Nanoscale mapping of hydrogen evolution on metallic and semiconducting MoS2 nanosheets

Tong Sunab, Hanyu Zhangc, Xiang Wangab, Jun Liuc, Chuanxiao Xiaoc, Sanjini U. Nanayakkarac, Jeffrey L. Blackburnc, Michael V. Mirkin*ab and Elisa M. Miller*c
aDepartment of Chemistry and Biochemistry, Queens College-CUNY, Flushing, NY 11367, USA
bGraduate Center of CUNY, New York, NY 10016, USA. E-mail: Michael.Mirkin@qc.cuny.edu
cMaterials and Chemical Science and Technology Directorate, National Renewable Energy Laboratory, Golden, CO 80401, USA. E-mail: Elisa.Miller@nrel.gov

Received 9th October 2018 , Accepted 27th November 2018

First published on 29th November 2018


Hydrogen evolution reaction (HER) on molybdenum disulfide (MoS2) nanosheets is enhanced for the metallic (1T) phase relative to the thermodynamically stable semiconducting (2H) phase. To measure this difference, we employ scanning electrochemical microscopy (SECM) for high-resolution mapping (<20 nm spatial resolution) of surface reactivity for mixed-phase and pure 2H-only MoS2 nanosheets. For mixed-phase MoS2 nanosheets, we find major differences in reactivity of the two phases for electron transfer involving ferrocenemethanol, allowing us to locate 1T and 2H regions and directly map the corresponding HER activity. In our measurements, we find that HER is immeasurably slow on the 2H basal plane and much faster on edges, whereas 1T portions are highly reactive across the entire portion. We also use scanning transmission electron microscopy-electron energy loss spectroscopy and scanning Kelvin probe microscopy to corroborate the phase domains and local workfunctions (surface potentials) within the MoS2 nanosheets; the mixed-phase MoS2 has a shallower workfunction compared to 2H MoS2, which could enable a greater driving force for H2 generation. This powerful combination of techniques for spatially mapping surface reactivity and correlated phase domains should be applicable to a broad range of materials for HER and other catalysis reactions.



Conceptual insights

Nanostructured MoS2 has recently been shown to be a good catalyst for H2 evolution, where the metallic phase has outperformed the thermodynamically stable semiconducting phase. We demonstrate for the first time that the two modes of scanning electrochemical microscopy (SECM) can be used to directly map the topography/phase as well as the catalytic reactivity of the same nanoscale area on the 2D catalyst surface. The use of two modes of SECM imaging enable probing specific active sites on the catalyst surface (edge, 1T/2H phase boundary) with the previously unattainable sub-20 nm spatial resolution and one-to-one correlation of electrochemical activity with specific surface features. The correlations between the topography/phase and surface reactivity for H2 generation were further elucidated by additional microscopies, where the lower energy workfunction measured on the mixed-phase nanosheets should provide a greater driving force to generate H2. This studies provides an advanced understanding of how the phase, metallic-to-semiconducting conversion, and surface energetics increases H2 evolution on MoS2 catalysts. In addition, the developed SECM concept is applicable to other nanoscale mechanistic studies of a wide range of layered materials. It can be used to explore new strategies for improving the activity and stability of 2D electrocatalysts through surface functionalization and phase engineering.

Dihydrogen (H2) is a carbon-free fuel with high volumetric energy density that can be readily transported.1 An attractive method for generating carbon-neutral H2 is via catalytic water splitting, and thus far expensive platinum-based catalysts are the most efficient for the hydrogen evolution reaction (HER).2 Alternative materials, such as transition metal dichalcogenides, have been investigated for HER, where molybdenum disulfide (MoS2) has attracted a lot of attention.3–8 In order for MoS2 to compete with Pt, it's catalytic activity for HER must be improved. Strategies towards this goal include doping, creating defect sites, exposing more edge sites by quantum confinement, straining, and phase engineering.9–19

