Coral-Shaped MoS2 Decorated with Graphene Quantum Dots

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Coral-shaped MoS2 Decorated with Graphene Quantum Dots Performed as a Highly Active Electrocatalyst for Hydrogen Evolution Reaction Bangjun Guo, Ke Yu, Honglin Li, Ruijuan Qi, Yuanyuan Zhang, Haili Song, Zheng Tang, Zi-Qiang Zhu, and Mingwei Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14035 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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ACS Applied Materials & Interfaces

Coral-shaped MoS2 Decorated with Graphene Quantum Dots Performed as a Highly Active Electrocatalyst for Hydrogen Evolution Reaction Bangjun Guo,† Ke Yu, *, †Honglin Li,† Ruijuan Qi,† Yuanyuan Zhang,† Haili Song,† Zheng Tang,† Ziqiang Zhu,† and Mingwei Chen‡,§, †

⊥

Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department

of Electronic Engineering, East China Normal University, Shanghai 200241, China ‡

State Key Laboratory of Metal Matrix Composites, School of Materials Science and

Engineering, Shanghai Jiao Tong University, Shanghai 200030, China §

WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

⊥

CREST, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan

ABSTRACT: We report a new CVD method to prepare coral-shaped monolayer MoS2 with a large amount of exposed edge sites for catalyzing hydrogen evolution reaction. The electrocatalytic activities of the coral-shaped MoS2 can be further enhanced by electronic band engineering via decorated with graphene quantum dot (GQD) decoration. Generally, GQDs improving the electrical conductivity of the MoS2 electrocatalyst. First-principle calculations

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suggest that the Coral MoS2@GQDs is a zero-gap material. The high electric conductivity and pronounced catalytically active sties give the hybrid catalyst an outstanding electrocatalytic performance with small onset overpotential of 95 mV and low Tafel slope of 40 mV/dec as well as excellent long-term electrocatalytic stability. The present work provides a potential way to design two-dimensional HER electrocatalysts through controlling the shape and modulating the electric conductivity.

KEYWORDS: coral shape, monolayer molybdenum disulfide, graphene quantum dots, electrocatalysis, hydrogen evolution reaction INDRODUCTION Developing clean, economical and renewable energy is a promising solution to solve the energy crisis, caused by the excessive reliance on fossil fuels energy.1−4 Among many possible alternatives, hydrogen produced by water splitting is one of the most reliable, convenient and clean energy resources.5−7 In particular, the electrochemical hydrogen evolution reaction (HER) has reached excellent performance by using platinum (Pt) group metals as catalysts.8,9 However, the scarcity and high price of Pt metals tremendously limit the further development. It remains challenging to exploit earth-abundant and inexpensive alternative materials, such as transition metal nitrides, selenides and sulfides, for high-efficiency H2 production.10−12 As a typical member of the transition metal disulfides, molybdenum disulfide (MoS2) has drawn significant attentions recently.13−18 The monolayer MoS2 particularly has attracted great interest for easily exposed edges, which are the main active sites for HER.19,20 MoS2 is prone to form two-dimensional (2D) morphology, which could be attributed to the intrinsic anisotropic structure.21,22 During the past few years, great efforts has been made to


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prove the high HER performance of single layer MoS2 by implementing chemical vapor deposition (CVD) synthesis method.20,23,24 Also several strategies have been developed to optimize their HER catalytic effects, such as developing the number of exposed active edge sites.25,26 However, traditional CVD methods commonly generate triangle-shaped monolayer MoS2 crystals (termed as “Triangle MoS2”) with less exposed edge sites and intrinsically lower catalytic activities.27 To overcome the obstacle, some methods have been developed, such as employing plasma exposure treatment on pristine triangle MoS2 monolayer to introduce more physical defects.28 While, the adopted treatment complicated the experimental procedure, and often leaded to decreased electrochemical stability of MoS2 as an electrocatalyst for HER. Herein, we reported the synthesis of the scattered coral-shaped monolayer MoS2 (termed as “Coral MoS2”) with a huge amount of exposed edges by an even simpler CVD method with pure MoS2 as only reactant source. The novel Coral MoS2 monolayers displays an observably improved catalytic performance compared with these previously reported triangle MoS2 nanosheets. Moreover, the efficiency of hydrogen evolution catalyzed by the coral MoS2 monolayers can be further increased by introducing graphene quantum dots (termed as “GQDs”) to form coral MoS2 composites (termed as “Coral MoS2@GQDs”). In general, GQDs improving the electrical conductivity of the MoS2 electrocatalyst.29,30 The mechanism of the excellent HER performance of the composited catalyst was studied by the first-principle calculations. The combination of the Coral MoS2 and GQDs is expected to greatly improve the electrochemistry activity by enhancing electrons transfer from graphene layer to MoS2 layer directionally.31 The calculation also reveals that the band structure can be effectively modified by compounding the two separated materials. The band gap of the composite Coral MoS2@GQDs clearly changes to zero, which lead to an


