Density Functional Calculations Research Articles

Electrochemically activated calcium sulfite for atrazine degradation: Performance, mechanism and kinetics

The conventional use of Na2SO3 in advanced oxidation processes leads to excessive dissolved SO32−, limiting the utilization of oxysulfur species. This study introduced an electrochemically activated CaSO3 (EC-CaSO3) process that exploited the slow-release property of CaSO3. The results showed that the removal ratios of several micropollutants were over 90 % within 10 min, with SO4•− as the primary oxidant. A mathematic model was further developed to simulate the radical conversion, analyze the degradation kinetics and optimize the operational parameters, using atrazine (ATZ) as the target pollutant. The continuous low concentration of SO32− promoted O2 evolution for oxysulfur species transformation and reduced SO4•− quenching. This resulted in 74 % of SO3•− converting to SO5•− and 84 % of SO32− contributing to SO4•− formation. In the initial 2 min, the matched electro-generation of SO3•− and O2 remarkably accelerated SO4•− generation, leading to 82 % removal of ATZ. As electrolysis continued, the reduced release rate of SO32− prevented excessive accumulation of SO32− and maintained high O2 evolution rate to drive SO4•− generation. Consequently, complete degradation of ATZ was achieved within 5 min, with the contribution of SO4•− exceeding 99 %. By investigating the influence of operational parameters on ATZ degradation and energy consumption, the cost-effective conditions (EE/O values < 2.2 kWh/m3/order) occurred at pH above 7, current densities of 60 to 90 A/m2 and initial CaSO3 dosages of 2 to 4 mM. Additionally, density functional calculation and LC-MS analysis confirmed the formation of less toxic intermediates through dealkylation and dechlorination-hydroxylation. These findings suggested that EC-CaSO3 process was a promising technology for oxidizing micropollutants.

Incorporation of Mo2C nanoparticles to pollen-derived 3D porous carbon as electrocatalyst for high performance lithium-sulfur batteries

The practical application and development of lithium‑sulfur (LiS) batteries are limited by the slow redox reaction kinetics and shuttle effect of polysulfide lithium (LiPSs). To address these issues, three-dimensional porous carbon (PC, derived from rapeseed pollen) modified with molybdenum carbide (Mo2C) nanoparticles is prepared by a simple and environment-friendly method. On the one hand, the PC-Mo2C host material can facilitate the uniform loading of sulfur and reduce its volume expansion due to its porous carbon structure. On the other hand, it mitigates the shuttle effect by effective chemical adsorption of LiPSs to form strong physical confinement and catalyze the conversion of LiPSs via the aid of the decorated Mo2C nanoparticles. Density functional theoretical calculations also illustrate the Mo2C nanoparticles are capable of inhibiting the shuttle of LiPSs and further improving the cycling stability. The results show that the PC-Mo2C/S electrode exhibits excellent electrochemical performance with high cycle stability (fading rate of 0.066 % per cycle at 0.2C and 0.054 % per cycle at 1C) and outstanding rate capacity (623.2 mAh g−1 at 5C).

Surface segregation on Ag@MNi (M = Pd, Pt, Rh, and Ru) core–shell nanocrystals for enhancing the oxygen reduction performance

Synthesis of cost-effective electrocatalysts for oxygen reduction reaction (ORR) has received wide attention due to their sluggish kinetics, which limits the development and application of fuel cell technique. In this work, we report a versatile synthetic approach to prepare a series of small-sized Ag@MNi (M = Pd, Pt, Rh, Ru) nanocrystals with core–shell structures by a simple solvothermal method. According to characterization analysis, surface segregation of Pd was proved in the outermost layer with the Pd-rich shell layer, which can regulate the adsorption energy of *OH. Electrochemical results show that the onset potential and half-wave potential of Ag@PdNi nanocrystals (Ag@PdNi NCs) are 1.005 and 0.913 V vs. RHE (reversible hydrogen electrode), respectively, with a Tafel slope of 55.31 mV dec-1 and strong ORR catalytic durability, which is similar to Ag@MNi (M = Pt, Rh, Ru) NCs and slightly higher than that of PdAg alloy nanoparticles (PdAg alloy NPs), PdNi alloy nanoparticles (PdNi alloy NPs), commercial Pd black and even commercial Pt/C, in alkaline medium. Density Functional (DFT) calculations show that the presence of Agcore effectively promotes the segregation of Pd atoms to the outer shell surface, where Ni provides better segregated substitution sites for Pd, which significantly improves the ORR activity and durability due to the electronic synergistic effect between Agcore nuclei and Pdshell can reduce the adsorption strength of OHads.

