Graphene has been recognized as an excellent lubrication material owing to its two-dimensional structure and weak interlayer interactions. However, most extant works concerning superlubricity ...involving graphene oxide have been limited to nanoscale or microscale dimensions (of the order of 1–10 μm). In present work, realization of a robust macroscale superlubricity state (μ = 0.0037), by taking advantage of the synergy effect of graphene-oxide nanoflakes (GONFs) and ethanediol (EDO) at Si3N4–SiO2 interfaces is reported. GONFs have been observed as being adsorbed on friction surfaces, thereby preventing direct contact between surface asperities. The extremely low shear stresses developed between these asperities contribute toward enhanced superlubricity and the resulting super-low wear. Meanwhile, the formation of partial-slip hydrodynamic boundary condition at the GONFs–EDO interface along with the formation of hydrated GONFs–EDO networks through hydrogen-bond interactions contribute to the generation of extremely low shear stresses of the liquid lubricating film. Such macroscale superlubricity provides a new approach toward realization of extremely low friction in GONFs through the synergy effect with liquids.
A simple one‐pot solvothermal method is reported to synthesize VS2 nanosheets featuring rich defects and an expanded (001) interlayer spacing as large as 1.00 nm, which is a ≈74% expansion as ...relative to that (0.575 nm) of the pristine counterpart. The interlayer‐expanded VS2 nanosheets show extraordinary kinetic metrics for electrocatalytic hydrogen evolution reaction (HER), exhibiting a low overpotential of 43 mV at a geometric current density of 10 mA cm−2, a small Tafel slope of 36 mV dec−1, and long‐term stability of 60 h without any current fading. The performance is much better than that of the pristine VS2 with a normal interlayer spacing, and even comparable to that of the commercial Pt/C electrocatalyst. The outstanding electrocatalytic activity is attributed to the expanded interlayer distance and the generated rich defects. Increased numbers of exposed active sites and modified electronic structures are achieved, resulting in an optimal free energy of hydrogen adsorption (∆GH) from density functional theory calculations. This work opens up a new door for developing transition‐metal dichalcogenide nanosheets as high active HER electrocatalysts by interlayer and defect engineering.
VS2 nanosheets featuring rich defects and an expanded (001) interlayer spacing as large as 1.00 nm are synthesized by a solvothermal method and demonstrate extraordinary kinetic metrics for the hydrogen evolution reaction. The electrocatalytic activity enhancement is attributed to the optimal free energy of hydrogen adsorption (∆GH) based on the molecular simulation.
Superlubricity, the state of ultralow friction between two sliding surfaces, has become a frontier subject in tribology. Here, a state-of-the-art review of the phenomena and mechanisms of liquid ...superlubricity are presented based on our ten-year research, to unlock the secrets behind liquid superlubricity, a major approach to achieve superlubricity. An overview of the discovery of liquid superlubricity materials is presented from five different categories, including water and acid-based solutions, hydrated materials, ionic liquids (ILs), two-dimensional (2D) materials as lubricant additives, and oil-based lubricants, to show the hydrodynamic and hydration contributions to liquid superlubricity. The review also discusses four methods to further expand superlubricity by solving the challenge of lubricants that have a high load-carrying capacity with a low shear resistance, including enhancing the hydration contribution by strengthening the hydration strength of lubricants, designing friction surfaces with higher negative surface charge densities, simultaneously combining hydration and hydrodynamic contribution, and using 2D materials (e.g., graphene and black phosphorus) to separate the contact of asperities. Furthermore, uniform mechanisms of liquid superlubricity have been summarized for different liquid lubricants at the boundary, mixed, and hydrodynamic lubrication regimes. To the best of our knowledge, almost all the immense progresses of the exciting topic, superlubricity, since the first theoretical prediction in the early 1990s, focus on uniform superlubricity mechanisms. This review aims to guide the research direction of liquid superlubricity in the future and to further expand liquid superlubricity, whether in a theoretical research or engineering applications, ultimately enabling a sustainable state of ultra-low friction and ultra-low wear as well as transformative improvements in the efficiency of mechanical systems and human bodies.
Several ionic liquids (ILs) are formed in situ with monovalent metal salts and ethylene glycol (EG). The macroscale superlubricity and antiwear properties of the ILs were studied between ceramic ...materials. Superlow coefficients of friction of less than 0.01 could be obtained for all ILs at silicon nitride (Si3N4) interfaces induced by tribochemical reactions. Notably, the IL (Li(EG)PF6) formed with LiPF6 and EG exhibited the greatest superlubricity and antiwear properties. The results of film thickness calculations and surface analysis showed that the lubrication regime during the superlubricity period was the mixed lubrication, and a composite tribochemical layer (composed of phosphates, fluorides, silica (SiO2), and ammonia-containing compounds), hydration layer, and fluid film contributed to superlubricity and wear protection. It was found that the small size of metal cations was beneficial for alleviating wear, and PF6 – anions exhibited the smallest friction and best antiwear performance at Si3N4 interfaces. This work studied the lubricity and antiwear properties of ILs with different cations and anions, enriching the range of alternative ILs for macroscale superlubricity and low wear, and is of importance to engineering applications.
