Coordination Shell Research Articles

Molecular Dynamics Simulations of Binary Phosphate Glass Using the ReaxFF Potential.

Due to the importance of the understanding of dissolution behavior of phosphate-based bioglasses (PBGs) in different biomedical applications, binary sodium and calcium phosphate glasses have been simulated for the first time using a newly developed ReaxFF force field and a standard melt-quench method with the LAMMPS classical molecular dynamics software. The partial radial distribution function of P-O within the first coordination shell indicated two distinct peaks corresponding to phosphorus bonding to NBO and BO, respectively, at distances consistent with those observed experimentally and a P-O coordination number of 4.0. Angular distribution functions were consistent with the experimental data. The calculated network connectivities are in good agreement with experimental data, and the detailed Qn distributions are broader than would be expected.

Modulating the Local Coordination Environment of M‐Nx Single‐Atom Site for Enhanced Electrocatalytic Oxygen Reduction

AbstractEfficient, durable, and economical oxygen reduction catalysts are key for practical applications such as fuel cells and metal–air batteries. Single atom catalysts (SACs) have attracted sustained and widespread attention owing to their unique electronic properties and exceptional atomic utilization, positioning them as promising electrocatalysts in energy conversion and storage. However, the symmetric charge distribution of the metal site in the symmetric M‐N4 configuration of SACs is not conducive to electron transfer and transport in electrocatalytic reactions, resulting in a low adsorption energy for oxygen reduction reaction (ORR) related species (*OH, *O, and *OOH), which severely limits the intrinsic activity of the electrocatalysts. To overcome this limitation and improve the activity and durability, heteroatom doping can effectively modulate the local coordination environment (LCE) of the metal atom, including the coordinating atoms, coordination shells and coordination number. These modifications have significantly improved the performance of carbon supported SACs with M‐N4 configuration in the ORR. Based on this, a thorough summary of the major progress made in recent years in adjusting the LCE of SACs through doping with heteroatoms is provided and a perspective on the future development is offered here.

Pectin alignment induced changes in ion solvation structure in ethylene carbonate-based liquid electrolytes

Classical molecular dynamics simulations are used to explore the impact of the alignment of pectin over the randomized configuration on ion solvation structure in pectin-loaded ethylene carbonate - lithium bis (trifluoromethanesulfonyl) imide electrolytes. Our study focuses on how biological macromolecules, specifically pectin, influence the behavior of liquid electrolytes, considering their applications in rechargeable batteries due to their ion solvation capabilities and ion transport characteristics. Aligned pectin causes a tightly packed first coordination shell of anions around lithium ions by weakening the long-ranged interactions beyond the first coordination shell compared to a random configuration. Consequently, the number of pectin oxygens around lithium decreases dramatically from 3 to 2, resulting in an overall diluted solvation shell containing fewer numbers of anions and pectin oxygens around lithium ions. With polymer alignment, the non-Gaussianity increases from 3.387 to 6.550 for lithium ions and from 0.475 to 0.621 for TFSI ions, reflecting a 90 % increase in dynamic heterogeneity for lithium ions and a 30 % increase for TFSI ions. Cation-cation correlations enhance ionic conductivity in randomized pectin, whereas isolated anion motion dominates in aligned pectin due to cation-pectin interactions. Our work not only highlights potential strategies for improving electrolyte performance in rechargeable batteries but also emphasizes the crucial role of molecular orientation in optimizing electrolyte properties, paving the way for more optimized and efficient battery technologies.

Advancing understanding of composition–structure–luminescent properties in laser glass through machine learnings

AbstractRare‐earth (RE)‐doped laser glasses meet urgent needs in national security and scientific fields, and their optimization has garnered extensive attention. However, the design of these laser glasses often relies excessively on trial‐and‐error experimentation, leading to significant costs and a lack of scientific guidance. Herein, we propose an integrated method that combines structural descriptors determined from molecular dynamics simulations, a self‐constructed luminescent database, and a machine learning algorithm to establish the composition–structure–luminescent property (CSLP) relationship. Using an Nd3+‐doped commercial silicate laser glass system as an example, the effectiveness of this approach has been demonstrated. The developed CSLP model enables highly accurate predictions of spectral properties, achieving a determination coefficient (R2) greater than 0.94, based on eight structural descriptors. The importance of different structural descriptors on spectral characteristics is ranked and thoroughly discussed, revealing an intrinsic relationship between the first and second coordination shells around RE ions and luminescent behaviors. Furthermore, the generic structural descriptors identified in the CSLP model can be extrapolated to other systems involving different network formers (e.g., silicate and phosphate) and modifier cations (e.g., Li, Na, K, Ba, and Ca). This capability facilitates the design of laser glasses tailored to specific targets, such as large emission cross‐sections, extended lifetimes, or reduced quenching effects.

