IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 22 (2011) 285714 (10pp) doi:10.1088/0957-4484/22/28/285714

Synthesis of WS2 nanostructures from thereaction of WO3 with CS2 and mechanicalcharacterization of WS2 nanotubecompositesM Tehrani1, C C Luhrs2,5, M S Al-Haik1, J Trevino3 and H Zea4

1 Department of Engineering Science and Mechanics, Virginia Tech, Blacksburg, VA 24061,USA2 Department of Mechanical and Aerospace Engineering, Naval Postgraduate School,Monterey, CA 93943, USA3 Department of Mechanical Engineering, University of New Mexico, Albuquerque,NM 87131, USA4 Departamento de Ingenierıa Quımica y Ambiental, Universidad Nacional de Colombia,Bogota, 11001, Colombia

E-mail: ccluhrs@nps.edu

Received 11 January 2011, in final form 19 May 2011Published 9 June 2011Online at stacks.iop.org/Nano/22/285714

AbstractTungsten disulfide (WS2) nanometer sheets, spheres, fibers and tubes were generated by asynthetic pathway that avoids the use of H2S as the source of sulfur and employs instead CS2

vapor, carried by an Ar or N2/H2 stream in a heated tubular furnace, for the reaction with WO3

precursor powders. The experiments were conducted at temperatures between 700 and 1000 ◦C,while the reaction times expanded between 30 min and 24 h. Characterization methods used toanalyze the products of the synthesis include TEM, SEM, XRD and EDX. We found a strongcorrelation between precursor and product microstructure, although the temperature andreaction times play a critical role in the products’ microstructural features as well. WS2

inorganic fullerene (IF) nanospheres are generated in a wide window of conditions, whilenanotubes and nanofibers are only produced at high temperatures or long reaction times. Aproposed growth mechanism based on the CS2 synthetic approach is presented.Nanoindentation and nano-impulse techniques were used to characterize the mechanicalproperties of polymer matrix–WS2 nanotube composites, finding them superior to equivalentSWCNT composites. The improvements in toughness of nanocomposites based on WS2 can beattributed to geometrical and morphological effects that assisted several tougheningmechanisms such as crack pinning and the formation of an immobilized polymeric interphasearound the nanotubes.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Nanomaterials consisting of various tungsten disulfide (WS2)structures have proven very useful in several applications in-cluding lubricants, catalysts, coatings and shock resistant com-

5 Author to whom any correspondence should be addressed.

posites [1–5]. WS2 tubes, cages and similar inorganic fullerenetype structures (IF-WS2) can be synthesized by diverseapproaches: laser ablation [6, 7], self-assembly [8], templatesynthesis [9, 10], hydrothermal reactions [11], metal–organicchemical vapor deposition (MOCVD) [12, 13], fluidized bedreactors [14–16], spray pyrolysis [17, 18], microwave inducedplasmas [19, 20] and gas–solid reactions [3, 6, 21]. The

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Nanotechnology 22 (2011) 285714 M Tehrani et al

different synthetic methods produce slightly different WS2

structures, depending on the precursor used and the rateof WS2 formation. (NH4)2WS4 thermal decomposition andWO3 reaction with sulfur-containing compounds are amongthe most common routes employed. All the reactions thatutilize WO3 as reactant have similar mechanisms; first WO3

is reduced with hydrogen, then shearing of atomic planesat the surface promotes the reaction of tungsten with sulfur-containing compounds and finally the sulfur atoms replace theoxygen ones to form WS2 [22]. Gas–solid reactions constitutea very simple approach to generate WS2; in most cases WO3

is reacted with a sulfur-containing compound (usually H2S)during extended periods of time at high temperature.

In this investigation, we employ a similar approach toproduce WS2 nanostructures except for the use of H2S. Westudied the reaction of WO3 with CS2 in diverse temperature–atmosphere–reaction time conditions to determine whichsettings will favor the generation of tubular, spherical or sheetlike nanostructures.

