Appl Phys ADOI 10.1007/s00339-009-5334-8

From beads-to-wires-to-fibers of tungsten oxide:electrochromic response

P.M. Kadam · N.L. Tarwal · P.S. Shinde · R.S. Patil ·H.P. Deshmukh · P.S. Patil

Received: 22 December 2008 / Accepted: 1 July 2009© Springer-Verlag 2009

Abstract Suitable host lattice and morphology for easy in-tercalation and deintercalation process are crucial require-ments for electrochromic device. In this investigation, theevolution of structural and morphological changes and theireffect on electrochromic (EC) properties of spray-depositedWO3 thin films are studied. Films of different morpholo-gies were deposited from an ammonium tungstate precur-sor solution using a novel pulsed spray pyrolysis technique(PSPT) on tin-doped indium oxide (ITO) coated glass sub-strates by varying quantity of spraying solution. Interestingmorphological transition from beads-to-wires-to-fibers as afunction of quantity of sprayed solution has been demon-strated. The porosity, crystallinity and “open” structures inthe films consisting of beads, wires, and fiber-like mor-phology enabled us to correlate these aspects to their ECperformance. WO3 films comprising wire-like morphology(20 cc spraying quantity) exhibited better EC properties both

P.M. KadamDepartment of Electronics, Smt. K.W. College, Sangli 416 416,Indiae-mail: kprakash5229@rediffmail.com

N.L. Tarwal · P.S. Shinde · P.S. Patil (�)Thin Film Materials Laboratory, Department of Physics,Shivaji University, Kolhapur 416 004, Indiae-mail: psp_phy@unishivaji.ac.in

P.S. Shindee-mail: pravinshindephysics@gmail.com

R.S. PatilDepartment of Physics, The New College, Kolhapur 416 012,India

H.P. DeshmukhDepartment of Physics, Y.M. College, Pune, India

in terms of coloration efficiency (42.7 cm2/C) and elec-trochemical stability (103 colored/bleached cycles) owingto their adequate open structure, porosity, and amorphicity,compared with the films having bead/fiber-like morphology.

PACS 61.05.Cp · 68.37.Hk · 64.70.Nd · 68.55.-a ·82.45.Yz

1 Introduction

Electrochromism is a phenomenon which involves persistentand reversible change of color in an electrochromic (EC)material by simultaneous injection of electrons and smallions (H+, Li+, Na+, etc.) [1]. EC materials have been at-tracting sustained interest during the last few years and showa great potential future [2, 3] in the display industries andwindow applications. Among various transition metal ox-ides, tungsten trioxide (WO3) has been predominantly ex-ploited owing to its cyclic stability and transmission in thebleached state combined with its high coloration efficiencyoccurring in the visible and infrared regions [1, 4]. Ab-sorption modulation is believed to be responsible for elec-trochromism in amorphous state of WO3, while colorationin crystalline phase occurs due to reflectance modulation[1, 5].

EC devices based on WO3 offer advantages such as lowpower consumption, open circuit memory effect, and a fullangle display and consequently find applications in smartwindows, reflectance variable mirrors, and information dis-plays [1, 6–9]. However, some drawbacks such as slow colorresponse and low contrast ratio have hindered their applica-tions on commercial level and allow scope for further re-search in improvement of these parameters. The easiest wayto override these drawbacks is to obtain large surface area

P.M. Kadam et al.

with adequate porosity and short diffusion distance of in-jecting ions [10]. The solid state diffusion of ions into anEC material is limited by two factors, the diffusion con-stant, which depends on the crystal structure of the mate-rial, and the length of the diffusion path, revealed by themicrostructure/porosity of the film [11]. In other words, thesurface morphology and microstructure of WO3 determineits EC performance. The films that possess an “open” struc-ture caused by extensive grain boundaries facilitate the in-tercalation of metal ions responsible for coloration [12, 13].The rate of intercalation depends largely on the structure ofthe WO3 films and thus the preparation conditions and themobility of the inserted atoms/ions in the film [14].

