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Electrochimica Acta 121 (2014) 240– 244

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

omparison of Structural and Electrochemical Properties of V2O5

hin Films Prepared by Organic/Inorganic Precursors

atma Pınar Gökdemir, Orhan Özdemir ∗, Kubilay Kutluıldız Technical University, Department of Physics, Istanbul, TURKEY

r t i c l e i n f o

rticle history:eceived 27 December 2013ccepted 27 December 2013

a b s t r a c t

Vanadium pentoxide thin films were produced from organic and inorganic precursors by sol gel dip-coating method. Fourier transform infrared spectroscopy and UV Vis spectroscopy were made to figure

vailable online 8 January 2014

eywords:anadium pentoxideyclic voltammetry

out structural properties of the films. Electrochemical properties were investigated by cyclic voltam-metry. The shape of the curves was in agreement with a typical diffusion controlled cyclic voltammogramsof amorphous V2O5 films for a reversible lithium ion intercalation/deintercalation process showingyellow–green–blue multi-electrochromism. V2O5 films, synthesized from organic precursor, indicatedlower band gap energy, higher charge capacity as well as homogeneous and low granule size comparedto inorganic route.

ip-coating

. Introduction

Vanadium oxide has received significant interest due to hav-ng potential for various industrial applications such as catalyst [1],athode material for lithium batteries [2], window for solar cells3], electro-optic switches [4], electrochromic devices [5] and gasensors [6]. The great interest drawn to V2O5-based thin films cane traced back to its important characteristics originates from its

ayered structure, together with the presence of multiple oxida-ion states of vanadium. The principal oxides of vanadium occur asingle valence in oxidation states from V2+ to V5+, in the form ofO, V2O3, VO2, and V2O5. However, the vanadium–oxygen phaseiagram also includes mixed valence oxides containing two oxida-ion states, such as V6O13 with V5+ and V4+ and a series of oxidesetween VO2 and V2O5 which contain V4+ and V3+ species. Theixed valence oxides form by introducing oxygen vacancy defects

nd if the number of oxygen vacancies exceeds a certain value,he vacancies tend to correlate and form crystallographic shearlanes and are subsequently eliminated by reorganization of V–Ooordination units. The result is a series of oxides with relatedtoichiometries, such as the Magnéli phases with VnO2n-1 and the

adsley phases with V2nO5n−2 formulas [7,8]. Different oxidationtates of vanadium can accommodate a large non-stoichiometryith oxygen vacancies as the basic point defects [9]. Consequently,

mall changes in the synthesis conditions can lead to significanteviations in the V2O5 stoichiometry. This has encouraged muchesearch for the production of vanadium pentoxide films with

∗ Corresponding author. Tel.: +90 212 383 4279; fax: +90 212 383 4234.E-mail address: ozdemir@yildiz.edu.tr (O. Özdemir).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.12.164

© 2014 Elsevier Ltd. All rights reserved.

relatively simple and low cost sol gel techniques that also offer largearea deposition with low temperature synthesis such as spray pro-cess [10], spin coating [11] or dip-coating [12]. Through mainly twochemical processes, either an inorganic route with metal salts inaqueous solutions or the metal-organic route with metal alkoxidesin organic solvents, V2O5 film can be easily synthesized. In bothcases, a reaction is initiated via hydrolysis to get reactive M–OHgroups that can be generated simply by adding water to an alkox-ide or by changing pH of an aqueous solution [13]. In the presentwork, structural, optical and electrochemical properties of vana-dium oxide thin films, synthesized by organic/inorganic precursors,were studied to demonstrate the impact of granule size on energyband gap and injected/extracted charge amount and experimentalfindings were interpreted according to quantum size effect.

2. Experimental

Inorganic sol was prepared by dissolving 0.5 g V2O5 powderin hydrogen peroxide (H2O2 15 wt %) at room temperature andthe resulting transparent orange sol was vigorously stirred for1 h at 70 ◦C in water bath on a magnetic stirrer yielding a darkred-brownish viscous solution. On the other hand, organic solwas prepared by introducing 7.65 ml vanadium (V) triisopropox-ide into 30 ml isopropyl alcohol and then solution was mixed for2 h at room temperature. 0.75 ml glacial acetic acid was introducedto the mixture and similarly stirred for 1 h, resulting a trans-parent orange solution. Without applying any aging process to

the sols, films were deposited on soda lime glass (SLG), Corn-ing glass and ITO coated glass substrates with a constant dippingspeed (100 mm/min). Prior to deposition, substrates were cleanedin an ultrasonic bath at 30 ◦C using acetone, isopropyl alcohol

himica Acta 121 (2014) 240– 244 241

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Table 1Optical band gap values (eV) of the films onto Corning glass substrates.

