John F. Smalley

Laser-induced temperature-jump coulostatics for the investigation of heterogeneous rate processes: theory and application

Abstract A coulostatic perturbation of an electrode surface effected by a laser-induced temperature jump is described. A thin foil or film electrode exposed to dielectric (air, or glass backing) on one side and solution on the other is irradiated by a laser pulse from the dielectric side. Non-reflected photons are absorbed in a thin layer of the metal at the metal/dielectric interface and thermalize virtually instantaneously. The high thermal diffusivity in the metal causes rapid heating of the metal/solution interface and a concomitant change in the open-circuit potential of the electrode. This change in potential and its subsequent relaxation(s) are analyzed quantitatively in terms of: (1) A junction potential between the (hot) electrode and the (cold) contact wire; (2) a change in the potential across the electrode double layer which can be effected by a change in the capacitance and/or by a change in dipole orientation (equivalent to a change in the potential of zero charge) and/or by charge transfer (electron transfer between the electrode and a redox system located at the outer Helmholtz plane (OHP) or inner Helmholtz plane (IHP), or ion transfer between the IHP and the OHP); (3) a Soret potential. For small temperature changes and a constant charge-transfer resistance the theoretical analysis yields an analytic equation. Using a flashlamp-pumped dye laser (width at half-max. ~ 0.35 μs) and a 25 μs Pt-foil electrode, the thermal relaxation phenomena are verified and calibrated experimentally and (as a demonstration) the method is used to evaluate the heterogeneous rate constant for ferri/ferrocyanide. The possibility of probing electrode responses in the sub-microsecond and sub-nanosecond time domains is discussed.

Interfacial bridge-mediated electron transfer: mechanistic analysis based on electrochemical kinetics and theoretical modelling

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

Evidence for adsorption of Fe(CN)63−/4− on gold using the indirect laser-induced temperature-jump method☆

Abstract The addition of K 3 Fe(CN) 6 and K 4 Fe(CN) 6 to a 1 M KF solution is shown to effect a significant change in the response of the open-circuit potential to a rapid change in the interfacial temperature. Using the indirect laser-induced temperature-jump (ILIT) method the interfacial temperature can be changed in a few nanoseconds—fast enough so that electron transfer between the electrode and Fe(CN) 6 3−/4− (adsorbed or in the bulk) does not have time to occur. The only explanation for a change in the ILIT response is that the Fe(CN) 6 3−/4− adsorbs on the gold surface and effects a change in the structure, and therefore the thermal response, of the double layer.

An indirect laser-induced temperature jump study of the influence of redox couple adsorption on heterogeneous electron transfer kinetics

Abstract The indirect laser-induced temperature jump technique is used to study the heterogeneous electron transfer kinetics of Fe(CN)63−/4−, Ru(NH3)63+/2+ (both in aqueous 1 M KF) and dimethylferrocene/dimethylferrocenium (Me2Fc, in 1 M LiClO4 in CH3OH) on Au electrodes. Evidence is obtained demonstrating not only that all three of these redox couples adsorb on Au but also that the behavior of the measured electron transfer kinetics for these couples is significantly perturbed from that expected for simple heterogeneous electron transfer reactions. The interpretation of the results of these measurements is, therefore, made much more complex and uncertain. Such complexity and uncertainty is greatly reduced by irreversibly attaching the redox moiety to the electrode surface as a part of a stable, organized structure (e.g. a self-assembled monolayer).

A serendipitous soret effect associated with a laser-induced interfacial temperature jump in an electrochemical system

Abstract A change in the interfacial temperature of an electrode causes a concomitant change in its open-circuit potential. A portion of this change in potential is due to the Soret potential, Δ V s , effected by Gibbs energy gradients of electrolyte ions associated with the extant temperature gradient. In our previously reported use of laser-induced interfacial temperature-jump (LIITJ) perturbations to investigate a variety of interfacial relaxation phenomena, it was necessary to characterize the temporal behavior of Δ V s , and related thermally induced concentration changes, Δ c j (x,t) , as a function of temperature, Δ T(x,t) . In the present work we present analytic and simulated analyses of the operative transport phenomena and show that Δ V s / Δ T (0, t ) and Δ c j (0, t )/ Δ T (0, t ) are constant for any electrolyte and independent of the temporal dependence of Δ T (0, t ) as long as Δ c j (0, t )/ c j o ⪡ 1 ( c j o = bulk concentration of the j th species) and ( t m c n > 10 −10 mol l −1 s ( t m = measurement time; c n = total ion concentration). This serendipitous result means that Soret phenomena will not introduce spurious relaxations into the LIITJ responses. The requisite conditions for this ideal behavior are established.

An informative subtlety of itemperature-jump or coulostatic responses for surface-attached species

Abstract Theoretical relationships are developed to describe the open-circuit responses associated with the indirect laser-induced temperature-jump (ILIT) method, a method for measuring fast electron-transfer rate constants of surface-attached redox species. The analysis is also applicable to data obtained using the coulostatic charge-injection method. The unique relationship between k m , the relaxation rate constant for the ILIT (or coulostatic) response, and E i , the potential at which the system is initially poised, exhibits a surprising sensitivity to the values of k 0 , E i 0 ′ (the standard rate constant and formal potential for the redox couple), α (the transfer coefficient in the Butler–Volmer equation) and γ (a dimensionless parameter which is directly proportional to the total surface concentration of the redox moiety). ILIT data for several examples of surface-attached ferrocene moieties confirm the theoretically predicted k m vs E i behavior. Values of E i 0 ′ and γ extracted from the ILIT data agree well with the values obtained from cyclic voltammetric data thereby confirming that the ILIT and cyclic voltammogram (CV) experiments are sampling the same ferrocene population.