Letter pubs.acs.org/Langmuir
Photoinduced Underwater Superoleophobicity of TiO2 Thin Films Yusuke Sawai,† Shunsuke Nishimoto,† Yoshikazu Kameshima,† Eiji Fujii,‡ and Michihiro Miyake*,† †
Department of Material and Energy Science, Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima-Naka, Kita-ku, Okayama 700-8530, Japan ‡ Industrial Technology Center of Okayama Prefecture, 5301 Haga, Kita-ku, Okayama 701-1296, Japan S Supporting Information *
ABSTRACT: The photoinduced wettabilities of water, nhexadecane, dodecane, and n-heptane on a flat TiO2 surface prepared by a sol−gel method-based coating were investigated. An amphiphilic surface produced by UV irradiation exhibited underwater superoleophobicity with an extremely high static oil contact angle (CA) of over 160°. The TiO2 surface almost completely repelled the oil droplet in water. A robust TiO2 surface with no fragile nanomicrostructure was fabricated on a Ti mesh with a pore size of approximately 150 μm. The fabricated mesh was found to be applicable as an oil/water separation filter.
1. INTRODUCTION In recent years, superhydrophobic, superhydrophilic, and superoleophobic surfaces under ambient atmosphere, generally defined as exhibiting a water contact angle (CAwater/air) > 150°, a CAwater/air < 5°, and an oil contact angle (CAoil/air) > 150°, respectively, have been investigated for their useful properties such as self-cleaning and oil/water separation.1−4 In addition to these types of extreme wettabilities under ambient atmosphere, underwater superoleophobicity (i.e., a CAoil/water > 150°) has become a new research focus as an important wettability for solid surfaces, due to the potential widespread applications of such surfaces, including self-cleaning and antioil fouling surfaces, fluidic microchips, oil/water separation filters, etc.5−17 Through Young’s equation, the CAoil/water on a flat surface, θOW, can be described as the following: γ cos θOA − γWA cos θ WA cos θOW = OA γOW (1)
example, underwater superoleophobicity can be achieved on hierarchical macromolecule−nanoclay hydrogels21 or aligned ZnO photocatalyst nanorod arrays.22 However, such underwater superoleophobic surfaces are difficult to prepare or are very fragile. There are few reports on robust and stable underwater superoleophobic surfaces that are easy to prepare and suitable for future practical applications to self cleaning and antioil fouling surfaces or oil/water separation filters. Titanium dioxide (TiO2) is a well-known chemically stable, nontoxic, relatively inexpensive photocatalyst material.23 The TiO2 surface undergoes dramatic wettability conversion by UV irradiation to become superhydrophilic with a CAwater/air of less than 5°, due to photocatalytic oxidation activity and photoinduced superhydrophilicity.24 The highly hydrophilic TiO2 flat surface is easily prepared and exhibits both antifogging and selfcleaning properties with excellent durability, such that it has already been used for various industrial items such as automobile side view mirrors and exterior tiles, glass, plastic films, and tent materials.25 However, reports on the wettability on the TiO2 surface have essentially just been measurements of the liquid in air, with the exception of a limited report that studied the adhesion and removal of wood extractive component emulsions on the TiO2 surface.26 There have been no reports on the underwater superoleophobicity on the TiO2 surface. Accordingly, we report the photoinduced underwater−oil wettability of TiO2 surfaces prepared by sol− gel method-based TiO2 coating on a glass slide and by calicination of a Ti plate. We also investigate whether the TiO2 surface is applicable as an oil/water separation filter.
where γOA is the oil/air interface tension, θOA is the CAoil/air, γWA is the water/air interface tension, θWA is the CAwater/air, and γOW is the oil/water interface tension. Since the surface tension of oil is much lower than that of water, we can infer from eq 1 that the higher the hydrophilicity in air becomes, the higher the underwater oleophobicity becomes.18 In fact, SiO2 surfaces specially cleaned with UV−O3 treatment reportedly showed both superhydrophilicity in air and underwater superoleophobicity.19 Surface structure is another important parameter for underwater superoleophobicity because a porous surface can trap water at the interface between an oil droplet and the surface, resulting in the enhancement of the CAoil/water.5 Previously, some underwater superoleophobic surfaces have been reported on micro/nano hierarchical structures, such as fish scale surfaces and fish scale-inspired surfaces.20 For © 2013 American Chemical Society
Received: April 15, 2013 Revised: May 22, 2013 Published: May 23, 2013 6784
dx.doi.org/10.1021/la401382g | Langmuir 2013, 29, 6784−6789
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Letter
Figure 1. SEM (upper) and AFM (bottom) images of the samples.
Table 1. Contact Angles of the Samples before and After UV Irradiation sol−gel method-based sample
calcined Ti plate sample
contact angle
before UV irradiation
after UV irradiation
before UV irradiation
after UV irradiation
CAwater/air CAheptane/air CAdodecane/air CAhexadecane/air CAheptane/water CAdodecane/water CAhexadecane/water
35.1 ± 3.1°
