Molecularly modified ultrathin Al2O3 layers as proton-conductive, oxygen-impermeable nanomembranes for catalytic surfaces

Abstract

Ultrathin inorganic oxide coatings can improve selectivity in photo- and electrocatalysis, but they also bury active sites and impede transport of the desired reactants. Here we quantify proton and O₂ permeability of 3–5 nm amorphous alumina (Al₂O₃) overlayers on poly-crystalline Pt using electrochemical impedance spectroscopy (EIS) and fourier-transform infrared reflection–absorption spectroscopy (FT-IRRAS). The apparent proton diffusivity amounts to ∼10⁻¹³ m² s⁻¹ in the atomic-layer-deposited (ALD) films. FT-IRRAS reveals hydrated AlOOH motifs whose presence correlates with the measured diffusion coefficients, highlighting their role as the dominant proton-transport pathways. The through-(Al₂O₃) film resistance is growing non-linear with thickness (17 → 37 Ω cm² for 3 → 4 nm) and becomes close to infinity at 5 nm. Embedding oligo(ethylene glycol) chains within the alumina reduces the through-film resistance to 2.6 Ω cm² at 3 nm. This is associated with enhancing proton access, albeit with a higher charge-transfer resistance (∼38 → 250 Ω cm²), consistent with diminished activity of the underlying Pt active sites. In O₂-saturated electrolyte the total impedance increases and the diffusion contribution moves below the measurement threshold (1 Hz), indicating preserved oxygen-blocking character. Practically, this sets different design priorities. For high-current electrocatalysis, performance is governed by the overlayer's area-specific resistance, which can be improved by molecular functionalization. In low-current photocatalysis, the ohmic resistance penalty is small, so maintaining (or boosting) the intrinsic activity of buried active sites is more important to justify selectivity gains from O₂ blocking.