Existing Form of Zinc Oxide and Phase Transformation for Zinc Oxide Encapsulated in Mesoporous Silica

Nanocomposites of ZnO encapsulated in mesoporous silica were prepared by wetness impregnation and calcination. The samples were characterized by X-ray diffraction, transmission electron microscopy, nitrogen adsorption–desorption isotherms, and X-ray photoelectron spectroscopy. The effects of ZnO content and thermal treatment on the existing form of ZnO as well as phase transformation were investigated. ZnO exists stably in the form of non-crystalline phase or cluster when crystallite size is small. With increasing ZnO content, as the size of ZnO reaches a critical size of crystalline phase, the non-crystalline ZnO or cluster transforms structurally to crystalline ZnO with low energy state. Besides, Zn2SiO4 was obtained by solid-state reaction between ZnO and mesoporous silica. The mesoporous silica acts as not only a reactant but also a diffusion barrier which inhibits the phase transformation from β-Zn2SiO4 to α-Zn2SiO4. The formation temperature of Zn2SiO4 is lower than that of conventional solid-state reaction because of the unique structure of mesoporous silica.