Polyethylenimine/Nitrogen-Doped Reduced Graphene Oxide/ZnO Nanorod Layered Composites for Carbon Dioxide Sensing at Room Temperature

In this study, we develop optimal layer-by-layer spray coatings of nitrogen-doped reduced graphene oxide (NrGO) and polyethylenimine (PEI) on the hydrothermally synthesized ZnO nanorods (ZNR) to fabricate a PEI/NrGO/ZNR sensor for CO2 sensing at room temperature (RT, 25 °C). Owing to the high aspect ratio of vertically aligned ZNR, it inhibits the agglomeration of 3D porous gas absorbing layers of NrGO and PEI during spray coatings. The PEI/NrGO, NrGO/ZNR, and PEI/NrGO/ZNR sensors show 1.22%, 3.82%, and 8.63% responses, respectively, in the presence of 5 vol % CO2 in the air (∼13% relative humidity) at RT. When exposed to 3, 5, 7, 9, and 11 vol % CO2, the responses of PEI/NrGO/ZNR sensor are measured to be 5.60%, 8.63%, 11.50%, 14.14%, and 16.98%, with an outstanding linear curve fitting of R2 = 0.9989. The gas sensing mechanism of the PEI/NrGO/ZNR layered nanocomposite sensor comprises a physicochemical adsorption process in which the adsorbed gas molecules on the 3D networked PEI/NrGO layers yield carbamate and carbonic acids via conventional acid–base- and base-catalyzed hydration, respectively. The excellent sensing performance is attributed to the synergistic effect from high electron mobility of the ZNR, hydrophobicity of the NrGO layer, and amine-rich PEI polymer. The NrGO, possessing a specific surface area of 147.8 m2/g with ultramicropores, aids the physisorption of CO2. Through a heat-assisted recovery technique, the sensor can be fully recovered to the baseline in each sensing cycle. In addition, the sensors show excellent stability along with reproducibility for consecutively tested cycles, thus having the potential to monitor the CO2 level in patients noninvasively.