Time-resolved XAS studies reveal sequential oxidative–reductive formation of Na-doped iridium oxide films with enhanced bio-stimulating performance

Abstract

We synthesize a Na-doped iridium oxide thin film via a sequential oxidative–reductive reaction in which the Ir³⁺ precursor undergoes a ligand exchange and oxidation process by sodium hypochlorite and oxygen to form Ir^>4+, followed by an immediate reduction step to produce Na-doped iridium oxide. Detailed reaction steps occurring in the wet chemical bath are validated by time-resolved X-ray absorption spectroscopy. Analysis using X-ray photoelectron spectroscopy indicates the composition to be Na_0.44IrO_1.55(OH)_1.55(H₂O)_0.73. In addition, the Na-doped iridium oxide reveals an amorphous structure with a smooth surface morphology. In electrochemical characterization, the Na-doped iridium oxide exhibits a significantly larger normalized charge storage capacity of 0.32 mC cm⁻² nm⁻¹ over that of sputtered iridium oxide (0.08 mC cm⁻² nm⁻¹). In a cell viability test, the Na-doped iridium oxide shows a negligible bio-toxicity. In short, the Na-doped iridium oxide demonstrates promising potential as a bio-stimulating electrode for implantable bio-electronics.