One-pot synthesis of nanosheet-assembled hierarchical MoSe2/CoSe2 microcages for the enhanced performance of electrocatalytic hydrogen evolution

School of Energy Science and Engineerin Technology of China, Chengdu 610054, Chi Institute of Chemical Materials, China A Mianyang 621900, China. E-mail: zhongpin Bristol Centre for Nanoscience and Quantu 1TR Bristol, UK † Electronic supplementary information ( MC11, Tafel plots of various samples, an included. See DOI: 10.1039/c5ra21638a ‡ These authors contribute equally. Cite this: RSC Adv., 2016, 6, 23


Introduction
As a clean energy source, hydrogen has been considered as a promising candidate to replace fossil fuels, and electrochemical water splitting is a promising and viable approach to produce H 2 . Highly active electrocatalysts used for the hydrogen evolution reaction (HER) are critical to reduce the energy consumption during the electrolysis of H 2 O into H 2 . Although platinum (Pt) and its alloys have been proven to be superior electrocatalysts in HER processes, 1-5 the scarcity of Pt limit their commercialisation. For this reason the development of alternative catalysts to replace the correspondent ones made from precious metals is an attractive research activity, both from the scientic and engineering point of view. 7 Over the past few years many potential alternatives to precious metal-based electrocatalysts have been developed, including transition metal suldes, [8][9][10][11][12] selenides, [13][14][15][16][17] borides, 18 carbides, 19 nitrides, 20 phosphides, 21,22 as well as a family of molecular catalysts. [23][24][25] Among all these alternatives a type of efficient electrocatalysts based on transition-metal dichalcogenide (TMD) has recently received signicant attention, owing to its exotic electronic structure and physical properties. 6,[26][27][28][29][30][31][32][33] TMD is prone to form two-dimensional (2D) sheets due to the graphite-like sandwich structure and the weak van der Waals interactions between two adjacent layers. [34][35][36][37] The 2D structure can offer both large surface area for ion adsorption and active sites on its edge. [38][39][40][41][42] Both experimental and theoretical studies have conrmed that the HER activity of electrocatalysts correlates with the number of catalytically active edge sites. [43][44][45][46][47][48][49][50][51] It has also proved that the edges of the TMD sheets play an important role in HER process. The edges have a large presence of unsaturated bonds and therefore unpaired electrons, whereas the basal surfaces of the sheets are catalytically inert. Therefore, the increase number of active sites by forming the nanosheets or nanoparticles with particular morphologies have been found to be an efficient pathway to enhance the HER activity. 8,13,28,31,43,44,49 However, with the introduction of the active edge sites the nanosized particles or sheets would intensively reduce the intrinsic electric conductivity of the catalysts, with a consequent degradation of the electrons transport during the catalytic process. [52][53][54] It is therefore critical to develop electrocatalysts with both active edges and good conductivity to achieve high HER performance. As an example, nanostructured MoS 2 exhibits poor overall HER performance due to its weak conductivity, 36 whereas a metallic 1T-MoS 2 catalyst with extremely low charge-transfer resistance shows highly enhanced HER activity. 55 High active HER electrocatalysts have also been developed by growing TMD nanocomposites on high conductive carbon materials. [52][53][54][56][57][58][59][60][61][62][63][64][65][66] In the experiments cited above the TMD induces a low value of over potential (OP) for the HER, while the high conductive carbon media provides a channel for electrons transportation. These phenomena gives evidence that a simultaneous optimization of the active sites and an increase of the conductivity of the electrocatalysts should be considered to achieve an overall improvement of the HER activity. Moreover, synergistic regulations have been found an effective way to enhance the HER process in hybrid electrocatalysts. [67][68][69][70][71][72] Recently Gao et al. have described perhaps the most efficient MoS 2 /CoSe 2 hybrid catalyst for HER. 73 In their study graphene-like MoS 2 nanosheets were grown on pre-synthesized CoSe 2 nanobelts. Within this particular morphology the active sites for HER can be provided by the MoS 2 nanosheets, while the dense CoSe 2 nanobelts provide a fast electrons transport channel.
