Hydrothermal synthesis of amorphous MoS2nanofiber bundles via acidification of ammonium heptamolybdate tetrahydrate
© to the authors 2007
Received: 16 March 2007
Accepted: 10 August 2007
Published: 1 September 2007
MoS2nanofiber bundles have been prepared by hydrothermal method using ammonium molybdate with sulfur source in acidic medium and maintained at 180 °C for several hours. The obtained black crystalline products are characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The PXRD pattern of the sample can be readily indexed as hexagonal 2H-MoS2. FTIR spectrum of the MoS2shows the band at 480 cm−1corresponds to the γas(Mo-S). SEM/TEM images of the samples exhibit that the MoS2nanofiber exist in bundles of 120–300 nm in diameter and 20–25 μm in length. The effects of temperature, duration and other experimental parameters on the morphology of the products are investigated.
KeywordsHydrothermal Nanofiber bundles MoS2 Citric acid
Nowadays, one-dimensional nanostructural materials (nanorods, nanowires, nanobelts and nanotubes) have attracted heightened attention because of their potential applications in nanodevices and functional materials . Continuous effort has been devoted to the investigation on the crystalline phases of the layered transition metal dichalcogenides, because of their unusual crystal structure and unique physical and chemical properties . In analogy to graphite, nanoparticles of many inorganic compounds such as MoS2, WS2 are not stable against folding, and can adopt nanotubular and fullerene-like structures . As transition metal sulfides, MoS2 has been the subject of significant research for applications including nonaqueous lithium batteries, catalytic hydride sulfurization of petroleum and wear resistance. For each of these applications, the important processes occur either at the surface or at the exposed edges of the MoS2 layers . MoS2 is a layered semiconductor (band gap = 1.2 eV, indirect) that resists oxidation even in moist air at temperatures up to 85 °C . This attribute makes MoS2 an attractive semiconductor material for nanoscience applications. Recent studies have suggested that reducing the size of the MoS2 crystals can improve their lubrication properties in bearings, O-rings or other heavy—wear applications .
Inorganic syntheses using organic surfactants are successfully applied for the preparation of various nanostructural materials [6, 7]. The subsequent template elimination without destruction of the mesoporous texture remains a difficult step . Various methods of preparation includes chemical vapor deposition, sol–gel processing, spray pyrolysis, co-precipitation, sonochemical synthesis, metathesis reactions, two step electrochemical synthesis etc are having been in practice [9–12]. Tenne and co-workers [13, 14] first reported the production of fullerene-like MoS2 nanotubes via the gas phase reaction between MoO3−x and H2S in a reducing atmosphere at elevated temperature (800–1000 °C). Rao and co-workers obtained MoS2 nanotubes by simple heating MoS3 in a stream of hydrogen under high temperature . Zelenski et al.  synthesized fibers and tubules of MoS2 by the thermal decomposition of ammonium thiomolybdate precursors at 450 °C. MoS2 nanotubes  were made by subliming 2H–MoS2 powder in presence of H2S at 1300 °C. Recently MoS2 nanorods  were synthesized using α-MoO3 nanorods with a mixture of H2S · H2 (15 vol% of H2S) for 4 h at several temperatures. However, almost all the methods to obtain MoS2 nanotubes and nanorods utilize high temperature gas-solid reaction under reducing gas atmosphere. Yumei et al. [1, 19] synthesized MoS2 nanorods and nanospheres by hydrothermal method using ammonium molybdate with Na2S in presence of NH2OH · HCl as oxidizing agent. Hydrothermal synthesis is becoming popular for environmental reason, since water is used as reaction solvent than organics. This method has been widely used to prepare nanostructures due to its simplicity, high efficiency and low cost.
In this article, as a part of our recent efforts on the investigation of 1-D materials , we report a simple low temperature hydrothermal method for preparation of MoS2 nanofiber bundles with morphology control. We examined the effect of precursors (ammonium molybdate, sodium sulphide and citric acid) concentration, duration and the temperature on the morphology of the product. Finally, we discuss appropriate conditions for producing MoS2 nanobundles.
