NiS2 interfaced nanosheets for highly efficient overall water splitting

Journal of Colloid and Interface Science 562 (2020) 363–369

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

W doping dominated NiO/NiS2 interfaced nanosheets for highly efficient overall water splitting Haohan Wang a, Tao Liu a, Kai Bao a, Jian Cao b, Jicai Feng a, Junlei Qi a,⇑ a b

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 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 27 August 2019 Revised 11 December 2019 Accepted 11 December 2019 Available online 12 December 2019 Keywords: W doping Electrocatalyst Free-standing Nanointerface NiO

a b s t r a c t Constructing high-efficiency electrocatalysts is vital towards electrocatalytic water splitting, but it remains a challenge. Although Ni-based materials have drawn extensive attention as highly active catalysts, the relatively limited electroactive sites in Ni-based catalysts still remains a great issue. In order to further boost the electrocatalytic performances, heteroatom doping and interface engineering are usually adopted for modification. Here, a new strategy is developed to construct W doped NiO/NiS2 interfaced nanosheets directly on carbon sheet, which is working as efficient and bifunctional electrocatalysts for overall water splitting. W doped NiO nanosheets are directly constructed on the carbon sheet by the hydrothermal and annealing processes. After that, W-NiO was subjected to Ar plasma assisted sulfuration treatment for forming W doped NiO/NiS2 interfaced nanosheets. Based on systematic investigations, we find that W doping can effectively induce the modified electronic structure of Ni to boost the intrinsic activities in NiO/NiS2. Further, forming NiO/NiS2 nanointerfaces can also provide rich electroactive sites and boost the charge transfer rate. Consequently, W doped NiO/NiS2 exhibits the much enhanced performances for overall water splitting. As a bifunctional electrode, W-NiO/NiS2 demonstrates a remarkable activity with a 1.614 V cell voltage at 10 mA cm 2 for overall water splitting. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (J. Qi). https://doi.org/10.1016/j.jcis.2019.12.044 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

With the increasing demands of energy, hydrogen is now received much attention owing to the renewable and environmental-friendly properties [1,2]. As a promising strategy to produce clean hydrogen, electrocatalytic water splitting offers a promising solution to energy crisis [1,2]. Electrocatalytic water

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splitting including two half reaction: two-electron-transfer hydrogen evolution reaction (HER) and a more sluggish four-electrontransfer oxygen evolution reaction (OER) [3,4]. Up to now, though Pt and RuO2/IrO2 are regarded as the perfect catalysts for HER and OER, respectively, the scarcity and the high price make it impossible for noble metals to achieve large-scale industry applications [5]. Even worse, some catalysts towards water splitting can facilitate only one side reaction, HER or OER, cannot achieving practical industry applications [6–8]. Thus, developing bifunctional catalysts with abundant resource becomes an urgent task. Recently, transition-metal-based catalyst, especially for Nibased materials such as NiO, NiSx and NiPx, are cost-effective and environment benign materials and have drawn extensive attention as highly active catalysts [9–11]. The satisfactory performances of Ni-based catalyst are primarily on the optimal electroactive sites on the surface, leading to a favorable condition for O, P, or S with intermediates in HER or OER process [10–12]. Unfortunately, the relatively limited electroactive sites in Ni-based catalysts remain a great issue, resulting in poor mass transfer and unsatisfied catalytic performances [13,14]. To solve these problems, considerable efforts have been devoted to improving catalytic performances through growing nanosized catalysts on conductive substrates [14,15], doping heteroatoms to modify DGH* [16,17], and preparing hybrid catalysts to tune its catalytic properties [18,19]. For example, Xu et al. fabricated phosphorus-doped cobalt-iron oxyhydroxide, which showed much enhanced OER activities [16]. In particular, interface engineering can be a promising strategy to enhance the activity of electrocatalysts [20,21]. In addition, abundant nanointerfaces are able to further promote host materials’ electrocatalytic performances owning to rich active sites and facilitated transfer rate, which is caused by mutual interaction between the interface [21–23]. For example, Guo et al. fabricated hollow Co3S4@MoS2 heterostructures, which showed good catalytic performances for both HER and OER [20]. Meanwhile, heteroatom doping is considered as one practical way to modify the electronic states of host materials, resulting in improved catalytic performances [16,17]. Furthermore, an additional approach for enhancing the catalytic performances is to integrate Ni-based materials with conductive and robust substrates, which can effectively boost electron-transfer rate and make sure the good mechanical strength [14,15]. In this study, we combine these strategies to design and synthesize W doped NiO/NiS2 interfaced nanosheets directly on carbon sheet as bifunctional water-splitting catalysts. It is discovered that W doping can lead to the modified electronic structure of Ni center for boosting the intrinsic activities of NiO/NiS2. Meanwhile, forming NiO/NiS2 nanointerfaces can also boost the charge transfer rate and provide rich electroactive sites. As a result, W-NiO/NiS2 catalysts exhibit a much faster electron rate so as to an excellent

