Removal of Residual Poly(vinylpyrrolidone) from Gold Nanoparticles Immobilized on SiO₂ by Ultraviolet-Ozone Treatment

Poly(vinylpyrrolidone) (PVP) is a widely used stabilizer or capping polymer in the colloidal syntheses of varying metal nanostructures (MNSs) with precisely controlled size and morphology but unfortunately cannot be completely removed by washing at the end of the syntheses. The residual PVP would itself remain as a contaminant or transform in later processing into other contaminants on the MNS surface, making it difficult to study the intrinsic surface properties of "clean" MNSs. We have shown earlier that ultraviolet-ozone (UVO) treatment in flowing air can be used to achieve a complete removal of residual PVP from SiO₂-immobilized Au nanoparticles (NPs) without changing the Au size and morphology but details about the chemistry of PVP degradation remain unexplored. We report herein a quantitative study on the chemistry of PVP degradation during the UVO treatment, by in situ monitoring the evolved gases with mass spectroscopy and ex situ measuring the surface residues at varying UVO-treatment durations with ultraviolet-visible diffuse reflection spectroscopy, infrared spectroscopy, and temperature-programmed oxidation techniques in differential thermogravimetry and mass spectroscopy modes. The residual PVP undergoes photochemical oxidative degradation dominated by C-C and C-N bond cleavages in the first hour of UVO treatment, producing overwhelmingly CO₂, N₂, and H₂O as the gaseous products, with PVP-derived carboxylic species as the prevailing intermediates maximizing at a duration of ca. 0.5 h. Further degradation slows with the increased oxidation degree of carbon skeleton and/or cross-linking reactions. The nature of PVP-derived residues and their degradation chemistry including kinetic characteristics in association with Au NPs and "pure" SiO₂ are discussed. These data could have important implications on manipulating the surface properties of MNSs by UVO treatment in many technological fields.