Metastable CeZrO_4-δ polymorph for high-sensitivity nitrogen dioxide detection

Accurate, real-time detection of nitrogen dioxide (NO₂) is essential for enforcing next-generation vehicle emission standards and ensuring environmental safety. Electrochemical devices offer a cost-effective solution; however, current sensing materials have limited sensitivity, stability, and selectivity. Here, we report a metastable polymorph of CeZrO_4-δ, stabilized by liquid-nitrogen quenching, as an NO₂-sensing material. The metastable tetragonal structure features surface-enriched Ce³⁺ species, oxygen-vacancy-related environments, and distorted tetragonal surface configurations, which collectively facilitate NO₂ adsorption and charge transport. The resulting sensor achieves a current response of 24,000% to 10 ppm NO₂, defined as [|(I_gas - I_air)/I_air|] × 100%, compared to 3300% for the equilibrium cubic phase, while maintaining stable operation under variable humidity and temperature. These findings demonstrate that metastable phases offer an effective route to tune sensing materials, and the current example provides a platform for sensitive and stable NO₂-resolved monitoring relevant to Euro 7-oriented emission diagnostics.