Ionic Equilibria for Synthesis of TiO2 Thin Films by the Liquid-Phase Deposition

The liquid-phase deposition (LPD) reaction and the deposition mechanisms for the preparation of hazy TiO2, transparent TiO2, and NH4TiOF3 films were investigated by quantitative 19F NMR analysis. Contrary to the conventional understanding, the 19F NMR spectra indicated that BF3(OH)− formed within a short reaction time, and the concentration of BF3(OH)− was always much higher than that of BF4 – during the LPD reaction. Quantitative analysis also indicated that several types of titanium complexes coordinated by fluoride ions dissolved in the LPD reaction solutions, but they were not detected by 19F NMR measurement because of rapid ligand substitution. A highly dense and transparent anatase TiO2 thin film was obtained at pH 3.5, and the growth of the particles comprising the films was accelerated by increasing the pH. Conversely, nucleation and crystal growth in the thin films were suppressed at lower pH. Furthermore, in the induction period of the LPD reaction, a shift of chemical equilibrium, which decreased the proton concentration, was observed, and we suggest that the necessary supersaturation state for nucleation is attained during this induction period.