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easier electronic transportation and higher electric conductivity. After composite with GQDs, the charge density also become different from the pure MoS2. Both experimental and theoretical studies demonstrated that the Coral MoS2@GQDs nanocomposite materials perform an outstanding catalytic activity for HER. EXPERIMENTAL SECTION Preparation of Coral MoS2. The Coral MoS2 sample was synthesized by a chemical vapor deposition (CVD) method. Pure MoS2 powder source (Sigma–Aldrich, 99% purity) in a quartz boat was placed in the center of a horizontal quartz tube with slidable furnace (OTF-1200X80SL 1200 °C with 8 in. tube diameter), and the insulating substrate (Si substrate with a 275 nm SiO2 top layer) was placed downstream far from the oven center in a cooler zone (at about 500 °C during growth). The system was initially pumped to a base pressure of 20 mTorr and flushed with the Ar carrier gas (about 50 sccm) repeatedly at room temperature to remove oxygen contamination. The growth progress was strictly controlled by the heating/cooling rates (Figure S1a) and sliding actions. At the preparing stage, the slidable furnace was fixed at the left side of quartz tube far away from the MoS2 source, the temperature was increased to 900 °C (10 °C /min), as illustrated in Figure S1b. Then the slidable furnace was pushed to the reaction zone immediately, and was held there for 20–24 min with the Ar carrier gas flowing (about 20 sccm) and the pressure was maintained at about 10 Torr, as illustrated in Figure S1c. After the evaporation stage, the slidable furnace was pulled back to the left side of quartz tube to cool down and the temperature of reaction zone decline rapidly, as illustrated in Figure S1d. The fast decline of temperature prevented subsequent over-growth of Coral MoS2.


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Preparation of GQDs. In a typical procedure,32,33 0.5 g of carbon nanofibers were added into a mixed solvent of 25 mL HNO3 (65%) and 75 mL H2SO4 (98%). Then the solution was sonicated and stirred for 24 h under a heating temperature of 100 °C. After thermal oxidation, a clear brown-red solution was obtained and cooled to room temperature. The mixture was diluted with deionized (DI) water (900 mL) and adjusted pH value to 7 with Na2CO3. The GQDs solution was further dialyzed with a dialysis bag (retained molecular weight: 3000 Da) for 3 days. Finally, GQDs solution with size of 5–10 nm was received. Synthesis of Coral MoS2@GQDs composite. A drop of GQDs solution was lightly dispersed onto the substrates area with monolayer Coral MoS2. The concentration of the as-prepared GQDs solution was about 10 mg L−1 and a drop solution dispersed onto the area about 1.0 cm−2, which means the dopant density of each drop step is about 5×10−4 mg cm−2. Then the composite substrates were placed onto a hot plate and evaporated to dryness (40 °C) under a cleanly sealed room. To make sure a resultful composite, the previous drop step and evaporation progress could be repeated on the same substrates. Preparation of Triangle MoS2. The single-layer Triangle MoS2 samples were synthesized by another CVD method with tube furnace equipped with a 1 in. diameter quartz tube. MoO3 (Sigma–Aldrich, 99% purity) powder and S powder were used as Mo source and S source, respectively. A quartz boat with MoO3 powder was put at the center of the high-temperature furnace. S powder was placed outside the hot zone at the upstream region of the furnace at lower temperature. Ar gas (50 sccm) was used to convey MoO3 vapor to the substrate. The growth pressure was set at 100 Torr. The furnace temperature was raised up to 850 °C, and after that the temperature was held for 30 mins, meanwhile the temperature of sulfur was raised up to 200 °C.


