Hierarchical 0D−2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting

Development of efficient electrocatalysts combining the features of low cost and high performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) still remains a critical challenge. Here, we proposed a facile strategy to construct in situ a novel hierarchical heterostructure composed of 0D−2D CoSe2/MoSe2 by the selenization of CoMoO4 nanosheets grafted on a carbon cloth (CC). In such integrated structure, CoSe2 nanoparticles dispersed well and tightly bonded with MoSe2 nanosheets, which can not only enhance kinetics due to the synergetic effects, thus promoting the electrocatalytic activity, but also effectively improve the structural stability. Benefiting from its unique architecture, the designed CoSe2/MoSe2 catalyst exhibits superior OER and HER performance. Specifically, a small overpotential of 280 mV is acquired at a current density of 10 mA·cm−2 for OER with a small Tafel slope of 86.8 mV·dec−1, and the overpotential is 90 mV at a current density of 10 mA·cm−2 for HER with a Tafel slope of 84.8 mV·dec−1 in 1 M KOH. Furthermore, the symmetrical electrolyzer assembled with the CoSe2/MoSe2 catalysts depicts a small cell voltage of 1.63 V at 10 mA·cm−2 toward overall water splitting.


INTRODUCTION
Hydrogen is a promising energy source that boasts a high power density and environmental friendliness; therefore, electrolysis of water is hotly pursued as a renewable, efficient, and pollution-free technique (Amiinu et al., 2017;Luo et al., 2018;Zhu et al., 2018). Electrocatalytic water splitting consists of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and electrocatalysts as the chemical reaction centers play a critical role in the water splitting electrolyzer. Although some noble metal oxide catalysts (RuO 2 and IrO 2 ) have high electrocatalytic performance for the OER and some noble metal catalysts (Pt and Ir) deliver good electrochemical property in the HER, the high cost and scarcity restrict their wide industrial application (Trasatti, 1972(Trasatti, , 1984. Therefore, noble-metal-free catalysts with high stability and efficiency are crucial to large-scale hydrogen production from water splitting. Currently, the OER activity in alkaline solution is the bottleneck in overall water splitting due to the sluggish kinetics arising from the multiproton-coupled electron transfer steps (Jamesh and Sun, 2018). In practice, the HER catalyst in the electrolyzer should be compatible with the OER catalyst and functions in the same medium. Hence, development of suitable bifunctional noble-metal-free electrocatalyts with both high HER and OER performance in alkaline media is of great significance.
In recent years, transition metal dichalcogenides (TMDs) have attracted significant research interests owing to their earthabundant reserves and acceptable activity for electrocatalytic HER (Xie et al., 2013;Zhang et al., 2017a;Xue et al., 2018;Wang et al., 2019). Particularly, layered MoSe 2 has been considered as a promising HER electrocatalyst because of its unique structure features and high electrochemical activity (Shi et al., 2015;Chen et al., 2018a;Zhang et al., 2018). Theoretical research has demonstrated that the Gibbs free energy for H atom absorption on the edge of MoSe 2 is lower than that of MoS 2 due to the more metallic nature of MoSe 2 , revealing the higher HER performance (Tang et al., 2014;Lai et al., 2017;Yang et al., 2018). In addition, it also has been experimentally confirmed that the unsaturated Se edges in MoSe 2 nanosheets are extremely active as the S edges in MoS 2 , which is responsible for the high HER activity (Jaramillo et al., 2007;Tang and Jiang, 2016). However, similar to MoS 2 , the HER activity of layered MoSe 2 is largely limited by its poor conductivity and serious aggregation or restacking during the synthesis procedure (Mao et al., 2015;Qu et al., 2015), inhibiting the practical application of MoSe 2 catalyst. Therefore, it is significant to improve the electrochemical activity of MoSe 2 -based catalyst. Recent works have shown that coupling MoSe 2 with other transition metal selenides and constructing heterostrucurted materials could be an effective approach to further enhance the electrochemical performance of MoSe 2 . For instance, Wang et al. found that the MoSe 2 @Ni 0.85 Se nanowire delivered enhanced kinetics and performance for HER in alkaline conditions due to the high density of active edges of MoSe 2 and the good conductivity of Ni 0.85 Se . Zhang et al. synthesized 3D MoSe 2 /NiSe 2 nanowires, which significantly enhanced HER activity with a low Tafel slope and overpotential in 0.5 M H 2 SO 4 , because the 3D structure affords more active sites . Liu et al. fabricated MoSe 2 -NiSe@carbon heteronanostructures and achieved glorious HER catalytic properties and excellent durability in both acidic and base conditions . In addition, the hierarchical mesoporous MoSe 2 @CoSe/N-C composite also exhibits outstanding HER activity (Chen et al., 2019b). Despite significant success, most of previous reports mainly focused on the improvement of HER performance, while the OER activity of MoSe 2 catalyst in alkaline media has been ignored. Hence, the rational design and fabrication of MoSe 2based bifunctional electrocatalysts with satisfactory activity and stability toward overall water splitting in alkaline solution still remain a big challenge.
In this work, we developed a facile in situ phase separation strategy to construct a novel hierarchical heterostructure consisting of 0D−2D CoSe 2 /MoSe 2 via the selenization of CoMoO 4 nanosheets supported on a carbon cloth (CC) (Figure 1). Due to the in situ phase transformation, CoSe 2 nanoparticles are uniformly anchored on MoSe 2 nanosheets in the integrated structure, which can not only enhance reaction kinetics because of the synergetic effects, thus boosting the electrocatalytic activity, but also effectively suppress the aggregation/restacking of MoSe 2 nanosheets, thereby improving the structural stability. Moreover, the hierarchical structure assembled by 0D−2D CoSe 2 /MoSe 2 could provide abundant active sites for the electrochemical reactions. As a result, the designed CoSe 2 /MoSe 2 architecture exhibits outstanding OER and HER performance in alkaline media. More specifically, a small overpotential of 280 mV is achieved at a current density of 10 mA·cm −2 for OER with a small Tafel slope of 86.8 mV·dec −1 , and the overpotential is 90 mV at a current density of 10 mA·cm −2 for HER with a Tafel slope of 84.8 mV·dec −1 in 1 M KOH. Moreover, the symmetrical electrolyzer assembled with the CoSe 2 /MoSe 2 catalysts delivers a small cell voltage of 1.63 V at 10 mA·cm −2 toward overall water splitting.

