Correlation between microstructure and optical

Applied Surface Science 257 (2010) 670–676

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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Review

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol–gel dip coating R. Mechiakh a,b,c,∗ , N. Ben Sedrine b , R. Chtourou b , R. Bensaha c a

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

a r t i c l e

i n f o

Article history: Received 21 March 2010 Received in revised form 22 July 2010 Accepted 1 August 2010 Available online 7 August 2010 Keywords: TiO2 Sol–gel Thin films Anatase Rutile Annealing

a b s t r a c t Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol–gel dip coating technique. TiO2 thin films prepared using a sol–gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO2 thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO2 thin films. The results show that the TiO2 thin films crystallize in anatase phase between 400 and 800 ◦ C, and into the anatase–rutile phase at 1000 ◦ C, and further into the rutile phase at 1200 ◦ C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results 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 between 1000 and 1400 ◦ C, the transmittance of the films was reduced significantly in the wavelength range of 300–800 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.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Preparation of the coating solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Preparation of TiO2 coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Morphological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. UV-VIS spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Spectroscopic ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ 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 [email protected] (R. Mechiakh). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.008

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1. Introduction TiO2 thin films are extensively studied because of their interesting chemical, electrical and optical properties [1,2]. TiO2 film in anatase phase could accomplish the photocatalytic degradation of organic compounds under the radiation of UV. So, it has a variety of application prospects in the field of environmental protection [3,4]. TiO2 thin film in rutile phase is known as a good blood compatibility material and can be used as artificial heart valves [5]. In addition, TiO2 films are important optical films due to their high reflective index and transparency over a wide spectral range [2]. During the two last decades, several methods have been used for the TiO2 thin films preparation, such as chemical vapor deposition [6], chemical spray pyrolysis [7], pulsed laser deposition [8] and sol–gel method [9]. In comparison with other methods, the sol–gel method has some advantages such as controllability, reliability, reproducibility and can be selected for the preparation of nano-structured thin films [9,10]. Sol–gel coating has been classified as two different methods such as dip and spin coating. The dip-coating has considerably been used for preparation TiO2 nanostructured thin films [11–13]. Experimental results have shown that the preparation of high transparent TiO2 thin film by dipcoating method needs to control morphology, thickness of the film and the anatase-to-rutile phase transformation [12,14]. The effect of particule size on the anatase-to-rutile transformation has been extensively investigated through thermodynamic or kinetic studies [15–17]. The pH value of the sol–gel system for the preparation of uniform nanoparticles of anatase titania from condensed TiO2 gel is a key factor for controlling the final particule size and shape of the product [17,18]. The temperature of anatase-torutile transition increases with the synthesized pH value [19–21]. The formation of a particular phase depends upon the nature of the starting material, its composition, deposition method and annealing temperature. In particular, the annealing temperature effect on the TiO2 thin films, can transform the structure from amorphous phase into crystalline anatase, and from anatase into rutile. There are various reports about the dependence of annealing temperature on structural and optical properties [22]. Numerous literature reports are dedicated to the fabrication of TiO2 thin films by sol–gel dip coating technique using many types of titanium alkoxides as precursors. Chrysicopoulou et al. [10] used titanium tetraethoxide as a precursor, ethanol as a solvent and HNO3 as a catalyst in the presence of a small amount of water. Ahn et al. [23] prepared the TiO2 thin layers by sol–gel process and their structural and optical properties were examined at various catalyst concentrations and calcination temperatures. Their thin films calcined at temperatures from 400 to 600 ◦ C are in anatase phase, and transform into the anatase-to-rutile phase at 800 ◦ C, and further into the rutile phase at 1000 ◦ C. The phase transformation temperature turns out to rely upon the concentration of catalyst HCl. The crystallite size of the films increases by increasing catalyst concentration and calcination temperature. In a previous study [24], we have found out the influence of the temperature on the optical and structural properties of TiO2 thin films, using tetrabutylorthotitanate as a precursor to prepare titanium solutions and thin films of TiO2 in their sol–gel process. The three-layered thin films crystallization starts at 350 ◦ C in anatase and brookite phases. For a higher number (10) of layers and an annealing temperature of 400 ◦ C, we have found [25,26] that the crystalline structure changes from anatase–brookite to rutile, which normally does not appear below 800 ◦ C as reported in literature [23]. In this paper, we report on the study of the surface morphology and optical properties of TiO2 thin films deposited on n-type Si (1 0 0) and sapphire substrates by sol–gel dip coating technique as a function of the preparation conditions. Structural and optical

