Comparison of photodegradation of methylene blue using various TiO2 films and WO3 powders under ultraviolet and visible-light irradiation

The photodegradation of methylene blue (MB) was investigated by UV–vis spectroscopy and liquid chromatography-mass spectrometry analyses. For this purpose, undoped, nitrogen-doped, and gold nanoparticle (AuNP)-doped TiO2 thin films for photodegradation were prepared, along with cuboid-shaped WO3 powder samples, some loaded with Pt and the others with Cu. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure analyses revealed that nitrogen atoms were incorporated into the TiO2 crystal both interstitially like KNO3 and NaNO2 and substitutionally like TiN. Mass analysis revealed that MB was degraded under visible-light irradiation via N-demethylation, cleavage of C–N and C–S bonds, and addition reactions of −OH and = O. Products with over m/z values greater than 400 were detected in the solutions degraded by the samples not doped with metals, but not in those degraded by metal-doped samples, which may have been due to the large adsorption capability of the metal-doped samples and their rapid degradation of MB.