Relationship between Hydrogen Evolution and Wettability for

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Relationship between hydrogen evolution and wettability for multiscale hierarchical wrinkles Woo-Bin Jung, Geun-Tae Yun, Yesol Kim, Minki Kim, and Hee-Tae Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19828 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Materials & Interfaces

Relationship between Hydrogen Evolution and Wettability for Multiscale Hierarchical Wrinkles

Woo-Bin Jung1, Geun-Tae Yun1, Yesol Kim1, Minki Kim1 and Hee-Tae Jung*,1

1National

Laboratory for Organic Opto-Electronic Materials, Department of Chemical and

Biomolecular Engineering (BK-21 plus), Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea

*Corresponding Author: [email protected]

Keywords: Transition metal dichalcogenides, hierarchical wrinkles, hydrogen evolution reaction, receding contact angle, MoS2, WS2

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Abstract Transition-metal dichalcogenides (TMD) are emerging 2D materials with potential use for the hydrogen evolution reaction (HER) because they express a desired binding energy with protons. To date, TMD-based HER catalytic performance has been enhanced mostly by chemical modification, such as introducing defects, doping, and phase control. Herein, we enhanced HER performance by precise control of wettability via hierarchical wrinkling. This hierarchical wrinkling confers tunability of the receding contact angle (2 ° ~ 30 °) by controlling the wavelength of the hierarchical wrinkles. Minimization of the receding contact angle is directly related to overpotential reduction on the MoS2 wrinkles through gas detachment from the catalytic surface. Unlike in previous studies, in this work we demonstrated the effect of wettability only without changing other parameters such as surface chemistry. We showed that our method can be applied to other TMD materials such as WS2. This study will contribute to future TMD-based catalyst applications, such as hydrogen evolution, CO2 reduction, and oxygen evolution.


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Introduction Transition-metal dichalcogenides (TMDs) have attracted much attention because of their superior properties, including inherent semiconduction, high catalytic performance, and highintensity photoluminescence1-4. Among the applications of these materials, the hydrogen evolution reaction (HER) using TMDs is of particular interest because of its provision of a proper binding energy with protons5-7. Two different approaches have been suggested to enhance the HER performance of TMDs: (1) control of the binding energy of the proton and HER catalyst and (2) structural and morphological control. Structural and morphological control is important because of the high surface area and wettability of TMDs. For example, the flower-shaped MoS2 nanostructure significantly enhanced HER performance by effective gas detachment, which is directly related to surface wettability8. In particular, the receding contact angle on the catalyst surface affected catalytic performance because the large receding angle generated large gas bubbles, which inhibited the catalytic reaction in dead spaces9. The fabrication of catalyst structures that express well-controlled wettability is one of the key factors that enhance their HER performance10-12. Wrinkling is one of the most effective ways to control the wettability of film-type materials because of its precisely controllable morphologies and ease of fabrication on large areas13, 14. A wrinkled structure is generated on shrink film or pre-strained film by strain relief, where wrinkles are formed by the instability of the skin layer and substrate15. To date, the precise effect of the wettability of MoS2 wrinkled structures on catalytic performance has not yet been investigated. Thus, a systematic study is required to fully understand the factors that impact the receding contact angle of TMDs, specifically, to understand how the wrinkled structure can tune the receding contact angle. Understanding the tunable receding contact angle of hierarchical wrinkles is crucial when attempting to fully exploit hydrogen evolution catalysis using wrinkled TMDs.



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In this study, we fabricated hierarchical MoS2 wrinkled structures with precise control of the wrinkle dimensions. Changes in the hierarchical wrinkle wavelength were found to control the wettability of MoS2, especially the receding contact angle, which is critical for gas detachment during hydrogen evolution. By controlling the degree and direction of strain during shrinking, we tuned the contact angle from 60° to 130° and achieved anisotropic wetting. Notably, the hierarchical MoS2 wrinkle expressed a reduced receding contact angle relative to that of a single wrinkle, which induced faster detachment of gas bubbles and resulted in enhanced hydrogen evolution performance compared with flat MoS2. Hierarchical wrinkles with controlled receding angles showed an overpotential reduction of 60 mV compared with that of primary wrinkles. Additionally, we demonstrated that our method can be used for another TMDs such as WS2.

