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Enabling Colloidal Synthesis of Edge-Oriented MoS2 with Expanded Interlayer Spacing for Enhanced HER Catalysis Yugang Sun, Farbod Alimohammadi, Dongtang Zhang, and Guangsheng Guo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05346 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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Enabling Colloidal Synthesis of Edge-Oriented MoS2 with Expanded Interlayer Spacing for Enhanced HER Catalysis Yugang Sun,†* Farbod Alimohammadi,† Dongtang Zhang†,‡ and Guangsheng Guo‡ †Department

of Chemistry, Temple University, 1901 North 13th Street, Philadelphia,

Pennsylvania 19122, USA ‡

Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and

Chemical Engineering, Beijing University of Technology, Beijing, 100124, P. R. China

KEYWORDS: Edge-oriented MoS2 nanosheets, interlayer-expanded MoS2 nanosheets, MoS2/graphene composite, microwave solvothermal reaction, HER catalysis

ACS Paragon Plus Environment

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ABSTRACT

By selectively promoting heterogeneous nucleation/growth of MoS2 on graphene monolayer sheets, edge-oriented (EO) MoS2 nanosheets with expanded interlayer spacing (~9.4 Å) supported on reduced graphene oxide (rGO) sheets were successfully synthesized through colloidal chemistry, showing the promise in low-cost and large-scale production. The number and edge length of MoS2 nanosheets per area of graphene sheets were tuned by controlling the reaction time in

the

microwave-assisted

solvothermal

reduction

of

ammonium

tetrathiomolybdate

[(NH4)2MoS4] in dimethylformamide. The edge-oriented and interlayer-expanded (EO&IE) MoS2/rGO exhibited significantly improved catalytic activity towards hydrogen evolution reaction (HER) in terms of larger current density, lower Tafel slope, and lower charge transfer resistance, compared to the corresponding interlayer-expanded MoS2 sheets without edge-oriented geometry, highlighting the importance of synergistic effect between edge-oriented geometry and interlayer expansion on determining HER activity of MoS2 nanosheets. Quantitative analysis clearly shows the linear dependence of current density on the edge length of MoS2 nanosheets.


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INTRODUCTION Increasing catalytic activity of non-Pt nanomaterials (e.g., oxides, chalcogenides, organicinorganic complex, etc.) towards electrochemical hydrogen evolution reaction (HER) represents a challenging but promising research direction because of the importance of hydrogen production in clean energy applications.1-3 Motivated by the traditional use of MoS2 nanostructures in the technological important hydrodesulfurization (HDS) process, nanostructured MoS2 has been actively explored as inexpensive catalyst to promote HER in recently years.4-21 Previous work have highlighted the importance of the intrinsic structural parameters including crystalline phase,5,16,22 edge site density/orientation,23-28 and interlayer spacing29-31 on determining HER catalytic activity of MoS2 nanostructures. For example, highly conducting 1T-phase MoS2 sheets (with metallic octahedral structure) exhibit improved HER catalytic activity compared to the semiconducting 2Hphase MoS2 sheets (with semiconducting trigonal prismatic structure) since both edges and basal surfaces in 1T MoS2 are catalytically active while only the edges of 2H MoS2 are active.16 However, the significant instability of the 1T MoS2 prevents its wide use in HER applications. As for the structurally stable 2H MoS2, HER activity can be enhanced by either exposing high density of active MoS2 edges23-28 or expanding the interlayer spacing of MoS2 nanostructures.29-31 It is even more promising to fabricate nanostructured MoS2 that can simultaneously exhibit both high-density edges and expanded interlayer spacing, which can synergistically improve HER activity of the MoS2. For instance, Wang et al. reported an approach relying on rapid sulfurization of thin ALD Mo films with hot S vapor, which produces MoS2 nanostructures with well-aligned edge-oriented (EO) geometry, which is favorable for pulling electrons from underneath support electrode (due to the high inlayer conductivity) to the active edge sites to promote HER. 26 In addition, intercalation of lithium in the EO MoS2 can expand the van der Waals gaps to further


