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2D WC single crystal embedded in graphene for enhancing hydrogen evolution reaction
Author’s Accepted Manuscript 2D WC Single Crystal Embedded in Graphene for Enhancing Hydrogen Evolution Reaction Mengqi Zeng, Yunxu Chen, Jiaxu Li, Haifeng Xue, Rafael G. Mendes, Jinxin Liu, Tao Zhang, Mark H. Rümmeli, Lei Fu www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30064-2 http://dx.doi.org/10.1016/j.nanoen.2017.01.057 NANOEN1771
To appear in: Nano Energy Received date: 4 November 2016 Revised date: 26 January 2017 Accepted date: 29 January 2017 Cite this article as: Mengqi Zeng, Yunxu Chen, Jiaxu Li, Haifeng Xue, Rafael G. Mendes, Jinxin Liu, Tao Zhang, Mark H. Rümmeli and Lei Fu, 2D WC Single Crystal Embedded in Graphene for Enhancing Hydrogen Evolution Reaction, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.01.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2D WC Single Crystal Embedded in Graphene for Enhancing Hydrogen Evolution Reaction Mengqi Zenga1, Yunxu Chena1, Jiaxu Lia, Haifeng Xuea, Rafael G. Mendesb2, Jinxin Liua, Tao Zhanga, Mark H. Rümmelib2, Lei Fua* College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China [email protected] ABSTRACT Electrochemical water splitting is regarded as one of the most economical and eco-friendly approaches for hydrogen revolution. Developing a low-cost and earth-abundant non-noble-metal catalyst will be of the most significance. Tungsten carbide (WC) is highly promising due to its platinum (Pt) -like behavior in surface catalysis. Here we first report a liquid metal solvent based co-segregation (LMSCS) strategy to fabricate a high uniformity of 2D WC crystals embedded in graphene by chemical vapor deposition (CVD) in one step. The 2D in-plane WC–graphene heterostructures (i-WC–G) are remarkably stable under an electro-catalytic environment and ensure good interfacial synergy between the 2D WC crystallites and graphene to achieve a more effective hydrogen evolution. The overpotential was as low as 120 mV and the Tafel slope was 38 mV/dec, which indeed exhibits outstanding catalytic potential among the reported 2D material systems. Our elegant and versatile approach allows the fabrication of other high-quality 2D transition metal carbides (TMCs) and their in-plane heterostructure, which will further promote practical catalytic applications of metal carbides. Graphicalabstract A liquid metal solvent based co-segregation strategy was proposed to fabricate a high uniformity of 2D WC crystals embedded in the few-layer graphene by chemical vapor deposition (CVD) in one step. In 1 2
These authors contributed equally to this work. IFW Dresden, P. O. Box 270116, 01069 Dresden, Germany 1
addition, the efficient catalytic ability of the 2D WC for the hydrogen evolution, for the first time, was experimentally exhibited. KEYWORDS WC, graphene, in-plane heterostructure, two-dimensionalization, HER
Introduction Hydrogen has been demonstrated to be a promising clean and economical energy source due to its advantages of high mass energy density and renewability. Direct electrochemical splitting of water provides an infinite potential method to generate hydrogen. The most effective hydrogen evolution reaction (HER) electrocatalysts are Pt-group metals. However, the natural scarcity and high costs of Pt limit the practical implementation. An advanced catalyst with low cost and earth abundance for the electrochemical HER is vigorously pursued. Transition metal carbides (TMCs) with excellent electric and modified chemical activity have aroused enormous attention as efficient electrochemical catalysts [1, 2]. And tungsten carbide (WC) is of particular interest due to its platinum (Pt) -like behavior in surface catalysis [3]. It is hoped that WC will develop into a highly competitive electro-catalyst for the development of the clean and renewable alternative energy sources. An ideal catalyst demands both the intrinsic catalytic potential and the charge transfer efficiency of the whole catalytic system [4]. The two dimensionalization and crystallization of the WC materials will provide the most effective reactive sites. Traditional synthesis approaches, such as the carbonization of W or W based precursors tend to yield bulk materials or nanoparticles with poor crystallinity, incomplete carbonization and impurities, all of which severely limit HER activity [5-7]. Exfoliated 2D TMC crystals possess inevitable defects or undesirable functional groups that can passivate the high-activity surface of the TMC crystals [8, 9]. Until now, the large-scale fabrication of pure and highly crystalline WC has not been demonstrated [10]. Moreover, currently available TMC powders or
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nanoparticles exhibit poor electrical connectivity due to their aggregation and exhibit incomplete anchoring with the supporting substrate [11, 12]. It was reported that the combination of crystalline WC with a conductive media could enhance its HER catalytic activity due to faster electron transfer and improved chemical/electrical coupling effects [12, 13]. Direct growth of 2D in-plane high-quality WC single crystals within a good conductor can be expected to provide synergy to simultaneously fully exhibit the intrinsic catalytic ability of the materials as well as efficient charge transfer within the entire catalytic system. Graphene with excellent conductivity [14], is easily produced by CVD, which makes it a good choice to combine with WC. However, a strictly in-plane heterostructure is quite hard to achieve on traditional solid substrates due to random nucleation and the chaotic layer stacking. Here, we first report a liquid metal solvent based co-segregation (LMSCS) strategy to effectively promote the simultaneous segregation of W and C atoms and fabricate an in-plane WC–graphene heterostructure (i-WC–G) in one step. The use of the as-produced i-WC–G in HER experiments confirm, for the first time, predictions that WC catalysts can be highly competitive as an electro-catalyst for energy production because of its excellent reactivity and superior chemical stability and, in addition, it uses environmentally friendly raw materials. The synergistic effects of WC-graphene heterostructures demonstrate its huge potential in HER catalysis. Experimental section CVD growth of 2D WC–graphene in-plane heterostructures (i-WC–G) on Ga–W substrates A commercial Ga pellet was divided into small droplets in hot ethanol. A droplet of Ga (10 mg) was then placed on a W foil (cut into 0.5×0.5 cm squares). Before loading Ga, the W foils were ultra-sonicated and rinsed with acetone, ethanol and deionized water prior to being dried under a nitrogen (N2) stream. The Ga pellets with a purity of 99.9999 wt.% and the W foils with a purity of 99.95 wt.% were purchased from Alfa Aesar China (Tianjin) Co. Ltd. and Shanghai Minor Metals Co. Ltd. The growth of i-WC–G was performed in a quartz tube furnace (HTF 55322C Lindberg/Blue M) under ambient pressure. The growth process consisted of three steps: (1) placing the Ga–W substrates in 3
the center region of the quartz tube. (2) heating the Ga–W substrates to 980 ∼ 1020˚C at a rate of 30˚C /min under the flow of Ar and H2; (3) exposing of the substrates to a carbon source at 1000˚C for 30 min under 300 sccm Ar, 30 sccm H2 and different flows of methane (CH4), ranging from 1 sccm to 5 sccm; (4) turn off the CH4 and cooling the substrates to room temperature rapidly under Ar and H2. Transferring the i-WC–G to the target substrates The process of transferring the samples to the target substrates involves spin-coating a poly(methyl methacrylate) (PMMA) film onto the substrate with i-WC–G grown on it and releasing the PMMA/sample film by etching out Ga in a diluted hydrogen chloride (1:1) for 1 h. This was followed by a rinsing in ultra-pure water to remove the metal ions. The PMMA layer was dissolved with hot acetone after the PMMA/sample film was transferred onto the target substrates, such as 300 nm SiO2/Si substrates and the quantifoils. Electrochemical Measurements All of the electrochemical measurements were carried out with a three-electrode system (sparged with pure N2, purity 99.999%) on an electrochemical workstation (CHI760E) at room temperature, using a 0.5×0.5 cm2 Ga–W foil with 2D i-WC–G directly grown on it as the working electrode, a Pt foil as a counter electrode, and a saturated calomel (SCE) as the reference electrode. In order to exclude the influence of the dissolved Pt that may redeposit onto the working electrode on the electrocatalytic process, XPS spectra were checked before and after the electrochemical characterization, as shown in Supplementary Figure S1. The reference electrode was calibrated by a reversible hydrogen electrode (RHE) in the electrolyte solution saturated with H2. All potentials were referenced to a reversible hydrogen electrode (RHE), E (RHE) = (0.059 ×pH + 0.276) V. Linear sweep voltammetry (LSV) was recorded in 0.1 M KOH (pH = 13) and 0.5 M H2SO4 (pH = 0.3), respectively. The scan rate of the linear sweep voltammetry was 5 mV/s to obtain the polarization curves. The loading of the electro-catalyst (WC) is 0.000936 mg/cm2 ~ 0.00222 mg/cm2. Commercial 20% Pt/C catalyst was used as a reference sample. The corresponding polarization curves with internal resistance (iR) compensation were adopted 4
