Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Lead-Free Halide Perovskite Nanocrystals for Light-Emitting Diodes

Lead-based halide perovskite nanocrystals (PeNCs) have demonstrated remarkable potential for use in light-emitting diodes (LEDs). This is because of their high photoluminescence quantum yield, defect tolerance, tunable emission wavelength, color purity, and high device efficiency. However, the environmental toxicity of Pb has impeded their commercial viability owing to the restriction of hazardous substances directive. Therefore, Pb-free PeNCs have emerged as a promising solution for the development of eco-friendly LEDs. This review article presents a detailed analysis of the various compositions of Pb-free PeNCs, including tin-, bismuth-, antimony-, and copper-based perovskites and double perovskites, focusing on their stability, optoelectronic properties, and device performance in LEDs. Furthermore, we address the challenges encountered in using Pb-free PeNC-LEDs and discuss the prospects and potential of these Pb-free PeNCs as sustainable alternatives to lead-based PeLEDs. In this review, we aim to shed light on the current state of Pb-free PeNC LEDs and highlight their significance in driving the development of eco-friendly LED technologies.

Mixed valence Sn doped (CH3NH3)3Bi2Br9 produced by mechanochemical synthesis

Bismuth halides with formula A₃Bi₂X₉, where A is an inorganic or organic cation, show desirable properties as solar absorbers and luminescent materials. Control of structural and electronic dimensionality of these compounds is important to yield materials with good light absorption and charge transport. Here we report mechanochemical reaction of (CH₃NH₃)₃Bi₂Br₉ with SnBr₂ at room temperature in air, yielding a material with strong absorption across the visible region. We attribute this to mixed valence doping of Sn(II) and Sn(IV) on the Bi site. X-Ray diffraction shows no secondary phases, even after heating at 200 °C to improve crystallinity. X-Ray photoelectron spectroscopy suggests the presence of Sn(II) and Sn(IV) states. A similar approach to dope Sn into the iodide analogue (CH₃NH₃)₃Bi₂I₉ was unsuccessful.

Triple Perovskite Nd1.5Ba1.5CoFeMnO9−δ-Sm0.2Ce0.8O1.9 Composite as Cathodes for the Intermediate Temperature Solid Oxide Fuel Cells

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd1.5Ba1.5CoFeMnO9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (RP) of NBCFM is 1.1273 Ω cm2~0.1587 Ω cm2 in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10−6 K−1 at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce0.8Sm0.2O2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10−6 K−1 to 15.25 × 10−6 K−1. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10−4 to 10−3 cm s−1 over the po2 range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm2 at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm2). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.

Theoretical Prediction of Mixed-Valence Layered Halide Perovskites Cs4M(IV)M(II)2X12 (M = Ge, Sn; X = Cl, Br)

Charge-ordered compounds (i.e., Cu⁺/Cu²⁺, Au⁺/Au³⁺, In⁺/In³⁺, Tl⁺/Tl³⁺, Sb³⁺/Sb⁵⁺, and Bi³⁺/Bi⁵⁺) have been widely explored because of their unique physical properties. Here, a new class of ⟨111⟩-oriented mixed-valence layered halide perovskites Cs₄M(IV)M(II)₂X₁₂ (M = Ge, Sn; X = Cl, Br) with C2/m, R-3m, and I41/amd space groups was predicted by first-principles calculations. Based on the decomposition enthalpy, the phonon spectrum, and the mechanical stability criteria, we found that Cs₄GeGe₂Cl₁₂ (C2/m and R-3m), Cs₄GeGe₂Br₁₂ (R-3m), and Cs₄GeGe₂Br₆Cl₆ (R-3m) exhibit thermodynamic, dynamical, and mechanical stability. The electronic structure calculations show that the predicted band gap of stable Cs₄Ge(IV)Ge(II)₂X₁₂ varies from 1.16 to 2.25 eV. And an isolated intermediate conduction band contributed by the Ge(IV) 4s states below the Ge(II)/Ge(IV) 4p states is observed in these compounds, which is similar to previously reported Cs₄CuSb₂Cl₁₂ but different from Cs₄CdM(III)₂Cl₁₂ (M = Sb, Bi). In addition, the calculated static dielectric constant and optical absorption coefficient of Cs₄GeGe₂Br₁₂ are close to those of typical double perovskites (e.g., Cs₂AgBiBr₆). We believe that our work enriches the family of mixed-valence halide perovskites and provides a new platform for potential optoelectronic semiconductor design.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs₃Bi₂Br₉, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr₂ to the solution-phase synthesis of Cs₃Bi₂Br₉ leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, ¹³³Cs and ¹¹⁹Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.

