In this paper, we report the synthesis of core− shell MoS2@CoO bifunctional electrocatalysts

Institute for Advanced Materials, South China Academy of Advanced Optoelectronics and National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

§Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, P. R. China

*^S Supporting Information

ABSTRACT: Core−shell nanostructured materials with synergetic effects have gained increasing attention for their widespread applications in electrochemical water-splitting field. However, for most electrocatalysts, the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can only work efficiently in neutral solution or alkaline solution, which hinders their further applications using untreated natural waters with a wide pH range, such as seawater (pH

∼ 7.5−8.5). In this paper, we report the synthesis of core−

shell MoS₂@CoO bifunctional electrocatalysts. The MoS₂@CoO-coated carbon cloth electrode demonstrates excellent electrocatalytic properties for both OER and HER with 325 and 173 mV at a current density of 10 mA cm⁻²and Tafel slopes of 129.9 and 83.0 mV dec⁻¹in 1 M KOH solutions, respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH = 7), showing good OER and HER activities for potential electrocatalytic water splitting applications in untreated natural waters.

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INTRODUCTION

Electrochemical water-splitting for clean energy production of hydrogen (H₂) and oxygen (O₂) can be one of the effective alternatives to fossil fuels, which has been increasingly exploited against environmental pollution, greenhouse effect, and energy shortage.¹⁻⁴H₂, as a clean and high-energy-density energy source, can promise to reduce carbon dioxide (CO₂) emission and cope with the problems of energy shortage. On the other hand, O₂can be applied in rechargeable metal−air batteries and regenerative fuel cells.⁵⁻⁷ Basically, water electrolysis devices require cost-effective electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with high current densities at low overpotentials.⁶ However, to the best of our knowledge, the most effective OER and HER electrocatalysts are ruthenium/

iridium oxides and platinum, respectively.^6,8 The limited reserves and high costs of these noble metals and noble metal oxides hinder their widespread applications. Further- more, most electrocatalysts can be only operated in alkaline mediums for OER and HER and a few electrocatalysts are capable of working under neutral conditions.^6,9−12Therefore, developing non-noble-metal bifunctional electrocatalysts with both high efficiency and ability to work in both alkaline and neutral solutions is highly urgent and also desirable.

In recent years, transition-metal dichalcogenides, transition- metal phosphides, transition-metal oxides, graphene-based nanocompounds, and carbon nitride-based materials have emerged as novel non-noble-metal-based OER or/and HER catalysts.¹³⁻²⁴ Especially, building bifunctional catalysts for both OER and HER with a desirable electrocatalytic synergetic effect working in a wide pH range has been one of the research focuses.^6,25Zhu et al. reported core−shell Co₈S₉@MoS₂with excellent HER and OER performances in 1 M KOH solutions due to their synergistic effect.²⁵Li et al. demonstrated that in situ oxidized CoO domains on CoSe₂nanobelts could produce a synergistic effect to enhance OER and HER in neutral media.²⁶Furthermore, the stabilities in their own systems have been greatly improved due to the synergistic effect. As far as we know, the synergistic effect of core−shell MoS₂@CoO for electrocatalytic water-splitting in alkaline and neutral media has not been reported.

Herein, we report the synthesis of bifunctional core−shell MoS₂@CoO electrocatalysts for water-splitting in alkaline and neutral solutions. The MoS₂@CoO consists of a CoO nanocrystal core wrapped by a shell of 2H-MoS₂ nanoflakes.

Received: November 15, 2018 Revised: February 20, 2019 Published: February 20, 2019 Downloaded via NANJING UNIV on April 3, 2019 at 14:00:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Such a core−shell hybrid nanostructure dramatically enhances the electrocatalytic performances compared to those of pure CoO nanocrystals and 2H-MoS₂ flakes. Specifically, the electrode of MoS₂@CoO-coated carbon cloth demonstrates excellent electrocatalytic properties for both OER and HER:

325 and 173 mV at a current density of 10 mA cm⁻²and Tafel slopes of 129.9 and 83.0 mV dec⁻¹in 1 M KOH solutions (pH

= 14), respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH = 7), showing good OER and HER activities for potential electrocatalytic water-splitting applications in untreated natural waters.

