Enhanced charge transport in ReSe

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This journal is © The Royal Society of Chemistry 2020 Chem. Commun.,2020,56, 305--308 | 305 Cite this:Chem. Commun.,2020,

56, 305

Enhanced charge transport in ReSe

₂

-based 2D/3D electrodes for eﬃcient hydrogen evolution

reaction†

Jing Li, âYuanwu Liu,âChen Liu,^bWentian Huang,âYing Zhang,âMinjie Wang,â Zhipeng Hou,âXin Wang,^cMingliang Jin,^cGuofu Zhou,^cXingsen Gao, â

Zhang Zhang *^acand Junming Liu ^ad

Ultra-high-density rhenium diselenide (ReSe2) nanoflakes were synthesized on a porous carbon cloth (PCC) by chemical vapor deposition (CVD). Besides the two-dimensional/three-dimensional (2D/3D) construction with more active catalytic sites, the small size eﬀect together with the interfacial C–Se bonding facilitated the electron transport between ReSe2 and PCC. Hence, the heat- treated ReSe2@PCC with enhanced charge transport is by far the best performance electrode for the hydrogen evolution reaction (HER) among the state-of-the-art ReX2-based electrodes.

An urgent demand for clean energy indeed exists for combating energy crisis and reducing environmental issues. At this moment, hydrogen (H2) seems to be the cleanest energy that has attracted much attention. Many kinds of catalysts have been extensively explored in order to improve the hydrogen evolution reaction (HER) in water electrolysis.^1–4Platinum (Pt) is considered to be the best-performing HER electro-catalyst due to its almost zero overpotential. However, the high cost and rarity of Pt hinder its large-scale applications.^5,6 Hence, the exploration for efficient and low-cost HER catalysts is essential to replace Pt. Recently, transition metal dichalcogenides (TMDs) are considered as potential candidates due to their earth-abundant nature and definite catalytic mechanism.^2,7,8

In general, the number of the active edge sites determines the HER catalysis of TMDs, whereas the basal surfaces are catalytically inert.^9,10Various methods have been developed to

decrease the size of TMDs for exposing more edge sites.⁹Among them, the use of a suitable three-dimensional (3D) substrate can realize the high-density growth of TMDs with more active sites.

A porous 3D structure possesses higher surface area than a less porous structure.^11–13Besides, TMDs should have high electron transport to promote the charge transfer rapidly between the electrode and the active sites, which can be improved by synthesizing TMDs directly on conductive substrates (carbon cloth, Au and graphene).^14–17Though the catalytic performance has indeed been improved with directly grown TMDs compared with the transferred one, there is still contact resistance between TMDs and the conductive substrate. In order to further enhance the HER performance, a TMD-based electrode with both high density of active sites and low contact resistance is still in the exploration process.

Recently, an ReX₂catalyst has been revealed to be a unique member among the TMD family, which has attracted much more attention in electro-catalytic applications due to its distorted 1T phase structure with diamond-shaped Re₄chains.^18–20However, the electro-catalytic performance of ReX2 is still in need of improvement. In this work, ultra-high-density ReSe2nanoflakes with much smaller two-dimensional (2D) sizes were uniformly synthesized on a porous carbon cloth (PCC) by chemical vapor deposition (CVD). An atmospheric post-heat treatment was developed to decrease the contact resistance of the ReSe2@PCC electrode, through which the interfacial C–Se bonding was generated to highly accelerate the electron separation efficiency.

The heat-treated ReSe2@PCC electrode possessed more exposed active sites and much enhanced charge transport, exhibiting the best HER performance among ReX₂-based electrodes so far;

moreover, it exhibited an overpotential of 140 mVvs.RHE at a current density of 10 mA cm ²and a Tafel slope of 64 mV dec ¹. The morphology and crystal structure were characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). To enhance the HER performance, more active sites on the catalyst should be exposed. Due to the larger special surface area, PCC was produced as a conductive

aGuangdong 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.

E-mail: [email protected]

bInstitute of Physical Science and Technology, Lanzhou University, Lan Zhou 510006, Lanzhou 730000, P. R. China

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

South China Normal University, Guangzhou 510006, P. R. China

dLaboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, P. R. China

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc08076g Received 15th October 2019,

Accepted 19th November 2019 DOI: 10.1039/c9cc08076g rsc.li/chemcomm

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306 | Chem. Commun.,2020,56, 305--308 This journal is © The Royal Society of Chemistry 2020 substrate for the growth of ReSe₂ (Fig. S1, ESI†). The rough

surface of the carbon fiber with high density pores can be clearly observed. Meanwhile, the porous carbon fiber maintains its basic shape, which should not aﬀect the whole electron conduction.

