Construction of Ag─Co(OH)2 Tandem Heterogeneous Electrocatalyst Induced Aldehyde Oxidation and the Co-Activation of Reactants for Biomass Effective and Multi-Selective Upgrading.

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) provides a feasible way for utilization of biomass resources. However, how to regulate the selective synthesis of multiple value-added products is still a great challenge. The cobalt-based compound is a promising catalyst due to its direct and indirect oxidation properties, but its weak adsorption capacity restricts its further development. Herein, by constructing Ag─Co(OH)2 heterogeneous catalyst, the efficient and selective synthesis of 5-hydroxymethyl-2-furanoic acid (HMFCA) and 2,5-furan dicarboxylic acid (FDCA) at different potential ranges are realized. Based on various physical characterizations, electrochemical measurements, and density functional theory calculations, it is proved that the addition of Ag can effectively promote the oxidation of aldehyde group to a carboxyl group, and then generate HMFCA at low potential. Moreover, the introduction of Ag can activate cobalt-based compounds, thus strengthening the adsorption of organic molecules and OH- species, and promoting the formation of FDCA. This work achieves the selective synthesis of two value-added chemicals by one tandem catalyst and deeply analyzes the adsorption enhancement mechanism of the catalyst, which provides a powerful guidance for the development of efficient heterogeneous catalysts.

Electrocatalytic Urea Synthesis via N2 Dimerization and Universal Descriptor.

Electrocatalytic urea synthesis through N2 + CO2 coreduction and C-N coupling is a promising and sustainable alternative to harsh industrial processes. Despite considerable efforts, limited progress has been made due to the challenges of breaking inert N≡N bonds for C-N coupling, competing side reactions, and the absence of theoretical principles guiding catalyst design. In this study, we propose a mechanism for highly electrocatalytic urea synthesis using two adsorbed N2 molecules and CO as nitrogen and carbon sources, respectively. This mechanism circumvents the challenging step of N≡N bond breaking and selective CO2 to CO reduction, as the free CO molecule inserts into dimerized *N2 and binds concurrently with two N atoms, forming a specific urea precursor *NNCONN* with both thermodynamic and kinetic feasibility. Through the proposed mechanism, Ti2@C4N3 and V2@C4N3 are identified as highly active catalysts for electrocatalytic urea formation, exhibiting low onset potentials of -0.741 and -0.738 V, respectively. Importantly, taking transition metal atoms anchored on porous graphite-like carbonitride (TM2@C4N3) as prototypes, we introduce a simple descriptor, namely, effective d electron number (Φ), to quantitatively describe the structure-activity relationships for urea formation. This descriptor incorporates inherent atomic properties of the catalyst, such as the number of d electrons, the electronegativity of the metal atoms, and the generalized electronegativity of the substrate atoms, making it potentially applicable to other urea catalysts. Our work advances the comprehension of mechanisms and provides a universal guiding principle for catalyst design in urea electrochemical synthesis.

Heterogeneous-Interface-Enhanced Adsorption of Organic and Hydroxyl for Biomass Electrooxidation.

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) provides an efficient way to obtain high-value-added biomass-derived chemicals. Compared with other transition metal oxides, CuO exhibits poor oxygen evolution reaction performance, leading to high Faraday efficiency for HMF oxidation. However, the weak adsorption and activation ability of CuO to OH- species restricts its further development. Herein, the CuO-PdO heterogeneous interface is successfully constructed, resulting in an advanced onset-potential of the HMF oxidation reaction (HMFOR), a higher current density than CuO. The results of open-circuit potential, in situ infrared spectroscopy, and theoretical calculations indicate that the introduction of PdO enhances the adsorption capacity of the organic molecule. Meanwhile, the CuO-PdO heterogeneous interface promotes the adsorption and activation of OH- species, as demonstrated by zeta potential and electrochemical measurements. This work elucidates the adsorption enhancement mechanism of heterogeneous interfaces and provides constructive guidance for designing efficient multicomponent electrocatalysts in organic electrocatalytic reactions.

Carrier doping-induced strong magnetoelastic coupling in 2D lattice.

The realization of intertwined ferroelasticity and ferromagnetism in two-dimensional (2D) lattices is of great interest for broad nanoscale applications but still remains a remarkable challenge. Here, we propose an alternative approach to realize the strongly coupled ferromagnetism and ferroelasticity by carrier doping. We demonstrate that prototypical 2D ß-PbO is dynamically, thermally and mechanically stable. Under hole doping, 2D ß-PbO possesses ferromagnetism and ferroelasticity simultaneously. Moreover, the robustness of ferromagnetic and ferroelastic orders is doping tunable. In particular, 2D ß-PbO features an in-plane easy magnetization axis that is coupled with the lattice direction, enabling the ferroelastic manipulation of the spin direction. Furthermore, the efficient ferroelastic control of the anisotropic optical property and spin splitting in 2D ß-PbO are also clarified. Our study highlights a new direction for 2D magnetoelastic research and enables the possibility for multifunctional devices.

