for Highly Catalytic E ﬃ ciency in Hydrogen Evolution Reaction

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Ultrathin Alumina Mask-Assisted Nanopore Patterning on Monolayer MoS

₂

for Highly Catalytic E ﬃ ciency in Hydrogen Evolution Reaction

Shaoqiang Su,

^†,‡,#

Qingwei Zhou,

^∥,#

Zhiqiang Zeng,

^‡

Die Hu,

^†,‡

Xin Wang,

^†,§

Mingliang Jin,

^†,§

Xingsen Gao,

^†,‡

Richard Nötzel,

^†,§

Guofu Zhou,

^†,§,⊥

Zhang Zhang,*

^,†,‡

and Junming Liu

^‡,∥

†National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics,

‡Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, and^§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, P. R. China

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

*^S Supporting Information

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 MoS₂are along the edges, whereas thermodynamically MoS₂favors the presence of a two-dimensional (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 MoS₂through an ultrathin 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, ultrathin alumina mask, nanopore arrays, catalytic eﬃciency, hydrogen evolution reaction

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INTRODUCTION

Among all of 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 Compared with the current H₂-generation techniques, which are mainly at the cost 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 that provides a high number of catalytically active sites with optimal intermediate hydrogen adsorption free energy to facilitate charge transfer and reduce the overpotential is a 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 a 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.^9,10Therefore, much attention has been paid to fabricating MoS₂nanostructures 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.^11,12 These 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

Received: December 18, 2017 Accepted: February 6, 2018 Published: February 6, 2018

www.acsami.org Cite This:ACS Appl. Mater. Interfaces2018, 10, 8026−8035

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restricts the catalytic efficiency. The increase in the edge atoms ratio is a feasible strategy to improve the catalytic efficiency, such as by the controllable growth of MoS₂nanoflakes, which remarkably improves the edge atoms ratio and the catalytic efficiency in the HER.^13,14

Vacancies and cracks have been introduced in the basal plane of MoS₂for improving the HER activity by plasma engineering and annealing in reductive atmospheres.^15,16 Although the defect engineering significantly enhances 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.²¹After defect engineering, the basal plane is likely to be divided into separate nanodomains by the uncontrolled defects and the crystal continuity is lost. This strongly reduces the conductivity due to poor interdomain electron transport and 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 in the charge transfer resistance from 124.6 to 8.8 Ω.²³Hence, 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 efficiency of 2-D MoS₂ in the HER, we activate 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 are applied to create different densities of nanopore arrays, which adjust the density of the edge length. The catalytically active sites on the edges of the nanopores are confirmed through Cu metal electrochemical deposition, and the HER properties of the porous monolayer 2H MoS₂ are further assessed via electrochemical characterizations. The introduction of high-density ordered nanopore arrays remarkably improve the cathodic current density and the exchange current density. We also demonstrate the similarity between the artificial edges and the pristine metallic edges of MoS₂in electrocatalysis. As a result, more material in the basal plane is turned to be catalytically active to facilitate the HER, with a much improved catalytic efficiency. The success in improving the catalytic efficiency of 2H MoS₂is considered to be universal for the basal plane modification of many other 2-D functional materials for exploring their edge-related properties and high-efficiency applications.

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EXPERIMENTAL SECTION

Synthesis of Monolayer 2H MoS₂.The monolayer 2H MoS2was synthesized using the chemical vapor deposition (CVD) on a silicon- on-insulator (SOI) substrate with a 300 nm thick SiO₂layer. The SOI substrates were cleaned by acetone, ethanol, deionized (DI) water, piranha solution, and DI water in that sequence and baked for 10 min at 150°C in air. Then, the substrate was placed face-down above a crucible containing 3 mg of MoO₃(99.95%, Aladdin) and loaded into a 4 cm diameter quartz tube three-zone CVD furnace. The whole process was performed at atmospheric pressure, using ultrahigh-purity Ar (99.999%) as the 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 and kept constant for 10 min. Then, the temperature was ramped up to 730°C, with a rate of 50°C/min. After 10 min at 730 °C, the furnace was cooled down with the heater

removed. The Arﬂow rate was 25 sccm (standard cubic centimeter per minute) when the temperature stayed at 730°C and 200 sccm during the temperature ramp up and cooling.