Results and discussion Figure 1a depicts the process of generating hierarchical MoS2 wrinkles. The synthesized MoS2 film was transferred onto a freestanding polystyrene (PS) substrate using poly(methyl methacrylate) as a support film16, 17. Subsequently, the prepared MoS2 on a PS substrate was heated to above the glass transition temperature of PS (120C) to shrink the PS substrate and to generate both primary and hierarchical wrinkles on MoS2. In this process, the areal strain was controlled by changing the heating time in the oven. The areal strain was calculated by ε = (A0 − Af)/A0, where A0 and Af are, respectively, the area before and after strain 13

relief. According to the equation of a wrinkle (𝜆 ≈ 2𝜋ℎ(𝐸𝑠 3𝐸𝑏)

, where 𝜆 is the

wavelength of the wrinkle and h is the skin thickness. Es and Eb are the respective moduli of the skin layer and bulk substrate), the wavelength of the V-MoS2 primary wrinkle at the first shrinking step is 1.5 μm at h1 = 38.6 ± 0.35 nm (Figure S1). After generation of the primary


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wrinkle, polyvinylpyrrolidone (PVP, Mw = 360,000 g/mol, Tg = 150°C) dissolved in ethanol was coated onto the primary MoS2 wrinkle with a primary strain, ε1, of 0.50. The PVP-coated MoS2 wrinkle was then shrunk until a secondary strain, ε2, of 0.50 was reached (εtotal = 0.75 from the first state) by heating above 130C. This resulted in a wrinkle structure with a larger wavelength due to the PVP layer that formed while maintaining the primary wrinkle. Finally, hierarchical MoS2 wrinkle structures composed of first (G1) and second (G2) wrinkles were obtained after rinsing of the PVP layer with ethanol. Although, in many previous works, hierarchical wrinkles have been reported, the wavelengths of first and second wrinkle could not be controlled independently.18-21 The hierarchical wrinkles from our method had two different wavelengths, which were controlled independently by changing the thicknesses of the MoS2 (h1) and PVP layers (h2). High-resolution transmission electron microscopy (HRTEM) images in Figure 1b show the atomic structures of each synthesized MoS2 film, where the side view of the stacked MoS2 layers shows a 0.63 ± 0.04 nm inter-layer distance. This vertical MoS2 (V-MoS2) has more active edge sites compared with planar MoS2 (P-MoS2), where vertical and planar MoS2 is vertically and planarly aligned MoS2 layers with respect to the film. The Raman spectrum in Figure 1c displays two typical peaks for V-MoS2 at 383.01 and 407.17 cm−1.22 Figure 1d presents representative SEM and magnified images of the fabricated hierarchical wrinkles, which indicate that the hierarchical wrinkles are composed of both G1 (first generation) and G2 (second generation) wrinkles with different wavelengths. The yellow line in the SEM images indicates G2 wrinkles that have a similar shape and larger wavelength compared with those of the G1 wrinkle.



Figure 1. Hierarchical wrinkle of vertical MoS2 (V-MoS2). (a) Process for fabricating hierarchical V-MoS2 wrinkles. (b) HR-TEM image of V-MoS2 shows side view of stacked MoS2 layers. (c) Raman shifts of V-MoS2 show two typical peaks: E2g1 (in-plane vibration) and A1g (out-of-plane vibration) (d) SEM images of hierarchical V-MoS2 wrinkles has two different wavelengths of G1 (right red lines) and G2 (left orange lines) wrinkles.

