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

Schrader, Imke; Warneke, Jonas; Neumann, Sarah; Grotheer, Sarah; Swane, Andreas Abildgaard; Kirkensgaard, Jacob J. K.; Arenz, Matthias; Kunz, Sebastian
The Journal of Physical Chemistry C, 2015
Abstract Image

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.

KEYWORDS: Colloids, Nanoparticles