Photoinduced gold recovery mediated by isopolymolybdate in strongly acidic HCl/NaCl solutions

The laser-induced formation of gold nanoparticles (AuNPs) was studied in hydrochloric acid solutions containing isopolymolybdate. Aqueous solutions of HAuCl4 (0.5 mM) in HCl (0.1–1.0 M)/NaCl media containing ethanol (0.5% v/v) were irradiated with 355 nm laser pulses in the absence or presence of molybdate. The reaction progress was probed by UV-vis spectroscopy. Without molybdate, the formation of AuNPs is significantly hindered because a large excess of Cl− renders the AuCl4− precursor resistant to direct photoreduction with ethanol. The incorporation of molybdate into solution effectively mediates the photoinduced formation of AuNPs in 0.1–0.5 M HCl. For this range of acidity, photoirradiation generates the molybdenum blue (MB) species from the reaction of photoexcited Mo36O1128− (polyoxoanions) with ethanol, followed by the reduction of AuCl4− with the MB species. At a higher acidity, the formation of AuNPs is virtually undetectable because the MB species are no longer generated due to the absence of anionic molybdate precursors