Charge States Of Atoms Research Articles

A comparative first-principles study on the physical properties of Gd2Zr2O7 weberite and pyrochlore

A comparative analysis of the physical properties of Gd2Zr2O7 weberite and pyrochlore is conducted using first-principles methods. The structural characteristics of Gd2Zr2O7 pyrochlore and weberite are examined at the atomic site, local coordination, and lattice parameter levels. The findings from ab initio molecular dynamics simulations and experimental data confirm the existence and stability of the Gd2Zr2O7 weberite structure at 300 K. The formation of cation antisite defects is calculated to be more facile in the weberite lattice compared to pyrochlore. The formation energy of vacancy defects is strongly correlated to the distinct defect configurations. The calculations further highlight that Gd2Zr2O7 weberite exhibits mechanical properties comparable to pyrochlore. The insulating nature, chemical bonding characteristics, and charge states of individual atoms in weberite and pyrochlore are elucidated through analysis of the partial density of states and Bader charges.

Theoretical study on the synergistic mechanism of Fe–Mn in sodium-ion batteries

This article conducts first-principles calculations to initially explore the construction of two configurations, NaFeO2 (NFO) and NaMnO2 (NMO), and studies the mixing enthalpies under different Fe–Mn ratios. The results indicate that NaFe3/8Mn5/8O2 (NFMO) exhibits the most thermodynamically stable structure. Subsequent calculations on the mixing enthalpies and volume changes during the sodium extraction process for NFO, NMO, and NFMO configurations are presented, along with the partial density of states (PDOS) and Bader charges of transition metals (TM) and oxygen. These calculations reveal the synergistic mechanism of Fe and Mn. Fe and Mn can engage in more complex electron exchanges during sodium extraction, optimizing the internal electron density distribution and overall charge balance, thereby stabilizing the crystal structure and reducing the migration of Fe3+ to the sodium layers during deep sodium extraction. The interaction between Fe’s 3d electrons and Mn’s 3d electrons through the shared oxygen atoms’ 2p orbitals occurs in the Fe–Mn–O network. This interaction can lead to a rebalancing of the electron density around Mn³⁺ atoms, mitigating the asymmetric electron density distribution caused by the d4 configuration of the lone Mn³⁺ and suppressing the Jahn-Teller effect of Mn3+. Moreover, the synergistic effects between Fe and Mn can provide a more balanced charge distribution, reducing extreme changes to the charge state of oxygen atoms and decreasing the irreversible oxygen release caused by anionic redox reactions during deep sodium extraction, thereby enhancing the material’s stability. This in-depth study of the interaction mechanism at the microscopic level when co-doping Fe and Mn offers valuable insights for the rational design and development of high-performance cathode materials.

Effects of Zr dopants on properties of PtNi nanoparticles for ORR catalysis: A DFT modeling.

Pt-based alloys, such as Pt3Ni, are among the best electrocatalysts for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Doping of PtNi alloys with Zr was shown to enhance the durability of the operating ORR catalysts. Rationalizing these observations is hindered by the absence of atomic-level data for these tri-metallic materials, even when not exposed to the fuel cell operation conditions. This study aims at understanding structure-property relations in Zr-doped PtNi nanoparticles as a key to their ORR function. In particular, we calculated, using a method based on density functional theory, the most stable chemical orderings of pristine and Zr-doped Pt3Ni particles containing over 400 atoms. We thus clarify (i) preferential location and charge states of Zr atoms in the Pt3Ni NPs; (ii) effect of doping Zr atoms on the stability of the Pt skin of the Pt3Ni NPs; (iii) charge redistribution induced by Zr dopants; (iv) layer-by-layer atomic ordering in the Pt3Ni/Zr NPs with the increasing Zr content; and (v) effect of Zr atoms on the adsorption energies of O and OH species as indicators of the ORR activity.

Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K.

