Structural Change of the Rutile–TiO2(110) - American Chemical Society

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Structural Change of the Rutile−TiO2(110) Surface During the Photoinduced Wettability Conversion Tetsuroh Shirasawa,*,†,‡ Wolfgang Voegeli,§ Etsuo Arakawa,§ Toshio Takahashi,§ and Tadashi Matsushita∥ †

National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan ‡ JST, PRESTO, Kawaguchi, Saitama 332-0012, Japan § Department of Physics, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan ∥ Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan 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. the superhydrophilicity.38−43 The other group is that a photoinduced intrinsic structural change of the TiO2 surface results in the superhydrophilicity. The authors who discovered the phenomenon proposed that the UV light generates an oxygen vacancy, which becomes an active site for the dissociation of adsorbed water molecules to OH groups which are hydrophilic in nature.44−48 After that, other papers suggested that the photoirradiation causes a reconstruction of the surface Ti−OH bonding configuration to increase the concentration of the OH group.9,49 The controversy arises from the difficulty in observing the molecular-scale processes involved in the wettability conversion. Conventional surface science techniques in vacuum conditions are often inadequate to study the wettability conversion process because the surface processes in vacuum may be different from those occurring in ambient conditions. Furthermore, it was reported that the hydrophilic state is a metastable state and rapidly recovers to the hydrophobic state in vacuum.50 Therefore, an in situ observation of the interface processes at the molecular scale in ambient conditions is helpful for understanding the mechanism. In the present study, the structural change of the rutile− TiO2(110) surface during the photoinduced wettability conversion was studied in a humid condition by using the Xray crystal truncation rod (CTR) scattering method, which is a

1. INTRODUCTION Since the discovery of the photoinduced dissociation of water on titanium dioxide (TiO2) in the early 1970s,1 TiO2 has been regarded as a promising photocatalytic material, and the photochemistry of TiO2 has attracted tremendous interest.2−5 The discovery of the photoinduced conversion of water wettability of TiO2 surfaces in the late 1990s6,7 further increased the significance of TiO2. It was found that the hydrophobic TiO2 surface can be converted to the superhydrophilic one by irradiation with UV light with an energy larger than the TiO2 bandgap of about 3 eV. This effect extends the range of the application of TiO2 to antifog coatings, selfcleaning, and building cooling.8,9 The interaction between water and the TiO2 surface is an essential building block involved in the wettability conversion process, and it has been the subject of intensive research. Among the TiO2 polymorphs, rutile is the most stable phase, and its (110) surface has the lowest surface energy.10 Therefore, a lot of experimental and theoretical studies were dedicated to understanding the molecular-scale processes occurring on the rutile (110) surface.11−37 However, revealing the interface processes buried in water is a challenging issue for both theory and experiment. There is no consensus about the mechanism of the wettability conversion, i.e., how the surface becomes superhydrophilic by the photoirradiation. The opinions about the mechanism can be roughly divided into two groups. Several papers claim that the photocatalytic decomposition of hydrophobic organic contaminants leads to © XXXX American Chemical Society

Received: August 21, 2016 Revised: December 8, 2016 Published: December 8, 2016 A

DOI: 10.1021/acs.jpcc.6b08448 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C powerful technique to analyze surface or interface structures nondestructively at the atomic scale in ambient conditions. We used the time-resolved CTR technique recently developed by the authors51 and observed evidence for the occurrence of an atomic-scale structural change during the UV-light irradiation. The atomic structures of the hydrophobic and hydrophilic surfaces were revealed by static CTR measurements. The results suggest that water molecules adsorbed on the fivecoordinated Ti atoms dissociate into OH groups by the photoirradiation, and the emitted protons hydroxylate the adjacent oxygen atoms to OH groups. The possible increase of the surface OH group population would lead to the superhydrophilicity.

