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Various strategies, including controls of morphology, oxidation state, defect, and doping, have been developed to improve the performance of Cu-based catalysts for CO2 reduction reaction (CO2RR), generating a large amount of data. However, a unified understanding of underlying mechanism for further optimization is still lacking. In this work, combining first-principles calculations and machine learning (ML) techniques, we elucidate critical factors influencing the catalytic properties, taking Cu-based single atom alloys (SAAs) as examples. Our method relies on high-throughput calculations of 2669 CO adsorption configurations on 43 types of Cu-based SAAs with various surfaces. Extensive ML analyses reveal that low generalized coordination numbers and valence electron number are key features to determine catalytic performance. Applying our ML model with cross-group learning scheme, we demonstrate the model generalizes well between Cu-based SAAs with different alloying elements. Further, electronic structure calculations suggest surface - negative center could enhance CO adsorption by back donating electrons to antibonding orbitals of CO. Finally, several SAAs, including PCu, AgCu, GaCu, ZnCu, SnCu, GeCu, InCu, and SiCu, are identified as promising CO2RR catalysts. Our work provides a paradigm for the rational design and fast screening of SAAs for various electrocatalytic reactions.

In the present study, a Bi$_2$WO$_6$/ZnWO$_4$ photocatalyst was successfully constructed by a modified hydrothermal synthesis method with different molar concentrations of Bi$_2$WO$_6$ with respect to ZnWO$_4$. The variation in molar concentrations of Bi$_2$WO$_6$ changed the photocatalytic properties of the Bi$_2$WO$_6$/ZnWO$_4$ catalyst. The synthesized Bi$_2$WO$_6$/ZnWO$_4$ photocatalyst was characterized by various techniques to decipher its structural and spectral properties. The interaction of Bi$^{2+}$ ionic charge carriers and many-body effects cause the band gap to narrow in Bi$_2$WO$_6$/ZnWO$_4$, as shown by PL analysis. The decrease in band gap energies (E$_g$) from 4.7 eV (ZnWO$_4$) to 3.5 eV (30% Bi$_2$WO$_6$/ZnWO$_4$) is beneficial because less energy is required to excite the valence electrons. The maximum degradation of Plasmocorinth B dye was found with 30% Bi$_2$WO$_6$/ZnWO$_4$ under UV irradiation. This increased activity of 30% Bi$_2$WO$_6$/ZnWO$_4$ can be attributed to the (i) synergistic effect in the bicrystalline framework of Bi$_2$WO$_6$ and ZnWO$_4$, (ii) the high close contact between Bi$_2$WO$_6$ and ZnWO$_4$, and (iii) the small crystallite size. The photocatalytic activity of synthesized Bi$_2$WO$_6$/ZnWO$_4$ photocatalyst shows its significant potential in water/ wastewater treatment application.

The presence of grain boundaries (GBs) has a great impact on the coefficient of thermal expansion (CTE) of polycrystals. However, direct measurement of local expansion of GBs remains challenging for conventional methods due to the lack of spatial resolution. In this work, we utilized the valence electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to directly measure the CTE of Σ5 and 45°GBs of SrTiO3 at a temperature range between 373 and 973 K. A CTE that was about 3 times larger was observed in Σ5 GB along the direction normal to GB plane, while only a 1.4 time enhancement was found in the 45° GB. Our result provides direct evidence that GBs contribute to the enhancement of CTE in polycrystals. Also, this work has revealed how thermodynamic properties are varied in different GB structures and demonstrated the potential of EELS for probing local thermal properties with nanometer-scale resolution.