To our knowledge, the use of CS2 to generate WS2

has been limited to methods that involve: (i) indirect routesthat require additional sulfur sources, (ii) intermediate stepsentailing the formation of H2S or (iii) the use of CS2 forthe generation of products with restricted shapes and sizes,such as plate like micrometer products. Examples of theseinclude: use of hydrothermal conditions along with ammoniaand water in order to produce H2S that will act as the sulfursource [23, 24], use of CS2 along with CCl4 as liquid media inwhich S and WO3 will be added to form a mixture that will thenbe heated and sonicated before being dried and heated underH2 atmosphere [3] and W metal powder and WO3 reactionwith flowing CS2 to generate plate like micrometer size WS2

products [25].Regarding the mechanical properties, WS2 has been

shown to stand shockwaves as high as 25 GPa and elevatedtemperatures without any significant structural degradationor phase change [2]. The single WS2 nanotube has anelastic modulus of 150–170 GPa and strain to fracture of15% [4]. These outstanding mechanical properties makeWS2 nanostructures very promising for structural applications.Although the WS2 single nanotube mechanical propertiesare not as high as those for single wall carbon nanotubes(SWCNTs), under similar conditions WS2 nanotubes haveshown to be more stable than SWCNTs when exposed to shockwaves [26]. While there are several studies discussing themechanical properties of WS2 structures [27, 28], there is nocited literature on investigating the mechanical aspects of WS2

nanocomposites at the nanoscale.We performed nanoindentation and nano-impulse tests on

composites made out of WS2 nanotubes with an epoxy matrix.The reduced modulus, nanohardness and low velocity impactresistance (dynamic hardness) of the WS2-nanotube-basedcomposites are measured and compared to the ones based onSWCNTs. Therefore, the current investigation provides, forthe first time, a comparative study of the impact resistanceand quasi-static nanoindentation properties for two polymericcomposite systems based on SWCNTs and WS2 nanotubes,respectively.

2. Experimental methods

2.1. The generation of WS2 structures

The window of temperatures and time conditions used inthis study was based on preliminary results that revealed theminimal temperature and time for which WS2 can be obtainedas a pure phase from the reaction of WO3 and CS2 (accordingto XRD analysis of products generated in diverse conditionswhere no WO3 reflections were detected).

Experiments were conducted between 700 and 1000 ◦Cwith treatments that extended from 30 min to 24 h. The gas–solid reaction was performed under inert (argon) and reducingatmospheres (N2/H2: 93%/7%). Flow rates of 20 slpm wereused. Commercial WO3 particles (both micron and nanosizes<100 nm (TEM), Sigma-Aldrich) were used as tungstenprecursor and CS2 (ACS reagent >99.9%, Sigma-Aldrich) assulfur source. The synthesis was carried out as follows. WO3

precursor nanoparticles were placed in a sintered alumina boat,then positioned into a quartz tube whose ends were equippedwith high temperature o-rings and stainless steel lids for gasintroduction and exhaust. A gas purge step was performedin order to assure that no oxygen was contained inside thereactor. The quartz tube was then introduced into a clamptubular furnace and the temperature was raised to the desiredset point. A stream of the carrier gas was then directed to thesurface of liquid CS2 which was contained in a trap locatedbetween the gas source and the furnace. The mixture of carriergas (Ar or N2/H2) with CS2 was introduced into the quartztube for the reaction to occur. The reactor for the synthesisis shown in figure 1. Once the reaction time was over, thevalve connecting the CS2 trap was closed and the powders wereallowed to cool only under carrier gas.

2.2. The preparation of WS2 nanotube composites

To examine the mechanical properties of WS2 nanotubepolymeric composites, WS2 nanotube epoxy composites wereprepared. SWCNT polymeric composites were prepared inidentical conditions for comparison. Short SWCNTs fromCheapTubes.com with a purity of >95%, outside diameter of1–2 nm and average length of 0.5–2 µm were used. The epoxycomprised PR2032 resin and PH3660 hardener (PTM&WIndustries, Inc.). This epoxy system is a medium viscositysystem, mostly used in structural applications. The systemcures at room temperature for 24 h. The properties of the epoxysystem are published at PTM&W Industries, Santa Fe Springs,CA 90670-4092, USA.

