The incorporation of ionic liquids in the two-step Stӧber process of gel formation and its application to perovskite crystal growth

Single-step catalysis has been extensively studied for the synthesis of silica ionogels. In contrast, the acid–base two-step catalysis process, also known as the two-step Stöber method, has attracted comparatively less attention. In this study, the two-step Stӧber synthesis of tetraethyl orthosilicate gels and ionogels incorporating eight imidazolium-based ionic liquids (ILs) are investigated. The applicability of the two-step Stӧber method for ionogel preparation, the influence of different ILs on the resulting gels, and the practical utility of these ionogels for counter-diffusion crystal growth were explored. Gelation results indicated that precursors synthesised with ethyl-imidazolium-based ILs failed to produce homogeneous gels due to reduced mutual solubility within the ethanol-rich solvent system. In contrast, hard, transparent gels were formed in the remained experiments incorporated with butyl-, hexyl- and octyl-imidazolium-based ILs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results revealed that both acid and base catalysts contributed to surface modification and morphology, as characteristics typical of both acid- and base-catalysed gels were observed within the same sample. Powder X-ray diffraction (PXRD) confirmed high crystallinity and purity for all crystals obtained from the gels, with minor impurity peaks detected in two experiments. These findings demonstrate that the two-step Stöber approach is an effective method for the preparation of silica ionogels, and that the resulting gels exhibit potential for application in counter-diffusion perovskite crystal growth. Graphical abstract: This composite image shows transmission electron microscopy of silica ionogel prepared via the two-step Stӧber process. The results show that both acidic and basic catalysts contribute to the morphology of final products, the typical neck-formation feature and large chunks of gel prepared in acidic condition as shown in (a) and (d), and the spherical nanoparticle formed in basic environment revealed in (b) and (c).