English

• Molybdenum disulfide (MoS₂) is one of the promising substitutions to Pt catalysts for electrochemically splitting water in hydrogen evolution reaction (HER)^[1]. Because the HER activities of MoS₂ nanosheets mainly originate from sulfide edges^[2-3], their practical applications therefore depend not only on activating their surfaces mainly exposed in inert (002) facets^[4-7] but also on improving the cycle stability of 2D MoS₂ nanosheets of only one to several atomic layers in thickness^[8]. Toward this end, scientists have strived to tailor the microstructures and composition of 2D MoS₂ nanosheets to further boost their HER activities^[9-15]. It is demonstrated that O-doping can activate inert basal (002) facets of semiconductor MoS₂ nanosheets in the 2H phase^[16]. 3D MoS₂ nanoboxes showed enhanced HER activities after being modified with MS (M: Ni and Co) shells onto their surfaces to form M-MoS₃ hybrids^[17-18]. The HER performance of the basal plane of MoS₂ can be highly improved by creating sulfur vacancies (MoS_2-x)^[19], which is attributed to the regulation of hydrogen adsorption-free energy (ΔG_H) for strained surfaces^[20]. Pető's group found that oxygen atoms spontaneously incorporate into the basal plane of MoS₂ single layers during ambient exposure, which results in substantially increasing the catalytic activity of the entire MoS₂ basal plane for HER^[21]. The HER performance from multilayered MoS₂ can be divided into two domains corresponding to "point defects" at low concentrations of surface sulfur vacancies and large regions under coordinated Mo atoms for high concentrations of surface sulfur vacancies^[22]. The extensive research activities, however, have mainly concentrated on tailoring the microstructures and composition of 2D MoS₂ nanosheets. To the best of our knowledge, there is no report in the literature on modulating the O-doping amount to activate 3D hierarchical MoS₂ materials.

  1.   Experimental

  1.1   Materials

  The main reagents included sodium molybdate dihydrate (Na₂MO₄·2H₂O, 98.0%, AR, Aladdin), hexadecyl trimethyl ammonium bromide (CTAB, 98.0%, AR, Aladdin), n-butanol (C₄H₉OH, 98.0%, AR, Aladdin), ethylene glycol ((CH₂OH)₂, 98.0%, AR, Aladdin), hydrochloric acid (HCl, 36%-38%, AR), thiourea (CH₄N₂S, 99.8%, AR, Aladdin). The deionized water (18.25 MΩ) was utilized for the entire experiment. All reagents were used without further purification.

  1.2   Synthesis of MoS₂ core-shell spheres with different O-doping amounts

  Precursors (MoS₂ core-shell superspheres) composed of quasi-molecular superlattices were first synthesized by performing a one-pot approach developed in our lab^[23-25]. Typically, 300 mL aqueous solution of Na₂MoO₄ (1 mmol, 0.242 0 g) was first added to 100 mL n-butanol solution of CTAB (15 mmol, 5.466 8 g) and stirred for 2 h. Ethylene glycol (50 mL) and concentrated hydrochloric acid (1.0 mL, 11.64 mol·L^-1) were then introduced into the reactor in order and stirred for another 2 h. Thiourea (15 mmol, 1.141 8 g) was added into the solution and further stirred for 3 h. The milky white mixture was finally transferred into an autoclave with a polytetrafluoroethylene (PTFE) liner and kept at 220 ℃ for 24 h. The black precipitates were separated by filtration, washed with distilled water and absolute ethanol three times each, and dried at 60 ℃ for 24 h to obtain monodispersed MoS₂ core-shell superspheres (0.127 5 g; yield: 85%). The precursor was subsequently annealed in Ar for 5 h by elevating the temperature to 900 ℃ at a heating rate of 20, 10, 5, and 2 ℃·min^-1 to obtain MoS₂ core-shell spheres assigned as MoS₂-O-20, MoS₂-O-10, MoS₂-O-5, and MoS₂-O-2, respectively.

