Sulphur-Depleted Monolayered Molybdenum Disulfide Nanocrystals for Superelectrochemical Hydrogen Evolution Reaction

Catalytically driven electrochemical hydrogen evolution reaction (HER) of monolayered molybdenum disulfide (MoS2) is usually highly suppressed by the scarcity of edges and low electrical conductivity. Here, we show how the catalytic performance of MoS2 monolayers can be improved dramatically by catalyst size reduction and surface sulfur (S) depletion. Monolayered MoS2 nanocrystals (NCs) (2–25 nm) produced via exfoliating and disintegrating their bulk counterparts showed improved catalysis rates over monolayer sheets because of their increased edge ratios and metallicity. Subsequent S depletion of these NCs further improved the metallicity and made Mo atoms on the basal plane become catalytically active. As a result, the S-depleted NCs with low mass (∼1.2 μg) showed super high catalytic performance on HER with a low Tafel slope of ∼29 mV/decade, overpotentials of 60–75 mV, and high current densities jx (where x is in mV) of j150 = 9.64 mA·cm–2 and j200 = 52.13 mA·cm–2. We have found that higher production rates of H2 could not be achieved by adding more NC layers since HER only happens on the topmost surface and the charge mobility decreases dramatically. These difficulties can be largely alleviated by creating a hybrid structure of NCs immobilized onto three-dimensional graphene to provide a very high surface exposure of the catalyst for electrochemical HER, resulting in very high current densities of j150 = 49.5 mA·cm–2 and j200 = 232 mA·cm–2 with ∼14.3 μg of NCs. Our experimental and theoretical studies show how careful design and modification of nanoscale materials/structures can result in highly efficient catalysis. There may be considerable opportunities in the broader family of transition metal dichalcogenides beyond just MoS2 to develop highly efficient atomically thin catalysts. These could offer cheap and effective replacement of precious metal catalysts in clean energy production

Experimental study on the variation law of methanol solubility in methanol-refined oil mixtures

ObjectiveMethanol, an emerging energy source, is vital for the energy transition. The batch transportation of methanol through refined oil pipelines reduce the long-distance methanol transportation cost and alleviate the low throughput of refined oil pipelines. During the batch transportation of methanol and refined oil, mixed oil is inevitable. The miscibility of methanol and refined oil as well as methanol residue in refined oil are crucial for selecting the separation and treatment processes of the mixed oil. MethodsA liquid-liquid phase equilibrium experimental setup was independently designed and built to investigate the effects of methanol volume fraction, water content, temperature, and other factors on the miscibility of methanol and refined oil. Gas chromatography-mass spectrometry (GC-MS) was employed to determine the methanol content in the gasoline/diesel layer, revealing the variation in methanol concentration in refined oil under different conditions. ResultsIn the temperature range of −10 °C to 50 °C, methanol and gasoline exhibited good miscibility. However, due to polarity differences, as the methanol volume fraction increased, their miscibility first decreased and then increased. Methanol and diesel had poor miscibility and were hard to fully mix, yet trace amounts of methanol could dissolve in the diesel layer. As the temperature dropped, methanol droplets precipitated in the diesel layer, causing turbidity. Simultaneously, as the methanol volume fraction in the mixture rose, the methanol concentration in the gasoline/diesel layer first increased and then decreased. In the gasoline layer, the methanol mass concentration reached a maximum of 69.91 g/L at a 30％ methanol volume fraction; in the diesel layer, it peaked at 17.24 g/L at a 50％ volume fraction. Lowering the temperature and increasing the water content of methanol significantly reduced methanol solubility in refined oil. As the temperature continued to fall, the methanol mass concentration in the gasoline and diesel layers decreased from 137.36 g/L to 61.84 g/L and from 17.24 g/L to 5.16 g/L, respectively. When the water content of methanol increased from 0.50％ to 1.25％, the corresponding mass concentrations in the gasoline and diesel layers decreased from 82.73 g/L to 48.65 g/L and from 30.73 g/L to 12.75 g/L, respectively. ConclusionThe solubility of methanol in refined oil is influenced by various factors, including methanol volume fraction, temperature, water content, and oil type. It is advisable to employ techniques such as distillation, gravity separation, and centrifugation for separating the mixed oil during the batch transportation of methanol and refined oil

