We report the first-principles electronic structure of BiVO4, a promising photocatalyst for hydrogen generation. BiVO4 is found to be a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band minimum. These interactions result in symmetric hole and electron masses. Implications for the design of ambipolar metal oxides are discussed.

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Band Edge Electronic Structure of BiVO4: Elucidating the Role of the Bi s and V d Orbitals

The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s2 lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn–Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO4), ferroelectrics (PbTiO3), multi-ferroics (BiFeO3) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.

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Stereochemistry of post-transition metal oxides: revision of the classical lone pair model.

Heterojunctioned BiOCl/Bi2O3, a new visible light photocatalyst

Three ternary oxides, SnWO4, PbWO4, and BiVO4, containing p-block cations with ns2np0 electron configurations, so-called lone pair cations, have been studied theoretically using density functional theory and UV−visible diffuse reflectance spectroscopy The computations reveal significant differences in the underlying electronic structures that are responsible for the varied crystal chemistry of the lone pair cations The filled 5s orbitals of the Sn2+ ion interact strongly with the 2p orbitals of oxygen, which leads to a significant destabilization of symmetric structures (scheelite and zircon) favored by electrostatic forces The destabilizing effect of this interaction can be significantly reduced by lowering the symmetry of the Sn2+ site to enable the antibonding Sn 5s−O 2p states to mix with the unfilled Sn 5p orbitals This interaction produces a localized, nonbonding state at the top of the valence band that corresponds closely with the classical notion of a stereoactive electron lone pair In compo

Structure and Bonding in SnWO4, PbWO4, and BiVO4: Lone Pairs vs Inert Pairs

Three polymorphs of Bi2O3 were selectively synthesized via solution-based methods. The phase structures of the as-prepared samples were confirmed by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). UV-vis diffuse reflectance spectroscopy was employed to study the optical properties of Bi2O3 polymorphs, and the band gaps were estimated to be 2.80, 2.48, and 3.01 eV for α-Bi2O3, β-Bi2O3, and δ-Bi2O3, respectively. The photocatalytic performances of the oxides were investigated by decomposing methyl orange and 4-chlorophenol under visible irradiation at room temperature. It was observed that β-Bi2O3 displayed much higher photocatalytic performance than N-doped P25. Among the three polymorphs of Bi2O3, the photocatalytic activities followed the order: β-Bi2O3 > α-Bi2O3 > δ-Bi2O3, which was in good accordance with the photoluminescence spectra measurement results. The synergistic effect of the crystal and electronic structures on the photocatalytic performances of Bi2O3 polymorphs was investigated. The much better photocatalytic activity of β-Bi2O3 was considered to be closely related to its smaller band gap, higher crystallinity and unique tunnel structure.

Synergistic effect of crystal and electronic structures on the visible-light-driven photocatalytic performances of Bi2O3 polymorphs

$\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is the thermodynamically stable phase of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ at room temperature. We have performed a theoretical and experimental investigation of its electronic structure using a combination of gradient corrected density functional theory (DFT), along with x-ray photoemission and $\mathrm{O}\text{\ensuremath{-}}K$ shell x-ray absorption and emission spectroscopies. We examine the nature of bonding in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ and in particular explore the nature of the stereochemically active Bi electron lone pair. The Bi $6s$ states are found to be concentrated at the bottom of the valence band but the states contributing to the lone pair on Bi are derived from the top of the valence band. Mixing between O $2p$ and Bi $6s$ states is found to be crucial in producing the asymmetric density on Bi. The role of the lone pair in the fast ion conductor $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is also investigated, through calculation of the electronic structure with ⟨100⟩, ⟨110⟩, and ⟨111⟩ alignment of oxygen vacancies. Alignment of the vacancies along ⟨100⟩ results in the most energetically favorable configuration of the $\ensuremath{\delta}$ phase, contrary to previous force field calculations and electrostatic arguments which favor the ⟨111⟩ alignment.

Russell G. Edgell

Papers

Electronic structure of the alpha and delta phases of Bi2O3: A combined ab initio and x-ray spectroscopy study