Nanoparticle Formation in the Flame Synthesis of Multicomponent Nanostructured Materials for Clean Energy Applications

This dissertation studies the initial stages of particle formation during the combustion synthesis of multicomponent nanomaterials. Combustion is commonly used for the production of nanomaterials at industrial scales and has advantages of high production rates, low waste generation, and scalability. However, there are limitations in being able to apply this synthesis approach to more advanced multicomponent systems. Gas to particle conversion in flames occurs through precursor decomposition, collisional growth, coagulation, condensation, and sintering. There is a fundamental gap in knowledge on the initial stages of particle formation in flames below 2nm due to measurement difficulties and instrumentation limitations. Using a high resolution differential mobility analyzer, the first mobility spectrum measurements of sub 2nm clusters were presented for single component TiO2 and multicomponent SiO2/TiO2 synthesis demonstrating the importance of discrete clusters during particle growth. Further insight into the nature of sub 2nm clusters were gained through the utilization of an atmospheric-pressure time-of-flight mass spectrometer to measure the chemical composition of sub 2nm clusters. During the combustion synthesis of TiO2, it was revealed that for negatively charged species, chemical ionization plays an important role in the formation of TiO2 clusters while large concentrations of high molecular weight organometallic Ti clusters could be measured. Using similar approaches for silica, intermediates of silica clusters could be identified. The main Si growth pathway was revealed to be through silicic acid clusters, where dehydration, hydrogen abstraction, and hydroxyl radical interactions persist through larger clusters of Si. With multicomponent systems of TiO2 and SiO2, evidence of mixed oxide cluster growth could be seen along with independent cluster growth. Mass mobility plots from tandem ion mobility spectrometry-mass spectrometry further revealed that metal oxide cluster masses were much larger than predicted by the widely utilized Kilpatrick relationship used for mass-mobility correlations. The role of ions during combustion synthesis was further studied through measurements of natively charged, charge conditioned, and neutral particle size distributions. Using current voltage measurements in an electric field, the concentration of ions and electric properties of flames were also used to provide insight into ion properties during combustion synthesis for TiO2 and SiO2. Finally, insight from previous studies on the mechanisms of particle formation were applied in developing a system for the synthesis of niobium doped TiO2 nanostructured thin films towards transparent conducting oxide applications. Using a flame aerosol reactor, highly conductive and transparent thin films could be synthesized while enhanced conductivity was achieved through controlled-niobium doping into the anatase crystal lattice of TiO2. Findings from this dissertation have revealed the complexity of gas to particle conversion mechanisms in the initial stages and are the first step towards developing the ability to design flame aerosol reactors for advanced materials synthesis