Construction of hierarchical Ni–Co–P hollow nanobricks with oriented nanosheets for efficient overall water splitting

Enlai Hu a, Yafei Feng a, Jianwei Nai b, Dian Zhao a, Yong Hu *a and Xiong Wen (David) Lou *b
aKey Laboratory of the Ministry of Education for Advanced Catalysis Materials, Department of Chemistry, Zhejiang Normal University, Jinhua 321004, P. R. China. E-mail: yonghu@zjnu.edu.cn
bSchool of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore. E-mail: xwlou@ntu.edu.sg; Web: http://www.ntu.edu.sg/home/xwlou/

Received 9th January 2018 , Accepted 12th March 2018

First published on 12th March 2018


Complex nano-architectures with ordered two-dimensional (2D) building blocks are a class of promising electrocatalysts for different electrochemical technologies. In this work, a novel template-engaged strategy followed by sequential etching and phosphorization treatments is demonstrated to fabricate open and hierarchical Ni–Co–P hollow nanobricks (HNBs) via the assembly of oriented 2D nanosheets. Benefiting from the unique nano-architectures with large electrolyte-accessible surface and abundant mass diffusion pathways, the as-prepared Ni–Co–P HNBs exhibit high electrocatalytic activity, which affords the current density of 10 mA cm−2 at low overpotentials of 270 mV and 107 mV for oxygen and hydrogen evolution reactions respectively, and excellent stability in an alkaline medium. Remarkably, when used as both the anode and cathode, a low cell voltage of 1.62 V is required to reach the current density of 10 mA cm−2, making the Ni–Co–P HNBs an efficient bifunctional electrocatalyst for overall water splitting.



Broader context

Hydrogen is a clean, efficient and renewable energy carrier. Electrochemical water splitting, in which the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are involved, is an environmentally-benign and economic route to generate hydrogen on a large scale. At present, considerable efforts are being devoted to fabricating robust earth-abundant catalysts as alternatives to the commercial catalysts mainly based on noble metals. Particularly, cost-efficient bifunctional catalysts with high activity for a full water splitting cell in the same electrolyte are of key importance, yet still under development. Here novel hollow nanobricks assembled from oriented Ni–Co–P nanosheets are constructed for highly efficient overall water splitting. Benefiting from the open and hierarchical structure, the catalyst not only exhibits outstanding OER performance but also decent HER performance in an alkaline electrolyte, thus functioning as a versatile electrode for efficient overall water splitting.

1. Introduction

The sustainable production of hydrogen through electrocatalytic water splitting is widely regarded as a promising approach for gaining renewable and clean energy.1 Efficient electrocatalysts are always required to accelerate the sluggish anodic oxygen evolution reaction (OER) and the fairly facile cathodic hydrogen evolution reaction (HER).2 Currently, the scarcity and high cost have been restricting the large-scale application of the noble-metal-based catalysts (e.g., RuO2/IrO2 for OER and Pt for HER). Therefore, earth-abundant alternatives have been extensively developed for the electrocatalytic water splitting, such as transition-metal oxides,3–7 (oxy)hydroxides,8–12 phosphates,13 and polymeric carbon nitride14 for OER, whereas chalcogenides,15–17 phosphides,18,19 carbides,20–22 and metal-free hybrids23 for the HER. Although significant progress has been achieved in this area, very few of these catalysts reported to date are capable of effectively catalyzing both the HER and OER in the same pH range.24–26 This is because active OER catalysts generally work well in a neutral or basic medium, whereas most HER catalysts perform better in an acidic medium. Thus, the searching for cost-efficient bifunctional catalysts with high activity for a full water splitting cell in the same electrolyte is of key importance, yet remains a big challenge.27 Recently, transition-metal phosphides are emerging as a class of promising bifunctional catalysts for the overall water splitting, as the P species could trap protons by acting as a base for HER, while facilitating the formation of peroxide intermediate for the OER.28 Nevertheless, the development of highly efficient phosphide catalysts is still insufficient as it requires some rational design of advantageous complex nanostructures.

Two-dimensional (2D) nanomaterials, e.g., nanosheets, have shown high specific surface areas and exotic electronic properties to be a class of promising catalysts for water splitting.9,10,15,29–31 Moreover, the intrinsic flexibility of 2D nanosheets makes them appealing building blocks for construction of various complex nanostructures for realizing improved performance or even generating novel properties.32–36 For example, an assembled graphene scroll can not only retain the same excellent conductivity as a graphene nanosheet but also exhibit superior mechanical properties.37 Among the strategies for the assembly of 2D nanosheets, the template-mediated approach is a fascinating one that could guide the assembly to form unique architectures that might hardly be obtained via a “self-assembly”, e.g., one-dimensional (1D) nanotubes38 and three-dimensional (3D) nanoboxes.39 However, this kind of methods generally results in assemblies constructed with randomly interlaced nanosheets. The fabrication of well-ordered or oriented nanosheets on templating structures/substrates is less reported due to the daunting synthetic challenge,40,41 let alone on anisotropic nanoparticles.

Herein, we present a template-assisted strategy to assemble oriented Ni–Co precursor 2D nanosheets on Ag2WO4 3D anisotropic cuboid particles. By subsequent heating, etching and phosphorization treatments, Ag2WO4 cuboid particles are selectively removed, while the Ni–Co precursor is converted to nickel–cobalt oxide (Ni–Co–O) and then phosphide (Ni–Co–P). Finally, open and hierarchical Ni–Co–P hollow nanobricks (HNBs) constructed with nanosheets can be fabricated. Benefiting from the structural merits including large exposed surface, abundant mass diffusion pathways, and the unique hierarchical and hollow structures, the as-prepared Ni–Co–P HNBs exhibit significantly enhanced electrocatalytic performance for overall water splitting in comparison with Ni–Co–P nanosheets.

