Friction on a single MoS2 nanotube
© Jelenc and Remskar; licensee Springer. 2012
Received: 9 January 2012
Accepted: 10 April 2012
Published: 10 April 2012
Friction was measured on a single molybdenum disulfide (MoS2) nanotube and on a single MoS2 nano-onion for the first time. We used atomic force microscopy (AFM) operating in ultra-high vacuum at room temperature. The average coefficient of friction between the AFM tip and MoS2 nanotubes was found considerably below the corresponding values obtained from an air-cleaved MoS2 single crystal or graphite. We revealed a nontrivial dependency of friction on interaction strength between the nanotube and the underlying substrate. Friction on detached or weakly supported nanotubes by the substrate was several times smaller (0.023 ± 0.005) than that on well-supported nanotubes (0.08 ± 0.02). We propose an explanation of a quarter of a century old phenomena of higher friction found for intracrystalline (0.06) than for intercrystalline slip (0.025) in MoS2. Friction test on a single MoS2 nano-onion revealed a combined gliding-rolling process.
PACS, 62.20, 61.46.Fg, 68.37 Ps
Inorganic solid lubricant molybdenum disulfide (MoS2) is a known lubricant, which has been applied extensively for decades. Its crystalline microstructures, tribological properties, and anti-corrosive mechanisms have been studied deeply . The easy mutual gliding of MoS2 layers along (001) basal planes and its surface inertness allow its low friction performance. Ultra-low friction coefficients as low as 0.003 between MoS2 flakes and MoS2 surfaces have been reported  and explained by an easy shear of basal planes of the crystal structure parallel to the sliding direction in accordance with the Amontons-Coulomb law.
MoS2 exists in two very different morphologies, both with a special effect on the tribology process. The usual platelike form, which can be synthesized or exploited as mineral, is widely used as an efficient dry lubricant or an oil or grease additive. Unfortunately, edges of layered crystals with high hardness are prone to oxidation which reduces the efficiency of lubrication, especially in humid environment. Thin flakes with a high active surface and with relatively low number of unsaturated bonds at edges are therefore preferable. The lubrication mechanism of MoS2 nanosheets, 50-nm thick, prepared by exfoliation and restacking, and added to liquid paraffin, was explained by the higher surface energy of MoS2 nanosheets, with better absorbance on the rubbing surfaces preventing them from a direct contact .
Curved, self-terminated shapes as nanotubes and fullerene-like particles (IF) with nano-onion morphology, firstly reported in 1992 on WS2  and one year later on MoS2 , brought 'elimination' of edges and immediately became intensively investigated with regard to their particular appropriateness for a new generation of lubricants. After the first enthusiastic suggestion of a possible rolling mechanism [6, 7], it was evidenced that under mechanical stress, the nanoparticles slowly deform and exfoliate, transferring WS2 nanosheets onto the underlying surfaces (third body effect), and continue to provide effective lubrication until they are totally exfoliated [8, 9]. Another positive effect of this new nano-lubricant is that the metal surface impregnated with IF nanoparticles does not seem to oxidize during the tribological test, although the coverage of the metal surface by the nanoparticles does not exceed 20% to 30%. This observation was explained by a lower temperature of the WS2-impregnated interface relating to the pure metal surface during the friction test. It was furthermore suggested that the WS2 nanoparticles may act as a kind of 'cathodic protection' against the oxidation of the metal surface, which prevents the oxidation of the metal surface .
Thin films of hollow MoS2 nano-onions, deposited by a localized high-pressure arc-discharge method, exhibited an ultra-low friction (an order of magnitude lower than for sputtered MoS2 thin films) and wear in nitrogen and at 45% humidity. The results were explained by the presence of curved S ± Mo ± S planes that prevent oxidation (absence of edges) and preserve the layered structure . Similarly, the experiments under boundary lubrication demonstrated that IF-MoS2 nanoparticles had significantly lower friction than the MoS2 films prepared by pulsed laser deposition . The effect was explained through orientation relationship of low friction (001) MoS2 planes with regard to the counter face and by chemical inertness of self-terminated layers.
At deviation from fully parallel orientation between the MoS2 basal plane, the cleavage takes place  with coefficient of friction (COF) of the order of 0.1; when intracrystalline shear between MoS2 layers took place, the COF was found unexpectedly higher (0.06) than when intercrystalline slip occurred (0.025).
In this work, we report on the first tribo-testing performed on a single MoS2 nanotube and on a single MoS2 nano-onion using atomic force microscopy (AFM) in ultra-high vacuum (UHV) at room temperature. The MoS2 nanotubes with incorporated MoS2 nano-onions were selected because no friction experiments are done yet on such morphology, which, besides unique inorganic peapod morphology, also answers health concerns regarding release of nanoparticles into the atmosphere. Nano-onions are safely stored inside long nanotubes in non-agglomerated stage and cannot become airborne. This particular morphology of MoS2 nanotubes named 'mama'-tubes is made clear using transmission electron microscopy and scanning tunneling microscopy (STM) with a special attention on surface and near surface structure. Dependency of friction on load was measured at different scan areas and angles with regard to the tube axis. Nanotubes with various adhesion supports to the substrate were tested with the aim to explain a large standard deviation in friction results using nanotubes as lubricants. Results explaining a quarter of a century old phenomena of higher friction was found between MoS2 counterparts when shear was taking place for intracrystalline slip than for intercrystalline slip. The results obtained on single MoS2 nanotubes are correlated with those obtained at the same conditions on MoS2 single crystal and on graphite. We present new evidence that a rolling mechanism of MoS2 fullerene-like nano-onions is possible at low loads.
