Invited Paper SE-TuA4
Wettability Controlled Surface of Photocatalytic Coatings : Application for Dynamic hydrophoboicity

Tuesday, October 21, 2008, 2:40 pm, Room 204

Highly hydrophobic coating attracts much attention for long time as antifouling surface because it provides low energy surface whose interaction with contamination is smaller. Especially hydrophobic coating with photocatalytic oxidation capability is expected to become an ideal antifouling surface. According to Young, Dupre and Girifalco-Good equations, substances that can be highly hydrophobic are restricted to polymer material. Therefore it is usually hard to obtain highly hydrophobic thin film with TiO2 photocatalyst because polymer material is oxidized by photocatalysis. Moreover TiO2 photocatalyst itself is hydrophilicized by photo illumination. Due to such reasons, there are not many reports of hydrophobic coating with photocatalyst. However the situation can be dramatically changed when the concept of dynamic hydrophobicity is introduced. Parameter of dynamic hydrophobicity such like sliding speed or sliding angle does not depend on not only thermodynamic stable parameters but it strongly depends on surface microstructure. In fact, we have been investigating the dynamic hydrophobicity on self-assembled monolayer surfaces and found that the dynamic hydrophobicity is strongly affected by the surface roughness of only a few nanometers. Therefore if we focus on dynamic hydrophobicity, the inorganic substances can become potential candidates for having higher hydrophobicity by controlling surface roughness. Inorganic materials are hardly oxidized by TiO2 photocatalysis, and this is a great advantage. Of course even though a high flat surface is obtained, a water droplet cannot form on the surface and the liquid does not slide off as a droplet. Therefore, we have to choose an inorganic material with the lowest possible hydrophilicity. Although most of the inorganic materials exhibit a higher surface energy as compared to organic materials, it was reported that HfO2 is less hydrophilic. Therefore, we have prepared HfO2 combined with a TiO2 composite film and examined the water sliding angle of the surface. The result shows that the surface processed by the optimized preparation procedure shows a water contact angle of around 70° and a sliding angle of 30° are observed. This surface also shows high photocatalytic decomposability against IPA. Also very recently, we have obtained TiO2 with highly dynamic hydrophobic surface by physical deposition process. It shows sliding angle of 17 ° and at the same time show fairly well photocatalytic activity.