Formate is an important liquid chemical, which can be produced by electrocatalytic carbon dioxide reduction reaction (CO2RR). Most of the metal catalysts for CO2RR to formate are toxic or noble metals, such as Cd, Hg, Pb, Sn and Pd, leading to the environmental pollution or increased production costs. Herein, we develop an environmentally friendly and low-cost NiZn alloy catalyst for CO2RR to formate. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) confirm the alloy structure of the prepared NiZn catalyst. As for a catalyst for CO2RR, the NiZn alloy exhibits the FEHCOO- (Faraday efficiency of HCOO-) of 36 ± 0.7% at −0.9 V vs. RHE in 0.1 M KHCO3, and remarkable stability for 40,000 s at −0.8, −0.9, −1.0 and −1.1 V vs. RHE, respectively. Theoretical calculation results indicate that the NiZn alloy exhibits the middle valence electron structure between the Zn and Ni metal, resulting in the favorable pathway for HCOOH formation but unfavorable for the hydrogen evolution reaction and CO production. The Ultraviolet Photoelectron Spectroscopy results verify the modulated valence electron structure for NiZn alloy as compared to Ni and Zn, consistent with the theoretical calculation results. This work provides new insights into design of alloy catalysts for CO2RR to formate.

Peer reviewed: True, The quasi-harmonic model proposes that a crystal can be modelled as atoms connected by springs. We demonstrate how this viewpoint can be misleading: a simple application of Gauss's law shows that the ion-ion potential for a cubic Coulomb system can have no diagonal harmonic contribution and so cannot necessarily be modelled by springs. We investigate the repercussions of this observation by examining three illustrative regimes: the bare ionic, density tight-binding and density nearly-free electron models. For the bare ionic model, we demonstrate the zero elements in the force constants matrix and explain this phenomenon as a natural consequence of Poisson's law. In the density tight-binding model, we confirm that the inclusion of localized electrons stabilizes all major crystal structures at harmonic order and we construct a phase diagram of preferred structures with respect to core and valence electron radii. In the density nearly-free electron model, we verify that the inclusion of delocalized electrons, in the form of a background jellium, is enough to counterbalance the diagonal force constants matrix from the ion-ion potential in all cases and we show that a first-order perturbation to the jellium does not have a destabilizing effect. We discuss our results in connection to Wigner crystals in condensed matter, Yukawa crystals in plasma physics, as well as the elemental solids.

Multi-principal element alloys (MPEAs) occur at or nearby the centre of the multicomponent phase space, and they have the unique potential to be tailored with a blend of several desirable properties for the development of materials of future. The lack of universal phase diagrams for MPEAs has been a major challenge in the accelerated design of products with these materials. This study aims to solve this issue by employing data-driven approaches in phase prediction. A MPEA is first represented by numerical fingerprints (composition, atomic size difference , electronegativity , enthalpy of mixing , entropy of mixing , dimensionless $$\Omega$$ Ω parameter, valence electron concentration and phase types ), and an artificial neural network (ANN) is developed upon the datasets of these numerical descriptors. A pyMPEALab GUI interface is developed on the top of this ANN model with a computational capability to associate composition features with remaining other input features. With the GUI interface, an user can predict the phase(s) of a MPEA by entering solely the information of composition. It is further explored on how the knowledge of phase(s) prediction in composition-varied $$\hbox {Al}_x$$ Al x CrCoFeMnNi and $$\hbox {CoCrNiNb}_x$$ CoCrNiNb x can help in understanding the mechanical behavior of these MPEAs. Graphic Abstract

Evidence of strong photoelectron-valence electron (PEVE) interactions has been observed in the anion photoelectron (PE) spectra of several lanthanide suboxide clusters, which are exceptionally complex from an electronic structure standpoint and are strongly correlated systems. The PE spectrum of Gd2O-, which should have relatively simple electronic structure because of its half-filled 4f subshell, exhibits numerous electronic transitions. The electron affinity determined from the spectrum is 0.26 eV. The intensities of transitions to excited states increase relative to the lower-energy states with lower photon energy, which is consistent with shakeup transitions driven by time-dependent electron-neutral interactions. A group of intense spectral features that lie between electron binding energies of 0.7 and 2.3 eV are assigned to transitions involving detachment of an electron from outer-valence σu and σg orbitals that have large Gd 6s contributions. The spectra show parallel transition manifolds in general, which is consistent with detachment from these orbitals. However, several distinct perpendicular transitions are observed adjacent to several of the vertical transitions. A possible explanation invoking interaction between the ejected electron and the high-spin neutral is proposed. Specifically, the angular momentum of electrons ejected from σu or σg orbitals, which is l = 1, can switch to l = 0, 2 with an associated change in the Ms of the remnant neutral, which is spin-orbit coupling between a free electron and the spin of a neutral.

