Surface Re-Esterification and Photo Sintering of Titania Xerogel Thin Films

Supraband gap light (<378 nm) was found to induce the hardening of films for surface-functionalized nanoparticles in titania xerogel thin films. Anatase nanoparticles prepared in acidic aqueous solutions and subsequently diluted in ethanol/water molar ratios >1 were re-esterified at the surface prior to UV exposure, as confirmed by Fourier Transform IR spectroscopy. Relative hardness tests demonstrated that sols having a high ethanol content, which are subsequently cast into thin films and subjected to UV light exposure under humid conditions, are capable of being hardened. The hardening effect was reduced relative to unexposed xerogel films at all tested humidity levels lacking UV exposure, indicating particle peptization without interparticle bonding. Contact angles significantly decreased for all films exposed to UV light. In contrast, films held in the dark did not change contact angles. Atomic force microscopy indicated nanoparticle growth for UV-exposed films at 40% relative humidity/40 °C. Adsorbed pore water is proposed to be necessary for initiating photoinduced hydrolysis reactions and maintaining capillary compressive forces between particles. Given an equivalent fluence (20 J cm^-2) above the band gap onset, harder films were obtained using successively shorter wavelengths, in agreement with measured absorptivity data for titania xerogel films. The data indicate that photoreactive sintering processes lead to permanent bond formation in our wetted xerogel films.