Ni3S2 Nanosheets in Situ Epitaxially Grown on Nanorods as High

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Ni3S2 nanosheets in-situ epitaxially grown on nanorods as high active and stable homojunction electrocatalyst for hydrogen evolution reaction Miaomiao Tong, Lei Wang, Peng Yu, Chungui Tian, Xu Liu, Wei Zhou, and Honggang Fu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03915 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Sustainable Chemistry & Engineering

Ni3S2 nanosheets in-situ epitaxially grown on nanorods as high active and stable homojunction electrocatalyst for hydrogen evolution reaction Miaomiao Tong, Lei Wang,* Peng Yu, Chungui Tian, Xu Liu, Wei Zhou, Honggang Fu* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin Xuefu Road, 150080, P.R.China. E-mail: [email protected],[email protected],[email protected]

ABSTRACT: Development of efficient noble metal-free electrocatalysts for accelerating the sluggish kinetics in the hydrogen evolution reaction (HER) has received a great deal of attention in electrolytic water splitting. Herein, we present a facile one-step solvothermal strategy for controllable constructing the homojunction structures of Ni3S2 nanosheets in-situ epitaxially grown on nanorods by using Ni foam as self-support substrate and nickel resource (Ni3S2/NF). In the synthesis, cetyltrimethyl ammonium bromide (CTAB) and hydrazine hydrate are used to controlling the formation of nanorods and nanosheets, respectively. The special 3D Ni3S2 nanorods@nanosheets homojunction could provide plentiful catalytically active sites, meanwhile, the intimate contact between Ni3S2 and Ni foam could enhance the long-term stability. The inevitable sulfur vacancies in the Ni3S2 could tune electronic structure of the surface and enhance the catalytic activity. The synergistic effect leads

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to the as-prepared Ni3S2/NF exhibits a superior HER performance with ηonset of 10.8 mV, η10 of 48.1 mV, and a Tafel slope of 88.2 mV dec−1 in alkaline electrolyte. Furthermore, it can continuously work for 10000 cycles with negligible activity loss. This work opens a new avenue for designing and synthesizing noble metal-free electrocatalysts with high activity and well stability towards HER. KEYWORDS: Ni3S2, homojunction, hydrogen evolution reaction, electrocatalytic, self-support

Introduction Presently, the rapid growth of fossil fuel depletion has caused not only the energy crisis but also the inevitable associated environmental issues, in particular, global warming and haze

[1, 2]

. For this reason, unflagging efforts have been devoted to the

exploitation and utilization of renewable and clean energy sources

[3, 4]

. With this

regard, hydrogen possesses the highest gravimetric energy density (142 MJ kg−1), can be considered as an ideal clean, high-efficient and renewable energy source [5, 6]. Thus far, the hydrogen evolution reaction (HER) of electrocatalytic water splitting plays the most effective and sustainable hydrogen production technology, in which electrocatalysts are mainly based on noble metal Pt

[7, 8]

. Although Pt-based catalysts

exhibit excellent energy transfer efficiency with low overpotential and low Tafel slop, the scarce and expensive nature of precious metals could severely hamper their practical utilization

[9-11]

. Therefore, novel catalysts for replacing noble metal Pt with

both high efficient and populist price have attracted immense research interest.


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Various metal compounds contained earth-abundant elements have been extensively investigated for HER, including early-transition metal or late-transition metal carbides selenides

[27-28]

[12-14]

, nitrides(Ni3N@Ni-Bi)

[15,16]

, phosphides

[17,18]

, sulfides

[19-26]

and their hybrids. Among them, transition metal sulfides, such as

layered transition metal compounds (WS2, MoS2) (Co(OH)2, CoS2, NiS, Ni3S2)

[37-41]

[29-36]

, iron-series compounds

, have obviously shown some inherent advantages

over other materials due to their relatively lower cost, environmentally friendly, rich valence state and easy to prepare. Relatively speaking, nickel sulfides, such as NiS, NiS2, Ni3S4, etc., have been rarely adopted as HER catalysts despite they widely used as electrode materials for supercapacitors and Li-ion batteries

[42,43]

