**1. Introduction**

590 Recent Advances in Nanofabrication Techniques and Applications

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Wettability is an important property governed by not only chemical composition, but also geometrical structure as well (Ichimura et al., 2000; Lai et al., 2009a; Wang et al., 1997). Super-wetting and antiwetting interfaces, such as superhydrophilic and superhydrophobic surfaces with special liquid-solid adhesion have recently attracted worldwide attention. Superhydrophilicity and superhydrophobicity are defined based on the conventional water contact angle experiment. If the contact angle is smaller than 5°, the surface is said to be superhydrophilic. Superhydrophobic refers to surface with contact angle greater than 150°. Such two extremely cases have attracted much interest due to their importance in both theoretical research and practical application (Lafuma & Quéré, 2003; Liu et al., 2010; Gao & Jiang, 2004).

In recent years, patterned thin films have received considerable attentions due to their interesting properties for a range of applications, such as optoelectronic devices, magnetic storage media, gas sensors, and fluidic systems. Compared to the conventional thin film technology, such as physical vapor deposition (Li et al., 2006; Zhang & Kalyanaraman, 2004), chemical vapor deposition (Jeon et al., 1996; Slocik et al., 2006) and sputtering (Rusponi et al., 1999), solution-based deposition method is becoming popular for the fabrication of patterning films due to the low temperature process under ambient environment, less energy and time consumption, and easier control of the experimental parameters (Lai et al., 2010a; Liu et al., 2007; Yoshimura & Gallage, 2008). Although traditional photolithographic technique is excellent for preparing sub-micrometer or even only sub-100-nanometer pattern (Cui & Veres, 2007; Li et al., 2009), it is a complex multi-step process (wafer cleaning; barrier layer formation; photoresist coating; soft-baking; mask alignment; exposure and development; and hard-baking) and needs to remove part of the film and all the photoresist used. Direct and selective assembly of nanostructured materials from precursors paves a new avenue for the fabrication of electronic optical microdevices.

Wetting micropatterns with different physical or chemical properties, without the need for ultra-precise positioning, have frequently been acted as templates for fabricating various functional materials in a large scale. The great difference in contact angle of the two extreme cases provides a potentially powerful and economical platform to directly and precisely construct patterned nanostructures in aqueous solution. In general, wetting micropatterns

Extremely Wetting Pattern by Photocatalytic Lithography and Its Application 593

the wetting template and functional composite nanostructure pattern (TiO2, ZnO, OCP and

Wettability of solid surfaces is a very important property of solid surface. Surfaces with extreme wetting properties, e.g. superhydrophilic and superhydrophobic, can be prepared by introducing certain rough structures on the originally "common" hydrophilic and hydrophobic surfaces. Various ways of preparing TiO2 semiconductor films on the different solid substrates have been developed, including sol-gel technique (Shen et al., 2005), sputtering (Takeda et al., 2001), chemical vapor deposition (Rausch & Burte, 1993), liquid phase deposition (Katagiri et al., 2007), hydrothermal (Yun et al., 2008), and electrochemical anodizing. Among them, the electrochemical anodizing is verified to be a convenient technique for fabricating nanostructured TiO2 films on titanium substrates (Lai et al., 2004, 2008b, 2009c; Gong et al., 2003). Moreover, the conductive titanium support substrate can be an advantage for fabricating functional material composites through electrochemical

