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Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 forphotoelectrochemical generation of hydrogen

Poonam Sharma a, Praveen Kumar a, Dinesh Deva c, Rohit Shrivastav b, Sahab Dass b,Vibha R. Satsangi a,*aDepartment of Physics & Computer Science, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra-282005-IndiabDepartment of Chemistry, Faculty of Science, Dayalbagh Educational Institute, Dayalbagh, Agra-282005-IndiacDST Unit on Nanosciences, Department of Chemical Engineering, Indian Institute of Technology Kanpur, India

a r t i c l e i n f o

Article history:

Received 5 April 2010

Received in revised form

28 June 2010

Accepted 2 July 2010

Available online 11 August 2010

Keywords:

Photoelectrochemical

Nanostructured

Iron oxide

Titanium dioxide

Spray-pyrolysis

Sol-gel

* Corresponding author. Tel.: þ91 9319104320E-mail address: vibhasatsangi@gmail.com

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.07.016

a b s t r a c t

Nanostructured semiconductor thin films of Zn-Fe2O3 modified with underlying layer of

Fe-TiO2 have been synthesized and studied as photoelectrode in photoelectrochemical

(PEC) cell for generation of hydrogen through water splitting. The Zn-Fe2O3 thin film

photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO2

film. A maximum photocurrent density of 748 mA/cm2 at 0.95 V/SCE and solar to hydrogen

conversion efficiency of 0.47% was observed for 0.89 mm thick modified photoelectrode in

1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the

films were characterized for various physical and semiconducting properties using XRD,

SEM, EDX and UVeVisible spectrophotometer. Zn-Fe2O3 thin films modified with Fe-TiO2

exhibited improved visible light absorption. A noticeable change in surface morphology of

the modified Zn-Fe2O3 film was observed as compared to the pristine Zn-Fe2O3 film. Flat-

band potential values calculated from MotteSchottky curves also supported the PEC

response.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction said requirements simultaneously. Compared to others,

Photoelectrochemical (PEC) splitting of water into hydrogen

and oxygen attracted researchers all over the world since the

generation of hydrogen using TiO2 as a photoanode by

Fujishima and Honda in 1972 [1]. The search has continued for

ideal PECwater splittingmaterial having the characteristics of

efficient sun light absorption, proper band edge energetic,

high quantum efficiency, practical durability and low cost

[2,3]. A wide range of oxide semiconductors like TiO2, a-Fe2O3,

ZnO, BaTiO3 andWO3 etc. has been thoroughly investigated in

PEC cell for the generation of hydrogen to get reasonable

efficiency at low cost [4e9]. However, no single semi-

conductingmaterial has yet been found to satisfy all the above

; fax: þ91 562 2801226.(V.R. Satsangi).

ssor T. Nejat Veziroglu. P

hematite (a-Fe2O3) is considered to be an attractive PEC

material, having desired property of narrow band gap

(approximately 2.2 eV), which in principle allows utilization of

a larger fraction of the solar spectrum, low cost, electro-

chemical stability and low toxicity [9e12]. But, the water

splitting efficiency for a-Fe2O3 is reported to be much lower

than the theoretical maximum efficiency of 12.9% [13]. This is

mainly due to the rapid electron-hole recombination in the

semiconductor resulting in short diffusion lengths of charge

carriers [14], slow surface reaction kinetics [5] and falloff in the

absorption cross-section of the material for wavelengths

approaching the band gap value. Additionally, the conduction

band edge of hematite is slightly below that of the Hþ/H2 redox

ublished by Elsevier Ltd. All rights reserved.

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 5 ( 2 0 1 0 ) 1 0 8 8 3e1 0 8 8 910884

potential; due to which an external electrical bias is needed to

generate hydrogen [15]. To overcome these problems and to

improve performance of hematite nanostructuring tech-

niques have been suggested and studied by several groups

[5,9,10,13]. However the reported value of the photocurrent

density is still much below to meet the challenge.

