Fabrication of reduced graphene oxide/metal (Cu, Ni, Co) nanoparticle hybrid composites via a facile thermal reduction method

Abstract

A facile thermal reduction method has been proposed for the fabrication of reduced graphene oxide/metal (e.g., Cu, Co, Ni) nanoparticle hybrid composites at 500 °C for 90 minutes under flowing argon due to the release of reductive gas by thermolysis of graphene oxide. The loading amount and dispersion of metal nanoparticles could be easily controlled via the mass ratio of graphene oxide/metal nitrate precursor and the calcination temperature. The results show that with the increase of graphene oxide/metal nitrate mass ratio, it is easier to obtain pure metallic nanoparticles with high dispersion and small nanoparticle size.

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It is well known that graphene has triggered a great deal of academic and commercial interest ever since its discovery by Geim and Novoselov in 2004.¹ Due to its extraordinary electronic properties,² high surface area,³ high optical transmittance,⁴ outstanding thermal conductivity^5,6 and high Young's modulus,⁷ graphene has been leading to a research boom in a wide range of areas, such as photoelectric devices,^8–10 chemical sensors,^7,11 electrochemical energy storage and conversion devices,^12,13 and environmental remediation.^14–16 Different kinds of graphene hybrids have shown special superiority over the host materials and other carbon-based materials.^12,17 Many metal oxides such as Fe₃O₄,¹⁸ SnO₂,¹⁹ Co₃O₄,²⁰ TiO₂,²¹ MnO₂ (ref. 22) and NiO (ref. 23) were widely employed to form composites with graphene, which all show better performance than the bulk materials. However, in the synthesis of all these hybrids, graphene oxide (GO)/graphene/reduced graphene oxide (rGO) were often only used as efficient supporter, where other nanoparticles or nanosheets (including metal oxide, metal sulfide) decorated. It seems that this “graphene” only functions as physical separator and supporter in the hybrid synthesis process.

In fact, researchers also have noticed that the carbon on graphene should be able to serve as sacrificial reductant. According to Fan et al.²⁴ and Yan et al.,²⁵ direct redox reaction between graphene and KMnO₄ can result in insoluble MnO₂, which deposits on the graphene surface. In our previous research, GO was also used as reductant to form MnO_x nanomaterials.^26,27 However, graphene/metal nanoparticle hybrids seem not so popular, and the published reports are mainly concerned about the noble metal nanoparticles (e.g., Au, Ag, Pt and Pd) supported on graphene nanosheets.^28–34 Since the transition metal nanoparticles (e.g., Ni, Co, Cu) have been widely considered as efficient catalyst substitutes for noble metals, many research have been focused on loading these non-noble metal nanoparticles on various supports, such as Al₂O₃, SiO₂ and C materials.³⁵ In addition, transition metal nanoparticles have also attracted remarkable interest in many other application fields because of the unique magnetic, surface and optical properties, as well as abundance and low cost.^36–38 However, relatively little interest has so far been paid to non-noble metal nanoparticles (e.g., Cu, Ni, Co) supported on rGO nanosheets.^13,39–41 Commonly, the supported metal catalysts were synthesized by firstly impregnating the metal precursors into supports and then the reduction using external reductants (e.g., H₂, NaBH₄, hydrazine), which is wasteful or dangerous. Therefore, it is still urgent to develop facile and controllable synthesis methods for rGO/metal nanoparticle hybrid composites.

As reported in literature, thermal reduction of graphene oxide under inert atmospheres would release reductive CO gas.^42,43 Thus, we herein try to explore the reducing power of GO to prepare several kinds of rGO/metal (e.g., Cu, Ni, Co) nanoparticle hybrid composites via a facile thermal reduction method. In this way, GO is used as CO resources for the reduction of metal precursors to form rGO/metal nanoparticle hybrid composites at high temperature under inert atmosphere. By controlling the mass ratio between GO and metal precursors, the particle size and loaded content of the metal nanoparticles can be monitored. We obtained rGO/Cu, rGO/Ni or rGO/Co nanoparticle hybrids via the thermal reduction method, which are meaningful for various areas (e.g., catalysis) because of the facile and controllable synthesis approach and the high dispersion of catalyst nanoparticles on the graphene carrier.

