Hydrothermal synthesis of amorphous MoS2nanofiber bundles via acidification of ammonium heptamolybdate tetrahydrate

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.


Introduction
Nowadays, one-dimensional nanostructural materials (nanorods, nanowires, nanobelts and nanotubes) have attracted heightened attention because of their potential applications in nanodevices and functional materials [1]. 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 [2]. In analogy to graphite, nanoparticles of many inorganic compounds such as MoS 2 , WS 2 are not stable against folding, and can adopt nanotubular and fullerene-like structures [3]. As transition metal sulfides, MoS 2 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 MoS 2 layers [1]. MoS 2 is a layered semiconductor (band gap = 1.2 eV, indirect) that resists oxidation even in moist air at temperatures up to 85°C [4]. This attribute makes MoS 2 an attractive semiconductor material for nanoscience applications. Recent studies have suggested that reducing the size of the MoS 2 crystals can improve their lubrication properties in bearings, O-rings or other heavy-wear applications [5].
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 [8]. 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][10][11][12]. Tenne and co-workers [13,14] first reported the production of fullerene-like MoS 2 nanotubes via the gas phase reaction between MoO 3-x and H 2 S in a reducing atmosphere at elevated temperature (800-1000°C). Rao and co-workers obtained MoS 2 nanotubes by simple heating MoS 3 in a stream of hydrogen under high temperature [15]. Zelenski et al. [16] synthesized fibers and tubules of MoS 2 by the thermal decomposition of ammonium thiomolybdate precursors at 450°C. MoS 2 nanotubes [17] were made by subliming 2H-MoS 2 powder in presence of H 2 S at 1300°C. Recently MoS 2 nanorods [18] were synthesized using a-MoO 3 nanorods with a mixture of H 2 S Á H 2 (15 vol% of H 2 S) for 4 h at several temperatures. However, almost all the methods to obtain MoS 2 nanotubes and nanorods utilize high temperature gas-solid reaction under reducing gas atmosphere. Yumei et al. [1,19] synthesized MoS 2 nanorods and nanospheres by hydrothermal method using ammonium molybdate with Na 2 S in presence of NH 2 OH Á 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 [20], we report a simple low temperature hydrothermal method for preparation of MoS 2 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 MoS 2 nanobundles.

Synthesis
The hydrothermal process was carried out like our previous report [21,22] to prepare MoS 2 nanofiber bundles. In a typical synthesis of MoS 2 nanofibers, sodium sulfide and H 2 S 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 ((NH 4 ) 6 Mo 7 O 24 Á 4H 2 O) was put into 25 mL distilled water under continuous stirring. After 10 min, 0.5 g citric acid monohydrate (C 6 H 8 O 7 Á H 2 O) 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. H 2 S 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.

Characterization
Powder X-ray diffraction data were recorded on Philips X'pert PRO X-ray diffractometer with graphite monochromatized Cu Ka 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 Ka 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. Figure 1a, b shows the PXRD patterns of the samples prepared using sodium sulfide and H 2 S as sulfur sources in acidic medium at 180°C for 48 and 24 h, respectively. It shows only a broad, weak envelope beginning at 2h = 30°a nd continuing out above 60°, the two maxima approximately located at the 100 and 110 positions of bulk 2H-MoS 2 [23]. The PXRD patterns resemble that of the singlelayer MoS 2 prepared by exfoliation method [24]. All the diffractions can be readily indexed as hexagonal 2H-MoS 2 [space group P6 3 /mmc (194)] with lattice constants a = 3.16116 Å and c = 12.2985 Å , identical to the reported data in the JCPDS cards (37-1492). At 180°C only poor crystals were produced. The absence of the 002 diffraction at 2h = 14.4°suggests that absence of stacking, thus the sample should be an aggregate of single sheets of MoS 2 [1,8,18,23,25].

