It is reported that the work function of Au and ZnO are 5.1 eV and 5.2 eV respectively Fig. 5. One-dimensional ZnO nanostructures fabricated by spinning solutions with different viscosity (a, b) sample 6 and (c, d) sample 7. Fig. 6. Schematic illustration for the possible formation mechanism of ZnO nanotubes. 524 X. Dong et al. / Ceramics International 42 (2016) 518–526 [25,26], and thus the Fermi energy level of Au is higher than that of ZnO.