Cage-Confinement Pyrolysis Route to Ultrasmall Tungsten Carbide

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Cage-Confinement Pyrolysis Route to Ultrasmall Tungsten Carbide Nanoparticles for Efficient Electrocatalytic Hydrogen Evolution Yan-Tong Xu,† Xiaofen Xiao,§ Zi-Ming Ye,† Shenlong Zhao,⊥ Rongan Shen,‡ Chun-Ting He,*,† Jie-Peng Zhang,*,† Yadong Li,*,‡ and Xiao-Ming Chen† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, China § Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Key Laboratory of High Performance Polymer-Based Composites of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China ⊥ CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China S Supporting Information *

nately, uncontrollable particle sintering/agglomeration is usually hard to avoid, especially for metal carbides,3,4 because ultrahigh thermolysis temperature (>1000 K) is necessary to overcome thermodynamic and kinetic barriers when carbon atoms incorporate into the metal lattice. In fact, traditional synthetic methods are very difficult to achieve metal carbide nanoparticles (NPs) with single-component/phase and uniform sizes smaller than 5 nm (Table S1). Despite nonsintered WC NPs have recently been successfully synthesized via a 6-step strategy,3,9b,15 developing facile and large-scale synthesis methods of stable ultrasmall WC NPs is still a great challenge. Herein, we propose a cage-confinement pyrolysis (CCP) strategy to produce ultrasmall WC NPs (Scheme 1).

ABSTRACT: The size-controlled synthesis of ultrasmall metal-based catalysts is of vital importance for chemical conversion technologies. Here, a cage-confinement pyrolysis strategy is presented for the synthesis of ultrasmall tungsten carbide nanoclusters/nanoparticles. An RHO type zeolitic metal azolate framework MAF-6, possessing large nanocages and small apertures, is selected to confine the metal source W(CO)6. High temperature pyrolysis gives tungsten carbide nanoclusters/nanoparticles with sizes ca. 2 nm, which can serve as an excellent electrocatalyst for the hydrogen evolution reaction. In 0.5 M H2SO4, it exhibits very low overpotential of 51 mV at 10 mA cm−2 and Tafel slope of 49 mV per decade, as well as the highest exchange current density of 2.4 mA cm−2 among all tungsten/molybdenum-based catalysts. Moreover, it also shows excellent stability and antiaggregation behavior after long-term electrolytic process.

Scheme 1. Comparison of the Cage-Confinement and NonConfinement Pyrolysis Methods for Synthesizing Nanocatalysts

O

ver the past decade, molybdenum- and tungsten-based materials have been considered to be promising alternative of the costly and low reserves noble metals (in particular platinum) for application in energy storage and conversion, especially in the field of electrocatalytic hydrogen evolution reaction (HER), which is the most economical and sustainable method for hydrogen industry.1 Actually, due to the unique electronic structures, Mo2C,2 WC,3 W2C,4 MoP,5 WN,6 MoS 2 , 7 WSe 2 ,8 and so on, 9 have demonstrated their considerable potential as substitutes to Pt. Thereinto, it has been confirmed that the electronic density of states of WC closely resembles that of platinum than that of W.10,11 Moreover, WC exhibits outstanding thermal/chemical stability,3,12 making a valuable candidate in catalysis industry. However, the lack of efficient methods to produce sizecontrolled WC with high activity has prevented its widespread application.4,13 To some extent, greatly reducing the particle sizes, especially preparation of nanoclusters or even single atoms, would be an effective way to obtain ultrahigh catalytic activities.14 Unfortu© 2017 American Chemical Society

Metal−organic frameworks (MOFs), possessing highly ordered porous structures, diversified metal/organic compositions and adjustable crystal morphologies, have been regarded as ideal reactive precursors for synthesizing well-defined nanocatalysts.16 Under different thermal conditions, MOFs Received: January 10, 2017 Published: April 5, 2017 5285

