The Application of Nano-MoS2 Quantum Dots as Liquid Lubricant Additive for Tribological Behavior Improvement

Elucidating the Role of Electron Transfer in the Photoluminescence of MoS2 Quantum Dots Synthesized by fs-Pulse Ablation

Herein, MoS2 quantum dots (QDs) with controlled optical, structural, and electronic properties are synthesized using the femtosecond pulsed laser ablation in liquid (fs-PLAL) technique by varying the pulse width, ablation power, and ablation time to harness the potential for next-generation optoelectronics and quantum technology. Furthermore, this work elucidates key aspects of the mechanisms underlying the near-UV and blue emissions the accompanying large Stokes shift, and the consequent change in sample color with laser exposure parameters pertaining to MoS2 QDs. Through spectroscopic analysis, including UV-visible absorption, photoluminescence, and Raman spectroscopy, we successfully unraveled the mechanisms for the change in optoelectronic properties of MoS2 QDs with laser parameters. We realize that the occurrence of a secondary phase, specifically MoO3-x, is responsible for the significant Stokes shift and blue emission observed in this QD system. The primary factor influencing these activities is the electron transfer observed between these two phases, as validated by excitation-dependent photoluminescence and XPS and Raman spectroscopies.

Biological effects of molybdenum(IV) sulfide nanoparticles and microparticles in the rat after repeated intratracheal administration

In this study, molybdenum(IV) sulfide (MoS2 ) nanoparticles (97 ± 32 nm) and microparticles (1.92 ± 0.64 μm) stabilized with poly (vinylpolypyrrolidone) (PVP) were administered intratracheally to male and female rats (dose of 1.5 or 5 mg/kg bw), every 14 days for 90 days (seven administrations in total). Blood parameters were assessed during and at the end of the study (hematology, biochemistry including glucose, albumins, uric acid, urea, high density lipoprotein HDL, total cholesterol, triglycerides, aspartate transaminase, and alanine transaminase ALT). Bronchoalveolar lavage fluid (BALF) analyses included cell viability, biochemistry (total protein concentration, lactate dehydrogenase, and glutathione peroxidase activity), and cytokine levels (tumor necrosis factor α, TNF-α, macrophage inflammatory protein 2-alpha, MIP-2, and cytokine-induced neutrophil chemoattractant-2, CINC-2). Tissues were subjected to routine histopathological and electron microscopy (STEM) examinations. No overt signs of chronic toxicity were observed. Differential cell counts in BALF revealed no significant differences between the animal groups. An increase in MIP-2 and a decrease in TNF-α were observed in BALF in the exposed males. The histopathological changes in the lung evaluated according to a developed classification system (based on severity of inflammation, range 0-4, with 4 indicating the most severe changes) showed average histopathological score of 1.33 for animals exposed to nanoparticles and microparticles at the lower dose, 1.72 after exposure to nanoparticles at the higher dose, and 2.83 for animals exposed to microparticles at the higher dose. In summary, it was shown that nanosized and microsized MoS2 can trigger dose-dependent inflammatory reactions in the lungs of rats after multiple intratracheal instillation irrespective of the animal sex. Some evidence indicates a higher lung pro-inflammatory potential of the microform.

Preparation of Multifunctional Nano-Molybdenum Disulfide and Its Tribological Properties in Water-Based Cutting Fluids

In order to meet the advanced requirements of the manufacturing industry, the use of water-based cutting fluids (WCFs) in metal processing is gradually increasing. However, their lubrication performance still needs to be improved considerably. Therefore, new multifunctional molybdenum disulfide nanoparticles (m-MoS2 NPs) were developed to improve the lubricating properties of WCFs. M-MoS2 NPs modified with silver nanoparticles were prepared by an in situ surface modification. The morphology and chemical composition of the m-MoS2 NPs were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Furthermore, the dispersion and bactericidal properties of m-MoS2 NPs with different weight percents in WCFs were also studied experimentally. The effect of m-MoS2 NPs concentration on friction properties and their friction mechanism were investigated in this research. The results revealed that the prepared m-MoS2 NPs were all nanoscale particles with a layered structure. The dispersion and bactericidal properties of m-MoS2 NPs in WCFs were better than those of MoS2 NPs. The best dispersion and bactericidal properties were observed with 1 wt % MoS2 NPs, as well as friction reduction and antiwear properties. During friction, the two friction surfaces were in the boundary lubrication state,and the prepared m-MoS2 NPs entered the friction contact zone along with the WCFs. A friction chemical reaction film rich in MoS2 and Ag NPs was formed on the friction surface to fill and repair the worn surface, exerting a good lubrication effect.

