Anatase or rutile TiO₂ nanolayer formation on Ti substrates by laser radiation: Mechanical, photocatalytic and antibacterial properties

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 fundamental 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 super-linear 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.