M. Homyonfer

Hollow nanoparticles of WS2 as potential solid-state lubricants

Solid lubricants fill a special niche in reducing wear in situations where the use of liquid lubricants is either impractical or inadequate, such as in vacuum, space technology or automotive transport. Metal dichalcogenides MX2 (where M is, for instance, Mo or W and X is S or Se) are widely used as solid lubricants. These materials are characterized by a layered structure with weak (van der Waals) inter-layer forces that allow easy, low-strength shearing,. Within the past few years, hollow nanoparticles (HNs) of MX2 with structures similar to those of nested carbon fullerenes and nanotubes have been synthesized,. Here we show that these materials can act as effective solid lubricants: HN-WS2 outperforms the solid lubricants 2H-MoS2 and 2H-WS2 in every respect (friction, wear and lifetime of the lubricant) under varied test conditions. We attribute the outstanding performance of HN-WS2 to its chemical inertness and the hollow cage structure, which imparts elasticity and allows the particles to roll rather than to slide.

Inorganic fullerene-like material as additives to lubricants: structure–function relationship

Abstract Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX2 (M=Mo, W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. The main goal of the present work was to study the tribological properties of IF–WS2 in comparison to 2H–WS2 and MoS2 platelets over a wide range of loads and sliding velocities. The size and shape distributions of the nanoparticles were studied by transmission electron microscopy (TEM). The average size of the IF–WS2 particles was 120 nm, while that of 2H–WS2 and 2H–MoS2 was 0.5 μm and 4 μm. The chemical reactivity of the different powders in an oxygenated atmosphere was verified by heating the powders in ambient atmosphere. The friction experiments were performed in laboratory atmosphere (humidity ∼50%) using a ring-block tester. Complementary information on the state of wear of the powders in the lubricating fluid and on the metal surfaces of the specimens was obtained using a combination of TEM, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It has been established that IF–WS2 nanoparticles appear to have excellent tribological properties in definite loading range in comparison to typical metal dichalcogenides. The oxidation of the IF particles and the wear track was essentially less than with solid lubricants made of platelets of the same chemical compound (WS2). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.

Synthesis and characterization of inorganic fullerene-like WSe2 material

Abstract Lately M0S2 and WS2 fullerene-like nanoparticles, generically termed inorganic fullerene-like (IF) materials, were reproducibly obtained by gas phase4,6 and solid state6 reactions and the growth mechanism of such structures was elucidated6. the present work deals with the synthesis of IF-WSe2 from tungsten oxide and selenium vapors. Thermodynamic calculations of the sulfidization and selenization processes are provided, and indicate salient differences between the two processes. Notwithstanding, the growth model of IF-WSe2 is similar to that of sulfide fullerene-like material. the IF-WSe2 was characterized by X-ray powder diffraction, transmission electron microscopy and high resolution transmission electron microscopy.

Photoelectrochemical studies with inorganic cage structures of metal dichalcogenides

Abstract Inorganic fullerene-like (IF) and nanotube nanoparticles of WS 2 and SnS 2 with an oxide core have been prepared by the reaction of the respective oxide nanoparticles with H 2 S in a reducing atmosphere. Ethanolic suspensions of these nanoparticles were prepared and they were subsequently deposited electrophoretically onto conductive substrates. To study their photoelectrochemical properties, the nanoparticles were immersed in a selenosulphate electrolyte and illuminated. The spectral response of the photoelectrodes was measured and the fundamental optical transitions were determined. The mechanism of charge excitation, conduction and transfer is discussed in view of the experimental results.

The Effect of Hollow Nanoparticles of WS2 on Friction and Wear

Abstract Friction and wear behavior of new nanomaterial-inorganic fullerene-like (IF) WS 2 supramolecules has been compared with layered solid lubricants MoS 2 and WS 2 . Lubrication mechanism of IF nanoparticles and tribochemistry of contact have been studied. The main advantages of IF nanoparticles lies in their round shape and the absence of dangling bonds. Velocity accommodation modes under friction with solid lubricant powders were considered. The main lubrication mechanism of contact with IF nanoparticles appears to be rolling friction.

Negative curvature in inorganic fullerene-like structure

Abstract Structures with negative curvature were predicted and experimentally observed in carbon nanoparticles belonging to an expanded class of fullerenes (named Schwartzites). to obtain negative curvature in such structures, it is necessary to dispose heptagons or octagons in the fullerene network which consists of hexagons and pentagons. This phenomenon allows one to construct complicated structures, of which only a few of were experimentally realized. In the present work, structures with negative curvature are shown to exist also in the recently discovered inorganic fullerene-like (IF) materials. A number of MoS2 nanoparticles with complicated topologies having negative curvature, are described.

SURFACE MODIFICATIONS: NANOSTRUCTURES AND NESTED POLYHEDRA GENERATED BY PULSING THE STM TIP

STM experiments were performed to study modifications on the surface of crystalline and amorphous transition metal chalcogenides. Pulses with different parameters were applied to the STM tip, at predefined locations, while scanning the surface of WSe2 and MoS3. Different types of structures were formed on the crystalline WSe2 surface: (a) holes, (b) mounds, (c) ringlike structures. The three types were formed at different ranges of voltage. In some cases local cleavage of the surface was observed. An explanation is offered for the creation of such a variety of structures. On the amorphous MoS3 surface the pulses created small, round mounds. The size of the structures grows with the voltage of the applied pulse. I–V characteristics taken on these structures show semiconducting behavior with a gap of 1.1 eV, similar to the gap of bulk MoS2. TEM images show that nested polyhedral structures are formed in the STM experiments, with sizes that agree with the observed structures. Similar round crystalline structures are formed on the surface of MoS2 grown under different conditions. This is the first time that such crystallization is observed by pulsing the STM tip.