Hydrophilic Sparse Ionic Monolayer‐Protected Metal Nanoparticles: Highly Concentrated Nano‐Au and Nano‐Ag “Inks” that can be Sintered to Near‐Bulk Conductivity at 150 °C

Abstract

Abstract Here, monolayer‐protected gold and silver nanoparticles with extremely high solvent dispersibility (over 200 mg mL −1 in water and glycols) and low coalescence temperature (approximately 150 °C, measured by the percolation transition temperature T p ) are developed, to reach conductivities better than 1 × 10 5 S cm −1 . These materials are suitable for inkjet and other forms of printing on substrates that may be solvent‐ and/or temperature‐sensitive, such as for plastic electronics, and as bus lines for solar and lighting panels. This is achieved using a new concept of the sparse ionic protection monolayer. The metal nanoparticles are initially protected by a two‐component mixed ligand shell comprising an ω‐functionalized ionic ligand and a labile ligand. These are selectively desorbed to give a sparse shell of the ω‐ionic ligands of ca. 25% coverage. Through a systematic study of different monolayer‐protected Au nanoparticles using FTIR spectroscopy, supported by XPS and DSC, it is shown that T p is not determined by thermodynamic size melting or by surface area effects, as previously thought, but by the temperature when ca. 80% of the dense monolayer is eliminated. Therefore, T p depends on the thermal stability and packing density of the shell, rather than the size of the metal core, while the solubility characteristics depend strongly on the exposed terminal group.