Assessment of Soft Ligand Removal Strategies: Alkylation as a Promising Alternative to High-Temperature Treatments for Colloidal Nanoparticle Surfaces

Advances in nanocrystal syntheses over the past decades now enable routine manipulation of particle shape, size, and chemistry with unprecedented precision. Building on decades of knowledge in colloidal chemistry, researchers have created notable high-performance, low-cost energy conversion catalysts. However, in almost all cases, controlling the surface chemistry—in particular, removing the surfactants used during the syntheses—is a challenge. Aggressive physical treatments, such as thermal pyrolysis and plasma etching, have become the methods of choice, but these processes test the material’s phase and chemical stability. In this work, we analyze softer methods to postsynthetically remove surfactants using chemical treatments. We performed various surface treatments to deprotect colloidal nanocrystals of a well-known electrocatalyst, manganese oxide (Mn3O4), passivated with hydrophobic ligands. We employed a suite of characterization techniques (transmission electron microscopy, infrared spectroscopy, voltammetry, and thermogravimetric analysis) to evaluate their surface chemistry. Although the four studied treatments (bases, alkylating agents, Lewis acids, or heat) could remove nearly all organic ligands, the surface chemistries of the treated nanomaterials differed significantly from treatment to treatment. These chemical differences arise from the reactions between the treatment agent and the surfactant or from side reactions of the agent with atmospheric water or carbon dioxide, leaving behind residues, such as hydrolyzed ligand removal agent, metal hydroxide, and metal carbonate, on the surface. From our study, we find that treatment using alkylating agents is the most promising approach for soft room-temperature stripping of nanocrystal ligands, producing Mn3O4 with the highest electrochemical activity.