Materials Research Bulletin 46 (2011) 1504–1509

Template synthesis and characterization of molybdenum disulfide nanotubules

Dongbo Yu, Yi Feng *, Yanfang Zhu, Xuebin Zhang, Bin Li, Huiqiang Liu

School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009, People’s Republic of China

A R T I C L E I N F O

Article history:

Received 20 September 2010

Received in revised form 7 April 2011

Accepted 19 April 2011

Available online 28 April 2011

Keywords:

A. Nanostructure

D. Microstructure

A B S T R A C T

Molybdenum disulfide nanotubules were prepared by thermal decomposition of ammonium

thiomolybdate ((NH4)2MoS4) precursors on anodized aluminum oxide template. Large quantities of

hollow MoS2 nanotubules with the bamboo-like structure were obtained. The morphology and

structures of MoS2 tubules were characterized by scanning electron microscopy, high-resolution

transmission electron microscopy, energy dispersive spectroscopy, electron diffraction and optical

absorption spectroscopy. MoS2 nanotubules completely reflected the three-dimensional structure of

nanopores in template. The properties of Mo–S chemical bonds in lattice structure and the wetting state

between porous surface and precursor have a great effect on the formation of sections in nanotubules,

the ridges in the nanopores also play a very special role of this formation.

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Materials Research Bulletin

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1. Introduction

Inorganic nanotubules and in particular nanotubules of thetransition metal dichalcogenides (e.g., MoS2, WS2) in analogy tocarbon nanotubules have been attracted considerable attention. Inrecent years, molybdenum disulphide has been the subject ofsignificant research for applications including catalytic [1,2] non-aqueous lithium batteries [3–5] and wear resistance [6–9].Currently the most intriguing application for the fullerene-likematerials is on the field of tribology, where 2H–MoS2 plateletshave already found substantial number of applications [10]. Thefact that the tribological behavior of MoS2 nanomaterials issuperior to that of carbon nanotubules in atmospheric environ-ment has already been proved very reliable. By analogy to graphite,nanoparticles of inorganic compounds such as MoS2, is a quasi-two-dimensional (2D) compound. Atoms within a layer are boundby strong covalent forces, while individual layers are held togetherby Van Der Waals interactions. The stacking sequence of the layerscan lead to the formation of either a hexagonal polymorph withtwo layers in the unit cell (2H), a rhombohedral with three layers(3R) or a trigonal with one layer (1T).

Tenne and co-workers [11,12] first reported the production offullerene-like MoS2 nanotubules via the gas phase reactionbetween MoO3�x and H2S in a reducing atmosphere at elevatedtemperature (800–1000 8C). MoS2 and WS2 nanotubules have beenprepared by simple heating MoS3 or (NH4)2MoS4 at 1200–1300 8Cunder a flow of hydrogen [13]. MoS2 nanotubules were synthesizedby thermal decomposition of ammonium thiomolybdate precur-

* Corresponding author. Tel.: +86 551 2902557; fax: +86 551 2901362.

E-mail address: fy123@mail.hf.ah.cn (Y. Feng).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.04.018

sors at 450 8C [14–16]. A number of bulk quantities of MoS2

nanotubules (NTs) have been fabricated by the chemical vapordeposition of a single source precursor based on tetrakis(diethylaminodithiocarbocarbomato) molybdate (IV) [17].

The growth in anodic aluminum oxide (AAO) is one of the mostpromising approaches owing to the tunable pore architecture,good mechanical strength and thermal stability. In this article, wedemonstrated another method via traditional two-step anodiza-tion to fabricate branched AAO nanopores with defined architec-tures. We report the formation of branched molybdenum disulfidenanotubules through thermal decomposition of (NH4)2MoS4

within the confined voids of a porous aluminum oxide membranetemplate. Carbon nanotubules [18,19] and various nanowires[20,21] with branched structures have been reported. Neverthe-less, little research has been reported on the synthesis of inorganicnanostructures with branched topologies. To the best of ourknowledge, the fabrication of multi-branched MoS2 nanotubuleshas seldom been reported. This concept has not yet been fullyexplored for one-dimensional layered metal dichalcogenidematerials, such as MoS2.

