Ultrathin Alumina Mask-Assisted Nanopore Patterning on Monolayer

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Ultra-thin Alumina Masks Assisted Nanopore Patterning on Monolayer MoS2 for Highly Catalytic Efficiency in Hydrogen Evolution Reaction Shaoqiang Su, Qingwei Zhou, Zhiqiang Zeng, Die Hu, Xin Wang, Mingliang Jin, Xingsen Gao, Richard Nötze, Guofu Zhou, Zhang Zhang, and Jun-Ming Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19197 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Ultra-thin Alumina Masks Assisted Nanopore Patterning on Monolayer MoS2 for Highly Catalytic Efficiency in Hydrogen Evolution Reaction Shaoqiang Su,

†‡∥

Qingwei Zhou,

⊥∥

‡

†‡

Zhiqiang Zeng, Die Hu, Xin Wang,

†§

Mingliang Jin,

†§

Xingsen Gao, †‡ Richard Nötze, †§ Guofu Zhou, †§¶ Zhang Zhang,* †‡Junming Liu⊥‡

† National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. ‡Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. §

Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. ⊥

Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

¶

Shenzhen Guohua Optoelectronics Tech. Co. Ltd., Shenzhen 518110, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Nanostructured molybdenum disulfide (MoS2) has been considered as one of the most promising catalysts in the hydrogen evolution reaction (HER), for its approximately intermediate hydrogen binding free energy to noble metals and much lower cost. The catalytically active sites of MoS2 are along the edges, while thermodynamically MoS2 favors the presence of a twodimensional (2-D) basal plane and the catalytically active atoms only constitute a small portion of the material. The lack of catalytically active sites and low catalytic efficiency impede its massive application. To address the issue, we have activated the basal plane of monolayer 2H MoS2 through an ultra-thin alumina mask (UTAM)-assisted nanopore arrays patterning, creating a high edge density. The introduced catalytically active sites are identified by Cu electrochemical deposition, and the hydrogen generation properties are assessed in detail. We demonstrate a remarkably improved HER performance as well as the identical catalysis of the artificial edges and the pristine metallic edges of monolayer MoS2. Such a porous monolayer nanostructure can achieve a much higher edge atom ratio than the pristine monolayer MoS2 flakes, which can lead to a much improved catalytic efficiency. This controllable edge engineering can also be extended to the basal plane modifications of other 2-D materials, for improving their edge-related properties. KEYWORDS: molybdenum disulfide, ultra-thin alumina mask, nanopore arrays, catalytic efficiency, hydrogen evolution reaction INTRODUCTION Among all the renewable energies, hydrogen (H2) is considered as one of the most potential energy forms for its highest mass energy density, storability and renewability.1–4 Comparing with


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the nowadays H2 generation techniques which are mainly at the price of fossil fuel consumption, electrochemical water splitting based on the electricity generated by wind, solar, or other renewable energy resources is a preferable way for scalable commercial production.5,6 For an efficient electrochemical water splitting process, an electrocatalyst which provides a high number of catalytically active sites with optimal intermediate hydrogen adsorption free energy to facilitate charge transfer and reduce the overpotential is prerequisite.7 Highly efficient catalysts achieve a better hydrogen evolution reaction (HER) performance with less material consumption due to a higher density of active sites per unit mass.5,8 Commonly, noble metals are used as electrocatalysts to improve the catalytic efficiency in the HER. However, the scarcity and high cost make them unsuitable for massive application. Recently, molybdenum disulfide (MoS2) has been considered to be one of the most promising candidates to substitute the noble metal catalysts in the HER process, due to its brilliant catalytic activity and much lower cost. It has been demonstrated that the HER active sites of MoS2 are located along the edges and, consequently, that the HER rate is proportional to the density of edge sites.9,10 Therefore, much attention has been paid to fabricating MoS2 nanostructures to increase the density of active sites, such as mesoporous MoS2 shaped by silicate templates and vertically aligned 2-D MoS2 through kinetically controlled growth.11,12These MoS2 nanostructures have preferentially exposed edges and enormously increased density of active sites, boosting the HER performance. However, thermodynamically, MoS2 favors the presence of a two-dimensional (2-D) HER inert basal plane, 10 which deeply limits the material utilization and consequently restricts the catalytic efficiency. The increase of the edge atoms ratio is a feasible strategy to improve the catalytic efficiency, such as by the controllable growth of MoS2