  1.3   Characterization

  The structures of the materials were characterized by using transmission electron microscopy (TEM, JEM-2100 operated at 200 kV, Japan). The field emission scanning electron microscopy (FESEM) was completed on a JSM-7001F operated at 15 kV. The thermogravimetric-differential thermal analysis (TG-DTA) of the materials was performed with a TG/DTA instrument (SDTQ600, Delaware) working at a constant heating rate of 2 ℃·min^-1 in Ar. To analyze the molecular structure and chemical composition of samples, FTIR spectra (Nicolet iS5, Thermo Fisher Scientific, USA) were collected in the range of 400-4 000 cm^-1. The XPS spectra of the materials were collected on a Thermo Scientific Escalab (ESCALAB 250Xi, Massachusetts) with monochromatized Al Kα X-ray as the excitation source.

  1.4   Assemblies of electrodes and electrochemical tests

  All electrochemical measurements were carried out at a CHI660E electrochemical workstation (Shanghai Chenhua Company) at room temperature. The precursor and as-prepared MoS₂ core-shell spheres were applied for assembling electrodes to investigate their HER performance in a standard three-electrode system with glassy carbon electrode coated with active materials as working electrode, Pt wire as counter- electrode, and Ag/AgCl electrode as reference electrode, respectively. The working electrodes were made by casting a colloid solution of active materials (1 mg·mL^-1) to glassy carbon electrodes of 3 mm diameter, then evenly dispersing 10 μL Nafion-water solution (0.01%, w/w) and finally keeping at room temperature overnight to dry their surfaces. The loading amount of the materials was 0.32 mg·cm^-2. All polarization curves were measured in 0.5 mol·L^-1 H₂SO₄ (pH=0) solution at a scan rate of 5 mV·s^-1 from -0.3 to 0.1 V (vs RHE). The closed cyclic voltammetry (CV) curves of the materials with different scanning rates from 20 to 200 mV·s^-1 were performed to investigate their double-layer capacitance in acid media (0.5 mol·L^-1 H₂SO₄). All electrochemical impedance spectroscopy (EIS) curves were measured in an acidic solution of 0.5 mol·L^-1 H₂SO₄ with an initial potential of 0.1 V (vs RHE) and a frequency of 10^-2-10⁶ Hz. The measured Tafel slope of 28 mV·dec^-1 for the Pt/C electrode agrees well with the values reported previously^[26].

  2.   Results and discussion

  Our group has developed a one-pot technique for producing functional materials with well-controlled size and shape, such as MoS₂ core-shell superspheres composed of quasi-molecular superlattices^[23]. The special molecular superlattices assembled with surfactant molecules (CTAB)₂S_y (y=1 or 2) and MoS₂ atomic layers offer a unique platform for us to further tailor their microstructures and compositions. The MoS₂ core-shell superspheres were therefore synthesized with the one-pot approach by simply enlarging the scale (Fig.S1, Supporting information) and applying it as precursors to produce MoS₂ core-shell spheres. While the precursor superspheres produced at an enlarged scale maintained a similar size and morphology (Fig.S2) in comparison with those reported previously, the oxygen doped into the precursors dramatically increased to ca. 23.1% based on XPS analyses (Fig.S3). The MoS₂ core-shell spheres with different O-doping amounts were produced by regulating the heating rates. The low- and high-magnification FESEM images (Fig.S4a-S4i) reveal that the as-prepared materials retained highly uniform spherical structures at the heating rate of 10, 5, and 2 ℃·min^-1, in contrast to the collapsed spheres at the heating rate of 20 ℃·min^-1 (Fig.S4j-S4l). The materials were further analyzed with XPS (Fig. 1a and Fig.S5-S7). Specifically, the Mo3d XPS spectra of MoS₂-O-20, MoS₂-O-10, MoS₂-O-5, and MoS₂-O-2 (Fig. 1a) mainly consist of two strong peaks at 229.08 and 232.28 eV, corresponding to Mo⁴⁺3d_5/2 and Mo⁴⁺3d_3/2 of 2H-MoS₂, respectively. A couple of weak peaks at 232.52 and 235.72 eV ascribe to Mo⁶⁺3d_5/2 and Mo⁶⁺3d_3/2 of MoO₃, indicating that oxygen atoms have incorporated into the lattices of 2H-MoS₂ nanosheets. The dynamic evolution of the O-doping amount (Fig. 1b) in MoS₂ core-shell spheres can be evaluated with the deconvolution of Mo element corresponding to MoO₃ and 2H-MoS₂ in Mo3d XPS spectra. It is interesting to find out that the O-doping amounts (Fig.S3) linearly declined from 23.1% of the precursors to 5.5% of MoS₂-O-5, as regulating the heating rate from 20 to 5 ℃·min^-1. The O-doping amount, however, approached a terrace at 5 ℃·min^-1, and it maintained at a similar level of 6.2% further lowering the rate to 2 ℃·min^-1. Compared to the precursors, the O-doping amount has been successfully modulated to a lowered level in all MoS₂ core-shell spheres after annealing at 900 ℃. It is convenient and effective to tune the O-doping amount in MoS₂ core-shell spheres by adjusting the heating rate. The lower rate results in a lower concentration of O doped in MoS₂ core-shell spheres. The inset in Fig. 1b shows TEM images of precursors and as-prepared materials. All of the particles inherit the core-shell spherical structures of precursors (Fig.S8-S10), except MoS₂-O-20 (Fig.S11) collapsed at a too-high heating rate. The EDS spectra of the materials also confirm the tuned O-doping amount and sulfur amount (Fig.S8e, S9e, S10e, S11e, and S12-S14). The results agree well with those from XPS. It is noteworthy that while all of the materials were annealed in Ar, the O-doping amount can be modulated without additional sulfur resources introduced into the processes. The decrease of the O-doping amount can be only attributed to the anion exchange reaction in situ taking place inside the precursors. Therefore, it is necessary to scrutinize the special microstructures of precursors to better understand the dynamic modulation of O-doping.

