Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting

Journal of Colloid and Interface Science xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting Wen-Zhuo Zhang a,1, Guang-Yi Chen a,⇑,1, Jian Zhao a,⇑, Ji-Cai Liang a,b, Li-Feng Sun a, Guang-Fei Liu a, Bao-Wei Ji a, Xiang-Yu Yan a, Jia-Rui Zhang a a b

School of Automotive Engineering, State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, China Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 7 September 2019 Revised 5 November 2019 Accepted 11 November 2019 Available online xxxx Keywords: Ni2P nanosheet array Cation vacancy Bifunctional electrocatalyst Overall water splitting

a b s t r a c t Hypothesis: The transition metal phosphide is one of the promising bifunctional electrocatalysts for overall water splitting. Moreover, the activity of phosphide catalysts can be further enhanced by the cationic vacancy engineering. Experiments: The self-growth Ni2P nanosheet arrays with abundant cationic vacancy defects (V-Ni2P/NF) has been synthesized via a facile multi-step reaction process involving hydrothermal, phosphorization and acid-etching of Mn which was doped in Ni2P nanosheets as a sacrificial dopant. Furthermore, the experimental studies and density functional theory (DFT) calculations were carried out to evaluate its electrochemical performance. Findings: The chemical and electrocatalytic property of Ni2P were successfully optimized by cationic vacancy engineering and the obtained V-Ni2P/NF catalyst exhibited superior bifunctional catalytic performance for both hydrogen evolution (HER) and oxygen evolution reaction (OER) compared to pristine Ni2P and Mn-doped Ni2P in alkaline electrolyte. The V-Ni2P/NF can afford the current density of 10 mA cm
2 at a small overpotential of 55 mV for HER and 250 mV for OER. Additionally, the water electrolysis device assembled by the V-Ni2P/NF electrode as both the anode and cathode just requires a small voltage of 1.59 V to achieve 10 mA cm
2 and shows no obvious attenuation for 50 h. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding authors. 1

E-mail addresses: [email protected] (G.-Y. Chen), [email protected] (J. Zhao). Wen-Zhuo Zhang and Guang-Yi Chen contributed equally to this work.

https://doi.org/10.1016/j.jcis.2019.11.039 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

2

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

1. Introduction Electrochemical water splitting into the hydrogen and oxygen has been widely regarded as a promising method to prepare sustainable, clean and environment friendly hydrogen (H2) energy [1–6]. The water electrolysis process is composed of the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), and both two reactions need catalysts to reduce the overpotential and improve the efficiency [4,7–9]. Currently, precious metal-based electrocatalysts (Pt, Ru/Ir oxides) are regarded as the state-of-the-art catalysts for HER and OER, respectively [10,11]. But the excessive price and scarcity nature seriously restrict their large-scale commercial application [12,13]. So exploring highly activity and affordable bifunctional catalysts to replace precious metal-based materials is highly desirable for water electrolysis industry. In the past decades, transition metal compounds including chalcogenides [14–17], phosphides [18–22], nitrides [23,24], and carbides [25,26] have raised much attention due to their good electrocatalytic activity to HER and/or OER. Among these materials, owing to their relatively high intrinsic conductivity, transition metal-based phosphides (TMPs) exhibit some immanent advantages compared to other catalysts [19,20,27–30]. Up to now, too much research attention have been put on the morphology tailoring and controllable synthesis [31,32]. Although many TMPs based catalysts with various morphologies and nanostructures such as CoP nanoparticles [33], porous Ni2P nanosheets [34], NiCoP nanocone [35], and Mo-doped Ni2P hollow nanoprism [36] have been successfully synthesized and shown decent HER and/or OER activity. However, the entire electrocatalytic performances such as the intrinsic moderate activity, reaction kinetic, and overpotentials to HER/OER still fail to compete with the precious metal-based catalysts. Besides the morphology, it is well known that the composition also play an important role on determining the catalytic performances [37,38]. Recent research works found that tailoring the composition by creating vacancy defects has exhibited promising results in HER and/or OER field due to the vacancy defects can catalytically motivate neighboring atoms and/or improve the interfacial charge transfer [39–42]. For example, Kwong et al developed a Fe-vacancy-rich FeP as an outstanding HER catalyst with 108 mV overpotential to afford 10 mA cm
2 in 1 M KOH [43]. Liu et al confirmed that the cationic vacancies in d-FeOOH nanosheets can create new catalytic active centers to induce the superior HER and OER activity [44]. Zhang et al demonstrated that the NiO nanorods with abundant oxygen vacancies showed the excellent water electrolysis performance, even better than the benchmark Pt and RuO2 catalysts [45]. Inspired by previous researches, here we put our research efforts on utilizing the collective effects of rational morphology design and vacancy defects engineering to enhance the overall water splitting performance of Ni2P catalyst in alkaline medium. The self-growth Ni2P nanosheet arrays on Ni foam with abundant surface cationic vacancy defects (named as V-Ni2P/NF) has been favorably fabricated by a facile multi-step reaction process. Based on the experimental data and density functional theory calculation (DFT), we demonstrate that the surface cationic vacancy defects formed by Mn doping and its subsequent removal by acidetching can bring more catalytic active sites with eligible unsaturated coordination to enhance intrinsic catalytic activity for HER and OER. Taking advantage of the high catalytic activity, porous structure with more surface active area, superior conductivity and mechanical stability, the obtained V-Ni2P/NF electrode shows outstanding bifunctional catalytic performance in alkaline electrolyte. It only needs a low overpotential of 55 mV and 250 mV to reach 10 mA cm
2 for HER and OER, respectively. Furthermore, the water electrolysis cell by using V-Ni2P/NF as both anode and