2. Results and discussion

The synthetic scheme is schematically illustrated in Fig. 1. Specifically, uniform cuboid Ag2WO4 solid nanobricks (SNBs) are employed as the templates, which are synthesized via a rapid microwave-assisted method (see Experimental procedures). In step I, the as-prepared Ag2WO4 SNBs are dispersed in a mixed solution containing ethanol and distilled water. Then certain amounts of Co(NO3)2, Ni(NO3)2, polyvinylpyrrolidone (PVP) and urea are added into the mixed solution before putting it in an oil bath at 90 °C for 10 h. As a result, Ag2WO4@Ni–Co precursor core–shell SNBs are obtained. These Ag2WO4@Ni–Co precursor core–shell nanostructures are then annealed in air at 300 °C for 2 h, followed by an etching process with nitric acid and ammonia sequentially for 10 min to remove the Ag2WO4 cores (step II). The use of nitric acid here is to promote the decomposition of the Ag2WO4 cores. Ammonia solution is then used to coordinate with released Ag+ species to form soluble [Ag(NH3)2]+ complex. As a result, the open and hierarchical Ni–Co–O HNBs assembled from nanosheets are obtained. Finally, these Ni–Co–O HNBs can be converted into the Ni–Co–P HNBs without too much structural change by reacting with NaH2PO2 as the phosphor source at 300 °C for 2 h in a nitrogen atmosphere (step III).
image file: c8ee00076j-f1.tif
Fig. 1 Schematic illustration of the construction of hierarchical Ni–Co–P hollow nanobricks.

The formation process of the Ni–Co–P HNBs is monitored by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The morphology and structure of the as-prepared Ag2WO4 SNBs are revealed in Fig. 2a and b. It can be seen that the product consists of cuboid solid particles with smooth surfaces and a highly uniform size, i.e., about 750 nm in length and about 270 nm in width/height. The powder X-ray diffraction (XRD) analysis (Fig. S1, ESI) confirms that the SNBs can be assigned to orthorhombic α-Ag2WO4 (JCPDS card No. 34-0061).42 After a chemical reaction as described above in step I (Fig. 1), the Ag2WO4 SNBs transform to the Ag2WO4@Ni–Co precursor core–shell nanostructures, as shown in Fig. 2c. Remarkably, the shell structures are constructed by the stacking of nanosheets on the four rectangular side surfaces of the templates. This unique oriented arrangement of the nanosheets might be originated from an epitaxial growth.40,41 The TEM image in Fig. 2d further confirms that the Ni–Co precursor nanosheets possess nearly vertical growth on the side surfaces of the Ag2WO4 SNBs, but leaving the top/bottom surfaces of the Ag2WO4 SNBs uncovered. Control experiments show that the reaction system with higher amounts of PVP can produce shorter Ag2WO4 particles but with similar size in width (Fig. S2, ESI). This suggests that PVP molecules might preferentially adsorb on the top and bottom surfaces of Ag2WO4 templates to impede the growth along this direction. In other words, these two PVP-blocked surfaces would hinder the nucleation of Ni–Co precursor 2D nanosheets. As a result, growth of Ni–Co precursor 2D nanosheets can only occur on the four exposed side surfaces of the Ag2WO4 templates. For comparison, Ni–Co precursor nanosheets are formed desultorily without the Ag2WO4 SNBs templates, as revealed by the FESEM observation (Fig. S3a and b, ESI). After the thermal and consequent etching treatments (step II in Fig. 1), the product becomes Ni–Co oxide (Ni–Co–O), similar to the spinal phase NiCo2O4 (JCPDS card No. 20-0781)43 as demonstrated by the XRD result (Fig. S4a, ESI). No noticeable XRD signals of Ag2WO4 can be detected, indicating the complete removal of the templates. In Fig. 2e, open and hierarchical Ni–Co–O HNBs with gaps among the stacking nanosheets can be clearly observed. Moreover, the entirety of the Ni–Co–O HNBs and their cavities well inherit the cuboid shape of the templates. The hollow interiors presented in the TEM image (Fig. 2f) further elucidate that the Ag2WO4 templates are completely dissolved. It is noteworthy that the etching process must be performed after the formation of the Ni–Co–O shell since the Ni–Co precursor is not stable in the nitric acid and ammonia. Similarly, Ni–Co–O nanosheets can be obtained from annealing the Ni–Co precursor nanosheets (Fig. S3c, d, and S4b, ESI).


image file: c8ee00076j-f2.tif
Fig. 2 FESEM and corresponding TEM images of the (a and b) as-prepared Ag2WO4 SNBs, (c and d) Ag2WO4@Ni–Co precursor core–shell SNBs, and (e and f) Ni–Co–O HNBs.