Contact and Tribological Study of Micro/Nano Groove Texture on the Surface of Gas Bearing Materials Based on Nanoscale

As a kind of sliding bearing, the gas bearing is widely used in high-speed rotating machinery. It realizes energy cleaning in the field of high-speed rotating machinery. In order to solve the problem of reducing the service life of gas bearings due to friction during startup and shutdown, we use micromachining technology to process groove textures with different groove widths on the surface of 0Cr17Ni7Al, a common material for gas bearings. A ball-disc friction contrast test is conducted under dry friction conditions with and without texture. The experiment shows that the lowest average friction coefficient of 0.8 mm texture is σ = 0.745. When the friction radius is 22.5 mm, the wear rate of 1.0 mm texture is the lowest at ω = 3.118 × 10-4mm3/N·mm. However, the maximum friction coefficient reached is σ = 0.898. Under the nanometer scale, the contact between friction pairs is fully analyzed. The influence mechanism of different groove widths, friction impacts and climbing heights on the friction and wear properties of the micromechanical groove texture on the surface of 0Cr17Ni7Al stainless steel is studied at the nano-fractal scale. The effects of different width grooves on the surface texture and tribological properties of the micromachine are studied.

Highly Efficient Electrocatalytic N2 Reduction to Ammonia over Metallic 1T Phase of MoS2 Enabled by Active Sites Separation Mechanism

The 1T phase of MoS2 has been widely reported to be highly active toward the hydrogen evolution reaction (HER), which is expected to restrict the competitive nitrogen reduction reaction (NRR). However, in this work, a prototype of active sites separation over 1T-MoS2 is proposed by DFT calculations that the Mo-edge and S atoms on the basal plane exhibit different catalytic NRR and HER selectivity, and a new role-playing synergistic mechanism is also well enabled for the multistep NRR, which is further experimentally confirmed. More importantly, a self-sacrificial strategy using g-C3 N4 as templates is proposed to synthesize 1T-MoS2 with an ultrahigh 1T content (75.44%, named as CNMS, representing the composition elements of C, N, Mo, and S), which yields excellent NRR performances with an ammonia formation rate of 71.07 µg h-1 mg-1 cat. at -0.5 V versus RHE and a Faradic efficiency of 21.01%. This work provides a promising new orientation of synchronizing the selectivity and activity for the multistep catalytic reactions.

Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review

Reducing power losses in engines is considered a key parameter of their efficiency improvement. Nanotechnology, as an interface technology, is considered one of the most promising strategies for this purpose. As a consumable liquid, researchers have studied nanolubricants through the last decade as potential engine oil. Nanolubricants were shown to cause a considerable reduction in the engine frictional and thermal losses, and fuel consumption as well. Despite that, numerous drawbacks regarding the quality of the processed nanolubricants were discerned. This includes the dispersion stability of these fluids and the lack of actual engine experiments. It has been shown that the selection criteria of nanoparticles to be used as lubricant additives for internal combustion engines is considered a complex process. Many factors have to be considered to investigate and follow up with their characteristics. The selection methodology includes tribological and rheological behaviours, thermal stability, dispersion stability, as well as engine performance. Through the last decade, studies on nanolubricants related to internal combustion engines focused only on one to three of these factors, with little concern towards the other factors that would have a considerable effect on their final behaviour. In this review study, recent works concerning nanolubricants are discussed and summarized. A complete image of the designing parameters for this approach is presented, to afford an effective product as engine lubricant.

An Evaluation of the Tribological Behavior of Cutting Fluid Additives on Aluminum-Manganese Alloys

The introduction of additives enhances the friction and wear reduction properties of cutting fluids (CFs) as well as aids in improving the surface quality of the machined parts. This study examines the tribological behavior of polymer-based and phosphorus-based additives introduced into cutting fluids for the machining of Al-Mn alloys. Ball-on-disc tests were used to evaluate the coefficient of friction (COF) and lubrication failure temperature to study the performance of the additives in the cutting fluids. Surface characterization was performed on the sliding tracks induced on the Al-Mn disc surfaces and used to propose the wear and friction reduction mechanisms. The polymer-based additive possessed a higher temperature at which lubrication failure occurred, displayed comparable COF at a lower temperature under certain conditions, and possessed a steadier tribological behavior. However, the phosphorus-based additive was observed to display lower COF and wear damage from 200 °C till failure. The lower COF values for the phosphorus-based additive at 200 °C corresponded with lower surface damage on the Al-Mn surface. The phosphorus-based additive’s performance at 200 °C could be attributed to the forming of a phosphorus-rich boundary layer within the sliding wear track, resulting in less surface damage on the Al-Mn surface and lower material transfer to the counterface steel ball surface.

A Review on MoS2 Energy Applications: Recent Developments and Challenges

Molybdenum disulfide (MoS2) is a promising transition metal dichalcogenide (TMD) that has exceptional electronic, magnetic, optical, and mechanical properties. It can be semiconducting, superconducting, or an insulator according to its polymorph. Its bandgap structure changes from indirect to direct when moving towards its nanostructures, which opens a door to bandgap engineering for MoS2. Its supercapacitive and catalytic activity was recently noticed and studied, in order to include this material in a wide range of energy applications. In this work, we present MoS2 as a future material for energy storage and generation applications, especially solar cells, which are a cornerstone for a clean and abundant source of energy. Its role in water splitting reactions can be utilized for energy generation (hydrogen evolution) and water treatment at the same time. Although MoS2 seems to be a breakthrough in the energy field, it still faces some challenges regarding its structure stability, production scalability, and manufacturing costs.