Recently, composite semiconductor thin films of different

band gap energies have gained considerable interest on

account of its modified optical and charge transportation

properties [16]. It is well accepted that the wide band gap

semiconductors generate a high photovoltage but exhibit low

photocurrent whereas, smaller band gap semiconductors can

utilize a larger fraction of the incident photons but generate

lower photovoltage [17]. Therefore, a device having multiple

band gap energy layers can cover broad range of solar spec-

trum. It is expected that combining best of a-Fe2O3 and TiO2 in

one photoelectrode, may provide a better and efficient PEC

system for generation of hydrogen. With this idea, this paper

presents the PEC study on nanostructured Zn doped a-Fe2O3

(Zn-Fe2O3) thin film, modified by introducing a layer of Fe

doped TiO2 (Fe-TiO2) below the hematite thin film. Zn-Fe2O3

and Fe-TiO2 thin films were chosen because of their good

optical and photoelectrochemical properties than pristine

a-Fe2O3 and TiO2 thin films [18,19]. Prepared photoelectrodes

were also characterized for their structural, electrical and

optical properties to assess the mechanism by which this

concept influences the photoelectrode performance.

Table 1 e Sample description.

S. No. Film thickness (mm) Acronym

Zn-Fe2O3 Fe-TiO2 Overallthickness

1 0.24 e 0.24 A1

2 0.24 0.32 0.56 A2

3 0.24 0.65 0.89 A3

4 0.24 0.97 1.21 A4

2. Experimental

All chemicals used in this study were analytical grade

reagents; Fe(NO3)3$9H2O (99.9%, Aldrich), Zn(NO3)2$6H2O

(99.9%, Aldrich), titanium tetra isopropoxide (TTIP, 97% Pure)

and diethanolamine were used to prepare the precursor

solution for Zn-Fe2O3 and Fe-TiO2 respectively.

2.1. Preparation of photoelectrode

2.1.1. Nanostructured TiO2 doped with ironNanostructured iron doped TiO2 thin films were prepared by

sol-gel spin coating, using titanium tetra isopropoxide,

ethanol, diethanolamine and 0.2 at.% Fe(NO3)3$9H2O (dopant).

The details of the method of preparation have been described

somewhere else [19]. In the present work Fe-TiO2 films of

thicknesses 0.32, 0.65 and 0.97 mm were deposited on con-

ducting glass substrate (SnO2:In) with one, two and three

layers, to study the effect of thickness of underlying Fe-TiO2

film on the PEC response of Zn-Fe2O3 films.

2.1.2. Nanostructured Zn-Fe2O3 modified with underlyinglayer of Fe-TiO2

The thin filmsofZn-Fe2O3wereprepared by spray depositionof

Zn-Fe2O3 thin film over the pre-deposited Fe-TiO2 thin films

using the spray-pyrolysis set-up (laboratory built & designed).

The spray precursor comprising of 0.15 M Fe(NO3)3$9H2O and

5.0 at.% Zn(NO3)2$6H2O (dopant) in de-ionized (D.I) water was

sprayed with air as carrier gas at a pressure 2 Kg/cm2 through

a pneumatic nebulizer with a nozzle diameter of 0.1 mm onto

(pre-deposited) Fe-TiO2 thin films, kept on substrate heater at

350 �C temperature,withnearlyone-third of its surface covered

with aluminiumfoil. The solutionwassprayed for a total period

of 20 s, in two successive spray of 10 s duration. The time gap of

3 min between each successive spray was left for thermal

hydrolysis of the films. In order to get uniform thin films, the

heightof thesprayingnozzle fromthesubstrateheaterandflow

rate of the spray precursor were maintained at 25 cm, and

0.54ml/min, respectively,during thedepositionprocess.Finally

all thefilmsweresubjected toannealing inair at 500 �C for 2h in

a Muffle furnace. The obtained Zn-Fe2O3 thin films were semi-

transparent reddish brown in appearance and strongly

adherent to the substrate. It is tomentionhere that thicknessof

Zn-Fe2O3 film was kept constant and thickness of underlying

Fe-TiO2 film was varied in the different samples. Average

thickness of the Zn-Fe2O3 films deposited for 20 s spray period

was 0.24 mm, as measured by alpha-step profilometer (Tencor

AlphaStep 500). Overall film thicknesswith otherdetails of four

set of samples prepared in this study have been summarized in

Table 1. All thefilmswere converted into photoelectrodesusing

copperwire, silverpasteandepoxy (Hysol, Singapore) for itsuse

as photoelectrode in PEC cell. The effective area of the photo-

electrode available for illumination was 1.0 cm2.

2.2. Characterizations

The surface morphology and elemental composition of the

samples were obtained by using a field emission scanning

electron microscope (FE-SEM), (Carl Zeiss SUPRA 40VP)

combined with Oxford energy dispersive X-ray analysis (EDX).

