Search

Following a Preliminary Assessment by the University of Queensland this erratum corrects the authorship of this article [1] by removing X.S. Zhao. The correct authorship list is as follows: Xi Xu, Chao Wang, Marco Fronzi, Xuehua Liu and Lei Bi.

The muon spin relaxation method is a powerful microscopic tool for probing the electronic states of materials by observing local magnetic field distributions on the muon. It often happens that a distribution of local magnetic fields shows an intermediate state between Gaussian and Lorentzian shapes. In order to generally describe these intermediate field distributions, we considered the convolution of two isotropic distributions in three dimensions and derived exact muon-spin relaxation functions which can be applied to all crossover regimes between Gaussian and Lorentzian.

Tailoring the electronic structure of the perovskite oxide could potentially allow dramatic improvements in the properties of cathode materials in proton-conducting solid oxide fuel cells (SOFCs). This has been demonstrated in the case of Mo-doped La0.5Sr0.5FeO3-δ, where the electronic structure of the La0.5Sr0.5FeO3-δ oxide has been changed with the Mo-doping, leading to a less strong metal-oxygen bond as well as a more active surface towards oxygen reduction. As a result, the more active oxygen atoms make the formation of oxygen vacancy and hydration that are critical for protonation more feasible. Furthermore, the electric field induced by Mo-doping provides an additional driving force for the movement of protons, accelerating the proton migrations in the oxide and thus improving the cathode performance. With the Mo-doped La0.5Sr0.5FeO3-δ as the cathode, a proton-conducting SOFC exhibits an impressive fuel cell output of 1174 mW cm−2 at 700 °C that surpasses most of the cells using similar types of cathodes. This study not only provides a proper cathode material without involving cobalt and barium elements but also gives an understanding of the design of the cathode by tailoring the electronic structures.

The electrochemical conversion of N2 to NH3 is an interesting research topic as it provided an alternative and energy-saving method compared with the traditional way of NH3 production. Although different materials have been proposed for N2 reduction, the use of defects in oxides was only reported recently and the relevant working mechanism was not fully revealed. In this study, Sr was used as the dopant for LaFeO3 to create oxygen vacancies, forming the Sr-doped LFO (La0.5Sr0.5FeO3-δ) perovskite oxide. The La0.5Sr0.5FeO3-δ ceramic oxide used as a catalyst achieves an NH3 yield of 11.51 μgh−1 mg−1 and the desirable faradic efficiency (F.E.) of 0.54% at −0.6 V vs reversible hydrogen electrode (RHE), which surpassed that of LaFeO3 nanoparticles. The 15N isotope labeling method was employed to prove the La0.5Sr0.5FeO3-δ catalyst had the function of converting N2 into NH3 under the electrolysis condition. The first principle calculations were used to investigate the mechanism at the atomistic level, revealing that the free energy barriers changed significantly with the introduction of oxygen vacancies that accelerated the overall nitrogen reduction reaction (NRR) procedure.

© 2020, The Author(s). Reducing our overwhelming dependence on fossil fuels requires groundbreaking innovations in increasing our efficiency in energy consumption for current technologies and moving towards renewable energy sources. Thermoelectric materials can help in achieving both goals. Moreover, because of recent advances in high-performance computing, researchers more increasingly rely on computational methods in discovering new thermoelectric materials with economically feasible performance. In this article, significant thermoelectric materials discovered through these computational methods are systematically reviewed. Furthermore, the primary computational tools that aid the design of the next-generation thermoelectric materials are introduced and discussed. These techniques include various levels of density functional theory, electronic transport simulations, and phonon calculations.

© 2020 American Chemical Society. High-performance thermoelectric oxides could offer a great energy solution for integrated and embedded applications in sensing and electronics industries. Oxides, however, often suffer from low Seebeck coefficient when compared with other classes of thermoelectric materials. In search of high-performance thermoelectric oxides, we present a comprehensive density functional investigation, based on GGA+U formalism, surveying the 3d and 4d transition-metal-containing ferrites of the spinel structure. Consequently, we predict MnFe2O4 and RhFe2O4 have Seebeck coefficients of ±600 μV K-1 at near room temperature, achieved by light hole and electron doping. Furthermore, CrFe2O4 and MoFe2O4 have even higher ambient Seebeck coefficients at ±700 μV K-1. In the latter compounds, the Seebeck coefficient is approximately a flat function of temperature up to ∼700 K, offering a tremendous operational convenience. Additionally, MoFe2O4 doped with 1019 holes/cm3 has a calculated thermoelectric power factor of 689.81 μW K-2 m-1 at 300 K and 455.67 μW K-2 m-1 at 600 K. The thermoelectric properties predicted here can bring these thermoelectric oxides to applications at lower temperatures traditionally fulfilled by more toxic and otherwise burdensome materials.

