Comparison of acid- and base-catalysed sol–gel synthesis for the <i>in situ</i> crystallisation of a perovskite

Confinement of ionic liquids (ILs) within solid matrices <i>via</i> sol-gel methods has been extensively studied to develop novel materials that retain the unique properties of the ionic liquids. In this study, the acid- and base-catalysed sol-gel synthesis of tetraethyl orthosilicate (TEOS) gels incorporating four ionic liquids: 1-butyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate, and 1-hexyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate was investigated. The influence of different methods, including catalysts and molar ratio, on the appearance, morphology and porosity of synthesised ionic silica gels was explored, along with their practical utility to grow bismuth halide perovskites <i>via</i> counter-diffusion in the resulting ionic gels. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen (N₂) gas sorption data showed that the acid-catalysed ionic gels showed a continuous three-dimensional (3D) network and contained a large number of micropores and mesopores (≤20 nm in diameter), whereas the base-catalysed ionic gels consisted of weakly connected nanoparticles with large voids. Powder X-ray diffraction (PXRD) confirmed that crystal formation was observed in all acid-catalysed ionic gels, while no crystals were found in base-catalysed ionic gels. These findings suggest that gels with a continuous solid-phase structure are better suited for crystal growth.