Segregation-Engineered Polarization Synchronizes CO₂ and Nitrate Reduction for Bias-Free Urea Synthesis

Overcoming the kinetic mismatch between CO₂ and NO₃ ^- reduction presents a central challenge for urea photoelectrochemical synthesis. Here, we develop segregation-engineered Si/Pd-Cu photocathodes where nanoscale phase segregations induce dual-level interfacial polarization. Cu-rich segregations favor Schottky-type band modulation, facilitating photogenerated electron extraction. Simultaneously, Pd-rich domains expose Pd^δ+-Cu^δ--like polarized sites that co-stabilize CO₂/NO₃ ^--derived intermediates, synchronizing their reduction kinetics for efficient C-N coupling. Under AM 1.5 G illumination, the optimized Si/1Pd-3Cu photocathode delivers urea with a remarkable faradaic efficiency up to ≈100% at 0 V vs. RHE, achieving an initial urea partial current density of 1.06 mA·cm^-2. Operando spectroscopies combined with theoretical calculations identify a Pd-rich governed, low-barrier C-N coupling pathway operating near the thermodynamic potential. Further integration into photovoltaic photoelectrochemical devices enables light-driven spontaneous urea synthesis without external bias. This work establishes segregation-programmed polarization in semiconductor/metal junctions as a powerful, general materials-design principle for mild and selective multielectron synthesis.