Structural and gas sensing properties of ultrafine Fe₂O₃ prepared by plasma enhanced chemical vapor deposition

Ultrafine Fe₂O₃ powders have been obtained using the plasma enhanced chemical vapor deposition (PECVD) method for the gas sensor application. Ferrocene is used as the precursor source in the PECVD powder preparation procedure. The structural properties of these ultrafine Fe₂O₃ powders have been systematically characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), and thermogravimetric analysis (TGA). DTA results show that the oxygen-rich atmosphere is more favorable than nitrogen for the burn-off of residual organic compounds in the precursor powders. It is interesting to note from TEM and X-ray diffraction patterns that β- and γ-Fe₂O₃ phases can co-exist with the α-Fe₂O₃ phase due to the size effect, depending upon processing conditions. The activation energy as a function of temperature and grain size is estimated. It is found that at lower temperatures, the activation energy for crystallization predominates; while at higher temperatures, the larger activation energy is attributed to the grain growth process. The gas sensitivities and dynamic conductance measurement for these ultrafine Fe₂O₃ gas sensors have also been investigated.