Titania nanoengineering towards efficient plasmonic photocatalysis: Mono- and bi-metal-modified mesoporous microballs built of faceted anatase

Preparation of porous micro-sized materials with solar response is urgent for fast commercialization of green technologies based on semiconductor photocatalysis. Here, mesoporous titania microballs composed of nanocrystalline anatase with exposed facets were prepared by a sub-zero sol-gel method followed by hydrothermal/alcohothermal crystallization. The mechanism of microballs formation and the influence of preparation conditions on the properties, and thus resultant photocatalytic activity, were investigated in detail. The photocatalytic performance was examined in four reaction systems under UV/vis irradiation, i.e., oxidative decomposition of methyl orange (MO) dye, hydrogen evolution, degradation of tetracycline (TC) antibiotic, and carbon dioxide reduction. Moreover, hydrogen generation was also examined under visible-light (vis) irradiation. Amorphous and faceted (octahedral- and decahedral-based) microballs were additionally modified with nanoparticles (NPs) of noble metals (Pt, Au, Ag, Pt/Au, Pt/Ag) for both UV-activity enhancement and vis response (i.e., plasmonic photocatalysis). It has been found that nano-architecture of microballs might be controlled by the ratio of alcohol to water used during hydrothermal/alcohothermal treatment. Accordingly, highly active microballs composed of pure octahedral- or decahedral-shaped anatase crystals, i.e., with {101} facets only (bipyramids) or eight {101} and two {001} facets, respectively, could be synthesized by a facile and environmental-friendly method. The versatility and high activity of faceted microballs have been confirmed in both oxidation and reduction reactions under UV and/or vis irradiation (comparable performance to that by famous P25). Decahedral-based samples exhibit usually higher photocatalytic activity than octahedral ones, despite worse photoelectronic properties (charge carriers' separation, electron transport capacity and photocurrent density), due to higher hydrophilicity. However, single type {101} facet photocatalyst (octahedral shape) is preferable for efficient CO2 adsorption and reduction into CO and CH4. Among noble metals, though platinum shows much higher positive effect on UV activity, gold is responsible for the highest activity under vis, which corresponds to the strongest plasmonic filed enhancement, as proven by finite difference time domain (FDTD) simulations. Concluding, micro-sized balls composed of faceted anatase are undoubtedly prospective photocatalyst for diverse environmental applications.