Dopant-free, polymorphic design of TiO₂ nanocrystals by flame aerosol synthesis
2011
Kho, Yung Kent | Teoh, Wey Yang | Mädler, Lutz | Amal, Rose
A dopant-free aerosol synthesis of highly crystalline TiO₂ nanoparticles (20–35nm) with tunable polymorphic content is demonstrated by rapid flame spray pyrolysis. By controlling precisely the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, anatase content as high as 96wt% (4wt% rutile) was obtained at high oxic flame conditions, while rutile content as high as 94wt% (6wt% anatase) was obtained under anoxic flames. The polymorphic variability lies within a narrow range of combustion equivalence ratios, that is, 1.0<Φ<1.5. Unlike any other flame aerosol syntheses, the anatase and rutile crystallite sizes were similar within each sample. Under highly oxic flame conditions (Φ<1.0), twinnings between anatase {011} planes could be observed, inferring oriented attachment taking place. Such mechanism could not, however, be seen under anoxic flame (Φ>1.0) possibly due to physical hindrance by surface carbonaceous content (typically <2wt%). The carbon content can be easily removed by short calcination without significantly affecting the surface areas and crystallite properties of the original TiO₂ nanocrystals, preserving hence its pristine state.
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