The crystalline phases of the thin film samples were analysed

by X-ray diffractometer (XRD), (PANalytical X’Pert PRO q e 2q

Diffractometer) with a Cu-Ka source. Crystallite sizes in the

prepared films were calculated from the XRD data using

Scherrer’s formula [20]. Optical absorption spectra were

recorded on the thin film samples using a UVeVisible spec-

trophotometer (Shimadzu, UV-2450).

2.3. Photoelectrochemical study

The PECmeasurement was performed in a three electrode cell

consisting of metal oxide semiconductor as working, satu-

rated calomel electrode (SCE) as reference, and platinum

mesh as counter electrode, all immersed in 1 M NaOH elec-

trolyte (Sigma Aldrich, 99.99% purity). The photocurrents of

the samples as a function of applied potential versus SCEwere

recorded using a potentiostat (PAR Model:Versa Stat II) and

a visible light source (150 W Xenon lamp, Bentham) with

illumination intensity of 150 mW/cm2. The resistivity for all

i n t e r n 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 5 ( 2 0 1 0 ) 1 0 8 8 3e1 0 8 8 9 10885

the samples was calculated from the slope of currentevoltage

characteristic curves under dark condition. The performance

of the photoelectrodes were also tested under simulated light

provided by 300 W solar simulator (Newport, Model: 91160) in

AM 1.5 condition with illumination intensity of 70 mW/cm2.

The light intensity of both the light sources was measured

using broadband powermeter (Melles Griot, Model:13PEM001).

The experimental procedure also involved the measurements

of the open circuit photovoltage (Voc) for all the samples.

Capacitance (Csc) at semiconductor/electrolyte junction was

measured by LCR meter (Agilent Technology, Model: 4263 B)

for varying electrode potentials, at AC signal frequency of

1 kHz. The flatband potential (Vfb) of semiconductor photo-

electrode was obtained from the intercept of MotteSchottky

plot using the following relation:

1

C2¼

�2

q303N

��Vapp � VFB � kT

q

�(1)

Here 3o is the permittivity of the vacuum, N is the carrier

density, Vapp is the applied potential, 3 is the dielectric

constant of the semiconductor, kT/q is the temperature

dependent term. The intercept of linear plot at C�2 ¼ 0 gives

the flatband potential. Hydrogen was collected and measured

at the counter electrode with the best performing photo-

electrode by the water displacement method. Solar to

hydrogen conversion efficiency was calculated for all the

samples under 1.5 AM solar simulator irradiation.

3. Results & discussion

3.1. X-ray diffraction

Fig. 1 represents XRD patterns of nanostructured Zn-Fe2O3,

Fe-TiO2 and modified thin films. The Peaks obtained at

2q¼ 24.02, 33.22, 42.9, 49.54, 56.0 and 57.8� are due to reflection

from the planes (012), (104), (202), (024), (211) and (018) of

hematite, respectively, indicating the existence of hematite

phase with rhombohedral structure. The peaks observed at

Fig. 1 e XRD pattern of various thin films prepared

(*corresponds to peaks of underlying SnO2:In coating of

substrate).

2q ¼ 25.3 and 48.0� are due to reflection from the planes (101)

and (200), respectively of the anatase phase of TiO2 with the

tetragonal structure. Additional weak peaks in the XRD

pattern of Zn-Fe2O3 thin film modified by Fe-TiO2 at 2q ¼ 28.6,

42.08, 59.28, 43.9 and 52.92� indicated the formation of some

mixed oxides, Ti9Fe3(Ti7Fe3)O3, FeTi2O5 and Ti4Fe2O0.4.

The formation of mixed phase could be explained by the

fact that during sintering, Fe3þ ions of Zn-Fe2O3 present at the

interface, diffused into the underlying Fe-TiO2 layer producing

a substitutional solid solution. In fact, as the radius of the two

ions Fe3þ (0.55 A) and Ti4þ (0.60 A) is approximately same [21],

the substitution of iron in the matrix of TiO2 is a favourable

process [22]. The average particle size of the a-Fe2O3 as

calculated from the XRD data using Debye Scherrer’s formula

was observed to increase from 25 nm for Zn-Fe2O3 to 37 nm for

modified Zn-Fe2O3 thin film.

3.2. UVeVisible absorption spectrum

Fig. 2 shows the UVeVisible absorption spectrum for all the

samples. An increase in the absorbance of Zn-Fe2O3 thin film

with increasing thickness of underlying Fe-TiO2 layer was

observed. It can be seen that underlying layer of Fe-TiO2

improved the visible light absorption of Zn-Fe2O3 thin films.

