A flexible sandwiched graphite nanoplatelets/copper nanowires/graphite nanoplatelets paper with superior electrical conductivity

Abstract

Flexible graphite nanoplatelets (GNPs)–copper nanowires (Cu NWs) composites of a sandwich-like structure were prepared by a simple filtration method. Cu NWs were sandwiched between GNPs films (G/NWs/G). The sandwiched structure results in conductivity up to 3.18 × 10⁵ S m⁻¹, higher than the reported graphene based papers and bulk graphite. The performance of the sandwiched papers is further improved to 8.65 × 10⁵ S m⁻¹ through mechanical pressing. Compared to the pure Cu NWs film, the sandwiched composite is oxidation resistant and stable in air, thus promising as candidates for high performance flexible electronics.

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Flexible and highly conductive paper-like materials have attracted much interest for various applications, such as energy storage devices^1–3 and electrodes.^4,5 Graphene based two dimensional materials, such as graphite nanoplatelets (GNPs), graphene oxide (GO) and reduced graphene oxide (rGO) have been popular starting materials for preparing highly conductive and flexible papers^6–11 because of their excellent electronic properties along the graphene layer.¹² GNPs obtained from acid intercalation and thermal expansion^13,14 were used to form conductive papers by filtering aqueous suspensions. The reported conductivity of GNPs based paper (5.00 × 10⁴ S m⁻¹)⁸ is much lower than that of graphene layer (2.00 × 10⁵ S m⁻¹),¹⁵ due to the incomplete exfoliation of GNPs and contact resistances between flakes. Moreover, the surfactants added into the aqueous suspensions for dispersing GNPs could degrade the electronic performance. Graphene oxide (GO), dispersing easily in water without additives, is widely used as precursor for rGO to prepare flexible and conductive papers. Thermal annealing and/or chemical reduction are required for restoring electronic conjugation to receive high conductivity. The performance of rGO based papers varies from 3.51 × 10⁴ S m⁻¹ to 1.39 × 10⁵ S m⁻¹, which is limited by the residual defects and oxygen functional groups.^6,9 It is reported that the reduction of GO could degrade the flexibility of produced films.⁶ In addition, the oxidation and reduction steps increase the complexity and costs of processing.

To further improve the conductivity of graphene based films, 1D metal nanowires are introduced.^16–18 Metal nanowires with high conductivity are widely used as transparent conductors and electrodes.^19–22 However, the fresh metal nanowires are easy to be oxidized in air, degrading the electrical performance.

Here we come up with novel sandwiched graphite nanoplatelets/copper nanowires/graphite nanoplatelets (G/NWs/G) paper with superior electrical conductivity through a simple filtration method. The sandwiched structure is chosen for two reasons: firstly, GNPs films not only provide extra pathways for electrons transfer, but also block the contacts between Cu NWs and oxygen, leading to oxidation resistance and stability of electrical properties; second, compared to the randomly hybrid structure, the sandwiched structure reduces the contact resistances as the electron scattering from interfaces between the Cu NWs and the GNPs is restricted greatly.

GNPs of large quantity were prepared by shear-assisted supercritical CO₂ exfoliation according to our previous work.²³ The large scale of the novel exfoliation method makes it ideally suited for many applications requiring a large amount of GNPs, such as effective fillers in composites^13,14 and paper-like materials with thickness varying from micrometers to millimeters.^8,10 The GNPs have been further treated in combinations of alcohol–water and acids in order to obtain well dispersed aqueous suspension according to the reported method.^10,24,25 The morphologies of produced GNPs are shown in Fig. 1. The scanning electron microscope (SEM) image (Fig. 1a) indicates thin layers with high aspect ratio. The number of graphene layers has been investigated by transmission electron microscope (TEM) (Fig. 1b). 90% of the produced exfoliated sheets are less than 10 layers and 70% are between 5 and 8 layers, which has been described elsewhere in our previous report.²³

Fig. 1 A SEM image (a) and a TEM image (b) of exfoliated graphite nanoplatelets.

