Persistent hydrophilicity for Titanium oxide ( TiO 2 ) thin films by Silicon oxide ( SiO 2 ) over nanolayers

SiO2 thin layers in thicknesses (1, 5, 10, 18 nm) on TiO2 thin layer in thickness 79 nm deposited by reactive RF sputtering technique. The deposited films were heat treated at temperatures (200, 400, 500, 600°C). The surface properties of thin films by atomic force microscopy (AFM), surface chemical composition by X-ray photoelectron spectroscopy (XPS) and self-cleaning effect in bi layers, were studied. In addition, enhanced hydrophilicity property in the films under the effect of annealed temperature and persistence without UV light illumination were evaluated.


INTRODUCTION
Titanium oxide exhibits hydrophilicity and self-cleaning properties as exposed to UV.As TiO2 surface is exposed to UV radiation, electron/hole pairs are created inducing an oxide-reduction reaction.To maintain the status of surface oxygen vacancies by OH groups that exist in atmospheric water, superhydrophilicity property (that is, water contact angle of near zero with surface) on the surface is generated (Wang et al., 1999;Sakai et al., 2001).However, a superhydrophilic surface is converted to an hydrophobic surface in the absence of UV illumination due to replacement of OH groups with atmospheric oxygen.As reported by other researchers (Machida et al., 1999;Ren et al., 2004;Guan, 2005;Lee et al., 2004;Maeda and Yamasaki, 2003;Yu et al., 2002) the addition of SiO2 to TiO2 enhances the hydrophilicity and it is maintained in dark without UV light radiation.According to X-ray photoelectron spectroscopy experiments, the formation of Ti-O-Si bond at the TiO2-SiO2 interface led to important changes in the electronic structure of over layer (Sanz et al., 1998).This changes enhanced the acid property of surface and result in improving hydroxyl groups at the surface of film (Guan et al., 2003(Guan et al., , 2005)), which led to increasing hydrophilicity and the surface of film maintained.In this work, SiO2 was more physical and chemically stable than TiO2 and also Si-OH surface bond was more stable than Ti-OH; also, bi-layer films was deposited with SiO2(top)/TiO2(under) (Komatsu et al., 1998;Guan et al., 2004).More recently, it was observed that SiO2-TiO2 interface is formed in the sufficiently high annealed temperatures (Permpoon et al., 2008).It was found that the thickness of SiO2 on TiO2 layer had important effect on increasing of hydrophilicity property (Hattori et al., 2000).In other researches, enhanced natural superhydrophilicity and its conservation in TiO2-SiO2 composite thin films or bi-layer films deposited in method of sol-gel at special temperatures and thickness have been investigated (Houmard et al.,2007;Liu et al., 2009).In this study, we deposited the SiO2/TiO2 bi-layer films by using radio frequency reactive magnetron sputtering at different thicknesses of SiO2 over layer and various annealed temperatures and evaluated the hydrophilicity and persistence in a dark place.

Fabrication of SiO2/TiO2 bilayer films
SiO2/TiO2 bilayer films were deposited on float glass substrates (Kaveh Glass Industry Group) by using radio frequency (RF) reactive magnetron sputtering.The base pressure of the sputtering chamber was ~10 -7 Torr (133322×10 -01 Pa).To grow the TiO2 (SiO2) target (with purity of 99.9%) was reactively sputtered at pressure of 100 m Torr (13.3322Pa) in an Ar/O2 (60/40) discharge gas.Thickness of the as-deposited-beneath TiO2 layer was considered 80 nm, while thickness of the as-deposited SiO2 over layer was adjusted 1, 5, 10 or 18 nm.Thickness of the deposited films was measured by using an interferential optical technique.The as-deposited bilayer films were then annealed at 200, 400, 500, 600°C in air for 30 min.