  Figure 1

  

  Figure 1.  (a) High-resolution Mo3d XPS spectra of MoS₂ core-shell spheres; (b) O-doping amounts in MoS₂ core-shell spheres evaluated with deconvolution of the XPS spectra

  Inset: TEM images of precursor superspheres and 3D MoS₂ core-shell spheres.

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  The TG-DTA profile (Fig. 2) of precursors revealed that they lost their weight by ca. 13.1% by elevating the temperature to 350 ℃, which can be attributed to the decomposition of organic surfactant molecules (CTAB)₂S_y (y=1 or 2) incorporating between MoS₂ atomic layers to form quasi-molecular superlattices. In addition, the characteristic absorptions ascribed to surfactant molecules also disappeared in the FTIR spectrum (Fig. 3) of MoS₂-O-2. The results are consistent with our previous observation on the formation of quasi-molecular superlattices consisting of (CTAB)₂S_y (y=1 or 2) and MoS₂ atomic layers. Therefore, we suggest that anion exchange reaction directly takes place in the heating process, because sulfur-related species could be released by the decomposition of (CTAB)₂S_y (y=1 or 2) and in-situ react with O-dopants. The higher the heating rate, the faster the decomposition of (CTAB)₂S_y, which results in a lower chance for in-situ performing anion exchange reaction. The O-doping amounts therefore linearly declined from 23.7% of precursor to 5.5% of MoS₂-O-5 with the decreasing of heating rate. Both the surfactant molecules incorporated into MoS₂ atomic layers and the initial amount of O-doped onto precursors could dominate the final level of O-doping amount together in the annealing process. The O-doping amount finally approached a terrace to MoS₂-O-5 and it cannot be further reduced, while the heating rate was lowered to 2 ℃·min^-1. Based on the in-situ anion exchange reaction, the amount of O has been successfully modulated by simply adjusting the heating rate. The finding could open a new pathway for tailoring the microstructures and composition of 3D hierarchical MoS₂ materials to further optimize their functionalities.