sample directallowed

indirectallowed

directforbidden

indirectforbidden

F.P. Gökdemir et al. / Electroc

nd deionized water, respectively. Deposited films were dried atoom temperature, and then heated at 150 ◦C for 20 min. Theemaining sols were left to dry at room temperature for thermo-ravimetric (TG) and differential thermal analyses (DTA), whichere performed with a SII Exstar 6000 TG/DTA 6300 model ther-al analysis system using an Al2O3 crucible in static air ambientith a heating rate of 10 ◦C/min. The structural properties of thelms were investigated by means of FTIR using a Bruker Ten-or27 model IR spectrophotometer. Optical properties of the filmsere examined by transmittance measurements using a PG Instru-ents T80 model UV/VIS spectrophotometer. Surface morphology

nd particle size of the films were analyzed through atomic forceicroscopy (AFM) with a Park Systems model device operating

t non-contact mode. Electrochemical analyses were carried outsing cyclic voltammetry experiments with an Autolab PGSTAT30odel potentiostat/galvanostat. These experiments were car-

ied out in a conventional three-electrode electrochemical cellhere a platinum and a silver wire were used as a counter and

eference electrode, respectively in a 1 molar (M) lithium perchlo-ate/propylene carbonate (LiClO4/PC) electrolyte.

. Results and discussion

Fig. 1 shows TG/DTA analyses for organic/inorganic precursorhat were carried out within the temperature range of 25 ◦C to

00 ◦C. For organic sol, initial weight loss took place up to 320 ◦Cith a large endotherm, associated with the volatilization and

ombustion of organic species. Second change around the 343 ◦Corresponds to the phase transition while other mass loss begins

ig. 1. TG/DTA curves of the coating solution powders prepared by a) inorganic b)rganic route.

inorganic 3.03 2.64 2.63 2.33organic 3.46 2.10 2.46 1.68

at 610 ◦C, reflecting melting point of V2O5. On the other side, forinorganic precursor, mass loss occurred in two stages: in the firststage (in between 20-120 ◦C), large endotherm was ascribed toloss of loosely bond water whilts as a second stage, appearingexothermal peak in TG curves, rapid loss of mass which took placeconcurrently with crystallization resulting orthorhombic V2O5 atthe proximity of 350 ◦C. In brief, water expulsion process occurredin two steps for the inorganic precursor and could be described as:V2O5 n·H2O→V2O5 0.5·H2O→V2O5 in TG/DTA analysis [13]. Fig. 2aexhibits both raw FTIR absorption spectra of the as-deposited films(denoted as symbol) and results of deconvolution processes (solidlines) to identify the IR modes. At first sight, each routes indicatedsimilar transmission spectra in IR analysis except the intensity.Complete IR analysis on the V2O5 films produced via inorganicroute were carried out in our previous work [14]. In here, exist-ence of water related species and common IR bonds related toV2O5 films are presented. Within this context, V2O5 films showtwo large bands at ∼1600 and ∼3400 cm−1. The peaks between1420 and 1650 cm−1 correspond to OH bending and OH-H stretch-ing from water [14,15]. Moreover, H2O and H3O+ bonds appeared at3362-3372 and 3200-3220 cm−1. In the 400-1100 cm−1, the V2O5film exhibits three characteristic vibration modes: V = O vibra-tions at 1017-1070 cm−1 [16,17], the V-O-V symmetric stretcharound 516 cm−1 [18,19] and the V-O-V asymmetric stretch at 752-756 cm−1 [20,21]. The IR spectra of V2O5 films on ITO substratesindicates that the group of bands presents below 600 cm−1 corre-sponds to the edge sharing 3V-OC stretching [22] and the bridgingV-OB-V deformations [23]. Peaks at 932 cm−1 and 1000 cm−1 corre-spond to V4+ = O and V5+ = O bands by indicating non stoichiometricV2O5 film while the band at 830-840 cm−1 shows disorder (oramorphous phase) of V2O5 film [24]. Transmittance curves for eachroute in the deposition of V2O5 films onto Corning glass substrateswere depicted in Fig. 2b. Within the visible region, V2O5 film, pro-duced via organic route, showed high transparency compared tothat of the film synthesized by inorganic route. To deduce theenergy band gap, optical absorption coefficient has to be deter-mined priorily. Therefore, the optical absorption coefficient of thefilm, ˛, was calculated through the relation

