Co-Doping Effects on the Electronic and Optical Properties of β-Ga2O3: A First-Principles Investigation.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Lithium metal batteries (LMBs) are among the most promising candidates for high energy density energy storage technologies [1]. In order to make LMBs commercially competitive with the other mature energy storage devices, several aspect hindering LMBs’ widespread application shall be resolved. In particular, the growth of Li dendrites can cause the LMBs performance decrease as well as severe safety problems associated with the short circuit. In this regard, the solid electrolyte interface (SEI), a nanoscopic layer formed between the metal electrode and the liquid electrolyte, controls the Li delivery to the surface of the negative electrode, where Li electrodeposition and dendrites formation occur. Thus, the understanding of Li diffusivity (e.g., pathways, energetics, etc.) through the SEI could enable the development of strategies towards safe, robust and long-lasting LMBs. For that reason, in this contribution, we present the results of a combined ab initio Density Functional Theory (DFT) calculations to study Li diffusion across the SEI and an extended phase-field model (PFM) for Li migration and electrodeposition, that is a step forward toward better understanding of Li dendrites formation and growth. The present DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package) code [2] with the plane wave basis sets and the projector augmented wave (PAW) [3] pseudo-potentials in the framework of Perdew-Burke-Ernzerhof sol (PBEsol) generalized gradient approximation (GGA) [4]. The migration barriers and diffusion coefficients are evaluated using the Nudged Elastic Band (NEB) method as implemented in VASP. To perform the NEB calculations, the minimum energy surfaces of each grain structure in SEI are first created, using a slab method. Then, the grain boundary (GB) structures are generated by forming stable interfaces between the minimum energy surfaces. After identifying the thermodynamically stable GBs, the lithium diffusion channels through them are identified, and the NEB calculations are performed. In the current study, we examine the migration barriers through the GBs of two major inorganic SEI grain structures such as Li2O and LiF. From the migration barriers, it could be observed that the diffusion through the GBs are almost two orders of magnitude faster than the compared diffusion coefficients through the bulk of the SEI structures. The evaluated diffusion coefficients and the free energies are then used to derive the thermodynamically consistent free energy density needed for the PFM model to simulate the filament growth and understand the distribution and evolution of stress fields during Li electrodeposition. The PFM [5] is developed employing MOOSE framework [6]. In the present work the effect of solid electrolyte interface (SEI) on the evolution of Li electrodeposits is captured by modeling the GB structures in SEI and diffusion through these GBs. The results from PFM reveal thatthe anisotropic nature of Li presence at the metal/SEI interface leads to uneven electrodeposition at the Li metal surface. In addition, an elastic deformation energy of the Li solid phase is included in the free energy functional of the PFM, which allows for monitoring the stress field evolution and its influence on Li filaments structure evolution. The present results also show that a significant stress is observed at the root of the Li electrodeposits and the triple junction between the GBs. This observation can guide the development of the experimental strategies for suppression of Li dendrites by lowering the stress field and also provide us more valuable information on the role of SEI as a transport medium, providing more scope for improvement on battery safety and performance.

Aluminum Nitrides (AlN) thin films have been deposited ‎on glass substrates using DC-magnetron sputtering technique for different back pressure. The piezoelectric and related properties of highly c-axis oriented AlN films fabricated by dc planar magnetron sputtering have been calculated. Experimental results show that highly c-axis oriented AlN films can be fabricated by dc planar magnetron sputtering. X-ray powder diffraction (XRD) technique shows that AlN thin films exhibit a hexagonal structure. Vienna Ab initio Simulation Package (VASP) within the framework of density functional theory (DFT) and generalized gradient approximation (GGA) was used to investigate the structural and electronic properties of hexagonal AlN structures. The experimental lattice parameters of the as prepared thin films are found to be in good agreement with ab-initio calculated parameters. UV–Vis spectrophotometer measurements are performed to investigate the optical properties of AlN thin films. We found that the refractive index of AlN thin films exhibits values ranging between 2.1 and 2.2. Furthermore, the elastic, piezoelectric and dielectric tensors of AlN crystal are calculated using VASP. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of AlN. The value of the principle component of electronic contribution to the static dielectric tensor of AlN is found to be ≈ 4.68 that is in good agreement with the experimental static dielectric constant. In addition, clamped-ion piezoelectric tensor is calculated. The diagonal components of the piezoelectric tensor are found to be e_33=1.784 C/m^2 and e_31=-0.8 C/m^2. The large values of the piezoelectric coefficients show that polar AlN crystal exhibits a strong microwave piezoelectric effect. Furthermore, the components of the elastic moduli tensor are calculated. The extraordinary electronic, optical, piezoelectric and elastic properties make AlN thin films potential candidates for several opto-electronic, elastic, dielectric and piezoelectric applications.

