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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Layer-controllable WS2-reduced graphene oxide hybrid nanosheets with high electrocatalytic activity for hydrogen evolution Jian Zhanga, Qi Wanga, Lianhui Wanga, Xing’ao Lia,*, Wei Huanga,b,*
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR01896J
In this paper, an efficient poly(vinylpyrrolidone) (PVP)-assisted hydrothermal method for in-situ growth of WS2 nanosheets with layer controllability on reduced graphene oxide (rGO) is reported. The layer number (from monolayer to ~25 layers) of the exfoliated WS2 can be controlled gradiently by adjusting the amount of PVP. The layer structure and morphology of the as prepared hybrids are confirmed by field emission scanning electron microscopy and high-resolution transmission microscopy. The X-ray diffraction, Raman, and X-ray photoemission spectroscopy of the obtained WS2-rGO hybrid nanosheets indicate highly crystallized structures, clear Raman shift and good layer-number dependent stoichiometry of the materials. Furthermore, These highly active and durable catalysts exhibit an electrocatalytic current density of 10 mA cm -2 at a small hydrogen evolution reaction (HER) overpotential (-170 mV) and a Tafel slope of 52 mV dec-1 with an excellent electrocatalytic stability (after 6 months storage).
1 Introduction Hydrogen is frequently considered as the perfect energy carriers that can be an alternative to fossil fuels in recent decades for its numerous virtues, such as high energy density and recyclability.1,2 As the most attractive molecular fuel, hydrogen obtained by electrocatalytic hydrogen evolution reaction (HER) from water is renewable and free of pollution. 3-5 Ideal electrocatalysts should be earth-abundant, decreased the overpotential of HER, and sufficiently stable with minimal decay in performance.6-8 Although the noble metal (such platinum) has long been an efficient candidate for HER for its advantage: chemical inertness, and high activity,9 more economic and competitive catalysts including transition metal sulfides, 10-14 carbides,15 boride,16 phosphides,17 have attracted great attention as electrocatalysts in recent years. Among these alternatives, WS2 has attracted increasing attention for high activity and the earth-abundant composition, leading to the development of various kinds of WS2 based HER electrocatalysts recently.18-21 As one of important member in the family of the 2 dimensional (2D) transition metal dichalcogenides (TMDs),22-24 atomically thin WS2 nanosheets (mono or few-layers) presents some unusual chemical, physical, and electronic properties
a. Key
Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China. E-mail: [email protected] (X. Li), Tel: +8602585866362 b. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing 211816, PR China. [email protected] (W. Huang).
compared with their bulk compound and therefore hold great promise for a variety of applications.25-28 Until now, atomically thin WS2 nanosheets can be synthesized by mechanical exfoliation,29 chemical Li-intercalation and exfoliation,30,31 and grown by chemical vapor deposition (CVD).32,33 Recently, layer-controlled WS2 layers have been synthesized by using WO3 and S as precursors via the CVD growth.34 Mono and few-layer nanosheets of WS2 prepared by this method show high crystallinity and reasonable electrical properties.35 By use of the mechanical exfoliation method, not only monolayer but also multilayer WS2 nanosheets can be obtained.36 Unfortunately, forced exfoliation only produces small-sized flakes. Besides, Shiva et al. reported a simple solidstate reaction method for few-layers of WS2 by heating tungstic acid and thiourea in N2 atmosphere at 773 K.37 The most popular strategy used for large scale manufacturing of TMD nanosheets may be the hydrothermal reaction. For example, oxygen-incorporated MoS2 sheets were prepared by a facile one-pot hydrothermal method by Xie’s group.10 Benefiting from the oxygen incorporation, efficient HER activity was achieved for the ultrathin nanosheets with a large cathodic current density of 126.5 mA cm−2 at an overpotential of 300 mV for HER application. A low temperature hydrothermal method was developed by Chhowalla et al.21 for the growth of WS2 nanosheets on the reduced graphene oxide (rGO) surface using tungsten chloride and thioacetamide as precursors. The asprepared hybrid nanosheets exhibit overpotential ranging from -150 ~ -200 mV with Tafel slope of 58 mV dec -1. Although their potential catalysts for HER has been realized more than 20 years ago,38 the absence of a layer-controllable WS2 nanosheet synthesis method has confined the study and application of WS2-rGO hybrids.39
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2 Experimental Preparation of GO nanosheets GO nanosheets were exfoliated from natural graphite by the modified Hummer's method reported elsewhere.44 The GO nanosheets were dispersed into deionized water under ultrasonic to form the solution with a concentration of 2.5 mg mL-1. Preparation of layer-controllable WS2-rGO hybrid nanosheets Layer-controllable WS2-rGO hybrid nanosheets were prepared by a one-pot hydrothermal reaction process. In a typical experiment, 20 mL of 2.5 mg mL -1 GO absolute ethanol was added to the mixture of 0.1 mmol WCl6 (Sigma-aldrich, 99.98%) and 0.6 mmol sodium diethyldithiocarbamate (NaS 2CN(C2H5)2, Shanghai Chemical Factory of China, AR) and the total volume of the solution was maintained at 36 mL. PVP (Shanghai Chemical Factory of China, AR) was then added to the mixture. After stirring for 0.5 h at room temperature by a magnetic stirrer, the solution was transferred to a 50 mL stainless steel autoclave, heated up to 220 °C and kept for 18 h. After cooling naturally, the product was filtered, washed with deionized water, and dried in a vacuum at 50 °C for 8 h. Three hybrids were synthesized, namely mWS2-rGO (m-mono), sWS2-rGO (sseveral) and dWS2-rGO (d-dozens), which were prepared with different precursor/surfactant ratios of 2 : 10, 2 : 5 and 2 : 1, respectively. Characterization of WS2-rGO hybrid nanosheets The field emission scanning electron microscopy (FE-SEM) images were taken on a scanning electron microscope (Hitachi S-4800). The transmission electron microscopy (TEM) images was conducted on a JEOL JEM-2100 microscope operating at 200 kV. X-ray diffraction (XRD) patterns were performed on a BRUKER D8 Advance X-diffractometer (Cu Ka radiation, 1.54056 °A). X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MK II with Mg Kα as the excitation source. The Brunauer-Emmett-Teller (BET) surface area was measured by the nitrogen gas adsorption-desorption method at 77 K using a TriStar II 3020. Raman spectra were conducted on the Renishaw in via Raman microscope with 488 and 514.5 nm lasers as the exciting radiation. Electrochemical measurements. Electrochemical measurements were performed using a threeelectrode system on an electrochemical workstation (CHI660B).