A theoretical study of geometrical structures, electronic properties, and spectral properties of single-atom transition metal-doped boron clusters MB24 (M = Sc, V, and Mn) is performed using the CALYPSO approach for the global minimum search, followed by density functional theory calculations. The global minima obtained for the VB24 and MnB24 clusters correspond to cage structures. Interestingly, the global minima obtained for the ScB24 cluster tend to a three-ring tubular structure. Population analyses and valence electron density analyses reveal that partial electrons on transition-metal atoms transfer to boron atoms. The localized orbital locator of MB24 (M = Sc, V, and Mn) indicates that the electron delocalization of ScB24 is stronger than that of VB24 and MnB24, and there is no obvious covalent bond between doped metals and B atoms. The spin density and spin population analyses reveal that MB24 (M = Sc, V, and Mn) have different spin characteristics which are expected to lead to interesting magnetic properties and potential applications in molecular devices. The calculated spectra indicate that MB24 (M = Sc, V, and Mn) has meaningful characteristic peaks that can be compared with future experimental values and provide a theoretical basis for the identification and confirmation of these single-atom transition metal-doped boron clusters. Our work enriches the database of geometrical structures of doped boron clusters and can provide an insight into new doped boron clusters.

Despite the fact that the positron annihilation has been used in biomedical applications, the detailed mechanism of the positron annihilation on biological molecules remains poorly understood so far. In this work, we investigated the positron binding and positron annihilation properties for both global minimum and hydrogen-bonded structures of 20 amino acid molecules using the multicomponent molecular orbital method. By regression analysis, we confirmed that positron affinity can increase with an increase of the permanent dipole moment of the parent amino acids as reported in previous studies, while the annihilation rate linearly increases with respect to the square root of positron affinity. By the one-particle property analyses for probabilities of electron–positron contacts, we found that delocalization characteristics of both electrons and positrons play key roles to enhance the positron annihilation rate arising from both the valence electrons in σ- and π-type molecular orbitals from 2p atomic orbitals but not from the highest occupied molecular orbital electrons, particularly for comparatively weakly bound positronic amino acid systems.

The development of advanced structural alloys with performance meeting the requirements of extreme environments in nuclear reactors has been long pursued. In the long history of alloy development, the search for metallic alloys with improved radiation tolerance or increased structural strength has relied on either incorporating alloying elements at low concentrations to synthesize so-called dilute alloys or incorporating nanoscale features to mitigate defects. In contrast to traditional approaches, recent success in synthesizing multicomponent concentrated solid-solution alloys (CSAs), including medium-entropy and high-entropy alloys, has vastly expanded the compositional space for new alloy discovery. Their wide variety of elemental diversity enables tunable chemical disorder and sets CSAs apart from traditional dilute alloys. The tunable electronic structure critically lowers the effectiveness of energy dissipation via the electronic subsystem. The tunable chemical complexity also modifies the scattering mechanisms in the atomic subsystem that control energy transport through phonons. The level of chemical disorder depends substantively on the specific alloying elements, rather than the number of alloying elements, as the disorder does not monotonically increase with a higher number of alloying elements. To go beyond our knowledge based on conventional alloys and take advantage of property enhancement by tuning chemical disorder, this review highlights synergistic effects involving valence electrons and atomic-level and nanoscale inhomogeneity in CSAs composed of multiple transition metals. Understanding of the energy dissipation pathways, deformation tolerance, and structural stability of CSAs can proceed by exploiting the equilibrium and non-equilibrium defect processes at the electronic and atomic levels, with or without microstructural inhomogeneities at multiple length scales. Knowledge of tunable chemical disorder in CSAs may advance the understanding of the substantial modifications in element-specific alloy properties that effectively mitigate radiation damage and control a material's response in extreme environments, as well as overcome strength-ductility trade-offs and provide overarching design strategies for structural alloys.

