Sulfurizing-Induced Hollowing of Co9S8 Microplates with Nanosheet

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Sulfurizing-Induced Hollowing of Co9S8 Microplates with Nanosheet Units for Highly Efficient Water Oxidation Huan Liu, Fei-Xiang Ma, Cheng-Yan Xu, Li Yang, Yue Du, Pan-Pan Wang, Shuang Yang, and Liang Zhen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00899 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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ACS Applied Materials & Interfaces

Sulfurizing-Induced Hollowing of Co9S8 Microplates with Nanosheet Units for Highly Efficient Water Oxidation Huan Liu,†,‡, ¶ Fei-Xiang Ma, †,‡,¶ Cheng-Yan Xu,*,†,‡ Li Yang,† Yue Du,†,§ Pan-Pan Wang,†,‡ Shuang Yang,†,‡ and Liang Zhen, *,†,‡,§ †

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

‡

MOE Key Laboratory of Micro-Systems and Micro-Structures Manufacturing, Harbin Institute of Technology, Harbin 150080, China §

School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen Graduate School,

Shenzhen 518055, China

KEYWORDS: Cobalt sulfide, electrocatalysis, oxygen evolution, hollow structures, nanosheets

ABSTRACT: Transition metal-based compounds are promising alternative non-precious electrocatalysts for oxygen evolution to noble metals-based materials. Nanosheets-constructed hollow structures can efficiently promote the electrocatalystic activity, mainly because of their largely exposed active sites. Herein, hierarchical Co9S8 hollow microplates with nanosheet building units are fabricated via sulfurization and subsequent calcination of pre-formed Co-glycolate microplates. Benefited from the advantages of hollow structure, nanosheet units and high Co3+ content, Co9S8 hollow microplates exhibit remarkable catalytic property for oxygen evolution reaction (OER) with low overpotential of 278 mV to reach current density of 10 mA cm-2, low Tafel slope of 53 mV dec-1 and satisfied stability. This construction method of Co9S8 hierarchical hollow microplates composing by nanosheets structure is an effective tactics for promoting OER performance of water splitting electrocatalysts.

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1. INTRODUCTION Oxygen evolution reaction (OER) is a crucial electrochemical reaction in many energy storage and conversion devices, such as regenerative fuel cells, rechargeable metal-air batteries and water splitting, all of which are considered as promising strategies to address energy consumption and environmental pollution.1-4 However, OER is a kinetically sluggish process and remains the bottleneck for related electrochemical applications, thus the development of efficient electrocatalyst is desperate needed.5,6 Although the state-of-art precious RuO2 and IrO2 deliver excellent OER activity, their high price and scarcity discourage their widespread applications.7,8 In recent years, tremendous endeavors have been made on the development of non-precious OER catalysts, such as transition metal (Fe, Co, Ni) based compounds, including oxides,9,10 layered double hydroxides,5, 11 carbides,12 sulfides, selenides etc. with low cost and high activities.13-15 Similar to the literature reported for nickel, iron-based compounds, the surface of cobalt compounds would convert to cobalt oxides/hydroxides under the strong oxidizing environment during the OER process,16 and the derived cobalt oxides/hydroxides usually exhibit better OER performance compared with the pristine cobalt compounds. As such, the unstable chemical structures of cobalt sulfides under ambient condition would facilitate their conversion to cobalt oxides/hydroxides. Therefore, cobalt sulfides, including CoS2, CoS and Co9S8, have drawn extensive attention as promising electrocatalysts for hydrogen evolution reaction (HER),17-19 another half reaction of water splitting, and their potential as efficient OER electrocatalysts has been also reported. For example, Amakrishna et al. designed CoS2/RGO-CNT nanocomposites with synergistic effect, yielding excellent HER electrocatalytic activity with an overpotential of 142 mV at current density of 10 mA cm-2 and good electrochemical stability.20 Cui et al. reported in situ electrochemical oxidation approach tuning of transition metal disulfides to oxides as highly efficient electrocatalysts for enhanced water oxidation.21 Dai et al. reported Co9S8/graphene hybrids with obviously enhanced ORR and OER electrocatalytic properties through N-doping, which effectivley tunes the electronic properties.22


