Stabilizing effect in nano-titania functionalized CdS photoanode for sustained hydrogen generation

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Stabilizing effect in nano-titania functionalized CdSphotoanode for sustained hydrogen generation

Alka Pareek a,b, Rahul Purbia a, Pradip Paik b, Neha Y. Hebalkar a,Hyun Gyu Kim c, Pramod H. Borse a,*a International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur PO,

Hyderabad 500 005, AP, Indiab School of Engineering Science and Technology, Hyderabad Central University, Gachibowli, Hyderabad 500046, AP,

IndiacDivision of High Technology Materials Research, Korea Basic Science Institute (KBSI), Busan 618-230,

Republic of Korea

a r t i c l e i n f o

Article history:

Received 24 October 2013

Received in revised form

17 December 2013

Accepted 30 December 2013

Available online xxx

Keywords:

Photoelectrochemical cell

Hydrogen generation

Spray deposited CdS

TiO2 nanoparticles

Stability of photoanode

Photocorrosion

* Corresponding author. Tel.: þ91 4024452426E-mail addresses: [email protected], pb

Please cite this article in press as: Pareek Ahydrogen generation, International Journ

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.12.1

a b s t r a c t

Efficient and stable photoanode has been fabricated by the surface functionalization of the

nanostructured film. For this, the surface of spray deposited CdS thin film was modified

through bi-functional molecule mediated chemisorption of TiO2 nanoparticles (NP).

Consequently, a systematic control over efficiency and photoanode stability against

corrosion has been investigated. An in-depth quantitative analysis of the photocorrosion of

these photoanodes is further studied using chronoamperometry, X-ray photoelectron

spectroscopy and induced coupled plasma spectroscopy. TiO2 NP modified photoanodes

show an enhanced efficiency and a stability. For photoelectrochemical (PEC) systems, the

stability factor (P

) has been defined for the first time based on the time dependent chro-

noamperometry, which clearly demonstrates thatP

modified >>P

bare. The modified pho-

toanode shows an improved Incident Photon to Current Efficiency of 22% than the bare CdS

(w8%) electrode. It gives an enhanced solar-to-hydrogen conversion efficiency of

STH w 0.7% w.r.t bare CdS (0.2%) under AM 1.5G solar simulator, at 0.2 V/SCE. Improved

stability of more than nine hours and enhanced efficiency is attributed to the controlled

passivation of CdS surface through TiO2 NP (5 nm), and inhibition of the charge recombi-

nation. Superior and stable performance of modified photoelectrode has been validated by

higher and stable hydrogen evolution over modified electrode.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

A sustainable and continuous hydrogen generation by pho-

toelectrochemical water-splitting reaction is highly desirable.

; fax: þ91 [email protected], pborse

, et al., Stabilizing effectal of Hydrogen Energy (

2014, Hydrogen Energy P85

It is known that photoelectrochemical and photocatalytic

hydrogen production through solar water-splitting is one of

the most promising renewable energy generation technolo-

gies [1e6]. The cadmium chalcogenide comprises as one of the

@gmail.com (P.H. Borse).

in nano-titania functionalized CdS photoanode for sustained2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.185

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

mailto:[email protected]



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http://dx.doi.org/10.1016/j.ijhydene.2013.12.185



Fig. 1 e Schematic presentation of NPT adsorption

mechanism on CdS using thioglycerol (TG) molecules.

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 x x x ( 2 0 1 4 ) 1e1 12

most important class of semiconductor in the photo-

electrochemical (PEC) research, as it is the most promising

candidate that exhibits a direct band-gap of 2.4 eV. Addition-

ally, it is considered an excellent visible light responsive ma-

terial [7,8] with well-suited band-edge positions for the

cleavage of water molecule into hydrogen and oxygen. Un-

fortunately, chalcogenide electrodes show poor photo corro-

sion resistance, and hence not capable of sustainable

hydrogen production though they yield high efficiency pho-

toanodes. In present work, we sought cadmium sulphide as

the photo anodic material for photoelectrochemical hydrogen

application, with a view to apply it for sustained PEC hydrogen

generation.

It is known that, chalcogenides suffer with a peculiar

problem of undesirable corrosion of electrode upon photo-

illumination [9,10]. This is because, when photogenerated

holes (hþ) in the valence band migrate to the surface, it

leads to the photocorrosion of chalcogenide electrode. This

phenomenon can be understood by following reaction:

CdSþ 4hþ þO2 þ 2H2O/Cd2þ þ SO2�4 þ 4Hþ (1)

In order to utilize suitable optoelectronic as well as pho-

toelectrochemical properties of CdS, it is highly desirable to

overcome or minimize its problem of photo-stability. In past,

there were few strategies implemented to suppress the pho-

tocorrosion. These strategies can be stated in following way.