electrocatalytic performance with low overpotentials at 116 mV (HER) and 263 mV (OER) in 10 mA cm 2. For practical applications, when W-NiO/NiS2 catalysts are directly used as cathode and anode for overall water splitting, W-NiO/NiS2 catalysts demonstrate remarkable activities with a 1.614 V voltage at 10 mA cm 2. 2. Results and discussion 2.1. Morphologies and nanostructures The fabrication process of W doped NiO/NiS2 is shown in Scheme 1. Firstly, W doped NiO nanosheets (W-NiO) were directly constructed on the carbon sheet by the hydrothermal and annealing processes. After that, W-NiO was subjected to Ar plasma assisted sulfuration treatment for fast forming W doped NiO/NiS2 interface nanosheets (W-NiO/NiS2). In addition, we synthesized different amounts of W doping and S by the similar processes, and investigated the effects of W and S amounts on the morphology, structure and electrocatalytic performances. For comparison, pure NiO and NiO/NiS2 nanosheets were also synthesized by similar processes. In the following discussion, we choose the optimum samples including pure NiO, W-NiO, NiO/NiS2 and W-NiO/NiS2 for detailed discussions. The nanostructures and morphologies of obtained samples are characterized by scanning electron microscope (SEM). As shown in Fig. 1a, NiO with smooth surfaces is distributed on carbon sheet. After introducing W element, the size of W-NiO is obviously reduced, as shown in Fig. 1b. With the rising dose of W source, the size of W-NiO nanosheet is gradually reduced, as shown in Fig. S1. After the sulfuration treatment, the morphologies of NiO/ NiS2 in Fig. 1c show the almost the same situation with NiO samples. Further, W-NiO/NiS2 maintains the nanosheet morphologies, as shown in Fig. 1d. And the unchanged nanostructures are further proved by the morphologies after different sulfuration treatments in Fig. S2. In addition, transmission electron microscopy (TEM) is also applied to characterize the nanostructure. The HRTEM images in Fig. 2b and 2c shows the lattice fringes could be indexed well to the (2 0 0) plane of NiS2 (JCPDS no. 89-1495) and (2 2 2) plane of NiO (JCPDS no. 89-5881). Compared with the W-NiO samples, some NiO phase was in-situ changed into NiS2 phase after sulfuration process, which constructed unique heterostructures with abundant nanointerfaces (Fig. 2b and c). Fig. 2d–h demonstrates the homogenously distribution of Ni, O, W and S over the selected area in the W-NiO/NiS2. As for X-ray diffraction (XRD) results in Fig. S3, all the NiO and W doped NiO samples can be corresponded well to the phases of NiO (JCPDS no. 89-5881), except for the peaks from carbon sheet. After sulfuration, the new appeared diffraction peaks of the samples are contributed to NiS2 phase (JCPDS no. 89-1495).

Scheme 1. The fabrication process of W doped NiO/NiS2 interfaced nanosheets.