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nanoflakes, which remarkably improved the edge atoms ratio and the catalytic efficiency in the HER. 13,14 Vacancies and cracks have been introduced in the basal plane of MoS2 for improving the HER activity by plasma engineering and the annealing in reductive atmospheres.15,16 Although the defect engineering significantly enhanced the electrocatalytic activity of 2-D MoS2, it is difficult to correlate the contributions of different defects with the improved HER performance due to the lack of uniformity. Besides the density of active sites, the electric conductivity of the catalysts has a significant influence on the electrocatalytic activity.17–20 In monolayer 2H MoS2, the electron conduction is along rather than through the basal plane.21 After defect engineering, the basal plane is likely divided into separate nanodomains by the uncontrolled defects and the crystal continuity is lost. This strongly reduces the conductivity due to the poor interdomain electron transport and the poor crystal quality.22,23 Xie et al. regulated the degree of disorder of MoS2 nanodomains and retained fast electron transport, which greatly improved the HER performance due to a remarkable decrease of the charge transfer resistance from 124.6 Ω to 8.8 Ω.23 Hence, the controllable and accurate edge engineering, which keeps the crystal continuity, is concluded to be a superior option to activate the basal plane of 2-D MoS2. In this paper, to enhance the catalytic effiency of 2-D MoS2 in the HER, we activated the basal plane of monolayer 2H MoS2 by uniform and ordered nanopore arrays patterning through ultrathin alumina mask (UTAM)-assisted ion beam etching (IBE). Two kinds of UTAMs were applied to create different densities of nanopore arrays, which adjusted the density of edge length. The catalytically active sites on the edges of the nanopores were confirmed through Cu metal electrochemical deposition, and the HER properties of the porous monolayer 2H MoS2 were further assessed via electrochemical characterizations. The introduction of high-density


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ordered nanopore arrays remarkably improved the cathodic current density and the exchange current density. We also demonstrated the similarity between the artificial edges and the pristine metallic edges of MoS2 in electrocatalysis. As a result, more material in the basal plane is turned to be catalytically active to facilitate the HER, with much improved catalytic efficiency. The success in improving the catalytic efficiency of 2H MoS2 is considered to be universal for the basal plane modification of many other 2-D functional materials for exploring their edge-related properties and high efficient applications. EXPERIMENTAL SECTION Synthesis of Monolayer 2H MoS2. The monolayer 2H MoS2 was synthesized using chemical vapor deposition (CVD) on silicon on insulator (SOI) substrates with a 300 nm thick of SiO2 layer. The SOI substrates were cleaned by acetone, ethanol, DI-water, piranha solution and DIwater in sequence and baked for 10 min at 150 °C in air. Then, the substrate was placed facedown above a crucible containing 3 mg of MoO3 (99.95%, Aladdin) and loaded into a 4-cmdiameter quartz tube three-zone CVD furnace. The whole process was performed at atmospheric pressure, using ultra-high-purity Ar (99.999%) as carrier gas. Another crucible containing 500 mg of sulfur (99.99%, Aladdin) was located upstream, 18 cm away from the growth substrate. The furnace temperature was ramped up to 300 °C with a rate of 20 °C min-1 and kept constant for 10 min. Then, the temperature was ramped up to 730 °C with a rate of 50 °C min-1. After 10 min at 730 °C, the furnace was cooled down with the heater removed. The Ar flow rate was 25 sccm (standard cubic centimeter per minute) when the temperature stayed at 730 °C and 200 sccm during temperature ramp up and the cooling.


