Graphitic Encapsulation and Electronic Shielding of Metal Nanoparticles to Achieve Metal-Carbon Interfacial Superlubricity

Macroscale, humidity-insensitive, and stable structural superlubricity achieved with hydrogen-free graphene nanoflakes

Achieving solid superlubricity in high-humidity environments is of great practical importance yet remains challenging nowadays, due to the complex physicochemical roles of water and concomitant oxidation on solid surfaces. Here we report a facile way to access humidity-insensitive solid superlubricity (coefficient of friction 0.0035) without detectable wear and running-in at a humidity range of 2-80%. Inspired by the concept of structural superlubricity, this is achieved between Au-capped microscale graphite flake and graphene nanoflake-covered hydrogen-free amorphous carbon (GNC a-C). Such GNC a-C exhibits reduced pinning effects of water molecules and weak oxidation, which demonstrates stable structural superlubricity even after air exposure of the surfaces for 365 days. The manufacturability of such design enables the macroscopic scale-up of structural superlubricity, achieving the leap from 4 μm × 4 μm contact to 3 mm ball-supported contact with a wide range of materials. Our results suggest a strategy for the macroscale application of structural superlubricity under ambient condition.

Macroscale Superlubricity on Nanoscale Graphene Moiré Structure-Assembled Surface via Counterface Hydrogen Modulation

Interlayer incommensurateness slippage is an excellent pathway to realize superlubricity of van der Waals materials; however, it is instable and heavily depends on twisted angle and super-smooth substrate which pose great challenges for the practical application of superlubricity. Here, macroscale superlubricity (0.001) is reported on countless nanoscale graphene moiré structure (GMS)-assembled surface via counterface hydrogen (H) modulation. The GMS-assembled surface is formed on grinding balls via sphere-triggered strain engineering. By the H modulation of counterface diamond-like carbon (25 at.% H), the wear of GMS-assembled surface is significantly reduced and a steadily superlubric sliding interface between them is achieved, based on assembly face charge depletion and H-induced assembly edge weakening. Furthermore, the superlubricity between GMS-assembled and DLC25 surfaces holds true in wide ranges of normal load (7-11 N), sliding velocity (0.5-27 cm -1s), contact area (0.4×104-3.7×104 µm2), and contact pressure (0.19-1.82 GPa). Atomistic simulations confirm the preferential formation of GMS on a sphere, and demonstrate the superlubricity on GMS-assembled surface via counterface H modulation. The results provide an efficient tribo-pairing strategy to achieve robust superlubricity, which is of significance for the engineering application of superlubricity.

Template-free scalable growth of vertically-aligned MoS2 nanowire array meta-structural films towards robust superlubricity

Two-dimensional (2D) molybdenum disulfide exhibits a variety of intriguing behaviors depending on its orientation layers. Therefore, developing a template-free atomic layer orientation controllable growth approach is of great importance. Here, we demonstrate scalable, template-free, well-ordered vertically-oriented MoS2 nanowire arrays (VO-MoS2 NWAs) embedded in an Ag-MoS2 matrix, directly grown on various substrates (Si, Al, and stainless steel) via one-step sputtering. In the meta-structured film, vertically-standing few-layered MoS2 NWAs of almost micron length (∼720 nm) throughout the entire film bulk. While near the surface, MoS2 lamellae are oriented in parallel, which are beneficial for caging the bonds dangling from the basal planes. Owing to the unique T-type topological characteristics, chemically inert Ag@MoS2 nano-scrolls (NSCs) and nano-crystalline Ag (nc-Ag) nanoparticles (NPs) are in situ formed under the sliding shear force. Thus, incommensurate contact between (002) basal planes and nc-Ag NPs is observed. As a result, robust superlubricity (friction coefficient μ = 0.0039) under humid ambient conditions is reached. This study offers an unprecedented strategy for controlling the basal plane orientation of 2D transition metal dichalcogenides (TMDCs) via substrate independence, using a one-step solution-free easily scalable process without the need for a template, which promotes the potential applications of 2D TMDCs in solid superlubricity.

