Cite this: RSC Advances, 2013, 3,10363

Growth of oriented single crystalline La-doped TiO2

nanorod arrays electrode and investigation ofoptoelectronic properties for enhancedphotoelectrochemical activity3

Received 12th February 2013,Accepted 11th April 2013

DOI: 10.1039/c3ra40746b

www.rsc.org/advances

Subha Sadhuab and Pankaj Poddar*abc

Fabrication of single crystalline, oriented one-dimensional (1D) rods or wires of titania on transparent

conducting oxide (TCO) substrates have enormous significance in the area of photoelectrochemical

research owning to their unique optoelectronic properties. It is possible to modify the electrical

conductivity and optoelectronic properties of titania by intentional inclusion of atomic impurity in the

material. Here for the first time, we have doped lanthanum homogeneously in TiO2 nanorod arrays. The

homogeneous distribution of lanthanum in titania lattice is confirmed by scanning transmission electron

microscopy (STEM) elemental mapping and line scanning analysis. After doping with lanthanum, there is a

negative shift of the flat-band potential of the TiO2 nanorods and the charge carrier density of the

nanorods is also improved. The energy-conversion efficiency of a dye-sensitized solar cell based on 4 mol%

La-doped nanorods is increased about 21% compared with the undoped one.

Introduction

Oriented one dimensional (1D) metal oxide structures directlygrown on substrates are of substantial interest due to theirwidespread application in fuel cells, Li-ion batteries, photo-electrochemical water splitting, solar cells etc.1–5 Thesenanorods or nanotubes based electrodes have been developedfor various optoelectronic applications due to large aspectratio and reduced grain boundary which favor efficienttransport of charge carriers. There is a lot of interest infabrication of 1D rods or wires of titania in desired crystallinefacets due to their unique optoelectronic properties. The singlecrystalline, oriented nanorods directly grown on transparentconducting oxide (TCO) substrates have a number of applica-tions in the area of solar photovoltaics and several otheroptoelectronic devices due to the enhanced electron transportand lower exciton recombination rates.5–10 By introducingdoping impurities in the lattice structure, electrical andmagnetic properties of these materials can be altered andmanipulated. Doping is routinely performed in bulk semi-conductors as well as nanoscale materials.11,12 Due to typical

electronic configuration of Ti4+ ions in TiO2, it is possible toalter its optoelectronic properties by preferential doping withs, p, d, or f block elements. Thus, incorporation of atomicimpurity in titania can tailor the band gap and electricalconductivity of the material.13 Usually, 1D nanorods aresynthesized in non-equilibrium conditions. Various hightemperature methods are used for this purpose. But it is verydifficult to homogeneously dope any 1D structure. At highgrowth temperature homogeneous doping is really a hard job.In contrast, low temperature solvothermal method is useful forsynthesis of doped nanostructures. The direct growth of singlecrystalline TiO2 nanorods on fluorine-doped tin oxide (FTO)coated glass substrates by hydrothermal and solvothermalmethod was first reported a few years ago14,15 followed by ageneral method to synthesize TiO2 rods on various sub-strates.5,16,17 The growth mechanism and dependence ofmorphology of titania nanorods on the surface chemistry ofsubstrates have also been studied.5 The same method is usefulfor direct synthesis of homogeneously doped nanorods onsubstrates. Due to the low synthesis temperature the physicalproperties of the substrates will also remain unaffected. It isreported that doping of lanthanum with titanium ion canincrease the oxygen vacancy of titania surface.18 In dye-sensitized solar cells (DSSCs) the carboxylic group of the dyemolecule is attached with titanium ion. More oxygen vacancyon the surface can led to more dye absorption which canenhance the photovoltaic performance of the cell. In thispaper, for the first time, we have doped various amount of

aPhysical & Material Chemistry Division, CSIR-National Chemical Laboratory, Pune,

411 008, India. E-mail: p.poddar@ncl.res.inbAcademy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg,

New Delhi, 110 001, IndiacCenter of Excellence on Surface Science, CSIR-National Chemical Laboratory, Pune,

