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Hierarchical, Dual Scale Structures of Atomically-thin MoS2 for Tunable Wetting Jonghyun Choi, Jihun Mun, Michael Cai Wang, Ali Ashraf, Sang-Woo Kang, and SungWoo Nam Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05066 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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Hierarchical, Dual Scale Structures of Atomicallythin MoS2 for Tunable Wetting Jonghyun Choi,1,† Jihun Mun,2,† Michael Cai Wang,1 Ali Ashraf,1 Sang-Woo Kang,*,2 and SungWoo Nam*,1,3 1

Department of Mechanical Science and Engineering, University of Illinois at Urbana-

Champaign, Urbana, Illinois 61801, United States. 2

Vacuum Center, Korea Research Institute of Standards and Science, Daejeon, 34113, Republic

of Korea. 3

Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign,

Urbana, Illinois 61801, United States. KEYWORDS: Molybdenum disulfide (MoS2), two-dimensional (2D) materials, surface coatings, tunable wettability, hierarchical patterning, nanoflowers, crumples

ABSTRACT: Molybdenum disulfide (MoS2), a well-known solid lubricant for low friction surface coatings, has recently drawn attention as an analog two-dimensional (2D) material beyond graphene. When patterned to produce vertically grown, nanoflower-structures, MoS2 shows promise as a functional material for hydrogen evolution catalysis systems, electrodes for

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alkali metal-ion batteries, and field-emission arrays. Whereas the wettability of graphene has been substantially investigated, that of MoS2 structures, especially nanoflowers, has remained relatively unexplored despite MoS2 nanoflower’s potential in future applications. Here, we demonstrate that the wettability of MoS2 can be controlled by multi-scale modulation of surface roughness through 1) tuning of the nanoflower structures by chemical vapour deposition synthesis and 2) tuning of microscale topography via mechanical strain. This multi-scale modulation offers broadened tunability (80-155°) compared to single-scale tuning (90-130°). In addition, surface adhesion, determined from contact angle hysteresis (CAH), can also be tuned by multi-scale surface roughness modulation, where the CAH is changed in range of 20-40°. Finally, the wettability of crumpled MoS2 nanoflowers can be dynamically and reversibly controlled through applied strain (~115-150° with 0-200% strain), and remains robust over 1,000 strain cycles. These studies on the tunable wettability of MoS2 will contribute to future MoS2based applications, such as tunable wettability coatings for desalination and hydrogen evolution.


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Recently, two-dimensional (2D) materials have drawn significant attention due to their extraordinary mechanical, electrical, and optical characteristics compared to bulk materials.1–3 Graphene has been the most extensively investigated 2D material, and thus the wettability of graphene has been studied substantially. For example, it has been reported that the wettability of graphene may be influenced by substrate effects, doping, and the number of layers.4–9 It is known that the commonly observed wettability of graphene in ambient conditions shows weak hydrophobicity due to airborne hydrocarbon adsorption (water contact angle (WCA) ~90°).10,11 Superhydrophobic surfaces (WCA greater than 150°) are desirable as they lead to higher efficiency in heat and mass transfer, condensation processes, self-cleaning, and waterproofing as compared to conventional surfaces.12–16 It is known that the hydrophobicity of graphene and other weak hydrophobic materials can be enhanced by increasing surface roughness.17,18 For example, Yang et al. reported enhancement of graphene surface roughness through nanoflowerstructuring, tuning the WCA of graphene up to 140°.19 Modulation of surface roughness can also be achieved by using pre-strained or thermoplastic substrates where the release of pre-strain or contractile deformation of thermoplastics induce crumpling of graphene.20–22 Besides graphene, molybdenum disulfide (MoS2), which is conventionally used as a solid lubricant,23 has been shown to have a 2D form analogous to graphene with even greater promise as a functional material for electronic devices and surface coatings.2,3 Motivated by atomicallythin MoS2’s potential, researchers have reported the wettability of nominally flat MoS2, such as the effect of hydrocarbon adsorption and layer number dependence.24,25 For example, the WCA of MoS2 has been established as ~ 90° in ambient, as hydrocarbon adsorption causes MoS2 to appear hydrophobic.25 However, the tunable wettability of atomically-thin MoS2 by the synthesis of vertically grown, nanoflower-structures or through crumpling has been relatively 3 ACS Paragon Plus Environment

