Space-Confined Growth of MoS2 Nanosheets within Graphite: The

Mar 14, 2014 - Xiaoli Zheng, Jianbo Xu, Keyou Yan, Hong Wang, Zilong Wang, and Shihe Yang* ... Technology, Clear Water Bay, Kowloon, Hong Kong...
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Space-Confined Growth of MoS2 Nanosheets within Graphite: The Layered Hybrid of MoS2 and Graphene as an Active Catalyst for Hydrogen Evolution Reaction Xiaoli Zheng, Jianbo Xu, Keyou Yan, Hong Wang, Zilong Wang, and Shihe Yang* Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: Since the electrocatalytic activity of layered molybdenum disulfide (MoS2) for hydrogen evolution reaction (HER) closely depends on its exposed edges, the morphology and size of the material are critically important. Herein, we introduce a novel solvent-evaporation-assisted intercalation method to fabricate the hybrid of alternating MoS2 sheets and reduced graphene oxide layers, in which the nanosize of the MoS2 nanosheets can be effectively controlled by leveraging the confinement effect within the two-dimensional graphene layers. Significantly, the resulting MoS2/reduced graphene oxide (RGO) composite shows excellent catalytic activity for HER characterized by higher current densities and lower onset potentials than the conventional pre-exfoliated RGO supported MoS2 nanosheets. Further experiments on the effect of oxidation degree of graphene, the crystallinity of MoS2, and the exposed active site density on the HER performance of the MoS2/RGO composites show that there is an optimum condition for the catalytic activity of HER due to a balance between the numbers of exposed active sites of MoS2 and the internal conductive channels provided by graphene.

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INTRODUCTION Hydrogen as a clean, efficient, and durable energy carrier has been intensively pursued to address the global issues of severe global energy shortage and environmental deterioration.1 Currently, platinum (Pt) and Pt-based alloys are the most efficient electrocatalysts for hydrogen evolution, but the high cost and scarcity of Pt-based metals hinder their widespread commercialization.2 Thus, the ongoing search for Pt-free catalysts for electrochemical hydrogen evolution reaction (HER) has attracted much attention. To the present, a variety of non-noble materials have been tested for catalyzing the electrochemical HER, including metal alloys,3−5 metal hydroxides (oxides),6−10 chalcogenides,11−13 and carbides14−16 as well as complexes.17−20 Molybdenum disulfide (MoS2), two-dimensional sheets of vertically stacked S−Mo−S interlayers, has attracted tremendous interest as a potential HER catalyst due to its exposed reactive sulfur sites at the edges.21,22 However, its inherent stacking feature among MoS2 layers severely decreases the amount of exposed active sites.23 Additionally, the conductivity along two vertically stacked S−Mo−S interlayers is extremely low, which is about 2 orders of magnitude lower than that of the intralayers.24,25 Therefore, it is critical to increase the exposed edge sites and hinder the stacking and aggregation of MoS2 layers. Nanosized MoS2 is considered to be a highly active electrocatalyst for HER due to the exposure of numerous reactive sulfur edge sites.26 Confinement of nanoparticles within conducting carbon materials, such as fullerene,27 carbon nanotubes,28 and graphene,29−34 has proven effective in restricting the size of nanoparticles down to the nanometer scale and thus improving © 2014 American Chemical Society

their catalytic properties. Pan and Bao reported that the catalytic activity and stability of nanosized catalysts confined in carbon nanotube channels have both been enhanced with respect to those anchored outside the nanotubes.28 Also it has been found that manganese intercalated graphene sheets exhibit higher relaxivity (by up to 2 orders) compared to that of paramagnetic chelate compounds.35 Graphene, which consists of sp2-bonded aromatic carbon sheets, holds great promise as a matrix to disperse and confine nanoparticles due to its outstanding properties such as two-dimensional (2D) layer structure, large specific surface area, high mechanical strength, and extraordinary electronic conductivity.36−40 Among the graphene family, graphene oxide (GO) has been considered as an effective substrate for the nucleation and subsequent growth of nanoparticles owing to the coupling interactions between nanoparticle precursors and oxygen-containing functional groups on the surface of GO sheets.41 Recently, intensive efforts have been devoted to the synthesis of MoS2/graphene and/or GO composites by solvothermal42−44 and microwave methods,45 which demonstrated high electrocatalytic activity in HER. However, up to now, the confinement of MoS 2 nanosheets within graphene layers has not been achieved on a large scale, and the effect of such confinement on the HER catalytic performance has not been reported yet. Thus, the preparation and characterization of space-confined and sizedcontrolled MoS2 nanosheets between graphene layers for Received: January 29, 2014 Revised: March 12, 2014 Published: March 14, 2014 2344