In the following phosphorization process (step III in Fig. 1), Ni–Co–O HNBs are converted into Ni–Co–P HNBs with the well-retained structures as proved by FESEM images in Fig. 3a and b. The XRD pattern of the product (Fig. S4a, ESI) manifests that all the diffraction peaks can be indexed to a hexagonal phase of NiCoP (JCPDS card No. 71-2336). No impurity peaks are detectable, indicating a complete transformation of Ni–Co–O to Ni–Co–P. It should be noted that a small amount of Ag (∼1.8% atomic ratio) is also detected by the EDX spectrum (Fig. S5, ESI). The interior void and hierarchical structures of the as-obtained Ni–Co–P particles are further revealed by a TEM image in Fig. 3c. The HRTEM image (Fig. 3d) shows clear lattice fringes with an interplanar spacing of 0.22 nm, corresponding to the (111) plane of the Ni–Co–P. The interplanar spacing of 0.24 nm corresponds to the (111) plane of metallic Ag, further confirming the existence of Ag in the product although Ag nanoparticles are only occasionally found during TEM observation. This indicates that some residual Ag+-containing compounds might absorb on the Ni–Co–O HNBs. During the phosphorization process, the highly reducing PH3 gas released from NaH2PO2 would easily reduce these absorbed Ag+ species to metallic Ag. The elemental mapping is performed on a single HNB (Fig. 3e) to reveal the spatial distribution of different elements in this unique nanostructure. As displayed in Fig. 3f–h, the mapping result exhibits a homogeneous distribution of the Ni, Co, and P elements throughout the whole HNB. With a similar phosphorization process, Ni–Co–O nanosheets can also be converted into Ni–Co–P nanosheets, as revealed by the FESEM and XRD results (Fig. S3e, f, and S4b, ESI).


image file: c8ee00076j-f3.tif
Fig. 3 (a and b) FESEM, (c) TEM, (d) HRTEM, and (e) STEM images of the hierarchical Ni–Co–P HNBs. Elemental mapping images of (f) Ni, (g) Co, and (h) P for the single Ni–Co–P HNB in (e).

X-ray photoelectron spectroscopy (XPS) measurements are performed to further confirm the chemical compositions and oxidation states of the Ni–Co–P HNBs. As presented by the high-resolution Ni 2p spectrum (Fig. S6a, ESI), the two main peaks at around 853.0 and 870.2 eV should be assigned to Ni 2p3/2 and Ni 2p1/2 in the Ni–P compound.44,45 The binding energy of 853.0 eV is quite close to that of metallic Ni (852.6 eV),46 indicating the existence of partially charged Ni species (Niδ+, δ is likely close to 0). Peaks at 856.3 and 874.7 eV along with two satellite peaks at 861.7 and 880.6 eV are well fitted with Ni 2p3/2 and Ni 2p1/2 of the oxidized nickel species, respectively.47,48 Similarly, as presented in Co 2p spectrum (Fig. S6b, ESI), the peak at 778.4 eV for Co 2p3/2 corresponds to Coδ+ in the Co–P compound,45 due to its slightly higher binding energy than that of metallic Co (778.2 eV).49 The peak at 781.2 eV can be ascribed to the oxidized Co species.50 The Co 2p1/2 peaks located at 793.2 and 797.8 eV can be attributed to the Coδ+ and the oxidized Co species in the Co–P compound, respectively.51 The corresponding satellite peaks of Co 2p3/2 and Co 2p1/2 are situated at 785.8 and 802.8 eV, respectively.52 The peaks at 129.1 and 129.9 eV in the P 2p spectrum (Fig. S6c, ESI) are attributed to P 2p3/2 and P 2p1/2 in reduced phosphorus of Ni–Co–P. The binding energy of the reduced phosphorus shows a slightly negative shift compared with that of the elemental P (130.0 eV),53 suggesting a small electron density transfer occurs from Ni/Co to P. The peak at 133.5 eV is assigned to PO43− or P2O5, resulting from the exposure of the product to air.45 Moreover, the incorporated Ag can be further detected by XPS (Fig. S6d, ESI). Peaks centered at 374.3 eV and 367.9 eV in the Ag 3d spectrum can be indexed to the metallic Ag0.54,55 These results further confirm the successful formation of the Ni–Co–P compound with some metallic Ag incorporated. The surface area and pore size of the Ni–Co–P HNBs and nanosheets are determined by N2 sorption measurements (Fig. S7, ESI). Accordingly, the Brunauer–Emmett–Teller (BET) specific surface area is 37.6 m2 g−1 for the Ni–Co–P HNBs and 29.4 m2 g−1 for the nanosheets. Clearly, there are three kinds of porosities within the hierarchical Ni–Co–P HNBs, including the micropores and mesopores among the oriented stacking nanosheets as well as the macropores derived from the open and hollow interior. With such a porous and hierarchical structure, not only the exposure of active sites but also the penetration of electrolyte can be promoted, which is expected to facilitate the electrochemical reactions.47,56

The electrocatalytic performance of the Ni–Co–P HNBs for OER is evaluated by using a standard three-electrode cell in 1.0 M KOH solution. Optimal OER activity is observed with a mass loading of 2.0 mg cm−2 on the Ni foam substrate (Fig. S8a, ESI). Fig. 4a shows that the Ni–Co–P HNBs exhibit lower onset potential and higher current density with the applied potential than the Ni–Co–P nanosheets. The overpotential required to drive the current density of 10 mA cm−2 is 270 mV for the Ni–Co–P HNBs versus 290 mV for the Ni–Co–P nanosheets. These results indicate the electrocatalytic OER activity of the Ni–Co–P HNBs is better than that of the Ni–Co–P nanosheets and comparable to those of metal phosphides or Ni-/Co-based catalysts (Table S1, ESI). It should be noted that the OER activity of the Ni foam used is also evaluated as a reference (Fig. S9a, ESI). The enhanced OER performance of the Ni–Co–P HNBs is further confirmed by a smaller Tafel slope compared with that of the nanosheets as revealed in Fig. 4b (76 mV dec−1versus 88 mV dec−1), indicating a favorable OER kinetics in the Ni–Co–P HNBs. The electrochemical impedance spectroscopy (EIS) is employed to further investigate the electrode kinetics. A smaller charge-transfer resistance is observed for the Ni–Co–P HNBs (Fig. S10a, ESI), which may account for its enhanced OER performance. The stability test of the Ni–Co–P HNBs for OER is conducted with cyclic voltammetry (CV) measurements. As shown in Fig. 4c, the decrease of the current density is negligible after 3000 cycles of successive CV scanning at a scan rate of 100 mV s−1. The long-term durability of the Ni–Co–P HNBs for OER was tested by a chronoamperometry measurement (inset of Fig. 4c), and about 93.5% of the initial current is retained over 20 h. These data indicate excellent stability of the Ni–Co–P HNBs towards the OER.


