The unique physicochemical properties of chalcogenides containing post-transition metals are often governed by the stereochemical activity of their ns2 lone pairs. A comparative structural analysis of bournonite (CuPbSbS3) and aikinite (CuPbBiS3) reveals that CuPbSbS3 has a larger unit cell volume (V = 552.58 & Aring;3) than CuPbBiS3 (V = 534.42 & Aring;3), despite antimony (Sb3+) being smaller than bismuth (Bi3+). This counterintuitive trend originates from the stronger lone pair effect of Sb3+, which induces a larger static distortion, leading to a tetrahedral SbS3L (l-lone pair) coordination, while Bi3+ exhibits a less asymmetric distortion corresponding to the more compact square-pyramidal BiS5 (or octahedral BiS5L) coordination. These structural differences are corroborated by temperature-dependent Raman spectroscopy, which captures distinct vibrational behaviors reflective of differing lattice dynamics. CuPbSbS3 exhibits higher anharmonicity, as quantified by anharmonicity coefficients extracted from Raman data. Electron localization function analysis and crystal orbital Hamilton population further confirm the higher activity of the Sb3+ lone pair compared to Bi3+. Interestingly, despite the lower atomic mass of Sb relative to Bi, both compounds exhibit nearly identical acoustic-phonon cutoff frequencies. Consequently, both CuPbSbS3 and CuPbBiS3 display partially suppressed Umklapp scattering and comparable low lattice thermal conductivities at high temperatures, despite their compositional and structural differences.

(Text in French) Semiconductors for photovoltaic and photoelectrochemical cells. The development of stable and efficient photoelectrochemical and photovoltaic cells benefits from the creation of new photoactive layers based on semiconductor materials and a detailed understanding of their physicochemical properties. This article aims to highlight the research conducted at ISCR in this field and related collaborative efforts. Topics covered will include photovoltaic solar cells based on halogenated perovskites as well as antimony triselenide, the design of photoelectrodes (photoanodes and photocathodes) through the assembly of inorganic molecular nanoclusters, and photoelectrodes based on silicon or III-V semiconductors epitaxially grown on silicon for water photoelectrolysis.

Identifying systems composed of earth-abundant elements capable of photoinduced electron-transfer represents a major breakthrough for the development of sustainable photocatalytic processes. Herein, we demonstrate that octahedral tungsten-halide clusters constitute a promising new class of robust photosensitizers, owing to their exceptional chemical stability, intense luminescence, and long-lived triplet excited states. Upon irradiation in aqueous solution, these species transfer an electron to polyoxometalates (POMs), well-known molecular catalysts capable of multielectron redox chemistry. More specifically, we investigate the photophysical properties of supramolecular systems combining the cluster-based motif [{W6Ii8}Cla6]2- and a series of archetypal POMs ([PW12O40]3-, [SiW12O40]4-, [BW12O40]5-, [P2W18O62]6-) as electron-acceptor units, in the presence of γ-cyclodextrin (γ-CD), which acts as both a supramolecular assembly scaffold and a stabilizing agent. The photoinduced electron-transfer process was investigated using steady-state and time-resolved optical spectroscopy, complemented by spectro-electrochemical measurements, which unambiguously confirm electron transfer from the excited cluster to the POMs. Furthermore, we demonstrate that both the ionic charge of the POMs and the presence of γ-CD significantly influence the efficiency of the process. This work highlights the dual role of γ-CD, which facilitates the association of two negatively charged inorganic units and stabilizes the charge-separated state by maintaining an appropriate spatial arrangement within the supramolecular assembly.

Lead halide perovskite solar cells, despite their outstanding optoelectronic properties, suffer from poor long-term stability under environmental stressors, limiting their industrial scalability. To address this, we explored photoelectrodes composed of...

Single-phase lamellar CIGS5 (Cu0.65In1.75Ga1.4S5, nominal composition) thin films were synthesized using co-evaporation. Structural, morphological and optoelectronic properties of these films have been investigated. The optical bandgap was calculated at approximately 2.1 eV, but extended band tails hinder an accurate determination. Such behavior may be related to the high density of point defects inherent in the crystal structure of CIGS5, which exhibits intrinsic cationic vacancies and mixed site occupancy. This results in compensated semiconducting behavior, with no clearly identified charge carriers. Nevertheless, photoelectrochemical measurements suggest an extremely weak excess of electrons. Diverse solar cell architectures with CIGS5 as absorber were fabricated. All exhibit very low efficiency. These poor performances are discussed and are attributed to the intrinsic optoelectronic properties of CIGS5 material in ambient conditions due to a charge compensating effect.

