The other one is formulated by Cassie and Baxter [32], which is generally valid for heterogeneous surfaces composed of air and a solid with hydrophobicity. Both models discuss

the surface wettability based on the surface roughness and geometry of materials. Our results indicate that in these TiO2 nanotubes of different selleck compound diameters (i.e., with different geometric factors), surface chemistry effects prevail in their surface wettability this website behavior. Figure 3 Optical images showing water droplets. On the as-grown (upper column), ScCO2-treated (middle column), and ScCO2-treated TiO2 nanotubes with UV light irradiation (lower column), respectively. Contact angles are denoted in the images. We attempt to elaborate the possible mechanism for the observed transitions in wettability in this study. First, we can almost exclude the possibility that the absorption of non-polar CO2 molecules on the nanotube surface leads to the hydrophobicity by the fact that the ScCO2-treated nanotubes still remain hydrophobic when kept in the atmosphere for more than 1 month. Another possibility is that newly forming functional groups on the nanotube surface during the ScCO2 process change the surface chemistry and wettability. Figure 4 shows the XPS surface analysis results,

in terms of the C 1s spectra, of the as-grown, ScCO2-treated, and ScCO2-treated TiO2 nanotubes of 100 nm in diameter with UV light irradiation, respectively. We find that the C-H signal in the as-grown sample becomes much stronger (more significantly than other not signals) after the ScCO2 treatment. It suggests that numerous C-H functional DMXAA clinical trial groups form on the TiO2 nanotube surface, possibly resulting from the reaction between the ScCO2 and TiO2·xH2O or Ti(OH)4. It has been

reported that the C-H functional groups are non-polar with a hydrophobic nature [33]. This can explain why the TiO2 nanotubes become hydrophobic after the ScCO2 treatment. In addition, it is well known that TiO2 can act as a photocatalyst under UV light irradiation [34]. The C-H functional groups can be effectively photo-oxidized on the TiO2 nanotubes under UV light irradiation [35]. Therefore, the ScCO2-treated nanotubes recover their surface wettability after being irradiated with the UV light. This also agrees with the XPS result that C-H signal diminishes in the UV light-irradiated sample. The Raman spectra in Figure 5 show a similar trend. The carbon-related Raman vibrations in the as-grown sample, including C-H bending, C-H stretching, and H-C-H bending modes [36, 37], become significantly stronger after the ScCO2 treatment and then diminish under UV light irradiation, indicating that the C-H functional groups indeed form on the nanotube surface and then are being photo-oxidized under UV light exposure. In addition, we find that almost no carbon-related Raman signals can be seen for the annealed TiO2 nanotubes before and after the ScCO2 treatment.