Ni3S4 microbars towards Ni3S4 polyhedrons for supercapacitor

Journal of Colloid and Interface Science 559 (2020) 115–123

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

Graphene oxide-drove transformation of NiS/Ni3S4 microbars towards Ni3S4 polyhedrons for supercapacitor Qin Hu a,b,1, Xuefeng Zou c,1, Yuhao Huang d, Yiqing Wei a,b, YaWang a,b, Feng Chen a,b, Bin Xiang a,b,⇑, Qibing Wu e, Wenpo Li a,b,⇑ a

Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China National-municipal Joint Engineering Laboratory for Chemical Process Intensification and Reaction, Chongqing 400044, China Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Normal University, Guiyang 550018, China d School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, China e State Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co. Ltd., Zunyi, Guizhou 563003, China b c

g r a p h i c a l a b s t r a c t In this work, we reported a facile one-step hydrothermal process to in-situ synthesis of Ni3S4@rGO composite by using a unique sulfur source 2mercaptopropionic acid and employing GO as an inducer.

a r t i c l e

i n f o

Article history: Received 27 June 2019 Revised 27 September 2019 Accepted 1 October 2019 Available online 8 October 2019 Keywords: Ni3S4@rGO In-situ growth

a b s t r a c t Ni3S4 is regarded as one of the promising electrode materials for energy storage, but the difficulty in obtaining its pure phase hinders its wide applications. In this work, we introduced a novel method to in-situ synthesize Ni3S4@reduced graphene oxide (rGO) composite, where graphene oxide (GO) was found to induce the oxidation of Ni2+ to Ni3+ and the morphology transformation from microbar to polyhedron during the hydrothermal process. The influence of the content and oxidation degree of GO on the phase composition and morphology of nickel sulfide is investigated. It is found that the oxygencontaining functional group of GO is responsible for the change of valence state, which thus drives the transformation of NiS/Ni3S4 towards Ni3S4. The obtained Ni3S4@rGO composite shows a high energy

⇑ Corresponding authors at: Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China. 1

E-mail addresses: [email protected] (B. Xiang), [email protected] (W. Li). Q. Hu and X. Zou contributed equally.

https://doi.org/10.1016/j.jcis.2019.10.010 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