Figure 1a shows a typical FESEM image of the titanium substrate before electrochemical anodization. The surface of the substrate was relatively smooth, with features of parallel polished ridges and grooves at the micron scale (Lai et al., 2010a). Figure 1b shows the top view SEM image of the typical TiO2 nanotube array film by anodizing under 20 V for 20 min. After anodization, shallow cavities as large as several micrometers in diameter were present on the surface of the sample. This is probably due to the anisotropic oxidation of the underlying Ti grains (Crawford & Chawla, 2009; Yasuda et al., 2007). From the high magnification image (Fig. 1c), it can be seen that vertically aligned TiO2 nanotubes with inner diameter of approximately 80 nm covered the entire surface including the shallow polygonal micropits. The side view image shows that the self-assembled layers of the TiO2 nanotubes were open at the top and closed at the bottom with thickness about 350 nm (inset of Fig. 1c). Water droplet can quickly spread and wet the as-grown vertically aligned TiO2 nanostructure film due to capillary effect caused by the rough porous structure, indicating such TiO2 structure film by electrochemical anodizing is superhydrophilic. A more hydrophobic behaviour, on the other hand, was obtained after coating the TiO2 film with fluoroalkyl silane. The inset of Fig. 1b shows the intrinsic contact angle (CA) on the asprepared vertically aligned TiO2 nanotube surface and its corresponding 1*H*,1*H*,2*H*,2*H*perfluorooctyltriethoxysilane (PTES, Degussa Co., Ltd.) modified surface is nearly 0o (superhydrophilic) and 156o (superhydrophobic), respectively. However, the CA for the "flat" TiO2 surface and its corresponding PTES modified sample is about 46o (hydrophilic) and 115o (hydrophobic), respectively. From these results, we know the top surface of the vertically aligned nanotubes has an amplification effect to make hydrophilic and hydrophobic surfaces become superhydrophilic and superhydrophobic, respectively. After UV irradiation for 30 min, the water CA on the TiO2 nanotube film and "flat" TiO2 film decreased to 0o and 26o, as a consequence of the photocatalytic activity of TiO2 films (Balaur et al., 2005; Lai et al., 2010a). Moreover, the sample showed hydrophobic character once again when it was treated with PTES. Therefore the surface can be reversibly switched between superhydrophobic and superhydrophilic by alternating SAM and UV photocatalysis on the rough TiO2 nanotube arrays (shown in Fig. 1d). Compared with the large wettability contrast on this type of rough surface (larger than 150o), the wettability of a "flat" TiO2 film can only be reversibly changed within the small range between 26o and 115o.

**2. Wettability on TiO2 nanostructures by electrochemical anodization** 

depositions to further improve their photoelectrochemical activities.

CdS) (Lai et al., 2009b, 2010a-d).

with low contact angle contrast (≤120o) on smooth substrates can be formed by photolithography (Falconnet et al., 2004; Kobayashi et al., 2011), microcontact lithography (Csucs et al., 2003; Kumar et al., 1992), colloidal patterning (Michel et al., 2002; Bhawalkar et al., 2010), electron beam lithography (Wang & Lieberman, 2003; Zhang et al., 2007), nanoimprint lithography (Jiao et al., 2005; Zhang et al., 2006), dip-pen nanolithography (Huang et al., 2010a; Lee et al., 2006; Xu & Liu, 1997), and so on. Among these methods, photocatalytic lithography employing semiconductors to photocatalytic decompose of organic monolayer is one of the most practical techniques because it able to accurately transfer an entire photomask pattern to a target substrate at a single exposure time under environmental condition (Bearinger et al., 2009; Lee & Sung, 2004; Nakata et al., 2010; Tatsuma et al., 2002; Wang et al., 2011). Moreover, it can greatly reduce the photoresist waste. The resolution of the patterning is greatly dependant on the mask alignment and light source exposure. Under optimal condition, a resolution of micrometer- or submicrometer-scale pattern of alkylsiloxane self-assembled monolayers can be achieved with UV light projection irradiation. Once patterned on the surface, organic monolayer has been applied in various ways to restrict corrosion or induce nanostructures growth. Firstly, patterned layer itself may serve as etching mask to protect the substrate to generate pattern with certain thickness/aspect ratio. Secondly, patterned organic layer may be employed as barrier to inhibit the liquid phase deposition of nanostructures to generate functional composite pattern with diverse shape and density. So far, only a few reports have been available on the fabrication and application of superhydrophilic–superhydrophobic patterning by photocatalytic lithography under ambient conditions (Lai et al., 2008a; Nishimoto et al., 2009; Zhang et al., 2007).

Upon UV irradiation, the electron-hole pairs in semiconductor TiO2 can be generated and migrated to its surface, where the hole reacts with OH- or adsorbed water to produce highly reactive hydroxyl radicals (Zhao et al., 1998). These hydroxyl radicals can further oxidize and decompose most organic compounds. Recently, we found that the pollutant solution can be rapidly decomposed on a nanotube array TiO2 film with UV irradiation (Lai et al., 2006, 2010b; Zhuang et al., 2007). Considering its effectiveness for the photocatalytic decomposition of organic compounds, the photocatalysis of such TiO2 nanotube film can be a promising way to decompose the low energy hydrophobic fluoroalkyl chains. So it is possible to achieve a conversion from superhydrophobicity to superhydrophilicity due to the amplification effect of the rough aligned nanotube structure. By using a patterned photomask to control the site-selective decomposition by UV light, that is photocatalytic lithography, superhydrophilic cells can be accurately transferred to a target substrate at a single exposure time under environmental condition. Therefore, these two types of extreme wettability coexist on the surface directly to make up of superhydrophilicsuperhydrophobic pattern.

