Formation of gold and gold sulfide nanoparticles and mesoscale intermediate structures in the reactions of aqueous HAuCl4 with sulfide and citrate ions

The effects of the molar ratio of sodium sulfide to chloroauric acid in the range of 0.5 to 5 and the time factor on the formation of the nanoparticles (NPs) of metallic Au, Au2S or their mixtures have been studied applying in situ and ex situ techniques (UV-Vis absorption spectroscopy, potentiometry, TEM, SPM, SERS, XPS). The products and intermediates have been compared with those for the reduction of chloroaurate with citrate ions and combinations of citrate and sulfide ions. An increase in the concentration of sulfide ions accelerates the reduction of Au(III) complexes but hinders the nucleation and growth of Au NPs, resulting in a prolonged period before the appearance of plasmon peaks. The electrochemical potential is not directly associated with the plasmon intensities, although the potential sharply decreases simultaneously with a blue shift of the near-IR peak emerging with the Na2S/HAuCl4 ratios of 0.5 to 1.5. It was concluded that the peak is due to longitudinal plasmon resonance of gold nanoplates. Au2S NPs, the nucleation of which is effectively inhibited, and probably some structures and fragments visible in TEM and AFM, including 2-5 nm Au NPs, crystallize in part outside the solutions. The evidence of partially liquid mesoscale structures comprising intermediate gold species as precursors of nanoparticles is presented, and their origin, ex situ transformation and role in the reaction mechanisms are discussed.