116 Phase composition Supercapacitor Cycle stability

Q. Hu et al. / Journal of Colloid and Interface Science 559 (2020) 115–123

storage capacity (1830 F g
1 at 2 A g
1), remarkably higher than the unpurified phase NiS/Ni3S4 (830 F g
1). Correspondingly, the assembled asymmetry supercapacitor indicates a high energy density of 37.3 Wh kg
1 at a power density of 398 W kg
1. More importantly, the capacitance retention reaches 91.4 % after 10,000 cycles at a current density of 2 A g
1. Thus, this research overcomes the difficulty of synthesizing the pure Ni3S4 phase, which provides a new available pathway for constructing highperformance electrode materials. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction To fulfill the growing needs of renewable and sustainable power sources, extensive efforts have been made to develop novel electrode active materials with excellent energy storage performances. Among them, transition metal chalcogenides (TMS) have aroused significant attention in energy storage fields including lithium ion battery, sodium ion battery, and supercapacitor due to their unique physical and chemical properties [1–5]. As a special class, nickel sulfides gain widespread researches not only because of its excellent theoretical capacity (450–880 mAh g
1) [6], but also its abundant phases (NiS, NiS2, Ni3S2, Ni3S4, Ni6S5, Ni7S6, Ni9S8) [7–9]. Despite its multiphase feature, only NiS and NiS2 are often investigated. However, the metastable Ni3S4 has attracted much less attention in view of the great difficulty in obtaining its pure phase by traditional solid state method under high temperature, always accompanied by NiS or NiS2 [10–12]. Researchers have found that Ni3S4 is unstable under high temperature, and it will break down to other phases when the ambient temperature exceeds 100 °C [13]. As a result, the electrochemical performance of Ni3S4 has not yet been completely revealed. Subsequently, various efforts such as controlling the reaction pH, the reactant mole ratios, and the precursor, adding capping agents, as well as varying the solvents ever attempted to synthesize Ni3S4 [13–17]. Ni3S4 is found in nature to be mineral polydymite and has a cubic spinel structure [11]. Previous reports have demonstrated that Ni3S4 shows excellent metallic behaviors due to its predominant carriers of electrons, which is closely related to the strong covalent effect between Ni:3d and S:3p orbitals [18–20]. What’s more, Ni3S4 is regarded as a promising active material for energy storage because of its desirable characteristics such as good redox reversibility, excellent theoretical capacitance (703 mAh g
1), and low cost. However, volume-expansion-induced capacity decay, poor rate performance, and weak electronic conductivity also can inhibit its electrochemical energy storage performances [7,14]. In order to remedy the above-mentioned defects of Ni3S4, various efforts such as hybridizing it with carbon materials and constructing its unique nanostructures have been made [16,21]. For instances, Sun [14] et al. prepared rose-like Ni3S4 by a two-step hydrothermal technique, finding it delivers a high specific capacitance of 1797.5 F g
1 at 0.5 A g
1 and reaches an capacitance retention rate of 93 % after 5000 cycles. Li [6] et al. obtained Ni3S4 nanorods/rGO by a high-temperature solution method for sodium ion battery, the composite shows excellent specific capacity (600 mAh g
1), superior rate capability, and long cycle life. Yang [19] et al. synthesized a Ni3S4 nanorods/nitrogen-doped graphene composite via a colloidal solution method, which displays a high electrocatalysis activity, and an advanced sodium ion battery capacity with ~670 mAh g
1 at 100 mA g
1. Thus, these of tactics mentioned above indeed can improve the electrochemical performances of Ni3S4. However, some defects such as ex-situ synthesis process, complicated procedures, and introduction of additives not only limit their applications, but also encumber the intrinsic strong electrochemical performances. To the best of our knowledge, insitu growth of Ni3S4 on graphene or its derivatives (e.g., rGO, GO)

is rarely reported, mainly because of the difficult synthesis of pure Ni3S4 phase. Different from the ex-situ synthesis, Ni3S4 directly growing on graphene materials will be helpful for optimizing its size or morphology, and strengthening its interaction with graphene sheets, thus bringing about more excellent performances [22–24]. Therefore, developing a simple and effective strategy to in-situ grow Ni3S4 on graphene sheets is critical but challenging. In this work, we report a facile one-step hydrothermal process to in-situ synthesis of Ni3S4/rGO composite, which is realized by using a unique sulfur source 2-mercaptopropionic acid and employing GO as an inducer. Different from previous many reports about GO which was usually employed as the template to controllably synthesize nanomaterials with special morphologies, or improve the conductivity. We for the first time find that GO has an oxidation effect on Ni2+, which thus can drive the transformation of NiS/Ni3S4 towards Ni3S4. As a result, the obtained Ni3S4/ rGO composite indicates a hugely improvement in specific capacity, reaching up to 1830 F g
1 at 2 A g
1, which is two times higher than the pristine NiS/Ni3S4 (830 F g
1). When fabricated into asymmetry supercapacitor, the device delivers a high energy density of 37.3 Wh kg
1 at a power density of 398 W kg
1, and possesses a long-term cycling stability (with a 91.4 % capacitance retention after 10,000 cycles). Our work demonstrates that the Ni3S4@rGO composite is a promising energy storage material for supercapacitor, which is superior to the pristine NiS/Ni3S4 and rGO as well as many of other metal sulfides. These findings provide some unique insights for the synthesis of Ni3S4-based materials or other composites. 2. Experimental 2.1. Chemicals Natural flake graphite (AR) was acquired from Qingdao Jinrilai graphene corporation. NaNO3 (AR) and 30 % of H2O2 (AR) were obtained from Chengdu Kelong reagent factory. KMnO4 (AR) and 98 % of H2SO4 (AR) were purchased from Chongqing Chuandong chemical group. NiCl26H2O (AR) was obtained from Guangdong Guanghua company. 2-mercaptopropionic acid (C3H6O2S, 96+ %) was purchase from Adamas. All the chemicals used in our experiments without further purification. 2.2. Preparation of GO GO was prepared from the graphite powder by an improved Hummers’ method. Firstly, 23.0 mL of concentrated sulfuric acid was added into the round-bottomed flask in an ice water bath to keep the temperature below 10 °C. Then, 0.5 g of sodium nitrate and 1 g of graphite powder were added into the sulfuric acid under stirring for 30 min. Successively, 3 g of KMnO4 was slowly dissolved into the above suspension under magnetic stirring. After 1 h, the mixed solution was transferred to a 35 °C water bath and stirred for 30 min. After that, 50.0 mL of deionized water (DI) was added drop by drop and kept stirring at 98 °C for 15 min. After that, the solution was diluted with 150.0 mL of DI, followed by

