Structure of the water-splitting photocatalyst oxysulfide α-LaOInS2 and ab initio prediction of new polymorphs

Flux Crystal Growth, Structure, and Optical Properties of LiLa3Ti2S3O6: An Oxysulfide Phase Derived from K2NiF4-Type Structure

Single crystals of a new titanium oxysulfide, LiLa3Ti2S3O6, were grown from a KI molten salt. Single-crystal X-ray diffraction analysis revealed that LiLa3Ti2S3O6 crystallizes in the space group Pnma with lattice parameters of a = 11.7319(4) Å, b = 3.94787(14) Å, and c = 20.6885(6) Å. In this structure, the one-dimensional chains of corner-sharing TiO5S octahedra are further corner-linked via equatorial and apical oxygen atoms to form unique corrugated two-dimensional perovskite-type layers in the ab plane with one octahedral thickness. These layers were intervened along the c-axis by the LaS rock-salt layers corrugated concomitantly with the perovskite-type layers, and LiO2S2 tetrahedral chains were located between these two types of two-dimensional layers. LiLa3Ti2S3O6 can be viewed as a modified K2NiF4-type structure with TiO5S octahedral layers stacked in a zigzag manner along the c axis. The oxysulfide has a direct-type band gap of 1.85 eV, based on UV-vis-NIR diffuse reflectance measurements. First-principles calculations showed that the conduction band minimum mainly consists of Ti 3d orbitals, and the valence band maximum consists of S 3p, O 2p, and Li 2s orbitals. The electronic structures near the Fermi level are similar to those of the structurally related photocatalytic oxysulfides Y2Ti2S2O5 and La5Ti2CuS5O7.

Iron Oxychalcogenides and Their Photocurrent Responses

We report here the results of an experimental investigation of the electronic properties and photocurrent responses of the CaFeOQ and La2O2Fe2OQ2 phases and a computational study of the electronic structure of polar CaFeOSe. We find that both CaFeOQ (Q = S and Se) have band gaps and conduction band edge positions compatible with light-driven photocatalytic water splitting, although the oxysulfide suffers from degradation due to the oxidation of Fe2+ sites. The higher O/Q ratio in the Fe2+ coordination environment in CaFeOSe increases its stability without increasing the band gap beyond the visible range. The photocurrent CaFeOSe shows fast electron-hole separation, consistent with calculated carrier effective masses. These results suggest that these iron oxychalcogenides warrant further study to optimize their stability and morphology for photocatalytic and other photoactive applications.

Designing Visible-Light Photoactive Thioapatites Using Thiovanadate Groups: The Ba5(VO4-αSα)3X (X = F, Cl, I) Phases

The new thioapatite Ba5(VO4-αSα)3X (X = F, Cl, I) series of compounds was prepared and characterized. Compared to known apatite phases built from unconnected vanadate VO4 groups separated by Ba2+ cations delimiting halide-filled channels, their crystal structure is built from mixed anion thiovanadate VO4-αSα, where V5+ is surrounded by both O and S, therefore exhibiting a triple anion lattice. Here, the strategy consisting in incorporating a chalcogenide anion aims at raising the valence band to bring the band gap to the visible range in order to reach photoactive materials under visible light. Both the halide anion nature and the S/O ratio impact the materials' photoconductivity. While the photocurrent response is comparable to that found in the recently investigated apatite phase Pb5(VO4)3I, a short carrier lifetime is detected as well as a shift of the activity toward the visible light. This apatite series combining thiovanadate and halide-filled channels opens new perspectives in the extended field of apatites and their applications.

Versatile Interplay of Chalcogenide and Dichalcogenide Anions in the Thiovanadate Ba7S(VS3O)2(S2)3 and Its Selenide Derivatives: Elaboration and DFT Meta-GGA Study

