Anatase or Rutile TiO2 Nanolayer Formation on Ti Substrates by Laser Radiation: Mechanical, Photocatalytic and Antibacterial Properties
Abstract:
A laser-induced oxidation method for the formation of a TiO2 layer on a Ti substrate was used. The TiO2 phase can be controlled by an Nd:YAG laser with basic frequency at an intensity I=52.8 MW/cm2 and three different doses. Dose D1=3.1x1020 phot/cm2 forms a TiO2 layer in the anatase phase, which possesses the highest photocatalytic, antibacterial and adhesion properties. As the laser dose increases, the TiO2 layer thickness increases from 40 nm to 100 nm, but the photocatalytic decomposition reaction constant decreases. The observed superlinear increase of the TiO2 layer thickness with the laser dose is explained by the presence of positive feedback during the irradiation process. The temperature rises with increasing of the thickness due to the interference-caused decrease of the reflection coefficient. As the thickness increases, TiO2 on Ti structure adhesion decreases from 800 mN to 400 mN due to the formation of a layer with a mixture of phases. The colonization intensities of P. Aeruginosa and S. Epidermidis bacteria decrease more than tenfold after TiO2 formation. These results are explained by the partial transformation of the TiO2 layer, formed in the anatase phase at dose D1, into the rutile one at doses D2 and D3 due to a deficit of O atoms caused by the low diffusion of O atoms in Ti. According to our experiments and calculations, using the Wagner oxide model, the laser technology can be used to form crystalline structures with a monolayer precision.