An in Situ Sulfidation Approach for the Integration of MoS2

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An In Situ Sulfidation Approach for the Integration of MoS2 Nanosheets on Carbon Fiber Paper and the Modulation of its Electrocatalytic Activity by Interfacing with nC60 Yun-Hyuk Choi, Jongbok Lee, Abhishek Parija, Junsang Cho, Stanislav V. Verkhoturov, Mohammed Al-Hashimi, Lei Fang, and Sarbajit Banerjee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01942 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016

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ACS Catalysis

An In Situ Sulfidation Approach for the Integration of MoS2 Nanosheets on Carbon Fiber Paper and the Modulation of its Electrocatalytic Activity by Interfacing with nC60

Yun-Hyuk Choi,1 Jongbok Lee,1 Abhishek Parija,1 Junsang Cho,1 Stanislav V. Verkhoturov,1 Mohammed AlHashimi,3 Lei Fang,*1,2 and Sarbajit Banerjee*1,2 1

Department of Chemistry and 2Materials Science and Engineering, Texas A&M University, College Station, Texas 77842-3012, USA; 3 Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar *E-mail: [email protected]; [email protected]

ABSTRACT Molybdenum disulfide (MoS2) is a promising earth-abundant and low-cost electrocatalyst for the hydrogen evolution reaction (HER). In this study, we describe a stepwise synthetic approach comprising vapor transport, reduction, and topochemical sulfidation for creating 3D arrays of MoS2 nanosheets directly integrated onto carbon fiber paper (CFP) substrates. The sulfidation process results in a high density of edge sites along both the edges and the basal planes of MoS2. The obtained materials characterized by a high density of exposed edge sites exhibit promising electrocatalytic performance including an overpotential (η10) of 245 mV at 10 mA/cm2, a Tafel slope of 81 mV/dec, and a turnover frequency (TOF) of 1.28 H2/s per active site at -0.2 V versus RHE in a 0.5 M acidic solution. The electrocatalytic properties of the MoS2 nanosheets are observed to be substantially enhanced by interfacing with solution-deposited buckminsterfullerene nanoclusters (nC60). A coverage of ca. 2% of nC60 yields a hybrid electrocatalyst exhibiting η10 of 172 mV, Tafel slope of 60 mV/dec, and TOF of 2.33 H2/s per active site at -0.2 V vs. RHE. The enhancement of electrocatalytic activity is found to derive from interfacial charge transfer at nC60/MoS2 p—n heterojunctions. The high conductivity of the interfacial layer formed as a result of charge transfer from nC60 to MoS2 is thought to substantially mitigate the limitations imposed by the poor basal plane conductivity of undoped MoS2. The hybrid catalysts illustrate an important design principle involving the use of structured interfaces to enhance the catalytic activity of low-dimensional materials.

KEYWORDS: molybdenum disulfide; fullerene; chemical vapor deposition; electrocatalyst; hydrogen evolution reaction

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1. INTRODUCTION Sunlight shining on earth is intermittent and thus a fundamental impediment to its meaningful utilization is the effective storage of solar energy.1-4 Water splitting, or the disproportionation of H2O into H2 and O2, represents a promising strategy but is non-trivial because it requires the concerted transfer of four electrons and four protons. In nature, the complex biological machinery for photosynthesis couples multiple platforms wherein the light harvesting, water oxidation, and proton reduction steps are each performed by discrete components. Consequently, there is much interest in hybrid structures wherein discrete components perform each of the individual reactions required for photocatalysis.5-8 A viable photocatalytic cycle can be constituted by coupling photocatalytic water oxidation with electrocatalytic hydrogen evolution. The latter hydrogen evolution reaction (HER), however, has been beset by a distinctive set of challenges.7,9-11 The Pt group metals are excellent catalysts for HER and evolve hydrogen at near-zero overpotentials in acidic media but are cost prohibitive and amongst the least abundant elements available to mankind. There has been a strong push to develop alternatives and some success has been achieved with MoS212-16 as well as transition metal phosphides.17,18 However, such materials tend to evolve H2 at high overpotentials as compared to Pt and thus waste much free energy. The electrocatalytic activity of MoS2 is mainly derived from catalytically active edge sites. In contrast, the basal planes are often thought to be catalytically inert with some exceptions.12,14 Furthermore, the low charge carrier mobility of MoS2 is an impediment to its use as an electrocatalyst. Consequently, there is room for improving the HER activity of MoS2 by modulating the edge state reactivity and ion/electron transport characteristics. In this work, we demonstrate a stepwise sulfidation approach for the direct integration of vertically aligned high-edge-density MoS2 onto conductive carbon fiber paper (CFP). Furthermore, we illustrate that interfacing a solution-deposited layer of buckminsterfullerene nanocluster (nC60) onto the basal planes of MoS2 allows for a substantial diminution of the HER overpotential. Considerable attention has focused on increasing the edge density of MoS2 in order to enhance its catalytic performance. For example, Wu et al. achieved a high density of active sites and promising HER performance characterized by a Tafel slope of 68 mV/dec for MoS2 nanosheets obtained from the mechanical ball-milling of MoO3 and elemental sulfur precursors.19 Xie et al. prepared defect-rich MoS2 nanosheets with a high density of active edge sites by a hydrothermal method using thiourea as the sulfur precursor.20 An alternative approach for the enhancement of HER activity involves chemical modification of the active sites of MoS2, which can potentially alter the enthalpy of hydrogen adsorption and the extent of hydrogen coverage via modulation of the edge electronic structure. For instance, Xie et al. demonstrated enhanced HER performance by incorporating oxygen within MoS2 nanosheets; the enhancement in HER activity is thought to derive from an improvement of the intrinsic electronic conductivity of the system.21 Other approaches that have been successful involve the fabrication of hybrid structures that interface MoS2 with graphene and carbon nanotubes (CNTs), wherein the conductive carbon nanomaterials provide alternative conduction pathways and facilitate electron transport between MoS2 edge-sites and conductive electrodes, thereby mitigating reliance on charge transport along the poorly conductive basal planes of MoS2.22-25 Most recently, Li et al. have activated the catalytically inert basal planes of MoS2 by introducing sulfur vacancies and strain, resulting in highly enhanced HER performance.26 Chemical vapor deposition is ubiquitously used to prepare well-crystallized MoS2 architectures, typically using molybdenum oxide or chloride precursors.15,26-29 A major drawback of this method as applied to the growth of MoS2 is that it necessitates the operation of several concurrent reactions. Consequently, the obtained MoS2 electrocatalyst samples are often plagued by poor size and shape homogeneity, with sparse substrate coverage.26,28,29 Furthermore, as a result of epitaxial relationships and preferred growth directions, much of the CVD literature reports the preparation of MoS2 flakes that 2


