Prospective Chalcohalide Perovskites: Pursuing (and Failing) the Synthesis of CsBiSCl2 Nanocrystals

Heavy pnictogen chalcohalides are often termed lead-free, perovskite-inspired materials. Despite theoretical predictions, incontrovertible experimental demonstrations of heavy pnictogen chalcohalides adopting a perovskite structure are lacking. Here we report our attempts to prepare CsBiSCl2 adopting a perovskite structure as colloidal nanocrystals. Synthesis of nanoscale materials can indeed rely on fast, nonequilibrium reactions and on large, eventually thermodynamically favorable surface energies, leading to the possibility of stabilizing kinetically trapped or metastable phases. However, we obtained no CsBiSCl2, but a mixture of nanocrystals of secondary phases, namely Cs3BiCl6 submicrometric polyhedra, Bi2S3 nanoscopic rods, and Cs3Bi2Cl9 nanoscopic dots, whose low polydispersity enabled an effective separation via size/shape selective precipitation. This work confirms that heavy pnictogen chalcohalides are hardly prone to adopting a perovskite structure. Nevertheless, chemistry at the nanoscale offers multiple possibilities for overcoming phase segregation and pursuing the synthesis of prospective mixed anion compound semiconductors.

van der Waals Stacking-Determined Polymorphs of Quasi-One-Dimensional BiSCl Grown by Chemical Vapor Deposition

We report chemical vapor deposition (CVD) synthesis of two quasi-one-dimensional (quasi-1D) polymorphs of BiSCl, denoted by y-BiSCl and r-BiSCl. The length of the CVD samples can reach about 0.4 mm. Such quasi-1D samples of the two polymorphs can be readily separated into individual pieces for either characterization or application. The two polymorphs can be clearly differentiated by Raman spectroscopy. First-principles calculations and group analysis are used to assign each Raman peak to the corresponding vibrational mode. Ultraviolet-visible measurements on solution grown thin-film samples reveal that the two polymorphs exhibit significantly different band gaps of 2.08 eV (y-BiSCl) and 1.81 eV (r-BiSCl). First-principles calculation further shows that the interatomic chain binding energy is 18.1 meV/Å2, confirming that the van der Waals stacking determines the difference in their band gaps. Our findings highlight the possibility of realizing the desired functionalities in quasi-1D materials by controlling stacking orientation.

Ultrathin, High-Aspect-Ratio Bismuth Sulfohalide Nanowire Bundles for Solution-Processed Flexible Photodetectors

In this study, a novel synthesis of ultrathin, highly uniform colloidal bismuth sulfohalide (BiSX where X = Cl, Br, I) nanowires (NWs) and NW bundles (NBs) for room-temperature and solution-processed flexible photodetectors are presented. High-aspect-ratio bismuth sulfobromide (BiSBr) NWs are synthesized via a heat-up method using bismuth bromide and elemental S as precursors and 1-dodecanethiol as a solvent. Bundling of the BiSBr NWs occurs upon the addition of 1-octadecene as a co-solvent. The morphologies of the BiSBr NBs are easily tailored from sheaf-like structures to spherulite nanostructures by changing the solvent ratio. The optical bandgaps are modulated from 1.91 (BiSCl) and 1.88 eV (BiSBr) to 1.53 eV (BiSI) by changing the halide compositions. The optical bandgap of the ultrathin BiSBr NWs and NBs exhibits blueshift, whose origin is investigated through density functional theory-based first-principles calculations. Visible-light photodetectors are fabricated using BiSBr NWs and NBs via solution-based deposition followed by solid-state ligand exchanges. High photo-responsivities and external quantum efficiencies (EQE) are obtained for BiSBr NW and NB films even under strain, which offer a unique opportunity for the application of the novel BiSX NWs and NBs in flexible and environmentally friendly optoelectronic devices.