Marginal blue shift in the absorption edge of Zn-Fe2O3 from

593 nm to 585 nm was observed for Zn-Fe2O3 thin film modi-

fied by Fe-TiO2. This increase in the absorbance and blue shift

in absorption edge may be attributed to the formation of new

mixed phases at the interface.

3.3. FE-SEM and EDX

The FE-SEM was carried out on all the samples. SEM micro-

graphs for some representative samples have been shown in

Fig. 3, which clearly indicate the formation of nanostructured

morphology. Fe-TiO2 thin film grown over the substrate

exhibited uniform granular structure consisting of particles in

the range 15e35 nm [Fig. 3(a)]. Whereas Zn-Fe2O3 thin film

exhibited the average particle size of 25 nm [Fig. 3(b)]. SEM

image of modified Zn-Fe2O3 film appears to be porous and

composed of relatively bigger particles of average sizew42 nm

35 0 40 0 45 0 50 0 55 0 6 0 0 65 0

0.00

0.25

0.50

0.75

1.00

1.25

)

u

.

a

(

e

c

n

a

b

r

o

s

b

A

Wavelength (nm)

A 1

A 2

A 3

A 4

Fig. 2 e Absorbance spectra for various thin films prepared.

Fig. 3 e SEM image of (a) Fe-TiO2 (b) Zn-Fe2O3 and (c) Zn-

Fe2O3 thin filmmodified with Fe-TiO2 of thickness 0.89 mm.

Inset shows the energy dispersive X-ray map of modified

Zn-Fe2O3 sample.

0 . 5 0 . 6 0 . 7 0 . 8 0 . 9 1 . 0

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

A 1

A 2

A 3

A 4

m

c

/

A

m

(

y

t

i

s

n

e

D

t

n

e

r

r

u

c

o

t

o

h

P

2 )

Applied Potential (V/SCE)

Fig. 4 e Photocurrent density versus applied potential

curve for various thin films under visible light illumination

using 150 W xenon lamp.

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 5 ( 2 0 1 0 ) 1 0 8 8 3e1 0 8 8 910886

[Fig. 3(c)]. The values of the grain sizes obtained by SEM

analysis were observed to be in good agreement with XRD

results. Inset of Fig. 3(c) showed the energy dispersive X-ray

image of the modified Zn-Fe2O3 thin film. Atomic percentage

of the Fe, Zn, Ti and O was 15, 1, 12 and 72.5% respectively.

FE-SEM with EDX microanalysis confirms uniform coating of

Zn-Fe2O3 over Fe-TiO2 thin film. Slightly higher content of

oxygen in the modified film can be attributed to the presence

of mixed phase in themodified film in addition to anatase and

hematite phase of Fe-TiO2 and Zn-Fe2O3 respectively.

3.4. Photoelectrochemical study

3.4.1. Currentevoltage characteristicsNanostructured Zn-Fe2O3 films modified with underlying

Fe-TiO2 were used as photoelectrode in PEC cell and cur-

rentevoltage characteristics were recorded under darkness

and illumination. The externally applied bias was varied from

�1.0 V/SCE (cathodic bias) to þ1.0 V/SCE (anodic bias). The

photocurrent density for all the samples was calculated by

subtracting dark current from current under illumination

Fig. 4 shows the photocurrent density versus applied potential

curves for all the thin film photoelectrodes. It is noted that

photocurrent density increased with increasing overall film

thickness up to 0.89 mm (for sample A3) of the modified

thin film, afterward it decreased. The observed value of

the photocurrent density for sample A3 was 700 mA/cm2 at

0.95 V/SCE, which is approximately ten times higher than

pristine Zn-Fe2O3 [Table 2]. Maximum photocurrent density

exhibited by sample A3 may be attributed to the many factors

like formation of mixed oxides at the interface, improved

absorption and coupled effect induced by the Fe-TiO2 film.

Film of optimized thickness seems to be capable of facilitating

efficient separation of photogenerated charge carriers and

their movement across the interface for photocurrent

improvement. Resistivity measurement indicated a reduction

in the value of the resistivity for the sample A3 (Table 2) which

may be another reason for enhanced photoresponse. The

decrease in the photocurrent density at film thickness more

than 0.89 mm can be explained using two important facts.

