Stabilization and superconductivity of AlB2-type nonstoichiometric molybdenum diboride by Sc doping

Molybdenum diboride is unique among transition metal diborides because it exists in both hexagonal (AlB₂-type) and rhombohedral structures. However, it is difficult to stabilize the superconducting AlB₂-type phase, which requires either extreme synthesis condition or suitable chemical doping. Here we report the structural and physical properties of Sc-doped nonstoichiometric molybdenum diborides (Mo_0.95Sc_0.05)_1-xB₂ and (Mo_1-ySc_y)_0.71B₂ prepared by the common arc melting method. The AlB₂-type phase is found to form over wide ranges of 0 ≤ x ≤ 0.29 and 0.025 ≤ y ≤ 0.30 for the first time, and bulk superconductivity with T_c up to 7.9 K is observed. T_c increases with increasing x in the (Mo_0.95Sc_0.05)_1-xB₂ series, but evolves nonmonotonically with varying y in the (Mo_1-ySc_y)_0.71B₂ series. Despite this contrast, T_c of both borides follows nearly the same linear dependence on the electron-phonon coupling constant, suggesting that it is mainly controlled by the electron-phonon interaction. In addition, the stabilization of AlB₂-type structure is attributed to the decrease in the number of d electrons as a consequence of Sc doping, which suggests that a similar effect may be achieved by substituting Mo with other d electron-poorer metal elements.     

Green synthesis of molybdenum-based nanoparticles and their applications in energy conversion and storage: A review

In the scope of the rapid technological advancements, nanoparticles (NPs) have gained prominence due to their excellent and tunable biological, and physicochemical properties. Nowadays, different methods are used for their synthesis. In particular, the green synthesis of metal precursors for the synthesis of NPs, represents a cost-effective, environmentally friendly, and hazardous chemical-free method for developing a large variety of NPs. By exploiting plant extracts, the production rate of NPs is relatively faster. Due to fossil reserves and high fuel consumption, renewable and clean energy materials are urgently needed to improve environmental sustainability. With outstanding electrochemical and physicochemical characteristics, molybdenum-based NPs (Mo-NPs) are gaining increasing attention in the fields of energy conversion and storage. Considering the significance of Mo-NPs synthesized from greener routes and their energy applications, it is necessary to review recent trends and developments in this field. This review summarizes important research studies and future research guidelines for the preparation of Mo-NPs through green routes and their applications to meet global energy and environmental demands. Moreover, future research directions are also highlighted to achieve sustainable greener precursors and Mo-NPs based energy storage devices.     

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties

The mechanical behavior of ZrB₂-MoSi₂ ceramics made of ZrB₂ powder with three different particle sizes and MoSi₂ additions from 5 to 70 vol% was characterized up to 1500 °C. Microhardness (12–17 GPa), Young’s modulus (450–540 GPa) and shear modulus (190–240 GPa) decreased with both increasing MoSi₂ content and with decreasing ZrB₂ grain size. Room temperature fracture toughness was unaffected by grain size or silicide content, whilst at 1500 °C in air it increased with MoSi₂ and ZrB₂ grain size, from 4.1 to 8.7 MPa m^½. Room temperature strength did not trend with MoSi₂ content, but increased with decreasing ZrB₂ grain size from 440 to 590 MPa for the largest starting particle size to 700–800 MPa for the finest due to the decreasing size of surface grain pullout. At 1500 °C, flexure strength for ZrB₂ with MoSi₂ contents above 25 vol% were roughly constant, 400–450 MPa, whilst for lower content strength was controlled by oxidation damages. Strength for compositions made using fine and medium ZrB₂ powders increased with increasing MoSi₂ content, 250–450 MPa. Ceramics made with coarse ZrB₂ displayed the highest strengths, which decreased with increasing MoSi₂ content from 600 to 450 MPa.     

Efficient overall water splitting over a Mo(IV)-doped Co3O4/NC electrocatalyst

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co₃O₄) supported on nitrogen (N)-doped carbon (x%Mo–Co₃O₄/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo⁴⁺) is found to be doped into Co₃O₄ by substituting cobalt ion (Co²⁺) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co₃O₄/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm^?2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co₃O₄/NC||4%Mo–Co₃O₄/NC displays a voltage of 1.62 V to deliver 10 mA cm^?2 in 1 M KOH. The Mo⁴⁺ dopant modulates the electronic structure of active cobalt ion (Co³⁺) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co₃O₄/NC, resulting in its superior bifunctional catalytic activity.     