Long Cu NWs sandwiched between GNPs films were synthesized by a hydrothermal method in large scale.²⁶ Glucose was used to reduce copper chloride under hexadecylamine (HDA) in aqueous environment. The SEM images present Cu NWs with high aspect ratio, length of tens micrometres and diameter from tens to hundreds nanometers (Fig. 2a and b), thus forming efficient percolating network for electrons transferring. X-ray diffraction (XRD) was conducted to investigate the microstructure of the products. Three diffraction peaks at 43.4°, 50.6°, and 74.3° correspond to the {111}, {200}, and {220} planes of face centered cubic (fcc) copper, agreeing well with the reports in previous literature.^22,26 No peaks from Cu₂O, CuO or other contaminations were observed, demonstrating the high purity and high quality of fresh products. The color of Cu NWs become dark after several days, while {111} planes of Cu₂O are detected by XRD, which could result from the oxidation of Cu NWs by the oxygen in air. The formed Cu₂O layer could degrade the electronic properties greatly.²²

Fig. 2 (a) Low magnification and (b) high magnification SEM images of prepared Cu NWs. (c) XRD of fresh prepared Cu NWs and Cu NWs exposed to air.

A simple sequential filtration method has been developed to prepare the G/NWs/G sandwiched paper. The scheme of the synthesis process is presented in Fig. 3a. GNPs paper with thickness of around 40 μm was formed by filtering the aqueous suspension produced before through a PVDF membrane (0.45 μm pore). After that, Cu NWs suspension in ethanol was filtered followed by a top GNPs film with thickness of around 20 μm. The formed sandwiched film was peeled off and dried in vacuum oven followed by annealing under H₂ (20% H₂ and 80% Ar) at 400 °C for 1 h. The annealing process is significant for reducing the junction resistance of Cu NWs, which could be attributed to the removal of organic residues and a thin oxide layer.²¹ The produced film is freestanding, uniform and flexible to bend into large angles, as shown in Fig. 3b. The cross section of the products was observed under SEM. Three layers of the sandwiched structure are clearly observed (Fig. 3c). The middle Cu NWs network provides percolating electrically conducting network, being protected by the bottom and top GNPs from oxygen in air. The Cu NWs and GNPs were further examined under the high magnification SEM, as presented in Fig. 3d and e.

Fig. 3 (a) Scheme of synthesis process for the G/NWs/G paper. (b) A micro image of the produced sample. (c) A SEM image of the cross section of the sandwiched paper. (d) A SEM image of the middle Cu NWs layer. (e) A SEM image of the GNPs films as the bottom layer.

The Cu NWs with different amounts were used to prepare a series of papers of conductivities measured by the four probe method with Kunde KDY-1 system (Guangzhou, P. R. China). With low Cu NWs loadings, the percolating network is not formed, and the conductivity of Cu NWs films is quite low. Therefore, the conductivity of samples is mainly from GNPs films (∼4.26 × 10³ S m⁻¹), as exhibited in Fig. 4a. When the Cu NWs form efficient pathways for electrons at around 10% vol, the resulting conductivities increase rapidly with the increase of Cu NWs amounts. The conductivity reaches up to 3.18 × 10⁵ S m⁻¹ with 25% vol Cu NWs (Fig. 4a). The GNPs at the bottom and top layers orientate randomly according to the cross section of SEM images (Fig. 3c and e), which could degrade the in-plane performance of the paper since electrons transfer along the graphene layer. In addition, the random orientations introduce pores, resulting loose stacking and electrons scattering. In order to align the GNPs, the films were further pressed under 5 MPa for 30 s.

Fig. 4 Conductivities of (a) as prepared and (b) pressed G/NWs/G papers measured as a function of Cu NW loading. (c) Conductivities of G/NWs/G and Cu NWs films as a function of time.

