Effect of synthesis conditions on photocatalytic activities of nanoparticulate TiO₂ thin films

In the present study, design of experiments (DOE) was used to find an optimal combination for the factors affecting the preparation, morphology and catalytic activity of nano-crystalline TiO₂ thin films synthesized by sol-gel method. Nine TiO₂ thin films were prepared onto indium-tin oxide (ITO) substrates by changing four operating parameters at three levels. The four operating parameters are the volume of 20% acetic acid aqueous solution, the volume of co-solvent alcohol, the amount of template reagent (triblock copolymer Pluronic P123), and the calcination temperature. The resulting films were characterized by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and Fourier transform-infrared (FT-IR) spectra. The results of DOE are examined by both the direct observation analysis (also called range analysis) and the analysis of variance (ANOVA) with 90 or 95% confidence intervals. The parameter settings have been optimized as the combination of precursor solution constitution of Ti(OC₄H₉)₄/20%HAC/EtOH/P123 (2 g/15 ml/8 ml/1.1 g), calcination temperature of 450 °C, calcination time of 3 h, and air reaction atmosphere. The thin film prepared under the optimal combination consists of ordered compact anatase nanoparticles with an average grain size of approximately 17.2 nm and an average roughness of 3.653 nm. The 1-h photodegradation efficiency of methyl orange (MO) in aqueous solution and toluene in gaseous phase is 98.9 and 100%, respectively. The factor analysis results of degradation efficiencies show that the calcinations temperature plays an important role in determining the photocatalytic activity of the prepared TiO₂ thin film. Furthermore, the surface morphology-dependent photocatalytic activity of the prepared thin films was also found.