Memristive Artificial Synapses Based on Brownmillerite for Endurable Weight Modulation.

Exploring a computing paradigm that blends memory and computation functions is essential for artificial synapses. While memristors for artificial synapses are widely studied due to their energy-efficient structures, random filament conduction in general memristors makes them less preferred for endurability in long-term synaptic modulation. Herein, the topotactic phase transition (TPT) in brownmillerite-phased (110)-SrCoO2.5 (SCO2.5) is harnessed to enhance the reversibility of oxygen ion migration through 1-D oxygen vacancy channels. By employing a heteroepitaxial structured 2-terminal configuration of Au/SCO2.5/SrRuO3/SrTiO3, the brownmillerite SCO2.5-based synapse artificial synapses are exploited. Demonstration of the TPT behavior is corroborated by comparing oxygen migration energy by density-functional theory calculations and experimental results, and by monitoring the voltage pulse-induced peak shift in the Raman spectra of SCO2.5. With the voltage pulse-driven TPT behaviors, it is reliably characterized by linear, symmetric, and endurable long-term potentiation and depression performances. Notably, the durability of the TPT-based weight control mechanism is demonstrated by achieving consistent and noise-free weight updates over 32 000 iterations across 640 cycles. Furthermore, learning performances based on deep neural networks and convolutional neural networks on various image datasets yielded very high recognition accuracy. The work offers valuable insights into designing memristive synapses that enable reliable weight updates in neural networks.

New Two-Dimensional Materials Obtained by Functionalization of Boron Graphdiyne Layers with Nickel

The decoration of hexagonal boron graphdiyne (BGDY) layers with Ni atoms has been investigated by density functional calculations. For one, two, and three Ni atoms per hexagon, the BGDY structure is approximately maintained. Decoration with six Ni atoms per hexagon leads to the formation of a novel, very stable two-dimensional material in which the hexagonal structure of BGDY is substantially distorted. The Ni-doped materials have a semiconductor character, and the electronic band gap width can be tailored by varying the amount of adsorbed Ni. BGDY-2Ni, BGDY-3Ni, and BGDY-6Ni have electronic band gaps promising for infrared detectors. This work shows that computer simulation helps to discover new materials by the functionalization of layered carbon materials with metal atoms.

Optical spectra of silver clusters and nanoparticles from 4 to 923 atoms from the TDDFT+U method

The localized surface-plasmon resonances of coinage-metal clusters and nanoparticles enable many applications, the conception and necessary optimization of which require precise theoretical description and understanding. However, for the size range from few-atom clusters through nanoparticles of a few nanometers, where quantum effects and atomistic structure play a significant role, none of the methods employed previously has been able to provide high-quality spectra for all sizes. The main problem is the description of the filled shells of d electrons which influence the optical response decisively. We show that the DFT+U method, employed with real-time time-dependent density-functional theory calculations (RT-TDDFT), provides spectra in good agreement with experiment for silver clusters ranging from 4 to 923 atoms, the latter representing a nanoparticle of 3 nm. Both the electron-hole-type discrete spectra of the smallest clusters and the broad plasmon resonances of the larger sizes are obtained. All calculations use the value of the effective U parameter that provides good results in bulk silver. The agreement with experiment for all sizes shows that the U parameter is surprisingly transferable. Our results open the pathway for calculations of many practically relevant systems including clusters coupled to bio-molecules or to other nano-objects.

Electric Field Influence on CO Clathrate Hydrates.