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expose catalytically inactive basal planes horizontal to a substrate.26,28,29 In order to address these problems, in this work, we illustrate a scalable approach for preparing homogeneously sized and well distributed MoS2 nanosheets integrated onto CFP with exposed catalytically active edge sites. Through the stepwise reduction and sulfidation of MoO3 by sulfur, we have prepared vertically oriented MoS2 nanosheets with exposed edge facets at the centimeter scale. The direct integration of edge-exposed MoS2 nanosheets onto CFP yields a 3D architecture with a high surface-to-volume ratio desirable for electrocatalytic applications.11 The inherent HER activity of the edge-sites of MoS2 is enhanced significantly by interfacing with nC60 nanoclusters as a result of the enhancement of the conductivity of MoS2 owing to charge transfer. 2. RESULTS AND DISCUSSION The stepwise vapor transport, reduction, and sublimation steps used to prepare edge-exposed MoS2 nanosheets on CFP are schematically illustrated in Figure 1a. In the first step, MoO3 nanosheets that are ca. 1—2 μm in lateral dimensions (Figure 1b) are deposited onto CFP by the vapor transport of MoO3 powder heated to 850°C. The SEM images in Figure 1b illustrate the homogeneous coverage, sharply faceted edge sites, and vertical growth orientation of the MoO3 nanosheets with respect to the CFP substrate. In the next step, reaction with sublimed sulfur at 400°C as per: 2MoO3 (s) + S (g) 2MoO2 (s) + SO2 (g)…(1) yields MoO2 nanosheets with retention of the vertical growth orientation, although the edges are slightly rounded. Finally, the topochemical sulfidation of MoO2 at 850°C as per: MoO2 (s) + 3S (g) MoS2 (s) + SO2 (g)…(2) yields faceted MoS2 nanosheets (Figure 1b) that are uniformly dispersed and vertically oriented across a large area (ca. 2 cm2) of the CFP. Figure S1 illustrates the morphologies of vapor transported MoO3 collected on a Si (100) substrate before and after stepwise reduction and sulfidation. These images further enable visualization of the vertical growth direction and high density of edge sites. Notably, the vertical growth orientation is achieved without mediation of a catalyst.30-32 Interestingly, the faceted MoO3 nanosheets are transformed to thicker rounded MoO2 discs upon reduction and finally converted to faceted MoS2 nanosheets during sulfidation. Figure 1c illustrates the edge topography of an individual MoS2 nanosheet. The edge geometries in large measure reflect the intrinsic crystal structures of the phases. Orthorhombic α-MoO3 crystallizes in a layered structure and thus faceted nanosheets are obtained comprising stacked layers (Figure 1b and Figures S1a and d). Reduction to monoclinic MoO2 yields rounded edges (Figure 1b and Figures S1b and e), whereas topochemical transformation to 2H-MoS2 again yields faceted structures reflecting the layered stacking of MoS2 sheets. The considerable lattice mismatch between MoO2 and MoS2 results in a substantial volume change, which creates a distinctive discontinuous motif characterized by faceted “clean” and discontinuous collapsed domains along the MoS2 basal planes (Figure 1c). The latter is important as it allows for exposure of an increased density of catalytically active edge-sites. Figure 2 corroborates the phase identification of the prepared materials based on X-ray diffraction (XRD) and Raman microprobe analysis. The XRD patterns acquired on CFP are dominated by the (002) reflections of the graphitic substrate. However, reflections corresponding to the deposited materials are discernible and are indexed to orthorhombic α-MoO3 (Joint Committee on Powder Diffraction Standards (JCPDS) 76-1003), monoclinic MoO2 (JCPDS 86-0135), and hexagonal 2H-MoS2 (JCPDS 87-2416) as shown in the figure (Figure 2a). Clearer phase assignment is enabled from the Raman spectra shown in Figure 2b since the graphitic D and G bands from the substrate are only observed above 1300 cm-1. The Raman bands of the nanosheets formed in the first step are well matched with the Raman active modes of orthorhombic α-MoO3 reported in the literature.33-35 The detailed Raman band assignments of the prepared α-MoO3 nanosheets are listed in Table S1 (Supporting Information). The Raman spectra of the nanodiscs formed by the reduction of the α-MoO3 nanosheet using sulfur are an excellent match for 3