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Transfer of the monolayer MoS2 and Coral MoS2@GQDs composite. The as-synthesized MoS2 and Coral MoS2@GQDs composite samples were transferred onto other substrates with a commonly used method. Firstly, poly (methyl methacrylate) (PMMA) thin film was spin-coated on the top of the Coral MoS2@GQDs/SiO2/Si substrate. Then, the SiO2 layer was etched by 2 M KOH solution and the PMMA/Coral MoS2@GQDs layer would lift off. After that, the PMMA/Coral MoS2@GQDs layer was then transferred onto a TEM grid or a clean glassy carbon electrode. Finally, the sample was air-dried and the PMMA was washed off with acetone and 2propanol. In our experiment, the transfers of Coral MoS2@GQDs composite samples were made after the synthesis of GQDs. Characterization. Raman and PL spectra of all samples were performed on a Jobin-Yvon LabRAM HR 800 micro-Raman spectrometer with a 488 nm laser. Morphology analysis of samples was inspected by atomic force microscopy (AFM, Digital Instruments Dimension 3100, Veeco). The field emission scanning electron microscopy (FE-SEM) images were carried out on a JEOL JSM-6700F SEM at an acceleration voltage of 3 kV and an in-lens detector. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a JEOL 2010 field emission electron microscope at a relative low voltage. X-ray photoelectron spectra (XPS) was collected on a Kratos Axis ULTRA X-ray photoelectron spectrometer, using a monochromatic Al Kα X-ray source and a hemispherical electron energy analyzer. Electrochemical Measurements. All of the electrochemical measurements were performed in a three-electrode system in N2 saturated 0.5 M H2SO4 electrolyte solution at room temperature. All samples were performed on the glassy carbon electrode (GCE, 3 mm in diameter) as working electrode, saturated Ag/AgCl (KCl-filled) and graphite rod (Sigma Aldrich) have been used as


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reference and counter electrodes, respectively. Linear sweep voltammetry (LSV) was tested with a scan rate of 5 mV s−1 from +0.2 to −0.6 V vs. RHE, using an electrochemical workstation (CHI660E, China). The electrochemical cycling stability of various samples was also conducted at a potential of 300 mV. The Nyquist plots (using the Autolab PGSTAT302N, Netherlands) were measured with frequencies ranging from 200 kHz to 0.1 Hz at an overpotential of 200 mV. Calculations. In order to model the Coral MoS2@GQDs composite structure, we considered the hybrid system of a monolayer of MoS2 on a monolayer graphene. The DFT based Vienna ab intio simulation package (VASP) was employed with the projector augmented wave (PAW) pseudopotential method in the framework of functional theory.34 First, the unit cell lattice parameters and geometries of pristine MoS2 were optimized until the forces on each atom are less than 0.01 eV/Å. A plane-wave basis energy cutoff of 500 eV, a 12×12×1 Γ-centered k-point grid, and 15 Å of vacuum were employed to converge in the total energy on the order of 1 meV per atom. The supercell for the hybrid MoS2@GQDs employed similar computational parameters. Band structure calculations were performed along the paths connecting the highsymmetry points: G (0, 0, 0), K (-1/3, 2/3, 0) and M (0, 0.5, 0) in the k-space.35 One thing to note is that the Mo edge is terminated with single S atom and that the Mo edge is not just a simple truncation of the bulk MoS2 structure. The Mo-edge termination with a single S atom matches that of the 50% sulfur covered Mo edge previously observed in model catalysts and predicted by DFT. We calculated the Gibbs free energies of different adsorption sites. It is commonly accepted that the criterion for a good HER catalyst is that the Gibbs free energy of adsorbed H should close to thermo-neutral (i.e., △G≈0). The results show that when an H atom adsorbed in the basal plane, Mo and S edges, the Gibbs free energies are calculated to be −1.73, −0.22 and 0.41 eV, respectively. Considering that the optimal value of △G should be around 0 eV, it is


