One‑pot rapid preparation of carbon‑coated Co–Cu alloy composites via the gaseous detonation method

composites but have little impact on the phase. The higher combustion heat of combustible gas would contribute to the preparation of carbon-coated Co– Cu/Cu nanoparticles with a core–shell structure and a larger particle size. The present study provides a novel means for the preparation of carbon-coated Co– Cu alloy nanomaterials. gaseous of acetylacetonate acetylacetonate a carbon and metal the mixture an explosion study the influence of explosion source on the phase and morphology of the nanopowders. The TEM and XRD analysis that the carbon-coated Co– Cu alloy of CNTs and carbon-coated Co–Cu/Cu nanoparticles with core– The explosion would the morphology of the carbon-coated Co–Cu alloy


Introduction
Co-Cu alloy nanoparticles are important catalysts, which can be widely applied in preparing industrial materials (like alcohol [1], biofuel [2], etc.), wastewater treatment [3], disease treatment [4], and so on. Scholars have explored a series of studies to improve the dispersion of Co-Cu alloy particles and ensure the stability of their catalytic activity. For instance, graphene-LaFeO 3 supported with Co-Cu alloys could improve the yield of alcohol from syngas [5], carbon-coated Co-Cu nanoparticles would ensure the catalytic activity [2,6,7], and the Co-Cu alloy nanoparticles on the support of CNTs could emerge good dispersion and catalytic activity [8]. However, the efficient and convenient preparation of Co-Cu alloy is still a research topic.
In recent years, systematic studies have been carried out on the efficient and rapid synthesis of CNTs Abstract Nano Co-Cu alloy is an important catalyst, which has an important practical value in the field of the petrochemical industry. In order to prepare carbon-coated Co-Cu alloy composites efficiently and conveniently, a gaseous detonation method was explored in the present paper. The mixture of cobalt acetylacetonate and copper acetylacetonate was used as a carbon source and metal source, and the mixture gas of hydrogen-oxygen and benzene-oxygen was used as an explosion source, respectively, to study the influence of explosion source on the phase and morphology of the nanopowders. The TEM and XRD analysis results indicated that the carbon-coated Co-Cu alloy composites were composed of CNTs and carbon-coated Co-Cu/Cu nanoparticles with coreshell structure. The explosion source would affect the morphology of the carbon-coated Co-Cu alloy and carbon-coated metal (Co, Cu, Fe) nanoparticles by the gaseous detonation method [9,10]. Although CNTs and core-shell structure nanoparticles have been successfully and rapidly prepared by the gaseous detonation, it is difficult to search the literature on the synthesis of carbon-coated metal alloy composites by the gaseous detonation method [11]. Because of the different sublimation or vaporization temperatures of the precursor, it is difficult to form an alloy phase in the detonation process. In our previous study, the carbon-coated Fe-Co alloy nanoparticles were attempted to be prepared using the gaseous detonation method; however, the Fe-Co alloy phase cannot be found in the XRD data, which means the exploration did not achieve the expected results [11]. After then, it was found that the precursor and explosion source are the key factors affecting the preparation of carbon-coated metal alloy composites. Therefore, it is interesting to study the preparation of carbon-coated Co-Cu alloy composites via the gaseous detonation method. In the present paper, the mixture of cobalt acetylacetonate and copper acetylacetonate was used as precursors and the influence of explosion source, mainly focusing on the combustion heat of combustible gas, on the phase and morphology of the carbon-coated Co-Cu alloy composites. It can provide an efficient and economical method to obtain carbon-coated Co-Cu alloy composites, also making it possible to study the properties of carbon-coated Co-Cu alloy nanomaterials.