Recently, one-dimensional (1D) nanostructures (wires,tubes, ribbons, and fibers) have attracted considerable inter-est due to their high aspect ratio, large surface areas, andunique physical properties. Ha et al. [15] reported the syn-thesis of 1D h-WO3. Zhang et al. [16] obtained micron-sizedmonoclinic and hexagonal h-WO3 tubes. Hollow tubular ortunneling morphologies of h-WO3 can directly be associ-ated with their possible usage as better intercalation hostmaterials, and their high porosity can be expected to ful-fill the demand of high efficiency and activity. MesoporousWO3 exhibits superior high-rate ion-insertion performancefor their short-diffusion length of lithium ions into EC films[17]. Leftheriotis and Yianoulis [18] obtained grape-likestructures emerging over the surface of the film. Films withsuch conglomerates incorporated into their structure havea high effective surface area, enhanced ion mobility, andimproved EC properties. Hence, the film structure and thesurface morphology play a decisive role in EC performance(coloration efficiency, optical modulation, reversibility, andkinetics of coloration–bleaching process involving diffusionof ions) of WO3, which in turn depend mainly on the typeof deposition method and the experimental parameters. Jo-raid [19] reported the highest coloration efficiency (η) of122 cm2/C for electron-beam-deposited crystalline WO3

films with transmission modulation (�T ) of 48% with H+as intercalating ions in 0.1 M H2SO4 electrolyte. However,with Li+ ions the highest value of η (∼110 cm2/C) wasobtained in 1 M LiClO4 + PC electrolyte with �T being77% [20]. The highest �T of 85% at λ = 642 nm and re-versibility of 95% was obtained for WO3 films prepared bysurfactant-assisted sol–gel method with relatively low valueof η (63.7 cm2/C) [21]. The highest value of diffusion co-efficient (D) of 0.85 × 10−9 cm2/s for intercalation of Li+ions into the spin-coated WO3 films was observed. How-ever, the �T at λ = 632.8 nm was 62.4% with drop in η at51.1 cm2/C [22]. Hence, looking at the recent state-of-the-art of the electrochromism for WO3 films, one can concludethat there is lot of scope for improvement of EC parametersand hence EC performance by monitoring the synthesis andapt film properties.

Many research articles have been devoted to a better un-derstanding of the correlation between the experimental pa-rameters and film properties, and precise control of the de-position process. However, little is known and is investi-gated so far about the effect of morphology on the elec-trochromism of WO3 film. Several methods such as wetchemistry methods [21], physical vapor deposition [23], andchemical vapor deposition [24] have been employed forgrowing WO3 films. Understanding materials, large area de-position, and low production cost are the prerequisites forcheap production of EC devices if the technology has to beused beyond narrow niche markets. The PSPT, in which thespray rate can be easily controlled by adjusting spray ON-OFF time, will be a suitable processing technique for theEC device fabrication. The PSPT, designed and patented byus [25] is less capital-intensive and also offers good-qualitythin films on large substrate area using low-price precursor.It also facilitates precise control over droplet residence time.

In our previous work [26], we reported synthesis of fi-brous reticulated morphology of WO3 and studied the ECphenomenon using H+ ions as intercalating agents exhibit-ing coloration efficiency of 34 cm2/C. However, in thispaper we have demonstrated variations in the spray de-posited WO3 morphologies that changes from micro-beads-to-wires-to-fibrous. Since the EC properties depend pro-foundly on the surface morphology and subsequent porosity,in-depth study has been undertaken to reveal the changesin the EC parameters associated with such morphologies.Therefore, it is aimed to prepare WO3 thin films at differ-ent spraying solution quantities by PSPT, to examine theirresultant morphologies and correlate them to the EC perfor-mance. The detailed EC characterizations revealed that thesamples with wire-like morphology are best suited to thesmart window application.

