Cation doping and oxygen vacancies in the orthorhombic FeNbO4 material for solid oxide fuel cell applications: A density functional theory study

The orthorhombic phase of FeNbO4, a promising anode material for solid oxide fuel cells (SOFCs), exhibits good catalytic activity toward hydrogen oxidation. However, the low electronic conductivity of the material specifically in the pure structure without defects or dopants limits its practical applications as an SOFC anode. In this study, we have employed density functional theory (DFT + U) calculations to explore the bulk and electronic properties of two types of doped structures, Fe0.9375A0.0625NbO4 and FeNb0.9375B0.0625O4 (A, B = Ti, V, Cr, Mn, Co, Ni) and the oxygen-deficient structures Fe0.9375A0.0625NbO3.9375 and FeNb0.9375B0.0625O3.9375, where the dopant is positioned in the first nearest neighbor site to the oxygen vacancy. Our DFT simulations have revealed that doping in the Fe sites is energetically favorable compared to doping in the Nb site, resulting in significant volume expansion. The doping process generally requires less energy when the O-vacancy is surrounded by one Fe and two Nb ions. The simulated projected density of states of the oxygen-deficient structures indicates that doping in the Fe site, particularly with Ti and V, considerably narrows the bandgap to ∼0.5 eV, whereas doping with Co at the Nb sites generates acceptor levels close to 0 eV. Both doping schemes, therefore, enhance electron conduction during SOFC operation.

Fabrication of ZnO Scaffolded CdS Nanostructured Photoanodes with Enhanced Photoelectrochemical Water Splitting Activity under Visible Light

CdS, characterized by its comparatively narrow energy band gap (∼2.4 eV), is an appropriate material for prospective use as a photoanode in photoelectrochemical water splitting. Regrettably, it encounters several obstacles for practical and large-scale applications, including issues such as bulk carrier recombination and diminished conductivity. Here, we have tried to address these challenges by fabricating a novel photoelectrode (ZnO/CdS) composed of one-dimensional ZnO nanorods (NRs) decorated with two-dimensional CdS nanosheets (NSs). A facile two-step chemical method comprising electrodeposition along with chemical bath deposition is employed to synthesize the ZnO NRs, CdS NSs, and ZnO/CdS nanostructures. The prepared nanostructures have been investigated by UV-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy. The fabricated ZnO/CdS nanostructures have shown enhanced photoelectrochemical properties due to the improvement of the semiconductor junction surface area and thereby enhanced visible light absorption. The incorporation of CdS NSs has been further found to promote the rate of the charge separation and transfer process. Subsequently, the fabricated ZnO/CdS photoelectrodes achieved a photocurrent conversion efficiency 3 times higher than that of a planar ZnO NR photoanode and showed excellent performance under visible light irradiation. The highest applied bias photon-to-current conversion efficiency (% ABPE) of about ∼0.63% has been obtained for the sample with thicker CdS NSs on ZnO NRs with a photocurrent density of ∼1.87 mA/cm2 under AM 1.5 G illumination. The newly synthesized nanostructures further demonstrate that the full photovoltaic capacity of nanomaterials is yet to be exhausted.

Exploring the Redox Properties of the Low-Miller Index Surfaces of Copper Tungstate (CuWO4): Evaluating the Impact of the Environmental Conditions on the Water Splitting and Carbon Dioxide Reduction Processes

Photocatalysis has gained significant attention and interest as an environmentally friendly and sustainable approach to the production of hydrogen through water splitting and the reduction and conversion of CO2. Copper tungstate (CuWO4) is a highly promising candidate for these applications owing to its appropriate bandgap and superior stability under different conditions. However, the redox behavior of the CuWO4 surfaces under different environments and their impact on the morphology of the material nanoparticles, as well as the electronic properties, remain poorly understood. In this study, we have employed density functional theory calculations to investigate the properties of the bulk and pristine surfaces of CuWO4 and how the latter are impacted by oxygen chemisorption under the conditions required for photocatalytic water splitting and carbon dioxide reduction processes. We have calculated the lattice parameters and electronic properties of the bulk phase, as well as the surface energies of all possible nonpolar, stoichiometric, and symmetric terminations of the seven low-Miller index surfaces and found that the (010) and (110) facets are the thermodynamically most stable. The surface-phase diagrams were used to derive the equilibrium crystal morphologies, which show that the pristine (010) surface is prominent under synthesis and room conditions. Our crystal morphologies suggest that the partially oxidized (110) surface and the partially reduced (011) surface may play an important role in the photocatalytic splitting of water and CO2 conversion, respectively. Our results provide a comprehensive understanding of the CuWO4 surfaces under the conditions of important photocatalytic applications.

A Review of Theoretical Studies on Carbon Monoxide Hydrogenation via Fischer-Tropsch Synthesis over Transition Metals

The increasing demand for clean fuels and sustainable products has attracted much interest in the development of active and selective catalysts for CO conversion to desirable products. This review maps the theoretical progress of the different facets of most commercial catalysts, including Co, Fe, Ni, Rh, and Ru. All relevant elementary steps involving CO dissociation and hydrogenation and their dependence on surface structure, surface coverage, temperature, and pressure are considered. The dominant Fischer-Tropsch synthesis mechanism is also explored, including the sensitivity to the structure of H-assisted CO dissociation and direct CO dissociation. Low-coordinated step sites are shown to enhance catalytic activity and suppress methane formation. The hydrogen adsorption and CO dissociation mechanisms are highly dependent on the surface coverage, in which hydrogen adsorption increases, and the CO insertion mechanism becomes more favorable at high coverages. It is revealed that the chain-growth probability and product selectivity are affected by the type of catalyst and its structure as well as the applied temperature and pressure.

Development of 2-in-1 Sensors for the Safety Assessment of Lithium-Ion Batteries via Early Detection of Vapors Produced by Electrolyte Solvents

Batteries play a critical role in achieving zero-emission goals and in the transition toward a more circular economy. Ensuring battery safety is a top priority for manufacturers and consumers alike, and hence is an active topic of research. Metal-oxide nanostructures have unique properties that make them highly promising for gas sensing in battery safety applications. In this study, we investigate the gas-sensing capabilities of semiconducting metal oxides for detecting vapors produced by common battery components, such as solvents, salts, or their degassing products. Our main objective is to develop sensors capable of early detection of common vapors produced by malfunctioning batteries to prevent explosions and further safety hazards. Typical electrolyte components and degassing products for the Li-ion, Li-S, or solid-state batteries that were investigated in this study include 1,3-dioxololane (C₃H₆O₂─DOL), 1,2-dimethoxyethane (C₄H₁₀O₂─DME), ethylene carbonate (C₃H₄O₃─EC), dimethyl carbonate (C₄H₁₀O₂─DMC), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium nitrate (LiNO₃) salts in a mixture of DOL and DME, lithium hexafluorophosphate (LiPF₆), nitrogen dioxide (NO₂), and phosphorous pentafluoride (PF₅). Our sensing platform was based on ternary and binary heterostructures consisting of TiO₂(111)/CuO(1̅11)/Cu₂O(111) and CuO(1̅11)/Cu₂O(111), respectively, with various CuO layer thicknesses (10, 30, and 50 nm). We have analyzed these structures using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), micro-Raman spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy. We found that the sensors reliably detected DME C₄H₁₀O₂ vapors up to a concentration of 1000 ppm with a gas response of 136%, and concentrations as low as 1, 5, and 10 ppm with response values of approximately 7, 23, and 30%, respectively. Our devices can serve as 2-in-1 sensors, functioning as a temperature sensor at low operating temperatures and as a gas sensor at temperatures above 200 °C. Density functional theory calculations were also employed to study the adsorption of the vapors produced by battery solvents or their degassing products, as well as water, to investigate the impact of humidity. PF₅ and C₄H₁₀O₂ showed the most exothermic molecular interactions, which are consistent with our gas response investigations. Our results indicate that humidity does not impact the performance of the sensors, which is crucial for the early detection of thermal runaway under harsh conditions in Li-ion batteries. We show that our semiconducting metal-oxide sensors can detect the vapors produced by battery solvents and degassing products with high accuracy and can serve as high-performance battery safety sensors to prevent explosions in malfunctioning Li-ion batteries. Despite the fact that the sensors work independently of the type of battery, the work presented here is of particular interest for the monitoring of solid-state batteries, since DOL is a solvent typically used in this type of batteries.

