**4. Application of wetting pattern**

598 Recent Advances in Nanofabrication Techniques and Applications

adjacent PTES molecule covered by the Cu grid can be remotely oxidized by a TiO2 nanotube photocatalyst or the diffusion, scattering, and diffraction of the incident light (Haick & Paz, 2001; Kubo et al., 2004). Therefore, to obtain a higher pattern resolution, the

optimized UV irradiated time in our case was controlled in the range of 10-30 min.

Fig. 5. (a) Optical micrograph of the as-obtained superhydrophilic-superhydrophobic pattern by focusing on the water droplet within the superhydrophilic regions. (b) Fluorescence microscopy image of the fluorescein probes on the as-prepared

Fig. 6. Fluorescence microscope images generated by blue light excitation on the different micropatterned templates with the adsorption of FITC-BSA, (a-1, a-2, a-3) the PTES template with a pH value of 7.5, 4.5 and 2.5, respectively; (b-1, b-2, b-3) the PTES-APTS template with

Based on the molecular self-assembly and photocatalytic lithography techniques, micropatterned templates of PTES or PTES-APTS(3-aminopropyltriethoxysilanes) with

superhydrophilic-superhydrophobic micropattern.

a pH value of 7.5, 4.5 and 2.5, respectively.

Uniform self-assembly of functional inorganic nanomaterials is a fundamental challenge. Nature adopts a superior approach in biomineralization, where "matrix" macromolecules induce nucleation of inorganic crystals at specific locations with controlled size and morphology, and sometimes even with defined growth orientation. We apply the biomimetic principles derived from liquid phase processes to the assembly of nanoscale functional materials into microscale systems. We carefully control surface wettability to promote etching or heterogeneous nucleation at designated superhydrophilic regions while completely suppress these processes elsewhere (superhydrophobic regions), therefore enable the controlled top-down or bottom-up assembly of inorganic nanomaterials directly from solution. Following this principle, arrays of crystalline TiO2 nanotube, ZnO nanorods, CdS semiconductor materials and octacalcium phosphate (OCP) biomaterials were nucleated and assembled directly from solution onto Ti substrates at the desired precise locations and then fabricated into arrays of photodetector or matrix devices for large-area microelectronic applications. This strategy of micropatterned nanocomposites will be helpful to develop various micropatterned functional nanostructured materials.

## **4.1 Template for preparing functional pattern**

Figure 7 shows an optical microscopy image of the TiO2 nanotube micropattern produced using a grid micropattern with different wet etching times (Lai et al., 2009b). A patterning with a clear outline was formed in a short time for 30 s (Fig. 7a). With an increase in the wet etching time (Fig. 7b and c), identical micropatterns with higher aspect ratios can be fabricated. When the etching was prolonged to 240 s (Fig. 7d), the size of the grids increased slightly, indicating that the PTES-SAM layer at the edge of the superhydrophobic lines is more easily etched as compared to the inner superhydrophobic area. This is due to the loose and disordered SAMs resulting from the scattered UV light photocatalytic degradation and the transfer of the active hydroxyl radicals at the edge of the grids. Moreover, the isotropic etching of the Ti substrate underneath the boundary leads to the collapse of the upper nanotube array structures. Therefore, the pattern can be obtained with a clear boundary in a short time after the wet etching in the aqueous solution, and the depth of the etching can be controlled simply by adjusting the etching time.

Extremely Wetting Pattern by Photocatalytic Lithography and Its Application 601

however, they can be easily removed by sonication. The vertical aligned TiO2 nanotube film with a length of about 1.53 µm shows no obvious change within the superhydrophobic regions (Fig. 8c), while the TiO2 nanotubes in the superhydrophilic regions are etched completely for 20 min (Fig. 8d), that is to say, a TiO2 micropattern with a high aspect ratio of ~20 can be obtained. This technique is particularly attractive in generating large-area functional nanostructure patterns in a high throughput fashion with a high aspect-ratio.

Fig. 9. SEM images of the patterned ZnO nanostructured micropatterns by liquid phase

nanorods, which were confined and grown within the superhydrophilic regions.

For practical application of thin film devices, the position and orientation-control of ZnO nanorods are very important because they directly relate to their physical and chemical performances (Koumoto et al., 2008; Masuda et al., 2006). In order to precisely control the spatial orientation of the ZnO nanostructures, we developed a new technique which is able to make the ZnO nanorods grow along the vertically aligned titania nanotubes rather by disordered deposition on the titania nanotube array surface. In this technique, resistance discrepancy was adopted to make the entrance of the tubes more conductive than the bottom of the tubes, to induce the epitaxial growth with spatial organization of uniform

Figure 9 shows representative top-view FESEM images of the ZnO/TiO2 micropatterns by liquid-phase deposition after different times. After growth for 30 min (Fig. 9a,b), the nucleation and growth of ZnO crystals with various morphologies and sizes (nanoparticles and nanorods) are sparsely dispersed within the predefined superhydrophilic regions. In the superhydrophobic regions, the nanotube structure is retained with its original morphology due to the indirect contact with the solution and the effective protection by the PTES monolayer. Upon further increase in the deposition time to 90 min (Fig. 9c,d), it was observed that ZnO nanorods were the predominant structural features. The average diameter of the grown ZnO nanorods increases greatly to about 400–800 nm in diameter and 3-5 µm in length, which may be due to lower nucleation rate and higher growing space for ZnO nanorods. The superhydrophilic microdots are almost covered with randomly lying ZnO nanorods. Moreover, there are different growth orientations and some connections into adjacent nanorods. It is of interest to note that a two dimensional (2D) pattern with smaller density of randomly packed ZnO nanorods, instead of the 3D pattern consisted of well-aligned vertical

deposition at 90oC for different times: (a, b) 30 min; (c, d) 90 min.

