Retardation of superhydrophobicity for the air plasma sprayed ceria coating surface

Abstract

Ceria has become a potential candidate for the preparation of superhydrophobic surfaces due to the apparent intrinsic hydrophobicity reported in recent years. In the present study, a CeO2 coating with bi-scale structures imitating lotus leaf surface was deposited by air plasma spraying (APS). It was found that the wettability of the coating surface exhibited a special delayed phenomenon, that the surface needed to be stored in the atmosphere for a period of time before it could change from superhydrophilicity in the fresh as-sprayed state to superhydrophobicity. To elucidate the mechanism, the research focused on analyzing the changes in the chemical composition of the surface. The results showed that adsorption effect was the main factor dominated the surface wettability. During atmospheric storage, the coating surface spontaneously adsorbed low surface free energy substances, mainly carboxylates containing alkane chain and alkanes, resulting in the transition of wettability. The bombardment of argon plasma jet to ceria during APS led to the valence state change of Ce atoms, resulting in the generation of coordinatively unsaturated metal cations acted as active adsorption sites, which was the reason for the spontaneous adsorption. Due to the restrictions on the location of the active sites and the limited concentration of airborne adsorbates, the spontaneous adsorption proceeded slowly, which led to the retardation of superhydrophobicity for the coating surface. The study laid a theoretical foundation for the preparation of CeO2 superhydrophobic coatings by APS, and provided further insights into the underlying mechanism of the apparent hydrophobicity of CeO2.