Improvement of Bi doping in ZnO nanocrystals by Co-doping with Al: Crystal geometry calculations and photocatalytic activity

In this work we demonstrate enhancement in visible-light photocatalytic activity (PCA) of ZnO nanoparticles (NPs) with minimal attenuation of visible light transmittance. This approach can benefit numerous optoelectronic and photocatalytic applications. ZnO NPs were p-n co-doped with Al and Bi to improve Bi doping into the ZnO crystal. Al- and/or Bi-doped ZnO was coprecipitated by ammonia from aqueous nitrate solutions of Zn²⁺, Al³⁺, and Bi³⁺, followed by microwave heating. Doping concentrations in Al- and Bi- doped ZnO (AZO and BZO) and Al/Bi co-doped ZnO (ABZO) were 1, 3, 5, and 7 mole %. The resulting NPs were characterized by XRD, TEM, EDS, BET, and UV-visible absorption. While EDS shows that almost all added Bi was incorporated into the ZnO, XRD analysis of BZO reveals formation of α-Bi₂O₃ as a secondary phase due to the poor Bi solubility in ZnO. Co-doping of Al with Bi suppressed α-Bi₂O₃ formation and increased Bi solubility in ZnO. XRD-based calculations of the lattice constants and deformation strain, stress, and energy all show insertion of Al and/or Bi into the crystal with different extents according to the dopants’ solubilities into ZnO. AZO and BZO NPs had E_g lowered by 0.05–1.39 eV and 0.30–0.70 eV, respectively, relative to ZnO. On the other hand, ABZO had E_g reductions of only 0.01–0.20 eV due to formation of acceptor-donor complex through co-doping. ABZO gave higher PCA enhancements with respect to E_g reductions (∆k_photo/–∆E_g) than either AZO and BZO, with values up to 370, 126, and 13 min^–1 eV^–1, respectively.