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Novel research avenues have been explored over the last decade on the use of transition metal carbides (TMCs) as catalytically active supports for metal nanoclusters, which display high catalytic activity despite the poor reactivity (or even inertness) of the bulk metal. It has been postulated that TMCs polarise the electron density of adsorbed metal particles in such a way that their catalytic activity ends up being superior to those dispersed on more traditional metal oxide supports. Herein, we investigate the structural and electronic properties of small clusters of precious metals (Rh, Pd, Pt and Au) and more affordable metals (Co, Ni and Cu) supported on TMCs with 1:1 stoichiometry (TiC, ZrC, HfC, VC, NbC, TaC, MoC and WC) by means of periodic Density Functional Theory calculations. Our high-throughput screening studies indicate that it is possible not only to have strongly bonded and stably dispersed metal nanoparticles on TMC surfaces, but also to manipulate their charge by carefully selecting elements with desired electronegativity for the host TMC and the metal cluster. By doing so, it is possible to tune the amount of charge density on the cluster hollow sites, which can facilitate the bonding of certain molecules. Moreover, we identify Pt, Pd and Rh clusters supported on hexagonal TMC (001) facets as the candidates with the highest potential catalytic activity -as estimated by the significant polarisation of the cluster electron density- and stability -as quantified by the strongly negative values of adsorption energy per atom and formation energy-.

Two-dimensional (2D) elemental metals have attracted considerable attention because of their fascinating physical properties and potential diverse applications. Information on the structures, mechanical properties, and dynamical stabilities of single atom thick phases of metallic elements in the periodic table has been previously reported. Here, the magnetic properties of 45 2D metals in three lattice structures (hexagonal, square, and honeycomb) were systematically explored using density functional theory calculations. Although it is well known that there are only five magnetic elements in their 3D counterparts, it is interesting to find that 18 of the 45 2D elemental metals are endowed with magnetism due to coordination number decreases and energy band narrowing of the out-of-plane orbitals. In particular, the honeycomb lattice is more likely to be magnetic than the square lattice. Most of the magnetic 2D metals are ferromagnetic and only a few are anti-ferromagnetic. It is further shown that the 2D metals with higher structural symmetry have smaller magnetic moments. These results provide a foundation for the further study of their magnetic/spintronic properties and their potential applications, such as in catalysis and gas sensing.

Lithium–selenium (Li–Se) batteries are considered a promising energy storage material due to their high electronic conductivity and volume capacity. However, the performance of Li–Se batteries is far away from commercial application, and few previous research studies achieved excellent performance of Li–Se batteries when the current density is higher than 5C (1C = 675 mA h g−1) to date. Recently non-noble metal single-atom catalysts have attracted extensive interest because of their low cost and maximum utilization, while introducing binuclear single atoms into Li–Se batteries is still a challenge. Herein, for the first time, a hierarchical porous carbon host with binuclear single-atom-sites FeN4 and Co2N3 with a PDA shell (APPC/Se@PDA) is prepared as the cathode for Li–Se batteries. The uniformly dispersed single-atom sites exhibit a synergistic effect by not only strengthening the binding force between C and Se species but also enhancing the transfer rate of electrons and Li+, which is demonstrated by electrochemical performance, ex situ X-ray photoelectron spectroscopy measurements and density functional theory (DFT) calculations. Moreover, the hierarchical pores and PDA shells provide physical confinement. As a result, APPC/Se@PDA exhibits a high specific capacity of 536 mA h g−1 at 0.2C after 220 cycles at a Se loading of 4 mg cm−2, and maintains a high reversible capacity of 645 mA h g−1 after 700 cycles at 2C with a low capacity attenuation of 0.0079% per cycle. Even at 10C and 23C, 420 mA h g−1 and 274 mA h g−1 are achieved after 1500 cycles and 2500 cycles, respectively. To the best of our knowledge, such an excellent performance at high current densities of Li–Se batteries has not been reported. Combining experiments and theoretical calculations, such a novel strategy broadens the future research of Li–Se batteries and further applications of non-noble metal single atoms.

Herein, UV/O3 treatment was imposed on the hydrothermally synthesized spinel NiCo2O4 urchin like structure to study its enhancement effect on the field electron emission properties. The FE studies revealed that UV/O3 treated NiCo2O4 possesses a 4.38 V μm−1 turn on field at 1 μA and an emission current density of 386 μA cm−2, whereas pristine NiCo2O4 retains 5.69 V μm−1 turn on field at 1 μA and 168 μA cm−2 emission current density. The enhancement in the FE characteristics of UV/O3 treated NiCo2O4 is attributed to the change in the electronic structure of the emitter due to UV/O3 treatment. Experimental data were supported through electronic structure simulations using Density Functional Theory technique. We have achieved the modification of the electronic properties of spinel NiCo2O4 due to the oxygen vacancy created by UV/O3 treatment. There is an enhancement of electronic states near the Fermi level due to oxygen vacancy. The computed work function reduces when oxygen vacancy is introduced on the surface of pristine NiCo2O4 which justifies the enhancement in the field electron emission after UV/O3 treatment. Due to O vacancy, there is charge gain by the 3d orbital of Ni and Co whereas there is a charge loss by the 2p orbital of the O atom near the vacancy. The results suggest UV/O3 treatment can be practiced in the field electron emission investigation to retain a good current density for practical applications.

Here, we combine the piezoelectric wurtzite ZnO and the ferroelectric (111) BaTiO3 as a hexagonal closed-packed structure and report a systematic theoretical study on the ferroelectric behavior induced by the interface of ZnO/BaTiO3 films and the transport properties between the SrRuO3 electrodes. The parallel and antiparallel polarizations of ZnO and BaTiO3 can lead to intrinsic asymmetric ferroelectricity in the ZnO/BaTiO3 superlattice. Using first-principles calculations we demonstrate four different configurations for the ZnO/BaTiO3/ZnO superlattice with respective terminations and find one most favorable for the stable existence of asymmetric ferroelectricity in thin films with thickness less than 4 nm. Combining density functional theory calculations with non equilibrium Green's function formalism, we investigate the electron transport properties of SrRuO3/ZnO/BaTiO3/ZnO/SrRuO3 FTJ and SrRuO3/ZnO/BaTiO3/SrRuO3 FTJ, and reveal a high TER effect of 581% and 112% respectively. These findings provide an important insight into the understanding of how the interface affects the polarization in the ZnO/BaTiO3 superlattice and may suggest a controllable and unambiguous way to build ferroelectric and multiferroic tunnel junctions.

