Microstructure And Energy Band Modulation Of Two-dimensional Metal Sulfides And NO₂ Sensing Properties At Room Temperature

As the rapid development of society,a large number of toxic and harmful gases such as NO₂ are discharged,which seriously pollutes the ecological environment and threatens the safety of human production and life.A gas sensor loaded with gas-sensing materials is an electronic device that can convert gas composition and concentration into an identifiable signal,enabling real-time and effective monitoring NO₂.However,traditional NO₂ sensors based on metal oxide materials require harsh working conditions,such as high temperature or light excitation,which is contrary to the future development trend of energy conservation and emission reduction.Therefore,research and development of new room-temperature NO₂ gas-sensing materials is the essential way to practice the sustainable development strategies.Two-dimensional（2D）nanomaterials with the properties of large specific surface area,high activity,and outstanding electrical properties,are ideal materials for developing low-temperature and even room-temperature NO₂ gas sensors.However,they still have some bottlenecks in gas-sensing properties,material structure,and sensing mechanisms.Taking the typical 2D metal chalcogenides（MCs）as the representative,in terms of gas sensing properties,they have an extremely long response/recovery time to NO₂ at room temperature（RT）,or even can not recover,and show a low response and sensitivity values and high detection limit;in the case of sensing mechanism,the intrinsic relationship among microstructure,energy band structure,and sensing properties is not clear enough;in terms of material structure,the heterointerface region of MCs-based heterostructure materials usually has a serious lattice mismatch which leads to low interfacial charge transfer efficiency,seriously degrading the gas-sensing efficacy of the heterostructure.To solve these problems,this dissertation employed the idea of heteroatom substitution to manipulate the microstructure and energy band regulation of typical Mo-and Sn-based sulfides in the MCs family,which were carried out by the doping and ion exchange,for achieving efficient NO₂ room-temperature sensing.The relationship between the structure and NO₂ sensing properties was investigated in detail.The contents of this dissertation are presented as follows:（1）Aiming at the problem that the response/recovery of MCs-based gas sensing materials is very slow and even irreversible at RT,the doping method of heteroatom substitution is employed.Specifically,nitrogen（N）atoms are used to replace the S atoms of MoS₂ to improve the surface activity and the energy band structure,realizing the shortened response/recovery times of MoS₂ to NO₂ at RT.N atoms were in-situ doped into MoS₂ lattices through the high-temperature solvothermal reactions using molybdenum and S sources with high N content.The p-type semiconducting N-doped MoS₂ nanosheets with different doping contents were prepared by adjusting the reaction temperature.The optimal N-doped MoS₂ showed the response value of23%and the response/recovery times of 18 s/72 s to 4 ppm NO₂.Compared with the slow responding and non-recovering of pristine MoS₂,the adsorption and desorption rates on N-doped MoS₂ were boosted by 118 and 514 times,respectively.Theoretical calculations and Hall-Effect tests indicate that the shortened response/recovery times of N-doped MoS₂ at RT are mainly attributed to the fact that N atoms as p-type dopants not only improve the surface activity,but also adjust the carrier concentration and mobility by improving the energy band structure of MoS₂.（2）Aiming at the problems of low response value and high detection limit,the defect structure was introduced on the basis of the doping strategy in the previous chapter.Accordingly,carbon（C）-doped SnS₂ nanosheets with riched S vacancies was synthesized.The reversible room-temperature NO₂ sensing with a high response value and a low detection limit was realized by the synergistic effect of doping and defects.Specifically,a solvothermal reaction,in which the common S source was replaced by an organic amino acid with abundant functional groups,was employed to in situ dope C atoms into the SnS₂ lattice while inducing the formation of S vacancies,for improving the electronic structure and surface activity of SnS₂.At room temperature,C-doped SnS₂ with riched S vacancies showed a response value of 700%to 4 ppm NO₂ with a detection limit as low as 10 ppb.Based on the ESR measurement and the analyses of bandgap and work function,the high-performance NO₂ sensing at RT was mainly ascribed to the synergistic effect of C doping and S vacancy.This synergistic effect not only improves the energy band structure of SnS₂（narrowed bandgap and elevated Fermi level）,but also increases the active sites to enhance the adsorption capacity of NO₂.（3）Motivated by the heteroatom doping methods in the previous two chapters,a liquid-phase ion-exchange strategy was developed to prepare a homogeneous single-phase SnS_1-xSe_x compound with tunable energy level,realizing a relative high response value and short response/recovery times to NO₂ at RT.It provides a new horizon for the development of high-performance MCs-based sensing materials at RT.The single-phase SnS_1-xSe_x solid solution were obtained by substituting the S atoms in SnS with Se atoms through the ion-exchange reaction,because the solubility product of metal selenides is smaller than that of metal sulfides.Adjusting the addition amount of Se precursor can realize that the content x of Se component in the compound can be continuously manipulated ranging from 0 to 1.At room temperature,the optimal SnS_0.6Se_0.4 crystal showed the response value of 165%and the response/recovery times of 32 s/263 s to 4 ppm NO₂.The detailed work function analysis revealed that the Fermi level of SnS_1-xSe_x crystals can be well tuned by the Se content in the crystals,which in turn changes the energy differenceΔE between the Fermi level of the crystals and the LUMO molecular orbital of NO₂.The increase ofΔE improve the charge transfer between the materials and NO₂ molecules,thereby increasing the response value of SnS_1-xSe_x to NO₂.（4）Aiming at the problems of high-degree lattice mismatch and inefficient charge transfer in the heterointerface of traditional heterostructures,the ion exchange reaction strategy in the previous chapter was extended to in situ construct SnS₂/SnSe₂lateral heterostructure for NO₂ room-temperature sensing materials.The result lays the foundation for the development of heterostructured materials with high-quality heterointerfaces.Specifically,the SnS₂/SnSe₂ lateral heterostructures with coherent interface were synthesized by the ion-exchange reaction between Se atoms and S atoms in SnS₂ using SnS₂ as a template in the liquid phase.By adjusting the Se/SnS₂molar ratio in the precursor,the content of SnSe₂ can be effectively regulated.the optimal SnS₂/SnSe₂ lateral heterostructures showed 24.5 times and 9.6 times higher response values,respectively,to 4 ppm NO₂ at RT than that of the common SnS₂/SnSe₂ heterostructures prepared by mechanical mixing and solvothermal deposition.The response time was also reduced by 72%and 89%,respectively,and recovery time was decreased by 49%and 80%,respectively.Thus,the high response value and rapid response/recovery NO₂ sensing at RT were achieved in the lateral heterostructure.The sensing mechanism is attributed to the SnS₂/SnSe₂ lateral heterostructure not only effectively modulating the energy band structure of SnS₂ and SnSe₂ components but also the coherent and lateral heterointerface significantly improving the built-in field to boosting the interfacial charge transfer efficiency.This dissertation provides a new idea for structural design and property improvement for the development of MCs-based materials for room-temperature high-sensitive NO₂ sensing,which paves the way for the research on the structure-function integration of 2D MCs-based nanomaterials.