Titanium finds more and more applications today. Over 96% of the world-wide use of titanium is in its oxide form, TiO₂ (titanium dioxide), thus creating a high demand for this material, due to a wide range of potential applications for environmental purposes [1]. It is extensively used for photo degradation of organic and inorganic pollutants [2], photovoltaic energy production [3], hydrogen production by water photo-splitting [4,5], and gas sensing [6,7]. This variety of applications is because of TiO₂ low cost, non-toxicity, as well as useful optical, physical, chemical, and electronic properties, including excellent transmittance of visible light, photo catalytic behavior, high dielectric constant, high refractive index, and high chemical stability [8,9].

It is well known that TiO₂ is an n-type semiconductor with an indirect energy band gap [6,10,11]. It generally crystallizes in three phases, the tetrahedral anatase (space group I41/amd, density=3.894 g/cm²), rutile (space group P4₂/mnm, density=4.25 g/cm²) and orthorhombic brookite (space group Pcab, density=4.12 g/cm²). The anatase and rutile phases belong to different space groups but both have a tetragonal crystal lattice. Rutile is the most stable form of TiO₂, whereas anatase and brookite are metastable and transform to the rutile phase upon heating [6,12-14]. Conventional methods to prepare TiO₂ thin films are pulsed laser deposition (PLD) [15,16], RF magnetron sputtering [17-19] plasma enhanced chemical vapor deposition [20], and sol-gel methods [6,15,21,22], among others. Focusing on the solgel method, it is one of the most attractive techniques for thin-film deposition, because of low cost, non-vacuum requirements, large area deposition, and low temperature processing. Furthermore, the sol-gel method produces thin films which have good homogeneity, excellent compositional control, and good electrical and optical properties [23].

The effect of the annealing temperature on TiO₂ powders has been often reported in the literature [11-13]. Mathpal et al. have studied the effect of annealing temperature on the structural and luminescence properties of TiO₂ powders [12]. Another study has investigated the effect of the annealing temperature on TiO₂ thin films, prepared by the sol-gel method and dip-coating techniques [22]. These studies have that the annealing temperature influences the structural and optical properties of TiO₂ thin films. Fewer articles investigate the phase transition (from anatase to rutile) of TiO₂ thin films obtained by the sol-gel and spin coating techniques with annealing temperature, as most articles study the properties of these films at lower temperatures [6].

Hence, we performed this work to understand the physical properties of TiO₂ sol-gel more systemically and deeply. The TiO₂ thin films are prepared by the sol-gel spin coating method and annealed between 40 and 900°C. The variation of structural properties of the thin film was studied through Raman, FTIR, PL spectroscopy and AFM microscopy. The crystalline structure, surface morphology and optical properties of the TiO₂ thin film were then studied and discussed. All results indicate the dependence of the formed phases (anatase, rutile, or mixed anatase-rutile phase) of TiO₂ thin films on the annealing temperature and indicate a phase transition which starts at 600°C. This simple and no toxic method provided us results similar to those given by other method more difficult and requires hard realization conditions, in one hand. In other hand, we have shown that TiO₂ thin films deposed in silicon substrates is a good agent to produce antireflecting coating for solar cells.

The precursor solutions of TiO2, prepared by the sol-gel method, were obtained using titanium (IV) isopropoxide (C₁₂H₂₈O₄Ti) as the starting material. The titanium (IV) isopropoxide was initially added in a mixture of ethanol and monoethanolamine (MEA) and the concentration of titanium (IV) isopropoxide was 0.5 M. The resultant solution was stirred at 80°C for 1 h to yield a clear and homogeneous solution. In order to prepare TiO₂ thin films, the obtained solution was deposited on silicon substrates by spin coating, with a rate of 3000 rpm for 30 s. The (100) silicon substrates were cleaned with acetone for 10 min, methanol for 10 min, rinsed with deionized (DI) water for 15 min, and then dried with nitrogen. After the spin-coating step, the films were pre-annealed at 300°C for 15 min. Finally, all films were annealed in air at different temperatures in the range of 400 to 900°C for 1 h.

The structural characteristics of the films were investigated by a Raman Jobin Yvon Lab RAM HR spectrometer, using the 632.8 nm irradiation from a He-Ne laser (at 3 mW) and Fourier transform infrared spectroscopy (FTIR). An atomic force microscope (AFM) (Nanoscope III) was used in tapping configuration (Veeco AFM head RTESP silicon pur) to scan an area of 2 μm × 2 μm. The obtained results were used to estimate the surface roughness and the grain size in the film. Standard software was used to calculate the root-mean-square (RMS) roughness and the grain size. The grain size was defined as the diameter of the grain at the middle of the height. Photoluminescence (PL) measurements were performed at room temperature using the fluorescence spectrophotometer (LS 45) with an excitation wavelength of 266 nm. Finally, optical reflectivity spectra were recorded with a Perkin Elmer Lambda 19 spectrophotometer in the 300-1200 nm range.

