Concentration-Mediated Band Gap Reduction of Bi₂MoO₆ Photoanodes Prepared by Bi³⁺ Cation Insertions into Anodized MoO₃ Thin Films : Structural, Optical, and Photoelectrochemical Properties

A secondary cation insertion technique to fabricate ternary Bi₂MoO₆ thin films with reduced optical band gaps and shallow valence bands by the controllable insertion of Bi³⁺ cations into anodized MoO₃ thin films has been established. Near-complete conversion of the MoO₃ thin film to a low-temperature-phase γ(L)-Bi₂MoO₆ thin film was achieved when the MoO₃ thin films were subject to hydrothermal treatment in a low Bi(NO₃)₃·5H₂O solution concentration. In contrast, a bilayered Bi₂MoO₆/MoO₃ thin film photoelectrode comprising predominantly a high-temperature-phase γ(H)-Bi₂MoO₆ oxide-electrolyte interface top region and a MoO₃ oxide-collector interface bottom region was formed when a high Bi(NO₃)₃·5H₂O solution concentration was utilized. UV-vis spectroscopy shows both the γ(L)-Bi₂MoO₆ (E_g = 2.7 eV) and γ(H)-Bi₂MoO₆ (E_g = 3.05 eV) thin films exhibit smaller band gaps than MoO₃ (E_g = 3.4 eV). For γ(L)-Bi₂MoO₆, the reduction in optical band gap was attributed to the formation of a higher-lying O 2p valence band maximum while, for the γ(H)-Bi₂MoO₆ thin film, hybridization of the Bi 6s orbitals with the O 2p valence orbitals lowers the potential of the valence band maximum, leading to the reduced band gap. Overall, the Bi₂MoO₆ thin films with the highest γ(L)-Bi₂MoO₆ concentration exhibited the highest photocurrent density. The photocurrent enhancement can be attributed to two main reasons: first, the trilayer Bi₂MoO₆/MoO₃ heterostructure obtained from the direct thin film assembly enables a smooth percolation of photoexcited charges from the surface generation sites to the charge collection sites at the Mo substrate, minimizing charge recombination losses; second, the MoO₆ octahedra-coordinated γ(L)-Bi₂MoO₆ possesses a wide conduction band enabling fast separation and migration of delocalized charges. The secondary cation insertion technique has potential as a universal method to prepare complex oxides with narrow band gaps and shallow valence bands from insertion-type oxides for solar energy applications.