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 gamma(L)-Bi2MoO6 thin film was achieved when the MoO3 thin films were subject to hydrothermal treatment in a low Bi(NO3)(3)center dot 5H(2)O solution concentration. In contrast, a bilayered Bi2MoO6/MoO3 thin film photoelectrode comprising predominantly a high-temperature-phase gamma(H)-Bi2MoO6 oxide-electrolyte interface top region and a MoO3 oxide-collector interface bottom region was formed when a high Bi(NO3)(3)center dot 5H(2)O solution concentration was utilized. UV-vis spectroscopy shows both the gamma(L)-Bi2MoO6 (E-g = 2.7 eV) and gamma(H)-Bi2MoO6 (E-g = 3.05 eV) thin films exhibit smaller band gaps than MoO3 (E-g = 3.4 eV). For gamma(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 gamma(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 gamma(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 gamma(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.