Structural properties

Raman spectra: Figure 1 shows the Raman spectra of TiO₂ thin films produced by the sol-gel method and deposited on silicon substrates, for various annealing temperatures. The spectra exhibit an active mode centered around 665 cm^-1, which is attributed to the Eg mode of the anatase phase [12,24]. For all annealing temperatures, Raman scattering reveals the presence of mixed (anatase and rutile) Raman active modes. The 665 cm^-1 Eg peak position is shifted to higher wavenumbers compared with literature values [24], which indicate a decrease of the particle size [25]. The spectra also exhibit three main phonon modes, B1g, Eg, and Ag, which appear at 140, 433, and 617 cm^-1, respectively. This result confirms the presence of a rutile phase TiO₂ [26,27]. The B2g mode of rutile TiO₂ centered at 822 cm^-1 appears clearly after annealing at 800°C, which shows that the rutile phase becomes gradually the dominant phase starting at 800°C, indicating that the anatase-rutile transition temperature should be less than 800°C. Both phases crystallize in a tetragonal structure [6,12-14]. Two strong peaks at 302 cm^-1 and 521 cm^-1 represent the Si-Si stretching mode of the substrate [28]. The peak at 939 cm^-1 is assigned to the stretching vibrations of Si-OH [29].

Figure 1: Raman spectra of TiO₂ thin films prepared on silicon substrates, annealed at different temperatures (400°C-900°C).

FTIR spectra: Figure 2 shows the absorbance spectra of annealed TiO₂ films at different temperatures (from 400-900°C) recorded by FTIR spectroscopy. The samples annealed at 400°C show a broad hump ranging from 621 nm to 2314 nm, which can be attributed to poor crystallinity and disappears by increasing the annealing temperature. The peak at 2360 cm^-1 results from adsorbed H₂O molecules, which were not completely removed after sol-gel growth. The peak at 1242 cm^-1 corresponds to the vibration mode of Ti-OH which is the terminal hydroxyl groups and it’s far below the top of the valence band of TiO₂ rutile phase [30,31]. This corresponding band starts to appear at 600°C and his intensity increases by increasing temperature. The peak at 436 cm^-1 corresponds to the O-Ti-O transverse mode of the anatase TiO₂ phase [32-34]. The band centered at 1072 cm^-1 appears for the samples treated at 600, 700, 800 and 900°C, and is attributed to Ti-O-Ti vibrations according to rutile phase [31,32]. We find that the intensity of this band increases with increasing annealing temperature. This indicates that the number of Ti-O-Ti vibrations is also growing with increasing annealing temperature.

Figure 2: Fourier transform infrared spectra of TiO₂ films annealed at different temperatures (400°C-900°C).

Morphological properties

To evaluate the surface roughness and the grain size of the TiO₂ films, an area of 2 μm × 2 μm has been scanned by AFM in tapping mode. The two-(2D) and three-dimensional (3D) AFM images for different annealing temperatures are shown in Figures 3a-3c. The sample grains become bigger when the temperature increases from 400°C to 900°C and the surface becomes rougher. The AFM 2D and 3D images show that the temperature affects the density and the size of TiO₂ grains significantly. The grains have been sorted according to their size in order to determine the surface density and grain diameter of the samples. Figures 4a-4c show the obtained histograms of the grain size distribution for three different annealing temperatures. From the AFM images we also find the average RMS roughness values which are 52, 35 and 97 nm for films annealed at 400, 600 and 900°C, respectively (Figure 5). Obviously, a significant increase of the grain size and RMS roughness is observed with the increase of the annealing temperature. This is due to the transformation of TiO₂ from the anatase to the rutile phase, which involves the coalescence of smaller particles into bigger ones [29,35]. The AFM results are in good agreement with the Raman and FTIR findings, which show the dependence of structural changes on thermal annealing: TiO₂ film crystallization is accompanied by densification, as observed on the AFM images.

Figure 3: 2D and 3D AFM images (2 μm x 2 μm) of TiO₂ films on silicon substrates annealed at (a) 400°C, (b) 600°C, and (c) 900°C.

Figure 4: Grain size distribution of TiO2 thin films on silicon substrates, annealed at (a) 400°C, (b) 600°C, and (c) 900°C, obtained from AFM measurements.