Copper-rich sulfides are promising materials for energy-conversion applications due to their environmental compatibility, cost-effectiveness, and earth abundance. Their structural diversity and chemical flexibility offer a fantastic playground for the discovery of new structures and compounds with various properties. We demonstrate that the presence of triangular copper in the ordered CdI2-type Zr2/3Cu1/3S2 layers of Cu2ZrS3 induces a high potential for polytypism in this sulfide. Two new layered polytypes, P31c(ABCB) and R3(ABC), are synthesized. Based on a comprehensive analysis by single-crystal X-ray diffraction, 3D ED and HRSTEM analyses, we highlight that these polytypes differ in their stacking sequences while sharing identical tetrahedral copper layers and ordered Zr2/3Cu1/3S2 layers. Both polytypes are characterized by large anisotropic vibrations of copper in triangular coordination. Low-temperature heat capacity and Raman spectroscopy, supported by phonon calculations, identify low-energy optical phonons associated with Cu atoms, which couple strongly to acoustic modes, yielding non-Debye behavior and extremely short phonon lifetimes. The R3 polytype displays intrinsically ultralow lattice thermal conductivity, reaching 0.35 W m-1 K-1 at 673 K, among the lowest reported in sulfides. Our findings provide new insights into the complex crystal chemistry and polytypism of copper-rich sulfides in relation to vibrational properties, opening new perspectives for the exploration of these materials.

Photoelectrodes based on transition metal clusters (MCs) are critical for advancing solar energy conversion, where precise control over film morphology and electronic properties is essential. While electrophoretic deposition (EPD) has been widely used for the design of MCs-based photoelectrodes, its intrinsic limitations-such as poor film homogeneity, crack formation, and limited microstructural control-compromise device performance. In this study, we demonstrate that electrodeposition (ED) of [{Mo6Ii 8}Ia 6]2- clusters produces homogeneous, crack-free films with the composition [{Mo6Ii 8}Ia 4(H2O)a 2}]& centerdot;xH2O. The ED process, driven by a two-step oxidation mechanism (1.1 and 1.6 V vs. Ag/AgCl), enables unprecedented control over film thickness (60-600 nm) and microstructure, resulting in increased photoconductivity (photocurrent 2-3 times higher and lower charge transfer resistance, R tc similar to 500 Omega). The films exhibit ambipolar conductivity and a quasi-pinned mid-gap Fermi level, confirming their potential for high-performance solar energy applications. Compared to EPD-derived films, ED films show enhanced charge extraction efficiency and minimized recombination losses, which are attributed to their defect-free microstructure and stable electronic properties. This work establishes ED as a superior method for fabricating high-quality MC-based photoelectrodes, addressing key limitations of traditional deposition techniques.

Metal sulfides have recently drawn significant interest from the scientific community due to their nontoxicity and abundance, making them suitable for a wide range of applications, including thermoelectric and optoelectronic technologies. Although numerous ternary metal sulfides have been reported in the literature, their crystal structures and physical properties remain largely unexplored. In the present work, we have synthesized bulk polycrystalline samples of AGeS3 (A = Pb/Sn) using mechanical alloying followed by spark plasma sintering and studied their crystal structures, microstructures, and thermal and vibrational properties in relation to computational modeling. The low lattice thermal conductivity in this class of compounds is mainly attributed to the weak interlayer bonding (2D character) due to the stereochemical activity of the lone pairs of Sn2+ in SnGeS3 and Pb2+ in PbGeS3. Importantly, we examine the nature of the chemical bonds in AGeS3 and elucidate the origin of distinct thermal conductivities in these two compounds despite having similar crystal structures. We show that the enhanced stereochemical activity of Sn2+ in SnGeS3, compared to Pb2+ in PbGeS3, leads to a more distinct two-dimensional character. This is demonstrated by stronger intralayer bonding, weaker interlayer interactions, and more prominent interlayer Sn-S antibonding states near the Fermi level. Anisotropic grain growth, observed in our TEM data, further supports this interpretation. Consequently, glass-like lattice thermal conductivity is observed in SnGeS3 while PbGeS3 exhibits crystalline-like thermal conductivity. These findings enrich the fundamental knowledge of crystal chemistry and thermal conduction relationships in metal sulfides and encourage further investigations into the design of materials for thermal management applications.

