Regular Article
Direct growth of vertically aligned ReSe
2nanosheets on conductive electrode for electro-catalytic hydrogen production
Jing Li
a, Qingwei Zhou
a,c, Chen Yuan
a, Pengfei Cheng
a, Xianbiao Hu
a, Wentian Huang
a, Xingsen Gao
a, Xin Wang
b, Mingliang Jin
b, Richard Nötzel
b, Guofu Zhou
b, Zhang Zhang
a,b,⇑, Junming Liu
a,caGuangdong 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, PR China
bNational Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, PR China
cLaboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, PR China
g r a p h i c a l a b s t r a c t
Vertically aligned two-dimensional ReSe2are directly grown on conductive substrates by a chemical vapor deposition method for enhanced electrocat- alytic hydrogen production.
a r t i c l e i n f o
Article history:
Received 22 April 2019 Revised 18 June 2019 Accepted 21 June 2019 Available online 21 June 2019
Keywords:
Electrocatalyst ReSe2nanosheets Chemical vapor deposition Hydrogen evolution reaction Conductive substrate
a b s t r a c t
Two-dimensional electrocatalysts with high-density active sites together with a good charge transfer to the conductive substrates can improve their hydrogen evolution reaction performances. Typically, a good contact between catalyst and electrode to enhance charge transfer can be achieved by way of direct growth. However, a controllable growth of vertically aligned two-dimensional ReSe2directly on conduc- tive substrates as working electrodes is not reported. In this work, for the first time, vertically aligned ReSe2nanosheets directly on conductive substrates (carbon cloth and glass carbon) are realized though a controllable chemical vapor deposition method. Compare to the way of transferring two-dimension ReSe2, the electrode with optimized growth of two-dimensional ReSe2exhibits superior hydrogen evolu- tion reaction performances.
Ó2019 Published by Elsevier Inc.
https://doi.org/10.1016/j.jcis.2019.06.073 0021-9797/Ó2019 Published by Elsevier Inc.
⇑ Corresponding author at: 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, PR China.
E-mail address:[email protected](Z. Zhang).
Contents lists available atScienceDirect
Journal of Colloid and Interface Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j c i s
1. Introduction
H2has been considered as a sustainable energy source and an ideal alternative for fossil fuels[1–4]. Electro-catalytic (EC) reduc- tion of water through hydrogen evolution reaction (HER) is an effi- cient and sustainable strategy for H2generation[5,6]. At present, platinum (Pt) is the most efficient HER catalyst due to its near- zero Gibbs free energy (DG(H*)) of absorbed H atoms[7]. However, the exorbitant price and scarcity hinder its practicability. Thus, it is necessary to find less expensive and more abundant materials to replace Pt. Recently, the relatively under-researched rhenium dis- elenide (ReSe2) has attracted more attentions[8–10], and the use of ReSe2 as a HER electrocatalyst has been explored [11,12]. Qi et al has achieved ReSe2 nanosheets (NSs) by one-pot synthetic method, and the as-prepared ReSe2NSs were coated onto the blank glass carbon electrode for HER [12]. Adopting a chemical vapor deposition (CVD) method, Jiang et al synthesized ReSe2flakes on non-conductive substrate, and the as-grown ReSe2 were transferred onto conductive substrates such as Au electrode for HER[11].
To enhance the HER performance of two-dimensional (2D) tran- sition metal dichalcogenides (TMDs), an ideal architecture of nanostructured films could be envisaged, comprising more exposed active sites together with a good electrical contact between 2D catalyst and electrode[13]. Apparently, more edges and surfaces of 2D catalyst can be exposed with its orientation being perpendicular to the substrate[14]. It has been well estab- lished that the metallic edges of MoS2are catalytically active sites for HER[15–17], although the active sites of ReSe2catalyst have not been confirmed directly neither by experimental nor theoreti- cal studies. The array of vertically aligned high-density nanosheets (NSs) exhibit more exposed active sites which are beneficial for the HER[14]. Besides, a good contact between catalyst and electrode can improve the charge transfer to further enhance the HER perfor- mance[18,19]. Deposition of liquid dispersion with the exfoliated 2D TMDs onto conductive substrates is accompanied with inevita- ble agglomeration and re-stacking. The transfer method does not allow for control over the alignment of 2D nanostructures as well as the formation of intimate electrical contact on conductive substrates, which have negative influences on the EC efficiency
[20–22]. Normally, a good contact between catalyst and electrode to enhance charge transfer can be achieved by way of a direct growth. Thus, a controllable growth of vertically aligned 2D ReSe2
directly on conductive substrates as working electrodes is pre- ferred for high-performance EC HER.
