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Fabrication of Core-Sheath NiCoP@FePx Nanoarrays for Efficient Electrocatalytic Hydrogen Evolution Mengxia Li, Xiaoxiao Liu, and Xianluo Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01191 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Sustainable Chemistry & Engineering

Fabrication of Core-Sheath NiCoP@FePx Nanoarrays for Efficient Electrocatalytic Hydrogen Evolution Mengxia Li, Xiaoxiao Liu and Xianluo Hu*

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.

Corresponding Author: Professor Xianluo Hu Email: [email protected]

Keywords: nanoarrays, atomic layer deposition, hydrogen generation, electrocatalysis, surface modification

ACS Paragon Plus Environment

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Abstract: Construction of effective, stable, eco-friendly and earth-abundant electrocatalysts that substitute noble metals for hydrogen evolution reaction (HER) is essential for developing renewable and clean energy. Here we report a layer-by-layer assembly route to fabricate coresheath structured nanoarrays composed of nickel cobalt phosphide and iron phosphide (NCP@FePx) on a conductive Ni wire by atomic layer deposition (ALD). The as-fabricated multimetallic phosphide exhibits high electrocatalytic activity in both alkaline and acidic media, with small overpotentials of 82.5 and 96 mV at 10 mA cm–2, respectively. The self-supported electrode displays superior long-term stability and favorable durability. The excellent activity is originated from the unique core-sheath structure, and the synergistic effects of the FePx sheath and the NCP core contributed to the enhanced activity. The present ALD-assisted layer-by-layer strategy may provide a general route for the controlled fabrication of nanostructured electrocatalysts with tunable compositions and surfaces.


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Introduction With the increasing energy crisis and environmental problem, the development of renewable energy has become the global consensus.1,2 Hydrogen, as a renewable secondary energy carrier, with high combustion value, high energy density and pollution-free characteristics, has been widely regarded as the ideal green energy fuel.3,4 Among various method for hydrogen generation, electrocatalytic splitting of water by hydrogen evolution reaction (HER) is an attractive way because of high energy conversion efficiency, which is promising for the future commercial hydrogen production.5,6 Currently, precious metals like platinum (Pt), palladium (Pd) with good conductivity, high electrocatalytic activity and good electrochemical stability have been widely used as the-state-of-the-art electrocatalyst for HER.7 However, their widespread applications are subject to the high cost and limited resource in the earth’s crust.8 Therefore, it is highly desirable to develop efficient and inexpensive HER electrocatalysts.9 Until now, a variety of catalysts based on non-precious transition metals, such as carbides,10– 14

sulfides,15–18 nitrides,19,20 selenides,21,22 and phosphides23–25 have been investigated. Among

these materials, transition metal phosphides (TMPs) have been extensively utilized as the electrocatalysts for hydrodesulfurization (HDS) and both HDS and HER work in a similar way. 26 Recent studies have demonstrated that TMPs (e.g. Ni2P,27–29 CoP,30–32 FeP,33,34 MoP,35–37 WP38–40) exhibit good electrochemical performances toward HER because of the good conductivity, durability, hydrogenase-like catalytic mechanism and moderate hydrogen bonding energy. In order to further improve the catalytic activity of TMPs, various strategies such as doping heteroatoms, immobilization of electrocatalysts on conductive substrates or nanostructuring have been developed.41,42 Recently, some researchers have demonstrated that alloying transition metal ions in monophosphides to form bimetal phosphides could improve the intrinsic electrocatalytic activity


