Ligand-Induced Chirality in ClMBA2 SnI4 2D Perovskite.

Chiral perovskites possess a huge applicative potential in several areas of optoelectronics and spintronics. The development of novel lead-free perovskites with tunable properties is a key topic of current research. Herein, we report a novel lead-free chiral perovskite, namely (R/S-)ClMBA2 SnI4 (ClMBA=1-(4-chlorophenyl)ethanamine) and the corresponding racemic system. ClMBA2 SnI4 samples exhibit a low band gap (2.12 eV) together with broad emission extending in the red region of the spectrum (∼1.7 eV). Chirality transfer from the organic ligand induces chiroptical activity in the 465-530 nm range. Density functional theory calculations show a Rashba type band splitting for the chiral samples and no band splitting for the racemic isomer. Self-trapped exciton formation is at the origin of the large Stokes shift in the emission. Careful correlation with analogous lead and lead-free 2D chiral perovskites confirms the role of the symmetry-breaking distortions in the inorganic layers associated with the ligands as the source of the observed chiroptical properties providing also preliminary structure-property correlation in 2D chiral perovskites.

Bismuth-Based Halide Perovskites for Photocatalytic H2 Evolution Application.

Metal halide perovskites (MHPs), in particular lead-based perovskites, have earned recognized fame in several fields for their outstanding optoelectronic properties, including direct generation of free charge carriers, optimal ambipolar charge carrier transport properties, high absorption coefficient, point-defect tolerance, and compositional versatility. Nowadays, this class of materials represents a real and promising alternative to silicon for photovoltaic technologies. This worthy success led to a growing interest in the exploration of MHPs in other hot research fields, such as solar-driven photocatalytic water splitting towards hydrogen production. Nevertheless, many of these perovskites show air and moisture instability problems that considerably hinder their practical application for photocatalytic water splitting. Moreover, if chemical instability is a problem that can be in part mitigated by the optimization of the chemical composition and crystal structure, the presence of lead represents a real problem for the practical application of MHPs in commercial devices due to environmental and healthcare issues. To successfully overcome these problems, lead-free metal halide perovskites (LFMHPs) have gained increasing interest thanks to their optoelectronic properties, comparable to lead-based materials, and their more eco-friendly nature. Among all the lead-free perovskite alternatives, this mini-review considers bismuth-based perovskites and perovskite derivatives with a specific focus on solar-driven photocatalysis application for H2 evolution. Special attention is dedicated to the structure and composition of the different materials and to the advantage of heterojunction engineering and the relative impact on the photocatalytic process.

Chiral Metal Halide Perovskites: Focus on Lead-Free Materials and Structure-Property Correlations.

Hybrid organic-inorganic perovskites (HOIPs) are promising materials in several fields related to electronics, offering long carrier-diffusion lengths, high absorption coefficients, tunable band gaps, and long spin lifetimes. Recently, chiral perovskites have attracted huge interest thanks to the possibility of further widening the applications of HOIPs. Chiral materials, being intrinsically non-centrosymmetric, display several attractive physicochemical properties, including circular dichroism, circularly polarized photoluminescence, nonlinear optics, ferroelectricity, and spin-related effects. Recent studies have shown that chirality can be transferred from the chiral organic ligands into the inorganic perovskite framework, resulting in materials combining the advantages of both chirality and perovskite superior optoelectronic characteristics. As for HOIPs for photovoltaics, strong interest is currently devoted towards the development of lead-free chiral perovskites to overcome any toxicity issue. While considering the basic and general features of chiral HOIPs, this review mainly focuses on lead-free materials. It highlights the first attempts to understand the correlation between the crystal structure characteristics and the chirality-induced functional properties in lead and lead-free chiral perovskites.

Nanocrystals perovskites photocatalyzed singlet oxygen generation for light-driven organic reactions.

Lead halide perovskite nanocrystals were prepared and used as photocatalysts for the in situ 1O2 generation to perform hetero Diels-Alder, ene and oxidation reactions with suitable dienes and alkenes. The methodology has been reasonably standardized and made applicable to a variety of olefinic substrates. The scope of the method is finely illustrated by the results in all the tested reactions, which allowed to obtain desymmetrized hydroxy-ketone derivatives, unsaturated ketones and epoxides. Some limitations were also observed especially in the case of the alkene oxidations as well as poor chemoselectivity was somewhere observed. 1O2 generated by lead halide perovskite nanocrystals as photocatalyst in organic reactions.

