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Applied Surface Science 257 (2010) 670–676

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

eview

orrelation between microstructure and optical properties of nano-crystallineiO2 thin films prepared by sol–gel dip coating

. Mechiakha,b,c,∗, N. Ben Sedrineb, R. Chtouroub, R. Bensahac

Département de Médecine, Faculté de Médecine, Université Hadj Lakhdar, Batna, AlgeriaLaboratoire de Photovoltaïque de Semi-conducteurs et de Nanostructures, Centre de Recherche des Sciences et Technologies de l’Energie, BP.95, Hammam-Lif 2050, TunisiaLaboratoire de Céramiques, Université Mentouri Constantine, Algeria

r t i c l e i n f o

rticle history:eceived 21 March 2010eceived in revised form 22 July 2010ccepted 1 August 2010vailable online 7 August 2010

eywords:iO2

ol–gel

a b s t r a c t

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanolas a solvent by sol–gel dip coating technique. TiO2 thin films prepared using a sol–gel process havebeen analyzed for different annealing temperatures. Structural properties in terms of crystal structurewere investigated by Raman spectroscopy. The surface morphology and composition of the films wereinvestigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra ofTiO2 thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry studywas used to determine the annealing temperature effect on the optical properties and the optical gap ofthe TiO2 thin films. The results show that the TiO2 thin films crystallize in anatase phase between 400and 800 ◦C, and into the anatase–rutile phase at 1000 ◦C, and further into the rutile phase at 1200 ◦C.

hin films

nataseutilennealing

We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphologyresults indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature.The TiO2 thin films have high transparency in the visible range. For annealing temperatures between1000 and 1400 ◦C, the transmittance of the films was reduced significantly in the wavelength range of300–800 nm due to the change of crystallite phase and composition in the films. We have demonstrated

as well the decrease of the optical band gap with the increase of the annealing temperature.

© 2010 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6712. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671

2.1. Preparation of the coating solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6712.2. Preparation of TiO2 coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6712.3. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671

3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6713.1. Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671

3.1.1. Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6713.1.2. Morphological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672

3.2. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6733.2.1. UV-VIS spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673

3.2.2. Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6743.2.3. Spectroscopic ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674

4. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 676

∗ Corresponding author at: Département de Médecine, Faculté de Médecine, Université Hadj Lakhdar, Batna, Algeria. Tel.: +213 773 96 31 47; fax: +213 32 45 14 30.E-mail address: raouf mechiakh@yahoo.fr (R. Mechiakh).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.08.008

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R. Mechiakh et al. / Applied Su

. Introduction

TiO2 thin films are extensively studied because of their inter-sting chemical, electrical and optical properties [1,2]. TiO2 filmn anatase phase could accomplish the photocatalytic degrada-ion of organic compounds under the radiation of UV. So, it has aariety of application prospects in the field of environmental pro-ection [3,4]. TiO2 thin film in rutile phase is known as a goodlood compatibility material and can be used as artificial heartalves [5]. In addition, TiO2 films are important optical films dueo their high reflective index and transparency over a wide spectralange [2].

During the two last decades, several methods have been usedor the TiO2 thin films preparation, such as chemical vapor depo-ition [6], chemical spray pyrolysis [7], pulsed laser deposition [8]nd sol–gel method [9]. In comparison with other methods, theol–gel method has some advantages such as controllability, reli-bility, reproducibility and can be selected for the preparation ofano-structured thin films [9,10]. Sol–gel coating has been clas-ified as two different methods such as dip and spin coating. Theip-coating has considerably been used for preparation TiO2 nano-tructured thin films [11–13]. Experimental results have shownhat the preparation of high transparent TiO2 thin film by dip-oating method needs to control morphology, thickness of the filmnd the anatase-to-rutile phase transformation [12,14].

The effect of particule size on the anatase-to-rutile transforma-ion has been extensively investigated through thermodynamic orinetic studies [15–17]. The pH value of the sol–gel system for thereparation of uniform nanoparticles of anatase titania from con-ensed TiO2 gel is a key factor for controlling the final particule sizend shape of the product [17,18]. The temperature of anatase-to-utile transition increases with the synthesized pH value [19–21].