˛ = 4�k/� (1)

where k is the extinction coefficient. It was obtained through afitting to the experimental data by OptiChar software by varyingrefraction film index (n), k and thickness of the film. In the anal-ysis, dispersive glass and surface as well as bulk inhomogeneitywere taken into account. The generated transmittance was alsoshown as solid line in Fig. 2b in which less than 1 discrepancy wasachieved in the fitting procedure. Hence, variation of n and k asfunction of � were given in Fig. 2c and d for inorganic and organicroute followed in the deposition of V2O5 films, respectively. Foreach case, the film refractive index at infinite wavelength, n∞, wasdetermined as “2” which was consistent with others where phys-ical deposition techniques were employed in the growth of V2O5film. On the contrary, energy band gap values (EG), seen in the insetof Fig. 2c and d for direct allowed case, were different from each

other; organic route yielded larger EG; 3.4 eV rather than 3.0 eV.For other possible transitions, the calculation was resumed and theresults were tabulated and given in Table 1. Though the presenceof various transitions, recent works on V2O5 films indicated direct

242 F.P. Gökdemir et al. / Electrochimica Acta 121 (2014) 240– 244

Fig. 2. a) FTIR spectra of the films synthesized by organic/inorganic precursor over different substrates, b) Optical transmission spectra of that of the films. Refractive indexand extinction coefficient variation as function of wavelength for c) inorganic and d) organic precursor deposited onto Corning glass. Note that direct allowed transition forthe films also given as inset in the figures.

Fig. 3. AFM images of the films prepared by a) inorganic b) organic route.

Table 2Anodic/cathodic charge values (mC/cm2) at different scan rates.

sample/scan rate (mV/s) 10 20 50 80 100

inorganic (Qa:Qc) 19.1:18.9 16.1:16.3 19.9:19.9 15.0:14.8 12.1:12.1organic (Qa:Qc) 31.7:31.7 31.3:31.1 28.1:28.1 27.3:27.2 28.0:27.9

F.P. Gökdemir et al. / Electrochimica Acta 121 (2014) 240– 244 243

F V and

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ig. 4. CVs of the films at a) 10 b) 20 c) 50 d) 80 e) 100 mV/s scan rates between -1

orbidden transition in which (˛h�)2/3 varies with (h�). Extrapo-ating such plot resulted the optical band gap value. It is obviousrom Table 1 that for each possible transition, estimated band gapor V2O5 film followed by inorganic route is less than that of thelm produced by organic route. Hence, its variation of V2O5 filmsroduced through inorganic/organic precursors might be due tohe presence of different granule size of the films. Indeed, AFMnalysis indicated homogenous and lower granule size of the V2O5lm, produced via organic precursor compared to inorganic one

Fig. 3). Although inorganic films have larger particle sizes, highererformance present in organic films might be originated fromifferent stoichiometry of the films [23]. Therefore, better intercala-ion/deintercalation capacity was anticipated for the film produced

+1 V in 1 M LiClO4/PC. In f), peak current changes of the films at these scan rates.

via organic precursor by CVs and depicted in Fig. 4. Both V2O5films exhibited multi electrochromism regarding to their layeredstructure and thickness with following reaction [25];

V2O5 + x(Li+ + e−) ⇔ LixV2O5 (2)

Inserted/extracted charge amounts were calculated using CVsand illustrated in Table 2. Randles-Sevcik plots of the films(for the first anodic and second cathodic peaks) were given inFig. 4f, referring a more reversible character with a linear depend-

ence for organic films. Also, it was deduced from the slope ofthese plots, Li ion diffusion coefficients seemed to be higher fororganic films. Moreover, calculated charges were high for organicfilms. Electrochemical analysis displayed organic films have better

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lectrochromic performance with higher charge capacities and bet-er reversibility.

. Conclusion

V2O5 films synthesized through organic precursor resultedigher intercalation/deintercalation charge capacity compared to

norganic precursor. Such results were the consequences of gran-le size of the films, verified by both AFM and UV-Vis. transmissionnalysis by comparing eventual grain size and optical energy bandap.

cknowledgements

This study is financially supported by Yildiz Technical Universityrojects under a contract of 2012-01-01-DOP02 and 2011-01-01-AP03. The authors thank to Assoc. Prof. Dr. Nevim SAN for FTIReasurements.

eferences

[1] R. Ramirez, B. Casal, L. Utrera, E. Ruiz-Hitzky, Oxygen reactivity in vanadiumpentoxide: electronic structure and infrared spectroscopy studies, J. Phys.Chem. 94 (1990) 8960–8965.

[2] C. Julien, E. Haro-Poniatowski, M.A. Camacho-López, L. Escobar-Alarcón, J.Jímenez-Jarquín, Growth of V2O5 thin films by pulsed laser deposition and theirapplications in lithium microbatteries, J. Mater. Sci. Eng. 65 (1999) 170–176.

[3] C.R. Aita, Y.L. Liu, M.L. Kao, S.D. Hansen, Optical Behavior of Sputter- DepositedVanadium Pentoxide, J. Appl. Phys. 60 (1986) 749–753.

[4] G.S. Nadkarni, V.S. Shirodkar, Experiment and Theory for Switching inAl/V2O5/Al devices, Thin Solid Films 105 (1983) 115–129.

