Controlling Nanostructure for Catalytic and Electrochemical Energy Storage Materials.

Abstract

Materials with precisely controlled nanostructures are needed to significantly enhance the efficiencies of next-generation chemical conversion and energy storage systems. This dissertation employs light and electrochemical techniques to control nanostructure of catalytic and electrochemical energy storage materials. We also define nanostructure-function relationships for three material systems. This information could help the design and synthesis of materials with superior performance. Single layer (SL), multilayer (ML), and wave-like (WL) hematite nanotube arrays (NA) were fabricated via the electrochemical anodization of iron foils. The films’ current responses during fabrication were tracked, allowing for the characterization of NA growth. Four distinct stages were identified: an ohmic response stage, an oxide film formation stage, a chemical dissolution stage, and a steady-state growth stage. Morphological and photoelectrochemical properties of the hematite electrodes were characterized and correlated with their photocatalytic performances. The IPCE of the WLNA at 350 nm was ~3 times that of the SLNA, and ~12 times that of the MLNA. Charge carrier transport and the active electrochemical surface area of the different morphologies were significant determinants of photocatalytic performance. Niobium pentoxide (Nb2O5) NA and planar electrodes were fabricated via a similar anodization technique. The Li+ intercalation behavior of the electrodes was characterized. NA electrodes exhibited a four-fold improvement in charge storage capacity and higher rate capabilities relative to planar electrodes due to larger surface areas and shorter ion diffusion lengths in the NA. Light of different wavelengths was used to control the photodeposition of noble metals on semiconducting tungsten trioxide. The metal nanoparticle sizes and weight loadings were functions of the illumination time, while geometries were controlled by the wavelength. Intrinsic variations in the plasmonic responses of the metals across the UV-vis spectrum allowed for control of their geometries. These photodeposited materials were evaluated for the selective hydrogenation of crotonaldehyde, a model a,b-unsaturated aldehyde; the results were correlated with key nanostructural properties of the noble metal particles. Regardless of photodeposition wavelength, all Pt materials were spherical and showed similar selectivities (~60%) for crotonaldehyde hydrogenation. Au/WO3 and Ag/WO3 catalysts exhibited high thermodynamic barriers for the dissociation of molecular hydrogen, and were ineffective for crotonaldehyde hydrogenation.