© 2020 Elsevier Ltd Electron beam induced etching (EBIE) has recently emerged as a promising direct-write nanofabrication technique. EBIE is typically assumed to proceed entirely through chemical pathways driven by electron-electron interactions. Here we show that knock-on (i.e., momentum transfer from electrons to nuclei) can play a significant role in EBIE, even at electron beam energies as low as 1.5 keV. Specifically, we calculate knock-on cross-sections for H, D, O and CO on the surface of diamond and show experimentally that they affect the kinetics of EBIE performed using oxygen, hydrogen and deuterium etch precursors. Our results advance basic understanding of electron-adsorbate interactions, particularly in relation to EBIE and the related techniques of electron beam-induced deposition and surface functionalisation.

© 2020, The Author(s). We examined the electronic property of Sb-doped Na0.785CoO2 using density functional calculations based on GGA+U formalism. We demonstrated that Sb dopants were the most stable when replacing Co ions within the complex Na0.875CoO2 lattice structure. We also showed that the SbCo dopants adopted the + 5 oxidation state introducing two electrons into the host Na0.875CoO2 compound. The newly introduced electrons recombined with holes that were borne on Co4+ sites that had been created by sodium vacancies. The elimination of Co4+ species, in turn, rendered Na0.875(Co0.9375Sb0.0625)O2 non-magnetic and diminished the compound’s thermoelectric effect. Furthermore, the SbCo dopants tended to aggregate with the Na vacancies keeping a minimum distance. The conclusions drawn here can be generalised to other highly oxidised dopants in NaxCoO2 that replace a Co.

© 2019 Elsevier Ltd and Techna Group S.r.l. A Pr-doping strategy was used to improve traditional strontium-doped lanthanum ferrite oxides for proton-conducting solid oxide fuel cells (SOFCs). Three different samples, La0.5Sr0.5FeO3-δ, La0.25Pr0.25Sr0.5FeO3-δ, and Pr0.5Sr0.5FeO3-δ,were successfully prepared. The Pr content was shown to have an obvious influence on the hydration ability of the materials. Hydration was improved at higher Pr-contents, suggesting a promising cathode performance. However, the improved hydration ability did not always lead to an increased fuel cell performance, and it was found that the fuel cell performed best when an appropriate Pr-doping amount was used that resulted in a good compromise between protonic and oxygen-ion conduction. As a result, the optimized composition La0.25Pr025Sr0.5FeO3-δ generated a high peak power density of 616 mW cm−2 and a low polarization resistance of 0.09 at Ω cm2 at 700 °C, which is an encouraging performance for a traditional cathode material.

© 2020 Wiley-VCH GmbH The screening of novel materials is an important topic in the field of materials science. Although traditional computational modeling, especially first-principles approaches, is a very useful and accurate tool to predict the properties of novel materials, it still demands extensive and expensive state-of-the-art computational resources. Additionally, they can often be extremely time consuming. A time and resource efficient machine learning approach to create a dataset of structural properties of 18 million van der Waals layered structures is described. In particular, the authors focus on the interlayer energy and the elastic constant of layered materials composed of two different 2D structures that are important for novel solid lubricant and super-lubricant materials. It is shown that machine learning models can predict results of computationally expansive approaches (i.e., density functional theory) with high accuracy.

© 2019 Elsevier Ltd CeO2 nanocubes with (100) surface orientation are successfully synthesized and found to facilitate the sinterability of CeO2 material. The CeO2 nanocubes show a much-improved sinterability relative to CeO2 nanoparticles prepared by the conventional citric-nitrate method. The nanocubes can be successfully sintered at a relatively low temperature of 1200 °C without using any sintering aids. In contrast, a pellet using conventional CeO2 nano-powder obtained from the conventional citric-nitrate method can be densified only after sintering at 1400 °C, which is 200 °C higher than that for the CeO2 nanocube sintering, although the starting particle size of both CeO2 samples is similar. Density functional theory indicates that the surface energy of the (100) plane is significantly higher than that of the (111) plane, which is the more typical surface presentation of conventional CeO2 particles. This high surface energy allows fast growth of the CeO2 nanocubes during sintering, contributing to their improved sinterability.