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

Numerical study of limits of neoclassical theory in the plateau regime in the presence of impurities

AbstractThis paper presents a numerical simulation to test the corrections to the radial electric field in the presence of impurities for tokamak devices whose main ions are in the plateau regime. The effects due to the presence of impurity density variation in a pedestal tokamak are known to generate conditions where conventional neoclassical theory is no longer valid. It is found that for a specific axisymmetric concentric magnetic field, the presence of steep radial gradients for both temperature and density translates in a correction to the radial electric field. We anticipate that comparisons to the Landreman–Fülöp–Guszejnov model display good agreement for both the analytic approximation and numerically solved value of the normalized density when low impurity concentration and atomic charge state cases are considered, coupled with values of the ordering parameter . Higher values of the latter reveal better agreement to the neoclassical Hazeltine–Hinton formula, suggesting a discrepancy with the validity threshold of the model.

Main group tin single atom catalyst supported by C5N monolayer for oxygen evolution reactions

Single atom catalysts (SACs) supported by carbonaceous materials present extraordinary activity for oxygen evolution reaction (OER), which usually contain transition metal atoms serving as active sites. However, when it turns to the SACs containing main group metal atoms, the OER performance is believed to be significantly decreased. We here proposed a strategy of designing SACs with p-block element of Sn as active sites, which exhibit excellent OER performance without nitrogen coordination of the metal centers. Density functional theory calculations demonstrate that atomically dispersed Sn atoms supported by the metallic monolayers of C5N can efficiently catalyze OER, of which the Sn center coordinated with four C atoms give rise to low overpotential of 0.54 V comparable or even lower than those given by the transition-metal-based SACs. Moreover, correlation between the overpotentials and some intrinsic properties of SACs, which demonstrates that the OER performance is not strongly correlated with the charge state of Sn atoms and Sn-N/C bond length. This study paves a new way of theoretically designing SACs containing main group elements toward OER or even broader range of electrochemical reactions.

Revisiting the scavenging activity of glutathione: Free radicals diversity and reaction mechanisms

Glutathione (GSH) is the most abundant low-molecular-mass thiolated compound in plants and animals, highly relevant in humans since it is one of the principal antioxidant agents capable of scavenging reactive oxygen species (ROS). In addition, it has a function in the immune system and in cell proliferation, and most recently it was discovered that it works against COVID-19. Considering the high flexibility of glutathione, with an atomic configuration and charge state susceptible to minimal changes in its chemical environment, it is of relevance to ascertain what is the scavenging capacity of each of the most probable glutathione species under physiological conditions. In order to find the site and mechanism that contributes the most to the antioxidant activity of GSH, density functional theory (DFT) calculations on multiple species of this molecule were performed in this work. Reactivity indexes, related to hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) mechanism were calculated for glutathione. In addition, Gibbs free energies of reaction (ΔG) were obtained for the reactions of GSH with several free radicals to confirm the thermochemical viability of each antioxidant mechanism and to avoid misleading conclusions in terms of considering energetically unfavorable reactions. The results confirm that HAT is the chemical route contributing the most to the antioxidant activity of GSH. However, at least for one radical of interest, SPLET should also be considered.

A comparative study of CO oxidation on Cu-doped C3N monolayer with N and C vacancies

In this study, we employ density functional theory based on first-principles calculations to investigate the CO oxidation performance of a Cu-doped C3N monolayer with N and C vacancies, which is a promising low-temperature and high-activity catalyst. We find that Cu dopants are more stably anchored at both the N and C vacancies of the C3N monolayer. To comprehensively understand the effect of N and C vacancies on CO oxidation, we investigate both the Eley–Rideal and Langmuir–Hinshelwood (LH) mechanisms. Our results indicate that the LH mechanism exhibits much lower barriers for the rate-limiting step of CO oxidation, suggesting its excellent performance in this process. Moreover, the charge state of Cu atoms and the asymmetric atomic interface at the C vacancy lead to a lower rate-limiting barrier for CO oxidation compared to that at the N vacancy (0.5 vs 0.71 eV). This study represents a promising strategy to enhance the activity and selectivity of CO oxidation by controlling the asymmetric atomic interface of single-atom catalysts.

