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Revealing the Heterogeneity of Large‐Area MoS2 Layers in the Electrocatalytic Hydrogen Evolution Reaction

Abstract The electrocatalytic activity concerning the hydrogen evolution reaction (HER) of micrometer‐sized MoS2 layers transferred on a glassy carbon surface was evaluated by scanning electrochemical cell microscopy (SECCM) in a high‐throughput approach. Multiple areas on single or multiple MoS2 layers were assessed using a hopping mode nanocapillary positioning with a hopping distance of 500 nm and a nanopipette size of around 55 nm. The locally recorded linear sweep voltammograms revealed a high lateral heterogeneity over the MoS2 sheet regarding their HER activity, with currents between −40 and −60 pA recorded at −0.89 V vs. reversible hygrogen electrode over about 4400 different measured areas on the MoS2 sheet. Stacked MoS2 layers did not show different electrocatalytic activity than the single MoS2 sheet, suggesting that the interlayer resistance influences the electrocatalytic activity less than the resistances induced by possible polymer residues or water layers formed between the transferred MoS2 sheet and the glassy carbon electrode.


Introduction
Two dimensional (2D) materials emerged in the last decades due to their unique properties, which open the way to new applications. Among those, transition metal dichalcogenides (TMDCs) were extensively studied for electrochemical applications, where they were used as electrocatalyst materials or to fabricate electrodes for different analyte detection. [1] Inspired by nature, where hydrogenase enzymes contain Mo, Fe and Ni, nanosized MoS 2 was predicted by theory to be an efficient hydrogen evolution reaction (HER) electrocatalyst almost 20 years ago. [2] Since then, many studies have confirmed the potential of MoS 2 as a highly active HER electrocatalyst. [3][4][5][6][7] Single MoS 2 layers show unique properties. They can be synthesised either in a bottom-up, e. g., chemical vapour deposition (CVD), [8][9][10] or top-down, e. g., exfoliation, approach. [11,12] While the latter generally offers high-quality 2D materials, it fails to produce large-scale single layers independ-ently of the substrate and hence lacks scalability. [10] Often, the transfer of MoS 2 layers on different supports is required, and recent studies indicate that this may also influence the properties of the MoS 2 . [13] Apart from the influence of the substrate itself, it was recently shown that intercalated water layers between the MoS 2 layers and the substrate substantially alter the MoS 2 ' electronic properties. [14,15] MoS 2 is seen as an abundant and lower-cost alternative for Pt as HER electrocatalyst. [1,16] Improving the electrocatalytic activity of MoS 2 was attempted by different methods, such as doping with other elements, [17,18] by introducing defects, [4] or by applying strain. [19] Multiple studies [4,[20][21][22][23] investigated the electrocatalytic activity of MoS 2 obtained via different synthesis routes at the macro-and nanoscale, leading to a debate about the location of active sites on MoS 2 . [24] While classical measurements performed at the macroscale can confirm an overall change in the electrocatalytic activity, they do not provide information about the active areas on the catalyst layer.
Scanning electrochemical cell microscopy (SECCM) demonstrated its power over the last decade in evaluating electrocatalyst materials at the nanoscale. [25,26] In SECCM, a nanopipette filled with electrolyte is approached to the surface to be investigated. Upon contact, the hanging nano-droplet forms an electrochemical cell that allows the exclusive electrochemical evaluation of the wetted area. By automatically retracting and repositioning the nanopipette at different electrode areas, a high-throughput evaluation of the sample surface is achieved in a relatively short time. [25] The technique is ideal for studying the lateral heterogeneity at the micro-and nanoscale and was used to evaluate electron-transfer rates or electrocatalytic activity of a broad range of materials such as Pt, [27] carbon nanotubes, [28] graphene, [29][30][31] MoS 2 , [21,32,33] pentlandite crystals, [34] Co(OH) 2 crystals, [35] high-enthropy alloys [36] etc.
Initial studies at the nanoscale, reported by Bentley et al., [32] showed that the bulk MoS 2 basal plane possesses a high activity towards HER, which significantly increases over the edge plane. The study of Takahashi et al. confirmed the existence of active sites on the basal space of small triangular MoS 2 layers having 130 nm in length. [21] They showed that on freshly prepared MoS 2, an increased activity could be observed for edges compared with the basal plane, a difference no longer visible when MoS 2 was exposed to air for several months. [21] Cabre et al. evaluated the electron transfer rate for TMDCs exfoliated from bulk crystals on Au substrates. [33] Plechinger et al. showed that CVD-grown MoS 2 could have a similar quality as exfoliated MoS 2 while having the advantage of larger and coherent single layers. [37] These large single layers enable low-resolution and non-localised measurement methods, such as X-ray photoelectron spectroscopy (XPS), and open the door for the investigation of various structural modification methods, e. g., ion-or electron irradiation and etching. In catalytic and electrochemical applications, large reproducible single layers with a high surface coverage are desirable to maximise the surface-tovolume ratio. [38] In the present work, we evaluated the surface of large areas of single-sheet MoS 2 grown by CVD, which were transferred on a glassy carbon (GC) electrode using the classical polymerassisted method. By probing 4941 different spots over an area of 30 μm × 40 μm (~4400 on the MoS 2 sheet), a variation of electrocatalytic HER activities was observed, and possible reasons for the increased lateral heterogeneity are discussed.