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increase its HER performance in terms of enhanced current density and reduced Tafel slope.27 This pioneering example clearly demonstrated that nanostructured MoS2 with both EO geometry and interlayer-expanded (IE) feature (which will be described as EO&IE MoS2 in the following content) is very promising for HER. Despite of the promise, only few successful examples have been reported for the synthesis of EO MoS2 with normal interlayer spacing (i.e., ~6.2 Å) through gas-phase sulfurization of thin Mo and Mo oxide films.25-28 The solid-vapor phase synthesis is costly, difficult to scale up in tube furnace, and incapable of forming IE MoS2. In this work, we propose a low-cost and scalable colloidal synthesis approach relying on microwave-assisted solvothermal reduction of (NH4)2MoS4 in dimethylformamide (DMF), which has been recently developed in our group to synthesize IE MoS2 nanostructures with interlayer spacing of ~9.4 Å in liquid solution.30 By carefully controlling reaction condition, in a very narrow temperature range the as-grown IE MoS2 nanosheets can form reticulated networks to support EO geometry when the nanostructured MoS2 is dried on flat substrates. Even though such edgeoriented assembly of IE MoS2 nanosheets efficiently exposes edges of some MoS2 nanosheets, a significant amount of MoS2 nanosheets still flatly laminate on the substrate surface. Moreover, the colloidal IE MoS2 nanosheets easily aggregate into follower-like microspheres (Fig. S1) when the reaction temperature is not precisely controlled in the narrow temperature window. Formation of microflowers makes it impossible to assemble the IE MoS2 naonsheets to exhibit EO geometry on supporting substrates. To tackle this challenge, graphene oxide (GO) monolayer sheets are added to the solvothermal reaction solution to selectively promote heterogeneous nucleation of MoS2 on the GO sheets, on which the as-grown MoS2 nanosheets can exhibit EO geometry due to the increased hydrophobicity of GO sheets associated with the conversion of GO to reduced GO (rGO). The non-wettability between the hydrophobic rGO sheets and the hydrophilic MoS 2


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nanosheets spontaneously forces the MoS2 nanosheets to grow out of the rGO surfaces, leading to the EO assembly of the IE MoS2 nanosheets on rGO monolayer sheets. Similar to the EO&IE MoS2 formed through the combination of solid-vapor phase reaction and electrochemical lithium intercalation,27 the EO&IE MoS2/rGO composite also exhibit HER activity much higher than the corresponding IE MoS2 microflowers and the bulk counterparts. This first-time success in synthesis of EO&IE MoS2 using colloidal chemistry highlights nucleation engineering in rational design and synthesis of high-performance HER catalysts.

RESULTS AND DISCUSSION Microwave-assistant solvothermal reduction of (NH4)2MoS4 in DMF usually produces microflowers (Fig. S1), each of which is composed of intertwined MoS2 nanosheets with expanded interlayer spacing, at elevated temperatures. The random orientations of individual MoS2 nanosheets in each microflower cause the difficulty to enable all MoS2 nanosheets to expose their catalytically active edges while each MoS2 nanosheet is still in direct contact with chargecollecting electrode, leading to a significantly low efficiency in utilizing active MoS2 edges. Adding oxidized graphene (GO) monolayer sheets to the reaction solution can change the nucleation/growth behavior of forming MoS2 nanosheets because the GO sheets can provide nucleation sites to promote heterogeneous nucleation of MoS2 on GO sheets. At the early reaction stage, the abundant oxygen-containing groups (e.g., -COO, -COOH, -COH, etc.)32 in GO sheets are beneficial for accelerating heterogeneous nucleation of MoS2 upon reduction of MoS42 ions at high temperatures. Due to the increased reducing ability of DMF at elevated temperatures, GO monolayer sheets are reduced to graphene (or rGO) ones by eliminating the oxygen-containing


