Materials Letters 62 (2008) 3558–3560

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Materials Letters

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Hydrothermal synthesis of hollow MoS2 microspheres in ionicliquids/water binary emulsions

Hao Luo a, Chao Xu a, Dingbing Zou a, Ling Wang a, Taokai Ying a,b,⁎a College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, PR Chinab Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, PR China

⁎ Corresponding author. Tel.: +86 579 82282780; fax:E-mail address: sky50@zjnu.cn (T. Ying).

0167-577X/$ – see front matter. Crown Copyright © 20doi:10.1016/j.matlet.2008.03.050

A B S T R A C T

A R T I C L E I N F O

Article history:

Hollow molybdenum disu Received 7 December 2007Accepted 20 March 2008Available online 28 March 2008

Keywords:Molybdenum disulfideIonic liquidsHollow spheresSemiconductors

lfide (MoS2) microspheres were synthesized in ionic liquids (1-butyl-3-methylimidazolium chloride, [BMIM]Cl)/water binary emulsions by the hydrothermal method at 180 °C for24 h. The optimum value of volumetric proportions (ILs/water) equaled 1:9. The structure and morphology ofproducts were characterized by means of X-ray powder diffraction (XRD), scanning electron microscope(SEM) and transmission electron microscopy (TEM). The experimental results gave the evidences that thesample is consists of hollow spheres 1.8 – 2.1 μm in diameter, and there are much sheet-like structures on thesurface of hollow MoS2 microspheres. In addition, ILs could be collected and reused for subsequent reactions.

Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction

Transition metal sulfide MoS2 is semiconductor material whichexhibits a layered structure consisting of covalently bound S–Mo–Strilayers, separated by a relatively large van der Waals gap. Theelectronic structure is such that band-edge excitation correspondslargely to a metal centered d–d transition [1,2]. Due to these features,the MoS2 compounds show numerous properties, such as catalysis,electrocatalysis, electrochemical intercalation, solid lubrication etc. Todate, there aremanymethods which have been reported to synthesizeMoS2, for instance, using gaseous reducing agent at high temperature[3–5], ultrasonic spray pyrolysis (USP) [6], sonochemical synthesis[7,8], hydrothermal synthesis [9–14], chemical solution route [15,16],microwave plasma route [17], spray drying process [18] etc. Amongthese approaches above, there are many kinds of MoS2 micro/nanomaterials with various morphologies, such as inorganic fullerene-likeand nanotubes, nanorods, nanowires, microspheres, hollow spheres[7,13,14,18] etc.

In recent decades, room temperature ionic liquids (ILs) haveattracted increasing interest in the context of green synthesis becauseof their unique chemical and physical properties of nonvolatility,nonflammability, thermal stability, and controlled miscibility [19].Recently, the applications of ILs in inorganic synthesis [20–22] are

+86 579 82282269.

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receiving much attention as well as the ILs used as solvents in organicchemical reactions. ILs cannot only act as reactionmedium for reactants,but can also be utilized as morphological templates for the products.

Novel hollow microspheres have been prepared in surfactant-stabilized oil/water emulsions [23]. According to this elicitation,Takuya Nakashima [20] described synthesis of hollow TiO2 micro-spheres in suitable ILs reaction systems. Then Jimin Du et al. [21]obtained hollow CaCO3 spheres in ILs. Later, Hollow carbon micro-spheres were synthesized in ILs [22]. In addition, Izaskun Uzcanga andhis co-workers have reported that the formation of hollow MoS2microspheres under ultrasound irradiation was observed for the firsttime [7]. Shi et al. obtained MoS2 hollow spheres in the mixture ofethanol and water [13]. Xu and Li [14] obtained nanocrystalline MoS2with hollow spherical morphology by the hydrothermal method.Herein, we report a route for synthesis of hollow MoS2 microspheresby means of redox reaction in ILs /water (1:9, v/v) binary emulsions atlow temperature (180 °C) for 24 h.

2. Experimental

The sodium molybdate (Na2MoO4·2H2O), thiourea (CS(NH2)2), areanalytical pure regents (AR) and purchased and used without furthertreatments. The ionic liquid (1-butyl-3-methylimidazolium chloride,[BMIM]Cl) was synthesized according to the literature [24].

In a typical synthesis, 0.72 g of Na2MoO4·2H2O (3 mmol) and0.69 g of thiourea (9 mmol) were dissolved in V1 ml deionized water.V2 ml of [BMIM]Cl was added into the solution under violent stirring

hts reserved.

Fig. 1. XRD patterns of as-synthesized MoS2 samples synthesized at 180 °C for 24 h in(a) water; (b) V2/V1 (ILs/water)=1/9.

Fig. 3. TEM image of hollow MoS2 microspheres.

3559H. Luo et al. / Materials Letters 62 (2008) 3558–3560

(V=V1+V2=60 ml). Then 12 mol/L HCl was dropped into the solutionwhile being stirred to adjust the pH value to less than 1. The solutionabove was put into 100 ml Teflon-lined stainless steel autoclave andsealed tightly. The autoclave was maintained at 160, 180, and 200 °Cfor 24 h, respectively [11], and then cooled to room temperaturenaturally. The black precipitates were collected through centrifugalmethod, washed with distilled water several times to remove theresidue of the reactants and ILs, and dried in the air to obtain the finalMoS2 powder. The ILs was collected.

