PHOTOCATALYTIC ACTIVITY OF AMORPHOUS-ANATASE MIXTURE OF TITANIUM(IV) OXIDE PARTICLES SUSPENDED IN AQUEOUS-SOLUTIONS

Titanium(IV) oxide (TiO2) powders of various amorphous-anatase composi tions were prepared by heat treatment (573-1073 K) of amorphous TiO2 i n air and characterized by differential scanning calorimetry (DSC), po wder X-ray diffraction (XRD), and BET surface area measurements. An ex othermic peak at ca. 723 K in the DSC pattern was assigned to the crys tallization of amorphous phase to anatase, and its heat was used to ev aluate the weight fraction of amorphous phase. The fraction of anatase crystallites (f(anatase)) was calculated as the remainder after the a morphous phase and contaminated water or organic residue. The XRD data showed that the size of anatase crystallites was slightly decreased b ut almost constant along with the increase in f(anatase), being consis tent with the small change in the BET surface area. These results sugg est that each amorphous particle transforms into an anatase crystallit e of similar size without sintering or crystal growth. The particles o f mixture of amorphous and anatase were used for three kinds of photoc atalytic reactions in deaerated aqueous suspensions: silver metal depo sition with oxidation of 2-propanol by bare powders, and dehydrogenati on of 2-propanol by ex situ and in situ platinized powders. For all th e reactions, the photocatalytic activity of amorphous TiO2 was negligi ble, increased almost linearly with f(anatase), and further improved b y calcination of completely crystallized powder. The negligible activi ty of amorphous TiO2 was attributed to recombination of photoexcited e lectron and positive hole at defects located on the surface and in the bulk of particles.