cathode can drive 10 mA cm
2 only at a low voltage of 1.59 V and run stably for a long time without attenuation, making it a promising substitute of noble-metal catalyst for effective overall water splitting. 2. Experimental section 2.1. Chemical and materials 50% manganese nitrate solution (Mn(NO3)2), ammonium fluoride (NH4F), potassium hydroxide (KOH), urea (CO(NH2)2), nickel nitrate hexahydrate (Ni(NO3)26H2O) and sodium hypophosphite (NaH2PO2) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals are all in analytical grade and used without further purification. Pt/C catalyst (20 wt%) was bought from Alfa Aesar. Deionized water was used throughout. Before experiment, the Ni foam (NF, thickness: 1 mm) was firstly treated by acetone and 1 M HCl and then rinsed with ethanol and deionized water in turn for three times. 2.2. Synthesis of electrocatalysts on Ni foam Typically, 5 mmol Mn(NO3)2, 7.5 mmol NH4F and 4 mmol urea were firstly dissolved in 35 ml deionized water to form a clear solution. Then, the aqueous solution and pre-cleaned Ni foam (2 cm  2 cm) were transferred into a Teflon-lined autoclave, heated to 120 °C and maintained at this temperature for 5 h. After natural cooling to 25 °C, the NF coated with the precursor of MnNi2P was taken out, cleaned with deionized water and thoroughly dried at 60 °C. After that, the Mn-Ni2P nanosheet arrays on NF were prepared by phosphatization of the precursor at 400 °C for 1 h under N2 atmosphere. In this phosphatization process, the NaH2PO2 as the phosphorus source was put into the porcelain boat at the upstream side and the NF with the precursor was placed at the downstream side which is 4 cm away from the NaH2PO2 powder. In order to create cationic vacancy defect, the above Mn-Ni2P sample was then immersed in 3 M HNO3 solution and acid etched for 15 min. Finally, the black color Ni2P nanosheet arrays with cationic vacancy defect on NF substrate was rinsed with distilled water, dried in an oven, and named as V-Ni2P/NF. For comparison, the pure Ni2P/NF sample was also prepared under the similar condition of Mn-Ni2P/NF sample except for 5 mmol Ni(NO3)2 was used to substitute Mn(NO3)2 during the hydrothermal process. 2.3. Characterizations The morphology of the samples were studied on the scanning electron microscopy (SEM, JEOL JSM-6330F), transmission electron microscopy (TEM, JEOL 2010, 200 kV). The X-ray diffraction (XRD) patterns were performed on Rigaku/Max-2550 with Co Ka radiation (k = 1.7890 Å). The element distribution were measured by Energy dispersive spectrometer (EDS) on JEOL JSM-6330F SEM. X-ray photoelectron spectroscopy (XPS) scans were carried on multifunctional imaging electron spectrometer (Thermo ESCALAB 250XI). 2.4. Electrochemical test The electrochemical measurements were performed by using the CHI660E workstation (Shanghai, Chenhua) in a typical threeelectrode system. For the HER and OER tests, the prepared catalytic electrode (1 cm  1 cm), graphite rod and saturated calomel electrode (SCE) were used as working electrode, counter electrode and reference electrode, respectively. Before the formal test, 50 cyclic voltammetric (CV) cycles were conducted between

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

0.1 ~
0.4 V vs. RHE towards HER in 1 M KOH and 1.2–1.6 V vs. RHE towards OER in O2-saturated 1 M KOH with a scan rate of 50 mV s
1. Then the polarization curves were obtained by linear sweep voltammetry (LSV) measurements with a scan rate of 2 mV s
1 for HER and OER. The potentials were all calibrated to reversible hydrogen electrode (RHE) using the following equation:

ERHE ¼ EMeasured þ 0:059  pH þ 0:242

ð1Þ

3

hydrogen which is calculated as
1/2 S0 (S0 refers to the entropy of gas phase H2 at standard conditions, 1 bar of H2 under pH = 0 at 300 K). The vibrational entropy of the adsorbed hydrogen is too small which can be ignored. Therefore the DGH can be taken according to Eq. (7) [51,52]:

DGH ¼ DEH þ 0:24eV

ð7Þ

Moreover, the polarization curves were all iRs compensated according to the following Eq. (2):

In alkaline electrolyte, the four one-electron paths as listed in the following Eqs. (8)–(11) were used to describe the overall OER process at the anode.

Ecorrected ¼ EMeasured
iRs

OH
þ  ¼ OH þ e


ð8Þ

OH þ OH
¼ O þ H2 O þ e


ð9Þ

ð2Þ

The Rs was obtained by the electrochemical impedance spectroscopy (EIS) at the open circuit potential within a frequency range from 105 to 0.01 Hz and the Rct was measured at the bias of
100 mV vs. RHE. Moreover, the electrochemical double-layer capacitance (Cdl) of catalysts were estimated by CV measurements from 0.1 to 0.2 V vs. RHE at different scan rate of 10, 20, 40, 60, 80, 100 mV s
1, followed by extracting the slope from the resulting (ja-jc)/2 vs. v plots (ja and jc represent the anodic and cathodic current densities at 0.15 V vs. RHE). The turnover frequency (TOF) values for HER were calculated by using the Eq. (3) [46]:

TOF ¼ jS=2Fn

ð3Þ

where j is the current density (A cm
2) at a given overpotential for HER, F is the Faraday constant (96485C mol
1), 2 is the number of electrons involved during the reaction, and n is the number of active sites. The number of active sites was quantified by the CV method with the potential range of
0.2 ~ 0.6 V vs. RHE at a scan rate of 50 mV/s in 1 M PBS solution (pH = 7) according to Hu et al. [47]. The surface charge Q was obtained as the half of the integrated charge over CV curve. Then the number of active sites can be described by n = Q/F.

O þ OH
¼ OOH þ e


ð10Þ

OOH þ OH
¼ O2 þ H2 O þ e


ð11Þ

where * refers to the surface active site and OH*, O* as well as OOH* represent the adsorbed intermediates on the surface active site. The adsorption energy of intermediates on the active sites were calculated using the following Eq. (12):

b N þ DGU DGN ¼ DEN þ G

ð12Þ

b N includes contributions where DEN is the adsorption energy and G from vibration energy and entropy of the adsorbate at 300 K. According to the previous report [53,54], the typical values of 0.35, 0.05 and 0.40 eV for OH*, O* and OOH* were employed. Additionally, considering the external bias, the DGU =
eU was used to correct the shifting energy (U = 0 and 1.23 V). The standard free energy change of each elementary step can be expressed as:

DG1 ¼ DGOH

ð13Þ

DG2 ¼ DGO
DGOH

ð14Þ

DG3 ¼ DGOOH
DGO

ð15Þ

DG4 ¼ DGO2
DGOOH

ð16Þ

DGO2 ¼ 4:92eV

ð17Þ

2.5. Theoretical calculation All the density function theory (DFT) calculations were conducted by the Cambridge Sequential Total Energy Package (CASTEP) in Material Studio, using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchangecorrelation functional [48,49]. The kinetic cutoff energy of 340 eV was assigned to the plane-wave basis set with a self-consistentfield energy tolerance of 1.0 e
6. The Brillouin zone integration was set with the 5  5  5 and 4  4  2 Monkhorst-Pack mesh k-points for the bulk and surface calculations, respectively. The core electrons were treated by the ultrasoft pseudopotentials [50]. For calculation of the free energy, the (1 1 1) facet was modeled with vacuum widths of 12 Å in the z-direction to separate the slabs. The adsorption energy of H* (DEH) and H2O on the catalyst surface (DEH2O) was obtained using the Eqs. (4) and (5):

