Mo2C Layer Research Articles

Heterostructuring the CO2-derived Mo2C layer with MoP2 via molten salt electrolysis for efficient hydrogen evolution reaction

Electrochemical fixation of CO2 in molten salts as carbides for catalyzing the hydrogen evolution reaction can contribute to carbon neutrality and value-added conversion of CO2 as well as facilitate the production of sustainable green hydrogen energy. However, the functional ability of the CO2-derived carbides still needs to be substantially improved. Herein, heterostructuring the CO2-derived Mo2C layer with MoP2 is realized via electro-splitting of CO2 in Ca3(PO4)2-containing molten salt. The as-designed Mo2C–MoP2 heterostructure layer presents significantly improved HER performances, namely overpotential being 109 mV @ 100 mA cm−2 and stability for 600 h @ 200 mA cm−2, greatly outperforming both the bare Mo2C layer and commercial Pt candidates. The superior performances of the Mo2C–MoP2 heterostructure are in one way attributed to the modified electronic structure that decrease the energy barrier of the Volmer rate-determining step for HER. In another way, the Mo2C–MoP2 dual-phase increases the hydrophilicity ability of the catalytic layer, accelerating the detachment of the H2 bubbles. The results can provide new insights for both value-added fixation of carbon dioxide and preparation of high-performance non-noble electrocatalyst.

Molten salt-assisted controlled synthesis of two-dimensional molybdenum carbide

The present investigation examines the carbothermal reduction synthesis of two-dimensional molybdenum carbide (2D Mo2C) using sodium carbonate (Na2CO3) as a molten salt diffusion promoter and sodium sulfide (Na2S) as a solid-state intercalation agent. Raw molybdenum disulfide (MoS2) powder undergoes carbothermal reduction facilitated by activated carbon and Na2CO3 above 800 °C to produce 2D Mo2C layers intercalated by Na2S. The diffusion of Na2S can be enhanced by the fluidity of molten salt Na2CO3, leading to the expansion of the Mo2C layer spacing to 29 nm under the influence of the temperature field. Na2S intercalation prevents layer shrinkage during cooling while molten Na2CO3 directs 2D growth, yielding 10 nm-thick sheets. The product maintains hexagonal β-Mo2C structure up to 950 °C with microflowers of accordion-shaped nanosheets. Ultrasonication exfoliates the weakly bound Mo2C layers into uniform, freely suspended flakes around 10–100 nm in lateral size. This work demonstrates the tuning of 2D Mo2C morphology in high-temperature reactions by utilizing molten salts. The principal results are the synthesis of micrometer-sized Mo2C sheets with controlled nanoscale thickness and uniform nanosheet dispersions, enabled by molten salt-directed diffusion of intercalated species. The major conclusion drawn is that solid-liquid synergistic diffusion can guide precision synthesis of layered nanomaterials.

Microstructure and finite element analysis of Mo2C-diamond/Cu composites by spark plasma sintering

Abstract Mo2C layer was generated on the diamond surface via vacuum micro-evaporating, which was used as the reinforcement particles to fabricate diamond/Cu composites by spark plasma sintering (SPS). The effect of evaporation parameters on the forming of Mo2C, and the holding time on diamond/Cu composites fabrication is studied. Combined with the experiment and finite element analysis (FEA), the holding time on diamond/Cu composites influence on the thermal conductivity (TC) of composites is further discussed. The results show that the Mo2C area on the diamond surface would gradually enlarge and cover the diamond surface evenly with the increment in evaporation time and temperature, better vacuum micro-evaporating parameters were given as 1,000°C for 60 min. The fractures in the diamond/Cu composites are mainly ductile fractures on copper and diamond falling out from the Mo2C interface. It was found that sintering time would significantly influence the dissipation property of diamond/Cu composites. A comprehensive parameter for SPS was obtained at 900°C, 80 MPa for 10 min, the relative density (RD) and TC of the composites obtained under the parameter were 96.13% and 511 W/(m K). A longer sintering time would damage the Mo2C interlayer and further decrease the bonding between copper matrix and diamond particles, which would lower the RD and TC of composites. It can be obtained from the comparison of simulation results and experimental results that the FEA result is closer to the experimental results due to the gaps with low heat conduction, and the air in the gaps is added in the simulation process.