Frictional characteristics of graphene layers with embedded nanopores

Graphite possessing extraordinary frictional properties has been widely used as solid lubricants. Interesting frictional characteristics have been observed for pristine graphene layers, for defective graphene, the frictional signal shows richer behaviors such as those found in topological defective graphene and graphene step edges. Recently discovered nanoporous graphene represents a new category of defect in graphene and its impact on graphene frictional properties has not yet been explored. In this work, we perform molecular dynamics simulations on the frictional responses of nanoporous graphene layers when slid using a silicon tip. We show that the buried nanopore raises maximum friction signal amplitude while preserving the stick-slip character, the size of the nanopore plays a key role in determining the maximum frictional force. Negative friction is observed when the silicon tip scanned towards the center of the nanopore, this phenomenon originates from the asymmetrical variation of the in-plane strain and the out-of-plane deformation when indented by the silicon tip. Moreover, the layer dependent frictional character is examined for the buried graphene nanopores, showing that increasing graphene layers weakens the effect of nanopore on the frictional signal.

Specific Features of Reactive Pulsed Laser Deposition of Solid Lubricating Nanocomposite Mo-S-C-H Thin-Film Coatings

This work investigates the structure and chemical states of thin-film coatings obtained by pulsed laser codeposition of Mo and C in a reactive gas (H₂S). The coatings were analysed for their prospective use as solid lubricating coatings for friction units operating in extreme conditions. Pulsed laser ablation of molybdenum and graphite targets was accompanied by the effective interaction of the deposited Mo and C layers with the reactive gas and the chemical states of Mo- and C-containing nanophases were interdependent. This had a negative effect on the tribological properties of Mo–S–C–H nanocomposite coatings obtained at H₂S pressures of 9 and 18 Pa, which were optimal for obtaining MoS₂ and MoS₃ coatings, respectively. The best tribological properties were found for the Mo–S–C–H_5.5 coating formed at an H₂S pressure of 5.5 Pa. At this pressure, the x = S/Mo ratio in the MoS_x nanophase was slightly less than 2, and the a-C(S,H) nanophase contained ~8 at.% S and ~16 at.% H. The a-C(S,H) nanophase with this composition provided a low coefficient of friction (~0.03) at low ambient humidity and 22 °C. The nanophase composition in Mo–S–C–H_5.5 coating demonstrated fairly good antifriction properties and increased wear resistance even at −100 °C. For wet friction conditions, Mo–S–C–H nanocomposite coatings did not have significant advantages in reducing friction compared to the MoS₂ and MoS₃ coatings formed by reactive pulsed laser deposition.

Nano Serpentine Powders as Lubricant Additive: Tribological Behaviors and Self-Repairing Performance on Worn Surface

Natural serpentine powders are applicable as additives for various lubricating oils. However, no uniform theories explain their tribological performance, lubrication, and wear mechanism, especially their self-repairing mechanism. Herein, the influence of different nano serpentine powders (NSPs) contents in liquid paraffin on the friction and wear characteristics of steel balls and the self-repairing process of NSPs on the worn surface were studied. Results show that the optimal amount of NSPs was 0.5 wt %. Relative to those of the base oil, the friction coefficients and wear spot diameters were reduced by 22.8% and 34.2%, respectively. Moreover, the long-term tribological test shows that the wear scar diameter decreased slightly after 3 h, reaching the state of dynamic balance between wear and repair. The outstanding tribological performance should be attributed to the formed bilayer tribofilm, the first layer of which contains nanoparticles surrounded by lubricants and the second layer of which contains nanoparticles compacted onto the surface of the steel ball.

Scalable Fabrication of Modified Graphene Nanoplatelets as an Effective Additive for Engine Lubricant Oil

The use of nano-additives is widely recognized as a cheap and effective pathway to improve the performance of lubrication by minimizing the energy loss from friction and wear, especially in diesel engines. In this work, a simple and scalable protocol was proposed to fabricate a graphene additive to improve the engine lubricant oil. Graphene nanoplates (GNPs) were obtained by a one-step chemical exfoliation of natural graphite and were successfully modified with a surfactant and an organic compound to obtain a modified GNP additive, that can be facilely dispersed in lubricant oil. The GNPs and modified GNP additive were characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy. The prepared GNPs had wrinkled and crumpled structures with a diameter of 10–30 µm and a thickness of less than 15 nm. After modification, the GNP surfaces were uniformly covered with the organic compound. The addition of the modified GNP additive to the engine lubricant oil significantly enhanced the friction and antiwear performance. The highest reduction of 35% was determined for the wear scar diameter with a GNP additive concentration of approximately 0.05%. The mechanism for lubrication enhancement by graphene additives was also briefly discussed.

Reactive Pulsed Laser Deposition of Clustered-Type MoSx (x ~ 2, 3, and 4) Films and Their Solid Lubricant Properties at Low Temperature

We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (-100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.