With applications in high performance electronics, photovoltaics, and catalysis, two-dimensional (2D) transition metal dichalcogenides (TMDCs) attract extensive attention due to their extraordinary physical properties. People have focused on TMDC-based materials for years, while the low mobility greatly hinders their further application. TMDC-based heterostructures with tunable band alignment have been experimentally confirmed to be feasible for photoelectronic devices or photocatalysts. Based on the density functional theory (DFT), there are four discoveries in this work: (1) we propose two new heterostructures based on BSe and MoS2/WS2 that have quite low mismatches and intrinsic type-II alignments. (2) Even though the VBM of BSe-MoS2 are completely contributed by BSe, the heterostructure is still endowed with a lower effective mass and a better transport characteristic in comparison with pristine structures. (3) A promoted absorption ability and a better transport characteristic oppose each other and the two characteristics cannot be obtained at the same time. (4) Tension strained structures can induce promoted light absorption in the solar spectrum and the predicted efficiency of the BSe-MoS2 bilayer can be as high as ∼19.3%, when the external electric field is applied. This theoretical survey proves that BSe-MoS2/WS2 with high flexibility and tunability are potential candidates for novel electronic devices and photocatalysts.

Perovskites are potential candidates for catalyst supports in biomass gasification to produce high-purity H2 due to their excellent redox properties. However, the significant mechanism of lattice oxygen release and migration in perovskites has not been clearly understood. In this work, the characteristics of surface oxygen release and subsurface oxygen migration in various LaAl-type perovskites were investigated by experiments and density functional theory calculations. Results show that the oxygen release capacity of La0.7Sr0.3AlO3−δ is considerable and that of Ni/La0.7Sr0.3AlO3−δ decreases slightly compared to the difficult occurrence of oxygen release in LaAlO3. Moreover, the rate-limiting step of oxygen release from pure LaAlO3 is determined to be the formation of O2 complex by two opposite O atoms. Sr doping reduces the charge of the outermost O atom, making oxygen release easy, and the desorption process of O2 becomes the rate-limiting step. After Ni loading, the strength of the surrounding Al–O bond increases, which raises the energy barrier and blocks the release of oxygen to some extent.

Currently, copper is the most active monometallic catalyst to generate hydrocarbon and oxygenated products. Nevertheless, the huge kinetic barrier of Cu surfaces for the formation of a C–C bond on the path toward C2+ products has remained unsolved to date. In this work, inspired by the successful growth of 2D borophene on Cu (111) surfaces [Nat. Nanotechnol., 2019, 14 (1), 44], we introduce a high density of one-dimensional copper/borophene interface sites for the C–C coupling reaction by using a framework of density functional theory calculations. Cu alone interacts very weakly with CO, while its synergistic interfacial interactions with boron atoms could significantly boost the CO binding strength, thus facilitating C–C coupling. The Cu–boron interface delivers an ultra-low kinetic energy barrier of only 0.42 eV for the CO dimerization step and a thermodynamic potential limiting step of only 0.61 V for the hydrogenation process toward ethanol production, inhibiting the hydrogen evolution reaction. The improved intrinsic catalytic activity can be ascribed to both the synergistic electronic and structural interactions between metal Cu and non-metal B atoms at the interface. Our work predicts an unprecedented family of low-dimensional nanohybrid catalysts towards the efficient electroreduction of CO to ethanol, thus providing a new class of guiding design principles for the broad experimentalist community.

A series of nonlinear optical chromophores A–D based on the diethylaminophenyl donor and tricyanofuran or phenyl-trifluoromethyl-tricyanofuran acceptors coupled through a tetraene bridge was synthesized and investigated. In particular, the donor and bridge sections of chromophores B and D were functionalized with a small chromophore alkylaniline cyanoacetate (B1) and dialkylaminobenzylidene malononitrile (D1) group, respectively, compared to the pentafluorobenzene group of chromophores A and C. Before poling, two small chromophores B1 and D1 with large dipole moment will greatly weaken the electrostatic interaction between chromophores by steric effect and electrostatic screening effect, thus increasing the poling efficiency. Although density functional theory calculations suggested that they have similar first-order hyperpolarizability, polymeric thin films doped with chromophores A–D exhibited the different r33 values of 115, 166, 213 and 276 pm V−1 at 1310 nm, respectively. The normalized r33 value of chromphores B and D was up to 7.72 × 10−19 and 17.25 × 10−19 pm cc per (V molecules), which is much higher than that of chromophores A and C. All these prove that introducing a small chromophore into the main chromophore can greatly improve the electro-optic coefficient of the chromophore by a rational molecular design.

Detailed information on structural, chemical, and physical properties of natural cleaved (10.4) calcite surfaces was obtained by a combined atomic force microscopy (AFM) and infrared (IR) study using CO as a probe molecule under ultrahigh vacuum (UHV) conditions. The structural quality of the surfaces was determined using non-contact AFM (NC-AFM), which also allowed assigning the adsorption site of CO molecules. Vibrational frequencies of adsorbed CO species were determined by polarization-resolved infrared reflection absorption spectroscopy (IRRAS). At low exposures, adsorption of CO on the freshly cleaved (10.4) calcite surface at a temperature of 62 K led to the occurrence of a single C-O vibrational band located at 2175.8 cm-1, blue-shifted with respect to the gas phase value. For larger exposures, a slight, coverage-induced redshift was observed, leading to a frequency of 2173.4 cm-1 for a full monolayer. The width of the vibrational bands is extremely small, providing strong evidence that the cleaved calcite surface is well-defined with only one CO adsorption site. A quantitative analysis of the IRRA spectra recorded at different surface temperatures revealed a CO binding energy of -0.31 eV. NC-AFM data acquired at 5 K for sub-monolayer CO coverage reveal single molecules imaged as depressions at the position of the protruding surface features, in agreement with the IRRAS results. Since there are no previous experimental data of this type, the interpretation of the results was aided by employing density functional theory calculations to determine adsorption geometries, binding energies, and vibrational frequencies of carbon monoxide on the (10.4) calcite surface. It was found that the preferred geometry of CO on this surface is adsorption on top of calcium in a slightly tilted orientation. With increased coverage, the binding energy shows a small decrease, revealing the presence of repulsive adsorbate-adsorbate interactions.

Accurate density functional theory calculations of the interrelated properties of thermoelectric materials entail high computational cost, especially as crystal structures increase in complexity and size. New methods involving ab initio scattering and transport (AMSET) and compressive sensing lattice dynamics are used to compute the transport properties of quaternary CaAl2Si2-type rare-earth phosphides RECuZnP2 (RE = Pr, Nd, Er), which were identified to be promising thermoelectrics from high-throughput screening of 20 000 disordered compounds. Experimental measurements of the transport properties agree well with the computed values. Compounds with stiff bulk moduli (>80 GPa) and high speeds of sound (>3500 m s−1) such as RECuZnP2 are typically dismissed as thermoelectric materials because they are expected to exhibit high lattice thermal conductivity. However, RECuZnP2 exhibits not only low electrical resistivity, but also low lattice thermal conductivity (∼1 W m−1 K−1). Contrary to prior assumptions, polar-optical phonon scattering was revealed by AMSET to be the primary mechanism limiting the electronic mobility of these compounds, raising questions about existing assumptions of scattering mechanisms in this class of thermoelectric materials. The resulting thermoelectric performance (zT of 0.5 for ErCuZnP2 at 800 K) is among the best observed in phosphides and can likely be improved with further optimization.

