DNA-assisted synthesis of nickel cobalt sulfide nanosheets as high-performance battery-type electrode materials

Journal of Colloid and Interface Science 528 (2018) 100–108

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Regular Article

DNA-assisted synthesis of nickel cobalt sulfide nanosheets as high-performance battery-type electrode materials Hanmeng Liu a,b, Yilin Wang a,c, Zhen Li a,c, Zhixia Yao a,c, Jing Lin a,d, Yujing Sun a,⇑, Zhuang Li a,⇑ a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China University of Science and Technology of China, Hefei, Anhui 230026, PR China c University of Chinese Academy of Sciences, Beijing 100049, PR China d College of Chemistry, Jilin Normal University, Siping, Jilin 136000, PR China b

g r a p h i c a l a b s t r a c t We demonstrate an environmental friendly strategy to synthesize the NiCo2S4 nanosheets by DNA-assisted.

a r t i c l e

i n f o

Article history: Received 26 March 2018 Revised 19 May 2018 Accepted 21 May 2018 Available online 22 May 2018 Keywords: NiCo2S4 DNA Nanosheets Battery-type material

a b s t r a c t Nickel-cobalt sulfide (NiCo2S4) nanosheets were successfully fabricated by an environment-friendly hydrothermal method with the assistance of DNA molecules. Different morphological samples were prepared by adjusting the concentrations of DNA. The NiCo2S4 nanosheets derived from 0.2 lg/mL DNA (denoted as DS2) exhibited a desirable mesoporous feature with superior electrochemical performance compared with other samples. As a battery-type electrode material, it exhibited a high specific capacity of 644C g
1 at the current density of 1 A/g, superior rate capability of 74.3% retention at 15 A/g and remarkable cycling stability of 90.5% after 1500 cycles. Thus, the electrode material of NiCo2S4 nanosheets assisted by DNA molecule offered great potential in eco-friendly energy storage device applications. Ó 2018 Published by Elsevier Inc.

1. Introduction Nowadays, the researchers are facing a huge challenge to develop environment-friendly and efficient energy storage systems, which can be applied to many devices that requires a steady ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Sun), [email protected] (Z. Li). https://doi.org/10.1016/j.jcis.2018.05.074 0021-9797/Ó 2018 Published by Elsevier Inc.

flow of power [1,2]. Supercapacitors (SCs), a high-power density of energy storage device, which are also called electrochemical capacitors (ECs), have attracted widespread attention in recent years, owing to their higher power density, fast charge/discharge rate, long life span, and low maintenance cost [3–7]. These overwhelming advantages make them accessible in various applications, such as military devices, intelligent instruments, hybrid electric vehicles and so on [8–10]. However, the unsatisfactory energy density

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which limits their applications needs to be further improved. Therefore, the researchers are developing in materials that might combine the high energy density of batteries with the long cycle life and short charging times of capacitors [10]. This hybrid or asymmetric device, consisting of capacitive electrode and a battery-type electrode, is not only higher in energy density than conventional capacitors, but also higher than batteries in power density [11,12]. Consequently, large numbers of researchers have concentrated on developing battery-type materials with faradaic behavior. Due to their low cost, low toxicity, and great flexibility in structure and morphology, transition metal oxides and their hydroxides have been widely used for fabricating energy storage devices [13–18]. Nevertheless, transition metal sulfides, especially the ternary nickel cobalt sulfide (NiCo2S4) exhibits much higher conductivity because of their smaller optical energy band gap in comparison with the corresponding ternary nickel cobalt oxide [4,5,14,19–24]. Metal sulfides undergo faradic redox reaction in the alkaline electrolyte (MS + OH
$ MSOH + e
, M = Ni, Co. . .), therefore, NiCo2S4 can achieve richer redox reactions and relatively high specific capacity than the corresponding single component sulfides or ternary metal oxides [24–28]. For the past few years, ternary metallic sulfides have been achieved with multiform and tunable morphologies in different ways, including nanoparticles, nanotubes, nanosheets and nanowires [14,25,28,29]. Various preparation methods and design ideas are used to fabricate NiCo2S4, in which the solution-based ion exchange reaction has been deemed as one of the most important chemical transformation methods [28,30]. For example, Lou’s group reported a novel ball-in-ball hollow structure of NiCo2S4 using a relatively facile hydrothermal method and subsequently adopting ion exchange method. The obtained NiCo2S4 performs high specific capacity and remarkable rate capability [5]. Similarly, Lu et al. synthesized three dimensional NiCo2S4 nanomaterials that the sulfur vacancy concentration and impurities of the materials can be adjusted by controlling the sulfurization process, exhibiting optimal electrochemical performance [31]. Although some progress has been made in NiCo2S4 electrode materials, low capacity is still inevitable shortcoming and needs further improvement. To improve the electrochemical performance of electrodes, the introducing biomaterials into nanomaterials is one of the effective strategies [32–34]. It is well-known that DNA molecules have been used as building blocks for construction of functionally sophisticated materials due to their unique structure and properties [33,35–37]. The double-helix structure and the negative charge in phosphate acid skeleton of DNA molecules make them become ideal template to design various nanostructures [37,38]. DNA molecules have been introduced to carbon, polymers, and metal oxides materials to improve their electrochemical performances