By quantum confining MoS2 and reducing it to the 1T phase from the thermodynamically stable (2H) phase, several groups obtained promising HER results.5,9,15,16,18,20 Researchers have shown very high HER activity at 2H MoS2 edges and within 1T MoS2, prompting the creation and study of morphologies with enhanced surface area and edge sites for 2H and a large portion of the 1T phase.19–22 Since the 1T phase is thermodynamically unstable, it reverts back to the 2H phase with time, and ongoing efforts are aimed at slowing or eliminating this 1T-to-2H conversion process.13,15,20,23,24 However, as the 1T phase reverts to the 2H phase, there is still significant uncertainty surrounding the catalytic contributions of isolated sheets of pure phase (1T or 2H) and various sites (edge, interior grain boundaries) within nanosheets having mixed-phase (2H and 1T). In such a dynamically evolving nanoscale system, a number of questions remain to be addressed that require simultaneous nanoscale resolution of morphology, electronic structure, and catalytic activity. Such questions include: how and where (spatially) does the conversion from 1T to 2H occur, and how can it be controlled? How do interior sites, edge sites, and domain boundaries evolve spatially in time, both in terms of phase and catalytic activity? Understanding such factors can provide important mechanistic insights into the kinetics and thermodynamics of HER on complex nanostructured catalysts.

In this communication, we use scanning electrochemical microscopy (SECM) to spatially probe the 1T and 2H domains within a nanosheet and how these domains lead to differences in HER activity and heterogeneous electron transfer rate throughout the nanosheet. SECM,25,26 scanning ion conductance microscopy,27 scanning electrochemical cell microscopy (SECCM),28–31 and scanning tunneling microscopy32 have previously been used to obtain nanoscale maps of catalytic activity; however, these types of studies on 2D MoS2 have been limited. Li et al. demonstrated with SECM that strained vacancies within 2D MoS2 led to higher HER activity.33 The HER activity has not been measured with high-spatial resolution across mixed-phase MoS2 nanosheets, directly showing the changes in activity at the edges, interior sites, and 2H/1T boundary. In SECM, the reactivity of electrocatalysts is characterized by scanning a small tip electrode above the catalyst's surface to measure local reactant and product fluxes and determine the local rates of specific heterogeneous reactions.25,26,34–39 Here we use well-characterized, polished nanodisk tips35,37,39 to map catalytic HER activity of mixed-phase MoS2 nanosheets with <20 nm lateral resolution. We pair the SECM probe with scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and scanning Kelvin probe microscopy (SKPM) to further characterize the 1T and 2H phase domains. Our results on mixed-phase MoS2 nanosheets indicate that the 1T phase tends to be surrounded by the 2H phase. Also, the edges of 2H MoS2 are active for HER while the interior sites have little to no activity; the HER activity is most prevalent within the 1T MoS2 and is essentially constant from edge to center.

We employ the feedback mode of SECM (Fig. 1A), with ferrocenemethanol (Fc) mediator to locate the 1T and 2H regions on the surface of MoS2 nanosheets and then map the corresponding reactivity toward HER using substrate generation/tip collection (SG/TC) mode (Fig. 1B; see ESI for detailed SECM discussion). Three experimental current vs. distance (iT vs. d) curves obtained over the indium doped tin oxide (ITO) surface and over 2H and mixed-phase MoS2 nanosheets are shown in Fig. S1A (ESI). The iTd curves obtained with the Fc mediator show major differences in electron-transfer reactivities, which result in the negative feedback at 2H MoS2 (curve 1), low positive feedback over ITO (curve 2), and significantly higher positive feedback at the 1T MoS2 surface (curve 3). These differences, which stem from high surface conductivity of metallic 1T MoS2 and low conductivity of the semiconductive 2H phase, are used to identify the phases in MoS2 nanosheets. A 2D color map of two 2H MoS2 nanosheets (Fig. S1B, ESI) on ITO shows uniformly negative feedback current over 2H MoS2 and small positive feedback current over ITO.


image file: c8nh00346g-f1.tif
Fig. 1 Schematic illustrating mechanism of SECM image contrast of MoS2 nanosheets. Schematic representation of (A) positive feedback produced by oxidation/reduction of Fc and (B) probing HER at the MoS2 surface in the SG/TC mode. Not to scale.

