Cavity-tunable Cu2O Spherical nanostructures and their NO2 gas sensing properties

Abstract

We report the preparation of cavity-controlled Cu2O nanospheres, having various mesoporous, hollow, and solid structures, by simply adjusting the OH- concentration and the release rate of Cu2+ with the assistance of polyvinyl pyrrolidone (PVP). It indicates that the OH- diffusion kinetics is the key factor that determines the morphology of the products. For [OH-] > 0.05 mol·L-1, the high chemical potential made them rapidly diffuse into the PVP micelle interiors. Adsorbed Cu2+ on the PVP produced Cu(OH)2, which was subsequently reduced to Cu2O. After re-crystallization, Cu2O solid spheres formed. For [OH-] < 0.025 mol·L-1, the OH- diffusion rate was reduced, and the Cu(OH)2 layer on the PVP micelles blocked diffusion into the interior. After re-crystallization, Cu2O hollow spheres had large cavities (~220 nm). For 0.025 mol·L-1 < [OH-] < 0.05 mol·L-1, hollow spheres with smaller cavities (30-60 nm) formed. When an aqueous NH3 solution was the OH- source, although the concentration of OH- is low, the small amount of Cu(OH)2 formed with the limited Cu2+ was not enough to block OH- diffusion into the micelles. The free NH3 and the low OH- concentration did not promote re-crystallization; thus, mesoporous Cu2O spheres were formed. We characterized NO2 gas sensing of the three structures. The porous structures exhibited more sensitivity than did the hollow or solid structures. Together with the specific surface area data, the improved gas sensitivity suggests that the open structure of the mesoporous spheres facilitates NO2 diffusion and O2 adsorption.