The XRD patterns were recorded by a Holland Philips X'PertPW3040/60 using Cu (Kα) radiation (λ=0.15418 nm) operating at40 kV and 40 mA with 2θ ranging from 10° to 70°. The SEM imageswere taken with Japan Hitachi S-4800 field-emission scanningelectron microscope. The TEM images were taken with a JEOL-2010transmission electron microscope at an accelerating voltage of 200 kV.

3. Results and discussions

Fig. 1 shows the XRD patterns of as-synthesized MoS2 samples. All the reflectionscan be readily indexed to hexagonal 2H (two-layer hexagonal) MoS2 (JCPDS 37-1492).The XRD pattern of the MoS2 microspheres (Fig. 1b) prepared in ionic liquid/water (1:9,v/v) binary emulsions shows a typical scan which is the characteristic of poorlycrystalline layered disulfides and similar to the scan reported by Wildervanck andJellinek for MoS2 produced by the decomposition of MoS3 [25, 26]. It exhibited a strong(002) maximum in low angle region and a broad envelope starting approximately at2θ=30o and lasting out to about 2θ=60o. This envelope contains the (100), (101), (102),(103), (006), (105), (106), (110), and (008) reflections. Among these reflections, thewell-defined maxima are at (100), (103), and (110) reflections. As shown in Fig. 1a, the MoS2samples prepared in pure water had a lower diffraction peak of (002), which indicatedthe poor-stacked layered structure of MoS2.

The size and morphology of MoS2 samples were observed by SEM (shown in Fig. 2).Fig. 2a showed that theMoS2 samples are uniformmicrospheres with diameters of 1.8 –

Fig. 2. SEM images of hollo

2.1 µm. Other SEM images (Fig. 2b and c) with the higher magnification offer a clearview of the surface morphology, which show distinctly that the MoS2 microspheres hadhollow structure and the surface of sample was constructed by sheet-like structures.Furthermore, the sample with hollow structure was proved by TEM (shown in Fig. 3).

Tian et al. [11] have reported the synthesis of MoS2 nanotubes and nanorods by ahydrothermal method at a low temperature (180 °C). According to their experimentalresults, reaction temperature played an important role in the morphology of theproducts. At 160 °C, only poor crystals were produced with almost no layer structure.Above 200 °C, the MoS2 layers became too thick to form tubes or rods difficultly. In ourexperiment, no product has been obtained at 160 °C. When the temperature waspromoted to 200 °C, the MoS2 became microspheres without hollow. In order toinvestigate the effect of ILs on the formation of hollow MoS2 microspheres, theexperiments were carried out by varying the value of volumetric proportions (V2/V1)while keeping other conditions unchanged. The results indicated that the optimumtemperature was 180 °C and the optimum value of volumetric proportions equaled 1:9(shown in Fig. 2). The reactions involved in the process can be shown as follows [12]:

4 Na2MoO4 þ 9 CS NH2ð Þ2þH2O

þ6 HCIYBMIM½ �CI=II2180 -C;24 h

O4 MoS2 þ Na2SO4 þ 18 NH3Aþ 6 NaClþ 9 CO2z

Izaskun Uzcanga et al. [7] have reported the formation of hollow MoS2microspheres using ultrasound irradiation. And they suggested that the hollow featuresare due to the interactions of the cavitation bubbles among them and with the surfacesof the horn and reaction vessel. On the research of the growth mechanism of thehollow-like structure using ionic liquids, Nakashima and Kimizuka [20] concluded theILs could form many micro-sized droplets in the solution under proper experimentalconditions. In our experiment, the reaction mechanism responsible for the formation ofhollow spheres can be explained as follows. The [BMIM]Cl consists of [BMIM]+ cation,which could form vesicle in water. Then the MoO4

2− ions of the precursor solution areabsorbed to the surface of these vesicles by the electrostatic interaction. This processprovides nucleation domains for the hydrothermal reaction between MoO4

2− and H2Sproduced by decomposition of thiourea. The fresh sheet-like MoS2 were produced andstacked together on the surface of vesicles during hydrothermal process. After reactionended, the black precipitates were washed with distilled water several times to remove

w MoS2 microspheres.

3560 H. Luo et al. / Materials Letters 62 (2008) 3558–3560

[BMIM]+ cations. Thus the hollow MoS2 microspheres can be formed. Based on theexperimental results, imidazolium molecules not only acted as the solvent, but also thestabilizing agent for the hollow microspheres.

4. Conclusions

In conclusion, hollow MoS2 microspheres were synthesized by thehydrothermal method in ILs/water binary emulsions at low tempera-ture. Further more, it was also found that sheet-like structures wereshaped on the surface of hollow MoS2 microspheres. Additionally, theILs has been reclaimed and reused for the subsequent reactions. Theproduct was obtained in high yield, because the ILs has the capabilityof extraction of metallic ion. On the one hand, the strategy describedhere is of particular interest for the synthesis of IF-MoS2 particles withhollow structure andmight be extended to other layeredmaterials. Onthe other hand, it has bright future in the development of cleanmanufacturing processes and in the remediation of sites contami-nated by an older generation of manufacturing technologies.

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