CNT films indicate conductivity advantages of incorporating metal nanoparticles

Flexible conductive sheets are expected to be used in elastic electronic equipment due to their superior conductivity and formability. In a study published in Scientific ReportsFour kinds of stretchable conductive sheets were made by combining and dividing metal nanoparticles on the surface of carbon nanotubes in a polydimethylsiloxane framework.

Study: Modeling and characterization of electrical conductivity on metal nanoparticles/carbon nanotubes/polymer composites† Image Credit: Rost9/Shutterstock.com

Preserving flexibility of carbon nanotubes

Carbon nanotubes (CNTs) are commonly used in electronic equipment, transducers, sensors and biomaterials due to their exceptional electrical, mechanical and thermal properties. Several stretchable conductive composites based on carbon nanotubes and flexible polymers such as polydimethylsiloxane (PDMS) have been thoroughly investigated.

The key to producing elastic polymeric composites with high conductivity is to develop a carbon nanotube conductive framework in a nearly isolated polymeric matrix. Increasing the CNT concentration of polymer composites can generally improve their conductivity, but it will ultimately decrease their flexibility.

Increasing the conductivity of carbon nanotubes rather than increasing their quantity is a more economical technique for preserving the flexibility of composites.

Ensuring high conductivity of carbon nanotubes for good flexibility

Numerous studies have been published so far on increasing the conductivity of nanoscale carbon nanotube/polymer composites. Adding metal nanoparticles (NPs) to the polymer framework is one of several techniques to increase the conductivity of the composite.

Another viable way to increase the conductivity of a carbon nanotube system is to coat it with high conductivity alloy or metal particles (such as gold and copper). Unfortunately, gold is too expensive for scalable use.

On the other hand, copper is considerably cheaper, but it is difficult to control the size of its nanoparticles (< 50 nm) and it is easily oxidized. The amount of gold used in synthesizing a gold-copper (AuCu) alloy can be reduced while maintaining the same percentage of coverage.

The size of nickel nanoparticles synthesized on CNTs can be precisely regulated. Only a sufficient number of metal nanoparticles deposited on the CNT surface can ensure good conductivity of the carbon nanotube system.

Schematic diagram of (a) Au/CNT and AuCu/CNT preparation process, (b) Ni/CNT preparation process, (c) Cu/CNT preparation process, and (d) Metal/CNT/PDMS films preparation process. © Wang, Y., Lu, S. et al. (2022).

Research methodology

This study fused nickel, gold, copper and AuCu alloy NPs on the surface of carbon nanotubes. The resulting metal/CNT was then dispersed in a polydimethylsiloxane matrix to create an elastic metal/CNT/PDMS composite sheet.

X-ray diffraction (XRD), scanning tunneling electron microscopy with high-angle annular dark field assays (HAADF-STEM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to analyze the microscale architecture of the samples.

The electrical conductivity of the resulting composites was determined using a four-point approach. Metal/CNT/PDMS elastic sheets showed electrical conductivity approximately two orders of magnitude higher than that of CNT/PDMS elastic sheets.

A computational framework based on the Metropolis methods and percolation theory was used to investigate the effect of metal nanoparticles on the conductivity of metal/CNT/PDMS sheets.

The main parameters influencing the conductivity of the developed metal/CNT/polymer composite were investigated based on the experimental and computational findings. In addition, the model estimated the electrical conductivity information of elastic composites by combining different metal or alloy NPs and modifying different frame characteristics of carbon nanotubes.

Key findings of the study

By combining uniformly distributed metal nanoparticles on the surface of carbon nanotubes, Au/CNT, Cu/CNT, Ni/CNT and AuCu/CNT were effectively produced. The findings of HAADF-STEM and XRD indicated that AuCu alloy particles with similar atomic ratios were fused on the surface of the nanotube.

Based on these findings, conductive elastic sheets on PDMS substrate with varied carbon nanotube loadings were produced. The findings of electrical conductivity testing revealed that the majority of metals/CNT/PDMS sheets produced had greater electrical conductivity as compared to CNT/PDMS sheets.

The addition of metal nanoparticles increased the electrical conductivity of carbon nanotubes and the tunneling effect at nanotube junctions. Unlike simple CNT junctions, electrons must penetrate metal nanoparticles from the nanotube at metal/CNT interfaces and then tunnel through the polymeric insulation layer to get to the next nanotube.

The computational findings revealed that the inherent conductivity and aspect ratio of CNTs, and the inherent conductivity, size and coverage ratio of metal nanoparticles were the primary parameters influencing the electrical conductivity of the metal/CNT/polymer sheet.

Numerical findings also showed that the conductivity of elastic composites can exceed 100 S/m in Au/CNT/PDMS, Cu/CNT/PDMS, AuCu/CNT/PDMS, Ag/CNT/PDMS or CuAg/CNT/PDMS films when the carbon nanotube concentration is eight wt%, opacity ratio of the metal nanoparticles is 100%, and the inherent conductivity of the nanotubes is 106 S/m.

Reference

Wang, Y., Lu, S. et al† (2022). Modeling and characterization of electrical conductivity on metal nanoparticles/carbon nanotubes/polymer composites. Scientific Reports† Available at: https://doi.org/10.1038/s41598-022-14596-x

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