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underexplored, despite its strong potential for fundamental research and device applications requiring superhydrophobic surfaces. Nanoflower-structured MoS2 has emerged as a key material system of interest, because of its potential applications as a catalyst for hydrogen evolution reactions (HER),26,27 electrodes for alkali metal-ion batteries,28,29 and field-emitters for flat panel displays.30 For example, Lu et al. reported that MoS2 nanoflowers exhibit enhanced hydrophobicity compared to flat MoS2, contributing the improvement of HER efficiency by rapidly driving off as-formed gas bubbles.31 However, the precise influence of the specific nanoflower morphology on wettability has yet to be investigated. Thus, a systematic study is required to fully understand the factors that impact the wettability of MoS2 nanoflowers, specifically how controlled synthesis of nanoflower morphologies at the nanoscale, and mechanical crumpling at the microscale can be used to tune the wettability. Understanding the tunable wettability of flat and three-dimensionally (3D) crumpled MoS2 nanoflower is crucial to fully exploit the potential of MoS2 in the aforementioned applications and MoS2-based tunable wettability coatings for desalination and hydrogen evolution processes. We demonstrate tunable wettability of MoS2 nanoflowers by multi-scale modulation of surface roughness through 1) tuning of nanoflower structures via metal-organic chemical vapour deposition (MOCVD) synthesis and 2) tuning of microscale crumples via mechanical deformation. Initially, MoS2 nanoflowers exhibit WCAs in range of 90-130° depending on morphology. Implementing crumples of MoS2 nanoflowers using thermoplastic or pre-strained substrates broadens the WCA tunability range to 80-155°, by providing multiscale-scale tunability at the micro and nanoscales. Furthermore, our multi-scale modulation allows the tuning of surface adhesion, represented by change of contact angle hysteresis (CAH). Such dual4 ACS Paragon Plus Environment

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scale, hierarchical patterning provides not only improved tunable wettability, but also superhydrophobicity.32,33 Finally, we demonstrate that tunability is preserved after multiple cycles of stretching and releasing of the underlying substrates. Our results will improve the knowledge of the wettability of MoS2, contributing the realization of the full potential of future MoS2-based applications. MoS2 nanoflowers were synthesized using MOCVD on SiO2/Si substrates up to the centimeter scale (Figure 1). Surface roughness and layer orientation can be modulated by varying synthesis pressure and time (Methods and Table S1 in Supporting Information). Five different types of nanoflowers were prepared (samples #1-5, Figure 1b). Samples #1-3 and #4-5 have layer orientation (θ) of 69.5±13.1° and 83.6±2.8° with respect to substrates, respectively (Figure 1a and Table S1), as characterized by transmission electron microscopy (TEM) (Figure S1). The surface roughness, characterized by atomic force microscopy (AFM), is in descending order within sample #1-3 and also within #4-5, where samples #1 (4) and 3 (5) possess the roughest and flattest surfaces in their respective groups. Figures 1c-d show their corresponding scanning electron microscopy (SEM) and AFM images. Raman spectroscopy characterizations show peaks at ~382 cm-1 and 409 cm-1 representing E12g and A1g modes, respectively (Figure S1a).34 In addition, X-ray photoelectron spectroscopy (XPS) characterizations for Mo(3d) and S(2p) regions are shown in Figure S1d-f. The Mo4+d5/2 and Mo4+d3/2 peaks in the Mo(3d) region and the 2p1/2 and 2p3/2 peaks in the S(2p) region further demonstrate the successful synthesis of MoS2 nanoflowers,24,35,36 with the stoichiometric ratio of Mo:S = 1:2.4. To systematically investigate the wettability of MoS2 nanoflowers as a function of surface roughness and layer orientation, the static WCAs of macroscopic and microscopic water droplets were measured using goniometry (Figure 2a) and environmental SEM (E-SEM, Figure 2b and 5 ACS Paragon Plus Environment