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groups. The chemical conversion of GO to rGO stops the continuous nucleation of MoS 2 on the rGO sheets. Therefore, continuous reduction of MoS42 ions only grow the existing MoS2 nuclei into larger MoS2 nanosheets. The non-wettability between MoS2 and rGO nanosheets (i.e., hydrophilic MoS2 versus hydrophobic rGO) forces the growing MoS2 nanosheets to protrude out of the rGO surfaces, enabling edge-oriented (EO) geometry of the as-grown MoS2 nanosheets. Such hybrid structures are described as MoS2/rGO in the following content. Fig. 1 presents typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of a product formed from a microwave solvothermal reaction at 260 oC for 2 h in the presence of GO sheets. It is clear that thin MoS2 nanosheets are uniformly deposited on the micron-sized rGO sheets to fully expose their edges (Fig. 1A and 1B). All the MoS2 nanosheets stand out of the rGO sheets at varying orientations with respect to the rGO surface. The TEM image recorded by aligning the electron beam perpendicular to the rGO surface shows that different MoS2 nanosheets exhibit different imaging contrasts and areas, confirming the varying orientations of individual MoS2 nanosheets (Fig. 1C). The MoS2 nanosheets perpendicular to the rGO surface enables the direct observation of layered structure and interlayer spacing of the MoS2 nanosheets with highresolution TEM (HRTEM) images (Fig. 1D). The spacing between two adjacent S-Mo-S layers is ~9.4 Å, a value much larger than that in bulk 2H MoS2 (6.15 Å). Raman spectrum of the product shown in Figure 1 exhibits characteristic peaks of both MoS2 nanosheets and rGO nanosheets (Fig. S2). Different from the symmetric G band peak at 1591 cm-1 for the GO nanosheets, the G band of the rGO nanosheets in the MoS2/rGO composite becomes asymmetric with the major peak shifting to a lower energy of 1578 cm-1 accompanied with a shoulder peak. Such variation of the G band is consistent with the reduction of GO to rGO.33


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Figure 1. (A, B) SEM and (C) TEM images of EO&IE MoS2/rGO sample synthesized from a reaction at 260 oC for 2 h. (D) HRTEM image of several EO&IE MoS2 nanosheets on a rGO nanosheet, highlighting the layered structure and exposed edges in the MoS2 nanosheets.

In the synthesis, the reaction solution is heated to the appropriate temperature at a fast ramp, which relies on strong absorption efficiency of microwave energy in ionic species (e.g., NH4+ and MoS42).30 The localized overheating can quickly reduce MoS42 ions to form MoS2 nuclei on GO sheets before the reaction solution reaches the setting temperature and the GO sheets are reduced to rGO sheets. A higher setting temperature requires a longer ramping period to enable the formation of more MoS2 nuclei on GO sheets, leading to a higher density of MoS2 nanosheets after


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continuous growth. Figs. 2A-C compare SEM images of the products synthesized at different solution temperatures (i.e., 200, 220 and 240 oC). The MoS2/rGO composite structures always exhibit EO&IE MoS2 nanosheets regardless of solution temperature in the range of 200-260 oC. The density of EO&IE MoS2 nanosheets significantly reduces as solution temperature decreases. Meanwhile, the average edge length of the MoS2 nanosheets only slightly varies. Quantitative analysis of the typical SEM images results in the dependence of density and edge length of the MoS2 nanosheets on reaction temperature (Fig. 2D). As temperature increases from 200 oC to 260 o

C, the average density of MoS2 nanosheets increases by 7.5 times (i.e., from 261 to 1960

nanosheetsm2), consistent with the accelerated nucleation rate at higher temperature and longer time required to reach the setting temperature. In contrast, the size of the MoS2 nanosheets decreases with reaction temperature. The mechanical flexibility and somehow random orientations of individual MoS2 nanosheets makes it unrealistically difficult to precisely measure either surface areas or edge lengths of the as-grown MoS2 nanosheets. Since there is no specific structural relationship between the MoS2 nanosheets and the supporting rGO nanosheets (i.e., no epitaxial relationship between MoS2 and rGO), it is reasonably considered that the growth of the MoS2 nanosheets is isotropic. Therefore, the overall length of the peripheral edges of each MoS2 nanosheet is proportional to its projected length in the SEM image (see Supporting Information). It is reasonable to use the projected edge length to study the relationship of catalytic performance on the overall peripheral edge length of the MoS2 nanosheets. In the following discussion, the measured edge length represents the projected edge length without any specifications. The projected average edge length of each MoS2 nanosheet decreases from 30.6  10.1 nm (220 oC) to 18.7  7.6 nm (260 oC). The larger number of MoS2 nuclei formed at higher temperature reduces the average size of the corresponding MoS2 nanosheets when the same amount of precursor is