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besides (NH4)2MoS4 and GO, the slurry also contains ∼10 mL of H2O. Then 40 mL of DMF (the volume ratio of DMF vs H2O is 4:1) and 0.5 mL of N2H4·H2O were added in the slurry. And the solution was further stirred for 20 min before being transferred to a 100 mL Teflon-lined autoclave and kept in an oven at 200 °C for 12 h. Then the product was centrifuged, washed with distilled water, and centrifuged repeatedly for many times to remove DMF. The final precipitate was redispersed in 10 mL of DI water and lyophilized. Pure MoS2 nanoparticles were prepared with the same procedure in the absence of GO. Electrochemical Measurements. First, 10 mg of a given catalyst sample and 0.1 mL of 5 wt % Nafion solution were mixed in 1.9 mL of ethanol by sonication for 20 min to form a homogeneous ink. Next, 5 μL of the catalyst ink was drop-cast onto a rotating disk electrode made of glassy carbon (4 mm in diameter, loading density ∼0.20 mg cm−2), which served as the working electrode. Linear sweep voltammetry (CHI 760 D potentiostat) with scan rate of 5 mV s−1 was conducted in 0.5 M H2SO4 solution using Ag/AgCl (ALS RE-1B) as the reference electrode and a Pt wire as the counter electrode. Electrolyte was degassed by bubbling N2 for 30 min prior to the start of each experiment. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at overpotential η = 170 mV from 106 to 0.02 Hz with an AC voltage of 5 mV. All the potentials reported in our work were against reversible hydrogen electrode (RHE) through RHE calibration. For conversion of the obtained potential (vs Ag/AgCl) to RHE, eq 1 was used.

electrocatalytic HER is highly warranted for both fundamental research and practical application. In this work, we report a facile way to confine MoS2 nanosheets between reduced GO (RGO) sheets, producing size-controlled MoS2/RGO composites for HER catalysis. In contrast with many previous methods that use the preexfoliated graphene or GO to make nanoparticle/graphene composites, the present work takes a two-step procedure: (1) intercalation of ammonium tetrathiomolybdate ((NH4)2MoS4) into GO assisted by solvent evaporation and (2) confined growth of MoS2 by solvothermal method to achieve nanosized MoS2/RGO composites. Here the graphene sheets are believed to play triple roles in the composite, that is, providing chemical functional groups for strong coupling between MoS2 precursor and graphene sheets, a confinement environment for MoS2 nucleation and growth, and charge transport pathway for high electrocatalytic activity. As expected, the resulting MoS2/RGO composite shows excellent catalytic activity of HER characterized by higher current densities and lower onset potentials than the conventional pre-exfoliated graphene supported MoS2 system. In addition, we also investigate the effect of oxidation degree of graphene, the crystallinity of MoS2, and the amount of exposed active sites on the HER performance of the MoS2/ RGO composites. The detailed study of the MoS2/RGO composite carried out here provides us with an insight into a novel method to the design and synthesis of graphene-based material for energy application.

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E RHE = EAg/AgCl + 0.059 pH + EAg/AgCl 0 (EAg/AgCl 0 = + 0.209 V)

(1)

Characterizations. Morphology analysis was conducted using a JEOL 6700F scanning electron microscope (SEM) at an accelerating voltage of 5 kV. Elemental mapping and electron energy dispersive spectroscopy (EDS) were performed with a JSM-6390 SEM. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were carried out on a JEOL 2010F microscope operating at 200 kV. X-ray diffraction (XRD) of the samples was characterized using a Philips high resolution X-ray diffraction system (model PW1825) with Cu Kα radiation (λ = 0.15406 nm). Raman spectra of powder samples were recorded on LabRAM HR Raman microscope with a laser excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) spectrum was collected on SSI SProbe XPS Spectrometer. Fourier transform infrared spectroscopy (FTIR) was taken on a BRUKER TENSOR27 instrument with 32 scans at a resolution of 2 cm−1 intervals.

EXPERIMENTAL SECTION

Materials. Natural graphite powder was purchased from Alfa Aesar (99.8%). Potassium permanganate (KMnO4, 99%), hydrogen peroxide (H2O2, 30%), ammonium tetrathiomolybdate ((NH4)2MoS4, 99.97%), hydrazine hydrate (N2H4·H2O, 98%), and nafion solution (5%) were purchased from Aldrich. Concentrated sulfuric acid (H2SO4, 98%), concentrated hydrochloric acid (HCl, 37%), and N,N-dimethylformamide (DMF, 99.8%) were purchased from VWR International S.A.S. The deionized (DI) water was prepared by the Millipore Milli-Q water purification system (18.2 MΩ). All reagents were used directly without further purification. Synthesis of Graphene Oxide (GO). GO with various degrees of oxidation were prepared from natural graphite powder by a modified Hummers method, including GO1, GO2, and GO3, which were obtained with different weight ratios of graphite to KMnO4 (1:1, 1:2, and 1:3), respectively. The detailed procedure was as follows: 0.5 g of graphite was put into a 100 mL round-bottom flask. A total of 15 mL of concentrated H2SO4 (98%) was added, and the mixture was stirred for 30 min in an ice water bath. Next, a certain amount of KMnO4 (0.5 g, 1 g, and 1.5 g) was slowly added to the flask, keeping the reaction temperature below 45 °C. Then, the solution was heated in an oil bath at 50 °C and allowed to stir for 3 h. Afterward, the flask was removed from the oil bath, and 150 mL of water and 10 mL of 30% H2O2 were added to end the reaction. Then, the suspension was centrifuged and washed with 5% HCl solution twice, followed by rinsing with a large amount of water repeatedly until the pH reached ∼7. The final precipitate was dispersed in 100 mL of water. In a control experiment, the prepared GO2 solution was sonicated for 1 h and centrifuged at 7000 r/min for 1 h. Then the black supernatant solution was obtained and referred to as e-GO2. The concentrations of GO were determined by weighting the quality of GO after drying 5 mL of the corresponding GO solutions, which were 1.7 mg/mL for GO1−GO3 and 1.0 mg/mL for e-GO2. Synthesis of MoS2/Reduced Graphene Oxide (MoS2/RGO) Composites. MoS2/RGO composites were prepared according to the procedures described by Li et al.42 and Cao et al.46 with some modifications. Briefly, 110 mg of (NH4)2MoS4 was dispersed and stirred in 30 mL of GO solution (50 mL of e-GO2 solution) at room temperature until a black homogeneous slurry was achieved. Here,