First strategy say, exploits the use of the electron rich species

viz. sacrificial redox scavengers such as halides [11e13], EDTA

[14], FeðCNÞ4�6 [11,15,16], S2� [11,17,18], SO2�3 [11,17,18] to con-

tent the problem. Such negative species combine with the

photogenerated holes, thereby inhibiting the photocorrosion.

Second onemakes use of the catalytic metal (as Pt, Ru and Au)

particle over the electrode surface; the population of holes is

minimized to reduce the anodic oxidation process [17,19] and

thus avoiding unnecessary oxidation of lattice S atoms. Third

option to address the problem is by isolating the semi-

conductor surface from electrolyte via. a polymer such as a

polystyrene, polyacrylic acid [20e23] or stable metal oxide

semiconductor such as ZnO, TiO2 [24e27]. Such polymer/

metal oxide layer acts as a protective layer over the semi-

conductor electrode, and consequently inhibits the photo-

corrosion process. Additionally, hybrid system of Cd

chalcogenides with carbon related compounds has been re-

ported [28e30], but there is no detailed and particular

emphasis on the stability studies. Among all the approaches,

surface passivation using metal oxide TiO2 coating has been

expected to bemost eligible as it is highly stable and economic

semiconductor system [31].

There have been reports on modification of TiO2 powder/

colloidal photocatalyst using CdS to enhance the visible

light induced water splitting [25e27,32e34] activity in TiO2.

However, a fewer reports exist on the modification of CdS

[35,36] films using TiO2, and specific focus on the modifica-

tion of nanostructured CdS film using TiO2 nanoparticles is

scarce in context to stability aspect of CdS electrode. It is

expected that the TiO2 led surface modification of an un-

stable chalcogenide (as CdS) photoanode surface can ideally

inhibit the photogenerated electronehole recombination

during the photoelectrochemical reactions, and thereby

Please cite this article in press as: Pareek A, et al., Stabilizing effechydrogen generation, International Journal of Hydrogen Energy (

yield an enhanced photoelectrochemical performance. In

addition, the passivation using such chemically stable ma-

terial yields a photocorrosion-proof electrode surface [35].

TiO2 can modify CdS film in two ways viz. physio-sorption

and chemisorption. Physio-sorption includes the formation

of TiO2 layer over CdS photoanode by making use of TiO2

precursor, which is followed by an annealing step [36].

Chemisorption, process involves the use of organic linkers,

an organic molecule [37e41], especially for creating a

chemical linking between the two material systems, say CdS

surface and TiO2 nanoparticles in the present case. As

already reported, chemisorption is an efficient way for the

adsorption of TiO2 nanoparticles on the CdS photoanode

surface [42]. Here an affinity of CdS and TiO2 towards thi-

ol(ReSH) and hydroxyl (eOH) groups respectively has been

useful to form a hybrid structure. In this chemical approach

an electron transfer and the consequent charge transport

properties mainly depends on the length of the bi-functional

(linker) molecule [43]. However, it is important to note that

lengthy bi-functional molecule causes hindrance in the

transfer of electrons from one semiconductor to other

semiconductor. There are various reports on the use of

linking agents for linking CdS nanoparticles to TiO2 base

electrodes [37e40]. In present work, we have attached the

CdS photoanode surface with “TiO2 nanoparticles” hence-

forth termed as NPT using thioglycerol (TG) as an organic

linker via. a simple adsorption technique. The procedure

can be easily described as shown in Fig. 1. We report here a

detail study of the synthesis, characterization and photo-

electrochemical properties of NPT modified CdS photo-

anodes. Reduction in the density of surface states of the CdS

surface by adsorption of NPT leads to a decrement in the

electronehole recombination process [44] thereby resulting

in an overall increase in PEC efficiency. More over TiO2 is a

stable metal oxide, which helps in the passivation of the

CdS photoanode surface and hence additionally provides

the photocorrosion stability. The work focuses on the sta-

bility improvement investigations over CdS photoanode

achieved via. NPT.

2. Experimental

2.1. Deposition of nanostructured CdS thin films

An automated spray pyrolysis setup has been used for the CdS

thin film deposition. Several optimizations were carried out to

achieve the nanostructured CdS thin film as reported earlier

t in nano-titania functionalized CdS photoanode for sustained2014), http://dx.doi.org/10.1016/j.ijhydene.2013.12.185



Table 1 e Name convention followed for NPTphotoanodes obtained after different treatmenttime.

