Shirley Turner

Objective: To characterize the morphology of voids in rutile nanoparticles using transmission electron microscopy techniques.

Problem: Characterization of the morphology of nanoparticles relative to their crystallographic orientation can be important for understanding the properties of these materials and for the development of new applications of the materials. For example, some crystallographic planes in catalyst particles are more reactive than other planes and therefore preferable for promoting some reactions. The characterization of nanoparticle morphology relative to crystallographic orientation is challenging, in part, due to the small size of the particles and due to difficulties in their manipulation.

In this study, TiO2 (rutile) nanoparticles containing voids are characterized. In previous work, it was shown that some rutile nanoparticles generated in a flame burner system contain central inclusions. These inclusions were characterized as voids or cavities using electron holography. Similar cavities have been noted in palladium nanoparticles. The morphology of these facetted cavities is of potential interest in determining volume changes in transformation reactions. If the cavities have reached equilibrium, the morphology is also of interest in determining relative surface energies of crystals. The morphology of cavities in nanoparticles had not been previously determined.

Approach: Rutile nanoparticles containing voids were characterized using selected area electron diffraction (SAED) and imaging by transmission electron microscopy (TEM). By TEM, only two-dimensional projections of particles are obtained and therefore many orientations of the particles were obtained to build up a three-dimensional model of morphology. In some limited cases, it was possible to tilt a sample around a major crystallographic axis, thereby simplifying the interpretation of morphology.

Figure 1: Proposed model for cavities in some rutile nanoparticles.

Results and Future Plans: Many of the voids are consistent with a prismatic morphology with dipyramid terminations. The prism consists of primarily four {110} faces with rounded or faceted corners between the primary faces. A major facet plane of the pyramids is (101). Figure 1 is a proposed model for the morphology of many of the voids. Future work will include use of a new sample holder for improved particle manipulation. The external morphology of some nanoparticles will be characterized with electron microscope techniques.

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