Due to a high aspect ratio and intrinsic van der Waalsattraction, the WS2 and carbon nanotubes form agglomeratedbundles and ropes. Ultrasonication has been shown to bean effective way of moderately dispersing carbon nanotubesin some low viscosity liquids [29]. Hence, the nanotubesamples (WS2 and SWCNTs) were independently addedto an ethanol solution 1:10 by weight, and sonicated for2 h using an ultrasonic cleaner at 40 kHz and 700 Wpower. The suspensions were then separately mixed withthe hardener and sonicated for one more hour, after whichvacuum was applied until the alcohol content was completely

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Figure 1. Schematic representation and image of WS2 synthesis setup for the direct reaction between WO3 and CS2.

evaporated. The nanotubes/hardener suspensions were addedto the resin. Further mixing and dispersion of the nanotubeswere performed by simultaneously using a mechanical stirrerand a sonicator for few minutes. Both the WS2 and SWCNTcomposites contained 3 wt% of the corresponding nanotubes.The short gelling time of the epoxy is a limiting factor forlonger dispersal of the nanotubes; therefore, the samples hadto be degassed right after the resin was added to prevent anyair bubbles from getting trapped in the nanocomposite. Thesamples were left to cure for 24 h at room temperature.

The procedure followed for the nanotube dispersion wasbased on our previous work [30, 31], where the methodologyproved effective in dispersing CNTs in the same epoxysystem. We limited the loading percentage to 3% withthe goal of having a point for comparison between the twocomposites while observing the percolation limit for theSWCNTs. Many researchers have posed a limit for dispersingSWCNTs in highly viscous material using sonication orcalendaring [32, 33]; it has been found that beyond a certainvolume fraction (usually around 3%), no further improvementsin the composite’s mechanical properties are achievedusing the dispersion methods previously mentioned. Athigher volume fractions, dispersion and alignment deterioratesignificantly.

2.3. Sample characterization

The samples were analyzed using a Scintag Pad V diffractome-ter/goniometer with Scintillation detector, Datascan software(Materials Data, Inc.) for diffractometer automation and datacollection, and Jade Software (Versions 9, also from MDI) fordata analysis.

TEM characterization was performed on a JEOL 2010high resolution transmission electron microscope (HRTEM)equipped with an EDS analysis detector. SEM analysis wasconducted on a Hitachi S-5200 Nano SEM working at 10 kV.

2.4. WS2 nanotube composite mechanical properties testing

Specimens of each SWCNT and WS2-nanotube-based com-posite were tested using a Nano Test 600 mechanicalnanocharacterization system (Micro Materials Ltd, UK).Nanoindentation tests were performed using a 5 µm radiusspherical diamond tip. A total of 10 load-controlled to 3 mNmaximum load indentations were carried out on differentspots on each nanocomposite specimen. The initial load andloading/unloading rates were set to 0.03 mN and 0.1 mN s−1

respectively. A holding period of 60 s at the peak load was usedand the data were corrected for thermal drift. These parameterswere chosen based on the results of the pre-nanoindentationtests as described in [34]. Oliver–Pharr analysis was used toextract the reduced modulus and hardness [35]. Contact areameasurements from indenting a fused silica reference sampleat different depths were utilized in the analysis.

A pendulum impulse impact test enables repetitiveimpacts to be produced at specified time intervals at the samelocation with precisely controlled force, using a solenoid andtimed relay. The nano-impulse module allows for repetitiveimpacts of the energized diamond probe into the materialsurface. The indenter tip was accelerated in less than a tenthof a second from a distance of 12.5 µm to the surface ofthe sample to produce a load of 3 mN. The repetitive impactwas performed 45 times in each spot with the diamond probebeing two seconds in contact and two seconds off the surface.

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Figure 2. Electron microscopy images of diverse WS2 nanostructures generated from the reaction of CS2, used as sulfur source, with WO3.(a) SEM image of nanotubes, (b) TEM image of nanofibers, (c) SEM micrograph of thin sheets, (d) TEM of cage like nanoparticles and(e) relative amount of each microstructure for samples produced under reducing atmospheres at diverse temperatures and times of treatment.

Measurement of the volume change between the first and laststrikes is used as a qualitative metric for the energy absorptionof the different samples. A total of five repetitive nano-impulsetests were carried out on each sample. Impact resistance is thenmeasured as the ability of the material to undergo less plasticdeformation under repetitive impact [36, 37].

3. Results and discussion

3.1. Microstructural features of WS2 products

The use of micrometer WO3 powder precursor and inertatmospheres for the reaction of WO3 with CS2, renderedWS2 micrometer plate like structures, thin sheets and

some nanofibers whose maximum lengths were <70 nm.Experiments performed employing short reaction times, lessthan 3 h, formed only extremely thin sheets of WS2, thatresembled the structure of carbon materials such as graphene.The generation of short fibers from micron WO3 and CS2

carried out under inert environments required the use of longerreaction times, of the order of 24 h, and produced mixtures ofstructures: a high yield of thin sheets and plates and very lowyields of fibers. Nanometer hollow spherical or cage like WS2

particles, with the occasional appearance of short fibers, wereobserved only when micrometer WO3 was reacted with CS2 inreducing atmospheres. No nanotubes were obtained by eitherthe use of inert atmospheres or micron size oxide precursors.

