Transparent, conducting films that are based on carbon nanotubes exploit the mechanical and electrical properties of the tubes for potential applications in areas such as structural coatings and optoelectronic devices. Geon-Woong Lee and colleagues at the Korea Electrotechnology Research Institute in Changwon1 have used a simple method to make films from carbon nanotubes and a silane binder that have wettability that can be tuned by changing the binder’s chemical structure. The combined properties of the films suggest that they could have extended uses for self-cleaning coatings, transparent film heating or electromagnetic interference shielding, amongst other applications.

The researchers functionalised multiwalled nanotubes (MWNTs) with hydroxyl groups and prepared a silane sol by sol-gel reaction of two silane compounds in water and ethanol. One silane compound contained only hydrophilic groups to provide mechanical strength and hydrophilic properties to the finished film. The other silane also contained an alkyl chain functionalised with many fluorine atoms, well known for their hydrophobic properties.

Fig. 1: Superhydrophobic conducting film of carbon nanotubes in a silane binder.

The two preparations were mixed together to produce a dispersed solution with the same appearance as ink. This solution was then spray-coated onto a surface to give a porous film of an MWNT network wrapped in silanes (Fig. 1). The addition of the silane binder produced a network of individual nanotubes, rather than bundles, which were obtained when no binder is used.

Lee and colleagues found that the both electrical resistance and transparency decreased as the film thickness increased. Additionally, the ratio of binder to MWNTs affected the sheet resistance.

The hydrophobicity of their films was increased by spraying more ink on the substrate. This is thought to be due to the creation of greater roughness, which is known to increase hydrophobicity. Adding silica nanoparticles to the ink also enhanced the hydrophobicity of the films. To make the films superhydrophilic, the researchers simply removed the fluorinated silane from the original recipe.

Joong Tark Han thinks there are exciting prospects ahead for their highly water repellent, conductive and transparent films, such as “transparent superhydrophobic film heaters, which could be used as automobile glass.” This group is working to commercialize the work in the near future.