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phonon modes of monoclinic MoO2 as reported previously in the literature.33,36 The sulfide structures on CFP show clear Raman signatures of 2-H MoS2 including Raman bands at 282, 377, and 404 cm-1, which can be ascribed to modes of E1g, E2g1, and A1g symmetry, respectively.37 X-ray photoelectron spectroscopy (XPS) analysis has further been performed by acquiring Mo 3d, O 1s, and S 2p core level spectra of each product, to investigate the evolution of the chemical composition (Figure 2c). The Mo 3d core level spectra are characterized by a distinctive doublet at 233.20 and 236.35 eV ascribed to the binding energies of Mo 3d5/2 and 3d3/2 states, respectively; these binding energies are characteristic of hexavalent molybdenum, verifying stabilization of the MoO3 phase. The O 1s singlet at 530.95 eV is further in good agreement with the value expected for an inorganic oxide.34,38 Upon reduction with sulfur, the XPS spectra for the nanodiscs shows a substantial alteration of the Mo 3d binding energies to 229.80 and 233.05 eV for the Mo 3d5/2 and 3d3/2 states, suggesting the stabilization of a tetravalent oxide of molybdenum. A remnant shoulder at 236.35 eV attributable to the binding energy of Mo 3d3/2 for hexavalent molybdenum indicates incomplete reduction.39 Corresponding features in the O 1s core level spectra at 530.75 and 531.85 eV, can be attributed to MoO2 and MoO3 respectively.40 Furthermore, a distinctive doublet is discernible in S 2p core level spectra at 162.75 and 163.80 eV and can be ascribed to S 2p3/2 and S 2p1/2 binding energies, respectively, revealing surface sulfidation forms some MoS2 even at a temperature of 400°C. The nanosheets after sulfidation at 850°C show Mo 3d core level spectra at 229.70 and 232.85 eV attributable to binding energies for Mo 3d5/2 and Mo 3d3/2, respectively; these values are characteristic of MoS2. The small shoulder at 226.95 eV is attributed to S 2s.22,23 A much more pronounced doublet is observed in S core level spectra at 162.65 and 163.75 eV assigned to S 2p3/2 and S 2p1/2 binding energies, respectively. These values verify the sulfidation of MoO2. A broad O 1s spectrum with a peak at 532.60 eV is attributed to surface-adsorbed oxygen species.40 The enthalpy of hydrogen adsorption on MoS2 edges has been estimated to be endothermic by ca. 0.08 eV and the extent of H-coverage is limited to one in four atoms at the edges of MoS2.12-14 Reducing the overpotential and increasing catalytic efficiency requires a further decrease of the hydrogen adsorption enthalpy and increase of the extent of H-coverage. One approach involves polarizing Mo—S bonds at the edges via electronic coupling with electron-donating or withdrawing moieties, ideally other semiconductors. Here, we have interfaced the faceted MoS2 nanosheets with nC60 clusters deposited from solution to prepare hybrid architectures. Upon solution deposition from chlorobenzene solution (nC60 of 0.5 mg/mL), nC60 clusters that are ca. 7 μm in diameter are deposited onto the fibers of CFP (Figure 3a). Figure 3b indicates that similar morphologies of nC60 were grown on the MoS2 nanosheets. Energy dispersive X-ray spectroscopy (EDS) maps (Figures 3c—e) acquired at C, Mo, and S elemental edges verify the co-localization of the C60 clusters atop the MoS2 nanosheets. Figure S2 of the Supporting Information further plots the corresponding EDS line profiles. The Raman spectra of the nC60 cluster and hybrid nC60/MoS2 architectures are shown in Figure 3f. Distinctive Raman modes of C60 are evidence in both spectra with bands assigned to phonons of Ag(1,2) and Hg(1-8) symmetry.41,42 Both MoS2 and C60 modes are discernible in the hybrid architecture. The coverage of nC60 clusters strongly depends on the concentration of the precursor solution as indicated by low-magnification FESEM images in Figure S3 of the Supporting Information. The size of the nC60 clusters increases with increasing concentration from 0.1 to 2.0 mg/mL. Notably, at a concentration of 0.1 mg/mL, the relatively small nC60 clusters are homogenously distributed throughout the sample (Figure S3c); however, upon increasing the concentration to 0.5 mg/mL (Figure S3a and b), the homogeneity is somewhat reduced. Upon increasing the precursor concentration to 2.0 mg/mL, the nC60 clusters are mostly present as large agglomerations that are rather sparsely distributed across the surface (Figure S3d). In the concentration range examined here, the clusters do not appear to form a continuous percolative network. In order to evaluate quantitatively the coverage of nC60 clusters for a precursor concentration of 0.5 mg/mL, the sample 4


