Molecular and supramolecular templating of silica-based nanotubes and introduction of metal nanowires

In this article, we describe the development and application of molecular and supramolecular templates for the fabrication silica nanotubes (NTs) and nanowires (NWs) and subsequent metallisation. In one first approach molecular templates such as nanofibres of [Pt(NH₃)₄](HCO₃)₂ salt crystals were used as templates. Pure silica as well as mixed silica/tin oxide NTs with incorporated Pt metal were synthesised. Tubular structures with lengths in the range of 2-5-μm and diameters between 50 and 800-nm were formed. Collaborative effects from templating crystals and a capping agent tetraethylorthosilicate are vital in obtaining Pt embedded SiO₂/stannic oxide NTs. This simple approach incorporates as much as about 40-wt.% of Pt directly inside the NTs. Pt NWs form directly during heat treatment in air by an autoreduction of the templating nanofibres. These NWs exhibit diameters of 15-35-nm and lengths up to 2-μm. Biotemplates such as cellulose nanocrystals isolated from biological sources are efficient supramolecular templates for the fabrication of inorganic one-dimensional structures. The fabrication of biotemplated silica NTs and NWs was performed via sol-gel processing. The biotemplated silica NTs and NWs exhibited diameters of around 15-nm and lengths up to 500-nm. Gold nanoparticles were deposited on the cellulose and SiO₂ coated cellulose nanocrystals to form metal NWs. The gold nanoparticles formed a nearly closed gold shell using weak reducing agents. Transmission electron microscopy and UV-Vis absorption spectroscopy confirmed successful deposition of the metal particles on the SiO₂ NWs. The fabrication of size-controlled metal NWs using Pt-derived molecular nanofibres and cellulose nanocrystals as supramolecular templates is of specific benefit for the design of metal NWs with various sizes and unique optical properties.