© 2019, The Author(s). Co3O4 nanocubes with exposed (001) planes were prepared and employed for use as first-generation Sr-doped LaMnO3 (LSM) cathodes in solid oxide fuel cells to improve the cell performance. Theoretical simulations suggest that the Co3O4 (001) plane has the smallest oxygen adsorption and oxygen dissociation energies compared with other planes, thus favouring cathode reactions in solid oxide fuel cells (SOFCs). Experimental studies consistently demonstrate that a cell using an LSM cathode made with Co3O4 nanocubes with selective (001) surfaces exhibits a peak power density of 500 mW cm−2 at 600 °C, while the power output for a cell using unselective (commercial) Co3O4 nanoparticles is only 179 mW cm−2 at the same temperature. The electrochemical study indicates that the use of Co3O4 nanoparticles with exposed (001) surfaces obviously accelerates the cathode reactions and thus decreases the polarisation resistance, which is the key to improving fuel cell performance. This study demonstrates the feasibility of using the crystal planes of metal oxides to improve the fuel cell performance and provides a new way to design SOFC cathodes.

© 2019 The hydrophobicity of CeO 2 surfaces is examined here. Since wettability measurements are extremely sensitive to experimental conditions, we propose a general approach to obtain contact angles between water and ceria surfaces of specified orientations based on density functional calculations. In particular, we analysed the low index surfaces of this oxide to establish their interactions with water. According to our calculations, the CeO 2 (111) surface was the most hydrophobic with a contact angle of Θ = 112.53° followed by (100) with Θ = 93.91°. The CeO 2 (110) surface was, on the other hand, mildly hydrophilic with Θ = 64.09°. By combining our calculations with an atomistic thermodynamic approach, we found that the O terminated (100) surface was unstable unless fully covered by molecularly adsorbed water. We also identified a strong attractive interaction between the hydrogen atoms in water molecules and surface oxygen, which gives rise to the hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water molecules on the lower-energy (111) surface stabilises oxygen vacancies, which are expected to enhance the catalytic activity of this plane. The findings here shed light on the origin of the intrinsic wettability of rare earth oxides in general and CeO 2 surfaces in particular and also explain why CeO 2 (100) surface properties are so critically dependant on applied synthesis methods.

© 2018 Elsevier B.V. Converting CO2 to fuels is required to enable the production of sustainable fuels and to contribute to alleviating CO2 emissions. In considering conversion of CO2, the initial step of adsorption and activation by the catalyst is crucial. In addressing this difficult problem, we have examined how nanoclusters of reducible metal oxides supported on TiO2 can promote CO2 activation. In this paper we present density functional theory (DFT) simulations of CO2 activation on heterostructures composed of clean or hydroxylated extended rutile and anatase TiO2 surfaces modified with chromia nanoclusters. The heterostructures show non-bulk Cr and O sites in the nanoclusters and an upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We show that the supported chromia nanoclusters can adsorb and activate CO2 and that activation of CO2 is promoted whether the TiO2 support is oxidised or hydroxylated. Reduced heterostructures, formed by removal of oxygen from the chromia nanocluster, also promote CO2 activation. In the strong CO2 adsorption modes, the molecule bends giving O–C–O angles of 127 - 132° and elongation of C–O distances up to 1.30 Å; no carbonates are formed. The electronic properties show a strong CO2–Cr–O interaction that drives the interaction of CO2 with the nanocluster and induces the structural distortions. These results highlight that a metal oxide support modified with reducible metal oxide nanoclusters can activate CO2, thus helping to overcome difficulties associated with the difficult first step in CO2 conversion.

© 2018 IEEE. We present density functional theory simulations of novel heterostructures composed of TiO2 rutile and anatase modified with nanoclusters of Bi2 O3, Cr2 O3 and ZrO2. These heterostructures are shown to adsorb and activate CO 2 ; this is characterised by strong adsorption energies, a bending of the CO 2 molecule and elongation of C-O distances in the molecule.

