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Large-area Buckled MoS Films on the Graphene Substrate Seon Joon Kim, Dae Woo Kim, Joonwon Lim, Soo-Yeon Cho, Sang Ouk Kim, and Hee-Tae Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01828 • Publication Date (Web): 04 May 2016 Downloaded from http://pubs.acs.org on May 12, 2016

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ACS Applied Materials & Interfaces

Large-area Buckled MoS2 Films on the Graphene Substrate Seon Joon Kim1, Dae Woo Kim1, Joonwon Lim2, Soo-Yeon Cho1, Sang Ouk Kim2, Hee-Tae Jung1,3,* 1

National Research Laboratory for Organic Opto-Electronic Materials, Department of Chemical

and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 2

National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale

Assembly, Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Korea 3

KAIST Institute for the Nanocentury

KEYWORDS MoS2, Graphene, Buckled structure, Wrinkle, Large-area, Hydrogen evolution reaction

ACS Paragon Plus Environment

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ABSTRACT

In this study, a novel buckled structure of edge-oriented MoS2 films is fabricated for the first time by employing monolayer graphene as the substrate for MoS2 film growth. Compared to typical buckling methods, our technique has several advantages: i) external forces such as heat and mechanical strain are not applied; ii) uniform and controllable buckling over a large area is possible; and iii) films are able to be transferred to a desired substrate. Dual MoS2 orientation was observed in the buckled film where horizontally aligned MoS2 layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS2 films, measured by performing water splitting tests in acidic environments, shows a reduced onset potential of -0.2 V vs. RHE compared to -0.32 V vs. RHE for pristine MoS2, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS2 structure, which may be extended to the formation of buckled structures in various 2D materials for future applications.


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Introduction Two-dimensional sheets of transition metal dichalcogenides (TMDs) have attracted considerable interest as isolated structures of atomically thin semiconductor films.1-3 The TMD material MoS2 has proven to be particularly interesting because it is readily prepared, low in cost, and provides favorable semiconducting properties. Two distinct MoS2 film structures have been synthesized to date in which the MoS2 molecules are aligned either vertically or horizontally with respect to the substrate. MoS2 films are commonly fabricated using chemical vapor deposition (CVD) methods in which a substrate is exposed to Mo precursors (Mo, MoO3, etc.) and S precursors (S8, H2S) at high temperatures.4-8 The MoS2 films possess a band gap of 1–2 eV, a high on/off ratio of 105, and a mobility of 100–200 cm2/V∙s, making the films good candidates for optoelectronic devices, such as transistors and photodetectors.9-12 The direct band gap provided by single-layer MoS2 films is beneficial in the design of high-performance electronics.13 In addition to the basal plane, which is planar in structure, defects and edges in MoS2 sheets provide active sites for chemical catalytic reactions.14-17 For example, the d-orbital in a Mo edge creates a metallic edge through which excess electrons are easily transferred to reduce protons to hydrogen gas in a hydrogen evolution reaction (HER) and to efficiently reduce CO2 in a carbon dioxide reduction reaction (CRR).14, 15, 18-21 S-terminated sites can increase the catalytic activity of hydrodesulfurization.22 Due to these properties, the low cost and abundance of the materials, and the ability to synthesize MoS2 films in large quantities, MoS2 is a promising candidate for replacing current noble catalysts, such as Pt/C. Recently, vertically aligned MoS2, MoSe2, and WS2 structures were realized by rapidly sulfurizing Mo, W precursors to synthesize edge-on-oriented sheets over a large area.23 Layers fabricated using this method consist of individual sheets aligned vertically with respect to the


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substrate, such that the edge of each sheet faces the top surface. The method may be readily tuned to alter the vertical orientations of the individual MoS2 sheets by controlling the Mo seed layer thickness and introducing appropriate post-treatments.24 Unlike planar MoS2 films, the outermost interface in MoS2 sheets is chemically active due to the exposed edge sites, which provide catalytic centers for efficient HER and gas detection.23, 25, 26 The structure and sheet orientation in a TMD film are critical for determining the catalytic performance. The facile control over the film orientation, as described above, suggests that the film fabrication methods may be improved to optimize the film structures for catalytic activity. Here, we report the fabrication of a buckled structure in edge-oriented MoS2 films over a large area simply by depositing monolayer graphene below the Mo film prior to rapid sulfurization via CVD. The vertically aligned MoS2 sheets formed near the surface across the entire film, whereas the MoS2 sheets near the graphene surface formed with a horizontal alignment. Our work demonstrates that a buckled surface morphology is critical to tuning and increasing the catalytic activity in a MoS2 film, as demonstrated by the highly enhanced HER performance. This method enables the integration of buckling traits into 2D two-dimensional materials and provides insight into the fabrication of films with diverse structures through a facile process involving manipulating the substrate properties.


