Controllable Synthesis of Ordered Mesoporous Mo2C@Graphitic

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Energy, Environmental, and Catalysis Applications

Controllable Synthesis of Ordered Mesoporous Mo2C@Graphitic Carbon Core-Shell Nanowire Arrays for Efficient Electrocatalytic Hydrogen Evolution Jiahui Zhu, Yan Yao, Zhi Chen, Aijian Zhang, Mengyuan Zhou, Jun Guo, Winston Duo Wu, Xiao Dong Chen, Yanguang Li, and Zhangxiong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04528 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

Controllable Synthesis of Ordered Mesoporous Mo2C@Graphitic Carbon Core-Shell Nanowire Arrays for Efficient Electrocatalytic Hydrogen Evolution Jiahui Zhu a, Yan Yao a, Zhi Chen a, Aijian Zhang a, Mengyuan Zhou a, Jun Guo b, Winston Duo Wu a, Xiao Dong Chen a, Yanguang Li c, and Zhangxiong Wu*a a

Suzhou Key Laboratory of Green Chemical Engineering, School of Chemical and

Environmental Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123, Jiangsu, China. b

Testing and Analysis Centre, Soochow University, Suzhou, Jiangsu 215123, China.

c

Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou,

Jiangsu 215123, China E-mail address *: [email protected]

ABSTRACT: Mo2C is a possible substitute to Pt-group metals for electrocatalytic hydrogen evolution reaction (HER). Both support-free and carbon-supported Mo2C nanomaterials with improved HER performance have been developed. Herein, distinct from prior research, novel

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ordered mesoporous core-shell nanowires with Mo2C cores and ultrathin graphitic carbon (GC) shells are rationally synthesized and demonstrated to be excellent for HER. The synthesis is fulfilled via a hard-templating approach combining in-situ carburization and localized carbon deposition. Phosphomolybdic acid confined in the SBA-15 template is first converted to MoO2, which is then in-situ carburized to Mo2C nanowires with abundant surface defects. Simultaneously, GC layer (the thickness is down to ~ 1.0 nm in most areas) is controlled to be locally deposited on Mo2C surface due to its strong affinity with carbon and catalytic effect on graphitization. Removal of the template results in the Mo2C@GC core-shell nanowire arrays with the structural properties well characterized. They exhibit excellent performance for HER with a low overpotential of 125 mV at 10 mA cm-2, a small Tafel slope of 66 mV dec-1 and an excellent stability in acidic electrolytes. The influences of several factors, especially the spatial configuration and relative contents of the GC and Mo2C components, on HER performance are elucidated with control experiments. The excellent HER performance of the mesoporous Mo2C@GC core-shell nanowire arrays originates from the rough Mo2C nanowires with diverse active sites and short charge-transfer paths, and the ultrathin GC shells with improved surface area, electronic conductivity and stabilizing effect on Mo2C.

KEYWORDS: Mesoporous material, molybdenum carbide, core-shell nanowire, nano-confining synthesis, hydrogen evolution reaction

The growing dependence on fossil fuels has accelerated their consumption and aggravated global warming and environmental pollution,1 leading to the enhanced demand on use of green and renewable energy sources. Hydrogen (H2) is considered as an ideal energy carrier as it has


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the highest gravimetric energy density without CO2 emission during combustion. Many resources such as water, fossil fuels and biomass can be used for H2 generation. Among them, electrochemical water splitting as a sustainable way of H2 production has attracted much attention, and the development of efficient electrocatalysts to reduce the overpotential for the H2 evolution reaction (HER) is crucial.2-7 Pt-based nanomaterials are regarded as the most effective electrocatalysts for HER.8-10 However, their high cost and scarcity hinder their widespread applications. Hence, extensive research has been dedicated to developing efficient noble-metalfree

electrocatalysts,

including

non-metal

electrocatalysts,

such

as

carbon-based

electrocatalysts,11 and transition metal electrocatalysts, such as metal dichalcogenides,12-16 phosphide,17 and carbides.1,

3, 18-22

Metal carbides, especially molybdenum carbide (Mo2C),

possessing the similar d-band electron structure as that of Pt, have been considered to be promising alternatives to Pt.21, 22, 23-26 The conventional way to synthesize Mo2C materials is the direct carburization of Mo metal, hydrides or oxides at fairly high temperature (> 1200 °C), leading to low surface areas, crystal impurities and low catalytic activities.27 To improve their catalytic performance, much research has been focused on the preparation of nano-sized counterparts to increase the density and accessibility of active sites.22, 25, 28-38 Two general approaches have been applied. The first one is to synthesize support-free nanoparticles and nanostructures.24, 28, 39 For example, Giordano et al. reported an effective “urea glass” route for synthesizing Mo2C nanoparticles with small size and high purity.24 Liao et al. reported the pyrolysis of a MoOx/amine nanowire precursor for the synthesis of nanoporous nanowires composed of Mo2C nanocrystallites.39 The second method is the use of carbons, such as porous carbons, carbon nanotubes and graphene sheets, to disperse Mo2C nanoparticles.26, 35, 40-47This can be achieved by using either one-step assembly or post-


