Direct observation of liquid pre-crystallization intermediates during the reduction of aqueous tetrachloroaurate by sulfide ions

Abstract

Phenomena preceding precipitation of gold(i) sulfide in the reaction of aqueous tetrachloroauric acid with sodium sulfide have been studied applying several time-resolved in situ techniques. As can be deduced from UV-vis absorption spectroscopy, soluble species ("pre-nucleation clusters") emerge within several seconds and slightly change with time; the spectra, along with the previous data for the immobilized products, are indicative of disordered matter with Au(I)-S bonding and the gap of about 1.5 eV. In situ tapping-mode atomic force microscopy (AFM) directly imaged "soft" droplet-like species from 20-50 nm to about 200 nm in the lateral size and the continuous film condensed on the highly oriented pyrolytic graphite (HOPG) support. Small-angle X-ray scattering (SAXS) revealed quasi-particles ("liquid clusters") growing to about 12 nm for 30 min, and slowly afterwards. Dynamic light scattering (DLS) was preferentially due to the larger species, although the liquid clusters within 20 nm comprise more than 80% of the total volume of the fluid products. The entities, which exist in solution for many hours before sudden gold sulfide sedimentation, are considered as "dense liquid" intermediates involved in a non-classical nucleation pathway.

Mineralization on polylactide/gelatin composite nanofibers using simulated body fluid containing amino acid

“…PNCs of 4.7 ± 1.4 nm in diameter were CaP ion-associated complexes of localized Ca 2+ , HPO 4 2− , H 2 PO 4 − ions, and well organized into stripes. The resulting solution full of well-organized PNCs was deemed as a kind of dense liquid [47]. The addition of amino acids demonstrated effects on the pre-nucleation process due to their electric charge characteristics.…”

Theories of nanoparticle and nanostructure formation in liquid phase

“…The concept of PNCs cannot fully explain the nature of ⋅ − p Au S H O m n 2 species [34]. According to theory, PNCs are stable solutes with a "molecular" character 609 6 In situ characterization techniques that emerge in aqueous solutions prior to nucleation, which differs from ion pairs and liquid and amorphous clusters.…”

“…Then, the fluid intermediates (referred to as "liquid clusters" after Refs. [27,[35][36][37] accumulating the NCs via density fluctuations emerge and grow to ∼12 nm within about 30 min, remaining in the solution for many hours [34].…”

“…(Source: Reproduced from Ref. [34] with permission from the PCCP Owner Societies.) 611 6 In situ characterization techniques and growth of colloidal NPs in solutions, such as the nucleation of Ag NPs [41] and the growth of Au NPs and ZnO NPs [19].…”

Colloidal and Deposited Products of the Interaction of Tetrachloroauric Acid with Hydrogen Selenide and Hydrogen Sulfide in Aqueous Solutions

“…Metallic Au NPs emerging at the molar ratios from 0.5 to 1.5 show a second localized surface plasmon resonance (LSPR) in the near-infrared region, which has been suggested to assign to gold-sulfide/gold core/shell particles [50], aggregation of gold nanoparticles [54], or, and this is most likely, to longitudinal plasmon resonance in non-spherical Au NPs [55]. Gold sulfide is the main final product at the Na 2 S/HAuCl 4 ratio higher than 2 but its formation is retarded, probably by slow nucleation and crystal growth of a metastable Au 2 S phase [55][56][57][58]. The reaction was concluded to proceed via non-classical pathway involving a series of pre-nucleation intermediates (10-40 nm "clusters" and submicrometer droplets made up of "dense liquid" due to spinodal decomposition of supersaturated solutions), whose existence was confirmed using in situ atomic force microscopy, dynamic light scattering (DLS) and other techniques [57,58].…”