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EXPERIMENTAL SECTION

Materials Synthesis. CoO was synthesized by a hydro- thermal method. Cobaltous acetate tetrahydrate (Co- (CH₃COO)₂·4H₂O, 1 mmol) served as a Co source, and triphenylphosphine (P(C₆H₅)₃, 10 mmol), as sacrificial agent, which were dissolved in 35 mL of oxidized oleylamine as an O source and a solvent. The mixture was stirred for 3 min with a glass rod to form a uniform system. Then, the mixture was poured into a reaction still and transferred to an oven at 180

°C for 18 h. Thefinal product was obtained by a centrifugation rate of 5000 rpm and washed with absolute ethyl alcohol three times, followed by drying naturally at room temperature for 10 h, and we finally obtained ∼50 mg CoO. Then, 20 mL of absolute ethyl alcohol was added into the as-prepared CoO;

then, 2H-MoS₂flakes with different masses were added in it.

The mixture was sonicated for 60 min and then annealed at 450°C in Ar gas to form core−shell MoS₂@CoO.

Co(CH₃COO)₂·4H₂O and 2H-MoS₂flakes were purchased from Shanghai Macklin Biochemical Co., Ltd. P(C₆H₅)₃ and oleylamine were purchased from Beijing Energy Engineering Technologies Co., Ltd. All of the chemicals were used without further purification.

Electrode Preparation. The carbon cloth electrode was cut into 3 × 10 mm² and orderly washed for 5 min by deionized water, acetone, and absolute ethyl alcohol with the assistance of sonication and blow-dried by N₂prior to use. The prepared MoS₂@CoO mixture was sonicated for 10 min and was dropped onto the carbon cloth to form a ∼2 mg cm⁻² catalyst work electrode. The fabrication processes of MoS₂and CoO electrodes were similar. Then, the electrodes were separately annealed at 450°C for 30 min in Ar (99.999%) to

remove solvents and make the catalysts contact the substrates tightly.

Characterization. Scanning electron microscopy (SEM) images were obtained with afield emission scanning electron microscope, ZEISS Gemini 500. Powder X-ray diffraction (XRD) patterns were obtained on Bruker D8-Advance using Cu Kα radiation, PANalytical X’Pert PRO. Raman spectrum was used with a Renishaw in Via Raman microscope.

Transmission electron microscopy (TEM, JEM-2100HR) and X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi) were also used to study the crystalline structure and surface composition.

Electrochemical Measurement. Electrochemical meas- urements were conducted with a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai). The electro- chemical cell was set up with a three-electrode system: a saturated calomel electrode (SCE) as the reference electrode, a Pt wire (OER) or carbon rod (HER) as the counter electrode, and the sample-coated carbon cloth electrode as the working electrode. The electrolyte solutions used were fresh 1 M KOH (pH = 14) or 1 M phosphate-buffered saline (PBS, pH = 7) or 1 M electrolyte adjusted by them both. Before measurements, the electrolyte solutions were saturated with N₂. Reversible hydrogen electrode potentials were calculated by potential vs RHE (V) = E vs SCE + E^θSCE + 0.059pH (V). All electrochemical data were presented withoutiRcompensation.

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RESULTS AND DISCUSSION

Electrocatalyst Preparation and Characterization.

The fabrication process of CoO nanocrystals was similar to a previous report.²⁷ Distinguishingly, we treated the hydro- thermal products in the atmosphere at room temperature. A two-step preparation route was applied to synthesize the core− shell MoS₂@CoO. First, the CoO nanocrystals were prepared by a hydrothermal method, and the core−shell MoS₂@CoO was further obtained by ultrasonication of a mixture solution of 2H-MoS₂ flakes and CoO nanocrystals. To verify the core− shell hybrid configuration, SEM and TEM were carried out. As shown in Figure 1a,b, with different magnifications, the as- prepared CoO nanocrystals have a big size distribution ranging from several tens to several hundreds of nanometers. By comparison, as shown in Figure 1c,d, besides the CoO nanocrystals having similar morphologies after sonication and Figure 1.SEM images of (a, b) CoO nanocrystals and (c, d) MoS₂@CoO with different magnifications. (e) TEM image of a single core−shell MoS₂@CoO; the marked surface area is illustrated in (f) HR-TEM image of multilayered 2H-MoS₂coating.

