Inorganic fullerene-like nanoparticles produced by arc discharge in

water with potential lubricating ability

J.J. Hu, J.E. Bultman, and J.S. Zabinski*

Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/MLBT), Wright-Patterson Air Force Base, Dayton, Ohio

45433-7750, USA

Received 3 November 2003; accepted 8 February 2004

KEY WORDS: inorganic fullerene-like, nano, MoS2, solid lubricant, arc discharge in water

Nano-structured particles such as inorganicfullerene-like (IF)-MoS2 and -WS2 have been studiedfor tribological applications in the past few years.These materials are inert and do not adhere tightly tosubstrate surfaces as does the lamellar form of thematerials. IF-MoS2 and -WS2 have been used effec-tively as solid lubricant additives for oil and were syn-thesized by solid–gas reactions [1,2]. They have alsobeen deposited by ablating MoS2 or WS2 targets withpulsed excimer laser radiation [3], and by localizedhigh-pressure arc discharge [4]. The advantages of IFnanoparticles include oxidation resistance, ultra-lowfriction surface and a wide environmental operatingrange [2,4–6]. The cost of making nano-materials is rel-atively high because of the use of vacuum chambersand gas handling systems that are used in their manu-facture. Simple arc-methods utilizing water or liquidnitrogen mediums permit continuous synthesis ofnanoparticles at relatively low-cost and the process isenvironmentally benign. The arc technique offers ahigh degree of flexibility in producing nano-structures.There have been recent reports of growing carbonnanotubes in liquid nitrogen [7], and carbon nano-onions in water [8]. Although graphite and MoS2 areboth common solid lubricants, nano-structured car-bons are less effective than MoS2 as lubricants. In thisstudy, the arc-method utilizing water was employed togrow some known, as well as new structures. Impor-tant results of our work are that we have observedIF-MoS2 nanoparticles from the arc discharge of MoS2rods in water and that these particles can have differentstructures. Therefore, the arc-method has significantpotential for producing lubricious nano-materials.

A water-based arc-discharge system, as shown infigure 1, was used to grow the nano-particles. The sys-

tem was connected with a DC power supply operatedat a maximum voltage of 30 V and current of 33 A.Both MoS2 and graphite rods have been used to growparticles. The end of a 6.35 mm diameter MoS2 rodwas sharpened to enhance arc discharge, while the flatend of graphite rods were used to initiate discharge.The difference between graphite and MoS2 electrodesis shown schematically at the bottom of figure 1. Mov-ing the anode close to the cathode ignited the arc, andthe arc was sustained by moving the anode holder witha micrometer. The discharge current between MoS2electrodes was smaller than graphite because the elec-trical resistance of MoS2 was higher. The length ofMoS2 rods was shortened to increase the dischargecurrent, which was not required for the graphite rods.IF-particles floated on water and were collected onholey carbon grids. Transmission electron microscopy(TEM) and X-ray energy-dispersive spectroscopy(EDS) were accomplished using a Philips CM200equipped with field emission gun, which provided acomplete coherent electron source and nm-sized probefor nano structure analyses.

The water-based arc device is quite flexible for mak-ing nanoparticles. TEM observations have found car-bon nano-onions, nanotubes and hollow-cagesproduced by the arc discharge of graphite rods inwater. The arrows in figure 2 indicate the differencesin the cores of carbon nanoparticles generated bywater-based synthesis. Because various carbon nanostructures have been widely investigated, we focus onthe details of the water-based arc method using MoS2rods to make the IF nanoparticles.

Large-sized particles produced during arc dis-charge of MoS2 rods mostly fell to the bottom ofthe tank. Examination by X-ray diffractions showedthat they were MoS2 crystals (i.e., large pieces of theelectrode). Material on the water surface consisted ofmembrane-like fragments embedded with particles up

*To whom correspondence should be addressed.

E-mail: Jeffrey.Zabinski@afrl.af.mil

1023-8883/04/1000–0543/0

Tribology Letters, Vol. 17, No. 3, October 2004 543

to sub-micron diameter as shown by the TEM imagein figure 3(a). There were significant amounts ofnano-sized particles as indicated by the arrows,which were difficult to see in comparison to the sub-micron ones. The number of small particles wasgreater than the number of large ones (comparingthose with diameters <100 nm to those >100 nm).The high fraction of small particles may be causedby the natural segregation of objects suspended inwater, where a high surface to volume ratio deter-mines whether an object sank or floated. Layer-structured MoS2, defined as a lamellar crystal, wasconfirmed to be hydrophobic and floatable in waterreferring to the Gaudin–Miaw–Spedden theory [9]. Ahigh density of nanoparticles has been commonlyobserved in the particles floating on water.