Results and Discussion
The single-layer MoS 2 sheet grown on a SiO 2 /Si substrate, as previously reported, [4] was transferred onto a GC electrode to enable mapping of the electrochemical activity of MoS 2 by SECCM. The GC electrode was used to establish the electrical connection (in the so-called bottom-contacted mode) and was further used as a working electrode (WE) in SECCM experiments. The mode of contacting the MoS 2 layers was shown to impact the measured electrocatalytic activity of MoS 2 by SECCM, since different pathways for the electrons transport are established. In top contact, the lateral electron transport along the layer occurs, while in bottom contact, the electron transfer occurs from the substrate to the upper MoS 2 sheet, where the reaction takes place. For example, in the case of bulk MoS 2, measuring with a top contact proved to be beneficial as compared to measuring with a bottom contact, as it prevented the recording linear sweep voltammograms (LSVs) with artefacts. [32] On the other hand, for single to few-layer MoS 2 , the bottom contact was successfully used to perform SECCM. [33] For the large singlesheet MoS 2 used in this study, the recorded LSVs during the SECCM measurement show no artefacts ( Figure S1), indicating that electrical conductivity between the MoS 2 sheet and the GC current collector is not limited.
SECCM measurements were performed with a capillary size of 55 nm in a hopping mode with a hopping step of 500 nm ( Figure 1).
Based on the recorded LSVs, current maps are derived by plotting the currents recorded at defined potentials. At lower overpotential [À 0.6 V vs. reversible hydrogen electrode (RHE), Figure S2], slightly higher currents are recorded on the GC compared with the MoS 2 sheet.
At overpotentials lower than À 0.6 V vs. RHE, we see a sudden increase in the currents recorded on the MoS 2 sheet compared to the GC (Figure 2, S2). Overall, over the MoS 2 sheet, we see a high variation of currents at À 0.89 V vs. RHE, ranging from À 20 to À 60 pA ( Figure S1), corresponding to current densities ranging from À 0.45 to À 1.36 A/cm 2 ( Figure S3), in agreement with the results of Bentley et al. [32] Exemplary LSVs from different locations are illustrated in Figure 2c. We compared the currents recorded on the GC at the beginning of the scan with those at the end of the scan to confirm the technical robustness of our experiment (white and black areas marked in Figure 2b. The histogram (Figure 2d) shows that on GC currents ranging between À 7 and À 24 pA are recorded, with an average of À 15 � 3 pA. Thus, we can confirm that no technical error occurred during the SECCM scan performed over 4941 landing sites, and the high current variation is caused by the MoS 2 structure only. Over the scan, lateral streaking can be observed over the scan direction, indicating changes in the droplet size. Such changes can be caused by differences in the surface's hydrophobicity/hydrophilicity due to residual polymers/water layers remaining on the MoS 2 , after its transfer.