Optical and Structural Property Tuning in Physical Vapor Deposited Bismuth Halides Cs3Bi2(I1-xBrx)9 (0 ≤ x ≤ 1).

Crystalline films of lead-free all-inorganic Cs3Bi2X9 (X = Br, I) perovskites have been deposited by radio frequency (RF)-magnetron sputtering providing high-quality, single-phase films as confirmed by structural, morphological, and optical property characterization. Progressive tuning of crystal structure characteristics and optical absorbance has been achieved in mixed Br/I phases Cs3Bi2(I1-xBrx)9 (0 ≤ x ≤ 1), highlighting a shift of the band gap from about 2.0 eV for Cs3Bi2I9 to 2.64 eV for Cs3Bi2Br9. X-ray diffraction and Raman scattering allowed defining the range of alloyed compositions where single-phase compositions are found. Finally, preliminary photocatalytic activity tests on the degradation of methylene blue provided solid data indicating the future possible exploitation of Bi-based perovskite derivative materials as active photocatalysts.

Water-Stable DMASnBr3 Lead-Free Perovskite for Effective Solar-Driven Photocatalysis.

Water-stable metal halide perovskites could foster tremendous progresses in several research fields where their superior optical properties can make differences. In this work we report clear evidence of water stability in a lead-free metal halide perovskite, namely DMASnBr3 , obtained by means of diffraction, optical and X-ray photoelectron spectroscopy. Such unprecedented water-stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr3 to g-C3 N4 , taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 µmol g-1 h-1 generated by the synergistic activity of the two composite costituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr3 and g-C3 N4 . The demonstration of an efficient photocatalytic activity for a composite based on lead-free metal halide perovskite in water paves the way to a new class of light-driven catalysts working in aqueous environments.

Enantiomeric Resolution and Absolute Configuration of a Chiral δ-Lactam, Useful Intermediate for the Synthesis of Bioactive Compounds.

During the past several years, the frequency of discovery of new molecular entities based on γ- or δ-lactam scaffolds has increased continuously. Most of them are characterized by the presence of at least one chiral center. Herein, we present the preparation, isolation and the absolute configuration assignment of enantiomeric 2-(4-bromophenyl)-1-isobutyl-6-oxopiperidin-3-carboxylic acid (trans-1). For the preparation of racemic trans-1, the Castagnoli-Cushman reaction was employed. (Semi)-preparative enantioselective HPLC allowed to obtain enantiomerically pure trans-1 whose absolute configuration was assigned by X-ray diffractometry. Compound (+)-(2R,3R)-1 represents a reference compound for the configurational study of structurally related lactams.

Investigation of Dimethylammonium Solubility in MAPbBr3 Hybrid Perovskite: Synthesis, Crystal Structure, and Optical Properties.

The possible existence of mixed methylammonium (MA)/dimethylammonium (DMA) lead bromide hybrid perovskites of general formula MA1- xDMA xPbBr3 (0 ≤ x ≤ 1) was investigated. A combined X-ray diffraction and solid-state nuclear magnetic resonance approach indicates that DMA can be incorporated up to about x = 0.30 while retaining the cubic lattice of MAPbBr3. By increasing the DMA content ( x), the absorption shows a progressive blue shift and the band gap moves from about 2.17 eV ( x = 0) to about 2.23 ( x = 0.30) with a concomitant slightly faster recombination in the mixed cation powders.

Chitosan Oleate Coated Poly Lactic-Glycolic Acid (PLGA) Nanoparticles versus Chitosan Oleate Self-Assembled Polymeric Micelles, Loaded with Resveratrol.

Chitosan oleate (CS-OA), a chitosan salt with amphiphilic properties, has demonstrated the ability to self-assemble in aqueous environment to give polymeric micelles useful to load poorly soluble drugs. More recently, CS-OA was proposed to stabilize nanoemulsions during the preparation by emulsification and solvent evaporation of poly lactic-glycolic acid (PLGA) nanoparticles (NPs) loaded with curcumin. Positive mucoadhesive behavior and internalization properties were demonstrated for these NPs attributable to the presence of positive charge at the NP surface. In the present paper, two CS-OA-based nanosystems, micelles and PLGA NPs, were compared with the aim of elucidating their physico-chemical characteristics, and especially their interaction with cell substrates. The two systems were loaded with resveratrol (RSV), a hydrophobic polyphenol endowed with anti-cancerogenic, anti-inflammatory, and heart/brain protective effects, but with low bioavailability mainly due to poor aqueous solubility. Calorimetric analysis and X-ray spectra demonstrated amorphization of RSV, confirming its affinity for hydrophobic domains of polymeric micelles and PLGA core of NPs. TGA decomposition patterns suggest higher stability of PLGA-NPs compared with polymeric micelles, that anyway resulted more stable than expected, considering the RSV release profiles, and the cell line interaction results.