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safe to conclude that the Mo-edge sites have the optimal HER activity. Next, when we added extra 0.06 e to the MoS2@GQDs composite, the △G significantly increases to −0.04 eV. It is remarkable that the △G of Mo-edge is approaching to thermo-neutral state. Therefore, it is safe to conclude that the HER activity should be improved when much more electrons involved in the corresponding electrochemical process for the main active site of MoS2 for HER is Mo-edge site that approaching to 0 eV. RESULTS AND DISCUSSION The morphology of Coral MoS2 was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Figure 1a shows a SEM image of Coral MoS2, illustrating scattered coral-shaped monolayer MoS2 with large amount of exposed edges. We also prepared other similar morphologies of Coral MoS2, which are exhibited in Figure S2 and S3 in the Supporting Information. It is obviously that the Coral MoS2 manufactured here is composed with large amount of MoS2 exposed edges. Figure 1b and 1c display the central and edge part of one independent Coral MoS2, respectively. One can see that the coral MoS2 is composed by MoS2 branches and each branch is made up with thinner coral twigs. The whole structure is well-organized. Edge morphology and the thickness of Coral MoS2 are illustrated by AFM images (Figure 1d). An apparent height of ~0.7 nm was observed, illustrating that the growth edges of the Coral MoS2 is almost monolayer.27,36 As shown in Figure 1e, the bright-field TEM image was also employed to demonstrate the unique morphology of the Coral MoS2, which agrees well with the observation of SEM and AFM. For comparison, details of over-grown Coral MoS2 (evaporated with a longer time) are also shown in Figure S4, S5 and S6. Moreover, we also prepared triangle-shaped MoS2 based on a traditional CVD method. The


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triangle-shaped MoS2 is confirmed a typical single-layered MoS2 by AFM and TEM. Morphologies of Triangle MoS2 are presented in Figure S7−S9, and the monolayer property of

Figure 1. Microscopic morphology of Coral MoS2. (a) SEM image of a coral-shaped MoS2. (b) and (c) Zoom-in SEM images of rectangle parts of (a). (d) AFM image showing a portion of a


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coral-shaped MoS2. The inset is the height profile along the marked line in (d). (e) Bright-field TEM image. (f) SEM images monitoring the growth process at different stages. Triangle MoS2 is also shown in Figure S10 for detail. Morphologies of the GQDs are also provided all in the Supporting Information presented by TEM images (Figure S11, S12) and AFM image (Figure S13). The coral-shaped monolayer MoS2 was synthesized by a simple one-step CVD method (see details in Figure S1). No catalyst was used in the novel MoS2 synthesis. The forming process of such special morphology could be clarified based on a fractal theory. Figure 1f illustrates the synthetic process of the unique coral-shaped MoS2. (i) Large numbers of MoS2 nuclei were formed, initially. (ii) Then these islands grew up along the two-dimensional space. (iii) When the nucleating stage had finished, some coral twigs began to sprout in a radial direction from the MoS2 islands. (iv)−(v) Coral twigs continued to expand, and then the coral branches took shape. (vi) Finally, the coral MoS2 was formed by a step-by-step fractal progress. As explained by the diffusion and chemistry limited aggregation (DCLA) model.37,38 The diffusive motion of vapored MoS2 crystalline grain is the main source for the growth of layered MoS2, while the low pressure and shrinking react time restrict the further growth of crystalline grain. Because the reactivity source is tiny here, the reaction time and pressure have remarkable influence on the formation progress according to the DCLA theory. It should be considered that the diffusion-limited branches should possess a universal fractal value which is the same as that of DCLA clusters. As the molecule grows to a certain size and the dendritic units can be replaced by a larger uniform coral shape, the entire surface will display the same fractal property at a large scale.39,40 As the reaction time went on, the small MoS2 pieces are deposited and the coral branches were shaped little by little, and the dispersive fractal structure was formed finally.41


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Figure 2. (a) HRTEM image of Coral MoS2. (b) SAED pattern recorded from (a). (c) HRTEM image around folded edges. (d) TEM image of Coral MoS2@GQDs. (e) and (f) HRTEM images of MoS2 and GQDs square parts of (d). High-resolution TEM (HRTEM) analysis was performed to investigate the atomic structure of the as-prepared MoS2 crystals. As shown in Figure 2a, the Coral MoS2 exhibits a periodic triangular packing arrangement of Mo atoms, demonstrating the high crystallinity of MoS2 materials.27,42,43 The corresponding selective area electron diffraction (SAED) also reveals that only one set of diffraction spots was obtained, suggesting the single-crystal nature of each branch of the Coral MoS2 (Figure 2b). Figure 2c shows the folded edges of the Coral MoS2, from which