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Fig. 1. SEM images of (a) NiO (b) W-NiO (c) NiO/NiS2 and (d) W-NiO/NiS2.

a

b ~ 0.241 nm interface (222) NiO

500 nm

e

~ 0.241 nm (222) interface NiO

interface ~ 0.284 nm (200) NiS2

~ 0.284 nm (200) NiS2

~ 0.284 nm (200) NiS2

~ 0.241 nm (222) NiO

2 nm

f

Ni

d

c

interface

g

W

200 nm

2 nm

h

O

S

Fig. 2. (a) TEM (b, c) HRTEM (d-h) element mappings of W-NiO/NiS2.

2.2. Surface states and chemical bonding Next, we also conducted X-ray photoelectron spectroscopy (XPS) measurements to further characterize the surface states in obtained samples. Ni 2p3/2 spectra NiO, W-NiO and W-NiO/NiS2 are shown in Fig. 3a. As for NiO, the peak at about 855.7 eV can be indexed to Ni2+, as well as a satellite peak at about 861.5 eV [13]. After introducing W element, the Ni2+ peak is slightly shifted to lower binding energy, leading to the improved electron density on Ni atom [24,25]. As for W-NiO/NiS2, two peaks at about 857.2 and 855.7 eV could be indexed to Ni3+ and Ni2+, as well as a satellite

peak [26]. Compared with the peak in W-NiO, the Ni2+ peak is slightly shifted to higher binding energy, demonstrating the strong electronic interaction in NiO/NiS2 nanointerfaces [27,28]. Further, the sulfuration process can lead to the presence of Ni3+, indicating the electron structure variation of the Ni center [29]. According to previous researches [8,30], Ni3+ possesses more 3d electron orbits and shows higher electronaccepting characteristics, which can effectively facilitate the electron transfer in alkaline solution, leading to higher electrochemical activities. Fig. 3b shows two peaks at about 35.6 and 37.6 eV from the 4f7/2 and 4f5/2, demonstrating the formation of W6+ [31]. Fig. 3c shows the peaks at about 162.8 and

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Fig. 3. XPS spectra of (a) Ni 2p3/2, (b) W 4f, (c) S 2p and (d) O 1s in obtained samples.

164.0 eV from S 2p3/2 and S 2p1/2, as well as a satellite peak [32]. As shown in Fig. 3d, three peaks at 530.1, 532.1 and 533.4 eV can be indexed to metal-O bonding, absorbed O and absorbed H2O on the surface of W-NiO/NiS2 [33]. Owing to nanosheet structure, rich active sites in nanointerfaces and modified electronic states by W doping, the W-NiO/NiS2 interfaced nanosheets are expected to achieve good electrocatalytic performances.

2.3. Electrochemical performances for HER The HER and OER performances of obtained samples are tested in a typical three-electrode configuration in 1 M KOH. Meanwhile, the catalytic performances of Pt/C and RuO2 are also tested. And the detailed HER properties are shown in Fig. 4. As displayed in Fig. 4a, W-NiO/NiS2 delivers an overpotential of 116 mV at 10 mA cm 2, which is competitive among recently reported lowcost electrocatalysts, as shown in Table S1. As shown in Fig. 4b, the Tafel slopes of NiO, W-NiO, NiO/NiS2, W-NiO/NiS2 and Pt/C are 454.5, 291.9, 120.0, 90.2 and 31.5 mV dec 1, respectively. This much lower Tafel slope in W-NiO/NiS2 indicates the faster HER kinetics [34–36]. In addition, the lowest transfer resistance in Nyquist plots for W-NiO/NiS2 also demonstrates the best electron conductivity [11,14]. The electrochemical active areas (ECSA) are evaluated by the electrochemical double-layer capacitance (Cdl) in Fig. 4d, since Cdl is linearly proportional to ECSA [11,15]. Obviously, W-NiO/NiS2 shows the largest Cdl value, suggesting the highest electroactive sites for HER reactions. As for multistep chronoamperometric curve in Fig. 4e, the potential remains steady in each step, demonstrating the good mass transport and mechanical performances for W-NiO/NiS2. Meanwhile, the longstability tests in Fig. 4f also exhibit that no obvious potential decay can be found for W-NiO/NiS2, revealing the robust active sites for HER tests. And XRD and SEM results in Fig. S6 demonstrate the phase and structure of W-NiO/NiS2 are maintained after

long-time HER tests, further suggesting the good durability. According to reported researches [37–40], Ni site shows the good water dissociation ability and W site possess the superior H adsorption ability. In addition, abundant nanointerfaces in WNiO/NiS2 are able to further promote host materials’ electrocatalytic performance owning to rich active sites and facilitated transfer rate, which is caused by mutual interaction between the interfaces. These reasons contribute to the lower overpotential and faster reaction kinetics for W-NiO/NiS2 in HER tests.

