Solid-state reduction of silica nanoparticles via oxygen abstraction from SiO4 units by polyolefins under mechanical stressing

Abstract

Metal oxides with an oxidation number lower than the highest often exhibit attractive functional properties. However, conventional chemical or thermal reduction of the stable oxides is often laborious and cannot be stopped at an appropriate level of reduction. Therefore, we here try to explore non-conventional reduction processes in a solid-state without external heating. Unique features of reduction processes of SiO₂ toward suboxides, SiO_x (1 ≤ x < 2), were made possible by milling fumed silica nanoparticles with polyolefins (POL), i.e., polypropylene (PP) or polyethylene (PE) and a fluorine-containing one, polyvinylidene difluoride (PVDF). We mainly examined the electronic and coordination states of Si by Si2p XPS spectra and ²⁹Si MAS NMR, respectively. They significantly differ from a similar commercial product obtained via a thermal route. Judging from the chemical shift of ²⁹Si MAS NMR as a criterion of the degree of reduction of SiO₂, the function of POL as a reductant is in the order PP ≈ PE > PVDF. Since the present solid-state reaction is free from the formation of unstable gaseous SiO as an intermediate, the products are free from the Si component in a Q⁰ state close to that of metallic Si. From these results we conclude that the present silicon suboxides obtained by co-milling silica with POL are closer to those defined as a random bonding model of SiO, than a random mixture model, the former being unachievable by a thermal process. The main mechanism of the present solid-state reduction is the oxygen abstraction from the SiO₄ units by the polarized POL, with its simultaneous oxidative decomposition up to the state of carbon. The reaction process is simple and scalable so that it may offer a new affordable fabrication method of silicon suboxides.