Structural Modulation And NO₂ Sensing Performance Of Two-dimensional Graphene And Chalcogenide Materials

The rapid development of human society and economy is along with tremendous energy consumption.Given that traditional fossil fuels are the dominant energy source,the produce of NO₂ and other gaseous pollutants is inevitable during energy consumption,which arouses serious environmental problems and is hazardous to human life.Using gas sensors for the real-time NO₂ monitor is an effective means of pollutant control.As gas-sensing technology becomes more intelligent and integrated,low-power and high-performance gas sensors are in high demand.In recent years,using the emerging two-dimensional（2D）materials instead of conventional metal oxides sensing material for the fabrication of low-power resistive gas sensors has become one of the hottest trends in the research of NO₂ gas-sensing.Among various 2D materials,the widely reported materials with outstanding NO₂sensing performance are 2D elemental materials（such as graphene,black phosphorus,and Te）and 2D metal chalcogenide materials（such as MoS₂,MoSe₂,and SnS₂）.The above types of 2D materials can be directly used for NO₂ sensing,but there are still problems such as low sensitivity,slow response/recovery,poor selectivity and stability,which need to be solved according to their structural characteristics.As of today,the heterojunction engineering of 2D materials（e.g.,SnO₂/rGO,SnO₂/SnS₂,rGO/SnS₂,etc.）is one of the most effective structural modulation strategies to enhance their NO₂ sensing performance.However,the NO₂ sensing performance of various 2D material heterojunctions still needs to be improved.For example,the metal oxides（MO_x）/graphene heterojunctions lack selective adsorption sites for NO₂,while the MO_x/2D metal chalcogenide heterojunctions suffer from low interfacial charge transfer efficiency,resulting in limited NO₂ sensitivity enhancement.Therefore,based on the defect modulation and heterojunction engineering strategies,rational structural modification of single-component 2D materials and their heterojunctions were carried out.From the perspectives of NO₂ adsorption enhancement and material electronic structure optimization,the NO₂ sensing performance of two-dimensional graphene and chalcogenides has been improved and the corresponding enhancement mechanisms were thoroughly investigated.The main research works are as follows:To address the low sensitivity of 2D metal chalcogenide material MoSe₂,defect modulation of MoSe₂ through the Se vacancy content optimization was carried out for NO₂ sensitivity improvement.The modulation of Se vacancy concentration based on oxygen passivation was achieved by thermal annealing of MoSe₂ in air.The KFM-based work function test demonstrated that the reduction of Se vacancy content leads to an elevated MoSe₂ Fermi level,which is in favor of the electron transfer from MoSe₂ to the electrophilic NO₂.As a result,MoSe₂ with optimal Se vacancy concentration showed a 6.3 times higher response to 1 ppm NO₂ than that of the pristine MoSe₂ at room temperature.Corresponding theoretical calculations showed that,by precisely modulating the Se vacancy content,the favorable adsorption of NO₂ on Se vacancy can be effectively utilized,while increasing the amount of charge transfer from MoSe₂ to NO₂,thus obtaining a greater change in resistance and improving NO₂ sensitivity.For the poor stability of 2D elemental Te,the Te@Se chalcogenide heterojunction with an elemental Se shell encapsulating Te nanoflake was prepared according to the heterojunction engineering strategy,which shed a light on the stability enhancement of elemental 2D gas-sensing materials.The elemental Se with the same crystal structure as Te capsulated on the surface of the Te nanoflake through epitaxial growth in solution,which effectively avoided the oxidation of Te nanosheets in air and thus enhanced the stability.In addition,the highly matched crystal structures of elemental Se and Te facilitated the charge transfer between them,thus generating an interfacial charge layer at the Te@Se heterointerface to modulate the material resistance.As a result,the Te@Se heterojunction exhibited higher NO₂sensitivity than that of Te nanoflake,the response of Te@Se to 1 ppm NO₂ was nearly four times higher than that of Te nanoflake,and an extremely low practical detection limit of 10 ppb was also obtained.To improve the response/recovery speed and the selectivity of elemental 2D material graphene,the MO_x/graphene heterojunction was engineered to enhance the response/recovery speed,and the defect modulation of graphene was then used to promote the adsorption and enrichment of NO₂ at the heterointerface,thus further enhancing the selectivity of MO_x/graphene heterojunction.The graphene oxide（GO）was functionalized by ethylenediamine（EDA）group,during which the EDA molecule was bonded with the epoxy group defect on GO,while GO was thermally reduced to rGO.The SnO₂/EDA-rGO heterojunction was then engineered by growing SnO₂ nanorods on the functionalization product EDA-rGO.The corresponding SnO₂/EDA-rGO sensor exhibited improved response/recovery speed and selectivity for NO₂ over the SnO₂/rGO heterojunction without EDA-functionalization.The response of SnO₂/EDA-rGO to 1 ppm NO₂ was 2.8 times higher than that of the SnO₂/rGO heterojunction.Theoretical calculations and work function measurement results showed that the EDA functionalization can enhance both chemical and electronic effects of the SnO₂/rGO heterojunction,and the synergistic enhancement of both effects is the main reason for the improved NO₂ gas-sensitive performance.Different from the heterojunctions constructed by elemental materials of the same main group,the MO_x/2D metal chalcogenide heterojunctions generally suffer from the lattice mismatch and the agglomeration of nanomaterials,resulting in low charge transfer efficiency at the heterointerface and limited enhancement of NO₂sensitivity for 2D compound materials.Using graphene as an interfacial charge transfer bridge was adopted to improve the interfacial charge transfer efficiency of the binary heterojunction of MO_x/2D metal chalcogenide materials,which led to a significant NO₂ sensitivity improvement.The SnO₂-rGO/SnS₂ ternary heterojunction with a sandwich structure was designed by introducing rGO as a charge transfer bridge in the SnO₂/SnS₂ binary heterojunction.Gas-sensing results showed that the SnO₂-rGO/SnS₂ ternary heterojunction sensor has a response of 1064 to 10 ppm NO₂,which was 18 times higher than that of the SnO₂/SnS₂ binary heterojunction.The work function and transient photocurrent tests showed that the interfacial bridging effect of rGO was the main reason for the improved charge transfer efficiency at the SnO₂/SnS₂ binary heterojunction.The overall gas-sensing performance of the SnO₂-rGO/SnS₂ ternary heterojunction sensor is excellent and has great potential for practical atmospheric environmental pollution monitoring and indoor air quality testing applications.