Substituent Engineering Couples Photocathode Interfacial Energetics with Ni-N4 Electronic Structure for Solar CO2 Reduction.

Molecular catalyst/semiconductor photocathodes require coordinated regulation of interfacial energetics and catalytic-site electronic structure for efficient solar CO2 reduction. Here, we show that peripheral substituent engineering in nickel phthalocyanines integrated on p-Si nanowires provides a molecular strategy to couple the interfacial response of p-Si/NiPc-X photocathodes with Ni-N4-centered CO2 activation. Frequency-dependent Mott-Schottky responses show a clearly resolved positive apparent interfacial capacitance-transition potential for NiPc-NH2/Si, while UPS and light/dark OCP measurements indicate an amino-substitution-associated interfacial electronic response. EIS and IMPS further reveal lower charge-transfer resistance and 92.7% charge-transfer efficiency for NiPc-NH2/Si, indicating improved photocarrier utilization. In parallel, Ni 2p XPS and Hirshfeld charge analysis indicate electron enrichment at the Ni-N4 center, and DFT calculations show that the thermodynamic free-energy requirement for *COOH formation decreases from 2.03 to 1.75 eV. These coupled interfacial and catalytic-site effects enable NiPc-NH2/Si to deliver 92.1% CO Faradaic efficiency and 2.8% applied-bias photon-to-current efficiency for solar CO2-to-CO conversion.