2 Experimental details

WO3 thin films were grown on tin-doped indium oxide(ITO) coated glass and bare glass substrates using a PSPT,the schematic of which is shown in Fig. 1. Details of the filmsynthesis are described elsewhere [26]. In brief, the precur-sor solution used for deposition of the films was preparedby dissolving pure WO3 powder in ammonia and distilledwater and then heated to 90°C, at which the formation ofammonium tungstate is completed. The substrate tempera-ture was maintained at 450°C during the deposition. Com-pressed air was used as a carrier gas for the precursor solu-tion delivery, and air flow rate was maintained at 15 cc/minduring the deposition. In order to avoid local cooling ef-fect of the substrate caused by compressed air, the spray-ing cycle of 7 s (spray-ON) followed by short periods of7 s without spraying (spray-OFF) was maintained through

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

out the deposition with the help of electromagnetic solenoid.The solution quantity was varied as 10, 20, 30, 40, 50, and60 cc, and the samples are denoted by W10, W20, W30, W40,W50, and W60, respectively. Film thickness was measuredusing weight difference method and confirmed by AmbiosXP-1 surface profiler. The films were transparent, uniform,and well adherent to the substrates whose adherence waschecked by a scotch tape test. The structural properties ofthe films were studied from X-ray diffraction (XRD) pat-terns recorded using an X-ray diffractometer (Philips, PW3710, Almelo, Holland) operated at 25 kV, 20 mA withCuKα radiation (λ = 1.5406 Å). The surface morphologyof the films was examined by analyzing the scanning elec-tron microscopy (SEM) images recorded using a scanningelectron microscope (Model JEOL-JSM-6360, Japan), op-erated at 20 kV. The optical transmittance was measuredusing a UV–Vis spectrophotometer (Systronics, model 119,Bangalore, India) in the wavelength range of 350–1000 nm.All electrochromic measurements were performed in anelectrolyte of 0.5 M LiClO4 in propylene carbonate (PC)in a conventional three-electrode arrangement comprisinggraphite as the counter electrode, WO3 film (2.5 cm2 ac-tive area) coated on ITO as the working electrode and sat-urated calomel electrode (SCE) serving as the referenceelectrode. The cyclic voltammetry (CV) and chronoamper-ometry (CA)/chronocoulometry (CC) experiments for elec-trochemical injection or extraction of electrons and Li+ions were conducted at the scan rate of 20 mV/s using aVersaStat-II (EG&G Make PAR 362), computer-controlledby M270 software. The potential was swept between anodic(+0.8 V) and cathodic (−0.5 V) cycle with respect to SCE.

Fig. 1 Schematic representation of pulsed spray pyrolysis technique(PSPT)

3 Results and discussion

All the WO3 films are adherent to the substrate, and the filmthickness estimated using the gravimetric technique variesfrom 251 to 741 nm. Figure 2 shows the XRD patterns ofthe WO3 thin films deposited on glass substrate, with differ-ent solution quantities. The samples W10 and W20 are foundto be amorphous. The occurrence of broad hump (in the 2θ

range of 15–38° centered at 25° is due to the glass sub-strate. The samples W30, W40, W50, and W60 exhibit poly-crystalline nature with well-defined reflexes along (110). Anadditional peak along (112) emerges for W40. Three morepeaks along (002), (220), and (310) are also observed forW50 and W60 samples. The planes correspond to the charac-teristic peaks of monoclinic WO3 phase in accordance withJCPDS No. 01-87-2396.