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deposited onto CFP has been examined by secondary ion mass spectrometry (SIMS) using 50 keV C602+ ions as the source. The C60 clusters are ca. 7 μm in diameter and cover ca. 2% of the total surface area of the carbon fibers of CFP (Figures S4 and S5 of the Supporting Information). Indeed, the SEM and SIMS measurements indicate that optimal concentrations of the C60 solution are necessary to maximize interfacial interactions and prevent crystallization into larger nC60 clusters. The electrocatalytic HER performance of CFP based samples with nC60 clusters alone, as-prepared 3D MoS2 nanosheets, and hybrid nC60/MoS2 architectures has been investigated in a 0.5 M aqueous solution of H2SO4, using a conventional three-electrode setup. Figure 4a depicts polarization curves, which have been corrected for ohmic potential drop (iR) losses (see Figure S6, Supporting Information). Bare CFP is contrasted as a control and is essentially catalytically inert towards HER. In contrast, nC60 (0.5 mg/mL) clusters on CFP exhibit a finite cathodic current density with an overpotential of 353 mV, reaching a current density of 10 mA/cm2 (η10) and a Tafel slope of 169 mV/dec (Figure 4b). The 3D faceted MoS2 nanosheets on CFP show HER activity with a η10 value of 245 mV and a Tafel slope of 81 mV/dec. Remarkably, interfacing the MoS2 nanosheets with nC60 results in a much lower overpotential. Hybrid nC60 (0.5 mg/mL)/MoS2 structures have a η10 value of 172 mV and a Tafel slope of 60 mV/dec. This result clearly indicates the synergistic enhancement of HER activity as a result of coupling between nC60 and MoS2. The high Tafel slope value of pristine nC60 on CFP (>120 mV/dec) indicates that HER proceeds through the Volmer mechanism, wherein proton reduction yielding hydrogen ad-atoms bound to the active sites represents the rate determining step.22 In contrast, the low Tafel slope values measured for as-prepared 3D MoS2 and hybrid nC60/MoS2 (60 and 80 mV/dec) suggest the operation of the VolmerHeyrovsky mechanism wherein the rate-determining steps involve both proton reduction and hydrogen desorption.22,24 It is noteworthy that the HER performance of the 3D array of MoS2 nanosheets with a high density of exposed edge-sites and their hybrid structures interfaced with nC60 are either higher or comparable to previously reported values for bulk or nanostructured MoS2. In addition, the hybrid materials reported here possess the advantages of well-defined architectures, conductive substrates, and scalability to centimeter-sized dimensions.20-26,43-46 Figure S7 (Supporting Information) contrasts the polarization curves of various concentrations of nC60 clusters either deposited directly onto CFP or interfaced with 3D MoS2 nanosheets on CFP. The cathodic current density of the neat nC60 cluster formed on CFP measured at -0.4 V versus RHE is gradually decreased from 22.6 to 18.5 to 14.6 mA/cm2 as the concentration of C60 deposition solution is increased from 0.1 to 0.5 to 2.0 mg/mL. With increasing concentration of C60 solution, the overpotential η10 is also increased from 331 to 353 to 363 mV. As noted above, the hybrid nC60/MoS2 electrocatalyst prepared using 0.5 mg/mL C60 deposition solution shows the best HER performance with the highest current density (J0.2V = 18.0 mA/cm2 at 0.2 V vs. RHE), lowest η10 value (172 mV), and the lowest Tafel slope (60 mV/dec). The nC60 (0.1 mg/mL)/MoS2 sample (J0.2V = 5.2 mA/cm2, η10 = 245 mV, and Tafel slope = 74 mV/dec) exhibits substantially worse performance that is analogous to the 3D MoS2 nanosheets without C60 hybridization (J0.2V = 5.0 mA/cm2, η10 = 245 mV, and Tafel slope = 81 mV/dec). At such low concentrations, the limited nC60 coverage likely limits the extent to which the edge reactivity is modulated. At substantially higher solution concentrations of C60, the HER performance is diminished as well. The nC60 (2.0 mg/mL)/MoS2 sample is characterized by values of J0.2V = 3.9 mA/cm2, η10 = 273 mV, and a Tafel slope = 80 mV/dec. We attribute the lack of synergistic enhancement in the latter case to the sparse and heterogeneous distribution of nC60 (2.0 mg/mL). The large agglomerations observed in Figure S3d (Supporting Information) suggest that the buckministerfullerene clusters are not effectively interfaced with MoS2, which likely perturbs the electronic coupling necessary for improved HER performance as described below.

