J. Cabezas Parra, T. Ayer, J.M. Notestein, D.F. Swearer
Northwestern University,
United States
Keywords: gold, nanocubes, photocatalysts, plasmonic, inverse catalysts
Summary:
Photocatalytic plasmonic nanoparticles have garnered interest in sustainable applications and have been reported as promising pathways for low-temperature, energy-efficient, light-driven catalytic reactions of industrial interest. Plasmonic materials are characterized by the collective oscillation of the conduction band electrons in response to external electromagnetic radiation. In the context of photocatalysis, these have been frequently utilized and studied in the form of metallic nanostructures supported on metal oxides (referred to as supported nanoparticles). However, it has been difficult to gain insight into the bulk oxide behavior during photocatalysis and how it might influence performance. For this reason, inverse plasmonic photocatalysts wherein a thin oxide layer coats the outside of the metal nanoparticles have been proposed as a platform that could enable closer fundamental study of the oxide’s influence and further understanding of how the plasmon affects it, which could better inform photocatalyst design. It is of particular interest to study how geometry affects the localized surface plasmon resonance (LSPR) of these photocatalysts, especially given the strong field enhancement (SFE) that typically occurs at corners due to electromagnetic confinement. Improving the intensity of the LSPR leads to more efficient energy transfer from absorbed radiation into chemical transformations. The goal of this project is to explore the influence of geometry on the photocatalytic performance of these inverse plasmonic photocatalysts, to study the metal-metal oxide interactions, and the effect of SFE on oxidation states and surface oxygen species. Given that the catalytic reactions of interest frequently occur at the interface between the metal and metal-oxide, it is expected that facet-selective coatings that corners uncovered would exhibit varying results. We have developed a reproducible synthesis method for colloidal monodisperse gold nanocubes for the purpose of producing metal nanoparticles with the correct geometries to exploit field enhancement effects, and specific surface functionalization required to deposit an oxide layer for the design of novel inverse plasmonic photocatalysts. This optimized procedure can be easily adjusted to tailor the dimensions and corner sharpness of the nanoparticles, and includes phase transfer and ligand exchange processes that enable them to remain stable in aqueous or organic solvents and be decorated with a variety of metal oxides. These template nanocubes can also be doped with platinum-group catalytic metals for enhanced reactivity via antenna-reactor-like systems. The synthesis and characterization methods, challenges with control of kinetic geometries and oxide-on-gold deposition, as well as potential applications of scientific and commercial interest are discussed. The current stage of this ongoing project focuses on developing colloidal atomic layer deposition (c-ALD) methods for full and partial oxide-coatings, promising a variety of possible decoration designs, tunability of the oxide thickness, composition, and (100) vs (111) facet selectivity. The conclusions drawn from optimizing and troubleshooting these c-ALD procedures will enable the synthesis of inverse photocatalysts with alternative plasmonic materials, such as earth-abundant copper, and alternative metal-metal oxide combinations suitable for different catalytic reactions of interest. Future directions include the implementation of novel laser-coupled transmission electron microscopy (TEM) techniques to study sintering impacts on the catalyst during reaction.