Stabilization of intrazeolitic cadmium telluride nanoclusters by ion exchange

Cadmium telluride nanoclusters were prepared by vapor-phase deposition of elemental tellurium in Na⁺-zeolite A, followed by partial exchange of the zeolite with aqueous Cd-(NO₃)₂, and reduction with hydrogen at 450 °C. The stability of the nanoclusters in environments that normally cause rapid Ostwald ripening or oxidation (air, water, and Br₂/ MeOH) was greatly enhanced by exchanging the Na⁺-zeolite with K⁺ after the Te° deposition and hydrogen reduction steps. Exchange of K⁺ for Na⁺ narrows the effective pore diameter of zeolite A from 4.0 to 3.3 Å, inhibiting the diffusion of larger atoms, ions, and molecules (Te°, Te^2-, and Br₂). Distinct absorption maxima in diffuse reflectance UV-visible spectra and sharp exciton peaks in low-temperature excitation spectra verified the presence of quantum-confined CdTe. These spectral features are largely unchanged when the material, in its K⁺-exchanged form, is exposed to air and water for periods of months. Under the same conditions, materials in the Na⁺ form are rapidly degraded. TEM micrographs of the K⁺-exchanged materials show 20-50 Å diameter nanoclusters dotted throughout the zeolite matrix. The partial loss of host crystallinity observed in X-ray diffraction patterns suggests that the process of cluster formation involves aggregation within the large cages of the zeolite and local destruction of the pore network.