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RESULTS AND DISCUSSION Layer Confined Synthesis of MoS2/RGO Nanosheet Composites. The concept for the layer confined synthesis of MoS2/RGO composite is illustrated in Scheme 1. The synthesis starts with graphite powder by mild oxidation and the resulting interlayer expansion. Then the expanded space allows MoS2 precursors to intercalate. Finally, in situ solvothermal reduction of the precursor leads to the MoS2 layers dispersed in and hybridized with the RGO layers. Figure 1 shows the general morphologies of the pure MoS2 and the MoS2/RGO2 hybrid synthesized by the solvothermal method. As shown in Figure 1a, the free MoS2 consists of largesized microspheres that are tightly aggregated together. When GO2 is incorporated (see Figure S1 in the Supporting Information), the MoS2 nanosheets are homogeneously dispersed on the surface of graphene, as shown in Figure 1b. The lateral size of the MoS2 nanosheets dispersed on the graphene sheets is in the range of 50−70 nm, and obvious ripples can be observed. No MoS2 microspheres are observed, indicating that the graphene sheets could be used as an efficient substrate for the nucleation and growth of MoS2 nanosheets. It 2345

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1:2.2, which is close to the MoS2 stoichiometry. The morphology of the MoS2/RGO2 composite was further examined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Figure 1c,d). The TEM image (Figure 1c) verifies that the ultrathin MoS2 nanosheets are homogeneously dispersed on the graphene sheets. The corresponding selected area electron diffraction (SAED) pattern (inset in Figure 1c) clearly shows two sets of diffraction signals: the two separated diffraction rings can be well-indexed as the hexagonal MoS2 phase with polycrystalline structure and corresponded to the (100) and (110) planes of MoS2, respectively. And the isolated diffraction spots belong to graphene sheets.47,48 The well-defined electron diffraction spots in the SAED pattern reveal that the transparent RGO sheet exhibit high crystalline graphene structure. And the intensity difference between {1100} and {2110} (I{1100} > I{2110}) is the unique characteristic of monolayer graphene,47,49 showing that the intercalation and solvothermal method could effectively produce a high quality of graphene. The highly exfoliated graphene sheets could increase the loading capacity of active materials and also provide efficient conducting network for rapid electronic transport during the electrocatalytic process. Furthermore, a typical lamellar structure of MoS2 with interlayer spacing of 0.63 nm can be observed from the HRTEM image (Figure 1d, the area designated by the square box in Figure 1c), and the MoS2 nanosheets mainly comprise less than 10 layers. To further garner the structural information of the samples, X-ray diffraction (XRD) and Raman spectroscopy were carried out. As shown in Figure 2A(a), no sharp peaks are observed for the free MoS2, indicative of the poorly crystallized MoS2 synthesized by the solvothermal method before annealing.50 The three broad and weak diffraction peaks centered at around 2θ = 15.0°, 33.8°, and 57.1° correspond respectively to the (002), (100), and (110) planes of the hexagonal MoS2 (2HMoS2, JCPDS 37-1492). From Figure 2A(b), we observe that the intensity of all the diffraction peaks of MoS2 in the MoS2/ GO2 hybrid decreases significantly accompanied by the appearance of a new broad peak centered at around 2θ = 25.1° ascribable to stacked RGO sheets.51 The decreased intensity of the diffraction peaks of MoS2 in the composite indicates that the incorporation of graphene considerably restrains the aggregation of layered MoS 2 during the solvothermal process, which leads to the growth of few-layered MoS2 nanosheets of poor crystallinity.48 Recently, it has been demonstrated that amorphous MoS2 exhibits higher HER activity than crystalline MoS2 as the amorphous structure affords a higher number of exposed edges.52−55 It appears that the nanometer sized, poorly crystalline MoS2 here can also provide ample edges and catalytically active sites as the amorphous MoS2 due to their confined environment between graphene layers. The pure GO2, MoS2, and MoS2/RGO2 composite were also characterized by Raman spectroscopy, and the characteristic peaks of MoS2 and graphene were revealed (see Figure 2B,C). As shown in Figure 2B, both GO2 and MoS2/RGO2 composites exhibit three dominant Raman peaks at 1349, 1578, and 2693 cm−1, which match very well with the D, G, and 2D bands of graphene, respectively. In general, the relative intensity ratio ID/IG is an indication of the graphene quality.56 The ID/IG value for the MoS2/RGO2 hybrid (ID/IG = 1.48) is smaller than that for GO2 (ID/IG = 1.60), indicating that the delocalized π conjugation is partially restored during the hybridization process. Beside the G-band,

Scheme 1. Schematic Representation of the Synthesis of MoS2/RGO Composite

Figure 1. Electron microscopy characterizations of the as-prepared MoS2 and MoS2/RGO samples. (a) SEM image of pure MoS2 nanoparticles without graphene sheets and (b) SEM and (c) TEM images of MoS2/RGO2 composite. Inset in (c) is the corresponding SAED pattern. (d) HRTEM of MoS2/RGO2 composite (zoomed from the red contour in Figure 1c), clearly showing the exposed edges of MoS2 nanosheets (indicated by the white arrows).