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loading methods.22, 36, 37, 41, 48 For example, Liu et al. reported that ultrasmall Mo2C nanoparticles embedded within N-rich carbon layers can be obtained via one-step pyrolysis of a mixture of ammonium molybdate and dicyandiamide.41 Moreover, MOF and MOF-derived material have been developed and demonstrated as promising materials for many electrochemical applications,49 Wu et al. demonstrated a MOF-assisted strategy, i.e. introducing guest metal species into MOFs as co-precursors followed by pyrolysis, for preparing mesoporous octahedrals consisting of MoCx nanocrystallites.36 The development of these metal carbide nanocatalysts have greatly improved their catalytic performance in HER. However, there are still challenging aspects for further development. Metal carbide nanoparticles can easily inter-grown and be aggregated, and are prone to be oxidized when exposed to air. The use of carbon supports improves their dispersion and stability, but often results in severe embedment that shields and/or reduces active sites. It is not clear how the spatial configuration of carbon and Mo2C components and their relative contents influence HER performance. To address these issues, distinct from previous research, we hypothesize that ordered mesoporous core-shell nanowire arrays composed of Mo2C nanowire cores and ultrathin graphitic carbon shells (denoted at Mo2C@GC) are a kind of ideal electrocatalysts for HER. Mesoporous materials can be constructed by using either soft or hard templating methods,5052

but the synthesis of mesoporous metal carbides is fairly challenging because of the large

volume shrinkages (> 77%) from metal precursors to carbides leading to easy structural collapse.53, 54 With mesoporous silicas as the hard templates, a few groups, including ourselves, have reported the synthesis of ordered mesoporous metal carbides.55-58 However, the construction of mesoporous Mo2C@GC core-shell nanowires has not been achieved. Herein, we demonstrate a well-designed hard templating process combining nano-confining carburization


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and in-situ localized carbon deposition for successful construction of novel mesoporous Mo2C@GC composites with the mesoporous silica SBA-15 as the template, phosphomolybdic acid (PMA) as the metal precursor and methane (CH4) as the agent for carburization and graphitic carbon deposition. A serious of mesoporous Mo2C@GC composites with different spatial configurations and relative contents can be controllably synthesized. The optimized mesoporous Mo2C@GC composites with rough Mo2C nanowire cores and ultrathin graphitic carbon shells exhibit outstanding HER performance because the former provides diverse active sites while the latter provides short and continuous electron transfer paths and stabilizes the former without shielding the active sites.

RESULTS AND DISCUSSION Concept and synthesis process of the ordered mesoporous Mo2C@GC core-shell nanowires Synthesis of the ordered mesoporous Mo2C@GC composites composed of rough Mo2C core nanowires intimately coated with thin graphitic carbon shells is realized by using the hard templating process combining in-situ carburization and localized graphitic carbon deposition. The mesopores of a silica template, typically SBA-15, are adopted as the nano-reactors (Scheme 1a). The mesopores can be fully filled with the PMA precursor via wet impregnation and solvent evaporation (Scheme 1b). Subsequently, the PMA precursor is thermally decomposed and reduced to MoO2 under a CH4/N2 atmosphere at a moderate temperature (Scheme 1c). The high conversion yield (~ 84.1 wt%) from PMA to MoO2 leads to the formation of continuous nanowires other than discrete nanoparticles, while the volume contraction (60 %) from PMA to MoO2 renders the length of the MoO2 nanowires much shorter than that of the mesopores


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(Scheme 1c). Afterwards, the MoO2 nanowires can be in-situ reduced and carburized to form Mo2C nanocrystallites at a higher temperature. Due to the mass loss (~ 20.3 wt%) and volume contraction (44.1 %) from MoO2 to Mo2C, the resultant Mo2C nanocrystallites are in the form of shorter and rough nanowires with abundant defects (Scheme 1d). Carbon can be preferentially deposited on Mo2C surfaces and molybdenum has a catalytic effect for graphitization. As a result, the evacuated space due to the volume contraction can be locally re-filled by epitaxial deposition of ultrathin graphitic carbon layers on the Mo2C surfaces, leading to the formation of unique intimate coreshell structures (Scheme 1d). Finally, removal of the SBA-15 template results in novel ordered Mo2C@GC core-shell nanowire arrays (Scheme 1e). Such a structure could be ideal for improving HER performance. The ordered mesoporous Mo2C@GC nanowire arrays possess uniformly distributed and abundant exposed active sites. The closely interconnected Mo2C nanowires can shorten the charge-transfer pathway during the HER process. The thin graphitic carbon shells can not only offer a high surface area and an improved electronic conductivity for rapid diffusion and transfer of electrons and ions, but also can stabilize the Mo2C cores to avoid deactivation and structure collapse during cyclic tests. Physicochemical properties of the ordered mesoporous Mo2C@GC core-shell nanowires Temperature is the determining factor to convert the PMA precursor to carbide in the CH4/N2 atmosphere. With a PMA/SBA-15 mass ratio of 3.3 (the PMA precursor fully occupies the pore volume of SBA-15), nearly pure MoO2 crystal phase (PDF#32-0671) is formed at 700 °C (Figure 1A, curve a), indicating that carburization and carbon deposition can hardly proceed at this temperature. At 750 °C, MoO2 is still the predominant crystal phase (Figure 1A, curve b), with the formation of a small fraction of carbide and deposited carbon. At 800 °C, β-Mo2C crystal phase (PDF#35-0787), which has been considered as the most active phase of


