This article describes the effect of the lateral size of graphene oxide (GO) on the humidity sensing properties of a GO-based sensor. The GO size effect on the humidity sensing performance was evaluated on gold electrodes drop-coated with either an ultralarge graphene oxide (UGO) sheet (lateral size = 47.4 ± 22.2 μm) or a small-sized graphene oxide (SGO) sheet (lateral size = 0.8 ± 0.5 μm). The in-plane conductance obtained from the UGO and SGO electrodes was found to increase by four orders of magnitude and by three orders of magnitude, respectively, upon exposure to relative humidity RH change from 7 to 100%. The maximal sensitivity (S) values of the UGO and SGO humidity sensors were determined to be S(UGO) = 4339 ± 433 and SSGO = 1982 ± 122. The GO size clearly influenced the overall proton conductivity, as evidenced by the activation enthalpy (Ea) required for proton conduction in UGO and SGO sheets: Ea (UGO) = 0.63 eV, Ea (SGO) = 1.14 eV. The UGO humidity sensor exhibited an excellent device performance with a high sensitivity and an ultrafast response/recovery time (0.2/0.7 s). Good humidity sensing stability was observed, with a variation of only ±4.6% over five days. The resistive-type UGO humidity sensor was capable of sensing the moisture on a fingertip at a distance of 0.5 mm with a sensitivity of 17.4 and a response/recovery time of 0.6 s/1.3 s. The excellent device performance of the UGO humidity sensor also permitted the determination of the position of a fingertip by detecting the fingertip moisture, hence offering a great potential for touchless display position interface applications.