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 and 25 V, respectively. Thefirst anodization lasted for at least 24 h and then the oxide layer was completely removed offby 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 thefirst, 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 and 30 nm, respectively. To obtain an UTAM, first, a thin layer of polystyrene (PS) (1 wt % PS/CHCl3solution) was spin-coated onto the anodized aluminum, followed by a 90 °C solidification. Then, the aluminum substrate was etched off with a mixture of CuCl₂and HCl solution (6.8 g CuCl₂+ 100 mL 37% HCl + 200 mL distilled water), and the remaining barrier layer was selectively etched offin 5 wt % H3PO4at 30°C for 30 min to obtain a through-hole UTAM. Also, the pore size of 30 nm can can be changed to 50 nm through an extra 5 min of chemical etching. Afterward, the UTAM was transferred onto the desired substrate with the 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 3×10⁻⁴mbar at room temperature.

The total etching time was 5 min. The whole etching 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 oﬀby 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, 1 wt % PS/CHCl₃solution) 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 immersed into 40% HF acid for a few seconds. After that, we slowly and vertically dipped the sample into deionized water, causing the PS/

MoS₂film to slip into the deionized water. Several minutes later, we transferred thefloatingfilm on glass carbon. After a pyrolysis process, the PS was completely removed. The heating process also enhanced the adhesion between MoS2and glassy carbon.

Characterizations.The nanostructures were characterized byﬁeld emission scanning electron microscopy (FE-SEM, ZEISS-Ultra55) and 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 reversible hydrogen electrode (RHE). Typical electrochemical characterizations were performed using linear sweep voltammetry from 0 to−0.6 V (vs RHE) with a scan rate of 50 mV/s. The electrochemical deposition of Cu was performed in a 1 M CuSO4solution by linearly sweeping from 0 to− 0.09 V (vs RHE) with a scan rate of 5 mV/s.

ACS Applied Materials & Interfaces Research Article

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RESULTS AND DISCUSSION

Large-area monolayer 2H MoS₂ﬂakes are synthesized using a chemical vapor deposition (CVD) method (see Figure S1).

Subsequently, as illustrated inFigure 1, the basal plane of the as-grown monolayer MoS₂is activated for enhancing the HER performance by an UTAM-assisted edge engineering. First, an UTAM is transferred onto the as-grown monolayer 2H MoS₂ (Figure 1a). Then, the IBE process is carried out to remove the exposed MoS₂with the assistance of UTAM (Figure 1b). After

the IBE process, the UTAM is selectively etched oﬀ, and the monolayer MoS₂ with the transferred pattern of ordered nanopore arrays is 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 1d, the numerous under-coordinated atoms at the pore edges are supposed to be catalytically active and adsorb a large number of hydrogen ions.

Figure 1.(a) Monolayer MoS2by CVD growth covered with UTAM. (b) Nanopatterning by the UTAM-assisted IBE process. (c) Nanopore arrays of patterned monolayer MoS2after the removal of UTAM. (d) Schematic illustration of enhanced HER performance.

Figure 2.(a) Top-view SEM image of triangular monolayer 2H MoS₂flakes 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 MoS₂flake. (d) AFM image of one MoS₂ flake with a thickness of∼0.7 nm, as measured along the red dotted line.