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the number of MoS2 layers can be identified.44 Both one and two layered MoS2 structure can be observed, indicating that few layered MoS2 crystals also exist in the Coral MoS2 in addition to the monolayer edges. This folded edge characterization is in line with the TEM result which is shown in Figure S6. The bright-field TEM image of Coral MoS2@GQDs composite exhibits the microstructure of monolayer Coral MoS2 loaded with a high density of recognizable GQDs (Figure 2d), which corresponds to the SEM results of composited Coral MoS2@GQDs as shown in Figure S14. Figure 2e and 2f show the lattice images of monolayer MoS2 (100) and GDQ (1120) taken from the marked regions in Figure 2d. A well-defined crystallographic relationship between the two phases can be identified from the HRTEM for DFT modeling. As shown in Figure 3a, Raman spectra were utilized to identify the layer number of MoS2 crystals by distinguishing the frequency differences of E2g1 peak (in-plane) and A1g peak (out-ofplane).45 The frequency differences (∆k) here is 19.2, 20.7, 23.1, and 26.8 cm−1 for the monolayer, bilayer, four-layer and few-layer MoS2, respectively. One can see that as the number of MoS2 layers increasing the ∆k is magnified gradually, which in line with the previous reports. After composited with GQDs, the frequency difference ∆k is slightly enlarged compared with the pure Coral MoS2, as shown in Figure 3b and Figure S15. Meanwhile, the full width at halfmaximum (fwhm) of D band and G band of the graphene in the Coral MoS2@GQDs also become broad (Figure S16). These differences in Raman spectra are mainly induced by the strong electron-phonon coupling by which the photo-excited electrons move from MoS2 to GQDs, and the photo-excited holes are trapped in the MoS2 layer.46,47 Because the MoS2 is a semiconductor material with a bandgap of ~1.76 eV, which can be easily generated by the laser. However, graphene is a zero-bandgap material, which means it can be slightly influenced by the laser. Thus, it is the MoS2 which is much active than the GQDs during this Raman progress, and


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the photo-excited electrons move from MoS2 layer to GQDs. Additional characterizations were acquired by photoluminescence (PL) emission spectra. The PL emission spectra of the Coral MoS2 consist of one major peak and one minor peak at around 678 and 627 nm, which have been defined as peak A and peak B, respectively (Figure 3c).48 The gradual decrease in the emission peak intensity accompanies with the increase of MoS2 layers. The monolayer region possesses the highest emission intensity for both Coral MoS2 and Triangle MoS2, while almost no emission can be detected from the few-layer regions. PL spectra of Coral MoS2@GQDs composites with different GQDs concentrations are demonstrated in Figure S17. For the representative peak A, an obvious red-shifting was obtained with the increasing of GQDs concentration, accompanying with the PL intensity decreases. Such phenomenon can be attributed to the change of the exciton density.49−51 X-ray photoelectron spectroscopy (XPS) was also adopted to investigate the valence states and chemical compositions of the Coral MoS2@GQDs. As shown in Figure 3d, the Coral MoS2@GQDs is composed of Mo, S, O and C elements. Compared with the pure Coral MoS2 (Figure S18), an intense graphitic C1s peak at 284.9 eV can be detected obviously. Highresolution XPS of C1s (Figure 3g) presents C−C, C−O, C=O, indicating the GQDs are functionalized with specific functional groups, meanwhile Mo 3d (Figure 3e) and S 2p (Figure 3f) show the characteristic peaks corresponding to Mo 3d5/2, Mo 3d3/2 orbitals and S 2p3/2, S 2p1/2 orbitals, respectively.52 These observations further confirm the fact that such unique Coral MoS2@GQDs is a synergetic constitution of the Coral MoS2 and the GQDs.


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Figure 3. (a) Raman spectra of Coral MoS2 at monolayer (1L), bilayer (2L) and few-layer (nL) regions. (b) Raman spectra of Coral MoS2, Coral MoS2@GQDs composite and pure GQDs. (c) PL spectra of Coral MoS2 at monolayer (1L), bilayer (2L) and few-layer (nL) regions. (d) XPS spectra of Coral MoS2@GQDs. High-resolution XPS spectra: (e) Mo 3d. (f) S 2p. (g) C 1s.


