High Purity Anatase TiO_2 Nanocrystals:Near Room-Temperature Synthesis,Grain Growth Kinetics,and Surface Hydration Chemistry

摘要

High purity,spherical anatase nanocrystals were prepared by a modified sol-gel method.Mixing of anhydrous TiCl_4 with ethanol at about 0 deg C yielded a yellowish sol that was transformed into phase-pure anatase of 7.7 nm in size after baking at 87 deg 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 deg C leads to an increase in grain size of the anatase nanoparticles to 32 nm.By varying the calcination time from 2 to 48 h at 300 deg 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_0t~me~(-E_a/RT) with a time exponent m = 0.286(+-9) and an activation energy of E_a = 32 +- 2 kJ-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.