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[3,39,40]. However, the synthetic methods are still timeconsuming and complex. Therefore, we proposed a facile and efficient method based on the predecessors’ works to achieve high performance. As far as we know, the nickel cobalt sulfide nanomaterial based on DNA molecule assisted-synthesis for electrode materials has not been reported. Herein, we report a facile strategy for fabricating NiCo2S4 nanosheets by DNA-assisted through an in-situ assembly procedure and subsequently hydrothermal treatment with ionexchange reaction. In this process, the pre-fabricated Ni-Co hydroxide precursors and S2
anion exchange reaction occur at the same time. We use thioacetamide (TAA) as the vulcanizing agent. The prepared NiCo2S4 nanosheets shows distinctive advantages when used as an electrode for supercapacitor. Firstly, the NiCo2S4 would exhibit higher electrochemical capacitive performance than the corresponding metal oxides. Secondly, crosslinked DNA molecules can improve its electrical conductivity as a role of building block for this nanostructure. Finally, the mesoporous feature of the NiCo2S4 nanosheets lead to a large electrode/electrolyte contact interface. The electrochemical tests indicate that this DNAassisted as-prepared NiCo2S4 nanosheet electrode material exhibits a high specific capacity of about 644 Cg
1 at a current density 1 A g
1. Remarkably, about 90.5% of the capacity is retained after 1500 change-discharge cycles, exhibiting an excellent electrochemical stability. These results suggest that the NiCo2S4 nanosheets based on DNA assisted-synthesis can act as high-performance electrode materials for hybrid energy storage devices. 2. Experimental 2.1. Materials Nickel chloride hexahydrate (NiCl26H2O, 99.95%, metal basis), cobalt chloride hexahydrate (CoCl26H2O, 99.95%, metal basis), ammonia solution (NH3H2O, 25%), potassium hydroxide (KOH) and absolute ethanol were purchased from the Beijing Chemical Co. (Beijing, China). Calf thymus DNA (fibers) was purchased from Sigma-Aldrich Chemical Co. Thioacetamide (TAA) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Nickel foam with a purity of 99.96% was purchased from Changsha Lyrun Material Co., Ltd., China. All those reagents were analytical grade and used without further purification. Ultrapure water (>18 MX) used throughout all the experiments was produced by an automatic distillation system (Shanghai, China). 2.2. Synthesis of Ni-Co-OH precursors Firstly, 10 mg of DNA was dissolved into 50 mL Tris-HCl buffer (pH = 8.23) and shook before use. Then, 100 mL of 0.034 M

Fig. 1. The scheme illustration of the DNA-assisted synthesis process for NiCo2S4 nanosheets.

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NiCl26H2O and 0.068 M CoCl26H2O light pink mixed solution was added to the beaker and stirred for 5 min. Next, the DNA solution with different volumes (10 lL, 100 lL, 500 lL) was added to the above solution and stirred for 8 min, respectively. Finally, 600 lL of ammonia solution was diluted 10-fold, added dropwise into

the above solution under vigorous stirring. After two hours, the obtained deep green suspension was centrifuged, washed several times with distilled water and ethanol, and dried at 60 °C. The pure Ni-Co hydroxide precursor was prepared in the same procedure without the addition of DNA.

Fig. 2. SEM images of DS0 (A), DS1 (C), DS2 (E) and DS3 (G). TEM images of DS0 (B), DS1 (D), DS2 (F) and DS3 (H).