© 2019 the Owner Societies. Interfacial metal-oxide systems with ultra-thin oxide layers are of high interest for their use in catalysis. The chemical activity of ultra-thin metal-oxide layers can be substantially enhanced compared to interfacial models with thicker oxide. In this study, we present a Density Functional Theory (DFT) investigation of the structure of ultra-thin rutile layers (one and two TiO2 layers) supported on TiN and the stability of water on these interfacial structures. The rutile layers are stabilized on the TiN surface through the formation of interfacial Ti-O bonds. Charge transfer from the TiN substrate leads to the formation of reduced Ti3+ cations in TiO2. The concentration of Ti3+ is proportionally higher in the ultra-thin oxide, compared to interfacial models with thicker oxide layers. The structure of the one-layer oxide slab is strongly distorted at the interface while the thicker TiO2 layer preserves the rutile structure. The energy cost for the formation of a single O vacancy in the one-layer oxide slab is only 0.5 eV with respect to the ideal interface. For the two-layer oxide slab, the introduction of several vacancies in an already non-stoichiometric system becomes progressively more favourable, which indicates the stability of the highly defective interfaces. Isolated water molecules dissociate when adsorbed at the TiO2 layers. At higher coverages, the preference is for molecular water adsorption. Our ab initio thermodynamics calculations show the fully water covered stoichiometric models as the most stable structure at typical ambient conditions. This behaviour is similar to that observed on thicker oxide in TiO2-TiN interfaces or pure TiO2 surfaces. In contrast, interfacial models with multiple vacancies are most stable at low (reducing) oxygen chemical potential values. The high concentration on reduced Ti3+ introduces significant distortions in the O-defective slab. Whereas, a water monolayer adsorbs dissociatively on the highly distorted 2-layer TiO1.75-TiN interface, where the Ti3+ states lying above the top of the valence band contribute to a significant reduction of the energy gap compared to the stoichiometric TiO2-TiN model. Our results provide a guide for the design of novel interfacial systems containing ultra-thin TiO2 with potential application as photocatalytic water splitting devices.

Copyright © 2018 American Chemical Society. We used density functional theory approach, with the inclusion of a semiempirical dispersion potential to take into account van der Waals interactions, to investigate the water adsorption and dissociation on cobalt sulfide Co9S8 and Co3S4(100) surfaces. We first determined the nanocrystal shape and selected representative surfaces to analyze. We then calculated water adsorption and dissociation energies, as well as hydrogen and oxygen adsorption energies, and we found that sulfur vacancies on Co9S8(100) surface enhance the catalytic activity toward water dissociation by raising the energy level of unhybridized Co 3d states closer to the Fermi level. Sulfur vacancies, however, do not have a significant impact on the energetics of Co3S4(100) surface.

© 2018 American Chemical Society. Diamond is an ideal material for a broad range of current and emerging applications in tribology, quantum photonics, high-power electronics, and sensing. However, top-down processing is very challenging due to its extreme chemical and physical properties. Gas-mediated electron beam-induced etching (EBIE) has recently emerged as a minimally invasive, facile means to dry etch and pattern diamond at the nanoscale using oxidizing precursor gases such as O2 and H2O. Here we explain the roles of oxygen and hydrogen in the etch process and show that oxygen gives rise to rapid, isotropic etching, while the addition of hydrogen gives rise to anisotropic etching and the formation of topographic surface patterns. We identify the etch reaction pathways and show that the anisotropy is caused by preferential passivation of specific crystal planes. The anisotropy can be controlled by the partial pressure of hydrogen and by using a remote RF plasma source to radicalize the precursor gas. It can be used to manipulate the geometries of topographic surface patterns as well as nano- and microstructures fabricated by EBIE. Our findings constitute a comprehensive explanation of the anisotropic etch process and advance present understanding of electron-surface interactions.

© 2018 The Royal Society of Chemistry. Utilising reversible oxygen redox in Na and transition metal oxides offers unprecedented opportunities for the design of high voltage, high capacity and affordable cathodes for application in rechargeable Na-ion batteries. Through a judicious materials search and theoretical investigations, we identified new compounds with excellent energy storage properties that rely on oxygen states for charge compensation during the redox reaction. According to our predictions, Na2-xMoO4 demonstrates a voltage of 4.743 V and an energy density of ∼617.3 W h kg-1. These values exceed the performance of most commercialised Na-ion cathode materials. Furthermore, both Na4-xZr5O12, demonstrating a voltage of 3.583 V, and Na1-xPd2PO3, demonstrating a voltage of 2.630 V, exhibit a meagre absolute volume change of ∼1% during the sodiation/desodiation process. Because of this minor volume change, these compounds are suitable for all-solid-state battery applications. An examination of the electronic structures of these compounds reveals that O states are always present at the top of the valence band regardless of the presence of 4d transition metal species or their oxidation states. This feature is attributed to the exceedingly substantial 4d-2p hybridisation over the entire valence band which also prevents the bonding of oxidised O ions in the desodiated compounds, thus preventing irreversible oxygen loss.

