Interfacial Synthesis And Assembly Of Inorganic Nanomaterials

In recent years, synthesis and assembly of noble metal and semiconductor nanomaterials and investigation of their properties have been attracted much attention due to their unique optical, electronic and catalytic properties different from the bulk and consequent potential applications. Langmuir monolayer technique is a very effective way in preparation and assembly of nanoparticles. In this thesis, varied nanostructures templated by Langmuir monolayer of functional amphiphilic molecules were obtained in one step via interfacial reaction at the air/water interface and characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffractometry (SAED) and UV-vis spectroscopy. The formation mechanism of these nanostructures were discussed. The study of this thesis mainly consists of four parts:I. Preperation of Ag nanoparticle-doped nanotubes and nanonetworks of poly(9-vinylcarbazole)(PVK). PVK-Ag composite nanotubes and nanonetworks were fabricated at the air/AgNO₃ aqueous solution interface through self-assembly of PVK thin films and reduction of silver ions irradiated with UV-light. One end of the formed nanotube is closed while the other is open. The diameter and wall thickness of the tubes are ca. 100 and 10-20 nm, respectively, and their length reaches over 10 micrometers. The average size of the silver nanoparticles dispersing in the walls of the tubes is 3.5 nm. The formed nano-networks bear homogeneous and well-aligned round pores which are doped with silver nanoparticles. The major factor that determines the formation of nanotube and nanonetwork is temperature, and coordination interaction between metal ions from the subphase and the carbazole groups of PVK molecules plays a key role, too. To the best of our knowledge, this is the first time to fabricate polymer composite nanotubes through a self-assembly process at the air/water interface. II. Fabrication of silver nanorings and circular arrays composed of silver nanoparticles. Hexagonal and round silver nanorings together with circular arrays of silver nanoparticles were formed via interfacial reduction using the template of PVK supramolecular systems under different temperatures at the air/water interface. The results showed that PVK formed circular structures under higher temperature which absorbed smaller silver nanocrystals generated by reduction, and hence silver nanorings were yielded. When the subphase temperature is lower, PVK molecules formed thicker round plates that can't be penetrated by UV-light, and therefore smaller nanoparticles aligned into circular arrays surrounding the round plates.III. Synthesis of polyhedral silver nanoplates at the air/water and solid/water interface. PVK molecules were spread onto the air/water interface, and then silver ions were reduced under UV-light irradiation at higher temperature. Polyhedral micron-sized single-crystalline nanoplates were observed after transferred with Formvar-coated carbon grids. However, more regular single-crystalline nanoplates were obtained when using carbon-coated copper grids, and the size of the nanoplates reached a few micrometers and varied with temperature. Further investigation indicated that the formation of the former is the result of reduction under UV-light irradiation, while the later can be possibly ascribed to the participation of solid substrates, that is, copper grids in reaction because carbon is conductive. Meanwhile, typical amphiphilic molecules such as arachidic acid were also spread on the surface of AgNO₃ aqueous solution, and after UV-light irradiation, different results were observed along with different copper grids. When carbon-coated copper grids were employed, single-crystalline silver nanoplates with varied shapes include triangle could be produced at room temperature, which is obviously related to the participation of copper. However, the nanoplates formed under the later condition are clearly smaller than those using PVK, which indicates the influence of monolayers at the interface on the generation of nanoplates. The case in which silver nanoplates could also be acquired without spreading amphiphilic molecules verifies further the participation of copper grids, although the nanoplates are irregular. Experimental results confirmed that (111) crystal plane of the nanoplates formed under different conditions are all parallel to the interface.Three kinds of results were obtained by using the same PVK-Ag⁺ system, as described above, which is primarily attributed to different dispersion states of PVK molecules at the air/water interface. Nanotubes and nanonetworks are the result of self-assembly of PVK molecules; Self-organized structures as templates lead to the production of silver nanorings and circular arrays of silver nanoparticles; while the yielding of polyhedral single-crystalline silver nanoplates bears upon nucleation and growth process of silver. Besides varied self-organized structures, PVK molecules could form into small aggregates and even disperse in a single-molecule form at the interface, especially at higher temperature. Silver particles will nucleate and grow in their inherent crystalline nature if there are not adequate PVK molecules influencing them. The basal plane of the nanoplates is (111) because the surface free energy of (111) crystal plane is the lowest.IV. Synthesis of PbS nanoparticles under PVK monolayers. Several PbS nanoparticles (including nanopyramids, square nanoplates, nanorods, etc.) were synthesized via interfacial reaction between Pb²⁺ and H₂S gas under PVK monolayers with PbCl₂ aqueous solution as subphase at the gas-liquid interface. It was demonstrated that all kinds of PbS nanoparticles have face-centered cubic (fcc) crystal structure. Triangular nanopyramids were formed at 20°C with (111) crystal plane parallel to the interface, while square nanoplates and nanorods were obtained at 30°C with (001) plane parallel to the interface. The conclusion can be drawn from the result that the formation of nanopyramids is attributed to lattice matching between 2D array of nitrogen atoms in PVK monolayer and (111) crystal plane of PbS. The causes of the formation of nanoplates and nanorods should be that the structure change of Langmuir monolayer with increasing temperature, which realizes lattice matching between 2D array of nitrogen atoms and (100) crystal face. The formation of PbS nanoparticles has been achieved in a controlled manner by modulating temperature and amount of reagents.