Electrochemical investigation of uranyl species reduction in alkaline oxalate electrolyte and microstructural characterization of deposited nanocrystalline UO2 thin films

In this study, electrochemical reduction behaviour of uranyl ions was investigated from an alkaline oxalate complex electrolyte. Detailed results of scan rates, pH and temperature dependent cyclic voltammetry were analysed to extract diffusion coefficients and to elucidate reduction mechanism. A two-step mechanism was established where the first step involving reduction of UO22+ to UO2+ was a slower and reversible in nature and the second step was proton assisted reduction of UO2+ to UO2. Detailed speciation study of the electrolyte confirmed [UO2(C2O4)3]4− as the predominant species in the pH range 5.0 to 8.5. XPS analyses of as-deposited oxide film confirmed about the formation of UO2 with a small percentage of UO3. For annealed films (at 973 K for 1 h), majority of oxide film was present in mixed valence state containing U6+ and U5+ with a small amount of U4+. High resolution transmission electron microscope (HR-TEM) examination of as-deposited film revealed the deposition of an ultra-nanocrystalline UO2 film with average gain size of 3.0 nm. However, annealed sample showed substantial grain growth associated with reduced inter-grain amorphous region. The average grain size obtained was 9.8 nm with lattice fringe spacing was 0.32 nm corresponding to the fcc UO2.