A DFT Study of Ruthenium fcc Nano-Dots: Size-Dependent Induced Magnetic Moments

Many areas of electronics, engineering and manufacturing rely on ferromagnetic materials, including iron, nickel and cobalt. Very few other materials have an innate magnetic moment rather than induced magnetic properties, which are more common. However, in a previous study of ruthenium nanoparticles, the smallest nano-dots showed significant magnetic moments. Furthermore, ruthenium nanoparticles with a face-centred cubic (fcc) packing structure exhibit high catalytic activity towards several reactions and such catalysts are of special interest for the electrocatalytic production of hydrogen. Previous calculations have shown that the energy per atom resembles that of the bulk energy per atom when the surface-to-bulk ratio < 1, but in its smallest form, nano-dots exhibit a range of other properties. Therefore, in this study, we have carried out calculations based on the density functional theory (DFT) with long-range dispersion corrections DFT-D3 and DFT-D3-(BJ) to systematically investigate the magnetic moments of two different morphologies and various sizes of Ru nano-dots in the fcc phase. To confirm the results obtained by the plane-wave DFT methodologies, additional atom-centred DFT calculations were carried out on the smallest nano-dots to establish accurate spin-splitting energetics. Surprisingly, we found that in most cases, the high spin electronic structures had the most favourable energies and were hence the most stable.

Water-Driven Synthesis of Deep-Blue Perovskite Colloidal Quantum Wells for Electroluminescent Devices

Perovskite colloidal quantum wells (QWs) are promising to realize narrow deep-blue emission, but the poor optical performance and stability suppress their practical application. Here, we creatively propose a water-driven synthesis strategy to obtain size-homogenized and strongly confined deep-blue CsPbBr₃ QWs, corresponding to three monolayers, which emit at the deep-blue wavelength of 456 nm. The water controls the orientation and distribution of the ligands on the surface of the nanocrystals, thus inducing orientated growth through the Ostwald ripening process by phagocytizing unstable nanocrystals to form well-crystallized QWs. These QWs present remarkable stability and high photoluminescence quantum yield of 94 %. Furthermore, we have prepared light-emitting diodes based on the QWs via the all-solution fabrication strategy, achieving an external quantum efficiency of 1 % and luminance of 2946 cd m^-2 , demonstrating state-of-the-art brightness for perovskite QW-based LEDs.

Oxygen diffusion in the orthorhombic FeNbO4 material: a computational study

ABO₄-type materials have shown significant potential for applications as luminescence and photocatalytic materials, and the orthorhombic FeNbO₄ (o-FeNbO₄) material has also shown excellent promise in catalytic electrodes, unlike other common ABO₄ materials. However, little computational work has been carried out on the o-FeNbO₄ structure, potentially because it is disordered and thus not straightforward to simulate. In this work, we first confirmed the accuracy of the force field parameters obtained from previous studies through optimizations carried out using the GULP code. Next, we found that one ordered configuration of the stoichiometric o-FeNbO₄ structure dominates when analysing the probabilities of cation disorder in three supercells (2 × 2 × 1, 2 × 1 × 2, and 1 × 2 × 2). We then studied the bulk properties of this selected o-FeNbO₄ through DFT calculations, including the lattice parameters, the mechanical properties and the electronic structures, where no remarkable differences were observed compared to the monoclinic FeNbO₄ structure. Finally, because oxygen mobility is key to the successful application of o-FeNbO₄ as an electrode material, we have simulated the diffusion pathways of oxygen through both the stoichiometric and non-stoichiometric structures, where the results show that the existence of oxygen vacancies enhances diffusion and the distribution of the Fe and Nb inside the lattice affects the energy barriers and could therefore impact the oxygen diffusion.

Carbon dioxide hydrogenation over the carbon-terminated niobium carbide (111) surface: a density functional theory study

Carbon dioxide (CO₂) hydrogenation is an energetic process which could be made more efficient through the use of effective catalysts, for example transition metal carbides. Here, we have employed calculations based on the density functional theory (DFT) to evaluate the reaction processes of CO₂ hydrogenation to methane (CH₄), carbon monoxide (CO), methanol (CH₃OH), formaldehyde (CH₂O), and formic acid (HCOOH) over the carbon-terminated niobium carbide (111) surface. First, we have studied the adsorption geometries and energies of 25 different surface-adsorbed species, followed by calculations of all of the elementary steps in the CO₂ hydrogenation process. The theoretical findings indicate that the NbC (111) surface has higher catalytic activity towards CO₂ methanation, releasing 4.902 eV in energy. CO represents the second-most preferred product, followed by CH₃OH, CH₂O, and HCOOH, all of which have exothermic reaction energies of 4.107, 2.435, 1.090, and 0.163 eV, respectively. Except for the mechanism that goes through HCOOH to produce CH₂O, all favourable hydrogenation reactions lead to desired compounds through the creation of the dihydroxycarbene (HOCOH) intermediate. Along these routes, CH₃* hydrogenation to CH₄* has the highest endothermic reaction energy of 3.105 eV, while CO production from HCO dehydrogenation causes the highest exothermic reaction energy of -3.049 eV. The surface-adsorbed CO₂ hydrogenation intermediates have minimal effect on the electronic structure and interact only weakly with the surface. Our results are consistent with experimental observations.

Recalibrating the calcium trap in amino acid carboxyl groups via classical molecular dynamics simulations

In order to use classical molecular dynamics to complement experiments accurately, it is important to use robust descriptions of the system. The interactions between biomolecules, like aspartic and glutamic acid, and dissolved ions are often studied using standard biomolecular force-fields, where the interactions between biomolecules and cations are often not parameterized explicitly. In this study, we have employed metadynamics simulations to investigate different interactions of Ca with aspartic and glutamic acid and constructed the free energy profiles of Ca²⁺-carboxylate association. Starting from a generally accepted, AMBER-based force field, the association was substantially over and under-estimated, depending on the choice of water model (TIP3P and SPC/fw, respectively). To rectify this discrepancy, we have replaced the default calcium parameters. Additionally, we modified the σ_ij value in the hetero-atomic Lennard-Jones interaction by 0.5% to further improve the interaction between Ca and carboxylate, based on comparison with the experimentally determined association constant for Ca with the carboxylate group of L-aspartic acid. The corrected description retrieved the structural properties of the ion pair in agreement with the original biomolecule - Ca²⁺ interaction in AMBER, whilst also producing an association constant comparable to experimental observations. This refined force field was then used to investigate the interactions between amino acids, calcium and carbonate ions during biogenic and biomimetic calcium carbonate mineralisation.

