Nanostructured Metal Sulfides: Classification,Modification Strategy,and Solar-Driven CO2 Reduction Application

Solar-driven conversion of CO₂ into high value-added fuels is expected to be an environmental-friendly and sustainable approach for relieving the greenhouse gas effect and countering energy crisis. Metal sulfide semiconductors with wide photoresponsive range and favorable band structures are suitable photocatalysts for CO₂ photoreduction. This review summarizes the recent progress on metal sulfide semiconductors for photocatalytic CO₂ reduction. First, the fundamentals, mechanisms and some principles, like product selectivity, of photocatalytic CO₂ reduction are introduced. Then, according to the elemental composition, the metal sulfide photocatalysts applied for CO₂ reduction are classified into binary (CdS, ZnS, MoS₂, SnS₂, Bi₂S₃, In₂S₃,Cu₂S, NiS/NiS₂, and CoS₂), ternary (ZnIn₂S₄, CdIn₂S₄, CuInS₂, Cu₃SnS₄, and CuGaS₂), and quaternary (Cu₂ZnSnS₄) systems, in which their crystal structures, photochemical characteristics, and photocatalytic CO₂ reduction applications are systematically demonstrated. Especially, the diverse modification strategies for improving the activity and product selectivity of photocatalytic CO₂ reduction on these metal sulfides are summarized. Finally, the current challenges and future directions for the development of metal sulfide photocatalysts for CO₂ reduction are proposed. This review is expected to serve as a powerful reference for exploiting high-efficiency metal sulfide photocatalysts for CO₂ conversion and furthering related mechanism understanding.     

R-reliable intervals and confidences for the lifetime of a parallel system

In this discussion, R-reliable intervals for the life-time of a parallel system, based on order statistics, are obtained. The failure distributions for the individual components are assumed to have densities of the form 1/θ_Xif(x_iθ_Xi) where X_i, for i = 1, 2, …, n, are the failure times for the components in the parallel system and θ_Xis are the scale parameters for the failure times X_i. A relationship between the confidence of the R-reliable interval for the total lifetime of the parallel system and the confidences of the R-reliable intervals for the lifetimes of the individual components is investigated. A bound for the confidence of the R-reliable interval for the total lifetime of the system is obtained. Examples for n=3 are presented.     

Spinodal decomposition and clustering in III/V alloys

The criteria for clustering and spinodal decomposition in III/V pseudobinary and quaternary solid alloys are examined. A chemical driving force for clustering and phase separation exists in some ternary and most quaternary alloys. However, single crystalline alloys are shown to be stabilized by the coherency strain energy inherent in any clustering or spinodal decomposition in alloys where lattice parameter is a function of composition. Analytical expressions are derived for T_s, the temperature above which no clustering or phase separation should occur. Most III/V pseudobinary and quaternary alloys are stable at all temperatures. Numerical techniques are used to calculate spinodal isotherms. Results are presented for the systems Ga_xIn_1−x As P_1−y, A1_xGa_1−xAs_ySb_1−y, and Ga_xIn_1−x As P_1−y. This work was supported by the Department of Energy, contract No. DE-AT 03-81 ER 10934.     

Complex Band Structures and Lattice Dynamics of Bi2Te3‐Based Compounds and Solid Solutions

Bi₂Te₃‐based compounds and derivatives are milestone materials in the fields of thermoelectrics (TEs) and topological insulators (TIs). They have highly complex band structures and interesting lattice dynamics, which are favorable for high TE performance as well as strong spin orbit and band inversion underlying topological physics. This review presents rational calculations of properties related to TEs and provides theoretical guidance for improving the TE performance of Bi₂Te₃‐based materials. Although the band structures of these TE materials have been studied theoretically and experimentally for many years, there remain many controversies on band characteristics, especially the locations of band extrema and the exact values of bandgaps. Here, the key factors in the theoretical investigations of Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and their solid solutions are reviewed. The phonon spectra and lattice thermal conductivities of Bi₂Te₃‐based materials are discussed. Electronic and phonon structures and TE transport calculations are discussed and reported in the context of better establishing computational parameters for these V₂VI₃‐based materials. This review provides a useful guidance for analyzing and improving TE performance of Bi₂Te₃‐based materials.     

