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Morphology-optimized interconnected Ni3S2 nanosheets coupled with Ni(OH)2 nanoparticles for enhanced hydrogen evolution reaction
Journal of Alloys and Compounds 827 (2020) 154163
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Morphology-optimized interconnected Ni3S2 nanosheets coupled with Ni(OH)2 nanoparticles for enhanced hydrogen evolution reaction Shuang Hao a, Qi Cao b, **, Liting Yang a, Renchao Che a, * a
Laboratory of Advanced Materials, Department of Materials Science and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai, 200438, China b Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Engineering Research Center for Pollution Treatment and Resource Utilization of Jiangsu Province, School of Energy and Environment, Southeast University, Nanjing, 210096, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2019 Received in revised form 2 February 2020 Accepted 3 February 2020 Available online 4 February 2020
Hydrogen evolution reaction (HER) performance of Ni3S2 electrocatalyst is associated with different morphologies and chemical compositions. In this work, nanosheets, nanorods and dendrite-like structure of Ni3S2 are synthesized and investigated as HER catalysts. Ni3S2 with interconnected nanosheets morphology exhibits the largest electrochemical active surface area, and thereby the lowest overpotential (288 mV vs. RHE) compared with Ni3S2 nanorods (314 mV vs. RHE) and dendrite-like Ni3S2 (303 mV vs. RHE) morphologies. Based on this, Ni(OH)2 species are further grown on the surface of nanosheets Ni3S2 and thus interconnected Ni3S2@Ni(OH)2 nanosheets have been obtained, demonstrating improved HER performance with decreased overpotential (237 mV vs. RHE) and superior stability. These results suggest that (i) the robust nanosheet structure of Ni3S2 can provide abundant space for contacting with electrolyte, which facilitates the mass transfer; and (ii) the formation of Ni3S2@Ni(OH)2 heterostructure is beneficial for water dissociation in HER process, which brings about accelerated HER kinetics. © 2020 Elsevier B.V. All rights reserved.
Keywords: Ni3S2 nanostructures Ni3S2@Ni(OH)2 heterostructure Hydrothermal reaction Electrocatalyst Hydrogen evolution reaction (HER)
1. Introduction The increasing depletion of fossil fuels and associated environmental issues have promoted the research on clean and renewable energy sources [1e4]. Till now, hydrogen (H2) has been widely regarded as a clean and efficient energy carrier for reduction of the dependence on fossil fuels [5e8], and electrochemical hydrogen evolution reaction (HER) is considered to be a green and effective approach for producing high-quality H2 [9e11]. Unfortunately, HER in alkaline media may suffer from sluggish kinetics and usually require large overpotentials that would lead to high power consumption [12,13]. Moreover, noble metal Pt-based materials, as the most efficient HER electrocatalysts, have been largely hindered by their scarcity and high cost [14,15]. Hence, the development of noble metal-free electrocatalysts for efficient HER is in urgent need. Nickel (Ni)-based materials have received widespread interest in electrocatalyst due to their cost-effective and earth-abundant
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Cao), [email protected] (R. Che). https://doi.org/10.1016/j.jallcom.2020.154163 0925-8388/© 2020 Elsevier B.V. All rights reserved.
advantages. On the one hand, Ni-based compounds show superior activity for HER. For instance, NiO is considered as one of the most promising HER catalysts due to its high activity while is hindered by the low electrical conductivity [16]. On the other hand, Ni-based bimetallic materials [17], for example the NiFe [18] and NiCu-based [19] catalysts, could offer synergistic effect allowing both high activity and conductivity. However, the complicated synthesis processes have impeded their practical applications. Among these electrocatalysts, nickel sulfide (Ni3S2) has been recognized as one of the best candidates for HER owing to its high conductivity and unique configuration [20,21]. However, Ni3S2 is still restricted by its unsatisfactory HER activity which can be caused by the crude morphology [22,23] and insufficient HER kinetics [24]. In this regard, the combination of Ni3S2 with other active materials has been proved to be an efficient strategy to enhance the intrinsic HER activity [25e28]. Nonetheless, the development of sophisticated Ni3S2-based electrocatalysts with high HER activity and robust stability still remains a daunting challenge. In this work, Ni3S2 electrocatalysts with different morphologies have been synthesized via hydrothermal reactions, with the morphology effect on HER meticulously studied. Compared with
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nanorods or dendrite-like structures, Ni3S2 with unique microsphere-like interconnected nanosheets structure exhibits the most favorable active-site exposure and thus the highest HER activity. Further, Ni(OH)2 was directly grown on Ni3S2 nanosheets. The as-generated interconnected Ni3S2@Ni(OH)2 nanosheets present decreased overpotential of 237 mV at 10 mA cm2 with accelerated HER kinetics and good durability for 20 h. The construction of Ni3S2@Ni(OH)2 heterointerface turns out to be beneficial for water dissociation, which brings about accelerated HER kinetics and enhanced HER activity. This work provides insights into the role that morphology plays in HER performance of Ni3S2 electrocatalysts, and proves that the formation of Ni3S2@Ni(OH)2 heterostructure could bring about enhanced and robust HER activity. 2. Experimental section 2.1. Preparation of Ni3S2/NF with different morphologies As-purchased nickel foam (NF, 99.7%, LONGSHENGBAO Co., Ltd.) was first pretreated for removal of oxidized Ni species on the surface. Briefly, pieces of NF (1 1.5 cm2) were sequentially washed by HCl (2 M), deionized (DI) water and ethanol under sonification for 20 min, and then dried in a vacuum oven at 60 C for 6 h. Next, Ni3S2/NF samples with different surface morphologies were prepared via a similar one-step hydrothermal reaction: (i) 50 mg of thioacetamide (TAA, CH3CSNH2, 99.0%) and 50 mg of nickel sulfate (NiSO4$6H2O, 98.5%), both purchased from Sinopharm Chemical Reagent Co., Ltd., were dissolved in DI water (30 mL) under stirring with NF for 1 h. Then the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 160 C for 4 h to form Ni3S2 interconnected nanosheets; (ii) Ni3S2 with dendrite-like morphology was synthesized via a similar route with the abovedescribed nanosheets, except that only 25 mg of NiSO4$6H2O was added for the hydrothermal reaction; (iii) Ni3S2 with nanorods morphology was prepared via the similar approach with the dendrite-like structure, except that a increased temperature of 180 C was applied for the hydrothermal reaction, instead of 160 C. Next, after each system has cooled down naturally to room temperature, the obtained Ni3S2/NF samples were washed by water and ethanol several times and dried in vacuum at 60 C for 6 h. 2.2. Preparation of hierarchical Ni3S2@Ni(OH)2/NF Hierarchical Ni3S2@Ni(OH)2/NF sample was prepared via a facile hydrothermal reaction. As-prepared Ni3S2 nanosheets/NF sample was immersed into pure water (30 mL) and stirred for 1 h. Then the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heat to 180 C for 80 min. Next, after it has cooled down naturally to room temperature, the obtained Ni3S2@Ni(OH)2/ NF sample was washed by DI water and ethanol several times and dried in vacuum at 60 C for 6 h. 2.3. Materials characterization Scanning electron microscopy (SEM) images were acquired by a Hitachi (Japan) S-4800 field-emission scanning electron microscope operated at an acceleration voltage of 1.0 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by a JEOL (Japan) JEM-2100F field-emission transmission electron microscope at 300 kV of the acceleration voltage. X-ray diffraction (XRD) measurements were conducted on a Bruker (Germany) D8 diffractometer with Ni-filtered Cu-Ka radiation in the 2q range of 10 e80 with the scanning rate of 10 min1. X-ray photoelectron spectroscopy (XPS) data were recorded
by an XPS spectrometer (Kratos, Axis Ultra Dld) with 150 W Al-Ka radiation. 2.4. Electrochemical measurements All the electrochemical measurements were performed on an electrochemical workstation (CHI660D) in 1.0 M KOH electrolyte with three-electrode configuration at 25 C. The electrolyte was deaerated before each measurement by purging with high-purity N2 gas for at least 30 min. Briefly, a Ag/AgCl electrode and a graphite rod were used as the reference electrode and counter electrode, respectively (Fig. S1). The diameter of the graphite rod is 6 mm, thus the exposed surface area of counter electrode to electrolyte is calculated to be 1.1413 cm2, as 1.5 cm of the height of the graphite rod was immersed under the liquid surface of electrolyte. The as-prepared samples were directly used as the working electrode. Potential values vs. reversible hydrogen electrode (RHE) were calculated according to the Eq. (1): E(RHE) ¼ E(Ag/AgCl) þ 0.0591 pH þ 0.197 V.