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toward HER.43 Hu and co-workers have developed nanowire-structure Ni-Co bimetallic phosphides on carbon cloth, featuring with novel HER catalytic performance.44 Binder-free electrocatalysts based on constructing the active materials on conductive substrates can efficiently avoid the reduced catalytic activity caused by using polymer binders.45,46 Moreover, It has been clarified that CoP nanowires exhibit the best electrocatalytic activity compared with the corresponding nanosheets and nanoparticles by exploring the dependence of their morphology on the electrocatalytic HER performance.47 Besides, the core-sheath nanostructures can efficiently improve the electrocatalytic performance of catalysts as a result of the synergistic effect generated by the core and sheath constituents.48 However, the electrocatalysts based on a core-sheath nanoarray structure for HER are seldom investigated because it is difficult to control the uniformity and thickness of the coating layer. Whereas, atomic layer deposition (ALD), as a promising deposition technique for manufacturing complex three-dimensional (3D) nanostructured materials by in situ self-controlled surface-chemistry reactions can solve the above problem perfectly.49,50 To date, ALD has been applied to design catalysts with a well-controlled, conformal and homogeneous coating layer.51,52 This unique strategy can improve the activity and stability of the catalysts. Also, it can significantly accelerate the illustration of the reaction mechanisms and structure-property relationships.33 Here we report on a layer-by-layer route to fabricate nickel cobalt phosphide encapsulated with iron phosphide (donated as NCP@FePx) nanoarrays that are grown on a nickel wire by ALD technique, serving as an active and robust electrocatalyst for hydrogen evolution reaction in acid and alkaline solutions. The multimetallic phosphide architecture is constructed by a facile and controllable method that includes three steps: firstly, Ni-Co precursor nanowire arrays are fabricated on the Ni wires by a hydrothermal process. Then, the outer sheath of the Fe2O3 layer is


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deposited on the surface by ALD. Finally, the core-sheath phosphides are obtained via in situ phosphorization. The unique electrode made of core-sheath NCP@FePx nanoarrays exhibits highly efficient catalytic activity and stability towards HER at both under acid and alkaline conditions. Notably, in alkaline (1 M KOH) and acid (0.5 M H2SO4) electrolytes, the multimetallic phosphide electrodes both show fascinating HER catalytic activities that are similar to the Pt sheet (1×1.5 cm2), with low onset potentials and small over-potentials of just 82.5, 96.1 mV at 10 mA cm–2 in alkaline and acid electrolytes, respectively. Meanwhile, the core-sheath NCP@FePx nanoarrays also exhibit superior long-term stability and favorable durability after 2000, 4000-time cyclic voltammetric sweeping or 20-h successive electrolyzing. Results and Discussion The fabrication procedure for the NCP@FePx electrode including three sequential steps as schematically shown in Figure 1 (see details in Experimental Section). For the first step, the precursor nanowire arrays of nickel cobalt based oxide (NCO) are directly grown on the Ni wires by a hydrothermal process. Ammonium fluoride provides anions to incorporate with nickel and cobalt ions, and urea hydrolyzes at high temperatures to produce hydroxide ions for the final formation of nickel cobalt hydroxide.53 The molar ratio of cobalt to nickel in the precursor is consistent with that of the reactants, since the cobalt hydroxide and nickel hydroxide possess the similar solubility at the same temperature. Subsequently, the nickel cobalt oxide@iron oxide (NCO@Fe2O3) nanowire arrays were obtained by ALD. Ferrocene (FeCp2), O3 were used as the Fe and O precursors, respectively. There are four continuous steps for the whole ALD process: FeCp2 dose - N2 purge- O2 dose- N2 purge. Especially, the thickness of the Fe2O3 coating layer is 0.017 nm per each deposition cycle. Thus, the NCO@Fe2O3 electrode with a coating layer of 6 nm can be achieved after 352 cycles. Finally, the NCO@Fe2O3 precursor was transformed into


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multimetallic phosphide NCP@FePx by thermal treatment at 350 °C in Ar atmosphere for 2 h using NaH2PO2 as the phosphor source. NaH2PO2 was decomposed gradually at 270 °C, generating the PH3 gas. The oxide layer reacted with PH3 and transformed into phosphide without destroying the morphology and structure of the precursor NCO@Fe2O3.

