Differences in Electrophysical and Gas Sensing Properties of Flame Spray Synthesized Fe_2O_3(γ-Fe_2O_3 and α-Fe_2O_3)

摘要

Nanoscaled Fe_2O_3 powders as candidates for gas sensing material for hydrogen detection were synthesized by the high temperature flame spray assisted combustion of ferrocene dissolved in benzene. X-ray diffraction (XRD) and selected area electron diffraction (SAED) show that the as prepared nanopowder consists of maghemite (γ-Fe_2O_3) with low crystallinity. Thermal post-treatment causes a phase transformation towards hematite (α-Fe_2O_3) accompanied by an increase in the crystallinity. Upon exposure to air and hydrogen at elevated temperatures, both phases show a significant variation of conductivity and activation energy-as evidenced by impedance spectra- and thus a favorable sensor response, surpassing even that of flame-synthesized nanocrystalline tin dioxide. The conductivity has been identified as of electronic origin, affected by trap states located in the region adjacent to grain boundaries. Quantitative analysis of the impedance spectra with equivalent circuits shows that the conductivity is thermally activated and affected by the interaction of hydrogen with the sensor material. The calculated Debye screening length of γ-Fe_2O_3 and α-Fe_2O_3 is about 27 nm and 16 nm, respectively, what contributes significantly to the sensitivity of the material. γ-Fe_2O_3 and α-Fe_2O_3 exhibit high sensor response towards hydrogen in a wide concentration range. γ-Fe_2O_3 shows n-type semiconducting behavior up to 573 K. α-Fe_2O_3 shows p-type semiconducting behavior, as reflected in the dynamic changes of the resistivity. For both sensor materials, 523 K was the optimal operating temperature.