1 DEH ¼ EðþHÞ
EðÞ
EðH2 Þ 2

ð4Þ

DEH2 O ¼ EðþH2 OÞ
EðÞ
EðH2 OÞ

ð5Þ

where E(*+M) refers to total energy of the H* and H2O adsorbed on the catalyst surface, E(*) refers to energy of the clean catalyst surface, and E(M) refers to the energy of H2O and H2 molecule in gas phase. Furthermore, the Gibbs free-energy of adsorbed H* on the catalyst surface (DGH) can be expressed as Eq. (6):

DGH ¼ DEH þ DEZPE
T DS

ð6Þ

where DEZPE refers to the difference in zero-point energy between the adsorbed hydrogen and hydrogen in gas phase, T refers to the temperature, and DS refers to the entropy change of adsorbed

Furthermore, the theoretical overpotential (g) can be obtained according to the following Eq. (18):

g ¼ max ðDG1 ; DG2 ; DG3 ; DG4 Þ=e
1:23ðV Þ

ð18Þ

3. Results and discussion As displayed in Scheme 1, the synthesis process of V-Ni2P/NF catalytic electrode involves three steps reaction including hydrothermal, phosphorization, and acid etching. Here the Ni foam was served as the substrate, reactant and collector at the same time. The Mn(NO3)2 was selected as another reactant because it can be decomposed to produce HNO3 under the hydrothermal process and the surface of Ni foam would be partly acid etched and released Ni2+ ions into the reaction system. During the hydrothermal process, the Ni2+ and Mn2+ can react with the hydrolysis products of urea and NH4F to form the self-growth Mn-doped Nicarbonate hydroxide nanosheet arrays precursor on Ni foam. Then the above precursor was converted into high-crystalline Mn-Ni2P nanosheet arrays through a high-temperature phosphorization process at 400 °C for 1 h. Finally, the resulting V-Ni2P/NF catalytic electrode was successfully prepared by engineering cationic vacancies on Ni2P nanosheets via a simple acid-etching treatment to remove Mn element in Mn-Ni2P. The possible growth mechanism

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

4

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

Scheme 1. Schematic synthesis procedure of the V-Ni2P/NF catalytic electrodes.

and chemical reactions involved in these steps can be illustrated as follows [35]: 2þ

NiðMnÞ

þ xF
! ½NiðMnÞF 

ðx
2Þ


COðNH2 Þ2 þ 2H2 O ! 2NHþ4 þ CO2
3 ½NiðMnÞF x 

ðx
2Þ


ð19Þ ð20Þ

þ 0:5ð2
yÞCO3 2
þ yOH
þ nH2 O

^  nH2 O þ xF
! Mndoped
NiðOHÞy ðCO3 Þ0:5ð2
yÞ A

ð21Þ

^  nH2 O þ NaH2 PO2 Mndoped
NiðOHÞy ðCO3 Þ0:5ð2
yÞ A 



400 C;N2

!

Mn
Ni2 P

Mn
Ni2 P þ 2HNO3 ! V
Ni2 P þ MnðNO3 Þ2

ð22Þ ð23Þ

The morphology of Mn-Ni2P/NF and V-Ni2P/NF were firstly characterized by SEM. As shown in Fig. 1a-b, the Ni foam was uniformly coated by the vertically aligned and interconnected MnNi2P nanosheets with a thickness of ~60 nm after the hydrothermal and phosphorization reactions. Owing to excellent mechanical property of the in-situ self-growth Mn-Ni2P, the nanosheet arrays was basically maintained after the acid-etching process and the morphology of V-Ni2P/NF electrode was similar to that of MnNi2P electrode except for the surface became rougher and porous, which is beneficial for the permeation of electrolyte and exposure of more catalytic active sites (Fig. 1c-d). The porous structure of V-Ni2P nanosheets was also confirmed by the TEM image shown in Fig. 1e. Moreover, from the HRTEM image (Fig. 1f), plenty of cationic vacancies and distinct nano-pore can be seen over the lattice of the Ni2P nanosheets, where the lattice fringe of 0.21 nm can be attributed to the (1 1 1) plane of Ni2P. The crystal structures of Mn-Ni2P/NF and V-Ni2P/NF samples were studied by XRD. As shown in Fig. 1g, except for the diffraction peaks from Ni foam, the other major peaks at about 47.7°, 52.3° (overlap with the diffraction peak of Ni), 55.6°, and 63.8° can be indexed to the diffractions of (1 1 1), (0 2 1), (2 1 0) and (3 0 0) planes of hexagonal Ni2P (JCPDS No. 65-3544). Need to be noted, there are not any diffraction peaks belong to manganese phosphide species in Mn-Ni2P/NF, demonstrating that Mn was doped into Ni2P. Further-

more, the EDS spectra were conducted to measure the element distribution and ingredient content of the Mn-Ni2P and V-Ni2P sample (In order to eliminate the effect of Ni foam, the powder samples were scratched from the Ni substrate). As shown in Fig. S1, the Mn-Ni2P sample was composed of the Ni, P, and Mn element with a Mn:Ni:P molar ratio of about 1:11.27:6.05. While no Mn element was detected in V-Ni2P and the molar ratio of Ni:P is about 1:0.56, indicating that the doped Mn element has been completely removed after the acid-etching process. Based on the above EDS results, it can be concluded that the ratio of cationic vacancy defects in the V-Ni2P catalyst is about 8.1%. Additionally, the element mapping (Fig. S2) shows that the Ni and P element are distributed uniformly in V-Ni2P/NF. For comparison, the pure Ni2P/ NF was also prepared and characterized by SEM and XRD. As illustrated in Fig. S3, the Ni2P/NF sample synthesized by introducing additional Ni2+ ions into the reaction system also presents the morphology of nanosheet arrays which is similar to the self-growth Mn-Ni2P/NF sample with minor difference. The chemical compositions and element valence states of MnNi2P/NF and V-Ni2P/NF samples were characterized by XPS. Not surprisingly, there is no any peak belong to Mn species can be observed in XPS survey spectra of V-Ni2P/NF (Fig. S4). The XPS data further confirms that Mn element has been completely eliminated by acid-etching, which is in accord with the above EDS result. The high-resolution Ni 2p spectrum (Fig. 2a) exhibits two peaks at ~853 and ~870 eV corresponding to the Ni 2p3/2 in Ni2P [55,56]. While the peaks at ~856 and ~874 eV with the satellite peaks at ~862 and ~880 eV can be assigned to the NiOx species, which due to the minor oxidation of Ni2P because of the exposure to air [56,57]. This surface oxidation phenomenon can also be observed in P 2p spectrum, the peaks at ~129 and ~134 eV are corresponding to the phosphide and phosphorus oxide species, respectively [58,59]. Compared to that of Mn-Ni2P/NF sample, the Ni 2p binding energy of V-Ni2P/NF is obviously moved to the higher energy side by 0.7 eV for 2p3/2, which indicates the electrons delocation of Ni active-sites because of the introduction of surface cationic vacancies. In addition, in the P 2p spectrum (Fig. 2b), the binding energy of V-Ni2P/NF is also obviously shifted to the lower energy side by 0.4 eV compared to that of Mn-Ni2P/NF, suggesting the presence of many negative charged Px
after the formation of cationic vacancy defects.

