Electrocatalysis on Edge-Rich Spiral WS2 for Hydrogen Evolution Prasad V. Sarma, Arijit Kayal, Chithra H. Sharma, Madhu Thalakulam, J. Mitra, and M. M. Shaijumon* School of Physics, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Thiruvananthapuram, Kerala 695551, India
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S Supporting Information *
ABSTRACT: Transition metal dichalcogenides (TMDs) exhibit promising catalytic properties for hydrogen generation, and several approaches including defect engineering have been shown to increase the active catalytic sites. Despite preliminary understandings in defect engineering, insights on the role of various types of defects in TMDs for hydrogen evolution catalysis are limited. Screw dislocation-driven (SDD) growth is a line defect and yields fascinating spiral and pyramidal morphologies for TMDs with a large number of edge sites, resulting in very interesting electronic and catalytic properties. The role of dislocation lines and edge sites of these spiral structures on their hydrogen evolution catalytic properties is unexplored. Here we show that the large number of active edge sites connected together by dislocation lines in the vertical direction for a spiral WS2 domain results in exceptional catalytic properties toward hydrogen evolution reaction. A micro-electrochemical cell fabricated by photo- and electron beam-lithography processes is used to study the electrocatalytic activity of a single spiral WS2 domain, controllably grown by chemical vapor deposition. Conductive atomic force microscopy studies show improved vertical conduction for the spiral domain, which is compared with monolayer and mechanically exfoliated thick WS2 flakes. The obtained results are interesting and shed light on the role of SDD line defects, which contribute to large number of edge sites without compromising the vertical electrical conduction, on the electrocatalytic properties of TMDs for hydrogen evolution. KEYWORDS: transition metal dichalcogenide, tungsten disulfide spirals, micro-electrocatalytic cell, conductive AFM, hydrogen evolution reaction
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asymmetric nanostructural growth, creating vertical conduction channels. The inclusion of axial screw dislocation in TMDs is an interesting concept where the bottom layer is spiraled up to form an atomically layered continuous spiral pyramid with abundant edge sites. The SDD growth yields fascinating morphologies, and subsequent strain field introduces exciting phenomena in TMDs.12,13 In bulk or few-layer van der Waals structures, the interlayer electronic coupling is weak, and hence the electronic conduction in the vertical direction is through electron tunneling. Due to this interlayer hopping of electrons, vertical conduction is compromised in few-layer domains.14 In spiral structures, the defect core is connected to all the layers through screw dislocations, and in that case, electronic conductivity is not limited by electron hopping. Anisotropic screw dislocation lines can carry electric current in a helical path irrespective of layer thickness, similar to that of an inductor, and hence, these spirals can be studied as a nanosolenoid.14,15 The continuous dislocation lines connect
here has been great progress in the use of renewable energy resources, and hydrogen is becoming a key player in the transformation of the global energy system. Large-scale production of hydrogen through water splitting requires the development of highly efficient and inexpensive catalysts, which still remains a challenge. Transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, and WSe2, with their thermoneutral adsorption energy, exhibit promising electrocatalytic properties toward hydrogen generation.1−3 Several strategies such as defect engineering, doping, increasing the edge concentration, etc., are shown to further enhance active catalytic centers in the TMD lattice.2,4−7 Despite preliminary understandings in defect engineering, insights on the role of various types of defects in electrocatalysis are limited. The defective structures are expected to show fundamentally different properties and might induce an additional doping effect that leads to the formation of catalytically active centers.8,9 The dearth of active catalytic sites and low interplanar conductivity are the key factors limiting the catalytic activity of bulk TMDs.10,11 Screw dislocation-driven (SDD) growth is a line defect, which unzips the lattice in a vertical direction and induces © XXXX American Chemical Society
Received: May 31, 2019 Accepted: August 20, 2019
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DOI: 10.1021/acsnano.9b04250 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. (A, B) AFM images of spiral WS2 domains and (C) AFM image along with the corresponding height profile of the monolayer WS2 domain, grown via CVD under controlled experimental conditions. (D) Schematic representation of spiral domain growth modes. The terms δ and α represent single-layer thickness of a WS2 sheet and interlayer spacing between WS2 layers, respectively. (E) Schematic of the atomic arrangement showing the distinct stacking sequence in spiral WS2. The tungsten atom in the top layer (red hexagon) is positioned at the center of the bottom layer hexagon (blue).
