서강대학교

초록/요약

The synthesis and characterization of one-dimensional (1D) semiconductor quantum-confined materials are important, because they have great potential as building blocks for nanoscale electronic devices and other novel applications. Among the known 1D semiconductor materials, quantum wires are the thinnest 1D quantum-confined materials. Examples of such quantum wires are rare. We have elucidated the interesting quantum confinement properties of the titanate (TiO32-) quantum wire which is regularly placed within ETS-10. It shows a length-dependent quantum confinement effect even at length scales longer than 50 nm. Its estimated effective reduced mass of exciton along the wire (？z) was smaller than 0.0006 me (me = rest mass of electron), which are much smaller than the reported smallest values (InSb: 0.014 me, single-walled carbon nanotube: 0.019 me) indicating much higher exciton mobility along the quantum wire than those of InSb (0.014 me) and single-walled carbon nanotube. The nature of electronic absorption of the titanate quantum wire was oxide-to-TiIV charge transfer, or ligand-to-metal charge transfer (LMCT). The stretching frequency of the titanate wire increases as the electron density of the wire increases. After elucidation of such important properties of the titanate quantum wire, it will be exciting if one could elucidate the physicochemical properties of the closely related vanadate (VO32-) quantum wire. In that sense, the discovery of vanadosilcate AM-6 by Anderson, Rocha, and coworkers in 1997 was a very important event since its structure adopts that of ETS-10 with VO32- quantum wires replacing TiO32- quantum wires. However, the AM-6 crystals had core ETS-10 crystals inside the AM-6, which made it difficult to obtain the quantum confinement effect of vanadate (VO32-) quantum wire. Recently, we succeeded in developing a method for the preparation of ETS-10 free pure AM-6 and in the case of VIVO32- quantum wire, the nature of the intense electronic absorption in the 3.0？5.5 eV region is V4+-to-O2- CT or metal-to-ligand CT (MLCT), in contrast to that of TiO32- quantum wire. Despite this development, however, we were unable to elucidate the important quantum confinement effect of vanadate (VO32-) quantum wire since we could not produce AM-6 in different sizes. We have introduced the method to produce AM-6 in various sizes and consequently the length dependent quantum confinement effects of vanadate (VO32-) quantum wires were elucidated. The estimated effective reduced mass of exciton along the wire (？z) is 0.0005 me (me = rest mass of electron), which is smaller than that of ETS-10: 0.0006 me. It also shows interesting length-dependent oscillator strength, magnetic susceptibility and relaxation time changes of vanadate (VO32-) quantum wire. Like ETS-10 the stretching frequency of vanadate (VO32-) quantum wire increases as the electron density of the wire increases. Microporous vanadium silicates are a new class of materials with potential for applications as ion-exchangers, gas separations, selective heterogeneous catalysts for acid-catalysed and oxidation reactions due to its high thermal stability and, in particular, due to the fact that the presence of a redox-active transition metal center. A new vanadium silicates has been synthesized under mild hydrothermal reaction condition, the structure is constructed from 4.2 ？ silicate (Si12O32)16- nanotube and square pyramidal oxovanadyl [O=VO4]2- units. The silicate (Si12O32)16- nanotubes are linked through the oxovanadyl [O=VO4]2- units to construct the open framework structure. The framework metal oxidation state has been converted V4+ to V5+ by simple treatment of Br2 solution in CCl4 at room temperature. The powder X-ray diffraction patterns indicate the structure remained intact even upon V4+ to V5+ conversion. The absorption spectra revealed that the nature of electronic absorption of as made sample was VIV-to-oxide charge transfer (MLCT), after framework metal oxidation the electronic absorption changed to oxide-to-VV charge transfer (LMCT). It is a very interesting example that the types of charge transfer (MLCT or LMCT) can be manipulated upon changing the oxidation states in a same compound. The V5+ sample showed the excellent catalytic activities for selective alcohol oxidation in compared to those of as made (V4+), V2O5 or other vanadium containing molecular sieves. It has also been examined how additional compounds can be formed by different combinations of the same building units (square pyramidal vanadium and tetrahedral silicon) and subsequently generalized our synthesis approach. We have obtained several novel open-framework vanadium silicates.