The formation of a particular phase depends upon the naturef the starting material, its composition, deposition method andnnealing temperature. In particular, the annealing temperatureffect on the TiO2 thin films, can transform the structure frommorphous phase into crystalline anatase, and from anatase intoutile. There are various reports about the dependence of annealingemperature on structural and optical properties [22].

Numerous literature reports are dedicated to the fabrication ofiO2 thin films by sol–gel dip coating technique using many typesf titanium alkoxides as precursors. Chrysicopoulou et al. [10] useditanium tetraethoxide as a precursor, ethanol as a solvent andNO3 as a catalyst in the presence of a small amount of water.hn et al. [23] prepared the TiO2 thin layers by sol–gel process and

heir structural and optical properties were examined at variousatalyst concentrations and calcination temperatures. Their thinlms calcined at temperatures from 400 to 600 ◦C are in anatasehase, and transform into the anatase-to-rutile phase at 800 ◦C, andurther into the rutile phase at 1000 ◦C. The phase transformationemperature turns out to rely upon the concentration of catalystCl. The crystallite size of the films increases by increasing cata-

yst concentration and calcination temperature. In a previous study24], we have found out the influence of the temperature on theptical and structural properties of TiO2 thin films, using tetrabutyl-rthotitanate as a precursor to prepare titanium solutions and thinlms of TiO2 in their sol–gel process. The three-layered thin filmsrystallization starts at 350 ◦C in anatase and brookite phases. Forhigher number (10) of layers and an annealing temperature of

00 ◦C, we have found [25,26] that the crystalline structure changesrom anatase–brookite to rutile, which normally does not appear

elow 800 ◦C as reported in literature [23].

In this paper, we report on the study of the surface morphol-gy and optical properties of TiO2 thin films deposited on n-typei (1 0 0) and sapphire substrates by sol–gel dip coating techniques a function of the preparation conditions. Structural and optical

Science 257 (2010) 670–676 671

evolutions with annealing temperature are investigated by AFM,Raman, ellipsometry, and UV-VIS spectrophotometer techniques.

2. Experimental

2.1. Preparation of the coating solutions

The sol solution was prepared by adding 1 mol of tetrabutyl-orthotitanate, TIP (Fluka), to a 1 mol beaker containing a mixtureof acetic acid (J.T. Baker) and butanol (Fluka) that had been mixedfor 5 min [26]. The mixture was vigorously stirred using a magneticstirrer during addition and for a further 60 min after addition ofthe precursor at room temperature. A gel film was formed on thesapphire substrate by dipping it into the solution and pulling it upat a constant rate of 0.6 cm s−1 by a dipping machine. This processis optimal for producing highly uniform coatings, by simple controlof the thickness through control of the speed of withdrawal fromthe coating solution. The gel films grown on the silicon substratecontain residual butanol and probably water from the condensationreaction. The dip coated silicon substrate was therefore left to dryat ambient temperature followed by heating at 100 ◦C in an ovenunder clean room environment for a minimum of 15 min.

2.2. Preparation of TiO2 coatings

A dip-coating apparatus made in our laboratory was used for thedepositions. The substrate was lowered into the coating solutionand then withdrawn at a regulated speed of 0.6 cm s−1. After eachcoating, the films were first dried at 100 ◦C for 15 min. The filmswere then heat-treated at different temperatures ranging between400 and 1400 ◦C with increasing temperature rate of 5 ◦C min−1 for2 h in furnace.