Highly Active Nanocrystalline ZnO and Its Photo-Oxidative Properties towards Acetone Vapor.

Zinc oxide is one of the well-known photocatalysts, the potential applications of which are of great importance in photoactivated gas sensing, water and air purification, photocatalytic synthesis, among others. However, the photocatalytic performance of ZnO strongly depends on its morphology, composition of impurities, defect structure, and other parameters. In this paper, we present a route for the synthesis of highly active nanocrystalline ZnO using commercial ZnO micropowder and ammonium bicarbonate as starting precursors in aqueous solutions under mild conditions. As an intermediate product, hydrozincite is formed with a unique morphology of nanoplates with a thickness of about 14-15 nm, the thermal decomposition of which leads to the formation of uniform ZnO nanocrystals with an average size of 10-16 nm. The synthesized highly active ZnO powder has a mesoporous structure with a BET surface area of 79.5 ± 4.0 m2/g, an average pore size of 20 ± 2 nm, and a cumulative pore volume of 0.507 ± 0.051 cm3/g. The defect-related PL of the synthesized ZnO is represented by a broad band with a maximum at 575 nm. The crystal structure, Raman spectra, morphology, atomic charge state, and optical and photoluminescence properties of the synthesized compounds are also discussed. The photo-oxidation of acetone vapor over ZnO is studied by in situ mass spectrometry at room temperature and UV irradiation (λmax = 365 nm). The main products of the acetone photo-oxidation reaction, water and carbon dioxide, are detected by mass spectrometry, and the kinetics of their release under irradiation are studied. The effect of morphology and microstructure on the photo-oxidative activity of ZnO samples is demonstrated.

Spectral and quantum mechanical investigation of calcium apatites isomorphically substituted in the anionic sublattice

The evolution of the valence band, the charge states of atoms, and the optical and vibrational spectra in Ca10(PO4)x(VO4)y(AsO4)z(OH)2 (x + y + z = 6) compound with three different types of oxygen tetrahedra have been studied by XPS, IR, and optical spectroscopy with the use of quantum mechanical calculation methods in the DFT approximation. It is shown that the occupied part of the valence band of compounds with three different types of tetrahedra has a pronounced band character with varying lengths of individual subbands. Two structural regions separated by energy are observed: the valence band's upper part and the valence band's lower part (subvalent states). The structure of the valence band's middle part, the region of valence states with energies from ∼ 13.0–19.0 eV, is weakly expressed. The sublattice of oxygen tetrahedra is decisive in forming the shape and the main features of the total density of electronic states of the compounds under study. The vibrations anharmonicity in the crystal lattice of such compounds can vary depending on the concentration of tetrahedra of a specific type. Such changes are local, and with the help of directed substitutions, it is possible to create the required spatial distribution of the anharmonic component over the crystal.The design of the calcium apatites structure by the method of isomorphic substitutions of (XO4) tetrahedra with tetrahedra of various types and with different ratios makes it possible to control the energy band gap width.

Room-Temperature Magnetism in 2D MnGa4 -H Induced by Hydrogen Insertion.

2D room-temperature magnetic materials are of great importance in future spintronic devices while only very few are reported. Herein, a plasma-enhanced chemical vapor deposition approach is exploited to construct the 2D room-temperature magnetic MnGa4 -H single crystal with a thickness down to 2.2nm. The employment of H2 plasma makes hydrogen atoms can be easily inserted into the MnGa4 lattice to modulate the atomic distance and charge state, thereby ferrimagnetism can be achieved without destroying the structural configuration. The as-obtained 2D MnGa4 -H crystal is high-quality, air-stable, and thermo-stable, demonstrating robust and stable room-temperature magnetism with a high Curie temperature above 620K. This work enriches the 2D room-temperature magnetic family and opens up the possibility for the development of spintronic devices based on 2D magnetic alloys.