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2.3. DNA-assisted synthesis of NiCo2S4 nanosheets About 30 mg of the above dark green precursors were redispersed into 40 mL ultrapure water, followed by addition of 50 mg of TAA. Then the mixture was transferred into a Teflon-lined stainless steel autoclave and kept at 120 °C for 6 h. After cooling to room temperature naturally, the black samples were washed several times with distilled water and ethanol, respectively, and dried at 60 °C. The black powders were obtained. To improve the crystallinity, the final product was annealed under N2 atmosphere at 300 °C for 2 h with a heating rate of 2 °C min
1. A series of NiCo2S4 nanosheets were fabricated using the different quantities of DNA in the first step of the synthesis, yielding the samples denoted as DS1 (0.02 lg/mL), DS2 (0.2 lg/mL) and DS3 (1 lg/ mL), respectively. The pure NiCo2S4 nanosheets without DNA was denoted as DS0. 2.4. Material characterizations Scanning electronic microscopy (SEM), and energy-dispersive X-ray (EDX) characterizations were obtained by S-4800 FE-SEM scanning electron microscope equipped with an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were obtained with a TECNAI G2 high-resolution transmission electron microscope operating at 200 kV. The X-ray diffraction (XRD) patterns were conducted using a Rigaku-D/max 2500 V X-ray diffractometer equipped with a Cu Ka radiation source (k = 1.54178 Å). X-ray photoelectron spectroscopy (XPS) studies were conducted with an ESCA-LAB-MKII spectrometer (VG Scientific, UK) with Al Ka X-ray radiation as the source for excitation. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Quadrach-rome Adsorption Instruments based on the Brunauer-Emmet-Teller (BET) theory. A CHI660A electrochemical workstation was used to investigate the electrochemical performance. 2.5. Electrochemical measurements Prior to the fabrication, Ni foam substrate (1 cm  1 cm) was cleaned ultrasonically in 3 M HCl, deionized water and ethanol sequentially each for 15 min to remove oxide layer of the surface and other impurities. The working electrode was prepared by mixing the as-obtained material, acetylene black and polyterafluoroethylene (PTFE) in a weight ratio of 75:20:5. Then, 500 lL ethanol was added to the above mixture and ultrasonicated to form a homogeneous mixture. Finally, the mixture was filled into the Ni foam and dried under 60 °C for 6 h. The mass loading of

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the sample was about 1 mg cm
2. The electrochemical measurements were investigated in a three-electrode cell configuration at room temperature on a CHI 660a electrochemical workstation. The Ni foam coated with active materials was used as working electrode, a platinum foil (1 cm  1 cm) and a saturate calomel reference electrode (SCE) were used as the counter and reference electrodes, respectively. 6 M KOH aqueous solution served as the electrolyte for all electrochemical measurements. The specific capacity (C) is calculated by the following equation:

C¼

IDt m

ð1Þ

where I is the discharge current, Dt is the discharge time, and m represents the mass of the electroactive material [41,42]. 3. Results and discussion Fig. 1 illustrates the environmental friendly fabrication scheme of a novel electroactive material based on DNA assisted-synthesis. Firstly, Ni2+ and Co2+ cations are absorbed on DNA frameworks, further react with OH
from ammonia solution to form Ni-Co-OH precursors on DNA molecules backbones. Secondly, the abundant S2
ions, which are released from the TAA solution at high temperature, exchange with OH
of the NiACoAOH precursors to form NiACoAS nanosheets. Finally, the product was annealed under N2 atmosphere at 300 °C for 2 h with a heating rate of 2 °C min
1 in order to improve the crystallinity of the sample. The morphologies and nanostructure features of different NiCo-S nanosheets derived from different DNA concentrations are examined by SEM and TEM as shown in Fig. 2. It can be obviously seen that the added different amounts of DNA show significant influence on the nanostructure of NiACoAS nanosheets. For DS0, no framework structure of DNA is observed, and the nanoparticles arrange in disorder (Fig. 2A and B). There are still a few disorder structures on the surface of DS1, while most of nanoparticles uniformly adsorb on the surface of DNA molecules (Fig. 2C and D). Unlike the other specimens, a homogenous porous structure can be seen on the surface of DS2, when the concentration of DNA is 0.2 lg/mL (Fig. 2E and F). However, with the increase of the DNA concentration, an irregular aggregation structure begins to appear on the surface of DS3, which is attributed to the self-assembly of excessive DNA molecules (Fig. 2G and H). It’s probably worthy pointing out that the nanosheets of DS2 are homogenous growing on the DNA linear structure, coils and networks. EDX analysis is conducted to detect the composition of the DS2, as shown in Fig. 3A. It proves the sample is composed of C, N, O, Co, Ni, P, Cl, S, Au, and Si elements. We suggest that the C, N, and P