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Fabrication of porous MoS2. The UTAM was prepared by a two-step anodization of aluminum foils.24,25 Generally, first, high-purity aluminum foils (99.999 %, Goodfellow Cambridge Limited) were annealed at 450 °C for 3 h in Ar atmosphere, before electropolishing in a mixture of HClO4 and C2H5OH (1:3 by volume) with a constant voltage of 20 V for 5 min. Then, a standard two-step anodization method was used, by using oxalic acid and sulfuric acid as electrolytes with a corresponding constant anodization voltage of 40 V and 25 V, respectively. The first anodization lasted for at least 24 h, and then the oxide layer was completely removed off by a wet chemical etching (a mixture of 1.8 wt.% chromic acid and 6 wt.% phosphoric acid) at 50 °C to obtain a textured surface on Al. The second anodization was conducted with the same electrochemical parameters as the first, with a 300 s oxidation. Oxalic acid and sulfuric acid were used as electrolytes for UTAM fabrication with two different pore densities, and the sizes of the pores can be 50 nm and 30 nm, respectively. To obtain an UTAM, first, a thin layer of polystyrene (PS) (1wt.% PS/CHCl3 solution) was spin-coated onto the anodised aluminium, followed by a 90 °C solidification. Then, the aluminum substrate was etched off with a mixture of CuCl2 and HCl solution (6.8 g CuCl2 + 100 ml 37 % HCl + 200 ml distilled water), and the remaining barrier layer was selectively etched off in 5wt% H3PO4 at 30 °C for 30 min to obtain through hole UTAM. And the pore size of 30 nm can can be changed to 50 nm through an extra 5 minites chemical etching. Afterwards, the UTAM was transferred onto the desired substrate with as-grown monolayer MoS2. The prepared sample was baked at 90 °C for 30 min that enabled the conformal contact between the UTAM and the MoS2. Before the IBE process, the PS was removed by rinsing in toluene several times. Then, the specimen was etched by Ar ion beam etching (MIBE-150C) in an ambient pressure of 3 ×10−4 mbar at room temperature. The total etching time was 5 min. The whole etching


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process was with a vertical incident ion beam to the substrate. The etching energy was set to a cathode current of 16.2 A, anode voltage of 50 V, plate voltage of 300 V, ion accelerating voltage of 250 V, neutralization current of 13 A, and bias current of 1.2 A. After the IBE process, the remaining UTAM was completely removed off by phosphoric acid (10 %) for 2 h at 60 °C. Transfer of MoS2. Based on some previous works,26–29 we realized the transfer of both monolayer MoS2 and porous MoS2. Briefly, a polystyrene (PS, 1wt.% PS/CHCl3 solution) layer was spin-coated onto the MoS2/SOI substrate. After the solidification, about 1 mm wide polymer strips at the edges of the SOI substrate were scratched off to expose the SiO2 surface. Then, the substrate was immerged into 40% HF acid for a few seconds. After that, we slowly and vertically dipped the sample into deionized water causing the PS/MoS2 film to slip into the deionized water. Several minutes later, we transferred the floating film on glass carbon. After a pyrolysis process, the PS was completely removed. The heating process also enhanced the adhesion between MoS2 and glassy carbon. Characterizations. The nanostructures were characterized by field emission scanning electron microscopy (FE-SEM, ZEISS-Ultra55), transmission electron microscope (TEM, JEOL JEM 2100). Raman measurements were carried out by a Renishaw inVia Raman system. The excitation wavelength was 532 nm. The electrochemical characterization was performed in 0.5 M H2SO4 using a CH Instrument electrochemical analyzer (CHI660E). A silver chloride electrode was used as the reference electrode and a carbon rod was used as the counter electrode. The potential shift of the reference electrode is calibrated to be -0.21 V vs RHE. Typical electrochemical characterizations were performed using linear sweep voltammetry from 0 V to − 0.4 V (vs RHE) with a scan rate of 50 mV/s. The electrochemical deposition of Cu was


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performed in a 1 M CuSO4 solution by linearly sweeping from 0 V to − 0.09 V (vs RHE) with a scan rate of 5 mV/s.

RESULTS AND DISCUSSION

Figure 1. (a) Monolayer MoS2 by CVD growth covered with UTAM. (b) Nanopatterning by the UTAM-assisted IBE process. (c) Nanopore arrays of patterned monolayer MoS2 after the removal off UTAM. (d) Schematic illustration of enhanced HER performance. Large-area monolayer 2H MoS2 flakes were synthesized using a chemical vapor depositon (CVD) method (see Figure S1). Subsequently, as illustrated in Figure 1, the basal plane of the asgrown monolayer MoS2 was activated for enhancing the HER performance by an UTAMassisted edge engineering. First, an UTAM was transferred onto the as-grown monolayer 2H


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MoS2 (Figure 1a). Then, the IBE process was carried out to remove the exposed MoS2 with the assistance of UTAM (Figure 1b). After the IBE process, the UTAM was selectively etched off, and the monolayer MoS2 with the transferred pattern of ordered nanopore arrays was achieved (Figure 1c). Via the UTAM-assisted IBE, the introduced nanopore arrays are well-shaped with controlled density and uniformity. After transferring to a desired substrate, as illustrated in Figure 1(d), the numerous under-coordinated atoms at the pore edges are supposed to be catalytically active and adsorb a large number of hydrogen ions.


