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

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 Sn²⁺-initiated phase-modulation strategy for the facile synthesis of a photothermal/photoelectronic phase with rhombohedral NiSe and Ni₃Se₂ (r-NiSe/r-Ni₃Se₂) 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-Ni₃Se₂. 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-Ni₃Se₂. 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 × 10²⁴ to 4.2 × 10²⁴ 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-Ni₃Se₂ 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-Ni₃Se₂||o-SnSe/h-NiSe/r-Ni₃Se₂ 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).