Semiconductor And Conductive Polymer Nanostructures Embedded
Semiconductor and Conductive Polymer NANOStructures Embedded
in Zeolite and Mesoporous Hosts
G. Tel’biz1, O. Shvets1, V. Ill’in1, M. Brodyn2, V. Voznyi2
1Institute of Physical Chemistry, National Acad. Sci. of Ukraine,
252039, prospekt Nauky 31, Kyiv, Ukraine
2Institute of Physics, National Acad. Sci. of Ukraine,
252022, prospekt Nauky 46, Kyiv, Ukraine
Conductive polymers (polyaniline) and semiconductor clusters (CdS, PbI2) have been synthesised in the three- and one-dimensional channels of zeolite FAU, MCM-41 and LTL. Stoichometetric, IR, and absence of bulk conductivity of the polymer/zeolite samples lead to the conclusion that polymer is formed insuide the host channei system. The polyaniline/zeolite and PbI2/zeolite represent a new class of materials containing semi- or conductor encapsulated in crystalline inorganic hosts with channel of molecular dimensions.
Host-guest interaction processes in the chemistry of zeolites are gaining increasing attention as powerful tools to incorporate various species into lattice cages [1-3]. The design and understanding of well-defined structures of nanometer dimensions is one the most challenging goals of contemporary solid state science. In particular, fundamental studies of the electronic structure and conduction mechanisms of conducting polymers [1] would benefit greatly from such structures, and developments along this lines could ultimately reduce the size of electronic circuitry to molecular dimensions. As semiconductor cluster size becomes comparable to or smaller than the Bohr radius of the exciton in bulk material, the electronic properties become remarkably different from the bulk ones. The lowest excitation energy of an electron–hole pair shifts considerably to higher values and oscillator strength increases due to the quantum confinement of carriers [2].
The effects of quantum confinement on the electronic properties have been observed earlier for semiconductor nanoclusters of diiferent types such as inclusions in glass matrix, colloidal particles, nanocrystallites in zeolite hosts.
We report here on the development of host-guest chemistry synthesis procedures and studies of some electronic properties for a number of practically unexplored systems: polyaniline (conductive polymer) in zeolites FAU and MCM-41 and semiconductor PbI2 in zeolite LTL. In the latter case the different behavior is revealed in comparison with PbI2 nanoclusters in other matrices.
A distinctive feature of polyaniline (PANI) among the conducting polymers is the that its conductivity is not only controlled by degree of chain oxidation but also by the level of protonation quinone diimine repeating units. As we have observed, if aniline is introduced into different zeolites and treated with oxidant, the color of resultant adducts changes. Such change is traditionally considered as an indication of polymerization). In contrast, no polymerization is observed with nonacidic forms of the zeolites.
The infrared spectra of the samples of both HNaY(FAU) and MCM shows typical components of the emeraldine salt form polyaniline at 1596, 1493, 1310, 1281 cm-1 and 1577, 1489, 1296,1220 ,respectively, as well as characteristic free zeolite modes at lower energies. The shift between IR bands of the PANI /zeolite composite and bulk PANI, such as between PANI/FAU and PANI/MCM is thought to result from various host-guest interactions in this systems.
The conductivity of FAU and MCM/PANI was 10 -7 - 10-8 S/cm, similar to the “virgin” zeolite conductivity and much lower than for the films of bulk PANI (10 -3 S/cm). These results demonstrate that polyaniline in zeolite is located inside the interconnected supercages in FAU and inside channels in MCM whereas no percolation develops on the external zeolite surfaces.
The remarkable feature in freshly prepared samples with PbI2 content 0.05 and more was the presence of rather narrow luminescence peak only slightly blue-shifted from Ebulk. Its temperature-dependent behavior as well as the comparison with data on PbI2 inclusions in SiO2 makes it possible to attribute this emission to large clusters possibly retaining the layer structure of bulk PbI2. In a surprising contrast with the behavior of PbI2 bulk phase in porous glass, this luminescence component was found to disappear gradually with time and/or after cw UV illumination.
Indeed, such view is supported when considering the structure of the objects under study. In LTL lattice column-like main channels include periodic spheroidal cages (approximately 0.71 nm in diameter) with “D sites” on their walls occupied by relatively large potassium cations. In the process of dehydration these cations move outside the channels - into “E sites” - thus leaving more space for PbI2 molecules to be introduced into spheroidal cages by adsorption from vapor phase. It is plausible to assume that at small loading densities PbI2 molecules are mainly distributed in isolated cages with large I- anions occupying opposite “D sites” on the walls and small Pb2+ cations in between. As loading densities become larger the probability increases that oligomer chain fragments are formed via interaction between PbI2 molecules residing in neighbouring cages when Pb2+ cations become localized in the plane of 12-membered “dividing” rings.
Aging is thought to result in the reverse chain of events. Namely, potassium cations tend to migrate back to “D-sites” due to hydration affecting the distribution of negative charge on oxygen atoms in 12-membered rings. In turn, this favors the fragmentation of long oligomer chains into smaller non-interacting components. The described behavior seems basically different from the situation, e.g. in the system “PbI2/zeolite Na-FAU» in the different lattice structure.
References
1. P.Enzel, T.Bein, Chem. Mater., 4, 819 (1992)
2. G.Stucky, J.MacDougall, Science, 669 (1990)
3. G.Tel’biz, O.Shvets, V.Gun’ko, et al. in: Stud. Surf. Sci and Cat., E.Weitcamp et al. (Eds.), 84, 1099 (1994)
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