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available to support the growth of MoS2. However, the edge length of the MoS2 nanosheets formed at 220 oC is slightly larger (by 1.9 nm) than that of the MoS2 nanosheets formed at 200 oC. Such reversed trend is ascribed to the fact that the reduction rate of MoS42 ions at 200 oC is too slow to complete the reduction of all MoS42 ions within 2 h. The results clearly demonstrate that the presence of GO monolayer sheets promotes heterogeneous nucleation of MoS2 to form EO MoS2 nanosheets on rGO sheets. The density and edge length of MoS2 nanosheets can be tuned by controlling reaction temperature. The EO feature of MoS2 nanosheets highlighted in Fig. 1 and 2 is remarkably different from previously reported MoS2/rGO hybrid structures, in which the MoS2 nanosheets exhibit random assembly on rGO surfaces.29,34-36

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Figure 2. SEM images of EO&IE MoS2/rGO synthesized from reactions at different temperatures: (A) 200 oC, (B) 220 oC, and (C) 240 oC. The reaction time was 2 h. (D) statistic analysis of the number of MoS2 nanosheets on 1 m2 rGO surface (blue columns) and the average edge length of one MoS2 nanosheet (red columns).

(002) rGO

Intensity (a.u.)

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MoS2/rGO 260 oC MoS2/rGO 240 oC MoS2/rGO 220 oC MoS2/rGO 200 oC Calculated XRD from c axis expanded cell 10

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2 (deg) Figure 3. X-ray diffraction patterns of EO&IE MoS2/rGO synthesized from reactions at different temperatures. The calculated pattern a MoS2 lattice with expansion along c axis is plotted for reference (green curve).

The crystalline structure of the synthesized EO&IE MoS2/rGO has been studied with X-ray diffraction (XRD) (Fig. 3). The XRD patterns of the MoS2/rGO composite structures formed at different reaction temperatures always exhibit two symmetric peaks with diploid relationship in


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low-angle region, i.e., 9.4o and 18.7o corresponding to the (002) and (004) reflections of layerstructured MoS2 with interlayer spacing of 9.4 Å, which is consistent to the HRTEM observations. It is worth noting that the (002) reflection peak at 9.4o is highly symmetric and much stronger than other reflection peaks regardless of the synthesis temperature, highlighting the high crystallinity in S-Mo-S layers. The formation of highly crystalline MoS2 lattices benefits the unique microwave solvothermal condition, excluding the possibility of forming defects in S-Mo-S layers, which usually significantly tweaks and weakens the (002) reflection peak.37 The expanded interlayer spacing is same to that of the nanostructured MoS2 synthesized in the absence of GO sheets (Fig. S3), indicating that the presence of GO sheets does not influence the structure of MoS2 nanosheets even though the EO geometry is promoted. The reflection peak at 32.7o exhibits an asymmetric profile, indicating the presence of stacking faults among S-Mo-S layers as a result of a-b plane gliding caused by intercalation of foreign species (e.g., DMF). Such stacking faults reduce the van der Waals interactions of the adjacent S-Mo-S layers to enhance HER catalytic activity.30 The rGO sheets without MoS2 exhibit a major XRD peak at 23.9o corresponding to the (002) reflections of closely stacked graphene sheets, confirming the conversion of GO to rGO under the microwaved solvothermal condition. In contrast, this XRD peak is absent in the EO&IE MoS2/rGO samples, indicating that the EO&IE MoS2 nanosheets protrude out of the rGO sheets to prevent the rGO sheets from restacking with well-defined periodicity. The absence of the XRD peak at 23.9o also confirms that the rGO nanosheets in the MoS2/rGO composite samples are dominated by monolayers because multiple-layer rGO nanosheets can exhibit XRD peak at 23.9o. The EO&IE MoS2 nanosheets on rGO sheets are expected to exhibit high electrocatalytic activity by taking the advantage of largely exposed edge sites, interlayer expansion of MoS 2, and high electric conductivity of rGO. The HER catalytic activity of the synthesized EO&IE


