Deciphering Photochemical Reaction Pathways of Aqueous Tetrachloroauric Acid by X-ray Transient Absorption Spectroscopy

Photolysis reaction pathways of [Au(III)Cl4]- in aqueous solution have been investigated by time-resolved X-ray absorption spectroscopy. Ultraviolet excitation directly breaks the Au-Cl bond in [Au(III)Cl4]- to form [Au(II)Cl3]- that becomes highly reactive within 79 ps. Disproportionation of [Au(II)Cl3]- generates [Au(I)Cl2]-, which is stable for <= 10 mu s. In contrast, intense near-infrared lasers photolyze water to generate hydrated electrons, which then reduce [Au(III)Cl4]- to [Au(II)Cl3]- at 5 ns. Hydrated electrons further induce a chain reaction from [Au(II)Cl3]- to [Au(0)Cl]- by successively removing one Cl-. The zero-valency Au anions quickly polymerize and condense to form Au nanoparticles, which become the dominating product after 400 s. Our results reveal that the condensation of zero-valency Au starts with dimerization of gold clusters coordinated with chloride ions rather than direct condensation of pristine Au atoms.