image file: c8ee00076j-f4.tif
Fig. 4 (a and d) Polarization curves and (b and e) Tafel plots of the hierarchical Ni–Co–P HNBs and Ni–Co–P nanosheets for (a and b) OER and (d and e) HER. (c and f) Polarization curves of the hierarchical Ni–Co–P HNBs initially and after 3000 CV scans for (c) OER and (f) HER. The insets in (c) and (f) are the chronoamperometry curves for OER and HER, respectively.

We have also investigated the HER performance of the Ni–Co–P HNBs in the same electrolyte with that for OER. After optimizing the loading mass of the catalyst on the Ni foam (Fig. S8b, ESI), the Ni–Co–P HNBs exhibit good electrocatalytic activity for HER with an overpotential of 107 mV at the current density of 10 mA cm−2 (Fig. 4d), which is 53 mV lower than that of the Ni–Co–P nanosheets and comparable to most of the recently reported congeneric catalysts (Table S2, ESI). Similarly, the intrinsic HER activity of the Ni foam used is confirmed to be much lower (Fig. S9b, ESI). The superior HER performance of the Ni–Co–P HNBs is further confirmed by a smaller Tafel slope of 46 mV dec−1 (Fig. 4e), compared with 53 mV dec−1 for the Ni–Co–P nanosheets. The favorable kinetics of the Ni–Co–P HNBs for HER should be attributed to their smaller charge-transfer resistance than the nanosheets (Fig. S10b, ESI). To estimate the stability of the Ni–Co–P HNBs for HER, successive CV scanning is conducted continuously for 3000 cycles (Fig. 4f). At the end of the measurement, the obtained polarization curve is similar to the initial one, with negligible change in the current density. The long-term durability of the Ni–Co–P HNBs for HER is also assessed by a chronoamperometry test (inset of Fig. 4f). The current density loss is about 4.3% over 20 h, indicating the good stability of the Ni–Co–P HNBs for HER in the alkaline medium. In addition, FESEM images of the Ni–Co–P HNBs harvested after the durability tests indicate good structural stability of the catalyst (Fig. S11, ESI).

According to the above evaluations, the Ni–Co–P HNBs modified electrode can serve as both the anode for OER and the cathode for HER. To verify the efficiency of Ni–Co–P HNBs as a bifunctional electrocatalyst, the overall water splitting measurement is carried out in a two-electrode cell. The polarization curves presented in Fig. 5a show that the Ni–Co–P HNBs afford a current density of 10 mA cm−2 at the cell voltage of 1.62 V, while the potential needs to rise to 1.75 V to reach the same current density in the case of the Ni–Co–P nanosheets. The inset of Fig. 5a shows a digital photo of the overall water splitting cell, where the generation of H2 and O2 bubbles can be clearly seen. Although the overall water splitting performance of the Ni–Co–P HNBs is still inferior to those of the noble-metal electrode materials, such as Pt/C/NF‖RuO2/NF (10 mA cm−2 at ∼1.53 V),57 it can be comparable or even superior to other transition-metal-based bifunctional catalysts (Table S3, ESI), such as NiCo2S4 NA/CC‖NiCo2S4 NA/CC (10 mA cm−2 at ∼1.68 V),58 NiS/NF‖NiS/NF (10 mA cm−2 at ∼1.64 V),57 and FeNi3N/NF‖FeNi3N/NF (10 mA cm−2 at ∼1.62 V).59 Stability is another vital parameter for the evaluation of electrocatalysts. The long-term durability test for overall water splitting is conducted by a chronoamperometry measurement. Remarkably, as presented in Fig. 5b, the Ni–Co–P HNBs are stable over 20 h with the current loss of only 6.6% for overall water splitting at the cell potential of 1.62 V.


image file: c8ee00076j-f5.tif
Fig. 5 (a) Polarization curves of the hierarchical Ni–Co–P HNBs and Ni–Co–P nanosheets for overall water splitting. The inset in (a) shows a photo of the overall water splitting cell. (b) The chronoamperometry curve of the hierarchical Ni–Co–P HNBs for overall water splitting at a voltage of 1.62 V.

It should be noted that the electrocatalytic activities of the Ni–Co–P HNBs, including OER, HER and overall water splitting, are also comparable to those of noble metal-based catalysts (Fig. S12, ESI), indicating that the as-prepared catalyst could be a promising alternative towards electrochemical water splitting. The excellent electrocatalytic activities of the Ni–Co–P HNBs should be attributed to the advantageous features from the unique oriented stacking 2D nanosheets and the 3D open and hollow nano-architectures. This could endow the Ni–Co–P HNBs with a higher electrochemical active surface area (ECSA; Fig. S13, ESI) and promote the diffusion of the reaction species, thus enhancing the electrocatalytic activity.56 Moreover, the highly open and hollow structures assembled from the nanosheets of the Ni–Co–P HNBs enable easy penetration of the electrolyte and release of the evolved gas bubbles, leading to the enhanced reaction kinetics and excellent electrochemical stability. Furthermore, the nanosheet building blocks are not separated but interconnected with each other, which helps to maintain good mechanical and electrical contacts.60

