Synergistic Cooperation of Rutile TiO₂ {002}, {101}, and {110} Facets for Hydrogen Sensing

An oriented TiO₂ thin film-based hydrogen sensor has been demonstrated to have excellent sensing properties at room temperature. The exposed high energy surface offers a low energy barrier for H₂ adsorption and dissociation. In this work, rutile TiO₂ with {101} and {002} facets exposed was controllably synthesized by adjusting the ethanol content of the hydrothermal solvent. The crystalline structure, morphologies, and H₂ sensing performance of the samples varied with the relative ratios of {002} and {101} facets. By increasing the ethanol content, the (002) orientation growth was enhanced and the (101) orientation growth was restrained, the size of the nanorods composing the thin film was reduced and the density of the film was increased. All of the prepared TiO₂ nanorod array film-based hydrogen sensors performed very well at room temperature. The TiO₂ hydrogen sensor with both {110} and {002} facets exposed gave a faster response, as well as better repeatability and stability than those with only {002} facets. Density functional theory simulations have been adopted to reveal the surface interaction of H₂ and the TiO₂ surface. The results suggested that H₂ tended to be adsorbed and dissociated on the (002) and (101) surface. There is a very small active barrier for atomic H to recombine into H₂ molecules on the (110) surface. Thin films with lower density, where more (110) surface is exposed, offered more space for H₂ regeneration, leading to shorter response and recovery times as well as higher sensitivity. The (002), (101), and (110) surfaces of rutile TiO₂ synergistically cooperated to complete the whole H₂ sensing process.