in the measurements. Onset overpotentials were determined based on the beginning of the linear regime in the Tafel plot. The electrochemical stability of the catalyst was conducted by cycling the potential from +0.3 to −0.3 V versus RHE at a scan rate of 100 mV/s. The Nyquist plots were obtained with frequencies ranging from100 kHz to 0.01 Hz with an amplitude of 10 mV at the open-circuit voltage. The impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The presence and identity of the generated H2 were confirmed by on-line gas chromatography (GC) analysis (Shimadzu, Tracera). The mixed gas went through the sampling loop continuously during longtime chronoamperometry by bubbling 5 sccm Ar to the gastight H-type electrolytic cell. In such a cell, a Ga–W foil covered with 2D i-WC-G heterostructure acted as the working electrode and a carbon paper was employed as the counter electrode, which were separated by a Nafion film. In addition, a reversible hydrogen electrode served as the reference electrode. The faradic efficiency (FE) of the electrocatalytic hydrogen evolution process was calculated by comparing the peak area of gas chromatography with the counterpart when using a certified standard gas. Characterization Optical images were taken with an optical microscope (Olympus DX51), and Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw in Via, 532 nm excitation wavelength). Scanning electron microscopy (SEM) images were obtained by Hitachi-S4800 and ZEISS Merlin Compact SEM. The XPS measurements were conducted with Thermo Scientific, ESCALAB 250Xi. The measuring spot size was 500 μm and the binding energies were calibrated by referencing the C 1s peak (284.8 eV). The XPS depth profiling was performed by Ar ionic bombardment to gradually remove the surface layers. Each etching takes about 200 seconds. The AFM images were taken with a NT-MDT Ntegra Spectra with samples transferred onto the 300 nm SiO2/Si. The transmission electron microscopy (TEM) images were obtained by an aberration-corrected high-resolution TEM system (FEI Titan 80-300) using an operating voltage of 80 kV and by a probe corrected high-resolution TEM system (Titan Probe corrected TEM, Titan G2 60-300) with the operating voltage of 300 kV. The 5
samples were transferred onto a quantifoil copper TEM grid for characterization. Results and Discussions The low melting-point metal, gallium (Ga), was chosen to serve as the liquid solvent to achieve the co-segregation of W and C atoms. Figure 1a shows the scanning electron microscope (SEM) image of ultrathin WC single crystals (the thickness is 1.4 nm, Figure 1b embedded in graphene on the Ga–W with high density, which in essence provides active sites comparable with the total atoms forming the materials in a HER reaction. Figure 1c shows the Raman spectrum of WC on SiO2/Si. The bands situated at around 700 and 800 cm–1 are attributed to W–C stretching modes, and are marked with “W1” and “W2”. The narrow peak widths of W1 and W2 indicate the samples are highly crystalline [15]. X-ray photoelectron spectroscopy (XPS) is utilized to further identify the components, as shown in Supplementary Figure S2. The characteristic carbide carbon peak for WC is observed at 282.8 eV [16]. A C 1s peak at 284.8 eV is also observed, which originates from the graphene surrounding the WC flakes and possibly carbon adsorbates from the atmosphere. The W 4f signals for the W foil sample are located at 31.45 and 33.7 eV, in agreement with literature [17]. For all samples, the W 4f peak locations are observed to be shifted by +0.2 to 0.8 eV higher in binding energy as compared to metallic W. The structure and morphology of the as-obtained WC grains are further characterized by transmission electron microscopy (TEM). Figure 1d shows a full view of the thin hexagonal WC crystal via scanning TEM (STEM). Energy dispersive X-ray (EDX) spectroscopy mapping measurements show peaks for the C–K, W–M, W–L edges and indicate that the crystal is comprised of W and C with a uniform distribution (Figure 1f, 1g and 1h). The darker color in Figure 1f corresponding to the C content is due to the underlying quantifoil support on the Cu grid. High resolution TEM (HRTEM) data highlight a highly crystalline structure with six-fold symmetry. In order to show the single crystallinity, selected-area electron diffraction (SAED) is conducted at different regions on an individual hexagonal WC single crystal, as shown in Supplementary Figure S3. All the patterns show the same six-fold symmetric diffraction points for different collection regions concomitant with a single WC crystal. The 6
interface structure of the i-WC–G is examined by Raman line scanning (Supplementary Figure S4) and TEM (Figure 2). The obtained spectra of graphene and WC are easily distinguished, confirming the 2D i-WC–G. Figure 2a presents a low-magnification TEM image of the 2D i-WC–G. Figure 2b shows a magnified TEM image of the WC–G interface, which exhibits intimate bonding. The inset in Figure 2b shows the fast Fourier transform (FFT) pattern obtained from the region marked by the green dash square and exhibits the six-fold symmetry of the WC crystal. Figure 2c shows a typical edge of the crystalline graphene, which verifies the existence of 2D graphene. Figure 2d–f show the SAED patterns sampled from the regions corresponding to the interface, individual WC and graphene, respectively. Both of the patterns in Figure 2e and 2f exhibit the six-fold symmetry single-crystal nature while the extracted interplanar spacings are different. In short, we confirm the presence of 2D i-WC–G. To demonstrate how the i-WC–G formed via our LMSCS process, Figure 3a–b shows a schematic diagram illustrating the growth mechanism. The process occurs as follows: when Ga is in a molten state at elevated temperature, it contains two components, ions and conductive electrons. The high-energy ions lead to intense thermal motion that makes it possible to escape from its original atomic cluster and join others. As a result, the space between the atomic clusters fluctuates, which allows them to serve as heteroatom containers [18]. At the interface of the W and Ga layer, W atoms pass through the interface and diffuse into the bulk phase of the liquid Ga due to the atomic thermal motion. Besides, the C atoms are catalytically decomposed by Ga, which then also diffuse into the liquid bulk. Thus, an appreciable quantity of decomposed C atoms was “dissolved” into the liquid bulk of the Ga. XPS depth profile is employed to demonstrate the “dissolution” of the C and W atoms in the liquid Ga bulk (Supplementary Figure S5). The co-segregation of the WC and graphene will have a competitive relationship, similar to the growth of Mo2C and graphene in previously reported work [19]. The growth temperature will determine the amount of heteroatoms in the liquid phase via affecting the decomposition of CH 4, and the embedding and diffusion of the heteroatoms (C and W) in the liquid bulk (Ga). Lower temperatures lead to i-WC–G with relatively less WC% coverage while the higher temperatures lead to i-WC–G with