We consider carbon monoxide (CO) confined in the hydrogen-bonded building blocks of sI and sII clathrate hydrates, viz., (512, 51262, 51264) cages, within the density functional theory-based calculations. We study their response to the applied electric fields in terms of changes in the geometrical parameters, dipole moment, HOMO-LUMO gap, and vibrational frequency shift. We examine the stability of CO clathrate hydrate building block cages and identify a possible indication of the field-induced release of CO.

High-performance fluoride removal by Fe/N co-doped microporous carbon: Mechanism of capacitive deionization with FeNx sites

Fluoride pollution is becoming increasingly serious with industrial growth. Controlling fluoride pollution have become important environmental issues of global concern. Transition metal nitrides are excellent CDI electrode materials due to their excellent conductivity and electrochemical stability. In this study, FeNx/C was synthesized as an electrode material for using the capacitive deionization (CDI) technique to achieve the effective removal of fluoride ions from wastewater. Its high specific surface area (463.278 mg/g) and high capacitance (44.29 F/g) were demonstrated and it had a high adsorption capacity of 158.33 mg/g. FeN3 and FeN4 were used as the main sites as capacitive deionization for removing fluoride from the water by chemisorption and hydrogen bonding. The higher adsorption energy by density functional calculation also indicated that the existence of Fe-N coordination bond effectively improved its electrosorption capacity. This study is beneficial to provide references and research ideas for removing fluoride in the CDI field.

Highly stable and active catalyst in fuel cells through surface atomic ordering.

Shape-controlled alloy nanoparticle catalysts have been shown to exhibit improved performance in the oxygen reduction reaction (ORR) in liquid half-cells. However, translating the success to catalyst layers in fuel cells faces challenges due to the more demanding operation conditions in membrane electrode assembly (MEA). Balancing durability and activity is crucial. Here, we developed a strategy that limits the atomic diffusion within surface layers, fostering the phase transition and shape retention during thermal treatment. This enables selective transformation of platinum-iron nanowire surfaces into intermetallic structures via atomic ordering at a low temperature. The catalysts exhibit enhanced MEA stability with 50% less Fe loss while maintaining high catalytic activity comparable to that in half-cells. Density functional calculations suggest that the ordered intermetallic surface stabilizes morphology against rapid corrosion and improves the ORR activity. The surface engineering through atomic ordering presents potential for practical application in fuel cells with shape-controlled Pt-based alloy catalysts.

From layered 2D carbon to 3D tetrahedral allotropes C12 and C18 with physical properties related to diamond: Crystal chemistry and DFT investigations

Mechanisms of changes from 2D to 3D (D = dimensionality) involving 2D C(sp2) trigonal paving to C(sp3) tetrahedral stacking are proposed through puckering of the 2D layers on one hand and interlayer insertion of extra C on the other hand. Such transformations, led to 3D hexagonal C12 and C18 allotropes respectively characterized by lon and bac topologies. Using density functional theory DFT calculations, the two allotropes were found cohesive and stable both mechanically (elastic properties) and dynamically (phonons). Comparisons of the physical properties with known uni C6 were established letting identify ranges of large Vickers hardness: HV (uni C6) = 89 GPa, HV (lon C12) = 97 GPa, and HV (bac C18) = 70 GPa. Whilst C6 was identified with acoustic phonons instability, C12 and C18 were found stable dynamically throughout the acoustic and optic frequency ranges. Furthering on the thermal properties the allotropes were characterized with a temperature dependence curve of the specific heat CV close to experimental data of diamond with best fit for novel C18. The electronic band structures reveal a small band gap of 1 eV for uni C6 and larger direct band gap of 3 eV for the two other 3D allotropes. Such modulations of the electronic and physical properties should open scopes of carbon research.

Hybrid Density Functional Study on the p-Type Conductivity Mechanism in Intrinsic Point Defects and Group V Element-Doped 2D β-TeO2.

