Selective synthesis of pure cobalt disulfide on reduced graphene oxide sheets and its high electrocatalytic activity for hydrogen evolution reaction
© Ahn et al. 2016
Received: 16 September 2015
Accepted: 13 January 2016
Published: 27 January 2016
We synthesized single-phase CoS2 on a large scale by adding graphene oxide of sufficient quantity via the hydrothermal method using cobalt acetate and thioacetamide as precursors; this produced the hybrid of CoS2 with reduced graphene oxide which exhibited high electrocatalytic activity in the hydrogen evolution reaction.
Considerable efforts have been made toward synthesizing transition metal dichalcogenide (TMD) nanomaterials due to their excellent electronic, optical, and mechanical properties [1, 2]. Among the more notable TMD compounds, MoS2 and WS2, which have electrical properties that can be changed from metallic to semiconducting by varying their crystal structure and the number of layers, have been extensively studied . In addition, the structure and general properties of pyrite-phase TMDs (FeS2, NiS2, and CoS2) have been investigated [3–6]. These materials have attracted interest for their potential applications; for example, FeS2 with a band gap of 0.95 eV has been used as an active layer in photovoltaic devices and NiS2 has been used as a Li storage material [6–9]. In particular, CoS2 has received considerable attention due to its metallic behavior, which makes it applicable as an electrocatalyst for oxygen reduction reactions and hydrogen evolution reactions (HERs) [3, 10, 11].
So far, MoS2 and WS2 nanostructures have been extensively explored as electrocatalysts for HER. The overpotentials of MoS2 and WS2 materials occur between −200 and −150 mV and their Tafel slopes fall in the range of 55–40 and 70–58 mV/dec, respectively [2, 12]. Recently, metallic CoS2 has been recognized for its potential as a viable HER catalyst like MoS2. An overpotential of −180 mV and a Tafel slope of 44.6 mV/dec have been measured for CoS2 films synthesized by gas-phase reactions . CoS2 micro- and nanowires have overpotentials of about −100 and –70 mV, respectively, and Tafel slopes of 58.0 and 51.6 mV/dec, respectively . However, CoS2 film is easily damaged by delamination during H2 evolution. Although CoS2 nanowires exhibited the best performance, they became delaminated within three hours. Accounting for a long-term stability, microwires have proven the most effective, despite their limited HER performance. The microstructured surface also helps to convey the H2 bubbles away from the electrode surface, thus maintaining the integrity of the catalyst for HER . On the other hand, the strong performance of CoS2 in HER may result from the oxidation state of sulfur in CoS2, since it is known that S2 2− exhibits higher HER efficiency than S2−. Chang et al. prepared MoSx with S2 2− on 3D Ni foam deposited with graphene layers for electrocatalytic hydrogen evolution and found that MoSx with S2 2− exhibits higher catalytic activity than MoS2 with S2− [13, 14]. Therefore, the HER activity of pure-phase CoS2 should be investigated since it is expected that CoS2 with S2 2− dimer will exhibit favorable performance in HER. As mentioned above, pure-phase CoS2 nanowires and microwires were obtained by the synthesis of cobalt hydroxide carbonate hydrate (Co(OH)(CO3)0.5·xH2O) nanowires and cobalt hydroxide (-Co(OH)2) microwires through a solution-based reaction followed by thermal sulfurization . Additionally, pure-phase CoS2 film has been synthesized by sulfurization of Co film: The substrate on which Co is deposited by an e-beam evaporator is located in the tube furnace and sulfur powder is vaporized by flowing Ar gas at 550 °C . All of the methods are two-step processes, which are limited in terms of mass production. Therefore, it would be worthwhile to establish a method for the large-scale synthesis of pure CoS2.
