The TiO2-N spectrum has a significant red shift compared to the spectrum of bare P25-TiO2 and has an absorption band at 600 nm resulting in greenish yellow colour of the sample. The yellow TiO2-RHSE sample shows a more pronounced light absorption in 340–420 nm wavelength range and a shoulder between 420 and 550 nm.
Silver and gold deposited samples show broad absorption plasmon bands with the maxima at 500 nm and 547 nm, respectively. Freshly prepared silver modified titanium dioxide has a yellow colour that turns brownish-yellow under ambient conditions due to the silver nanoparticles becoming oxidized. The gold nanoparticles cannot be oxidized in air and hence there is a persistent purplish powder.
Although it is obvious from these spectra that the colourful samples are able to absorb visible light this does not mean necessarily that the absorbed photons can be used for the generation of charge carriers.
3.2. Photocatalytic activity under UV and visible light irradiation
The irradiation experiments were carried out with two different types of lamps. Their spectra are displayed in Fig. 3b. The fluorescent tubes emit mostly near-UV light (maximum at 365 nm) with some visible light as well. The spectrum of our metal halide lamp was slightly modified by circulating a NaNO2 cut-off filter solution (1 M) in the thermostating jacket of the photoreactor in order to provide completely UV free visible light irradiation (λ > 400 nm).
3.2.1. Phenol and oxalic acid degradation under UV irradiation
Fig. 4a shows only the most important photocatalytic decay curves of phenol with UV irradiation (in order to provide better visibility for the data points). All initial decomposition rate values are summarized in Table 1. The highest degradation rate was found for the TiO2-FH sample and it exceeded the photocatalytic activity of Aeroxide P25 titanium dioxide. The activity of P25-TiO2 was significantly reduced by the deposition of gold and silver. Iodine, nitrogen and iron doped samples showed very low activity under UV irradiation for phenol.