Se

SYa

b

a

ARR2AA

KCPC

1

iaoipgSip[epscaoaot

0d

Electrochimica Acta 56 (2011) 2762–2768

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

ynthesis of cadmium sulfide spongy balls with nanoconduits forffective light harvesting

.A. Vanalakara, S.S. Mali a, R.C. Pawara, N.L. Tarwala, A.V. Moholkara, Jin A. Kimb,e-bin Kwonb, J.H. Kimb, P.S. Patil a,∗

Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, Maharashtra, IndiaDepartment of Materials Science and Engineering, Chonnam National University, Gwangju 500 757, South Korea

r t i c l e i n f o

rticle history:eceived 29 November 2010eceived in revised form1 December 2010

a b s t r a c t

Thin films of cadmium sulfide (CdS) consisting of spongy balls with nanoconduits have been chemi-cally synthesized at 70 ◦C from an aqueous alkaline bath onto soda lime glass and fluorine-doped tinoxide (FTO)-coated glass substrates. The synthesized spongy balls of CdS were characterized using X-ray

ccepted 22 December 2010vailable online 30 December 2010

eywords:dS spongy balls with nanoconduitshotoelectrochemical properties

diffraction (XRD), UV–vis spectroscopy, scanning electron microscopy (SEM), Fourier transform Ramanspectroscopy (FT-Raman) and X-ray photoelectron spectroscopy (XPS). The XRD pattern revealed the for-mation of CdS particles with a cubic crystal structure. SEM micrographs show that the spongy ball-likemorphology is composed of nanoconduits. Such spongy balls with nanoconduits that contain numer-ous nanowalls are a facile way to trap light. The light absorption path length of a photon increases inthe nanoconduits and thus can participate in multiple scattering and absorption. This is beneficial for

and i

hemical bath deposition effective light harvesting

. Introduction

Metal chalcogenides (sulfide, selenide and telluride) have beenntensively studied over the past sixty years due to their potentialpplications in photoelectrochemical (PEC) cells, gas sensors andther optical devices [1,2]. Cadmium sulfide (CdS) is one of the mostmportant members of the metal chalcogenide family and has greatotential applications in solar cells because of its particular bandap (2.4 eV), which covers the solar spectrum in the visible region.everal techniques have been used to deposit thin CdS films, includ-ng electrodeposition [3], chemical bath deposition (CBD) [4], sprayyrolysis techniques (SPT) [5], chemical vapor deposition (CVD)6], vacuum evaporation methods [7] and sputtering [8]. However,fficient, low temperature and low-cost deposition methods forreparing thin films for technological industrial applications aretill continually sought worldwide. CBD is a soft solution processapable of producing high-quality thin films at relatively low costsnd temperatures [9]. The CBD process involves the transportationf reactant ions, adsorption, surface diffusion, reaction, nucleation

nd growth. Surface morphology can be tuned by adjusting the pHf the growth solution, the deposition time, the temperature andhe reagent concentrations.

∗ Corresponding author. Tel.: +91 95231 2690571.E-mail address: psp phy@unishivaji.ac.in (P.S. Patil).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.12.080

mprovement in the power conversion efficiency of solar cells.© 2010 Elsevier Ltd. All rights reserved.

Recent investigations have highlighted an attractive approachof light trapping and scattering and its effective harvesting to boostthe energy conversion efficiency of solar cells. The term “light-trapping” refers to the redistribution of incoming light to newdirections while maximizing its absorption within the solar cell[10]. This approach has been applied in solar cells by roughen-ing the surface to bounce the light inside the cell [11–15]. Lightscattering is another approach that can be used to increase theefficiency of solar cells by utilizing optical enhancement effects.Light scattering affects the transport behavior of light by changingor extending the distance that light travels within the photoelec-trode. Hence, the light-harvesting efficiency is improved due to theincreased interaction between the incident photons and nanocrys-tallites [16–19]. In this paper, attempts were made to engineer themorphology of CdS thin films to accomplish light scattering andharvesting and to maximize optical absorption. Their subsequenteffects on enhancing photocurrent and efficiency were examined.

