The influence of Cu, Fe, Co, and Cr transition metal ion impurities on the optical absorption spectra of polydisperse powders of nanocrystalline TiO2 rutile and anatase synthesized in identical experimental conditions is studied. For all rutile samples, no spectral shift of the absorption edge is observed, while a significant red shift of the absorption edge for modified anatase is detected. The shift value correlates well with the relative position of the impurity levels in the TiO2 band gap. The effect of the impurities on the rutile optical absorption is observed only in the region 3,0 ÷ 4,5 eV, whereas the modified anatase absorption changes significantly in the region 3,0 ÷ 5,5 eV. The band gap remains practically unchanged for modified rutile, whereas it decreases for modified anatase, being the largest for A/Cr and A/Fe samples. The photocatalytic activity of rutile and anatase samples in the reaction of photodestruction of organic dye safranine under UV irradiation is investigated. It is shown that the transition metal impurities in rutile, at their high concentration, become additional recombination centers for photoexcited electron-hole pairs that leads to the inhibition of photocatalysis. Increase in the rate of photocatalysis in the presence of modified anatase correlates well with the increasing optical absorption of modified anatase in the region 4,0 ÷ 5,5 eV.
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