The newly discovered two-dimensional (2D) β-TeO2 possesses extraordinarily high p-type carrier mobility and demonstrates immense potential in the electronics field. However, current research on its p-type conductivity mechanisms and the modifications of element doping remains relatively insufficient. In this study, the intrinsic point defects and extrinsic element doping in monolayer β-TeO2 are comprehensively analyzed to probe the potential sources of the intrinsic p-type conductivity and the extrinsic p-type doping possibility in 2D β-TeO2 through hybrid density functional calculations. Our results reveal that the vacancy defects with low formation energies have deep transition levels and thus cannot be used as sources of unintentional p-type conductivity in 2D β-TeO2. The investigations and discussions via Group V element doping modifications in 2D β-TeO2 indicate that bismuth (Bi) doping can easily and significantly enhance the p-type conductivity of 2D β-TeO2 under the presence of O-rich, which can be achieved experimentally. Furthermore, Bi doping can significantly increase carrier mobility without seriously affecting the electronic structure. The finding shows that the Bi element is an ideal dopant candidate for a p-type modification in 2D β-TeO2. Our calculations pave an alternative strategy to achieve the realization of superior p-type conductivity in 2D β-TeO2.

Importance of Orbital Invariance in Quantifying Electron-Hole Separation and Exciton Size.

A fundamental tenet of quantum mechanics is that properties should be independent of representation. In self-consistent field methods such as density functional theory, this manifests as a requirement that properties be invariant with respect to unitary transformations of the occupied molecular orbitals and (separately) the unoccupied molecular orbitals. Various ad hoc measures of excited-state charge separation that are commonly used to analyze time-dependent density-functional calculations violate this requirement, as they are based on incoherent averages of excitation amplitudes rather than expectation values involving coherent superpositions. As a result, these metrics afford markedly different values in various common representations, including canonical molecular orbitals, Boys-localized orbitals, and natural orbitals. Numerical values can be unstable with respect to basis-set expansion and may afford nonsensical results in the presence of extremely diffuse basis functions. In contrast, metrics based on well-defined expectation values are stable, representation-invariant, and physically interpretable. Use of natural transition orbitals improves the stability of the incoherent averages, but numerical values can only be interpreted as expectation value in the absence of superposition. To satisfy this condition, the particle and hole density matrices must each be dominated by a single eigenvector so that the transition density is well described by a single pair of natural transition orbitals. Counterexamples are readily found where this is not the case. Our results suggest that ad hoc charge-transfer diagnostics should be replaced by rigorous expectation values, which are no more expensive to compute.

Electronegativity Effects on Conformational Stability Using Bent's Rule: From Simple Molecules to Acetylcholine

Hybridization and electronegativity are fundamental concepts in chemistry connected by Bent's rule. This rule explains many aspects of the structural chemistry and reactivity of organic and inorganic compounds. Over decades, the application of Bent's rule has expanded, demonstrating its wide-ranging utility in elucidating molecular stability due to the substitution of highly electronegative atoms. This study successfully leverages Bent's rule to explain conformational energy difference in acetylcholine case using density-functional calculations. We used butane (Group 2) and butanone (Group 3) families to model the head of ACh+ and substituted one of the carbons with highly electronegative atoms: B, C, N, and O. This enabled us to evaluate the effects of electronegativity, as well as the presence of carbonyl groups, on their conformational stability and s character. There were three highlighted results. First, our calculations result in the s character are consistent with Bent's rule. Second, the high conformational energy differences can be attributed to the changes of s character in C2–Z bond: linear in Group 2 and exponential in Group 3, indicating the strong contribution of the carbonyl group. Third, the key determining factor in ACh+ conformational stability is the carbonyl group, which also strongly contributes to the solute-solvent interactions. Therefore, our study can be further applied to other similar molecules, potentially leading to broader applications in chemistry, especially for understanding ACh+ stability for Alzheimer’s Disease treatment.