Tunable, Wide-Temperature, and Macroscale Superlubricity Enabled by Nanoscale Van Der Waals Heterojunction-to-Homojunction Transformation

Achieving macroscale superlubricity of van der Waals (vdW) nanopowders is particularly challenging, due to the difficulty in forming ordered junctions before friction and the friction-induced complex contact restructuration among multiple nanometer-sized junctions. Here, a facile way is reported to achieve vdW nanopowder-to-heterojunction conversion by graphene edge-oxygen (GEO) incorporation. The GEO effectively weakens the out-of-plane edge-edge and in-plane plane-edge states of the vdW nanopowder, leading to a coexistent structure of nanoscale homojunctions and heterojunctions on the grinding balls. When sliding on diamond-like carbon surfaces, the ball-supported structure governs macroscale superlubricity by heterojunction-to-homojunction transformation among the countless nanoscale junctions. Furthermore, the transformation guides the tunable design of superlubricity, achieving superlubricity (µ ≈ 0.005) at wide ranges of load, velocity, and temperature (-200 to 300 °C). Atomistic simulations reveal the GEO-enhanced conversion of vdW nanopowder to heterojunctions and demonstrate the heterojunction-to-homojunction transformation superlubricity mechanism. The findings are of significance for the macroscopic scale-up and engineering application of structural superlubricity.

Hydrogen-Enhanced Catalytic Conversion of Amorphous Carbon to Graphene for Achieving Superlubricity

The solid-state conversion of amorphous carbon into graphene is extremely difficult, but it can be achieved in the friction experiments that induce macroscale superlubricity. However, the underlying conversion mechanisms remain elusive. Here, the friction experiments with Cu nanoparticles and (non-hydrogen (H) or H) a-C in vacuum, show the H-induced conversion of mechanical to chemical wear, resulting in the a-C's tribosoftening and nanofragmentating that produce hydrocarbon nanoclusters or molecules. It is such exactly hydrocarbon species that yield graphene at hydrogen-rich a-C friction interface, through reaction of them with Cu nanoparticles. In comparison, graphene isn't formed at Cu/non-H a-C friction interface. Atomistic simulations reveal the hydrogen-enhanced tribochemical decomposition of a-C and demonstrate the energetically favorable graphitization transformation of hydrocarbons on Cu substrates. The findings are of importance to achieve solid-state transformation between different carbon allotropes and provide a good strategy to synthesize other graphitic encapsulated catalysts with doped elements.

Effect of the Graphitization Mechanism on the Friction and Wear Behavior of DLC Films Based on Molecular Dynamics Simulations

Whether a graphitization mechanism can control the low-friction behavior of DLC films is still controversial. In this paper, we establish the molecular dynamics model of the DLC film with graphene (DLC-GR-DLC) by LAMMPS and study the influence of the graphitization mechanism on the friction and wear behavior of the DLC film. The friction force of the DLC-GR-DLC model in the running-in stage is significantly smaller than that of the DLC film and then gradually increases to the same size as that of the DLC film. Further analysis indicates that the graphitization mechanism could indeed reduce the shear stress of the friction interface when graphene remains intact. However, the curling and breaking of the graphene structure will lead to an increase in shear force at the friction interface.

The Poynting Vector Field Generic Singularities in Resonant Scattering of Plane Linearly Polarized Electromagnetic Waves by Subwavelength Particles

We present the results of a study of the Poynting vector field generic singularities at the resonant light scattering of a plane monochromatic linearly polarized electromagnetic wave by a subwavelength particle. We reveal the impact of the problem symmetry, the spatial dimension, and the energy conservation law on the properties of the singularities. We show that, in the cases when the problem symmetry results in the existence of an invariant plane for the Poynting vector field lines, a formation of a standing wave in the immediate vicinity of a singularity gives rise to a saddle-type singular point. All other types of singularities are associated with vanishing at the singular points, either (i) magnetic field, for the polarization plane parallel to the invariant plane, or (ii) electric field, at the perpendicular orientation of the polarization plane. We also show that in the case of two-dimensional problems (scattering by a cylinder), the energy conservation law restricts the types of possible singularities only to saddles and centers in the non-dissipative media and to saddles, foci, and nodes in dissipative. Finally, we show that dissipation affects the (i)-type singularities much stronger than the (ii)-type. The same conclusions are valid for the imaginary part of the Poynting vector in problems where the latter is regarded as a complex quantity. The singular points associated with the formation of standing waves are different for real and imaginary parts of this complex vector field, while all other singularities are common. We illustrate the general discussion by analyzing singularities at light scattering by a subwavelength Germanium cylinder with the actual dispersion of its refractive index.

