Effect of Nb doping on the phase transition and optical properties of sol–gel TiO2 thin films

Various content Nb-doped TiO2 thin films were prepared by sol–gel process. XRD analysis shows that the existence of crystalline TiO2 in anatase and rutile form depends on the Nb content in the examined samples. It is observed that Nb promotes the anatase to rutile phase transition but has a depression effect on the anatase grain growth. It is found that incorporation of about 4 at.% of Nb completely transforms anatase TiO2 to the rutile form at a calcination temperature as high as 900 °C. The mechanism is proposed. Optical analyses show that the films have an average of 60% transmission in visible region. The energy gap values using Tauc's formula have also been estimated. The band gap of rutile Ti1−xNbxO2 solid solutions increases with increasing x.

Research highlights▶ It is found that TiO2 phase transformation is affected by temperature and dopants. ▶ In our work, Nb promotes the anatase to rutile phase transition. This is different from others’ works. ▶ We analyze optical properties. In our work, the band gap of rutile Ti1−xNbxO2 solid solutions increases with increasing x.