Typical layered-TMD materials like MoSe 2 and CoSe 2 have been extensively investigated for HER due to their low-cost and electrochemical stability. 13,50,60,61,74,75 Molybdenum chalcogenides is prone to evolve into a hierarchical structure composed of ultrathin nanosheets, 49 while cobalt chalcogenides tend to form simpler shapes such as nanobelt 76,77 or nanoparticles. These different structural congurations imply the existence of different crystallization kinetics in these two types of TMD composites synthesized via hydrothermal, solvothermal or colloidal methods. We can therefore postulate that in a one-pot synthesis of the Mo/Co-based hybrid composite the crystallization and the morphology of the product would be signicantly affected by these two different crystallization mechanisms, which may lead to the formation of Mo/Co-based hybrid composite with a hierarchical structure and an improved HER. We have therefore tried, for the rst time, to synthesize a novel MoSe 2 /CoSe 2 hybrid catalyst via a simplied one-pot hydrothermal method. Under optimal conditions and suitable Mo/Co ratio we have successfully synthesized a MoSe 2 /CoSe 2 microcage with a unique nanosheetassembled hierarchical morphology. This MoSe 2 /CoSe 2 nanocomposite has exhibited an evident HER enhancement in terms of highly reduced onset overpotential, Tafel slope, and increased cathodic current. The improvement of HER observed in the MoSe 2 /CoSe 2 hybrid catalysts can be attributed to both the unique morphology and the synergetic coupling between MoSe 2 and CoSe 2 . 76 The simplicity of the one-pot hydrothermal method adopted, as well as the enhanced HER activity due to the hierarchical structure observed in the MoSe 2 /CoSe 2 hybrid catalyst open a new way to explore superior hybrid electrocatalysts for HER.

Materials
Analytical grade ammonium molybdate tetrahydrate, cobaltous acetate tetrahydrate SeO 2 powder, and ethanediamine were bought from SIGMA-ALDRICH and were used as received without further purication.

Synthesis
The synthesis of the MoSe 2 /CoSe 2 composite with Mo : Co ¼ 1 : 1 (noted as MC11) was performed using 0.17 mmol ammonium molybdate tetrahydrate, 1.2 mmol cobaltous acetate tetrahydrate and 4.8 mmol SeO 2 powder dissolved in 10 mL distilled water under vigorous stirring for 30 min to form a homogeneous solution. Subsequently, 25 mL of ethanediamine were added with continuing stirring for 15 min at room temperature. The nal solution gave a milky color. The solution was then transferred into a 40 mL Teon-lined stainless steel autoclave and maintained at 200 C for 20 h. The reaction system was then allowed to cool down to room temperature, the obtained black precipitates were collected by centrifugation at 8000 rpm for 7 min, washed with distilled water and ethanol at least 3 times, and then dried at 60 C in the air. To obtain the nal products, the above collections were annealed at 500 C for 1 h in owing N 2 atmosphere. For comparison, pure MoSe 2 , CoSe 2 , and other two MoSe 2 /CoSe 2 composites with Mo and Co mole ratios of 3 : 1 (noted as MC31) and 1 : 3 (noted as MC13), were prepared using the same procedure, only with different ratios of Mo and Co salts.

Electrochemical measurement
All electrochemical measurements were performed in a threeelectrode system attached to an electrochemical workstation (VSP (Bio-Logic SAS)). The electrodes containing the asprepared catalysts were manufactured using the following approach. Firstly 20 mg of catalyst powder and 150 mL of Naon solution (Sigma Aldrich, 5 wt%) were dispersed in a 5 mL waterethanol solution with a volume ratio of 4 : 1 by sonication lasting 30 min to form a homogeneous ink. Subsequently 5 mL of the dispersion were loaded onto a glassy carbon electrode of 3 mm diameter (loading 0.285 mg cm À2 ). Finally, the as-prepared catalyst lm was dried in the air at room temperature. Linear sweep voltammetry (LSV) with a scan rate of 2 mV s À1 was performed in 0.5 M H 2 SO 4 using an Ag/AgCl (in saturated KCl solution) electrode as the reference electrode, and a Pt plate as the counter electrode. All the potentials were calibrated to a reversible hydrogen electrode (RHE). LSV was recorded by sweeping the potential from À0.7 to 0.2 V vs. RHE at room temperature. Cyclic voltammetry (CV) was conducted for 1000 cycles between À0.3 and 0.2 V vs. RHE at 50 mV s À1 for the stability test. The Nyquist plots were measured with frequencies ranging from 200 kHz to 100 mHz at an overpotential of 250 mV. The impedance data were then tted to a simplied Randles circuit to extract the series and charge-transfer resistances.