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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 tetrabutylorthotitanate, TIP (Fluka), to a 1 mol beaker containing a mixture of acetic acid (J.T. Baker) and butanol (Fluka) that had been mixed for 5 min [26]. The mixture was vigorously stirred using a magnetic stirrer during addition and for a further 60 min after addition of the precursor at room temperature. A gel film was formed on the sapphire substrate by dipping it into the solution and pulling it up at a constant rate of 0.6 cm s−1 by a dipping machine. This process is optimal for producing highly uniform coatings, by simple control of the thickness through control of the speed of withdrawal from the coating solution. The gel films grown on the silicon substrate contain residual butanol and probably water from the condensation reaction. The dip coated silicon substrate was therefore left to dry at ambient temperature followed by heating at 100 ◦ C in an oven under 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 the depositions. The substrate was lowered into the coating solution and then withdrawn at a regulated speed of 0.6 cm s−1 . After each coating, the films were first dried at 100 ◦ C for 15 min. The films were then heat-treated at different temperatures ranging between 400 and 1400 ◦ C with increasing temperature rate of 5 ◦ C min−1 for 2 h in furnace. 2.3. Characterization The Raman spectra were recorded at room temperature using a Jobin-Yvon Labram HR combined Raman-IR microanalytical spectrometer equipped with a motorized xy stage and autofocus. The spectra were generated with 17 mW, 632.8 nm He–Ne laser excitation and were dispersed with the 1800 g/mm grating across the 0.8 m length of the spectrograph. The laser power was 9 mW on the sample surface. The spectral resolution of this apparatus is estimated to be less than 0.5 cm−1 for a slit aperture of 150 ␮m and a confocal hole of 300 ␮m. Morphological study was performed using atomic force microscopy (AFM) in tapping mode configuration by a Topometrix TMX 2000 Explorer AFM. Optical properties of the films were examined by a UV-VIS spectrophotometer (UV3101PC). Spectroscopic ellipsometry (SE) experiment was performed at room temperature using an automatic ellipsometer SOPRA GES5. The system uses a 75-W xenon lamp, a rotating polarizer, an autotracking analyser, a double monochromator, a photomultiplier tube and a GaInAs photodiode as detectors. Data were collected in the 0.25–1.5 ␮m region with the step of 0.005 ␮m, at incidence angle of  = 75◦ . 3. Results and discussion 3.1. Structural properties 3.1.1. Raman spectroscopy Fig. 1 shows the Raman spectra in the range of 100–700 cm−1 of TiO2 films annealed at different temperatures: 400, 600 and 800 ◦ C on sapphire substrates. At 400 ◦ C (Fig. 1(a)), the spectra show symmetric vibration modes (A1g + 2B1g + 3Eg ) of tetragonal anatase phase identified at 142 (Eg ), 197 (Eg ), 394 (B1g ), 515 (B1g ), and

