Structural control and photocatalytic properties of photocatalytic TiO2 thin films prepared using two-step deposition including radical-assisted sputtering

TiO2 films were deposited using a two-step growth process, in which an ultra-thin nucleation layer was first produced by reactive-mode sputtering, followed by a thicker growth layer by radical-assisted sputtering. The effect of the structure of the nucleation layer on the density, crystallinity and photocatalytic properties of the final films was investigated. The kinetic energy of sputtered particles reaching the substrate during the first growth step was also considered. This was found to affect the amount of three-dimensional island growth that occurred, and thus the number of seed particles and gaps between them. The optimum structure is found to be one in which few seed particles have undergone island growth, so that there are a large number of gaps. This not only produces a dense final structure, but the crystallinity is improved due to chemical annealing by radicals during the second growth step, leading to a film with excellent photocatalytic properties. This indicates the importance of an initial structure that facilitates absorption and diffusion of radicals.