DOI:10.30919/esmm5f112

Received: 03 Dec 2018
Revised: 13 Dec 2018
Accepted: 13 Dec 2018
Published online: 14 Dec 2018

Ultrathin MoSe2 Nanosheets Coated on Hollow Carbon Spheres as Efficient Hybrid Catalyst for Hydrogen Evolution Reaction

 

Qin Yuan,1 Ruijing Wang,1 Qunlong Wang,2 Peng Sun,1 Rongrong Nie1 and Xuefeng Wang1*

1Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China

2Shanghai Joyworld Innovative Materials Technology Co. LTD, Shanghai 200433, China.

 

Abstract

Two-dimensional transition metal chalcogenide is a promising non-precious metal electrocatalyst for hydrogen evolution reaction (HER); however, the improvement of an inefficient catalytic activity is still a major challenge at present due to their limited active sites and low conductivity. In this work, we present ultrathin MoSe2 nanosheets were coated on hollow carbon spheres (HCS) surface through strong interaction with surface hydrophilic functional groups OH to fabricate a core-shell HCS@MoSe2 hybrid catalyst. The MoSe2 structure with nanosheets could effectively expose more active edges and the HCS core further provides a beneficial conductivity. Finally, the obtained composite shows the remarkable HER activity with a small Tafel slope of 74 mV dec-1, a small overpotential of 200 mV at a current density of 10 mA cm-2, and an excellent long-term durability. Therefore, this Pt-free and highly efficient electrocatalyst shows its great potential for renewable hydrogen production.

Table of Content

Ultrathin MoSe2 coated hollow carbon sphere hybrid catalysts show remarkable HER activity.

 

 

Keywords:Molybdenum diselide; Hollow carbon spheres; Hydrogen evolution reaction


1. Introduction


     Transition metal chalcogenides (TMDs), composed of covalently bonded X-M-X (M= Mo, W; X= S, Se, Te) layers by van der waals forces, have received much attention in energy conversion and storage fields due to their tunable band structure and intrinsic electrochemical properties.1-2 Among them, MoS2 and its hybrids are the most popular electrocatalytic materials that have been widely investigated for the hydrogen evolution reaction (HER).3-6 The MoSe2 materials, which have the similar electronic structure and physicochemical properties as MoS2, have gradually attracted the general interest.7-8 However, bulk MoSe2 is a poor catalyst for HER because of limited catalytic active sites.9 Recent works focus on increasing the MoSe2 active sites and improving its conductivity,10-12 and several strategies were demonstrated to enhance intrinsic HER catalytic activity. For example, Lai et al. used bacterial cellulose-derived carbon nanofibers to assist the uniform growth of few-layered MoSe2 nanosheets (CBC/MoSe2), which effectively increased the active sites and conductivity of MoSe2 for HER.13 Wang et al. prepared MoSe2 on a reduced grapheme oxide (rGO)/polyimide (PI) composite substrate by electrochemical deposition, which showed superior catalytic acitivity in HER.14 Deng et al synthesized N-MoSe2/VG shell/core arrays that remarkably boost their HER activity by means of the improved conductivity and rich catalytic active sites.15 These composites of MoSe2 are assembled onto the conductive carbon matrix. Recently hollow carbon spheres have been demonstrated to be an attractive carbon materials morphology in ion batteries and supercapacitor, since the unique hollow structure could offer a large surface area and minimize the diffusion distances of electrolyte ions.16-19 For example, hierarchical MoS2 shell supported on carbon spheres as an anode material for LIBs manifest high specific capacity and good rate capability.20

      In this study, we successfully prepared a composite with the hollow carbon spheres as a core and uniform MoSe2 nanosheets as shell (denoted as HCS@MoSe2). Encouragingly, this unique composite with ultrathin MoSe2 nanosheets uniformly grown on HCS exhibits excellent catalytic activity for HER with a small overpotential and Tafel slope because the ultrathin MoSe2 nanosheets covered the whole surface of HCS through strong interaction without obvious agglomeration, and the HCS core with a high conductivity compensates the low electron transfer of the TMDs.

