Photoelectrochemical degradation of selected organic substances on FeO photoanodes: a comparison with TiO.

The photoelectrochemical degradation of selected aromatic substances, acid orange 7 (AO7), salicylic acid (SA), benzoic acid (BA), and 4-chlorophenol (4-CP) was studied on hematite (α-FeO) and compared with titanium dioxide (TiO), both deposited as thin films on conducting substrates (FTO/glass). Batch type reactors were used under backside and front side illumination. Electrical bias was applied on the semiconducting electrodes, such that only valence band processes leading to oxidative pathways were followed. The initial Faradaic efficiency, f, of degradation processes was determined from the UV-Vis absorbance decrease of the starting materials. f for 1 mM AO7 degradation in 0.01 M sulphuric acid was found to be 7.5%. When the pH of the solution was neutral (pH 7.2) or alkaline (pH 13), f decreased to 1.7%. For 1 mM SA, f was 6.2% on hematite photoanodes and 6.1% on titanium dioxide. For 1 mM benzoic acid and 4-chlorophenol, f was an order of magnitude lower, but only on hematite. This is ascribed to the lack of OH· radical formation on hematite, which seems to be essential for the photooxidation of these compounds.