Schematic picture of surfaces. The white, gray, and black balls represent oxygen, titanium, and hydrogen atoms, respectively. The larger white balls represent top-layer atoms called bridging oxygen, which form a one-dimensional row along . The second top layer , , and atoms are three-coordinated in-plane oxygen, five-coordinated titanium, and six-coordinated titanium atom, respectively. A water molecule tends to adsorb at the vacancy site, . The arrows W and B represent the proposed diffusion paths of one of the H atoms in , corresponding to dissociation of adsorbed .
(a) Stereoscopic view of the slab simulating surface. Balls indicate the constituent atoms as described in Fig. 1. The rectangular parallelepiped represents the unit cell used in the calculation, where the slab is sandwiched between vacuum regions. (b) Top view of surface. , , and surface unit cells are indicated by the dashed lines. The large white circles are top-layer atoms, while the black circles denote H atoms adsorbed on . The other circles are O (white) and Ti (gray) atoms located at the second top layer.
Schematic top view of the diffusion paths for H atom on surface. Atom kinds are distinguished with the same circles as in Fig. 2(b). A water molecule is first adsorbed at position A, i.e., site. Dissociation of such can occur by diffusion of one of the H atoms from A to B, C, D, and E points, corresponding to the paths of , , , , , and in the text. As a result of the dissociation from A to B, a pair of hydroxyl units is formed. Splitting reaction of the pair can occur by diffusing H atom from B to and E points, corresponding to the paths of , , and .
Calculated adiabatic potentials of H-atom diffusion on surface along (a) straight-line paths and (b) relay-point paths. The alphabetical symbols, A to R, represent the surface positions shown in Fig. 3. The reaction coordinate represents the hydrogen-diffusion length along each path, corresponding to the distance between two hydrogen atoms at two points. The energies are measured relative to the hydrogen configuration A.
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