A study of pre-nucleation intermediary structures arising in the reaction of H2PtCl6 with sodium sulfide
Recently, a series of experimental findings which are difficult to understand within the frame of classical nucleation theory (CNT) have been established. In particular, a formation of extra-ordinary large-scale (in comparison with the size of classical nuclei) pre-nucleation species has been observed in a range of processes such as crystallization of calcium carbonate, calcium phosphate, silica gel formation, and protein crystallization. According to in situ tapping-mode atomic force microscopy (AFM), the liquid droplet-like species from 10-50 to 200 nm and more in lateral size were arisen during the reaction, which is in good agreement with time-resolved dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) data. In this contribution, we try to examine the pre-nucleation process taking place in a similar reaction between aqueous hexachloroplatinic acid (3•10-4 M) and sodium sulfide with molar ratio of 3-to-1 at room temperature. In comparison to analogous gold species, the platinum chlorocomplexes has been suggested to be more inert akin to ligand exchange. Moreover, in contrast to relatively well examined gold sulfide, platinum sulfides, both PtS and PtS2, whether in bulk or nanoscale form, are poorly studied, and the data related to their structure and electronic properties are controversial. X-ray absorption spectra were acquired in fluorescence mode on BM23 beamline of the ESRF (Grenoble, France). Energy discrimination was accomplished with double-slit crystal-analyzer using Si(111) reflections. Energy scale was calibrated using Pt foil as a standard. LII,III-edge Pt spectra were measured with energy-dispersive silicon Vortex detector situated at 90° toward incident X-ray beam, with Lα1-line was being used. Experimental data were fitted, fixing the Debay-Waller factors, and varying interatomic Pt-Cl and Pt-S distances, using the Demeter ver. 0.9.25 software suite. As fit results suggest, the replacement of Cl atoms by sulfur in the Pt first coordination sphere takes place during the 1st hour of the reaction, although some Cl ligands persist within the sphere even in 7.5 hours after the start of reaction. The nearest ligand environment of Pt in akin to Pt-S distances and coordination numbers can be assigned to PtS2-like one, with the second and more distant spheres are rather strongly disordered. Dynamic light scattering (DLS) data obtained with Malvern Nanosizer ZS instrument shows rapid growth of scatterers up to 150-200 nm for ~60 minutes after the start of reaction. Then, after relatively slow stage of scatterer enlargement, hydrodynamic diameter (Dh) of scatterers reached its maximum. And finally, a slow decrease in Dh was observed. Interestingly, no precipitate forms, which draws explanation of the phenomena observed within the frame of classical nucleation theory inconsistent. In conclusion, we propose a model, which includes a rapid formation of PtxSy species within liquid clusters growing up to 150-200 nm and forming ‘dense droplets’, within which nucleation (and nucleation associated with it) takes place giving rise to solid particles, 12-15 nm in size.