Composition Gradients in Sputtered Ti-Au Alloys: Site-Selective Au Decoration of Anodic TiO2 Nanotubes for Photocatalytic H2 Evolution

Au nanoparticles at the surface of TiO2 can improve the photocatalytic H2 generation performances because of their electron transfer cocatalytic ability. The key to maximize the cocatalytic effect is a fine control over Au nanoparticle size and placement on the photocatalyst, in relation to parameters such as the TiO2 morphology, illumination wavelength and pathway, and light penetration depth in the photocatalyst. Here we present an approach for site-selective intrinsic decoration of anodic TiO2 nanotubes (TNs) with Au nanoparticles: we produce, by Ti and Au cosputtering, Ti–Au alloy layers that feature compositional gradients across their thickness; these layers, when anodized under self-ordering electrochemical conditions, can form Au-decorated TNs where the Au nanoparticle density and placement vary according to the Au concentration profile in the metal alloy substrates. Our results suggest that not only the amount but also the Au cocatalyst placement can strongly affect the photocatalytic H2 evolution performance of the TN layers. We demonstrate that when growing Au-decorated TNs, the use of Ti–Au substrates with a suitable Au compositional gradient enables a higher photocatalytic H2 generation performance compared to TNs classically grown with a homogeneous cocatalyst decoration. As a side effect, a proper placement of the cocatalyst nanoparticles allows for reducing the amount of noble metal without dumping the H2 evolution activity.