Physical characteristics of nanostructure TiO2

Abstract

Titanium dioxide (TiO2) in a colloidal suspension of nanocrystalline TiO2 particles form provided by SP-300NA Cristal Company, France, was treated to be fine powder used as the starting material to prepare thin films deposited on glass, quartz, and (NaCl) single crystal substrates through conventional thermal evaporation technique, using high vacuum coating unit supplied by a quartz crystal monitor. TiO2 as one of the basic ceramic materials has found a variety of applications in industry and in our daily life. The particle size of Titanium dioxide (TiO2) system, especially when reduced to nano regime, has a great potential to offer remarkable improvement in physical, mechanical, optical, biological and electrical properties. TiO2 powder and thin films were analyzed for its phases using X-ray powder diffraction (XRD) technique. Crystallite size, morphology and grain growth were determined using high-resolution transmission electron microscopy (HR-TEM) and field emission scanning electron microscope. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA/DTG) were used to study the thermal properties. Fourier transformation Infrared (FTIR) technique used to identify the molecular structure of TiO2 powder and thin film. FT-IR bands revealed that the spectrum of the as-deposited film does not change with evaporation indicating that the thermal evaporation technique is a good one to obtain undissociated and stoichiometric TiO2 films. High –Resolution Transmission Electron Microscope (HRTEM), showed nanoparticles of as-deposited TiO2 anatase thin films of average size, 19 nm. TiO2 anatase thin films of thickness 263nm annealed for 4 hours at different temperatures affected both the structure of the films, as indicated from the thermal analyses results. The surface topography and nanoparticles shape of TiO2 thin films as deposited and annealed at the temperatures 473, 673 and 873K were studied using the scanning electron microscope. The films showed good uniformity, crack free surface and nanoparticles with small ellipsoidal shape dispersing with high separations. XRD measurements showed that the average crystallite size of the powder TiO2 anatase is 30.266 nm. At 873K, the amorphous film showed well defined TiO2 anatase phase of 33.5 nm. The indentation experiments were carried out by using micro hardness tester showed the absence of monotonic behaviour and irregular thermally induced oscillations in the curves of the film hardness, HVf versus the annealing temperature. This behavior may be due to the combined effects of the applied factors; the temperature, the load and the dwell time, on the obtained HVf. Also the indentation size effect index (ISE), m, was calculated as 1.6 for the TiO2 films indicating that the hardnesss depends on indentation size. An ANN program and back propagation algorithm (Rprop) were used to model the nonlinear relationships for the composite hardness with the relative indentation depth, β and the film hardness with time at different temperature. The simulation and predicted values of the ANNs are in accordance with the experimental data. The performance of the ANN model was also tested at non-trained data and was found to be in good agreement with the experimental data. A basic repository on the domain knowledge of the indentation process verified the expected variations of micro-hardness according to the conditions applied on hardness measurements. The optical properties of TiO2 were investigated in the wavelength range of 250–2500 nm at normal incidence by using spectrophotometric measurements. The spectral behaviour of the refractive index, n, for TiO2, shows an anomalous dispersion in the wavelength range 250–820 nm, and normal dispersion region in the wavelength range 820–2500 nm. The parameters calculated from the analysis of the normal dispersion curve were obtained using the single oscillator model. The type of optical transition of the as-deposited and annealed films was found to be indirect allowed band-gap . The estimated non-linear refractive index, the band gap , and the calculated values of the nonlinear third-order optical susceptibility, χ(3), increased up to 1073K then decreased above this temperature. The electrical conductivity measured in air which increased on increasing the working temperature, and the resistivity of the TiO2 film increased with increasing film thickness. The curves show two stages and the resistance of the heat treated films after deposition exceeded that of the as-prepared films. The influence of increasing temperature contributes less electrical conductivity for the annealed samples than that for the as-prepared samples. The activation energies for TiO2 thin films at T<333 K and T>333K are considered as a change from extrinsic to intrinsic conduction. Annealing TiO2 films increased the activation energy. The conduction mechanisms of the electron transfer in the TiO2 thin films were explained by applying Seto model with the several modifications proposed by Baccarani.