Self-Healing Superhydrophobic CeO2 Coating Prepared By Anodic Deposition
Authors: Nakayama Katsutoshi , Hiraga Takuya , Zhu Chunyu , Tsuji Etsushi , Aoki Yoshitaka , Habazaki Hiroki 
Published: February 2020

Superhydrophobic surfaces, in which water contact angle is >150o and the contact angle hysteresis is less than 10°, have attracted much attention, because of their superior properties including self-cleaning, anti-biofouling, corrosion resistance, icephobicity, and oil/water separation. In order to obtain superhydrophobicity, highly rough surfaces have been coated with an organic layer with a low surface energy. In the last decades a lot of studies have been conducted on the fabrication and application of superhydrophobic surfaces, but the superhydrophobic surfaces have not been used practically, because of poor durability of superhydrophobicity. Thus, there is a strong demand for the preparation of durable superhydrophobic surfaces.
Recently, it was reported that the surface of rare-earth oxides (REOs) prepared by a magnetron sputtering method exhibits hydrophobicity even without organic coating [1]. If an organic thin coating on the oxide surface is not necessary for superhydrophobicity, it can be expected that the robustness of superhydrophobic surface is enhanced and more durable superhydrophobicity is obtained. In the present study, we have prepared a CeO2 coating on Type 304 stainless steel by an anodic deposition method, which is more suitable for practical application rather than PVD techniques.
Prior to the anodic deposition, Type 304 stainless steel plates was electropolished in a mixed solution of HClO4 and ethylene glycol at 20 V below 10°C or roughened by electrochemical etching in solution containing 1.2 wt% HNO3 and 3.6 wt% HCl at a constant current density of 10 kA m-2 or 100 A m-2 for 400 s or 60 s at 313 K, respectively. The electrochemical etching was also conducted on Type 304 stainless steel mesh. The anodic deposition was conducted at a constant current density of 10 A m-2 in solution containing 0.01 mol dm-3 Ce(NO3)3 and 0.05 mol dm-3 hexamethylenetetramine for 1 h at 333 K. After deposition, the surface wettability was evaluated by static contact angle measurement for water (4 μL). The measurements were performed after exposure to laboratory air for various periods of time. The surface and cross-sectional morphology were observed by FE-SEM and TEM, respectively. Surface composition were evaluated XRD, XPS and GDOES.
SEM observation of the surface of the CeO2 coating on the flat stainless steel plate revealed that the coating consisted of densely packed CeO2 nanoparticles (10-15 nm in diameter). It was also found from TEM observation that the thickness of the CeO2 layer was approximately 60 nm. The coating consists of a crystalline CeO2 single phase and the species derived from substrate (Fe, Cr and Ni) were practically absent, as confirmed by XRD and GDOES, respectively.
The water contact angle of the coating on the flat stainless steel plate was only 20° immediately after deposition. This is contrast to the hydrophobicity of magnetron-sputtered CeO2 surface. However, the water contact angle increased gradually with time of air exposure, and reached approximately 105o after three days. This means CeO2 surface changed from hydrophilic to hydrophobic. In contrast, water contact angle on anodized aluminum surface remained hydrophilic even after three days. XPS surface analysis revealed that this wetting behavior was due to adsorption of contaminant hydrocarbon species on the CeO2 surface from air.
In order to enhance the water repellency, surface roughness was introduced on stainless steel by electrochemical etching. The etching in a mixed solution of HNO3 and HCl produced a number of etch pits; the sized of the pits extended to several tens micrometers at 10 kA m-2. The CeO2 coating on the etched stainless steel was also hydrophilic initially, but the water contact angle increased to ~130° after air exposure. The superhydrophobic surface, which showed the static contact angle >150° and contact angle hysteresis of less than 10°, was obtained when CeO2 was coated on the etched stainless steel mesh.
The superhydrophobicity of the present CeO2 coating on the etched stainless steel mesh was associated with the formation of contaminant hydrocarbon layer. This means that even if the hydrocarbon layer is removed, the layer will be regenerated and the superhydrophilicity will be healed. In order to examine the self-healing property, the CeO2 coating on the etched stainless steel mesh was treated with O2 plasma and exposed in air. We found that the water contact angle reduced to ~0° after O2 plasma treatment, but recovered to ~160° after air exposure. This self-healing was repeated at least four times. Thus, the CeO2 coated stainless steel is promising as a self-healing superhydrophibic material.
Reference
[1] G. Azimi, R. Ghiman, H. Kwon, A. T. Paxson and K. K. Varanasi, Nat. Mater.
12, 315 (2013).

Figure 1