Au nanoparticles (NPs) with sizes of 4, 8, 18 and 26 nm, as well as their corresponding Au@SiO2 core-shell NPs, were selectively synthesized. The prepared plasmonic NPs were loaded onto Pt/TiO2 in order to investigate their size-dependent plasmonic effect in the photocatalytic CO2 conversion reaction. In evolving CH4 from CO2 under 365 nm LED lamp irradiation, the loading of the Au or Au@SiO2 NPs does not appreciably influence the photocatalytic activities of Pt/TiO2, whereas under the co-irradiation of 365 and 530 nm LED lamps, their photocatalytic activities are remarkably increased, clearly indicating that the enhancement of the catalytic activities is caused by the localized surface plasmon resonance (LSPR) effect of the Au and Au@SiO2 NPs. With respect to their catalytic efficiency, the optimum sizes of the Au NPs are determined to be 18 nm for both the bare Au and Au@SiO2 NPs. In addition, the Pt/TiO2/Au@SiO2 systems show significantly higher catalytic activities than Pt/TiO2/Au, due to the enhanced LSPR caused by the coverage of the SiO2 shells over the Au NPs. Particularly, Pt/TiO2/Au18@SiO2 exhibits the highest activity in evolving CH4, which is estimated to be 3.1 times that of the bare Pt/TiO2. (C) 2016 Elsevier B.V. All rights reserved.