Hierarchically porous CuO hollow spheres fabricated via a one-pot template-free method for high-performance gas sensors

CuO hollow spheres with hierarchical pores, that is, quasi-micropores (1.0-2.2 nm), mesopores (5-30 nm), and macropores (hollow cores, 2-4 μm), have been synthesized via a simple one-pot template-free method. The CuO hollow spheres also show a hierarchical architecture, namely, the primary CuO nanograins, the quasi-single-crystal nanosheets assembled by nanograins, and the spheres composed of the nanosheets. A mechanism involving an "oriented attachment" growth step followed by an "Ostwald ripening" process has been proposed for the hierarchical structure and pore formation of the typical CuO hollow spheres. With such unique hierarchical pores and architecture, the CuO hollow spheres display excellent sensing performance toward H ₂S as gas sensing material, such as low detection limit of 2 ppb, high sensitivity at parts per billion level concentration, broad linear range, short response time of 3 s, and recovery time of 9 s. The excellent performance is ascribed to a synergetic effect of the hierarchical structure of the unique CuO spheres: the quasi-micropores offer active sites for effectively sensing, the mesopores facilitate the molecular diffusion kinetics, and the macropores serve as gas reservoirs and minimize diffusion length, while good conductivity of the quasi-single-crystal nanosheets favors fast charge transportation, which contribute to the high sensitivity, quick response, and recovery of the H ₂S sensor, respectively.