Novel CdS spongy balls with nanoconduits were prepared at70 ◦C using the CBD method on fluorine doped tin oxide (FTO)substrates. Although a great variety of CdS nanostructures weresynthesized, only a few of these species have been employedin solar cells studies. The morphologies of CdS nanoparticles,

nanorods, nanoflowers, marigold flowers, and nanospheres havebeen investigated for their photovoltaic properties. The short cir-cuit current (JSC) ranges from 0.063 to 3.51 mA/cm2, and the opencircuit voltage (VOC) ranges from 210 to 580 mV, with the highestefficiency reported as 2.2% for 0.3 cm2 [20–26]. We have inves-

S.A. Vanalakar et al. / Electrochimica Acta 56 (2011) 2762–2768 2763

tb((

2

2n

wuucst

Fig. 1. X ray diffraction pattern of CdS spongy balls thin film.

igated the morphology of a novel spongy ball that has not yeteen explored. This material remarkably improves the PEC JSC4.17 mA/cm2) and efficiency (1.42%) for solar cells with larger areas1.2 cm2).

. Experimental details

.1. Materials and preparation of CdS spongy balls withanoconduits

All chemicals were purchased from s.d. fine-chemicals and usedithout further purification. Cadmium sulfate (CdSO4·H2O) wassed as a cadmium (Cd) source, and thiourea (H N·CS·NH ) was

2 2sed as a sulfur (S) source. Liquor ammonia (NH3) was used as aomplexing agent. The preparative parameters, including precur-or concentration, deposition time and temperature, were variedo yield good quality CdS thin films. The standard recipe was as

Fig. 2. The Raman spectrum of CdS spongy balls thin film.

Fig. 3. The room temperature optical absorption spectrum of CdS spongy balls thinfilm. Inset shows the optical band gap energy spectra.

follows: aqueous ammonia (NH4OH) was added to a 1 M CdSO4solution to maintain a pH of 11 in the solution. The initially turbidsolution became transparent with the addition of excess ammonia.Following this, 1 M thiourea (H2N·CS·NH2) was added to the abovesolution. The CdS films were deposited by dipping the glass and FTOsubstrates into the solution at 70 ◦C for 10, 20 and 30 min, denotedas CdS10, CdS20 and CdS30, respectively. The deposited films wererinsed with distilled water and allowed to dry at room temperatureovernight.

2.2. Characterizations

The structural properties of the CdS thin films were studiedusing an X-ray diffractometer (Philips, PW 3710, Almelo, Holland)operating at 25 kV and 20 mA with CuK� radiation (1.5406 A). TheFT-Raman spectra of the films were recorded in the spectral range250–1000 cm−1 using an FT-Raman spectrometer (Bruker Multi-RAM, Germany) that employed a Nd:YAG laser source with anexcitation wavelength of 1064 nm and a resolution of 4 cm−1. Theoptical absorbance was measured using a UV–vis spectrophotome-ter (UV1800, Shimadzu, Japan). The surface morphology of the filmswas examined by SEM (Model JEOL-JSM-6360, Japan), operatedat 20 kV. Field emission SEM (JSM-6701F model) was employedto further examine the CdS morphology. The thickness of theresulting CdS films was measured using a surface profiler (AmbiosXP-1). The surface morphology and roughness of the films wereobserved using atomic force microscopy (AFM, Digital Instrument,nanoscope III) operated at room temperature. The chemical compo-sition and valence states of the constituent elements were analyzedby XPS, Physical Electronics PHI 5400, USA, with a monochromaticMg-K� (1254 eV) radiation source.

2.3. J–V measurements

The J–V characteristics of the films were recorded usinga semiconductor characterization system (SCS-4200 Keithley,Germany) with a two-electrode configuration under a halogen

lamp (28 mW/cm2). The following cell configuration was used torecord J–V plots:

Glass/FTO/CdS/Na2S–NaOH–S/G.