Single atom catalysis has emerged as a powerful technique for catalysis due to its outstanding performance and atom economy. Controlling the hybridization of the atom with its environment is crucial in determining the selectivity and/or yield of the reaction. However, the single atom environment is usually ill-defined and hard to predict because the pyrolysis process used in preparing SACs damages the original status of the precursors in the catalyst preparation. A molecular engineering approach to synthesize single atom catalysts (SACs) on a heterogeneous template provides a strategy to make SACs with a highly uniform coordinating environment. Herein, we report the preparation of a molecular engineered Pd single atom catalyst with a pre-defined M-N3C1 coordination (Pd-N3C1-SAC) using a structure-rigid Pd-N3C1 porphyrin as the precursor, which shows more efficient Suzuki coupling compared with the SAC with Pd-N4 coordination. The origin of the high activity of the Pd-N3C1-SAC is revealed through density functional theory calculations, where a lower reaction barrier for the rate-determining oxidative addition is identified.

How the local atomic environment that relates to the coordination ligand, crystalline structure, and substrate composition influences the catalytic performance of a single-atom catalyst (SAC) has not been fully explored. Here, we theoretically design Ru SAs supported on composition-adjustable inert Cu oxide substrates and investigate their electrocatalytic performance in comparison with that of well-studied bulk Ru and Ru SAs coordinated by nitrogen. Density functional theory (DFT) calculations reveal that Ru SAs supported on Cu oxides (SAs/CuxOy) cause a significant up-shift of the d-band center of Ru SAs in the order of Ru/Cu2O(111) > Ru/Cu(111) > Ru/CuO(111) > Ru–N > Ru(0001), highlighting the superiority of the weak SA-substrate interaction originating from the appropriate coordination environment. Using the nitrogen reduction reaction (NRR) as a probe, the DFT results show that the change of SA electronics due to the use of the CuxOy substrate leads to facile N2 adsorption which reduces alkaline HER and H* poisoning. Following the theoretical prediction, atomically dispersed Ru on Cu oxides is prepared for the first time and exhibits outstanding catalytic activity and selectivity towards the NRR with an NH3 yield rate of 42.4 μg mgcat.−1 h−1 and a faradaic efficiency up to 14.1% at +0.05 V vs. the reversible hydrogen electrode. Finally, we propose new descriptors that help rapidly screen SACs for alkaline NRR. This study represents a successful illustration of the “theoretical design – experimental verification – theoretical generalization” pathway, featuring a slight change in the local SA atomic environment for substantial catalytic performance enhancement.

Atomic layer deposition (ALD) has scarcely been utilized in large-scale manufacturing and industrial processes due to its low productivity, even though it possesses several advantages for improving the device performance. The major cause of its low productivity is the slow growth rate, which is determined by the amount of chemisorbed precursor. The slow growth rate of ALD has become even more critical due to the introduction of heteroleptic-based precursors for achieving a higher thermal stability. In this study, we investigated the theoretical and experimental chemisorption characteristics of the Ti(CpMe5)(OMe)3 precursor during the ALD of TiO2. By density functional theory calculations, the relationship between the steric hindrance effect and the chemistry of a chemisorbed precursor was revealed. Based on the calculation result, a way for improving the growth per cycle by 50% was proposed and demonstrated, successfully.

The structural, electronic and optical properties of a new van der Waals heterostructure, C2N/g-ZnO, composed of C2N and g-ZnO monolayers with an intrinsic type-II band alignment and a direct bandgap of 0.89 eV at the Γ point, are extensively studied using first-principles density functional theory calculations. The results indicate that the special optoelectronic properties of the constructed heterostructure mainly originate from the interlayer coupling and electron transfer between the C2N and g-ZnO monolayers, and the photogenerated electrons and holes are located on the C2N and g-ZnO layers, respectively, which reduces the recombination probability of the electron–hole pairs. According to Bader charge analysis, there are 0.029 electrons transferred from g-ZnO to C2N to form a built-in electric field of ∼9.5 eV at the interface. Furthermore, the tunability of the electronic properties of the C2N/g-ZnO heterostructure under vertical strain and electric field is explored. Under different strains, the type-II band alignment properties of the heterostructure are retained and the vertical compressive strain has a greater influence on the bandgap modulation than the vertical stretching strain. The implemented electric field also does not change the type-II band alignment but changes the bandgap of the heterostructure from 1.30 to 0.58 eV when the electric field strength varies from −0.6 to 0.6 V A−1. In addition, the absorption spectrum of the C2N/g-ZnO heterostructure under solar light is also studied. The absorption range of the heterostructure varies from the ultraviolet to near-infrared region with the absorption intensity in the order of 105 cm−1. All of these studies indicate that the C2N/g-ZnO heterostructure has excellent electronic and optical properties and promising applications in nanoelectronics and optoelectronics.

Accurate simulation of semiconductor nanowires (NWs) under strain is challenging, especially for bent NWs. Here, we propose a simple yet efficient unit-cell model to simulate strain-mediated bandgap modulation in both straight and bent NWs. This is with consideration that uniaxlly bent NWs experience continuous compressive and tensile strains through their cross-sections. A systematic investigation of a series of III-V and II-VI semiconductors NWs in both wurtzite and zinc blende polytypes is performed using hybrid density functional theory methods. The results reveal three common trend in bandgap evolution upon application of strain. Existing experimental measurements corroborate with our predictions concerning bandgap evolution as well as direct-indirect bandgap transitions upon strain. By examining the variation of previous theoretical studies, our result further highlights the significance of geometrical relaxtion in NW simulation. This simplified model is expected to be applicable to investigations of the electronic, optoelectronic, and sensorial properties of all semiconductor NWs.

We report a facile synthesis of an Ir surface-enriched IrCo alloy catalyst and its excellent activity which outperforms commercial Pt/C for the hydrogen evolution reaction (HER) in acidic media showing fast kinetics. The as-synthesized catalyst exhibits low overpotentials of merely 9 mV and 29.3 mV to afford current densities of 10 and 100 mA cm−2, respectively, a turnover frequency of 1.25 s−1, a low Tafel slope of 23.7 mV dec−1, and stability for 100 h at a high current density of 100 mA cm−2 in 0.5 M H2SO4 electrolyte. Experiments and density functional theory (DFT) calculations indicate that the enrichment of Ir atoms on the outer surface of IrCo alloys is responsible for the effectively optimized free energy of hydrogen adsorption, which boosts the kinetics and HER performance. Our finding provides an insight into the metal surface enrichment on alloy nanoparticles (NPs).

A 3D/2D g-C3N4/ZnIn2S4 hollow spherical heterostructure was constructed by growing modified ZnIn2S4 nanosheets on the surface of g-C3N4 microspheres. This composite material can not only take advantage of the hollow structure with a stronger light absorption capacity and more active sites, but its heterostructure can also increase the efficiency of photogenerated electron migration and separation, thereby improving the serious photogenerated electron recombination of two separate materials. The unique structural superiority led to excellent photocatalytic CO2 reduction performance, where a higher CO generation rate reaching 7368.7 μmol g−1 h−1 by using IZIS–CN100 was undoubtedly strong evidence. The heterojunction formation process and the subsequent electronic changes on both theoretical and experimental aspects of the combination of electrochemical tests and density functional theory (DFT) were verified. Finally, we proposed a feasible photocatalytic mechanism and brought forth new ideas in the application of materials, structural control, and the combination of theories and experiments.