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MoS2/rGO samples have been evaluated in N2-saturated 0.5 M H2SO4 aqueous solution and compared to the state-of-the-art Pt/C catalyst, microflowers of IE MoS2 nanosheets, and rGO sheets. The Ohmic potential drop (iR) losses from the electrolyte resistance are corrected. The glassy carbon (GC) electrodes modified with varying catalysts steadily rotate at 1,600 rpm to efficiently remove H2 bubbles generated on the electrode surfaces. Fig. 4A compares the cathodic polarization curves for the GC electrodes modified with different catalysts shown in Figs. 1, 2 and Fig. S1. The rGO sheets are essentially inert to catalyze HER while all MoS 2 samples are highly active. Cathodic scan reveals that the onset potential of H2 evolution occurs at approximate –117 mV versus RHE (reversible hydrogen electrode) for the EO&IE MoS2/rGO synthesized at 260 oC, beyond which the catalytic reduction current increases sharply (black curve, Fig. 4A). In contrast, EO&IE MoS2/rGO samples synthesized at lower temperatures (i.e., 240 oC, 220 oC, and 200 oC) require larger overpotentials (i.e., 129 mV, 141 mV, and 150 mV, respectively) to initiate H2 evolution (Fig. 5A). The slight difference in onset overpotentials (η0) for different EO&IE MoS2/rGO catalysts might be ascribed to the difference in MoS2/rGO interfaces (more like a Schottky metal/semiconductor junction) formed at different temperatures. In contrast, the freestanding IE MoS2 microflowers synthesized without rGO initiates H2 evolution reaction only at even higher overpotentials than its corresponding EO&IE MoS2/rGO sample formed at the same temperature, e.g., 172 mV and 141 mV for IE MoS2 and EO&IE MoS2/rGO formed at 220 oC, respectively. Such difference in onset overpotential indicates that rGO sheets can lower energy barrier for electron transfer from GC electrodes to MoS2 nanosheets. The interlayer expansion in the microwave-synthesized MoS2 nanosheets also accounts for the low onset overpotentials for HER. Fig. 4A shows that a sharp increase in current density appears once the onset potential is exceeded. Since only MoS2 nanosheets in the EO&IE MoS2/rGO samples are active towards HER,


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the current density (j) is normalized against both mass of MoS2 (Fig. S4) and geometrical area of the electrode to compare the HER activity of different catalysts. The HER kinetics can be determined from the dependence of the logarithmic current density on overpotential (η). The slope in the linear region of (log j – η), i.e., Tafel slope, represents a typical parameter to describe the kinetic metrics of a catalyst. The Tafel slope of 43.5 mV dec-1 has been measured for the EO&IE MoS2/rGO sample synthesized at 220 oC, which is much smaller than that (i.e., 81.4 mV dec-1) of the corresponding freestanding IE MoS2 nanosheets, clearly highlighting the importance of EO feature on promoting HER reaction kinetics on MoS2 edge sites (Fig. 4B). In contrast, Tafel slopes of the EO&IE MoS2/rGO samples synthesized at different temperatures are essentially the same. Such similarity indicates that the HER reaction kinetics at the MoS2 edges of an EO&IE MoS2/rGO sample is independent of the synthesis temperature once the applied bias potential exceeds the corresponding onset overpotential. As a result, the difference in current density of different EO&IE MoS2/rGO catalysts is ascribed to the charge transfer resistance from GC electrode through the rGO sheets and MoS2 nanosheets to the catalytically active MoS2 edge sites. These comparisons indicate that electrochemical HER on the EO&IE MoS2/rGO catalysts can be represented with an equivalent circuit consisting of a diode (determined by the MoS2/rGO interface), a resistor corresponding to solution resistance (Rsol), a resistor corresponding to charge transfer resistance (Rct), and a capacitor (Fig. S5).