Characterization X-ray diffraction (XRD) was performed on a Bruker DX-1000 diffractometer with Cu Ka radiation (l ¼ 1.54182 A). Scanning electron microscope (SEM) were recorded on a Hitachi S-4800. Transmission electron microscopy (TEM) was carried out using a Carl Zeiss SMT Pvt Ltd, Libra 200 FE. High-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX) spectroscopic and elemental mapping analysis were all performed on a JEOL JEM-ARF200F. X-ray photoelectron spectra (XPS) were measured using an ESCALAB MK II.

Results and discussions
The novel hierarchical nanosheet-assembled microcages composed of MoSe 2 /CoSe 2 nanosheets were synthesized via a facile one-pot hydrothermal method with standard reagents, including ammonium molybdate tetrahydrate, cobaltous acetate tetrahydrate, SeO 2 powder as the precursors, and ethylenediamine as the surfactant. In our hydrothermal process, the MoSe 2 is prone to form a ball-like nanosheet with loose morphology, while the CoSe 2 would ted to form dense nanoparticles with irregular shapes. On the opposite, the MoSe 2 / CoSe 2 composite with the specic Mo/Co ratio would lead to a unique hierarchical nanosheet-assembled microcages.
As shown in Fig. 1a, the XRD peaks from the pure MoSe 2 agree well with the standard pattern of hexagonal MoSe 2 (JCPDS card no. 29-0914), in which the clear broadening of the diffraction peaks indicates the ultrane crystallization at nanoscale level. As can be observed in Fig. 1b, the fabricated MoSe 2 shows a typical ball-like morphology composed by ultrathin nanosheets with an average thickness of 16.8 nm. The XRD patterns from CoSe 2 are however characterized by the combination of orthorhombic CoSe 2 (JCPDS card no. 53-0449) and cubic CoSe 2 (JCPDS card no. 89-2002), demonstrating therefore a polymorphic composition. It should be noticed that, when comparing with the XRD of the MoSe 2 , the presence of sharper diffraction peaks in the CoSe 2 XRD spectra imply an improved degree of crystallization with larger grain sizes, which is also evident in the SEM images of Fig. 1b and c. As expected, all diffraction peaks in the MoSe 2 /CoSe 2 composite XRD patterns can be separated from either MoSe 2 or CoSe 2 compounds, demonstrating a phase purity of the synthesized product. Besides, we note that the peaks match well the ones related to the CoSe 2 , while the peaks consistent with the ones shown by the pristine MoSe 2 are broadened. These characteristics reveal that the CoSe 2 compound tends to aggregate and to form larger grain sizes, while the MoSe 2 retains its nanosized distribution in the MoSe 2 /CoSe 2 composite. The chemical composition of the as-synthesized MoSe 2 /CoSe 2 composite were investigated by XPS and EDX spectroscopic analysis, of which the results are shown in the ESI. † From the observation of the XPS spectra (Fig. S1a †) it is possible to notice that the whole XPS spectrum features three main elements, including Mo, Co and Se in the composite. Furthermore, two characteristic peaks arising from the Mo 3d 3/2 and Mo 3d 5/2 orbitals are located at 232.0 eV and 228.9 eV (Fig. S1b †), which conrm that the Mo is in its Mo(IV) state. 11,49 The peaks of Co 2p 1/2 at 793.5 eV and Co 2p 3/2 at 778.5 eV (Fig. S1c †) indicate the presence of Co-Co, 78 which is commonly present in CoSe 2 with Co/Se/Co layered structure. The peaks from Se 3d at 55.1 eV and 54.5 eV (Fig. S1d †) conrm the presence of a À2 oxidation chemical state for the Se. 79,80 The EDX results given in Fig. S2 † further conrms the presence of the atomic ratio of the three composition elements (Mo, Co, and Se) used in the experiments. Fig. 2 shows SEM and TEM images related to the MoSe 2 / CoSe 2 composite with various Mo/Co ratio. It can be observed that the synthesized MoSe 2 /CoSe 2 composites demonstrate totally different morphologies from that of either MoSe 2 or CoSe 2 when using combinations of Co and Mo salts under the same reaction condition. The MC31 and MC11 products form hierarchical tube-like nanosheet-assembled microcage, (Fig. 2a  and b). These microcages aggregate with a uniform size distribution, with an average diameter of 1.1 mm and length of 3.1 mm for MC31, and 1.65 mm and 3.37 mm for the MC11 compound. The MC13 with higher Co ratio evolves into an irregular shape, but still demonstrates a cage-like morphology (Fig. 2c). The detailed hierarchical structure of the MoSe 2 /CoSe 2 composites is highlighted in the inset of Fig. 2b. The