In order to reveal the correlation effect on the electronic properties in particular 5f electron occupation numbers of Pu/U ions in a (Pu,U) mixed oxide-PuUO4, a first principles calculation is performed by using density functional theory (DFT) plus a dynamical mean field theory (DMFT) scheme with the spin-orbit coupling (SOC) and on-site Coulomb repulsion for correlation effect due to localized Pu/U 5f orbitals. Results demonstrate that Pu/U 5f electron occupation numbers in the ground state of PuUO4 are mainly composed of 5f4/5f5 and 5f2/5f3 configurations, and exhibiting the intermediate occupation (IO) numbers with average 5f occupation numbers of about nf = 4.879 and 2.423 for Pu and U ions, respectively, irrespective of different Pu and U lattice sites in PuUO4. Pu 5f j = 5/2 and j = 7/2 components are in moderately and weakly correlated states, respectively, while U 5f j = 5/2 and j = 7/2 manifolds are both in weakly correlated states. jj and LS coupling schemes are feasible for Pu and U 5f electrons, respectively. In order to directly compare with the experimental angle-resolved photoemission spectrum (ARPES), we also estimate the momentum-resolved electronic spectrum function for this system.

Localizing charge carriers close to catalytically active sites can reduce the charge recombination in photoexcited polymeric carbon nitride (PCN), which thus potentially increases its photocatalytic H2 generation. However, this strategy has not been applied to enhance the photocatalytic performance of PCN. Herein density functional theory (DFT) calculations unravel that N defect (Ndefect) engineered tri-s-triazine units form a highly localized mid-gap state in PCN, which can localize the π electrons for efficient separation of electrons and holes. Moreover, Ndefect engineered tri-s-triazine units impart the lowest Gibbs free energy barrier of −0.129 eV for the reduction of H+ to adsorbed H, thereby serving as the catalytically active sites. Enlightened by theoretical results, the Ndefect engineered tri-s-triazine units were created via post oxidation of PCN in an electronic furnace. Such PCN with N defects offers an increased photocatalytic H2 generation under visible light exposure. The highest H2 generation rate is up to 98.1 μmol h−1, 3.8 times higher than that by using pristine PCN (26.1 μmol h−1). This work manifests the significance of localizing charge carriers close to the catalytically active sites for increased photocatalytic performance of PCN.

Ab initio simulations of a multi-component alloy using density functional theory (DFT) were combined with experiments on thin films of the same material using X-ray photoelectron spectroscopy (XPS) to study the connection between the electronic and atomic structures of multi-component alloys. The DFT simulations were performed on an equimolar HfNbTiVZr multi-component alloy. Structure and charge transfer were evaluated using relaxed, non-relaxed, as well as elemental reference structures. The use of a fixed sphere size model allowed quantification of charge transfer, and separation into different contributions. The charge transfer was generally found to follow electronegativity trends and results in a reduced size mismatch between the elements, and thus causes a considerable reduction of the lattice distortions compared to a traditional assumption based on tabulated atomic radii. A calculation of the average deviation from the average radius (i.e. the so-called delta-parameter) based on the atomic Voronoi volumes gave a reduction of delta from ca. 6% (using the volumes in elemental reference phases) to ca. 2% (using the volumes in the relaxed multi-component alloy phase). The reliability of the theoretical results was confirmed by XPS measurements of a Hf22Nb19Ti18V19Zr21 thin film deposited by sputter deposition. The experimentally observed core level binding energy shifts (CLS), as well as peak broadening due to a range of chemical surroundings, for each element showed good agreement with the calculated DFT values. The single solid solution phase of the sample was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) including energy dispersive spectroscopy (EDS) with nm-resolution. These observations show that the HfNbTiVZr solid solution phase is non-ideal, and that chemical bonding plays an important part in the structure formation, and presumably also in the properties. Our conclusions should be transferable to other multi-component alloy systems, as well as some other multi-component material systems, and open up interesting possibilities for the design of material properties via the electronic structure and controlled charge transfer between selected metallic elements in the materials. Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2018-04834, 2019-05403, 2018-05973, 2019-05487]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Faculty Grant SFO at LiU [2009 00971]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; RFBRRussian Foundation for Basic Research (RFBR) [20-02-00178]; Swedish e-Science Research Centre (SeRC)

The incorporation of a certain amount of Cl ions into methylammonium lead iodide (MAPbI3) perovskite films and how these incorporated Cl ions affect the structural and electronic properties of these films have been an intensively studied topic. In this study, we comprehensively investigated Cl incorporation in MAPbI3 at the atomic scale by a combined study of scanning tunneling microscopy, X-ray photoelectron spectroscopy, ultraviolet and inverse photoemission spectroscopy, density functional theory and molecular dynamics calculations. At a Cl concentration of 14.8 ± 0.6%, scanning tunneling microscopy images confirm the incorporation of Cl ions on the MAPbI3 surface, which also corresponds to the highest surface stability of MAPbI3 found from the viewpoint of both thermodynamics and kinetics by density functional theory and molecular dynamics calculations. Our results show that the Cl concentration is crucial to the surface bandgap and stability of MAPbI3.

Chemical looping combustion (CLC) technology is an innovative energy conversion technology that employs oxygen carriers (OC), typically metal oxides, to burn fossil fuels with a minimal carbon footprint. The performance of OCs can be enhanced by the support on which they are deposited through two mechanisms acting at different scales, viz., microstructural and synergetic effects. In this work, the synergetic effect of NiO supported on TiO2 in reaction with hydrogen as a fuel is studied using density functional theory (DFT). Changes in the energetics of the NiO-hydrogen reaction are explained as a consequence of the interaction between the TiO2 support and NiO. The results indicate that the electronic interaction of the TiO2 support with NiO lowers the energy of intermediate states and the energy of the reaction. The effect of TiO2 increases with the creation of more O vacancies as the reaction proceeded. This enhanced reactivity of the NiO-hydrogen reaction is attributed to both an electronic effect of TiO2 and a geometric effect due to O vacancy creation. The synergetic effect of the support on the OC reactions at the atomic level reported here can pave the path to differentiate the electronic and geometric effects and establish the knowledge for the rational design of OC and support systems.

