**5. Summary and outlook**

608 Recent Advances in Nanofabrication Techniques and Applications

Fig. 16b shows the photocurrent spectra of the couple CdS/TiO2 nanotube array electrode prepared under different electrodeposition times. It is apparent that the pure TiO2 nanotube array samples have a photo-response wavelength lower than 400 nm due to its band-gap of 3.2 eV (curve a). The decoration of CdS nanospheres with a smaller energy band-gap (2.4 eV) can significantly extend the photo-response range from 380 nm to about 500 nm.Moreover, the CdS modified TiO2 nanotube array electrodes can also greatly increase the photocurrent response under UV light, especially for the samples obtained under 2 min electrodeposition (curve b), which thus would be the optimal deposition time. This is attributed to the uniform dispersed CdS nanospheres with suitable size decorated onto the TiO2 nanotubes. This allows for more efficient electron transfer and lower electron-hole recombination rate which leads to enhanced light harvesting at the directly grown CdS/TiO2 heterojunctions. With the increase of time (curve c and d), more CdS particles with bigger size started to randomly distribute on top of TiO2 nanotube arrays. Such composite nanostructures would weaken the light absorption of the uniform CdS/TiO2 heterojunction underlayer, which has resulted in a lower photocurrent in both UV and

(b)

Fig. 16. (a) Typical SEM images of the CdS micropattern; (b) Photocurrent spectra of micropatterned CdS film on TiO2 nanotube array electrode. (curve a-d): pure TiO2; 2 min; 3 min; and 5 min. (c) Superhydrophilic region; (d) superhydrophobic region. EDX spectrum of

the corresponding superhydrophilic (e) and superhydrophobic regions (f).

visible light region.

Extremely wetting micropattern (superhydrophilic/superhydrophobic) on TiO2 nanostructure surface by using SAM technique and photocatalytic lithography has been studied intensely as it provides a cost effective template to construct well defined functional composited pattern. Numerous potential applications have also been proposed and investigated in biomedical, sensors and micro-nano devices. We believe that the photocatatlytic lithography patterning technique presented in this chapter should be general to create micro-scale wetting pattern on other semiconductor substrates and these developments will open the door for more widespread application of the wetting pattern in practical fields.