can be attributed to the interaction between the MoS2 precursors and the oxygen-containing functional groups on the graphene.42 In particular, MoS2 nanosheets are successfully intercalated into the graphene layers (indicated by the black arrow in Figure 1b), and the size of MoS2 nanosheets can be effectively controlled with a relatively large amount of exposed edges. The elemental mapping images shown in Figure S2 (Supporting Information) also confirm the homogeneous distribution of sulfur (S) and molybdenum (Mo) elements across the whole graphene sheets. And the corresponding elemental compositions exhibit that the atomic ratio of Mo:S is 2346

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Figure 2. XRD and Raman spectra of the as-prepared samples. (A) XRD patterns of (a) MoS2 and (b) MoS2/RGO2 composite. (B) Raman spectra of (a) GO2 and (b) MoS2/RGO2 composite. (C) Raman spectra of (a) MoS2 and (b) MoS2/RGO2 composite in the range of 350−450 cm−1.

Figure 3. XPS of GO2 and MoS2/RGO2 composites. (a) High resolution C 1s spectrum of GO2. High resolution (b) C 1s, (c) Mo 3d, and (d) S 2p spectra of MoS2/RGO2 composite. The atomic percentage was determined to be O 12.4%, C 70.5%, Mo 5.9%, and S 11.2%.

the 2D-band is also a signature of graphitic sp2 materials.57 The increased intensity of the 2D band for the MoS2/RGO2 hybrid further proves the effective restoration of the graphene structure. The Raman spectrum in Figure 2C(a) in the informative range from 350 to 450 cm−1 reveals the characteristic peaks of MoS2. The two dominant peaks of pure MoS2 at 376 and 403 cm−1 are associated with the inplane E12g and out-of-plane A1g vibrational modes of the hexagonal MoS2, respectively.25 For the MoS2/RGO2 hybrid, the corresponding two peaks are at 376 and 400 cm−1 (Figure 2C(b)), respectively. It has been reported that the difference between the Raman peak frequencies of E12g and A1g exhibits a stepwise decrease with decreasing number of MoS2 layers.58 Therefore the decreased frequency difference for the MoS2/ RGO2 hybrid compared with the pure MoS2 confirms that the MoS2 nanosheets in the hybrid are ultrathin with mainly single or few layers of the materials. Moreover, the relative integrated intensities of the two modes of E12g and A1g can provide useful information about the terminated structure of the MoS2 nanosheets: a higher intensity ratio of E12g mode than that of A1g mode indicates the formation of terrace surface-terminated structure, whereas the edge-terminated structure is correlated

with a lower intensity of E12g mode than that of A1g mode.25 Applying this judgment to the situation here, we have formed the edge-terminated MoS2 nanosheets with single or few layers within the graphene layers by the layer confined growth method, which appears to increase the exposed edge sites. Finally, by comparing the Fourier transform infrared spectra (FTIR) of the raw material (NH4)2MoS4 and the MoS2/RGO2 hybrid (see Figure S3 in the Supporting Information), we notice the absence of the characteristic peaks of the symmetric (456 cm −1) and antisymmetric (477 cm −1) ν(Mo−S) vibrations of the MoS42− anion in the latter, again clearly indicating the complete reduction/removal of the raw material during the hybridization process.59 The atomic valence states and the composition of the MoS2/ RGO2 composite were characterized by X-ray photoelectron spectroscopy (XPS). Figure 3a shows the C 1s XPS spectrum of GO, where four different peaks are corresponding to CC/ CC (285.0 eV), CO (286.9 eV), CO (287.9 eV), and COOH (288.8 eV) groups, respectively. For the MoS2/RGO2 composite (Figure 3b), the intensities of all C 1s peaks pertaining to the carbons bonding to oxygen decrease dramatically, signaling the effective removal of the oxygen2347

