Formation of Nano-Porous Structures on Oxide Semiconductor Surface by Photoetching Technique

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Photoetching is an unique technique for tailoring the semiconductor surface. The effect of photoetching (photoelectrochemical etching) on photoinduced hydrophilic conversion of titanium dioxide (TiO_2) film electrodes prepared by thermal oxidation of Ti plate has been studied. We have found that photoetching of TiO_2 film results in the formation of nano-porous structure depending upon preparation condition (oxidation temperature), photoetching condition (electrode potential) and crystallographic orientation of the TiO_2 grain. Photoetching of the film oxidized at temperature higher than 900℃ makes nano-porous structure consisting of (100) plane of rutile structure of TiO_2 and improves the photoinduced hydrophilic conversion property. Photoetching under anodic polarization increases the rate of photoinduced hydrophilic conversion, while the film photoetched under weak anodic polarization does not show the photoinduced hydrophilic conversion property. We have also examined the photoelectrochemical properties of photoetched Cr, Sb doped visible light sensitive TiO_2 electrode. It is important to increase visible-light sensitivity of TiO_2 from a viewpoint of solar energy utilization. After UV light photoetching, roughness of the surface increases and convexo-concave structures with several tens of nanometer size were observed. Whereas after photoetching of VS light illumination, the steps are selectively undissolved and convexo-concave structures with about one hundred nanometer size were observed on its terrace surface. After photetching with UV and VS light, the photoresponse for VS light region increases up to 4 % of IPCE (incident photon to current conversion efficiency). On the contrary the photoresponses for UV region decrease in both cases.