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Figure 3. IF-WS2 spherical nanoparticles with average diametersclose to 50 nm and hollow structures tend to agglomerate to formhollow micron size spheres. (a) SEM image of WS2 nanospheresforming clusters and (b) WS2 hollow spherical nanostructuresobserved by TEM.

WO3 micron-sized powder did not react at low temperaturesfor short periods of time.

When starting with WO3 nanoparticles as precursor,the nanoscale features in the starting materials aided thegeneration of WS2 products with features in the nanometerrange. Electron microscopy observations of samples generatedby the WO3 nanoparticle reaction with CS2 corroborated theexistence of diverse WS2 nanostructures: tubes, spherical IFparticles, fibers and thin sheets. Figure 2 presents electronmicroscopy images of some of those features: (a) nanotubes,with diameters in the nanoscale but lengths in the micronscale, with hollow centers and multiwall structures, (b) shortfibers of no more than 70 nm in length and with a differentappearance than nanotubes, no hollowness observed, wereoccasionally identified as well, (c) thin sheets of WS2 werespotted under certain experimental conditions, the thickness ofthe sheets was in most cases smaller than 3 nm, and (d) cagelike nanoparticles, with diameters of the order of 50–70 nm andthe characteristic interplanar spacing for IF WS2 nano-onions.Figure 2(e) presents the relative amounts of each of thesemicrostructural features for diverse experimental conditionsusing reducing atmospheres. These relative amounts werecalculated from SEM micrographs; images of multiple regionsof each sample were obtained and the percentages of areaoccupied by nanoparticles, nanotubes, fibers or thin sheets withrespect to the total determined.

Figure 4. XRD analysis of nanotube samples found after 3 h ofreaction at 900 ◦C shows reflections that can be identified astungstenite, WS2.

Similar features to those of the thin sheets presentedherein were observed in quasi-two-dimensional graphene likeWS2 structures, whose synthesis was recently accomplished byexfoliation and intercalation routes [38].

The use of reducing atmospheres greatly increased theamount of inorganic fullerene type and nanotubular structuresgenerated. The microstructure of the synthesized particles wasprofoundly affected by the temperature and the duration of theexperiment as well. Thin sheets were more apparent whenthe WO3 nano-powder synthesis ran for short periods of time.Nanotubes were more abundant when the WO3 was exposedfor times longer than 6 h and/or temperatures equal to 800 ◦Cor higher.

Spherical particles were observed to form during all ofthe time/temperature combinations and had the tendency toform micrometer size clusters. WS2 nanofibers co-existedamong these other structures; although never seen as thepredominant microstructure when reducing conditions wereused. Nanotubes were the most abundant microstructural typefor samples annealed for 24 h.

The most abundant structures generated from the reactionof CS2, used as sulfur precursor, and WO3 under reducingconditions were IF-WS2 spherical nanoparticles. Thesecage like materials possessed different morphologies such asperfect spheres, semispherical particles or structures in whichcrystalline directions were evident and resulted in polyhedralshapes. IF type WS2 nanoparticles were prone to agglomerateand, while nanometer features were still observable, theclusters acquired hollow micron size spherical shapes. AnSEM image of WS2 nanospheres as part of a cluster ispresented in figure 3(a). The hollow center of the WS2

spherical nanostructures was consistently observed by TEM.See figure 3(b).

XRD analysis of the samples revealed that the completetransformation to WS2 occurs after 3 h of reaction in reducingenvironment at the CS2 vapor–gas flow conditions used.Figure 4 shows the x-ray diffraction pattern of a sample inwhich nanotubular structures dominated the microstructure.One could observe that the main peaks corresponded totungstenite and only a small intensity peak located close to 24◦(*) remained for WO3.

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Figure 5. (a) WS2 nanotubes are commonly observed by SEM insamples that also present micron size agglomerates of IF-WS2,(b) the nanotubes’ lengths are of the order of several micrometers,with average diameters of 60 nm, (c) the tubes’ growth proceeds inmultiple directions (not aligned).

