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Electrochromic thin  lms of sodium intercalated vanadium(V) oxidexerogels: Chemical bath deposition and characterization

Metodija Najdoski a,b,⁎, Violeta Koleva c, Sasho Stojkovikj a,b, Toni Todorovski a,1

a Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, POB 162, Arhimedova 3, 1000 Skopje, Republic of Macedoniab Research Center for Environment and Materials, Macedonian Academy of Sciences and Arts, Krste Misirkov 2, 1000 Skopje, Republic of Macedoniac Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, G. Bonchev Str. Bldg. 11, 1113 So a, Bulgaria

a b s t r a c ta r t i c l e i n f o

 Article history:Received 26 February 2015Revised 20 July 2015Accepted in revised form 21 July 2015Available online 29 July 2015

Keywords:

Thin  lmsVanadium(V) oxide xerogelsChemical synthesisElectrochromismOptical propertiesElectrochemical properties

An optimized chemical bath method is applied to obtain well-structured thin   lms with compositionNa0.33V 2O5·nH2O (n = 1 and 1.3). The method is based on a controlled precipitation reaction that takes placein the system of sodium metavanadate and diethyl sulfate at 85 °C. The  lm structure, morphology and thechanges occurring during prolonged aging are examined by XRD, IR spectroscopy, TG-DTA, SEM and AFM. Theelectrochemical and electrochromic properties are studied by cyclic voltammetry and UV –vis spectroscopy.The as-deposited thin  lms are characterized with high optical transmittance varying between 40 and 70% atthe 500 nm visible region in dependence on  lm thickness. The Na0.33V 2O5·nH2O thin lms exhibit stable elec-trochemical cycling combined with relatively high electrochromic activity. The reproducibility of the transmit-tance variance of 55% after 500 cycles in the electrochromic cell is a promising result for the potentialapplication of Na0.33V 2O5·nH2O thin lms in electrochromic devices.

© 2015 Published by Elsevier B.V.

1. Introduction

Vanadium(V) oxide and derived compounds have been extensivelystudied due to valuable chemical and physical properties which deter-minea widerange of applicationsin catalysis, high-energy lithium batte-ries anda variety of electricand optical devices. Thesynthetic proceduresat ambient conditions usually produce hydrated vanadium(V) oxides,V 2O5·nH2O, known as xerogels which adopt layered structures withV 2O5 layers and interstitial water molecules [1–3]. Vanadium(V) oxidexerogels like crystalline V 2O5 are typical intercalation compounds withmultiple valence state of vanadium which enables redox-dependentproperties [4–6]. Due to the high intercalation capacity (for instance, alithium intercalation capacity about 1.4 times larger than that of crystal-line V 2O5) [7] they have a great potential for applications like reversiblecathodes for lithium batteries [8,9], micro-batteries [6], supercapacitors[10], electrodes [11] and humidity sensors [12]. The reversible cationintercalation/deintercalation within the xerogel framework is concomi-tant with reversible reduction/oxidation of V(V) to V(IV) or to a lower

valence vanadium state giving rise to easy color changes: yellow(V(V)), blue (V(IV)), green (V(III) or a mixture of V(V) and V(IV)) andviolet (V(II)) [2,6]. The multi-colored electrochromism demonstratedby V 2O5·nH2O xerogels makes them very attractive since it providesthe opportunity to extend the range of functions of the electrochromicmaterials. Vanadium(V) oxide xerogels under the form of thin lms onelectroconductive glass substrates have been used in electrochromicdevices [13,14], electrochromic mirrors [14],  “smart windows” designedfor architectural purposes to control light transmittance [15–17] andcontrolled reectance mirrors for vehicles [18].

The thin  lm properties, including V 2O5·nH2O xerogels, are wellknown to depend essentially on its microscopic characteristics [19,20]such as structure, crystallinity and morphology, which can be governedby the deposition method and the deposition parameters (kind andconcentration of the precursors, rate of deposition, temperature, pres-sure, etc.). Therefore, the choice of the suitable synthetic procedure isa powerful tool for control and optimization of the material properties.

In this regard, we have recently developed a simple chemical bathdeposition method to obtain well-dened ammonium intercalatedvanadium(V) oxide xerogels with the compositions (NH4)xV 2O5·1.3H2O (x = 0.15 and 0.30) [21,22]. The method is based on the directacidication of NH4VO3 solutions by acetic acid at different tempera-tures between50 and85 °C. Through rational selection of thedepositionparameters like vanadium concentration, temperature and depositiontime, (NH4)xV 2O5·1.3H2O thinlms exhibiting high values of the trans-mittance variance (ΔT ) of 55% at 400 and 900 nm were designed.

Surface & Coatings Technology 277 (2015) 308–317

⁎   Corresponding author at: Institute of Chemistry, Faculty of Natural Sciences andMathematics, Ss. Cyril and Methodius University, POB 162, Arhimedova 3, 1000 Skopje,Republic of Macedonia.