ACS Applied Materials & Interfaces

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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 top-view SEM image ofFigure 2a, large-area high- density MoS₂ flakes are observed on the SOI substrate, exhibiting well-defined triangular shapes with mostly 20−30μm side lengths. Figure 2b shows that the MoS₂ flakes maintain their triangular shapes well after the transfer to a conductive substrate. To clarify the homogeneous monolayer growth, Raman mapping was carried out. MoS₂has two typical Raman peaks corresponding to the E_2g¹ and A_1g 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 MoS₂flake is demonstrated inFigure 2c, which is colored based on the frequency difference of the E_2g¹ and A_1g 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) confirms the thickness of one monolayer of the CVD grown MoS₂flake of 0.7 nm deduced from the scan along the red dotted line.^33,34 All of the morphology characterizations demonstrate both high quality of the as-grown monolayer 2H MoS₂ flakes and nondestructive nature of the transfer process.

To activate the basal plane, the monolayer 2H MoS₂flakes are patterned by UTAM-assisted IBE. To observe the edge engineering, the UTAM is completely removed after IBE. As shown inFigure 3a,c, ordered nanopore arrays are patterned on the basal plane of the monolayer MoS₂flakes. As shown in the corresponding magnified SEM images (Figure 3b,d), by adjusting the density of the nanopore arrays using different UTAMs, low-density porous (LDP), and high-density porous (HDP) monolayer 2H MoS₂flakes are obtained. The pore sizes of the different UTAMs are adjusted to be the same with an average diameter of 50 nm. Statistically, the LDP and HDP monolayer MoS₂flakes increase the density of edge length from 0.788μm/μm²of the pristine MoS₂flakes to 23.22 and 63.08 μm/μm², respectively. With the UTAM-assisted edge engineer-

ing, the nanopore arrays patterning can be also achieved on a large-area monolayer MoS₂(seeFigure S2).

To clarify the crystalline structure of the patterned MoS₂, the LDP monolayer MoS₂ﬂakes are transferred onto a carbonﬁlm 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 is investigated by high-resolution (HR) TEM shown inFigure 3f.

The single-crystalline basal plane of monolayer 2H MoS₂with a round edge is clearly recognized, whereas no atoms remain inside the pore area. This confirms that well-defined nanopore edges are introduced into the basal plane and the remaining parts of MoS₂are 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,h, the Raman and PL spectra reveal some differences after patterning with different UTAMs. After the porous patterning, the decrease in 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 are introduced during the patterning.³⁶⁻³⁸

As shown in Figure 4, the chemical states and phase information of the MoS₂ by CVD growth before and after UTAM-assisted nanopatterning are characterized by X-ray photoelectron spectroscopy (XPS) analysis. After Shirley background subtraction, the Mo 3d, S 2s, and S 2p peaks are deconvoluted to show the contributions from 1T and 2H. In Figure 4a, the peaks at around 229 and 232 eV correspond to Mo⁴⁺ 3d_5/2 and Mo⁴⁺ 3d_3/2 of 2H MoS₂, respectively. After Figure 3.(a, c) Top-view SEM images of low-density porous (LDP) and high-density porous (HDP) monolayer 2H MoS2flakes, and (b, d) the corresponding magnified SEM images. (e) TEM image of the LDP monolayer 2H MoS2formed 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) photoluminescence (PL) spectra of LDP, HDP, and pristine monolayer 2H MoS2flakes.

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UTAM-assisted nanopatterning, as illustrated in Figure 4b,c, two additional peaks appear and shift to the lower binding energies of 228.2 and 231.6 eV for LDP MoS₂and to 228.6 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 MoS₂S 2p_1/2and S 2p_3/2, which appear at 163 and 161.9 eV, respectively. 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 nanopatterning. By integrating the areas in Figure 4b,c, the metallic MoS₂take a proportion of 59% in HDP MoS₂, which is

16% higher than the proportion in LDP MoS₂. There are no characteristic peaks of Al³⁺at around 74 eV (seeFigure S3c),⁴³ which conﬁrms the complete removal of the UTAM. Because the characteristic peaks of Mo⁶⁺ could not be observed at around 236 eV (seeFigure S3b),⁴⁴⁻⁴⁶both the LDP and HDP MoS₂are not oxidized after nanopatterning.