Before analyzing the effect of hierarchical wrinkles on wettability, we prepared both P-MoS2 and V-MoS2 wrinkled structures (Figure S1) by controlling the Mo thickness to verify the effect of h1 on the contact angle. Figure S2a and S2b show SEM images of strain-controlled wrinkles of P-MoS2 and V-MoS2. As the strain increases, the amplitude of the P-MoS2 wrinkle increases while the wavelength is at ~200 nm. Simultaneously, the static contact angle (θW) can be tuned by controlling the strain. Here, θW on the P-MoS2 wrinkle increased from 68° to 94° as the strain increased from 0 to 0.75 (Figure S2c). On the other hand, Figure S2d shows


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that θW of V-MoS2 widened in the range of 59°–128°. These results indicate that θW is tuned by the strain and thickness of MoS2. Furthermore, the contact angle hysteresis (θH) increased as θW increased with the strain, confirming the presence of the Wenzel state over the entire strain regime (Figure S2c and S2d). θH is calculated by the following the equation: θH = advancing contact angle (θA) − receding contact angle (θR). This means that the tunable adhesion of water to MoS2 wrinkles occurs via mechanical wrinkling. The increase in θH is attributed to the enhanced surface area, where the liquid–air–solid composite configuration is energetically preferred in the Wenzel state among wetting models (ref). Here, θR decreases from 26.5° to 12° as the strain increases; hence, we hypothesize that hierarchical wrinkling can reduce the receding contact angle by a greater amount because of the larger surface area. To precisely control the receding contact angle by changing the wavelength of the hierarchical wrinkles, we used different concentrations of the PVP solution (CPVP) from 0 wt% to 5 wt%. The hierarchical wrinkles had two different wavelengths (G1 and G2), which were controlled independently by h1 and h2. Figure 2a shows SEM images of the fabricated hierarchical wrinkles with different PVP concentrations (CPVP) for h1 = 38.6 ± 0.35 nm. The yellow lines in the SEM images indicate G2 wrinkles that have a similar shape to but a different wavelength from those of a G1 wrinkle. With control of CPVP from 0 wt% to 5 wt%, G2 wrinkles appeared at 3 wt%, and the G2 wavelength increased from 5 μm to ~10 μm as CPVP increased from 3 wt% to 5 wt%. A critical CPVP exists for generating ness for embedding the existing wrinkled structures16.

hierarchical wrinkles with controlled wavelengths, which is

due to insufficient PVP thick We found that the transition of primary wrinkles to hierarchical wrinkles reduced the receding contract angle. Figure 2b and 2c show the receding contact angle as function of CPVP. The resulting θR of the hierarchical wrinkles are tunable by control of CPVP. For the primary wrinkles, the receding contact angles were unchanged at around 12°. On the other hand, the



receding contact angle decreased from 14° to 6° as CPVP increased from 2 wt% to 5 wt%, which is the range found for hierarchical wrinkles. At CPVP = 2 wt%, hierarchical wrinkles with a G2 wavelength of 5 μm generated an impregnated Cassie state, which resulted in an increase of the receding contact angle. After this point, the receding contact angle began to decrease as CPVP increased because of the wetting transition from the impregnated Cassie state to the Wenzel state. In general, the increase of the pattern pitch induces the change from Cassie state to Wenzel state, so the solid/liquid fraction (fSL) increases simultaneously. Figure 2d shows a schematic of the relationship between the receding contact angle and gas detachment from the surface. During the HER, hydrogen gas is generated by the catalyst and forms gas bubbles on the surface of the MoS2. We can regard the air–liquid–solid interface of the gas bubble as the receding state of the liquid. When a large receding contact angle is present, indicating that the adhesion force between the liquid and solid is weak, the gas bubble grows to a large size as the air–solid contact area increases. On the other hand, when the receding contact angle is small, the contact angle at the air–liquid–solid interface becomes smaller as the gas bubble grows on the surface. Thus, the adhesive contact area is too small, and a small gas bubble detaches. Since the grown gas bubble on the catalytic surface is dead space for the catalytic reaction, fast detachment of gas bubble aids the use of entire catalytic surfaces.