Perovskite-inspired materials for photovoltaics and beyond-from design to devices

Lead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials in other optoelectronic applications, namely light-emitting diodes, photocatalysts, radiation detectors, thin film transistors and memristors. Finally, the prospects and key challenges faced by the field in advancing the development of perovskite-inspired materials towards realization in commercial devices is discussed.

Structure and Optical Properties of Layered Perovskite (MA)2PbI2-xBrx(SCN)2 (0 ≤ x < 1.6)

The layered perovskite (MA)₂PbI₂(SCN)₂ (MA = CH₃NH₃⁺) is a member of an emerging series of compounds derived from hybrid organic-inorganic perovskites. Here, we successfully synthesized (MA)₂PbI_2-xBr_x(SCN)₂ (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN^- anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)₂PbI₂(SCN)₂ tolerates a wide range of bromide substitution toward tuning the band gap energy.

2D layered all-inorganic halide perovskites: recent trends in their structure, synthesis and properties

Recently, halide perovskites have appeared as a superior class of materials for diverse applications, mainly in optoelectronics and photovoltaics. Perovskite halides are broadly classified as hybrid organic-inorganic and all-inorganic analogues depending on the chemical nature of the A cation in the ABX3-type structure. Immense progress has already been achieved in halide perovskites focusing mainly on the hybrid equivalents and all-inorganic three-dimensional (3D) structures, however all-inorganic two-dimensional (2D) layered halide perovskites are relatively new and their nanostructures have gained significant attention in the last few years. In this minireview, we presented a discussion on the recently developed all-inorganic 2D layered halide perovskites highlighting their crystal structure, synthetic methodologies, chemical transformations, and optical properties. We have demonstrated a significant number of examples of Pb-free 2D halide perovskite nanostructures. Strategies for the shape-controlled synthesis of nanostructures and their excitonic properties are discussed in detail. Thermal conductivity and thermoelectric properties are emphasized along with the magnetic properties of layered transition-metal based perovskites. We have also mentioned the recent examples of all-inorganic 2D halide perovskites as photocatalysts for solar-driven CO2 reduction. Finally, we have concluded the article with an outlook for the further progress in 2D all-inorganic halide perovskites toward the structural diversity and prospective new applications.

Self-Powered and Broadband Lead-Free Inorganic Perovskite Photodetector with High Stability

Metal halide perovskite materials have opened up a great opportunity for high-performance optoelectronic devices owing to their extraordinary optoelectronic properties. More than lead halide ones, stable and nontoxic bismuth halide perovskites exhibit more promise in their future commercialization. In this work, we developed for the first time photodetectors based on full-inorganic Cs₃Bi₂I_9-xBr_x perovskites and modulate their performance by varying x in the composition systematically. Among those self-powered photodetectors, those based on Cs₃Bi₂I₆Br₃ shows the best performance with excellent photosensitivity of 4.1 × 10⁴ at zero bias as well as the responsivity and detectivity reaching 15 mA/W and 4.6 × 10¹¹ Jones, respectively. More strikingly, the full-inorganic perovskite photodetectors exhibit excellent stability in the ambient environment and can maintain over 96% of the initial value after 100 days owing to the high stability of the core perovskite film. The paper definitely paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

Material Design and Optoelectronic Properties of Three-Dimensional Quadruple Perovskite Halides

The discovery of new halide perovskite-type structures could favor the exploration of optoelectronic materials, as in the case of double perovskites applied in solar cells, light-emitting diodes, and X-ray detectors. In this work, we propose a strategy for designing quadruple perovskites by heterovalent cation transmutation from double perovskites. Two stable quadruple perovskite halides, i.e., Cs₄CdSb₂Cl₁₂ and Cs₄CdBi₂Cl₁₂, with a vacancy-ordered three-dimensional (3D) crystal structure were predicted through symmetry analysis and density functional theory (DFT) calculations. The title perovskite halides are also electronically 3D with direct forbidden bandgaps. Following the indication provided by the DFT results, Cs₄CdSb₂Cl₁₂ and Cs₄CdBi₂Cl₁₂ as unique quadruple perovskites were successfully synthesized by a solvothermal method. The steady-state photoluminescence (PL) shows wide emission, while the transient PL exhibits carrier recombination lifetime on the order of microseconds at low temperature. The quadruple perovskite halides provide an alternative platform for promising optoelectronic material design in addition to simple and double perovskites.

Tunable optical absorption in lead-free perovskite-like hybrids by iodide management

Tunable optical absorption in lead-free perovskite-like hybrids by iodide management.