CdS, clusters in zeolites

A recent, well-defined example of such clusters is the existence of CdS superclusters in zeolites (86,87). The authors prepared very small CdS clusters in zeolites. In zeolite Y, for example, there are sodalite cages (5... 

W. Chen, Z. Liu, Z. Wang, Some new observation on the formation and optical properties of CdS clusters in zeolite-Y, Solid State Commun. 100 (1996) 101-104. 

Fig. 13. Diffuse reflectance UV/Vis spectra of bulk CdS and CdS clusters in zeolite Y, after [301]. The spectral intensity at 350 nm increases with increasing loading...

Figure 3. The hyperlattice arrangement of CdS clusters in adjacent sodalite units of the zeolite Y. (hatched = Cd open = S).

Until recent work by Zhen et al. [ 149,150], there has been little evidence for the formation of mixed metal clusters in zeolites. Through single crystal XRD, they studied the reaction of zinc vapour with Cd-X and Tl-X (FAU) and identified a unique range of possible cadmium-zinc and thallium-zinc clusters. Some evidence has also been presented that the reaction of various alkali metals with zeolites containing the cations of a different metal may result in the formation of mixed metal clusters [68, 71, 151], but, for the most part, these products remain poorly characterized. 

In the preparation of zeolite-entrapped CdS, considerable attention has been paid to the ncd/nj ratio. A marked non-stoichiometry of zeolite-hosted metal sulfide particles has been found for CdS nanoparticles by X-ray photoelectron spectroscopy [345]. After sulfidation of a zeolite X sample that is partially ion-exchanged with Cd ions, the binding energies of Cd 3ds/2 electrons decrease by about 0.3 - 0.5 eV in dependence on the diameter of the CdS nanoparticles formed. The shift originates from the replacement of ionic interactions between the Cd + ions and the zeolitic framework oxygen by more covalent (Cd -S ) bonds. However, due to the larger effective masses of the electrons and holes in CdS (m, eff = 0.42 nie, mn, eff = 0.18 m ) [339], the absorption of CdS clusters in the pores of zeohtes is less affected by the zeoUte framework than that of PbS clusters. However, the effect of the zeolite framework on the excited-state relaxation processes, i. e., the luminescence behavior of the CdS clusters, can be very large. 

One can see that with decreasing average thickness of the silica grain boundaries below about 1 run the intensity of the PL strongly decreases. The investigations being currently done in our laboratory should decide if this decrease is due to a delocalization of the electrons and holes. Such an effect has been described for CdS clusters trapped in zeolites [41]. To optimize the electroluminescence from nc-Si films will require to find the optimum compromise. 

Quantization effects have not only been observed with colloidal solutions, but also with nano crystals deposited on Pt or SnOa [38,39] and with semiconductor clusters (CdS,PbS) produced in zeolites [40,41]. The same effect has been found with thin single crystalline semiconductor layers (super lattices) (see e.g. Refs. [42,43]. In this case the quantization occurs only in one dimension, and the shift of energy states is much less, i.e, in the order of 0.1 eV, whereas the shift in a colloidal particle (quantization in 3 dimensions) amounts to several eV. 

Raman spectra gave evidence for quantum size effects in CdS clusters confined in zeolite pores.433 The Raman spectra of nanocrystals of CdSi xSex... 

Since all of the CdS clusters reside in the sodalite cages of the zeolite Y framework, the larger supercages of the structure are still available for absorption of substrate molecules - in this case olefins for photo- oxidation via electron transfer. Colloidal CdS in free solution has been used for such oxidations previously(19) and in a competitive oxidation of styrene and 1,1-diphenylethylene we find that unconfined bulk CdS will effect oxidation in a ratio of 1 2 for these two olefins (irradiation at 365nm). In the zeolite confined system we find however that the ratio becomes 1 1 ie a slight shift in selectivity toward the smaller substrate as may be expected on the basis of size/diffusion effects. From the viewpoint of the enzyme mimic, we have here a system... 

CdS, PbS, CdSe, and ZnS clusters arc the semiconductor compounds studied most extensively in zeolites.[39,40] In addition, ternary systems such as ZnCdS have attracted the interest of many researchers.[41] An ordered array of cadmium clusters assembled in zeolite A was prepared in an attempt to produce heavy metal clusters with different distributions and gcometries.[42] (Fig. 8)... 

The samples were quite stable under the electron beam and their images are consistent with a cubic array of high nuclearity clusters occupying most of the sodalite cages, separated by nearly empty a-cages. However, Cd clusters located not only in the sodalite cages but also in the 8-rings of zeolites result in different properties. [43]... 

Careful analysis of both powder X-ray dilfraction and EXAFS spectroscopic data located the cadmium sulfide as (CdS)4 cubes occupying the space within sodalite cages, with the Cd ions coordinated to framework oxygen atoms (Fig. 6.10). Furthermore, the clusters were observed to order between adjacent sodalite cages, to give superclusters or a superlattice structure. In subsequent work, a variety of compounds and elements have been prepared as well-defined clusters within zeolite frameworks, including metal oxides, selenides and phosphides, and these have been studied mainly with the view of determining the effects of cluster size on optical and electronic properties. 

Figure 4-14. Structures proposed for (CdS)4 clusters in the sodalite cages of zeolite A. [217] Reproduced from Science with permission of the American Assodation for the Advancement of Sdence (see also color plates).

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