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larger WC% coverage (Supplementary Figure S6a–c). Slower cooling rates result in thicker WC flakes with higher coverage (Supplementary Figure S6d), which is similar to reported work on graphene deposition over high carbon-solubility metals [20]. In order to achieve the best performance for HER using our obtained i-WC–G, we employ a growth temperature of 1000˚C and a rapid cooling strategy to achieve the i-WC–G with relatively high WC% coverage. From the valence analysis of each element along a surface normal to the direction (cross-section) on Ga, the co-segregation process can be demonstrated. At the topmost surface (Figure 3c), the C 1s band contains a strong graphitic peak centered at 284.8 eV, originating from the graphene and possible carbon adsorbates from the atmosphere. There is also a relatively strong peak centered at 289.0 eV, originating from a C–O bonding structure. Noticeably, the characteristic carbide peak for WC is observed at 282.8 eV, indicating the presence of WC. After removal of the surface layer, the impurity adsorbates and graphene layer were almost entirely eliminated. Thus, the relative intensity of the carbide peak for WC became stronger. The XPS W4f band also confirms the existence of WC. As one goes deeper into the bulk, the carbide C1s peak shows a continuously decreasing relative intensity while the pure carbon C1s peak exhibits a stable intensity (Figure 3c, 3e). In addition, for W 4f band, the carbide peak also shows a continuously decreasing intensity with depth, and more importantly, the metallic W 4f peaks are present and show a marked increase with depth (Figure 3d, 3f). Therefore, one can infer that the W and C atoms indeed diffuse into the bulk of the liquid metal and i-WC–G forms via the co-segregation of the dissolved atoms. A series of i-WC–G with different WC% coverage (40% ~ 95%) are investigated for their potential to catalyze the HER in the acid conditions (0.5 M H2SO4). The polarization curves for these i-WC–G are shown in Figure 4a. Strikingly, the sample with 70% coverage required only ∼120 mV to achieve a 10 mA/cm2 current density. Compared with other tungsten carbide with various morphologies (Supplementary Table S1), our as-prepared i-WC–G heterostructure exhibits the lowest overpotential for catalyzing HER. Notably, the loading amount of the WC in our i-WC–G heterostructure with 70%
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WC coverage is as low as 0.00164 mg/cm2, which is far lower than that of the other tungsten carbide catalysts that exhibit the similar catalytic ability (0.16~2.2 mg/cm2) [21, 22]. It means that our ultrathin 2D WC with perfect crystallinity with graphene connecting exhibits the highest efficiency for HER. Other mainstream 2D TMDs for catalyzing HER were also summarized in Supplementary Table S2. Remaining the same, our i-WC–G heterostructure still exhibits the top catalytic ability. Therefore, we can convincingly point out that the catalytic property of the 2D i-WC-G heterostructure is superior to literature 2D materials. In addition, the current-time measurement during the electrocatalytic process under the constant overpotentials was combined with the on-line gas chromatography (GC) analysis to extract the faradaic efficiency (FE) (Supplementary Figure S7). The FE of the electrocatalytic hydrogen evolution process was calculated by comparing the peak area of gas chromatography with the counterpart when using a certified standard gas. We obtained the current-time curve during the electrocatalytic hydrogen evolution process under the overpotentials of 0.1 V and 0.2 V, respectively. The as-extracted corresponding FE is 97.2% and 96.3%, respectively, thus demonstrates that the current is from hydrogen evolution. In addition, the as-prepared 70% i-WC–G is also employed as the catalyst for the alkaline HER in 0.1 M KOH solution. The polarization curve after iR-correction and the corresponding Tafel plot of 2D i-WC–G with the WC coverage of 70% were shown in Figure S8. The overpotential needed to drive the current density of 10 mA/cm2 in the electrocatalytic process is ∼225 mV. The linear portion of the Tafel plot is fitted to the Tafel equation, yielding Tafel slope of ~108 mV/dec. This well demonstrated the catalytic potential of the as-proposed 2D i-WC–G in the alkaline HER field. To elucidate the HER mechanism of our i-WC–G heterostructure for catalyzing, Tafel plots were fitted to Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope (Figure 4b). The Tafel slope is an inherent property of the catalyst that is determined by the rate-limiting step of the HER. The determination and interpretation of the Tafel slope are important for elucidation of the elementary steps involved. Having a very high Hads coverage, the HER on a Pt surface is known to
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proceed through the Volmer–Tafel mechanism, and the recombination step is the rate-limiting step at low overpotentials [23–25]. The Tafel slope of commercial Pt-C was ~115 mV/dec in the overpotential range beyond +/– 50 mV, in which range the back reactions, such as hydrogen oxidation reaction (HOR), could be eliminated. The value of the Tafel slope for Pt-C was in agreement with the reported ones, thus supporting the validity of our electrochemical measurements (Figure 4b) [26, 27]. The Tafel slope of i-WC–G with 70% coverage was 38 mV/dec, which is lower than the related state-of-the-art 2D HER catalysts (including various tungsten carbides), as shown in Supplementary Table S1 and Table S2. We can suggest that the hydrogen evolution on our i-WC–G heterostructure electrode probably proceeds via a Volmer–Heyrovsky mechanism, where the electrochemical desorption is the rate-limiting step, which is similar to the reported tungsten carbide catalysts [28]. The detailed mechanism analysis of the i-WC–G heterostructure for electrocatalyzing HER was offered in the supplementary information. We attribute the HER activity of i-WC–G heterostructure to the interfacial synergy effect between high-quality 2D WC and graphene. Firstly, it’s the catalytic reactivity of WC comes from the filling of the d-states at the Fermi level of W by alloying with carbon [29]. Only high crystallinity WC can have a positive effect on HER activity (as shown in Supplementary Table S1), since defects, which act as the initial sites would inevitably form passive oxides in aqueous environments and therefore decrease the catalytic performance [30]. In addition, the two dimensionality of the materials helps provide large active surface areas with high crystallinity, in which the number of the active sites is comparable with the total atoms forming the materials. Thus, the HER performance gradually improves as the coverage of WC augments (at least initially), as shown in Figure 4a. Secondly, the graphene can effectively promote the HER catalytic performance of the 2D high crystalline WC. Although the intrinsic graphene doesn’t show catalytic activity for catalyzing the HER, it can effectively optimize the catalytic performance of the WC due to the interfacial synergy. On one hand, dual electrical-behavior regulation at the interface between WC and graphene can be achieved. As demonstrated by the experimental data, i-WC–G heterostructure exhibited more excellent catalytic performance than that of pure WC (as seen
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in Figure 4c). The i-WC–G with 70% coverage exhibited more excellent catalytic performance, of which the overpotential is as low as 120 mV. While for the pure WC with 70% coverage, the overpotential is 189 mV, in which such contrast experiment confirmed the function of graphene. It has been reported that TMCs possess the advantages of superior electric conductivity [31]. The intrinsically metallic nature of WC forms ohmic contacts at the interfaces with graphene which unlike Schottky contacts, and thus, does not require an additional overpotential to overcome the barrier. The extremely high semi-metallic character of the linked graphene at the interface facilitates electron transport at the electro-catalyst surface [14]. We can attribute this to the hybridization of metallic WC with the in-plane conductive graphene and thus achieve dual electrical-behavior regulation [4]. To profoundly elucidate the interface mechanism that facilitates electron transport underlying the superior HER activity of our i-WC–G heterostructure, we executed a series of electrochemical impedance spectroscopy (EIS) measurements at an overpotential of 150 mV, 200 mV and 250 mV respectively (Figure 4d). The impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The Nyquist plots show a small charge transfer resistance (12.6 Ω) of i-WC–G heterostructure at 150 mV. And when at 200 mV, the value will be as low as 5.83 Ω, which is much lower than that of pure WC (24.9 Ω) under the same measurement conditions. When at 250 mV, the value even can decrease to 2.26 Ω. The significantly reduced impedance will undoubtedly afford markedly faster charge transfer and further promote the HER kinetic process. In brief, we could powerfully confirm that the electrical regulation between WC and the linked graphene in an interconnected network, that is a synergy effect, can contribute to an enhanced catalytic performance for HER. The electron conjugation across the 2D basal surface is favorable, and exhibits fast inter-domain electron transport (illustrated in Figure 4f). While for the more traditional configuration of stacked and aggregated heterostructures formed by hydrothermal methods (Figure 4e), the charge transfer will have a higher potential barrier at the interfaces. Moreover, the stacking of each component will result in a decrease of the effective catalytic crystal face. On the other hand, graphene also helps facilitate the