Mo2C interface layer: effect on the interface strength and cutting performance of diamond/Fe-Ni-WC composites

The weak interfacial strength between Fe-based matrix and diamonds causes diamond particles to fall off prematurely, significantly affecting the cutting efficiency of diamond composites. This work aimed to optimize the interfacial microstructure and mechanical properties of the diamond/Fe-Ni-WC composites by coating a Mo2C layer on the diamond particles. The Mo2C-coated diamonds were prepared via the molten salt method. Diamond/Fe-Ni-WC composites were sintered by the hot-pressing process under a vacuum. The bending strength of composites with Mo2C-coated diamonds was improved from 804 to 996 MPa with the increased coating time. The energy dispersive spectroscopy scanning indicated that the Mo2C coating layer changed the interfacial composition between the matrix and the diamonds from Fe–C alloy to Mo2C, improving the interface strength. Moreover, the diamond protrusion heights of composites were raised by adding a Mo2C interface, resulting in an improvement in the cutting efficiency of impregnated diamond tools.

Substrate effect on hydrogen evolution reaction in two-dimensional Mo2C monolayers

The physical and chemical properties of atomically thin two-dimensional (2D) materials can be modified by the substrates. In this study, the substrate effect on the electrocatalytic hydrogen evolution reaction (HER) in 2D Mo2C monolayers was investigated using first principles calculations. The isolated Mo2C monolayer shows large variation in HER activity depending on hydrogen coverage: it has relatively low activity at low hydrogen coverage but high activity at high hydrogen coverage. Among Ag, Au, Cu, and graphene substrates, the HER activity is improved on the Ag and Cu substrates especially at low hydrogen coverage, while the effects of the Au and graphene substrates on the HER activity are insignificant. The improvement is caused by the charge redistribution in the Mo2C layer on the substrate, and therefore the HER activity becomes high for any hydrogen coverage on the Ag and Cu substrates. Our results suggest that, in two-dimensional electrocatalysis, the substrate has a degree of freedom to tune the catalytic activity.

Mechanical Properties and Interface Microstructure of Fe-Ni-Wc-Based Impregnated Diamond Composite Reinforced by Mo2c Layer

Mechanical Properties and Interface Microstructure of Fe-Ni-Wc-Based Impregnated Diamond Composite Reinforced by Mo2c Layer

Fabrication of a novel molybdenum carbide composite coating with double-layer structure on cast iron via in situ solid-phase diffusion

The improvement of comprehensive properties including hardness, toughness, adhesion strength, and wear resistance of the hard coating is highly desirable for various applications. Herein, a new strategy involving in situ solid-phase diffusion (ISSD) is utilized to fabricate a novel molybdenum carbide composite coating (MCCC) with double-layer structure on cast iron (CI). MCCC is composed of two layers, the outermost layer (Layer I) is a completely dense Mo2C layer, and just beneath it is a Fe3Mo3C(Si) composite transitional layer (Layer II) which connects the outer layer and the CI substrate. The squares of the thickness of the Layer I, Layer II, and MCCC are found to be proportional to the ISSD time, thus following the classic parabolic law. The hardness, elastic modulus, and fracture toughness of the coating surface reach 24.6 ± 0.5 GPa, 483.5 ± 3.1 GPa, and 3.0 ± 0.1 MPa·m1/2. With the evolution of the microstructure in the coating along the thickness direction, the hardness and elastic modulus gradually decrease, while the fracture toughness gradually increases. Furthermore, the result of scratch test shows that the adhesion strength between Layer I and Layer II is approximately 93.4 N, while the adhesion strength between the coating and the substrate is greater than 93.4 N. Therefore, this work provides a new route for the fabrication of MCCC with double-layer structure capable of combining superhardness, excellent toughness, and superior adhesion strength.