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Geometric structure plays an important role in the active sites of electrocatalysts, because OER are taken place on the surface and/or near-surface of catalyst. Two-dimensional (2D) FeNiS2 ultrathin nanosheets with abundant active sites and high electrical conductivity show high electrocatalytic activity towards OER.23 Hierarchical hollow nanostructures, especially those consisting of nanosheet units, have received more attention because they could not only inherit the advantages of the 2D structures, but also create additional structural merits, including enlarged surface area, which is beneficial for exposing more active sites, as well as robust stability.24,25 Such favorable structures would accelerate the OER processes. Thus, fabricating hierarchical hollow structures is an effective strategy to enhance the electrocatalytic properties of cobalt sulfides. However, there are limited research progresses on the synthesis of hierarchical hollow structures of cobalt sulfides toward OER. In this work, we constructed hierarchical Co9S8 hollow microplates with nanosheet units through sulfurization and subsequent calcination of Co-glycolate microplates as schematically shown in Scheme 1. Sulfurization of Co-glycolate microplates under solvothermal condition using thioacetamide (TAA) as S source leads to formation of Co1-xS hollow microplates, which were then converted to Co9S8 hollow microplates upon calcination under Ar atmosphere. Due to the intrinsic crystallographic anisotropy of hexagonal Co1-xS phase, thin nanosheets were grown on the hollow microplates, forming unique nanosheet-on-microplate architecture. The two-dimensional nanosheets morphology was well retained during the transformation from hexagonal Co1-xS to cubic Co9S8. Such unique hierarchical nanosheet-onmicroplate hollow structures were supposed to expose more surface active sites, effectively boosting the OER performance of Co9S8.


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Scheme 1 Schematic illustration of the fabrication procedure of hierarchical Co9S8 hollow microplates.

2. EXPERIMENTAL SECTION 2.1 Chemicals Cobalt acetate tetrahydrate, thioacetamide (TAA), potassium hydroxide (KOH), methanol, ethyl alcohol absolute and ethylene glycol were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP, WM: ~29000) and Nafion solution (5 wt%) were purchased from SigmaAldrich Chemical Reagent Co., Ltd. 2.2 Synthesis Synthesis of Co-glycorate precursor microplates. In a typical synthesis, 1 mmol cobalt acetate tetrahydrate (250 mg), 125 mg PVP and EG (20 ml) were added into a 50 ml three-necked flask with magnetic stirring for 20 min. The reaction temperature was heated up to 190 oC in 10 minutes. After reaction for 1 h, the solution was naturally cooled to room temperature. The pink product was centrifuged and rinsed with ethanol for several times to remove PVP on the microplates surface, then the Co-glycolate microplates were obtained. The porous Co3O4 microplates were prepared by calcining the Co-glycolate precursor microplates in ambient atmosphere at 350 oC for 2 h. Synthesis of Co1-xS and Co9S8 hierarchical hollow microplates. Hierarchical Co1-xS hollow microplates were obtained via an easy sulfuration process. 37.57 mg thioacetamide was dissolved in 10 ml absolute