DOI: 10.1021/jacs.7b00165 J. Am. Chem. Soc. 2017, 139, 5285−5288

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Journal of the American Chemical Society

located at about 26°, which can be indexed by the (002) plane of graphite (JCPDS, no. 75-1621). Except the carbon matrix, the PXRD of WC@NPC exhibited several weak and broad peaks around 31.5°, 35.6°, and 48.3°, corresponding to the (001), (100), and (101) planes of WC (JCPDS, no. 51-0939). X-ray photoelectron spectroscopy (XPS) was used to characterize the compositions and chemical state of each element. For WC@NPC, two strong signals corresponding to W 4f7/2 and W 4f5/2 of WC are found at 31.4 and 33.6 eV (Figure 1b),3 and three small peaks at 34.7, 36.9, and 38.1 eV can be assigned to W 4f orbits of surface WOx passivation layer. Nevertheless, an in situ quick alkaline rinsing treatment can be applied to remove these surface oxides without impacting the bulk carbide structure.3,25 The presence of nitrogen atoms with different chemical states in both WC@NPC and NPC can be found by deconvoluting N 1s signals into three subpeaks at 397.8, 400.3, and 401.5 eV (Figure S8).20a,26 The morphologies and nanostructures of the pyrolyzed samples were further examined by scanning electron microscopy (SEM) and TEM. WC@NPC and NPC appear as polyhedral porous particles remaining the crystal morphology of MAF-6 (Figures 2a, S9−S11). Ultrasmall WC NPs

can be easily, selectively and homogeneously converted into their corresponding inorganic/carbon materials.17 Unfortunately, the obtained metal NPs still scarcely have sizes smaller than 5 nm.9a,18 As a subclass of MOFs, zeolitic metal-azolate frameworks (MAFs) usually show very large cage-like pores interconnected by very small apertures, which may be suitable for isolating metal sources as guests and effectively avoid their diffusion/agglomeration during thermal pyrolyzing treatments.19 Moreover, the rich N atoms in MAFs can be beneficial for producing N-doped carbon to stabilize small metal clusters.16a,17b RHO-[Zn(eim)2] (MAF-6, Heim = 2ethylimidazolate, Figure S1a), a metal−organic zeolite with hydrophobic nanocavities (d = 1.84 nm) and flexible small apertures (average diameter = 0.76 nm), as well as high thermal/chemical stability,20 was selected as a CCP host for the metal source W(CO)6 (molecular diameter, ca. 0.92 nm).21 Considering the high volatility of metal carbonyl complexes,22 low-temperature vapor adsorption can be applied to prepare the CCP precursor. The desired CCP precursor W(CO)6@MAF-6 was obtained by vapor adsorption treatment (MAF-6 and W(CO)6 mixture sealed and heated at 85 °C for 24 h). The powder X-ray diffraction (PXRD) pattern of W(CO)6@MAF-6 shows characteristic diffraction peaks of MAF-6 instead of those of W(CO)6 (Figure 1a). Fourier transform infrared (FT-IR)

Figure 1. (a) PXRD patterns of all the corresponding materials; (b) high-resolution W 4f XPS spectra of WC@NPC.

spectroscopy of W(CO)6@MAF-6 showed characteristic absorbance peaks of both MAF-6 and W(CO)6 (Figure S2). Moreover, transmission electron microscopy (TEM) showed no obvious impurity (Figure S3), illustrating that W(CO)6 was encapsulated in the cages rather than surface deposition or mechanical mixing. Thermogravimetry curve of W(CO)6@ MAF-6 (Figure S4) showed 20 wt % loss before the decomposition of the MAF-6 host, being less than the amount of adsorbed W(CO)6 (23%), which indicated the decomposition of W(CO)6 and confinement of tungsten inside MAF6. W(CO)6@MAF-6 was then pyrolyzed to yield tungsten carbide protected by or embedded in N-doped nanoporous carbon (denoted as WC@NPC, Figures 1a, S5, and S6). For comparison, guest-free MAF-6 was also pyrolyzed to form Ndoped nanoporous carbon (denoted as NPC, Figure 1a). Note that the Zn(II) ions are reduced to Zn(0) and vaporized under high temperature.17b,23 N2 adsorption isotherms of WC@NPC and NPC revealed similar pore size distributions with the coexistence of both micropore and mesopore (Figure S7). The Brunauer−Emmett−Teller (BET) surface area of WC@NPC is 800 m2/g, which is greatly larger than most of inorganic− carbon composite materials derived from non-MOF-based precursors.24 As shown in Figure 1a, there is a broad peak