Towards atomic-scale smooth surface manufacturing of β-Ga2O3 via highly efficient atmospheric plasma etching

Abstract The highly efficient manufacturing of atomic-scale smooth β-Ga2O3 surface is fairly challenging because β-Ga2O3 is a typical difficult-to-machine material. In this study, a novel plasma dry etching method named plasma-based atom-selective etching (PASE) is proposed to achieve the highly efficient, atomic-scale, and damage-free polishing of β-Ga2O3. The plasma is excited through the inductive coupling principle and carbon tetrafluoride is utilized as the main reaction gas to etch β-Ga2O3. The core of PASE polishing of β-Ga2O3 is the remarkable lateral etching effect, which is ensured by both the intrinsic property of the surface and the extrinsic temperature condition. As revealed by density functional theory-based calculations, the intrinsic difference in the etching energy barrier of atoms at the step edge (2.36 eV) and in the terrace plane (4.37 eV) determines their difference in the etching rate, and their etching rate difference can be greatly enlarged by increasing the extrinsic temperature. The polishing of β-Ga2O3 based on the lateral etching effect is further verified in the etching experiments. The Sa roughness of β-Ga2O3 (001) substrate is decreased from 14.8 to 0.057 nm within 120 s, and the corresponding material removal rate reaches up to 20.96 μm/min. The polished β-Ga2O3 displays significantly improved crystalline quality and photoluminescence intensity, and the polishing effect of PASE is independent of the crystal faces of β-Ga2O3. In addition, the competition between chemical etching and physical reconstruction, which is determined by temperature and greatly affects the surface state of β-Ga2O3, is deeply studied for the first time. These findings not only demonstrate the high-efficiency and high-quality polishing of β-Ga2O3 via atmospheric plasma etching but also hold significant implications for guiding future plasma-based surface manufacturing of β-Ga2O3.

A deep equivariant neural network approach for efficient hybrid density functional calculations

Hybrid density functional calculations are essential for accurate description of electronic structure, yet their widespread use is restricted by the substantial computational cost. Here we develop DeepH-hybrid, a deep equivariant neural network method for learning the hybrid-functional Hamiltonian as a function of material structure, which circumvents the time-consuming self-consistent field iterations and enables the study of large-scale materials with hybrid-functional accuracy. Our extensive experiments demonstrate good reliability as well as effective transferability and efficiency of the method. As a notable application, DeepH-hybrid is applied to study large-supercell Moiré-twisted materials, offering the first case study on how the inclusion of exact exchange affects flat bands in magic-angle twisted bilayer graphene. The work generalizes deep-learning electronic structure methods to beyond conventional density functional theory, facilitating the development of deep-learning-based ab initio methods.

Fermi surface and Lifshitz transitions of a ferromagnetic superconductor under external magnetic fields

Lifshitz transitions are being increasingly recognized as significant in a wide variety of strongly correlated and topological materials, and understanding the origin and influence of Lifshitz transitions is leading to deeper understanding of key aspects of magnetic, transport, or quantum critical behavior. In the ferromagnetic superconductor UCoGe, a magnetic field applied along the c axis has been shown to induce a series of anomalies in both transport and thermopower that may be caused by Lifshitz transitions. The need to understand the subtleties of the relationship between magnetism, superconductivity and a heavy-electron Fermi surface in the ferromagnetic superconductors makes it important to explore if and why a series of magnetic-field-induced Lifshitz transitions occurs in UCoGe. Here we report magnetic susceptibility measurements of UCoGe, performed at temperatures down to 45 mK and magnetic fields (μ0H||c) up to 30 T. We observe a series of clearly defined features in the susceptibility, and multiple sets of strongly field-dependent de Haas-van Alphen oscillations, from which we extract detailed field dependence of the quasiparticle properties. We complement our experimental results with density functional theory band structure calculations, and include a simple model of the influence of magnetic field on the calculated Fermi surface. By comparing experimental and calculated results, we determine the likely shape of the Fermi surface and identify candidate Lifshitz transitions that could correspond to two of the features in susceptibility. We connect these results to the development of magnetization in the system. Published by the American Physical Society 2024

