Chemical transformation of hollow coordination polymer particles to Co3O4 nanostructures and their pseudo-capacitive behaviors

Inorganica Chimica Acta 427 (2015) 266–272

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Chemical transformation of hollow coordination polymer particles to Co3O4 nanostructures and their pseudo-capacitive behaviors Zhihui Xu a, Bo Lv a, Xiaobo Shi b, Lixian Chen a, Kuaibing Wang a,⇑ a b

Department of Chemistry, College of Science, Nanjing Agricultural University, Nanjing 210095, PR China College of Agriculture, Nanjing Agricultural University, Nanjing 210095, PR China

a r t i c l e

i n f o

Article history: Received 30 October 2014 Received in revised form 24 December 2014 Accepted 5 January 2015 Available online 12 January 2015 Keywords: Coordination polymer particles Cobalt oxides Pseudo-capacitor Precursors Cyclic voltammetry Electrochemical impedance spectroscopy

a b s t r a c t A straightforward approach has been developed to fabricate Co3O4 nanostructures based on hollowstructured coordination polymer precursors, which have been synthesized from Co2+ and organic building blocks. The coordination polymer precursors and the transformation products have characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The pseudo-capacitive behavior of Co3O4 nanostructures has been conducted by cyclic voltammetry, galvanostatic charge–discharge studies and electrochemical impedance spectroscopy. The result suggests that porous Co3O4 rods have smaller charge transfer resistance and faster ion diffusion rate in comparison with Co3O4 particles, and show better cycle properties at charging–discharging intensity of 3 A g 1. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction To date, crystalline or amorphous coordination polymer particles (CPPs) made from functional building blocks with different metal ions, such as Zn(II), Cu(II) and In(III) ions, have received a great deal of attention because of their useful applications in gas storage, antibacterial, optics, and drug delivery [1–6]. Currently, chemical transformation from crystalline or amorphous CPPs has emerged a useful method for functional metal oxides materials with pre-defined morphologies [7]. Moreover, a methodology that involves straightforward chemical transformation and continuous thermal annealing of the colloidal CPPs has been formulated for generating hybrid metal oxide materials with tunable chemical compositions [8]. Therefore, annealing from CPPs provides a way to access a variety of metal oxides including hybrid ones, something that should facilitate their eventual use in practical applications. Among these metal oxides, Co3O4 is an extremely low-cost and environmental friendly oxide material and is known for its unique electronic and magnetic properties, especially has been applied in pseudo-capacitors as electrode material and proved to be a potential alternate to expensive RuO2 [9–12]. Although several methodologies have been developed for the preparation of Co3O4 with a

⇑ Corresponding author. Tel./fax: +86 025 84396697. E-mail address: [email protected] (K. Wang). http://dx.doi.org/10.1016/j.ica.2015.01.008 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

variety of morphologies and dimensions, interesting however, the synthesis of Co-based oxides materials in the utilization of hollow-structured Co-based CPPs as precursors has not been attempted to date. Herein, we report, as a candidate, on the synthesis of Co3O4 and C/CoO hybrids via straightforward chemical transformation from simple Co-carboxylate CPPs. The obtained product are then characterized by thermal analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX). In addition, the pyrolysis products of Co3O4 from CPPs show a morphology-dependent phenomenon and thus displaying intense electrochemical capacitive properties. The transformation method, that is thermal decomposition from CPPs, has potential advantages, including high selectivity, high product purity and yield, and no need for a complex procedure. 2. Experimental 2.1. Synthesis of Co-based CPPs Solvents and all other chemicals were obtained from commercial sources and used as received unless otherwise noted. Firstly, the organic building blocks 3,5-diaminobenzoic acid (HDABA, 7.60 g) and NaOH (2.16 g) with the molar ratio of 1:1 were dissolved in deionized water to form 0.5 M NaDABA aqueous solution. In a typical synthesis procedure, 0.5 M NaDABA (8 mL) aqueous

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

solution was introduced into 0.5 M Co(OAc)2 (4 mL) aqueous solution drop by drop in water and dimethylformamide solvents (H2O/ DMF, 10/10, v:v) under vigorous stirring, and a large number of black precipitate occurred immediately. After stirring for 20 min, the product, namely CPP-1, was collected by centrifugation, and washed several times with DMF and water. Similar to CPP-1, replacement of solvent system with water and ethanol (H2O/EtOH, 10/10, v:v), keeping other synthetic parameters unchanged, can obtain a dark grey precipitate and denoted as CPP-2. 2.2. Synthesis of Co3O4 nanostructures and carbon/CoO hybrids For synthesis of Co3O4 nanostructures, the as-prepared precursors of CPP-1 and CPP-2 were firstly ground and paved on container bottom, respectively. The precursors were heated in air to 450 °C and maintained for 1 h, and allowed to cool down to room temperature spontaneously. While carbon/CoO hybrids were obtained by annealing the precursor of CPP-2 under N2 atmosphere at 450 °C for 30 min and the heating rate of the furnace was kept at 1 °C/min. 2.3. Electrode preparation The working electrodes were prepared as follows. The mixture containing 80 wt.% Co3O4, 15 wt.% acetylene black and 5 wt.% polytetrafluoroethylene (PTFE) was well mixed, and then was incorporated in nickel foam (1 cm  1 cm), and the typical mass load of electrode materials ranged in 2–4 mg. 2.4. Methods and Measurements X-ray diffraction (XRD) data were collected on a Bruker D8 Advance instrument using Cu Ka radiation (k = 1.54056 Å) at room temperature. The morphology of the as-prepared samples and the corresponding energy dispersive X-ray (EDX) spectroscopy were obtained by using a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) images were captured on a JEOL JEM-1011 instrument microscopy at an acceleration voltage of 100 kV. Fourier-transform

267

infrared (FTIR) spectra were obtained on a Bruker Vector 22 FT-IR spectrophotometer by using KBr pellets. Thermogravimetric analysis (TGA) was performed on a Pyris 1 DSC thermal analyzer (PerkinElmer Corp.) from 25 °C up to 700 °C with a heating rate of 10 °C min 1. The electrochemical measurements were carried out by an electrochemical analyzer system, CHI660E (Chenhua Instrument, Shanghai, China) in a three-compartment cell with a platinum plate counter electrode, an Ag/AgCl reference electrode and a working electrode. The electrolyte was a 1 M KOH aqueous solution and electrochemical impedance spectroscopy measurements of as-synthesized samples were conducted at open circuit voltage in the frequency range of 100 kHz to 10 mHz. 3. Results and discussion Co-based CPPs, denoted as CPP-1 and CPP-2, were firstly synthesized from a solution containing both Co metal ions and the deprotonated organic linker NaDABA, using a precipitation method. Taking advantage of the outstanding reactivity and thermal behavior of CPPs, cobalt oxides with unique morphologies have been formulated and prepared using the following process; i) CPPs with different morphology can be obtained by adjusting the solvent parameters, and ii) a final annealing process of preprepared CPPs that engenders decomposition of the CPPs and formation of metal oxides. Thermal treatment of CPP-1 and CPP-2 at 450 °C in air then results in the formation of cobalt oxides, namely Co3O4-1 and Co3O4-2, respectively. In addition, thermal annealing of CPP-2 under N2 surroundings can afford a carbon/CoO composite (C/CoO). CPP-1 was obtained from a solvent system of H2O/DMF (10/10, v:v) and firstly characterized by FE-SEM as shown in Fig. 1a. The generating porous clusters were constructed by a variety of inhomogeneous particles with sizes of 12–97 nm and these particles aggregated together as displayed in Fig. 1b. As compared, CPP-2, synthesized from H2O/EtOH system (10/10, v:v), showed a uniform nanoflake arrays structure (Fig. 1c). The arrays exhibited a hierarchically hollow structure consisting of interconnected nanoflakes with mean thickness of 15 nm, as illustrated in Fig. 1d. Notably,

Fig. 1. (a and b) SEM images of particle-like CPP-1 cluster. (c and d) SEM mages of CPP-2 with uniform nanoflake arrays structure.

268

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

Fig. 2. (a) XRD pattern for particle-like CPP-1. (b) XRD pattern for CPP-2 nanoflake arrays. (c) TGA curves of as-prepared CPPs, where black(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) line represents CPP-1 and the red one represent CPP-2. (d) IR spectra of as-prepared CPP-1 and CPP-2.

the nanoflakes arrange vertically and form a net-like array with pores of 50–500 nm, which is a common morphology that usually appears in several Ni-based materials [13,14]. The chemical compositions of the resulting CPPs were then determined using conventional methods, including XRD spectroscopy, TGA and IR spectroscopy (Fig. 2). The XRD spectra of CPP-1 suggested an amorphous state (Fig. 2a), which is a common phenomenon for synthesizing CPPs in poor solvent of DMF [3,4,6]. As to CPP-2, as shown in Fig. 2b, the observed peaks were weak and can be assigned to a semi-crystalline state. The significant difference may be due to the solvent-induced effect that the different

polarity of solvent can lead to different assembly and stacking modes of Co-carboxylate metal–organic frameworks, because the change of dipole–dipole interactions would induce different kinetic control and initial speciation [15]. It can be supported from the result of TGA, as shown in Fig. 2c, which revealed two different decomposition curves. CPP-2 was stable up to 350 °C after an initial weight loss of 20% due to solvent liberation (water and ethanol) in the 50–100 °C temperature range, while CPP-1 very gradually decomposed the water and DMF in the temperature range of 50–250 °C, which was in good agreement with the opinion of solvent-induced effect on the resulting morphology.

Fig. 3. XRD patterns of the annealing product for Co3O4-1 and Co3O4-2.

Fig. 4. EDX patterns for the nanostructures of Co3O4-1 and Co3O4-2.

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

269

Fig. 5. (a and b) SEM images for the porous Co3O4-1 structure. (c and d) TEM images show the detailed Co3O4-1 struture.

The formation of coordination polymers from metal ions and carboxylate-functionalized ligands is well known in transition-metal coordination chemistry, and they can be conveniently characterized by IR spectroscopy [16]. The characteristic bands of carboxylate group in resulting CPPs were both shown at 1566 cm 1 for antisymmetric stretching and 1391 cm 1 for symmetric stretching. The absence of the expected bands at 1685–1715 cm 1 for the protonated carboxylate groups illustrates the complete deprotonation after the formation of CPPs (Fig. 2d). Co3O4-1 and Co3O4-2 were then prepared by annealing the corresponding CPPs of CPP-1 and CPP-2 at 450 °C for 1 h in air. The structure characterization of the annealing product was shown in Fig. 3. Fig. 3 displayed a typical XRD pattern of the product. All

the diffraction peaks matched exactly with the spinel Co3O4 phase (JCPDS 42-1467), and the intensity of the peaks indicated the higher crystallinity of the Co3O4 product in contrast with their CPPs precursors. The chemical composition of the resulting Co3O4 products was confirmed by EDX spectroscopy (Fig. 4), in which only cobalt and oxygen atoms were observed. The atom concentration of Co and O atoms for Co3O4-1 and Co3O4-2 was 55.78% and 43.22%, and 55.47% and 44.53% separately. The atom ratio was both close to 3:4, which is in accordance with the XRD results. The size and morphology of the resulting cobalt oxide particles were then characterized by FE-SEM and TEM, respectively. Fig. 5a and b observed that Co3O4-1 possessed short rod-like motifs with rough surfaces, which underwent a significant change from

Fig. 6. (a and b) SEM images for the porous Co3O4-2 structure. (c and d) TEM images show the Co3O4-2 particles.

270

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

Fig. 7. (a and b) TEM images for C/CoO composites structure which inherited the morphology of CPP-2 precursor. (c) XRD pattern for C/CoO composites. (d) EDX pattern for C/ CoO composites.

Fig. 8. (a and b) Cyclic voltammetry curves of Co3O4-1 and Co3O4-2 at various scan rates of 5–200 mV/s. (c) Cyclic voltammetry curves for Co3O4-1 and Co3O4-2 at scan rate of 10 mV/s. (d) Charge–discharge curves for Co3O4-1 and Co3O4-2 at the same current density of 1 A g 1.

nanoparticles clusters to short-rods in comparison with the original precursor CPP-1. TEM images suggested that the rod-like motifs were porous after calcination from CPPs with lengths and widths of 100–150 nm and 30–90 nm separately (Fig. 5c and d).

Similar to Co3O4-1, the morphology of Co3O4-2 displayed ricelike particles (Fig. 6a and b) and was not inherited from their original precursor of CPP-2. TEM images suggested that the rice-like particles were ununiformed and ranged randomly with sizes of

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

271

Fig. 9. (a) Electrochemical impedance spectra (EIS) of electrodes of Co3O4-1 and Co3O4-2 at room temperature. (b) An equivalent circuit for the as-prepared Co3O4 samples. (c) Cycling test of as-prepared Co3O4-1 sample at a constant current of 3 A g 1.

5–110 nm, as vividly illustrated in Fig. 6c and d. The result suggests that new nucleus are generated during calcination reaction from amorphous or semi-crystalline CPPs, and these seeds aggregate together, reassemble, grow directionally and finally anneal to form metal oxides with different shapes. This transformation phenomenon is usual, but the process is different from the reported mechanism on annealing from CPPs [17]. In the present system, firstly, the high annealing temperature may result in reset of nuclei, and more favorite facets are exposed in crystal surface to minimize the surface tension, and therefore self-organizing into new morphology. Secondly, the high annealing temperature in calcination process has an influence on the transformation profile during the nucleation process and the subsequent growth kinetics in different planes. Notably, the C/CoO composite was also generated through the chemical transformation method, which was carried out at 450 °C for 30 min under N2 surroundings. Unlike Co3O4 particles, the morphology of C/CoO composite inherited part of flake-like shape from original CPP-2 and these flakes arranged into a flower-like motif, as shown in Fig. 7a. It is noteworthy that some lamellar structure was arranged in parallel, others were vertical arrangement (Fig. 7b).The XRD result and EDX pattern suggested the CoO nature and composited with carbon to form the hybrids, as separately depicted in Fig. 7c and d. These results suggest that the chemical transformation methodology has great advantages on synthesizing inorganic materials with high purity, simple procedure and special morphology. Co3O4 has been extensively researched as the electrode material for energy storage, such as lithium-ion batteries and pseudocapacitors [9–12,18,19]. In this paper we studied the electrochemical properties of as-prepared Co3O4 products by applying them as the active materials for pseudo-capacitor electrode. The measurements were conducted using cyclic voltammetry (CV) in 1 M KOH electrolyte with the voltage window in 0–0.5 V. The obtained CV curves for as-prepared Co3O4 are displayed in Fig. 8a and b. The

CV curves are nearly symmetrical and show two pairs of redox peaks. The broad redox reaction peaks which come from the redox processes of Co3O4/CoOOH/CoO2, are characters of the electrochemical pseudo-capacitors from reversible faradaic redox reactions occurring within the electro-active materials [20,21]. The area calculated from the CV curve for Co3O4-1 is apparently much larger than that of Co3O4-2, which indicates that porous Co3O4-1 could provide fast ion and electron transfer, benefiting for the electrochemical performance (Fig. 8c). Chronopotentiometry measurements further confirmed the suggestions. The shapes of the charge–discharge curves show the characteristics of pseudo-capacitor, as depicted in Fig. 8d, which are in line with the result of the CV curves. Both samples present two variation ranges during the charge and discharge steps and the increase in charging time represents the higher capacitance of Co3O4-1 rods. The specific capacitance of Co3O4-1 and Co3O4-2 at a current density of 1 A g 1 is calculated to be 80 and 34 F g 1 separately, which is much smaller than some reported data on Co3O4 [11,12]. Even so, we believe that more functional Co3O4 materials with distinct structures, transformed from other pre-defined Co-CPPs through choosing proper organic linker, could serve as promising candidates in electrochemical capacitor fields. Electrochemical impedance spectroscopy (EIS) further confirmed the CV results. The bulk solution resistance of two asprepared samples was measured to be 2.23 and 1.09 X separately, however, the charge-transfer resistance was calculated (by using ZsimpWin software) to be 1.04 and 1.36 X, respectively (Fig. 9a). The result clearly demonstrates the smaller electrochemical reaction impedance of the Co3O4-1 electrode, indicating that porous Co3O4-1 rods have smaller charge transfer resistance and faster ion diffusion rate, which are consistent with the electrochemical results above. Notably, the EIS data is fitted by an equivalent circuit, which consists of a bulk solution resistance (Rs), a chargetransfer resistance (Rct), a pseudo-capacitive element (Cp) from the redox process of Co3O4, and a constant phase element (CPE)