The hydrothermal process was carried out like our previous report [21, 22] to prepare MoS2 nanofiber bundles. In a typical synthesis of MoS2 nanofibers, sodium sulfide and H2S gas were used as sulfur sources in order to investigate the source effect on the morphology of the product.
In the first trail 1.235 g ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24 · 4H2O) was put into 25 mL distilled water under continuous stirring. After 10 min, 0.5 g citric acid monohydrate (C6H8O7 · H2O) was added to the above solution. To this solution mixture, 0.312 g sodium sulfide crystals were added. The initial greenish black color was changed to red after 1h stirring on a magnetic stirrer. In the subsequent trail, 0.5 g ammonium heptamolybdate tetrahydrate was put into 25 mL distilled water under continuous stirring. H2S gas is passed in to the warmed acidified with dilute HCl solution for about 5–8 min.
The sealed 25 mL Teflon lined autoclaves containing above solution mixture were placed for hydrothermal treatment at 180 °C for several hours. The autoclaves were cooled and the resulted black solid was retrieved from the solution by centrifugation, washed with distilled water followed by ethanol to remove the ions possibly remaining in the end product, and finally dried in air.
Powder X-ray diffraction data were recorded on Philips X’pert PRO X-ray diffractometer with graphite monochromatized Cu Kα radiation (( = 1.541 Å). The Fourier transform infrared spectrum of the samples was collected using Nicollet FTIR spectrometer. Scanning electron micrograph images were taken with JEOL (JSM—840 A) scanning electron microscope. X-ray photoelectron spectroscopy was carried out on an ESCA-3 Mark II X-ray photoelectron spectrometer, (VG Scientific, UK) using Al Kα radiation (1486.6 eV) as the exciting source. The morphology of the samples was observed using (H-800EM Japan Hitachi) transmission electron microscope (TEM) equipped with EDS (Kevex Sigma TM Quasar, USA) to measure the elements contained in the samples.
Results and discussion
XPS data of Mo(3d) and S(2p)
Binding energy (eV)
Peak height (Intensity)
Figure 4b shows S(2p) doublet peak (2p3/2 and 2p1/2) at 161.25 and 162.94 eV respectively in which 2p3/2 and 2p1/2 orbital peaks coincide with each other. It is to be noted that the peak positions of S(2p) and Mo(3d5/2) for MoS2 is in good agreement with he results on MoS2 .
The chemical shifts of Mo(3d5/2) and S(2p) in MoS2 nanoparticle is +1.6 and −2.2 eV in comparison to elemental Mo and S . The electronegativity of S being 2.5 is greater than that of Mo being 1.8. Therefore there occurs some partial electron transfer from molybdenum to sulfur. As a result, molybdenum becomes positively charged as shown by the increase in binding energy and sulfur becomes negatively charged as shown by the decrease in binding energy.
Step 3 is a reduction reaction, in which an intermediate phase MoCl4 is produced , and step 4 is a sulfureted reaction under hydrothermal conditions.
Nanofiber of MoS2bundles with 150–300 nm in diameters and 20–25 μm in length have been successfully synthesized by a simple low temperature hydrothermal method. PXRD pattern indicated the amorphous MoS2was made up of single layer and layer stacking had not taken place. The nanofiber self assembled into bundles and the nanofiber bundle had the same growth direction. SEM study reveals the formation of bundles of well-aligned MoS2nanofiber with very high aspect ratio. Since the MoS2nanomaterials are often very difficult to prepare and the search for more simple routine is quite valuable with regard to energy-saving and less pollution. It can be demonstrated that the optimum conditions for preparing MoS2nanofiber are at 180 °C for 48 h using citric acid and 180 °C for 24 h by passing H2S in presence of HCl. Under these conditions the morphology of the products resembles to one another.
The author G. T. Chandrappa thankful to the Department of Science and Technology, NSTI Phase-II, New Delhi, Government of India for financial support to carryout this research work. We also thank the Central Facility, Dept. of Physics and Dept. of Metallurgy, Indian Institute of Science, Bangalore for collecting XRD data and SEM images.
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