Self-cleaning coatings: Present and Future
Self-cleaning materials have considerable attention for both their unique properties and practical applications in energy and environmental areas. Since the effect of surface photoinduced superhydrophilicity was discovered in 1997 (Fig. 1) the TiO2 based photocatalysts have gained considerable attention.
Figure 1. TiO2 nano-coating: UV-induced superhydrophilic conversion.
Vigorous applied studies of the effect have resulted in the development of self-cleaning and anti-fogging glass, self-cleaning coating for exterior and interior surfaces for buildings, tunnels, and road shields and, so on. Despite to wide applications the understanding of mechanisms and reasons yielding the observation or non-observation of this effect on some metal oxide surfaces are still under discussion.
The present study explored the effects of several factors (namely, wetting, light intensity, spectral variation of the actinic light, heating, surface acidity) on the hydrophilic conversion of the surface of TiO2 and ZnO nanocoatings. The experimental dependences of the efficiencies of photoinduced hydrophilic surface conversion on the intensity and wavelength of the actinic light clearly indicate the role of electronic photoexcitation in hydrophilic surface transformation. Particularly, the maximum extrema in spectral dependence of the efficiency of photoinduced hydrophilic conversion correspond to the energies of the first indirect and first direct electronic band-to-band transitions in TiO2. At the same time, temperature dependence and the effect of the surface acidity on the hydrophilic behavior of the TiO2 surface demonstrate the importance of the multi-layer hydrate structure in both the original hydrophilicity of the surface and the direction of the photoinduced hydrophilic conversion. Estimation of the surface energy alteration under photoexcitation suggests that only specific surface sites (10-3 ¸ 10-4 monolayer) are responsible for the effect of photoinduced superhydrophilicity of TiO2 surface.
Meanwhile, the vast majority of studies on this matter have suggested that the surface photoinduced hydrophilicity is dictated by the structural changes of the water multi-layer at the surface of metal oxides. In our previous works on study of hydrated TiO2 and ZnO coatings we have proposed following mechanism of the primary photoexcitation processes responsible for both photostimulated adsorption and photoinduced transformation of surface into new hydrophilic state:
S(S+) + h(e) → S+(S) (1)
S+(S) + e(h) → S(S+) (2)
H → S(S+) (3)
S+(S) → H (4)
The determinative step is a generation of photoholes (h) and photoelectrons (e), which can be trapped by the hole (S) and/or the electron (S+) surface sites responsible for hydrophilic conversion (photoactivation) (1). There are also photodeactivation (2) and non-photostimulated deactivation (4) of these active surface sites (S+ or/and S). The step (3) corresponds to the conversion of the surface into a new hydrophilic state (H).
It would be very useful to determine which type of photocarriers, photoholes or/and photoelectrons, rules the photoinduced hydrophilic conversion on surface of certain material. In turn, knowing this type one can predict and create the surface with proper hydrophilic properties under light. To form of the heterojunction structures comprising TiO2 and other narrow bandgap semiconductor material (ZnO, CdS, WO3) and to study the photoinduced hydrophilic transformations for these TiO2-topped surfaces of formed structures under light of different spectral composition seems to be promising way to figure out the mechanism of photoinduced hydrophilic transition and to control the wettability of solids’ surfaces.
Several strategies to improve the efficiency of photocatalytic and self-cleaning properties are demonstrated. One of these is to create composite films. Figure 2 illustrates the photoinduced electron transfer in heterostructures at the photoexcitation in intrinsic absorption region of titanium dioxide.
Figure 2. Schematic illustration of photoinduced electron transfer in TiO2/ZnO, TiO2/CdS, TiO2/WO3 heterojunctions.
It is seen that in these cases there is a domination of photoholes at the surfaces of the TiO2/ZnO and TiO2/WO3 systems while the surface of the TiO2/CdS heterostructure should be possessed of photoelectrons. Moreover, irradiation of layered heterojunctions by visible light (intrinsic absorption region of cadmium sulfide and tungsten oxide (VI)) is supposed to lead to charge separation, and therefore, to increase photoelectrons or photoholes on the TiO2-topped surfaces of TiO2/CdS, TiO2/WO3 heterojunctions, respectively.
Another topic concerning self-cleaning nano-coating is to real time manipulation of hydrophilic/hydrophobic properties of semiconductor-based nano-coating surfaces. It can be caused by different factors. Photoinduced hydrophilic conversion is a promising way to do this. In recent years many works are aimed at finding a method of controllable switching between hydrophilicity and hydrophobicity of the surface. The hydrophilic surface state is generally determined by its energy. The surface energy changing can be realized in several different ways. Here we report the ability to control the surface wettability of zirconium dioxide nano-coatings by changing the composition of actinic light. Such unique photoinduced hydrophilic behavior of ZrO2 surface is ascribed to the formation of different active surface states under photoexcitation in intrinsic and extrinsic ZrO2 absorption regions. The sequential effect of different actinic lights on the surface hydrophilicity of zirconia is found to be repeatable and reversibly switchable from highly hydrophilic state to more hydrophobic state. Observed light-controllable reversible and reproducible switching of hydrophilicity opens new possible ways for the application of the ZrO2 based materials.