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ethanol and stirred 15 min at room temperature. And then Co-glycolate precursor microplates were added into the above solutions with continuous stirring. Thereafter, the above solution was removed into a Teflon-lined stainless-steel autoclave and heated up to 120 oC for 12 h. The sample was centrifuged and rinsed with ethanol repeatedly. Co9S8 hollow microplates were obtained via annealed the Co1-xS hierarchical hollow microplates under Ar atmosphere at 350 oC for 2 h. 2.3 Materials Characterization The morphology and structure of the products was characterized using a FEI Quanta 200F field emission scanning electron microscope (SEM) and JEOL JEM-2100 microscope with accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/Max-γB diffractometer with Cu Kα radiation. X-ray photoelectron spectrometry (XPS) of the product was obtained on a VG Kα Probe spectrometer (Thermo Fisher Scientific) with Al Ka radiation as the excitation source. 2.4 Electrochemical measurements The electrocatalytic activity of OER was performed in a three-electrode system (Wavedrive 20, PINE Research Instrumentation). Ag/AgCl electrode and Pt wire were used as the reference and counter electrode, respectively. A rotating disk electrode (RDE) (PINE Research Instrumentation) with glassy carbon (GC) electrode (5 mm in diameter) dispersed with catalyst samples were used as working electrode with a rotation speed of 1600 rmp. 3 mg of electrocatalyst materials was dispersed in the mixed solution containing 750 uL methanol and 250 uL DI water, with 15 uL 5 wt.% Nafion as the binder. After sonication for 5 min, 21 uL of catalyst ink was distributed onto the GC electrode surface uniformly and dried naturally at room temperature. The loading amount of catalysts is 0.37 mg cm-2. The working electrode was cycled about 20 times to gain the steady CV curves. The linear sweep voltammetry (LSV) with scan rate 2 mV/s was used to get the polarization curves. Chronopotentiometry was conducted under a constant potential at 1.58 V vs. RHE. All the tests were carried out at room temperature. The


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Tafel slope was calculated from polarization curves by the Tafel equation. The overpotentials (η) are calculated based on the formula η = EAg/AgCl + 0.196 V + 0.059pH − 1.23 V. The pH value of 1 M KOH solution was 13.69. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at η = 0.28 V from 100 KHz to 0.01 Hz with an AC amplitude of 5 mV on a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Corp., China). The effective surface areas of catalysts materials were compared by estimating their electrochemical double layer capacitances (Cdl) with CV. CV curves were performed at a potential range of 0.2–0.3 V vs. RHE where no obvious electrochemical features corresponding to the Faradic current were observed. The capacitive currents at 0.26 V were plotted against the scan rate.

3. RESULTS AND DISCUSSION Co-glycolate precursor microplates were first fabricated by a polyol refluxing route as described in the experimental section. Polyvinyl pyrrolidone (PVP) and cobalt acetate tetrahydrate (Co(Ac)2·4H2O) were dissolved completely and refluxed in EG solvent, of which PVP coordinated with Co2+ and as an effective regulator to introduce the formation of Co-glycolate microplates.26 After refluxed for 1 h, uniform Co-glycolate microplates formed with lateral size and thickness of 2–3 μm and 200–300 nm, respectively (Figures S1 and S2). The formation process of Co-glycolate microplates at different reaction time was monitored by SEM (Figure S3a-c). The Co-glycolate microplates were converted from nanospheres to microplates with increasing reaction time. The sample synthesized without PVP consisted of irregular particles (Figure S3d), suggesting PVP plays a critical role in the formation of Coglycolate microplates. In the subsequent sulfuration step, Co-glycolate microplates could be readily converted to hierarchical Co1-xS hollow microplates. The sulfuration of Co-glycolate microplates were conducted under solvothermal conditions using TAA as S source at 120 oC for 12 h. As the inward diffusion rate of S2- anions is slower than the outward transport rate of Co2+ species released from Co-glycolate