2.4. Electrochemical performances for OER To eliminate the effects of redox peaks on the overpotential calculation, we choose the negative current in the corresponding cyclic voltammetry curves for detailed discussions. As shown in Fig. 5a, W-NiO/NiS2 shows a good OER activity with an overpotential of 263 mV at 10 mA cm 2, which outperforms those in NiO, W-NiO and NiO/NiS2, and even comparable with that of RuO2. And the OER activity for W-NiO/NiS2 is comparable or even much better than that of recently reported electrocatalysts (Table S2). The Tafel slopes in Fig. 5b show that W-NiO/NiS2 shows the lower value of 76.6 mV dec 1, demonstrating the enhanced OER kinetics. In addition, W-NiO/NiS2 shows the best electron conductivity (Fig. 5c) and larges electroactive sites (Fig. 5d) for OER tests. As shown in Fig. 5e, no decay in every step in the chronoamperometric tests indicates the excellent mass transport of W-NiO/NiS2. The long-term stability toward OER in Fig. 5f also proves the good durability of W-NiO/NiS2 in OER tests. The XRD pattern in Fig. S8a shows that only NiO phase is remained after long-term OER tests. And SEM results in Fig. S8b shows the nanosheet morphology show no obvious changes. In addition, Fig. S8c shows the S element is remained after long-term OER tests, suggesting the presence of surface S residues. According to reported researches [8,41], metal sulfides are not stable in OER tests, where corresponding oxides

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Fig. 4. (a) HER polarization curves, (b) the corresponding Tafel plots, (c) Nyquist plots and (d) corresponding ECSA calculated from Cdl of obtained samples. (e) Consecutive multi-step chronoamperometric tests and (f) chronopotentiometry measurements for W-NiO/NiS2 (without iR correction).

Fig. 5. (a) OER polarization curves, (b) the corresponding Tafel plots, (c) Nyquist plots and (d) corresponding ECSA calculated from Cdl of obtained samples. (e) Consecutive multi-step chronoamperometric tests and (f) chronopotentiometry measurements for W-NiO/NiS2 (without iR correction).

or hydroxides should be the actual electroactive species for OER. And the surface S residues in oxides or hydroxides can lower the adsorption free energy difference in OER intermediates, leading to the enhanced OER performances [42]. Meanwhile, cation doping can effetively modify the reaction free energy at rate-determining steps, tune electronic structures and lead to more active sites for OH adsorption [43,44]. Consequently, these reasons contribute to enhanced OER properties of W-NiO/NiS2.

2.5. Electrochemical performances for overall water splitting Finally, a water-splitting device is constructed by W-NiO/NiS2 as both electrodes, as shown in Fig. 6. The W-NiO/NiS2||W-NiO/ NiS2 only requires a voltage of 1.614 V at 10 mA cm 2 (Fig. 6a). When comparing with reported bifunctional electrocatalysts in Table S3, such low voltage is competitive. And the measured voltage is very close to the calculated voltage in Fig. 6b. The slight

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Fig. 6. (a) Polarization curves of W-NiO/NiS2||W-NiO/NiS2 and Pt/C||RuO2 for overall water splitting. (b) The calculated and measured voltage for overall water splitting. (c) The chronopotentiometry measurements for W-NiO/NiS2||W-NiO/NiS2 (without iR correction).

deviation could be attributed to difference in different measuring systems. In addition, W-NiO/NiS2||W-NiO/NiS2 shows no obvious voltage decay in 10 and 50 mA cm 2 (Fig. 6c), demonstrating the good stability.