Enhanced visible-light-assisted photocatalytic hydrogen generation by MoS2/g-C3N4 nanocomposites

A facile, one-pot, solvothermal synthesis of MoS₂ microflowers (S1) and the heterostructures MoS₂/g-C₃N₄ with varying ratios of 1:1 (S2), 1:2 (S3) and 1:3 (S4) exhibiting enhanced visible-light-assisted H₂ generation by water splitting has been reported. The compounds were thoroughly characterized by PXRD, FESEM, HRTEM, EDS, UV–vis and XPS techniques. FESEM and HRTEM analyses showed the presence of microflowers composed of nano-sized petals in case of pure MoS₂ (S1), while the MoS₂ microflowers covered with g-C₃N₄ nanosheets in case of MoS₂/g-C₃N₄ heterostructure, S4. XPS analysis of S2 showed the presence of 2H phase of MoS₂ with g-C₃N₄. The Eosin-Y/dye-sensitized visible-light-assisted photocatalytic investigation of the samples in the absence of any noble metal co-catalyst revealed very good water splitting activity of MoS₂/g-C₃N₄ heterostructure, S2 with hydrogen generation rate of 1787 μmol h⁻¹g⁻¹ which is about 6 and 40 times higher than pure MoS₂ and g-C₃N₄ respectively. The relatively higher catalytic activity of the heterostructure, S2 has been ascribed to the efficient spatial separation of photo-induced charge carriers owing to the synergistic interaction between MoS₂ and g-C₃N₄. A possible mechanism for the Eosin-Y-sensitized photocatalytic H₂ generation activity of MoS₂/g-C₃N₄ heterostructures has also been presented. The enhanced activity of S2 was further supported by fluorescence measurements. Thus, the present study highlights the importance of non-noble metal based MoS₂/g-C₃N₄ heterojunction photocatalysts for efficient visible-light-driven H₂ production from water splitting.     

Three-dimensional Ni2P–MoP2 mesoporous nanorods array as self-standing electrocatalyst for highly efficient hydrogen evolution

Electrochemical hydrogen evolution reaction (HER) via the splitting of water has required electrocatalysts with cost-effectiveness, environmentally friendliness, high catalytic activity, and superior stability to meet the hydrogen economy in future. In this context, we report the successful synthesis of self-standing mesoporous Ni₂P–MoP₂ nanorod arrays on nickel foam (Ni₂P–MoP₂ NRs/3D-NF) through an effective phosphidization of the corresponding NiMoO₄ NRs/3D-NF. The as-synthesis Ni₂P–MoP₂ NRs/3D-NF, as an efficient HER electrocatalyst, exhibits small overpotential of 82.2 and 124.7 mV to reach current density of 10 and 50 mA cm⁻², a low Tafel slope of 52.9 mV dec⁻¹ and it retains its catalytic performance for at least 20 h in alkaline condition. Our work also offers a new strategy in designing and using transition metal phosphide-based 3D nanoarrays catalysts with enhanced catalytic efficiency for mass production of hydrogen fuels.     

Efficient hydrogen evolution catalyzed by amorphous molybdenum sulfide/N-doped active carbon hybrid on carbon fiber paper

Among numerous noble-metal-free electrocatalysts, molybdenum sulfides are recognized as promising candidates for hydrogen evolution reaction (HER). Owing to abundant under-coordinated sulfur atoms serving as catalytically active centers, intensive spectra studies have revealed that the HER property of amorphous molybdenum sulfide (a-MoS_x) is superior to crystal molybdenum sulfide (c-MoS₂). In this work, nitrogen-doped active carbon (NAC) is obtained through plasma treatment and amorphous MoS_x/NAC hybrid catalyst films are electrodeposited on carbon fiber papers (CFP) which are employed as porous three-dimensional electrodes with low resistivity. The incorporation of NAC increases the electrochemically active surface area, enhances the electron transport, and facilitates the reaction kinetics. Moreover, the functionality durability benefits from synergistic effect between a-MoS_x and NAC. Thus, the obtained hybrid catalyst delivers the excellent HER activity, requiring only 203 mV overpotential to achieve a geometrical current density of 100 mA cm^?2 with a Tafel slope of ～43.9 mV per decade.     

