Highly Sensitive Humidity Sensor Based on Freestanding Graphene Oxide Sheets for Respiration and Moisture Detection

Graphene oxide (GO) is regarded as one of the best materials for humidity sensing owing to its unique structure (defect enriched) and excellent adsorption properties. Herein, GO was prepared by modified Hummers' method for the fabrication of highly sensitive humidity sensors. The strong diffraction peak at 10.06° in the x-ray diffraction (XRD) pattern and increased ID/IG ratio (0.96) for GO obtained from Raman spectroscopy indicate the formation of GO. The Brunauer–Emmett–Teller (BET) analysis for GO revealed a surface area of 17 m2/g and a pore radius of 20 Å. The surface morphology and topography of GO sheets were examined by field emission scanning electron microscopy, transmission electron microscopy and atomic force microscopy, respectively. For humidity sensing application, a device was prepared by drop casting of GO on interdigitated electrodes and its sensitivity was investigated at room temperature (26 °C) under a different relative humidity (RH) level of 11–97% RH. The GO sensor exhibited high sensing response at 11–97–11% RH with good response (13 s)/recovery time (32 s). Notably, the sensor displayed very small humidity hysteresis (0.26%), excellent repeatability and long-term stability (1.13% decrease). Moreover, the proposed sensor quickly responded to different human respiration including nose breath, normal/fast (exercise) mouth breathing, and human skin moisture (sweat) as well as simple spoken words. This finding indicates that flexible, freestanding GO sheets have broad applications in smart humidity sensing devices and medical care treatment.