Water Wettability of an Ultrathin Layer of Silicon Oxide Epitaxially Grown on a Rutile Titanium Dioxide (110) Surface

Abstract

Water wettability of a SiO2 layer fabricated on a rutile TiO2(110) surface was examined. The TiO2 wafer was annealed at 1273 K in a quartz case used as a silicon oxide vapor source. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and frequency modulation atomic force microscopy (FM-AFM) analysis indicated that the monolayer of the stishovite SiO2 with a rutile structure was formed on the TiO2 surface. The water contact angle on the SiO2 monolayer was approximately 0° when the layer was cooled in the laboratory air for 6 h to room temperature and reached 15° after a further 24 h exposure to the laboratory air. The fused SiO2 surface and the SiO2-free TiO2(110) surface showed water contact angles of 37° and 32°, respectively, after being annealed at 1273 K and subsequently cooled for 6 h in the laboratory air. The high hydrophilicity of the SiO2 monolayer was attributed to the high density of hydrophilic surface OH groups. The O 1s XPS spectra showed that the density of the OH groups on the SiO2 monolayer was four times as high as that on the SiO2-free TiO2(110) surface. The formation of OH groups was probably promoted by the surface O vacancies originating from 9% lattice mismatch in the rutile structure between SiO2 and TiO2.