We demonstrate an alternative, data-driven approach to uncovering structure-property relationships for the rational design of heterobimetallic transition-metal complexes that exhibit metal-metal bonding. We tailor graph-based representations of the metal-local environment for these complexes for use in multiple linear regression and kernel ridge regression (KRR) models. We curate a set of 28 experimentally characterized complexes to develop a multiple linear regression model for oxidation potentials. We achieve good accuracy (mean absolute error of 0.25 V) and preserve transferability to unseen experimental data with a new ligand structure. We also train a KRR model on a subset of 330 structurally characterized heterobimetallics to predict the degree of metal-metal bonding. This KRR model predicts relative metal-metal bond lengths in the test set to within 5%, and analysis of key features reveals the fundamental atomic contributions (e.g., the valence electron configuration) that most strongly influence the behavior of these complexes. Our work provides guidance for rational bimetallic design, suggesting that properties, including the formal shortness ratio, should be transferable from one period to another.

Owing to the unique structural, electronic, and physico-chemical properties, molybdenum clusters are expected to play an important role in future nanotechnologies. However, their ground states are still under debate. In this study, the crystal structure analysis by particle swarm optimization (CALYPSO) approach is used for the global minimum search, which is followed by first-principles calculations, to detect an obvious dimerization tendency in Mon (n = 2−18) clusters when the 4s and 4p semicore states are not regarded as the valence states. Further, the clusters with even number of atoms are usually magic clusters with high stability. However, after including the 4s and 4p electrons as valence electrons, the dimerization tendency exhibits a drastic reduction because the average hybridization indices Hsp, Hsd, and Hpd are reduced significantly. Overall, this work reports new ground states of Mon (n=11, 14, 15) clusters and proves that semicore states are essential for Mon clusters.

Controlling the oxide ion (O2−) concentration in oxides is essential to develop advanced ionic devices, i.e. solid oxide fuel cells, smart windows, memory devices, energy storage devices, and so on. Among many oxides several transition metal (TM)-based perovskite oxides show high oxide ion conductivity, and their physical properties show high sensitivity to the change of the oxide ion concentration. Here, the change in the oxide ion concentration is shown through the overlayer deposition on the SrFe0.5Co0.5O2.5 (SFCO) oxygen sponge film. We grew SFCO films followed by the deposition of two kinds of complex oxide films under exactly the same growth conditions, and observed the changes in the crystal structure, valence states, and magnetic ground states. As the NSMO overlayer grows, strong evidence of oxidation at the O K edge is shown. In addition, the Fe4+ feature is revealed, and the electron valence state of Co increased from 3 to 3.25. The oxide ion concentration of SFCO changes during layer growth due to oxidation or reduction due to differences in chemical potential. The present results might be useful to develop advanced ionic devices using TM-based perovskite oxides.

Electrocatalytic nitrogen reduction reaction (NRR) is a promising method for sustainable production of NH3, which provides an alternative to the traditional Haber-Bosch process. However, the poor Faraday efficiency caused by N≡N triple bond activation and competitive hydrogen evolution reaction (HER) have seriously hindered the application of NRR. In this work, a novel strategy to promote NRR through boron-transition-metal (TM) hybrid double-atom catalysts (HDACs) has been proposed. The excellent catalytic activity of HDACs is attributed to a significant difference of valence electron distribution between boron and TMs, which could better activate N≡N bonds and promote the conversion of NH2 to NH3 compared with boron or metal single-atom catalysts and traditional double-atom catalysts (DACs). Hence, by means of DFT computations, the stability, activity, and selectivity of 29 HDACs are systematically investigated to evaluate their catalytic performance. B-Ti@g-CN and B-Ta@g-CN are screened as excellent nitrogen-fixing catalysts with particularly low limiting potentials of 0.13 and 0.11 V for NRR and rather high potentials of 0.54 and 0.82 V for HER, respectively. This work provides a new idea for the rational design of efficient nitrogen-fixing catalysts and could also be widely used in other catalytic reactions.

Single lanthanide atoms and molecules are promising candidates for atomic data storage and quantum logic due to the long lifetime of their magnetic quantum states. Accessing and controlling these states through electrical transport requires precise knowledge of their electronic configuration at the level of individual atomic orbitals, especially of the outer shells involved in transport. However, no experimental techniques have so far shown the required sensitivity to probe single atoms with orbital selectivity. Here we resolve the magnetism of individual orbitals in Gd and Ho single atoms on MgO/Ag(100) by combining X-ray magnetic circular dichroism with multiplet calculations and density functional theory. In contrast to the usual assumption of bulk-like occupation of the different electronic shells, we establish a charge transfer mechanism leading to an unconventional singly ionized configuration. Our work identifies the role of the valence electrons in determining the quantum level structure and spin-dependent transport properties of lanthanide-based nanomagnets.