All measurements were performed in 50 mL of 0.5View M HArticle (pH 2SO4Online DOI: 10.1039/C5NR01896J = 0.157, purged with H2, 99.999%) electrolyte. 6 mg of WS2-rGO powder and 50 μL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 2 mL water-isopropanol solution with volume ratio of 3:1 by sonicating for 2 h to form a homogeneous dispersion. Then 10 μL (40 μg catalyst) of the dispersion was loaded onto a glassy carbon electrode as the working electrode (loading ~0.562 mg cm−2 catalyst), a graphite rod (Alfa Aesar, 99.9995%) as a counter electrode, and Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode. All of the potentials were calibrated to a reversible hydrogen electrode (RHE). The electrochemical stability of the catalyst was evaluated by cycling the electrode 3000 times each cycle started at 0.20 V and ended at -0.24 V vs. RHE with a scan rate of 50 mV s-1. The Nyquist plots were measured in the frequency range 10 -1 to 105 Hz with an overpotential of 200 mV.
3 Results and discussion
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Herein, we develop a simple but effective hydrothermal process to fabricate layer-controllable WS2-rGO hybrid nanosheets. The thickness of WS2 nanoflakes can be systematically modulated (mono, several, and dozens-layered) by controlling the amount of surfactant, poly(vinylpyrrolidone) (PVP). This is one of the first reports on layer controlled of WS 2 nanosheets by use of single source precursors (W(S 2CN(C2H5)2)6) for growth of ultrathin WS2 nanoflakes controlled gradiently on rGO. M(S2CN(C2H5)x (M = metallic element) has been employed to synthesis various metal sulfides, such as CdS nanorods 40 and Bi2S3 nanotubes.41 The as-prepared samples exhibit good catalytic activity and durable for HER, matching the activity of MoS2/rGO42 and MoS2/N-Doped CNT.43
It has been demonstrated that ultrathin nanomaterials can be prepared by ionic45 and nonionic surfactant46 assist. Herein, we synthesis WS2-rGO hybrid nanosheets by a simple one-pot hydrothermal process in the presence of PVP as surfactant. The strategy for the fabrication of the layer-controllable WS2-rGO hybrid nanosheets is illustrated in Fig. 1. The synthesis started with dispersing single source precursor W(S2CN(C2H5)2)6 and GO. WS2 nanocrystals in-situ nucleate and grow on the surface of the GO from the decomposing of W(S2CN(C2H5)2)6, while GO nanosheets transformed to rGO. Meanwhile, the layer number of WS2 can be controlled from ~1 layer to ~25 layers by adjusting the amount of PVP. For a relatively low ratio of precursor/surfactant (2 : 10), atomically thin WS2 are capped completely by sufficient PVP ligands. With the increasing of the ratio, several and dozens-layer WS2 nanosheets can be detected on the basal planes of rGO.
Fig. 1 Schematic illustration of the fabrication of the layercontrollable WS2-rGO hybrid nanosheets. Due to the functional groups on the basal plane of GO, single source precursors W(S2CN(C2H5)2)6 decompose, nucleate, and subsequently grow to WS2 nanocrystal on this active substrate. Fig. 2 (a-c) shows FESEM images of mono-to multi-layered WS2rGO hybrid sheets (mWS2-rGO, sWS2-rGO, and dWS2-rGO, respectively) which synthesized via hydrothermal reaction at 260 °C for 18 h. It can be seen that the as-prepared dWS2 has a
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compact combination with GO, which exhibit nanosheets structure with a thickness of ~20 nm (~ 30 layers), as shown in Fig. 2a. In contrast, the sWS2-rGO composites has a layered appearance obviously and the average thickness of the sWS 2 is ~5 nm (about 7 layers of WS2) with the increase of the surfactant (PVP). Finally, when the precursor/surfactant ratios reached to 2 : 10, a curved leaf-like appearance with the thickness of ~1 nm can be detected in Fig. 2c. The novel layer controlled WS2-rGO nanosheets can be easily obtained by controlling the amount of surfactant according to the SEM observation. XRD analysis of the WS 2-rGO hybrids show high crystalline hexagonal structure (JCPDS Card no 84-1398) without any other impurities, as shown in Fig. 2d. The slightly shift of the WS2 (002) peak to lower angle indicates an expanded interlayer distance between the adjacent sheets due to the twisting of the WS2 crystal structure.47 For comparison, the (002) peaks gradually disappear with the increasing of surfactant dosage from the XRD patterns, consisting with the lack of substantial stacking of layers.44
Fig. 2 SEM images of (a) dWS2-rGO (dozens layers), (b) sWS 2rGO (several layers) and (c) mWS2-rGO (mono layer) hybrid nanosheets. XRD patterns of (d) WS2-rGO hybrids. Further insights into the detailed microstructure of the WS 2rGO hybrid nanosheets were obtained from TEM and HRTEM images, as shown in Fig. 3. For the dWS2-rGO sample, WS2 nanosheets with an average thickness of ~20 nm lie vertically on the surface of rGO (Fig. 3a), which is consistent with the SEM images (Fig. 2a). WS2 nenosheet anchored on the rGO can be detected with even much more compact in Fig. 3b (sWS2-rGO) and c (mWS2-rGO) compared to image of dWS2-rGO. No large WS2 sheets could be found according to the images contrast, indicating that the sheet-to-sheet assembly should be relatively uniform. More important, this close assembly possesses the unique feature that WS2 sheets will have maximum electrical contact with rGO, which could result in high conductivity of the hybrids.48 The HRTEM images as shown in Fig. 3 (c-e), indicate layered WS2 with gradually reduced number of layers (~25 layers for dWS2-rGO, ~7 layers for sWS2-rGO, and mono layer for mWS2-rGO), which provide a visual evidence for the layercontrollable WS2 nanosheets. Compared to the layer spacing of
WS2 as shown in Fig. 3e, it is reasonable to infer that dark Viewthese Article Online DOI: monolayered 10.1039/C5NR01896J contrast lines are the standing edges of the WS 2 nanosheets in Fig. 3f. More details of the center and the edge of the hybrid including lattice spacing (0.27 nm) and interlayer distances (0.62 nm) can be revealed from Fig. 3g to i, which agrees well with the XRD results. The ring-like mode in the selected area electron diffraction (SAED) pattern confirms the presence of polycrystalline WS2 with high quality hexagonalphase in the hybrid nanosheets (the inset of Fig. 3g).49 The HRTEM results clearly determinate the relationship between the amount of PVP and the WS2 layer number in the hybrid nanosheets.