via the chemical vapor deposition route under controlled experimental conditions. Atomic force microscopy (AFM) images shown in Figure 1A and B clearly illustrate the pyramidal morphology of spiral domains with step edges and dislocation center in comparison to the smooth terrace surface of their monolayer counterpart (Figure 1C). The spiral growth results from the accidental uplifting of the grain boundary during the initial growth process, leading to the formation of vertically mismatched edges with a Burgers vector equivalent to the thickness of a monolayer.20,21 The overall growth mechanism for the pyramid-shaped WS2 flakes seems to be complex and is not very clear. Nevertheless, several such pyramidal growths reported in 1D and 2D nanomaterials are explained based on classical crystal growth theory, where the supersaturation of the system drives different growth modes such as screw-dislocation-driven, layer-by-layer (LBL), and dendritic growth.22 If the deposition is carried out on a foreign substrate, the nucleation will never be homogeneous. There will always be some defect sites with higher chemical potential such as the emerging point of screw dislocations and embedded foreign atoms, which are more active for crystal growth compared with the remaining part of the substrate.23 Here, the spirally stacked WS2 pyramidal structures are believed to follow the SDD growth model, which is the most favorable mode of growth at low supersaturation.24 The anisotropic pyramidal growth is continued in this fashion by self-perpetuating steps of SDD layers. Screw dislocations developed under moderate supersaturation conditions create step edges (slipped planes) in the bottom layer. The accidental uplifting of a grain boundary triggered by different growth rates of various edge terminations, which is due to the high
the entire edge sites together, which can drastically enhance the active sites for catalysis. Although there have been few attempts to study the single-domain electrocatalysis on monolayer TMD domains toward hydrogen evolution reaction (HER),16−19 there have not been any studies reported on the electrocatalytic properties of defect-rich spiral TMD domains. The role of dislocation lines and edge sites of these spiral structures on electrocatalysis toward hydrogen generation is an unexplored topic, and hence it will be interesting to explore single-domain electrocatalysis on these peculiar structures with abundant edge sites spirally wound in the vertical direction. In this work, we show that the large number of active edge sites connected together by dislocation lines in the vertical direction for a spiral WS2 domain results in exceptional catalytic properties toward hydrogen evolution reaction. A microelectrochemical cell fabricated via the photo- and e-beam lithography process is used to study the electrocatalytic activity of a single spiral WS2 domain, controllably grown by chemical vapor deposition (CVD). Conductive atomic force microscopy (c-AFM) studies show improved vertical conduction for the self-perpetuating spiral domain, which is compared with monolayer and mechanically exfoliated thick WS2 flakes. The obtained results are interesting and shed light on the importance of SDD line defects, which contribute to large number of edge sites without compromising the vertical electrical conduction, on the electrocatalytic properties of spiral TMDs for hydrogen evolution.
RESULTS AND DISCUSSION Synthesis and Characterization of Spiral WS 2 Domains. WS2 spiral and monolayer domains are synthesized B
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Figure 2. (A) HRTEM image obtained from the top of the WS2 spiral domain (marked area in B). (B) Bright field TEM (BF-TEM) image of a WS2 spiral domain. Inset shows corresponding SAED pattern, which reveals single-crystal characteristics. (C) HRTEM image obtained from the monolayer region of the spiral domain, as marked in B. (D) Bright field TEM (BF-TEM) image from the spiral WS2 domain. Inset shows corresponding SAED pattern. (E, F) Dark field images obtained from various reciprocal lattice points as marked on the SAED pattern.
Figure 3. (A) Raman spectra and (B) photoluminescence spectra collected from monolayer, spiral, and mechanically exfoliated WS2 domains. PL map obtained using 532 nm excitation laser from (C) monolayer, (D) spiral, and (E) thick mechanically exfoliated WS2 domains, recorded at the wavelengths as mentioned in the respective images. Insets of C−E are optical images of corresponding WS2 flakes.
concentration of precursors, initiates spiral growth, and this process is schematically represented as step 1 in Figure 1D. The uneven surface of the substrate, which is caused by the partial etching of SiO2/Si with piranha and plasma treatment, also catalyzes the spiral growth. The unsaturated sulfur edges
in the slipped plane act as the nucleation sites for further addition of precursor atoms, leading to the growth of the second layer on top of the bottom layer, which is shown schematically in Figure 1D (steps 2, 3). Once the slip plane is formed with vertically mismatched edges, the growth process C
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Figure 4. (A) Schematic representation of experimental setup for c-AFM measurements. (B) Current map and (C) topography of CVDgrown WS2 spiral domain recorded simultaneously in contact mode. (D) AFM height profile taken along the black dashed line in C. (E) IV characteristics and (F) dI/dV spectra obtained at different points (1, 2, 3) within a single domain, as marked by blue circles on the current map in C.
