Browsing by Subject "electrochemistry"

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    Field-Effect Modulation of Electrochemical Reactions on Ultrathin Semiconductor Electrodes using High-k Dielectric Layer
    (2023-08) Wang, Yuxin
    This thesis focuses on the modulation of heterogeneous charge transfer on ultrathin semiconducting electrodes for enhanced electrochemical reaction kinetics. Instead of changing the chemical composition of a semiconductor, a physical approach is adopted to modulate the solution electrochemistry on ultrathin semiconductor working electrodes by altering the electronic occupation of the electrode. A high-k local gate field-effect transistor (FET) structure is used in this study to realize large modulation ability under low applied gate voltage (V_G). Specifically, the structure in this thesis is fabricated with Pd as the local gate, HfO2 as the dielectric, and an ultrathin ZnO semiconductor (~ 5 nm) as the working electrode. By back-gating this metal/insulator/semiconductor (MIS) FET, charge carriers electrostatically induced by the back-gate lead to band edge shift on the ultrathin semiconductor within the space charge region, thereby manipulating charge transfer kinetics at the electrode/electrolyte interface. The use of HfO2 with a high dielectric constant boosted the injected charge density in the semiconductor, and realized low operation V_G (< 10 V). Cyclic voltammograms (CV) are used to probe reactivity modulation of several outer-sphere redox couples in ionic liquids, demonstrating a continuous and reversible control of electrochemical reactions. Furthermore, steady-state voltammetry was carried out on outer-sphere redox species at back-gated ultrathin ZnO working electrodes to determine electron transfer rate constants (k_ET) as a function of V_G. The Pd/HfO2/ZnO stack's enhanced gating power allowed observation of a non-monotonic dependence of k_ET on V_G, revealing the inherent density of redox acceptor states in solution. This work highlights the independent tuning of electrochemical kinetics at ultrathin working electrodes byV_G, irrespective of the conventional working electrode potential. Additionally, the shift of working electrode potential by back-gating is studied by probing the working electrode potential (V_ZnO) with respect to fixed reference. By this approach, band edge shift on the ultrathin ZnO electrode (∆δ) by back-gating can be determined. This also allows the estimation of quantum capacitance (C_Q), representing the density of states distribution, and separation of electric double layer charging potential (∆ϕ_EDL) at the ZnO/electrolyte interface from the band edge shift (∆δ). In conclusion, the field-effect modulation approach in this thesis essentially constitutes a unique four-electrode electrochemical system, which offers a valuable platform to investigate electronic properties and charge transport on semiconductor electrodes.
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    Geobacter sulfurreducens inner membrane cytochrome transcriptional and phenotypic data
    (2021-06-15) Joshi, Komal; Chan, Chi Ho; Bond, Daniel R; dbond@umn.edu; Bond, Daniel R; Bond Lab
    Geobacter sulfurreducens utilizes extracellular electron acceptors such as Mn(IV), Fe(III), syntrophic partners, and electrodes that vary from +0.4 to −0.3 V vs. Standard Hydrogen Electrode (SHE), representing a potential energy span that should require a highly branched electron transfer chain. Here we describe CbcBA, a bc-type cytochrome essential near the thermodynamic limit of respiration when acetate is the electron donor. Mutants lacking cbcBA ceased Fe(III) reduction at −0.21 V vs. SHE, could not transfer electrons to electrodes between −0.21 and −0.28 V, and could not reduce the final 10% – 35% of Fe(III) minerals. As redox potential decreased during Fe(III) reduction, cbcBA was induced with the aid of the regulator BccR to become one of the most highly expressed genes in G. sulfurreducens. Growth yield (CFU/mM Fe(II)) was 112% of WT in ∆cbcBA, and deletion of cbcL (an unrelated bc-cytochrome essential near −0.15 V) in ΔcbcBA increased yield to 220%. Together with ImcH, which is required at high redox potentials, CbcBA represents a third cytoplasmic membrane oxidoreductase in G. sulfurreducens. This expanding list shows how metal-reducing bacteria may constantly sense redox potential to adjust growth efficiency in changing environments.