Effect of surfactant concentration on the morphology of Mo_xS_y nanoparticles prepared by a solvothermal route

1 Introduction

Molybdenum disulphide (MoS₂) is the most studied dichalcogenide material regarding its catalytic performances and lubricants properties [1], [2], [3], [4], [5] due to its sandwich interlayer structure formed by stacking of the (S-Mo-S) layers in the direction [001]. These layers are loosely bounded by van der Waals forces, which accounts for easy cleavage of layers in the [001] direction [6]. As a useful semiconductor, molybdenum disulfide generally crystallizes with a hexagonal lamellar structure (S-Mo-S) with sulfur layers located between Mo atoms [7] accordingly. MoS₂ has received considerable attention thanks to related exceptional properties [8]. Thus, number of MoS₂ preparation methods have been reported to permit obtaining various morphologies including nanowires, nanotubes [9], [10], [11], microscale spherical [12], fullerene-like [13], hollow nanospheres [14]. These methods included gas-phase reactions [15], sulfidation of oxide [16], thermal decomposition of precursors [17], sonochemical process [18], hydrothermal and solvothermal methods [10], [19] etc. Note that special conditions and complex apparatus were involved without allowing the control of the size and the morphology of the resulting nanoparticles. This challenging issue has been addressed by adding surfactants to the synthesis reaction depending on the targeted shapes [7]. Nicole Berntsen et al. have prepared nanostructured MoS₂ by using solvothermal conditions without and with surfactant (CTAB) permitting to obtain MoS₂ string-like morphology with developed surface area [20]. Also, Zhuangzhi Wu et al. have successfully synthesized MoS₂ nanospheres with an average diameter of 100 nm using polyethylene glycol surfactant-assisted route [21]. The latter promotes the formation of nanospheres through binding effect, which prevents also MoS₂ crystallization of, yielding to layer stacking decrease and textural stabilization.

The aim of the present work is to study the effect of surfactant on the synthesis and the control of morphology and size of Mo_xS_y nanomaterials. The adopted surfactant-assisted solvothermal route developed in the present work to fabricate Mo_xS_y nanomaterials at lower temperature (180°C) for 24 h. We chose a cationic surfactant, (CTAB (C₁₆H₃₃)N(CH₃)₃Br))), widely used for these types of synthesis. The obtained results were compared with our previous works [11], [14] reporting successful preparation of different MoS₂ morphologies (hollow nanospheres, nanotubes) only by optimizing experimental conditions and electrolyte concentration. The present article highlights the role of surfactant and its concentration on the structure and morphology of the grown Mo_xS_y nanomaterials.

2 Materials and methods

3 Results and discussion

The as-prepared materials were first characterized by XRD. Figure 1 displays XRD patterns of the products obtained according to the aforementioned four different syntheses in addition to those corresponding to crystalline 2H-MoS₂ and the bulk CTAB for shake of comparison. It is shown the presence of two phases, corresponding, respectively, to a crystalline phase identified by the intense peaks, and a second one recognized by broad peaks (Figure 1A–D) typical for nanostructured phase [7], [22]. The intense peaks are, probably, due to the presence of some CTAB amount, absorbed on the sample surface. Thus, it seems that under the used experimental conditions, the formation of crystalline MoS₂ phase is far from complete, because of the required synthesis or calcinations temperatures higher than 700°C [8]. On the other hand, the comparison of the intense peaks positions observed in the diffraction patterns of the four samples with the XRD patterns of the bulk CTAB illustrate an excellent similarity. In fact, the complete decomposition of the used surfactant needs temperature around 230°C which is higher than that used for the solvothermal synthesis (180°C). This might be the reason for the remaining intact surfactant, found absorbed on the nanoparticles surface even after excessive washing with water and acetone [20]. However, the broad peak of low intensity observed around 2θ=12° in the XRD patterns of the four synthesized materials, can be reasonably ascribed to the (002) plane of MoS₂-2H. These features are indicative of the amorphous nature of the obtained materials and indicate the low stacking as well as the highly disordered packing of their MoS₂ layers [23], [24].

Figure 1:

X-ray diffraction patterns of samples A, B, C and D, CTAB bulk and MoS₂-2H.