Mind the Interface Gap: Exposing Hidden Interface Defects at the Epitaxial Heterostructure between CuO and Cu2O

Well designed and optimized epitaxial heterostructures lie at the foundation of materials development for photovoltaic, photocatalytic, and photoelectrochemistry applications. Heterostructure materials offer tunable control over charge separation and transport at the same time preventing recombination of photogenerated excitations at the interface. Thus, it is of paramount importance that a detailed understanding is developed as the basis for further optimization strategies and design. Oxides of copper are nontoxic, low cost, abundant materials with a straightforward and stable manufacturing process. However, in individual applications, they suffer from inefficient charge transport of photogenerated carriers. Hence, in this work, we investigate the role of the interface between epitaxially aligned CuO and Cu₂O to explore the potential benefits of such an architecture for more efficient electron and hole transfer. The CuO/Cu₂O heterojunction nature, stability, bonding mechanism, interface dipole, electronic structure, and band bending were rationalized using hybrid density functional theory calculations. New electronic states are identified at the interface itself, which are originating neither from lattice mismatch nor strained Cu-O bonds. They form as a result of a change in coordination environment of CuO surface Cu²⁺ cations and an electron transfer across the interface Cu¹⁺-O bond. The first process creates occupied defect-like electronic states above the valence band, while the second leaves hole states below the conduction band. These are constitutional to the interface and are highly likely to contribute to recombination effects competing with the improved charged separation from the suitable band bending and alignment and thus would limit the expected output photocurrent and photovoltage. Finally, a favorable effect of interstitial oxygen defects has been shown to allow for band gap tunability at the interface but only to the point of the integral geometrical contact limit of the heterostructure itself.

Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 μM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H₂ gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H₂ gas. A dual-mode nanosensor is shown to detect UV/H₂ gas simultaneously for selective detection of H₂ during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Catalytic formation of oxalic acid on the partially oxidised greigite Fe3S4(001) surface

Greigite (Fe₃S₄), with its ferredoxin-like 4Fe-4S redox centres, is a naturally occurring mineral capable of acting as a catalyst in the conversion of carbon dioxide (CO₂) into low molecular-weight organic acids (LMWOAs), which are of paramount significance in several soil and plant processes as well as in the chemical industry. In this paper, we report the reaction between CO₂ and water (H₂O) to form oxalic acid (H₂C₂O₄) on the partially oxidised greigite Fe₃S₄(001) surface by means of spin-polarised density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+U-D2). We have calculated the bulk phase of Fe₃S₄ and the two reconstructed Tasker type 3 terminations of its (001) surface, whose properties are in good agreement with available experimental data. We have obtained the relevant phase diagram, showing that the Fe₃S₄(001) surface becomes 62.5% partially oxidised, by replacing S by O atoms, in the presence of water at the typical conditions of calcination [Mitchell et al. Faraday Discuss. 2021, 230, 30-51]. The adsorption and co-adsorption of the reactants on the partially oxidised Fe₃S₄(001) surface are exothermic processes. We have considered three mechanistic pathways to explain the formation of H₂C₂O₄, showing that the coupling of the C-C bond and second protonation are the elementary steps with the largest energy penalty. Our calculations suggest that the partially oxidised Fe₃S₄(001) surface is a mineral phase that can catalyse the formation of H₂C₂O₄ under favourable conditions, which has important implications for natural ecosystems and is a process that can be harnessed for the industrial manufacture of this organic acid.

Al2O3/ZnO Heterostructure-Based Sensors for Volatile Organic Compounds in Safety Applications

Monitoring volatile organic compounds (VOCs) in harsh environments, especially for safety applications, is a growing field that requires specialized sensor structures. In this work, we demonstrate the sensing properties toward the most common VOCs of columnar Al₂O₃/ZnO heterolayer-based sensors. We have also developed an approach to tune the sensor selectivity by changing the thickness of the exposed amorphous Al₂O₃ layer from 5 to 18 nm. Columnar ZnO films are prepared by a chemical solution method, where the exposed surface is decorated with an Al₂O₃ nanolayer via thermal atomic layer deposition at 75 °C. We have investigated the structure and morphology as well as the vibrational, chemical, electronic, and sensor properties of the Al₂O₃/ZnO heterostructures. Transmission electron microscopy (TEM) studies show that the upper layers consist of amorphous Al₂O₃ films. The heterostructures showed selectivity to 2-propanol vapors only within the range of 12-15 nm thicknesses of Al₂O₃, with the highest response value of ∼2000% reported for a thickness of 15 nm at the optimal working temperature of 350 °C. Density functional theory (DFT) calculations of the Al₂O₃/ZnO(1010) interface and its interaction with 2-propanol (2-C₃H₇OH), n-butanol (n-C₄H₉OH), ethanol (C₂H₅OH), acetone (CH₃COCH₃), hydrogen (H₂), and ammonia (NH₃) show that the molecular affinity for the Al₂O₃/ZnO(1010) interface decreases from 2-propanol (2-C₃H₇OH) ≈ n-butanol (n-C₄H₉OH) > ethanol (C₂H₅OH) > acetone (CH₃COCH₃) > hydrogen (H₂), which is consistent with our gas response experiments for the VOCs. Charge transfers between the surface and the adsorbates, and local densities of states of the interacting atoms, support the calculated strength of the molecular preferences. Our findings are highly important for the development of 2-propanol sensors and to our understanding of the effect of the heterojunction and the thickness of the top nanolayer on the gas response, which thus far have not been reported in the literature.

AuCo nanoparticles: ordering, magnetisation, and morphology trends predicted by DFT

The rapid development of applications relying on magnetism at the nanoscale has put a spotlight on nanoparticles with novel morphologies that are associated with enhanced electronic and magnetic properties. In this quest, nanoalloys combining highly magnetic cobalt and weakly reactive gold could offer many application-specific advantages, such as strong magnetic anisotropy. In the present study, we have employed density functional theory (DFT) calculations to provide a systematic overview of the size- and morphology-dependence of the energetic order and magnetic properties of AuCo nanoparticles up to 2.5 nm in diameter. The core-shell icosahedron was captured as the most favourable morphology, showing a small preference over the core-shell decahedron. However, the magnetic properties (total magnetic moments and magnetic anisotropy) were found to be significantly improved within the L1₀ ordered structures, even in comparison to monometallic Co nanoparticles. Atom-resolved charges and orbital moments accessed through the DFT analysis of the electronic level properties permitted insight into the close interrelation between the AuCo nanoparticle morphology and their magnetism. These results are expected to assist in the design of tailored magnetic AuCo nanoalloys for specific applications.