For example, in the region marked with pink in Figure 2b, the recorded currents at À 0.89 V vs. RHE range between À 20 to À 40 pA, while in the red marked area higher currents between À 40 to À 60 pA are recorded. Locally, in an area of a few 1-2 μm 2 , the MoS 2 sheet shows a relatively homogeneous catalytic activity, but significant differences are observed over extended ranges, as seen in Figure 2. SEM analysis of the transferred MoS 2 layers revealed the presence of regions where additional triangular-shaped MoS 2 layers with~1 μm length are present on the MoS 2 single layers (Figure 2e and Figure 3a).
During the SECCM scan, several landings occurred on top of the multi-layer MoS 2 regions leading to LSVs recorded on a single or multiple MoS 2 layers. The footprint of the SECCM landing spot on the multilayered MoS 2 is marked by a red circle in the SEM image and the corresponding current with a red square in the resulting SECCM color map in Figure 3b. We do not observe significant differences between the single-layer or the multi-layer MoS 2 activities by comparing the recorded currents at the same potential. This indicates that the recorded , where no difference was noticeable either in the HER electrocatalytic activity or the Tafel slope for single, two, or three layers of MoS 2 . [21] Based on the analysis of the areas containing single or multilayered MoS 2, we can conclude that the heterogeneity as observed in Figure 2 is not due to the presence of MoS 2 multilayers. Polymer residues may still be present on top of the sample, equally affecting single and multi-layers. Another possible factor is the existence of a water layer/gas bubbles between the transferred MoS 2 and the GC surface, as we already reported previously. [15] These intercalated water layers are commonly found in 2D samples exposed to ambient conditions. They occur in exfoliated samples as well, are difficult to remove and are known to affect the electronic properties. [39][40][41] Tafel analysis can provide information regarding the reaction mechanism and was extensively used to identify the rate-limiting step in electrocatalytic driven processes. [42] The Tafel slopes were automatically derived in the log(i)-range 2.5 to 3.5 and plotted similar to the activity maps (Figure 4a). To ensure a good quality of the data fitting used to derive the Tafel plots, we checked the value of the coefficient of determination (R 2 ) automatically calculated for each measured plot (Figure 4b). R 2 values ranging from 0.9-1 were obtained for most of the points (~95 %). The points with values outside this range were removed (white points in Figures 4a, 4b).
The distribution of calculated Tafel slopes over the SECCM scan is presented as a histogram in Figure S4, where Tafel slopes with values ranging between 80-120 mV/dec for the majority of measured points. These values indicate the Volmer step or, according to predicted models, the Heyrovsky step at high coverages rates as the rate-determining step. [42] For a limited number of points (mainly located on the GC surface), we obtained higher values of the Tafel slopes than those theoretically expected. Overall, the recorded Tafel plots support the HER activity of the basal planes. They agree with several other reports, which show that the basal planes of pristine MoS 2 are present on the MoS 2 single layers (Figure 2e and Figure 3a) and are not highly elctrocatalytically active but similar to the ones of Au and Cu.. [32]