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modification of the charge distribution and asymmetry spin of the WC. It is noted that i-WC–G heterostructure with too large WC coverage also exhibits relatively poor catalytic performance. When the coverage is larger than 80%, the overpotential needed to drive the current density of 10 mA/cm 2 increases instead, which is attributed to another role that graphene plays in the HER. The (0001) surface of WC had an interaction with H, as recorded by the hydrogen binding energies (HBE) of –0.99 eV [32], which would subsequently lead to a relatively poor HER performance because of the foreseeable difficulty of hydrogen release. However, when the C atoms of the graphene linked with WC crystals, the charge density distribution and asymmetry spin of the pure WC would be modified, thus adjusting the interaction with H+ and further leading to a favorable endothermic ΔGH* for the adsorption and desorption of hydrogen [17]. Therefore, for such a synergetic couple, the role of WC and graphene in such a 2D in-plane heterostructure is both important. Besides, for our i-WC–G heterostructure catalyst, the reproducible polarization plot after 5000 cycles always exhibit good durability of the electro-catalyst, as shown in the Supplementary Figure S9. Conclusion In summary, we demonstrate for the first time the one-step synthesis of 2D i-WC–G suitable for HER, which shows excellent catalytic performance for HER. The two dimensionalization of high quality WC crystals provides a high surface area ensuring efficient catalytic reactivity at each exposed crystal face. The interfacial synergy of WC with graphene facilitate the electron transfer promotion at the interface and across the whole catalytic surface. This efficient design is achieved by a LMSCS strategy, in which the co-segregation of WC and graphene is adjusted in a controlled manner through the unique solvent effect of the liquid metal bulk. The method could be extended to other 2D TMCs to further develop the TMC family in the field of catalysis. Acknowledgements The research was supported by Natural Science Foundation of China (Grants 51322209, 21473124,
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21673161) and Sino-German Center for Research Promotion (Grants GZ 871). We thank Prof. Wei Luo and Prof. Lin Zhuang for the electrochemical measurements. Mengqi Zeng and Yunxu Chen contributed equally to this work. Appendix A. Supplementary information Supplementary data
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Mengqi Zeng received her B.S. from Wuhan University in 2013, and continued her studies as a Ph.D candidate under the supervision of Prof. Lei Fu in the College of Chemistry and Molecular Sciences at Wuhan University. Her current research interests are the controllable growth, assembly and transfer of two-dimensional materials on the liquid metal catalyst. Yunxu Chen received her B.S. from Xinyang Normal University in 2015, and continued her studies as a Ph.D candidate under the supervision of Prof. Lei Fu in the College of Chemistry and Molecular Sciences at Wuhan University. Her current research interests are the catalyst design for the growth of transition metal carbides.
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Prof. Lei Fu received his B.S. degree in chemistry from Wuhan University in 2001. He obtained his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences in 2006. After obtaining his Ph.D., he worked as a Director's Postdoctoral Fellow at the Los Alamos National Laboratory, Los Alamos, NM (2006~2007). Thereafter, he became an Associate Professor of Peking University. In 2012, he joined Wuhan University as a Full Professor. His research interests cover two-dimensional materials and energy devices.
Figure 1. Characterizations of the WC single crystals embedded in graphene. (a) SEM image of the WC single crystals embedded in graphene. (b) AFM image of the WC single crystal and the thickness analysis. (c) Raman spectrum of WC. (d) STEM image of the WC single crystal. (e) HRTEM atomic image of the WC single crystal and the SAED patterns. (f–h) EDX elemental mapping of (f) C–K, (g) W–M and (f) W–L edges.
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Figure 2. TEM characterizations of the 2D i-WC–G. (a) TEM image of i-WC–G. HRTEM image of (b) the interface between WC and graphene and (c) edge of the graphene. (d–f) SAED patterns sampled at different regions. The SAED patterns of graphene were outlined by white dash circle to mark the difference compared with that of WC.
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Figure 3. The mechanism of the formation of 2D i-WC–G on a Ga–W substrate. (a) Schematic drawing and (b) cross-view of a designed liquid metal solvent, Ga–W substrate for CVD growth of 2D i-WC–G. (c) XPS C 1s peaks and (d) W 4f peaks from surface, subsurface and bulk along the surface normal direction on Ga–W. (e) Changes of the C 1s core-level signal and (f) W 4f core-level signal with respect to depth in the Ga–W.
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Figure 4. The catalytic performance of the 2D i-WC–G on a Ga–W substrate and the schematic diagrams of the catalytic process. (a) The polarization curves for 2D i-WC–G with different WC% coverages, the inset shows the relationship between the overpotential and the coverage. (b) Tafel plots corresponding to 2D i-WC–G with different WC% coverages and commercial Pt catalyst (20 wt % Pt on Vulcan carbon black). (c) The comparison of the HER catalytic performance for pure Ga substrate, pure WC on Ga, Pt and i-WC–G heterostructure on Ga. (d) EIS characterization of i-WC–G heterostructure and pure WC during the electro-catalytic process. Schematic illustration of the electron transport during the HER process via the stacked and aggregated heterostructures as the catalyst (e) and the 2D in-plane heterostructures as the catalyst (f).
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Highlights 1. A novel liquid metal solvent based co-segregation (LMSCS) strategy was employed to directly fabricate 2D WC single crystals embedded in graphene film, i.e. a unique in-plane heterostructure, over a large scale in one step via chemical vapor deposition (CVD). 2. This 2D in-plane WC–graphene heterostructure (i-WC–G) was firstly applied in HER. Basing on the high crystallinity of the WC and utilizing the interfacial synergistic catalytic effects, the 2D i-WC–G heterostructure exhibited excellent electrocatalytic activity for the HER 3. The overpotential was as low as 120 mV and the Tafel slope was 38 mV/decade, which indicated higher performance and efficiency than mainstream 2D HER catalysts. 4. The reproducible polarization plot even after 5000 voltammetry (CV) cycles highlighted the excellent durability of our developed 2D i-WC–G heterostructure. 5. Such a versatile synthesis approach will allow the fabrication of other high-quality 2D transition metal carbides (TMCs) and their embedding in in-plane structures and this will promote the practical catalytic application of metal carbides.
Graphical abstract A liquid metal solvent based co-segregation strategy was proposed to fabricate a high uniformity of 2D WC crystals embedded in the few-layer graphene by chemical vapor deposition (CVD) in one step. In addition, the efficient catalytic ability of the 2D WC for the hydrogen evolution, for the first time, was experimentally exhibited.
