Scanning electrochemical microscopy with conducting polymer probes: Validation and applications

Scanning electrochemical microscopy (SECM) allows spatially and temporally resolved measurements of a broad range of reactive surfaces and specimens, typically using electrochemically active metal probes. While conducting polymers (CPs) present several analytical properties of interest due to their chemical versatility, potentially enabling the measurement of ionic fluxes as well as redox processes, they have not been widely used as probe materials for SECM. CPs can be modified and fine-tuned to improve experimental parameters and they can be easily prepared by electrodeposition. In this paper, we show a new type of CP probe for SECM that retains the spatial resolution of conventional metal probes and introduces the possibility to exploit a wide range of ionic and redox systems. Poly-3,4-ethylenedioxythiophene (PEDOT) was electrochemically deposited on flat and recessed Pt microdisks to generate CP SECM probes. To demonstrate their usefulness, an insulating substrate with conducting features was imaged. Well-defined SECM feedback images were observed for both the CP well-probe and the Pt probe, proving the efficiency of the new electrode to image redox reactions. Additionally, an organosulfur compound was used as mediator taking advantage of the electrocatalytic effect PEDOT has on the molecule's kinetics. Finally, these probes were also used in a mediator-less fashion, taking advantage of the ion flux required to electrochemically oxidize the PEDOT deposit. We investigated the impact of anion size and concentration on current-distance relationships for SECM probe positioning. CP probes pose exciting prospects for the imaging and measurement of combined redox and ionic processes in energy materials.

Quantifying Wavelength-Dependent Plasmonic Hot Carrier Energy Distributions at Metal/Semiconductor Interfaces

Hot carriers generated from the nonradiative decay of localized surface plasmons are capable of driving charge-transfer reactions at the surfaces of metal nanostructures. Photocatalytic devices utilizing plasmonic hot carriers are often based on metal nanoparticle/semiconductor heterostructures owing to their efficient electron-hole separation ability. The rapid thermalization of hot carriers generated at the metal nanoparticles yields a distribution of carrier energies that determines the capability of the photocatalytic device to drive redox reactions. Here, we quantify the thermalized hot carrier energy distribution generated at Au/TiO₂ nanostructures using wavelength-dependent scanning electrochemical microscopy and a series of molecular probes with different redox potentials. We determine the quantum efficiencies and oxidizing power of the hot carriers from wavelength-dependent reaction rates and photocurrent across the metal/semiconductor interface. The wavelength-dependent reaction efficiency tracks the surface plasmon resonance spectrum of the Au nanoparticles, showing that the reaction is facilitated by plasmon excitation, while the responses from molecules with different redox potentials shed light on the energy distribution of the hot holes generated at metal nanoparticle/semiconductor heterostructures. The results provide important insight into the energies of the plasmon-generated hot carriers and quantum efficiencies of plasmonic photocatalytic devices.

Surface Interrogation Scanning Electrochemical Microscopy for a Photoelectrochemical Reaction: Water Oxidation on a Hematite Surface

To understand the pathway of a photoelectrochemical (PEC) reaction, quantitative knowledge of reaction intermediates is important. We describe here surface interrogation scanning electrochemical microscopy for this purpose (PEC SI-SECM), where a light pulse to a photoactive semiconductor film at a given potential generates intermediates that are then analyzed by a tip generated titrant at known times after the light pulse. The improvements were demonstrated for photoelectrochemical water oxidation (oxygen evolution) reaction on a hematite surface. The density of photoactive sites, proposed to be Fe⁴⁺ species, on a hematite surface was successfully quantified, and the photoelectrochemical water oxidation reaction dynamics were elucidated by time-dependent redox titration experiments. The new configuration of PEC SI-SECM should find expanded usage to understand and investigate more complicated PEC reactions with other materials.

Quantifying photothermal heating at plasmonic nanoparticles by scanning electrochemical microscopy

Photothermal heating at metal nanoparticles results from the non-radiative decay of localized surface plasmons. The local heat generation enhances the mass transport rate of redox molecules and causes a shift in their formal potential, both of which can impact an electrochemical process at the nanoparticle interface. Here we present a methodology for probing the surface temperature at a plasmonic nanoparticle substrate using scanning electrochemical microscopy (SECM). Light is used to excite a plasmonic substrate electrode, while an ultramicroelectrode tip is positioned close to the substrate to read out both the mass transfer rate and concentration profile of the redox molecules. The measured mass transfer rate and the shift in the equilibrium potential provide a quantitative value of the temperature increase at the substrate surface, which is verified by simulations using a mass transfer model coupled with heat dissipation. The developed SECM approach is suitable for probing heat generation at a variety of both plasmonic and non-plasmonic nanostructures.