MoS2 nanotubes for nanotribology testing were dispersed in ethanol using ultrasound bath and were drop casted onto freshly cleaved highly ordered pyrolytic graphite (HOPG) surface. The substrate consisted of atomically flat terraces separated by cleavage steps of different, uncontrolled height. The sample was transferred into a UHV chamber (base pressure in a range of 10-10 mbar) equipped with a commercial AFM/STM (VT-AFM, Omicron Nanotechnology GmbH, Taunusstein, Germany). UHV-AFM investigations were carried out at room temperature. Single crystal silicon tip (NT-MDT, Moscow, Russia), CSG10, Sb-doped, n-type, force constant = 0.1 N/m, and curvature radius typically 10 nm, was used as AFM probe. The friction measurements were made on a top of every single nanotube. The AFM was operated in contact mode, and hence, the cantilever deflections included both the topographic and lateral force information from sample surfaces. The topographic images were used to measure the size of MoS2 mama-tubes, while recording the lateral deflection of the cantilever during the scanning of the 'friction images' was obtained. Measurement of the frictional force was done for at least three step increments of normal load at a given location. Lateral force image was averaged over the scanned area for both trace and retrace scans. The friction versus normal force was fitted with a straight line whose slope is the friction coefficient. Accuracy of a particular measurement was obtained from least-square fit, while mean values are shown with standard deviations.
Results and discussion
The counterparts in the experiments consisted of the AFM tip and the top surface of nanotubes. The Hertzian contact improves parallel orientations between surface and subsurface MoS2 layers of thin-walled nanotubes and prevents cleavage. The contact pressure and area are dependent on the sharpness of the tip. However, when the tip scans the overside surface of a tube, cleavage of surface layers is expected, and transfer of the layered materials onto the tip could take place. The tip shape was therefore regularly reconstructed using the Scanning Probe Image Processor (SPIP) before and after each friction test. While pristine AFM tip before the friction test revealed an 8 ± 1-nm curvature radius in x-direction, after the test at the 3-nN load, the calculated radius was 17 ± 1 nm and increased up to 25 nm after several friction tests. Although one could expect low values of COF due to tip contamination, the value of COF over 14 measurements on the same tube, 330 nm in diameter using the same tip, was found to be 0.07 ± 0.01. The transferred material enlarged the tip wideness, but due to easy gliding, it was pushed out of the contact area, leaving the top tip intact.
Friction was measured either on areas much smaller than the diameter of the tubes, which was determined from height profiles or on areas larger than the tube diameters, but smaller than the curvature radius of the AFM tip. No convincing influence on friction by a direction of scanning with regard to a tube axis was found. The COF (μ) was determined using the Coulomb law, which says that friction force (friction) is proportional to the applied load, i.e., the normal force (Fn). The COF is an empirical property of the contacting materials, in our case, of AFM tip and MoS2 nanotube, or MoS2 fullerene-like nano-onion, MoS2 single crystal or graphite. Accuracy of COF for a particular measurement was obtained from least-square fits. COF was determined in accordance with the known procedure .
Structure and surface morphology of MoS2 nanotubes
Encapsulated MoS2 fullerene-like nano-onions range from a few nanometers in diameter up to more than 100 nm (Figure 1a). Their shape can be quasi-spherical or partially facetted (Figure 1b). The STM reveals a modulation of topography at a nanometer scale (Figure 1c) explained as undulations of the (001) MoS2 basal planes with buried plane edges. This unique morphology appears due to a minimization of surface energy during the process of the nanotube formation, when quasi one-dimensional Mo6S2I8 needles (Figure 1d) are transformed to the curved two-dimensional MoS2 layers. Thickness of the walls is typically around 10 nm.
Friction measured on single MoS2 nanotubes laid flat on graphite (HOPG)
Friction measured on single MoS2 nanotubes with weak interaction with the underlying substrate
Although a trivial explanation, the interaction strength between the nanotube and underlying substrate, which changes the value of COF for several times, has a drastic influence on the use of nanomaterials as lubricants. The effect explains the still confusing phenomena  of higher friction typical of intracrystalline slip (0.06) than of intercrystalline slip (0.025) obtained for thin and flat MoS2 crystals. The values of COF show an also unexpected similarity, revealing a mechanism of friction, which is beyond the shape effects. On the first view, and neglecting role of defects, a shear (intracrystalline slip) should not differ from intercrystalline slip. It should be even less energetically costive due to known easy shear of basal planes of the MoS2 crystal structure parallel to the sliding direction leading to superlubricity , but just the opposite trend was reported . Our results reveal that the interaction strength between the nanotube and underlying substrate plays a crucial role in the intercrystalline slip. Weak interaction prevents that the shear deformations would contribute to the energy cost in the friction process. The energy released during the friction process cannot dissipate to the substrate as easy as in intracrystalline slip, which is what results in lower consumption of energy by gliding and therefore to lower COF. Local electronic perturbation is also possible. Periodic crystal potential and electron dipole oscillations intensified by breaking of bonds between atoms of counterparts are at the origin of electron-phonon coupling; therefore, a local increase of temperature is expected.
