Vertically oriented MoS2 nanosheets with vast exposed edges for enhanced electrocatalytic hydrogen evolution

Molybdenum disulfide (MoS2), a typical two-dimensional transition metal dichalcogenides, is a promising candidate for electrochemical water splitting catalysis due to its ultrahigh special area and highly exposed active edge sites. The main challenges restricted the wider application for MoS2-based nanomaterials are the complex preparation process and the high overpotential. Here, we design a novel and facile sealed vessel to synthesize vertical oriented MoS2 nanosheets electrocatalyst with vast exposed edges. Benefiting from the unique vertically-oriented structural and compositional characteristics, the MoS2 nanosheets with 10-20 layers exhibits superior hydrogen evolution reaction (HER) performance with a small overpotential of 135.2 mV at a current density of 10 mA∙cm-2 and low Tafel slope of 82.5 mV∙dec-1 as well as extraordinary catalytic stability over 5000 cycles. Importantly, the sealed vessel reactor system may open up a versatile and potential synthetic way to construct various morphologies and structure of metal dichalcogenides for high-performance energy storage and conversions devices.


Introduction
2D layered transition metal dichalcogenides (TMDCs) owing to their exciting electrochemical and optoelectronic properties, which have inspired a broad range of fundamental and practical interests in electrochemical energy storage and conversion as well as optoelectronic device [1,2]. Particularly, MoS 2 has emerged as a new possibility beyond commercial 20 wt% Pt/C catalyst in facilitating efficient electrochemical hydrogen evolution [3,4]. However, like many other metal sulfides-based catalysts, MoS 2 -based catalysts are still plagued with two main challenges: the complicated synthesis process and high overpotential caused by a small quantity active edge sites and the low intrinsic electric conductivity, impede its broad range of applications [3]. Up to now, Pt-based electrocatalysts exhibit most excellent electrocatalytic activity, but the scarcity and high-cost seriously restricted their widespread applications [5,6]. Therefore, the research for an earth-abundant, high catalytic activity and commercial water-splitting MoS 2 -based electrocatalyst is still imperative.
For this purpose, enormous synthetic method and strategies have been explored to design and midificate catalytic functionality of MoS 2 . One of the major synthesis methods is chemical vapour deposition (CVD) [7], which can effectively control morphological of MoS 2 , introduce sulfur vacancy sites and strain or disorder [8]. However, the use of H 2 S or H 2 /N 2 forming gas always cause environmental pollution and resource waste in an open system. Moreover, additional electronic and structural modification always need high precision experimental equipment. Another major strategy is solution-processed approaches [9], which can engineer various morphologies and structure of MoS 2 to enhance their intrinsic activity and conductivity by atomic-scale engineering [10] or coupling with conductive substrate, such as carbon nanosheet [11], carbon nanotube [12], graphene [13], or 3D metals [14]. Whereas, it still needs to be pointed out that solution-processed approaches is normally complicated, uncontrollable and require multiple process [15]. In addition to, electrodeposition is also an important strategy. For example, Kibsgaard [16] et al. engineered MoS 2 mesoporous films with doublegyroid via template-assisted electrodeposition to maximally expose active edge sites due to the existence of the predominant inert (0001) basal plane with relatively larger Gibbs free energy [8]. Nevertheless, removing the template may influence the structure of MoS 2 . Therefore, seeking to simple and high-yield approaches to synthesize hierarchical MoS 2 -based nanomaterial is requisite [17,18].
Here, we design a novel and facile sealed vessel to synthesize MoS 2 -based nanostructures and investigate their electrocatalytic performance. The sealed vessel is a significant and attractive synthetic method due to its onestep preparation via mixed precursors without organic solvents or toxic gases. By virtue of sealed vessel, we first have designed and synthesized vertically oriented MoS 2 nanosheets with enormous exposed edges, which exhibits remarkable electrochemical performance with small Tafel slope of 82.5 mV•dec -1 and an overpotential of 135.2 mV at a current density of 10 mA•cm -2 as well as extraordinary catalytic stability over 5000 cycles.

Preparation of MoS2 nanosheets
The overall synthesis approach involves the following steps. In brief, first, all reaction source were mixed with pestle for several minutes under ambient conditions where 0.2 g H 3 P(Mo 3 O 10 ) 4 •xH 2 O was used as the molybdenum source, and 0.3 g sulfur powder as the sulfur source. Subsequently, the mixed precursor was transferred into and sealed a homemade sealed vessel in the Ar-filled glove box. Finally, the sealed vessel was heated at 600 °C for 2 hours with a heating ramp rate of 10°Cꞏmin -1 under inert atmosphere to obtain MoS 2 nanosheets. After the sealed vessel natural cooling to room temperature in the furnace, The final product was gained for further analysis.