Fig. 3. EDX image of DS2 (A) and HRTEM image of DS2 (B).

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The crystal phase of the as-synthesis sample prepared by 0.2

lg/mL DNA is investigated by XRD measurement, as shown in

Fig. 4. Typical XRD patterns of DS2 (upper) and the standard spectrum of NiCo2S4 from the JCPDS card (no. 20-0782) (lower).

elements may be attributed to DNA molecule, and Cl element may due to the residual of the initial reaction chlorides. Besides, the Au and Si elements are inevitably from tests process. The HRTEM image in Fig. 3B exhibits the lattice fringes with inter-planar distances of 0.283 nm and 0.332 nm, which can be indexed to the (3 1 1) and (2 2 0) crystal planes of the cubic NiCo2S4 [15,24].

Fig. 4. The diffraction peaks at 16.3°, 26.8°, 31.6°, 38.4°, 50.5° and 55.3° correspond to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes, and can be indexed to the nanostructure cubic phase of NiCo2S4 (JCPDS Card no. 20-0782). Nonetheless, the sample is composed of NiCo2S4, which is consistent with the HRTEM. The chemical composition and chemical state of NiCo2S4 nanosheets have been studied using XPS and the corresponding spectra are presented in Fig. 5A–D. The full survey XPS spectrum of the DS2 corresponds to elements S, N, C, O, Ni, and Co (in Fig. 5A). The element N, a part of C and O may come from DNA molecules. The Co 2p and Ni 2p emission spectrums can be best fitted into two spin-orbit doublets and two shake-up satellites by using a Lorentzian-Gaussian fitting method. In detail, the Co 2p spectrum at binding energies of 781.3 eV and 797.3 eV are ascribed to Co2+, while the binding energies at 779.84 eV and 795.5 eV are characteristics of Co3+. The peaks at 786.5 eV and 802.9 eV are assigned to the two shake-up satellite peaks (in Fig. 5B) [4,24]. Furthermore, the fitting Ni 2p peaks are shown in Fig. 5C, two kinds of nickel species containing Ni2+ and Ni3+ can be observed. The peaks located at 853.9 eV and 870.8 eV are indexed to Ni2+, and the binding energies at around 873.2 and 855.7 eV of Ni 2p peaks indicate the existence of Ni3+. Likewise, the peaks at around 861.5 eV and 880.6 eV are attributed to the shake-up satellite peaks of Ni 2p spectrum [15,43]. Similarly, in the S 2p spectrum (in Fig. 5D), the binding energy at 162.9 eV matches well with metal-sulfur bonding (NiAS and CoAS bonding), and the binding energy at

Fig. 5. XPS surveys: full spectra (A); high resolution XPS spectra: Co 2p (B), Ni 2p (C) and S 2p (D) of the sample DS2.

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Fig. 6. N2 adsorption–desorption isotherms and PSD (inset) of DS2 and DS0 (A). Specific surface area (BET)-dependence on the different concentrations of DNA (B).

161.7 eV corresponds to the S2
in low coordination state at the surface [9,31]. Above all, the near-surface of the NiCo2S4 nanosheets are consisted of Ni2+, Ni3+, Co2+, Co3+ and S2
elements by means of XPS analysis. The nitrogen adsorption-desorption technique is used to figure out the specific surface area, pore size and its distribution in different samples, as shown in Fig. 6. It is obviously found that the isotherm curves are belong to type Ⅳ, and the hysteresis loops are

observed in the range of 0.5–1.0P/P0, indicating the presence of mesoporous structures in the as-synthesized samples (Fig. 6A). The pore size distribution (PSD) is ranging from 10 to 40 nm, which is calculated by Barrett-Joyner-Halenda (BJH) method (the inset of Fig. 6A). The Brunauer-Emmett-Telller (BET) surface areas of different samples are calculated and shown in Fig. 6B. It is clearly seen that the BET surface area strongly relies on the concentration of DNA. When the concentration of DNA is 0.2 lg/mL, the highest

Fig. 7. Electrochemical performance of different samples: CV curves of at the scan rate of 5 mV/s (A), the galvanostatic charge and discharge curves at a current density of 1 A/ g (B), the specific capacity as a function of current density of different samples (C), the EIS Nyquist plots of the NiCo2S4 nanosheets, the inset is the enlarged high-frequency region of the EIS spectrum (D).

