Effect of cobalt concentration on structural and optical band gap of TiO2/CoOX thin films

Cobalt incorporation in TiO 2 thin films enables modulation of optical and electronic properties across distinct regimes, ranging from substitutional doping at low concentrations to secondary phase formation at higher levels. In this work, CoO x –TiO 2 thin films were deposited by reactive magnetron sputtering using a mosaic Ti–Co target, allowing the growth of films with controlled cobalt concentration gradients under identical plasma conditions. Structural and chemical analyses (XRD and XPS) confirmed the coexistence of Co-doped TiO 2 with CoO and Co 3 O 4 phases, reflecting the transition from substitutional incorporation to phase-segregated regimes. AFM and SEM/EDS analyses revealed smooth, homogeneous surfaces with uniform elemental distribution. Optical measurements showed a systematic red shift of the absorption edge, accompanied by a reduction of the indirect band gap from 2.9 eV to 1.8 eV as cobalt content increased. This band gap narrowing is attributed to Co-related impurity states, oxygen-vacancy–induced defects, and contributions from secondary phases, which introduce localized states within the band gap. Overall, the results demonstrate that mosaic-target sputtering is an effective single-step strategy for investigating composition-dependent phenomena in Co–TiO 2 systems, offering a scalable approach for tailoring optoelectronic properties across both doping and phase-segregated regimes.