2764 S.A. Vanalakar et al. / Electrochimica Acta 56 (2011) 2762–2768

trum o

aea(smttf

3

wcnittco

C

TT

Fig. 4. XPS analysis of CdS spongy balls thin film (a) survey spec

The CdS films (average area 1.2 cm2) and graphite rod (aver-ge area 1.2 cm2) were employed as the working and counterlectrodes, respectively. The distance between the photoelectrodend counter electrode was 0.5 cm. An aqueous 1 M polysulphideNa2S + S + NaOH) solution was used as the redox electrolyte. Mea-urements for the power output characteristics and J–V plots wereade at fixed intervals after waiting a sufficient amount of time for

he system reach equilibrium (both in the dark and under illumina-ion). A plot of log J vs. V was used to calculate the junction idealityactor in dark.

. Results and discussion

The precipitation of metal chalcogenides in CBD occurs onlyhen the ionic product exceeds the solubility product of metal

halcogenides (CdS in our case) [27]. Combinations of ions formuclei on the substrate and in the solution, which results in precip-

tation. The film growth takes place via ion-by-ion condensation ofhe materials or by adsorption of colloidal particles from the solu-

ion onto the substrate. The complexing agents (like NH3) help toontrol the reaction rate. The formation of CdS thin films using NH3ccurs via following steps [28,29]:

dSO4 → Cd2+ + SO42−Cadmiumsulfatedissociation (1)

able 1he values of solar cell parameters for CdS10, CdS20 and CdS30 films.

Sr. No. Sample JSC (mA/cm2) VOC (mV) Imax (mA/cm2) Vmax (mV)

1 CdS10 1.475 371.96 0.877 293.882 CdS20 3.305 375.11 1.440 232.643 CdS30 4.179 452.98 1.660 235.16

f the CdS sample (b and c) narrow range scans for the Cd and S.

NH4OH → NH3 + H2OAmmoniadissociation (2)

Cd2+ + 4NH3 → [Cd(NH3)4]2+Complexformation (3)

CS(NH2)2 + 2OH− → S2− + CH2N2 + 2H2OThioureadecomposition

(4)

Cd2+ + S2− → CdSFormationofCdS. (5)

In strong basic solution, CdS is formed after an intermediateformation of hydroxide:

nCd2+ + 2n(OH)− → [Cd(OH)2]n (6)

[Cd(OH)2]n + nS2− → nCdS + 2nOH−. (7)

A typical XRD pattern for CdS30 films is shown in Fig. 1. Thecomparison of the observed XRD pattern with the standard JCPDSdata (80-0019) confirms the formation of a CdS phase with cubic

crystal structure. The lattice parameter ‘a’ of CdS30 is calculatedusing the following equation:

1d2

= h2 + k2 + l2

a2(8)

RS (�) RSh (�) Ideality factor (n) FF (%) Efficiency (�) in %

161 759 1.70 47 0.9298 134 1.94 27 1.2095 100 2.01 21 1.42

S.A. Vanalakar et al. / Electrochimica Acta 56 (2011) 2762–2768 2765

F n SEMb s withr cation

Tr

f

D

wcioa

ig. 5. SEM microstructure of CdS spongy balls thin films; (a and b) low magnificatioall with nanoconduits; (d–f) the high magnification SEM image of CdS spongy ballespectively; (g) low magnification FESEM image of CdS30 sample; (h) high magnifi

he mean value of a = 5.810 A is in good agreement with theeported value, a = 5.811 A.

Further, the average crystallite size is estimated using Scherrer’sormula, given below in Eq. (9):

= k�

ˇ cos �(9)

here � = 1.5406 A, k is the dimensionless constant (0.95), ˇ is theorrected broadening of the diffraction line measured at half ofts maximum intensity (taken in radians by multiplying a factorf �/360), D is the diameter of the crystallite and � is the diffractionngle. The calculated crystallite is approximately 18 nm. Addition-

images show the formation of CdS spongy balls over the substrate; (c) CdS spongynanoconduits formed by numerous nanowalls for sample CdS10, CdS20 and CdS30,FESEM image of CdS30 sample.

ally, the broadened peaks indicate the nanocrystalline nature of thefilms.

