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Surface-Induced 2D/1D Heterostructured Growth of ReS

₂

/CoS

₂

for High-Performance Electrocatalysts

Yuanwu Liu, Jing Li, Wentian Huang, Ying Zhang, Minjie Wang, Xingsen Gao, Xin Wang, Mingliang Jin, Zhipeng Hou,* Guofu Zhou, Zhang Zhang,* and Junming Liu

Cite This:ACS Appl. Mater. Interfaces2020, 12, 33586−33594 Read Online

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^sı Supporting Information

ABSTRACT: Two-dimensional/one-dimensional (2D/1D) het- erostructures have received much attention from researchers for their abundant catalytically active sites and low contact resistance due to formation of chemical bonds at the interface. The investigation of such heterostructures, however, is confined to lattice-matched materials, which severely limits the material candidates. Herein, we demonstrate a lattice-mismatched 2D/1D heterostructured electrocatalyst consisting of 2D ReS₂nanosheets and 1D CoS₂ nanowires. We propose that the higher surface energy of the CoS₂nanowire and the lattice mismatch between 1D and 2D units are crucial for the growth process of ReS₂nanosheets.

More importantly, the terminal S²⁻exposed on the surface of CoS₂ nanowires serves not only as the nucleus of ReS₂nanosheets but

also as a bridge to enhance electron transport efficiency. Thus, the ReS₂/CoS₂ heterostructures show outstanding hydrogen evolution reaction performance. This work is of general interest for the design of complex multidimensional nano-heterostructures with outstanding functionalities.

KEYWORDS: 2D/1D, electrocatalyst, ReS₂, CoS₂, hydrogen evolution reaction

1. INTRODUCTION

Low-dimensional nanostructured materials have found im- portant applications in catalysis because of their large specific surface area and increased structure stability.^1,2 However, for single-dimensional nanostructured materials, the limited number of catalytic sites hinder further improvement of their catalytic performance. To overcome the limits on the number of catalytic sites, researchers have increasingly focused their interest on multidimensional nano-heterostructures (MDNHSs) in view of their greater catalytic activity area, which improves the catalytic performance.³⁻⁶ Moreover, MDNHSs can inherit the merits while also mitigating the drawbacks of each component element. These features are of great significance to both fundamental physics and catalytic science. As a result, MDNHSs are becoming an important area of focus for further development of catalysis.

To be an efficient catalyst, MDNHSs require not only high conductivity to realize a rapid transfer of electrons but also abundant catalytic sites.⁷ To date, zero-dimensional (0D) nanostructures are generally introduced to increase the number of catalytic sites in MDNHSs because their catalytic activity area is the largest among low-dimensional structures. In such 0D-based MDNHSs, however, contact resistance is relatively high because of the lack of chemical bonding and strong orbital hybridization at the interface.⁸⁻¹⁰ This feature dramatically

decreases electron transmission efficiency between materials with different numbers of dimensions, which limits further improvement of catalytic performance. It has been reported that electrons transfer faster through chemical bonds than van der Waals forces between two materials.¹¹Thus, an ideal way to enhance electron transmission efficiency is to connect materials with different numbers of dimensions using chemical bonds at their interface. Typically, one-dimensional (1D) nanostructures possess large surface areas, and there are a large number of unsaturated bonds on the surface, which facilitate the growth of other materials.⁷ At the same time, 1D nanostructures have the characteristics of fast and directional electron transmission, which is conducive to the transmission of electrons to materials of other dimensions.¹²Hence, from the perspective of heterostructure design, the epitaxial growth of 2D materials on 1D materials is achieved by forming bridge bonds at the interface, which can effectively reduce contact resistance. Meanwhile, 2D/1D heterostructures also possess a

Received: February 16, 2020 Accepted: July 3, 2020 Published: July 3, 2020 Downloaded via NANJING UNIV on October 10, 2020 at 11:17:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

the performance of a mixture of the two materials. However, these heterostructures are confined to lattice-matched materials,^5,14,15 which severely limits the material candidates for 2D/1D heterostructures. To extend 2D/1D systems, lattice-mismatched heterostructures must be investigated.