Synthesis of CoMoO 4 Nanosheet
Firstly, a pristine carbon cloth (CC) was treated with nitric acid solution overnight, subsequently ultrasonicated in deionized (DI) water and dried in an oven at 80 • C for 2 h. After that, 1 mmol cobalt acetate, 1 mmol ammonium molybdate, 2 mmol urea, and 5 mmol ammonium fluoride were dissolved in 30 mL DI water, followed by ultrasonication for 30 min. Then, the homogeneous solution was poured into a 50-mL Teflon-lined stainless autoclave with the CC kept at 150 • C for 6 h. After cooling to room temperature, the CC was washed with DI water for several times and dried in a vacuum freeze-dryer overnight. Finally, the obtained sample was treated at 400 • C for 2 h with a ramp rate of 5 • C min −1 in an argon atmosphere.

Synthesis of CoSe 2
The CoSe 2 was prepared through two steps. Firstly, the treated CC was immersed in a 0.1 M Co(NO 3 ) 2 solution for the electrodeposition of Co (Yang et al., 2015). Then, the collected sample was reacted with 0.5 g selenium powder under an Ar/H 2 (90%/10%) atmosphere at 450 • C for 1 h.

Synthesis of MoSe 2
Firstly, MoS 2 was prepared via hydrothermal reaction with the CC at 200 • C for 12 h, followed by heating at 400 • C for 2 h to form MoO 3 . Then, the obtained MoO 3 was reacted with 0.5 g selenium powder at 450 • C for 1 h under an Ar/H 2 (90%/10%) atmosphere.

Preparation of Pt/C
Four milligrams of 20% Pt/C and 20 µL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1) and then sonicated to form a uniform solution. Finally, the 1 * 1 cm 2 FIGURE 1 | Schematic illustration of the fabrication of CoSe 2 /MoSe 2 .
CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

Preparation of RuO 2
Four milligrams of RuO 2 and 20 µL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1), and then the sample was sonicated to form a uniform solution. Finally, the 1 * 1 cm 2 CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

CHARACTERIZATION
The phase composition of the samples were characterized by X-ray diffraction (XRD, Bruker D8A A25), and the chemical states were determined through X-ray photoelectron spectroscopy (XPS, ESCALB 250Xi). The morphology and microstructure were recorded via field emission scanning electron microscopy (FE-SEM, FEI Nova NANOSEM 400) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100 UHR STEM).