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10.0 mL 30 % H2O2. Subsequently, the mixture was centrifuged and washed with 5 % HCl (500.0 mL) solution for several times. After that, the mixture was washed with DI to near neutrality. Finally, the product was dried at 35 °C for 24 h. 2.3. Preparation of directly reduced graphene oxide (DrGO) 20 mg of GO was dispersed in 10.0 mL of distilled water by sonication for 1 h, after which it was transferred into a 50.0 mL of Teflon-lined autoclave and kept at 200 °C for 24 h. After that, the mixture was centrifuged and rinsed with DI and ethanol for several times and dried at 50 °C for 12 h in a vacuum oven. The synthesized sample was labeled as DrGO. 2.4. Synthesis of Ni3S4@rGO composites and NiS/Ni3S4 The Ni3S4@rGO composite was prepared by a facile one-step hydrothermal method. In a typical process, 2, 5, 10, 20 30, and 50 mg of GO (products denoted as NiS/Ni3S4@rGO-2, NiS/Ni3S4@rGO-5, NiS/Ni3S4@rGO-10, Ni3S4@rGO-20, Ni3S4@rGO-30, and Ni3S4@rGO-50) were dispersed in 10.0 mL of DI by ultrasonication for 1 h, respectively. Next, 0.237 g of NiCl26H2O and 0.318 g of C3H6O2S were dissolved into 20.0 mL of DI, and then the mixture was added into the GO suspension drop by drop under vigorous magnetic stirring for 30 min. Subsequently, the hybrid solution was transferred into a 50.0 mL of Teflon-lined autoclave and kept at 200 °C for 24 h. After cooling to room temperature, these of samples were centrifuged and rinsed with DI and ethanol for several times and dried at 50 °C for 12 h in a vacuum oven. NiS/Ni3S4 was produced by a similar route without GO. To investigate the effect of oxygen-containing functional groups of GO on the composition of product, the GO was substituted with 20 mg DrGO through a similar route, and the obtained product was defined as NiS/Ni3S4@DrGO-20. 2.5. Characterization These composites were characterized by a powder X-ray diffraction (XRD-600) patterns (2h ranging from 10° to 90°). Raman spectra were performed on a Lab Ram HR Evolution spectrophotometer. Valence distribution of elements and bond features were obtained by X-ray photoelectron spectroscopy (XPS, ESCA Lab 250). The microstructures of these samples were observed using field emission scanning electron microscope instrument (FESEM, JSM7800F) and high-resolution transmission electron microscopy (HRTEM, FEI Talos F200S G2), and the distribution of elements was detected by an energy dispersive spectrometer (EDS). 2.6. Electrochemical measurement The electrochemical performances of these samples were evaluated by a CHI760E electrochemical workstation (Shanghai Chenhua, China) in a 2 M KOH electrolyte. The working electrode was prepared by directly pressing the prepared material into the nickel-foam (1  1 cm2), and the mass loading of nickel sulfides is about 1.2 mg cm
2. In a three-electrode system, the asprepared nickel sulfide electrode was employed as the working electrode, Pt electrode (2  2 cm2) and saturated calomel electrode served as the counter and reference electrode, respectively. Cyclic voltammetry (CV) curves were carried out in the potential range of
0.1 to 0.6 V, and the scan rates are 5, 10, 20, 30, 40 and 50 mV s
1, respectively. The galvanostatic charge/discharge (GCD) measurements were conducted in the potential window of 0– 0.4 V at current densities of 2, 3, 5, 8, 10, and 20 A g
1, respectively. Specific capacitance of the active material was calculated according to the following equation:

C ¼ I  Dt=ðm  Dv Þ

ð1Þ
1

where C, I, Dt, and m represent the specific capacitance (F g ), discharge current (A), discharge time (s), and the mass of active material (g), respectively. The energy density and power density of the device are calculated as follows:

E ¼ C  ðDv Þ2 =7:2

ð2Þ

P ¼ 3600  E=Dt

ð3Þ
1

where C represents the specific capacitance (F g ), Dv is the operating voltage (V), Dt is the discharge time (s), E (W h kg
1) and P (W kg
1) represent the energy density and power density, respectively. 3. Results and discussions NiS/Ni3S4 composite was prepared by a one-step hydrothermal process using NiCl26H2O and C3H6O2S as the nickel and sulfur source, respectively. Interestingly, the introduction of 20 mg GO in reaction can drive the transformation of NiS/Ni3S4 hybrid towards Ni3S4. Fig. 1a indicates the product obtained without GO as precursor is a mixed phase, consisting of hexagonal b-NiS (JPCDS No. 12-0041), hexagonal ɑ-NiS (JPCDS No. 02-1280), and cubic Ni3S4 (JPCDS No. 43-1469), in which the b-NiS is the major phase. Differently, when introducing 20 mg GO in hydrothermal process, the diffraction peaks observed in the Ni3S4@rGO-20 sample are closely indexed to the standard pattern of pure Ni3S4 without almost impurities, and the unobvious peak of rGO was observed at 26°. This predicts that GO plays a key role in phase transformation process. To investigate the role of GO, various amounts of GO were added during the hydrothermal process. After introducing GO, we find that the characteristic peaks of the b-NiS and ɑ-NiS phases are greatly weakened, while the characteristic peaks of Ni3S4 become prominent. When the content of GO is  20 mg, only Ni3S4 phase was observed (Fig. S1a). Therefore, it can be deduced that the oxygen-containing functional group of GO may favor the oxidation of Ni2+ to Ni3+ state, which gives rise to the generation of Ni3S4. To confirm our conjecture, 20 mg GO in reactant was substituted with DrGO. As a result, the obtained product presents a mixed phase harboring Ni3S4 and a-NiS phases (Fig. S1b). Given these, it can be concluded that the oxygen-containing groups of the introduced GO are critical for realizing the transformation of NiS/Ni3S4 hybrid towards Ni3S4. The plausible mechanism can be expressed as follows (Fig. 2). Firstly, the ionization of the oxygen-containing groups of GO in aqueous solution makes it present negative charge, and then the positively charged Ni2+ is absorbed on the negatively charged GO sheets via the electrostatic effect. Secondly, the empty 3 d orbital of nickel ion makes it bond with the sulfur atom with lone pair electrons from C3H6O2S. As a result, the available -SH groups in C3H6O2S could coordinate with Ni2+ to form primary complexes [25]. Subsequently, the SAH bond could be broken into S2- and the Ni2+ could be partially oxidized to Ni3+ by the oxygen-containing groups of GO under high temperature. After then, Ni2+ and Ni3+ could react with S2- to form Ni3S4, and GO was reduced to rGO simultaneously. As shown in Fig. 1b, the Raman technique was employed to investigate the defect degree of rGO. Typically, the observed two peaks at approximately 1342 cm
1 and 1591 cm
1 are designated as the D-band (ID) and G-band (IG) peaks of carbon in rGO, respectively [26]. The ID/IG value of GO is calculated to be 0.94. Compared with GO, the increased ID/IG value for rGO (1.10) suggests an increased defect degree. Interestingly, we find that rGO in Ni3S4@rGO-20 composite indicates highly improvement on the defect degree (ID/IG: 1.44), obviously higher than that of the bare rGO.