51621091) and Natural Science Foundation of Heilongjiang Province of China (YQ2019E023) is highly appreciated.

3. Conclusion

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.12.044.

On the basis of the previously reported modification approaches [16,17,20,21], this work has demonstrated the synergistic effect of heteroatom doping and interface engineering for enhancing the electrocatalytic performances of W doped NiO/NiS2 nanosheets. As a result, serving as bifunctional catalysts, the W-NiO/NiS2 catalysts demonstrate remarkable activities with a 1.614 V voltage at 10 mA cm 2, as well as good stability, which is a promising candidate for next generation of HER and OER catalyst. Further, such low voltage for overall water splitting is competitive among reported catalysts [45–48]. The outstanding catalytic performances can be attributed to the following merits: (1) The introduction of W element can effectively modulate the electron density of Ni center for boosting the intrinsic activities. (2) The rich nanointerfaces between NiO and NiS2 can also induce strong electron interaction and thus boost charge transfer rate. (3) Constructing catalysts directly on conductive and robust substrates can make sure electrical connection and mechanical strength, which is better than those prepared by organic binders [2,49]. In this work, the fundamental knowledge may contribute to rationally design bifunctional catalysts by the synergistic effect of heteroatom doping and interface engineering. CRediT authorship contribution statement Haohan Wang: Investigation, Data curation, Formal analysis, Writing - original draft. Tao Liu: Investigation, Data curation. Kai Bao: Investigation, Data curation. Jian Cao: Resources, Funding acquisition. Jicai Feng: Resources. Junlei Qi: Conceptualization, Writing - review & editing, Resources, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The support from the National Natural Science Foundation of China (Grant Nos 51575135, 51622503, U1537206 and

