Bi NPs were prepared by MW assisted method using ascorbic acid as reducing agent.
MW-Bi NPs were very uniform and the average size was about 70 nm.
MW-Bi possessed good visible light absorption ability and the band gap was 1.77 eV.
The photocatalytic efficiency of MW-Bi was 2.7 times that of SV-Bi.
The enhanced photocatalytic activity of MW-Bi was ascribed to its small size effect.