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Figure 4. Electrochemical measurements for the Triangle MoS2, Coral MoS2, Over-grown Coral MoS2 and Coral MoS2@GQDs. (a) Polarization curves. (b) Corresponding Tafel plots. (c) Nyquist plots surveying at an overpotential of 200 mV. (d) Polarization curves initial cycles and after 2000 cycles. (e) Cyclic voltammograms of Coral MoS2@GQDs at a scan rate from 20 to 200 mV s-1. (f) The ratio of the capacitive currents measured at 0.20 V vs RHE for the Coral MoS2@GQDs, Coral MoS2, Over-grown Coral MoS2 and Triangle MoS2 catalysts.


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The HER electrocatalytic activities of the as-prepared samples were evaluated in 0.5 M H2SO4 solution (N2 saturated) with a typical three-electrode system. The polarization curves in Figure 4a show that the onset overpotential of Coral MoS2@GQDs (95 mV) is obviously smaller than that of pure Coral MoS2 (124 mV), over-grown Coral MoS2 (178 mV) and Triangle MoS2 (205 mV). The calculations of the onset overpotentials are detailed in Figure S19. Meanwhile, the applied overpotential of Coral MoS2@GQDs is about 120 mV at current density of 10 mA cm−2, much lower than that of Coral MoS2 (173 mV), over-grown Coral MoS2 (217 mV) and Triangle MoS2 (286 mV). Moreover, the polarization curve also shows a rapid rise of current density between 100 mV and 150 mV for the Coral MoS2@GQDs. The Tafel plots were generally adopted to elucidate the electron transfer kinetics.53 The corresponding Tafel slopes of Coral MoS2, over-grown Coral MoS2 and Triangle MoS2 are 63, 74 and 82 mV/dec, respectively (Figure 4b). However, the Tafel slope of Coral MoS2@GQDs is only 40 mV/dec, which is one of the lowest value of MoS2 based catalysts reported in the literature. Such a smaller Tafel slope suggests that the electrocatalyst proceeds via a combined Volmer-Heyrovsky mechanism for hydrogen evolution.54 We also make a comparison for different kind of MoS2-based materials performed as HER electrocatalyst in Table S1. Table 1. Calculated values of TOF for Coral MoS2@GQDs, Coral MoS2, Over-grown Coral MoS2 and Triangle MoS2. Cdl (mF cm-2)

Relative roughness factor

Surface sites (cm-2)

J(η = 200 mV) (mA cm-2)

TOF (H2/s)

Coral MoS2@GQDs

5.11

85

9.9×1016

86

2.7

Coral MoS2 Over-grown Coral MoS2

2.24

37

4.3×1016

24

1.8

0.96

16

1.9×1016

7.4

1.2

Triangle MoS2

0.64

10

1.2×1016

2

0.5


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Electrochemical impedance spectroscopy (EIS) was performed to probe the ionic migration process and the electrode kinetics during the HER process. Representative Nyquist plots (overpotential of 200 mV) are presented in Figure 4c, and the width of the semicircles indicate the charge-transfer resistance (RCT) of the HER process.55 An apparent decrease of electrical resistance values from 1500 Ω (Triangle MoS2), 610 Ω (over-grown Coral MoS2), 120Ω (Coral MoS2) to 24 Ω (Coral MoS2@GQDs) was obtained. This phenomenon can be interpreted by the enhancement of charge transfer with more exposed edge sites and the improvement of interdomain conductivity by GQDs. Stability is also an important parameter to evaluate the quality of catalysts. To investigate the long-term cycling stability of the catalysts developed in this study, continuous cyclic voltammetry was conducted at a scan rate of 0.1 V s−1 between −0.55 and +0.3 V vs RHE (Figure 4d). After 2000 cycles, there is only negligible change of the polarization curves for the Coral MoS2@GQDs as well as for the pure Coral MoS2, while the Triangle MoS2 shows obvious degradation after the testing, which confirms the better stability of our intentionally designed electrocatalyst. The electrochemical durability of MoS2 catalysts was further examined using chronoamperometry (j−t) at a constant overpotential of 300 mV. As shown in Figure S20, a relatively stable current densities of about 150 mA cm−2, 60 mA cm−2 and 30 mA cm−2 were observed for Coral MoS2@GQDs, Coral MoS2 and over-grown Coral MoS2 through 30000 s continuous operation. Figure S21 also shown the SEM images of such coral-shaped MoS2@GQDs after 500 HER cycles. As can be seen, both the central and edge parts could maintain stable coral-shaped morphology after a long-time cycle. All of the results above prove that Coral MoS2@GQDs possesses excellent long-term stability. To evaluate the effective surface area of the as-prepared electrocatalytic materials, cyclic voltammograms (Figure 4e and Figure S22) and double-layer capacitances (Cdl) were measured


