TiO2 nanowires growth mechanism via thermal oxidation assisted by Au nanoparticles

• A growth mechanism for nanowires produced by seed-assisted oxidation is presented. • The Ti interstitials migrate from the metallic Ti substrate through the TiO 2 layer to the Au nanoparticle on the surface. • Nanometric heterojunction at the Au nanoparticle/TiO 2 interface improves the growth rate locally. • The Ti underlayer acts as a source of Ti interstitials while a thick TiO 2 layer inhibits the nanowires' growth. Nanowires (Nws) play a crucial role in titanium dioxide's photo-physical and photochemical applications, such as photovoltaics, photocatalysis, and water splitting. Among the techniques employed to synthesise nanostructures, the Au seed-assisted thermal oxidation is the simplest and the most intriguing. TiO 2 NWs, with Au-nanoparticles (AuNPs) on the tip, can be obtained by annealing in air, a thin Au film deposited on a metallic Ti. Au-NPs, formed during the annealing, act as seeds for NWs growth, while the oxide layer, formed during oxidation, inhibits the Nws growth. An in-depth investigation is conducted on the effects of various substrates, oxide layer thickness, and oxidation time. We propose a diffusion mechanism involving the transport of Ti interstitials from the metallic Ti substrate, through the TiO 2 , to the surface. Moreover, high-resolution conductive AFM analysis at the AuNp/TiO 2 interface suggests the formation of a nanometric heterojunction. The heterojunction electric field promotes a localised enrichment of Ti interstitials close to AuNps, thereby enhancing the oxidation rate and resulting in the creation of TiO 2 nanowires.