fn d

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(SrT. X-

Infrared techniques. The porous SrTiO3 particles exhibits strong rst order Raman scattering which is nor-mally absent in bulk SrTiO3 crystal due to breaking of symmetry. A green emission around 536 nmobserved in the photoluminescence spectrum indicates the existence of self trapped excitons (STE). A

able e

proved more than 10% [57].In DSSCs, the choice of materials depends on its conduction

band and density of states that allows effective electronic couplingwith the dye energy level to facilitate charge separation and min-imize recombination. Additionally, the material must have highinternal surface area to maximize light absorption by the dyemonolayer with good electrical conductivity to the substrate. Also

e of thermistors,superconducting. In the coctahedra

dination similar to the titanium arrangement in the anataand can be loosely thought of as a doped TiO2 structureTiO2 and SrTiO3 have similar band gap value of 3.2 eV, bat-band potential of SrTiO3 is greater than that of anatase TiO2.The conduction band of SrTiO3 lie 0.2 eV above the conductionband of anatase and hence SrTiO3 is expected to produce excellentphotovoltage. The suitable band position of SrTiO3 makes it as apromising material for the development of DSSCs [17,18].

In the present work, we have adopted a facile hydrothermalsynthesis of SrTiO3 and its effect on the performance of the DSSCsusing the low cost organic Eosin Yellow (EY) dye as a sensitizer has

Corresponding author. Tel.: +91 9443918384.

Journal of Alloys and Compounds 586 (2014) 456461

Contents lists availab

a

.e lE-mail address: vr.optics1@gmail.com (V. Ramakrishnan).those of the traditional silicon-based cells. In 1991, a low-costDSSC with conversion efciency of 7% was discovered by Oreganand Gratzel, based on visible light sensitizing material with TiO2nanoparticles. Recently the conversion efciency has been im-

with important applications in the manufacturmultilayer capacitors, electro-optical devices andquantum interference devices (SQUID) [15,16]SrTiO3, the titanium atoms are arranged in 6-fold0925-8388/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.10.012ase ofl coor-se TiO2. Bothut thelar cell technology as one of the extensive research area today. Sil-icon based solar cells are the dominant photovoltaic devicebecause of its high light conversion efciency but it is still expen-sive compared to the conventional grid electricity resources. Forthis reason developments on potentially cheaper solar cells basedon the thin lms have been made [14]. Dye-sensitized solar cells(DSSCs) have received considerable interest recently because theyare cost-effective and eco friendly with efciencies comparable to

region [814]. So there has been continuous interest to increasethe power conversion efciencies of DSSCs by incorporating then-type metal oxides such as TiO2, ZnO, SnO2, Nb2O5 and SrTiO3.

Among the different types of metal oxides, the ternary oxidestrontium titanate (SrTiO3) has more structural similarities withanatase titanium dioxide (TiO2). SrTiO3 is a cubic structured perov-skite material having ABO3 stoichiometry with a space group ofPm3m and lattice parameter of 3.9 . It is a wide band gap materialKeywords:Hydrothermal methodStrontium titanateMesoporousRaman spectrumPhotoluminescencePhotoanode

1. Introduction

The increasing demand for renewsandwich type Eosin Yellow sensitized solar cell is prepared using porous SrTiO3 exhibits excellent diodecharacteristics with improved photovoltage of 0.73 V.

2013 Elsevier B.V. All rights reserved.

nergy has made the so-

metal oxides employed for the fabrication of DSSCs have solarabsorption at ultra violet (UV) region, whereas the dye moleculesare only responsible for the absorption of visible and near- infraredReceived in revised form 13 September 2013Accepted 2 October 2013

The eld emission scanning electron microscope reveals the highly ordered mesoporous particles with apore size of 42 nm. The vibrational spectra of SrTiO3 are analyzed by Raman and Fourier TransformA facile hydrothermal synthesis of SrTiO3application

P. Jayabal a, V. Sasirekha b, J. Mayandi c, K. JeganathaaDepartment of Laser Studies, School of Physics, Madurai Kamaraj University, MaduraibDepartment of Physics, Avinashilingam University, Coimbatore 641043, IndiacDepartment of Material Science, School of Chemistry, Madurai Kamaraj University, MadCenter for Nanoscience and Nanotechnology, Bharathidasan University, Tiruchirapalli 6

a r t i c l e i n f o

Article history:Received 3 July 2013

a b s t r a c t

Porous strontium titanateand titanium isopropoxide

Journal of Alloys

journal homepage: wwwor dye sensitized solar cell

, V. Ramakrishnan a,21, India

i 625021, India24, India

iO3) is synthesized by the hydrothermal interaction of strontium acetateray diffraction conrms the formation of cubic structured SrTiO3 particles.

le at ScienceDirect

nd Compounds

sevier .com/locate / ja lcom

been reported. The use of organic dyes, which are much cheaperthan the conventional ruthenium complexes, as photosensitizerin DSSC has several advantages over metal complex dyes like high-er absorption coefcient and user friendly. Moreover the adsorbeddye can be easily removed by sintering in air which leads to recycleuse of the photo-anode. In addition to this, we have also studiedthe various structural and optical properties of SrTiO3. Theobtained SrTiO3 is porous nature with better properties in manyaspects than ordinary SrTiO3 because of its high specic surfacearea. This property improves light absorption drastically andbrings better power conversion efciencies.