The thicknesses of the sandwiched samples before and after compression have been measured (see ESI†). The thickness change of GNPs film is more than that of the Cu NWs film, which indicates that the GNPs layer is aligned effectively. It is reported that the alignment of GNPs is helpful for improving the tensile modulus and strength of the paper.⁸ The conductivities of the same sample before and after pressing have been measured (Fig. 4a and b). After compression, the volume loadings of Cu NWs show an increase because of the change of sample densities (see ESI†). The conductivity of the aligned GNP films increased to 2.27 × 10⁴ S m⁻¹, and the performance of the pressed sandwiched sample is higher than that of the as prepared samples (Fig. 4b), that is 8.65 × 10⁵ S m⁻¹, much higher than the reported graphene based conductive papers (Table 1). The reported GNPs paper has a conductivity of 5.00 × 10⁴ S m⁻¹.⁸ The performance is improved to 8.50 × 10⁴ S m⁻¹ for the acid treated GNPs.¹⁰ The reported highest conductivity of rGO paper is 1.83 × 10⁵ S m⁻¹, achieved by electro-spray deposition followed by high temperature and pressure treatments.¹¹ 1D silver nanowires have been used to randomly combine with graphene sheets, leading to the conductivity (3.19 × 10⁵ S m⁻¹)¹⁶ similar to that of our sandwiched paper. In terms of the reported randomly hybrid paper, the CVD grown graphene flakes of 1.09 × 10⁵ S m⁻¹ were used, and the conductivity increases around 3 times by hybridization with the silver nanowires. Although the performance of the CVD graphene is high, the production process is highly cost. For the fresh G/NWs/G paper, the conductivity increases more than 70 times after introducing the Cu NWs networks. The large enhancement of the sandwiched sample indicates that the sandwiched structure could be more effective for improving electrical performance compared to the randomly hybrid structure since the contacts resistances from electrons scattering at the interface of 1D metal nanowire and GNPs is reduced. Stability of the G/NWs/G paper was compared with that of the exposed Cu NWs film (Fig. 4c). After being exposed in air for 5 days, the conductivity of the sandwiched paper remains stable, while the conductivity of Cu NWs film drops because of the oxidative layer formed by oxygen.

Table 1 Electrical conductivities of the graphene based papers

Starting materials	Preparation	Conductivity (S m^−1)	Reference
rGO	Filtration; annealing	4.45 × 10^4	7
rGO	Filtration; annealing	3.51 × 10^4	6
rGO	Filtration	1.39 × 10^5	9
rGO	Electro-spray deposition	1.83 × 10^5	11
GNPs	Filtration	5.00 × 10^4	8
Acid treated GNPs	Filtration	8.50 × 10^4	10
CVD graphene, silver nanowires	Filtration: hybrid randomly	3.19 × 10^5	16
GNPs, copper nanowires	Filtration: sandwiched structure	8.65 × 10^5	Our work

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Conclusions

In summary, the flexible conductive papers composed of graphite nanoplatelets and 1D metal nanowires with a novel sandwiched structure were prepared, with the highest conductivity among the previously reported graphene based papers. Cu NWs provide percolating electrically conducting network. Besides acting as electron pathways, GNPs protect the Cu NWs from the air, which leads to the G/NWs/G paper of stable performance. The prepared samples are promising in applications such as flexible energy conversion and storage devices.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21322609, 21576289 and 51401239), the Ministry of Science and Technology of China (no. 2011CBA001000), the Science Foundation Research Funds provided to New Recruitments of China University of Petroleum, Beijing (2462014QZDX01) and Thousand Talents Program.