2.3. Characterization

The Raman spectra were recorded at room temperature using aJobin-Yvon Labram HR combined Raman-IR microanalytical spec-trometer equipped with a motorized xy stage and autofocus. Thespectra were generated with 17 mW, 632.8 nm He–Ne laser exci-tation and were dispersed with the 1800 g/mm grating across the0.8 m length of the spectrograph. The laser power was 9 mW onthe sample surface. The spectral resolution of this apparatus is esti-mated to be less than 0.5 cm−1 for a slit aperture of 150 �m and aconfocal hole of 300 �m. Morphological study was performed usingatomic force microscopy (AFM) in tapping mode configuration by aTopometrix TMX 2000 Explorer AFM. Optical properties of the filmswere examined by a UV-VIS spectrophotometer (UV3101PC). Spec-troscopic ellipsometry (SE) experiment was performed at roomtemperature using an automatic ellipsometer SOPRA GES5. Thesystem uses a 75-W xenon lamp, a rotating polarizer, an auto-tracking analyser, a double monochromator, a photomultiplier tubeand a GaInAs photodiode as detectors. Data were collected in the0.25–1.5 �m region with the step of 0.005 �m, at incidence angleof � = 75◦.

3. Results and discussion

3.1. Structural properties

3.1.1. Raman spectroscopy−1

Fig. 1 shows the Raman spectra in the range of 100–700 cm

of TiO2 films annealed at different temperatures: 400, 600 and800 ◦C on sapphire substrates. At 400 ◦C (Fig. 1(a)), the spectra showsymmetric vibration modes (A1g + 2B1g + 3Eg) of tetragonal anatasephase identified at 142 (Eg), 197 (Eg), 394 (B1g), 515 (B1g), and

672 R. Mechiakh et al. / Applied Surface Science 257 (2010) 670–676

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1000 ◦C. The increase of roughness can be interpreted as the phasechange, and the increase of grain size (Table 1).

Table 1The influence of annealing temperature on grain size and roughness of TiO2 film.

Temperature (◦C) Size grain (nm) RMS (nm)

ig. 1. Raman spectra of the TiO2 thin films obtained after three dippings andnnealing temperatures of 400 (a), 600 (b) and 800 ◦C (c). A = anatase.

39 cm−1 (Eg). The observed band positions are in good agreementith the previous reports for anatase phase [27].

At 600 ◦C (Fig. 1(b)), one observe a very intense band at 142 cm−1

nd low intensity bands at 194, 394, 514, and 639 cm−1. The posi-ions of these bands correspond to those obtained by Djaoued etl. [28] at temperature 600 ◦C on quartz substrate. They are alsobserved by other authors, in particular by Balaji et al. [29], Math-ws et al. [30] on quartz and glass substrates between temperatures00 and 600 ◦C, and are allotted to the anatase phase. For thennealing temperature of 800 ◦C (Fig. 1(c)), we note a weak dis-lacement towards high frequencies of the anatase bands situatedround 143, 196, 395, 516, and 639 cm−1. This can be explainedy the presence of strain in the films. In fact, the strain may beue to the difference of the thermal expansion coefficients betweenhe film and the substrate. It is well known [31] that the anatasetructure is transformed into rutile at an annealing temperature of00 ◦C. However, for our samples annealed at temperatures from00 to 800 ◦C, no bands corresponding to the rutile phase arebserved. In spite of the absence of this band, we cannot concludehe absence of the rutile phase, because it can be present in a smallmount and represented by a low total intensity of the signal. Lastly,t is noticed that the Raman bands become sharper with anneal-ng. As an example, one note that the full width at half maximumFWHM) of the peak located at 142 cm−1 decreases from 11.01 to.07 cm−1 between 600 and 800 ◦C, thus representing an increase

n the size of crystallites.By increasing the annealing temperature (Fig. 2), the anatase

eak intensities decrease and the rutile peaks appear. At 1000 ◦CFig. 2(a)), a mixed anatase–rutile phase is observed and repre-ented by the humps located at 444 and 606 cm−1. We also clearlyee that the anatase peaks nearly disappear, while the rutile peakntensities drastically increase.

For the annealing temperature of 1200 ◦C (Fig. 2(b)), the anatasehase is completely transformed into the rutile phase. The rela-ive Raman spectrum is composed of three broad bands around35, 444 and 609 cm−1. These bands clearly indicate the presencef rutile phase. Consequently, a phase transition from anatase-o-rutile occurs in the temperature range 1000–1200 ◦C. For annnealing temperature of 1400 ◦C (Fig. 2(c)), the three bands (235,45 and 609 cm−1) become more intense.