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Figure 4. (A) Normalized polarization curves for HER on modified glassy carbon electrodes comprising EO&IE MoS2/rGO catalysts synthesized at different temperatures, IE MoS2 microflowers and rGO sheets synthesized at 220 oC, and a commercial Pt/C catalyst. The potential sweep rate was 5 mV s-1. (B) Tafel plots and (C) Nyquist plots of the corresponding catalysts of (A). (D) Comparison of polarization curves of the EO&IE MoS2/rGO sample synthesized at 260 C before (black) and after (red curve) 3,000 potential cycles between 0.3 and 0.3 V versus RHE

o

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The electrochemical impedance spectroscopy (EIS) (Fig. 4C) has been used to extract the charge transfer resistance (Rct) in the EO&IE MoS2/rGO catalysts synthesized at varying temperatures, i.e., 70.83, 48.67, 26.79, 14.14  for the samples formed 200, 220, 240, and 260 oC, respectively. The value of Rct can influence the current density when the overpotential is above the


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onset overpotential (i.e., ηη0>0). The EO&IE MoS2/rGO catalysts with lower Rct always exhibit higher current density at the same value of (ηη0), e.g., the sample formed at 260 oC performs with the highest current density while the sample formed at 200 oC shows the lowest current density (Fig. 5B). The dependence of current density on reaction temperature is consistent with that of the edge length density (i.e., overall MoS2 edge length on 1 m2 rGO surface) of the MoS2 nanosheets (Fig. 5B), i.e., the catalyst with a higher MoS2 edge length density exhibits a higher current density. It is worth pointing out that the current density of different MoS2/rGO samples at the same offset overpotential (ηη0) exhibits a linear relationship with the MoS2 edge length density regardless of the synthesis temperature and the size of MoS2 nanosheets (Fig. 5C), highlighting that the charge transfer resistance (Rct) is mainly dominated by the MoS2 edges rather than the MoS2 basal planes in the EO&IE MoS2 nanosheets. The weak dependence of current density on the MoS2 basal planes indicates the lack of in-plane defects, which are usually catalytically active. Therefore, it is preferred to control synthesis condition (e.g., temperature, precursor concentration, etc.) to increase nuclei density and size of individual MoS2 nanosheets, both of which are beneficial for achieving high MoS2 edge length density to improve current density.


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Figure 5. (A) Onset overpotentials (0) required to initiate HER for EO&IE MoS2/rGO synthesized at different temperatures. (B) Current density at the same offset overpotential (0) for different EO&IE MoS2/rGO catalysts. The MoS2 edge length density (i.e., total MoS2 edge length with unit of m on 1 m2 rGO surface) of different catalysts are also plotted for comparison. (C) The linear relationship between current density and the MoS2 edge length density for the EO&IE MoS2/rGO catalysts.


