Raman microprobe study of nanophase TiO 2 and oxidation-induced spectral changes
- PDF / 634,679 Bytes
- 7 Pages / 593.28 x 841.68 pts Page_size
- 15 Downloads / 171 Views
A Raman microprobe study of as-compacted nanophase TiO2 was carried out to investigate the spatial inhomogeneity of its anatase and rutile phases. Also, changes in the observed Raman spectra (line shifts and broadening) were investigated as a function of annealing at temperatures up to 600 °C in argon or air. Microscopic phase inhomogeneity is observed and Raman spectral changes are shown to result from inhomogeneous oxygen deficiency in the nanophase TiO2. The line positions corresponding to the Raman active Eg modes in both anatase and rutile are found to be sensitive to this oxygen deficiency and are potential quantitative indicators of such deviations from stoichiometry.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURES
Nanophase materials,1 with their nanometer-scale grain structures and properties that are often significantly different and improved relative to their coarsergrained counterparts, have stimulated much recent interest.2'3 The capability to synthesize such materials via the assembly of atomic clusters formed using the gas condensation method4'5 has opened up a wide range of opportunities for both interesting scientific studies and novel applications. The characterization of these new materials has already utilized a variety of materials research tools, including conventional macroscopic Raman spectroscopy. Previous macroscopic Raman measurements6 on nanophase TiO2 showed that both the rutile and anatase phases of TiO2 were present in the samples. A large inhomogeneity in the anatase phase was observed, with the majority of the sample being dominated by rutile. The anatase phase could be completely removed from the material by annealing the samples at temperatures above 875 °C. It was also shown that the grain size within the as-compacted sample had no effect on the Raman spectrum, and it was suggested that intragrain defects due to oxygen deficiency would be the dominant mechanism in affecting the line shapes of the Raman lines of nanophase TiO2. With these considerations in mind, we began our investigation into the microscopic spatial inhomogeneity of the Raman spectrum of as-compacted nanophase TiO2 samples. Previous measurements7 using the nanoindenter technique indicated significant microscopic inhomogeneities in the mechanical properties (hardness, strain-rate sensitivity) of nanophase TiO2, which were attributed to microstructural inhomogeneities in these materials.
The nanophase TiO2 was prepared using the gas condensation method. In this method as applied to TiO2,8 atomic clusters of Ti are first formed by evaporation of Ti into pure He gas at about 0.5 kPa and then oxidized by the rapid introduction of about 2 kPa of O2 into the gas condensation chamber. The resulting TiO2 particles are then consolidated in situ, under highvacuum conditions (^10~6 Pa), at a pressure of 1.4 GPa. The as-compacted samples used in the present study were disks 9 mm in diam and 0.3-0.5 mm thick, with average grain sizes of about 12 nm. The Raman instrumentation consisted of a Coherent Innova 70 Ar + ion laser and
Data Loading...
