Adsorption of O₂, H₂, CO, NH₃, and NO₂ 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 O 2, H 2, CO, NH 3, and NO 2 . 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 H 2 molecules are physisorbed while CO, NH 3, and NO 2 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 NH 3 ) tend to enhance the concentration of major carriers (electrons), whereas the electron-acceptor molecules (O 2 and NO 2 ) tend to reduce the concentration. Moreover, we find that O 2 and NO 2 can dissociate at the V O sites through filling the V O with one atomic O originated from the adsorbates. The dissociation of O 2 is exothermic and barrierless while the dissociation of NO 2 is also exothermic but entails a small activation barrier (0.49 eV).