The effects of radio frequency atmospheric pressure plasma and thermal treatment on the hydrogenation of TiO₂ thin film

The effects of radio frequency (RF) atmospheric pressure (AP) He/H₂ plasma and thermal treatment on the hydrogenation of TiO₂ thin films were investigated and compared in this work. The color of the original TiO₂ film changes from white to black after being hydrogenated in He/H₂ plasma at 160 W (gas temperature ~381 ℃) within 5 min, while the color of the thermally treated TiO₂ film did not change significantly even in pure H₂ or He/H₂ atmosphere with higher temperature (470 ℃) and longer time (30 min). This indicated that a more effective hydrogenation reaction happened through RF AP He/H₂ plasma treatment than through pure H₂ or He/H₂ thermal treatment. The color change of TiO₂ film was measured based on the Commission Internationale d'Eclairage L^*a^*b^* color space system. Hydrogenated TiO₂ film displayed improved visible light absorption with increased plasma power. The morphology of the cauliflower-like nanoparticles of the TiO₂ film surface remained unchanged after plasma processing. X-ray photoelectron spectroscopy results showed that the contents of Ti³⁺ species and Ti–OH bonds in the plasma-hydrogenated black TiO₂ increased compared with those in the thermally treated TiO₂. X-ray diffraction (XRD) patterns and Raman spectra indicated that plasma would destroy the crystal structure of the TiO₂ surface layer, while thermal annealing would increase the overall crystallinity. The different trends of XRD and Raman spectra results suggested that plasma modification on the TiO₂ surface layer is more drastic than on its inner layer, which was also consistent with transmission electron microscopy results. Optical emission spectra results suggest that numerous active species were generated during RF AP He/H₂ plasma processing, while there were no peaks detected from thermal processing. A possible mechanism for the TiO₂ hydrogenation process by plasma has been proposed. Numerous active species were generated in the bulk plasma region, accelerated in the sheath region, and bumped toward the TiO₂ film, which will react with the TiO₂ surface to form OVs and disordered layers. This leads to the tailoring of the band gap of black TiO₂ and causes its light absorption to extend into the visible region.