Effect of annealing temperature on wettability of TiO2/PDA thin films

Abstract

TiO2/PDA thin films are promising candidates for surface coating, wettability is a key factor that influences their applications in surface engineering. In this work, the wettability of TiO2/PDA thin films annealed at different temperatures was investigated. It is found that with the increase of annealing temperature during the preparation process, the wettability of TiO2/PDA thin films first increases and then decreases. The underlying mechanism is attributed to the thin film structure evolution caused by increasing the annealing temperature. The raise of annealing temperature results in larger TiO2 grain size and surface roughness, making the film structure more compact. As a result, the optical band gap of TiO2/PDA thin film decreases, which therefore enhance the light absorption of films and increase the amount of photogenerated holes. The lattice oxygen in TiO2 film can be oxidized to O2 molecule by the trapped photogenerated holes. As O2 molecules release from the surface, oxygen vacancies are generated and occupied easily by water molecules, which make the surface hydrophilic. Therefore, the increase of photogenerated holes results in more oxygen vacancies and then the improvement of hydrophilicity. However, the annealing at 600 °C causes the decomposition of PDA in the thin films and the anatase-to-rutile phase transition in TiO2, which leads to the inverse changes in the wettability.