Review on synthetic approaches and PEC activity performance of bismuth binary and mixed-anion compounds for potential applications in marine engineering

Photoelectrochemical (PEC) technology in marine engineering holds significant importance due to its potential to address various challenges in the marine environment. Currently, PEC-type applications in marine engineering offer numerous benefits, including sustainable energy generation, water desalination and treatment, photodetection, and communication. Finding novel efficient photoresponse semiconductors is of great significance for the development of PEC-type techniques in the marine space. Bismuth-based semiconductor materials possess suitable and tunable bandgap structures, high carrier mobility, low toxicity, and strong oxidation capacity, which gives them great potential for PEC-type applications in marine engineering. In this paper, the structure and properties of bismuth binary and mixed-anion semiconductors have been reviewed. Meanwhile, the recent progress and synthetic approaches were discussed from the point of view of the application prospects. Finally, the issues and challenges of bismuth binary and mixed-anion semiconductors in PEC-type photodetection and hydrogen generation are analyzed. Thus, this perspective will not only stimulate the further investigation and application of bismuth binary and mixed-anion semiconductors in marine engineering but also help related practitioners understand the recent progress and potential applications of bismuth binary and mixed-anion compounds.

Growth of BiSBr Microsheet Arrays for Enhanced Photovoltaics Performance

In this study, single-crystalline BiSBr is synthesized using a solution-based approach and conducted a systematic characterization of its photoelectric properties and photovoltaic performances. UV photoelectron spectroscopy and density functional theory (DFT) calculations reveal that BiSBr is an indirect p-type semiconductor, characterized by distinct positions and compositions of the valence band maximum and conduction band minimum. The BiSBr single crystal microrod features a significant electrical conductivity of 14 800 S m-1 along the c-axis, denoting minimal carrier resistance in this direction. For photovoltaic performance assessment, the authors successfully fabricated two homogeneous BiSBr films on TiO2 porous substrates: A microsheet array film via physical vapor deposition (PVD) and solvothermal treatment, and a BiSBr microsheet film via PVD and thermal treatment. The solar cell, comprising a BiSBr microsheet array film with an architecture of fluorine-doped tin oxide FTO/TiO2/BiSBr/(I3 -/I-)/Pt, demonstrated a power conversation efficiency of 1.40%, ≈11 times that of BiSBr microsheet film counterpart. These preliminary results underscore the potential of BiSBr microsheet arrays, producible through low-cost solution processes, as adept light absorbers, enhancing photovoltaic efficiency through effective light scattering and promoting efficient electron-hole separation and transport.

Potential application of bismuth oxyiodide (BiOI) when it meets light

Bismuth oxyiodide (BiOI) is a kind of typical two-dimensional (2D) material that has been increasingly developed alongside other 2D materials such as graphene, MXenes, and transition-metal dichalcogenide. However, its potential applications have not been widely whispered compared to those of other 2D materials. Using its distinctive properties, BiOI can be used for various applications, especially when it meets sunlight and other light-related electromagnetic waves. In this present review, we discuss the developments of BiOI and challenges in the applications for photodetector and light-assisted sensors, photovoltaic devices, optoelectronic logic devices, as well as photocatalysts. We start the discussion with a basic understanding and development of BiOI, crystal structure, and its properties. The synthesis and further development, such as green synthesis and its challenges in the synthesis-suited industry, as well as device integration, are also explained together with a plausible strategy to enhance the feasibility of BiOI for those various applications. We believe that the provided discussion and perspectives will not only promote BiOI to be one of the highly considered 2D materials but can also assist recent graduates in any materials science discipline and inform the senior scientists and industrial-based stakeholders of the latest advances in bismuth oxide and mixed-anion compounds.

Mist Chemical Vapor Deposition of Bi13S18I2 for Photoelectrochemical-type Photodetection

Bismuth-based ternary compounds have attracted much attention owing to their various merits, such as low toxicity and tunable electrical and optical properties. However, these compounds are yet to be understood due to the lack of suitable targets limited by immature synthesis techniques. In this work, we aimed at the synthesis, properties investigation, and photodetection application of Bi13S18I2. Mist chemical vapor deposition was adopted for the deposition of the Bi13S18I2 thin film for the first time. The deposition mechanism was discussed from the perspective of crystal phase and surface morphology. Based on the Bi13S18I2 thin film synthesized at optimal temperature, we constructed a photoelectrochemical-type photodetector. The photodetection performance was evaluated from the points of electrolyte composition, working temperature, and bias voltage. This study would pave the way for the controllable synthesis and applications of bismuth-based ternary compounds.

