Effect of stabilizer on structural, optical and electrochemical properties of sol–gel derived spin...

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Solar Energy Materials & Solar Cells 88 (2005) 47–64

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Effect of stabilizer on structural, optical andelectrochemical properties of sol–gel derived spin

coated TiO2 films

Amita Vermaa, S.B. Samantaa, A.K. Bakhshib, S.A. Agnihotrya,�

aNational Physical Laboratory, Electronic Materials Division, Dr. K.S. Krishnan Road, New Delhi 110012, IndiabDepartment of Chemistry, University of Delhi, Delhi 110007, India

Available online 2 December 2004

Abstract

Thin films of TiO2 were prepared using two different sol–gel routes. The two routes employed

diethanolamine (DEA) and acetylacetone as stabilizing agents with titanium isopropoxide

(Ti(OPri)4) in ethanol as the deposition solution. The densification at 500 1C achieved the

nanophase TiO2 films, which were investigated by performing structural, optical and

electrochemical studies. Ion storage capacity and transmission measurements showed superior

response of the films derived from DEA. Between the films obtained from the two routes, the

appearance of the rutile phase at lower temperature for the film synthesized using DEA was

predicted on the basis of the thermal analysis of the corresponding xerogel. The nanocrystalline

nature of the films was evident from the X-ray diffraction, atomic force microscopy, and

scanning electron microscopy. The films deposited from both the stabilizers exhibited

electrochromism in 1 M LiClO4-propylene carbonate electrolyte on cathodic polarization.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Sol–gel; Electrochromism; Electrochromic electrode; TiO2; Anatase

1. Introduction

Sol–gel process involving hydrolysis and condensation of the metal alkoxidesfollowed by treatment at elevated temperature causing polymerization to produce

see front matter r 2004 Elsevier B.V. All rights reserved.

.solmat.2004.10.006

nding author. Tel: +91 11 25742610x2283; fax: +91 11 25726938.

dress: [email protected] (S.A. Agnihotry).

www.elsevier.com/locate/solmat

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A. Verma et al. / Solar Energy Materials & Solar Cells 88 (2005) 47–6448

the oxide network formation has attracted a great deal of investigations. Tailormaking of the film properties, introduction of porosity in films, chemical andmechanical stability of films, low process cost and possible processing at lowtemperature are some of the few advantages offered by the sol–gel technology.Titanium oxide (TiO2) films are achievable from numerous techniques such assol–gel process [1–5], ion beam sputtering, chemical vapor deposition, activatedreactive evaporation, etc. TiO2 is a wide band gap semiconductor and can absorbapproximately 5% of sunlight, the lower quantum yield has limited its applications.Numerous studies have been focused to improve the photoresponse by dyesensitizing [6], depositing noble metals, surface modification, and doping. Highdielectric constant (e) has contributed its application in micro electronic devices forexample in capacitors and metal–dielectric–semiconductor devices. Other numeroustechnological applications of TiO2 coatings include its prospective use as a materialfor smart windows [7], antireflective coatings [8,9], optical filters [10], in catalysis andsolar cells. Electrochromism in TiO2 electrodes shows dependence on the crystallinestructure with the anatase phase demonstrating this phenomenon. The anatase phasehas ‘zigzag’ rows of large vacant sites for accommodation of H+ and Li+.

The present study reports the preparation of TiO2 coatings by sol–gel spinningtechnique based on two different precursor solutions employing (i) diethanolamine(DEA), (ii) acetylacetone and acetic acid as the stabilizing agents to preventprecipitation of titanium isopropoxide by ethanol. Metal alkoxides are susceptible tonucleophillic attack and are highly reactive due to electronegative OR groupsstabilizing the metal in its highest oxidation state. Acetylacetone is used to stabilizemetal alkoxides by ligand modification. It behaves as a chelating agent and stabilizesthe sol by decreasing the reactivity of the alkoxide. Similarly acetic acid acts as acatalyst and chelating agent in addition to acting as a modifier of the molecularstructure of coating sol. Ethanolamines yield suitable precursor sols for spin coatingsof the oxides by controlling the rate of hydrolysis of the metal alkoxides. DEA isused as a stabilizing agent and inhibitor for the precipitation of oxides on hydrolysisof the alkoxides.