Time of treatment NPT/CdS

3 min NPT-a

7 min NPT-b

15 min NPT-c

30 min NPT-d

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 4 ) 1e1 1 3

[45]. The automated computer interface system has ability to

control the inert gas pressure, precursor flow rate, substrate

temperature and the film thickness. Equal amount of solu-

tions of CdCl2 (0.1 M) & (NH2)2CS (0.1 M) were used in double

distilled water to prepare the precursor for film deposition.

Fluorine doped tin oxide; FTO (Pilkington TCO-15) with re-

sistivity 12Ucm has been used as substrate with PEC active

area of 2 cm2. The distance between nozzle and substrate was

maintained at 23 cm throughout the experiment.

2.2. Modification of CdS photoanode

After the film deposition, the surface of nanostructured CdS

photoanode was modified by NPT via. thioglycerol as organic

linker. Optimum concentration of thioglycerol (TG) was used

for complete adsorption of NPT on the CdS photoanode sur-

face. The concentration of TG is optimized by varying its

volume ratio in the range 0.1e1.0 (Fig. S.I. 1). It was found that

very low TG concentration (<0.1 v/v) was insufficient to attach

enough nanoparticles, whereas when higher concentration

(>>0.1 v/v) of TGwas used it deteriorated the photoanode, and

the CdS films peeled off (Fig. S.I. 2) from the FTO substrate. Our

optimization shows that stable film and the best PEC perfor-

mance can be achieved at TG concentration of 0.1 v/v%, which

is used in present study.

Further, NPT adsorption was carried-out on the TG

treated CdS photoanode as described in previous paragraph.

For this, NPT were synthesized using TiCl4 precursor in

aqueous medium via. known procedure in literature [46,47].

Precisely titanium tetrachloride is dissolved in the distilled

water at ice-cold temperature and further mixed with

ammonia solution. Thus obtained mixture was stirred for an

hour and refluxed for w18 h. As received precipitate was

collected by centrifugation and washed in distilled water to

remove any un-reacted impurities, and then air-dried. The

NPT (5 nm) were characterized (Fig S.I.3 and S.I.4) and used

for linking on the CdS surface by simple adsorption method.

Briefly, the as-prepared CdS thin films were treated with

optimum concentration of TG solution and later with NPT

dispersion (anatase 0.5 wt.%) so that NPT can be linked on

CdS photoanode surface in the manner shown in Fig. 1. Ef-

fect of amount of NPT adsorption on CdS surface was

studied based on the treatment times viz. 5 min (NPT a),

7 min (NPT b), 15 min (NPT c) and 30 min (NPT d) and named

as NPT a-d, henceforth throughout the manuscript, unless

specified.

The process was repeated with NPT dispersion made by

using rutile TiO2 (0.5 wt.%) dispersion. This was done to study

the “effect of TiO2 phase on the stabilization of CdS electrode”.

The PEC performance of these modified photoanodes was

considered for further optimization (Fig. S.I. 5). Expectedly,

anatase phase NPT shows efficient PEC performance. The NPT

adsorption on TG treated CdS thin films were carried out for

different time intervals, to understand the “effect of TG treat-

ment time” on the PEC performance. Further, its effect on the

optical properties is also studied to validate if TG adsorption

has taken place over the CdS film and that it does not degrade

the PEC performance. A typical case of 7 min treatment time

(Fig. S.I.6) has been shown in the transmission spectra that

revealed the optimum coverage of NPT.

Please cite this article in press as: Pareek A, et al., Stabilizing effecthydrogen generation, International Journal of Hydrogen Energy (

2.3. Characterization

Surface morphology of bare nanostructured CdS and NPT

treated CdS thin films were observed using field emission

scanning electron microscopy (Hitachi model S4300SE/N)

operated at 20 kV. Energy-dispersive X-ray spectroscopic (EDS)

analysis was done for the elemental characterization of the

films. X-ray diffraction pattern were obtained by using Bruker

AX D8 XRD diffractometer equipped with CuKa X-ray gun. The

scan is taken in the range of 2q ¼ 10e70� with an increment of

0.010 per second at the grazing angle of incidence. X-ray

photoelectron spectroscopy (O-micron model) has been used

for elemental and compositional analysis. The transmission

spectra were recorded using UVeVis spectrometer (Perkin

Elmer, lambda 650). The presence of Cd2þ ions were detected

from Induced coupled plasma spectroscopic (ICP-OES VARIAN

720-ES) studies.

The surfacemodified CdS thin film electrodes of area 2 cm2

are used for PEC measurements and the results were con-

verted to obtain the current-density accordingly. The photo-

electrochemical analysis was carried out using

electrochemical works station (PARSTAT 2273 Model). A two

electrode configuration with the modified photoanode as

working electrode and graphite as counter electrode is used

for measurement, whenever cell performance was desirable.