Investigations of Li metal and ionic compounds through experimental and theoretical spectroscopy has been of tremendous interest due to their prospective applications in Li-metal and Li-ion batteries. Li K-edge soft X-ray absorption spectroscopy (sXAS) provides the most direct spectroscopic characterization; unfortunately, due to the low core-level energy and the highly reactive surface, Li-K sXAS of Li metal has been extremely challenging, as evidenced by many controversial reports. Here, through controlled and ultra-high energy resolution experiments of two kinds of in-situ prepared samples, we report the intrinsic Li-K sXAS of Li-metal that displays a prominent leading peak, which has never been revealed before. Furthermore, theoretical simulations show that the Li-K sXAS is strongly affected by the response of the valence electrons to the core-hole due to the low number of valence electrons in Li. We successfully reproduce the Li-K sXAS by state-of-the-art calculations with considerations of a number of relevant parameters such as temperature, resolution, and especially contributions from transitions which are forbidden in the so-called single-particle treatment. Such a comparative experimental and theoretical investigation is further extended to a series of Li ionic compounds, which highlight the importance of considering the total and single-particle energies for obtaining an accurate alignment of the spectra. Our work provides the first reliable Li-K sXAS of Li metal surface with advanced theoretical calculations. The experimental and theoretical results provide a critical benchmark for studying Li surface chemistry in both metallic and ionic states.

Probing the transition from a metallic state to a molecular state in gold nanoparticles is fundamentally important for understanding the origin of surface plasmon resonance and the nature of the metallic bond. Atomically precise gold nanoclusters are desired for probing such a transition based upon a series of precise sizes with X-ray structures. While the definition of the metallic state in nanoclusters is simple, that is, when the HOMO-LUMO gap (Eg) becomes negligibly small (Eg < kBT, where kB is the Boltzmann constant and T the temperature), the experimental determination of ultrasmall Eg (e.g., of kBT level) is difficult, and the thermal excitation of valence electrons apparently comes into play in ultrasmall Eg nanoclusters. Although a sharp transition from nonmetallic Au246(SR)80 to metallic Au279(SR)84 (SR: thiolate) has been observed, there is still uncertainty about the transition region. Here, we summarize several criteria on determining the metallic state versus the molecular (or nonmetallic) state in gold nanoclusters, including (1) Eg determined by optical and electrochemical methods, (2) steady-state absorption spectra, (3) cryogenic optical spectra, (4) transient absorption spectra, (5) excited-state lifetime and power dependence, and (6) coherent oscillations in ultrafast electron dynamics. We emphasize that multiple analyses should be performed and cross-checked in practice because no single criterion is definitive. We also review the photophysics of several gold nanoclusters with nascent surface plasmon resonance. These criteria are expected to deepen the understanding of the metallic to molecular state transition of gold and other metal nanoclusters and also promote the design of functional nanomaterials and their applications.

Influences of Ta and Ni/Ti ratio on transformation temperatures, microstructures, and super-elastic properties of NiTiTa alloys were investigated. It is found that: (1) increasing Ni/Ti ratio increases the solubility limit of Ta in NiTi matrix and suppresses the formation of Ta-rich secondary phases. 4 at.% Ta can be dissolved in alloys with Ni/Ti ratio of ~ 1.1; (2) high Ni/Ti ratio promotes the formation of Ti2Ni3 phase, which is suppressed by increasing Ta additions; (3) phase transformation temperatures increase with Ta content at a rate of ~ 70–80 °C/at.% Ta, but rapidly decrease with increasing Ni/Ti ratio; (4) super-elastic properties of NiTiTa alloys can be optimized by adjusting residual cold work, Ta content, and Ni/Ti ratio; and finally, (5) phase transformation temperatures of NiTiTa alloys decrease with increasing valence electron concentration in general.

