Enhanced photocatalytic activity of TiO₂@mercapto-functionalized silica toward colored organic dyes

Core–shell-type TiO₂@mercaptopropyl-functionalized silica (MPS) particles were prepared for enhanced photocatalytic performance of TiO₂. The MPS particles were adopted in the composite-type photocatalyst design so as to enhance the photo-catalytic activity through the chemical interaction between chemically modified substrate particles and organic dye molecules, although they do not have photocatalytic activity. Three organic dyes, rhodamine B, rhodamine 6G, and methyl orange, were selected for testing dye adsorption characteristics and photocatalytic activities of the TiO₂@MPS particles, commercial P25, and unsupported TiO₂ nanosols. Rhodamine B and rhodamine 6G dyes were adsorbed much more on the TiO₂@MPS particles than P25, by factors of 6 and 43, respectively, while methyl orange molecules were adsorbed more readily on P25. Such difference in the dye adsorption characteristics of the two photocatalysts may be related with available functional groups and the dyes. This would lead to enhanced photocatalytic degradation of the dyes because of combined chemical and physical adsorption mechanisms. Toward the two rhodamine dyes, the TiO₂@MPS particles show significantly enhanced photocatalytic activity per TiO₂ than P25 under both UV and visible light irradiation. Especially under visible irradiation, the degradation rate constants of rhodamine B and rhodamine 6G molecules per TiO₂ were higher, 18 and 45 times, respectively, for the TiO₂@MPS particles than for P25. Toward methyl orange dye, the degradation rate constant per TiO₂ was lower for the TiO₂@MPS particles than for P25 under UV irradiation but it was eight times higher under visible irradiation. The TiO₂@MPS particles were recyclable and their photocatalytic activity did not change at all at least for three repeated photo-degradation tests.