Electric-field-assisted swirl-flame synthesis of high-porosity nanostructured titania (TiO2) films

Nanostructured mesoporous metal-oxide films can be used in various applications, including dye-sensitized solar cells based on titania. Optimization of the properties of these films is crucial in improving their efficiency. Nanostructured TiO2 films with high uniformity and porosity are grown in a stagnation swirl flame setup under an applied electric field. The effects of external electric-field magnitude and polarity are studied for different substrate temperatures and precursor loading concentrations. The results show considerable differences in film characteristics, for differing electric fields, with more columnar structures and higher porosities under low voltages up to +/-400 V. The films have higher packing density at higher voltages of +/-800 V. At low substrate temperatures, the morphology and structure are more prominent owing to less on-substrate sintering of the nanoparticles. At low voltages, oppositely-charged particles will be attracted to the substrate increasing the electrophoretic velocity but decreasing the in-flame agglomeration; while at high voltages, the particles will be repelled and stay in the flame longer, thus increasing the in-flame agglomeration. A simple model is proposed which predicts the trend for deposition of particles and formation of nanostructured TiO2 films of a given morphology by balancing the effects of thermophoresis, electrophoresis, and Brownian motion of the particles. The model's trend for packing density agrees with the experiments.