Scanning electrochemical microscopy investigation of the photodegradation kinetics of 4-chlorophenol sensitised by TiO2 films

Scanning electrochemical microscopy (SECM) has been used to investigate quantitatively the photodegradation kinetics of 4-chlorophenol (4-CP), in aerated and oxygenated aqueous solutions, at back-illuminated TiO2 films deposited from a suspension of Degussa P25 TiO2 powder at the end of an optical fibre. A potentiometric Ag/AgCl ultramicroelectrode (UME), positioned at a known distance above the TiO2 film, was used to monitor directly the Cl− production from the photodegradation of 4-CP. The effects of both light intensity and 4-CP concentration were investigated and the associated kinetics were determined. Amperometric measurements with a Pt UME demonstrated that O2 was depleted significantly adjacent to the TiO2 surface under these conditions. A theoretical model, which employs a Langmuir–Hinshelwood type kinetic equation, has been developed to interpret the kinetics of the photodegradation process and determine the associated quantum efficiency.