Unveiling the Structural Properties, Optical Behavior, and Thermoelectric Performance of 2D CsSn2Br5 Halide Obtained by Mechanochemistry.

Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn2Br5 specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn2+ lone electron pair (5s2) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr4] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately ∼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (∼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 104 µV K-1 at 350 K, which evolves down to negative values of -3.0 × 103 µV K-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m-1 K-1.

Molecular adsorption on coinage metal subnanoclusters: A DFT+D3 investigation.

Gold and silver subnanoclusters with few atoms are prominent candidates for catalysis-related applications, primarily because of the large fraction of lower-coordinated atoms exposed and ready to interact with external chemical species. However, an in-depth energetic analysis is necessary to characterize the relevant terms within the molecular adsorption process that can frame the interactions within the Sabatier principle. Herein, we investigate the interaction between Agn and Aun subnanoclusters (clu, n = 2-7) and N2 , NO, CO, and O2 molecules, using scalar-relativistic density functional theory calculations within van der Waals D3 corrections. The onefold top site is preferred for all chemisorption cases, with a predominance of linear (≈180°) and bent (≈120°) molecular geometries. A larger magnitude of adsorption energy is correlated with smaller distances between molecules and clusters and with the weakening of the adsorbates bond strength represented by the increase of the equilibrium distances and decrease of molecular stretching frequencies. From the energetic decomposition, the interaction energy term was established as an excellent descriptor to classify subnanoclusters in the adsorption/desorption process concomitant with the Sabatier principle. The limiting cases: (i) weak molecular adsorption on the subnanoclusters, which may compromise the reaction activation, where an interaction energy magnitude close to 0 eV is observed (e.g., physisorption in N2 /Ag6 ); and (ii) strong molecular interactions with the subnanoclusters, given the interaction energy magnitude is larger than at least one of the individual fragment binding energies (e.g., strong chemisorption in CO/Au4 and NO/Au4 ), conferring a decrease in the desorption rate and an increase in the possible poisoning rate. However, the intermediate cases are promising by involving interaction energy magnitudes between zero and fragment binding energies. Following the molecular closed-shell (open-shell) electronic configuration, we find a predominant electrostatic (covalent) nature of the physical interactions for N2 ⋯clu and CO ⋯clu (O2 ⋯clu and NO⋯clu), except in the physisorption case (N2 /Ag6 ) where dispersive interaction is dominant. Our results clarify questions about the molecular adsorption on subnanoclusters as a relevant mechanistic step present in nanocatalytic reactions.

Detailed Structural Features of the Perovskite-Related Halide RbPbI3 for Solar Cell Applications.

All-inorganic lead halide perovskites like CsPbBr3, CsPbI3, or RbPbI3 are good replacements for the classical hybrid organic-inorganic perovskites like CH3NH3PbI3, susceptible to fast degradation in the presence of humid air. They also exhibit outstanding light absorption properties suitable for solar energy applications. Here, we describe the synthesis of RbPbI3 by mechanochemical procedures with green credentials, avoiding toxic or expensive organic solvents; this specimen exhibits excellent crystallinity. We report neutron powder diffraction data, essential to revisit some subtle structural features around room temperature (200-400 K). In all these regimes, the orthorhombic Pnma crystal structure is characterized by the presence along the b direction of the crystal of double rows of edge-sharing PbI6 octahedra. The lone electron pairs of Pb2+ ions have a strong stereochemical effect on the PbI6 octahedral distortion. The relative covalency of Rb-I versus Pb-I bonds shows that the Pb-I-related motions are more rigid than Rb-I-related vibrations, as seen in the Debye temperatures from the evolution of the anisotropic displacements. The optical gap, measured by diffuse reflectance UV-vis spectroscopy, is ∼2.51 eV and agrees well with ab initio calculations. The thermoelectric Seebeck coefficient is 3 orders of magnitude larger than that of other halide perovskites, with a value of ∼117,000 µV·K-1 at 460 K.

Unveiling the Structural Behavior under Pressure of Filled M0.5Co4Sb12 (M = K, Sr, La, Ce, and Yb) Thermoelectric Skutterudites.

Skutterudite-type compounds based on □Co4Sb12 pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8a voids (in the cubic space group Im3̅) with different elements (alkali, alkali earth, and rare earth) helps to reduce the thermal conductivity and thus increases the thermoelectric performance. A systematic characterization by synchrotron X-ray powder diffraction of different M-filled Co4Sb12 (M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range ∼0-12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli (B0) were calculated for all the filled skutterudites, yielding unexpected results. A lattice expansion due to the filler elements fails in the description of the Bulk moduli. Topochemical studies of the filler site environment exhibited a slight disturbance and an increased ionic character when the filler is incorporated. The mechanical properties by means of Bulk moduli resulted in being sensitive to the presence of filler atoms inside the skutterudite voids, being affected by the covalent/ionic exchange of the Co-Sb and Sb-Sb bonds.

α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties.

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag2WO4. Theses α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag2-2xZnxWO4 models. The electronic properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were linked to distortion effects and oxygen vacancies (VOx) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were explained by means of distortional effects and oxygen vacancies (VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag+ cations was four.

Potentiated electron transference in α-Ag2WO4 microcrystals with Ag nanofilaments as microbial agent.

