Swift tuning from spherical molybdenum microspheres to hierarchical molybdenum disulfide nanostructures by switching from solvothermal to hydrothermal synthesis route
© Korea Nano Technology Research Society 2017
Received: 23 June 2017
Accepted: 15 September 2017
Published: 29 September 2017
Herein, we report the synthesis of metallic molybdenum microspheres and hierarchical MoS2 nanostructures by facile template-free solvothermal and hydrothermal approach, respectively. The morphological transition of the Mo microspheres to hierarchical MoS2 nanoflower architectures is observed to be accomplished with change in solvent from ethylenediamine to water. The resultant marigold flower-like MoS2 nanostructures are few layers thick with poor crystallinity while spherical ball-like molybdenum microspheres exhibit better crystalline nature. This is the first report pertaining to the synthesis of Mo microspheres and MoS2 nanoflowers without using any surfactant, template or substrate in hydro/solvothermal regime. It is opined that such nanoarchitectures of MoS2 are useful candidates for energy related applications such as hydrogen evolution reaction, Li ion battery and pseudocapacitors. Inquisitively, metallic Mo can potentially act as catalyst as well as fairly economical Surface Enhanced Raman Spectroscopy (SERS) substrate in biosensor applications.
In the family of molybdenum compounds, metallic molybdenum and molybdenum disulfide are the least and the most explored entities as far as their synthesis is concerned. Curiously, research on molybdenum nanoparticles has not been adequately pursued as compared to the noble metals such as gold and silver though it is cheaper and readily available and has the potential of being used in catalytic, refractory plasmonic as well as in biosensing applications. There are sporadic reports pertaining to the synthesis of molybdenum nanostructures [1–8]. Mostly, molybdenum nanostructures have been generated by various plasma techniques such as RF plasma , DC magnetron sputtering [2, 3], electron cyclotron resonance plasma  etc. Recently, nanocomposites of Mo-polyphenylene sulfide (PPS) and Mo-MoO3-PPS have also been reported by novel polymer-inorganic solid-state reaction method . However, synthesis of molybdenum nanostructures using facile solvothermal route has not been reported so far.
Molybdenum sulfide is an exotic class of materials exhibiting very interesting properties in its 0D, 1D, 2D and 3D forms [10–14]. Among different molybdenum sulfide compounds, molybdenum disulfide (MoS2) is the most important and is being researched fundamentally, computationally and experimentally across the globe. Especially, hierarchical nanostructures of MoS2 are very important from the standpoint of their applications in diverse fields revolving around biology and electronics as they simultaneously possess bulk nature due to a bigger size as well as quantum confinement effects due to nanosheet-like nature of petals. Owing to such attributes, MoS2 nanoflowers have been produced using different methods such as hydro/solvothermal route, sol–gel route, chemical process, chemical vapor deposition, etc. [15–22]. There are some reports pertaining to hydrothermal synthesis of MoS2 nanostructures with flower-like morphology for vivid applications [11, 17, 18, 23–26]. Herein, we present simple one-pot protocol dealing with synthesis of metallic molybdenum microspheres and hierarchical MoS2 nanostructures by reasonably scalable solvothermal and hydrothermal technique, respectively, just by imparting a change of solvent.
2 Experimental details
All chemicals were of reagent grade and were used as received. The synthesis of molybdenum disulfide was carried out through hydrothermal route. In typical procedure, 1 mM of ammonium heptamolybdate tetrahydrate was added in 30 ml of deionized water (DIW) in a beaker and stirred for 20 min till it is dissolved. Similarly, in another beaker, 3 mM of thiourea was taken in 30 ml of DIW and stirred for 20 min with final molar ratio of Mo:S precursor being 1:3. Both the solutions were mixed and subjected to rigorous stirring for another 20 min. The resultant solution was then transferred into 100 ml Teflon-lined stainless-steel autoclave which was properly sealed and placed in an oven for hydrothermal treatment at 200 °C for 9 h. The oven is allowed to cool naturally to room temperature. Subsequently, the resulting black solid was retrieved from the solution by centrifugation, washed with distilled water followed by ethanol two times to remove the ions possibly remaining in the end product, and finally dried at 60 °C for 6 h, respectively.
Similar reaction was repeated in another set of experiment using mixture of ethylenediamine and DIW in 5:1 volume ratio (solvothermal route) keeping all other experimental conditions the same. The samples prepared corresponding to the hydrothermal and solvothermal routes are labeled as MS9 and Mo9, respectively. The structural information on virgin powder samples was obtained using X-ray diffraction (Bruker D8 Advance) technique. The diffraction angle 2(θ) was varied between 10 and 80° range and the observed XRD peaks were compared with standard JCPDS cards. The surface morphological features of the samples were investigated by field emission scanning electron microscopy (FESEM) using HITACHI S-4800. The powder sample was directly placed on the conducting carbon film and was coated with a thin Au–Pd film by sputtering to avoid the effects due to charging of the sample. The fine-scale microstructure of the samples was examined by field emission transmission electron microscopy (FETEM) with JEM-2200FS (JEOL, Japan), at an acceleration voltage of 200 kV. The samples for FETEM were prepared by dispersing fine powder of the resultant product in isopropyl alcohol. A drop of dispersion was then transferred to carbon coated grid for further analysis. The Brunauer–Emmett–Teller (BET) surface area measurements were performed using a NOVAtouch LX1 surface area and pore size analyzer from Quantachrome Instruments, USA.
3 Results and discussion
The TEM images disclose the predominant formation of flower-like nanostructures in the case of MS9 sample (Fig. 4a). At higher-magnification, we could observe that the petals, which are fragile in appearance, encompass few layers of MoS2 (Fig. 4b). The lattice image (Fig. 4c) indicates that the petal at the twist is made up of around 20 layers with the d value as 6.2 Å which matches with the available reports of few layered thick MoS2 nanostructures . The BET surface area for Mo and MoS2 nanostructures was calculated to be 2.31 and 18.62 m2/g, respectively. Higher surface area observed in case of MoS2 nanoflowers is attributable to its flower like morphology.
Synthesis of hierarchical nanostructures of MoS2 has been accomplished by simple hydrothermal route without resort to usage of surfactant/substrate/template while the solvothermal route using ethylenediamine as solvent leads to formation of metallic Mo microspheres under similar reaction conditions. Direct oxidation of ammonium molybdate to Mo in presence of thiourea via solvothermal route is a salient aspect of our work which needs new understanding of super-critical conditions controlled chemical reactions. The obtained hierarchical nanostructures of MoS2 possess very high surface area due to the presence of petal-like surface features. Such high surface area in MoS2 nanostructures along with lesser degree of resistance due to stacking layers can be useful for applications in hydrogen evolution reaction (HER) and pseudocapacitors. While metallic Mo can be explored as novel catalyst and possibly as the economical SERS substrate in biosensor and refractory plasmonic applications, it may be noted that the relevant literature data on such studies is not readily available to the best of our knowledge. Therefore, our efforts in these application-oriented directions are underway.
All authors have contributed to the writing of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Availability of data and materials
The authors have no data to share since all data are shown in the submitted manuscript.
This research was financially supported by the Ministry of Electronics and Information Technology of Indian Government and the National Research Foundation of Korea (2016K1A4A3914691 and IITP-2016-R0992-16-1021). Dr. Dinesh Amalnerkar is grateful to the Ministry of Science, ICT and Planning of Korean Government for financial support through the Brain-Pool Program of KOFST.
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