Facile Synthesis of Novel Networked Ultralong Cobalt Sulfide

Nov 13, 2015 - Ultralong cobalt sulfide (CoS1.097) nanotube networks are synthesized by a simple one-step solvothermal method without any surfactant o...
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Facile Synthesis of Novel Networked Ultralong Cobalt Sulfide Nanotubes and Its Application in Supercapacitors Sangui Liu,†,‡ Cuiping Mao,†,‡ Yubin Niu,†,‡ Fenglian Yi,†,‡ Junke Hou,†,‡ Shiyu Lu,†,‡ Jian Jiang,†,‡ Maowen Xu,*,†,‡ and Changming Li†,‡ †

Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Beibei, Chongqing 400715, P. R. China ‡ Chongqing Key Laboratory for Advanced Materials & Technologies of Clean Energies, Beibei, Chongqing 400715, P. R. China S Supporting Information *

ABSTRACT: Ultralong cobalt sulfide (CoS1.097) nanotube networks are synthesized by a simple one-step solvothermal method without any surfactant or template. A possible formation mechanism for the growth processes is proposed. Owing to the hollow structure and large specific area, the novel CoS1.097 materials present outstanding electrochemical properties. Electrochemical measurements for supercapacitors show that the as-prepared ultralong CoS1.097 nanotube networks exhibit high specific capacity, good capacity retention, and excellent Coulombic efficiency. KEYWORDS: solvothermal synthesis, nanotubes, cobalt sulfides, electrochemistry, supercapacitor

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with porous hierarchical structure.6−8 To date, the special structure materials of CoSx with various morphologies have been synthesized, such as CoS nanospheres and nanowires,9 flowerlike CoS1.097,10 hierarchical wormike CoS2,11 CoS2 ellipsoids, 12 yolk−shell Co 9 S 8 microspheres, 13 roselike Co9S8,14 Co3S4 nanotubes,15 Co3S4 nanosheets.16 It is worth noting that CoSx tubes and hollow nanocrystals are mostly prepared by the high temperature thermally sulfidized process or two-step anion exchange process.17−21 However, the byproduct of pungent H2S in the high-temperature synthesis and tedious procedure are the point at issue. And it is highly desirable to explore alternative environmentally friendly and efficient approaches. Herein, for the first time, we developed a simple one-step solvothermal method with ethylene glycol (EG) and aqueous for synthesizing the ultralong CoS1.097 nanotube networks assembled by interlaced nanoflakes. The hierarchical hollow structured CoS1.097 shows a large specific area of 67.8 m2 g−1. The electrochemical properties are studied as supercapacitor

n recent years, functional materials with designed hierarchical micro/nanostructure have attracted huge interest from materials scientists and chemists, because of their characteristic structure and potential for various applications, such as catalytic, magnetic, electronic, optical, and energy systems.1−4 Many strategies have been employed to obtain the hierarchical structure materials with internal or outside porosity. For example, bioinspired method was used to construct metal oxides with 3D hierarchical structures, including ZnO, TiO2, SnO2, and BiVO4.5 In addition, organic surfactants and mesoporous silica or carbon materials have been extensively used as soft templates and hard templates to synthesize porous hierarchical functional materials, such as Co1−xS wormlike microtubes,6 polyhedral CoS2,7 CoS polyhedral nanocages.8 However, one drawback of the above strategies is the tedious procedure, of which the preparation and removal of templates. Novel efficient and simple methods for preparation of porous hierarchical functional materials are anticipant. As one of the most complicated metal chalcogenides, cobalt sulfides (CoSx), with various phases and stoichiometric compositions, have attracted particular attention because of their unique physical and chemical properties, especially that © XXXX American Chemical Society

Received: September 15, 2015 Accepted: November 13, 2015

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DOI: 10.1021/acsami.5b08716 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a−e) FESEM images, (f) TEM image, (g) HRTEM image, and (h) SAED pattern of nanotube networks CoS1.097 obtained from EG aqueous solution.

Figure 2. (a) EDX elemental mapping and (b, c) XPS spectra of nanotube networks CoS1.097.