High-Temperature Structural Evolution in the Ba3Mo(1- x)W xNbO8.5 System and Correlation with Ionic Transport Properties.

The evolution of the hybrid structure between 9R hexagonal perovskite and palmierite in the entire Ba3Mo(1- x)W xNbO8.5 solid solution (where x = 0, 0.25, 0.5, 0.75, and 1) was probed in the 100-900 K range by synchrotron high-resolution powder diffraction. Each sample exhibits a chemical-dependent structural model in the low-temperature regime (from 100 to 500 K) in which 9R and palmierite structures compete each other, the former being progressively favored as tungsten replaces molybdenum. Above 500 K, unit cell parameters and metal site occupancies start to converge toward a similar structural arrangement that is completely reached at 900 K. In fact, at this temperature, the entire solid solution discloses comparable unit cell and an almost enterely occupied M1 site, with a structure that is much closer to palmierite rather than 9R polytype. The present crystallographic results well explain the behavior of the material's bulk ionic conductivity, whose temperature evolution for different compositions depends from the contribution of tetrahedral units proper of the palmierite structure.

Origin of colossal magnetoresistance in LaMnO3 manganite.

Phase separation is a crucial ingredient of the physics of manganites; however, the role of mixed phases in the development of the colossal magnetoresistance (CMR) phenomenon still needs to be clarified. We report the realization of CMR in a single-valent LaMnO3 manganite. We found that the insulator-to-metal transition at 32 GPa is well described using the percolation theory. Pressure induces phase separation, and the CMR takes place at the percolation threshold. A large memory effect is observed together with the CMR, suggesting the presence of magnetic clusters. The phase separation scenario is well reproduced, solving a model Hamiltonian. Our results demonstrate in a clean way that phase separation is at the origin of CMR in LaMnO3.

Chemical Structures of Specific Sodium Ion Battery Components Determined by Operando Pair Distribution Function and X-ray Diffraction Computed Tomography.

To improve lithium and sodium ion battery technology, it is imperative to understand how the properties of the different components are controlled by their chemical structures. Operando structural studies give us some of the most useful information for understanding how batteries work, but it remains difficult to separate out the contributions of the various components of a battery stack (e.g., electrodes, current collectors, electrolyte, and binders) and examine specific materials. We have used operando X-ray diffraction computed tomography (XRD-CT) to study specific components of an essentially unmodified working cell and extract detailed, space-resolved structural information on both crystalline and amorphous phases that are present during cycling by Rietveld and pair distribution function (PDF) methods. We illustrate this method with the first detailed structural examination of the cycling of sodium in a phosphorus anode, revealing surprisingly different mechanisms for sodiation and desodiation in this promising, high-capacity anode system.

Correlation between Deposition Parameters and Hydrogen Production in CuO Nanostructured Thin Films.

In this article, we report a systematic investigation of the role of (i) substrate temperature, (ii) oxygen partial pressure, and (iii) radio frequency (rf) power on the crystal structure and morphology of CuO nanostructured thin films prepared by means of rf-magnetron sputtering starting from a Cu metal target. On selected films, photocatalytic tests have been carried out in order to correlate the structural and morphological properties of the thin films prepared under different conditions with the photocatalytic properties and to find out the key parameters to optimize the CuO nanostructured films. All of the synthesized films were single-phase CuO nanorods of variable diameter between 80 and 200 nm. Better-aligned rods were obtained at relatively low substrate temperatures and from low to intermediate oxygen partial pressures, resulting in more efficient photocatalytic activities. Our investigation suggests a relevant role of the crystallographic orientation of the CuO tenorite film on the photocatalytic activity, as demonstrated by the significant improvement in H2 evolution for highly oriented films.

FA0.8MA0.2SnxPb1-xI3 Hybrid Perovskite Solid Solution: Toward Environmentally Friendly, Stable, and Near-IR Absorbing Materials.

We report the first investigation addressing the synthesis and characterization of the FA0.8MA0.2SnxPb1-xI3 solid solution showing a complete solubility of Sn on the Pb-site leading to cubic single-phase materials. The explored composition shows excellent phase stability and absorbance in the near-IR spectral region.