The synthesis of 2H MoS2 nanosheets is detailed in the SI. Briefly, we deposit the solution-exfoliated (predominantly 1T) nanosheets onto ITO substrates and thermally anneal the nanosheets in N2 to fully convert them to 2H. A micrometer-scale SG/TC mode image (Fig. 2A) shows significant HER activity for the edge of a 2H flake. The feedback mode image obtained with the Fc mediator and a smaller tip (a = 18 nm) shows higher resolution on the edge of a flake (Fig. 2B). The hydrogen evolution activity of the same area is mapped with the same tip in SG/TC mode (Fig. 2C), and the high-resolution map clearly demonstrates that the HER activity is confined to the flake edge, whose reactive width appears to be <50 nm. The true width of the reactive edge may be significantly narrower than that in Fig. 2C due to the diffusional broadening effect, which is observed for any redox species generated at the substrate and is characteristic of this technique.25,26


image file: c8nh00346g-f2.tif
Fig. 2 Imaging topography and HER activity of a pure 2H MoS2 nanosheet on the ITO surface with SECM. (A) Submicrometer-scale SG/TC image of HER over a 2H MoS2 flake. (B) The high-resolution feedback mode image of the boundary between 2H MoS2 and ITO as well as the corresponding (C) SG/TC mode (HER activity) images of a flake edge. See ESI section for experimental conditions.

Fig. 2A and C demonstrate that the HER rate of the basal 2H surface is immeasurably slow (i.e., essentially indiscernible from that of the catalytically inert ITO surface). Fig. S2 (ESI) shows a high-resolution feedback mode and SG/TC mode image of another 2H MoS2 nanosheet, which has similar HER results. Our HER results qualitatively agree with the conclusions of Zhang et al. and Bentley et al.,22,28 with both studies finding higher reactivity for 2H MoS2 edges relative to basal sites, although both of these prior studies have lower spatial resolution than realized in our study. Specifically, the experimental setup in Zhang et al. was unable to isolate edge and interior site reactivity, and the resolution in Bentley et al. was sub-micron, which would prohibit this SECCM experimental configuration from directly measuring the edges and basal sites of our MoS2 nanosheets. Importantly, the <20 nm resolution achieved here via SECM enables us to conclusively and directly distinguish between edge and interior HER activity without relying upon on indirect measurements for MoS2 nanosheets.

The mixed-phase MoS2 nanosheets are prepared and deposited onto ITO (additional details are provided in the ESI). XPS studies (Fig. S3, ESI) taken at NREL one week after synthesis confirm that the nanosheets are mixed-phase, with both 2H (23%) and 1T (77%) phases co-existing within the population of nanosheets.20,40 Additional characterization of the mixed-phase MoS2 nanosheets can be found in our recent publication.20 SECM measurements show that the MoS2 flakes (aged one to four weeks after delivery to CUNY-Queens) contain both the 2H and 1T phases within one nanosheet, which is consistent with a recent report by Girish.41 The activity maps of the mixed-phase MoS2 nanosheets obtained with the Fc redox mediator (Fig. 3A–C) point to the presence of the 1T phase. In a relatively low-resolution image of the mixed-phase nanosheet (Fig. 3A), the 2H portion gives negative feedback (iT ≈ 13 pA) while the 1T portion produces higher positive current feedback (iT ≈ 23 pA) compared to the ITO substrate (iT ≈ 18 pA). The 2D color map of the same area shows the 1T phase sandwiched by the 2H phase (Fig. 3B). The conversion is not symmetric and appears to occur along straight lines. This conversion is likely caused by a sliding of the S plane and has a “ripple effect” along chemical bonds.42 The zoomed-in map (Fig. 3C) emphasizes the abruptness of the 1T-to-2H conversion boundary. In agreement with the approach curves in Fig. S1A (ESI), unlike the 2H portion, positive feedback is measured across the entire 1T portion and not just the boundary. Note that the shipping and additional aging of the mixed-phase nanosheets for the SECM measurements may have increased the 2H[thin space (1/6-em)]:[thin space (1/6-em)]1T ratio compared to the ratio determined from XPS.