Surface effects on deuterium permeation through vanadium membranes

Dense vanadium-based membranes offer high permeability and perfect selectivity to hydrogen isotopes, maintain favorable neutronic properties, and are compatible with liquid metals such as PbLi. These properties make vanadium membranes a promising fusion fuel cycle technology for processes such as tritium extraction from PbLi and exhaust processing. Surface contamination has a deleterious effect on the gas-phase hydrogen permeation through vanadium, and the reported permeabilities range from 10-14 to 10-7 mol m-1 s-1 Pa-0.5. Thin dense films of palladium applied to clean vanadium surfaces enable a consistently high hydrogen permeability. In this study, uncoated vanadium resulted in deuterium permeabilities ranging from 2.8 × 10-11 to 6.4 × 10-9 mol m-1 s-1 Pa-0.5 at 300 °C–700 °C, respectively. Post-test analysis revealed a VOx surface layer and VCx subsurface layer formed on the feed side, while the as-received surface oxide dissolved leaving a submonolayer oxide on the permeate surface. The Pd-coated V resulted in a maximum deuterium permeability of 2.1 × 10-7 mol m-1 s-1 Pa-0.5 at 375 °C upon activation of the Pd surface by oxidation and reduction. The deuterium permeation declined upon heating to 500 °C due to intermetallic diffusion between the Pd and V. The Mo2C-coated V resulted in deuterium permeabilities ranging from 2.7 × 10-10 to 1.8 × 10-9 at 500 °C–700 °C, respectively, and a post-test analysis found the carbon in the Mo2C layer had dissolved into the V near the interface.

Controlled growth of Mo2C pyramids on liquid Cu surface

Precise spatial control of 2D materials is the key capability of engineering their optical, electronic, and mechanical properties. However, growth of novel 2D Mo2C on Cu surface by chemical vapor deposition method was revealed to be seed-induced 2D growth, limiting further synthesis of complex Mo2C spatial structures. In this research, we demonstrate the controlled growth of Mo2C pyramids with numerous morphologies, which are characterized with clear terraces within the structures. The whole evolution for Mo2C pyramids in the coursed of CVD process has been detected, posing significant potential in probing growth mechanism. The formation of the Mo2C pyramids arises from the supersaturation-induced nucleation and concentration-gradient driven diffused growth of a new Mo2C layer on the edged areas of intrinsic ones, as supported by STEM imaging. This work provides a novel Mo2C-based pyramid structure and further reveals a sliding growth mechanism, which could offer impetus for the design of new 3D spatial structures of Mo2C and other 2D materials.

Mo-interlayer-mediated thermal conductance at Cu/diamond interface measured by time-domain thermoreflectance

The interfacial thermal conductance (G) in Cu/diamond composites is generally determined by a deduction from thermal conductivity measurement of the composites. In this article, we prepare a Cu/Mo/diamond sandwich structure by magnetron sputtering to directly measure G between Cu and diamond using time-domain thermoreflectance and to probe the effect of the interlayer on G. The interlayer is tuned to a 105 nm-thick Mo layer on a 5 nm-thick Mo2C layer, to a full Mo2C layer, and to a Mo2C layer on a mixed Mo2C and fullerene layer with annealing temperature. The partial conversion of Mo interlayer benefits the thermal transport between Cu and diamond and a maximum G value of 132 MW m−2 K−1 is obtained for the case of 105 nm-thick Mo layer on 5 nm-thick Mo2C layer. This study highlights the importance of appropriate interlayer insertion to mediate thermal transport at Cu/diamond interface for thermal management applications.