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Fig. 3 TEM and HRTEM images of dWS2-rGO (a, d, and g), sWS2rGO (b, e, and h), and mWS2-rGO (c, f, and i) hybrid nanosheets. Inset: SAED pattern of g. High-resolution XPS measurements are then employed to analyze the atomic species and bonding characteristics of the mWS2-rGO hybrid nanosheets (Fig. 4a-c). The W4f core-level spectra (Fig. 4a) reveals two strong peaks at 34.75 and 32.31 eV, which identified as W4f5/2 and W4f7/2 levels for the W4+ state in WS2, respectively. Besides, another weak peak observed at a higher binding energy (37.88 eV) is attributed to W5p3/2.50 The S2p core-level spectra (Fig. 4b) exhibits two peaks at 161.86 and 163.35 eV, which are indexed to S2p1/2 and S2p3/2, respectively.50 The C1s spectra as shown in Fig. 4c consists two types of carbon atoms that one peak centered at 284.56 eV is ascribed to the presence of sp2 C without oxidation in the basal of rGO, the other weak peak (285.52 eV) detected at the shoulder indicates the presence of a trace amount of oxygencontaining functional groups (C-O) according to the previous reports.51 The adsorption-desorption isotherm of WS2-rGO samples displays a type IV adsorption branch with a H3 hysteresis loop as shown in Fig. 4d. As expected, the BET surface area of mWS2-rGO sample is calculated to be 139 m 2 g−1, which is much higher than those of sWS2-rGO (128 m2 g-1), dWS2-rGO
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Fig. 4 Core-level W4f (a), S2p (b), and C1s (c) XPS spectra of mWS2-rGO. Nitrogen adsorption-desorption isotherm (d) of layer-controllable WS2-rGO hybrid nanosheets. Although Raman spectroscopy has become a very powerful tool for identifying the number of layer in TMDs and examining the changes in material properties with thickness, such as MoS2 and WS2, resonant Raman scattering in layer-controllable WS2rGO hybrid nanosheets have not been observed previously. Representative Raman spectra of layered WS 2-rGO hybrids with different WS2 thickness are shown in Fig. 5a, which is tested at 514 nm (the excitation wavelength, λexc). The G (1589.8 cm-1) and D (1348.2 cm-1) bands associated with the E2g phonon of sp2-bonded carbon atoms and the defects in rGO, respectively.53 The nearly equal difference values in wavenumber indicate that the amount of PVP have almost no effect to the GO of three samples which prepared in the same process. For the Raman fingerprints of WS2, two strong peaks are observed from both the out of plane A1g and the in plane E12g vibration modes (overlap with longitudinal acoustic mode (2LA)) around 400 cm-1, which belong to the four Raman active modes of the WS2 (Fig. 5e). Several interesting characteristics as a function of film thickness are summarized from Fig. 5b. Most strikingly, the A1g mode blueshifts, while the E12g mode redshifts slightly with increasing sample thickness. This blueshifts can attribute to the increase in van der Waals interactions between layers. On the other hand, anomalous behavior of the E12g mode can assign to the stronger dielectric screening of the long range Coulomb interactions between the effective charges in thicker samples.54 The fact that the WS2 nanosheets possess monolayered thickness in the as-prepared mWS2-rGO sample can be demonstrated the ratio of peaks intensity of E12g and A1g (~2.2) when 514.5 nm laser used as excitation source.48 Besides, 417.3 cm-1 for A1g vibration is also agreement with the value reported for mono-layered WS2 well.55 From the view of the frequency difference between A1g and E12g modes (Δ = A1g− E12g), the values
-1 of Δ in the three samples are 64.5, 66.8, and 68.2 View Article cm Online, DOI: 10.1039/C5NR01896J respectively, indicating the number of layers (monolayer for mWS2-rGO, ~7 layers for sWS2-rGO, and ~25 layers for dWS2rGO). In addition to the excitation at 514 nm, λexc=488.5 nm has also been employed to describe the Raman features of the mWS2-rGO as shown in Fig. 5c. The individual contributions from first-order and second-order can be clearly separates by multi-peak Lorentzian fitting. For 488.5 nm, it is remarkable that many second-order peaks weaken or disappear compared with those observed for the λexc= 514.5 nm. Accordingly, the number of layers in as-synthesized WS2-rGO hybrid nanosheets are identified unambiguously through the systematical analysis of the Raman results.