[Ba4X][In19S32] (X = Cl, Br): two quaternary metal chalcohalides exhibiting remarkable photocurrent responses

Inorganic metal chalcohalides, as significant semiconductor materials, have emerged as promising candidates for photoelectric applications. Herein, a new type of quaternary chalcohalide, [Ba4X][In19S32] (X = Cl, Br), has been discovered using the high-temperature halide salt flux method. Single-crystal X-ray diffraction analysis reveals that they are isostructural and crystallize in the tetragonal space group I41/amd (no. 141) featuring the octahedral hole formed by six [InS4]5- tetrahedra filled with a [ClBa4]7+ polycation, surrounded by a three-dimensional covalent framework formed by interconnecting [InS6]9- octahedra through corner-sharing and edge-sharing. Moreover, [Ba4Cl][In19S32] and [Ba4Br][In19S32] exhibit wide optical bandgaps of 2.70 eV and 2.46 eV, respectively, and moderate birefringences (0.044 @ 2100 nm and 0.042 @ 2100 nm, respectively). Specifically, [Ba4X][In19S32] (X = Cl, Br) display remarkable photocurrent responses under simulated solar-light illumination, implying their potential for photocatalytic applications. Theoretical calculations were employed to understand the interrelationship between the optical properties and electronic structure. The study on the synthesis and structure-property relationship analysis of inorganic metal chalcohalides provides new insight into the exploration of promising photoelectric materials.

Influence of crystal structure and composition on optical and electronic properties of pyridinium-based bismuth iodide complexes

This study investigates the impacts of structure and composition on the optical and electronic properties of a series of pyridinium-based bismuth iodide complexes. Organic substrates with various functional groups, such as 4-aminopyridine (4-Ampy), 4-methylpyridine (4-Mepy), 4-dimethylaminopyridine (4-Dmapy), and 4-pyridinecarbonitrile (4-CNpy) with different electron-donating and electron-withdrawing groups at the para position of the pyridine ring were employed. Crystallographic analysis reveals various bismuth iodide structures, including 1D chains and discrete 0D motifs. The optical band gap of these materials, identified via diffuse reflectance spectroscopy (DRS) and verified with density functional theory (DFT) calculations, is influenced by the crystal packing and stabilising interactions. Through a comprehensive analysis, including Hirshfeld surface (HS) and void assessment, the study underscores the influence of noncovalent intermolecular interactions on crystal packing. Spectroscopic evaluations provide insights into electronic interactions, elucidating the role of electron donor and acceptor substituents within the lattice. Thermogravimetric differential thermal analysis (TG-DTA) indicates structural stability up to 250 °C. Linear sweep voltammetry (LSV) reveals significant conductivity in the range of 10-20 mS per pixel at 298.15 K. X-ray absorption spectroscopy (XAS) at the Bi L3 edge indicates a similar oxidation state and electronic environment across all samples, underscoring the role of bismuth centres surrounded by iodides.

A study of the Rashba effect in two-dimensional ternary compounds ABC monolayers (A = Sb, Bi; B = Se, Te; C = Br; I)

The structure and electronic and spintronic properties of two-dimensional (2D) ternary compounds ABC (A = Sb, Bi; B = Se, Te; C = Br; I) monolayers are investigated using the first-principles method. The ABC monolayers possess typical Janus structures with a considerable potential gradient normal to the surface, inducing intrinsic Rashba spin splitting (RSS) at the conduction band minimum near the Γ point. Among them, the splitting strength of the BiSeI monolayer is the largest and its Rashba coefficient can reach 1.84 eV Å. The projected energy band of the BiSeI monolayer suggests that the RSS state is mainly rooted in the Bi-p_z orbital. The RSS strength can be modulated by applying the in-plane strain. The tensile strain can improve the RSS strength, which is ascribed to the increase of the potential gradient normal to the surface. These results indicate that these 2D ternary compounds have great potential for application in tunable spintronic devices.

Emerging Chalcohalide Materials for Energy Applications

Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as "chalcohalides". As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists. This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic-inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials. We then proceed to discuss their optoelectronic properties, band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.

Colloidal Bismuth Chalcohalide Nanocrystals

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition-dependent band gap spanning the visible spectral range and absorption coefficients exceeding 105  cm-1 . The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar-simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology.

