OICC Press International Nano Letters 2228-5326 3 1 (December 2013) 2013 11 14 Local structure modification and phase transformation of TiO2 nanoparticles initiated by oxygen defects, grain size, and annealing temperature 10.1186/2228-5326-3-55 EN Biswajit Choudhury Department of Physics, Tezpur University, Napaam, Assam, 784028, IN Amarjyoti Choudhury Department of Physics, Tezpur University, Napaam, Assam, 784028, IN Journal Article 2013 11 14 Abstract TiO 2 nanoparticles acquire complete crystalline anatase phase on thermal treatment of as-prepared anatase TiO 2 at 450°C. Anatase-rutile mixed phase and rutile phase are achieved by annealing anatase TiO 2 at 700°C and 950°C respectively. The anatase-rutile mixed phase TiO 2 has 87.8% rutile phase. This signifies that the percentage of rutile fraction in mixed phase can be tailored by changing the annealing temperature. As-prepared anatase TiO 2 with a crystallite size of 5 nm has a positive strain ( η ) of 0.0345, which is due to the presence of oxygen defects on the surface and on the grain boundary. Removal of defects releases the strain and relaxes the lattice to its normal state, and thus, a negative strain η of (−) 0.0006 is observed in complete rutile phase. The interface between nearest anatase crystallites and between anatase and rutile crystallites contains oxygen vacancies that act as nucleation site for the growth of rutile nuclei. These oxygen defects are responsible for the broadening of the Raman E g peak of anatase and for the shortening of the phonon lifetime in a 5-nm-sized anatase nanocrystallite. Removal of defects decreases the Raman peak width and increases the phonon lifetime in a larger rutile crystallite. The long lifetime of phonon in a larger rutile crystallite is due to temperature-dependent anharmonic phonon coupling.