In this work, for the first time, vertically aligned ReSe2
nanosheets have been synthesized successfully which can directly grow on conductive substrates (carbon cloth and glass carbon), through a low temperature (450 ) CVD method. The morphology and density of ReSe2nanosheet arrays are adjustable by different CVD growth conditions. The high-density ReSe2NSs with vertical orientation result in a significant increase of active edge sites.
And the electrodes by direct growth of ReSe2NSs on conductive substrates exhibit enhanced HER performances compared with the one with transferred method.Furthermore, the optimal growth of ReSe2NSs on carbon cloth as a working electrode exhibits supe- rior HER performance, together with an overpotential of 265 mV at a current density of 10 mA/cm2, a high capacitance of the double layer (Cdl) of 9.6 mF/cm2and a low Tafel slope of 69 mV/dec.
2. Results and discussion
To synthesize ReSe2NSs, rhenium trioxide (ReO3) and Selenium (Se) were used, with an Ar/H2 mixture as the carrier gas (see Fig. S1). ReO3is chosen as precursor, since ReO3can ensure a high crystal quality with a relatively low growth temperature[8]. While ascending to a certain temperature (about 400°C), ReO3starts to decompose into rhenium heptoxide (Re2O7), which has an extre- mely high volatility and can ensure an efficient growth on carbon substrates using an improved CVD method. As schematically illus- trated inFig. 1a, the ReSe2NSs grew vertically on curved surfaces of carbon cloth by way of a CVD method. As shown inFig. 1b–d, the scanning electron microscope (SEM) images demonstrate mor- phologies of the carbon cloth (CC) substrate with and without growth of ReSe2NSs. Compared with the bare carbon cloth sub- strate (Fig. 1b), the ReSe2NSs grow uniformly on the curved sur- faces with a high-density (Fig. 1c). In the high magnification SEM image ofFig. 1d, we further confirm the vertical growth orientation of ReSe2NSs. Meanwhile, the growth of ReSe2on a flat substrate of black glass carbon (BGC) is also with high-density, good uniformity
Fig. 1.(a) Schematic illustration of ReSe2nanosheets CVD growth process on a cylindrical conductive substrate of carbon cloth. SEM images of (b) bare carbon cloth, (c), (d) synthesized ReSe2nanosheets at 450°C with low and high magnifications, respectively.
and vertical orientation (seeFig. S2). The high-density ReSe2NSs with vertical alignment form complex there-dimensional (3D) nanostructures on the conductive substrate, which should be a promising architecture as EC electrode with more active sites for catalytic reaction[15–17].
Generally, the lower chemical reactivity of Se makes Se-based TMDs more difficult to be synthesized than the S-based TMDs.
Without reducing gas H2introduced into the CVD system, no ReSe2
could be synthesized[23]. Therefore, we emphasize on the effect of H2content on the CVD growth. As shown inFig. S3a and S3d, there is no ReSe2growth without H2. As the H2content increases, it is obvious that the corresponding morphologies of ReSe2grown on carbon cloth are quite different (seeFig. S3), since an appropriate amount of H2is essential for adjusting the growth thickness and nucleation density of ReSe2[8]. Furthermore, we have investigated the time effect on the ReSe2growth with a fixed H2gas flow of 3 sccm. Initially, the growth of ReSe2NSs is parallel to the substrate (seeFig. S4a). Subsequently, the spontaneous vertical growth is caused by out-of-plane Re-Re bonds which cause under- coordinated Se atoms to stand out from the ReSe2 basal plane [24]. With the increase of growth time, the density of ReSe2NSs is higher accordingly (seeFig. S4b–d). Thus, the morphology and the density of ReSe2 NSs grown on a conductive substrate are adjustable by the H2content and the growth time, respectively.