Figure 5: Surface roughness of TiO2 films on silicon substrates, annealed at (a) 400°C, (b) 600°C, and (c) 900°C, obtained from AFM measurements.

Optical properties

Photoluminescence: PL spectra of the samples were recorded at room temperature in the wavelength range from 300 to 700 nm (400- 700 nm shown in Figure 6). The overall PL intensity increases between 400°C and 500°C. The highest PL intensity for the sample annealed at 500°C is probably due to self-trapped exciton recombination, which is a combined effect of defect centres generated from oxygen vacancies and particle size [12]. For all higher temperatures, the overall PL intensity decreases systematically with annealing temperature, even though a remarkable change of relative PL intensities for the samples annealed at 600°C and 700°C is observed. Moreover, for 600°C to 900°C the PL intensity for wavelengths shorter than 522 nm is lower than the intensity for longer wavelengths. This behavior is probably due to a mixed anatase-rutile phase. As shown in Figure 6, the PL spectra present a peak at 486 nm (2.55 eV) and a broad band around 585 nm (2.11 eV). The 585 nm band is attributed to the anatase phase and the 486 nm peak to the rutile phase [36]. The emission at 486 nm can be attributed to charge transfer from Ti3+ to the oxygen anion in a Tio⁸⁻₆ complex, associated with oxygen vacancies at the surface. There are three types of defects (oxygen vacancies, Ti⁴⁺ and Ti³⁺ interstitials), among which the formation of defects associated with Ti³⁺ is energetically more favorable. Therefore, this type of defect may be formed predominantly during the crystallization of anatase TiO₂, indicating this band originates from intrinsic rather than surface states [12,37]. As Chung et al. indicate, that, when the structure is a mixture of anatase and rutile phases at low oxygen flow ratio, additional electron states in the band gap are generated, which results in the occurrence of the 585 nm band [36]. In addition, a new radiative transition occurs, which leads to a new PL peak at 486 nm at the rutile phase, increasing with annealing temperature [38].

Figure 6: Room temperature PL spectra of TiO2 thin films on silicon substrates annealed at different temperatures (400°C-900°C).

The 465 nm peak and the two overlapping peaks centered at 539 nm are attributed to defect states due to oxygen vacancies [12]. The PL intensity of these peaks decreases as the annealing temperature increases; this result indicates that oxygen vacancies disappear gradually with annealing temperature.

The phase transition (anatase to rutile) starts at 600°C and the mixed anatase-rutile phase appears at higher temperatures, as we can see by comparing the intensities of the two characteristics bands of anatase and rutile phases (585 nm and 486 nm, respectively). The intensity of the anatase band at 585 nm decreases and then gradually disappears for higher annealing temperatures, while the intensity of the rutile band at 486 nm still exists with annealing temperature, therefore we can speak about phase transition. In addition, the development of other broad bands around the 539 nm peak may indicate the presence of other shallow traps and defect states inside the band gap and mainly originate from the morphology distribution of the nanostructures [12].

Reflectivity: The reflectivity of the silicon substrate and TiO₂ films on silicon for different annealing temperatures has been measured in the wavelength range from 300 to 1200 nm. Figure 7 indicates that the reflectivity of the films decreases from 37 to 13%, by comparing it with silicon reflectivity, in the region from 400 to 800 nm. The reduced reflectivity is usually explained by secondary reflections of the incident light at the sidewalls between grains. This reflectivity decrease indicates an efficient control of the antireflecting behavior of TiO₂ thin films [39,40].

Figure 7: Reflectivity spectra of TiO₂ films, annealed at different temperatures (400°C-900°C).

Titanium oxide thin films were deposited on silicon substrates by using the sol-gel spin coating method and annealed at different temperatures (400°C-900°C). The Raman, AFM, and FTIR results indicate the dependence of the formed phases (anatase, rutile, or mixed anatase-rutile phase) of the TiO₂ films on the annealing temperature. The films annealed from 400°C to 900°C show the PL peaks at 486 nm (2.55 eV) according to rutile phase, while the 585 nm band (2.11 eV) confirms the presence of the anatase and disappears gradually for temperatures higher than 600°C. Furthermore, the deposition of TiO₂ thin films allows the control of the antireflecting properties of silicon substrate between 13% and 37% in the 400-800 nm region.

Part of this work has been done at National Hellenis Research Foundation. Theoretical and Physical Chemistry Institute, 48 Vasileos Konstantinou Ave., 11635 Athens, Greece and financed by the Tunisian Ministry of Higher Education and Scientific Research.