Preparation Of Ldhs-Based Photofunctional Materials And Investigation On The Two-Dimensional Confinement Effect

Photofunctional materials are a class of photoactive compounds which can absorb, store, transmit and utilize light energy. Layered photofunctional materials with two-dimensional ordered structure have attracted great attention due to their application in the fields of illumination, microlasers, sensors and light emitting diodes (LEDs).Layered double hydroxides (LDHs) are a family of 2D inorganic layered materials with versatility in chemical composition and structural morphology. The LDHs can be exfoliated into positively-charged mono-layer nanosheets that serve as building block for the construction of various 2D-organized functional materials. By adjusting the charge density and chemical composition of host layers or the assembly method of the guest molecules, the photofunctional performance can be finely tuned. In this dissertation, both experimental and theoretical studies are employed to study the host-guest interaction and the confinement effect imposed by LDHs, which are benefical for the design and construction of advanced photofunctional materials. The main contents are described as follows:1. Layer-by-layer assembly has been employed to construct multi-color emission ultrathin films (UTFs) by assembly of LDH nanosheets with blue-, green-and red-emitted chromophores. The hybrid UTFs exhibit ordered heterogeneous structure, uniform morphology and controllable thickness. Based on rational selection of chromophore guests and assembly sequence, the fluorescence emission color of the UTFs can be tuned through the whole visible-light region. Especially, finely controlled white emission UTFs can be successfully realized with the color coordinates at (0.322,0.324). Moreover, the UTFs displays intelligent photoluminescence behavior, i.e., white, orange, or red emission can be obtained for the same UTF by simply changing the excitation light, which is expected to show potential use in colorimetric sensors, photonic switches and multiband filters.Ordered (Au@PAAS/LDH)n UTFs were fabricated by the assembly of LDH nanosheets and modified Au nanoparticles to achieve advanced surface enhanced Raman scattering (SERS) in the solid state. The as-prepared UTFs display a periodic long-range ordered structure and dense immobilization of Au NPs within the LDH interlayer microenvironment. Tightly accommodated Au NPs with 2.80 nm in x-z and 2.16 nm in x-y planes can be constructed due to the aid of LDH nanosheets, giving rise to an enlarged electromagnetic field for SERS detection. The vibration of molecules in this electromagnetism field can be markedly intensified, and the concentration down to 10-9 M for R6G is successfully detected. Versatility of this system was further verified toward different solvents (acetone, formamide,1,2-dichloroethane (DCE), ethanol and water) and various dye molecules (methylene blue, crystal violet, congo red, nile blue and acid red). Moreover, these hybrid UTFs also demonstrate good reproducibility and stability.2. Au nanoclusters (Au NCs) have been immobilized onto 2D LDH nanosheets via a layer-by-layer self-assembly process, and the as-prepared Au NCs/LDH UTFs exhibit an ordered structure with dense anchoring of Au NCs. The electrophilic effect of LDH nanosheets effectively confine the Au NCs, resulting in reduced non-radiative transition and enhanced fluorescence performances. The confinement effect of LDH nanosheets imposes an increased emissive Au(Ⅰ) units as confirmed by XPS and periodic density functional theoretical simulations, which contributes to largely improved QYs (from 2.69% to 14.11%) and prolonged fluorescence lifetime (from 1.84μs to 14.67μs). In addition, the Au NCs/LDH UTFs exhibit temperature-responsive photoluminescence (PL) and electrochemiluminescence (ECL) performances with good sensitivity and stability, as a result of the well-organized immobilization of Au NCs and corresponding Au-LDH interactions.Furthermore, negatively-charged Au NCs are localized onto the surface of exfoliated MgAl-, CoAl- and CoNi-ELDH nanosheets respectively with a high density of Au NCs. The as-prepared Au NCs/MgAl-ELDH hybrid material exhibits largely-enhanced fluorescence performances compared with pristine Au NCs (quantum yield:19.05% vs.2.60%; lifetime:13.99 μs vs.1.99 μs). Both experimental and theoretical investigations reveal that the electron mobility of Au NCs anchoring to the MgAl-ELDH nanosheets is greatly depressed, and the excited electron transfer is forbidden. The excited electrons of Au NCs can be effectively confined by the MgAl-ELDH nanosheets, resulting in the electron-hole recombination and significantly enhanced fluorescence behavior in the Au NCs/MgAl-ELDH system; but this is not feasible in the cases of CoAl-ELDH and CoNi-ELDH system due to the mismatching of forbidden zone between CoAl-ELDH (or CoNi-ELDH) and Au NCs. Moreover, Au NCs/MgAl-ELDH hybrid material exhibits excellent bio-imaging performance in intracellular environment with good stability, anti-photobleaching property and biocompatibility, which may find potential use in bio-imaging and cell labeling.