Effect of Co2+ doping on Raman spectra and the phase transformation of TiO2:Co2+ nanowires

Co2+-doped anatase TiO2 nanowires were successfully synthesized by hydrothermal method. Inhibitory effect of Co2+-doping on the anatase to rutile phase transformation was investigated by X-ray diffraction, Raman scattering. The replacement of Ti4+ ions with Co2+ ions led to a slight expansion of the unit cell volume and a shift and broadening of all the characteristic Raman scattering modes of the anatase TiO2:Co2+ nanowires. The absorption spectra exhibited the band-band transitions of TiO2 host and the transitions within Co2+ ions located in octahedral field of the anatase TiO2 nanowires. Co2+-doping reduced both the indirect and the direct band gaps. Photoluminescence spectra of the undoped anatase TiO2:Co2+ nanowires showed the transitions between the bands and the transitions related to surface defect states, oxygen vacancies. The Co2+-doping caused only the quenching phenomenon of emission of the TiO2 nanowires.