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Carbon nanotube-silicon (CNT-Si)-based heterojunction solar cells (HJSCs) are a promising photovoltaic (PV) system. Herein, few-layer black phosphorus (FL-BP) sheets are produced in N-methyl-2-pyrrolidone (NMP) using microwave-assisted liquid-phase exfoliation and introduced into the CNTs-Si-based HJSCs for the first time. The NMP-based FL-BP sheets remain stable after mixing with aqueous CNT dispersion for device fabrication. Due to their unique 2D structure and p-type dominated conduction, the FL-BP/NMP incorporated CNT-Si devices show an impressive improvement in the power conversion efficiency from 7.52% (control CNT-Si cell) to 9.37%. Our density-functional theory calculation reveals that lowest unoccupied molecular orbital (LUMO) of FL-BP is higher in energy than that of single-walled CNT. Therefore, we observed a reduction in the orbitals localized on FL-BP upon highest occupied molecular orbital to LUMO transition, which corresponds to an improved charge transport. This study opens a new avenue in utilizing 2D phosphorene nanosheets for next-generation PVs.

© 2017 The Royal Society of Chemistry. Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here, we performed density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we found that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculated the photoluminescence (PL) line shape for this defect and found that it reproduces a number of key features in the experimental PL lineshape.

First-principles molecular dynamics simulations have been employed to analyse the proton diffusion in cubic BaZrO3 perovskite at 1300 K. A non-linear effect on the proton diffusion coefficient arising from an applied isometric strain up to 2 % of the lattice parameter, and an evident enhancement of proton diffusion under compressive conditions have been observed. The structural and electronic properties of BaZrO3 are analysed from Density Functional Theory calculations, and after an analysis of the electronic structure, we provide a possible explanation for an enhanced ionic conductivity of this bulk structure that can be caused by the formation of a preferential path for proton diffusion under compressive strain conditions. By means of Nudged Elastic Band calculations, diffusion barriers were also computed with results supporting our conclusions.

Work on the design of new TiO2 based photocatalysts is described. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metal oxides. Density functional theory (DFT) level simulations are compared with experimental work synthesizing and characterizing surface modified TiO2 . DFT calculations are used to show that nanoclusters of metal oxides such as TiO2 , SnO/SnO2 , PbO/PbO2 , ZnO and CuO are stable when adsorbed at rutile and anatase surfaces, and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. The origin of the red shift and the fate of excited electrons and holes are determined. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. Comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations are described. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.

© 2014 Elsevier B.V. All rights reserved. To investigate the kinetics of the formation of solute cluster structures in some of the Mg ternary alloys, we perform a first-principles analysis of some fundamental quantities in doped Mg lattices. We calculate interaction energies between vacancy and solute atoms in both Mg hexagonal close packed (HCP) and face-centered cubic (FCC) crystal structures. In particular, we consider in this work Al, Gd, Y, and Zn solute atoms. Also, to understand the diffusion mechanism, we calculate vacancy-assisted diffusion for Mg and solute atoms in HCP and FCC lattices using the nudge elastic band method.

We present an ab initio investigation of the interaction between methane, its dehydrogenated forms and the cerium oxide surface. In particular, the stoichiometric CeO2(111) surface and the one with oxygen vacancies are considered. We study the geometries, energetics and electronic structures of various configurations of these molecules adsorbed on the surface in vacuum, and we extend the analysis to realistic environmental conditions. A phase diagram of the adsorbate-surface system is constructed and relevant transition phases are analyzed in detail, showing the conditions where partial oxidation of methane can occur. © 2014 The Royal Society of Chemistry.

Using density functional theory, we characterized the in-plane binding between L12-type solute clusters in Mg-M-RE (M = Al, Zn; RE = Y, Gd) long-period stacking ordered (LPSO) structures. The difference between the Al and Zn concentrations within the clusters determines whether the intercluster interaction is attractive or repulsive. Incomplete in-plane ordering observed experimentally in Mg-Zn-Y LPSO structures was suggested to be caused by the unlinked nature of the clusters owing to their significant inward contraction. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In the present work we present a Density Functional Theory approach to investigate the structural and electronic properties of the low index ZrO 2-CeO 2 interface. Optimizations of the crystal geometry for separate ZrO 2 and CeO 2 bulks as well as the interfaces are carried out and the structural morphology is analyzed. The energy of formation of the oxygen vacancies is analyzed at different values of the lattice parameter, in order to verify its dependency on the strain. This eventually allows us to identify the vacancy concentration difference between bulks and interfaces. Activation energy of the oxygen migration is also calculated in the optimized bulk as well as under strain condition as at the interfaces level, to identify eventual preferential migration channel. The effect of doping on the lattice geometry is analyzed for the low index interfaces in order to verify its influence on the morphologic disorder and consequently on vacancy concentration. ©The Electrochemical Society.