image file: c8nh00346g-f3.tif
Fig. 3 Imaging redox and HER activity of mixed-phase MoS2 nanosheets. (A) Feedback mode image of a flake on ITO obtained with Fc redox mediator and (B) corresponding 2D color map. (C) Zoom-in area showing a more detailed picture of boundaries between ITO, 2H MoS2, and 1T MoS2 based on feedback current of Fc and (D) HER SG/TC line profiles across the same area of the substrate as in (C). See ESI section for experimental conditions.

SG/TC mode is used to extract the line profiles (Fig. 3D) to quantify the HER activity over the area of the mixed-phase MoS2 nanoflake that is also imaged in the feedback mode (Fig. 3C). By comparing these two figures, one can see that the 1T phase is very active for HER and that the 2H MoS2 edges also exhibit significant activity toward HER. The tip current over the 2H interior is low but measurable, unlike the SG/TC images of the single-phase 2H MoS2 flake (Fig. 2C and Fig. S2B, ESI). This difference can likely be attributed to the large amount of H2 generated at the 1T MoS2 surface where diffusion can produce a broad concentration profile extending to the imaged 2H portion of the flake. This detailed information cannot be extracted from the lower resolution SG/TC image (Fig. S4, ESI), which appears to suggest that the entire nanosheet is HER active, and the signal is dominated by the flux of hydrogen from the 1T phase.

Our SECM measurements suggest that the observed spatial differences in SECM current densities arise from spatial variation of 1T/2H phases within a nanosheet, but do not serve as a direct measurement of the spatial variation in atomic bonding environment. To address this issue, we turn to STEM and EELS measurements to confirm the phases within MoS2 nanosheets. Taking advantage of the high-spatial resolution of STEM and high-energy resolution of EELS, we identify the electron energy difference of sulfur at a MoS2 nanosheet edge and center. Fig. 4A shows the STEM-bright field (STEM-BF) image for an individual MoS2 flake that has been aged for three weeks to foster the partial transition from 1T to 2H. The contrast varies across the nanosheet, indicating possible defects and thickness differences across the flake.43,44 The normalized EELS spectra of the S–L2,3 edge at two spots (indicated in Fig. 4A) are displayed in Fig. 4B, where the lower energy feature (∼161–164 eV, S 2p3/2 + S 2p1/2 states → S 3p) is the pre-edge and the higher energy feature (∼170–176 eV, S 2p3/2 + S 2p1/2 states → S 4d) is the primary edge.45 The S–L2,3 transition is ∼2 eV higher energy at the nanosheet edge than in the center. These results are consistent with the edge being the 2H phase and the interior being the 1T phase, an interpretation supported by XPS data (Fig. S3, ESI), which show the S 2p binding energy shifts to higher energy upon converting from 1T to 2H.


image file: c8nh00346g-f4.tif
Fig. 4 Nanoscale characterization of MoS2 nanosheets. (A) STEM-BF image of an individual mixed-phase MoS2 nanosheet. (B) EELS spectra of the mixed-phase MoS2 nanosheet at the edge (blue dot in (A)) and center (red dot in (A)). Intensities are normalized to the average between 170–180 eV. (C) SKPM measurements were taken before and after conversion to 2H-only phase, where the MoS2 nanosheets were deposited on a highly-conducting silicon substrate. Figure shows apparent height profiles (top) and corresponding surface potential profiles (bottom) of representative mixed-phase and 2H-only MoS2 nanosheets (additional SKPM images in Fig. S6, ESI).

Further support for this phase assignment stems from different electron exposure times for the EELS measurements. The center of the MoS2 EELS data does not change as a function of electron exposure time, whereas the MoS2 edge EELS data shifts to lower energy following longer electron exposure times (Fig. S5, ESI). Several studies have demonstrated that the electron beam used in STEM measurements can supply enough electrons to induce the 2H to 1T phase transition.46–48 Thus, the shift to lower energy at the MoS2 edge observed in Fig. S5 (ESI) is consistent with conversion from the 2H to 1T phase due to excess negative charge supplied by the electron beam. The EELS results on mixed-phase MoS2 nanosheets are thus consistent with our SECM data, suggesting that the 1T phase is surrounded by the 2H phase.