Conclusion
The lateral heterogeneity of large single layers of MoS 2 grown by CVD and subsequently transferred to a GC was evaluated by SECCM in a hopping mode using a nm-sized nanopipette. The activity maps revealed a high lateral heterogeneity which becomes significant over longer ranges. Since no difference in activity was recorded for single and multi-layer MoS 2 , we attribute the observed lateral heterogeneity to the presence of polymer residues or the presence of a water layer/gas bubbles between the MoS 2 sheet and the GC formed during the transfer process. The Tafel slopes and the recorded current densities indicate a moderate HER activity of the large-area MoS 2 transferred on the GC, similar to that of the exfoliated MoS 2 layers reported in literature.

Synthesis of Large-Scale MoS 2 and Transfer on the GC
Large-scale single-layer MoS 2 was synthesised in a custom CVD process. The precursor, a droplet of ammonium heptamolybdate (AHM) solution, was placed on a piece of silicon with a 300 nm thick oxide layer. Subsequently, the sample was dried at 300°C for 24 min, and a seeding promotor (1 % sodium cholate acid solution dissolved in H 2 O) was applied using a spin-coater. Each precursor, sulfur being the second, is placed into a separate crucible and is positioned in a furnace with different heating zones. The sulfur crucible is placed into the first heating zone, while the crucible with the silicon piece is placed downstream into the second heating zone. An optimised heating profile makes sure that each precursor enters the gas phase simultaneously. During the process, argon flows through the quartz tube to suppress chemical reactions with air, e. g., oxidation. After 30 min, the samples can be retrieved. To transfer the grown MoS 2 flakes onto the glassy carbon substrate, poly(methyl methacrylate) (PMMA) was spin-coated onto the samples. Then, the sample was placed into a 0.1 M KOH bath, slowly etching the SiO 2 and separating the PMMA/MoS 2 layer from the silicon piece. After that, the resulting layer of PMMA/MoS 2 was transferred into a water bath to clean it, and it could then be scooped off with the glassy carbon substrate. The glassy carbon was purchased (HTW Hochtemperatur-Werkstoffe GmbH) in a polished condition. The roughness of the glassy carbon surface was measured in an atomic force microscope (AFM) to 2.5 � 0.5 nm. In the last step, acetone was applied to the PMMA/MoS 2 layer to dissolve the remaining PMMA.

Scanning Electrochemical Cell Microscopy
Single barrel pipettes with nanometer-sized tip openings were fabricated by laser pulling single barrel quartz glass capillaries using a CO 2 where E Ag/AgCl is the applied potential to the working electrode versus the QRCE, 0.210 V is the standard potential of the Ag j AgCl j 3 M KCl reference electrode, and E OCP is the correction factor of the open circuit potential measured against the commercial reference electrode (31 mV). 0.059 is the result of (RT) · (nF) À 1 , with R the universal gas constant, T the temperature (25°C = 298°K), F the Faraday constant and n equals 1.  Electrochemical measurements were performed in a home-built SECCM workstation. [43] A sample holder supporting the GC/MoS 2 sample is mounted on a 3-axis step motor microscrew system (Owis) and controlled by an L-Step PCIe controller card (Lang). This enables a coarse positioning and first approach of the pipette tip to approximately 50 μm above the surface.
The filled single barrel capillary with the QRCE is mounted at a 3axis piezo nanopositioner (P-611.3S nanocube, Physik Instrumente). For fine positioning of the pipette tip above the surface, an analog amplifier (E-664, Physik Instrumente) is used.
In the electrochemical experiments, the MoS 2 sample transferred on the GC is used as WE and the Ag/AgCl-wire as QRCE in a 2electrode system. The electrochemical cell is established after approaching the MoS 2 /GC sample towards the nanopipette having a hanging drop. Once the MoS 2 /GC makes contact with the hanging drop, an electrochemical cell is formed. The surface contact is detected by measuring the current between the WE and the QRCE, stopping the approach immediately after a current threshold of À 2 pA is recorded using a low noise current amplifier (DLPCA-200, FEMTO). The size of the formed electrochemical cell depends on the capillary opening which is estimated based on the size of the nanopipette opening.
Using a hopping-mode measurement protocol a spatial resolution was possible by measuring 61 × 81 individual spots with a distance of 500 nm, resulting in a map of 30 × 40 μm.

Scanning Electron Microscopy
For the characterisation of the nanopipette tip (size measurement of the opening) and the analysis of the landing spots during the SECCM scan, a Quanta 3D ESEM (FEI) microscope was used.