2. Experimental

2.1. Chemicals

     Absolute ethanol, NaOH (99 %), formaldehyde (37 wt%), ammonia solution (25 wt%), Na2MoO4 (99.0 %), selenium (99.9 %), hydrazine hydrate (N2H4H2O, 80 wt%) solution, resorcinol (99.5 %), ethylorthosilicat (98 %) were used without further purification. All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2. Preparation of Hollow Carbon Spheres

       Ammonia aqueous solution (3 mL, 25 wt%) was added to a mixture of 10 mL deionized water and 70 mL ethanol. After the solution was stirred for 30 minutes, 3.46 ml ethylorthosilicate (TEOS), 0.4 g resorcinol and 0.56 ml formaldehyde solution (37 wt%) were added to the above solution and stirred for 24 h at room temperature to form the solid SiO2@SiO2 products. Secondly, the product was collected by centrifugation and washed with absolute ethanol and distilled water several times, and vacuum-dried. The solid samples were carbonized by heating at 700℃ for 2 h under argon gas flow in a tube furnace. Finally, the SiO2 templates were removed by treating the material in 2M sodium hydroxide for several hours, then the final HCS were also obtained by centrifugation and washed with distilled water before dried in vacuum at 80 ºC for 24 h.

2.3. Fabrication of HCS@MoSe2

      Hollow carbon spheres (0.1 g) were added to the mixed solution which included 20 ml distilled water and 30 ml ethanol. After ultrasonic dispersion for 1 h, Na2MoO4 (0.4188 g) was added to the solution. Separately, selenium powder (0.3158 g) was dissolved in 10 ml of hydrazine hydrate solution in air atmosphere. The mixture was continuously stirred until a red-colored solution was obtained, and then the hydrazine hydrate-Se solution was slowly dropped into 50 ml of Na2MoO4 mixture solution at room temperature with rigorously stirring. The mixture turned dark red and the ratio of selenium and molybdenum is 2:1. The as-prepared precursor solution was transferred into the autoclave, sealed, and then heated at 200 °C for 12 h in an electric oven. The autoclave was then naturally cooled down to room temperature. After that, the black products were filtered off and washed with DI water several times to remove the residuals and then dried at 60 °C overnight. For comparison, MoSe2 nanosheets were prepared through the same method in the absence of hollow carbon spheres.

2.4. Characterization

       The morphology, structure, and composition of the electrode materials were characterized by field emission scanning (FE-SEM; Hitachi S-4800, Japan) and transmission (TEM; JEOL, JEM-2100, Japan) electron microscopes. Powder X-ray diffraction (XRD) patterns of the as-prepared samples were recorded using a Bruker Focus D8 with Cu Ka radiation. Raman spectra were collected using a Renishaw Invia Raman microscope with a 514 nm laser under ambient conditions. FTIR spectra were recorded at 1 cm-1 resolution on a Bruker Vertex 70V with 0.1 cm-1 accuracy using a three-reflection attenuated total reflection (ATR) accessory with a ZnSe internal reflection element (Harrick Scientific Products) and X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD).

2.5. Electrochemical measurements

       Electrochemical measurements were performed on a CHI 660D electrochemical workstation. All measurements were tested in 0.5 M H2SO4 using a traditional three-electrode system with a graphite rod counter electrode, Hg/Hg2Cl2 (in saturated KCl solution) reference electrode and glassy carbon working electrode of 0.5 cm in diameter. The sample (4 mg) was first ultrasonically dispersed in 0.4 ml of Nafion solution (5 wt%), afterwards, the suspension (~10 µL) was attached onto a glass carbon (GC) electrode as a working electrode (loading about 0.196 mg cm-2) and the as-prepared catalyst coating was dried at room temperature for at least 0.5 h. Linear sweep voltammetry curves (LSV) were obtained at a sweep rate of 5 mV s-1 from -0.34 V to 0.06 V (vs. RHE). The pure MoSe2 and commercial 10 wt% Pt/C were tested in the way described above for comparison. Electrochemical impedance spectroscopy (EIS) measurements were collected from 100 kHz to 0.1 Hz with an amplitude of 5 mV at an overpotential of 250 mV, and the EIS data was fitted by the ZView software. In electrochemical analysis, all the potentials were related to reversible hydrogen electrode (RHE) according to equation:

E (RHE) = E (Hg/Hg2Cl2) + (0.242 +0.059 pH) V

Where E(RHE) is the reversible hydrogen electrode potential, E(Hg/Hg2Cl2) is the obtained potential versus saturated calomel electrode potential.

 

Scheme 1  Schematic illustration of the synthesis of HCS@MoSe2.