E-mail addresses: metonajd@pmf.ukim.mk (M. Najdoski), vkoleva@svr.igic.bas.bg(V. Koleva), sashostojkovikj@gmail.com (S. Stojkovikj),  toni.todorovski@irbbarcelona.org(T. Todorovski).

1 Present address: Institute for Research in Biomedicine, Parc Cientíc de Barcelona,08028 Barcelona, Spain.

http://dx.doi.org/10.1016/j.surfcoat.2015.07.041

0257-8972/© 2015 Published by Elsevier B.V.

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However, the use of the same simple synthetic procedure in the caseof initial NaVO3, i.e. acidication of NaVO3 solution with acetic acid at75 °C (pH = 3), leads to the formation of unstructured amorphousthin lms as we have previously established [23]. In that case a furtherthermal treatment at 400 °C was needed in order to obtain crystallinelms which representa two-phase mixture of sodium vanadium oxidessuch as NaV 6O15 and Na1.1V 3O7.9. Thus prepared thin lms exhibited in-suf ciently high ΔT  values of about 20% in the voltage range of ± 2 V.

The present work is focused on the examination of well-structuredhydrated sodium vanadium oxide thin lms with electrochromic prop-erties. Such thin  lms are prepared by an optimized chemical bathmethod that ensures one-step deposition of the thin   lms at lowtemperature. For the purpose we have applied a different synthetic ap-proach: instead of a direct acidication, we have used here an indirectacidication through the hydrolysis of diethyl sulfate present in thechemical bath. This controlledprecipitation reaction gives rise to thede-position of thin lmsof vanadium(V) oxide xerogel with composition of Na0.33V 2O5·H2O having well-organized layered structure in the nano-scale region. These lm characteristics are advantageous to achievingelectrochemical stability and highΔT value of 55%which is reproduciblefor 500 cycles. The changes during the lm aging are studied in respectto the  lm structure, V(V) reduction and electrochromic effect.

It is worth emphasizing that the as-prepared well-structuredNa0.33V 2O5·H2O composition (solid or  lm) is highly benecial forfurther obtaining a variety of chemical compositions. Thus, once ob-tained it can be used to produce either lower hydrated xerogelsNa0.33V 2O5·nH2O (0.3  b  n  b  1) or a single phase of NaV 6O15 by thermaltreatment at an appropriatetemperature.All thesecompositionshavinga layered or tunnel structure can serve as host matrices for intercalationprocesses which open opportunities for different applications.From thispoint of view the difference with the previously studied (NH4)xV 2O5·1.3H2O compositions is obvious: their thermal treatment produces thewell studied V 2O5.

2. Material and methods

Thin lm deposition is performed onto commercially available glasssubstrates. They are coated with a conductive, transparent thin layer of SnO2:F (FTO) with 80% optical transparency in the visible spectrum andelectrical resistance of 10–20 Ω/cm2. Before deposition, the substrateswere cut into pieces with the dimensions 40 mm × 25 mm × 2 mmand cleaned in the following order: with detergent, alkaline solution,1:1 diluted hydrochloric acid, hexane, acetone and rinsed with deion-ized water and dried at room temperature. Commercial diethyl sulfate(Sigma-Aldrich), propylene carbonate (Sigma-Aldrich), lithium per-chlorate (Sigma-Aldrich), sodium metavanadate min 98 wt.% (CarloErba), ethanol 96% (Alkaloid), sodium hydroxide (Merck), hydrochloricacid (Merck), hexane (Merck) and acetone (Merck) are used withoutfurther purication.

 2.1. Preparation of the thin lms

The lms are deposited from a chemical bath with optimized com-position and process conditions. The stock solution for preparation of the chemical bath is obtained by heating at 65 °C of 0.50 g sodiummetavanadate and 250 ml deionized water. The chemical bath solutionis prepared in a 120 ml beaker by mixing 100 ml sodium metavanadatesolution (0.016 M) and 0.5 ml of diethyl sulfate. The addition of diethylsulfate results in a change of the solution color from yellow to darkorange. The cleaned substrates are vertically supported to the wall of the beaker with the non-conductive side facing the wall. The depositionsystem is then heated up to 85 °C (deposition temperature) with con-tinuous stirring and the achieved temperature and stirring are main-tained during the deposition time. pH of the chemical bath during the

deposition is about 5.5. The beginning of the deposition reaction is

observed as the appearance of turbidity of the liquid phase which isdue to the formation of a solid substance (precipitate). Just at this mo-ment the substrate is removed from the chemical bath and quickly,but carefully, is wiped with cotton soaked with ethanol (96%). This pro-cedure is needed in order to remove weakly sticking grains while thewell adherent grains remain on the surface. The substrate is thenreturned into the chemical bath at the same position and the depositiontime starts to be measured. In such a manner we have prepared thin

lms for 5, 10 and 15 min deposition times. To prepare a thicker 

lm,the lm already obtained for the 15 min deposition time is reinsertedinto a fresh chemical bath for a further 15 min (2 × 15 min) which isdone 5 min after the beginning of the deposition reaction in the secondchemical bath. Thus the obtainedlm will be further designated as thatprepared for the 30 min deposition time. By the deposition procedureused the lms are deposited on the both sides of the substrates. To re-move the thin  lm from the non-conductive side of the substrate, thissite is carefully wiped with cotton wetted with 2 M aqueous solutionof sodium hydroxide. Finally, the as-deposited thin  lms are wipedwith cotton soaked with ethanol, rinsed with ethanol and left verticallyto dry at room temperature. The thin lms prepared for the 5 to 15 mindeposition time have yellow color, while the thicker lm obtained forthe 30 min deposition time has yellow-brown color.

