ABSTRACT
TiO2, among wide band gap semiconductors, is an efficient photocatalyst for organic compound photodegradation both on aerated surfaces and in aqueous suspensions (Choi et al., 2007; Chong et al., 2010). TiO2 is a relatively inexpensive semiconductor which exhibits high photocatalytic activity, non-toxicity and stability in aqueous solutions, etc. (Choi et al., 2010; Sakkas et al., 2010). Furthermore, the synthesis of mesoporous nanocrystalline anatase TiO2 particles, films or membranes has extended their use in environmental remediation. Ultrafine TiO2 powders with high particle surface area have good photocatalytic activity since reactions take place on the surface of the photocatalyst. On the other hand, powders can easily agglomerate in larger particles and as a consequence adverse phenomena to their photocatalytic activity are observed. Sol-gel method, employing appropriate templating techniques or hydrothermal treatment of titanium alkoxides are among common techniques used nowadays to fabricate efficient nanocomposite TiO2 powders and films (Barbé et al., 1997; Stathatos et al., 2004). However, TiO2 is usually used as disperse catalyst for its high catalytic surface area and activity (Litter, 1999). Nevertheless, TiO2 powders cannot easily be recovered from aqueous systems when they are used for water treatment. Highly dispersed TiO2 particles in suspension are difficult to handle and remove after their application in water and wastewater treatment. Recently, many research studies have been carried out to immobilize TiO2 catalyst onto various substrates as thin films and membranes (Li et al., 2008; Wang et al., 2009). Despite of their lower catalytic surface area there is an increase of the catalytic activity and utilization because of the extension of their field of applications. The specific surface area, particle morphology and possible aggregation, phase composition and number of -OH surface groups are among the most critical parameters for high photocatalytic activity of the as-prepared immobilized powders and films. Another approach to enhance the photocatalytic properties of the catalysts is the promotion of their porous structures.