Scheme 1 illustrates the synthesis route of rGO/metal nanoparticle hybrid composites via the thermal reduction method. Firstly, metal ions (e.g., Cu²⁺, Ni²⁺, Co²⁺) were adsorbed onto the GO surface because of the electrostatic interaction between metal ions and the functional groups (carboxyl, hydroxyl and epoxy groups) on GO surface in aqueous solution. Then, metal hydroxides were formed on the surface of GO after the addition of NaOH precipitant. After centrifuged and washed by deionized water for several times, the composites were treated at 500 °C for 90 min under flowing argon. The metal precursors were reduced during the thermal treatment by CO gas released from the thermolysis of GO. The final products were referred as rGO/Cu-x-y, rGO/Ni-x-y or rGO/Co-x-y, in which x-y is the mass ratio of GO to metal nitrate precursors. Full details on the synthesis and characterizations are given in the ESI.†

Scheme 1 Schematic diagram of the formation of rGO/metal nanoparticle hybrid composites via a thermal reduction method.

In the XRD pattern of rGO/Cu-1-2 (Fig. 1a), there are three main peaks at 2θ = 43.3°, 50.4° and 74.1°, corresponding to the miller indices (111), (200) and (220) of metallic Cu with a face-centered cubic structure (JCPDS Card no. 04-0836), respectively. However, there are also some small diffraction peaks which can be ascribed to other copper species, such as Cu₂O (JCPDS Card no. 34-1354) and CuO (JCPDS Card no. 48-1548), indicating that Cu²⁺ is not fully reduced. With the increased mass ratio of GO/Cu(NO₃)₂·2.5H₂O to 2-1 (Fig. 1b), the diffraction intensity of metallic Cu decreases and there are almost no diffraction peaks for Cu₂O and CuO, indicating that copper precursor in this composite was completely reduced to metallic Cu during the thermal treatment under flowing argon at 500 °C for 90 min. In addition, the broad diffraction peak at around 26° can be indexed to the (002) plane of the orderly stacked carbon nanosheets of rGO, indicating the reduction of GO during the thermal treatment.⁴⁴ It is proposed here that the metallic Cu was formed via the reduction of copper precursor by the reductive CO released from the thermolysis of GO. In the synthesis process of rGO/Cu hybrid composites via the thermal reduction method, no external reductive gases (e.g., H₂ and CO) were used and it could provide a facile, safe and economical synthesis method for rGO supported metal composites. What's more, rGO can be retained in the hybrid composites to improve the properties and performance via easily controlling the concentration of reactants.

Fig. 1 XRD patterns for rGO/Cu hybrid composites: (a) rGO/Cu-1-2 and (b) rGO/Cu-2-1.

The TEM images shown in Fig. 2 reveal the particle size and dispersion of metallic Cu in the rGO/Cu hybrid composites. For rGO/Cu-1-2 as shown in Fig. 2a, there are some individual and big Cu or copper oxide particles with size up to several hundred of nanometers (i.e., 500 nm), which may be due to the small mass ratio of rGO/Cu(NO₃)₂·2.5H₂O (i.e., 1-2). The FESEM image shown in Fig. 3a also demonstrates the existence of individually big Cu or copper oxide particles in rGO/Cu-1-2 hybrid composite. In sample rGO/Cu-1-1 (Fig. 2b), even though the particle size of Cu or copper oxide is still not uniform, these nanoparticles decorated completely on the surface and/or inside the rGO framework, which can be attributed to the improved dispersion and chelating ability by the increased mass ratio of GO/Cu(NO₃)₂·2.5H₂O. The FESEM image of rGO/Cu-1-1 in Fig. 3b also clearly shows the non-uniform nanoparticle size in this composite. If further increasing the mass ratio of GO/Cu(NO₃)₂·2.5H₂O to 2-1, it would result in the formation of highly dispersed metallic Cu nanoparticles with a narrow size distribution of 50–100 nm inside the rGO framework, as demonstrated by its TEM (Fig. 2c) and FESEM (Fig. 3c) results. Further increasing the mass ratio of GO/Cu(NO₃)₂·2.5H₂O to 5-1 would lead to the formation of even smaller Cu nanoparticles with size of 5–15 nm on the surface of rGO nanosheets, as shown in Fig. 2d and 3d. These results indicate that highly dispersed Cu nanoparticles with a narrow size distribution into rGO framework can be obtained in sample rGO/Cu-2-1 via the facile thermal reduction method.

