Photodeposition has emerged as a promising technique to grow metallic nanoparticles (NPs) on semiconductor supports, through reduction of metal ions by photoexcited electrons in the support. Recombination with holes is a competing process that can be suppressed, by including a hole scavenger to extract holes. Various studies have indicated the importance of including a hole scavenger, but the exact role and interactions of hole scavengers during photodeposition remains unclear. In particular, the effect of various hole scavengers on the resulting material properties are not well understood e.g., optical response, particle size, particle distribution, and growth kinetics of the deposited NP. Controlling synthesis is crucial for optimizing optical properties and thus applications in photocatalytic performance. In this work we have investigated the influence of the presence and type of four alcohol-based hole scavengers on the photodeposition of gold (Au) on zinc oxide (ZnO).

Ex-situ High-Resolution (Scanning) Transmission Electron Microscopy combined with Energy Dispersive X-ray spectroscopy (HR-(S)TEM-EDX) results show that without a hole scavenger, a broad particle size and shape distribution (20 ± 15 nm) and less nucleation sites are obtained. The optical response of Au NPs prepared without a hole scavenger show two Au Localized Surface Plasmon Resonance (LSPR) peaks, one located at ~ 515 nm and one at ~ 575 nm. From electromagnetic simulations we suspect that the LSPR peak at ~ 515 nm arises due to smaller, low-aspect-ratio NPs, whereas the LSPR peak at ~ 575 nm is due to a few large NPs with a high aspect ratio. In contrast, the use of different alcohol-based scavengers results in a uniform spatial distribution of monodisperse NP sizes of 9 ± 3 nm, leading to a well-defined Au LSPR peak centered at ~ 540 nm. Unique in-situ UV-Vis spectroscopy measurements reveal the formation of an adsorbed intermediate of Au on ZnO under dark equilibrium conditions, which reacts upon UV irradiation. These measurements also show that the photodeposition of Au on ZnO in the presence of ethanol as a hole scavenger leads to a faster nucleation and substantially accelerates particle growth compared to the non-scavenged case.

These results show that in the presence of a hole scavenger, efficient charge separation generates a high density of photogenerated electrons. Combined with numerous nucleation sites leads to the formation of many small Au NPs. In contrast, in the absence of a hole scavenger fewer charge carriers result in fewer nucleation events, which lead to a limited number of larger Au NPs. This work highlights how in-situ UV-Vis spectroscopy monitoring provides mechanistic and kinetic insight, enabling optimization of photodeposition and tailoring of Au/ZnO nanocomposite properties for photocatalysis.