The precipitate from the chemical bath is separated by vacuumltration, washed with ethanol and dried in air at room temperaturefor 3–4 h. The fresh precipitate has brown color.

3. Characterization of the thin lms

The composition and structure of both thin  lms and precipitatefrom the chemical bath were examined using a Rigaku Ultima IV X-raydiffractometer with CuKα   radiation. The thermal studies (TG andDTA) were carried by a LABSYS™   Evo apparatus (SETARAM) in atemperature interval of up to 500 °C in an airow at a heating rate of 10 °C/min. Infrared spectra were recorded with a Perkin-Elmer System2000 infrared interferometer using KBr disks.

The morphology of the thinlmswas observed by scanning electron

microscopy (JEOL JSM-5510). The topography of the lm surfaces wasexamined by AFM in taping mode at room temperature usingNanoScopeV system (Veeco Instruments Inc.).

In-situ optical spectra of the thin  lms were recorded by a VarianCary 50 Scan spectrophotometer ranging from 350 to 900 nm at volt-ages in the range of ±2.5 V. The electrochromic cell was a home-madecell of 3 mm thick window glass. The cell is actually a Vis cuvette on asquared glass base with holes that allow the cuvette to be attached tothe spectrophotometer. The electrochromic cell was used as a two-electrode system: one electrode was a blank FTO substrate and theother electrode was FTO substrate with a thin   lm. The distancebetween the electrodes was about 1 cm and 1 M LiClO4 in propylenecarbonate (PC) was used as an electrolyte (30 ml). The surface of eachelectrode was about 8 cm2. The prolonged cycling up to 500 cycles

was performed in the same two-electrode electrochromic cell with al-ternative square pulse voltage of +2.5 V and switching time of 60 s.

The electrochemical behavior of Na0.33V 2O5∙H2O thin lms were ex-amined by cyclic voltammetry in 1 M LiClO4 (PC) in a conventionalthree-electrode cell using a micro AUTOLAB II equipment (Eco-Chemie)in the potential range initially between −2.5 and +2.5 V, and then re-duced to−1 and +1V. The preparedthin lm is the working electrode,the reference electrode is Ag/AgCl (3 M KCl) and the auxiliary electrodeis a platinum wire. The CV curves are recorded at 10 and 50 mV/sscanning rates.

The   lm thickness was measured by a Alpha Step D-100prolometer (measuring parameters: stylus force 5 mg, length 8 mm,range 10 μ m and speed 0.07 mm/s). All as-deposited Na0.33V 2O5·nH2Oxerogels thin lms with different thickness have passed an adhesion

tape test.

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4. Results and discussion

4.1. Chemical consideration for  lm formation

The chemistry of the deposition process is based on a controlled pre-cipitation reaction resulting fromthe acidication of the NaVO3 solutionthat occurs in the presence of diethyl sulfate. Themain idea for the pre-cipitation process was taken from a previous study of one of the authors

[24]. Above 65 °C the hydrolysis of diethyl sulfate takes place accordingto the reaction:

CH3CH2Oð Þ2SO2  aqð Þ þ 4H2O lð Þ→ 2CH3CH2OH aqð Þ þ 2H3Oþ aqð Þ þ SO4

2− aqð Þ:

The concentration of H3O+ gradually increases (decreasing pH to

about 5.5) and the conditions for precipitation of vanadium(V) oxidexerogels are fullled [6].

4.2. Composition and structure of the thin lms

Fig. 1 shows the X-ray powder diffraction (XRD) patterns of the as-deposited  lm (Fig. 1a) and the precipitate from the chemical bath, socalled the brown sample (Fig. 1b).

Besides the peaks due to the FTO substrate (PDF 46–1088) the twopatterns are very similar, except the difference in the intensities of some peaks, for instance at 25.49, 29.85, 47.31 and 50.54° (2θ). Ingeneral, sucha difference is reasonable and it is mainlydue to the occur-rence of texture in thin lm growth [25]. The similarity between theXRD patterns evidences the same phase composition of the  lm andprecipitate. This is of importance since we are able to undertake somestudies that require more amount of the sample (for example TG-DTAanalyses) using the precipitate and the obtained data are then referredto the lm.