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S2), respectively. The WCAs of MoS2 nanoflowers measured by E-SEM are in the range of 90130°, slightly higher compared to those measured by goniometer. This can be attributed to the reduced gravitational effect on smaller, lower mass droplets and the continuous growth of water droplets in condensation mode, causing the drop to continuously advance during measurement resulting in an increased (advancing) WCA. As expected, rougher MoS2 nanoflowers (#1,4) exhibit higher WCAs compared to their flat counterparts (#3,5) (Figure 2c).17,18 Moreover, comparing samples #2 and 4 (which possess similar surface roughnesses), it was found that nanoflowers with perpendicular orientations (θ ~ 83.6±2.8°, sample #4) exhibit higher WCAs compared to tilted orientation nanoflowers (θ ~ 69.5±13.1°, sample #2). Another important measure of surface wettability is CAH, which represents the adhesion between water and a surface. The WCA values have a range between advancing (maximal) and receding (minimal) WCAs, arising from the molecular rearrangements at solid-liquid interfaces after they come into contact.37 Thus, CAH is calculated from the difference between the advancing and receding WCAs, which were measured using the dynamic sessile drop method: inflating and deflating the volume of droplets, respectively. Figure 2d compares the advancing and receding WCAs of all samples; in the roughness region of 1-10 nm, samples with rougher surfaces (e.g., #1) show higher hysteresis compared to flat samples (e.g., #3). This result is consistent with the theoretically simulated result of Johnson and Dettre, where CAH increases with increasing surface roughness in low roughness regions.38 In addition to the modulation of WCA via tuning the nanoscale growth morphology, another degree of structural tunability at the microscale was enabled by mechanically induced crumpling. We selected samples #1 and 3 (the roughest and flattest, respectively) to compare the effect of additional microscale structural tuning. To achieve crumples, as-synthesized MoS2 6 ACS Paragon Plus Environment

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nanoflowers were transferred onto polystyrene (PS) substrates (Figure 3a, top panel), using poly(methyl methacrylate) (PMMA) as a support film.39 Details of the transfer process are provided in Methods and Figure S3. Subsequently, thermally activated contractile transformation of PS was exploited by heating above the glass transition temperature of PS (~100 °C) to obtain additional microscale crumples of MoS2 nanoflowers (Figure 3a,b).21,40,41 The AFM (Figure 3d,f) and SEM images (Figure S4) show the development of crumples as a function of the macroscopic compressive strain in the PS. As a result, the WCAs of the dual-scale patterned MoS2 increase accordingly (Figure 3c,e,g) as the root-mean-square (RMS) roughness measured by AFM increases up to 250 nm (for sample #1) at 60% compressive strain (Figure S5). Here, the range of achievable WCAs is widened to 80-155°, resulting from the dual-scale modulation of surface roughness. Bare PS substrate without MoS2 nanoflowers does not show significant change of morphology and WCA upon crumpling (Figure S6), demonstrating that the wide tunability of WCAs results from crumpling of MoS2 nanoflowers. Furthermore, the CAH of crumpled sample #1 (the roughest) becomes smaller (~ 20°, Figure 3c) compared to the as-synthesized MoS2 nanoflowers without crumpling (~ 40°, Figure 2d). This demonstrates the tunable adhesion of water to MoS2 nanoflowers via mechanical crumpling. This reduced CAH is attributed to the crumpling induced enhanced density of micro and nanostructures,42 where liquid-air-solid composite configuration is energetically preferred (see Supporting Information).38 In contrast, sample #3, which is close to flat MoS2, shows negligible tunability of CAH. We note that both the as-synthesized and crumpled MoS2 nanoflowers exhibit droplet pinning (Figure S7), due to the petal effect in the Cassie impregnating wetting regime.43,44 This petal effect exhibits relatively high CAH compared to the lotus effect (CAH ~ 5°), which causes droplets to pin on the surface instead of sliding. 7 ACS Paragon Plus Environment

