Core-Shell Nanorods of SnS-C and SnSe-C: Synthesis andCharacterization

Vilas G. Pol,* Swati V. Pol, and Aharon Gedanken

Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center forAdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel

ReceiVed December 27, 2007. In Final Form: January 26, 2008

The straightforward, efficient, solventless, RAPET (reactions under autogenic pressure at elevated temperature)approach was explored for the fabrication of core-shell nanomaterials. Carbon-encapsulated SnS and SnSe nanorodswere synthesized by a one-step thermal decomposition of tetramethyltin in the presence of either S or Se powder ina closed reactor at 700°C for 40 min, under their autogenic pressure in an inert atmosphere. The powder X-raydiffraction measurements provided structural evidence for the formation of pure orthorhombic phases of SnS or SnSeparticles. The Raman spectroscopy measurements ensured that the nature of the coated carbon was semigraphitic. Thescanning electron micrographs verified the 1D morphology of the formed SnS and SnSe chalcogenides, and theirstoichiometry was confirmed by EDAX measurements. The HR-TEM micrographs distinguished between core andshell morphologies. The nitrogen gas adsorption on the surface of core-shell nanostructures was determined by BETsurface area analysis. The plausible mechanism for the creation of chalcogenide cores (SnS or SnSe) with a carbonshell was elucidated.

Introduction

In the past few years, nanostructures of metal chalcogenidessuch as CdS, CdSe, ZnS, SnS, etc. have attracted considerableinterest owing to their application in optical, electronic, andsuperconductor devices.1,2 Among these materials, SnS isparticularly important because of its interesting properties andpotential technological applications. Owing to the versatilecoordinating ability of tin and sulfur, tin sulfide shows a varietyof phases, such as SnS, Sn2S3, Sn3S4, Sn4S5, and SnS2.3 SnS isa narrow band gap IV-VI semiconductor with an optical bandgap of 1.08 eV. In addition, the orthorhombic herzenbergitemodification of SnS possesses a layered structure, a structuralanalogue of black phosphorus.4 The narrow band gap and theinteresting structural property of SnS make it a potential candidatefor a solar absorber in thin film solar cells and a near-infrareddetector5 as a photovoltaic material6 as well as in a holographicregistrar system.7 Thin films of SnS have shown a conversionefficiency in photovoltaic devices similar to those found forsilicon films (up to 25%).8 There are many traditional methodsfor the preparation of SnS, namely, chemical vapor deposition,9

electrochemical deposition,10 molecular beam epitaxy,11 spraypyrolysis,12 a solvothermal method,13 etc. Recently, 1D nano-structures have gained extensive interest due to their contribution

to the understanding of fundamental concepts and potentialtechnological applications.14 The surfactant-assisted chemicalroute was used to synthesize SnS nanowires.15 In our recentwork, the microwave-assisted synthesis of SnS nanoflakes andtheir electrochemical performance as Li-inserting materials16

showed a stable reversible capacity higher than 600 mAh/g withSnS electrodes.

SnSe has attracted the attention of many researchers becauseof its high absorption coefficients, which are useful foroptoelectronic applications17 and memory switching devices.18

As a direct semiconductor with a band gap of 1.0 eV, SnSe isan orthorhombic solid which transforms to a tetragonal structureat ∼830 °C. Traditionally, SnSe can be synthesized throughelemental combination at high temperature or precipitate fromaqueous solutions. However, these procedures require either ahigh-energy input or toxic reagents such as H2Se.17 A low-temperature route to a nanocrystalline SnSe precipitate has beenproposed by Wang19 et al. SnSe crystals with various morphol-ogies have been grown from organic20 and aqueous21 solutions,as well as by using a variety of other solvents.22 Laminar SnSecrystals were synthesized by chemical vapor transport23 and byfilms of SnSe made by heating vacuum-deposited Sn in closecontact with chemically deposited selenium.24

Very recently, a netlike SnS/carbon nanocomposite film anodematerial was studied for lithium ion batteries, and the effects of

* To whom correspondence should be addressed. E-mail: vilaspol@gmail.com.

