Up- and Down-Conversion Cubic Zirconia and Hafnia Nanobelts

One-dimensional (1D) metal oxide nanomaterials, such as zinc oxide (ZnO) nanowires and nanobelts (or nanoribbons), have recently attracted immense attention due to their sizeand shape-dependent optical, mechanical, and electronic properties. In stark contrast, investigations of 1D zirconia (ZrO2) and hafnia (HfO2) nanomaterials remain unexploited largely because of the formidable challenges associated with the fabrication of these structures with controlled dimensions and crystal phases. ZrO2 and HfO2 materials typically exhibit three primary polymorphs (monoclinic, tetragonal, and cubic) and have important technological applications as catalyst supports, oxygen detectors, hightemperature fuel cell electrolytes, gate dielectric in metaloxide semiconductor devices, and optical waveguides. Apart from a few rare examples of rod-like polycrystalline structures formed via porous alumina templates or by using an inverse microemulsion technique, most of the ZrO2 nanomaterials obtained thus far are best classified as nanoparticles and thin films. Here, we introduce a direct and scalable approach, based upon a thermal decomposition method under normal atmospheric pressure, for growing well-defined single-crystalline cubic ZrO2 and HfO2 nanobelts with controllable structural compositions and novel optical properties. To the best of our knowledge, these nanomaterials provide the first evidence of ZrO2 and HfO2 crystals with belt-like morphology. In a typical synthesis of the ZrO2 precursors, a solution of ZrCl4 (1.6 g, 7 mmol) and YCl3 6H2O (0.3 g, 1 mmol) in ethanol (10mL) was first treated with sodium hydroxide (1.5 g, 38 mmol) to yield a viscous gel mixture. The mixture was transferred into a 15mL Teflon-lined autoclave and heated at 140 8C for 24 h. Upon cooling the resulting precipitate was