2. EXPERIMENTAL METHOD We used a commercial rutile−TiO2(110) wafer (Shinkosha Co., Ltd.) as the sample. The sample was 10 mm × 10 mm in size, and the thickness was 0.5 mm. The surface of the sample was cleaned as follows. First, the sample was ultrasonically washed with acetone and subsequently washed with ultrapure water and then dipped in 20% HF, in order to remove surface impurities.52 After rinsing with pure water, the sample was annealed in oxygen atmosphere of ∼102 Pa at 1000 K for 1 h to obtain an atomically flat surface with a typical terrace size of 200 nm as confirmed by an atomic force microscopy (AFM) observation. The sample exhibited a clear 1 × 1 low-energy electron diffraction (LEED) pattern. The surface exhibited water contact angle of a few tens of degrees before UV-light irradiation. The surface chemical composition analysis by Auger electron spectroscopy measurements indicated carbon-containing contamination with a coverage of 0.4 monolayer (ML) (1 ML is defined as 4 atoms per 1 × 1 surface unit cell). Such a carbon-containing contamination is usually observed for airexposed surfaces.43,52 In the following CTR scattering analysis, no trace of the carbon contaminants was detected, indicating that the contaminants were disordered and did not contribute to the diffraction signals. We believe the existence of the contaminants does not change our main conclusions. The time-resolved X-ray CTR scattering measurements were performed at beamline NW2A of the Photon Factory Advanced Ring at KEK. The experimental layout is schematically shown in Figure 1a. The heart of this method is the use of a wavelength-dispersive laterally convergent synchrotron X-ray beam instead of a monochromatic collimated X-ray beam used in conventional CTR scattering measurements. Such an X-ray beam is produced by a curved crystal polychromator which is similar to those used in wavelength-dispersive X-ray absorption spectroscopy measurements.53 In the CTR scattering measurements, the use of the X-ray beam and a two-dimensional detector allows us to measure a wide range of the scattering intensity profiles along a CTR simultaneously in a short time (∼1 s) without rotating the sample or detector.51 It is suitable for in-operando studies of structural changes involved in surface/interface processes. Details of the technique are described elsewhere.51 In the present experiments, we used a tapered undulator X-ray source to produce a high-flux wavelength-dispersive X-ray beam with a smooth spectrum.54 The X-ray spectrum was about 10 times stronger than that in our previous report, in which a bending magnet X-ray source was used.51 We used two flat mirrors coated with Rh to eliminate higher-order harmonics and a flat-bent Rh-coated mirror for vertical focusing. The mirror system was located upstream of the curved crystal polychromator. The poly-

Figure 1. (a) Illustration of the time-resolved CTR measurement during UV-light irradiation on the rutile−TiO2(110) surface. (b) Time evolution of CTR scattering intensity at the anti-Bragg point (0 0 1) during the X-ray irradiation. (c) The scattering intensity profiles of the (0 0) rod under the UV-light irradiation (λ = 365 nm, 80 mW/cm2). (d) Time evolutions of the normalized intensity, integrated around the anti-Bragg point (0 0 1) as indicated in (c), under the different UV light powers.

chromator was a double-side-polished Si wafer with a typical thickness of 0.2 mm, and the Si (111) reflection was used in the transmission geometry. The energy range of the X-ray beam was 16−23 keV. A typical beam size at the sample position was 0.15 mm (fwhm) in both the vertical and horizontal directions. The scattered X-rays were detected with a two-dimensional pixel array detector (PILATUS-100K, DECTRIS Ltd.). As the UV light source we used a commercial mercury light source REX-250 (ASAHI SPECTRA USA Inc.). The UV light was monochromatized to 365 nm with a band-pass filter with a bandwidth of 10 nm in fwhm. The UV light completely bathed the sample surface. For the quantitative structural analysis of the hydrophobic and hydrophilic surfaces, we performed conventional monochromatic CTR scattering measurements at beamline 4C of the Photon Factory at KEK. The X-ray energy was 11 keV. We used the conventional four-circle diffractometer installed at the beamline and a PILATUS-100K as the detector. Data processing of the two-dimensional scattering data was performed according to the previous report.55 We obtained five inequivalent CTR profiles on the hydrophobic surface in the dark. The data acquisition time was about 12 h. After that, we irradiated the surface with the UV light (80 mW/cm2) for 30 min and then obtained six inequivalent CTR profiles. The power of UV light is much larger than the threshold of ∼20 mW/cm2 for the wettability conversion.44 During the whole measurement, the sample was exposed to the UV light. The data acquisition time for the photoirradiated surface was about 20 h. After the measurement, we confirmed that the water contact angle became ∼0°. The sample was put in a wet N2 or He gas flow in all the CTR scattering measurements. The N2 or He gas flow was led through a gas washing bottle with a tissue paper (KimWipes, Kimberly-Clark Co.) wet with distilled water upstream of the sample cell. We note that in the time-resolved measurements the high-flux undulator X-ray source (several 1012 photons/ mm2/s) affected the surface structure and changed the CTR profiles. Figure 1b shows the CTR intensity change caused by B

