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

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 degrees 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 degrees C showed the highest sensitivity to ethanol because of good contact, porous structure, good crystallinity, high surface-to-volume ratio and open space.