Nesting Co3Mo Binary Alloy Nanoparticles onto Molybdenum Oxide

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Energy, Environmental, and Catalysis Applications

Nesting Co3Mo Binary Alloy Nanoparticles onto Molybdenum Oxide Nanosheet Arrays for Superior Hydrogen Evolution Reaction Jiyi Chen, Yuancai Ge, Qianyi Feng, Peiyuan Zhuang, Hang Chu, Yudong Cao, William R. Smith, Pei Dong, Mingxin Ye, and Jianfeng Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19148 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Materials & Interfaces

Nesting Co3Mo Binary Alloy Nanoparticles onto Molybdenum Oxide Nanosheet Arrays for Superior Hydrogen Evolution Reaction Jiyi Chen1, Yuancai Ge1, Qianyi Feng1, Peiyuan Zhuang1, Hang Chu1, Yudong Cao1, William R. Smith2, Pei Dong2, Mingxin Ye1*, and Jianfeng Shen1* 1 2

Institute of Special Materials and Technology, Fudan University, Shanghai 200433, P. R. China Department of Mechanical Engineering, George Mason University, VA 22030, USA

Abstract: Transition-metal alloy has attracted a great deal of attention as an alternative to Pt-based catalysts for hydrogen evolution reaction (HER) in alkaline. Herein, a facile and convenient strategy to fabricate Co3Mo binary alloy nanoparticles nesting onto molybdenum oxide nanosheet arrays on nickel foam is developed. By modulating the annealing time and temperature, the Co3Mo alloy catalyst displays a superior HER performance. Owing to substantial active sites of nanoparticles on nanosheets, as well as the intrinsic HER activity of Co3Mo alloy and no use of binder, the obtained catalyst requires an extremely low overpotential of only 68 mV at 10 mA cm-2 in alkaline, corresponding Tafel slope of 61 mV dec-1. At the same time, the catalyst demonstrates excellent stability during the long-term measurements. The density function theory calculation provides a deeper insight into the HER mechanism, unveiling that the active sites on Co3Mo-based catalyst are Mo atoms. This strategy of combining catalytic active species with hierarchical nanoscale materials can be extended to other applications and provides a candidate of non-noble metal catalysts for practical electrochemical water splitting. Keywords: cobalt molybdenum alloy, nanoparticles, nanosheet arrays, hydrogen evolution reaction, alkaline

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1. Introduction With the development of society, there has been a growing demand for sustainable and clean energy sources due to unacceptable long-term pollution and inadequate supplies of fossil fuels. Hydrogen, as an alternative to the current conventional energy sources, has showed a lot of attractive virtues. Its oxidation product (water and zero CO2 emission) is environmental and eco-friendly.1 In addition, it has the highest gravimetric energy density of existing energy sources. Because of the sufficient electrical energy from the renewable power (wind, sunlight, tides, etc.) and the ultrapure hydrogen production, electrolysis-based hydrogen production has been widely accepted as an appropriate approach by more and more scientists.2 For years, such promising technology which achieves the mutual conversion between hydrogen energy and electricity has been greatly improved.3 In order to accelerate the sluggish kinetic process of the electrode reaction, the electrode is modified by electrocatalysts which also enhance the efficiency of the reaction. With respect to hydrogen evolution reaction (HER), which occurs at the cathode, platinum (Pt) and Pt-based electrocatalysts still remain the benchmark catalysts capable of driving HER close to ideal thermodynamic conditions.4,5 Pt-based materials, however, are scarce and high-cost on the earth. Thus, these shortcomings severely hinder its application on industrial scale. Recent years have seen a great improvement of hydrogen production on other lowcost alternative electrocatalysts. Among them, transition-metal-based (TMB) electrocatalysts displays the superior performance. For example, metal sulfides,6,7 metal phosphides,8,9 metal oxides,10 metal selenides,11 metal carbides,12,13 metal nitrides,14,15 and metal alloys16,17 are the branches of TMB categories. Interestingly, the morphologies of the catalysts also have a tremendous influence on their corresponding HER performance. In principle, if a catalyst has superior activity towards HER, it could come from one or more from the following four possible aspects: 1) there are a large number of active sites of catalysts; 2) a proper binding ability of reaction intermediates with the active sites, which will lead to good intrinsic activity; 3) tantivy mass transfer of reactants (H+/H2O) and products (H2/OH-) occurs between the surface of catalysts 2 ACS Paragon Plus Environment