(1) Bowers, M. J., II; McBride, J. R.; Rosenthal, S. J.J. Am. Chem. Soc. 2005,127, 15378.

(2) Dhas, N. A.; Zaban, A.; Gadenken, A.Chem. Mater. 1999, 11, 806.(3) Jiang, T.; Ozin, G. A.J. Mater. Chem. 1998, 8, 1099.(4) Wiedemeier, H.; von Schnering, H. G.Z. Kristallogr. 1978, 148, 295.(5) Pramanik, P.; Basu, P. K.; Biswas, S.Thin Solid Films1987, 150, 269.(6) Sing, J. P.; Bedi, R. K.Thin Solid Films1991, 199, 9.(7) Berger L. I., Ed.Semiconductor Materials; CRC Press: New York, 1997.(8) Nair, M. T. S.; Nair, P. K.Semicond. Sci. Technol.1991, 6, 132.(9) Shibata, T.; Miura, T.; Kishi, T.; Nagai, T.J. Cryst. Growth1990, 106,

593.(10) Ghazali, A.; Zainal, Z.; Hussein, M. Z.; Kassim, A.Sol. Energy Mater.

Sol. Cells1998, 55, 237.(11) Nnebesny, K. W.; Collins, G. E.; Lee, P. A.; Chau, L. K.; Danziger, J.;

Osburn, E.; Armstrong, N. R.Chem. Mater.1991, 3, 829.(12) Ortiz, A.; Lopez, S.Semicond. Sci. Technol.1994, 9, 2130.(13) Hu, H.; Yang, B.; Zeng, J.; Qian, Y.Mater. Chem. Phys.2004, 86, 233.

(14) Jana, N. R.; Gearheart, L.; Murphy, C. J.J. Phys. Chem. B2001, 105,4065.

(15) Liu, Y.; Hou, D.; Wang, G.Chem. Phys. Lett.2003, 379, 67.(16) Patra, C. R.; Odani, A.; Pol, V. G.; Aurbach, D.; Gedanken, A.J. Solid

State Electrochem.2007, 1,1 186.(17) Wang, C.; Li, Y. D.; Zhang, G. H.; Zhuang, J.; Shen, G. Q.Inorg. Chem.

2000, 39, 4237.(18) Naik, P. G.; Desai, C. F.; Shah, R. C.; Siddiqui, S. S.Proc. SPIE- Int.

Soc. Opt. Eng. 1996, 2733, 547.(19) Wang, W.; Geng, Y.; Quian, Y.; Wang, C.; Liu, X.Mater. Res. Bull.

1999, 34, 403.(20) Zhan, W.; Liu, J.; Yu, S.; Zhong, C.; Chen, X.; Zhao, H.; Quian, Y.J.

Cryst. Growth2001, 223, 1.(21) Zhang, W.; Yang, Z.; Liu, J.; Zhang, L.; Hui, Z.; Yu, W.; Quian, Y.; Chen,

L. J. Cryst. Growth2000, 217, 157.(22) Li, B.; Xie, Y.; Huang, J.; Quian, Y.Inorg. Chem. 2000, 39, 2061.(23) Agarwal, A.J. Cryst. Growth1998, 183, 347.(24) Bindu, K.; Nair, P. K.Semicond. Sci. Technol.2004, 19, 1348.

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10.1021/la7040532 CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 03/26/2008

a 3D netlike carbon structure25 were described. The uniformlysurrounding carbon on the SnS nanoparticles guaranteed thecontact that acted as a buffer matrix to alleviate the volumeexpansion of the Li-Sn alloy, providing enough paths for theelectrolyte to reach the SnS active material during the discharge-charge process.25 One U.S. patent (by Hu¨ner et al.) claimed thatSnS-carbon-based nanocomposites are good solid lubricants.26

Therefore, there is a need to develop a scalable process that cancoat carbon on SnS/SnSe particles that show promising elec-trochemical properties. 1D nanomaterials are even more promis-ing, since they facilitate electron transport.