Complex formation of 1D-coordination poly-mers based on arendiyl-bisphosphinic acid

Within the framework of the density functional theory and the ab initio pseudopotential, we have obtained the spatial distributions of the valence electrons density, the electron energy spectra and the atom charge states of 1D-coordination polymers based on arendiyl-bisphosphinic acids and metal ions. Formation of one-dimensional polymer chains takes place under participation of strong intermolecular bonds.

Synchrotron-based operando X-ray diffraction and X-ray absorption spectroscopy study of LiCo0.5Fe0.5PO4 mixed d-metal olivine cathode

Lithium-ion batteries based on high-voltage cathode materials, such as LiCoPO4, despite being promising in terms of specific power, still suffer from poor cycle life due to the lower stability of common non-aqueous electrolytes at higher voltages. One way to overcome this issue might be decreasing the working potential of the battery by doping LiCoPO4 by Fe, thus reducing electrolyte degradation upon cycling. However, such modification requires a deep understanding of the structural behavior of cathode material upon lithiation/delithiation. Here we used a combination of operando synchrotron-based XRD and XAS to investigate the dynamics of d-metal local atomic structure and charge state upon cycling of LiCo0.5Fe0.5PO4 mixed d-metal olivine cathode material. Principal components analysis (PCA) of XAS data allowed the extraction of spectra of individual phases in the material and their concentrations. For both Co and Fe two components were extracted, they correspond to fully lithiated and delithiated phases of LixMPO4 (where M = Fe, Co). Thus, we were able to track the phase transitions in the material upon charge and discharge and quantitatively analyze the M2+/M3+ electrochemical conversion rate for both Fe and Co. Rietveld's refinement of XRD data allowed us to analyze the changes in the lattice of cathode material and their reversibility upon (de)lithiation during cycling. The calculation of DFT and Bader charge analysis expects the oxygen redox procedure combined with d-metals redox, which supplements iron charge variations and dominates at high voltages when x < 0.75 in LixCoFePO4.

Onsite and intersite electronic correlations in the Hubbard model for halide perovskites

Halide perovskites (HPs) are widely viewed as promising photovoltaic and light-emitting materials for their suitable band gaps in the visible spectrum. Density functional theory (DFT) calculations employing (semi)local exchange-correlation functionals usually underestimate the band gaps for these systems. Accurate descriptions of the electronic structures of HPs often demand higher-order levels of theory such as the Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional and GW approximations that are much more computationally expensive than standard DFT. Here, we investigate three representative types of HPs, ABX3 halide perovskites, vacancy-ordered double perovskites (VODPs), and bond disproportionated halide perovskites (BDHPs), using DFT+U+V with onsite U and intersite V Hubbard parameters computed self-consistently without a priori assumption. The inclusion of Hubbard corrections improves the band gap prediction accuracy for all three types of HPs to a similar level of advanced methods. Moreover, the self-consistent Hubbard U is a meaningful indicator of the true local charge state of multivalence metal atoms in HPs. The inclusion of the intersite Hubbard V is crucial to properly capture the hybridization between valence electrons on neighboring atoms in BDHPs that have breathing-mode distortions of halide octahedra. In particular, the simultaneous convergence of both Hubbard parameters and crystal geometry enables a band gap prediction accuracy superior to HSE for BDHPs but at a fraction of the cost. Our work highlights the importance of using self-consistent Huabbard parameters when dealing with HPs that often possess intricate competitions between onsite localization and intersite hybridization.

Reconstruction of Cooperite (PtS) Surfaces: A DFT-D+U Study.