To reveal the activation of the basal plane for HER, the large- area monolayer MoS₂ ﬂakes with nanopore arrays patterning are transferred onto glassy carbon, and an electrodeposition of Cu is performed to identify the catalytically active sites. The HER process is thought to follow the so-called Volmer− Figure 4.XPS spectra of (a) the MoS₂without nanopatterning, (b) the LDP MoS₂, and (c) the HDP MoS₂.

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Heyrovsky mechanism.⁴⁷ The intermediate hydrogen adsorbs on the active sites, accepting electrons or combining with other intermediate hydrogen to generate H₂.⁴⁸Hence, it is feasible to identify the active sites using elements that have similar binding free energy as the intermediate hydrogen, with copper ions as a good choice.²²As illustrated inFigure 5a, the nanopore edges are supposed to be the catalytically active sites. Similar to the intermediate hydrogen adsorbing on the active sites, the Cu²⁺

ions are supposed to be absorbed on the edges. However, after accepting the 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 MoS₂on a glassy carbon substrate. The nanopores have round-shaped edges with bright contrasts due to the agglomeration eﬀect of the electrons.²¹ After Cu electrodeposition, as shown inFigure 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,⁴⁹the ordered Cu nanoring arrays tend to align in small domains as colored inFigure 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 sites for Cu²⁺ ions. Therefore, during the HER, the intermediate hydrogen are adsorbed on the nanopore edges being the catalytically active sites.

To assess the HER performance, electrochemical characterizations have been performed in a three-electrode electrochemical system with a 0.5 M H₂SO₄electrolyte. The pristine and patterned monolayer MoS₂ flakes are transferred onto glassy carbon as the working electrodes. As schematically illustrated inFigure 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. InFigure 6b, the electrode of pristine monolayer MoS₂ flakes exhibit a relatively poor HER performance and a sluggish current response to the applied potentials. In contrast, the UTAM-assisted edge engineering significantly improves the HER performance of the monolayer MoS₂flakes. The HDP electrode exhibits a current density of 10 mA/cm²at 385 mV, which is 124 mV lower compared to that of the pristine MoS₂. Also, the potential will be 500 and 560 mV, respectively, when the HDP and LDP electrode exhibits a current density of 100 mA/cm². To evaluate the influence of edge engineering on electrode kinetics, the linear portions of the overpotential (measured voltage versus RHE for the HER at pH 0) versus current response are fitted to the 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 MoS₂between the areas with and without UTAM-assisted IBE.

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Tafel equation (Figure 6c). Obviously, the UTAM-assisted IBE results in decrease in the Tafel slope from 187 mV/dec for the pristine monolayer MoS₂ ﬂakes to 122 mV/dec and to 109 mV/dec for the LDP and HDP MoS₂ﬂakes, respectively. The current density depends much less on the overpotential 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 the Mo based catalysts, mainly because the mass of our patterned MoS₂ loaded on the glassy carbon for electrochemical measurement is much less than that of 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 the patterned edge is random, which means that not all of the edge atoms are in the unsaturated state that is suitable for catalyzing the HER.

Finally, the exchange current density, determined by extrapolating the overpotential in the Tafel plot to 0, is 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 length per unit mass is improved to 7.44 ×10⁶and to 2.02 × 10⁷m/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 exchange current density is improved to 1.7 and 2.8 μA/cm²from 1.1 μA/cm², indicating an improved activation of the electrocatalytic reaction.¹⁰Moreover, the activation of the basal plane is accompanied by a decrease in the MoS₂ area coverage, confirming the improved material utilization (Figure 6d, blue line). To be more pellucid, these parameters were summarized in the Supporting InformationTable 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 nanostructure for the HER.

To investigate the electrode kinetics, electrochemical impedance spectroscopy is performed at an overpotential of 450 mV, as shown in Figure 7a,b. The measured data can be ﬁtted to an equivalent circuit (inset inFigure 7a), consisting of constant phase elements associated with the catalyst MoS₂, charge-transfer resistances from MoS₂to the redox couple in electrolyte (R_ct), and the overall series resistance (R_overall).