Figure 2. (a) TEM, (b) HRTEM, (c) elemental mapping, and (d) particle-size distribution of WC@NPC. (Inset: TEM images under lower magnification of the corresponding samples.)

homogeneously embedded in carbon substrate can be found under higher magnification (Figure 2a). The measured particlesize distribution has an average diameter of ca. 2 nm (Figure 2d), being much smaller than most reported metal carbides (Table S1). Judging from the high-resolution TEM (HRTEM) image (Figure 2b), the interplanar distance of the tiny particle is 2.5 Å, being consistent with the (100) planes of WC, as well as surface covering with graphitic carbon with 3.4 Å interplane spacing of the (002) crystal lattice. The homogeneous distribution and contents of C, N and W in WC@NPC were identified by energy-dispersive X-ray spectroscopy (EDS, Table S2) and the corresponding elemental mapping (Figure 2c). 5286

DOI: 10.1021/jacs.7b00165 J. Am. Chem. Soc. 2017, 139, 5285−5288

Communication

Journal of the American Chemical Society By directly adding W(CO)6 into the reaction solution during MAF-6 synthesis, we obtained another tungsten-containing MOF precursor (denoted as W(CO)6 /MAF-6), which appeared almost the same as W(CO)6@MAF-6 in the aspects of appearances and PXRD patterns (Figure 1a). However, thermogravimetry of W(CO)6/MAF-6 implied lower W(CO)6 loading for the obviously less weight loss (Figure S4). This sample was also pyrolyzed with the same condition as for W(CO)6@MAF-6 to give a sample denoted as W@NPC. Despite the similar graphite degrees according to the Raman spectra (Figure S12), W@NPC shows huge differences in the components, morphologies, and metal particle sizes compared to WC@NPC because W(CO)6/MAF-6 decomposed and sintered into relatively large metal W NPs (mainly 5−55 nm, Figure S13), as judged by PXRD (Figure 1a), SEM (Figure S14), (HR)TEM (Figures S15 and S16), and XPS spectra (Figure S17). Specifically, three sharp peaks, implying the larger particle diameter, at 38.4°, 44.6°, and 64.9° can be indexed as the (111), (200), and (220) planes of cubic tungsten phase (JCPDS, no. 88-2339). Moreover, the carbon substrate of W@ NPC was partly collapsed, which probably resulted from W agglomeration. Such huge disparities between WC@NPC and W@NPC could be attributed to the different distributions of W(CO)6 and confinement effect in their precursors. On one hand, W(CO)6 molecules were well confined and isolated within the cages in W(CO)6@MAF-6 (Figure S1b). With the rising of temperature, W(CO)6 molecules were decomposed into several W atoms within cages, stabilized on their high reactivity state, in favor of bonding with C/N atoms from the original MAF-6 framework due to confinement of the cage. In contrast, W(CO)6 entered barely into the cage of W(CO)6/MAF-6 via in situ mixing (cyclohexane filling the cage prior to W(CO)6 for the recognition and template effect20a), so that it appeared at the mesodefects of MAF-6 crystals despite its lower loading. Consequently, W atoms from the decomposition of W(CO)6 tend to aggregate to form more stable and larger W NPs quickly rather than react with carbon framework. We then explored the electrocatalytic HER performances of the as-mentioned samples in 0.5 M H 2 SO 4 . As the corresponding HER polarization curves shown in Figures 3a, S18, and S19, NPC displayed negligible catalytic activity, whereas W@NPC shows weak HER activity similar to levels observed in other metal tungsten based catalysts.27 Remarkably, WC@NPC shows exceptionally high HER catalytic activity (Figures S20 and S21) in the aspects of exchange current density (j0, 2.4 mA cm−2), overpotential (η), and Tafel slope (49 mV per decade, Figure 3b) among all tungsten-based catalysts (Table S3). It starts hydrogen evolution near the thermodynamic potential (0 mV), which is comparable to the superior performance of commercial Pt/C catalyst. The η at current density of 10 mA·cm−2 is as low as 51 mV. Besides, j0 is twice the value of commercial Pt/C catalyst, and much better than all the other tungsten/molybdenum-based catalysts (Table S3). To investigate the electrochemical stability of WC@NPC, an i−t curve was obtained with a constant potential of 50 mV (vs RHE) for 5 h (Figure 3c).3,15 Despite a minor current drop at the beginning, the current gradually stabilizes afterward, suggesting excellent stability and durability of WC@NPC, which can also be confirmed by the well retained particle size after the HER test (Figures S22 and S23). Unlike other tiny NPs, the unexpected stability of the ultrasmall WC nano-

