Hierarchically Ordinated Two-Dimensional MoS 2 Nanosheets on Three-Dimensional Reduced Graphene Oxide Aerogels as Highly Active and Stable Catalysts for Hydrogen Evolution Reaction

: Hydrogen gas (H 2 ) is being intensively proposed as a next-generation clean energy owing to the depletion of fossil fuels. Electrochemical water splitting is one of the most promising processes for hydrogen production. Furthermore, many efforts focusing on electrochemical water splitting have been made to develop low-cost, electrochemically active, and stable catalysts for efﬁcient hydrogen production. MoS 2 has emerged as an attractive material for developing catalysts for the hydrogen evolution reaction (HER). Hence, in this study, we design hierarchically ordinated two-dimensional (2D) MoS 2 nanosheets on three-dimensional (3D) reduced graphene oxide (rGO) (H-2D/3D-MoS 2 -rGO) aerogel structures as a new class of electrocatalysts for the HER. We use the one-pot hydrothermal synthesis route for developing high-performance electroactive materials for the HER. The as-prepared H-2D/3D-MoS 2 -rGO contains a unique 3D hierarchical structure providing large surface areas owing to the 3D porous networks of rGO and more active sites owing to the many edge sites in the MoS 2 nanosheets. In addition, the H-2D/3D-MoS 2 -rGO structure exhibits remarkable electrochemical properties during the HER. It shows a lower overpotential than pure MoS 2 and excellent electrochemical stability owing to the large number of active sites (highly exposed edge sites) and high electrical conductivity from the rGO structure.


Introduction
Hydrogen gas (H 2 ) is a promising renewable and inexhaustible energy source for replacing generally used fossil fuels [1][2][3]. In particular, the hydrogen evolution reaction (HER) process is crucial for hydrogen production. Platinum and other noble metal-based materials have proved to be the most attractive electrocatalysts for the HER, with high activity and durability in acidic or basic media [4][5][6][7]. However, in terms of the production cost of H 2 as per the price of Pt catalysts, the production of H 2 from noble Pt materials is not effective [8,9]. Therefore, in recent years, transition metal oxides/sulfides have emerged as effective alternatives to Pt and Pt-based electrocatalysts. Numerous studies have been devoted to enhancing the electrochemical properties of transition metal oxides/sulfides/phosphides/ carbides [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. For example, Shen and Yin's research groups reported that the bimetallic nickel-cobalt phosphide shows improved HER activity and stability, due to its unique morphology with nanosheets on the 3D integrated framework, providing a large surface area and many active sites for HER [24]. In a recent works, among many HER catalyst candidates, two-dimensional MoS 2 , an earth-abundant material, with its unique layered structure and chemical properties, has shown attractive HER catalytic activity with the corresponding electrolyte ion adsorption and desorption energies [10,11]. However, the basal plane of pure MoS 2 is catalytically inert, and only the edge S sites, occupying a small area in pure MoS 2 , are catalytically active [29][30][31][32][33][34]. Therefore, increasing the number of active edges for forming large electrolyte ion adsorption/desorption sites and enhancing the electrical conductivity of pure MoS 2 for facile charge transport are the two main strategies for obtaining high HER catalytic activities.
Structural engineering of MoS 2 through the miniaturization of its size, shape, and dimensions is an effective and viable strategy for dramatically increasing the total number of edge sites, which are related to catalytic active sites. Mo-edge sites of MoS 2 can provide high electrochemical activity and exhibit the low free energy of hydrogen adsorption (∆G H * ) compared to inert adsorption sites at the basal plane of MoS 2 [10,31,[35][36][37]. Therefore, many studies have focused on vertically grown MoS 2 sheets, MoS 2 nanostructures of different shapes, and various MoS 2 heterostructures for achieving a high density of edge sites [31,[38][39][40][41][42][43]. Concurrently, controlling the conductivity of structurally engineered MoS 2 can synergistically improve its overall catalytic performance during the HER. In this regard, highly conductive materials such as carbon-based materials (e.g., graphene, carbon nanotubes) can be used as a subclass of conducting networks or substrates, because these materials can facilitate fast charge carrier transport, thus enhancing the HER activity [39,[44][45][46][47][48]. In particular, reduced graphene oxide (rGO) is one of the most attractive materials for producing conducting templates because of its chemical stability, high conductivity, and excellent fabrication compatibility with solution-based processes [49][50][51]. Moreover, rGO can function as an excellent backbone network for constructing threedimensionally dispersed catalytic materials [52][53][54][55]. However, it is challenging to develop the appropriate synthetic routes and fabrication steps for structural engineering of MoS 2 using conductive carbon materials, such as rGO, to achieve highly dispersed catalytic materials. These unique engineering approaches can be particularly beneficial in terms of enhancing the surface area, ion diffusion paths, and conductivity of the catalysts used for the HER.