In this paper we report on the single-step synthesis of carbon-encapsulated SnS or SnSe nanorods by the thermal decompositionof tetramethyltin in the presence of either S or Se powder in aclosed reactor at 700°C for 40 min, under their autogenic pressurein an inert atmosphere. To avoid the use of highly toxic H2S andH2Se gases, elemental sulfur or selenium powder was chosen tofabricate core-shell nanostructures. A systematic morphological,compositional, and structural characterization was carried out,after which a plausible mechanism for the creation of SnS orSnSe nanorods with carbon core-shell nanostructures waselucidated. Recently, the synthetic process for carbon-coatedZnS and ZnSe nanoparticles27awas reported, since it can reducetheir toxicityandmakethemamenableforbiologicalapplications.27b

Therefore, the present RAPET (reactions under autogenic pressureat elevated temperature) synthetic strategy might exploit afavorable route to synthesize various carbon-encapsulatedchalcogenide nanostructures. In the RAPET process, the thermalenergy is provided to break the bonds between the elementspresent in the chemical precursor, followed by the segregationof the formed gaseous or critical phases during cooling in aninert atmosphere at a certain temperature under its autogenicpressure. Previously, such a one-stage, efficient, and simplesteconomic RAPET process has been successfully employed forthe synthesis of various fascinating nanostructures.28

Experimental Section

Synthesis of Carbon-Encapsulated SnS and SnSe Nanorods.Tetramethyltin [(CH3)4Sn,d ) 1.29] and Se and S powders werepurchased from Aldrich with>99% purity and used as received. Ina typical synthesis of SnS nanorods coated with carbon (SnS-C),1.08 g of tetramethyltin (TMT) is mixed with 0.193 g of S powderand introduced into a 4 mL stainless steel (SS) reactor at roomtemperature in a nitrogen-filled glovebox. The filled SS reactor isclosed tightly with the other plug and placed at the center of thetube’s furnace. The temperature of the furnace is raised to 700°Cat a rate of 40oC/min, and the temperature is maintained at 700°Cfor 40 min. The SS reactor, heated to 700°C, is gradually cooled(∼4 h) to room temperature and opened. A 0.979 g portion of aSnS-C gray powder with a 77.4% yield is obtained.

For the synthesis of SnSe nanorods coated with carbon (SnSe-C), 1.08 g of TMT is mixed with 0.474 g of Se powder. The analogousreaction parameters are employed as above, and 1.12 g of a SnSe-Cblack-gray product is obtained with a 72.4% yield. The structural,morphological, and elemental analysis is carried out for the obtainedproducts without further processing or purification. Using the RAPET

approach, metal oxides29(V2O3, Nb2O5), dielectric materials30(Co-ZrO2, ZrTi2O6), ceramics31(SiO2, ZrO2), and magnetic nanoparticles32

(Ni, Co, Fe, Fe3O4) were found to be encapsulated in a carbon shell.Interestingly, in all of the above cases, zero-dimensional core-shellmorphologies are obtained. However, in the present system,distinctive core-shell 1D nanorods of SnS/SnSe uniformly coveredby carbon are obtained.

Characterization. Various analytical techniques are used toidentify and characterize the as-prepared SnS-C and SnSe-C core-shell nanostructures. To understand the purity of the crystal structure(the interatomic distance and angle) of the as-prepared products, theX-ray diffraction patterns are measured with a Bruker AXS D*Advance powder X-ray diffractometer [using Cu KR (λ ) 1.5418Å) radiation]. An Olympus BX41 (Jobin-Yvon-Horiba) Ramanspectrometer is employed, using the 514.5 nm line of an Ar ion laseras the excitation source to analyze the nature of the coated carbon.Since TMT contains C and H in addition to metal, C, H, N, and S-Oanalyses are conducted for the as-prepared products on an Eager 200CHNS-O analyzer. The core-shell nanostructures are distinguishedby employing high-resolution transmission electron microscopy (HR-TEM; JEOL 2010) at 200 kV. The overall morphology of the materialsis investigated by high-resolution scanning electron microscopy (HR-SEM; JSM, 7000 F). A Micromeritics (Gemini 2375) surface areaanalyzer is used to measure the surface area of the samples.Additionally, the determination of nitrogen gas adsorption on thesurfaces of the chalcogenides is revised.