Figure 1. Schematic illustration of the fabrication of NCP@FePx nanowire arrays on the Ni wires. The morphology and structure of the products are characterized by scanning electron microscopy (SEM). It is clearly shown that the nanowire arrays are uniformly covered on the surface of the Ni wires in macro-scale (Figure 2a, inset), compared with the surface of the bare nickel wire (Figure S1). The Ni-Co based precursor nanowire arrays have an average diameter of about 100 nm and lengths up to 5 μm (Figure 2a). It is worth noting that the morphology and structure of the NCO@Fe2O3 nanowires after ALD are nearly identical to the former one due to the high conformity and homogeneity of the ALD technique (Figure 2b). After phosphorization, the similar morphology of NCP@FePx nanowires can still be well retained (Figure 2c and 2d), compared with the primary NCO and NCO@Fe2O3 precursors. The core-sheath structure can be illustrated explicitly by transmission electron microscopy (TEM). As shown in Figure 3a, the precursor of the Ni-Co-based nanowire is naked without a coating layer before ALD (Figure S2).


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However, there exists a homogeneous and coherent coating layer (the thickness is about 6–7 nm) on the surface of the Ni-Co-based nanowire after ALD (Figure 3b). Interestingly, the thickness of the outer sheath of NCP@FePx has been increased to 17 nm (Figure 3c), which is much thicker than that before phosphorization. This increased thickness could result from the variation in the layer density and the oxidation of superficial phosphide. Such an amorphous sheath could protect the inner phosphide from etching during the catalytic process. Besides, the internal core of the NCP@FePx nanowire is stacked with lots of uniform nanoparticles, which is obviously different from the material before phosphorization (Figure 3c, inset). The obscure circles in the selectedarea electro diffraction (SAED) pattern of NCP@FePx (Figure 3d) imply the poor crystallinity of the NCP@FePx nanowire after encapsulation, corresponding to the crystal planes (101), (200), (102), (210) of nickel cobalt phosphide, respectively. Nevertheless, from the SAED patterns of NCO and NCO@Fe2O3, the bright spots in Figure S3a indicate the high crystallinity of NCO. After coating a Fe2O3 layer on the surface of NCO by ALD, the spots become blurry and aligned as circles (Figure S3b), indicating that the coating layer is amorphous. The high-resolution TEM (HRTEM) images indicate that the inter-planar distances of 0.278 and 0.187 nm correspond to the (101), (210) planes of hexagonal NiCoP (Figure 3e).


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Figure 2. SEM images of (a) NCO, (b) NCO@Fe2O3, (c,d) NCP@FePx.

The crystal structure was confirmed by powder X-ray diffraction (XRD) (Figure S4). The peaks at 44.5°, 51.8°and 76.4°correspond to the (111), (200), (220) planes of the Ni wire (PDF No.65-2865), respectively, while the peak at 41.0°is consistent with the (111) plane of hexagonal NiCoP (PDF No.71-2336). As shown in the energy dispersive X-ray spectra (EDS) (Figure S5), the atomic ratio of Fe:Ni:Co is close to 0.8:1:3 and P is much larger than them, which reveals that the outer sheath and inner core have been converted into phosphides. In order to further explore the structure of NCP@FePx, EDS mapping (Figure 3f and 3g) under the TEM mode was used to demonstrate the elemental composition and distribution in the NCP@FePx nanowire. It is obvious that Co and Ni elements are distributed homogeneously in the inner part of the nanowires and Fe elements are well coated on the outside of the Ni-Co-based phosphide. Besides, the phosphor


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elements exist in the whole part of the nanoarray structure, suggesting that the phosphorization process is both successfully operated in the inner and outer layers of the nanowires. The presence of oxygen in the material is originated from the contact with air. The line scan result describes the distribution of each element at different positions of an individual nanowire. Figure 3h clearly displays that the concentration of Fe is initially increased, and deceased in the external sheath, but almost disappears at the interior part of the NCP@FePx nanoarray. In contrast, the concentrations of Ni and Co begin to increase until the position is at the internal part, and the phosphor is always in the steady state. Those results demonstrate the favorable encapsulation of the iron phosphides and the thickness of the outer sheath layer, which is consistent with the TEM and EDS results (Figure 3).

Figure 3. Structural characterization of nanowire arrays. (a–c) TEM images of NCO, NCO@Fe2O3, and NCP@FePx. (d) SAED pattern of NCP@ FePx. (e) HRTEM image of NCP@FePx. (f,g) EDS elemental mapping of Ni, Co, Fe, P in the NCP@FePx. (h) Line scan of NCP@ FePx.


