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/H-2 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/H-2 plasma at 160 W (gas temperature similar to 381 degrees C) within 5 min, while the color of the thermally treated TiO2 film did not change significantly even in pure H-2 or He/H-2 atmosphere with higher temperature (470 degrees C) and longer time (30 min). This indicated that a more effective hydrogenation reaction happened through RF AP He/H-2 plasma treatment than through pure H-2 or He/H-2 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/H-2 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.