Synthesis of MoS2 and MoO3 hierarchical nanostructures using a single-source molecular precursor

Powder Technology 253 (2014) 347–351

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Synthesis of MoS2 and MoO3 hierarchical nanostructures using a single-source molecular precursor Tianxi Wang a,⁎, Jing Li b, Gaoli Zhao c a b c

School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China School of Chemistry and Chemical Engineering, Xuzhou Institute of Technology, Xuzhou 221111, China Academic Affairs Office, Henan Institute of Science and Technology, Xinxiang 453003, China

a r t i c l e

i n f o

Article history: Received 10 June 2013 Received in revised form 14 November 2013 Accepted 1 December 2013 Available online 8 December 2013 Keywords: Molybdenum trioxide and disulfide Nanostructures Powder technology

a b s t r a c t MoS2 nanoflake clusters were synthesized via hydrothermal treatment of an air-stable, easily obtained singlesource molecular precursor (molybdenum diethyldithiocarbamate oxide, Mo((C2H5)2NCS2)2O2) in deionized water at 200 °C for 24 h; and orthorhombic phase MoO3 (α-MoO3) nanoplate clusters were obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h. The obtained products were characterized by X-ray diffraction, Xray photoelectron spectroscopy, Raman spectroscopy and field emission scanning electronic microscopy. Besides, the comparative synthesis experiment by direct pyrolysis of ammonium peramolybdate suggested that MoS2 nanoflake clusters might play a template role in the formation of α-MoO3 nanoplate clusters. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Nanostructured MoS2 and MoO3 have shown intriguing physical and chemical properties as well as exciting prospects for applications in lubrication, electrochemistry, catalysis, sensor, optics and electronics, etc. [1–23]. In particular, two-dimensional (2D) MoS2 and MoO3 can exhibit morphology- and thickness/size-dependent physical and chemical properties, which have attracted considerable scientific and practical interest [1–3]. For instance, when the thickness of 2D MoS2 is reduced to only one layer, the bandgap of MoS2 can change from indirect to direct, and monolayer MoS2 can exhibit strong fluorescence [1]. 2D MoO3 can offer enhanced properties such as increased carrier mobility, tunability of band structures, and enhanced thermal and mechanical properties [1–3]. Furthermore, the construction of MoS2 and MoO3 hierarchical nanostructures with 2D building blocks (e.g., nanoflake, nanoplate and nanosheet) can further enhance their functionality in interesting ways [4]. Hence, it is of great importance to design the synthesis methods for 2D MoS2 and MoO3 nanomaterials. The synthesis of inorganic nanomaterials using single-source molecular precursors can control the microstructures and compositions of the obtained products [24–35]. By employing a proper single-source molecular precursor, combined with hydrothermal decomposition or other pyrolysis processes [24–35], nanostructured products could be obtained usually under the conditions much milder than those adopted in the conventional solid-state synthesis. Molybdenum diethyldithiocarbamate oxide (Mo((C2H5)2NCS2)2O2) is an air-stable, easily obtained,

⁎ Corresponding author. Tel.: +86 13849363420. E-mail address: [email protected] (T. Wang). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.12.005

inexpensive, solid metallorganic compound, and it has been used as a single-source precursor to prepare MoS2 powders by thermolysis in N2 atmosphere [35]. Herein, we report first the synthesis of MoS2 nanoflake clusters via hydrothermal treatment of Mo((C2H5)2NCS2)2O2 in deionized water at 200 °C for 24 h, then the synthesis of MoO3 nanoplate clusters by heating MoS2 nanoflake clusters in air at 350 °C for 5 h. The obtained products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and field emission scanning electronic microscopy (FESEM). Besides, for comparison, the synthesis of MoO3 by direct pyrolysis of ammonium peramolybdate was also conducted. 2. Experimental All the chemical reagents were of analytical grade and bought from Sinopharm Chemical Reagent Co., Ltd. 2.1. Synthesis 2.1.1. Synthesis of molybdenum diethyldithiocarbamate oxide (Mo((C2H5)2NCS2)2O2) 4 mmol ammonium peramolybdate ((NH4)6Mo7O24·4H2O) and 20 mmol sodium diethyldithiocarbamate ((C2H5)2NCS2Na·3H2O) were firstly dissolved in 200 ml of deionized water, respectively. These two solutions were mixed in a 500 ml beaker and magnetically stirred for 6 h, then the reaction solution was kept stationary under the ambient condition for 6 h. The resulting yellow precipitate was filtered, washed with deionized water, and dried in vacuum at 60 °C for 12 h.