The hydrothermal reaction is known as a facile method for the large-scale synthesis of CoS2. However, producing pure-phase CoS2 is quite difficult because of the complex stoichiometric compositions of cobalt sulfides such as CoS2, CoS, Co9S8, and Co1−xS that form when the hydrothermal synthesis of CoS2 is attempted using precursors such as cobalt chloride (as cobalt precursor) and thioacetamide, sodium thiosulfate, etc. (as sulfur precursors) [15–20]. Indeed, a mixture of cobalt sulfides has been synthesized using the hydrothermal method. For example, Huang et al. found that the flower-like cobalt sulfides prepared by a solvothermal method included a mixture of CoS and Co9S8 (9 %) . Qian et al. then found that as-synthesized cobalt sulfides consisted of CoS2, Co9S8, and Co3S4 when using cobalt chloride in toluene . Furthermore, most reports employed only X-ray diffraction (XRD) for the characterization of CoS2 [15–20]. However, in-depth characterization using TEM and XPS as well as XRD is required for the identification of CoS2, due to the existence of various stoichiometric compositions of cobalt sulfides. In addition, impurities such as oxygen can occur in CoS2 since it is susceptible to oxidation in air . Thus, further research is required to allow for the characterization and large-scale production of pure CoS2 with S2 2− dimer.
Herein, we report on such a large-scale synthesis of pure-phase CoS2 on rGO by a one-pot hydrothermal reaction using cobalt acetate and thioacetamide precursors. We demonstrate that GO concentrations of greater than 2 mg/mL are critical in supporting the synthesis of pure-phase CoS2 and inhibiting the oxidation of the CoS2 surface. Furthermore, we provide detailed structural analyses of the synthesized CoS2 and investigate its electrocatalytic activity in HER, thereby demonstrating a long-term stability.
2.1 Synthesis of GO solution
GO was prepared from graphite powder by the modified Hummers’ method.
2.2 Hydrothermal synthesis of CoS2/rGO hybrid
GO solution was added to the mixture of cobalt acetate tetrahydrate (9 mmol) and thioacetamide (90 mmol) and the total volume of the solution was adjusted to 400 mL for all reactions. The solution was transferred to a 500 mL Teflon-lined stainless steel autoclave, heated up to 240 °C, and kept for 24 h. After cooling naturally, the product was filtered, washed with DI water, and dried in vacuum at 60 °C for 12 h. During the hydrothermal process, GO was converted to rGO and CoS2/rGO was formed.
The samples were characterized with field emission scanning electron microscopy (Hitachi, S4800), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100 F with probe-Cs corrector, 200 kV), X-ray diffraction [Rikgaku RU-200 diffractometer equipped with Ni-filtered Cu Kα radiation (40 kV, 100 mA, λ = 0.15418 nm)], and X-ray photoelectron spectroscopy (K-alpha, ThermoFisherwith monochromatic Al Kα radiation as the X-ray source). The rGO concentration of samples was characterized with element analyzer (Thermo Scientific, Flash 2000).
2.4 Electrochemical measurements
Electrocatalytic measurements were carried out using a 3-electrode cell and a 0.5 M sulfuric acid (H2SO4) electrolyte solution. A graphite rod (Sigma Aldrich) and Ag/AgCl electrode (Wonatech) were used as counter electrode and reference respectively. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE) using platinum wires as working and counter electrodes. Materials were dispersed in deionized water at 4 mg/mL and sonicated for 1 h. The ink was then drop-casted onto glassy carbon electrodes of 3 mm diameters (loading 285 µg/cm2) and capped by Nafion (0.5 %, 3 µL, Sigma Aldrich). Linear sweep voltammetry was performed with a 5 mV/s scan rate using a Zive SP2 potentiostat from Wonatech and the electrodes were cycled at least 40 cycles prior to any measurement. The polarization curves were iR-corrected. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 1 MHz to 0.1 Hz.
