Low-Temperature Sintering of L-Alanine-Functionalized Metallic Copper Particles Affording Conductive Films with Excellent Oxidative Stability

Here, the alpha amino acid L-alanine is employed as both a capping and stabilizing agent in the aqueous synthesis of submicron-sized metallic copper particles under ambient atmospheric conditions. The reduction of the copper(II) precursor is achieved using L-ascorbic acid (vitamin C) as the reducing agent. The nature of the complex formed between L-alanine and the copper(II) precursor, pH of the medium, temperature, and the relative proportion of capping agent are found to play a significant role in determining the size, shape, and oxidative stability of the resulting particles. The adsorbed L-alanine is shown to act as a barrier imparting excellent thermal stability to capped copper particles, delaying the onset of temperature-induced aerial oxidation. The stability of the particles is complemented by highly favorable sintering conditions, rendering the formation of conductive copper films at significantly lower temperatures (T <= 120 degrees C) compared to alternative preparation methods. The resulting copper films are well-passivated by residual surface L-alanine molecules, promoting long-term stability without hindering the surface chemistry of the copper film as evidenced by the catalytic activity. Contrary to the popular belief that ligands with long carbon chains are best for providing stability, these findings demonstrate that very small ligands can provide highly effective stability to copper without significantly deteriorating its functionality while facilitating low-temperature sintering, which is a key requirement for emerging flexible electronic applications.