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X-ray photoelectron spectroscopy (XPS) is utilized to investigate the surface chemical composition and electronic properties. The survey spectrum of the precursor NCO (Figure S6a) demonstrates the existence of Ni, Co, and O elements clearly. After coating the Fe2O3 atomic layers on NCO by ALD, there exist Fe peaks in the survey spectrum of NCO@Fe2O3 (Figure S6b). That indicates that the Fe2O3 atomic layer has been coated on the surface of NCO successfully, which agrees well with the TEM images of NCO@Fe2O3 (Figure 3b). The survey spectrum of NCP@FePx (Figure S6c) indicates the presence of Ni, Co, Fe, and P elements. Figure 4a, 4b and 4c show the high-resolution XPS spectra of Ni, Co, Fe 2p. For the Ni 2p spectrum of NCO nanowires, two main peaks at 856.1 and 873.6 eV correspond to the Ni 2p3/2 (Ni2+) and Ni 2p1/2 (Ni3+), respectively, while the one at 861.6 eV is related to the shakeup satellite peak. The Co 2p spectrum of NCO was well fitted with two spin-orbit doublets at 781.2 and 797.3 eV, and they can be assigned to the characteristic binding energies (BEs) for Co2+ and Co3+, and two shakeup satellites. The Ni and Co elemental characteristic peaks of NCO@Fe2O3 and NCP@ FePx present a large decrease and slight shift after the ALD process, which also proves that the coating Fe2O3 layer has been successfully grown on the surface of NCO and NCP. As for the Fe 2p spectrum of NCO@Fe2O3 (Figure 4d), there are three peaks, Fe 2p2/3, Fe 2p1/2, and one satellite, centering at 711, 720, and 725 eV, respectively, in accordance with the electronic state of Fe2O3. Raman spectra of NCO@Fe2O3 are shown in the Figure S7. The peaks at 340, 500, 640 cm-1 are consistent with the previous reports on -Fe2O3.54 In the Fe 2p spectrum (Figure S6d) of NCP@FePx, the peak at 712.5 eV can be assigned to oxidized Fe species arising from superficial oxidation of FePx exposed to air, while the peak at 725.4 eV is assigned to the Fe 2p1/2 (Fe3+). The characteristic peaks of Fe2O3 have been changed after phosphorization, which can also prove the transformation from


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oxide to phosphide. Figure 4d shows the high-resolution XPS spectrum for P 2p in NCP@FePx, in which the peaks at 135.8 and 130.2 eV are attributed to P 2p1/2 and P 2p3/2, respectively. The minor one matches well to the P3− and the predominate one is attributed to PO43−, resulting from the exposure to oxygen in air. The element valence state of uncoated NCP has also been given in the Figure S8.

Figure 4. (a–c) XPS spectra of the Ni, Co, Fe 2p of NCO (blue curves), NCO@Fe2O3 (red curves), NCP@FePx (black curves). (d) High-resolution XPS spectra of P 2p of NCP@FePx.

The electrocatalytic HER performance of the NCP@FePx nanowire arrays has been evaluated in 1 M KOH and 0.5 M H2SO4 electrolytes by using a typical three-electrode setup with a scan rate of 10 mV s–1, respectively. Commercial Pt sheet (1×1.5 cm2), NCO, NCO@Fe2O3, NCP (all loaded on the Ni wire with the same surface area) and bare Ni wire were also examined under the