Nanoscale Accepted Manuscript
(92 m2 g-1), and other reported WS2 based hybrid nanosheets (10~20 m2 g-1 21 and 98.2 m2 g-1 52). These results are consistent with the observed morphological changes (SEM and TEM images).
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Fig. 5 Raman spectra (a) for layer-controllable WS2-rGO hybrid nanosheets (mWS2-rGO, sWS2-rGO, and dWS2-rGO,) using a laser with 514.5 nm wavelength. The magnified image (b) in (a). Raman spectra from the mWS2-rGO hybrid nanosheets (c), using different laser excitation (514.5 and 488 nm, respectively), including Lorentzian peak fits. Structural model of WS 2 viewed along the b-axis (d) and (e) the four Raman active modes of the WS2 bilayer structure. The electrocatalytic activity of the layer-controllable (mono, several, and dozens- layered) WS2-rGO hybrid nanosheets toward HER was investigated in 0.5 M H 2SO4 solution using a typical three electrode setup, as described in the experimental section. The polarization curves (i-R corrected) from electrodes made from the as-prepared hybrids materials yielded overpotentials ranging from -100 ~ -150 mV suggesting the superior HER activity, as shown in Fig. 6a. Notably, the overpotentials required for the mWS2-rGO electrode to produce current densities of 10 and 20 mA cm-2 are 170 and 210 mV, respectively. These overpotentials consistent with previous reports, which focus on ultrathin WS 2 electrocatalysts with similar testing condition, including single-component WS2 nanoflakes56 and WS2 nanosheets anchored on rGO. 21 In contrast, both sWS2-rGO and dWS2-rGO shows slight lower HER activity, with overpotentials of 195 and 202 mV for 10 mA cm -2, respectively, indicating that the WS2-rGO hybrids with monolayered WS2 is crucial to enhance the electrocatalytic activity for
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the HER. In addition, the polarization curve of glassy carbon testifies no HER activity. Fig. 6b shows Tafel plots for exfoliated WS2-rGO samples and Pt. The Tafel plots (after i-R correction) display a slope of 78 mV dec-1 for dWS2-rGO, 70 mV dec-1 for sWS2-rGO, and 52 mV dec-1 for mWS2-rGO, respectively. Compared to the value of ~40 mV dec-1 observed with MoS2 (similar structure with WS2) loaded on rGO,57 Volmer-Heyrovsky reaction pathway should be involved to produce hydrogen for mWS2-rGO hybrids. The reason of the much lower Tafel slope value (52 mV dec-1) can attributed to the formation of an interconnected conducting bridge for rapid electron transport between mono-layered WS2 and rGO.57 In order to further reveal the advantage of the gradient exfoliated WS2-rGO, electrochemical impedance spectroscopy (EIS) was employed to investigate the electrode kinetics under HER process. The electrical equivalent circuit diagram given in Fig. 6c (inset) was used to model the solid liquid interface, and the experimental data were well fitted in Fig. 6, where a constant phase element (CPE) was employed. As shown in Fig. 6c, the representative Nyquist plots display a remarkably decreased charge transfer resistance (Rct) for mWS2-rGO (18 Ω) compared to sWS2-rGO (46 Ω) as well as dWS2-rGO (52 Ω). In addition to larger BET surface area (Fig. 4d), enhanced conductivity of mono-layered WS2 anchored on rGO may improve the catalytic performance.
conditions, even after 6 months storage under air View atmosphere. Article Online DOI: HER 10.1039/C5NR01896J The remarkable durability as well as superior activity of the mono-layered WS2 nanosheets coupled with rGO may making it promising HER catalyst for practical applications.
Conclusions
Nanoscale Accepted Manuscript
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In summary, layer-controllable WS2-rGO hybrid nanosheets have been synthesized by a hydrothermal process for the first time. As a result, the optimized catalyst (mWS2-rGO) exhibits remarkable HER activity with a current density of 10 mA cm−2 at a small overpotential of 170 mV. The enhanced electrocatalytic performance for HER can be attributed to the high surface area, abundant active edges, and more importantly, an interconnected conducting bridge between mono-layered WS2 and rGO. Moreover, impressively small overpotential (~100 mV), a low Tafel slope (52 mV dec-1), and remarkable durability (after 6 months storage) have been achieved. With the excellent HER performance and low cost, the WS 2-rGO hydrids catalyst can compete with the extensively studied MoS2-rGO well and will provide inspirations for production myriad of layer modulated TMD hybrids.
Acknowledgements
We acknowledge the financial support from the National Basic Research Program of China (2012CB933301, 2014CB648300), the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), the National Synergistic Innovation Center for Advanced Materials (SICAM), the Natural Science Foundation of Jiangsu Province, China (BM2012010), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), and the National Natural Science Foundation of China (51172110, 51372119, 81273409, 61136003, 51173081).
