Facile surfactant-assisted synthesis of CTAB-incorporated MoS₂ ultrathin nanosheets for efficient hydrogen evolution

Abstract

Molybdenum disulfide (MoS₂) has emerged as a promising electrocatalyst for catalyzing protons to hydrogen via the so-called hydrogen evolution reaction (HER). In order to enhance the HER activity, CTAB (cetyltrimethyl ammonium bromide)-incorporated MoS₂ ultrathin nanosheets with high specific surface area and more catalytic active sites have been successfully synthesized via a facile surfactant-assisted hydrothermal approach. Adding CTAB to the hydrothermal process can effectively regulate both structural and electronic benefits by controllable disorder engineering and simultaneous CTAB incorporation in MoS₂ catalysts, leading to a higher specific surface area, more catalytic active sites and better electrical conductivity for improving HER activity. The optimized CTAB-incorporated MoS₂ ultrathin nanosheets exhibit excellent activity for HER with a small onset potential of 88 mV, a low Tafel slope of 55 mV dec⁻¹, and relatively good stability.

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Introduction

The ever increasing global demand for energy and environmental pollution due to burning of fossil fuels has stimulated considerable effort to exploit renewable and clean energy alternatives.¹ Hydrogen is seen as an alternative fuel that is secure, sustainable and clean.² The hydrogen economy is a circulating system in which electricity is obtained from hydrogen fuel cells, and byproducts of water are dissociated into hydrogen and oxygen. The methods to dissociate water are generally divided into two categories. One is water splitting, by which water can be oxidized or reduced using photochemical reactions. The other is electrochemically driven water dissociation which involves the HER and the oxygen evolution reaction (OER). Efficient catalysts for HER are the key to achieve optimal performance.³ Usually, an efficient electrocatalyst is required to accelerate the HER rate due to the multi-electronic nature of reduction of protons to form dihydrogen (2H⁺ + 2e⁻ → H₂).⁴ Up to now, the most effective electrocatalysts for HER are based on Pt-group metals, which are capable of catalyzing HER at a significant rate with almost no overpotential. However, as we know, Pt-group metals are very scarce in the earth and extremely expensive.⁵ Scientists have been trying to explore other cheap and abundant materials which are also very active for HER to substitute them. Meanwhile, HER is more likely to occur in acidic medium than in alkaline or neutral condition.⁶ Therefore, it is still urgent to develop low cost, acid-stable HER electrocatalysts.⁷

So far, various candidates have been explored to replace Pt-based catalysts, including MoS₂,⁸ WS₂,^9,10 FeS,² MoSe₂,^11,12 MoP,¹³ MoC,^14–16 and so on. Among these candidates, MoS₂ has received tremendous attention due to the earth abundant composition and high activity, leading to the development of various kinds of MoS₂-based HER electrocatalysts in the form of crystalline or amorphous states, and even in molecular mimics.^17,18 As a typical transition-metal dichalcogenide (MX₂), MoS₂ is a layered structure consisting of S–Mo–S arrangements connected to each sheet by van der Waals forces. Layered materials expose their basal plane which has low roughness and is chemically inert. Moreover, the catalytic activity of MoS₂, according to computational studies and direct experimental comparison reported recently, has been verified to stem mainly from the edges rather than the basal surface.¹⁶ Therefore, the unsaturated sulfur atoms on the edges play a crucial role in HER catalysis. Thus, developing nano-sized MoS₂ with more exposed edges is considered as an efficient strategy to create excellent electrocatalysts for HER.^19–21 In consequence, a lot of efforts have been focused on achieving this in the past few years.^20–27

For example, Xie's group presented a hydrothermal route to prepare defect-rich MoS₂ nanosheets with excessive supply of thiourea and illustrate its enhanced electrocatalytic performance for HER.²⁴ Recently, polyaniline with the morphology of nanowire was employed to construct the hierarchical integrative hybrid with MoS₂, and the dense and approximate vertical growth of MoS₂ nanosheets exposing abundant active edges with the participation of polyaniline, which has excellent HER performance.²⁵ In addition, Sung's group synthesized edge-exposed MoS₂ nano-assembled structures and engineered the energetic issues using rather large particles, the resulting assembled sphere exhibiting a high HER activity.²³

However, the synthesis of nano-sized edge-terminated MoS₂ is quite difficult owing to thermodynamic issues, which hinders the application of the material. The edge-terminated sites are highly energetic, and are unstable relative to the basal plane. Due to thermodynamic stability, small nanoparticles form into fullerene-like structures which have few exposed edge sites. Designing the MoS₂ structure both enlarging the active center and increasing the stability is critical issue for application of MoS₂ as a catalyst for HER.