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Experimental Section Preparation and Transfer of Monolayer Graphene Cu foils (25 µm thick, Goodfellow, 99.99% purity) were placed in a 4 inch diameter quartz tube. After creating a vacuum atmosphere, an 8 sccm H2 flow was introduced, the furnace was heated to 1040°C, and the film was annealed for 1 hour prior to CVD growth to remove residual oxides. A 50 sccm CH4 flow was then injected into the chamber, in addition to the H2 flow, over 20 minutes to grow graphene on the Cu foil. The furnace was then rapidly cooled to room temperature under H2 to retrieve the graphene-grown Cu foil. The graphene layer was then coated with a PMMA support layer by spin-casting, and the assembly was annealed at 180°C for 3 minutes to remove solvents and strengthen the polymer film integrity. The Cu substrate was dissolved using an iron(III) chloride (FeCl3) aqueous solution, and the PMMA–graphene assembly was thoroughly washed several times with deionized water before transferring onto the SiO2/Si wafer. After drying to remove water, the PMMA layer was dissolved in acetone to complete the transfer.

Electrochemical Water Splitting Test (Hydrogen Evolution Reaction, HER) The electrochemical tests were conducted by transferring the MoS2 films onto a glassy carbon electrode. The MoS2 films on the SiO2/Si wafers were first coated with a PMMA film that acted as a support during the transfer process. The SiO2 layer was etched away using a 2 M KOH aqueous solution heated to 80°C to detach the MoS2 film from the substrate. The films were thoroughly washed with deionized water multiple times and transferred onto a glassy carbon electrode. After drying, the supporting PMMA layer was dissolved in acetone to complete the


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transfer process. The HER performance was tested using a 0.5 M H2SO4 electrolyte solution and a three electrode setup. A reversible hydrogen electrode (RHE) was used as the reference electrode, and graphite was used as the counter electrode. LSV was performed at a scan rate of 2 mV/s, with the exception of the stability tests, which were performed at 50 mV/s.

Equipment and Characterization The surface morphologies were analyzed using scanning electron microscopy (SEM, FEI Nova230), atomic force microscopy (AFM, Park Systems). The MoS2 film properties were analyzed by Raman spectroscopy (Raman, Horiba Jobin Yvon ARAMIS) using a 514 nm visible-range laser. Grazing incidence X-ray diffraction (GIXRD) studies were performed at the Pohang Accelerator Laboratory (PAL, Pohang, Korea) using a beam size of 200µm x 500µm and a Rayonix 2D MAR 165 detector. Cross-sections of the samples were prepared using a focused ion beam (FIB, FEI Helios Nanolab 450 F1), and high-resolution images of the cross-sections were collected by transmission electron microscopy (TEM, FEI Tecnai TF30 ST).


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Results and Discussion The overall procedure used to fabricate buckled edge-oriented MoS2 films is presented in Fig. 1a. First, monolayer graphene is grown on a Cu foil (25 µm thick, Goodfellow, 99.99% purity) using a CVD method at 1040°C under 8 sccm H2 and 50 sccm CH4 at low pressures.27 The graphene monolayer was then transferred to a SiO2 (300 nm)/Si wafer for further processing using a conventional poly(methyl methacrylate)(PMMA) method. High-quality monolayer graphene was grown using this method, as indicated by the small ID/IG ratio of 0.085, and a sharp 2D peak with a FWHM of 35 cm–1 was observed in the Raman spectrum (Fig. S1, Supporting Information).27 The graphene domain structure was imaged using the liquid crystal birefringence method (Fig. S2, Supporting Information), revealing that the graphene formed domains that were smaller than 10 µm, typical of CVD-grown graphene domains.28-30 After the graphene transfer step, molybdenum (Mo) films of various thicknesses (2–20 nm) were deposited onto the graphene layer using e-beam deposition methods to obtain a smooth Mo film. The Mo films were then reacted with sulfur using CVD methods to synthesize the MoS2 film. Substrates bearing the deposited Mo films were place in the downstream 2 inch diameter quartz tube chamber of a twochamber furnace, and sulfur powders were placed on a crucible in the upstream furnace chamber. Vacuum conditions were applied, and 50 sccm Ar was injected into the chamber. Initially, the Mo films in the downstream furnace were heated for 20 minutes to 770°C. Sulfur powders were then heated to 220°C and maintained at that temperature for 20 minutes to fabricate MoS2. The furnace was then rapidly cooled to room temperature under Ar to retrieve the sample. In this study, MoS2 films were grown on both graphene-covered SiO2 wafers and on bare SiO2 wafers to compare the effects of the underlying graphene layer on the growth of the MoS2 layer. MoS2 films were successfully grown on both substrates, as indicated by the Raman spectra, which