Finally, we aim to corroborate our SECM measurements by evaluating the local phase changes across MoS2 nanosheets using scanning Kelvin probe microscopy (SKPM), which measures local surface potential variations. We expect to measure a surface potential difference between the two phases, which is directly related to the local workfunction and, therefore, its reactivity towards HER. Details of the SKPM methodology and analysis are included in the ESI. We examine the SKPM of mixed-phase MoS2 (aged two weeks and stored at room temperature or under refrigeration to prevent conversion to 2H) and thermally annealed MoS2 (2H-only) on a highly-conducting Si substrate – imaging both surface morphology (apparent height) and the surface potential simultaneously.49–51 The MoS2 nanosheets are 1–3 nm tall and 200–400 nm wide (Fig. 4C (top) and Fig. S6, ESI). For each apparent height profile over mixed-phase and 2H-only MoS2, the corresponding surface potential profiles are shown (Fig. 4C (bottom) and Fig. S6, ESI).

For the mixed-phase MoS2, we measure ∼+250 (±50) mV surface potential difference between the probe tip and the mixed-phase MoS2; for the 2H-only MoS2, we measure ∼+50 (±50) mV. This change in surface potential difference corresponds to the mixed phase MoS2 possessing a shallower workfunction (i.e., the Fermi level is closer to the vacuum level) compared to 2H-only MoS2. Since HER activity is related to the chemical potential of electrons in the MoS2 nanosheets, the measured differences in local workfunction observed for mixed-phase and 2H-only nanosheets is consistent with the improved HER activity observed in SECM for the mixed-phase MoS2. In our SKPM images of the mixed-phase MoS2, we do not observe spatially-varying surface potential changes across individual nanosheets, possibly due to a combination of the smaller amount of the 2H phase combined with the averaging effect from the tip size.

Conclusions

In conclusion, we use high-resolution SECM to map the HER activity of solution-exfoliated MoS2 nanosheets on ITO. Fully converted 2H MoS2 nanosheets have edges that are active for HER, while the interior is essentially inactive for HER. High spatial resolution (<20 nm) is essential for correctly discerning between the edge and basal areas in 2H MoS2. Furthermore, we show that (before annealing or time-dependent conversion fully back to 2H) solution-exfoliated MoS2 nanosheet populations are not composed of nanosheets that are fully in either the 1T or 2H phase but instead are mixed-phase within the nanosheets. In this SECM measurement on mixed phase MoS2 nanosheet, conversion appears to proceed from the outside of a nanosheet inwards and along straight lines, which is likely due to the sliding of the S plane. By combining high-resolution SECM data with XPS, STEM-EELS, and SKPM, we gain additional insight about the mixed-phase MoS2, such as the mixed phase has a shallower local workfunction compared to the 2H MoS2, which could change the thermodynamic driving force for H2 generation. This study highlights the importance of fully converting the 2H MoS2 phase into the 1T phase and stabilizing this phase for increased HER activity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The support of the SECM work by the National Science Foundation (CHE-1416116; MVM) is gratefully acknowledged. The MoS2 exfoliation, XPS, SKPM, and STEM-EELS work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program. The views expressed in the article do not necessarily represent the views of the Department of Energy or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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Footnotes

Electronic supplementary information (ESI) available: Experimental section for sample preparation and characterization. There are additional figures for the feedback mode SECM responses, 2D SECM color maps of the topography and reactivity of 2H MoS2 nanosheets, XPS of the mixed-phase and 2H MoS2 nanosheets, low-resolution SG/TC map of HER on mixed-phase MoS2 nanosheets, STEM-EELS spectra of mixed MoS2 nanosheets, and SKPM of mixed-phase and 2H MoS2 nanosheets. See DOI: 10.1039/c8nh00346g
Authors contributed equally to the work.

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