(1)
Polarization curves were measured at a scan rate of 5 mV s1. Double layer capacitance (Cdl) was measured using cyclic voltammetry (CV) scanning from 0.1 V to 0.2 V vs. RHE at different scan rates from 5 to 30 mV s1, with an interval of 5 mV s1. The electrochemical impedance spectroscopy (EIS) measurements were carried out within the frequency range of 0.1e100 kHz at an overpotential of 250 mV vs. RHE. 3. Results and discussion Different morphologies of the Ni3S2 micro/nanostructures grown on NF were first revealed by SEM characterization. Fig. 1a and b exhibit the nanorods morphology of Ni3S2 grown on NF. The average length of the nanorod is about 2 mm. Meanwhile, asobtained Ni3S2 with dendrite-like morphology is shown in Fig. 1c and d, where such micro-dendrite structure seems to be ripened from small Ni3S2 nanoparticles. Fig. 1e and f displays the morphology of as-obtained nanosheets Ni3S2. It can be observed that these Ni3S2 nanosheets are interconnected and spontaneously organized, forming hierarchical microsphere assemblies. The driving force for the formation of dendrite-like or nanosheets Ni3S2 is originated from the initial concentration of nickel source (Ni2SO4) in the reaction, which determines the nuclei generation and growth. Particularly, higher initial concentration of Ni2SO4 will lead to higher supersaturation degree, thus generating more nuclei with smaller sizes, which tend to grow into nanocrystalline structures with large surface area (i.e. nanosheets) during the consequent Ostwald ripening process. On the contrary, the lower amount of nuclei with larger size tend to grow into larger bulk crystals at micrometer scale (i.e. dendrites). Next, XRD patterns were recorded to verify the chemical composition of as-obtained Ni3S2 with different morphologies. As can be identified from Fig. 2, the characterization diffraction peaks of the three Ni3S2/NF samples are well consistent with the standard peaks of hexagonal Ni3S2 (JCPDS No. 44e1418). Meanwhile, diffraction peaks at ca. 32.2 , 35.7, and 48.8 can be indexed to the (300), (021), and (131) planes of NiS (JCPDS No. 12e004), confirming the existence of NiS phase in Ni3S2 nanorods. The approximate proportion of NiS phase in the nanorods/NF sample is calculated to be 25% via rietveld refinement analysis of the XRD pattern (Fig. S2). These results suggest that slightly increased temperature from 160 C to 180 C could lead to the phase transformation from hexagonal Ni3S2 to NiS, thus relatively low temperature is more favorable for synthesizing pure-phase Ni3S2
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Fig. 1. SEM images recorded at different magnifications of as-prepared Ni3S2 electrocatalysts with (a, b) nanorods, (c, d) dendrite-like and (e, f) nanosheets morphologies.
Fig. 2. XRD patterns of as-prepared Ni3S2 electrocatalysts with different morphologies.
catalysts [29]. Besides, the other three strong diffraction peaks appearing at ca. 44.4 , 51.6 , and 76.1 could be attributed to Ni (JCPDS No.01e1258) substrate. The polarization curve of each sample is displayed in Fig. 3a, which demonstrates that the nanosheets require the lowest overpotential of 288 mV vs. RHE to afford a current density of 10 mA cm2. This outperforms both the Ni3S2 nanorods (314 mV) and dendrite-like Ni3S2 electrocatalyst (303 mV), respectively. Further, Tafel plots were calculated based on the polarization curves to reveal the HER kinetics, as shown in Fig. 3b. It is clear that nanosheets of Ni3S2 performs the lowest Tafel slope (120 mV dec1), implying the most accelerated kinetics of electrocatalytic HER process. In addition, the electrochemical active surface area (ECSA) for HER of each Ni3S2 electrocatalyst was investigated as well by measuring double-layer capacitance (Cdl) in non-faradaic current region of 0.1e0.2 V vs. RHE (Fig. S3)[30]. As exhibited in Fig. 3c, the Ni3S2 nanosheets/NF electrode possesses the highest Cdl of 14.7 mF cm2, compared with the nanorods (11.8 mF cm2) and dendrite-like (13.2 mF cm2) Ni3S2 electrocatalysts. Such result demonstrates that the Ni3S2 electrocatalyst with interconnected nanosheets morphology should have the most favorable exposure of active sites. Moreover, it has been reported that NiS, compared to Ni3S2, is rather inactive for HER [24], which has been detected in
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Fig. 3. (a) Polarization curves recorded at the scan rate of 5 mV s1, (b) corresponding Tafel plots, (c) the estimation of Cdl, and (d) Nyquist plots (insert shows the equivalent circuit) of the prepared Ni3S2 electrocatalysts with different morphologies.
Ni3S2 nanorods by XRD characterization mentioned above (Fig. 2). Therefore, the pure phase Ni3S2 together with larger ECSA induced more favorable active site exposure, have brought about superior HER performance of the Ni3S2 electrocatalyst with nanosheets morphology. In addition, EIS measurements were carried out to further investigate the HER performance of these Ni3S2/NF electrodes. The charge-transfer resistance (RCT) and solution resistance (Rs) can be obtained from the semicircles in the high- and lowfrequency range of the Nyquist plots, respectively. The electron transfer process at the interface is analyzed based on the EIS results [31,32]. The apparent electron transfer rate constant was calculated using Eq. (2) and summarized in Table S1 with other electrochemical parameters:
kapp ¼
RT F 2 RCT AC
(2)
where R is the universal gas constant; T is the temperature (298 K); F is the Faraday constant; RCT stands for the charge-transfer resistance; A and C represent the surface area of the electrode and concentration of the electrolyte, respectively. The smallest RCT and highest kapp values of Ni3S2 nanosheets/NF together suggest its highest electron transfer kinetics. Based on the hierarchical microsphere-like interconnected Ni3S2 nanosheets, Ni(OH)2 was grown on the surface to form heterostructure. The chemical composition and morphology of Ni3S2@Ni(OH)2/NF sample were investigated by XRD and SEM analysis. Fig. 4a shows the XRD patterns of original Ni3S2 nanosheets/NF and Ni3S2@Ni(OH)2 nanosheets/NF samples, where the distinct diffraction peaks of Ni3S2 (JCPDS No. 44e1418) could be identified clearly in both samples. Meanwhile, the characteristic
peaks appearing at ca. 19.2 , 33.1, and 38.5 are attributed to the (001), (100), and (101) planes of Ni(OH)2 (JCPDS no.14e0117), which confirms the generation of Ni(OH)2 nanoparticles. It can be observed from the SEM images in Fig. 4b and Fig. S9 that asgenerated Ni(OH)2 nanoparticles are successfully decorated on the surface of Ni3S2 nanosheets. For comparison, different amount of DI water (20 mL and 40 mL) added in the hydrothermal reaction to prepare Ni3S2@Ni(OH)2/NF has been investigated as well. The severe structure collapses of the interconnected Ni3S2 nanosheets observed in the 20 mL and 40 mL case (Fig. S4) help to further confirm that 30 mL of DI water is suitable for the formation of hierarchical Ni3S2@Ni(OH)2/NF with proper decoration of Ni(OH)2 species and superior morphology retention. In addition, HRTEM image of original Ni3S2 nanosheets (Fig. S5) displays well-resolved lattice fringe with interplanar spacing d ¼ 0.29 nm, which corresponds to (110) plane of hexagonal Ni3S2 phase [33,34]. Further, the HRTEM image as well as the corresponding SAED pattern of Ni3S2@Ni(OH)2 are shown in Fig. 4c, in which a clear boundary can be observed between Ni(OH)2 and Ni3S2, as labeled by a red dashed line. The interplanar spacing of 0.14 nm and 0.18 nm in the outer part could be attributed to (111) and (102) planes of Ni(OH)2, while the d-spacing of 0.19 nm and 0.20 nm in the inner part are attributed to (113) and (202) planes of Ni3S2. These results confirm that the Ni(OH)2 particles have been directly grown on the surface of Ni3S2 nanosheets, which is in accordance with the XRD results. Furthermore, XPS was employed to investigate the chemical environment of as-prepared electrocatalysts. The XPS survey spectrum of Ni3S2@Ni(OH)2 is shown in Fig. 5a, verifying the existence of Ni, S and O elements. Specifically, high-resolution Ni 2p spectrum shown in Fig. 5b can be decomposed into two peaks locating at ca. 853.7 eV and 871.3 eV, attributing to Ni 2p1/2 and Ni
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Fig. 4. (a) XRD patterns of Ni3S2 nanosheets/NF and Ni3S2@Ni(OH)2/NF electrode; (b) SEM image and (c) HRTEM image and the corresponding SAED pattern of the obtained Ni3S2@Ni(OH)2 electrocatalyst.