In the present studies properties of the sol–gel spin coated TiO2 thin films obtainedfrom the above-mentioned two precursor sols have been investigated. Thedecomposition process of the precursor sol was followed to determine the formationof crystalline phases. The influence of sol composition on the microstructure of thecorresponding films was examined from AFM, SEM, X-ray diffraction, and Fouriertransform-infrared spectroscopic experiments.

2. Experimental

2.1. Preparation of precursor sol (1)

The starting solution was prepared by mixing DEA with 10 ml of ethanol(C2H5OH) followed by the addition of titanium tetraisopropoxide (TTIP). Afterstirring this solution for 30 min, additional 10 ml of ethanol was added and the

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A. Verma et al. / Solar Energy Materials & Solar Cells 88 (2005) 47–64 49

stirring was continued for 3 h at room temperature. The relative volume ratio of eachchemical in the precursor sol was TTIP: DEA: C2H5OH ¼ 3: 1: 20. The resultant solwas stable for a period of 7 days followed by the occurrence of gelation. Completedrying of the gel thus obtained under ambient conditions caused shrinkage of thenetwork resulting into the xerogels.

2.2. Preparation of precursor sol (2)

The coating solution was prepared by dissolving TTIP in two thirds of the totalethanol volume while stirring, followed by the addition of glacial acetic acid(CH3COOH). The solution was magnetically stirred for duration of 15 min with theremaining ethanol being added and the stirring continued further for another 90 min.The white precipitates obtained were dissolved by drop wise addition of acetylacetone (acac) while stirring. The relative volume ratio of each chemical in thestarting solution was TTIP: acac: C2H5OH: CH3COOH ¼ 1.25: 1: 19: 0.006. Theprecursor sol was stable for duration of few hours and became a xerogel rapidly.

2.3. Preparation of TiO2 thin films

Acetone washed and dried transparent conducting oxide (SnO2: F) coated glasssubstrates and microglass slides were spin coated by precursor sol (1) and (2) at3000 rpm for 35 s. The films obtained were annealed at 500 1C for 5 min in air,inheriting high adherence to the substrates, transparency, chemical and mechanicalstability. An important noticeable observation regarding the films deposited withoutthe stabilizers is the poor adherence of such films emphasizing the importance of thestabilizing agent towards the improvement of the adherence, transparency and thequality of the corresponding films.

2.4. Characterization of films and xerogels

Transmission spectra of the films in the range of 300–800 nm were recorded usingUV 3101 PC Shimadzu Spectrophotometer. The transmission profiles in the coloredand bleached states were obtained in situ in a two-electrode electrochemical cell(built with two flat fused quartz windows) comprising TiO2 films and Pt counterelectrode on application of constant current density of 1 mA cm�2. XRD patterns ofthe films were recorded in the 2y range from 20 to 701 with a Rigaku–Rotaflex RAD-B. Thermal studies (DTA) on xerogels were performed at a heating rate of5 1C min�1 in an inert atmosphere in the temperature range of room temperature to600 1C using a Rigaku Thermoflex PTC-10A. FTIR spectra of the xerogels andthermally treated xerogels were recorded in the wavenumber range of 400–4000 cm�1

on a Perkin Elmer FT-IR Spectrometer SPECTRUM 2000 Spectrophotometer.While spectra of films were recorded in the reflection mode, the spectra for xerogelswere recorded in the transmission mode in the form of KBr pellets. Electrochemicalmeasurements on the TiO2 electrodes vs. platinum in the liquid electrolyte wereperformed on a computer controlled setup consisting of a He–Ne laser source

