Frequency modulation atomic force microscope observation of TiO_2(110) surfaces in water

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Rutile titanium dioxide (TiO_2) (110) surfaces were examined in water using a frequency modulation atomic force microscope. On the surfaces cleaned by Ar^+ sputtering and annealing in ultrahigh vacuum, step-terrace structure was observed. The inlets at the steps and the pits on the terraces indicated erosion of the surface in water. Strings extended to the [001] direction were occasionally observed in the topography images and assigned to the clusters of the H_2O molecules. The tip experienced a repulsive force when the vertical tip position of z was less than 6 nm from the surface, and the force oscillated at z at less than 2 nm. The repulsive force originated from the disruption of the hydrogen bonding network of H_2O molecules formed on the hydrophilic sputter-annealed surface. The oscillatory force arose from structural alternate order-disorder transitions of the H_2O molecules at the gap between the tip and the TiO_2 surfaces. On the TiO_2 surface annealed in air, no strings were observed in the topography images. The tip experienced an attractive force before experiencing a repulsive force in its approach to the surface. Oscillatory behavior was not observed in the force curve. The air-annealed TiO_2 and tip surfaces were both hydrophobic and attracted to each other to expel the H_2O molecules from their gap. Ordering the H_2O molecules at the gap between the two hydrophobic surfaces was entropically unfavorable.
2010-05-28