Natural and persistent superhydrophilicity of SiO_2/TiO_2 and TiO_2/SiO_2 bi-layer films

摘要

Sol-gel SiO_2/TiO_2 and TiO_2/SiO_2 bi-layer films have been deposited from a polymeric SiO_2 solution and either a polymeric TiO_2 mother solution (MS) or a derived TiO_2 crystalline suspension (CS). The chemical and structural properties of MS and CS bi-layer films heat-treated at 500 ℃ have been investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscospy. Water contact angle measurements show that MS SiO_2/TiO_2 and CS TiO_2/SiO_2 bi-layer films exhibit a natural superhydrophilicity, but cannot maintain a zero contact angle for a long time over film aging. In contrast, CS SiO_2/TiO_2 bi-layer films exhibit a natural, persistent, and regenerable superhydrophilicity without the need of UV light. Superhydrophilic properties of bi-layer films are discussed with respect to the nature of the TiO_2 single-layer component and arrangement of the bi-layer structure, i.e. TiO_2 underlayer or overlayer.