Abstract: Background, aim, and scope Nowadays, glass fogging and building exterior pollution have influenced all aspects of our social lives. For instance, fogging of rearview mirror would affect visibility and increase the occurrence of traffic accidents; the fouling of building walls and sculptures would affect appearance and even corrode the material. Traditionally the surface cleaning was performed manually, which is expensive and high risky, especially for skyscrapers cleaning. Recently, the photo-induced hydrophilic film, as one of the special wetting surface, has emerged as a self-cleaning surface that can be used for anti-fouling function with low maintenance cost and operation risk. The good water wettability can be obtained under solar light, which can effectively reduce the refraction and total reflection to achieve the anti-fogging effect. In addition, the dust and pollutants can be removed thoroughly by rainfall to prevent surface fouling. In this background, it is necessary to conduct a systematical review on photocatalytic hydrophilic film. Materials and methods In this review, we focused on the following aspects: (1) the hydrophilic theory of photocatalytic film; (2) the evaluation methods of hydrophilicity; (3) the relationship between photocatalytic activity and hydrophilicity; (4) the strategies of improving hydrophilicity; and (5) the application level of photocatalytic hydrophilic film. Results Firstly, taking TiO2 for example, four typical hydrophilic theories of photocatalytic film are summarized as following: (1) 2D/3D capillary effect: simulating spike-decorated tubes to porous materials with different cohesion and adhesive force of water inside and outside of thin tubes, the water molecules tend to infiltrate or rise in the thin tubes and crack of spikes, which macroscopically results in water wettability on the film surface; (2) photo-induced generation of surface vacancies: oxygen vacancies can be formed at the two coordinated oxygen bridging sites near the surface under UV irradiation, and the oxygen vacancies can increase the amount of hydroxyl radicals formed by the chemisorbed water molecules; (3) photo-induced reconstruction of Ti-OH bonds: the photo-generated holes (h+) of TiO2 can transport to the surface and be captured by lattice oxygen, which weaken the binding energy between Ti and lattice oxygen so that water molecules can rupture this bond resulting in the formation of new Ti-OH bonds; (4) photo-oxidation of adsorbed hydrocarbon: the adsorbed organic contaminants are decomposed by photocatalysis to obtain UV-induced hydrophilic surface that the water droplet slides to the clean surface and spreads completely. Secondly, the evaluation method for analyzing hydrophilic surface of photocatalytic film is described by measuring water contact angle, referring to ISO 15989. According to the international standard, the droplet of water (1~2 μL) is suspended at the end of the syringe needle followed by transferring to the specimen surface, and ten preferable points on the specimen are selected for contact angle measurements through contact angle meter to calculate the average as contact angle of the specimen. Thirdly, referring to several articles, the relationship between photocatalytic activity and hydrophilicity of photocatalytic film can promote each other. Furthermore, according to the influencing factors of hydrophilicity and four typical hydrophilic theories, strategies to improve hydrophilicity of photocatalytic film are summarised and generalized into regulating chemical composition and morphology structures. Finally, the practical applications of photocatalytic hydrophilic films are summarized. Many famous foreign companies (such as Pilkington, TOTO and PPG) have concentrated on investigating and manufacturing anti-fogging and self-cleaning products in the mid-1990s, while the relevant research started relatively late in China since 2000. So far, a few commercial products based on hydrophilic TiO2 films have gradually developed, including anti-fogging and self-cleaning glass. For example, the self-cleaning glass produced by Zhongke Nanotechnology Engineering Center Co.,Ltd., has been applied in the National Grand Theatre and car showrooms to realize self-cleaning effect. Disscusion For hydrophilic theories of photocatalytic film, 2D/3D capillary effect pays attention to surface morphology of films, while the other three theories lay emphasis on hydrophilic transformation under UV irradiation. ISO 15989 is the international standard to measure water-contact angle of films, and it is worth noting that the horizontal plane and measurement points should be chosen carefully to ensure data reliability. Photocatalytic activity and hydrophilicity of photocatalytic films can promote mutually. On one hand, the photocatalytic film can decompose organic contaminants and the water droplet slides to the clean surface to improve hydrophilicity. On the other hand, the improvement of hydrophilicity can increase hydroxyl groups on the surface of photocatalytic film to facilitate pollutant decomposition. The above mentioned strategies can improve hydrophilicity of photocatalytic films. For regulating chemical composition, the hydrophilicity can be effectively improved by widening light-response range and enhancing photocatalytic activities. For regulating morphology structures, the hydrophilicity can be promoted by constructing the porous or rough surface. Hence, the former strategy is suitable for practical applications by coating photocatalytic hydrophilic materials on glass and building surface, but the latter with rigorous process is mainly applied in the laboratory research. Combining photocatalysis with hydrophilic surface can facilitate practical applications, which is matured and industrialized in foreign countries. Although domestic research advances have been achieved in laboratory, it’s still insufficient in industrialization. Conclusions The photocatalytic hydrophilic films can realize anti-fogging and self-cleaning surfaces to overcome the disadvantages of traditional methods. Thus it shows promising application prospects in many fields. However, in terms of hydrophilic films in practical applications, some problems are still needed to be solved, including low utilization of sunlight, weak adhesion, short durability of photoinduced super-hydrophilicity, difficulties in obtaining super-hydrophilic surface at low temperature for industrialization. Recommendations and perspectives Therefore, it is needed to focus on solving the above problems, which will play an important role in accelerating the progress and leading the future development trend of photocatalytic hydrophilic films in anti-fogging and self-cleaning applications.
Keywords: Photocatalytic films; hydrophilicity; anti-fogging; self-cleaning