搜索结果：Journal of materials chemistry. A

Piezoelectric energy harvesting is a process in which energy in the form of kinetic movements can be harvested and converted into useful electrical energy using piezoelectric materials. Metal-organic frameworks (MOFs) have a huge potential for piezoelectric energy harvesting owing to their high flexibility, structural tunability, and very low dielectric constants due to their high porosity. The piezoelectric constant d relevant for piezoelectric energy harvesting depends on the piezoelectric constant e and the flexibility of the structure (i.e. mechanical properties). The mechanical properties of MOFs have previously been extensively studied but the piezoelectric constant e was never explored for MOFs. In this work, we generate a database of piezoelectric properties, specifically e for around ∼1608 previously synthesized non-centrosymmetric MOF structures. The calculations were performed using the density functional perturbation theory (DFPT) method. The highest piezoelectric constant e obtained in this work is approximately ∼2.76 C m-2, which is significantly higher than that of the flexible organic piezoelectric polymer polyvinylidene fluoride (PVDF) and its copolymers. In this work, we analyze and identify structural factors that influence the values of the piezoelectric constant for high-performing MOFs. Based on that, a series of guidelines for the design of MOF structures that can lead to a high piezoelectric constant e are presented. One class of high-performing piezoelectric MOFs is based on polar patterns of O-(short)-Mo-(long)-O unequal bond length, reminiscent of ferroelectric inorganic oxides. This class could have potential for ferroelectricity, meaning that the bond length pattern could be reversed by external electrical field. We substantiate this by showing experimentally via SHG-microscopy that the O-(short)-Mo-(long)-O unequal bond lengths are indeed malleable by external conditions.

The microporous nature of metal-organic frameworks (MOFs) often limits their capacity to incorporate large molecular guests, such as organometallic catalysts. In this work, we demonstrate a defect-engineering strategy for the Zr-based MOF UiO-66 to generate hierarchical pore structures capable of hosting the bulky Lehn-type complex [Re(bpy-4-COOH)(CO)3Cl]. By introducing missing-linker and missing-cluster defects-both during synthesis and through a selective ligand removal (SeLiRe) process-we modulate the framework's pore structure and volume. Using a post-synthetic modification approach, 2,2'-bipyridine-4-carboxylic acid (bpy-4-COOH) is anchored into the MOF structure via solvent-assisted ligand incorporation, followed by complexation with [Re(CO)5Cl]. A comprehensive suite of characterization techniques including TGA, Ar-physisorption, STEM-EDX, solid-state NMR, XAS and other spectroscopic methods confirmed the formation and uniform distribution of the Re-complex within the MOF porosity. Our results show that the introduced defects and the associated creation of mesoporosity are essential for successful incorporation of the large Re-complex, while nearly defect-free UiO-66 cannot be modified with the ligand post-synthetically. The use of the SeLiRe process enables us to gain reasonable control over the amount of the Re-complex inside the MOF and leads to a homogeneous distribution throughout the particles. Photocatalytic CO2RR experiments show CO as the main product with high selectivity when using TEOA as a sacrificial agent. This work demonstrates the potential of engineering hierarchical porosity in MOFs for immobilizing large, catalytically active molecular species in a stable and well-defined environment.

Battery research often encounters the challenge of determining chemical information, such as composition and elemental oxidation states, of a layer buried within a cell stack in a non-destructive manner. Spectroscopic techniques based on X-ray emission or absorption are well-suited and commonly employed to reveal this information. However, the attenuation of X-rays as they travel through matter creates a challenge when trying to analyze layers buried at depths exceeding hundred micrometers from the sample's surface. In the context of battery research, the limited escape depth of X-rays often necessitates the design of experiment-specific cells with thinner inner layers, despite the risk that these tailored cells may not exactly replicate the cycling behavior of larger commercial cells. Muon-induced X-ray emission (MIXE) is a non-destructive spectroscopic technique that involves implanting negative muons into a sample and detecting the highly energetic muonic X-rays generated when these muons are captured by the sample's atoms. By virtue of the high energy of muonic X-rays, the depth of analysis of MIXE greatly exceeds that of other X-ray based techniques. In this article, we introduce the technique and lay the groundwork for employing MIXE in future in situ/operando analyses of batteries. We demonstrate that MIXE can detect nearly all elements, including low atomic number ones such as Li. Additionally, we establish the quantitative nature of MIXE through the precise determination of LiNi x Mn y Co1-x-y O2 (NMC) electrode stoichiometries. Finally, we demonstrate that MIXE enables the acquisition of depth-resolved chemical information from a 700 μm thick cell, in good agreement with simulation results.

