Felted nanotubes form strong networks

Materials World magazine
1 Jan 2018

A wet chemical infiltration process allows carbon nanotubes to be combined with other materials without losing properties, forming a network that can enhance battery power. Kathryn Allen reports. 

Carbon nanotubes (CNTs) can now be combined with other materials without losing valuable properties. CNTs have been felted into a 3D network with other materials using a wet chemical infiltration process, as detailed by a team of researchers from the Functional Nanomaterials working group at Kiel University, Germany, and the University of Trento, Italy, in Nature Communications. 

While CNTs are lightweight, highly conductive and stable, when combined with other materials for industrial applications they can lose some of their beneficial properties or not combine successfully. This latest research saw scientists mix the CNTs with water and drip them onto a porous ceramic material made of zinc oxide, forming a CNT tube-zinc oxide composite. 

This material can be used as filling for conductive plastics, in battery and metal-free filter technology, nanoelectronics, sensors and medical implants. The CNT tubes can also be used as a scaffold material for cell growth as the open pore structure allows cells to grow inside the network. 

The felting process 

This wet chemical infiltration approach offers well-controlled reaction conditions, desired scalability and tuneable composition. However, it also allows a template to be used, which, under normal wet chemistry methods, is not possible. Using this method, the team were able to produce 3D CNT networks with an open pore structure (pore sizes up to several micrometres in diameter), which is beneficial for applications requiring a large and accessible surface area, with the potential for more reactive sites. 

Fabian Schütt, first author of the research paper Hierarchical self-entangled carbon nanotube tube networks and academic staff member at the Functional Nanomaterials Group, explained the process. ‘The wet chemical infiltration process is based on highly porous ceramic networks made of tetrapodal zinc oxide particles having a porosity greater than 93%. These templates have open pores that can be up to several micrometres in diameter. Combining the high porosity of the ceramic template with its superhydrophilic surface properties leads to large capillary forces upon contact with the CNT ink, allowing the liquid to be easily infiltrated and rapidly absorbed. During drying, the CNTs will be deposited on the template in a self-organised fashion.’ 

Schütt notes that, due to the stabilising agents in the CNT dispersion and the thin film of water, surfaces already covered with CNTs are not likely to attach others. Thereby, a thin, self-entangled coating of CNTs is formed around the ceramic template. 

Felting the CNTs in this way also affords the structures high electrical conductivity, due to the use of a template and the self-entangled network surrounding it, as Schütt explains. ‘Due to the interconnected tetrapodal shape, it already forms a 3D percolation network. Therefore, only a few CNTs are needed to form one on this pre-existing network. To achieve the same conductivity without using the template, it would be necessary to align all the CNTs or use a higher amount to form the same number of conductive pathways. Additionally, only a few dead ends – unconnected tetrapodal arms – exist and thus, nearly all the surface of the template contributes to the conductivity.’

The structure also has high mechanical strength due to the self-entangled individual CNTs. The compressive stress applied to the ceramic composite template is converted into tensile stress. Schütt said, ‘The interwoven CNT networks can be compared to the so-called 2D buckypapers – sheets consisting of randomly arranged nanotubes, which are held together only by felting and van-der-Waals forces. Basically, this is what we are doing, but we found a new method to produce buckypaper networks arranged in a 3D fashion. Those buckypapers are well known to be strong in tension, since all nanotubes are interwoven with each other and thus it is hard to pull an individual tube out of the net.’ 

A tube of tubes

The team dissolved the ceramic template via a chemical etching process, using a diluted aqueous hydrochloric acid solution, leaving a 3D network of tubes, each made up of a layer of CNTs. The size of the network produced was a cylinder with a height and diameter of 6mm, but the size is adjustable, up to several centimetres cubed in volume. According to Schütt, ‘The infiltration process is based on strong capillary forces of the porous ceramic template, a homogenously thin and self-entangled CNT coating over several millimetres (up to some centimetres) in penetration depths can be achieved. The size and geometry of the template can be adjusted. Therefore, by using a high-quality dispersion especially designed for this process, a scale-up could be easily realised.’ 

Due to the large hollow space inside these networks, the CNT tubes can be filled with a polymer, allowing them to be combined with other materials, but not alter their molecular structure or lose properties. Furthermore, the infiltration process is not limited to producing CNT tubes, potentially working with other wet-chemically dispersed nanoscale structures, such as graphene or polymers. 

To read the paper in full visit, go.nature.com/2iXUONu