Facile fabrication of reduced graphene oxide encapsulated copper spherical particles with 3D architecture and high oxidation resistance

Abstract

Reduced graphene oxide nano-sheets encapsulated copper spherical particles (Cu@rGO) were obtained through a simple electrostatic self-assembly process followed by chemical reduction. The oxidation resistance of Cu spherical particles was considerably enhanced by the coating of rGO nano-sheets. XRD results show that the core–shell structured Cu@rGO did not exhibit any sign of oxidation in air after storage at room temperature for 70 days or after heat treatment at 130 °C for 1.5 h. Furthermore, the rGO wrapped on the surface of Cu particles could transfer the two-dimensional rGO nano-sheets into three-dimensional (3D) networks. The Cu@rGO particles with 3D structure might have promising potential applications in thermal management, electrically conductive interconnection, electrodes, etc.

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1. Introduction

Copper–carbon composite materials have attracted increasing attentions for various applications including electric devices, electrodes, catalysts, etc.^1–10 Compared to the high cost of Ag, cheap Cu has similar high electrical and thermal conductivity. However, Cu is easily oxidized even under ambient atmospheres, which would hinder its more widespread applications. Polymers, metals and inorganic materials have been investigated as oxidation protective layers of copper surfaces.^11–13 Among them, the two-dimensional carbon nanomaterial of graphene is an ideal material for coating to prevent Cu oxidation and retain its excellent thermal and electrical properties due to the unique properties of graphene, including super thermal and electric conductivity, extreme thinness, high mechanical flexibility and strength.^14–18 On the other hand, the two-dimensional graphene nano-sheets might be deformed into a three-dimensional (3D) architecture through coating on the other materials. In some cases, such as electrodes of electrochemical devices or thermal interface materials, a 3D structure is expected for graphene stacking.^14,19 The 3D structure would help make full utilization of the large specific surface area nature of graphene nano-sheets or change the simple in-plane conductive path of graphene into a 3D conductive network.

CVD graphene materials have been applied for coating Cu nanoparticles or Cu foil, which exhibited excellent oxidation tolerance.^20–22 Graphene encapsulated metallic copper nanoparticle composite could be also obtained by metallic potassium and potassium borohydride/ethylenediamine reduction of CuCl₂–graphite intercalation which prepared at high temperature of 528 °C for long time of 7 days.²³ But these processes are high cost and at high operating temperature, complex, or dangerous. The low temperature solution phase preparation of graphene from graphene oxides (GO) or graphite seems more advantaged in terms of the cost and large scale production.^24,25 For example, uncapped metal nanoparticles on graphene could be obtained by microwave-reduction of graphene oxide and metal ion in solution with tuning catalytic, electrical, and Raman properties.²⁶ The composite of graphene nano-sheets supported copper nanoparticles was prepared by the in situ far IR-assisted chemical reduction of a mixture containing graphene oxide and copper(II) ions to a well interconnected hybrid network with high performances as the electrode of sensors.²⁷

Herein, we report a simple low temperature solution process to fabricate reduced graphene oxide (rGO) nano-sheets coated Cu spherical particles. The rGO coatings could serve as the protective shells for prevent the Cu spherical particles from oxidation. In addition, the two-dimensional rGO nano-sheets could transfer into 3D structure when they cover on the surface of Cu spherical particles.

2. Experimental

2.1. Materials

Copper spherical particles were bought from Aladdin reagent Co., Ltd. Graphite powder, 3-aminopropyltriethoxysilane (APTES), and methylbenzene (A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultra pure water (18 MΩ cm) was used in the experiments.

2.2. Preparation of graphene oxide (GO)

GO nano-sheets were prepared using a modified Hummers method from graphite powders.^28,29 In a typical reaction, 1 g graphite, 1 g NaNO₃, and 46 mL concentrated H₂SO₄ were mixed under agitation. Next, 6 g KMnO₄ was gradually added (1 g 10 min⁻¹) into the above mixture while keeping the temperature below 20 °C (ice bath). Then, the solution was stirred for about 90 min at 35 °C. Subsequently, 80 mL water was added, and the stirring continued for 30 min as the temperature was raised to 90 °C. Finally, 80 mL water was added, followed by the slow addition of 10 mL H₂O₂. And the colour of solution changed from brown to yellow. The product was washed repeatedly in 5% HCl solution until sulfate could not be tested with barium chloride. Centrifugation at 8000 rpm (15 min) with water was used until the pH was 4–6 for the production.

2.3. Preparation of reduced graphene oxide nano-sheets encapsulated Cu spherical particles (Cu@rGO)

Cu spherical particles (0.1 g) were dispersed into 9.9 mL methylbenzene via ultrasonication. Then 0.1 mL APTES was added into the above solution and stirred at room temperature for 24 h. The product was washed with ethanol several times.