Single-Atom Catalysts for H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Reaction

Oxygen reduction reaction via the two-electron route (2e⁻ ORR) provides a green method for the direct production of hydrogen peroxide (H₂O₂) along with in situ utilization. The effective catalysts with high ORR activity, 2e⁻ selectivity, and stability are essential for the application of this technology. Single-atom catalysts (SACs) have attracted intensively attention for H₂O₂ electrosynthesis owing to the unique geometric and electronic configurations. In this review, the mechanism and theoretical predictions for 2e⁻ ORR over SACs are first introduced. Then, the recent advances of various SACs for the electrosynthesis of H₂O₂ are documented. And the correlation between the central atom, coordination atoms, and coordination environment of SACs and the corresponding electrocatalytic ORR performance including activity, selectivity, and stability are emphatically analyzed and summarized. Finally, the major challenges and opportunities regarding the future design of SACs for the H₂O₂ production are pointed out.     

Optimizing the Charge Carrier Dynamics of Thermal Evaporated TexSe1-x Films for High-Performance Short-Wavelength Infrared Photodetection

Short-wavelength infrared photo-sensing materials are dominated by germanium, indium gallium arsenide, indium antimonide, and mercury cadmium telluride. However, the complex fabrication process and high production cost hinder their widespread applications. Recently, Te_xSe_1-x has shown great potential for infrared photodetection, but Te_xSe_1-x-based devices are still suffering from extremely high dark current and poor device performance. In this work, high-quality Te_xSe_1-x films are fabricated by thermal evaporation and low-temperature annealing. The optoelectronic properties of the Te_xSe_1-x thin films are systematically investigated and optimized. The absorption spectrum is carefully tuned to cover the broad range from 300 to 1600 nm by modulating the ratio of Te and Se. Photodiodes based on the optimized Te_xSe_1-x thin films are also fabricated, and achieve high responsivity, reduced dark and low noise current density, and a fast response time of <850 ns. Then, prototypical devices based on Te_0.65Se_0.35 thin films are demonstrated for optical communications, indicating the great potential for next-generation, low-cost short-wavelength infrared photodetection.     

Large‐Scale Synthesis and Characterization of TiO2‐Based Nanostructures on Ti Substrates

Na₂Ti₆O₁₃ nanoplates, nanowires, and continuous nanowire network films are hydrothermally formed on a large scale directly on Ti substrates for the first time. The morphology of the formed Na₂Ti₆O₁₃ nanostructures can be easily tuned by varying the experimental parameters of temperature, reaction duration, and the NaOH concentration. Our study demonstrates that the synthesized Na₂Ti₆O₁₃ nanostructures are easily converted into H₂Ti₃O₇ nanostructures—a desirable precursor for the fabrication of various TiO₂‐based nanomaterials—with shape preservation, by an ion‐exchange process. Anatase, a mixture of anatase and rutile, and rutile TiO₂ nanowires are formed when the H₂Ti₃O₇ nanowires are annealed at 450, 600, and 750 °C, respectively. The optical properties and the photocatalytic activity of H₂Ti₃O₇ nanowires and of the TiO₂‐based nanomaterials are also addressed. The approach described in this study provides a simple and novel method for the large‐scale synthesis of various TiO₂‐based nanostructured materials that grow directly on Ti substrates and are ready for a wide range of practical applications, such as the photodegradation of wastewater.     

