Coumarin-based quantification of hydroxyl radicals and other reactive species generated on excited nitrogen-doped TiO2

In order to extend the photoactivity of titanium dioxide into the visible region, highly porous nitrogen-doped TiO2 catalysts (NTiO2) were successfully synthesized by a modified co-precipitation method with ammonium hydroxide as a nitrogen source. Different approaches such as dosing order of the reagents and temperature of the synthesis, calcination period and temperature were tested to examine the optimum outcome regarding photocatalytic OH radical formation under UV and visible light. Coumarin as a traditional probe for this purpose was applied; measuring the luminescence of 7-hydroxycoumarin produced in the reaction with OH radicals, beside the formation of other hydroxylated derivatives. Only a few percentages of the coumarin molecules reacted with OH radicals, while most of them underwent reactions with other photogenerated species such as electrons (anaerobic/aerobic) and superoxide anion radicals (aerobic). Accordingly, our observations suggest that coumarin can also be used as a probe to quantify the formation of other reactive species. The results were obtained from the difference between the amounts of degraded coumarin and hydroxylated coumarin derivatives formed during photocatalytic experiments. These coumarin-based quantifications of photoactivity was applied for the characterization of the prepared nitrogen-doped TiO2 catalysts (NTiO2). In addition, material analysis (X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, and Fourier transform infrared spectroscopy) showed that crystallinity and nitrogen content were found to be crucial features in the photocatalytic performance of the catalysts.