3. Results and Discussion

3.1. Synthesis and structural characterization

     The preparation process of core-shell HCS@MoSe2 hybrids is displayed in Scheme 1, which involves a three-step method. Firstly, the synthesized SiO2@SiO2/RF by modified Stober method21 were carbonized by heating at 700 °C for 2 h under argon gas flow in a tube furnace leading to the formation of SiO2@SiO2/C. Secondly, the SiO2 templates were removed by treating the material in 2 M sodium hydroxide for several hours and the HCS formed subsequently. Finally, ultrathin MoSe2 nanosheets are grown on the HCS based via the hydrothermal treatment at 200 °C to form an HCS@MoSe2 core-shell structure. Fig. 1a and 1b show the scanning electron micrograph (SEM) morphology of SiO2@SiO2/C and HCS, respectively. It can be seen that the SiO2@SiO2/C has a spherical morphology with a diameter of approximately 300 nm, and HCS obtained after etching of the SiO2 keeps an initial structure of the template. It has been reported in the literature that there are abundant hydrophilic functional groups on the surface of HCS,22 which indicated that HCS offer an attractive supporter for the attachment of MoSe2 nanosheets. After the solvothermal process involving NaMoO4 as the Mo source and Se powder as the Se source, the curly MoSe2 nanosheets were uniformly decorated on the surface of the HCS (Fig. 1c), which is beneficial for the exposure of MoSe2 active edges. In contrast, the pure MoSe2 exhibited a pleated spherical configuration with an agglomerated structure (Fig. S1), indicating that the existence of HCS could avoid the stacking of MoSe2 nanosheets. From Fig. 1d, it could be found that the petal-like MoSe2 nanosheets were also inserted into the broken cavity of the hollow carbon framework. Moreover, the element mapping images in Fig. 1e show a homogeneous distribution of the Mo and Se elements on the surface of HCS. Similar hollow structure of HCS was also observed in transmission electron microscopy (TEM) images in Fig. 2a, and its outer shell layer is about 10 nm. In order to further confirm the structure of core-shell HCS@MoSe2, the high resolution transmission electron microscopy (HRTEM) was carried out. As shown in Fig. 2c, d, the surface of core HCS was covered by a thin layer of MoSe2 nanosheets without aggregation, and the interlayer spacing of 0.65 nm was in accordance with the (0 0 2) facet of MoSe2. It is worth noting that the layered MoSe2 nanosheets were composed of about 2-8 monolayers, which favor to expose more active sites and greatly enhance the HER catalytic performance.

      Crystal structures of the as-synthesized core-shell HCS@MoSe2, pure MoSe2 and HCS were studied by XRD as shown in Fig. 3a. As for pure MoSe2, the diffraction peaks at 2θ=13.7°, 27.6°, 34.4°, 41.9°, 47.5° and 56.9° can be respectively indexed to the (0 0 2), (0 0 4) (1 0 2), (0 0 6), (1 0 5) and (0 0 8) diffraction planes of the hexagonal 2H-MoSe2 phase (JCPDS:29-0914), revealing the high purity of MoSe2 nanosheets23-24. The XRD pattern of core-shell HCS@MoSe2 hybrids show diffraction peaks corresponding with (0 0 2), (1 0 2), and (0 0 8) crystal facets of hexagonal 2H-MoSe2 phase, indicating that the HCS were completely covered by the petal-like MoSe2 nanosheets.

Fig. 1  SEM images for the prepared (a) SiO2@SiO2/RF (b)HCS and (c,d) HCS@MoSe2; (e) EDX mapping images (the element of C, Se, Mo) of HCS@MoSe2 hybrids.

Fig. 2 TEM images of (a) HCS and (b,c) HCS@MoSe2; (d) HRTEM images of HCS@MoSe2.

 

     Raman spectroscopy was employed to further study the structure of the HCS@MoSe2. The typical peaks of pure MoSe2 at 238 and 281 cm-1 can be observed in the spectra (Fig. 3b), which corresponds to the out-of-plane A1g mode and in-plane E2g1 mode of MoSe2, respectively.25 In addition, the D and G bands of the HCS were located at 1335 and 1591 cm-1 respectively.26 From the Raman spectrum of HCS@MoSe2, the characteristic peaks corresponding to MoSe2 nanosheet and the carbon D and G bands with the HCS were all observed, confirming the successful integration of the MoSe2 nanosheets with HCS templates.