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Figure 2. Tunable wettability of hierarchical MoS2 wrinkles. (a) SEM images of primary wrinkles (CPVP = 0, 1 and 2 wt%) and hierarchical wrinkles (CPVP = 3, 4 and 5 wt%) (b) Dependence of the receding contact angle on CPVP. (c) Optical images of the receding contact angles on different wrinkle morphologies. (d) Gas detachment from the different wetting surfaces. During hydrogen evolution, hydrogen gas bubble grows up on the catalyst surface. Three-phase interface of electrolyte, catalyst and hydrogen gas bubble is similar to receding wetting behavior. In large receding contact angle, gas bubble can grow up easily. In small receding contact angle, gas bubble cannot grow up and rapidly detached with small bubble size. To verify the relationship between the receding contact angle and HER, we compared hydrogen evolution on MoS2 hierarchical wrinkles with different G2 wavelengths. Figure 3a shows the controlled G2 wavelength as a function of CPVP, which differs from the results shown in Figure 2 because the MoS2 wrinkles were fabricated on a gold layer and used as electrodes for hydrogen evolution. Because of this gold layer, the G1 and G2 wavelengths increased relative to those of the wrinkles composed of only MoS2. The G2 wavelength of a MoS2–Au wrinkle increased from 5.8 μm to 14.6 μm as CPVP increased from 3 wt% to 15 wt%. The inset SEM images show the resulting hierarchical wrinkling, where G1 and G2 wrinkles coexist while possessing different wavelengths. To deconvolute the effect of microscale wrinkles on the receding contact angle, we tested primary MoS2 wrinkles with a tunable microscale wavelength, which were fabricated by coating PVP onto flat MoS2 before shrinking. Figure S3a shows that the wavelength of a MoS2 wrinkle increases from 19.5 μm to 25.5 μm as CPVP increases from 1 wt% to 15 wt%. Figure



S3b and S3c suggest that the performance in terms of the overpotential and Tafel slope degrade as the wavelength increases. As a result, both the receding contact angle and overpotential are proportional to the wavelength, which means that only microscale wrinkles without hierarchical structuring cannot reduce the receding contact angle, resulting in a degraded HER performance. On the other hand, hierarchical MoS2 wrinkles exhibited a greatly enhanced performance with a lower over-potential than that of flat and primary MoS2 wrinkles (Figure S4 and Figure 3b). Notably, hierarchical MoS2 wrinkles fabricated with a CPVP of 5 wt% showed the most enhanced HER performance (blue line), with an overpotential of 225 mV at −10 mA/cm2, which is lower than that for primary MoS2 wrinkles (282 mV, black line). The corresponding Tafel plots show that the hierarchical wrinkles have a lower Tafel slope than primary wrinkles (Figure 3c). More specifically, hierarchical MoS2 wrinkling decreases the Tafel slope from 102 mV/dec for primary wrinkling to 66.9 mV/dec with the use of 5 wt% CPVP. Plotting the Tafel slopes is a useful method for evaluating the rate-limiting mechanism of the HER. In an acidic electrolyte, a three-step mechanism has been suggested. These steps involve a primary discharge step (Volmer reaction),23 𝐻3𝑂 + + 𝑒 ― → 𝐻𝑎𝑑𝑠 + 𝐻2𝑂 (1) followed by a desorption step (Heyrovsky reaction), 𝐻𝑎𝑑𝑠 + 𝐻3𝑂 + + 𝑒 ― → 𝐻2 + 𝐻2𝑂 (2) or a recombination step (Tafel reaction), 𝐻𝑎𝑑𝑠 + 𝐻𝑎𝑑𝑠→ 𝐻2 (3) The rate-limiting mechanism is determined by the surface coverage of the adsorbed hydrogen, Hads, on the catalyst surface. In the case of high surface coverage, respective Tafel slopes of 30 mV/dec or 40 mV/dec indicate whether the Heyrovsky or Tafel mechanism is dominant, thereby acting as the rate-limiting step. In the case of low coverage, the Volmer mechanism is


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dominant and the Tafel slope is 120 mV/dec. In the case of primary MoS2 wrinkles with a Tafel slope of 102 mV/dec, the dominant mechanism is the Volmer reaction (equation 1), and the primary discharge step is the rate-limiting step23. Hierarchical MoS2 wrinkling with a Tafel slope of 66.9 mV/dec was a result of a Volmer–Heyrovsky (equations 1 and 2) HER mechanism. These higher Tafel slopes compared with Pt/C (30 mV/dec)24 were attributed to a low Hads on the MoS2 surfaces. From these results, the hierarchical wrinkles showed a better HER performance than did the primary wrinkles, and different G2 wavelengths resulted in different HER performances. To directly compare the receding contact angle and HER performance, we plotted the overpotentials at −10 mA/cm2 against the G2 wavelength (Figure 3d). The receding contact angle decreases from 17° for primary wrinkling to 5° for hierarchical wrinkling with a G2 wavelength of 8 μm. When the G2 wavelength was increased to more than 8 μm, the receding contact angle increased to over 15°. Interestingly, the overpotential was observed to follow the tendency of the receding contact angle, and the smallest overpotential was obtained at a G2 wavelength of 8 μm. Because there is only a G2 wavelength difference without any difference in the catalyst density and materials, we can conclude that the receding contact angle can affect the HER performance.



