Facile quantization of semiconductor compounds in the zeolite: characterization of quantum dots of Zn₄S⁶⁺ and Zn₂S²⁺ in zeolite Y

The extra-framework zinc sulfide cationic clusters, Zn₄S⁶⁺ and Zn₂S²⁺, have been introduced into zeolite Y (FAU, Si/Al = 1.56). The |Zn_24.5Na₁₂(Zn₄S⁶⁺)_0.5(Zn₂S²⁺)_5.5|[Si₁₁₇Al₇₅O₃₈₄]-FAU was prepared by allowing aqueous 0.1 M Na₂S solution to flow past a |Zn_37.5|[Si₁₁₇Al₇₅O₃₈₄]-FAU at 294 K for 2 days. Its structure was determined by single-crystal synchrotron X-ray diffraction techniques. The crystallographic study showed that one sulfide ion at the center of the sodalite cavity coordinates to four Zn²⁺ ions at site I’ to give a centered-tetrahedral cationic cluster, Zn₄S⁶⁺, and the other sulfide ion opposite 6-ring in the sodalite cavity bridges between two Zn²⁺ ions at site I’ to give a cationic cluster with bent arrangement, Zn₂S²⁺, in 6.3% and 68.8% of the sodalite cavity of zeolite Y, respectively. Through high-resolution transmission electron microscope image, zinc sulfide quantum dots (QDs) group with the size of one unit cell (ca. 3 nm) in which hundreds were distributed was identified. By using UV–vis diffuse reflectance spectroscopy, it was confirmed that the zinc sulfide QDs occupied in the structure of zeolite Y exhibited enhanced optical activity because of the quantum size effect compared with that of bulk zinc sulfide. In this study, a method for quantizing semiconducting compounds in the cavity of zeolite Y was presented more easily, and its properties were investigated in more depth. The results proved that zeolite could be applied and used as a quantum container for the quantization of semiconductor compounds.