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containing functional groups by the solvothermal treatment. In addition, the position of the C 1s line of the composite attributed to CC/CC (284.6 eV) is downshifted by ∼0.4 eV compared to that of the pristine GO2 (285.0 eV), indicating charge transfer from graphene to MoS2,60 which may improve the catalytic activity. The high-resolution XPS of the composite in the Mo 3d region is shown in Figure 3c. It can be deconvoluted into five peaks, and one of those at 226.1 eV actually corresponds to S 2s of MoS2. The two main intense Mo 3d5/2 (228.8 eV) and Mo 3d3/2 (232.7 eV) components are characteristic of MoS2, while the high binding energy peak of Mo 3d (235.9 eV) corresponds to MoO3 or MoO42−, which may result from the oxidation of the catalyst sample in air.60,61 The shoulder at around the binding energy of 230.0 eV indicates that Mo6+ is partially reduced and Mo5+ is present in the sulfide intermediate phases.44,62,63 Sulfur species were determined from the high-resolution XPS S 2p spectrum (Figure 3d). The main doublet located at binding energies of 161.8 and 163.5 eV corresponds to the S 2p3/2 and S 2p1/2 lines of MoS2.61 Meanwhile, the binding energy at 164.8 eV suggests the existence of bridging disulfides S22− and/or apical S2− ligands, which may be related to high activity HER species.64 The high-energy component at 168.7 eV can be assigned to S4+ species in sulfate groups (SO32−), and these groups could locate at edges of MoS2 layers, which were still present even after washing with soluble salts.53,60 It has been reported that the MoSx stoichiometry can be determined by the difference between the binding energies of Mo 3d5/2 and S 2p3/2.65 Accordingly, the MoSx stoichiometry in the composite is determined to be MoS1.85, suggesting that MoS2 nanosheets containing a very small amount of oxidized molybdenum are obtained after the solvothermal treatment. Combining the results of SEM, TEM, XRD, Raman, and XPS presented above, the novel sandwiched structure with ultrathin, nanosized, lowcrystallinity MoS2 nanosheets locked up within well-exfoliated graphene layers can be established. Such a structure could significantly increase the amount of exposed active edge sites without compromising the internal network conductivity for the highly active electrocatalysis. Mechanistic Investigation of the Nanosheet Hybridization Mechanism. In order to reveal the growth mechanism of the confinement grown MoS2/RGO composites during the solvothermal process, control experiments were performed by varying some reaction conditions. Figure 4 shows the corresponding SEM and TEM images. For convenience, hereafter, we designate the various MoS2/graphene composites as “MoS2/RGOx”, where x refers to the oxidation degrees of GO (from 1 to 3) and 1−3 is the weight ratio of graphite to KMnO4 added during the oxidation process. As shown in Figure 4a,b, when the pre-exfoliated GO2 (e-GO) is used as the substrate for MoS2 growth, MoS2 nanosheets with large sizes (lateral size 90−100 nm) aggregate severely on the crinkly graphene sheets (MoS2/e-RGO2 composite). The overlapping and coalescing of the graphene sheets by MoS2 nanosheets would decrease the contact area with the electrode and also hinder the formation of the interconnected conducting network, thus restricting the rapid electronic transport needed for the catalytic reaction processs. However, when GO1 is used as the graphene source, a large number of free MoS2 microspheres together with small-sized MoS2 nanosheets (30 nm, indicated by black arrows) are decorated on the surface of graphene (MoS2/RGO1 composite, Figure 4c,d). The MoS2 microspheres may be formed due to the deficiency of oxygen-

Figure 4. Electron microscopy characterizations of MoS2/RGO composites prepared under different conditions. (a) SEM and (b) TEM images of MoS2/RGO2 composite prepared with pre-exfoliated GO2 (MoS2/e-RGO2 composite), (c) SEM and (d) TEM images of MoS2/RGO1 composite, and (e) SEM and (f) TEM images of MoS2/ RGO3 composite.

containing functional groups on the GO1, while the small-sized MoS2 nanosheets may be grown in limited interspace between the graphene sheets with a low degree of oxidation. When the oxidation degree of GO is increased to GO3, it can be clearly seen that higher densities of MoS2 nanosheets (lateral size in 50−60 nm) are supported on the graphene surface (MoS2/ RGO3 composite, Figure 4e,f) in contrast to the case of MoS2/ RGO2 composite (Figure 1b,c), indicating that the GO3 sheets provide more abundant functional groups for the nucleation and subsequent growth of MoS2 nanosheets. Figure 5 shows the XRD patterns and Raman and XPS spectra of the as-prepared MoS2/e-RGO2, MoS2/RGO1, and MoS2/RGO3 composites. Compared with Figure 2 and Figure 3, the chemical structures and compositions of the MoS2/RGO composites do not exhibit significant changes. As shown in Figure 5A, the XRD patterns of the MoS2/e-RGO2 and MoS2/ RGO3 composites mainly show the same peak as that of MoS2/ RGO2 composite but different from that of the MoS2/RGO1 composite. The MoS2/RGO1 composite (Figure 5A(b)) shows a peak at 2θ = 26.2°, which is fairly close to that of graphite powder, indicating that the graphite is not fully oxidized into GO at low oxidation degree. The Raman spectra of the composites shown in Figure 5 B,C also exhibit characteristic peaks of reduced GO and typical MoS2 vibrational modes in the range of 350 to 450 cm−1. Table 1 summarizes the ID/IG ratio of raw GO samples and the corresponding MoS2/RGO composites as well as the Raman shift of E12g and A1g modes of MoS2 and MoS2/RGO composites. The increased ID/IG ratio 2348

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Figure 5. XRD, Raman, and XPS characterizations of MoS2/RGO composites prepared under different conditions. (A) XRD patterns, (B, C) Raman spectra, and high resolution (D) C 1s, (E) Mo 3d, and (F) S 2p XPS spectra of (a) MoS2/e-RGO2, (b) MoS2/RGO1, and (c) MoS2/RGO3 composites.

morphologies of MoS2/RGO composites were obtained by changing the preparation conditions and, thus, may lead to diverse performances. On the basis of the above characterization results, the mechanism for the space-confined synthesis of MoS2/RGO composites can be devised as shown in Scheme 2. When the oxidation degree was low, such as in GO1 (Scheme 2a), the number of nucleation sites was limited and the resulting graphene sheets tended to be thick. As a result, large-sized MoS2 microspheres together with small-sized MoS2 nanosheets were decorated on the thick RGO1 sheets. When GO2, a moderately oxidized graphene, was used as the substrate for the MoS2 growth (Scheme 2b), there were sufficient oxygencontaining functional groups for attaching the MoS2 precursors without the actual exfoliation. When the mixed solution was moderately stirred at room temperature to allow a gradual water evaporation, the precursors were eventually intercalated between the graphene layers.66 The subsequent conversion process then yielded the target MoS2 nanosheets confined between the simultaneously reduced GO layers. If intentionally exfoliated by sonification, aggregated MoS2/e-GO2 products were generally formed. When the well-exfoliated GO2 (e-GO2) was used as the substrate (Scheme 2c), the thin graphene sheets tended to curl up together and finally large-sized MoS2 sheets were overlapped and coalesced on the curled graphene sheets. Nevertheless, when the oxidation degree was high such as in GO3 (Scheme 2d), high densities of MoS2 nanosheets were supported on the graphene sheets due to increased oxygen-containing functional groups. However, the graphene framework after introducing the great amount of oxygencontaining functional groups cannot be restored efficiently during the reduction process, so the electrical conductivity of the MoS2/RGO3 may be severely destroyed. In the composite formation process described above, the cosolvent DMF was added to the aqueous solutions to slow down the hydrolysis reaction rates.67 Here the GO2 could not only act as a substrate for the nucleation and growth of MoS2 but also confine the size of the MoS2 nanosheets and prevent them from severe aggregation. Reciprocally, the growth of MoS2 nanosheets could help to separate the graphite layers and

