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Title: Studies in sol-gel chemistry
Author: Gardener, Martin
ISNI:       0000 0001 3490 1059
Awarding Body: Nottingham Trent University
Current Institution: Nottingham Trent University
Date of Award: 2000
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The aim of this work was twofold. Firstly the incorporation of thermochromie bis(diammine)copper(II) chloride complexes and the derived diammoniumtetrachlorocuprate(II) salts, into silica sol-gel materials was investigated examining the differences in thermochromie behaviour including mechanisms of interaction between the free compounds and the doped materials. Secondly the aqueous sol-gel chemistry of zinc acetate was studied examining the nature of condensed phases from low (O.IM) and high (saturated, 1.38M) concentration solutions. The effect of low levels of sulphate ions on the condensation products produced from saturated solutions was also investigated. Finally zinc oxide thin films were prepared from dispersions of zinc hydroxyacetate materials for acoustic wave devices. Incorporation of bis(diammine)copper(II) chloride complexes in silica sol-gel by a two-step acid/ base process resulted in a material with differences in optical properties characteristic of a restriction of in-plane ligand motion arising from hydrogen bonding of soluble silica species to the hydrogen atoms of the -NH; groups. Thermal treatment of the materials to 80°C for 1 hour gave a similar purple -> blue colour transition as for the free complexes. For bis(ethanediammine) and bis( 1,2-propanediammine) complexes this resulted from the loss of water followed by chloride ion coordination to the Cu(II) ions while the bis(N,N'-diethylethylenediammine) complex changed from light blue to green as the bulky ligands crowd the axial positions and prevent coordination of the chloride. Prolonged thermal treatment of the doped silicas (24 hours) resulted in irreversible colour changes not found for the free compounds with bis(ethanediammine) and bis(l,2-propanediammine) complexes becoming green while the bis(N,N'-diethylethylenediammine) complex became yellow. This transition was found to result fiom the complex migrating into and being trapped in regions of strong electronic influence in the silica resulting in a twisting of the in-plane ligands towards a tetrahedral geometry. Incorporation of bis(I,2-propanediammine) into colloidal silica gel gave light blue coloured samples as differences in surface chemistry of the matrix resulted in a different degree of geometric distortion. Doping of alkoxide silica sol-gel with 1,2-diammoniumpropanetetrachlorocuprate salts resulted in a pale green, transparent material that became yellow when thermally treated to 80°C for 1 hour corresponding to the colour for the low temperature phase to high temperature phase colour found for the free compound. The colouration of the high temperature phase could be preserved by desiccation of the material and was reversed upon exposure to humidity. Thermal treatment of the silica for longer periods (24 hours) gave deep green coloured samples as a proton was transferred intermolecularly to the matrix with coordination of the resulting -NHz groups to the Cu(II). Cooling and rehydration of these samples under ambient conditions gave light blue coloured samples with optical characteristics similar to those of solutions of the complex isolated from appropriately acidified solutions of the bis(l,2-propanediammine)copper(II) chloride complex containing two ammonium and two ammine groups. These samples became deep green when heated to 80°C for I hour even though the free compound did not show any thermochromie behaviour hence suggesting interaction with the silica. Thermal treatment of the samples to 150°C for I hour gave an irreversible red/ brown colouration. The reason for this is not clear but could not be attributed to decomposition of the complex. Condensation products obtained from low (O.IM) concentration zinc acetate solution were found to be dependant upon the pH of the solution and pKa of the alkali used for precipitation. A critical pH of 10.6 was found experimentally, and explained by theoretical calculations using the Partial Charge Model, above which ionic dissociation of acetate from zinc occurred resulting in formation of crystalline hydrous zinc oxide phases. Precipitation with ammonia formed rhombohedral particles of e-Zn(0H)2 (Wulfingite) from the 2-dimensional growth of [Zn(OH2)4 (OH)2]° precursors plus a small amount of zinc oxide. Increasing the pK, of the alkali (KOH or NaOH) resulted in zinc oxide (zincite) formation through deprotonation of the acidic bridging hydroxyl groups formed during olation. Condensation below pHIO.6 resulted in an amorphous, polymeric material based upon bridging acetate ligands and bridging hydroxyls. The slow reaction rates at the low Zn^^ concentration resulted in a polymer structure consisting of acetates with O-C-O angles close to that of free acetate with minimum strain. The polymeric structure was formed via a reaction pathway that was found to be independent of pH with solid phases resulting only from condensation between h=2 precursors. Addition of small amounts of sulphate ions (ca. ISO /': 3400Zn^^) to saturated solutions of zinc acetate prior to ammonia addition resulted in zinc oxide formation when dispersions of zinc hydroxyacetate were warmed to temperatures above 40°C following a washing process to remove contaminating ions. A semi-qualitative catalytic mechanism for sulphate induced zinc oxide formation has been proposed to account for the observed results and is supported by calculations using the Partial Charge Model. An initial study into the use of dispersions of zinc hydroxyacetate materials for zinc oxide thin film and acoustic wave device preparation showed that aqueous sol-gel routes to zinc oxide thin films are feasible although the coatings prepared on silica substrates were capable of supporting bulk acoustic waves and not surface acoustic waves.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Inorganic chemistry