Scanning transmission electron microscopy (STEM) was used to analyze the crystal structure of ReSe2NSs. Fig. 2a is a top-view schematic lattice structure of 2D layered ReSe2. We can clearly rec- ognize the a- and b-axis, as well as the Re41D chains belonging to the distorted phase. A high-resolution TEM (HRTEM) image of ReSe2 nanosheet is presented in Fig. 2b. The defect-free lattice structure and the corresponding sharp SAED pattern (inset of Fig. 2b) confirm its high crystal quality by way of CVD growth.
Fig. 2c is an enlarged view of the selected area (marked in Fig. 2b), where we can index the directions of in-plane (0 1 0) b-axis and out-of-plane (1 0 0) a-axis corresponding to Fig. 2a.
EDX mapping was utilized to identify the composition and its 2D uniformity. The cross-sectional HRTEM image (seeFig. S5c) along a nanosheet edge reveals a typical multi-layered 2D nanostructure of ReSe2with a thickness of 8–10 nm, as determined by an inter- layer (0 0 1) spacing of 6.2 Å[25].Fig. 2d is the high-angle annular dark field (HAADF) STEM image of a ReSe2nanosheet. Highly uni- form distributions of Se and Re in the nanosheet can be observed, respectively, as shown in the elemental EDX mapping images of Fig. 2e and f. The chemical stoichiometry was deduced to be about 1:2 from EDX spectrum (seeFig. S5d), which is consistent with the X-ray photoelectron spectroscopy (XPS) observation. All these characterizations indicate a highly crystallined 2D ReSe2nanosheet growth by the CVD method.
Fig. 2.Crystal structure and composition of the ReSe2NSs. (a) Top-view schematic lattice structure of a 2D layered ReSe2with marked Re41D chains. (b) HRTEM image of a 2D ReSe2nanosheet and inset of corresponding selected area electron diffraction (SAED) pattern, and (c) an enlarged view of selected area with atomic resolution of Re41D chains. (d) HAADF- STEM image of a ReSe2nanosheet; (e) and (f) the corresponding elemental energy dispersive x-ray (EDX) mapping of Se and Re, respectively. (g) X-ray diffraction patterns of the ReSe2nanosheets grown on carbon cloth, and (h), (i) the corresponding XPS spectra of Se 3d and Re 4f, respectively.
The as-synthesized ReSe2NSs grown on carbon cloth were fur- ther investigated by X-ray diffraction (XRD), Raman spectrum and XPS. Due to the low crystal symmetry of ReSe2, Raman spectro- scopic characterization (532 nm laser) reveals the presence of more than 10 distinctive peaks in the range of 100–300 cm 1, which significantly exceeds the number of peaks observed in con- ventional TMDs such as MoS2[11]. As shown inFig. S5a, a series of Raman modes perfectly matched the spectra of mechanically exfo- liated 2D ReSe2nanoflakes[10]. Among these peaks, the character- istic Raman peak at 123 cm–1corresponds to an Eg-like mode, and the peaks at 158 and 172 cm 1correspond to Ag-like modes. The XRD patterns of the ReSe2nanosheets on carbon cloth substrates were also presented in Fig. 2g, where the 0 0 1 peak of ReSe2
nanosheets were observed, indicating the vertical growth of ReSe2
nanosheets. Meanwhile, as shown in the full XPS spectrum (see Fig. S5b), three elements of Re, Se and C can be observed. Generally, the signals of C come from the carbon cloth substrate. With regard to the Se and Re signals, their high-resolution XPS spectra were
analyzed. As plotted inFig. 2h, two distinctive peaks for the bond- ing configurations of Se are located at 56.1 and 55.7 eV, respec- tively, corresponding to the Se 3d5/2 and 3d3/2. As shown in Fig. 2i, the core-level peaks corresponding to Re 4f7/2and 4f5/2 are located at 41.7 and 44.3 eV, respectively. These spectrums are consistent with the bulk ReSe2[10]. In addition, the ratio of Re to Se acquired from XPS survey is nearly 1:2.26, suggesting that the CVD-grown ReSe2NSs are reasonably stoichiometric.