, insomuch the

unperfect electrocatalytic activity results in most of the present catalysts could not fulfill the high standard demand of HER. The high-efficient electrocatalysis of nickel sulfides for HER should satisfy the following criteria simultaneously: (i) the component should possess high intrinsic electrocatalytic activity for HER; (ii) the special structures facilitate to the rapid transfer of reactants and electrons; (iii) a strong adhesion must exist between the catalytic active components and current collector, which could prevent the peeling off during long-time operation. In this respect, special nanostructured metal sulfides directly grow on current collectors to construct 3D networks will endow the advantages of large active surface areas, shorten electron- and ion-transport pathways. Homojunction is the junction composed of the same composite with different nanostructures. It could greatly facilitate to the electron charge due to the lattice match and promote the kinetics ion



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diffusion because of the unique 3D structures compared to the single nanostructures for HER

[32]

. For instance, 1D nanorods could facilitate to the electron transfer by

shortening the pathways, meanwhile, 2D nanosheets possess plenty of electrocatalytic active sites by providing large surface area. As a result, the 3D homojunctions combined of 1D nanorods and 2D nanosheet would exhibit a better HER activity compared with the single nanorods and nanosheets. However, the special 3D nanorods@nanosheets metal compound homojunctions generally synthesized from a tedious and complicated two-step method. Thereby, it is still a challenge to synthesize nickel sulfides nanorods@nanosheets structure by a facile one-step route. Besides the nanostructures of electrocatalysts, the intrinsic activity of the active sites is another crucial adjective factor that affects the overall catalytic performance

[42]

. It is well

known that the existence of vacancies in the electrocatalysts can tune the electronic properties of the surface, which could tune the catalytic activity

[43]

. For Ni3S2, the

main valence state for Ni and S are +2 and –2, respectively, so the inevitable sulfur vacancy in the Ni3S2 could result in tuning electronic structure of the surface and enhancing the catalytic activity for HER. Herein, we present an in-situ growth strategy for synthesis of Ni3S2 nanosheets in-situ epicatically grown on 1D nanorods derived from Ni foam (Ni3S2/NF) via a facile one-step solvothermal route in the existence of hydrazine hydrate, cetyltrimethyl ammonium bromide (CTAB) and sulfur powder, in which Ni foam not only serves as the self-support substrate but also provides the resource of Ni atoms. During the synthesis, CTAB firstly control the formation of Ni3S2 nanorods at relative


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lower solvothermal temperature, and then hydrazine hydrate facilitated to the epitaxial growth of Ni3S2 nanosheets on Ni3S2 nanorods with the improvement of solvothermal temperature. As Pt-free catalyst, the as-prepared Ni3S2/NF exhibited very promising electrochemical catalytic activities for the HER in 1.0 M KOH, with an overpotential of 48.1 mV at 10 mA cm−2 and a Tafel slope of 88.2 mV dec−1. Experimental Synthesis of Ni3S2/NF Ni3S2/NF was obtained by a simple solvothermal strategy. Typically, a piece of NF (2 × 3 cm2) was continuously treated with 1 M HCl, acetone and deionized water several times. Then, the pre-treated NF was immersed to the mixture containing 56 mL deionized water and 4 mL hydrazine hydrate containing 22 mmol cetyltrimethyl ammonium bromide (CTAB) and 4 mmol sulfur powder in a 100 mL Teflon-lined stainless autoclave. After reactions at 160 oC for 12h, the resulting product was washed with ethanol for three times and then dried in vacuum at 40 oC, so Ni3S2/NF could be synthesized (the Ni3S2 content is about 0.9 mg cm−2). In order to investigate the effect of reaction temperature on the nanostructures and performance of the products, different solvothermal temperatures of 140, 150, 170 and 180 oC were employed

to

synthesize

Ni3S2/NF-140,

Ni3S2/NF-150,

Ni3S2/NF-170

and

Ni3S2/NF-180, respectively. In parallel to this, the samples synthesized without using hydrazine hydrate and CTAB were also prepared for comparison, respectively, denoted as Ni3S2/NF-R and Ni3S2/NF-S. The synthetic conditions for all the compared samples were listed in Table S1.