411 008, India

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra40746b

RSC Advances

PAPER

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lanthanum homogeneously in TiO2 nanorod arrays directlygrown on FTO substrates. The homogeneous distribution oflanthanum in titania lattice is confirmed by scanningtransmission electron microscopy (STEM) elemental mappingand line scanning analysis. The optical characteristics of theas-synthesized nanorods have been thoroughly studiedthrough various experiments like UV-visible-DRS, photolumi-nescence and Raman spectroscopy. The change of flat-bandpotential (VFB) and carrier density of the nanorods with dopinghas also been calculated through Mott–Schottky measure-ments. The photon to electron conversion efficiency of theseLa-doped TiO2 nanorods has also been measured afterabsorbing the nanorods in dye and fabricating liquid-junctiondye-sensitized solar cells where these dye adsorbed nanorodswere used as the photoanode.

Materials and methods

(A) Synthesis of TiO2 and La-doped TiO2 nanorods on FTOsubstrates

La-doped and undoped TiO2 nanorods were synthesized onpolycrystalline FTO (fluorine doped tin oxide, sheet resistance= 8 ohm/square) coated glass substrates through solvothermalchemistry. For the doped one, titanium butoxide (TBOT)(purity ¢97%, Aldrich Co.) and lanthanum(III) nitrate hexahy-drate (La(NO3)3?6H2O) (purity 99.99% Aldrich Co.) were usedas precursor. In a typical synthesis process, first the substrateswere ultrasonically cleaned in a mixed solution of deionisedwater, acetone and isopropanol and then dried in N2 flow. 30mL H2O and 30 mL concentrated (35%) HCl was stirred for 10min in a 100 mL Teflon vessel followed by the drop-wiseaddition of 1 mL titanium butoxide and various amounts ofLa(NO3)3?6H2O solution and stirring for another 10 min atroom temperature. The substrates were placed horizontallywithin the Teflon vessel and the vessel was loaded inside a 100mL autoclave for the hydrothermal reaction at 150 uC for 20 hin a furnace. After synthesis, the autoclave was cooled to roomtemperature and the substrates were washed vigorously withDI water and then dried in air.

(B) Fabrication of DSSC

To fabricate a solar cell, at first, the as-synthesized TiO2

nanorods on FTO substrate were annealed in air at 400 uC for30 min. Then it was cooled to 80 uC and was immersed in a 0.5mM ethanolic solution of N-719 dye (cisbis(isothiocyanato)-bis(2,29-bipyrridyl-4-49-dicarboxylato)-ruthenium(II)bis-tetrabu-tylammonium) (as-received from Solaronix Co.) for 24 h inorder to enable sufficient adsorption of the dye which will actas the sensitizer in the presence of light. After completing theadsorption of the dye, the substrate was washed very carefullywith ethanol and dried for 1 h. After that a platinum coatedFTO substrate which will be used as the counter electrode wasplaced above the dye adsorbed FTO substrate and anelectrolyte containing 0.6 M 1-hexyl-2-3-dimethylimidazoliumiodide, 0.1 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine inmethoxyacetonitrile was injected into the space between theanode and the cathode to complete the assembly of the DSSC.

(C) Characterization techniques

In order to confirm the crystalline phase of TiO2 nanorods,X-ray diffraction (XRD) study was performed using aPANalytical X9PERT PRO instrument and the iron-filtered Cu-Ka radiation (l = 1.5406 Å) in the 2h range of 10–80u with a stepsize of 0.02u. To analyze the shape and size of the synthesizednanorods scanning electron microscopy (FEI Quanta 200environmental scanning microscope) and high-resolutiontransmission electron microscopy (FEI (model Tecnai F30)high-resolution transmission electron microscope (HRTEM)equipped with a field emission source operating at 300 kVvoltage to image TiO2 nanocrystals on carbon-coated copperTEM grid) were done. Optical properties of the as synthesizedTiO2 nanorods were investigated by UV-visible spectrophot-ometer. UV-Vis-DRS spectroscopy measurements were per-formed on a Jasco UV-vis-NIR (Model V570) dual beamspectrometer operated at a resolution of 2 nm. PL spectrawere acquired using a Fluorolog Horiba Jobin Yvon fluores-cence spectrophotometer, equipped with a 400 W Xe lamp asan excitation source and a Hamamatsu R928 photomultipliertube (PMT) as a detector. Raman spectroscopy measurementswere recorded at room temperature on an HR 800 Ramanspectrophotometer (Jobin Yvon, Horiba, France) using mono-chromatic radiation emitted by a He–Ne laser (633 nm),operating at 20 mW. Mott-Schottky measurements wereobtained in 3 M KCl solution at 298 K at frequency 10 KHzusing a Solartron S1287 Electrochemical Interface and 1255 BFrequency Response Analyzer (Solartron Analytical, UK).Current–voltage characteristics were calculated by irradiatingthe cell with 100 mW cm22 (450 W xenon lamp, Orielinstrument), 1 sun AM 1.5 G filter was used to simulate thesolar spectrum. The active area of the cell was 0.5 cm2. Thephotocurrent was measured by using a Keithley 2400 source.