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

5

Fig. 1. SEM images of (a,b) Mn-Ni2P/NF and (c,d) V-Ni2P/NF. (e) TEM image and (f) HRTEM image of V-Ni2P/NF. (g) XRD patterns of V-Ni2P/NF and Mn-Ni2P/NF.

Fig. 2. High-resolution XPS spectra of (a) Ni 2p and (b) P 2p for V-Ni2P/NF and Mn-Ni2P/NF.

The HER activities of as-prepared integrated electrodes were then investigated to shed light on the important role of cationicvacancy defects playing in electrocatalytic field. Before the formal test, 50 cyclic voltammetric (CV) circles were conducted to achieve the steady state. The LSV curves of V-Ni2P/NF, Mn-Ni2P/NF and Ni2P/NF are given in Fig. 3a. It can be obviously seen that the V-Ni2P/NF sample only needs a little overpotential of 81 mV to afford the current density of 10 mA cm
2, which is better than the other two prepared electrodes and many reported non-noble metal based HER catalysts (Table S1). The intrinsic activity of the V-Ni2P/NF was further evaluated in terms of the apparent turnover frequency (TOF). The TOF values are calculated in accordance with the electrochemical active sites which are proportional to the surface charges (Fig. S5a). As given in Fig. S5b, the TOF value of V-Ni2P/ NF (1.85 s
1) at the overpotential of 200 mV is much better than that of Mn-Ni2P/NF (0.94 s
1) and Ni2P/NF (0.81 s
1), demonstrating that V-Ni2P/NF possesses higher intrinsic activity by the formation of cationic vacancy defects compared to Mn-Ni2P/NF and Ni2P/ NF catalysts. Furthermore, the electrochemically active surface area of as-prepared electrodes were estimated by the Cdl calculated on the basis of CV measurement. The calculated Cdl values of V-Ni2P/NF, Mn-Ni2P/NF and Ni2P/NF electrodes are 31, 22 and 11 mF cm
2, respectively (Fig. S6). The favorable HER kinetic of V-Ni2P/NF catalytic electrode was firstly proven by a smaller tafel slope of 48 mV dec
1 compared to Mn-Ni2P/NF (60 mV dec
1) and Ni2P/NF (71 mV dec
1) (Fig. 3b) and further verified by the EIS test (Fig. 3c). The charge transfer resistance (Rct) of V-Ni2P/NF is obviously smaller than that of Mn-Ni2P/NF and Ni2P/NF (Table S2), suggesting the introduction of cationic vacancies could accelerate the charge transfer rate during the HER process. The sta-

bility is another important parameter to estimate the performance of the catalysts in practical applications. As displayed in Fig. 3d, the potential remained nearly unchanged during the chronopotentiometry (CP) measurement which carried out at the current density of 10 mA cm
2 for 50 h. The polarization curve was also recorded after the CP test. As shown in Fig. S7, the HER activity was only slightly declined compared to the initial state, indicating the favorable HER catalytic activity and long-time stability. The remarkable HER stability of V-Ni2P/NF also can be supported by SEM, EDS and XRD analyses. Fig. S8 clearly reveals that the morphology, composition, and crystal structure of V-Ni2P/NF preserved very well after long-term HER process, which can be attributed to the mechanical and chemical stability of the self-growth V-Ni2P nanosheets on Ni foam substrate. Besides the HER activity, the OER activity of as-prepared catalytic electrodes were also investigated by the same threeelectrodes system in O2-saturated 1 M KOH electrolyte. It can be easily observed that the self-growth V-Ni2P/NF catalytic electrode performs the excellent OER performance with small overpotential (250 mV) at 10 mA cm
2 and low tafel slope (81 mv dec
1) (Figs. 4a and S9), which was not only better than the prepared Mn-Ni2P/NF (280 mV) and Ni2P/NF (360 mV) samples but also many reported none noble-metal based catalysts (Table S3). Moreover, the CP curve (Fig. 4b) and corresponding LSV curve (Fig. S10) after the CP measurement show the negligible degradation of potential after 50 h, indicating that the V-Ni2P catalytic electrode also exhibited the remarkable OER stability. In addition, due to the mechanical stability of the self-growth V-Ni2P nanosheets on Ni foam, the morphology and micro-structure of V-Ni2P/NF preserved very well after long-time OER process (Fig. S11a,b). However, the XRD result

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

6

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

Fig. 3. HER performance of as-prepared electrode in 1 M KOH. (a) LSV curves, (b) corresponding tafel slopes and (c) Nyquist plots of V-Ni2P/NF, Mn-Ni2P/NF and Ni2P/NF electrode (insert: the equivalent circuit used to fit the experiment data). (d) Long-term HER stability test of V-Ni2P/NF at 10 mA cm
2 (without iR correction).

Fig. 4. OER performance of as-prepared electrode in 1 M KOH. (a) LSV curves of V-Ni2P/NF, Mn-Ni2P/NF and Ni2P/NF electrode. (b) Long-term OER stability test of V-Ni2P/NF at 10 mA cm
2 (without iR correction).

clearly showed that the diffraction peaks belong to Ni2P were vanished and two new diffraction peaks belong to nickel oxide hydroxide (JCPDS NO.40-1179) were appeared, indicating that Ni2P was oxidized during the OER process (Fig. S11d). This change also can be verified by the EDX test (Fig. S11c). The P peak is virtually disappeared while the intensity of O peak is obviously increased. This phenomenon is also consistent with previous reports of phosphide-based catalysts for OER [58,60,61]. In order to further understand the inherent relationship between the cationic vacancy defects and outstanding activity for HER and OER, the density functional theory (DFT) simulations were carried out. Generally, the HER reaction in alkaline electrolyte can be divided into three states including the initial state of H2O adsorption, the intermediate state of catalyst-H* and the final state of H2 desorption. Thus, the adsorption energy of H* and H2O on the surface of different catalysts were evaluated, respectively. Based on the TEM and XRD analysis, the V-Ni2P (1 1 1), Mn-Ni2P (1 1 1) and Ni2P (1 1 1) plane with the adsorption of H* and H2O were modeled and shown in Figs. S12–14. Currently, the Gibbs free-energy of adsorbed H* (DGH) is regard as an important parameter to evaluate the capability for catalyzing hydrogen generation. The excellent