This study is a framework proposal for understanding the antimicrobacterial effect of both α-Ag2WO4 microcrystals (AWO) synthesized using a microwave hydrothermal (MH) method and α-Ag2WO4 microcrystals with Ag metallic nanofilaments (AWO:Ag) obtained by irradiation employing an electron beam to combat against planktonic cells of methicillin-resistant Staphylococcus aureus (MRSA). These samples were characterized by X-ray diffraction (XRD), FT-Raman spectroscopy, ultraviolet visible (UV-vis) measurements, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM). The results reveal that both AWO and AWO:Ag solutions have bacteriostatic and bactericidal effects, but the irradiated sample is more efficient; i.e., a 4-fold of the MRSA planktonic cells as compared to the nonirradiated sample was observed. In addition, first principles calculations were performed to obtain structural and electronic properties of AWO and metallic Ag, which provides strong quantitative support for an antimicrobacterial mechanism based on the enhancement of electron transfer processes between α-Ag2WO4 and Ag nanoparticles.

Structural Features and Optical Properties of All-Inorganic Zero-Dimensional Halides Cs4PbBr6-xIx Obtained by Mechanochemistry.

Despite the great success of hybrid CH3NH3PbI3 perovskite in photovoltaics, ascribed to its excellent optical absorption properties, its instability toward moisture is still an insurmountable drawback. All-inorganic perovskites are much less sensitive to humidity and have potential interest for solar cell applications. Alternative strategies have been developed to design novel materials with appealing properties, which include different topologies for the octahedral arrangements from three-dimensional (3D, e.g., CsPbBr3 perovskite) or two-dimensional (2D, e.g., CsPb2Br5) to zero-dimensional (0D, i.e., without connection between octahedra), as the case of Cs4PbX6 (X = Br, I) halides. The crystal structure of these materials is complex, and their thermal evolution is unexplored. In this work, we describe the synthesis of Cs4PbBr6-xIx (x = 0, 2, 4, 6) halides by mechanochemical procedures with green credentials; these specimens display excellent crystallinity enabling a detailed structural investigation from synchrotron X-ray powder diffraction (SXRD) data, essential to revisit some features in the temperature range of 90-298 K. In all this regime, the structure is defined in the trigonal R3̅c space group (#167). The presence of Cs and X vacancies suggests some ionic mobility into the crystal structure of these 0D halides. Bond valence maps (BVMs) are useful in determining isovalent surfaces for both Cs4PbBr6 and Cs4PbI6 phases, unveiling the likely ionic pathways for cesium and bromide ions and showing a full 3D connection in the bromide phase, in contrast to the iodide one. On the other hand, the evolution of the anisotropic displacement parameters is useful to evaluate the Debye temperatures, confirming that Cs atoms have more freedom to move, while Pb is more confined at its site, likely due to a higher covalency degree in Pb-X bonds than that in Cs-X bonds. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy shows that the optical band gap can be tuned depending on iodine content (x) in the range of 3.6-3.06 eV. From density functional theory (DFT) simulations, the general trend of reducing the band gap when Br is replaced by I is well reproduced.

Quantitative evaluation of the surface stability and morphological changes of Cu2O particles.

Cu2O low-index surfaces periodic models have been simulated based on density functional theory. The calculated surfaces energies allowed estimating the morphology by means of the Wulff theorem as well as the investigation of possible paths of morphological changes. Therefore, systematic morphology diagrams and change paths according to the energy modulation in relation to the surfaces stabilizations were elaborated. The applicability of this strategy was exemplified by comparing the obtained results with experimental available data from the literature. The morphology diagrams with the quantitative energetic point of view can be used as a guide to support experimental works in order to understand the relation between surface interactions and crystal growth.

Europium doped zinc sulfide: a correlation between experimental and theoretical calculations.

This paper presents the correlation among electronic and optical property effects induced by the addition of different concentrations of europium (Eu3+) in zinc sulfide (ZnS) by microwave-assisted solvothermal (MAS) method. A shift of the photoluminescence (PL) emission was observed with the increase of Eu3+. The periodic DFT calculations with the B3LYP hybrid functional were performed using the CRYSTAL computer code. The UV-vis spectra and theoretical results indicate a decrease in behavior of the energy gap as a function of dopant concentration. Therefore, new localized states are generated in the forbidden band gap region, the new states increase the probability of less energy transitions which may be responsible for a red shift in the PL bands spectrum.

Structural and electronic analysis of the atomic scale nucleation of Ag on α-Ag2WO4 induced by electron irradiation.

In this work, we utilise a combination of theory, computation and experiments to understand the early events related to the nucleation of Ag filaments on α-Ag2WO4 crystals, which is driven by an accelerated electron beam from an electron microscope under high vacuum. The growth process and the chemical composition and elemental distribution in these filaments were analysed in depth at the nanoscale level using TEM, HAADF, EDS and XPS; the structural and electronic aspects were systematically studied in using first-principles electronic structure theory within QTAIM framework. The Ag nucleation and formation on α-Ag2WO4 is a result of the order/disorder effects generated in the crystal by the electron-beam irradiation. Both experimental and theoretical results show that this behavior is associated with structural and electronic changes of the [AgO2] and [AgO4] clusters and, to a minor extent, to the [WO6] cluster; these clusters collectively represent the constituent building blocks of α-Ag2WO4.

Photocatalytic activity of semiconductor sulfide heterostructures.

This paper reports a theoretical and experimental study of the heterostructure photocatalytic activity in a CdS or ZnS and CdS@ZnS decorated system prepared by a microwave assisted solvothermal (MAS) method. A theoretical model of the decorated system was created in order to analyze the electronic transition mainly in their interface. The results show that CdS and ZnS interfaces produce an electron charge transfer from the CdS electron-populated clusters to the ZnS hole-populated clusters which helps to enhance the photocatalytic activity of the CdS@ZnS decorated system.