Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films
Introduction
Research on the semiconductor photocatalyst represented by TiO2 started with Honda–Fujishima effect at the beginning of the 1970s [1]. Until now, various studies have been performed on the applications of environmental purification [2], [3], [4], [5], [6] and the mechanism [7], [8], [9], [10], [11].
Recently, it has been found that TiO2 presents hydrophilic and amphiphilic properties [12], [13]. Superhydrophilic property of the surface allows water to spread completely across the surface rather than remaining as droplets. The result is TiO2-coated glass which is antifogging and self-cleaning [14]. Transparent self-cleaning TiO2 films on glass substrates have a high potential for practical applications such as mirrors, window glasses, windshields of automobiles, etc. [15], [16], [17]. For example, making use of the idea of cleaning by a stream of water, coated windows can be cleaned by rainfall.
In the case of a film which consists of only TiO2, the contact angle of water almost becomes zero during UV irradiation. However, it is found that the contact angle goes up and is restored comparatively quickly in a dark place. It is desirable that the contact angle rises slowly in a dark place, and stays low for a long time, because if actual use is considered, it is not always irradiated by UV light, such as sunlight in rainy or cloudy days. It was found out that by adding SiO2, the contact angle of water is low immediately after production, and the maintenance of hydrophilicity in a dark place is also good [18], [19], [20]. Not only the improved hydrophilic property but also improved photocatalysis have been found for TiO2/SiO2 composite films [21], [22], [23].
One of the most interesting aspects of TiO2 is that the photocatalysis and hydrophilicity can take place simultaneously on the same surface even though the mechanisms are completely different [2]. This is the reason that the film has a self-cleaning effect. However, very little attention has been focused on the relationship between the photocatalysis, hydrophilicity and self-cleaning effect. The synergetic effect of photocatalysis and hydrophilicity is very important in sustaining the self-cleaning effect. In this paper, we report the relationship between photocatalysis and photo-generated hydrophilicity of TiO2/SiO2 composite films. The effects of photo-generated hydrophilicity, photocatalysis and self-cleaning are discussed in this study.
Section snippets
Film preparation
The sol–gel method is used to prepare the TiO2/SiO2 composite films. Tetraethylorthosilicate (TEOS, 1 mol) in 20 mol ethanol is hydrolyzed containing 0.2 mol HCl for 1 h used as TEOS precursor solution. Then 1 mol tetrabutylorthotitanate (TBOT) is dissolved in a solution of 58 mol ethanol used as TBOT precursor solution. Then TEOS and various amounts of TBOT precursor solution are mixed together and further quantities of HCl catalyst are added (TBOT/HCl=1:0.5 in molar ratio). The content in
Thickness and microstructure of the films
The entire thickness of the samples ranges between 120 and 130 nm in two dipping cycle. The specific results are shown in Table 1.
XRD patterns obtained from these films are shown in Fig. 1. XRD measurement shows that all the films are anatase crystal structure. Rutile peak is not seen at all the composite films.
The smaller the crystal cells, the broader the diffraction peak. With the addition of SiO2, the peaks gradually become broad. That is, the particle size of TiO2 becomes smaller with the
Discussions
The following results can be deduced from the experiments: (1) the amount of adsorbed organic substance decreases with the increasing content of SiO2 in TiO2 films, up to about 30–40 mol% SiO2 addition, where it is lowest. (2) Photocatalytic activity is closely related to hydrophilicity. Both activities reinforce each other and maintain the self-cleaning effect.
Conclusions
In this research, the film, in which SiO2 is added to TiO2, is prepared on the silicate glass by sol–gel method, and the relationship between photocatalysis and hydrophilicity, and their influence on self-cleaning effect are investigated. The following knowledge is obtained:
- 1.
Depending upon the content of SiO2 in TiO2, TiO2/SiO2 film surface can have more photocatalytic activity and less superhydrophilic activity, or vice versa. In these experiments, the optimum photocatalytic character can be
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