Quantitative Evaluation of the Photoinduced Hydrophilic Conversion Properties of TiO₂ Thin Film Surfaces by the Reciprocal of Contact Angle

Abstract

Kinetics on the photoinduced hydrophilic conversion processes of the TiO2 surface was investigated. First, we show that there exist linear relationships between the reciprocal of the contact angle and the UV irradiation time and use the slope of this straight line as the hydrophilic conversion rate, which is independent of the initial value of the contact angle. Second, we examine the relationships between the reciprocal of the contact angle and the reconstruction of the surface hydroxyl groups and show that the reciprocal of the contact angle corresponds to the density of the surface hydroxyl groups reconstructed by UV irradiation. Finally, the dependence of the hydrophilic conversion on various parameters such as the incident UV intensity, the wavelength of irradiated light, and the concentration of the hole scavenger are quantitatively investigated with the proposed hydrophilic conversion rate. These results show that the photoinduced hydrophilic conversion proceeds competitively with the photocatalytic oxidation process on the TiO2 surface under UV irradiation. The comparison of the hydrophilic conversion rate with the rate for the reverse process clarifies that the critical contact angle is obtained when the reconstruction of the surface hydroxyl groups and their relaxation process are equilibrated under UV irradiation.

Modifications of surface materials and their effects on cleanability as studied by radiochemical methods

“…The observed effects of UVradiation on cleanability of TiO 2 -coated ceramic surfaces were greatest on rough surfaces, implying that increasing roughness increases the surface area available for photo-induced phenomena (Mellott et al 2006). According to the results the contact angles of TiO 2 -coated surfaces decreased under UV-radiation, in accordance with earlier studies (Watanabe et al 1999, Fujishima et al 2000b, Nakajima et al 2000, Sakai et al 2003). The results of cleanability studies showed that organic particle soil was removed more efficiently after UV-radiation than without UV treatment, whereas UV-radiation did not affect the removal of oil soil.…”

“…It generates two separate photo-induced phenomena: a photocatalytic phenomenon and a superhydrophilic phenomenon. In many studies, the contact angles of TiO 2 coated surfaces decreased with the illumination time of UV-light (Watanabe et al 1999, Nakajima et al 2000, Fujishima et al 2000b, Sakai et al 2003. Increasing crystallite size was reported to increase photoactivity (Jung and Park 2004).…”

UV Irradiation Induced Transformation of TiO2 Nanoparticles in Water: Aggregation and Photoreactivity

“…Subsequently, water molecules dissociated to compensate for these oxygen vacancies and produced surface hydroxyl groups. 27,28 Indeed, most of oxygen vacancies were converted to adsorbed hydroxyl radicals and then released during this process. Quantitative analysis of the three bands revealed a faster formation rate of the bridging hydroxyls than the terminal hydroxyls (Figure 3b).…”

Superlyophilic Interfaces and Their Applications

“…[4,[121][122][123] After UV irradiation, oxygen vacancies are created on the surface which induce the conversion of Ti 4+ sites to Ti 3+ , and the Ti 3+ sites can capture dissociated water to form hydrophilic water multilayers. According to the lateral force microscopy (LFM) image, after UV irradiation, hydrophilic (bright) nanodomains and the remaining oleophilic (dark) areas on rutile TiO 2 (100) face can be observed (Figure 5b).…”