High-performance hydrogen evolution electrocatalysis by layer-controlled MoS₂ nanosheets

Abstract

Hydrogen is considered as an important clean energy carrier for the future, and electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production. As a potential alternative to Pt-based catalysts in hydrogen evolution reaction (HER), two-dimensional (2D) molybdenum sulfide (MoS₂) nanomaterials have attracted enormous research interest, while the structure control for high-performance HER electrocatalysis remains a considerable challenge due to the lack of efficient preparation techniques. Herein, we reported a one-pot chemical method to directly synthesize 2D MoS₂ with controllable layers. Multiple-layer MoS₂ (ML-MoS₂), few-layer MoS₂ (FL-MoS₂) and single-layer MoS₂ coating on carbon nanotubes (SL-MoS₂-CNTs) can be efficiently prepared through the modulation of experimental conditions. The enhanced catalytic activity in HER is demonstrated by reducing the layer number of MoS₂ nanosheets. Remarkably, the optimized SL-MoS₂-CNTs sample showed long-term durability with an accelerated degradation experiment even after more than 10 000 recycles, and high HER activity with an onset overpotential of only ∼40 mV vs. RHE. This study introduces a novel, cheap and facile strategy to prepare layer-controlled 2D MoS₂ nanosheets in a large quantity, and is expected to broaden the already wide range energy applications of 2D MoS₂ nanosheets.

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Context

Two-dimensional (2D) molybdenum sulfide (MoS₂) nanomaterials have attracted significant research interest because of their intriguing structural and electronic properties as well as numerous potential applications in many energy fields, such as electronic devices,^1–3 lithium batteries^4–6 and electrocatalytic hydrogen evolution from water.^7–10 Bulk MoS₂ consists of several single-layer MoS₂ nanosheets, which are linked by van der Waals force, and each sheet is made up of a hexagonal layer of molybdenum atoms between two hexagonal layers of sulfur atoms. Previous studies indicate that few-layer, especially single-layer, MoS₂ possesses distinctively different physical and chemical properties compared with their bulk counterparts, viz. stronger photoluminescence,¹¹ greater luminescence quantum efficiency¹² and higher on–off ratio of transistors.^2,13 Therefore, extensive efforts have been devoted for the preparation of few-layer and single-layer MoS₂ nanosheets. Presently, there are mainly two strategies: top-down methods, including mechanical exfoliation,^14,15 chemical exfoliation,^16,17 electrochemical lithium intercalation,^18,19 laser thinning²⁰ and ball milling,²¹ and bottom-up methods, including chemical vapor deposition on substrates^22,23 and chemical synthesis.^18,24,25 Although significant progress has been made by several research groups, controlling the layer numbers of these materials still remains a considerable challenge. In particular, for the chemical synthesis methods in the liquid phase, reaction-intermediates are usually unstable and tend to form quasi-0D nanoparticles or 3D bulk materials during the preparation process.^26,27

Herein, we report a facile and scalable strategy for the production of MoS₂ nanosheets with controllable layers via the direct chemical reaction of hexaammonium heptamolybdate ((NH₄)₆Mo₇O₂₄) and carbon disulfide (CS₂) under mild conditions. The layer number of MoS₂ nanosheets can be efficiently controlled by modulating the experimental conditions, i.e. synthesis under H₂O-free environment for multiple-layer MoS₂ (ML-MoS₂), and synthesis process under H₂O-assisted environment for few-layer MoS₂ (FL-MoS₂). On the basis of this method, we can further prepare single-layer (SL) MoS₂ coating on carbon nanotubes (SL-MoS₂-CNTs) by adding CNTs in the precursors. This method can easily scale up because the throughput is only limited by the size of the autoclave (for more details, see the Experimental section in ESI†). The significantly enhanced catalytic activity is demonstrated in a hydrogen evolution reaction (HER) by reducing the layer number of MoS₂ nanosheets. Remarkably, the optimized SL-MoS₂-CNTs samples showed long-term durability and high HER activity with an onset overpotential of only ∼40 mV vs. RHE.

The morphology and structure of MoS₂ nanosheets were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). One can see that FL-MoS₂ exhibits a typical 3D flower-like morphology, which consists of MoS₂ nanosheets (Fig. 1a and b). Further, the high resolution (HR) TEM image shows that these nanosheets mainly contain 1–5 layers (L) (Fig. S1d†) with a layer distance of 0.62 nm, corresponding to the (002) plane of MoS₂ (Fig. 1c). The sub-angstrom resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1d and S2a†) reveal that the honeycomb arrangement of the atoms extend over the entire nanosheet of FL-MoS₂, and the corresponding energy dispersive X-ray (EDX) mapping images reveal that the distribution of molybdenum atoms and sulfur atoms in the entire nanosheet is very homogeneous (Fig. 1e and f and S2b and S2c†), further confirming the structural feature of MoS₂ nanosheets. In comparison, the ML-MoS₂ presents an irregular aggregation (Fig. S3a†) with an obvious thicker nanosheet structure compared with FL-MoS₂, i.e. the average layer number is ca. 6 L for ML-MoS₂ and 3 L for FL-MoS₂, as obtained from the statistical analysis of the HRTEM images in Fig. 2b and S1d and S3d.† In addition, when CNTs are added during synthesis, it is interesting to find that MoS₂ nanosheets tend to continuously coat on CNTs (Fig. 1g–i), and most of the layer numbers (more than 70%) are 1 L (Fig. 2b and S4d†).

