Silicon nanowire aqueous dispersions for processing into macroscopic network materials

Nanowires and other high aspect ratio nanoparticles are building blocks to form network materials in formats such as films, sheets, fibres and electrodes that thus bridge the nano and macro scales. The assembly of nanowire network materials is enabled by a new floating catalyst chemical vapour deposition synthesis method that produces crystalline silicon nanowires (SiNW) on a scale of grams per day. Here, we produce SiNW dispersions in water by sonication through steric and electrostatic stabilisation of the negatively charged particles in basic pH or with cationic surfactants. Negative charge arises from the 1.3 nm-thin native oxide layer. Some permanent aggregates are found as a consequence of cross-links between the thin oxide at the surface of adjacent SiNWs. Removing them by centrifugation yields SiNW dispersions of 52 μg mL^-1. Processing into macroscopic materials is demonstrated as transparent films and as freestanding sheets. In the sheets, the SiNWs are predominately aligned parallel to the sheet thickness, as a paper-like SiNW solid with tensile strength above 10 MPa, modulus above 1 GPa and toughness of 0.5 J g^-1.