Valley polarization caused by crystalline symmetry breaking.

In two-dimensional (2D) hexagonal lattices with inversion asymmetry, time-reversal (T) connected valleys are at the center of current valleytronic research. In order to trigger valley polarization, dynamical processes and/or magnetism have been considered. In this work, we propose a new mechanism, valley-contrasting sublattice polarization (VCSP), to polarize valleys by reducing the crystalline symmetry that connects the valleys. In our mechanism, significant valley polarization could be readily generated without magnetism, an electric field, or an optical process. Based on tight-binding model analysis and first-principle calculations, the control of valley polarization via crystalline symmetry can be successfully realized in concrete LaOBiS2 polytypes with Peierls-like structure distortion. Our results provide an unprecedented possibility for exploring valley-contrasting physics.

Hydroxyl-Boosted Nitrogen Reduction Reaction: The Essential Role of Surface Hydrogen in Functionalized MXenes.

MXenes, an emerging family of two-dimensional (2D) metal carbides and nitrides, have been demonstrated to be effective nitrogen reduction reaction (NRR) catalysts. So far, most of the theoretical studies toward NRR are based on bare MXenes; however, the structural stabilities are questionable. In this work, we studied the NRR process on several synthesized MXenes (Ti2C, V2C, Cr2C, Zr2C, Nb2C, Mo2C, Hf2C, and Ta2C) with hydroxyl (OH) termination since the structures are preferred under NRR operating conditions as per Pourbaix stability diagrams. It is found that OH plays an essential role in tuning the NRR chemistry, as a new surface-hydroxylation mechanism. Different from the widely accepted NRR mechanism where only protons are involved in the reaction, hydrogen (H) atoms from surface hydroxyl could be captured by the intermediate and participate into the NRR, while the remaining H vacancy can subsequently be self-repaired by the protons under the applied potential. The cooperative effect of surface hydroxylation can effectively boost the NRR, while Mo2C(OH)2 stands out with the most favorable limiting potential of -0.62 V and highest selectivity. Moreover, new scaling relationships based on the H vacancy energy are established, elucidating the possibility for structure-activity tuning. This study not only elaborates the essential role of surface OH functionalization in evaluating NRR performance but also affords new insights into advance sustainable NH3 production.

High-Throughput Screening of Synergistic Transition Metal Dual-Atom Catalysts for Efficient Nitrogen Fixation.

Great enthusiasm in single-atom catalysts (SACs) for the nitrogen reduction reaction (NRR) has been aroused by the discovery of metal-Nx as a promising catalytic center. However, the poor activity and low selectivity of available SACs are far away from the industrial requirement. Through the first-principles high-throughput screening, we find that Fe-Fe distributed on graphite carbon nitride (Fe2/g-CN) can manipulate the binding strength of the target reaction species (compromises the ability to adsorb N2H and NH2), therefore achieving the best NRR performance among 23 transition metal (TM) centers. Our results show that Fe2/g-CN achieves a high theoretical Faradaic efficiency of 100% and, impressively, the lowest limiting potential of -0.13 V. Particularly, multiple-level descriptors shed light on the origin of NRR activity, achieving a fast prescreening among various candidates. Our predictions not only accelerate discovery of catalysts for ammonia synthesis but also contribute to further elucidate the structure-performance correlations.

Molybdenum Nitride Electrocatalysts for Hydrogen Evolution More Efficient than Platinum/Carbon: Mo2N/CeO2@Nickel Foam.

To produce hydrogen economically by electrolysis of water, one needs to develop a non-precious-metal catalyst that is as efficient as platinum metal. Here, we prepare such a catalyst by growing a layer of Mo2N over a layer of CeO2 deposited on nickel foam (NF) [hereafter, Mo2N /CeO2@NF] and show that the activity of this self-supported catalyst for hydrogen evolution in 1.0 M KOH is more efficient than that of the Pt/C electrode, achieving a current density of 10 mA/cm2 at a fairly low overpotential of 26 mV. Furthermore, after a long-time electrochemical stability test for 24 h at a fixed current density, the overpotential needed to attain a current density of 10 mA/cm2 is increased only by 6 mV, implying the huge potential of this method to prepare a super HER activity electrode for water splitting.