Figure 3. HER performance on MoS2 hierarchical wrinkles. (a) The G2 wavelength is tunable depending on CPVP. (b) LSV curves and (c) Tafel slopes with varying CPVP from 0 wt% (only primary wrinkle) to 15 wt%. (d) Relationship between the overpotential and receding contact angle for tunable G2 wavelengths.

To verify that our method is not limited to MoS2, we examined the HER performance of WS2 with hierarchical wrinkling. Figure 4 shows that hierarchical WS2 wrinkles can be fabricated using the same method; the overpotential was observed to decrease on the surface with a low receding contact angle. Figure 4a shows a scheme for generating primary and hierarchical WS2 wrinkles by the same method used for MoS2 wrinkles. Here, WS2 was synthesized by the chemical vapor deposition method. Figure 4b shows the representative two Raman shift peaks for WS2 film (E2g1 = 354.4 cm−1 and A1g = 419.1 cm−1)25. SEM images in Figure 4c show primary and hierarchical wrinkling with different CPVP values, which resulted in a controlled G2 wavelength. Whereas the primary WS2 wrinkling achieved without a PVP coating consisted of only G1 wrinkles, G2 wrinkles appeared from CPVP = 3 wt% and the G2 wavelength increased up to 10 μm. Figure 4d–4f show the results of the HER, where the overpotential of hierarchical wrinkling decreased relative to that of primary wrinkling (CPVP = 0 wt%); the Tafel


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slope also decreased from 122 mV/dec to 70 mV/dec. From the observed enhanced performance due to WS2 hierarchical wrinkling, we can correlate the overpotential with the receding contact angle on WS2. The receding contact angle decreased from 17° for primary wrinkling to 7° for hierarchical wrinkling with 6.5 µm G2 wavelength. The minimum recorded overpotential for the receding contact angle was at a G2 wavelength of 6.5 μm, and it increased after this point. From this result, the relationship between the receding contact angle and overpotential was verified for a material other than MoS2. Therefore, we concluded that hierarchical wrinkling may be used to enhance the HER performance of various materials by reducing the receding contact angle.

Figure 4. Enhanced HER performance for WS2 hierarchical wrinkles. (a) Process of achieving WS2 hierarchical wrinkles. (b) Typical Raman shifts of WS2. (c) SEM images of WS2 hierarchical wrinkles with a tunable G2 wavelength. (d) LSV curves and (e) Tafel slopes for each sample. (f) The same trend for the overpotential and receding contact angles with a controlled G2 wavelength was observed as for MoS2. In conclusion, we have demonstrated the tunable wettability of TMDs (MoS2 and WS2) via the modulation of strain and hierarchical wrinkling wavelength to achieve effective hydrogen gas



detachment during HER. We controlled the wavelength of the nanoscale and microscale wrinkles independently using a sacrificial PVP layer. Multiscale modulation of the hierarchical wrinkles allowed tuning of receding contact angle in the range of 2–30°. In terms of the HER, hierarchical wrinkling with a lower receding contact angle showed better HER performance than did flat and primary wrinkling because of faster hydrogen gas detachment from the catalyst surface. Our study will provide new insights into the use of multiscale wrinkling and the control of surface wettability, contributing to the advancement of catalysts utilizing filmtype materials.