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and electrolyte; 4) the electrode made by catalysts has good electrical conductivity.3 Based on the above considerations, nanosheet, as one of the typical morphologies with large specific surface area, which will result in more exposed active sites and much faster mass transfer rate of gas bubbles, has been studied thoroughly.18-23 By using nitrogen doped Ni3S2 nanosheet arrays, Kou achieved a relatively low overpotential of 155 mV at 10 mA cm-2 in 1.0 M KOH electrolyte.21 Another work also used ultrathin feroxyhyte (δ-FeOOH) nanosheets with rich Fe-vacancies as the cathode materials, which obtained an overpotential of 108 mV at current density of 10 mA cm-2.22 On the other hand, nanoparticles (NPs) of TMB catalysts also play a pivotal role towards HER. It is evident that NPs have a high number of exposed active sites and remarkable intrinsic catalytic activity, but these NPs generally aggregated during the synthesis process.24 However, there are some strategies to overcome this problem.25-29 It is worth noting that metal alloy NPs perform especially well among these sorts of catalysts. As Shen and coworkers demonstrated, nickel-copper (NiCu) alloy NPs encapsulated into graphitic shells showed a superior HER activity of reaching 10 mA cm-2 with an overpotential of 74 mV.28 In light of the reports mentioned above, herein we design a facile and convenient strategy to fabricate cobalt-molybdenum (Co3Mo) alloy NPs nesting onto molybdenum oxide nanosheet arrays for HER by annealing the precursor cobalt molybdate (CoMoO4) nanosheet arrays in H2 atmosphere. The in situ topotactic conversion from CoMoO4 nanosheet arrays to molybdenum oxide nanosheet arrays followed by the formation of Co3Mo alloy nanoparticles can efficiently avoid the aggregation of nanoparticles and maintain the morphology of support substrate - the vertically aligned nanosheet arrays. By controlling the reduced time and temperature, the amounts of NPs were modulated. Owing to the large specific surface area of nanosheet arrays and substantial active sites of NPs, as well as the intrinsic HER activity of Co3Mo alloy, the obtained binder-free catalysts successfully achieve an extremely low overpotential of only 68 mV at 10 mA cm-2. Through chronopotentiometry tests, the catalyst displayed outstanding durability at a current density of 10 mA cm-2 for 20 h. Moreover, through density functional theory 3 ACS Paragon Plus Environment


(DFT), the mechanism of this remarkable HER catalyst was uncovered that the actual active sites of Co3Mo alloy are the Mo atoms with the smallest adsorption Gibbs free energy of hydrogen atoms (ΔGadsH*). 2. Experimental Section Materials. Nickel foam (Ni foam, 99.8 wt%, 1 mm in thickness), cobalt nitrate hydrate (Co(NO3)2·6H2O), sodium molybdate hydrate (Na2MoO4·2H2O), acetone, deionized (DI) water, 3 M hydrochloric acid (HCl) and ethanol (99.7 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pt/C (20 wt.% Pt loading on carbon) was bought from Macklin. All chemicals were used directly without purification. Before being used as the substrate, Ni foam (2 × 4 cm2) was pretreated in 3 M HCl, DI water, and acetone with the assistance of ultrasonication for 15 minutes respectively. Synthesis of Cobalt Molybdate on Ni Foam (CMO@NF). Precursor CoMoO4 nanosheet arrays on Ni foam were synthesized by a facile hydrothermal method. 1 mmol Na2MoO4·2H2O was dissolved uniformly in 15 ml DI water by magnetic stirring. Then 15 ml ethanol was added into the above solution, followed by adding 1 mmol Co(NO3)2·6H2O and stirring for 20 min to obtain a violet suspension. The dried pretreated Ni foam was placed against the wall of autoclave, immersed in the above suspension and sequentially sonicated for 15 minutes. Then the autoclave was sealed and maintained at 160 C for 6 h. After cooling to room temperature, the substrate was taken out and cleaned by ultrasonication for 5 min to remove the loosely attached products in DI water, dried at 60 C for 6 h in vacuum oven. The loading amounts of CoMoO4 nanosheet on Ni foam was about 1 mg cm-2. Synthesis of Reduced Cobalt Molybdate on Ni Foam (RCMO@NF). A piece of Ni foam covered with hierarchical CoMoO4 nanosheet arrays was put into a tube furnace and annealed under H2/Ar (5:95) atmosphere at 350 C for 2 h to obtain 3502h. The heating rate was 5 C min-1. Similarly, 400-2h, 500-2h, 600-2h, 500-3h, 5004h, 500-5h were synthesized under the same conditions, except for the temperature and holding time. 4 ACS Paragon Plus Environment