Appendix A. Supplementary material

References [1] H.D.M. Follmann, I. Messias, M.N. Queiroz, R.A. Araujo, A.F. Rubira, R. Silva, Designing hybrid materials with multifunctional interfaces for wounddressing, electrocatalysis, and chemical separation, J. Colloid Interf. Sci. 533 (2019) 106– 125. [2] X. Wei, Y. Zhang, H. He, D. Gao, J. Hu, H. Peng, L. Peng, S. Xiao, P. Xiao, Carbonincorporated NiO/Co3O4 concave surface microcubes derived from a MOF precursor for overall water splitting, Chem. Commun. 55 (2019) 6515–6518. [3] Y. Guo, J. Tang, Z. Wang, Y. Kang, Y. Bando, Y. Yamauchi, Elaborately assembled core-shell structured metal sulfides as a bifunctional catalyst for highly efficient electrochemical overall water splitting, Nano Energy 47 (2018) 494– 550. [4] C. Zhang, W. Zhang, N.E. Drewett, X. Wang, S.J. Yoo, H. Wang, T. Deng, J. Kim, H. Chen, K. Huang, S. Feng, W. Zheng, Integrating catalysis of methane decomposition and electrocatalytic hydrogen evolution with Ni/CeO2 for improved hydrogen production efficiency, ChemSusChem 12 (2019) 1000– 1010. [5] Y. Liu, F. Yang, W. Qin, G. Yang, Co2P@NiCo2O4 bi-functional electrocatalyst with low overpotential for water splitting in wide range pH electrolytes, J. Colloid Interf. Sci. 534 (2019) 55–63. [6] D. Zhao, H. Liu, X. Wu, Bi-interface induced multi-active MCo2O4@MCo2S4@PPy (M=Ni, Zn) sandwich structure for energy storage and electrocatalysis, Nano Energy 57 (2019) 363–370. [7] Y. Li, J. Yin, L. An, M. Lu, K. Sun, Y. Zhao, D. Gao, F. Cheng, P. Xi, FeS2/CoS2 interface nanosheets as efficient bifunctional electrocatalyst for overall water Splitting, Small 14 (2018) 1801070. [8] Z. Dai, H. Geng, J. Wang, Y. Luo, B. Li, Y. Zong, J. Yang, Y. Guo, Y. Zheng, X. Wang, Q. Yan, Hexagonal-phase cobalt monophosphosulfide for highly efficient overall water splitting, ACS Nano 11 (2017) 11031–11040. [9] U.D. Silva, J. Masud, N. Zhang, Y. Hong, W.P.R. Liyanage, M.A. Zaeem, M. Nath, Nickel telluride as a bifunctional electrocatalyst for efficient water splitting in alkaline medium, J. Mater. Chem. A 6 (2018) 7608–7622. [10] Y. Wang, Y. Li, L. Ding, Z. Chen, A. Ong, W. Lu, T.S. Herng, X. Li, J. Ding, NiFe (sulfur)oxyhydroxide porous nanoclusters/Ni foam composite electrode drives a large-current-density oxygen evolution reaction with an ultra-low overpotential, J. Mater. Chem. A 7 (2019) 18816–18822. [11] T. Liu, A. Li, C. Wang, W. Zhou, S. Liu, L. Guo, Interfacial electron transfer of Ni2P-NiP2 polymorphs inducing enhanced electrochemical properties, Adv. Mater. 30 (2018) 1803590. [12] Z. Fang, L. Peng, Y. Qian, X. Zhang, Y. Xie, J.J. Cha, G. Yu, Dual tuning of Ni-Co-A (A = P, Se, O) nanosheets by anion substitution and holey engineering for efficient hydrogen evolution, J. Am. Chem. Soc. 140 (2018) 5241–5247. [13] J. Nai, Y. Lu, L. Yu, X. Wang, X.W. Lou, Formation of Ni-Fe mixed diselenide nanocages as a superior oxygen evolution electrocatalyst, Adv. Mater. 29 (2017) 1703870. [14] X. Zhang, J. Li, Y. Yang, S. Zhang, H. Zhu, X. Zhu, H. Xing, Y. Zhang, B. Huang, S. Guo, E. Wang, Co3O4/Fe0.33Co0.66P interface nanowire for enhancing water oxidation catalysis at high current density, Adv. Mater. 30 (2018) 1803551. [15] Z. Li, W. Niu, L. Zhou, Y. Yang, Phosphorus and aluminum codoped porous NiO nanosheets as highly efficient electrocatalysts for overall water splitting, ACS Energy Lett. 3 (2018) 892–898.

