Synthesis, growth mechanism and thermal stability of copper nanoparticles encapsulated by multi-layer graphene
Introduction
Metal nanomaterials have received tremendous scientific and practical interest due to their unique properties and novel applications compared to their bulk counterparts [1], [2], [3], [4], [5]. A typical example is the potential applications of metal nanoparticles in ink-jet printing technology. Ink-jet printing technology is considered as a promising alternative to the traditional time-consuming, expensive and environment-unfriendly lithography technology for the fabrication of circuit boards in the electronic industry [6], [7]. In this novel technology, conductive inks have now been widely considered as the most critical factor, and the suspensions of metal nanoparticles seem to be the most likely candidates for the preparation of conductive inks [8], [9]. Owing to their high conductivity and excellent non-oxidizing properties, noble metal nanoparticles, such as silver and gold, have so far been the most active research objects in the past years [8], [9], [10]. However, their high cost prevents them from widespread practical applications. As a result, the synthesis of non-noble metal nanoparticles with decent thermal stability and oxidation resistivity has now become of great interest from a scientific and industrial point of view.
Copper (Cu) nanoparticles, due to their relatively low cost and high electrical conductivity, exhibit high potential for replacing the noble metal nanoparticles used in conductive inks [11], [12]. However, bare Cu nanoparticles exposed to air at room temperature will be oxidized to form Cu2O within several hours [13], [14]. Therefore, the problem of their poor oxidation resistance must be resolved before they can be applied into practice. Recently, Cu nanoparticles encapsulated by carbon (C) shells, i.e., C/Cu shell/core nanoparticles, have attracted intense interest, because the C shells can serve as the shields to protect the Cu nanocores from oxidation [15], [16], [17], [18], [19], [20], [21]. Nevertheless, as these C shells are usually amorphous and/or too thick, their conductivity is typically quite poor [20]. As a result, these C/Cu nanoparticles are not suitable for producing conductive inks.
It is well-known that multi-layer graphene has high conductivity [22] and can be used as excellent conducting electrodes [23], [24]. Therefore, a shell of multi-layer graphene can serve as the dual role of protecting the metal nanocore from oxidation [25], [26], [27] and conductively connecting the neighbor metal nanocores [28]. This suggests that Cu nanoparticles encapsulated by multi-layer graphene, i.e. graphene/Cu shell/core nanoparticles, should be an ideal material for fabricating conductive inks. For example, Luechinger et al. have successfully fabricated ink-printed electrical circuits using self-manufactured copper/carbon particles [29]. However, the graphene formed on the surface of Cu is usually single-layer, rather than multi-layer; because its formation process is based on the adsorption–diffusion mechanism instead of the dissolution–precipitation mechanism [30]. According to the adsorption–diffusion mechanism, the pre-formed single-layer C atoms can passivate the Cu surface, and dramatically hinder the formation of multi-layer graphene on Cu [30], [31]. Furthermore, the growth temperature of graphene on Cu is typically about 1000 °C, which has been above the melting point of Cu nanoparticles [32]. Therefore, it has been very challenging to synthesize graphene/Cu shell/core nanoparticles.
In this paper, we report the large-scale fabrication of Cu nanoparticles encapsulated by multi-layer graphene by using a one-step metal-organic CVD at 600 °C. A novel coalescence mechanism was proposed to explain the formation of the graphene/Cu shell/core nanoparticles at the relatively low temperature. Both differential scanning calorimetry and thermogravimetric (DSC–TG) and high-resolution transmission electron microscopy (HRTEM) analyses showed that the graphene/Cu shell/core nanoparticles exhibited excellent thermal stability.
Section snippets
Experimental
The graphene-encapsulated Cu nanoparticles were synthesized in a horizontal tube furnace with a vacuum pump system, as shown in Fig. 1. Analytical copper(II) acetylacetonate (Cu(acac)2, Aldrich Chemical Co., 97%) powder was loaded into a quartz boat installed in the evaporating region with a temperature of 150 °C (upstream) in the furnace. The gaseous Cu(acac)2, formed in the evaporating region, was then transported to the reaction region of 600 °C (midstream) by the carrier gas H2 (flow: 200
Morphological and structural characteristics of the graphene/Cu shell/core nanoparticles
The stainless-steel wool, after taken out from the furnace tube, was found to be fully covered with black powder. Typically, several grams of this black powder could be produced in one test run. Fig. 2a presents the typical SEM image of the black powder synthesized at 600 °C, showing that the powder consists of nanosized particles. The inset in Fig. 2a shows the optical photo of the powder in black after shaken off from the stainless-steel wool. High-magnification SEM imaging (Fig. 2b) further
Conclusions
We have developed a low-cost, high-yield and one-step metal-organic CVD to fabricate Cu nanoparticles encapsulated by multi-layer graphene at 600 °C. The synthesized graphene/Cu shell/core nanoparticles have single-crystalline Cu nanocores with an average diameter of 18 nm and multi-layer graphene shells with a thickness of 1–2 nm. The obtained shell/core nanoparticles were formed by the coalescence of Cu and C atoms within the C/Cu nanoagglomerates, which originated from the agglomeration of C/Cu
Acknowledgements
This study is supported by the National Natural Science Foundation of China (Grant Nos. 50825102, 50804057, 51074188, 51071178, 50721003, and 50823006) and the Postdoctoral Science Foundation of Central South University.
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