Burgers vector b.14 The variation in diffuse scattering intensity can be expected for various reciprocal lattice points. Different planes will satisfy different diffraction conditions, and the planes that satisfy g·b = 0 do not show any diffraction intensity.14 Dark field TEM images recorded from the edges of the spiral domains (Figure S1) clearly showed diffuse electron scattering from the edges (Figure S1C). Since the spiral structure is an extension of the monolayer, the 2H structure is not preserved in spirals, and the growth follows AA′ stacking order, which is confirmed through highresolution TEM (HRTEM) analysis.14 This stacking leads to an axial asymmetry in the structure, which induces broken inversion symmetry in the lattice, causing significant contributions in optical and electronic properties.12,28,29 Further, Raman and photoluminescence spectroscopy measurements were performed on spiral and monolayer WS2 domains along with a mechanically exfoliated thick film (∼40 layers), for comparison. Raman spectra show characteristic E12g and A1g peaks around 355 and 418 cm−1, respectively (Figure 3A). The strong PL peak centered at ∼630 nm results from the direct transition of the A exciton,30 and the observed peak energy (∼1.97 eV) is in agreement with the previously reported values for monolayer WS2.31−33 The PL peak for the spiral domain is found to be slightly shifted to higher wavelengths (640 nm) with diminished intensity (Figure 3B). PL from mechanically exfoliated flakes (ME) (∼34 nm thick) is also studied for better comparison. PL spectra of ME WS2 flake showed a less intense peak at ∼692 nm, corresponding to direct excitonic transition (∼1.8 eV) and a very intense peak corresponding to the indirect band transition at 870 nm (1.4 eV). Interestingly, the PL peak corresponding to lower energy indirect excitonic transition is absent in spiral domains, which could be due to the fact that
continues up to an interface or a nodal point, forming a spiral.24 The terms α and δ mentioned in Figure 1D represent a single-layer thickness of a WS2 sheet and interlayer spacing between WS2 layers, respectively. The lateral step-velocity determines the terrace width, and it is governed by the availability of precursors.24 The observed closely packed terrace in synthesized WS2 spirals (Figure 1B) indicates that the growth is under sulfur-rich conditions. Further, transmission electron microscopy (TEM) has been used to identify the atomic arrangements in the spiral domains (Figure 2). HRTEM images recorded from the area marked on the top layer (Figure 2A) and bottom layer (Figure 2C) of the WS2 spiral domain (Figure 2B) clearly reveal different atomic arrangements in the top and bottom layers, thus confirming a distinct stacking sequence in spiral domains.12 Both the layers are in the same direction, and the top layer is translated by a lattice vector a. The tungsten atom in the top layer is positioned at the center of the bottom layer hexagon (Figure 1E).25 It is evident from the HRTEM image (Figure 2A) that the most stable Bernal stacking is not conserved in spirals but follows AA′ stacking, which resembles a rhombohedral structure.25 To draw further insights on the edge sites of spiral domains, we performed dark field TEM studies on the same domain as studied using bright field TEM (Figure 2D). From various reciprocal lattice points marked on the selected area electron diffraction (SAED) pattern (inset of Figure 2D), the corresponding dark field images are recorded (Figure 2E,F). Since the crystal lattice around the dislocation and the edges are distorted or strained,26 these strained areas exhibit high electron density that causes significant diffused electron scattering.27 Hence, the electron scattering from edges will be more compared to the terrace parts. Also, the magnitude of the scattering will be high if reciprocal lattice vector g is parallel to D
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Figure 5. (A) Schematic and (B) photograph of the electrochemical microcell assembly used for electrocatalytic measurements. (C) Polarization curves and (D) Tafel plots obtained from spiral, monolayer, and mechanically exfoliated WS2 flakes. The WS2 spiral domain, with a large number of edge sites connected together by dislocation lines in the vertical direction, exhibits enhanced electrocatalytic properties toward HER with a low Tafel slope and onset potential compared to the CVD-grown monolayer domain and mechanically exfoliated thick WS2 flake.