From the compositional point of view, the incorporated small amount of Ag might also have some positive effect on the catalytic activity of Ni–Co–P, we have established related models (Fig. S14, ESI) and carried out some primitive density functional theory (DFT) calculations for the HER Gibbs free-energy change (ΔGH*) as well as the water adsorption energy (ΔGH2O*) in alkaline condition. Fig. 6a presents the ΔGH* pattern of the Ag-incorporated Ni–Co–P and Ni–Co–P catalysts. The Ni–Co–P has a relatively lower ΔGH* value, quite far from the ideal ΔGH* of 0 eV. Upon the incorporated of Ag, the ΔGH* changes to a value that is relatively close to 0 eV for the catalyst-H* state, indicating its faster proton/electron-transfer and then faster hydrogen release rate during the HER process.61,62 Additionally, the calculated water adsorption energies for the Ag-incorporated Ni–Co–P and Ni–Co–P are shown in Fig. 6b. In comparison to the pristine Ni–Co–P, the water adsorption energy is much larger for the Ag-incorporated Ni–Co–P, implying a more favorable formation of the catalyst-H2O (initial state) on the surface of Ag-incorporated Ni–Co–P, which could further promote the HER reaction.63 All these calculated results manifest that the Ni–Co–P would possess an enhanced HER activity with the incorporation of Ag.


image file: c8ee00076j-f6.tif
Fig. 6 (a) Calculated free-energy change and (b) water adsorption energies for the Ni–Co–P and Ag-incorporated Ni–Co–P.

3. Conclusion

In summary, we have first developed a strategy for organizing 2D nanosheets into an oriented stacking on 3D anisotropic nanostructured templates. After the facile etching of the templates and subsequent phosphorization process, open and hierarchical Ni–Co–P hollow nanobricks (HNBs) can be fabricated with shells assembled from ordered nanosheets. Compared with irregular Ni–Co–P nanosheets, the Ni–Co–P HNBs are demonstrated as a more efficient bifunctional electrocatalyst for overall water splitting, which needs overpotentials of 270 mV and 107 mV for OER and HER respectively to afford the current density of 10 mA cm−2 in an alkaline medium. Furthermore, the Ni–Co–P HNBs electrode exhibits excellent overall water splitting activity with a potential of 1.62 V to reach the current density of 10 mA cm−2. The excellent durability also suggests that the Ni–Co–P HNBs are a promising bifunctional electrocatalyst for the overall water splitting.

4. Experimental

4.1 Material preparation

Synthesis of Ag2WO4 SNBs. In a typical procedure, 680 mg of AgNO3 and 500 mg of PVP (Mw = ∼58 K) were dispersed in 20 mL of deionized (DI) water with the assistance of sonication to obtain solution A. 136 mg of Na2WO4 and 500 mg of PVP were dissolved into 15 mL of DI water to get solution B. Solution B was then dropped into solution A slowly within 15 min with vigorous magnetic stirring, and a grey suspension was obtained. The grey mixture was placed in a microwave refluxing system and irradiated at 400 W for 20 min. The final product was collected by centrifugation, washed with DI water and anhydrous ethanol, and then dried at 65 °C for 12 h.
Synthesis of Ag2WO4@Ni–Co precursor core–shell SNBs. In a typical procedure, 100 mg of the as-prepared Ag2WO4 SNBs, 600 mg of Co(NO3)2·6H2O, 300 mg of Ni(NO3)2·6H2O, 1 g of urea and 1 g of PVP were added into a round-bottom flask and dispersed in a mixture containing 70 mL of anhydrous ethanol and 10 mL of DI water by sonication. Then the mixture was refluxed in an oil bath at 90 °C for 10 h under the magnetic stirring. After the solution was cooled down to room temperature, the final product was collected by centrifugation, washed with DI water and anhydrous ethanol for several times, and then dried at 65 °C for 12 h.
Synthesis of hierarchical Ni–Co–O HNBs. In a typical procedure, 20 mg of the as-prepared Ag2WO4@Ni–Co precursor core–shell SNBs was firstly annealed at 300 °C in air for 2 h with a heating rate of 2 °C min−1 to produce Ag2WO4@Ni–Co–O hybrid SNBs. The resulting annealed product was then dispersed into 10 mL distilled water followed by adding 10 mL of 1.0 M HNO3 solution dropwise for 10 min and then 10 mL of NH3·H2O. After 10 min of ultrasonication, the Ag2WO4 templates can be removed. The final product was collected by centrifugation, washed with DI water and anhydrous ethanol for several times, and then dried at 65 °C for 12 h.
Synthesis of hierarchical Ni–Co–P HNBs. In a typical procedure, 20 mg of the as-prepared hierarchical Ni–Co–O HNBs and 400 mg of NaH2PO2 were placed at two separate positions in a porcelain crucible with a cover, and NaH2PO2 was at the upstream side of the furnace. The sample was annealed at 300 °C for 2 h with a heating rate of 2 °C min−1 in N2 atmosphere. The Ni–Co–P HNBs were collected after cooling to ambient temperature under nitrogen atmosphere.
Synthesis of Ni–Co–P nanosheets. The irregularly assembled Ni–Co–P nanosheets were prepared via a similar method with the synthesis of the hierarchical Ni–Co–P HNBs except adding the Ag2WO4 SNBs.

4.2 Characterizations

The crystalline phase of the products was analyzed by powder XRD measurements with a Philips PW3040/60 X-ray diffractometer using Cu–Kα radiation and performed at a scanning rate of 0.06° s−1. The morphologies were examined by FESEM conducted on a Hitachi S-4800 scanning electron microanalyzer with an accelerating voltage of 15 kV. The microstructure of the products was further characterized by TEM and HRTEM measured at 200 kV with a JEM-2100F field-emission TEM. The surface compositions of the samples were analyzed with XPS, using an ESCALab MKII X-ray photoelectron spectrometer with Mg Kα X-ray as the excitation source. N2 sorption isotherms were performed at 77 K on a Micrometrics ASAP 2020 surface area and porosity analyzer and the samples were degassed in vacuum at 200 °C for 4 h before the measurements.

