The role of silver nanoparticles on silver modified titanosilicate ETS-10 in visible light photocatalysis

摘要

Nanoparticles of noble metals, such as silver and gold, have been investigated as one way to hinder the recombination of electrons and holes produced by irradiated semiconductors. However, the exact role silver plays in hindering electron-hole recombination is unclear. In order to assess the role of ionic silver, Ag~+, and metallic silver, Ag~0, on the potential photocatalytic activity of a titanosilicate ETS-10 semiconductor (i.e., a means to extend light absorption into the visible range, a means to form a Schottky barrier to hinder electron-hole recombination, as a catalyst for the production of oxygen radicals, as a means to capture electron through species reduction, etc.) the photosensitization of methylene blue (MB) was investigated under visible light irradiation. Visible light irradiation was chosen so as to decouple the effects silver nanoparticles play in this photosensitized reaction from the complexities of electron-hole pair dynamics that may be inherent to ETS-10. Ag~+ and Ag~0 were investigated in both their free-floating forms and utilizing ETS-10 as a substrate. All forms of silver were produced by AgNO3 dissociation. Ion exchange of as-synthesized ETS-10 from AgNO3 solutions was utilized to prepare Ag~+-modified ETS-10 (Ag~+-ETS-10), and H2 reduction of Ag~+-ETS-10 was used to prepare Ag~0 nanopartide-modified ETS-10 (Ag~0-ETS-10). In all preparations, the morphology and crystal structure of ETS-10 remained unchanged. Ag~+-ETS-10 samples showed a progressive absorption edge red shift in the UV region with increasing sample Ag~+ content. Unlike as-synthesized ETS-10 and Ag~+-ETS-10 samples, Ag~0-ETS-10 samples showed absorbance in the visible region. The absorption bands at ~350 nm and ~460 nm observed for Ag~0-ETS-10 were attributed to the surface plasmon resonance of Ag~0 nanoparticles on the surface of ETS-10 crystals. In contrast to as-synthesized ETS-10, which was essentially inactive in this illumination range, all Ag~+-ETS-10 and Ag~0-ETS-10 samples showed apparent enhanced activity for the degradation of MB under visible light (420-630 nm) irradiation. These results suggest that silver nanoparticles provide a "portal" for oxygen (i.e., oxygen radicals) to interact with MB radicals by providing a medium for interfacial charge transfer of an electron to oxygen adsorbed on the silver, with ETS-10 providing a reservoir for MB molecules and their radicals, and a path for injected electrons to reduce Ag~+.