Increase of Polymerization Yield on Titania by Surface Reduction

The surface-assisted Ullmann reaction, widely employed in the synthesis of covalent organic networks, molecular spin chains, and graphene nanoribbons, has been extensively studied on noble metal surfaces, being widely accepted that the metal surface acts as a catalyst during the reaction. However, the development of routes to obtain covalent molecular systems on nonmetallic substrates represents a key technological advancement. Recent experiments demonstrate molecular coupling reactions on transition metal oxide surfaces, suggesting alternative reaction mechanisms, because the high oxidization state of metal atoms forming the substrate impairs the catalytic activity. Here, we use synchrotron-based photoelectron spectroscopy to follow the temperature dependence of the polymerization of a reference precursor (4,4-dibromo-p-terphenyl precursor) on reduced and nearly stoichiometric rutile TiO2(110) surfaces. Throughout the reaction, we carefully monitor the concentration of precursor molecules, polymers, and surface defects. We find that while some polymerization takes place regardless of the amount of surface reduction, the reaction yield increases on highly reduced TiO2(110) substrates. The spectroscopic lines of evidence concurrently indicate that the polymerization is driven by the surface segregation of interstitial Ti atoms, whose diffusion is favored in reduced crystals.