2p3/2, respectively. This result is in consistence with the Ni 2p spectrum of Ni3S2 in previous reports [35]. Particularly, peaks locating at 863.7 eV and 880.1 eV are attributed to NieO bonds, revealing the formation of Ni(OH)2 species on the outer surface of Ni3S2[36]. In S 2p spectrum as shown in Fig. 5c, the characterist ic peaks at 161.5 eV and 162.9 eV can be assigned to S2 ions in Ni3S2[37]. In addition, as shown in Fig. 5d, the O 1s spectrum can be decomposed into three peaks at 528.3 eV, 529 eV, and 530.2 eV, which can be denoted as O1, O2, and O3 peak, respectively. Accordingly, the identification of O1 represents the formation of metal-oxygen bonds (NieO), and O2 represents oxygen ions (OH). O3 stands for the water (H2O) adsorption on materials surface [38]. For comparison, the XPS spectra of the original Ni3S2/NF sample are exhibited in Fig. S6. The higher intensity of O1 and O2 can be attributed to the formation of Ni(OH)2. Therefore, it is verified that the Ni3S2@Ni(OH)2 heterostructure has been successfully grown on NF substrate. The electrocatalytic performance of the Ni3S2@Ni(OH)2/NF electrode was measured using the same three-electrode system. As shown in Fig. 6a, the Ni3S2@Ni(OH)2 electrocatalyst exhibits
improved HER performance with decreased overpotential from 288 mV of pristine Ni3S2 to 237 mV vs. RHE. The Tafel plots, as shown in Fig. 6b, are applied to study the HER mechanism of the Ni3S2@Ni(OH)2. Typically, the HER process in alkaline media could be divided into two steps [39,40]: (i) Volmer step stands for water dissociation and generation of adsorbed hydrogen (Had) on active sites (M), as described below in Eq. (3); (ii) Heyrovesky electrochemical desorption step (Eq. (4)) and Tafel recombination step (Eq. (5)). H2O þ M þ e / M-Had þ OH
(3)
H2O þ M-Had þ e / M þ H2 þ OH
(4)
M-Had þ M-Had / 2 M þ H2
(5)
For original pure-phase Ni3S2 nanosheets, the higher Tafel slope (120 mV dec1) indicates the sluggish kinetics of Volmer reaction. With the introduction of Ni(OH)2, the Ni3S2@Ni(OH)2 nanosheets exhibit decreased Tafel slope of 109 mV dec1, suggesting
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Fig. 5. XPS results of the obtained Ni3S2@Ni(OH)2 electrocatalyst: (a) the survey spectrum, as well as the (b) Ni 2p, (c) S 2p and (d) O 1s narrow-scan spectra.
Fig. 6. (a) Polarization curves recorded at the scan rate of 5 mV s1, (b) Tafel plots, and (c) estimation of Cdl of pure Ni3S2/NF and Ni3S2@Ni(OH)2/NF electrodes, respectively; (d) Time-dependent current density curve of the Ni3S2@Ni(OH)2/NF electrode at a fixed overpotential of 237 mV affording 10 mA cm2.
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accelerated Volmer reaction for water dissociation. It has been reported that owing to the strong OH adsorption ability of Ni(OH)2 in HER process, Ni(OH)2 can help to cleave the HOeH bonds in water dissociation, thus H2O molecules can be effectively dissociated into H* and OH- intermediates [41,42]. Therefore, the Ni(OH)2 on the surface could facilitate the generation of H* intermediates in Volmer reaction, bringing about improved HER kinetics of the Ni3S2@Ni(OH)2 electrocatalyst. Moreover, according to the Cdl calculations shown in Fig. 6c, Ni3S2@Ni(OH)2 possesses increased Cdl of 20.3 mF cm2 than pure Ni3S2 nanosheets (14.7 mF cm2). This result implies that as-prepared Ni3S2@Ni(OH)2 nanosheets can provide abundant surface for contacting with electrolyte, which facilitates the mass transfer for HER process. In addition, EIS result (Fig. S7) reveals the smaller RCT (2.49 U) and faster kapp, indicating the faster electron transfer and higher HER kinetics of Ni3S2@Ni(OH)2/NF as well. The HER stability of Ni3S2@Ni(OH)2 was further demonstrated by time-dependent current density (I-t) curves under the static voltage of 237 mV to give 10 mA cm2. Fig. 6d shows the superior current density retention for 20 h (127%) of the Ni3S2@Ni(OH)2 catalyst, which surpasses the original pure Ni3S2 nanosheets (77.5%) (Fig. S8). This slight improvement can be attributed to the more sufficient infiltration of electrolyte on Ni3S2@Ni(OH)2/NF electrode surface during the stability test. To further understand the electrocatalytic stability of the electrode, the chemical composition and morphology of Ni3S2@Ni(OH)2/NF after 20 h I-t test were characterized. The XRD patterns in Fig. 7a compare the Ni3S2@Ni(OH)2 before and after the test, demonstrating good
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maintenance in chemical composition of the catalyst after the stability rest. Besides, the morphology of Ni3S2@Ni(OH)2e20 h is displayed in Fig. 7bed and Fig. S9, where well retained interconnected nanosheets structure can be clearly observed, proving that the Ni3S2@Ni(OH)2 catalyst is highly resistant against structural agglomeration or collapse during long-term operation. Therefore, with the changeless structure and chemical composition, the Ni3S2/Ni(OH)2 nanosheets could display robust electrocatalytic stability. Overall, the advantages of the Ni3S2/Ni(OH)2 electrocatalyst could be summarized as the following aspects: (i) The hierarchical microsphere-like interconnected nanosheets structure is favorable for active-site exposure, which promotes the mass transfer; (ii) Ni3S2@Ni(OH)2 heterointerface accelerates the HER kinetics due to the enhanced water dissociation; (iii) The optimized nanosheets structure and chemical composition are in favor of robust HER stability, bearing efficient hydrogen production for 20 h.
4. Conclusions In summary, nanosheets, nanorods and dendrite-like structures of Ni3S2 are synthesized and investigated as HER catalysts. Compared with the nanorods and dendrite-like morphologies, the optimized Ni3S2 electrocatalyst with microsphere-like interconnected nanosheets structure can provide more active-site exposure, which is beneficial to the HER activity. Meanwhile, the construction of Ni3S2@Ni(OH)2 heterointerface on pristine interconnected nanosheets of Ni3S2 could further reduce the
Fig. 7. (a) XRD patterns of the Ni3S2@Ni(OH)2/NF electrode before and after stability test; (b) SEM image of the surface morphology of the Ni3S2@Ni(OH)2/NF electrode after stability test; (ced) HRTEM images of the Ni3S2@Ni(OH)2 electrocatalyst after stability test. Insert in panel (c) shows the corresponding SAED pattern.
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overpotential to 237 mV, accelerate HER kinetics to 109 mV dec1, and achieve robust stability affording 20-hour HER operation with unchanged chemical composition and surface structure. Overall, this work deepens the study of the cost-effective Ni3S2-based electrocatalysts, taking advantages of optimized morphology and combination of active Ni(OH)2 species, and thus achieves enhanced electrocatalytic HER performance. 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. CRediT authorship contribution statement Shuang Hao: Conceptualization, Investigation, Data curation, Methodology, Formal analysis, Writing - original draft. Qi Cao: Project administration, Supervision, Methodology, Validation, Writing - review & editing. Liting Yang: Investigation. Renchao Che: Conceptualization, Resources, Funding acquisition, Validation, Project administration, Writing - review & editing. Acknowledgments This work was supported by Ministry of Science and Technology of China (973 Project No. 2018YFA0209102), and National Natural Science Foundation of China (11727807, 51725101, 51672050, and 61790581). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154163. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (6861) (2001) 332e337. [2] F. Gong, W. Wang, H. Li, D. Xia, Q. Dai, X. Wu, M. Wang, J. Li, D.V. Papavassiliou, R. Xiao, Solid waste and graphite derived solar steam generator for highlyefficient and cost-effective water purification, Appl. Energy 261 (2020) 114410. [3] Q. Cao, J. Yu, K. Yuan, M. Zhong, J.J. Delaunay, Facile and large-area preparation of porous Ag3PO4 photoanodes for enhanced photoelectrochemical water oxidation, ACS Appl. Mater. Interfaces 9 (23) (2017) 19507e19512. [4] C. Li, Q. Cao, F. Wang, Y. Xiao, Y. Li, J.J. Delaunay, H. Zhu, Engineering graphene and TMDs based van der Waals heterostructures for photovoltaic and photoelectrochemical solar energy conversion, Chem. Soc. Rev. 47 (13) (2018) 4981e5037. [5] R.F. Service, The hydrogen backlash, Science 305 (5686) (2004) 958e961. [6] L. Yu, I.K. Mishra, Y. Xie, H. Zhou, J. Sun, J. Zhou, Y. Ni, D. Luo, F. Yu, Y. Yu, S. Chen, Z. Ren, Ternary Ni2(1-x)Mo2xP nanowire arrays toward efficient and stable hydrogen evolution electrocatalysis under large-current-density, Nano Energy 53 (2018) 492e500. [7] Q. Cao, Y.F. Cheng, H. Bi, X. Zhao, K. Yuan, Q. Liu, Q. Li, M. Wang, R. Che, Crystal defect-mediated band-gap engineering: a new strategy for tuning the optical properties of Ag2Se quantum dots toward enhanced hydrogen evolution performance, J. Mater. Chem. 3 (40) (2015) 20051e20055. [8] K. Yuan, Q. Cao, X. Li, H.-Y. Chen, Y. Deng, Y.-Y. Wang, W. Luo, H.-L. Lu, D.W. Zhang, Synthesis of WO3@ZnWO4@ZnO-ZnO hierarchical nanocactus arrays for efficient photoelectrochemical water splitting, Nano Energy 41 (2017) 543e551. [9] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X.W. Lou, Y. Xie, Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution, Adv. Mater. 25 (40) (2013) 5807e5813. [10] 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) 29e36. [11] J. Yu, Q. Cao, Y. Li, X. Long, S. Yang, J.K. Clark, M. Nakabayashi, N. Shibata, J.J. Delaunay, Defect-rich NiCeOx electrocatalyst with ultrahigh stability and low overpotential for water oxidation, ACS Catal. 9 (2) (2019) 1605e1611.
