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2 Thin Solid Films 520 (2012) Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: An investigation of super-hydrophilic properties of TiO 2 /SnO 2 nano composite thin films Mansoor Farbod, Saide Rezaian Physics department, Shahid Chamran University, Ahvaz, Iran article info abstract Article history: Received 9 December 2010 Received in revised form 24 September 2011 Accepted 28 September 2011 Available online 6 October 2011 Keywords: TiO 2 /SnO 2 Nano composite Sol gel Dip coating Hydrophilicity A sol gel dip coating technique was used to fabricate TiO 2 /SnO 2 nano composite thin films on soda-lime glass. The solutions of SnO 2 and TiO 2 were mixed with different molar ratios of SnO 2 :TiO 2 as 0, 3, 4, 6, 8, 9, 10.5, 13, 15, 19.5, 25 and 28 mol.% then the films were prepared by dip coating of the glasses. The effects of SnO 2 concentration, number of coating cycles and annealing temperature on the hydrophilicity of films were studied using contact angle measurement. The films were characterized by means of scanning electron microscopy, X-ray diffraction and atomic force microscopy measurements. The nano composite thin films fabricated with 8 mol.% of SnO 2, four dip coating cycles and annealing temperature of 500 C showed super-hydrophilicity Elsevier B.V. All rights reserved. 1. Introduction In the last decade, the fabrication of nanostructures within the size range of 100 nm has attracted considerable attention due to their potential applications in science and technology. Nanostructures of TiO 2, because of their unique properties, have been most widely used materials for the nano technological studies. TiO 2 thin films prepared by different methods [1 6] have been extensively studied due to their practical applications in various industrial areas such as production of self-cleaning and anti-fogging surfaces [7]. These phenomena arise due to the photocatalytic and hydrophilic properties of TiO 2. By increasing the hydrophilicity and wettability character of a surface which depends on surface microstructure, surface chemical composition and on geometry of the surface, water can spread better over the surface and improves the cleaning character of the surface. Many attempts are now made to increase the hydrophilicity of TiO 2 films [8 10]. It seems that the hydrophilicity can be improved by doping of TiO 2 [8,11 13]. Also TiO 2 films show a high hydrophilicity under ultraviolet irradiation [5,14,15]. In this study, TiO 2 /SnO 2 nano composite thin films on soda-lime glass were fabricated using a sol gel dip coating technique and showed super hydrophilicity without irradiation of UV light. The properties of the films were studied by scanning electron microscopy Corresponding author. Tel./fax: address: farbod_m@scu.ac.ir (M. Farbod). (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and the hydrophilicity of the films by contact angle measurements. 2. Experimental 2.1. Preparation of thin films Preparation of the TiO 2 /SnO 2 composite sols In order to prepare TiO 2 /SnO 2 nano composite precursor solution, TiO 2 and SnO 2 sols were first made separately. Tetra-n-butyl titanate, absolute ethanol and deionized water in the molar ratios of 1:82:3 were used respectively for TiO 2 sol preparation. Tetra-n-butyl titanate ((C 4 H 9 O) 4 Ti) was added drop wised to the absolute ethanol under vigorous stirring. After 30 minutes stirring at room temperature, 2 ml of HNO 3 was mixed with 20 ml deionized water and was added to the solution to adjust the ph=3.5. After 4 h stirring, a transparent yellowish solution was obtained which was stable for more than two months at various concentrations. SnO 2 sol was prepared by dissolving stannic chloride in absolute ethanol, followed by vigorous stirring and refluxing for 2 h at 80 C. The molar ratios of SnCl 4. 5(H 2 O), absolute ethanol were chosen 1:62 respectively. The SnO 2 sol was obtained by stirring the solution for 24 h at room temperature. The composite sols were then prepared by mixing of TiO 2 and SnO 2 solutions with different molar ratios of SnO 2 to TiO 2 as 0, 3, 4, 6, 8, 9,10.5, 13, 15, 19.5, 25 and 28 mol.% followed by stirring at room temperature for 15 min. All of the composite sols were transparent yellowish /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.tsf

