Synthesis Of MoSe₂ Hierarchical Architectures From Interlayer-Expanded Nanosheets For Energy Device Applications

As a typical transition metal dichalcogenides ?MX₂?,MoSe₂ nanosheets are considered to have promising applications in sodium-ion batteries ?SIBs? and electrocatalysis for hydrogen generation, owing to their advantages of 2D layered structure, large interlayer distance, ultrathin thickness of monolayers, and abundant active sites. However, the performances correlate with the morphology, phase,composition, defect, and interlayer distance. In this thesis, we focus on design and synthesis of novel MoSe₂ nanostructures, and improve their electrochemical performance as electrodes in SIBs and hydrogen evolution reaction ?HER? by modulating the morphology, phase, composition, defect, and interlayer distance. Main results are summarized as follows:1. We report for the first time a simple solvothermal method to synthesize assembled 1T-MoSe₂ nanosheets, which possess expanded ?002? interlayer spacings as large as 1.17 nm with an 81% expansion as compared to that ?0.646 nm? of the bulk counterpart.The 1T-MoSe₂ nanosheets exhibit striking kinetic metrics for the hydrogen evolution reaction ?HER? with a low onset potential of 60 mV and a small Tafel slope of 78 mV dec^-1, which are better than those of the 2H-MoSe₂ counterpart with a normal interlayer spacing of 0.64 nm. The outstanding electrocatalytic activity is attributed to the high concentration of the metallic 1T phase as well as the expanded interlayer distance,contributing to increasing the number and catalytic activity of the MoSe₂ catalysts.2. S-doped 2H-MoSe₂ ?2H-MoS₂xSe2-2x? mesoporous nanospheres assembled from several-layered nanosheets are synthesized by sulfurizing freshly-prepared 1T-MoSe₂ nanospheres and serve as a robust host material for sodium storage. The sulfuration treatment is found to be beneficial for removing surface/interface contamination of insulating organic contaminants and conversion of the 1T phase to 2H phase with improved crystallinity and electrical conductivity. These results in significantly enhanced sodium storage performance, including charge/discharge capacity, first Coulombic efficiency, cycling stability, and rate capability. Coupled with benefits from the in-situ carbon modification and its mesoporous morphology,the 2H-MoS₂xSe2-2x?x=0.21? nanosphere anode can maintain a reversible capacity of 407 mA h g^-1 after 100 cycles with no observable capacity fading at a high current density of 2.0 A g^-1 This value is much higher than those with anode fabricated with the freshly-prepared 1T-MoSe₂ (95 mA h g^-1) and the annealed 2H-MoSe₂ (144 mA h g^-1) samples. As the current density rises from 0.05 to 5.0 A g^-1 ?100-fold increase?, the discharge capacity retention is significantly increased from 39% before sulfuration to 65% after sulfuration.The superior electrochemical performance of the 2H-MoS₂xSe2-2x electrode suggests a promising way to design advanced sodium host materials by surface/interface engineering.3. Hierarchical nanotubes constructed from interlayer-expanded MoSe₂ nanosheets are synthesized by a solventhermal method for the first time. The nanotubes with lengths of 1-2?m and diameters of 180-200 nm have two closed tips. Formation mechanism of the unique nanotubes is carefully investigated and effect of molar ratios of ethonal to octylamine on MoSe₂ morphologies is revealed. The hierarchical nanotubes are demonstrated to have superior sodium storage performance. They can maintain a reversible capacity of 378 mAh g^-1 after 120 cycles at the current density of 500 mA g^-1. The excellent electrochemical performance is attributed to both the interlayer expansion that can effectively decrease diffusion barriers of sodium ions and the hierarchical nanotube morphology that plays a key role in relaxing volume-change-induced mechanical stresses during sodiation/desodiation.