Microstructured materials synthesis via salt-assisted ultrasonic spray pyrolysis

Micro- and nanostructured materials are a very important part of today's technology due to their unique physical, optical, magnetic, and electrical properties. Many methods of producing these materials, however, require expensive precursors or templates and complicated, multi-step procedures. Ultrasonic spray pyrolysis (USP) is an industrially-scalable technique that has been shown to yield relatively monodisperse sub-micron particles in a continuous process using inexpensive and, often, environmentally-friendly precursors. The incorporation of an inert or reactive salt into the precursor solution allows for structural modification of the final product. Inert salts can phase separate as either liquids or solids (depending on the furnace temperature and the melting point of the salt or salt mixture) to provide an in situ template for formation of the product. The salt can then be easily removed from the product by washing and potentially reused. Reactive salts will decompose in the furnace creating gases which can increase the microporosity of products in USP. The work described in this dissertation shows the versatility of salt-assisted USP for making a wide variety of microstructured materials and highlights a few applications of the materials formed, including superhydrophobic surfaces, energy storage, and controlled drug release and hyperthermia. First, salt-assisted USP is used to make roughened ZnO microspheres. By controlling the amount of NaCl in the precursor solution, the agglomerated ZnO nanoparticles will form microspheres with varying degrees of roughness. The size of the particles formed is also easily controlled by adjusting the concentration of the precursors in the precursor solution. By controlling both the size and the roughness of the ZnO particles formed, the hierarchical structure of a film of these particles can be controlled. This has consequences for the wetting properties of the film. Specifically, hierarchically structured films are known to stabilize the Cassie-Baxter state and encourage superhydrophobic (or superhygrophobic) behavior. Very high surface area iron oxide microspheres can also easily be produced from USP. The crystallinity of the final product can be controlled by adjusting the precursor solution. When aqueous Fe3+ salts react with a weak base (e.g., Na2CO3), they can form high molecular weight iron polymer which is stable in solution (so-called Spiro-Saltman balls). Using the Spiro-Saltman precursor, USP yields high surface area (~300 m2/g) crystalline microspheres. If iron chloride is used in the place of iron nitrate, hollow spheres are obtained which have a lower surface area (~100 m2/g). Mixing different ratios of iron nitrate and iron chloride gives products with intermediate morphologies and surface areas. These iron oxide microspheres were tested as lithium-ion battery anodes. Salt-assisted USP has previously been used to make carbon spheres with a wide variety of morphologies. There is no predictive understanding, however, of why one morphology is formed over another. This is especially true when dealing with salt mixtures. As a simple system, sucrose was pyrolyzed with different ratios of sodium nitrate and sodium chloride. It was shown that the salt ratio dictates the morphology of the product whereas the furnace temperature (i.e., the phase of the salt solution) has little to no effect. With the addition of iron/nitrogen precursors, porous Fe/N/C microspheres were made and tested as oxygen reduction catalysts in fuel cells. Finally, microspheres containing superparamagnetic iron oxide nanoparticles are synthesized using USP. Magnetic silica microspheres were previously made using iron precursors. By introducing, cobalt, manganese, and copper precursors into the system, ferrite nanoparticles are formed within the silica microspheres. These nanoparticles are more stable at higher temperatures than the iron oxide nanoparticles. The silica microspheres, however, are not porous. Porous carbon microspheres with iron oxide nanoparticles can be made using USP. Unlike the silica microspheres, the carbon-based microspheres do not require annealing after production. The carbon microspheres have been shown to undergo magnetic heating and have shown slow drug release after loading with ibuprofen, making these spheres a promising candidate for a multifunctional biomedical device that incorporates hyperthermia, controlled drug release, and imaging contrast.