Oxychalcogenides are emerging as promising alternative candidates for a variety of applications including for energy. Only few phases among them show the presence of Q-Q bonds (Q = chalcogenide anion) while they drastically alter the electronic structure and allow further structural flexibility. Four original oxy(poly)chalcogenide compounds in the system Ba-V-Q-O (Q = S, Se) were synthesized, characterized, and studied using density functional theory (DFT). The new structure type found for Ba₇V₂O₂S₁₃, which can be written as Ba₇S(VS₃O)₂(S₂)₃, was substituted to yield three selenide derivatives Ba₇V₂O₂S_9.304Se_3.696, Ba₇V₂O₂S_7.15Se_5.85, and Ba₇V₂O₂S_6.85Se_6.15. They represent original multiple-anion lattices and first members in the system Ba-V-Se-S-O. They exhibit in the first layer heteroleptic tetrahedra V⁵⁺S₃O and isolated Q^2- anions and in the second layer dichalcogenide pairs (Q₂)^2- with Q = S or Se. Selenide derivatives were attempted by targeting the selective substitution of isolated Q^2- or (Q₂)^2- (in distinct layers) or both by selenide, but it systematically led to concomitant and partial substitution of both sites. A DFT meta-GGA study showed that selective substitution yields local constraints due to rigid VO₃S and pairs. Experimentally, incorporation of selenide in both layers avoids geometrical mismatch and constraints. In such systems, we show that the interplay between the O/S anionic ratio around V⁵⁺, together with the presence/nature of the dichalcogenides (Q₂)^2- and isolated Q^2-, impacts in unique manners the band gap and provides a rich background to tune the band gap and the symmetry.

An unusual O2-/F- distribution in the new pyrochlore oxyfluorides: Na2B2O5F2 (B = Nb, Ta)

Two new oxyfluorides with a pyrochlore-type structure, Na₂Nb₂O₅F₂ and Na₂Ta₂O₅F₂, have been obtained, for which the XRD crystallographic study coupled with ¹⁹F solid state NMR reveals an unusual O/F distribution. Both materials are n-type semiconductors exhibiting photoconductive properties.

Sulfide-Based Photocatalysts Using Visible Light, with Special Focus on In2S3, SnS2 and ZnIn2S4

Sulfides are frequently used as photocatalysts, since they absorb visible light better than many oxides. They have the disadvantage of being more easily photocorroded. This occurs mostly in oxidizing conditions; therefore, they are commonly used instead in reduction processes, such as CO2 reduction to fuels or H2 production. Here a summary will be presented of a number of sulfides used in several photocatalytic processes; where appropriate, some recent reviews will be presented of their behaviour. Results obtained in recent years by our group using some octahedral sulfides will be shown, showing how to determine their wavelength-dependent photocatalytic activities, checking their mechanisms in some cases, and verifying how they can be modified to extend their wavelength range of activity. It will be shown here as well how using photocatalytic or photoelectrochemical setups, by combining some enzymes with these sulfides, allows achieving the photo-splitting of water into H2 and O2, thus constituting a scheme of artificial photosynthesis.

A high dimensional oxysulfide built from large iron-based clusters with partial charge-ordering

Herein we report the original Ba₁₀Fe_7.75Zn_5.25S₁₈Si₃O₁₂ oxysulfide which crystallizes in a new structural type. Contrary to the usual oxychalcogenides, it crystallizes with a non-centrosymmetric 3D spatial network structure built from large magnetic clusters consisting of twelve (Fe^2+/3+/Zn)S₃O tetrahedra decorating a central Fe²⁺S₆ octahedron and exhibiting a spin glass state.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi₂TiO₄F₂ depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

Aurivillius oxyfluoride Bi₂TiO₄F₂ shows a promising photocatalytic behavior, which has been related to its polar structure, but polar properties have been difficult to reproduce in bulk samples. We explore a possible anion order and how this could give polar structures when combined with other distortions. We highlight the influence of strain on anion order and therefore the properties in these systems.

Oxysulfide Ba5(VO2S2)2(S2)2 Combining Disulfide Channels and Mixed-Anion Tetrahedra and Its Third-Harmonic-Generation Properties

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. In particular, heteroleptic environments around transition metals allow tuning of the polarity or band-gap engineering for instance. We present the original oxysulfide Ba₅(VO₂S₂)₂(S₂)₂, the fifth member in the quaternary system Ba-V-S-O. It exhibits the mixed-anion building units V⁵⁺O₂S₂ and isolated disulfide pairs (S₂)^2-. The structure is solved by combining single-crystal and powder X-ray diffraction and transmission electron microscopy. First-principles calculations were combined in order to highlight the anion roles. In particular, our density functional theory study shows that the 3p states of the disulfide pairs dictate the band gap. In this study, we point out anionic tools for band-gap engineering that can be useful for the design of phases for numerous applications. Finally, third harmonic generation (THG) was measured and compared to the large THG observed for Cu₂O, which reveals the potential for nonlinear-optical properties that should be further investigated.