Table 1. Intensity Ratios of D- and G-Bands (ID/IG) of Raw GO Samples and the Corresponding MoS2/RGO Composites and Raman Shift of E12g and A1g Modes of MoS2 and MoS2/RGO Composites sample

ID/IGa

sample

ID/IGa

E12g (cm−1)

A1g (cm−1)

GO2 e-GO2 GO1 GO3

1.55 1.58 1.43 1.65

MoS2 MoS2/RGO2 MoS2/e-RGO2 MoS2/RGO1 MoS2/RGO3

 1.47 1.51 1.33 1.60

375.5 375.5 377.3 375.8 377.3

403.5 400.6 403.3 404.3 401.3

a

The ID/IG intensity ratios were calculated by the ratios of peak areas of D- and G- bands. The peak areas of D- and G- bands were obtained by fitting to the Lorentzian function with baseline corrections.

of the MoS2/RGO composites compared to that of the raw GO samples shows that the GO is successfully reduced during the solvothermal process. The ID/IG ratio also indicates that the amount of oxygen-containing functional groups in raw GO increase from GO1 to GO3 and these groups can enhance the interaction between the graphene and MoS2 precursors. Then the functional groups in raw GO can facilitate the nucleation and subsequence growth of MoS2 nanosheets on the graphene surface, which agree well with the SEM and TEM results shown in Figures 1 and 4. Beside MoS2/RGO1 composite, the difference between the Raman peak frequencies of E12g and A1g of the other MoS2/RGO composites all decrease compared with that of MoS2, indicating that the interaction between the Mo precursors and graphene can efficiently avoid the aggregation of MoS2. Figure 5D−F shows the high resolution XPS spectra of the MoS2/RGO composites, and the binding energies and elemental compositions of the samples were summarized in Tables S1 and S2 (Supporting Information). The related oxygen contents in the composites agree well with the Raman results. And the least content of MoS2 in MoS2/ RGO1 composite is consistent with the electron microscope observation. Therefore, based on the above characterizations, the as-prepared MoS2/e-RGO2, MoS2/RGO1, and MoS2/RGO3 composites contain the same composition and chemical structure as the MoS2/RGO2 composite. However, the different 2349

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Scheme 2. Schematic Illustration for the Synthesis of Various MoS2/RGO Composites Based on Graphite Oxidation, OxidationInduced Interlayer Expansion/Exfoliation, Evaporation-Assisted Intercalation, and Space-Confined Growtha

a

Low oxidation: a small number of nucleation sites lead to the formation of large MoS2 particles mixed with graphite. Medium oxidation: appropriate number of nucleation sites for evaporation-assisted intercalation and space-confined growth of highly dispersed MoS2 nanosheets between graphene layers, but intentional exfoliation leads to the formation of big blobs of MoS2/e-RGO2 composites. High oxidation: a large number of nucleation sites lead to easy exfoliation and the formation of small MoS2 nanoparticle decorated single layer graphene oxides.

restrain them from stacking together during the solvothermal process. Electrocatalytic HER Activity of the MoS2/RGO Nanosheet Hybrids. To investigate the catalytic activity of the MoS2/RGO2 composite toward HER in comparison with MoS2 and other composites of MoS2/e-RGO2, MoS2/RGO1, and MoS2/RGO3, the samples were deposited on glassy carbon electrode with a loading of 0.20 mg cm−2 and measured in N2saturated 0.5 M H 2 SO 4 solution in a three-electrode configuration (see Experimental Section for details). Figure 6A shows the polarization curves within a cathodic potential window in the range of 0 to −0.4 V vs RHE. As shown in Figure 6A(a) and Figure S4 (Supporting Information), the free MoS2 microspheres or RGO2 alone both exhibit little HER activity. By contrast, the HER activities of MoS2/RGO composite catalysts are higher than that of free MoS2 microspheres. From Figure 6A(b), it can be seen that the MoS2/RGO2 composite catalyst shows a small onset overpotential of ∼140 mV for the hydrogen evolution, beyond which the cathodic current density rises rapidly under more negative potentials. The onset overpotential of MoS2/RGO2 composite is much smaller than that of other MoS2/RGO composites, suggesting the prominent catalytic activity of the MoS2/RGO2 composite for HER.68,69 Moreover, the MoS2/ RGO2 composite displays the largest cathodic current density among all the tested samples, which is 23 mA cm−2 at 200 mV, amounting to 115 A g−1 normalized by the loading mass. The good catalytic behavior of MoS2/RGO2 composite may arise from the synergetic effect of major exposed active edge sites and good conductive channels provided by such MoS2/RGO2 composite. As shown in Figure 6A(c−e), polarization curves of MoS2/e-RGO2, MoS2/RGO1, and MoS2/RGO3 composites exhibit rather poor HER performance with high onset overpotential (150−175 mV) and low cathodic current density. Especially, the MoS2/e-RGO2 composite, although displaying a comparable onset overpotential with that of the MoS2/RGO2 composite, delivered a much smaller cathodic current density,