Therefore, in this work, we developed a simple hydrothermal method to synthesize hierarchically ordinated two-dimensional (2D) MoS 2 nanosheets on three-dimensional (3D) rGO (H-2D/3D-MoS 2 -rGO) aerogels. The proposed H-2D/3D-MoS 2 -rGO heterostructures were fabricated via a one-pot and facile hydrothermal synthesis route. Moreover, through freeze-drying, the original 3D hydrogel structures were maintained without shrinkage deformation of H-2D/3D-MoS 2 -rGO as it developed a unique light and hard aerogel structure during the drying process. The as-prepared H-2D/3D-MoS 2 -rGO structures exhibited a large surface area and excellent HER catalytic activity because of the large number of active sites (highly exposed edge sites), highly dispersed MoS 2 nanosheets in the rGO networks, and high electrical conductivity, thus ensuring favorable electrochemical performance during the HER.  Figure 2a presents an illustration of the assembled H-2D/3D-MoS 2 -rGO structures. The thin MoS 2 nanosheets are uniformly distributed on the backbone of the 3D rGO networks, which provide hierarchical structures. As mentioned in the introduction, a well-engineered MoS 2 -based HER catalyst with a large surface area and high density of edge sites is most important for achieving enhanced electrochemical activity in the HER. As shown in Figure 2a, the fabricated H-2D/3D-MoS 2 -rGO electrode will show the superior electrochemical properties owing to its unique hierarchical structures consisting of the MoS 2 nanostructures and 3D porous rGO network, which can provide the enlarged surface area and active sites for HER, respectively. In addition, although the H-2D/3D-MoS 2 -rGO has a highly porous network, the as-obtained H-2D/3D-MoS 2 -rGO aerogel sustained an 8 mL glass vial (approximately 9 g) without any shape deformation (left side of Figure 2b). Moreover, the sample is very light (total weight of 239 mg and density of 45.1 mg cm −3 ), enough so to be placed on a whole foxtail. These mechanical features are most important for the design and development of durable and light electrode materials from an economic perspective in the energy industry. Figure 1 schematically illustrates the fabrication steps of the hierarchically ordinated H-2D/3D-MoS2-rGO aerogels using the one-pot hydrothermal synthesis route. Figure 2a presents an illustration of the assembled H-2D/3D-MoS2-rGO structures. The thin MoS2 nanosheets are uniformly distributed on the backbone of the 3D rGO networks, which provide hierarchical structures. As mentioned in the introduction, a well-engineered MoS2-based HER catalyst with a large surface area and high density of edge sites is most important for achieving enhanced electrochemical activity in the HER. As shown in Figure  2a, the fabricated H-2D/3D-MoS2-rGO electrode will show the superior electrochemical properties owing to its unique hierarchical structures consisting of the MoS2 nanostructures and 3D porous rGO network, which can provide the enlarged surface area and active sites for HER, respectively. In addition, although the H-2D/3D-MoS2-rGO has a highly porous network, the as-obtained H-2D/3D-MoS2-rGO aerogel sustained an 8 mL glass vial (approximately 9 g) without any shape deformation (left side of Figure 2b). Moreover, the sample is very light (total weight of 239 mg and density of 45.1 mg cm -3 ), enough so to be placed on a whole foxtail. These mechanical features are most important for the design and development of durable and light electrode materials from an economic perspective in the energy industry.  