Photocatalytic Degradation of Rhodamine B Dye and Hydrogen Evolution by Hydrothermally Synthesized NaBH4—Spiked ZnS Nanostructures

Metal sulphides, including zinc sulphide (ZnS), are semiconductor photocatalysts that have been investigated for the photocatalytic degradation of organic pollutants as well as their activity during the hydrogen evolution reaction and water splitting. However, devising ZnS photocatalysts with a high overall quantum efficiency has been a challenge due to the rapid recombination rates of charge carriers. Various strategies, including the control of size and morphology of ZnS nanoparticles, have been proposed to overcome these drawbacks. In this work, ZnS samples with different morphologies were prepared from zinc and sulphur powders via a facile hydrothermal method by varying the amount of sodium borohydride used as a reducing agent. The structural properties of the ZnS nanoparticles were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. All-electron hybrid density functional theory calculations were employed to elucidate the effect of sulphur and zinc vacancies occurring in the bulk as well as (220) surface on the overall electronic properties and absorption of ZnS. Considerable differences in the defect level positions were observed between the bulk and surface of ZnS while the adsorption of NaBH₄ was found to be highly favourable but without any significant effect on the band gap of ZnS. The photocatalytic activity of ZnS was evaluated for the degradation of rhodamine B dye under UV irradiation and hydrogen generation from water. The ZnS nanoparticles photo-catalytically degraded Rhodamine B dye effectively, with the sample containing 0.01 mol NaBH₄ being the most efficient. The samples also showed activity for hydrogen evolution, but with less H₂ produced compared to when untreated samples of ZnS were used. These findings suggest that ZnS nanoparticles are effective photocatalysts for the degradation of rhodamine B dyes as well as the hydrogen evolution, but rapid recombination of charge carriers remains a factor that needs future optimization.

Understanding the Interactions between Triolein and Cosolvent Binary Mixtures Using Molecular Dynamics Simulations

Biodiesel is one of the emerging renewable sources of energy to replace fossil-fuel-based resources. It is produced by a transesterification reaction in which a triglyceride reacts with methanol in the presence of a catalyst. The reaction is slow because of the low solubility of methanol in triglycerides, which results in low concentrations of methanol available to react with triglyceride. To speed up the reaction, cosolvents are added to create a single phase which helps to improve the concentration of methanol in the triglyceride phase. In this study, molecular dynamics simulations are used to help understand the role of cosolvents in the solvation of triglyceride (triolein). Six binary mixtures of triolein/cosolvent were used to study the solvation of triolein at 298.15 K. Results of 100 ns simulations at constant temperature and pressure to simulate mixing experiments show that in the first 10 ns all the binary mixtures remain largely unmixed. However, for the cosolvents that are fully miscible with triolein, the partial densities across the simulation boxes show that the systems are fully mixed in the final 10 ns. Some solvents were found to interact strongly with the polar part of triolein, while others interacted with the aliphatic part. The radial distribution functions and clustering of the solvents around triolein were also used as indicators for solvation. The presence of cosolvents also influenced the conformation of triolein molecules. In the presence of solvents that solubilize it, triolein preferred a propeller conformation but took up a trident conformation when there is less or no solubilization. The results show that tetrahydrofuran is the best solvent at solubilizing triolein, followed by cyclopentyl methyl ether, diethyl ether, and hexane. With 1,4-dioxane, the solubility improves with an increase in temperature. The miscibility of a solvent in triolein is aided by its ability to interact with both the polar and nonpolar parts of triolein.

Catalytic Reduction of Carbon Dioxide on the (001), (011), and (111) Surfaces of TiC and ZrC: A Computational Study

We present a computational study of the activity and selectivity of early transition-metal carbides as carbon dioxide reduction catalysts. We analyze the effects of the adsorption of CO₂ and H₂ on the (001), (011), and metal-terminated (111) surfaces of TiC and ZrC, as carbon dioxide undergoes either dissociation to CO or hydrogenation to COOH or HCOO. The relative stabilities of the three reduction intermediates and the activation energies for their formation allow the identification of favored pathways on each surface, which are examined as they lead to the release of CO, HCOOH, CH₃OH, and CH₄, thereby also characterizing the activity and selectivity of the two materials. Reaction energetics implicate HCO as the key common intermediate on all surfaces studied and rule out the release of formaldehyde. Surface hydroxylation is shown to be highly selective toward methane production as the formation of methanol is hindered on all surfaces by its barrierless conversion to CO.

A DFT Mechanistic Study on Base-Catalyzed Cleavage of the β-O-4 Ether Linkage in Lignin: Implications for Selective Lignin Depolymerization

The detailed mechanism of the base-catalyzed C-C and C-O bond cleavage of a model compound representing the β-O-4 linkage in lignin is elucidated using DFT calculations at the M06/6-31G* level of theory. Two types of this linkage have been studied, a C2 type which contains no γ-carbinol group and a C3 type which contains a γ-carbinol. Cleavage of the C2 substrate is seen to proceed via a 6-membered transition structure involving the cation of the base, the hydroxide ion and the α-carbon adjacent to the ether bond. The reaction with KOH has the lowest activation barrier of 6.1 kcal mol⁻¹ with a calculated rate constant of 2.1 × 10⁸ s⁻¹. Cleavage of the C3 substrate is found to proceed via two pathways: an enol-formation pathway and an epoxide-formation pathway. The first path is the thermodynamically favored pathway which is similar to the pathway for the C2 substrate and is the preferred pathway for the isolation of an enol-containing monomer. The second path is the kinetically favored pathway, which proceeds via an 8-membered transition state involving a hydrogen hopping event, and is the preferred pathway for the isolation of an epoxide-containing monomer. The KOH-catalyzed reaction also has the lowest activation barrier of 10.1 kcal mol⁻¹ along the first path and 3.9 kcal mol⁻¹ along the second path, with calculated rate constants of 2.4 × 10⁵s⁻¹ and 8.6 × 10⁹s⁻¹ respectively. Overall, the results provide clarity on the mechanism for the base-catalyzed depolymerization of lignin to phenolic monomers. The results also suggest both NaOH and KOH to be the preferred catalysts for the cleavage of the β-O-4 linkage in lignin.

A first-principles study of CO2 hydrogenation on Niobium-terminated NbC (111) surface

As a promising material for the reduction of Greenhouse gas, Transition metal carbides which are highly active in the hydrogenation of CO2 are mainly considered. In this regard, the reaction mechanism of CO2 hydrogenation to useful products on the Nb-terminated NbC (111) surface is investigated by applying density functional theory calculations. The computational results display that formation of CH4 , CH3OH and CO are more favored than other compounds, where CH4 is the dominant product. In addition, the findings from reaction energies reveal that the preferred mechanism for CO2 hydrogenation is thorough HCOOH * where the largest exothermic reaction energy releases during HCOOH * dissociation reaction (2.004eV). The preferred mechanism of CO2 hydrogenation towards CH 4 production is CO2 *→ t,c-COOH *→ HCOOH *→ HCO *→ CH2O *→ CH2OH *→ CH2 *→ CH3 *→ CH4 * where CO2 * → t,c-COOH * → HCOOH * → HCO * → CH2O * → CH2OH * → CH3OH * and CO2 * → t,c-COOH * → CO * are also found as the favored mechanisms for CH3 OH and CO productions thermodynamically, respectively. During the mentioned mechanisms the hydrogenation of CH2O * to CH2OH * has the largest endothermic reaction energy of 1.344 eV. It is also found from the electronic properties calculations that Nb-terminated NbC (111) is a suitable catalyst for CO2 hydrogenation where adsorption and activation of CO2 and also desorption of final products can be easily done on the surface.

