The influence of TiO2 powder and film on the photodegradation of methyl orange

Recent investigation has showed that the semiconductor nano-powder or thin film, such as titanium dioxide suspended in water, can utilize ultraviolet light for photocatalytic oxidation of organic pollutants, such as dyes. The photocatalytic activity of TiO2 (powder or thin film) was tested in the reaction of photodegradation of methyl orange (MO). The effect of the initial concentration of dye, the pH, the amount and the type of TiO2 have been investigated. The films were prepared by doctor blade technique, and were annealed in air at various temperatures ranging from 300 to 500 degrees C. The thermal treatment increases the surface roughness of thin films. The best film (without fissures), with higher pores and small grains (150 nm) was the annealed one at 500 degrees C. The correlation between the measured contact angle and the films roughness estimated from atomic force microscopy (AFM) shows that a low contact angle (wetting behavior) corresponds to a higher porosity (a fractured structure, rich in edges and corners), and consequently to high surface energy. Kinetic analyses indicate that the photodegradation rates of dyes can usually be approximated as first-order kinetics for degradation mechanisms. Adsorption is a prerequisite for the TiO2-assisted photodegradation, and the extent of degradation has been discussed in terms of the Langmuir-Hinshelwood model. Thin-film coating of photocatalyst may solve the problems of leaching and separation. At a low concentration of MO (0.0125 mM) the efficiency of MO photodegradation is comparable in the case of thin film and powder of TiO2. (C) 2008 Elsevier B.V. All rights reserved.