Microwave plasma synthesis of Nanopowders

Bare and coated TiO2 nanoparticles with particle sizes d < 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation (PAC) measurements of the electric quadrupole interaction (QI) at the probe nucleus Ta-181 on the metal site of TiO2 at temperatures 290 <= T <= 1450 K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO2 was found. Anatase appears to be the stable form of nanocrystalline TiO2 below d similar to 10 nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d < 5 nm after synthesis to d > 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO2. The data suggest a critical grain size of d similar to 10 nm for the onset of the ordering process. The spectra of coarse-grained TiO2 are reached at a particle size d >= 30 nm. In n-TiO2 coated with Al2O3 and ZrO2 both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO2 cores and in the ZrO2 coating of n-TiO2/ZrO2 at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO2 kernels and the coatings. (