The composition, structure and the formation mechanism of the solid-electrolyte interphase (SEI) in lithium-based (e.g., Li-ion and Li-metal) batteries have been widely explored in the literature. However, very little is known about the ions transport through the SEI, specifically through their grain boundaries (GBs) of the inorganic inner layers, and the corresponding mechanism for Li dendrites nucleation. Understanding the underlying ions diffusion processes across the inorganic components of SEI could lead to significant progress, enabling the performance increase and improving the mitigating strategies of dendritic growth and other safety aspects of these batteries. For this reason, in this work, we present the results of a combined ab initio Density Functional Theory (DFT) calculations to study Li diffusion across the SEI and Li nucleation at the Li metal/SEI junction and an extended phase-field model (PFM) for Li migration and electrodeposition that is a step forward toward better understanding of Li dendrites formation and growth.The present DFT calculations are performed using the VASP (Vienna Ab initio Simulation Package)1 code with the plane wave basis sets and the projector augmented wave (PAW)2 pseudo-potentials in the framework of Perdew-Burke-Ernzerhof sol (PBEsol)3 generalized gradient approximation (GGA)4. The migration barriers and diffusion coefficients are evaluated using the Nudged Elastic Band (NEB) method, as implemented in VASP.The major GB structures of SEI that are of interest are the ones between LiF/LiF, Li2O/Li2O and the mixed GBs between LiF/Li2O slabs. The energy barriers for the diffusion of Li vary significantly depending upon the structure of these channels, with the general trend being that Li diffusion in the GB is generally faster than in the neighboring crystalline regions within the grain interiors. The diffusion through LiF/Li2O slabs has the lowest barriers whereas GBs of LiF/LiF slabs has the highest. To understand the role of these GBs in dendrite nucleation, the SEI/electrode interface is analyzed for its stability. The energetics from the DFT analyses depend heavily on the grain structures, with (LiF/LiF)/Li grain structures being most stable and (LiF/Li2O)/Li being the least stable. The interfacial energies in the Li/SEI interfaces show that the interface between (LiF/Li2O)/Li has the least critical length (~1.6 Å) for stable dendritic growth and most favorable for the initiation of crack in the SEI.To quantify the influence of the SEI on the evolution of Li electrodeposits at the continuum scale, we performed the PFM calculations. The PFM is developed employing MOOSE framework5, and the SEI model includes the typical GB structures within 20 nm thickness of the SEI grains. The data from the DFT as described above is used for Li diffusion through the SEI and for mechanical properties of GB and grains. The results from PFM reveal that the anisotropic nature of Li presence at the metal/SEI interface leads to uneven electrodeposition at the Li metal surface. Also, the results allow for monitoring the stress field evolution and its influence on Li filament structure. The present results also show that significant stress is observed at the root of the Li electrodeposits and the triple junction between the GBs.

This work describes adsorption and internal vibrational motion of the water monomer on the Cu(110) surface. We have computed the vibrational wave numbers for the adsorbed water using anharmonic variational calculations previously applied to the gas phase molecule. The three-dimensional potential energy surface for the vibrational motion of the water molecule has been computed using density functional theory (DFT) with periodic boundary conditions, generalized gradient approximation (GGA), plane wave basis sets and projector augmented wave (PAW) potentials as implemented in the Vienna ab initio simulation package (VASP). The data points on the potential energy surface have been fitted into an analytical model and the eigenvalues of the resulting Hamiltonian have been computed variationally.

High performance planar-heterojunction (PHJ) perovskite (CH3NH3PbI3) solar cells fabricated through low-temperature annealing are demonstrated. Simple spin-coating with an optimized solvent washing process readily forms homogeneous and crystalline perovskite thin films. The perovskite films fabricated via this solvent washing process show a low dependence on annealing temperature in achieving high crystallinity and large grain size, prerequisites for high efficiency perovskite solar cells. The solar cell device fabricated by solvent washing and 100 °C annealing exhibited a high power conversion efficiency (PCE) over 14% with high short circuit current density (JSC) of 19.3 mA cm−2 and fill factor (FF) of 0.80. More importantly, the device annealed at low temperature (<90 °C) also yields high PCEs of over 12%. This enables us to fabricate flexible solar cells at low-temperatures with promising PCE as high as 9.43%. This study demonstrates that this optimized solvent washing process is highly relevant for low-cost roll-to-roll (R-2-R) processing of high performance perovskite solar cells.