2. Experimental procedures

2.1. Synthesis of aluminum oxide templates

The anodic alumina membrane was fabricated following thetwo-step anodization process of Al foils in acid solutions. Pure Al(99.999%) foils were annealed at 500 8C for 4 h under argonatmosphere and then electrochemically polished in a mixture ofHClO4 and ethanol (1:5 in vol.) at 20 V. The size of the pores couldbe varied from 50 to 200 nm, depending on the anodization acidused or the anodization voltage. After 4–5 h anodization, the

Fig. 1. SEM images of top and side view of AAO membrane prepared under different anodization conditions (a) and (b) AAO membrane anodized in 0.3 M oxalic acid at 50 V (c)

and (d) AAO membrane anodized in 0.3 M phosphoric acid at 120 V.

D. Yu et al. / Materials Research Bulletin 46 (2011) 1504–1509 1505

sample was immersed in a mixture of phosphoric acid and chromicacid to remove the porous aluminum oxide formed in the firstanodization. The remaining Al foil was anodized for second timeunder the same condition for 12 h or longer according to thedesired depth. In addition, both the first and second anodizationwas carried out in ice-water bath to keep the temperature at 0 8Cduring the whole process. Then the sample was etched using CuCl2solution to remove the aluminum substrate. The pore bottom wassubsequently opened and widened by chemical etching in 5 wt%phosphoric acid at room temperature for more than 2 h. The

Fig. 2. SEM images of MoS2 nanotubular arrays grown in different templates (a) template

acid at 120 V (c) and (d) corresponding EDS spectra.

scanning electron microscopy (SEM) images provide informationon the top and side view of AAO membrane prepared underdifferent anodization conditions. Fig. 1a and b show thenanochannels of AAO membrane anodized in 0.3 M oxalic acidat 50 V and diameters of about 70 nm, respectively. While those ofAAO membrane anodized in 0.3 M phosphoric acid at 120 V inFig. 1c and d were branched with diameters of about 240 nm. Theanodization process became unstable due to the high voltage,resulting in the growth of branched nanopores. The structure ofbranches depended mainly on the electrolyte and the anodic

s anodized in 0.04 M oxalic acid at 80 V (b) templates anodized in 0.3 M phosphoric

Fig. 3. XRD of as-prepared MoS2 nanotubules.

D. Yu et al. / Materials Research Bulletin 46 (2011) 1504–15091506

voltage and the number of branches increased as the anodicvoltage varies.

2.2. Fabrication of MoS2 nanotubules

MoS2 nanotubules were prepared via a modified method ofZelenski and Dorhout. The solution of the (NH4)2MoS4 thiomo-lybdate precursor was prepared in the solvent of dimethylsulfoxide (DMSO) by magnetic stirring to ensure that the solidhas dissolved completely in the red-brown solution. AAOtemplates were immersed into precursor solution for appropriatetime and then dried at 70 8C until the solvent was evaporatedcompletely. Repeated dipping and drying was performed beforethe loaded templates were placed into a horizontal tubularfurnace. A mixture of 10% H2/N2 atmosphere with a total flow of100 ml/min was ventilated into the tube while increasing thetemperature to 450 8C at a rate of 10 8C/min and keeping for 2 h.

Different AAO membranes were applied as templates forsynthesis of MoS2 nanotubules by pyrolytic process under thesame conditions. The morphologies and structures of MoS2

nanotubules were characterized by X-ray diffraction (XRD, D/MAX-2500VC/PC), a scanning electron microscope (SEM, Sir-ion200), electron diffraction, a transmission electron microscope(TEM, H800 at 200KV) and a high-resolution scanning electronmicroscope (HRTEM, JEOL2010 at 200KV, Ll = 20.08 mm nm).Specimen for (SEM) was affixed to a thick film of epoxy resin andthen repeated rinsed in 3 M NaOH to dissolve the aluminatemplate. This yielded an ensemble of MoS2 nanotubulesprotruding from the substrate surface like the bristles of brush.The surface of this sample was sputtered with Au prior to SEMimage. In order to prepare specimens for TEM and HRTEM, MoS2