In this chapter, we firstly discuss the wettability on TiO2 nanostructure film by electrochemical anodization. Secondly, we demonstrate using a novel synthetic process to prepare wetting pattern with a high contrast (superhydrophilic–superhydrophobic) on TiO2 nanotube structured film by a combination of SAM technique and photocatalytic lithography. The resultant micropattern has been characterized with scanning electron microscopy, optical microscopy, electron probe microanalyzer and X-ray photoelectron spectroscopy. Finally, we focus on the technological details and potential future application of wetting template to induce and direct the assembly of functional nanostructure to form uniform micropatterns. For example, the patterning, biomedical and sensing application of

with low contact angle contrast (≤120o) on smooth substrates can be formed by photolithography (Falconnet et al., 2004; Kobayashi et al., 2011), microcontact lithography (Csucs et al., 2003; Kumar et al., 1992), colloidal patterning (Michel et al., 2002; Bhawalkar et al., 2010), electron beam lithography (Wang & Lieberman, 2003; Zhang et al., 2007), nanoimprint lithography (Jiao et al., 2005; Zhang et al., 2006), dip-pen nanolithography (Huang et al., 2010a; Lee et al., 2006; Xu & Liu, 1997), and so on. Among these methods, photocatalytic lithography employing semiconductors to photocatalytic decompose of organic monolayer is one of the most practical techniques because it able to accurately transfer an entire photomask pattern to a target substrate at a single exposure time under environmental condition (Bearinger et al., 2009; Lee & Sung, 2004; Nakata et al., 2010; Tatsuma et al., 2002; Wang et al., 2011). Moreover, it can greatly reduce the photoresist waste. The resolution of the patterning is greatly dependant on the mask alignment and light source exposure. Under optimal condition, a resolution of micrometer- or submicrometer-scale pattern of alkylsiloxane self-assembled monolayers can be achieved with UV light projection irradiation. Once patterned on the surface, organic monolayer has been applied in various ways to restrict corrosion or induce nanostructures growth. Firstly, patterned layer itself may serve as etching mask to protect the substrate to generate pattern with certain thickness/aspect ratio. Secondly, patterned organic layer may be employed as barrier to inhibit the liquid phase deposition of nanostructures to generate functional composite pattern with diverse shape and density. So far, only a few reports have been available on the fabrication and application of superhydrophilic–superhydrophobic patterning by photocatalytic lithography under ambient conditions (Lai et al., 2008a;

Upon UV irradiation, the electron-hole pairs in semiconductor TiO2 can be generated and migrated to its surface, where the hole reacts with OH- or adsorbed water to produce highly reactive hydroxyl radicals (Zhao et al., 1998). These hydroxyl radicals can further oxidize and decompose most organic compounds. Recently, we found that the pollutant solution can be rapidly decomposed on a nanotube array TiO2 film with UV irradiation (Lai et al., 2006, 2010b; Zhuang et al., 2007). Considering its effectiveness for the photocatalytic decomposition of organic compounds, the photocatalysis of such TiO2 nanotube film can be a promising way to decompose the low energy hydrophobic fluoroalkyl chains. So it is possible to achieve a conversion from superhydrophobicity to superhydrophilicity due to the amplification effect of the rough aligned nanotube structure. By using a patterned photomask to control the site-selective decomposition by UV light, that is photocatalytic lithography, superhydrophilic cells can be accurately transferred to a target substrate at a single exposure time under environmental condition. Therefore, these two types of extreme wettability coexist on the surface directly to make up of superhydrophilic-

In this chapter, we firstly discuss the wettability on TiO2 nanostructure film by electrochemical anodization. Secondly, we demonstrate using a novel synthetic process to prepare wetting pattern with a high contrast (superhydrophilic–superhydrophobic) on TiO2 nanotube structured film by a combination of SAM technique and photocatalytic lithography. The resultant micropattern has been characterized with scanning electron microscopy, optical microscopy, electron probe microanalyzer and X-ray photoelectron spectroscopy. Finally, we focus on the technological details and potential future application of wetting template to induce and direct the assembly of functional nanostructure to form uniform micropatterns. For example, the patterning, biomedical and sensing application of

Nishimoto et al., 2009; Zhang et al., 2007).

superhydrophobic pattern.

the wetting template and functional composite nanostructure pattern (TiO2, ZnO, OCP and CdS) (Lai et al., 2009b, 2010a-d).