2. Experimental

2.1. Materials

Strontium acetate Sr(CH3COO)2 (SigmaAldrich), titanium isopropoxideTi[OCH(CH3)2]4 (TIP, 97%, SigmaAldrich), ethyl alcohol EtOH (>99.7% Merk), citricacid (SigmaAldrich), poly ethylene glycol (PEG, SigmaAldrich), Eosin Yellow(SigmaAldrich) and ammonia solution (Ficher) were purchased and used withoutfurther purication.

2.2. Hydrothermal method

The strontium titanate (ST) was synthesized by hydrothermal method usingstrontium acetate and titanium isopropoxide as the starting materials. Initially tita-

(ITO) using the doctor blade technique. The photo anode was prepared by immers-ing the SrTiO3/ITO lm into the 0.5 mM of EY dye solution for 24 h. A sandwich typesolar cell was assembled with dye sensitized SrTiO3 electrode as anode and Ptcoated ITO electrode as the cathode with standard iodide electrolyte solution as amediator. The area of SrTiO3 working electrode was 1 cm2 (1 cm 1 cm). Theschematic representation of DSSC fabrication is shown in Fig. 1.

2.4. Characterization techniques

The structural analysis was carried out by powder X-ray diffractometer with CuKa radiation (k = 1.54 ). The surface morphology and composition of the SrTiO3particles were examined by eld emission scanning electron microscope (FE-SEM,Carl ZeissSigma) equipped with the energy dispersive X-ray spectrometer (EDX,Oxford instruments-INCAx). The IR spectrum of the sample was recorded in therange of 4004000 cm1 using Shimadzu FT-IR spectrometer. The Raman scatteringmeasurements were performed in 180 backscattering geometry by using a HoribaJobin Yvon LabRamHR800 equipped with a CCD detector. The sample was excitedby 633 nm emission from a HeNe laser and the accuracy of the wavenumberwas about 0.3 cm1. The photoluminescence at room temperature was excited bythe 325 nm line from a HeCd laser. The photoluminescence spectra were analyzedby a Horiba Jobin Yvon LabRamHR800 with a CCD detector. The absorbance spectrawere measured using the Shimadzu (UV2450) UVVIS spectrophotometer. Orielclass-A solar simulator (91195A, Newport) was used as a light source. A com-puter-controlled Autolab PGSTAT302N electrochemical workstation was employedfor current voltage (IV) measurements.

3. Results and discussion

P. Jayabal et al. / Journal of Alloys and Compounds 586 (2014) 456461 457nium isopropoxide was weighted and dissolved in ethanol under stirring condition(solution A). Strontium acetate was dissolved in water at 80 C for 30 min (solutionB). Then the solution B was cooled to room temperature and it was slowly pouredinto solution A with a constant stirring for 1 h. The pH of the mixed solution wasadjusted to 11 using ammonia solution. Finally the solution was transferred intothe Teon lined stainless steel autoclave vessel. The sealed vessel was heated to120 C for 12 h in a furnace and cooled down to room temperature. The resultantprecipitate was washed with ethanol and dried at 100 C for 5 h. The dried powderwas calcinated at 900 C for 6 h to eliminate the impurities. The prepared powderwas divided into two equal parts, one part of the powder was used to characterizethe structural and optical properties and other part of the powder was used to fab-ricate the solar cell.

2.3. Preparation of electrodes and cell assembly

In the solar cell fabrication, synthesized SrTiO3 powder was made into a pasteby grinding 0.5 g of SrTiO3 powder in 2:1 solution of poly ethylene glycol (PEG)and citric acid. Then the paste was coated on the transparent conducting oxideFig. 1. Schematic illustratioThe powder XRD technique was used to investigate the phase ofSrTiO3. Fig. 2 shows the XRD pattern of SrTiO3 particles with thediffraction peaks of (100), (110), (111), (200), (211), (200) and(310). All the diffraction peaks can be indexed to cubic Perovskitestructure of SrTiO3 with lattice constant a = 3.90 . The obtainedplanes are in good agreement with the literature value (JCPDS cardNo. 89-4934). From the XRD spectrum it is clear that the synthe-sized particles do not have any trace of impurities [1922].