According to Ref. [32], the anatase-to-rutile phase transforma-ion takes place at temperatures from 600 to 700 ◦C, however, inhis work, the phase transformation occurs at 1000 ◦C. This dis-repancy may be due to the difference of TiO2 crystallite structurend size. Djaoued et al. [28] reported that rutile peaks appeared at

Fig. 2. Raman spectra of the TiO2 thin films obtained after three dippings andannealing temperatures of 1000 (a), 1200 (b) and 1400 ◦C (d). R = rutile, * = substrate.

800 ◦C when diethanolamine (DEA) was used as a catalyst, whilerutile peaks appeared at 900 ◦C when polyethyleneglycol (PEG)was employed as a catalyst [33]. Therefore, the crystalline phasetransition temperature depends on the catalyst used in the solpreparation.

3.1.2. Morphological propertiesIn order to confirm the crystallized structure studied in the pre-

vious paragraph by Raman spectroscopy, we propose atomic forcemicroscopy (AFM) surface imaging analysis.

In Fig. 3, we present 2D and 3D AFM images of the TiO2 filmsprepared on silicon substrates, corresponding to the annealingtemperatures of 400, 600, 700, 800 and 1000 ◦C.

Fig. 3(a) and (b) shows respectively the surface morphologyof TiO2 films annealed at 400 and 600 ◦C. At 400 ◦C anneal-ing temperature, the surface morphology indicates a porousand fine structure with small size grains in anatase phase.However at 600 ◦C (Fig. 3(b)), larger anatase crystal grains areobserved.

It can be seen that the thin films annealed from 700 to 1000 ◦C(Fig. 3(c)–(e)) show the same grain shape, but an increase in thegrain size. These figures show as well the increase of the inter-grainporosity and roughness with increasing the annealing temperature.

The TiO2 films root mean square (RMS) roughness analysis hasbeen carried out. As shown in Fig. 4, the RMS follows a similar evo-lution as the surface morphology observations. The RMS roughnessincreases slowly from 0.880 to 4.235 nm when the annealing tem-perature increases from 400 to 700 ◦C, then reaches 7.270 nm at

400 5.174 0.880600 27.275 1.634700 32.25 4.235800 43.032 7.7041000 41.565 7.270

R. Mechiakh et al. / Applied Surface Science 257 (2010) 670–676 673

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ig. 3. AFM surface morphology images of the TiO2 thin films of obtained after thre

.2. Optical properties

.2.1. UV-VIS spectroscopy

Fig. 5 shows the TiO2 thin film UV–vis spectra for different

nnealing temperatures, in the wavelength range of 300–1500 nm.iO2 thin films annealed at temperatures from 400 to 800 ◦Cepresent high transparency coefficients in the visible rangef 400–800 nm (Fig. 5(a–c)). For higher annealing temperatures

ings and annealing temperatures of 400 (a), 600 (b), 700 (c) 800 (d) and 1000 (e).

from 1000 to 1400 ◦C (Fig. 5(d)–(f)), the transmittance of TiO2thin films is considerably reduced in the wavelength range of400–900 nm. One can conclude that the films turn out to be

opaque for annealing temperatures above 1000 ◦C. This is duefirstly, to the absorption increase resulting from the phase trans-formation anatase-to-rutile, and secondly, to the light scatteringincrease with crystallite size and particle clustering. Moreover,non-stoichiometric films would be formed at high annealing

674 R. Mechiakh et al. / Applied Surface Science 257 (2010) 670–676

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Fig. 4. The influence of annealing temperature on roughness of TiO2 film.

emperatures. These results are in good accordance with those ofaman spectroscopy.

In the wavelength range of 500–1500 nm, interference fringesre also visible. In addition it is important to notice the redshiftf the TiO2 thin film absorption edges by increasing the annealingemperature. This result is related to the TiO2 thin film band gapnergy change due to the phase transformation [34].

.2.2. ReflectanceThe reflectance spectra recorded for the TiO2 samples annealed

t different temperatures are shown in Fig. 6. For the annealingemperatures of 800 and 1000 ◦C, the reflectance reaches 90% inhe visible region. Increase in the annealing temperature causeshe respective decrease in the thickness and increase of reflectanceercentage. It has been previously observed that the film thick-ess decreases with increasing the annealing temperature and dippeed.