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2.1.3. Synthesis of MoO3 Nanostructured α-MoO3 was obtained by heating 300 mg of the hydrothermally synthesized MoS2 in air at 350 °C for 5 h. 2.2. Characterization The obtained products were characterized by XRD (German Bruker AXS D8 ADVANCE X-ray diffractometer), Raman (Britain Renishaw Invia Raman spectrometer, excitation at 532 nm, 3 mW), XPS (American Thermo-Scientific ESCALAB 250 XPS system, Al Kα radiation and adventitious C 1s peak (284.6 eV) calibration), and SEM (Japan Hitachi S-4800 FESEM).

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Fig. 3. XRD pattern of the product synthesized via hydrothermal treatment of Mo((C2H5)2NCS2)2O2 in deionized water at 200 °C for 24 h.

All the XRD peaks of the precursor in Fig. 1 can be indexed to monoclinic phase molybdenum diethyldithiocarbamate oxide (Mo((C2H5)2NCS2)2O2), according to the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 25-1978. Besides, the FESEM

Mo 3p3/2 Mo 3p1/2 Intensity (a. u.)

2.1.2. Synthesis of MoS2 500 mg of Mo((C2H5)2NCS2)2O2 was placed into a 40 ml Teflon-lined stainless steel autoclave, and 32 ml of deionized water was added with stirring. The autoclave was sealed and maintained at 200 °C for 24 h, then cooled to room temperature naturally. The resulting precipitates were filtered, washed with deionized water and ethanol, and dried in vacuum at 100 °C for 4 h.

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2θ (deg.) Fig. 1. XRD pattern of the precursor synthesized through the reaction between ammonium peramolybdate and sodium diethyldithiocarbamate in deionized water under the ambient condition.

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Fig. 1 shows the XRD pattern of the precursor synthesized through the reaction between ammonium peramolybdate and sodium diethyldithiocarbamate in deionized water under the ambient condition.

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Binding energy (eV) Fig. 2. FESEM image of the Mo((C2H5)2NCS2)2O2 precursor.

Fig. 4. XPS spectra of the as-synthesized MoS2.

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2θ (deg.) Fig. 6. XRD pattern of the product obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h.

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no. 76-1003). Besides, the Raman spectrum of this product exhibits obvious peaks at around 221, 247, 292, 340, 378, 470, 666, 821 and 997 cm− 1 in the wavenumber range of 200 to 1100 cm− 1 (Fig. 7), which are also in agreement with the reported Raman spectra of αMoO3 [39–43]. The Raman peak at 997 cm−1 is assigned to the terminal oxygen (Mo_O) stretching mode, which results from an unshared oxygen [39–43]. The peak at 821 cm−1 can be assigned to the double coordinated oxygen (Mo\O\Mo) stretching mode, which results from corner-sharing oxygen between two MoO6 octahedra [39–43]. The peaks at 666 and 470 cm−1 can be assigned to the edge sharing of triply coordinated oxygen (Mo\O3) stretching mode, which results from edge-shared oxygen between three octahedral [39–43]. The peaks at 340 and 378 cm−1 can be assigned to Mo3\O and Mo_O bending modes [39–43], respectively. The peak at 292 cm−1 represents the bending mode for the double bond (Mo_O) vibration [39–43]. Both the peaks at 221 and 247 cm−1 represent the bending mode of Mo2\O [39–43]. Fig. 8 shows the XPS spectra of the α-MoO3 obtained by heating MoS2 nanoflake clusters in air at 350 ºC for 5 h. The survey XPS spectrum in Fig. 8 reveals that this product is composed of Mo and O elements, except for the adventitious C. No peak characteristic of S2− (e.g., around 162.0 eV) is observed in Fig. 8, suggesting the complete conversion of MoS2 into MoO3 after being heated in air at 350 °C for 5 h. Mo 3p XPS spectrum shows that the binding energies of Mo 3p3/2 and Mo 3p1/2 are 398.8 and 416.3 eV (Fig. 4), respectively, which can be assigned to Mo6 + in MoO3 [44–46]. The O 1s XPS

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Raman shift (cm-1) Fig. 5. FESEM images of the as-synthesized MoS2.