There have been insistent demands for effective lubricants as well as additives for reducing friction and wear in various applications. Recently, graphene oxide (GO) and nanodiamond (ND) have been ...observed to be promising materials to reduce friction and wear in tribological studies. Herein, a novel lubricant has been developed by using GO and ND together as lubricant additives in water in order to enhance tribological performance. The tribopairs employed were Si3N4 balls and Si wafers, and the applied normal load was 5–80 mN. The lowest friction coefficient of approximately 0.03 was obtained for water with 0.1 wt.% GO and 0.5 wt.% ND, while the corresponding depth of wear track was as low as about 5 nm. Analysis of the tribological mechanisms elucidated that the sliding-induced nanostructured tribofilm, the low shearing resistance between graphene sheets, and the possible ball bearing effect of ND all contributed to the remarkable tribological behaviors of the lubricant.
•GO and ND are used simultaneously as solid additives in water.•Low friction coefficient of about 0.03 and tiny wear track depth of about 5 nm were obtained.•The sliding-induced tribo-films were investigated by TEM to reveal the tribological mechanisms.
Abstract
Spin–orbit torque has recently been intensively investigated for the purposes of manipulating the magnetization in magnetic nano-devices and understanding fundamental physics. Therefore, the ...search for novel materials or material combinations that exhibit a strong enough spin-torque effect has become one of the top priorities in this field of spintronics. Weyl semimetal, a new topological material that features open Fermi arc with strong spin–orbit coupling and spin–momentum locking effect, is naturally expected to exhibit an enhanced spin-torque effect in magnetic nano-devices. Here we observe a significantly enhanced spin conductivity, which is associated with the field-like torque at low temperatures. The enhancement is obtained in the
b
-axis WTe
2
/Py bilayers of nano-devices but not observed in the
a
-axis of WTe
2
/Py nano-devices, which can be ascribed to the enhanced spin accumulation by the spin–momentum locking effect of the Fermi arcs of the Weyl semimetal WTe
2
.
Hydrogenated amorphous carbon (a-C:H) is capable of providing a near-frictionless lubrication state when rubbed in dry sliding contacts. Nevertheless, the mechanisms governing superlubricity in a-C:H ...are still not well comprehended, mainly due to the lack of spatially resolved structural information of the buried contact surface. Here, we present structural analysis of the carbonaceous sliding interfaces at the atomic scale in two superlubricious solid lubricants, a-C:H and Si-doped a-C:H (a-C:H:Si), by probing the contact area using state-of-the-art scanning electron transmission microscopy and electron energy-loss spectroscopy. The results emphasize the diversity of superlubricity mechanisms in a-C:Hs. They suggest that the occurrence of a superlubricious state is generally dependent on the formation of interfacial nanostructures, mainly a tribolayer, by different carbon rehybridization pathways. The evolution of such anti-friction nanostructures highly depends on the contact mechanics and the counterpart material. These findings enable a more effective manipulation of superlubricity and developments of new carbon lubricants with robust lubrication properties.
In present work, a superlubricity phenomenon of phosphoric acid (H3PO4) was found under ambient conditions. An ultralow friction coefficient of about 0.004 between glass/Si3N4 and sapphire/sapphire ...tribopairs was obtained under the lubrication of a phosphoric acid aqueous solution (pH 1.5) at high contact pressure (the maximum pressure can reach about 1.65 GPa) after a running-in period of about 600 s. The experimental results indicate that the superlow friction state was very stable for more than 3 h. In such a state, solidlike films formed on the two sliding surfaces, which are hydrates of phosphoric acid with a hydrogen-bonded network according to the Raman spectrum. The superlubricity mechanism is mainly attributed to the hydrogen bond effect that forms a hydrated water layer with low shearing strength, and the dipole–dipole effects that form an interfacial Coulomb repulsion force also make some contributions to low friction. This work may help us to introduce a new approach to superlubricity and may lead to the wide application of superlubricity in future technological and biomedical areas.
The robust liquid superlubricity of a room-temperature ionic liquid induced by tribochemical reactions is explored in this study. Here, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ...(EMIMTFS) could realize stable superlubricity (μ < 0.01) with water at the interfaces of Si3N4/SiO2. A superlow and steady friction coefficient of 0.002–0.004 could be achieved under neutral conditions (pH of 6.9 ± 0.1) after 600 s of running-in process. Various factors that could affect superlubricity were explored, including concentration of EMIMTFS, sliding speed, applied load, and volume of the lubricant. The results reveal that superlubricity can be achieved with EMIMTFS aqueous solution under a broad scope of conditions. The results of surface analysis show that a steady composite tribochemical layer comprising EMIMTFS, silica, ammonia-containing compounds, and sulfides was formed by tribochemical reactions between EMIMTFS and Si3N4 during the running-in period. The film thickness calculation reveals that the achieved superlubricity is in a mixed lubrication regime that comprises boundary lubrication and thin film lubrication. The superlubricity state is governed by a firm composite tribochemical layer, a molecular adsorption layer (electric double layer of EMIMTFS), and a fluid layer. The liquid superlubricity achieved by the ionic liquid is helpful for the development of new ionic liquids with superlubricity characteristics and is of great significance for scientific understanding as well as engineering applications.
The chemokine receptor CCR5 plays a vital role in immune surveillance and inflammation. However, molecular details that govern its endogenous chemokine recognition and receptor activation remain ...elusive. Here we report three cryo-electron microscopy structures of G
protein-coupled CCR5 in a ligand-free state and in complex with the chemokine MIP-1α or RANTES, as well as the crystal structure of MIP-1α-bound CCR5. These structures reveal distinct binding modes of the two chemokines and a specific accommodate pattern of the chemokine for the distal N terminus of CCR5. Together with functional data, the structures demonstrate that chemokine-induced rearrangement of toggle switch and plasticity of the receptor extracellular region are critical for receptor activation, while a conserved tryptophan residue in helix II acts as a trigger of receptor constitutive activation.
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