Results and Discussion

The X-ray diffraction pattern of the as-prepared SnS-C productfabricated during 40 min of a RAPET reaction between TMTand S at 700°C is presented in Figure 1a. The reflection linescan be readily indexed to those of the herzenbergite (SnS)orthorhombic phase (Pbnm62 space group) of tin sulfide withlattice constantsa ) 4.32 Å, b ) 11.19 Å, andc ) 3.98 Å(Powder Diffraction File (PDF) no. 39-354). The XRD patternconfirms the formed pure orthorhombic phase of SnS, althoughno reflection lines are obtained for the coated carbon. Figure 1bpresents the wide-angle XRD pattern of a SnSe-C nanocompositeprepared at 700°C for 40 min by reacting TMT with Se powderunder an inert atmosphere inside an SS reactor. The reflectionlines can be indexed with an orthorhombic (Pnma62 space group)SnSe phase, with lattice constantsa ) 11.5 Å,b ) 4.15 Å, andc ) 4.44 Å. The peak intensities and positions match well withPDF no. 3-65-6459. No extra peaks indicating impurities areobserved, which substantiates the claim that this technique issuitable for the fabrication of pure SnSe. The graphitic carbonsignal is not observed, which might be due to its amorphous orsemicrystalline nature. This is further understood by Ramanspectroscopy and discussed in the following section.

The SEM image in Figure 2a demonstrates the overallmorphology of a SnS-C sample. Rodlike particles with differentdiameters (40-200 nm) and lengths of more than micrometersize are observed. Figure 2b also reveals∼100 nm diameter with1-2 µm size long nanorods of the SnSe-C sample. The imagesof the SnS-C and SnSe-C samples are indistinct, and toovercome this problem, TEM measurements were conceded.The compositional analysis and chemical purity of the samples

(25) LI, Y.; Tu, J. P.; Huang, X. H.; Wu, H. M.; Yuan, Y. F.Electrochem.Commun.2007, 9, 49.

(26) Huner, R.; Melcher, B.; Milczarek, R.; Kienleitner, H. U.S. Patent 6,-303,545 B1, 2001.

(27) (a) Pol, S. V.; Pol, V. G.; Gedanken, A.J. Phys. Chem. C2007, 111,13309. (b) Fu, A.; Gu, W.; Larabell, C.; Alivisatos, A. P.Curr. Opin. Neurobiol.2005, 15, 568.

(28) (a) Pol, V. G.; Pol, S. V.; Gedanken, A.J. Phys. Chem. B2005, 108, 6121.(b) Pol, S. V.; Pol, V. G.; Frydman, A.; Churilov, G. N.; Gedanken, A.J. Phys.Chem. B 2005, 109, 9495. (c) Pol, S. V.; Pol, V. G.; Seisenbaeva, G.; Kessler,V. G.; Gedanken, A.Chem. Mater. 2004, 16, 1793. (d) Pol, S. V.; Pol, V. G.;Gedanken, A.AdV. Mater. 2006, 18, 2023.

(29) (a) Pol, S. V.; Pol, V. G.; Gedanken, A.ChemsEur. J.2004, 10, 4467.(b) George, P. P.; Pol, V. G.; Gedanken, A.Nanoscale Res. Lett.2007, 2, 17.

(30) (a) Pol, V. G.; Pol, S. V.; Gedanken, A.; Kessler, V. G.; Seisenbaeva, G.A.; Sung, M.; Asai, S.J. Phys. Chem. B2005, 109, 6121. (b) Pol, S. V.; Pol, V.G.; Grinblat, Y.; Selvan, R. K.; Kessler, V. G.; Spijksma, G. I.; Seisenbaeva, G.A.; Gedanken, A.J. Phys. Chem. C2007, 111, 2484.

(31) (a) Pol, V. G.; Pol, S. V.; George, P. P.; Markovsky, B.; Gedanken, A.J. Phys. Chem. B2006, 110, 13420. (b) Pol, S. V.; Pol, V. G.; Seisenbaeva, G.A.; Kessler, V. G.; Gedanken, A.Chem. Mater.2004, 16, 1793.