semimetallic edge states in spiral structures favor nonradiative recombination of indirect-band excitons.11,25,34 Our dark field TEM images (Figure 2E) also show that the edge sites are more electron rich compared to terrace sites. While the PL intensity map of the monolayer domain exhibited more or less uniform intensity throughout the domain (Figure 3C), the spiral domain showed diminished intensity at the spiral center (Figure 3D). PL map of the mechanically exfoliated WS2 domain is also shown for comparison (Figure 3E). The optical images of these flakes are given in the inset. X-ray photoelectron spectroscopy has been further used to confirm the composition of WS2. High-resolution X-ray photoelectron spectroscopy (XPS) spectra obtained from a CVD-grown WS2 sample show peaks at 32.9 eV, 35.1, and 38.5 eV, corresponding to W 4f7/2, W 4f5/2, and W 5p3/2 states of tungsten. Sulfur S 2p peaks are observed at 162.2 and 163.5 eV, which are attributed to S 2p3/2 and S 2p1/2, respectively (Figure S2). To obtain insights on the peculiar photoluminescence observed in spiral WS2, arising from their semimetallic edge sites, c-AFM measurements were performed using gold-coated AFM tips (Figure 4A). CVD-grown spiral domains were transferred onto gold-coated (5 nm Cr and 50 nm Au) SiO2/Si substrates through a poly(methyl methacrylate) (PMMA) transfer technique (see Experimental Methods). Electronic properties were measured by applying bias to the semiconductor through a Au contact pad while keeping the c-AFM tip at virtual ground. The nanometric dimension of the tip ensures high spatial resolution of the recorded current across the spiral. Figure 4B shows the current map recorded at +1.2 V bias, acquired simultaneously with topography. The current map shows a relative variation in current between 0 and 20 nA, with brighter regions representing higher current. Importantly,
the core of the spiral that harbors screw dislocation defects shows the highest current, despite its higher thickness. The topography of the above studied WS2 spiral structure (Figure 4C) shows a central nucleation core with a height of ∼77 nm, as evidenced in the lateral line scan (Figure 4D). To explore the local electrical properties of these spiral domains, point IV characteristics were recorded at three points (blue circles in Figure 4B) of the spiral, for bias sweeps between ±2.5 V, as shown in Figure 4E. While points 1 and 3 lie on the thinner regions of the flake, point 2 lies at the core. The error bars in IV characteristics indicate variation across multiple IV sweeps. All IV’s show rectifying behavior, representative of a Schottky junction formed between the semiconducting WS2 surface and the Au c-AFM probe, displaying a rectifying ratios between 5 and 50 at ±1 V bias. The difference between the metal work function (φM) and semiconductor electron affinity (χS) determines the junction barrier height (φb = φM − χS).35 Interestingly, the IV’s show that the junction carries significantly higher current under positive sample bias, which would render the Schottky junction reverse biased, since these WS2 flakes are understood to be ntype doped36 with an electron affinity of ∼4 eV.37 This is quite contrary to observations made on their macroscopic planar counterparts.38−41 Transport across a Schottky junction is contributed to by both thermionic emission (TE) and tunneling. Details of the electron transport mechanism discussing this anamalous behavior is presented in Supporting Information Section 1, and the schematic band diagram of the junction under both forward and reverse bias is shown in Figure S3. Also, Figure S4A shows the topography of another spiral structure exhibiting a spiral layer thickness of ∼0.74 nm along with the current map (Figure S4B) and correlated line scans (Figure S4C) across topography and current map E
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is facilitated by lowering the contact resistance.16,19 The high contact resistance limits Volmer reaction, and hence, this will be the rate-limiting step.16 The mechanically exfoliated flake (ME-WS2) shows the least catalytic activity, which can be attributed to the charge transfer kinetics. Potential correction of quasi-reference electrode to RHE has been done by performing a platinum response in acidic media. The potential-corrected polarization curves and CVs of platinum response with respect to various reference electrodes are shown in Figure S11. Raman spectroscopy measurements were performed before and after HER studies on WS2 domains, which revealed no change in the observed peaks, confirming the stability of the catalyst domains (Figure S12). The studies conducted by the Manish Chhowalla and Norskov groups demonstrated that the 2H basal planes of MoS2 could be activated for electrocatalysis by introducing sulfur vacancies.16,42 Various groups reported defects in CVDgrown domains and the modifications in electronic band structure due to these vacancies.43,44 Sulfur or metal vacancies