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.

We conducted a study of electronic structures and chemical bonding of gaseous ThO₂⁻ and ThO₃⁻ using velocity-map imaging and ab initio calculations. The electron affinity of neutral ThO₂ molecule is reported for the first time with the value of 1.21(5) eV. We obtained a vibrationally resolved photoelectron spectroscopy of ThO₂⁻ and observed the symmetric stretching frequency of 824(40) cm⁻¹ for neutral molecules. One hot band transition is observed in the spectrum of ThO₂⁻, which allows the measurement of symmetric stretching mode for ThO₂⁻. The ground state of ThO₂⁻ is ²A₁ with C_2v symmetry: the detachment of an electron from the singly occupied molecular orbital (SOMO) results in the ground state of ThO₂. Kohn-Sham molecular orbital analyses reveal an σ and two weak π bonds for Th–O multiple bonds in ThO₂. Global minimum search methodology combined with quantum chemical calculations are used to find the minima of ThO₃ and ThO₃⁻, and the adiabatic detachment energy of ThO₃⁻ is calculated to be 3.26 eV at the coupled cluster with singles and doubles plus perturbative triples level. Our theoretical calculations suggest that the ground state of ThO₃ is ¹A′ with a symmetry of C_s, while the most stable ThO₃⁻ is ²A₁ with C_2v symmetry; thus, the transition from ThO₃⁻ to ThO₃ undergoes a significant geometry reorganization. Molecular orbital analyses suggest that the SOMO of ThO₃⁻ is mainly participated by O 2p and O to Th back donation was found in HOMO-2 molecular orbital. This investigation will shed some light on the understanding of covalent bonding in Th-contained molecules.

In contrast to the normal LIAD method, the enhanced signals were obtained using substrate-enhanced LIAD source (SE-LIAD) being about 5-fold to 14-fold enhancement.