Structural Change of the Rutile–TiO₂(110) Surface During the Photoinduced Wettability Conversion

Abstract

The surface structural change of the rutile–TiO2(110) during the UV-light-induced wettability conversion was studied with atomic resolution using the X-ray crystal truncation rod (CTR) scattering method. We confirmed that an atomic-scale surface structural change occurs during the UV-light irradiation by using time-resolved CTR profile measurements. Quantitative structural analysis on static CTR data, which were measured before and after the conversion, shows that on the hydrophobic (nonphotoirradiated) surface the five-coordinated Ti atom is covered with an O atom likely in a form of water molecule, for which the bridging O atom is not likely hydroxylated, and that large atomic positional fluctuations occur on the hydrophilic (photoirradiated) surface possibly due to the photoinduced proton transfer from the intact water molecule to the bridging oxygen atom. The resulting surface OH groups might be active sites for water adsorption to make the surface superhydrophilic.

Real-time Observation of Interface Atomic Structures by an Energy-Dispersive Surface X-ray Diffraction

“…Based on the determined atomic structures and chemical states provided by previous reports, we proposed a mechanism of the wettability transition as shown in Figure 5 [17]. The UV illumination can induce a proton transfer from the intact water at the OT site to the non-hydroxylated oxygen at the OB site [47] [ Figure 5(b)].…”

“…Theoretical studies are also challenging because the energy hierarchy of the water adsorption phases depends on the calculation conditions [42,43]. We studied the possible structural change of the TiO2 surface during the wettability transition in an ambient condition by SXRD measurements [17]. We studied the rutile-TiO2(110) surface, as it has the lowest surface energy among the TiO2 polymorphs surfaces [44].…”

“…The atomic structures of the hydrophobic and superhydrophilic surfaces were quantitatively analyzed using static CTR data [17]. The atomic positions and Ti−O and O−O bond lengths indicate that; on the hydrophobic surface, the five-coordinated Ti atom was terminated with an O atom [denoted as OT in Figure 5(a)] which is likely in a form of water, the bridging O atom (OB) were not hydroxylated, and the surface was partially covered with adsorbed water molecules (AW) located at a lattice site (the site occupancy was ~0.3).…”

“…The single-acquisition CTR data allows the quantitative determination of the out-of-plane atomic structure, which is a big advantage over the high-energy monochromatic methods described above. In this review, two examples are presented to demonstrate the capability for the time-resolved quantitative structure analysis: the photo-induced wettability transition of the rutile-TiO2(110) surface [17] and an electrochemical reaction on the Pt(111) electrode surface [18].…”

“…A possible mechanism of the photoinduced wettability transition of the rutile-TiO2(110) surface[17]. (a) The hydrophobic surface is covered with O atom (OT) located atop the five-coordinate Ti atom, bridging O atom (OB), and ordered adsorbed water molecule (AW).…”