  Figure 2

  

  Figure 2.  TG-DTA curves of precursor superspheres

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  Figure 3

  

  Figure 3.  FTIR spectra of precursor super spheres and MoS₂-O-2

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  The electrochemical performance of materials was evaluated in acid media to investigate the effect of the O-doping amount on their HER activities. Fig. 4a shows the linear sweep voltammetry (LSV) curves of precursors, MoS₂-O-2, MoS₂-O-5, MoS₂-O-10, and MoS₂-O-20. Impressively, MoS₂-O-2 and MoS₂-O-5 electrodes had similar polarization profiles and both presented the highest current density in comparison with the precursors, MoS₂-O-10, and MoS₂-O-20 electrodes. The very close overpotentials of MoS₂-O-2 (246 mV) and MoS₂-O-5 (247 mV) at 10 mA·cm^-2 were the lowest, compared to MoS₂-O-10 (259 mV), MoS₂-O-20 (283 mV), and precursors (385 mV). Fig. 4b shows the Tafel curves of the electrodes. Compared to precursor (184 mV·dec^-1), MoS₂-O-10 (102 mV·dec^-1), and MoS₂-O-20 (123 mV·dec^-1), MoS₂-O-2 and MoS₂-O-5 also showed the lowest Tafel slopes (90 and 91 mV·dec^-1, respectively). The TEM and HRTEM images (Fig.S15) showed the structural stability of MoS₂-O-2 after the LSV tests. In addition, the MoS₂-O-2 and MoS₂-O-5 electrodes also exhibited superior catalysis activities in comparison with MoS₂ micro boxes consisting of single crystalline- oriented MoS₂ nanosheets with overpotential of 300 mV at 10 mA·cm^-2 and Tafel slope of 134 mV·dec^-1, as well as MoS₂/C hollow spheres with overpotential of 479 mV at 10 mA·cm^-2 and Tafel slope of 274 mV·dec^{-1 [27]}. The double-layer capacitance (C_dl) (Fig. 4c) was also calculated from CV curves to estimate the electrochemically active surface area (ECSA) (Fig.S16) of the different electrodes. The MoS₂-O-2 electrodes exhibited the largest C_dl (5.7 mF·cm^-2), and MoS₂-O-20 delivered the lowest C_dl value. The EIS spectra of the materials (Fig. 4d) can help to further understand the electrode kinetics over a frequency range from 100 kHz to 0.1 Hz at an amplitude of 5 mV. The spectra all consist of a depressed semicircle of charge transfer resistance (R_ct) in the high-frequency region and a slope of Warburg impedance (Z_w) in the low-frequency region. The MoS₂-O-2 electrode exhibited the lowest R_ct, compared to others with higher O-doping amounts, suggesting that a suitable O-doping amount can substantially boost the charge transfer from catalyst to electrode. The slope of the MoS₂-O-2 electrode at the low-frequency region was higher than 45°, indicating that MoS₂-O-2 can store charges efficiently. The cycle stabilities of MoS₂-O-2 electrodes were also evaluated by continuously conducting linear CV. The potential and current density of the electrodes all remained unchanged before and after 2 000 cycles at a scan rate of 20 mV·s^-1 (Fig.S17). The excellent long-term cycling stabilities of the electrodes can be attributed to the stable 3D hierarchical structures of the materials. The results indicate that the electrochemical performance of MoS₂ core-shell spheres is closely related to the O-doping amount, and it can be optimized and highly improved by tuning the O-doping amount and tailoring their 3D hierarchical structures. O-doping can affect the electrochemical performance of MoS₂ materials significantly.

  Figure 4

  

  Figure 4.  (a) LSV curves, (b) Tafel plots, (c) C_dl values, and (d) EIS spectra of materials

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  3.   Conclusions

  In summary, based on the in-situ anion exchange reaction, the O-doping amount of MoS₂ core-shell spheres can be tuned dynamically by regulating the heating rate. The lower rates result in lower O-doping amounts. While it is still a challenge to thoroughly understand the mechanism of modulating the O-doping amount and further tune it to a lower level, this approach may open a novel pathway to modulate diverse dopants onto 3D hierarchical materials through creating molecular superlattices to perform in-situ reactions.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  (a) High-resolution Mo3d XPS spectra of MoS₂ core-shell spheres; (b) O-doping amounts in MoS₂ core-shell spheres evaluated with deconvolution of the XPS spectra

  Inset: TEM images of precursor superspheres and 3D MoS₂ core-shell spheres.

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  Figure 2  TG-DTA curves of precursor superspheres

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  Figure 3  FTIR spectra of precursor super spheres and MoS₂-O-2

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  Figure 4  (a) LSV curves, (b) Tafel plots, (c) C_dl values, and (d) EIS spectra of materials

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