Abstract
Monodispersed MoS2 ultrathin nanosheets have been successfully fabricated by a facile hydrothermal process assisted by ionic liquid Brij56. The effect of Brij56 on the morphology and structure of MoS2 has been obviously observed. XRD shows that the as-prepared MoS2 assisted by Brij56 has the weak and broad peak of (002) planes, which implies the small size and well dispersed structure of MoS2 nanosheets. TEM and SEM images reveal that MoS2 ultrathin nanosheets have small size and few stacking layers with the adding of Brij56. HRTEM images prove that MoS2 appears to have a highly monodispersed morphology and to be monolayer ultrathin nanosheets with the length about 5–8 nm, which can provide more exposed rims and edges as active sites for hydrogen evolution reaction. Brij56 has played a crucial role in preparing monodispersed MoS2 ultrathin nanosheets as excellent electrocatalysts. The growth mechanism of monodispersed MoS2 has been discussed in detail.
1. Introduction
Hydrogen as a promising sustainable energy carrier has attracted much attention owing to the increasing environmental pollution and the limited fossil fuel [1]. The ideal fashion of hydrogen production is to adopt photoelectrochemical [2] or electrochemical [3] route. However, Pt as highly active electrocatalyst for hydrogen evolution reaction (HER) [4] has a high price and limited resources, which largely prevent the wide utilization of HER. Therefore, replacing the novel metals with earth-abundant elements represents future development of the electrocatalysts for HER [5].
MoS2 has been widely investigated as a promising substitute for Pt due to its unique properties and abundant reserve [6]. Recent research has shown that the active sites of MoS2 for HER are highly dependent on the exposed defects of the rims and edges [7]. However, as a typical two-dimensional (2D) transition-metal sulfide, MoS2 has analogous layered structure of graphene, which results in severe stacking owing to the high surface energy and interlayer van der Waals attraction [8]. Moreover, the unsaturated sulfur atoms of MoS2 can improve the discharge reaction and form S−H bonds, thus leading to hydrogen evolution easily [9]. Hu’s group prepared the amorphous MoS3 particles with catalytically active S2− and superior catalytic activity [10]. Therefore, designing highly active MoS2 with more rims and edges sites has been a challenge by a facile process [11].
The ionic liquids have been used as structure-directing agent or dispersion solvents to prepare novel nanomaterials including the 2D nanosheets [12, 13]. The most exposed basal planes of MoS2 are the thermodynamically stable (002) planes, which have poor activity for HER [14]. In our previous work, ionic liquid Brij56 [C16H33-(OCH2-CH2)10OH] has been used to synthesize MoS2 nanoflowers [15]. However, the severe stacking and large size of MoS2 nanoflowers would decrease the exposed active sites for HER, which may be attributed to the Na2MoO4 as precursor. Thus, preventing the growth along (002) plane by changing reactants and experimental conditions can be expected to improve the exposure of the active sites of MoS2 for HER [16].
In this work, ammonium thiomolybdate (ATTM) has been chosen as precursor owing to its in situ decomposition [17] in comparison to sulfidation from Na2MoO4. And suitable pH and reductant HONH3Cl have also been used to control the structure of MoS2. Therefore, monodispersed MoS2 ultrathin nanosheets have been prepared by a facile hydrothermal synthesis assisted by ionic liquid Brij56. The effect of Brij56 concentration on the structure and size of MoS2 has been investigated in detail. The growth mechanisms of monodispersed MoS2 under the conditions of ATTM and Brij56 are also discussed.
2. Experimental
ATTM were synthesized according to the previous literature [18]. Then 1 mL or 5 mL Brij56 and 0.550 g ATTM were added to 20 mL of deionized water at about pH 10 under stirring for 3 h. Then, an appropriate amount of HONH3Cl was added. The obtained solution was transferred into a Teflon stainless steel autoclave. Hydrothermal reaction was carried out at 240°C for 24 h. The as-prepared samples were washed and dried at 80°C for 24 h in a vacuum oven. Compared with the absence of Brij56, MoS2 was also synthesized under otherwise identical conditions.
Crystallographic information of all samples was investigated with X-ray powder diffraction (XRD, X’Pert PRO MPD, Cu KR). The morphology of the samples was examined with scanning electron microscopy (SEM, Hitachi, S-4800) and high-resolution analytical transmission electron microscopy (HRTEM, JEM-2100UHR, 200 kV). Selected area electron diffraction (SAED) was used to examine the samples’ crystallinity.
