Comparing Photoelectrochemical Methanol Oxidation Mechanisms for Gold versus Titanium Nitride Nanoparticles Dispersed in TiO2 Matrix

Using nanostructured TiO2 as the common matrix, TiN/TiO2 and Au/TiO2 photocatalysts are compared in their activity for photoelectrochemical methanol oxidation testing both visible and UV excitation. We demonstrate that both TiN and Au nanoparticles (NPs) act as photo-sensitizers extending TiO2 activity into the visible region of white light. Our results highlight two major differences between the TiN/TiO2 and Au/TiO2 photocatalysts. First, potential bias is required to initiate visible light-induced CH3OH oxidation at the TiN/TiO2 interface that forms an ohmic junction. Second, the reaction at the TiN/TiO2 interface is most likely driven by the highly reactive holes generated by interband transitions in TiN NPs at wavelengths shorter than 630 nm, while plasmonic effect peaking at 580 nm is thought to be a major driving force for the CH3OH oxidation on the Au/TiO2 material. Under UV excitation, an increase in the saturation photocurrent vs. bare TiO2 is observed both for TiN/TiO2 and Au/TiO2 photocatalysts. We attribute this behavior to (1) the participation of highly reactive holes generated by interband transitions in TiN NPs in photoelectrochemical CH3O H oxidation, and (2) better electron-hole separation at the Au/TiO2 interface due to back transfer of photoexcited electrons from the conduction band of TiO2 to Au. Theoretical modeling is performed to assist with interpretation of optical and photoelectrochemical results. (c) 2019 The Electrochemical Society.