Fig. 4. SEM and corresponding TEM images of MoS2 nanotubular arrays with differe

nanotubules synthesized by templates anodized in 0.3 M oxalic acid at 50 V (c) and (d) bra

(f) branched MoS2 nanotubules by templates anodized in 0.3 M phosphoric acid at 120

nanotubules were released by completely dissolving alumina in5 M NaOH solution, and then separated from solution bycentrifugation. After washing for several times with distilledwater, the nanotubules were well dispersed in ethanol. UV–Visabsorption spectrum of MoS2 nanotubules were recorded with aspectrometer (SolidSpec-3700 Series) from 200 nm to 800 nm.

3. Results and discussion

Fig. 2a and b illustrates the typical SEM images of denselypacked, erected MoS2 nanotubular array after dissolving AAO

nt morphologies grown in different templates (a) and (b) monodispersed MoS2

nched MoS2 nanotubules by templates anodized in 0.04 M oxalic acid at 80 V (e) and

V.

Fig. 5. Typical TEM images of MoS2 nanotubules.

D. Yu et al. / Materials Research Bulletin 46 (2011) 1504–1509 1507

templates. They reveal that almost each pore of the aluminamembrane is filled with nanotubules. The MoS2 nanotubulesprefer to tangle together without the support of the AAO templateand large-scale nanotubules are congregated into some clusters.The corresponding EDS analysis result was shown in Fig. 2c and dprovides qualitative evidence for the presence of Mo and S in thenanotubules. Because of the small quantity of MoS2 nanotubules,the testing are examined on a copper-net (using in TEM), which isthickly covered by close bed of nanotubules. Fig. 3 shows thepowder X-ray diffraction (XRD) spectrum of as-prepared MoS2

nanotubules, where the strongest peaks can be assigned to thehexagonal phase (2H), some miscellaneous peaks have beensmoothed as well. Fig. 4 depicts SEM and corresponding TEMimages of MoS2 nanotubular arrays with bamboo- like structurevia different templates. SEM images were shown in Fig. 4a and bindicates that MoS2 nanotubules with diameter of 65 nm inaverage were straight and arranged uniformly by the restriction ofthe template pores. Some chipped walls are observed on the tubesurfaces, showing the hollow structure of the nanotubules (Fig. 4amarked by arrow). Fig. 4c and e depicts the multi-branched MoS2

nanotubules growth in templates anodized in 0.04 M oxalic acid at80 V with average diameter of about 120 nm and in 3 Mphosphoric acid at 120 V with average diameter of about240 nm, respectively. It suggests that multi-branched MoS2

nanotubules can be obtained not only in templates anodized inphosphoric acid, but also in those anodized in oxalic acid. It can beobserved that these nanotubules are closed in sections showing theso-called bamboo structure as the TEM images shown in Fig. 4b, dand f. It is noted that each node has different lengths, and they areclosed and connected with each other in the tip. Alternatively,these nanotubules can be seen as a line of nanocapsules assembledtogether during their formation processes. Isolated stacking ofnanocapsules along the nanotubular structures indicates thatindividual nanocapsules most probably grow from different nuclei.

An enlarged image of the branched nanotubules shown inFig. 4e makes us to distinguish different types of ramificationwith diameters slightly smaller to main tubes. Some of thebranches are short and closely packed with saw-like appearancelabeled by arrow B while others are long and Y-shaped labeled byarrow A. To obtain more details about the structure andformation mechanism of as-synthesized nanotubules in ourproduct, a typical Y-shaped nanotubule shown in Fig. 5a agreeswell with the images obtained by SEM in Fig. 1d. The uniqueshapes of MoS2 produced by our reactions likely reflect the shapeof the thiomolybdate precursor left behind in the template afterthe solvent evaporated, so it has been found out that thenanotubules in Fig. 4e have the same shape as the nanopores intemplates marked by a pane in Fig. 1d. It suggests that theformations we observed within the tubules of the MoS2 resultedfrom the evaporation dynamics of the solvent-precursor mixturerather than any annealing process of the MoS2 that had beenformed [14]. So it is reasonable to conclude that MoS2

nanotubules perfectly copy the three-dimensional structure ofthe AAO templates controlling through the anodic voltage andelectrolyte adopted for anodization [22,23]. The dimensions ofthe nanotubules are confined by the pores of AAO template, sothe length and average diameter of MoS2 nanotubules corre-spond to the diameter and thickness of the membrane.