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value of the surface area can achieve to 38.09 m2 g
1. Other samples’ isotherm curves and pore size distributions (PSD) are illustrated in Fig. S1 (Supporting Information) for comparison. The electrochemical performance of the as-fabricated samples is evaluated by using a three-electrode cell in the 6.0 M KOH aqueous electrolyte in a potential window from
0.1 to 0.4 V. The comparison of cyclic voltammetry curves (CV) of the NiCo2S4 nanosheets derived from different DNA concentrations as electrodes at the scan rate of 5 mV/s is shown in Fig. 7A. It clearly shows that the sample of DS2 demonstrates the highest faradic redox peak intensity and the CV area, which indicates that the DS2 has significantly improved specific capacity and shows the best electrochemical activity compared to other samples. In addition, the impact of Ni foam on electrochemical test can be neglected, which can be seen from the comparative map of their CV curves as shown in Fig. S2 (Supporting Information). The galvanostatic charge-discharge (GCD) curves show the different samples in the potential window of
0.1 to 0.4 V at the current density of 1 A/g (Fig. 7B). The potential plateaus of all GCD curves can be visible, revealing the battery-type characteristics of such materials. According to Brousse et al. point of capacitors, it would be more appropriate to describe charging storage performance of the battery-type electrodes with capacity [44]. Obviously, DS2 delivers the highest capacity due to the longest discharging time among all as-synthesized samples. In more detail, the specific capacity calculated from discharging curves of different samples at different current densities are plotted in Fig. 7C. Noticeably, the specific capacity of the NiCo2S4 electrode derived from 0.2 lg/mL DNA is 644C g
1 at a current density of 1 A/g, which is the highest among all samples and is consistent with CV results. The electrochemical performance is evidently superior to those of the

NiCo2S4 electrode derived from the none DNA (Fig. S3 in Supporting Information) and other concentrations of DNA (Fig. S4 and S5 in Supporting Information). Therefore, these results illustrate that the concentration of DNA has a great influence on morphologies and the electrochemical performances of the final products. The DS2 with proper concentration of DNA has the highest surface area and mesoporous structure, which provide more sites for ions adsorption and expand electrode/electrolyte contact interface. This is also revealed by electrochemical impedance spectroscopy (EIS) measurement conducted at the potential of 5 mV in the frequency range of 0.01–105 Hz. The obtained Nyquist plot is shown in Fig. 7D. The intersection of the plots at the x-axis represents the solution resistance (RS), which includes the following three parts: the resistance of electrolyte solution, the intrinsic resistance of the electroactive material itself, and the contact resistance at the interface of the electroactive material/current collector [45]. Obviously, the RS of DS2 is 0.78 X (the inset of Fig. 7D), which is even lower than other samples and indicates the superior electronic conductivity of DS2. The semicircles, whose diameters represent the charge transfer resistance (Rct), are not obvious in the high frequency regions, implying that the charge transfer resistance is far less than the diffusion resistance [15]. The slope of the curves in the low frequency region represent the Warburg impedance and all curves are relatively steeper, which demonstrates that electrolyte ions can be close to electroactive materials easily. It reflects that the electrochemical performance of DS2 is optimal. The representative CV curves of the NiCo2S4 electrode of DS2 at various scan rates ranging from 5 to 40 mV s
1 are presented in Fig. 8A. It is noted that all the CV curves have a similar shape, indicating that the electrode is suitable for fast faradic redox reaction. The obvious redox reaction peaks are battery-type

Fig. 8. Electrochemical characterization of DS2: CV curves at different scan rates (A), the galvanostatic charge and discharge curves at different current densities (B), the specific capacity (j) and capacity retention ( ) as a function of different current densities (C), the cycling performance at current density of 2 A g
1 (D).

