

thickness of: A-200 nm, B-500 nm and C-1000 nm. Inset

shows Tauc plot of CdS-B film that was used for band gap

estimation.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 4 39

D ¼ 0:9lb cos q

(2)

where, D-particle size, l-wavelength, q-diffraction angle and

b-FWHM. As displayed in Table 1, the crystallite size was

found to increase from A, B to C as 22 nm, 28 nm and 28.9 nm

respectively. The EDS spectrum of film-B confirmed that Cd

and S exist in the stoichiometric ratio of 1:1 (Fig. SI.5).

Fig. 4 shows the optical transmission spectra for the CdS

films A to C, indicating the absorption edge for Film B at

wavelength longer than 500 nm. Film B exhibited a very low

transmission than Films A and C, confirming its high

absorptivity toward visible light photons. Films A & C display

a systematic variation in the transmission with respect to

increase in the thickness from film A to C. The band gap (Eg)

was estimated using Tauc plots analysis. Inset of Fig. 4 shows

Tauc plot for the typical film B revealing its direct band gap as

2.4 eV, which is in accordance with the known reports [19,25].

In order to study the film surface morphology and to measure

the film thickness, morphological and cross-sectional studies

were carried out. Fig. 5 shows the surface morphology of the

films A to C along with their respective cross-sectional view,

exhibiting respective distinct features. Interestingly the

surface of film B exhibited certain aligned rod like nano-

structure throughout the surface. On the other hand the films

with other thicknesses viz. film A (and C) display very smooth

(and highly rough) surface respectively. The changes in the

film thickness are clearly visible from the cross-sectional

view, indicating the film thickness of 200 nm, 500 nm and

1000 nm for films A, B and C respectively. In view of achieving

rod like features in film B, it was further investigated with

special focus on its PEC property.

3.2. X-ray photoelectron spectroscopy (XPS)

Elemental characterization of the films was carried out using

XPS. Typical XPS analysis is shown in Fig. 6 for film B. As

clearly seen in Fig. 6(a), the survey scan indicates the presence

of Cd 3d & S 2p lines along with the remnant carbon peak. The

region wise scan of S 2p and Cd 3d states is shown in Fig. 6(b)

and (c) respectively. A detail analysis using de-convolution of

peaks indicated that S 2p can be fitted with two peaks, one at

B.E. w160.9 eV, and other being at 162.2 eV. These peaks can

be respectively attributed to the S2� of bulk S atoms [28] and to

S2�ions of surface S-atoms. Further, Fig. 6(c) displays the

peaks of Cd 3d5/2 and Cd 3d3/2 centered at about 404.69 eV and

411.5 2 eV, validating the spin-orbit separation of 6.83 eV. The

doublet-separation can be attributed to Cd2þ of CdeS bond

[29]. The attempts to de-convolute, doublet peak did not yield

any additional component, thus ruling out the existence of

CdeO over the film surface. This result is in accordance with

the XRD studies, which validate that no oxide phase is present

in the film. It is also evident from EDS study that these films

display Cd:S ratio of 1:1 (Fig. SI.5). The XPS study clearly

demonstrates that, the film consists of a stoichiometric

composition of CdS crystal structure.

3.3. Photoelectrochemical studies and hydrogenproduction

Photoelectrochemical analysis is very important to identify

and quantify the performance of a photoanode. Fig. 7(a)

displays the high transient photocurrents from various pho-

toanodes; while film-B being the highest among all. In all the

Fig. 5 e FESEM images showing morphology and film cross-section for the film thickness of (a) 200 nm (Film-A), (b) 500 nm

(Film-B), and (c) 1000 nm (Film-C).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 440

cases, the photoanodic current showed a rapid (wms) increase

when light illumination was made ON, however it decayed to

a much lower value (steady state) in few tens of seconds. The

exponential shape of the transient part possessing certain

time constant is known to be function of e the applied bias,

incident light intensity and the redox species present in the

electrolyte. Thus the time constant can be correlated with the

reaction kinetics. Such faster decay kinetics reveals the larger

recombination rates in the photoanode, where once the

charges acquire an equilibrium a steady state value is reached

by the system. The steady state value was found to be highest

(137 mA/cm2) in case of film-B indicating that film deposited at

350 �C yields efficient photoanode due to its nanostructured

morphology. It is noteworthy that film-B displays maximum

absorption which contributes to its improved PEC

performance.