Characterizations X-ray diffraction (XRD) patterns were performed on a Rigaku D/ max-IIIB diffractometer using Cu Kα (λ=1.5406 Å) with the accelerating voltage of 40 kV and the applied current of 20 mA). Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 instrument operating at 5 kV. Transmission electron microscopy (TEM) experiments were obtained with a JEOL JEM-2100 electron microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a VG ESCALABMK II with a Mg KR achromatic X-ray source. Scanning Kelvin probe (SKP) measurements (SKP5050 system, Scotland) were tested in ambient atmosphere, and a gold electrode was used as the reference electrode. The work function (φ) is calculated by the following formula: jAu -

hAu 1000

= j -

h 1000

in which φAu is the work function of Au, ηAu= −239.75 eV. Electrochemical tests All electrochemical performances were tested on CHI660 electrochemical workstation by a standard three-electrode system, in which the synthetic Ni3S2/NF composite was used as working electrode, a graphite rod and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. In all measurements, the SCE reference electrode was calibrated with RHE according to the formula in 1.0 M KOH electrolyte, E(RHE) = E(SCE) + 1.059 V. Polarization curves were recorded by using linear sweep voltammetry (LSV) with a scan rate of 5 mV s−1. All the LSV curves have been treated by iR-corrected. Cyclic


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voltammogram (CV) were tested by different scan rates from 20~200 mV s−1. The ohmic resistance spectra of all the electrodes were measured by AC impedance spectroscopy and all currents presented were corrected against the ohmic potential drop. During the stability test, the catalysts were recorded by cyclic voltammetry (CV) scan between +0.20 and –1.1 V versus RHE at a scan rate of 100 mV s−1 in 1.0 M KOH for 10000 cycles. After that, the LSV curves were recorded. Results and discussion The morphology and structure characterization of Ni3S2/NF The Ni3S2/NF could be synthesized by a facile solvothermal strategy for direct sulfidization of NF with sulfur in a solvothermal system in the presence of CTAB and hydrazine hydrate. In the solvothermal reaction, the NF not only serve as a support material but also serve as the resource of Ni atoms for in-situ formation the special structures of Ni3S2 nanorods@nanosheets homojunction. The crystalline phase of the prepared Ni3S2/NF was measured by X-ray diffraction (XRD) pattern. As shown in Fig. 1, the three strong diffraction peaks located at 44.9o, 52.3o, and 76.7o are ascribed to the (111), (200) and (220) faces of metallic nickel (JCPDS 03-1051), respectively, resulting from the Ni foam [35]. The other peaks at 22.3o, 31.6o, 38.3o, 50.3o, and 55.5o can be corresponded to the (101), (110), (003), (113) and (122) planes of Ni3S2 (JCPDS 44-1418)

[45]

, respectively, indicating the formation of Ni3S2/NF. Besides, no

other obvious peaks can be observed, implying no impurity exist in the sample.



Fig. 1 XRD pattern of the synthetic Ni3S2/NF.

Scan electron microscopy (SEM) image in Fig. S1 indicates NF with a smooth surface. It can be seen that the complete coverage of NF with Ni3S2 nanorods from the low magnification SEM image of the Ni3S2/NF (Fig. 2A). The enlarged SEM image (Fig. 2B, 2C) further displays the Ni3S2 nanorods with a diameter of about 150~200 nm. The transmission electron microscopy (TEM) image further demonstrates the microstructure of Ni3S2 nanorods coated by nanosheets (Fig. 2D). The special nanostructures could be favorable for the rapid transfer of electrons and ions, resulting in the well electrocatalytic performance. In addition, the high-resolution TEM (HRTEM) images in Fig. 2E and 2F shows both the nanosheets and nanorods exhibit clear lattice fringes with interplane distances of 0.28 and 0.41 nm could be observed obviously, corresponding to the (021) and (101) planes of Ni3S2, respectively. It is demonstrated the formation of Ni3S2 nanorods@nanosheets homojuncion.


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Fig. 2 (A-C) SEM, (D) TEM and (E, F) the corresponding HRTEM images of the enlarged red frame area from (D) for Ni3S2/NF.

X-ray photoelectron spectra (XPS) could be further revealed the composition of the synthetic sample. The Ni 2p, S 2p, and O 1s XPS spectra were revealed in Fig. 3A. The Ni 2p high-resolution XPS spectrum in Fig. 3B shows two strong peaks at 855.2 and 872.7 eV corresponded to 2p3/2 and 2p1/2 of Ni2+

[46, 47]

, The peak at 852.7 eV is

ascribed to the characteristic of Ni3S2 [48, 49], Besides, the extra two peaks at 860 and 879 eV indicated the existence of nickel oxide species, demonstrating the sulfur vacancy maybe exist in the Ni3S2 [40]. The high-resolution XPS spectrum of S2p in Fig. 3C reveals the 2p3/2 and2p1/2 of S2− at 161.7 eV and 162.8 eV

[50]

, respectively.

