Synthesis and characterization of gold and gold-cuprous oxide nanostructures

Abstract

In the past few years, substantial efforts have been invested into the synthesis and characterization of plasmonic gold nanostructures owing to their unique size and shape-dependent physical and chemical properties. Gold (Au) nanostructures (NSs) are of great interest for scientific research because of their attractive applications in numerous fields, built upon their interesting surface plasmon resonance (SPR) features and biocompatibility. Corresponding to these fascinating features, multifaceted Au NSs have been synthesized using a quaternary ammonium cationic surfactant, methyltrioctylammonium chloride (Aliquat 336), as a shaping and stabilizing agent. Transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy analyses confirm the existence of Aliquat 336 stabilized NSs that are demonstrated to achieve minimal ligand density in the form of monomolecular layer onto the Au surface. Thermogravimetric analysis (TGA) and dynamic light scattering (DLS) experiments have been performed to quantify the ligand density on the surface of Au. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements are accomplished to determine the structure and binding of ligand molecules to the Au surface. Zeta potential (+24.3 mV) of the nanoparticles (NPs) shows that the particles are positively charged and sufficiently stable in nature. The quats surfactant also manipulates the growth of extremely elongated Au nanorods (aspect ratio within 10-57) and nanowires following one-step hydrothermal syntheses. A pronounced change in the shapes of Au NSs strongly depends on the growth parameters including ligand contents, reaction temperature and reaction duration. As-synthesized Au NSs i.e. multi-faceted and cubic nanoparticles are coated with cuprous oxide to form Au-Cu2O core-shell nano-morphologies in which efficient shape evolution of the Cu2O shell is achieved through fine adjustment of the ratio H2O:NH2OH∙HCl. The effect of particle morphology and shell thickness on the optical properties of truncated-octahedra, cuboctahedra and nanoflowers Au-Cu2O having sizes within 90-230 nm shows that the SPR band of the Au-core shifts progressively to red with increasing shell thickness. A comparative study to correlate the photoluminescence (PL) analyses of core-shell nanostructures with their photocatalytic activities towards the decomposition of methyl orange shows that truncated-octahedra and nanoflowers, bounded by (111) facets, are photocatalytically more active. The results are in good agreement with the PL analysis in that cuboctahedra with more (100) catalytically inactive sites reveal a comparatively sharp emission peak.