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The Transformation Pathways of Mo6S2I8 Nanowires into Morphology-Selective MoS2 Nanostructures Marko Virsˇek,† Matthias Krause,‡ Andreas Kolitsch,‡ Alesˇ Mrzel,† Ivan Iskra,† Srecˇo D. Sˇkapin,† and Maja Remsˇkar*,† Jozef Stefan Institute, JamoVa cesta 39, SI-1000 Ljubljana, SloVenia, and Forschungszentrum Dresden-Rossendorf, Institut fu¨r Ionenstrahlphysik und Materialforschung, D-01314 Dresden, Germany ReceiVed: February 10, 2010; ReVised Manuscript ReceiVed: March 12, 2010
Nanomaterials prepared by the sulfurization of Mo6S2I8 nanowires and the time and temperature dependence of the transformation process are investigated by high-resolution transmission electron microscopy, X-ray powder diffraction, and wavelength-dependent Raman spectroscopy. Depending on the temperature, coaxial MoS2 nanotubes or MoS2 “mama”-tubes are formed after 2 h of sulfurization. Using a few minutes of sulfurization time, core-shell nanowires composed of well-ordered MoS2 layers covering a Mo6S2I8 core are formed, proving an outside-to-inside transformation process. The crystallinity of the three MoS2 nanostructures increases with increasing transformation temperature, i.e., in the sequence from MoS2/Mo6S2I8 core-shell structures via coaxial MoS2 tubes to the MoS2 “mama”-tubes. The analysis indicates a different nature of the defects in the MoS2-based nanomaterials, originating from the sulfurization of the Mo6S2I8 than in the conventional MoS2 platelike crystals. A correlation between the Raman spectroscopic parameters and the defect density in MoS2 is identified. 1. Introduction The transition-metal dichalcogenide nanotubes and the fullerene-like (IF) structures were the first closed-cage noncarbon nanoparticles discovered. They closely resemble the structures of multiwall carbon nanotubes1 and carbon onions.2 The first IF nanomaterials, polyhedra and cylinders of WS2, were obtained by annealing tungsten films in a reducing H2S atmosphere.3 Until now, transition-metal IF materials were also prepared by the sulfurization of transition-metal oxides,4 metal chlorides5 or metal-chalcogen-halogens6 in a H2S/H2/inert gas atmosphere, by a chemical transport reaction,7 the thermal decomposition of trisulfides8 or ammonium thiomolybdate molecular precursors,9 using the template method,10 hydrothermal synthesis,11 by irradiation in transmission electron microscope (TEM)12 and by short electrical pulses on trisulfide using scanning tunneling microscopy.13 From among the many potential applications of IF nanomaterials,14 MoS2 and WS2, in particular, show great promise in tribology as solid lubricants and as oil and grease additives. Like in carbon, where carbon nanobuds15 and carbon peapods16 have been reported, combinations of cylindrical and spherical-like morphologies were recently shown to exist in IF materials. First, IF nanobuds were reported in the WS2 system, where WS2 nanotubes decorated with IF nanoparticles nucleated on the outside surface corrugations were prepared by the sulfurization of W5O14 nanowires.17 The first inorganic nanopods, i.e., IF nanoparticles growing inside multiwall nanotubes, were prepared in the MoS2 system by the sulfurization of Mo6S2I8 nanowires, which are members of the MoSxIy family.18 They form at temperatures above 900 °C and are called MoS2 * Corresponding author: Solid State Physics Department, Jozef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia; tel: 386 1 4773 728; fax: 386 1 4773 191; e-mail:
[email protected]. † Jozef Stefan Institute. ‡ Forschungszentrum Dresden-Rossendorf, Institut fu¨r Ionenstrahlphysik und Materialforschung.