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The Journal of Physical Chemistry C the undulator X-ray beam. The time scale of the X-ray irradiation effect was on the order of 103 s. Since the time scale is 1 order of magnitude larger than that of the UV-light irradiation effect, the X-ray irradiation effect would not significantly affect the following results. The X-ray irradiation effect was not observed in the monochromatic X-ray CTR measurements with the bending magnet X-ray beam whose photon flux density is 1 order of magnitude smaller than the undulator X-ray beam. Hereafter, we define the nonprimitive rectangular unit cell for the surface, in accordance with the previous CTR study.11 The lattice parameters are a = c = √2a0 = 6.4972 Å and b = c0 = 2.9587 Å, where a0 and c0 are the tetragonal lattice parameters of the bulk crystal. The directions of the basis vectors a, b, and c are, respectively, the [−110], [001], and [110] directions of the bulk crystal (see Figure 4a). In the structural analysis, the atomic position, mean-square displacement (Debye−Waller factor), and occupancy of the adsorption site were optimized by nonlinear least-squares fitting to reproduce the measured CTR data. We imposed the p2mm symmetry in the structural relaxation.

Figure 2. (a) Scattering intensity profiles of the (0 1) rod of the rutile−TiO2(110) surface during the UV-light irradiation (λ = 365 nm, 87 mW/cm2), measured by the time-resolved method. (b) Time evolutions of the normalized intensity at the selected points indicated in (a).

Figure 2b, the time evolutions of the scattering intensity at L = 1.34, 1.65, and 1.85 indicate that the intensity change was not uniform along the rod; namely, the intensity increased at L = 1.34 and decreased at L = 1.85, and it hardly changed at L = 1.65. Such a nonuniform change of the CTR profile cannot be caused by only the inhomogeneity of surface structure, demonstrating the occurrence of an atomic-scale structural change during the photoirradiation. 3.2. Structure of Hydrophobic Surface. The atomicscale structure of the hydrophobic surface was revealed by quantitative structural analysis. The CTR profiles measured from the hydrophobic surface are shown as red circles in Figure 3. The oxygen rods (1 0) and (0 1) have broad peaks at around (1 0 2) and (0 1 3). These are similar to those measured on the surface immersed in deionized water, where a laterally ordered water molecular layer is formed.11 It was reported that such a profile feature appears when the surface is exposed to moisture.12 The appearance of the water-induced features in the present study is reasonable because the sample was put in a wet gas flow. The best-fit structural model obtained by nonlinear least-squares fitting is schematically depicted in Figure 4a, and the optimal values of structural parameters are listed in Table 1. The CTR profiles calculated for the best-fit structural model are shown as red lines in Figure 3. The goodness of fit is χ2 = 6.2, where a value of 1 means a perfect fit taking into account the experimental uncertainty. The best-fit surface is terminated with an oxygen atom located at the bridging site (denoted as OB in Figure 4a) and covered with an oxygen atom (denoted as OT) located atop of the five-coordinated Ti atom (Ti2), and another oxygen atom representing an adsorbed water molecule (AW) is located at the hollow site in between the OB and OT rows. The best-fit structural model is different from the clean surface in ultrahigh vacuum (UHV), which is terminated with OB and fivecoordinated Ti and is covered with neither OT nor AW.58−62 The present model is very similar to the surface immersed in deionized water11 except for the number of adsorption sites for AW. In ref 11, Zhang et al. showed three adsorption sites for AW: the atop site of OB (denoted as AW1 in ref 11), the hollow site in between the OB and OT rows (AW2), and a bridging site between OB and OT (AW3). In the present analysis, we tested all the individual adsorption sites and their combinations and found that only the model of the AW2 site gives a good fit to the experimental data with a physically reasonable structure. In ref 11, the other two adsorption sites are located at higher positions than AW2. The absence of the two adsorption sites in our study might be due to the difference in the ambient condition: our sample was placed in a wet gas flow, while the sample in ref 11 was completely immersed in water. This

