Polarization Conforms Performance Variability in Amorphous Electrodeposited Iridium Oxide pH Sensors: A Thorough Surface Chemistry Investigation
Author:
Marsh Paul1, Huang Mao-Hsiang1ORCID, Xia Xing1, Tran Ich2, Atanassov Plamen34ORCID, Cao Hung156ORCID
Affiliation:
1. Department of Electrical Engineering and Computer Science, University of California Irvine, Irvine, CA 92697, USA 2. Irvine Materials Research Institute, University of California Irvine, Irvine, CA 92697, USA 3. Department of Chemical and Biomolecular Engineering, University of California Irvine, Irvine, CA 92697, USA 4. Department of Materials Science and Engineering, University of California Irvine, Irvine, CA 92697, USA 5. Department of Biomedical Engineering, University of California Irvine, Irvine, CA 92697, USA 6. Department of Computer Science, University of California Irvine, Irvine, CA 92697, USA
Abstract
Electrodeposited amorphous hydrated iridium oxide (IrOx) is a promising material for pH sensing due to its high sensitivity and the ease of fabrication. However, durability and variability continue to restrict the sensor’s effectiveness. Variation in probe films can be seen in both performance and fabrication, but it has been found that performance variation can be controlled with potentiostatic conditioning (PC). To make proper use of this technique, the morphological and chemical changes affecting the conditioning process must be understood. Here, a thorough study of this material, after undergoing PC in a pH-sensing-relevant potential regime, was conducted by voltammetry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Fitting of XPS data was performed, guided by raw trends in survey scans, core orbitals, and valence spectra, both XPS and UPS. The findings indicate that the PC process can repeatably control and conform performance and surface bonding to desired calibrations and distributions, respectively; PC was able to reduce sensitivity and offset ranges to as low as ±0.7 mV/pH and ±0.008 V, respectively, and repeat bonding distributions over ~2 months of sample preparation. Both Ir/O atomic ratios (shifting from 4:1 to over 4.5:1) and fitted components assigned hydroxide or oxide states based on the literature (low-voltage spectra being almost entirely with suggested hydroxide components, and high-voltage spectra almost entirely with suggested oxide components) trend across the polarization range. Self-consistent valence, core orbital, and survey quantitative trends point to a likely mechanism of ligand conversion from hydroxide to oxide, suggesting that the conditioning process enforces specific state mixtures that include both theoretical Ir(III) and Ir(IV) species, and raising the conditioning potential alters the surface species from an assumed mixture of Ir species to more oxidized Ir species.
Funder
a startup fund from the Henry Samueli School of Engineering, UC Irvine, NSF CAREER Graduate Assistance in Areas of National Need
Subject
Electrical and Electronic Engineering,Biochemistry,Instrumentation,Atomic and Molecular Physics, and Optics,Analytical Chemistry
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