The band gap of the samples was determined by the equation:

g = 1239.8/� (1)

here Eg is the optical band gap (eV) and � (nm) is the wavelengthf the absorption edge in the spectrum [35].

Table 2 shows the obtained optical band gap of the TiO2 thinlms annealed at different temperatures. It is seen that the optical

ig. 5. Transmission spectra of the TiO2 thin films obtained after three dippings andnnealing temperatures of 400 (a), 600 (b), 800 (c), 1000 (d), 1200 (e) and 1400 ◦Cf).

Fig. 6. Reflectance spectra of the TiO2 thin films obtained after three dippings andannealing temperatures of 400 (a), 600 (b), 700 (c), 800 (d) and 1000 (e).

band gap decreases from 3.51 to 3.33 eV by increasing annealingtemperature from 400 to 800 ◦C. The decrease of the TiO2 opticalband gap with annealing temperature might be the result of thechange in film density and increase in grain size. At a temperatureexceeding 1000 ◦C, where the anatase-to-rutile phase transforma-tion takes place, the optical band gap of the thin films decreasesconsiderably because the rutile phase has lower optical band gapcompared to the anatase phase. These results are in accordancewith those of Raman spectroscopy.

3.2.3. Spectroscopic ellipsometrySpectroscopic ellipsometry (SE) determines the complex

reflectance ratio � defined in terms of the standard ellipsometricparameters and� as [36]:

� = rprs

= (tan ) · ei� (2)

where rp and rs are the reflection coefficients for light polarizedparallel (p) and perpendicular (s) to the sample’s plane of incidence,respectively.

Fig. 7 shows the ellipsometric measurement (scatters) andmodelling (lines), in terms of tan and cos�, as a function of wave-length, for TiO2 annealed samples at 600, 800 and 1000 ◦C. The aimof the present ellipsometric study is to determine the annealingtemperature effect on the optical properties and the optical gap ofthe TiO2 thin films prepared with sol–gel technique. The best ellip-sometric model used in this work consists of a four-phase model:(Si substrate/TiO2/TiO2 mixed with void/ambient).

The Forouhi interband model [37] is based on the quantumtheory of absorption. The formulation is applicable to amor-phous semiconductors and dielectrics, crystalline semiconductors,dielectrics and metals throughout the interband region. A gen-

eral expression of the extinction coefficient k is deduced addinga finite lifetime for the excited state to which the electron trans-fers due to photon absorption. The optical band gap Eg is identifiedto the energy for which k(E) has its absolute minimum. Thenthe refractive index n(E) is deduced from the Kramers–Kroning

Table 2The influence of annealing temperature on the TiO2 film optical band gap.

Temperature (◦C) Optical band gap (eV)

400 3.51600 3.49700 3.48800 3.33

1000 3.30

R. Mechiakh et al. / Applied Surface Science 257 (2010) 670–676 675

Ff

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ig. 7. Refractive index (a) and extinction coefficient (b) as a function of wavelength,or TiO2 annealed samples at 600, 800 and 1000 ◦C.

ispersion relations. The n and k value are given by the followingxpressions:

(E) = N∞ +∑ BqE + Cq(

E − Bq)E + Cq

(3)

(E) =∑ A(E − Eg)2

( ) (4)

E − Bq E + Cq

here N∞ is the refractive index at high energy, Bq is a first interme-iate parameter calculated by (A/Q )(−(B2/2) + EgB− E2

g + C) where

= 0.5√

4C − B2, Cq is a second intermediate parameter calcu-

able 3iO2 best-fit ellipsometric (SE) modelling parameters as a function of the annealing temp

SE fit parameters

Layer 1 Thickness (�m)Void composition

Layer 2 Thickness (�m)N∞Eg (eV)P1 peak A

BC

P2 peak ABC

P3 peak ABC

Standard deviation �

Fig. 8. Ellipsometric measurement (scatters) and modelling (lines), in terms of tan and cos�, as a function of wavelength, for TiO2 annealed samples at 600, 800 and1000 ◦C.