More importantly, recent reports suggest that a suitable lattice mismatch for heterogeneous materials may effectively improve catalytic performance by adjusting the electronic structure at the interface.¹⁶

In this work, we report the synthesis of a 2D/1D lattice- mismatched heterostructure based on a 1D CoS₂nanowire and 2D ReS₂ nanosheet via a chemical vapor deposition (CVD) method. X-ray photoelectron spectroscopy (XPS) results prove that the existence of terminal S²⁻at the 2D/1D interface plays an important role in improving electron transfer efficiency and enhancing electrical conductivity of the ReS₂/CoS₂. The ReS₂/ CoS₂ heterostructures exhibited remarkable hydrogen evolu- tion reaction (HER) performance with a small overpotential of

−114 mV at 10 mA/cm²and−196 mV at 100 mA/cm². Our results offer new insights for both construction of 2D/1D heterostructures and synthesis of high-performance electro- catalysts in water splitting.

2. EXPERIMENTAL SECTION

2.1. Reagents.All regents were of the analytical grade and were used without further purification. Cobalt nitrate hexahydrate (Co (NO₃)₂·6H2O, 99%), urea (CO (NH₂)₂, 99%), sublimed sulfur (S, 99.99%), and sodium hypophosphite (NaH2PO2, 99%) were purchased from Aladdin. Rhenium trioxide (ReO3, 99.9%) was purchased from Alfa Aesar. Ammoniumfluoride (NH4F, 98%) was obtained from Energy Chemical. Deionized (DI) water was supplied with a Millipore system.

2.2. Synthesis of CoS2and CoP Nanowires. CoS₂ nanowires were prepared according to the previous works.^17,18In brief, 1.576 g of Co(NO3)2·6H2O, 1.2 g of urea, and 0.222 g of NH4F were dissolved in 35 mL of DI water and then agitated for 30 min. The obtained solution and carbon cloth (CC) (1×4 cm²) were placed into a 50 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 120°C for 6 h. After the autoclave cooled down to room temperature, the Co precursor was taken out and thoroughly washed with DI water and ethanol several times alternatively and then dried at 60 °C for 6 h. Subsequently, the Co precursor was placed in an alumina boat, and the other alumina boat containing 1 g of sulfur was placed at the upstream of the tube furnace. The two alumina boats were calcined at 500°C for 2 h with a heating speed of 2°C/min under Arflow and then cooled down to room temperature naturally.

Finally, the prepared sample was immersed in carbon disulfide (CS2) for 10 min and washed with DI water and ethanol several times. The synthetic procedures for CoP nanowires are the same as the ones for CoS₂. The only difference is that the NaH₂PO₂ was used in the calcined step instead.

2.3. Synthesis of ReS₂/CoS₂and ReS₂/CoP.S (500 mg) in an alumina boat was placed in the central hot zone 1 where the temperature would reach 200°C. ReO3(2 mg) in an alumina boat

was analyzed by X-ray diffraction (XRD, Bruker D8 ADVANCE) with a sweep speed of 4°min⁻¹. Raman spectroscopy was performed on a Renishaw 42K864 system with a 532 nm excitation laser. XPS measurements were characterized using Thermo Fisher Scientific K- Alpha⁺.

2.5. Electrochemical Measurements.Electrochemical measure- ments were performed using an electrochemical workstation at room temperature (CHI 760C, CH Instruments Inc.) in a 0.5 M H2SO4

aqueous solution. Ag/AgCl (in a 3 M KCl solution) and a graphite rod were used as reference and counter electrodes, respectively. The prepared ReS₂/CoS₂heterostructure on the CC was directly used as a working electrode without any binder. Without special emphasis, the current densities were evaluated by the geometric surface area of ReS₂/CoS₂. All voltages were corrected to the reversed hydrogen electrode according to the following equation:ERHE=EAg/AgCl+ 0.059 pH + 0.205. The polarization curves were acquired by sweeping the potential from 0 to−0.4 V [vsreversible hydrogen electrode (RHE)]

with a scan rate of 5 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out with an amplitude of 10 mV and a frequency range from 100 kHz to 0.1 Hz. Cyclic voltammetry (CV) curves at different scanning rates were used to estimate the electrochemical double layer capacitance (EDLC) of the prepared samples. Stability tests were measured by the potential between 0 and−0.4 V (vsRHE) at a sweep rate of 100 mV/s for 1000 cycles at room temperature, and linear sweep voltammetry (LSV) was recorded at a scanning rate of 5 mV/s before and after the 1000 cycles. All the electrochemical measurements have beeniR-corrected, whereiis the test current and Ris the compensation resistance.