Na2S (0.1 M) & Na2SO3 (0.01 M) is used as an electrolyte with a

view to maintain an inherent stability of CdS. A Hgearc lamp

(Oriel) of power 500 W equipped with 470 (�10) nm band-pass

filter delivering an output power of 50 mW/cm2 is used for PEC

measurements. The infrared rays were cut-off using water

filter before falling on the filter. During the hydrogen evolution

measurements, hydrogen was detected by gas chromatog-

raphy (GC 2010 plus), while the PEC reactionwas carried-out at

0.2 V (vs. SCE) bias. In order to study the solar hydrogen pro-

duction, the solar simulator (Oriel Model 91160) equipped

with A.M.0, A.M.1.0 and an AM 1.5 Global (Newport) filter was

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

case.

3. Results and discussions

Photoanode were named according to the adsorption time of

TiO2 nanoparticles on the CdS photoanodes, and are displayed

in Table 1. In the following study, NPT-b, the optimal photo-

anode has been used as photoanode throughout, to study the

structural, optical, morphological and the charge transfer

properties of modified photoanodes w.r.t bare CdS

photoanodes.




Table 2 e Absorption edge position, and bandgapvariation for NPT photoanodes.

Sample name Absorption edge (nm) Bandgap (eV)

CdS 496 2.50

CdS-TG 491 2.52

CdS-NPT 496 2.50


3.1. Structural and optical characterization ofunmodified and modified CdS photoanodes

Fig. 2(a) shows X-ray diffraction spectra of the bare CdS and

the modified photo electrodes. All the films exhibits hexago-

nal phase that belongs to P63MC (JCPDS file No. 00-006-0314)

space group. The peaks due to FTO substrate can be easily

observed indicating the existence of thin film over FTO sub-

strate at the background of the film. Peaks related to TiO2 were

not found in the spectra. The crystallite size was found to be

22 nm, as estimated from debye Scherrer equation. The study

indicated the formation of nanostructure film over FTO under

the given conditions. Further, the nanostructured CdS films

were used for the fabrication of modified electrode.

Fig. 2(b) shows the UVeVIS absorption spectra of bare CdS,

and modified CdS thin films. Bare CdS and TG modified films

showed very similar behaviour, whereas CdS/NPT showed a

blue shift in their absorptionwith respect to the bare film. The

result of optical analysis is presented in Table 2. The study

confirms that the optical property of the CdS film remain

unaffected after adsorption of TG. On the contrary, a small

blue-shift in the band-gap of other two modified samples is

attributed to the existence of nano-sized TiO2 in case of NPT

treated CdS photoanodes [45].

Fig. 2 e (a) XRD patterns and (b) absorbance spectra for the

comparison of bare CdS photoanodes with NPT modified

CdS photoanodes.


3.2. Morphological characterization of modifiedphotoanodes

Fig. 3 shows the schematic electrode representation and sur-

face morphologies of the bare and modified CdS films, along

with their EDS spectra. Bare CdS film displays a nano-

structured surface (Fig. 3(a)) consisting of randomly aligned

rods throughout, which undergoes filler-type morphology

changes, upon the NPT treatment, as seen in the SEM of

Fig. 3(b). When TG (i.e. S–R–OH) is brought in vicinity of CdS

film surface, the thiol groups strongly react with the substrate,

and distort or fill-up the cavities near the rods yielding an

asymmetrically scattered rods all over the CdS surface. The

NPT treatment further, leads to the linking of dangling hy-

droxyl ions of TG to that of interactive ions of NPT. Presence of

NPT can be clearly seen in the form of spherical particles

(white colour) on the CdS surface as seen in Fig. 3(b). The

detection of Ti in EDS further validates the existence of linked

TiO2 nanoparticles over the CdS film surface. This adherent

NPT remains over the film even after washing of film in

solvent.

Fig. 3 e Schematic representation, EDS spectra and FESEM

images of (A) bare CdS photoanode; and (B) NPT modified

CdS photoanode.





This is in agreement with the earlier report [43] that S-

terminated CdS surface has affinity towards the thiol-group,

whereas TiO2 towards the hydroxyl group of TG. This study

demonstrates that we have achieved NPT modified nano-

structured CdS films. In order to, finally validate this model in

a quantitative manner; we further performed the XPS study

for the sake of completeness. The film surface studies (XPS

Fig. S.I.7eS.I.9) does confirm the proposed model (shown in

Figs. 1 and 3) and support the results of above section. Surface

studies clearly reveal the existence of Ti in the modified films.

3.3. Photoelectrochemical measurements of modifiedphotoanodes

The purpose of nanoparticle mediated surfacemodification of

CdS is to achieve an improved photoanode. Consequently, in

order to investigate the PEC behaviour of CdS and modified

CdS photoanodes, PEC characterization of respective photo-

electrodes was carried out. Fig. 4 shows the comparative

chronoamperometric curves for NPT modified CdS photo-

electrodes w.r.t. the bare CdS photoelectrode. An improve-

ment in the efficiency with the adsorption of NPT on CdS

surface is confirmed from the above studies.

