CNTs materials

Materials Letters 236 (2019) 179–182

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Gaseous detonation synthesis of Co@C nanoparticles/CNTs materials Tiejun Zhao a, Xiaohong Wang a, Xiaojie Li a,b, Yang Wang a, Xinhua Song a, Honghao Yan a,⇑ a b

Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, China

a r t i c l e

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Article history: Received 27 July 2018 Received in revised form 22 September 2018 Accepted 16 October 2018 Available online 17 October 2018 Keywords: Carbon materials Co@C Carbon nanotubes Gaseous detonation Raman

a b s t r a c t Co@C/CNTs magnetic carbon nanomaterials were fabricated by a gaseous detonation method with cobalt (III) acetylacetonate (Co(acac)3) as precursor and a mixture gas of hydrogen and oxygen as explosion source. The current work investigates how proportion of oxygen in the mixture gas affects the morphologies, phases, and degree of graphitization of Co@C/CNTs nanomaterials. The characterization of transmission electron microscopy indicates that the samples were consisted of core-shell nanoparticles and nanotubes, and the proportion of oxygen had little influence on the morphologies of the samples. Xray diffraction and Raman spectra analysis demonstrate that the core-shell particles and nanotubes were Co@C and CNTs, respectively. The cobalt nanoparticles generated from the decomposition of Co(acac)3 would be oxidized to CoO when the proportion of oxygen was greater than 50% in the mixture gas. Though the degree of graphitization of Co@C/CNTs nanomaterials was little affected by the proportion of oxygen, the particle size of Co@C increased with the increase of the proportion of oxygen in the mixture gas, simultaneously, the carbon matrix was decreased. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction At first, detonation method was used to prepare nanodiamonds [1]. Because of its simple operation, high efficiency and low cost, detonation method has become a novel method for preparing nanomaterials. At present, the detonation diamonds have been produced commercially. With the development of detonation method, carbon-coated metal (Fe, Co, Ni, et al) nanoparticles [2], carbon nanosheets [3], metallic oxides (SiO2, TiO2) [4], and Al2O3diamonds composite nanoparticles [5] have been prepared via explosive detonation method. However, the detonation samples need complex purification process. The samples prepared by gaseous detonation method are much cleaner than that obtained from explosive detonation method. In gaseous detonation method, combustible gas and oxygen/air are used as explosion source to prepare nanomaterials. Currently, many kinds of carbon materials with high purity have been prepared via gaseous detonation method [6–8]. Nevertheless, there are few literatures report the preparation of carbon-coated cobalt nanoparticles (Co@C) by using gaseous detonation method. Therefore, hydrogen-oxygen mixture gas was used as explosion source to prepare Co@C/CNTs in present work, and the influence of the proportion of oxygen in hydrogen-

⇑ Corresponding author at: Address: No. 2 Linggong Road, Ganjingzi District, Dalian City, Liaoning Province 116024, PR China. E-mail address: [email protected] (H. Yan). https://doi.org/10.1016/j.matlet.2018.10.105 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.

oxygen gas on the morphology and the degree of graphitization of the samples was studied in detail. 2. Experimental section The operation process of preparing Co@C/CNTs was similar to the fabrication of Fe@C by gaseous detonation as described in literature [6]. 2 g of Co(acac)3 powders were placed uniformly in gaseous detonation tube. After vacuuming, hydrogen and oxygen were filled in the tube with a molar ratio of 3:1, 2:1, and 1:1, respectively. The mixture gas was ignited when the temperature of the tube was 165 °C for 10 min. Finally, the samples were collected and marked as S1, S2, and S3, respectively. The phases of the samples were characterized by X-ray diffractometer (XRD) (Rigaku D/MAX 2400, Cu Ka, radiation) with the scanning angle ranging from 20° to 80°. A transmission electron microscopy (TEM) (Tecnai F30, FEI, US) was used to identify the morphologies of the samples. The Raman properties of the samples were assessed via a DXR Smart Raman spectrometer (Thermo Fisher, US). 3. Results and analysis Fig. 1(a) shows three sharp peaks around 44°, 51.5°, and 75.8° are corresponding to the (1 1 1), (2 0 0), and (2 2 0) crystal planes of Co, respectively. The intensity of (1 1 1) crystal plane increased

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Fig. 1. XRD patterns of samples (a) and EDX pattern of S2. (S1, n(H2):n(O2) = 3:1; S2, n(H2):n(O2) = 2:1; S3, n(H2):n(O2) = 1:1).