ReSe₂was directly grown on PCC by using the CVD method. As shown in the overview SEM image of Fig. 1a, a nanostructured ReSe2 layer completely covers the surface of PCC with high uniformity. In the high-magnification SEM images (Fig. 1b and c), we confirm that the out-layer is composed of ultra-high-density free-standing ReSe2nanoflakes with small 2D sizes in the range of tens of nanometers. Compared with the free-standing ReSe2

nanosheets grown on CC, the ReSe2 nanoflakes grown on PCC

at the same CVD conditions possessed distinctly diﬀerent morphologies (Fig. S2, ESI†). The much smaller 2D size and the much higher growth density should result in significantly increased active sites of ReSe₂. The crystal structure of the ReSe₂ nanoflakes was investigated by a spherical aberration- corrected transmission electron microscope (ACTEM). The unique crystal structure of ReSe₂with Re₄chains can be clearly recognized in Fig. 1d. The single crystal quality of the ReSe₂ nanoflakes fabricated by CVD was also confirmed from the high-resolution TEM (HR-TEM) image and its corresponding SAED pattern (Fig. S3a, ESI†). The thickness of the ReSe2

nanoflakes is about 2.5–4 nm, corresponding to the 4–8 layers of sandwiched Se–Re–Se (Fig. S3b, ESI†). Besides, the highly uniform distributions of C, Se and Re were verified from energy dispersion X-ray spectroscopy (EDX) elemental mappings (Fig. S4, ESI†). The chemical stoichiometric ratio of Re to Se was calculated to be 1 : 2.16.

In order to decrease the contact resistance, the as-synthesized ReSe₂@PCC underwent a suitable post-heat treatment in air. The bonding states of C, Se, Re and O before and after the heat treatment were analyzed by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 2a, obviously, two new peaks appear for the heat-treated sample. One peak appearing at 288.7 eV corresponded to the CQO bonding caused by air oxidation; the other one at 286.4 eV was ascribed to the formation of the C–Se bonding,²¹which suggested the improvement in the interfacial bonding between the ReSe2nanoflakes and the PCC electrode through the post-heat treatment. Furthermore, as illustrated in Fig. 2b, the O 1s peak at 532.6 eV can be attributed to water absorption, and a new peak at 531.8 eV for the heat-treated sample corresponds to the CQO bonding, which causes the Fig. 1 Morphology and crystal structure of ReSe2nanoflakes grown on

PCC. (a–c) SEM images at different magnifications. (d) HAADF-STEM image of ReSe2with marked Re4chains.

Fig. 2 (a–d) High-resolution XPS spectra of C, Re, Se and O in ReSe₂@PCC, respectively. (e) Raman and (f) XRD spectra of ReSe₂nanoflakes before and after the heat treatment.

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This journal is © The Royal Society of Chemistry 2020 Chem. Commun.,2020,56, 305--308 | 307 narrowing of the O 1s peak width.²² Furthermore, as shown

in Fig. 2c and d, two distinctive peaks can be observed for both Se (Se 3d_5/2and 3d_3/2) and Re (Re 4f_7/2and 4f_5/2). Due to the formation of the C–Se bonding, the XPS peaks of Re and Se for the heat-treated ReSe₂@PCC have redshifts compared with that of the as-prepared one. In addition, there is no peak of the Re–O bonding, indicating that the oxygen in air could not react with ReSe₂during the post-heat treatment. The stoichiometric ratios of Se to Re were calculated to be almost the same (about 2.2) before and after the heat treatment (Table S1, ESI†), indicating the compositional stability of ReSe2in the heating process. Due to the low symmetry of the crystal structure, there are more than ten distinctive Raman peaks for 2D ReSe2(Fig. 2e). Among these peaks, the Eg-like (123 cm ¹) and Ag-like (158 cm ¹) are the characteristic ones. Their peak positions were confirmed to be the same before and after the heat treatment, suggesting that no phase transition happened during the heating process. There are two characteristic XRD peaks of ReSe2(JCPDS 97-002-6256), corresponding to the (001) basal plane and the (0 21) plane on the upward growth direction of the ReSe₂nanoflakes.²³As illustrated in Fig. 2f, the diffraction peaks from the (001) and (0 21) planes remained invariable due to the stable crystal structure. Therefore, these analyses mainly manifested that interfacial C–Se bonding was fulfilled through the post-heat treatment without any change in ReSe2.

The HER performance was investigated through a three- electrode system in a 0.5 M H2SO4solution. Fig. 3a illustrates that the ReSe2@PCC electrode has a lower overpotential of 200 mV at the current density of 10 mA cm ²compared with the ReSe2@CC electrode (270 mV). We had presented that the

ReSe2 nanoflakes grown on PCC had more active sites than those grown on CC. The heat-treated ReSe2@PCC electrode has a further decreased overpotential of 140 mV due to the improvement in C–Se bonding, which can effectively promote the charge transport between ReSe₂ and PCC. The temperature of the post-heat treatment also had an influence on the HER performance (Fig. 3b).

