Titanium dioxide (TiO2), one of the basic ceramic materials, has found a variety of applications in industry and in our daily life. It has been shown that particle size reduction in this system, especially to nano regime, has the great potential to offer remarkable improvement in physical, mechanical, optical, biological and electrical properties. This thesis reports on the synthesis and characterization of the nanocrystalline TiO2 ceramic in details. The study selected a simple sol-gel synthesis process, which can be easily controlled and reproduced. Titanium tetraisopropoxide, isopropanol and deionized water were used as starting materials. By careful control of relative proportion of the precursor materials, the pH and peptization time, TiO2 nanopowder was obtained after calcination at 400oC. The powder was analyzed for its phases using X-ray powder diffraction (XRD) technique. Crystallite size, powder morphology and lattice fringes were determined using high-resolution transmission electron microscopy (HR-TEM). Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were used to study the thermal properties. As-synthesized powder was uniaxially compacted and sintered at elevated temperature of 1100-1600oC to investigate the effects of sintering on nano powder particles, densification behavior, phase evolution and mechanical properties. Microstructure evolution as a function of sintering temperature was studied by scanning electron microscopy (SEM). The results showed that 400oC was an optimum calcination temperature for the as-synthesized TiO2 powder. It was high enough to achieve crystallization, and at the same time, helped minimize the thermal growth of the crystallites and maintain nanoscale features in the calcined powder. After calcination at 400oC (3 h), XRD results showed that the synthesized nano-TiO2 powder was mainly in single anatase phase. Crystallite size was first calculated through XRD, then confirmed by HR-TEM, and found to be around 5~10 nm. The lattice parameters of the nano-TiO2 powder corresponding to this calcination temperature were calculated as a=b=0.3853 nm, c=0.9581 nm, α=β=γ=90o through a Rietveld refinement technique, which were quite reasonable when comparing with the literature values. Considerable amount of rutile phase had already formed at 600oC, and the phase transformation from anatase to rutile fully completed at 800oC. The above rutilization process was clearly recorded from XRD data, and was in good corresponding to the DSC-TGA result, in which the broad exothermic peak continued until around 800oC. Results of the sintered TiO2 ceramics (1100oC-1600oC) showed that, the densification process continued with the increase in sintering temperature and the highest geometric bulk sintered density of 3.75 g/cm3 was achieved at 1600oC. The apparent porosity significantly decreased from 18.5% to 7.0% in this temperature range, the trend of which can be also clearly observed in SEM micrographs. The hardness of the TiO2 ceramics increased with the increase in sintering temperature and the maximum hardness of 471.8±30.3 HV was obtained at 1600oC. Compression strength increased until 1500oC and the maximum value of 364.1±10.7 MPa was achieved; after which a gradual decrease was observed. While sintering at ambient atmosphere in the temperature range of 1100oC-1600oC helped to improve the densification, the grain size also increased. As a result, though the sintered density at 1600oC was the highest, large and irregular-shaped grains formed at this temperature would lead to the decrease in the compression strength.

Due to recent advances in computational hardware and code accessibility, state-of-the-art calculations are currently employed to investigate materials at the nanoscale with varying levels of accuracy. As such, this dissertation highlights a series of materials ranging from one-dimensional wires, to reactive surfaces, to bulk crystals. Initial characterizations for all considered materials are carried out using density functional theory where additional approximations are utilized to obtain more complex quantities. For Millon's salt, first-principles calculations confirm a quasi-one-dimensional description where the metallic backbone influences electronic properties while hydrogen-bonding between ligands results in structural stability. We show that valence band dispersion can be controlled via strain or ligand substitution, pointing to tunable hole-carrier possibilities. Optical properties are also addressed with respect to experimental and theoretical findings. Our focus then shifts to titanium dioxide, a popular and promising photocatalyst. Specific nitrogen doping on the anatase (001) surface introduces intra gap states accessible for photoactivation in the visible. The additional presence of a fluorine dopant or oxygen vacancy enhances the density of these particular states available for transitions. Titanium dioxide also has experimentally displayed involvement in carbon dioxide reduction mechanisms. From first-principles calculations, anatase (001) surfaces containing an oxygen vacancy exhibit an increased potential for carbon dioxide to undergo reduction due to an exposed titanium atom in comparison to the pristine case. Other binding configurations on both types of surfaces suggest the existence of alternative conversion pathways. As a recently realized plasmonic material, titanium nitride proves advantageous in relation to more traditional materials, e.g., gold or silver; one of the main factors stems from its tunable permittivity. We investigate this aspect by theoretically incorporating defects into titanium nitride, which introduces a systematic approach to control plasmonic activity over a broad frequency range. Finally, lifetimes of hot-electrons, originating from plasmonic decay, for instance, possess finite lifetimes in titanium nitride, as well as in other similar materials, that are described by electron-electron interactions through the electron self-energy. Average lifetimes resemble those obtained with a free electron gas model while details of the band structure influence lifetime behavior. Calculations exploring factors affecting these lifetimes are presented.

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