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molybdenum carbides for HER, and a very weak peak assigned to graphitic carbon can be observed in the representative sample [email protected] (Figure 1A, curve c), indicating a complete conversion of the PMA precursor to carbide and deposition of carbon. The crystal size of Mo2C is calculated to be ~ 8.6 nm which is very close to the mesopore diameter (~ 9.7 nm) of the SBA-15 template, indicating the graphitic carbon layer is very thin and the growth of the Mo2C is well confined in the mesopores. At 850 and 900 °C, the β-Mo2C crystal phase is still present (Figure 1A, curves d and e) with the diffraction peak at ~ 26 ° becoming stronger (inset in Figure 1A), indicating that more carbon is deposited at a higher temperature (Figure 1B and Table S1). TG analysis of the samples reveals weight increases starting at ~ 300 °C followed by weight loss starting from ~ 500 °C (Figure 1B), attributed to the oxidation of the Mo2C (or MoO2 for the samples obtained at low temperatures) and deposited graphitic carbon, respectively. For the representative sample [email protected] (Figure 1B, curve c, and Table S1), the contents of the Mo2C and carbon are calculated to be ~ 80 and 20 wt%, respectively. The volume contraction (~ 44.1 %) from MoO2 to Mo2C leads to release of pore space (Scheme 1d). Theoretically, complete re-fill of deposited carbon into this space gives a graphitic carbon content of ~ 22 wt%, very close to the measured value (20 wt%), verifying that the deposited carbon is predominantly deposited at the Mo2C surfaces to form core-shell structures (Scheme 1d, e). The corresponding Raman spectrum (Figure 2A, a) shows a D band at 1318 cm-1 and a G band at 1586 cm-1 with an intensity ratio of 0.81, indicating a high graphitization degree. Furthermore, the Mo 3d XPS spectrum (Figure 2B) can be fitted into three pairs of peaks. The predominant pair of peaks at 228 and 231.2 eV is assigned to Mo2C, while the two minor pairs are ascribed to MoO2 (229.3, 232.8 eV) and MoO3 (232.3, 235 eV), respectively. The presence of molybdenum oxides is


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caused by partially oxidized Mo2C surfaces. The C 1s spectrum (Figure 2C) shows a predominant peak at 284.8 eV assigned to graphitic carbon and two minor peaks at 286.2 and 288.9 eV assigned to C-O and C=O moieties, respectively. Only a weak carbide carbon peak at 282.8 eV can be observed in the C1s XPS spectrum, which is probably because of the very low content of carbide carbon, the shielding of the surface carbon layer and possible surface oxidation of Mo2C. Besides, XPS measurement reveals that the signal intensity of phosphorous is very weak (nearly noisy) in the sample, indicating the formation of phosphide can be neglected under the synthesis conditions. The representative sample [email protected] shows a wheat-like morphology and an ordered hexagonal mesostructure (Figure 3a-c), indicating a successful mesostructure replication. The overall geometrical particle sizes (~ 3.0 µm) of the product are much smaller than those (~ 6.8 µm) of the template (Figure S1), which is due to the dramatic volume contraction (~ 77.4 %) from the PMA precursor to Mo2C. High-resolution SEM (HRSEM) (Figure 3c) and TEM images (Figure 3d) show the presence of regularly arranged nanowires of ~ 9.1 nm in diameter periodically gapped by nano-channels of ~ 4.2 nm in size. The nanowires are not smooth, but rough and interconnected one another (Figure 3c), which may provide diverse and accessible active sites. Importantly, the nanowires are in a core/shell configuration (Figure 3d-f), with the inner cores composed of well crystalized Mo2C, as revealed by the clear lattice fringes with interlayer distances of 0.23 and 0.237 nm assigned to the (101) and (002) planes of β-Mo2C, and the outer shells composed of very thin (down to ~ 1.0 nm in thickness in most areas) graphitic carbon layers as revealed by the lattice fringes with a spacing of 0.337 nm assigned to the (002) plane. The selected-area electron diffraction (SAED) pattern (Figure 3g) displays individual bright spots connected with concentric rings, indicating that overall the