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

639 cm−1 (Eg ). The observed band positions are in good agreement with the previous reports for anatase phase [27]. At 600 ◦ C (Fig. 1(b)), one observe a very intense band at 142 cm−1 and low intensity bands at 194, 394, 514, and 639 cm−1 . The positions of these bands correspond to those obtained by Djaoued et al. [28] at temperature 600 ◦ C on quartz substrate. They are also observed by other authors, in particular by Balaji et al. [29], Mathews et al. [30] on quartz and glass substrates between temperatures 200 and 600 ◦ C, and are allotted to the anatase phase. For the annealing temperature of 800 ◦ C (Fig. 1(c)), we note a weak displacement towards high frequencies of the anatase bands situated around 143, 196, 395, 516, and 639 cm−1 . This can be explained by the presence of strain in the films. In fact, the strain may be due to the difference of the thermal expansion coefficients between the film and the substrate. It is well known [31] that the anatase structure is transformed into rutile at an annealing temperature of 800 ◦ C. However, for our samples annealed at temperatures from 600 to 800 ◦ C, no bands corresponding to the rutile phase are observed. In spite of the absence of this band, we cannot conclude the absence of the rutile phase, because it can be present in a small amount and represented by a low total intensity of the signal. Lastly, it is noticed that the Raman bands become sharper with annealing. As an example, one note that the full width at half maximum (FWHM) of the peak located at 142 cm−1 decreases from 11.01 to 8.07 cm−1 between 600 and 800 ◦ C, thus representing an increase in the size of crystallites. By increasing the annealing temperature (Fig. 2), the anatase peak intensities decrease and the rutile peaks appear. At 1000 ◦ C (Fig. 2(a)), a mixed anatase–rutile phase is observed and represented by the humps located at 444 and 606 cm−1 . We also clearly see that the anatase peaks nearly disappear, while the rutile peak intensities drastically increase. For the annealing temperature of 1200 ◦ C (Fig. 2(b)), the anatase phase is completely transformed into the rutile phase. The relative Raman spectrum is composed of three broad bands around 235, 444 and 609 cm−1 . These bands clearly indicate the presence of rutile phase. Consequently, a phase transition from anataseto-rutile occurs in the temperature range 1000–1200 ◦ C. For an annealing temperature of 1400 ◦ C (Fig. 2(c)), the three bands (235, 445 and 609 cm−1 ) become more intense. According to Ref. [32], the anatase-to-rutile phase transformation takes place at temperatures from 600 to 700 ◦ C, however, in this work, the phase transformation occurs at 1000 ◦ C. This discrepancy may be due to the difference of TiO2 crystallite structure and size. Djaoued et al. [28] reported that rutile peaks appeared at

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

800 ◦ C when diethanolamine (DEA) was used as a catalyst, while rutile peaks appeared at 900 ◦ C when polyethyleneglycol (PEG) was employed as a catalyst [33]. Therefore, the crystalline phase transition temperature depends on the catalyst used in the sol preparation.

3.1.2. Morphological properties In order to confirm the crystallized structure studied in the previous paragraph by Raman spectroscopy, we propose atomic force microscopy (AFM) surface imaging analysis. In Fig. 3, we present 2D and 3D AFM images of the TiO2 films prepared on silicon substrates, corresponding to the annealing temperatures of 400, 600, 700, 800 and 1000 ◦ C. Fig. 3(a) and (b) shows respectively the surface morphology of TiO2 films annealed at 400 and 600 ◦ C. At 400 ◦ C annealing temperature, the surface morphology indicates a porous and fine structure with small size grains in anatase phase. However at 600 ◦ C (Fig. 3(b)), larger anatase crystal grains are observed. 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 the grain size. These figures show as well the increase of the inter-grain porosity and roughness with increasing the annealing temperature. The TiO2 films root mean square (RMS) roughness analysis has been carried out. As shown in Fig. 4, the RMS follows a similar evolution as the surface morphology observations. The RMS roughness increases slowly from 0.880 to 4.235 nm when the annealing temperature increases from 400 to 700 ◦ C, then reaches 7.270 nm at 1000 ◦ C. The increase of roughness can be interpreted as the phase change, and the increase of grain size (Table 1).

Table 1 The influence of annealing temperature on grain size and roughness of TiO2 film. Temperature (◦ C)

Size grain (nm)

RMS (nm)

400 600 700 800 1000

5.174 27.275 32.25 43.032 41.565

0.880 1.634 4.235 7.704 7.270

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Fig. 3. AFM surface morphology images of the TiO2 thin films of obtained after three dippings and annealing temperatures of 400 (a), 600 (b), 700 (c) 800 (d) and 1000 (e).

