N-Heterocyclic Carbene Monolayers on Metal-Oxide Films: Correlations between Adsorption Mode and Surface Functionality

Abstract

N-Heterocyclic carbene (NHC) ligands have been self-assembled on various metal and semimetal surfaces, creating a covalent bond with surface metal atoms that led to high thermal and chemical stability of the self-assembled monolayer. This study explores the self-assembly of NHCs on metal-oxide films (CuO<i>_x</i>, FeO<i>_x</i>, and TiO<i>_x</i>) and reveals that the properties of these metal-oxide substrates play a pivotal role in dictating the adsorption behavior of NHCs, influencing the decomposition route of the monolayer and its impact on work function values. While the attachment of NHCs onto CuO<i>_x</i> is via coordination with surface oxygen atoms, NHCs interact with TiO<i>_x</i> through coordination with surface metal atoms and with FeO<i>_x</i> via coordination with both metal and oxygen surface atoms. These distinct binding modes arise due to variances in the electronic properties of the metal atoms within the investigated metal-oxide films. Contact angle and ultraviolet photoelectron spectroscopy measurements have shown a significantly higher impact of F-NHC adsorption on CuO<i>_x</i> than on TiO<i>_x</i> and FeO<i>_x</i> , correlated to a preferred, averaged upright orientation of F-NHC on CuO<i>_x</i>.