Study On Synthesis And Assembly Behavior Of Metal Oxide Colloidal Nanocrystals

Self-assembly refers to the process of orderly arrangement of building blocks through weak interactions,and colloidal nanocrystals can form long-range ordered supra-structures through the self-assembly process.The study of nanocrystal self-assembly depends highly on the development of inorganic nanocrystal synthetic methods.Until now,the size,morphology and crystal phase of nanocrystals could be precisely controlled during synthesis,and anisotropic nanocrystals such as nanorods,nanowires,nanosheets and multipodal nanocrystals have been obtained.The assembly of anisotropic nanocrystals enriches the structural diversity of nanocrystal superlattices.Since the functions of nanocrystal assemblies are closely related to their geometry,the understanding of the assembly pathways for such complex nanocrystals is insufficient,and the assembly mechanism remains unclear.Therefore,this project aims to explore the controllable synthesis and assembly behavior of anisotropic nanocrystals,and to reveal the influencing mechanism.The research content mainly includes the following three parts:(1)Controllable synthesis of manganese oxide nanocrystal.During the synthesis of manganese oxide colloidal nanocrystals,octadecene was used as solvent and oleic acid was used as ligand.The size and crystal phase of the nanocrystals were controlled by adding additional stearyl alcohol.When the mass of stearyl alcohol increased from 0.5 g to 2.0 g,the size of the nanocrystals decreased from～13 nm to～8 nm.The amount of stearyl alcohol continued to increase,the size of the nanocrystals increased,and the crystal phase changed from MnO to Mn3O4.The nanocrystals formed a two-dimensional ordered structure by solvent evaporation,and the distance between the nanocrystals was statistically found to be about 3.5 nm,which was larger than that of the oleic acid-coated nanocrystals,indicating the participation of stearyl alcohol on the nanocrystal surface.(2)Study on the assembly behavior of manganese oxide nanocrystals.The synthesized manganese oxide nanocrystals are truncated octahedral with two crystal faces {111} and {100},and the surface curvature of {100} is larger and the Angle of the organic ligand on the surface is larger.When a trace amount of water(420 ppm)was added to the solvent,the water molecules easily penetrated the ligand layer on the crystal face with larger curvature on the nanocrystals,and mediated the assembly of the nanocrystals into rhombic dodecahedrons through hydrogen bonding interactions.Under anhydrous conditions,the alkyl chains on the surface of nanocrystals assemble to form polyhedral supercrystals such as triangular platform or octahedrons through van der Waals interactions.When the water content was 232 ppm,the nanocrystals self-assembled to form tetramer-like supercrystals.As the nanocrystal concentration increases,larger and more complex nanocrystal supercrystals form.In addition,the effects of temperature,nanocrystal size and poor solvent were explored,and it was found that only the presence of water molecules affected the assembly behavior of nanocrystals.(3)Synthesis and assembly of Gd2O3 nanosheets.By controlling the thermal decomposition temperature and aging time,the transition of Gd2O3 nanocrystals from tripod shape to triangular flake shape was realized.During the assembly of Gd2O3 nanosheets,trace amounts of water are introduced.Due to the large curvature of the vertices and edges of Gd2O3 nanosheets,the opening angle of the surface alkyl chains is large,which is conducive to the penetration of water molecules.The Gd2O3 nanosheets assemble to form a two-dimensional structure through hydrogen bonding interactions between water molecules in the alkyl chains.In the absence of water molecules in the solution,Gd2O3 nanosheets assemble to form longrange ordered one-dimensional nanofibrous structures through the interaction of alkyl chains.When the nanocrystals were assembled by the method of solvent destabilization,the trace amount of water mediated the assembly of Gd2O3 nanosheets to form a sol.Under anhydrous conditions,the assembly of Gd2O3 nano sheets was in a gel state.In conclusion,the two surface capping agents can realize the simultaneous and controllable synthesis of nanocrystal size and crystal phase.For anisotropic nanocrystals,the opening angle of the ligand molecules is larger in the parts with larger surface curvature,which is conducive to the penetration of water molecules.When water molecules are close to each other,the hydrogen bonds between water molecules are restored,and the hydrogen bond interactions and the van der Waals interactions between alkyl chains mediate the assembly of anisotropic nanocrystals.