Size-dependent plasmonic effects of M and M@SiO2 (M = Au or Ag) deposited on TiO2 in photocatalytic oxidation reactions

Monodispersed Au nanoparticles (NPs) with sizes of 18, 26 and 34 nm and Ag NPs with sizes of 17, 25 and 33 nm, as well as their corresponding M@SiO2 core-shell NPs (M=Au or Ag), were selectively synthesized. The prepared individual plasmonic NPs were then loaded onto TiO2 and their size-dependent plasmonic effects in photocatalytic oxidation reactions were systematically analyzed. In the removal reactions of the organic compounds, aqueous salicylic acid (SA) and aniline, under UV-vis light irradiation, both the Au and Ag NPs significantly enhanced the catalytic activity of TiO2, while the smaller NPs were more effective. Although the Ag NPs show stronger LSPR effect than the Au NPs, the overall catalytic activity of the Ag/TiO2 systems was not higher than Au/TiO2, and their catalytic activity variation according to the size of plasmonic NPs was more sensitive for Ag/TiO2. The observed results seem to be due to the LSPR sensitization effect. It was also found that the M@SiO2 NPs were significantly more effective than the bare M NPs in enhancing the photocatalytic activity of TiO2 and the size-dependent plasmonic effects of the M@SiO2 NPs were quite different from those of the bare M NPs. In regrading to catalytic activity enhancement, the optimum sizes of the Au and Ag NPs for the Au@SiO2 and Ag@SiO2 NPs were determined to be 26 and 25 nm, respectively. Moreover, the Ag@SiO2 NPs, which generate stronger LSPR, were more effective than the Au@SiO2 NPs. Consequently, Ag25@SiO2/TiO2 demonstrates the highest activity in decomposing SA and aniline, which is 3.8 and 2.5 times, respectively, that of the bare TiO2. (C) 2017 Elsevier B.V. All rights reserved.