Intrinsic effects of thickness, surface chemistry and electroactive area on nanostructured MoS2 electrodes with superior stability for hydrogen evolution

Compaction of Hydrophobic Molybdenum Disulfide Coatings for Promoting Tribological Behaviors on Engineering Steel

Ambient environment has a crucial effect on the lubrication performance of molybdenum disulfide (MoS₂) coatings. In this work, we fabricated porous MoS₂ coatings via a facile optimized aerosol-assisted chemical vapor deposition (AACVD) method. It is found that the obtained MoS₂ coating demonstrates outstanding antifriction and antiwear lubrication performance with the coefficient of friction (COF) and wear rate as low as 0.035 and 3.4 × 10^-7 mm³/Nm in lower humidity (15 ± 5)%, respectively, which is comparable to the lubrication ability of pure MoS₂ in vacuum. In addition, the hydrophobic property of porous MoS₂ coatings is suitable for infusing lubrication oil to achieve stable solid-liquid lubrication in higher humidity (85 ± 2)%. The composite lubrication system shows excellent tribological behavior in both dry and wet environments, which will alleviate the sensitivity of the MoS₂ coating to the environment and ensure the service life of the engineering steel in complex industrial backgrounds.

A Low-Temperature Synthetic Route Toward a High-Entropy 2D Hexernary Transition Metal Dichalcogenide for Hydrogen Evolution Electrocatalysis

High-entropy (HE) metal chalcogenides are a class of materials that have great potential in applications such as thermoelectrics and electrocatalysis. Layered 2D transition-metal dichalcogenides (TMDCs) are a sub-class of high entropy metal chalcogenides that have received little attention to date as their preparation currently involves complicated, energy-intensive, or hazardous synthetic steps. To address this, a low-temperature (500 °C) and rapid (1 h) single source precursor approach is successfully adopted to synthesize the hexernary high-entropy metal disulfide (MoWReMnCr)S2 . (MoWReMnCr)S2 powders are characterized by powder X-ray diffraction (pXRD) and Raman spectroscopy, which confirmed that the material is comprised predominantly of a hexagonal phase. The surface oxidation states and elemental compositions are studied by X-ray photoelectron spectroscopy (XPS) whilst the bulk morphology and elemental stoichiometry with spatial distribution is determined by scanning electron microscopy (SEM) with elemental mapping information acquired from energy-dispersive X-ray (EDX) spectroscopy. The bulk, layered material is subsequently exfoliated to ultra-thin, several-layer 2D nanosheets by liquid-phase exfoliation (LPE). The resulting few-layer HE (MoWReMnCr)S2 nanosheets are found to contain a homogeneous elemental distribution of metals at the nanoscale by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) with EDX mapping. Finally, (MoWReMnCr)S2 is demonstrated as a hydrogen evolution electrocatalyst and compared to 2H-MoS2 synthesized using the molecular precursor approach. (MoWReMnCr)S2 with 20% w/w of high-conductivity carbon black displays a low overpotential of 229 mV in 0.5 M  H2 SO4 to reach a current density of 10 mA cm-2 , which is much lower than the overpotential of 362 mV for MoS2 . From density functional theory calculations, it is hypothesised that the enhanced catalytic activity is due to activation of the basal plane upon incorporation of other elements into the 2H-MoS2 structure, in particular, the first row TMs Cr and Mn.

Facile and Scalable Synthesis of Self-Supported Zn-Doped CuO Nanosheet Arrays for Efficient Nitrate Reduction to Ammonium

CuO has been regarded as a promising catalyst for the electrochemical reduction of nitrate (NO₃^-RR) to ammonium (NH₃); however, the intrinsic activity is greatly restricted by its poor electrical property. In this work, self-supported Zn-doped CuO nanosheet arrays (Zn-CuO NAs) are synthesized for NO₃^-RR, where the Zn dopant regulates the electronic structure of CuO to significantly accelerate the interfacial charge transfer and inner electron transport kinetics. The Zn-CuO NAs are constructed by a one-step etching of commercial brass (Cu₆₄Zn₃₆ alloy) in 0.1 M NaOH solution, which experiences a corrosion-oxidation-reconstruction process. Initially, the brass undergoes a dealloying procedure to produce nanosized Cu, which is immediately oxidized to the Cu₂O unit with a low valence state. Subsequently, Cu₂O is further oxidized to the CuO unit and reconstructed into nanosheets with the coprecipitation of Zn²⁺. For NO₃^-RR, Zn-CuO NAs show a high NH₃ production rate of 945.1 μg h^-1 cm^-2 and a Faradaic efficiency of up to 95.6% at -0.7 V in 0.1 M Na₂SO₄ electrolyte with 0.01 M NaNO₃, which outperforms the majority of the state-of-the-art catalysts. The present work offers a facile yet very efficient strategy for the scale-up synthesis of Zn-CuO NAs for high-performance NH₃ production from NO₃^-RR.

Strategies to improve electrocatalytic performance of MoS2-based catalysts for hydrogen evolution reactions

Electrocatalytic hydrogen evolution reactions (HERs) are a key process for hydrogen production for clean energy applications. HERs have unique advantages in terms of energy efficiency and product separation compared to other methods. Molybdenum disulfide (MoS2) has attracted extensive attention as a potential HER catalyst because of its high electrocatalytic activity. However, the HER performance of MoS2 needs to be improved to make it competitive with conventional Pt-based catalysts. Herein, we summarize three typical strategies for promoting the HER performance, i.e., defect engineering, heterostructure formation, and heteroatom doping. We also summarize the computational density functional theory (DFT) methods used to obtain insight that can guide the construction of MoS2-based materials. Additionally, the challenges and prospects of MoS2-based catalysts for the HER have also been discussed.

Mussel-Inspired Interfacial Modification for Ultra-Stable MoS2 Lubricating Films with Improved Tribological Behavior on Nano-Textured ZnO Surfaces Using the AACVD Method

Friction-associated energy loss and mechanical wear leading to failure is a major problem in industries. To mitigate this, the design and testing of novel lubricants is important. Here, we show the facile one-pot synthesis of ZnO/ZnO nanorods (NRs) and MoS2 films on pre-treated glass substrates via aerosol-assisted chemical vapor deposition. The bearing capacity and wear life of ZnO film/ZnO NRs/MoS2 films were improved due to the lubricant retention capabilities of the NRs. Modification of the ZnO/MoS2 nano-arrays using polydopamine (PDA) allowed the realization of robust and ultra-stable solid lubricants through the triple action of chemical chelation, layered materials, and nanotexture, especially under heavy load conditions. Compared with pristine MoS2, the adhesion and bearing strength of the composite film increased by 11 and 30 times, respectively, while the coefficient of friction and wear rate decreased by 94 and 85%, respectively. This is because the chelation between the transition metal and the groups in the interlayer PDA was fully utilized to improve the interface compatibility, which significantly improves the robustness of ZnO NRs and the adhesion of MoS2. This allowed a stable and firm mechanical lock between the substrate, lubricant films, and the steel ball. It demonstrated a convenient method to achieve the antifriction and anti-wear of solid lubricating materials by PDA interface modification for practical industrial applications.