Recent progress on doped ZnO nanostructures for visible-light photocatalysis

Global environmental pollution and energy supply demand have been regarded as important concerns in recent years. Metal oxide semiconductor photocatalysts is a promising approach to apply environmental remediation as well as fuel generation from water splitting and carbon dioxide reduction. ZnO nanostructures have been shown promising photocatalytic activities due to their non-toxic, inexpensive, and highly efficient nature. However, its wide band gap hinders photo-excitation for practical photocatalytic applications under solar light as an abundant, clean and safe energy source. To overcome this barrier, many strategies have been developed in the last decade to apply ZnO nanostructured photocatalysts under visible light. In this review, we have classified different approaches to activate ZnO as a photocatalyst in visible-light spectrum. Utilization of various nonmetals, transition metals and rare-earth metals for doping in ZnO crystal lattice to create visible-light-responsive doped ZnO photocatalysts is discussed. Generation of localized energy levels within the gap in doped ZnO nanostructures has played an important role in effective photocatalytic reaction under visible-light irradiation. The effect of dopant type, ionic size and its concentration on the crystal structure, electronic property and morphology of doped ZnO with a narrower band gap is reviewed systematically. Finally, a comparative study is performed to evaluate two classes of metals and nonmetals as useful dopants for ZnO nanostructured photocatalysts under visible light.