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displayed a sharp A1g peak at 383 cm–1 and an E12g peak at 408 cm–1 (Fig. S3, Supporting Information). Buckled MoS2 films were able to be further transferred onto arbitrary substrates without damage, such as polystyrene (PS) films (Fig. S4, Supporting Information), showing their potential to be applied to flexible devices.

Figure 1. Fabrication of buckled MoS2 films at a graphene interface. (a) Schematic illustration of the process used to fabricate buckled edge-oriented MoS2 films. Monolayer graphene was grown on a Cu foil via CVD and was then


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transferred onto a Si wafer coated with SiO2. A thin Mo film between 2 nm and 20 nm was then deposited on top of the graphene layer using electron beam (e-beam) deposition. The buckled vertically aligned MoS2 films were obtained through rapid sulfurization in a CVD chamber at 770°C. (b–e) Scanning electron microscopy (SEM) images of the edge-oriented MoS2 films deposited onto graphene using various Mo seed layer thicknesses: (b) 2 nm, (c) 5 nm, (d) 10 nm, or (e) 20 nm. The wrinkle features were readily observed in films prepared with a Mo seed layer thickness exceeding 10 nm. (f) Image of a 12 mm x 12 mm large-area buckled MoS2 film. (g) Lowmagnification SEM image of a buckled MoS2 film, revealing a uniform structure over a large area.

The influence of graphene on the morphology of the CVD-grown MoS2 film was monitored by collecting scanning electron microscopy (SEM) images of the MoS2 films prepared on the graphene layer or on the SiO2 (300 nm)/Si substrate using different Mo seed layer thicknesses: 2, 5, 10, and 20 nm. As reported previously, MoS2 films grown on SiO2/Si substrate yielded a smooth surface with a mean roughness of less than 10 nm.23 These films lacked any type of buckled structure, regardless of the Mo seed layer thickness (2–20 nm, see the Fig. S5, Supporting Information). On the other hand, although uniform MoS2 films could be prepared using 2 nm Mo seed layer on graphene (Fig. 1b), small buckled structures began to appear in the films prepared with a 5 nm Mo seed layer on graphene (Fig. 1c). Remarkable buckled features were observed in the MoS2 films prepared on graphene using a Mo seed layer exceeding 10 nm (Fig. 1d–1e). The degree of buckling increased as the thickness increased, and chain-like morphologies were observed in the MoS2 films prepared on graphene using the 20 nm thick Mo films (Fig. 1e). The buckled MoS2 films were uniformly fabricated on graphene over a large area with a small number of defects, as shown in Fig. 1f–1g and Fig. S6, Supporting Information. The average full-width of each wrinkle was 0.47 µm for the 10 nm Mo film, 0.69 µm for 15 nm Mo film, and 1.05 µm for the 20 nm Mo film, indicating a nearly linear correspondence between the film thickness and the wrinkle width (Fig. S7, Supporting Information). The wrinkle heights ranged from 400 nm for the 10 nm Mo film to nearly 1 µm for the 20 nm Mo film, as determined from the height profiles measured using atomic force microscopy (AFM) (see Fig. S8,


