Preparation and characterization of nanocrystalline titania powders by sonochemical synthesis

Nanocrystalline mesoporous titania powders were synthesized by hydrolyzing titanium isopropoxide in ethanol–water mixtures which were ultrasonically treated without using any templates or chemicals. Titanium isopropoxide–ethanol mixture was added dropwise to a water–ethanol mixture placed in an ultrasonic bath. The properties of the sonochemically synthesized powder were compared with those of the powders prepared without ultrasonic treatment along with Degussa P-25 titania powder. The phase structure, crystallite size, surface area, particle size, powder density were determined and sintering behavior was analyzed in this work. The nanotitania powder prepared during ultrasonic induced hydrolysis (TiO2-U) was determined to be formed from a mixture of anatase and brookite phases at 25 °C. The brookite phase in nanotitania powder prepared without ultrasonic treatment (TiO2-NoU) was detected at 70 °C. The anatase–rutile phase transformation was completed in the 500–700 °C range for both powders. The average crystallite sizes of the powders at 25 °C were determined as 10 and 5 nm for TiO2-NoU and TiO2-U, respectively. The surface area decreased from 238 to106 m2/g for TiO2-NoU and from 287 to 82 m2/g for TiO2-U when the calcination temperature was increased from 200 to 500 °C. The evolution of the N2 adsorption–desorption behavior with calcination temperature and the corresponding pore size distributions/volumes was attributed to the formation of closely packed submicron aggregates during powder synthesis and calcination. The sintering behavior was concluded to be controlled by 7–10 nm crystallites and the submicron aggregates. The determination of the densification behavior of titania powders prepared by different methods with various levels of dopants may prove to be very useful for a better understanding of the phase/pore structure evolution which is crucial for a significant number of applications.