Insights from density functional theory calculations into the effects of the adsorption and dissociation of water on the surface properties of zinc diphosphide (ZnP2) nanocrystals

Zinc phosphides (ZnP₂ and Zn₃P₂) are emerging absorber materials for photovoltaic applications owing to their abundancy and non-toxic nature. Herein, we provide a comprehensive characterisation of the surface structure, composition, stabilities, morphology, and electronic properties of both bare and hydrated/hydroxylated low-Miller index surfaces of β-ZnP₂ by means of density functional theory (DFT) calculations. Mechanistic insights into the fundamental aspects of water adsorption and dissociation, including the adsorption geometries, energetics, and structural parameters along the reaction path are systematically characterised. The stabilities of the surfaces under dry and wet conditions are discussed in detail and the predicted phase diagrams for the water adsorption are presented. Using calculated surface energies, we have derived the equilibrium morphology of the β-ZnP₂ nanocrystals under vacuum and upon hydration or hydroxylation. Atomic-level insights into the origin of the incipient oxidation of β-ZnP₂ surfaces are provided through analysis of Bader charges, which reveal that the Zn sites to which H₂O and OH species are bound undergo oxidation due to the transfer of charge to the adsorbed species. Adsorption-induced changes to the electronic properties before and after hydration/hydroxylation were characterised by the work function and partial density of states. The results highlight the need for protection of β-ZnP₂ nanocrystals against possible oxidation in the presence of water through post-synthesis organic functionalisation.

Density Functional Theory Study of Ethylene Carbonate Adsorption on the (0001) Surface of Aluminum Oxide α-Al2O3

Surface coating is one of the techniques used to improve the electrochemical performance and enhance the resistance against decomposition of cathode materials in lithium-ion batteries. Despite several experimental studies addressing the surface coating of secondary Li-ion batteries using α-Al₂O₃, the reactivity of the material toward the electrolyte components is not yet fully understood. Here, we have employed calculations based on the density functional theory to investigate the adsorption of the organic solvent ethylene carbonate (EC) on the major α-Al₂O₃(0001) surface. During adsorption of a single EC molecule, it was found that it prefers to bind parallel to the surface through its carboxyl oxygen. As the surface coverage (θ) was increased up to a monolayer, we observed larger adsorption energies per EC molecule (E _ads/N _EC) for parallel interactions and a reduction for perpendicular interactions. We also noted that increasing the surface coverage with both parallel and perpendicularly interacting EC molecules led to a decrease of the surface free energies and hence increased stability of the α-Al₂O₃(0001) surface. Despite the larger E _ads/N _EC observed when the molecule was placed parallel to the surface, minimal charge transfer was calculated for single EC interactions and at higher surface coverages. The simulated scanning tunneling microscopy images are also presented for a clean corundum α-Al₂O₃ surface and after adsorption with different coverages of parallel and perpendicularly placed EC molecules.

Magnetic structure and exchange interactions in pyrrhotite end member minerals: hexagonal FeS and monoclinic Fe7S8

Iron mono-sulphides, or pyrrhotites, are minerals present in the Earth's crust and mantle as well as major magnetic constituents of several classes of meteorites, thus are of interest to a wide range of disciplines including geology, geophysics, geochemistry, and material science. Despite displaying diverse magnetic properties as a result of iron vacancy ordering, the underlying exchange mechanism has not been quantified. This study presents an examination of the electronic and magnetic properties for the two pyrrhotite group end members, hexagonal FeS and monoclinic Fe₇S₈(4C superstructure) by means of density functional theory coupled with a Heisenberg magnetic model. The easy magnetization axes of FeS and Fe₇S₈are found to be positioned along the crystallographicc-direction and at an angle of 56° to thec-direction, respectively. The magnetic anisotropy energy in Fe₇S₈is greatly increased as a consequence of the vacancy framework when compared to FeS. The main magnetic interaction, in both compounds, is found to be the isotropic exchange interaction favouring antiferromagnetic alignment between nearest-neighbouring spins. The origin of the exchange interaction is elucidated further following the Goodenough-Kanamori-Anderson rules. The antisymmetric spin exchange is found to have a minor effect in both compounds. The theoretical findings presented in this work thus help to further resolve some of the ambiguities in the magnetic features of pyrrhotites.

Arc Synthesis, Crystal Structure, and Photoelectrochemistry of Copper(I) Tungstate

A little-studied p-type ternary oxide semiconductor, copper(I) tungstate (Cu₂WO₄), was assessed by a combined theoretical/experimental approach. A detailed computational study was performed to solve the long-standing debate on the space group of Cu₂WO₄, which was determined to be triclinic P1. Cu₂WO₄ was synthesized by a time-efficient, arc-melting method, and the crystalline reddish particulate product showed broad-band absorption in the UV-visible spectral region, thermal stability up to ∼260 °C, and cathodic photoelectrochemical activity. Controlled thermal oxidation of copper from the Cu(I) to Cu(II) oxidation state showed that the crystal lattice could accommodate Cu²⁺ cations up to ∼260 °C, beyond which the compound was converted to CuO and CuWO₄. This process was monitored by powder X-ray diffraction and X-ray photoelectron spectroscopy. The electronic band structure of Cu₂WO₄ was contrasted with that of the Cu(II) counterpart, CuWO₄ using spin-polarized density functional theory (DFT). Finally, the compound Cu₂WO₄ was determined to have a high-lying (negative potential) conduction band edge underlining its promise for driving energetic photoredox reactions.

Predicting the Membrane Permeability of Fentanyl and Its Analogues by Molecular Dynamics Simulations

The lipid membrane is considered a crucial component of opioid general anesthesia. The main drug used for the induction and maintenance of opioid anesthesia is fentanyl and its various analogues. However, these drugs have different clinical effects, and detailed atomic-level insight into the drug–membrane interactions could lead to a better understanding how these drugs exert their anesthetic properties. In this study, we have used extensive umbrella sampling molecular dynamics simulations to study the permeation process of fentanyl and three of its analogues into a variety of simple phospholipid membrane models. Our simulations show that we can accurately predict the permeability coefficients of these drug molecules, which is an important process in understanding how pharmaceuticals reach their molecular targets. We were also able to show that one phospholipid provides more accurate predictions than other lipids commonly used in these types of permeation studies, which will aid future studies of these types of processes.

How bulk and surface properties of Ti4SiC3, V4SiC3, Nb4SiC3 and Zr4SiC3 tune reactivity: a computational study

We present several in silico insights into the MAX-phase of early transition metal silicon carbides and explore how these affect carbon dioxide hydrogenation. Periodic density functional methodology is applied to models of Ti4SiC3, V4SiC3, Nb4SiC3 and Zr4SiC3. We find that silicon and carbon terminations are unstable, with sintering occurring in vacuum and significant reconstruction taking place under an oxidising environment. In contrast, the metal terminated surfaces are highly stable and very active towards CO2 reduction. However, we predict that under reaction conditions these surfaces are likely to be oxidised. These results are compared to studies on comparable materials and we predict optimal values for hydrogen evolution and CO2 reduction.

