ELECTROCATALYTIC AND OPTOELECTRONIC BEHAVIOR OF COLLOIDALLY FUNCTIONALIZED TRANSITION METAL (HYDR)OXIDES
Transition metal oxides and hydroxides have utility in electrochemical catalysis and electronic applications due to their unique structural and electronic properties. In this dissertation, transition metal (hydr)oxides were studied in two veins which highlight the versatility of these nanomaterials: electrochemical CO oxidation and electron-doping. This work demonstrates how the structure and electronics of transition metal (hydr)oxides impact their electronic and catalytic applications.
In order to move toward renewable energy sources, fuel cells have been sought out as an alternative electrical energy source. Platinum has been utilized as a catalyst in fuel cells due to its high catalytic behavior. However, the small amounts of carbon monoxide found in hydrogen gas will slowly saturate the platinum surface and effectively poison the catalyst. Fortunately, previous work has shown transition metal hydr(oxy)oxides of Ni, Co, Mn, and Fe on a Pt surface decreased the overpotential for CO oxidation by working in a bifunctional manner by activating H2O on the oxide surface while reacting with CO bound to platinum.
In our work, we developed a synthetic method for depositing a monolayer metal halide shell onto platinum nanoparticles. Using a solution-phase method, a series of colloidal first-row and post-transition metal halides on Pt were studied in electrochemical CO oxidation. Through this work, the CO oxidation activity was strongly dependent on the identity of the metal oxide shell and a ~200 mV shift towards lower overpotential relative to platinum was observed. Furthermore, this synthetic method allowed us to access core-shell nanostructures that had not been previously reported and study them as potential catalysts in fuel cells.
In more recent work, transition metal (hydr)oxides were investigated for their electron- doping properties. Ambipolar doping of transition metal (hydr)oxides is highly desirable for applications in electronic and optoelectronic devices. However, developing a transition metal oxide that is stable to both n-type defects (oxygen vacancies) and p-type defects (metal vacancies) is difficult. NiO and CoO are known p-type semiconductors with high conductivities; however, very little is known about their n-doped form.
By turning to layered hydrotalcite and brucite Co(OH)2 and Ni(OH)2, we discovered that the n-doped form is accessible. By treating the metal hydroxide nanosheets with electron donor, n-butyllithium, the conductivity increased from 10-12 S/cm to 10-6 S/cm. Using an array of X-ray spectroscopic techniques, the nanoplatelets were found to compensate doped-electrons through lithium adsorption, oxygen vacancy formation, and metal-centered reduction. This work highlights the importance of understanding the role of crystal structure, defect formation, and structural environment in accessing ambipolar forms of transition metal (hydr)oxides.