Investigation of the H2O sensing mechanism of DC-operated chemiresistors based on graphene oxide and thermally reduced graphene oxide

Graphene oxide (GO) is a promising material for H2O vapour sensing. However, the H2O sensing mechanisms are still under investigation especially in the case of thermally reduced GO. To this purpose, planar devices were fabricated by spin-coating graphene oxide on glass substrates. Ultra high response to H2O was recorded but poor repeatability and stability over time were also noted. Three different degrees of thermal reduction were applied to improve material stability. An inverse change of resistance was observed for reduced graphene oxide compared to pure graphene oxide upon interaction with H2O. The sensing mechanisms that govern GO and reduced GO behavior were studied based on DC measurements. In the case of GO, strong ionic conductivity was proposed whereas in the case of reduced GO mixed electronic/ionic with the leading mechanism affected by H2O percentage in air, degree of material reduction, and sensor working temperature. Finally, it was found that by promoting one sensing mechanism over the other, improved operating humidity range of the sensor can be achieved.