From the above XPS spectra, it can be analyzed that the main valence state for Ni and S in the Ni3S2 are +2 and –2, respectively, so the sulfur vacancies would exist in the



Page 10 of 30

Ni3S2. The O 1s spectrum in Fig. 3D exhibits two peaks at 531.0 and 532.1 eV, respectively, which can be originated from the hydroxyl oxygen of the adsorbed water and surface adsorbed oxygen on the surface of the Ni3S2 [51, 52], respectively. It is further demonstrated the sulfur vacancy sites would occupy by the adsorbed oxide species, which can tune electronic structure of the surface and enhance the catalytic activity for HER.

Fig. 3 XPS spectra of the synthetic Ni3S2/NF: (A) Wide spectrum; High-resolution XPS spectra of (B) Ni 2p, (C) S 2p and (D) O1s.

Formation mechanism of Ni3S2/NF To

investigate

the

formation

mechanism

of


Ni3S2

nanorod@nanosheet

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homojunction structures in the Ni3S2/NF, the compared synthetic experiments without using hydrazine hydrate and CTAB were also prepared for comparison, respectively, denoted as Ni3S2/NF-R and Ni3S2/NF-S. The XRD patterns in Fig. S2 demonstrated that the Ni3S2 crystalline phase still could form in the conditions of only using CTAB and hydrazine hydrate, respectively. TEM images in Fig. S3 indicate that only the Ni3S2 nanorods and nanosheets could be obtained when only using CTAB and hydrazine hydrate in the synthetic process, respectively. This is further demonstrated CTAB and hydrazine hydrate play indispensable roles in the controllable formation of nanorod

and

nanosheet

structures

for

the

special

structures

of

Ni3S2

nanorod@nanosheet, respectively. In view of the above, the Ni3S2/NF derived from different solvothermal temperatures in the range of 120~180 oC were also synthesized (see Table S1). The XRD patterns in Fig. S4 indicate the Ni3S2 phase could be formed under the different solvothermal temperatures. It is further proved that the diameter of the Ni3S2 nanorods is increased with the improvement of solvothermal temperature ascan be seen from the SEM image in Fig. S5. In order to identify the growth process of nanosheets, TEM images were also shown in Fig. S6. It is obviously observed that the Ni3S2 nanorod structures first form at the solvothermal temperature of 120 oC, then the Ni3S2 nanosheet structures gradually epitaxially grown on Ni3S2 nanorods with the improvement of solvothermal temperature, resulting in the formation of Ni3S2 nanorods@nanosheets homojunction owing to the lattice match. Based on the above analyses, the formation mechanism of Ni3S2 nanorods@nanosheets homojunction should be illustrated as follows (Scheme 1): First, the active Ni atom in



the Ni foam could form Ni3S2 nanorods under the condition of relative lower solvothermal temperature under the structural direction effect of CTAB. Then, based on the lattice match, the epitaxial growth of Ni3S2 nanosheets would occur on the surface of Ni3S2 nanorods with the promoting of solvothermal temperature due to the effect of hydrazine hydrate, lead to the formation of Ni3S2 nanorods@nanosheets homojunction. The 3D homojunction maybe exhibit excellent HER catalytic activity.

Scheme 1 The proposed formation mechanism of Ni3S2/NF.

Electrocatalytic activities of Ni3S2/NF toward HER The unique structures of Ni3S2 nanorodes@nanosheets homojunction grown on conducting Ni foam matrix could not only provide abundant active sites but also accelerate electron transfer for promoted HER. The electrocatalytic performance of the synthetic Ni3S2/NF for the HER was evaluated in a 1.0 M KOH electrolyte by using the typical three-electrode system. As comparison, the Ni3S2 nanorods coated on NF (denoted as Ni3S2-NF, the XRD pattern and SEM image were shown in Fig. S7 and S8, respectively), the commercial 20 % Pt/C catalyst coated on NF (denoted as Pt/C-NF) and the blank NF were also tested. During the test, the comparable


