Dark-blue titanium dioxide: Effect of phenothiazine on structural and optical properties of nanocrystalline anatase TiO2

We studied the effect of the phenothiazine (PTZ) surface doping on the structural and optical properties of nanocrystalline titanium dioxide powder with a single anatase phase (A-TiO2). We found that the mixing and interaction of PTZ and A-TiO2 leads to the intense dark-blue color of PTZ-doped A-TiO2 (A/PTZ). The appearance of new bands in the Fourier transform IR spectrum indicated the formation of PTZ(center dot+) radical cations and reduced Ti3+ species as well as extra oxygen vacancies (V-o) in the TiO2 matrix. The UV-vis absorption spectrum of A/PTZ exhibited an increase in absorption in the visible region ( > 400 nm). The doping of A-TiO2 with PTZ causes a noticeable redshift of the absorption edge and a significant narrowing of the band gap by 0.24 and 0.49 eV for direct and indirect electronic transitions, respectively. The photoluminescence spectra show that the photoluminescence originating from Ti3+ and [V-o-Ti3+] states for A/PTZ is stronger than for A-TiO2 , but the excitonic photoluminescence of A/PTZ is quenched. An increased number of surface defects in A/PTZ can essentially increase the nonradiative charge recombination, and therefore may considerably enhance the photoactivity of the dark-blue TiO2 in the visible range of the spectrum.