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precursor, the unequal diffusion of reacting species would generate a void space inside the core after the sulfuration completed.27-29 The formation of thin Co1-xS nanosheets on the surface of Co-glycolate microplates was attributed to the crystallographic anisotropy of hexagonal phase Co1-xS. The diffusion rate of S2- anions is a crucial factor that determines the hollowing process. TAA has a strong chelating effect with Co2+ in the Co-glycolate, therefore can induce a slow release rate of S2- ions.30 Since the reaction rate was slow, tiny nanosheets appeared on the microplates surface. When Na2S was used as S source instead TAA, solid microplates consisted with tiny nanoparticles were formed, proving the chelation of TAA with Co2+ has a critical role for the formation of hierarchical hollow structure (Figure S4). XRD patterns of Co1-xS sample was presented in Figure 1a. The diffraction peaks at 30.7o, 35.5o, 47.1o and 54.8 o can be assigned to hexagonal phase Co1-xS (JCPDS no. 42-0826), and the additional diffraction peak at 10.7o was attributed to the residual Co-glycolate precursor in the microplates.31 After sulfuration, SEM images clearly showed the hollow morphology of Co1-xS, which kept the size and shape of precursor Coglycolate microplates (Figure 2a and b). The hierarchical hollow microplates were then characterized by transmission electron microscope (TEM). Since hexagonal phase Co1-xS belongs to an anisotropy structure, many random two-dimensional nanosheets grow on the surface of hollow microplates to from a unique hierarchical structure with hollow interiors (Figure 2d and e). In addition, all the diffraction rings in the selected-area electron diffraction (SAED) pattern were readily indexed to the hexagonal Co1-xS phase (Figure 2f).


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Figure 1 XRD patterns of (a) Co1-xS and (b) Co9S8. The standard patterns of Co1-xS and Co9S8 are also presented.

In the subsequent procedure, the prepared Co1-xS hierarchical hollow microplates were calcined in inert atmosphere at 350 o for 2 h to increase the crystallinity, and the new phase Co9S8 were obtained. In Figure 1b, all the diffraction peaks of the calcined product can be readily indexed to the cubic Co9S8 phase (JCPDS no. 19-0364, space group: Fm3m) well, and the crystallinity of calcined product was much better than that of Co1-xS sample. During the calcination, the residual Co-glycolate in the Co1-xS sample would decompose and react with S released from S-rich Co1-xS, resulting in the formation of Co9S8 phase. The morphological information of hierarchical Co9S8 hollow microplates was gained from SEM and TEM images (Figure 2c, g and h). The fabricated hierarchical Co9S8 hollow microplates perfectly inherited the hollow structure of Co1-xS microplates upon further calcination. The diameter and thickness of Co9S8 hierarchical hollow microplates was kept about 2–3 um and 200–300 nm, respectively (Figure S5). The nanosheets densely grown on the outside surface of hollow microplates were about 10–20 nm in thickness. This unique hierarchical hollow structure with thin nanosheets grown on the outside surface of hollow microplates would greatly expose abundant active sites as OER electrocatalyst and benefit for the transportation of electrochemical reaction species. SAED pattern in Figure 2i exhibits a


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polycrystalline nature, and all the diffraction rings readily indexed to the cubic Co9S8 phase, in agreement with the XRD result. The element mapping confirms that Co and S elements were uniformly dispersed in the microplates (Figure S6).

Figure 2 (a, b) SEM images of hierarchical Co1-xS hollow microplates; (c) SEM image of an individual Co9S8 microplate showing the growth of thin nanosheets on the microplate surface. (d-f) TEM images and corresponding SAED pattern of Co1-xS hollow microplates; (g-i) TEM images and corresponding SAED pattern of Co9S8 hollow microplates.

Considering the advantages of large exposed catalytic active sites, both of Co9S8 and Co1-xS hierarchical hollow microplates were evaluated as electocatalysts for water oxidation. For comparison, porous Co3O4 microplates were also prepared upon calcination of Co-glycolate precursor microplates in ambient atmosphere at 350 oC for 2 h (Figures S7 and S8). The polarization curves of Co9S8, Co1-xS hollow