The Journal of Physical Chemistry C

annealing, most surfaces of the CoO are covered by a rough outside layer, which consists of ultrasonically broken MoS₂ nanoflakes. The core−shell hybrid nanostructures have larger specific surface areas than the pure nanocrystals, which could probably improve the corresponding electrocatalytic perform- ances.^25,28,29Moreover, the two types of distinct morphologies of CoO nanocrystals can be recognized as octahedron and spherical polyhedron. These two types of nanocrystals have large portions of flat facets, which should facilitate the wrapping by two-dimensional (2D) MoS₂nanoflakes. A single core−shell MoS₂@CoO was illustrated in the TEM image of Figure 1e, with a typical spherical polyhedron CoO core. A selected thin surface area was marked to further perform the high-resolution (HR) TEM observation (as shown in Figure 1f). Clearly, a multilayered 2D MoS₂tightly covered the CoO surface. The lattice fringes with a measured lattice spacing of 0.62 nm correspond to the (002) planes of 2H-MoS₂.³⁰The microscopy observations indicate that the ultrasonic mixing

and annealing can successfully transform CoO nanocrystals and 2H-MoS₂flakes into the core−shell MoS₂@CoO hybrid system.

To further verify the crystalline structure and valence state of elements, XRD, Raman, and XPS measurements were conducted. Figure 2a presents XRD patterns of 2H-MoS₂ flakes, as-prepared CoO nanocrystals, and core−shell MoS₂@ CoO, respectively. 2H-MoS₂flakes gave 15 strong diffraction peaks, corresponding to the (002), (004), (100), (101), (102), (103), (006), (105), (106), (110), (008), (107), (108), (203), and (116) planes of 2H-MoS₂ (PDF#37-1492), respectively.

Four strong diffraction peaks corresponding to (111), (200), (220), and (311) planes (PDF#65-2902) can be observed in CoO nanocrystals. After the ultrasonic mixing, the two strongest diffraction peaks corresponding to (002) and (103) planes of 2H-MoS₂flakes and all the diffraction peaks of CoO nanocrystals can be found in core−shell MoS₂@CoO. Raman spectroscopy was also conducted to further identify the Figure 2.(a) XRD patterns of MoS2, CoO, and MoS2@CoO. (b) XPS measurements of MoS2@CoO transferred on SiO2/Si substrate.

Figure 3. (a) OER polarization curves of different electrodes in N2-saturated 1 M KOH solution with a scan rate of 5 mV s⁻¹ and (b) corresponding Tafel plots. (c) HER polarization curves of different electrodes in N₂-saturated 1 M KOH solution with a scan rate of 2 mV s⁻¹and (d) corresponding Tafel plots.

composition (seeFigure S1). The characteristic peak positions of 2H-MoS₂were located at 384 and 410 cm⁻¹, and the ones belonging to CoO were found at 555, 675, and 1087 cm⁻¹, respectively.^31,32Furthermore, the existence of both MoS₂and CoO on the surface of the core−shell hybrid was confirmed by the XPS measurements illustrated inFigure 2b. Obviously, the peaks at 796.4 and 781.2 eV were attributed to the binding energies of Co 2p_1/2and Co 2p_3/2. The satellite peaks at 785.6 and 802.9 eV suggested the Co 2p came from Co²⁺ in CoO.^26,33 This confirmed the existence of Co²⁺ on MoS₂@ CoO. The O 1s spectrum was presented, with the peak of 530.1 eV belonging to O²⁻and the peak at 532.7 eV attributed to O−Si binding energy on the SiO₂/Si substrate.³⁴The peaks of 232.8 and 229.5 eV in Mo 3d spectrum correspond to Mo 3d_5/2and Mo 3d_3/2, respectively, and the peak of 226.4 eV was assigned to S 2s.³⁵ The binding energies of S at 163.4 and 161.8 eV correspond to S 2p_1/2and 2p_3/2, respectively.³⁵The XPS results were well consistent with the XRD and Raman characterizations.