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Figure 2. (a) Top-view SEM image of triangular monolayer 2H MoS2 flakes by CVD on SOI. (b) Top-view SEM image of the transferred MoS2 flakes on a conductive substrate, showing the well maintained morphology. (c) Raman mapping of one MoS2 flake. (d) AFM image of one MoS2 flake with a thickness of ~0.7 nm, as measured along the red dotted line. Before the UTAM-assisted IBE, the chemical vapor deposition (CVD) grown monolayer 2H MoS2 flakes have been observed by scanning electron microscope (SEM). As shown in the topview SEM image of Figure 2a, large-area high-density MoS2 flakes are observed on the SOI substrate, exhibiting well-defined triangular shapes with mostly 20~30 µm side lengths. Figure 2b shows that the MoS2 flakes maintained their triangular shapes well after the transfer to a conductive substrate. To clarify the homogeneous monolayer growth, Raman mapping was carried out. MoS2 has two typical Raman peaks corresponding to the E12g and A1g modes, which are closely related with the layer number and can be used to determine the thickness of 2-D MoS2. The Raman mapping of one MoS2 flake is demonstrated in Figure 2c, which is colored based on the frequency difference of the E12g and A1g Raman peaks. The uniform 20 cm-1 frequency difference indicates monolayer growth and high homogeneity of the MoS2 flake.30–32 Being consistent with the Raman mapping, the atomic force microscopy (AFM) image (Figure 2d) confirmed the thickness of one monolayer of the CVD grown MoS2 flake of 0.7 nm deduced from the scan along the red dotted line.33,34 All the morphology characterizations demonstrate both the high quality of the as-grown monolayer 2H MoS2 flakes and the non-destructive nature of the transfer process.


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Figure 3. (a), (c) Top-view SEM images of low density porous (LDP) and high density porous (HDP) monolayer 2H MoS2 flakes, and (b), (d) the corresponding magnified SEM images. (e) TEM image of the LDP monolayer 2H MoS2 formed by UTAM-assisted IBE, and (f) high resolution (HR) TEM image of the pore edge (marked in (e)), the inset shows the corresponding selected area electron diffraction (SAED) pattern. (g) Raman and (h) PL spectra of LDP, HDP and pristine monolayer 2H MoS2 flakes. In order to activate the basal plane, the monolayer 2H MoS2 flakes were patterned by UTAMassisted IBE. To observe the edge engineering, the UTAM was completely removed after IBE. As shown in Figure 3a and 3c, ordered nanopore arrays were patterned on the basal plane of the monolayer MoS2 flakes. As shown in the corresponding magnified SEM images (Figure 3b and 3d), by adjusting the density of the nanopore arrays using different UTAMs, low density porous (LDP) and high density porous (HDP) monolayer 2H MoS2 flakes were obtained. The pore sizes of the different UTAMs were adjusted to be the same with an average diameter of 50 nm. Statistically, LDP and HDP monolayer MoS2 flakes increase the density of edge length from 0.788 µm/µm2 of the pristine MoS2 flakes to 23.22 µm/µm2 and 63.08 µm/µm2, respectively. With