induce doping effects in the TMD lattice, which is energetically more favorable for H+ adsorption.45 The defects and vacancies in the WS2 lattice of the spiral and monolayer domains, as clearly revealed from the HRTEM image (Figure 2C), are believed to largely contribute to the electrocatalytic performance of these structures.46,47 Spiral domains which are grown through the defect-driven mechanism are rich in edge sites and are expected to exhibit interesting electronic and catalytic properties. The HER data obtained for spiral WS2 domains are further compared with various other single-domain electrocatalytic HER data from the literature (Table S1). The current map shown in Figure 4 indicates the spatial inhomogeneity in the current distribution in a spiral WS2 flake. The central bright region in the c-AFM image illustrates that the domain center is highly conducting compared to the remaining part of the domain. This indicates that in spirals the charge transfer is not limited in the vertical direction and the electronic conduction from the substrate to the catalyst surface is unhindered. However, c-AFM studies showed that monolayers exhibit higher vertical conductivity (Supporting Information Figure S5). The reduced vertical conduction in spiral domains is due to the path resistance experienced by the charge carriers due to the enhanced vertical distance (thickness of the spiral) that the carrier has to cover before reaching the current collector compared to monolayer domains. Highly conducting edges in spiral domains will enhance the nonradiative recombination, as observed in their photoluminescence properties. The defect lines in spirals, which connect all the catalytically active edges together, are electronically conducting. Hence, the electrocatalytic activity of the spiral domain is superior to that of monolayers.
sections. The current map shows that the edges of the spiral carry higher current than the adjoining plateau regions. Together, the results presented in Figure 4 and S4 indicate that the regions harboring defects in the SDD domains, i.e., the central core and the spiral edges, carry higher current compared to the planar terraces, likely originating from the higher local conductivity of the regions. We also performed the c-AFM experiment on a mono/ bilayer thick CVD-grown WS2 domain and a 34 nm thick mechanically exfoliated WS2 flake. Topography, current map, and local IV characteristics are shown in the Supporting Information (Figures S5 and S6; exact determination of mono/ bilayer domain thickness is quite dif f icult due to the surface roughness of the substrate). The exfoliated sample (ME-WS2) shows poor conductivity perpendicular to its plane compared to the mono/bilayer sample. Being a van der Waals (vdW) layered structure, electron transport in exfoliated WS2 multilayers vertical to its plane happens through interlayer tunneling.14 As a result, thick exfoliated WS2 flakes exhibit poor vertical transport compared to that in the mono/bilayer domain. The enhanced transport observed in the spiral WS2 compared to the exfoliated one is likely due to the interconnecting nature of the WS2 layers in the case of the former. A comparison of vertical electron transport of monolayer, mechanically exfoliated flake and spiral WS2 is presented in Figure S7. Electrochemical Measurements. Electrochemical measurements were performed on monolayer, spiral, and mechanically exfoliated WS2 domains using microcell devices fabricated by using standard lithography techniques. The contacts to WS2 were realized using photolithography followed by Cr/Au metallization. The contacts to the WS2 domains are Schottky in nature, and it is difficult to achieve contact resistance less than 1 kΩ with 2H structures.16 The schematic of steps involved in device fabrication is illustrated in Figure S8, and the respective optical images of the WS2 domain during various steps are shown in Figure S9. Following the electrode deposition, a PMMA layer was coated and a small window was unwrapped by e-beam lithography to expose the WS2 domain alone covering the gold contacts. This ensures that the gold contacts do not contribute to the electrocatalysis. Optical images of various domains and a photograph of the fabricated device are shown in Figure S10. Schematic and photograph of the electrochemical microcell assembly are shown in Figure 5A and B, respectively. For measuring HER activity, a drop of 0.5 M H2SO4 is placed on top of the device, and a platinum wire (as a counter electrode) and AgCl-coated Ag wire (as the pseudoreference electrode) are positioned in contact with the drop of the electrolyte using micropositioners and a microscope (Figure 5B). Polarization and Tafel plots are recorded from spiral, monolayer, and mechanically exfoliated WS2 domains, shown in Figure 5C and D, respectively. The spiral domain showed an overpotential of 560 mV (vs Ag/AgCl quasi-reference) to attain a 0.02 μA μm−2 (20 nA μm−2) current density, while the monolayer sample required 642 mV (vs Ag/AgCl quasi-reference) to attain the same current. Tafel slope of spiral and monolayer WS2 domains are found to be 81 and 121 mV dec−1, respectively. Even though both the materials are in 2H phase, the spiral domain displays enhanced HER performance with a smaller Tafel slope and lower onset potential compared with the monolayer counterpart. Usually, 2H basal planes are less active in catalysis unless the charge injection to the basal planes
