Metal Oxide Nanoparticles Synthesized by Nonthermal Plasma for Electrochemical Applications

Metal oxide nanocrystals can be synthesized by colloidal processing and gas-phase processing routes. High-quality nanocrystals produced by colloidal routes typically have surface ligands. Transport of electrons and ions among nanocrystals is hindered by surface ligand, and degrades related electrical performance. Gas-phase processing, for example combustion and thermal plasma, can make free-standing nanocrystals. However, nanocrystals produced by combustion method are heavily agglomerated, which further limits the usage of nanoparticles in electrochemical applications. Nonthermal plasma, as an emerging attractive route, has been initially focused on the synthesis of elemental group IV materials, and extended to include the synthesis of multicomponent materials, such as metal oxide nanocrystals. As majority of nanoparticles in the nonthermal plasma are negatively charged, the mutual repulsion between nanocrystals can suppress their agglomeration.In this thesis, nonthermal plasma, combined with an inertial impaction deposition system, have been used to synthesize thin films composed of metal oxide nanoparticles, such as ZnO and SnO2, and doped-metal oxide nanoparticles (Sb-SnO2), without any post-treatment, a feat that colloidal routes are not well-suited for. During these experiments, a new processing concept was developed to synthesize doped metal-oxide nanocrystals by nonthermal plasma with a uniform composition distribution and minimum phase segregation of dopant elements. The concept involves two sequential stages and is termed as a dual-plasma process. This synthetic route can be used to make highly conductive and porous films composed of Sb-SnO2 (ATO) nanocrystals in a single step. With these unique features, ATO nanocrystals, which are thermally and chemically robust, are seen as a promising catalyst support material for electrochemical applications. In the context of electrochemical windows, a porous ATO structures can be used as a framework to electrochemically deposit an amorphous WO3 layer to fabricate WO3-ATO nanocomposites. The formation of heterojunction nanostructure is proposed as a potential method to improve electrochromic performance. The conductive network formed by ATO nanocrystals among the WO3 nanoparticles can provide electrical pathways for electrons and promote the electron transport rate, and increase the color switching speed