Colloidal platinum nanoparticles are obtained via a surfactant-free polyol process in alkaline ethylene glycol. In this popular synthesis, ethylene glycol functions as solvent and reducing agent. The preparation procedure is known for its reproducibility to obtain 1-2 nm nanoparticles, but at the same time the controlled preparation of larger nanoparticles is challenging. A reliable size control without the use of surfactants is a fundamental yet missing achievement for systematic investigations. In this work it is demonstrated how the molar ratio between NaOH and the platinum precursor determines the final particle size and how this knowledge can be used to prepare and study in a systematic way supported catalysts with defined size and Pt to carbon ratio. Using small-angle X-ray scattering, transmission electron microscopy, infrared spectroscopy, X-ray absorption spectroscopy, pair distribution function and electrochemical analysis it is shown that changing the NaOH/Pt molar ratio from 25 to 3, the Pt nanoparticle size is tuned from 1 to 5 nm. This size range is of interest for various catalytic applications, such as the oxygen reduction reaction (ORR). Supporting the nanoparticles onto a high surface area carbon, we demonstrate how the particle size effect can be studied keeping the Pt to carbon ratio constant, an important aspect that in previous studies could not be accomplished.
The preparation of colloidal nanoparticles in alkaline ethylene glycol is a powerful approach for the preparation of model catalysts and ligand-functionalized nanoparticles. For these systems the term "unprotected" nanoparticles has been established because no strongly binding stabilizers are required to achieve stable colloids. Irrespective of this fact, the particles must be considered as being covered by adsorbates, as otherwise the particles would coalesce and precipitate. The identification of these protecting adsorbate species is however still under debate and is the scope of the present study. "Unprotected" Pt and Ru nanoparticles were characterized by NMR spectroscopy, which does not evidence the presence of any C−H containing species bound to the particle surface. Instead, the colloids were found to be covered by CO, as demonstrated by IR spectroscopy. However, analysis of the stretching mode reveals the presence of a second species. On the basis of the spectroscopic characterization this species is concluded to be OH − , and it is demonstrated that the applied synthesis route results only in stable colloids if OH − is present within the reaction mixture. IR spectroscopy reveals that the CO coverage increases as the NaOH concentration used in the precursor solution is decreased. However, even at the lowest for the synthesis suitable OH − concentration the surface was found to be covered by both species. Finally, the effect of the OH − concentration on the particle size distribution was studied. The maximum was found to shift to larger particle diameters as the OH − concentration is lowered which is accompanied by broadening of the size distribution.
In this communication we present an investigation of the influence of light on the formation of platinum (Pt) nanoparticles (NPs) using standard polyol synthesis reagents at room temperature. It is demonstrated that instead of thermal treatment, UV-light can be used for particle formation thus opening new pathways for the synthesis of NPs with defined size distribution.The polyol based method for the synthesis of NPs has been shown to be a powerful approach to produce well-defined colloids with narrow size distributions. In several reports it was demonstrated that this method is particularly favorable for an effective and versatile synthesis of Pt-based catalysts [1] and the design of tailored ligand-functionalized NPs.[2] Despite the fact that various types of well-defined, catalytically active Pt and Pt-alloy NPs were obtained [1c] , further development of synthesis methods and understanding the size control mechanism is of significance. In order to get more insights into Pt NP formation, we used steady state absorption and fluorescence spectroscopy that provide information on the chemical species present during the particle formation process. In figure 1 series of UV-VIS absorption and emission spectra are shown monitoring the time evolution of a synthesis reaction mixture while stored at room temperature (RT) and exposed to daylight. As the reaction starts, the first absorption spectrum recorded immediately after creating the reaction mixture by mixing the H2PtCl6-EG (hexachloroplatinic acid dissolved in ethylene glycol) and NaOH-EG (sodium hydroxide dissolved in ethylene glycol) solutions indicates the unreduced PtCl6(-2) complex with an absorption peak at 268 nm, in line with previous reports [3] . At the same time no emission in the 375 -550 nm range is observed. The 268 nm absorption feature starts to disappear during the first hours while holding the precursor solution at RT, indicating a reaction of the PtCl6 (-2) complex. After a relatively fast stage of the PtCl6 (-2) reaction (~70 hours for complete disappearance), a slow evolution of absorption bands around 305 nm and 360 nm as well as the formation of emissive reaction products appears (with two main excitation bands around 305 nm and 360 nm, see supporting information (SI), Figure S1). In concert with the appearance of new absorption bands after ~350 hours the colloid color gradually changes from light yellow to brownish-yellow, indicating that the growth and formation of Pt NPs has started. Also a broad slope between 250 nm and 450 nm can be seen that increases during the whole reaction and is reported to be related to the formation of Pt NPs as well [4] . Red curve: PtNPs suspension; pink curve: Pt NPs re-dispersed in EG; brown curve: supernatant of Pt NP suspension; green curve: difference between red and pink curve (the peak shift most likely is related to a shift in OH -concentration during the re-dispersion procedure) [5] .
Unprotected colloidal nanoparticle suspensions are promising building blocks for various types of catalysts. The work featured on the Back Cover shows that exposing a Pt precursor reaction mixture at room temperature to light leads to the formation of well‐defined, unprotected Pt nanoparticles, whereas no particles are formed in the dark. This suggests that light can be used instead of thermal heat for the synthesis of unprotected Pt nanoparticles, providing a new synthetic route. More information can be found in the Communication by M. Arenz, T. Vosch et al. on page 104 in Issue 2, 2016 (DOI: 10.1002/cnma.201500118).
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