TiO2–Au nanocomposite materials embedded in polymer matrices and their application in the photocatalytic reduction of nitrite to ammonia

Abstract

Titanium dioxide–gold nanocomposite materials ((TiO₂–Au)_nps) were prepared and embedded in methyl functionalized silicate sol–gel (MTMOS) and Nafion (Nf) matrices. The nanocomposite materials were characterized by UV-vis absorption spectra, scanning electron micrographs (SEM) and high resolution transmission electron micrographs (HRTEM). When gold nanoparticles were deposited on TiO₂ ((TiO₂–Au)_nps) and immobilized in MTMOS silicate sol–gel and Nafion matrices, the band-gap (E_bg) of the TiO₂ shifted to lower energy. During the photocatalytic experiment, the (TiO₂–Au)_nps incorporated polymer matrices improved the photocatalytic reduction of nitrite ions to ammonia, owing to the effective interfacial charge transfer process in the presence of a hole scavenger, oxalic acid. Further increase in the photocatalytic reduction of nitrite to ammonia was observed by incorporating a [Ni(teta)]²⁺ complex into the MTMOS/(TiO₂–Au)_nps or Nf/(TiO₂–Au)_nps film. The immobilization of (TiO₂–Au)_nps in a functionalized silicate sol–gel or Nafion matrix is advantageous for the preparation of solid-phase photocatalyst film and to design a solid–solution system leading to the physical separation of the catalyst from the solution and the products in contrast compared to the colloidal photocatalyst system. The Au_nps deposited on TiO₂ act as an e⁻ sink which promotes efficient interfacial electron transfer from TiO₂ to the substrate upon irradiation. The ultimate contact between (TiO₂–Au)_nps and Ni(II) complex in the film efficiently promotes the electron transfer from Au_nps to Ni(II) complex leading to the formation of an intermediate Ni(I) complex, which facilitates the efficient catalytic reduction of nitrite to ammonia.