A current barrier to the practical applications of metal-organic frameworks (MOFs) is the vast quantity of organic solvents required for their preparation under dilute solvothermal conditions. Herein, we report the rapid, ambient-temperature, and high-concentration (up to 1.0 M) aqueous syntheses of three families of salicylate-based MOFs: M2(dobdc) (M = Mg, Co, Ni, Zn; dobdc4- = 2,5-dioxido-1,4-terephthalate), M2(dobpdc) (M = Mg, Co, Ni, Zn; dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), and M2(m-dobdc) (M = Mg, Co, Ni; m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate). High-concentration MOF formation is accomplished by incorporating NaOH to deprotonate the linker molecules in situ, generally avoiding the crystallization of phases with partially protonated linkers favored under high-concentration solvothermal conditions. 77 K N2 surface area measurements confirm that the MOFs (especially Zn-based frameworks) demonstrate comparable or enhanced surface areas relative to traditionally prepared materials. Furthermore, this method enables the first synthesis of Zn2(m-dobdc), which does not form under standard solvothermal conditions. This material exhibits higher CO2 uptake and ideal adsorbed solution theory (IAST) CO2/N2 selectivities compared to the canonical framework Zn2(dobdc), highlighting the utility of aqueous high-concentration methods to facilitate the discovery of porous materials with improved gas sorption properties. Overall, our findings offer a practical and general alternative to dilute solvothermal syntheses of salicylate-based MOFs, paving the way for their production and implementation in industrial settings.

Numerous challenges exist in fully understanding current lithium-ion battery (LIB) technology and commercializing "beyond LIBs" which could help support reaching net-zero carbon emissions in the future. These highly complex systems undergo many dynamic processes at different time and length scales, including ion conduction, interphase formation, and degradation, that can be challenging to capture with traditional characterization tools. As a result, scanning probe microscopy (SPM) has become an invaluable platform for enhancing the understanding of these complex and important processes. SPM can be used to obtain topographical, mechanical, electrical, and electrochemical information on a wide range of materials in a variety of environments, including in situ and operando studies. In this perspective, we briefly describe the operating principles of LIBs and a number of relevant SPM techniques, followed by presenting recent highlights of SPM's unique capabilities as a characterization tool for battery systems. Finally, we offer recommendations for the improvement of SPM studies of battery materials as well as future outlooks.

The production of green hydrogen via photoelectrochemical water splitting has the potential to play a vital part in the decarbonization of our energy economy. To commercialize this technology, the stability of the current photoelectrode materials needs to be improved. Therefore, it is essential to understand the processes causing degradation and define suitable figures of merit. This work investigates the degradation mechanisms of oxynitride electrodes focusing on TiO2 necked LaTiO2N particle-based photoanodes. Their degradation behaviour was assessed by chronoamperometries at 1.23 V vs. RHE in basic electrolyte. We identified two current decay processes based on a semi-empirical correlation between the measured chronoamperometries and a sum of two exponential decay terms. The time constant of the second exponential function is proposed as an alternative figure of merit for quantifying the stability of oxynitride based photoanodes. The applicability of this figure of merit to a wider range of oxynitride-based photoanodes and measurement conditions is demonstrated by evaluating previously reported chronoamperometries. By in depth analysis before and after chronoamperometry using STEM-EDX/EELS, HREM, ICP-MS, and XPS, we find experimental evidence that the performance decrease of the LaTiO2N photoanodes is caused by a combination of surface oxidation and cocatalyst dissolution.