(32) (a) Pol, S. V.; Pol, V. G.; Frydman, A.; Churilov, G. N.; Gedanken, A.J. Phys. Chem. B2005, 109, 9495. (b) Pol, S. V.; Pol, V. G.; Gedanken, A.Eur.J. Inorg. Chem.2007, 2089.

5136 Langmuir, Vol. 24, No. 9, 2008 Pol et al.

were performed by EDAX. The EDAX results were recordedfrom different parts of the SnS-C and SnSe-C samples, andthe representative spectra are presented in Figure 2c,d. Thesefigures indicate that the core-shell chalcogenides are stoichio-metric SnS and SnSe, calculated from quantitative analysis datawithin the experimental error. For the SnS-C sample, theelementary analysis revealed the formation of near-stoichiometricSnS (52 atom % Sn and 48 atom % S). Here, for bothchalcogenides, the additional signals for the presence of carbonare verified. The double-sided carbon tape was used as a supportfor mounting the sample on a copper disk. However, the thicklayer of samples (>50µm) was mounted so that the carbon wasmeasured from only the sample and not from the support. Forthe study of the exact amount of carbon present in the sample,further CHNS-O analysis is required, and the obtained amountsare discussed in the mechanism section.

The N2 adsorption-desorption isotherm of a SnS-C sampleis presented in Figure 3a. The measured BET surface area is 16m2/g, and a total pore volume of 0.028 cm3/g is recorded. TheBET surface area analysis technique is also employed for thedetermination of nitrogen gas adsorption on the surface of theSnS-C sample at room temperature. The maximum (17.5 cm3/g) nitrogen gas adsorption is recorded at 0.995 relative pressure(757.54 mmHg) at liquid nitrogen temperature (77 K) and canbe seen in Figure 3a. For the temperature, 77 K, the nitrogenadsorption on SnS-C is 17.5 cm3/g. Therefore, at roomtemperature (298 K), the amount of adsorbed nitrogen on theSnS-C sample is 67.7 cm3/g. Interestingly, the adsorption-desorption hysteresis does not close at zero relative pressure.This suggests that some of the nitrogen remains adsorbed on thesurfaces of the SnS-C nanorods. We have compared the nitrogenadsorption of similar core-shell nanostructures with analogoussurface areas. The wormlike nanostructure of WS2 coated withcarbon produced a BET surface area of 17 m2/g and a total pore

volume of 0.0797 cm3/g. The amount of adsorbed nitrogen onthe anisotropic WS2 particles33embedded in the carbon is 195.65cm3/g.

The N2 adsorption-desorption isotherm for the SnSe-Csample is presented in Figure 3b. A measured BET surface areaof 8.2 m2/g and a total pore volume of 0.00022 cm3/g are recorded.The maximum (12 cm3/g) nitrogen gas adsorption is recordedat 0.994 relative pressure (757.54 mmHg) at liquid nitrogentemperature (77 K). Therefore, at room temperature (298 K), theamount of adsorbed nitrogen on the SnSe-C sample is 46.5cm3/g. The measured surface area of SnS-C nanorods is morethan that of the SnSe-C nanorods.

The representative transmission electron micrographs of aSnS-C sample are presented in Figure 4. The two smallestnanorods were chosen for the TEM imaging for the confirmationof the core-shell as well as the length of the nanorods. Figure4a shows two parallel core-shell nanorods, where the dark 1DSnS core is surrounded by faint carbon layers. The strong whiteline (over focus) differentiates between the two nanorods of SnS-C. The diameters of the SnS-C nanorods presented in Figure4a are∼40 nm. Figure 4b presents the HR-TEM image of asingle nanorod at its edge. The cores of SnS are uniformly coveredwith a∼12 nm thick carbon layer. The disordered lattice planesof carbon around the dark SnS core can be seen. The hugeinterlayer spacing is noticed for the core; however, it is a falseimage due to aberrations. An HR-TEM micrograph is presentedin Figure 4c, after concentration on the SnS core, revealing theperfect single-crystalline orthorhombic structure of the SnS core.The measured spacing of the crystallographic planes is 0.28 nm(Figure 4c), which corresponds to the{111} lattice fringes of theorthorhombic structure of SnS (PDF no. 39-354). The growthdirection of the nanorod is parallel to the axis of the{111} plane.This is very consistent with the XRD results, which also supportthe maximum growth along the [111] direction. The SAED(selected area electron diffraction) pattern taken from the areaof the encapsulated nanoparticle can be indexed as single-crystalline SnS (Figure 4d) along the (120), (101), and (200)zone axes.

