Rita Rosentsveig

Friction mechanism of individual multilayered nanoparticles

Inorganic nanoparticles of layered [two-dimensional (2D)] compounds with hollow polyhedral structure, known as fullerene-like nanoparticles (IF), were found to have excellent lubricating properties. This behavior can be explained by superposition of three main mechanisms: rolling, sliding, and exfoliation-material transfer (third body). In order to elucidate the tribological mechanism of individual nanoparticles in different regimes, in situ axial nanocompression and shearing forces were applied to individual nanoparticles using a high resolution scanning electron microscope. Gold nanoparticles deposited onto the IF nanoparticles surface served as markers, delineating the motion of individual IF nanoparticle. It can be concluded from these experiments that rolling is an important lubrication mechanism for IF-WS2 in the relatively low range of normal stress (0.96±0.38 GPa). Sliding is shown to be relevant under slightly higher normal stress, where the spacing between the two mating surfaces does not permit free rolling of the nanoparticles. Exfoliation of the IF nanoparticles becomes the dominant mechanism at the high end of normal stress; above 1.2 GPa and (slow) shear; i.e., boundary lubrication conditions. It is argued that the modus operandi of the nanoparticles depends on their degree of crystallinity (defects); sizes; shape, and their mechanical characteristics. This study suggests that the rolling mechanism, which leads to low friction and wear, could be attained by improving the sphericity of the IF nanoparticle, the dispersion (deagglomeration) of the nanoparticles, and the smoothness of the mating surfaces.

Synthesis of fullerene-like MoS2nanoparticles and their tribological behavior

Further understanding of the growth mechanism and the detailed structure of fullerene-like MoS2 (IF-MoS2) nanoparticles was achieved by using a new kind of reactor. The annealed nanoparticles consist of >30 closed layers and their average diameter is 50–80 nm although a small (<5%) fraction of larger IF nanoparticles was discernible. The majority of the nanoparticles are found to have an oval (pitta-bread or flying-saucer) shape rather than being quasi-spherical. The (002) peak of the powder diffraction pattern reveals only a small (0.3%) shift to lower angles as compared to the bulk (2H) phase. This observation suggests that the structure of the nanoparticles produced in the present reactor is more relaxed as compared to the previously synthesized IF-MoS2 powder, which exhibited up to 2% shift. The present reactor also permitted scaling up of the production of the IF-MoS2 to more than 0.6 g/batch. Impregnation of such nanoparticles in metallic coatings is shown to endow these surfaces with excellent tribological behavior, which suggests numerous applications.

Controlled Doping of MS2 (M=W, Mo) Nanotubes and Fullerene‐like Nanoparticles

Doping of semiconductor nanocrystals and nanowires with minute amounts of foreign atoms plays a major role in controlling their electrical, optical, and magnetic properties. In the case of carbon nanotubes, subsequent doping with oxygen and potassium leads to a p-type and n-type behavior, respectively. In another work, VOx nanotubes were transformed from spin-frustrated semiconductors to ferromagnets by doping with either electrons or holes. Calculations indicated that nand p-type doping of multiwall MoS2 nanotubes (INT) could be accomplished by substituting minute amounts of the Mo lattice atoms with either Nb (p-type) and Re (n-type), respectively. Substituting (< 0.1 at%) molybdenum by rhenium atoms and sulfur by halogen atoms was shown to produce n-type conductivity in MoS2 crystals. To synthesize rhenium-doped nanoparticles (NP) and nanotubes both in situ and subsequent doping methods were used. Figure 1a shows the quartz reactor used for in situ synthesis of rhenium doped MoS2 NP with fullerene-like structure (Re:IF-MoS2). The formal Re concentration was varied from 0.02 to 0.7 at%. The precursor RexMo1 xO3 (x< 0.01) powder was prepared in a specially designed auxiliary reactor (see Supporting Information). Evaporation of this powder takes place in area 1 at 770 8C (Figure 1 a). The oxide vapor reacts with hydrogen gas in area 2 (Figure 1a) at 800 8C which leads to a partial reduction of the vapor and its condensation into Re-doped MoO3 y nanoparticles. The resulting NP react with H2/H2S gas in area 3 at 810–820 8C to produce reduced oxide nanoparticles engulfed with a few closed layers of Re:MoS2, which protect it against ripening into bulk 2H-MoS2. [7] To complete this oxide to sulfide conversion a long (25–35 h) annealing process at 870 8C in the presence of H2S and forming gas (H2 10 wt %; N2) was performed. At the end of this diffusion-controlled process a powder of Re-doped MoS2 NP with a fullerene-like (IF) structure (Re:IF-MoS2) was obtained. In addition, doping of IF-WS2 NP and INT-WS2 was subsequently carried out by heating the pre-prepared IF/INT in an evacuated quartz ampoule also containing ReO3, or ReCl3 and iodine. In the case of ReCl3, both the rhenium and the chlorine atoms (substitution to sulfur atoms) served as ntype dopants. Typical high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) micrographs of the Re-doped fullerene-like NP are shown in Figure 1b. The Re:IF-MoS2 consists of about 30 closed (concentric) MoS2 layers. No impurity, such as oxides, or platelets (2 H) of MoS2 could be found in the product powder. The line profile and the Fourier analyses (FFT) (inset of Figure 1b) show an interlayer spacing of 0.627 nm (doped). Furthermore, the layers seem to be evenly folded and closed with very few defects and cusps, demonstrating the Re-doped NP to be quite perfectly crystalline. HRTEM did not reveal any structural changes even for the samples with high Re concentration (0.71 at%). However, owing to its quasispherical shape and size, this analysis cannot completely rule-out the presence of a small amount of the ReS2 phase in the IF NP. Figure 1c shows a typical TEM image of Re(Cl) (post synthesis) doped multiwall WS2 nanotube. There is no [*] L. Yadgarov, Dr. R. Rosentsveig, Dr. A. Albu-Yaron, Prof. R. Tenne Department of Materials and Interfaces, Weizmann Institute Rehovot 76100 (Israel) E-mail: reshef.tenne@weizmann.ac.il