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In order to examine the mechanistic basis for the observed modulation of electrocatalytic properties upon interfacing with nC60, electrochemically active surface areas (ECSA) of all the samples have been estimated by measuring the double-layer capacitance (Cdl) from cyclic voltammetry (CV) data across a potential range with no Faradaic current.46-48 The voltammograms have been collected at various scan rates (20—100 mV/s) in the potential range of 0.10—0.30 V versus RHE, where the current is preponderantly due to the charging of the double layer (and not due to proton reduction). Figures S8 and S9 of the Supporting Information depict CV curves acquired for nC60, 3D nanosheets of MoS2, and hybrid nC60/MoS2 architectures with various C60 concentrations. The differences (Δj) of anodic and cathodic current densities at 0.20 and 0.23 V versus RHE for each CV plot is shown as a function of the scan rate in Figures S8d and S9e. The slope of each Δj versus scan rate plot is equal to a value of 2Cdl. The ECSA has been obtained from the ratio of the measured Cdl with respect to the specific capacitance of flat crystalline MoS2 (ca. 66.7 μF/cm2).46-48 The resulting Cdl and ECSA values are displayed as a function of C60 concentration in Figure 4c. Significantly, the Cdl and ECSA of nC60 on CFP and hybrid nC60/MoS2 on CFP are respectively lower and higher than those of 3D MoS2 nanosheets on CFP, and are decreased with increasing C60 concentration. From these results, it can be inferred that the nC60 clusters formed on CFP or MoS2/CFP are increasingly agglomerated and crystallized with increasing C60 concentration in solution, which is consistent with the morphologies observed by SEM in Figure S3 of the Supporting Information. Furthermore, the Cdl and ECSA of hybrid nC60/MoS2 appear to be the sum of those of nC60 and 3D MoS2 nanosheets on CFP. However, an increased concentration of electrochemically active sites does not necessarily translate to increased HER activity since the nC60 clusters alone are much less active as compared to the 3D MoS2 architectures. The decrease of Cdl and ECSA with increasing C60 concentration of the precursor solution leads only to a slight deterioration of the cathodic current density for nC60/CFP and the Tafel slope is mostly preserved, indicating that the changes in Cdl, ECSA, and the resulting number of active sites do not fundamentally alter the HER mechanism (i.e., Volmer reaction in the neat C60) and rate. These two sets of observations suggest that the improved HER performance observed for the hybrid nC60 (0.5 mg/mL)/MoS2 electrocatalyst is derived from an intrinsic enhancement of the inherent catalytic activity of MoS2 for HER rather than an increase in the number of active sites upon C60 deposition. The turnover frequency (TOF), defined as the number of H2 molecules evolved per active site per unit time, is an essential parameter to contrast the inherent catalytic activity of different systems.46-48 The TOF can be calculated using the expression TOF = JNA/2Fn(ECSA), where J is the current density, NA is Avogadro’s number, 2 represents the stoichiometric number of electrons consumed at the electrode during HER, F is Faraday’s constant, n is the number of active sites (1.164 × 1015 cm-2) on a flat surface of crystalline MoS2,47 and ECSA is the electrochemically active surface area of the electrode. Figure 4d plots the TOF (per active site) of the 3D MoS2/CFP and hybrid nC60 (0.5 mg/mL)/MoS2 structure prepared on CFP in the applied potential range of -0.1 to -0.3 V versus RHE; in this regime, the HER is controlled by electrode kinetics with minimal influence from other effects. The measured TOF of the hybrid nC60 (0.5 mg/mL)/MoS2 structure at -0.2 V (2.33 H2/s per active site) is nearly twice as high as that of 3D MoS2 nanosheets (1.28 H2/s per active site) on CFP. These results highlight the synergistic enhancement of the inherent catalytic activity of the edge sites of the MoS2 nanosheets upon nC60 hybridization. The 3D hybrid architectures constructed on mesoporous CFP clearly represent viable electrocatalysts. In order to further investigate a possible origin of the enhanced HER performance observed for the hybrid nC60 (0.5 mg/mL)/MoS2 structure, electrochemical impedance measurements have been performed at various potentials between 10 and -250 mV by sweeping the frequency from 200 kHz to 100 mHz with an AC amplitude of 10 mV (Figure S10, Supporting Information). Figure 5a shows the Nyquist plots of as-prepared 3D MoS2 nanosheets and hybrid nC60/MoS2 architectures prepared on CFP 6


