Titanium dioxide (TiO2) has been widely investigated as photocatalyst because of its environmentally and economically advantages with high chemical stability, earth abundant and biocompatible properties. However, its large band gap for the activity to only UV light region, and the high recombination rate of photogenerated electron and hole pairs have to be overcome to utilize effectively sunlight, and to enhance the photocatalytic performance. Recent enormous efforts to overcome the above-mentioned drawbacks have resulted in the one-dimensional TiO2 nanotubes, nanofibers and nanorods to suppress the carrier recombination, and/or the heterojunction structure of TiO2 with another semiconductor to achieve larger separation of the photogenerated electron and hole, as well as the modification of TiO2 nanoparticles with gold clusters to expand the light conversion from UV to visible and near-infrared region. Another interesting approach on black TiO2 nanoparticles succeeded in narrowing the band gap of pure white TiO2 nanoparticles, however, it required the hard treatment condition of a 2 MPa hydrogen atmosphere at ca. 200 °C for 5 days (X. Chen, et al., Science, 331 (2011) 746). In order to synthesize the hydrogenated TiO2 nanoparticles, the thermal treatment under hydrogen and plasma treatment have mostly relied on the reduction of TiO2 nanoparticles. But, these processes have drawbacks such as high temperature over 1,000 °C, vacuum system for hydrogen plasma and long treatment time. After thermal treatment, the sintered nanoparticles should be crush to follow multistep processes. Thus, alternative approaches have been highly demanded in the reduction of TiO2 with keeping its nano-sized particle.On the other hand, non-equilibrium plasma, in which the electron temperature is very high and deviates from the ion temperature, is an attractive reaction field because it can achieve low temperature processes in material synthesis and other applications. In particular, when plasma is generated in a liquid, reaction processes below the boiling point of the liquid become possible, and a special reaction field can be expected to be formed. In a so-called solution plasma using a bipolar pulse power supply (O. Takai, Pure Appl. Chem., 80 (2008) 2003), the solvent was water, and plasma was generated in an air-fed environment within solvent. Therefore, the surface modification of TiO2 nanoparticles in a solution plasma reaction field to introduce oxygen vacancy on the sub-surface while maintaining the nano-size has succeeded in improving photocatalytic activity (S. Pitchaimuthu, C. Terashima et al., ACS Omega 3 (2018) 898). The present study focused to treat pristine TiO2 nanoparticles by the discharge in water-based solution and to investigate the material properties as well as the photocatalytic activities for decomposing organics.
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