Size dependence of thermal stability of TiO2 nanoparticles

Anatase TiO2 nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to 125 m(2)/g. The size effects on the stability of TiO2 in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700-800 degreesC. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ/mol with the decrease of initial anatase particle size from 23 to 12 nm. The decreased thermal stability in finer nanoparticles was primarily due to the reduced activation energy as the size related surface enthalpy and stress energy increased. (C) 2004 American Institute of Physics.