Results and discussion

In order to confirm the crystalline phase of La-doped TiO2

nanorods, XRD study was done. From the XRD, it is confirmedthat the different mol% La-doped TiO2 nanorods grown onFTO substrates are in rutile phase. In Fig. 1, the XRD patternsof various amounts of La-doped and undoped TiO2 nanorodssynthesized on FTO coated glass substrates, FTO coated glasssubstrate and PDF file # 21-1276 have been compared. It isclear from the diffraction peaks that the nanorods are highlyoriented with respect to the substrate along the (002) plane. Ifthe intensity of the (002) peak is compared with the standardTiO2 data (PDF data file #21-1276), it is found that the relativeintensity of (002) peak is increased y10 times from y10% to100%. From XRD pattern, it is confirmed that almost no peak-shift takes place as the amount of doping is very little. Thus,the presence of impurity phases such as La2O3 or La2Ti2O7 canbe ruled-out which can be further proved from Raman andTEM analysis. As the size of La3+ (0.115 nm) ion is larger thanTi4+ (0.068 nm) ion, it is quite difficult for La3+ ions to enterinto the lattice structure of TiO2. So, here the probability offormation of structural defect is less and for that reason lattice

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parameters of doped TiO2 nanorods will not change andalmost no distortion will take place. In Fig. S1 in the ESI3 thezoom view of the XRD patterns has been shown. In the zoomview too there is no noticeable peak shift in the dopednanorods. So from the XRD pattern it can be confirmed thatthe lattice parameters of the TiO2 nanorods remain unchangedafter doping with lanthanum due to mismatch of ionic radius.

Fig. 2 and 3 represent the top and cross sectional view SEMimages of various amounts of La-doped TiO2 nanorods grownon FTO substrates respectively. From the images it isconfirmed that the nanorods are densely packed and perfectlyoriented in the out-of-plane direction with respect to FTO. Thesubstrates are evenly covered by the nanorods. It is clear fromthe figure that the top facets of the rods are square shaped

which also supports the expected growth habit of tetragonalcrystal. From the horizontal and cross sectional view of theimages it is seen that the top surface of the as-synthesized rodsare quite rough but the side walls of the nanorods are relativelysmooth. A closer look of the images also exposed that thenanorods are consisted of a number of thinner nanowires. It isfound that with increase in the amount of lanthanum dopantthe density of the nanorod arrays decreased gradually. Theaverage diameter, length and density of the as-grown nanorodarrays have been determined from the SEM images. Theresults are summarized in Table S1 in the ESI3. From an earlierreport it was found that La-dopant can inhibit the growth ofTiO2 nanocrystals.19 So, it can be assumed that as the amountof dopant concentration is increased in the growth solution,the nucleation density of the seed layer is decreased slowly andthus the compactness of the rods, grown on FTO substrateswas minimized. Further, from EDX analysis the chemicalstoichiometry of the TiO2 nanorods was estimated and thecalculated data is shown in Table S2 (ESI3).

In order to study the formation and composition of thenanorods in detail, further structural characterization wascarried out through TEM. Fig. 4 shows the TEM and HRTEMimage of an individual representative 2 mol% La-doped TiO2

nanorod grown on FTO substrate. In Fig. 4(C) lattice fringeswith interplanar spacing d101 = 0.24 nm corresponding to therutile phase of titania can be identified clearly. No core/shelltype structure was seen. Sharp and clean selected area electrondiffraction pattern (SAED) for the nanorod grown on FTOsubstrate (Fig. 4(B)) was visualized nicely along [11̄0] zone axiswhich was the inveterate evidence of the single crystallinity ofthe nanorods. The fast Fourier transform (FFT) pattern(Fig. 4(D)) of the high resolution phase contrast image(Fig. 4(C)) also confirms the single crystallinity of thenanorods. In Fig. 4(E) the reciprocal lattice points not

Fig. 2 SEM images of (A) 1 (B) 2 (C) 3 and (D) 4 mol% La-doped TiO2 nanorodsgrown on FTO coated glass substrates (top view).