Methods Preparation of MoS2 and WS2. MoS2 and WS2 films with different atomic structures with different thicknesses of Mo and W were synthesized by chemical vapor deposition (CVD). Mo and W were deposited on a SiO2/Si wafer by e-beam evaporation. Mo and W films were loaded in the furnace for sulfurization. Sulfur (purchased from Sigma-Aldrich) was loaded in the heating zone at 200 °C, which was before the Mo and W films heating zone (770 °C). The quartz tube was evacuated to 50 mTorr and then purged with argon (Ar) at 50 sccm for 30 min. For the heating process to grow MoS2 and WS2, Mo and Wfilms were heated to 770 °C for 30 min and maintained at this temperature for 15 min. After the growth of MoS2 and WS2, the furnace tube was rapidly cooled to room temperature. The final MoS2 and WS2 atomic structures depended on the Mo and W thickness: planar WS2 and MoS2 from 2 nm thickness and vertical MoS2 from 15 nm thicknesses. Fabrication of wrinkles. To form wrinkles of MoS2 and WS2 on the polystyrene (PS) substrate, these 2D materials were transferred onto PS by a PMMA-assisted method. The TMD-PS bi-layered film was heated at a temperature greater than 135 C for different periods


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to control the amount of strain. After strain relief, the TMD-PS bi-layered film was rapidly cooled to room temperature. To calculate the strain of the PS film, the areas of a drawn box were measured, and the strain was determined using the equation ε = (A0 − Af)/A0. The strain was controlled up to 0.80 by control of the heating time in the oven. Characterization. Scanning electron microscopy (SEM; S-4800, Hitachi) images were recorded to observe the morphology of the wrinkled structure. The feature dimension of the hierarchical wrinkles was determined from the average measurement of SEM and AFM images with using image processing program (imageJ and XEI program of Park systems). For quantitative measurement of the wavelength, fast Fourier transformation (FFT) was performed to AFM images and the wavelength was calculated. Especially, for the G2 wavelength of hierarchical wrinkle, the SEM images have to be processed by ImageJ program. At first, using ImageJ program, line profile across the several waves of G2 wrinkles in SEM image can be plotted and the wavelength (valley to valley) can be measured in the graph. Finally, we measured several valley-to-valley distances and calculated average G2 wavelength. Raman spectroscopy (Aramis, Horiba Jobin Yvon) was used to detect the peaks typical of WS2 and MoS2. The spectrum was obtained by exposure of each film to a 514 nm laser beam. Electrochemical experiments. To verify the advantages of the wrinkles as a catalyst, the catalytic activity of MoS2 wrinkles in hydrogen evolution was examined. The reaction was performed in a custom-made three-electrode electrochemical cell. MoS2, platinum (Pt) wire, and a Ag/AgCl electrode were respectively used as the working, counter, and reference electrodes. To prepare MoS2 and WS2 wrinkles for the working electrode, MoS2 and WS2 wrinkles on a stainless-steel plate was prepared using a silver paste that was passivated by polymers. Electrolytes were prepared using 0.5 M H2SO4 Solution. Cyclic voltammetry curves were recorded between 0 V and −0.8 V versus reversible hydrogen electrode (RHE) at a scan rate of 5 mV/sec.



Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and future Planning, Korea (MSIP, NRF2018R1A2B3008658) and the KAIST GCORE(Global Center for Open Research with Enterprise) grant funded by the Ministry of Science and ICT (N11180214). Associated content Supporting Information available: The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxx Details about experimental procedures. Figure S1-S7, reporting synthesis of materials and supporting experimental results of prepared primary and hierarchical wrinkles. References 1. Baugher, B. W.; Churchill, H. O.; Yang, Y.; Jarillo-Herrero, P., Intrinsic electronic transport properties of high-quality monolayer and bilayer MoS2. Nano Lett 2013, 13 (9), 4212-6. 2. Abbasi, P.; Asadi, M.; Liu, C.; Sharifi-Asl, S.; Sayahpour, B.; Behranginia, A.; Zapol, P.; Shahbazian-Yassar, R.; Curtiss, L. A.; Salehi-Khojin, A., Tailoring the Edge Structure of Molybdenum Disulfide toward Electrocatalytic Reduction of Carbon Dioxide. ACS Nano 2017, 11 (1), 453-460. 3. Chou, S. S.; Sai, N.; Lu, P.; Coker, E. N.; Liu, S.; Artyushkova, K.; Luk, T. S.; Kaehr, B.; Brinker, C. J., Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide. Nat Commun 2015, 6, 8311. 4. Kang, D. H.; Pae, S. R.; Shim, J.; Yoo, G.; Jeon, J.; Leem, J. W.; Yu, J. S.; Lee, S.; Shin, B.; Park, J. H., An Ultrahigh-Performance Photodetector based on a Perovskite-Transition-Metal-Dichalcogenide Hybrid Structure. Adv Mater 2016, 28 (35), 7799-806. 5. Behranginia, A.; Asadi, M.; Liu, C.; Yasaei, P.; Kumar, B.; Phillips, P.; Foroozan, T.; Waranius, J. C.; Kim, K.; Abiade, J.; Klie, R. F.; Curtiss, L. A.; SalehiKhojin, A., Highly Efficient Hydrogen Evolution Reaction Using Crystalline Layered ThreeDimensional Molybdenum Disulfides Grown on Graphene Film. Chemistry of Materials 2016, 28 (2), 549-555. 6. Chua, X. J.; Luxa, J.; Eng, A. Y. S.; Tan, S. M.; Sofer, Z.; Pumera, M., Negative Electrocatalytic Effects of p-Doping Niobium and Tantalum on MoS2 and WS2 for the Hydrogen Evolution Reaction and Oxygen Reduction Reaction. ACS Catalysis 2016, 6 (9), 5724-5734. 7. Lee, S. C.; Benck, J. D.; Tsai, C.; Park, J.; Koh, A. L.; Abild-Pedersen, F.; Jaramillo, T. F.; Sinclair, R., Chemical and Phase Evolution of Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production. ACS Nano 2016, 10 (1), 624-32. 8. 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" MoS(2) nanostructured electrodes. Adv Mater 2014, 26 (17), 2683-7, 2615.


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9. Kim, J. Y.; Lim, J.; Jin, H. M.; Kim, B. H.; Jeong, S. J.; Choi, D. S.; Li, D. J.; Kim, S. O., 3D Tailored Crumpling of Block-Copolymer Lithography on Chemically Modified Graphene. Adv Mater 2016, 28 (8), 1591-6. 10. Choi, J.; Mun, J.; Wang, M. C.; Ashraf, A.; Kang, S. W.; Nam, S., Hierarchical, Dual-Scale Structures of Atomically Thin MoS2 for Tunable Wetting. Nano Lett 2017, 17 (3), 1756-1761. 11. Bhimanapati, G. R.; Hankins, T.; Lei, Y.; Vila, R. A.; Fuller, I.; Terrones, M.; Robinson, J. A., Growth and Tunable Surface Wettability of Vertical MoS2 Layers for Improved Hydrogen Evolution Reactions. ACS Appl Mater Interfaces 2016, 8 (34), 22190-5. 12. Tao, L.; Duan, X.; Wang, C.; Duan, X.; Wang, S., Plasma-engineered MoS2 thin-film as an efficient electrocatalyst for hydrogen evolution reaction. Chem Commun (Camb) 2015, 51 (35), 7470-3. 13. Ahmed, S. F.; Rho, G.-H.; Lee, K.-R.; Vaziri, A.; Moon, M.-W., High aspect ratio wrinkles on a soft polymer. Soft Matter 2010, 6 (22). 14. Chen, Y. C.; Crosby, A. J., High aspect ratio wrinkles via substrate prestretch. Adv Mater 2014, 26 (32), 5626-31. 15. Budday, S.; Andres, S.; Walter, B.; Steinmann, P.; Kuhl, E., Wrinkling instabilities in soft bilayered systems. Philos Trans A Math Phys Eng Sci 2017, 375 (2093). 16. Jung, W. B.; Cho, K. M.; Lee, W. K.; Odom, T. W.; Jung, H. T., Universal Method for Creating Hierarchical Wrinkles on Thin-Film Surfaces. ACS Appl Mater Interfaces 2018, 10 (1), 1347-1355. 17. Jiao, L.; Fan, B.; Xian, X.; Wu, Z.; Zhang, J.; LIu, Z., Creation of Nanostructures with Poly(methyl methacrylate)-Mediated Nanotransfer Printing. J Am Chem Soc 2008, 130, 12612-12613. 18. Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.; Genzer, J., Nested self-similar wrinkling patterns in skins. Nat Mater 2005, 4 (4), 293-7. 19. Vandeparre, H.; Gabriele, S.; Brau, F.; Gay, C.; Parker, K. K.; Damman, P., Hierarchical wrinkling patterns. Soft Matter 2010, 6 (22). 20. Lin, G.; Chandrasekaran, P.; Lv, C.; Zhang, Q.; Tang, Y.; Han, L.; Yin, J., Self-similar Hierarchical Wrinkles as a Potential Multifunctional Smart Window with Simultaneously Tunable Transparency, Structural Color, and Droplet Transport. ACS Appl Mater Interfaces 2017, 9 (31), 26510-26517. 21. Lin, G.; Zhang, Q.; Lv, C.; Tang, Y.; Yin, J., Small degree of anisotropic wetting on self-similar hierarchical wrinkled surfaces. Soft Matter 2018, 14 (9), 1517-1529. 22. Feng, Y.; Zhang, K.; Wang, F.; Liu, Z.; Fang, M.; Cao, R.; Miao, Y.; Yang, Z.; Mi, W.; Han, Y.; Song, Z.; Wong, H. S., Synthesis of Large-Area Highly Crystalline Monolayer Molybdenum Disulfide with Tunable Grain Size in a H2 Atmosphere. ACS Appl Mater Interfaces 2015, 7 (40), 22587-93. 23. Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H., MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J Am Chem Soc 2011, 133 (19), 7296-9. 24. Ma, L.; Ting, L. R. L.; Molinari, V.; Giordano, C.; Yeo, B. S., Efficient hydrogen evolution reaction catalyzed by molybdenum carbide and molybdenum nitride nanocatalysts synthesized via the urea glass route. Journal of Materials Chemistry A 2015, 3 (16), 83618368. 25. Zhao, X.; Ma, X.; Sun, J.; Li, D.; Yang, X., Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS(2) Nanodots for Hydrogen Evolution. ACS Nano 2016, 10 (2), 2159-66.



