Surface engineering of Au nanostructures for plasmon-enhanced electrochemical reduction of N₂ and CO₂ into urea in the visible-NIR region

The photoelectrochemical reduction of CO₂ and N₂ (N₂CO₂RR) is a promising method of producing urea under ambient conditions since highly active and stable electrocatalysts are desired. Plasmonic metals have attracted considerable attention due to their enhanced electrochemical activity at visible and near-infrared wavelengths (NIR). Herein, the morphology of Au was tuned to spherical nanoparticles, nanorods, and nanosheets by utilizing a variety of structure-directing agents. Among them, Au nanosheets (Au NSs) can absorb a broad spectrum of NIR wavelengths, enabling electrochemical reduction of N₂ into NH₃, with high yield rates and higher Faradic efficiency (FE) than most of the N₂RR results reported. In addition, a distal associative pathway for N₂RR into NH₃ has been established over Au NSs. Additionally, the Au NSs photocathode demonstrates high stability over a period of 10 consecutive runs. In addition, this work provides a guide to fabricating highly stable photocathodes that convert N₂ and CO₂ into urea. Au NSs photocathode achieves a maximum urea yield rate of 98.5 µg_ureamg_cat^-1h⁻¹ and FE of 22.7% at −0.7 V vs. RHE. Results show that the N₂ and CO₂ is the primary factor for urea production, whereas reducing NO₃^– and HCO₃^– contributes significantly to the total urea yield rate. Density functional theory calculations (DFT) reveal that Au NSs play a crucial role in promoting N₂ and CO₂ adsorption, activation, and stimulating the coupling reaction between C-N to form urea by the distal mechanism. As a result, this work opens up the possibility of developing hybrid catalytic systems for simultaneously reducing nitrate-containing wastewater and CO₂, thus producing urea-rich treated water for agricultural use and achieving carbon neutrality.