Close-packed colloidal SiO₂ as a nanoreactor: Generalized synthesis of metal oxide mesoporous single crystals and mesocrystals

We report a generalized "immobilized crystallization in silica nanoreactor" (ICSR) strategy for the synthesis of an extensive series of well-defined and high-quality metal oxide mesoporous single crystals (SnO₂, TiO₂, and CeO₂ MSCs) and mesoporous mesocrystals (CeO₂ and ZrO₂ MMCs) with varying morphologies, sizes, and phases. Close-packed colloidal SiO₂ is used as a nanoreactor, a peculiar reaction medium in which immobilized nucleation and crystallization have been systematically studied. High hydrophilicity of residual Si-OH groups facilitates surface adsorption and pore filling of precursor solution, leading to spontaneous nucleation and subsequent crystal growth in the reactor template. The silica template merely serves a faithful negative replication without interfering in the crystallization process but with an added advantage of avoiding crystal aggregation without the need for surfactant. The universality of the ICSR strategy is demonstrated by synthesizing MSCs and MMCs of different materials and different pore sizes. The great value of the as-obtained MSCs and MMCs is exemplified by a case study on the conspicuous gas-sensing activities of the SnO₂ MSCs. With 3D-connected mesopores and a single-crystalline framework, the highest gas-sensing activity is achieved when high-energy facets are maximally exposed. Overall, this work has provided insights and strategies for the rational fabrication of MSC and MMC materials and opened unprecedented opportunities for studying their structure - property relationship.