Gas sensing properties and electrical resistance of Langmuir-Blodgett iron oxide nanoparticle arrays

Nanoparticle (NP) sensors are prepared by Langmuir-Blodgett deposition of colloid Fe₂O₃ or CoFe₂O₄ NPs onto alumina substrates. The thickness of the deposit is 1, 2, 4 or 7 NP monolayers (ML). NP arrays are analysed by GIXRD, SEM, SAXS, DLS and XANES. The average NP diameters are 6.4 nm and 7.6 nm for Fe₂O₃ and CoFe₂O₄, respectively. The response current of the sensor is measured at ≈ 400 °C in the mixtures of dry air with the monitored CO and NO₂ gases. The best relative response is 38 for 500 ppb of NO₂. The resistance of NP arrays was studied between 250 and 500 °C. With 2 ML Fe₂O₃ deposit the sheet resistance decreases with increasing temperature from 10¹¹ to 10⁹ Ω. Activation energy of the semiconductor type conductivity is 0.75 eV. High gas monitoring sensitivity of NP sensors is due to the big effective surface of Fe₂O₃ and CoFe₂O₄ NP arrays.