WS2 nanotubes were frequently observed by SEM analy-sis in samples that also presented micron size agglomerates ofIF-WS2, i.e. reaction times of 12 h and temperatures of 900 ◦C,figure 5. The nanotubes’ lengths were of the order of severalmicrometers, with average diameters of 60 nm.

3.2. Proposed growth mechanisms

Tenne and co-workers have identified the basic steps in theWS2 generation from WO3 and H2S [16, 22, 39, 40]. The

Table 1. Nanoindentation test results.

WS2-based compositeSWCNT-basedcomposite

Hardness (GPa) 0.190 06 ± 0.01363 0.173 88 ± 0.00240Reduced modulus (GPa) 4.438 ± 0.234 3.487 ± 0.054

first step is usually correlated with the oxide reduction, whichpromotes shearing of surface atomic planes, followed bya sulfidization step that forms closed shell sulfide layers.Subsequent propagation of hydrogen within the particulatestructure will reduce further oxide layers, followed by asulfidization process of the same, with an inward reactionfront.

As mentioned earlier, CS2 will react with WO3 to formWS2 in inert atmospheres although nanotubes will not begenerated under such conditions. In order to form nanotubesusing CS2, reducing environments are required, in the caseof our experiments supplied by a mixture of 7% H2 in N2.In a similar fashion to that reported for H2S-based reactions,the synthesis of WS2 nanotubes and nanospheres from CS2

implies a delicate balance between alternate reduction andsulfidization steps. TEM observations of nanotubes evidencedthis process: the nanotubes presented multiwall structuresthat were commonly divided in sections in which the internalwalls collapsed forming caps at regular spacings, while theexternal walls’ growth continued. The nanotubes’ internalwalls seem to form the closed structures prior to the growthof the external layers, as TEM observations of the tubes tipsrevealed, supporting the idea of oxide material condensing insuch points, followed by its reduction and sulfidization, seefigure 6. In contrast, synthesis performed in inert atmosphereswill favor the exfoliation of layers and formation of thin WS2

sheets.Electron microscopy images of the nanotubes show that

they are not aligned with each other and they seem to containsegments that change their growth direction multiple times,suggesting a spiral or step winding growth.

3.3. Mechanical properties

Fractured surfaces of the nanocomposites were coated witha thin layer of gold and observed under SEM. SWCNT andWS2 nanotube dispersions are presented in figures 7(a) and (b).Figure 7(c) shows two representative nanoindentation curvesfor the two composites based on SWCNT and WS2 nanotubes.

Table 1 presents the results of Oliver–Pharr analysis [35]of the nanoindentation tests. The results were averaged over 10tests for each sample. The neat sample has a reduced modulusof 3.47 ± 0.25 GPa and hardness of 0.1918 ± 0.016 GPa [41].

These results revealed that the hardness and Young’smodulus of the WS2-based composite are 10% and 27%better than the corresponding values for the SWCNT-based composite. Sample nano-impulse tests of the twonanocomposites are shown in figures 8(a) and (b) while cratevolume changes are presented in table 2.

Based on this analysis the impact resistance of the WS2-based composite is roughly 28% higher than the correspondingvalue for the SWCNT-based composite.

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Figure 6. TEM observations of the nanotube samples show: (a) areasof the sample in which nanotubes grow from clusters of WS2

semispherical particles that do not present evidence of crystallinecomponents, (b) the tubes present multiwall structures thatcommonly are divided into sections in which the internal wallscollapse forming caps while the external walls are alwayscontinuous. (c), (d) The nanotubes’ internal walls seem to form theclosed structures prior to the growth of the external layers, asevidenced in the tubes’ tips.

The strengthening mechanisms in the epoxy systemsstrongly rely upon the degree of dispersion of the nanotubesas well as the extent of cohesive interactions between thetubes and polymer chains [29, 42]. According to the SEM

Table 2. Nano-impulse test results.

Crate volume change (nm3)

SWCNTs-based composite (1.69 ± 0.12) ×1011

WS2-based composite (1.21 ± 0.15) ×1011

micrographs shown in figures 7(a) and (b) a reasonable state ofdispersion for both nanocomposite systems exists, under whichthe WS2 performs better than the SWCNTs in both quasi-staticand high strain rate conditions.