3.2. Optical properties 3.2.1. UV-VIS spectroscopy Fig. 5 shows the TiO2 thin film UV–vis spectra for different annealing temperatures, in the wavelength range of 300–1500 nm. TiO2 thin films annealed at temperatures from 400 to 800 ◦ C represent high transparency coefficients in the visible range of 400–800 nm (Fig. 5(a–c)). For higher annealing temperatures

from 1000 to 1400 ◦ C (Fig. 5(d)–(f)), the transmittance of TiO2 thin films is considerably reduced in the wavelength range of 400–900 nm. One can conclude that the films turn out to be opaque for annealing temperatures above 1000 ◦ C. This is due firstly, to the absorption increase resulting from the phase transformation anatase-to-rutile, and secondly, to the light scattering increase with crystallite size and particle clustering. Moreover, non-stoichiometric films would be formed at high annealing

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Fig. 6. Reflectance spectra of the TiO2 thin films obtained after three dippings and annealing temperatures of 400 (a), 600 (b), 700 (c), 800 (d) and 1000 (e). Fig. 4. The influence of annealing temperature on roughness of TiO2 film.

temperatures. These results are in good accordance with those of Raman spectroscopy. In the wavelength range of 500–1500 nm, interference fringes are also visible. In addition it is important to notice the redshift of the TiO2 thin film absorption edges by increasing the annealing temperature. This result is related to the TiO2 thin film band gap energy change due to the phase transformation [34]. 3.2.2. Reflectance The reflectance spectra recorded for the TiO2 samples annealed at different temperatures are shown in Fig. 6. For the annealing temperatures of 800 and 1000 ◦ C, the reflectance reaches 90% in the visible region. Increase in the annealing temperature causes the respective decrease in the thickness and increase of reflectance percentage. It has been previously observed that the film thickness decreases with increasing the annealing temperature and dip speed. The band gap of the samples was determined by the equation: Eg = 1239.8/

(1)

where Eg is the optical band gap (eV) and  (nm) is the wavelength of the absorption edge in the spectrum [35]. Table 2 shows the obtained optical band gap of the TiO2 thin films annealed at different temperatures. It is seen that the optical

band gap decreases from 3.51 to 3.33 eV by increasing annealing temperature from 400 to 800 ◦ C. The decrease of the TiO2 optical band gap with annealing temperature might be the result of the change in film density and increase in grain size. At a temperature exceeding 1000 ◦ C, where the anatase-to-rutile phase transformation takes place, the optical band gap of the thin films decreases considerably because the rutile phase has lower optical band gap compared to the anatase phase. These results are in accordance with those of Raman spectroscopy. 3.2.3. Spectroscopic ellipsometry Spectroscopic ellipsometry (SE) determines the complex reflectance ratio  defined in terms of the standard ellipsometric parameters and  as [36]: =

rp = (tan rs

) · ei

(2)

where rp and rs are the reflection coefficients for light polarized parallel (p) and perpendicular (s) to the sample’s plane of incidence, respectively. Fig. 7 shows the ellipsometric measurement (scatters) and modelling (lines), in terms of tan and cos , as a function of wavelength, for TiO2 annealed samples at 600, 800 and 1000 ◦ C. The aim of the present ellipsometric study is to determine the annealing temperature effect on the optical properties and the optical gap of the TiO2 thin films prepared with sol–gel technique. The best ellipsometric 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 quantum theory of absorption. The formulation is applicable to amorphous semiconductors and dielectrics, crystalline semiconductors, dielectrics and metals throughout the interband region. A general expression of the extinction coefficient k is deduced adding a finite lifetime for the excited state to which the electron transfers due to photon absorption. The optical band gap Eg is identified to the energy for which k(E) has its absolute minimum. Then the refractive index n(E) is deduced from the Kramers–Kroning Table 2 The influence of annealing temperature on the TiO2 film optical band gap.