3. RESULTS AND DISCUSSION

In the above 2D/1D lattice-mismatched heterostructure, CoS₂ was chosen as the 1D component for several reasons. First, as a representative of Co-based materials and pyrite-phase tran- sition metal disulfides (TMDs), pyrite CoS₂ is inherently a conductive metal, as opposed to semiconducting pyrites (NiS₂, FeS₂), which makes it a unique advantage as an electrocatalyst material.¹⁹Second, CoS₂nanowires can expose more catalyti- cally active sites compared with bulk CoS₂crystals. Third, the terminal S²⁻of CoS₂has extremely high reactivity, which may serve as the nucleation site for lattice-mismatched materials.^6,20 As an emerging 2D material, the catalytic properties of ReS₂ are significantly different from those of traditional TMDs because of its weak interlayer coupling.²¹ Such a weak interlayer coupling feature not only allows it to expose more edge sites and basal planes, but also promote the diffusion of electrolyte ions between layers.²²More importantly, S²⁻on the surface of CoS₂ has high reactivity, which can serve as the nucleation site for other materials. For example, Zhou et al.

successfully synthesized a WS₂−CoS₂ electrocatalyst. The formation of WS₂−CoS₂is due to the high reactivity of S²⁻

on the surface of CoS₂, which can react with (NH₄)₂WS₄to synthesize WS₂, thus forming a Co−S−W interface.²³Hence, ReS₂ nanosheets were expected to grow on CoS₂ nanowires

because the S²⁻on the CoS₂nanowires, with its high reactivity, should serve as a nucleation site for the growth of ReS₂into a nanosheet. Moreover, both the CoS₂ nanowires and ReS₂ nanosheets crystallize into the cubic crystal and triclinic systems,^24,25respectively, and their lattice constants have a big difference. Hence, they are ideal materials for investigating lattice-mismatched 2D/1D heterostructures in catalysis.

The ReS₂(2D)/CoS₂(1D) heterostructure has been synthe- sized through a two-step process, as schematically illustrated in Figure 1a−c. First, CC is chosen as the growth substrate for CoS₂ nanowires. Because of the oxygen-containing functional groups on its surface, CC can bond with the Co precursor in a hydrothermal process, thus allowing the growth of CoS₂ nanowire arrays. High-density ReS₂ nanosheets are then grown on CoS₂ nanowires via CVD because the enormous specific surface area of a nanowire provides abundant nucleation sites, and the terminal S²⁻ in CoS₂ has high reactivity.²⁶The morphologies are investigated by SEM. CC is composed of high-density carbon fibers with an average

diameter of 7μm (Figure 1d). High-density CoS₂ nanowires with lengths of 7−8 μm are uniformly distributed on the cylindrical surface of the carbon fibers (see Figures 1e and S2a). In the high-magnification inset of Figure 1e, CoS₂ nanowires have a rough surface morphology, and their diameter decreases from bottom to top. As shown inFigure 1f, by using a CVD method, freestanding 2D ReS₂nanosheets, which are mostly hexagonal-shaped having side lengths less than 100 nm, are synthesized on the CoS₂nanowires to form a 2D/1D heterostructured system. Furthermore, high-density ReS₂ nanosheets can grow uniformly on CoS₂ nanowires by adjusting the growth time of ReS₂nanosheets (Figure S2b,c).

Meanwhile, the thickness of a ReS₂nanosheet is about 3 nm, corresponding to five layers of sandwiched S−Re−S. Clearly, compared with other low-dimensional hybrid systems, such a 2D/1D heterostructure achieves a much higher specific surface area, which could expose more catalytically active sites.²⁷

The crystal structures are first investigated by XRD. As illustrated in Figure 2a, a characteristic peak at 14.6° Figure 1.Schematic illustration for the fabrication of heterostructured ReS₂/CoS₂composites. (a) Wovenfibers of bare CC. (b) CoS2nanowire arrays on CC by a hydrothermal process and sulfurization. (c) Heterostructured ReS₂/CoS₂on CC by CVD. Low-magnification SEM images of the (d) woven CC, (e) CoS₂nanowire arrays and (f) ReS₂/CoS₂corresponding to (c). Insets in (d−f) are the corresponding high-magnification SEM images of a carbonfiber, a CoS2nanowire, and ReS₂/CoS₂, respectively.

