Transformation of an Inert Insulator into a Photocatalyst via Causal Strain-Defect Interplay in Nanoscale SrSO₄

The activation of wide-bandgap insulators represents an attractive route for developing scalable photocatalysts from earth-abundant materials. Herein, we demonstrate that synthesizing SrSO₄ nanoparticles (∼12 nm) induces compressive lattice strain, which thermodynamically drives the formation of oxygen vacancies, ultimately transforming the inert insulator SrSO₄ into an efficient photocatalyst. Contrary to the conventional expectation that compressive strain directly narrows the bandgap, we reveal that the primary role of compressive strain is to significantly lower the formation energy of oxygen vacancies (from 4.1 to 2.89 eV under 4% strain). The resulting vacancies introduce midgap states that effectively reduce the bandgap, enhance photon absorption, and act as electron traps to promote charge separation. The causal strain-defect synergy optimizes the surface reactivity, facilitating reactant adsorption and reactive oxygen species generation, ultimately achieving a NO oxidation efficiency of 55.82%, which markedly outperforms the bulk SrSO₄ (13.64%). Combined experimental and theoretical evidence confirms efficient NO activation and stepwise oxidation to nitrate. This work establishes a rational design strategy, where lattice strain is employed to deliberately engineer active defects, thereby creating a new paradigm for the activation of earth-abundant inorganic insulators for sustainable solar energy conversion.