CuInTe2 is a promising semiconductor with a tunable bandgap of 1.0-1.2 eV, enabling it to efficiently absorb sunlight and convert it into usable energy. Following this development, characterization of its structural and electronic properties is currently underway. In this study, the Vienna Ab Initio Simulation Package (VASP) with density functional theory (DFT) and plane-wave basis sets was used to investigate the structural and electronic properties of both neutral and anionic clusters. For (CuIn)nTe2 and ((CuIn)nTe2)− (n = 1–8) clusters, geometric optimization revealed the lowest-energy isomers, all of which adopt cubic chalcopyrite structures. According to the results, the low-lying energy geometry of Cu2In2Te2 and (CuInTe2)− clusters exhibit their maximum relative stability. The (CuIn)nTe2 thin-film experimental finding of 1.85 eV is a good match with their mean HOMO–LUMO gaps of 1.652 eV and 2.464 eV. Binding energy per atom increases with cluster size, although the HOMO–LUMO gap breaks at n = 5, most likely as a result of bond-specific interactions and orbital hybridization. The Cu2In2Te2 cluster stands out with maximum HOMO–LUMO gap and dissociation energy, consistent with its enhanced stability. Adiabatic ionization potentials decrease with cluster size, indicating growing metallic character, while dissociation energies show odd–even oscillations but overall increase as size grows. Partial charge density analysis shows that both neutral and anion clusters are significant for semiconductor applications, including photovoltaic cells and related devices.

Layer-dependent electronic and magnetic properties of SrMnO3/LaAlO3 superlattices

A density-functional study of the structural, electronic, and vibrational properties of Ti8C12 metallocarbohedrynes with relevance to ultrafast time-resolved spectroscopy

Quantum confinement of electrons in atomic chains provides the most powerful and versatile means to control electronic, optical, magnetic and thermoelectric properties of materials needed to make diodes, spin valves and optical labels. Furthermore, the alloying of metallic atoms in different compositions produces novel mechanical, electronic and chemical behaviours in bimetallic chains as well as in other structures. This motivated us to perform theoretical investigations on the structure, stability, magnetic and electronic properties of bimetallic atomic chains of Au–Ag and Au–Pt, by using Vienna ab-initio simulation package (VASP), which is based on the density functional theory (DFT) within generalised gradient approximation. We have used tension and cohesive energy criteria to assess the stability of the Au–Ag and Au–Pt atomic chains. A comparison between the computed cohesive energies of various possible structures are made to suggest the most probable chain structures that can occur in break junction experiments. Our computed results suggest that the ground state of the Au–Ag and Au–Pt atomic chains should have zig-zag geometry. Furthermore, the most favoured chain structures that can be formed at the last stage of nanowires stretching are: (i) an atomic chain with alternate arrangement of equal number of Au and Ag / Pt atoms and (ii) an atomic chain where two Ag / Pt atoms are separated by one Au atom. Our results on the electronic band structure and optical properties suggest that the Au–Ag atomic chain could be of semiconducting nature, while the most stable Au–Pt chain is metallic in nature. A spin-polarised calculation with the inclusion of spin–orbit coupling shows that the Au–Pt atomic chains are magnetic, if the number of Au atoms is not more than the number of Pt atoms.

Structural, electronic and optical properties of concentration dependent indium (In) doped ZnO monolayer are studied in the framework of density functional theory. We have considered the various concentration of In (6.25, 12.50 and 18.75 atomic (at.) %) in ZnO monolayer. We have used Vienna Ab initio Simulation Package (VASP) with Projected Augmented Wave (PAW) pseudopotentials and generalized gradient approximation - Perdew, Burke and Ernzerhof (GGA-PBE) functional. Substitution of Zn with In atom is energetically favorable. The large sized In atom as compared to that of Zn induces the stress in ZnO monolayer. To reduce the stress, In atom protrudes out from the plane of ZnO monolayer. The computational model under DFT calculations with ultrasoft pseudopotential and PBE exchange correlation functional, showed that In atom remains in the plane of ZnO monolayer. Similar behavior is observed with the use of full-potential linearized augmented plane wave (FLAPW)-GGA approach. The effects of pseudopotential and exchange correlation functional are observed in terms of structural deformation in In-doped ZnO monolayer. It effectively modulates the band structure and optical properties. The computational codes, potentials, and exchange correlation functionals are playing major role in investigating the structural and electronic properties of various nanostructures.

