Hydrophilicity transition of the clean rutile TiO2 (1 1 0) surface

We present contact angle measurements of water on single-crystal rutile TiO2 (1 1 0) surfaces, exposed to ambient air, or protected in dry air. Our measurements indicate that the surfaces exposed to ambient air are hydrophobic, with a contact angle of θ = 61(5)°. However, the well-protected dry surface also exhibits some hydrophobic tendency, with θ = 32(5)°. It is known that UV irradiation transforms both surfaces superhydrophilic, with θ = 0° [R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432]. We also present preliminary X-ray crystal truncation rod measurements on the hydrophobic TiO2 (1 1 0) surface, and of the effect of UV illumination on the surface.