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Preparation of Pt/C. Commercial Pt/C was selected as benchmark catalyst for HER. Based on previous work,30 5.9 mg Pt/C powder was dispersed in a mixed solution consisted of 990 µL ethanol and 10 µL Nafion (5 wt.%) under ultrasonication until to obtain a black homogeneous suspension. Then, 84 µL suspension was slowly dropped on the surface of a pretreated Ni foam (0.5 × 0.5 cm2) and dried. The area mass loading of Pt/C was calculated to be about 2 mg cm-2. Characterizations. The morphologies of the catalysts were filmed by fieldemission scanning electron microscopy (FESEM, Tescan, MAIA3 XMH) at an accelerating voltage of 15 kV. Energy dispersive X-ray spectroscopy mapping (EDXmapping) images were obtained using Bruker Xflash 6|30. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced X-ray diffractometer with Cu K radiation ( = 0.154 nm). The selected-area electron diffraction (SAED) patterns, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning transmission electron microscopy (STEM) and EDX-mapping images of 500-4h were characterized by FEI Tecnai G2 F20. The TEM and HRTEM images of the specimen of 500-2h, 500-3h, 500-5h were obtained on a JEM-2100 with an accelerating voltage of 200 kV. Before each TEM test, the catalysts were immersed in ethanol and ultrasonicated to collect nanosheets suspension for characterization. The chemical elements analysis was performed by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C ESCA system using Mg anode). Raman spectra were conducted on a Dilor LABRAM-1B multichannel confocal microspectrometer with 514 nm laser excitation. Thermogravimetric analysis (TGA) was accomplished using an TG209 F1Iris. Electrochemical measurements. The HER electrochemical measurements were tested using a three-electrode system on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd) with 1 M KOH electrolyte at room temperature, unless specifically indicated. The as-prepared samples were used as the working electrode directly, whose area immersed in electrolyte was 0.5 × 0.5 cm2, while saturated calomel electrode (SCE) and polished graphite rod were applied as the 5 ACS Paragon Plus Environment


reference electrode and counter electrode. Prior to each test, Nitrogen gas (N2) was injected into the electrolyte to remove the dissolved oxygen. During the test, the flow of N2 gas was maintained as well. Linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s-1. The polarization curves were corrected regarding the iR compensation due to the existence of the electrolyte ohmic resistance (compensated level 85 %). According to previous work, there’s a definite connection between electrochemical surface area (ECSA) and electrochemical double layer capacitances (Cdl).8 Therefore, cyclic voltammetry (CV) was conducted in a range of -0.7 V to -0.8 V (vs. SCE) at series of scan rate (ranged from 10 to 100 mV s-1). As for the stability of the working electrode, chronopotentiometry was performed at a current density of 10 mA cm-2 for 20 h. Electrochemical impedance spectroscopy (EIS) measurements were carried out using Metrohm Autolab PGSTAT302N under the same system from 10000 to 0.1 Hz with an AC voltage of 10 mV (vs. reversible hydrogen electrode (RHE)). With regard to the use of SCE and 1 M KOH as the reference electrode and electrolyte respectively, all potentials were calibrated to RHE by adding 1.0672 V (0.2412 + 0.059 × pH). DFT calculation. DFT calculations were performed by Cambridge Serial Total Energy Package (CASTEP) code in the Materials Studio Software.31 The generalized gradient approximation Perdew-Burke-Ernzerh (PBE) exchange correlation functional was employed. The Brillouin zone was sampled 3 × 5 × 1 k-point grid was based on Monkhorst-Pack method. And the semiempirical dispersion correction of Grimme scheme was adopted. The applied cut-off energy was 550 eV. The hydrogen adsorption energy was calculated with (2 0 1) slabs of Co3Mo with a vacuum of 10 Å. Structures were optimized under Broyden-Fletcher-Goldfarb-Shanno schemes. The convergences of energy, maximum displacement, and maximum force were set as 5.0 × 10-6 eV atom−1, 5.0 × 10−4 Å, and 0.01 eV Å−1, respectively.8 The differential hydrogen chemisorption energy (ΔEadsH) of active sites was calculated by ΔEadsH = Eslab+H – Eslab – 1/2 × EH2, where Eslab+H is the total DFT energy for the slabs with a hydrogen atom, Eslab is the total DFT energy for the slabs. And the 6 ACS Paragon Plus Environment

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EH2 is the DFT energy of a H2 molecule in the gas phase. According to previous works, the Gibbs free energy (ΔGadsH*) was calculated from ΔEadsH + ΔEZPE − TΔSH, where the ΔEZPE and ∆SH are the difference in zero point energy and the entropy difference between the adsorbed state and the gas phase, respectively.5 3. Results and Discussion

3.1. Characterization of Nanostructure and Composition Our in situ synthesis of Co3Mo alloy NPs involved two straightforward steps, as illustrated in Figure 1. Briefly, the CoMoO4·3/4 H2O nanosheet arrays were grown on a pre-treated Ni foam (2 × 4 cm2) via hydrothermal reaction firstly by using cobalt nitrate as the Co source and sodium molybdate as the Mo source (namely CMO@NF). Then, the as-prepared CMO@NF precursor was put into a tube furnace and annealed under H2/Ar atmosphere. The process included two reactions: 1) the dehydration of CoMoO4·3/4H2O nanosheet; 2) the reduction of metal salt to binary metal alloy and metal oxide. During the procedure, both Co atoms and Mo atoms diffused outward despite the different diffusion rate.32 As a result, Co3Mo alloy NPs formed on the surface of nanosheets, leaving the template topotactic conversed to molybdenum oxide nanosheets.