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Supporting Information). These results indicated that the underlying graphene substrate played a critical role in determining the morphology of the MoS2 films synthesized via CVD. The layered structures of the buckled MoS2 films were characterized by collecting the twodimensional diffraction patterns using grazing incidence X-ray diffraction (GIXRD). Figs. 2a–2d show the GIXRD patterns collected from MoS2 films prepared on graphene using various Mo seed layer thicknesses: 2 (Fig. 2a), 5 (Fig. 2b), 10 (Fig. 2c), or 20 nm (Fig. 2d). The MoS2 (002) peak was observed at q=1, corresponding to an interlayer spacing of 6.3 Å and indicating that the MoS2 films were well-ordered and compactly stacked (Fig. 2e). Although the interlaying spacing (q=1) was uniform throughout all samples, the (002) peak orientation depended on the Mo seed layer thickness. A Mo seed layer thickness of 2 nm yielded a strong (002) intensity only in the out-of-plane direction (qz=1), and the intensity was negligible in the in-plane direction. On the other hand, the peak intensity of (002) was the highest in the in-plane direction (qxy=1), with a slight (002) peak in the out-of-plane direction and a diffuse ring corresponding to (002) for Mo seed layer thicknesses of 5, 10, and 20 nm. These results indicated a transition in the orientations of the MoS2 sheets from horizontal to vertical as the MoS2 layer thickness increased. A mixture of planar and vertical structural orientations was observed in the case of the thick MoS2 films prepared on graphene above 10 nm. The 2D GIXRD patterns obtained from the MoS2 films on the SiO2 wafers were collected (Fig. S9, Supporting Information) and revealed the absence of a dominant alignment in the 2 nm thick film. The MoS2 sheets were completely vertically oriented in the layers with a thickness exceeding 5 nm. The effect of the Mo seed layer thickness on the MoS2 orientation was determined using Raman spectroscopy (Fig. 2f). The ratio of the E12g intensity to the A1g intensity decreased from 0.86 for a 2 nm thick Mo film to 0.43 for a 20 nm thick Mo film (Fig. S10, Supporting


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Information), indicating that the out-of-plane excitation, typical of vertically aligned MoS2 films, became prominent in the thicker films.24 The Raman spectra were collected from different local areas on a buckled MoS2 film, as shown in Fig. 2g. Although the peak positions in the MoS2 films on a SiO2 wafer appeared at 383 cm–1 and 409 cm–1, the peak positions in the planar regions of the MoS2 films prepared on graphene were red-shifted by 2 cm–1 to 381 cm–1 and 407 cm–1. The peak positions of the MoS2 wrinkles, in particular, were largely red-shifted by 5 cm–1 to 378 cm–1 and 405 cm–1. Because the local stress induces a red shift in the MoS2 Raman peaks, the results indicate that graphene applies strain to the upper MoS2 film, with the highest stress observed on the buckled regions of the MoS2 film. 31

Figure 2. Vertical alignment in the buckled edge-oriented MoS2 films. (a-d) Grazing incidence X-ray diffraction (GIXRD) patterns collected from the buckled edge-oriented MoS2 films prepared on graphene with various Mo seed layer thicknesses: (a) 2 nm, (b) 5 nm, (c) 10 nm, (d) 20 nm. (e) Interlayer spacing calculated from the GIXRD patterns for the 2 and 10 nm thick Mo films. All MoS2 samples displayed an interlayer spacing of 0.63 nm. (f) Raman peaks of the edge-oriented MoS2 films of various thicknesses prepared on graphene. (g) Raman peaks of the edge-oriented MoS2 films displaying a variety of textures, including the flat films prepared on a SiO2 wafer (black), the planar regions of the buckled films prepared on graphene (red), and the buckled region in the films prepared on graphene (blue).


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The stacked MoS2 sheet structures in the buckled structure were directly visualized using transmission electron microscopy (TEM) applied to a cross-sectional cut of the buckled area prepared by cutting the MoS2 film grown on graphene from a 20 nm thick Mo seed film using a focused ion beam (FIB), as shown in Fig. 3. Fig. 3a shows a cross-sectional SEM image of a MoS2 film grown on graphene from a Mo seed layer 20 nm thick. The MoS2 films were coated with carbon and Pt to conserve the structure of the buckled MoS2 film during the FIB process and to enhance the image contrast during TEM measurements. It is noteworthy that small amount of pores resulting from the delamination of MoS2/graphene films exist in the structure, which is likely to be due to the relatively low adhesion energy between graphene and SiO2 during buckling. The TEM images of the highlighted regions shown in Fig. 3a and Fig. 3b revealed that the MoS2 sheets were well aligned along the entire curvature of the buckled film without noticeable defects. High-resolution TEM (HRTEM) images of three areas within the buckled region were collected ((i), (ii), (iii)), and are shown as magnified images in Fig. 3c. The inset (i) of Fig. 3c reveals that the bottom MoS2 sheets near the graphene interface (buckled cavity side, ~7 nm from the bottom) assumed a long-range planar alignment with respect to the bottom substrate, whereas the majority of the sheets near the top surface were vertically aligned, and all individual MoS2 sheets had a spacing of 0.63 nm. Identical orientation behaviors were observed in the insets (ii) and (iii), demonstrating that the MoS2 film orientation was uniform across the entire buckled film. These results agreed with the GIXRD results, which indicated the presence of a mixture of horizontally and vertically aligned MoS2 sheets, as shown in Fig. 2. The mixed sheet alignment was also observed in the planar area indicated in Fig. 3a and in the Fig. S11, Supporting Information. The buckled MoS2 films grown on the graphene surface formed films in which the MoS2 sheets were oriented horizontally at film thicknesses of less than 7 nm