The vertically aligned ReSe2 NSs grown on conductive sub- strates can be potentially considered as EC-HER electrodes. EC- HER performances of the ReSe2NSs with optimized growth condi- tions (30 min CVD with a 3 sccm H2flow) were investigated in a typical three-electrode electrochemical cell with a 0.5 M H2SO4
electrolyte. As illustrated in the polarization curves ofFig. 3a, onset overpotentials at a current density of 10 mA/cm2 for the ReSe2/- substrate electrodes are much lower than those of the blank sub- strates besides Pt, which demonstrates the significantly enhanced EC-HER activity by the introduction of ReSe2NSs. Moreover, the
Fig. 3.(a) Polarization curves and (b) corresponding Tafel plots of different ReSe2/substrate electrodes in a 0.5 M H2SO4electrolyte. (c) Difference in current density at 0.04 V vs. RHE plotted against scan rate fitted to a linear regression for the estimation of Cdl. (d) Nyquist plots of ReSe2/substrate at a 140 mV overpotential. (e) Polarization curves and (f) corresponding Cdlof ReSe2/CC electrodes with different ReSe2growth times in a 0.5 M H2SO4.
HER current density of ReSe2/substrate electrode is strongly depen- dent on the way of integrating with the conductive substrates.
Specifically, onset overpotentials of about 265 and 303 mV were observed for electrodes of ReSe2NSs grown on carbon cloth (ReSe2/ CC) and on glass carbon (ReSe2/BGC), while a much bigger one about 369 mV is required for the electrode of transferred ReSe2
NSs on glass carbon (ReSe2/BGC). Obviously, direct growth of ReSe2/substrate electrodes exhibit a great enhancement of HER, due to the more intimate electrical contacts and higher exposed active sites. Furthermore, The ReSe2grown on carbon cloth demon- strates a better EC-HER activity than ReSe2grown on glass carbon, since the 3D structure of carbon cloth substrate provides a larger specific surface area for the growth of ReSe2.
Tafel slope, an important indicator for intrinsic property in catalysis, can be introduced to make sense of the rate-limiting steps involved in HER[26]. Typically, the Tafel equation of
g
= a + blog(j) can be used, whereg
, a, b, and j are the overpotential, Tafel constant, Tafel slope and current density, respectively.Fig. 3b illustrates the Tafel plots of ReSe2/CC, ReSe2/GC, transferred ReSe2/GC and Pt electrodes. The Pt reference has a Tafel slope of 32 mV/dec. Compared with 280 mV/dec from the transferred ReSe2/GC, the ReSe2/CC and ReSe2/BGC electrodes have Tafel slopes of 69 and 120 mV/dec, respectively, indicating the much improved HER efficiency by the direct CVD growth.
To estimate the effective surface area of the solid-liquid inter- face, capacitance of the double layer (Cdl) was measured using a cyclic voltammetry (CV) method (seeFig. S6), which is expected to be linearly proportional to the effective surface area[27]. As plotted inFig. 3c, the Cdlof ReSe2/CC (9.6 mF/cm2) is much higher than the ones of ReSe2/GC (0.55 mF/cm2) and transferred ReSe2/GC (0.44 mF/cm2). We assume that the Cdlis also associated with the active surface sites, and a higher Cdlcoincides with the excellent EC activity for HER. To better elucidate the electrode kinetics, electro- chemical impedance spectroscopy (EIS) was performed at an
overpotential of 210 mV. As plotted inFig. 3d, the measured data fitted to an equivalent circuit (inset ofFig. 3d). The Nyquist plots reveal a decrease in the charge transfer resistance (Rct) from 673Xfor transferred ReSe2/GC to 435.1X for ReSe2/GC. Mean- while, we observed a remarkable decrease of Rct from 435.1Xto 142.4Xfor ReSe2/CC. In general, the larger Rct mainly results from the worse contact between the transferred ReSe2and conductive substrate, which attenuates the electrical conductivity. Therefore, the Nyquist plots also indicate that the direct CVD growth of ReSe2
on carbon cloth has the better HER performance.
