Study on the growth mechanism of monolayer and few-layer hexagonal boron nitride films on copper foil.

During the process of synthesizing h-BN on Cu foil via chemical vapor deposition (CVD), low-pressure CVD (LPCVD) typically synthesizes monolayer h-BN films, whereas atmospheric pressure CVD (APCVD) yields few-layer h-BN films. Herein, a growth mechanism for monolayer and few-layer h-BN on Cu foil is proposed using first-principles calculations: Cu(111) passivated h-BN hinders the diffusion of B and N atoms at the subsurface of Cu(111), preventing sufficient transportation of B and N atoms to the existing h-BN layer, thereby leading to the formation of monolayer h-BN in LPCVD. For APCVD, the edges of h-BN are passivated by H, which decreases the barrier energy for the diffusion of B and N atoms on the Cu(111) subsurface, and B and N atoms can easily migrate from the subsurface of Cu(111) to its surface, resulting in the nucleation of h-BN between the existing h-BN and Cu(111), and leading to the formation of few-layer h-BN films. This work provides a theoretical basis at the atomic scale for further understanding the growth of monolayer and few-layer h-BN films on Cu foil.

Composite interfaces of g-C3N4 fragments loaded on a Cu substrate for CO2 reduction.

Designing an electrocatalyst with high efficiency and product selectivity is always crucial for an electrocatalytic CO2 reduction reaction (CO2RR). Inspired by the great progress of two-dimensional (2D) nanomaterials growing on Cu surfaces and their promising CO2RR catalytic efficiencies at their interfaces, the unique performance of Cu-based 2D materials as high-efficiency and low-cost CO2RR electrocatalysts has attracted extensive attention. Herein, based on density functional theory (DFT) calculations, we proposed a composite structure of graphitic carbon nitride (g-C3N4) fragments loaded on a Cu surface to explore the CO2RR catalytic property of the interface between g-C3N4 and the Cu surface. Three composite interfaces of C3N4/Cu(111), C3N4/Cu(110) and C3N4/Cu(100) have been studied by considering the reaction sites of vertex nitrogen atoms, edge nitrogen atoms and the nearby Cu atoms. It was found that the C3N4/Cu interfaces where nitrogen atoms contact the Cu substrate present competitive CO2RR activity. Among them, C3N4/Cu(111)-N3 exhibited a better activity for CH3OH production, with a low overpotential of 0.38 V. For HCOOH and CH4 production, C3N4/Cu(111)-Cu and C3N4/Cu(100)-N1 have overpotentials of 0.26 V and 0.44 V. The electronic analysis indicates the electron transfer from the Cu substrate to the g-C3N4 fragment and mainly accumulates on the nitrogen atoms of the interface. Such charge accumulation can activate the adsorbed CO bond of CO2 and lead to lower energetic barriers of CO2RR. DFT calculations indicate that the boundary nitrogen sites reduced the energy barrier of *CHO, which is crucial for CO2RR, compared with that of the pristine Cu surface. Our study explores a new Cu-based electrocatalyst and indicates that the C3N4/Cu interface can enhance the activities and selectivity of CO2RR and open a new strategy to design high-efficiency electrocatalysts for CO2RR.

A 2D-0D-2D Sandwich Heterostructure toward High-Performance Room-Temperature Gas Sensing.

The construction of two-dimensional (2D) van der Waals (vdW) heterostructures over black phosphorus (BP) has been attracting significant attention to better utilize its inherent properties. The sandwich of zero-dimensional (0D) noble metals within BP-based vdW heterostructures can provide efficient catalytic channels, modulating their surface redox potentials and therefore inducing versatile functionalities. Herein, we realize a 2D WS2-Au-BP heterostructure, in which Au nanoparticles are connected between BP and WS2 via ionic bonds. The ultralow conduction band minimum position, the reduced adsorption energies of O2, and the increased dissociation barrier energy of O2- into 2O contribute greatly to improving the long-term stability of BP in the air. The formation of heterostructures can reduce the potential barrier energy in target gas molecules, thus enhancing the absorption energy and charge transfer. Taking the paramagnetic NO2 gas molecules as a representative, a stable response magnitude of 2.11 to 100 ppb NO2 is achieved for 80 days, which is far larger than the initial responses of most BP-based materials. A practical gas sensing system is also developed to demonstrate its real-world implementation. This work provides a promising demonstration of 0D noble metal within 2D BP-based vdW heterostructure for simultaneously improving the long-term stability and room-temperature reversible gas sensing.

ZnO-Au@ZIF-8 core-shell nanorod arrays for ppb-level NO2 detection.

