稀有金属(英文版) 2018,37(12),1014-1020

Few-layered ReS₂ nanosheets grown on graphene as electrocatalyst for hydrogen evolution reaction

Han Gao Hu-Hu Yue Fei Qi Bo Yu Wan-Li Zhang Yuan-Fu Chen

State Key Laboratory of Electronic Thin Films and Integrate Devices, Univrsity of Electronic Science and Technology of China

School of Optoelectronic Engineering, Chongqing University of Posts and Telecommunications

作者简介：Fei Qi,e-mail: 493290586@qq.com;*Yuan-Fu Chen,e-mail: yfchen@uestc.edu.cn;

收稿日期：13 September 2017

基金：financially supported by the National Natural Science Foundation of China (No. 51372033);the National High Technology Research and Development Program of China (No. 2015AA034202);the Program of the Ministry of Education of China for Introducing Talents of Discipline to Universities (No. B13042);

Few-layered ReS₂ nanosheets grown on graphene as electrocatalyst for hydrogen evolution reaction

Han Gao Hu-Hu Yue Fei Qi Bo Yu Wan-Li Zhang Yuan-Fu Chen

State Key Laboratory of Electronic Thin Films and Integrate Devices, Univrsity of Electronic Science and Technology of China

School of Optoelectronic Engineering, Chongqing University of Posts and Telecommunications

Abstract：

Few-layered ReS₂ two-dimensional(2 D) semiconductor nanosheets directly nucleated and grown on reduced graphene oxide(RGO) were synthesized through a facile hydrothermal method. Compared with bare ReS₂, the ReS2/RGO hybrid delivers much better electrocatalytic activity for hydrogen evolution reaction(HER) in acidic media. It exhibits a lower Tafel slope of 107.4 mV dec^-1and a larger current density of-5.2 mA·cm^-2 at-250 mV(vs. RHE),compared with ReS₂(152.7 mV·dec^-1,-3.1 mA·cm^-2). The ReS₂/RGO hybrid has a unique architecture constructed by highly conductive and porous RGO internetworks, which guarantees easy electrolyte infiltration and efficient charge transfer and provides sufficient active edge sites, resulting in enhanced HER performance. The present synthesis approach can be extended to synthesize other 2 D-semiconductor-based composites for energy storage and catalytic devices.

Keyword：

ReS₂/RGO; Hydrothermal method; Electrocatalyst; Hydrogen evolution reaction;

Received： 13 September 2017

1 Introduction

As green,clean and sustainable power,hydrogen energy produced from water splitting via hydrogen evolution reaction (HER) has become one of the research hot spots [ 1, 2, 3] .Recently,the transition metal dichalcogenides(TMDs) as electrocatalytic materials,such as MoS₂ [ 4, 5] ,MoSe₂ [ 6, 7, 8] ,ReSe₂ [ 9] ,WSe₂ [ 10, 11, 12, 13, 14, 15] ,WS₂ [ 16, 17, 18, 19] ,NiSe₂ [ 3, 20, 21, 22] ,CoSe₂ [ 23, 24, 25, 26, 27, 28, 29] and PtSe₂ [ 30] ,have attracted increasing attention due to their excellent HER performances.As a new member of the TMDs family,rhenium disulfide (ReS₂) has many distinctive features and exhibits great potential for future novel device applications due to unusual structure and unique anisotropic properties [ 31] .

Recently,some efforts have been devoted to studying the electrocataly tic activity of ReS ₂ for HER.Fujita et al. [ 32] reported the electrocataly tic activity of chemically exfoliated ReS₂ nanosheets.Gao et al. [ 33] reported the vertical ReS₂ nanosheets after post-lithiation with good electrocataly tic activity.However,the low electrical conductivity of ReS₂ still restrains its application in HER.So,it is crucial to rationally design a ReS₂ hybrid with high conductivity and sufficient HER active sites and synthesize it via a facile method.Carbon materials with excellent conductivity,such as carbon nanotubes and graphene,have been widely used to improve the conductivity of active materials for HER [ 12, 13, 21, 28] .Among them,reduced graphene oxide (RGO) has been widely used to construct highly conductive and porous networks,which can be used to load active electrocatalyst.Therefore,it is significant to design and synthesize a conductive and porous ReS₂/RGO composite to remarkably enhance the conductivity and improve the HER performance.Nevertheless,up to now,there is no report on the synthesis and electrocatalytic activity of ReS₂/RGO composite yet.