Nanocompression of individual multilayered polyhedral nanoparticles

Inorganic layered materials can form hollow multilayered polyhedral nanoparticles. The size of these multi-wall quasi-spherical structures varies from 4 to 300 nm. These materials exhibit excellent tribological and wear-resisting properties. Measuring and evaluating the stiffness of individual nanoparticle is a non-trivial problem. The current paper presents an in situ technique for stiffness measurements of individual WS(2) nanoparticles which are 80 nm or larger using a high resolution scanning electron microscope (HRSEM). Conducting the experiments in the HRSEM allows elucidation of the compression failure strength and the elastic behavior of such nanoparticles under uniaxial compression.

Study of the growth mechanism of WS2 nanotubes produced by a fluidized bed reactor

Metal dichalcogenide nanotubes and in particular those of WS2 were shown to exhibit some unique physical and chemical properties, which offer numerous applications for this kind of nanophase material. Using a fluidized bed reactor (FBR), WS2 nanotubes were obtained in substantial amounts recently, rendering a systematic study of their properties possible. The FBR synthesized nanotubes are multiwalled (5–7 layers); open-ended; long (<0.5 mm), and with diameters of 15–20 nm. They are therefore distinguishable from the previously reported WS2 nanotubes which were shorter, bulkier and with closed ends. Careful analysis by various electron microscopy techniques is used in the present study to shed some light on the growth mechanism of these newly synthesized nanotubes. The proposed growth mechanism model differs markedly from the previously reported mechanisms of formation of both fullerene-like WS2 nanostructures and inorganic nanotubes of WS2.

Dependence of the Absorption and Optical Surface Plasmon Scattering of MoS2 Nanoparticles on Aspect Ratio, Size, and Media

The optical and electronic properties of suspensions of inorganic fullerene-like nanoparticles of MoS2 are studied through light absorption and zeta-potential measurements and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect band gap transition, a new peak is observed at 700-800 nm. This spectral peak has not been reported previously for MoS2. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggests that this transition arises from a plasmon resonance.

Observation of a Burstein–Moss Shift in Rhenium-Doped MoS2 Nanoparticles

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.

Interaction Between Selected MoS2 Nanoparticles and ZDDP Tribofilms

Nanoparticles based on transition metal dichalcogenides (TMD) are considered to hold great promise as boundary lubricating additive/material for improving friction and wear of engineering functional surfaces. However, TMD nanoparticles cannot provide a comprehensive surface protection against oxidation, corrosion or sludge control. Therefore, the current lubricant developments may still have to depend on conventional additives such as zinc dialkyl dithiophosphate (ZDDP), and it is essential to understand the interaction of nanoparticles with such additives in order to explore how these nanoparticles could be commercially employed in fully formulated lubricants. This paper examines the tribological properties of three different nanoparticles: inorganic fullerene-like MoS2, rhenium-doped MoS2 and MoS2 nanotubes in steel and steel with preformed ZDDP tribofilm surfaces using a pin-on-disc-type tribometer under reciprocating sliding conditions. The resulting tribofilms have been evaluated using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, transmission electron microscopy and atomic force microscopy. The results show that although the nanoparticles are able to reduce friction in all cases, the resulting tribofilm composition and morphology, and their lubricating mechanisms are significantly different. The MoS2 nanoparticles and nanotubes show good synergism with ZDDP, and tribofilms formed from nanoparticles exhibit improved friction and wear properties compared to that typically formed from ZDDP.

Behavior of solid lubricant nanoparticles under compression

Inorganic fullerene-like materials have been identified as being of potentially utmost importance for many industrial applications. MoS2 and WS2 hollow nanoparticles have been identified as strong candidates for tribological applications such as solid lubricants. The main goal of this work was to evaluate the mechanical properties of solid lubricant particles in ensemble under hydrostatic pressure. The behavior of nanopowders under compression has been described on the basis of constitutive models of continuum mechanics. The model will be applied to an isotropic compaction of copper (well-studied medium), fullerene-like (IF-WS2) nanoparticles and a natural powder of 2H-WS2 platelets. The morphology of individual nanoparticles and nanoparticle ensembles will be examined and discussed. Another aspect of this work was to study the applicability and limitations of the proposed constitutive model for the understanding of the tribological behavior of solid lubricant nanoparticles. Compression with the maximal pressure (500 MPa) showed that the shape of the IF nanoparticles is preserved. The dominant mechanism of damage was found to be the delamination or peeling-off of the external sheets of hollow nanoparticles. Strong destruction of 2H-WS2 platelets was observed under compression.

A magnetic resonance study of MoS2 fullerene-like nanoparticles

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) investigation of inorganic fullerene-like MoS(2) nanoparticles. Spectra of bulk 2H-MoS(2) samples have also been measured for comparison. The similarity between the measured quadrupole coupling constants and chemical shielding anisotropy parameters for bulk and fullerene-like MoS(2) reflects the nearly identical local crystalline environments of the Mo atoms in these two materials. EPR measurements show that fullerene-like MoS(2) exhibits a larger density of dangling bonds carrying unpaired electrons, indicative of them having a more defective structure than the bulk sample. The latter observation explains the increase in the spin-lattice relaxation rate observed in the NMR measurements for this sample in comparison with the bulk 2H- MoS(2) ones.