The dried APTES modified Cu spherical particles (0.1 g) were dispersed into 30 mL water with the aid of ultrasonication, followed by the addition of 20 mL aqueous GO nano-sheets suspension (1 mg mL⁻¹). After mildly stirring for 1–3 h at room temperature, 2.6 mL N₂H₄·H₂O (85%) was added into the solution to reduce the GO nano-sheets. The rGO nano-sheets encapsulated Cu spherical particles were finally obtained after centrifugation and washing with ultrapure water.

2.4. Material characterization

The morphologies of samples were examined by a field emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM 450) and transmission electron microscope (TEM FEI Tecnai F20 microscope). The crystalline structure was investigated by X-ray power diffraction (Rigaku D/Max 2500, Japan) with Cu Kα radiation (λ = 1.541874 Å). XPS analysis was recorder on a PHI 5800 XPS system, where Al Kα excitation source was used. A Renishaw RM-1000 micro-Raman spectrometer was used to measure the Raman spectra of samples with the 514.53 nm line of an argon laser. Fourier transform infrared (FTIR) spectra were recorded with a FTIR spectrophotometer (Vertex70 FT-IR-spectrometer). Ultraviolet-visible spectroscopy (UV-vis) of the samples were recorded on an UV-vis-NIR spectrometer (Shimadzu UV-3600).

3. Results and discussion

Fig. 1 schematically illustrated the preparation of rGO nano-sheets encapsulated Cu spherical particles. At first, copper spherical particles were modified with APTES to graft the amino groups.^30,31 Next, the obtained APTES modified Cu spherical particles assembled with negatively charged GO nano-sheets by electrostatic attraction. Finally, hydrazine hydrate was used to reduce the GO to obtain the rGO nano-sheets encapsulated Cu spherical particles. SEM images in Fig. 2(a and b) showed that the pristine Cu particles were spherical and the diameter was about 0.2–1 μm. Fig. 2(c and d) displayed that the surface of the Cu spherical particles were covered with rGO nano-sheets. When the two-dimensional rGO nano-sheets encapsulated Cu spherical particles, they were changed to 3D architecture as shown in Fig. 2(c and d) and 3(a). The deformed rGO nano-sheets could serve as the cover layers for the encapsulation of Cu spherical particles and outer linked nano-sheets for linking the neighbour particles. Additionally, the rGO shells exhibited wrinkled and rough textures, which were associated with the flexible and extremely thin nature of graphene.³¹ The typical core@shell structure of Cu@rGO was also observed in TEM images (Fig. 3b). It could be seen that rGO nano-sheet shells were firmly coated around the surface of Cu spherical particles.

Fig. 1 Schematic illustration of the preparation of Cu spherical particles encapsulated by reduced graphene oxide nano-sheets.

Fig. 2 SEM images of (a and b) bare Cu spherical particles, (c and d) rGO nano-sheets encapsulated Cu spherical particles.

Fig. 3 TEM images of (a and b) rGO nano-sheets encapsulated Cu spherical particles.

Fig. 4a and b showed the FTIR spectrums of GO and Cu@rGO, respectively. The broad peak between 3660–3420 cm⁻¹ were associated with the presence of hydroxyl groups due to the adsorbed water and structural hydroxyl groups of graphite oxides.³² The bands at 3200 and 1620 cm⁻¹ were assigned to the vibration of adsorbed water molecules and epoxide groups.^32,33 While for Cu@rGO (Fig. 4b), the above peaks were decreased in intensity. Furthermore, the peaks located at 1726 and 1220 cm⁻¹ in Fig. 4a, which oriented from the stretching vibration of carboxyl groups on the edges of the layer planes or conjugated carbonyl groups in the GO,^32,33 nearly disappeared in Cu@rGO. The results indicated an effective reduction of GO.

Fig. 4 FTIR spectra of GO (a) and rGO nano-sheets encapsulated Cu spherical particles (b).

UV-vis spectra of as-synthesized samples were shown in Fig. 5. The typical absorption peak of GO at 229 nm corresponds to π–π* transitions of the aromatic C=C of GO. After chemical reduction, Cu@rGO displayed the red-shift peaks at 270 nm, indicating the formation of graphite-like conjugated structure.³⁴ Cu@rGO also showed another absorption peak around 600 nm, which could be explained to the presence of Cu.³⁵

Fig. 5 UV-vis spectra of GO and rGO nano-sheets encapsulated Cu spherical particles.

