Preparation And Gas Sensing Properties Of Typical Metal Oxide/Graphene Composites

The existence and concentration of flammable,explosive and harmful gases exceed the standard,which seriously threatens human production and life.In order to protect personal safety and property from loss,the monitoring of flammable,explosive and harmful gases becomes very important.Improving the sensor's sensitivity,selectivity,response-recovery time and other gas-sensing properties to the target gas is of great significance for accident prevention.In this paper,we used flake graphite as raw material,and modified Hummers method was used to prepare graphite oxide?GO?with the dominant hydroxyl content.As the degree of oxidation increases,the content of oxygen-containing functional groups gradually increases.When the mass ratio of potassium permanganate to graphite is 3.5,the maximum average cation exchange capacity?CEC?of GO measured by formaldehyde condensation method and distillation method is 459.82 mmol/100g and 125.01 mmol/100g.In the determination of GO CEC by formaldehyde condensation method,there was no significant change of-OH content in GO structure,while in the process of heating distillation,GO was reduced by distillation method.Combining the GO CEC value with the change of the structure of the cation exchange products,it was concluded that the formaldehyde condensation method is more suitable for determining the GO CEC value than the distillation method.SnO₂/rGO nanocomposites with excellent gas sensing performance to NH₃ at room temperature were prepared by precipitation-calcination method.When the mass ratio of GOs to SnO₂ is 1.0%,the gas sensing performance of the composite is the best.At room temperature,when the NH₃ concentration was 5 ppm and 100 ppm,the sensitivity of the SnO₂/rGO?1.0%?sensor reached 15.0%and 49.6%,respectively,and the response and recovery time to 100 ppm NH₃ were shorter at 21 s and 204 s,respectively.The sensitive mechanism of NH₃ at room temperature is the result of the combined action of the chemical reaction between NH₃ and O₂^-on the surface of SnO₂,the p-n heterojunction produced by rGO and SnO₂,and the ionization of dissolved NH₃ into conductive ions.Based on the best process for preparing SnO₂/rGO composites,PdO-SnO₂/rGO nanocomposites with better gas sensitivity to NH₃ at room temperature were prepared.When the molar ratio of Pd²⁺to Sn⁴⁺was 0.5%,the composite has the best gas sensing performance. At room temperature,the sensitivity of PdO?0.5%?-SnO₂/1G sensor to 100 ppm NH₃ was57.4%,and the response and recovery time were 23 s and 114 s,respectively.At room temperature,the NH₃ gas sensing performance of PdO-SnO₂/rGO nanocomposites is higher than that of SnO₂/rGO nanocomposites,which is caused by the formation of p-n heterojunction between PdO and SnO₂ and the formation of adsorbed oxygen ions promoted by PdO as catalyst.Fe₂O₃/rGO composite was directly prepared by hydrothermal method.The composite has good humidity sensitivity to medium and high humidity.When the mass ratio of GOs to Fe₂O₃ is 1.0%,the humidity sensitivity of Fe₂O₃/rGO?1.0%?sensor at room temperature is the best at medium and high humidity,and the humidity sensitivity of the composite decreases with the increase of rGO composite.The sensitivity of Fe₂O₃/rGO?1.0%?sensor to93.6%RH was 97.7%,and the response recovery time was 5 s and 31 s respectively.The humidity sensing mechanism of the sensor at medium and high humidity is that the water molecules in low RH replace O₂^-and form H₃O⁺and OH^-in medium and high RH,the OH^-structure of the sensitive material is connected to multi-layer water molecules through hydrogen bond,so as to promote proton transfer between adjacent water molecules,reduce the resistance of the sensor and rGO provide a large number of adsorption sites for water molecules.Based on the best process of preparing Fe₂O₃/rGO composite,Pd-Fe₂O₃/rGO composite with excellent gas sensitivity to H₂ was prepared by one-step hydrothermal method.When the molar ratio of Pd²⁺to Fe³⁺is 0.5%,the sensitivity of composite to H₂ increases first and then decreases with the increase of working temperature.The results show that when the sensor is operated at 180?,the sensitivity to H₂ is the highest,and the sensitivity of Pd?0.5%?-Fe₂O₃/rGO sensor to 1000 ppm H₂ has reached 78.4%,and the response recovery time is short,which are 19 s and 6 s.At 180°C,the mechanism of Pd?0.5%?-Fe₂O₃/rGO composites having excellent gas sensitivity to H₂ is the chemical reaction between O^-on the surface of Fe₂O₃ and H₂ and H,and the existence of Pd-Fe₂O₃ heterojunction and rGO,which together increase the gas sensing properties of the composite for H₂.