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measured at -150 mV. The Nyquist plots are fitted to an equivalent circuit model comprising the following elements: an ohmic resistance (Rs), a charge-transfer resistance (Rct), constant phase element (Q), and a Warburg constant (W). The obtained Rct values are plotted as a function of potential in Figure 5b. The kinetics of electrocatalytic HER on the different electrode samples can be evaluated based on their respective Rct values with a lower Rct value corresponding to a faster reaction rate.23,24 The resulting Rct values reveal a similar trend as the η10 and Tafel slope values deduced from the cathodic current density in polarization curves (Figures 4a and S7); specifically, the Rct values of 3D MoS2/CFP and hybrid nC60/MoS2 prepared on CFP are nearly two orders of magnitude lower than those of nC60/CFP. Furthermore, the lowest Rct values are obtained for the hybrid nC60 (0.5 mg/mL)/MoS2 structure. Taken together, these results suggest that the enhanced HER performance observed upon interfacing with nC60 derive in large measure from the increased conductance of the hybrid constructs when C60 is appropriately interfaced with MoS2. Indeed, recent ab initio density functional theory calculations of C60/MoS2 constructs are particularly instructive in understanding the nature of the interface formed between these two semiconductors. Gan et al. have determined that the lowest energy configuration for these heterostructures corresponds to the hexagonal rings of C60 situating directly above S sites on the basal planes of MoS2 resulting in buckministerfullerene molecules being able to rotate freely on the surface.49 This configuration yields a Type-II interface with charge depletion from C60 and charge accumulation on MoS2 estimated to be ca. 0.055 e- per C60 unit.49 This directional charge transfer is thought to be key to the reduced resistance of the hybrid constructs. Indeed, the Type-II alignment has been further verified by recent theoretical and experimental studies of C60/MoS2 hybrids.50 Chen and colleagues have predicted that the valence band edge of MoS2 (-4.5 eV) resides lower than that of C60 (-3.8 eV), resulting in charge transfer and electron accumulation on MoS2 when the two semiconductors are interfaced. Upon application of an electric field, the steadily increasing electron density in MoS2 reduces the junction-barrier height, further allowing facile electron tunneling and transport and giving rise to conductive pathways along the interfaces of the resulting C60/MoS2 p—n heterojunctions.50 Therefore, based on the measured TOF, deduced resistance values, and charge transfer resistance values extrapolated from EIS data, the enhanced HER performance of the hybrid nC60 (0.5 mg/mL)/MoS2 structure likely derives from a charge transfer mechanism. The nC60 clusters donate electron density to MoS2 and give rise to a conductive interfacial layer that is much more effective at charge transport as compared to the relatively insulating basal planes of MoS2. Such charge transfer may also polarize the Mo—S bonds reducing the enthalpy of hydrogen adsorption. Notably, this mechanism, essentially invoking interfacial doping of MoS2, is quite distinct from hybrid MoS2/carbon nanotube and MoS2/graphene heterostructures wherein the latter components actually form conductive pathways for electron transport between the CFP electrodes and the catalytically active edge sites, thereby mitigating the poor transport characteristics of the basal planes of 2H-MoS2. The role of interfacial doping is further underscored by the dependence of HER performance on the concentration of the C60 precursor solution and the morphology of the nC60 clusters. Agglomerated C60 clusters that are inhomogeneously dispersed across the MoS2 basal planes will be ineffective at modulating the electronic structure of MoS2 through electron transfer. To assess the long-term stability of nC60 (0.5 mg/mL)/CFP, 3D MoS2/CFP, and hybrid nC60 (0.5 mg/mL)/MoS2 on CFP as electrocatalysts for HER, CV sweeps have been performed for 1000 cycles in a 0.5 M aqueous solution of H2SO4 in the range between -0.2 and 0.2 V versus RHE at a scan rate of 100 mV/s. The polarization curve for the 3D MoS2 on CFP is almost exactly superimposable upon the initial data suggesting no degradation in performance (Figure S11). In contrast, after 1000 cycles, the hybrid nC60/MoS2 catalysts show a slight increase of the overpotential η10 to 181 mV and the Tafel slope is 7