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Nevertheless, crumpled nanoflowers show reduced droplet adhesion compared to as-synthesized MoS2 nanoflowers. As a result, a droplet cannot be easily placed onto the crumpled nanoflower surface with a pipette (Supporting Information and Movie S1). Based on the aforementioned multi-scale tunability, we demonstrate that the wettability of our MoS2 nanoflowers can be dynamically modulated and is reversible by controlling the microscale crumples using a stretchable elastomer substrate. We transferred MoS2 nanoflowers onto 200% biaxially pre-strained Ecoflex substrates. As the pre-strain is released, compression and crumpling of the nanoflowers will result. Subsequent re-stretching of the EcoFlex substrates reverses crumpling of the nanoflowers, allowing the dynamic change of WCA of nanoflowers. We measured WCAs while applying and/or releasing biaxial strains up to 200% (Figure 4a and S8-9), both of which exhibit similar trends and values. A slight reduction (~ 5°) is observed between the WCA of an as-transferred MoS2 nanoflower and a sample fully re-stretched to 200% strain, i.e. until it has resumed a flat configuration. The WCAs of nanoflowers on Ecoflex show similar trends as the WCAs of crumpled nanoflowers on PS (Figure 3), where the WCA increases with the crumpling of MoS2 nanoflowers. We show representative droplet images and corresponding SEM images of crumpled nanoflowers at each strain value in Figure S10-12. The discrepancy in WCAs of MoS2 nanoflowers on Ecoflex and PS can be attributed to the different roughness and surface energy values of these substrates, which affect the roughness and quality of transferred nanoflowers. Finally, we demonstrate the robustness of the crumpled nanoflowers by applying up to 1,000 stretch and release cycles (Figure 4b). The maximum applied strain during cycling was set relatively low as 180%, with a higher applied pre-strain of 250%, to prevent potential damage to nanoflowers which occurs on full re-stretching. The measurement of static WCA was performed 8 ACS Paragon Plus Environment

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at the 180% strain segment of each stretch and release cycle. The static WCA values obtained during stretch/release cycles show negligible change compared to the initially measured WCA (within ± 3°) (Figure 4b and S13) indicating the robustness and consistency of strain-tunable wettability. Furthermore, the SEM images of nanoflowers after 1,000 cycles do not show observable damage (inset of Figure 4b), further confirming the durability of our crumpled nanoflowers. These results represent a significant step toward tunable wettability of MoS2 via multi-scale mechanical modulation of surface roughness. Although tunable wettability has been studied by changing surface roughness via chemical synthesis19 or crumpling20 with graphene, these studies have used only modulation of single-scale features (either micro or nanostructure), whereas multiscale structural hierarchy provides enhanced and stable control of wettability.32,42 Compared to the direct growth method on patterned substrates to realize different levels of hydrophobicity, mechanical crumpling allows reconfigurable modulation of the roughness as a function of the strain in substrates, and thus achieves reversible, dynamic, and facile control of the wettability using a variety of soft and stretchable substrates. Our study provides appropriate degree of hydrophobicity for various potential applications, such as waterproof electronic devices with superhydrophobicity (WCA > 150°), or medical applications with reduced hydrophobicity (WCA < 100°) for effective contact with biological substances.45-46 Furthermore, our crumpled MoS2 nanoflower can be used as tunable wettability coating for separation membranes and hydrogen evolution systems.47-49 Finally, our study expands the knowledge of tunable wettability of generic 2D materials, improving applications based on molybdenum diselenide (MoSe2),50 tungsten disulfide (WS2),51 and vanadium disulfide (VS2) monolayers.52


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In conclusion, we have demonstrated tunable wettability of MoS2 nanoflowers via modulation of multi-scale roughness. We controlled the nanoscale and microscale topographies of MoS2 nanoflowers using chemical synthesis and mechanical deformation, respectively. Multiscale modulation of surface roughness allowed for the achievement of tunable WCAs in the range of 80-155°. The crumpled MoS2 nanoflowers show reduced CAH compared to assynthesized nanoflowers, demonstrating the capability of tuning the adhesive force between water and nanoflowers. In addition, wettability of MoS2 nanoflowers could be dynamically modulated using a stretchable substrate and demonstrated robustness during multiple cycles of stretch and release. Our study will provide new insight into the design of rough surfaces and control of surface wettability, contributing the advancement of future devices utilizing MoS2based surfaces.