Electric Field-Driven Interfacial Alloying for in Situ Fabrication of Nano-Mo2C on Carbon Fabric as Cathode toward Efficient Hydrogen Generation

A binderless composite cathode for efficient electrocatalytic hydrogen evolution reaction (HER), Mo2C-decorated carbon cloth (denoted as CC/MC), is simply fabricated via a novel and unique strategy which involves a solid-solid phase interfacial electrochemical reaction between carbon fiber and bulk-MoS2 in molten NaCl-KCl (700 °C). MoS2, evenly coated on carbon cloth (CC), is electrochemically reduced in situ and readily reacts with the carbon fibers of CC current collector to form a Mo2C nanoparticle layer. The experiment and calculation results show that the applied electric field results in a declining migration barrier of Mo vacancies in Mo2C lattice, which promotes the diffusion of Mo atoms into carbon across the interfacial Mo2C layer, thereby impelling the combination of Mo with C in depth. The electrochemical tests indicate that the optimized cathode (CC/MC-2) exhibits a small overpotential of 134.4 mV at 10 mA cm-2 and stays stable for HER in acidic media. The catalytic capacity for N2 reduction of CC/MC-2 is analyzed. In addition, a Ni-doped Mo2C-modified carbon fabric electrode with enhanced HER activity (η10 = 96.6 mV) can be prepared through a similar process.

Carbide-free one-zone sulfurization method grows thin MoS2 layers on polycrystalline CVD diamond

The last few decades faced on the fabrication of advanced engineering materials involving also different composites. Here, we report on the fabrication of few-layer molybdenum disulfide on top of thin polycrystalline diamond substrates with a high specific surface area. In the method, pre-deposited molybdenum coatings were sulfurized in a one-zone furnace at ambient pressure. As-prepared MoS2 layers were characterized by several techniques including grazing-incidence wide-angle X-ray scattering, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. We found out that the initial thickness of Mo films determined the final c-axis crystallographic orientation of MoS2 layer as previously observed on other substrates. Even though it is well-known that Mo diffuses into diamond at elevated temperatures, the competing sulfurization applied effectively suppressed the diffusion and a chemical reaction between molybdenum and diamond. In particular, a Mo2C layer does not form at the interface between the Mo film and diamond substrate. The combination of diamond high specific surface area along with a controllable layer orientation might be attractive for applications, such as water splitting or water disinfection.

Reliability of SiC Schottky Diodes with Mo2C Electrode

High current density electrical test was performed on SiC Schottky diodes. The shift in the Schottky barrier height was suppressed by adopting Mo2C as an electrode. Physical analysis revealed an epitaxial growth of Mo2C on SiC surface, which may be the origin of the stability. Introduction SiC Schottky barrier diodes (SBD) are used for high efficiency power conversion circuit owing to its high breakdown voltage, low ON-resistance and fast operation speed. However, pure metal and SiC surface reacts during high temperature annealing, forming an inhomogeneous interface [1]. Therefore, there is a concern in the diode characteristics change with time. We have shown diode characteristics with high temperature stability by using Mo2C as Schottky electrodes [2]. In this report, electric characteristics of SiC-SBD with Mo or Mo2C electrode when this devices is applied current of high current density and TEM analysis of Mo2C/SiC interface with high temperature stability are reported. Experiments A SiC wafer used in this study is an epitaxially grown 4°-off n-type 4H-SiC (0001) with a donor concentration of 1015cm-3 on a SiC substrate. Mo or Mo2C electrodes were sputter-deposited and patterned by reactive ion etching. The Mo and Mo2C samples were annealed in 500 or 850oC, respectively. Diode characteristics were measured before and after flowing a current density of 100A/cm2 for 500 s. Results and discussions The diode characteristics before and after the electrical current test are shown in fig. 1. Although increase in the Schottky barrier height was observed in both diodes, the amount of shift was suppressed with the Mo2C diode. This fact suggests that local interface reaction due to inhomogeneous current flow was suppressed. Moreover, only a slight degradation in the ideality factor was suppressed with the Mo2C electrode. Transmission electron microscope image of the diode, shown in fig. 2, revealed an epitaxial growth of a Mo2C layer on the SiC surface. The reliable diode characteristics might be originated from the sharp and uniform interface. Conclusion An electrical current test was performed on SiC Schottky diodes with Mo2C electrodes. A stable Schottky barrier height with a robust ideality factor was observed. TEM analysis revealed an epitaxial growth of Mo2C on the SiC surface with a sharp and uniform interface.