Observing Fig. 5b carefully, we could find that the produce wehave obtained is not tubules, much more likes films, and at least itis an incomplete tubule. In the case of template with porediameters more than 300 nm, the produce is largely existed in theform of curved imperfect films rather than tubes. To sum up, as thetemplate pores becomes large enough, intact tubule cannot beformed any longer and the morphology of tubules is out of controland films of MoS2 nanostructures are preferred, i.e., intact tubule

cannot be formed any longer, yet the overall appearance is in relatewith the topologies of nanopores.

High-magnification images of MoS2 nanotubules prepared viatemplates with diameters of about 65 nm and 120 nm areillustrated in Fig. 6. It can be revealed from Fig. 6a and b thatthe as-synthesized MoS2 nanotubules are multi-walled withuneven wall thickness. We know that the pores of templateprepared at 80 V are much larger than those at 50 V, surfacetension of (NH4)2MoS4 precursor is so different that the innersurface of porous wall only can be coated by a small quantity of theprecursor, and with the decrease of diameter of pores, less quantityof the precursor loads into the pores. The wall thickness increaseswith diameters, more definitely, wall thicknesses are about 10 nm,20 nm and 30 nm while the diameter varies among 65 nm, 120 nmto 240 nm. MoS2 nanotubules with diameters of 65 nm consist of5–11 layers while those of 120 nm are composed of 11–24 layers.The average interplanar distance at the edge of MoS2 nanotubulesaccording to Fig. 5a and b is about 6.39 A. The discontinuous andasymmetrical walls indicate MoS2 nanotubules are imperfect withdefects due to inaccessibility of optimal growth conditions, such aslow temperature, rough surface of the template pores and so on.Also, as is shown in Fig. 6c, the electron diffraction pattern of thenanotubules shows the strong MoS2 (0 0 2), (1 0 0) and (1 1 0)reflections and the weak (1 0 3), (2 0 0) and (2 1 3) ones,respectively, suggesting that the crystallographic orientations ofthe MoS2 are randomly distributed with turbostatic structureswith the (0 0 2) planes running parallel to the wall.

Fig. 6. HRTEM images of MoS2 nanotubes with different diameters (a) 65 nm (b) 120 nm (c) the corresponding electron diffraction pattern from nanotubules with diameter of

120 nm (d) a typical TEM image of nanotubules with incomplete sections.

8007006005004003002000.5

0.6

0.7

0.8

0.9

1.0

Absorbance

wavelength/nm

Fig. 7. The optical absorption spectrum of tubules prepared after dissolving away

the template.

D. Yu et al. / Materials Research Bulletin 46 (2011) 1504–15091508

The observed bamboo-like morphologies are similar to themorphologies of carbon nanotubules prepared by firing poly(furfuryl alcohol) in aluminum oxide templates at 900 8C [24], butno explanation was offered for this observation in the report. BNnanotubules were synthesized by heat-treatment of borazineoligomers using alumina porous template [25], and the bamboo-like morphology was attributed to the wetting behavior of theprecursor. It has been proposed that the imperfections in thesurface of the template have a great effect on wetting by creating ameniscus along the pores which creates the sections of MoS2

tubules when fired [14]. After the templates were dipped into theprecursor solution, the denseness of the precursor remnant alongthe length of the template nanopores varied with the unevennessof pore diameter and the roughness of the internal pores. Theprecursor was coated on these microcosmic ridges along nano-channel, which were formed in the producing process of AAO; andthe initial state of plugged blockages was performed, it can betestified in Fig. 6d labeled by circles. The reason that finally thesections were not formed maybe attribute to the less quality of Moand S, so the ridges may provide the sufficient reactant of necessity.After reacting in reducing atmosphere, a heterogeneous nucleationand the growth of crystalline would occur on the surface of pores,and then the MoS2 bamboo-like nanotubules were formed. It isclear to notice that the smoothness of inner porous wall in thetemplates have a great effect on the morphology of the MoS2

nanotubules. The assumption the reaction is under the idealcondition that the pores of template are even and the inner wall ofpores are smooth enough, the tubules we would obtain should beuniform, straight and no section in channels could be accepted.