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(l ¼ 632.8 nm), a Si photodetector together with a versatile micro-controllerbased ECD characterization unit. Multiple step potential cycling wasperformed by applying a square wave potential of amplitude 71.5 V at afixed frequency of 0.0011 Hz. Cyclic voltammetric studies were performed on acomputer controlled OMNI potentiostat. All measurements were performedin an electrolyte of 1 M LiClO4 in propylene carbonate (PC) in a threeelectrode arrangement comprising TiO2 film as the working electrode, aplatinum counter electrode and Ag/AgCl/KCl serving as the reference electrode.The CV experiments were conducted using scan rate of 20 mV s�1 and potentialwas swept between cathodic (1.0 V) to anodic (1.0 V) vs. Ag/AgCl/KCl. Thesurface morphology of the films was observed using scanning electron microscopyon a JEOL JSM 840 scanning electron microscope and atomic forcemicroscopic images for the films were obtained on a Nanoscope II instrument.The thickness measurements were performed with a Rank Taylor–Hobson Talystepinstrument.

3. Results and discussion

3.1. Optical properties

Fig. 1 shows the transmission spectra in the UV–Visible spectral region of theTiO2 films deposited using two different stabilizing agents. In the wholevisible spectral region, the films exhibited good transparency with the transmissionas high as 81.4% and 75.7% at 550 nm for the DEA and acetylacetone derived

Fig. 1. Transmission spectra of TiO2 films derived from the two different stabilizing agents: DEA (- - -)

and acac (—).

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films respectively. Also, the shift in the absorption edge was observed to beinsignificant for the TiO2 films derived from either of the two stabilizing agentssuggesting almost similar optical energy gap for the two films. The diminishedtransmission in the latter film is attributed to light scattering in consequence of itslarger grain size and greater thickness. A single spin resulted in a thickness of 400and 520 nm respectively for the films deposited from DEA and acetylacetone.Additionally, the lower refractive index (1.89 at l ¼ 546.1 nm) of the former film isindicative of higher level of porosity, thereby explaining the phenomenon ofenhanced transmission. The index of refraction for the film obtained fromacetylacetone was determined to be 1.94. The extinction coefficients of the filmsamounted to a same value of 0.009.

3.2. Structural analysis

The X-ray diffraction patterns presented in Fig. 2 display coexistence of bothamorphous and crystalline TiO2 phases proved by the simultaneous presence of thebroad hump in the low 2y region demonstrating short-range order or amorphicityand diffraction peaks assigned to the anatase phase. The film deposited using DEAwas characterized by the appearance of a sharp Bragg peak at 2y ¼ 25.141 (CuKa)featuring the miller index of (1 0 1) indicating the formation of crystalline tetragonalanatase phase of TiO2 [3]. The grain size evaluated from the Scherrer equation alongthe (1 0 1) plane amounted to about 79.4 nm. The film deposited from acetylacetoneshowed the peaks characteristic of the anatase phase at 2y ¼ 25.231 and 48.091(CuKa) oriented along (1 0 1) and (2 0 0) planes respectively. The latter film inheritedgrains 160.8 nm in dimension. Dinh et al. [11] have also carried out the thermal

Fig. 2. X-ray diffraction patterns of the TiO2 films obtained from the two stabilizing agents: (a) DEA and

(b) acac.

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treatment of the dip coated TiO2 films using DEA at 480 1C for 3 h with the resultantappearance of the anatase phase possessing an average grain size of 38 nmalong the (1 0 1) plane. The discrepancy observed in our results and the oneobtained by Dinh et al. is the outcome of the deposition technique employedfor the preparation of the films and the duration of the thermal treatment.Buscema et al. [12] deposited TiO2 films using acetylacetone employing thedip coating technique and have shown the onset of crystallization to the anatasephase in the films at a temperature of 400 1C. The authors also remarked theappearance of the rutile phase in the films at an annealing of 800 1C. Buscema andcoworkers have observed lower threshold temperature for the coexistence of theanatase and the rutile phases in the thermally treated xerogels explained by them tobe the outcome of the substrate, which stabilizes the anatase phase. XRDinvestigations on the films deposited employing spin coating technique from a solcontaining acetic acid by Ozer et al. [2] has revealed the crystallinity to appear at andabove an annealing temperature of 350 1C whereas at a lower temperatureamorphicity prevails.