H. Wang et al. / Journal of Colloid and Interface Science 562 (2020) 363–369 [16] H. Xu, J. Wei, C. Liu, Y. Zhang, L. Tian, C. Wang, Y. Du, Phosphorus-doped cobaltiron oxyhydroxide with untrafine nanosheet structure enable efficient oxygen evolution electrocatalysis, J. Colloid Interf. Sci. 530 (2018) 146–153. [17] J. Li, D. Wu, J. Iocozzia, H. Du, X. Liu, Y. Yuan, W. Zhou, Z. Li, Z. Xue, Z. Lin, Achieving efficient incorporation of p-electrons into graphitic carbon nitride for markedly improved hydrogen generation, Angew. Chem. Int. Ed. 131 (2019) 2007. [18] J. Shan, T. Ling, K. Davey, Y. Zheng, S. Qiao, Transition-metal-doped RuIr bifunctional nanocrystals for overall water splitting in acidic environments, Adv. Mater. 31 (2019) 1900510. [19] X. Liu, J. Iocozzia, Y. Wang, X. Cui, Y. Chen, S. Zhao, Z. Li, Z. Lin, Noble metalmetal oxide nanohybrids with tailored nanostructures for efficient solar energy conversion, photocatalysis and environmental remediation, Energy Environ. Sci. 10 (2017) 402–434. [20] Y. Guo, J. Tang, H. Qian, Z. Wang, Y. Yamauchi, One-pot synthesis of zeolitic imidazolate framework 67-derived hollow Co3S4@MoS2 heterostructures as efficient bifunctional catalysts, Chem. Mater. 29 (2017) 5566–5573. [21] A. Wu, Y. Xie, H. Ma, C. Tian, Y. Gu, H. Yan, X. Zhang, G. Yang, H. Fu, Integrating the active OER and HER components as the heterostructures for the efficient overall water splitting, Nano Energy 44 (2018) 353–363. [22] J. Yin, Y. Li, F. Lv, M. Lu, K. Sun, W. Wang, L. Wang, F. Cheng, Y. Li, P. Xi, S. Guo, Oxygen vacancies dominated NiS2/CoS2 interface porous nanowires for portable Zn-air batteries driven water splitting devices, Adv. Mater. 29 (2017) 1704681. [23] L. An, Y. Li, M. Luo, J. Yin, Y.-Q. Zhao, C. Xu, F. Cheng, Y. Yang, P. Xi, S. Guo, Atomic-level coupled interfaces and lattice distortion on CuS/NiS2 nanocrystals boost oxygen catalysis for flexible Zn-air batteries, Adv. Funct. Mater. 27 (2017) 1703779. [24] L. Wen, J. Yu, C. Xing, D. Liu, X. Lyu, W. Cai, X. Li, Flexible vanadium-doped Ni2P nanosheet arrays grown on carbon cloth for an efficient hydrogen evolution reaction, Nanoscale 11 (2019) 4198–4203. [25] X. Yu, S. Zhang, C. Li, C. Zhu, Y. Chen, P. Gao, L. Qi, X. Zhang, Hollow CoP nanopaticle/N-doped graphene hybrids as highly active and stable bifunctional catalysts for full water splitting, Nanoscale 8 (2016) 10902– 10907. [26] Y. Liu, S. Jiang, S. Li, L. Zhou, Z. Li, J. Li, M. Shao, Interface engineering of (Ni, Fe) S2@MoS2 heterostructures for synergetic electrochemical water splitting, Appl. Catal. B: Environ. 247 (2019) 107–114. [27] Y. Yang, K. Zhang, H. Lin, X. Li, H.C. Chan, L. Yang, Q. Gao, MoS2-Ni3S2 heteronanorods as efficient and stable bifunctional electrocatalysts for overall water splitting, ACS Catal. 7 (2017) 2357–2366. [28] L. An, J. Feng, Y. Zhang, R. Wang, H. Liu, G. Wang, F. Cheng, P. Xi, Epitaxial heterogeneous interfaces on N-NiMoO4/NiS2 nanowires/nanosheets to boost hydrogen and oxygen production for overall water splitting, Adv. Funct. Mater. 29 (2019) 1805298. [29] H. Zhang, H. Jiang, Y. Hu, Y. Li, Q. Xu, S. Petr, C. Li, Tailorable surface sulfur chemistry of mesoporous Ni3S2 particles for efficient oxygen evolution, J. Mater. Chem. A 7 (2019) 7548–7552. [30] W. Ma, R. Ma, C. Wang, J. Liang, X. Liu, K. Zhou, T. Sasaki, A superlattice of alternately stacked Ni-Fe hydroxide nanosheets and graphene for efficient splitting of water, ACS Nano 9 (2015) 1977–1984. [31] S.Y. Yi, Y.G. Song, J.Y. Park, J.M. Suh, G.S. Kim, Y. Shim, J.M. Yuk, S. Kim, H.W. Jang, B. Ju, C. Kang, Morphological evolution induced through a heterojunction of W-decorated NiO nanoigloos: synergistic effect on high-performance gas sensors, ACS Appl. Mater. Interfaces 11 (2019) 7529–7538. [32] Z.Y. Wang, Y.F. Dong, H.J. Li, Z.B. Zhao, H.B. Wu, C. Hao, S.H. Liu, J.S. Qiu, X.W. Lou, Enhancing lithium-sulphur battery performance by strongly binding the

[33] [34]