The morphologies and microstructures of the prepared products were examined by FESEM and TEM, displayed in Figure 1 and Figure S2. Bewitchingly, the morphologies of samples synthesized with different conditions are distinctive. As shown in Figure S2a, b, the product obtained from EG solution do not reveal a specific morphology, while the sample obtained from aqueous presents sphere-like structure with a size of about 1 μm, assembled by interleaving nanoplates (Figure S2c, d). Interesting morphologies and microstructures are illustrated in Figure 1, of which the FESEM images (Figure 1a−e) show that CoS1.097 formed in EG aqueous solution has a novel network structure of ultralong nanotubes constructed by interconnected flakelike subunits. Figure 1a reveals the ultralong structure beyond our measurement, whereas Figure 1b displays the network architecture according to the conspicuous knots (the red circles) and Figure 1c shows its interconnected nanoflakes

electrodes, and the unique CoS1.097 networks present a high specific capacity of 764 F g−1, good capacity retention of 85% after 500 cycles and excellent Coulombic efficiency of almost 100%. The crystalline structure of the as-prepared materials were examined by XRD. As shown in Figure S1, all of the diffraction peaks for three samples obtained from different solutions can be indexed to hexagonal phase CoS1.097, which match well with the standard XRD pattern (JCPDS No.19−0366) and the corresponding crystal faces are (211), (204), (213), (220), (215), (306), (330), and (606), respectively. The XRD patterns cannot be matched with any other phases of CoSx, including CoS, CoS1.035, CoS2, Co3S4, Co4S3, and Co9S8, and no obvious impurity peaks are detected. Thus, the pure CoS1.097 is produced by the simple one-step solvothermal method. B

DOI: 10.1021/acsami.5b08716 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Formation Process of Nanotube Networks CoS1.097

Figure 3. Electrochemical performance of cobalt sulfide electrodes: (A) cyclic voltammograms at 10 mV s−1 and (B) galvanostatic charge−discharge curves at 2 A g−1 of samples obtained from (a) EG aqueous solution, (b) EG and (c) aqueous; (C) CV curves at various scan rates and (D) galvanostatic discharge curves at various current densities of nanotube networks CoS1.097 electrode; (E) specific capacitances at various current densities and (F) Coulombic efficiency of the nanotube networks CoS1.097 and discharge cycling at a current density of 2 A g−1 of the three obtained CoS1.097 electrodes.

with 5−7 nm in thickness. The hollow structure can be clearly seen from the selected high-magnification FESEM of top view (Figure 1d) and side view (Figure 1e). TEM was employed to confirm its microstructure. As shown in Figure 1f, the obvious contrast between dark edge and pale center certifies the criss-crosses networks and hollow structure.

The inner diameter of the tube is about 200 nm, with a wall thickness about 100 nm. The representative HRTEM image (Figure 1g) shows the lattice fringe with an interplanar spacing of 0.193 nm and corresponds to the (306) lattice plane. The selected area electron diffraction (SAED) pattern (Figure 1h) with bright diffraction rings suggests the polycrystalline nature C