Both patterns display comparatively small number of diffractionpeaks, about ten peaks, all broad. The   rst peak centered at 7.89°(d = 11.20 Å) has much higher intensity than the other peaks locatedabove 23° (2θ scale). The comparison with the patterns from PDF data-base showed that our diffractograms do not correspond exactly to any

known X-ray powder patterns of vanadium compounds. However,

there is a great similarity with the patterns of V 2O5·nH2O xerogel(PDF 40–1296) with the difference that our patterns exhibit additionalpeaks. From the literature survey on vanadium oxides [6,26–29] it iswellknown that the X-ray patternsof V 2O5·nH2O xerogel arecharacter-ized by a small numberof 3–5 broad peaksfrom theseries of 00l reec-tions (missing  002  peak). Moreover, the intensity of the  rst orderdiffraction  001   located generally around  d  = 11.5 Å is much largerthan that of the other terms. The structure of xerogel was resolved by

Petkov et al. [29]. The xerogel is found to be an assembly of doubleV 2O5 sheet forming slabs that arestacked along thec -axis of a monoclin-ic unit cell. The slabs are separated by water molecules. The basaldistance between the layers depends on the amount of water andincreases by step of about 2.8 Å for each water layer: 11.55 Å forn ≈ 1.5–1.6 and 8.75 Å for  n ≈ 0.5 [30]. Due to the layered structure,vanadium(V) oxide gels are able to intercalate a wide variety of inorganic and organic guest species without change in the one-dimensional stacking of thelayers [6,31–35]. It was reported that the in-tercalation of cations like Na+ and TMA+ (tetramethyl ammonium)into the xerogel leads to the appearance of extra diffraction peaks inthe diffraction patterns of M0.3V 2O5·1.5H2O [31]. Durupthy et al. [31]have suggested that the observation of  hkl set of reections (instead of 00l only) is related to the loss of ordered stacking of the double V 2O5layers. It is important thatour diffractograms resemble to a great extentthe patterns given in the above paper. Based on all above we considerthat our synthesis product in the form of precipitate or  lm is Na+

intercalated vanadium(V) oxide xerogel, NaxV 2O5·nH2O, so that theoxidation number of vanadium in our samples becomes less than 5.

Theincorporation of cations between the layers of the gels is a resultof ion-exchange reactions with the acid protons of the gels, so theamount of the intercalated ions is around 0.3–0.4 per mole of V 2O5[31,33–36]. For the sodium ions the equilibrium amount is found to be0.33 [32]. Concerning our compositions (precipitate and  lm) we haveone more argument in favor of the above ratio: during the annealingat 400 °C they completely transform into a single phase of monoclinicNaV 6O15 (Na0.33V 2O5) (PDF 86–120) without any additional diffractionpeaks (XRDpatters arenot shown)which conrms a Na:Vratioof 1:6 inthe as-prepared compositions.

The formation of vanadium(V) oxide xerogel is further supported byIR spectroscopy (Fig. 2). It is clearly seen thatthe spectral characteristicsof the synthesis product are different from those of the initial reagentNaVO3   (Fig. 2a). The IR spectra of the as-deposited scraped   lm(Fig. 2b) and precipitate (Fig. 2d) are practically identical (within thelimit of the experimental resolution), thus conrming the same phasecomposition of the two samples.

TheIR spectra(Fig. 2b, d) aredominated by strongabsorptions in the1020–400 cm−1 region associated with the vibrations of the vanadium–oxygen framework. The band at 1012 cm−1 is attributed to thestretching vibration of terminal V _O groups (shorter V \\O bond), theband at 763 cm−1 is due to asymmetric stretching vibrations of thebridged V \\O\\V units (longer V \\O bonds), and the band at514 cm−1 is assigned to the V \\O\\V symmetric stretch mixed with

bending vanadium oxygen vibrations   [36–38].   The absorption at917 cm−1 is likely to be associated with a V _O stretch stronglyperturbed by the water molecules [38]. In addition, a weak band near1230 cm−1 appears but its origin is unclear (Fig. 2). The presence of water molecules in the V 2O5·nH2O xerogel is clearly manifested bythe bands at 3590 cm−1 (shoulder) and 3400 cm−1 (OH stretchingvibrations) and at 1614 cm−1 (HOH bending vibration) (Fig. 2b, d).The band near 3600 cm−1 has been assigned to water molecules nearlyfree of hydrogen bonding, which are presumably directly bonded tovanadium through their oxygen atom   [37–39]. The band below3580 cm−1, in our samples at 3400 cm−1 has been attributed to thewater molecules hydrogen bonded with the oxygen either of V 2O5 [37,40] or of other H2O molecules [37–39]. There is a third kind of watermolecules, that also give rise to a νOH band around 3600 cm−1 since

they are not involved in hydrogen bonds [37,38]. These water molecules

Fig. 1. XRD patterns of (a) as-deposited lm, (b) brown precipitate and (c) greenish

precipitate.

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The sensitivity of the vanadium(V) oxide xerogels toward reductionunder storage which is accompanied by progressive color change ingreen after some weeks or months was commented on by Livage [6],but the explanation of this phenomenon is still not clear. Unfortunately,we are not able to specify what species are responsible for the electrontransfer process leading to the V(V) reduction. However, consideringthe easy hydrolysis of the weakly bound water as found in  [43] wecould suppose a possible role of the water molecules and protons in

the reduction process.On the other hand, our IR absorption studies give valuable informa-tion that the degreeof the reduction of V(V)in Na0.33V 2O5·H2O xerogeldoes not increase during the prolonged aging at ambient conditions.