FT-Raman spectroscopy is a sensitive method of analysis ofnanometer-sized crystals [30,31]. The Raman spectrum of the CdSfilm is shown in Fig. 2. The cubic structured CdS belongs to theT2

d(F43m) space group, which has normal lattice vibration modes

given by

�opt = A1 + 2B1 + E1 + 2E2 (10)

where A1, E1 and E2 are Raman active and B1 is forbidden. TheRaman spectrum of the CdS film exhibits a well-resolved bandat 302 cm−1, corresponding to the first order scattering of the

2766 S.A. Vanalakar et al. / Electrochimica Acta 56 (2011) 2762–2768

Fig. 6. AFM image of CdS spongy balls thin film.

Fig. 7. (a) Photocurrent-density–voltage curves for sample CdS10, (b) photocurrent-density–voltage curves for sample CdS20 and (c) photocurrent-density–voltage curvesfor sample CdS30.

S.A. Vanalakar et al. / Electrochimica Acta 56 (2011) 2762–2768 2767

Fig. 8. CdS-polysulphide interface formed in the nanoconduits of a spongy ball. CdS nanowalls, in contact with polysulfide electrolyte, become depleted of electrons (showna CdSs n evep

lbnlt2(cit

tipaibeiw

Ce3dsbelpsfs4iT

s uncompensated donor ions in gray). The built-in-potential is generated at eacheparation (shown as dotted lines). Light trapping via multiple scattering-absorptioathway of photogenerated electrons is also shown.

ongitudinal optical (LO) phonon mode and the second-orderand at approximately 599 cm−1. CdS can have both hexago-al wurtzite and cubic zinc blended structures. The zone-center

ongitudinal–optical A1 (LO) phonon frequency for both struc-ures is nearly 305 cm−1 [32–36]. The FWHM of the 1 LO peak is1.64 cm−1. Although this large width indicates poor crystallinitylack of long-range order) in the films, the well-defined peak indi-ates the crystalline nature of the material. Hence, the large FWHMn the present case can be attributed to a polycrystalline effect inhe as-deposited film.

Fig. 3 shows the room temperature optical absorption spectra ofhe CdS films recorded in the range of 500–1100 nm without takingnto account scattering and reflection losses. The optical absorptioneak at ∼515 nm gives a value of 2.4 eV for the band gap energy. Andditional absorption hump in the visible region for the CdS30 filmsndicates the light scattering effect. Similar types of results haveeen reported for ZnO thin films [37,38]. The light scattering abilitynhances the photon absorption ability of CdS30 film, resulting in anncrease in the short-circuit current density, which is in accordance

ith previous reports. [39,40].Fig. 4(a) shows the survey spectrum of the CdS sample CdS30. The

and O peaks stem mainly from atmospheric contamination due toxposure of the sample to air. The unambiguous presence of the Cdd doublet signal clearly shows the formation of CdS. Fig. 4(b and c)epicts narrow range scans for the Cd and S peak region of the sameamples. The binding energies obtained from the XPS analyses haveeen corrected, taking into account the specimen charging and ref-rencing C 1s at 284.88 eV. The two-peak structure in the Cd 3d coreevel arises from the spin-orbit interaction with the Cd 3d5/2 peakosition at 403.75 eV and the 3d3/2 at 410.48 eV. It is clear from thepectral graph that Cd 3d exhibits narrow, well-defined features

or the doublet structure. This suggests that Cd atoms appear topecifically bond to S atoms. The XPS binding energy of Cd 3d3 at04.14 eV and the S 2p at 160.89 eV is indicative of the CdS chem-

stry. These results agree well with those reported in the literature.he peak at 1111.04 eV is due to the auger electron of Cd [41].

nanowalls-polysulfide electrolyte interface, which enforces an electron-hole pairnts in the nanoconduits is shown as progressively dotted arrows. Highly schematic

