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

Structured platinum nanoclusters Ptn (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 Ptn clusters (n = 2–8), coalescence into mesocrystals, and transformation into Pt nanocrystals. The "quantum" size range of Ptn (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 transmission electron microscopy and low-frequency UV Raman spectra show the initial stages of Pt atoms assembled into Ptn 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.