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We demonstrate permanent water trapping in reduced graphene oxide (rGO) after high relative humidity (RH) exposure. For this purpose, we grew graphene oxide films via spin-coating on glass substrates followed by thermal reduction. The electrical resistance of the planar device was then measured. We observed that resistance is significantly increased after water vapor exposure and remains stable even after 250 days in ambient conditions. Various techniques were applied to desorb the water and decrease (recover) the material's resistance, but it was achieved only with low temperature thermal annealing (180 0 C) under forming gas (H 2 /N 2 mixture). The permanent effect of water absorption was also detected by x-ray photoelectron spectroscopy. Reduced graphene oxide (rGO) has attracted a strong research interest for gas sensing applications in recent years due to its unique electrical and chemical characteristics 1,2,3. Its main advantages among others are good chemical stability over time and ease of functionalization contributing to satisfying selectivity between various analytes 4. The gas sensing mechanism _____________________________

Graphene oxide (GO) is a promising material for H 2 O vapour sensing. However, H 2 O sensing mechanisms are still under investigation especially in the case of thermally reduced GO. To this purpose, planar devices were fabricated by spincoating graphene oxide on glass substrates. Ultra high response to H 2 O 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 H 2 O. The sensing mechanisms that govern GO and reduced GO behaviour 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 H 2 O 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.

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