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2022Boosting Efficient Ammonia Synthesis over Atomically Dispersed Co-Based Catalyst via the Modulation of Geometric and Electronic Structures Yanliang Zhou†, Congying Wang†, Xuanbei Peng†, Tianhua Zhang, Xiuyun Wang, Yafei Jiang, Haifeng Qi, Lirong Zheng, Jianxin Lin and Lilong Jiang Yanliang Zhou† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Congying Wang† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Xuanbei Peng† National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Tianhua Zhang National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Xiuyun Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 , Yafei Jiang Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055 , Haifeng Qi Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 , Lirong Zheng Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 , Jianxin Lin National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 and Lilong Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002 https://doi.org/10.31635/ccschem.021.202100912 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ammonia (NH3) synthesis at mild conditions is of great significance, while the significant bottleneck of this process is the activation of N2 to realize the desired NH3 synthesis performance, which requires deep insight and rational design of active sites at the atomic level. Here, were synthesized atomically dispersed Co-based catalysts with different Co-N coordination numbers (CNs) to explore the coordination-sensitive NH3 synthesis reaction for the first time. Our studies showed that Co-based catalysts increased the NH3 synthesis rate gradually with a decrease in CN. The Co-N2 catalyst exhibited the highest NH3 synthesis rate of 85.3 mmol gCo−1 h−1 at 300 °C and 1 MPa, which outperformed most of the previously reported Co-based catalysts. Various characterizations and theoretical calculations demonstrated that atomically dispersed Co catalyst with low CN could generate more unoccupied Co 3d charges and tetrahedral cobalt(II) sites. The unoccupied Co 3d charge, in turn, promoted the electron donation from the Co active center to the antibonding π-orbital (π*) of N2 and expedites N2 hydrogenation. Furthermore, the Co-N2 catalyst with more tetrahedral cobalt(II) sites could effectively facilitate the desorption of N-containing intermediate species (such as *NH3 and *N2H4) to obtain a high NH3 synthesis rate. Download figure Download PowerPoint Introduction Ammonia (NH3) is not only an essential raw material for the synthesis of nitric acid, fertilizer, ammonium salt, and so on but also a promising renewable energy carrier.1,2 The state-of-the-art manufacture of NH3 comes from the Haber–Bosch (HB) process using iron-based catalysts that require harsh reaction conditions (400–500 °C, 10–30 MPa), thereby suffering from high energy consumption and environmental pollution problems.3–5 Therefore, it is urgent to develop effective routes that are both energy-saving and environment-friendly. Most recently, electric power generated from renewable energy sources is widely used for hydrogen (H2) production via water electrolysis. It becomes economically acceptable to NH3 synthesis from renewable H2 and N2 via an electrolysis-driven HB (eHB) process.6 The eHB route is attractive for realizing efficient utilization of renewable energy and a "carbon-free" society. However, the major challenge of NH3 synthesis lies in the activation of N2 (945 kJ mol−1) at mild conditions to realize desired NH3 synthesis rate,7 which would require a deep insight into the reaction mechanism; therefore, the development of advanced, efficient catalysts that can produce NH3 at mild conditions is beneficial. Currently, industrial applied Fe- or Ru-based catalysts that exist as nanoparticles usually obey the dissociative mechanism,8,9 requiring harsh conditions to dissociate the stable N≡N triple bond. In contrast to the N2 dissociative mechanism, most of the enzyme-catalyzed N2 fixation processes followed the associative mechanism, wherein the N≡N triple bond breaks stepwise after partial hydrogenation of the N2 molecule.10,11 Notably, the dissociation energy of the N–N bond (297 kJ/mol) from *N2H4 intermediates is less than a third of that from the N≡N triple bond. Consequently, the exploitation of advanced catalysts that obey the N2 hydrogenation process instead of direct N2 dissociation is promising for realizing the desired NH3 synthesis performance at mild conditions. Further, NH3 synthesis is a typical structure sensitivity reaction, where the pathway of N2 activation is closely related to the geometric and electronic structures of active sites.8,12 For instance, theoretical studies have demonstrated that N2 could preferentially undergo hydrogenation via an associative pathway other than direct dissociation on Ru(0001) surface,13 RhCo3 clusters,12 and Co3Mo3N catalysts.14 In addition, very recently, a limited number of researches have provided us with experimental evidence that the N2 hydrogenation to N2Hx species is preferred over atomically dispersed Ru and Co, as well as Li-promoted Ru catalysts.6,15,16 Among them, atomically dispersed catalysts present well, having the potential for NH3 synthesis at mild conditions, as the direct N2 dissociation is difficult while the N2 hydrogenation is feasible