Hydrophilicity and material characterizations
Surface hydrophilicity of the bi layer films was evaluated by measuring water contact angle on surface of the films.Experiments were performed under an ambient condition using a digital camera (with 2 mega pixel resolution) and suitable software (LB-ADSA).Before any experiment, surfaces of the films were washed with distilled water, acetic acid and ethanol.Then, a distilled water droplet was dropped on the film and water contact angle was measured with accuracy better than ±5°.Then, the superhydrophilic samples (with the contact angle < 5° stored within plastic covers in dark to check any variation in the water contact angle of the films by elapsing the time (up to six weeks) under no UV irradiation.Super-hydrophilicity of the films was also studied by measuring the water contact angle under UV light illumination.In this work, a 6-W mercury (Philips) UV source with wavelengths ≥254 nm was used.To evaluate self cleaning effect, surface of the films was contaminated by the special oil and after exposure to UV illumination.To study the surface topography of the films, a Park Scientific Model CP-Research (VEECO) atomic force microscopy (AFM) was utilized at a non-contact mode.To investigate the surface chemical states of the films, X-ray photoelectron spectroscopy (XPS) was used.The XPS data were acquired by using a hemispherical analyzer with an AL K X-ray source operating at energy of 1486.6 eV and a vacuum better than 10 -7 Pa.The binding energy of the XPS peaks was calibrated by fixing the C(1s) peak to 285 eV.SDP Ver.4.0 software was applied to deconvolute and analyze the XPS peaks.The peaks were deconvoluted by using Gaussian-Lorentzian components (90 to 10%) with a constant FWHM for each component after a Shirley background subtraction.The elemental compositions of the films were determined using area ratio of the deconvoluted XPS peaks and the sensitivity factor (SF) of each element in XPS.

Hydrophilicity and self-cleaning
Figure 1 shows hydrophilicity of the SiO2/TiO2 thin films(with various thicknesses of the SiO2 nanolayer ranging from 1 to 18 nm) annealed at the different temperatures of 200, 400, 500, 600°C, as compared to that of the as-deposited film, in the absence of UV irradiation.The as-deposited films in all thicknesses of SiO2 nanolayer showed contact angles >35°.In case of the SiO2/TiO2 with thickness of 1 nm, very good hydrophilicity with contact angle ~5° at annealed temperatures of 400, 600°C and also thickness of 18 nm at annealed temperatures of 500, 600°C was observed.With regard to the above results, the annealing temperature in SiO2 various thicknesses play an important role in enhanced hydrophilic property without UV irradiation.
To check the self-cleaning property of the bilayer films, we selected the films which initially presented a hydrophilic property with the water contact angle <20 in the absence of UV irradiation (based on the results of Figure 1).Then, the films were contaminated by the oil layer and exposed to the UV irradiation for 4 h.Table 1 shows water contact angle on surface of the contaminated SiO2/TiO2 bilayers before and after UV irradiation.It is seen that the oil contamination resulted in increasing the water contact angle of the films from >5° around 30°, indicating disappearing of the superhydrophilic property of the films.However, after 4 h UV irradiation the films showed their natural superhydrophilic property, indicating the role of the beneath TiO2 layer as an effective photocatalyst for removing the oil contamination layer.
In order to study persistence of the superhydrophilicity of the film photocatalitically under UV irradiation, the water contact angles were measured in one week time intervals as shown in Figure 2. The SiO2(5 nm)/TiO2 annealed at 500°C was the longest superhydrophilicity (up to six weeks) among the samples.It should be noted that the superhydrophilicity of these films reappeared  In Figure 2, the water contact angles with surface of the films were represented at the time of removing of the superhydrophilicity property and the contact angles were measured 5° at the times of before.

AFM analysis
To investigate the effects of heat treatment on surface topography and consequently on the hydrophilicity of the films morphology, AFM was utilized.Figure 3 shows AFM image of the SiO2(5 nm)/TiO2 films (the films with the longest superhydrophilicity) before and after annealing at 500°.
For evaluating the effect of surface roughness on the hydrophilicity of the thin films, the water contact angles measured on surfaces of the SiO2(5 nm)/TiO2 films were compared with the water contact angles modified through Wenzel's equation (Wenzel, 1949) (the contact angles in which the effect of surface roughness was eliminated), as shown in Figure 4.It is seen that although surface roughness could be partially effective in decreasing the measured contact angle especially at annealing temperatures ≥400°C, it was not the main parameter describing the superhydrophilicity of films and its variation.In fact, surface chemical composition can be considered as one of the important parameters in this regard, as discussed in the following.

