Surface Chemistry of “Unprotected” Nanoparticles: A Spectroscopic Investigation on Colloidal Particles

Abstract

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.

UV‐Induced Synthesis and Stabilization of Surfactant‐Free Colloidal Pt Nanoparticles with Controlled Particle Size in Ethylene Glycol

“…Most reports stress that the reaction occurs only in strictly alkaline solution, therefore an excess of NaOH is typically used for the syntheses . However, based on recent results we proposed that the use of no or low NaOH concentrations should allow for tuning the size of surfactant‐free NPs . Here we demonstrate that this postulation was correct, but the use of UV light instead of thermal energy is needed as a driving force in order to kinetically inhibit sintering processes.…”

“…However, organic compounds that play a role in the reaction mechanism and NP stabilization might be observable using IR. Consistent with previous results by Schrader et al., the negative pointing absorption peaks around 2900 cm −1 can be attributed to ethylene glycol and the peak around 1650 cm −1 relates to water. In addition, the IR data shows a peak around 1600 cm −1 which could be due to C=C containing compounds that are produced as the reaction media becomes more alkaline and Pt NPs are present.…”

“…Pt is known as an efficient catalyst and most likely helps to produce more fluorescent compounds than in the control reaction (orange spectrum in Figure B), in agreement with Gao et al . In cases of higher NaOH concentrations (0.25 m or 0.5 m ), an additional small peak around 2015 cm −1 is observed in the IR spectra, which corresponds to CO adsorbed on the surface of Pt NPs . The same peak could not be observed in reaction mixtures without NaOH even when the initial concentration of Pt salt (and the expected amount of particles produced) was increased 30 times (data not shown).…”

Synthesis Mechanism and Influence of Light on Unprotected Platinum Nanoparticles Synthesis at Room Temperature

“…360 nm). c) Detailed investigation of different absorption bands that appeared by the end of formation of Pt NPs after ≈2000 h. 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 curves (the peak shift most likely is related to a shift in OH − concentration during the re‐dispersion procedure) …”

“…EDS (see Supporting Information) confirmed that the observed NPs are indeed Pt. Comparing daylight‐induced NP formation at RT to our standard heat‐induced Pt NP synthesis (see Figure d), the former results in an average particle size that is slightly larger and exhibits a somewhat broader size distribution as a consequence of different precursor concentrations and slower reaction kinetics (see discussion of particle growth in the Supporting Information). In previous work, UV‐light‐induced Pt NP formation has been reported in combination with strongly binding ligands, which limit catalytic applications .…”