Highly Efficient Purification of Multicomponent Wastewater by Electrospinning Kidney-Bean-Skin-like Porous H-PPAN/rGO-g-PAO@Ag+/Ag Composite Nanofibrous Membranes.

Due to the complexity of harmful wastewater components, environmental and multifunctional materials are required for sewage purification. In this paper, a novel kidney-bean-skin-like hydrophilic porous polyacrylonitrile/reduced graphene oxide-g-poly(amidoxime)-loaded Ag+ (H-PPAN/rGO-g-PAO@Ag+/Ag) composite nanofiber membrane was fabricated by combining electrospinning and hydrolysis methods. The spinning solution was pumped at a rate of 0.4 mL/h with the voltage set at a constant value of 23 kV. Then, some of the -CN groups switched to hydrophilic -COOH groups via a hydrolysis method, which acts as a linker of GO-g-PAN, Ag+, and the polyacrylonitrile (PAN) matrix. A further step of chelation and thermal treatment were used for generating Schottky junctions between rGO-g-PAO@Ag+ and Ag. After five-cycle tests, it exhibited outstanding mechanical properties ensuring the filtration and purification performance of the H-PPAN/rGO-g-PAO@Ag+/Ag composite nanofiber membrane (i.e., the tensile strength was still 7.21 MPa, and the elongation was 61.53%) for simulated wastewater. The methods of thermal treatment and high-pressure Hg lamp irradiation promoted the reduction of GO to rGO and Ag+ to Ag particles, which endows the final product H-PPAN/rGO-g-PAO@Ag+/Ag with excellent photocatalytic and bactericidal properties. Its catalytic efficiency for dyes benzoic acid (BA), Rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) was up to 99.8, 98, 95, and 91%. The antibacterial rate was 100% against Escherichia coli and 99% against Staphylococcus aureus. More importantly, the photocatalytic and antibacterial PAN-based nanofiber membrane can be simply scaled up, which provides the membrane with great potential in highly efficient wastewater treatment and augmenting water supply.

Metal-Organic Framework/Polythiophene Derivative: Neuronlike S-Doped Carbon 3D Structure with Outstanding Sodium Storage Performance.

Herein, a metal-organic framework (MOF)/polythiophene (PTh)-derived S-doped carbon is successfully designed and prepared employing zeolitic imidazolate frameworks (ZIF-8/ZIF-67) and thiophene (Th) as precursors. The S-doped carbon presents a neuronlike three-dimensional network structure (3DSC). The 3DSC delivers extra-high capacities (225 mAh/g at 5000 mA/g after 3000 cycles) and excellent endurance ability of current changes when applied in Na-ion batteries (SIBs). Moreover, when the 3DSC-700 anode is coupled with a sodium vanadium phosphate cathode to construct a Na-ion full cell, after 50 cycles, a high capacity of ∼229.64 mAh/g is obtained at 100 mA/g. Electrochemical impedance spectroscopy analysis, density functional theory calculations, and pseudocapacitance contributions are adopted to investigate the excellent sodium storage mechanism of the 3DSC electrode. A new idea has been provided in this work to open up the possibility of MOF materials and carbon-based materials applications in SIBs in the future.

Metal-Free B@g-CN: Visible/Infrared Light-Driven Single Atom Photocatalyst Enables Spontaneous Dinitrogen Reduction to Ammonia.

Conversion of naturally abundant dinitrogen (N2) to ammonia (NH3) is one of the most attractive and challenging topics in chemistry. Current studies mainly focus on electrocatalytic nitrogen reduction reaction (NRR) using metal-based electrocatalysts, while metal-free and solar-driven photocatalysts have been rarely explored. Here, on the basis of the "σ donation-π* back-donation" concept, single B atom supported on holey g-CN (B@g-CN) can serve as metal-free photocatalyst for highly efficient N2 fixation and reduction under visible and even infrared spectra. Our results reveal that N2 can be efficiently activated and reduced to NH3 with extremely low overpotential of 0.15 V and activation barrier of 0.61 eV, lower than most of metal-based NRR catalysts, thereby guaranteeing low energy cost and fast kinetics of NRR. The inherent properties of B@g-CN, such as centralized spin-polarization on the B atom, efficient prohibition of competitive hydrogen evolution reaction (HER), and reduced exciton binding energy, are responsible for the high selectivity and Faradaic efficiency for NRR under ambient conditions. Moreover, for the first time, we theoretically disclose that the external potential provided by photogenerated electrons for NRR/HER endowing B@g-CN spontaneous NRR and inaccessible HER. This work may provide a promising lead for designing efficient and robust metal-free single atom catalysts toward photocatalytic NRR under visible/infrared spectrum.