Furthermore, Transmission electron micrographs of the prepared samples are displayed in Figures 2–5. It is observed for the sample obtained from synthesis A a non-uniformity in shape and size of the formed particles as well as the presence of small aggregates of spherical particles and some nanospheres of poor dispersion in size in addition to amorphous clusters. In the case of sample B, TEM images reveal the presence of some amorphous clusters and hollow tubular structures having a wall formed of several layers, their size distribution is ranging from 10 to 15 nm, while their length varies from 40 to 80 nm. However, for sample C, it is observed the formation of nanotubes with a filled structure indicating the development of highly developed morphology as compared with samples A and B. The observed tubes have a diameter distribution ranging between 500 and 800 nm and a length extending to a few microns, in addition to the presence of spheres with diameter around 0.6 microns (Figure 4). Of interest, the obtained nanotubes represent 50% with respect to the total morphologies observed by TEM for this sample, an interesting value as compared to result reported in literature (not exceeding 30%) [25]. These nanotubes have also been studied by HRTEM to observe their layer structure. During the analysis by HRTEM, the tubular structures begin to degrade (Figure 6). The filled structure is destroyed, until the disappearance of their tubular shape, probably as a result of the high energy of the electron beam (200 kV). Furthermore, the TEM images of the particles obtained by the synthesis D allows to observe the presence of highly aggregated and very poorly dispersed tubular cluster (Figure 5). These clusters have cracks in the ends indicating their instability.

Figure 2:

TEM image of the spherical particles obtained from sample A.

Figure 3:

TEM image of amorphous clusters obtained from sample B.

Figure 4:

TEM image of nanotubes obtained from sample C.

Figure 5:

TEM image of tubular cluster obtained from sample D.

Figure 6:

HRTEM image of the nanotubes obtained from sample C.

The amount of CTAB used in reaction seems to play an important role in determining the obtained morphology. With a moderate surfactant concentration, tubular structures, with a relatively high percentage, are obtained (synthesis C), however with low or high concentrations, only amorphous clusters are observed. Moreover, subsequent EDX analysis of the obtained nanotubes reveals the presence, in addition to molybdenum and sulfur, of several other chemical elements (C, O, Na) thus proving the contamination of the samples structure and confirming the results obtained by XRD (Supplementary Information S.1). These impurities are due, probably, to the surfactant used during the solvothermal syntheses. The atomic S/M ratio is equal to 2.5.

According to the structural characterization of the samples prepared by the solvothermal syntheses assisted by CTAB, we identified the relationship between the amount of surfactant and the nature of the formed morphologies, thus confirming the observations reported in the literature [6], [26]. Furthermore, the synthesized materials exhibit non-homogeneity in their chemical composition and crystallinity (confirmed by XRD and EDX results) probably due to the intrinsic nature of CTAB which is difficult to remove by reiterative washing with water and acetone [20], [27]. However, and for a deeper study of these materials, nanotubes obtained from synthesis C were selected to be characterized by IR, TGA and XPS.

The IR spectra of the nanotubes and bulk CTAB are compared in Figure 7. We can observe, easily, the similarity between the two infrared spectra, which is probably due to the presence of CTAB particles absorbed on the surface of the Mo_xS_y material [28], a result in good agreement with those obtained by XRD and EDX. However, for sample C (Figure 7B), a band at 484 cm⁻¹ assigned to the Mo-S stretching mode of vibrations is observed, as well as the band at 530 cm⁻¹ scribed to bridging S₂²⁻species [11]. The recorded IR spectrum indicate also the presence of IR band at 646 cm⁻¹ corresponding to sulphate groups, commonly present in amorphous or well dispersed MoS₂ samples resulting from sample surface oxidation upon contact with air [29].

Figure 7:

IR spectra of (A) Bulk CTAB and (B) the nanomaterials obtained from sample C.

Figure 8 shows the thermogravimetric analysis of the nanomaterials, obtained from sample C, carried out under helium. The TGA curve exhibits a first weight loss of about 5.5% in the range of 25~160°C, attributed to the removal of residual solvent and the desorption of physisorbed water from the surface of the sample. Also, TGA curve shows another mass loss that occurs between 200 and 550°C, and corresponds to 17.1%. This weight loss can be attributed to the dehydroxylation of the sample, the removal of NH₄⁺ cations resulting from the decomposition of (NH₄)₂CO₃ [30], as well as the decomposition of CTAB particles which are absorbed in the structure of MoS₂ tubes, and which can be eliminated at about 300°C [31].

Figure 8:

Thermogravimetric curve of the nanomaterials obtained from sample C.

Further XPS analysis of the MoS₂ solids prepared according to the C synthesis route are compiled in Table 2. The binding energies (BE) of Mo 3d, S 2p and O 1s as well as the surface composition for the Mo_xS_y nanomaterials are shown. All binding energies were referenced to the C 1s peak of the adventitious carbon set a 284.6 eV.