DOI: 10.1021/acsami.5b08716 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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gradually evolve into longer interconnected nanotubes with the voids expanding. In addition, plenty of nanoplates begin to take shape and intersect randomly. As the Ostwald ripening process continues, the plates become thinner and thinner, and develop into interlaced nanoflakes. The inner diameter of the tubes is approximately equal to the diameter of initial nanorods. Moreover, the connections between those nanotubes become tight and intricate. The architecture of ultralong CoS1.097 nanotube networks are formed. A proposed mechanism is illustrated in Scheme 1. Many compounds of CoSx, such as CoS, CoS2, Co3S4, Co9S8, Co1−xS and their mixture, have been investigated as electrode materials for supercapacitors and batteries. Be inspired, we studied the electrochemical properties of our CoS1.097 nanotube networks as an electrode for supercapacitors and expected that its novel criss-cross structure assembled by interlaced nanoflakes would benefit the performance. Furthermore, we also measured the electrochemical properties of the CoS 1.097 products prepared from aqueous solution and EG solution. Figure 3A shows the typical CVs of three CoS1.097 samples at 10 mV s−1 in 2 M KOH aqueous electrolyte. Two pairs of redox peaks can be seen in the CV curves. And the CV curve of CoS1.097 networks electrode shows a more obvious box-like shape than those of other two electrodes, indicating the coexistence of an electric double-layer capacitance and pseudocapacitive capacitance. Obviously, the area of CV curve for CoS1.097 networks is apparently larger, representing a higher capacitance. Compared to those of other two electrodes (Figure 3B), the galvanostatic discharge time of CoS1.097 network electrode is also significantly longer. Thus, above data unambiguously prove that CoS1.097 networks affords higher specific capacity, which is possibly due to the more slack structure and larger specific surface area. The typical CV curves of CoS1.097 networks electrode at various scan rates are shown in Figure 3C. At lower scan rates of 10 and 20 mV s−1, the redox peaks can be observed in the CV curves. And there exhibit a shoulder peak at around 0.35 V and a corresponding cathodic peak at about 0.15 V. The characteristics of the CV curves and the redox peak potentials are agreement with those reported CoSx.6,10 Figure 3D shows the discharge curves of CoS1.097 network electrode measured at different current densities in the potential range of 0−0.4 V (vs SCE). Consistent with the CV curves, the plateaus in the discharge curves appear at about 0.35 and 0.15 V, implying the existence of Faradaic processes. The specific capacitance values are calculated to be approximately 764, 686, 597, 400, and 237 F g−1 at 0.5, 1, 2, 5, and 10 A g−1, respectively, which are much higher than those of other two CoS1.097 materials (Figure 3E). Remarkably, the specific capacitance is 400 F g−1 even at a high current density of 5 A g−1, which is much higher than that of CoS1.097 obtained from aqueous and so close to that of CoS1.097 obtained from EG solution at 1 A g−1. In addition, a high specific capacitance value of 237 F g−1 can be retained when the current density increases to 10 A g−1, testifying its good rate capability. Figure 3F shows Coulombic efficiency and the galvanostatic discharge cycling performance evaluated at 2 A g−1. After 500 cycles, the value of specific capacitance is still as high as 550 F g−1, which is about 85% of the initial capacitance. The capacitance retention after 500 cycles is about 79% (from 438 to 350 F g−1) and 77.5% (from 420 to 317 F g−1) of CoS1.097 obtained from EG and aqueous, respectively. Moreover, the Coulombic efficiency is almost 100% during the whole cycling, indicating the excellent

