| Abstract |
The synthesis of new micro- and meso-porous solids with particular
physiochemical properties which arise from the inorganic framework, is an important field of research worldwide. An important development was the synthesis of organized mesoporous aluminosilicates with controlled pore diameter reported by Mobil research group in the early ‘90. Following this discovery, various
mesostructured metal oxides based on Vanadium, Titanic and Zirconium, have been reported using very the surfactant-assisted methodology. Whereas mesoporous oxidic materials are considered very promising for applications in catalysis and gas separation/storage, they do not exhibit interesting optical, electronic or photonic properties. This is because the inorganic framework is insulator. The combination of optoelectronic or semiconducting properties with porosity in one material could open
the pathway for advanced applications including photocatalysis, molecular detectors, non-linear optical systems and quantum photonic solids. Therefore, the development of non-oxidic porous solids based on metal-chalcogenides (S, Se, Te) have attracted
significant attention during the last year.
A promising methodology that has been applied towards the development of mesoporous chalcogenides is based on the cooperative self-assembly of molecular anions of metal-chalcogenides such as[MQ4]4-, [M2Q6]4-, [M4Q10]4-, [M’Q3]3- (M= Ge4+, Sn4+ M’= Ga3+, In3+, Sb3+ and Q=S,
Se, Te), transition metal linking cations (Mn2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Ga3+, In3+, Sb3+, Sn4+, Pt2+, Pd2+)
and surfactant molecules acting as templates. In all cases the solids are mesostructured and not mesoporous and this is because the removal of the organic molecules from the pore space leads to structural collapse. In almost all cases the reactions carried out in
formamide.
Despite the fact that well-ordered hexagonal and cubic mesostructured chalcogenides have been reported, in many cases the chemical composition is not well defined. This is in contrast with the fact that well defined molecular building units (e.g. [SnSe4]4- or [Sn2Se6]4-) have been used for their synthesis. From preliminary 119Sn NMR measurements was shown that in formamide complex equilibrium takes place that lead to chemical transformation of the inorganic species. For example the anions SnSe44- dissolved in formamide are dimerized forming SnSe44-and Se2-. In
contrast, similar reactivity was not observed in water. Therefore, the synthesis and characterization of mesostructured chalcogenides derived from aqueous solution may lead to a better understanding of the mechanism of formation of these materials and eventually may allow the design of novel architectures with accessible porosity.
In the present research work, the synthesis of surfactant templated
mesostructured semicoducting solids based the chalcogenide anions [SnSe4]4- and [Sn2Se6]4- was studied for first time in aqueous solution. These particular anions in the presence of surfactant molecules and linking metal cations Zn2+ result in the
formation mesostructured solids These are semiconductors with energy gap in the range 2.0-2.3eV. The degree and the type of pore organization (hexagonal, cubic or lamellar) is controlled by a) the nature of the surfactant in terms of both chain length
and head group type and b) the ratio of SnSe44-/[Sn2Se6]4- on the initial mixture. In particular, in the case of the dimeric anions [Sn2Se6]4- , the materials shows a hexagonal pore organization, while the use of SnSe44- or a mixture of both lead to
materials with cubic Ia-3d pore symmetry. All solids were fully characterized with a variety of experimental techniques, including powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). The
optical properties were studied with solid state UV-vis/near IR diffuse reflectance spectroscopy.
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Σύνθεση, χαρακτηρισμός και μελέτη των οπτοηλεκτρονικών νέων ημιαγωγών νανοδομημένων υλικών