Experimental
The carbon-coated Co-Cu alloy composites were prepared via the explosion of the mixture of combustible gas and oxygen. In the present paper, the mixture of cobalt acetylacetonate and copper acetylacetonate was used as a precursor, which was mixed evenly in an agate mortar according to the molar ratio of n(Co):n(Cu) = 2:1. Hydrogen-oxygen and benzene-oxygen mixtures were used as explosion source, respectively, to explore the influence of explosion source on the phase and morphology of carboncoated Co-Cu alloy composites. The detonation experiments were carried out in a closed detonation tube as shown in Fig. 1.
For the hydrogen-oxygen detonation experiment, the precursor powders were evenly placed in the detonation tube along the axis direction and vacuumed the tube after sealing. Then, hydrogen and oxygen were filled into the tube in a molar ratio of 1:1 when the temperature was 165 °C. The mixed gas was ignited by an electric spark after 10 min; finally, the generated powders were collected and marked as S1.
For the benzene-oxygen detonation experiment, the precursor powders were evenly dispersed in the detonation tube along the axis direction and vacuumed the tube after sealing. When the temperature was 165 °C, benzene was injected into the tube, and then oxygen was filled into the tube according to the molar ratio of n(benzene):n(hydrogen) = 1:5. Ten minutes later, the mixed gas was ignited by an electric spark. Finally, the obtained powders were collected and marked as S2.
The phase composition was tested by an X-ray diffractometer (Rigaku D/MAX 2400, Cu Kα) with a scanning angle in a range of 20-80°. The morphology was measured by transmission electron microscope (Tecnai F30, FEI). Figure 2 shows the phase composition analysis, and three sharp diffraction peaks in S1 and S2 can be found. The diffraction peaks at 43.69, 50.98, and 74.68° are assigned to Co 0.52 Cu 0.48 (PDF#50-1452) (111), (200), and (220), respectively [12], which indicates the Co-Cu alloy was prepared by the gaseous detonation method. The crystallite size of the Co-Cu alloy was calculated by the Scherrer equation [13] Table 1. The average crystallite size of Co 0.52 Cu 0.48 in S1 is 11.5 nm and that in S2 is 15.7 nm, which demonstrates that the crystallite size of the Co-Cu alloy would be affected by the explosion source, and the combustible gas with higher combustion heat would contribute to the larger crystallite size. The unnoticeable diffraction peaks at 43.3, 50.4, and 74.1° are Cu (PDF#04-0836) (111), (200) and (220), respectively, which means the samples contain some pure copper particles. As for the diffraction peaks of cobalt, they are significantly different from Co-C materials prepared via the gaseous detonation method [14], which illustrates no pure cobalt in the samples. Although the diffraction peaks of carbon cannot be seen in Fig. 2, it is impossible to determine whether the samples contain pure carbon, or it is probably amorphous. The diffraction peaks of carbon are so weak, which can also be found in our previous study [11,15], while the carbon structure can be seen in the TEM images. Therefore, it is necessary to further clarify the existence of the carbon phase and morphological characteristics of the samples by TEM characterization. Figure 3 shows the TEM images of two samples. CNTs and nanoparticles can be clearly seen in Fig. 3a, and the nanoparticles are core-shell structures with obvious agglomeration as shown in Fig. 3a and b. The core-shell nanoparticles should be Co-Cu alloys or copper core and carbon shell according to the XRD analysis results and literature [9,16]. Therefore, the  Fig. 2 XRD patterns of carbon-coated Co-Cu alloy composites. (S1, prepared by hydrogen-oxygen detonation; S2, prepared by benzene-oxygen detonation)  core-shell nanoparticles would be carbon-coated Co-Cu/Cu(Co-Cu/Cu@C) nanoparticles. Though it is difficult to determine the phase of the metal particle at the top of the CNTs, it can be inferred from our previous work [9,11], literature [17], and XRD characterization results that the metal nanoparticle should be Co-Cu alloys. The XRD analysis results show that there is no pure cobalt in samples. The sample obtained by benzene-oxygen detonation is also composed of CNTs and Co-Cu/Cu@C nanoparticles as shown in Fig. 3c; however, the number of CNTs is significantly less than that in sample S1. What is more, the core-shell structure of Co-Cu/Cu@C nanoparticles in S2 is much clearer than that in S1 (as shown in Fig. 3d). The particle size of Co-Cu/Cu@C nanoparticles was measured and counted, and the statistical results are shown in Fig. 4. It can be found that most of the particles in S1 are between 10 and 20 nm in size and that of S2 is in the range of 15-20 nm, which is a little larger than S1. It is well known that temperature plays an important role in the growth of CNTs and carbon-coated metal nanoparticles. Under the same conditions, such as precursor, initial conditions, zero-oxygen equilibrium, etc., it is more difficult to prepare CNTs by the detonation of combustible gas with high combustion heat [18]. In addition, for the Co-Cu alloy catalyst, the reaction temperature higher than 900 °C is detrimental to the growth of CNTs [17]. Most importantly, after the detonation reaction in the detonation tube, the detonation wave will decay into a combustion wave, and a low-temperature environment would be maintained for a short time [19], which contributes to the growth of CNTs. This phenomenon can also be confirmed by our previous studies [10,18]. Therefore, for cobalt acetylacetonate and copper acetylacetonate, the precursors consumed a lot of heat during decomposition; CNTs and core-shell nanoparticles will be obtained by the detonation of the combustible gas with low combustion heat under zero oxygen balance or high combustion heat gas under negative oxygen balance.

Conclusions
Carbon-coated Co-Cu alloy composites were successfully prepared in one pot via the gaseous detonation method. The XRD analysis and TEM characterization results indicate that the nanopowders consisted of CNTs and core-shell particles, and the core-shell particles should be Co-Cu@C nanoparticles or Cu@C nanoparticles. The particle size and morphologies of carbon-coated Co-Cu alloy composites are affected by the combustion heat of combustible gas. Under the same condition, the combustible gas with high combustion heat is conducive to the preparation of larger particles, but the number of CNTs in the nanopowders would be reduced. For high-combustion heat gas, like benzene, CNTs also can be obtained under negative oxygen conditions with cobalt acetylacetonate as the precursor.

Conflict of interest The authors declare no competing interests.
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