4.3 Fabrication of electrodes

The Ni–Co–P (or Ni–Co–O) electrodes were prepared according to a previous report.56 Briefly, 70 wt% Ni–Co–P powder, 20 wt% carbon black and 10 wt% polymer binder (polyvinylidene fluoride, PVDF) were dispersed in n-methyl-pyrrolidone (NMP) solvent to produce homogeneous slurry. The prepared slurry was then pasted on a Ni foam substrate of ∼1.0 cm2 in area and dried at 80 °C for 12 h under vacuum. To determine the optimal electrocatalytic performance of the hierarchical Ni–Co–P HNBs, polarization curves were first obtained with different mass loading of the hierarchical Ni–Co–P HNBs from about 1.0 mg cm−2 to 2.5 mg cm−2, and the optimal mass loading was determined to be about 2 mg cm−2 (Fig. S8, ESI). All the Ni foam substrates were cleaned by 6.0 M HCl solution, ethanol and distilled water with assistance of ultrasonication for several minutes before utilization.

4.4 Electrochemical measurements

All electrochemical experiments were performed in a three-electrode system at room temperature using a bipotentiostat workstation (WD20-BASIC Metrohm PGSTAT101). A carbon rod was used as the counter electrode, and a saturated calomel electrode (SCE) was utilized as the reference electrode. The catalyst electrodes were used as the working electrodes to evaluating the HER or OER performance. The electrolyte was 1.0 M KOH (pH = 13.6) bubbled with oxygen (or nitrogen) for 30 min prior to OER (or HER) measurements. All the potentials were converted to the potentials referring to the reversible hydrogen electrode (RHE, saturated KCl), according to E(RHE) = E(SCE) + 0.059 pH + 0.242. EIS measurements were conducted on a Zennium electrochemical workstation (ZAHNER, Germany) in the range of 105 to 0.01 Hz with 10 mV sinusoidal perturbations, and all current densities were corrected against ohmic potential drop. To promote the mass transport and removal of the generated gas bubbles in the process of chronoamperometry measurements, the electrolyte was vigorously stirred by a stir bar.

4.5 Computational methodology

Free energies were calculated by density functional theory (DFT) based on Vienna Ab-initio simulation package (VASP).64–66 We selected generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional for the exchange–correlation energy and projector augmented wave (PAW) potentials.67,68 Moreover, the kinetic energy cutoff was set to 350 eV, the total energies were converged to 1 × 10−5 eV, and the convergence criterion for the residual forces on the atoms was set to 0.05 eV Å−1 during the relaxation. The k-mesh of 3 × 3 × 2 was employed for the Brillouin zone integrations. The Dudarev method was used to further accurately describe the strong Coulomb repulsion between the localized Ni and Co 3d electrons.69 We hence set the effective onsite Coulomb interaction values according to the literature: 6.4 eV for Ni and 4.0 eV for Co with J = 1.0 eV.70 All the calculations were conducted on (001) surface of Ni–Co–P and Ag-incorporated Ni–Co–P slab model with 2 × 2 super cell. The surfaces were constructed as slab consists of three layers and a vacuum layer of 15 Å thicknesses. The adsorption sites were selected as Co–Co bridge site in the calculations (Fig. S14, ESI). The free energy change for H* adsorptions were determined as follows:
ΔGH = EtotalEsurEH2/2 + ΔEZPETΔS
where Etotal is the total energy for the adsorption state, Esur is the energy of pure surface, EH2 is the energy of H2 in gas phase, ΔEZPE is the zero-point energy change and ΔS is the entropy change.71

The total energies for water adsorptions were calculated as follows:

Eads = Eadsorbate/slabEadsorbateEslab

A more negative energy suggests a more stable adsorption.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Y. Hu acknowledges financial support from the Natural Science Foundation of China (21671173), Zhejiang Provincial Natural Science Foundation of China (LR14B010001), and the Zhejiang Provincial Public Welfare Project (2016C31015).

