Abstract
The reactions of aqueous gold complexes with H2Se and H2S are important for transportation and deposition of gold in nature and for synthesis of AuSe-based nanomaterials but are scantily understood. Here, we explored species formed at different proportions of HAuCl4, H2Se and H2S at room temperature using in situ UV-vis spectroscopy, dynamic light scattering (DLS), zeta-potential measurement and ex situ Transmission electron microscopy (TEM), electron diffraction, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Metal gold colloids arose at the molar ratios H2Se(H2S)/HAuCl4 less than 2. At higher ratios, pre-nucleation “dense liquid” species having the hydrodynamic diameter of 20–40 nm, zeta potential −40 mV to −50 mV, and the indirect band gap less than 1 eV derived from the UV-vis spectra grow into submicrometer droplets over several hours, followed by fractional nucleation in the interior and coagulation of disordered gold chalcogenide. XPS found only one Au+ site (Au 4f7/2 at 85.4 eV) in deposited AuSe, surface layers of which partially decomposed yielding Au0 nanoparticles capped with elemental selenium. The liquid species became less dense, the gap approached 2 eV, and gold chalcogenide destabilized towards the decomposition with increasing H2S content. Therefore, the reactions proceed via the non-classical mechanism involving “dense droplets” of supersaturated solution and produce AuSe1−xSx/Au nanocomposites.
Highlights
The Au–Se bonding is more favorable than the Au–S so one could expect a higher rate of nucleation of AuSe and shorter lifetime of liquid intermediates
At the ratios higher than 3, pre-nucleation dense liquid entities of 20–40 nm in the average hydrodynamic diameter and negative zeta potential grow into submicrometer droplets with time; gradual nucleation and coagulation of disordered gold chalcogenide within the droplets were faster with H2 S
Surface layers of amorphous gold–selenide AuSe, which form upon drying the reaction media, tend to decompose yielding metallic gold nanoparticles capped with elemental selenium; no metal gold was found in the bulk precipitate using X-ray diffraction (XRD) diffraction
Summary
Mixed silver–gold sulfides and selenides, including nanoparticulate ones, have attracted intense interest as minerals and natural sources of precious metals [1,2,3,4,5,6,7,8,9,10,11,12,13,14] and due to high ionic conductivity, tunable optical characteristics, thermoelectric and other properties promising for materials applications [15,16,17,18,19]. Au(Te,Se,S) phases in intergrowths with native gold have been found in the high-sulfidation epithermal deposits formed from acidic fluids[14]. Rabenau and Schulz [31] and other authors [34,35] have assumed that the gold in the first sites is Au3+ , while Au+ is located in the linear environment, and each Se atom is linked with one Au+ and two Au3+
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