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in Figure 4f. Furthermore, the per-site turn over frequencies (TOF) of the few layer MoS2 and Coral MoS2@GQDs catalysts was also calculated with an overpotential of 200 mV (see Calculation of TOF in Supporting Information for details). As shown in Table 1, Coral MoS2@GQDs shows a TOF of 2.7 H2 s−1 per active site, which is much higher than the pure triangle MoS2 catalyst (0.5 H2 s−1), indicating the superior HER catalytic activities of Coral [email protected] In order to survey the electronic structures of monolayer MoS2 and Coral MoS2@GQDs composite, the band structures were calculated. The calculated band gap of pure MoS2 monolayer is 1.76 eV as presented in Figure 5a, which is very close to the previously reported values.57,58 Figure 5b shows the electronic structure of the MoS2@GQDs composite. It reveals that the band structure can be effectively modified by compounding the two separated materials. The band gap of the composite clearly changes to zero, which lead to an easier electronic transportation and higher electric conductivity of Coral MoS2@GQDs and thereby the kinetics of electrocatalysis for HER. These results agree well with the experimental HER and EIS measurements. The charge density was also calculated to investigate the electron configuration of the Coral MoS2@GQDs. As shown in Figure 5c, all atoms are held at the same positions as they are in the integral structure in computing of individual MoS2 and graphene. The pale green and yellow regions represent charge depletion and accumulation, respectively. The charge transfer occurs mainly between the contact area and in the vicinity of the interface from graphene layer to MoS2. As shown in Figure 5d, the main feature of the Coral MoS2@GQDs composite is that electrons transfer from graphene layer to MoS2 directionally and results in the formation of an “electron rich” MoS2.


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Figure 5. (a) and (b) The band structures of MoS2 and Coral MoS2@GQDs. The red circles represent graphene states. (c) The 3D different charge density distribution of the Coral MoS2@GQDs. The pale green and yellow regions represent the charge depletion and accumulation space. (d) Adsorption configurations of an H atom on the Mo-edge sites. CONCLUSIONS In summary, we have developed a novel method to fabricate Coral MoS2@GQDs with a large amount of exposed edges and a zero band gap. The composite catalyst show outstanding HER catalytic activities, including a smaller onset overpotential, lower Tafel slope, low applied potential at a current density of 10 mA cm−2 and excellent cycling stability. The superior performance is attributed to the abundant MoS2 active edge sites and the enhanced charge transfer, providing an opportunity to co-regulate both structural and electronic benefits for HER. Therefore, the controllable engineering and accurate design of the Coral MoS2@GQDs


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electrocatalyst may provide a feasible candidate for low-cost electrochemical H2 production. This work also reveals a novel approach to improve the activities of monolayer MoS2 based electrocatalysts. ASSOCIATED CONTENT Supporting Information Figure S1 (schematic view of Coral MoS2 growth), Figures S2-S18 (SEM, TEM, AFM, Raman, PL and XPS of different Triangle MoS2, Coral MoS2 and Coral MoS2@GQDs materials), Figures S19-S21 (detailed electrochemical analysis of synthetic materials). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Tel.: +86 21 54345198; Fax: +86 21 54345119. E-mail address: [email protected] (Ke Yu) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by financial support from the National Science Foundation of China (Grants 61574055, 61474043, 61425004), and the Open Project Program of Key Laboratory of Polar Materials and Devices, MOE (Grant No. KFKT20140003), BJG was sponsored by ECNU Outstanding Doctoral Dissertation Cultivation Plan of Action (No. YB2016032). MWC was


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TOC 44x35mm (300 x 300 DPI)


Coral-Shaped MoS2 Decorated with Graphene Quantum Dots

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