Enhanced photocatalytic degradation of tetramethylammonium on silica-loaded titania

Abstract

Photocatalytic degradation (PCD) of tetramethylammonium (TMA) in water was studied using both pure TiO2 and silica-loaded TiO2 (Si–TiO2). Use of Si–TiO2 catalyst prepared from commercial TiO2 powder by a simple method developed in this work enhanced the PCD rate of TMA considerably. The Si/Ti atomic ratio of 18% was found to be an optimum in photoactivity and the calcined sample was more efficient than the uncalcined one. Several factors were noted to be responsible for the higher photoefficiency of Si–TiO2 catalyst. Si–TiO2 calcined at 700 °C did not show any sign of change in the crystalline structure from that of uncalcined pure TiO2. The increased thermal stability of Si–TiO2 enabled the bulk defects to be removed at high temperatures without forming the inactive rutile phase, which may partly contribute to the higher photoactivity. The most outstanding characteristics of Si–TiO2 is its surface charge modification. Loading silica on to a titania surface made the surface charge highly negative, which was confirmed by zeta potential measurements. The enhanced electrostatic attraction of cationic TMA onto the negatively charged Si–TiO2 surface seems to be the main reason for the enhanced photoactivity of Si–TiO2. As a result of this surface charge change, the TMA PCD rate with Si–TiO2 exhibited a maximum around pH 7 whereas the PCD with pure TiO2 was minimized at pH 7. The X-ray photoelectron spectroscopic analysis showed the formation of SiOx on the TiO2 surface but the diffuse reflectance UV spectra indicated no significant difference in the band gap transition between pure TiO2 and Si–TiO2. In addition, the diffuse reflectance IR spectra showed the presence of more surface OH groups on Si–TiO2 than on pure TiO2, which may also contribute to the higher photoactivity of Si–TiO2 through generating more OH radicals upon UV illumination.