Sorption of trivalent europium Eu(III) by Na+-substituted bentonite: Mechanistic insight and stabilization effect

The sorption of Eu(III) by Na+-substituted bentonite (Na-bentonite) was investigated as a function of pH and NaNO3 concentration ([NaNO3]0). At pH < ∼7.5, Eu(III) sorption decreased with the increasing [NaNO3]0, whereas at pH > ∼7.5, it remained nearly complete, independent of [NaNO3]0. Our thermodynamic model indicated that the sorption at pH < ∼7.5 occurred via a combination of cation exchange and surface complexation, with the former diminishing as [NaNO3]0 increased. Meanwhile, the sorption at pH > ∼7.5 was primarily due to surface complexation. By X-ray diffraction, the incorporation of hydrated Eu(III) in the interlayers of montmorillonite increased its lattice spacing and crystallinity along the c-axis, where cation exchange was predominant. Also, Eu LIII-edge X-ray absorption spectroscopy revealed that the sample dominated by cation exchange had an absorption edge energy and coordination structure similar to aqueous Eu(III), indicating outersphere complex formation. In other cases, Eu(III) sorption was characterized by innersphere complexation, as indicated by increased covalency and the presence of more pronounced second coordination shells. Importantly, the analysis of dissolved Si and Al suggested that the increased stability of Na-bentonite was likely due to the surface complexation of Eu(III) with aluminol groups at the edges, especially at higher surface coverages. Given its high sorption capacity for Eu(III) and stabilization effect mediated by sorption, Na-bentonite could be serve as an effective backfill material and migration barrier for containing actinides in nuclear waste repositories.

The effect of water molecule configurations on the type of core ionized site: O1s core ionization of ascorbic acid in water

The analysis of photoelectron spectra can be a complex and uncertain process, primarily due to the overlap of the peaks in the spectra. This overlap can lead to inaccuracies in the reported ionization energies derived from the binding energy positions of the visible peaks. This constraint explains the absence or inaccurate reporting of ionization energy values for certain biomolecules in the articles. Reliable theoretical methods are required for the interpretation of experimental results. In the present work, the overlap of photoelectron spectrum peaks is of particular importance, given the similarity between the molecular oxygen signatures of ascorbic acid (AA) and water. We used the SAC-CI method, based on the ONIOM model, to examine how the first coordination shell of water molecules affected the O1s core ionization energy of AA at 298 and 350 K temperatures. Our investigation shows that water’s photoelectron spectrum covers the O1s spectrum of AA.

Atomic-Level Structure of the Organic-Inorganic Interface of Colloidal ZnO Nanoplatelets from Dynamic Nuclear Polarization-Enhanced NMR.

Colloidal semiconductor nanoplatelets (NPLs) have emerged as a new class of nanomaterials that can exhibit substantially distinct optical properties compared to those of isotropic quantum dots, which makes them prime candidates for new-generation optoelectronic devices. Insights into the structure and anisotropic growth of NPLs can offer a blueprint for their controlled fabrication. Here, we present an atomic-level investigation of the organic-inorganic interface structure in ultrathin and stable benzamidine (bza)-supported ZnO NPLs prepared by the modified one-pot self-supporting organometallic approach. High-resolution transmission electron microscopy analysis showed a well-faceted hexagonal shape of ZnO NPLs with lateral surfaces terminated by nonpolar (101̅0) facets. The basal surfaces are flat and well-formed on one side and corrugated on the other side, which indicates that the layer-by-layer growth in the thickness of the NPLs likely occurs only in one direction via the expansion of 2D islands on the surface. The ligand coordination modes were elucidated using state-of-the-art dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy supported by density functional theory chemical shift calculations. Specifically, it was found that (101̅0) nonpolar facets are stabilized by neutral L-type bza-H ligands with hydrogen bond-supported η1-coordination mode, while polar (0001) and (0001̅) facets are covered by μ2-coordinated X-type anionic bza ligands with different conformations of aromatic rings. Moreover, the ligand packing on (101̅0) lateral facets was determined using 13C natural abundance (∼1.1%) homonuclear dipolar correlation experiments. Overall, an in-depth understanding of the growth mechanism and the unique bimodal X-type/L-type ligand coordination shell of ZnO NPLs is provided, which will facilitate further design of anisotropic nano-objects.