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sample is polycrystalline, while each Mo2C nanocrystal is probably single crystalline, which can be observed in some areas of the sample (Figure 3f). Furthermore, the dark-field scanning TEM (STEM) image (Figure 3h) and the corresponding elemental maps (Figure 3i, 3j) and energy dispersive X-ray (EDX) spectrum (Figure 3k) confirm the uniform distribution of C and Mo elements in the sample. The representative sample [email protected] shows type IV N2 sorption isotherms (Figure 4A, curve a), typical of a mesoporous material. The specific surface area and pore volume are 37 m2 g-1 and 0.11 cm3 g-1, respectively, much lower than those (532 m2 g-1 and 1.10 cm3 g-1) of the SBA-15 template because of the lower mesostructural ordering and much higher density of Mo2C than that of silica. The pore size distribution curve is relatively wide with broad peaks centered at ~ 4.2 and 16-32 nm (Figure 4B, curve a), ascribed to the mesopores generated from the silica pore walls and the pore voids within and among the Mo2C nanowire arrays, respectively. No obvious microporosity exists in the sample owing to the high crystallinity and compact structure between the Mo2C and carbon components. HER performance of the ordered mesoporous Mo2C@GC core-shell nanowires In a three-electrode cell, with a graphite rod and a Ag/AgCl electrode as the counter and reference electrodes and a 0.50 M H2SO4 as the electrolyte, the electrochemical double-layer capacitance (Cdl) value for the presentative sample [email protected] is estimated to be 7.25 mF cm-2 (Figure S2), indicating a high active electrochemical surface area. The representative catalyst [email protected] displays an excellent HER activity (Figure 5A, curve b) with an onset potential of ~ 58 mV at a current density of 1.0 mA cm-2, and an overpotential of ~ 125 mV at a current density of 10 mA cm-2. Such a superior activity is better than or comparable to those


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of state-of-the-art nanostructured molybdenum carbides reported in literature (Table S2),25, 33, 3539, 41, 43, 48, 59-65

and close to the performance of the commercial 20 wt% Pt/C catalyst with an

onset potential of ~ 6.0 mV and an overpotential of ~ 38 mV at a current density of 1.0 and 10 mA cm-2, respectively (Figure 5A, curve a, and Table 1). Impressively, the current density of the [email protected] catalyst is comparable to that of the Pt/C catalyst at the overpotential of ~ 180 mV. Besides, Their Tafel slopes (Figure 5B, curves a and b) are 42 and 66 mV dec-1, respectively, indicative of very close HER kinetic rates. Furthermore, after 3000 cycles, a negative shift of only ~ 15 mV can be observed for the [email protected] catalyst (Figure 5D), indicative of an excellent cycling stability. SEM and TEM images (insets a and b in Figure S3A) show that after the cycling test the core-shell nanowire array structure remains with only slight structure corrosion observed. The Raman spectrum after the cyclic test (Figure S3B) also reveals only a slightly increased ID/IG value, indicating minor carbon defects can be induced due to the oxidation of the surface carbon layers during the CV cyclic test. In addition, the timedependent current density (I-t) measurement (Figure S3A) acquired at a fixed overpotential of 125 mV (vs. RHE) reveals only slight decays of current density after 10000 s, indicating a good stability of the [email protected] catalyst. Influencing factors on material property and HER performance The superior HER performance of the ordered mesoporous Mo2C@GC core-shell nanowires (i.e. the representative sample [email protected]) can be attributed the combination of the special structure configuration (Mo2C nanowire core@ultrathin graphitic carbon shell) with abundant active sites and the ordered mesostructure with a high surface area in the sample. In order to validate the assumption, a series of control experiments were conducted and the results were compared and discussed as described below.


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The templating effect As compared with the representative sample [email protected], the control sample Mo2C@GC-NP-800 prepared without the use of the SBA-15 template under the same conditions shows a pure β-Mo2C crystal phase (Figure S4A). However, it shows a far inferior HER performance (Figure 5A, curve c), with significantly high overpotentials of 215 and 309 mV at a current density of 1.0 (onset) and 10 mA cm-2, respectively. The Tafel slope is up to ~ 108 mV dec-1 (Figure 5B, curve c). The poor performance is because this control sample is composed of larger (17.5 nm calculated from the wide-angle XRD pattern shown in Figure S4A) and severely aggregated Mo2C nanocrystals (Figure S5). It has a very low surface area and a small pore volume of only about 2.6 m2 g-1 and 0.02 cm3 g-1, respectively. Besides, the control sample has a much lower carbon content (2.0 wt%) (Figure S4B) than that (20 wt%) of the representative sample [email protected], indicating that the epitaxial growth of graphitic carbon occurs preferentially on Mo2C surfaces and thus a high Mo2C surface area results in more carbon disposition. The electrochemical impedance spectra (EIS) (Figure 5C) at an overpotential of 85 mV (vs. RHE) of the representative catalyst [email protected] was fitted into an equivalent circuit (Figure S6), including a bulk solution resistance (Rs) of ~ 15 Ω, a constant phase element (CPE), and a charge transfer resistance (Rct) of ~ 76 Ω which is determined from the semicircle registered at low frenquencies (high Z´) Compared to the control Mo2C@GC-NP-800 catalyst, the [email protected] catalyst displays a significantly smaller charge-transfer impedance. This indicates that the special core-shell configuration of the former catalyst with nanocrystalline Mo2C nanowires as the core and graphitic carbon as the shell can greatly facilitate electron transfer.36, 38 Therefore, the templating synthesis is essentially important to obtained desirable material properties for better HER performance.