3.3. Thermal analysis

The structural changes in the xerogels were followed by differential thermalanalysis (Fig. 3). The xerogel from DEA showed endothermic peak positioned at94 1C indicating loss of remnant solvent and water molecules. The transition ofamorphous TiO2 to the anatase phase was evident from the exotherm at 323 1C.Additionally, the intense exothermic peak centered at 561 1C provides evidence for

Fig. 3. Differential thermal analysis of the as-dried xerogels derived using the two stabilizing agents:

(a) DEA and (b) acac (Heating rate: 5 1C/min, nitrogen atmosphere).

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the transformation of the anatase phase in TiO2 to the rutile phase [12]. Thethermogram of the acetylacetone derived xerogel was distinguished by theappearance of the endotherm centered at 227 1C reflecting the removal of solvent,water molecules and the decomposition of the organic moieties. The transition of theamorphous TiO2 to the anatase phase was evident from the exotherm at 482 1C. Theabsence of exothermic peak assignable to rutile phase transition is indicative of lowerthreshold temperature for the formation of crystalline rutile phase in the film derivedfrom DEA. The assignment of the exotherms in the DTA patterns of the twoxerogels is further supported by the XRD results of the corresponding xerogelsthermally treated at 500 1C for 5 min, (not shown here) which show the formation ofanatase phase only.

3.4. FTIR spectroscopic investigations

3.4.1. FTIR spectra of xerogels

The FTIR spectra of the xerogels recorded in the form of KBr pellets in thetransmission mode are illustrated in Fig. 4. The xerogel obtained utilizing DEAshows (Fig. 4(a)) band typical of symmetric N–H stretching at 3401 cm�1. Due to theappearance of O–H stretching in the similar wavenumber region, the band at3401 cm�1 involves mixed contributions from both N–H stretching and O–Hstretching vibrational modes. The band appearing at 1623 cm�1 is ascribed to theN–H deformation. The bands at 1458 and 1384 cm�1 signify C–H deformation. At

Fig. 4. FT-IR transmission spectra of the as-dried xerogels derived using the two stabilizing agents:

(a) DEA and (b) acac.

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1081 cm�1 a band characteristic of C–O stretching or C–H deformation (coupled)appeared. The C–C stretching was evident from the presence of band at 960 cm�1.Out-of-plane C–H deformation was revealed from the band at 670 cm�1. Themedium intensity band at 466 cm�1 is attributed to Ti–O–Ti stretching mode ofvibration. The xerogel from acetylacetone (Fig. 4(b)) featured CQO stretchingmode with a partial single bond character at 1601 cm�1 due to the enol ring instructure (I) and a band at 1526 cm�1 assigned to CQC stretching of the enol ring(structure I). The bands at 1384 and 1360 cm�1 are characteristic of C–Odeformation. The appearance of the bands at 1273, 1183, 1023 and 931 cm�1 are asignature of C–O stretching or C–H deformation (coupled). The out-of-plane C–Hdeformation was evident in the xerogel from the bands at 824 and 661 cm�1. Thebands at 584 and 440 cm�1 are assigned respectively to Ti–O and Ti–O–Ti stretchingvibrational modes. The shift in the position of the bands in the xerogel assigned tothe carbonyl group in acetylacetone, which appear at 1726 and 1706 cm�1 ensuresthe interaction of the stabilizer with the bonding including titanium and oxygen. Thisobservation is suggestive of a reaction between acetylacetone and titaniumisopropoxide.

Further thermal treatment at 500 1C for 5 min resulted into TiO2 network with thecomplete absence of bands signifying the two stabilizers. The spectrum originatingfrom DEA derived xerogel (Fig. 5(a)) exhibited a broad band positioned at 523 cm�1

assigned to Ti–O stretching vibrational mode. The xerogel obtained fromacetylacetone demonstrated Ti–O–Ti stretching vibration centered at 481 cm�1

(Fig. 5(b)). The values depict to the increased growth of the polymeric chains in thelatter thermally treated xerogel. Additionally, thermal treatment at elevatedtemperature of 500 1C ensured the transformation of Ti–O bonds prevalent in thelatter xerogel completely to Ti–O–Ti bonds.

