Crystal structure tailoring of Au–Cu alloy NPs using the embedding film matrix as template

Abstract

Au–Cu alloy nanoparticles (NPs) were grown in three different dielectric film matrices by the sol–gel method in order to see the influence of the embedding matrices on the crystal structure of the alloy NPs. The chosen film matrices were TiO₂ (anatase; tetragonal), ZrO₂ (cubic) and SiO₂ (amorphous). Au and Cu salts (1 : 1 molar ratio) doped MO₂ (M = Ti, Zr and Si) sols were prepared keeping the total metal concentration same (M : MO₂ = 1 : 9) in all the three cases, and used them for coating deposition on silica glass substrates. Heat-treatment of the films in air at 500 °C resulted in removal of organics, formation of metallic Au NPs and matrix crystallization (tetragonal and cubic in cases of TiO₂ and ZrO₂, respectively; SiO₂ remained amorphous). Further heat-treatment in reducing atmosphere (10% H₂–90% Ar/500 °C) induced the reduction of Cu ions with simultaneous formation of Au–Cu alloy NPs. In cases of TiO₂ and ZrO₂ films, formation of equiatomic Au₁Cu₁ alloy NPs were completed whereas Au₃Cu₁ alloy NPs was found to be formed in SiO₂ under the similar experimental conditions. Structural analyses confirmed the formation of ordered face centred tetragonal (fct) and disordered face centred cubic (fcc) Au–Cu NPs in cases of TiO₂ and ZrO₂ matrices, respectively. Thus tetragonal (anatase TiO₂) and cubic (ZrO₂) structures of the embedding film matrices acted as templates to nucleate and subsequent growth of the Au–Cu order (fct) and disorder (fcc) alloy nanocrystals, respectively. Cubic Au₃Cu₁ alloy NPs was also formed in amorphous SiO₂; however, without any structural influence of the matrix alloy formation could not be completed at such a low temperature.