It is well known that the mechanical properties ofSWCNTs are much higher compared to those of WS2. Forexample the Young’s modulus for WS2 nanotubes was foundto be in the range of 140–175 GPA [7] compared to thatof SWCNTs, ∼1.2 TPA [8]. The ultimate strength ofWS2 nanotubes with an average diameter of 10 nm wasfound to be in the range of 3.7–16.3 GPa [9] comparedto 250 GPa for SWCNTs [8]. Despite the superiorstrength and elastic properties of the SWCNTs [43, 44],these properties are not directly transmitted to their derivedpolymeric composites. Dispersion and deagglomeration aretypically dominating factors in limiting the enhancement ofmechanical properties [4].

Furthermore, several mechanisms that stem from thesize and morphology of the nanofillers control the overallfracture toughness of the nanocomposites. For example, it iswell recognized that materials that can exhibit crack pinningtypically will possess higher fracture resistance. A crackpinning mechanism has been invoked for nanoparticles [45].This is analogous to the restricted movement of dislocationsthrough metals by incorporating much harder and strongerparticles (dispersion hardening). The crack pinning occurswhen the toughening particles in the polymeric composite arelarger than the crack-opening displacement. It is likely thatthe SWCNTs (1–2 nm in diameter) are much smaller thanthe crack-opening displacement and thus they are unlikely tocause crack pinning. WS2 nanotubes, on the contrary, aremuch larger (70 nm diameter) than SWCNTs and thus they aremore likely to pin the cracks’ movement and thus enhance thetoughness.

Another factor that plays a role in enhancing the toughnessof the nanocomposite is the formation of an interphase orimmobilized layer of polymer around the agglomerates. Thislayer can exhibit thicknesses anywhere in the range of tens ofnanometers to 1.4 µm [46]. SWCNTs tend to agglomeratemore than WS2 nanotubes upon introducing them into theepoxy matrix; therefore their interparticle distance in thederived nanocomposites is larger even for equal loadings.However, for WS2, as a consequence of a better dispersion, theimmobilized interphase may be present throughout the epoxymatrix causing an improved crack resistance.

4. Conclusions

The work outlined in this paper shows that the synthesisof WS2 nanostructures can be successfully performed usingCS2 as sulfur providing agent, in both inert and reducing

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Figure 7. Scanning electron micrograph of (a) SWCNT/epoxy and (b) WS2/epoxy nanocomposite fracture surfaces. (c) Loading/unloadinghysteresis of the two nanocomposite systems, WS2 and SWCNTs, respectively.

Figure 8. Sample nano-impulse tests of (a) WS2 and (b) SWCNTnanocomposites, respectively.

atmospheres. The former environment used with micron sizeWO3 precursors yields mainly plate like and extremely thinWS2 sheets. Long reaction times, i.e. 24 h, for the same inertatmosphere produce spherical IF nanoparticles and very fewfibers. No nanotubes are generated from micron precursorsin inert environments. The precursor microstructure playeda large role in determining the outcome regarding the newlyformed WS2 features, such as the particles’ shapes and sizes.

Use of WO3 precursor nano-powders along with thereducing environments was indispensable in order to generatesignificant amounts of IF WS2 nanospheres and nanotubes.The microstructure of the synthesized particles was greatlyaffected by the temperature and the duration of the experiment.Nanotubes were detected when the WO3–CS2 reactionoccurred for times longer than six hours and/or temperaturesequal to 800 ◦C or higher. Nanosheets were more apparentwhen the WO3 nano-powder synthesis ran for short periodsof time. Nanospheres were formed during most of thetime/temperature combinations and have the tendency toform micrometer size clusters. Nanofibers were observedalong diverse structures and were never the predominantmicrostructure. The growth mechanisms are similar to theones reported for H2S-based reactions, with reduction andsulfidization process dominating the growth of the nanotubularstructures.

Nanoindentation tests revealed that the WS2-basedcomposite hardness and reduced modulus are 10% and

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27% better than the corresponding values for the SWCNT-based composite, respectively. The nano-impulse testsindicate that the impact resistance of the WS2-nanotube-based composite is roughly 28% higher compared to theSWCNT-based composite. Despite the superior propertiesof SWCNTs, several processing (dispersion), geometricaland morphological effects can restrict their enhancementof the fracture toughness. WS2-based nanocomposites,on the contrary, due to the ease of dispersion and thedimension induced mechanisms such as crack pinning andimmobilized epoxy interphase, are tougher than the SWCNT-based composites.

Acknowledgments

The authors greatly acknowledge the support of the NationalScience Foundation (NSF) Awards # EEC-0741525 and NSF-CMMI-0846589.