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The temperature effect With a fixed PMA/SBA-15 mass ratio of 3.3, for the resultant [email protected] samples obtained at 700 ~ 900 °C (with a gap of 50 °C), the HER activities of the samples obtained at 700 and 750 °C are very poor (Figure 6A, curves a and b). This is due to the intrinsic low HER activity of MoO2 which is the only or predominant crystal phase in corresponding samples (Figure 1A, curves a and b). On the other hand, with the temperature increased from 800 to 900 °C, the HER activity of the resultant samples also decreases significantly (Figure 6A, curves c-e), i.e., the overpotential at 10 mA cm-2 increases dramatically from 125 to 235 mV. Wideangle XRD patterns (Figure 1A, curves c-e) and TG curves (Figure 1B, curves c-e and Table S1) show that the carbon content increases considerably with the ramp of temperature, indicating that the Mo2C nanocrystals could be imbedded within a carbon matrix with increased thickness and density. HRSEM images (Figure S7) reveals that the geometric sizes of the resultant samples obtained at 850 and 900 °C become larger and the surfaces becomes increasingly dense with much less-resolved surface nanopores, indicating that the higher temperatures result in extra carbon deposition in the pore space without Mo2C. The deposited carbon can deeply wrap the Mo2C nanocrystallites. Besides, the growth of Mo2C nanocrystals at higher temperatures can lead to partial destroy of the mesostructure. As a result, the density and accessibility of Mo2C active sites, and the surface areas and porosities are all largely deteriorated for the samples obtained at higher temperstures, leading to the poorer HER performance. Therefore, the appropriate temperature (i.e. 800 °C) is a key factor to obtain the special ordered mesoporous nanowire arrays with Mo2C cores and ultrathin graphitic carbon shells to significantly enhance HER performance. The Mo2C/carbon structural configuration effect


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To figure out how the relative contents and the different spatial configurations of the Mo2C and carbon components, i.e. coll-shell nanowires with higher carbon contents (Scheme S1) and carbon nanowire supported Mo2C nanoparticles (Scheme S2) influence the HER performance, a series of Mo2C@GC samples were obtained by using a series of PMA/SBA-15 mass ratios from 3.3 down to 0.05 at the optimized temperature of 800 °C. It is found that β-Mo2C crystal phase is formed with a wide range of PMA/SBA-15 mass ratios from 3.3 to 0.1 (Figure 7A, curves a e), while α-MoC crystal phase (PDF #08-0384) is formed only with a very low PMA/SBA-15 mass ratio of 0.05 (Figure 7A, curve f). On the other hand, the diffraction peak at ~ 26 ° becomes gradually more intensive with the decrease of PMA/SBA-15 mass ratio, indicating gradually increased amount of carbon is deposited. TG analyses reveal that the deposited carbon amount increases from 20 to 93 wt% with the decrease of the PMA/SBA-15 mass ratio from 3.3 to 0.05 (Figure 7B and Table 1). The temperature ranges for oxidation of Mo2C (or MoC) and carbon are partially overlapped so that the weight increase due to oxidation of Mo2C (or MoC) is not obvious in the cases of relatively low contents of Mo2C (or MoC) (Figure 7B, curves c-f). Besides, the oxidation of carbon occurs at a relatively lower temperature for the sample with a higher content of Mo2C, presumably due to molybdenum oxide catalyzing combustion of carbon. Interestingly, the intensity ratios of the D and G bands in the Raman spectra of the samples increases with the decrease of Mo2C content (Figure 2A), indicating that molybdenum has a catalytic effect for graphitization. All the mesoporous Mo2C@GC-800-y samples obtained from different PMA/SBA-15 mass ratios can duplicate the morphology of the SBA-15 template (Figure S8). With the decrease of the mass ratio, the domain sizes of the resultant products become larger and their surfaces gradually become denser and more smooth, suggesting that gradually increased contents of


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carbon are deposited in the mesopores of the template and on the external surfaces of the template. Notably, the [email protected] and [email protected] samples are composed of ordered mesoporous Mo2C@GC core-shell nanowire arrays (Figure S8a, b). The difference is that the carbon shells are thicker for the latter sample (Figure S9a, b), in accordance with the TG results that the latter has a higher carbon content (Figure 7B). Distinctly, the sample Mo2C@GC-800-1 shows the presence of an ordered mesoporous carbon matrix supporting Mo2C nanoparticles (Figure S9c, Scheme S2). The other samples obtained with further deceased PMA/SBA-15 mass ratios show the presence of Mo2C nanoparticles supported on carbon matrix with poor mesostructure orderings (Figure S9d, e), indicating without sufficient support from metal carbides, the deposited carbon itself cannot fully replicate and support the ordered mesostructure after the removal of the template. With the PMA/SBA-15 mass ratio gradually decreased from 3.3 to 1.0, the surface area and pore volume of the resultant samples increase sharply and the pore size distribution become more narrow (Figure 5A, B, curves a-c, and Table 1) due to the well-replicated mesostructure composed of decreasing amount of Mo2C. With a further decrease of PMA/SBA-15 mass ratio from 1.0 to 0.5, the surface area and the pore volume both decrease and the pore size distribution becomes broader (Figure 4A, B, curves c, d). This is because the product has a low mesostructural ordering as a result of the disappearance of Mo2C nanowires to support the mesostructure. Further decreases of the PMA/SBA-15 mass ratio from 0.5 down to 0.05 lead to products with increased surface areas and pore volumes (Figure 4A, B, curves e, f, and Table 1) due to the gradually decreased Mo2C amount in the products. The Mo2C@GC-800-y samples show significantly different HER performances. With the continuous decrease of the PMA/SBA-15 mass ratio, the resultant Mo2C@GC samples shows