Electrocatalytic OER and HER Performances. To optimize the electrocatalytic performances of core−shell MoS₂@CoO in a N₂-saturated 1 M KOH solution, different mass ratios of MoS₂to CoO were analyzed (seeFigure S2a).

Clearly, the MoS₂@CoO electrocatalyst with a mass ratio of CoO/MoS₂= 50:8 displayed the best electrocatalytic perform- ance, with a loading density of 2.0 mg cm⁻² on the carbon cloth. Meanwhile, the electrocatalytic performances of the working electrodes with different loading densities of the optimal MoS₂@CoO were compared (seeFigure S2b) and the loading density of 2.0 mg cm⁻² corresponds to the best electrocatalytic performance, indicating an optimal coverage of the electrocatalyst on the carbon cloth.

First, we evaluated the OER activities from different electrodes of core−shell MoS₂@CoO, CoO nanocrystals and 2H-MoS₂ flakes with the same loading density of approx- imately 2 mg cm⁻² on carbon cloth, in a N₂-saturated 1 M KOH solution at a 5 mV s⁻¹scan rate. For comparison, RuO₂ and bare carbon cloth were also used as reference electrodes.

Figure 3a presents their polarization curves without iR corrections. Normally, both carbon cloth and MoS₂ flake- coated electrodes showed poor OER performances and the OER performance of the CoO-coated electrode was much worse than that of RuO₂ and MoS₂@CoO electrodes.

However, the carbon cloth electrode coated with the optimal MoS₂@CoO displayed a significantly enhanced OER perform- ance, with an onset overpotential of 270 mV and overpotential of 325 mV at 10 mA cm⁻². In comparison, to reach the same current density of 10 mA cm⁻², the CoO electrode, RuO₂, 2H-

MoS₂ flake electrode, and bare carbon cloth required overpotentials of 399, 269, 500+, and 500+ mV, respectively.

Tafel slopes were calculated by the Tafel equationη=blog(j/

(mA cm⁻²)) + a, where η, b, j, and a correspond to the overpotential, the Tafel slope, the current density, and the exchange current density, respectively.¹⁶As plotted in Figure 3b, the MoS₂@CoO electrode demonstrates the smaller Tafel slope of 83.0 mV dec⁻¹ compared with MoS₂@CoO, CoO, MoS₂, and carbon cloth electrodes, showing a comparable performance to that of RuO₂. Besides, stability of the MoS₂@ CoO electrode in OER was investigated (seeFigure S3). After 1000 cycles of potential sweeps in 1 M KOH solution, the polarization curve exhibits a current density loss of about 30%

(vs RHE = 1.7 V) compared with the initial one. The OER current density after 10 h also declined to 80.1% of its starting value. SEM images before the 1st potential sweep and after the 1000th potential sweep were compared (see Figure S4).

Clearly, a part of the catalysts detached from the surfaces of the carbon cloth after 1000 potential sweeps. Meanwhile, the anodic corrosion of MoS₂may have a negative influence on the electrode stability during the OER process.³⁶Besides, XPS and XRD measurements after the 1000 cycles of potential sweeps (seeFigures S5 and S6) have been made to confirm that the element valence and hybrid composition do not change after 1000 potential sweeps.

Their electrocatalytic HER performances were also evaluated in 1 M KOH solution with a scan rate of 2 mV s⁻¹. As illustrated inFigure 3c,d, to reach a current density of 10 mA cm⁻², the carbon cloth, MoS₂, CoO, MoS₂@CoO, and Pt wire electrodes required overpotentials of 500+, 357, 272, 173, and 46 mV, respectively, corresponding to Tafel slopes of 500.1, 235.4, 176.4, 129.9, and 41.3 mV dec⁻¹. Normally, the commercial Pt wire demonstrated the best HER performances.

However, compared to those of the CoO and MoS₂electrodes, both electrocatalytic OER and HER performances of the core− shell MoS₂@CoO electrode were significantly enhanced.