Accurate and efficient calculations of absorption spectra of molecules and materials are essential for the understanding and rational design of broad classes of systems. Solving the Bethe–Salpeter equation (BSE) for electron–hole pairs usually yields accurate predictions of absorption spectra, but it is computationally expensive, especially if thermal averages of spectra computed for multiple configurations are required. We present a method based on machine learning to evaluate a key quantity entering the definition of absorption spectra: the dielectric screening. We show that our approach yields a model for the screening that is transferable between multiple configurations sampled during first principles molecular dynamics simulations; hence it leads to a substantial improvement in the efficiency of calculations of finite temperature spectra. We obtained computational gains of one to two orders of magnitude for systems with 50 to 500 atoms, including liquids, solids, nanostructures, and solid/liquid interfaces. Importantly, the models of dielectric screening derived here may be used not only in the solution of the BSE but also in developing functionals for time-dependent density functional theory (TDDFT) calculations of homogeneous and heterogeneous systems. Overall, our work provides a strategy to combine machine learning with electronic structure calculations to accelerate first principles simulations of excited-state properties., Machine learning can circumvent explicit calculation of dielectric response in first principles methods and accelerate simulations of optical properties of complex materials at finite temperature.

The manifestation of the structural and magnetic properties of Co substituted TiFe2 is investigated using powder X-ray diffraction, magnetization and density functional theory calculations. The alloys TiFe2 and TiFeCo crystallize in the hexagonal structure (P63/mmc) with a reduction in the lattice parameters of TiFeCo (by about 0.51% in a and 0.64% in c) when compared to TiFe2. On the other hand, TiCo2 crystallizes in the cubic structure (Fdm). A structural transition from hexagonal to cubic is anticipated for a composition with x ∈ [0.5, 1]. The non-collinear antiferromagnetic (AFM) spin structure (formed by 6h Fe atoms) of TiFe2 with Neel temperature TN ∼ 275 K is reported at zero magnetic field H. Meanwhile, a magnetic field-induced collinear antiferromagnetic spin structure is suggested by magnetization measurements and supported by density functional theory calculations. The magnetization of TiFeCo shows a weak-ferromagnetic (FM)-like transition around 204 K, followed by a broad hump at 85.5 K and H = 200 Oe. Ferromagnetic interactions are weakened, causing the hump to disappear due to the possible transfer of electrons between Fe and Co. TiCo2 shows compensated ferrimagnetism with magnetization of the order of 10−5μB f.u.−1 and a linear increase of M with H at 5 K. The presence of a non-collinear AFM spin structure in TiFe2, a reduced magnetic moment in TiFeCo due to the charge transfer between Co and Fe, and compensated ferrimagnetism in TiCo2 promise a rich phase diagram of Ti(Fe1−xCox)2 alloys and the possible potential of these alloys for use in spintronics applications.

Defect control is a promising way to enhance the electrocatalysis performance of metal oxides. Oxygen vacancy enriched NiCo2O4 was successfully prepared using cold plasma. Oxygen as a plasma-forming gas introduces oxygen vacancies via electron etching. The concentration of oxygen vacancies can be controlled by different plasma-forming gas. CoO, which formed on the plasma samples, is beneficial for quick charge transfer and electrocatalytic performance. A high amount of nitrogen atoms of up to 10.1% was doped on NiCo2O4 because of the enriched oxygen vacancies and improved the stability of the oxygen defects and the conductivity of the catalyst. Electrocatalytic studies showed that the plasma-induced N-doped NiCo2O4 shows enhanced electrocatalytic performance for the oxygen reduction reaction (ORR). It shows a typical four-electron process that considerably improves the current density and onset potential. The HO2- % was as low as 0.59% and current density was 4.9 mA cm-2 at 0.2 V (Vs. RHE) on the plasma-treated NiCo2O4. Calculations based on density functional theory reveal the mechanism for the promotion of the catalytic ORR activity via plasma treatment. This increases the electron density near the Fermi level, reducing the work function, and changing the position of the d-band center.

Hydroxyl radicals (OH*) play a crucial role in atmospheric chemistry and biological processes. In this study, Born-Oppenheimer molecular dynamics simulations are performed under ambient conditions for a hydroxyl radical in a water nanodroplet containing 191 water molecules. Density functional theory calculations are performed at the BLYP-D3 level with some test calculations at the B3LYP-D3 level. In two 150 ps trajectories, either with OH* initially located in the interior region or at the surface of the water nanodroplet, the OH* radical ends up in the subsurface layer of the nanodroplet, which is different from the "surface preference" predicted from previous empirical force field simulations. The solvation structure of OH* contains fluctuating hydrogen bonds, as well as a two-center three-electron hemibond in some cases. The mobility of OH* is enhanced by hydrogen transfer, which has a free energy barrier of ∼4.6 kcal mol-1. The results presented in this study deepen our understanding of the structure and dynamics of OH* in aqueous solutions, especially around the air-water interface.

The in-plane anisotropy of monolayer InSe plays a critical role in the application of photodetectors. In this work, through nonequilibrium Green's function density functional theory (NEGF-DFT) and time-dependent density functional theory (TD-DFT) calculations, we investigated the anisotropic quantum transport in darkness and under linearly polarized light, and explored the role of surface-bound excitons in the anisotropic photocurrent. The anisotropic dark quantum transport is attributed to different potential barriers in the zigzag and armchair orientations (Id-zig/Id-arm = 1.2 × 102). Linearly polarized photocurrent calculations show that the extinction ratio reaches a maximum value of 105.67. Moreover, surface-bound exciton calculations via TD-DFT revealed that the strong anisotropic photocurrent derives from surface-bound excitons generated in the In 5pz, Se 4pz, and Se 4dz2 orbitals. InSe shows tremendous potential for use in field-effect transistors, flexible nano- and optoelectronics, and polarized light devices.

We investigated the electronic and structural properties of the infinite linear carbon chain (carbyne) using density functional theory (DFT) and the random phase approximation (RPA) to the correlation energy. The studies are performed in vacuo and for carbyne inside a carbon nano tube (CNT). In the vacuum, semi-local DFT and RPA predict bond length alternations of about 0.04 {\AA} and 0.13 {\AA}, respectively. The frequency of the highest optical mode at the $\Gamma$ point is 1219 cm$^{-1}$ and about 2000 cm$^{-1}$ for DFT and the RPA. Agreement of the RPA to previous high level quantum chemistry and diffusion Monte-Carlo results is excellent. For the RPA we calculate the phonon-dispersion in the full Brillouine zone and find marked quantitative differences to DFT calculations not only at the $\Gamma$ point but also throughout the entire Brillouine zone. To model carbyne inside a carbon nanotube, we considered a (10,0) CNT. Here the DFT calculations are even qualitatively sensitive to the k-points sampling. At the limes of a very dense k-points sampling, semi-local DFT predicts no bond length alternation (BLA), whereas in the RPA a sizeable BLA of 0.09 {\AA} prevails. The reduced BLA leads to a significant red shift of the vibrational frequencies of about 350 cm$^{-1}$, so that they are in good agreement with experimental estimates. Overall, the good agreement between the RPA and previously reported results from correlated wavefunction methods and experimental Raman data suggests that the RPA provides reliable results at moderate computational costs. It hence presents a useful addition to the repertoire of correlated wavefunction methods and its accuracy clearly prevails for low dimensional systems, where semi-local density functionals struggle to yield even qualitatively correct results., Comment: 7 pages, 5 figures