HER catalyst usually possesses the absolute value of DGH = 0, which is the most thermal-neutral state [51,62]. As shown in Fig. 5a, the (1 1 1) surface of V-Ni2P exhibits a DGH value of
0.18 eV, which is much close to Pt (
0.09 eV) [52,63] and more thermo-neutral than Mn-Ni2P (1 1 1) (
0.25 eV) and Ni2P (1 1 1) (
0.39 eV), implying the favorable adsorption-desorption ability of the intermediate adsorbed hydrogen on V-Ni2P. The H2O adsorption energy on the catalysts were also studied to evaluate the adsorption capability of H2O. The V-Ni2P shows a much higher adsorption energy of H2O (1.24 eV) than that of Mn-Ni2P (0.88 eV) and Ni2P (0.40 eV), indicating that the H2O can be simply adsorbed on the surface of V-Ni2P to facilitate the HER process. While during the OER processes, the typically four reaction processes were involved containing the adsorption of OH*, O* and OOH*. Similar to the HER process, the Gibbs free-energy for intermediate adsorption species is also crucial to determine the OER activity. So the V-Ni2P (1 1 1), MnNi2P (1 1 1) and Ni2P (1 1 1) plane with the adsorption of OH*, O* and OOH* (Figs. S15–17) were constructed to calculated the Gibbs free-energy at the electrode potential U = 0 V (Table S4) and 1.23 V (vs. RHE). As shown in Fig. 5b, all the Gibbs free-energy corresponding to each reaction steps present the uphill tendency under 0 V

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

7

Fig. 5. Mechanism investigation. (a) Gibbs free-energy of adsorbed H* and adsorption energy of H2O on the catalyst surface. Gibbs free-energy of OH*, O* and OOH* adsorption on the catalyst surface at the electrode potential (b) U = 0 V and (c) U = 1.23 V. (d) Total DOS calculated for the catalyst.

Fig. 6. Overall water electrolysis performance of as-prepared electrode in 1 M KOH. (a) LSV curves of V-Ni2P/NF, Mn-Ni2P/NF and Ni2P/NF electrode. (b) The long-term stability test of V-Ni2P/NF at 10 mA cm
2 for overall water splitting.

(vs. RHE), indicating that the additional energy is needed to trigger the reaction. Moreover, the change of Gibbs free-energy (DG) of VNi2P for each step is also calculated. In the four reaction step, the third step which converts O* to OOH* requires the highest free energy change (DG3 = 2.32 eV), indicating it is the rate controlling step. Interestingly, the third step of Mn-Ni2P and Ni2P are also the rate controlling step. Based on the above Eq. (18), the theoretical overpotential is obtained from max(DG). Thus the theoretical overpotential of V-Ni2P is 1.09 V, much lower than that of Mn-Ni2P (1.51 V) and Ni2P (1.74 V). This order is completely consistent with the test results. When the equilibrium potential (U = 1.23 V) was applied, the |DG| of V-Ni2P at each reaction step is much lower than Mn-Ni2P and Ni2P, suggesting a favorable adsorption process of intermediates on V-Ni2P (1 1 1) (Fig. 5c). This result clearly demonstrates that the cationic vacancy on the Ni2P surface play a key role on the OER activity. Additionally, the distribution of the density of states (DOS) are also given in Fig. 5d. The electron density of V-Ni2P at the fermi level (Ef) is much higher than Mn-Ni2P and Ni2P, suggesting that the cation vacancy can optimize the electronic structure to enhance the electrical conductivity of the catalyst.

Combining the above experimental and DFT results, it can be definitely concluded that the V-Ni2P/NF catalytic electrode shows both superior HER and OER catalytic ability in alkaline medium. Therefore, the self-growth V-Ni2P/NF should be a promising electrochemical catalyst for overall water electrolysis. A twoelectrode water electrolysis device by employed V-Ni2P/NF as both anode as well as cathode was packaged and performed in 1 M KOH solution. As shown in Fig. 6a, V-Ni2P/NF||V-Ni2P/NF system only requires a low cell voltage of 1.59 V to achieve the current density of 10 mA cm
2, which is better than that of Mn-Ni2P/NF||Mn-Ni2P/ NF (1.64 V) and Ni2P/NF||Ni2P/NF system (1.68 V). Furthermore, the V-Ni2P/NF||V-Ni2P/NF system also exhibited an impressive stability performance during overall water splitting. The voltage of the electrolytic device at 10 mA cm
2 only slightly increased after 50 h (Fig. 6b). 4. Conclusions In summary, the self-growth Ni2P nanosheet arrays on Ni foam with abundant cationic vacancy defects have been successfully

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

8

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

prepared and directly used as the catalytic electrode for bifunctional water electrolysis. The electrochemical tests show that the V-Ni2P/NF catalytic electrode only needs a small voltage of 1.59 V to afford 10 mA cm
2 and stably operates for at least 50 h in alkaline electrolyte. This outstanding bifunctional catalytic capability of the V-Ni2P/NF electrode is mainly ascribed to the collective effects of the following three merits: (1) The monolithic integration of the V-Ni2P nanosheet arrays on Ni foam retains better electrical connection and mechanical property compared to those prepared by using extra binders [64,65] or synthesized by introducing extra Ni2+ in the reaction solution [66,67], so more efficient pathways for electron and mass transportation are provided, as well as the collapse and agglomeration of nanostructures can be avoided. (2) The creation of abundant cationic-vacancy defects on the surface of Ni2P nanosheets can effectively reduce the energy barriers of H2O adsorption and optimize the adsorption process of intermediates during the HER and OER. Moreover, the cationic-vacancy defects can also optimize the electronic structure to improve the electrical conductivity. (3) The porous structure on the surface of the V-Ni2P nanosheets generated during the acid-etching process can not only increase the active specific surface area but also facilitate the infiltration of electrolyte during the overall water electrolysis. The above research results demonstrate the promising potential of the prepared V-Ni2P/NF to replace precious metalbased catalysts for the practical production of clean hydrogen energy. Declaration of Competing Interest We declare that we have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper, and no financial interests or personal relationships which may be considered as potential competing interests. Acknowledgements The authors are grateful to the Fundamental Research Funds for the Central Universities of China (No. DUT17LK01) and the National Natural Science Foundation of China (No. 51575088). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.039. References [1] L. Yan, Y. Sun, E. Hu, J. Ning, Y. Zhong, Z. Zhang, Y. Hu, Facile in-situ growth of Ni2P/Fe2P nanohybrids on Ni foam for highly efficient urea electrolysis, J. Colloid Interf. Sci. 541 (2019) 279–286. [2] F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao, W.A. Goddard III, S. Chen, Z. Ren, High-performance bifunctional porous non-noble metal phosphide catalyst for overall water splitting, Nat. Commun. 9 (2018) 2551. [3] N. Mahmood, Y. Yao, J.W. Zhang, L. Pan, X. Zhang, J.J. Zou, Electrocatalysts for hydrogen evolution in alkaline electrolytes: mechanisms, challenges, and prospective solutions, Adv. Sci. 5 (2018) 1700464. [4] K.L. Yan, J.F. Qin, Z.Z. Liu, B. Dong, J.Q. Chi, W.K. Gao, J.H. Lin, Y.M. Chai, C.G. Liu, Organic-inorganic hybrids-directed ternary NiFeMoS anemone-like nanorods with scaly surface supported on nickel foam for efficient overall water splitting, Chem. Eng. J. 334 (2018) 922–931. [5] N.K. Chaudhari, H. Jin, B. Kim, K. Lee, Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting, Nanoscale 9 (2017) 12231. [6] Y. Zheng, Y. Jiao, A. Vasileff, S.Z. Qiao, The hydrogen evolution reaction in alkaline solution: from theory, single crystal models, to practical electrocatalysts, Angew. Chem. Int. Ed. 57 (2018) 7568–7579. [7] G.B. Darband, M. Aliofkhazraei, A.S. Rouhaghdam, Facile electrodeposition of ternary Ni-Fe-Co alloy nanostructure as a binder free, cost-effective and durable electrocatalyst for high-performance overall water splitting, J. Colloid Interf. Sci. 547 (2019) 407–420.

