Influence of crystallization conditions and of gaseous ammonia treatment on mesoporous TiO2 properties

Abstract

After surfactant removal by ethanol extraction, amorphous mesoporous titania, synthesized through a soft templating method have been crystallized under different atmosphere: synthetic air, nitrogen followed by exposition to oxygen flow and pure nitrogen at 300, 350 or 380 °C. The photocatalysis efficiency towards methyl orange degradation of the materials crystallized under oxidizing conditions increases with the calcination temperature thanks to the enhancement of crystallinity and the growth of anatase particles. However, if samples are heated under nitrogen followed by exposition to oxygen flow the mesopores collapse during the transformation of the amorphous titania walls at 380 °C. By contrast, under pure nitrogen atmosphere the photodegradation of the dye is not governed by the anatase particles but it is related to the specific surface area of the photocatalyst. A good compromise between avoiding the collapse of the mesostucture and improving photocatalytic efficiency consists in crystallizing the amorphous TiO2 under synthetic air at 380 °C.In the synthesis procedure, to avoid the mesostucture collapse upon surfactant removal, a treatment of the hybrid mesophase under ammonia atmosphere is needed to complete the condensation of the titania precursor. Because of the possible interactions between ammonia and amorphous titania but also with surfactant, a wormhole-like structure is obtained, when the hybrid mesophase is exposed either at high NH3 pressure or for a long time. Conditions under which this step is performed also strongly affect the photocatalytic efficiency of the crystallized TiO2. Our results show that when the ammonia pressure is increased or when the treatment is prolonged, interaction of NH3 with amorphous titania through Lewis adsorption promotes the nucleation of rutile. Therefore, upon calcination less amorphous TiO2 particles can be transformed into nanosized anatase. Consequently, a decay of the photocatalytic efficiency is noted.