3.4.2. FTIR spectra of films

Fig. 6 shows the FTIR spectra in the reflection mode of the as-depositedTiO2 films derived from the two stabilizing agents. The presence ofband at 3350 cm�1 affirms the existence of water in the DEA derived as-depositedfilm (Fig. 6(a)) and signifies the O–H stretching vibrational mode. The band at1601 cm�1 proved the presence of N–H deformations in the film. Thebands positioned at 1443 and 1374 cm�1 in the diethanolamine derived film areattributed to C–H deformation. The C–O stretching or C–H deformation(coupled) appeared at 1070 cm�1. A weak band at 957 cm�1 is a signature ofC–C stretching in the film. The presence of Ti–O–Ti polymeric chains wasconfirmed by two weak bands at 426 and 405 cm�1. It is noteworthy that the bandsprevalent in the corresponding xerogel show similar positioning, though theirintensity is much stronger. For the film obtained from acetylacetone, the bandcentered at 1620 cm�1 (Fig. 6(b)) signifies the CQO stretching mode of vibration inthe enol ring structure, while the bands at 1542 and 1521 cm�1 are ascribedto the C ¼ C stretching band of the unidentate structure (I). The shift in the positionof the bands prevalent in acetylacetone assigned to the carbonyl group stretching at1726 and 1706 cm�1 confirms the interaction of acetylacetone as a chelating ligand in

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Fig. 5. FT-IR transmission spectra of the xerogels thermally treated at 500 1C for 5 min derived using the

two stabilizing agents: (a) DEA and (b) acac.


the as deposited acetylacetone derived film.

The absence of the band near 1700 cm�1 in the as-deposited film affirms thepresence of enolic form in acetylacetone (structure II) and absence of the ketostructure III, thereby confirming the existence of a monodentate structure having thetitanium atom bonded to one of the oxygen atoms in acetylacetone [13]. Its enolicform contains hydroxyl groups, which react with the titanium isopropoxide [14] asindicated below:

TiðOPriÞ4 þ acacH ! TiðOPriÞ3ðacacÞ þ PriOH (1)

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Fig. 6. FT-IR reflection spectra of the as deposited TiO2 films derived using the two stabilizing agents:

(a) DEA and (b) acac.


The bands at 1115 and 1052 cm�1 prove the existence of C–O stretching or C–Hdeformation (coupled). The presence of Ti–O and Ti–O–Ti polymeric chains wasclearly evident from the bands centered respectively at 783 and 462 cm�1.Additionally, the same film showed the presence of water and featured the O–Hstretching at 3166 cm�1.

Upon annealing treatment at 500 1C, the film derived from DEA featured a sharpband at 846 cm�1 (Fig. 7(a)) assigned to the longitudinal optical (LO) mode ofcrystalline anatase phase in TiO2. The reported wavenumber values of anatase andrutile phases in TiO2 are �840 and �800 cm�1 respectively. The LO mode ofamorphous TiO2 is known to appear at �870 cm�1. The slight blue shift in theband’s position is an indication of the existence of TiO2 also in the amorphous phase,which is evident from the XRD investigations, which show the coexistence of bothcrystalline and amorphous phases of TiO2. The transverse optical (TO) mode of theanatase phase in TiO2 appeared at 431 cm�1 for the film obtained from DEA. Evenupon thermally treating the film some amount of water is retained in the film, whichis confirmed from the band centered at 3594 cm�1. The band at 1583 cm�1 affirmed

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Fig. 7. FT-IR reflection spectra of the TiO2 films annealed at 500 1C for 5 min derived using the two

stabilizing agents: (a) DEA and (b) acac.


the in plane bending motion of water molecules. Surprisingly, the OH stretchingmode was not very strong in the acetylacetone-derived film, though in plane bendingmotion could be observed at 1572 cm�1 (Fig. 7(b)). The significantly high intensityand the sharpness of the band at 831 cm�1 assigned to the LO mode of anatase phasein the same film is an affirmation of the XRD results, which show the film to inheritlarger grain size. The red shift in the LO mode of TiO2 is a manifestation of the factthat higher degree of crystallinity prevails in the film, which is in agreement with thechronoamperometric results showing a low ion insertion capacity of the film owingto difficult Li ion diffusion in a highly ordered film. The TO mode of TiO2 appearedat 436 cm�1 for the film obtained from acetylacetone. The band appearing at458 cm�1 is ascribed to the Ti–O–Ti stretching mode. The assignments of the bandsprevalent in the as deposited and annealed films are also depicted in Table 1.

