Facile Synthesis of Highly Dispersed Co₃O₄ Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NOx Gas Sensor

Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co₃O₄@BP-PEI composite was designed and synthesized using a hydrothermal method. The size of the highly dispersed Co₃O₄ nanoparticles (NPs) on the FL-BP NSs can be controlled. The BP-C5 (190 °C for 5 h) sensor, with 4-6 nm Co₃O₄ NPs on the FL-BP NSs, exhibited an ultrahigh sensitivity of 8.38 and a fast response of 0.67 s to 100 ppm of NO x at room temperature in air, which is 4 times faster than the response of the FL-BP NS sensor, and the lower detection limit reached 10 ppb. This study points to a promising method for tuning properties of BP-based composites by forming air-stable overlayers and highly dispersed metal oxide NPs for use in high-performance gas sensors.