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

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 μs. 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.