[35] [36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

369

discharge products on amino-functionalized reduced graphene oxide, Nat. Commun. 5 (2014) 5002. P. Kuang, M. He, B. Zhu, J. Yu, K. Fan, M. Jaroniec, 0D/2D NiS2/V-MXene composite for electrocatalytic H2 evolution, J. Catal. 375 (2019) 8–20. Y. Zhu, G. Chen, Y. Zhong, W. Zhou, Z. Shao, Rationally designed Hierarchically structured tungsten nitride and nitrogen-rich graphene-like carbon nanocomposite as efficient hydrogen evolution electrocatalyst, Adv. Sci. 5 (2018) 1700603. 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. Y. Zheng, Y. Jiao, A. Vasileff, S. Qiao, The hydrogen evolution reaction in alkaline solution: from theory, single crystal models, to practical electrocatalysts, Angew. Chem. Int. Ed. 57 (2018) 7568. D. Strmcnik, P.P. Lopes, B. Genorio, V.R. Stamenkovic, N.M. Markovic, Design principles for hydrogen evolution reaction catalyst materials, Nano Energy 29 (2016) 29–36. X. Liu, Y. Jiao, Y. Zheng, K. Devey, S. Qiao, A computational study on Pt and Ru dimers supported on graphene for the hydrogen evolution reaction: new insight into the alkaline mechanism, J. Mater. Chem. A 7 (2019) 3648–3654. J. Zhang, X. Bai, T. Wang, W. Xiao, P. Xi, J. Wang, D. Gao, J. Wang, Bimetallic nickel cobalt sulfide as efficient electrocatalyst for Zn-air battery and water splitting, Nano-Micro Lett. 11 (2019) 2. W. Chen, J. Pei, C. He, J. Wan, H. Ren, Y. Wang, J. Dong, K. Wu, W. Cheong, J. Mao, X. Zheng, W. Yan, Z. Zhuang, C. Chen, Q. Peng, D. Wang, Y. Li, Single tungsten atoms supported on MOF-derived N-doped carbon for robust electrochemical hydrogen evolution, Adv. Mater. 30 (2018) 1800396. X. Zhang, F. Meng, S. Mao, Q. Ding, M.J. Shearer, M.S. Faber, J. Chen, R.J. Hamers, S. Jin, Amorphous MoSxCly electrocatalyst supported by vertical graphene for efficient electrochemical and photoelectrochemical hydrogen generation, Energy Environ. Sci. 8 (2015) 862–868. T. Wang, G. Nam, Y. Jin, X. Wang, P. Ren, M.G. Kim, J. Liang, X. Wen, H. Jang, J. Han, Y. Huang, Q. Li, J. Cho, NiFe(Oxy) hydroxides derived from NiFe disulfides as an efficient oxygen evolution catalyst for rechargeable Zn-air batteries: the effect of surface S residues, Adv. Mater. 30 (2018) 1800757. Y. Sun, K. Xu, Z. Wei, H. Li, T. Zhang, X. Li, W. Cai, J. Ma, H.J. Fan, Y. Li, Strong electronic interaction in dual-cation-incorporated NiSe2 nanosheets with lattice distortion for highly efficient overall water splitting, Adv. Mater. 30 (2018) 1802121. Z. Cui, Y. Ge, H. Chu, R. Baines, P. Dong, J. Tang, Y. Yang, P.M. Ajayan, M. Ye, J. Shen, Controlled synthesis of Mo-doped Ni3S2 nano-rods: an efficient and stable electro-catalyst for water splitting, J. Mater. Chem. A 5 (2017) 1595– 1602. G. Ou, P. Fan, H. Zhang, K. Huang, C. Yang, W. Yu, H. Wei, M. Zhong, H. Wu, Y. Li, Large-scale hierarchical oxide nanostructures for high-performance electrocatalytic water splitting, Nano Energy 35 (2017) 207–214. X. Zhang, S. Zhang, J. Li, E. Wang, One-step synthesis of well-structured NiS– Ni2P2S6 nanosheets on nickel foam for efficient overall water splitting, J. Mater. Chem. A 5 (2017) 22131–22136. 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. R. Huang, W. Chen, Y. Zhang, Z. Huang, H. Dai, Well-designed cobalt-nickel sulfide microspheres with unique peapod-like structure for overall water splitting, J. Colloid Interf. Sci. 556 (2019) 401–410. 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. 55 (2019) 10896–10899.