[12] N. Jiang, L. Bogoev, M. Popova, S. Gul, J. Yano, Y. Sun, Electrodeposited nickelsulfide films as competent hydrogen evolution catalysts in neutral water, J. Mater. Chem. 2 (45) (2014) 19407e19414. [13] N. Cheng, Q. Liu, A.M. Asiri, W. Xing, X. Sun, A Fe-doped Ni3S2 particle film as a high-efficiency robust oxygen evolution electrode with very high current density, J. Mater. Chem. 3 (46) (2015) 23207e23212. [14] N. Liu, Y. Guo, X. Yang, H. Lin, L. Yang, Z. Shi, Z. Zhong, S. Wang, Y. Tang, Q. Gao, Microwave-assisted reactant-protecting strategy toward efficient MoS2 electrocatalysts in hydrogen evolution reaction, ACS Appl. Mater. Interfaces 7 (42) (2015) 23741e23749. [15] H. Yan, Y. Xie, Y. Jiao, A. Wu, C. Tian, X. Zhang, L. Wang, H. Fu, Holey reduced graphene oxide coupled with an Mo2N-Mo2C heterojunction for efficient hydrogen evolution, Adv. Mater. 30 (2) (2018). [16] S. Lu, M. Hummel, Z.R. Gu, Y. Gu, Z.S. Cen, L. Wei, Y. Zhou, C.Z. Zhang, C. Yang, Trash to treasure: a novel chemical route to synthesis of NiO/C for hydrogen production, Int. J. Hydrogen Energy 44 (31) (2019) 16144e16153. [17] Y. Wei, M. Song, L. Yu, F. Meng, J. Xiao, Hydroxyl-promoter on hydrated Ni(Mg, Si) attapulgite with high metal sintering resistance for biomass derived gas reforming, Int. J. Hydrogen Energy 44 (36) (2019) 20056e20067. [18] J. Lian, Y. Wu, H. Zhang, S. Gu, Z. Zeng, X. Ye, One-step synthesis of amorphous NieFeeP alloy as bifunctional electrocatalyst for overall water splitting in alkaline medium, Int. J. Hydrogen Energy 43 (29) (2018) 12929e12938. [19] X. Yang, W. Xu, S. Cao, S. Zhu, Y. Liang, Z. Cui, X. Yang, Z. Li, S. Wu, A. Inoue, L. Chen, An amorphous nanoporous PdCuNi-S hybrid electrocatalyst for highly efficient hydrogen production, Appl. Catal. B Environ. 246 (2019) 156e165. [20] T.-W. Lin, C.-J. Liu, C.-S. Dai, Ni3S2/carbon nanotube nanocomposite as electrode material for hydrogen evolution reaction in alkaline electrolyte and enzyme-free glucose detection, Appl. Catal. B Environ. 154e155 (2014) 213e220. [21] T. Zhu, L.L. Zhu, J. Wang, G.W. Ho, In situ chemical etching of tunable 3D Ni3S2 superstructures for bifunctional electrocatalysts for overall water splitting, J. Mater. Chem. 4 (36) (2016) 13916e13922. [22] Y. An, B. Huang, Z. Wang, X. Long, Y. Qiu, J. Hu, D. Zhou, H. Lin, S. Yang, Constructing three-dimensional porous Ni/Ni3S2 nano-interfaces for hydrogen evolution electrocatalysis under alkaline conditions, Dalton Trans. 46 (32) (2017) 10700e10706. [23] D.Y. Chung, J.W. Han, D.H. Lim, J.H. Jo, S.J. Yoo, H. Lee, Y.E. Sung, Structure dependent active sites of NixSy as electrocatalysts for hydrogen evolution reaction, Nanoscale 7 (12) (2015) 5157e5163. [24] N. Jiang, Q. Tang, M. Sheng, B. You, D.-e. Jiang, Y. Sun, Nickel sulfides for electrocatalytic hydrogen evolution under alkaline conditions: a case study of crystalline NiS, NiS2, and Ni3S2 nanoparticles, Catal. Sci. Technol. 6 (4) (2016) 1077e1084. [25] Q. Che, Q. Li, Y. Tan, X. Chen, X. Xu, Y. Chen, One-step controllable synthesis of amorphous (Ni-Fe)S-x/NiFe(OH)(y) hollow microtube/sphere films as superior bifunctional electrocatalysts for quasi industrial water splitting at large-current-density, Appl. Catal. B Environ. 246 (2019) 337e348. [26] F.J. Sarabia, P. Sebastian Pascual, M.T.M. Koper, V. Climent, J.M. Feliu, Effect of the interfacial water structure on the hydrogen evolution reaction on Pt(111) modified with different nickel hydroxide coverages in alkaline media, ACS Appl. Mater. Interfaces 11 (1) (2019) 613e623. [27] Y. Zhou, C.F. Sun, X.G. Yang, G. Zou, H.J. Wu, S.Q. Xi, Flame-like Ni(OH)2 strongly promotes the dissociation of water and can be used to produce an excellent hybrid electrocatalyst for the hydrogen evolution reaction in alkaline media, Electrochem. Commun. 91 (2018) 66e70. [28] F. Meng, M. Song, B. Song, Y. Wei, Q. Cao, Y. Cao, Enhanced degradation of Rhodamine B via alpha-Fe2O3 microspheres induced persulfate to generate reactive oxidizing species, Chemosphere 243 (2019) 125322. [29] Y. Tan, W.D. Xue, Y. Zhang, D.X. He, W.J. Wang, R. Zhao, Solvothermal synthesis of hierarchical alpha-NiS particles as battery-type electrode materials for hybrid supercapacitors, J. Alloys Compd. 806 (2019) 1068e1076. [30] T. Wu, X. Zhu, G. Wang, Y. Zhang, H. Zhang, H. Zhao, Vapor-phase hydrothermal growth of single crystalline NiS2 nanostructure film on carbon fiber cloth for electrocatalytic oxidation of alcohols to ketones and simultaneous H2 evolution, Nano Res. 11 (2) (2018) 1004e1017. [31] E.P. Randviir, A cross examination of electron transfer rate constants for carbon screen-printed electrodes using Electrochemical Impedance Spectroscopy and cyclic voltammetry, Electrochim. Acta 286 (2018) 179e186. [32] R.S. Moakhar, S. Masudy Panah, M. Jalali, G.K.L. Goh, A. Dolati, M. Ghorbani, N. Riahi-Noori, Sunlight driven photoelectrochemical light-to-electricity conversion of screen-printed surface nanostructured TiO2 decorated with plasmonic Au nanoparticles, Electrochim. Acta 219 (2016) 386e393. [33] S. Hao, J. Liu, Q. Cao, Y. Zhao, X. Zhao, K. Pei, J. Zhang, G. Chen, R. Che, In-situ electrochemical pretreatment of hierarchical Ni3S2-based electrocatalyst towards promoted hydrogen evolution reaction with low overpotential, J. Colloid Interface Sci. 559 (2020) 282e290. [34] D. Yang, L. Cao, L. Feng, J. Huang, K. Kajiyoshi, Y. Feng, Q. Liu, W. Li, L. Feng, G. Hai, Formation of hierarchical Ni3S2 nanohorn arrays driven by in-situ generation of VS4 nanocrystals for boosting alkaline water splitting, Appl. Catal. B Environ. (2019) 257. [35] L. Zeng, K. Sun, X. Wang, Y. Liu, Y. Pan, Z. Liu, D. Cao, Y. Son g, S. Liu, C. Liu, Three-dimensional-networked Ni2P/Ni3S2 heteronanoflake arrays for highly enhanced electrochemical overall-water-splitting activity, Nano Energy 51 (2018) 26e36. [36] S. Min, C. Zhao, Z. Zhang, G. Chen, X. Qian, Z. Guo, Synthesis of Ni(OH)2/RGO
S. Hao et al. / Journal of Alloys and Compounds 827 (2020) 154163 pseudocomposite on nickel foam for supercapacitors with superior performance, J. Mater. Chem. 3 (7) (2015) 3641e3650. [37] J. Yu, Y. Du, Q. Li, L. Zhen, V.P. Dravid, J. Wu, C.Y. Xu, In-situ growth of graphene decorated Ni3S2 pyramids on Ni foam for high-performance overall water splitting, Appl. Surf. Sci. 465 (2019) 772e779. [38] J.T. Ren, G.G. Yuan, C.C. Weng, L. Chen, Z.Y. Yuan, Uniquely integrated Fedoped Ni(OH)2 nanosheets for highly efficient oxygen and hydrogen evolution reactions, Nanoscale 10 (22) (2018) 10620e10628. [39] Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher, X. Wang, Vertically oriented MoS2 and WS2 nanosheets directly grown on carbon cloth as efficient and stable 3dimensional hydrogen-evolving cathodes, J. Mater. Chem. 3 (1) (2015) 131e135.