Modified MXene: promising electrode materials for constructing Ohmic contacts with MoS2 for electronic device applications.

The development of MoS2-based electronic devices is dependent on finding electrode materials suitable for forming Ohmic contacts with MoS2. For this purpose, we carried out density functional theory and nonequilibrium Green's function calculations to investigate the possibility of using pristine and modified MXene (Ta2C/Ta2CF2/Ta2C(OH)2) monolayers as the electrode materials in such devices. These calculations indicated the formation of chemical bonds at the MoS2/Ta2C interface, and resulting strong orbital hybridization between the MoS2 and Ta2C components, but relatively weak interactions for MoS2/Ta2CF2 and MoS2/Ta2C(OH)2. Ohmic contacts were observed in all three cases. Transport properties were further simulated by modeling two-probe field effect transistors (FETs) with Ta2C/Ta2CF2/Ta2C(OH)2 as electrodes. Interestingly, these simulations indicated the formation of Ohmic contacts between Ta2CF2/Ta2C(OH)2 electrodes and the channel, but an n-type Schottky barrier for the Ta2C electrode. Furthermore, we found the resistance of the FET based on MoS2/Ta2C(OH)2 to be half of that based on MoS2/Ta2CF2. The results of our study not only revealed promising electrode materials for forming full Ohmic contacts with MoS2 monolayers in FET devices, but also validated the effective role of a small-molecule fragment as a buffer layer in realizing Ohmic contacts between metal and semiconductor.

SnO2/Reduced Graphene Oxide Interlayer Mitigating the Shuttle Effect of Li-S Batteries.

The short cycle life of lithium-sulfur batteries (LSBs) plagues its practical application. In this study, a uniform SnO2/reduced graphene oxide (denoted as SnO2/rGO) composite is successfully designed onto the commercial polypropylene separator for use of interlayer of LSBs to decrease the charge-transfer resistance and trap the soluble lithium polysulfides (LPSs). As a result, the assembled devices using the separator modified with the functional interlayer (SnO2/rGO) exhibit improved cycle performance; for instance, over 200 cycles at 1C, the discharge capacity of the cells reaches 734 mAh g-1. The cells also display high rate capability, with the average discharge capacity of 541.9 mAh g-1 at 5C. Additionally, the mechanism of anchoring behavior of the SnO2/rGO interlayer was systematically investigated using density functional theory calculations. The results demonstrate that the improved performance is related to the ability of SnO2/rGO to effectively absorb S8 cluster and LPS. The strong Li-O/Sn-S/O-S bonds and tight chemical adsorption between LPS and SnO2 mitigate the shuttle effect of LSBs. This study demonstrates that engineering the functional interlayer of metal oxide and carbon materials in LSBs may be an easy way to improve their rate capacity and cycling life.

Group†IV Monochalcogenides MX (M=Ge, Sn; X=S, Se) as Chemical Anchors of Polysulfides for Lithium-Sulfur Batteries.

Although rechargeable lithium-sulfur batteries are considered as advanced energy systems, their practical implementation is impeded by many factors, in particular the rapid capacity fade and low Coulomb efficiency caused by the shuttle effect. To overcome this problem for achieving longer cycle life and higher rate performance, anchoring materials for lithium polysulfides are highly desirable. In this work, for the first time, we report phosphorene-like MX (M=Ge, Sn; X=S, Se) monolayers as promising anchoring materials to restrain the lithium polysulfides shuttling. Our study provides fundamental selection criteria for the effective suppression of the polysulfides shuttling. Adsorption calculations reveal that polysulfide capture by the MX is through chemisorption with a suitable range of adsorption energies. Morever, we show that excellent surface diffusion of Li and polysulfides endow a fast charge/discharge rate for lithium-sulfur batteries. Graphene with desirable electronic properties is constructed to improve the electrical conductivity in the new graphene@MX heterostructures. Based on the strong anchoring ability, improved rate capability, and enhanced conductivity, MX-based composites hold great promise as an anchoring material for high-energy lithium-sulfur batteries.

Self-assembled supramolecular system PDINH on TiO2 surface enhances hydrogen production.

