A graphene oxide (GO)–porous anodic alumina (PAA) bilayer system: How GO dispersion regulates the lower RH detection limit to near zero in conjugation with PAA

Abstract

In RH-humidity sensors, improving the lower detection limit (LOD) with high sensing responsiveness is an unsolved problem to date. A novel humidity sensor utilizing a bilayer system has been presented, wherein a layer of graphene oxide (GO) is vertically deposited onto a porous anodic alumina (PAA) substrate. For 1 mg ml⁻¹ amount of GO flake density, both the GO and PAA layers work synergistically throughout the humidity range simultaneously pushing the lower detection limit to 0.9% RH. When the density of GO flakes exceeds 1 mg ml⁻¹, the synergetic activity ceases, and the sensor exclusively operates under high relative humidity conditions due to the dominance of the GO layer alone. Exploration of such flipping activity is well described with a suitable mechanism. The said GO dispersion sensor system shows an exceptional capacitive response throughout the humidity range (0.9–99% RH) with an ultrafast response of 0.8 s, excellent sensitivity of 1178.76 pF/% RH, and negligibly small hysteresis of 0.129%. The exploration of humidity sensing using a unique combination of two hygrometric materials, namely GO and PAA, has not been previously investigated in order to enhance the lower detection limit and sensitivity. There exists significant potential for reducing the limit of detection (LOD) through manipulation of the pore shape of the porous anodic alumina (PAA) layer, as well as the thickness of the GO layer.