Mo 3d spectra corresponding to the sample C reveals that the Mo 3d doublet presents a complex signal. The Mo 3d peak can be decomposed in four components located at 232,5, 235,7, 237,5 and 240,5 eV (Figure 9A). The BE of the first Mo 3d5/2 and Mo 3d3/2 components are in good agreement with the value reported for Mo in MoS₂ [32], while the other two components correspond to the Mo in Mo⁶⁺ in an oxidized state [33], [34]. On the other hand, The S 2p spectra of Mo_xS_y nanomaterials shows the presence of three S 2p3/2 components, the first one, located at 163,2 eV, is attributed to S²⁻ ions in MoS₂, whereas the second, observed at 168,7 eV, corresponds to elemental sulfur in its S_n form [35] while the third component corresponds to SO₄² originating, probably, from an air oxidation of the nanotubes surface, which confirms the non-homogeneity of the sample and the presence of impurities in the structure of the analyzed material (results already confirmed by XRD, EDX and FT-IR). The S/Mo ratio, determined from the relative amount of Mo (MoS₂) and S (MoS₂), is equal to 1.79, a value near to that of the commercial MoS₂ (Table 2).

Figure 9:

X-ray photoelectron spectra of (A) Mo 3d and (B) S 2p for the nanomaterials obtained from sample C.

In the light of the above described results, we notice the effect of increasing the mass of the surfactants on the formation of spherical or tubular structures. Obtaining these morphologies is the result of the self-aggregation of surfactant molecules in micelles, which play the role of template around which the inorganic species condense. This self-aggregation into micelles occurs only when their concentration exceeds the threshold of the critical micelle concentration (CMC) beyond which the surfactant molecules are organized to minimize the interactions between the hydrophobic and hydrophilic parts and reduce the solvent/surfactant interface [36]. In fact, this dual nature gives them both an attraction and repulsion for the solvent which induces the formation of these self-assemblies. Therefore, we assume that the synthesis assisted by surfactants of MoS₂ nanoparticles, with well-defined morphologies, is based on the ability of these surfactant molecules to the formation of micelles in the reaction medium. In consideration of the results mentioned in literature [37], [38] and in the present study, the synthesis mechanism is assumed to be as shown in Figure 10.

Figure 10:

Proposed mechanism for the formation of MoS₂ nanotubes from the assembly of spherical micelles.

• The surfactant molecules in the neighborhood of the CMC spontaneously form spherical micelles.

• The MoS₂ germs begin to form during the synthesis reaction.

• Spherical micelles tend to approach to reduce the free energy, and thus form more stable elongated structures.

• These elongated micelles play the template role needed for the growth of MoS₂ germs giving rise to nanotubes formation.

However, the elimination of surfactant from the obtained structure remains the most difficult step for the release and stabilization of formed morphologies. Typically, this removal is achieved by calcination of the obtained materials at relatively high temperatures (between 500 and 600°C) under flow of air or inert gas [39]. This finding may explain the presence of crystalline phase and impurities in the structure of prepared materials (identified by XRD and EDX) that were difficult to remove under lower drying temperature (60°C).

4 Conclusion

Mo_xS_y nanoparticles with different morphologies have been synthesized successfully using surfactant-assisted solvothermal route. The obtained structures depend, primarily, on the surfactant concentration. Comprehensive structural characterizations carried out show that at low surfactant concentration, small aggregates of spherical particles and some nanospheres of poor dispersion in size as well as amorphous clusters are observed, however, at moderate concentration, a mixture of nanotubes and nanospheres is formed. Nonetheless, at higher surfactant concentration, aggregated and very poorly dispersed tubular cluster with cracks in their ends are observed.

It should be pointed out that the obtained nanotubes represent 50% of the total morphologies observed by TEM, which represents an interesting value compared to those mentioned in literature. Nevertheless, non-homogeneity of the chemical composition and poor size and particle morphology dispersion was observed for all the synthesized materials. Therefore; the improvement and simplification of the surfactant-assisted solvothermal synthesis of MoS₂ become paramount, to provide MoS₂ nanomaterials with homogeneous composition and morphology, by studying the effect of the experimental conditions and the reaction kinetic on the formation of stables micelles and the interactions between surfactant and the used additives. Moreover, the removal of the organic template (that involves a calcination step either by air or under vacuum) has to be widely studied to ensure the subsequent elimination of the template without destruction of the formed nanostructure.