[8] A. Sivanantham, P. Ganesan, L. Estevez, B.P. McGrail, R.K. Motkuri, S. Shanmugam, A stable graphitic, nanocarbon-encapsulated, cobalt-rich coreshell electrocatalyst as an oxygen electrode in a water electrolyzer, Adv. Energy Mater. 8 (2018) 1702838. [9] X. Xu, Y. Deng, M. Gu, B. Sun, Z. Liang, Y. Xue, Y. Guo, J. Tian, H. Cui, Large-scale synthesis of porous nickel boride for robust hydrogen evolution reaction electrocatalyst, Appl. Surf. Sci. 470 (2019) 591–595. [10] J. Wang, Z. Wei, S. Mao, H. Li, Y. Wang, Highly uniform Ru nanoparticles over N-doped carbon: pH and temperature-universal hydrogen release from water reduction, Energy Environ. Sci. 11 (2018) 800–806. [11] C. Meng, T. Ling, T.Y. Ma, H. Wang, Z. Hu, Y. Zhou, J. Mao, X.W. Du, M. Jaroniec, S.Z. Qiao, Atomically and electronically coupled Pt and CoO hybrid nanocatalysts for enhanced electrocatalytic performance, Adv. Mater. 29 (2017) 1604607. [12] J.N. Tiwari, S. Sultan, C.W. Myung, T. Yoon, N. Li, M. Ha, A.M. Harzandi, H.J. Park, D.Y. Kim, S.S. Chandrasekaran, W.G. Lee, V. Vij, H. Kang, T.J. Shin, H.S. Shin, G. Lee, Z. Lee, K.S. Kim, Multicomponent electrocatalyst with ultralow Pt loading and high hydrogen evolution activity, Nat. Energy 3 (2018) 773. [13] J. Xu, S. Murphy, D. Xiong, R. Cai, X.K. Wei, M. Heggan, E. Barborini, S. Vinati, R. E.D. Borkowski, R.E. Palmer, L. Liu, Cluster beam deposition of ultrafine cobalt and ruthenium clusters for efficient and stable oxygen evolution reaction, ACS Appl. Energy Mater. 1 (2018) 3013–3018. [14] L. Li, J. Li, L. Liu, X. Wang, Y. Gao, Y. Zhou, One pot assembly of vertical embedded MoS2/Graphene heterostructure and its high performance for hydrogen evolution reaction, ACS Appl. Energy Mater. 2 (2019) 1413–1418. [15] J. Du, R. Wang, Y.R. Lv, Y.L. Wei, S.Q. Zang, One-step MOF-derived Co/Co9S8 nanoparticles embedded in nitrogen, sulfur and oxygen ternary-doped porous carbon: an efficient electrocatalyst for overall water splitting, Chem. Commun. 55 (2019) 3203–3206. [16] X. Shang, J.F. Qin, J.H. Lin, B. Dong, J.Q. Chi, Z.Z. Liu, L. Wang, Y.M. Chai, C.C. Liu, Tuning the morphology and Fe/Ni ratio of a bimetallic Fe-Ni-S film supported on nickel foam for optimized electrolytic water splitting, J. Colloid Interf. Sci. 523 (2018) 121–132. [17] W.K. Gao, J.F. Qin, K. Wang, K.L. Yan, Z.Z. Liu, J.H. Lin, Y.M. Chai, C.G. Liu, B. Dong, Facile synthesis of Fe-doped Co9S8 nano-microspheres grown on nickel foam for efficient oxygen evolution reaction, Appl. Surf. Sci. 454 (2018) 46–53. [18] H. Zhao, Z.Y. Yuan, Transition metal-phosphorus-based materials for electrocatalytic energy conversion reactions, Catal. Sci. Technol. 7 (2017) 330–347. [19] Q. Yan, T. Wei, J. Wu, X. Yang, M. Zhu, K. Cheng, K. Ye, K. Zhu, J. Yan, D. Cao, G. Wang, Y. Pan, Self-supported FeNi-P nanosheets with thin amorphous layers for efficient electrocatalytic water splitting, ACS Sustainable Chem. Eng. 6 (2018) 9640–9648. [20] Y. Yan, X.R. Shi, M. Miao, T. He, Z.H. Dong, K. Zhan, J.H. Yang, B. Zhao, B.Y. Xia, Bio-inspired design of hierarchical FeP nanostructure arrays for the hydrogen evolution reaction, Nano Res. 11 (2018) 3537–3547. [21] D.Y. Chung, S.W. Jun, G. Yoon, H. Kim, J.M. Yoo, K.S. Lee, T. Kim, H. Shin, A.K. Sinha, S.G. Kwon, K. Kang, T. Hyeon, Y.E. Sung, Large-scale synthesis of carbonshell-coated FeP nanoparticles for robust hydrogen evolution reaction electrocatalyst, J. Am. Chem. Soc. 139 (2017) 6669–6674. [22] J. Xu, J.P.S. Sousa, N.E. Mordvinova, J.D. Costa, D.Y. Petrovykh, K. Kovnir, O.I. Lebedev, Y.V. Kolenko, Al-induced in situ formation of highly active nanostructured water-oxidation electrocatalyst based on Ni-phosphide, ACS Catal. 8 (2018) 2595–2600. [23] Z. Chen, Y. Ha, Y. Liu, H. Wang, H. Yang, H. Xu, Y. Li, R. Wu, In situ formation of cobalt nitrides/graphitic carbon composites as efficient bifunctional electrocatalysts for overall water splitting, ACS Appl. Mater. Interfaces 10 (2018) 7134–7144. [24] C. Zhu, Z. Yin, W. Lai, Y. Sun, L. Liu, X. Zhang, Y. Chen, S.L. Chou, Fe-Ni-Mo nitride porous nanotubes for full water splitting and Zn-Air batteries, Adv. Energy Mater. 8 (2018) 1802327. [25] N. Han, K.R. Yang, Z. Lu, Y. Li, W. Xu, T. Gao, Z. Cai, Y. Zhang, V.S. Batista, W. Liu, X. Sun, Nitrogen-doped tungsten carbide nanoarray as an efficient bifunctional electrocatalyst for water splitting in acid, Nat. Commun. 9 (2018) 924. [26] C. Lu, D. Tranca, J. Zhang, F.R. Hernandez, Y. Su, X. Zhuang, F. Zhang, G. Seifert, X. Feng, Molybdenum carbide-embedded nitrogen-doped porous carbon nanosheets as electrocatalysts for water splitting in alkaline media, ACS Nano 11 (2017) 3933–3942. [27] M. Zhuang, X. Ou, Y. Dou, L. Zhang, Q. Zhang, R. Wu, Y. Ding, M. Shao, Z. Luo, Polymer-embedded fabrication of Co2P nanoparticles encapsulated in N, P-doped graphene for hydrogen generation, Nano Lett. 16 (2016) 4691– 4698. [28] X. Cao, D. Jia, D. Li, L. Cui, J. Liu, One-step co-electrodeposition of hierarchical radial NixP nanospheres on Ni foam as highly active flexible electrodes for hydrogen evolution reaction and supercapacitor, Chem. Eng. J. 348 (2018) 310–318. [29] G.F. Chen, T.Y. Ma, Z.Q. Liu, N. Li, Y.Z. Su, K. Davey, S.Z. Qiao, Efficient and stable bifunctional electrocatalysts Ni/NixMy (M= P, S) for overall water splitting, Adv. Funct. Mater. 26 (2016) 3314–3323. [30] M. Ledendecker, S.K. Calderon, C. Papp, H.P. Steinruck, M. Antonietti, M. Shalom, The synthesis of nanostructured Ni5P4 films and their use as a nonnoble bifunctional electrocatalyst for full water splitting, Angew. Chem. Int. Ed. 54 (2015) 12361–12365. [31] U.K. Sultana, A.P. OMullane, Electrochemical formation of amorphous molybdenum phosphosulfide for enabling the hydrogen evolution reaction in alkaline and acidic media, ACS Appl. Energy Mater. 1 (2018) 2849–2858.