on single-atom sites.15 Meanwhile, atomically dispersed catalysts with definite structures offer the possibility to investigate the geometric and electronic effects on NH3 synthesis at the atomic level. Differing from nanoparticle catalysts, in which the particle size and crystal face usually play an important part in the reactivity, catalytic activity of atomically dispersed catalysts is mainly determined by the coordination environment of single-atom sites.17–19 However, up to now, the modulation of the coordination structure of single-atom active sites to optimize the catalytic activity of NH3 synthesis has not yet been researched. Moreover, the effects of geometric and electronic structures in these single-atom sites on the dynamic transformation of intermediates remain unclear. Herein, we synthesized a series of atomically dispersed Co-Nx (x = 2∼4) catalysts through the pyrolysis of nitrogen-anchored Co-based precursors. The coordination numbers (CNs) of Co-N over the as-prepared catalysts were modulated by varying the calcination temperature. The coordination-sensitive NH3 synthesis reaction in the case of Co-based catalyst is disclosed through a combination of a suite of elaborated experimental characterizations and theoretical calculations. The geometric and electronic structures of Co active sites could be simultaneously tuned via the modulation of Co-N CNs, which are closely related to the N2 activation and desorption of N-containing intermediate species (such as *N2H4 and *NH3), thus resulting in different catalytic behaviors in the NH3 synthesis. This finding is beneficial to rationally design advanced Co-based catalysts by regulating the geometric and electronic structures to realize efficient NH3 synthesis at mild conditions. Experimental Methods Catalyst preparation Synthesis of hollow N-doped porous carbon spheres support The synthetic method of hollow N-doped porous carbon spheres (HNPCSs) support was referenced in previous work.20 Briefly, NH3 solution (10 mL) and ethanol (240 mL) were dissolved in deionized water (80 mL) and stirred evenly at room temperature (RT). Then, tetraethyl orthosilicate (11.2 mL) was added to this solution and stirred further for 1 h. Subsequently, poloxamer, resorcinol, and formaldehyde (2.24 mL) were added to the above solution and continually stirred for 0.5 h. After the addition of melamine (1.26 g) and formaldehyde (1.68 mL), the mixture was treated at 100 °C for 24 h. After filtration and washing, the sample obtained was calcined under an Ar atmosphere at 700 °C for 2 h. Finally, the sample was etched by hydrofluoric acid (HF) solution to remove the SiO2 substrate. After filtration, washing, and drying, the resulting substance was herein referred to as HNPCSs support. Synthesis of Co-Nx and Co-NPs catalysts In detail, HNPCSs (120 mg) and cobalt phthalocyanine (CoPc; 80 mg) was dispersed in dimethylformamide (DMF; 120 mL), respectively, and the mixtures were stirred for 2 h. Then the CoPc and DMF mixed solution was added slowly to the HNPCSs suspension. After stirring for 24 h, the sample was dried in a vacuum dryer at 60 °C for 12 h. Finally, the samples were pyrolyzed under Ar atmosphere at different temperatures of 450, 550, and 650 °C for 2 h, denoted as Co-N4, Co-N3, and Co-N2, respectively. The synthetic procedure used for the Co-NPs catalyst was similar to that of Co-N2, except that the calcination was performed at 700 °C for 10 h. Synthesis of catalysts as reference The Cs-Co/MgO catalyst was synthesized by an impregnation method. In detail, a certain amount of cobalt nitrate hexahydrate and cesium nitrate was dissolved in deionized water. MgO support was added to this solution, and the mixture was stirred evenly. After impregnation at RT for 24 h, the mixture was dried at 80 °C for 24 h. The acquired catalyst was denoted as Cs-Co/MgO, in which the content of Co was the same as that over Co-N2, and the molar ratio of Cs to Co was ∼1.0. A Fe-based catalyst was purchased commercially from Fujian SJ. FD. Co. Ltd. (Fuzhou, Fujian, China) for comparative studies. NH3 synthesis performance test The NH3 synthesis activities over the as-synthesized catalysts were tested in a pressurized fixed bed reactor. In detail, the catalyst (0.2 g, 20–30 mesh) was loaded into the stainless steel reactor (inner diameter 10 mm). Before measurement, the catalysts were treated in a 25%N2–75%H2 feed gas at 300 °C for 2 h. Subsequently, the catalyst performance was evaluated in 300 °C at a weight hourly space velocity (WHSV) of 60,000 mL g−1 h−1 and 1 MPa. The outlet NH3 was trapped by diluted H2SO4 solution and then detected by ion chromatography (DIONEX, ICS-600; Thermo Scientific, Waltham, MA, USA). The turnover frequency (TOFCo) was calculated based on the total molar of Co metal using the following equation: TOF Co = r n M (1)where, r represents the NH3 synthesis rate (mol g−1 s−1), nM is the molar amounts of Co obtained from inductively coupled plasma (ICP) analysis. Catalyst characterization X-ray diffraction (XRD) measurement was recorded on a PANalytical X'Pert Pro diffractometer (PANalytical, Almelo, The Netherlands) using a Cu Kα radiation source. N2 adsorption/desorption isotherms were collected on an ASAP 2020 apparatus (MicroMetric, Lincolnshire, United Kingdom). Elemental analysis (EA) was conducted with a Vario EL-Cube instrument (Elementar, Jiangsu, China) to measure the N contents of the samples. The Co contents of the catalysts were measured by ICP atomic emission spectroscopy (ICP-AES) analysis equipped with an Ultima 2 spectrometer (Perkin-Elmer, Waltham, MA, USA). Catalyst morphologies were detected on scanning electron microscopy (SEM; a Hitachi Model S-4800; Hitachi, Tokyo, Japan). Transmission electron microscopy (TEM) images of the catalysts were obtained on a JEM-2010 instrument (JEOL Ltd., Tokyo, Japan). Aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) analysis was performed on a JEOL JEM-ARM 200 F instrument (JEOL Ltd., Tokyo, Japan). Raman spectroscopy was measured on a multichannel modular triple Raman system (Renishaw Co., London, United Kingdom). Ultraviolet photoelectron spectroscopy (UPS) was measured using a helium resonance lamp offering He I (hν = 21.2 eV) photons. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, Shanghai, China) with a monochromatic Al Kα source (hν = 1486.6 eV) as the X-ray source. The XPS binding energy was referenced to the C 1s peak at 284.6 eV of adventitious carbon. Electron-paramagnetic resonance (EPR) test was performed on an E500 spectrometer (Bruker-BioSpin, Shanghai, China) with a 100 KHz magnetic field modulation at RT. UV–vis diffuse reflection spectrum (DRS) data were obtained from a Perkin Elmer Lambda 750s UV–vis spectrometer (Perkin-Elmer, Waltham, MA, USA). The productions of NH3 synthesis were trapped at the aqueous solution containing sulfuric acid and para-(dimethylamino) benzaldehyde for the UV–vis DRS measurement. X-ray absorption spectroscopy (XAS), containing X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses were performed at the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF), China. Spectra were collected at the Co K-edge in transmission mode with a Si (111) double crystal monochromator. The Co foil and CoPc samples were used as references. Density functional theory (DFT) calculations First-principle calculations were performed using the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE),21 as implemented in the Vienna ab initio Simulation Package (VASP 5.4.4).22 The valence electronic states of all atoms were expanded in a plane-wave basis set with a cutoff energy of 400 eV, and gamma points were used for Brillouin zone integration. All atoms were allowed to relax until the forces fell below 0.02 eV Å−1. The energy convergence criterion was set to 10−5 eV. Grimme D3 method with the zero-damping function was used to describe van der Waals interactions.23 The graphene supercell with a surface periodicity of 10 × 10, including 200 atoms as a basis, was employed to construct the Co-Nx (x = 4, 3, and 2) moieties. A vacuum space of 20 Å was used in the normal direction of the graphene plane to ensure negligible interaction between the mirror images. Results and Discussion Structure characterization The process for the synthesis of Co-Nx catalysts with different Co-N CNs is depicted in Figure 1a. The atomically dispersed Co-Nx catalysts were prepared by CoPc pyrolysis supported on HNPCSs precursor. The CN of Co-Nx catalysts was modulated by varying the pyrolysis temperature (as shown in the "Catalyst preparation" section). To highlight the coordination structure of atomically dispersed Co center, Co nanoparticle loaded on the same support (Co-NPs) was prepared for comparison. EXAFS analyses of the Co K-edge were conducted to demonstrate the coordination structure of Co-Nx catalysts. The Co K-edge EXAFS spectra of as-synthesized catalysts and reference Co foil and CoPc, are illustrated in Figure 1b. An apparent peak located at 1.5 Å for the as-synthesized Co-Nx samples was attributed to Co-N coordination, compared with CoPc reference.24 Meanwhile, no typical peaks for Co-Co coordination at 2.2 Å were visible, indicating the atomically dispersed Co atoms on these Co-Nx catalysts. Besides, the peak intensity of Co-N coordination decreased with increased pyrolysis temperature, suggesting a decrease in Co-N CN. Based on fitting results, the average CNs of Co-N of the three samples were 3.9, 3.0, and 2.1 (Table 1), and thus, the samples were denoted as Co-N4, Co-N3, and Co-N2, respectively. Additionally, the fitting result of Co K-edge EXAFS spectra of Co-N2 based on our hypothetical model was consistent with the experimental result (Figure 1c), demonstrating the validity of the depicted models. In the case of Co-NPs, the Co-Co coordination existed predominant beside the weak Co-N coordination, indicating that Co entities were distributed mainly in the form of nanoparticles. Figure 1 | (a) Schematic diagram of the preparation process of Co-Nx with different CNs. (b) Co K-edge EXAFS spectra of as-prepared samples. (c) EXAFS spectra and the corresponding curve-fitting results of Co-N2 catalyst. Download figure Download PowerPoint Table 1 | EXAFS Data Fitting Results of the As-Synthesized Catalysts Samples Shell CN R (Å)a σ2 × 102 (Å2)b ΔE0 (eV)c R-Factor (%) Co foil Co-Co 12.0 2.49 0.7 6.3 0.9 Co-N4 Co-N 3.9 1.92 0.4 8.1 0.07 Co-N3 Co-N 3.0 1.91 0.3 7.2 0.09 Co-N2 Co-N 2.1 1.89 0.3 −1.1 0.8 Co-NPs Co-N 2.0 2.03 0.7 −1.8 0.7 Co-Co 2.1 2.49 0.7 −1.8 0.7 Note: The accuracy of parameters: CN, ±20%; R, ±1%; σ2, ±20%; ΔE0, ±10%. The fitting range in k- and R-space is 3.0–12.3 Å−1 and 1.0–2.7 Å, respectively. aR, bonding