inner surface of the microcage is dense and smooth, while the external surface is composed by multi-fold nanosheets with a thickness of 5-10 nm. The elemental mapping characterization (Fig. S3 †) shows a uniform distribution of both Mo and Co, indicating conucleation and crystallization of the MoSe 2 and CoSe 2 during the hydrothermal reaction. The co-growth mechanism leads to a thorough mix of MoSe 2 and CoSe 2 at nanoscale level, which can facilitate the synergic effect between MoSe 2 and CoSe 2 by inducing defects formation. What is interesting, the much more strong colour of Mo element in Fig. S3 † may imply that MoSe 2 prefer to grow on the surface of the microcage forming the nanosheets, while Co prone to crystalize in the inner part of the microtube. The formation of the unique hierarchical nanosheet-assembled microcage can be reasonably attributed to the competing nucleation and growth processes and different crystallization kinetics existing in molybdenum-and cobaltbased chalcogenides. Although the precise growth mechanism of the nanosheets assembled hierarchical microcage is not completely clear yet, the highly tunable structure induced by merely altering the Mo/Co ratio is a very encouraging evidence for further exploration of the Mo/Co hybrid nanocomposite.  Fig. 2f features a clear narrow strip distribution of the MoSe 2 phase in the main CoSe 2 . These results also prove that the MoSe 2 /CoSe 2 composite is constituted by quasiamorphous MoSe 2 and polymorphic CoSe 2 , which is consistent with the ndings from the XRD analysis. Besides, defects are also observed along the MoSe 2 nano-strip. These defects act as active sites for HER, while the dense and smooth inner surface of the microcage improves signicantly its conductivity.
To investigate the HER performance of the synthesized MoSe 2 /CoSe 2 composite, the electrochemical measurements were carried out on the bare GC electrode in a 0.5 M H 2 SO 4 solution using a typical three-electrode setup. For comparison, HER catalytic measurements using MoSe 2 , CoSe 2 , Pt and GC were also performed. All measurements were carried out at the same optimized loading weight of 0.285 mg cm À2 . As shown in Fig. 3 and S4, † all the products related to the MoSe 2 /CoSe 2 composites exhibit a superior HER performance when compared to the ones of the pristine MoSe 2 and CoSe 2 . The onset overpotential (h) of MC11 is 110 mV, obviously lower than the 200 mV of the pure MoSe 2 , and the 140 mV of CoSe 2 . MC11 shows an extremely large cathodic current density of 90.4 mA cm À2 at h ¼ 350 mV, which is 28 times larger than the one observed in MoSe 2 , and also larger than the 60.2 mA cm À2 observed in CoSe 2 . From Fig. S4a † it is quite evident that the cathodic current density of 90.4 mA cm À2 at h ¼ 350 mV for MC11 is one of the best electrochemical performances of the MoSe 2 /CoSe 2 composite catalysts, suggesting a superior HER activity of the MoSe 2 /CoSe 2 catalyst when using the optimized composition. The corresponding Tafel plots of the above HER catalysts are shown in Fig. 3b and S4b, † in which the dotted line along the linear sections have been tted according to the Tafel equation. The MoSe 2 /CoSe 2 composite possesses smaller Tafel slopes (73 mV per dec for MC11, 85 mV per dec for MC31, and 93 mV per dec for MC13) than the ones typical of pure MoSe 2 (144 mV per dec) and CoSe 2 (95 mV per dec), therefore highlighting a faster increasing rate of H 2 generation with increasing h values. The Tafel slope can also be adopted for the assessment of the rate-determining step in the HER process. 49,79 In acidic medium, the whole HER process includes the following three steps. First, H + is adsorbed onto the catalyst surface through a discharge step with Volmer reaction (H 3 O + + e / H ads * + H 2 O, b z 120 mV per dec, with b being the Tafel slope). Then the unstable H ads * migrates towards a more stable position on the catalyst surface with Spillover reaction: (H ads * / H ads , b z 60 mV per dec). Finally, an electrochemical desorption step or a recombination step is needed to generate H 2 generation with Heyrovsky reaction (H ads + H 3 O + + e / H 2 + H 2 O, b z 40 mV per dec), or Tafel reaction (H ads + H ads / H 2 , b z 30 mV per dec). 79 The Tafel slope in the range of 73-93 mV per dec for the MoSe 2 / CoSe 2 composite indicates that the Volmer or Spillover reaction may probably be the rate-determining step in the HER process.