Whereas the vibrations of the V \\O units are not affected, somechanges are observed in the OH stretching mode region (Fig. 2). In thegreenish samples the intensity of the band around 3600 cm−1 is in-creased and the band near 3440 cm−1 is upshifted (Fig. 2c, e) comparedto the respective bands in the yellow/brown samples (Fig. 2b, d).Obviously, some changes related to the amount and state of the watermolecules in the gel occur during storage and to elucidate this phenom-enon we examined the thermal behavior of the greenish sample aged20 days (Fig. 3). As seen the TG curves for the fresh and aged samplesdiffer considerably from each other. Firstly, the total mass loss for thegreenish sample is increased to 10.67% (vs. 8.73% in brown sample)which leads to a higher water content of 1.3 mol. This water amountis distributed in the following proportions: 1.1 mol (mass loss of 9.13%) are released between 40 and 205 °C (rststrong endothermicef-fect at 175 °C), about 0.1 mol (mass loss of 1.32%) between 205 and270 °C and the last amount of 0.1 mol between 270 and 320 °C. Thecomparison in the proportions between the two samples shows thatthe greenish sample contains more amount of water molecules thatare removed below 200 °C than the brown sample, i.e. more amountof weakly bonded water molecules. Therefore, the  “extra” water mole-cules accommodated during the storage are weakly bonded. This nd-ing can explain the observed increase in the intensity of the  ν(OH)band at 3595 cm−1 in the IR spectrum of the aged samples.

From the above data it follows that the vanadium reduction duringaging of our Na0.33V 2O5·H2O is accompanied by an increase in thewater contentto Na0.33V 2O5·1.3H2O. Such phenomenon hasbeen previ-ously reported for reduced xerogels [44]. Babonneau et al. [44] havefound that the reduced gels having V(IV)/V(V) of 16% contain 2.5 H2Oper V 2O5 instead of 1.6–1.8 H2O for gels with usual V(IV)/V(V) about 1to 4%.

4.3. Morphology of Na0.33V  2O5·H  2O thin lms

As described in the literature [6,31] vanadium(V) oxide xerogelsnormally exist in the form of long ribbons. SEM images of two thinlms prepared for the 15 and 30 min deposition times are depicted inFig. 4.

Among the studiedlms these are the lms exhibiting the best opti-cal properties (see Section 4.5) and their thickness determined by theprolometer is about ~150 and~300 nm (for the 15 and 30 min deposi-tion times, respectively). The cross-sectional SEM image of the  lm forthe 30 min deposition time (Fig. 4d) gives a thickness of about 250 nm.

The chemical bath deposition method produces thin  lms whichsurface is completely covered with the deposited material (Fig. 4a, band c). Both  lms are dense and without any porosity. The SEM imageof the thinner  lm recorded at a higher magnication (Fig. 4b) clearlyshows the granular structure of the  lm. It is composed of nanograins,spherical and elongated, with sizes between 50 and 200 nm. Besideswell separated nanograins, randomly oriented ribbon-like units (about200 nm wide and 1–1.5 μ m long) are also visible (Fig. 4a).

The observations by SEM are supported by AFM data for the sameNa0.33V 2O5∙H2O thin  lms (Fig. 5). The 2D surface topography of thethinner   lm (Fig. 5a) shows nanograins with sizes in the range of 50–150 nm. In addition, there are elongated ribbon-like units with thefollowing dimensions: width from 150 to 300 nm and length from 0.7to 2 μ m. The longer deposition time ensures the growth of the grainsand, expectedly, the thicker lm (Fig. 5b) exhibits larger grains withsizes from 250 to 700 nm as well as larger ribbons reaching to 1  μ m

Fig. 4. SEM images of Na0.33V 2O5∙H2O thin  lms with different thickness: (a) and (b) ~150 nm, (c) ~300 nm and (d) cross-section view of the thin lm with ~300 nm thickness.

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wide and 3 μ m long. Moreover, AFM data reveal that the lms exhibitcomparatively high surface roughness with amplitude of the grainheight at around 600 nm for the thinner  lm and 250–500 nm for thethicker one, i.e. the thickness is not uniform throughout the wholelm area. The 3D images, and especially those for the thinner   lm(Fig. 5c), clearly illustrate the way for the formation of the ribbons.We can see that the ribbons arise as a result of the coalescence of 

nanograins in a preferred direction. We suppose that only the grainshaving the most suitable orientation and that are situated closely toeach other are able to coalesce and thus to form long ribbon-like units.Moreover, we can also speculate, that a possible growth mechanism of Na0.33V 2O5∙H2O thin  lms is the  “island” mechanism (Fig. 5d) in whichthe coalescenceof the islands generates ribbon-like units and the latter,at a certain point, will form the compact  lm layer.