Figure 1. Schematic illustration of the synthesis procedures of RCMO@NF In order to obtain the best HER activity, different anneal times and temperatures were investigated. Under different anneal conditions, the morphology of CMO@NF precursor and reduced CoMoO4 nanosheet arrays on Ni foam (RCMO@NF) were filmed by FESEM. As demonstrated in Figure 2, pristine CMO@NF shows a great deal of nanosheets vertically standing on Ni foam (Figure 2a) with a smooth surface. After adjusting the temperature to 350 C for 2 h, the change of nanosheets between CMO@NF and 350-2h (RCMO@NF annealed under different condition was named by temperature-time) can be seen in Figure 2b, as there was some loss of structure on the edges of nanosheets. The reason for this phenomenon is the dehydration of H2O.33 According to the images of the EDX-mapping (Figure S1, Supporting Information), Co, Mo, and O elements homogeneously disperse within the nanosheets of 350-2h. Additionally, the EDX-spectrum image of 350-2h in Figure S2 in the Supporting Information indicates that crystallized water was totally dehydrated, becuase the inset table report shows that the atom ratio of Co, Mo, O is about 1:1:4, which is fully consistent with the atom ratio of CoMoO4. Furthermore, the result of TGA (Figure S3) shows that there are two mass losing platform around 100 C and 300 C respectively 8 ACS Paragon Plus Environment

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corresponding to the losing of absorped water and crystallized water. And the total mass losing pecent (7%) approximately equals to the proportion of hydrated water of CoMoO4·3/4 H2O (3/4 × 18 ÷ 232.37, the 1% deviation represents the proportion of absorped water), confirming once again that the crystallized water disappeared. When the anneal temperature transformed to 400 C and 500 C, some nanoparticles appeared on the bare surface of nanosheets. Compared to 500-2h (Figure 2e), however, NPs of 400-2h (Figure 2c) were much smaller and the amounts were less. As for 600-2h (Figure 2d), it seemed that the surface of nanosheets was totally covered by bulk alloy. Possibly induced by the over-high temperature, it was conspicuous that the nanosheet arrays substrate cracked into pieces. Based on the principles of a good catalyst mentioned above, all these changes of morphology might lead to the decrease of electrochemical catalytic activity. And in fact, the activity of 600-2h towards HER was indeed worse than that of 400-2h. In summary, 500 C is the most appropriate temperature to obtain the proper size of alloy NPs. Keeping the temperature at 500 C, with reaction time of 2 h, 3 h, and 4 h, the amounts of CoMo alloy NPs on nanosheets gradually increased and the morphology of bottom nanosheet arrays remained stable (Figure 2 e,f,g). This phenomenon could be explained by topotactic conversion in precedent work.17 When the annealing time was prolonged to 5 h, the surface of nanosheets was totally covered by nanoparticles as shown in Figure 2h. In addition, 500-5h faced the same problem as 600-2h and a new challenge: not only did the cracks of the nanosheet arrays substrate, but so did a few aggregation of NPs. The synergetic influence resulted in the deterioration of catalytic activity. What aligned with the expectations was that the catalytic activity of 500-4h was the best due to the most suitable distribution of alloy nanoparticles and the intact nanosheet arrays substrate. The EDX-mapping of 500-4h, collected by ultrasonication in ethanol, in the Supporting Information Figure S4 indicates the dispersion of Co, Mo, and O elements. From a large scale, the enrichment of metal elements was not evident, which will be re-characterized in nanoscale via EDX-mapping of TEM.



Figure 2. SEM images of (a) pristine CMO@NF, (b) 350-2h, (c) 400-2h, (d) 600-2h, (e) 500-2h, (f) 500-3h, (g) 500-4h, (h) 500-5h. Scale bars of inset in (b) 200 nm, (c) 200 nm, (d) 500 nm, (e) 500 nm, (f) 500 nm, (g) 200 nm and (h) 200 nm. Moreover, in the Supporting Information Figure S5, XRD patterns between CMO@NF and residual powder, which were gathered after hydrothermal reaction, prove that the chemical composition of the residual powder and the nanosheet arrays on Ni foam are of the same materials in virtue of the similar peaks of two XRD patterns, except for two strong peaks at 44.5, 51.8 from Ni foam (JCPDS No. 04-0850). At the same time, the sharp peaks of residual sample located at ~13, 17.7, 25.4, 27.3, 29.6, 33.2 and 34.5 match well with the previously reported XRD pattern, which is indexed to triclinic CoMoO4·3/4H2O.33 These comparisons certify that the nanosheets vertically aligned on the surface of Ni foam are hydrate CoMoO4. After the topotactic conversion from CoMoO4 nanosheets to molybdenum oxide nanosheets decorated with CoMo alloy NPs, the product of 500-4h was also characterized by XRD. After transformation, the peaks of CoMoO4·3/4H2O vanished, proving that the CoMoO4 had been absolutely conversed. In Figure 3a, the appearance of peaks at ~20, 40.6 and 46.5 is indexed to the (100), (200) and (201) planes of Co3Mo alloy, displayed as a purple perpendicular line (JCPDS No. 29-0488). The two main areas, circled by dotted line in the XRD pattern, match well with the molybdenum oxide (MoO3, JCPDS No. 47-1081, displayed as a blue perpendicular line; Mo4O11, JCPDS No. 05-0337, displayed as a red perpendicular line). The peaks centering at ~25, 25.9, 26.5, 33.4, 34.2, and 35.0 _