3. Results and Discussion
XRD patterns of the as-synthesized MoS2 under different concentration of Brij56 are shown in Figure 1(a). MoS2 without Brij56 has strong peaks corresponding to (002), (100), (103), and (110) reflections, respectively, consistent with the standard diffraction file of MoS2 (JCPDS 37-1492). With the using of Brij56, the peaks of (002) and (103) of MoS2 decrease remarkably (1 mL Brij56) and almost disappear (5 mL Brij56), which indicates that Brij56 strongly prevents the growth of (002) and (103) planes. And MoS2 with the absence of (002) implies the low crystallinity and monolayer structure of MoS2 [19]. The slight right shift of (002) peak could be attributed to the distortion of lattice in MoS2. The (100) and (110) planes keep stable, indicating the good stability of MoS2, which can be confirmed by the selected area electron diffraction (SAED). As shown in Figures 1(b), 1(c), and 1(d), the SAED patterns show clearly more and more invisible rings corresponding to (002) and (103) with the adding of Brij56, which well agrees with the results of XRD.
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The size and morphology of as-prepared MoS2 have been observed by SEM and TEM (Figure 2). Figures 2(a) and 2(d) show that MoS2 assisted by 0 mL Brij56 has large aggregation and severe stacking, which imply less rims and edges of MoS2. Figure 2(b) shows that the large aggregation of MoS2 decreases and MoS2 has some loose microstructure (in Figure 2(e)) when using 1 mL Brij56. The effect of Brij56 on morphology of MoS2 has been proved. Next, as shown in Figure 2(c), MoS2 assisted by 5 mL Brij56 became smaller with the size of about 20 nm. And porous structures of MoS2 have been observed. Figure 2(f) confirms the loose porous structure and smaller size of MoS2, indicating less stack layers and more rims and edges sites of MoS2 with the increasing of concentration of Brij56.
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HRTEM images with higher magnification prove the change tendency of monodispersed MoS2 (Figure 3). Figure 3(a) shows that MoS2 assisted by 0 mL Brij56 has the length of more than 100 nm and very severe stacking, indicating less rims and edges. Figure 3(b) shows that MoS2 assisted by 1 mL Brij56 has obviously decreasing length of about 20–30 nm and less stacking layers with the larger interlayer spacing. The results indicate that Brij56 has prevented the rapid growth along (002) planes and the severe stacking of MoS2. Figure 3(c) shows that monodispersed MoS2 assisted by 5 mL Brij56 has the length of about 5–8 nm and appears to have monolayer structure, which is corresponding to the more porous structure of MoS2 (in Figure 2(c)). The decreasing size and less stacking of MoS2 would provide more defects sites. In addition, monodispersed MoS2 ultrathin nanosheets are usually curly and bent to some extent on the rims and edges, which means more active sites [20].
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The formation mechanisms of monodispersed MoS2 assisted by Brij56 have been discussed in Figure 4. Firstly, nonionic surfactant Brij56 as a dispersant will form a stable spherical micelles system for homogeneously dispersing under stirring. The size of the micelles will decrease with the increasing of the concentration of Brij56. The micelles could provide nucleation domains for in situ decomposition of , which may be helpful for size controlling and the growth along (002) direction of MoS2. During the hydrothermal process, Brij56 micelles tend to be hydrophobic and increase the viscosity of micellar solution, which favors the monodispersed growth of MoS2 [20]. MoS2 with more monodispersed structure under the high concentration of Brij56 could provide rich active sites.
4. Conclusions
Monodispersed MoS2 ultrathin nanosheets with more active sites for HER have been fabricated by a facile hydrothermal process assisted by Brij56. MoS2 assisted by Brij56 has weak and broad peak of (002), indicating small size and well dispersed structure. SEM and TEM images reveal that highly dispersed MoS2 nanosheets have been obtained with the increasing of the concentration of Brij56. Monodispersed MoS2 assisted by 5 mL Brij56 has the length of about 5–8 nm and a monolayer structure, which provide more rims and edges sites. The curly structure of monodispersed MoS2 ultrathin nanosheets would also expose more active sites. The facile hydrothermal synthesis assisted by Brij56 has been a good route for excellent MoS2 electrocatalysts for HER.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (nos. U1162203 and 21106185) and the Fundamental Research Funds for the Central Universities (15CX05031A).