Fig. 2.

Formation mechanism and reactivity of the hydrogenated TiO₂ shell. a Atomic structures of four key intermediates—initial state, sur3-1 state, sur3-2 state, and sub state—in the surface subsurface diffusion of a H atom. In the sur3-1 (sur3-2) state, atomic hydrogen is adsorbed on a surface three-fold O with the hydrogen pointing toward the vacuum (toward the bulk). In the sub (or final) state the hydrogen is adsorbed at a subsurface O atom. The O atom is red, H is gray blue, and the pink-red sticks represent the TiO₂ system. Red arrows indicate the three steps (Steps 1–3) for the diffusion of H (marked by the green dot circle) into TiO₂. b Potential energy profile for the surface subsurface diffusion of atomic hydrogen. Energies are relative to the initial state, and ‘TS’ indicates transition states. Blue and green bars represent energies in the absence and presence of adsorbed water, respectively. c Schematic representation of the photocatalytic process: under UV light, the photoexcited electron–hole pairs can split water to form hydrogen and oxygen. d Modified photocatalytic process described in this work: the hydrogen atoms diffuse first into TiO₂ (blue arrow) to form the hydrogenated TiO₂ shell on the NP^14,15 before they are desorbed into water to form the hydrogen bubble. e, f Energy barriers and the desorption energies for the process of H₂ formation on the TiO₂(101) surface or subsurface as a function of the H/O ratio σ in the inner TiO₂, respectively