The Directed Assembly Of Multi-Dimensional Layered Double Hydroxides-Based Materials And Their Enhanced Functionalities

Layered double hydroxides (LDHs) are a large class of typical inorganic layered host materials. The tunable metal ions in a wide range without altering the structure of LDHs and the anion exchange property make them attractive candidates as catalysts or catalyst precursors, ion exchangers, absorbents, optical materials, and electroactive materials for energy storage and conversion. However, conventional methods for LDH preparation generally give poor control over the morphology, particle size, and surface area, which significantly limits practical applications of LDHs materials. To meet specific requirements in applications, the assembly of sophisticated multi-dimentional LDHs-based architectures (with controllable morphology, orientation, and dimensionality) combined with other functional components is required. Therefore, the organization of LDHs nanomaterials into ID,2D, or3D hierarchical architectures could lead to a new generation of functional materials and devices.Self-assembly or directed assembly of discrete nanostructures into organized patterns provides a new route to the construction of functional materials. The anisotropic layered structure of LDHs makes them as attractive building blocks for the fabrication of advanced materials with tunable nanostructures from nanometer-to micrometer-scale as well as enhanced functionalities. In this dissertation, we have explored the construction of new types of LDHs nanostructures by the organization of LDHs nanoplatelets into ID,2D, or3D hierarchical architectures by virtue of the electric-field, magnetic-field, and template directed assembly methods, respectively. The relationships between the LDHs architectures and trigging assembly conditions are studied rationally. Moreover, several functional materials have been developed, which demonstrate superior properties in photocatalysis, electrochemical energy storage and conversion, photo-electrochemical catalysis as well as adsorption/separation of biomolecules. The detailed contents are listed as follows:(1) The synthesis of LDHs and their modifications. We investigated the traditional coprecipitation synthesis, modification, and exfoliation process of LDHs nanoparticles, which provides the basis for the subsequent directed assembly of LDHs materials. Typically, ZnTi-LDHs with different Zn/Ti ratio have been synthesized by coprecipitation of zinc and titanium salts from homogeneous solution, which were demonstrated as visible-light photocatalysts. However, the obtained ZnTi-LDHs displays limited absorption of visible light. In order to further improve their photo-functionalities, we demonstrated the design and fabrication of organic dye and inorganic carbon quantum dots co-sensitized ZnTi-LDHs nanocomposite. The resulting co-sensitized ZnTi-LDHs exhibits largely enhanced photocatalytic oxygen generation performance from water.(2) Electric-field directed assembly of1D LDHs nanoarrays. We studied the electric-field directed assembly of LDHs nanoplatelets onto the surface of metal oxides nanowires. Well-aligned hierarchical nanoarrays containing metal oxides core and LDHs nanoplatelets shell have been synthesized via a facile electrosynthesis method. The resulting ZnO@CoNi-LDH core-shell nanoarray exhibits promising behavior in photoelectrochemical water splitting, giving rise to a largely enhanced photocurrent density as well as stability, much superior to those of ZnO-based photoelectrode. For instance, the photocurrent density is about three times larger than that of pristine ZnO NWs at+0.5V and seven times at+1.0V, demonstrating the definitely enhanced PEC performance in water splitting by introducing CoNi-LDH shell. This is attributed to the successful integration of photogenerated electron-hole separation originating from the ZnO core and the excellent electrocatalytic activity of LDH shell. This work provides a facile and cost-effective strategy for the fabrication of multifunctional nanoarrays with a hierarchical structure, which can be potentially used in energy storage and conversion devices.(3) Magnetic-field directed assembly of2D LDHs films. The ordered ultrathin film based on iron(III) porphyrin and LDHs nanosheets has been fabricated via the layer-by-layer (LBL) method trigged by the electrostatic interaction. The film modified electrode demonstrates a couple of well-defined reversible redox peaks attributed to Co(Ⅲ)/Co(Ⅱ), with iron(Ⅲ) porphyrin serving as an efficient mediator for facilitating the electron transfer. Moreover, it displays satisfactory electrocatalytic behavior toward H2O2with a wide linear range of response, high sensitivity and low detection limit. In addition, magnetic films based on CoFe-LDH nanoplatelets and porphyrin anions were fabricated by the LBL assembly technique with the assistance of an external magnetic field, which show largely enhanced electrochemical behavior and magnetic anisotropy. The ordered ultrathin films fabricated via the magnetic field assisted LBL method exhibit excellent electrocatalytic performance toward glucose with superior sensitivity, selectivity and stability, in comparison with those assembled without magnetic field assistance.(4) Templates directed assembly of3D LDHs microspheres. We also demonstrate the assembly of LDHs nanoplatelets into hierarchical microspheres by using template directed method. Firstly, hierarchical LDHs microspheres with tunable interior structure were synthesized by a facile and cost-effective surfactant-templated method. SEM and TEM images reveal that the obtained microspheres display a three-dimensional architecture with hollow, yolk-shell and solid interior structure, respectively, with continuous changes in specific surface area and pore-size distribution. The hollow MgFe-LDH microspheres exhibit excellent electrocatalytic oxidation of ethanol in alkaline fuel cell, including high activity, long-term durability and cycling stability, owing to the significantly improved faradaic redox reaction and mass transport. Secondly, core-shell LDH microspheres with tunable interior architecture have also been synthesized by in situ growth on the surface of hard templates. The obtained LDHs microspheres display a three-dimensional architecture with core-shell, yolk-shell, and hollow interior structure respectively. The hollow NiAl-LDH microspheres exhibit excellent pseudocapacitance performance, including high specific capacitance and rate capability, good charge/discharge stability and long-term cycling life. Finally, three-component microspheres containing SiO2-coated FeO4magnetite core and an LDHs nanoplatelet shell have been synthesized via an in situ growth method. The resulting Fe3O4@SiO2@NiAl-LDH microspheres display3D core-shell architecture with flowerlike morphology, large surface area, and uniform mesochannels. The Ni2+cations in the NiAl-LDH shell provide docking sites for histidine and the core-shell microspheres exhibit excellent performance in the selective adsorption and separation of a histidine (His)-tagged green fluorescent protein with high capacity, selectivity and stability.