Adsorption of O₂, H₂, CO, NH₃, and NO₂ on ZnO Nanotube: A Density Functional Theory Study

Abstract

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 (VO), as well as its potential application as a sensor for gas molecules O2, H2, 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 VO 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, O2 and H2 molecules are physisorbed while CO, NH3, and NO2 are molecularly chemisorbed. With the VO 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 VO sites through filling the VO 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).