CONCLUSIONS In summary, we have demonstrated a controlled CVD process for the SDD growth of spiral WS2 domains, which exhibit a large number of edge sites compared to CVD-grown monolayer domains. c-AFM studies were carried out to understand the vertical electrical transport in these domains, which showed that the vertical electronic conductivity of spirals is unconstrained and edges are more conducting compared to terrace parts of the spiral domain. With an aim to understand the effect of edge sites resulting from the spiral morphology on their electrocatalytic properties toward hydrogen generation, we have carried out electrochemical studies on F
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a single spiral WS2 domain using a micro-electrochemical cell. The WS2 domains having spiral morphology exhibited enhanced electrocatalytic properties toward HER with a low Tafel slope and onset potential compared to a CVD-grown monolayer domain and a mechanically exfoliated thick WS2 flake. Enhanced catalytic activity for the spiral domain is believed to have originated from the increased number of active edge sites connected together by dislocation lines in the vertical direction without compromising the vertical charge transport. The obtained results are very interesting and clearly reveal the importance of edge sites on hydrogen generation electrocatalysis.
M. M. Shaijumon: 0000-0001-5745-4423 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS P.V.S. acknowledges UGC (F.2-16/2009 (SA-I) 28 APR 2015), Govt. of India, for the financial support. C.H.S. acknowledges CSIR, Govt. of India, for the research fellowship. M.M.S. acknowledges financial support from the Science and Engineering Research Board (SERB), Department of Science & Technology, Govt. of India (EMR/2017/000484), and Indian Institute of Science Education and Research (IISER) Thiruvananthapuram, Kerala, India. M.M.S. is grateful to the Indo-US Science and Technology forum (IUSSTF) for the Indo-US Joint R&D network center [IUSSTF/JC-071/2017]. J.M. acknowledges financial support from UGC, Government of India, UKIERI, the Royal Academy of Engineering (UK), and the Newton Bhabha Fund, UK.
EXPERIMENTAL METHODS Chemical Vapor Deposition of Spiral WS2. WS2 spirals were controllably synthesized on a SiO2/Si substrate by chemical vapor deposition.11 WO3 powder drop casted on a cleaned SiO2/Si act as tungsten source, and 500 mg of sulfur powder kept at the upstream end of the quartz tube acts as sulfur source. The deposition was carried out under an argon atmosphere at 900 °C. The obtained WS2 spirals were characterized using SEM, Raman spectroscopy, TEM, PL, and AFM. Electrocatalytic Studies. Microelectrodes were fabricated using photolithography with Shipley S1813 polymer. A photoresist is initially coated onto the as-grown CVD substrate, and using photolithography, patches for electrodes were opened. This was followed by the deposition of Cr/Au electrodes using a thermal evaporation method. After the microelectrode deposition, photopolymer is removed from the substrate. To ensure that the deposited electrodes do not contribute to the electrocatalysis, the entire substrate is coated with PMMA polymer. Later, using e-beam lithography, the WS2 domain is exposed and rest of the substrate remains masked under PMMA. The catalytic measurements were carried out using an Agilent B2901 potentiostat. HER studies were performed in 0.5 M H2SO4 electrolyte using a AgCl-coated Ag wire quasi-reference electrode and Pt wire counter electrode. Linear sweep voltammetry technique was performed for recording catalytic studies in the negative potential region (0 to −0.7 V). Conductive-AFM Measurements. As-grown CVD substrates were spin-coated by PMMA 950A polymer at 3000 rpm for 40 s. Later, these domains were kept floating on 3 M aqueous KOH solution for slow etching of a SiO2 layer. Once the PMMA layer is lifted out from the substrate, the PMMA layer is transferred to DI water to remove KOH residue from the sample domains. After that, these layers are scooped onto Cr/Au-coated SiO2/Si substrate. Then acetone is used to remove the PMMA layer, and later these substrates are used for c-AFM measurements, which were carried out using a Bruker atomic force microscope with gold-coated AFM tips. TEM Sample Preparation. After PMMA coating and KOH treatment, once the PMMA layer is lifted out from the Si/SiO2 substrate, the layers were scooped onto a TEM grid. The PMMA layer is then washed and removed from the TEM grid using acetone.