“mama”-tubes.6 With the same procedure, MoS2 coaxial nanotubes are obtained at a lower sulfurization temperature of around 800 °C.19 The coaxial tubes are composed of a larger number of separated parts of the coaxial nanotube walls, while in “mama”-tubes one nanotube wall is decorated from the inside by IF particles. While scanning electron microscopy (SEM) studies showed that the outer dimensions of the particles are conserved when the Mo6S2I8 nanowires are transformed to coaxial tubes or to “mama”-tubes, two typical grainlike building blocks can be observed with transmission electron microscopy (TEM): nanotube walls and spherical IF particles.6,19 In the direction of the MoS2 c-axis, the nanotube walls have a typical width of around 15 nm. The diameters of IF particles inside the “mama”-tube walls range between a few tens of nanometers to more than 200 nm, with the average IF wall thickness in the direction of the local c-axis being about one-third of the diameter. Besides TEM and SEM, X-ray diffractometry (XRD) and Raman spectroscopy are powerful methods to elucidate the structure and the defects of dichalcogenide nanotubes and fullerene-like particles. XRD line broadening is a standard technique to quantify effective crystallite sizes by applying the Debye-Scherrer equation. More recently, Raman spectroscopy has been applied to detect and to quantify the morphological and structural properties as well as defects in IF-MoS220–23 and IF-WS2 nanostructures.24 So far, little is known about the sulfurization pathways of Mo6S2I8 nanowires into nanostructures with different structures and morphologies. This knowledge is, on one hand, important for up-scaling of the synthesis of the MoS2 “mama”-tubes and MoS2 coaxial nanotubes, of which currently gram quantities can be produced per day. Moreover, it is a precondition for the stabilization of hybrid materials, which combine the structural, electronic, and optical properties of Mo6S2I8 nanowires and MoS2 nanotubes. For example, they have the potential for use
10.1021/jp101298g 2010 American Chemical Society Published on Web 03/24/2010
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Figure 1. HRTEM images of core-shell Mo6S2I8/MoS2 nanowires.
in the charge separation of nanostructured photoelectrochemical solar cells by tuning the MoSxIy band gap.25 Here, the temperature and time dependence of the sulfurization process of Mo6S2I8 nanowires as well as the structural properties of the formed nanostructures are studied by highresolution transmission electron microscopy (HRTEM), XRD, and Raman spectroscopy. Well-defined core-shell MoS2/MoSxIy nanowires were prepared and characterized. The nature and the density of the structural defects in three different MoS2-based nanostructures is analyzed and a uniform picture of the structural defects is proposed. 2. Experimental Section 2.1. Synthesis. As previously reported,6 the Mo6S2I8 precursor nanowires were prepared in sealed quartz ampules (10 cm in length and 2.2 cm in diameter) directly from elements at 1045 °C. The 72 h reaction time resulted in needles up to several millimeters long with diameters varying from a few nanometers to a few hundred nanometers. The Mo6S2I8 phase was confirmed by electron diffraction and XRD. The fibrous nanowire material was collected after the nonreacted iodine had been evaporated. The 25 mg of material was put into a quartz boat, which was inserted into the quartz tube, 60 cm in length and 2.8 cm in diameter, equipped with gas inlet and outlet connections. In order to prevent oxidation, the flow of argon was established for 15 min, and then a reaction-gas mixture flow was introduced. The reaction-gas mixture is composed of 1-2% H2S, 1% H2, and Ar. After another 15 min, the reactor was inserted directly into the preheated furnace. Different temperatures between 498 and 964 °C were chosen for the sulfurization, which lasted for 2 h. At the temperature of 867 °C, sulfurization for different times (2-120 min) was performed. At the end of the process, the hot tube was removed from the furnace and cooled in the reaction gas flow followed by an Ar gas flow for 15 min to avoid oxidation. 