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[40] 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 (40) (2016) 5530e5537. [41] Q. Xu, H. Jiang, H. Zhang, Y. Hu, C. Li, Heterogeneous interface engineered atomic configuration on ultrathin Ni(OH)2/Ni3S2 nanoforests for efficient water splitting, Appl. Catal. B Environ. 242 (2019) 60e66. [42] X. Wang, R. Liu, Y. Zhang, L. Zeng, A. Liu, Hierarchical Ni3S2-NiOOH heteronanocomposite grown on nickel foam as a noble-metal-free electrocatalyst for hydrogen evolution reaction in alkaline electrolyte, Appl. Surf. Sci. 456 (2018) 164e173.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Morphology-optimized interconnected Ni3S2 nanosheets coupled with Ni(OH)2 nanoparticles for enhanced hydrogen evolution reaction Shuang Hao a, Qi Cao b, **, Liting Yang a, Renchao Che a, * a
Laboratory of Advanced Materials, Department of Materials Science and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai, 200438, China b Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Engineering Research Center for Pollution Treatment and Resource Utilization of Jiangsu Province, School of Energy and Environment, Southeast University, Nanjing, 210096, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2019 Received in revised form 2 February 2020 Accepted 3 February 2020 Available online 4 February 2020
Hydrogen evolution reaction (HER) performance of Ni3S2 electrocatalyst is associated with different morphologies and chemical compositions. In this work, nanosheets, nanorods and dendrite-like structure of Ni3S2 are synthesized and investigated as HER catalysts. Ni3S2 with interconnected nanosheets morphology exhibits the largest electrochemical active surface area, and thereby the lowest overpotential (288 mV vs. RHE) compared with Ni3S2 nanorods (314 mV vs. RHE) and dendrite-like Ni3S2 (303 mV vs. RHE) morphologies. Based on this, Ni(OH)2 species are further grown on the surface of nanosheets Ni3S2 and thus interconnected Ni3S2@Ni(OH)2 nanosheets have been obtained, demonstrating improved HER performance with decreased overpotential (237 mV vs. RHE) and superior stability. These results suggest that (i) the robust nanosheet structure of Ni3S2 can provide abundant space for contacting with electrolyte, which facilitates the mass transfer; and (ii) the formation of Ni3S2@Ni(OH)2 heterostructure is beneficial for water dissociation in HER process, which brings about accelerated HER kinetics. © 2020 Elsevier B.V. All rights reserved.
Keywords: Ni3S2 nanostructures Ni3S2@Ni(OH)2 heterostructure Hydrothermal reaction Electrocatalyst Hydrogen evolution reaction (HER)
1. Introduction The increasing depletion of fossil fuels and associated environmental issues have promoted the research on clean and renewable energy sources [1e4]. Till now, hydrogen (H2) has been widely regarded as a clean and efficient energy carrier for reduction of the dependence on fossil fuels [5e8], and electrochemical hydrogen evolution reaction (HER) is considered to be a green and effective approach for producing high-quality H2 [9e11]. Unfortunately, HER in alkaline media may suffer from sluggish kinetics and usually require large overpotentials that would lead to high power consumption [12,13]. Moreover, noble metal Pt-based materials, as the most efficient HER electrocatalysts, have been largely hindered by their scarcity and high cost [14,15]. Hence, the development of noble metal-free electrocatalysts for efficient HER is in urgent need. Nickel (Ni)-based materials have received widespread interest in electrocatalyst due to their cost-effective and earth-abundant
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Cao), [email protected] (R. Che). https://doi.org/10.1016/j.jallcom.2020.154163 0925-8388/© 2020 Elsevier B.V. All rights reserved.
advantages. On the one hand, Ni-based compounds show superior activity for HER. For instance, NiO is considered as one of the most promising HER catalysts due to its high activity while is hindered by the low electrical conductivity [16]. On the other hand, Ni-based bimetallic materials [17], for example the NiFe [18] and NiCu-based [19] catalysts, could offer synergistic effect allowing both high activity and conductivity. However, the complicated synthesis processes have impeded their practical applications. Among these electrocatalysts, nickel sulfide (Ni3S2) has been recognized as one of the best candidates for HER owing to its high conductivity and unique configuration [20,21]. However, Ni3S2 is still restricted by its unsatisfactory HER activity which can be caused by the crude morphology [22,23] and insufficient HER kinetics [24]. In this regard, the combination of Ni3S2 with other active materials has been proved to be an efficient strategy to enhance the intrinsic HER activity [25e28]. Nonetheless, the development of sophisticated Ni3S2-based electrocatalysts with high HER activity and robust stability still remains a daunting challenge. In this work, Ni3S2 electrocatalysts with different morphologies have been synthesized via hydrothermal reactions, with the morphology effect on HER meticulously studied. Compared with
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nanorods or dendrite-like structures, Ni3S2 with unique microsphere-like interconnected nanosheets structure exhibits the most favorable active-site exposure and thus the highest HER activity. Further, Ni(OH)2 was directly grown on Ni3S2 nanosheets. The as-generated interconnected Ni3S2@Ni(OH)2 nanosheets present decreased overpotential of 237 mV at 10 mA cm2 with accelerated HER kinetics and good durability for 20 h. The construction of Ni3S2@Ni(OH)2 heterointerface turns out to be beneficial for water dissociation, which brings about accelerated HER kinetics and enhanced HER activity. This work provides insights into the role that morphology plays in HER performance of Ni3S2 electrocatalysts, and proves that the formation of Ni3S2@Ni(OH)2 heterostructure could bring about enhanced and robust HER activity. 2. Experimental section 2.1. Preparation of Ni3S2/NF with different morphologies As-purchased nickel foam (NF, 99.7%, LONGSHENGBAO Co., Ltd.) was first pretreated for removal of oxidized Ni species on the surface. Briefly, pieces of NF (1 1.5 cm2) were sequentially washed by HCl (2 M), deionized (DI) water and ethanol under sonification for 20 min, and then dried in a vacuum oven at 60 C for 6 h. Next, Ni3S2/NF samples with different surface morphologies were prepared via a similar one-step hydrothermal reaction: (i) 50 mg of thioacetamide (TAA, CH3CSNH2, 99.0%) and 50 mg of nickel sulfate (NiSO4$6H2O, 98.5%), both purchased from Sinopharm Chemical Reagent Co., Ltd., were dissolved in DI water (30 mL) under stirring with NF for 1 h. Then the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 160 C for 4 h to form Ni3S2 interconnected nanosheets; (ii) Ni3S2 with dendrite-like morphology was synthesized via a similar route with the abovedescribed nanosheets, except that only 25 mg of NiSO4$6H2O was added for the hydrothermal reaction; (iii) Ni3S2 with nanorods morphology was prepared via the similar approach with the dendrite-like structure, except that a increased temperature of 180 C was applied for the hydrothermal reaction, instead of 160 C. Next, after each system has cooled down naturally to room temperature, the obtained Ni3S2/NF samples were washed by water and ethanol several times and dried in vacuum at 60 C for 6 h. 2.2. Preparation of hierarchical Ni3S2@Ni(OH)2/NF Hierarchical Ni3S2@Ni(OH)2/NF sample was prepared via a facile hydrothermal reaction. As-prepared Ni3S2 nanosheets/NF sample was immersed into pure water (30 mL) and stirred for 1 h. Then the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heat to 180 C for 80 min. Next, after it has cooled down naturally to room temperature, the obtained Ni3S2@Ni(OH)2/ NF sample was washed by DI water and ethanol several times and dried in vacuum at 60 C for 6 h. 2.3. Materials characterization Scanning electron microscopy (SEM) images were acquired by a Hitachi (Japan) S-4800 field-emission scanning electron microscope operated at an acceleration voltage of 1.0 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by a JEOL (Japan) JEM-2100F field-emission transmission electron microscope at 300 kV of the acceleration voltage. X-ray diffraction (XRD) measurements were conducted on a Bruker (Germany) D8 diffractometer with Ni-filtered Cu-Ka radiation in the 2q range of 10 e80 with the scanning rate of 10 min1. X-ray photoelectron spectroscopy (XPS) data were recorded
by an XPS spectrometer (Kratos, Axis Ultra Dld) with 150 W Al-Ka radiation. 2.4. Electrochemical measurements All the electrochemical measurements were performed on an electrochemical workstation (CHI660D) in 1.0 M KOH electrolyte with three-electrode configuration at 25 C. The electrolyte was deaerated before each measurement by purging with high-purity N2 gas for at least 30 min. Briefly, a Ag/AgCl electrode and a graphite rod were used as the reference electrode and counter electrode, respectively (Fig. S1). The diameter of the graphite rod is 6 mm, thus the exposed surface area of counter electrode to electrolyte is calculated to be 1.1413 cm2, as 1.5 cm of the height of the graphite rod was immersed under the liquid surface of electrolyte. The as-prepared samples were directly used as the working electrode. Potential values vs. reversible hydrogen electrode (RHE) were calculated according to the Eq. (1): E(RHE) ¼ E(Ag/AgCl) þ 0.0591 pH þ 0.197 V.