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PII: DOI: Reference:
S2211-2855(17)30064-2 http://dx.doi.org/10.1016/j.nanoen.2017.01.057 NANOEN1771
To appear in: Nano Energy Received date: 4 November 2016 Revised date: 26 January 2017 Accepted date: 29 January 2017 Cite this article as: Mengqi Zeng, Yunxu Chen, Jiaxu Li, Haifeng Xue, Rafael G. Mendes, Jinxin Liu, Tao Zhang, Mark H. Rümmeli and Lei Fu, 2D WC Single Crystal Embedded in Graphene for Enhancing Hydrogen Evolution Reaction, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.01.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
2D WC Single Crystal Embedded in Graphene for Enhancing Hydrogen Evolution Reaction Mengqi Zenga1, Yunxu Chena1, Jiaxu Lia, Haifeng Xuea, Rafael G. Mendesb2, Jinxin Liua, Tao Zhanga, Mark H. Rümmelib2, Lei Fua* College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China [email protected] ABSTRACT Electrochemical water splitting is regarded as one of the most economical and eco-friendly approaches for hydrogen revolution. Developing a low-cost and earth-abundant non-noble-metal catalyst will be of the most significance. Tungsten carbide (WC) is highly promising due to its platinum (Pt) -like behavior in surface catalysis. Here we first report a liquid metal solvent based co-segregation (LMSCS) strategy to fabricate a high uniformity of 2D WC crystals embedded in graphene by chemical vapor deposition (CVD) in one step. The 2D in-plane WC–graphene heterostructures (i-WC–G) are remarkably stable under an electro-catalytic environment and ensure good interfacial synergy between the 2D WC crystallites and graphene to achieve a more effective hydrogen evolution. The overpotential was as low as 120 mV and the Tafel slope was 38 mV/dec, which indeed exhibits outstanding catalytic potential among the reported 2D material systems. Our elegant and versatile approach allows the fabrication of other high-quality 2D transition metal carbides (TMCs) and their in-plane heterostructure, which will further promote practical catalytic applications of metal carbides. Graphicalabstract A liquid metal solvent based co-segregation strategy was proposed to fabricate a high uniformity of 2D WC crystals embedded in the few-layer graphene by chemical vapor deposition (CVD) in one step. In 1 2
These authors contributed equally to this work. IFW Dresden, P. O. Box 270116, 01069 Dresden, Germany 1
addition, the efficient catalytic ability of the 2D WC for the hydrogen evolution, for the first time, was experimentally exhibited. KEYWORDS WC, graphene, in-plane heterostructure, two-dimensionalization, HER
Introduction Hydrogen has been demonstrated to be a promising clean and economical energy source due to its advantages of high mass energy density and renewability. Direct electrochemical splitting of water provides an infinite potential method to generate hydrogen. The most effective hydrogen evolution reaction (HER) electrocatalysts are Pt-group metals. However, the natural scarcity and high costs of Pt limit the practical implementation. An advanced catalyst with low cost and earth abundance for the electrochemical HER is vigorously pursued. Transition metal carbides (TMCs) with excellent electric and modified chemical activity have aroused enormous attention as efficient electrochemical catalysts [1, 2]. And tungsten carbide (WC) is of particular interest due to its platinum (Pt) -like behavior in surface catalysis [3]. It is hoped that WC will develop into a highly competitive electro-catalyst for the development of the clean and renewable alternative energy sources. An ideal catalyst demands both the intrinsic catalytic potential and the charge transfer efficiency of the whole catalytic system [4]. The two dimensionalization and crystallization of the WC materials will provide the most effective reactive sites. Traditional synthesis approaches, such as the carbonization of W or W based precursors tend to yield bulk materials or nanoparticles with poor crystallinity, incomplete carbonization and impurities, all of which severely limit HER activity [5-7]. Exfoliated 2D TMC crystals possess inevitable defects or undesirable functional groups that can passivate the high-activity surface of the TMC crystals [8, 9]. Until now, the large-scale fabrication of pure and highly crystalline WC has not been demonstrated [10]. Moreover, currently available TMC powders or
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nanoparticles exhibit poor electrical connectivity due to their aggregation and exhibit incomplete anchoring with the supporting substrate [11, 12]. It was reported that the combination of crystalline WC with a conductive media could enhance its HER catalytic activity due to faster electron transfer and improved chemical/electrical coupling effects [12, 13]. Direct growth of 2D in-plane high-quality WC single crystals within a good conductor can be expected to provide synergy to simultaneously fully exhibit the intrinsic catalytic ability of the materials as well as efficient charge transfer within the entire catalytic system. Graphene with excellent conductivity [14], is easily produced by CVD, which makes it a good choice to combine with WC. However, a strictly in-plane heterostructure is quite hard to achieve on traditional solid substrates due to random nucleation and the chaotic layer stacking. Here, we first report a liquid metal solvent based co-segregation (LMSCS) strategy to effectively promote the simultaneous segregation of W and C atoms and fabricate an in-plane WC–graphene heterostructure (i-WC–G) in one step. The use of the as-produced i-WC–G in HER experiments confirm, for the first time, predictions that WC catalysts can be highly competitive as an electro-catalyst for energy production because of its excellent reactivity and superior chemical stability and, in addition, it uses environmentally friendly raw materials. The synergistic effects of WC-graphene heterostructures demonstrate its huge potential in HER catalysis. Experimental section CVD growth of 2D WC–graphene in-plane heterostructures (i-WC–G) on Ga–W substrates A commercial Ga pellet was divided into small droplets in hot ethanol. A droplet of Ga (10 mg) was then placed on a W foil (cut into 0.5×0.5 cm squares). Before loading Ga, the W foils were ultra-sonicated and rinsed with acetone, ethanol and deionized water prior to being dried under a nitrogen (N2) stream. The Ga pellets with a purity of 99.9999 wt.% and the W foils with a purity of 99.95 wt.% were purchased from Alfa Aesar China (Tianjin) Co. Ltd. and Shanghai Minor Metals Co. Ltd. The growth of i-WC–G was performed in a quartz tube furnace (HTF 55322C Lindberg/Blue M) under ambient pressure. The growth process consisted of three steps: (1) placing the Ga–W substrates in 3
the center region of the quartz tube. (2) heating the Ga–W substrates to 980 ∼ 1020˚C at a rate of 30˚C /min under the flow of Ar and H2; (3) exposing of the substrates to a carbon source at 1000˚C for 30 min under 300 sccm Ar, 30 sccm H2 and different flows of methane (CH4), ranging from 1 sccm to 5 sccm; (4) turn off the CH4 and cooling the substrates to room temperature rapidly under Ar and H2. Transferring the i-WC–G to the target substrates The process of transferring the samples to the target substrates involves spin-coating a poly(methyl methacrylate) (PMMA) film onto the substrate with i-WC–G grown on it and releasing the PMMA/sample film by etching out Ga in a diluted hydrogen chloride (1:1) for 1 h. This was followed by a rinsing in ultra-pure water to remove the metal ions. The PMMA layer was dissolved with hot acetone after the PMMA/sample film was transferred onto the target substrates, such as 300 nm SiO2/Si substrates and the quantifoils. Electrochemical Measurements All of the electrochemical measurements were carried out with a three-electrode system (sparged with pure N2, purity 99.999%) on an electrochemical workstation (CHI760E) at room temperature, using a 0.5×0.5 cm2 Ga–W foil with 2D i-WC–G directly grown on it as the working electrode, a Pt foil as a counter electrode, and a saturated calomel (SCE) as the reference electrode. In order to exclude the influence of the dissolved Pt that may redeposit onto the working electrode on the electrocatalytic process, XPS spectra were checked before and after the electrochemical characterization, as shown in Supplementary Figure S1. The reference electrode was calibrated by a reversible hydrogen electrode (RHE) in the electrolyte solution saturated with H2. All potentials were referenced to a reversible hydrogen electrode (RHE), E (RHE) = (0.059 ×pH + 0.276) V. Linear sweep voltammetry (LSV) was recorded in 0.1 M KOH (pH = 13) and 0.5 M H2SO4 (pH = 0.3), respectively. The scan rate of the linear sweep voltammetry was 5 mV/s to obtain the polarization curves. The loading of the electro-catalyst (WC) is 0.000936 mg/cm2 ~ 0.00222 mg/cm2. Commercial 20% Pt/C catalyst was used as a reference sample. The corresponding polarization curves with internal resistance (iR) compensation were adopted 4