ToC graphics


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Figure 1. Hierarchical wrinkle of vertical MoS2 (V-MoS2). (a) Process for fabricating hierarchical V-MoS2 wrinkles. (b) HR-TEM image of V-MoS2 shows side view of stacked MoS2 layers. (c) Raman shifts of V-MoS2 show two typical peaks: E2g1 (in-plane vibration) and A1g (out-of-plane vibration) (d) SEM images of hierarchical V-MoS2 wrinkles has two different wavelengths of G1 (right red lines) and G2 (left orange lines) wrinkles.



Figure 2. Tunable wettability of hierarchical MoS2 wrinkles. (a) SEM images of primary wrinkles (CPVP = 0, 1 and 2 wt%) and hierarchical wrinkles (CPVP = 3, 4 and 5 wt%) (b) Dependence of the receding contact angle on CPVP. (c) Optical images of the receding contact angles on different wrinkle morphologies. (d) Gas detachment from the different wetting surfaces. During hydrogen evolution, hydrogen gas bubble grows up on the catalyst surface. Three-phase interface of electrolyte, catalyst and hydrogen gas bubble is similar to receding wetting behavior. In large receding contact angle, gas bubble can grow up easily. In small receding contact angle, gas bubble cannot grow up and rapidly detached with small bubble size.


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Figure 3. HER performance on MoS2 hierarchical wrinkles. (a) The G2 wavelength is tunable depending on CPVP. (b) LSV curves and (c) Tafel slopes with varying CPVP from 0 wt% (only primary wrinkle) to 15 wt%. (d) Relationship between the overpotential and receding contact angle for tunable G2 wavelengths.



Figure 4. Enhanced HER performance for WS2 hierarchical wrinkles. (a) Process of achieving WS2 hierarchical wrinkles. (b) Typical Raman shifts of WS2. (c) SEM images of WS2 hierarchical wrinkles with a tunable G2 wavelength. (d) LSV curves and (e) Tafel slopes for each sample. (f) The same trend for the overpotential and receding contact angles with a controlled G2 wavelength was observed as for MoS2.


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Relationship between Hydrogen Evolution and Wettability for

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