Facile solvothermal synthesis of bimetallic CoMoS₂ and NiMoS₂ nanospheres

1 Introduction

Transition metal chalcogenides especially molybdenum disulfide (MoS₂) are of significant interest from a scientific and technological point of view [1]. These materials are known as typical graphene-like structures formed by stacking of the (S-Mo-S) layers in the direction (001) [2]. The layers are loosely bounded to each other by van der Waals forces allowing the easy cleavage of S-Mo-S in the direction (001) as well as having interesting chemical properties related to the anisotropy degrees. It is was pointed out for chalcogenide materials that, in addition to the composition and atom arrangement, tuning the dimensionality and morphology, as nanospheres, nanosheets, nanofibers, nanotubes, etc. are of interest for determining the potential for application in the next generation of nanoelectronic devices [3], lubricants additives [4–8] and catalysis [9–12].

In the latter case, MoS₂ is often used in hydrotreating processes due to its higher activity for hydrodesulfurization (HDS), hydrodenitrogenation, methanation and the synthesis of light hydrocarbons from CO hydrogenation [13]. The single layer S-Mo-S slabs of MoS₂ crystallites were considered to be of significant importance towards catalytic performances [14] that were enhanced, also, by using transition metals such as cobalt (Co) or nickel (Ni) as promoters [15]. The catalytic behavior was associated with the possibility of metal-S anti-bonding electrons removal from Co and Ni and their transfer to Mo, i.e. the ability of Co and Ni to formally reduce Mo in the binary oxides [16]. Further improvement of the catalytic activity was also correlated with a higher reactive surface at the nanometric scale due to the observed increase of MoS₂ slabs curvature, providing more predisposed active sites for the hydrodesulfurization of 4,6-dimethyldibenzothiophene [17, 18]. Moreover, the role of bimetallic amorphous Co-Mo-S catalysts in the hydrogenation-dehydration (HYD) and deoxygenation (DDO) of phenols was explained by the Rim-Edge model involving an increased number of layers in stacks and shortened slab length yielding to a higher p-cresol conversion with enhanced toluene selectivity [19].

So far MoS₂ nanomaterials have been obtained using various synthesis methods namely, thermal decomposition of precursors [20], sulfidation of oxides [21], high temperature solid-state reactions [22], soft chemical synthesis including sol-gel [23], solvothermal and hydrothermal techniques [18] as well as the recently reported co-precipitation method [24]. Nevertheless, none of these methods addressed the synthesis route of well-defined bimetallic spherical particles.

Herein, we report a simpler one-step solvothermal method for the preparation of the transition bimetallic particles as a continuation of our previous work regarding the preparation of homogeneous hollow MoS₂nanospheres [25]. NiMoS₂ and CoMoS₂ nanospheres were successfully prepared at low temperature with a relatively short reaction time, without surfactant, by addition of salts of Ni and Co in the MoS₂ nanospheres synthesis. To the best of our knowledge, such innovative procedure enabling simpler preparation of transition bimetallic nanospheres has never been reported before.

2 Materials and methods

3 Results and discussion

The prepared MMoS₂ samples (M=Ni or Co) were first characterized by TEM analysis that revealed the formation of almost original sphere-like morphologies with irregular sizes ranging from 450 nm to 1.2 μm, having a filled look and strong contrast in their inside (Figures 1 and 2). Further observation with HRTEM images (Figure 3) permitted the observation of the nanospheres outer layers composed of several folded leaflets characteristics of highly disordered structures [18]. The sheets located at the edges of the nanospheres (NiMoS₂ and CoMoS₂) are very short as compared to the elongated long sheets, previously observed for MoS₂ nanospheres [25]. This phenomenon is apparently correlated with the presence of Ni or Co in MoS₂structure that was reported to cause a decrease in sheets length and an increase of stack layers [17].

Figure 1:

TEM image of NiMoS₂ nanospheres.

Figure 2:

TEM image of CoMoS₂ nanospheres.

Figure 3:

High resolution TEM of the unsupported nanospheres (A) NiMoS₂ and (B) CoMoS₂.

Subsequent EDX analysis of the bimetallic NiMoS₂ and CoMoS₂ nanospheres shows the only presence of Ni, Co, Mo and S without any significant impurities (S1). Furthermore, the quantification of EDX peaks indicates an S/Mo atomic ratio of 2.2 for both NiMoS₂ and CoMoS₂ materials, whereas values near to 0.7 and 0.6 were obtained for Ni/Mo and Co/Mo (S2), respectively. Note that the values obtained from these peaks do not fit a well-defined compound formulation and match the chemical composition given by EDX analysis only in certain areas of the sample.

