Tailoring the D‐band Center of Cobalt via Heterojunction Engineering With Interfacial Electric Field: Modulation to High‐Performance α‐Fe ₂ O ₃ @Co ₃ O ₄ Formaldehyde Sensors for Indoor Air Monitoring

ABSTRACT Gas sensors are efficient for environmental pollution monitoring. Studies have shown that heterojunction engineering is highly promising for gas sensing by modulating the interfacial electric field (IEF), yet its regulation mechanism on the electronic structure of surface‐active sites remains insufficiently understood. Herein, we construct a tailored IEF in α‐Fe 2 O 3 @Co 3 O 4 heterojunctions to upshift Co 3 + sites’ d‐band center, optimizing target gas adsorption. The material exhibits an ultrahigh formaldehyde response (26.5 to 1 ppm), rapid response‐recovery, and 100 ppb detection capability. In situ/ex situ characterizations and theoretical calculations confirm the IEF enhances Co 3 + sites’ adsorption by increasing antibonding orbital occupancy. As a core thermodynamic parameter characterizing the energy variation of the system, enthalpy holds great significance in this work: its enthalpy change can quantitatively reflect the energy variation in the adsorption process, provide thermodynamic evidence for the upshift of the d‐band center and the increased occupancy of antibonding orbitals, and clarify the spontaneous direction and intensity of the reaction as well as the intrinsic thermodynamic mechanism underlying the enhanced adsorption capability induced by the IEF. Integrating with a MEMS array and wireless device enables real‐time monitoring and cloud transmission, laying a foundation for indoor environmental health monitoring.