The optimal temperature should be around 2201C. We observed the attenuated HER performance at 2401C due to the decrease in the ReSe₂nanoflakes by oxidation (Fig. S5, ESI†). Correspondingly, as shown in Fig. 3c, the heat-treated ReSe2@PCC electrode has a smaller Tafel slope of 64 mV dec ¹compared with ReSe2@PCC (104 mV dec ¹) and ReSe2@CC (151 mV dec ¹).

The interfacial reaction and electron transfer kinetics under HER conditions were investigated by electrochemical impedance spectroscopy (EIS). As plotted in Fig. 3d, the heat-treated ReSe2@ PCC has the lowest charge transfer resistance (Rct= 4.51O) due to the C–Se bonding, indicating the fastest electro-catalytic reaction rate. Furthermore, the corresponding electrochemical double-layer capacitances (C_dl) were measured to estimate the electrochemical effective surface areas (ECSA) (Fig. 3e). Obviously, the heat-treated electrode possessed the highestC_dlof 11.5 mF cm ², which also corresponded to the largest ECSA. Long-time stability is also a crucial factor to assess an electro-catalyst. To testify the stability of heat-treated ReSe₂@PCC, a long period (40 000 s) current density–

time (J–t) test was conducted at an overpotential of 130 mV (Fig. 3f). The current density of ReSe2@PCC declined within 10 000 s due to the transient state²⁴and adherent H2bubbles.²⁵ In the enlarged view, the undulation of the current curve is due to the persistent absorption and release of H2bubbles. To illustrate the changes in the catalyst after the long-time stability test,

Fig. 3 (a) The polarization curves of pure PCC, ReSe₂@CC, ReSe₂@PCC, heat-treated ReSe₂@PCC. (b) ReSe₂@PCC with diﬀerent post-heat temperatures. (c) The corresponding Tafel slopes. (d) EIS Nyquist plots at a 140 mV overpotentialvs.RHE, and (e) the ratio of current density with various scan rates. (f) The stability test of heat-treated ReSe₂@PCC at a 130 mV overpotentialvs.RHE; inset is the enlarged view.

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308 | Chem. Commun.,2020,56, 305--308 This journal is © The Royal Society of Chemistry 2020 we compared the surface morphologies of ReSe2@PCC before

and after the electrochemical test with a slight change (Fig. S6a and b, ESI†). Their Raman and XRD spectra were also compared, where no peak shifts or new peaks were observed after theJ–ttest (Fig. S6c and d, ESI†). Based on the electrochemical performances of the state-of-the-art ReX₂ catalysts listed in Table 1,^26–31we achieved the best HER performance so far with the heat-treated ReSe₂@PCC electrode.

Ultra-high-density free-standing ReSe₂ nanoflakes were synthesized on a porous carbon cloth by chemical vapor deposition. The ReSe2@PCC electrode throughout a proper post-heat treatment could not only maintain structural and compositional stability, but also improve the formation of interfacial bonding. Besides the 2D/3D construction with more active catalytic sites, the small size eﬀect together with the interfacial C–Se bonding facilitated the charge transport between ReSe2and PCC. We expect that this work can provide a new strategy for the synthesis of other TMD-based high-performance HER catalysts.

This work was supported by National Key R&D Program of China (2016YFB0401501), Science and technology project of Guangdong Province (No. 2015B090913004), Cultivation project of National Engineering Technology Center (Grant no. 2017B090903008), Science and Technology Program of Guangzhou (No. 2019050001), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007), Guangdong National Science Foundation (No. 2018A030313377, No.

2016A030308019) and Xijiang R&D Team (X. W.).

Conflicts of interest

The authors declare no competing financial interests.

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Table 1 Electrochemical performances of ReX2-based catalysts

Samples Z(mVvs.RHE) forj= 10 mA cm ² Tafel slope (mV dec ¹) Ref.

ReSe2microspheres 300 (j= 8 mA cm ²) 67.5 26

ReS₂microspheres 300 (j= 3 mA cm ²) 143.3 26

Sunflower-shaped monolayer ReSe₂ 270 76 27

V Re-ReS₂ 147 69 28

T@Td ReS₂(iR correction) 173 69.5 29

Vertically ReS₂(Li-intercalation) 200 84 30

Light-treated ReS₂(iR correction) 167 77 31

ReSe2nanoflakes on porous carbon cloth 200 108 This work

Treated ReSe₂nanoflakes on porous carbon cloth 140 64 This work

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