Notes and references 1 Fig. 6 (a) Polarization curves (iR-corrected) and (b) corresponding Tafel plots (iR-corrected) of various samples (including glassy carbon and Pt for comparison) as indicated. (c) Nyquist plots of different samples (inset: electrical equivalent circuit diagram) and (d) Durability test for mWS 2-rGO hybrid nanosheets after 3000 CV cycles and 6 months storage under air atmosphere. Besides the activity in HER, another significant criterion for electrocatalyst selection is good durability. To investigate the long-term cycling durability of the mWS2-rGO hybrid nanosheets, accelerated degradation testing were performed in 0.50 M H2SO4. As shown in Fig. 6d, excellent durability of mWS2rGO can be revealed from the comparison of the polarization curves before and after 3000 CV cycles. More important, the mWS2-rGO sample exhibited negligible decay (
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Zhang, Q. Wang, L. Wang, X. Li and W. Huang, Nanoscale, 2015, DOI: 10.1039/C5NR01896J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Layer-controllable WS2-reduced graphene oxide hybrid nanosheets with high electrocatalytic activity for hydrogen evolution Jian Zhanga, Qi Wanga, Lianhui Wanga, Xing’ao Lia,*, Wei Huanga,b,*
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR01896J
In this paper, an efficient poly(vinylpyrrolidone) (PVP)-assisted hydrothermal method for in-situ growth of WS2 nanosheets with layer controllability on reduced graphene oxide (rGO) is reported. The layer number (from monolayer to ~25 layers) of the exfoliated WS2 can be controlled gradiently by adjusting the amount of PVP. The layer structure and morphology of the as prepared hybrids are confirmed by field emission scanning electron microscopy and high-resolution transmission microscopy. The X-ray diffraction, Raman, and X-ray photoemission spectroscopy of the obtained WS2-rGO hybrid nanosheets indicate highly crystallized structures, clear Raman shift and good layer-number dependent stoichiometry of the materials. Furthermore, These highly active and durable catalysts exhibit an electrocatalytic current density of 10 mA cm -2 at a small hydrogen evolution reaction (HER) overpotential (-170 mV) and a Tafel slope of 52 mV dec-1 with an excellent electrocatalytic stability (after 6 months storage).
1 Introduction Hydrogen is frequently considered as the perfect energy carriers that can be an alternative to fossil fuels in recent decades for its numerous virtues, such as high energy density and recyclability.1,2 As the most attractive molecular fuel, hydrogen obtained by electrocatalytic hydrogen evolution reaction (HER) from water is renewable and free of pollution. 3-5 Ideal electrocatalysts should be earth-abundant, decreased the overpotential of HER, and sufficiently stable with minimal decay in performance.6-8 Although the noble metal (such platinum) has long been an efficient candidate for HER for its advantage: chemical inertness, and high activity,9 more economic and competitive catalysts including transition metal sulfides, 10-14 carbides,15 boride,16 phosphides,17 have attracted great attention as electrocatalysts in recent years. Among these alternatives, WS2 has attracted increasing attention for high activity and the earth-abundant composition, leading to the development of various kinds of WS2 based HER electrocatalysts recently.18-21 As one of important member in the family of the 2 dimensional (2D) transition metal dichalcogenides (TMDs),22-24 atomically thin WS2 nanosheets (mono or few-layers) presents some unusual chemical, physical, and electronic properties
a. Key
Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China. E-mail: [email protected] (X. Li), Tel: +8602585866362 b. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing 211816, PR China. [email protected] (W. Huang).
compared with their bulk compound and therefore hold great promise for a variety of applications.25-28 Until now, atomically thin WS2 nanosheets can be synthesized by mechanical exfoliation,29 chemical Li-intercalation and exfoliation,30,31 and grown by chemical vapor deposition (CVD).32,33 Recently, layer-controlled WS2 layers have been synthesized by using WO3 and S as precursors via the CVD growth.34 Mono and few-layer nanosheets of WS2 prepared by this method show high crystallinity and reasonable electrical properties.35 By use of the mechanical exfoliation method, not only monolayer but also multilayer WS2 nanosheets can be obtained.36 Unfortunately, forced exfoliation only produces small-sized flakes. Besides, Shiva et al. reported a simple solidstate reaction method for few-layers of WS2 by heating tungstic acid and thiourea in N2 atmosphere at 773 K.37 The most popular strategy used for large scale manufacturing of TMD nanosheets may be the hydrothermal reaction. For example, oxygen-incorporated MoS2 sheets were prepared by a facile one-pot hydrothermal method by Xie’s group.10 Benefiting from the oxygen incorporation, efficient HER activity was achieved for the ultrathin nanosheets with a large cathodic current density of 126.5 mA cm−2 at an overpotential of 300 mV for HER application. A low temperature hydrothermal method was developed by Chhowalla et al.21 for the growth of WS2 nanosheets on the reduced graphene oxide (rGO) surface using tungsten chloride and thioacetamide as precursors. The asprepared hybrid nanosheets exhibit overpotential ranging from -150 ~ -200 mV with Tafel slope of 58 mV dec -1. Although their potential catalysts for HER has been realized more than 20 years ago,38 the absence of a layer-controllable WS2 nanosheet synthesis method has confined the study and application of WS2-rGO hybrids.39
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2 Experimental Preparation of GO nanosheets GO nanosheets were exfoliated from natural graphite by the modified Hummer's method reported elsewhere.44 The GO nanosheets were dispersed into deionized water under ultrasonic to form the solution with a concentration of 2.5 mg mL-1. Preparation of layer-controllable WS2-rGO hybrid nanosheets Layer-controllable WS2-rGO hybrid nanosheets were prepared by a one-pot hydrothermal reaction process. In a typical experiment, 20 mL of 2.5 mg mL -1 GO absolute ethanol was added to the mixture of 0.1 mmol WCl6 (Sigma-aldrich, 99.98%) and 0.6 mmol sodium diethyldithiocarbamate (NaS 2CN(C2H5)2, Shanghai Chemical Factory of China, AR) and the total volume of the solution was maintained at 36 mL. PVP (Shanghai Chemical Factory of China, AR) was then added to the mixture. After stirring for 0.5 h at room temperature by a magnetic stirrer, the solution was transferred to a 50 mL stainless steel autoclave, heated up to 220 °C and kept for 18 h. After cooling naturally, the product was filtered, washed with deionized water, and dried in a vacuum at 50 °C for 8 h. Three hybrids were synthesized, namely mWS2-rGO (m-mono), sWS2-rGO (sseveral) and dWS2-rGO (d-dozens), which were prepared with different precursor/surfactant ratios of 2 : 10, 2 : 5 and 2 : 1, respectively. Characterization of WS2-rGO hybrid nanosheets The field emission scanning electron microscopy (FE-SEM) images were taken on a scanning electron microscope (Hitachi S-4800). The transmission electron microscopy (TEM) images was conducted on a JEOL JEM-2100 microscope operating at 200 kV. X-ray diffraction (XRD) patterns were performed on a BRUKER D8 Advance X-diffractometer (Cu Ka radiation, 1.54056 °A). X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MK II with Mg Kα as the excitation source. The Brunauer-Emmett-Teller (BET) surface area was measured by the nitrogen gas adsorption-desorption method at 77 K using a TriStar II 3020. Raman spectra were conducted on the Renishaw in via Raman microscope with 488 and 514.5 nm lasers as the exciting radiation. Electrochemical measurements. Electrochemical measurements were performed using a threeelectrode system on an electrochemical workstation (CHI660B).