To address such issue,herein,for the first time,a facile hydrothermal method was present to synthesize a porous ReS₂/RGO composite constructed by few-layered ReS₂nanosheets directly nucleated and grown on RGO.As an electrocatalyst,ReS₂/RGO exhibits much better electrocatalytic performance than bare ReS_2.The electrocatalytic activity and mechanism were further discussed.

2 Experimental

2.1 Synthesis of ReS2 and ReS2/RGO

ReS₂/RGO was prepared as the following procedures.Firstly,100 mg graphene oxide (GO) was dispersed into60 ml deionized (DI) water.Secondly,536 mg ammonium perrhenate,417 mg hydroxylamine hydrochloride and685 mg thiourea were dissolved in the GO solution and stirred for 2 h.The mixed solution was transferred to a100-ml autoclave and heated at 240℃for 24 h.The prepared ReS₂/RGO was washed with DI water and ethanol several times.After drying at 60℃for 4 h,the final product was obtained by drying.As comparison,the bare ReS₂ was obtained by the process procedures similar to ReS₂/RGO.

2.2 Morphology and structure characterizations

The surface morphology and structure of bare ReS₂ and ReS₂/RGO were represented by scanning electron microscope (SEM,JSM-7000F) and transmission electron microscope (TEM,Tecnai F20).The crystalline structure was performed by X-ray diffractometer (XRD,Rigaku D/MAX-rA).

2.3 Electrocatalytic measurements

The electrocatalytic measurements were carried out using a three-electrode system on an electrochemical station (CHI660D).Platinum (Pt) wire was used as the counter electrode,and saturated calomel electrode (SCE) calibrated by reversible hydrogen electrode (RHE) was used as the reference electrode,E_RHE=E_sCE+0.25 V.To prepare the testing electrodes,4 mg ReS₂/RGO was dispersed in 1 ml water/ethanol (3:1) along with 50μl Nafion (5 wt%in water and 1-propanol) with 30-min sonication.Subsequently,5μl slurry was dropped onto a blank glassy carbon electrode used as the working electrode.Finally,the modified electrode was dried.Prior to the measurements,the electrolyte was degassed by bubbling with N₂ for 1 h.The polarization curves were obtained by seeping the potential from 0.10 to-0.35 V (vs.RHE) at a sweep rate of 5 mV·s^-1.The Tafel plots were acquired by evolution of the polarization curves.For the electrochemical impedance spectroscopy (EIS),the frequency was applied from0.01 Hz to 100.00 kHz at a given overpotential.The stability was evaluated by continuous cyclic voltammetry(CV) tests between-0.35 and 0.10 V (vs.RHE) at a scan rate of 100 mV·s^-1 for 3000 cycles.

3 Results and discussion

3.1 Microstructure analysis

The crystalline structure was analyzed by XRD.As shown in Fig.la,the reflection peaks of as-synthesized ReS₂/RGO located at 14.5°,32.7°,44.6°,57.8°,corresponding to the lattice planes of (100),(002),(300),(122)(JCPDS82-1379),respectively,are clearly observed [ 34] .The Raman spectrum of ReS₂/RGO is shown in Fig.1b.Two characteristic Raman modes at 161.7 cm^-1 (E_g) and211.4 cm^-1 (A_g) are associated with ReS₂.In addition,some other Raman characteristic peaks are also present in the range of 100-400 cm^-1,which is related to the unique crystal structure of ReS₂ (E_g-like).In addition,the two typical peaks are located at 1358 and 1596 cm^-1,which can be assigned to D and G bands of RGO.XRD and Raman data confirm the presence of ReS₂/RGO [ 35, 36] .

SEM images of ReS₂ and ReS₂/RGO were performed.From Fig.2a,the bare ReS₂ has a microsphere structure with a size of 1-2μm.ReS₂ microspheres exhibit a poor dispersion and notable agglomeration.SEM morphology of ReS₂/RGO composite is shown in Fig.2b.Two-dimensional (2D) layered ReS₂ nanosheets are directly nucleated and grown on RGO nanosheets during hydrothermal reaction.

The morphology and structure of ReS₂/RGO were further investigated by TEM.From Fig.2c,one can clearly observe that many ultrathin nanosheets are curly.This characteristic is advantageous to fast transfer for charges.From the high-resolution TEM (HRTEM) image shown in Fig.2d,one can observe a 0.62-nm lattice spacing which is assigned to the (100) crystal plane of ReS_2.TEM results are consistent with XRD results.