Tunneling electroresistance (TER) is the change in tunneling resistance induced by ferroelectric polarization reversal in ferroelectric tunnel junctions (FTJs), and how to achieve a giant TER has always been a central topic in the study of FTJs. In this work, by considering the NaTiO3/BaTiO3/LaTiO3 junction with asymmetric polar interfaces as an example, we propose a novel scheme to realize a giant TER based on the reversible partial metallization of ferroelectric barrier upon the switching of ferroelectric polarization. Density functional theory calculations indicate that high on-state and low off-state conductances are obtained and the TER ratio is as high as 3.20 × 108% due to the reversible partial barrier metallization, which leads to a great difference in the effective tunneling barrier widths. The reversible partial barrier metallization, accompanied by the ferroelectric polarization reversal, is driven by the parallel or anti-parallel alignment of the depolarization electrical field of the ferroelectrical barrier and a strong built-in electrical field cooperatively contributed by the asymmetric polar interfaces and the difference in the work functions of the two leads. The findings suggest a feasible scheme for constructing promising high performance FTJ memory devices by combining both asymmetric polar interfaces and substantially different work functions.

Catalysts play a critical role in the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) for energy storage, conversion, and utilization. Herein, first-principles density functional theory (DFT) calculations demonstrated that single-metal-atom (Fe, Co, or Ni) sites can bind to the surface of 2D WO2, enhancing the adsorption of intermediates involved in the OER/ORR. Furthermore, it was found that the single-metal-atom-doped 2D WO2 achieves the smallest OER and ORR overpotentials of 0.42 V and 0.40 V, respectively, which are comparable to those of IrO2 or Pt-based catalysts. This predicts the excellent OER/ORR catalytic activities of the single-metal-atom (Fe, Co, or Ni) doped 2D WO2, which would be a promising bifunctional catalyst for fuel cells, water splitting, and metal–air batteries.

Combined with density functional theory (DFT) calculations, a substitutional heteroatom-doping approach is employed to design asymmetric local structures based on highly ordered mesoporous MoO2 nanostructures. Such synergistic strategies on increasing both the number and intrinsic activity of active sites jointly lead to a significant water oxidation performance boost.

In this study, the adsorption of gas molecules, such as O2, NH3, CO, CO2, H2O, NOx (x = 1, 2) and SO2, on Janus Te2Se monolayer has been investigated by means of density functional theory (DFT) calculations. We show that Janus Te2Se monolayer is preferable for SO2 and NOx molecules with suitable adsorption strength and apparent charge transfers. We further calculated the current-voltage (I-V) curves using the nonequilibrium Green's function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V curves before and after NOx (SO2) adsorption on Janus Te2Se. Thus, we predict that Janus Te2Se could be a promising candidate for SO2 and NOx sensors with high selectivity and sensitivity. Moreover, the effect of strain on the gas/substrate adsorption systems was also studied, implying that the strained Janus Te2Se monolayer could enhance the sensitivity and selectivity to SO2 and NO2. The adsorbed SO2 and NO2 on Janus Te2Se could escape by releasing the applied strain, which indicates that the capture process is reversible. Our study widens the application of Janus Te2Se not only as piezoelectric materials, but also as a potential gas sensor or capturer of SO2 and NOx with high sensitivity and selectivity.

Metallocarbohedrenes or metcars belong to one of the classes of stable nanoclusters having a specific stoichiometry. In spite of the available theoretical and experimental studies, the structure of pristine Ti8C12 metcar is still uncertain. We study the geometric structure of a titanium metcar, Ti8C12, together with its electronic properties and chemical activity towards adsorption and activation of CO2 molecule by means of density functional theory. Our results suggest that the CO2 molecule is strongly adsorbed and undergoes a significantly high degree of activation onto the Ti8C12 metcar. The migration of charge from titanium metcar to CO2 molecule attributes the high degree of activation of this molecule. In the infrared vibrational spectra for CO2 molecule adsorbed onto Ti8C12, we find a new signal which is absent in the corresponding spectra for gaseous CO2. In addition to adsorption energy, we also estimate the energy barrier for the dissociation of CO2 molecule to CO and O fragments on a Ti8C12 cluster. As a whole, this work reveals the ground state geometry of Ti8C12 metcar and highlights the role of this metcar in CO2 adsorption and activation, which are the key steps in designing potential catalysts for CO2 capture and its conversion to industrially valuable chemicals.

Dietary supplementation is prevalent in our daily life, in which tablet form plays an important role due to its long-term stability and cost-effectiveness. In the pharmaceutical industry, coordination polymer-based tablets are widely produced for dietary supplementation. The mechanical properties of such dietary supplements are very crucial for the tableting process and need to be broadly studied. In this study, the mechanical properties of a copper supplement, namely copper glycinate hydrate (Cu[O2CCH2NH2]2·H2O), have been studied via high-pressure synchrotron powder X-ray diffraction and density functional theory calculations. This material is a 1D coordination polymer, and adjacent chains are linked via hydrogen bonding in the structure. The measured bulk modulus (Ko) is ∼13.9 GPa, which is comparable to that of small organic drug crystals and far smaller than that of inorganic dietary supplements. The calculated Young's moduli (13.55–35.75 GPa) indicate a significant elastic anisotropy, and the directional dependent Young's modulus can be substantiated with the underlying atomic structure. Our experimental and calculated results provide useful quantitative information for tableting and formulating this dietary supplement in the pharmaceutical industry.

Tailoring the d-band center is an effective method to promote the hydrogen evolution reaction (HER) activity of electrocatalysts. Herein, we demonstrate that the d-band center can be tuned through a hole-creating method to enhance pH-universal HER activity using CoP as a basic platform. The density functional theory (DFT) calculation reveals that the d-band center and the valence electron number of Co sites around the holes are upshifted, which boosts H adsorption. In addition, introducing holes into the nanosheets of CoP can optimize the ΔGH* of the Co atoms around the holes. Inspired by the DFT results, a soft template method was developed to synthesize hole-rich CoP nanosheets. With the advantages of hole-rich and unique nanosheet features, the hole-rich CoP nanosheets show low HER overpotentials with only 84 and 94 mV to achieve a current density of 10 mA cm−2 in both acidic and alkaline media, better than that of the other dimensional counterparts. In addition, the hole-rich CoP nanosheets also show satisfactory stability after long-term HER tests. This strategy of regulating the d-band center is expected to be extended to other transition metal phosphide electrocatalysts for enhancing the HER performance.

Catalysts that boost the oxygen reduction reaction (ORR) are highly needed for fuel cells and metal–air batteries. Herein, we report the preparation of surface-vacancy-rich PtFe interconnected nanowires as an excellent ORR catalyst. When utilized in both alkaline and acidic electrolytes, this hierarchically structured catalyst shows high mass activities at 0.9 V versus the reversible hydrogen electrode (RHE) (alkaline electrolyte: 3.65 A mgPt−1 and acid: 1.10 A mgPt−1), high half-wave potential (alkaline electrolyte: 0.959 V vs. RHE and acid electrolyte: 0.924 V vs. RHE), and high stability (almost no activity loss after 10 000-cycle accelerated durability testing). Density functional theory calculations reveal that the presence of vacancies on the Pt-skin provides rich active ORR sites that can weaken the bonding strength and adsorption of O-containing intermediates. This paper presents a facile strategy to assemble efficient hierarchically structured electrocatalysts with rich surface defects for devices such as fuel cells.