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same condition for comparison. Figure 5a and 5d exhibit the linear sweep voltammetry (LSV) curves (vs. RHE) of the NCP@FePx nanowire and other control samples in alkaline and acid media. It is shown that the bare Ni wires give negligible catalytic activity in both alkaline and acid media. This phenomenon suggests that the measured electrocatalytic HER activities are mainly originated from the multimetallic phosphides rather than the Ni wire substrate. As shown in Figure 5a, the Pt sheet presents the most excellent electrocatalytic HER activity with a small onset potential and overpotentials of only 20.5 mV at 10 mA cm–2 (Figure 6a), while the precursors of NCO, NCO@Fe2O3 and the bare Ni wire show exactly poor electrocatalytic activity in 1 M KOH solutions. However, the phosphides of NCP and core-sheath structure NCP@FePx both have a large improvement of catalytic activity toward HER after phosphorization. Besides, the NCP@FePx catalyst performs Pt-like catalytic activity toward HER. The NCP@FePx electrode only needs 82.5 mV but the NCP electrode needs 177.5 mV to drive 10 mA cm–2 (Figure 6a). The overpotentials of the core-sheath NCP@FePx catalyst at 20 mA cm–2 and 30 mA cm–2 are about 120 and 137.5 mV. In Figure 5d, the LSV curves tested in the electrolyte of 0.5 M H2SO4 are in well consistent with those curves in Figure 5a. For the commercial Pt sheet, the overpotential at 10 mA cm–2 is only 40 mV. The core-sheath NCP@FePx electrode exhibits highly efficient HER activity close to that of the Pt sheet and only needs overpotential of 96 mV to obtain 10 mA cm–2. However, the uncoated NCP needs 177.6 mV which is higher than that of the coated NCP@FePx. For comparison, the NCO, NCO@Fe2O3 and the bare Ni wire have been explored, but they display undesirable HER activity which is similar to that in 1 M KOH solutions. As seen from Figure 6a, the overpotential of NCP@FePx at 10 mA cm–2 in alkaline is lower than that in acidic media, indicating that NCP@FePx exhibits more highly catalytic activity and affinity toward the alkaline electrolyte.


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The Tafel slope is generally applied to identify the catalytic mechanism of HER. There exist three primitive steps in the HER process: (1) the Volmer reaction (electrochemical hydrogen adsorption): H3O+ + e− → Hads + H2O; (2) the Heyrovsky reaction (electrochemical desorption): H3O+ + e− + Hads→ H2 + H2O (39.4 mV dec–1); (3) the Tafel reaction (chemical desorption): Hads + Hads → H2 (29.6 mV dec–1).55–57 Figure 5b displays the Tafel plots of different samples in 1 M KOH. The Pt sheet displays a Tafel slopes of 41 mV dec–1, which is corresponding to the Volmer– Heyrovsky mechanism. The NCP@FePx exhibits a Tafel slope of 69.1 mV dec–1, suggesting that the HER catalytic process is dominated by the Volmer–Tafel mechanism. The Ni wire, NCO, NCP and NCO@Fe2O3 possess Tafel slopes of 102.1, 91.5, 74.49 and 182.7 mV dec–1. These catalysts follow the similar Volmer–Tafel mechanisms except NCO@Fe2O3. While for the acidic environment, as shown in Figure 5e, the Pt sheet follows the Volmer–Heyrovsky mechanism with a Tafel slope of 30.4 mV dec–1. The NCP@FePx, NCP, NCO exhibit Tafel slopes of 50.1, 68.7, 99.56 mV dec–1, respectively. Such Tafel slopes suggest that those catalysts follow the same Volmer–Tafel mechanisms. The NCO@Fe2O3 and Ni wire have Tafel slopes of 145, 156.5 mV dec–1, which belong to the more complicated mechanisms. As shown Figure S9, the NCP@FePx establishes the homogeneous catalytic mechanisms by comparing their Tafel slopes in 1 M KOH and 0.5 M H2SO4 electrolytes. The long-term stability and durability at the constant current density are also curial for an ideal HER electrocatalyst. To this end, we have examined the stability (see Figure 5c and 5f) of the NCP@FePx electrode by cyclic voltammetry at an accelerated scan rate of 100 mV s–1 in alkaline and acidic media. After the degradation test for 2000 and 4000 cycles, the final polarization curves exhibit a little attenuation compared with the initial ones. The chronoamperometry analyses (Figure S10) of the NCP@FePx electrodes indicate that they can


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maintain the current densities after 20 h at the fixed overpotentials. The NCP@FePx electrode can still preserve 85.4% current densities in 1 M KOH while only 70% in 0.5 M H2SO4 over 20 h. Therefore, the NCP@FePx electrocatalyst is more corrosion-resistant and stable in the alkaline electrolyte, in contrast to the acidic electrolyte.