in the measurements. Onset overpotentials were determined based on the beginning of the linear regime in the Tafel plot. The electrochemical stability of the catalyst was conducted by cycling the potential from +0.3 to −0.3 V versus RHE at a scan rate of 100 mV/s. The Nyquist plots were obtained with frequencies ranging from100 kHz to 0.01 Hz with an amplitude of 10 mV at the open-circuit voltage. The impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The presence and identity of the generated H2 were confirmed by on-line gas chromatography (GC) analysis (Shimadzu, Tracera). The mixed gas went through the sampling loop continuously during longtime chronoamperometry by bubbling 5 sccm Ar to the gastight H-type electrolytic cell. In such a cell, a Ga–W foil covered with 2D i-WC-G heterostructure acted as the working electrode and a carbon paper was employed as the counter electrode, which were separated by a Nafion film. In addition, a reversible hydrogen electrode served as the reference electrode. The faradic efficiency (FE) of the electrocatalytic hydrogen evolution process was calculated by comparing the peak area of gas chromatography with the counterpart when using a certified standard gas. Characterization Optical images were taken with an optical microscope (Olympus DX51), and Raman spectroscopy was performed with a laser micro-Raman spectrometer (Renishaw in Via, 532 nm excitation wavelength). Scanning electron microscopy (SEM) images were obtained by Hitachi-S4800 and ZEISS Merlin Compact SEM. The XPS measurements were conducted with Thermo Scientific, ESCALAB 250Xi. The measuring spot size was 500 μm and the binding energies were calibrated by referencing the C 1s peak (284.8 eV). The XPS depth profiling was performed by Ar ionic bombardment to gradually remove the surface layers. Each etching takes about 200 seconds. The AFM images were taken with a NT-MDT Ntegra Spectra with samples transferred onto the 300 nm SiO2/Si. The transmission electron microscopy (TEM) images were obtained by an aberration-corrected high-resolution TEM system (FEI Titan 80-300) using an operating voltage of 80 kV and by a probe corrected high-resolution TEM system (Titan Probe corrected TEM, Titan G2 60-300) with the operating voltage of 300 kV. The 5
samples were transferred onto a quantifoil copper TEM grid for characterization. Results and Discussions The low melting-point metal, gallium (Ga), was chosen to serve as the liquid solvent to achieve the co-segregation of W and C atoms. Figure 1a shows the scanning electron microscope (SEM) image of ultrathin WC single crystals (the thickness is 1.4 nm, Figure 1b embedded in graphene on the Ga–W with high density, which in essence provides active sites comparable with the total atoms forming the materials in a HER reaction. Figure 1c shows the Raman spectrum of WC on SiO2/Si. The bands situated at around 700 and 800 cm–1 are attributed to W–C stretching modes, and are marked with “W1” and “W2”. The narrow peak widths of W1 and W2 indicate the samples are highly crystalline [15]. X-ray photoelectron spectroscopy (XPS) is utilized to further identify the components, as shown in Supplementary Figure S2. The characteristic carbide carbon peak for WC is observed at 282.8 eV [16]. A C 1s peak at 284.8 eV is also observed, which originates from the graphene surrounding the WC flakes and possibly carbon adsorbates from the atmosphere. The W 4f signals for the W foil sample are located at 31.45 and 33.7 eV, in agreement with literature [17]. For all samples, the W 4f peak locations are observed to be shifted by +0.2 to 0.8 eV higher in binding energy as compared to metallic W. The structure and morphology of the as-obtained WC grains are further characterized by transmission electron microscopy (TEM). Figure 1d shows a full view of the thin hexagonal WC crystal via scanning TEM (STEM). Energy dispersive X-ray (EDX) spectroscopy mapping measurements show peaks for the C–K, W–M, W–L edges and indicate that the crystal is comprised of W and C with a uniform distribution (Figure 1f, 1g and 1h). The darker color in Figure 1f corresponding to the C content is due to the underlying quantifoil support on the Cu grid. High resolution TEM (HRTEM) data highlight a highly crystalline structure with six-fold symmetry. In order to show the single crystallinity, selected-area electron diffraction (SAED) is conducted at different regions on an individual hexagonal WC single crystal, as shown in Supplementary Figure S3. All the patterns show the same six-fold symmetric diffraction points for different collection regions concomitant with a single WC crystal. The 6
interface structure of the i-WC–G is examined by Raman line scanning (Supplementary Figure S4) and TEM (Figure 2). The obtained spectra of graphene and WC are easily distinguished, confirming the 2D i-WC–G. Figure 2a presents a low-magnification TEM image of the 2D i-WC–G. Figure 2b shows a magnified TEM image of the WC–G interface, which exhibits intimate bonding. The inset in Figure 2b shows the fast Fourier transform (FFT) pattern obtained from the region marked by the green dash square and exhibits the six-fold symmetry of the WC crystal. Figure 2c shows a typical edge of the crystalline graphene, which verifies the existence of 2D graphene. Figure 2d–f show the SAED patterns sampled from the regions corresponding to the interface, individual WC and graphene, respectively. Both of the patterns in Figure 2e and 2f exhibit the six-fold symmetry single-crystal nature while the extracted interplanar spacings are different. In short, we confirm the presence of 2D i-WC–G. To demonstrate how the i-WC–G formed via our LMSCS process, Figure 3a–b shows a schematic diagram illustrating the growth mechanism. The process occurs as follows: when Ga is in a molten state at elevated temperature, it contains two components, ions and conductive electrons. The high-energy ions lead to intense thermal motion that makes it possible to escape from its original atomic cluster and join others. As a result, the space between the atomic clusters fluctuates, which allows them to serve as heteroatom containers [18]. At the interface of the W and Ga layer, W atoms pass through the interface and diffuse into the bulk phase of the liquid Ga due to the atomic thermal motion. Besides, the C atoms are catalytically decomposed by Ga, which then also diffuse into the liquid bulk. Thus, an appreciable quantity of decomposed C atoms was “dissolved” into the liquid bulk of the Ga. XPS depth profile is employed to demonstrate the “dissolution” of the C and W atoms in the liquid Ga bulk (Supplementary Figure S5). The co-segregation of the WC and graphene will have a competitive relationship, similar to the growth of Mo2C and graphene in previously reported work [19]. The growth temperature will determine the amount of heteroatoms in the liquid phase via affecting the decomposition of CH 4, and the embedding and diffusion of the heteroatoms (C and W) in the liquid bulk (Ga). Lower temperatures lead to i-WC–G with relatively less WC% coverage while the higher temperatures lead to i-WC–G with