Notes and references

1. L. Hu, J. W. Choi, Y. Yang, S. Jeong, F. L. Mantia, L. Cui and Y. Cui, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 21490–21494.
2. X. M. Liu, Z. D. Huang, S. W. Ho, B. Zhang, P. C. Ma, M. M. F. Yuen and J. K. Kim, Compos. Sci. Technol., 2012, 72, 121–144.
3. J. Zhang, X. Wang, J. Ma, S. Liu and X. Yi, Electrochim. Acta, 2013, 104, 110–116.
4. C. Wang, D. Li, C. O. Too and G. G. Wallace, Chem. Mater., 2009, 21, 2604–2606.
5. J. Xiao, L. Wan, S. Yang, F. Xiao and S. Wang, Nano Lett., 2014, 14, 831–838.
6. H. Chen, M. B. Muller, K. J. Gilmore, G. G. Wallace and D. Li, Adv. Mater., 2008, 20, 3557–3561.
7. W. Huang, X. Ouyang and L. L. Lee, ACS Nano, 2012, 6, 10178–10185.
8. H. Wu and L. T. Drzal, Carbon, 2012, 50, 1135–1145.
9. X. Lin, X. Shen, Q. Zheng, N. Yousefi, L. Ye, Y. W. Mai and J. K. Kim, ACS Nano, 2012, 6, 10708–10719.
10. Z. L. Hou, W. L. Song, P. Wang, M. J. Meziani, C. Y. Kong, A. Anderson, H. Maimaiti, G. E. LeCroy, H. Qian and Y. P. Sun, ACS Appl. Mater. Interfaces, 2014, 6, 15026–15032.
11. G. Xin, H. Sun, T. Hu, H. R. Fard, X. Sun, N. Koratkar, T. B. Tasciuc and J. Lian, Adv. Mater., 2014, 26, 4521–4526.
12. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
13. X. Tian, M. E. Itkis, E. Bekyarova and R. C. Haddon, Sci. Rep., 2013, 3, 1710.
14. A. Yu, P. Ramesh, M. E. Itkis, E. Bekyarova and R. C. Haddon, J. Phys. Chem. C, 2007, 111, 7565–7569.
15. Z. S. Wu, W. Ren, L. Gao, J. Zhao, Z. Chen, B. Liu, D. Tang, B. Yu, C. Jiang and H. M. Cheng, ACS Nano, 2009, 3, 411–417.
16. J. Chen, H. Bi, S. Sun, Y. Tang, W. Zhao, T. Lin, D. Wan, F. Huang, X. Zhou, X. Xie and M. Jiang, ACS Appl. Mater. Interfaces, 2013, 5, 1408–1413.
17. I. N. Kholmanov, S. H. Domingues, H. Chou, X. Wang, C. Tan, J. Y. Kim, H. Li, R. Piner, A. J. G. Zarbin and R. S. Ruoff, ACS Nano, 2013, 7, 1811–1816.
18. I. N. Kholmanov, C. W. Magnuson, A. E. Aliev, H. Li, B. Zhang, J. W. Suk, L. L. Zhang, E. Peng, S. Hossein Mousavi, A. B. Khanikaev, R. Piner, G. Shvets and R. S. Ruoff, Nano Lett., 2012, 12, 5679–5683.
19. A. R. Rathmell and B. J. Wiley, Adv. Mater., 2011, 23, 4798–4803.
20. D. Zhang, R. Wang, M. Wen, D. Weng, X. Cui, J. Sun, H. Li and Y. Liu, J. Am. Chem. Soc., 2012, 134, 14283–14286.
21. H. Im, S. H. Jung, J. Jin, D. Lee, J. Lee, D. Lee, J. Y. Lee, I. D. Kim and B. S. Bae, ACS Nano, 2014, 8, 10973–10979.
22. S. Han, S. Hong, J. Ham, J. Yeo, J. Lee, B. Kang, P. Lee, J. Kwon, S. S. Lee, M. Y. Yang and S. H. Ko, Adv. Mater., 2014, 26, 5808–5814.
23. L. Li, J. Xu, G. Li, X. Jia, Y. Li, F. Yang, L. Zhang, C. Xu, J. Gao, Y. Liu and Z. Fang, Chem. Eng. J., 2016, 284, 78–84.
24. L. M. Veca, M. J. Meziani, W. Wang, X. Wang, F. S. Lu, P. Y. Zhang, Y. Lin, R. Fee, J. W. Connell and Y. P. Sun, Adv. Mater., 2009, 21, 2088–2092.
25. L. L. Tian, P. Anilkumar, L. Cao, C. Y. Kong, M. J. Meziani, H. J. Qian, L. M. Veca, T. J. Thorne, K. N. Tackett, T. Edwards and Y. P. Sun, ACS Nano, 2011, 5, 3052–3058.
26. M. Mohl, P. Pusztai, A. Kukovecz and Z. Konya, Langmuir, 2010, 26, 16496–16502.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c5ra23070e

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