Fig. 1 (a) SEM image of FL-MoS₂. (b) TEM image of FL-MoS₂. (c) HRTEM image of FL-MoS₂ with inset showing a layer distance of 0.62 nm. (d) HAADF-STEM image of FL-MoS₂ with the inset showing a honeycomb arrangement of MoS₂; blue and yellow dots represent Mo and S, respectively. The scale bar in the inset is 0.5 nm. The area with the dashed circles shows the defects in the nanosheets. (e) and (f) EDX mapping images of Mo and S for FL-MoS₂ corresponding to figure (d). (g)–(i) HRTEM images of SL-MoS₂-CNTs.

Fig. 2 (a) XRD patterns of SL-MoS₂-CNTs, FL-MoS₂ and ML-MoS₂ in comparison to bulk MoS₂. (b) Intensity ratio of the MoS₂ (002) peak and MoS₂ (110) peak (I_(002)/(100)) for bulk MoS₂, ML-MoS₂, FL-MoS₂ and SL-MoS₂-CNTs in XRD, and the average layer number of ML-MoS₂, FL-MoS₂ and SL-MoS₂-CNTs obtained from the statistical analysis by HRTEM in Fig. S1, S3 and S4,† respectively. (c) Raman spectra of SL-MoS₂-CNTs, FL-MoS₂ and ML-MoS₂ in comparison to bulk MoS₂.

X-ray diffraction (XRD) and Raman spectroscopy were used to further analyze the layer structure of these MoS₂ samples. As observed in the XRD patterns in Fig. 2a, all the samples exhibited the typical (002), (100), (103) and (110) planes of hexagonal 2H-MoS₂. Because the (002) reflection of MoS₂ is the characteristic peak for the c axis and (100) for the ab plane, we can take the intensity ratio between (002) and (100) peaks (I_(002)/(100)) of MoS₂ to assess the layer number variation in the series of these samples. One can see the ratio I_(002)/(100) of bulk MoS₂ (22.67), ML-MoS₂ (1.69), FL-MoS₂ (0.85) and SL-MoS₂-CNTs (0.28) decrease gradually (Fig. 2b), indicating that the layer number of these MoS₂ samples also decreases. In addition, all the Raman spectra (Fig. 2c) of these MoS₂ samples showed two characteristic peaks at 390 and 415 cm⁻¹ corresponding to the E_2g¹ and A_1g modes of the hexagonal MoS₂ crystal, respectively. It is reported that the frequency of A_1g mode will have a red-shift as the layer number of MoS₂ decreases.^28–30 As expected, an obvious red-shift of A_1g modes was observed from bulk MoS₂ to FL-MoS₂, indicating that the layer number decreases gradually in these samples, which is in good agreement with the XRD and HRTEM analysis. Note that there was a slight blue-shift of the A_1g modes from FL-MoS₂ to SL-MoS₂-CNTs, which was probably due to the presence of a strain introduced by the increase in curvature of MoS₂ nanosheets when coating onto the CNTs.^31,32

The above mentioned results demonstrate that MoS₂ nanosheets with controllable layers have been successfully synthesized via a one-step reaction between (NH₄)₆Mo₇O₂₄ and CS₂ with appropriately chosen conditions, and we propose a possible mechanism for this method. As depicted in Scheme 1, the direct reaction of (NH₄)₆Mo₇O₂₄ and CS₂ will form the small 2D domains of MoS₂ and then assemble into multiple-layer MoS₂ nanosheets due to the interaction via the van der Waals force between the layers. When the reaction was assisted with H₂O, the layer-assembling was partly hindered due to the adsorbed H₂O molecules around them, hence leading to thinner nanosheets. When CNTs are introduced in the H₂O-assisted reaction, the CNTs can serve as templates to immobilize the small MoS₂ domains during the reaction, which further hinders the layer-assembling and finally leads to the formation of a single-layer MoS₂ nanosheets coating on the surface of the CNTs. The mechanism of MoS₂ nanosheets growing on CNTs here should be similar with that on the other templates, e.g. graphene.^33,34

Scheme 1 Schematic illustration for the direct chemical synthesis of multiple-layer MoS₂ (ML-MoS₂), few-layer MoS₂ (FL-MoS₂) and single-layer MoS₂ coating on carbon nanotubes (SL-MoS₂-CNTs) by modulating the experimental conditions.