Firstly, although increase in the thickness enhances the

absorption (Fig. 2), yet it may increase the rate of recombina-

tion of photogenerated carriers by increasing the distance

traveled by the photogenerated carriers to migrate towards

the surface, thereby reducing the photocurrent [23]. Secondly,

this decrease in photocurrent density may occur due to the

Table 2 e The Values of flatband potential, resistivitymeasured in dark and photocurrent density for varioussamples with 150 W Xenon lamp.

S.No.

Acronym Flatbandpotential(V/SCE) in

Dark

Resistivity(�105 U cm)

Photocurrentdensity (mA/cm2) at

0.95 V/SCEmeasured with

visible light source

1 A1 �0.57 16.0 72

2 A2 �0.90 9.9 413

3 A3 �0.93 9.8 700

4 A4 �0.76 9.8 294

i n t e r n 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 5 ( 2 0 1 0 ) 1 0 8 8 3e1 0 8 8 9 10887

increased charge recombination at a large number of grain

boundaries present in thicker film, which results in the loss of

charge carrier during their transport within the film from

collection at back ITO contact [24,25].

The hydrogen generated during the PEC reaction was

collected by water displacement method at the Pt counter

electrode and measured at the interval of 10 min with

best performing photoelectrode of 0.89 mm thickness i.e. for

sample A3. Rate of hydrogen generationwas 1.2ml cm�2h�1 at

0.95 V/SCE, which was observed to be constant as shown in

Fig. 5. The nature of current with time during hydrogen

collectionhasbeenshownin the insetofFig. 5.Constantnature

of the current during hydrogen collection clearly reflects the

stable nature of the photoelectrode in electrolyte. To test

stability of the photoelectrode in the electrolyte observations

were repeated for 15 times. The physical appearance of the

sample remainedunaltered andhydrogenproduction ratewas

observed to be reproducible, showing excellent stability of the

photoelectrode in the electrolyte.

Since the modified photoelectrode comprised of two

materials i.e. Zn-Fe2O3 with absorption in visible region and

Fe-TiO2 with absorption in UV region, therefore, the use of

a light source having actual sun conditions would be of more

relevance with respect to the absorption characteristics of the

0 600 1200 1800 2400 3000 3600 4200

0 600 1200 1800 2400 3000 3600 4200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0

-2.2

-2.4

Cu

rren

t (m

A)

Time (Second)

)

L

m

(

d

e

t

c

e

l

l

o

c

n

e

g

o

r

d

y

H

Time (second)

V app

= 0.95 V/SCE

Fig. 5 e Volume of Hydrogen Collected with time for

sample A3 of thickness 0.89 mm. Inset shows the current

versus time curve recorded during hydrogen collection.

photoelectrode. Hence, apart from ordinary visible light

source, performance of all the photoelectrodes in PEC cell

were also studied under AM 1.5 solar simulator and resulted

photocurrent density curves have been shown in Fig. 6. The

maximum photocurrent density of 748 mA/cm2 at 0.95 V/SCE

externally applied bias was observed for sample A3. The open

circuit photovoltage (Voc) values were measured for all the

samples and summarized in Table 3. An increased value of

photovoltage for the modified Zn-Fe2O3 photoelectrode was

observed as compared to the pristine Zn-Fe2O3 thin film.

3.5. MotteSchottky study

MotteSchottky curves obtained for all the samples under

darkness have been presented in Fig. 7. All the samples

exhibited positive slopes, indicating the semiconductor

thin films to be of n-type. Flatband potential is an important

factor in deciding the photoresponse of material. More nega-

tive the value of flatband potential, better is the ability of

the semiconductor film to facilitate the charge separation in

PEC application [26]. Calculated values of flatband potentials

for all the samples have been given in Table 2. It was found

to increase from �0.57 V/SCE for pristine Zn-Fe2O3 to

�0.93V/SCE for sampleA3. Themaximumvalueof theflatband

potential obtained for sample ‘A3’ also supports the best

photocurrent density exhibited by this sample.

3.6. Efficiency calculation

The solar to hydrogen conversion efficiency by the water

splitting reaction was calculated for all the samples with

solar simulated light source at AM 1.5 conditions using the

following equation [27]:

hð%Þ ¼ Jp��1:23� Vapp

��I0�� 100 (2)

where the photocurrent density, Jp is in mA/cm2, Io is input

intensity of light source, Vapp ¼ Vmea�Voc, where Vmea is the

electrode potential (V/SCE) of the working electrode at which

the photocurrent was measured under illumination and Voc is

0 . 5 0 . 6 0 . 7 0 . 8 0 . 9 1 . 0

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

m

c

/

A

m

(

y

t

i

s

n

e

D

t

n

e

r

r

u

c

o

t

o

h

P

2 )

Applied Potential (V/SCE)

A 1

A 2

A 3

A 4

Fig. 6 e Photocurrent density versus applied potential

curve for various thin films under simulated light using 1.5

AM solar simulator.

