One-pot graphitic encapsulation of Fe₇C₃ nanoparticles enabling enhanced photothermal performance

Here, we report a one-pot solvothermal synthesis that enables Fe₇C₃ nanoparticles (NPs) to direct the in situ formation of conformal graphitic carbon shells under comparatively mild conditions. Comprehensive structural analyses reveal that the resulting Fe₇C₃@C nanostructures consist of a crystalline carbide core encapsulated within a few-layer graphitic carbon shell, with small γ-Fe₂O₃ NPs by-products. Systematic variation of reaction parameters shows that both a reducing atmosphere and the Fe₇C₃ phase are essential for promoting graphitization, while Fe₅C₂ and Fe₃C NPs prepared under analogous solvothermal conditions develop only amorphous carbon shells. This structural feature directly translates into superior photothermal behavior: Fe₇C₃@C NPs exhibit enhanced photothermal heating under 808 nm irradiation, even after 10 irradiation cycles, and a photothermal conversion efficiency (PCE) exceeding those of other iron carbides and several noble-metal nanostructures. These findings establish Fe₇C₃ as a uniquely capable phase for promoting in situ carbon ordering and highlight phase-dependent surface chemistry as a powerful tool for designing high-performance photothermal materials.