A plausible mechanism for the formation of a chalcogenidecore with carbon shell nanostructures is elucidated in the followingsection. The thermal decomposition of TMT might have led tocomplete dissociation into Sn atoms and CH3 radicals at 700°Cwith subsequent transformation of the latter into methane andacetylene/polyines. The latter polymerize then on the surface ofnanowires, forming a core-shell structure. When tungsten(VI)isopropoxide34a in 2-propanol is decomposed in an analogousRAPET reaction, similar 1D core-shell WO3-C nanowires wereobtained.

Indeed, the RAPET of TMT in the absence of S or Se yieldeda Sn-C core-shell structure. That work is beyond the scope ofthis paper; it will be published elsewhere. The calculated amounts(wt %) of Sn, S, C, and H in the reaction mixture [(CH3)4Sn+S, using a Sn:S molar ratio of 1:1] are 56.31, 15.21, 22.79, and5.69, respectively. The calculated mass of a metal Sn (in 1.265g of [(CH3)4Sn and S] reaction mixture is 0.712 g, while 0.70g is accounted for in the 0.98 g of the SnS-C product. ElementalC, H, N, and S analysis measurements are conducted for theSnS-C product. The measured C, H, and S amounts are 8%,0.2%, and 19%, respectively. The amount of carbon is reduced(∼11%), and the remaining hydrogen amount is almost negligible

(33) Pol, V. G.; Pol, S. V.; Perkas, N.; Gedanken, A.J. Phys. Chem. C2007,111, 134.

(34) (a) Pol, S. V.; Pol, V. G.; Kessler, V. G.; Gedanken, A.New J. Chem.2006, 30, 370. (b) Guha, P.; Kar, S.; Chaudhuri, S.Appl. Phys. Lett. 2004, 85,3851.

Figure 1. XRD patterns of a thermally decomposed mixture of (a)TMT and S and (b) TMT and Se at 700°C for 40 min under an inertatmosphere in a closed SS reactor.

Core-Shell Nanorods of SnS-C and SnSe-C Langmuir, Vol. 24, No. 9, 20085137

in the SnS-C product. The total yield of the SnS-C product is77.4%. This means that some byproducts such as hydrocarbonsare being formed. The following reaction is proposed for theformation of SnS and carbon as the solid products:

The growth of the crystals with some preferred structure orplanes is governed by the surface energy of the planes in thatparticular condition, and the planes with a lower surface energytend to dominate over the others. This growth process is calledshape-selective surface adsorption.34b The final shape of theobtained crystals is determined by the competitive growth along

different crystalline directions.35The initial nucleation conditionof the crystal growth process has a crucial role in shaping theSnS crystals. From our HR-TEM and XRD observations, theSnS particle growth along the directions parallel to the [111]direction is preferred. SnS is a layered anisotropic material witha double layer of Sn and S perpendicular to thec-axis.36 Thebonding between these layers is much weaker, as shown bylarger interatomic distances and by a (001) cleavage plane.37

This provides the possibility of growth into 1D crystals underthe appropriate thermodynamic conditions.