To confirm the 3D hierarchical structure of the H-2D/3D-MoS 2 -rGO, we carried out SEM analysis. Figure 2c presents an SEM image of the as-obtained H-2D/3D-MoS 2 -rGO samples. Extremely thin nanostructured rGO sheets with uniform and smooth surface morphology form an interconnected 3D microscale porous network. Moreover, small nanoscale MoS 2 sheets with sizes of 200-300 nm are uniformly distributed and deposited on the 3D porous networks of rGO. These unique interconnected 2D/3D structures might have resulted from the one-pot hydrothermal synthetic route and fast freeze-drying procedure, during which the H-2D/3D-MoS 2 -rGO maintained its original aerogel structure without any deformation. Next, the elemental distributions of the H-2D/3D-MoS 2 -rGO samples were identified via EDX elemental mapping analysis (Figure 2d). The Mo, S, and C elements are clearly observed throughout the sample, confirming that these elements are uniformly distributed among the hierarchical structures. The highly ordered structure and dispersion of MoS 2 might have provided a large surface area and many active edge sites to improve the overall electrochemical performance during the HER. Contrary to the H-2D/3D-MoS2-rGO, the pure MoS 2 synthesized without rGO appeared as micron-sized particles (Figure 3a), indicating that pure MoS 2 particles could have relatively low specific surface areas. In fact, To confirm the 3D hierarchical structure of the H-2D/3D-MoS2-rGO, we carried out SEM analysis. Figure 2c presents an SEM image of the as-obtained H-2D/3D-MoS2-rGO samples. Extremely thin nanostructured rGO sheets with uniform and smooth surface morphology form an interconnected 3D microscale porous network. Moreover, small nanoscale MoS2 sheets with sizes of 200-300 nm are uniformly distributed and deposited on the 3D porous networks of rGO. These unique interconnected 2D/3D structures might have resulted from the one-pot hydrothermal synthetic route and fast freeze-drying procedure, during which the H-2D/3D-MoS2-rGO maintained its original aerogel structure without any deformation. Next, the elemental distributions of the H-2D/3D-MoS2-rGO samples were identified via EDX elemental mapping analysis (Figure 2d). The Mo, S, and C elements are clearly observed throughout the sample, confirming that these elements are uniformly distributed among the hierarchical structures. The highly ordered structure and dispersion of MoS2 might have provided a large surface area and many active edge sites to improve the overall electrochemical performance during the HER. Contrary to the H-2D/3D-MoS2-rGO, the pure MoS2 synthesized without rGO appeared as micron-sized particles (Figure 3a), indicating that pure MoS2 particles could have relatively low specific surface areas. In fact, Brunauer-Emmett-Teller (BET) analysis (Figure 3b) revealed that the specific surface area of the H-2D/3D-MoS2-rGO was 42.2 m 2 g -1 , which is approximately 2.7 times higher than that of pure MoS2 (15.5 m 2 g -1 ).   Figure 4b shows the Raman spectra of the as-prepared samples. For the H-2D/3D-MoS 2 -rGO and pure rGO samples, two representative peaks can be identified at~1348 and 1583 cm −1 , which correspond to the characteristic D and G bands, respectively [39,56]. For the H-2D/3D-MoS 2 -rGO and pure MoS 2 samples, two peaks were clearly observed at~379 and~405 cm −1 , which are in accordance with the in-plane (E 1 2g ) and out-of-plane (A 1g ) modes, respectively [39,57]. It is highly expected that the H-2D/3D-MoS 2 -rGO electrode will contribute to the improved electrochemical properties in HER due to its excellent structural features from the crystalline MoS 2 and rGO. To clearly reveal the presence of the corresponding elements, we carried out X-ray photoelectron spectroscopy (XPS) analysis. Figure 4c presents the high-resolution XPS spectra of the Mo 3d, S 2p, and C 1s regions with the fitted peak components. First, the Mo 3d doublet is clearly observed at~229.4 eV (Mo 4+ 2d 5/2 ) and at~232.6 eV (Mo 4+ 2d 3/2 ) [33,44,48,56,57]. Moreover, the two peaks at binding energies of~163.5 and~162.3 eV correspond to S 2+ with the p spin-orbit splitting, indicating the existence of MoS 2 phase [33,44,48,57]. In addition, the XPS spectrum of C 1s for the H-2D/3D-MoS 2 -rGO was deconvoluted into the four peaks located at around 284.5 eV (C-C), 285.6 eV (C-O), 287.0 eV (C=O), 288.6 eV (O=C-O), and 290.8 eV [44,48,56,58]. In particular, the intensive peak at around 284.5 eV corresponded to the C-C bond, revealing that the GO particles were converted to the rGO network. Thus, a comparison of the SEM, EDX, XRD, Raman, and XPS results provides direct evidence for the formation of the hierarchically ordinated H-2D/3D-MoS 2 -rGO aerogels.