Figure 3. (a) HER polarization curves at 5 mV s−1 in 0.5 M H2SO4; (b) Tafel curve calculated by η = a lg |j| + b; (c) i−t curve of WC@ NPC (inset: HER LSV curves before and after stability test); (d) free energy of H* adsorption on different surfaces by DFT calculation. RHE = reversible hydrogen electrode.

clusters/NPs in WC@NPC should be mainly due to the graphite protective overlayers. To understand better the difference of WC@NPC and W@ NPC as catalysts for HER, their free energies (ΔGH*) for H* (intermediate state from H+ to H2 during HER) adsorption were studied by density functional theory (DFT) calculations. Observing from HRTEM, WC NPs were mostly coated with 1−2 graphitic shells which probably promote H* adsorption synergistically.28 In this regard, we investigated 6 model structures for H* adsorption (Figure S24): W3C3 cluster, W6 cluster, Graphene, N-Graphene (N-doped graphene), W3C3@ Graphene, and W3C3@N-Graphene. Besides, we considered that H* might be adsorbed on the graphene side or the metal side for those combination structures. Although the W3C3/W6 clusters used for simulations are much smaller than the real particles used for experiments, it would not undermine the reliability of calculations for the investigation by this simple model.29 For comparison, the free energy of H* adsorbed on Pt has also been investigated, which can serve as a reference with all the other ones.30 As shown in Figure 3d, the ΔGH* values for W, WC, Graphene, and N-Graphene were calculated as −1.10, −0.93, +1.71, and 0.68 eV, respectively, meaning that WC and NGraphene are more appropriate HER catalysts than their counterparts as demonstrated in other investigations.10,31 On the other hand, these values still deviate a lot from that (−0.24 eV) of the ideal material Pt. Interestingly, when W3C3 clusters bond with graphene or N-doped graphene, the values of ΔGH* on both sides drastically decreased, indicating synergistic electronic effect between the two low-activity components.29,31 Particularly, for the W3C3−graphene composite, the ΔGH* on the graphene side and metal side can be cut down to much more moderate values of −0.27 and −0.41 eV, respectively, being very close to that (−0.24 eV) of the ideal material Pt and these smaller free energies agree well with the supreme experimental HER performance of WC@NPC. In summary, by using a zeolitic MAF as sacrificing host/ template, uniform carbon-protected WC nanoclusters/NPs in average sizes of 2 nm have been successfully prepared. Significantly, this catalyst exhibits platinum-like HER activity 5287

DOI: 10.1021/jacs.7b00165 J. Am. Chem. Soc. 2017, 139, 5285−5288

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in 0.5 M H2SO4 and much better exchange current density. The excellent catalytic performance is attributed to the ultrasmall size of WC NPs and synergy of graphene shells. In principle, the simple and low-cost CCP strategy may be extended to synthesize other NPs/nanoclusters for substituting noble metal catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00165. Detailed experimental description, supporting figures and tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Jie-Peng Zhang: 0000-0002-2614-2774 Yadong Li: 0000-0003-1544-1127 Xiao-Ming Chen: 0000-0002-3353-7918 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “973 Project” (2014CB845602), NSFC (21225105, 21290173, 21473260, 21521091, 21131004, 21390393, and U1463202), and National Postdoctoral Program for Innovative Talents (BX201600195).

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DOI: 10.1021/jacs.7b00165 J. Am. Chem. Soc. 2017, 139, 5285−5288