3.5. Electrochemical measurements

3.5.1. Cyclic voltammetric studies

The capability of the films to reversibly intercalate/deintercalate the Li ions wasdetermined by this technique. Fig. 8 shows the voltammograms of the TiO2 films

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Table 1

Assignments of the bands (cm�1) in the FT-IR spectra of the as-deposited (as-dep) and annealed (D) TiO2

films derived from the two stabilizing agents

TiO2 (as-dep,

DEA)

TiO2 (D, DEA) TiO2 (as-dep,

acac)

TiO2 (D,

acac)

Band assignment Refs.

3350 3594 3166 — O–H str. [15]

— — 1620 — CQO str. [13]

1601 — — — N–H def. [15]

— 1583 — 1572 O–H def. [15]

— — 1542, 1521 — CQC str. [13]

1443, 1347 — 1406 — C–H def. [15]

1070 — 1115, 1052 — C–O str. or C–H def.

(coupled)

[15]

957 — — — C–C str. [15]

— 846 — 831 Eu, LO mode of

anatase

[16]

— — 783 — A2u, LO mode of

anatase

[16]

426, 405 499 462 458 Ti–O–Ti str. [17]

— 431 — 436 Eu, TO mode of

anatase

[16]

Fig. 8. Cyclic voltammograms of TiO2 films obtained from the two stabilizing agents: (a) acac and

(b) DEA.


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derived from the two different stabilizing agents. A higher cathodic peak currentobtained for the film deposited utilizing DEA at 71 V is correlated by the presenceof grains comparatively smaller in size i.e. 79.4 nm, as the diffusion of the Li ions ismore facile in a highly disordered film. Addition of acetylacetone induced the growthof grains, which caused reduction in cathodic peak current due to difficult Li iondiffusion in a comparatively more ordered film. The voltammograms werereproducible and retraced the same path even after 102 cycles implying the goodelectrochemical reversible behavior of the films. Also, within the potential window of71 V, the voltammograms were devoid of anodic peak potentials. The CVsregistered within the potential range of 71.5 V showed characteristic anodic andcathodic peak maxima exhibiting a relative ease in the diffusion process at a highervoltage application. Upon application of 71 V, the diffusion coefficients in theDEA and acetylacetone derived films amounted respectively to 1.35� 10�11

and 4.43� 10�12 cm2 s�1 at a scan rate of 20 mV s�1 as evaluated using theRandles–Sevcik equation [18] described below:

Ip ¼ 0:4463nF ðnF=RTÞ1=2 CD1=2v1=2; (2)

where Ip ¼ peak current density (A cm�2), n ¼ number of electrons involved in theredox reaction, F ¼ Faraday constant, R ¼ gas constant, T ¼ temperature,C ¼ concentration of Li+ in the liquid electrolyte (mol cm�3), V ¼ scan rate(Vs�1), D ¼ diffusion coefficient (cm2 s�1).

The trend observed at 71.5 V was reversed with the film obtained fromacetylacetone inheriting a value of 2.01� 10�10, while the DEA derived filmdisplayed the diffusion coefficient value of 1.22� 10�10 cm2 s�1. The incoherenceobserved is an indication of the fact that beyond a certain potential, the crystallitesize does not seem to govern the diffusion rate and the intercalation process is muchmore facile even in the film demonstrating a higher grain size. Using the cyclicvoltammetric technique, Ottaviani et al. [19] arrived at an apparent diffusioncoefficient value of 3.7� 10�10 cm2 s�1 at 25 1C for the sputtered TiO2 films havingthe anatase crystalline phase. The cathodic peak potentials for the acetylacetone andDEA derived films were determined to be 1.5 and 1.44 V respectively. Similarly, theanodic peak potentials for the former and the latter film amounted respectively to1.06 and 1.10 V. A better reversibility for the intercalation and deintercalationprocesses was observed for the film derived from DEA owing to a lower difference inthe diffusion coefficient values during the Li ion insertion and extraction reactions.In addition, the lower cathodic peak potential shows the intercalation reaction toproceed at a relatively lower potential application in the film derived from DEA.Further a minor hump at �1.0 V observed for both the films is suggestive of theintercalation reaction to proceed in two different species and in all likelihoodprovides evidence in favor of the existence of Ti in+3 and+4 oxidation states.