Mo2C coating on diamond: Different effects on thermal conductivity of diamond/Al and diamond/Cu composites

Mo2C submicron layer coated diamond particles prepared by a molten salts route with Mo powder as the starting material were used as the filler in Cu- and Al- matrix composites. The microstructure and thermal property of the composites prepared by a vacuum pressure infiltration method were investigated. When introducing a 500nm thick Mo2C layer, the thermal conductivity of the composites with different matrix presented different performance. A high thermal conductivity (657Wm−1K−1) was obtained in diamond/Cu composites owing to the improved interfacial bonding and lower interfacial thermal resistance, while the thermal conductivity of diamond/Al composites decreased from 553Wm−1K−1 to 218Wm−1K−1 when introducing the Mo2C layer, which can be attributed to the formation of harmful granule-phase (Al12Mo) at the interface of diamond and aluminum. This work provides a promising approach to improve performance of diamond reinforced metal matrix composites by selecting carbide as an interface modifier.

Electrochemical Synthesis of Nanomaterials in Molten Salts

For the first time by electrochemical synthesis from NaCl-KCl-NaF-GdF3-KBF4 melt at temperature 1023 K and with utilization of molybdenum cathode the gadolinium hexaboride nanotubes were synthesized. Most likely GdB6 nanotubes formation is ruled by intercalation mechanism. Gadolinium hexaboride lattice is the framework of boron atom octahedrons, in gaps of which the gadolinium atoms are situated. These gaps are spacious enough to host the metal atoms without deformation and even with reserve of vacant space. Electrochemical synthesis of GdB6 occurs at potentials negative enough for alkali metal cations electroreduction. So alkali metal cations intercalate into the emptiness of hexaboride lattice where cations are reduce in situ. The metal atoms have larger radius than its cations so they provoke mechanical strain. At the certain value of the strain GdB6 splits with nanotubes formation. Nanoneedles of TaO with tetragonal crystal lattice were obtained by electrolysis of CsCl melt, containing monooxofluoride complexes of tantalum. It was determined the correlation between the redox potential of molten system, temperature and stability of tantalum monoxide. Multi-layered tantalum boride structures were produced via electrolysis in the NaCl-KCl-NaF-K2TaF7-KBF4 melt. The overall thickness of the layers, synthesized under direct current regime, was 30-50 μm, and they consisted of numerous fine layers (nanolayers). A new generation of highly active and stable catalytic nanostructured coatings for the water-gas shift reaction by high temperature electrochemical synthesis in molten salts has been developed. The steady-state reaction rates for the Mo2C/Mo coatings were higher than those for the bulk Mo2C and commercial Cu/ZnO/Al2O3 catalysts over in the temperature range explored. The catalytic activity is enhanced by at three orders of magnitude comparing to that of the pure Mo2C phase. The methanation reaction was completely suppressed in the whole temperature range studied on the Mo2C/Mo systems. The catalytic activity remained constant during 5000 hours on-stream. The coatings were also stable during the thermal cycling, while the activity of commercial catalysts tends to decrease with time. A novel microstructured reactor/heat-exchanger containing eight sections with a cross-section of 10 mm x 10 mm and a length of 100 mm has been designed and constructed. Each section of the reactor contains flat, perforated Mo plates and Mo wires with a diameter of 250 mm and a length of 100 mm coated with a porous Mo2C layer. Nanoneedles of silicon were obtained in chloride-fluoride melts containing K2SiF6 by potentiostatic electrolysis which accompanied by reaction of disproportionation.