Short-Range Order of Methylammonium and Persistence of Distortion at the Local Scale in MAPbBr3 Hybrid Perovskite.

Short-range investigation by means of variable-temperature neutron total scattering and pair distribution function analysis revealed that the local environment around the methylammonium (MA) cation in MAPbBr3 hybrid perovskite is maintained through the different phase transitions observed as a function of temperature. In addition, the orthorhombic distortion of the lattice is present at any temperature. Local structure around MA changes from static to configurationally averaged or dynamic with temperature but the local structure of the low-temperature orthorhombic phase is preserved.

CH3NH3SnxPb1-xBr3 hybrid perovskite solid solution: synthesis, structure, and optical properties.

We report the synthesis and characterization of a MASnxPb1-xBr3 (MA = methylammonium; nominal x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) solid solution. The original synthetic method developed allowed one to obtain single-phase materials with homogeneous Sn/Pb distribution. All of the samples prepared are cubic, and the unit cell linearly decreases with increasing x value. The optical response indicates a linear trend (Vegard's law) of the band gap with increasing Sn content from 2.20 eV (x = 0) to 1.33 eV (x = 1), thus extending light absorption into the near-IR.

Structure-properties correlations in Fe chalcogenide superconductors.

The focus of this review article is on the PbO-type Fe chalcogenides that recently emerged as a new class of superconductors. The fundamental correlations between structural details and chemical compositions are discussed together with their influence on the electronic properties.

Rotational Dynamics of Organic Cations in Formamidinium Lead Iodide Perovskites.

We report results from quasi-elastic neutron scattering studies on the rotational dynamics of formamidinium (HC[NH2]2+, FA) and methylammonium (CH3NH3+, MA) cations in FA1-xMAxPbI3 with x = 0 and 0.4 and compare it to the dynamics in MAPbI3. For FAPbI3, the FA cation dynamics evolve from nearly isotropic rotations in the high-temperature (T > 285 K) cubic phase through reorientations between preferred orientations in the intermediate-temperature tetragonal phase (140 K < T ⩽ 285 K) to an even more complex dynamics, due to a disordered arrangement of the FA cations, in the low-temperature tetragonal phase (T ⩽ 140 K). For FA0.6MA0.4PbI3, the dynamics of the respective organic cations evolve from a relatively similar behavior to FAPbI3 and MAPbI3 at room temperature to a different behavior in the lower-temperature phases where the MA cation dynamics are a factor of 50 faster as compared to those of MAPbI3. This insight suggests that tuning the MA/FA cation ratio may be a promising approach to tailoring the dynamics and, in effect, optical properties of FA1-xMAxPbI3.

Preparation of Heterojunctions Based on Cs3Bi2Br9 Nanocrystals and g-C3N4 Nanosheets for Photocatalytic Hydrogen Evolution.

Heterojunctions based on metal halide perovskites (MHPs) are promising systems for the photocatalytic hydrogen evolution reaction (HER). In this work, we coupled Cs3Bi2Br9 nanocrystals (NCs), obtained by wet ball milling synthesis, with g-C3N4 nanosheets (NSs), produced by thermal oxidation of bulk g-C3N4, in air. These methods are reproducible, inexpensive and easy to scale up. Heterojunctions with different loadings of Cs3Bi2Br9 NCs were fully characterised and tested for the HER. A relevant improvement of H2 production with respect to pristine carbon nitride was achieved at low NCs levels reaching values up to about 4600 µmol g-1 h-1. This work aims to provide insights into the synthesis of inexpensive and high-performing heterojunctions using MHP for photocatalytic applications.

Cubic or Not Cubic? Combined Experimental and Computational Investigation of the Short-Range Order of Tin Halide Perovskites.

Tin-based metal halide perovskites with a composition of ASnX3 (where A= MA or FA and X = I or Br) have been investigated by means of X-ray total scattering techniques coupled to pair distribution function (PDF) analysis. These studies revealed that that none of the four perovskites has a cubic symmetry at the local scale and that a degree of increasing distortion is always present, in particular when the cation size is increased, i.e., from MA to FA, and the hardness of the anion is increased, i.e., from Br- to I-. Electronic structure calculations provided good agreement with experimental band gaps for the four perovskites when local dynamical distortions were included in the calculations. The averaged structure obtained from molecular dynamics simulations was consistent with experimental local structures determined via X-ray PDF, thus highlighting the robustness of computational modeling and strengthening the correlation between experimental and computational results.