Thermally reduced graphene oxide showing n- to p-type electrical response inversion with water adsorption

Graphene oxide (GO) is regarded as one of the potential candidates for gas and humidity sensor applications owing to its excellent water adsorption capabilities. However, there is a lack of understanding the response of GO to humidity. Some authors believe the water adsorption leads to p-type response while other think it to be n-type. The present work demonstrates the mechanism of water adsorption and n- to p-type response inversion of GO to humidity. The GO suspension was synthesised by modified Hummers' method and then drop-casted onto the substrates having already patterned gold interdigital electrodes (IDE). The sensors can detect wide humidity range (5-95 %RH) with a low voltage 0.1 V leading to low power consumption < 10(-5) W. It is also found that rGO samples annealed at temperature below 150 degrees C show n-type (e.g. Rair/RRH -1.29 x 10(5) for rGO-40) while the samples annealed above this temperature demonstrate p-type humidity response (e.g. RRH/Rair -40 for rGO-150). The static test response time is 8 s, while recovery time is 13 s with excellent long-term stability and RH accuracy value (+/- 4% RH). Further characterization are performed to elucidate the physio-chemical model of the sensing mechanism as well as n- to p-type conductivity inversion under humidity.