In this study, we have successfully synthesized the CTAB-incorporated MoS₂ ultrathin nanosheets (denoted as CTAB–MoS₂) using a facile surfactant-assisted hydrothermal approach, which exhibit excellent HER performances including a small onset potential, a low Tafel slope and relatively good stability. In comparison with the product without CTAB, the layer number of CTAB-incorporated MoS₂ ultrathin nanosheets decreased, the lattice expanded and the electrical conductivity improved. Furthermore, the structure of the obtained CTAB-incorporated MoS₂ ultrathin nanosheets has been characterized by various techniques.

Experimental

Synthesis of various MoS₂

The CTAB-incorporated MoS₂ ultrathin nanosheets were prepared by a facile hydrothermal route. Typically, 1.25 mmol Na₂MoO₄·2H₂O, 2.5 mmol CH₃CSNH₂ and 0.05 mmol CTAB were dissolved in 60 mL distilled water under vigorous stirring to form a homogeneous solution at room temperature. After being stirred for 30 min, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at different temperatures (160 °C, 180 °C, 200 °C) for 30 h. Then the reaction system was allowed to cool down to room temperature naturally. The obtained products were collected by centrifugation, washed with distilled water and ethanol, and dried at 60 °C. The synthesis approach of the MoS₂ with no CTAB is similar to the above process. Just not add CTAB at 180 °C. At 160 °C, we didn't get any product except an orange solution. With the increase of the synthesis temperature, black products are obtained in the Teflon-liner.

Characterization

The crystalline structures of the samples were characterized using X-ray powder diffraction (XRD Rigaku D-max-γA XRD with Cu Kα radiation, λ = 1.54178 Å) from 5° to 90°. SEM images were collected using scanning electron microscopy (SEM, JSM-6700F from JEOL). TEM images and HRTEM images were obtained using transmission electron microscopy (TEM, FEI Tecnai G20). The surface areas were tested by nitrogen adsorption/desorption analysis (Automated Physisorption and Chemisorption Analyzer, micromeritics ASAP 2020). The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI 550 spectrometer with Al Kα (1486.6 eV) as the X-ray source. The Fourier transform infrared spectroscopy (FTIR) was taken on a Nicolet 510P instrument.

Electrochemical measurements

4 mg of various MoS₂ were dispersed in 1 mL water–ethanol solution with volume ratio of 3 : 1 containing 30 μL Nafion (5% wt) by sonicating for 1 h to form homogeneous slurry. The addition of a small amount of Nafion could effectively improve the distribution of the catalyst and enhance its binding onto the electrode surface.²⁶ Then, 5 μL of the slurry (containing 20 μg of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter with a catalyst loading of 0.285 mg cm⁻². The MoS₂ modified GCE was then dried at room temperature. Each modified GCE was loaded with the same amount of catalyst. All electrochemical measurements were conducted on an Autolab PGSTAT302N in a standard three-electrode system at room temperature. Linear sweep voltammetry (LSV) with scan rate of 5 mV s⁻¹ was conducted in 0.5 M H₂SO₄ (purged with pure Ar) using a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as the counter electrode. The various MoS₂ modified glassy carbon electrode was as the working electrode. The Nyquist plots were measured with frequencies ranging from 100 kHz to 0.1 Hz at η = 100 mV. All the potentials were calibrated to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059pH) V. Experimental data were analyzed and fitted with the software NOVA 1.8.

