Thermally- and photoinduced changes in the water wettability of low-surface-area silica and titania

The surface properties of silica and titania are mainly determined by the presence, density, and type of terminal hydroxyl groups (Si-OH "silanol" and Ti-OH "titanol."). Thermal treatment at elevated temperatures causes dehydroxylation on both surfaces, confirmed by streaming potential and ToF-SIMS measurements. The magnitude of the zeta potential markedly decreases after heat treatment, but the IEP is not affected. The intensity ratio MOH+/M+ (M = Si or Ti), which reflects the surface density of OH groups, also decreases noticeably after high-temperature treatment. The mechanism is condensation of adjacent silanol/titanol groups into siloxane/titanoxane bonds. Ultraviolet light (lambda = 254 nm) has little effect on silica but rapidly induces hydrophilicity on titania surfaces. There is a strong correlation between the amount of hydrocarbons adsorbed on the surface and the density of titanol groups (thence the water contact angle). The effect of UV radiation can be entirely attributed to photolytic decomposition of organic contaminants. Dehydroxylated titania and silica (at 1050 degrees C) show very different wetting behavior: silica is moderately hydrophobic (water contact angle of about 40 degrees), while titania is hydrophilic (0 degrees). This dissimilarity can be explained with a simple model estimating the van der Waals and acid-base interfacial interactions.