In the present case, elongated, rod-shaped SnS prefers to forman anisotropic core, and carbon surrounds the core in the laterstage. Similarly, the thermolysis of W(CO)6 in the presence ofelemental S, in a closed SS reactor at 750°C under an inertatmosphere, produced WS2 nanoworms33 embedded in carbon.Additionally, under equivalent reaction conditions, the thermaldecomposition of VO(OC2H5)3 and MoO(OMe)4 produced V2O3

and MoO2 cores, respectively, with a carbon shell. In all theabove-mentioned cases, the process is kinetically controlled;therefore, in situ formed WS2 worms or V2O3 nanoparticles29a

or MoO2 nanoparticles38 show a higher solidification rate thancarbon to form the core of the composite. Carbon, having aslower solidification rate, forms the shell layer. The coated carbonis mostly found to be a mixture of ordered and disordered carbonlayers since the thermal decomposition of the precursor solutionwas carried out at low temperature (Figure 5). This reasoningapplies also to the SnSe-C product and results in a SnSe core,which solidifies first, and the carbon, which solidifies later,creating the shell.

(35) Lee, S. M.; Cho, S. N.; Cheon, W.AdV. Mater. 2003, 15, 441.(36) Biswas, S.; Kar, S.; Chaudhuri, S.Appl. Surf. Sci.2007, 253, 9259.(37) Albers, W.; Hass, C.; Van der Masesn, E.J. Phys. Chem. Solids1960,

15, 306.(38) Pol, S. V.; Pol, V. G.; Kessler, V. G.; Seisenbaeva, G. A.; Sung, M. G.;

Asai, S.; Gedanken A.J. Phys. Chem. B2004, 108,6322.

Figure 2. SEM images of the (a) SnS-C sample and (b) SnSe-C sample and EDS of the (c) SnS-C sample and (d) SnSe-C sample.

Figure 3. BET surface area measurements of the (a) SnS-C sampleand (b) SnSe-C sample.

(CH3)4Sn+ S f SnS+ C + CxHyv

5138 Langmuir, Vol. 24, No. 9, 2008 Pol et al.

To comprehend the nature of covered carbon in the SnS-Csample, a Raman spectroscopy measurement is performed. TheRaman spectrum is characteristic of disordered graphitic carbonwith nanosized graphitic crystals. The D band is associated withgraphitic edge planes and defects in the graphitic lattice. Therelative intensity of the D and G bands is generally used toestimate the size of graphitic clusters in the hexagonal27 plane,La. The D and G bands of carbon show an intensity ratio ofID/IG

) 0.92 for the SnS-C sample. The first D peak is located at1337 cm-1 and originates from the in-plane lattice vibrations.The detected nongraphitic layers might be disordered becausethe growth temperature (700°C) is not high enough to improvethe local order of the formed carbon layers. In our earlier39report,we coated similar semigraphitic carbon on VOx and tested it as

a Li-insertion material. The performance of such electrodes interms of reversible capacity39 and rates was much better thanthat of electrodes containing nanoparticles of VOx without acarbon coating, since in our case carbon coating enables goodelectrical contact between the VOx particles.

In conclusion, the thermal decomposition of TMT in thepresence of either S or Se powder in a closed reactor at 700°C(40 min) under their autogenic pressure in an inert atmosphereproduces carbon-encapsulated SnS and SnSe nanorods. XRDmeasurements confirmed the orthorhombic structures of SnSand SnSe nanomaterials. The HR-TEM technique differentiatedbetween core and shell morphologies. The nitrogen gas adsorptionon the surface of core-shell nanostructures was determined byBET surface area analysis. The plausible mechanism for theformation of 1D chalcogenide cores (SnS or SnSe) withsemigraphitic shell nanostructures was elucidated. The Ramanspectroscopy measurements ensured the nature of the coatedcarbon.

Acknowledgment. A.G. thanks the IsraeliMinistryofScience,Culture snd Sport for supporting this research through thestrategic-generic program.

LA7040532

(39) Odoni, A.; Pol, V. G.; Pol, S. V.; Aurbach, D.; Gedanken A.AdV. Mater.2006, 18, 1431.

Figure 4. Transmission electron micrographs of (a) a SnS-C sample and (b) the edge of the SnS nanorod shown at high resolution, (c)HR-TEM of the SnS core, and (d) electron diffraction pattern of a SnS-C sample.

Figure 5. Raman spectrum of carbon from the Sn-C sample.

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