Figure 2.(a) XRD patterns of CoS₂nanowires and the ReS₂/CoS₂heterostructure. (b) Raman spectra of CoS₂nanowires and the ReS₂/CoS₂ heterostructure. The migration occurs only on the Co−S bond, indicating that the growth of ReS₂nanosheets affects the electron cloud of the Co− S bond. (c) XPS survey spectra of CoS₂nanowires and the ReS₂/CoS₂heterostructure on CC. High-resolution XPS scans of (d) Co 2p, (e) Re 4f, and (f) S 2p electrons of CoS₂and ReS₂/CoS₂, respectively.

corresponds to the (001) planes of ReS₂ (JCPDS no. 52- 0818). The other peaks are highly consistent with the planes of a pyrite CoS₂structure (JCPDS no. 41-1471).²⁸Moreover, the vibration modes of molecular bonds are investigated by studying the Raman spectrum (Figure 2b). The Raman peaks at 162.5, 213.4, and 304.2 cm⁻¹ correspond to the in- plane (E_g), out-of-plane (A_g), and like-in-plane (E_g-like) vibration modes of ReS₂, respectively.²⁹Additionally, the two peaks at 286.3 and 390.6 cm⁻¹correspond to in-plane (E_g) and out-of-plane (A_g) vibration modes of Co−S, respectively.³⁰ Clearly, the Raman signatures of both E_gand A_gmodes of the Co−S bond appearing in ReS₂/CoS₂ have blue shifts compared with those seen in pure CoS₂. The Raman spectra indicate that the electron cloud density of S atoms on the surface of CoS₂ may be influenced by the growth of ReS₂. Therefore, we assume that S²⁻exposed on the surface of CoS₂ nanowires played an important role in the initial growth of ReS₂. All tests indicate that ReS₂/CoS₂ heterostructures are successfully prepared.

The surface electronic state and chemical composition are characterized by using XPS.Figure 2c demonstrates the whole survey spectra of the ReS₂/CoS₂ heterostructure and CoS₂ nanowires on CC, where the characteristic peaks of Co, O, S, and C can be distinguished. In the binding energy range of 0− 500 eV, we can observe the existence of Re 4f, Re 4p, and Re 4d only in the ReS₂/CoS₂sample. Generally, O and C should be ascribed to the CC and the oxygen-containing functional groups attached on the surface of CC.³¹ In a high-resolution XPS scan of the ReS₂/CoS₂(seeFigure S2d), compared with the one of CC, the peak at 284.8 eV corresponds to C 1s, and the new peak appearing at 286.4 eV is ascribed to the C−S bond.³²The XPS observation of the C−S bond suggests that CoS₂ and CC substrates are connected by a covalent bond, which is beneficial for electron transport in CoS₂/CC and for the material’s HER performance. In the high-resolution XPS spectra of the Co region (Figure 2d), the Co 2p peaks in ReS₂/ CoS₂ shift to a lower binding energy compared with the corresponding ones in CoS₂. Such a phenomenon indicates an increase in the electron cloud density in CoS₂ after

incorporating with ReS₂². With regard to the Re region (Figure 2e), the peaks at 42.2 and 44.7 eV are ascribed to the binding energy of Re 4f_7/2and Re 4f_5/2of ReS₂, respectively.³³ In the S region (Figure 2f), two new peaks appear at 162.3 and 163.7 eV, which correspond to the binding energy of the Re−S bond.³⁴ Moreover, the peaks at 162.9 and 164.1 eV are assigned to S 2p_3/2and S 2p_1/2of the terminal S²⁻in CoS₂,^20,35 and the two peaks shift toward lower binding energy in the ReS₂/CoS₂sample. We assume that the peak shift of S 2p is probably caused by the electron redistribution around S atoms at the interface of ReS₂/CoS₂, suggesting the existence of a coupling interface between CoS₂and ReS₂.³⁶

To further investigate the interface of the ReS₂/CoS₂ heterostructure, scanning TEM (STEM) is used.Figure 3a is a low-magnification TEM image of the ReS₂/CoS₂ hetero- structure. High-density freestanding ReS₂ nanosheets are distributed on CoS₂ nanowires. Furthermore, as shown in the right side of Figure 3a, the high-angle annular dark-field (HAADF) TEM image and the corresponding (EDX) elemental mappings are analyzed. We notice that the Re and S elements are distributed more widely than the Co element, suggesting that high-density ReS₂ nanosheets grow on the surface of the CoS₂ nanowire. A small freestanding ReS₂ nanosheet growing on a CoS₂nanowire is displayed inFigure 3b. Clearly, the 2D crystal planes of ReS₂are in a hexagonal shape. A corresponding schematic atomic diagram of the ReS₂/ CoS₂ heterostructure is illustrated in the right of Figure 3b.