According to above SEM and TEM analysis,the bare ReS₂ is microsphere structure,while ReS₂/RGO hybrid is nanosheet structure.That means,after the introduction of RGO,the functional group and defects on the surface of the graphene oxide can act as nucleation sites for ReS₂ growth and inhibit the stacking and reunion of ReS₂ nanosheets.

Fig.1 a XRD pattern of ReS₂/RGO and b Raman spectrum of ReS₂/RGO

Fig.2 SEM images of a ReS₂ (inset for higher magnification) and b ReS₂/RGO;c TEM image and d HRTEM image of ReS₂/RGO

3.2 Electrocatalytic performance

The electrocatalytic properties of ReS₂/RGO were tested in a standard three-electrode cell setup in 0.5 mol·L^-1 H₂SO₄solution.The elec trocataly tic characteristics of ReS₂ and ReS₂/RGO are shown in Fig.3.Linear sweep voltammetry(LSV) plots in Fig.3a show the polarization curves of ReS₂ and ReS₂/RGO at 5 mV·s^-1 with current density normalized by geometric surface area.Obviously,the ReS₂/RGO delivers a large current density of-5.2 mA·cm^-2 at-0.25 V (vs.RHE) and a low onset potential about-100 mV (vs.RHE),which are both superior to those of ReS₂.

Tafel slop is currently used to make further exploration of the HER performance,and it corresponds to HER overpotential and the logarithm of current density (η=a+blg|j|,whereηis the overpotential,j is the current density,and b is the Tafel slope) [ 37] .As shown in Fig.3b,the Tafel slopes of ReS₂ and ReS₂/RGO are 152.1 and107.4 mV·dec^-1,respectively.Obviously,the Tafel slope of ReS₂/RGO is much smaller than that of ReS₂.

From LSV curve (Fig.3a),the current density of ReS₂/RGO and ReS₂ is-5.2 and-3.1 mA·cm^-2 at-250 mV(vs.RHE),respectively,which increases by 67%.The greater current density of catalyst stands for the better HER performance.Meanwhile,the Tafel slope of the ReS₂/RGO is smaller than that of ReS₂.This means that ReS₂/RGO is advantageous for practical applications,since it will lead to a quicker increment of HER rate with overpotential rising.From above results,it can be speculated that RGO may play a significant role in improving the electrocatalytic HER performances of ReS₂.RGO with high electrical conductivity enhances the conductivity of the ReS₂/RGO hybrid and accelerates the rate of exchange electrons,thus enhancing the HER performance.

Fig.3 a Polarization curves and b corresponding Tafel plots of ReS₂ and ReS₂/RGO

The process of HER in acid solution is as follows:Primitively,the proton gets an electron to turn into adsorption state of hydrogen atoms on the electrode surface;this process is the Volmer step (H₃O++e^-→H_ads+H₂O).If the Volmer step is the rate control step,the Tafel slope corresponds to 120 mV·dec^-1.Afterward,the reaction will carry from two aspects because of the different catalytic properties of various materials.One of the aspects is the Heyrovsky step that combines the adsorption hydrogen atoms with the protons of electrolyte(H₃O⁺+H_ads+e^-→H₂↑+H₂O).Another aspect is the Tafel step,the adsorption hydrogen atoms combine in pairs to generate hydrogen on the catalyst (H_ads+H_ads→H₂↑).If the Heyrovsky or Tafel step is the ratecontrolling step,the Tafel slope will become 40 or30 mV·dec^-1,correspondingly [ 35, 38] .The Tafel slopes of ReS₂/RGO and ReS₂ are 107.4 and 152.1 mV·dec^-1,respectively.So,it is obvious that ReS₂/RGO pertains to Volmer-Heyrovsky combination mechanism.It suggests that ReS₂/RGO hybrid as an electrocatalyst exhibits much faster HER kinetics and better electrocatalytic behavior compared with ReS₂.

3.3 Elec trocataly tic stability

The catalytic stability is also crucial to the practical applications of electrocatalyst.To examine the HER stability,the polarization curves of ReS₂/RGO were performed by taking the continuous cyclic voltammetry(CV) test.As shown in Fig.4a,the polarization curve of ReS₂/RGO after 3000 CV cycles from-0.35 to 0.10 V(vs.RHE) at a scan rate of 100 mV·s^-1 nearly overlaps to the initial curve,suggesting excellent long-term cycling stability.In order to further indicate the superior stability of the ReS₂/RGO,time dependence of current density at-0.31 V (vs.RHE) was performed (Fig.4b),the current density nearly does not change in a larger current density(-10 mA·cm^-2) for 22 h.Through the proof of the two patterns above,the HER stability provides ReS_a/RGO great feasibility for hydrogen production.

