Trace Nickel Activated Biphasic Core‐CuO _ii /Shell‐CuO _i Secondary Microspheres Enable Room Temperature Parts‐Per‐Trillion‐Level NO ₂ Detection

Design and fabrication of room temperature (RT) high‐performance sensors based on novel structures using low‐cost and straightforward approaches are not only technologically challenging but scientifically intriguing. Herein, biphasic CuO‐based core‐CuO _i /shell‐CuO _ii secondary microspheres are in situ activated by trace nickel and are prepared via phase separation and self‐assembly. Detailed X‐ray powder diffraction, X‐ray photoelectron spectra, UV, Raman spectra, and density functional theory calculations confirm the formation of biphasic isomorphous crystals. Owing to these unique structural properties, optimum RT performance is achieved for 1.758 wt% trace nickel with response (588.0%), sensitivity (1.37% parts‐per‐billion (ppb) ⁻¹ ), short response time (27 s) for 1000 ppb NO ₂ , practical 16 ppb and theoretical 80 ppt LODs, excellent selectivity, and long‐term stability (over ten cycles), which are very outstanding among those reported. Interestingly, Schottky barriers were created between CuO _i ‐/CuO _ii ‐based cores and shells with the same crystal structures but different lattice constants, which are in situ doped with trace nickel simultaneously. Schottky barriers, catalytic, and spill‐over effects can be optimized by trace nickel doping to achieve optimum performance. Furthermore, a deep understanding of the NO ₂ sensing mechanisms based on isomorphous biphases generated via phase separation, self‐assembly, and trace‐Ni element activation is proposed for the first time, informative to the scientific community for fabricating high‐performance nanosensors.