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electrodes have a catalyst loading of approximately 0.9 mg cm−2 as that of the synthetic Ni3S2/NF electrode. The linear sweep voltammetry (LSV) curves of the four electrodes on the reversible hydrogen electrode (RHE) scale are illustrated in Fig. 4A and the results are displayed in Table 1. In stark contrast, the Ni3S2/NF shows a relative lower onset overpotential (ηonset) of 10.8 mV and overpotential at the representative current density of 10 mA cm−2 (η10) of 48.1 mV than that of the Ni3S2-NF (97.1 mV, 125.9 mV) and NF (74.5 mV, 127.4 mV), indicating the higher reaction activity of Ni3S2/NF, which is comparable to that of the Pt/C-NF (0, 10.6 mV). Tafel slope is directly associated with the HER reaction kinetics of electrocatalysts, accordingly, the corresponding Tafel plots for the four electrodes are shown in Fig. 4B. Ni3S2/NF exhibits a Tafel slope of 88.2 mV dec−1, implying an HER route may be controlled by Heyrovsky mechanism (H2O + Hads + e− → H2↑ + OH−), where Hads denotes the hydrogen atom adsorbed on the surface of catalyst [44]. It should also be noted that the Tafel slope of Ni3S2/NF is higher than that of Pt/C-NF (53.6 mV dec−1) but much smaller than those of the Ni3S2-NF (119.3 mV dec−1) and NF (148.1 mV dec−1). The lower Tafel slope of Ni3S2/NF demonstrates its faster kinetics and a higher catalytic activity for HER as compared with other samples. This indicates the special homojunction structure of Ni3S2 nanorods@nanosheets could expose the highest effective active sites, which is responsible for the excellent HER electrocatalytic performance. Besides, the Ni3S2/NF exhibits larger η10 and smaller Tafel slope than those of the Ni3S2/NF-R and Ni3S2/NF-S (Fig. S9), further demonstrating the nanorods@nanosheets homojunction can effectively promote the



kinetics of electron transport and ion diffusion compared with the single nanorod and nanosheet structures.

Fig. 4 Electrochemical performances: (A) LSV curves at a scan rate of 5 mV s−1 with iR-corrected. (B) The corresponding Tafel plots of Ni3S2/NF, Ni3S2-NF, Pt/C-NF and NF. (C) CV curves of Ni3S2/NF at different scanning rates of 20−200 mV s−1 in the potential window of 0.1−0.2 V (inset: linear fitting of the capacitive currents obtained at 0.15 V). (D) EIS Nyquist fitting plots of Ni3S2/NF, Ni3S2-NF, Pt/C-NF and NF at the open circuit potential with a amplitude of 5 mV, inset is the corresponding equivalent circle for fitting.


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Table 1 The HER peformances for all the comparable electrodes.

ηonset

η10

Tafel slope

Cdl

(mV)

(mV)

(mV dec−1)

(µF cm−2)

Ni3S2/NF

10.8

48.1

88.2

13.6

Pt/C-NF

0

10.6

53.6

18.0

Ni3S2-NF

97.1

125.9

119.3

1.3

NF

74.5

127.4

148.1

0.8

samples

Owing to the H+ would react with electrons on the catalyst to form H atoms in the HER process, so an excellent catalyst must have the ability to capture more electrons for facilitating HER process

[45]

. The surface work function could directly represent

the ability of catalyst to capture electrons, and the scanning Kelvin probe (SKP) test is an effective technology to estimate work function. As shown in Fig. 5, the work function values of Ni3S2/NF, Pt/C-NF, Ni3S2-NF and NF are about 4.9, 5.1, 4.7, and 4.6 eV, respectively. The work function of Ni3S2/NF is higher than Ni3S2-NF and NF. It is also close to that of Pt/C, implying the enhanced ability of Ni3S2/NF to capture electrons, which could lead to the excellent electrocatalytic activity towards HER. The higher work function of Ni3S2/NF is attributed to the well contact between Ni3S2 nanorods@nanosheets and NF by an in-situ growth strategy facilitate to the rapid electron transfer.



Fig. 5 The work function drawings of (A) Ni3S2/NF, (B) Pt/C-NF, (C) Ni3S2-NF and (D) NF.