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti3SiC2 laminated composite is prepared by spark plasma sintering at 1300°C under a pressure of 50MPa. Al powder is used as sintering aid to assist the formation of Ti3SiC2. The fabricated composites were annealed at 800, 1000 and 1150°C under vacuum for 5, 10, 20 and 40h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo2C layer, AlMoSi layer and Ti5Si3 solid solution layer are formed during sintering. Experimental results show that the Mo/Ti3SiC2 layered composite prepared in this study has good interfacial phase stability up to at least 1000°C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150°C for 10h, cracks formed at the interface.

Preparation of a C/C–Cu composite with Mo2C coatings as a modification interlayer

To improve the wettability between molten copper and carbon and prepare C/C–Cu composites, Mo2C layers were fabricated throughout the internal surface of a C/C preform in molten salt (NaCl–KCl) with ammonium paramolybdate. The Mo2C layers obtained showed a thickness of approximately 1μm and uniformly covered the internal pore surface of the C/C preform. Mo2C layer formation transformed the non-wettable copper/carbon interface into Mo2C/carbon and wettable Cu/Mo2C interfaces. Good interfacial bonding was observed between Mo2C/C and Cu/Mo2C, and the carbon near the Mo2C layer showed an evidently high degree of orientation. Compared with the C/Cu composite currently used, the C/C–Cu composite prepared in this work showed higher flexural and impact strengths, as well as electrical resistivity reaching the same order of magnitude as that of pure copper.

In-situ fabrication of MoSi2/SiC–Mo2C gradient anti-oxidation coating on Mo substrate and the crucial effect of Mo2C barrier layer at high temperature

Abstract MoSi2/SiC–Mo2C gradient coating on molybdenum was in situ prepared with pack cementation process by two steps: (1) carburizing with graphite powder to obtain a Mo2C layer on Mo substrate, and (2) siliconizing with Si powder to get a composite MoSi2/SiC layer on the upper part of Mo2C layer. The microstructure and elemental distribution in the coating were investigated with scanning electron microscopy (SEM), back scattered electron (BSE), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). Cyclic oxidation tests (at 500 °C, 1200 °C, 1400 °C and 1600 °C) demonstrated excellent oxidation resistance for the gradient composite coating and the mass loss was only 0.23% in 60 min at 1600 °C. XRD, EPMA, thermal dynamic and phase diagram analyses indicated that the Mo2C barrier layer played the key role in slowing down the diffusion of C and Si toward inner Mo substrate at high temperature and principally this contributed to the excellent anti-oxidation for Mo besides the outer MoSi2/SiC composite layer.

Effects of temperature and Mo2C layer on stress and structural properties in CVD diamond film grown on Mo foil

Polycrystalline diamond films have been prepared by hot-filament-assisted chemical vapor deposition (HFCVD) on Mo foils. The morphology, growth rate, phase composition, element distribution and residual stress of the films at different temperature were investigated by field-emission scanning electron microscopy, Raman spectrum, field emission electron probe microanalysis and X-ray diffraction. Results show that the residual stress of the diamond films is compressive. The thermal stress plays a decisive role in the total stress, while the intrinsic stress can change the trend of the total stress. The residual stress of substrate gradually changes from tensile stress to compressive stress with the increase of the deposited temperature. A Mo2C interlayer is formed during deposition process, and this layer has an important influence on the stresses of films and substrates.

Effect of molybdenum as interfacial element on the thermal conductivity of diamond/Cu composites

As a carbide forming additive, molybdenum gets maximum benefit of high thermal conductivities of diamond and copper. Coating Mo on the diamond surface can promote the interfacial bonding in diamond/Cu composites. A Mo coated diamond/Cu composite with high thermal conductivity of 726W/mK is obtained, which is achieved by a thin nano-sized Mo2C layer. Mo coated diamond or un-coated diamond reinforced copper composites have been made by pressure assisted liquid copper infiltration method. The interfacial configuration, the bulk thermal conductivity and the evolution of interfacial thermal resistance are presented, indicating that the good adhesion at the interface can decrease the thermal boundary resistance and thus increase the thermal conductivity of the composites.