High purity anatase TiO2 nanocrystals:: Near room-temperature synthesis, grain growth kinetics, and surface hydration chemistry

High purity, spherical anatase nanocrystals were prepared by a modified sol-gel method. Mixing of anhydrous TiO4 with ethanol at about 0 degrees C yielded a yellowish Sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 degrees C for 3 days. This synthesis route eliminates the presence of fine seeds of the nanoscale brookite phase that frequently occurs in low-temperature formation reactions and also significantly retards the phase transformation to rutile at high temperatures. Heating the as-is 7.7 nm anatase for 2 h at temperatures up to 600 degrees C leads to an increase in grain size of the anatase nanoparticles to 32 rim. By varying the calcination time from 2 to 48 h at 300 degrees C, the particle size could be controlled between 12 and 15.3 nm. The grain growth kinetics of anatase nanoparticles was found to follow the equation, D-2 - D-0(2) = k(0)t(m)e((-Ea/RT)) with a time exponent m = 0.286(+/- 9) and an activation energy of E-a = 32 +/- 2 kJ center dot mol(-1). Thermogravimetric analysis in combination with infrared and X-ray photoemission spectroscopies has shown the anatase nanocrystals at different sizes to be composed of an interior anatase lattice with surfaces that are hydrogen-bonded to a wide set of energetically nonequivalent groups. With a decrease in particle size, the anatase lattice volume contracts, while the surface hydration increases. The removal of the surface hydration layers causes coarsening of the nanoparticles.