The use of photonic crystals (PCs) is gaining interest regarding the interaction between light and organized structures. However, there are limitations to their use, such as production cost, rate, and substrate size. This study demonstrates an efficient and cost-effective fabrication of an opal structure based on the electrophoretic deposition (EPD) of self-made silica particles. The process allows one to reduce the fabrication time compared to classical evaporation or sedimentation methods, which can take days (evaporation) or weeks to months (sedimentation), while this method takes only 2 min. Silica particles with a diameter of less than 300 nm exhibit a markedly low sedimentation rate. Nevertheless, when they form a close-packed structure, they display a partial band gap in the visible region. The resulting opals are iridescent, from blue to red, and display additional angle-dependent colors. Furthermore, the synthesis of silica particles was completed within a 2 h time frame. Their washing, including a centrifugation step, did not result in the formation of aggregates. Such particles tend to break the long-range order in the PCs.

Sphalerite-derivative copper-based sulfides have emerged in the past decade as serious candidates for cost-efficient and environmentally friendly thermoelectric applications. In order to investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties in Cu5Sn2S7, a recently discovered degenerate semiconductor with an ordered monoclinic structure, we have synthesized a series of Zn-substituted Cu5-xZnxSn2S7 sphalerite derivatives. For 0 <= x <= 0.4, a monoclinic sphalerite derivative phase is mainly formed, while for 0.4 < x <= 1, two sphalerite derivatives with monoclinic and tetragonal symmetries coexist. By substituting Zn for Cu in Cu5-xZnxSn2S7, we observe a substantial increase of the thermoelectric figure of merit zT up to 0.36 at 673 K in the biphasic region around x = 0.8. This improvement mainly arises from the strong reduction of the lattice thermal conductivity, itself driven by the coexistence of two phases with distinct sphalerite S subnetworks and enhanced cationic disorder in the tetragonal phase with respect to the monoclinic phase. The role of percolation between the grains in the electrical conductivity in the biphasic samples is also considered. The relationships between this complex crystal chemistry and the electrical and thermal transport properties are addressed in detail by using a combination of synchrotron X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy, first-principles calculations, and transport property measurements.

Metal chalcogenide semiconductors are being widely investigated for applications in solar energy conversion, such as photovoltaics and visible light photocatalysis. Herein, an initial assessment of potentialities of new lamellar chalcogenides named CIGSn is provided, while comparing them with that of the well-known CIGS chalcopyrite. The main difference between CIGS and CIGSn compounds concerns their electronic properties and more precisely the nature of charge carriers. Cu0.32In1.74Ga0.84S4 (CIGS4) is an n-type semiconductor, unlike the chalcopyrite CuIn0.7Ga0.3S2 (CIGS) that is a p-type semiconductor. More noticeable, Cu1.44In2.77Ga0.76S6 (CIGS6) and in a lesser extent Cu0.65In1.75Ga1.4S5 (CIGS5), exhibit an ambipolar character with a slight predominance of electron transport. The Fermi levels of all lamellar CIGSn compounds are similar (-4.5 eV) and higher that of the chalcopyrite CIGS (-5.1 eV). In addition, the charge carrier densities of CIGSn compounds (1014 - 1017 cm-3) are significantly lower than that of CIGS (1020 cm-3), which is consistent with their higher resistivity. Photoluminescence measurements and OCP decays suggest much more in-gap defect states in the lamellar compounds. These results suggest that CIGSn compounds would not be suitable for photovoltaic applications. Nevertheless, their energy bands show an interesting positioning, with respect to redox potentials involved in water splitting and CO2 reduction. In addition, ambipolarity could enhance the efficiency of catalytic reactions, because a type of minority charge carriers does not limit the charge transport.