Fig. 4 shows the effect of surface modification of CdS

photoanode with NPT on the PEC properties. As can be clearly

seen in case of NPT-d photoanode, the photocurrent increases

by 3.7 times than that generated by the bare CdS photoanode.

Above photocurrent improvement is evaluated using simple

differential function of I as defined by dI (dI¼ IModified film � IBare

Fig. 4 e Comparative study of photocurrent variation with

NPT treatment time for the NPT photo-electrodes with

respect to bare CdS, (dI [ IModified film e IBare CdS).


CdS). The values of dI as shown in Fig. 4 clearly indicate an

enhancement in the photocurrent of modified photoelectrode

w.r.t. bare CdS photoelectrode. Similarly there is a systematic

increase in the efficiency for NPT-a, NPT-b and NPT-c photo-

anodes. This enhancement in the efficiency with TiO2 can be

attributed to the reduction of density of surface states on CdS

surface with the adsorption of NPT [36].

Electrochemical impedance spectroscopy was carried out

to understand the charge transfer mechanism between pho-

toelectrode/electrolyte interfaces. All themeasurements were

performed in dark conditions with an applied frequency of

10 kHz and ac small signal amplitude of 10mV Fig. 5 shows the

comparison of the Nyquist plots of CdS and NPT modified

photo-electrodes. A semicircle in the Nyquist plot clearly de-

picts that the charge transfer process at the CdS/electrolyte

interface is dominant and capacitive in nature [36]. The

experimental data were fitted using Z-View software. Fig. 5(d)

shows an equivalent circuit deduced from the analysis of

corresponding Nyquist plot.

In equivalent circuit, Rs represents an ohmic resistance of

electrolyte. Rt and Ct are the resistance and capacitance of the

electrode electrolyte interface. As can be seen in Fig. 5(a) the

diameter of the semicircle of Nyquist plot corresponding to

NPT modified photoelectrode is much greater than that of

bare CdS photoelectrode. It is important to note the recom-

bination time, which is equal to the product of resistance of

the solid interface and the capacitance (sr ¼ Rt � Ct) [36]. The

NPT modified photoanode shows much higher recombination

lifetime than bare CdS, implying reduction in the recombi-

nation process of charge carriers in modified electrodes.

Hence, these electrodes show an improved PEC performance.

3.4. Photoelectrochemical stability studies of NPTmodified electrodes

Implementation of sustainable hydrogen energy for energy

applications necessitates achieving a stable and an efficient

photoanode. Stability remains one of the most important

factors that controls life of a photoanode, and ultimately even

of PEC cell. In general, stability depends on several physical,

chemical and other factors [48]. Accordingly, in order to,

quantitatively describe stability of a photoanode, we have

defined a new parameter (Sphotoelectrode) which is the function

of several dependent parameters (physical/chemical/others)

as described below:

Sphotoelectrode ¼ 4ðphysicalÞ+4ðchemicalÞ+4ðothersÞ (2)

Where, f (physical) e physical properties of electrodes

correlated to stability viz. crystal structure, orientation,

thickness, area, geometry.

f (chemical) e pH of electrolyte, electrolyte composition,

concentration of electrolyte.

f (others) e temperature, time, stirring and photocurrent

density.

It is imperative that stability (Sphotoelectrode) is multiple-

parameter dependent function involving various processes,

and thus it is difficult to describe it quantitatively. However, in




Fig. 5 e Electrochemical impedance studies of (a) bare CdS, NPT modified photoanode (b) only NPT modified photoanode (c)

bare CdS photoanode (d) equivalent circuit and corresponding values of charge transfer resistance and capacitance values.

Fig. 6 e Chronoamperometry of bare CdS and NPT treated

photoanodes for different adsorption time for 2.5 h under

470 nm filter (Pin w 40 mW/cm2). Inset shows the

chronoamperometric of bare CdS andmost stable electrode

NPT-b under solar simulator (Pin w 80 mW/cm2) for 9 h.


order to simplify its quantitative estimation in present study,

we have considered the variation of photocurrent with time as a

means to estimate the time-dependent stability of a photo-

anode; accordingly, we propose “photoanode-stability” to be a

simple function of photocurrent density and illumination

time, assuming other parameters constant:

X

photoelectrode

% ¼ DCdS � Dmodified

DCdS� 100%

0Dmodified/DCdS;

Pphotoelectrode/0

Dmodified/0;P

photoelectrode/1

(3)

Where Dphotoelectrode is slope of “I vs. t” curve of the photo-

electrode, given by:

D ¼ vIvt

¼ Iðt2Þ � Iðt1Þt2 � t1

Here, I(t1) is photocurrent at time ‘t1’and I(t2) is photocur-

rent at time ‘t2’.