Interfacial Polarization Phenomena in Compressed Nanowires of SbSI

The systematic studies of the extrinsic Maxwell–Wagner–Sillars polarization process in compressed antimony sulfoiodide (SbSI) nanowires are carried out by dielectric spectroscopy. The dielectric response is studied in temperature (100≤T≤350) K and frequency (10−3≤f≤106) Hz ranges. Dielectric functions commonly used for the analysis of dielectric spectra related to intrinsic polarization processes were applied in the elaboration of experimental data. It was found that the respective “semi-circles” in the Cole–Cole-type plots display a characteristic pear-like shape for the ferroelectric phase. On the other hand, the data for the paraelectric phase form symmetrical arcs. This response is effectively parametrized using the experimental Cole–Davidson and Cole–Cole functions fitted to the data obtained for the ferroelectric and paraelectric phases, respectively. It is deduced that the particular shape of spectra in the ferroelectric phase is due to spontaneous polarization, which is responsible for an asymmetric broadening of relaxation functions related to the interfacial polarization.

Controlled hydrothermal synthesis of BiOxCly/BiOmBrn/g-C3N4 composites exhibiting visible-light photocatalytic activity

The first systematic synthesis of bismuth oxychloride/bismuth oxybromide/graphitic carbon nitride (BiO_xCl_y/BiO_mBr_n/g-C₃N₄) nano-composites used a controlled hydrothermal method. The structure, morphology and characteristic of BiO_xCl_y/BiO_mBr_n/g-C₃N₄ photocatalyst were measured by XRD, UV-vis-DRS, FT-IR, FE-TEM, FE-SEM-EDS, PL, BET, HR-XPS and EPR. Under visible light irradiation, the photodegradation activity was evaluated for the decolorization of crystal violet (CV) and 2-hydroxybenzoic acid (2-HBA) in aqueous solution. The catalytic performance showed that, when using sample BB2C1-4-250-30 wt% g-C₃N₄ composite as a photocatalyst, the best reaction-rate-constant (k) was 0.071 h^-1. It was 1.5 times higher than the k value of BB2C1-4-250 as a photocatalyst. From the scavenging effect of various scavengers, the results of EPR showed that reactive OH was the main scavenger, while O₂^-, h⁺ and ¹O₂ were the second scavenger in CV degradation. In this study, a possible photodegradation mechanism was proposed and discussed. In this work, our method of BiO_xCl_y/BiO_mBr_n/g-C₃N₄ preparation could be used for future mass production and the BiO_xCl_y/BiO_mBr_n/g-C₃N₄ composite materials could be applied to the environmental pollution control in future.

Alternative Lone-Pair ns2 -Cation-Based Semiconductors beyond Lead Halide Perovskites for Optoelectronic Applications

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent-ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures

We report the colloidal synthesis of a series of surfactant-stabilized lead chalcohalide nanocrystals. Our work is mainly focused on Pb4S3Br2, a chalcohalide phase unknown to date that does not belong to the ambient-pressure PbS-PbBr2 phase diagram. The Pb4S3Br2 nanocrystals herein feature a remarkably narrow size distribution (with a size dispersion as low as 5%), a good size tunability (from 7 to ∼30 nm), an indirect bandgap, photoconductivity (responsivity = 4 ± 1 mA/W), and stability for months in air. A crystal structure is proposed for this new material by combining the information from 3D electron diffraction and electron tomography of a single nanocrystal, X-ray powder diffraction, and density functional theory calculations. Such a structure is closely related to that of the recently discovered high-pressure chalcohalide Pb4S3I2 phase, and indeed we were able to extend our synthesis scheme to Pb4S3I2 colloidal nanocrystals, whose structure matches the one that has been published for the bulk. Finally, we could also prepare nanocrystals of Pb3S2Cl2, which proved to be a structural analogue of the recently reported bulk Pb3Se2Br2 phase. It is remarkable that one high-pressure structure (for Pb4S3I2) and two metastable structures that had not yet been reported (for Pb4S3Br2 and Pb3S2Cl2) can be prepared on the nanoscale by wet-chemical approaches. This highlights the important role of colloidal chemistry in the discovery of new materials and motivates further exploration into metal chalcohalide nanocrystals.