Effect of coverage on the magnetic properties of -COOH, -SH, and -NH2 ligand-protected cobalt nanoparticles

Implementation of magnetic nanoparticles in biomedicine requires their passivation, which often comes at a cost of diminished magnetic properties. For the design of nano-agents with targeted magnetic behaviour, it is important to distinguish between ligands which can improve desired performance, and those that reduce it. Carboxylic acid-, thiol-, and amine-protected cobalt nanoparticles were studied by density functional theory calculations to model the impact of ligand coverage on the magnetic properties. The simulations show that the functional group, arrangement, and coverage density of the ligand coating control both the total magnetic moment and magnetic anisotropy energy of the nanoparticle, as well as the distribution of local spin magnetic moments across the metallic core. Captured effects of ligand binding on the orbital moments of cobalt atoms were insignificant. Out of the three ligand families, only carboxylic acid coatings increased the magnetic moments of cobalt nanoparticles, while amines and thiols quenched them. Calculated anisotropy energies of protected nanoparticles consistently increased with the growing ligand density, reaching the highest values for a 100% coverage of both carboxylic acid and thiol coatings. However, the binding nature of the two functional groups showed opposite impacts on the d-states of interacting cobalt atoms. This study has thus established important principles for the design of biocompatible magnetic nanocomposites, highlighting different routes to achieve desired magnetic behaviour.

TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and Volatile Organic Compounds: An Experimental and Theoretical Investigation

TiO₂/Cu₂O/CuO multi-nanolayers highly sensitive toward volatile organic compounds (VOCs) and H₂ have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultrathin TiO₂ films were deposited by spray pyrolysis on top of sputtered-annealed Cu₂O/CuO nanolayers to enhance their gas sensing performance and improve their protection against corrosion at high operating temperatures. The prepared heterostructures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet visible (UV-vis) and micro-Raman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses between 20 and 30 nm (Cu₂O/CuO nanolayers) exhibited a high response and an excellent selectivity to ethanol vapor after thermal annealing the samples at 420 °C. The results obtained at an operating temperature of 350 °C demonstrate that the CuO/Cu₂O nanolayers with thicknesses between 20 and 30 nm are sensitive mainly to ethanol vapor, with a response of ∼150. The response changes from ethanol vapors to hydrogen gas as the thickness of the CuO/Cu₂O nanolayers changes from 50 to 20 nm. Density functional theory-based calculations were carried out for the geometries of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) heterostructures and their sensing mechanism toward alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu₂O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO₂(111) facets, all of which terminate in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best-matching lattices. Despite the impact of the Cu₂O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data, supported by the computational results, are important in understanding the use of the multi-nanolayered films tested in this work as reliable, accurate, and selective sensor structures for the tracking of gases at low concentrations.

Structural and electronic properties of Cu4O3 (paramelaconite): the role of native impurities

Hybrid density functional theory has been used to study the phase stability and formation of native point defects in Cu4O3. This intermediate copper oxide compound, also known as paramelaconite, was observed to be difficult to synthesize due to stabilization issues between mixed-valence Cu1+ and Cu2+ ions. The stability range of Cu4O3 was investigated and shown to be realized in an extremely narrow region of phase space, with Cu2O and CuO forming readily as competing impurity phases. The origin of p-type conductivity is confirmed to arise from specific intrinsic copper vacancies occurring on the 1+ site. Away from the outlined stability region, the dominant charge carriers become oxygen interstitials, impairing the conductivity by creating deep acceptor states in the electronic band gap region and driving the formation of alternative phases. This study further demonstrates the inadequacy of native defects as a source of n-type conductivity and complements existing experimental findings.

The role of surface oxidation and Fe-Ni synergy in Fe-Ni-S catalysts for CO2 hydrogenation

Increasing carbon dioxide (CO₂) emissions, resulting in climate change, have driven the motivation to achieve the effective and sustainable conversion of CO₂ into useful chemicals and fuels. Taking inspiration from biological processes, synthetic iron-nickel-sulfides have been proposed as suitable catalysts for the hydrogenation of CO₂. In order to experimentally validate this hypothesis, here we report violarite (Fe,Ni)₃S₄ as a cheap and economically viable catalyst for the hydrogenation of CO₂ into formate under mild, alkaline conditions at 125 °C and 20 bar (CO₂ : H₂ = 1 : 1). Calcination of violarite at 200 °C resulted in excellent catalytic activity, far superior to that of Fe-only and Ni-only sulfides. We further report first principles simulations of the CO₂ conversion on the partially oxidised (001) and (111) surfaces of stoichiometric violarite (FeNi₂S₄) and polydymite (Ni₃S₄) to rationalise the experimentally observed trends. We have obtained the thermodynamic and kinetic profiles for the reaction of carbon dioxide (CO₂) and water (H₂O) on the catalyst surfaces via substitution and dissociation mechanisms. We report that the partially oxidised (111) surface of FeNi₂S₄ is the best catalyst in the series and that the dissociation mechanism is the most favourable. Our study reveals that the partial oxidation of the FeNi₂S₄ surface, as well as the synergy of the Fe and Ni ions, are important in the catalytic activity of the material for the effective hydrogenation of CO₂ to formate.

A Perspective on Modelling Metallic Magnetic Nanoparticles in Biomedicine: From Monometals to Nanoalloys and Ligand-Protected Particles

The focus of this review is on the physical and magnetic properties that are related to the efficiency of monometallic magnetic nanoparticles used in biomedical applications, such as magnetic resonance imaging (MRI) or magnetic nanoparticle hyperthermia, and how to model these by theoretical methods, where the discussion is based on the example of cobalt nanoparticles. Different simulation systems (cluster, extended slab, and nanoparticle models) are critically appraised for their efficacy in the determination of reactivity, magnetic behaviour, and ligand-induced modifications of relevant properties. Simulations of the effects of nanoscale alloying with other metallic phases are also briefly reviewed.

The mechanism underlying the functionalisation of cobalt nanoparticles by carboxylic acids: a first-principles computational study

The promise of biocompatible magnetic nanoparticles with high magnetic saturation in the implementation as drug carriers and hyperthermia agents has generated significant interest in functionalised cobalt nanoparticles. Carboxylic acid coatings on metallic nanoparticles have been shown as an attractive option owing to their respectable stability and biocompatibility. However, only limited information is available on the molecular mechanism leading to the formation of such protective coatings. In this study, ab initio molecular dynamics simulations have been used to unravel the functionalisation mechanism starting from a neutral cobalt cluster and valeric acid molecules. Three stages were detected in the coating process: (i) rapid initial adsorption of acid molecules, (ii) simultaneous adsorption of new molecules and dissociation of those already interacting with the cluster, and, finally, (iii) grouping of dissociated hydrogen atoms and subsequent desorption of acid molecules. The fate of the hydrogen atoms was probed through a combination of static and dynamic ab initio modelling approaches, which predicted H2 generation with favourable energetics. A better understanding of the functionalisation and interaction mechanisms will aid the rational design of biocompatible cobalt nanoparticles for various applications.