Fig. 7. Optical micrograph images of the time-resolved evolution process of the resulting vertical aligned TiO2 nanotube micropattern on a superhydrophilic-superhydrophobic template by wet chemical etching in 0.1 wt% HF solution: (a) 30, (b) 60, (c) 120, and (d) 240 s.

Fig. 8. (a) The SEM micrograph of the ordered vertical aligned TiO2 nanotube array pattern by vapor-condense etching for 10 min. Magnified images of the corresponding (b) superhydrophilic-superhydrophobic boundary, (c) superhydrophobic area, and (d) superhydrophilic area. The inset figure shows the corresponding cross-sectional image.

The micropattern with a higher aspect-ratio can also be fabricated by a developed vapor etching technique. As water evaporated, the vapor containing HF dewetted the superhydrophobic lines while condensing on the superhydrophilic grids to etch the vertical aligned TiO2 nanotube layer. Figure 8a and b shows the as-prepared TiO2 micropattern with a clear boundary by water vapor etching for 10 min. This resulted from the selective condensing of the vapor from the water solution containing 5 wt% HF in the superhydrophilic regions. Some collapsed residue (indicated by the black arrow) covers the boundary due to the rapid etching of the bottom nanotubes and the resultant bubbles;

Fig. 7. Optical micrograph images of the time-resolved evolution process of the resulting vertical aligned TiO2 nanotube micropattern on a superhydrophilic-superhydrophobic template by wet chemical etching in 0.1 wt% HF solution: (a) 30, (b) 60, (c) 120, and (d) 240 s.

Fig. 8. (a) The SEM micrograph of the ordered vertical aligned TiO2 nanotube array pattern

The micropattern with a higher aspect-ratio can also be fabricated by a developed vapor etching technique. As water evaporated, the vapor containing HF dewetted the superhydrophobic lines while condensing on the superhydrophilic grids to etch the vertical aligned TiO2 nanotube layer. Figure 8a and b shows the as-prepared TiO2 micropattern with a clear boundary by water vapor etching for 10 min. This resulted from the selective condensing of the vapor from the water solution containing 5 wt% HF in the superhydrophilic regions. Some collapsed residue (indicated by the black arrow) covers the boundary due to the rapid etching of the bottom nanotubes and the resultant bubbles;

by vapor-condense etching for 10 min. Magnified images of the corresponding (b) superhydrophilic-superhydrophobic boundary, (c) superhydrophobic area, and (d) superhydrophilic area. The inset figure shows the corresponding cross-sectional image. however, they can be easily removed by sonication. The vertical aligned TiO2 nanotube film with a length of about 1.53 µm shows no obvious change within the superhydrophobic regions (Fig. 8c), while the TiO2 nanotubes in the superhydrophilic regions are etched completely for 20 min (Fig. 8d), that is to say, a TiO2 micropattern with a high aspect ratio of ~20 can be obtained. This technique is particularly attractive in generating large-area functional nanostructure patterns in a high throughput fashion with a high aspect-ratio.

Fig. 9. SEM images of the patterned ZnO nanostructured micropatterns by liquid phase deposition at 90oC for different times: (a, b) 30 min; (c, d) 90 min.

Figure 9 shows representative top-view FESEM images of the ZnO/TiO2 micropatterns by liquid-phase deposition after different times. After growth for 30 min (Fig. 9a,b), the nucleation and growth of ZnO crystals with various morphologies and sizes (nanoparticles and nanorods) are sparsely dispersed within the predefined superhydrophilic regions. In the superhydrophobic regions, the nanotube structure is retained with its original morphology due to the indirect contact with the solution and the effective protection by the PTES monolayer. Upon further increase in the deposition time to 90 min (Fig. 9c,d), it was observed that ZnO nanorods were the predominant structural features. The average diameter of the grown ZnO nanorods increases greatly to about 400–800 nm in diameter and 3-5 µm in length, which may be due to lower nucleation rate and higher growing space for ZnO nanorods. The superhydrophilic microdots are almost covered with randomly lying ZnO nanorods. Moreover, there are different growth orientations and some connections into adjacent nanorods. It is of interest to note that a two dimensional (2D) pattern with smaller density of randomly packed ZnO nanorods, instead of the 3D pattern consisted of well-aligned vertical nanorods, which were confined and grown within the superhydrophilic regions.

