Combination of electrophoresis and electro-flocculation for the formation of adhering IrO₂ pH sensing films

We demonstrate a simple scheme to produce dense and adhering IrO₂ thin films on ITO substrates via a bipolar pulsing process combining both electrophoresis and electro-flocculation. Our fabrication steps entail the synthesis of negatively-charged IrO₂ nanoparticles in size of 3.5 nm using NaClO as an oxidizer in conjunction with Na₃IrCl₆, citric acid, and NaOH. Subsequently, multiple bipolar potentiostatic pulses are imposed in which alternating positive and negative potentials engender the electrophoresis of IrO₂ nanoparticles, followed by their immediate coalescence facilitated by proton-catalyzed electro-flocculation. The resulting IrO₂ thin films reveal impressive physical properties in surface uniformity, robust structural integrity, and strong adhesion to the substrate. The as-synthesized IrO₂ thin film exhibits an amorphous nature with a pH sensing slope between −73.6 and −71.5 mV/pH, characteristic of super-Nernstian behavior. In contrast, the annealed IrO₂ sample demonstrates a crystalline rutile phase with a pH sensing slope between −49.5 and −54.7 mV/pH, consistent with typical Nernstian response. Our approach provides a new route for the formation of strong and adhering oxide films from their colloidal constituents.