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and vertically at film thicknesses exceeding 7 nm, as described in the schematic illustration shown in Fig. 3d. For comparison, a cross-sectional image of MoS2 grown on a bare SiO2 wafer was examined (Fig. S12, Supporting Information). In this sample, only vertically aligned sheets were observed, with an interlayer spacing of 0.63 nm. No planar sheets were observed, unlike in the MoS2 film grown on graphene.

Figure 3. Cross-section of a buckled edge-oriented MoS2 film. (a) SEM cross-sectional image of a buckled edgeoriented MoS2 film grown on graphene (using a Mo seed layer thickness of 20 nm). (b) Transmission electron microscopy (TEM) image of the area highlighted in (a). (c) High-resolution TEM images of the highlighted regions (i), (ii), (iii) in (b). The MoS2 sheet orientation structure was uniformly propagated across all areas of the film. The orientations of the MoS2 sheets are evident, with a d-spacing of 0.63 nm. (d) Schematic illustration of the observed MoS2 sheet alignment along the buckled structure.


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The previous results indicate that the graphene substrate significantly influenced the alignment and structure of the MoS2 layers. First, the horizontal alignment of the MoS2 sheets near graphene likely minimized the strain between the graphene and MoS2 basal plane, unlike the strain associated with an interface between graphene and the MoS2 edge sites. Growth in the vertical MoS2 sheets proceeds inward from the top surface. The high surface energy of the sheet edges and the accumulated stress near the graphene interface induces the direction of sheet propagation to redirect horizontally. Curved continuous MoS2 layers with an abrupt orientation change (Fig. 3c) reduce both the surface energy difference at the graphene–MoS2 interface and the accumulated film stress. The buckled MoS2 structure arises from the internal stress in the film that built up during the growth process, in which accumulated stress on the buckled MoS2 was able to be observed from the Raman shift in Fig. 2g. Previous studies have shown that diamond-like carbon and graphitic films evolve to form buckled structures once the internal stress exceeds a critical threshold.32,

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The resulting telephone cord and hexagonal wrinkle

structures reported in previous studies resemble the MoS2 buckled structure observed in this work. The wrinkle width has been shown to be proportional to the film thickness during typical buckling delamination processes, as seen in our case (Fig. S6, Supporting Information).32,

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These results support the statement that the buckled structures originated from the internal stress in the MoS2 film.


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Figure 4. Electrochemical HER performance of the edge-oriented MoS2 films. (a) Linear sweep voltammetry (LSV) curves were collected from the MoS2 films prepared from different Mo seed layer thicknesses. LSV curves obtained from the MoS2 films grown on a SiO2 wafer (dashed line), buckled MoS2 films grown on graphene (solid line), and non-buckled MoS2 films grown on graphene (dotted line). (c) The Tafel plots obtained from (b). (d) Stability tests of the buckled MoS2 films. The LSV curves overlapped after 1000 cycles.

The significance of the large-area buckled MoS2 films was demonstrated by measuring the catalytic activity of the buckled vertically aligned MoS2 films in the context of HER reactions, in which our films were used as the active electrode. The fabricated MoS2 films were transferred onto a glassy carbon electrode, and the HER tests were performed in a three-electrode configuration using 0.5 M H2SO4 as the electrolyte. The linear sweep voltammetry (LSV) curves collected from the MoS2 films on the SiO2 wafers are shown in Fig. 4a for Mo seed layer thicknesses of 2, 10, and 20 nm. The onset voltage of the planar MoS2 films (Mo 2 nm) was –0.5 V vs. RHE, and the onset voltage of the vertically aligned MoS2 films (10 nm, 20 nm Mo seed