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The edge-oriented feature of the EO&IE MoS2 on rGO sheets openly exposes the catalytically active MoS2 edges to solution, completely eliminating the influence of reactive species diffusion on current density. This is confirmed by the almost ideal semispheres of EIS for all EO&IE MoS2/rGO catalysts (Fig. 4C). In contrast, the EIS of the freestanding IE MoS2 microflowers exhibits a non-semispherical profile, indicating the significant contribution of reactive species diffusion to the overall resistance (green curve, Fig. 4C). The difficulty in diffusing reactive species to the catalytically active MoS2 edges is ascribed to the flower-like geometry, which prevent many edges from openly exposing to solution. Therefore, the edge-oriented feature and high MoS2 edge length density in the EO&IE MoS2/rGO catalysts are crucial for achieving high performance toward HER. In addition, the catalysts are stable for low-term operation. For instance, Fig. 4D compares the polarization curves of the 260 oC MoS2/rGO sample before and after 3000 cyclic voltammetry cycles. Only minor decay is observed at the high overpotential region, where hydrogen bubbles can form. By taking the advantage of unique heterogeneous nucleation and growth processes of MoS2 on GO sheets and the unique reaction kinetics in the microwave solvothermal reaction, MoS2 nanosheets with expanded interlayer spacing of 9.4 Å have been successfully grown on rGO sheets to show EO geometry that effectively exposes catalytically active MoS2 edges toward electrolyte. This success represents the first-time synthesis of EO&IE MoS2 nanosheets with rational control through colloidal chemistry, which is promising for scaling production. The very similar Tafel slopes of the HER polarization curves for varying EO&IE MoS2/rGO catalysts synthesized at different temperatures indicates that the HER reaction kinetics at the EO&IE MoS2 edges is independent of the synthesis temperature. Such similarity of MoS2 edges enables us to precisely


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correlate catalytic activity and MoS2 edge length density, which can be tuned by controlling reaction temperature, showing a linear dependence of current density on MoS2 edge length density. Compared with the freestanding IE MoS2 nanosheets without EO feature, the EO&IE MoS2/rGO catalysts exhibit significantly improved HER activity in terms of higher current density, lower onset overpotential, and lower Tafel slope, highlighting the importance of EO feature in promoting catalytic activity of MoS2 nanosheets. These understandings shed light on rational design of highperformance HER catalysts that can be achieved by controlling nucleation and growth kinetics of colloidal chemistry.

EXPERIMENTAL SECTION Chemicals Graphite powders, sodium chloride (NaCl, ≥99.0 %), sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3, 99+%), potassium permanganate (KMnO4, 99+%), hydrogen peroxide (H2O2, 30% in water), concentration aqueous solution hydrochloric acid (HCl, 37%), N,N-dimethylformamide (DMF, 99.9%), ammonium tetrathiomolybdate ((NH4)2MoS4, 99.95%) were purchased from Fisher Chemical. All chemical were used as received without further purification. Synthesis of graphene oxide (GO) nanosheets GO nanosheets were synthesized using the Hummers’ method38 with slight modifications. In a typical synthesis, 1 g of graphite powders were mixed with 20 g of NaCl crystals in an agate mortar, followed by grinding with a pestle for 30 min. The resulting fine powder mixture was transferred to a vacuumed filtration apparatus and washed with plenty of water to remove NaCl.


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The remaining graphite fine powders were placed in an oven set at 60 oC to be dried overnight. The dry powders were transferred to a 250-ml round bottom flask. To the flask was slowly added 23 ml of concentrated H2SO4 (98%). The suspension was continuously stirred at room temperature for 24 h. To this dispersion was added 100 mg of NaNO3 crystals that were completely dissolved in 5 min. 6 g of KMnO4 powders were then slowly added to the dispersion while a continuous stirring was kept for 30 min at room temperature to completely dissolve the KMnO4 powders. The dispersion was then carefully heated up to 90 oC and the temperature was maintained for 90 min. In the next step, to the resulting dispersion was slowly added 40 ml of deionized (DI) H2O, i.e., 3 ml of H2O was added followed by a 5-min interval until 40 ml of H2O was completely added. Continuous stirring was maintained for additional 50 min. To this dispersion was slowly and sequentially added 140 ml of H2O and 10 ml of 30% H2O2. After the dispersion was cooled down to room temperature, it was then centrifuged at 6000 revolutions per minute (rpm) and washed with an aqueous solution of 5% HCl. The collected powders were re-dispersed in 100 ml of H2O with assistance of ultrasonication for 30 min, resulting in a complete exfoliation of the oxidized graphite powders into GO nanosheets. Finally, the resulting GO dispersion was subjected to centrifugation at 5000 rpm for 5 min. The brown supernatant was collected and dried at room temperature, producing exfoliated monolayered GO nanosheets. The solids concentrated at the bottom of centrifuge tube were disposed. Growth of edge-oriented and interlayer-expanded (EO&IE) MoS2 nanosheets on the reduced GO (rGO) nanosheets