Detailed parameters are listed inTable S2 in the Supporting Information. The Nyquist plots reveal remarkable decreases in the charge transfer resistance (R_ct) from 667.4Ωfor the MoS₂ flakes to 234.6Ωfor the LDP MoS₂and 57.3Ωfor the HDP MoS₂, respectively. Such a difference indicates that the patterned metallic edges could facilitate the charge transfer and improve the HER performance. To estimate the effective surface area of the solid−liquid interface, the capacitance of the double layer (C_dl) is 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 C_dl is improved from 0.99 μF for pristine MoS₂flakes to 5.25μF for LDP MoS₂and 8.86μF for HDP MoS₂, which suggests that the effective surface area has been improved after the nanopatterning. The specific activities per unit mass of HDP MoS₂and LDP MoS₂are both improved a lot than the MoS₂ flakes (Table S3 in the Supporting Information), which can also indicate the improved density of active sites.

The durability of the patterned MoS₂electrodes in an acidic environment is also characterized. A 10 000 s continuous HER test of HDP MoS₂has been provided as shown inFigure 7d, at a static overpotential of 390 mV vs RHE. As shown in the inset, a typical serrate time-dependent curve is obtained, which is caused by the repeated bubble accumulation and release processes. The current density exhibits only a slight degradation 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 MoS2area coverage ratio (blue) and MoS₂edge length density (red).

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after the period of 10 000 s, which is mainly caused by the consumption of H⁺ in the electrolyte.

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CONCLUSIONS

In conclusion, to enhance both the HER performance and material utilization, we have activated the basal plane of the monolayer 2H MoS₂ through an UTAM-assisted nanopore arrays patterning. The high-density catalytically active edges were introduced into the basal plane of the monolayer MoS₂ with good controllability. The similarity between the nanopore edges and the pristine metallic edges of the monolayer MoS₂in electrocatalysis is veriﬁed, and the catalytic eﬃciency for the HER is enormously improved. This controllable edge engineering can be applied to other 2-D materials, improving their edge- related electrical, magnetic, or optical properties.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.7b19197.

Illustration of the CVD growth of monolayer 2H MoS₂ ﬂakes (Figure S1); nanopore arrays patterning on a large- area monolayer MoS₂ observed under SEM and TEM (Figure S2); X-ray photoelectron spectroscopy (Figure S3); cyclic voltammograms (Figure S4); edge density

and exchange current density of MoS₂ before and after nanopatterning (Table S1); electrochemical parameters of MoS₂ before and after nanopatterning (Table S2);

speciﬁc activity parameters of MoS₂ before and after nanopatterning (Table S3) (PDF)

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

ORCID

Shaoqiang Su:0000-0001-6443-3938

Qingwei Zhou:0000-0002-9102-1266

Xingsen Gao:0000-0002-2725-0785

Zhang Zhang:0000-0001-6287-502X

Junming Liu:0000-0001-8988-8429 Author Contributions

#S.S. and Q.Z. contributed equally.

Funding

This work was supported by the National Key R&D Program of China (2016YFB0401501), Guangdong Innovative Research Team Program (No. 2013C102), Science and Technology project of Guangdong Province (No. 2015B090913004), the G u a n g d o n g N a t i o n a l S c i e n c e F o u n d a t i o n ( N o . 2014A030313434), the Pearl River S&T Nova Program of Figure 7.(a) Nyquist plots of diﬀerent samples, showing the electrode kinetics at 450 mV (vs RHE). Theﬁtted 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 rateﬁtted to a linear regression allowing for the estimation ofC_dl. (d) Time dependence of current density under a static potential of 390 mV (vs RHE), inset is an enlargement of the area denoted by the dash circle.

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Guangzhou (No. 201506010019), and the Innovation Project of Graduate School of South China Normal University.

Notes

The authors declare no competingﬁnancial interest.

The manuscript was written through contributions of all of the authors. All of the authors have given approval to the ﬁnal version of the manuscript.

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