Surface Stabilization of High-Pressure TiO ₂ Polymorph via High-Energy Ball Milling: Boosting Noble-Metal-Free CO ₂ Photoreduction

High-energy ball milling (HEBM) is employed to stabilize the high-pressure TiO2–II polymorph as nanocrystallites anchored to anatase surfaces, producing a controllable polymorphic mixture that markedly enhances CO2 photoreduction to CH3OH in aqueous media without noble metals or cocatalysts. The resulting architecture features TiO2–II intimately interfaced with strained anatase and a high density of extended defects (grain boundaries, phase interfaces, and dislocation terminations) hosting reactive surface species with modified electronic properties. This defect-rich configuration provides high-affinity CO2 adsorption and activation sites. Both bulk and surface are profoundly restructured under the extreme nonequilibrium conditions of HEBM, which reproduce high-pressure transformation pathways at ambient conditions. These results highlight a green, scalable strategy for defect and polymorph engineering in TiO2, enabling targeted surface chemistry design to improve photocatalytic CO2 conversion.