ZnO-Au@ZIF-8 core-shell heterostructures were prepared by ZIF-8 encapsulation of sacrificial ZnO-Au nanorods. Because of the catalytic activity of the Au nanoparticles and the sieving effects of the ZIF-8, the ZnO-Au@ZIF-8 heterostructures showed an outstanding response of 1.8 to 5 ppb NO2, and exhibited higher selectivity, stability, anti-humidity and fast response and recovery properties. The combination of the gas-selective catalytic activity of noble metals with the MOF filter used in this work can be easily extended to synthesize other types of MOS@MOF sensors, opening a new avenue for the detection of hazardous gases.

Enhance Hydrogen Evolution Reaction Performance via Double-Stacked Edges of Black Phosphorene.

Based on the density functional theory (DFT) calculations, we explored the structures and HER catalytic properties of reconstructed and double-stacked black phosphorene (BP) edges. Ten bilayer BP edges were constructed by the double stacking of three typical monolayer edges, i.e., zigzag (ZZ) edge, armchair (AC) edge, skewed diagonal (SD) edge, and their reconstructed derivatives with their layer's configurations, edge deformations and thermodynamic stabilities were discussed. Based on these edges, five chemical sites on four bilayer BP edges were selected to be promising candidates for a HER catalyst, which present higher HER activities than that of Pt(111). Besides, among these four edges, two edges have even lower energetic barriers for the Tafel reaction. Compared with the monolayer edges, these selected bilayer BP edges confirm the remarkable enhancement of the HER catalytic properties, which can be attributed to their unique edge structures and the enhanced electronic densities after the hydrogen adsorptions. Finally, the thermostability of these edges at room temperature has also been proved by the DFT-MD simulations. This theoretic study deepens our fundamental understanding of the double-stacked edge structures of the BP and provides a new way for the rational design of highly efficient and noble-metal-free HER catalysts.

High Detectivity of PbS Films Deposited on Quartz Substrates: The Role of Enhanced Photogenerated Carrier Separation.

PbS films grown on quartz substrates by the chemical bath deposition method were annealed in an O2 atmosphere to investigate the role of oxygen in the sensitization process at different annealing temperatures. The average grain size of the PbS films gradually increased as the annealing temperature increased from 400 °C to 700 °C. At an annealing temperature of 650 °C, the photoresponsivity and detectivity reached 1.67 A W-1 and 1.22 × 1010 cm Hz1/2 W-1, respectively. The role of oxides in the sensitization process was analyzed in combination with X-ray diffraction and scanning electron microscopy results, and a three-dimensional network model of the sensitization mechanism of PbS films was proposed. During the annealing process, O functioned as a p-type impurity, forming p+-type PbS layers with high hole concentrations on the surface and between the PbS grains. As annealing proceeds, the p+-type PbS layers at the grain boundaries interconnect to form a three-dimensional network structure of hole transport channels, while the unoxidized p-type PbS layers act as electron transport channels. Under bias, photogenerated electron-hole pairs were efficiently separated by the formed p+-p charge separation junction, thereby reducing electron-hole recombination and facilitating a higher infrared response.

Quasi-one-dimensional phosphorene nanoribbons grown on silicon by space-confined chemical vapor transport.

Phosphorene nanoribbons (PNRs) combine the flexibility of one-dimensional (1D) nanomaterials with the large specific surface area and the edge and electron confinement effects of two-dimensional (2D) nanomaterials. In spite of the substantial advances in bulk black phosphorus (BP) manufacturing, achieving PNRs without degradation is still a big challenge. In this work, we present a strategy for the space-confined chemical vapor transport synthesis of quasi-one-dimensional surface-passivated monocrystalline PNRs on a silicon substrate. The growth mechanism of the PNRs is proposed by combining experimental results and DFT calculations, indicating that the P4 molecules can break, restructure, and epitaxially nucleate on the surface of the Au3SnP7 catalyst, and finally prefer to grow along the zigzag (ZZ) direction to form PNRs. The low gas flow rate and an appropriate phosphorus molecule concentration allow the growth of PNRs with structural integrity, which can be regulated by the amount of red phosphorus and the confined space.

CuO-decorated MOF derived ZnO polyhedral nanostructures for exceptional H2S gas detection.

