Direct synthesis of nanostructured TiO2 films with controlled morphologies by stagnation swirl flames

A novel premixed swirl flame in stagnation point geometry is used to synthesize uniform, high-quality nanostructured TiO2 films at growth rates of 20–200 nm/s in a single step. The roles of precursor concentration and substrate temperature in controlling film morphology and characteristics are investigated. Increasing precursor concentration, for a given substrate temperature, significantly increases the packing density of the nanoporous film. The specific surface area of the film is mainly dependent on substrate temperature, where two distinct regimes, i.e. in-flame-agglomeration at low temperature and on-substrate-sintering at high temperature, specify film properties. A simplified deposition model for the formation of the nanoporous film structure is proposed, correlating penetration distance of thermophoretically-driven Brownian particles into the film with the resultant morphology. The model predicts the packing density in excellent agreement with experiments, thereby clarifying the complex roles of precursor concentration and substrate temperature. Finally, increasing substrate temperature, at fixed precursor concentration, changes the packing density little, but considerably decreases specific surface area, as sintering becomes dominant among controlling mechanisms.

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► Synthesis of TiO2 thin films is done by novel stagnation swirl flames.
► Precursor concentration plays a significant role in controlling packing density.
► Specific surface area of film depends on substrate temperature.
► Simplified deposition model is built on basis of thermophoretic and Brownian motion.