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gradually deteriorated HER activities (Figure 6B and Table 1). The [email protected] catalyst shows the lowest onset overpotential and the smallest overpotential at a current density of 10 mA cm-2 (Figure 6B, curve a, Table 1), as well as the lowest Tafel slope (Figure 6C, curve a). Moreover, it shows the smallest charge-transfer resistance (Figure 6D). The [email protected] and [email protected] catalysts both present core-shell structures, but the latter sample shows a relatively inferior HER performance (27 and 26 mV more negative for the overpotentials at 1.0 and 10.0 mA cm-2) despite of its higher surface area and larger pore volumes (Figure 4A and Table 1). This result indicates that thicker carbon shells can reduce the accessibility of the active Mo2C component and increase the resistance of electron transfer (Figure 6D, curves a and b). With the PMA/SBA-15 mass ratio decreased from 2.5 to 1.0, the resultant sample shows an increased charge resistance (Figure 6D, curve c) and thus the HER activity further decreses (14 and 19 mV from the overpotentials at 1.0 and 10.0 mA cm-2) (Figure 6B, curve c). With the PMA/SBA-15 mass decreased from 1.0 to 0.25, there are significantly shape decreases in HER activities for the resultant Mo2C@GC samples (Figure 6B, curves c-e), in accordance with the dramatic increases in charge resistance (Figure 6D, curves c-e). This indicates that the ordered mesoporous structure in the former catalyst (Figure S9c) is advantageous over the disordered ones in the latter samples (Figure S9d, e) for supporting the Mo2C component with better HER performance. There is another shape decrease in HER activity for the samples obtained with a decreasing PMA/SBA-15 mass ratio from 0.25 to 0.05 (Figure 6B, curves e, f), indicating that the Mo2C or MoC nanoparticles severely imbedded in carbon matrix are hardly accessibly for HER. On the other hand, all the mesoporous Mo2C@GC samples show better HER performance than that of the control sample Mo2C@GC- NP-800 catalysts in spite of its significantly higher


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content of Mo2C, indicating the mesostructure is essential for increasing active site density, shortening electron transfer path and facilitating mass transportation.

CONCLUSIONS In summary, by using a hard templating approach that combines in-situ carburization and localized graphitic carbon deposition, a series of Mo2C/GC composites with different relative Mo2C/GC contents and two distinct structure spatial configurations, i.e. core-shell nanowires and GC-supported Mo2C nanoparticles, have been obtained by adjusting the temperature and PMA/SBA-15 mass ratio. Under optimized synthetic conditions, i.e. a temperature of 800 °C and a PMA/SBA-15 mass ratio of 3.3, rough interconnected Mo2C nanowires can be formed in the template, and ultrathin GC of ~ 1.0 nm thick can be locally deposited on the Mo2C surfaces due to their strong affinity with carbon and catalytic effect on graphitization, leading to the formation of the ordered mesoporous Mo2C@GC composites with Mo2C nanowires as the cores and ultrathin GC as the shells, and a high surface area (37 m2 g-1) and uniform mesopores (~ 4.2 nm). The mesoporous Mo2C@GC core-shell nanowire arrays exhibit remarkable HER performance with low overpotentials of 58 and 125 mV at a current density of 1.0 (onset) and 10.0 mA cm-2, respectively, a low Tafel slope of 66 mV dec-1, and an excellent cyclic stability under acidic conditions. The performance is close to that of a commercial 20 wt% Pt/C electrocatalyst, and better than those of most reported Mo2C-based nanomaterials. Control experiments have elucidated that the Mo2C@GC core-shell nanowire arrays have several key advantages for HER, namely, the interconnected rough Mo2C core nanowires providing diverse active sites and short charge-transfer pathways, the ultrathin GC shells improving electronic conductivity with low


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charge transfer resistance and stabilizing the Mo2C core nanowires, and the ordered mesoporous structure offering a space for fast mass transportation and easy accessibility to the active sites.

EXPERIMENTAL SECTION Chemicals The triblock copolymer Pluronic P123 (Mw = 5800, EO20PO70EO20) was purchased from SigmaAldrich. Phosphomolybdic acid (denoted as PMA, H3Mo12O40P·xH2O), tetraethoxysilane (TEOS), hydrochloric acid (HCl, 37 wt%) and hydrofluoric acid (HF, 40 wt%) were purchased from Sinopharm Chemical Reagent Co. The methane (purity > 99.999 %) and nitrogen (purity > 99.999 %) gases were purchased from Linde Electronic Specialty Gases (Suzhou) Co., Ltd. The Nafion solution (5.0 wt%) was purchased from DuPont. The commercial Pt/C (20 wt% Pt on carbon black) catalyst was purchased from Johnson Matthey. All the chemicals were used as received without further purification. Synthesis process of the catalysts The mesoporous silica template SBA-15 was synthesized according to previous literature.66 The ordered mesoporous Mo2C@graphtic carbon (denoted as Mo2C@GC) core-shell nanowire arrays were synthesized via a nano-confined chemical vapor deposition (CVD) process that combines carburization and carbon deposition. For a typical synthesis, ~ 0.50 g of the mesoporous silica SBA-15 template was dispersed in 12.0 mL of ethanol. Then, an ethanol solution (0.10 ~ 6.60 mL) containing 0.025 ~ 1.65 g of PMA was added. The mixture was magnetically stirred (150 rpm) at room temperature until the ethanol solvent was fully evaporated. Then the obtained


