Absorption vs. action spectra analysis of NF-codoped TiO2 photocatalysts

The photocatalytic behaviour of a series of ammonium fluoride-doped TiO2 photocatalysts was investigated in the decomposition of formic acid and acetic acid in aqueous suspensions and in the gas phase mineralization of acetaldehyde. Very similar photoactivity trends, usually increasing with increasing the calcination temperature for a given nominal dopant amount, were obtained for the two test reactions. Moderately doped TiO2 calcined at 700°C, consisting of pure anatase, was the best performing photocatalyst in all test reactions. In order to shed light on the origin of the enhanced photoactivity of such materials under visible light, acetic acid photo-oxidation was investigated systematically as a function of irradiation wavelength, by collecting so-called action spectra. By comparing the shapes of the action spectra with those of the absorption spectra of the investigated photocatalysts a model is proposed, based on spectral features deconvolution, which allows a clear distinction between inactive light absorption, ascribed to nitrogen doping, and effective photoactivity in acetic acid decomposition, possibly consequent to extrinsic absorption originating from surface oxygen vacancies or surface defects. Recent studies performed with F-doped TiO2 materials fully confirm this attribution.