Understanding the role of Zn2+ in surfactant-free layered silicate delamination: Exfoliation of magadiite

We investigated the effect of Zn2+ treatment on the exfoliation of layered silicate magadiite, as a model system, to better understand the role of zinc in surfactant-free zeolite exfoliation. Samples of magadiite and ball-milled magadiite were exfoliated via treatment with aqueous Zn(NO3)2 solution, and the resulting materials were characterized via powder X-ray diffraction, nitrogen physisorption, high-angle annular dark-field scanning transmission electron microscopy, and infrared spectroscopy. Zn2+ treatment of magadiite increases its external surface area by 40%, while Zn2+ treatment of ball-milled magadiite increases its external surface area by 109%. Acid-washing removes Zn(O)x(OH)y colloids and further increases external surface area, leading to a total surface area increase of 150% for magadiite after Zn2+ treatment and acid-washing, and 182% for ball-milled magadiite after Zn2+ treatment and acid-washing, by exposing surface area that was not accessible after Zn2+ treatment due to pore blocking. We propose a mechanism of exfoliation to account for these surface-area increases, which involves the Zn(O)x(OH)y colloids growing and forcing layers apart selectively at the grain boundaries. Ball milling not only makes existing grain boundaries more accessible, but it also creates new grain boundaries, resulting in more efficient exfoliation.