Covalent organic frameworks (COFs) have developed as efficient and selective adsorbents to mitigate 99TcO4 - contamination. However, the eco-friendly and scalable production of COF-based adsorbents for the removal of 99TcO4 - has not yet been reported. This study explores the potential of a cationic COF (TpDB-COF) synthesized via a green hydrothermal method, achieving gram-scale yields per batch, thereby addressing a significant limitation of existing COF production methods. The TpDB-COF demonstrates an exceptional stability in strongly acidic conditions (2 weeks in 3 M HNO3), as well as in various organic solvents, making it suitable for harsh nuclear waste environments. Adsorption experiments using ReO4 - as a surrogate for 99TcO4 - show rapid adsorption kinetics, reaching nearly 100% removal efficiency within 1 min (with initial concentration of 28 ppm at a solid-to-liquid ratio of 1 g L-1), a maximum adsorption capacity of 570 mg g-1 and excellent stability. Moreover, the COF maintains high selectivity for ReO4 - even in the presence of competing anions such as SO4 2- and NO3 -. These findings highlight that the hydrothermal synthesis is an effective method to synthesize COF adsorbents for efficient removal of 99TcO4 - and offers a sustainable approach for practical applications.

The zeolitic imidazole framework-8 (ZIF-8) is a crystalline porous material that has been widely employed as template to fabricate porous nitrogen-doped carbons with high microporosity via thermal treatment at high temperatures. The properties of the carbon scaffold are influenced by the pore structure and chemical composition of the parent ZIF. However, the narrow pore size distribution and microporous nature from ZIF-8 often results in low mesopore volume, which is crucial for applications such as energy storage and conversion. Here we show that insertion of N-heterocyclic amines can disrupt the structure of ZIF-8 and dramatically impact the chemical composition and pore structure of the nitrogen-doped carbon frameworks obtained after high-temperature pyrolysis. Melamine and 2,4,6-triaminopyrimidine were chosen to modify the ZIF-8 structure owing to their capability to both coordinate metal ions and establish supramolecular interactions. Employing a wide variety of physical characterization techniques we observed that melamine results in the formation of a mixed-phase material comprising ZIF-8, Zn(Ac)6(Mel)2 and crystallized melamine, while 2,4,6-triaminopyrimidine induces the formation of defects, altering the pore structure. Furthermore, the absence of heterocyclic amine in the ZIF-8 synthesis leads to a new crystalline phase, unreported to date. The thermal conversion of the modified ZIFs at 1000 °C leads to nitrogen-doped carbons bearing Zn moieties with increased surface area, mesopore volume and varying degree of defects compared to ZIF-8 derived carbon. This work therefore highlights both the versatility of heterocyclic amines to modify the structure of framework materials as well as their role in tuning pore structure in nitrogen-doped carbons, paving the way to targeted design of high-performance electrodes for energy storage and conversion.

Na-ion batteries are sustainable, low-cost alternatives to Li-ion batteries. However, their limited energy density has hindered a widespread adoption. Among positive electrode materials, polyanionic compounds approaching the performances of LiFePO4 are being investigated. The Na3V2(PO4)2F3 family of phosphate fluorides in particular has demonstrated sufficient specific capacity at high operating voltage. Combined with remarkable capacity retention and power capabilities, it entered applications in power tools. However significant concerns exist about the availability of vanadium. To find alternatives, we explored the substitution of V with other transition metals. We considered Ti, Cr, Mn, Fe, Co, Ni, Mo, Zr and Nb using first-principles calculations based on density functional theory with the r2SCAN functional. For all compounds, we investigated in detail the expected operational voltage, as well as the structural characteristics and Na+ mobility via nudged-elastic band calculations (NEB). Most metals yield too high voltages for operation within the stability window of common electrolytes, with the notable exceptions of Mn and Mo that show promising voltages over the reversible (de)intercalation of 3 Na/f.u. In all cases, the electrochemical operation is found to occur with small volume change (maximum 6% for Mn) and the computed migration barriers remain similar to vanadium's ones. Finally, we propose potential synthesis reactions for all compounds and calculate their Gibbs free energy. The never-before reported Co-, Mn- and Mo-based compounds are predicted to be synthesizable. Our work suggests the existence of novel promising positive electrode materials for Na-ion batteries, and it suggests potential synthetic routes to experimentally achieve them.

