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(NO 3 ) 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 Br 2 /MeOH) was greatly enhanced by exchanging the Na + −zeolite with K + after the Te 0 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 0, Te 2-, and Br 2 ). 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.