Combined Experimental and Theoretical Study of the Competitive Absorption of CO2 and NO2 by a Superbase Ionic Liquid

A superbase ionic liquid (IL), trihexyltetradecylphosphonium benzimidazolide ([P66614][Benzim]), is investigated for the capture of CO2 in the presence of NO2 impurities. The effect of the waste gas stream contaminant on the ability of the IL to absorb simultaneously CO2 is demonstrated using novel measurement techniques, including a mass spectrometry breakthrough method and in situ infrared spectroscopy. The findings show that the presence of an industrially relevant concentration of NO2 in a combined feed with CO2 has the effect of reducing the capacity of the IL to absorb CO2 efficiently by ∼60% after 10 absorption-desorption cycles. This finding is supported by physical property analysis (viscosity, 1H and 13C NMR, and X-ray photoelectron spectroscopy) and spectroscopic infrared characterization, in addition to density functional theory (DFT) calculations, to determine the structure of the IL-NO2 complex. The results are presented in comparison with another flue gas component, NO, demonstrating that the absorption of NO2 is more favorable, thereby hindering the ability of the IL to absorb CO2. Significantly, this work aids understanding of the effects that individual components of flue gas have on CO2 capture sorbents, through studying a contaminant that has received limited interest previously.

Behavior of S, SO, and SO3 on Pt (001), (011), and (111) surfaces: A DFT study

In the hybrid sulfur (HyS) cycle, the reaction between SO₂ and H₂O is manipulated to produce hydrogen with water and sulfuric acid as by-products. However, sulfur poisoning of the catalyst has been widely reported to occur in this cycle, which is due to strong chemisorption of sulfur on the metal surface. The catalysts may deactivate as a result of these impurities present in the reactants or incorporated in the catalyst during its preparation and operation of the HyS cycle. Here, we report a density functional theory investigation of the interaction between S, SO, and SO₃ with the Pt (001), (011), and (111) surfaces. First, we have investigated the adsorption of single gas phase molecules on the three Pt surfaces. During adsorption, the 4F hollow sites on the (001) and (011) surfaces and the fcc hollow site on the (111) surface were preferred. S adsorption followed the trend of (001)_4F > (011)_4F > (111)_fcc, while SO adsorption showed (001)_4F > (011)_bridge/4F > (111)_fcc and SO₃ adsorption was most stable in a S,O,O bound configuration on the (001)_4F > (011)_4F > (111)_fcc sites. The surface coverage was increased on all the surfaces until a monolayer was obtained. The highest surface coverage for S shows the trend (001)_S = (111)_S > (011)_S, and for SO it is (001)_SO > (011)_SO > (111)_SO, similar to SO₃ where we found (001)_SO3 > (011)_SO3 > (111)_SO3. These trends indicate that the (001) surface is more susceptible to S species poisoning. It is also evident that both the (001) and (111) surfaces were reactive toward S, leading to the formation of S₂. The high coverage of SO₃ showed the formation of SO₂ and SO₄, especially on the (011) surface. The thermodynamics indicated that an increased temperature of up to 2000 K resulted in Pt surfaces fully covered with elemental S. The SO coverage showed θ ≥ 1.00 on both the (001) and (011) surfaces and θ = 0.78 for the (111) surface in the experimental region where the HyS cycle is operated. Lower coverages of SO₃ were observed due to the size of the molecule.

A surface oxidised Fe–S catalyst for the liquid phase hydrogenation of CO2

A surface oxidised Fe–S catalyst enhances the liquid phase conversion of CO₂ to formate under mild hydrothermal conditions.

CO2 reduction to acetic acid on the greigite Fe3S4{111} surface

Acetic acid (CH3-COOH) is an important commodity chemical widely used in a myriad of industrial processes, whose production still largely depends on homogeneous catalysts based on expensive rare metals. Here, we report a computational study on the formation of CH3-COOH from carbon dioxide (CO2) as an alternative chemical feedstock on the {111} surface of the low-cost greigite Fe3S4 catalyst. We have used density functional theory calculations with a Hubbard Hamiltonian approach and long-range dispersion corrections (DFT+U-D2) to simulate the various stages of the direct combination of C1 species of different composition to produce glyoxylic acid (CHO-COOH) as a key intermediate in the formation of CH3-COOH. Three reaction mechanisms are considered: (i) the main pathway where the direct formation of the C-C bond takes place spontaneously, followed by a step-wise reduction of CHO-CHOO to CH3-COOH; and the competitive pathways for the non-promoted and H-promoted elimination of hydroxy groups (OH) and water (H2O), respectively from (ii) the carboxyl; and (iii) the carbonyl end of the glyoxylate intermediates. The thermodynamic and kinetic profiles show that the energies for the intermediates on the main pathway are very similar for the two catalytic sites considered, although the activation energies are somewhat larger for the exposed tetrahedral iron (FeA) ion. In most cases, the intermediates for the deoxygenation of the carboxylic acid are less stable than the intermediates on the main pathway, which suggests that the molecule prefers to lose the carbonylic oxygen. The suitable surface properties of the Fe3S4{111} surface show that this material could be a promising sustainable catalyst in future technologies for the conversion of CO2 into organic acid molecules of commercial interest.

Controlling the Lithium Intercalation Voltage in the Li(Mn1-xNix)2O4 Spinel via Tuning of the Ni Concentration: a Density Functional Theory Study

ABSTRACT LiMn2O4 spinel is a promising cathode material for secondary lithium-ion batteries. Despite showing a high average voltage of lithium intercalation, the material is structurally unstable, undergoing lowering of the crystal symmetry due to Jahn-Teller distortion of the six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the structural stability of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of the Ni incorporation and its effect on the electrochemical properties of this spinel material are not yet known. In this work, we have employed density functional theory calculations with a Hubbard Hamiltonian (DFT+u) to investigate the thermodynamics of cation mixing in the Li(Mn1_xNix)2O4 solid solution. Our results suggest LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000 K, in agreement with experiments. We also found that the configurational entropy is much lower than the maximum entropy at 1000 K, indicating that higher temperatures are required to reach a fully disordered solid solution. A maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition, which is very close to the experimental value. The temperature was found to have a negligible effect on the Li intercalation voltage of the most stable composition. The findings reported here support the application of LiMn1.5Ni0.5O4 as a suitable cathode material for lithium-ion batteries, with a highly stable voltage of intercalation under a wide range of temperatures. Keywords: Spinel, equilibrium concentration, mixing thermodynamics, solid-state chemistry and lithium voltage of intercalation.

H-FER-Catalyzed Conversion of Methanol to Ethanol and Dimethyl Ether: a First-Principles DFT Study

ABSTRACT Methanol adsorption and dehydration reactions within zeolites represent important steps in the catalytic conversion process to form long-chain hydrocarbons. Herein, first-principles density functional theory (DFT) is employed in the determination of methanol adsorption and conversion in ferrierite (FER), where we predict the fundamental adsorption geometries and energetics of methanol adsorption. The methanol molecule is shown to physisorb at all explored binding sites, stabilized through hydrogen-bonded interactions with the acid site atOmeth-Hframbond distances ranging from 1.33-1.51 A. We demonstrate that the zeolites' adsorption capability is affected by the silicon/aluminium ratio, with stronger adsorptions predicted in the material with silicon to aluminium fractions of 5 than 8. The adsorption strength is also found to vary depending on the tetrahedral binding site, with the T1O2 site yielding the most stable methanol adsorption structure in the Si/Al ratio = 5(Eads = -22.5 kcal mol-1), whereas the T1O1 site yields the most stable adsorption geometry (Eads = -19.2 kcal mol-1) in the Si/Al ratio = 8. Upon translational and rotational motion, methanol is protonated resulting in the breaking of its C-O bond to form a methoxy species bound to the framework oxygen (O-CH3 distance of 1.37 A), whereas the water molecule is stabilized at the acid site through H-bonding (Owat-H = 2.0 A). Further reaction between the methoxy species and a second methanol molecule results in the formation of ethanol and protonated dimethyl ether, with adsorption energies of -42 and -25 kcal mol-1, respectively. The results in this study provide atomistic insight into the effect of acidity of the FER zeolite on the adsorption and conversion of methanol. Keywords: Zeolites, ferrierite, methanol adsorption, acid sites, density functional theory (DFT).

