Fabrication Of Ultra-Small Nickel-Based Nanocrystal/Carbon Materials Based On Self-Assembly Of Methacrylate

Ultra-small nanocrystals exhibit unique physicochemical properties that are extremely sensitive to size and composition due to their large proportion and coordination unsaturated properties of surface atoms,which play an important role in the fields of adsorption,catalysis,energy storage and so on.At present,ultra-small nanocrystals are mostly prepared by liquid phase synthesis,but the surface atomic coordination are largely saturated with protective groups on the surface,resulting in the limited application.While the physical method requires specific instruments and is difficult to solve the production bottleneck.Therefore,it is of great significance to study how to efficiently synthesize ultra-small nanocrystals with high activity and controllable composition.In this thesis,methacrylate anion（MAA^-）was selected as the building block to prepare two-dimensional nickel-based coordination polymers based on the desolvation mechanism.Using nickel-based coordination polymers as precursors and according to the design requirements of dye adsorbent,alkaline water electrolysis catalyst for hydrogen production and supercapacitor electrode,three kinds of ultra-small nanocrystal/carbon composites were precisely constructed by one-step thermal decomposition,and their performance enhancement mechanism were clarified.This study provides a new idea for the controllable preparation of ultra-small（<5 nm）multicomponent nanocrystals.Ni（MAA）₂ nanosheets were self-assembled by MAA^-and Ni²⁺ions in water/ethanol system at normal temperature and pressure.Using Ni（MAA）₂nanosheets as precursors,NiO/C nanosheets with about 3 nm NiO nanocryatals were prepared by one-step thermal decomposition based on the limiting effect of in situ synthesis.Its maximum adsorption capacity for Congo red（CR）anionic dye was up to 2000 mg g^-1,which was 3-4 times of that of similar materials reported in literatures,and it was easy to be recycled.Through comparative experiments,it was proved that the performance enhancement mechanism of CR adsorption by NiO/C nanosheets was mainly due to the significant improvements of the adsorption sites and intrinsic adsorption activity caused by the ultra-small size and good dispersion of NiO nanocrystals.Furthermore,the method of constructing nanocrystals based on the limiting effect of M（MAA）₂ coordination polymers was extended to the preparation of unloaded ultra-small metal oxide nanocrystals.ZnO and Ca O nanocrystals with particle size of about 2 nm were prepared using Zn（MAA）₂ and Ca（MAA）₂nanoribbons as precursors,respectively,indicating the universality of this method.The in-depth analysis of fine structure showed that the thickness of Ni（MAA）₂nanosheets was only four molecular layers.Using ultrathin nanosheets as precursors,further based on the confined effect and the mechanism of autocatalysis and self-reduction during the thermal decomposition process,the nanocapsule structure composed of 3-4 nm Ni/NiO heterojunction coated with carbon nano onions was accurately constructed by controlling the thermal decomposition conditions.The Ni/NiO/C nanocapsules were used for alkaline electrocatalytic decomposition of water to produce hydrogen,the overpotential at the current density of 10 m A cm^-2was only 80 m V,which was better than the performance of most reported Ni/NiO catalysts in literatures.Through comparative experiments and the regulation of Niⁿ⁺content or particle size,it was clarified that the enhancement mechanism of catalytic performance came from the synergistic effects of carbon nano onions,ultra-small NiO nanocrystals rich in Ni³⁺ions and ultra-small Ni nanocrystals.In addition,Pd/Ni/NiO/C nanocapsules were constructed by introducing trace Pd²⁺ions into Ni（MAA）₂ nanosheets uniformly based on the HSAB theory.The loading of Pd nanocrystals was only 2.1 wt%,while the overpotential of nanocapsules decreased by nearly 30 m V at the current density of 10 m A cm^-2.Through extending the self-assembly method of mono-metal coordination polymers to that of bimetallic coordination polymers（BMCPs）,Ni_xCo_1-x（MAA）₂BMCPs with adjustable morphology and composition were prepared.Based on the confined effect and the mechanism of autocatalysis and self-reduction during the thermal decomposition process of Ni_xCo_1-x（MAA）₂ nanosheets,Ni/Co/CoNiO₂/C nanocapsules for supercapacitors were constructed by one-step thermal decomposition.And the optimum C_s value was 1200 F g^-1 at the current density of 1A g^-1.Through comparative experiments,the performance enhancement mechanism was discussed and mainly because the ultra-small multicomponent nanocrystals with adjustable bimetal ratio had abundant oxidation states and were conducive to the transport and diffusion of electrolyte ions.An asymmetric supercapacitor（ASC）was further assembled,which provided an energy density of 36.8Wh kg^-1 at the power density of 850 W kg^-1.Due to the protective effect of nanocapsules,the ASC devices showed excellent long-term cyclic stability,and the capacity retention rate was as high as 94.96%after 20000 cycles.In addition,the construction method of BMCPs based on MAA^-was extended to the combination of Cu²⁺,Zn²⁺,Mg²⁺and Co²⁺ions,proving the universality of construction method of M_xM_y-MAA BMCPs.The composition and structure of BMCPs can be regulated by adjusting the feeding ratio of bimetallic elements and the degree of desolvation.