Phase-Selective Precrystallization Enables the Topological Conversion of Uniform Mesoporous Titanium Nitride

Nonoxide ceramic materials offer excellent thermal, electrical, and mechanical properties; however, their integration with mesoporosity remains fundamentally constrained by high-temperature conversion reactions, which inherently disrupt the mesoscopic order through lattice reconstruction and grain coarsening. Herein, we report a phase-selective precrystallization conversion strategy in which mesostructured rutile TiO₂ intermediates are directly constructed, enabling a mesoscale topology-preserving oxide-to-nitride conversion to mesoporous titanium nitride (TiN) microspheres at temperatures as low as 700 °C, substantially below the conventional nitridation conditions (∼1100 °C). Kinetic and thermodynamic analyses reveal that rutile precrystallization simultaneously lowers the activation barrier and shifts the driving force for nitridation, thereby bypassing the conventional anatase-rutile-TiN pathway that disrupts the mesoscopic order. Assisted by a transient carbon scaffold that kinetically suppresses grain coarsening, the conversion proceeds with an exceptionally small volume contraction (∼7.3%), yielding mesoporous TiN frameworks with a high surface area (82 m² g^-1), continuous crystalline pore walls (mean pore size of 17.5 nm), and radial mass-transport pathways. When employed as conductive supports for Ir catalysts, the resulting mesoporous TiN enables strong electronic coupling and structural stabilization under acidic oxygen evolution conditions, delivering high activity (1.64 V at 1.0 A cm^-2) and long-term durability over 500 h in proton-exchange membrane water electrolysis at ultralow Ir loadings (∼0.15 mg_Ir cm^-2). This work establishes a phase-selective pathway via precrystallization as a promising strategy for reconciling mesoporosity and crystallinity in nonoxide ceramics.