Metal atom clusters are well-defined nanoscale objects containing a precise number of metal atoms and ligands. Face-capped cluster units of the type [{M6L8i}L '(a)(6)] (M = Mo, Re, L = S, Se or I, L ' = Cl, Br, I, CN or H2O) exhibit unique optical and electronic properties that make them relevant building blocks for the rational design of nanomaterials using nanoarchitectonic concepts. Photoelectrodes based on Mo-6 and Re-6 clusters with various compositions obtained by deposition of uniform layers of those building blocks onto semiconducting surfaces were recently reported. Remarkably, high quality interfaces were formed not only between building blocks but also between the building blocks and the semiconducting surfaces. On the one hand, layers based on active {Mo6I8i} cluster cores exhibit an ambipolar behavior like carbon nanotube, graphene and transition metal chalcogenides. On the other hand, mixing the two types of {Re6S8i} and {Re6Se8i}-based building blocks enables the creation of micro-(p-n) junctions with enhanced photogenerated current intensity. Herein, we report new advances in the design of photoelectrodes using heterometallic Re4Mo2 cluster-based building blocks. The association of Mo and Re in {Re(4)Mo(2)Q(8)(i)} cluster cores (Q = S and Se) leads to electronic properties and absorption properties significantly different from those of homometallic {Mo6I8i} and {Re(6)Q(8)(i)}. Indeed, beyond different molecular orbital diagrams, the {Re(4)Mo(2)Q(8)(i)} cluster-based units exhibit 22 valence electrons per cluster (VEC) whereas the VEC value for {Mo6I8i} and {Re(6)Q(8)(i)} cluster units is 24. The mixing of rhenium and molybdenum within the same heterometallic cluster enables not only the optical and transport properties of the active layers to be optimized but it also enables the position of the energy levels to be tuned. This appears very appealing for band alignment engineering in order to design optimized photoelectrodes for solar energy conversion. We show herein that the energy levels of the photoelectrodes built on {Re(4)Mo(2)Q(8)} cluster-based layers immobilized on FTO surfaces are compatible with the photoelectrochemical water splitting.

Mixed-anion compounds, which incorporate multiple types of anions into materials, displays tailored crystal structures and physical/chemical properties, garnering immense interests in various applications such as batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed studies regarding correlations between crystal structure, chemical bonding, and thermal/vibrational properties are rare for these compounds, which limits the exploration of mixed-anion compounds for associated thermal applications. In this work, we investigate the lattice dynamics and thermal transport properties of the metal chalcohalides, CuBiSCl2. A high-purity polycrystalline CuBiSCl2 sample, successfully synthesized via modified solid-state synthetic method, exhibits a low lattice thermal conductivity of 0.9-0.6 W m-1 K-1 from 300 to 573 K. By combining various experimental techniques including 3D electron diffraction with theoretical calculations, we elucidate the origin of low lattice thermal conductivity in CuBiSCl2. The stereo-chemical activity of the 6s2 lone pair of Bi3+ favors an asymmetric environment with neighboring anions involving both short and long bond lengths. This particularity often implies weak bonding, low structure dimensionality, and strong anharmonicity, leading to low lattice thermal conductivity. In addition, the strong two-fold linear S-Cu-S coordination with weak Cu – Cl interactions induces large anisotropic vibration of Cu or structural disorder, which enables strong phonon-phonon scattering and decreases lattice thermal conductivity. The investigations into lattice dynamics and thermal transport properties of CuBiSCl2 broadens the scope of the existing mixed-anion compounds suitable for the associated thermal applications, offering a new avenue for the search of low thermal conductivity materials in low-cost mixed-anion compounds.

Mixed-anion compounds, which incorporate multiple types of anions into materials, display tailored crystal structures and physical/chemical properties, garnering immense interest in various applications such as batteries, catalysis, photovoltaics, and thermoelectrics. However, detailed studies regarding correlations among crystal structure, chemical bonding, and thermal/vibrational properties are rare for these compounds, which limits the exploration of mixed-anion compounds for associated thermal applications. In this work, we investigate the lattice dynamics and thermal transport properties of the metal chalcohalide, CuBiSCl2. A high-purity polycrystalline CuBiSCl2 sample exhibits a low lattice thermal conductivity (κL) of 0.9-0.6 W/(m·K) from 300 to 573 K. By combining various experimental techniques, including three-dimensional (3D) electron diffraction, with theoretical calculations, we elucidate the origin of low κL in CuBiSCl2. The stereochemical activity of the 6s2 lone pair of Bi3+ favors an asymmetric environment with neighboring anions involving both short and long bond lengths. This particularity often implies weak bonding, low structure dimensionality, and strong anharmonicity, leading to a low κL. In addition, the strong 2-fold linear S-Cu-S coordination with weak Cu···Cl interactions induces a large anisotropic vibration of Cu, which enables strong phonon-phonon scattering and decreases κL. The investigations into lattice dynamics and thermal transport properties of CuBiSCl2 broaden the scope of the existing mixed-anion compounds suitable for the associated thermal applications, offering a new avenue for the search for low thermal conductivity materials in low-cost mixed-anion compounds.