References

[1] Espino J, Alvarez L, Ornelas C, Rico J L, Fuentes S,Berhault G and Alonso G 2003 Comparative study of WS2

and Co(Ni)/ WS2 HDS catalysts prepared by ex situ/in situactivation of ammonium thiotungstate Catal. Lett. 90 71–80

[2] Zhu Y Q et al 2005 Shock-absorbing and failure mechanisms ofWS2 and MoS2 nanoparticles with fullerene-like structuresunder shock wave pressure J. Am. Chem. Soc. 127 16263–72

[3] Yang H et al 2006 Synthesis of inorganic fullerene-like WS2

nanoparticles and their lubricating performanceNanotechnology 17 1512–9

[4] Kaplan-Ashiri I and Tenne R 2007 Mechanical properties ofWS2 nanotubes J. Cluster. Sci. 18 549–63

[5] Rosentsveig R et al 2009 Fullerene-like MoS2 nanoparticlesand their tribological behavior Tribol. Lett. 36 175–82

[6] Nath M, Govindaraj A and Rao C N R 2001 Simple synthesisof MoS2 and WS2 nanotubes Adv. Mater. 13 283–6

[7] Schuffenhauer C et al 2005 Synthesis of fullerene-like tantalumdisulfide nanoparticles by a gas-phase reaction and laserablation Small 1 1100–9

[8] Remskar M et al 2001 Self-assembly ofsubnanometer-diametersingle-wall MoS2 nanotubes Science 292 479–81

[9] Whitby R L D et al 2002 Complex WS2 nanostructures Chem.Phys. Lett. 359 68–76

[10] Deng H, Chen C, Peng Q and Li Y 2006 Formation oftransition-metal sulfide microspheres or microtubes Mater.Chem. Phys. 100 224–9

[11] Shang Y, Xia J, Xu Z and Chen W 2005 Hydrothermalsynthesis and characterization of quasi-1-D tungstendisulfide nanocrystal J. Dispers. Sci. Technol. 26 635–9

[12] Zink N, Therese H A and Pansiot J 2008 In situ heating TEMstudy of onion-like WS2 and MoS2 nanostructures obtainedvia MOCVD Chem. Mater. 20 65–71

[13] Zink N, Pansiot J and Kieffer J 2007 Selective synthesis ofhollow and filled fullerene-like (IF) WS2 nanoparticles viametal–organic chemical vapor deposition Chem. Mater.19 6391–400

[14] Rosentsveig R, Margolin A, Feldman Y, Popovitz-Biro R andTenne R 2002 Bundles and foils of WS2 nanotubes Appl.Phys. A 74 367–9

[15] Rosentsveig R, Margolin A, Feldman Y, Popovitz-Biro R andTenne R 2002 WS2 nanotube bundles and foils Chem. Mater.14 471–3

[16] Margolin A, Rosentsveig R, Albu-Yaron A,Popovitz-Biro R and Tenne R 2004 Study of the growthmechanism of WS2 nanotubes produced by a fluidized bedreactor J. Mater. Chem. 14 617–24

[17] Bastide S et al 2004 Synthesis of inorganic fullerenes andnanoboxes of MOS2 and WS2 by spray pyrolysis Abst.Papers Am. Chem. Soc. 228 200-COLL U482

[18] Bastide S, Duphil D, Borra J and Levy-Clement C 2006 WS2

closed nanoboxes synthesized by spray pyrolysis Adv. Mater.18 106–9

[19] Vollath D and Szabo D V 1998 Synthesis of nanocrystallineMoS2 and WS2 in a microwave plasma Mater. Lett.35 236–44

[20] Brooks D J, Douthwaite R E, Brydson R, Calvert C,Measures M G and Watson A 2006 Synthesis of inorganicfullerene (MS2, M = Zr, Hf and W) phases using H2S andN-2/H-2 microwave-induced plasmas Nanotechnology17 1245–50

[21] Chen J, Li S, Gao F and Tao Z 2003 Synthesis andcharacterization of WS2 nanotubes Chem. Mater. 15 1012–9

[22] Tenne R, Homyonfer M and Feldman Y 1998 Nanoparticles oflayered compounds with hollow cage structures (inorganicfullerene-like structures) Chem. Mater. 10 3225–38

[23] Chen X, Wang X, Wang Z, Yu W and Qian Y 2004 Directsulfidization synthesis of high-quality binary sulfides (WS2,MoS2, and V5S8) from the respective oxides Mater. Chem.Phys. 87 327–31

