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 (Eg = 2.7 eV) and γ(H)-Bi2MoO6 (Eg = 3.05 eV) thin films exhibit smaller band gaps than MoO3 (Eg = 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.