Plasmonic Photocatalysis with Metal-Semiconductor Hybrid Nanostructures

Abstract

Photocatalysis with plasmonic nanoparticles (NPs) is emerging as an attractive strategy to make and break chemical bonds in a sustainable fashion. However, the fast relaxation dynamics of the photoexcited charge carriers in plasmonic NPs has often led to poor photocatalytic conversions. To overcome these limitations, the separation and extraction of photoexcited hot charge carriers must be faster than the thermalization process. This demands the integration of rationally chosen materials to construct hybrid plasmonic photocatalysts. Heterostructure containing plasmonic metal and semiconductor NPs is one such hybrid material that could suppress the recombination of photoexcited charge carriers, thereby improving the overall performance of plasmonic photocatalysis. In this work, the enhanced photocatalytic activity of gold nanoparticle – titanium dioxide metal-semiconductor heterostructure (Au-TiO2) is used for the efficient regeneration of nicotinamide (NADH) cofactors under visible-light irradiation. The modification of plasmonic AuNPs with n-type TiO2 semiconductor enhanced the charge separation process under visible-light excitation, because of the Schottky barrier formed at the Au-TiO2 heterojunction. This led to a 12-fold increment in the photocatalytic activity of plasmonic AuNP in regenerating NADH cofactor. Detailed mechanistic studies revealed that the integration of TiO2 semiconductor into plasmonic AuNPs led to the modification of the reaction pathway as well. Au-TiO2 hybrid photocatalyst followed a less-explored light-independent pathway, in comparison to the conventional light-dependent path followed by the sole AuNP photocatalyst. The stark difference in the reaction mechanism with sole AuNP and Au-TiO2 hybrid photocatalyst also profoundly impacted the reaction yield, where the NADH regeneration yield reached ~70 % in the light-independent pathway under optimized conditions. Furthermore, the change in reaction mechanism also alleviated the issues of product degradation under prolonged irradiation, as is commonly seen in photocatalytic NADH regeneration in literature. Thus, our study emphasizes the rational choice of components in hybrid nanostructures in dictating the photocatalytic activity and the underlying reaction mechanism in plasmon-powered chemical transformations.