Preparation, structure and properties of gold-based 'ruby' sodium silicate glass: A model metal-glass nano-composite

Abstract Gold ruby glass has been well-known for its deep red color dating back to Roman times but the role of gold nano-particles (AuNPs) in the creation of color was revealed only in 20th century. Glass alchemists discovered early on that addition of tin promotes the development of color. However, the underlying mechanism, which relies on the availability of electrons for oxidation/reduction and growth of AuNP above a minimum size, remains unclear. Additionally, very little information is available on how the electrical properties of such glasses are affected by the presence of gold nanoparticles or tin doping. In this dissertation, a gold-doped sodium trisilicate glass has been investigated as a model for metal-glass nano-composite in order to unravel the origin of the optical, electrical, and thermal properties of ruby glass in terms of atomistic structure and microstructure. This study also provides an insight on the dynamics of electrons in silicate glasses.We investigated the development of the AuNPs by extended X-ray absorption fine structure (EXAFS), in situ X-ray absorption near edge structure (XANES) analysis, and optical spectroscopy of sodium trisilicate glass doped with ≤ 0.1mol% of gold (as HAuCl4) and varying amounts of SnO 2 (0.005-0.1 mol%). The samples were prepared by the conventional melt-quench technique followed by thermal treatment. XANES was measured at the Au L3 -edge and Sn K-edge while heating the glass up to 550 °C. Development of the ruby color was followed concurrently with in situ optical spectroscopy; the in situ spectroscopy showed a red-shift and a blue-shift of the surface plasmon resonance peak due to AuNPs forming in samples with varying concentration of SnO2 with heat treatment. The XANES and EXAFS indicated transformation of ionic gold to metallic gold, and conversion from Au-O to Au-Au bonds. SnO2 doped samples had lower metallic gold i.e. Au-Au bonds in the as-quenched state, but also had a shorter incubation time and a faster growth of AuNPs during the heat treatment process. The tin remained as Sn4+ ions in the distorted octahedral site surrounded by mainly oxygen throughout i.e. both before and after the heat treatment. The addition of SnO2 helped gold dissolve in the glass matrix during melting, while acting as a nucleating agent at lower temperatures.Based on above results, a new model for the formation of AuNPs controlled by electron (FACE) availability is presented. The model suggests that the electrons involved in the reduction process through Au+1 + e-  Au0 control the growth process and in tin-free glass they mainly come from NBO. To prove this concept, additional electrons were generated by X-ray irradiation. The results showed a faster growth of AuNPs by the availability of these additional electrons, confirming that the reduction of gold ions is the rate limiting process that required availability of electrons. In addition, the X-ray irradiation produced the ruby color more effectively if the sample was irradiated during heating instead of before heating as the irradiation created new defects. However, the effect of the reduction process from X-irradiation diminished when higher SnO2 was present in the glasses. After the reduction stage, the growth of AuNPs, Au0 + Au0  AuNP, was controlled by the diffusion of gold atoms and Ostwald ripening of particles.The major factors that determine the influence of SnO2 on the AuNP formation include: i) Au+1 is more uniformly distributed in glass matrix of the as-quenched sample than in the tin-free sample, ii) the electrons associated with SnO_6^(2-) unit are more mobile than those associated with the NBOs of silicate network, and iii) SnO2 lowers the surface energy as a surfactant so that AuNPs may grow at a faster rate. The experimental results are in excellent qualitative agreement with the proposed FACE model.From scanning transmission electron microscope observations, spherical and non-spherical AuNPs were found in the glass matrix after annealing. In general, the average size of gold nanoparticles increased with heat treatment, as expected from the growth process. A smaller size and narrower size distribution of AuNPs were observed in the sample with higher SnO2 concentration. Different orientations (i.e. multiple grains) were found in several AuNPs, possibly originating from the coalescence of two particles. Some AuNPs appeared to have a core-shell structure but the energy dispersive X-ray spectroscopy did not show higher Sn, Na or Cl concentration in or surrounding the AuNPs.The Open Z-scan technique showed nonlinearity which demonstrated an optical limiting behavior. The samples with higher gold amounts showed a higher nonlinear absorption coefficient (β). The addition of tin to the samples caused the deep ruby red color within the glass but there was a lower β in these samples. Moreover, the tin-doped samples showed a negative or a positive β depending on the intensity of laser. Finally, the addition of both 0.1 mol% gold and 0.1 mol% tin increased the DC electrical conductivity, which originated from the sodium ion transport, as compared to the undoped base glass. The activation energy of DC conductivity decreased when the amount of tin increased with the gold content kept constant. The Na+ ion transport appeared to be different for the two types of dopings: Au doping caused general expansion of molar volume whereas tin doping enhanced the polarizability of the medium.