New insights into CdS quantum dots in zeolite-Y

When dry Cd²⁺-exchanged zeolites Y are exposed to dry H ₂S under a rigorously anhydrous condition, CdS quantum dots (QDs) are formed in the supercages of zeolite-Y regardless of the loading levels of CdS from 0.01% to 32% and regardless of the Si/Al ratio of zeolite-Y between 1.8 and 2.5. The absorptions with the maximums (λ_max) ≤ 290 nm are assigned as those arising from isolated CdS QDs with the sizes smaller than or equal to the size of a supercage (1.3 nm); the absorptions with λ_max between 290 and 380 nm are assigned as those arising from interconnected CdS QDs that were formed by the interconnection of isolated CdS QDs through the supercage windows; and the absorptions with λ _max > 400 nm are assigned as those arising from mesosized (3-10 nm) CdS QDs residing in or on the surfaces of amorphous aluminosilicate. The H⁺ ions alone, which are generated during the formation of CdS, do not destruct the zeolite-Y framework causing the formation of amorphous aluminosilicate. Instead, the water-induced agglomeration of isolated and interconnected CdS QDs to mesosized CdS QDs in the presence of H⁺ ions leads to the destruction of the zeolite-Y framework. The size of the interconnected CdS QD which is formed by moisture adsorption increases as the loaded amount of CdS increases for a given zeolite and as the size of the zeolite host increases. The presence of a tetraethylammonium ion in each supercage not only gives rise to the formation of very small QDs within zeolites Y but also prevents the zeolite framework from destruction.