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Title: Concentration-mediated band gap reduction of Bi₂MoO₆ photoanodes prepared by Bi³⁺ cation insertions into anodized MoO₃ thin films : structural, optical, and photoelectrochemical properties
Authors: Lou, Shi Nee
Amal, Rose
Scott, Jason
Ng, Yun Hau
Keywords: Engineering::Materials
Issue Date: 2018
Source: Lou, S. N., Amal, R., Scott, J. & Ng, Y. H. (2018). Concentration-mediated band gap reduction of Bi₂MoO₆ photoanodes prepared by Bi³⁺ cation insertions into anodized MoO₃ thin films : structural, optical, and photoelectrochemical properties. ACS Applied Energy Materials, 1(8), 3955-3964.
Journal: ACS Applied Energy Materials
Abstract: 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.
ISSN: 2574-0962
DOI: 10.1021/acsaem.8b00675
Rights: © 2018 American Chemical Society. All rights reserved.
Fulltext Permission: none
Fulltext Availability: No Fulltext
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