Considering that H2S is a hazardous gas that poses a significant risk to people's lives, research into H2S gas sensors has garnered a lot of interest. This work reports a CuO/ZnO multifaceted nanostructures(NS) created by heat treating Cu2+/ZIF-8 impregnation precursors, and their microstructure and gas sensing characteristics were examined using various characterization techniques (XRD, XPS, SEM, TEM, and BET). The as-prepared hollow CuO/ZnO multifunctional nanostructures had a high gas response value (425@50 ppm H2S gas), quick response and recovery times (57/191s @20 ppm), a low limit of detection (1.6@500 ppb H2S), good humidity resistance and highly selective towards H2S gas. The hollow CuO/ZnO multifaceted nanostructures possessed enhanced gas sensing capabilities which may be related to their porous hollow nanostructures, the manufactured p-CuO/n-ZnO heterojunctions, and the spillover effect between CuO and H2S.

Metal-organic frameworks-derived In2O3/ZnO porous hollow nanocages for highly sensitive H2S gas sensor.

The detection of hydrogen sulfide (H2S) is critical because of its potential harm and widespread presence in the oil and gas sectors. The zeolitic imidazolate framework-8 (ZIF-8) derived ZnO nanostructures manufactured as gas sensors have exceptional sensitivity and selectivity for H2S gas. In/Zn-ZIF-8 template material was synthesized by a simple one-step co-precipitation method followed by thermal annealing in air. The heat treatment resulted in In2O3/ZnO nanostructures with mixed heterostructures. The crystal structure (XRD), morphology (SEM/TEM), chemical state (XPS), surface area (BET), etc were investigated to ascertain the nature of the as-prepared material. SEM imagery revealed that the as-prepared In2O3/ZnO sensitive material had a microstructure of porous hollow nanocages with an average particle size of about 200 nm, which is beneficial to the diffusion and adsorption of gas molecules. The gas sensing performance test results of the In2O3/ZnO hollow nanocages show that their response to H2S gas is significantly improved 67.5 @50 ppm H2S (about 11 times that of pure ZnO nanocages) at an optimal temperature of 200 °C, better selectivity, lower theoretical detection limit and good linearity between gas concentration and response values. The enhanced gas sensing feat to H2S gas is mainly attributed to the formation of n-n heterojunction and the wide surface area of the newly formed In2O3/ZnO porous hollow nanocages.

Facile fabrication of nanoflower-like WO3/WS2 heterojunction for highly sensitive NO2 detection at room temperature.

Realizing efficient detection of ultra-low concentrations of hazardous gases contributes to air pollution monitoring, ecosystem and human health protection. Herein, we firstly fabricated the nanoflower-like WO3/WS2 composites by a facile process to highly sensitively detect NO2 at room temperature. The WO3 content in the WO3/WS2 composites can be adjusted by altering the calcination temperature, and the WO3 nanoparticles disperse uniformly on the WS2 surface, forming the WO3/WS2 heterojunction. The room-temperature responses of WO3/WS2 composites gradually climb with the NO2 concentration increasing from 0.005 to 5 ppm, and the WW-280 and WW-300 composites possess the optimal gas sensitivity when the NO2 concentrations are lower and higher than 100 ppb, respectively. In particular, the two WO3/WS2 composites present the limitation of detection (LOD) of ≤ 5 ppb, and they exhibit the excellent selectivity, good reproducibility and long-term stability towards NO2. A possible gas sensing mechanism was also proposed from the point of views of gas adsorption, redox reactions and electron transfer. The appropriate WO3 content and molar ratio of hexagonal to monoclinic WO3, and the formation of WO3/WS2 p-n heterojunction can contribute to the high sensitivity of WO3/WS2 composite to various concentrations of NO2. This work offers a promising gas sensing material for room-temperature detection to low concentrations of NO2.

Efficient detection of hazardous H2S gas using multifaceted Co3O4/ZnO hollow nanostructures.

The rapid increases in environmental hazardous gases have laid dangerous effects on human health. The detection of such pollutants gases is mandatory using various optimal techniques. In this paper, porous multifaceted Co3O4/ZnO nanostructures are synthesized by pyrolyzing sacrificial template of core-shell double zeolitic imidazolate frameworks (ZIFs) for gas sensing applications. The fabricated exhibit superior gas sensor response, high selectivity, fast response/recovery times, and remarkable stability and sensitivity to H2S gas. In particular, the multifaceted Co3O4/ZnO nanostructures show a maximum response of 147 at 100 ppm of H2S under optimum conditions. The remarkable gas sensing performances are mainly ascribed to high porosity, wide surface area multifaceted nanostructures, presence of heterojunctions and catalytic activity of ZnO and Co3O4, which are beneficial for H2S gas sensors industry.

Enhanced NO2 gas-sensing performance by core-shell SnO2/ZIF-8 nanospheres.