Nature of the Poynting Vector Field Singularities in Resonant Light Scattering by Nanoparticles

Singularities of the Poynting vector field subwavelength patterns in resonant light scattering by nanoparticles are discussed and classified. There are two generic types of the singularities, namely, (i) the singularities related to the vanishing of the magnetic (and/or electric) field at the singular points and (ii) the singularities related to the formation of standing waves in proximity to the singular points. The connection of these types of singularities to the topology of the singular points, space dimension (3D vs. 2D), and energy conservation law are revealed. In particular, it is shown that in 2D cases in non-dissipative media, the energy conservation reduces the possible types of generic singular points to saddles and centers only. In 3D cases, a universal expression connecting different components of the Poynting vector and valid for any generic singularities is derived and numerically checked for various types of singular points.

Macroscale Robust Superlubricity on Metallic NbB2

Robust superlubricity (RSL), defined by concurrent superlow friction and wear, holds great promise for reducing material and energy loss in vast industrial and technological operations. Despite recent advances, challenges remain in finding materials that exhibit RSL on macrolength and time scales and possess vigorous electrical conduction ability. Here, the discovery of RSL is reported on hydrated NbB₂ films that exhibit vanishingly small coefficient of friction (0.001-0.006) and superlow wear rate (≈10^-17 m³ N^-1 m^-1 ) on large length scales reaching millimeter range and prolonged time scales lasting through extensive loading durations. Moreover, the measured low resistivity (≈10^-6 Ω m) of the synthesized NbB₂ film indicates ample capability for electrical conduction, extending macroscale RSL to hitherto largely untapped metallic materials. Pertinent microscopic mechanisms are elucidated by deciphering the intricate load-driven chemical reactions that generate and sustain the observed superlubricating state and assessing the strong stress responses under diverse strains that produce the superior durability.

Concurrent Oxidation-Reduction Reactions in a Single System Using a Low-Plasma Phenomenon: Excellent Catalytic Performance and Stability in the Hydrogenation Reaction

The catalytic activity and stability of metal nanocatalysts toward agglomeration and detachment during their preparation on a support surface are major challenges in practical applications. Herein, we report a novel, one-step, synchronized electro-oxidation-reduction "bottom-up" approach for the preparation of small and highly stable Cu nanoparticles (NPs) supported on a porous inorganic (TiO₂@SiO₂) coating with significant catalytic activity and stability. This unique embedded structure restrains the sintering of CuNPs on a porous TiO₂@SiO₂ surface at a high temperature and exhibits a high reduction ratio (100% in 60 s) and no decay in activity even after 30 cycles (>98% conversion in 3 min). This occurs in a model reaction of 4-nitrophenol (4-NP) hydrogenation, far exceeding the performance of most common catalysts observed to date. More importantly, nitroarene, ketone/aldehydes, and organic dyes were reduced to the corresponding compounds with 100% conversion. Density functional theory (DFT) calculations of experimental model systems with six Cu, two Fe, and four Ag clusters anchored on the TiO₂ surface were conducted to verify the experimental observations. The experimental results and DFT calculations revealed that CuNPs not only favor the adsorption on the TiO₂ surface over those of Fe and AgNPs but also boost the adsorption energy and activity of 4-NP. This strategy has also been extended to the preparation of other single-atom catalysts (e.g., FeNPs-TiO₂@SiO₂ and AgNPs-TiO₂@SiO₂), which exhibit excellent catalytic performance.

Tribochemistry of superlubricating amorphous carbon films

Tribochemistry refers to a series of physical and chemical reactions that occur at a sliding interface under friction action, and the tribological properties of materials also change significantly. Understanding the effect of tribochemical reactions on the tribological properties of materials is important for controlling the structure and composition of materials by chemical means and promoting the engineering application of materials. This study primarily introduces the tribochemical reactions of diamond-like carbon (DLC) films during the friction process in different environments and the relationship between tribochemistry and the tribological properties of DLC films. From this, the study proposes strategies to achieve the superlubricity of DLC films through tribochemistry. Finally, challenges and countermeasures in the engineering application of DLC films are discussed.