XRD analysis was used for the phase identification of the prepared bimetallic NiMoS₂ and CoMoS₂ nanospheres using normalized diffraction patterns of hexagonal molybdenum disulfide (MoS₂-2H) and those of MoS₂ nanospheres obtained in our previous work, for the sake of comparaison [25]. As illustrated in Figure 4, all Mo-based sulfide nanospheres exhibited broad diffraction peaks indicating the presence of a disordered phase, characteristic of nanoscale particles [11] compared to a commercial MoS₂-2H powder known for its very crystallized structure. It is worth noting the presence of the peak at 2θ=15° characteristic of crystalline MoS₂ basal planes (002) in the diffraction patterns of both bimetallic nanospheres (Figure 4B–C) while it is very low for MoS₂ nanospheres (Figure 4A). Other peaks of low intensity at 2θ=36° and 59° associated, respectively, with (100) and (110) planes of MoS₂-2H , also appear on the diffractograms of both bimetallic nanospheres [28]. In the case of NiMoS₂ (Figure 4B), the XRD pattern shows new peaks of low intensity at 2θ=31° and 55° that could be attributed to the presence of Ni₃S₄according to [18]. Three other minor peaks are also observed at 2θ=30°, 35° and 46° and correspond to NiS [29]. According to the literature, these peaks are detectable for (Ni/Ni+Mo) ratios above 0.4, further increase of the Ni amount enhances the formation of NiS and Ni₃S₄ species [18].

Figure 4:

XRD patterns of MoS₂-2H and unsupported nanospheres (A) MoS₂, (B) NiMoS₂ and (C) CoMoS₂.

However, in the case of CoMoS₂(Figure 4C), the XRD pattern reveals the presence of two small features at 2θ=29.5° and 52° which are likely due to Co₉S₈ species that overlaps with the peaks corresponding to poorly crystallized MoS₂, which are detected for (Co/Co+Mo) ratios starting from 0.2 [30]. The presence of such new peaks in the case of bimetallic sulfide nanomaterials might be attributable to the possible formation of NiS, Ni₃S₄ and Co₉S₈ resulting from lower dispersion of Ni and Co [31]. So far, the recorded XRD patterns do not revealed any sign of crystalline structures of mixed bimetallic sulfide nanospheres corresponding to Ni-Mo-S or Co-Mo-S phases, probably because these structures (Mo-M-S phase, M=Ni or Co) are present as very small nanocrystallites that cannot be detected by the used XRD method [32].

The chemical structure of bimetallic NiMoS₂ and CoMoS₂ nanospheres was also investigated with IR spectroscopy (Figure 5) using the IR bands recorded below 2500 cm^-1 [33, 34]. The latter spectra were found to exhibit very similar low intensity IR bands centered at 480 and 550 cm^-1 that can be assigned to the elongation mode of Mo-S [35] and the vibrational mode of S-S [36], respectively. Additional IR bands at 1045, 1150 and 1445 cm^-1 denoted the presence of sulfates species [35]. The relatively intense band located at 641 cm^-1 is likely due to the presence of the SH group, resulting from the dissociation of the hydrogen, while the IR band located around 830 cm^-1can be assigned to the elongation mode of nitrate ions, resulting from the possible decomposition of nitrate salts [37]. Note that the presence of the latter species remains even after the samples being washed several times with water and acetone.

Figure 5:

IR spectrum of the prepared nanospheres (A) NiMoS₂ and (B) CoMoS₂.

In the case of CoMoS₂nanospheres, the band observed at 1550 cm^-1, usually attributed to ammonium ions (Figure 5B), is probably due to an impurity resulting from the decomposition of the precursor – ammonium heptamolybdate (AHM) [38]. It should be pointed out that, for NiMoS₂ nanospheres, the presence of NiMoS phase is confirmed by its IR bands located at 2120 and 2083 cm^-1 [39].

Figure 6 shows the TGA profile of the prepared nanospheres, which present a continuous weight loss of about 20% between 25°C and 600°C through two stages. The first one occurring between 25°C and 150°C, corresponding to 5.5% and 5.7%, for NiMoS₂ and CoMoS₂, respectively, is associated with the removal of residual solvent and physisorbed water from the samples. The second weight loss proceeding between 200 and 480°C, which corresponds to 15.8% and 15.1% for NiMoS₂ and CoMoS₂, respectively, is due to the samples dehydroxylation and dissociation of nitrate salts used in the solvothermal synthesis [40], in agreement with the aforementioned IR spectroscopy results. Similar thermal behavior was reported in the literature as resulting from the decomposition of (NH₄)₂CO₃, eliminated by the thermal treatment at this temperature range [41], as well as the rupture of weakly bonded surface oxygen giving rise to nanocrystals recrystallization [42].