国产特殊装备用润滑脂的研制

为满足特殊装备使用中的润滑需求,选取聚α-烯烃和酯类油的混配油作为基础油,锂皂作为稠化剂,以纳米MoS_2作为极压抗磨剂,月桂酰胺丙基氧化胺作为助分散剂,月桂酰胺丙基氧化胺与二硫化钼质量比为1∶5,选用酚胺混合型抗氧剂,加入防锈剂C 1%,通过直接皂化的方法制备了一种新型润滑脂,纳米MoS_2和月桂酰胺丙基氧化胺在皂化反应阶段加入。该润滑脂的各项理化性能与现用商品润滑脂相当,且解决了现用润滑脂固体添加剂的团聚和沉降问题,提高了润滑脂的分散稳定性,综合性能优良,可以满足某特殊装备的润滑使用需求。     

Investigation of the versatility of SPES membranes customized with sulfonated molybdenum disulfide nanosheets for DMFC applications

Sulfonated poly(ether sulfone) (SPES) based proton exchange membranes (PEMs) are fabricated using sulfonated molybdenum disulfide (S-MoS₂) nanosheets via facile solution casting method. SPES (DS = 30%) and S-MoS₂ are synthesized and sulfonation is evidently observed in FTIR and XRD analysis. The anchoring of sulfonic acid group on exfoliated molybdenum disulfide (E-MoS₂) and elemental composition of S-MoS₂ are confirmed by XPS spectrum. Physico-chemical characteristics such as ion-exchange capacity (IEC), water uptake, swelling ratio and oxidation stability are found to be increases after the addition of S-MoS₂ into SPES matrix. Increment in S-MoS₂ content in SPES matrix decreases the surface contact angle due to the increase in hydrophilicity. Further, the dispersing ability of S-MoS₂ in SPES matrix is evidently shown by an increase in surface roughness, tensile strength and thermal stability of the SPES/S-MoS₂ nanocomposite membranes. On the whole, SPES/S-MoS₂-1 membrane showed the highest proton conductivity of 5.98 × 10⁻³ Scm⁻¹, selectivity of 19.6 × 10⁴ Scm⁻³s, peak power density of 28.28 mWcm⁻² and lesser methanol permeability of 3.05 × 10⁻⁸ cm²s⁻¹. The strong interfacial interaction between SPES and S-MoS₂ in nanocomposite membranes create strong hydrogen bond network to facilitate the proton conduction pathway via both vehicle and Grotthuss type mechanisms. Overall results suggested that the SPES/S-MoS₂ nanocomposite membranes are superior and appropriate alternative for commercially high-cost Nafion® membranes for use in renewable direct methanol fuel cell (DMFC) devices.     

Size-controlled synthesis of Mo powders via hydrogen reduction of MoO2 powders with the assistance of Mo nuclei

The size-controlled preparation of Mo powders is always a challenge and important task in the molybdenum metallurgy. In the current study, Mo powders with controllable sizes are synthesized by hydrogen reduction of MoO₂ powders with the assistance of Mo nuclei in the range of 900–1100 °C. The influences of the particle sizes of Mo nuclei, the additive amount as well as reaction temperature on the morphology and particle sizes of the final products are studied. For the hydrogen reduction of MoO₂ without any additive, the obtained Mo powders always have large particle sizes. However, the addition of small amounts of nuclei in MoO₂ can help Mo nucleate dispersedly, and the growth of Mo could be also controlled by adjusting the sizes of added nuclei, amount of addition and the reaction temperature. With the addition of Mo nuclei, the different sizes of Mo powders with the good dispersity can be prepared. As adding commercial Mo powders with the particle size of about 2.03 μm, the micron-sized Mo powders ranged from 2.11 μm to 3.25 μm could be prepared. While for the case of adding ultrafine Mo nuclei of about 170 nm, Mo powders from 0.28 μm to 0.88 μm can be obtained. Moreover, the more the amounts of nuclei added and the lower the reaction temperature (in the range of 900–1100 °C) is, the smaller the particle size of the prepared Mo powder will be. The current method is a facile and feasible method, and is potential to be used for industrial production of Mo powder with controllable particle sizes.