Understanding the initial stages of precious metals precipitation: Nanoscale metallic and sulfidic species of gold and silver on pyrite surfaces

The nanoscale products deposited on pyrite at room temperature from 0.1 mM tetrachloroaurate or silver nitrate aqueous solutions and colloidal solutions (sols) of Au0, Ag0 and gold sulfide nanoparticles, which were prepared beforehand by the reduction of the Au(III) and Ag(I) complexes with sodium citrate, borohydride or sulfide, have been studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), scanning tunneling microscopy (STM) and tunneling spectroscopy (STS). The quantity of metals spontaneously immobilized from the solutions is basically higher than from the sols and depends on the reaction conditions. In particular, the surface concentration of gold passes a maximum at the initial molar ratio Na2S to HAuCl4 of about 2 in the sulfidic sols. Ag and Au sulfide phases deposit as plain islands of 10–30 nm in the lateral size while elemental metals form nanoparticles. Although XPS showed predominant lines of metallic gold in all the cases, XAS in the transmission mode revealed that disordered gold sulfide is the main product after the interaction of pyrite with the media with Na2S to HAuCl4 ratios more than 1.5–2. The discrepancy was rationalized in terms of decomposition of surface layer of Au2S yielding Au0 in air and the spectrometer vacuum. Silver spontaneously deposits preferentially as Ag2S, and metallic nanoparticles arise with increasing reaction time; the precipitation of Ag nanoparticles (NPs) from their sols results also in some silver sulfide. A sharp current surge was found in tunneling spectra at silver sulfide and Ag NPs under positive or both positive and negative threshold biases of several tenths of a volt, respectively. The effect is most likely due to an increase in the local conductance (known as resistive switching) as a result of enrichment of acanthite (α-Ag2S) by Ag producing conducting argentite (β-Ag2S) and then Ag0. The switching allows identifying Ag0 and Ag2S phases on nanoscale; it can play an important role in surface reactions of silver in the ore deposition and processing.