distance. bσ2, the Debye–Waller factor. cΔE0, inner potential shift. SEM ( Supporting Information Figure S1) and TEM images (Figures 2a and 2b) simultaneously displayed uniform microspheres with the hollow structure of Co-Nx and Co-NPs catalysts. The average diameter of the microspheres was ∼150 nm of these samples. The high-resolution TEM (HR-TEM) images (Figure 2c and Supporting Information Figure S2) did not show any Co nanoparticles sights, implying a high dispersion of Co species as clusters or single atoms over Co-Nx catalysts. We further used the AC-HAADF-STEM characterization to inspect the Co species over Co-Nx catalysts. We found that plenty of highly dispersed Co single atoms were visible over Co-N2 (Figure 2d), Co-N3 (Figure 2e), and Co-N4 (Figure 2f) catalysts. In comparison with Co-Nx catalysts, distinct nanoparticles were observed over the Co-NPs catalyst ( Supporting Information Figure S3). Combined with the EXAFS results, we deduced that atomically dispersed Co-Nx catalysts with different Co-N CNs were synthesized successfully. Figure 2 | (a and b) TEM and (c) HR-TEM images of Co-N2 catalyst. AC-HAADF-STEM images of (d) Co-N2, (e) Co-N3, and (f) Co-N4 catalysts. Download figure Download PowerPoint The XRD patterns ( Supporting Information Figure S4) showed that all Co-Nx displayed a broad diffraction peak at ∼26.4°, to the crystal plane of carbon The highly was by Raman measurement. The Raman spectra ( Supporting Information Figure displayed that samples peaks at and corresponding to the and of or carbon and Furthermore, the intensity of the to of these samples were ( Supporting Information Table that the coordination structure has a on carbon Notably, the diffraction peaks on the Co-N4 sample are attributed to the CoPc suggesting an of CoPc species and the of Co-N4 which with the fitting results of XRD diffraction peaks were related to Co or CoPc species in the Co-N3 and Co-N2 samples. to the above EXAFS and our demonstrated that the pyrolysis of CoPc would form atomically dispersed Co sites over Co-Nx catalysts after the pyrolysis of the at 700 °C for 10 h, the of diffraction peaks at and in Co-NPs catalyst were to the (111) and crystal of Co respectively. The average size of Co nanoparticles over Co-NPs was to the of (111) crystal plane via which with the particle observed in the HR-TEM ( Supporting Information Figure S3). showed that the Co contents in these samples were distributed in the range of ( Supporting Information Table to the results, the N contents of the synthesized Co catalysts ( Supporting Information Table S1) decreased with increased pyrolysis temperature. N2 ( Supporting Information Table S1) results showed that the surface of Co-Nx catalysts increased from to g−1 with increased pyrolysis temperature. However, at a high pyrolysis temperature, the surface of Co-NPs catalyst decreased to of the geometric and electronic structure was performed to insight into the of different CNs on the geometric and electronic structure of Co sites. to the results (Figure the peak intensity in the range of eV was of the geometric of Co that tetrahedral or The from the tetrahedral of Co sites were to to this The intensity of this peak over Co-N2 was than that of Co-N3 or Co-N4, indicating that more Co were at the tetrahedral sites in the Co-N2 Besides, the of the absorption in the images (Figure could be used to the states of Co A of absorption from Co-N4 to Co-N2 was indicating that the valence of Co species decreased with decreased CN of the Co-Nx catalysts. Figure | Co K-edge spectra and the images in the range of (a) eV and (b) eV. Download figure Download PowerPoint Further, XPS were performed to obtain the valence of Co and N species over the as-synthesized catalysts. shown in Figure the Co spectra were fitting into the peaks with binding of and eV, to and peaks of surface Co were observed on these catalysts, except for Co-NPs, which with the results of XRD and The atomic ratio of over the Co-N4, Co-N3, and Co-N2 catalysts determined from XPS results were and (Table This finding was supported by the of Co entities over Co-N2 as shown in the (Figure results, that the decreased CN have been promoted to generate more unoccupied Co 3d over the Co-N2 results were further by The spectra ( Supporting Information Figure showed that the of and over these catalysts were attributed to an electron in the of and carbon We found that the intensity of increased with the of CN, indicating that more on the Co-N2 catalyst. The N 1s spectra (Figure three of N and N over Co-Nx the comparison of the content of different N species from the peak (Table we observed that the N content increased from to while the N content decreased from to with of CN over Co-Nx catalysts. This result that the atomically dispersed Co mainly with the these demonstrated that the geometric and electronic structures of the atomically dispersed Co catalyst were modulated through different CNs of the in turn, the catalytic activity the NH3 synthesis. Figure | (a) Co and (b) N 1s XPS spectra of the as-synthesized catalysts. Download figure Download PowerPoint Table 2 | The XPS Spectra Results of the As-Synthesized Catalysts Samples N (%) N (%) N (%) (%) (%) (%) Co-N4 24 60 Co-N3 12 Co-N2 20 Co Catalyst activity and Figure the NH3 synthesis rate over the as-synthesized catalysts with a 25%N2–75%H2 gas and a of mL