To further evaluate the EC-HER performance of ReSe2/CC elec- trode, current density-potential (j-E) relationships of the elec- trodes with different ReSe2 growth times were illustrated in Fig. 3e. The onset overpotentials are 320, 298, 265 and 295 mV which is in consistent with 10, 20, 30 and 40 mins of ReSe2growth times, respectively. As the growth time increases from 10 to 30 mins, the onset overpotential is decreasing due to the verified increased density of ReSe2NSs grown on carbon cloth. Neverthe- less, with the growth time further up to 40 mins, the onset overpo- tential ascends higher than the one of 30 mins. We assume that the overgrowth of ReSe2on carbon cloth would hinder the transfer of electrons and attenuate the corresponding HER performance. Fur- thermore, the corresponding Cdl of 2.5, 4.9, 9.6 and 8.8 mF/cm2 were confirmed in Fig. 3f, respectively. Likewise, Cdl of 9.6 mF/
cm2 with the 30 min ReSe2 growth indicates the optimal HER performance.
A high-performance EC device should also possess a better sta- bility. As plotted inFig. 4a, the stability of optimal ReSe2/CC elec- trode has been examined by a current density-time (J-t) measurement at an overpotential of 260 mV in a 10,000 s duration.
A typical serrate line shape was confirmed in the inset of enlarged curve, which could be attributed to the alternate processes of bub- ble accumulation and release[27]. To illustrate changes of the cat- alyst after stability test, XPS spectra were compared before and
Fig. 4.(a) Stability of the ReSe2/CC electrode at overpotential of 260 mV, and inset is enlarged curve of the selected portion denoted by the square. (b) XPS spectra of the ReSe2/CC electrode at the initial stage and after stability test. (c), (d) The corresponding SEM images of the ReSe2/CC electrode at the initial stage and after stability test, respectively.
after the J–ttest (Fig. 4b). The XPS peak positions of Se and Re were confirmed to be invariable after the test, as well as their peak shapes. The stoichiometric ratios of Re to Se were almost the same (seeTable S1). In addition, as shown inFig. 4c and d, we compared the morphology, density and alignment of the ReSe2NSs grown on CC substrate at the initial stage and after the electric test. Only a few spots on the substrate growth area were covered with detached ReSe2NSs after the 10,000 s test. Thus, the long-term sta- bility of ReSe2/CC electrode was achieved by the direct CVD method.
3. Conclusions
The morphology and density of the ReSe2NSs are adjustable by the CVD growth conditions. The direct growth of ReSe2NSs on con- ductive electrode can expose more active sites and facilitate the electron transfer to improve EC performance. As a consequence, the direct growth of vertically aligned ReSe2nanosheets delivers more excellent EC-HER performance than the transferred ReSe2
one. We expect that this work can provide a simple synthetic method to prepare high-performance EC electrodes in the field of renewable energy.
Declaration of Competing Interest
The authors declare no competing financial interests.
Acknowledgments
This work was supported by National Key R&D Program of China (2016YFB0401501), Cultivation project of National Engineer- ing Technology Center (grant no. 2017B090903008), Xijiang R&D Team (X.W.). National Natural Science Foundation of China (51561135014), The Guangdong Natural Science Foundation (2018A030313377), Guangdong Provincial Key Laboratory of Optical Information Materials and Technology Grant (2017B030301007), Program for Chang Jiang Scholars and Innova- tive Research Teams in Universities (IRT_17R40), and MOE Interna- tional Laboratory for Optical Information Technologies and the 111 Project.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.06.073.
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