All measurements were performed in 50 mL of 0.5View M HArticle (pH 2SO4Online DOI: 10.1039/C5NR01896J = 0.157, purged with H2, 99.999%) electrolyte. 6 mg of WS2-rGO powder and 50 μL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 2 mL water-isopropanol solution with volume ratio of 3:1 by sonicating for 2 h to form a homogeneous dispersion. Then 10 μL (40 μg catalyst) of the dispersion was loaded onto a glassy carbon electrode as the working electrode (loading ~0.562 mg cm−2 catalyst), a graphite rod (Alfa Aesar, 99.9995%) as a counter electrode, and Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode. All of the potentials were calibrated to a reversible hydrogen electrode (RHE). The electrochemical stability of the catalyst was evaluated by cycling the electrode 3000 times each cycle started at 0.20 V and ended at -0.24 V vs. RHE with a scan rate of 50 mV s-1. The Nyquist plots were measured in the frequency range 10 -1 to 105 Hz with an overpotential of 200 mV.
3 Results and discussion
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Herein, we develop a simple but effective hydrothermal process to fabricate layer-controllable WS2-rGO hybrid nanosheets. The thickness of WS2 nanoflakes can be systematically modulated (mono, several, and dozens-layered) by controlling the amount of surfactant, poly(vinylpyrrolidone) (PVP). This is one of the first reports on layer controlled of WS 2 nanosheets by use of single source precursors (W(S 2CN(C2H5)2)6) for growth of ultrathin WS2 nanoflakes controlled gradiently on rGO. M(S2CN(C2H5)x (M = metallic element) has been employed to synthesis various metal sulfides, such as CdS nanorods 40 and Bi2S3 nanotubes.41 The as-prepared samples exhibit good catalytic activity and durable for HER, matching the activity of MoS2/rGO42 and MoS2/N-Doped CNT.43
It has been demonstrated that ultrathin nanomaterials can be prepared by ionic45 and nonionic surfactant46 assist. Herein, we synthesis WS2-rGO hybrid nanosheets by a simple one-pot hydrothermal process in the presence of PVP as surfactant. The strategy for the fabrication of the layer-controllable WS2-rGO hybrid nanosheets is illustrated in Fig. 1. The synthesis started with dispersing single source precursor W(S2CN(C2H5)2)6 and GO. WS2 nanocrystals in-situ nucleate and grow on the surface of the GO from the decomposing of W(S2CN(C2H5)2)6, while GO nanosheets transformed to rGO. Meanwhile, the layer number of WS2 can be controlled from ~1 layer to ~25 layers by adjusting the amount of PVP. For a relatively low ratio of precursor/surfactant (2 : 10), atomically thin WS2 are capped completely by sufficient PVP ligands. With the increasing of the ratio, several and dozens-layer WS2 nanosheets can be detected on the basal planes of rGO.
Fig. 1 Schematic illustration of the fabrication of the layercontrollable WS2-rGO hybrid nanosheets. Due to the functional groups on the basal plane of GO, single source precursors W(S2CN(C2H5)2)6 decompose, nucleate, and subsequently grow to WS2 nanocrystal on this active substrate. Fig. 2 (a-c) shows FESEM images of mono-to multi-layered WS2rGO hybrid sheets (mWS2-rGO, sWS2-rGO, and dWS2-rGO, respectively) which synthesized via hydrothermal reaction at 260 °C for 18 h. It can be seen that the as-prepared dWS2 has a
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compact combination with GO, which exhibit nanosheets structure with a thickness of ~20 nm (~ 30 layers), as shown in Fig. 2a. In contrast, the sWS2-rGO composites has a layered appearance obviously and the average thickness of the sWS 2 is ~5 nm (about 7 layers of WS2) with the increase of the surfactant (PVP). Finally, when the precursor/surfactant ratios reached to 2 : 10, a curved leaf-like appearance with the thickness of ~1 nm can be detected in Fig. 2c. The novel layer controlled WS2-rGO nanosheets can be easily obtained by controlling the amount of surfactant according to the SEM observation. XRD analysis of the WS 2-rGO hybrids show high crystalline hexagonal structure (JCPDS Card no 84-1398) without any other impurities, as shown in Fig. 2d. The slightly shift of the WS2 (002) peak to lower angle indicates an expanded interlayer distance between the adjacent sheets due to the twisting of the WS2 crystal structure.47 For comparison, the (002) peaks gradually disappear with the increasing of surfactant dosage from the XRD patterns, consisting with the lack of substantial stacking of layers.44
Fig. 2 SEM images of (a) dWS2-rGO (dozens layers), (b) sWS 2rGO (several layers) and (c) mWS2-rGO (mono layer) hybrid nanosheets. XRD patterns of (d) WS2-rGO hybrids. Further insights into the detailed microstructure of the WS 2rGO hybrid nanosheets were obtained from TEM and HRTEM images, as shown in Fig. 3. For the dWS2-rGO sample, WS2 nanosheets with an average thickness of ~20 nm lie vertically on the surface of rGO (Fig. 3a), which is consistent with the SEM images (Fig. 2a). WS2 nenosheet anchored on the rGO can be detected with even much more compact in Fig. 3b (sWS2-rGO) and c (mWS2-rGO) compared to image of dWS2-rGO. No large WS2 sheets could be found according to the images contrast, indicating that the sheet-to-sheet assembly should be relatively uniform. More important, this close assembly possesses the unique feature that WS2 sheets will have maximum electrical contact with rGO, which could result in high conductivity of the hybrids.48 The HRTEM images as shown in Fig. 3 (c-e), indicate layered WS2 with gradually reduced number of layers (~25 layers for dWS2-rGO, ~7 layers for sWS2-rGO, and mono layer for mWS2-rGO), which provide a visual evidence for the layercontrollable WS2 nanosheets. Compared to the layer spacing of
WS2 as shown in Fig. 3e, it is reasonable to infer that dark Viewthese Article Online DOI: monolayered 10.1039/C5NR01896J contrast lines are the standing edges of the WS 2 nanosheets in Fig. 3f. More details of the center and the edge of the hybrid including lattice spacing (0.27 nm) and interlayer distances (0.62 nm) can be revealed from Fig. 3g to i, which agrees well with the XRD results. The ring-like mode in the selected area electron diffraction (SAED) pattern confirms the presence of polycrystalline WS2 with high quality hexagonalphase in the hybrid nanosheets (the inset of Fig. 3g).49 The HRTEM results clearly determinate the relationship between the amount of PVP and the WS2 layer number in the hybrid nanosheets.