The NH3 synthesis rate in the case of Co-N2 was 85.3 gCo−1 h−1 at mild conditions of 300 °C and 1 MPa, than over Co-N3 gCo−1 and Co-N4 gCo−1 An NH3 synthesis rate of gCo−1 h−1 was acquired over Co-NPs, which is more to over the atomically dispersed Co-Nx catalysts. Meanwhile, the NH3 synthesis rate over the carbon as support was mmol h−1 at 300 °C and 1 MPa, as shown in Supporting Information Table suggesting that the of support to the catalytic activity of Co-Nx could be results that NH3 synthesis is a coordination-sensitive reaction, and the Co-N2 catalyst with the CN the highest NH3 synthesis rate at mild conditions. It has been reported that the addition of a electronic to Co-based catalysts could NH3 synthesis performance, referred to as an electronic Herein, we the of to the highly active Co-N2 catalyst on NH3 synthesis It showed that the NH3 synthesis rate over was that of the (Figure The typical Cs-Co/MgO and the Fe-based catalysts were also performed and evaluated for comparison ( Supporting Information Table 300 °C and 1 MPa, the NH3 synthesis rate over Co-N2 mmol gCo−1 was than of Cs-Co/MgO mmol and Fe-based mmol catalysts, indicating the performance of NH3 synthesis activity over Co-N2 catalyst. For based on the total number of Co active sites on these catalysts, were calculated and depicted in Figure Notably, the of Co-N2 and were and respectively, which outperformed most of the previously reported Co-based catalysts in Supporting Information Table Figure | (a) NH3 synthesis rate based on Co content over as-synthesized catalysts at 1 and 300 (b) TOF was calculated on the total number of Co atoms basis over the catalysts. (c) N2 and (d) H2 reaction over Co-N2 and Co-NPs catalysts. Download figure Download PowerPoint A test of the Co-N2 catalyst was conducted at 300 °C and 1 MPa. shown in Supporting Information Figure the NH3 synthesis rate a stable of mmol gCo−1 h−1 a The used catalyst was further tested using HR-TEM and EXAFS shown in the Supporting Information Figure no or at different were observed over the used Co-N2 catalyst. Moreover, the Co K-edge EXAFS spectrum of the used Co-N2 catalyst ( Supporting Information Figure showed that existed the Co-N coordination, while no Co-Co coordination was indicating the of a highly stable Co-N2 active in the process of NH3 synthesis. the of these demonstrated the of Co-N2 catalyst in NH3 synthesis at mild conditions. studies studies were conducted to demonstrate the of N2 and H2 activation over the Co-Nx catalysts. The reaction of N2 and H2 usually as related to the and the of hydrogen For most catalysts, the reaction of N2 is to suggesting that the could be the direct dissociation of the N≡N triple bond. shown in Figure the reaction of N2 over Co-N2 was than that of over low reaction of N2 has been reported over Ru-based catalysts supported by or where the of bond than direct N≡N triple bond dissociation was Additionally, the reaction of H2 on Co-NPs was (Figure The demonstrated that the of H2 on Co to hydrogen

Herein, a series of molecular actuators based on the crystals of (E)-2-(4-fluorostyryl)benzo[d]oxazole (BOAF4), (E)-2-(2,4-difluorostyryl)benzo[d]oxazole (BOAF24), (E)-2-(4-fluorostyryl)benzo[d]thi...

The structural, electronic band structure and optic properties of the Ni doped MgSiP2 chalcopyrite compound have been performed by using first-principles method in the density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke and Ernzerhof (PBE) is used for the exchange and correlation functional. The present lattice constant (a) follows generally the Vegard’s law. The electronic band structure, total and partial density of states (DOS and PDOS) are calculated. We present data for the frequency dependence of imaginary and real parts of dielectric functions of Ni doped MgSiP2. For further investigation of the optical properties the reflectivity, refractive index, extinction coefficient and electron energy loss function are also predicted. Our obtained results indicate that the lattice constants, electronic band structure and optical properties of this compound are dependent on the substitution concentration of Ni.

Using the first-principles calculations based on density functional theory (DFT), the structural, elastic, electronic, and vibrational properties of LiAl have been explored within the generalized gradient approximation (GGA) using the Vienna ab initio simulation package (VASP). The results demonstrate that LiAl compound is stable in the NaTl-type structure (B32) at ambient pressure, which is in good agreement with the experimental results and there is a structural phase transition from NaTl-type structure (B32) to CsCl-type structure (B2) at around 22.2 GPa pressure value. The pressure effects on the elastic properties have been discussed and the elastic property calculation indicates that the elastic instability could provide a phase transition driving force according to the variations relation of the elastic constant versus pressure. To gain further information about this, we also have investigated the other elastic parameters (i.e., Zener anisotropy factor, Poisson’s ratio, Young’s modulus, and isotropic shear modulus). The electronic band structure, total and partial density of states, phonon dispersion curves, and one-phonon density of states of B2 and B32 phases are also presented with results.