J. Cabezas Parra, E.R. Newmeyer, J. North, M. Hershey, D.F. Swearer
Northwestern University,
United States
Keywords: photocatalysis, copper, hydrotalcites, plasmonics, layered double hydroxides
Summary:
Photocatalytic supported plasmonic copper nanoparticles have garnered interest for sustainable applications and have been reported as promising pathways for low-temperature, energy-efficient light-driven catalytic reactions of industrial interest. Commonly synthesized via co-precipitation to produce a hydroxycarbonate precursor that is then calcined and reduced under a positive hydrogen flow, copper nanoparticles supported on mixed metal oxides are obtained. In this research, the catalyst precursors were studied to improve understanding of support-nanoparticle interactions and the role of metal oxides supports in photocatalytic performance. Cu/Zn/Al, Cu/Mg/Al, and Cu/Ce/Al hydroxycarbonate precursors were synthesized at varying compositional ratios by constant-pH co-precipitation and subsequently characterized by diffuse reflectance UV-Vis spectroscopy (DRUVS) to confirm the presence of plasmonic nanoparticles after calcining and reducing. Powder X-Ray Diffraction (pXRD) and X-Ray photoelectron spectroscopy (XPS) were employed to determine the identity of the precursors, and morphological properties were examined by scanning electron microscopy (SEM) and surface area analysis. The hydroxycarbonate precursors were determined to be identifiable minerals of known structures and properties. These results show that supported plasmonic copper nanoparticles can be sourced from known synthetic minerals, introducing a unique perspective in photocatalyst design and new research avenue in plasmonics. The localized surface plasmon resonance (LSPR) and morphologies of the supported copper nanoparticles depended on the composition of the metal oxides resulting from calcining the precursors, which were subject to the mineral identities of the precursors that were, in turn, contingent on the distinct experimental synthesis ratios. Mixed metal oxides can affect the dielectric environment around the nanoparticles, and oxygen vacancies and surface states can modify electron density and polarizability in the nanoparticles affecting peak placement and intensity. Copper-bearing hydrotalcites, specifically, resulted in mixed metal oxide supports that favored considerably higher surface areas and plasmonic nanoparticles with stronger LSPR responses. The relationship between mixed metal oxides and LSPR intensity presented in this study is relevant to advances in photocatalysis. Such insight can contribute background information about the materials used in the design of these photocatalysts, potentially expediting their design and tailoring for specific reactions. Well-informed decisions can be made about synthesis conditions preemptively, as a general description and estimated effectiveness of the supported nanoparticles can be predicted in association to the identified precursor mineral. This contrasts with the time-intensive prior need to solely rely on post-synthesis characterization to gain insight and recursively modify the experimental conditions. The relationships between precursors, mixed metal oxide compositions, and the behavior of copper nanoparticles are discussed. Manuscript in preparation.