2D MoS₂ based nanomaterials have been regarded as a promising non-precious catalysts for hydrogen evolution reaction (HER), and have stimulated great research interest in the past few years.^35–41 Previous experimental and computational studies indicated that the activity of hydrogen evolution reaction (HER) originates from the edges and defects of MoS₂ nanosheets.^{8,35,36,42,43} It implies that increasing the edges and defects in MoS₂ nanosheets will efficiently promote the HER activity. In this study, we find that reducing the layer number of MoS₂ nanosheets will introduce mass defects in these nanosheets, as observed in Fig. 1d and S4,† implying that the HER activity could be enhanced by reducing the layer number of MoS₂ nanosheets. Therefore, we investigate the effect of layer numbers of MoS₂ nanosheets on the HER activity and expect that the optimization of MoS₂ catalysts can enhance the HER performance. The HER electrochemical measurements were performed by a typical three-electrode setup in 0.1 M H₂SO₄. As shown in Fig. 3a, all the prepared MoS₂ samples exhibited high activity compared with bulk MoS₂ and blank glassy carbon (GC) electrode. Moreover, the HER activity increases significantly as the layer number of MoS₂ nanosheets reduces, i.e. the HER onset overpotential is ∼200 mV for bulk MoS₂, ∼110 mV for ML-MoS₂ and ∼50 mV for FL-MoS₂ (see Table S1†). Remarkably, the polarization curve of SL-MoS₂-CNTs shows an onset overpotential of only ∼40 mV vs. RHE, beyond which the cathodic current increased rapidly under more negative potentials. Further, the overpotential at a current density of 10 mA cm⁻² is 236 mV, showing that SL-MoS₂-CNTs is amongst the most active non-precious HER catalysts in an acidic medium.^37,43–47 In addition, the HER exchange current density (j₀), normalized by the mass loading of these catalysts can be favorably compared to that of some reported outstanding MoS₂-based catalysts (Table S2†). The high HER activity of SL-MoS₂-CNTs may arise from the increasing defects in the single layer, as well as the enhanced electrical conductivity of CNTs that promotes electron transfer during the reaction. The Tafel slope is often used to reveal the inherent property of HER electrocatalysts: a smaller Tafel slope will lead to a faster increment of the HER rate with increasing overpotential.⁸ As shown in Fig. 3b, the Tafel slope of SL-MoS₂-CNTs is 63 mV dec⁻¹, much smaller compared with FL-MoS₂ (87 mV dec⁻¹) and ML-MoS₂ (110 mV dec⁻¹), further confirming that SL-MoS₂-CNTs possess higher HER activity.

Fig. 3 (a) Polarization curves of SL-MoS₂-CNTs, FL-MoS₂, and ML-MoS₂ in comparison with bulk MoS₂, CNTs, 40% Pt/C and blank GC electrode. (b) Tafel plots of ML-MoS₂, FL-MoS₂, SL-MoS₂-CNTs and 40% Pt/C. (c) Durability measurements of SL-MoS₂-CNTs and FL-MoS₂. The polarization curves recorded initially and after 10 000 CV sweeps between +0.57 and −0.13 V (vs. RHE). (d) Potential values recorded initially and after every 1000 CV sweeps for SL-MoS₂-CNTs and FL-MoS₂ at 1 mA cm⁻² and 10 mA cm⁻².

Durability is another important criterion to evaluate a HER electrocatalyst. Herein, accelerated degradation experiments were adopted to conduct the evaluation. After every 1000 cyclic voltammetric (CV) sweeps between −0.13 and +0.57 V (vs. RHE), a polarization curve was recorded. From Fig. 3c, one can see that the SL-MoS₂-CNTs retains almost the same activity as the initial, even after 10 000 recycles. In contrast, FL-MoS₂ exhibits an obvious degradation in the activity. Moreover, from the potential values recorded at the same current density of 1 mA cm⁻² and 10 mA cm⁻² with different CV sweeps in Fig. 3d, one can see that SL-MoS₂-CNTs shows higher durability with negligible increase in overpotential compared with FL-MoS₂. Considering that more edges and defects will be introduced in MoS₂ nanosheets as the layer number are reduced, the structural stability of MoS₂ will reduce and usually lead to a decrease in the catalytic durability. However, in this study, the single-layer MoS₂ on CNTs shows even higher durability than free-standing few-layer nanosheets. We deduce that the introduction of CNTs may enhance the interaction of CNTs and MoS₂ nanosheets, which can efficiently hinder the degradation of HER durability.

In summary, we introduce a facile and efficient chemical method to directly synthesize 2D MoS₂ nanosheets with controllable layers. Within the method, both free-standing MoS₂ nanosheets and single-layer MoS₂ coating on CNTs can be efficiently prepared via the modulation of experimental conditions. These MoS₂ nanosheets exhibit a significant layer number effect on the hydrogen evolution reaction (HER), i.e. the HER activity will be enhanced by reducing the layer number. Remarkably, the optimized SL-MoS₂-CNTs samples showed long-term durability with more than 10 000 recycles and high HER activity with an onset overpotential of only ∼40 mV vs. RHE. These resulting layer-controlled MoS₂ nanosheets are expected to find potential applications as non-precious HER catalysts as well as in other fields such as electronics and lithium batteries.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 21303191 and 51390474) and the strategic Priority Research Program of the Chinese Academy of Sciences (no. XDA09030100), and thank Prof. Ting Yu at Nanyang Technological University for fruitful discussions on Raman analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05614k

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