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microplates and porous Co3O4 microplates were compared in Figure 3a, in which the polarization curves of RuO2, Pt/C and bare GC electrode were plotted as reference. All the catalyst loading amount is 0.37 mg cm-2 on the rotating disk electrode, and electrolyte is 1 M KOH solution. The working electrode was firstly cycled about 20 times to gain the steady CV curves. The polarization curves of GC electrode and Pt/C suggested the poor OER activity (Figure 3a). The onset potential of Co9S8 was about 1.45 V vs. RHE, smaller than that of Co1-xS (1.48 V vs. RHE) and Co3O4 (1.54 V vs. RHE). The overpotential to drive a current density of 10 mA cm-2 for Co9S8 hierarchical hollow microplates was 278 mV, which is lower than the value for benchmarking RuO2 (232 mV). Moreover, the values of Co1-xS hollow microplates and porous Co3O4 microplates were about 300 and 385 mV, respectively. In Figure 3b, the slopes of Tafel for Co9S8 and Co1-xS are both 53 mV dec-1, smaller than that of porous Co3O4 (56 mV dec-1) and previous reports.32,33 Low Tafel slopes of Co9S8 hollow microplates suggested fast deprotonating of OH- in the OER.34,35 In the case of Tafel slopes, the Co9S8 and Co1-xS hierarchical hollow microplates enabled faster oxygen evolution than Co3O4 porous microplates. Combined with the polarization curve in Figure 3a, it is clear that the Co9S8 sample was more active than Co1-xS and Co3O4 porous microplates for OER in 1 M KOH (basic) solution. To better understand the reasons for higher electrocatalytic activity of Co9S8 hollow microplates, EIS measurements for Co9S8, Co1-xS and Co3O4 electrodes in frequency range of 100 KHz to 0.01 Hz were carried out, and the results were shown in Figure 3c. The depressed semicircle in the high-frequency region corresponds to charge transfer resistance (Rct).36 The Rct of Co9S8 electrode (~12.1 Ω) was much lower than that of Co1-xS (~17 Ω) and Co3O4 (~350 Ω) electrodes at overpotential of 0.51 V, suggesting Co9S8 hierarchical hollow microplates possess higher electrical conductivity and much faster electron transfer process during the electrochemical reaction.37 The improved electrical conductivity of Co9S8 hierarchical hollow microplates was mainly attributed to the high intrinsic conductivity of Co9S8 itself as


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well as enhanced the crystallinity of Co9S8 hierarchical hollow microplates, which facilities the OER kinetics. 80

a

b

)

Co1-xS -2

Co1-xS

Co3O4

Overpotential (V)

Current density (mA cm

0.40

Co9S8

60

20% Pt/C GC RuO2

40

-2

10 mA cm

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Co9S8

0.35

56 mV dec

-1

53.7 mV dec

0.30

-1

53 mV dec

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Co3O4

30

0 0

200

400

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10 0 0

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2.0

50

)

Co9S8

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Co9S8

40

d

800

Co1-xS

1.2 -2

Current density (mA cm

50

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Log (j/ mA cm

Potential (V vs. RHE)

c

Co3O4

-1

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-Z'' (ohm)

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-2

125 mF cm

Co1-xS

2.0

Co3O4

1.5 -2

42.5 mF cm

1.0 0.5 -2

3 mF cm

0.0 0

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Z' (ohm)

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v (mV/s)

Figure 3 (a) Polarization curves, (b) Tafel plots, (c) electrochemical impedance spectra at overpotential of 280 mV, and (d) charging current density difference (Δj = ja−jc) plotted against scan rate of Co1-xS and Co9S8 hollow microplates and Co3O4 porous microplates.

The electrochemical surface area is also an important factor that affects the performance of electrocatalysts. To reveal the difference in catalytic performance for the three electrode materials (Co9S8, Co1-xS and Co3O4), electrochemical double layer capacitances (Cdl) achieved from cyclic voltammetry test (Figure S9) were used to compare the electrochemically active surface area (ECSA) of different catalysts, as shown in Fig. 3d. The Cdl of Co9S8 hollow microplates was calculated to be 125 mF cm-2, much larger than that of Co1-xS (42.5 mF cm-2) and Co3O4 (3 mF cm-2), suggesting better ion