Fig. 2 TEM images of rGO/Cu hybrid composites: (a) rGO/Cu-1-2, (b) rGO/Cu-1-1, (c) rGO/Cu-2-1, (d) rGO/Cu-5-1.

Fig. 3 FESEM images of rGO/Cu hybrid composites: (a) rGO/Cu-1-2, (b) rGO/Cu-1-1, (c) rGO/Cu-2-1, (d) rGO/Cu-5-1.

In this research, besides Cu(NO₃)₂·2.5H₂O as precursor for rGO/Cu composite, we also investigated several other transition metal precursors, such as Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O. The XRD patterns of rGO/Ni-1-1 (Fig. 4a) and rGO/Ni-3-1 (Fig. 4b) indicate the formation of metallic Ni with face-centered cubic phase (JCPDS Card no. 03-1051) in these two hybrid composites, further demonstrating the reductive power of GO during the thermal treatment under flowing argon at high temperature. However, there are traces of NiO (JCPDS Card no. 78-0643) in both rGO/Ni-1-1 and rGO/Ni-3-1, which can be attributed to the not high enough calcination temperature or not long enough treatment time for the reduction of Ni²⁺ by CO from the thermolysis of GO.^45,46 The XRD pattern of rGO/Co-3-1 in Fig. 4c also indicates the formation of metallic Co with cubic phase (JCPDS Card no. 15-0806) in this composite. In addition, the diffraction peak at around 26° for the (002) plane of rGO were observed in all these three samples, indicating the formation of rGO via the thermal reduction method.

Fig. 4 XRD patterns: (a) rGO/Ni-1-1, (b) rGO/Ni-3-1, (c) rGO/Co-3-1.

Similar to the results of rGO/Cu and rGO/Ni hybrid composites, decreasing the mass ratio of rGO/Co(NO₃)₂·6H₂O to 1-1 would lead to the incomplete reduction of cobalt precursor, as indicated by XRD results of rGO/Co-1-1 in Fig. 5a. Cobalt monoxide (CoO) (JCPDS Card no. 43-1004) is the main phase in rGO/Co-1-1 hybrid composite via the thermal reduction method after treated at 500 °C for 90 min under flowing argon. Increasing the calcination temperature to 600 °C would increase the formation of metallic Co, however, there is still existence of CoO, which may be due to the limited amount of CO gas released from the thermolysis of GO when the mass ratio of GO/Co(NO₃)₂·6H₂O is 1-1. The particle size of Co/CoO is within 50–200 nm in rGO/Co-1-1 hybrid obtained from 500 °C (Fig. 5b), while the particle size of Co/CoO is even bigger in rGO/Co-1-1 obtained from 600 °C (Fig. 5c) due to the crystal growth with temperature.

Fig. 5 XRD patterns (a) and FESEM images (b and c) of rGO/Co-1-1 composite after calcination at 500 °C (b) or 600 °C (c) for 90 min under flowing argon.