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layer) showed an enhanced value of –0.32 V vs. RHE. The enhanced catalytic performance of the vertically aligned MoS2 films as compared to that of the planar films arose from the high density of catalytically active edge sites on the surface, as discussed in previous studies.23 The LSV curves obtained from the MoS2 films prepared using a 10 nm thick Mo seed layer on a SiO2 (dashed) supporting a graphene layer (solid) are shown in Fig. 4b. The catalytic activities of the buckled MoS2 films on graphene increased significantly as the onset voltage was decreased from –0.32 V vs. RHE (MoS2 on SiO2 wafer) to –0.2 V vs. RHE. The enhanced catalysis was also observed in the corresponding Tafel plots, which revealed a Tafel slope that decreased from 267 mV/dec (MoS2 on SiO2 wafer) to 160 mV/dec (Fig. 4c). A similar trend was observed for the MoS2 film grown using a 20 nm thick Mo seed layer (Fig. S13, Supporting Information), however with a smaller enhancement, which might be due to the larger pore size inside the wrinkles. Electrochemical impedance spectroscopy was also performed to compare the electron transfer kinetics (Fig. S14, Supporting Information) where the charge transfer resistance greatly decreases from 600~700 ohms for MoS2 on SiO2 to 250 ohms for buckled MoS2 on graphene. The buckled MoS2 films operated stably over 1000 cycles, during which the LSV curves barely changed, as shown in Fig. 4d. Also at a continuous operation over 12 hours, buckled MoS2 films showed a good current density retention at applied potentials (Fig. S15, Supporting Information). The enhanced catalytic activity of the buckled MoS2 films most likely arose from the high surface area and the high surface roughness. The buckled structures a few hundred nanometers in height, formed using the 10 nm thick Mo seed layer, provided a surface area that was 112% of the surface area expected of a planar film covering the same area. The larger surface area provided more reaction sites, and the entire buckled area was catalytically active as the MoS2 sheets were vertically aligned across the entire area. In addition to increasing the surface area, a


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rough MoS2 surface has been shown previously to facilitate H2 bubble detachment, thereby enhancing the overall HER performance.34 As previous works have also utilized the conductive nature of graphitic substrates for enhanced catalytic activity35, a non-buckled MoS2 film on graphene was prepared in order to address the influence of the additional graphene layer itself as a conductive electrical pathway. Non-buckled MoS2 films transferred onto a monolayer graphene substrate displayed a poor HER performance, indicating that the influence of substrate conductivity is negligible, as shown in Fig. 4b.

Conclusion In summary, we synthesized a new buckled MoS2 structure through rapid sulfurization of a Mo seed layer supported by a monolayer graphene-coated substrate. The presence of graphene induced the formation of a large-area delamination-assisted buckled structure, the dimensional features of which could be readily controlled by manipulating the Mo seed layer thickness. Compared to conventional buckling techniques, our method has several advantages: First, i) external forces, such as mechanical strain or heat, are not applied here; ii) uniform, controllable buckling over a large area is readily achievable; and iii) the film may be transferred to an arbitrary substrate. Spectroscopy and cross-sectioned electron microscopy images revealed that most of the MoS2 sheets were vertically oriented, with the edge sites exposed to the surface, whereas the layers near the graphene interface were horizontally oriented. The alignment structure was propagated uniformly across the entire sample for both the buckled and planar regions, indicating that the buckling process did not damage the sheet orientations. The buckled MoS2 films showed an enhanced HER performance compared with the smooth films, indicating


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that the rough morphology of the novel structure provided a higher catalytic activity. The approach to preparing the buckled structure may be applicable to other two-dimensional materials. Buckled structures are anticipated to be useful for the development future devices with enhanced performances.


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ASSOCIATED CONTENT Supporting Information. Additional images and measurements on vertically grown MoS2 sheets and the graphene substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] (Prof. H.-T. Jung) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF), funded by the Ministry

of

Science,

ICT,

and

Future

Planning,

Korea

(MSIP,

Grant

No.

2015R1A2A1A05001844), Global Frontier Research Center for Advanced Soft Electronics (No. 2014M3A6A5060937, MSIP), and the Climate Change Research Hub of KAIST (No. N01150139).


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[34] Lu, Z.; Zhu, W.; Yu, X.; Zhang, H.; Li, Y.; Sun, X.; Wang, X.; Wang, H.; Wang, J.; Luo, J.; Lei, X.; Jiang, L. Ultrahigh Hydrogen Evolution Performance of Under-Water “Superaerophobic” MoS2 Nanostructured Electrodes. Adv. Mater. 2014, 26, 2683-2687. [35] Sun, J.; Memon, M. A.; Bai, W.; Xiao, L.; Zhang, B.; Jin, Y.; Huang, Y.; Geng, J. Controllable Fabrication of Transparent Macroporous Graphene Thin Films and Versatile Applications as a Conducting Platform. Adv. Funct. Mater. 2015, 25, 4334-4343.


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Table of Contents Graphic


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Films on the Graphene Substrate - ACS Publications - American

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