Insight into the complexation of uranium with Bacillus sp. dwc-2 by multi-spectroscopic approaches: FT-IR, TRLF and XAFS spectroscopies

The micromechanisms of the interactions between uranium and microorganisms, especially chemical bonds and bonding numbers between uranium and microbe are not yet clear. In this work, uranium coordination with functional groups in/on Bacillus sp. dwc-2 has been rationally explored by X-ray absorption fine structure (XAFS), Fourier transform infrared (FT-IR), time-resolved laser-induced fluorescence (TRLF) spectroscopies. The results indicate that under acidic condition, 66.8 % of O atoms in coordination shell of U(VI) equatorial plane are provided by organic phosphoryl groups via monodentate coordination, 18.8 % and 15.5 % are provided by water molecules and carboxyl groups via monodentate and bidentate coordination, respectively. While under alkaline condition, these proportions change to 53.1 %, 34.4 % and 22.0 %, respectively. Under neutral condition, U(VI) is mainly deposited by complexing with inorganic phosphates released by microbial cells. Under anaerobic condition, partial U(VI) can be reduced to U(IV) which primarily complex with phosphorus containing groups via monodentate and bidentate coordination, existing as amorphous solids or coordination polymers. This is the first exploration of the binding mechanism between uranium and microorganisms at a semi-quantitative level under different conditions. Our studies will further enhance the contribute to the understanding of coordination mechanism of uranium with microorganism, and demonstrate the potential bioremediation value of this bacterium.

1H, 13C, 15N NMR, and DFT Studies on Complex Formation of Zinc(II) Ion with Ethylenediamine in Ionic Liquid [C2mIm][TFSA

In bis(trifluoromethylsulfonyl)amide (TFSA-)-based ionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA- ([C2mIm][TFSA]), the complex formation equilibria of zinc(II) ion (Zn2+) with ethylenediamine (EN) have been investigated. An EN molecule may coordinate with Zn2+ as a bidentate ligand. First, the formation of Zn2+-EN complexes in [C2mIm][TFSA] was confirmed from the difference of 1H and 13C NMR chemical shift values of EN molecules between [C2mIm][TFSA]-EN binary solvents and the 0.1 mol dm-3 Zn(TFSA)2/[C2mIm][TFSA]-EN solutions as a function of EN mole fraction xEN. Second, the stability constants of Zn2+-EN complexes formed in the IL were determined from the concentration ratio [EN]/[Zn2+] dependence of 15N NMR chemical shift values of the TFSA- N atom in the Zn2+/IL-EN solutions. In the IL, mono-, bis-, and tris-EN complexes are successively formed by 1:1 replacement of TFSA- anions coordinated with Zn2+ by EN molecules with increasing EN content. Third, 1H and 13C NMR measurements with the help of density functional theory (DFT) calculations were made on [C2mIm][TFSA]-EN binary solvents as a function of xEN to clarify key interactions to the mechanism of the complex formation. Fourth, the stability constants of Zn2+-EN complexes in the IL were compared with those in aqueous solutions. It was suggested that the hydrogen bonding of the EN molecule with the imidazolium ring H atoms and the TFSA- O atoms reduces the stability of the mono-EN complex in the IL. In contrast, the intracomplex hydrogen bonds between EN and TFSA- in the first coordination shell contribute to the higher stability of the bis-EN complex in the IL than that in aqueous solutions. The difference in the stability constants between the tris-EN complexes and hexaacetonitrile complexes, where acetonitrile (AN) molecules act as monodentate ligands, was interpreted in terms of the higher electron donicity of EN. Finally, to verify the present evaluation, the experimental 13C NMR chemical shift values of EN molecules in the solutions were compared with the theoretical values calculated by DFT using the stability constants determined.