with the increase of the proportion of oxygen in the mixture gas. However, when the molar ratio of hydrogen to oxygen was 1:1, CoO appeared in S3 indicating some Co nanoparticles were oxidized to CoO. Three weak peaks at 36.25°, 42.18°, and 61.37° were related to the (1 1 1), (2 0 0), and (2 2 0) crystal plane of CoO, demonstrating that the hydrogen cannot afford the reaction with oxygen, and the Co and C nanoparticles, generated from the decomposition of Co(acac)3, might react with the remaining oxygen. Fig. 1(b) exhibites the EDX pattern of S2. It can be found that S2 contained C, Co, O, Si, and Cu elements, in which Cu element was from the film used in the measurement, and Si element might be relating to the collection of sample. The O element was attributed to the oxidation of Co by oxygen; however, it was too few to be checked by XRD analysis. The morphologies of the three samples were evaluated by TEM, as shown in Fig. 2. Fig. 2(a) shows that S1 was consisted of nanoparticles and nanotubes, and the nanoparticles were core– shell structure with obvious agglomeration. It can be determined

that the core-shell structure should be Co@C nanoparticles with Co-core and carbon-shell. The size of Co@C ranged from several nanometers to 33 nm. The nanotubes, as marked with a red arrow, appeared in S1 though they were not so many. It was worth noting that some smaller Co nanoparticles were embedded in the carbon wall. The HR-TEM images of CNTs and Co@C nanoparticles are displayed in Fig. 2(b). It is clear to see that the CNTs were multiwalled carbon nanotubes with 17 nm and 5 nm in external diameter and internal diameter, respectively. The lattice fringes were obvious and the lattice spacing was about 0.39 nm. The coreshell structure of Co@C was clear, and the lattice spacing of Co core was 0.22 nm relating to the (1 1 1) crystal plane of Co. The coreshell and tube structures were more distinctive in S2. The Co nanoparticles were encapsulated by carbon layer, and some Co nanoparticles were embedded in a carbon matrix. The particle size of Co ranged from 20 to 60 nm, which was much larger than that in S1. The CNTs grew from the edge of the carbon matrix with the catalysis of Co nanoparticles. The graphite layer of CNTs and

Fig. 2. TEM and HR-TEM images of samples. (a, c, and e, the TEM images of S1, S2, and S3; b, d, and f, the HR-TEM images of S1, S2, and S3.).

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Raman spectrum of sample S3 Fig. 3. Raman spectra of three samples. (a), n(H2):n(O2) = 3:1; (b), n(H2):n(O2) = 2:1; (c), n(H2):n(O2) = 1:1.).

Co@C was indistinct, however, the lattice fringes of Co was distinct (as shown in Fig. 2(d)). The morphologies of S3 were similar to S2, whereas the carbon matrix was much smaller than that in the S2, which was contributed to the reaction between carbon and oxygen. The particle size of Co was larger than that in S2. In Fig. 2(f), it can be found a carbon cap in the CNTs, which was not found in S1 and S2. Raman spectroscopy is a useful technique to analyze the degree of ordering of structure or the defects in graphitic-like materials. Fig. 3 shows that three samples exhibited a peak 186 cm 1 relating to the A1g radial breathing modes (RBM) of carbon. The peaks around 1349 cm 1 were contributed to the disorder graphite structure (D-band) caused by the defects or the disorder of graphitic materials [9]. The peaks around 1592 cm 1 were related to the G-band caused by tangential modes, which represent the well order structure associated with sp2 carbon atoms in graphite [10]. Generally, the value of ID/IG is used to determine the degree of graphitization of carbon materials. The ID/IG values of S1, S2, and S3 were 1.01, 1.06, and 1.05, respectively, which meant the degree of graphitization of three samples was low. The results were consistent with the degree of graphitization of Fe@C prepared by gaseous detonation method [7]. The rapid detonation reaction cannot provide much time for carbon transforms to graphite, therefore, the ID/IG value was large. The peaks 467, 510, 667 cm 1 were associated with the Raman spectra of Co3O4 [11]. It was no doubt that the Co atoms were reacted with oxygen even in a large molar ratio of hydrogen to oxygen (S1, n(H2):n(O2) = 3:1). In Costa’s study, the Raman spectra intensity of Co3O4 decreased with the increase of temperature [11]. Though the temperature in detonation reaction can reach thousands of Kelvin, the duration limited the reduction of Co3O4.

4. Conclusions Co@C/CNTs nanomaterials were prepared via gaseous detonation method using Co(acac)3 as precursor, and hydrogen-oxygen gas as explosion source. The influence of the proportion of oxygen in hydrogen-oxygen gas on phases, morphologies, and Raman properties of Co@C/CNTs was studied. The samples were characterized by XRD, TEM and Raman spectrometer. It can be found that the nanomaterials consisted of Co@C and CNTs, and the CoO phase presented when the molar ratio of hydrogen to oxygen was 1:1. The size of Co@C increased with the increase of the proportion of oxygen in hydrogen-oxygen gas, meanwhile, the carbon matrix was reduced. The proportion of oxygen in hydrogen-oxygen gas had little effect on the morphologies and the degree of graphitization of Co@C/CNTs. Though the degree of graphitization of Co@C/ CNTs nanomaterials was low, some graphite layers can be seen clearly in samples. Acknowledgements This project was financially supported by the National Natural Science Foundation of China (Nos. 10872044, 11672067, and 11672068). References [1] P.S. Decarli, J.C. Jamieson, Formation of Diamond by Explosive Shock, Science 133 (346) (1961) 1821–1822. [2] X.Q. Li, X.J. Li, X.H. Wang, X.C. Pan, H.H. Yan, Characterization of carbonencapsulated permalloy nanoparticles prepared through detonation, Mater Res Express 4 (7) (2017) 075024.

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