Surface and Coatings Technology 173 (003) 19 3 Enhanced effect and mechanism of SiO addition in super-hydrophilic property of TiO films a, c b Kaishu Guan *, Baojun Lu, Yansheng Yin a Research Institute of Chemical Equipment, East China University of Science and Technology, Shanghai 0037, PR China b Key Laboratory for Liquid Structure and Heredity of Ministry of Education, Shandong University, Jinan 50061, PR China c Laiyang Agricultural College, Shandong 6500, PR China Received 4 October 00; accepted in revised form17 March 003 Abstract Relationship between the effect and the amount of SiO addition on the photo-generated hydrophilicity of TiO films is investigated. X-ray diffraction, UV Vis spectrumand Fourier transforminfrared spectroscopy are used to determine filmbehavior and the hydrophilicity is assessed by contact angle measurement. It is found that addition of 40 mol% SiO to TiO is most effective for reducing the contact angle of water. The contact angle of water can maintain low for a long time in dark place. SiO addition has a suppressive effect on the crystal growth of TiO in calcination and enhances the transmittance of the composite films. SiO and TiO form single oxide particles in the films, but a part of complex oxide may be formed. Ti O Si bonds increase the acidity which induces the increasing of the hydroxyl content in the composite films. 003 Elsevier Science B.V. All rights reserved. Keywords: Super-hydrophilic property; TiO ysio composite films; Sol gel 1. Introduction Research on the semiconductor hydrophilicity represented by TiO was reported in Wang s report about amphiphilic surfaces in 1997 w1x. The super hydrophilic property of the surface allows water to spread completely across the surface, rather than remaining as droplets. The result is TiO -coated glass which is antifogging and self-cleaning. Transparent super-hydrophilic self-cleaning TiO films on glass substrates have a high potential for practical applications such as in mirrors, window glasses, windshields of automobiles, and so on wx. Then, in the case of a filmwhich consists of only TiO, the contact angle of water becomes almost 0 during UV irradiation. However, it was found that the contact angle goes up and is restored comparatively early in a dark place. It is desirable that the contact angle rises slowly in a dark place, and maintains a low for a long time, because in practical applications, it *Corresponding author. Tel.: q86-16453055; fax: q86-16453810. E-mail address: llzmh@sohu.com (K. Guan). cannot always be irradiated by UV light, such as sunlight. It was found that by adding SiO, the contact angle of water was low from immediately after production, and the maintenance of hydrophilicity in a dark place was also good w3x. However, little attention has been paid to the physical and chemical characteristics, especially the surface acidity of the SiO ysio compos- ite films. In this paper, we report the effect of SiO addition on photo-generated hydrophilicity of TiO films from sol solutions. The mechanism of enhanced hydrophilicity of the TiO SiO composite films are discussed.. Experimental process One mole tetraethylorthosilicate (TEOS) in 0 mol ethanol containing 0.-mol HCl for 1 h is hydrolyzed and used as TEOS precursor solution. Then 1 mol tetrabutylorthotitanate (TBOT) is dissolved in a solution of 58-mol ethanol and used as TBOT precursor solution. Then TEOS and various amounts of TBOT precursor solution are then mixed together and further quantities of HCl catalyst are added (TBOT:HCls1:0.5 in molar 057-897/03/$ - see front matter 003 Elsevier Science B.V. All rights reserved. doi:10.1016/s057-897(03)0051-8 转载
