Photocatalytic degradation of methylene blue on nanocrystalline TiO2: Surface mass spectrometry of reaction intermediates

Photocatalytic degradation reactions of methylene blue on nanocrystalline TiO2 (nc-TiO2) films were studied in situ by surface mass spectrometric techniques including secondary ion mass spectrometry, laser desorption direct ion mass spectrometry, and laser desorption/laser photoion mass spectrometry. The parent ion of methylene blue and/or its reduced form leucomethylene blue was observed with little fragmentation by all methods prior to ultraviolet exposure. The surface composition changed upon ultraviolet (UV) irradiation in air, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signal diminished and intermediate reaction products such as sulfoxides and sulfones were detected from the TiO2 surface. After prolonged UV irradiation these species also vanished and the methylene blue appeared to be almost completely mineralized.