Timely detection of harmful, poisonous and air pollutant gases is of vital importance to the protection of human beings from exposure to rigorous gases. The development of gas-sensing devices based on sphere-like porous SnO2/ZIF-8 nanocomposites is required to overcome this challenge. Nanostructures with high surface area, more porosity and hollow interior provide plenty of active cites for high responses in metal oxide gas sensors. The engineered gas sensors have excellent sensing sensitivity (164), rapid response and recovery times (60, 45 s), and favorable selectivity for NO2 gases under 300 °C. Consequently, NO2 gas sensors based on core-shell SnO2/ZIF-8 nanospheres are regarded viable capacity industrial applicants.

Theoretical Study on P-coordinated Metal Atoms Embedded in Arsenene for the Conversion of Nitrogen to Ammonia.

The conversion of gaseous N2 to ammonia under mild conditions by artificial methods has become one of the hot topics and challenges in the field of energy research today. Accordingly, based on density function theory calculations, we comprehensively explored the d-block of metal atoms (Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Ru, Rh, W, and Pt) embedded in arsenene (Ars) for different transition systems of phosphorus (P) coordination as potential electrocatalysts for N2 reduction reaction (NRR). By adopting a "two-step" strategy with stringent NRR catalyst screening criteria, we eventually selected Nb@P3-Ars as a research object for a further in-depth NRR mechanism study. Our results show that Nb@P3-Ars not only maintains the thermodynamic stability at mild temperatures but also dominates the competition with the hydrogen evolution reaction when used as the electrochemical NRR (e-NRR) catalyst. In particular, while the NRR process occurs by the distal mechanism, Nb@P3-Ars has a low overpotential (0.36 V), which facilitates the efficient reduction of N2. Therefore, this work predicts the possibility of Nb@P3-Ars as an e-NRR catalyst for reducing N2 from a theoretical perspective and provides significant insights and theoretical guidance for future experimental research.

Internal electric field induced S-scheme heterojunction MoS2/CoAl LDH for enhanced photocatalytic hydrogen evolution.

The requisite interfacial contact of heterojunction photocatalysts has a significant contribution in separation of interfacial charge carriers for photocatalytic hydrogen (H2) evolution in a more efficient manner. Herein, an internal electric field (IEF)-induced S-scheme system comprised of two-dimensional (2D) CoAl layered double hydroxide (LDH) and 2D molybdenum disulfide (MoS2) was constructed via a simple hydrothermal process. In the presence of visible-light irradiation, the 2D/2D MoS2/CoAl LDH hybrid demonstrates eightfold greater photocatalytic H2 generation rate as compared with that of CoAl LDH. The mechanism was investigated in the light of the results of the X-ray photoelectron spectroscopy (XPS) and work-function calculated by density functional theory (DFT) simulation, and the improved activity was ascribed to that the rapid detachment of the electron-hole (e--h+) combinations and high redox ability, both are simultaneously realized in MoS2/CoAl LDH hybrid with a 2D/2D S-scheme charge transfer mechanism induced by the IEF across interface of the MoS2 and CoAl LDH. Furthermore, favorable 2D/2D structure and better H* adsorption behavior of MoS2/CoAl LDH also promoted the improvement of water reduction performance. This work is a valuable guideline in developing of IEF-induced S-scheme photocatalysts with 2D/2D architecture for improved photocatalytic performance.

Single atom-doped arsenene as electrocatalyst for reducing nitrogen to ammonia: a DFT study.

Due to the wide application of NH3 in the energy and chemical industry, the rational design of a highly efficient and low-cost electrocatalyst for nitrogen fixation at moderate conditions is highly desirable to meet the increasing demand for sustainable energy production in the modern society. Herein, we have systematically studied the catalytic performance of transition metal (TM) atom (i.e., V, Cr, Fe, Co, Cu, Ru, Pd, Ag, Pt, Au)-doped arsenene nanosheet, a new two-dimensional (2D) nanomaterial in VA group, as a heterogeneous catalyst for nitrogen reduction reaction (NRR). By density functional theory (DFT) calculation and systematic theoretical screening, our study predicts that the systems of V-, Fe-, Co- and Ru-doped arsenene have promising potentials as NRR electrocatalysts with high-loading TM and highly stable adsorption of N2 molecule. Particularly, the V-doped system exhibits two feasible configurations for N2 adsorption and an ultralow overpotential (0.10 V) via the enzymatic pathway, which is very competitive among similar reported electrocatalysts. This theoretical study not only extends the electrocatalyst family for nitrogen fixation, but also further deepens our physical insights into catalytic improvement, which can be expected to guide the rational design of novel NRR catalysts.