The XPS results of GO and Cu@rGO were shown in Fig. 6. The C1s XPS spectrum of GO nano-sheets indicated the successful oxidation of graphite (Fig. 6a). The four decomposed peaks were corresponded to carbon atoms in different functional groups: the non-oxygenated ring C (284.5 eV), the COC/COH groups (286.3 eV), carbonyl groups (C=O, 287.2 eV), and the carboxylate carbon (C(O)OH, 289.3 eV).^36–38 Cu@rGO also exhibited the similar functional oxygen functionalities while their peaks became weaker (Fig. 6b), indicating a considerable degree of deoxygenation process accompanies the reduction of graphene oxide.^36,38 The oxygen-containing groups were still detected in the Cu@rGO since the complete reduction of GO might be difficult according to the theoretical analysis.³⁹

Fig. 6 The C1s XPS spectra for (a) graphene oxide nano-sheets and (b) Cu@rGO.

Fig. 7 showed the Raman spectroscopy of Cu@rGO, Cu@GO and GO nano-sheets. Two first order characteristic peaks were observed in GO: the D peak (1358 cm⁻¹) and G peak (1605 cm⁻¹). The D band corresponded to the breathing mode of the k-point phonons of A_1g symmetry and the G band originated from the E_2g phonon of C_sp².⁴⁰ Cu@GO displayed the similar D peaks (1357 cm⁻¹) and G peaks (1602 cm⁻¹). While after a chemical reduction process, the G peaks of Cu@rGO was shifted toward lower Raman shift values (1598 cm⁻¹), and an increased D/G intensity ratio was observed from 0.99 to 1.15. The changes suggested a decrease in the average size of the sp² domains.^36,40 The decrease of the average size of the sp² domains might be ascribed to the creation of new smaller graphitic domains, which were smaller in size, but more numerous in number after reduction.³⁶ All the above FTIR, UV-vis, XPS and Raman results indicated an effective reduction of GO nano-sheets at room temperature. The second order peak of Cu@rGO appeared between 2300 and 3200 cm⁻¹. This peak could be fitted using Lorentzian profiles (inset images of Fig. 7) and the region can be divided into 2D, D + G, and 2D′ peaks at 2689, 2933, and 3164 cm⁻¹ respectively. The blue shifted 2D peak and the broad peak (2300–3200 cm⁻¹) also indicated an increasing number of graphene layers according to the previous reports.^41,42 Unlike planar graphene nano-sheets, such D + G and 2D′ peaks were assigned to nonplanar graphitic layers,^20,43 which might imply that the deformation of rGO nano-sheets around the surface of Cu spherical particles.

Fig. 7 Raman patterns of GO, Cu@GO and Cu@rGO, inset: second order region fitted using Lorentzian profiles.

XRD results in Fig. 8 indicated the oxidation resistance ability of rGO coated Cu and bare Cu spherical particles. Bare Cu and rGO coated Cu spherical particles stored in air at room temperature for 30 days and 70 days were named Cu-30, Cu@rGO-30, Cu@rGO-70, respectively (Fig. 8a). Obviously, after coating with rGO nano-sheets, Cu@rGO did not exhibit any sign of oxidation after 70 days. However, a little peak attributed to Cu₂O (111) at 36.4° was observed in bare Cu spherical particles only after 30 days (Fig. 8a), suggesting the oxidation of bare Cu spherical particles (4Cu + O₂ → 2Cu₂O). The thermal stability of Cu spherical particles and Cu@rGO were also tested by XRD (Fig. 8b). The formation of cuprous oxide on the surface of bare Cu spherical particles occurred quickly at 90 °C within 1 h. For Cu@rGO, the rGO nano-sheets shell protected Cu core from oxidation even at 130 °C for 1.5 h in the air. The results indicated that the easily oxidized Cu spherical particles could be efficiently protected to inhibiting oxidation in air atmosphere by rGO nano-sheets coating. Considering the high electrical and thermal conductivity of Cu and graphene nano-sheets in plane, the composite of rGO nano-sheets encapsulated copper spherical particles is expected to possess excellent oxidation-tolerant stability and high electrical, thermal conductivity in 3D directions for potential applications in thermal management, electrically conductive interconnect, and electrodes, etc.

Fig. 8 XRD patterns of bare Cu and Cu@rGO after different conditions (a) room temperature for 30 days, 70 days; (b) annealed at 90, 130 °C.

4. Conclusions

Cu spherical particles were successfully encapsulated in rGO nano-sheets by a facile electrostatic self-assembly method followed by chemical reduction at room temperature. rGO nano-sheets served as a protective layer for copper spherical particles, leading to the rGO nano-sheets coated Cu spherical particles with high oxidation resistance. Cu spherical particle was then as a skeleton to transfer the two-dimensional to three-dimensional architecture of rGO nano-sheets. The core–shell Cu@rGO composite with 3D architecture and high oxidation resistance would be a promising material for wide potential applications.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21203236), Guangdong and Shenzhen Innovative Research Team Program (no. 2011D052, KYPT20121228160843692).

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