Nanostructures In Zeolite Cages

An example of the efficacy of thermoluminescence to provide information on nanostructures is provided by studies of cadmium sulfide (CdS) clusters introduced into the cages of zeolite-Y. This material, which is found in nature as the mineral faujasite (Na2,CaXAl2SÍ4)Oi2 • 8H20, is cubic in structure with lattice constant a = 2.474 nm. It has a porous network of silicate (SÍO4) and alumínate (AIO4) tetrahedra that form cube-octahedral cages ~0.5nm in diameter, called sodalite cages, since they resemble those found in the mineral sodalite Na4Al3SÍ30i2Cl. The Al and Si atoms are somewhat randomly distributed in their assigned lattice sites. The sodalite cages have entrance windows ~0.25nm in diameter. There are also larger supercages with diameter ~1.3nm, and ~0.75-nm windows. Figure 8.35 shows a sketch of the structure with tetrahedrally bonded Cd^ cubic clusters occupying die sodalite cages, and with the supercage in the center empty.

As the CdS is introduced into the sodalite, it initially tends to enter the sodalite cages shown in Fig. 8.35, but it can also form clusters in the supercages, especially for higher loading. In addition, clusters of CdS in near-neighbor pores can connect to form larger effective cluster sizes. Thus, as the loading increases, the average cluster size also increases. The ultraviolet (200-400 nm) and blue-green-yellow range

Figure 8.35. Sketch of zeolite-Y structure showing six sodalite cages (diameter ~0.5nm) occupied by CdtS« tetrahedral clusters, and one empty supercage (diameter ~1.3nm) in the center. (From H. Herron and Y. Wang, in Nanomateriats, Synthesis, Properties and Applications, A. S. Edelstein, ed., IOP, Bristol, UK, 1996, p. 73.)

Figure 8.35. Sketch of zeolite-Y structure showing six sodalite cages (diameter ~0.5nm) occupied by CdtS« tetrahedral clusters, and one empty supercage (diameter ~1.3nm) in the center. (From H. Herron and Y. Wang, in Nanomateriats, Synthesis, Properties and Applications, A. S. Edelstein, ed., IOP, Bristol, UK, 1996, p. 73.)

Figure 8.36. Reflectance spectra of CdS clusters in zeolite-Y, with CdS loadings of 1,3,5, and 20wt% corresponding to curves 1, 2, 3, and 4, respectively. [From W. Chen et al., J. Lumin. 71, 151 (1997).]

Wavelength(nm)

Figure 8.36. Reflectance spectra of CdS clusters in zeolite-Y, with CdS loadings of 1,3,5, and 20wt% corresponding to curves 1, 2, 3, and 4, respectively. [From W. Chen et al., J. Lumin. 71, 151 (1997).]

Figure 8.37. Dependence of thermoluminescent and phosphorescent intensities of CdS clusters in zeolite-Y on the CdS loading. [From W. Chen et al., J. Lumin. 71, 151 (1997).]

CdS loading(wt%)

Figure 8.37. Dependence of thermoluminescent and phosphorescent intensities of CdS clusters in zeolite-Y on the CdS loading. [From W. Chen et al., J. Lumin. 71, 151 (1997).]

(400-600 ran) optical absorption spectra shown in Fig. 8.36 exhibit a shift to the red (toward longer wavelengths) as the loading increases from 1 to 5% in zeolite-Y, as expected from the increase in the average cluster size with greater loading. At 20% loading the spectrum corresponds to that of bulk CdS, suggesting that some bulk phase has formed outside the zeolite pores. The photoluminescence is low for low CdS loading, increases in intensity as CdS is added, then falls off again for high loadings, as shown in Fig. 8.37. In contrast to this, the thermoluminescence glow curve intensity is highest for low loading, and decreases in intensity as the loading increases, as indicated by the data presented in Figs. 8.34 and 8.37. This is explained by the presence of trapped carriers introduced into the CdS clusters during sample processing. These carriers are detrapped by the thermal energy added to the sample near the temperature 375 K of the thermoluminescence peak in Fig. 8.34, the temperature where the thermal energy kBT equals the trap depth. The smaller clusters associated with low loading have more surface states, and hence more electrons to detrap and contribute to the glow peak. The increase in quantum confinement characteristic of smaller clusters also contributes to the increase in recombination probability, with the resulting enhanced thermoluminescence.

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