Absorption vs. action spectra analysis of TiO2 photocatalysts doped or co-doped with p-block elements

The photocatalytic behavior of an extended series of NH4F-doped TiO2 photocatalysts, labeled as D series, has been thoroughly investigated in both liquid and gas phase reactions. Focusing in particular into the possible activation of these doped materials under visible light, the photocatalytic oxidation of acetic acid was investigated as a function of the irradiation wavelength, by collecting so-called action spectra. A comparison between the shapes of these latter and those of the absorption spectra of the investigated photocatalysts allowed us to clearly distinguish between inactive light absorption and effective photoactivity. XPS and EPR analysis of NH4F-doped materials, even if calcined at 700 °C, revealed the presence of residual nitrogen-containing species, that might be responsible for the spectral features and/or the photoactivity of NH4F-doped TiO2, which should thus be regarded as N and F co-doped materials. Aiming at better clarifying the role of fluorine and/or nitrogen as TiO2 dopants, a series of doped photocatalysts was prepared employing HF instead of NH4F as dopant source, thus avoiding the co-presence of nitrogen in the material. At the same time the effects of the co-presence of p-block elements boron and fluorine in titania were also explored, employing the photocatalytic oxidative decomposition of transparent substrates such as formic acid and acetic acid as main test reactions in aqueous suspension. The general trend of reaction rate increase with increasing the calcination temperature of full-anatase doped materials in liquid phase test reactions under full lamp irradiation, especially in the case of the HF-doped series, was in perfect agreement with the results obtained with the NH4F-doped TiO2 series. This remarkable similarity suggests that the beneficial effect obtained for doped samples calcined at higher temperature should be mainly related to the structural modifications induced by fluorine doping rather than by nitrogen doping. Moreover, a detailed action spectra analysis showed that only for photocatalysts of the HF-doped and BF-codoped TiO2 series a progressively higher calcination temperature ensured a better apparent quantum efficiency in the near UVA region, whereas boron doping did not produce this kind of effect. This is in perfect agreement (in both maximum position and relative value) with the photoefficiency increase observed for the NH4F-doped series and provides another unequivocal confirmation of the hypothesis that fluorine (and not nitrogen) is mainly responsible for the observed photoactivity increase in the UVA region of the investigated doped TiO2 photocatalysts.