Nitrogen Dioxide Sensing Using Multilayer Structure of Reduced Graphene Oxide and α-Fe₂O₃

Multilayers consisting of graphene oxide (GO) and α-Fe₂O₃ thin layers were deposited on the ceramic substrates by the spray LbL (layer by layer) coating technique. Graphene oxide was prepared from graphite using the modified Hummers method. Obtained GO flakes reached up to 6 nanometers in thickness and 10 micrometers in lateral size. Iron oxide Fe₂O₃ was obtained by the wet chemical method from FeCl₃ and NH₄OH solution. Manufactured samples were deposited as 3 LbL (GO and Fe₂O₃ layers deposited sequentially) and 6 LbL structures with GO as a bottom layer. Electrical measurements show the decrease of multilayer resistance after the introduction of the oxidizing NO₂ gas to the ambient air atmosphere. The concentration of NO₂ was changed from 1 ppm to 20 ppm. The samples changed their resistance even at temperatures close to room temperature, however, the sensitivity increased with temperature. Fe₂O₃ is known as an n-type semiconductor, but the rGO/Fe₂O₃ hybrid structure behaved similarly to rGO, which is p-type. Both chemisorbed O₂ and NO₂ act as electron traps decreasing the concentration of electrons and increasing the effective multilayer conductivity. An explanation of the observed variations of multilayer structure resistance also the possibility of heterojunctions formation was taken into account.