Adsorption of O₂, H₂, CO, NH₃, and NO ₂ on ZnO nanotube: A density functional theory study

Using density functional theory (DFT), we have investigated the structural and electronic properties of a prototype ZnO (6,0) zigzag single-walled nanotube (SWNT) with and without oxygen vacancy (V_o), as well as its potential application as a sensor for gas molecules O₂, H ₂, CO, NH₃, and NO₂. The DFT calculation shows that the defect-free ZnO (6,0) SWNT is semiconducting with a direct band gap larger than that of bulk ZnO. By introducing the V_o defects, localized impurity states are induced above the valence band maximum while the Fermi level is lifted. As such, the defect-containing ZnO (6,0) SWNT becomes an n-type semiconductor. On the sidewall of a defect-free ZnO (6,0) SWNT, O ₂ and H₂ molecules are physisorbed while CO, NH ₃, and NO₂ are molecularly chemisorbed. With the V _o defects, the binding interaction between gas molecules and the ZnO nanotube becomes stronger. The electron-donor molecules (CO and NH₃) tend to enhance the concentration of major carriers (electrons), whereas the electron-acceptor molecules (O₂ and NO₂) tend to reduce the concentration. Moreover, we find that O₂ and NO₂ can dissociate at the V_o sites through filling the V_o with one atomic O originated from the adsorbates. The dissociation of O₂ is exothermic and barrierless while the dissociation of NO₂ is also exothermic but entails a small activation barrier (0.49 eV). © 2008 American Chemical Society.