Fig.4 a Durability test for ReS₂/RGO after 3000 CV cycles and b time dependence of current density under static potential of at-0.31 V (vs.RHE)

Fig.5 CV plots of a ReS₂/RGO and b ReS₂;c estimated C_d1 data of ReS₂/RGO and ReS₂ calculated from CV plots;d Nyquist plots (0.1 HZ-1.0 MHz) of ReS₂/RGO and ReS₂

3.4 Reasons for HER performance enhancement

In order to further understand the catalytic activity,CV measurements were employed to estimate the electrochemical surface area (ECSA) using the electrochemical double-layer capacitance (C_d1) values of ReS₂ and ReS₂/RGO,as shown in Fig.5c.The C_d1 can be calculated by the relationship of the current at different scan rates(20-200 mV·s^-1) [ 39] .Figure 5a,b shows the cyclic voltammograms of ReS₂/RGO,and ReS₂,obtained in the range of 0.1-0.2 V (vs.RHE),respectively.Figure 5c shows the current density (Δj=(ja-jc/2) dependence of the scan rates at 0.15 V (vs.RHE),where ja and jc represent the upper and lower current density,respectively).The C_d1value is the slope of the linear line.From Fig.5c,the C_d1 of ReS₂/RGO is 4.05 mF·cm^-2,which is larger than that of bare ReS₂ (0.94 mF·cm^-2).The huge promotion of C_d1suggests that ReS₂/RGO has much large ECS A and more active sites for HER [ 3] .Figure 5d shows EIS results,where the X axis represents the real component of impedance and Y axis the imaginary part of impedance.The charge transfer resistance (R_ct) of ReS₂/RGO is obviously smaller than that of ReS₂/RGO since the RGO improves the conductivity of ReS₂/RGO hybrid.The smaller R_ct value suggests that ReS₂/RGO has a quicker charge transfer at the interface between electrocatalyst and electrolyte in comparison with ReS₂.

Compared with ReS₂,the main reason why ReS₂/RGO has more excellent performance of hydrogen evolution is that RGO can not only provide a conductive,porous skeleton,but also form a close connection with ReS₂.These are favorable to the infiltration of electrolyte and electronic transmission.In addition,ReS₂ growth on the surface of graphene can provide the larger active surface area and more active site,which is consistent with the data of Fig.5.

The excellent HER activity,fast HER kinetics and longterm stability of ReS₂/RGO can be simply interpreted as follows.The ReS₂/RGO hybrid has a unique porous structure constructed by few-layered ReS₂ nanosheets and conductive RGO,which guarantees its good HER performance:(1) ReS₂ can be favorable to provide abundant active sites and facilitate the electrolyte penetration;(2)ReS₂ grows on the RGO backbone,which facilitates rapid charge transportation;(3) the C_d1 calculated from CV curves demonstrates that the ReS₂/RGO has much larger C_d1 than ReS₂,which further confirms that ReS₂/RGO has many more electrocatalytic active sites;(4) the lower charge transfer resistance suggests that after introducing RGO,the conductivity of the whole electrode is effectively enhanced and the charge transportation ability between the electrocatalyst and electrolyte is improved.

4 Conclusion

In summary,ReS₂/RGO hybrid was synthesized via a facile one-pot process based on hydrothermal reaction,which demonstrates good electrocatalytic activity for HER.Compared with bare ReS₂,the ReS₂/RGO hybrid shows superior HER performance in acidic media.It has a lower onset potential of-100 mV (vs.RHE),a higher current density of-5.2 mA-cm^-2 at-250 mV (vs RHE) and a smaller Tafel slope of 107.4 mV·dec^-1.The inimitable architecture constructed of ReS₂ and RGO makes electrocatalytic activity stable even after 3000 cycles.The high catalytic performance,stability,facile and low-cost synthesis method make ReS₂/RGO hybrid promising to be a suitable substitute for precious metals catalysts for HER.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51372033),the National High Technology Research and Development Program of China (No.2015AA034202) and the Program of the Ministry of Education of China for Introducing Talents of Discipline to Universities (No.B13042).

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