Design and preparation of a hybrid ferroelectric material through ethylene glycol covalently grafted to Kaolinite

Kaolinite, with a chemical formula of Al₂Si₂O₅(OH)₄, is an abundant and broadly available layered clay mineral. The aluminosilicate monolayer of kaolinite is composed of [SiO]₆ macrorings on one side and gibbsite aluminol groups [Al(OH)₃] on the other side. In this study, ethylene glycol (EG) molecules were covalently grafted to the inner surfaces of kaolinite via etherification between EG hydroxyl and gibbsite aluminol groups to obtain a hybrid material, covalently grafted kaolinite (denoted as K-EG-cg). Commonly, the preparation of K-EG-cg via the conventional heating and stirring method takes longer time (ca. 16 hours); however, only ca. 6 hours are required to achieve K-EG-cg using the solvothermal reaction; moreover, the intercalation efficiency or ratio of the product obtained via the solvothermal reaction is comparable to that obtained using the conventionally heating and stirring method. Infrared spectroscopy, thermogravimetric (TG) analysis, and powder X-ray diffraction were performed; the measurements clearly demonstrate that K-EG-cg is a covalently grafted product and not the result of the physical intercalation of kaolinite with EG (abbr. K-EG). Moreover, the hybrid material K-EG-cg showed much higher deintercalation temperature as compared to K-EG. The dielectrics of K-EG-cg was investigated, indicating that this hybrid material showed intrinsic ferroelectric nature, with the spontaneous polarization P_S ≈ 0.018 μC cm^-2, remanent polarization P_r ≈ 0.015 μC cm^-2, and coercive field E_C ≈ 1.045 kV cm^-1 at room temperature. This study provides a fresh impetus to achieve kaolinite-based hybrid functional materials via the covalent grafting approach, which can overcome the disadvantage of thermal instability of the intercalated kaolinite.