Optimizing the Intermediates Adsorption by Manipulating the Second Coordination Shell of Ir Single Atoms for Efficient Water Oxidation

AbstractThe precise regulation of single‐atom catalysts (SACs) with the desired local chemical environment is vital to elucidate the relationship between the SACs structure and the catalytic performance. The debate on the effect of the local coordination environment is quite complicated even for the SACs with the same composition and chemical nature, calling for increased attention on the regulation of the second coordination shell. For oxide‐supported SACs, it remains a significant challenge to precisely manipulate the second coordination shell of single atoms supported on oxides due to the structural robustness of oxides. Here, Ir single atoms were anchored on NiO supports via different bonding strategies, resulting in the diverse Ir−O−Ni coordination numbers for Ir sites. Specifically, Ir1/NiO, Ir1−NiO, and Ir1@NiO SACs with increasing Ir−O−Ni coordination numbers of 3, 4, and 5 were synthesized, respectively. We found that the activity of the three samples towards oxygen evolution reaction (OER) exhibited a volcano‐shaped relationship with the Ir−O−Ni coordination number, with Ir1−NiO showing the lowest overpotential of 225 mV at 10 mA cm−2. Mechanism investigations indicate that the moderate coordination number of Ir−O−Ni in Ir1−NiO creates the higher occupied Ir dz2 orbital, weakening the adsorption strength for *OOH intermediates and thereby enhancing the OER activity.

Optimizing the Intermediates Adsorption by Manipulating the Second Coordination Shell of Ir Single Atoms for Efficient Water Oxidation.

The precise regulation of single-atom catalysts (SACs) with the desired local chemical environment is vital to elucidate the relationship between the SACs structure and the catalytic performance. The debate on the effect of the local coordination environment is quite complicated even for the SACs with the same composition and chemical nature, calling for increased attention on the regulation of second coordination shell. For oxide-supported SACs, it remains a significant challenge to precisely manipulate the second coordination shell of single atoms supported on oxides due to the structural robustness of oxides. Here, Ir single atoms are anchored on NiO supports via different bonding strategies, resulting in the diverse Ir-O-Ni coordination numbers for Ir sites. Specifically, Ir1/NiO, Ir1-NiO, and Ir1@NiO SACs with increasing Ir-O-Ni coordination numbers of 3, 4, and 5 were synthesized, respectively. We found that the activity of the three samples towards oxygen evolution reaction (OER) exhibited a volcano-shaped relationship with the Ir-O-Ni coordination number, with Ir1-NiO showing the lowest overpotential of 225 mV at 10 mA cm-2. Mechanism investigations indicate that the moderate coordination number of Ir-O-Ni in Ir1-NiO creates the higher occupied Ir dz2 orbital, weakening the adsorption strength for *OOH intermediates and thereby enhancing the OER activity.

Long-range interactions driving neighboring Fe–N4 sites in Fenton-like reactions for sustainable water decontamination

Actualizing efficient and sustainable environmental catalysis is essential in global water pollution control. The single-atom Fenton-like process, as a promising technique, suffers from reducing potential environmental impacts of single-atom catalysts (SACs) synthesis and modulating functionalized species beyond the first coordination shell. Herein, we devised a high-performance SAC possessing impressive Fenton-like reactivity and extended stability by constructing abundant intrinsic topological defects within carbon planes anchored with Fe−N4 sites. Coupling atomic Fe−N4 moieties and adjacent intrinsic defects provides potent synergistic interaction. Density functional theory calculations reveal that the intrinsic defects optimize the d-band electronic structure of neighboring Fe centers through long-range interactions, consequently boosting the intrinsic activity of Fe−N4 sites. Life cycle assessment and long-term steady operation at the device level indicate promising industrial-scale treatment capability for actual wastewater. This work emphasizes the feasibility of synergistic defect engineering for refining single-atom Fenton-like chemistry and inspires rational materials design toward sustainable environmental remediation.

Temperature-induced structure evolution in monocrystalline and polycrystalline cobalt via molecular dynamics simulations

The structure transition of metallic melt strongly depends on temperature and significantly influences the comprehensive properties. However, observing the structure change in experiments is still challenging. Here, molecular dynamics is used to study the melting process and microstructural evolution in single crystal and polycrystal cobalt (Co). The results indicate that the melting process of single crystal structure starts from 1870 K and lasts for a very short period, while the polycrystal melts from about 1760 to 1870 K. In polycrystal Co, the melting initially occurs in grain boundaries, and the melting temperature shows a positive correlation with grain size. An interesting solidification phenomenon occurs on the surface of big grains in the beginning of melting process. The coordination number increasing from 12 to about 13.4 near melting point proves the local expansion of the first coordination shell, indicating the structural evolution from long-range order to short-range order in the continuous heating process. The common neighbor sub-cluster index and Voronoi polyhedron demonstrate the short-range icosahedron structures, while these polyhedrons become polydisperse and isolated in Co liquid. The findings ignite the investigation of the liquid structure origin of crystal materials and extend the understanding of the atomic structure evolution in melting.