4.4. Electrochemical properties

Firstly, we performed CV measurements in large potential range of ±2.5 V with a blank substrate. As expected, we observed only oxygenreduction that begins around−1.5 V and high polarization of the work-

ing electrode in therange of ±1 V with ~0 mA/cm2

current density. This

allows us to perform electrochemical analysis in the relevant voltagerange of ±1 V.

Fig. 6 shows cyclic voltammograms withve scans (10 mV/s) of twoas-deposited lms with different thickness. Both cyclic voltammogramsexhibit three pairs of oxidation/reduction peaks. It is essential that theshape and position of the respective redox peaks are very close for thetwo lms (they differ within ±0.04 V) which reect the same electro-

chemical processes. The anodic peaks (A1, A2, A3) appear at −0.15,0.16 and 0.74 V and the cathodic peaks (C1, C2, C3) are at −0.50,−0.16, and 0.38 V (Fig. 6a). The A2/C2 pair is the most intense one.The color changes observed in the three-electrode cell are the same asthose in the two-electrode electrochromic cell.

The survey from the literature shows that thecyclic voltammogramsof vanadium(V) oxide xerogels display one [45], two [46] and threeredox pairs [2]. The presence of one redox pair has been attributed tothe amorphous nature of the lms [45]. According to Benmoussa et al.[46] the appearance of two redox pairs reects the crystalline natureof the lms. The observation of three redox pairs in our  lms is consis-tent withdata of Costa et al. obtained at thesame scanning speed for va-nadium oxide gel prepared on  exible polyethylene terephthalate/indium tin oxide electrodes [2]. It should be mentioned that the general

view of the CV curves presented by Costa et al.  [2]  also having an

Fig. 5. AFM imagesof Na0.33V 2O5∙H2O thinlms: (a) and (b) are 2D images with prolesof lmswith150and 300 nm thickness, respectively; (c) and (d)are3D images of lmswith150and 300 nm thickness, respectively.

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intensive A2/C2 redox pair as well as the voltage ranges for the anodic(from −0.23 to 0.64 V) and cathodic (from −0.8 to 0.3 V) peaks arevery similar to our data.

The calculation of the exchanged charge showed that the valuesfor the extracted charge for the thin   lm with 150 nm thickness is65.5 mC/cm2 and for the  lm with 300 nm thickness is 75.5 mC/cm2.The inserted charge values are much lower, 19.9 mC/cm2 and22.3 mC/cm2 respectively. The higher values for the exchanged charge

for thicker lms are expected due to their higher capacity.The electrochemical reversibility for 200 cycles was monitored at

scanning rate of 50 mV/s for the  lm with 300 nm thickness (Fig. 7).In conformity with the higher scanning rate all CV curves in  Fig. 7display one broad pair of oxidation/reduction peak instead of thethree pairs well-separated at the lower scanning rate (Fig. 6b). Theoxidation/reduction pair remains stable during the cycling with a slightshift of the peak maximum to lower potentials: from0.48 V/−0.44Vforthe 5th scan to 0.43 V/−0.41 V for the 200th scan. Moreover, the peakmaxima move to higher potentials than those of the most intensiveA2/C2 pair at the lower rate. It is also observed that the peak area(exchanged charges) decreases during the cycling and this process is

more pronounced up to the 100th cycle. Then, the electrochemicalreversibility appears to be stabilized.

As mentioned previously, over time, the as-deposited lms changedits original yellow/yellow-brown color into greenish color. Fig. 8 com-pares the CV curves (up to 20 scans) of a fresh  lm (yellow-brown)and an aged 20 days lm (brown-greenish) with the same thickness.

Opposite to our expectations, the comparison shows that electro-chemical behavior of the fresh and aged  lm during the initial scans

does not differ considerably. In both cases there are three cathodicand three anodic peaks, however, the  rst anodic peak A1 is very welldistinguished in the fresh  lm, while in the aged one it appears as ashoulder to the second A2 peak. The corresponding peak potentialsshow differences within ±0.06 V.

With increasing scan number a clear difference between the freshand aged  lm is observed (Fig. 8). During the longer cycling somechanges occur and the fresh lm is more essentially concerned. It isclearly seen (Fig. 8a) that after 15 scans some peaks disappear in theCV curves of the fresh   lm. The A3/C3 pair surely disappears, theA1/C1 pair strongly diminishes and the A2 peak is shifted to more pos-itive valuesof thepotential. Thus, thecyclicvoltammograms of thefreshlm transform from a system with three redox peaks to a system withonly one redox pair, A2/C2, which is characteristic of amorphous  lms[47]. At the same time the agedlm exhibits more stable cycling behav-ior and only the redox A1/C1 pair decreases in intensity. Regarding theexchanged charges, going from the 1st to the 20th scans for the freshlm the extracted charge slightly decreases from 41 to 37 mC/cm2,while the decrease in the inserted charge is more pronounced (from15 to 8 mC/cm2). The lm aging causes a reduction mostly of the ex-tracted charges (from 34 to 30 mC/cm2 for 5th and 20th scans, respec-tively), while the inserted charges are similar to those for the freshlm (from 14 to 8 mC/cm2 for 5th and 20th scans), respectively.

