Synthesis of nano-sized tungsten oxide particles encapsulated in a hollow silica sphere and their photocatalytic properties for decomposition of acetic acid using Pt as a co-catalyst

Nano-sized tungsten oxide (WO3) particles, each of which was encapsulated as a core in a hollow silica sphere (WO3@SiO2), were synthesized using calcium tungstate particles as the starting material. The calcium tungstate particles, each of which was covered with a silica shell, were converted to tungstic acid by nitric acid treatment and then to WO3 by heat treatment to obtain WO3@SiO2. A hollow space was formed in WO3@SiO2 between the WO3 core and the SiO2 shell as a result of shrinkage of WO3 during the heat treatment. The thus-obtained WO3@SiO2 was 40 nm in diameter, the WO3 core was 10 nm in diameter, and the silica shell, which was permeable to gas and liquid, was 10 nm in thickness. WO3@SiO2 absorbed visible light to the wavelength of 454 nm, which enabled photocatalytic reaction under visible light; Pt was loaded on the WO3 cores in the photocatalytic reactions. In contrast to Pt-loaded bulk WO3 photocatalysts without an SiO2 shell, Pt-loaded WO3@SiO2 showed continuous and complete decomposition of gaseous acetic acid in air under visible as well as UV irradiation.