Energy storage chemistry: Atomic and electronic fundamental understanding insights for high-performance supercapacitors

The scarcity of fuels, high pollution levels, climate change, and other major environmental issues are critical challenges that modern societies are facing, mostly originating from fossil fuels-based economies. These challenges can be addressed by developing green, eco-friendly, inexpensive energy sources and energy storage devices. Electrochemical energy storage materials possess high capacitance and superior power density. To engineer highly efficient next-generation electrochemical energy storage devices, the mechanisms of electrochemical reactions and redox behavior must be probed in operational environments. They can be studied by investigating atomic and electronic structures using in situ x-ray absorption spectroscopy (XAS) analysis. Such a technique has attracted substantial research and development interest in the field of energy science for over a decade. The mechanisms of charge/discharge, carrier transport, and ion intercalation/deintercalation can be elucidated. Supercapacitors generally store energy by two specific mechanisms—pseudocapacitance and electrochemical double-layer capacitance. In situ XAS is a powerful tool for probing and understanding these mechanisms. In this Review, both soft and hard x rays are used for the in situ XAS analysis of various representative electrochemical energy storage systems. This Review also showcases some of the highly efficient energy and power density candidates. Furthermore, the importance of synchrotron-based x-ray spectroscopy characterization techniques is enlightened. The impact of the electronic structure, local atomic structure, and electronically active elements/sites of the typical electrochemical energy storage candidates in operational conditions is elucidated. Regarding electrochemical energy storage mechanisms in their respective working environments, the unknown valence states and reversible/irreversible nature of elements, local hybridization, delocalized d-electrons spin states, participation of coordination shells, disorder, and faradaic/non-faradaic behavior are thoroughly discussed. Finally, the future direction of in situ XAS analysis combined with spatial chemical mapping from operando scanning transmission x-ray microscopy and other emerging characterization techniques is presented and discussed.