2.2. X-ray Diffractometry. Samples were monitored by XRD using a Bruker AXS D4 Endeavor diffractometer (Karlsruhe, Germany) with Cu KR1 radiation and a Sol-X energy dispersive detector within the angular range 2Θ from 6° to 73° with a step size of 0.04° and a collection time of 3-4 s. The samples were rotated during the measurement at a constant speed of 6 rpm. The diffractometer limitations together with the specimen-height inaccuracy limit the accuracy of the plane distance determination to ∼1%. 2.3. Raman Spectroscopy. Raman spectra were recorded in a micro-Raman 180° backscattering configuration on a Labram HR spectrometer equipped with a holographic 1800 lines/mm grating and a liquid-nitrogen-cooled CCD detector (HoribaJobin-Yvon, France). For the excitation a He-Ne 632.8 nm laser and a frequency-doubled Nd:YAG 532 nm laser were used. The spectral resolution, determined by the width of three CCDpixels, was 0.9 cm-1 for the red and 1.5 cm-1 for the green laser line. Stray-light rejection was provided by the holographic notch filters. Using a sufficiently long thermal equilibration of the spectrometer and the use of the Si(100) F2g line at 521 cm-1
as an external standard a wavenumber accuracy of (1 CCD pixel was assured. The laser power was roughly 100 µW with a CCD accumulation time of 20-30 min. Under these conditions any heating or degradation effects were excluded. During each measurement, a small number of nanowires were in range of the laser focus and thus contributed to the signal. 2.4. High-Resolution Transmission Electron Microscopy. HRTEM of the Mo6S2I8/MoS2 core-shell nanowires synthesized at 867 °C was performed using a 200 keV Jeol instrument (JEM2100). 3. Results 3.1. High-Resolution Transmission Electron Microscopy. The first HRTEM study of nanowires prepared for the sulfurization conditions leading to core-shell Mo6S2I8/MoS2 nanowire material is presented. Figure 1 shows the HRTEM images of different nanowires sulfurized for 2 min at 867 °C, prepared within the same batch. About 6-12 layers of MoS2 are observed to form in the outer side, showing directly the outside-to-inside transformation of the Mo6S2I8 precursor. A well-defined boundary is observed in (a), with a limited transition region between the MoS2 and the precursor. In (b), below the first surface layers, some spherelike hollow or material-deficient areas are observed that could represent the nucleation sites for IF-like particles. A splitting of the nanowire interior by additional MoS2 walls is seen in (c), leading to the formation of structures typical for coaxial MoS2 nanotubes. In the transition region between the two phases, strong diffusion of the material must be occurring in order to accommodate structures like split walls or fullerenelike particles. Though the conditions in the experiment were carefully controlled, the local temperature and atmosphere conditions at 867 °C, as measured in a control region close to the reactor, have a large impact on the morphology of a particular nanostructure. A further optimization of the parameters is needed for the synthesis of large quantities of structures, shown in Figure 1a, representing well-defined core-shell Mo6S2I8/MoS2 nanowires with potentially interesting optical and electronic properties. 3.2. X-ray Diffractometry. The XRD spectra of the precursor Mo6S2I8 nanowires, a reference MoS2 powder (Aldrich, 0 can contribute to the Raman spectrum due to spatial confinement by the crystallite boundaries or
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TABLE 2: Relative Broadening of XRD Lines, Temperature Dependence of Broadening (f(T)), and Effective Particle Size for Samples Sulfurized at Different Temperaturesa relative broadening reflection
ordering direction
MoS2
964 °C
867 °C
800 °C
705 °C
498 °C
f(T)
101 103 105 100 110 002
between c axis and basal plane
1 1 1 1 1 1
7.1 10.9 11.7 2.5 2.8 2.3
6.6 11.1 11.6 2.9 3.6 3.2
6.8 11.5 13.7 3.5 3.5 3.2
7.4 11.3 12.9 3.5 4.0 4.8
no data no data no data no data no data 11.7
1.1 1.1 1.1 1.4 1.4 2.1
in x-y basal plane | c axis
effective particle size (nm) 5 19-25 (5) 13-25
a Effective particle size is given for lowest and highest temperatures; for the 002 line the data for the sample prepared at 498°C are given in parentheses.