Recently, metal sulfides have begun to receive attention as potential cost-effective materials for thermoelectric applications beyond optoelectronic and photovoltaic devices. Herein, based on a comparative analysis of the structural and transport properties of 2D PbSnS2 and 1D PbSnS3, we demonstrate that the intrinsic effects that govern the low lattice thermal conductivity (κL) of these sulfides originate from the combination of the low dimensionality of their crystal structures with the stereochemical activity of the lone-pair electrons of cations. The presence of weak bonds in these materials, responsible for phonon scattering, results in inherently low κL of 1.0 W/m K in 1D PbSnS3 and 0.6 W/m K in 2D PbSnS2 at room temperature. However, the nature of the thermal transport is quite distinct. 1D PbSnS3 exhibits a higher thermal conductivity with a crystalline-like peak at low temperatures, while 2D PbSnS2 demonstrates glassy thermal conductivity in the entire temperature range investigated. First-principles density functional theory calculations reveal that the presence of antibonding states below the Fermi level, especially in PbSnS2, contributes to the very low κL. In addition, the calculated phonon dispersions exhibit very soft acoustic phonon branches that give rise to soft lattices and very low speeds of sounds.

Copper-rich sulfides are very promising for energy conversion applications due to their environmental compatibility, cost effectiveness, and earth abundance. Based on a comparative analysis of the structural and transport properties of Cu3BiS3 with those of tetrahedrite (Cu12Sb4S13) and other Cu-rich sulfides, we highlight the role of the cationic coordination types and networks on the electrical and thermal properties. By precession-assisted 3D electron diffraction analysis, we find very high anisotropic thermal vibration of copper attributed to its 3-fold coordination, with an anisotropic atomic displacement parameter up to 0.09 Å2. Density functional theory calculations reveal that these Cu atoms are weakly bonded and give rise to low-energy Einstein-like vibrational modes that strongly scatter heat-carrying acoustic phonons, leading to ultralow thermal conductivity. Importantly, we demonstrate that the 3-fold coordination of copper in Cu3BiS3 and in other copper-rich sulfides constituted of interconnected CuS3 networks causes a hole blockade. This phenomenon hinders the possibility of optimizing the carrier concentration and electronic properties through mixed valency Cu+/Cu2+, differently from tetrahedrite and most other copper-rich chalcogenides, where the main interconnected Cu-S network is built of CuS4 tetrahedra. The comparison with various copper-rich sulfides demonstrates that seeking for frameworks characterized by the coexistence of tetrahedral and 3-fold coordinated copper is very attractive for the discovery of efficient thermoelectric copper-rich sulfides. Considering that lattice vibrations and carrier concentration are key factors for engineering transport phenomena (electronic, phonon, ionic, etc.) in copper-rich chalcogenides for various types of applications, our findings improve the guidelines for the design of materials enabling sustainable energy solutions with wide-ranging applications.

The mixing of rhenium and molybdenum within the same heterometallic cluster enables to modulate optoelectronic properties of the photo-active layers. Such {Re 4 Mo 2 Q 8 }-based photoelectrodes appear promising for the photoelectrochemical water splitting.

The mixing of rhenium and molybdenum within the same heterometallic cluster enables to modulate optoelectronic properties of the photo-active layers. Such {Re4Mo2Q8}-based photoelectrodes appear promising for the photoelectrochemical water splitting.

A series of new cluster compounds with {Re4Mo2S8} and {Re3Mo3S8} cores has been obtained and investigated. The clusters with different Re/Mo ratios were isolated as individual compounds, which made it possible to study their spectroscopic and electrochemical properties. The geometry of the new clusters was studied using a combination of X-ray diffraction analysis, XAS and quantum chemical DFT calculations. It was shown that the properties of the new clusters, such as the number and position of electrochemical transitions, electronic structure and change in geometry with a change in charge, are similar to the properties of clusters based on the {Re4Mo2Se8} and {Re3Mo3Se8} cores described earlier.