The second method for improving its photocatalytic efficiency includes dye-sensitizing,
compositing of semiconductors with a narrow band gap,
noble metal doping, non-metal (e.g. N, C, S) doping, and transition metal doping, etc. [16e20]. Among these, doping with metal, non-metal
or noble metal is an effective method to modify the physical and photocatalytic properties of ZnO photocatalyst in order to extend its practical applications.