Typically, the growth rates along different crystal planes are considered to be decisive in determining the crystal shape.³⁷In the high-resolution STEM image of Figure 3c (inset is the enlarged view on ReS₂), a distinct interface between the ReS₂ nanosheet and CoS₂nanowire is revealed. We confirm that Re₄ units formed the 1D Re−Re chains in the ReS₂nanosheet, and the two characteristic lattice spacings of 0.34 and 0.31 nm could be ascribed to (100) planes along theb-axis and (010) planes along thea-axis of ReS₂, respectively.³⁸In general, the rapid crystal growth of (100) planes is dominant, while the growth rate in the (010) direction is relatively slow.^39,40Thus, the freestanding growth of ReS₂ should result in anisotropic Figure 3.(a) Low-magnification TEM image of heterostructured ReS₂/CoS₂. The HAADF and elemental mapping images of Co, S, and Re are taken from the white boxed region. (b) HRTEM image of interfacial area on ReS₂/CoS₂on the left and the corresponding schematically atomic structure on the right. (c) STEM image of interfacial area between the ReS₂nanosheet and CoS₂nanowire. Inset is the high-magnification view of ReS₂. (d) High-magnification view along the interface, with marked lattice distortion of ReS₂.

2D crystal shapes. However, we observe the freestanding growth of 2D hexagonal ReS₂nanosheets on CoS₂nanowires, which should be related to a 2D/1D heterostructured growth mechanism. ReS₂and CoS₂belong to triclinic and cubic crystal systems, respectively.^41,42 According to the calculation of the semiconductor heterojunction formula,⁴³the lattice mismatch of the semiconductor heterojunction composed of these two substances is 15.22% (see Calculation of Lattice Mismatch section in theSupporting Information). During the formation of a heterostructure, the internal energy at the interface increases because of the large lattice mismatch.⁴⁴As marked in Figure 3d, we observe clear lattice distortion of ReS₂near the interface of ReS₂/CoS₂, which is a direct proof of the increased internal energy of freestanding ReS₂ nanosheets grown on CoS₂ nanowires. The corresponding fast Fourier transform (FFT) images of the interface region and the noninterface region are compared (see Figure S4), indicating the lattice distortion at the interface.

Typically, the terminal S²⁻ have been exposed on the surfaces of CoS₂.^6,45,46 Using XPS, Han et al. demonstrated that S²⁻exist at the edge of CoS₂and is highly catalytic.¹⁸Hou et al. confirmed the existence of terminal S²⁻ between MoS₂ and CoS₂, which acted like a bridge to transfer electrons more efficiently.⁴⁷In our XPS survey (Figure 2f), the terminal S²⁻

are also confirmed on the surfaces of CoS₂ nanowires. To clarify the effect of terminal S²⁻on the freestanding growth of 2D hexagonal ReS₂ nanosheets, cobalt phosphide (CoP) nanowires with a similar crystalline structure are selected as a control model of heterostructured growth of ReS₂/CoP (see Figure S5 inSupporting Information).^48,49