3 Results and discussion
Experimental conditions for CoS2/rGO synthesis with various concentrations of GO
Concentration of GO (mg/mL)
Ratio of GO:Co2+ (mg:mmol)
CoS2 + Co3S4 + Co3O4 + CoS
CoS2 + Co3S4 + CoS
CoS2 + CoS
CoS2 + CoS
Figure 1a shows the X-ray diffraction (XRD) spectra of the products according to GO concentration. When GO is not added, there are not only reflections of CoS2 such as the (200), (220), and (311) planes, but also reflections for CoS, Co3S4, and Co3O4. This result is in good agreement with previous reports stating that multiphase cobalt sulfides are usually synthesized in these conditions [15, 16]. Note that CoS was partially observed along with CoS2 for a 1 mg/mL GO concentration (Fig. 1a, JCPDS 75-0605). This result is consistent with the TEM results, which indicate a d-spacing corresponding to the (100) plane of hexagonal-phase CoS within the CoS2/rGO products (Additional file 1: Figure S4, space group P63/mmc; a = 3.384 Å, c = 5.16 Å). In addition, our XRD pattern for CoS2/rGO did not show signs of oxidation, while the non-GO Co3S4 showed a pattern for Co3O4 (Fig. 1a, JCPDS 42-1448; JCPDS 80-1543). Furthermore, we calculated the average crystal size by Scherrer formula using the (200) plane. The results show 36.65 nm and 33.82 nm for the CoS2/rGO (2.3 mg/mL) and CoS2/rGO (1 mg/mL), respectively. These results are well matched with the SEM data in Fig. 3. Note that XRD spectra for CoS2/rGO samples with various GO concentrations in Table 1 are shown in Additional file 1: Figure S5 and pure-phase CoS2 was obtained when GO concentration is higher than 2 mg/mL. A representative Raman spectrum of CoS2/rGO (2.3 mg/mL of GO) is shown in Fig. 1b. Characteristic peaks for CoS2 at 288, 388, and 484 cm−1 corresponding to the Eg, Ag, and Tg(3) modes, respectively , and the D and G bands of rGO, can be clearly seen.
To verify the oxidation states of the elements in the CoS2/rGO products, we measured their X-ray photoelectron spectroscopy (XPS) spectra (Fig. 2). The peaks for the binding energy of Co 2p3/2−1/2 appeared at 778.8 and 794 eV for the CoS2/rGO sample with 1 mg/mL GO and at 779 and 794.2 eV for the sample with 2.3 mg/mL GO, indicating an oxidation state of Co2+ (Fig. 2a) . The peaks around 786 and 803 eV are associated with the shake-up type peaks of the 2p3/2-1/2 and peaks at 782.3 and 798 eV are associated with Co3+ from the mixture of cobalt sulfides without GO . For the case of S 2p3/2-1/2, doublet peaks appear at 162.9 and 164 eV for the CoS2/rGO with 1 mg/mL GO sample and at 162.6 and 163.9 eV for the CoS2/rGO with 2.3 mg/mL GO sample, indicating the presence of S2 2− (Fig. 2b) . Note that there is an additional peak at ~161.8 eV for S2− in the CoS2/rGO with 1 mg/mL GO sample and cobalt sulfides without rGO . Although CoS2 is easily oxidized in air as mentioned above , our results show no peak for oxidation in any of the CoS2/rGO samples, while there is a substantial oxidation peak at 169.1 eV for the non-GO sample in Fig. 2b, which is consistent with the XRD spectra. Consequently, the XPS results confirm the pure-phase nature of the CoS2 for GO concentration of 2.3 mg/mL.
In summary, we successfully synthesized CoS2/rGO with pure-phase CoS2 on rGO sheets via hydrothermal reaction. The CoS2/rGO hybrid materials exhibited high catalytic activity for HER: overpotential of −150 mV versus RHE and a Tafel slope of 48 mV/dec for CoS2/rGO (2.3 mg/mL GO). Thus, the present study demonstrates the large-scale synthesis of CoS2/rGO hybrids with a long-term stability and strong HER performance.
This work was supported by the NRF Grant (NRF-2014R1A2A2A01007136) and a Grant (Code No. 2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning, Korea and by the framework of Indus-try Convergence Fundamental Technology Development Program (10050509, MOTIE, Korea) of Korea Evaluation Institute of Industrial Technology by the Ministry of Trade, Industry and Energy, Korea.
The authors declare that they have no competing interests.
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