XPS analyses
To determine the chemical state and surface stoichiometry of the layers, the samples were analyzed by XPS.The survey scans of the SiO2/TiO2 thin films (with the SiO2 thicknesses of 1 and 5 nm annealed at 500 and 600°C, respectively) are presented in Figure 5.The survey spectra show the presence of Si, Ti, O, Na, C on surface of the films.In the XPS survey scans, Silicon related to the SiO2 overlayer was obvious clearly.Also, Titanium obtained from the TiO2 underlayer was observed on the surface of the films and therefore can be an expected possibility of the mixed oxide (Ti-O-Si) on the surface of the films.In this case, we will discuss it in  The Ti(2p) peaks of the as-deposited and annealed SiO2(5 nm)/TiO2 thin films are shown in Figure 6.There are two peaks corresponding to the Ti(2p3/2) and Ti(2p1/2) components at binding energies of 458.58 and 464.18 eV, respectively.Deconvolutions of the Ti(2p3/2) peaks indicated that the as-deposited films contained a slight amount of Ti 3+ chemical state (with Ti 3+ / Ti 4+ ratio of 0.003), while after annealing at 500°C, no significant trace relating to the presence of Ti 3+ was found.The binding energies relating to the Ti 3+ and Ti 4+ (as the predominant chemical state of the films) were considered at 456.7 and 458.5 eV, respectively (Kumar et al., 2000).
For studying the chemical state of the Si in thin films, the Si(2p) peaks was recorded and deconvoluted, as shown in Figure 7.Each peak was deconvoluted into two components (2p3/2, 2p1/2) with separation energy of 0.6 eV and area ratio of 2 which is calculated from the splitting theory of 2p levels.Figure 7 shows the Si(2p) spectra of the as-deposited and annealed SiO2(5 nm)/TiO2 films.Taking into consideration the binding energy 103.2 eV for pure SiO2 and in comparison with the binding energy values at our samples, compound structure of SiO2 thin layer was concluded (Netterfield et al., 1989;Babapour et al., 2006).As shown in Figure 7 and Table 2 with increasing of the annealing temperature, the binding energy values of Si(2p) enhanced to the higher energy values.This shift in the Si(2p) binding energy indicates forming of a mixture oxide (Ntterfield et al., 1989;Tachibana, 2000).The results of our XPS analysis for the Ti(2p) and Si(2p) peaks were summarized in Table 2.
The O(1s) spectra of the as-deposited and annealed SiO2(5 nm)/TiO2 bi-layer films are presented in Figure 8.The O(1s) peak of the as-deposited sample was deconvoluted into two components at binding energies of 530.88 and 532.71 eV.The two deconvoluted peaks were assigned to formation of Ti-O-Ti (peak A) and Si-O-Si (peak B).However, another peak (peak C) was found in the deconvoluted O(1s) peak of the film annealed at 500°C at the binding energy of 531.32 eV which was assigned to formation of Ti-O-Si bond in the annealed film, consistent with the previous reports (Permpoon et al., 2006;Yamashita et al., 1998;Lassaletta et al., 1995;Lin et al., 2002;Gallas et al., 2002;Almeida, 1998).Our results demonstrated that the naturally superhydrophilicity of the annealed films in the absence of UV irradiation and the persistence of the superhydrophilicity can be assigned to formation of this bond (Ti-O-Si bond) in the SiO2-TiO2 nanocomposite.Our XPS analysis also showed that the heat treatment resulted in increasing the concentration of Ti on surface of the films (Table 2), and consequently, increasing the

Figure 1 .
Figure 1.Water contact angle on surface of the as-deposited and annealed SiO2/TiO2 bilayer films in the absence of UV irradiation at room temperature for various thicknesses of the SiO2 layer: a) 1, b) 5, c) 10, and d) 18 nm.

Figure 4 .
Figure 4. ) Measured and ◊) calculated contact angles (based on Wenzel's equation) on surface of the SiO2(5 nm)/TiO2 films annealed at the different temperatures.

Table 1 .
Results of the self-cleaning experiments.

Table 2 .
XPS data of the SiO2/TiO2 thin films for different temperatures.