Layered mixed-anion oxides are considered potential candidates for thermoelectric materials because they typically possess the advantages of oxides (high-temperature stability, low toxicity, and the use of cost-effective elements) and layered mixed-anion compounds (strong phonon anharmonicity and bonding heterogeneity). In this paper, we predicted the thermoelectric performance of environmentally friendly layered mixed-anion oxides Bi2MO4Cl (M = Y, La, and Bi) using density functional theory (DFT) calculations. The results show that Bi3O4Cl and Bi2LaO4Cl exhibit ultra-low average lattice thermal conductivities of less than 0.3 W m-1 K-1 at 1000 K, which are attributed to the combined effects of heavy atoms, weak ionic bonding, strong phonon anharmonicity, and low structural symmetry. In addition, the weak ionic bonding significantly inhibits out-of-plane heat transfer, resulting in the lattice thermal conductivity in the out-of-plane direction being the lowest compared to other directions. As a result, under dopable conditions, the predicted p-type maximum average ZT of Bi3O4Cl reaches 2.20 at 1000 K, which is superior to the thermoelectric performance of currently known environmentally friendly thermoelectric materials, and the predicted p-type maximum ZT of Bi2LaO4Cl is over 4 in the out-of-plane direction. These results illustrate the potential for the excellent thermoelectric performance of Bi2MO4Cl (M = Y, La, and Bi), and also highlight the application potential of layered mixed-anion compounds in achieving low lattice thermal conductivity and enhancing thermoelectric performance.

Solid-state oxygen ion batteries (OIBs) are a novel technology for electrochemical energy storage, based on the exchange of oxygen between two mixed conducting oxide electrodes via an oxide ion-conducting electrolyte. Suitable electrode materials not only require good ionic and electronic conductivity, but also a highly variable oxygen non-stoichiometry δ to chemically store large amounts of charge. Another desirable characteristic for anodes is good material stability down to very reducing oxygen chemical potentials. This work focuses on the exploration of La0.5Sr0.5Cr0.2Mn0.8O3-δ and its electrochemical and defect chemical properties, with particular focus on its applicability in anodes of oxygen ion batteries. Thin film model cells were prepared by pulsed laser deposition (PLD) of electrodes on 100-oriented Y:ZrO2 single crystals. These planar half-cells were sealed with ZrO2 and glass to inhibit oxygen exchange with the atmosphere. Electrode capacities of up to 930 mAh cm-3 were achieved and confirmed to be stable over more than 70 cycles at 400 °C between -0.07 V and -2.07 V vs. 1 bar O2. Charge/discharge curves revealed the existence of two plateaus at -0.8 V and -1.4 V. Further, electrochemical impedance measurements on samples with microelectrodes were employed to study the chemical capacitance C chem, oxygen diffusion coefficient, and ionic resistivity of La0.5Sr0.5Cr0.2Mn0.8O3-δ over the same range of potentials. High resolution C chem vs. oxygen chemical potential measurements revealed two clearly separated peaks, indicating two separate redox processes, which correspond to the two distinct plateaus found in the charge/discharge curve. A defect chemical model (Brouwer diagram) was developed, based on a two stage transition: Mn4+ → Mn3+ → Mn2+. The model can quantitatively explain the location of both peaks in the chemical capacitance curve and the corresponding plateaus of the charge/discharge curve. Furthermore, X-ray photoelectron spectroscopic measurements of the Mn3+ → Mn2+ transition fully confirmed this model. Altogether, this study showed that La0.5Sr0.5Cr0.2Mn0.8O3-δ is a highly promising anode material for oxygen ion batteries operating at high voltages.

Here, we investigated the separate and combined effects of guanidinium (Gua+) and/or thiocyanate (SCN-) ions on the opto-electronic properties of mixed Sn-Pb perovskites, which are used as absorber layers in photovoltaics. Therefore, we spin-coated Cs0.25FA0.75Sn0.5Pb0.5I3 thin films with GuaI, Pb(SCN)2 or GuaSCN, in the absence or presence of SnF2. By comparing the (micro)structural and opto-electronic properties of the perovskite films, we elucidated the functions of both ions. We found that SCN- suppresses tin oxidation and doping, reduces crystal defects and improves the carrier transport properties, regardless of SnF2 addition. We demonstrate that this is due to coordination with Sn2+ and scavenging of Sn4+ in the spin-coating solution, resulting in a pile-up of SnO x at the film surface. Gua+ is incorporated to a limited extent into the 3D cubic perovskite structure. Gua+ cannot suppress tin oxidation and doping, making this additive useless without SnF2. Conversely, when combined with SnF2, Gua+ enhances the carrier transport properties. Combining Gua+ and SCN- until a maximum addition of 4 mol% and SnF2 results in large grains and pinhole-free films with superior charge carrier transport properties, leading to a substantial increase in the pseudo-open circuit voltage of 50 mV. Addition of >4 mol% GuaSCN leads to the formation of Gua-based 2D perovskites, including Gua x FA2-x Sn y Pb1-y I4 and Gua x FA3-x Sn y Pb2-y I7, which do not improve the carrier dynamics. In short, we observe a synergistic effect on addition of Gua+ and SCN- ions, which leads to improved structural and opto-electronic properties, which is promising for implementation in solar cells.

