Foamed single-crystalline anatase nanocrystals exhibiting enhanced photocatalytic activity

Abstract

A novel and facile approach based on an aqueous reaction of ammonium fluotitanate with an alkaline substance was developed in this work to prepare foamed single-crystals (about 50 nm) of anatase TiO₂ exhibiting excellent photocatalytic performances. It is shown that multicrystalline and rice-shaped anatase nanoparticles (around 100 nm in length, 30 nm in width), which have a foamed microstructure and F-dopants, were initially precipitated as a precursor, which can then be readily transformed by calcination at 500 °C into foamed anatase single crystals (about 50 nm in size) with significant amounts of entrapped nanopores (4–10 nm). A fluorine-assisted dehydration/assembly mechanism was believed to be responsible for the formation of such a peculiarly nano-structured precursor, and recrystallization may account for its transformation into the foamed single crystals. Both the precursor nanoparticles and the resultant single crystals were found to have narrowed bandgaps. Though the precursor has already shown good photocatalytic performance in the decomposition of methyl orange, the calcination derived single crystals exhibited even higher activity due to their better crystallinity, smaller size, single crystalline nature and hollow structure. The alkaline substance used for precipitation was found to influence the microstructure and consequently photoactivity of the anatase nanoparticles. It is also shown that, among all the obtained anatase nanomaterials, the foamed single-crystalline nanocrystals, precipitated with sodium hydroxide and having the highest pore density, exhibited photocatalytic activities far superior to Degussa P25, especially under simulated sunlight irradiation. The results obtained herein may shed light on the photoactivity improvement of anatase TiO₂ and investigations of other foamed functional nanostructures.