Borates And Uranyl Compounds With Optical And Photoelectrical Functions: Synthesis, Structures And Properties

Non-centrosymmetric structure is a prerequisite for a crystal to exhibit efficientbulk second-order nonlinear optical (NLO) effects that can be exploited in themanufacture of second harmonic generator (SHG), electro-optical, andphotorefractive devices. A number of non-centrosymmetric compounds have beenfound to be very useful as NLO materials and efforts have also been made to searchfor new NLO crystals during the past two decades. Because the polar orientation ofinorganic triangular groups in crystals can lead to acentric structures andconsequently favors NLO properties, as evidenced by borates containing triangularBO3 units, more and more attention has been directed to crystalline boratecompounds as potential NLO materials. On the other hand, the search for materials simultaneously involving two ormore different physical properties is of special interest because multifunctionalmaterials have a new breadth of applications. The photovoltaic effect, generallybased on semiconductor materials that feature a compromise between large band gapand strong absorption, and as well a low recombination rate of the photo-generatedcharge carriers, is the most fundamental energy conversion process. In combinationwith the so-called "fuel-cell-technology", photovoltage and photocatalysis are ofenormous scientific significance for getting clean energy and storing energy. It is ofspecial interest to explore semiconductor materials because of their promising iABSTRACT Jilin University Ph. D. Thesisphotovoltaic and photocatalysis properties of converting light directly into electricity,or energy stored in chemical fuels such as hydrogen. Photocatalysis attractsincreasing interest as a potentially clean and renewable source for hydrogen fuel bysplitting water into H2 and O2 using solar energy, as well as potential applications forthe green degradation of organic pollutants due to the great environmental impact. By using hydrothermal technique, a novel chiral lead borate (Pb6B11O18(OH)9)has been prepared. The structure of the compound has been solved by single crystalX-ray diffraction analysis. There exist infinite helical chains and finite chainfragments built up from BO4 and BO3 units. In the compound the Pb2+ cations arelocated in the space between adjacent polyborate anionic chains, compensating thenegative charges of the chains. The chiral feature of this lead borate is unique, andsince it lacks symmetry center the compound exhibits distinct NLO properties. Twonew compounds – strontium borate SrBO2(OH) and barium polyborateBa3B6O9(OH)6 have been prepared. In the structure of SrBO2(OH), the Sr2+ ion issurrounded by eight oxygen atoms, and the borate ion is identified as an individual[BO2(OH)]2- group. The structure of Ba3B6O9(OH)6 can be regarded as constructedfrom anionic borate chains, the negative charges of which are compensated by theBa2+ cations residing between the adjacent chains. The unusual feature of thiscompound can also be described in terms of infinite chains of B3O3 six-memberedrings sharing B atoms, which are all tetrahedrally coordinated by oxygen atoms. A chiral zinc borate Zn2(OH)BO3, which exhibits nonlinear optical,photoluminescent and photoelectronic properties, has been successfully preparedhydrothermally. The structure of the compound has a ZnO matrix and the electronicstructure of the compound can be regarded as that for ZnO modified by the presenceof B atoms. The luminescence properties of the Zn2(OH)BO3 were characterized byexcitation and emission spectra. Photovoltage signals were observed for theas-synthesized compound and the samples calcined at various temperatures bysurface photovoltage spectroscopy (SPS) and field-induced surface photovoltage iiABSTRACT Jilin University Ph. D...