Accordingly, the slope of Iet curve for CdS (6CdS) and

modified photoanode (6modified) were calculated from Fig. 6.

Subsequently, stability (Ʃ) is estimated for each modified

photoanode w.r.t the bare CdS photoanode. It can be noted

that Fig. 6 displays the comparison of chronoamperometric

curves for various electrodes for the duration of 2.5 h, while

inset of Fig. 6 indicating the time-dependent photocurrent

generation from NPT-b for around 9 h, under simulated solar

radiation. Fig. 6 clearly demonstrates that the NPT-b is the

most stable photoanode, as it displays a stable photocurrent.

These curves clearly reveal that, if the decay in photocurrent

of photoanode is comparatively faster as in case of bare CdS, it

indicates a poor stability (Sphotoelectrode). However, if there is no

decay in the photocurrent as seen for themodified photoanode

(say NPT-b-red curve) then it indicates the photoanode has 100

% stability. It is noteworthy that in order to quantify slope


(6CdS), it is necessary to relate it with the angle subtended by

the decay curve with X-axis (as shown in Fig. S.I.11). This can

be shown as below.

As, mathematically the slope is given by:

D ¼ tan j0j ¼ 0zstable photoelectrode

j ¼ 450zunstable photoelectrode

An ideal stable photoanode exhibits y¼ c behaviour in the I

vs. ‘t’ curve i.e. straight line with slope, D ¼ 0 (j ¼ 0). Deviation

from ideal behaviour leads to the reduction in the photocurrent,

which shows a deterioration of the photoanode due to the

photocorrosion process. This can be clearly understood from





the Fig. S.I.11 that shows angle j. On the basis of above pro-

posed theory, we have considered the analysis of I vs. ‘t’

characteristics as shown in Fig. 6. Table 3 summarizes the

variation in the stability (Sphotoelectrode) as deduced from

equation-3, for the modified photoanodes. It can be seen that

NPT-b shows significant stability (P

modified) than bare CdS

(P

CdS). This clearly shows a correlation of the stable NPT-b

behaviour with the maximum S of 100%.

Further, It can be noted NPT-b can be concluded as with

optimum loading, and not those with higher loading, say like

NPT-d. NPT-d shows a delayed equilibration with time,

possibly due to the fact that only few layers of NPT can be

adherent to CdS surface by getting linked to hydroxyl group of

TG. During electrochemical measurement extra TiO2 layers

comes out in the electrolyte with time until it reaches an op-

timum amount which can be adherent to CdS surface. This

leads to reduction in current up to a specific duration of time

and then it becomes stable.

After studying the stability of different NPT modified

electrodes under 470 nm filter for 2.5 h, further stability

studies were done for the most stable electrode i.e. NPT-b

under AM 1.5 solar simulator for 9 h as shown in inset of Fig. 6.

The above electrode is further found to be stable for 9 h,

without any deterioration in the electrode performance. Sta-

bility factor is calculated for NPT-b under solar simulator

measurements for 9 h as mentioned in Table 3 (NPT-b S).

Quantitative analysis of electrode and electrolyte was done

to further prove the suppression of photocorrosion. X-ray

photoelectron spectroscopy was performed for the analysis of

deterioration of photoelectrode by photocorrosion process.

In a PEC cell hydrogen is produced by photoelectrons

generated during illumination of photoelectrode given by

reaction:

2Hþ þ 2e�/H2 (4)

And holes generated are consumed to oxidize sulphide

ions into elemental sulphur. This process known as photo-

corrosion process given by reaction [49]:

S2� þ 2hþ/S (5)

This elemental sulphur combineswith sulphite ion present

in electrolyte (Na2S þ Na2SO3) and forms thiosulphate ion:

Sþ SO2�3 /S2O

2�3 (6)

Table 3 e Change in the photocurrent (I) with time (t) asslope-O, and stability (Sphotoelectrode) parameter asevaluated during the present study for differentphotoanodes.

Photoelectrode Dphotoelectrode � 10�3 aPphotoelectrode%

CdS bare 2.30 0

NPT-a 1.90 17

NPT-b 0.00 100

NPT-c 0.63 72

NPT-d 0.62 73

NPT-b S 0.00 100

a Pis estimated after 2.5 h of the PEC measurements for all elec-

trode except NPT-b S, Sphotoelectrode for NPT-b S was specifically

tested for longer time of 9 h.


So the presence of thiosulphate on the electrode is the

evidence of photocorrosion reaction occurring during illumi-

nation of photoelectrode.

