The high speed channel electrode applied to heterogeneous kinetics: the oxidation of 1,4-phenylenediamines and related species in acetonitrile

Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes: Field-Effect Modulation of Heterogeneous Electron Transfer Rate Constants by 30× with Enhanced Gate Capacitance

We report steady-state voltammetry of outer-sphere redox species at back-gated ultrathin ZnO working electrodes in order to determine electron transfer rate constants kET as a function of independently controlled gate bias, VG. We demonstrate that kET can be modulated as much as 30-fold by application of VG ≤ 8 V. The key to this demonstration was integrating the ultrathin (5 nm) ZnO on a high dielectric constant (k) insulator, HfO2 (30 nm), which was grown on a Pd metal gate. The high-k HfO2 dramatically decreased the required VG values and increased the gate-induced charge in ZnO compared to previous studies. Importantly, the enhanced gating power of the Pd/HfO2/ZnO stack meant it was possible to observe a nonmonotonic dependence of kET on VG, which reflects the inherent density of redox acceptor states in solution. This work adds to the growing body of literature demonstrating that electrochemical kinetics (i.e., rate constants and overpotentials) at ultrathin working electrodes can be tuned by VG, independent of the conventional electrochemical working electrode potential.

Voltammetric Monitoring of Transient Hydrodynamic Flow Profiles in Microfluidic Flow Cells

We consider the transition to steady-state flow in the inlet region of a hydrodynamic channel cell and show that a microelectrode positioned within this inlet region allows chronoamperometric results to be recorded, from which information about the extent of the development of the flow profile may be deduced as well as information about the precise dimensions of the microfluidic channel.

Marcus Theory of Outer-Sphere Heterogeneous Electron Transfer Reactions: Dependence of the Standard Electrochemical Rate Constant on the Hydrodynamic Radius from High Precision Measurements of the Oxidation of Anthracene and Its Derivatives in Nonaqueous Solvents Using the High-Speed Channel Electrode

The validity of Marcus theory for outer-sphere heterogeneous electron transfer for the electro-oxidation of a range of anthracene derivatives in alkyl cyanide solvents is investigated. The precision measurement of these fast electron transfers (k(0) >or= 1 cm s(-1)) is achieved by use of the high-speed channel electrode and, where necessary, fast-scan cyclic voltammetry. First, the solvent effect on the rate of electron transfer is studied by considering the first oxidation wave of 9,10-diphenylanthracene in the alkyl cyanide solvents: acetonitrile, propionitrile, butyronitrile, and valeronitrile. Second, the variation of k(0) for a series of substituted anthracenes is investigated by analyzing the voltammetric response of the one-electron oxidations of 9-phenylanthracene, 9,10-dichloroanthracene, 9-chloroanthracene, 9,10-dicyanoanthracene, 9-cyanoanthracene, 9-nitroanthracene, 9,10-diphenylanthracene, and anthracene in acetonitrile. It is shown that the rate of electron transfer of a single compound in different alkyl cyanides is determined by the longitudinal dielectric relaxation properties of the solvent, while differences in rate between the substituted anthracenes in acetonitrile can be quantitatively rationalized by considering their relative hydrodynamic radii. This makes possible the accurate prediction of electron-transfer rates for a molecule by interpolation of rate constants known for related molecules.