TABLE 3: Selected Raman Parameters of the MoS2 Nanostructures after 2 h of Sulfurization at Different Temperaturesa 523 nm laser
633 nm laser
process T (°C)
E2g freq (cm-1)
fwhm E2g (cm-1)
A1g freq (cm-1)
fwhm A1g (cm-1)
498 705 800 837 869 981 MoS2
381.7 382.8
7.4 5.0
407.3 408.5
5.9 3.6
8 13
383.0 382.9 384.0
3.5 3.1 1.6
408.7 408.6 409.6
3.0 2.6 2.2
21 25 100 (def.)
a
effective particle size (nm)
2LA(M) freq (cm-1)
A2u freq (cm-1)
intensity ratio 2LA(M)/A2u
451.6 453.1 453.4 454.5
463.7 465.1 465.4 466.2
2.32 1.55 1.54 1.42
454.0 455.4
465.9 466.5
1.30 0.95
Effective particle size is calculated from the E2g line.
defects. The quantification of the “effective particle size” using this model can be estimated from the E2g line fwhm following the approximation of formula 120
L)
4π √3q
(1)
For the estimation, a linear relationship is adopted for the phonon wave vector: q ) k ∆ω. Here, k ) 0.145 × 10-7 and ∆ω is the line broadening in cm-1 given by the difference of the experimental fwhm of the E2g mode and the value of 1.1 cm-1. This is estimated from the MoS2 single-crystal data (fwhm for E2g is 1.6 cm-1) using eq 1 and taking the average effective particle size from the XRD data (≈100 nm). The effective particle size derived from the E2g probes ordering in the x-y basal plane and the results agree well with the XRD data for the 110 lines. The E2g line width is found to be larger than that of the A1g in the transformed samples, which is just the opposite of the platelike MoS2. Also, a smaller broadening is observed for the A1g lines compared to the E2g mode when samples with a decreasing process temperature are compared. The A1g mode normal coordinate is directed along the c axis and the E2g mode along the basal plane. The larger E2g line width temperature dependence is consistent with the proposed point defects, which bring disorder into the basal plane. Later, an identification of the resonance Raman parameter, which could most reliably correlate to the defect density in MoS2, was investigated. In the frequency range between 350 and 495 cm-1 Lorentz area fitting revealed seven lines. Besides the A1g and E2g modes also shoulders at their low-frequency sides were found with a position about 5 cm-1 below their neighboring first-order lines. Following the notation in24 we refer to these shoulders as D-A1g and D-E2g. The position of the so-called dispersive peak is found at around 419 cm-1. The intense peak around 460 cm-1 was assigned in the literature to contributions from a second-order process of the zone-edge 2LA
(M) phonon at ∼454 cm-1 and the resonance-condition-mediated A2u Raman inactive mode at ∼466 cm-1.20 Although imperfect fits for a structure around 460 cm-1 have been achieved taking only two Lorentz area lines, the Raman frequencies of the two most intense features were independent of the fit model, and thus a simple two-peak model was chosen. For these seven lines, various intensity ratios were calculated and the 2LA(M)/A2u intensity ratio was found to be the only one which could be correlated as a monotonic function to the process temperature and, therefore, to the defect density. In Table 3 the model fitting results are shown for the 2LA(M) and A2u lines. The decreasing sulfurization temperature causes (a) a red shift of both lines and (b) an increase of the 2LA(M)/A2u intensity ratio. Although the first-order line broadening can also be correlated with the defect density, the 2LA(M)/A2u intensity ratio exhibits a higher dynamic range. Compared to this work, a much higher 2LA(M)/A2u