According to K. Hashimoto and co-workers, the wettability of TiO2 surfaces can be altered by irradiation. Surfaces become hydrophilic (water contact angle, ∼0°) after UV irradiation and gradually revert to a more hydrophobic state (contact angles, 50−70°) when left in the dark or exposed to visible light. Such changes have been observed for both anatase and rutile surfaces (single crystals and polycrystals) and presumably are not directly related to the photocatalytic activity of TiO2. We report here similar changes of the contact angle of water on flat titania wafers and on densely packed layers of micron-sized titania particles. Wettability changes can be reversibly cycled, and the effect is rather robust. The hydrophobic-to-hydrophilic conversion is faster than the inverse one (at comparable UV and visible light intensities). The contact angle change observed on wafers (∼50°) is twice as large as that on the particle layer, but this is mainly related to the effect of roughness. Using a photomask, we have patterned successfully regular arrays of hydrophobic circles (10 μm in diameter) on a hydrophilic TiO2 matrix and vice versa. These findings are of significant interest for the design of intelligent surfaces even though the detailed mechanism of the transition is yet to be elucidated.

Irradiating single-walled carbon nanotubes can lead to heat generation or ignition. These processes could be used in medical and industrial applications, but the poor solvent compatibility and high aspect ratios of nanotubes have led to concerns about safety. Here, we show that certain functionalized fullerenes, including polyhydroxy fullerenes (which are known to be environmentally safe and to have therapeutic properties) are heated or ignited by exposure to low-intensity (<10(2 ) W cm(-2)) continuous-wave laser irradiation. We also show that polyhydroxy fullerenes and other functionalized fullerenes can be transformed into single-walled nanotubes, multiwalled nanotubes and carbon onions without the presence of a catalyst by exposure to low-intensity laser irradiation in an oxygen-free environment. To demonstrate the potential usefulness of these processes in applications, we disrupted animal cells dosed with polyhydroxy fullerenes by exposing them to a near-infrared laser for a few seconds, and also ignited an explosive charge in contact with a particle of carboxy fullerenes.

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