Mechanisms of Highly Efficient Photocatalytic Pollutant Degradation by Au/TiO2 Janus Nanoparticles

Semiconductor nanoparticles partially coated with metals have been widely used to degrade contaminants in water, but the physical mechanisms underlying degradation are poorly understood, limiting their real-world implementation. Here, we reveal the degradation mechanisms that dominate when gold-coated titanium dioxide (Au/TiO2) “Janus” nanoparticles (JNPs) are irradiated with monochromatic ultraviolet light (254 nm and 365 nm wavelengths) to degrade 1,4-dioxane, a carcinogenic model pollutant. To do so, we performed experiments with ultraviolet light at different wavelengths with and without radical quenching, extensive JNP characterization (SEM, XRD, EDS, DLS, and UV-Vis), and 3D simulations of self-propulsion and light-matter interactions. We traced the enhanced photocatalytic activity of Au-coated JNPs to both increased light absorption due to Au acting as an optical antenna, and inhibited recombination of photogenerated electrons and holes. These two effects increase the production of hydroxyl radicals, accelerating the degradation of 1,4-dioxane. The reduced electron/hole recombination is due to two factors: the Schottky barrier that forms between Au and TiO2 (which drives photogenerated electrons from TiO2 into the metal), and stoichiometric changes in the TiO2 that accompany gold sputtering which facilitate electron sequestration by the metal. In contrast, self-propulsion and surface plasmon resonance play at most a minor role.