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changed to 65 mV/dec. The observed changes are suggestive of the partial loss of C60 clusters upon prolonged electrocatalytic cycling, which likely disrupts some of the interfacial charge transfer and thereby disrupts charge transport between the CFP substrate and active catalytic edges. Future work will focus on devising covalent means of tethering C60 to the MoS2 basal planes in order to improve the long-term stability. 3. CONCLUSIONS In summary, we have developed a scalable, stepwise vapor transport, reduction, and topochemical sulfidation approach that yields vertically oriented 2H-MoS2 nanosheets with a high density of exposed edge sites directly integrated onto conductive CFP substrates. The polycrystalline nanosheets are homogeneously dispersed across centimeter scales. They are characterized by a mixture of faceted as well as discontinuous collapsed edges within the basal planes induced as a result of the volume expansion accompanying topochemical sulfidation. The 3D MoS2 nanosheets on CFP exhibit an overpotential η10 value of 245 mV and a Tafel slope of 81 mV/dec, comparable or better than values reported in the literature for bulk or nanostructured MoS2 and show complete retention over 1000 HER cycles. The 3D MoS2 nanosheets have been further interfaced with nC60 clusters by a facile solutiondeposition method. The hybrid structures show greatly enhanced HER activity with an overpotential η10 value of 172 mV and a Tafel slope of 60 mV/dec when the deposition concentration of C60 was 0.5 mg/mL. This condition corresponds to a ca. 2% coverage of the MoS2 nanosheets by nC60 clusters. The improved activity of the hybrid catalysts is seen to derive not from an increase of the electrochemically active surface area, but from the interfacial doping of MoS2 induced by charge transfer from the nC60 clusters to MoS2 that results from the relative positioning of their valence and conduction band edges. An optimal coverage of nC60 with a homogeneous distribution is necessary for such interfacial doping. More generally, this work proved the concept of using structured interfaces to enhance the catalytic activity of low-dimensional materials. Future work will focus on covalent tethering of fullerene units onto the basal planes of MoS2 in order to improve the long-term stability. 4. EXPERIMENTAL SECTION 4.1. Preparation of MoS2 nanosheets, C60 clusters, and their hybrid structures. The CVD processes were performed using a 1-inch-diameter horizontal cold-wall quartz tube furnace equipped with gas flow controls. In the first step to prepare MoO3 nanosheets, 15.0 mg of MoO3 powder (Sigma-Aldrich, purity ≥99.5%) was placed within an alumina boat, which was placed at the center of tube. A bare CFP substrate (Toray Paper 120) with dimensions of 7 cm × 1 cm size was placed downstream from the MoO3 source at a distance of 15 cm from the alumina boat. After an initial Ar purge for 30 min, the MoO3 powder was heated to 850°C at a ramp rate of 20°C/min and transported under a 68.3 sccm Ar flow at 1 atm. After holding at 850°C for 10 min, the furnace was allowed to cool naturally to room temperature. Subsequently, MoO3 nanosheets integrated onto ca. 2 cm2 areas of the CFP were recovered. Such nanosheets were reproducibly formed at a distance of ca. 18—20 cm from the alumina boat. The MoO3-deposited CFP was cut to dimensions of 4 cm × 1 cm thereby preserving margins on all sides. This substrate was then placed at the center of the tube furnace but downstream at a distance of 20 cm from an alumina boat containing 100 mg of elemental sulfur powder (Alfa Aesar, 99.5% purity). Next, after purging with Ar, the reactor was heated to a temperature of 400°C at a ramp rate of 20°C/min under an Ar flow of 100 sccm at 1 atm to facilitate the reaction of sublimed sulfur with the MoO3 nanosheets. After holding at 400°C for 20 min, the furnace was then naturally cooled to room temperature. Subsequently, a final CVD step was performed by replacing the spent sulfur in the alumina boat with an additional 100 mg of fresh elemental sulfur. The reactor was heated to 850°C at a ramp 8


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ACS Catalysis

rate of 20°C/min under a 100 sccm flow of Ar at 1 atm for 20 min after which the furnace was allowed to naturally cool to room temperature. The CFP paper was then removed from the center of the furnace for characterization and electrocatalytic evaluation. In order to prepare hybrid nC60/MoS2 structures on CFP, C60 powder (Strem Chemicals Inc., 99.9% purity) was dissolved in chlorobenzene at concentrations of 0.1, 0.5, and 2.0 mg/mL, respectively. The MoS2/CFP (as well as bare CFP as a control) were immersed within the chlorobenzene solutions for 1 min and then removed. Subsequently, the samples were annealed at 160°C for 10 min under a flowing Ar atmosphere. 4.2. Structural characterization. The morphology of the prepared materials was examined by field-emission scanning electron microscopy using a JEOL JSM-7500F instrument. The edge-sites of MoS2 flakes harvested from the MoS2/CFP sample by ultrasonication for 1 h in toluene were examined by high-resolution transmission electron microscopy using a JEOL JEM-2010 instrument operated at an accelerating voltage of 200 keV. Phase assignment was performed with the help of X-ray diffraction using a Bruker D8-Advance instrument equipped with a Cu Kα source (λ = 1.5418 Å) as well as by Raman microprobe analysis using a Jobin-Yvon HORIBA LabRAM HR800 instrument coupled to an Olympus BX41 microscope. Raman spectra were collected with excitation from the 514.5 nm line of an Ar-ion laser; the laser power was kept below 10 mW to minimize photooxidation. The chemical composition and oxidation states of MoO3, MoO2, and MoS2 prepared on CFP were investigated by X-ray photoelectron spectroscopy (XPS, Omicron XPS) with Mg Kα radiation (1253.6 eV). Energy calibration was achieved by setting the C1s line from adventitious hydrocarbons to 284.8 eV. The elemental composition of the C60 clusters deposited on CFP and MoS2deposited CFP was examined by energy-dispersive X-ray spectroscopy (EDS) coupled to the FE-SEM system. The coverage of C60 (0.5 mg/mL) clusters deposited on CFP was measured on a custom-made secondary ion mass spectrometer (SIMS) using C602+ projectiles with an energy of 50 keV as the source.51 4.3. Electrochemical characterization. The HER performance of the prepared materials was evaluated using a three-electrode cell with the help of a Bio-Logic potentiostat (SP-200). All of the measurements were performed in a 0.5 M aqueous solution of H2SO4 purged with N2 gas. MoS2/CFP, C60/CFP, and the hybrid structures prepared on CFP were individually used as the working electrodes. A saturated calomel electrode (SCE) and a Pt plate were used as reference and counter electrodes, respectively. The potential versus SCE (ESCE) was converted to the potential versus the reversible hydrogen electrode (RHE) (ERHE) using the relation ERHE = ESCE + 0.279 V.52 Polarization curves for HER were measured using linear sweep voltammetry (LSV) in the range between 0.1 and -0.4 V versus RHE at a scan rate of 8 mV/s. The polarization curves were corrected for the ohmic potential drop (iR) losses, where R is the series resistance of the electrochemical cell as determined by electrochemical impedance spectroscopy (EIS) measurements. EIS measurements were performed in the range between 200 kHz and 50 mHz using an AC amplitude of 25 mV. The EIS measurements for obtaining the charge-transfer resistance (Rct) values were performed at various potentials between 10 and -250 mV by sweeping the frequency from 200 kHz to 100 mHz using an AC amplitude of 10 mV. In order to estimate the electrochemically active surface area (ECSA) of the samples, the double-layer capacitance (Cdl) of the samples was determined by cyclic voltammetry (CV) in the potential range of 0.10—0.30 V versus RHE at scan rates between 20—100 mV/s. ASSOCIATED CONTENT Supporting Information FE-SEM images of samples prepared on a flat Si(100) substrate; electron micrographs showing the edge-sites of MoS2 basal planes; phonon mode assignments of Raman bands measured for MoO3; EDS 9


