Molecular and atomic adsorption of hydrogen on nanotubes: An ab initio study

Molecular and atomic adsorption of hydrogen on anatase TiO 2 nanotubes is investigated using density functional theory. Hydrogen molecules are found to physisorb nondissociatively on the Ti atoms, with little effect on the electronic property of the nanotubes. Differently, a semiconductor-to-metal transition occurs when the hydrogen atoms chemisorb only on the twofold-coordinated O atoms. Further increasing hydrogen concentration will first lead to a ‘heterolytic adsorption mode’ where the adjacent Ti atoms can also adsorb the hydrogen atoms, and finally the full adsorption in both molecular and atomic forms. The observed adsorption-dependent electronic properties of TiO 2 nanotubes is useful for their applications in hydrogen sensors.