Bulk-Direct Synthesis of TiO₂Nanoparticles by Plasma-Assisted Electrolysis with Enhanced Photocatalytic Performance

Abstract

A new plasma-assisted electrolysis method has been developed to synthesize amorphous TiO 2 nanoparticles and exploited for the enhanced photocatalytic performance. The method is simple, environmentally friendly, produces nanoparticles directly from bulk metal, and is suitable for mass production. The process was conducted in low-concentration nitric acid electrolyte under a voltage of 450 V, the minimum necessary to produce plasma on the anode surface. The average nanoparticle size was tuned between 16 and 28 nm by controlling electrolyte concentration within the range of 5 to 15 mM. The production rate increased with time, with the maximum of 11.27 g/h. The amorphous TiO 2 nanoparticles were calcined at various temperatures to determine the crystalline structures and to compare their photocatalytic effects. The structure ranged from pure anatase to rutile under various calcination temperatures; the anatase-rutile mixed phase produced at 600 • C showed the highest catalytic performance, with 94% degradation of methylene blue within 30 min owing to a synergetic effect between the phases. This liquid-phase plasma-assisted electrolysis method can pave the way for large-scale synthesis of highly pure metal-based ceramic nanoparticles with narrow size distributions. Titanium dioxide is a semiconducting material that shows chemical stability and noble characteristics. For several decades, it has been extensively studied due to its physical and chemical properties such as wide band gap and efficient photon electron transfer, which allow its application in photocatalysis, 1-3 antireflection coating, 4-6 water splitting, 7-9 and dye-sensitized solar cells. 10-13 Among these, its photocatalytic effect has attracted the most attention; TiO 2 possesses powerful organic decomposition properties under ultraviolet light.Formation of the three main crystal phases of TiO 2 , namely anatase, rutile, and brookite, depends on the synthesis method and calcination conditions. Generally, mixed phases of anatase and rutile show the best photocatalyst performance due to a synergistic effect. Among the pure phases, anatase yields more efficient catalytic performance than rutile because it has a larger band gap (anatase 3.2 eV, rutile 3.0 eV). The wider band gap reduces light absorption and raises the maximum of the valence band to a higher level that is closer to adsorbed molecules' redox potentials. Then, it facilitates electron transfer from the TiO 2 surface to molecules due to the increased oxidation potential of the electrons. This enhances the decomposition of organic molecules absorbed on the surface of the TiO 2 .14 The photocatalytic performance originating from absorbed ultraviolet light is also affected by photogenerated charge carriers such as electrons and holes. 15 Highly excited holes in the valence band diffuse into the outer layer of a particle and react with adsorbed water molecules on the surface, creating reactive •OH radicals. In addition, electrons remaining in the conduction band produce oxide radicals on the parti...

One-Stage Method for Removing Dyes under the Action of Underwater Plasma and Ferrites of Cobalt, Nickel, and Titanium

“…The next step was to obtain oxide structures of D elements. Aqueous solutions of metal salts and electrode materials in buffer solutions (to generate conductivity) were used as starting materials [11][12][13][14][15][16][17][18][19]. The results obtained showed that the method is quite promising, but to obtain pure structures it is necessary to wash or calcine the synthesis products.…”