Synthesis and Characterization of (3D/2D) α-Fe₂O₃/Graphene Heterostructure for Application as a Toxic Gas Sensor

Metal semiconducting oxides (MOXs) have been widely employed in gas sensors due to their potential features, such as high sensitivity, selectivity, rapid gas detection, and stability. N-type MOXs (e.g. SnO₂, WO₃, ZnO, and α-Fe₂O₃) have been successfully applied as sensing elements in gas sensor devices for the detection of a variety of toxic gases, such as NO₂, NH₃, O₃, and H₂S. Among the n-type MOXs, hematite (α-Fe₂O₃) nanostructures have gained attention due to their high oxygen ion mobility at the material surface, excellent chemical and thermal stability under ambient atmosphere, and low cost. Despite its advantages, few studies have investigated the sensing activity of hematite. Therefore, efforts have been paid to improving the sensing performance of α-Fe₂O₃ by using distinct strategies, such as combination with other components including noble metals, semiconductors, and carbon-based materials. Among these, graphene has received considerable attention due to its unique two-dimensional (2D) structure, high electron conductivity and mobility, and high surface area. Also, various MOxs may be combined with GO and/or rGO resulting in a self-assembled heterostructure with unique properties for the most diverse applications. Regarding gas sensing applications, hydrophobic graphene-based materials have been cited as an effective way to be employed in gas sensor devices leading to high sensitivity and selectivity even in high humidity conditions. Motivated by these considerations, we report here a versatile approach towards the production of advanced materials based on (3D/2D) α-Fe₂O₃/graphene sheets heterostructure to be applied as a resistive NO₂ gas sensor. The effect of different amounts of addition ratios of graphene on the sensing performance has been studied. The α-Fe₂O₃/Gr heterostructures were obtained by hydrothermal method and the crystalline phase, morphological, and surface properties were studied by XRD, FESEM, TEM and XPS techniques. Additionally, we successfully combined experimental and DFT calculations to understand the role played by the heterointerface and the chemical environment along the exposed surfaces α-Fe₂O₃/Graphene sheets in the sensing properties towards NO₂ gas. The gas sensing experiments show that the α-Fe₂O₃/graphene heterostructure gas sensor has better sensing performance for NO₂ than the pristine graphene, such as high response, excellent selectivity, and low detection limit. The effects of temperature and moisture on the NO₂ sensitivity of α-Fe₂O₃/graphene heterostructures were also investigated. The optimal operating temperature for the highest gas response is around 150 °C, and the formation of α-Fe₂O₃/graphene heterostructure improved the NO₂ sensitivity against moisture. Furthermore, the selectivity was also evaluated by exposing the heterostructures to different gases such as O₃, NH₃, and CO. These findings show that (3D/2D) α-Fe₂O₃/Graphene sheets heterostructures have a remarkable potential for practical applications as NO₂ gas sensor in environmental monitoring devices.