ELECTROCHEMICAL MEASUREMENTS
All samples made use of a three-electrode system performed by a biologic VSP300 type electrochemical workstation (Biologic Science Instruments, France). The sample of CoSe 2 /MoSe 2 was put on the electrode holder as the working electrode with a mass loading of 4 mg/cm 2 , the saturated calomel electrode (SCE) was the reference electrode, and a carbon rod served as the counter electrode. The electrolyte was 1 M KOH solution with saturated N 2 . Linear sweep voltammetry (LSV) was characterized by polarization curves of OER with a scanning rate of 5 mV s −1 from 0 to 0.8 V vs. SCE. Similarly, the polarization curves of HER were determined under the same condition from 0 to −0.8 V vs. SCE. The potentials were standardized by a reversible hydrogen electrode (RHE) as shown in the following: E (RHE) = E (SCE) + 0.059 × pH with instrument automatic 85% iR compensation. The electrochemically active surface area (ECSA) was calculated by cyclic voltammetry (CV) performed from −0.3 to −0.2 V vs. SCE with different scanning rates of 40, 60, 80, 100, and 120 mV s −1 . Electrochemical impedance spectroscopy (EIS) measurements were conducted by biologic VMP3 (Biologic Science Instruments, France) from 100 KHz to 0.1 Hz. The overall water-splitting electrolyzer was performed with CoSe 2 /MoSe 2 as electrodes and 1 M KOH as the electrolyte. Figure 2A presents the FE-SEM image of the as-prepared CoMoO 4 precursor, which presents uniform nanosheets (with a lateral size of 2 µm) perpendicularly grown on the CC substrate with high coverage. After a selenization process, the obtained CoSe 2 /MoSe 2 sample well maintains the pristine morphology of the CoMoO 4 precursor ( Figure 2B). Moreover, the high-magnification SEM image further reveals that lots of nanoparticles are well dispersed on the surface of the nanosheet (Figure 2C), implying the structure and phase evolution during the selenization treatment. The elemental maps in (Figure 2D) show that Mo, Co, and Se are uniformly distributed throughout the nanosheets. In addition, the low-resolution TEM images in (Figures 2E,F) display that nanoparticles are uniformly distributed on the nanosheet during the thermal reduction procedure, forming the 0D/2D structure. Furthermore, the highresolution TEM (Figure 2G) shows the lattice fringes of 0.26 nm and 0.65 nm corresponding to the (111) and (002) planes of CoSe 2 and MoSe 2 , respectively (Qu et al., 2016;Liu et al., 2017), demonstrating the successful formation of the CoSe 2 /MoSe 2 after the selenization reaction.