Figure 6:

TGA curves of (A) NiMoS₂ and (B) CoMoS₂ nanospheres.

The surface of the prepared bimetallic sulfide nanospheres were characterized by XPS. Figure 7 shows the XPS spectrum of Mo3d for both bimetallic sulfide nanomaterials. The Mo3d doublet can be decomposed into two Mo3d5/2 components with peaks centered at 228.3 eV and 231.5 eV for both NiMoS₂ and CoMoS₂ samples, as illustrated in Figure 7. The binding energy of the first component at 228.3 eV corresponds to Mo(IV) species in MoS₂ [43] whereas the second peak centered at 231.5 eV reveals the presence of oxidized Mo species [in the form of Mo(V)], resulting from the sample contamination either by air [44] or to the formation of MoS_xO_y compounds resulting from the oxygen exposition during the synthesis process [38, 45]. Another Mo3d3/2 component of very low intensity illustrated by a peak near to 234 eV for both NiMoS₂ and CoMoS₂ nanospheres (Figure 7), can be attributed to oxidized Mo [in the form of Mo(V)] [38].

Figure 7:

XPS spectra of Mo3d level for nanospheres (A) NiMoS₂ and (B) CoMoS₂.

Figure 8 shows XPS spectra of sulfur containing species as indicated by the S2p doublet where the first binding energy located at 161.6 eV corresponds to S^2- ion of MoS₂ whereas those centered at 168.3 and 168.5 eV for NiMoS₂ and CoMoS₂, respectively, are associated with sulfate species contained in the bimetallic sulfide nanospheres. These results are in good agreement with IR results (Figure 6) and suggest that the surface of the prepared nanospheres is oxidized upon air exposure. The amounts (atomic percentage) of these species determined by XPS are respectively, 5.2 and 6.1% for NiMoS₂ and CoMoS₂ samples.

Figure 8:

XPS spectra of S2p level for nanospheres (A) NiMoS₂ and (B) CoMoS₂.

Further analysis of the chemical state of MMoS₂ nanospheres as revealed by Ni2p and Co2p spectra is shown in Figure 9. The photoelectron Ni2p3/2 peak located at 852.3 eV (Figure 9A) is attributed to NiS [46], whereas the peak of Co2p3/2, attributed to Co₉S₈ species [47, 48], is observed at 777.8 eV in the case of CoMoS₂ nanospheres (Figure 9B). The presence of sulfides species (NiS and Co₉S₈) strongly suggests that the Ni and Co reaction is not completed in mixed phase MMoS₂ (M=Ni or Co). So far, there is no evidence for the formation of mixed phase NiMoS or CoMoS. Nevertheless, the presence of sulfides (NiS, Co₉S₈) on the surface of bimetallic material is suggested to result from poor dispersion of the promoters into the MoS₂ structure [49].

Figure 9:

XPS spectra of (A) Ni2p level for NiMoS₂ nanospheres and (B) Co2p level for CoMoS₂ nanospheres.

Hence, considering the obtained XRD and XPS data, it is suggested that, Ni or Co atoms are likely located at the edges of the prepared nanomaterials. As compared to its crystalline structure, the disordered structure of MoS₂ nanospheres contains surface vacant sites that might allow easy insertion of these atoms. The latter have a tendency to remain located along the MoS₂ edges forming an active outer layer. On the one hand, it should be noted that the use of relatively high concentration of Ni and Co precursor in the synthesis reaction resulted in a poor dispersion of these atoms in the MoS₂ structure. On the other hand, the presence of these atoms in MoS₂ edges might also cause a reduction, or even depletion in the number of sites, yielding to the formation of new sulfide NiS, Ni₃S₄ and Co₉S₈.