For practical application of thin film devices, the position and orientation-control of ZnO nanorods are very important because they directly relate to their physical and chemical performances (Koumoto et al., 2008; Masuda et al., 2006). In order to precisely control the spatial orientation of the ZnO nanostructures, we developed a new technique which is able to make the ZnO nanorods grow along the vertically aligned titania nanotubes rather by disordered deposition on the titania nanotube array surface. In this technique, resistance discrepancy was adopted to make the entrance of the tubes more conductive than the bottom of the tubes, to induce the epitaxial growth with spatial organization of uniform

Extremely Wetting Pattern by Photocatalytic Lithography and Its Application 603

The typical SEM image of TiO2 nanotube array surface before and after the deposition of OCP film by electrochemical technique for 5 min is shown in Fig. 11a,b. The superhydrophobic-superhydrophilic micropatterned TiO2 was used as a micro-template to selectively deposit nano-OCP crystals on the superhydrophilic regions by an electrochemical deposition to form a special micropatterned nano-OCP. The deposition electrolyte was consisted of 0.042 mol/L Ca(NO3)2 and 0.025 mol/L NH4H2PO4. The pH value was adjusted to approximately 4.2 with 0.05 mol/L NaOH solution. The precipitation was carried out galvanostatically at a cathodic current of 0.5 mA cm-2 under 67.5oC for a certain time (Wang et al., 2008). It can see that quasi-perpendicular ribbon-like crystals of several hundred nanometers in width are uniformly grown on the TiO2 nanotube array surface. Fig. 11c shows a typical fluorescence microscope of the superhydrophilic-superhydrophobic micropattern on TiO2 nanotube surface. As can be seen, the green dot patterns are clearly imaged through the fluorescence contrast between the UV-irradiated superhydrophilic and photomasked superhydrophobic regions. The photoirradiated dot exhibiting a uniformly stronger fluorescence against the surrounding dark background is due to the highly affinity to solution resulting in the absorption of fluorescent probes into the irradiated nanotube array films. Therefore, a clear well-defined fluorescence pattern based on the superhydrophobic-superhydrophilic pattern is obtained. Fig. 11d displays the identical patterning of OCP biomaterials deposited on the superhydrophilic-superhydrophobic patterns on TiO2 nanotube array surface. It is obvious that the size of the white OCP dots is equal to that of the superhydrophilic area on template, indicating the deposited regions were only located within the superhydrophilic dots where photocatalytic degradation of

Fig. 11. SEM images of the (a) TiO2 nanotube array film fabricated by electrochemical anodization; (b) OCP nanostructure layer on TiO2 nanotube array film by electrochemical

superhydrophobic template (c) and patterned OCP thin films selectively deposited in pre-

deposition for 5 min. Optical fluorescence pattern of the superhydrophilic-

defined superhydrophilic regions by electrochemical deposition for 5 min (d).

**4.2 Biomedical arrays** 

PTES-SAMs was performed.

ZnO nanorods along the direction of the nanotubes. The quasi-perpendicular ZnO nanorods nucleate and grow uniformly and selectively throughout the superhydrophilic regions of the TiO2 nanotube surface by electric field assisted deposition at 90oC for 3 min, while no nanorods are observed in the superhydrophobic regions (Fig. 10a,b) (Lai et al., 2010a). The EDS spectra also reveal that the presence of Zn, Ti and O elements on the superhydrophilic regions, while the elemental components in the superhydrophobic areas are only Ti and O. The inset of Fig. 10b shows the hexagonal end facet of a vertically aligned ZnO nanorod with a diameter about 100–150 nm growing on top of the TiO2 nanotube array surface. Therefore, the density, size and orientation of ZnO nanorods are very sensitive to the presence of electric fields. A 3D AFM profile image (Fig. 10c) shows that the microscopic structure of the ZnO crystal deposition consisted in dense column arrays, which are induced and directed by the wettability template. The thickness of vertical ZnO nanorod film is in the range of 800–900 nm. Furthermore, the three dimensional confocal microscopy image (Fig. 10d) also shows that the growth of the ZnO nanorod pattern is identical with the superhydrophilic/superhydrophobic template.

On the basis of the versatile superhydrophilic-superhydrophobic template, we can successfully control the growth of ZnO nanostructures in the superhydrophilic regions under mild reaction conditions and in the absence of seed and noble metal catalyst. In the superhydrophobic regions, the growth is suppressed. This special template can be utilized to generate different nanostructured ZnO patterns with clearly defined edges. Hence, it is expected that this novel micropatterned technique based on the superhydrophilicsuperhydrophobic template will become a powerful tool for fabricating various types of micropatterned nanomaterials and devices.

Fig. 10. (a,b) Typical SEM images of the vertically aligned ZnO nanorods selectively grown on superhydrophilic patterning regions by the developed electric field assisted deposition technique at 90oC for 3 min. The inset in (a) shows the side view SEM image of the corresponding ZnO nanorod micropattern. The inset in (b) shows the higher magnified SEM image of a ZnO nanorod with hexagonal end facet. (c) 3D AFM image of the ZnO/TiO2 micropattern. (d) Confocal microscopy image of the perpendicular ZnO nanorod array.