Morphology of WO3 thin films deposited onto the ITOcoated glass substrates is examined using SEM. Figure 3ashows clear formation of micron-sized beads nucleated ontothe substrate for a sprayed solution quantity of 10 cc. Thebeads appear to be of 0.1–0.3 μm size with apparentlyno agglomeration. The indication of onset of interlacingof the beads suggests that the wires are originated fromthese beads. These beads thus further grow into wires of0.25μm thick, for 20 cc solution, as shown in Fig. 3b. Uponincreasing the solution quantity to 30 cc, fibrous reticu-lated morphology is observed (Fig. 3c–f). Further increasein solution quantity leads to (i) increment in the width offibers from 0.5 to 3.0 μm decreasing the population den-sity, (ii) branched reticulates, and (iii) extended orderedfibers. Such novel morphology facilitates the control oversurface area and porosity/open structures, thus affectingthe ion insertion kinetics (ion diffusion length and time,ionic mobility, etc.) that may lead to enhanced EC perfor-mance.

Fig. 2 X-ray diffraction patterns of the as-deposited WO3 thin filmsdeposited on glass substrate at 450°C, (a) W10, (b) W20, (c) W30,(d) W40, (c) W50, and (e) W60

P.M. Kadam et al.

Fig. 3 Scanning electronmicroscopy images of the WO3thin films deposited onto theITO-coated glass substrates at450°C, showing differentmorphological features such asbeads, wires, or fibers forsamples: (a) W10, (b) W20,(c) W30, (d) W40, (e) W50, and(f) W60

Figure 4(a–f) shows the cyclic voltammograms (CVs) forall the samples recorded at the scan rate of 20 mV/s in0.5 M LiClO4 + PC electrolyte with linear potential sweepbetween +0.8 and −0.5 V versus SCE. The arrows indicatescan direction of CV curves. The background current is ofthe order of 10 μA. The nature and shape of CVs illustratingreduction upon intercalation and oxidation upon deintercala-tion processes are similar to the reported WO3 samples [18,26, 27]. However, the magnitudes of terminal cathodic andanodic peak current densities vary with the sprayed solutionquantity and reaction kinetics. Moreover, the area of the CV,which is a measure of the charge intercalated into the film, islarger for W20 sample and is smaller for other samples (W10,W30–W60). This can be attributed to the decrease in ion mo-bility in other samples [18]. The cathodic current density in-creases from 1.01 to 1.75 mA/cm2 with increase in sprayedsolution quantity for samples W10–W60. This indicates thationic intercalation depends on the amount of active massdeposited onto the substrate. However, marginal change in

the magnitude of anodic peak currents (0.266 mA/cm2 forW10 and 0.09 mA/cm2 for W60) indicate that deinterca-lation process does not follow the same behavior with themass deposited. W20 and W30 samples have distinguish-able anodic peak which apparently wanes for other sam-ples. This reveals that for W40, W50, and W60 samples,deintercalation process becomes kinetically sluggish. Thereason is not yet clear, but the resultant morphology, to-gether with amorphicity of W20 and W30 samples, facil-itates (de)intercalation kinetics by coupling adequate de-gree of porosity with open structure. Various authors havehighlighted importance of loose/open structure and porosityof the WO3 samples that augment overall EC performance[12–14, 16–18, 28–30].

Findings from CV studies are further substantiated withchronocoulometry (CC) curves recorded in 0.5 M LiClO4 +PC electrolyte upon application of a step potential of ±0.5 Vversus SCE for 10 s as shown in Fig. 5. From CC experi-ments it is seen that there is gradual increment in interca-

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

Fig. 4 Cyclic voltammogramsof the WO3 samples: (a) W10,(b) W20, (c) W30, (d) W40,(e) W50, and (f) W60, recordedin 0.5 M LiClO4 + PCelectrolyte. The potential isswept from +0.8 to −0.5 Vversus SCE at the scan rate of20 mV/s

lated charge density, while deintercalated charge density de-creases for W10, W20, W30, W40, W50, and W60 samples.This observation leads to the conclusion that the wire-likemorphology is beneficial for the reversible (de)intercalationkinetics (W20 sample), whereas relatively fibrous and denserfilms (W30 to W60) favor intercalation process, but deinter-calation process becomes kinetically sluggish. These resultsecho the aforementioned findings from CV.

