Immobilizing a Lehn-Type Catalyst with Nitrocyclocondensation Chemistries: CO₂ Reduction on Silicon Hybrid Photoelectrodes

We present the first examples of installing a redox reporter and CO₂ reduction (CO₂R) catalyst onto freshly HF-etched Si (photo)electrodes using a solution-phase nitrocyclocondensation (NCC) reaction. Previous studies detailed the mechanism of NCC reactions on Si under ultrahigh vacuum conditions but have not applied these chemistries to generate functional (photo)electrodes. Installing 4-nitrophenyl ferrocene (4-NpFc) molecules directly onto degenerately doped p⁺Si allowed us to evaluate the effects of different NCC reaction conditions on coverage (e.g., 3.6 ± 1.2 × 10^-11 mol/cm²) and electrochemical reversibility. The 4-NpFc molecules remained immobilized under the reducing potentials (-2.0 V vs Fc^+/0 in acetonitrile) needed to drive molecular CO₂R catalysts. Installing 4-nitroaniline (4-NA) molecules onto low-doped pSi allowed us to couple Lehn-type Re(I) catalysts with pendant carboxylic acids. The CO Faradaic efficiency of CO production under 1-sun illumination (-2.15 V vs Fc^+/0) was 23% in CO₂-saturated electrolyte, a value that surpasses previous examples of CO₂R catalyst monolayers on Si. These two examples highlight the versatility of solution-phase NCC attachment chemistries for generating functional Si (photo)electrodes.