New Insights into CdS Quantum Dots in Zeolite−Y
journal contributionposted on 02.04.2020, 14:33 by Nak Cheon Jeong, Hyun Sung Kim, Kyung Byung Yoon
When dry Cd2+-exchanged zeolites Y are exposed to dry H2S 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 (λmax) ≤ 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 λmax 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 λmax > 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.