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Fig. 3 TEM and HRTEM images of dWS2-rGO (a, d, and g), sWS2rGO (b, e, and h), and mWS2-rGO (c, f, and i) hybrid nanosheets. Inset: SAED pattern of g. High-resolution XPS measurements are then employed to analyze the atomic species and bonding characteristics of the mWS2-rGO hybrid nanosheets (Fig. 4a-c). The W4f core-level spectra (Fig. 4a) reveals two strong peaks at 34.75 and 32.31 eV, which identified as W4f5/2 and W4f7/2 levels for the W4+ state in WS2, respectively. Besides, another weak peak observed at a higher binding energy (37.88 eV) is attributed to W5p3/2.50 The S2p core-level spectra (Fig. 4b) exhibits two peaks at 161.86 and 163.35 eV, which are indexed to S2p1/2 and S2p3/2, respectively.50 The C1s spectra as shown in Fig. 4c consists two types of carbon atoms that one peak centered at 284.56 eV is ascribed to the presence of sp2 C without oxidation in the basal of rGO, the other weak peak (285.52 eV) detected at the shoulder indicates the presence of a trace amount of oxygencontaining functional groups (C-O) according to the previous reports.51 The adsorption-desorption isotherm of WS2-rGO samples displays a type IV adsorption branch with a H3 hysteresis loop as shown in Fig. 4d. As expected, the BET surface area of mWS2-rGO sample is calculated to be 139 m 2 g−1, which is much higher than those of sWS2-rGO (128 m2 g-1), dWS2-rGO
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Fig. 4 Core-level W4f (a), S2p (b), and C1s (c) XPS spectra of mWS2-rGO. Nitrogen adsorption-desorption isotherm (d) of layer-controllable WS2-rGO hybrid nanosheets. Although Raman spectroscopy has become a very powerful tool for identifying the number of layer in TMDs and examining the changes in material properties with thickness, such as MoS2 and WS2, resonant Raman scattering in layer-controllable WS2rGO hybrid nanosheets have not been observed previously. Representative Raman spectra of layered WS 2-rGO hybrids with different WS2 thickness are shown in Fig. 5a, which is tested at 514 nm (the excitation wavelength, λexc). The G (1589.8 cm-1) and D (1348.2 cm-1) bands associated with the E2g phonon of sp2-bonded carbon atoms and the defects in rGO, respectively.53 The nearly equal difference values in wavenumber indicate that the amount of PVP have almost no effect to the GO of three samples which prepared in the same process. For the Raman fingerprints of WS2, two strong peaks are observed from both the out of plane A1g and the in plane E12g vibration modes (overlap with longitudinal acoustic mode (2LA)) around 400 cm-1, which belong to the four Raman active modes of the WS2 (Fig. 5e). Several interesting characteristics as a function of film thickness are summarized from Fig. 5b. Most strikingly, the A1g mode blueshifts, while the E12g mode redshifts slightly with increasing sample thickness. This blueshifts can attribute to the increase in van der Waals interactions between layers. On the other hand, anomalous behavior of the E12g mode can assign to the stronger dielectric screening of the long range Coulomb interactions between the effective charges in thicker samples.54 The fact that the WS2 nanosheets possess monolayered thickness in the as-prepared mWS2-rGO sample can be demonstrated the ratio of peaks intensity of E12g and A1g (~2.2) when 514.5 nm laser used as excitation source.48 Besides, 417.3 cm-1 for A1g vibration is also agreement with the value reported for mono-layered WS2 well.55 From the view of the frequency difference between A1g and E12g modes (Δ = A1g− E12g), the values
-1 of Δ in the three samples are 64.5, 66.8, and 68.2 View Article cm Online, DOI: 10.1039/C5NR01896J respectively, indicating the number of layers (monolayer for mWS2-rGO, ~7 layers for sWS2-rGO, and ~25 layers for dWS2rGO). In addition to the excitation at 514 nm, λexc=488.5 nm has also been employed to describe the Raman features of the mWS2-rGO as shown in Fig. 5c. The individual contributions from first-order and second-order can be clearly separates by multi-peak Lorentzian fitting. For 488.5 nm, it is remarkable that many second-order peaks weaken or disappear compared with those observed for the λexc= 514.5 nm. Accordingly, the number of layers in as-synthesized WS2-rGO hybrid nanosheets are identified unambiguously through the systematical analysis of the Raman results.