       To reveal the interaction between MoSe2 nanoparticles and HCS in the core-shell HCS@MoSe2, the bonding characterization of the hybrids by ATR-FTIR analysis of HCS and HCS@MoSe2 are shown in Fig. 3c. In the region of 750-2200 cm-1, the strong absorption bands for HCS at 1670 and 1114 cm-1 are ascribed to the C=O stretching vibrations of organic solutions and C-O(H) stretching vibrations peak, which exhibits the existence of the surface hydrophilic functional groups OH on the as-prepared HCS. Whereas the absorption bands at 897 cm-1 and 790 cm-1 can be assigned to C-C stretching and C-H bending modes, respectively. For the HCS@MoSe2 hybrids, the characteristic absorption peaks are consistent with HCS except the C-O(H) stretching peak (1100 cm-1) , which shows broadened and red-shifted feature obviously. This phenomenon implies the oxygen (O) in C-O is bonding to the metal molybdenum (Mo) in MoSe2, making the interaction between carbon (C) in HCS and oxygen (O) in OH functional groups above mentioned become weaker, resulting in red-shift of C-O(H) stretching vibration. Correspondingly, it can be seen a new peak at 950 cm-1 attributed to the formation of Mo-O bonds.27 This formation of the surface hydrophilic functional groups on HCS is not only beneficial to the tight attachment of MoSe2 nanosheets on HCS for stability, but also conducive to electronic transmission.

     In order to further analyze the chemical composition and valence states of the HCS@MoSe2, XPS spectrum was carried out in Fig. 4. The survey spectrum in Fig. 4a indicates the presence of Mo, Se, C, and O elements in the composite, which is in good agreement with the EDS results. C is originated from HCS core, and O is originated from the Mo-O bonds or partial surface oxidation of the MoSe2 nanoparticles. The high resolution spectrum of C1s exhibits three peaks at 283.0 eV , 284.6 eV and 286.1 eV are ascribed to C-O, C-C and C-Mo respectively,28 indicating the combination of HCS and MoSe2 mainly depends on Mo-C and Mo-O bonding.29 As depicted in Fig. 4c, two characteristic peaks located at 228.7 eV and 232.0 eV can be assigned to Mo 3d5/3 and Mo 3d2/3 spin orbit peaks of MoSe2, which indicated that Mo is basically in the Mo (IV) state.30 The small peak at 235.9 eV can be ascribed to Mo6+ from MoO42-. Meanwhile, the Se 3d can be deconvoluted into Se 3d5/2 and Se 3d3/2 at 54.3 eV and 55.1 eV, indicating the state of Se2-.31 Moreover, the detailed compositional analysis reveals that the surface Mo:Se atomic is 12.99:6.42, which is close to the formula of MoSe2.

Fig. 3  XRD patterns (a) and (b) Raman spectra of HCS, pure MoSe2 nanosheets, and HCS@MoSe2, (c)ATR-FTIR spectra of HCS and HCS@MoSe2.

 

Fig. 4  XPS spectra of HCS@MoSe2 composite: (a) survey spectrum, (b) C 1s, (c) Mo 3d, (d) Se 3d.

3.2. Electrocatalytic properties evaluation

     The HER measurements of core-shell HCS@MoSe2 hybrids, the pure MoSe2 and HCS were performed in a three-electrode setup using a H2SO4 solution (0.5 M) at room temperature. In general, an ideal HER catalyst is capable of providing a high current at a low overpotential, along with a low onset potential. Firstly, the electrochemical activity of different materials was measured by LSV in Fig. 5a. As expected, the Pt/C (10 wt%) catalyst exhibited extremely high HER activity with a near zero onset potential and a large current density. On the contrary, the HCS have nothing to be with HER activity since there is almost no reduction current even at the electrode potential reaching to 0.35 V (vs. RHE). Pure MoSe2 exhibited poor HER activity with the lower catalytic current and more negative onset potential due to its severe aggregation and low conductivity. The cathodic polarization curve of the HCS@MoSe2 hybrids shows the best current density and exhibit a lower overpotential of 200 mV at a current density of 10 mA cm-2, which is much smaller than that of pure MoSe2 (10 mA cm-2 at 300 mV) and better than reported MoSe2 based electrocatalysts in acidic solution (Table 1).

 

Table 1. Comparison of the HER activity of HCS@MoSe2 with the reported MoSe2 based electrocatalysts in acidic solution.

Catalyst

Electrolyte

Current density

j (mA cm-2)

Corresponding overpotential

(mV)

Refs.

MoSex/Mo

10

270

32

MoS2(1−x)Se2x

10

273

33

MoSe2/CC

6

220

34

MoSe2 nanoflower

10.96

300

35

HCS@MoSe2

10

200

This work

 