Our experiments were performed in ultra-high vacuum, but the results can help to explain poorly understood instantaneous increase of friction in cases when MoS2 is exposed to water vapor. We propose the possible explanation that water condensates in nanovoids among MoS2 flakes even at low vapor pressure and in accordance with Kelvin equation , it remains there, where by surface tension, it bonds the MoS2 flakes together and influence the interaction strength between them. The intercrystalline slip becomes energetically more costly, and consequently, the friction increases.
Friction and angle of scanning
Deformations of the MoS2 nanotube wall
We have also tested bending of the nanotube wall under the AFM tip as a function of the position of imbedded nano-onions. The wall is so thin that one can expect that a deformation of the wall under AFM load is larger in areas without 'support' of nano-onions. The line profile (Figure 5a) over two nanoparticles indeed reveals a 'camel' shape (Figure 5c). Measuring the distance between the top of the peak (A) and the valley between two peaks (B) at different loads in a range between 3 and 33 nN shows that the distance (i.e., the peak height or the valley depth) changes in a linear way with the load (Figure 5d) revealing the elastic deformation.
Friction measured on a single MoS2 fullerene-like nano-onion
Based on values of the COF, the nanotubes can be divided to those with a strong interaction between the nanotube and the underlying substrate, and to 'semi' self-standing ones. The term semi is used because the measured area was located at the site where the tube was detached from the substrate, although other parts of the same nanotube were well-supported by the substrate. The average COF value of the supported nanotubes is found to be 0.08 ± 0.02, while COF of semi self-standing tubes is a few times lower, i.e., 0.02 ± 0.005. It is known that friction is related to the bonding energy between the surfaces, mainly to the adsorption energy of the sliding surfaces on each other . Moreover, the atomic layers below the gliding surfaces contribute to the dissipation process significantly . Our finding that the substrate, which is for the nanotubes' diameter apart from the studied contact change friction between the nanotubes and the AFM tip, is in line with the model, which considers plastic deformation of the contact as a prime actor in dissipation. In semi self-standing tubes, such dissipation of energy in limited, intracrystalline shear stress is minimized and the tubes behave rigidly. Lower energy is released in the friction process, what together with dragging process explains very low COF.
It is important to note that although the higher COF is found on well-supported nanotubes, this value (0.08 ± 0.02) is still much below the average COF obtained at the same testing conditions on air-cleaved MoS2 single crystal (0.115 ± 0.003) and on HOPG (0.16 ± 0.01). Results found on single crystals are in the range of published friction data for vacuum conditions (different testing methods), 0.15 to 0.3 for MoS2  and 0.06 to 0.3 for graphite , and are higher than the friction coefficient on freshly cleaved MoS2 single crystal (0.023 ± 0.001) measured with Si3N4 AFM tip . It is interesting to note that the last value perfectly matches our results obtained on weakly supported nanotubes.
Friction measurements on a single MoS2 fullerene-like nano-onion revealed in a range of accuracy the same COF (0.077 ± 0.004) as found on well-supported MoS2 nanotubes. This indicates that the shape difference between cylindrical and spherical geometry does not play the dominant role in friction mechanism of MoS2.
For the first time, friction was measured on a single MoS2 nanotube and on a single MoS2 nano-onion, both on HOPG substrate. Experiments were performed in UHV at room temperature. We found that the coefficients of friction between silicon AFM tip and MoS2 nanotube or MoS2 nano-onion are much below the relevant values for flat single crystal MoS2 or graphite. Further, we found indications that friction at the nanoscale depends strongly on interaction strength between the nanotube and underlying substrate, which is what is explained with shear deformation and dissipation of energy. The MoS2 nanotubes with high interaction strength revealed up to four times larger COF (0.08 ± 0.02) than weakly supported tubes with COF of 0.023 ± 0.005. The results explain the contradictory old phenomena of higher friction typical for intracrystalline slip than for intercrystalline slip. They contribute to understanding of typically, highly scattered results using nanomaterials as lubricants. We evidenced that a rolling mechanism of MoS2 fullerene-like nano-onions is possible at low loads.
JJ, BSc is connected with the Solid State Physics Department, Jozef Stefan Institute and Centre of Excellence NAMASTE. MR is a senior researcher at the Solid State Physics Department, Jozef Stefan Institute and Centre of Excellence NAMASTE.
This work was financed by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia, by the FOREMOST project of the European Union Sixth Framework Program under grant number NMP3-CT-2005-515840 and by the Centre of Excellence NAMASTE.
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