The synthesized spongy balls of CdS were examined by SEM andFE-SEM. Fig. 5(a–h) shows the SEM images of the CdS10, CdS20 andCdS30 films. The low magnification SEM images in Fig. 5(a) and (b)show the formation of spongy balls of CdS over the entire substrate.The SEM image in Fig. 5(c) shows spongy balls of CdS with nanocon-duits. The magnified SEM images of the CdS10, CdS20 and CdS30films are shown in Fig. 5(d–f). The spongy balls of CdS are formedand made up of nanoconduits and exhibit numerous nanowalls thatare interconnected. Such walls, with thicknesses of ∼35 nm, areclearly seen in the FE-SEM images in Fig. 5(g and h). Due to thepresence of these nanowalls, the spongy balls of CdS exhibit highereffective surface areas for light absorption. The balls also scatterthe light, multiplying it into the photoanode and hence improvingthe absorption. The light absorption path length of photons can beincreased as they are trapped in the nanoconduits. These mecha-nisms boost the PEC performance of the CdS electrode. AFM images(Fig. 6) of CdS samples reveal a uniform, crack-free and denselypacked microstructure with nanoconduits. The surface roughnessof the film was calculated to be 1037 nm. The AFM image replicatesthe 3D morphology and allows for matching to the SEM image.

Fig. 7(a–c) shows the J–V characteristics of CdS thin films CdS10,CdS20 and CdS30. In all samples, the J–V characteristics in the darkdisplay ideal diode-like rectifying characteristics for the fabricatedPEC cells. Upon illumination, the J–V curves shift in the IVth quad-rant, indicating the generation of electricity, which is typical of solarcell characterization. The magnitude of the JSC was 1.47, 3.30 and4.17 mA/cm2 for the CdS10, CdS20 and CdS30 films, respectively.The corresponding VOC was found to be 372, 375 and 453 mV,respectively. This observation reveals that the JSC was increasedthree-fold, whereas there was no subsequent rise in the VOC. How-ever, the shape of the J–V curves changed substantially. The nearly

ideal bulging-type J–V curve for the CdS10 sample begins to sagtoward the origin for the CdS20 and CdS30 films, with a subsequentdecrease in the fill factor (FF). The FF depends on the series resis-tance (RS) and shunt resistance (RSh). The RS is due to the resistanceof the metal contacts, Ohmic losses in the front surface of the cell,

2 himic

ibeiactceC

tohaitow

(

(

(

(

f

I

wkibtWtf2

4

utctTnl

[[

[

[[[

[[

[[

[[[[

[[[[

[[

[[[[[

[

[[[

768 S.A. Vanalakar et al. / Electroc

mpurity concentration and junction depth. Ideally, the RS shoulde 0 �. The RSh represents the loss due to surface leakage along thedge of the cell or due to crystal defects. Ideally, the RSh should benfinite. The values of RS and RSh are calculated from J–V curves andre given in Table 1. RS varies slightly, from 161 to 95 �, while RShhanges drastically, from 759 � for the CdS10 sample to 100 � forhe CdS30 sample. The variation in RSh seems to be dominant in ourase. The consequences of decreasing RSh and FF with conversionfficiency (�) are shown in Table 1. The highest � is 1.42% for thedS30 sample.

The observed values of photocurrent in our samples were largerhan those observed in other CdS samples with compact planerr porous morphologies [23–26,42]. The photocurrent depends onow efficiently the photogenerated carriers in the semiconductorsre harvested. This results from two main collection mechanisms,ncluding the separation of the carriers in the space charge field andhe diffusion of the carriers toward the interface. The hike in JSC inur samples seems to be due to the spongy ball-like morphology,hich induces the following light harvesting phenomena:

1) Nanoconduits with interconnected nanowalls increase the sur-face area in contact with the redox electrolyte.

