Local structure modification and phase transformation of TiO2 nanoparticles initiated by oxygen defects, grain size, and annealing temperature

TiO2 nanoparticles acquire complete crystalline anatase phase on thermal treatment of as-prepared anatase TiO2 at 450°C. Anatase-rutile mixed phase and rutile phase are achieved by annealing anatase TiO2 at 700°C and 950°C respectively. The anatase-rutile mixed phase TiO2 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 TiO2 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 Eg 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.