Synthesis And Gas Sensing Properties Of Metal Sulfide Quantum Dots/Reduced Graphene Oxide Composites

For gas sensor devices,miniaturization has become an irresistible trend,so the room temperature detection performance of gas sensing materials is particularly important.Metal sulfide quantum dots（QDs）have demonstrated excellent potential for gas detection at room temperature,and the addition of rGO can effectively solve the key problems of poor long-term stability and the loss of electrical properties of quantum dots.Two metal sulfide QDs/rGO gas sensing materials are successfully synthesized:PbS QDs/rGO and CuSbS₂ QDs/rGO composites.The phase and morphology of the composites are analyzed by XRD,FESEM,HRTEM,Raman,FTIR,etc.,and the gas sensing properties toward ammonia at room temperature are investigated as well.The gas sensing mechanisms are explored by characterizations like I-V,DRIFT,UPS,UV-vis,ICP,etc.The experimental results show that the two metal sulfide QDs/rGO composites have exceptional room temperature ammonia sensing performance,which could be attributed to the abundant active sites for gas adsorption and internal optimized electron transport path.The main contents are as following:1.The PbS QDs/rGO composites were prepared by one-step liquid phase method.The characterization results show that the average size of PbS QDs is 8 nm,which in situ grow on rGO via C-S chemical bonds.A unique three-dimensional"nano-cacoon"structure is formed due to the ultrasound treatment.The NH₃response test was performed on the PbS QDs/rGO composites.The results show that the composites possess a significant improvement in the room temperature detection ability of NH₃ compared to pure PbS QDs and rGO.It has a response of3.7 to 1000 ppm NH₃ at room temperature and a minimum detection limit of 750ppb with good selectivity.The enhancement of the sensing properties is mainly attributed to the synergistic effect in the composites:The Schottky barrier is formed at the interface of the two materials,so the small variation of the work function during the gas sensing reaction greatly changes the resistance of the composites,thereby improving the gas responses.Additionally,the quantum dots have high surface energy and numerous defects,which could promote more NH₃ adsorption as the active center of the reaction.The free electrons released from the reaction could transfer from PbS QDs to rGO through chemical bonds,which fully utilizes the excellent electron transport characteristics of rGO.It is also more conducive to the conversion of chemical signals to electrical signals on the surface of composites,enhancing the gas detection capability.2.The CuSbS2 QDs/rGO composites were prepared by thermal injection method.The HRTEM characterization of the composites illustrates that the average size of CuSbS2 QDs is 5.3 nm,which is evenly distributed on the rGO layer with chemical interaction.This structure is obviously advantageous for the exposure of surface active sites and improvment of gas responses at room temperature.The NH3detection capability of CuSbS2 QDs/rGO composites was measured at room temperature.The results show that the NH3 sensing performance of the composites is obviously improved compared with pure CuSbS2 QDs and rGO.It possesses a response of 1.42 to 250 ppm NH3 at room temperature,a minimum detection limit of 500 ppb and an average response time of 50 s with good selectivity.Besides,the introduction of visible light further enhances gas responses.The sensing mechanisms of the composites are investigated by various characterizations:Physical and chemical adsorptions occur simultaneously on the surface of the composites,while hydroxyl groups and S vacancies（acidic sites）play the role of active sites to promote the adsorption of NH3.The adsorbed NH3 reacts with the pre-adsorbed O₂^-on the surface,eventually forming NO molecules.The free electrons released by the reaction transfer from the CuSbS2 QDs to the rGO,and finally transport rapidly on the rGO,exerting its excellent electron transport characteristics.