_ 0) Surface is non-polar and is modelled by a symmetric slab with a (2×2) surface cell, which consists of 14 atomic layers (112 atoms without H adsorption). Each atomic layer has 4 Zn and 4 O atoms (Figure ). A vacuum layer of 15Å was added to the surface to separate the slab from its periodic images. The first Brillouin zone (BZ) were sampled using a 2×1×1 Monkhorst-Pack k-point grid. The plane wave cutoff was set to 800 eV. Surfaces with 0, 1, 2 and 4 H atoms adsorbed were used to simulate 0%, 25%, 50% and 100% H coverage. During the geometry optimization, the first four atomic layers were fully relaxed until the Hellman-Feynman force is less than 0.01eV/Å, while the rest were fixed to simulate the bulk.