3 M. Farbod, S. Rezaian / Thin Solid Films 520 (2012) Dip coating Soda lime glass was used as the substrate for thin films. They were carefully cleaned in acetone, ethanol and double distilled water. The TiO 2 /SnO 2 films formed on the substrates were prepared from the above TiO 2 /SnO 2 solution by dip-coating process at ambient atmosphere. The speed of dipping and withdrawing the substrate was 2 mm/s. The substrate was relaxed in the solution for 10 s then pulled up at the same speed. After each coating, the films were allowed to dry at room temperature for 10 min. The procedure was repeated to obtain multiple coatings. The coated substrates were dried in a vertical position for 30 min in an ambient atmosphere. The films were finally annealed at different temperatures ranging from 400 to 600 C for 1 h with the heating and cooling rates of 5 C min 1 in air. Contact angle (deg.) SnO 2 :TiO 2 molar ratio (%) Fig. 1. Water contact angle of films with 2 coating cycles, heat-treated at 450 C and different doping levels Film characterization The characterization of films was performed by measuring SEM using a Leo, 1455 VP scanning electron microscope operating at 20 kv and XRD using a PW-1840 Philips diffractometer (Bragg- Brentano configuration) at room temperature utilizing Cu K α radiation with the wavelength of A. The peak position and intensities were obtained between with a velocity of 0.02 /s. The uniformity and surface morphology of the prepared films were characterized by using a DME DS95-50E atomic force microscopy in the operating AC dynamic mode. The cantilever was an Al coated one with the length of 160 μm and tip height of μm Hydrophilic property measurement The hydrophilic property of thin films was evaluated by measuring the static contact angle between de-ionized water and films at ambient conditions at room temperature. The pictures of droplets were taken using a digital camera (Canon power shot G9) and a goniometer was used to determine the contact angle. In order to ensure repeatability, the volume of the attached water droplets was always chosen the same. Also, the contact angle was measured for droplets which were placed at different positions of the films and the averaged value was adopted as the contact angle. Then the effect of SnO 2 content, the number of dip coating cycles and the annealing temperature on the hydrophilicity of films were investigated. 3. Results and discussions 3.1. Effect of SnO 2 content on hydrophilicity As mentioned in Section several films with different molar ratios at.% of SnO 2 :TiO 2 were prepared. The measured contact angles of the films with two coating cycles and heat treated at 450 C against the SnO 2 content are shown in Fig. 1. The results indicated that the lowest contact angle was found for the films with SnO 2 doping levels of mol.%. These doping levels were used to investigate the other effects Effect of the annealing temperature on hydrophilicity Again three sets of films with molar ratios of 8, 9 and 10.5 at.% of SnO 2 :TiO 2 and 4 coating cycles were prepared. The films then were heated at 400, 500 and 600 C for 60 min. The heating and cooling rates were chosen 5 C min 1. By measuring the contact angles for the different films which are plotted in Fig. 3, it was found that the films which were annealed at 500 C had lowest contact angle and surprisingly less than 5, showing that highly hydrophilic states were achieved. The contact angle for the film with molar ratio of 8 at.% of SnO 2 :TiO 2 and 4 coating cycles was 0, demonstrating super-hydrophilicity. These conditions are in fact the optimal conditions for preparation of superhydrophilic films. Fig. 4 shows a picture of super-hydrophilicity of such a film. It can be seen from Fig. 4 that the water has completely spread over the film surface. For the films with the contact angle less than 5, the antifogging property was observed on the steamed surface. The hydrophilicity is believed to relate to the density of surface hydroxyl of the TiO 2 films [10,15]. Using X-ray photoelectron spectroscopy experiment, Yu et al. [10] have shown that the Hydroxyl groups exist in the coating films. They believe that the H 2 O can chemically and physically adsorb on the surface of TiO 2 coating films and react with the surface to form Ti OH. By increasing the hydroxyl groups, van der waals forces and hydrogen bonds interactions between H 2 O and OH increase so water can easily spread across the surface and the hydrophilic property increases. It seems that the hydrophilicity of the films can be improved to a limited extent by doping of SnO 2. The dip-coated TiO 2 /SnO 2 films in this study seem to have all the properties required for superhydrophilicity. Four times dip-coating, doped with molar ratios of at.% of SnO 2 :TiO 2 and annealed at 500 C gives the most promising results of super-hydrophilicity TiO 2 films. We found also that the prepared TiO 2 composite films showed coloration phenomenon. Their colors strongly depended on the film thickness. As the thickness of 3.2. Effect of coating cycle times on hydrophilicity In order to investigate the effect of film thickness on hydrophilicity, three sets of films were prepared with molar ratios of 8, 9 and 10.5 at.% of SnO 2 :TiO 2. Each set consisted of three samples with different coating cycles of 2, 4 and 6. The annealing temperature was chosen again 450 C. The measured contact angles are plotted against the number of coating cycles are shown in Fig. 2. As can be observed, the films with 4 coating cycles exhibit the lowest contact angle and so the highest hydrophilicity. Fig. 2. Effect of film thickness on the contact angle of TiO 2 /SnO 2 films heat-treated at 450 C and different SnO 2 concentrations.

