"Liquid-phase calcination" of colloidal mesoporous silica nanoparticles in high-boiling solvents

We report on a novel high temperature liquid phase "calcination" method with trioctylphosphine oxide (TOPO), tri-n-octylamine (TOA), and squalene for removing the template and strengthening the silica network in colloidal mesoporous silica (CMS) nanoparticles. For such materials, the common calcination procedure in air would result in strong agglomeration, thus preventing their use in colloidal suspensions. The highest efficiency of the new approach is obtained by thermal calcination in TOPO at only 275 °C, as shown by an increasing degree of silica condensation, and the retention of the high colloidal stability of the CMS nanoparticles. Moreover, we also show the ability of the TOPO treatment to remove the template, thus saving a preparation step. The resulting CMS nanoparticles retain the ordered mesostructure, high porosity, and large surface area of the original mesoporous nanoparticles, while showing a much greater degree of silica condensation and high stability. The concept of "liquid calcination" represents a powerful general approach for the preparation of stable colloidal porous nanoparticles.