lated by A/Q ((E2g + C)(B/2) − 2EgC), A, B, C, N∞ and Eg are treated as

fitting parameters.We have found that the Forouhi and Bloomer dispersion law

[37,38] better describes the TiO2 refractive index and extinctioncoefficient, where Eg is the optical band gap, N∞ is the refractiveindex value at high energy, A, B and C are the fitting parame-ters relative to the three absorption peaks. The effective mass

approximation [39] consisting of a mixing of the found TiO2 opticalproperties and void, which physically describes the surface rough-ness of the films. This procedure allows as well to find the layerthickness with high accuracy. After finding the best dispersion law,

erature.

Annealing temperature

600 ◦C 800 ◦C 1000 ◦C

0.0246 0.0005 0.06850.14 0.29 0.18

0.0420 0.0727 0.05742.3091 2.1355 2.79333.4506 3.2790 3.09720.1648 0.1118 1.31087.8784 8.2528 3.9854

15.6580 17.1930 16.6700.0003 0.0003 −0.03151.8315 1.8272 1.69430.8415 0.8360 0.90190.1427 0.6653 0.00142.8534 13.2970 0.0247

−62.6770 −292.0840 −0.55330.002 0.002 0.007

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76 R. Mechiakh et al. / Applied Su

he fitting procedure consists of changing the different parame-ers and find the ones that minimizes the standard deviation �. Thetandard deviation measures the deviation between the theoreticalodel and the measurement. Fig. 7(a) and (b) shows, respectively,

he results of the ellipsometric analysis in terms of refractive indexn) and extinction coefficient (k), as a function of wavelength, foriO2 samples annealed at 600, 800 and 1000 ◦C. The best-fit param-ters are listed in Table 3. It is important to notice that � is very lowlower than 1%), leading to a good fitting procedure and accurateiO2 optical properties (see Fig. 8). On the one hand, we can con-rm using another optical technique (SE) that the annealing affectshe optical band gap, which is found to decrease by increasing theemperature. Within the 2% error with respect to the optical bandap, the obtained Eg are in good agreement with the reflectanceesults. On the other hand, we can confirm the AFM obtained RMSoughness that is found to increase with the annealing tempera-ure. This is reflected in the void composition that reaches 29% forhe annealing temperature of 800 ◦C, which is also found to havehe highest roughness value of 7.702.

. Conclusion

Sol–gel-based nano-crystalline TiO2 thin films were prepared bymploying tetrabutyl-orthotitanate as a precursor and the effectf treatment temperature on their structural and optical proper-ies was examined. The TiO2 thin films annealed at temperaturesrom 400 to 800 ◦C are in anatase phase, and transform into thenatase-to-rutile phase at 1000 ◦C, and further into the rutile phaset 1200 ◦C. The crystallite size of the films increases by increasinghe annealing temperature. According to AFM imaging, all films fab-icated are uniform, and their density and crystallinity are increasedith increasing the annealing temperature. The surface morphol-

gy results reveal that the rutile films are denser than the anatasehase. The roughness of the TiO2 thin films increases by increasinghe annealing temperature. The deposited TiO2 thin films revealhigh transparency in the visible range. The transmittance of thelms annealing between 1000 and 1400 ◦C is prominently reduced

n the wavelength range of 400–1000 nm because of enhancedbsorption as a result of the change of crystallite phase and com-osition in the films and the scattering effect originating from

ncreased grain size. The ellipsometric analysis confirms the resultsound by Raman, AFM and UV–vis techniques in terms of surface

orphology and optical band gap. The optical properties of the

lms are found to be closely related to the microstructure and crys-allographic structure which depend on the annealing temperature.

In this study, we have successfully fabricated TiO2 thin filmsith desired structural and optical properties by sol–gel dip coatingethod using the titanium alkoxide as a starting material.

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Science 257 (2010) 670–676

Acknowledgement

The authors want to thank the Research and Technology Centerof Energy in Tunisia, for AFM, Ellipsometry, and spectrophotometermeasurements.

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