_

are assigned to the (200), (11 1 ), (111), (020), (10 2 ) and (102) facets of MoO3, 10 ACS Paragon Plus Environment

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respectively.34 And the peaks at 25.6, 26.7, 27.3, 32.1, 32.3, 32.9, 33.1, 33.6, 34.5, 36.3, and 37.0 are ascribed to (601), (102), (610), (601), (212), (502), (020), (312), (810), (602) and (221) planes of Mo4O11. Thus, these results illustrate that the product 500-4h consists of Co3Mo, MoO3 and Mo4O11. Another important thing to note is that the two peaks of Ni foam are so strong that the peaks of Co3Mo, MoO3 and Mo4O11 around these two peaks are almost concealed. Therefore, the powder of reduced CoMoO4 (RCMO) was characterized by XRD. When the reduction time was 2 h, CoMoO4 was reduced to Co2Mo3O8 and Co (Figure S6a). As the reaction time was prolonged to 4h, the peaks of Co3Mo appeared, demonstrating the production of alloy (Figure S6b). As shown in Figure S7, the possible reason for the different constitution of RCMO powder and RCMO nanosheet arrays may be the distinct morphology of the rod-like CMO powder. However, with the formation of Co3Mo alloy, the precursor turned into nanosheet (Figure S7b), which is familiar with the RCMO nanosheet, manifesting the conversion from cobalt molybdate to molybdenum oxide nanosheet.

Figure 3. a) XRD pattern, b) TEM image, c,d) HRTEM images, e) SAED pattern, f) dark-field scanning transmission electron microscopy (STEM) image, and g-i) corresponding STEM elemental mapping of Co, Mo, and O of 500-4h. Inset of (f) is the layered STEM graphic of nanoparticles of 500-4h. 11 ACS Paragon Plus Environment


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To further demonstrate the structure and morphology of the catalysts, TEM and HRTEM were applied to analyze the RCMO nanosheets. In Figure 3b, the TEM image of 500-4h shows that the spheral alloy nanoparticles scattered on the surface of a single layered nanosheet in a relatively dense way. HRTEM images (Figure 3c,d) display a series of clear lattice fringes. The interplanar distance of 0.195 nm on the nanoparticle corresponds to the (201) facet of Co3Mo alloy, and the lattice fringes of 0.214 nm, 0.177 nm, and 0.216 nm on the nanosheet are assigned to the (220), (400), and (310) planes of MoO3. The reason why these identification of discrete Co3Mo alloy NPs and MoO3 nanosheets differ from the XRD results is that these facets’ corresponding XRD peaks are concealed by two ultra-strong peaks of Ni foam, which was explained earlier in this paper. Moreover, the corresponding SAED pattern (Figure 3e) shows that the _

diffraction rings match well with (201) plane of Co3Mo, (220) (302) plane of MoO3, and (221) plane of Mo4O11, respectively. The inset of Figure 3f is the layered STEM graphic of nanoparticles of 500-4h. Noticeably, from the images of EDX-mapping of layered graphic (Figure 3g, h), the nanoparticles are composed of Co and Mo elements, while the nanosheet on the bottom layer is constituted by O element and Mo element. It needs to be mentioned that the EDX-mapping of Mo element on bottom layer nanosheet was not provided. The number of Co atoms on nanoparticles is much greater than that of Mo atoms, therefore the Co/Mo ratio of nanoparticles is higher than 1. For 500-2h, 500-3h, and 500-5h, TEM and HRTEM were also conducted, showed in Figure S8, Figure S9, and Figure S10. It is apparent that the number of NPs raises gradually when the annealing time is increased. When the time was prolonged to 5 h, the nanosheet was entirely covered by NPs. As shown in HRTEM images, the interplanar spacing and diffraction rings of nanoparticles are assigned to the planes of Co3Mo. XPS was applied to further support the existence of Co3Mo alloy, MoO3 and Mo4O11. In Figure S11a in the Supporting Information, the image confirms the presence of Co, Mo and O elements in 500-4h sample. The high-resolution spectrum of O 1s (Figure S11b) indicates that the only peak at 531.2 eV is assigned to Mo-O.32 As demonstrated in Figure S11c, the high-resolution Mo 3d pattern of 500-4h could be deconvoluted into 12 ACS Paragon Plus Environment

