A DFT study of gas molecules adsorption on the anatase (0 0 1) nanotube arrays

First-principles density functional theory (DFT) calculations have been performed on the structural and electronic properties of anatase (0 0 1) TiO2 nanotube arrays (TNTAs) without and with the adsorption of various gas molecules including O2, H2, CO, NO, H2O, CO2, NO2, N2O, NH3 and CH4 in details. Our study shows that the (12,0) and (18,0) TNTAs are semiconductors with indirect band gaps higher and lower than that of the bulk TiO2, respectively. The adsorption of the same gas molecule on (18,0) TNTAs is relatively stronger than that on (12,0) TNTAs due to the surface curvature effect. H2O and NH3 are chemisorbed while other gases are physisorbed. In addition, all the studied gases are charge donors to the TNTAs. The electronic and transport properties of both TNTAs are sensitive to the adsorption of certain gases such as O2, NO and NO2. In contrast, other gases have little effect on modifying the electronic structures of TNTAs.