The peak disappearing in the CV curves with the increasing scannumber is observed by other scientists in the cases of vanadium oxidexerogels [2,48] but the origin of this phenomenon is still not clear.Several possible reasons are commented in the literature. The deactiva-tion of some redox sites of the   lm could be related either toamorphization of the   lm   [45]   or to crystalline phase transition

Fig. 6. Fivecyclicvoltammograms of Na0.33V 2O5∙H2O thinlms with thickness:(a) 150nmand (b) 300 nm at a scanning rate of 10 mV/s.

Fig. 7. Cyclic voltammograms to 200scansof thinlm with300 nm thickness ata scanning

rate of 50 mV/s.

Fig. 8. Cyclic voltammograms of thinlmswith300 nm thickness: (a) freshlm, 1st to 4th

and 16th

and 20th

scans, (b) aged lm, 5th

, 7th

, 10th

, 15th

and 20th

scans (10 mV/s).

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triggered by the oxidation/reduction cycles [2] or to changes in mor-phology and microstructure of the  lm leading to the formation of amore  “homogeneous”  lm [48].

The observed redox peaks in the potential range ±1 V forNa0.33V 2O5∙H2O thin lms are related to step-wise reduction of V(V) toV(IV) concomitant with formation of different crystalline states LixV 2O5and, accordingly reversible V(IV) oxidation and Li-deintercalation[46,49,50].

4.5. Optical properties

Optical transmittance spectra in the 350–900 nm spectral range of the as-deposited thin  lms are given in Fig. 9. It is important that thefour  lms exhibit very similar dependence of the optical transmittanceon the wavelength, but the overall transmittance decreases with thelm thickness. This is expected since more states in the thicker  lmare available for the photons to be absorbed. The thin  lms have rela-tively high transmittance (low absorbance) at wavelengths longerthan 550 nm as the thinner  lms (obtained for 5, 10 and 15 min depo-sition time) exhibit transmittance between 70 and 90%, while the thicklm (30 min deposition time) exhibits a lower transmittance between55 and 70%. The marked reduction in the transmittance at wavelengthsshorter than 550 nm is associated with the fundamental absorption. It isessential that the absorption edge for the fourlms appears in a narrowinterval of wavelengths (365–385 nm). This means that the  lms withdifferent thicknesses should have very close values of the optical bandgap which implies the same or very close stoichiometry regarding thedifferent vanadium sites. In this regard we have also compared the IR spectra of the scraped lms obtained for the5, 15 and 30min depositionandwe have notfound anyvariationin thepositions andintensity of thevibrational bands in comparison with the IR spectrum presented inFig. 2b. Considering these data we believe that the change of the  lmcolor with the time of deposition from yellow to yellow-brown shadeis mainly related to the  lm thickness and grain growth evidenced byAFM analysis (Fig. 5a, b) rather than the same changes in thevanadiumoxidation state during the deposition.

The optical transmittance of Na0.33V 2O5∙H2O thin   lms in the

350–900 nm range is recorded in a two-electrode electrochromic cellusing LiClO4/PC electrolyte. Our previous studies on (NH4)xV 2O5·1.3H2O thin  lms in the voltage range between ±1 and ±2.5 V haveshown that the increased voltage results in increasing transmittancevariance (ΔT ) and shorter bleaching and coloration response times [30].

It is important to notice that the current densities achieved in theused two-electrode electrochromic cell are around 0.27 mA/cm2 at+2.5 V (at the highest voltage), but these values are still much lowerthan those achieved in the three-electrode cell (CV measurements):around 1.5 mA/cm2 ata signicantly lower voltage of 0.3 V. This fact ex-plains the longer response time (Fig. 10) observed in the two-electrodeelectrochromic cell.

The response time (τ ) is calculated as the time required for the thinlm to change itscolor fromyellow/brown to obtain90% [51] ofthebluecolor (τ c,90%  —  coloration response time) or vice versa, to obtain 90% of the yellow/brown color (τ b,90%  — bleaching response time). It is seenin Fig. 10 that thebleachingresponsetimeτ b,90% is 10min,whilethe col-oration response timeτ c,90% is 17.5 mini.e. thereduction process follow-edwith Li+-intercalation is almost two times slower than the oxidationprocess followed with Li+-deintercalation.