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larger WC% coverage (Supplementary Figure S6a–c). Slower cooling rates result in thicker WC flakes with higher coverage (Supplementary Figure S6d), which is similar to reported work on graphene deposition over high carbon-solubility metals [20]. In order to achieve the best performance for HER using our obtained i-WC–G, we employ a growth temperature of 1000˚C and a rapid cooling strategy to achieve the i-WC–G with relatively high WC% coverage. From the valence analysis of each element along a surface normal to the direction (cross-section) on Ga, the co-segregation process can be demonstrated. At the topmost surface (Figure 3c), the C 1s band contains a strong graphitic peak centered at 284.8 eV, originating from the graphene and possible carbon adsorbates from the atmosphere. There is also a relatively strong peak centered at 289.0 eV, originating from a C–O bonding structure. Noticeably, the characteristic carbide peak for WC is observed at 282.8 eV, indicating the presence of WC. After removal of the surface layer, the impurity adsorbates and graphene layer were almost entirely eliminated. Thus, the relative intensity of the carbide peak for WC became stronger. The XPS W4f band also confirms the existence of WC. As one goes deeper into the bulk, the carbide C1s peak shows a continuously decreasing relative intensity while the pure carbon C1s peak exhibits a stable intensity (Figure 3c, 3e). In addition, for W 4f band, the carbide peak also shows a continuously decreasing intensity with depth, and more importantly, the metallic W 4f peaks are present and show a marked increase with depth (Figure 3d, 3f). Therefore, one can infer that the W and C atoms indeed diffuse into the bulk of the liquid metal and i-WC–G forms via the co-segregation of the dissolved atoms. A series of i-WC–G with different WC% coverage (40% ~ 95%) are investigated for their potential to catalyze the HER in the acid conditions (0.5 M H2SO4). The polarization curves for these i-WC–G are shown in Figure 4a. Strikingly, the sample with 70% coverage required only ∼120 mV to achieve a 10 mA/cm2 current density. Compared with other tungsten carbide with various morphologies (Supplementary Table S1), our as-prepared i-WC–G heterostructure exhibits the lowest overpotential for catalyzing HER. Notably, the loading amount of the WC in our i-WC–G heterostructure with 70%
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WC coverage is as low as 0.00164 mg/cm2, which is far lower than that of the other tungsten carbide catalysts that exhibit the similar catalytic ability (0.16~2.2 mg/cm2) [21, 22]. It means that our ultrathin 2D WC with perfect crystallinity with graphene connecting exhibits the highest efficiency for HER. Other mainstream 2D TMDs for catalyzing HER were also summarized in Supplementary Table S2. Remaining the same, our i-WC–G heterostructure still exhibits the top catalytic ability. Therefore, we can convincingly point out that the catalytic property of the 2D i-WC-G heterostructure is superior to literature 2D materials. In addition, the current-time measurement during the electrocatalytic process under the constant overpotentials was combined with the on-line gas chromatography (GC) analysis to extract the faradaic efficiency (FE) (Supplementary Figure S7). The FE of the electrocatalytic hydrogen evolution process was calculated by comparing the peak area of gas chromatography with the counterpart when using a certified standard gas. We obtained the current-time curve during the electrocatalytic hydrogen evolution process under the overpotentials of 0.1 V and 0.2 V, respectively. The as-extracted corresponding FE is 97.2% and 96.3%, respectively, thus demonstrates that the current is from hydrogen evolution. In addition, the as-prepared 70% i-WC–G is also employed as the catalyst for the alkaline HER in 0.1 M KOH solution. The polarization curve after iR-correction and the corresponding Tafel plot of 2D i-WC–G with the WC coverage of 70% were shown in Figure S8. The overpotential needed to drive the current density of 10 mA/cm2 in the electrocatalytic process is ∼225 mV. The linear portion of the Tafel plot is fitted to the Tafel equation, yielding Tafel slope of ~108 mV/dec. This well demonstrated the catalytic potential of the as-proposed 2D i-WC–G in the alkaline HER field. To elucidate the HER mechanism of our i-WC–G heterostructure for catalyzing, Tafel plots were fitted to Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope (Figure 4b). The Tafel slope is an inherent property of the catalyst that is determined by the rate-limiting step of the HER. The determination and interpretation of the Tafel slope are important for elucidation of the elementary steps involved. Having a very high Hads coverage, the HER on a Pt surface is known to
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proceed through the Volmer–Tafel mechanism, and the recombination step is the rate-limiting step at low overpotentials [23–25]. The Tafel slope of commercial Pt-C was ~115 mV/dec in the overpotential range beyond +/– 50 mV, in which range the back reactions, such as hydrogen oxidation reaction (HOR), could be eliminated. The value of the Tafel slope for Pt-C was in agreement with the reported ones, thus supporting the validity of our electrochemical measurements (Figure 4b) [26, 27]. The Tafel slope of i-WC–G with 70% coverage was 38 mV/dec, which is lower than the related state-of-the-art 2D HER catalysts (including various tungsten carbides), as shown in Supplementary Table S1 and Table S2. We can suggest that the hydrogen evolution on our i-WC–G heterostructure electrode probably proceeds via a Volmer–Heyrovsky mechanism, where the electrochemical desorption is the rate-limiting step, which is similar to the reported tungsten carbide catalysts [28]. The detailed mechanism analysis of the i-WC–G heterostructure for electrocatalyzing HER was offered in the supplementary information. We attribute the HER activity of i-WC–G heterostructure to the interfacial synergy effect between high-quality 2D WC and graphene. Firstly, it’s the catalytic reactivity of WC comes from the filling of the d-states at the Fermi level of W by alloying with carbon [29]. Only high crystallinity WC can have a positive effect on HER activity (as shown in Supplementary Table S1), since defects, which act as the initial sites would inevitably form passive oxides in aqueous environments and therefore decrease the catalytic performance [30]. In addition, the two dimensionality of the materials helps provide large active surface areas with high crystallinity, in which the number of the active sites is comparable with the total atoms forming the materials. Thus, the HER performance gradually improves as the coverage of WC augments (at least initially), as shown in Figure 4a. Secondly, the graphene can effectively promote the HER catalytic performance of the 2D high crystalline WC. Although the intrinsic graphene doesn’t show catalytic activity for catalyzing the HER, it can effectively optimize the catalytic performance of the WC due to the interfacial synergy. On one hand, dual electrical-behavior regulation at the interface between WC and graphene can be achieved. As demonstrated by the experimental data, i-WC–G heterostructure exhibited more excellent catalytic performance than that of pure WC (as seen
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in Figure 4c). The i-WC–G with 70% coverage exhibited more excellent catalytic performance, of which the overpotential is as low as 120 mV. While for the pure WC with 70% coverage, the overpotential is 189 mV, in which such contrast experiment confirmed the function of graphene. It has been reported that TMCs possess the advantages of superior electric conductivity [31]. The intrinsically metallic nature of WC forms ohmic contacts at the interfaces with graphene which unlike Schottky contacts, and thus, does not require an additional overpotential to overcome the barrier. The extremely high semi-metallic character of the linked graphene at the interface facilitates electron transport at the electro-catalyst surface [14]. We can attribute this to the hybridization of metallic WC with the in-plane conductive graphene and thus achieve dual electrical-behavior regulation [4]. To profoundly elucidate the interface mechanism that facilitates electron transport underlying the superior HER activity of our i-WC–G heterostructure, we executed a series of electrochemical impedance spectroscopy (EIS) measurements at an overpotential of 150 mV, 200 mV and 250 mV respectively (Figure 4d). The impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. The Nyquist plots show a small charge transfer resistance (12.6 Ω) of i-WC–G heterostructure at 150 mV. And when at 200 mV, the value will be as low as 5.83 Ω, which is much lower than that of pure WC (24.9 Ω) under the same measurement conditions. When at 250 mV, the value even can decrease to 2.26 Ω. The significantly reduced impedance will undoubtedly afford markedly faster charge transfer and further promote the HER kinetic process. In brief, we could powerfully confirm that the electrical regulation between WC and the linked graphene in an interconnected network, that is a synergy effect, can contribute to an enhanced catalytic performance for HER. The electron conjugation across the 2D basal surface is favorable, and exhibits fast inter-domain electron transport (illustrated in Figure 4f). While for the more traditional configuration of stacked and aggregated heterostructures formed by hydrothermal methods (Figure 4e), the charge transfer will have a higher potential barrier at the interfaces. Moreover, the stacking of each component will result in a decrease of the effective catalytic crystal face. On the other hand, graphene also helps facilitate the
