First-principles study of electronic structures and photocatalytic activity of low-Miller-index surfaces of ZnO

First-principles calculations have been performed to investigate the electronic structures and optical properties of the main low-Miller-index surfaces of ZnO: nonpolar (101¯0) and (112¯0) surfaces as well as polar (0001)-Zn and (0001¯)-O surfaces. According to the structure optimization results, there are similar relaxation behaviors for the (101¯0) and (112¯0) surfaces, both with a strong tilting of the surface Zn-O dimers and an obvious contraction of the surface bonds. For the polar surfaces, the surface double layers both tend to relax inwards, but the largest relaxation is found on the (0001¯)-O surfaces. The calculated band gaps are 0.56, 0.89, 0.21, and 0.71 eV for (101¯0), (112¯0), (0001)-Zn and (0001¯)-O surfaces, respectively. For the nonpolar (101¯0) and 112¯0 surfaces, the Fermi levels locate at the valence band maximum, which are similar to that of bulk ZnO. The surface states in the conduction band lead to the increased Fermi level and cause the n-type conduction behavior for (0001)-Zn surface. For the (0001¯)-O surface, the Fermi level shifts down a little into the valence band, leading to the p-type conduction behavior. From the optical properties calculations, absorption regions of all the four surfaces are quite wide and the main absorption peaks locate in the UV region. For the (0001)-Zn surface, it has the strongest absorptions in the near UV-light range and a remarkable red-shift phenomenon of the absorption edge. This indicates that (0001)-Zn surface has the highest photocatalytic activity among the four surfaces as the low excitation energy is required theoretically. The computed results are in accordance with the experimental observations.