In Situ Observation of Ferroin and Ferriin in Vicinity of ITO Electrode Surface by Slab Optical Waveguide Spectroscopy

Abstract

Slab optical waveguide (SOWG) spectroscopy was carried out for an in situ observation of ferroin and ferriin formed at an electrode/solution interface during electrolysis on an indium-tin oxide (ITO) electrode. A new spectroelectrochemical cell for SOWG spectroscopy was developed to observe in situ absorption spectra of species in the vicinity of the ITO electrode surface. A cyclic voltammogram (CV) of 5 mmol·dm⁻³ ferroin in 1 mol·dm⁻³ potassium nitrate aqueous solution was obtained as the potential swept from +1.4 to +0.4 V and then from +0.4 to +1.4 V. Furthermore a cathodic and anodic peaks appeared at +0.82 and +0.92 V, respectively. The SOWG spectra were obtained every 0.05 mV during the sweep. The SOWG spectra at the potential of +0.4 and +1.4 V were identical to the UV-VIS spectra of ferroin and ferriin, respectively. The concentration profiles near the electrode during cyclic voltammetry of ferroin were discussed based on the absorbance at 510 and 650 nm on SOWG spectra. Furthermore, in situ real-time monitoring of absorption during potential step experiment of ferroin was made at various incident angles from 70 to 77° using SOWG spectroscopy.