Exploring the mechanisms of metal co-catalysts in photocatalytic reduction reactions: Is Ag a good candidate?

• Well-defined TiO2-supported Au, Ag, Aucore–Agshell nanoparticles as photocatalysts.
• Au nanoparticles enable charge transfer to H+ thus promote the H2 evolution.
• Ag nanoparticles enable charge transfer to O2 promoting other selective reactions.
• Aucore–Agshell nanoparticles exhibit non-selective behavior for reduction reactions.

Metal co-catalysts are essential for enhancing photocatalytic performance, especially in reduction reactions using semiconductor photocatalyst materials as a consequence of the reduced recombination kinetics of charge carriers by spatial charge separation. Generally Au, Pd, Pt, and their alloys are more promising candidates than Ag for photocatalytic H2 evolution experiments, although Ag can trap more electrons having more negative reduction potential than that of Au, Pd, and Pt. Here we have synthesized and examined well-defined Au, Ag, and core-shell structured Au–Ag nanoparticles as co-catalysts for TiO2 in photocatalytic H2 evolution. By varying the dissolved oxygen in the reaction suspension, we found that selective photocatalytic reduction can be achieved by fine tuning the co-catalyst materials. Whilst Au NPs are superior for proton reduction, Ag NPs exhibits excellent performance for oxygen reduction. All core-shell structured Au–Ag NPs show non-selectivity in photocatalytic reduction of proton and oxygen.

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