ASSEMBLY OF URANYL PEROXIDE NANOCLUSTERS ON SOLID SURFACES: PRELIMINARY RESULTS

Uranyl peroxide nanoclusters are polyoxometalates that may by synthesized to have a wide range of chemical compositions, sizes and topologies. The readily tunable nature of these clusters makes them exciting prospects for use in future nuclear fuel cycles and in the successful management of uranium in the environment. In this work, we attempt to develop an understanding of the interaction between different species of uranyl peroxide nanoclusters (e.g., U₆₀, U₂₄, and U₂₄P) and different solid surfaces of potential geo-environmental significance (e.g., muscovite, calcite, zeolite and glass). Surface imaging using atomic force and scanning electron microscopy has shown that certain clusters can readily assemble into a complex array of well-formed, micron-scale geometrical shapes on different surfaces. For instance, the U₆₀ nanocluster can form both large (up to 10-15 μm in cross-section) hexagonal and rectangular shapes on the surface of muscovite, whereas U₂₄ forms small (to <5 μm in cross-section) squares. On glass, however, U₆₀ is commonly observed to form complex dendritic growth features in addition to squares and hexagons. Imaging by atomic force microscopy also reveals that nanocluster surface structures consistently grow in a uniform, layer-by-layer manner, where the height of each layer corresponds to the diameter of the cluster being deposited; no spiral growth has ever been observed. Further, timed experiments show that growth occurs very rapidly (i.e., within minutes) where, in the case of U₆₀ nanocluster deposition, up to 25 layers may be deposited per minute. In addition, current results suggest that rates at which surface features grow may be related to the point group symmetry of the nanocluster: those with the highest symmetry (e.g., U₆₀, point group 4/m-32/m) are deposited much faster than those with lower symmetries (U₂₄P, point group 2/m2/m2/m).