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039

W.-Z. Zhang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx [32] L.A. Stern, L. Feng, F. Song, X. Hu, Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles, Energy Environ. Sci. 8 (2015) 2347–2351. [33] D.H. Ha, B. Han, M. Risch, L. Giordano, K.P.C. Yao, P. Karayaylali, Y.S. Horn, Activity and stability of cobalt phosphides for hydrogen evolution upon water splitting, Nano Energy 29 (2016) 37–45. [34] Q. Wang, Z. Liu, H. Zhao, H. Huang, H. Jiao, Y. Du, MOF-derived porous Ni2P nanosheets as novel bifunctional electrocatalysts for the hydrogen and oxygen evolution reactions, J. Mater. Chem. A 6 (2018) 18720–18727. [35] J. Li, G. Wei, Y. Zhu, Y. Xi, X. Pan, Y. Ji, I.V. Zatovsky, W. Han, Hierarchical NiCoP nanocone arrays supported on Ni foam as an efficient and stable bifunctional electrocatalyst for overall water splitting, J. Mater. Chem. A 5 (2017) 14828– 14837. [36] Q. Wang, H. Zhao, F. Li, W. She, X. Wang, L. Xu, H. Jiao, Mo-doped Ni2P hollow nanostructures: highly efficient and durable bifunctional electrocatalysts for alkaline water splitting, J. Mater. Chem. A 7 (2019) 7636–7643. [37] X. Wang, W. Ma, C. Ding, Z. Xu, H. Wang, X. Zong, C. Li, Amorphous multielements electrocatalysts with tunable bifunctionality toward overall water splitting, ACS Catal. 8 (2018) 9926–9935. [38] Z. Wu, D. Nie, M. Song, T. Jiao, G. Fu, X. Liu, Facile Synthesis of Co-Fe-B-P nanochains as efficient bifunctional electrocatalysts for overall water-splitting, Nanoscale 11 (2019) 7506–7512. [39] A.Y. Lu, X. Yang, C.C. Tseng, S. Min, S.H. Lin, C.L. Hsu, H. Li, H. Idriss, J.L. Kuo, K. W. Huang, L.J. Li, High-sulfur-vacancy amorphous molybdenum sulfide as a high current electrocatalyst in hydrogen evolution, Small 12 (2016) 5530– 5537. [40] J. Bao, X. Zhang, B. Fan, J.J. Zhang, M. Zhou, W. Yang, X. Hu, H. Wang, B. Pan, Y. Xie, Ultrathin spinel-structured nanosheets rich in oxygen deficiencies for enhanced electrocatalytic water oxidation, Angew. Chem. Int. Ed. 127 (2015) 7507–7512. [41] Z. Xiao, Y. Wang, Y.C. Huang, Z. Wei, C.L. Dong, J. Ma, S. Shen, Y. Li, S. Wang, Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting, Energy Environ. Sci. 10 (2017) 2563– 2569. [42] D. Gao, B. Xia, Y. Wang, W. Xiao, P. Xi, D. Xue, J. Ding, Dual-native vacancy activated basal plane and conductivity of MoSe2 with high-efficiency hydrogen evolution reaction, Small 14 (2018) 1704150. [43] W.L. Kwong, E.G. Espino, C.C. Lee, R. Sandstrom, T. Wagberg, J. Messinger, Cationic vacancy defects in iron phosphide: a promising route toward efficient and stable hydrogen evolution by electrochemical water splitting, ChemSusChem 10 (2017) 4544–4551. [44] B. Liu, Y. Wang, H.Q. Peng, R. Yang, Z. Jiang, X. Zhou, C.S. Lee, H. Zhao, W. Zhang, Iron vacancies induced bifunctionality in ultrathin feroxyhyte nanosheets for overall water splitting, Adv. Mater. 30 (2018) 1803144. [45] T. Zhang, M.Y. Wu, D.Y. Yan, J. Mao, H. Liu, W.B. Hu, X.W. Du, T. Ling, S.Z. Qiao, Engineering oxygen vacancy on NiO nanorod arrays for alkaline hydrogen evolution, Nano Energy 43 (2018) 103–109. [46] J. Xu, T. Liu, J. Li, B. Li, Y. Liu, B. Zhang, D. Xiong, I. Amorim, W. Li, L. Liu, Boosting the hydrogen evolution performance of ruthenium clusters through synergistic coupling with cobalt phosphide, Energy Environ. Sci. 11 (2018) 1819–1827. [47] D. Merki, S. Fierro, H. Vrubel, X. Hu, Amorphous molybdenum sulfide films as catalysts for electrochemical hydrogen production in water, Chem. Sci. 2 (2011) 1262–1267. [48] Y. Wu, X. Liu, D. Han, X. Song, L. Shi, Y. Song, S. Niu, Y. Xie, J. Cai, S. Wu, J. Kang, J. Zhou, Z. Chen, X. Zheng, X. Xiao, G. Wang, Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis, Nat. Commun. 9 (2018) 1425. [49] J. Li, M. Yan, X. Zhou, Z.Q. Huang, Z. Xia, C.R. Chang, Y. Ma, Y. Qu, Mechanistic insights on ternary Ni2-xCoxP for hydrogen evolution and their hybrids with graphene as highly efficient and robust catalysts for overall water splitting, Adv. Funct. Mater. 26 (2016) 6785–6796. [50] W. Gao, M. Yan, H.Y. Cheung, Z. Xia, X. Zhou, Y. Qin, C.Y. Wong, J.C. Ho, C.R. Chang, Y. Qu, Modulating electronic structure of CoP electrocatalysts towards