intensity-ratio enhancement was observed by Frey et al., which was correlated to the crystallite size. The accompanying first-order line broadening was, however, similar to that from our work and thus lacking in sensitivity. A consistent trend of the 2LA(M)/A2u intensity ratio correlated with the sulfurization temperature, thus showing the sensitivity of this parameter to small changes in the defect density in the MoS2. Additional information on the nature of defects could be extracted from selective line broadening of the first-order Raman lines. Concluding Remarks The Raman spectra of Mo6S2I8 are reported here for the first time and representative fingerprints used for the characterization of the transformation of Mo6S2I8 nanowires into morphologyselective MoS2 nanostructures are defined. The transformation reaction proceeds rapidly, giving a yield of g99% at about 867 °C within a reaction time of 15 min, and the estimated rate of the outside-to-inside diffusion-limited transformation process is 3-4 nm/min. The process is highly sensitive to the initial conditions in the reactor, when the crucial transformation occurs
Pathways of Nanowires to Nanostructures and the product morphology is formed. An additional sulfurization time completes the transformation process, enhances only slightly the product’s crystallinity, but has no effect on the morphology. If the process is stopped after a few minutes, welldefined core-shell MoS2/Mo6S2I8 nanowires are formed with potentially interesting properties due to the well-defined boundary between the two phases. A temperature-independent level of stacking faults and dislocations is revealed. The existence of interstitial iodine point defects, the density of which is decreasing with increasing temperature, is shown. The effective particle size estimation shows that the local properties of these MoS2 nanostructures differ from those of the platelike MoS2 crystals in a higher density of dislocations and stacking faults at higher process temperatures, additional point defects at lower process temperatures, and smaller grainlike structures in the whole transformation temperature range. The 2LA(M)/A2u intensity ratio was identified as the most reliable Raman parameter for determining the general defect density in MoS2, while broadening of the first-order lines demonstrated the sensitivity to the nature of the defects. Acknowledgment. The authors thank Dr. Gintautas Abrasonis (Forschungszentrum Dresden-Rossendorf (FZD)) and Dr. Martin Chambers (Jozef Stefan Institute (JSI)) for valuable hints and discussion. This work is funded by the European Union’s Sixth Framework Program (FOREMOST project under Contract NMP3-CT-2005-515840). References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Ugarte, D. Nature 1992, 359, 707. (3) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (4) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature 1993, 365, 113. (5) Schuffenhauer, C.; Popovitz-Biro, R.; Tenne, R. J. Mater. Chem. 2002, 12, 1587–1591. (6) Remskar, M.; Mrzel, A.; Virsek, M.; Jesih, A. AdV. Mater. 2007, 19, 4276–4278. (7) Remskar, M.; Skraba, Z.; Cleton, F.; Sanjines, R. Appl. Phys. Lett. 1996, 69, 351–353. (8) Nath, M.; Rao, C. N. R. J. Am. Chem. Soc. 2001, 123, 4841–4842. (9) Chen, J.; Li, S. L.; Xu, Q.; Tanaka, K. Chem. Commun. 2002, 16, 1722. (10) Santiago, P.; Ascencio, J. A.; Mendoza, D.; Perez-Alvarez, M.; Espinosa, A.; Reza-SanGermaan, C.; Schabes-Retchkiman, P.; CamachoBragado, G. A.; Jose-Yacamaan, M. Appl. Phys. A: Mater. Sci. Process. 2004, 78 (4), 513–518.
