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 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 × 10⁵ 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.