Correlation between the sensitivity and the hysteresis of humidity sensors based on graphene oxides

We report on the correlation between the sensitivity and the sorption/desorption hysteresis of thin-film humidity sensors based on graphene oxides (GO). Both properties are systematically investigated by the conductance change of thin-film humidity sensors as well as the resonance frequency shift of quartz crystals coated with GO with varied pH. GO-based humidity sensors made at pH 3.3 present a lower level of sensitivity (2.1 ± 0.4 μS/%RH) and hysteresis-induced error (2.8 ± 0.1%RH) whereas those made at pH 9.5 present an increased level in both sensitivity (12.3 ± 2.2 μS/%RH) and hysteresis-induced error (3.7 ± 0.6%RH) by conductance measurements in the humidity range from 10%RH to 90%RH. Such correlation between two properties is also observed when the thickness and the functionality of GO films are varied. The correlation in GO-based humidity sensors is consistent with the correlation determined by a quartz crystal microbalance, indicating that the actual change of the water content in GO films underlies such behaviour. We discuss a possible mechanism for the observed correlation between the sensitivity and the hysteresis based on charged states on GO surface and their interactions with water molecules by using Fourier transform infrared (FT-IR) spectroscopy, water contact angle measurements, and response/recovery time measurement. Our findings would provide a new insight for the development of humidity sensors based on GO and its derivatives.