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Fig. 1. (a) XRD patterns. (b) Raman spectra of rGO, NiS/Ni3S4, and Ni3S4@rGO-20. Ni 2p XPS spectra of (c) Ni3S4@rGO-20 and (d) NiS/Ni3S4.

Fig. 2. Schematic illustration of the synthesis of NiS/Ni3S4 microbars and Ni3S4 polyhedrons.

This phenomenon signifies the interaction between GO and Ni3S4 [27]. Similar result was observed in other prepared samples (Fig. S1c). It is worthy to note that the rich defects have been demonstrated to be benefit for the electrochemical process [23,28–31]. Thus, the obtained Ni3S4@rGO may indicate a special electrochemical performance. Ni 2p XPS spectra of Ni3S4@rGO-20 and NiS/Ni3S4 further prove the oxidation of Ni2+ to Ni3+ (Fig. 1c and d). As shown in Fig. 1c, the two peaks observed at 861.3 eV and 879.9 eV are designated as the shakeup satellites of Ni 2p spectrum. Specifically, the two peaks

centered at 852.6 eV and 856.1 eV deconvoluted from the Ni 2p3/2 and the two peaks located at 869.8 eV and 874.1 eV separated from Ni 2p1/2 are assigned to Ni2+and Ni3+ [32,33], respectively, implying the existence of abundant elemental valence states in the as-prepared hybrid. As shown in Fig. 1d, similar characteristic peaks can be found at NiS/Ni3S4 spectrum. Of note is that the peaks of Ni3+ observed for Ni3S4@rGO-20 exhibit stronger signal and larger integral area than that of NiS/Ni3S4, while the peaks of Ni2+ in Ni3S4@rGO-20 show lower intensity than that of NiS/Ni3S4. This indicates the higher content of Ni3+ in Ni3S4@rGO-20, thus further

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verifying the oxidation effect of GO on Ni2+. The survey, C 1s, and S 2p high-resolution spectra were presented in Fig. S2. The presence of C, O, Ni, and S elements was revealed by the survey spectrum (Fig. S2a). The peaks observed at 284.6 eV, 285 eV, and 286.1 eV can be attributed to C@C, CAO, and C@O bonds, respectively (Fig. S2b), which originates from the remaining oxygencontaining functional group of rGO [34,35]. The corresponding S 2p spectra of Ni3S4@rGO-20 and NiS/Ni3S4 were presented in Fig. S2c and d, respectively. SEM technology was employed to take a further insight into the morphology of the as-synthesized samples. Fig. 3a and b show the radically aggregated bar-like NiS/Ni3S4 with a predominant diameter around 1 lm. After introducing 20 mg of GO, micron bars disappear. Instead, Ni3S4 polyhedrons with an average size of 500 nm are uniformly anchored on the skeleton of rGO (Fig. 3c and d), indicating GO has an essential effect on the crystal growth of Ni3S4. The SEM images of products synthesized with various GO contents were shown in Fig. S3. The morphological evolution process was described in detail in the supporting information. As shown in Fig. S4a, C, O, S, and Ni elements are evenly distributed in Ni3S4@rGO-20 sample, indicating good interaction between Ni3S4 and rGO. The microstructure of NiS/Ni3S4 and Ni3S4@rGO-20 was detected by TEM. As shown in Fig. 3e, TEM image also observes the bar-like microstructure of that of NiS/Ni3S4, which is in agreement with SEM. As seen in Fig. 3f (1,2,3), high-resolution TEM images reveal that the lattice fringe of 0.25 nm is indexed to the (0 2 1) crystal plane of NiS, while 0.24 nm and 0.22 nm are indexed to the (4 0 0) and (2 2 0) crystal planes of Ni3S4. Differently, a polyhedral-like substance decorated onto wrinkled rGO sheet was observed. And only the lattice fringe of 0.22 nm which is indexed to the (2 2 0) crystal plane of Ni3S4 was detected. Thus, these also fully demonstrate the transformation of NiS/Ni3S4 to Ni3S4. To evaluate the electrochemical performance of the prepared samples, a series of electrochemical processes including CV and GCD curves were carried out by a three-electrode system in a 2 M aqueous KOH. As shown in Fig. 4a, the CV curves of the Ni3S4@rGO-20 were performed within a potential window of
0.1 to 0.6 V. It is found that with the increase of scan rate, a slight redox

