A facile fluorine-mediated hydrothermal route to controlled synthesis of rhombus-shaped Co3O4 nanorod arrays and their application in gas sensing

Abstract

We have successfully synthesized rhombus-shaped Co₃O₄ nanorod (NR) arrays via a facile fluorine-mediated hydrothermal route involving the formation of Co(OH)F as precursor and then thermal conversion to porous Co₃O₄. The hydrothermal temperature was critical to the rhombic shape of the nanostructures. At low temperatures, the Co²⁺ ions interacted with F⁻ anions to form CoF⁺ complexes and then reacted with OH⁻ to form rhombus-shaped Co(OH)F. Above 100 °C, the disproportionation of formaldehyde from hydrolysis of hexamethylenetetramine (HMT) hindered the formation of Co(OH)F precursor. In this process, CO₃²⁻ anions direct the dissolution–recrystallization process instead of F⁻ anions due to stronger affinity of CO₃²⁻ to Co²⁺. During the transformation from the precursor to the oxide, various annealing temperatures affect the gas sensing performance. The synthesized rhombus-shaped Co₃O₄ NR arrays gas sensor annealed at 450 °C showed the highest sensitivity to ethanol because of good contact, porous structure, good crystallinity, high surface-to-volume ratio and open space.