Construction of hierarchical trimetallic organic framework leaf-like nanostructures derived from carbon nanotubes for gas-sensing applications.

Unique trimetallic organic material (TMOM)-based nanostructures combined with the new architectures of metal-organic frameworks (MOFs) are promising candidates for gas-sensing applications. This work is the first to successfully convert MOF nanomaterials into nano-porous carbon through carbon nanotubes (CNT) catalytic reaction via a simple and facile hydrothermal method. The leaf-like nanostructures exhibit a high surface-to-volume ratio of 363 m2 g-1. The TMOM nanostructures were subsequently exposed to different types of target gases for a wide range of gas concentrations at different operating temperatures. The carbon nanotubes (TMOM-CNT) hybrid nanocomposites were characterized using X-ray powder diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, energy dispersion spectrum analysis, thermo-gravimetric analysis, and transmission electron microscopy. The fabricated Zn-Co-Ni MOF@CNT sensors exhibit high selectivity and gas-sensing response toward H2S gas at an optimal temperature of 325 °C for 100 ppm. These superior gas-sensing performances reveal that the Zn-Co-Ni MOF@CNT sensors with a unique leaf shape exhibit potential applications for the environment applications in gas sensor industry.

Na-Ions Diffusion Impacts Supercapacitor Performance for Amaryllis-like NiCo2O4 Nanostructures.

NiCo2O4 nanomaterials with exceptional electrochemical performances are synthesized via a simple and low-cost method. The synthesized nanostructures exhibit a high specific surface area of 121.52 m2 g-1 and excellent specific capacitance of 2498.49 F g-1 at a current density of 2 A g-1, an energy density of 79 Wh kg-1, and power density of 3570 W kg-1. The remarkable cycling stability of 92.61% retention after 5000 cycles demonstrates that NiCo2O4 nanomaterials have a potential for practical application in energy storage devices. The Na+ ion diffusion (by VASP) affirms a low activation energy barrier for Na ion intercalations onto the electrode material, illuminating excellent electrochemical performances.

Influence of Interaction Components on the NOx Adsorption Properties of Cu-Ba-Ce Composite Catalysts.

For further investigating the role of components in copper-cerium-barium composite oxide catalysts, a series of noble metal-free catalysts with different barium contents were synthesized by citric acid method. The prepared catalysts were characterized by X-ray diffraction, Brunauer-Emmett- Teller method, scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy. Catalysts were evaluated by NO oxidation, NOx adsorption, and NOx temperature-programmed desorption. The results showed that barium content significantly affected the catalysts' properties. With barium addition, interactions among components considerably changed; hence, the microstructure and performance of catalysts were distinctly different. In general, the interactions of barium-copper and barium-cerium were not conducive to catalyst adsorption. Adsorption performance derived from barium carbonate was superior to those of the other two components.

Highly robust and flexible n-type thermoelectric film based on Ag2Te nanoshuttle/polyvinylidene fluoride hybrids.

Highly robust and flexible n-type thermoelectric (TE) films based on Ag2Te nanoshuttle/polyvinylidene fluoride were prepared by a solution-processable method without a surfactant. A good power performance of over 30 µW (m K2)-1 at room temperature was achieved. Moreover, the synthesized fabrics also exhibited potential for application in flexible electronic devices with negligible performance change after 1000 bending cycles.

Native point defects on hydrogen-passivated 4H-SiC (0001) surface and the effects on metal adsorptions.

With the continued expansion of silicon carbide's (SiC) applications, atomistic understanding on the native point defects of its surfaces, particularly on those of the hydrogen-passivated (HP) 4H-SiC (0001) surface, becomes imperative. Using first-principles calculations, the structures and formation energies of several typical native point defects (e.g., ISi, IC, VSi, VC, and SiC) on the (0001) HP-surface of 4H-SiC were systematically explored, including the effects of the unit cell size, environmental condition, charge state, and hydrogen incorporation. Furthermore, their adsorptions of Ag (Mo) atom on these defective sites were systematically investigated. The formation energies of these defects in the HP-surface, clean surface, and bulk SiC were concluded together with their thermodynamic concentrations in the HP-surface estimated. The influences of these defects to metal (Ag, Mo) adsorptions of HP-surfaces were concluded. Based on these conclusions, the wettability improvement between the metal liquid and ion (Ag or Mo) implanted SiC substrates in the previous studies can be well understood at the atomistic scale. This study provides a theoretical guideline to SiC surface modification for the production of metal-SiC composites, brazing of SiC with metals, fabrication of electronic devices, or the growth of two dimensional nanofilms.