(Invited) Large Electromechanical Coupling in Zr-Doped Ceria

The catalytic, ion-transport and chemo-mechanical properties of Zr-doped ceria have been the subjects of broad ranging investigation by the materials research community. Yet, until recently, electromechanical coupling in ZrxCe1-xO2-δ has not been studied: ZrxCe1-xO2-δ ceramics were not expected to be electromechanically active. We have investigated the electrostriction (ES) effect, a second order electromechanical response, i.e., strain, u , is proportional to M·E 2, where E is the applied electric field and M is the longitudinal electrostriction strain coefficient. Large electrostriction in ceria has generally been associated with oxygen vacancies (Vo) which provide charge compensation for aliovalent dopants or for cerium reduction (Ce3+). The electrostriction effect induced by oxygen vacancies is restricted to frequency <1 Hz and is characterized by low saturation strain (|usat| <15 ppm).By contrast, isovalent doping with Zr (unexpectedly) results in a large electrostriction strain coefficient, reaching |M|=10-16 m2/V2 for x=0.1. This effect persists to frequency ≥3 kHz with |u| ≥220 ppm, making Zr0.1Ce0.9O2 competitive with the best commercial electrostrictor (PMN-PT15), but with ~100 times lower dielectric constant and three-fold higher elastic modulus. XAS data, DFT modelling and ab initio molecular dynamics (AIMD) calculations demonstrate that, unlike the permanent, but orientationally labile, elastic dipoles (i.e. local distortions with symmetry lower than that of the host lattice) formed by oxygen vacancies, elastic dipoles formed by Zr-doping are dynamic. In the absence of an E-field, [ZrO8]-local bonding units remain, on average, centered with respect to the second (cation) coordination shell. Due to bond anharmonicity, however, displacement of Zr by an anisotropic E-field requires less energy than displacement of the host cations, resulting in a large dynamic elastic dipole. This polarizable elastic dipole gives rise to large electrostrictive strain and constitutes the first example of non-classical electrostrictors (NCES) relying solely on substitutional point defects. In fact, a relatively small, isovalent dopant cation, such as Zr, along with nearest neighbor anions, vibrating anharmonically in a relatively large cage (compared to that of the host cation) may in fact facilitate electromechanical coupling.|M| of Zr-doped ceria increases exponentially with Zr content for x=0-0.1, suggesting that the contribution of Zr-ions to electrostrictive strain may not be simply additive. Zr-doping also increases the relative dielectric permittivity at 100Hz, from ~26 (x=0), to ~220 (x=0.1) and lowers the elastic modulus from 227GPa (x=0) to 214GPa (x=0.1), even though the number of chemical bonds remains unchanged. AIMD calculations report that stiffness for moving [CeO8]-local bonding units is essentially isotropic and is 2-2.4 times higher than for [ZrO8]. Stiffness for moving [CeO8] first nearest neighbor to Zr, is only slightly decreased from that in the bulk. These results can provide the theoretical basis for the reduction of the Young’s modulus and increase in the dielectric permittivity with Zr doping.When the concentration of Ce3+ is ≥ 100 ppm in ZrxCe1-xO2-δ, ( i.e., the material has not been completely re-oxidized), accompanied by the formation of oxygen vacancies (VO) for charge compensation, electrical conductivity is increased and electrostriction is suppressed. In addition, by co-doping Zr0.1Ce0.9O2 with 0.5mol% of aliovalent cations - Ca, Sc, Yb or La - we observed that the aliovalent dopant reduces the electrostriction strain coefficient by more than an order of magnitude and restores the values of Young’s modulus and dielectric permittivity to values close to those of undoped ceria. Since all these co-dopants, irrespective of valence and ionic radius, lead to a similar result, we concluded that the species responsible for the suppression of electrostriction in Zr doped ceria must be the oxygen vacancies. This finding is supported by XAS measurements and AIMD calculations. Fourier transform of Zr K-edge EXAFS spectra reveal that, even though the molar ratio ZrCe:VO is 40:1 in the co-doped compounds, oxygen vacancies nevertheless succeed in introducing enhanced disorder into the second coordination shell (cation) of Zr. DFT modelling predicts that a [ZrO7-VO] local bonding unit is stiff and asymmetrically distorts adjacent unit cells, leading to an elastic interaction length in the lattice between Zr-ions ≥ two-and-a-half-unit cells, which, in the absence of oxygen vacancies, can allow limited collective motion. However, such collective motion does not lead to a phase transition even at 123 K, implying that interaction between Zr-ions is neither sufficiently strong nor sufficiently long-range to produce freezing of the displacement, an effect that has been observed for perovskite relaxors.

Mössbauer analysis of (FeNiCrSiB) amorphous alloy ribbons

The fabrication of iron-based amorphous alloys was executed through the utilization of the melt-spinning technique, followed by a characterization employing X-ray diffraction (XRD), scanning electron microscopes (SEM) and transmission Mössbauer spectroscopy (MS). The studies were performed on Fe76-xNixCr4Si8B12 (where x = 0, 4, 12, and 30) metallic glasses in the form of ribbons. Mössbauer spectroscopy enables the exploration of the localized surroundings of iron (Fe) atoms within the glassy state, revealing alterations in the amorphous structure attributed to variations in the addition of Nickel (Ni). The broad lines in the Mössbauer spectra of ferromagnetic amorphous ribbons are a result of the distribution of non-equivalent iron sites and interatomic distances. The line intensity ratio suggests a preferential orientation of iron moments within the ribbon plane. The results derived from Mössbauer analysis indicate that the amorphous alloys feature bimodal distributions of the hyperfine fields. Changes in the nickel content within the alloys impact the disorder in the as-cast state and concurrently influence parameters such as the average hyperfine field <Hhyp>, the average radius R of the first coordination shell, and the number of nearest neighbor Fe atoms.

Nitrogen doping retrofits the coordination environment of copper single-atom catalysts for deep CO2 reduction

