Electroprecipitation of Nanometer-Thick Films of Ln(OH)₃ [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

We report investigations of the deposition of nanometer-thick Ln(OH)3 films (Ln = La, Ce, and Lu) and their effect on outer-sphere and inner-sphere electron-transfer reactions. Insoluble Ln(OH)3 films are deposited from aqueous solutions of LaCl3 onto the surface of 12.5 μm radius Pt microdisk electrodes during water or oxygen reduction. Both reactions produce interfacial OH-, which complexes with Ln3+, resulting in the precipitation of Ln(OH)3. Surface analyses by scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, and atomic force microscopy indicate the formation of a 1-2 nm thick uniform film. Outer-sphere electron-transfer reactions (Ru(NH3)63+ reduction, FcMeOH oxidation, and Fe(CN)64-/3- oxidation/reduction) were investigated at Ln(OH)3-modified electrodes of different film thicknesses. The results demonstrate that the steady-state transport-limited current for these reactions decreases with an increase in the film thickness. Moreover, the degree of blockage depends upon the redox species, suggesting that the Ln(OH)3 films are free from pinholes greater than the size of the redox molecules. This suggests that the films are either ionically conducting or that electron tunneling occurs across these thin layers. A similar blocking effect was observed for the inner-sphere reductions of H2O and O2. We further demonstrate that the thickness of La(OH)3 films can be controlled by anodic dissolution. Additionally, we show that La3+ lowers the supersaturation of dissolved H2 required to nucleate a stable nanobubble.