2) Effective light absorption via trapping and scattering in thenanoconduits. This increases the absorption path length of lightand thereby increases the interaction between light and CdSnanocrystallites. Hence, the optical absorption and energy har-vesting efficiency of CdS spongy balls are enhanced (Fig. 8).

3) The 35-nm-thick nanowalls essentially act as a space charge(depletion) region, which absorbs and creates an electron-holepair and separates them effectively due to the built-in poten-tials at the nanowall-electrolyte interface, shown by dottedlines in Fig. 8.

4) The interconnected nanowalls serve as direct pathways for pho-togenerated electrons and reduce their loss.

The ideality factor ‘nd’ of prepared CdS films is determined fromollowing diode equation (Eq. (11)):

= I0(eqV/ndkT − 1) (11)

here I0 is the reverse saturation current, V is forward bias voltage,is Boltzmann’s constant, T is ambient temperature in Kelvin and nd

s an ideality factor. The ideality factor is determined under forwardias and is normally found to be between 1 and 2 depending onhe relation between diffusion current and recombination current.

hen the diffusion current is more than the recombination current,hen the ideality factor becomes 1. In the reverse case, the idealityactor becomes 2. The ideality factor was found to be 1.70, 1.94 and.01 for samples CdS10, CdS20 and CdS30, respectively.

. Conclusions

The nanocrystalline CdS thin films were successfully depositedsing the facile chemical bath deposition method. X-ray diffrac-ion studies revealed that the synthesized CdS thin films have a

ubic structure with improved crystallinity. SEM images showedhat the spongy balls possess a nanoconduit-like morphology.hese nanoconduits consist of densely packed interconnectedanowalls. The increase in effective surface area, which enhances

ight-trapping, and the intense scattering light that is not absorbed,

[

[[

[

a Acta 56 (2011) 2762–2768

resulting in a longer effective path for light to travel through thenanowalls, improves light harvesting. The improved crystallinityand surface area lead to scattering and light-trapping and result inan enhanced PEC.

Acknowledgments

The authors are thankful to the UGC for financial support underthe UGC-DSA-I (2010-15) and UGC-ASIST programs.

References

[1] S. Mokrushin, Y. Tkachev, Kolloidn Z. 23 (1961) 438.[2] G. Kitaev, A. Uritskaya, S. Mokrushin, Russ. J. Phys. Chem. 39 (1965) 1101.[3] P.K.M. Bandaranayake, P.V.V. Jayaweera, K. Tennakone, Sol. Energy Mater. Sol.

Cells 76 (2003) 57.[4] J.N. Ximello-Quiebras, G. Contreras-Puente, J. Aguilar-Herna, G. ndez, A.

Santana-Rodriguez, R. Arias-Carbajal, Sol. Energy Mater. Sol. Cells 82 (2004)263.

[5] D.P. Amalnerkar, K. Yamaguchi, T. Kajita, H. Minoura, Solid State Commun. 90(1994) 3.

[6] H. Uda, H. Yonezawa, Y. Ohtsubo, M. Kosaka, H. Sonomura, Sol. Energy Mater.Sol. Cells 75 (2003) 219.

[7] S. Ray, R. Banerjee, A.K. Barua, J. Appl. Phys. 19 (1980) 1889.[8] B.S. Moon, J.H. Lee, H. Jung, Thin Solid Films 299 (2006) 511.[9] Y.J. Chang, C.L. Munsee, G.S. Herman, J.F. Wager, P. Mugdur, D.H. Lee, C.H. Chang,

Surf. Interface Anal. 37 (2005) 398.10] D. Redfield, Appl. Phys. Lett. 25 (1974) 647.11] L. Ae, D. Kieven, J. Chen, R. Klenk, Th. Rissom, Y. Tangand, M.Ch. Lux-Steiner,

Prog. Photovolt. Res. Appl. 18 (2010) 209.12] R.H. Franken, R.L. Stolk, H. Li, C.H.M. van der Werf, J.K. Rath, R.E.I. Schropp, J.