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five components at 228.77 eV (Mo0 3d5/2), 230.34 eV (Mo4+ 3d5/2), 232.07 eV (Mo6+ 3d5/2), 233.18 eV (Mo4+ 3d3/2), 235.17 eV (Mo6+ 3d3/2).17,32,35 The deconvolution of Co 2p in 500-4h (Figure S11d, Supporting Information) shows binding energy peaks at 779.5 eV and 795.6 eV are assigned to Co0 2p3/2 and Co0 2p1/2, respectively. The other two peaks located at 784.3 eV and 802.3 eV belong to the oxidized Co, which was reported in previous work due to the increment of Co content in alloy.36,37 The XPS peaks of metallic state of Co0 and Mo0 authenticate the formation of Co3Mo alloy. And the peaks which belong to Mo4+ and Mo6+ come from the oxidized Mo in Mo4O11 and MoO3. Raman spectroscopy is a useful technology for material characterization. Hence Raman scattering analysis was conducted here in the range of 200-1100 cm-1, given in Figure S12 in the Supporting Information. The strongest peaks at 931 cm-1 for both CMO@NF and 500-4h correspond to the symmetric stretching mode of Mo-O.38-40 In nearby area, there are two peaks observed at 876 and 816 cm-1 in both patterns, which are assigned to the asymmetric stretching mode of oxygen in O-Mo-O bond.39-41 For CMO@NF, the distinct peak at 335 cm-1 is attributed to the symmetric stretching mode of Co-O-Mo bond, which was not detected on 500-4h.38-40 This difference demonstrates that the composition of 500-4h nanosheet substrate is molybdenum oxide rather than Co2Mo3O8 appearing in XRD patterns of RCMO powder (Figure S6). The slight shifts of results possibly come from the slight variation in morphology and the structural order-disorder degree of the materials. Besides, they fluctuate in a reasonable range.39 For 500-4h, the distinct peaks at 371 and 667 cm-1 belong to the scissoring modes of O-Mo-O and the triply coordinated oxygen (Mo3-O) stretching mode resulted from edge-shared oxygen atoms to three adjacent octahedral, indicating the existence of MoO3 and Mo4O11.42,43 The peaks between 470 and 550 cm-1 are assigned to NiO, which may come from the Ni foam substrate.44 It is noticed that alloy with an atomic vibration could not be detected by Raman spectrum.32 In conclusion, all these results certify that during the annealing procedure, CoMoO4 hydrate nanosheet arrays on Ni



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foam (CMO@NF) in situ topotactic transformed to Co3Mo binary alloy nanoparticles adhered to molybdenum oxide nanosheet arrays.

3.2. Electrochemical Characterization of HER Catalytic Activity To investigate the HER catalytic activity of the obtained catalysts, bare Ni foam (Bare NF), CMO@NF, and RCMO@NF annealed under various conditions were directly applied as the working electrode in a standard three-electrode system in alkaline solution (1 M KOH). As a comparison, commercial Pt/C was measured, using Nafion as a binder. The measurement results of LSV are displayed in a way of polarization curves in Figure 4a. Notably, Pt/C catalyst and 500-4h have the best catalytic activity (grey dotted curve) for delivering a relatively high current density when then value of overpotential is small. Moreover, the curves reveal a law that with the annealing time and temperature going up, the activities of RCMO@NF present a rising trend. Two exceptions were 600-2h and 500-5h, of which activity was even worse than 400-2h and 500-3h, respectively. In general, Tafel slope (obtained from Tafel plots, η versus log j) and the overpotential at a current density of 10 mA cm-2 are two main parameters to evaluate a catalyst’s electrochemical performance towards HER. The lower the overpotential and the Tafel slope, the higher the growth of HER rate and the energy efficiency.45 Besides, there is a consensus that the kinetics of HER could be expressed by three principle reactions, and the equations were demonstrated below.46 H2 O + e− → Hadsorbed + OH− (Volmer reaction)

(1)

H2 O + e− + Hadsorbed → H2 + OH− (Heyrovsky reaction) Hadsorbed + Hadsorbed → H2 (Tafel reaction)

(2) (3)

The Tafel slope is usually used as an indicator of the rate determining step and provides profound insights into the kinetic mechanism of HER. As shown in Figure 4b, the Tafel slope of commercial Pt/C is 33.5 mV dec-1, which is consistent with the previously reported result.32 As for 500-4h, the Tafel slope is as low as 61.3 mV dec-1, which is much lower than 500-3h (84.0 mV dec-1), 500-5h (107.0 mV dec-1), 500-2h (109.3 mV dec-1), 400-2h (119.9 mV dec-1), and 600-2h (111.5 mV dec-1). According 14 ACS Paragon Plus Environment