REFERENCES (1) Tsai, C.; Chan, K.; Abild-Pedersen, F.; Nørskov, J. K. Active Edge Sites in MoSe2 and WSe2 Catalysts for the Hydrogen Evolution Reaction: A Density Functional Study. Phys. Chem. Chem. Phys. 2014, 16, 13156−13164. (2) Ouyang, Y.; Ling, C.; Chen, Q.; Wang, Z.; Shi, L.; Wang, J. Activating Inert Basal Planes of MoS2 for Hydrogen Evolution Reaction Through the Formation of Different Intrinsic Defects. Chem. Mater. 2016, 28, 4390−4396. (3) Damien, D.; Anil, A.; Chatterjee, D.; Shaijumon, M. M. Direct Deposition of MoSe2 Nanocrystals Onto Conducting Substrates: Towards Ultra-efficient Electrocatalysts for Hydrogen Evolution. J. Mater. Chem. A 2017, 5, 13364−13372. (4) Yu, Y.; Huang, S.-Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553−558. (5) Sarma, P. V.; Tiwary, C. S.; Radhakrishnan, S.; Ajayan, P. M.; Shaijumon, M. M. Oxygen Incorporated WS2 Nanoclusters With Superior Electrocatalytic Properties for Hydrogen Evolution Reaction. Nanoscale 2018, 10, 9516−9524. (6) Hai, X.; Chang, K.; Pang, H.; Li, M.; Li, P.; Liu, H.; Shi, L.; Ye, J. Engineering the Edges of MoS2 (WS2) Crystals for Direct Exfoliation into Monolayers in Polar Micromolecular Solvents. J. Am. Chem. Soc. 2016, 138, 14962−14969. (7) Gopalakrishnan, D.; Damien, D.; Shaijumon, M. M. MoS2 Quantum Dot-Interspersed Exfoliated MoS2 Nanosheets. ACS Nano 2014, 8, 5297−5303. (8) Wang, S.; Robertson, A.; Warner, J. H. Atomic Structure of Defects and Dopants in 2D Layered Transition Metal Dichalcogenides. Chem. Soc. Rev. 2018, 47, 6764−6794. (9) Noh, S. H.; Hwang, J.; Kang, J.; Seo, M. H.; Choi, D.; Han, B. Tuning the Catalytic Activity of Heterogeneous Two-Dimensional Transition Metal Dichalcogenides for Hydrogen Evolution. J. Mater. Chem. A 2018, 6, 20005−20014. (10) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv. Mater. 2016, 28, 6197−6206. (11) Sarma, P. V.; Patil, P. D.; Barman, P. K.; Kini, R. N.; Shaijumon, M. M. Controllable Growth of Few-Layer Spiral WS2. RSC Adv. 2016, 6, 376−382. (12) Fan, X.; Jiang, Y.; Zhuang, X.; Liu, H.; Xu, T.; Zheng, W.; Fan, P.; Li, H.; Wu, X.; Zhu, X.; Zhang, Q.; Zhou, H.; Hu, W.; Wang, X.; Sun, L.; Duan, X.; Pan, A. Broken Symmetry Induced Strong Nonlinear Optical Effects in Spiral WS2 Nanosheets. ACS Nano 2017, 11, 4892−4898.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04250. HRTEM, dark field TEM, conductive AFM details, micro-electrochemical device fabrication details, and optical images of fabricated microdevices (PDF)
AUTHOR INFORMATION Corresponding Author
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DOI: 10.1021/acsnano.9b04250 ACS Nano XXXX, XXX, XXX−XXX