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the UTAM-assisted edge engineering, the nanopore arrays patterning can be also achieved on a large-area monolayer MoS2 (see Figure S2). To clarify the crystalline structure of the patterned MoS2, the LDP monolayer MoS2 flakes were transferred onto a carbon film coated porous copper net for transmission electron microscopy (TEM) observations. The monolayer MoS2 with nanopore arrays patterning is shown in Figure 3e. The nanopores are arranged in a hexagonal close packed pattern with interpore distance of 100 nm. The pore diameter is about 50 nm, being consistent with the SEM observations. The pore edge area was investigated by high-resolution (HR) TEM shown in Figure 3f. The single crystalline basal plane of monolayer 2H MoS2 with a round edge is clearly recognized, while no atoms remained inside the pore area. This confirms that well-defined nanopore edges were introduced into the basal plane and the remaining parts of MoS2 were well protected by the UTAM. The inset in Figure 3f is the corresponding selected area electron diffraction (SAED)

pattern of 2H MoS2. The sharp and highly symmetrical spots pattern

indicates the well-kept single crystallinity after the UTAM-assisted IBE. Moreover, as illustrated in Figure 3g and 3h, Raman and PL spectra reveal some differences after the patterning with different UTAMs. After the porous patterning, the decrease of peak intensities in both Raman and PL spectra indicate the material loss of MoS2. Besides, with the introduction of more edges, the red shift of the Raman peaks reflects the enhancement of disordered atom vibrations.35 Two peaks are recognized in the PL spectra at ～620 and～670 nm, which are attributed to the B1 and A1 excitons of MoS2, respectively. The red shift of the PL peak (～670 nm) also is in line that new edges were introduced during the patterning.36–38


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Figure 4. XPS spectra of (a) the MoS2 without nanopatterning, (b) the LDP MoS2 and (c) the HDP MoS2.


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As shown in Figure 4, chemical states and phase information of the MoS2 by CVD growth before and after UTAM assisted nanopatterning were characterized by X-ray photo-electron spectroscopy (XPS) analysis. After Shirley background subtraction, the Mo 3d, S 2s, and S 2p peaks were deconvoluted to show the contributions from 1T and 2H. In Fig. 4a, the peaks at around 229 eV and 232 eV correspond to Mo4+ 3d5/2 and Mo4+ 3d3/2 of 2H-MoS2, respectively. After UTAM-assisted nanopatterning, as illustrated in Fig 4b and c, two additional peaks are appeared and shifted to the lower binding energies of 228.2 eV and 231.6 eV for LDP Mo S2 and to 228.6 eV and 231.8 eV for HDP MoS2. In the S 2p region of the spectra, two additional peaks are extracted besides the doublet peaks of 2H-MoS2 S 2p1/2, and S 2p3/2, which appear at 163 eV and 161.9 eV. The additional peaks are similar to the ones of 1T-MoS2.39–42 Generally, the appearance of 1T phase indicates that the metallic edges of MoS2 have been created by the nanopatterning. By integrating the areas in Fig. 4b and c, the metallic MoS2 take a proportion of 59 % in HDP MoS2, which is 16 % higher than the propotion in LDP MoS2. There are no characteristic peaks of Al3+ at around 74 eV (see Figure S3b),43 which confirms the complete removal of the UTAM. Since the characteristic peaks of Mo6+ could not be observed at around 236 eV,44–46 both the LDP and HDP MoS2 were not oxidized after the nanopatterning.


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Figure 5. (a) Schematic illustration of catalytically active sites located by Cu electrodeposition. (b) Top-view SEM image of porous monolayer MoS2 transferred on a glass carbon substrate, and (c) after Cu electrodeposition. (d) Large-area of Cu nanoring arrays with the short range ordering from the UTAM. (e) Cu electrodeposition boundary on a monolayer MoS2 between the areas with and without UTAM-assisted IBE.


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In order to reveal the activation of the basal plane for HER, the large-area monolayer MoS2 flakes with nanopore arrays patterning were transferred onto glassy carbon, and an electrodeposition of Cu was performed to identify the catalytically active sites. The HER process was thought to follow the so-called Volmer–Heyrovsky mechanism.47 The intermediate hydrogen adsorbs on the active sites, accepting electrons or combining with other intermediate hydrogen to generate H2.48 Hence, it is feasible to identify the active sites using elements which have a similar binding free energy as the intermediate hydrogen, with copper ions as a good choice.22 As illustrated in Figure 5a, the nanopore edges are supposed to be the catalytically active sites. Similar to the intermediate hydrogen adsorbing on the active sites, Cu2+ irons are supposed to absorb on the edges. However, after accepting electrons, the Cu metal cannot perform desorption. Thus, the Cu nanorings are presumed to electrodeposit on the round edges. Figure 5b is the top-view SEM image of the LDP monolayer 2H MoS2 on glassy carbon substrate. The nanopores have round-shaped edges with bright contrasts due to the agglomeration effect of electrons.21 After Cu electrodeposition, as shown in Figure 5c, Cu nanoring arrays are observed along the pore edges with a thickness of about 20 nm. Large-area LDP monolayer MoS2 leads to the uniform Cu nanoring arrays. Due to the short-range ordering nature of the UTAM,49 the ordered Cu nanoring arrays tend to align in small domains as colored in Figure 5d. As plotted in Figure 5e, on monolayer MoS2, there is a clear boundary of Cu electrodeposition between the area with and without UTAM-assisted patterning. Obviously, the pristine basal plane has no binding sitesfor the Cu2+ ions. Therefore, during the HER, the intermediate hydrogen adsorbs on the nanopore edges being the catalytically active sites.


