On the true morphology of highly photoactive anatase TiO2 nanocrystals

Abstract

Nanosized titania photocatalysts were prepared from a TiOSO4·2H2O–Na2CO3 mixture via high-energy milling for 5min, and subsequent annealing at a temperature range of 300–700°C/1h. Photoactivity of all samples monitored by electron paramagnetic resonance (EPR) spectroscopy was correlated with phase composition, crystallinity, crystal size and morphology of the annealed products. The anatase nanocrystals with an XRD average size of 24nm and well-faceted bipyramidal morphology exposed mainly by {101} type planes obtained on heating at 700°C, exhibited markedly higher photoactivity than the Aeroxide® P25 TiO2 standard and sample annealed at 600°C. The rapid increase in photoactivity was assigned to complete crystallization of the sample, optimal size, but mainly to the well-developed bipyramidal morphology of anatase nanocrystals and enlargement of the {101} type plane area due to final growth of bipyramids in the 〈001〉 direction.The weak beam dark field (WBDF) transmission electron microscopy (TEM) method was used to characterize morphology of the nanocrystals. Based on the thickness changes along the electron beam incidence inside the individual nanocrystals yielded by WBDF images, it was demonstrated that the mixture of polyhedral shaped anatase projections, such as square-like, hexagonal-like, rhomboid-like, rhombic-like, and rectangular-like prevailing in bright-field (BF) images, corresponds exclusively to tetragonal bipyramidal shape projected along different crystallographic directions. WBDF images of the most often occurring anatase nanocrystal projections are summarized and compared with both the schematic depictions of similarly oriented anatase bipyramids and that recorded in BF images. The difficulties of 3D morphology assessment from the projected nanocrystal shapes are highlighted. Attention is paid to the determination of facets limiting the nanocrystals, using the high resolution (HR)TEM method.