Effect of ion bombardment on low-temperature growth of TiO2 thin films in DC reactive sputtering with two sputtering sources

Low-temperature, high-rate deposition of photocatalytic TiO2 films is the key to their widespread practical adoption. Although several methods are available for achieving high deposition rates at low temperature, the obtained films have poor crystallinity because of insufficient control over the energy of deposited atoms and surface migration. Here we investigated the feasibility of using radio-frequency (RF) bias sputtering to enhance surface migration by precisely examining the effect of the induced ion bombardment on the crystallinity of films grown from a crystallized TiO2 seed layer. Deposition experiments were conducted using DC reactive sputtering with two sputtering sources and an unheated substrate with an RF bias of 0–70 V. The results clearly showed that RF bias indeed encouraged crystal growth and thus afforded films with improved photocatalytic properties. The largest mean crystallite size was obtained at an RF bias voltage of ~ 50 V; the obtained film exhibited excellent hydrophilicity and had a contact angle of less than 2.0° after 1 h of UV irradiation. Further increase in the bias voltage to above 60 V, however, led to a sudden degradation in crystallinity and photocatalytic properties, probably because of the significant damage caused at such high ion energies.