Ultra-long SiO2 and SiO2/TiO2 tubes embedded with Pt nanoparticles using magnus green salt as templating structures

Abstract

For the first time, magnus green salt (MGS, [Pt(NH3)4][PtCl4]) fibers precipitated by solvent modification have been employed as a structure-directing modifier to synthesize single silica and silica/titania microtubes via a sol–gel process. In the case of titania tubes, tetraethylorthosilicate must be used as a capping agent to hinder the aggregation of primary MGS fibers and to serve as a protective layer against thermal stress during the metal salt fiber reduction. This implies that SiO2/TiO2 tubes result. The synthesized tubular materials were imaged by scanning and transmission electron microscopy, while their composition was determined by energy dispersive X-ray analysis and thermogravimetric analysis. Crystallinity and thermal stability of the tube walls were studied using X-ray diffraction analysis. The obtained oxide tubes possess high aspect ratios (80–200) because they are up to 60 μm in length, but only 300–700 nm in thickness. The key aspects of the synthesis approach are that the templating MGS fibers control the internal diameter of the oxide tubes, while the synthesis conditions control their wall thickness. The suggested method is a simple approach which produces, at low temperatures, very long oxide tubes with a very high amount of Pt (48–51 wt%) directly incorporated inside the tubes. To the best of our knowledge, filling of SiO2 or SiO2/TiO2 nanotubes with such a dense population of Pt metal nanoparticles has not been demonstrated so far; our own experiments with [Pt(NH3)4](HCO3)2 as templating salt formed only tubes containing about 40 wt% Pt and were only about 20 μm long. The now formed more Pt-rich tubes are expected to have vivid applications in (photo)catalysis and in fabricating novel devices, such as nano- or sub-microcables.