Densifying carbon nanotubes on assembly surface by the self-contraction of silk fibroin

Highlights: • Self-contraction of silk fibroin induces localized densification of carbon nanotubes. • Carbon nanotubes can be densified at and below the micrometer scale. • The tensile strength of CNT fibers and narrow ribbons are increased remarkably. High densification of carbon nanotubes (CNTs) is important for high utilization efficiency of their superior properties in macroscopic assemblies. However, the conventional "top-down" compressing strategies have met problems to modify CNT assemblies at and below the micrometer scale. Here we report a molecular way to strap CNTs together via the self-contraction of silk fibroin (SF) during its drying process, resulting in a localized densification below the micrometer scale. Importantly, after the thermal removal of SF molecules, the densified assembly was well maintained. The SF-induced densification increased the average strength from 355 MPa to 960 MPa for CNT fibers, and from 1.45 GPa to 1.82 GPa for CNT ribbons, which contain much more CNTs on the surface.