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67]

9

enhanced hydrogen evolution by Ce chemical doping in both acidic and basic media, Nano Energy 38 (2017) 290–296. B. Liu, Y.F. Zhao, H.Q. Peng, Z.Y. Zhang, C.K. Sit, M.F. Yuen, T.R. Zhang, C.S. Lee, W.J. Zhang, Nickel-cobalt diselenide 3D mesoporous nanosheet networks supported on Ni foam: an all-pH highly efficient integrated electrocatalyst for hydrogen evolution, Adv. Mater. 29 (2017) 1606521. Y. Zheng, Y. Jiao, Y. Zhu, L.H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S.Z. Qiao, Hydrogen evolution by a metal-free electrocatalyst, Nat. Commun. 5 (2014) 3783. K. Hu, M. Wu, S. Hinokuma, T. Ohto, M. Wakisaka, J.I. Fujita, Y. Ito, Boosting electrochemical water splitting via ternary NiMoCo hybrid nanowire arrays, J. Mater. Chem. A 7 (2019) 2156–2164. L.C. Seitz, C.F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H.Y. Hwang, J.K. Norskov, T.F. Jaramillo, A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction, Science 353 (2016) 1011–1014. W. Li, D. Xiong, X. Gao, W.G. Song, F. Xia, L. Liu, Self-supported Co-Ni-P ternary nanowire electrodes for highly efficient and stable electrocatalytic hydrogen evolution in acidic solution, Catal. Today 287 (2017) 122–129. J. Xiao, Q. Lv, Y. Zhang, Z. Zhang, S. Wang, One-step synthesis of nickel phosphide nanowire array supported on nickel foam with enhanced electrocatalytic water splitting performance, RSC Adv. 6 (2016) 107859– 107864. J. Zheng, W. Zhou, T. Liu, S. Liu, C. Wang, L. Guo, Homologous NiO//Ni2P nanoarrays grown on nickel foams: a well matched electrode pair with high stability in overall water splitting, Nanoscale 9 (2017) 4409–4418. X. Wang, W. Li, D. Xiong, D.Y. Petrovykh, L. Liu, Bifunctional nickel phosphide nanocatalysts supported on carbon fiber paper for highly efficient and stable overall water splitting, Adv. Funct. Mater. 26 (2016) 4067–4077. J. Xu, Y. Liu, J. Li, I. Amorim, B. Zhang, D. Xiong, N. Zhang, S.M. Thalluri, J.P.S. Sousa, L. Liu, Hollow cobalt phosphide octahedral pre-catalysts with exceptionally high intrinsic catalytic activity for electro-oxidation of water and methanol, J. Mater. Chem. A 6 (2018) 20646–20652. Y. Tan, H. Wang, P. Liu, Y. Shen, C. Cheng, A. Hirata, T. Fujita, Z. Tang, M. Chen, Versatile nanoporous bimetallic phosphides towards electrochemical water splitting, Energy Environ. Sci. 9 (2016) 2257–2261. X. Cheng, Z. Pan, C. Lei, Y. Jin, B. Yang, Z. Li, X. Zhang, L. Lei, C. Yuan, Y. Hou, A strongly coupled 3D ternary Fe2O3@Ni2P/Ni(PO3)2 hybrid for enhanced electrocatalytic oxygen evolution at ultra-high current densities, J. Mater. Chem. A 7 (2019) 965–971. H. Shu, D. Zhou, F. Li, D. Cao, X. Chen, Defect engineering in MoSe2 for the hydrogen evolution reaction: from point defects to edges, ACS Appl. Mater. Interfaces 9 (2017) 42688–42698. Q. Zhou, Z. Shen, C. Zhu, J. Li, Z. Ding, P. Wang, F. Pan, Z. Zhang, H. Ma, S. Wang, H. Zhang, Nitrogen-doped CoP electrocatalysts for coupled hydrogen evolution and sulfur generation with low energy consumption, Adv. Mater. 30 (2018) 1800140. B. Sun, S. Yang, Y. Guo, Y. Xue, J. Tian, H. Cui, X. Song, Fabrication of molybdenum and tungsten oxide, sulfide, phosphide (MoxW1-xO2/MoxW1-xS2 / MoxW1-xP) porous hollow nano-octahedrons from metal-organic frameworks templates as efficient hydrogen evolution reaction electrocatalysts, J. Colloid Interf. Sci. 547 (2019) 339–349. X. Wei, Y. Zhang, H. He, L. Peng, S. Xiao, S. Yao, P. Xiao, Carbon-incorporated porous honeycomb NiCoFe phosphide nanospheres derived from a MOF precursor for overall water splitting, Chem. Commun. 57 (2019) 10896–10899. R. Wu, B. Xiao, Q. Gao, Y.R. Zheng, X.S. Zheng, J.F. Zhu, M.R. Gao, S.H. Yu, A janus nickel cobalt phosphide catalyst for high-efficiency neutral-pH water splitting, Angew. Chem. Int. Ed. 57 (2018) 15445–15449. W. Zhang, G. Chen, J. Zhao, J. Liang, G. Liu, B. Ji, L. Sun, Fe-doped CoNi0.5P hierarchical arrays as efficient bifunctional electrocatalysts for overall water splitting: Evolution of morphology and coordination of catalytic performance, ChemistrySelect 4 (2019) 6744–6752.

Please cite this article as: W.-Z. Zhang, G. Y. Chen, J. Zhao et al., Self-growth Ni2P nanosheet arrays with cationic vacancy defects as a highly efficient bifunctional electrocatalyst for overall water splitting, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.039