Characterizations
Morphology was investigated field-emission scanning electron microscopy (Hitachi FESEM S-4700) equipped with EDS. Transmission electron microscopy (TEM, TECNAI G2 F20) was used to test the microstructures. The Raman was performed using Renishaw RW100 Micro-Raman system system with an excitation laser line of 514.6 nm in ambient air environment. Crystal structure of the materials was performed by a Grazing Incident Angle XRD (GIAXRD) with Cu Kα (λ = 0.154 nm) as the radiation source XRD (Bruker D2). The Xray photoemission spectroscopy (XPS ESCALAB-250) equipped with Al (K-alpha) source was carried out to obtain the bonding information of the elements and Co doping contents.

Electrochemical measurements
Electrochemical measurements were performed with an electrochemical analyzer (CHI760D) under ambient conditions. Rotation disk electrodes of grassy carbon (GC, PINE, 4 mm diameter) modified with different catalysts were rotated at 2000 rpm. In a typical modification process, the GC surface was polished to a mirror finishing followed by a thorough cleaning with H 2 O in ultrasonication bath. 2 mg of the synthesized material were dispersed in mixed solvent: 360 μL waterethanol (v:v=3:1) and 40 μL Nafion solution (5wt%) followed by ultrasonincation for 120 min, forming a homogeneous suspension with a concentration of 5mgꞏmL -1 . 10 μL of catalyst dispersion was drop into a cleaned GC electrode with pipet and the solvent was evaporated at room temperature (≈0.40 mgꞏcm -2 ). In the measurement, the modified GC electrodes were acted as working electrode with a graphite rod as counter electrode, and Ag/AgCl (3 M KCl) as reference electrode in the three-electrode electrochemical cell. An aqueous solution of 0.5 M H 2 SO 4 with a pH value of 0.1 was used as electrolyte and purged with N 2 gas for at least 30 min before measurements. The HER performance was investigated using linear sweep voltammetry (LSV) under potentials between 0.1 and -0.6 V versus RHE at the scan rate of 5 mVꞏs -1 . The stability of catalysts was examined by consecutive LSV scans over 5000 cycles. All the data here were collected without iR compensation and all the potentials were calibrated with respect to a RHE by the equation E (vs. RHE) = E 0 (Ag/AgCl)+E (vs. Ag/AgCl) +0.059PH.  The TEM and high-resolution TEM images (Figure 2) show that the MoS 2 nanosheets possess the layer number of 10-20. So thin nanosheets are favorable for the exposure of active sites. Figure 3a shows that all the identified peaks can be uniquely indexed to a hexagonal phase of MoS 2 (JCPDS card no. 37-1492). The Raman spectrum (Figure 3b) shows two characteristic peaks located at 373.2 and 401.2 cm -1 , which are assigned to the E 1 2g and A 1g band of MoS 2 material, respectively. Figure 3c shows that the peak positions of the Mo 3d 3/2 and Mo 3d 5/2 binding energies of Mo 4+ are located around 232.18 and 228.87 eV, respectively. Figure 3d shows two peaks located at 163.49 and 162.29 eV for the S 2p XPS spectrum, corresponding to the characteristic peaks of S 2p 1/2 and S 2p 3/2 , respectively. The above results fully demonstrate the formation of hexagonal MoS 2 .  Figure 4a shows the typical cathodic polarization curve of MoS 2 nanosheets, which shows a small overpotential of only 135.2 mV at the current density of 10 mA•cm -2 . To understand the detailed underlying mechanism of HER activity of catalysts, Tafel plots curves are acquired, as shown in Figure 4b, which shows a small Tafel slope of ~ 82.5 mV•dec -1 and indicates its HER mechanism follows Volmer-Heyrovsky reaction where electrochemical desorption is the rate-determining step. To further evaluate the stability of the catalysts, consecutive LSV scans are carried out. As shown in Figure 4c, after 5000 cycles, the LSV curves show very small shifts, further demonstrating the high stability of the obtained electrocatalysts.

Results and discussion
The excellent HER performance may be attributed to the few-layered and the vertically oriented nanostructure, which can expose more active sites, decrease the distance of electron transfer, and well maintain the material stability.

Conclusions
Here, we have designed a novel hermetic container to one-step synthesize vertically oriented MoS 2 nanosheets with vast exposed edges. As expected, as HER electrocatalysts, the MoS 2 nanosheets exhibit remarkable electrochemical performance with high current density, low Tafel slope, and persistent stability in acid solution. The dramatically improved electrocatalytic performance of the MoS 2 nanosheets is attributed to its unique structure, such as vertically orientation, few-layered structure, and vast exposed edges. Importangtly, Compared to other synthetic approach, the method with hermetic sealed vessel is facile and easily reproducible for synthesizing the MoS 2 nanosheets by utilizating low-cost precursors, which could be extended to other materials for better large-scale production of electrochemical energy storage and conversions devices.