Structure, optical absorption and electronic states of Zn⁺ ion implanted and subsequently annealed sol-gel anatase TiO₂ films

Zn⁺ or both Zn⁺ and O⁺ ions were implanted in porous anatase TiO₂ films prepared by sol-gel method and subsequently annealed in N₂ or O₂ atmosphere. The results were compared with that obtained after Ar⁺ ion implantation and subsequent annealing. The samples were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED) and X-ray diffraction (XRD). Optical absorption and electronic states were characterized by photothermal deflection spectrometry (PDS) and electron energy loss spectrometry (EELS). The porous film was densified by Zn⁺ ion implantation up to the ion penetration depth. After the subsequent annealing at 800 °C the phase transformation from anatase to rutile accompanied with grain growth up to the film thickness was observed. In addition, the phase transformation was not induced by the annealing up to 800 °C with or without preceding Ar⁺ ion implantation. Thus, the implanted impurity Zn assisted the phase transformation. Annealing in O₂ tends to reduce the rate of phase transformation and create ZnTiO₃. Optical absorption above the photon energy of 2.9 eV was increased remarkably by the Zn⁺ or Zn⁺ and O⁺ ion implantation and subsequent annealing. EELS spectra of the Zn⁺ implanted and annealed sample is consistent with the results of PDS. The change in the optical absorption above 2.9 eV is due to the phase transformation.