Anodic TiO2 nanotube membranes: Site-selective Pt-activation and photocatalytic H2 evolution

Anodic TiO2 nanotube membranes of various thicknesses (1.5-60 μm) fixed on FTO slides were site-selectively decorated with Pt nanoparticles at only their upper extremity. Their photocatalytic H2 evolution ability (with and without Pt) is investigated under UV light (325 nm) illumination through the FTO slide. The irradiation side is opposite to the Pt decorations in order to avoid Pt shading effects. This design is key to study the H2 evolution in relation to membrane thickness, light absorbance, and electron diffusion length in the nanotubes. The Pt decoration improves the H2 evolution, and the photocatalytic enhancement, quantified as ratio of H2 evolution rate with Pt vs. that without Pt, varies with the membrane thickness. The highest photocatalytic enhancement is for 5-10 μm-long nanotubes, owing to maximized photon harvesting and efficient electron trapping/transfer to reactants. This is enabled by a short pathway of electron diffusion towards Pt. The photocatalytic enhancement drops dramatically for membrane >30 μm while membranes of intermediate lengths (∼10-30 μm) show similar photocatalytic enhancement. This can be ascribed to an electron diffusion length in Pt-TiO2 nanotubes of 20-30 μm.