The optical transmittance spectra recorded at ±2.5 V with aswitching time of 20 min of four Na0.33V 2O5∙H2O thin  lms preparedfor different deposition times are shown in Fig. 11. It isseen thatthe de-pendence of the transmittance of the reduced forms on wavelengthpasses through a maximum at 530 nm (Fig. 11a) and 560 nm(Fig. 11b, c, d). The oxidized forms exhibit a steep increase in the trans-mittance up to 600 nmwithfurther gradualincreases up to 900 nm. It isimportant that the fundamental absorption edge for both reduced andoxidized  lms exhibits a red shift (to lower energy) with the increasein the lm thickness. The red shift of the absorption edge reects a de-crease in the optical band gap and this effect has been attributed tothe increase in the grain size and effective decrease in the imperfections

at the grain-boundary regions [52].Transmittance variance (ΔT ) dened as: ΔT  = T (bleached)− T (colored)

is used to quantify the electrochromic effect of the prepared thin  lms.For all thin   lms the maximum   ΔT   value is obtained at 900 nm(Fig. 11). The three thinner  lms exhibit close ΔT  values of 32–37%,while for the thicker  lm having 300 nm thickness  ΔT  reaches 55%.For the latter  lm the optical response during a prolonged cycling upto 500 cycles was also examined and the corresponding optical trans-mittance spectra are included in Fig. 11d. As seen the oxidized state iscompletely recovered during the cycling and the transmittance curveafter 500 cycles well matches the initial curve. Some changes occur inthe reduced states between the 5th and 100th cycles resulting in an in-creasein thetransmittance between 400and 500nm andshiftof theab-sorption edge to lower wavelengths (high energy) as well as a slight

decrease in the transmittance between 500 and 700 nm. The observedblue shift of the absorption edge reects a blue shift of the opticalband gapand this shift could be related to thechange of thelm micro-structure andincrease in the imperfections during thelithium intercala-tion process [53]. Further changes between 100th and 500th cycles arenot observed, so that the reduced state is stabilized which is evidentfrom the overlap of the corresponding transmittance curves.

Therefore, the data for the optical stability of the thin  lm after the100th cycle in the electrochromic cell are in line with the data for itselectrochemical stability. Between 700 and 900 nm the transmittanceof the reduced forms in all cycles shows the same trend as the initialscan. It is very important that the  ΔT  value after 500 cycles is 53% at900 nm i.e. it retains practically unchanged which is a demonstrationof the good optical stability of as-deposited   lms. The comparison

with the literature data for electrochromic vanadium(V) oxide xerogelsFig. 9. Optical transmittance spectra of as-deposited thin  lms with time of deposition.

Fig. 10. Responsetimeof Na0.33V 2O5∙H2O thin lm with 300nm thickness at 900 nm at±2.5 V.

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[47,51,54,55] shows that the obtained results are promising and canserve as a base for further design of electrochromic devices.

As was mentioned before, the as-deposited yellow/yellow-brown

lms turn greenish tint over time. Fig. 12 shows the change in the opti-cal transmittance over time (up to 20 months) of the best  lm with300 nm thickness. The optical transmittance spectra evidence thataging does not affect the transmittance in the 350–550 nm spectral re-gion. As expected from the greenish shade of the aged  lm, however, agradual decrease of the  lm transmittance at wavelengths longer than550 nm (i.e. an increased absorption of the red light) is observed asthe transmittance reduction in 20 days aging is about 10% at 900 nm.It is also seen that the transmittance reduction is most signicant upto 10th day while the  lm structure still stabilizes (about 8%) and afterthat it is only about 2%. Further decrease of the transmittance is not re-corded even for a prolonged aging of 20 months (Fig. 12). It is very im-portant, however, that when such a  lm aged 20 days is placed in theelectrochromic cell (Fig. 12b) it is completely recovered in its oxidized

state and thus ΔT  retains its high value (56%).

5. Conclusions

Well-structured nano-sized thinlms withcompositionsNa0.33V 2O5·nH2O (n = 1 and 1.3) are successfully deposited by an optimized chem-ical bath method where the acidication of NaVO3   solution occursthrough the hydrolysis of diethyl sulfate. The thin lms demonstrate ap-preciable optical transmittancevarying between 40 and70% at 500nm independence on the  lm thickness. The lm structure and morphology,electrochemical behavior and electrochromic activity of fresh and agedthin  lms are studied. It is established that the long-term aging of thelms does not affect their electrochromic properties. The thin  lms ex-hibit maximum transmittance variance (ΔT ) at 900 nm which varies be-

tween 32 and 55% depending on the lm thickness. The electrochemical

Fig. 11. In-situ optical transmittance spectra of Na0.33V 2O5∙H2O thin  lms prepared for different deposition times: (a) 5 min, (b) 10 min, (c) 15 min and (d) 30 min; after 5 cycles in(a, b, c) and up to 500 cycles in (d).

Fig. 12. Optical transmittance spectra of as-deposited thin  lm with 300 nm thickness:

(a) overtime and(b) brown-greenish lm aged20 daysplacedin the electrochromiccell.

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reversibility and electrochromic reproducibility with ΔT  retention up to500 cycles have beendemonstrated. These characteristicscan be attribut-ed to the well-dened structure and nanosized morphology of the thinlms provided by the suitable preparation method. The results obtainedare good prerequisite for further application of Na0.33V 2O5·nH2O thinlms in different electrochromic devices.

 Acknowledgment

The authors thank the Bulgarian Academy of Sciences and theMacedonian Academy of Sciences and Arts for the  nancial supportand the Alexander von Humboldt Foundation for providing the electro-chemical equipment.

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