Photosensitizing Properties of Thiolated Gold Clusters. Exploring Beyond Plasmonics

Metal nanoparticles possess unusual catalytic and optical properties. Specifically, larger size particles exhibit plasmonic effects as they interact with incident photons. However, the plasmon resonance activity ceases as the size of metal nanoparticles decrease. However, such small size nanoparticles with few atom metal clusters continue to exhibit photoactivity in the visible. The gold clusters containing 10-25 metal atoms exhibit molecular like properties with relatively long lived excited state (lifetime ∼ 800 ns). When a mesoscopic TiO₂ film sensitized by Au_x-GSH NCs was used as the photoanode in a photoelectrochemical cell, we can generate photocurrent under visible light irradiation with a photoconversion efficiency of 2%. The reversible reduction (E⁰= -0.63 V vs. RHE) and oxidation (E⁰= 1.51 V vs. RHE) potentials make them suitable as visible sensitizers for driving the water splitting reaction. Additionally, sensitizing Pt/TiO₂ nanoparticles with Au_x-GSH NCs has been explored in an aqueous slurry system and irradiating with visible light resulted in hydrogen production. These metal nanoclusters which serve as a new class of visible photosensitizers exhibit significantly higher photoactivity than the larger nanoparticles and thus offer new opportunities as light harvesting assemblies.