Fig. 3 SEM images of (A) 1 (B) 2 (C) 3 and (D) 4 mol% La-doped TiO2 nanorodsgrown on FTO coated glass substrates (cross-sectional view).

Fig. 1 XRD patterns of pure TiO2 and La-doped TiO2 nanorods with variousamounts of doping. 1La_TiO2, 2La_TiO2, 3La_TiO2 and 4La_TiO2 denote thesamples where the added amount of La(NO3)3?6H2O was 1, 2, 3 and 4 mol%with respect to molar amount of TBOT.

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corresponding to the c-plane were removed to further analyzethe lattice structure. The remaining lattice points and theinverse FFT (IFFT) image are shown in Fig. 4(E) and 4(F)respectively. The IFFT image revealed a more clear structurewith high crystallinity and minimal distortion.

STEM elemental mapping was done in order to inspect thedistribution of lanthanum in the nanorods. In this technique,a two dimensional map of the samples can be visualized nicelywhere the relative location of individual element of aparticular material can be spotted. Here through colorintensity the relative amount of the element present can bejudged. Fig. 5 shows the STEM image of 2 mol% La-dopedTiO2 nanorod and corresponding elemental mapping of the assynthesized nanorod. In this figure the dark field image of anindividual nanorod and the elemental maps of Ti–K and La–Mare shown. The dark field image of the nanorod provides areference to locate the particles in the elemental map. Fromthe elemental mapping it was confirmed that lanthanum wasdistributed homogeneously throughout the rods. Thus from

STEM and line scanning analysis (Fig. 5(D)) it can be provedthat the as-synthesized TiO2 nanorods were homogeneouslydoped with lanthanum and no secondary phase was present.

In order to study the effect of La-doping on the opticalproperties of TiO2 nanorods the UV-vis-DRS spectra of thedoped and undoped nanorods was measured. All the samplesshowed sharp absorption edge around 420 nm (Ebg y 3.0 eV).It is found that the shape of the curve remained almostunchanged but with doping the amount of absorptionincreased (Fig. 6A). As discussed earlier due to huge sizedifference of La3+ and Ti4+ ions it is difficult to incorporateLa3+ ions in TiO2 lattice and that is the main reason for notchanging the absorption curve shape with doping. Theincrease in absorption may be due to the presence of surfacestate caused by oxygen vacancy.20,21 As lanthanum andtitanium are in different valence states as a consequence,oxygen vacancies can be generated. All the doped and undopedsamples exhibit a wide range of absorption below 420 nm. Thiscan be due to band–band electron transition of the rutile TiO2

at its band gap energy i.e. around 3 eV. The tail around 420–500 nm can be attributed to the surface state.21 So it can besaid that lanthanum doping did not give rise to any newspectral phenomena. Assuming that the rutile TiO2 nanorodspossess a direct transition the band edge energy of the as-synthesized nanorods are calculated by plotting (ahn)2 againsthn (where a is absorption coefficient and hn is photon energy).After calculating the band gap from the intercepts (Fig. 6B) it isfound that with doping a little amount of red shift takes placefrom 3.0 eV to 2.9 eV.

Fig. 4 (A) TEM image and (B) selected area electron diffraction pattern of 2mol% La-doped single TiO2 nanorod grown on FTO coated glass substrate. (C)HRTEM image of the nanorod. (D) FFT pattern of the nanorod. (E) Reciprocallattice points not corresponding to the c-plane has been removed. (F) Results ofIFFT based on the lattice points shown in (E).

Fig. 5 (A) STEM image of a representative 2 mol% La-doped TiO2 nanorod. (B,C) The corresponding elemental mapping of (B) Ti–K (C) La–M (100 nm scale barfor all). (D) Line scanning analysis across the single nanorod indicated by the lineas shown in the inset (scale bar 50 nm).