Results and Discussion
To evaluate their electrochemical performance, the H-2D/3D-MoS 2 -rGO and pure MoS 2 samples were applied directly as working electrodes. Moreover, a Pt/C electrode was used as the reference electrode to objectively compare the performances of the assynthesized H-2D/3D-MoS 2 -rGO and pure MoS 2 electrodes. The corresponding HER polarization linear sweep voltammetry (LSV) curves and Tafel plots of both samples are shown in Figure 5a,b, respectively. Notably, the H-2D/3D-MoS 2 -rGO sample exhibits a relatively low overpotential of 286 mV at a current density of 10 mA cm −2 compared to pure MoS 2 . To further understand the improved catalytic behavior of the H-2D/3D-MoS 2 -rGO structure, the Tafel plots and calculated Tafel slopes for the H-2D/3D-MoS 2 -rGO and pure MoS 2 samples were derived from the polarization curves. As observed in Figure 5b, H-2D/3D-MoS 2 -rGO delivers a Tafel slope of 77 mV dec −1 , which is obviously lower than that that of pure MoS 2 (125 mV dec −1 ) and other MoS 2 -based literatures (summarized in Table 1) [53,[56][57][58][59]. The improved performance during the HER could be attributed to the increased number of active sites resulting from the hierarchically designed structure utilizing the 2D MoS 2 and 3D porous networks of rGO, where the ion adsorption and desorption processes could occur. Moreover, the high electrical conductivity of 3D rGO could have enhanced the overall electrochemical performance (Figure 5c). Finally, cycling tests of H-2D/3D-MoS 2 -rGO were conducted to investigate its electrochemical stability during the HER. As shown in Figure 5d, the H-2D/3D-MoS 2 -rGO electrode shows only a slight HER activity loss (~7 mV increase in the overpotential at a current density of 10 mA cm −2 ) for 20 h. In addition, the initial performance of H-2D/3D-MoS 2 -rGO was well maintained even after the 40 h electrochemical test (Figure 5e), confirming its high stability in acidic media for the HER.  To evaluate their electrochemical performance, the H-2D/3D-MoS2-rGO and pure MoS2 samples were applied directly as working electrodes. Moreover, a Pt/C electrode was used as the reference electrode to objectively compare the performances of the assynthesized H-2D/3D-MoS2-rGO and pure MoS2 electrodes. The corresponding HER polarization linear sweep voltammetry (LSV) curves and Tafel plots of both samples are shown in Figure 5a,b, respectively. Notably, the H-2D/3D-MoS2-rGO sample exhibits a relatively low overpotential of 286 mV at a current density of 10 mA cm −2 compared to pure MoS2. To further understand the improved catalytic behavior of the H-2D/3D-MoS2-rGO structure, the Tafel plots and calculated Tafel slopes for the H-2D/3D-MoS2-rGO and pure MoS2 samples were derived from the polarization curves. As observed in Figure 5b, H-2D/3D-MoS2-rGO delivers a Tafel slope of 77 mV dec -1 , which is obviously lower than that that of pure MoS2 (125 mV dec -1 ) and other MoS2-based literatures (summarized in Table 1) [53,[56][57][58][59]. The improved performance during the HER could be attributed to the increased number of active sites resulting from the hierarchically designed structure utilizing the 2D MoS2 and 3D porous networks of rGO, where the ion adsorption and desorption processes could occur. Moreover, the high electrical conductivity of 3D rGO could have enhanced the overall electrochemical performance (Figure 5c). Finally, cycling tests of H-2D/3D-MoS2-rGO were conducted to investigate its electrochemical stability during the HER. As shown in Figure 5d, the H-2D/3D-MoS2-rGO electrode shows only a slight HER activity loss (~7 mV increase in the overpotential at a current density of 10 mA cm -2 ) for 20 h. In addition, the initial performance of H-2D/3D-MoS2-rGO was well maintained even after the 40 h electrochemical test (Figure 5e), confirming its high stability in acidic media for the HER.