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6463 (11) Lavayen, V.; Mirabal, N.; O’Dwyer, C.; Santa Ana, M. A.; Benavente, E.; Sotomayor Torres, C. M.; Gonzalez, G. Appl. Surf. Sci. 2007, 253 (12), 5185–5190. (12) Jose-Yacaman, M.; Lorez, H.; Santiago, P.; Galvan, D. H.; Garzon, I. L.; Reyes, A. Appl. Phys. Lett. 1996, 69, 8–1065. (13) Homyonfer, M.; Mastai, Y.; Hershfinkel, M.; Volterra, V.; Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 7804. (14) Tenne, R. Nat. Nanotechnol. 2006, 1, 103. (15) Nasibulin, A. G.; Pikhitsa, P. V.; Jiang, H.; Brown, D. P.; Krasheninnikov, A. V.; Anisimov, A. S.; Queipo, P.; Moisala, A.; Gonzalez, D.; Lientschnig, G.; Hassanien, A.; Shandakov, S. D.; Lolli, G.; Resasco, D. E.; Choi, M.; Toma´nek, D.; Kauppinen, E. I. Nat. Nanotechnol. 2006, 2, 156–161. (16) Smith, B. W.; Luzzi, D. E. Chem. Phys. Lett. 2000, 321 (1-2), 169–174. (17) Remskar, M.; Virsek, M.; Jesih, A. Nano Lett. 2008, 8 (1), 76–80. (18) (a) Remskar, M.; Mrzel, A.; Skraba, Z.; Jesih, A.; Ceh, M.; Demsar, J.; Stadelmann, P.; Levy, F.; Mihailovic, D. Self-assembly of subnanometerdiameter single-wall MoS2 nanotubes. Science 2001, 292, 479. (b) Vrbanic, D.; Remskar, M.; Jesih, A.; Mrzel, A.; Umek, P.; Ponikvar, M.; Jancar, B.; Meden, A.; Novosel, B.; Pejovnik, S.; Venturini, P.; Coleman, J. C.; Mihailovic, D. Air-stable monodispersed Mo6S3I6 nanowires. Nanotechnology 2004, 15 (5), 635. (19) Remskar, M.; Virsek, M.; Mrzel, A. Appl. Phys. Lett. 2009, 95 (13), 133122. (20) Frey, G. L.; Tenne, R.; Matthews, M. J.; Dresselhaus, M.; SDresselhaus, G. Phys.ReV. B 1999, 60 (4), 2883. (21) Virsˇek, M.; Krause, M.; Kolitsch, A.; Remsˇkar, M. Phys. Status Solidi B 2009, 246 (11-12), 2782. (22) Virsˇek, M.; Jesih, A.; Milosˇevic´, I.; Damnjanovic´, M.; Remsˇkar, M. Sur. Sci. 2007, 601 (13), 2868. (23) Pratap, R.; Gupta, R. K. Phys. Status Solidi A 1972, 9 (4), 15. (24) (a) Krause, M.; Virsˇek, M.; Remsˇkar, M.; Salacan, N.; Fleischer, N.; Chen, L.; Hatto, P.; Kolitsch, A.; Mo¨ller, W. Chem. Phys. Chem 2009, 10 (13), 2221–2225. (b) Krause, M.; Virsˇek, M.; Remsˇkar, M.; Kolitsch, A.; Mo¨ller, W. Phys. Status Solidi B 2009, 246 (11-12), 2786. (25) Thomalla, M.; Tributsch, H. J. Phys. Chem. B 2006, 110, 12167. (26) Perrin, C.; Sergent, M. J. Chem. Res., Synops. 1983, 38. (27) Srolovitz, D. J.; Safran, S.; Homyonfer, M.; Tenne, R. Phys. ReV. Lett. 1995, 74 (10), 1779. (28) Feldmanm, Y.; Waserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. (b) Yang, D. S.; Sandoval, J.; Divigalpitiya, W. M. R.; Irwin, J. C.; Frindt, R. F. Phys. ReV. B 1991, 43, 14. (29) (a) Ja¨ger-Waldau, A.; Lux-Steiner, M. Ch.; Bucher, E.; Ja¨gerWaldau, G. Conference Record of the 23rd IEEE PhotoVoltaic Specialists Conference; IEEE: New York, 1993, 597-203. (b) Ballif, C.; Regula, M.; Schmid, P. E.; Remsˇkar, M.; Sanijes, R.; Ley, F. Appl. Phys. A 1996, 62, 543. (c) Ja¨ger-Waldau, A.; Lux-Steiner, M. Ch.; Bucher, E. Solid State Phenom. 1994, 37-38, 479. (30) Congeduti, A.; Nardone, M.; Postorino, P. Chem. Phys. 2000, 256, 117–123. (31) Ichinokawa, T.; Hamaguchi, I.; Hibino, M.; Kirschner, J. Surf. Sci. 1990, 231 (3), L189. (32) Perrin, C.; Sergent, M. J. Chem. Res., Synops. 1983, 38. (33) (a) Li, H.; Li, W. J.; Ma, L.; Chen, W. X.; Wang, J. M. J. Alloy Compd. 2009, 471 (1-2), 442.
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