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peak shift can still be observed, which mainly stem from the insufficient diffusion of electrolyte ions at high scan rates. Noteworthily, the profile shape of CV curves at different scan rates slightly changes, which indicates a good reversible redox, predicting a good energy storage behavior. Fig. 4b depicts the GCD curves of Ni3S4@rGO electrode at the current density of 2, 3, 5, 8, 10, and 20 A g
1. It is found that all of GCD curves harbor a pair of charge/discharge platforms, which further certifies the typical faradic redox behavior observed in CV curves. Additionally, all GCD curves at every charge/discharge current density give obvious symmetry, demonstrating Ni3S4@rGO as electrode active materials possesses good electrochemical redox reversibility. Similar CV and GCD curves were observed for the electrode fabricated by NiS/Ni3S4 (Fig. 4c and d). In a comparison, Ni3S4@rGO-20 depicts a larger profile area than NiS/Ni3S4, illuminating its better energy storage capacity. In terms of rGO, as electrode active materials, its CV and GCD curves only gives small profile area, indicating its low energy storage capacity. Graphene and its derivatives such as rGO have been proved to possess low energy storage capacity in positive potential range [36–39]. Thus, benefit from rGO, the obtained Ni3S4@rGO-20 indeed shows obviously improvement in energy storage capacity. According to Eq. (1), the specific capacitance of the fabricated Ni3S4@rGO-20 electrode was calculated to be 1830, 1620, 1425, 1300, 1225, and 950 F g
1 at the current density of 2, 3, 5, 8, 10, and 20 A g
1, respectively (Fig. 4e). However, the specific capacitance of NiS/ Ni3S4 is only 830, 735, 663, 600, 575, and 450 F g
1 at 2, 3, 5, 8, 10, and 20 A g
1, respectively (Fig. 4e). Thus, Ni3S4@rGO-20 as electrode active materials shows obviously higher energy storage capacity than NiS/Ni3S4. As compared with the as-reported researches, our synthesized Ni3S4@rGO-20 indicates a relatively high energy storage capacity (Table 1). Additionally, it is found that as the current density increases from 2 to 20 A g
1, the specific capacitance decreases, but can still maintain 51% at 20 A g
1, delivering its good rate performance. This is mainly because of the presence of the wrinkled rGO which promotes the charge transfer between Ni3S4 and electrolyte ions, and provides more ions channels for the permeability of ions [40]. Meanwhile, similar

Fig. 3. (a, b) SEM images of NiS/Ni3S4. and (c,d) Ni3S4@rGO-20. (e) TEM and (f, 1, 2, 3) HRTEM images of NiS/Ni3S4. (g) TEM and (h, i) HRTEM images of Ni3S4@rGO-20.

