Femtosecond to nanosecond spectroscopy of transition metal-doped TiO2 particles

The transient absorption properties of TiO2 particles were studied by femtosecond transient absorption spectroscopy. For the transition metal (Cr and/or Fe)-doped samples the transient absorption spectra were observed to shift to the shorter wavelengths with the time while for the non-doped sample the position of the absorptions did not change. The spectral shifts were the highest for the sample doped by both Cr and Fe. The transient absorption spectra right after excitation had a broad absorption band with the flat top in 550-650 nm region, whereas at 1 ns delay the maximum of the transient absorption shifted to 510-530 nm area for Cr- and Fe-doped samples. The observed new absorption bands were attributed to the trapped holes. For comparison, activity measurements for the same samples were performed in photocatalytic reactor system. Doping reduced the photocatalytic activity of the samples. This can be interpreted in favour of the importance of the hot and shallow trapped holes for the photocatalytic activity, which gains another relaxation channel of trapping by the doping transition metals.