Hydrogenation of TiO2 nanosheets and nanoparticles: typical reduction stages and orientation-related anisotropic disorder

Abstract

Nanostructured black TiO₂ materials prepared through a hydrogenation process have evoked significant interest in solar energy harvesting and conversion technologies due to their strong light absorption and utilization performances. Herein, by combining RMD simulations and DFT calculations, we systematically unravel the structural evolution mechanisms during the hydrogenation process of the nanosheets and nanoparticles of three different TiO₂ phases: anatase, brookite, and rutile. Our results demonstrate that the reduction of TiO₂ nanosheets occurs in three typical stages, namely the rapid escaping of free water and inner O atom diffusion, the slow free-water formation and the surface disorder, and eventually the dramatic stress deformation of the sheets. When it comes to the reduction of nanoparticles, orientation-relative anisotropic disorder has been observed. It is found that the preferable disordered surface is along {001} for anatase, {100} for brookite and {001} for rutile, causing anisotropic stress deformation. The surface reactivity of these three structures follows the order: rutile > anatase > brookite for both nanosheets and nanoparticles. Besides, the reduction rates are prominently dependent on the temperature, pressure, and H₂ content. We expect that these results would provide theoretical guidance for the rational preparation of black TiO₂ to enhance the development of clean energy.