Thermodynamics of the Atomic Distribution in Pt3Pd2, Pt2Pd3 and their Corresponding (111) Surfaces

ABSTRACT In this study, we have developed solid-state models of platinum and palladium bimetallic catalysts, Pt3Pd2 and Pt2Pd3, which are rapidly thermally annealed at 800 °C. These models were constructed by determining all the unique atomic configurations in a 2x2x1 supercell, using the program Site-Occupation Disorder (SOD), and optimized with the General Utility Lattice Program (GULP) using Sutton-Chen interatomic potentials. Each catalyst had 101 unique bulk models that were developed into surface models, which were constructed using the two-region surface technique before the surface energies were determined. The planes and compositions with lowest surface energies were chosen as the representative models for the surface structure of the bimetallic catalysts. These representative models will now be used in a computational study of the HyS process for the production of hydrogen. Keywords: HyS process, platinum, palladium, solid-state, catalyst, Site-Occupation Disorder.

Density Functional Theory Study of the Adsorption of Oxygen and Hydrogen on 3d Transition Metal Surfaces with Varying Magnetic Ordering

ABSTRACT We have employed density functional theory (DFT) calculations to investigate the adsorption of molecular oxygen and hydrogen on 3d transition metal (TM) surfaces with varying ordered magnetic structures in the bulk, namely ferromagnetic Fe(110), Co(0001), Ni(111) and diamagnetic Cu(111). The trend observed in the energies of adsorption was compared with the magnetic moment of the cell using the d-band centre model of chemisorption and the Stoner model of magnetic energy. As the gap between the d-band centre and the Fermi level of the TM decreases, more antibonding orbitals are present above the Fermi level and thus unoccupied, leading to stronger binding. Correspondingly, the shift in the d-band centre decreases the density of states (DOS) at the Fermi level giving rise to the ordered magnetic structure. Keywords: d-Band centre, chemisorption, Hedvall effect, magnetism.

Competitive Adsorption of H2O and SO2 on Catalytic Platinum Surfaces: a Density Functional Theory Study

ABSTRACT Platinum has been widely used as the catalyst of choice for the production of hydrogen in the hybrid sulphur (HyS) cycle. In this cycle, water (H2O) and sulphur dioxide (SO2) react to form sulphuric acid and hydrogen. However, the surface reactivity of platinum towards H2O and SO2 is not yet fully understood, especially considering the competitive adsorption that may occur on the surface. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the competitive effect of both H2O and SO2 on the Pt (001), (011) and (111) surfaces. Comparing the adsorption of a single H2O molecule on the various Pt surfaces, it was found that the lowest adsorption energy (Eads = -1.758 eV) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the molecular adsorption on the (011) surface (Eads = -0.699 eV) and (111) surface (Eads = -0.464 eV). For the molecular SO2 adsorption, the trend was similar, with the lowest adsorption energy (Eads = -2.471 eV) obtained on the (001) surface, followed by the (011) surface (Eads = -2.390 eV) and (111) surface (Eads = -1.852 eV). During competitive adsorption by H2O and SO2, the SO2 molecule will therefore preferentially adsorb onto the Pt surface. If the concentration of SO2 increases, self-reaction between two neighbouring SO2 molecules may occur, leading to the formation of sulphur monoxide (SO) and -trioxide (SO3) on the surface, which could lead to sulphur poisoning of the Pt catalytic surface. Keywords: Platinum, water, sulphur dioxide, hydrogen, adsorption, density functional theory.

DFT+U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co3O4

ABSTRACT Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co3O4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co3O4. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions. Keywords: Materials chemistry, cobalt oxides, cobalt metal, spinel, density functional theory, on-site Coulomb correction.

Lysine-arginine advanced glycation end-product cross-links and the effect on collagen structure: A molecular dynamics study

The accumulation of advanced glycation end‐products is a fundamental process that is central to age‐related decline in musculoskeletal tissues and locomotor system function and other collagen‐rich tissues. However, although computational studies of advanced glycation end‐product cross‐links could be immensely valuable, this area remains largely unexplored given the limited availability of structural parameters for the derivation of force fields for Molecular Dynamics simulations. In this article, we present the bonded force constants, atomic partial charges and geometry of the arginine‐lysine cross‐links DOGDIC, GODIC, and MODIC. We have performed in vacuo Molecular Dynamics simulations to validate their implementation against quantum mechanical frequency calculations. A DOGDIC advanced glycation end‐product cross‐link was then inserted into a model collagen fibril to explore structural changes of collagen and dynamics in interstitial water. Unlike our previous studies of glucosepane, our findings suggest that intra‐collagen DOGDIC cross‐links furthers intra‐collagen peptide hydrogen‐bonding and does not promote the diffusion of water through the collagen triple helices.

Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes

CuBi₂O₄ exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi₂O₄ has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s-O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d-O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d-O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi₂O₄ derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi₂O₄ may result from a direct but forbidden Cu d-d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi₂O₄ is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi₂O₄-based photoelectrodes.

Single CuO/Cu2O/Cu Microwire Covered by a Nanowire Network as a Gas Sensor for the Detection of Battery Hazards

In this study, a strategy to prepare CuO/Cu₂O/Cu microwires that are fully covered by a nanowire (NW) network using a simple thermal-oxidation process is developed. The CuO/Cu₂O/Cu microwires are fixed on Au/Cr pads with Cu microparticles. After thermal annealing at 425 °C, these CuO/Cu₂O/Cu microwires are used as room-temperature 2-propanol sensors. These sensors show different dominating gas responses with operating temperatures, e.g., higher sensitivity to ethanol at 175 °C, higher sensitivity to 2-propanol at room temperature and 225 °C, and higher sensitivity to hydrogen gas at ∼300 °C. In this context, we propose the sensing mechanism of this three-in-one sensor based on CuO/Cu₂O/Cu. X-ray diffraction (XRD) studies reveal that the annealing time during oxidation affects the chemical appearance of the sensor, while the intensity of reflections proves that for samples oxidized at 425 °C for 1 h the dominating phase is Cu₂O, whereas upon further increasing the annealing duration up to 5 h, the CuO phase becomes dominant. The crystal structures of the Cu₂O-shell/Cu-core and the CuO NW networks on the surface were confirmed with a transmission electron microscope (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED), where (HR)TEM micrographs reveal the monoclinic CuO phase. Density functional theory (DFT) calculations bring valuable inputs to the interactions of the different gas molecules with the most stable top surface of CuO, revealing strong binding, electronic band-gap changes, and charge transfer due to the gas molecule interactions with the top surface. This research shows the importance of the nonplanar CuO/Cu₂O layered heterostructure as a bright nanomaterial for the detection of various gases, controlled by the working temperature, and the insight presented here will be of significant value in the fabrication of new p-type sensing devices through simple nanotechnology.