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Figure 6. (a) Schematic illustration of the enhanced HER by the edge engineering. (b) Polarization curves for different working electrodes. (c)

Tafel plots for different working

electrodes. (d) Plots of exchange current density versus MoS2 area coverage ratio (blue) and MoS2 edge length density (red). In order to assess the HER performance, electrochemical characterizations have been performed in a three-electrode electrochemical system with a 0.5 M H2SO4 electrolyte. The pristine and patterned monolayer MoS2 flakes were transferred onto glassy carbon as working electrodes. As schematically illustrated in Figure 6a, by the UTAM-assisted patterning, the introduced high-density nanopore edges being the catalytically active sites adsorb a large amount of intermediate hydrogen and efficiently generate H2. In Figure 6b, the electrode of pristine


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monolayer MoS2 flakes exhibited a relatively poor HER performance and sluggish current response to applied potentials. In contrast, the UTAM-assisted edge engineering significantly improves the HER performance of the monolayer MoS2 flakes. The HDP electrode exhibits a current density of 10 mA cm −2 at 385 mV, which is 124 mV lower compard to the pristine MoS2. And the potential will be 500 mV and 560 mV, respectively, when the HDP and LDP electrode exhibits a current density of 100 mA cm −2. To evaluate the influence of edge engineering on electrode kinetics, the linear portions of the over potential (measured voltage versus RHE for the HER at pH 0) versus current response were fitted to the Tafel equation (Figure 6c). Obviously, the UTAM-assisted IBE results in decrease in the Tafel slope from 187 mV dec−1 for the pristine monolayer MoS2 flakes to 122 mV dec−1 and to 109 mV dec−1 for the LDP and HDP MoS2 flakes, respectively. The current density depends much less on the over potential after the nanopore arrays patterning. In general, the HER performance of our working electrodes is still inferior to the commercial Pt/C catalyst and the state-of-the-art HER results of Mo-based catalysts. Since the mass of our patterned MoS2 loaded on the glassy carbon for electrochemical measurement is much less than most Mo-based catalysts. On the other hand, the nanopatterning method in our paper can only control the relatively macro nanostructure, and in fact, the chemical state of atoms at patterned edges is random, which means that not all the edge atoms are in the unsaturated state that is suitable for catalyzing the HER. Finally, the exchange current density, determined from extrapolating the overpotential in the Tafel plot to zero, was plotted versus both the edge length density and the area coverage ratio of MoS2 (Figure 6d). After patterning of the high-density nanopore arrays, the edge lengh per unit mass was improved to 7.44*106 m/mg and to 2.02*107 m/mg for the LDP and HDP MoS2 flakes, respectively, from 2.53*105 m/mg for the pristine MoS2 flakes (Figure 6d red line). The


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exchange current density was improved to 1.7 µA/cm2 and 2.8 µA/cm2 from 1.1 µA/cm2, indicating the improved activation of the electrocatalytic reaction.10 Moreover, the activation of the basal plane was accompanied by a decrease of the MoS2 area coverage, confirming the improved material utilization (Figure 6d blue line). To be more pellucid, these parameters were summerised in the suppoting infoemation Table S2. The nanopore arrays patterning leads to both improved HER performance and less material consumption, which reflects the high catalytic efficiency of such a porous nanostructures for the HER.