Notes and references

  1. Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov and T. F. Jaramillo, Science, 2017, 355, eaad4998 CrossRef PubMed .
  2. Y. Liang, Y. Li, H. Wang and H. Dai, J. Am. Chem. Soc., 2013, 135, 2013 CrossRef CAS PubMed .
  3. H. Hu, B. Guan, B. Xia and X. W. Lou, J. Am. Chem. Soc., 2015, 137, 5590 CrossRef CAS PubMed .
  4. L. Han, X.-Y. Yu and X. W. Lou, Adv. Mater., 2016, 28, 4601 CrossRef CAS PubMed .
  5. L. Trotochaud, J. K. Ranney, K. N. Williams and S. W. Boettcher, J. Am. Chem. Soc., 2012, 134, 17253 CrossRef CAS PubMed .
  6. J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao-Horn, Science, 2011, 334, 1383 CrossRef CAS PubMed .
  7. J. Nai, B. Y. Guan, L. Yu and X. W. Lou, Sci. Adv., 2017, 3, e1700732 CrossRef PubMed .
  8. H. Liang, F. Meng, M. Cabán-Acevedo, L. Li, A. Forticaux, L. Xiu, Z. Wang and S. Jin, Nano Lett., 2015, 15, 1421 CrossRef CAS PubMed .
  9. F. Song and X. Hu, J. Am. Chem. Soc., 2014, 136, 16481 CrossRef CAS PubMed .
  10. F. Song and X. Hu, Nat. Commun., 2014, 5, 4477 CAS .
  11. J. Nai, H. Yin, T. You, L. Zheng, J. Zhang, P. Wang, Z. Jin, Y. Tian, J. Liu, Z. Tang and L. Guo, Adv. Energy Mater., 2015, 5, 1401880 CrossRef .
  12. S.-H. Ye, Z.-X. Shi, J.-X. Feng, Y.-X. Tong and G.-R. Li, Angew. Chem., Int. Ed., 2018, 57, 2672 CrossRef CAS PubMed .
  13. M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072 CrossRef CAS PubMed .
  14. T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2014, 53, 7281 CrossRef CAS PubMed .
  15. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274 CrossRef CAS PubMed .
  16. F. Wang, Y. Li, T. A. Shifa, K. Liu, F. Wang, Z. Wang, P. Xu, Q. Wang and J. He, Angew. Chem., Int. Ed., 2016, 55, 6919 CrossRef CAS PubMed .
  17. J.-X. Feng, J.-Q. Wu, Y.-X. Tong and G.-R. Li, J. Am. Chem. Soc., 2018, 140, 610 CrossRef CAS PubMed .
  18. E. J. Popczun, J. R. McKone, C. G. Read, A. J. Biacchi, A. M. Wiltrout, N. S. Lewis and R. E. Schaak, J. Am. Chem. Soc., 2013, 135, 9267 CrossRef CAS PubMed .
  19. E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis and R. E. Schaak, Angew. Chem., Int. Ed., 2014, 53, 5427 CrossRef CAS PubMed .
  20. F.-X. Ma, H. B. Wu, B. Y. Xia, C.-Y. Xu and X. W. Lou, Angew. Chem., Int. Ed., 2015, 54, 15395 CrossRef CAS PubMed .
  21. H. B. Wu, B. Y. Xia, L. Yu, X.-Y. Yu and X. W. Lou, Nat. Commun., 2015, 6, 6512 CrossRef CAS PubMed .
  22. Y. Zheng, Y. Jiao, Y. Zhu, L. H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec and S. Z. Qiao, Nat. Commun., 2014, 5, 3783 Search PubMed .
  23. J.-X. Feng, H. Xu, S.-H. Ye, G. Ouyang, Y.-X. Tong and G.-R. Li, Angew. Chem., Int. Ed., 2017, 56, 8120 CrossRef CAS PubMed .
  24. H. Shi, H. Liang, F. Ming and Z. Wang, Angew. Chem., Int. Ed., 2017, 56, 573 CrossRef CAS PubMed .
  25. Y. Zhu, W. Zhou, Y. Zhong, Y. Bu, X. Chen, Q. Zhong, M. Liu and Z. Shao, Adv. Energy Mater., 2017, 7, 1602122 CrossRef .
  26. L.-A. Stern, L. Feng, F. Song and X. Hu, Energy Environ. Sci., 2015, 8, 2347 CAS .
  27. I. Roger, M. A. Shipman and M. D. Symes, Nat. Rev. Chem., 2017, 1, 0003 CrossRef CAS .
  28. S. Anantharaj, S. R. Ede, K. Sakthikumar, K. Karthick, S. Mishra and S. Kundu, ACS Catal., 2016, 6, 8069 CrossRef CAS .
  29. J. Xie, X. Zhang, H. Zhang, J. Zhang, S. Li, R. Wang, B. Pan and Y. Xie, Adv. Mater., 2017, 29, 1604765 CrossRef PubMed .
  30. J. Miao, F.-X. Xiao, H. B. Yang, S. Y. Khoo, J. Chen, Z. Fan, Y.-Y. Hsu, H. M. Chen, H. Zhang and B. Liu, Sci. Adv., 2015, 1, e1500259 Search PubMed .
  31. Y. Liu, X. Hua, C. Xiao, T. Zhou, P. Huang, Z. Guo, B. Pan and Y. Xie, J. Am. Chem. Soc., 2016, 138, 5087 CrossRef CAS PubMed .
  32. Z. Li, Z. Liu, H. Sun and C. Gao, Chem. Rev., 2015, 115, 7046 CrossRef CAS PubMed .
  33. J. E. ten Elshof, H. Yuan and P. Gonzalez Rodriguez, Adv. Energy Mater., 2016, 6, 1600355 CrossRef .
  34. X. Huang, C. Tan, Z. Yin and H. Zhang, Adv. Mater., 2014, 26, 2185 CrossRef CAS PubMed .
  35. Z. Sun, T. Liao, Y. Dou, S. M. Hwang, M.-S. Park, L. Jiang, J. H. Kim and S. X. Dou, Nat. Commun., 2014, 5, 3813 CAS .
  36. Z. Lai, Y. Chen, C. Tan, X. Zhang and H. Zhang, Chem, 2016, 1, 59 CAS .
  37. W. Wan, Z. Zhao, H. Hu, X. Hao, T. C. Hughes, H. Ma, L. Pan and J. Qiu, Carbon, 2014, 76, 46 CrossRef CAS .