Strange-metal behavior without fine tuning in PrV2Al20

Strange-metal behavior observed in the praseodymium-based heavy-fermion material PrV2Al20 has been tentatively interpreted in the framework of proximity to a quantum critical point (QCP) associated with quadrupolar ordering. Here, we demonstrate that an alternative, natural explanation exists without invoking a QCP, in terms of the unconventional nature of the quadrupolar Kondo effect taking place in non-Kramers ions. Using a combination of density-functional theory calculations and analytical arguments, we construct a periodic Anderson model with realistic parameters to describe PrV2Al20. We solve the model using dynamical mean-field theory preserving the model symmetries and demonstrate the non-Fermi liquid strange-metal behavior stemming from the two-channel nature of the quadrupolar Kondo effect. Our calculations provide an explanation for the puzzling temperature dependence in the magnetic susceptibility and provide a basis for analyzing future photoemission experiments. Published by the American Physical Society 2024

Flower-like NiFe layered double hydroxide/Bi2WO6 S-scheme heterojunction for photodegradation of ciprofloxacin under visible light

Photocatalytic degradation stands as an effective remedy to solve the global environmental pollution caused by organic pollutants. In this study, we synthesized a NiFe layered double hydroxide/Bi2WO6 (NF-LDH/BWO) Step-scheme (S-scheme) heterojunction. The attachment of NF-LDH particles on flower-like BWO increased the active sites and the interface area between two pure materials. The construction of LDH/BWO heterojunction endowed with improved light absorption capacity and boosted separation efficiency of photo-generated carriers. Consequently, compared with BWO and NF-LDH, the NF-LDH/BWO composites showed enhanced photoactivities. At NF-LDH mass ratio of 10%, the optimal degradation rate of ciprofloxacin (75.9%) within 60 min under visible illumination was achieved, 1.43- and 9.37-fold those of BWO and NF-LDH, respectively. In addition, the NF-LDH/BWO composite also demonstrated excellent cyclic stability. The S-scheme mechanism was proposed based on both experimental and density functional calculation results.

Structural, optoelectronic, and magnetic properties of Q‑carbon studied by hybrid density functional theory ab initio calculations and experiment

This work presents a theoretical study of the structural, optoelectronic, and magnetic properties of Q‑carbon, utilizing hybrid functionals within density functional theory (DFT) ab initio calculations. Moreover, experiments were conducted to measure structural parameters, dielectric functions, and magnetic properties, using various techniques. From the DFT simulations, structural, electronic, and optical properties have been simulated. The band gap energy was calculated using a hybrid approach, which combine HSE functional with different values of exact Hartree-Fock (HF) exchange (α). We propose a theoretical investigation of cubic and quasi-cubic forms of Q‑carbon. Our findings indicate that a non-cubic Q‑carbon cell is energetically more stable and possesses higher cell mass density and bulk modulus compared to a cubic cell. Calculations indicated that Q‑carbon exhibits semiconductor behavior; an indirect band gap of 3.10 eV and a direct band gap of about 3.4 eV was obtained for α = 0.44, in good agreement with experimental values. The density of states analysis allowed us to identify the p orbital of sp2 hybridized atoms as being prevalent on the top of the valence band and in the lowest energy conduction band states. Magnetic moment values of 0.37 μB and 0.41μB was found in two sp2 bonded C atoms, in accordance with the experimental observation. Several optical properties were calculated. The theoretical findings are compatible with the experimental results shown here and available in the literature, with excellent agreement found between the calculated optical band gap and that obtained from absorption measurements.

Switching of electrochemical selectivity due to plasmonic field-induced dissociation

Electrochemical reactivity is known to be dictated by the structure and composition of the electrocatalyst-electrolyte interface. Here, we show that optically generated electric fields at this interface can influence electrochemical reactivity insofar as to completely switch reaction selectivity. We study an electrocatalyst composed of gold-copper alloy nanoparticles known to be active toward the reduction of CO2 to CO. However, under the action of highly localized electric fields generated by plasmonic excitation of the gold-copper alloy nanoparticles, water splitting becomes favored at the expense of CO2 reduction. Real-time time-dependent density functional tight binding calculations indicate that optically generated electric fields promote transient-hole-transfer-driven dissociation of the O─H bond of water preferentially over transient-electron-driven dissociation of the C─O bond of CO2. These results highlight the potential of optically generated electric fields for modulating pathways, switching reactivity on/off, and even directing outcomes.