Hydrogenation and hydrogen diffusion at the anatase TiO2(101) surface.

Hydrogenation of TiO2 enhances its visible photoabsorption, leading to efficient photocatalytic activity. However, the role of hydrogen has not been fully understood. The anatase TiO2(101) surface treated by hydrogen ion irradiation at 500 eV was investigated by photoemission spectroscopy and nuclear reaction analysis. Hydrogen irradiation induces an in-gap state 1-1.6 eV below the Fermi level and a downward band bending of 0.27 eV. The H depth profile at 300 K shows a surface peak with an H amount of (2.9 ± 0.3) × 1015 cm-2 with little concentration in a deeper region. At 200 K, on the other hand, the H depth profile shows a maximum at about 1 nm below the surface corresponding to an H amount of (6.1 ± 0.3) × 1015 cm-2 along with a broad distribution extending to 50 nm at an average concentration of 0.8 at. %. These results show that H diffusion in anatase TiO2 is much faster than in rutile TiO2 [Y. Ohashi, J. Phys. Chem. C 123, 10319-10324 (2019)]. The H diffusion coefficient at 200 K is determined to be 2.7 ± 0.1 × 10-13 m2 s-1.

Proton Configuration in Water Chain on Pt(533).

In this paper, a wetting behavior of Pt(533) is studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under an ultrahigh-vacuum condition at 145 K. The imaginary parts of the surface nonlinear susceptibility (Imχ(2)) of the H-bonded OH stretching region are successfully obtained for submonolayer water coverage that show negative bands indicating H-down (proton pointing to the substrate) configurations both for the water at the step and at the terrace. The growth manner of the Imχ(2) signal with coverage and the results of an isotopic dilution are consistent with a model in which a one-dimensional (1D) chain at the step forms a "zigzag" structure that contains H-down orientations. This finding resolves the previous controversy in the literature concerning the proton configuration in the 1D water chain at the step.

Blackening of titanium dioxide nanoparticles by atomic hydrogen and the effect of coexistence of water on the blackening.

A fast blackening process of titanium dioxide nanoparticles by exposing to atomic hydrogen was studied by estimating the color of the nanoparticles. The whiteness of TiO2 decreased exponentially with time, which suggests a first-order reaction between atomic H and surface oxygen, whose rate constant is proportional to the ambient pressure of H2. The rate constant increases as the temperature of nanoparticles at exposing to atomic hydrogen. The structure and size of nanoparticles were estimated by the X-ray diffraction (XRD), which shows that a part of anatase transferred to rutile and the crystal sizes of both anatase and rutile increased by hydrogenation above 600 K. The blackening of TiO2 halfway stopped under the condition of the similar partial pressure of water with hydrogen. This suggests the presence of reverse reaction between H2O and oxygen vacancy, whose reaction rate constant is proportional to the partial pressure of H2O.