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Fig. 4. (a) CV curves of Ni3S4@rGO-20 electrode at various scanning rates. (b) GCD curves of Ni3S4@rGO-20 electrode at different current densities. (c) CV curves of the asprepared rGO, NiS/Ni3S4, and Ni3S4@rGO-20 electrodes at 5 mV s
1. (d) GCD curves of the as-prepared rGO, NiS/Ni3S4, and Ni3S4@rGO-20 electrodes at 2 A g
1. (e) Specific capacitance of the rGO, NiS/Ni3S4, and Ni3S4@rGO-20 electrodes at different current densities. (f) The specific capacitances of Ni3S4@GO composites incorporated rGO with different amounts at 2 A g
1.

Table1 Comparison table. Sample

Capacitance

Current Electrolyte density

Ni3S4@rGO polyhedrons (This work) Rose-like Ni3S4 [14] Graphene-Coupled Flower-Like Ni3S2 [3] Ni3S4@amorphous MoS2nanospheres [12] 3D Ni3S4nanosheet [8] NiS hierarchical hollow cubes [35] NiCo2S4-rGO composites [27] Ni3S2@b-NiS [41] NiCo2S4@CoS2 [42] square rod-like NiS2 [43]

1830 F g
1 1535 F g
1 1315 F g
1 1440.9 F g
1 1213 F g
1 874.5 F g
1 1107 F g
1 1158 F g
1 1565 F g
1 1020.2 F g
1

2 2 1 2 2 1 1 1 1 1

A A A A A A A A A A

g
1 g
1 g
1 g
1 g
1 g
1 g
1 g
1 g
1 g
1

2M 2M 1M 6M 3M 2M 1M 6M 2M 6M

KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH

phenomenon as the previous reports was observed that with increasing the content of rGO in the Ni3S4@rGO composites, the specific capacitance increases first and then decreases [3,27]. Of

note is that the specific capacitances of these samples incorporated with rGO are all higher than the pristine NiS/Ni3S4, demonstrating that the addition of rGO is helpful for improving the specific capacitance. When the content of rGO increases to 20 mg, the obtained Ni3S4@rGO gives the maximum specific capacitance. Further, an asymmetric supercapacitor (ASC) was fabricated by employing the Ni3S4@rGO-20 as the positive electrode, the rGO as the negative electrode, the filter paper as separator, and 2 M KOH solution as the electrolyte. The rGO was prepared by a flameinduced method as reported previously [44]. Fig. 5a displays the CV curves of rGO and Ni3S4@rGO-20 electrode at a scan rate of 10 mV s
1, which illustrates that the operating potential window of this ASC device can be extended to 1.6 V. To optimize the electrochemical properties, it is critical to balance the charge stored between the positive and negative electrode (q+ =q
). Typically, the mass ratio of positive and negative electrodes can be obtained from the equation:

Q. Hu et al. / Journal of Colloid and Interface Science 559 (2020) 115–123

mþ =m
¼ C
 DV
=C þ  DV þ

ð4Þ

where m means the electrode mass, C is the specific capacitance, and DV refers to the potential range for the positive/negative material. The specific capacitances of positive and negative electrodes are calculated to be 1830 F g
1 and 224 F g
1 at 2 A g
1, respectively. Thus, the optimal mass ratio of positive/negative electrode is 0.306. The loading mass of Ni3S4@rGO-20 and rGO are 1.6 mg and 5.2 mg, respectively. Fig. 5b gives the CV curves of the Ni3S4@rGO-20//rGO device at different scan rates in a voltage window of 0–1.6 V. It is noteworthy that all CV curves exhibit quasirectangular shapes, which can be regarded as the contribution of both electrical double-layer capacity and pseudocapacitance. In addition, no distortion was observed in these of CV curves even

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when the scan rate increases to 80 mV s
1, indicating the excellent electrochemical behavior. The GCD curves of Ni3S4@rGO-20//rGO device in Fig. 5c display almost symmetric shapes, which demonstrates its good redox reversibility and outstanding columbic efficiency. As shown in Fig. 5d, the specific capacitance of the device was calculated to be 105.0, 96.9, 88.7, 80.0, 71.3, 65.0, and 56.3 F g
1 at 0.5, 1, 2, 4, 6, 8, and 10 A g
1, respectively. As shown in Fig. 5e, the device shows preeminent capacitance retention, up to 91.4 % after 10,000 cycles at 2 A g
1, indicating its excellent cycling stability. Notably, the capacitance increases after 3000 cycles, which can be attributed to the activation process of Ni3S4@rGO-20 [45]. Furthermore, the ASC device shows a good couloumbic efficiency in a durable cycling process, which demonstrates good redox reversibility of Ni3S4@rGO-20.

