“Clickable” Metal-Oxide Nanomaterials Surface-Engineered by Gas-Phase Covalent Functionalization with Prop-2-ynoic Acid

The surface properties of metal-oxide nanomaterials (MONMs) can be tuned by reacting them with a variety of organic compounds. Reactions of the oxides with functionalized carboxylic (R–COOH) or phosphonic acids (R–PO(OH)2) and various approaches utilizing silylation (such as with R–Si(X)3, where the X could be Cl or −OCH3) are often used to deliver the target modifier R to the oxide surface. However, the liquid-phase reactions between metal oxides and these agents often cause agglomeration or multilayer growth, morphology change, or surface etching. In this paper, we report a novel approach that circumvents all of these problems. The proposed two-step functionalization approach utilizes exposure of the oxide materials to prop-2-ynoic acid (HC≡C–COOH, propiolic acid) in the gas phase as a first step. The second step can then utilize the created C≡C for post-modification that introduces any predesigned functionality to the surface via “click” chemistry with azides (R–N3). Gas-phase exposure of prop-2-ynoic acid was carried out onto surfaces of different MONMs (ZnO nanorods, CuO nanowires, TiO2 nanoparticles, and CeO2 nanoparticles) under medium vacuum (×10–2 Torr), and this step was demonstrated to preserve the nanostructure of all the materials studied. The surface modification with “click” chemistry was tested via Cu(I)-catalyzed reaction of benzyl azide, with an added bonus of the CuO materials not requiring the presence of the copper catalyst. The combination of microscopic and spectroscopic investigations including scanning electron microscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance spectroscopy was used to follow the process and to compare with the traditional liquid-phase modification schemes.