Constructing organic-inorganic hybrids is one of the hopeful strategies to improve photocatalyst performance. In this study, perylene-3,4,9,10-tetracarboxylic diimide (PDINH) and commercial TiO2 P25 are chosen as raw materials to construct a PDINH/TiO2 organic-inorganic hybrid, which has higher photocatalytic H2 production activity and photocurrent intensity than pure PDINH and TiO2, respectively. The apparent quantum efficiency for H2 production over 0.5%PDINH/TiO2 reaches as high as 70.69% using irradiation at 365 nm. Moreover, XRD, DRS, HRTEM, FT-IR, and XPS are used to characterize the crystal structure, optical absorption, morphology, molecular structure, and chemical bonds, as well as the elemental and chemical states of PDINH/TiO2 organic-inorganic hybrid. The interfaces between PDINH and TiO2, which largely determine photocatalytic performance, is also analyzed systematically. Furthermore, charge density difference (Δρ) is used to analyze the electron-ion interactions of PDINH and TiO2, and reveals that substantial charge transfer occurs from PDINH to TiO2.

Tunable Schottky contacts in MSe2/NbSe2 (M = Mo and W) heterostructures and promising application potential in field-effect transistors.

The performance of electronic and optoelectronic devices based on two-dimensional (2D) materials could be significantly affected by the electrical contacts. In search of low-resistance contacts with transition-metal dichalcogenides (TMDs), we combine density functional calculations with quantum transport simulations to investigate the structural and electronic properties of the van der Waals (vdW) heterostructures MSe2/NbSe2 (M = Mo and W). The formation of a p-type Schottky contact at the MSe2/NbSe2 interface with small Schottky barriers (0.37 eV for MoSe2/NbSe2 and 0.18 eV for WSe2/NbSe2) is demonstrated. The low Schottky barrier heights indicate a low contact resistance, which is beneficial for electron injection and low-resistance. Remarkably, we demonstrate that the Schottky barrier can be effectively tuned via the application of vertical compressive pressure, an external electrical field and tensile strain. Finally, the results are supported by quantum transport simulation, which further proves the highly transparent contacts and promising application potential in field-effect transistors (FET). Therefore, our formalism and findings not only provide insights into the MSe2/NbSe2 interfaces but also help in the design of MSe2 monolayer-based devices.

Evolution of the linear band dispersion of monolayer and bilayer germanene on Cu(111).

The structural and electronic properties of germanene are always strongly modulated by the hybridization effects with metal substrates. In order to see what will happen when a buffer layer is introduced in-between germanene and metal substrates, we study the structural and electronic properties of the recently synthesized monolayer and bilayer germanene on Cu(111) though first-principles calculations. Our results show that the monolayer germanene on Cu(111) displays a nearly flat configuration and interface states form between the Ge pz and Cu sp-like states, with the Ge π states maintaining the Dirac character. For bilayer germanene on Cu(111), interactions with Cu(111) are reduced due to germanene inter-layer interactions, which is beneficial for the transfer of germanene. In comparison with the bottom germanene layer, the Ge pz character of the upper germanene layer can be maintained near the Fermi level. Since the linear band dispersion is at the heart of the novel quantum phenomenon, our results will facilitate research into the synthesis, extraordinary quantum properties, and applications based on the two-dimensional (2D) germanium system.

Sc2 C as a Promising Anode Material with High Mobility and Capacity: A First-Principles Study.

Two-dimensional (2D) Sc2 C, an example of a MXene, has been attracting extensive attention due to its distinctive properties and great potential in applications such as energy storage. In light of its high capacity and fast charging-discharging performance, Sc2 C exhibits significant potential as an anode material for lithium- and sodium-ion batteries. Herein, a systematic investigation of Li/Na atom adsorption and diffusion on Sc2 C planes was performed based on density functional calculations. The metallic character of pristine and adsorbed Sc2 C ensures desirable electric conductivity, which indicates the advantages of 2D Sc2 C for lithium- and sodium-ion batteries. A significant charge transfer from the Li/Na atoms to Sc2 C is predicted, which indicates the cationic state of the adatoms. In addition, the diffusion barriers are as low as 0.018 and 0.012 eV for Li and Na, respectively, which illustrates the high mobility and cycling ability of Sc2 C. In particular, each formula unit of Sc2 C can adsorb up to two Li/Na atoms, which corresponds to a relatively high theoretical capacity of 462 or 362 mAh g-1 . The average electrode potential was calculated to be as low as 0.32 and 0.24 V for stoichiometric Li2 Sc2 C and Na2 Sc2 C, respectively, which makes Sc2 C attractive for the overall voltage of the cell. Herein, our results suggest that Sc2 C could be a promising anode candidate for both lithium-ion and sodium-ion batteries.