Keeping this in view, XPS analysis is carried out for

photoelectrode before and after 2.5 h of PEC measurements.

Fig. 7 shows the region wise XPS spectra of sulphur in CdS

and NPT coated CdS photoanodes before and after 2.5 h of

photoelectrochemical measurements. As shown in Fig. 7(a)

there is a thiosulphate peak observed at 163.3 eV after 2.5 h

of PEC measurements (highlighted with red colour in

Fig. 7(a) which is not seen before measurements in XPS

spectra. It clearly confirms photocorrosion reaction

occurred during measurements. Fig. 7(b) shows the XPS

spectra of NPT modified CdS photoelectrode before and after

9 h of PEC measurements. In Fig. 7(b) there is no thio-

sulphate peak observed after 2.5 h of PEC measurements.

This clearly demonstrates the suppression of photo-

corrosion reaction in NPT modified films. There is a peak at

164.2 eV in CdS-NPT electrodes before measurements which

corresponds to S of thiol group present in thioglycerol which

is absent in bare CdS as expected.

For further confirmation of inhibition of photocorrosion

and to know the extent of film dissolution in the electrolyte,

induced coupled plasma (ICP) elemental analysis is done. As

already known [44] during photocorrosion process, deterio-

ration of CdS photoelectrode takes place because of which

Cd2þ ions would be present in an electrolyte. So concentration

of Cd2þ ions in electrolyte is investigated before and after 2.5 h

of PEC measurements. The detection of concentration of Cd

ions in electrolyte is shown in Table 4.

Expectedly, negligible concentration of Cd2þ is observed in

electrolyte used for CdS-NPT photoanodes, rather than elec-

trolyte tested for bare CdS photoanodes. This shows stability

Fig. 7 e X-ray photoelectron spectroscopy of (a) bare CdS

electrode before (CdS-BM) and after (CdS-M); (b) CdS-NPT-b

electrode before (CdS/NPT-BM) and after (CdS/NPT-M)

taken after 2.5 h of PEC measurement.




Table 4 e Cd2Dion concentration detected in therespective electrolyte taken after the PECmeasurement ofrespective electrodes. ICP technique was used for thischaracterization.

Samplename

Measurement time (h) aConcentrationof Cd2þ ions(ppm) � 10�2

Stability

CdS 0 (Before measurement) 0.00 Unstable

2.5 (After measurement) 1.46

CdS/NPT 0 (Before measurement) 0.00 Stable

2.5 (After measurement) 0.81

a ICP measurement was also carried out for electrolyte before any

PEC measurement (as received), no Cd2þ was detected.

Fig. 8 e IPCE measurement of bare CdS and NPT-b photo

electrode.

Fig. 9 e Chronoamperometry of bare CdS and NPT-b

photoelectrode under AM 1.5 solar simulator at 0.2 V/SCE.

Inset shows JeV curves under same source.


of CdS photoelectrode is much improved by the passivation of

its surface by making use of nano-TiO2 layer.

These results demonstrated by above quantitative

elemental analyses are inline with the estimated stability

factor specially described in this study. Thus the work clearly

confirms the achievement of highly stable and efficient CdS

photoelectrode attained by NPT modification.

3.5. Efficiency studies of stabilized electrode

3.5.1. Incident photon to current conversion efficiencyIt is interesting to note at this point that NPT modified

electrode not only show an increased stability but also

exhibit an improved photocurrent generation as shown by

NPT-b. This is well understood by efficiency studies on NPT-

b photo electrodes. Incident photon to current efficiency

(IPCE) is one of the most important characterizations for PEC

devices. It describes the photocurrent collected per incident

photon flux as a function of illumination wavelength. IPCE

was calculated for bare CdS and NPT-b modified electrodes

using the expression [50].

IPCE ¼ 1240 � JphðmAcm�2ÞPðmWcm�2Þ � lðnmÞ

Where Jph the photocurrent density in mA cm�2, P is power of

source in mW cm�2 and l is wavelength in nm.

IPCE calculated for NPT-b electrodes is found to be 22%

which is much enhanced than that of bare CdS photoanode

(w8%) at 470 nm as shown in Fig. 8. Greater IPCE of NPT-b

electrodes demonstrates its improved efficiency of convert-

ing incoming radiation into hydrogen from water, with an

assumption that all the electrons produced during illumina-

tion were used for the evolution of hydrogen.