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Fig. 7 shows the photoluminescence property of the assynthesized doped and undoped TiO2 nanorods grown onFTO. It is known that if stable chemical state of dopant hashalf filled or full filled outer electron configuration thennormally it will not generate a different PL phenomena as it isunable to capture electrons.22 So it is found that TiO2 and La-doped TiO2 exhibit similar type of PL signals with nodifference in curve shape. The doped and undoped nanorods

show strong and wide PL signal at a range from 400 to 500 nmat 380 nm excitation wavelength. The reason behind this widerange of PL emission may be attributed to binding exci-tons.19,20 The peak around 415 nm (y2.98 eV) is due to band–band emission of rutile TiO2 nanorods as the photoexcitedelectrons radiatively return back from conduction band tovalence band.20 The peak around 464 nm is the characteristicpeak of rutile TiO2.23 The reason for this emission signal isdue to metal–ligand charge transfer transition from Ti4+ tooxygen anion in TiO6

82 complex. Although the doped andundoped sample exhibit almost similar type curve shape butthe PL intensity of the peaks can be heavily influenced as aresult of the change in surface structure of the material due toalternation of oxygen vacancy.22,24 It is found that the PLintensity increased gradually as the amount of La-dopantincreased. This trend is followed till the amount of La contentis 3 mol%. But after that the intensity of the PL peaks decreaseas the amount of dopant increases. The decrease in PLintensity can be due to the presence of a large number of Ti–O–La bonds which can decrease the content of surface oxygenvacancies. Moreover the effective area of TiO2 for absorbinglight can be less.19

Fig. 8 shows the Raman spectra of TiO2 and La-doped TiO2

nanorods at room temperature. In both pure and dopedsamples the characteristic peaks for rutile TiO2 nanocrystalswere found which again supports the formation of rutilestructure. The two Raman active fundamental modes, Eg

(y444 cm21) and A1g (y610 cm21) and the second order effectat y244 cm21 due to multiple phonon vibration of B1g modecan be clearly distinguished.25,26 It is found that in La-dopedsample the principle peak at 444 cm21 gradually shifted tohigher frequency (y448 cm21) up to 3 mol% and then again aslight red shift takes place at 4 mol% (y442 cm21). This peakshift can be a cause to an increase in oxygen vacancy whichcan also be proved from PL spectra.24 It also indicates that theTi–O bond is slightly shorter and stronger due to bondcontraction in case of lanthanum doping. No peak of La–O isseen in the spectra which supports the XRD result. It can be

Fig. 6 (A) UV-Visible DRS spectra, (B) the dependence of the absorptioncoefficient as a function of energy plot for pure TiO2 and La-doped TiO2

nanorods with various amounts of doping.

Fig. 7 PL spectra, for pure TiO2 and La-doped TiO2 nanorods with variousamounts of doping.

Fig. 8 Raman spectra, for pure TiO2 and La-doped TiO2 nanorods with variousamounts of doping.

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due to interstitial doping of lanthanum. For that reason La isincorporated in the interstitial site of TiO2 lattice.

Electrochemical impedance measurement was done tostudy the role of La-doping on the transport properties ofthe nanorods. Fig. 9 shows the Mott–Schottky plot of TiO2, 2and 3 mol% La-doped TiO2 nanorods. All the samples show apositive slope which is the property of the n-type semiconduc-tor.27 Compared to TiO2 nanorods the doped one showedsmaller slopes which indicate that with doping charge carrierdensity increases. The flat-band potential (VFB) and carrierdensity of the doped and undoped nanorods were calculatedfrom Mott–Schottky equation.

1/C2 = (2/e0ee0Nd)[(V 2 VFB) 2 kT/e0]

where e0 is the fundamental charge constant, e the dielectricconstant of TiO2, e0 is the permittivity of vacuum, Nd the donordensity, V the electrode applied potential, VFB the flat- bandpotential and kT/e0 is a temperature-dependent correction term. Itwas found from the calculation that with increase in dopingconcentration donor density increases. The calculated results areshown in Table 1. As no evidence of core/shell structure was foundfrom TEM and STEM analysis the possibility of carrier concentra-tion gradient can be ruled-out.

