Photoelectrochemical degradation of selected organic substances on Fe2O3 photoanodes: a comparison with TiO2

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-Fe₂O₃) and compared with titanium dioxide (TiO₂), 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, f₀, of degradation processes was determined from the UV-Vis absorbance decrease of the starting materials. f₀ 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), f₀ decreased to 1.7%. For 1 mM SA, f₀ was 6.2% on hematite photoanodes and 6.1% on titanium dioxide. For 1 mM benzoic acid and 4-chlorophenol, f₀ 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.

WO3 thin films for photoelectrochemical purification of water

Tungsten trioxide thin films on transparent substrates (glass and F:SnO(2) or ITO-coated glass) were prepared by layer-by-layer brush painting and spin-coating using organic precursors. Well-crystallized WO(3) with monoclinic structure was formed on all substrates after annealing at 500 degrees C or above. The dense semiconducting films are specular and transparent outside the band-gap. Their photoactivity in junctions with aqueous electrolytes extends up to 470 nm, with incident photon to current conversion efficiencies around 0.9 at 313 nm and up to 0.1 at 436 nm. Films of 10 cm x 10 cm were used for the study of solute degradation reactions in a thin-film reactor under backside illumination. Dilute aqueous solutions of model substances for contaminants like oxalic acid were decomposed under continuous flow using broadband UVA illumination and electrical bias. Operation under solar illumination was also feasible. The advantage over operation without bias (conventional photocatalysis) prevailed for all decomposition reactions studied.

Photoelectrocatalytic degradation of 4-chlorophenol and oxalic acid on titanium dioxide electrodes

Photocatalytically active thin TiO(2) films were produced by spin-coating or dip-coating an alkoxy precursor onto a transparent conducting electrode substrate and by thermal oxidation of titanium metal. The thin films were used to study the photoelectrocatalytic or photoelectrochemical degradation of oxalic acid and 4-chlorophenol (4-CP) under near UV (monochromatic, 365 nm) light irradiation. Degradation was monitored by a variety of methods. In the course of oxalic acid degradation, CO(2) formation accounted for up to 100% of the total organic carbon degradation for medium starting concentrations; for the degradation of 4-CP, less CO(2) was detected due to the higher number of oxidation steps, i.e. intermediates. Incident-photon-to-current conversion efficiency, educt degradation and product formation as well as Faradaic efficiencies were calculated for the degradation experiments. Quantum yields and Faradaic efficiencies were found to be strongly dependent on concentration, with maximum values (quantum yield) around 1 for the highest concentrations of oxalic acid.