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The synthesis was similar to the microwave-assisted solvothermal reaction developed in our group30 except the existence of rGO nanosheets that provide heterogeneous nucleation sites to promote the formation of EO&IE MoS2 nanostructures. In a typical synthesis, 3 mg of GO nanosheets were dispersed in 6 ml of DMF with assistance of ultrasonication for half hour. To this dispersion was added 10 mg (NH4)2MoS4, which was dissolved with assistance of magnetic stirring for 20 min under ambient condition. The DMF dispersion containing both GO and (NH4)2MoS4 was transferred to a 10-ml reaction vessel that was sealed with a septa cap and placed in a microwave reactor (Monowave 300, Anton Parr). The dispersion was then quickly heated to an appropriate temperature (e.g., 200, 220, 240, and 260 °C) in a period of ~10 s and the temperature was maintained for 2 h to complete the reaction. The reaction dispersion was then quickly cooled down to room temperature with pressurized air flow. The products were then collected via centrifugation at 8000 rpm for 15 min followed by washing with DI water and ethanol for five times. The products were then dried in an oven set at 60 °C for 4h. Characterization X-ray diffraction (XRD) patterns were recorded on a Bruker D8 x-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). Scanning electron microscopy (SEM) images were collected with a FEI Quanta 450 FEG microscope operated at an accelerated voltage of 20kV and high vacuum mode. The SEM microscopy was equipped with X-MaxN 50 spectrometer (Oxford Instruments) for energy dispersive x-ray (EDX) analysis. Transition electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were recorded with JEOL JEM-1400 microscope and a JEOL 2010F(s) microscope, respectively. The samples were prepared by drop-casting appropriate amount of ethanol dispersion of the products on silicon wafers (for XRD measurement and SEM


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imaging) or carbon-coated copper grids (for TEM imaging), following by drying in fume hood and at room temperature. Electrochemical characterization Electrochemical measurements were performed with an electrochemical analyzer (CH Instrument CHI604E) at room temperature. Rotation disk electrodes of glassy carbon (GC) (PINE, 5 mm diameter) modified with different catalysts were rotated at 1600 rpm. In a typical modification process, the GC surface was polished to a mirror finishing followed by a thorough cleaning with DI H2O in ultrasonication bath. An appropriate amount of the synthesized MoS2 were dispersed in a mixed solvent of ethanol:water (3:1 v/v) in the presence of 5 wt% Nafion followed by ultrasonincation for 30 min, forming a dispersion with a concentration of 5 mg/ml. 10 ul of a catalyst dispersion (containing 50 g of catalyst) was delivered to a cleaned GC electrode with pipette and the solvent was evaporated at room temperature. In the measurement, the modified GC electrodes were used as working electrode, graphite was used as counter electrode, and a Ag/AgCl reference electrode was used in the three-electrode electrochemical cell. An aqueous solution of 0.5 M H2SO4 was used as electrolyte and degassed by bubbling pure nitrogen for at least half hour before measurements. The electrochemical impedance spectroscopy (EIS) analysis was performed at an overpotential of 200 mV over a frequency range from 5 MHz to 5 mHz with the amplitude of the sinusoidal voltage of 5 mV. All the measurements were carried out at room temperature.


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ASSOCIATED CONTENT Supporting Information. Analysis details for determining density and edge length of MoS2 nanosheets in EO&IE MoS2/rGO samples, Raman spectra of GO and EO&IE MoS2/rGO sample, SEM images and XRD pattern of freestanding IE MoS2 synthesized without rGO support, Weight percentage of MoS2 in EO&IE MoS2/rGO samples, Equivalent circuit corresponding to HER on ET&IE MoS2/rGO catalysts.

AUTHOR INFORMATION Corresponding Author [email protected] (Y.S.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Y.S. acknowledges the startup support from Temple University. REFERENCES (1)

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9.4 Å

MoS2

rGO

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