Simultaneous in situ reflectance and probe beam deflection measurements at solid electrode-aqueous electrolyte interfaces

A method is herein described for performing simultaneous in situ normal incidence reflectance spectroscopy (ΔR/R, λ = 633 nm) and probe beam deflection (PBD) measurements at solid electrodes in aqueous electrolytes, while scanning the potential linearly between two prescribed limits. Results obtained for Au in 0.1 M HClO₄ and for Pt in both 0.1 M HClO ₄ and 0.1 M NaOH were found to be in excellent agreement with those reported in the literature for each individual spectroelectrochemical technique under otherwise similar conditions. Data collected for Pt electrodes in CO-saturated 0.1 M HClO₄ revealed rather sudden changes in both ΔR/R and PBD signals in the voltammetric region where the characteristic sharp peak associated with the oxidation of adsorbed CO occurs. This behavior was ascribed, respectively, to oxide formation (ΔR/R) and to changes in the electrolyte composition in region neighboring the electrode, involving predominantly the acid concentration (PBD). In contrast, CO oxidation on Pt in 0.1 M NaOH yielded a PBD response consistent with formation of solution-phase carbonate via the reaction of the product, CO₂, with hydroxyl ion. The exquisite sensitivity of ΔR/R and PBD to interfacial phenomena was further illustrated using a monolayer of hemin irreversibly adsorbed on glassy carbon surfaces in 0.1 M Na₂B₄O₇ (pH ∼9.2). For this system, ΔR/R was found to be proportional to the relative fractions of hemin and its reduced counterpart, while the PBD signal could be correlated with corresponding variations in the electrolyte concentration induced by the surface-bound redox process.