of the nanotube networks CoS1.097. Moreover, the BET surface area and the porous structure of these three different nanostructured CoS1.097 materials were analyzed by nitrogen adsorption−desorption measurements. The results (Figure S3) show that the nanotube networks CoS1.097 has a much larger surface area of 67.8 m2 g−1 than that of the other two samples obtained from aqueous and EG (20.5 and 40.9 m2 g−1, respectively), whereas their characteristics of pore size distribution are barely different. Further investigations of nanotube network structures of asprepared materials obtained from EG aqueous solution were employed by energy-dispersive X-ray spectroscopy (EDX) and XPS. Elemental mapping (Figure 2a) shows that both elemental Co and elemental S are homogeneously distributed over the nanotubes, and the atom ratio of S to Co is 1.103, which corresponds to the XRD patterns. Figure 2b, c display the XPS spectra of the Co and S regions. In the Co 2p spectrum (Figure 2b), the peak centered at about 778.5 eV agrees with the binding energies of sulfided Co−S, whereas another peak at higher binding energies could be attributed to a satellite signal.5,22 Figure 2c shows the spectrum of S 2p region, the two peaks at 161.7 and 162.6 eV can be assigned to the S 2p3/2 and S 2p1/2 of Co−S binding, respectively. The peak at high energy side is ascribed to Co−S−thiolate binding.2 The Co 2p peaks and the S 2p peaks in the XPS patterns are characteristic of cobalt sulfide. The above results demonstrate that ultralong CoS 1.097 nanotube networks have been successfully fabricated via a simple one-step solvothermal method. To reveal the growth mechanism of hierarchical CoS1.097 nanotube network architectures, we performed a series of timedependent experiments by harvesting products at different reaction times of 3, 6, 9, and 18 h, respectively. As shown in Figure S4, all of XRD patterns of these products are in good agreement with CoS1.097 phase. In the initial time, Co2+ and Lcysteine homogeneously disperse in aqueous EG solution. It is well-known that cysteine molecule, in which there are many functional groups (−NH2, −COOH, and −SH), has a strong tendency to coordinate with inorganic cations and metals. The L-cysteine functions as not only S source, but also the shapecontrolling agent. According to previous reports, Co2+ can react with cysteine to form initial precursor complexes.9,23,24 It is possible that the amino group reacts with carboxyl group of neighboring cysteine molecule. Otherwise, in this reaction system, because of the polar characteristics of H2O and (HOCH2)2 and the interaction of cysteine with EG and water, the precursor complexes prefer to oriented aggregate together to produce cross-shaped CoS1.097 nanorods (∼200 nm in diameter) after solvothermal treatment for 3h. In the aqueous solution or EG solution, the initial CoS1.097 particles prefer aggregating to form sphere-like structure, because of the higher surface energy of the nanoparticles and the interaction of hydrogen bonds between the tiny particles.9 With the increasing of reaction time, these nanorods begin to get long and crooked; meanwhile, some voids appear inside the nanorods and many embedded nanoplates and small particles can be observed on the surfaces. This probably is a dissolution−recrystallization process (Ostwald ripening). The center area inside the nanorods dissolve into system. Coordinating with cysteine molecule, tiny CoS1.097 nuclei recrystallize to form nanoplates at high-energy sites provided by the surfaces. Similar process were reported in the synthesis of CoSx.10,25,26 After a reaction time of 9 h, these nanorods D

DOI: 10.1021/acsami.5b08716 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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the National Natural Science Foundation of China (No. 21063014 and 21163021) and Fundamental Research Funds for the Central Universities (SWU 113079, XDJK2014C051).

reversibility of pseudocapacitive reactions in the CoS1.097 networks. In addition, as shown in Table S1, the electrochemical performances of CoS1.097 electrode is quite good, which is favorable for considering both the high specific capacitance with good cycling stability and rate capability. The practical performance of the electrode in a full-cell setup was further evaluated by fabricating a cobalt sulfide networksactive carbon-based asymmetric capacitor. The mass ratio of positive and negative electrode is optimized to be about 0.3 (see Experimental Section in the Supporting Information and Figure S5). The parameters of full-cell, such as CV curves, rate capability, cycling stability, and the Ragone plot relating the energy density to power density, are given in Figure S6. The calculated specific capacitance of the device is 107 F g−1 at 1 A g−1 in the potential range from 1.5 to 0 V. It seems likely that this capacitance value could be further increased by improving the negative active carbon electrode. And the energy density decreases from 33.4 to 17.1 W h kg−1 as the power density increases from 0.75 to 7.5 kW kg−1. Additionally, the cycling stability was performed at a constant current density of 5 A g−1. As shown in Figure S6D, the capacitance retention is 91.7 and 83.3% after 2000 cycles and 5000 cycles, respectively. These results render it promising as one of the attractive candidates for energy storage. In summary, we report the synthesis of ultralong CoS1.097 nanotube networks via a one-step solvothermal method. In comparison with the reported methods, of which most are template/surfactant-assistance or two-step anion exchange strategies, our approach is very simple and efficient. More importantly, the resulting CoS1.097 networks with high specific area present high specific capacitance, good rate capacity and excellent Coulombic efficiency. The outstanding performances are benefiting from its unique porous hollow structure constructed by interlaced nanoflakes. We anticipate that the novel structured CoS1.097 could be used for other applications and the simple process could be extended to fabricate other novel structured metal sulfides.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08716. Detailed description of experimental methods, Figures S1−S6, and Table S1 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M. W. Xu). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under cstc2011 pt-sy90001, Start-up grant under SWU111071 from Southwest University (Chongqing, China) and Technology Commission under cstc2012gjhz90002. The work is supported by grants from E

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DOI: 10.1021/acsami.5b08716 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX