Synthesis and electrochemical properties of mesoporous carbon supported well–dispersed cobalt oxides nanoparticles - Nguyen Van Tu

Journal of Science and Technology 55 (1B) (2017) 230–237 SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF MESOPOROUS CARBON SUPPORTED WELL–DISPERSED COBALT OXIDES NANOPARTICLES Nguyen Van Tu1, 2, *, Shuang Yang2 1Institute for Chemistry and Material, 17 Hoang Sam Street, Nghia Do Ward, Cau Giay District, Ha Noi, Viet Nam 2School of Material Science and Engineering, Wuhan University of Technology 122 Luoshi Road, Wuhan, P. R. China *Email: nguyenvantu882008@yahoo.com Received: 30 December 2016; Accepted for publication: 6 March 2017 ABSTRACT In this article, well–dispersed cobalt oxide nanoparticles supported on mesoporous carbon (CMK–3) have been successfully synthesized. The composites were characterized by field emission scanning electron microscopy, transmission electron microscopy, X–ray diffraction and nitrogen adsorption–desorption analysis. The results have confirmed that, at a cobalt loading of 15 wt%, the composites have not only retained mesoporous structure of the support but also shown a good control of dispersed cobalt oxide nanoparticles with size of ~4 nm. The electrochemical property tests for the synthesized samples have shown significant improvement compared to the blank carbon (CMK–3) without cobalt oxide incorporation. Keywords: CMK–3, cobalt oxide, nanoparticles. 1. INTRODUCTION The recent advances in nanoscience and nanotechnology have provided an impetus for the development of new hybrid porous nanostructures, especially metal oxide nanoparticles (NPs) confined in porous carbons. Hybrid porous nanocomposites have received much attention because of their unique properties including well–controlled pore structures, high surface areas, and large and tunable pore sizes [1]. The small metal oxide particles and with their porous structure can significantly enhance the utilization of active materials and shorten the transport/diffusion path of ions and molecular entities. Thus, porous carbon–based nanocomposites have many potential applications including electrodes [2], catalysts [3, 4] and adsorbents [5]. The use of mesoporous carbon incorporated nanosized metal oxide materials have attracted huge attention as they have demonstrated many unique advantages for supercapacitor and secondary lithium battery applications. Cobalt metal and its oxides have been heavily studied for applications such as supercapacitors [6 – 8], anode materials in Li–ion rechargeable batteries [9], solid–state sensor Nguyen Van Tu, Shuang Yang 231 [10], heterogeneous catalysts [11], solar energy absorbers [12], and electrochromic devices [13]. In strategies for enhancement of performance in applied catalysis or in energy conversion based on cobalt oxides, the utilization of carbon–based materials such as activated carbon, carbon nanotubes (CNTs) and graphene nanosheet (GNS) can be a meaningful and useful method [9, 14 – 16]. Consider the unique physicochemical properties offered by these carbon materials, in particular ordered mesoporous carbons (OMCs), are regarded as ideal hosts for host–guest hybrid porous nanocomposites due to the more favorable mass transfer processes within the pores [2, 17]. Hu and co–workers [18] developed silica–supported porous carbon nanomembrane (SS–CNM) as a conductive support for the electroactive Co3O4. Their aim was to fabricate regularly packed nanorods in the mesochannels. This type of SS–CNM was prepared from a self–assembled monolayer of an enediyne compound on the surface of mesoporous silica followed by Bergman cyclization and carbonization. Meanwhile, in situ combination of carbon and guest precursors was considered as an alternative because of the relative simple and controllable synthesis protocols. Zhao’s group used an in situ route to synthesize ZSM–5 microspheres composed of well–dispersed uniformly sized Fe3O4 NPs [19] and ordered mesoporous Pt@graphitic carbon hybrids [20]. This approach shows clear advantages to control the guest size and dispersion. One limitation is that a reduced utilization and activity of guests may be resulted because they usually situate inside the walls of hosts In order to achieve a better control of metal oxide NPs, mesoporous carbon supported well– dispersed cobalt oxides NPs have been synthesized with ordered mesoporous carbon CMK–3 as the matrix and cobalt nitrate hexahydrate as the cobalt oxides precursor through ammonia post– treated impermanent route. The particle size and dispersion of cobalt oxides were investigated. The electrochemical properties of the prepared composites were also discussed. 2. EXPERIMENTAL METHODS 2.1. Material synthesis Mesoporous ordered carbon, CMK–3 (diameter of 3.88–5.0 nm, 99 % purity, BET surface area of 1200–1300 m2 g–1) was purchased from Nanjing XF NANO Co. Ltd (China). Cobalt nitrate hexahydrate was purchased from Sigma–Aldrich (≥ 99.9 % purity). A predetermined amount of CMK–3 was first dispersed in ethanol, followed by the addition of a 20 wt% ethanolic solution containing cobalt nitrate hexahydrate. The mixture was continuously stirred at room temperature until the solvent was fully evaporated. After drying the impregnated sample in the vacuum oven overnight, it was brought into contact with ammonia vapor that was generated by heating an ~14 wt% ammonia solution at 60 °C for 3 h. This procedure hydrolyzes the adsorbed cobalt nitrate in a controlled manner prior to high temperature calcination. After this step, the sample was washed with ethanol, dried at 45 °C under vacuum overnight, and finally calcined under an argon atmosphere at 400~500 °C for 2 h. The obtained sample was denoted as xCo@C_T, where x and T stand for the loading of cobalt in wt% and the calcination temperature, respectively. The cobalt loading in this work was calculated based on the Co in its zero valent form. 2.2. Characterization Field emission scanning electron microscopy (FESEM) was carried out on an FEI Nova NanoSEM 450 FEGSEM and an FEI Magellan 400 high resolution SEM. Both SEMs are Synthesis and electrochemical properties of mesoporous carbon supported well–dispersed 232 equipped with in–lens detectors and directional concentric backscattered detectors. Selected backscattered electron (BSE) images were analyzed by Image–Pro Plus 5.0 software to provide particle size information. Transmission electron microscope (TEM) images were acquired with an FEI Tecnai F20 transmission electron microscope. Powder X–ray Diffraction (XRD) analysis was performed on a Philips PW1130 powder diffractometer with CuKα radiation (λ = 1.5405 Å) at 40 kV and 25 mA. Nitrogen sorption data was collected at 77 K by using a Micromeritics Tri– star adsorption analyzer. The electrochemical experiments were performed on Autolab PGSTAT30 (Eco Echemie B.V. company) with a three–electrode electrochemical cell system that consisted of a working electrode, a platinum counter electrode and an Ag/AgCl reference electrode. The distance between the working electrode and the counter electrode was 2.5 cm. The working electrode was prepared by mixing the powder composite and polytetrafluoroethylene (PTFE, 10 wt%) at a weight percent ratio of 9:1 in isopropyl alcohol. The resultant slurry was then coated onto the 1cm2 nickel foam which was used as the current collector. The coated nickel foam was finally pressed at a pressure of 10 MPa for 5 minutes and subsequently dried at 80 °C under vacuum for 12 h. Cyclic voltammetry was carried out in the potential range (–0.4 V, 0.4 V) in a 5 M KOH electrolyte at room temperature [16]. All electrochemical measurements were measured at New energy of Laboratory, School of Material Science and Engineering, Wuhan University of Technology. 3. RESULTS AND DISCUSSION 3.1. Structure and morphology Figure 1a shows scanning electron microscopy (SEM) image of 10Co@C sample calcined at 400 °C. A clean surface and channel structure similar to the CMK–3 template was observed which indicated the well–preserved porous carbon structure after the impregnation and the calcination processes. Figure 1b shows the corresponding TEM image of 10Co@C_400. The fact that only continuous stripes with no aggregated particles were observed, suggested a uniform distribution of the precursor inside the channels of carbon. The BET specific surface area and total pore volume of CMK–3 are 1250 m2g–1 and 1.36 cm3g–1, respectively (Table 1). After Co was loaded, the BET specific surface area and total pore volume of 10Co@C_400 reduce to 1080 m2g–1 and 1.09 cm3g–1, respectively. It suggests that the cobalt precursors likely consume the space in the micro and mesopore, causing the reduction of the surface area and pore volume. Based on the total pore volume of CMK–3 and the theoretical assumption of using the cobalt precursor solution to completely fill all the pores, the maximum possible amount of cobalt was calculated. This corresponded to a final structure with a cobalt loading of 15 wt%. However, even after the cobalt loading was increased to 15 wt%, apart from the carbon channel stripes, no clear particles were observed in the SE image (Figure 1c), suggesting uniform distribution of cobalt oxide particles and well–retained carbon structure. Figure 1d shows its TEM image. Channel–like structures can be easily observed having tiny particles present, revealing that the increment of cobalt loading has boosted particle development. 15Co@C_400 showed a BET specific surface area of 1000 m2g–1 which was lower than 10Co@C_400 (Table 1). It was noteworthy from the nitrogen adsorption analysis data that both 10Co@C_400 and 15Co@C_400 show significant reductions of pore volume both in the meso and microrange Nguyen Van Tu, Shuang Yang 233 compared to CMK–3. This suggests that the cobalt precursors were likely occupying the space in meso and micropore. Figure 1. Scanning electron microscopy (a) and transmission electron (b) images of 10Co@C_400; Scanning electron microscopy (c) and transmission electron (d) images of 15Co@C_400. Table 1. Porosity parameters of carbon support and xCo@C_T composites. Sample name BET specific surface area (m2g–1) Micropore volume (cm3g–1) Mesopore volume (cm3g–1) Total pore volume (cm3g–1) Pore size (nm) CMK–3 1250 0.52 0.84 1.36 3.52 10Co@C_400 1080 0.45 0.64 1.09 3.48 15Co@C_400 1000 0.41 0.56 0.97 3.35 15Co@C_500 1160 0.48 0.66 1.14 3.50 As the calcination temperature was increased to 500 °C, NPs of cobalt oxides could be identified in the SEM image of 15Co@C_500 (Figure 2a). To further investigate the NPs dispersion, concentric backscattered electron analysis was performed. Figure 2b shows the corresponding BSE image of 15Co@C_500. The observation of continuous stripes with well– dispersed bright spots suggests the uniform distribution of NPs inside the carbon channels. Cobalt oxide particles are identified as bright spots in the BSE images. Figure 2c shows the TEM image of 15Co@C_500. The NPs were observed well–dispersed with uniform size of ~ 4 nm. These results demonstrate that our synthesis approach has successfully and uniformly incorporated the cobalt oxide NPs into the meso–channels of the CMK–3 carbon. Moreover, 15Co@C_500 sample shows higher specific surface area of 1160 m2g–1 than that of 15Co@C_400. Considering of the temperature adopted for calcination, we believe that at the Sy 23 tem tem po tha pe nat an sug de 3.2 CM exh of beh nthesis and 4 perature of perature of re volume fo t metal oxid Figure 2. Sca The XRD aks emerged ure of this d 42.20° ca gested tha composition F Electroche The resul K–3 and 1 ibited a rec 10 mVs–1 avior. How electrochem 400 °C, mo 500 °C, the r gas adsorp e particles in nning electro patterns of at 23.4° a material. Fo n be indexe t CoO was of the cobal igure 3. Pow mical prop ts obtained f 5Co@C_50 tangular sha (Figure 4a) ever, with a ical properti st of the cob decomposi tion. 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