0 K. Guan et al. / Surface and Coatings Technology 173 (003) 19 3 Table 1 Contact angle of the surface of TiO SiO films Fig. 1. Schematic of contact angle of a droplet. ratio). The content (mol%) of SiO is varied from0, 10, 0, 30 and 40 to 50%, respectively. 3 Glass plates (5=75= mm ) are used as the support substrates. Glass plates are washed in water, acetic acid and ethanol, respectively. The coatings are prepared on the supports by a dip-coating method. Gel coatings are dried at 00 8C for 30 min and then heat-treated in air, the temperature is elevated slowly ( 8Cmin ) to 650 8C and kept at 650 8C for 1 h. Repeating the cycles fromwithdrawing to heating increases the thickness of the films. Thus, we obtained the samples for examination. The hydrophilic property of the composite films is evaluated by measuring the contact angle for water droplets under UV irradiation by a 0-W ultraviolet light. Then these films are characterized using X-ray diffraction (XRD), Ultraviolet Vis spectrophotometer (UV Vis) and Fourier transforminfrared spectroscopy (FTIR). 3. Results 3.1. Contact angle of water in the films Contact angle measurements are made in air using the sessile-drop method. A droplet is injected on the surface using 1-ml micro-injector. Assuming that the geometry of the sessile drop is a spherical section (as shown in Fig. 1), the contact angle can be estimated by measuring the spreading diameter of the contact circle using a reading-microscope. The error in contact angle measurements from such an approximation is mainly attributed to the deviation of the actual geometry of the drop froma perfect sphere. The relative error in contact angle is -5% w4,5x. Table 1 shows the contact angle of water on the films after 1-h UV irradiation and in a dark place for 4 h. The contact angle of the sample which consists of only TiO is approximately 108. It reduces to below 58 for up to 40 mol% SiO addition. However, the contact angle is the same level for 10 and 50 mol% SiO addition with the pure TiO sample. After putting samples for 4 h in a dark place, the contact angle goes up from10 to 308 with only TiO sample, and from 3 to 68 with 40 mol% SiO addition. The results show that with the addition of 40 mol% SiO, the contact angle of the filmincreases very slowly and can maintain super-hydrophilic state for a long time in the dark. SiO content (mol%) 0 10 0 30 40 50 Contact angle (8) 10 10 7 5 3 9 1 h UV irradiation Contact angle (8) 30 16 15 7 6 13 In dark place for 4 h 3.. XRD spectra of the films XRD patterns which were obtained fromfilms are shown in Fig.. XRD measurement of TiO (Fig. a) shows a polycrystalline structure of anatase. As shown in Fig. b and c, only (101) peak is obvious, other peaks which exist as the sample of a pure TiO gradually disappear. The rutile peak is not seen both in the pure TiO and in the TiO ysio composite films w,6x. The smaller the crystal cells, the broader the diffraction peak. With the increasing of SiO content, the peak gradually becomes broad. That is, it means that the particle size of TiO becomes smaller with the addition of SiO. As for the sample with SiO addition, contact between TiO particles is barred by SiO or Ti O Si bonds during the growth progress in calculation w7,8x. The grain growth of TiO crystal is suppressed in calculation and particle size is maintained as in the original state. 3.3. Thickness and UV Vis spectra of the films According to the weight, square and density of the deposited films, the average thickness of each dipping cycle can be calculated. The entire thickness of the samples ranges between 10 and 130 nm in two dipping cycles. The specific results are shown in Table. Fig.. XRD patterns of the TiO ysio containing SiO (a) 0%; (b) 0%; (c) 40%.
K. Guan et al. / Surface and Coatings Technology 173 (003) 19 3 1 Table Thickness of the films for different contents of SiO with two dipping cycles SiO content (mol%) 0 10 0 40 50 Thickness (nm) 130 130 14 10 10 Fig. 3 shows the UV Vis spectra of (a) soda lime glass substrate and soda lime glass coated TiO films containing SiO of (b) 0%, (c) 10%, (d) 0% and (e) 40% by repeating two coating cycles, respectively. As can be seen fromfig. 3a, the absorption edge of soda lime glass is approximately 80 nm. This is caused by the adsorption of soda lime glass, because SiO has no adsorption edge in the ultraviolet band. Fig. 3b shows that the absorption edge of a TiO filmis approximately 330 nm. In fact, the band gap of TiO is 3. ev. So its absorption edge should be approximately 387.5 nm. However, the crystallite size of TiO films prepared by the sol gel method is smaller, owing to quantum size effects, the band gap of TiO become larger, therefore, its absorption edge (330 nm) shows a blue shift w7,9x. The adsorption edges of the composite films (as shown in Fig. 3c e) are approximately 310 30 nm and show a slightly increased blue shift, compared with the pure TiO film, which can be partly attributed to the fact that the TiO crystallite size in composite films is smaller than that in a pure TiO film. On the other hand, on adding SiO the bandgap could reduce in wavelength due to SiO content in the film. The transmittance of the soda lime glass is the highest and reaches to 90% over the visible light region, and drops to approximately 70% with the pure TiO film. The transmittance of the composite films containing SiO (as shown in Fig. 3c e) is approximately 80%. The oscillation of the curves between 800 and 380 nmis due to the interference between the filmand the substrate w8x. The thickness of the sample of pure TiO film is almost the same as that of TiO ysio films. So the result shows that the transmittance of the films with the addition of SiO is higher than that of a pure TiO film. The higher the content of SiO, the higher transmit- tance of the films, which results from the decreasing of light scattering of small crystallite w7,8x. Fig. 3. UV Vis transmission spectra of (a) soda lime glass, coated TiO containing SiO of (b) 0%; (c) 10%; (d) 30%; and (e) 40% films by repeating two coating cycles, respectively. Ti O bands. As can be seen in Fig. 4b and c, two new absorption peaks are seen at 1050 and 950 cm, which are ascribed to the asymmetric stretching vibrations of the Si O Si and Ti O Si bands, respectively. As can be seen in Fig. 4, with increasing of SiO content, the intensities of the absorption peaks at 1050 and 950 cmincrease, which can be deduced that SiO and TiO formnot only single oxide particles but also complex oxide ones. 3.4. FTIR spectra Fig. 4 shows the FTIR spectra of films. A broad absorption peak is seen at 3000 3800 cm, which is assigned to the stretching modes of O H bands and related to surface absorbed water w7,9x. The peak at approximately 1600 cm is attributed to the bending vibration of H O H bond, which is assigned to the chemisorbed water. The peak at approximately 440 cmdue to the stretching vibrations of Ti O Ti and Fig. 4. FTIR spectra of TiO ysio containing SiO (a) 0%; (b) 0%; (c) 40%, respectively.
K. Guan et al. / Surface and Coatings Technology 173 (003) 19 3 Fig. 5. Schematic of surface hydroxyl converses for TiO. The peak at 3000 3800 cmrelated to the chemi- cally and physically absorbed H O also increases with increasing of SiO content, which is assigned to the fact that the addition of SiO has the capability of absorbing water in air w7,9x. 4. Discussion The length of Ti O bonds is small and different (0.1937 and 0.1964 nmfor anatase structure, 0.1944 and 0.1988 nmfor rutile structure) in the structure of TiO. Molecular polar becomes stronger because of imbalance of Ti O bonds which make it easy to dissociative water adsorption and formhydroxyl on TiO surface. The higher the specific surface area, the more the hydroxyl content on the surface. However, with increasing of the heat-treatment temperature, the specific surface area and hydroxyl content decrease, which can be expressed as in Fig. 5. The surface hydroxyl(i) converses by (1) forming unstable surface dioxide(ii). The unstable surface dioxide (II) can then hydrate and converse to (I) by (). When the heat-treatment temperature is over 350 8C, (II) will react by (3) forming lattice oxygen which does not dissociate adsorbed water. The special