RESULTS AND DISCUSSION
To investigate phase evolution during the selenization process, the crystal structure and phase composition of the obtained samples were characterized by X-ray diffraction (XRD) analysis ( Figure 3A). The diffraction peaks of CoMoO 4 precursor (in the black line) can be well indexed to the CoMoO 4 phase (JCPDS No: 21-0868) . After the thermal reduction, some new diffraction peaks can be observed. The diffraction peaks at around 13.7 • , 27.6 • , 31.4 • , and 37.8 • can be assigned to the MoSe 2 phase (JCPDS No: 77-1715) (Qu et al., 2016), while the other peaks could be attributed to the phase of CoSe 2 (JCPDS No:53-0449) . The XRD result clearly manifests the successful phase separation of the CoSe 2 and MoSe 2 from the CoMoO 4 precursor via the selenization process.
X-ray photoelectron spectroscopy (XPS) measurement was carried out to analyze the composition and chemical state of as-prepared samples. (Figure 3B) illustrates the high-resolution Co 2p peaks at 778.8 eV (Co 2p 3/2 ), 793.7 eV (Co 2p 1/2 ), 780 eV (Co 2p 3/2 ), and 796 eV (Co 2p 1/2 ), corresponding to CoSe 2 and cobalt-oxide bond, while those peaks at 784.1 eV and 801.5 eV are the satellite peaks (Mu et al., 2016;Wang et al., 2017b;Gao et al., 2018). Furthermore, the fine Mo 3d XPS spectrum (Figure 3C) shows the main peaks at 228.8 eV and 231.9 eV, which represent the Mo 3d 5/2 and Mo 3d 3/2 of MoSe 2 (Wang et al., 2018a,b). Additionally, the peak located at 230 eV can be ascribed to the Se 3s of MoSe 2 . The Se 3d XPS spectrum ( Figure 3D) displays the characteristic of CoSe 2 and MoSe 2 at 54.5 eV and 55.4 eV in agreement with the Se 3d 5/2 and Se 3d 3/2 , respectively . Moreover, the peak at around 59.8 eV is confirmed to correspond to the selenium-oxygen bond (Kong et al., 2014). According to these results, the selenization process induced the phase separation from CoMoO 4 into the nanoscale CoSe 2 and MoSe 2 .
To understand the effects of the structure and composition of prepared catalyst on the electrochemical performance, several CoSe 2 /MoSe 2 catalysts were collected at different selenization temperatures and the HER and OER performance were evaluated by LSV analysis (Figure S2). It can be seen that the CoSe 2 /MoSe 2 sample obtained at 450 • C (CoSe 2 /MoSe 2 -450) possesses better electrocatalytic properties than other counterparts, which can be ascribed to its superior structure. As shown in (Figure S3), with the selenization temperature increasing, the size of nanoparticles on the surface of nanosheets increased as well, indicating higher crystallinity. Generally, larger particle size will reduce the active surface of catalyst (Zhang et al., 2017b;Chen et al., 2019a). Therefore, when the selenization temperature elevated to 500 • C (CoSe 2 /MoSe 2 -500), the catalytic performance slightly declined owing to its larger particle size and lower active surface. In addition, (Figure S4) displays the composition of the CoSe 2 /MoSe 2 catalysts achieved at a different selenization temperature. As can be seen, when the selenization process proceeded at low temperature, the obtained CoSe 2 /MoSe 2 catalyst has poor MoSe 2 phase and low crystallinity, which are responsible for the poor electrochemical catalytic performance of the catalysts (CoSe 2 /MoSe 2 -350 and CoSe 2 /MoSe 2 -400). Therefore, the catalyst synthesized at 450 • C shows the best performance, benefiting from the appropriate crystal structure and phase composition.
The electrochemically active surface area (ECSA) of asprepared catalyst was evaluated by the double-layer capacitance (C dl ), which was measured by CV in a non-Faradaic reaction potential range (Deng et al., 2015). The C dl values of the CoSe 2 /MoSe 2 (1.6 mF cm −2 ) is higher than those of CoSe 2 (0.63 mF cm −2 ) and MoSe 2 (0.8 mF cm −2 ), as shown in (Figure 5), suggesting more active sites of the CoSe 2 /MoSe 2 catalyst. Furthermore, the smaller R ct value for the CoSe 2 /MoSe 2 catalyst in the EIS measurement ( Figure S5) implies the promoted charge transfer and boosted kinetics, which can be ascribed to the abundant interfaces and synergetic effect between the CoSe 2 and MoSe 2 .
The structural stability is another significant parameter for catalysts in HER and OER. (Figures 4C,F) show a galvanostatic for CoSe 2 /MoSe 2 catalyst in both the HER and OER processes. The morphology and composition of the catalyst after galvanostatic cycling are characterized by SEM and XPS. The CoSe 2 /MoSe 2 could well inherit the pristine sheet-like structure, demonstrating good structural stability. In addition, the fine XPS spectra of the Co 2p, Mo 3d, Se 3d acquired from the sample of CoSe 2 /MoSe 2 after galvanostatic measurement confirm the reservation of CoSe 2 and MoSe 2 (Figure S6), indicating phase stability during the electrochemical reactions.

CONCLUSION
In summary, a novel hierarchical 0D−2D Co/Mo selenide was developed by a facile in situ phase separation strategy. Benefiting from its unique structure and composition, the constructed CoSe 2 /MoSe 2 catalyst exhibits small η 10 of 280 mV and 90 mV and Tafel slopes of 86.8 mV·dec −1 and 84.8 mV·dec −1 for OER and HER, respectively. Furthermore, the electrolyzer comprising CoSe 2 /MoSe 2 as the bifunctional catalyst shows a small water splitting cell voltage of 1.63 V at a current density of 10 mA·cm −2 . This work provides insights into rational design and development of economical and valid bifunctional catalysts for overall water splitting.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
LX implemented the experiment, analyzed the data and wrote the article. HS, XL, XZ, and BG participated in the formulation of the experimental scheme. YZ, KH, and PC revised the article.

FUNDING
This work was financially supported by the National Natural Science Foundation of China (Nos. 51572100, 61434001, and 31500783).