Visualizing light-induced dynamic structural transformations of Au clusters-based photocatalyst via in situ TEM

Ultrasmall gold (Au) clusters have been regarded as one of the prototypes materials for solar energy conversion due to their unique strong molecular-like light absorption properties. However, the light-induced aggregation of Au clusters into nanoparticles is one of the most important factors that restricts its application in photocatalysis. Although Au clusters aggregation has been widely demonstrated, the underlying mechanism for cluster fusion is still unclear due to the lack of experimental evidence. Herein, we report the direct observation of Au clusters on TiO2 nanosheets aggregating when used as visible light photocatalysts for the reduction of nitroaromatics. Through in situ high-resolution transmission electron microscopy (TEM), the coexistence of two fusion mechanisms of Au clusters on TiO2 under ultraviolet-visible (UV-Vis) light irradiation in air is identified, i.e., the migration and coalescence (MC) and Ostwald ripening (OR). Additionally, the correlation between the photostability of Au clusters and reaction atmospheres has been investigated, among which Au clusters have higher stability in an inert N2 atmosphere or vacuum than the oxidizing atmospheres (i.e., air and O2). These results indicate the inherent stability of Au cluster during photocatalysis, and instability comes from the consuming of ligand layer. This work not only discloses the underlying mechanism of Au cluster sintering but also provides guidelines for enhancing metal clusters-based photocatalysts stability.