We have studied soft X-ray photoabsorption in the doubly deprotonated gas-phase oligonucleotide [dTGGGGT–2H]2−. The dominating decay mechanism of the X-ray induced inner shell vacancy was found to be Auger decay with detachment of at least three electrons, leading to charge reversal of the anionic precursor and the formation of positively charged photofragment ions. The same process is observed in heavy ion (12 MeV C4+) collisions with [dTGGGGT–2H]2− where inner shell vacancies are generated as well, but with smaller probability. Auger decay of a single K-vacancy in DNA, followed by detachment of three or more low energy electrons instead of a single high energy electron has profound implications for DNA damage and damage modelling. The production of three low kinetic energy electrons with short mean free path instead of one high kinetic energy electron with long mean free path implies that electron-induced DNA damage will be much more localized around the initial K-shell vacancy. The fragmentation channels, triggered by triple electron detachment Auger decay are predominantly related to protonated guanine base loss and even loss of protonated guanine dimers is tentatively observed. The fragmentation is not a consequence of the initial K-shell vacancy but purely due to multiple detachment of valence electrons, as a very similar positive ion fragmentation pattern is observed in femtosecond laser-induced dissociation experiments., A K-shell vacancy in DNA that is induced by a (therapeutically relevant) soft X-ray of MeV carbon ion, decays by Auger processes accompanied by emission of at least 3 low energy electrons.

The crystal structure and physical properties of Nb25Mo5+xRe35Ru25-xRh10 (0 ≤ x ≤ 10) and Nb5Mo35-yRe15+yRu35Rh10 (0 ≤ y ≤ 15) high-entropy alloys (HEAs) have been studied by X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements. The results show that the former HEAs with valence electron concentration (VEC) values of 6.7–6.9 crystallize in a noncentrosymmetric cubic α-Mn structure, while the latter ones with VEC values of 7.1–7.25 adopt a centrosymmetric hexagonal close-packed (hcp) structure. Despite different structures, both series of HEAs are found to be bulk superconductors with a full energy gap, and the superconducting transition temperature Tc tends to decrease with the increase of VEC. Nevertheless, the Tc values of the hcp-type HEAs are higher than those of the α-Mn-type ones, likely due to a stronger electron phonon coupling. Furthermore, we show that VEC and electronegativity difference are two key parameters to control the stability of α-Mn and hcp-type HEAs. These results not only are helpful for the design of such HEAs, but also represent the first realization of structurally different HEA superconductors without changing the constituent elements.

This paper highlights the relationship between changes in structural and morphological features, electronic structure and exanging of time mechanical treatment at microbraker (MBT). Scaning electron microscopy revealed a change in the morphological features of nanoscale powders. From the comparison of SEM images of 0.8SiO2/0.2Al2О3 mixture before and after MBT, it is established that due to MBT, the agglomerates of the initial components are simultaneously crushed with perfect mixing particles of oxides between each other and the formation of new agglomerates with a denser structure. The increase in processing time leads to an increase in the density of the nanocomposite. The effect of time of mechanical treatment on the structural parameters and phase composition of mixtures of silicon dioxide and titanium were studied using the method of X-ray structural analysis. The established agglomeration is accompanied by a change in the lattice parameter c with a change in the regions of coherent scattering of crystalline Al2О3. Ultra-soft X-ray emission spectroscopy was used to study the distribution of Op-, Sisd- and Alsd- valence electrons in 0.8SiO2/0.2Al2О3 powder mixtures after the different time of mechanicall treatment. An increase in atomic charges has been measured and can be explained by the transfer of electrons from Si and Al to O atoms in split Opπ-binding states.

The structural properties and radial distributions of the valence electron in different excited levels of Na atom (n = 3–5, l = 0–4; n and l being the principal and orbital angular momentum quantum numbers, respectively) under impenetrable spherical confinement have been studied, where the interaction between the frozen core and the valence electron is mimicked by a model potential available in the literature. The effect of the core on the valence electron has been investigated by estimating the structural properties of Na10+ ion under similar confinement. Scaled radial densities at the nucleus and related ratios are presented for a few excited states of the valence electron of Na atom and the corresponding analytic results have been tested numerically.