3.5.2. Multiple step chronoamperometry

The ion storage capacity (ISC) defined as the amount of Li ion intercalated/deintercalated per unit area of the films were determined on immersing the film in

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Fig. 9. Transmittance variation of TiO2 films obtained using the two stabilizing agents: (a) acac and

(b) DEA as a function of time at l ¼ 632.8 nm for a potential step between 3.5 and �3.5 V vs. Pt in a 1 M

LiClO4/PC electrolyte at 0.0011 Hz.


1 M LiClO4—PC electrolyte against Pt counter electrode upon application of 71.5 Vat a fixed frequency of 0.0011 Hz. The TiO2 grain size was once again observed toinfluence the ISC of the corresponding films significantly. The lower ISC obtainedfor the film deposited from acetylacetone (5.23 mC cm�2) is interpreted in terms ofthe larger grain size observed from the XRD results. A comparatively higher ISC forthe film derived using DEA (6.18 mC cm�2) is a manifestation of the lower grain sizeof TiO2 in the film in agreement with the voltammograms showing a higher cathodicpeak current for the latter film. On application of 73.5 V at a frequency of0.0011 Hz, the evaluated coloration (tc) and bleaching (tb) times for the former andthe latter films from Fig. 9 respectively were tc ¼ 230 s, tb ¼ 11 s and tc ¼ 168 s,tb ¼ 5 s. The slower response times for the former film are accounted by its lowervalue of the ion storage capacity.

3.5.3. Electrochromic properties of TiO2 films

On being subjected to cathodic polarization by application of constant current of1 mA cm�2, the films demonstrated the electrochromic phenomenon exhibiting agrayish coloration, with the better electrochromic response observed for the filmdeposited from DEA. The possible explanation for this feature lies in the higher ISCfor the corresponding film, even though the thickness for the film derived fromacetylacetone was higher. This observation is best explained in terms of higherrefractive index for the latter film, thereby causing decreased porosity and less openstructure. Additionally, the larger grain size calculated for the film obtained fromacetylacetone also leads to a more compact structure. Fig. 10 shows the transmission

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Fig. 10. Transmission characteristics in the colored and bleached states of TiO2 films derived using the

two stabilizing agents: (- - -) DEA and (—) acac in the visible spectral region on application of constant

current density of 1 mA cm�2 vs. Pt in a 1 M LiClO4/PC electrolyte.


characteristics of the films in the intercalated (colored) and deintercalated (bleached)states in the UV–Visible spectral region. The optical modulation amounted to 37%and 24% at 550 nm respectively for the DEA and acetylacetone derived films. Thecoloration efficiencies (Z) for the TiO2 films derived from the DEA and acetylacetonerespectively amounted to 5.72 and 5.55 cm2 C�1. For the amorphous and crystallineTiO2 films derived using poly peroxotitanic acid obtained from Ti(OBun)4 in H2O2

solution, Hu et al. [20] arrived at coloration efficiency values at 630 nm of 9.93 and7.36 cm2 C�1 for the TiO2 films fired respectively at 150 1C and 450 1C. The authorsalso remarked a higher bleaching efficiency and lower coloration efficiency for theanatase film in comparison to the amorphous film. The lower value of colorationefficiency observed in the present work is in consequence of the fact that the films arefired at a relatively higher temperature of 500 1C, also the values are reported at awavelength of 550 nm. In general a rise in the coloration efficiency was evident withrespect to the wavelength. The equation used for the determination of colorationefficiency is described below:

Zðcm2 C�1Þ ¼ logðTbleach=T coloredÞ=Q=A (3)

where T(bleach/colored) ¼ Transmittance in bleached/colored state at l ¼ 550 nm andQ/A ¼ ion insertion capacity (C cm�2).