The hydrogen abstraction (HB) and addition reactions (HD) by H radicals are examined on a series of polycyclic aromatic hydrocarbon (PAH) monomers and models of quasi-surfaces using quasi-classical trajectory (QCT) method. The QCT results reproduce the rate constants of HB reactions on PAH monomers from density function theory (DFT) in the range of 1500-2700 K. The PAH size has a minor impact on the rates of HB reactions especially at temperatures beyond 2100 K. By contrast, HD reactions have a clear size dependence and a larger PAH yields a higher rate. It is also found that the preferred reaction pathway changing from HB to HD reactions at ~1900 K. The rates of surface HB and HD reactions exceed those in the gas phase by nearly a factor of magnitude. Further analysis on the detailed trajectory of QCT method reveals that about 50% of the surface reactions can be attributed to the events of surface diffusion, which depends on the local energy transfer in the gas-surface interactions. However, this phenomenon is not preferred in PAH monomers as expected. Our finding here highlights the misinterpretation of surface reactions as the product of the first collision between gaseous species and particle surface, and surface diffusion induced reactions should be accounted for in the rates of surface HB and HD reactions. Rate constants of HB and HD reactions on each reactive site (surface zig-zag, surface free-edge and pocket free-edge sites) are calculated by QCT method, which are recommended for the further development of surface chemistry models in soot formation.

Metal–organic frameworks (MOFs) are composed of metal nodes connected by organic linkers. With the massive amount of possible metal and linker combinations, it is critical to develop structure–property relationships. However, a major impediment to developing these relationships is that MOFs often have multiple metal coordination structures in a single crystal. Here, we report periodic and cluster density functional theory (DFT) calculations analyzing the coordination structures of MOF-808 and sulfated MOF-808 and their physical and chemical properties. For MOF-808, we determined coordination structures by comparing computationally determined lattice constants with experimental values and then used these coordination structures to simulate N2 and Ar adsorption isotherms. Our simulated average N2 and Ar uptakes agree very well with the experimental values. For the sulfated MOF-808, which has been proposed to be a superacidic material, we determined the impact of coordination structure on acidity. Surprisingly, our results based on calculated proton affinities suggest a 1025 range in acid site strength depending on the coordination structure, with only a few sites having high acidity. The use of vibrational frequencies and other property-based methods for determining relative acidity fail. Our analysis revealed that the acidity of sulfated MOF-808 is unlikely controlled by a sulfate–water hydrogen bond that was previously proposed. Instead, we show that there is a strong correlation between conjugate base stability and proton affinity to rationalize acidity of protons in a single coordination structure model as well as across all coordination structures.

The modification of carbon nitride (CN) for visible light photocatalytic CO2 reduction is an important issue to explore in the context of energy and environmental challenges. The current work describes the fabrication of a butterfly-shaped photocatalyst derived from a heterojunction between the electron-rich organic monomer 2,5-dibromothiophene (TP) and a CN framework via copolymerization (UCN-TP). The successful incorporation of TP within CN means that it acts as a nucleophilic spare entity that provides electron solidity (π-conjugation) via charge allocation and increases the photocatalytic activity of CN (under visible light illumination). Density functional theory (DFT) data provide additional information on the amalgamation of TP inside CN and the reduced band gap. The best sample UCN-TP0.06 generated 68.18 μmole of CO and 18.52 μmole of H2 by photoreduction, demonstrating the highest photocatalytic activity. Moreover, this remarkable modification of the chemical configuration, band gap and surface area encouraged the good detachment of light-produced photons, thereby rendering it the best candidate for the photoreduction of CO2.

By means of density functional theory and unbiased structure search computations, we systematically investigated the stability and electronic properties of a new Ga2O2 monolayer. The phonon spectra and ab initio molecular dynamics simulations show that the Ga2O2 monolayer is dynamically and thermally stable. Moreover, it also shows superior open-air stability. In particular, the Ga2O2 monolayer is an indirect semiconductor with a wide band gap of 2.752 eV and high hole mobility of 4720 cm2 V−1 s−1. Its band gap can be tuned flexibly in a large range by applied strain and layer control. It exhibits high absorption coefficients (>105 cm−1) in the ultraviolet region. The combined novel electronic properties of the Ga2O2 monolayer imply that it is a highly promising material for future applications in electronics and optoelectronics.

Rechargeable zinc ion batteries (ZIBs) have received significant attention from the scientific community as an alternative to lithium ion batteries (LIBs) for large-scale energy storage systems owing to their high safety and low cost. However, the lack of a suitable cathode material limits their practical implementation. By using density functional theory (DFT)-based computations, we have herein investigated the electronic structure of a very recently synthesized phenanthroline-based covalent organic framework (PACOF) to lend support for its applicability as a promising cathode material for ZIBs. We have analyzed the diffusion barriers, open-circuit voltages (OCVs), and storage capacity of this COF. Thereafter, inspired by some very recent experimental research works, we have predicted a new framework based on quinone and phenanthroline (QPACOF), which is found to exhibit better electrochemical performance as a cathode material for ZIBs compared with PACOF. Our predicted framework (QPACOF) exhibits almost twice the storage capacity shown by PACOF. Moreover, the OCVs of QPACOF are higher than that of PACOF, ensuring a larger cell voltage for QPACOF. Finally, we have proposed a possible synthetic route to experimentally synthesize our predicted model system (QPACOF).

Conductive metal–organic frameworks (MOFs) are relevant as active materials for many applications, including electrocatalysis, chemiresistive sensing, and energy storage. However, the structure–activity relationship toward electrocatalysis in conductive MOFs needs to be further explored. Herein, a new strategy of electronic structure modification is proposed by introducing phthalocyanine with extra metal sites into conductive MOFs. Density Functional Theory (DFT) calculations predict that these MOFs possess excellent conductivity with a narrow band-gap. Electrochemical tests and d-band center calculations confirm the highest activity of NiPc–Ni among the MOFs, indicating that both Ni–N4 and Ni–O4 sites are active for the OER. Moreover, PDOS analysis and TOF calculations indicate that the electronic structure interaction between Ni–O4 and Ni–N4 sites improves their intrinsic OER activity. This work provides an effective strategy for modulating the electronic structure and revealing the structure–activity relationship in 2D conductive MOFs toward electrocatalysis.

Surface-enhanced Raman spectroscopy (SERS) coupled with density functional theory (DFT) computations can characterise the adsorption orientation of a molecule on a nanoparticle surface. When using DFT to simulate SERS on a silver surface, one typically employs an atom (Ag), ion (Ag+), or cluster (Agx or Agx+) as the model surface. Here, by examining the nucleobase 2,6-diaminopurine (2,6-DAP) and then generalising our strategy to three other molecules, we show that employing silver oxide (Ag2O) as the model surface can quantitatively improve the accuracy of simulated SERS.

