From spindle-like beta-FeOOH nanoparticles to alpha-Fe2O3 polyhedral crystals: shape evolution, growth mechanism and gas sensing property

American football-shaped, angular American football-shaped and cubic alpha-Fe2O3 micro-crystals with different gas-sensing properties have been achieved by an in situ pH-dependent hydrothermal synthesis using beta-FeOOH nanospindles as the sacrificing template. XRD, SEM and TEM have been employed to characterize the products. It was found that the morphology and phase evolution occurred in the hydrothermal process. With the increase of the pH value of the hydrothermal system, the morphology evolves from spindle to American football, angular American football and cube, whereas the crystal sizes decrease from 1-1.2 mm to 140 nm. The effect of the reaction time on the phase and morphology of the final products has been studied and a possible dissolution-recrystallization growth mechanism is proposed. Furthermore, the gas sensing properties of the prepared alpha-Fe2O3 crystals were investigated for a series of organic vapors at different operation temperatures and vapor concentrations, demonstrating that their gas sensing behavior was influenced by the different morphologies and that the as-prepared alpha-Fe2O3 polyhedral crystals exhibit excellent gas sensing selectivity to acetone vapor. The experimental results revealed that the gas-sensing performance of the alpha-Fe2O3 crystals is closely related to the morphology of the samples.