Ta₂O₅Nanotubes Obtained by Anodization: Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen Production

Freestanding tantalum oxide nanotubes (Ta 2 O 5 NTs) were easily fabricated by controlling only the electrolyte temperature during anodization in a sulfuric acid solution. When the electrolyte temperature decreased, the adherence of NTs to the Ta substrate increased. High electrolyte temperatures facilitated formation of freestanding NTs. Thermal treatment of the freestanding Ta 2 O 5 NTs below 750 °C resulted in an amorphous structure. The orthorhombic crystalline phase appeared only at temperatures higher than 750 °C. The effect of thermal treatment on the crystalline structure and morphology of Ta 2 O 5 NTs showed that the NTs retained their tubular shape up to 800 °C. In addition, it was shown that the crystallinity of the NTs was enhanced from 11% to 34% by increasing the treatment time for the NTs at 800 °C from 0.5 to 1 h. High crystallinity and low surface contamination increased the photocatalytic activity of the freestanding NTs for hydrogen production by water splitting using a water/ethanol solution under UV radiation. The sample annealed at 800 °C for 1 h showed the highest photocatalytic activity for hydrogen generation. Additionally, changes to the physicochemical properties of the surface and bulk of the photocatalyst showed decreased selectivity for minor products (C 2 H 4 and C 2 H 6 ).