Figure 6. Electrochemical measurements in a cathodic potential window. (A) Polarization curves, (B) Tafel plots, and (C) Nyquist plots of (a) MoS2, (b) MoS2/RGO2, (c) MoS2/e-RGO2, (d) MoS2/ RGO1, and (e) MoS2/RGO3 composites modified electrode. The inset in part C is the equivalent electrical circuit. (D) Stability for the MoS2/ RGO2 composite modified electrode with initial polarization curve (black curve) and after 500 cycles (red curve) in 0.5 M H2SO4 solution. The scan rate is 100 mV s−1 and scan region ranges from 0 to −0.4 V vs RHE.

which can be attributed to the aggregated structure of the MoS2/e-RGO2 composite, hence limiting the exposure of the active sites and blocking the internal conducting channels. Besides, the poor HER activity of MoS2/RGO1 and MoS2/ RGO3 composites further verifies that not only the exposed active sites of MoS2 but also the conductivity of graphene sheets are the dominant factors in enhancing the HER performance. Tafel plots obtained from overpotential versus log current density (η vs log j) were shown in Figure 6B, which can be used 2350

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for quantitative kinetics analysis of the HER.11 The Tafel slopes can be determined by fitting the linear portions of the Tafel plots to the Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope). The values of Tafel slope yielded from Figure 6B are ∼107, ∼41, ∼53, ∼44, and ∼45 mV per decade for MoS2, MoS2/RGO2, MoS2/e-RGO2, MoS2/RGO1, and MoS2/RGO3, respectively. Remarkably, the Tafel slope of ∼41 mV per decade for MoS2/RGO2 composite is comparable to that reported in the literature for MoS2/ graphene catalysts (∼41 mV per decade42 and ∼42 mV per decade43). Interestingly, although the Tafel slopes of MoS2/eRGO2, MoS2/RGO1, and MoS2/RGO3 composites are lower than that of MoS2/RGO2 composite, the values are still higher than some reported MoS2 based composites (∼58 mV per decade45 and ∼69 mV per decade 70 ), suggesting the interspatial-confinement growth method could be a promising way to prepare low-cost and high efficient HER catalyst. The interface reactions and electrode kinetics of MoS2/RGO composite catalysts in HER can be further demonstrated by electrochemical impedance spectroscopy (EIS).43 The Nyquist plots of the samples and the corresponding electrical equivalent circuit diagram are given in Figure 6C. The impedance parameters by fitting the EIS responses are listed in Table 2,

RGO1 < MoS2/e-RGO2, which is in accordance with the HER results shown in Figure 6A. It has been well established that the Rct is related to the electrocatalysis kinetics and a lower Rct value corresponds to a faster reaction rate. In contrast, Cdl depends on the electrode surface area and a higher interfacial Cdl means larger active surface area of the electrode.71 Thus, the fast charge transfer during the electrocatalytic reaction coupling with large exposed active surface area could contribute to the superior electrocatalytic activity of the MoS2/RGO2 composite. Furthermore, to probe HER stability of the MoS2/RGO2 composite, we cycled the catalyst continuously for 500 cycles between 0 and −0.4 V vs RHE at 100 mV S−1. At the end of cycling, the catalyst was measured at the same applied potential range as the initial test. As shown in Figure 6D, the current density of MoS2/RGO2 composite catalyst decreases slightly to 95% of the initial value after the continuous 500 cycling, indicating that the MoS2/RGO2 composite catalyst exhibits excellent HER activity with reasonable long-term stability. The catalyst poisoning or the delamination of the catalyst from the electrode may contribute to the slight loss in catalytic activity.53,69 We stress that the main advantage of using a mildly oxidized GO as the conducting substrate is a good balance between the sufficient junction points for relatively high catalyst loading, the high dispersion of the nanosized MoS2, and the excellent conductive network. Our results indicate that graphene can provide an intriguing confinement environment for the growth of the catalyst that is highly HER active. Thus, the MoS2/RGO composite as an archetypical catalyst proffers guidelines to design and synthesize graphene-based materials for energy applications. It has been reported that amorphous MoS2 exhibits efficient catalytic performance for HER in that it contains a higher number of exposed edges and more active unsaturated sulfur atoms than the crystalline ones.54 In order to investigate the effect of MoS2 crystallinity on the HER activity, we compared the catalytic activity of the as-prepared MoS2/ RGO2 composite with that of the annealed ones. Figure S5 in the Supporting Information shows the XRD patterns of MoS2/ RGO2 composites after annealing in a H2/Ar (5 vol % H2) atmosphere at 450 and 800 °C for 1 h, respectively. The intensity of the (002) peak for MoS2 increases as the annealing temperature rises, showing the crystallinity is increased after annealing treatment. The polarization curves of as-prepared and annealed MoS2/RGO2 composites are shown in Figure 7A. The current densities of the MoS2/RGO2 composites significantly decrease with increasing annealing temperature. The corresponding Tafel and Nyquist plots are shown in Figure 7B,C,