Results and discussion
Chemical analysis using EDS indicates the presence of Mo and S (elements of MoS 2 ), and no other elements existed as observed in Fig. 2. Furthermore, the quantification of the peaks shows that the atomic ratio of S to Mo is 1.92, which is very close to the stoichiometric MoS 2 . From the IR spectrum (Fig. 3), it is observed that the band at 480 cm -1 corresponds to c as (Mo-S). The bands above 600 cm -1 correspond to Mo-O vibrations and they indicate that oxygen is present in similar types of coordination as in MoO 3 . We assign the band at 941 cm -1 to the c as (Mo-O) vibrations of terminal (-Mo=O) group and 693 cm -1 to c(Mo-O) vibrations. The frequency of the vibration is shifted to the lower wavenumber compare to its standard value in crystalline MoO 3 , due to the presence of reduced Mo centers and sulfur as a ligand [26].
X-ray photoelectron spectra of Mo(3d) and S(2p) core level regions of as-prepared nanoparticles are shown in Fig. 4a and b and  sets of spin-orbit doublet. Accordingly, the high intense Mo(3d 5/2 ) peaks at 229.1 and low intense Mo(3d 3/2 ) peak at 232.1 eV could be attributed to MoS 2 . Whereas lower binding energy shoulder peak at 226.25 eV is because of S(2s) [27]. Figure 4b shows S(2p) doublet peak (2p 3/2 and 2p 1/2 ) at 161.25 and 162.94 eV respectively in which 2p 3/2 and 2p 1/2 orbital peaks coincide with each other. It is to be noted that the peak positions of S(2p) and Mo(3d 5/2 ) for MoS 2 is in good agreement with he results on MoS 2 [28].
The chemical shifts of Mo(3d 5/2 ) and S(2p) in MoS 2 nanoparticle is +1.6 and -2.2 eV in comparison to elemental Mo and S [29]. 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.
MoS 2 nanofiber of about 120-300 nm in diameter and 20-25 lm in length are identified from SEM images (Fig. 5). They are well aligned and occur in-group of bundles with very high aspect ratio. It can be seen that the nanofiber self assembled into bundles and the nanofiber in a bundles had the same growth direction. The morphology and structure of individual MoS 2 nanorods have been characterized in further detail using TEM, which are shown in Fig. 6. It can be seen that the nanorods are straight and uniform along their entire length and thickness of about up to 300 nm. The ends of the nanorods are broken. Figure 6d is the high resolution TEM image of the MoS 2 nanorods and this image shows solid nanorods are uniform in thickness.
The reduction of transition metal ions by surfactant is useful for the production of ultra fine particles of metal sulfides. Unfortunately, molybdic compound reduction chemistry is quite complex and the nature of the products strongly depends on the reaction conditions. For example, Afanasiev et al. show that the reduction of aqueous ammonium molybdate at different temperatures and conditions yields binary sulfides such as MoS 5.6 , MoS 6 , and MoS 3 as well as binary oxides such as MoO 3 and MoO 2 . In fact, the mechanism differs markedly in aqueous and in nonaqueous solutions. In order to disclose the transition  [30][31][32] on phase of products was investigated. The possible reaction mechanism can be written as Step 1 is a sulfureted reaction, in which an intermediate phase (NH 4 ) 2 MoS 4 is produced [33], and step 2 is a reduction reaction under hydrothermal conditions. Thus by the addition of 1 g of citric acid or without citric acid, sulfureted reaction and reduction reaction do not react completely so that the products consist of much MoO 3 phase as shown in the Fig. 1c, d. Therefore during the formation of MoS 2 nanofiber bundles, citric acid plays a very prominent role. Figure 7a, b gives the SEM images of the product prepared with and without citric acid. We evaluate the effect of temperature and duration on the morphology of the products. But there was no interesting morphology of the product obtained. The morphology of the product prepared at 180°C for 24 h (Fig. 8) using H 2 S resembles with the morphology of the product prepared at 180°C for 48 h (Fig. 5) using sodium sulfide as sulfur source. Here, there is a regular arrangement of MoS 2 fibers takes place to form bundles. The thickness of the fiber is found to be about 300 nm and length is about 12 lm. From the SEM images, we observed that the length of all fibers is found to be same with sharp ends having voids in each MoS 2 bundles.
The possible reaction mechanism can be written when H 2 S as sulfur source is Step 3 is a reduction reaction, in which an intermediate phase MoCl 4 is produced [19], and step 4 is a sulfureted reaction under hydrothermal conditions. Figure 9a, b shows the SEM images of MoS 2 prepared at 180°C for 16 and 20 h. When the duration is increased to 48 h, nanorods (Fig. 9c) are obtained. The thickness of the nanorods is found to be in the range of 200-600 nm.

Conclusion
Nanofiber of MoS 2 bundles with 150-300 nm in diameters and 20-25 lm in length have been successfully synthesized by a simple low temperature hydrothermal method. PXRD pattern indicated the amorphous MoS 2 was 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 MoS 2 nanofiber with very high aspect ratio. Since the MoS 2 nanomaterials are often very difficult to prepare and the search for more simple routine is quite valuable with regard to energysaving and less pollution. It can be demonstrated that the optimum conditions for preparing MoS 2 nanofiber are at 180°C for 48 h using citric acid and 180°C for 24 h by passing H 2 S in presence of HCl. Under these conditions the morphology of the products resembles to one another.