(1)
Polarization curves were measured at a scan rate of 5 mV s1. Double layer capacitance (Cdl) was measured using cyclic voltammetry (CV) scanning from 0.1 V to 0.2 V vs. RHE at different scan rates from 5 to 30 mV s1, with an interval of 5 mV s1. The electrochemical impedance spectroscopy (EIS) measurements were carried out within the frequency range of 0.1e100 kHz at an overpotential of 250 mV vs. RHE. 3. Results and discussion Different morphologies of the Ni3S2 micro/nanostructures grown on NF were first revealed by SEM characterization. Fig. 1a and b exhibit the nanorods morphology of Ni3S2 grown on NF. The average length of the nanorod is about 2 mm. Meanwhile, asobtained Ni3S2 with dendrite-like morphology is shown in Fig. 1c and d, where such micro-dendrite structure seems to be ripened from small Ni3S2 nanoparticles. Fig. 1e and f displays the morphology of as-obtained nanosheets Ni3S2. It can be observed that these Ni3S2 nanosheets are interconnected and spontaneously organized, forming hierarchical microsphere assemblies. The driving force for the formation of dendrite-like or nanosheets Ni3S2 is originated from the initial concentration of nickel source (Ni2SO4) in the reaction, which determines the nuclei generation and growth. Particularly, higher initial concentration of Ni2SO4 will lead to higher supersaturation degree, thus generating more nuclei with smaller sizes, which tend to grow into nanocrystalline structures with large surface area (i.e. nanosheets) during the consequent Ostwald ripening process. On the contrary, the lower amount of nuclei with larger size tend to grow into larger bulk crystals at micrometer scale (i.e. dendrites). Next, XRD patterns were recorded to verify the chemical composition of as-obtained Ni3S2 with different morphologies. As can be identified from Fig. 2, the characterization diffraction peaks of the three Ni3S2/NF samples are well consistent with the standard peaks of hexagonal Ni3S2 (JCPDS No. 44e1418). Meanwhile, diffraction peaks at ca. 32.2 , 35.7, and 48.8 can be indexed to the (300), (021), and (131) planes of NiS (JCPDS No. 12e004), confirming the existence of NiS phase in Ni3S2 nanorods. The approximate proportion of NiS phase in the nanorods/NF sample is calculated to be 25% via rietveld refinement analysis of the XRD pattern (Fig. S2). These results suggest that slightly increased temperature from 160 C to 180 C could lead to the phase transformation from hexagonal Ni3S2 to NiS, thus relatively low temperature is more favorable for synthesizing pure-phase Ni3S2
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Fig. 1. SEM images recorded at different magnifications of as-prepared Ni3S2 electrocatalysts with (a, b) nanorods, (c, d) dendrite-like and (e, f) nanosheets morphologies.
Fig. 2. XRD patterns of as-prepared Ni3S2 electrocatalysts with different morphologies.
catalysts [29]. Besides, the other three strong diffraction peaks appearing at ca. 44.4 , 51.6 , and 76.1 could be attributed to Ni (JCPDS No.01e1258) substrate. The polarization curve of each sample is displayed in Fig. 3a, which demonstrates that the nanosheets require the lowest overpotential of 288 mV vs. RHE to afford a current density of 10 mA cm2. This outperforms both the Ni3S2 nanorods (314 mV) and dendrite-like Ni3S2 electrocatalyst (303 mV), respectively. Further, Tafel plots were calculated based on the polarization curves to reveal the HER kinetics, as shown in Fig. 3b. It is clear that nanosheets of Ni3S2 performs the lowest Tafel slope (120 mV dec1), implying the most accelerated kinetics of electrocatalytic HER process. In addition, the electrochemical active surface area (ECSA) for HER of each Ni3S2 electrocatalyst was investigated as well by measuring double-layer capacitance (Cdl) in non-faradaic current region of 0.1e0.2 V vs. RHE (Fig. S3)[30]. As exhibited in Fig. 3c, the Ni3S2 nanosheets/NF electrode possesses the highest Cdl of 14.7 mF cm2, compared with the nanorods (11.8 mF cm2) and dendrite-like (13.2 mF cm2) Ni3S2 electrocatalysts. Such result demonstrates that the Ni3S2 electrocatalyst with interconnected nanosheets morphology should have the most favorable exposure of active sites. Moreover, it has been reported that NiS, compared to Ni3S2, is rather inactive for HER [24], which has been detected in
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Fig. 3. (a) Polarization curves recorded at the scan rate of 5 mV s1, (b) corresponding Tafel plots, (c) the estimation of Cdl, and (d) Nyquist plots (insert shows the equivalent circuit) of the prepared Ni3S2 electrocatalysts with different morphologies.
Ni3S2 nanorods by XRD characterization mentioned above (Fig. 2). Therefore, the pure phase Ni3S2 together with larger ECSA induced more favorable active site exposure, have brought about superior HER performance of the Ni3S2 electrocatalyst with nanosheets morphology. In addition, EIS measurements were carried out to further investigate the HER performance of these Ni3S2/NF electrodes. The charge-transfer resistance (RCT) and solution resistance (Rs) can be obtained from the semicircles in the high- and lowfrequency range of the Nyquist plots, respectively. The electron transfer process at the interface is analyzed based on the EIS results [31,32]. The apparent electron transfer rate constant was calculated using Eq. (2) and summarized in Table S1 with other electrochemical parameters:
kapp ¼
RT F 2 RCT AC
(2)
where R is the universal gas constant; T is the temperature (298 K); F is the Faraday constant; RCT stands for the charge-transfer resistance; A and C represent the surface area of the electrode and concentration of the electrolyte, respectively. The smallest RCT and highest kapp values of Ni3S2 nanosheets/NF together suggest its highest electron transfer kinetics. Based on the hierarchical microsphere-like interconnected Ni3S2 nanosheets, Ni(OH)2 was grown on the surface to form heterostructure. The chemical composition and morphology of Ni3S2@Ni(OH)2/NF sample were investigated by XRD and SEM analysis. Fig. 4a shows the XRD patterns of original Ni3S2 nanosheets/NF and Ni3S2@Ni(OH)2 nanosheets/NF samples, where the distinct diffraction peaks of Ni3S2 (JCPDS No. 44e1418) could be identified clearly in both samples. Meanwhile, the characteristic
peaks appearing at ca. 19.2 , 33.1, and 38.5 are attributed to the (001), (100), and (101) planes of Ni(OH)2 (JCPDS no.14e0117), which confirms the generation of Ni(OH)2 nanoparticles. It can be observed from the SEM images in Fig. 4b and Fig. S9 that asgenerated Ni(OH)2 nanoparticles are successfully decorated on the surface of Ni3S2 nanosheets. For comparison, different amount of DI water (20 mL and 40 mL) added in the hydrothermal reaction to prepare Ni3S2@Ni(OH)2/NF has been investigated as well. The severe structure collapses of the interconnected Ni3S2 nanosheets observed in the 20 mL and 40 mL case (Fig. S4) help to further confirm that 30 mL of DI water is suitable for the formation of hierarchical Ni3S2@Ni(OH)2/NF with proper decoration of Ni(OH)2 species and superior morphology retention. In addition, HRTEM image of original Ni3S2 nanosheets (Fig. S5) displays well-resolved lattice fringe with interplanar spacing d ¼ 0.29 nm, which corresponds to (110) plane of hexagonal Ni3S2 phase [33,34]. Further, the HRTEM image as well as the corresponding SAED pattern of Ni3S2@Ni(OH)2 are shown in Fig. 4c, in which a clear boundary can be observed between Ni(OH)2 and Ni3S2, as labeled by a red dashed line. The interplanar spacing of 0.14 nm and 0.18 nm in the outer part could be attributed to (111) and (102) planes of Ni(OH)2, while the d-spacing of 0.19 nm and 0.20 nm in the inner part are attributed to (113) and (202) planes of Ni3S2. These results confirm that the Ni(OH)2 particles have been directly grown on the surface of Ni3S2 nanosheets, which is in accordance with the XRD results. Furthermore, XPS was employed to investigate the chemical environment of as-prepared electrocatalysts. The XPS survey spectrum of Ni3S2@Ni(OH)2 is shown in Fig. 5a, verifying the existence of Ni, S and O elements. Specifically, high-resolution Ni 2p spectrum shown in Fig. 5b can be decomposed into two peaks locating at ca. 853.7 eV and 871.3 eV, attributing to Ni 2p1/2 and Ni
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Fig. 4. (a) XRD patterns of Ni3S2 nanosheets/NF and Ni3S2@Ni(OH)2/NF electrode; (b) SEM image and (c) HRTEM image and the corresponding SAED pattern of the obtained Ni3S2@Ni(OH)2 electrocatalyst.