Fig. 5. (a) CV curves of Ni3S4@rGO-20 and rGO electrodes at a scan rate of 10 mV s
1. (b) CV curves of the assembled device tested at various scan rates. (c) GCD curves of the Ni3S4@rGO-20//rGO device at different current densities. (d) Specific capacitances of Ni3S4@rGO-20//rGO device at different current densities. (e) Cycling performance of Ni3S4@rGO-20//rGO device at a current density of 2 A g
1; (f) Ragone plot of Ni3S4@rGO-20//rGO device. (g) Potical image of LEDs lighted up by two devices in series.

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Fig. 5 (continued)

The energy density and power density are two key parameters for evaluating the practical application of a ASC device. Fig. 5f shows the Ragone plot of Ni3S4@rGO-20//rGO. It delivers a maximum energy density of 37.3 Wh kg
1at a power density of 398 W kg
1, and still reaches 20.0 Wh kg
1 even at the high power density of 8000 W kg
1, which is superior than many previously reported energy storage devices, such as NiS//CNFs (34.9 Wh kg
1 at 387.5 W kg
1) [46], CMTs-1000/Ni2CoS4//AC (28.1Wh kg
1 at 753 W kg
1) [47], NiS/PEDOT:PSS with DEG electrode (21.78 Wh kg
1 at 3611.11 W kg
1) [48], Ni3S4(S2)//AC (18.625 Wh kg
1 at 150 W kg
1) [14], MnO2//FCO (12.36 Wh kg
1 at 6760 W kg
1) [49], VS2//C-Fe/PANI (27.8 Wh kg
1 at 2991 W kg
1) [50], and NiCo2O4//AC (24.5 Wh kg
1 at 175 W kg
1) [51]. Additionally, two such serially connected supercapacitors can even power 16 LEDs for more than 5 min (Fig. 5g), suggesting a promising application of the prepared Ni3S4@rGO-20. 4. Conclusion In summary, we developed a novel strategy for the phasecontrolled synthesis of Ni3S4@rGO composite. Our work shows that NiS/Ni3S4 microbars can be transformed to Ni3S4 polyhedrons in the presence of GO during the hydrothermal process. The phase transformation can be considered as an oxidation effect of the oxygen-containing functional groups of GO, which drives the conversion of Ni2+ to Ni3+ state. It is found that the content and oxidation degree of GO play key roles in the synthesis of highly pure Ni3S4. Owing to high crystallinity of Ni3S4, rich defects in rGO, and in-situ synthesis-induced structural feature, the Ni3S4@rGO20 exhibits an obviously stronger energy storage capacity with 1830 F g
1 at 2 A g
1 than that of pristine NiS/Ni3S4 (830 F g
1). The ASC device assembled with Ni3S4@rGO-20 and rGO indicates an energy density of 37.3 Wh kg
1 at a power density of 398 W kg
1, accompanied by a good cycling performance with 91.4 % capacitance retention after 10,000 cycles at 2 A g
1. Its practical application has been further demonstrated by powering 16 LEDs for more than 5 min. Compared with previous reports about the phase-controlled synthesis of nickel sulfides [6,8,14,52], we in-situ synthesized high-performance Ni3S4@rGO composite and firstly demonstrate the oxidation effect of GO on metal ions. This research sheds lights on the in-situ synthesis of metal sulfides on graphene or its derivatives, especially for metal sulfides with high oxidation state. Declaration of Competing Interest The authors declare no competing interests.

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