Materials Discovery of Stable and Nontoxic Halide Perovskite Materials for High‐Efficiency Solar Cells

Two critical limitations of organic–inorganic lead halide perovskite materials for solar cells are their poor stability in humid environments and inclusion of toxic lead. In this study, high‐throughput density functional theory (DFT) methods are used to computationally model and screen 1845 halide perovskites in search of new materials without these limitations that are promising for solar cell applications. This study focuses on finding materials that are comprised of nontoxic elements, stable in a humid operating environment, and have an optimal bandgap for one of single junction, tandem Si‐perovskite, or quantum dot–based solar cells. Single junction materials are also screened on predicted single junction photovoltaic (PV) efficiencies exceeding 22.7%, which is the current highest reported PV efficiency for halide perovskites. Generally, these methods qualitatively reproduce the properties of known promising nontoxic halide perovskites that are either experimentally evaluated or predicted from theory. From a set of 1845 materials, 15 materials pass all screening criteria for single junction cell applications, 13 of which are not previously investigated, such as (CH₃NH₃)_0.75Cs_0.25SnI₃, ((NH₂)₂CH)Ag_0.5Sb_0.5Br₃, CsMn_0.875Fe_0.125I₃, ((CH₃)₂NH₂)Ag_0.5Bi_0.5I₃, and ((NH₂)₂CH)_0.5Rb_0.5SnI₃. These materials, together with others predicted in this study, may be promising candidate materials for stable, highly efficient, and nontoxic perovskite‐based solar cells.     

Elastic and optoelectronic properties of novel Ag3AuSe2 and Ag3AuTe2 semiconductors

Ag₃AuSe₂ and Ag₃AuTe₂ are interesting class of semiconducting materials. Here, elastic and opto-electronic properties of Ag₃AuSe₂ and Ag₃AuTe₂ semiconductors are studied in detail using density functional theory. Different schemes are selected to treat the exchange-correlation effects. The unit cell of the compounds is fully optimized and calculated cell constants are found in agreement to the existing experimental data. The calculated elastic constants and elastic moduli reveal that the compounds possess ductile nature and are elastically stable. It is found that both compounds are direct bandgap semiconductor with bandgap value of 1.009 eV for Ag₃AuSe₂ and 0.551 eV for Ag₃AuTe₂. As the compounds have narrow and direct bandgaps, therefore optically active. The optical properties like reflectivity, absorption coefficient, energy loss function, refractive index including and complex dielectric function are studied in detail. The direct band gap nature of these compounds make them useful candidate for different devices applications.     

A.c. loss measurements in high-tc superconductors

Low a.c. loss is a key issue for electrical engineering applications of high-T_c superconductors at industrial frequencies. Different material options are characterized by three different a.c. loss measurement techniques which approach as much as possible the application conditions of the conductors. Melt-textured-growth YBa₂Cu₃O_7−x bulk samples, suitable for magnetic bearings, are characterized in an external a.c. field by magnetization measurements. Melt-casted Bi₂Sr₂Ca₂Cu₃O_8−x samples and powder-in-tube Bi₂Sr₂Ca₂Cu₃O_10−x tapes are suitable for current leads or power cables. In these applications the conductors are exposed to a.c. transport currents. The associated losses in self field are measured by a very sensitive electrical measurement technique. Finally, a calorimetric method is necessary when larger conductors (for instance Bi₂Sr₂Ca₂Cu₃O_8−x tubes) are tested under transport currents I_t, generating a transverse magnetic field H α I_t, as encountered in magnet windings for SMES, transformers or generators. The results show that the a.c. losses are sufficiently low for self field applications at industrial frequencies and a comparison of the different high T_c superconductors is given. The results show further that the a.c. losses are essentially hysteretic and can be modeled using the Bean model.     

An internal calibration technique for pseudobinary systems by auger electron spectroscopy

