Evaluation of microwave plasma oxidation treatments for the fabrication of photoactive un-doped and carbon-doped TiO2 coatings

The photoactivity of both un-doped and carbon-doped titanium dioxide (TiO2) coatings has been widely reported. In this paper, the use of a microwave plasma as a novel oxidation treatment for the fabrication of these coatings is evaluated. The photoactivity performance of the microwave plasma-formed coatings is benchmarked against those fabricated through air furnace oxidation as well as those deposited using reactive magnetron sputtering. The un-doped and carbon-doped TiO2 coatings were prepared respectively by microwave plasma-oxidizing titanium metal sheets and sputter deposited titanium carbide thin films. The resulting oxides were characterized using XPS, XRD, FEG-SEM, and optical profilometry. The oxide layer thicknesses achieved over the 15 to 45 minute oxidation times were in the range of 0.15 to 3.44 µm. These coatings were considerably thicker than those obtained by air furnace oxidation. The microwave plasma-formed oxides also exhibited significantly higher surface roughness values compared with the magnetron-sputtered coatings. The photoactivity performance of both un-doped and carbon-doped coatings was assessed using photocurrent density measurements. Comparing the un-doped TiO2 coatings, it was observed that those obtained using the microwave plasma oxidation route yielded photocurrent density measurements that were 4.3 times higher than the TiO2 coatings of the same thickness that were deposited by sputtering. The microwave plasma-oxidized titanium carbide coatings did not perform as well as the un-doped TiO2 probably due to the presence of un-oxidized carbide in the coatings, which reduced their photoactivity.