Figure 5. Linear sweep polarization curves of NCO, NCO@Fe2O3, NCP, NCP@FePx, Pt sheet, Ni wire for HER in (a) 1 M KOH and (d) 0.5 M H2SO4 with a scan rate of 10 mV s–1. The corresponding Tafel plots of catalysts in (b) 1 M KOH and (e) 0.5 M H2SO4. Stability test of NCP@FePx after 2000, 4000 cycles at a scan rate of 100 mV s–1 in (c) 1 M KOH and (f) 0.5 M H2SO4.

Electrochemical impedance spectroscopy (EIS) analysis was employed to reveal the HER kinetics of catalysts in alkaline media. From Figure 6b, the Nyquist plots imply that NCP and NCP@FePx have much smaller charge transfer resistance (Rct) than NCO and NCO@Fe2O3. In particular, NCP@FePx has the minimum Rct after the ALD process, indicating that ALD can


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provide an effective way to improve the Faradaic process and HER kinetics. The effective surface area (ECSA) is proportional to the double-layer capacitance (Cdl) which can be determined by CVs at different scan rates. As shown in Figure 6c and d, the NCP@FePx nanowires exhibit a much higher Cdl of 46.9 mF cm–2 than NCP of 20.9 mF cm–2. It obviously indicates that NCP@ FePx can expose more active sites compared with uncoated NCP. The CVs of NCP, NCO and NCO@Fe2O3 have also been presented in Figure S11 and S12. On the whole, the lower Rct and larger electrochemical surface area contribute to the higher HER activity of NCP@FePx. The smaller Rct would be originated from the transfer of electron from inner core to the amorphous sheath.58

Figure 6. Comparison of (a) overpotential at 10 mA cm–2 between Pt, NCP@FePx, NCP in alkaline and acid electrolytes. (b) EIS Nyquist plots of NCO, NCO@Fe2O3, NCP, NCP@FePx at overpotential of 200 mV. (c) Cyclic voltammograms of NCP@FePx within the range of no faradaic reactions. (d) Plots of variation of double-layer charging currents at 0.15 V as a function of scan rate for NCP and NCP@FePx.


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In order to investigate the ALD effect on the improved HER activity, we have coated a series of Fe2O3 layers with tunable thickness on the surface of Ni-Co based precursors. The coating layers of 4, 6, and 10 nm were achieved by adjusting the ALD cycles. For the purpose of selecting the preferable core-sheath structure, we have examined the HER activity of these electrode with different thickness before and after phosphorization. As shown in Figure S13, the NCP@FePx electrode with a 6-nm coating layer exhibits the best HER catalytic activity among these three electrodes after phosphorization. The surface structure and properties of the products play a crucial key in catalytic activity. For the HER process, electrons on the surface of the catalyst is important for the formation of hydrogen by combining with the absorbed H+. The electron density and electronic potential distribution at the outside sheath can be modulated by the metal core due to the electron penetration.59 This bimetal phosphide encapsulated metal phosphide structure is similar to the Pd@Pt core-sheath structure, in which the electronic transport from the higher work function of the Pd core to the lower work function of the Pt sheath.58 Then the accumulation of electrons on the surface can promote the formation of hydrogen.60 Furthermore, the thickness of outside layers has a substantial influence on the HER activity.61 Therefore, in the NCP@FePx core-sheath nanostructure, the electronic structure of the outermost amorphous layer can be regulated by the inner core because of the electron transport, and there exist more defects in the amorphous Fe2O3 sheath which could further lower the gas desorption energy barrier for higher HER activity.62 This has been proved by the LSV curves of FePx and NCP@FePx in Figure S14. It is evident that the HER performance of iron phosphide without NCP is much inferior to the NCP and NCP@FeP x. However, the NCO@Fe2O3 electrodes with coating layers of different thickness almost show little HER activity in the potential window. That may be because transition metal oxides are unsuitable