Electrochemically active surface area (ECSA) is an effective method for estimating the relative activity of electrocatalysts. To evaluate the ECSA of catalysts under the working conditions, its double-layer capacitance (Cdl) from cyclic voltammetry (CV) curves at different scan rates of 20~200 mV s−1 between 0.10 and 0.34 V versus a RHE had been calculated. The principle of the potential window selection is that the current is mainly capacitive (Fig. 4C and Fig. S10). The inset in Fig. 4C shows a well linear correlation when the current density at 0.15 V is plotted against the scan rate. As shown in Table 1, the Ni3S2/NF exhibits the largest specific capacitance of 13.6 µF cm−2 compared with that of the Ni3S2-NF and NF, which accords with the observations of the smallest Tafel plots of Ni3S2/NF. Moreover, the Ni3S2/NF shows


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much better electrocatalytic activity than the Ni3S2/NF-140, Ni3S2/NF-150, Ni3S2/NF-170 and Ni3S2/NF-180 (see Fig. S11, S12 and Table S2), proving the synthetic condition could greatly affect the structure and performance. Notably, the present Ni3S2/NF exhibits much better HER performance than most of the reported non-precious catalysts, such as Ni3S2–based compounds and Ni3S2–based self-supporting materials (Table S3), indicating the promising application for substituting of Pt/C catalyst. Electrochemical impedance spectroscopy (EIS) is an effective means to study the electrode kinetics and interface reactions during the electrocatalytic reaction process. The EIS Nyquist fitting plots are shown in Fig. 4D. The small series resistances (Rs) of 3.7 Ω and charge transfer resistances (Rct) of 8.5 Ω for Ni3S2/NF are lower than those of the Ni3S2-NF (7.0 Ω, 9.5 Ω), indicating an ultrafast Faradaic process and a superior HER kinetics. It is further illustrated that the intimate contact between Ni3S2 and NF could shorten the pathways for electron transport, resulting in the increased electrical conductivity. Cyclic stability is a significant and effective way to demonstrate the practical application of catalyst. The long-term durability test was carried out in the potential range of −0.25 to 0.1 V at the scan rate of 50 mV s−1 for 10000 cycles. It also can be seen that the Ni3S2/NF could still keep the initial structure after 10000 cycles (Fig. S13). As shown in Fig. 6A, LSV curves of Ni3S2/NF display no obvious variation loss after 10000 cycles, indicating the well cyclic stability. On the contrary, Pt/C-NF exhibits a nonnegligible negative shift after cycling (Fig. 6B). The time-dependent current density (i-t) curve further demonstrates the well durability of Ni3S2/NF. This excellent stability of Ni3S2/NF is attributed to the intimate contact between Ni3S2 and NF by an in-situ strategy would make the active components hard to fall off during the long-term test.



Fig. 6 LSV curves for (A) Ni3S2/NF and (B) Pt/C-NF before and after 10000 cycles.

Conclusions In summary, the Ni3S2 nanorods@nanosheets homojunction nanostructures were prepared on NF substrate (Ni3S2/NF) by a simple controllable solvothermal strategy. The collection effects of the special 3D nanostructures and the well conductivity endow Ni3S2/NF with outstanding catalytic activity (ηonset = 10.8 mV, η10 = 48.1 mV), faster kinetics (Tafel plots of 88.2 mV dec−1) and excellent stability (almost no


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degradation after 10000 cycles) towards HER, which is superior to most non-noble metal, including transition metal-based catalysts and non-metal catalysts. The great HER activity can be ascribed to the exposure of abundance active sites originated from the nanorods@nanosheets structure, while the homojunction of Ni3S2 nanorods@nanosheets and the intimate contact between Ni3S2 and NF not only could shorten the electron transport pathway but also could be responsible for the well stability. Besides, the existence of sulfur vacancy in the Ni3S2 could tune electronic structure of the surface and enhance the catalytic activity for HER. Our present work provides new opportunity towards rational design of high-efficiency and cost-effective Pt-free electrocatalysts for water electrolysis technologies. Acknowledgements We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21631004, 21771059, 21401048, 21571054, 51672073), the Natural Science Foundation of Heilongjiang Province (B2017008), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016016), the Harbin science

and

technology

innovation

talents

research

Foundation

(2015RAQXJ057).

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Int. Ed. 2015, 54, 6325−6329. DOI: 10.1002/anie.201501419.



TOC

The homojunction of Ni3S2 nanosheets epitaxially grown on Ni3S2 nanorods could exhibit better activity and stability towards HER than most of the present non-precious metal-based electrocatalysts.


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Ni3S2 Nanosheets in Situ Epitaxially Grown on Nanorods as High

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