Partial sulfurization of oxide fine particles and their application to visible-light absorbable photocatalysts

The sulfurization behavior of metal transition oxides, TiO₂, SrTiO₃ and BaTiO₃ was carried out by using a quartz-type thermobalance in CS₂ atmosphere. As was to be expected, nanometric oxides had different sulfurization behaviors compared with micrometric ones. These differences were ascribed mainly to the particle size, the content of adsorbed water and the defects present in the oxides such as OH groups. A considerable reduction of the starting temperature of sulfurization as well as the ending sulfurization temperature was observed when the particle size decreased. In lesser extent, the content of adsorbed water on oxides also produced changes in the behavior of the oxides, e.g. an excess of them produced a slow sulfurization of the surface since the molecules of adsorbed water are located on the same sites where CS₂ molecules are absorbed. During the calcination, the removal of adsorbed water and OH groups delayed the starting sulfurization temperature of bulk oxides therefore the content of this defect favored the sulfurization via the formation of intermediates compounds by reaction between CS₂ and OH groups and/or the generation of vacancies during heating which give a highly reactive surface to CS₂. The highly reactive surface favored the sulfurization at low temperatures below the starting temperature of formation of metal sulfides. As was observed, partially sulfurized oxides were formed at temperature below 500 °C without formation of metal sulfides and keeping the initial structure of the oxide. Thus, the partial replacement of sulfur by oxygen was the reason why the optical responses in the visible light region and photocatalytic activity of photooxidation increased at these sulfurization temperatures.