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

Temperature (◦ C)

Optical band gap (eV)

400 600 700 800 1000

3.51 3.49 3.48 3.33 3.30

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

dispersion relations. The n and k value are given by the following expressions: n(E) = N∞ +

k(E) =









Bq E + Cq



(3)

E − Bq E + Cq

A(E − Eg )2



(4)

E − Bq E + Cq

where N∞ is the refractive index at high energy, Bq is a first intermediate parameter calculated by (A/Q )(−(B2 /2) + Eg B − Eg2 + C) where



Q = 0.5

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

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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 and 1000 ◦ C.

lated by A/Q ((Eg2 + C)(B/2) − 2Eg C), 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 extinction coefficient, where Eg is the optical band gap, N∞ is the refractive index value at high energy, A, B and C are the fitting parameters relative to the three absorption peaks. The effective mass approximation [39] consisting of a mixing of the found TiO2 optical properties and void, which physically describes the surface roughness of the films. This procedure allows as well to find the layer thickness with high accuracy. After finding the best dispersion law,

Table 3 TiO2 best-fit ellipsometric (SE) modelling parameters as a function of the annealing temperature. SE fit parameters

Annealing temperature 600 ◦ C

Layer 1

Thickness (␮m) Void composition

Layer 2

Thickness (␮m) N∞ Eg (eV) P1 peak

P2 peak

P3 peak

Standard deviation 

A B C A B C A B C

800 ◦ C

1000 ◦ C

0.0246 0.14

0.0005 0.29

0.0685 0.18

0.0420 2.3091 3.4506 0.1648 7.8784 15.6580 0.0003 1.8315 0.8415 0.1427 2.8534 −62.6770 0.002

0.0727 2.1355 3.2790 0.1118 8.2528 17.1930 0.0003 1.8272 0.8360 0.6653 13.2970 −292.0840 0.002

0.0574 2.7933 3.0972 1.3108 3.9854 16.670 −0.0315 1.6943 0.9019 0.0014 0.0247 −0.5533 0.007

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the fitting procedure consists of changing the different parameters and find the ones that minimizes the standard deviation . The standard deviation measures the deviation between the theoretical model and the measurement. Fig. 7(a) and (b) shows, respectively, the results of the ellipsometric analysis in terms of refractive index (n) and extinction coefficient (k), as a function of wavelength, for TiO2 samples annealed at 600, 800 and 1000 ◦ C. The best-fit parameters are listed in Table 3. It is important to notice that  is very low (lower than 1%), leading to a good fitting procedure and accurate TiO2 optical properties (see Fig. 8). On the one hand, we can confirm using another optical technique (SE) that the annealing affects the optical band gap, which is found to decrease by increasing the temperature. Within the 2% error with respect to the optical band gap, the obtained Eg are in good agreement with the reflectance results. On the other hand, we can confirm the AFM obtained RMS roughness that is found to increase with the annealing temperature. This is reflected in the void composition that reaches 29% for the annealing temperature of 800 ◦ C, which is also found to have the highest roughness value of 7.702. 4. Conclusion Sol–gel-based nano-crystalline TiO2 thin films were prepared by employing tetrabutyl-orthotitanate as a precursor and the effect of treatment temperature on their structural and optical properties was examined. The TiO2 thin films annealed at temperatures from 400 to 800 ◦ C are in anatase phase, and transform into the anatase-to-rutile phase at 1000 ◦ C, and further into the rutile phase at 1200 ◦ C. The crystallite size of the films increases by increasing the annealing temperature. According to AFM imaging, all films fabricated are uniform, and their density and crystallinity are increased with increasing the annealing temperature. The surface morphology results reveal that the rutile films are denser than the anatase phase. The roughness of the TiO2 thin films increases by increasing the annealing temperature. The deposited TiO2 thin films reveal a high transparency in the visible range. The transmittance of the films annealing between 1000 and 1400 ◦ C is prominently reduced in the wavelength range of 400–1000 nm because of enhanced absorption as a result of the change of crystallite phase and composition in the films and the scattering effect originating from increased grain size. The ellipsometric analysis confirms the results found by Raman, AFM and UV–vis techniques in terms of surface morphology and optical band gap. The optical properties of the films are found to be closely related to the microstructure and crystallographic structure which depend on the annealing temperature. In this study, we have successfully fabricated TiO2 thin films with desired structural and optical properties by sol–gel dip coating method using the titanium alkoxide as a starting material.

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