4 1956 M. Farbod, S. Rezaian / Thin Solid Films 520 (2012) Fig. 3. Effect of annealing temperature on the water contact angle of TiO 2 /SnO 2 film with 4 coating cycles and different molar% of SnO 2. the films increased by increasing the number of coating cycles, the coloration became faint, while the films remained still transparent SEM characterization of thin films SEM images obtained from different composites indicated that the film composite grain sizes are in nanometer range. Fig. 5 shows SEM image of pure TiO 2 and composite films prepared under the same conditions. SEM observations showed that grain sizes of composite samples are about 40 nm which are less than that of the pure samples AFM observations Uniformity of the prepared TiO 2 /SnO 2 thin films and the surface morphology of the films were observed by atomic force microscopy Fig. 5. SEM image of a) puretio 2 film, b) TiO 2 /SnO 2 film with molar ratio of 8 at.% of SnO 2 : TiO 2, 2 coating cycles and annealed at 500 C. (AFM). Fig. (6a) and (b) show the 2D AFM images of pure TiO 2 and composite films prepared using 1 and 4 coating cycles respectively, annealed at 500 C for 1 h. The AFM image of surface topology of the pure TiO 2 and the composite film prepared under optimal doping with 4 coating cycles and annealed at 500 C were taken. The data of these images along the profile lines and same images for the films prepared with one coating cycle indicated that the films with 4 coating cycles were more uniform than those with one coating cycle. It has also been found that the composite films were more uniform than the pure samples XRD patterns Fig. 4. Pictures of water droplets on the TiO 2 /SnO 2 films with molar ratio of 8 at.% of SnO 2 : TiO 2, 4 coating cycles and heat treated at a) 400 C, b) 500 C, c) 600 C. The XRD measurements were performed for all prepared films under different conditions. Figs. 7 and 8 show the XRD patterns of pure TiO 2 film and TiO 2 /SnO 2 composite films with molar ratio of 8 at.% of SnO 2 :TiO 2 and various coating cycles, annealed at 500 C. As can be seen from the figures, we could not obtain diffraction peaks from the films prepared by 4 and even 20 coating cycles due to very thin character of the films. So we had to increase the number of coating cycles to 50, in order to perform phase identification. The phase identification showed both the pure TiO 2 and the TiO 2 / SnO 2 composite form anatase structure. It is worth noticing that both TiO 2 and SnO 2 patterns have some peaks at 25.3, 55 and 73 so we cannot observe the trace of SnO 2 in the x ray pattern of the composite film. No trace of Na was observed in the x ray pattern of the samples which could migrate from the soda lime glass substrate and diffuse in the films. The energy dispersive X-ray results showed some small trace of Na in the films with two coating cycles. Also our observation showed that the diffraction peak width of the TiO 2 /SnO 2 composite is broader than that of the pure TiO 2 which indicates that the average size of nano particles in the composite is smaller than that of the

5 M. Farbod, S. Rezaian / Thin Solid Films 520 (2012) Fig. 6. AFM images of TiO 2 films with molar SnO 2 content of 0 (a) and 8 at.% (b), prepared using 1 and 4 coating cycles and annealed at 500 C for 1 h. Fig. 7. XRD pattern of Pure TiO 2 films with 4 and 50 coating cycles, annealed at 500 C. Fig. 8. XRD pattern of TiO 2 /SnO 2 film with molar ratio of 8 at.% of SnO 2 with 20 and 50 coating cycles, annealed at 500 C. pure TiO 2. This can be attributed to the existence of SnO 2 that reduces the growth rate of TiO 2 particles. The reason for such a phenomenon is that the charges of tin ions are transferred to the TiO 2 sol particles in the composite causing an increase of the surface charge of the TiO 2 particles sol and keeping the particles away from each other. In this way the particles are most likely formed in smaller sizes. 4. Conclusion Super-hydrophilic TiO 2 /SnO 2 nano composite thin films were prepared using dip coating technique. The hydrophilicity was investigated by contact angle measurements. The results showed that the super-hydrophilicity is achieved for the composite films with molar ratio of 8 at.% of SnO 2 :TiO 2, 4 coating cycles and annealed at 500 C. Such films had anatase phase structure and based on SEM observations, the nanoparticles sizes of the composite films were smaller than that of the pure TiO 2. The composite films were more uniform than the pure TiO 2 films, which were revealed by AFM measurements. Acknowledgment The authors acknowledge Shahid-Chamran University of Ahvaz for financial support of this work and Mr Rostami Kia for his help.

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