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accessibility for electrolyte, much more exposed catalytic active sites and larger ECSA in Co9S8 hollow microplates than in Co1-xS and Co3O4 microplates. The polarization curves from Figure 3a normalized the activity to ECSA were shown in supporting information (Figures S10 and S11). The Cdl of Co9S8 in Figure 3d is larger than Co1-xS and Co3O4, implying more accessible exposed active sites in Co9S8. The exchange current densities were then normalized to ECSA for comparing the intrinsic OER activity per active site of these catalysts. As shown in Figure S11, Co9S8 shows highest normalized current densities at lower overpotential. These results confirm that Co9S8 process highest intrinsic OER activity with more exposed active sites. The high ECSA of Co9S8 was attributed to the unique hierarchical hollow structure and improved electron conductivity. Compared with the previous literature, the overpotential and Tafel slope values of hierarchical Co9S8 hollow microplates were lower than most non-precious cobalt-based electrocatalysts for OER in basic solution, such as Co3S4 hollow nanosheets, Co-S nanosheets on the CTs/CP, Co-S grown on Ti mesh, MoS2 composite with Co9S8/CNFs, Co-P, CoO (Table S1). The remarkable OER activity of Co9S8 was mainly attributed to the structural advantages of nanosheets grown on hollow microplates, which exposed considerable surface active sites to boost the catalytic activity and facilitated the ion transfer process during the electrochemical reactions. Another important criterion for electrocatalysts is the overpotential stability, which was evaluated by chronopotentiometry in this work (Figure 4). After running for 20 h at a fixed overpotential of 350 mV, the current density of Co9S8 catalyst slowly declined from 31.70 to 20.93 mA cm-2 and the catalystic performance has a minor positive shift in polarization curve (inset in Figure 4), demonstrating good stability of Co9S8 hierarchical hollow microplates during the OER process.


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-2

-2

Current density (mA cm )

80

Current density (mA cm )

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70 60 50 40 30

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Co1-xS

η = 350mV

50 40

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after 20h

20 10 0 1.2

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Time (h) Figure 4 Chronoamperometry curves (j-t) at a constant overpotential of 350 mV of Co1-xS and Co9S8 hollow microplates. Inset is polarization curves of the Co9S8 hollow microplates before and after 20 h stability test.

Understanding the mechanism for the high OER activity of Co9S8 hierarchical hollow microplates is critical for designing efficient electrodes. The first two CV scans of Co9S8 electrode are shown in Figure S12. During the first cycle process, an obvious electro-transfer peak is observed at about 1.1 and 1.3 V (vs. RHE) assigned to the Co+/Co2+, Co2+/Co3+ redox couple.38 Then, the irreversible oxidation peaks disappeared in the subsequent CV scans. Similar reactions occurred in OER for other cobalt compound based catalysts, and the formation of cobalt oxides/hydroxides (such as CoII(OH)2, CoIIIO(OH) and CoO2) was widely accepted after this activation with cycling.39,40 Generally, as OER electrocatalysts, the surface of cobalt compounds convert to cobalt oxides/hydroxides under strong oxidizing environment during the OER, and the derived cobalt oxides/hydroxides usually exhibit better OER performance compared with the pristine cobalt compounds.41-43 In order to verify the conversion of Co9S8 to cobalt oxides/hydroxides, SEM, TEM and X-ray photoelectron spectrometry (XPS) analysis of hierarchical Co9S8 hollow microplates after OER test at 1.58


