Aqueous Isolation of 17-Nuclear Zr/Hf Oxide Clusters during the Hydrothermal Synthesis of ZrO2 /HfO2

Abstract

Novel 17-nuclear Zr/Hf oxide clusters ({Zr₁₇ } and {Hf₁₇ }) are isolated from aqueous systems. In the clusters, Zr/Hf ions are connected through μ₃ -O, μ₃ -OH, and μ₂ -OH linkages into a pinwheel core which is wrapped with SO₄ ^2- , HCOO^- , and aqua ligands. Octahedral hexanuclear Zr/Hf oxide clusters ({Zr₆ }_oct and {Hf₆ }_oct ) are also isolated from the same hydrothermal system by decreasing the synthesis temperature. Structures, synthetic conditions, vibrational spectra, and ionic conductivity of the clusters are studied. Structural studies and synthesis inspection suggest that formation of {Zr₆ }_oct and {Zr₁₇ } involves assembly of the same transferable building blocks, but the condensation degree and thermodynamic stability of the products increase with hydrothermal temperature. The role of {Zr₆ }_oct and {Zr₁₇ } in the formation of ZrO₂ nanocrystals are then discussed in the scenario of nonclassical nucleation theory. In addition, the Zr oxide clusters exhibit ionic conductivity owing to the mobility of protons. This study not only adds new members to the Zr/Hf oxide cluster family, but also establishes a connection from Zr⁴⁺ ions to ZrO₂ in the hydrothermal preparation of zirconium oxide nanomaterials.

Keywords: IVB elements; ZrO2; crystallization mechanisms; metal oxide clusters; prenucleation clusters.