Of course, it could not be convincing to explain the formation ofsection all attributing to the irregular shape of the pores and thewetting state in the pores. Each section along tubule consists oftwo domes vis-a-vis and the thickness of the knots is about twice of

the wall thickness. It seems that the transecting regions created bywetting would never have such inerratic structure, and therewould be the possibility only having one dome in each section, andeven the transaction region has two domes, it would have neverreached for such large percentage, some underlying factors shouldalso be considered. The heterogeneous nucleation of MoS2

resulting from the roughness of surface of inner porous wall leadsto the elastic strain among chemical bonds during the reactionprocess, while the deviation from planarity of the nanoclusterduring the early stages of the formation of a hollow cage structureis an androgenic process due to the elastic strain involved in tiltingthe chemical bond, and the tendency of curvature is performed

D. Yu et al. / Materials Research Bulletin 46 (2011) 1504–1509 1509

after a defect is generated in the ridge regions that the stressbetween two neighboring sulfur atoms during folding is largerthan the energy of the Mo–S chemical bond [8]. Therefore theformation of these two domes could be suggested to be the releaseof accumulative effect of the stain among chemical bonds where adefect exists.

Optical absorption measurement on MoS2 nanotubules isrecorded on an ultraviolet–visible (UV–Vis) absorption spectros-copy at room temperature. Fig. 7 shows the UV–Vis absorptionspectra of MoS2 nanotubules generated in templates anodized in0.3 M H3PO4 at 120 V. It can be noticed that the nanotubulesdisplay an intense and broad absorption from 350 to 550 nm with amaximum centered at about 480 nm, as well as weaker absorbancein the red region. The position of the peak in the spectrum arereproducible and the appearance of the spectrum is similar to thatof bulk MoS2 [12]. Zelenski and Dorhout also reported that thespectra of the nanotubules formed in the template which was notdissolved away from the MoS2 fibers as it acted as a sample holder,showed a maximum centered at 380 nm in the blue region and twomuch weaker absorptions at 604 and 660 nm [14]. It can beconcluded that the optical absorption spectra of MoS2 nanotubulesare strongly related to the morphology. More detailed studies forthe spectral behavior of nanoscale particles should be madelaunched.

4. Conclusion

In summary, the synthesis of nanotubules of MoS2 has beendemonstrated by thermal decomposition of ammonium thiomo-lybdate precursor in the confined pores of aluminum oxidetemplates. The shape of the tubules depends on the state ofthiomolybdate precursor left behind in the template after thesolvent evaporated, smoothness of porous inwall and wettabilityof precursor solution has an influence upon the microcosmicmorphologies of nanotubules. The diameter of these hollowtubules was controlled by homologous pores and the quantityof remnant MoS2 was responsible for wall thickness of MoS2

nanotubules. It has been noted that when the diameter of poresreaches a critical value, tubule size and morphology is partly lost,the product of reaction is more like films of MoS2 rather thantubules. It is obvious to observe that the nanotubules we haveproduced are bamboo-like structures and the average interplanardistance of MoS2 nanotubules was a little larger than the standardcrystalline MoS2. The formation of sections in nanotubules is

greatly affected by the Mo–S chemical bond and the wettingbehavior between precursor solution and the surface of porousinwall. The nanotubules display an intense and broad absorption atabout 480 nm in the optical absorption spectrum.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (N.O. 60979017 and 91026018), andthe Natural Science Foundation of Anhui Province of China (GrantN.O. 11040606M50).

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