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to the expected Tafel slope of 29.5, 39, and 118 mV dec-1 relating to different ratedetermining steps of HER, the Tafel slope value of 500-4h demonstrates that HER over this catalyst proceeds through Volmer-Heyrovsky mechanism.47-49 Figure 4c presents the overpotentials required for all tested catalysts at a current density of 10 mV cm-2. Remarkably, the overpotential of 500-4h is only 68 mV, which is better than state-ofthe-art Pt-free HER catalysts (Table S1, Supporting Information) in alkaline such as NiCu nanoalloys@NiCuN@N-Carbon (93 mV),16 Co NPs@N-CNTs@rGO (108 mV),26 monolayer MoS2@Carbon sheets (126 mV),50 and oxygen-incorporated Co2P (160 mV).51 On the other hand, 500-3h (75 mV) is lower than 500-5h (78 mV), while 400-2h (117 mV) is lower than 600-2h (182 mV). All these results indicate that 500-4h outperforms other RCMO@NF catalysts towards HER. In addition, 500-3h and 400-2h show a better activity than 500-5h and 600-2h respectively.

Figure 4. a) LSV polarization curves in 1 M KOH, b) corresponding Tafel slopes c) overpotential at a current density of 10 mA cm-2, d) calculated Cdl in 1 M KOH, e) EIS Nyquist plots, f) Long-term stability tests of 500-4h. The inset of (e) is the simulation of equivalent circuit of the working electrode and (f) is the SEM image of 500-4h after a 20 h electrochemical HER test. To further unveil the origin of the enhanced electrochemical performance, ECSA and EIS measurements were carried out in the same three-electrode system in 1 M KOH. 15 ACS Paragon Plus Environment


For there being a function relation between ECSA values and Cdl,52 CV was adopted to characterize the active surface area in non-faradaic region, as depicted in Figure S13 in the Supporting Information. It is found that 500-4h exhibits the largest Cdl (27.2 mF cm-2) among all RCMO@NF catalysts (Figure 4d), suggesting that 500-4h’s accessible active sites are much more than others. As a result, the ECSA value of 500-4h is the highest among all samples, which could reach to 170 cm2 (Figure S14b). More detailed Cdl and ECSA information is given in Figure S14 in the Supporting Information. It is evident that the high ECSA value is resulted from the high specific surface area of molybdenum oxide nanosheet arrays and appropriate amounts of Co3Mo alloy nanoparticles without aggregation, compared to 500-5h, 500-3h, 500-2h and 400-2h. In order to eliminate the influence of large area of 3D electrode Ni foam, the LSV polarization curves of 500-2h, 500-3h, 500-4h and 500-5h were normalized to the ECSA values, as shown in Figure S15a. And the corresponding Tafel slope of these four samples in Figure S15b are similar. According to the theory of catalyst’s performance mentioned in Introduction section, this result demonstrates that these four samples possess the same intrinsic activity, which come from the Co3Mo nanoparticles. The reasons for the distinct performance of 500-2h, 500-3h, 500-4h and 500-5h are the different active site numbers (ESCA values) and the disparate electrical conductivity of electrode measured by EIS. Nyquist plots in Figure 4e are fitted with Randles equivalent circuit to obtain the resistance of the device. The inset circuit is the diagram of Randles circuit. Rs is the resistance of electrolyte, while Rct is the charge transfer resistance.6 The width of semicircle represents the value of Rct and indicates the overall catalytic kinetic effects.32 500-4h displays the smallest Rct of 3.0 Ω, while the values for 500-3h, 500-5h, 500-2h, and 400-2h are higher (5.4 Ω, 7.52 Ω, 7.3 Ω, and 11.5 Ω). The lower the resistance, the faster the reaction rate towards HER. In conclusion, all these results confirm that 5004h is the best catalyst for HER in alkaline among this series of RCMO@NF catalysts with small overpotential, low Tafel slope and good conductivity (small Rct). The deterioration of catalytic activity of 600-2h and 500-5h come from the cracks of 16 ACS Paragon Plus Environment

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nanosheet arrays substrate and aggregation of alloy nanoparticles, which lead to the decrement of the number of active sites (ECSA) and the conductivity (EIS). For HER, the long-term stability was another crucial property for any practical application. In Figure 4f, 500-4h was tested at 10 mA cm-2 in 1 M KOH for 20 h. After a period of reaction, the overpotential tended to be stable. The inset SEM image displays the morphology of 500-4h after the long-term stability test, indicating that the structure preserved well after 20 h stability test. Figure S13 in the Supporting Information shows the LSV polarization curves before and after the long-term stability test. It is found that the HER performance after stability test was almost the same. Interestingly, the decrement of overpotential (Figure 4f) at a current density of 10 mA cm-2 and the increment of Tafel slope (Figure S16), which was similar to the result of a previously reported work,53 corresponded to the rearrangement on the surface of active sites or the separation of a few oxidized alloy nanoparticles from nanosheet presumably. In addition, in situ electrochemical oxidation was conducted to further determine the effective catalytic site. After the electrochemical oxidation, the performance of 500-4h decreased sharply (Figure S17). This result confirms that Co3Mo alloy is the actual catalytic site. 3.3. Density Function Theory (DFT) Calculation on Gibbs Free Energy of HER In order to further reveal the mechanism of the remarkable HER catalyst in alkaline solution, theoretical calculation was applied to in-depth investigate the reaction steps of HER by using DFT. In general, according to the three principle reaction steps mentioned