The color change in the TiO2 films is explained by the double injection of electronand the Li ion in accordance with the equation mentioned below:

TiO2 þ xLiþ þ xe�2 LixTiO2

Transparent Grayish blue(4)

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The determination of x in LixTiO2 compound for the two films was possibledue to the known values of inserted charge of the saturated colored states and thethickness of the corresponding films. The DEA-derived film was inherited with thethickness of 400 nm. The volume of the unit area of the film amounted to a value of40� 1018 A3. Following the crystallographic parameters of TiO2, the number of unitcells in the unit area of the film equals 293� 1015. Therefore, the number of Ti ions inthis value was evaluated to be 11.7� 1017. Since, the number of electrons and the Liions are equal, one can obtain the number of Li ions from the total inserted chargeand the electronic charge values. The number of the Li ions was determined to be1.78� 1017. From the atomic ratio of the number of Ti and the Li ions, the value of x

was evaluated to be 0.152. The value of x in the LixTiO2 compound for the filmderived from acetylacetone amounted to 0.14. The faint coloration is clearly evidentfrom the low coefficient (x) value of Li charge in the compound in the two films. Thehigher optical modulation observed for the film obtained from DEA is wellcorroborated by comparatively higher x value compared to the acetylacetone-derived film.

3.6. Morphological properties

The AFM images of the films obtained from the two different stabilizing agentsare indicated in Fig. 11. In conformity with the XRD results, the DEA-derived filmwas endowed with the average grain size of 60.5 nm and roughness of the order of1.2 nm. The acetylacetone-derived film was characterized by grain size approximately70.2 nm in dimension with the level of roughness approaching 4.53 nm. Theincreased degree of roughness is in consequence of the increased grain size of thecorresponding film. The inconsistency between the grain sizes measured from XRDand the AFM studies is explained on the basis of the high resolution of the latter

Fig. 11. AFM images of the TiO2 films obtained from the two stabilizing agents: (a) DEA and (b) acac.

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Fig. 12. SEM micrographs of the TiO2 films derived from the two stabilizing agents: (a) DEA and

(b) acac.


technique in addition to the fact that the TiO2 grain sizes evaluated by the AFMstudy are not the mean average values owing to the presence of grains with varyingdimensions.

The SEM micrographs presented in Fig. 12 in coherence with the AFM resultsindicated larger grains approaching a size of 200 nm for the acetylacetone-derivedfilm. For the film derived from DEA grains approximately 100 nm in dimension wereobserved. Additionally, the films were visually seen to be crack free andhomogeneous and owing to the crystalline nature, the films were closely packedand featured a highly dense structure.

4. Conclusions

Synthesis of sol–gel process derived nanophase TiO2 films from titaniumisopropoxide in ethanol, in the presence of stabilizing agents (i) diethanolamine(DEA) and (ii) acetic acid and acetylacetone was performed. Crack free filmswith good adherence and quality were obtained from both the routes. Thetransition of anatase to the rutile phase occurred at a relatively lowertemperature in the TiO2 xerogel obtained from the former route, with thelatter xerogel being characterized by the absence of the exothermic peak assignedto the rutile phase formation up to 600 1C. The topographical informationobtained from the AFM and SEM micrographs was correlated directly to the ionstorage capacity (ISC). The larger grain size observed for the acetylacetone derivedfilm resulted in a lower ion storage capacity (ISC) and cathodic peak current for thefilm owing to the difficult diffusion in an relatively more ordered film. The higherISC observed for the DEA-derived film led to better electrochromic activity inthe corresponding film with the coloration efficiency approaching a value of5.72 cm2 C�1 at 550 nm.

ARTICLE IN PRESS


Acknowledgments

Besides the financial support from Council of Scientific & Industrial Research(AV) and the Ministry of Non Conventional Energy Sources (MNES), Governmentof India, characterization facility extended by University Science InstrumentationCentre, University of Delhi is gratefully acknowledged.

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