An internal calibration technique has been developed for the quantitative analysis of pseudobinary systems by means of Auger electron spectroscopy without the need for stan-dards of known composition. This technique has been used for analyzing PbO-In₂O₃ two-phase films and A1N-A1₂ O₃ , AlAs-GaAs, and GaP-InP solid solutions. The technique is applicable to systems of the type (AC_m) (BC_n)_1-x if the Auger intensities measured for elemens A, B, and C are given by the linear relationships I = xI_A ^**, I_B = (l-x)I_B ^*, and I_C = xI_C ^** + (l-x)I_{C C} ^* where I_A ^** and I_C ^** are the inten-sities for pure AC_m and I_B ^* and I* are the intensities for pure BC_n . To determine whether this criterion is satisfied, a series of Auger measurements is made on regions of dif-ferent composition within a single specimen (e.g., on speci-mens in which a concentration variation exists across the surface or surfaces exposed by repeated sputter-etching of a sample with an in-depth composition gradient), and plots of I_A vs I_B and I_C/I_B vs I_A/I_B are made. If these plots are linear over tne range of compositions investigated, it is assumed that the above linear relationships are valid over the whole range of the pseudobinary system. It is then possible to construct linear calibration plots of I_A, I_B , and I_C vs x for the whole composition range.     

Recent Advances in Atomic‐Level Engineering of Nanostructured Catalysts for Electrochemical CO2 Reduction

Electrochemical reduction of CO₂ into value‐added chemicals provides a promising approach to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. As the CO₂ molecule is chemically inert and the reaction kinetics is sluggish, efficient electrocatalysts are thus highly required for promoting the conversion of CO₂. With great efforts devoted to improving the catalytic performance, the development of electrocatalysts for CO₂ reduction has gone from bulk metals with poor control to nanostructures with atomic precision. Nanostructured electrocatalysts with atomic precision are believed to be capable of combining the advantages of heterogeneous and homogenous catalysts. In this review, the recent advances in designing nanostructured electrocatalysts at the atomic level for boosting the catalytic performance toward CO₂ reduction and revealing the structure–property relationship are summarized. The challenges and opportunities in the near future are also proposed for paving the development of electrocatalytic CO₂ reduction.     

Photosensitive structures based on CuIn<Subscript>5</Subscript>Te<Subscript>8</Subscript> single crystals: Development and properties

A new ternary compound is synthesized for the first time, and bulk CuIn₅Te₈ single crystals are grown by directed crystallization of near-stoichiometric melt. It is established from X-ray diffraction patterns of grown crystals that they exhibit the structure of imperfect chalcopyrite with parameters of the unit cell of CuIn₅Te₈, which were close to those known for the CuInTe₂ ternary compound with the composition index n = 0. First, photosensitive structures are fabricated based on CuIn₅Te₈ crystals, and photosensitivity spectra are obtained for them; it is shown that it is possible to achieve broadband photosensitivity under illumination of the barrier side of these crystals. From the analysis of photosensitivity spectra, the character of band-to-band transitions and corresponding energies of these transitions in CuIn₅Te₈ are determined. This opens up prospects to use this new semiconductor in photoconverters of solar radiation.     

Extraction of optimized parameters for Si0.6Ge0.4 material and SPP mode propagation through Si0.6Ge0.4/Ag/Si0.6Ge0.4 waveguide

The Lorentz model and modified Debye model （MDM） parameters forSi0.6Ge0.4 are presented. A nonlinear optimiza- tion algorithm is developed. The obtained parameters are used to determine the complex relative permittivity of Si0.6Ge0.4, and compared with the experimental data for validation. Finally the obtained parameters are used to analyze the properties of symmetric surface plasmon polariton （SPP） mode propagation in a dielectric-metal-dielectric （DMD） material constructed with silver （Ag） and Si0.6Ge0.4 for further verifying the extracted parameters.     

Infrared transmission spectra of Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript>Te (x = 0.04) crystals

Three indicators (T₁₀₀₀, T₅₀₀₀, and T₁₀₀₀/T₅₀₀₀) are used to appraise the infrared (IR) transmission spectra for Cd_1−xZn_xTe (CZT) slices. By comparing the values of these three indicators, four typical types of IR spectra are characterized for CZT crystals. The CZT crystals possessing the four types of IR spectra are different in microstructures, especially the densities and sizes of Te precipitates, the free carrier concentrations, and the resistivities. Mechanisms for the elimination of tiny and dense Te precipitates are given by analyzing the variation of the IR transmittance in the range of 500–5000 cm⁻¹ during the annealing process.     

