The Apparent Surface Free Energy of Rare Earth Oxides is Governed by Hydrocarbon Adsorption

The surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic, while others reporting hydrophobic. Quantitative studies exist supporting each conclusion, showing either intrinsic hydrophobicity, or transition from hydrophilicity to hydrophobicity due to adsorption of volatile organic compounds (VOCs). Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to VOC adsorption. We show that for indoor ambient atmospheres as well as well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water on CeO2, Er2O3 and Yb2O3 increase with exposure time. To study the free interface, we examined the surfaces comprehensively with X-ray photoelectron microscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings.