Controllable Synthesis, Composition Tuning And Energy Device Application Of MoS_2xSe_2（1-x） Nanotubes

Transition metal dichalcogenides (MX2) have attracted considerable attention due to the advantages of 2D layered structure,2D permable channels, ultrathin thickness, and large specific area. They are condidered as promising applications in sodium ion batteries, electrocatalyst for hydrogen evolution reaction, and photodetection devices. In this thesis, hierarchical nanotubes assembled from MoS2xSe2(1-x) nanosheets were synthesized and demonstrated as roubst host materials for sodium storage applications. Main results are summarized as follows:1. Hierarchical nanotubes consisting of 2D monolayer MoS2 and carbon (MoS2:C) interoverlapped superstructure nanosheets have been synthesized, in which the MoS2 and carbon layers are alternately sandwiched. The hierarchical architectures assembled from the MoS2:C superstructures are beneficial for:(i) providing substantially expanded (002) interlayer spacing (0.98 nm) of 2H-MoS2 which facilitates fast Na+ insertion/extraction reaction kinetics, (ii) improving electrical conductivity of MoS2 by carbon monolayer insertion with ideal heterointerface contact, (iii) preventing aggregation of MoS2 nanosheets, and (iv) accommodating volume change upon sodiation/desodiation. The superstructure nanotubes are demonstrated as a robust anode material for sodium storage with superior electrochemical performance. They deliver a high rate-capability and maintain discharge capacities of 295 and 187 mA h g-1 at high current densities of 10.0 and 20.0 A g-1, respectively. Furthermore, they show durable cycling life (capacity retention of 101.3%,108.2% and 107.8% after 200 cycles at current densities of 0.2,0.5 and 1.0 A g-1, respectively, in comparison to those of the 2nd cycles), and an initial Coulombic efficiency as high as 84%. The MoS2:C superstructure nanotubes perform among the best of current MoS2-based electrode materials.2. Carbon-doped Mo(Se0.85S0.15)2 (i.e.Mo(Se0.85S0.15)2:C) hierarchical nanotubes have been synthesized by selenizing the MoS2:C nanotubes and serve as a high-performance anode material of sodium ion battaries. The hierarchical nanotubes with diameters of 300 nm and wall thicknesses of 50 nm consist of numerous 2D layered nanosheets, and can act as a robust host for sodiation/desodiation cycling. The Mo(Se0.85S0.15)2:C hierarchical nanotubes deliver a discharge capacity of 360 mA h g-1 at a high current density of 2000 mA g-1 and keep 81.8% capacity retention as compared to that at the current density of 50 mA g-1, showing superior rate capability. Comparing with the 2nd cycle discharge capacities, the nanotube anode can maintain capacities of 102.2%,101.9% and 97.8% after 100 cycles at current densities of 200, 500 and 1000 mA g-1, respectively. The hollow interior, hierarchical organization, layered structure and carbon doping are beneficial for fast Na+-ion and electron kinetics and responsible for the stable cycling performance and high rate capabilities.