To obtain the chemical diffusion coefficient of Li+ inWO3, CVs at different scan rates are recorded. Figure 6shows the cyclic voltammograms recorded at different scanrates for the sample W20. The area of the voltammogramincreases with increasing scan rate, and the anodic peak cur-rent values shift towards higher potentials. Such behavior islikely and is similar to equivalent results reported by oth-ers [22, 26, 31]. Inset of Fig. 6 shows the plot of peak cur-rent density versus square root of scan rate for W20 sample.The anodic peak current values in the CVs of all sampleshave been used to determine the diffusion coefficient dur-ing intercalation and deintercalation of Li+ ions accordingto Randles–Sevcik equation (1) [31],

ip = 0.4463nFCA

√nFνD

RT(1)

Fig. 5 Chronocoulometry curves of the WO3 samples: (a) W10,(b) W20, (c) W30, (d) W40, (e) W50, and (f) W60, recorded in 0.5 MLiClO4 + PC electrolyte upon application of step potential of ±0.5 Vversus SCE for 10 s

where ip is the peak current density (mA/cm2), n is thenumber of electrons involved in the reaction of the redoxcouple, F is Faraday’s constant (96,485 C), C is the con-centration of active ions in the solution (M), A is the work-ing electrode area (cm2), ν is the rate at which the potential

P.M. Kadam et al.

is swept (V/s), D is the diffusion coefficient (cm2/s), R isthe universal gas constant (8.314 J/K), and T is the absolutetemperature (K). Linear variation of ip with root scan rate in-

Fig. 6 Cyclic voltammograms of W20 sample recorded in 0.5 MLiClO4 + PC electrolyte at different scan rates with a potential win-dow from +0.8 to −0.5 V versus SCE. Inset shows the plot of anodicpeak current density versus square root of scan rate for the sample W20,interpolated through the (0,0) axis for determination of diffusion co-efficient

dicates that the cyclic process is diffusion controlled. Left-heriotis et al. [31] showed that diffusion coefficient deter-mined from Randles–Sevcik method is more credible whenobtained at lower scan rate (2 mV/s or lower). Therefore,the curve in the inset of Fig. 6 is interpolated through (0,0)

axis to estimate the value of ip and to determine the diffu-sion coefficient at the scan rate of 2 mV/s. The value of D

for sample W20 determined at 2 and 100 mV/s scan rates is1.32 × 10−11 and 17.74 × 10−11 cm2/s, respectively. Left-heriotis et al. [31] obtained similar D values for the electron-beam-deposited WO3 films.

Figure 7 shows the transmission spectra of all the WO3

samples in their colored and bleached states recorded inthe wavelength range of 350–1000 nm after electrochemi-cal polarization in 0.5 M LiClO4 + PC electrolyte upon ap-plication of ±0.5 V versus SCE for 10 s. The sample W10

having bead-like morphology exhibit relatively less opticalmodulation (�T ) due to excess porosity and less active de-posited mass onto the substrate. The visible transmittance ofthe W20 sample at λ = 630 nm in its bleached state is 80%.The sample W20 exhibits better electrochromism, illustratedby intense blue coloration upon cathodic polarization. Uponcoloration (i.e., with increasing Li+ intercalation), the visi-ble transmittance at λ = 630 nm is attenuated to 48%, show-

Fig. 7 Optical transmissionspectra of the WO3 samples:(a) W10, (b) W20, (c) W30,(d) W40, (e) W50, and (f) W60 intheir colored and bleached statesrecorded in the wavelengthrange of 350–1000 nm

From beads-to-wires-to-fibers of tungsten oxide: electrochromic response

ing �T of 50%. This attenuation in case of amorphous WO3

films is believed to be due to the absorption by W5+ byforming tungsten bronze (LixWO3−x) after breaking up thecontinuous random network of W–O–W [32].