By extrapolating the Tafel plots as shown in Fig. 3c it is possible to obtain the exchange current densities (j 0 ). All the electrochemical parameters are summarized in Table 1. We can observe that MoSe 2 /CoSe 2 exhibits larger exchange current density compared to the pristine MoSe 2 and CoSe 2 . We should also note that, although CoSe 2 exhibits a signicantly better HER behaviour than the MoSe 2 , the MC13 compound with the highest CoSe 2 proportion has the worst performance amongst all the MoSe 2 /CoSe 2 composite catalysts. On the opposite, the MC11 with a medium Co content demonstrates a remarkable exchange current density of 117.5 mA cm À2 , which is larger than the one obtained in the other compounds and almost 3.7 and 2.75 times larger than MoSe 2 and CoSe 2 , respectively. The Nyquist plots of the EIS response provide further considerations about the electrode kinetics under HER. As can be observed from Fig. 3d and S4 † all Nyquist plots consist of a tiny portion of circle plus a slash line. The charge-transfer resistance (R CT ) and series resistance (R S ) (as listed in Table 1) have been extracted from the Nyquist plots by tting of the circle arc portion with the simplied Randles equivalent circuit 6 using the EC-lab soware. The data listed in Table 1 show a remarkable decrease of the R CT and R S in all the CoSe 2 /MoSe 2 composite samples, which implies an enhancement of the inter-domain conductivity. 6 In particular, the R CT decreases from 770 U to $2 U with the introduction of CoSe 2 into MoSe 2 , and the improved conductivity is directly corresponding to the highly enhanced HER properties. We can therefore conclude that the enhancement of the HER activity observed in the MoSe 2 /CoSe 2 composite can be attributed to its unique hierarchical morphology, as well as to the synergic effect between the MoSe 2 and CoSe 2 nanocomposites. The increased conductivity originated by the assembled microcages contributes to the larger cathodic current, and at the same time efficient active edge sites were induced by the hybridization of the MoSe 2 and CoSe 2 at nanoscale, which plays an important role in reducing the OP in the HER process.
Cycling stability has been tested by continuous cyclic voltammetry between À0.5 V and 0.2 V (vs. RHE) at 50 mV s À1 for 1000 cycles in 0.5 M H 2 SO 4 . As shown in Fig. 4, the current density of the as-synthesized tube-like MoSe 2 /CoSe 2 composite nanosheets exhibits no obvious degradation, which suggests the presence of a good cycling performance and stability. The negligible degradation of the current density might be caused by the consumption of H + or the remaining H 2 bubbles on the surface of the catalyst, which hinder the reaction. 28,49

Conclusion
We have explored the possibility of developing a facile one-pot hydrothermal method to synthesize hierarchical nanosheetassembled MoSe 2 /CoSe 2 microcages for electrocatalytic HER. Compared against pristine MoSe 2 and CoSe 2 , the MoSe 2 /CoSe 2 nanocomposite shows signicantly enhanced HER catalytic activity with a lower onset overpotential of 75 mV, larger cathodic current, and a smaller Tafel slope. The improvement of the HER performance can be attributed to the increase of conductivity introduced via the nanosheets-assembled microcages, and the defects generated by the MoSe 2 /CoSe 2 hybridization at nanoscale. The facile synthesis procedure and the easily controllable hierarchical structure generated open a new way to develop superior hybrid catalysts for HER.