3.5.2. Solar to hydrogen conversion efficiencyTo evaluate hydrogen generating performance of NPT-b elec-

trode under standard solar conditions, chronoamperometric

and Currentepotential (JeV) measurements were carried out

under AM 1.5 solar simulator as shown in Fig. 9. Solar to

hydrogen efficiency (STH) was calculated for bare CdS and the

most stable modified electrode (NPT-b) at 0.2 V/SCE using

following expression [51].

hð%Þ ¼�Vrev � Vapp

� � JpI0

� 100


Here Vrev is standard reversible potential i.e. 1.23 V, I0 is

power intensity of source i.e. 80 mW/cm2 and

Vapp ¼ Vmeas � Voc

Whereas, Voc is open circuit potential and Vmeas is the po-

tential at which photocurrent measurements are carried out

with respect to a reference electrode (SCE).

Open circuit potential for bare CdS and NPT-b electrode is

found to be �0.7 eV and �0.4 eV respectively as measured

using J-V curves without illumination (Fig. S.I. 10). Corre-

spondingly STH efficiencies calculated for bare CdS and NPT-b

photoanodes are found to be 0.2% and 0.7% respectively.

3.6. Stable and efficient hydrogen evolution frommodified photoanode

Finally in order to validate stability and efficiency of modified

CdS photoanode w.r.t hydrogen production, following studies

were carried out. Accordingly, accumulated hydrogen was

detected for bare CdS as well as for NPT-b electrodes at

different time intervals. Expectedly, NPT-b shows a very high




Fig. 10 e The evolved hydrogen with reaction time

demonstrates that the modified photoanode (NPT-b) stably

generates 25 times higher hydrogen gas under simulated

solar radiation (AM 1.5) as compared to the unmodified

CdS.


hydrogen evolution than bare CdS. Fig. 10 clearly demon-

strates that themodified photoanode (NPT-b) stably generates

25 times higher hydrogen gas under simulated solar radiation

(AM 1.5) as compared to the unmodified CdS. It is interesting

to note that during first 20 min, modified electrode generates

noticingly higher hydrogen gas as compared to the bare CdS

electrode. However, for longer accumulation times, very high

magnitude of hydrogen gas is evolved. The comparison of the

time dependent behaviour of CdS and modified photoanode

reveals that during the initial phase of PEC reaction, photo-

corrosion of the bare CdS was seemingly very less significant.

During later part of the PEC reaction both photocorrosion

and hydrogen productions are competitive processes, as

correlated from the detected Cd/S and generated photocur-

rent as well as evolved hydrogen gas. Thus in the case of bare

CdS, photocorrosion was found to be dominating process as

shown in earlier section (Fig. 7 & Table 3). This leads to bare

CdS yielding negligibly low hydrogen. In contrast, in case of

the modified photoanodes the photocorrosion was inhibited,

and at the same time photocurrent generation was domi-

nating process as supported by elemental analysis (Fig. 7 &

Table 4) and time dependent photocurrent measurement

(Fig. 6 & Table 3). Consequently in line with our earlier dis-

cussion a reduced charge-recombination is the main param-

eter that is responsible for yielding higher efficiency for

modified photoanodes. This behaviour was consistent even

for longer time of PEC reaction. Hence, modified NPT-b comes

out to be an efficient and a stable photoelectrode for hydrogen

generation via. cleavage of water molecule. This is one of the

desired achievements for CdS photoelectrode, which has been

specifically demonstrated, in present work.

4. Conclusions

Very stable CdS photoanodes have been fabricated by spray

pyrolysis film deposition technique. The modification of


“aligned rod-like nanostructured CdS” bare electrodes by

adsorption of TiO2 (5 nm) nanoparticles yields 3.7 times

enhanced photocurrent generation than bare CdS electrode.

Themodified electrode generated 25 timesmore hydrogen gas

than the bare CdS. An enhanced IPCE of 22%has been reported

for TiO2 modified photoanode rather than bare CdS (w8%).

Modified photoanode shows improved STH of 0.7% than that

of bare CdS (w0.2%). Enhancement in photocurrent displayed

by nanoparticle TiO2 modified CdS photoanode is attributed to the

reduction in the electronehole recombination process. Spe-

cifically, photochemical-stability is studied by quantitative

elemental analysis using XPS and time dependent photocur-

rent. Stability factor (P

) that has been defined for the first time

has been quantified for the bare and the modified electrodes.

It is found thatP

modified(100%) achieved is much better thanPCdS (72%) when tested for a typical duration of 2.5 h of PEC

reaction time. Thus, obtainedP

-factor for modified electrode

demonstrates an improved stability for nano-TiO2 modified

photoelectrode. Such high efficiency and stability is attributed

to its passive behaviour over photo-corrosive CdS surface.

Acknowledgement

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

Venkatesh, for their help during film characterization.

Permission granted by Director, ARCI to publish the work is

also gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2013.12.185.

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Stabilizing effect in nano-titania functionalized CdS photoanode for sustained hydrogen generation

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