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yellow powders (denoted as PMA@SBA-15) were dried under vacuum at 50 °C for ~ 3.0 h. Subsequently, ~ 0.25 g of the PMA@SBA-15 composite was transferred into a tube furnace for thermal treatment. The thermal treatment process involved three stages. First, the tube furnace was heated from room temperature to 600 °C with a ramp rate of 2 °C min-1 under flowing N2 (50 ~ 55 mL min-1). Then, a CH4 flow (30 mL min-1) was introduced into the furnace, and the temperature was increased to the targeted temperature (700 ~ 900 °C, with a gap of 50 °C) and held isothermal at the targeted temperature for 2.0 h. Afterwards, the furnace was allowed to cool down naturally. When the temperature of the furnace was cooled down to 600 °C, the CH4 flow was turned off. Black powders (denoted as Mo2C@GC@SBA-15) were obtained after the thermal treatment. The Mo2C@GC@SBA-15 composites were immersed in a 10.0 wt% HF solution for 12.0 h to remove the silica template to obtain the final ordered mesoporous Mo2C@GC core-shell nanowire arrays, which were denoted as Mo2C@GC-x-y, where x stands for the heating temperature in °C and y for the mass ratio of PMA and SBA-15, respectively. With a PMA/SBA-15 mass ratio of 3.3, the pore volume of SBA-15 can theoretically be fully filled with PMA. Besides, in order to study the effect of mesostructure on HER activity, without the use of SBA-15 template, a control sample (denoted as Mo2C@GC-NP-800) was also synthesized by directly treating the PMA precursor by using the same experimental procedure as described above. Measurement and characterizations Powder wide-angle X-ray diffraction (XRD) patterns were recorded by using a Bruker D2 Phaser diffractometer (Germany) with Cu Kα radiation. Scanning electron microscopy (SEM) images


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were taken on a Hitachi S4700 or a S8010 electron microscope. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G-20 or a JEOL 2100F microscope operated at 200 kV. Thermogravimetric (TG) analyses were conducted on a Mettler Toledo TGA/DGC 3+ analyzer from 25 to 800 °C with a ramp rate of 10 oC min-1 under flowing O2 (20 mL min-1). X-ray photoelectron spectra (XPS) were measured on a Thermo Fisher ESCALAB 250Xi spectroscope calibrated by C 1s at 284.6 eV. N2 adsorption-desorption isotherms were measured at -196 °C on a Micromeritics ASAP 2020 analyzer. Before the tests, all the samples were degassed under high vacuum at 180 °C for 8.0 h. Electrode preparation and electrochemical measurements About 2.0 mg of a certain active material was dispersed in a mixture composed of ethanol (0.40 mL), water (0.10 mL) and 5.0 wt% Nafion solution (10 µL), and then ultrasonicated for 30 min at room temperature to obtain a homogeneous ink. Then, 10 µL of the ink was dropped on a glassy carbon electrode with a diameter of 5.0 mm and dried at ambient conditions for 2.0 h, leading to a catalyst loading level of ~ 0.2 mg cm-2. A standard three-electrode cell with a graphite rod as the counter electrode and a Ag/AgCl electrode as the reference electrode was used to investigate the HER activities of as-synthesized electrocatalysts. The HER activity was evaluated by linear sweep voltammetry (LSV) scans conducted on a rotating disk electrode with a rotating speed of 1600 rpm, and a scan rate of 5.0 mV s-1 in a potential window of 0.115 ~ 0.485 V (vs. the reverse hydrogen electrode, RHE) in a 0.50 M H2SO4 electrolyte saturated with N2. Before the LSV scans, all the electrocatalysts were activated by cyclic voltammetry measurement with a scan rate of 100 mV s-1 in a potential window of 0.10 ~ 0.40 V for 10 cycles. All the reported potentials were referenced to RHE. During the tests, the IR-compensated function was adopted, but all reported polarization curves were original without any correction.


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The stability of the catalysts was investigated at a scan rate of 100 mV s-1 for 3000 cycles. The electrochemical impendence spectroscopy (EIS) measurement was carried out from 10 kHz to 0.01 Hz at -85 mV (vs. RHE) with an amplitude of 5 mV.

