Manipulating semiconductor-mediated photocatalysis by controlling the morphology of metal cocatalyst in Ag/TiO2 nanocomposites.

Abstract

This research focuses on synthesizing silver (Ag)/titanium dioxide (TiO2) photocatalysts by either chemical reduction or photodeposition. Ag particles act as cocatalysts by trapping photoexcited electrons and suppressing electron-hole recombination in TiO2. The hypothesis was that the shape and surface morphology of Ag cocatalyst could affect the activity and selectivity of a photocatalytic process. Truncated Ag nanoprisms on TiO2 were synthesized by either chemical reduction or photodeposition. Only chemical reduction produced truncated Ag nanoprisms of consistent shape and attachment to TiO2. This is a seed-mediated synthesis, where Ag seeds were deposited on TiO2 first to provide strong anchoring, then Ag was additionally deposited to form truncated nanoprims. In a different approach, Ag was deposited on TiO2 through photodeposition using the same corresponding chemicals except reducing agent. While photodeposited Ag nanoparticles appear to strongly adhere to TiO2, they grow big and multifaceted or rounded. The shape-guiding and size limiting effects of the capping ligands are virtually absent in photodeposition. The performance of the synthesized Ag/TiO2 nanocomposites was tested in the photocatalytic reduction of m-dinitrobenzene in non-aqueous solution under visible light of 400-405nm wavelength. When prepared with the same amount of Ag by weight of TiO2, the two types of catalysts behave differently. Truncated triangular Ag nanoprisms deposited by chemical reduction, where the majority of the exposed surfaces on cocatalyst are of {111} close-packed type, exhibit 100% conversion and 100% selectivity for the reduction of m-dinitrobenzene to m-nitroaniline. Photodeposited irregular Ag nanoparticles on TiO2, where multiple facets are exposed on cocatalyst, exhibit 100% conversion of mdinitrobenzene into two products: m-nitroaniline (selectivity 15%) and m-phenylenediamine (85%). This demonstrates that more uniform surfaces of the metal cocatalyst lead to more selective conversion.