A series of heteroleptic Ir(iii) complexes composed of two cyclometalated C^N ligands and one dipyrrinato ligand used as an ancillary ligand are synthesized and characterized. With the introduction of a fluorine atom, phenyl ring or diphenylamino group into both C^N ligands and by keeping the ancillary ligand unchanged, these Ir(iii) dipyrrinato phosphors do not show an obvious shift in their emission bands. They exhibit emissions extending well into the near-infrared region with an intense band located at around 685 nm in both photo- and electroluminescence spectra, and the deep red to near-infrared organic light emitting diodes (OLEDs) based on them afforded a maximum external quantum efficiency of 2.8%. Density functional theory (DFT) calculations show that both the electronic contributions on the lowest unoccupied molecular orbitals (LUMOs) and the highest energy semi-occupied molecular orbitals (HSOMOs) are mainly localized on the dipyrrinato ligand, indicating that the ancillary ligand, which remains unchanged in this series, exhibits a lower triplet state energy in the iridium phosphors than those involving the C^N ligands. Therefore a switch from "(C^N)2Ir" to dipyrrinato ligand-based emission is observed in these iridium(iii) complexes.

Titanium-based materials have been considered to be promising materials for many years. The structures and properties of TixOy and TiC at the nanoscale are important for studying the mechanism of formation of their nanoparticles. In this work, the density functional theory (DFT) method was used to calculate the structures of (TixOy)n (n = 1-5) and (TiC)n (n = 1-10) clusters. Based on these calculations, the formation pathways from (TiO2)n clusters to (TiO)n and (TiC)n clusters via carbon reduction were investigated. The results show that the pathway via (Ti2O3)n or (Ti3O5)n is the most likely pathway for the transformation of the (TiO2)n cluster to the (TiO)n cluster. The (TiO)n cluster is not the final product in the reduction process of (TiO2)n with C, and it can be finally transformed into (TiC)n clusters via various TinOxCy clusters via reaction with C. In addition, the (TiO2)n clusters can be transformed into (TiC)n clusters directly via various TixOyCz clusters.

Two-dimensional (2D) rhenium disulfide (ReS2) has been attracting immense interest as a highly promising hydrogen evolution reaction (HER) electrocatalyst recently. However, the HER catalytic active sites of ReS2 are still limited to the finite edges of the nanosheets and the basal-plane is catalytically inert. How to activate the in-plane catalytic activity of ReS2 accordingly becomes the sticking point to further boost its H2 evolution efficiency. Here, the fluorinated ReS2 hierarchical structure assembled as ultrathin nanosheets is proved to exhibit excellent HER activity, where the optimal sample of ReS2-F5.93 shows a low overpotential of 142 mV vs. RHE at a current density of 10 mA cm−2, a Tafel slope of 64 mV dec−1, and significantly improved stability. The enhanced HER performance is mainly attributed to the synergistic effects of increased in-plane active sites and remarkably accelerated charge transfer kinetics. The density functional theory calculation results reveal that F doping can yield the optimal H* adsorption Gibbs free energy and meanwhile increase the electronic states near the Fermi level for accelerated electron transfer. This work can broaden the horizon for designing efficient HER electrocatalysts via activating the in-plane catalytic activity of transition metal dichalcogenides (TMDs).

Wide-gap oxide materials including gallium oxide and aluminium oxide have been attracting much interest due to their tremendous potential for application in power devices. In this work, a new III-oxide material utilizing chalcogen and oxygen mixing to form Al2(O1−xSex)3 III-oxychalcogenides is explored and studied. First-principles density functional theory calculations using hybrid functionals were applied to investigate the effect of Se on the electronic and structural properties of corundum Al2(O1−xSex)3 alloys. Analysis on formation enthalpies indicated that Al2(O1−xSex)3 alloys exhibit a large miscibility gap, rendering a single-crystal or metastable phase achievable if the Se content remains 8% or less. Our investigations on electronic properties show that Se addition leads to a significant reduction of the bandgap energy of Al2O3, providing a remarkably large energy bandgap tuning range of 7.46 eV. In addition, band alignment studies reveal a type-I band offset for the Al2(O1−xSex)3/Al2O3 structure across the entire composition range, in which the valence band offset to conduction band offset ratio reaches as high as 0.7 : 0.3 when the Se content is less than 6%. Interestingly, although adding Se leads to a significant upward movement of the valence band in Al2(O1−xSex)3 alloys, the downward movement of the conduction band becomes dominant when the Se content increases. Overall, our work reveals the peculiar properties of mixed-anion AlOSe III-oxychalcogenides that show potential values for technological applications. More importantly, our initial work here carves out an uncharted territory in III-oxychalcogenides that requires further investigations from both experimental and theoretical approaches.

Doping with nitrogen atom is an effective way to modify the electronic and magnetic properties of graphene. In this paper, we studied the effect of the number of dopant atoms on the electronic and magnetic properties of the two most common nitrogen bond configurations in N-doped graphene, that is, graphitic and pyridinic, using density functional theory (DFT). We found that the formation of graphitic and pyridinic configurations can initiate the transition of the electronic properties of graphene from semimetal to metal with n-type conductivity for the graphitic configuration and p-type conductivity for the pyridinic configuration. The formation of a bandgap-like structure was observed in both configurations. The bandgap increased with the increase in the number of dopant atoms. We also observed that the formation of graphitic configuration did not cause a transition to the magnetic properties of graphene even though the number of dopant atoms was increased. In the pyridinic configuration, the increase in the number of dopant atoms caused graphene to be paramagnetic, with the remarkable total magnetic moment of 0.400 μB per cell in the pyridinic-N3 model. This study provides a deeper understanding of the modification of electronic and magnetic properties of N-doped graphene by controlling the bond configuration and the number of nitrogen dopants.

Two novel complexes were constructed by adsorption of two typical organic molecules, including one electron-donating molecule (TTF = tetrathiafulvalene) and one electron-withdrawing molecule (TCNQ = tetracyanoquinodimethane), on the surface of black phosphorus quantum dots (BPQDs). There exist considerable charge transfer and strong non-covalent interaction between organic molecules and BPQDs. The results of density functional theory (DFT) calculations show that the static first and second hyperpolarizabilities can be significantly enhanced by introducing TTF/TCNQ groups on BPQDs. Apart from static nonlinearity, hyper-Rayleigh scattering (HRS) first hyperpolarizability shows a remarkable value at a dispersion frequency of 1907 nm. Besides, the frequency-dependent first hyperpolarizability increases significantly while going from the gas to solvent phase. BPQDs–TTF presents a strong dipolar character, whereas BPQDs–TCNQ presents a relatively larger octupolar behavior. Compared with BPQDs–TTF, BPQDs–TCNQ exhibits better stability and larger first hyperpolarizability. This work highlights the superiority of combining TCNQ and BPQDs to construct high-performance nonlinear optical (NLO) molecules.