Synthesis of H-2D/3D-MoS 2 -rGO
To synthesize the H-2D/3D-MoS 2 -rGO aerogels, commercially available GO was first modified from graphite powder (Graphene supermarket, Ronkonkoma, NY, USA) using the Hummers method to increase its surface area [60][61][62]. Three grams of graphite powder, 3 g of sodium nitrate (NaNO 3 , Sigma-Aldrich, Saint Louis, MO, USA), and 100 mL of sulfuric acid (H 2 SO 4 , Sigma-Aldrich, Saint Louis, MO, USA) were mixed, and then 10 g potassium permanganate (KMnO 4 , Sigma-Aldrich, Saint Louis, MO, USA) was slowly mixed in an ice bath. Next, the solution was continuously stirred to oxidize the graphite powder for 1 h at 95 • C. The resulting solution was diluted with distilled water, and 5 mL of hydrogen peroxide (30% H 2 O 2 , Sigma-Aldrich, Saint Louis, MO, USA) was added to the solution. Finally, to purify, the resulting solution was rinsed with 5% hydrochloric acid (HCl, Sigma-Aldrich, Saint Louis, MO, USA) and D.I water to obtain the graphite oxide (GO). The surface-treated GO was dispersed in distilled water at a concentration of 5 mg mL −1 . The H-2D/3D-MoS 2 -rGO structures were prepared via one-pot hydrothermal synthesis. First, 309 mg sodium molybdate dihydrate (Na 2 MoO 4 ·2H 2 O, Sigma-Aldrich, Saint Louis, MO, USA) and 971 mg thiourea (CH 4 N 2 S, Sigma-Aldrich, Saint Louis, MO, USA) were dissolved in deionized water and 17.5 mL of the GO solution. The mixture was transferred into a Teflon-lined stainless-steel autoclave (iNexus, Inc., Seongnam-si, Gyeonggi-do, Korea) and then kept at 200 • C for 12 h. After natural cooling, we obtained the H-2D/3D-MoS 2 -rGO hydrogels. Finally, to form the H-2D/3D-MoS 2 -rGO aerogels, the hydrogel was freeze-dried for 48 h using a freeze dryer (FDB-5503, OPERON, Gimpo-si, Gyeonggi-do, Republic of Korea).

Characterization and Electrochemical Measurements
The structural features and elemental distributions of the as-prepared samples were analyzed using field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy (FE-SEM and EDX, Gemini SEM 300, ZEISS). The Brunauer-Emmett-Teller (BET) surface areas of the synthesized samples were analyzed through nitrogen sorption measurements (Belsorp mini X, MicrotracBEL Corp.). The crystalline structures of the as-prepared samples were examined via powder X-ray diffraction (XRD Miniflex 600, RIGAKU) and Raman spectroscopy (AXIS NOVA, Kratos, Korea Basic Science Institute-Jeonju Center). The surface chemical states of the H-2D/3D-MoS 2 -rGO were characterized by X-ray photoelectron spectroscopy (XPS, AXIS NOVA, Kratos, Korea Basic Science Institute-Jeonju center). Electrochemical analyses of the as-prepared samples were carried out using a potentiostat (PGSTAT302N, Metrohm, Autolab) in a three-electrode system with a graphite rod (Metrohm, Autolab) as the counter electrode and an Ag/AgCl electrode (in saturated 3 M KCl, Metrohm, Autolab) as the reference electrode in a 0.5 M H 2 SO 4 electrolyte solution (Sigma-Aldrich, Saint Louis, MO, USA). The loading mass on the glass carbon working electrode for pure MoS 2 , H-2D/3D-MoS 2 -rGO, and Pt/C is 150 µg total cm −2 , 150 µg total cm −2 , and 20 µg Pt cm −2 , respectively.

Conclusions
In summary, we successfully designed and developed hierarchically ordinated 2D MoS 2 nanosheets on 3D rGO aerogels via a one-pot hydrothermal synthesis route to achieve improved catalytic performance during the HER. Through various microscopic and spectroscopic investigations, we confirmed that the as-prepared H-2D/3D-MoS 2 -rGO has a 3D hierarchical structure, which can provide large surface areas owing to the 3D porous networks of rGO and more active sites owing to the many edges of the MoS 2 nanosheets. Therefore, the developed H-2D/3D-MoS 2 -rGO exhibited good electrochemical performance during the HER, showing a lower overpotential than pure MoS 2 and high electrochemical stability. Thus, it is expected that the as-prepared H-2D/3D-MoS 2 -rGO structures can be used as highly active and stable electrode materials in acidic HER systems.

Conflicts of Interest:
The authors declare no conflict of interest.