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 TiO 2 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 H 2 . 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 TiO 2 halfway stopped under the condition of the similar partial pressure of water with hydrogen. This suggests the presence of reverse reaction between H 2 O and oxygen vacancy, whose reaction rate constant is proportional to the partial pressure of H 2 O.