From platinum atoms in molecules to colloidal nanoparticles: A review on reduction, nucleation and growth mechanisms

Platinum (Pt) is one of the most studied materials in catalysis today and considered for a wide range of applications: chemical synthesis, energy conversion, air treatment, water purification, sensing, medicine etc. As a limited and non-renewable resource, optimized used of Pt is key. Nanomaterial design offers multiple opportunities to make the most of Pt resources down to the atomic scale. In particular, colloidal syntheses of Pt nanoparticles are well documented and simple to implement, which accounts for the large interest in research and development. For further breakthroughs in the design of Pt nanomaterials, a deeper understanding of the intricate synthesis-structures-properties relations of Pt nanoparticles must be obtained. Understanding how Pt nanoparticles form from molecular precursors is both a challenging and rewarding area of investigation. It is directly relevant to develop improved Pt nanomaterials but is also a source of inspiration to design other precious metal nanostructures. Here, we review the current understanding of Pt nanoparticle formation. This review is aimed at readers with interest in Pt nanoparticles in general and their colloidal syntheses in particular. Readers with a strongest interest on the study of nanomaterial formation will find here the case study of Pt. The preferred model systems and characterization techniques used to perform the study of Pt nanoparticle syntheses are discussed. In light of recent achievements, further direction and areas of research are proposed.

195Pt NMR and Molecular Dynamics Simulation Study of the Solvation of [PtCl6]2- in Water-Methanol and Water-Dimethoxyethane Binary Mixtures

The experimental ¹⁹⁵Pt NMR chemical shift, δ(¹⁹⁵Pt), of the [PtCl₆]^2- anion dissolved in binary mixtures of water and a fully miscible organic solvent is extremely sensitive to the composition of the mixture at room temperature. Significantly nonlinear δ(¹⁹⁵Pt) trends as a function of solvent composition are observed in mixtures of water-methanol, or ethylene glycol, 2-methoxyethanol, and 1,2-dimethoxyethane (DME). The extent of the deviation from linearity of the δ(¹⁹⁵Pt) trend depends strongly on the nature of the organic component in these solutions, which broadly suggests preferential solvation of the [PtCl₆]^2- anion by the organic molecule. This simplistic interpretation is based on an accepted view pertaining to monovalent cations in similar binary solvent mixtures. To elucidate these phenomena in detail, classical molecular dynamics computer simulations were performed for [PtCl₆]^2- in water-methanol and water-DME mixtures using the anionic charge scaling approach to account for the effect of electronic dielectric screening. Our simulations suggest that the simplistic model of preferential solvation of [PtCl₆]^2- by the organic component as inferred from nonlinear δ(¹⁹⁵Pt) trends is not entirely accurate, particularly for water-DME mixtures. The δ(¹⁹⁵Pt) trend in these mixtures levels off for high DME mole fractions, which results from apparent preferential location of [PtCl₆]^2- anions at the borders of water-rich regions or clusters within these inherently micro-heterogeneous mixtures. By contrast in water-methanol mixtures, apparently less pronounced mixed solvent micro-heterogeneity is found, suggesting the experimental δ(¹⁹⁵Pt) trend is consistent with a more moderate preferential solvation of [PtCl₆]^2- anions. This finding underlines the important role of solvent-solvent interactions and micro-heterogeneity in determining the solvation environment of [PtCl₆]^2- anions in binary solvent mixtures, probed by highly sensitive ¹⁹⁵Pt NMR. The notion that preferential solvation of [PtCl₆]^2- results primarily from competing ion-solvent interactions as generally assumed for monatomic ions, may not be appropriate in general.

Intrinsic 37/35Cl and 18/16O isotope shifts in 195Pt and 103Rh NMR of purely inorganic Pt and Rh complexes as unique spectroscopic fingerprints for unambiguous assignment of structure

Well-resolved intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope shifts (where M = ¹⁹⁵Pt or ¹⁰³Rh) are visible in the ¹⁹⁵Pt NMR peak profiles of relatively kinetically inert [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) complexes, their corresponding hydroxido [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions, and [RhCl_n(H₂O)_6-n]^3-n (n = 3-6) complexes in aqueous solutions at ca. 293 K. Although some such isotope effects have been previously reported, there are very limited published data in the open literature, and the first systematic studies of such intrinsic ¹ΔM(^37/35Cl) and ¹ΔM(^18/16O) isotope effects are reviewed in this perspective. In high magnetic-field NMR spectrometers, the ¹⁹⁵Pt and ¹⁰³Rh NMR peak profiles acquired within a relatively narrow temperature range (288-300 K) constitute unique 'spectroscopic fingerprints', which allow unambiguous structural assignment in solution. Available data for Pt(iv) and Rh(iii) complexes give rise to intrinsic isotope ¹Δδ¹⁹⁵Pt/¹⁰³Rh(^37/35Cl) profiles, which are extraordinarily sensitive to the structure of a particular complex or its geometric isomer. The profiles of aquated Pt(iv) and Rh(iii) complexes in acidic solutions may be resolved at either an isotopologue level only or at both an isotopologue and an isotopomer level depending on the structure. By contrast, in the series of [PtCl_6-n(OH)_n]^2- (n = 1-6) anions, ¹Δδ¹⁹⁵Pt(^37/35Cl) isotope shifts are resolved only at an isotopologue level. Relatively larger ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts obtained by the partial ¹⁸O enrichment of both the [PtCl_n(H₂O)_6-n]^4-n (n = 1-6) and [PtCl_6-n(OH)_n]^2- (n = 1-6) series give rise to remarkable ¹⁹⁵Pt NMR peak profiles showing both ^37/35Cl and ^18/16O shifts. In the [PtCl_6-n(OH)_n]^2- (n = 1-5/6) anions a typical NMR peak profile spanning ∼2 ppm only may be resolved at both the isotopologue and isotopomer levels, depending on whether ^18/16OH^- ions are coordinated trans to chloride ions or not. The potential utility of such ¹Δ¹⁹⁵Pt(^37/35Cl) and ¹Δ¹⁹⁵Pt(^18/16O) isotope shifts in selected practical applications involving such complexes is briefly illustrated.