Chemistry of Materials最新文献

Melt-grown, highly crystalline CsPbBr₃ has been intensely investigated as a semiconductor for direct hard radiation detection. While the phase purity and crystallinity of the CsPbBr₃ ingots are assessed by X-ray diffraction and optical microscopy, the overall quality of the material is ultimately judged by the performance of the final device. The iterative evaluation of crystal quality would greatly benefit from broadening readily accessible structural methods. In this work, we establish nuclear quadrupole resonance (NQR) spectroscopy as a versatile, noninvasive technique for evaluating the quality of melt-grown CsPbBr₃ ingots. We show that in addition to its inherent utility for probing the local environment around a quadrupolar nucleus, NQR spectroscopy is highly sensitive to crystal orientation and crystallinity, as further supported by ab initio calculations. The key spectroscopic descriptors (linewidth and integrals) can thus be correlated with both macroscopic and microscopic structural features, thereby establishing a robust and rapid method for evaluating crystal quality. Customized resonators can accommodate large ingots and enable measurements directly in the quartz ampule used for melt growth, as well as semiautomated spatial mapping of spectroscopic features across the ingots. For instance, we show that removing the impurities collected near the top of the ingot and subsequent recrystallization improve the homogeneity and overall crystallinity of the samples, highlighting the need for multiple purification steps. We also observe that different crystallographic orientations of crystal domains along the ingot are obtained and preserved in cut crystal disks. These findings pave the way for integrating NQR spectroscopy as a practical, noninvasive tool for in-line or in-situ crystal quality control and guided sample selection.

The NaFeNb(PO₄)₃, NFNP, material has been designed as a candidate anode material for sodium-ion batteries, as in its pristine form it combines the presence of Fe(III) and Nb(V)─available for possible reduction upon Na insertion─allowing for the formal introduction of 3 Na ions at reasonable potentials, and the robust NASICON structure with open channels for Na migration. The NFNP material has been successfully obtained by the solid-state route and fully characterized in terms of structure and transport properties by means of diffraction, XAS, and DFT analysis. Although promising, the electrochemical testing reveals that the initially satisfactory results in terms of capacity and Coulombic efficiencies fade upon cycling. The in-depth operando investigation, with the implementation of in situ XRD and XAS, unveiled a phase transition upon cycling; this involves the formation and accumulation of a low-symmetry secondary phase delivering lower capacity related to the Nb redox couples.

Semiconducting polymers are being explored for electrochemical and photoelectrochemical energy transformation and storage applications. For these applications, it is critical to understand how ion insertion from the electrolyte into polymer electrodes modulates the polymer electronic structure and electron doping levels. This study explores electrochemical cation insertion in the n-type conjugated redox polymer P90, composed of alternating naphthalene diimide (NDI) acceptor and bithiophene (T2) donor units, where the NDI units are functionalized with heptaethylene glycol (HEG, 90%) and 2-octyl dodecyl (OD, 10%) side chains. By combining in situ techniques (UV–vis absorption and Raman spectroscopies with electrochemistry), structural analysis using ex situ grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) calculations, we reveal that dications enable negative polaron and bipolaron formation in the P90 at less reducing potentials while supporting more bipolaron formation than the monocations; moreover, larger dications with smaller hydrated radii increase the maximum P90 electron doping level. We also determine that the monocations lead to more thermodynamically stabilized polarons compared with the dications. These findings highlight the critical role of cation identity in tuning electrochemical charging, charge stabilization, and electronic structure of n-type conjugated redox polymers, providing guidance on the rational design of polymer-based (photo)electrochemical applications.

Lead-free iodide double perovskites are an interesting class of materials since they combine a relatively low toxicity (compared to the lead counterpart) with the small band gap typical of iodide-based perovskite structures. Their reported number is small due to their lower structural stability compared to the chloride and bromide analogues; hence, their synthesis is difficult. The structural constraints that limit stability, on the other hand, can be much relieved in layered organic–inorganic perovskites. Following this line of thought, we report here a successful fast precipitation route to iodide layered (C_nH_(2n+1)NH₃)₄AgBiI₈ (n = 10, 12, and 14) double perovskites that borrow concepts from the synthesis of colloidal nanocrystals. X-ray diffraction studies revealed for these compounds a monoclinic crystal structure containing edge-sharing-alternating [AgI₆] and [BiI₆] octahedra. These materials have experimental band gaps of 2.1 eV, as also corroborated by theoretical calculations. We have also investigated their phase transitions by thermal analysis and temperature-dependent diffraction and found them to be similar to their lead-based layered perovskite counterparts.

MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides with the general formula of M_n+1X_nT_x (where M represents an early transition metal, X is C and/or N, and T_x is the surface functional groups), offer exceptional tailorability in structure, composition, and surface chemistry. Among them, nitrogen-containing MXenes have enhanced electronic and optical properties compared to their carbon analogs. Yet, challenges in synthesizing them have made nitride and carbonitride MXenes the least explored class. Herein, we report the synthesis of Ti₃Al(C_2–yN_y) MAX phases using a high-aluminum method to minimize oxygen impurities as well as other competing binary and ternary phases. Therein, subsequent etching and delamination of Ti₃Al(C_2–yN_y) MAX phases into Ti₃(C_2–yN_y)T_x MXenes were done using a coupled HF/HCl/LiCl method. Systematic variation of X-site chemistry (Ti₃C₂T_x, Ti₃C_1.75N_0.25T_x, Ti₃C_1.5N_0.5T_x, Ti₃C_1.25N_0.75T_x, and Ti₃CNT_x) enabled direct correlations between chemistry and optoelectronic properties. Increased nitrogen content leads to increased preference for halogen terminations, stronger light-matter interaction, blue-shifted optical absorbance, and decreased electrical conductivity. Despite these variations, the work function remains nearly constant across all compositions, indicating that it is primarily dictated by M and T_x chemistries. These findings demonstrate that solid-solution carbonitride MXenes provide a platform to independently control optical and electronic behaviors, offering opportunities for MXene-based optoelectronic and energy applications.

The ability to control the composition and defect density of zeolites is gaining attention based on their impact in a wide range of catalysis, adsorption, and separation processes. This study focuses on the use of demetalation as a strategic approach for defect engineering and tailoring acid strength wherein we venture beyond conventional aluminosilicate zeolites to assess the impact of heteroatom substitution. We selected three commercially relevant zeolite structures (CHA, MFI, beta) with distinct pore networks and prepared isostructures by replacing aluminum with boron, gallium, and zinc. Demetalation was carried out using either deionized water or mild acid treatment to effectively remove metals without using toxic chemicals, expensive reagents, or harsh conditions. A combination of experiments and density functional theory (DFT) calculations confirm the efficiency of demetalation follows the order B > Zn > Ga > Al, with beta exhibiting the highest degree of metal removal. The judicious selection of heteroatom leads to distinct defect sites and silicon-to-metal gradients in demetalated zeolite crystals, dictating the extent to which crystallinity is retained. DFT calculations reveal that boron removal is the only thermodynamically favorable process under both pH-neutral and acidic conditions, which allows for almost complete removal of metal from zeolites without appreciably compromising their structural integrity. Catalytic tests using methanol-to-hydrocarbons (MTH) as a benchmark reaction reveal several benefits of demetalation over one-pot syntheses. Dealumination of CHA leads to the introduction of mesopores, which enhances mass transport, leading to prolonged catalyst lifetime and increased cumulative methanol turnover. We also show that the same process for zeolite MFI tailors its acidity in ways that cannot be emulated by direct synthesis of catalysts with equivalent Si/Al composition. The collective findings from this study provide valuable insights into the efficiency and extent of demetalation for a broad class of heteroatom-substituted zeolites; and also highlight demetalation as an alternative to conventional techniques for effectively tuning the physicochemical properties of zeolites.

Intermetallic compounds are attractive candidates for hydrogen storage applications. This study investigates the thermodynamics of hydrogen absorption in binary AB₂ Laves phases with the C15 crystal structure. First-principles calculations, cluster expansion models, and statistical mechanics simulations are employed to determine the pressure–composition isotherms for two prototypical Laves phases: ZrMo₂ and ZrV₂. Our calculations show that ZrMo₂ accommodates hydrogen exclusively within A₂B₂ coordinated tetrahedral sites. In contrast, ZrV₂ accommodates hydrogen over both A₂B₂ and AB₃ coordinated tetrahedral sites. Finite-temperature simulations reveal that hydrogen atoms can occupy neighboring edge-sharing tetrahedra and are separated by a distance close to the Switendick criterion in ZrV₂. The occupation of both interstitial site types increases the hydrogen storage capacity of ZrV₂ as compared to ZrMo₂. Building on this insight, we perform a high-throughput search of binary C15 Laves phases and identify several promising candidates that can accommodate hydrogen across multiple interstitial sites. The results of this study provide chemical guidelines for tuning the hydrogen storage capacity of intermetallic compounds.