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image in Fig. 2 shows that the Mo((C2H5)2NCS2)2O2 precursor consists of irregular particles. The XRD pattern of the product synthesized via hydrothermal treatment of Mo((C2H5)2NCS2)2O2 in deionized water at 200 °C for 24 h is shown in Fig. 3, which is similar to that of low crystallinity MoS2 [36]. Moreover, no XRD peaks of the Mo((C2H5)2NCS2)2O2 precursor can be observed in Fig. 3, indicating that the Mo((C2H5)2NCS2)2O2 precursor has been completely decomposed into MoS2 and other byproducts upon hydrothermal treatment in deionized water at 200 °C for 24 h. The elemental composition and chemical state of the as-synthesized MoS2 were also analyzed by XPS. The survey XPS spectrum (not shown here) reveals that this sample is composed of Mo and S elements. The Mo 3p XPS spectrum shows that the binding energies of Mo 3p3/2 and Mo 3p1/2 are 395.0 and 412.6 eV (Fig. 4), respectively, which correspond to Mo4+ in MoS2 [37,38]. The S 2p XPS spectrum shows that the binding energy of S 2p3/2 is 162.0 eV (Fig. 4), which corresponds to S2− in MoS2 [37,38]. Besides, the molar ratio of Mo:S was determined to be 1:2.1, also close to the stoichiometry of MoS2. Fig. 5(a) and (b) shows the low- and high-magnification FESEM images of the as-synthesized MoS2, respectively. The morphology of the as-synthesized MoS2 (Fig. 5(a) and (b)) was completely different from that of the Mo((C2H5)2NCS2)2O2 precursor (Fig. 2), suggesting the complete decomposition of the Mo((C2H5)2NCS2)2O2 precursor upon hydrothermal treatment in deionized water at 200 °C for 24 h. The as-synthesized MoS2 comprises clusters of interleaving, slightly bending nanoflakes, whose thickness is about 22 nm. Fig. 6 shows the XRD pattern of the product obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h. It displays only the XRD peaks characteristic of orthorhombic phase α-MoO3 (JCPDS card

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Fig. 7. Raman spectra of the as-synthesized MoO3.

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spectrum shows that the binding energy of O 1s is 530.8 eV (Fig. 4), which can be assigned to O2− in MoO3 [44 46]. Fig. 9(a) and (b) shows respectively the low- and high-magnification FESEM images of the α-MoO3 obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h, whereas Fig. 9(c) shows the FESEM image of the MoO3 obtained by direct pyrolysis of (NH4)6Mo7O24·4H2O in air at 350 °C for 5 h. It can be seen from Fig. 9(a) and (b) that the α-MoO3 obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h comprises clusters of interleaving nanoplates, whose thickness is about 25 nm. In contrast, the α-MoO3 obtained by direct pyrolysis of

4. Conclusions MoS2 nanoflake clusters were synthesized via hydrothermal treatment of Mo((C2H5)2NCS2)2O2 in deionized water at 200 °C for 24 h,

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(NH4)6Mo7O24·4H2O in air at 350 °C for 5 h comprises irregular particles (Fig. 9(c)). The results in Fig. 9 suggest that MoS2 nanoflake clusters may play a template role in the formation of α-MoO3 nanoplate clusters.

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526 Fig. 9. (a and b) Low- and high-resolution FESEM images of the α-MoO3 obtained by heating MoS2 nanoflake clusters in air at 350 °C for 5 h, and (c) FESEM image of the MoO3 obtained by direct pyrolysis of (NH4)6Mo7O24·4H2O in air at 350 °C for 5 h.

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