The TEM (Fig. 6a) and FESEM (Fig. 6b) images of rGO/Ni-1-1 indicate the formation of Ni nanoparticles with a wide particle size distribution, which may be due to the poor dispersion ability of the small amount of GO. The HRTEM image inset of Fig. 6a shows clear lattice fringes and the d spacing of 0.19 nm corresponds to the (111) plane of metallic Ni. The TEM (Fig. 6c) and FESEM (Fig. 6d) images of rGO/Ni-3-1 show highly dispersed metallic Ni nanoparticles with size of ca. 10–30 nm on the surface of rGO sheets, which can be attributed to the improved dispersion and chelating ability of increased GO/Ni(NO₃)₂·6H₂O mass ratio. When the mass ratio of GO/Co(NO₃)₂·6H₂O is 3-1, metallic Co nanoparticles with size smaller than 30 nm could also be highly dispersed into the rGO framework, as demonstrated by its TEM (Fig. 6e) and FESEM (Fig. 6f) results. The HRTEM image inset of Fig. 6e shows clear lattice fringes and the d spacing of 0.20 nm corresponds to the (111) plane of metallic Co.

Fig. 6 TEM and FESEM images for rGO/Ni and rGO/Co hybrid composites: (a and b) rGO/Ni-1-1, (c and d) rGO/Ni-3-1, (e and f) rGO/Co-3-1. Inset are HRTEM images.

The synthesis parameters and particle size distribution of these as-prepared samples were summarized in Table 1. The particle size distribution which was evaluated from the TEM and SEM images indicates that the particle size of metal (oxide) nanoparticles decreased but more homogeneous with the increase of the mass ratio of GO/metal nitrate.

Table 1 Synthesis parameters and particle size distribution of samples

Sample name	Mass ratio of GO/metal nitrate	Calcination conditions			Particle size (nm)
		Temperature (°C)	Time (min)	Gas
rGO/Cu-1-2	1-2	500	90	Ar	100–500
rGO/Cu-1-1	1-1	500	90	Ar	50–200
rGO/Cu-2-1	2-1	500	90	Ar	50–100
rGO/Cu-5-1	5-1	500	90	Ar	5–15
rGO/Ni-1-1	1-1	500	90	Ar	10–200
rGO/Ni-3-1	3-1	500	90	Ar	10–30
rGO/Co-1-1	1-1	500	90	Ar	50–200
rGO/Co-3-1	3-1	500	90	Ar	10–30

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To further confirm the reduction of GO, FT-IR spectrum of GO, rGO/Cu-2-1, rGO/Ni-3-1 and rGO/Co-3-1 are characterized, as displayed in Fig. 7. In the FT-IR spectra of GO (Fig. 7a), the characteristic absorption peaks at around 1050, 1400, 1620 and 1720 cm⁻¹ can be ascribed to the stretching vibration of C–O, O–H deformation, the adsorbed H₂O deformation, and C=O in carboxylic acid moieties, respectively.⁴⁷ However, in the FT-IR spectra of rGO/Cu-2-1 (Fig. 7b), rGO/Ni-3-1 (Fig. 7c) and rGO/Co-3-1 (Fig. 7d) obtained by the thermal reduction method, almost no characteristic absorption peaks for C–O, C=O and O–H groups were observed, demonstrating that GO was thermally reduced.

Fig. 7 FT-IR spectra: (a) GO, (b) rGO/Cu-2-1, (c) rGO/Ni-3-1 and (d) rGO/Co-3-1 hybrid composites.

Conclusions

Summarily, in this study, we have demonstrated that GO can be used as solid reductive agent to prepare rGO/metal (e.g., Cu, Ni, Co) nanoparticle hybrid composites with high dispersion via a facile thermal reduction method at 500 °C under flowing argon without using any external reductive agents (such as CO, H₂, NaBH₄). The loading amount and dispersion of metal nanoparticles can be easily controlled by altering the mass ratio of GO/metal precursor. The results in this study could provide a facile and controllable synthesis process for rGO/metal nanoparticle hybrid composite, which would broaden the synthesis approach of rGO related nanomaterials and various applications, such as ORR electrocatalyst.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details including synthesis and characterizations. See DOI: 10.1039/c5ra08670a

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