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V vs. RHE for 20 h were performed. After cycling for 20 h, hollow microplates kept the original feature very well while the thinner nanosheets disappeared (Figure S13, Figure 5a and b). SAED pattern reveals that the cubic type Co9S8 is transformed into cobalt oxides/hydroxides phase after OER test for 20 h (Figure 5c). The EDS elemental mapping proves that Co9S8 electrode after OER test for 20 h is mainly composed by Co and O (Figure S14). In addition, the electronic state of Co, S, O in Co9S8 electrocatalyst after 20 h stability test were investigated by XPS. The peaks of Co 2p spectrum at around 778.9 and 793.8 eV, and O 1s spectrum at 530.2 eV, which can be indexed to cobalt (oxy)hydrates, demonstrate the surface oxidation of Co9S8 during the OER (Figure 5d and f).21,44 In addition, the S 2p characteristic peaks disappeared in the Co9S8 after OER stability test (Figure 5e), further verifies the Co9S8 were converted to cobalt (oxy)hydrates totally during the OER process under strong oxidizing environment. The O 1s spectrum of the Co9S8 after 20 h stability test at 530.2 eV is the oxygen from cobalt oxides/hydroxides, different from the abundant surface hydroxyls before OER stability test. XRD patterns of the sample after cycling for 20 h were displayed in the supporting information (Figure S15). No obvious characteristic peaks from Co9S8 are observed In addition, the difference in electronic state of Co1-xS and Co9S8 hierarchical hollow microplates were also investigated by XPS (Figure 5d). The core level of Co 2p3/2 doublet peaks at 778.5, 793.0 eV and 2p1/2 at 780.7, 796.5 eV were attributed to Co3+ and Co2+ oxidation state in both Co9S8 and Co1-xS hollow microplates, respectively. 45, 46 The peak area ratio of Co3+ and Co2+ suggests that there are more Co3+ species on the surface of Co9S8 than Co1-xS. It was reported that Co3+ in Co3O4 can serve as the real catalytic active centre for OER.32 Co3+/Co2+ ratio in Co9S8 were larger than in Co1-xS, which is one reason for better OER performance of Co9S8 than Co1-xS. In the S 2p spectrum (Figure 5e), the first two peaks were attributed to the typical S 2p3/2 of metal-sulfide bonds and covalent S-C bonds in Co1-xS and Co9S8 hierarchical hollow microplates.47,19 This result further verified that Co9S8 was obtained by calcining the hierarchical Co1-xS hollow microplates. While the peaks at 167.5 eV in S 2p spectrum were indexed to


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SO42-, which the SO42- originated from sulfur partly oxidized on the material surface. The O 1s spectrum of the Co1-xS and Co9S8 at 531.1 eV were the abundant surface hydroxyls (Figure 5f). The XPS survey spectra of Co1-xS and Co9S8 hierarchical hollow microplates suggested that high proportion Co3+ species can endow Co9S8 hollow microplates as an excellent OER electrocatalyst.

Figure 5 (a-c) TEM images and SAED pattern of Co9S8 electrocatalyst after 20 h stability test; (d-f) High resolution spectra of Co1-xS, Co9S8 and Co9S8 electrocatalysts after 20 h stability test: (d) Co 2p; (e) S 2p; (f) O 1s.

4. CONCLUSIONS In summary, efficient Co9S8 OER electrocatalyst with unique structure of nanosheets grown on the outside surface of hollow microplates were successfully fabricated by a two-step strategy. The hierarchical Co9S8 hollow microplates with extensive catalytic active sites and good electron conductivity exhibit superior electrocatalytic properties for oxygen evolution. The OER performance of Co9S8 hollow microplates, with low overpotential (278 mV) and Tafel slopes (53 mV dec-1), were superior to most of the reported cobalt based compounds. The remarkable activities should be ascribed to the Co3+ rich and


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favorable hollow architecture of Co9S8, which was verified from related characterization. This work proves that Co9S8 is highly efficient and electrocatalytic performance stable for water oxidation, and rational design and engineering of hollow nanostructures would be benefit to enhance its electrochemical performance. ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Detail of chemical structure characterization and electrocatalytic performance of the as-prepared samples. (PDF) AUTHOR INFORMATION

Corresponding Author E-mail address for Cheng-Yan Xu: [email protected]. E-mail address for Liang Zhen: [email protected].

Author Contributions H. Liu and F.X. Ma contributed to this work equally.

Funding Sources No funding support the research of this manuscript.


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Graphic Abstract


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Sulfurizing-Induced Hollowing of Co9S8 Microplates with Nanosheet

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