It should also be noted that the surface of bimetallic sulfide nanospheres contains a significant amount of oxygen representing about 37.4 and 26.1% for NiMoS₂ and CoMoS₂, respectively. This is explained, as previously mentioned by the surface partial oxidation upon air exposure during the synthesis reaction as indicated by the presence of dangling bonds on disordered surface [50]. TPR study was applied in order to investigate the behavior of metal-sulfur bonds during heating under hydrogen flow [51]. The TPR profiles obtained for NiMoS₂ and CoMoS₂ nanospheres were compared to that obtained for MoS₂ nanospheres prepared in our previous study [25]. The superposition of the TPR curves (Figure 10) obtained for MoS₂, NiMoS₂ and CoMoS₂ nanospheres samples show two well-separated sets of peaks. In the case of NiMoS₂ and CoMoS₂ nanospheres, the TPR profiles showed significant changes in peaks position and intensity, indicating different reduction stages compared to the TPR profile obtained with MoS₂ nanospheres. The latter exhibits, two main peaks centered at 255 and 556°C, while the sets of peaks corresponding to bimetallic sulfide nanomaterials are shifted to lower temperatures, located at 200 and 450°C, and 217 and 480°C for CoMoS₂ and NiMoS₂respectively.

Figure 10:

TPR patterns of unsupported MoS₂, CoMoS₂ and NiMoS₂ nanospheres.

It is worth noting that the addition of Ni and Co to the MoS₂ nanospheres seems to enhance the reduction process through the decomposition into atomic hydrogen [52] as well as the reduction of Ni and Co probably to metallic form.

As compared to MoS₂nanospheres, the TPR peaks located at lower temperatures obtained with bimetallic sulfide NiMoS₂ and CoMoS₂ suggest that the presence of Ni or Co weakens the strength of the metal-sulfur bonding [53, 54]. This behavior is due to the reduction by H₂, which might accordingly, promote the catalytic activity [27, 32]. The TPR peaks located at a higher temperature are likely associated with metal reduction (Mo and M: Co or Ni). Accordingly, the involved reduction process of the bimetallic nanospheres might proceed as follow:

1. Surface reduction

   MMoS2+x→<200°CxH2MMoS2+xH2S

   MMoS2→∼200°CH2MMoS∗+H2S

2. Mass reduction

   MMoS∗→~450°CH2M+Mo+H2S

In fact, the first reduction of MMoS₂ (M: Ni or Co) to MMoS* is related to weak binding of sulfur in these materials compared to non-promoted MoS₂ (which is reduced at higher temperatures). Hence the addition of promoters (Ni, Co) decreases the strength of the sulfur-metal bonding [53, 54] and yields to the subsequent reduction of MMoS* as indicated by the formation of metallic elements (Mo, Ni, Co). It is generally accepted that the elimination of sulfur following the reduction by H₂ generates vacant sites that are of importance with respect to the catalytic activity [27, 32]. This statement is justified by the binding energy explanation based on TPR analysis, correlating catalytic activity to the sulfur-metal binding strength [48].

All the samples resulting from the TPR analysis have been subjected to additional XRD analysis in order to provide evidence of a possible phase transition. The obtained patterns (Figure 11) revealed the presence of refined and well resolved diffraction peaks as compared to those of the non reduced samples with poor crystalline structure (Figure 5). This phenomenon is mainly due to the presence of new peaks attributed to metals (Mo, Co, Ni) and metal oxides such as MoO₃, NiO, CoO. The former peaks confirm the complete reduction of a portion of Mo, Co and Ni, while those associated with the oxide phases were identified based on the aforementioned data obtained by FT-IR and XPS analysis. The complete elimination of these species by reduction was not achieved in the studied temperature range because of complexes thermodynamic limitations [48].

Figure 11:

XRD patterns after TPR of nanospheres (A) NiMoS₂ and (B) CoMoS₂.

4 Conclusion

In the present work, we highlight a simple one-pot solvothermal method for the synthesis of NiMoS₂ and CoMoS₂ nanospheres of well-defined shape and morphology, with a diameter ranging between 450 nm and 1 μm. The prepared nanomaterials have been comprehensively characterized using different techniques. The XRD patterns indicated the formation of poorly crystallized bimetallic NiMoS₂ and CoMoS₂ nanomaterial whereas TEM and HRTEM observations showed the presence of bimetallic nanospheres outer layers composed of several folded leaflets characteristic of highly disordered structures. Moreover, a decrease in the length of the sheets was observed as well as an increase in the stack of layers of the obtained bimetallic samples compared to MoS₂ nanospheres (prepared in our previous work). TPR experiments carried out with NiMoS₂ and CoMoS₂ nanomaterials revealed important changes in TPR profiles, as compared to MoS₂ nanospheres, indicating a significant shift of the reduction peaks to lower temperatures accompanied by an increased H₂ consumption as a result of the weakening of the metal sulfur bonding strength. The easily introduced method for the preparation of well-defined bimetallic nanospheres might be of interest for different applications particularly in catalysis.