Photoelectric phenomena in the Cu (Al, In)/p-CuIn3Se5 Schottky barriers

Structures are formed on the p-CuIn₃Se₅ crystals and photoelectric phenomena in the Cu/p-CuIn₃Se₅, Al/p-CuIn₃Se₅, and In/p-CuIn₃Se₅ Schottky barriers are studied. The spectra of quantum efficiency for photoconversion in new structures were obtained for the first time. The characteristics of the interband transitions are discussed, and the CuIn₃Se₅ band gap is determined. It is concluded that CuIn₃Se₅ crystals can be used in the fabrication of high-efficiency broadband photoconverters of optical radiation.     

Three‐Dimensional Co3O4@MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage

In this paper, a highly ordered three‐dimensional Co₃O₄@MnO₂ hierarchical porous nanoneedle array on nickel foam is fabricated by a facile, stepwise hydrothermal approach. The morphologies evolution of Co₃O₄ and Co₃O₄@MnO₂ nanostructures upon reaction times and growth temperature are investigated in detail. Moreover, the as‐prepared Co₃O₄@MnO₂ hierarchical structures are investigated as anodes for both supercapacitors and Li‐ion batteries. When used for supercapacitors, excellent electrochemical performances such as high specific capacitances of 932.8 F g^?1 at a scan rate of 10 mV s^?1 and 1693.2 F g^?1 at a current density of 1 A g^?1 as well as long‐term cycling stability and high energy density (66.2 W h kg^?1 at a power density of 0.25 kW kg^?1), which are better than that of the individual component of Co₃O₄ nanoneedles and MnO₂ nanosheets, are obtained. The Co₃O₄@MnO₂ NAs are also tested as anode material for LIBs for the first time, which presents an improved performance with high reversible capacity of 1060 mA h g^?1 at a rate of 120 mA g^?1, good cycling stability, and rate capability.     

Technique for controlled adjustment of bubble collapse field in epitaxial garnet films by etching

Post growth control of bubble collapse field through film thickness adjustment be etching in phosphoric acid is discussed. The etching process is simple, accurate, and rapid. The etching properties for Gd₃Ga₅O₁₂ and Y_2.6Sm_0.4Fe_3.8Ga_1.2012 are characterized and presented. Special emphasis is placed on the etching of several iron garnet films of two compositions, one gallium substituted and the other containing calcium and germanium, to illustrate the reproducibility and flexibility of the etching technique. Results on 11 epitaxial films are listed with their pertinent bubble domain parameters and etch conditions. Also, experimental results are given for five films etched simultaneously, illustrating the adaptability of the technique for multifilm etching. Yield data from 605 etched films are presented which show that only eight were rejected for reasons related to the etching process.     

Probing Molecular and Crystalline Orientation in Solution‐Processed Perovskite Solar Cells

The microstructure of solution‐processed organometallic lead halide perovskite thin films prepared by the “gas‐assisted” method is investigated with synchrotron‐based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH₃NH₃PbI₃ crystallites and CH₃NH₃⁺ cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH₃NH₃PbI₃ crystallites in 200–250 nm thick films prepared on both planar TiO₂ and mesoporous TiO₂. Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO₂ (but not on mesoporous TiO₂) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO₂ morphology. NEXAFS measurements on all samples found that CH₃NH₃⁺ cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH₃NH₃PbI₃ layer deposited on planar TiO₂ in particular indicates the absence of any preferential orientation of CH₃NH₃⁺ cations within the CH₃NH₃PbI₃ unit cell for as‐prepared layers, that is, without any electrical poling.     

Photosensitivity of the structures based on quinary solid solutions of the isoelectronic series of germanium

The temperature dependence of resistivity and the spectrum of optical absorption are investigated for (Cu₂GaSe₃)_0.6(3GaAs)_0.4 single crystals. Photosensitive structures of several types (Schottky barriers, welded structures, and photoelectrochemical cells) have been developed. The rectification and the photovoltaic effect are observed for the structures obtained. The revealed specific features of photosensitivity in structures and possibilities for their practical application as photoconverters are also discussed.