Kaolinite as one of the most widely distributed and commonly utilized minerals has an increasing demand in various fields, including coatings, adsorption, catalysis, rubber, and biomedicine. The utilization of kaolinite is affected by various factors like particle size, layer thickness, and specific surface area. Kaolinite nanosheets as a type of two-dimensional (2D) nanomaterial exhibit novel properties that encompass quantum size effects, surface effects, small size phenomena, as well as macroscopic quantum tunneling effects. The existing preparation process of kaolinite nanosheets has some notable challenges, including high thickness, small specific surface area, and severe damage to the crystal structure, all of which constrain their potential applications in industries. This study introduced a process for the preparation of kaolinite nanosheets, including dimethyl sulfoxideintercalation, ball milling exfoliation, ultrasonic dispersion, and microwave drying techniques. The structural and morphological changes of kaolinite during the DMSO intercalation and subsequent ball milling exfoliation processes were analyzed.

Initially, kaolinite of 5 g was accurately weighed and thoroughly ground in an agate mortar. Afterwards, the ground powder was mixed with 30 ml of DMSO and distilled water in a 100 ml beaker. The mixed suspension in the beaker was stirred magnetically at 66 ℃ for 400 min. After the suspension was centrifugated, the subsequent precipitate was subjected in an oven at 90 ℃ to yield the DMSO-kaolinite intercalation complex (DK). Subsequently, the DK of 10 g was mixed with 20 ml of distilled water. The mixed suspension with sodium hexametaphosphate as a dispersant was ground in a planetary ball mill with zirconia balls of 1-3 mm at 300 r·min^-1 for 3 h. The resulting ground suspension was centrifuged at 4 000 r·min^-1 for 5 min, and the precipitate was rinsed with ethanol for three times. Finally, the sample was placed in a microwave dryer at 700 W for 10 min, yielding kaolinite nanosheets (BDK). The structural and morphological transformations of kaolinite throughout the intercalation and ball milling exfoliation processes were thoroughly characterized by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), BET surface area measurement (BET), scanning electron microscopy (SEM), and atomic force microscopy (AFM).

The XRD patterns of kaolinite nanosheets prepared indicate that DMSO intercalates into the layers of kaolinite, resulting in an expansion of the interlayer spacing to 1.12 nm. The diffraction peak shape of the BDK sample exhibits no significant alterations, indicating the preservation of kaolinite crystallinity with a crystallinity index value of 1.06. The FTIR analysis reveals that DMSO molecules formed bonds with the hydroxyl groups present on the internal surface of kaolinite. The peak intensities and positions of the BDK sample remain unchanged, compared to the raw kaolinite material, signifying the preservation of kaolinite's fundamental crystal structure throughout the process. The SEM and AFM images reveal that the thickness of the BDK nanosheets is diminished to approximately 15 nm, accompanied by a decrease in particle size. The nanosheets exhibit an average length ranging from 500 to 900 nm, distributing uniformly. The BET analysis further demonstrates that the specific surface area increases from 9.28 m²·g^-1 to 26.62 m²·g^-1, having a nearly threefold enhancement. Compared to the grinding method, the DMSO intercalation-ball milling-ultrasonic-microwave drying method achieves a significant exfoliation effect in a shorter time. The kaolinite nanosheets obtained through this process retain the basic crystal structure of kaolinite with a nanoscale vertical dimension (i.e., 1-100 nm). The kaolinite nanosheets prepared through this process can be used as coating fillers.

The kaolinite nanosheets with intact crystal shape, thin layers, and large specific surface area were prepared by DMSO intercalation-ball milling-flaking-ultrasonic dispersion-microwave drying techniques. The optimized parameters for DMSO intercalation and ball milling flaking were intercalation temperature of 66 ℃, intercalation time of 400 min, volume ratio of DMSO to H₂O of 30 ∶ 3, ball milling time of 3.1 h, ball milling rotational speed of 300 r·min^-1, and sodium hexametaphosphate additive of 0.22 g. The average particle size of the kaolinite nanosheets prepared under the optimum condition was 0.77 μm. The characterization analysis showed that DMSO molecules bonded with the hydroxyl groups on the inner surface of kaolinite, and the basic crystal structure of kaolinite was maintained after ball milling. The thickness of ultrafine kaolinite nanosheets reduced to approximately 15 nm, with a uniform distribution of the layers and a well-maintained crystalline morphology. The specific surface area expanded to 26.62 m²·g^-1. The interlayer force of kaolinite was reduced by DMSO intercalation, while maintaining its basic crystal shape. Low-speed ball milling separated the kaolinite layer, while maintaining the intact layer structure.