Ab Initio Characterization And Experimental Study On Graphene Based Formaldehyde Sensors

Formaldehyde(HCHO),which is one of the most common indoor air pollutants,has been categorized as a carcinogen for human being according to the US Environment Protection Agency(EPA).Among various HCHO detecting methods,chemoresistive sensor has attracted tremendous attention due to its long life-time,simple structure,low cost,inherent miniaturization,great compatibility with integrated circuit(IC)technology,etc.The basic principle of a common chemoresistive sensor is the adsorption and desorption of gas molecules on sensing materials.It has been extensively demonstrated that sensing materials play a crucial role in determining the overall performances of sensors.Graphene has been demonstrated as a promising sensing material for gas sensors.And reduced graphene oxide(rGO)is commonly used to fabricate graphene based HCHO sensors because of its low production cost,fine-tuning of structure and properties(e.g.,conductivity and dispersibility in water).On the other hand,the residual oxygen-containing groups in rGO could significantly affect the adsorption of gas molecules,thereby influencing the sensing behaviors(e.g.,response,selectivity,and reversibility).In this work,the effects of oxygen containing groups on the response of rGO to HCHO were investigated both theoretically and experimentally.Density functional theory(DFT)calculations were performed to calculate the adsorption energy(Eads)and charge transfer(?Q)for the adsorption of HCHO on pristine graphene(PG),rGO with epoxides(rGO-O),rGO with hydroxyl group(rGO-OH)and rGO with carboxyl group(rGO-COOH).Results indicate that the incorporation of oxygen-containing groups leads to an obvious increase of Eads and ?Q,with an order of carboxyl group>hydroxyl group>epoxides>pristine graphene.According to the sensing mechanism,the enhancement of Eads and ?Q could increase the variation of concentration of charge carries,the conductance change of sensing materials,and hence the sensor response.Experimental measurements show that with the decreasing C/O atomic ratio from 16.2 to 6.6,the sensor response to 1 ppm HCHO increases from 0.10%to 0.73%,confirming the DFT calculation results.Furthermore,carboxyl functionalized reduced graphene oxide(rGO-COOH-T200)were obtained by converting epoxides and hydroxyl groups into carboxyl groups."With a certain C/O atomic ratio of?6.6,rGO with 6.80%carboxyl groups exhibits a distinctly larger response to 0.2-3 ppm HCHO,compared with the counterpart with 3.09%carboxyl groups.This work provides an atomic-level understanding of the sensing principle of graphene based gas sensor.The as-obtained insights into the effects of oxygen-containing groups on the response of rGO to HCHO could be instructive in preparing rGO-based HCHO sensors for advanced performances.