Browsing by Author "Tucic, Aleksandar"

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Wet chemical synthesis and characterization of organic/TiO2 multilayers
(2008) Tucic, Aleksandar; Aldinger, Fritz (Prof. Dr.)
The low-temperature deposition of oxide-base thin films from solution induced by organic templates is inspired by the process of biomineralization. Biominerals, i.e. inorganic materials synthesized by living organisms, show highly controlled micro- and nanostructures and in many cases physical properties superior to their manmade counterparts. In bio-inspired processes thin oxide films can be deposited from aqueous solutions on organic self-assembled monolayers or polyelectrolytes (PE). Liquid flow deposition (LFD) for the synthesis of TiO2 is based on the continuous flow of a precursor solution along the substrate. Whereas the concentration of the precipitating species within the reaction solution decreases with increasing deposition time, LFD provides a means to keep the concentration within the solution constant. Consequently, also the growth rate of the film is not affected by such aging effects. The deposition technique for the synthesis of PE layers is based on the electrostatic attraction between oppositely charged polyions layer by layer. The spontaneous sequential adsorption of dissolved anionic and cationic polyelectrolytes leads to the formation of ordered multilayer assemblies on a solid substrate. In this work, both techniques were combined in order to synthesize composite, multilayer PE/TiO2 thin films by wet chemical processing is investigated. The main aim is to mimic the architecture of nacre, which is present for instance in sea-shells in order to achive ceramic-based system with enhanced mechanical performances. The properties of the deposited films were characterized by means of SEM, XRD, AFM in order to establish the optimum parameters of the reaction process concerning the film homogeneity, thickness, structure and surface roughness. Polyelectrolyte (PE) films were synthesized applying the layer-by-layer deposition technique. The film thickness was determined applying AFM, TEM cross-sections and the quartz-crystal microbalance (QCM) technique. AFM investigations of the surface morphology of the PE layers showed densely packed globular aggregates of the deposited polymer. In order to investigate the dependence of the morphology and structure of the TiO2 films on the surface modification, depositions on Si substrates modified with polyelectrolytes (PE) were carried on. Thickness of the deposited TiO2 films, estimated by SEM cross-sections, is slightly higher than that of films deposited unmodified Si. Homogeneous films with the same microstructure were deposited on unmodified Si substrate and on PE-covered silicon substrates. Composite PE/TiO2 films were synthesised by applying the layer-by-layer deposition technique for the synthesis of PE layers and the static deposition techniques for synthesis of TiO2 layers. Auger electron spectroscopy (AES) was used to determine the concentrations of Ti and O (as the main constituent of the inorganic phase), C (as the main constituent of the organic phase) and Si (substrate), as a function of depth below the film surface. The AES profile clearly demonstrates the presence of a multilayered structure of alternating TiO2-enriched and C-enriched layers; i.e. it provides proof for the existence of an ordered composite structure of well-defined inorganic and organic layers. SEM, TEM and STEM cross-sections were used for the characterization of the microstructure of the multilayers. Analytical TEM investigations were performed using the energy-dispersive X-ray spectroscopy (EDX) and electron-energy loss spectroscopy (EELS) method. The nanoindentation technique was employed to determine the mechanical properties of obtained composite films, with the emphasis on their hardness and Young’s modulus. The comparison of the nanoindentation data obtained from the TiO2 single layer and the (PE/TiO2)2 and (PE/TiO2)3 multilayer samples reveals that the incorporation of organic layers improves the mechanical properties of CBD-derived TiO2 films. This enhanced mechanical performances can be attributed to the hardening by the differences in shear modulus between the organic and the inorganic phase and the interaction between the incorporated TiO2 particles and the PE, which is stronger then the one between the TiO2 particles within the oxide layers.