From Single Atoms to Nanocrystals: Photoreduction of [PtCl6]2- in Aqueous and Tetrahydrofuran Solutions of PVP

Structured platinum nanoclusters Pt-n (n = 5-30) capped by poly(N-vinylpyrrolidone) (PVP) have unique and highly attractive properties as potential selective catalysts. We show that the assembly of Pt mononuclear compounds in aqueous and tetrahydrofuran (THF) solutions under UV irradiation proceed via several steps: formation of linear Pt clusters (n = 2- 8), coalescence into mesocrystals, and transformation into Pt nanocrystals. The quantum size range of Pt-n (n = 5-100) clusters is intermediate between those clusters with molecular properties and those with metallic properties. The PVP cage acts as a nano reactor and can hinder diffusion of photoexcited Pt atoms. The diffusion of the Pt from the polymer cage is strongly affected by the hydrophobic or hydrophilic property of the solution. An aqueous solution of [PtCl6](2-) + PVP transforms into noncrystalline aggregates of molecules of less than 1.5-2 nm in diameter, whereas in THF solution Pt nanocrystals increase proportional to the UV irradiation time up to 10 nm in diameter. Dynamic imaging by high-resolution microscopy and low-frequency UV Raman spectra show the initial stages of Pt atoms assembled into Pt-n clusters. The assignment of the Raman bands is supported by density functional theory calculations. The proposed scheme of photoinduced reactions suggests the coupling of coordinatively unsaturated Pt ions inside the amidate-rich polymeric stabilizer.