  38. Y. M. Chen, X. Y. Yu, Z. Li, U. Paik and X. W. Lou, Sci. Adv., 2016, 2, e1600021 Search PubMed .
  39. X.-Y. Yu, H. Hu, Y. Wang, H. Chen and X. W. Lou, Angew. Chem., Int. Ed., 2015, 54, 7395 CrossRef CAS PubMed .
  40. J. Chen, X.-J. Wu, Y. Gong, Y. Zhu, Z. Yang, B. Li, Q. Lu, Y. Yu, S. Han, Z. Zhang, Y. Zong, Y. Han, L. Gu and H. Zhang, J. Am. Chem. Soc., 2017, 139, 8653 CrossRef CAS PubMed .
  41. P. Falcaro, K. Okada, T. Hara, K. Ikigaki, Y. Tokudome, A. W. Thornton, A. J. Hill, T. Williams, C. Doonan and M. Takahashi, Nat. Mater., 2017, 16, 342 CrossRef CAS PubMed .
  42. Z. Lin, J. Li, Z. Zheng, J. Yan, P. Liu, C. Wang and G. Yang, ACS Nano, 2015, 9, 7256 CrossRef CAS PubMed .
  43. P. Li and H. C. Zeng, Adv. Funct. Mater., 2017, 27, 1606325 CrossRef .
  44. H. Liang, A. N. Gandi, D. H. Anjum, X. Wang, U. Schwingenschlögl and H. N. Alshareef, Nano Lett., 2016, 16, 7718 CrossRef CAS PubMed .
  45. C. Du, L. Yang, F. Yang, G. Cheng and W. Luo, ACS Catal., 2017, 7, 4131 CrossRef CAS .
  46. A. P. Grosvenor, M. C. Biesinger, R. S. C. Smart and N. S. McIntyre, Surf. Sci., 2006, 600, 1771 CrossRef CAS .
  47. Y. Li, J. Liu, C. Chen, X. Zhang and J. Chen, ACS Appl. Mater. Interfaces, 2017, 9, 5982 CAS .
  48. M. Kong, Z. Wang, W. Wang, M. Ma, D. Liu, S. Hao, R. Kong, G. Du, A. M. Asiri, Y. Yao and X. Sun, Chem. – Eur. J., 2017, 23, 4435 CrossRef CAS PubMed .
  49. S. Valeri, A. Borghi, G. C. Gazzadi and A. di Bona, Surf. Sci., 1999, 423, 346 CrossRef CAS .
  50. X. Yang, A.-Y. Lu, Y. Zhu, M. N. Hedhili, S. Min, K.-W. Huang, Y. Han and L.-J. Li, Nano Energy, 2015, 15, 634 CrossRef CAS .
  51. X. Chen, M. Cheng, D. Chen and R. Wang, ACS Appl. Mater. Interfaces, 2016, 8, 3892 CAS .
  52. L. Danni, C. Tao, Z. Wenxin, C. Liang, M. A. Abdullah, L. Qun and S. Xuping, Nanotechnology, 2016, 27, 33LT01 CrossRef PubMed .
  53. H. Li, H. Li, W.-L. Dai, W. Wang, Z. Fang and J.-F. Deng, Appl. Surf. Sci., 1999, 152, 25 CrossRef CAS .
  54. J. Li, Y. Xie, Y. Zhong and Y. Hu, J. Mater. Chem. A, 2015, 3, 5474 CAS .
  55. S. Wang, H. Qian, Y. Hu, W. Dai, Y. Zhong, J. Chen and X. Hu, Dalton Trans., 2013, 42, 1122 RSC .
  56. X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang and Z. Lin, Angew. Chem., Int. Ed., 2016, 55, 6290 CrossRef CAS PubMed .
  57. W. Zhu, X. Yue, W. Zhang, S. Yu, Y. Zhang, J. Wang and J. Wang, Chem. Commun., 2016, 52, 1486 RSC .
  58. D. Liu, Q. Lu, Y. Luo, X. Sun and A. M. Asiri, Nanoscale, 2015, 7, 15122 RSC .
  59. B. Zhang, C. Xiao, S. Xie, J. Liang, X. Chen and Y. Tang, Chem. Mater., 2016, 28, 6934 CrossRef CAS .
  60. H. Wang, H.-W. Lee, Y. Deng, Z. Lu, P.-C. Hsu, Y. Liu, D. Lin and Y. Cui, Nat. Commun., 2015, 6, 7261 CrossRef CAS PubMed .
  61. J.-S. Li, Y. Wang, C.-H. Liu, S.-L. Li, Y.-G. Wang, L.-Z. Dong, Z.-H. Dai, Y.-F. Li and Y.-Q. Lan, Nat. Commun., 2016, 7, 11204 CrossRef CAS PubMed .
  62. P. Chen, T. Zhou, M. Zhang, Y. Tong, C. Zhong, N. Zhang, L. Zhang, C. Wu and Y. Xie, Adv. Mater., 2017, 29, 1701584 CrossRef PubMed .
  63. P. Chen, K. Xu, S. Tao, T. Zhou, Y. Tong, H. Ding, L. Zhang, W. Chu, C. Wu and Y. Xie, Adv. Mater., 2016, 28, 7527 CrossRef CAS PubMed .
  64. G. Kresse and J. Furthmüller, Comput. Mater. Sci., 1996, 6, 15 CrossRef CAS .
  65. G. Kresse and J. Furthmüller, Phys. Rev. B: Condens. Matter Mater. Phys., 1996, 54, 11169 CrossRef CAS .
  66. G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 49, 14251 CrossRef CAS .
  67. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865 CrossRef CAS PubMed .
  68. G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 59, 1758 CrossRef CAS .
  69. O. Bengone, M. Alouani, P. Blöchl and J. Hugel, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 16392 CrossRef CAS .
  70. C. Xia, P. Li, A. N. Gandi, U. Schwingenschlögl and H. N. Alshareef, Chem. Mater., 2015, 27, 6482 CrossRef CAS .
  71. Y. Zheng, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2015, 54, 52 CrossRef CAS PubMed .

Footnotes

Electronic supplementary information (ESI) available: Additional morphology and composition characterizations, electrochemical characterization, and DFT calculations. See DOI: 10.1039/c8ee00076j
These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018