Cooperite (PtS) is one of the main sources of platinum in the world and has not been given much attention, in particular from the computational aspect. Besides, the surface stability of cooperite is not fully understood, in particular the preferred surface cleavage. In the current study, we employed computer modeling methods within the plane-wave framework of density functional theory with dispersion correction and the U parameter to correctly predict the bulk and surface properties. We reconstructed and calculated the geometries and surface energies of (001), (100), (101), (112), (110), (111), and (211) cooperite surfaces of stoichiometric planes. The Pt d-orbitals with U = 4.5 eV and S p-orbitals with U = 5.5 eV were found optimum to correctly predict a band gap of 1.408 eV for the bulk cooperite model, which agreed with an experimental value of 1.41 eV. The PtS-, Pt-, and S-terminated surfaces were investigated. The structural and electronic properties of the reconstructed surfaces were discussed in detail. We observed one major mechanism of relaxation of cooperite surface reconstructions that emerged from this study, which was the formation of Pt-Pt bonds. It emanated that the (110) and (111) cooperite surfaces underwent significant reconstruction in which the Pt2+ cation relaxed into the surface, forming new Pt-Pt (Pt2 2+) bonds. Similar behavior was perceived for (101) and (211) surfaces, where the Pt2+ cation relaxed inward and sideways on the surface, forming new Pt-Pt (Pt2 2+) bonds. The surface stability decreased in the order (101) > (100) ≈ (112) > (211) > (111) > (110) > (001), indicating that the (101) surface was the most stable, leading to an octahedron cooperite crystal morphology with truncated corners under equilibrium conditions. However, the electronic structures indicated that the chemical reactivity stability of the surfaces would be determined by band gaps. It was found that the (112) surface had a larger band gap than the other surfaces and thus was a chemical stability competitor to the (101) surface. In addition, it was established that the surfaces had different reactivities, which largely depended on the atomic coordination and charge state based on population atomic charges. This study has shown that cooperite has many planes/surface cleavages as determined by the computed crystal morphology, which is in agreement with experimental X-ray diffraction (XRD) pattern findings and the formation of irregular morphology shapes.

Effect of Li+, Mg2+, and Al3+ Substitution on the Performance of Nickel Ferrite-Based Hydroelectric Cells

Defect engineering in nickel ferrite has been performed to enhance the power output of the hydroelectric cell (HEC). Oxygen vacancies can be tuned in the ferrite using different valence element substitutions to enhance water dissociation by hydroelectric cells. In this work, Li+, Mg2+, and Al3+ substituted at the nickel site in nickel ferrite (NiFe2O4) are synthesized and studied for hydroelectric cell devices. Introducing these Li+, Mg2+, and Al3+ substituents (of a different atomic radius and valence charge state) in nickel ferrite leads to the generation of oxygen vacancy differently, which is discussed thoroughly in this work. In comparison to Mg and Al, lithium substitution lowers the overall cationic charge in nickel ferrite, leading to formation of a high number of oxygen vacancies for overall ionic charge compensation. Lithium-substituted nickel ferrite (NLFO) has a high number of defects (oxygen vacancies, etc.) compared to Mg-substituted (NMFO) and Al-substituted (NAFO) nickel ferrite, confirmed by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. The highest peak area of oxygen vacancies in the O 1s core-level spectra has been obtained for the case of NLFO among NLFO, NMFO, and NAFO. These vacancy sites provide a large number of adsorption sites for water molecule adsorption and dissociation. A high lattice strain (9.83 × 10–4) is induced in NLFO calculated by the Williamson–Hall plot. The porous nature of all samples has been confirmed from FESEM and BET analysis. The charge transfer processes in dry and wet HECs have been analyzed by fitting an equivalent RC circuit in the Nyquist spectra. The lithium-substituted nickel ferrite HEC with rich oxygen vacancies generates a high short-circuit current of ∼58.7 mA, which is ∼2 times higher than that of NMFO HEC and ∼3 times higher than that of NAFO HEC.

