Abstract
In this study, we report a simple two-step dip coating” process to fabricate reproducible chemiresistive NO2 gas sensor based on graphene materials. This gas sensor is able to operate at room temperature with good sensitivity. Herein, we use polymer-capped graphene nanoplatelets suspension prepared by liquid phase exfoliation（LPE）and reduced graphene oxide（rGO） approach as sensing material. In LPE method, we choose harmless polyvinylpyrrolidone（PVP） as intercalator and water as solvent. With the help of high-power sonication, the hydrophobic graphite powder is exfoliated into PVP-capped graphene nanoplatelets aqueous dispersion（PVP-graphene）. Characterized by X-ray photoelectron spectroscopy, transmission electron microscope and Raman spectroscopy, PVP-graphene nanoplatelets features boundary defects and lateral dimension of a few hundred nanometers. For rGO, firstly, we use modified Hummer’s method to produce graphene oxide. Then for the reduce process, the commonly-used hydrazine, which is known to be extremely dangerous and harmful is replaced by less-harmful thiourea as the reducing agent. Through a series of characterization, home-made rGO features both defects and oxygen functional groups. . Meanwhile, PVP is also used as the dispersant to fabricate aqueous dispersion（PVP-rGO） The sensing device is prepared by two-step dip coating. The first step uses commercial cationic surfactant to modify the substrate’s surface to facilitate the chemisorption of graphene materials in the subsequent step. Vacuum heat treatment is used as recovery method for sensors. Both sensing curves obtained from PVP-graphene and PVP-rGO are found to be well fit using double exponential model, which indicates multiple sensing sites exist on the graphene materials PVP-rGO sensing fitting result shows that the slow reaction(R2), which is dominated by defects or functional groups, accounts for total response relatively higher than PVP-graphene. The result shows that oxygen functional groups can provide additional sensing sites and thus improve the sensitivity. The calibration lines of these NO2 sensors are obtained by using Freundlich adsorption equation.