ASSOCIATED CONTENT Supporting Information Schematic illustrations of Mo2C@GC composites with different spatial configurations; HESEM images of SBA-15 template; XRD, TG curve and SEM image of Mo2C@GC-NP-800; SEM images and components contents obtained from TG analysis of Mo2C@GC composites obtained from different temperature; SEM images and TEM images of Mo2C@GC composites obtained from different PMA/SBA-15 mass ratio; HER performance compared with reported state-of-theart nanostructured molybdenum carbides catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author E-mail address: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports from the National Natural Science Foundation of China (Nos. 21501125, 21506135), the Natural Science Foundation of Jiangsu Province (BK20150312), the Young Thousand Talented Program (2015), the Suzhou Municipal Science and Technology Bureau (N310904116), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, and the project of scientific and technologic infrastructure of Suzhou (SZS201708) are much appreciated. Z. Wu acknowledges the start-up fund from Soochow University. ORCID: J. Zhu: 0000-0002-2922-6762; Y. Li: 0000-0003-0506-0451; Z. Wu: 0000-0002-2899-6474 Scheme 1. Schematic illustration of the synthetic process of for the ordered mesoporous Mo2C@GC core-shell nanowire arrays. It involves the loading of PMA into the mesopores of the template SBA-15 (a) to form a PMA@SBA-15 composite (b), conversion of PMA to MoO2 to form a MoO2@SBA-15 composite (c), carburization to form a Mo2C@GC@SBA-15 composite (d), and template removal to form the final Mo2C@GC composite (e), respectively.


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Figure 1. Wide-angle XRD patterns (A) and TG curves (B) of the various samples obtained with a fixed PMA/SBA-15 mass ratio of 3.3 at a heating temperature of 700 (a), 750 (b), 800 (c), 850 (d), and 900 ºC (e), respectively.


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Figure 2. Raman spectra (A) of the samples [email protected] (a), [email protected] (b), Mo2C@GC-800-1 (c), and Mo 3d, (B) and C 1s (C) XPS spectra of the representative catalyst [email protected].


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Figure 3. HRSEM (a-c), TEM (d), HRTEM (e, f) images, SAED pattern (g) taken on the area shown in (d), dark-field STEM image (h) and the corresponding elemental maps of C (i) and Mo (j), and EDX spectrum (k) of the representative sample [email protected] obtained with a PMA/SBA-15 mass ratio of 3.3 at 800 ºC.


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Figure 4. N2 sorption isotherms (A) and the corresponding pore size distribution curves (B) of the various samples obtained at fixed temperature of 800 ºC with a PMA/SBA-15 mass ratio of 3.3 (a), 2.5 (b), 1.0 (c), 0.5 (d), 0.25 (e), 0.05 (f), respectively.


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Figure 5. Polarization curves (A), the corresponding Tafel plots (B) and the EIS Nyquist plots (C) of the commercial 20% Pt/C catalyst (a), the representative sample [email protected] (b) obtained with a PMA/SBA-15 mass ratio of 3.3 and the control sample Mo2C@GC-NP-800 (c) obtained without the use of SBA-15 template, respectively. (D) is the polarization curves of the representative sample [email protected] before (solid line) and after (dash line) 3000 cycles.


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Figure 6. Polarization curves (A) of the various samples obtained with a fixed PMA/SBA-15 mass ratio of 3.3 at a temperature of 700 (a), 750 (b), 800 (c), 850 (d), and 900 ºC (e), respectively, and polarization curves (B), the Tafel plots (C) and the EIS Nyquist plots (D) of [email protected] (a), [email protected] (b), Mo2C@GC-800-1 (c), [email protected] (d), [email protected] (e), [email protected] (f) obtained with different PMA/SBA-15 mass ratios at 800 ºC.


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Figure 7. Wide-angle XRD patterns (A) and TG curves (B) of the various samples obtained at a fixed temperature of 800 ºC with a PMA/SBA-15 mass ratio of 3.3 (a), 2.5 (b), 1.0 (c), 0.5 (d), 0.25 (e), and 0.05 (f), respectively.


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Table 1. Summary of the physicochemical properties and the key parameters for HER performance of the SBA-15 template, the various Mo2C@GC samples obtained at a fixed temperature of 800 ºC with a series of PMA/SBA-15 mass ratios and the control samples.

a

Sample

SBET a (m2/g)

Vt b (cm3/g)

Dc (nm)

Mo2C content (wt%) d

GC content (wt%) d

Onset potential (mV) e

Overpotential at 10 mA cm−2 (mV)

[email protected]

37

0.11

4.2, 16 ~ 32

80

20

58

125

66

[email protected]

111

0.21

4.2, 8.0 ~ 32

76

24

85

151

74

[email protected]

317

0.40

4.5

49

51

99

170

75

[email protected]

262

0.19

2.2

30

70

128

212

81

[email protected]

281

0.21

2.3

21

79

133

212

80

[email protected]

406

0.27

2.0

7

93

205

312

109

Mo2C@GCNP-800

2.60

0.02

3.7~ 20.4

98

2.0

215

309

108

20% Pt/C

167

0.30

50

/

/

6.0

38

42

SBA-15

480

1.08

9.7

/

/

/

/

Tafel slope (mV dec-1)

/ b

The specific surface area obtained the BET method with the adsorption data over a relative pressure range of 0.03 ~ 0.2. The total pore volume

obtained from the adsorbed amount at a relative pressure of 0.995. adsorption branch.

d

c

The peak mesopore size obtained from the BJH model by using the

The mass percentage obtained from the TG curves under flowing oxygen.

e

The onset potential is referred to as the

overpotential at a current density of 1.0 mA cm−2.


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