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modification of the charge distribution and asymmetry spin of the WC. It is noted that i-WC–G heterostructure with too large WC coverage also exhibits relatively poor catalytic performance. When the coverage is larger than 80%, the overpotential needed to drive the current density of 10 mA/cm 2 increases instead, which is attributed to another role that graphene plays in the HER. The (0001) surface of WC had an interaction with H, as recorded by the hydrogen binding energies (HBE) of –0.99 eV [32], which would subsequently lead to a relatively poor HER performance because of the foreseeable difficulty of hydrogen release. However, when the C atoms of the graphene linked with WC crystals, the charge density distribution and asymmetry spin of the pure WC would be modified, thus adjusting the interaction with H+ and further leading to a favorable endothermic ΔGH* for the adsorption and desorption of hydrogen [17]. Therefore, for such a synergetic couple, the role of WC and graphene in such a 2D in-plane heterostructure is both important. Besides, for our i-WC–G heterostructure catalyst, the reproducible polarization plot after 5000 cycles always exhibit good durability of the electro-catalyst, as shown in the Supplementary Figure S9. Conclusion In summary, we demonstrate for the first time the one-step synthesis of 2D i-WC–G suitable for HER, which shows excellent catalytic performance for HER. The two dimensionalization of high quality WC crystals provides a high surface area ensuring efficient catalytic reactivity at each exposed crystal face. The interfacial synergy of WC with graphene facilitate the electron transfer promotion at the interface and across the whole catalytic surface. This efficient design is achieved by a LMSCS strategy, in which the co-segregation of WC and graphene is adjusted in a controlled manner through the unique solvent effect of the liquid metal bulk. The method could be extended to other 2D TMCs to further develop the TMC family in the field of catalysis. Acknowledgements The research was supported by Natural Science Foundation of China (Grants 51322209, 21473124,
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21673161) and Sino-German Center for Research Promotion (Grants GZ 871). We thank Prof. Wei Luo and Prof. Lin Zhuang for the electrochemical measurements. Mengqi Zeng and Yunxu Chen contributed equally to this work. Appendix A. Supplementary information Supplementary data
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Mengqi Zeng received her B.S. from Wuhan University in 2013, and continued her studies as a Ph.D candidate under the supervision of Prof. Lei Fu in the College of Chemistry and Molecular Sciences at Wuhan University. Her current research interests are the controllable growth, assembly and transfer of two-dimensional materials on the liquid metal catalyst. Yunxu Chen received her B.S. from Xinyang Normal University in 2015, and continued her studies as a Ph.D candidate under the supervision of Prof. Lei Fu in the College of Chemistry and Molecular Sciences at Wuhan University. Her current research interests are the catalyst design for the growth of transition metal carbides.
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Prof. Lei Fu received his B.S. degree in chemistry from Wuhan University in 2001. He obtained his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences in 2006. After obtaining his Ph.D., he worked as a Director's Postdoctoral Fellow at the Los Alamos National Laboratory, Los Alamos, NM (2006~2007). Thereafter, he became an Associate Professor of Peking University. In 2012, he joined Wuhan University as a Full Professor. His research interests cover two-dimensional materials and energy devices.
Figure 1. Characterizations of the WC single crystals embedded in graphene. (a) SEM image of the WC single crystals embedded in graphene. (b) AFM image of the WC single crystal and the thickness analysis. (c) Raman spectrum of WC. (d) STEM image of the WC single crystal. (e) HRTEM atomic image of the WC single crystal and the SAED patterns. (f–h) EDX elemental mapping of (f) C–K, (g) W–M and (f) W–L edges.
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Figure 2. TEM characterizations of the 2D i-WC–G. (a) TEM image of i-WC–G. HRTEM image of (b) the interface between WC and graphene and (c) edge of the graphene. (d–f) SAED patterns sampled at different regions. The SAED patterns of graphene were outlined by white dash circle to mark the difference compared with that of WC.
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Figure 3. The mechanism of the formation of 2D i-WC–G on a Ga–W substrate. (a) Schematic drawing and (b) cross-view of a designed liquid metal solvent, Ga–W substrate for CVD growth of 2D i-WC–G. (c) XPS C 1s peaks and (d) W 4f peaks from surface, subsurface and bulk along the surface normal direction on Ga–W. (e) Changes of the C 1s core-level signal and (f) W 4f core-level signal with respect to depth in the Ga–W.
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Figure 4. The catalytic performance of the 2D i-WC–G on a Ga–W substrate and the schematic diagrams of the catalytic process. (a) The polarization curves for 2D i-WC–G with different WC% coverages, the inset shows the relationship between the overpotential and the coverage. (b) Tafel plots corresponding to 2D i-WC–G with different WC% coverages and commercial Pt catalyst (20 wt % Pt on Vulcan carbon black). (c) The comparison of the HER catalytic performance for pure Ga substrate, pure WC on Ga, Pt and i-WC–G heterostructure on Ga. (d) EIS characterization of i-WC–G heterostructure and pure WC during the electro-catalytic process. Schematic illustration of the electron transport during the HER process via the stacked and aggregated heterostructures as the catalyst (e) and the 2D in-plane heterostructures as the catalyst (f).
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Highlights 1. A novel liquid metal solvent based co-segregation (LMSCS) strategy was employed to directly fabricate 2D WC single crystals embedded in graphene film, i.e. a unique in-plane heterostructure, over a large scale in one step via chemical vapor deposition (CVD). 2. This 2D in-plane WC–graphene heterostructure (i-WC–G) was firstly applied in HER. Basing on the high crystallinity of the WC and utilizing the interfacial synergistic catalytic effects, the 2D i-WC–G heterostructure exhibited excellent electrocatalytic activity for the HER 3. The overpotential was as low as 120 mV and the Tafel slope was 38 mV/decade, which indicated higher performance and efficiency than mainstream 2D HER catalysts. 4. The reproducible polarization plot even after 5000 voltammetry (CV) cycles highlighted the excellent durability of our developed 2D i-WC–G heterostructure. 5. Such a versatile synthesis approach will allow the fabrication of other high-quality 2D transition metal carbides (TMCs) and their embedding in in-plane structures and this will promote the practical catalytic application of metal carbides.
Graphical abstract A liquid metal solvent based co-segregation strategy was proposed to fabricate a high uniformity of 2D WC crystals embedded in the few-layer graphene by chemical vapor deposition (CVD) in one step. In addition, the efficient catalytic ability of the 2D WC for the hydrogen evolution, for the first time, was experimentally exhibited.
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