The Preparation and Characterisation of Graphene and Its Analogues

The studies in this thesis give deep insights on the large scale preparation of graphene and the fabrication and properties of novel monolayered quantum dots (QDs). Graphene has received remarkable attention due to its interesting physical and chemical properties. Among various preparations for graphene, the solvothermal deoxidation of graphene oxide (GO) is highly attractive as it potentially offers a relatively economical and scalable manufacturing route for use in industrial applications. Unfortunately, the deep deoxidation of GO and highly dispersable reduced GO (rGO) are difficult to achieve using this approach, although the reasons for this deoxidation remain unclear. This thesis shows that the agglomeration/self-assembly of partially reduced GO (p-rGO) sheets in the solvothermal deoxidation reaction suppresses the deep deoxidation of GO and led to low dispersibility/electrical conductivity of the product. By tuning the surface energy of the solvent to minimize the surface enthalpy of the dispersion, these technical problems can be ameliorated and full deoxidation of GO with high dispersibility and electrical conductivity achieved. In this thesis, an alternative novel and effective route to fabricating graphene QDs (GQDs, lateral size ~ 20 nm) is also described. This technique of delaminating layered structures has also been developed to produce monolayered QDs of boron nitride (BN, lateral size of ~ 10 nm), tungsten disulfide (WS2, lateral size ~ 8-15 nm) and molybdenum disulfide (MoS2, lateral size of ~ 8-20 nm). This has opened up many opportunities in studying these interesting materials with reduced dimensions, with new behaviours and properties emerging from the various QDs. The zigzag edges of GQDs led to the appearance of new band gaps and give strong blue-green luminescence centred at 420 nm wavelength (quantum yield of ~7.6%). In monolayered BN QDs, carbene-replaced zigzag edges, carbon-replaced N vacancy point and BOx- (x = 1 and 2) species added new luminescence at around 425 nm wavelength (quantum yield of ~2.5%). Strong luminescence was created by the reduced dimensions of WS2 and MoS2 monolayered QDs causing them to became direct semiconductors. The reduced lateral dimensions also caused marked quantum confinement effects to arise, such as large blue shifts in absorption features of BN, WS2 and MoS2 monolayered QDs. The formation of monolayered WS2 and MoS2 QDs also led to their valance bands being split by giant spin-orbit coupling effects to a far greater degree than is observed form monolayered sheets. The studies suggest strongly that these features are likely to be tunable with lateral dimensions, which makes the QDs potentially very interesting for applications. Although these uses may include spintronics, optoelectronics and even quantum computing, their application in biology is demonstrated by all the monolayered QDs being used as non-toxic fluorescent labels in confocal microscopy of biological cells

Effects of boron content on microstructure and wear properties of FeCoCrNiBx high-entropy alloy coating by laser cladding

The FeCoCrNiBx high-entropy alloy (HEA) coatings with three different boron (B) contents were synthesized on Q245R steel (American grade: SA515 Gr60) by laser cladding deposition technology. Effects of B content on the microstructure and wear properties of FeCoCrNiBx HEA coating were investigated. In this study, the phase composition, microstructure, micro-hardness, and wear resistance (rolling friction) were investigated by X-ray diffraction (XRD), a scanning electron microscope (SEM), a micro hardness tester, and a roller friction wear tester, respectively. The FeCoCrNiBx coatings exhibited a typical dendritic and interdendritic structure, and the microstructure was refined with the increase of B content. Additionally, the coatings were found to be a simple face-centered cubic (FCC) solid solution with borides. In terms of mechanical properties, the hardness and wear resistance ability of the coating can be enhanced with the increase of the B content, and the maximum hardness value of three HEA coatings reached around 1025 HV0.2, which is higher than the hardness of the substrate material. It is suggested that the present fabricated HEA coatings possess potentials in application of wear resistance structures for Q245R steel