Synthesis of Ni@SiO₂ Nanotube Particles in a Water-in-Oil Microemulsion Template

SiO2 nanotubes containing Ni nanoparticles were synthesized in a template nonionic surfactant water-in-oil (w/o) microemulsion, and were characterized by bright-field TEM. The highly defined cylindrical nanotube cavities had a uniform diameter of 12–13 nm; however cavity lengths were shown to be strongly dependent on aging time before silica precursor addition, hydrazine concentration, and synthesis temperature. Depending on synthesis conditions, Ni@SiO2 (i.e., core@shell) nanotubes attained lengths up to 2 μm. A hypothesis is advanced that gas phases form and remain entrained in reverse-micelles, effecting both their elongation and also the exclusion of agglomerating silica species to outer micellar regions, thereby causing cavity formation. SiO2 shell thicknesses could be controlled in the range of 5.1 to 12.4 nm by simple variation of the amount of silica precursor used. Furthermore, H2 chemisorption measurements demonstrated 14–15% Ni dispersions, and N2 physisorption analyses showed intraparticle pores on the order of 1 nm. These results confirm the accessibility of active metal sites via diffusion through the porous silica shells. Therefore the reported Ni@SiO2 materials make an interesting addition to the developing class of M1@M2Ox yolk–shell catalytic materials.