Flame synthesis of semiconductor photocatalysts for solar photochemical conversion

摘要

A range of UV-light active TiO2 and visible-light active BiVO4 photocatalysts were designed and synthesized by flame technology. The polymorphic design of dopant-free TiO2 nanoparticles with tunable phase compositions and closely controlled crystallite sizes using flame spray pyrolysis (FSP) has been successfully demonstrated. By controlling the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, highly crystalline TiO2 nanoparticles (20–35 nm) having 6-96 wt.% anatase (remaining as rutile) was obtained. The wide range of anatase-rutile compositions with a narrow variation in crystallite sizes has allowed the exclusive investigation of the effect of TiO2 polymorphic content on photocatalytic H2 production and electron transport properties in dye-sensitized solar cells (DSSCs). Synergistic effects, where mixed anatase-rutile phase exhibited much higher H2-evolution activity, were observed and attributed to more efficient charge separation because of the migration of electrons across the phase junction. Formations of highly reducing hydroxymethyl radicals during the simultaneous oxidation of methanol, which inject additional electrons to the TiO2 conduction band, were also identified. In DSSCs, electron diffusion coefficient was found to increase with increasing anatase content, while the electron lifetime depended on anatase-rutile composition, as well as the semiconductor trap conditions. Under the same trapping effect, rutile-rich TiO2 exhibited longer electron lifetime. The different charge transport behavior in anatase and rutile was attributed to the crystallographic arrangement in the unit cell. Despite the difference in charge transport behavior, photochemical conversion efficiency of anatase was found to be similar to that of rutile in thin film (3-4 m) solar cell devices. The last part of the work concerns the development of visible-light active BiVO4 photocatalysts. Structural evolution from amorphous to crystalline scheelite-tetragonal subsequently to scheelite-monoclinic was obtained by in situ flame annealing during FSP synthesis. Increased crystallinity and scheelite-monoclinic content with increased filter temperature had systematically improved the photocatalytic O2-evolution activity. Treatment of flame-made BiVO4 under aqueous nitric acid with addition of small amount of Bi and V promoted crystallization and transformation to pure scheelite-monoclinic phase, reducing charge-trapping defects, therefore giving 5-fold improvement in the photocatalytic O2 evolution.