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substitutes for HER electrocatalysts but much more favorable to oxygen evolution reaction (OER). Besides, the coverage of Fe2O3 layers block the exposure of active sites in NCO for HER, and NCO exhibits superior HER catalytic performance than that of Fe2O3. Thus, controlling the appropriate thickness of the coating layer also plays an important role in the HER performance of catalysts. Furthermore, it should be noted that the multi-metal synergistic function of the interface between two different phases may also play an important role in enhancing the HER performance of catalysts. Previous studies have confirmed that the electrocatalytic performance of core-sheath nanostructures can be improved owing to the synergistic effects between the core and sheath components.48 Therefore, the hybrid structure made of nickel cobalt phosphide and iron phosphide may also have the similar synergistic effects for the HER catalysis. The formation of bimetal phosphide-metal phosphide nanointerface can generate strong electron transfer between Ni, Co and Fe through the intermediate phosphor atoms bonded to these metals,63 leading to a low hydrogen adsorption and the improvement of HER electrocatalytic performance. Moreover, the 3D interconnected network nanowire can offer larger open spaces and surface areas which can facilitate the diffusion of electrolytes and hydrogen gas bubbles evolving. The direct adhesion of nanowires to the conductive Ni wire can also provide pathways for efficient charge transfer during the catalytic process. Conclusions In summary, we have fabricated nanoarrays comprising a sheath of amorphous iron phosphide that encapsulates a core of homogenous nickel cobalt phosphide through an ALD-based layer-bylayer assembly route. The resulting NCP@FePx electrode exhibits superior HER catalytic activity both in acidic and alkaline electrolytes. Coating an iron-containing layer on nickel cobalt


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phosphide through ALD has an important effect on the HER performance. In general, the improved HER activity of NCP@FePx mainly benefits from the synergistic effects of the core@sheath structure, large surface area, and facile diffusion of the reactants and products. This study not only paves the way towards the development of multimetallic, high-performance and cost-effective HER electrocatalysts with a core-sheath structure that can be used for large-scale hydrogen production, but also demonstrates the use of the ALD technique for controllably designing nanostructured materials for energy and environmental applications. Experimental section Materials: Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), cobaltous nitrate (Co(NO3)2·6H2O), ammonium fluoride (NH4F), urea (CO(NH2)2) are used for the preparation of the precursors. Ferrocene (C10H10Fe) is the iron source for preparing the coating layer of Fe2O3 by ALD. NaH2PO2 is used for phosphorization. All chemical reagents used in the experiments are of analytical grade without further purification and purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (China). Synthesis of the NCO nanoarrays: In order to remove the organics and eliminate the thin surface oxide layer. Ni wires (Ф 0.02 cm × 6 cm) were firstly cleaned by acetone, 1 M (dilute) HCl, deionized (DI) water and ethanol for 15 min before use, respectively. In the typical synthesis, 25.5 mg of Ni(NO3)2·6H2O, 79.0 mg of Co(NO3)2·6H2O, 53.75 mg of NH4F and 135 mg of urea were dissolved in 30-mL deionized (DI) water under vigorous magnetic stirring for 30 min to form a transparent pink solution at room temperature. Then the uniform solution was transferred to a 50 ml Teflon-lined stainless autoclave in which a Ni wire has been immersed and reacted together at