During the initial stage of nucleation, unsaturated S atoms on the surface of CoS₂have extremely high reactivity and thus can easily react with rhenium heptoxide (Re₂O₇) decomposed from ReO₃.⁵⁰ Hence, we deduce that the terminal S²⁻ could facilitate the nucleation of ReS₂. Meanwhile, P atoms exposed on the surface of CoP are replaced by S atoms to obtain the CoP_xS_(1−x) structure under an S atmosphere at 500 °C.⁵¹ Theoretically, the content of S should be 0.68%, based on the content of Re (0.34%) in ReS₂/CoP (seeFigure S6e). In fact, the S content is measured to be 1.39%, which suggests the existence of substituted S. The subsequent growth stage can be divided into early growth stage and final growth stage. As schematically illustrated in the growth model of Figure 4a, when 2D ReS₂ grows on CoS₂ with a rough surface, the internal energy is dramatically increased in the early growth stage, which inhibits planar 2D growth. On a CoP surface, however, because of a relatively smooth surface (see Figure 4b), ReS₂tends to planar 2D growth. Experimentally, in the early growth stage (5 min), freestanding growth of 2D ReS₂ nanosheets was observed only on CoS₂nanowires (seeFigure S6a−d). Normally, the interfacial stress has an influence on the crystal morphology of 2D materials.^52,53 In the final growth stage, the accumulated internal energy near the heterointerface is released, for example, the lattice distortion observed near the ReS₂/CoS₂heterointerface (Figure 3d). In general, the growth rate along (100) planes is higher than that along (010) and (1−10) planes. However, Hafeez et al.found that the growth of crystal planes with a higher growth rate was much inhibited at these interfaces because of the excessive internal energy.⁴⁰ Thus, as schematically illustrated in thefinal stage ofFigure 4a, the growth rate in the (100) direction is somewhat reduced compared to those in (010) and (1−10) directions. Gradually, hexagonal ReS₂ nanosheets appeared with increased 2D size

(seeFigure S7a). Ghoshalet al.reported that unstable regions would gradually appear in the center of planar 2D ReS₂, leading to the appearance of out-of-plane unsaturated S atoms.⁵⁴ The unsaturated S is reactive and can initiate freestanding 2D growth on the planar one. In fact, during the final growth stage (10 min), freestanding 2D ReS₂ nanosheets also appear on the CoP nanowires and are mostly in irregular crystal shapes (see Figure S7b), which can be ascribed to the excessive growth of (100) planes.⁵⁴

LSV measurements are performed to investigate the HER performances of ReS₂/CoS₂ electrocatalysts synthesized on CC. A typical three-electrode setup is used in 0.5 M H₂SO₄at a scan rate of 5 mV/s. As illustrated in Figure 5a, the HER activities with different ReS₂growth times (10, 15, 20, and 25 min) are compared with bare CC, CoS₂@CC, and Pt/C electrodes. The performance of the Pt/C is better than most reported ones in the literatures (Table S1). As expected, the pure CC electrode exhibits almost no HER activity, while CoS₂@CC demonstrates excellent HER activity.⁴⁶ The HER activities of all ReS₂/CoS₂@CC samples are further enhanced beyond that of CoS₂@CC. The enhanced HER performance should be attributed to the S²⁻ at the ReS₂/CoS₂ interface, which can improve the electron transmission efficiency at the interface. Moreover, the band diagram of ReS₂/CoS₂can help to understand the improvement of electronic efficiency at the interface. InFigure S8, when these two substances come into contact with each other, a Schottky junction is formed.⁸ Ideally,E_f‑ReS2 >E_f‑CoS2, and electrons will move from ReS₂to CoS₂. In the electrochemical test, a reverse bias will be applied to the electrode. Therefore, electrons willflow from CoS₂to ReS₂. However, the electron must cross the Schottky barrier, and the electron transmission efficiency will decrease.¹¹As for our samples, ReS₂ and CoS₂ are connected by S²⁻ on the surface of CoS₂instead of simple contact. The formation of chemical bonds will change the electronic structure at the interface, thereby reducing the potential barrier.^55,56Therefore, Figure 4.Schematic illustration of the three-dimensional (3D) crystal structure and growth mechanism of freestanding ReS2nanosheets. (a) In the early growth stage, ReS₂ nanosheets grow out of the plane because of the rough surface of CoS2, while in thefinal growth stage, the formation of hexagonal ReS2nanosheets is attributed to interface stress. (b) On CoP, in the early growth stage, ReS₂nanosheets grow parallel to the surface of CoP, while in thefinal growth stage, the formation of irregular ReS₂ nanosheets should be attributed to the excess growth rate along the (100) plane.

the electron transport will be enhanced by the presence of S²⁻. To prove that the enhanced HER performance is not only derived from the intrinsic activity of ReS₂, ReS₂nanosheets are directly synthesized on CC, and their HER performance is tested (see Figure S9). In Figure S9d, the overpotentials of ReS₂/CC and CoS₂/CC are much smaller than that of ReS₂/ CoS₂at 10 mA/cm². Compared with the HER performance of ReS₂/CoS₂, the performance of ReS₂ or CoS₂alone is poor.