    The excellent performance of HCS@MoSe2 hybrids from the uniformly distribution of MoSe2 nanosheets on the surface of HCS, which exposes much more active edges and provides abundant catalytic sites for the HER process, thus leading to obtain optimized HER catalytic activity. The Tafel slope, which can be deduced from the Tafel equation:

h = blog(j) + a

where h is the overpotential, j is the current density and b is the Tafel slope. The Tafel slope is always correlated with the reaction pathway and the adsorption type,36 which reveals inherent property of catalyst materials and indicates the rate-determining step in the whole HER process. Besides, a small Tafel slope is preferred as it means a faster increase of hydrogen generation rate with increasing overpotential applied.37 Tafel slopes for the HCS@MoSe2 electrode was about 74 mV dec-1, which was smaller than that of the pure MoSe2 (100 mV dec-1) in Fig. 5b, indicating that the HCS@MoSe2 electrode has a faster kinetics of hydrogen generation. According to the mechanism of hydrogen evolution, the HER process mainly contains three possible elementary reactions in acidic medium. The Volmer reaction can also be called discharge reaction (H++e-→Hads), which usually has a high Tafel slope of 120 mV dec-1; Heyrovsky reaction (electrochemical desorption): Hads+H++e-→H2 and Tafel reaction (Recombination): Hads+Hads→H2 may also occur in the HER process, at which the Tafel slope value will be significantly reduced to 40 mV dec-1 and 30 mV dec-1 respectively.38 Therefore, the HER process of the HCS@MoSe2 electrode can be considered as the Volmer-Heyrovsky mechanism based on the principle of HER dynamics, and the Volmer reaction is used as the rate determining step.

    Electrochemical impedance spectroscopy (EIS) is an important technique for studying the interfacial reaction and electron transfer kinetics. To understand the electrode kinetics during HER, the measurement was performed for HCS@MoSe2 hybrids and pure MoSe2. Fig. 5c displays the Nyquist plots of two catalysts and a facile Randle equivalent circuit is plotted in the inset to model the impedance data, in which the observed semicircle is primarily attributed to the charge transfer resistance (Rct) of the H+ reduction at the electrode-electrolyte interface. A lower Rct corresponds to a faster electron transfer. The Rct of HCS@MoSe2 hybrids is about 8.9 Ω, which is remarkably reduced as compared to those of pure MoSe2 (52 Ω). This demonstrates that HCS core dramatically enhance the electron transfer.

     Electrochemical effective surface area (ECSA) is commonly used to evaluate the active sites of electrocatalysts. To further understand the enhanced catalytic activity of HCS@MoSe2 hybrids, the double-layer capacity (Cdl) is measured by simple cyclic voltammetry (Fig. S3), which has been proved to be an effective parameter to estimate the electrochemically active surface area of the catalyst.39 The positive and negative current density differences (ΔJ=ja-jc) are plotted at a given potential of 0.3V versus RHE against the scan rate, where half of the slope value is Cdl in Fig. 5d. It is worth noting that the HCS@MoSe2 hybrids (4.1 mF cm-2) have almost twice the capacitance of pure MoSe2 nanosheets (2.3 mF cm-2), which means that the growth of MoSe2 nanosheets uniformly on HCS can increase the number of active electrocatalytic sites and improve catalytic performance.

Fig. 5 (a) LSV polarization curves for HCS, pure MoSe2 nanosheets, and HCS@MoSe2 hybrid modified GCE in 0.5 M H2SO4 solution. Scan rate: 5 mV s−1; (b)The Tafel plots of the commercial Pt/C, pure MoSe2 nanosheets and HCS@MoSe2 respectively; (c)EIS Nyquist plots for pure MoSe2 nanosheets and HCS@MoSe2 modified electrodes; The inset shows the simplified Randles equivalent circuit; (d) Variation of double-layer charging currents at 0.3 V as a function of the voltage scan rate.

 

   The durability is another major concern for an electrocatalyst. A potential long-term cycling is performed by taking continuous cyclic voltammograms (CVs) at a sweep rate of 0.1 V s-1 for 1000 cycles from -0.34V to 0.06V (vs. RHE). The current density of the HCS@MoSe2 hybrids decreased slightly compared with its initial state, demonstrating its good stability during long term cycling.

Fig. 6 Stability test of the HCS@MoSe2 composite.

 

4. Conclusions

     In summary, the MoSe2 nanosheets were successfully grown on HCS by a three-step method, which displayed a highly efficient HER performance. The MoSe2 nanosheets were uniformly distributed on HCS which expose more active sites, and the HCS core provide a beneficial conductive environment for fast electron-transfer. When tested as electrode materials for HER, HCS@MoSe2 have a small Tafel slope of 74 mV dec-1, a small overpotential of 200 mV at a current density of 10 mA cm-2, and a good long-term durability. Therefore, our research provides a promising pathway for synthesis of efficient electrocatalysts, which have the potential to replace Pt-based catalysts.

 

Supporting Information

Additional Figs. S1-S3.

Acknowledgements

The authors gratefully acknowledge the financial support offered by NSFC Grants (21373152).

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