TiH Hydride Formed on Amorphous Black Titania: Unprecedented Active Species for Photocatalytic Hydrogen Evolution

Amorphous structures are often good catalysts for their large varieties of exposed surface sites, but the characterization of the catalytic sites has long been a great challenge for both theory and experiment. One such interesting example is the recently synthesized "black TiO2" via the hydrogenation of TiO2 nanoparticles, which has an amorphous shell and crystalline core, and exhibits high hydrogen evolution reaction (HER) activity in the visible-light photocatalytic water splitting. Here, we utilize our recently developed theoretical tool, namely, the stochastic surface walking global optimization with neural network potential followed by first-principles validation, to quantitatively determine the thermodynamic phase diagram of TiO2 in contact with H2 at different temperatures and pressures. Among a number of anatase surfaces, we show that, only on a ridged anatase (112) surface, a local high H coverage, 0.69 ML, can be gradually built up accompanied by the surface amorphization. This high H coverage not only renders the black color of the amorphous TiO2 but also provides an unprecedented low-energy reaction channel for the HER: a transient Ti–H hydride becomes likely to form on exposed Ti atoms, and its reaction with neighboring OH has much lower barrier, i.e., 0.6 eV, compared to the traditional H coupling channel via two surface OH groups (barrier >1.6 eV). Our results not only provide deep insights into the surface species and reactions that are unique to amorphous materials, but also demonstrate that the global sampling with neural network potential holds a great promise for solving complex structures under realistic reactions.