Internal strain-driven bond manipulation and band engineering in Bi2-x Sb x YO4Cl photocatalysts with triple fluorite layers.

In extended solid-state materials, the manipulation of chemical bonds through redox reactions often leads to the emergence of interesting properties, such as unconventional superconductivity, which can be achieved by adjusting the Fermi level through, e.g., intercalation and pressure. Here, we demonstrate that the internal 'biaxial strain' in tri-layered fluorite oxychloride photocatalysts can regulate bond formation and cleavage without redox processes. We achieve this by synthesizing the isovalent solid solution Bi2-x Sb x YO4Cl, which undergoes a structural phase transition from the ideal Bi2YO4Cl structure to the Sb2YO4Cl structure with (Bi,Sb)4O8 rings. Initially, substitution of smaller Sb induces expected lattice contraction, but further substitution beyond x > 0.6 triggers an unusual lattice expansion before the phase transition at x = 1.5. Detailed analysis reveals structural instability at high x values, characterized by Sb-O underbonding, which is attributed to tensile strain exerted from the inner Y sublayer to the outer (Bi,Sb)O sublayer within the triple fluorite block - a concept well-recognized in thin film studies. This concept also explains the formation of zigzag Bi-O chains in Bi2MO4Cl (M = Bi, La). The Sb substitution in Bi2-x Sb x YO4Cl elevates the valence band maximum, resulting in a minimized bandgap of 2.1 eV around x = 0.6, which is significantly smaller than those typically observed in oxychlorides, allowing the absorption of a wider range of light wavelengths. Given the predominance of materials with a double fluorite layer in previous studies, our findings highlight the potential of compounds endowed with triple or thicker fluorite layers as a novel platform for band engineering that utilizes biaxial strain from the inner layer(s) to finely control their electronic structures.

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

The band structure offers fundamental information on electronic properties of solid state materials, and hence it is crucial for solid state chemists to understand and predict the relationship between the band structure and electronic structure to design chemical and physical properties. Here, we review layered oxyhalide photocatalysts for water splitting with a particular emphasis on band structure control. The unique feature of these materials including Sillén and Sillén-Aurivillius oxyhalides lies in their band structure including a remarkably high oxygen band, allowing them to exhibit both visible light responsiveness and photocatalytic stability unlike conventional mixed anion compounds, which show good light absorption, but frequently encounter stability issues. For band structure control, simple strategies effective in mixed-anion compounds, such as anion substitution forming high energy p orbitals in accordance with its electronegativity, is not effective for oxyhalides with high oxygen bands. We overview key concepts for band structure control of oxyhalide photocatalysts such as lone-pair interactions and electrostatic interactions. The control of the band structure of inorganic solid materials is a crucial challenge across a wide range of materials chemistry fields, and the insights obtained by the development of oxyhalide photocatalysts are expected to provide knowledge for diverse materials chemistry.

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

Post-transition metal cations with a lone pair (ns2np0) electronic configuration such as Pb2+ and Bi3+ are important components of materials for solar-to-energy conversion. As in molecules like NH3, 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 Bi3+ site is realized by isovalent substitutions with Y3+ by considering its similar ionic radius and absence of a lone pair. The quaternary photocatalyst Bi2YO4X is singled out as a candidate for Bi substitution from a survey of the coordination environments in Y-O compounds. The introduction of Bi3+ to the undistorted Y3+ site in Bi2YO4X 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.

Metal-free carbazole-thiophene photosensitizers designed for a dye-sensitized H2-evolving photocatalyst in Z-scheme water splitting.

Dye-sensitized photocatalysts with molecular dyes and widegap semiconductors have attracted attention because of their design flexibility, for example, tunable light absorption for visible-light water splitting. Although organic dyes are promising candidates as metal-free photosensitizers in dye-sensitized photocatalysts, their efficiency in H2 production has far been unsatisfactory compared to that of metal-complex photosensitizers, such as Ru(II) tris-diimine-type complexes. Here, we demonstrate the substantial improvement of carbazole-thiophene-based dyes used for dye-sensitized photocatalysts through systematic molecular design of the number of thiophene rings, substituents in the thiophene moiety, and the anchoring group. The optimized carbazole-thiophene dye-sensitized layered niobate exhibited a quantum efficiency of 0.3% at 460 nm for H2 evolution using a redox-reversible I- electron donor, which is six-times higher than that of the best coumarin-based metal-free dye reported to date. The dye-sensitized photocatalyst also facilitated overall water splitting when combined with a WO3-based O2-evolving photocatalyst and an I3-/I- redox shuttle mediator. The present metal-free dye provided a high dye-based turnover frequency for water splitting, comparable to that of the state-of-the-art Ru(II) tris-diimine-type photosensitizer, by simple adsorption onto a layered niobate. Thus, this study highlights the potential of metal-free organic dyes with appropriate molecular designs for the development of efficient water splitting.