Structure, phase transformations, and defects of HfO_2 and ZrO_2 nanoparticles studied by ~(181)Ta and ~(111)Cd perturbed angular correlations, ~1H magic-angle spinning NMR, XPS, and x-ray and electron diffraction

摘要

Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO_2 and ZrO_2 synthesized in a microwave-plasma process have been investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), and perturbed angular correlation (PAC) spectroscopy. The PAC technique was used to measure the electric quadrupole interactions (QIs) of the nuclear probes~(181)Ta and ~(111)Cd in nanocrystalline HfO_2 and ZrO_2 as a function of temperature. For comparison, the QI of ~(181)Ta in the bulk oxides was determined in the same temperature range 300 K ≤ T ≤ 1550 K. The oxygen-metal ratio of the as-synthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4 ≤ x ≤ 1.8. A hydrate surface layer with a hydrogen content of 5-10 wt %, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by ~1H magic-angle spinning NMR. XRD and TEM show that bare n-ZrO_2, Al_2O_3-coated n-ZrO_2, and Al_2O_3-coated n-HfO_2 are synthesized in the tetragonal or cubic modification with a particle size d ＜ 5 nm, whereas bare n-HfO_2 is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO_2 is Q_A=32{5) kJ/mol. Coating with Al_2O_3 stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While TEM micrographs of the native nanoparticles reveal a well-ordered cation sublattice, the observation of a broad QI distribution in the PAC spectra suggests a high degree of disorder of the oxygen sublattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature QI measurements. Hafnia nanoparticles persist in the monoclinic (m) phase up to T ≤ 400 K. In coated n-ZrO_2/Al_2O_3, the monoclinic and tetragonal (t) phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic (t or c) n-ZrO_2 presents a sharp m → t transition. A "defect" component involving 30%-40% of the probe nuclei appears in the 181Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ≥ 1200 K. The temperature dependence of this component can be reproduced by assuming that Ta impurities in hafnia and zirconia may trap electrons at low temperatures. The observation that the defect component appears only in nanoparticles with diameter d ＜ 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies (V_O) and/or V_O-hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion is supported by the absence of a size effect for ~(111)Cd probes in HfO_2 and ZrO_2. The temperature dependence of the Ta defect fraction is consistent with a Ta~+ impurity level at E_d～0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.