The microstructure and morphology of the synthesized parti-cles were analyzed by eld emission scanning electron microscopy(FE-SEM). Fig. 3 shows the FE-SEM micrograph of spherical shapedSrTiO3 particles and the histogram of the particle size distribution.The spherical shaped particles show sponge-like mesoporous mor-phology as visibly noticed in the FE-SEM image with the pore sizen of DSSC fabrication.

structure with space group of Pm3m O1h

. According to the grouptheoretical analysis, the phonons of SrTiO3 have three fold degen-erate optical modes and thus vibration modes are 3F1u + F2u, wherethe F2u is neither IR nor Raman active and F1u is only IR active. Inthe bulk SrTiO3, the rst order Raman modes are symmetricallyforbidden due to the selection rule qo = 0 near the center of theBrillouin zone. The rst order Raman lines have been seen in SrTiO3due to the breaking of selection rule (qo 0). Further strain, grainboundaries, oxygen vacancies and impurity atoms are also ex-pected to play an important role for the activation of rst order Ra-man scattering. Generally rst order optical phonons, TO1, areobserved between 85 and 120 cm1. The hard modes TO2 + LO1(two modes have very close wavenumbers), LO3, TO4 and LO4 areexpected to occur at 180, 480, 550 and 800 cm1 respectively. LOand TO represent the longitudinal and transverse optical phononsand are numbered (TO1,2,3) according to increasing wavenumbers[2630]. In the present case, rst order and second order Ramanbands are observed. The bands observed at 116, 161, 184, 436,554 and 799 cm1 are attributed to rst order modes and theirwavenumbers are assigned to TO1, LO1, TO2, LO3, TO4 and LO4 pho-nons respectively. The bands at 224, 282 and 727 cm1 are due to

Fig. 2. XRD pattern of SrTiO3.

458 P. Jayabal et al. / Journal of Alloys and Compounds 586 (2014) 456461of 42 nm [23]. The average size of the particles is estimated as620 nm. In order to conrm the chemical composition of SrTiO3,a quantitative elemental analysis was carried out by EDAX spec-trometer attached to the SEM. Fig. 4 shows the EDAX spectrumof SrTiO3 and inset shows the weight percentage of the elements.The stoichiometric ratios of main metallic components of SrTiO3are Sr(20.78%), Ti(20.85%) and O(58.37%). The results indicate thatthe weight percentage (wt.%) of Sr, Ti and O in the SrTiO3 by EDAXbasically agree with the designed compositions.

Fig. 5 exhibits the FT-IR spectrum of the SrTiO3. The vibrationalband at 633 cm1 corresponds to the stretching vibration of SrOand band at 570 cm1 is ascribed to TiO stretching vibrations inTiO6 groups. The weak peak at 432 cm1 is due to the TiO bendingvibrations. As the sample was calcinated to 900 C there was notrace of impurities like Sr2TiO4, SrCO3 and TiO2 in the FT-IR spec-trum which also agrees with the XRD result as shown in Fig. 2[24,25].

The phenomenon of inelastic scattering of light is a useful non-destructive method to investigate the structural changes of thePerovskite materials. Fig. 6 shows the room temperature Ramanspectrum of SrTiO3. At room temperature the SrTiO3 has a cubicFig. 3. FE-SEM image of SrTiO3 (Ithe second order modes. In particular, the appearance of strongrst order modes at room temperature suggests that the cubicstructure lost its inversion symmetry. The breaking of inversionsymmetry can occur due to the presence of nanoscopic impuritiesincorporated in the crystalline lattice and signatures associatedwith frozen dipole moments [31]. As we have not observed Egmode and it can be suggested there are no frozen dipole momentsat the surfaces, which may break the inversion symmetry of thecrystal. The polar TO2 phonon will interact with polarization uc-tuations due to the presence of defect-induced ferroelectric polarregions. This interaction will lead to the observation of a Fano lineshape for the polar TO2 phonons. In our case we could not see apronounced Fano asymmetric line shape for the TO2 phononappeared at 181 cm1. This suggests that there is no contributionfrom nanoscopic impurities. For our SrTiO3 mesoporous material,the loss of inversion of symmetry may arise from the presence ofnanopores which may break the surface symmetry. The porousstructure of the material enables large interaction area so thatthe enhanced rst order modes can be clearly seen [32].

Fig. 7 shows photoluminescence spectrum of SrTiO3. In mostcases oxygen vacancies are the main sources for the luminescencenset) particle size histogram.

process. In this case a broad luminescence band at room tempera-

The experimentally observed higher photo voltage is ascribed tothe more negative positioned conduction band of SrTiO3 [40]. On

Fig. 4. EDAX spectrum of SrTiO3 (inset) weight percentage of elements.