These single crystalline oriented nanorods grown on FTOsubstrates can effectively act as photoanode in sensitized solarcells. Due to the single crystalline structures of these nanorodsrapid and efficient transfer of electrons take place from thesensitizer to collecting conducting substrate. Moreover, as thenanorods are directly synthesized on the transparent sub-strate, so more incident light can be passed through the

backside of the photoelectrode and hence able to generatemore photoelectrons. Fig. 10 shows the current density–voltage curves of the assembled solar cell under 100 mWcm22 illumination. The average performance characteristics ofthe cells in terms of short-circuit photocurrent density (JSC),open-circuit voltage (VOC), fill factor (FF) and photoelectricconversion efficiency (g) are summarized in Table 2. It is foundthat with doping VOC value increases from 0.704 V to 0.749 V.The increase in open circuit voltage is due to the elevation offlat band potential as conduction band edge of the doped TiO2

nanorods is shifted towards more negative potential. The JSC

value also increases from 2.38 mA cm22 to 3.81 mA cm22 afterdoping. The plausible reason for increment of current densitycan be attributed to an increase of loading of more amount ofdye. The photon to electron conversion efficiency of the TiO2

nanorods was increased y21% after doping with lanthanum.It is known that lanthanum doping can increase the oxygenvacancy of titania surface. The amount of dye adsorption canalso be increased with increase in oxygen vacancy which mightbe a reason for this efficiency improvement. However, it is verydifficult to know the precise reason for the increase in energy-conversion efficiency of DSSC, as betterment of the efficiencyof a cell depends not only on the shape, size and inherentproperties of the metal oxide materials used but also onvarious other parameters.

Fig. 9 Mott–Schottky plots measured at a frequency of 10 kHz at 298 K in 3 MKCl solution for TiO2 and 2 and 3 mol% La-doped TiO2 nanorods.

Table 1 Flat band potential (VFB) and donor density (Nd) Of TiO2 and La-dopedTiO2 Nanorods

Samples VFB (V) Nd (1018) cm23

TiO2 20.532 0.561La_TiO2 20.615 1.392La_TiO2 20.735 6.413La_TiO2 20.832 8.734La_TiO2 20.862 8.75

Fig. 10 Current density vs. potential curves for dye sensitized solar cellfabricated from pure TiO2 and La-doped TiO2 nanorods grown on FTO glasssubstrates.

Table 2 Photovoltaic properties of DSSCs assembled with TiO2 and La-dopedTiO2 nanorods

Samples JSC (mA cm22) VOC (V) FF (%) g (%)

TiO2 2.38 0.704 60.48 1.391La_TiO2 3.69 0.789 52.95 1.542La_TiO2 3.74 0.793 53.48 1.583La_TiO2 3.81 0.794 54.29 1.644La_TiO2 3.28 0.749 55.22 1.69

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Conclusion

In summary, single crystalline oriented La-doped TiO2

nanorods have been synthesized on FTO substrates.Homogeneous doping of lanthanum throughout the nanorodshas been proved by STEM analysis. Detailed optical studies ofthe as synthesized doped and undoped nanorods have beendone through UV, PL and Raman spectroscopy.Electrochemical impedance measurement was done to inves-tigate the band edge position and to know the trend ofelectron carrier density in the doped nanorods. The La-dopedTiO2 nanorods have been successfully used as a photoanode ina DSSC. Photon to electron power conversion efficiency wasincreased by 21% after doping the TiO2 nanorods with 4 mol%lanthanum. This simple hydrothermal synthesis method canbe applied considerably to homogeneously dope any metalcations with titanium ion. Systematic investigation of dopingof metal cations with Ti4+ ions for synthesis of homogeneouslydoped single crystalline nanorods directly grown on substrateswill provide valuable information for designing high-perfor-mance photoelectrochemical devices.

Acknowledgements

PP acknowledges support from the Young Scientist Awardgrant from the Council for Scientific and Industrial Research(CSIR) in Physical Sciences and a separate grant from theDepartment of Science & Technology (DST), India (DST/INT/ISR/P-8/2011). SS acknowledges the support from the Councilof Scientific and Industrial Research, India (CSIR) forproviding the Senior Research Fellowship. Authors alsoacknowledge the seed funds from their institute.

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