Besides, its electrocatalytic activities for both OER and HER were compared with those of other representative non-noble- metal-based bifunctional electrocatalysts (see Table S1).^{2,3,16,37−40} After 1000 cycles of potential sweeps in 1 M KOH solution (seeFigure S7a), the electrocatalytic perform- ance had declined by about 10% (vs RHE =−0.4 V) than that of the initial one, indicating relatively stable HER activity of core−shell MoS₂@CoO electrocatalyst when working in alkaline solutions, and the HER could be maintained with a negligible degradation for 10 h (seeFigure S7b).

We further investigated electrocatalytic performances of the MoS₂@CoO electrode when working in solutions with pH Figure 4.(a) OER and (b) HER performances of MoS₂@CoO and bare carbon cloth electrodes in 1 M electrolytes with different pH values.

The Journal of Physical Chemistry C

alkaline solutions. The bare carbon cloth presented negligible HER activity, and an overpotential of 176 mV was required for the MoS₂@CoO electrode to arrive at 10 mA cm⁻²in 1 M PBS (pH = 7.0). Besides, stability of the MoS₂@CoO electrode in 1 M PBS was also investigated. The OER performance was gradually attenuated during the 10 h (seeFigure S8), whereas the HER performance was slightly increased (seeFigure S9).

In general, its electrocatalytic activities for both OER and HER have obvious advantages compared to those of other non- noble-metal bifunctional electrocatalysts working in neutral solutions (pH = 7.0) (seeTable S2).11,12,26,41,42

To explain the electrocatalytic mechanism, electrochemical impedance spectroscopy (EIS) had been conducted to analyze the electrode kinetics at 1.6 V vs RHE during the OER process.

Nyquist impedance plots presented three semicircles of different diameters (see Figure S10), which correspond to MoS₂, CoO, and MoS₂@CoO from large to small, respectively, and the inset stands for the equivalent circuit. According to our previous work, the semicircles correspond to charge-transfer resistances (R_ct) at the electrocatalyst/electrolyte interface, which usually reflect the catalytic activity.³⁰ Accordingly, the charge-transfer resistance for MoS₂@CoO was only 6.5 Ω, which is smaller than that of pure CoO (10.2Ω) and MoS₂ (15.6Ω). Since a lower charge-transfer resistance and smaller Tafel slope indicate higher electrical conductivity and faster reaction kinetics, the MoS₂@CoO electrode demonstrated excellent electrocatalytic performances.

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CONCLUSIONS

In summary, bifunctional electrocatalysts of core−shell MoS2@ CoO have been first synthesized through a simple two-step method. The optimized MoS₂@CoO-coated carbon cloth electrode demonstrates excellent electrocatalytic performances for both OER and HER in 1 M KOH, with 325 and 173 mV at a current density of 10 mA cm⁻²and Tafel slopes of 83.0 and 129.9 mV dec⁻¹, respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH = 7).

The non-noble-metal-based core−shell MoS2@CoO electro- catalyst was realized to enhance both OER and HER activities with a capability of working in both alkaline and neutral solutions, making it potential for applications in untreated natural water.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acs.jpcc.8b10954.

Raman spectrum analysis; engineering the optimal ratio and loading of MoS₂@CoO; OER and HERj−t(current density vs time) curves of MoS₂@CoO in 1 M KOH

*E-mail:zzhang@scnu.edu.cn. Phone: +8613660357067.

ORCID

Pengfei Cheng:0000-0002-2848-8891

Qingwei Zhou:0000-0002-9102-1266

Xiaozi Lin:0000-0002-4410-558X

Lingling Shui:0000-0001-8517-1535

Xingsen Gao:0000-0002-2725-0785

Guofu Zhou:0000-0003-1101-1947

Zhang Zhang:0000-0001-6287-502X

Junming Liu:0000-0001-8988-8429 Notes

The authors declare no competingfinancial interest.

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ACKNOWLEDGMENTS

This work was supported by the National Key R&D Program of China (Grant No. 2016YFB0401501), Guangdong In- novative Research Team Program (Grant No. 2013C102), the G u a n g d o n g N a t i o n a l S c i e n c e F o u n d a t i o n ( N o . 2018A030313377), Cultivation project of National Engineer- ing Technology Center (Grant No. 2017B090903008), Xijiang R&D Team (X.W.), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No.

2017B030301007), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), MOE International Laboratory for Optical Information Technologies, and the 111 Project.

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