Fig. 6. Raman spectrum of SrTiO3.

P. Jayabal et al. / Journal of Alloys and Compounds 586 (2014) 456461 459ture suggests multiphonon process. The relaxation occurs byinvolving the participation of numerous states within the bandgap. The origin of this luminescence is assigned to the self-trapped-excitons (STE) formed in between the conduction and va-lance band of the SrTiO3. As this type of luminescence is observedin many compounds having titanate octahedra, it is not related toany particular impurities. When the electrons are excited to theconduction band by the use of UV light they form small polaronsthat interact with the holes to produce STEs. The recombinationof STEs causes the visible emission. The emission spectrum resem-bles the Gaussian distribution and the calculated FWHM is 96.2 nmfor the peak centered at 536 nm [3335].

The absorption spectra of EY, SrTiO3/ITO lm and dye coatedSrTiO3/ITO lm are shown in Fig. 8. It is clearly evident that thedye has visible absorption between 450570 nm centered at530 nm. The strong absorption in the visible region gives an ideathat it has an ability to harvest light energy in most of the visiblespectrum. Also it is a suitable sensitizer for the wide band-gapmaterials [36]. After the injection of dye molecules into the porousSrTiO3 particles, the dye molecules are adsorbed to the particles.Now the energy level of SrTiO3/EY system is affected andconsequently the band gap energy is altered. From the Fig. 8, it isFig. 5. FT-IR spectrum of SrTiO3 pellet.apparent that before dye loading SrTiO3 lm has maximumabsorption at 350 nm whereas the dye loaded lm has absorptionin 350 nm as well as 540 nm. This suggests that a new band formedin the visible region is due to the adsorption of dye molecules. Thisinteraction might be as ester-like linkage of the carboxyl group ofEY and oxide layer by a simple chemisorption. This can be evi-dently noticed in the absorption spectrum of SrTiO3/EY system.As a result, the dye molecules enable SrTiO3 to absorb visible light[37,38].

The current densityvoltage (IV) plot of DSSC fabricated withEY dye is shown in Fig. 9. The detailed photo electrochemicalparameters of DSSCs such as short circuit current density (Jsc), opencircuit voltage (Voc), ll factor (FF) and power conversion efciency(PCE) are measured under simulated sunlight at an intensity of62.5 mW cm2. The device shows good photovoltaic performancewith Jsc = 4.4 104 A/cm2, Voc = 0.73 V, FF = 0.55 and efciencyg = 0.51%. The photovoltage observed in the EY sensitized SrTiO3cell is as good as ruthenium complex sensitized SrTiO3 cell [39].Fig. 7. Photoluminescence spectrum of SrTiO3 excited at 325 nm.

andFig. 8. UVVIS absorption spectra of SrTiO lm, EY and dye coated SrTiO .460 P. Jayabal et al. / Journal of Alloysthe other hand, the observed lower current density may be due tothe reverse saturation current. Because of the permeation of elec-trolyte solution into the mesoporous SrTiO3 lm and even intothe ITO surface, the recombination of the charge carriers fromthe conduction band of SrTiO3 to the electrolyte causes decreasein Jsc. Thus, the increase of dark current decreases Jsc [41]. As thethickness of the lm is larger, the distance which electrons travelthrough the SrTiO3 lm increases, thus electronhole recombina-tion is more likely takes place and consequently the parameterssuch as FF, Jsc decrease drastically. The metal free organic dyemay also possibly increase the recombination strongly [42]. Thelow value of photocurrent induced by the cell may cause the over-all efciency to be low. The better efciency can be achieved bymodifying the scheme of working electrode, good choice of dyeand miniaturizing the lm thickness.

4. Conclusions

The cubic phase mesoporous SrTiO3 has been successfullysynthesized by a facile hydrothermal approach. The FE-SEMimage shows spherical shaped particles with highly ordered

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Acknowledgements

We thank UGC-UPE programme of Madurai Kamaraj University,for providing micro-Raman and solar simulator facilities andDST-PURSE programme for the nancial support. One of theauthors KJ acknowledges DST-nanomission for the partial nancialsupport.

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P. Jayabal et al. / Journal of Alloys and Compounds 586 (2014) 456461 461

A facile hydrothermal synthesis of SrTiO3 for dye sensitized solar cell application1 Introduction2 Experimental2.1 Materials2.2 Hydrothermal method2.3 Preparation of electrodes and cell assembly2.4 Characterization techniques

3 Results and discussion4 ConclusionsAcknowledgementsReferences