2p3/2, respectively. This result is in consistence with the Ni 2p spectrum of Ni3S2 in previous reports [35]. Particularly, peaks locating at 863.7 eV and 880.1 eV are attributed to NieO bonds, revealing the formation of Ni(OH)2 species on the outer surface of Ni3S2[36]. In S 2p spectrum as shown in Fig. 5c, the characterist ic peaks at 161.5 eV and 162.9 eV can be assigned to S2 ions in Ni3S2[37]. In addition, as shown in Fig. 5d, the O 1s spectrum can be decomposed into three peaks at 528.3 eV, 529 eV, and 530.2 eV, which can be denoted as O1, O2, and O3 peak, respectively. Accordingly, the identification of O1 represents the formation of metal-oxygen bonds (NieO), and O2 represents oxygen ions (OH). O3 stands for the water (H2O) adsorption on materials surface [38]. For comparison, the XPS spectra of the original Ni3S2/NF sample are exhibited in Fig. S6. The higher intensity of O1 and O2 can be attributed to the formation of Ni(OH)2. Therefore, it is verified that the Ni3S2@Ni(OH)2 heterostructure has been successfully grown on NF substrate. The electrocatalytic performance of the Ni3S2@Ni(OH)2/NF electrode was measured using the same three-electrode system. As shown in Fig. 6a, the Ni3S2@Ni(OH)2 electrocatalyst exhibits
improved HER performance with decreased overpotential from 288 mV of pristine Ni3S2 to 237 mV vs. RHE. The Tafel plots, as shown in Fig. 6b, are applied to study the HER mechanism of the Ni3S2@Ni(OH)2. Typically, the HER process in alkaline media could be divided into two steps [39,40]: (i) Volmer step stands for water dissociation and generation of adsorbed hydrogen (Had) on active sites (M), as described below in Eq. (3); (ii) Heyrovesky electrochemical desorption step (Eq. (4)) and Tafel recombination step (Eq. (5)). H2O þ M þ e / M-Had þ OH
(3)
H2O þ M-Had þ e / M þ H2 þ OH
(4)
M-Had þ M-Had / 2 M þ H2
(5)
For original pure-phase Ni3S2 nanosheets, the higher Tafel slope (120 mV dec1) indicates the sluggish kinetics of Volmer reaction. With the introduction of Ni(OH)2, the Ni3S2@Ni(OH)2 nanosheets exhibit decreased Tafel slope of 109 mV dec1, suggesting
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Fig. 5. XPS results of the obtained Ni3S2@Ni(OH)2 electrocatalyst: (a) the survey spectrum, as well as the (b) Ni 2p, (c) S 2p and (d) O 1s narrow-scan spectra.
Fig. 6. (a) Polarization curves recorded at the scan rate of 5 mV s1, (b) Tafel plots, and (c) estimation of Cdl of pure Ni3S2/NF and Ni3S2@Ni(OH)2/NF electrodes, respectively; (d) Time-dependent current density curve of the Ni3S2@Ni(OH)2/NF electrode at a fixed overpotential of 237 mV affording 10 mA cm2.
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accelerated Volmer reaction for water dissociation. It has been reported that owing to the strong OH adsorption ability of Ni(OH)2 in HER process, Ni(OH)2 can help to cleave the HOeH bonds in water dissociation, thus H2O molecules can be effectively dissociated into H* and OH- intermediates [41,42]. Therefore, the Ni(OH)2 on the surface could facilitate the generation of H* intermediates in Volmer reaction, bringing about improved HER kinetics of the Ni3S2@Ni(OH)2 electrocatalyst. Moreover, according to the Cdl calculations shown in Fig. 6c, Ni3S2@Ni(OH)2 possesses increased Cdl of 20.3 mF cm2 than pure Ni3S2 nanosheets (14.7 mF cm2). This result implies that as-prepared Ni3S2@Ni(OH)2 nanosheets can provide abundant surface for contacting with electrolyte, which facilitates the mass transfer for HER process. In addition, EIS result (Fig. S7) reveals the smaller RCT (2.49 U) and faster kapp, indicating the faster electron transfer and higher HER kinetics of Ni3S2@Ni(OH)2/NF as well. The HER stability of Ni3S2@Ni(OH)2 was further demonstrated by time-dependent current density (I-t) curves under the static voltage of 237 mV to give 10 mA cm2. Fig. 6d shows the superior current density retention for 20 h (127%) of the Ni3S2@Ni(OH)2 catalyst, which surpasses the original pure Ni3S2 nanosheets (77.5%) (Fig. S8). This slight improvement can be attributed to the more sufficient infiltration of electrolyte on Ni3S2@Ni(OH)2/NF electrode surface during the stability test. To further understand the electrocatalytic stability of the electrode, the chemical composition and morphology of Ni3S2@Ni(OH)2/NF after 20 h I-t test were characterized. The XRD patterns in Fig. 7a compare the Ni3S2@Ni(OH)2 before and after the test, demonstrating good
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maintenance in chemical composition of the catalyst after the stability rest. Besides, the morphology of Ni3S2@Ni(OH)2e20 h is displayed in Fig. 7bed and Fig. S9, where well retained interconnected nanosheets structure can be clearly observed, proving that the Ni3S2@Ni(OH)2 catalyst is highly resistant against structural agglomeration or collapse during long-term operation. Therefore, with the changeless structure and chemical composition, the Ni3S2/Ni(OH)2 nanosheets could display robust electrocatalytic stability. Overall, the advantages of the Ni3S2/Ni(OH)2 electrocatalyst could be summarized as the following aspects: (i) The hierarchical microsphere-like interconnected nanosheets structure is favorable for active-site exposure, which promotes the mass transfer; (ii) Ni3S2@Ni(OH)2 heterointerface accelerates the HER kinetics due to the enhanced water dissociation; (iii) The optimized nanosheets structure and chemical composition are in favor of robust HER stability, bearing efficient hydrogen production for 20 h.
4. Conclusions In summary, nanosheets, nanorods and dendrite-like structures of Ni3S2 are synthesized and investigated as HER catalysts. Compared with the nanorods and dendrite-like morphologies, the optimized Ni3S2 electrocatalyst with microsphere-like interconnected nanosheets structure can provide more active-site exposure, which is beneficial to the HER activity. Meanwhile, the construction of Ni3S2@Ni(OH)2 heterointerface on pristine interconnected nanosheets of Ni3S2 could further reduce the
Fig. 7. (a) XRD patterns of the Ni3S2@Ni(OH)2/NF electrode before and after stability test; (b) SEM image of the surface morphology of the Ni3S2@Ni(OH)2/NF electrode after stability test; (ced) HRTEM images of the Ni3S2@Ni(OH)2 electrocatalyst after stability test. Insert in panel (c) shows the corresponding SAED pattern.
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overpotential to 237 mV, accelerate HER kinetics to 109 mV dec1, and achieve robust stability affording 20-hour HER operation with unchanged chemical composition and surface structure. Overall, this work deepens the study of the cost-effective Ni3S2-based electrocatalysts, taking advantages of optimized morphology and combination of active Ni(OH)2 species, and thus achieves enhanced electrocatalytic HER performance. 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. CRediT authorship contribution statement Shuang Hao: Conceptualization, Investigation, Data curation, Methodology, Formal analysis, Writing - original draft. Qi Cao: Project administration, Supervision, Methodology, Validation, Writing - review & editing. Liting Yang: Investigation. Renchao Che: Conceptualization, Resources, Funding acquisition, Validation, Project administration, Writing - review & editing. Acknowledgments This work was supported by Ministry of Science and Technology of China (973 Project No. 2018YFA0209102), and National Natural Science Foundation of China (11727807, 51725101, 51672050, and 61790581). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154163. References [1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (6861) (2001) 332e337. [2] F. Gong, W. Wang, H. Li, D. Xia, Q. Dai, X. Wu, M. Wang, J. Li, D.V. Papavassiliou, R. Xiao, Solid waste and graphite derived solar steam generator for highlyefficient and cost-effective water purification, Appl. Energy 261 (2020) 114410. [3] Q. Cao, J. Yu, K. Yuan, M. Zhong, J.J. Delaunay, Facile and large-area preparation of porous Ag3PO4 photoanodes for enhanced photoelectrochemical water oxidation, ACS Appl. Mater. Interfaces 9 (23) (2017) 19507e19512. [4] C. Li, Q. Cao, F. Wang, Y. Xiao, Y. Li, J.J. Delaunay, H. Zhu, Engineering graphene and TMDs based van der Waals heterostructures for photovoltaic and photoelectrochemical solar energy conversion, Chem. Soc. Rev. 47 (13) (2018) 4981e5037. [5] R.F. Service, The hydrogen backlash, Science 305 (5686) (2004) 958e961. [6] L. Yu, I.K. Mishra, Y. Xie, H. Zhou, J. Sun, J. Zhou, Y. Ni, D. Luo, F. Yu, Y. Yu, S. Chen, Z. Ren, Ternary Ni2(1-x)Mo2xP nanowire arrays toward efficient and stable hydrogen evolution electrocatalysis under large-current-density, Nano Energy 53 (2018) 492e500. [7] Q. Cao, Y.F. Cheng, H. Bi, X. Zhao, K. Yuan, Q. Liu, Q. Li, M. Wang, R. Che, Crystal defect-mediated band-gap engineering: a new strategy for tuning the optical properties of Ag2Se quantum dots toward enhanced hydrogen evolution performance, J. Mater. Chem. 3 (40) (2015) 20051e20055. [8] K. Yuan, Q. Cao, X. Li, H.-Y. Chen, Y. Deng, Y.-Y. Wang, W. Luo, H.-L. Lu, D.W. Zhang, Synthesis of WO3@ZnWO4@ZnO-ZnO hierarchical nanocactus arrays for efficient photoelectrochemical water splitting, Nano Energy 41 (2017) 543e551. [9] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X.W. Lou, Y. Xie, Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution, Adv. Mater. 25 (40) (2013) 5807e5813. [10] 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) 29e36. [11] J. Yu, Q. Cao, Y. Li, X. Long, S. Yang, J.K. Clark, M. Nakabayashi, N. Shibata, J.J. Delaunay, Defect-rich NiCeOx electrocatalyst with ultrahigh stability and low overpotential for water oxidation, ACS Catal. 9 (2) (2019) 1605e1611.
