Band Gap Narrowing by Suppressed Lone-Pair Activity of Bi³⁺

Abstract

Post-transition metal cations with a lone pair (ns 2 np 0 ) electronic configuration such as Pb 2+ and Bi 3+ are important components of materials for solar-to-energy conversion. As in molecules like NH 3 , the lone pair is often stereochemically active in crystals, associated with distorted coordination environments of these cations. In the present study, we demonstrate that suppressed lone pair stereochemical activity can be used as a tool to enhance visible light absorption. Based on an orbital interaction model, we predict that a centrosymmetric environment of the cations limits the orbital interactions with anions, deactivates the lone pair, and narrows the band gap. A high-symmetry Bi 3+ site is realized by isovalent substitutions with Y 3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi 2 YO 4 X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y−O compounds. The introduction of Bi 3+ to the undistorted Y 3+ site in Bi 2 YO 4 X results in a narrowed band gap, as predicted theoretically and confirmed experimentally. The orbital interaction controlled by site symmetry engineering offers a pathway for the further development of post-transition metal compounds for optoelectronic applications.

Internal strain-driven bond manipulation and band engineering in Bi2−xSbxYO4Cl photocatalysts with triple fluorite layers

“…It is notable that the bandgaps for 0.2 ≤ x ≤ 0.8 are among the smallest of oxychloride photocatalysts, with the exception of Bi 2.8 Y 0.2 O 4 Cl with 2.0 eV. 36–38…”

Dual Polarization Strategy for Boosting Electron–Hole Separation toward Overall Water Splitting within Ferroelectric β-AIBIIIO2 (BIII = P3+, As3+, Sb3+, and Bi3+ for Lone Pairs)

“…This is also evident from Figure d that there is a significant reduction in the band gap of type II and type III. Notably, the presence of 6s 2 lone pairs in both Tl + and Bi 3+ plays an important role in the band gap, resulting in β-TlBiO 2 possessing the narrowest band gap among the calculated materials. A detailed analysis of the band gap difference is presented in the Supporting Information.…”

Band engineering of layered oxyhalide photocatalysts for visible-light water splitting

“…The symmetry of the middle cation in the triple fluorite layer is important for tuning the VBM of Bi 2 MO 4 Cl when it is occupied by Bi with lone pair electrons. 95 While the middle Bi in Bi 3 O 4 Cl is significantly displaced from the center of the cube formed by oxygen, the solid solution Bi 2 (Bi x Y 1− x )O 4 Cl ( x ≤ 0.5), retaining high tetragonal symmetry, has Bi in a centrosymmetric environment. The high symmetry suppressed the “secondary” lone pair interaction of the middle Bi (the right of Fig.…”