Modified graphene oxide/Nafion composite humidity sensor and its linear response to the relative humidity

Graphene oxide (GO) and its derivatives have been applied to the humidity sensing area in recent years due to its abundant hydrophilic functional groups, and proved to be ultrasensitive to the humidity changing. But the stability of the sensors is always an obstacle to their application. Besides, the long-term linearity is seldom studied. In this work, diamine modified GO (MGO) has been synthesized and mixed with Nafion polymer to form a hybrid film, and applied to the humidity sensors. We have applied two measurement methods by either fitting of the impedance spectra to extract charge transfer resistance (R) or measurement of the impedance under a certain frequency, to evaluate the response to the relative humidity (RH). Both MGO and its composite show good sensitivity to the RH under the two testing methods The R shows good linearity at the full testing RH range even after two months, while the impedance under a certain frequency can't keep the linear response to the RH after certain period of time. The addition of Nafion decreases the R and increases the sensitivity of MGO based sensor. Linearity of the sensor is improved by the combination of MGO and Nafion. Long-term testing results show the linearity of the composite sensors changes over time but always keeps a better linearity than the pure MGO sensor.