Sn 2+ ‐Induced In Situ Phase Modulation Enabling a Highly Active o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 Multifunctional Heterostructure for Photo‐Thermal/Electronic Urea Water Splitting

ABSTRACT The construction of highly active sites and the simultaneous establishment of photothermal/photoelectronic phases represent an emerging paradigm for boosting small‐molecule‐assisted water splitting. Herein, we report an effective Sn 2+ ‐initiated phase‐modulation strategy for the facile synthesis of a photothermal/photoelectronic phase with rhombohedral NiSe and Ni 3 Se 2 (r‐NiSe/r‐Ni 3 Se 2 ) as a precursor. The strategy drives an in situ phase transition of r‐NiSe to hexagonal NiSe (h‐NiSe), along with the generation of orthorhombic SnSe (o‐SnSe), ultimately forming the multifunctional heterostructure o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 . Theoretical calculations reveal that h‐NiSe lowers the urea oxidation reaction (UOR) energy barrier relative to r‐NiSe (0.745 eV vs. 0.901 eV), while the o‐SnSe enhances both light harvesting and photothermal/photoelectronic functionalities of the o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 . Unlike traditional UOR electrocatalysts, its photothermal effect promotes urea adsorption, offsets the endothermic enthalpy of UOR, and accelerates electron/mass‐transfer kinetics. Concurrently, the photoelectronic effect enhances the charge‐carrier density from 1.4 × 10 24 to 4.2 × 10 24 cm −3 , lowers the UOR activation energy from 48.4 to 9.7 kJ mol −1 . Capitalizing on these synergistic advantages, the o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 delivers exceptional UOR activity, achieving 10, 500, and 1000 mA cm −2 at merely 1.28, 1.34, and 1.37 V, respectively. When implemented in a urea‐assisted water splitting electrolyzer, the o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 ||o‐SnSe/h‐NiSe/r‐Ni 3 Se 2 device requires only 1.34 and 1.79 V to sustain 100 and 500 mA cm −2 , respectively, outperforming the conventional HER||OER electrolyzer (1.61 and 1.99 V).