Sonochemical Surface Grafting Using Diazonium Salts

Surface modification via the attachment of organic molecules with specific end functionalities has attracted significant attention, driven by applications in chemical sensors and biosensors, electrocatalysis, and molecular electronics. Among the available approaches, electrochemical reduction of diazonium salts to form surface-bound organic thin films is one of the most widely used methods for chemically modifying metals, carbon materials, and semiconductors. Here, we report a nonelectrochemical approach to diazonium-derived thin-film formation, in which surfaces are immersed in a dilute diazonium solution (1 mM) containing traces of water and dimethyl sulfoxide and exposed to probe ultrasonication (400 W, 24 kHz). Ultrasonication induces cavitation, generating a transient reducing environment, primarily via hydrogen radicals formed upon bubble collapse, which enables diazonium reduction at the surface. Prior to reduction, interfacial electrostatics regulate the local concentration of the cationic diazonium precursor near the surface by promoting or hindering its pre-accumulation at the surface. For semiconducting Si-H, this electrostatic effect can be controlled by the doping type, with p-type and n-type Si producing opposite surface potentials and therefore different degrees of diazonium accumulation. Thus, sonication supplies the hydrogen radicals required for reduction, whereas substrate electrostatics determine precursor pre-concentration and thereby influence the extent of grafting. This method enables grafting of organic thin films across diverse substrates and establishes interfacial electrostatics as a promising control parameter for surface reactions involving charged species.