This work reports the thermal and electron beam stabilities of a series of isostructural metal-organic frameworks (MOFs) of type MFM-300(M) (M = Al, Ga, In, Cr). MFM-300(Cr) was most stable under the electron beam, having an unusually high critical electron fluence of 1111 e- Å-2 while the Group 13 element MOFs were found to be less stable. Within Group 13, MFM-300(Al) had the highest critical electron fluence of 330 e- Å-2, compared to 189 e- Å-2 and 147 e- Å-2 for the Ga and In MOFs, respectively. For all four MOFs, electron beam-induced structural degradation was independent of crystal size and was highly anisotropic, although both the length and width of the channels decreased during electron beam irradiation. Notably, MFM-300(Cr) was found to retain crystallinity while shrinking up to 10%. Thermal stability was studied using in situ synchrotron X-ray diffraction at elevated temperature, which revealed critical temperatures for crystal degradation to be 605, 570, 490 and 480 °C for Al, Cr, Ga, and In, respectively. The pore channel diameters contracted by ≈0.5% on desorption of solvent species, but thermal degradation at higher temperatures was isotropic. The observed electron stabilities were found to scale with the relative inertness of the cations and correlate well to the measured lifetime of the materials when used as photocatalysts.

Sodium-based batteries are gaining increasing attention due to the abundant availability of sodium, among these, sodium-metal batteries (SMBs) are a promising solution. However, the development of suitable electrode materials for SMBs remains a significant challenge. Organic materials such as two-dimensional covalent organic frameworks (2D COFs) have emerged as promising electrodes due to their vast versatility, although their sodium storage mechanism remains poorly understood. In this work, the sodium storage mechanism of a β-ketoenamine anthraquinone-based COF (DAAQ-TFP), employed as a cathode for SMBs, is unveiled through a combination of electrochemical, physicochemical, and computational studies. In contrast with previous studies suggesting a capacitive storage mechanism, our results reveal a combination of pseudocapacitive and faradaic processes. Molecular dynamic simulations combined with ex situ X-ray diffraction studies confirmed that sodium storage occurs via interaction with carbonyl groups located within the COF channels rather than through intercalation between the COF layers. Moreover, electrode calendering experiments demonstrate that the faradaic contribution is governed by the porous COF structure. Finally, the redox inactivity of the carbonyl groups of the β-keto units is demonstrated through both computational and electrochemical measurements. These results further reinforce the role of anthraquinone units as the sole active sites responsible for sodium storage in this type of organic electrode materials.

Photocatalytic nanomaterials combining organic dyes and inorganic semiconductor nanoparticles (NPs) are extensively investigated for light-driven production of solar fuels and for conversion of organic feedstocks. However, their applications for the valorization of abundant raw materials by exploiting low-energy visible light remain limited. In this study, we report a facile preparation of TiO2 nanoparticles sensitized with a quinacridone (QA) industrial pigment for the aqueous oxidation of glycerol to glyceraldehyde with red light (λ = 620 nm), reaching 47.5 ± 5.0 μmol gNP -1 h-1 of productivity and 80% selectivity in the presence of TEMPO co-catalyst. The hybrid material outperforms the single components and shows recyclability up to at least 5 additional times under red light while maintaining intact productivity; furthermore, it demonstrates versatility by operating also under green, yellow or white light irradiation. We believe that this work will provide a new avenue for using industrial pigment-sensitized materials in photocatalysis exploiting low energy light, providing novel strategies for the future development of this field.