Fig. 7(b) shows the graph of variation of photocurrent-

density and solar-to-hydrogen (STH) efficiency of the photo-

anodes (A to C). Both the curves display a concurrent trend in

variation with the thickness, indicating maximum efficiency

of film-B. Ideally, the hydrogen generation is directly propor-

tional to the photocurrent (J ). The current is generated, when

the photo-induced electrons are available for the H2 evolution

reaction, where the reaction can be given by the following

equation:

2Hþ þ 2e�/H2ðgasÞ (3)

It indicates that during PEC reaction, the photo generated

electrons react with Hþ-ions at the counter electrode to yield

H2 gas. The high photocurrent (137 mA/cm2) of film-B is thus

responsible for achieving a maximum solar-to-hydrogen

(STH) efficiency (w0.20%) at an applied bias of 0.2 V vs Ag/

AgCl in it than other films.

The solar-to-hydrogen conversion efficiency (h) was

calculated by using following relation [30]:

h% ¼�Jp�1:23� Vapp

��

I0� 100% (4)

Fig. 6 e XPS spectrum of 500 nm thick CdS film (Film-B)

showing survey scan (a) and region wise scan for Cd 3d

level (b) and S 1s level (c).

Fig. 7 e (a) Plots showing variation of photocurrent and

hydrogen generated in an electrolyte (Na2S(0.01 M) &

Na2SO3 (0.02 M)) at 0.2 V over CdS photoanodes; and (b)

respective chronoamperometric scans for the films A to C.

Fig. 8 e IPCE of CdS thin films deposited at different

thicknesses.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 4 41

where Vapp is the absolute value of applied potential that can

be expressed as:

Vapp ¼ Vmeas � Voc

where Vmeas and Voc are applied potential and open circuit

potential respectively and I0 is power density of source used in

mW/cm2. This yields an overall efficiency. In order to under-

stand the performance of a photoanode material system

following section is discussed.

3.4. IPCE measurements of CdS photoanodes

Photoanode performance can be analyzed by studying the

Incident photon-to-current efficiency (IPCE) which is given by

the relation [31]:

IPCE ¼ 1240 � Il � P � 100% [5]

IPCE is also known as wavelength dependent efficiency.

Fig. 8 shows the IPCE variation with the film thickness. IPCE

was highest for film-B which is 8.5% at around 500 nm. All the

films showed considerate IPCE values in thewavelength range

of 400e700 nm as expected in case of CdS system. Film-A

showed a poor performance as compared to the other two

films. Incidentally, the film-C shows higher IPCE values in the

range of 600e700 nm, but film-B showed highest IPCE-values

in the range of 450e600 nm. The behavior is also in accor-

dance with the photo-absorption capability of the respective

films as shown in optical characterization.

3.5. MotteSchottky studies

The behavior of a photoanode is not only dependent on the

physical properties of film, but is also decided by the charge

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 442

kinetics occurring at the semiconductor/electrolyte interface.

The charge transport properties can be studied by the elec-

trochemical characterizationoffilmsspecificallybyestimation

of the donor density and by the flat band potential measure-

ment. The value of the flat band potential (VFB) was calculated

using MotteSchottky (MeS) relation [8] and the barrier capac-

itance were measured at 1 kHz in the range of �1.2 to 1.2 V (vs

Ag/AgCl) in dark. Fig. 9 shows the MeS plots indicating the flat

bandpotential values of�1.08V,�0.502Vand�0.188V for film

C, A and B respectively. The donor density was found to

increase with the increase in the thickness (A to C) from

1.0� 1017 cm�3 to 1.5� 1017 cm�3. The reasonof large change in

the value of VFB with thickness can be attributed to creation of

new donor levels [32]. Film B showed high flat band potential,

and comparatively a high donor density value, which probably

contributes to improve its PEC performance.

Fig. 10 e (a) Chronoamperometry of CdS photo-electrodes

deposited over different substrate areas viz. B1-1.5 cm2,

B2-12 cm2 and B3-37.5 cm2, under solar simulator at 0.2 V

bias; (b) graph shows the linear variation of photocurrent

with film area.

3.6. Large area film deposition and PEC performance

In order to evaluate the PEC behavior of large area films

deposited in present work, PEC measurements were carried

out over these films of different electrode area (1.5e37.5 cm2).

Fig. 10(a) shows the results of chronoamperometric

measurements over different photo-electrodes B1 (1.5 cm2),

B2 (12 cm2) and B3 (37.5 cm2), which are summarized as graph

of photocurrent vs the photoanode areas in Fig. 10(b). The

photocurrent was generated when the light was turned ON

validating the PEC behavior of the films. Importantly it can be

seen that the variation in the photocurrent followed a linear

function with respect to electrode area, indicating the supe-

rior quality of the film. Unlike this observation, in practice the

large area films often suffer by the problem of film inhomo-

geneity, film non-uniformity which in turn expected to yield

poor photocurrent-density in a large area film. The linear fit

clearly demonstrates that the film retains an expected

photocurrent density for all the films as discussed in more

details in next section.