The samples W30 to W60 exhibit low-average visibletransmittance in the bleached state and less �T after col-oration. Scattering of light becomes predominant for suchsamples with fibrous morphology and hence these samplesexhibit low-average visible transmittance. Less optical mod-ulation is due to inefficient coloration process caused byinadequate porosity and open structure. �T of 19% is ob-served for PSPT grown WO3 films (prepared relatively atlow substrate temperature of 300°C) for a spraying solutionquantity of 20 cc [26],

η = �ODλ=630 nm

Qi

= log(Tb/Tc)

q/A(2)

where Tb and Tc are the transmittances of the film in thebleached and colored states, respectively. High value of η ofEC film means that EC device should have adequate opti-cal regulation upon less intercalation of charges and caus-ing a better stability and reproducibility of colored/bleached(c/b) cycles. The value of η determined at λ = 630 nm from(1) is 19 cm2/C for W10, being optimum for W20 (i.e.,42.7 cm2/C) and then decreased to 14 cm2/C for W60 sam-ple. The η in WO3 thin films is strongly dependent on thestoichiometry, grain size, morphology, and the presence ofany long range order. The relatively high W5+ content inW20 sample owing to presence of open structures and ade-quate degree of porosity is responsible for its superior col-oration efficiency than other samples. The η value obtainedfor W20 sample is better than that of earlier reported val-ues of 34 and 42 cm2/C, respectively, for PSPT grown [26]and conventional spray pyrolysis methods in 0.1 M H2SO4

electrolyte [33, 34]. The relationship of porosity, ionic dif-fusion, and open structure of WO3 thin films to enhance ECproperties have also been reported by others [35, 36].

The electrochemical stability of all the WO3 samples isalso studied in 0.5 M LiClO4 + PC. Figure 8 shows an over-lay of the CV curves recorded after 1st and 1000th c/b cyclesfor W20 sample. The shape of the CV remains practicallyunchanged over extended cycling up to 103 c/b cycles. How-ever, in case of W30–W60, the inserted charges accumulateafter every c/b cycle due to sluggish deintercalation kineticsof these samples and therefore are relatively less stable thanW20 sample. Hence, the W20 sample having wire-like mor-phology is better candidate for ECDs due to superiority of η

and electrochemical stability.

4 Conclusions

To summarize, WO3 thin films with various interestingmorphologies are deposited by a pulsed spray pyrolysis

Fig. 8 Overlay of cyclic voltammograms for the W20 sample in0.5-M LiClO4 + PC electrolyte recorded after 1st and 1000th col-ored/bleached cycles, demonstrating the electrochemical stability ofthe film

technique at 450°C using ammonium tungstate. Films areadherent to the substrates having film thickness of 251–741 nm. Films having beads and wires are x-ray amor-phous, while those having fibrous morphology are polycrys-talline with monoclinic crystal structure. The visible trans-mittance, porosity, and crystallinity vary with the quantityof sprayed solution. An interesting morphological transfor-mation from beads-to-wires-to-fibers has been establishedby varying the solution quantity. The electrochromic per-formance of WO3 films with their different morphologiesis investigated in 0.5 M LiClO4 + PC electrolyte by cyclicvoltammetry. Morphology plays decisive role in determin-ing the electrochemical activity of the films; it is betterfor wire-like morphology (20-cc spraying quantity) and be-comes sluggish for fiber-like morphology. WO3 film com-prising wire-like morphology is superior both in terms ofcoloration efficiency (42.7 cm2/C) and electrochemical sta-bility (103 colored/bleached cycles), owing to adequate de-gree of porosity and “open” structures coupled with amor-phicity.

Acknowledgements Authors wish to acknowledge the UniversityGrants Commission, New Delhi, India for the financial support throughthe UGC-DRS-II phase, ASIST programme, and Department of Sci-ence and Technology through DST-FIST programme.

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