Results and discussion

The structural information was investigated by XRD. Fig. 1 shows the XRD patterns of various MoS₂ samples. Briefly, lower synthesis temperature is the prerequisite for the formation of CTAB-incorporated MoS₂ ultrathin nanosheets. At high temperature (200 °C), pure molybdenum disulfide (denoted as MoS₂-200) that all the diffraction peaks match the hexagonal phase of MoS₂ (2H-MoS₂) was synthesized. With the decrease of synthesis temperature, the incorporation of CTAB induced the splitting of the typical peak of (002). A low borden peak at 9.14° (marked by #1) and a low borden peak at 15.28° (marked by #2) are observed, which can be attributed to the expansion of interlayers. Moreover, calculated by the Scherrer equation, the basal spacing increases to about 3.3 Å compared to the standard pristine MoS₂, which can be attributed to the incorporation of CTAB.^28,29 It can be inferred that CTAB-incorporated MoS₂ was synthesised with the assistance of surfactant CTAB at 180 °C. Then, a test experiment without CTAB at 180 °C was carried out to investigate the effect of surfactant in the process of synthesising MoS₂. As shown in Fig. 1, a weak and borden (002) diffraction peak can be clearly observed which is agree well with 2H-MoS₂. All of the peaks of this MoS₂ sample (denoted as MoS₂-180) are significantly broadened as well as CTAB–MoS₂, suggesting bad crystallinity of CTAB–MoS₂ and MoS₂-180. In summary, all of above phenomena indicates that temperature play a key role in the formation of CTAB–MoS₂.

Fig. 1 XRD patterns of CTAB–MoS₂, MoS₂-180, and MoS₂-200.

The morphologies and size of CTAB–MoS₂, MoS₂-180, and MoS₂-200 are characterized by SEM. The low- and high-magnification SEM images (Fig. 2) clearly reveals the great difference among various MoS₂ samples. The low-magnification SEM image (Fig. 2A₁) indicates that CTAB–MoS₂ is consisted of uniform amorphous curl nanosheets. The lateral size of the nanosheets is in the range of 100–200 nm, and obvious ripples and corrugations can be observed from the high-magnification SEM image (Fig. 2A₂), which can bring in more exposed sulfur edges sites. However, MoS₂-180 and MoS₂-200 are composed of large quantities of uniform three-dimensional (3D) flower-like microspheres (Fig. 2B₁ and C₁), which have much big difference with CTAB–MoS₂ nanosheets. It can be clearly seen that the yield of product with 3D flower-like spheres was almost 100%. The SEM images with higher magnification (Fig. 2B₂ and C₂) present a clear view of the surface morphology of microsphere. It reveals that the entire structure of the architecture is built from several dozen nanosheets. These nanosheets were connected to each other through the center to form 3D flower-like structures. As shown in Fig. 2B₂ and C₂, the diameter of MoS₂-180 microspheres is in the range of 300–700 nm, while the diameter of MoS₂-200 microspheres is in the range of 500–1000 nm that is bigger than MoS₂-180. These phenomena further conform that the surfactant couldn't effect at high temperature and the microspheres could grow bigger under higher temperature. By comparing CTAB–MoS₂ and MoS₂-180, we can find that the morphology of the product has undergone great change with the incorporation of CTAB. These results illustrate that CTAB plays a key role for the formation of the CTAB-incorporated MoS₂ ultrathin nanosheets.

Fig. 2 SEM images of CTAB–MoS₂ (A₁, A₂), MoS₂-180 (B₁, B₂), and MoS₂-200 (C₁, C₂).

In order to further study the effects of CTAB, we investigate the microstructure of the CTAB–MoS₂ and MoS₂-180 by TEM and HRTEM. TEM image verifies CTAB–MoS₂ has two-dimensional (2D) amorphous ultrathin nanosheets structure (Fig. 3A₁). Besides, dense interconnected ripples and corrugations can also be observed. These ripples and corrugations suggest the edge-rich feature of the ultrathin MoS₂ nanosheets. Fig. 3B₁ shows solid sphere structure of MoS₂-180. We can see MoS₂-180 is bigger than CTAB–MoS₂ about four to seven times. Furthermore, it is clearly observed that the layer number of CTAB–MoS₂ is less than MoS₂-180 (Fig. 3A₂ and B₂). The interlayer distance between CTAB–MoS₂ measured from Fig. 3A₃ is about 0.95 nm while MoS₂-180 have a well-layered structure with a d (002) = 0.63 nm (Fig. 3B₃). These results are in accordance with that from the XRD. Interplanar spacing of 2.5 and 2.7 Å can be observed from Fig. 3A₄ and B₄, which is consistent with the d spacing of (100) planes of MoS₂.