[5] S.F. Cogan, N.M. Nguyen, S.J. Perrotti, R.D. Rauph, Optical properties of elec-trochromic vanadium pentoxide, J. Appl. Phys. 66 (1989) 1333–1337.

[6] S. Zhuiykov, W. Wlodarski, Y. Li, Nanocrystalline V2O5–TiO2 thin-films for oxy-

gen sensing prepared by sol–gel process, Sens. Actuators B 77 (2001) 484–490.

[7] S. Surnev, M.G. Ramsey, F.P. Netzer, Vanadium oxide surface studies, Progressin Surface Science 73 (2003) 117–226.

[8] S. Beke, A review of the growth of V2O5 films from 1885 to 2010, Thin SolidFilms 519 (2011) 1761–1771.

[

[

a Acta 121 (2014) 240– 244

[9] R.J.D. Tilley, Defect Crystal Chemistry and Its Applications, Blackie Glasgow,London, 1987.

10] A. Ashour, N.Z. El Sayed, Physical properties of V2O5 sprayed films, Journal ofOptoelectronics and Advanced Materials 11 (2009) 251–256.

11] F.N. Dultsev, L.L. Vasilieva, S.M. Maroshina, L.D. Pokrovsky, Structural and opti-cal properties of vanadium pentoxide sol–gel films, Thin Solid Films 510 (2006)255–259.

12] A. Cremonesi, D. Bersani, P.P. Lottici, Y. Djaoued, R. Bruning, Synthesis andstructural characterization of mesoporous V2O5 thin films for electrochromicapplications, Thin Solid Films 515 (2006) 1500–1505.

13] J. Livage, Optical and electrical properties of vanadium oxides synthesized fromalkoxides, Coord. Chem. Rev. 190-192 (1999) 391–403.

14] O. Ozdemir, F.P. Gokdemir, U.D. Menda, P. Kavak, A.E. Saatci, K. Kutlu, Nano-crystal V2O5 nH2O sol-gel films made by dip-coating, AIP Conf. Proc. 1476(2012) 233–240.

15] N. Ozer, C.M. Lampert, Electrochromic performance of sol-gel depositedWO3–V2O5 films, Thin Solid Films 349 (1999) 205–211.

16] Y. Iida, Y. Kanno, Doping effect of M (M = Nb, Ce, Nd, Dy, Sm, Ag, and/or Na)on the growth of pulsed-laser deposited V2O5 thin films, J. Mat. Pro. Tech. 209(2009) 2421–2427.

17] F.P. Gokdemir, U.D. Menda, P. Kavak, A.E. Saatci, O. Ozdemir, K. Kutlu, Influenceof water expulsion on structural properties of V2O5 nH2O sol-gel films, AIPConf. Proc. 1476 (2012) 279–284.

18] F. Huguenin, M.J. Giz, E.A. Ticianelli, R.M. Torresi, Structure and properties ofa nanocomposite formed by vanadium pentoxide containing poly(N-propanesulfonic acid aniline), J. of Pow. Sources 103 (2001) 113–119.

19] A.E. Saatci, F.P. Gokdemir, U.D. Menda, P. Kavak, O. Ozdemir, K. Kutlu, Ionic con-duction in different hydrated V2O5 films, AIP Conf. Proc. 1476 (2012) 289–295.

20] Y. Yang, Q. Zhu, A. Jin, W. Chen, High capacity and contrast of elec-trochromic tungsten-doped vanadium oxide films, Solid State Ionics 179 (2008)1250–1255.

21] A. Jin, W. Chen, Q. Zhu, Y. Yang, V.L. Volkov, G.S. Zakharova, Electrical andelectrochemical characterization of poly (ethylene oxide)/V2O5 xerogel elec-trochromic films, Solid State Ionics 179 (2008) 1256–1262.

22] C.V. Ramana, O.M. Hussain, B.S. Naidu, P.J. Reddy, Spectroscopic characteriza-tion of electron-beam evaporated V2O5 thin films, Thin Solid Films 305 (1997)219–226.

23] M.B. Sahana, C. Sudakar, C. Thapa, G. Lawes, V.M. Naik, R.J. Baird, G.W. Auner,R. Naik, K.R. Padmanabhan, Electrochemical properties of V2O5 thin filmsdeposited by spin coating, Mat. Sci. and Eng. B 143 (2007) 42–50.

24] S.H. Lee, H.M. Cheong, M.J. Seong, P. Liu, C.E. Tracy, A. Mascarenhas, J.R. Pitts,S.K. Deb, Raman spectroscopic studies of amorphous vanadium oxide thin films,Solid State Ionics 165 (2003) 111–116.

25] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, TheNetherlands, 1995.