PhD Seminar - Shayan Angizi, September 23, 1:30pm

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Title: The Role ofEnvironmental Parameters in The Interactions of Aqueous Electrolytes withGraphene Solid State Devices

Date: Friday, September 23, 2022

Time: 1:30-2:30

Room:  ABB A404

Host: Peter Kruse

Abstract: Since thediscovery of thermodynamically stable monolayer graphene, it has succeeded inovertaking a number of conventional materials in chemical sensing applicationsdue to its exceptional chemical and electrical properties. In addition to beingelectrically conductive, graphene also has a large surface area whichfacilitates faster electronic interaction with analytes. In spite of graphene'sinherent potential for chemical sensing, its application to aqueouselectrolytes has been limited by an incomplete understanding of itsinteractions with the electrolytes’ environmental parameters. This thesisfocuses on mechanisms through which graphene-based solid-state sensors (i.e.,chemiresistors, Schottky diodes) respond to changes in aqueous electrolytes.Multiple environmental parameters, including pH, ionic strength,oxidation-reduction potential, as well as a target analyte (free chlorine), arechosen to examine their impacts on the performance of devices.

 To begin, graphene's pH responsewas explored, showing that its pH sensitivity is strongly defect-dependent. Thegraphene defectivity was determined with the aid of Raman spectroscopy andX-ray photoelectron spectroscopy (XPS). As revealed by measurements of theoxygen-to-carbon ratio (O/C) in XPS and the D-band/G-band intensity ratio (ID/IG)in Raman, graphene responds to pH in two main defectivity regions. In a lowdefect region, the graphene surface was shown to mainly interact withcorresponding ions (i.e., H3O+ and OH)through an electrostatic gating effect. However, in the high defect region, theresponse is dominated by protonation-deprotonation of oxygen-based functionalgroups. Therefore, the modulation of defectivity results in the change in pHresponsivity. According to this result, we demonstrate that thermally reducedgraphene oxide could be highly pH-sensitive to the pH range of 3-10 bydominating the defect induced pH response.

 Aside from pH, the impacts ofchanges in ionic strength, DO, and ORP of the electrolytes were investigated.We demonstrate that graphene chemiresistive devices can be used to investigatedeviations in experimental screening lengths from the theoretical Debye length.We also present an overview of ion arrangements in the proximity of graphene,emphasizing the importance of DO in the Stern layer.

 Lastly, the development of anultra-sensitive water quality sensor was shown by utilizing monolayer graphenein Schottky diodes. For the case study, free chlorine, a primary disinfectantof water, was chosen as the target analyte. Schottky diodes are demonstrated tooffer sensitivity and LOD values competitive with current literature whenenvironmental parameters are taken into account. I believe that this thesisprovides a deeper understanding of graphene's applicability in aqueous mediaand opens new research avenues in graphene/aqueous interfacial interactions.

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