Efficient ZnO-based visible-light photocatalysts were synthesized via a low-cost method by increasing the Co concentration at the outermost surface of ZnO:Co thin film under an annealing process. The simply adjusted Co concentration gradient (≈1.5% increase) into the surface of ZnO:Co was driven by thermal diffusion. Co ions substitute Zn sites in the ZnO lattice. The band gap narrowed due to sp-d exchange interactions between the conduction band electrons and the d electrons of Co. The annealing process enhances the grain size and the degree of crystallinity of the diffused Co/ZnO:Co, which contributed to the absorption edge red-shift.

Thermal Diffusion of Co into Sputtered ZnO:Co Thin Film for Enhancing Visible-Light-Induced Photo-Catalytic Activity. Kao, C.Y., Liao, J.D., Chang, C.W., Wang, R.Y.: Applied Surface Science, 2011, 258[5], 1813-8

Table 3

Chemical diffusion of Co in ZnO single crystals