Polyhydroxy Fullerene for Synthesizing Noble Metal Nanoparticles

Abstract

The redox properties of fullerene and its derivatives enable them to function as electron relays through either oxidation or reduction. Polyhydroxy fullerene (PHF), also known as fullerol or fullerenol, is highly water-soluble derivative and a well-known antioxidant. We have found that PHF is also very effective in reducing noble metal precursors and stabilizing metal nanoparticles by forming covalent M-O bonds. Traditionally, noble metal nanoparticles, such as gold are synthesized using a reducing agent like sodium borohydride (NaBH4) or sodium citrate at elevated temperatures and alkaline pH, with stabilizers added during nucleation to control the size and dispersion of the nanoparticles. While sodium citrate is sometimes used as a single reducing and stabilizing agent, it is not an effective stabilizer and is often replaced with polymers to improve colloidal dispersion. In contrast, PHF can be used as a single reducing and stabilizing agent for metal nanoparticles. The nanoparticles were synthesized by simply mixing the metal precursors with PHF in an aqueous phase and stirring at room temperature. The size and morphology of synthesized nanoparticles were characterized with high-resolution transmission electron microscopy, aberration corrected TEM, and dynamic light scattering. These nanoparticles were monodisperse with size ranging from 2 nm to 100 nm in diameter, which could be controlled by changing the concentration of the two reactants. We utilized multiple tools to investigate the mechanism of reduction and nanoparticle formation. We propose a kinetic agglomerate-dissociation mechanism to describe the formation of gold nanoparticles using PHF. Kinetic agglomerates form instantaneously upon mixing the two reactants due to electrostatic interactions. During the reduction process reactive carbonyl peaks were observed with electron energy loss spectroscopy. The carbonyl moieties were observed only after agglomerate formation and subsequently changed to Au-O bonds between PHF and synthesized gold nanoparticles, which was confirmed with Fourier-transform infrared spectroscopy and density-functional theory calculations, and X-ray photoelectron spectroscopy. Further, we demonstrated that the synthesis of gold nanoparticles using PHF can be affected by pH due to changes in hemiketal structure of PHF. Compared to citrate ions, PHF-coated gold nanoparticles are more stable, remaining so for over two years. The novel PHF-mediated synthesis method can also be applied to other noble metals like platinum, palladium, and silver and create bimetallic nanoparticles. The surface PHF coating on noble metal nanoparticles integrates the properties of both metal and carbon nanoparticles, resulting in superior functional applications. Our findings open up new opportunities for metallic nanoparticle preparation methods, properties, and applications.