structure of surface hydroxyl makes it easy to composite with other oxide and form improved composite oxide. SiO and TiO formsingle oxide particles in the films w3x, but a part of complex oxide may be formed due to the existence of Ti O Si vibration in the IR spectrum w10x. The Ti O Si bonds formation and utilization can be imagined as follows. Binary metal oxides are well-known to have surface acidity. A model proposed by Tanabe et al. w11,1x assumes that the dopant oxide s cation enters the lattice of its host oxide and retains its original coordination number. Since the dopant cation is still bonded to same number of oxygen even though the oxygen atoms are now of a new coordination, a charge imbalance is created. The charge imbalance must be satisfied. So bronsted sites (extra protons) are expected to formwhen the charge imbalance is negative. Lewis sites are expected to formwhen the charge is positive. The charge imbalance is calculated for each individual bond to the dopant cation and multiplied by the number of bonds to the cation. For the structure of TiO SiO, where TiO is the major component oxide, in the interfaces of TiO SiO, silica atoms may enter the titanium lattice. The coordination numbers of the positive elements in the component single oxides remain 4 for Si and 6 for Ti when they are mixed, whereas those of negative elements should be 3 and, respectively. The four positive charges of the silicon atomare distributed to four bonds, i.e. a positive charge is distributed to each bond, while the two negative charges of oxygen atom are distributed to three bonds, i.e. yy3 of a valence unit is distributed to each bond. The difference in charge for one bond is q1yy3sq1y3, and for all the bonds the valence unit of q1y3=4sq4y3 is excess. In this case, the Lewis acidity is assumed to appear upon the presence of an excess of positive charge. The schematic chart is shown in Fig. 6. Surface acidity is thought to take the formof stronger surface hydroxyl groups. The stable hydroxyl groups on the surface is beneficial in maintaining the hydrophilicity which can explain why the water contact angle increases slowly and remains low for a long time in the dark for the composite films. On the other hand, the decreased grain size in the composite films can also be helpful in increasing the Fig. 6. Schematic model of TiO SiO structure and charge difference.
K. Guan et al. / Surface and Coatings Technology 173 (003) 19 3 3 hydrophilicity, due to the increased quantumeffect of TiO. 5. Conclusion In summary, the films in which SiO is added to TiO is produced on the glass surface. The mechanism of the enhanced super-hydrophilicity for the films with SiO addition is analyzed. The following results are obtained: 1. By adding SiO to TiO, the contact angle during UV irradiation can be lowered, and the super-hydrophilic state can be maintained for a long time.. The optimum range existed when the amount of SiO to TiO was approximately 40 mol% which is the most effective for the reduction of the contact angle of water. 3. SiO and TiO formsingle oxide particles in the films, but a minor part of complex oxide may be formed as the bonds of Ti O Si. The complex oxide enhances the surface acidity, which results in the increasing of the hydroxyl content in the composite films, with the consequence that the hydrophilicity and the capability to hold absorbed water of the composite films are increased and can be maintained for a long time in a dark place. References w1x R. Wang, K. Hashimoto, A. Fujishima, et al., Nature 388 (31) (1997) 431 43. wx T. Watanabe, S. Fukayama, M. Miyauchi, A. Fujishima, K. Hashimoto, J. Sol Gel Sci. Technol. 19 (000) 61 76. w3x M. Machida, K. Norimoto, T. Watanabe, K. Hashimoto, A. Fujishima, J. Mater. Sci. 34 (1999) 569 574. w4x D.J. Ryley, B.H. Khoshaim, J. Coll. Inter. Sci. 59 () (1977) 43 48. w5x A. Fenga, B.J. McCoy, Z.A. Munir, D. Cagliostro, Mater. Sci. Eng. A 4 (1998) 50 56. w6x K. Miyashita, S. Kuroda, T. Ubukata, T. Ozawa, H. Kubota, J. Mater. Sci. 36 (001) 3877 3884. w7x J. Yu, J.C. Yu, G. Zhong, J. Han, Q. Zhao, J. Mater. Sci. Lett. 0 (001) 1745 1748. w8x J.C. Yu, J. Yu, W. Ho, J. Zhao, J. Photochem. Photobiol. A: Chem. 148 (00) 331 339. w9x M. Jung, J. Sol Gel Sci. Technol. 19 (000) 563 568. w10x Z. Liu, R.J. Davis, J. Phys. Chem. 98 (1994) 153 161. w11x K. Tanabe, T. Sumiyoshi, K. Shibata, T. Kiyoura, J. Kitagawa, Bull. Chem. Soc. Jpn. 47 (5) (1974) 1064 1066. w1x M. Itoh, H. Hattori, K. Tanabe, J. Catal. 35 (1974) 5 31.