Nanoscale Accepted Manuscript
(92 m2 g-1), and other reported WS2 based hybrid nanosheets (10~20 m2 g-1 21 and 98.2 m2 g-1 52). These results are consistent with the observed morphological changes (SEM and TEM images).
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Fig. 5 Raman spectra (a) for layer-controllable WS2-rGO hybrid nanosheets (mWS2-rGO, sWS2-rGO, and dWS2-rGO,) using a laser with 514.5 nm wavelength. The magnified image (b) in (a). Raman spectra from the mWS2-rGO hybrid nanosheets (c), using different laser excitation (514.5 and 488 nm, respectively), including Lorentzian peak fits. Structural model of WS 2 viewed along the b-axis (d) and (e) the four Raman active modes of the WS2 bilayer structure. The electrocatalytic activity of the layer-controllable (mono, several, and dozens- layered) WS2-rGO hybrid nanosheets toward HER was investigated in 0.5 M H 2SO4 solution using a typical three electrode setup, as described in the experimental section. The polarization curves (i-R corrected) from electrodes made from the as-prepared hybrids materials yielded overpotentials ranging from -100 ~ -150 mV suggesting the superior HER activity, as shown in Fig. 6a. Notably, the overpotentials required for the mWS2-rGO electrode to produce current densities of 10 and 20 mA cm-2 are 170 and 210 mV, respectively. These overpotentials consistent with previous reports, which focus on ultrathin WS 2 electrocatalysts with similar testing condition, including single-component WS2 nanoflakes56 and WS2 nanosheets anchored on rGO. 21 In contrast, both sWS2-rGO and dWS2-rGO shows slight lower HER activity, with overpotentials of 195 and 202 mV for 10 mA cm -2, respectively, indicating that the WS2-rGO hybrids with monolayered WS2 is crucial to enhance the electrocatalytic activity for
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the HER. In addition, the polarization curve of glassy carbon testifies no HER activity. Fig. 6b shows Tafel plots for exfoliated WS2-rGO samples and Pt. The Tafel plots (after i-R correction) display a slope of 78 mV dec-1 for dWS2-rGO, 70 mV dec-1 for sWS2-rGO, and 52 mV dec-1 for mWS2-rGO, respectively. Compared to the value of ~40 mV dec-1 observed with MoS2 (similar structure with WS2) loaded on rGO,57 Volmer-Heyrovsky reaction pathway should be involved to produce hydrogen for mWS2-rGO hybrids. The reason of the much lower Tafel slope value (52 mV dec-1) can attributed to the formation of an interconnected conducting bridge for rapid electron transport between mono-layered WS2 and rGO.57 In order to further reveal the advantage of the gradient exfoliated WS2-rGO, electrochemical impedance spectroscopy (EIS) was employed to investigate the electrode kinetics under HER process. The electrical equivalent circuit diagram given in Fig. 6c (inset) was used to model the solid liquid interface, and the experimental data were well fitted in Fig. 6, where a constant phase element (CPE) was employed. As shown in Fig. 6c, the representative Nyquist plots display a remarkably decreased charge transfer resistance (Rct) for mWS2-rGO (18 Ω) compared to sWS2-rGO (46 Ω) as well as dWS2-rGO (52 Ω). In addition to larger BET surface area (Fig. 4d), enhanced conductivity of mono-layered WS2 anchored on rGO may improve the catalytic performance.
conditions, even after 6 months storage under air View atmosphere. Article Online DOI: HER 10.1039/C5NR01896J The remarkable durability as well as superior activity of the mono-layered WS2 nanosheets coupled with rGO may making it promising HER catalyst for practical applications.
Conclusions
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In summary, layer-controllable WS2-rGO hybrid nanosheets have been synthesized by a hydrothermal process for the first time. As a result, the optimized catalyst (mWS2-rGO) exhibits remarkable HER activity with a current density of 10 mA cm−2 at a small overpotential of 170 mV. The enhanced electrocatalytic performance for HER can be attributed to the high surface area, abundant active edges, and more importantly, an interconnected conducting bridge between mono-layered WS2 and rGO. Moreover, impressively small overpotential (~100 mV), a low Tafel slope (52 mV dec-1), and remarkable durability (after 6 months storage) have been achieved. With the excellent HER performance and low cost, the WS 2-rGO hydrids catalyst can compete with the extensively studied MoS2-rGO well and will provide inspirations for production myriad of layer modulated TMD hybrids.
Acknowledgements
We acknowledge the financial support from the National Basic Research Program of China (2012CB933301, 2014CB648300), the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), the National Synergistic Innovation Center for Advanced Materials (SICAM), the Natural Science Foundation of Jiangsu Province, China (BM2012010), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), and the National Natural Science Foundation of China (51172110, 51372119, 81273409, 61136003, 51173081).
Notes and references 1 Fig. 6 (a) Polarization curves (iR-corrected) and (b) corresponding Tafel plots (iR-corrected) of various samples (including glassy carbon and Pt for comparison) as indicated. (c) Nyquist plots of different samples (inset: electrical equivalent circuit diagram) and (d) Durability test for mWS 2-rGO hybrid nanosheets after 3000 CV cycles and 6 months storage under air atmosphere. Besides the activity in HER, another significant criterion for electrocatalyst selection is good durability. To investigate the long-term cycling durability of the mWS2-rGO hybrid nanosheets, accelerated degradation testing were performed in 0.50 M H2SO4. As shown in Fig. 6d, excellent durability of mWS2rGO can be revealed from the comparison of the polarization curves before and after 3000 CV cycles. More important, the mWS2-rGO sample exhibited negligible decay (