Hence, the enhanced HER performance is ascribed to the introduction of ReS₂ and derived from a synergistic effect between ReS₂and CoS₂.⁵⁷Specifically, the highest-performing ReS₂/CoS₂-20 (seeFigure S11a) has overpotentials of 114 and 196 mV at 10 and 100 mA/cm², respectively, which are the values closest to those of the Pt/C (28, 79 mV). Catalytically active sites of 2D ReS₂existed on both the plane edges and the basal planes, which are ascribed to unsaturated S and Re−Re bonds, respectively.⁵⁸⁻⁶⁰We observe that both size and density of freestanding ReS₂ nanosheets on CoS₂ nanowires are gradually increased over thefirst 20 min (seeFigure S10a−d), introducing more active sites to improve HER activity.

However, with additional growth time, the HER activity of ReS₂/CoS₂-25 decreased sharply, corresponding to the agglomeration of high-density ReS₂ nanosheets (see Figure S10e).

The Tafel slopes are analyzed byfitting the linear portions of polarization curves with the Tafel equation (η=blogj+a).⁶¹ As illustrated inFigure 5b, the Tafel slope of ReS₂/CoS₂-20 was 63.7 mV/dec, which is lower than those for CoS₂(117.5 mV/dec), ReS₂/CoS₂-10 (109.5 mV/dec), ReS₂/CoS₂-15 (63.8 mV/dec), and ReS₂/CoS₂-25 (109.3 mV/dec). This slope is the closest to that of commercial Pt/C (43.17 mV/

dec). Generally, a lower Tafel slope reflects more favorable HER kinetics, suggesting that a larger current density per unit area can be obtained under a lower overpotential. To compare the electrochemical active surface area (ECSA) of electrodes, EDLC (C_dl) are analyzed by CV at different scan rates (see Figure S11b−f). As displayed in Figure 5c, the C_dl of ReS₂/

CoS₂-20 was 69.48 mF/cm², which is larger than those of bare CoS₂(12.24 mF/cm²), ReS₂/CoS₂-10 (33.94 mF/cm²), ReS₂/ CoS₂-15 (39.33 mF/cm²), and ReS₂/CoS₂-25 (32.01 mF/

cm²). ECSA can be calculated according to the Randles− Sevcik equation

C C ECSA C

40 F cm per cm

dl s

dl 2

ECSA

μ 2

= = _{− −}

(1) whereC_s(40μF/cm²) is the specific capacitance of a smooth surface.⁶²With this equation, the ECSA of ReS₂/CoS₂-20 is calculated to be 1737 cm², which is larger than those of CoS₂ (306 cm²), ReS₂/CoS₂-10 (848 cm²), ReS₂/CoS₂-15 (983 cm²), and ReS₂/CoS₂-25 (800 cm²). From the tendency of ECSA with increased ReS₂ growth time, we deduce that the agglomeration of ReS₂ nanosheets can greatly reduce the number of active sites for Re−Re bonds on the plane surface.

The enhanced HER activities of ReS₂/CoS₂ heterostruc- tured electrocatalyst are further confirmed by EIS measure- ments at −0.2 V. Typically, R_ct denotes the charge-transfer resistance for HER at the electrode/electrolyte interface. The approximate semicircle curves are presented in the Nyquist plots ofFigure 5d, where the diameter reflectsR_ctduring the process of HER.R_ctvalues of bare CoS_2,ReS₂/CoS₂-10, ReS₂/ CoS₂-15, ReS₂/CoS₂-20, and ReS₂/CoS₂-25 were 15.31, 4.86, 3.41, 2.10, and 4.85 Ω, respectively. The ReS₂/CoS₂-20 electrode has the lowest resistance, which suggests that it experienced faster HER kinetics than other electrodes.

