A strategy for ultrasensitive and selective detection of methylamine using p-type Cr₂O₃: Morphological design of sensing materials, control of charge carrier concentrations, and configurational tuning of Au catalysts

An ultrasensitive and selective trimethylamine (TMA) sensor was fabricated by the urea-assisted uniform loading of Au nanoparticles (∼5 nm) on Cr₂O₃ yolk-shell spheres prepared by ultrasonic spray pyrolysis. The response (resistance ratio) of the Cr₂O₃ yolk-shell spheres uniformly loaded with fine Au nanoparticles to 5 ppm TMA was 200.9 at 225 °C, which was significantly higher than those of pure Cr₂O₃ yolk-shell spheres (24.6) and Cr₂O₃ yolk-shell spheres loaded with heavily agglomerated Au particles (9.2). The detection limit of Cr₂O₃ yolk-shell spheres uniformly loaded with Au nanoparticles was as low as 4.3 ppb. The unprecedentedly high TMA response is explained and discussed in relation to the gas-accessible morphology of the yolk-shell spheres, and the catalytic and electronic promotion of the gas sensing reaction by both Cr₂O₃ and well-dispersed Au nanoparticles.