Sterical ligand stabilization of nanocrystals versus electrostatic shielding by ionic compounds: a principle model study with TEM and XPS

Abstract

Colloidal metal nanoparticles are usually fabricated via the reduction of metal salt precursor compounds in liquid phase. To prevent agglomeration, organic capping agents are used. Commonly, it is neglected that ionic compounds may coadsorb when present in the mixture. However, we shall show that ionic adsorbates play a key role in size and growth control. We present a universal case study on gold nanoparticles with various amine ligands to reveal two competitive stabilization mechanisms via quantitative X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Gold nanocrystals were obtained starting with a single phase room temperature synthesis. The stabilization turned out to be a combination of sterical and electrostatic shielding depending on the ligand molecule properties. We can adjust the ratio of both contributions via simple liquid phase ligand exchange procedures at moderate temperatures as shown with XPS. Apparently, ions further have the power to steer different ripening processes. HR-TEM studies proved that there is no influence on nanoparticle morphology during heat treatment or ligand exchange. Introducing a lack of stabilization by weaker sterical ligands offers an auspicious new way for the synthesis of porous nanomaterials. The novel findings illustrate that electrostatic stabilization by coadsorbed ionic compounds can play a crucial role in understanding various experimental results and thus the colloidal synthesis of nanomaterials in general appears in a new light.