ACS Catalysis

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line profile of the hybrid structure; SIMS analysis; Nyquist plots, polarization and CV curves of samples with different concentrations of C60; and stability testing of electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. Fang) *E-mail: [email protected] (S. Banerjee). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge support from the Research Corporation for Science Advancement through a Scialog Award. The preparation of MoS2 nanostructures was supported in part by a New Directions Award from the American Chemical Society Petroleum Research Fund. This work was supported in part by the National Priorities Research Program award (NPRP9-160-2-088) from the Qatar National Research Fund.

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Figure 1. (a) Illustration of the stepwise process comprising vapor transport, reduction, and sulfidation used to grow MoS2 nanosheets on CFP. (b) FESEM images (i)—(iii) indicating homogeneous distribution of (i) MoO3 nanosheets, (ii) MoO2 nanodiscs, and (iii) MoS2 nanosheets grown on the textured CFP substrate. (iv)-(vi) illustrate high-magnification SEM images depicting the edge geometries and orientation. (c) (i) High-magnification SEM image of an individual MoS2 nanosheet. (ii) Low-magnification TEM image of a nanosheet depicting the locations of “clean” well-faceted and “collapsed” edges. (iii) HRTEM image of a clean edge, and (iv) HRTEM image of a discontinuous collapsed edge.

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Figure 2. (a) XRD patterns, (b) Raman spectra (514.5 nm laser excitation) of MoO3 nanosheets (blue), MoO2 nanodiscs (red), and MoS2 nanosheets (black) prepared on CFP. (c) XPS spectra indicating Mo 3d, O1s, and S 2p binding energies.

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Figure 3. FESEM images of nC60 (0.5 mg/mL) clusters deposited on (a) CFP and (b) MoS2 nanosheets prepared on CFP. EDS elemental maps of (c) C, (d) Mo, and (e) S acquired along the demarcated region of (b). (f) Raman spectra acquired at 514.5 nm laser excitation for nC60 (0.5 mg/mL) clusters and hybrid nC60 (0.5 mg/mL)/MoS2 structures on CFP.

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Figure 4. (a) Polarization curves and (b) Tafel plots measured for nC60 (0.5 mg/mL), as-prepared 3D MoS2 nanosheets, and hybrid nC60 (0.5 mg/mL)/MoS2 architectures deposited on CFP contrasted to data acquired for bare CFP. The data have been acquired in aqueous solutions of 0.5 M H2SO4 using a threeelectrode assembly. (c) Double-layer capacitances (Cdl) and electrochemically active surface areas (ECSA) plotted as a function of C60 concentration for nC60 on CFP (black), 3D nanosheets of MoS2 (blue), and hybrid nC60/MoS2 prepared on CFP (red). (d) Turnover frequencies of the neat MoS2 and hybrid C60 (0.5 mg/mL)/MoS2 prepared on CFP.

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Figure 5. (a) Nyquist plots of as-prepared 3D MoS2 nanosheets and hybrid nC60/MoS2 architectures prepared on CFP measured at –150 mV versus RHE; the inset depicts the corresponding equivalent circuit model used to extract the charge-transfer resistance. (b) The charge-transfer resistance (Rct) for nC60, as-prepared 3D MoS2 nanosheets, and hybrid nC60/MoS2 architectures prepared on CFP are plotted as a function of potential.

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Table of Contents (TOC)

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An in Situ Sulfidation Approach for the Integration of MoS2

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