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Figure 1. Basic characterization of as-synthesized MoS2 nanoflowers. (a) Schematic illustration of as-synthesized MoS2 nanoflowers on SiO2/Si. (b) Photos of as-synthesized MoS2 nanoflower chips. Samples produced with longer synthesis time (samples #1-3) have tilted structures (θ = 69.5±13.1°) compared to those with shorter synthesis time (#4-5, θ = 83.6±2.8°) (Table S1). (c) SEM and (d) AFM images of as-synthesized MoS2 nanoflowers. Scale bars: 200 nm (c) and 1 µm (d). Values below (d) indicate the RMS roughness and tilting angles of each sample.


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Figure 2. Tunable wettability of MoS2 nanoflowers by manipulation of the nanostructure. (a) Macroscopic WCA measurement using goniometer. Scale bars: 500 µm. (b) Microscopic WCA measurement by water condensation using E-SEM. Scale bars: 5 µm. (c) Tunable static WCAs as measured by gonimeter and E-SEM at different roughness values. (d) Advancing/receding WCAs of MoS2 nanoflowers depending on the nanostructure.


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Figure 3. Tunable wettability of MoS2 nanoflowers by manipulation of the microstructure. (a) Photos of as-transferred (upper panel) and crumpled (lower panel) MoS2 nanoflowers on a PS substrate. Scale bars: 1 cm. (b) Schematic illustrations (upper panel) and SEM images (lower panel) of crumpled MoS2 nanoflowers via thermal shrinkage of a PS substrate. Scale bars: 500 nm (lower left) and 100 nm (lower right). (c) Tunable advancing/static/receding WCAs of MoS2 nanoflowers as a function of macroscopic compressive strain. Triangles denote the #1 13 ACS Paragon Plus Environment

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nanoflowers (the roughest), whereas circles represent the #3 nanoflowers (the flattest). AFM height (d) and droplet images (e) of #1 nanoflower sample at each compressive strain. Scale bars: 1 µm (d) and 500 µm (e). AFM height (f) and droplet images (g) of #3 nanoflower sample at each compressive strain. Scale bars: 1 µm (f) and 500 µm (g).


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Figure 4. Dynamic measurement of wettability as a function of substrate stretching. (a) Wettability of sample #1 on Ecoflex as a function of biaxial tensile strain. The sample was transferred onto a 200% pre-strained Ecoflex, followed by releasing the pre-strain to induce crumpling. Measurement was performed while the biaxial tensile strain was applied. Inset: SEM images of sample #1 on Ecoflex at crumpled (left) and stretched (right) states. Scale bars: 5 µm. (b) Preservation of wettability during 1,000 cyclic biaxial stretching and releasing. Inset: SEM images of 180% re-stretched sample #1 on Ecoflex after first cycle (left) and 1,000 cycles (right). Scale bars: 5 µm.


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ASSOCIATED CONTENT Supporting Information. Methods and supporting ﬁgures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by Industrial Technology Innovation Program funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) (No.10062161, Direct low-temperature synthesis of two-dimensional materials and heterostructure on flexible substrate for next-generation highmobility electronic devices). This work was also supported by the Air Force Office of Scientific Research/Asian Office of Aerospace Research Development (AFOSR/AOARD) Nano Bio Info Technology (NBIT) Phase III Program (AOARD-13-4125), and the Early Career Faculty grant


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(NNX16AR56G) from NASA’s Space Technology Research Grants Program. Experiments were carried out in part in the Frederick Seitz Materials Research Laboratory Central Research Facilities, Micro and Nano Technology Laboratory, and the Beckman Institute Imaging Technology Group at the University of Illinois at Urbana−Champaign. The authors acknowledge insightful discussion with S. Robinson and S. MacLaren.


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