Controlling surface segregation and tuning silver ion release of reactively sputtered Ag/TiOx nanocomposites
Reactively sputtered Ag/TiOx nanocomposite coatings show high potential for antibacterial applications. However, previous studies on such coatings showed unavoidable segregation of silver out to the surface, which results in direct exposure of the potentially harmful Ag nanoparticles (NPs) to the environment and could also lead to a rapid depletion of the Ag source. Moreover, potentially harmful silver NPs are directly exposed to the environment without encapsulation. Tailoring the silver ion release is thus highly promising for enhancing the antibacterial applications of reactively sputtered Ag/TiOx coatings. The present work is concerned with the influence of sputter parameters on the surface morphology and silver ion release properties of reactively sputtered Ag/TiOx nanocomposite coatings. Since surface segregation of silver in such nanocomposites has not been investigated systematically, here we show under which conditions of the reactive sputtering process silver surface segregation occurs and how it can be controlled to obtain Ag/TiOx nanocomposites with a defined morphology. Silver NPs were embedded in or coated on TiOx thin films for 3D and 2D system purposes. Titanium oxide is known for its biocompatibility, transparency, low coat and good dielectric properties. DC reactive magnetron sputtering is selected because it not only provides controllable structures, sizes and morphologies, and allows formation of films with controllable stoichiometry, but it is also straightforward to implement in industrial-scale production with relatively high deposition rates and large area coatings. A series of titanium oxide thin films containing different silver filling factors were synthesized by a DC reactive magnetron co-sputtering method at different total pressure and oxygen conditions. The aim was to minimize Ag oxidation in conjunction with controlled surface segregation. We present an investigation about the influence of total pressure, silver filling factor, and titanium oxide barriers on the surface morphology evolution. Furthermore, the relation between the morphology of the nanocomposites and their optical properties is presented. To gain additional insight into the role that oxygen and plasma conditions played in silver surface segregation, 2D layer-by-layer structures were deposited and the silver surface segregation behavior was investigated by x-ray photoelectron spectroscopy (XPS) as well. The potential of silver ion release is studied by immersion samples in water for several different periods of time. Atomic absorption spectroscopy (AAS) is used to measure the concentration of silver ion in water. Different approaches to tailor the silver ion release are studied in this work. Tuning silver ion release by varying the total gas pressure, oxygen flow rate, silver concentration, barrier deposition rate and barrier thickness is presented. In addition, the kinetics of silver ion release in Ag/TiOx nanocomposites is discussed as well.