272

Z. Xu et al. / Inorganica Chimica Acta 427 (2015) 266–272

to account for the double layer capacitance, as shown in Fig. 9b. Since it is important for an electrochemical capacitor material to have good cycling performance, an endurance test was conducted to examine the life of porous Co3O4-1 rods by using charging– discharging cycles at 3 A g 1. Fig. 9c reveals that porous Co3O4-1 electrode has good cycle properties as an excellent electrode material for electrochemical capacitors and the specific capacitance even grow a little larger in the first 100 cycles, which might be due to an electrochemical activation of electrode, as electrolytes in general require a certain period of time to penetrate the entire inner space of an active electrode material [22]. 4. Conclusion Porous Co3O4 nanostructures with distinct morphologies have been achieved through chemical transformation methodology. The precursors display particles and nanoflakes array motifs, which is constructed from Co2+ reacting with deprotonated organic linker NaDABA, and are both hollow-structured. This chemical transformation process has potential advantages, including simple procedure, high product purity and yield, and the morphology of precursor can easily manipulated by documenting or designing distinct organic ligands and synthetic parameters. In order to prove the straightforward process, the C/CoO composite is also generated through this transformation approach. Furthermore, the pseudocapacitive behavior of Co3O4 nanostructures have been investigated, and the result shows porous Co3O4-1 rods display better cycling performances as excellent electrode materials for electrochemical capacitors.

References [1] M. Oh, C.A. Mirkin, Nature 438 (2005) 651. [2] D. Tanaka, A. Henke, K. Albrecht, M. Moeller, K. Nakagawa, S. Kitagawa, J. Groll Nat. Chem. 2 (2010) 410. [3] C. Jo, H.J. Lee, M. Oh, Adv. Mater. 23 (2011) 1716. [4] I. Imaz, M.R. Martínez, L.G. Fernández, F. García, D.R. Molina, J. Hernando, V. Puntes, D. Maspoch, Chem. Commun. 46 (2010) 4737. [5] P. Horcajada, C. Serre, M. Vallet-Regí, M. Sebban, F. Taulelle, G. Férey, Angew. Chem., Int. Ed. 45 (2006) 5974. [6] Y.M. Jeon, G.S. Armatas, D. Kim, M.G. Kanatzidis, C.A. Mirkin, Small 5 (2009) 4650. [7] K. Wang, X. Ma, Z. Zhang, M. Zheng, Z. Geng, Z.L. Wang, Chem. Eur. J. 19 (2013) 7084. [8] W. Cho, Y.H. Lee, H.J. Lee, M. Oh, Adv. Mater. 23 (2010) 1720. [9] H. Pang, F. Gao, Q. Chen, R.M. Liu, Q.Y. Lu, Dalton Trans. 41 (2012) 5862. [10] M.B. Zheng, J. Cao, S.T. Liao, J.S. Liu, H.Q. Chen, Y. Zhao, W.J. Dai, G.B. Ji, J.M. Cao, J. Tao, J. Phys. Chem. C 113 (2009) 3887. [11] R.B. Rakhi, W. Chen, M.N. Hedhili, D. Cha, H.N. Alshareef, ACS Appl. Mater. Interfaces 6 (2014) 4196. [12] S.G. Kandalkar, C.D. Lokhande, R.S. Mane, S.H. Han, Appl. Surf. Sci. 253 (2007) 3952. [13] J. Zhu, J. Jiang, J. Liu, R. Ding, H. Ding, Y. Feng, G. Wei, X. Huang, J. Solid State Chem. 184 (2011) 578. [14] M. Salavati-Niasari, A. Khansari, F. Davar, Inorg. Chim. Acta 362 (2009) 4937. [15] K. Wang, Z. Geng, M. Zheng, L. Ma, X. Ma, Z.L. Wang, Cryst. Growth Des. 12 (2012) 5606. [16] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim Nature 423 (2003) 705. [17] S. Jung, W. Cho, H.J. Lee, M. Oh, Angew. Chem. Int. Ed. 48 (2009) 1459. [18] Y.G. Li, B. Tan, Y.Y. Wu, Nano Lett. 8 (2008) 265. [19] X.W. Lou, D. Deng, J.Y. Lee, J. Feng, L.A. Archer, Adv. Mater. 20 (2008) 258. [20] S. Palmas, F. Ferrara, A. Vacca, M. Mascia, A.M. Polcaro, Electrochim. Acta 53 (2007) 400. [21] G.L. Wang, D.X. Cao, C.L. Yin, Y.Y. Gao, J.L. Yin, L. Cheng, Chem. Mater. 21 (2009) 5112. [22] C.C. Hu, K.H. Chang, T.Y. Hsu, J. Electrochem. Soc. 155 (2008) F196.