Fig. 3 TEM and HRTEM images of CTAB–MoS₂ (A₁–A₄), MoS₂-180 (B₁–B₄).

Fig. 4 displays the FTIR spectrum of CTAB–MoS₂. FTIR can not only reflect in the inorganic bonding of groups, but also can indicate the existence of the organic groups. Two peaks at 3443 cm⁻¹ and 1638 cm⁻¹ are attributed to the stretching vibration and bending vibration of the sample surface water.³⁰ Two characteristics peaks of CTAB at 2917 cm⁻¹ and 2849 cm⁻¹ are associated with the asymmetric and symmetric stretching vibration of the –CH₂-groups.³¹ And the characteristic absorption peak at 1402 cm⁻¹ is attributed to the ammonium group of CTAB. The absorption peak of 1124 cm⁻¹ is due to stretching vibration of C–N bond.^32,33 All these results can confirm the incorporation of CTAB to MoS₂. The CTAB-incorporated MoS₂ ultrathin nanosheets were synthesised by the facile hydrothermal approach.

Fig. 4 FTIR spectrum of CTAB–MoS₂.

The chemical states of Mo and S in the CTAB-incorporated MoS₂ ultrathin nanosheets and MoS₂-180 are analyzed by XPS technique. For the CTAB–MoS₂, it can be observed that two characteristic peaks arising from 229.0 eV and 232.3 eV are attributed to the Mo 3d_5/2 and Mo 3d_3/2 binding energies for a Mo(IV) oxidation state (Fig. 5A), while the corresponding peaks for the S 2p_3/2 and S 2p_1/2 orbitals of divalent sulfide ions (S²⁻) are observed at 161.8 eV and 163.1 eV (Fig. 5B).³⁴ In addition to the XPS peaks for the MoS₂ structure, another set of peaks for S and Mo are also presented (Fig. 5A and B). The observation of Mo 3d_5/2 and Mo 3d_3/2 binding energies at 233.6 eV and 230 eV suggest the presence of Mo(V).³⁵ Meanwhile, the S 2p_3/2 and S 2p_1/2 energies at 162.6 eV and 163.9 eV suggest the existence of bridging S₂²⁻ or apical S²⁻, which result from the unsaturated S atoms, as indicated in panels A and B in Fig. 5.³⁶ However, the Mo 3d and S 2p regions (Fig. 5C and D) of MoS₂-180 are different from the CTAB–MoS₂. Two characteristic peaks of Mo 3d_3/2 and Mo 3d_5/2 orbitals are located at 232.6 eV and 229.3 eV. Besides, the corresponding peaks for the S 2p_3/2 and S 2p_1/2 are observed at 162.0 eV and 163.3 eV (Fig. 5B). These Mo 3d and S 2p spectra are consistent with Mo(IV) and −2 oxidation state of sulfur, suggesting the lack of bridging S₂²⁻ or apical S²⁻. These observations indicate that the CTAB–MoS₂ ultrathin nanosheets prepared are featured with more active sites stemming from the coordinately unsaturated sites and sulfur atoms,²¹ which are suspected to have better HER performance.

Fig. 5 XPS spectra of CTAB–MoS₂: (A) Mo 3d, (B) S 2p and MoS₂-180: (C) Mo 3d, (D) S 2p.

Finally, the Brunauer–Emmett–Teller (BET) surface area of CTAB–MoS₂ is measured to be 51.35 m² g⁻¹, much higher than that of MoS₂-180 (18.25 m² g⁻¹) (Fig. 6). Since higher surface area of ultrathin nanosheets morphology can result in a higher accessible reactive sites. Therefore, better HER performance can be expected from these rich edges and active unsaturated sulfur atoms of CTAB–MoS₂ ultrathin nanosheets.

Fig. 6 Nitrogen adsorption–desorption isotherms of the as-synthesized CTAB–MoS₂ nanosheets and MoS₂-180 microspheres.

In order to verify our expectation, electrochemical measurements of various MoS₂ samples were performed in 0.5 M H₂SO₄ solution by LSV to investigate the HER activity. All measurements were carried out at the same optimized loading weight of 0.285 mg cm⁻² using a three-electrode setup. As a reference point, we also performed measurements using a commercial Pt catalyst (20 wt% Pt on Vulcan carbon black) exhibiting high HER catalytic performance (with a near zero overpotential).³⁷ Since the 20% Pt/C catalyst was used, the Pt content was calculated as 57 μg cm⁻².³⁸ As shown in Fig. 7A, CTAB–MoS₂ exhibit excellent activity for the HER with a small onset overpotential of approximately 88 mV for HER. This onset overpotential is much smaller than that of MoS₂-180 (163 mV), which suggests the good catalytic activity of CTAB–MoS₂. Moreover, cathodic current density is considered as an important evaluating criterion for HER activity. As shown in Fig. 7A, CTAB–MoS₂ exhibits an extremely large cathodic current density with 21 mA cm⁻² at an overpotential of 200 mV, which is much bigger than MoS₂-180 (2 mA cm⁻²). The good catalytic behavior of CTAB–MoS₂ ultrathin nanosheets may arise from the unique CTAB-incorporated ultrathin nanosheets structure which brings in more additional active edge sites. To obtain further insight into the HER on CTAB–MoS₂, Tafel plots of various samples are investigated (Fig. 7B). The resulting Tafel slope of CTAB–MoS₂ is 55 mV dec⁻¹, which can matches with several earlier reports for MoS₂ catalysts.^39,40 In the absence of the incorporation of CTAB, the Tafel slope of MoS₂-180 is 163 mV dec⁻¹. The small Tafel slope of the CTAB–MoS₂ ultrathin nanosheets is advantageous for practical applications, since it will lead to a faster increment of HER rate with increasing overpotential.⁴¹ These results further verify that the incorporation of CTAB plays an important role in enhancement of the HER performance. Furthermore, the Tafel slope of 55 mV dec⁻¹ in this work suggests that the HER takes place via a rapid Volmer reaction followed by a rate-limiting Heyrovsky step and the Volmer–Heyrovsky mechanism is operative in the HER catalyzed by CTAB–MoS₂, according to the classical theory for the HER in acidic media.^39,42 To gain a better understanding of the improvement mechanism of the CTAB–MoS₂, electrochemical impedance spectroscopy (EIS) measurements were also performed. Fig. 7C shows the representative Nyquist plots of the impedance of different catalysts at an overpotential of 100 mV, including CTAB–MoS₂ and MoS₂-180. It is clear that CTAB–MoS₂ exhibits one capacitive semicircle, indicating that the corresponding equivalent circuit for the HER was characterized by one time constant and the reaction is kinetically controlled. Moreover, the diameter of the semicircle for the CTAB–MoS₂ is very small, indicating a smaller charge transfer resistance of about 8.5 Ω. The charge transfer resistance is smaller than MoS₂-180, which can be attributed to the incorporation of CTAB enhancing the electrical conductivity. The small resistance of the CTAB–MoS₂ lead to better HER activity.

Fig. 7 (A) Polarization curves of various samples, (B) corresponding Tafel plots, (C) Nyquist plots of the various MoS₂ catalysts, (D) stability test for CTAB–MoS₂ catalyst in 0.5 m H₂SO₄ with scan rate of 50 mV s⁻¹.

Finally, stability is another significant criterion by which to evaluate a catalyst. A long term cycling test in acidic environment was also performed to assess the electrochemical stability. Fig. 7D displays the polarization curves of CTAB–MoS₂ before and after 1000 cycles. A little decay of the cathodic currents indicates the good stability of the CTAB-incorporated MoS₂ ultrathin nanosheets in a long-term electrochemical process.

Conclusions

In summary, we put forward a novel strategy to synthesize CTAB-incorporated MoS₂ ultrathin nanosheets using a facile hydrothermal approach. The incorporation of CTAB into MoS₂ lammer structures induces better electrical conductivity. The existence of unsaturated active sulfur ligands and edge-rich ultrathin layers result in the exposure of additional catalytically-active sites. These lead to outstanding catalytic performance for the HER. We expect that this success of applying surfactant to increase active sites may open up a potential pathway for designing more efficient MoS₂-related catalyst for HER in the future.

Acknowledgements

This work was financially supported by Natural Science Foundation of China (Grant no. 51272115) and Natural Science Foundation of Shandong Province (no. ZR2012EMM001), A Project of Shandong Province Higher Educational Science and Technology Program (J13LA10) and A Project of Shandong Province Higher Educational Science and Technology Program (J14LA15).

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