Appl. Phys. 102 (2007) 014503.13] J. Krc, F. Smole, M. Topic, Prog. Photovolt. Res. Appl. 11 (2003) 429.14] E. Garnett, P. Yang, Nano Letters 10 (2010) 1082.15] S.B. Rim, S. Zhao, S.R. Scully, M.D. McGehee, P. Peumans, Appl. Phys. Lett. 91

(2007) 243501.16] X. Sheng, J. Zhai, L. Jiang, D. Zhu, Appl. Phys. A 96 (2009) 473.17] Q. Zhang, T.P. Chou, B. Russo, S.A. Jenekhe, G. Cao, Adv. Funct. Mater. 18 (2008)

1654.18] T.P. Chou, Q. Zhang, G.E. Fryxell, G. Cao, Adv. Mater. 19 (2007) 2588.19] Q. Zhang, T.P. Chou, B. Russo, S.A. Jenekhe, G. Cao, Angew. Chem. Int. Ed. 47

(2008) 2402.20] M.E. Calixto, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 59 (1999) 65.21] S. Tiwari, S. Tiwari, Sol. Energy Mater. Sol. Cells 90 (2006) 1621.22] H. Jia, Y. Hu, Y. Tang, L. Zhang, Electrochem. Commun. 8 (2006) 1381.23] H.S. Hilal, R.M.A. Ismail, A.E. Hamouz, A. Zyoud, I. Saadeddin, Electrochim. Acta

54 (2009) 3433.24] J.K. Dongre, V. Nogriya, M. Ramrakhiani, Appl. Surf. Sci. 255 (2009) 6115.25] J.K. Dongre, M. Ramrakhiani, J. Alloy Compd. 487 (2009) 653.26] S.B. Patil, A.K. Singh, Appl. Surf. Sci. 256 (2010) 2884.27] K.L. Chopra, R.C. Kainthla, D.K. Pandya, A.P. Thakoor, Phys. Thin Films 12 (1982)

167.28] M. Froment, D. Lincot, Electrochim. Acta 40 (1995) 1293.29] H. Moualkia, S. Hariech, M.S. Aida, N. Attaf, E.L. Laifa, J. Phys. D: Appl. Phys. 42

(2009) 135404.30] Y. Lida, M. Furukawa, T. Aoki, T. Sakai, Appl. Spectrosc. 52 (1986) 73.31] M. Li, Z. Feng, G. Xiong, P. Ying, Q. Xin, J. Li, Phys. Chem. B 105 (2001) 8107.32] A. Badr, K.M. AbdEl-Kader, R.M. Khafagy, J. Appl. Poly. Sci. 92 (2004) 1984.33] R.C.C. Leite, S.P.S. Porto, Phys. Rev. Lett. 17 (1966) 10.34] F. Chen, R.J. Zhou, L.G. Yang, N.M. Wang, H.Z. Chen, J. Phys. Chem. C 112 (2008)

1001.35] Z.Q. Wang, J.F. Gong, J.H. Duan, H.B. Huang, S.G. Yang, X.N. Zhao, R. Zhang, Y.W.

Du, Appl. Phys. Lett. 89 (2006) 033102.36] G.Z. Shen, C. Lee, J. Growth Des. 5 (2005) 1085.37] J.C. Lytle, A. Stein, Annu. Rev. Nano Res. 1 (2006) 1.38] Z.S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Coord. Chem. Rev. 48 (2004)

1381.

39] S. Nishimura, N. Abrams, B.A. Lewis, L.I. Halaoui, T.E. Mallouk, K.D. Benkstein, J.

van de Lagemaat, A.J. Frank, J. Am. Chem. Soc. 125 (2003) 6306.40] L.I. Halaoui, N.M. Abrams, T.E. Mallouk, J. Phys. Chem. B 109 (2005) 6334.41] M. Karimi, M. Rabiee, V. Moztarzadeh, M. Tahriri, M. Bodaghi, Curr. Appl. Phys.

9 (2009) 1263.42] S. Tiwari, S. Tiwari, Cryst. Res. Technol. 41 (2006) 78.