Table 2. EIS Parameters of MoS2 and MoS2/RGO Composites Prepared at Various Conditions sample

Rs (Ω)

Rct (Ω)

Cdl (mF)

MoS2 MoS2/RGO2 MoS2/e-RGO2 MoS2/RGO1 MoS2/RGO3 MoS2/RGO2-450 MoS2/RGO2-800

8.4 8.0 10.5 10.3 10.8 9.0 10.7

7,150 107 395 315 148 990 1950

0.25 2.82 1.50 2.04 2.11 1.35 0.75

where the ohmic series resistance (Rs) was assigned to the intercept of the semicircle on the real axis, and the charge transfer resistance (Rct) and the corresponding interfacial capacitance (Cdl) were fitted by the semicircle of the Nyquist plot with Zsimpwin software in terms of the equivalent circuit model shown in the inset in Figure 6C. The Rs of all the samples falls in the range of 8−11 Ω, manifesting the good conductivity of the electrolyte. It is noted that the MoS2/RGO2 composite presents the lowest Rct and highest Cdl. In addition, the charge transfer resistances (Rct) of MoS2/RGO composites follow the sequence MoS2/RGO2 < MoS2/RGO3 < MoS2/

Figure 7. Electrochemical measurements of MoS2/RGO2 composite catalysts annealed after various temperatures. (A) Polarization curves, (B) Tafel plots, and (C) Nyquist plots of (a) as-prepared MoS2/RGO2 composite and MoS2/RGO2 composite after annealing at (b) 450 °C and (c) 800 °C in a H2/Ar (5 vol % H2) atmosphere. 2351

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and synthesis of a potential matrix for cost-effective catalysts in electrochemical hydrogen production.

respectively. The resulting Tafel slopes of the MoS2/RGO2 composites annealed at 450 and 800 °C are 55 and 65 mV decade−1, respectively, which are much higher than that of the as-prepared MoS2/RGO2 composite. Moreover, the Rct of the annealed composites increases sharply with the increase of the annealed temperature (listed in Table 2). And the corresponding Cdl decreases to 1.35 and 0.75 mF when increasing the annealed temperature to 450 and 800 °C, respectively. It has been reported that amorphous MoS2 possesses a huge number of unsaturated sulfur atoms on the surface and annealing would increase particle size and crystallinity, resulting in a decreased number of unsaturated sulfur atoms and thus a lowered HER activity.64 Therefore, the decreased HER activity of the MoS2/ RGO2 composite with increasing annealing temperature appears to be a result of a large loss of the unsaturated sulfur atoms on the MoS2 surfaces. In addition, to obtain further insight into the effect of exposed edge active sites on the HER performance, MoS2/ RGO2 composite prepared at low concentration of GO2 (25 mg, MoS2/L-RGO2) was tested toward HER (see Figure S6 in the Supporting Information). As shown the SEM image in Figure S6A (Supporting Information), the loading amount of MoS2 nanosheets on the graphene sheets of MoS2/L-RGO2 composite is much higher than that of MoS2/RGO2 composite, indicating that larger amount of exposed active sites existing in MoS 2/L-RGO2 composite. However, the MoS 2/L-RGO2 composite exhibits inferior HER activity (lower current density, lower Tafel slope (46 mV per decade) and higher Rct (155 Ω)) than that of the MoS2/RGO2 composite as shown in Figure S6B−D (Supporting Information). The difference in catalytic activity between MoS2/RGO2 and MoS2/L-RGO2 composites may be ascribed to the internal conductivity of the composite, where the high density of the MoS2 nanosheets in MoS2/LRGO2 composite may block the efficient charge transfer among the conductive graphene skeleton during the catalytic process. Thus, the superior catalytic activity of the as-prepared MoS2/ RGO2 composite can be attributed to the combined effects of abundance of exposed active edge sites and excellent internal conductive channels.

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ASSOCIATED CONTENT

S Supporting Information *

Assignments of the XPS spectra and elemental composition of MoS2/RGO composites, SEM characterization of the GO, elemental mapping images of MoS2/RGO2 composite, FTIR of (NH4)2MoS4 and MoS2/RGO2 composite, polarization curve of RGO2 , and XRD patterns of annealed MoS2/RGO 2 composite, SEM image of MoS2/L-RGO2 composite and its HER performance. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding Author. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the HK-RGC General Research Funds (GRF No. HKUST 605710 and 606511). REFERENCES

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CONCLUSION In summary, a solvent-evaporation-assisted intercalation and subsequent solvothermal treatment method has been put forward to the synthesis of MoS2 nanosheets/RGO composites. The mildly oxidized GO can not only provide the oxygencontaining functional groups for MoS2 precursor attachment but also control the size of MoS2 nanosheets due to the space confinement effect among the GO layers. The polycrystalline structure of nanosized MoS2 nanosheets leads to exposure of additional active edge sites, thus resulting in high electrocatalytic activity. With the merits of the sandwiched structure, MoS2/RGO2 composite exhibits excellent HER activity with small onset overpotential of 140 mV, a large cathodic current density, and small Tafel slope of 41 mV per decade. Moreover, the effect of oxidation degree of graphene, the crystallinity of MoS2, and the amount of exposed active sites on the HER performance of the MoS2/RGO composites have been investigated. The excellent electrochemical properties of MoS2/RGO2 composite could be ascribed to the combined effects of abundance of exposed active edge sites and excellent internal electrical conductivity. The detailed study of the MoS2/ RGO composite provides us with further insight into the design 2352

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