[24] Afanasiev P 2008 Synthetic approaches to the molybdenumsulfide materials C. R. Chim. 11 159–82

[25] Shin W, Yoon D H and Kim S J 1998 Synthesis of tungstendisulfide solid lubricant by solid–gas reactions J. Mater. Sci.Lett. 17 1731–4

[26] Zhu Y Q et al 2003 Shock-wave resistance of WS2 nanotubesJ. Am. Chem. Soc. 125 1329–33

[27] Roy M, Koch T and Pauschitz A 2010 The influence ofsputtering procedure on nanoindentation and nanoscratchbehaviour of W–S–C film Appl. Surf. Sci. 256 6850–8

[28] Voevodin A A, O’Neil J P and Sabinski J S 1999Nanocomposite tribological coatings for aerospaceapplications Surf. Coat. Technol. 116 36–45

[29] Song Y S and Youn J R 2005 Influence of dispersion states ofcarbon nanotubes on physical properties of epoxynanocomposites Carbon 43 1378–85

[30] Choi E S et al 2003 Enhancement of thermal and electricalproperties of carbon nanotube polymer composites bymagnetic field processing J. Appl. Phys. 94 6034–9

[31] Garmestani H et al 2003 Polymer–mediated alignment ofcarbon nanotubes under high magnetic fields Adv. Mater.15 1918–21

[32] Jin L, Bower C and Zhou O 1998 Alignment of carbonnanotubes in a polymer matrix by mechanical stretchingAppl. Phys. Lett. 73 1197–9

[33] Gojny F H, Wichmann M H G, Kopke U, Fiedler B andSchulte K 2004 Carbon nanotube-reinforcedepoxy-composites: enhanced stiffness and fracturetoughness at low nanotube content Compos. Sci. Technol.64 2363–71

[34] Tehrani M, Safdari M and Al-Haik M S 2011Nanocharacterization of creep behavior of multiwall carbonnanotubes/epoxy nanocomposites Int. J. Plast. 27 887–901

[35] Oliver W C and Pharr G M 1992 An improved technique fordetermining hardness and elastic modulus using load anddisplacement sensing indentation experiments J. Mater. Res.7 1564–83

[36] Beake B D and Smith J F 2004 Nano-impact testing-aneffective tool for assessing the resistance of advancedwear-resistant coatings to fatigue failure and delaminationSurf. Coat. Technol. 188 594–8

9

Nanotechnology 22 (2011) 285714 M Tehrani et al

[37] Beake B D, Goodes S R, Smith J F and Gao F 2004 Nanoscalerepetitive impacttesting of polymer films J. Mater. Res.19 237–47

[38] Matte H S et al 2010 MoS2 and WS2 analogues of grapheneAngew. Chem. Int. Edn 49 4059–62

[39] Rothschild A, Sloan J and Tenne R 2000 Growth of WS2

nanotubes phases J. Am. Chem. Soc. 122 5169–79[40] Tenne R and Redlich M 2010 Recent progress in the research of

inorganic fullerene-like nanoparticles and inorganicnanotubes Chem. Soc. Rev. 39 1423–34

[41] Al-Haik M S, Trinkle S, Momotyuk O, Roeder B T,Kemper K and Hussaini M Y 2007 Nanocharacterization ofproton radiation damage on magnetically oriented epoxy Int.J. Polym. Anal. Charact. 12 413–30

[42] Zhang S G et al 2003 Use of functionalized WS2 nanotubes toproduce new polystyrene/polymethylmethacrylatenanocomposites Polymer 44 2109–15

[43] Yakobson B I and Avouris P 2001 Mechanical properties ofcarbon nanotubes Carbon Nanotubes (Topics in AppliedPhysics vol 80) pp 287–327

[44] Kaplan-Ashiri I et al 2004 Mechanical behavior of individualWS2 nanotubes J. Mater. Res. 19 454–9

[45] Johnsen B B, Kinloch A J, Mohammed R D, Taylor A C andSprenger S 2007 Toughening mechanisms ofnanoparticle-modified epoxy polymers Polymer 48 530–41

[46] Ragosta G, Abbate M, Musto P, Scarinzi G and Mascia L 2005Epoxy-silica particulate nanocomposites: chemicalinteractions, reinforcement and fracture toughness Polymer46 10506–16

10