Figure 7. (a) Nyquist plots of different samples. showing the electrode kinetics at 450 mV (vs RHE), The fitted curves are presented by solid lines. (b) Enlargement of (a) for better comparison. (c) The differences in current density at 0.15 V vs. RHE plotted against scan rate fitted to a linear regression allowing for the estimation of Cdl. (d) Time dependence of current


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density under a static potential of 390 mV (vs RHE), inset is an enlargement of the area denoted by the dash circle. To investigate the electrode kinetics, electrochemical impedance spectroscopy (EIS) is performed at an overpotential of 450 mV as shown in Figure 7a and b. The measured data can be fitted to an equivalent circuit (inset in Figure 7a), consisting of constant phase elements (CPE) associated with the catalyst MoS2, charge-transfer resistances from MoS2 to the redox couple in electrolyte (Rct), and the overall series resistance (Roverall). Detailed parametes were listed in Table S2 in the supporting information. The Nyquist plots reveal remarkable decreases of the charge transfer resistance (RCT ) from 667.4.Ω of the MoS2 flakes to 234.6 Ω of the LDP MoS2 and 57.3 Ω of the HDP MoS2, respectively. Such a difference indicates the patterned metallic edges could facilitate the charge transfer and imorove the HER performance. To estimate the effective surface area of the solid-liquid interface, the capacitance of the double layer (Cdl) was measured using a simple cyclic voltammetry method (see Figure S4) , which is expected to be linearly proportional to the effective surface area.50,51 As shown in Figure 7c, the Cdl was improved from 0.99 µF of pristine MoS2 flakes to 5.25 µF of LDP MoS2 and 8.86 µF of HDP MoS2, which suggests that the effective surface area has been improved after the nanopatterning. The specific activities per unit mass of HDP MoS2 and LDP MoS2 are both improved a lot than the MoS2 flakes (Table S3 in Supporting Information), which can also indicate the improved density active sites. The durability of the patterned MoS2 electrodes in an acidic environment was also characterized. A 10000 seconds continuous HER test of HDP MoS2 has been provided as shown in Figure 7d, at a static overpotentia of 390 mV vs RHE. As shown in the inset, a typical serrate time- dependent

curve was obtained, which was caused by the repeated bubble


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accumulation and release processes. The current density exhibits only a slight degradation after the period of 10000 seconds, which is mainly caused by the consumption of H+ in the electrolyte.

CONCLUSIONS In conclusion, to enhance both the HER performance and material utilization, we have activated the basal plane of monolayer 2H MoS2 through an UTAM-assisted nanopore arrays patterning. High-density catalytically active edges were introduced into the basal plane of monolayer MoS2 with good controllability. The similarity between the nanopore edges and the pristine metallic edges of monolayer MoS2 in electrocatalysis was verified, and the catalytic efficiency for the HER was enormously improved. This controllable edge engineering can be applied to other 2-D materials, improving their edge-related electrical, magnetic or optical properties.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Figure S1. Illustration of the CVD growth of monolayer 2H MoS2 flakes. Figure S2. (a) Nanopore arrays patterning on a large-area monolayer MoS2 observed under SEM. (b) Nanopore arrays patterning on a large-area monolayer MoS2 observed under TEM.


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Figure S3. (a) X-ray photoelectron spectroscopy of samles befor and after the nano patterning, (b) the enlarge view of the area in (a) which contain the characteristic peaks of Mo6+ at around 236 eV. (c) the enlarge view of the area in (a) which contain the characteristic peaks of Al3+ at aroud 74 eV. Figure S4. Cyclic voltammograms in the region of 0.1‒0.2 V vs. RHE for the (a) CVD growth of MoS2 flakes. (b) LDP MoS2. (c) HDP MoS2. Table S1. Edge density and exchange current density of MoS2 before and after nanopatterning. Table S2. Electrochemical Parameters of MoS2 before and after nanopatterning. Table S3. Specific activity parameters of MoS2 before and after nanopatterning. AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Zhang Zhang). 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.

Funding Sources This work was supported by the National Key R&D Program of China (2016YFB0401501), Guangdong Innovative Research Team Program (Nos.2013C102), Science and technology project of Guangdong Province (No.2015B090913004), the Guangdong National Science Foundation (No.2014A030313434), the Pearl River S&T Nova Program of Guangzhou


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Ultrathin Alumina Mask-Assisted Nanopore Patterning on Monolayer

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