Study On Controllable Synthesis And Electrochemical Energy Storage Properties Of Two - Dimensional Sn And Nb - Based Chalcogen Nanostructures

In this thesis, several two-dimensional (2D) Sn-and Nb-based chalcogenides nanostructures (2D crystals), such as highly crumpled ultrathin SnS2nanosheets (NSs), pancake-like SnSe2nanodisks (NDs), hexagonal mixed-phase SnSe-SnSe2NDs, near monolayer SnSe NSs and ultrathin NbSe2NSs, have been controllablely synthesized via non-hydrolytic system thermal synthesis or decomposition methods by selecting proper precursors and high boiling-point solvent. The compositions, microstructures and crystallinitine degree of these2D nanostructures are characterized by EDS, XRD, XPS, FT-IR, FE-SEM, AFM, TEM and HRTEM. The corresponding growth mechanisms of these2D crystals have been proposed on the basis of a series of control experiments. Moreover, the electrochemical energy storage performances of these2D crystals were evaluated by using them as electro-active materials. And some flexible all-solid-state energy storage devices have been fabricated according to the experimental results. The detailed results are given below:(1) Highly crumpled ultrathin SnS2NSs have been successfully synthesized via a simple solid-liquid phase chemical route, i.e. thermal decomposition of pre-synthesized single-sourece precursors (Sn2Cl4(Tu)5-2H2O) in oleic acid (OA) that serves as solvent and capping reagent. A series of control experiments demonstrate that the reaction temperature has a great impact on the component and phase structure of the final product. By controlling reaction temperature, pure hexagonal phase SnS2NSs, mixed-phse SnS-SnS2NSs and pure orthorhombic phase SnS NSs can be easily obtained. Using those NSs as active mateirals, their pseudo-capacitive performances are evaluated in6M KOH electrolyte. The results demonstrate that the mass specific capacitances of pure SnS2NSs and the cycling stabilitiesare much higher than those of mixed phase SnS-SnS2NSs and pure orthorhombic phase SnS NSs.(2) The SnS2NSs with different thickness or layer numbers have been prepared by using element sulfur and cysteine as the sulfide sources respectively and OA as the solvent and capping reagent. To describe simply, these two kinds of NSs are named as SnS2-1and SnS2-2, respectively. The desired supercapacitors (SCs) are fabricated by using these NSs as active materials. The influence of the thickness or layer numbers on the pseudo-capacitive performances of the SnS2NSs based SCs are investigated. The results exhibit that reducing the thickness or layer number of the SnS2NSs can increase the surface electro-active sites and facilitate to improve electrochemical energy storage capacity.(3)2D tin selenide nanostructures, including pure SnSe2nanodisks (NDs), mixed-phase SnSe-SnSe2NDs and pure SnSe nanosheets (NSs), have been synthesized by reacting SnCl2and trioctylphosphine (TOP)-Se in the presence of borane-tert-butylamine complex (BTBC) and1,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone. By utilizing the interplay of TOP and BTBC and changing only the amount of BTBC, the phase-controlled synthesis of2D tin selenide nanostructures are realized for the first time. Phase-dependent pseudocapacitive behavior is observed for the resulting2D nanostructures. The specific capacitances of pure SnSe2NDs (168F g-1) and SnSe NSs (228F g-1) are much higher than those of mixed-phase SnSe-SnSe2NDs and other reported materials (e.g., G, G-MnO2or Mn3O4hybrids, G-CoS2composites, CoS, TiN mesoporous spheres, etc.); thus, these tin selenide materials were used to fabricate flexible, all-solid-state supercapacitors with polyvinyl alcohol/H2SO4gel as the separator and solid electrolyte. Devices fabricated with these two tin selenide materials exhibited high areal capacitances, good cycling stabilities, excellent flexibilities and desirable mechanical stabilities, which were comparable to or better than those reported recently for other solid-state devices based on G and3D GeSe2nanostructures. Additionally, the rate capability of the SnSe2NDs device was much better than that of the SnSe NSs device, indicating that SnSe2NDs are promising active materials for use in high-performance, flexible, all-solid-state supercapacitors.4. The ultrathin NbSe2NSs have been successfully synthesized by thermal treatment of cheap NbCls solid and powder Se precursors in the presence of BTBC and oleylamine (OLA) under temperature-programming mode. The thickness of the obtained NbSe2NSs is about3-4nm measured from their turnup edges. Except for the molar ratio of precursors, reaction temperature and reaction time, the used BTBC has a great impact on the formation of such NbSe2NSs. Without BTBC and keeping other conditions constant, no NbSe2NSs can be formed and the obtained product is Nb2Se9nanorods (NRs) with the diameter of30-60nm. This result implies that BTBC is not only a reducing reagent but also a structure-directing reagent, which may cover on the c-axis of NbSe2and passivate its growth along this direction, beneficial to form desired2D NSs. In6M KOH electrolyte, the pseudo-capacitive performance of the obtained NbSe2NSs is evaluated by cyclic voltammetry (CVs) and galvostatic charge-discharge test,compared with Nb2Se9NRs, the NbSe2NSs also exhibit higher electrochemical stability and capacitance rention capability. Even after1000continuous cycles, the NbSe2NSs electrode still can reserve87%of its initial capacitance. This work demonstrates that NbSe2NSs can serve as an attractive active material among niobium selenide nanostructure family for use in high-performance SCs.