Unique Properties of Selectively Formed Zirconia Nanostructures

Zirconia has attracted considerable attention because of its diverse practical applications in fuel-cell technology, its use as a catalyst or catalyst support, its ability to combine with sulfuric acid to form extremely strong acids, and its use as an oxygen sensor and as a possible high-dielectric-constant material for large-scale integrated circuits or as a gate dielectric in metal-oxide semiconductor devices. However, the optical properties of this semiconductor, especially its photoluminescence (PL) properties, are seldom reported. Because these PL properties, if properly harnessed, might play an important role in the future improvement of informationstorage devices, the characterization and enhancement of PL from ZrOx may have important implications. Here, we expand the potential of these optical properties. We report not only on the formation of nanometer-sized ZrOx structures which combine into fractal forms, but also on the first synthesis procedure for preparing zirconia-based nanoshells and hollow nanospheres. Our synthesis procedure complements recent spray-pyrolysis studies, which produce hollow nanospheres in the size range 1–10 lm. The formation and growth of nanoshells and nanospheres can greatly dominate the formation of other less-defined zirconia-based nanostructures under certain experimental conditions. The nanoshells produced do not result from solvent-induced nanosphere disruption and they have smooth, as opposed to fractured, surfaces. In contrast to zirconia-based nanoparticles and nanospheres, we find that these “metal-oxide” nanoshell configurations demonstrate a significantly enhanced PL that may result from the geometry of the ultrathin nanoshell walls. In Figures 1a–c, we present representative transmission electron microscopy (TEM) images corresponding to the irregularly shaped and distorted spheroidal structures typically formed under a wide range of experimental conditions. These spheroidal structures readily combine into the fractal-like structure depicted in Figure 1c. In contrast, Figures 2a–e present representative TEM images corresponding to the ZrOx nanoshell and nanospherical forms, which we generated under a set of specific conditions outlined in the Experimental section. These structures should be compared to the nanostructures depicted in Figure 1, which are formed over a much wider range of conditions. Figure 2a represents an overview of nanoshell and nanosphere formation accompanied by the formation of a few hollow elliptical forms. Figure 2b presents a closer view obtained from a spectrum taken of material formed under similar conditions demonstrating both thick(∼ 20 nm) and thin(∼ 5 nm) walled shell formation. We found that thick-walled shells can be generated and grown in regions of slower condensation between a source reactant oxidant nozzle(ZrCl4) containing crucible and a downstream cold finger at ∼ 20 °C. More rapid condensation at the cold finger, enhanced by a larger temperature gradient relative to the reactant oxidation zone, produces thinner-walled shells and some hollow spheres. Figure 2c represents the arrested formation of a hollow nanosphere as it grows from a thick-walled nanoshell in the region (downstream) intermediate to the reaction/oxidation zone and the system cold finger. By examining product formation across this region under appropriate conditions, it is possible to observe several degrees of growth. Provided that initially forming shells diffuse to the cold finger they can be surrounded by a thin-walled shell (Fig. 2d). Figure 2d exemplifies several multishelled/-walled configurations oriented about a central nanosphere where the outer nanoshell is thin-walled. Figure 2e demonstrates a smaller nanosphere located within a thick-walled, nearly nanospherical shell-like structure. A closer view of the hollow nanosphere groupings reveals that they can be interconnected by a permeable channel. In Figures 3a–c Raman spectra taken for the ZrOx nanoshelland hollow-nanosphere configurations can be compared. Figure 3a corresponds to the Raman spectrum and curve fit for ZrOx nanoshells (see Fig. 2b). The curve-fit spectrum is dominated by features associated with crystalline tetragonal (472 620 cm) and monoclinic ZrOx. The features associated with the spectrum of crystalline tetragonal nanoshells are shifted from those of ZrOx powder (Fig. 3c), where they are located at 477.5 and 616.45 cm. The Raman spectrum observed for samples generated under conditions that favor the formation of nanospheres (Fig. 3b) appears to correspond to a broad peak located near 570 cm. This result suggests that the crystallinity associated with the nanoshells is lost in sphere formation (Figs. 2c–e), as these structures are amorphous in character. We have not yet obtained evidence that a crystalline structure can be associated with either the irregular nanostructures of Figure 1 or the formed nanospheres. For comparison, Figure 3c shows the corresponding Raman spectrum of bulk ZrOx powder. The observed features are in agreement with the observations of previous workers. Not C O M M U N IC A TI O N S