Electron Paramagnetic Resonance of Alkali Metal Atoms and Dimers on Ultrathin MgO.

Electron paramagnetic resonance (EPR) can provide unique insight into the chemical structure and magnetic properties of dopants in oxide and semiconducting materials that are of interest for applications in electronics, catalysis, and quantum sensing. Here, we demonstrate that EPR in combination with scanning tunneling microscopy (STM) allows for probing the bonding and charge state of alkali metal atoms on an ultrathin magnesium oxide layer on a Ag substrate. We observe a magnetic moment of 1 μB for Li2, LiNa, and Na2 dimers corresponding to spin radicals with a charge state of +1e. Single alkali atoms have the same charge state and no magnetic moment. The ionization of the adsorbates is attributed to charge transfer through the oxide to the metal substrate. Our work highlights the potential of EPR-STM to provide insight into dopant atoms that are relevant for the control of the electrical properties of surfaces and nanodevices.

Strong electronic interaction of indium oxide with palladium single atoms induced by quenching toward enhanced hydrogenation of nitrobenzene

The realization of efficient and fully controllable synthesis of single atom catalysts is an exciting frontier, yet still challenging in the modern catalysis field. Here we describe a straightforward high-temperature quenching approach to precisely construct isolated palladium atoms supported over cubic indium oxide, with individual palladium atoms coordinated with four neighboring oxygen atoms. This palladium catalyst achieves exceptional catalytic efficiency in the selective hydrogenation of nitrobenzene to aniline, with more than 99% chemoselectivity under almost 100% conversion. Moreover, it delivers excellent recyclability, anti-CO poisoning ability, storage stability, and substrate tolerance. DFT calculations further reveal that the high catalytic activity stems from the optimized electronic structure and the charge states of palladium atoms in the defect-containing indium oxide. Our findings provide an effective approach to engineering single atom catalysts at the atomic level and open the door to a wide variety of catalytic reactions.

Repeatable Photoinduced Insulator-to-Metal Transition in Yttrium Oxyhydride Epitaxial Thin Films

Currently, no known material retains its photoinduced metallic conductivity over a long time while also exhibiting repeatable metal-to-insulator recovery. Here, we demonstrate such a highly repeatable photoinduced insulator-to-metal transition in yttrium oxyhydride (YOxHy) epitaxial thin films. The temperature (T) dependence of the electrical resistivity (ρ) of the films transforms from the insulating to the metallic state (dρ/dT > 0) under ultraviolet laser illumination. The YOxHy film recovers its original insulating state when heated (125 °C) under an Ar atmosphere and regains metallic conductivity when subsequently subjected to ultraviolet laser illumination again, showing a repeatable photoinduced insulator-to-metal transition. First-principles calculations show that the itinerant carriers originate from the variations in the charge states of the hydrogen atoms that occupy octahedral interstitial sites. This study indicates that tuning the site occupancy (octahedral/tetrahedral) of the hydrogen atoms exerts a significant effect on the photoresponse of metal hydrides.

Electronic structure and stability of hexanuclear complex [Cu6(hfa)4(dpm)4(OH)4

Interatomic interactions and charge state of atoms in hexanuclear complex [Cu6(hfa)4(dpm)4(OH)4] have been studied by the methods of quantum chemistry and X-ray photoelectron spectroscopy (XPS). The shape and energy position of the Cu2p3/2-lines indicate that all copper atoms state could be interpreted as Cu2+. It was also analyzed the quantum chemical and XPS data on atomic charge distribution for monoligandic bis-β-diketonate Cu(hfa)2 and Cu(dpm)2 complexes. The comparison of calculated and experimental data for monoligandic and hexanuclear complexes revealed the electronic density redistribution on the oxygen and copper atoms in 1 due to the charge transfer interaction between Cu(hfa)2- and Cu(dpm)-fragments with participation of the bridging OH-groups, which was confirmed by NBO analysis.