Mixed metal oxide nanowires via solid state alloying.

Abstract

Mixed metal oxide materials with composition control find applications in energy conversion and storage processes such as heterogenous catalysis, photoelectrochemical catalysis, electrocatalysis, thermal catalysis, and lithium-ion batteries. Mixed metal oxides and/or complex oxides with composition control and in one-dimensional form as nanowires could be interesting to various catalysis applications due to control on single crystal surfaces, active sites, acidity versus basicity site density, and oxygen vacancies. The major challenge is to synthesize mixed metal oxide nanowires beyond binary oxides with composition control. In this dissertation, solid state alloying of binary oxide nanowires with solid and liquid precursors is studied to obtain mixed metal oxide nanowires. Solid state alloying studies were conducted using either solid precursors of hydroxides or liquid precursor of nitrates mixed with the already synthesized binary metal oxide nanowires, dried and solid state diffused under inert atmosphere to achieve the ternary mixed metal oxides or solid solutions of mixed metal oxide nanowires. First and foremost, porous nanowires of binary oxides have been found to be more beneficial for alloying experiments. Several experiments using different precursors and solutes into commonly available binary oxides such as Zinc Oxide (ZnO), Alumina (Al2O3) and Titania (TiO2) were conducted to understand the underlying mechanism. Alloying elements included copper, zirconium, cobalt, nickel, and alkali metals. Experiments suggest that the alloying into nanowires were uniform irrespective of the uniformity of contact with precursor. Results also yielded higher solubilities of solutes into nanowires compared to those predicted from solubility in bulk materials. Specifically, the solubility of solute copper obtained is more than 8 at% for titania and alumina nanowire materials. Based on the thermodynamic phase diagram, this solubility is beyond the thermodynamic bulk solubility for these materials at these process conditions. Extended solubility could be potentially attributed to the available higher surface energy at nanoscale, surface free energy minimization, and thermodynamic stabilization. A mechanism is proposed for alloying with nanowires which suggests that the reaction of precursors with metal oxide nanowires is necessary for solute’s diffusion and alloying. The electrocatalytic behavior of porous tin oxide nanowires was investigated using electrochemical reduction of CO2 to formate. It is concluded that the high-density grain boundary on the nano wire structure is a primary factor in the observed enhancement of the selectivity, rate of HCOOH formation, and associated minimization of the H2 evolution reaction during the electrochemical reduction of CO2. Formic acid formation begins at lower overpotential (350