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120 °C for 6 h. After cooling down naturally, the product was taken out and washed with ethanol and water for several times. Synthesis of the NCO@Fe2O3 nanoarrays by ALD: The obtained NCO nanowire arrays were coated with an atomic layer of Fe2O3 (thickness in 6 nm) by ALD, ferrocene (FeCp2) and O3 were used as the Fe and O precursors, respectively. The temperature of the FeCp2 precursor was maintained at 150 °C and the subsequent deposition process was conducted at 250 °C. The thickness for the Fe2O3 coating layer can be determined by controlling the number of the ALD Fe2O3 cycles. Three different thickness of coating layers (4, 6, and 10 nm) has been obtained by 235, 352, and 588 ALD cycles, respectively. Synthesis of NCP@FePx nanoarrays: In the annealing process, the Ni wire was placed in the downstream side and 0.28 g of NaH2PO2 was placed in the upstream side of the porcelain boat. Subsequently, the samples were heated at 350 °C for 120 min under Ar atmosphere (50 sccm). For comparison, the uncoated nickel cobalt phosphide (NCP) was also synthesized under the same phosphorization condition, except for only adding the uncoated NCO precursors. Synthesis of FePx by ALD: The Ni wires without NCO nanowire arrays were coated with an atomic layer of Fe2O3 (thickness in 6 nm) by ALD as illustrated before. Then, the FePx product was obtained by the phosphorization (the condition is similar to NCP@ FePx). Structural characterizations: The morphology and microstructure of the products were characterized by field-emission scanning electron microscope (FE-SEM, FEI Sirion 200). Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, selected-area electron diffraction (SAED), and energy disperse X-ray spectrum (EDS) were obtained by the field-emission transmission electron microscope (Tecnai G2 F30, Holland)


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equipped with an energy spectrometer system. For TEM characterizations, the Ni-supported samples were ultrasonicated in ethanol, and the TEM specimen was prepared by using the supernatant. The powder X-ray diffraction (XRD) patterns were collected on a PANalytical MultiPurpose Diffractometer with Cu Kα1 irradiation (λ = 1.5406 Å) operating at 40 kV and 40 mA for crystallographic characterization. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VG MultiLab 2000 system (Thermo VG Scientific) using a monochromatic Al Kα X-ray source for surface and valence states analysis. Raman measurements were performed with a Raman Microscope employing a laser with an excitation wavelength of 325 nm. Electrochemical Measurements: All the HER electrochemical measurements were evaluated in a standard three-electrode glass cell system connected to a CHI 760E Electrochemical workstation (CHI Instruments, Shanghai Chenhua Instrument Corp., China) at 25 °C. A saturated calomel electrode (SCE) was used as the reference electrode and a graphite rod was employed as the counter electrode, respectively. The electrodes made of NCO/NW, NCP/NW, NCP@FePx /NW, NCO@Fe2O3/NW were performed as the working electrode. Before the electrochemical measurements, we clamped the nickel wire with a platinum electrode holder as the current collector and the platinum electrode holder was not immersed in electrolytes. 0.5 M H2SO4 and 1 M KOH solutions were used as the electrolytes, and especially all the electrolytes were saturated by hydrogen before and during the experiments in which the flow rate of H2 is 20 sccm. For each electrochemical test, the volume of electrolytes is 20 mL. Linear sweep voltammetry was performed at a scan rate of 10 mV s–1. A platinum sheet (1 × 1.5 cm2) electrode was employed as the benchmark in acid and basic media for comparison. The area of all the working electrodes is about 0.1884 cm2. All the polarization curves were obtained with IR-corrected/ compensation. All


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the potentials reported in this work were converted reversible hydrogen electrode (RHE) according to the following equation (Equation 1): E (versus RHE) = E (versus SCE) + 0.2415 V + 0.059 × pH (1) All the presented data are the stable-state ones after several cycles’ activation. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of Ni wire, TEM images of NCO and NCO@Fe2O3, XRD patterns of NCP@FePx, Raman spectra of NCO@Fe2O3, EDS spectra of NCP@FePx, XPS spectra of catalysts, cyclic voltammograms, stability curves, Tafel plots of catalysts, and HER performances catalysts (PDF) Corresponding Author Professor Xianluo Hu, Email: [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. Acknowledgments This work is supported by National Natural Science Foundation of China (Nos. 51772116, 51522205, and 51472098), and the fund for Academic Frontier Youth Team of HUST. The authors thank Analytical and Testing Center of HUST for XRD, SEM, and TEM measurements.

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shell system toward synergetic electrocatalytic water splitting. Adv. Mater. 2015, 27, 4752-4759, DOI: 10.1002/adma.201501969.


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For Table of Contents Use Only

Core-sheath NiCoP@FePx nanoarrays are designed and synthesized as an efficient catalyst for electrocatalytic hydrogen evolution.


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Fabrication of Core-Sheath NiCoP@FePx ... - ACS Publications

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