Electrochemical stability is also a key factor for a high- performance electrodes. In Figure 5e, ReS₂/CoS₂-20 synthe- sized on CC exhibits a much improved stability at 10 mA/cm² for a 10 h duration. The excellent stability is due to the fact that ReS₂/CoS₂heterostructures can prevent the oxidation of active S atoms on the surface of CoS₂ nanowires. Moreover, Figure 5f demonstrates two polarization curves for ReS₂/CoS₂- 20 before and after a 1000-cycle stability test. The negligible change verifies that an ReS₂/CoS₂ heterostructure directly synthesized on CC serves as a stable and high-performance Figure 5.(a) HER polarization curves of CC, CoS₂, ReS₂/CoS₂-10, ReS₂/CoS₂-15, ReS₂/CoS₂-20, ReS₂/CoS₂-25, and Pt/C electrodes in 0.5 M H₂SO₄with a potential sweep rate of 5 mV/s and (b) corresponding Tafel plots. (c) Dependence of current on the scan rate with different double layer capacitances. (d) Nyquist plots of CoS₂, ReS₂/CoS₂-10, ReS₂/CoS₂-15, ReS₂/CoS₂-20, and ReS₂/CoS₂-25 electrodes. (e) Electrochemical stability of ReS₂/CoS₂-20 measured by chronopotentiometry at 10 mA/cm². (f) HER polarization curves of ReS₂/CoS₂-20 before and after 1000 cycles for the stability test.

HER electrode. Based on the electrochemical performances of state-of-the-art ReS₂and CoS₂catalysts listed inTable S2, we have achieved the best HER performance with ReS₂/CoS₂. In addition, three batches of ReS₂/CoS₂-20 are prepared and tested for the electrochemical performance (see Figure S12), indicating a high reproducibility of the catalytic effect.

Another benchmarking methodology can be used to quickly evaluate the performance of catalysts.⁶³ The benchmarking parameters for HER catalysts in 0.5 M H₂SO₄ can be all compared in one plot (seeFigure S13). The best catalyst with high catalytic activity and good stability should operate at a low potential and appear near the diagonal dashed line. Clearly, ReS₂/CoS₂-20 demonstrates the best HER performance among the catalysts, which is also consistent with the result shown in Figure 5a,f. In addition, we calculate its roughness factor (RF) according toeq 2.

RF A ECSA

geometric area

=

(2) Ageometric areais 1 cm². ReS₂/CoS₂-20 has the largest RF (1737), indicating the largest contact area with the electrolyte.

4. CONCLUSIONS

In summary, high-density ReS₂nanosheets with a controllable hexagonal 2D shape are grown freestanding on CoS₂ nanowires via a CVD method, forming a hybrid 2D/1D heterostructured electrocatalyst. The corresponding growth mechanism for the 2D/1D heterostructure is proposed. We also demonstrate that S²⁻ at the 2D/1D interface work as a bridge to enhance the electron transport efficiency of ReS₂/ CoS₂. Therefore, such a heterostructured electrocatalyst, with merits of both 1D and 2D components, can significantly improve HER performance. This work offers new insights for the construction of 2D/1D systems and high-performance electrocatalysts in water splitting.

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ASSOCIATED CONTENT

*^sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.0c02951.

Calculation of lattice mismatch, figures showing schematic illustrations of lattice mismatch, SEM images, XPS spectrum, TEM images, STEM images, FFT images, XRD patterns, EDX elemental mapping, Raman spectrum, CV curves, HER performances of CoS₂-based catalysts and Pt/C, band alignment diagram, and the benchmarking methodology (PDF)

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AUTHOR INFORMATION Corresponding Authors

Zhipeng Hou−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China; orcid.org/0000-0003-4935-2149;

Email:houzp@m.scnu.edu.cn

Zhang Zhang− Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics and National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou

510006, P. R. China; orcid.org/0000-0001-6287-502X;

Email:zzhang@scnu.edu.cn Authors

Yuanwu Liu−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

Jing Li−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R.

China

Wentian Huang−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

Ying Zhang−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

Minjie Wang−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

Xingsen Gao−Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China; orcid.org/0000-0002-2725-0785 Xin Wang−National Center for International Research on

Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China; orcid.org/0000-0002-4771-8453 Mingliang Jin−National Center for International Research on

Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China

Guofu Zhou−National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China; orcid.org/0000-0003-1101-1947 Junming Liu− Guangdong Provincial Key Laboratory of

Quantum Engineering and Quantum Materials, Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China; Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, P. R. China;

orcid.org/0000-0001-8988-8429 Complete contact information is available at:

https://pubs.acs.org/10.1021/acsami.0c02951

Author Contributions

Y.L. conducted the experiments and wrote this paper; Z.Z.

designed the experiments and guided the writing, Z.H. guided the writing; J.L., W.H., Y.Z., and M.W. helped to analyze experimental results; X.G., X.W., and M.J. offered important advices and convenient tests; Z.Z., G.Z., and J.L. funded the experiments.

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