Metal-organic frameworks (MOFs) are porous, crystalline materials with high surface area, adjustable porosity, and structural tunability, making them ideal for diverse applications. However, traditional experimental and computational methods have limited scalability and interpretability, hindering effective exploration of MOF structure-property relationships. To address these challenges, we introduce, for the first time, a category-specific topological learning (CSTL), which combines algebraic topology with chemical insights for robust property prediction. The model represents MOF structures as simplicial complexes and incorporates elemental categorizations to enable balanced, interpretable machine learning study. By integrating category-specific persistent homology, CSTL captures both global and local structural characteristics, rendering multi-dimensional, category-specific descriptors that support a predictive model with high accuracy and robustness across eight MOF datasets, outperforming all previous results. This alignment of topological and chemical features enhances the predictive power and interpretability of CSTL, advancing understanding of structure-property relationships of MOFs and promoting efficient material discovery.

Solid-state NMR spectroscopy, when combined with first-principles density functional theory (DFT) calculations, offers a highly sensitive probe of atomic-scale structure and dynamics in solid-state ion conductors, enabling the characterisation of subtle features that govern ionic conductivity. However, current approaches for interpreting NMR spectra rely on a comparison with static DFT reference calculations, which are inadequate for materials exhibiting fast ion dynamics such as lithium battery solid electrolytes. Here, using room-temperature NMR measurements and first-principles calculations, we show that the standard static-structure approach fails to reproduce the experimental 35Cl isotropic chemical shift (δ iso) of the fast Li-ion conductor Li6PS5Cl and substantially overestimates the quadrupolar coupling constant (C Q). We show that this discrepancy can be resolved using only ten DFT calculations by sampling relaxed configurations representative of Li-ion diffusion from machine-learning molecular dynamics. Compared with vibrational motion, Li-ion hopping around Cl is shown to dominate the motional averaging through reorientation of the NMR tensors. This study therefore provides an efficient computational method to resolve the complexities of the NMR spectra of Li6PS5Cl, which can be widely applied to other ion-conducting solids.

Probing the local structure and chemistry of wide-bandgap amorphous oxide thin films remains challenging due to the limitations of lab-based spectroscopy. This work integrates X-ray photoelectron spectroscopy (XPS), hard X-ray photoemission spectroscopy (HAXPES), molecular dynamics simulations using machine-learning interatomic potentials, density-functional theory (DFT) calculations, and classical electrostatic modeling of final-state core-ionization effects in Al atoms to uncover the structure and chemistry of amorphous alumina polymorphs made with atomic layer deposition (ALD). DFT calculations using the ΔKohn-Sham method supported the interpretation of final-state effects and validated electrostatic model assumptions. Shifts in the measured Auger parameters were interpreted as extra-atomic relaxation energies, revealing sensitivity to the local coordination environment. Structural disorder and thermal fluctuations were found to govern the distribution of extra-atomic relaxation energies, suggesting that cryo-XPS can isolate and reveal intrinsic structural building blocks of amorphous oxides. Simulated heating and annealing demonstrated that Auger parameter shifts can serve as indicators of phase decomposition in H-supersaturated ALD amorphous alumina. These findings provide a pathway for comprehensive interpretation and predictive modeling of XPS spectra in amorphous wide-bandgap oxides.

Experimental screening of Metal Organic Frameworks (MOFs) for separation applications can be costly and time-consuming. Computational methods can provide many benefits in this process, as expensive compounds and a wide range of operating conditions can be tested while crucial mechanistic insights are gained. TAMOF-1, a recently developed MOF, stands out for its exceptional stability, robustness and cost-effective synthesis. Its good CO2 uptake capacity makes it a promising agent for flue gas separation applications. In this work, we combine experiments with simulations at the atomistic and numerical level to investigate the adsorption and separation of CO2 and N2. Using Monte Carlo simulations, we accurately reproduce experimental adsorption isotherms and elucidate the adsorption mechanisms. TAMOF-1 effectively separates CO2 from N2 because of preferential binding sites near Cu2+ atoms. To assess separation performance in equilibrium at different conditions along the entire isotherm pressure range, adsorbed mole fractions, selectivities, and the trade-off between selectivity and uptake (TSN) are calculated. The dynamic separation performance is assessed by breakthrough experiments and numerical simulations, demonstrating efficient dynamic separation of CO2 and N2, with CO2 being retained in the column.