Solvents are central to metal–organic framework (MOF) solvothermal synthesis. However, how solvent–MOF interplay impacts MOF stabilization individually and across polymorphs is not well understood. To address this knowledge gap, here we perform data-driven analysis of 20,532 heats of adsorption at dilute conditions (ΔH_ads) and 447 free energies of solvation (ΔF_sol) for four solvents, namely, dimethylformamide (DMF), water (H₂O), methanol (MeOH), and n-hexane (C6) (which was used as a control). To accelerate data collection, we developed a protocol to extrapolate ΔF_sol from calculations with the MOFs only partially solvated. Free energies were obtained via thermodynamic integration. We found ΔF_sol and ΔH_ads to be only moderately correlated due to solvent–solvent interactions coming into play when the MOF is solvated. In any case, trends in ΔF_sol were ultimately explained on the basis of solvent kinetic diameter and polarity, as well as MOF void fraction (V_f) and functionalization polarity. For instance, the correlation between ΔF_sol and V_f was one of the strongest correlations presented in this study (more so as the solvent size increases), indicating that small-pore MOFs are more easily stabilized by solvation than large-pore MOFs. We also found that solvation-induced MOF stabilization became more pronounced as solvent kinetic diameter (polarity) decreased (increased). We found differences in this solvation-induced stabilization between polymorphs capable of overcoming inherent (i.e., in vacuum) differences in polymorph stability, causing the most stable polymorph to “switch.” We found the probability to cause “switches” to increase as solvent kinetic diameter (polarity) decreased (increased). Inspection of multivariate linear regression coefficients suggested that differences in solvation-induced stabilization in polymorphs can be primarily explained by their differences in density, V_f, and, to a lesser extent, volumetric surface area.

Operando synchrotron-based Fourier transform infrared (SR-μFTIR) microspectroscopy takes advantage of the high brilliance of synchrotron radiation to collect high-quality spectra with a superior signal-to-noise ratio (S/N) in subsecond timeframes. This technique achieves a spatial resolution finer than ten micrometers, enabling real-time operando mapping of electrode surfaces during battery operation, even under high C rate. The combination of both high temporal and spatial resolution makes (SR-μFTIR) a powerful tool for investigating dynamic electrochemical processes at the microscale. Operando SR-μFTIR microspectroscopy can be applied to the study of electrode materials, electrolytes, and electrode–electrolyte interfaces. It is especially valuable for the elucidation of reaction mechanisms taking place in noncrystalline and/or lightweight element-based electrodes, such as organic electrode materials. Herein, we show how a simple modification of the commercially available ECC-Opto-Std (ELCELL) cell allows unraveling the potential of operando SR-μFTIR microspectroscopy for investigating organic electrodes. This setup is applied to study the reaction mechanism of polyimide derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) in lithium, sodium, and calcium cells. During the charge/discharge process of the polyimide, a reversible change in the carbonyl bands intensities is observed with the concomitant appearance of two main new bands. Density functional theory calculations assign these bands to competing enolation/carbonylation processes with direct interactions between the aromatic ring and alkaline metal ions present in the electrolyte. Furthermore, the enhanced spectral resolution of synchrotron radiation provides a more detailed insight into the stepwise mechanism pathway in Na cells, as well as rate-dependent variations in the reaction mechanism.

The thermal evolution of LiNi_xMn_yCo_zO₂ (NMC) lithium-ion battery electrode materials is examined at various states of charge (SOC) or lithium concentrations for a variety of Ni:Mn:Co ratios or electrode compositions. Synchrotron X-ray diffraction (XRD) combined with Rietveld analysis shows the onset decomposition temperatures of phases, decomposition products, lattice parameters, and phase fractions as a function of composition and SOC. SOC impacts the lattice parameters of the NMC phase, where a collapse of the c-axis in the NMC phases is noted due to lithium extraction. Among the compositions examined, the low-Ni NMC111 uncycled sample (NMC111 0% SOC) exhibited the highest thermal stability, with a decomposition temperature approximately 250 °C higher than that of NMC532 0% and NMC811 0%. When the SOC exceeds 50% (i.e., more than 0.4 mol of Li ions extracted), the influence of Ni content on the decomposition temperature becomes negligible, with decomposition occurring around 250–300 °C for all compositions. Ni content also affects the decomposition pathways: NMC111 tends to first form a TM₃O₄-type phase, where TM represents transition metals, before transforming into a TMO-type phase, whereas most of the NMC811 samples directly decompose into the TMO phase. The presence of metallic phases was confirmed by both XRD and thermogravimetric-differential scanning calorimetry (TGA-DSC) analysis, as a result of heating under inert conditions. The TGA-DSC results suggest that metallic phase formation is favored at lower SOC in samples with a higher Ni content. This work provides comprehensive insight into the thermal degradation pathways of NMC materials as a function of composition, SOC, and temperature.