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H. Liu et al. / Journal of Colloid and Interface Science 528 (2018) 100–108 Table 1 Electrochemical performance of some reported NiACoAS-based composites prepared by various methods in three-electrode systems. Sample NCS nanoplates NCS nanotubes

Specific capacity (C g
1)

Synthesis method sacrificial template method sacrificial template method

218.5 at 1 A g


1

559.8 at 1 A g


1


1

NCS microsphere

hydrothermal

384 at 2 A g

NCS microtubes

hydrothermal

574 at 1 A g
1

NCS hollow tubular

hydrothermal and template method

523 at 2 A g
1
1

NCS@MnO2 heterostructure

hydrothermal

286.4 at 1 A g

NCS/AMC

hydrothermal

260.4 at 0.6 A g
1

NCS nanosheets

hydrothermal and template method

644 at 1 A g
1

Rate ability

Cycling life

Ref.

53.2% 1–20 A g
1 50.3% 0.2–5 A g
1 92% 2–20 A g
1 83.1% 1–10 A g
1 68% 2–20 A g
1 78.6% 1–10 A g
1 88.1% 1–10 A g
1 74.3% 1–15 A g
1

81% after 1000 cycles 63% after 1000 cycles –

[20]

84.8% after 5000 cycles 94% after 20,000 cycles 86.8% after 2000 cycles 87.4% after 2000 cycles 90.5% after 1500 cycles

[48]

[46] [47]

[26] [49] [50] This work

CoS þ OH
¢ CoSOH þ e


ð2Þ

CoSOH þ OH
¢ CoSO þ H2 O þ e


ð3Þ

electrochemical performance. It exhibited a good specific capacity of 644 C g
1 at a current density of 1 A/g, superior rate capability of 74.3% retention at 15 A/g and remarkable cycling stability of 90.5% after 1500 cycles. All these attractive results confirmed the outstanding electrochemical performances of the NiCo2S4 nanosheets based on DNA-assisted. What’s more, this eco-friendly method would open a new green chemistry pathway to fabricate other metal sulfides using for next-generation energy storage devices.

NiS þ OH
¢ NiSOH þ e


ð4Þ

Acknowledgements

characteristics of sulfide materials, which are primarily attributed to fast and faradaic redox processes of Co2+/Co3+/Co4+ and Ni2+/Ni3+ interacted with OH
from electrolyte ions, illustrated by the following equations [5,24].

It can be seen that the anodic and cathodic peaks move towards higher and lower potentials, respectively, which are attributed to polarization of the electrodes. Fig. 8B presents the galvanostatic charge-discharge measurement results at different current densities ranging from 1 to 15 A/g. The distinct voltage plateaus in the GCD curves indicate the presence of faradaic redox reaction and phase transitions of the DS2. The specific capacity calculated from GCD curves is the function of current densities as shown in Fig. 8C. The specific capacity reaches up 644, 610, 582, 545 and 479 C g
1 at a current density of 1, 2, 4, 8 and 15 A/g, respectively. It is noteworthy that about 74.3% of the specific capacity is retained when the current density increases from 1 to 15 A/g. These results indicate the DS2 exhibit incredible rate capability and remarkable electrochemistry performance compared to other samples. The cycling performance of DS2 electrode is also evaluated by repeated charging-discharging measurement at a current density of 2 A/g, as shown in Fig. 8D. The initial specific capacity is 610 C g
1, and the value decreases to 552 C g
1 with 90.5% retention after continuous cycling for 1500 cycles. As displayed in Table 1, we compare the performance of this work with some reported nickel cobalt sulfide (NCS) materials, the NiCo2S4 nanosheets exhibit outstanding specific capacity and long cycling life as the electrode material. This NiCo2S4 nanosheets can undergo a fast, faradaic reaction and prove a short ion diffusion path on the presence of DNA, which is promising to be an alternative material for the energy storage devices due to its fascinating electrochemical properties. 4. Conclusions In summary, NiCo2S4 nanosheets were successfully fabricated by an environment-friendly hydrothermal method with the participation of DNA. It was found that the morphologies and electrochemical performances of the NiCo2S4 materials can be tailored by changing the concentration of DNA solutions. The DS2 derived from 0.2 lg/mL of DNA solution showed the optimal

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