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

Photolysis reaction pathways of [Au(III)Cl] in aqueous solution have been investigated by time-resolved X-ray absorption spectroscopy. Ultraviolet excitation directly breaks the Au-Cl bond in [Au(III)Cl] to form [Au(II)Cl] that becomes highly reactive within 79 ps. Disproportionation of [Au(II)Cl] generates [Au(I)Cl], which is stable for ≤10 μs. In contrast, intense near-infrared lasers photolyze water to generate hydrated electrons, which then reduce [Au(III)Cl] to [Au(II)Cl] at 5 ns. Hydrated electrons further induce a chain reaction from [Au(II)Cl] 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.