[12] N. Jiang, L. Bogoev, M. Popova, S. Gul, J. Yano, Y. Sun, Electrodeposited nickelsulfide films as competent hydrogen evolution catalysts in neutral water, J. Mater. Chem. 2 (45) (2014) 19407e19414. [13] N. Cheng, Q. Liu, A.M. Asiri, W. Xing, X. Sun, A Fe-doped Ni3S2 particle film as a high-efficiency robust oxygen evolution electrode with very high current density, J. Mater. Chem. 3 (46) (2015) 23207e23212. [14] N. Liu, Y. Guo, X. Yang, H. Lin, L. Yang, Z. Shi, Z. Zhong, S. Wang, Y. Tang, Q. Gao, Microwave-assisted reactant-protecting strategy toward efficient MoS2 electrocatalysts in hydrogen evolution reaction, ACS Appl. Mater. Interfaces 7 (42) (2015) 23741e23749. [15] H. Yan, Y. Xie, Y. Jiao, A. Wu, C. Tian, X. Zhang, L. Wang, H. Fu, Holey reduced graphene oxide coupled with an Mo2N-Mo2C heterojunction for efficient hydrogen evolution, Adv. Mater. 30 (2) (2018). [16] S. Lu, M. Hummel, Z.R. Gu, Y. Gu, Z.S. Cen, L. Wei, Y. Zhou, C.Z. Zhang, C. Yang, Trash to treasure: a novel chemical route to synthesis of NiO/C for hydrogen production, Int. J. Hydrogen Energy 44 (31) (2019) 16144e16153. [17] Y. Wei, M. Song, L. Yu, F. Meng, J. Xiao, Hydroxyl-promoter on hydrated Ni(Mg, Si) attapulgite with high metal sintering resistance for biomass derived gas reforming, Int. J. Hydrogen Energy 44 (36) (2019) 20056e20067. [18] J. Lian, Y. Wu, H. Zhang, S. Gu, Z. Zeng, X. Ye, One-step synthesis of amorphous NieFeeP alloy as bifunctional electrocatalyst for overall water splitting in alkaline medium, Int. J. Hydrogen Energy 43 (29) (2018) 12929e12938. [19] X. Yang, W. Xu, S. Cao, S. Zhu, Y. Liang, Z. Cui, X. Yang, Z. Li, S. Wu, A. Inoue, L. Chen, An amorphous nanoporous PdCuNi-S hybrid electrocatalyst for highly efficient hydrogen production, Appl. Catal. B Environ. 246 (2019) 156e165. [20] T.-W. Lin, C.-J. Liu, C.-S. Dai, Ni3S2/carbon nanotube nanocomposite as electrode material for hydrogen evolution reaction in alkaline electrolyte and enzyme-free glucose detection, Appl. Catal. B Environ. 154e155 (2014) 213e220. [21] T. Zhu, L.L. Zhu, J. Wang, G.W. Ho, In situ chemical etching of tunable 3D Ni3S2 superstructures for bifunctional electrocatalysts for overall water splitting, J. Mater. Chem. 4 (36) (2016) 13916e13922. [22] Y. An, B. Huang, Z. Wang, X. Long, Y. Qiu, J. Hu, D. Zhou, H. Lin, S. Yang, Constructing three-dimensional porous Ni/Ni3S2 nano-interfaces for hydrogen evolution electrocatalysis under alkaline conditions, Dalton Trans. 46 (32) (2017) 10700e10706. [23] D.Y. Chung, J.W. Han, D.H. Lim, J.H. Jo, S.J. Yoo, H. Lee, Y.E. Sung, Structure dependent active sites of NixSy as electrocatalysts for hydrogen evolution reaction, Nanoscale 7 (12) (2015) 5157e5163. [24] N. Jiang, Q. Tang, M. Sheng, B. You, D.-e. Jiang, Y. Sun, Nickel sulfides for electrocatalytic hydrogen evolution under alkaline conditions: a case study of crystalline NiS, NiS2, and Ni3S2 nanoparticles, Catal. Sci. Technol. 6 (4) (2016) 1077e1084. [25] Q. Che, Q. Li, Y. Tan, X. Chen, X. Xu, Y. Chen, One-step controllable synthesis of amorphous (Ni-Fe)S-x/NiFe(OH)(y) hollow microtube/sphere films as superior bifunctional electrocatalysts for quasi industrial water splitting at large-current-density, Appl. Catal. B Environ. 246 (2019) 337e348. [26] F.J. Sarabia, P. Sebastian Pascual, M.T.M. Koper, V. Climent, J.M. Feliu, Effect of the interfacial water structure on the hydrogen evolution reaction on Pt(111) modified with different nickel hydroxide coverages in alkaline media, ACS Appl. Mater. Interfaces 11 (1) (2019) 613e623. [27] Y. Zhou, C.F. Sun, X.G. Yang, G. Zou, H.J. Wu, S.Q. Xi, Flame-like Ni(OH)2 strongly promotes the dissociation of water and can be used to produce an excellent hybrid electrocatalyst for the hydrogen evolution reaction in alkaline media, Electrochem. Commun. 91 (2018) 66e70. [28] F. Meng, M. Song, B. Song, Y. Wei, Q. Cao, Y. Cao, Enhanced degradation of Rhodamine B via alpha-Fe2O3 microspheres induced persulfate to generate reactive oxidizing species, Chemosphere 243 (2019) 125322. [29] Y. Tan, W.D. Xue, Y. Zhang, D.X. He, W.J. Wang, R. Zhao, Solvothermal synthesis of hierarchical alpha-NiS particles as battery-type electrode materials for hybrid supercapacitors, J. Alloys Compd. 806 (2019) 1068e1076. [30] T. Wu, X. Zhu, G. Wang, Y. Zhang, H. Zhang, H. Zhao, Vapor-phase hydrothermal growth of single crystalline NiS2 nanostructure film on carbon fiber cloth for electrocatalytic oxidation of alcohols to ketones and simultaneous H2 evolution, Nano Res. 11 (2) (2018) 1004e1017. [31] E.P. Randviir, A cross examination of electron transfer rate constants for carbon screen-printed electrodes using Electrochemical Impedance Spectroscopy and cyclic voltammetry, Electrochim. Acta 286 (2018) 179e186. [32] R.S. Moakhar, S. Masudy Panah, M. Jalali, G.K.L. Goh, A. Dolati, M. Ghorbani, N. Riahi-Noori, Sunlight driven photoelectrochemical light-to-electricity conversion of screen-printed surface nanostructured TiO2 decorated with plasmonic Au nanoparticles, Electrochim. Acta 219 (2016) 386e393. [33] S. Hao, J. Liu, Q. Cao, Y. Zhao, X. Zhao, K. Pei, J. Zhang, G. Chen, R. Che, In-situ electrochemical pretreatment of hierarchical Ni3S2-based electrocatalyst towards promoted hydrogen evolution reaction with low overpotential, J. Colloid Interface Sci. 559 (2020) 282e290. [34] D. Yang, L. Cao, L. Feng, J. Huang, K. Kajiyoshi, Y. Feng, Q. Liu, W. Li, L. Feng, G. Hai, Formation of hierarchical Ni3S2 nanohorn arrays driven by in-situ generation of VS4 nanocrystals for boosting alkaline water splitting, Appl. Catal. B Environ. (2019) 257. [35] L. Zeng, K. Sun, X. Wang, Y. Liu, Y. Pan, Z. Liu, D. Cao, Y. Son g, S. Liu, C. Liu, Three-dimensional-networked Ni2P/Ni3S2 heteronanoflake arrays for highly enhanced electrochemical overall-water-splitting activity, Nano Energy 51 (2018) 26e36. [36] S. Min, C. Zhao, Z. Zhang, G. Chen, X. Qian, Z. Guo, Synthesis of Ni(OH)2/RGO
S. Hao et al. / Journal of Alloys and Compounds 827 (2020) 154163 pseudocomposite on nickel foam for supercapacitors with superior performance, J. Mater. Chem. 3 (7) (2015) 3641e3650. [37] J. Yu, Y. Du, Q. Li, L. Zhen, V.P. Dravid, J. Wu, C.Y. Xu, In-situ growth of graphene decorated Ni3S2 pyramids on Ni foam for high-performance overall water splitting, Appl. Surf. Sci. 465 (2019) 772e779. [38] J.T. Ren, G.G. Yuan, C.C. Weng, L. Chen, Z.Y. Yuan, Uniquely integrated Fedoped Ni(OH)2 nanosheets for highly efficient oxygen and hydrogen evolution reactions, Nanoscale 10 (22) (2018) 10620e10628. [39] Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher, X. Wang, Vertically oriented MoS2 and WS2 nanosheets directly grown on carbon cloth as efficient and stable 3dimensional hydrogen-evolving cathodes, J. Mater. Chem. 3 (1) (2015) 131e135.
9
[40] 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 (40) (2016) 5530e5537. [41] Q. Xu, H. Jiang, H. Zhang, Y. Hu, C. Li, Heterogeneous interface engineered atomic configuration on ultrathin Ni(OH)2/Ni3S2 nanoforests for efficient water splitting, Appl. Catal. B Environ. 242 (2019) 60e66. [42] X. Wang, R. Liu, Y. Zhang, L. Zeng, A. Liu, Hierarchical Ni3S2-NiOOH heteronanocomposite grown on nickel foam as a noble-metal-free electrocatalyst for hydrogen evolution reaction in alkaline electrolyte, Appl. Surf. Sci. 456 (2018) 164e173.