Table 3 e The values of photocurrent density and solar tohydrogen conversion efficiency for various samples with1.5 AM solar simulator.

S.No.

Acronym Photocurrentdensity (mA/

cm2) at 0.95 V/SCE measured

with solarsimulator

Open circuitphotovoltagemeasuredwith solarsimulator(mV/SCE)

Solar tohydrogenconversionefficiency, h(%) at 0.95 V/

SCE

1 A1 3 75 0.02

2 A2 507 89 0.27

3 A3 748 90 0.47

4 A4 312 88 0.15

0 . 1 5 0 . 3 0 0 . 4 5 0 . 6 0 0 . 7 5 0 . 9 0 1 . 0 5 1 . 2 0 1 . 3 5

0.0

0.1

0.2

0.3

0.4

0.5

n

o

i

s

r

e

v

n

o

C

n

e

g

o

r

d

y

H

o

t

r

a

l

o

S

(

y

c

n

e

i

c

i

f

f

E

)

%

Thickness ( m )

Fig. 8 e Solar to hydrogen conversion efficiency versus film

thickness curve for various thin films.

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 5 ( 2 0 1 0 ) 1 0 8 8 3e1 0 8 8 910888

the electrode potential (V/SCE) of the same working electrode

at open circuit condition under same illumination (AM 1.5

solar simulator) and in the same electrolyte. The efficiency

calculations were made for all the samples at 0.95 V/SCE and

are given in Table 3. The solar to hydrogen conversion effi-

ciency as a function of film thickness has been plotted and

shown in Fig. 8. The highest value of efficiency of 0.47% was

exhibited with the sample A3 having thickness of 0.89 mm and

is approximately twenty times larger than pristine film of

Zn-Fe2O3. There are very few reports available on the calcu-

lation of solar to hydrogen conversion efficiency for similar

metal oxide structures. Liu et al. [28], Miller et al. [10] and

Kale et al. [29] has reported Solar to hydrogen conversion effi-

ciency for TiO2/SnO2, WO3/SnO2 (hybrid photoelectrode) and

TiO2/In2S3/CdSe composite systems as 4.73%, 0.7% and 0.13%

respectively. Light source used, although not specified, seems

tobeanordinaryUVeVisiblesource toexcite thewidebandgap

material SnO2, WO3, TiO2 and not the solar simulator. Since,

the ultimate goal of the PEC research is to design and develop

a system working efficiently under the solar energy illumina-

tion, w0.47% efficiency obtained with solar simulated light

source in thiswork formodifiedZn-Fe2O3film is anappreciable

value. Thus, this study clearly indicates that PEC system

obtained by using Zn-Fe2O3 photoelectrode modified with

underlying Fe-TiO2 layer, is more efficient for generation of

hydrogen using solar energy as compared to other similar

systems. Further work on this line is underway.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.00

0.03

0.06

0.09

0.12

0.15

Applied Potential (V/SCE)

C

/

1

2

0

1

x

2

1

F

(

2

-

m

c

4 )

A 1

A 2

A 3

A 4

Fig. 7 e MotteSchottky curves for various thin films.

4. Conclusion

The present study showed that the Zn-Fe2O3 thin film modi-

fied with underlying Fe-TiO2 is a better photoelectrode in PEC

cell for splitting of water to generate hydrogen using solar

energy, as compared to single material photoelectrode. The

maximum solar to hydrogen conversion efficiency of 0.47%

was exhibited by modified Zn-Fe2O3 photoelectrode with

overall thickness 0.89 mm. The improved photoresponse of

this photoelectrode may be attributed to the efficient separa-

tion of photogenerated charge carriers at the interface,

reduction in resistance and improved light absorption ability.

Acknowledgments

The authors gratefully acknowledge the partial financial

support obtained for thiswork by the Department of Science&

Technology, New Delhi under the project no: SR/S2/CMP-47/

2005 and by the University Grant Commision, New Delhi

under the project no: 37-128/2009.

r e f e r e n c e s

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