Utilization of PEC concept for energy generation is next

important salient feature of the present work. The open

Fig. 9 e MotteSchottky curves for CdS deposited at

different thicknesses.

circuit potentials and the short circuit currents were

measured in direct sun for these films under unbiased

conditions. Fig. 11 shows the generated open circuit potential

and short circuit currents from these films of variable elec-

trode areas. The photographs of actual films used for the

electrode formation are shown in the inset of the Fig. 11. It is

important to mention that achieving such efficient large area

films entrusts the spray pyrolysis deposition technique as an

economic and industrially viable technique, that displays high

potential for generating solar conversion photoanodes.

4. Importance of large area film in energyapplication

Deposition of an efficient large area thin film is a crucial factor

with respect to its commercial applicability. The deposition of

a large area photoelectrode can amplify the photovoltaic

output, provided the physico-chemical properties of the

Fig. 11 e Variation of open circuit potential (Voc) and short

circuit current (Isc) with photoanode area for B1-1.5 cm2,

B2-12 cm2, B3-37.5 cm2 and B4-81 cm2 electrodes

measured directly under sun, and photograph of the

respective films in the inset. Themeasurements were done

under natural sunlight between 11a.m.e2 p.m. at ARCI,

Hyderabad (Andhra Pradesh, India; 78.47�E, 28.28�N), with

sunlight irradiation density of 0.020 W cmL2.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 3 6e4 4 43

physical property of the film are retained. In fact practically

the drift in the photocurrent density with increase in the film

deposition area is generally observed, where the deterioration

in PEC property is attributed to the poor quality of the large

film area. Ideally the photocurrent density of large area pho-

toanode is expected to be constant and can bemeasure of film

quality. Accordingly, in an n-type semiconducting material,

the photocurrent produced is directly proportional to the area

of photoelectrode which is given by following expression [33]:

i ¼ n � F �A � k0f � nsc � Co [6]

where n is carrier density, F is Faraday constant, A is area, nscis concentration of electrons at the interface, C0 is Capaci-

tance, kf0is heterogeneous rate constant. The linear variation

fit of photocurrent with area clearly validates the excellent

film properties as shown in Fig. 10(b).

Sustained hydrogen production needs the attention with

respect to scale-up feature of any photoanode. In present

report, the photocurrents generated were found to be directly

proportional to the photoanode area as shown in Fig. 10(b).

This is an important achievement with respect to scale-up the

photoanodes for energy conversion. A step further, the large

area photoanodes exhibited a correlateable high open circuit

potential and short circuit current variation in the photoanode

area (see Fig. 11), indicating that there is an immense scope of

utilization of SPD methodology for making efficient photo-

anodes. There are quite a few reports on study of large area

photoanodes [34,35], where it has been reported that with an

increase in the PEC active area, practically yields lowered

photocurrent density [36] thereby hampering the large area

applicability. Such poor performance is owing to the increased

electrical resistance of the conducting substrate as well as the

defects formation in the film. Thus it is highly desirable to

achieve an adherent, defect free and pinhole free large-area

film for the photoelectrode fabrication. Spray pyrolysis is the

easiest method to deposit adherent large-area thin films with

defects-free surface morphology. The improved photoanode

performance of large area photoanode clearly implies that

spray pyrolysis is an economic and reliable methods best

suited for commercialization of large area photoanodes for

energy generation.

5. Conclusions

Adherent and uniform large-area nanorod structured CdS

films were deposited using spray pyrolysis deposition. The

film deposition parameters viz. deposition temperature, rate

of film deposition etc., were optimized, and later these opti-

mized conditions were used for large area (37.5 cm2) film

deposition over FTO substrate. These films were used to PEC

hydrogen generation application. Film thickness of 500 nm

was found to yield best PEC performance yielding solar-to-

hydrogen (STH) efficiency of 0.20%. Superior photoanode

performance is attributed to the nanorod structured

morphology of the film. PEC performance of large area CdS

films were studied and high photocurrent of 5 mA was

observed for active electrode area 37.5 cm2. It has been

demonstrated that the formation of high quality large-area

film yields enhanced PEC H2 production and electrical power

generation, using a very simple economic film deposition

method.

Acknowledgments

The authors wish to thank Mr. K. Ramesh Reddy and L. Ven-

katesh, for their help during film characterization. Authors are

also thankful to Dr. Neha Y. Hebalkar for her help in X-ray

photoelectron spectroscopy. Permission granted by Director,

ARCI to publish the work is also gratefully acknowledged.

Appendix A. Supporting information

Supporting information related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2012.10.057.

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