Gas Sensing Properties Of 2-Dimensional RGO And RGO Based Nanocomposites At Room Temperature

Graphene with 2D structure has attracted more and more attention as a gas sensitive material due to its six-element circular structure and zero-band gap with Dirac points.It has shown significant sensitivity to NH3 and NO2 at room temperature.In order to meet the market demand of low energy,high response,low detection limit,etc.graphene based two-dimensional room temperature gas sensor needs to be promoted.Reduced graphene oxide(rGO)with two-dimensional structure,high electrical conductivity and various surface functional groups,has provided a great possibility to improve its sensitive performance by adjusting the types and quantities of surface groups.In addition,it is more likely to hybridize rGO with other semiconductor by chemical synthesis to form heterojunctions.RGO based heterojunction has shown better properties combining the advantages of 2-dimensional rGO and the composite semiconductor.However,systematic research on sensitive mechanism of rGO based sensor is still scarce at present.And,sensitivity and speed of response/recovery of rGO based gas sensors still need to be improved urgently.In this paper,the gas sensitive mechanism of rGO based gas sensor has been studied,mainly focusing on the role of oxygen functional groups.And,adsorption capacities of functional groups for NO2 and NH3 molecules have been verified by the first principles calculation.Then,the gas-sensitive performance and response mechanism of p-n heterojunction formed by p-type rGO and traditional n-type metal oxides have been studied.A high-performance NO2 gas sensor has been developed by figuring out the relationship between changes of functional group in heterojunction and gas-sensitive properties.Besided,in view of the recovery difficulty of RGO-based ammonia sensor,a p-p homotypic heterojunction of rGO and 2D WS2 nanosheet has been proposed,and its room-temperature gas-sensitive properties have been studied.At last,through adjusting functional groups on the surface of rGO,sulfonated rGO and WS2 composite semiconductor has been introduced to improve the sensitivity to ammonia at room temperature.Gaussian computational modeling based on DFT was used to analyze the charge exchange mechanism between sulfonates and gas molecules.The main contents and conclusions of this paper are as follows:1.In order to investigate the influence of oxygen functional groups on the gas sensitive properties of rGO,three different reduction methods(thermal reduction,chemical reduction and mixed reduction)have been selected to reduce graphene oxide.It has been found that there are three types of oxygen-containing groups in rGO,such as-OH,C-O-C and C-OOH,by XPS analysis,and the content of oxygen-containing species has a significant influence on the sensitivity of rGO to NO2 at room temperature.Sample with more-OH species has shown better NO2 sensitivity and faster speed of response.This is because the proportions of hydroxyl group and epoxy group in rGO are larger,and hydroxyl group has stronger adsorption capacity on NO2 compared with epoxy group.This has been further verified by calculations based on Density Functional Theory(DFT),which showed that the model of graphene with hydroxyl group had higher binding energy with NO2 gas molecule.2.The formation of heterojunctions with metal oxides is an effective method to improve the sensitivity of rGObased gas sensors.The p-n heterojunction formed by rGO and traditional semiconductor,SnO2 or WO3,has been found to reduce the working temperature effectively.But sensing mechanism of rGO/MOx heterojunctions still needs systematic study and elaboration further.Gas-sensitive rGO/SnO2 and rGO/WO3 heterojunctions have been synthesized through a one-step hydrothermal method.RGO/SnO2,which is more sensitive to NO2,has been chosed to evaluate the effects of surface functional groups and conductive types of heterogeneous junctions on their gas-sensitive properties at room temperature.The microstructure,functional groups and gas-sensitive properties of the hydrothermal synthesis SnO2 and rGO/SnO2 heterojunctions were performed.And the hydrothermal synthesis rGO/SnO2 heterojunctions exhibited rapid response and recovery signals to 2～8 ppm NO2 at room temperature.After post-treatment at about 100℃,the conductive type of the rGO/SnO2 heterojunction is converted from p-type to n-type,which is due to the further reduction of rGO and the reduce of adsorbed water on the surface of the rGO/SnO2 heterojunction.The n-type rGO/SnO2 heterojunction performed higher sensitivity and faster response recovery to NO2 at room temperature,while the response to 8 ppm NO2 was 78%and the response/recovery time was 100/200 s..3.The gas-sensitive mechanism of the p-p type heterojunction synthesized by rGO and another p-type 2D sensitive material is worthy of further study to improve the performance of graphene-based room temperature sensors.The p-p type rGO/WS2 heterojunctions obtained by one-step hydrothermal synthesis has been investigated to have good sensitivity to 10～50 ammonia in an environment close to room temperature(33.5℃),and the response to 10 ppm NH3 could reach 121%.This is attributed to higher proportion of hydroxyl groups on the surface of rGO after the synthesis of heterojunctions,which had better adsorption capacity to ammonia molecules compared with other oxygen functional groups.And the WS2 nanoflakes doped in the heterojunction has also played signifcant role in the enhanced response and the extra Lewis acid active centers.Lewis acid centers in heterojunctions were more likely to adsorb Lewis alkaline ammonia molecules.In addition,sensor equipped with rGO/WS2 heterojunctions has shown excellent selectivity and good long-term stability,indicating a potential to be employed as a room-temperature ammonia sensor.4.The rGO/WS2 heterojunction showed high sensitivity to ammonia,but its response/recovery speed was still too slow(over 600 s),which would affect the detection efficiency.A new surface functional group(-SO3H)was introduced into the rGO surface by pre-sulfonation process in an ice bath,and the sulfonated-rGO/WS2 heterojunction was further synthesized by hydrothermal process.The introduction of sulfonate improved the room temperature ammonia sensitivity of rGO-based p-p heterojunction significantly:the sensitivity of sulfonated heterojunction to 10～50 ppm ammonia was about three times that of the unsulfonated sample,and the response/recovery time after sulfonated heterojunction was shortened by half.This could be benefit from the introduction of-SO3H,which is sensitive to ammonia and adsorbs/desorbs gas molecule quickly.DFT calculation of ammonia adsorption on sulfonated graphene sheet has been performed to illustrate the adsorption capacity of-SO3H group.In addition,I-V characteristics,selectivity and stability of sensor based on S-rGO/WS2 have also been investigated to verify that it can be a promising candidate in ammonia rapid detection at room temperature.