Preparation And Characterization Of Low-dimensional Schiff-base Nanostructures At The Solid/Liquid Interface

Covalent surface nanostructures are a type of structure that is constructed directly on the surface by surface reactions.These covalently linked nanostructures can facilitate efficient charge transport and therefore have potential applications in molecular electronics,optoelectronic devices,catalysis,and sensors.The key challenge in the field of nanotechnology is the construction of long-range ordered covalently linked nanostructures with high surface coverage,low-cost and simple methods.Dynamic covalent reaction,such as Schiff base reaction,is a kind of reversible reaction,which can be controlled by adjusting external conditions to obtain a thermodynamically stable structure,hence preparation of ordered covalent nanostructures.In this thesis,we use Schiff base reaction as the reaction system to synthesize a series of low-dimensional covalent nanostructures at the solid/liquid interface by designing precursors with different structures and regulating the reaction conditions.Scanning tunneling microscopy(STM)and theoretical calculations are utilized to characterize the structure and influencing factors of the surface polymers.The controllable preparation of Schiff base one-dimensional polymers(1DPs)and hierarchically cross-linked structures synergistically with covalent and non-covalent interactions was achieved at the octanoic acid/highly oriented pyrolytic graphite(HOPG)interface.We found that molar ratio of the precursors and monomer structure played a vital role in the formation of the assembling structures.By controlling the molar ratio of monomers,either a two-dimensional supramolecular network or 1DPs can be controllably obtained at the octanoic acid/HOPG interface.The supramolecular network could also be transformed to covalent linked polymers by annealing the sample to 100 ℃.Hierarchical networks,constructed by non-covalent bond stabilized cross junctions of covalent one-dimensional molecular wires,are synergistically formed at the liquid/solid interface through in situ on-surface condensation of 2,5-bis(octyloxy)terephthalaldehyde and 3,3’-dihydroxybenzidine.The effect of monomer concentration and structure on this hierarchical cross-linked structure was studied in detail by STM.Our investigation demonstrates that high concentrations favor the formation of hierarchical cross-linked structures.The octoxyl group of 2,5-bis(octyloxy)terephthalaldehyde and hydroxyl group of 3,3’-dihydroxybenzidine,as well as octanoic acid,play a critical role in the formation of hierarchical cross-linked structures,which has been proved by both STM experiments and DFT simulations.Selecting a triangular aromatic aldehydes and linear diamines as the reaction monomers,a large-area,highly-ordered Schiff base 2DPs with pore sizes ranging from 1.8 nm to 5.2 nm were synthesized at the solid/liquid interface by adjusting the monomer concentration.This is the simplest way to prepare 2DPs.The relationship between the quality of Schiff base 2DPs and the concentration of monomer was revealed by STM.The X-ray photoelectron spectroscopy proves that the 2DPs obtained at the solid/liquid interface is connected by imine bonds.Isomorphic 2DPs with pore sizes of 3.7 nm and 4.4 nm were obtained at the solid/liquid interface by selecting aldehyde and amine monomers with the same backbone size but carryin g different functional groups.In addition,the influence of solvent,temperature,and monomer structure on the formation of Schiff base 2DPs was also investigated.Based on the preparation of Schiff base 1DPs and 2DPs at the octanoic acid/HOPG interface,we used STM to study the dynamic covalent chemistry of Schiff base polymers at the solid/liquid interface.By adjusting the molar ratio of the components,the composition on the surface can be regulated.The result shows that the surface selectively stabilizes the molecules with stronger adsorption ability and capability to form ordered assemblies.The amine-amine exchange process at the solid/liquid interface was also studied using STM.The results show that diamines with strong adsorptive capacity can replace diamines with weak adsorptive capacity out of the polymer chains,but the opposite process is difficult to happen.In addition,2DP heterojunctions were successfully constructed on the surface.In addition,the effect of surface defects on the formation of Schiff base 1DPs and 2DPs was studied,including the effect of defect density and removal of surface defects.3,5-bis-tert-butylbenzenediazonium was grafted as a surface defect by cyclic voltammetry.The density of surface defects were controlled by the concentration of 3,5-bis-tert-butylbenzenediazonium.Schiff base coupling of 2,5-bis(octyloxy)terephthalaldehyde(OTPA)with p-phenylenediamine(PDA)yields a large scale ordered 1DPOTPA-PDA on the modified HOPG with a low surface density of grafted species.As the density of defects increases,the degree of polymerization of the polymer and the area of the ordered domain gradually decrease s.Similarly,for 2DPs,the ordered 2DPs were formed on the modified HOPG with a low surface density of grafted species.As the surface density of defects increases,the orderliness of the network is destroyed,and the size of ordered domain decreases.The study of 2DPs with different pore sizes shows that the surface density of defects has a great influence on the growth of 2DPs with larger pore size.The removal of surface defects results in the decomposition of 1DP grown on the low density modified HOPG surface and 2DPs grown on the high density modified HOPG surface,while having little effect on the 2DPs growth on the low density modified HOPG surface.However,Schiff base polymers can regenerate in the defect removed area,which is mainly due to the spontaneity and reversibility of Schiff base reaction.