High-resolution TEM imaging showed that somemulti-wall-encapsulated nanoparticles were producedas shown in figure 3(b)–(d). The closed shells exhibiteda polyhedral projection of roughly 8 nm long as shownin figure 3(b), and of 15 nm diameter as shown in fig-ure 3(c). Figure 3(d) displays a couple of large- andsmall-encapsulated nanoparticles. The larger particles

have increasingly more sides on its shell and form asmooth rim at low energy. The fringe spacing betweentwo neighboring layers of the shell is about 0.6 nm,which approximately equals the distance between twobasal planes in the hexagonal MoS2 unit cell. The mea-surements were in agreement with previous data onIF-MoS2 and -WS2 structures, which were synthesizedin vacuum and purge gas. In some particles, two orthree layers of MoS2 wrapped a solid core, which haddark TEM image contrast as shown in figure 3(b)–(d).Using an electron beam at 1.0 nm spot size for EDSchemical analyses, core and shell composition wasstudied. A plot of sulfur and molybdenum composi-tions is provided in figure 4. The core is encapsulatedin a multi-wall MoS2 cage and has a relatively highconcentration of Mo, with the atomic ratio of Mo to Sbeing approximately 1:0.83. The EDS measurementson the shell of layer structured MoS2 show a muchhigher concentration of S than that in the core. Thehighest reading of S content has been detected in theregion surrounding the nanoparticles. It has not yetbeen confirmed whether some MoS2 decomposed toMoS2)x during arc discharge, or whether unbalanceddiffusion of Mo and S occurred. Sen et al. reported anexcess amount of W metal encapsulated within theWS2 cage, which was produced by means of laser abla-tion of pressed WS2 target under argon flow [10]. Theybelieve that sulfur loss gave rise to metallic nanoparti-cles. Amorphous MoS3, reduced Mo and W oxideshave been the only known materials in IF cage struc-tures before that study [11]. Our results establish thatthe water-based arc method can produce new type ofIF encapsulates filled with high concentration of Mo.The size range of the IF nanoparticles is 5–30 nm asmeasured by TEM, which is much smaller than parti-cles obtained from other synthesis methods.

The mechanism responsible for the formation of IF-MoS2 is proposed based on the layered crystal struc-ture and arc discharge process. Arc discharge creates ahot plasma zone between the two electrodes, andresults in MoS2 pieces bombarded or burned off from

Figure 1. Water-based arc discharge system for preparing nano-

particles.

Figure 2. Carbon nanoparticles produced by arc discharge of graphite rods in water: (a) large nanoparticle with small core; (b) small nano-

particle with large core.

544 J.J. Hu et al./IF nanoparticles produced by arc discharge in water

the rods. Because of the weak van der Waals bondsbetween adjacent sulfur sheets and strong inter-layercovalent SAMoAS bonds, high temperature or thermalstress could commence bending or faceting of MoS2fragments within or near the hot plasma zone. Thosestructures stabilize by closing themselves, and cool asthey travel through the temperature gradient zonecaused by water and water vaporization. Although the

hypothesis is supported by our current results, moreexperimental data is required for unambiguous inter-pretation.

Besides the closed structures, TEM observationsshowed that some uncompleted products from the arcdischarge of the MoS2 rods were in the water. Thewhite arrows shown in figure 5(a) and (b) indicatesome small fragments of the layer-structured MoS2with a fringe spacing of �0.6 nm. The nanoparticleshown in figure 5(a) has one opening edge that is notcovered with MoS2 layers. Part of the particle doesnot have a shell for some reason. As shown in fig-ure 5(b), an unwrapped nanoparticle at �10 nm diam-eter is adjacent to a wrapped one.

In conclusion, the present results suggest that thewater-based arc method is a simple economical way tosynthesize some familiar nano materials as well as newtypes of lubricious IF nanoparticles. Increasing voltageand power increases the discharge current for MoS2rods. Sharpening and shortening the rod helps to gen-erate a larger plasma volume and consequently moreIF nanoparticles. Further modification to the water-discharge system is underway, and a better separationprocess is required for purifying the IF nanoparticles.

0

02

04

06

08

001

Ato

m %

SoM

Core Shell

Figure 4. EDS chemical analysis of the core and shell that show S

and Mo compositions in the histogram.

Figure 3. Inorganic fullerene-like nanoparticles produced by arc discharge of MoS2 rods in water: (a) materials floating on water surface as

observed in TEM; (b), (c) and (d) high-resolution TEM images of multi-wall-encapsulated nanoparticles with a solid core.

J.J. Hu et al./IF nanoparticles produced by arc discharge in water 545

The tribological properties of the IF nanoparticleswere determined over a wide environmental range, andwill be reported in a separate publication (J.J. Hu andJ.S. Zabinski, to be submitted). The results demon-strate that the IF nanoparticles provide enhanced fric-tion characteristics over macro particles and sputteredfilms.

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Figure 5. (a) Partially wrapped and (b) unwrapped nanoparticles in the synthesis of inorganic fullerene-like MoS2.

546 J.J. Hu et al./IF nanoparticles produced by arc discharge in water