above,

the

catalytic

activity is

strongly associated

with

the

adsorpotion/desorption Gibbs free energy of hydrogen atoms (ΔGH*) on the active sites of catalysts. What’s more, the value of adsorption Gibbs free energy of hydrogen atoms (ΔGadsH*) is widely used as an indicator to describe the activity towards HER, which should be as small as possible to close to zero.8,54-56 Apparently, Pt has the optimal ΔGadsH* value that has been demonstrated in many previously reported works.32,47,57 In this work, we applied DFT as well to disclose the active sites of Co3Mo-based catalyst. The detailed calculation information would be given in the following experimental 17 ACS Paragon Plus Environment


section. As depicted in Figure 5a, the crystal structure of Co3Mo is of closed-packed arrangement and the layers of the slabs are corresponded to (201) plane, in which there are four possible active sites, Mo atom, Co-1 atom, Co-2 atom, and Co-3 atom, respectively. Depend on the values comparison in Figure 5b, the Mo atom possesses far smaller ΔGadsH* (-0.05149 eV) than other three Co atoms, which are -0.57009 eV (Co1), -0.57341 eV (Co-2), and -0.22478 eV (Co-3). Thus, this result indicates that the active site for HER is the Mo atom.

Figure 5. a) (201) surface of Co3Mo alloy, b) Calculated free energy diagram for hydrogen adsorption on (201) surface of different surficial atoms. Mo and Co are not good catalysts for HER in volcano plot.47 However, by alloying with Co, the Mo atoms obtain a notably enhanced activity. The synergistic effect of regulating the ΔGadsH* is resulted from the electron-transfer between Co and Mo in the Co3Mo alloy.58 Based on the theory of work function (Φ), the ΦM (work function of metal) of Co is 4.7 eV and Mo 4.3 eV. The difference between these two values would lead to the different d-band electron density states.59 When the voltage is applied across the electrode, the electrons tend to transfer from the Mo atoms to Co atoms. As a result, Mo atoms can absorb hydrogen atoms efficiently, which is consistent with the results of DFT.


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4. Conclusion In summary, Co3Mo binary alloy nanoparticles nesting onto molybdenum oxide nanosheet arrays on Ni foam were synthesized via a facile topotactic conversion method as an efficient catalyst for hydrogen evolution reaction. The in situ topotactic conversion from CoMoO4 nanosheet arrays to molybdenum oxide nanosheet arrays followed by the formation of Co3Mo alloy nanoparticles can efficiently avoid the aggregation of nanoparticles and maintain the morphology of support substrate - the vertically aligned nanosheet arrays. By modulating the annealing time and temperature, the obtained catalyst achieved an outstanding HER performance in 1 M alkaline solution, reaching a current density of 10 mA cm-2 with an overpotential of 68 mV. At the same time, the catalyst exhibited excellent long-term stability. The superior performance of 500-4h could be attributed to four main aspects: 1) the remarkable intrinsic activity of Co3Mo alloy; 2) a large number of active sites, provided by alloy nanoparticles; 3) tantivy mass transfer of reactants (H2O) and products (H2/OH-) between the surface of catalyst and electrolyte, due to the particularity of nanosheet arrays structure; 4) good conductivity of binder-free catalyst and intact nanosheet arrays substrate. On the basis of DFT calculation, it is uncovered that the actual active sites of Co3Mo alloy are the Mo atoms. In all, this work designs a strategy for combining catalytic active species and hierarchical nanoscale materials, which could be extended to a lot of applications, and provides an alternative of non-precious metal catalyst for practical electrochemical water splitting. Associated Content Supporting Information Available: The additional characterizations; EDXmapping and EDX-spectrum of 350-2h; TGA of CoMoO4 hydrate powder; EDXmapping of 500-4h; XRD patterns of CMO@NF, CoMoO4 hydrate powder and RCMO powder; SEM images of CMO powder and RCMO powder; TEM and HRTEM images of 500-2h, 500-3h, 500-5h; XPS of 500-4h; Raman spectra of RCMO@NF and 500-4h; CV curves and corresponding Cdl and ESCA values of Ni Foam, CMO@NF and all RCMO@NF; LSV curves and corresponding Tafel slopes 19 ACS Paragon Plus Environment


normalizing to the ECSA; LSV curves before and after long-term stability test of 5004h; LSV curves of 500-4h before and after electrochemical oxidation; HER performance of previous reports compared with this work.

Author Information Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]. Notes The authors declare no competing ﬁnancial interest.

Acknowledgements J. F. Shen acknowledges the financial support from National Natural Science Foundation of China (51202034).

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Nesting Co3Mo Binary Alloy Nanoparticles onto Molybdenum Oxide

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