The electrocatalytic CO2 reduction reaction (CO2RR) represents an effective way to address energy crises and environmental issues by converting CO2 into valuable chemicals. Single-atom catalysts (SACs) can achieve excellent catalytic activity in CO2RR. However, the study of CO2RR on SACs still poses significant challenges, especially in terms of controlling the selectivity towards the deep product such as CH4 and CH3OH. Herein, we employ density functional theory (DFT) calculations to investigate the CO2RR on Cu SAC supported on N-doped graphene (Cu-N/C) and explore the role of N dopants on the CO2RR performance. Compared to Cu SACs supported on N-doped defective graphene with double vacancy (Cu-N/C-DV), Cu SACs supported on N-doped defective graphene with single vacancy (Cu-N/C-SV) can effectively convert CO2 into the deeply reduced C1 products, including CH4 and CH3OH, thus further indicating that Cu-N/C-SV has a stronger interaction with ∗CO, which is conducive to the deep reduction of ∗CO. Increasing the coordination number of N atoms or the proximity of doping site to the Cu active site can effectively enhance the stability of catalyst and promote the adsorption of ∗CO on Cu-N/C-SV. However, this also increases the free energy of the formation of ∗CHO intermediate. The results suggest that CuC3-Nm, which contains a N atom in the second coordination shell (meta-position) of Cu SACs, has the best electrocatalytic performance of CO2RR in terms of both selectivity and catalytic activity, not only contributing to an in-depth understanding of the reaction mechanism of CO2RR on SACs but also providing insights into the design of SACs for efficient CO2RR.

The structure of water from hot to supercooled temperatures based on analysis of experimental X-ray scattering data

An increasing number of studies support the existence of a liquid–liquid critical point (LLCP) in water, implying that under ambient conditions, water is a supercritical mixture of low-density (LDL) and high-density liquid (HDL) structures. Here, we analyse X-ray scattering data in terms of this model. Small-angle X-ray scattering data shows that anomalies are seen at temperatures as high as 360 K. By assuming the local structure in LDL-domains resembles that of the corresponding amorphous solid, we estimate the population of LDL-domains at different temperatures from pair-correlation functions (PCFs). By subtracting the LDL contribution, we isolate the PCF of the HDL component. The temperature-dependent HDL-PCFs show a feature around 4 Å in the supercooled regime, interpreted as interstitials resulting from collapse of the second coordination shell. We define the first coordination shell as neighbours up to 3.3 Å, based on an isosbestic point in the running coordination number. Our analysis shows that the HDL-domains have on average two strongly and two weakly H-bonded neighbours. The LDL population in TIP4P/2005-water is less temperature dependent than in real water, and the HDL component contains more structures with three strongly H-bonded neighbours, leading to a slight overestimation of the first peak of the PCF.

Amino-Induced Modulation of Electronic State and Neighboring Site Distance through Second Shell Boosted Catecholase-Mimicking Activity of Electron-Rich Cu Center.

Boosting the biomimetic catalytic activity of nanozyme is important for its potential application. One common strategy to achieve this goal mainly focused on manipulating the electronic state of metal site through the first coordination shell to modulate the adsorption/desorption strength of related reactant, intermediate and/or product, but remained challenging. Taking Cu-based catecholase-mimicking nanozyme for example, this work herein reports a different strategy involving amino-induced modulation of electronic state through the second shell to raise the electron density of Cu site, which further triggers the repulsion effect between neighboring geminal Cu centers to increase the Cu─Cu distance. The resulting nanozyme with electron-rich Cu site (DT-Cu) presents a lower work function and an upshifted d-band center in comparison with its counterpart (i.e., relatively electron-deficient TA-Cu), which promotes the electron transfer and enhances the adsorption strengths of Cu site for O2, catechol and H2O2 intermediate. The longer Cu─Cu distance of DT-Cu accelerated the O─O bond dissociation of H2O2 intermediate. This expedites the oxygen reduction process during catecholase-like catalysis, which together with the enhanced O2/H2O2/catechol adsorption corporately boosts the catecholase-like activity of DT-Cu.

Short-Range Order in Gallium Solid Solutions in α-Iron

Short-range order in soft magnetic FeGa alloys containing from 3 to 25 at % Ga was studied using nuclear gamma resonance (Mössbauer) spectroscopy. The Mössbauer spectra were analyzed via fitting with subspectra corresponding to different configurations of the neighborhoods of the Fe atoms with Ga in the first and second coordination shells. It has been shown that in samples of alloys containing from 3 to 17 at % gallium, the short-range order is almost independent of the heat treatment conditions (quenching from a paramagnetic state or exposure in a ferromagnetic state) and is characterized by the presence of pairs of Ga atoms in the position of the second neighbors (B2-type clusters). At a Ga content of 17 to 21 at %, the portion of B2 clusters turns out to be significantly higher after quenching than after annealing, which correlates with the observed effect of heat treatment on the magnitude of magnetostriction. As the Ga concentration (21–25 at %) further increases, the observed features in the distribution of Ga atoms indicate the appearance and growth of D03 phase regions.