SAXS and XAFS In-Situ temperature dependent study of gold nanostructures synthesis and stability

FERNANDO PSCHUNDER; GIOVANETTI, LISANDRO J; CRISTIAN HUCK-IRIART; HOPPE, CRISTINA E.; MA. S. ÁLVAREZ CARIMEDO; F. G. REQUEJO; JOSE MARTIN RAMALLO LOPEZ

Campinas

Congreso; 31st RAU (Annual Users Meeting - LNLS/CNPEM); 2021

Ultrathin gold nanowires (AuNWs) have attracted great interest due to their unique conductivity properties making them suitable candidates for a myriad of applications. Many relatively simple syntheses protocols have been reported using gold salts and surfactants in organic media, in general HAuCl4⋅3H2O and oleylamine (OAm) in hexane, resulting in micrometer long wires with sub 5 nm diameters. Comparing similar reported procedures, the OAm/Au molar ratio appears to be of crucial importance to lead the synthesis toward thin wires with high efficiency or spherical nanoparticles, although the effect of the synthesis temperature is still a factor to address systematically 1 2 3 . Moreover, temperature stability studies of AuNWs are fundamental for future developments and applications.Here we present a small angle x-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS) in situ temperature dependent study of an already reported AuNWs4 synthesis. We also studied the stability of the final nanostructures in solution in the range of 20 – 90ºC. SAXS and EXAFS measurements were complemented with transmission electron microscopy (TEM) images. OAm/Au molar ratio was kept at 40, well within the nanowires high yield range, and the synthesis temperatures varied from 20 to 80ºC without stirring. It was found that the synthesis evolves toward the formation of nanowires when the temperature is kept below 40°C, and ununiform nanoparticles started to be produced above this temperature. In addition, the AuNW were found to be stable up to 70ºC, when the correlation between AuNWs began to decrease and above 80°C the nanowires were disassembled to generate correlated nanoparticles. In situ XAFS measurement at Au L3 edge showed an acceleration in the gold reduction rate from Au(3) to Au(1) with higher temperatures, and the Fourier transformed of EXAFS spectra showed how the gold coordination transitions from Au-N to Au-Au at earlier times.