3. RESULTS AND DISCUSSION 3.1. Time-Resolved CTR Scattering Measurements under UV-Light Irradiation. In the specular reflection CTR [(0 0) rod] a decrease in the intensity was observed particularly at the anti-Bragg point (0 0 1) during the photoirradiation (Figure 1c). It is consistent with the previous report which compares the (0 0) rod scattering profiles measured before and after the photoirradiation.12 The anti-Bragg point, located at the middle of two bulk Bragg peaks in the rod, significantly decreases with an increase of structural inhomogeneity such as surface roughness.56 In the present case, the surface lost a uniform atomic arrangement during the photoirradiation, as shown later. The structural inhomogeneity decreased the intensity of anti-Bragg point. The result is consistent with AFM observations which indicate that the surface wettability becomes inhomogeneous after the photoirradiation.6,7 The rate of the intensity decrease depends on the UV-light power as shown in Figure 1d, indicating that the phenomenon is a photoinduced effect. The rate of intensity decrease, estimated from the first 50 s, is 0.8 × 10−2/s for the UV-light power of 41 mW/cm2, and the rate increases to 1.1 × 10−2/s for 80 mW/ cm2. The intensity decrease was not obvious when the power was lower than about 20 mW/cm2. The threshold value is consistent with that of the previous water contact angle measurement.44 After the CTR measurements we conducted water contact angle measurements and confirmed that the samples showing the clear intensity decrease exhibited a contact angle of ∼0°. We note that the rate of intensity decrease also depends on surface quality. Particularly, the intensity decrease was not clearly observed on dirty samples which were not cleaned beforehand, indicating that the phenomenon is intrinsic to the TiO2 surface. More direct evidence for the occurrence of a surface structural change was obtained from the measurements of the (0 1) rod. There is no scattering contribution from bulk Ti atoms to the (0 1) rod, and therefore the so-called “oxygen rod” is more sensitive to the surface structure.57 The measurements were performed with grazing incidence conditions (glancing angle 0.5°) to increase the surface sensitivity. In Figure 2a, the CTR profile measured at the photoirradiation time of 400 s is clearly different from that measured at 10 s. In C

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Figure 3. Measured CTR scattering amplitudes and calculated ones for the best-fit structural models for the nonirradiated and photoirradiated rutile−TiO2(110) surface.

about the OT site. Previous DFT studies claimed that the intact molecular adsorption on the OT site is stable beneath thick water films, while dissociative adsorption can coexist when the surface is not fully covered with water.15−19 However, recent DFT studies show that the relative energy of the associative adsorption to the dissociative adsorption varies as a function of the number of the TiO2 layers in the calculations and conclude that the water does not dissociate at any coverage.13,20 On the experimental side, it was widely accepted that the associative adsorption at OT is dominant on a surface terrace21,22 and that the dissociative adsorption occurs locally at the O B vacancies23,24 and step edges.25 However, recent studies show water molecules can partially dissociate even at a defect-free surface terrace when the water coverage is below 1 ML.26−28 The partial dissociation would be an indication that the difference in the adsorption energy is marginal at such a low coverage. In summary, a reasonable view gained from the previous reports is that the associative adsorption at the OT site is predominant over the dissociative one in ambient conditions. Our results agree with this idea as shown below. It is known that the Ti−O bond length at the surface varies according to the oxygen species, i.e., O, OH, and H2O, and thus it can be used as a chemical fingerprint. In the present analysis, the derived Ti2−OT length is 2.09 ± 0.03 Å (Table 1) which agrees with the result of the previous CTR study of 2.13 ± 0.03 Å.11 According to photoelectron diffraction (PhD) studies, the bond length is 2.21 ± 0.02 Å29 and 1.85 ± 0.08 Å27 for H2O and OH, respectively. Our result is much closer to that for the

interpretation is supported by a previous CTR study which shows that the degree of lateral ordering of the adsorbed water increases with the surface wetness.12 In the present analysis, the derived occupancy of the AW2 site is 0.32 ± 0.03 (see Table 1), indicating that the hydrophobic surfaces with a typical water contact angle of a few tens of degrees12 still adsorb a fractional amount of water molecules. The number of adsorbed water might increase on the photoirradiated surface as will be discussed in Section 3.4. The planar average electron density profile calculated for the best-fit structural model is shown as a red line in Figure 4c. In the profile, the peaks of OT and AW are located at 2.05 and 3.5 Å above the topmost TiO plane, and the AW has a lower density than OT. It is similar to the mass density profiles reported in the recent density functional theory (DFT) studies.13,14 These studies showed that the OT site is nearly fully occupied by water molecules with a high degree of lateral ordering, and that a less-ordered second water layer is distributed at 3.0−4.5 Å above the topmost TiO layer. In the present analysis, we could not uniquely determine the positions of H atoms or even their presence because H atoms are almost transparent to X-rays. However, we interpreted the OT and AW as oxygen atoms of intact water, according to the agreement with the DFT studies and the discussion below. In order to discuss our results in detail, we would like to refer to previous studies about the water-TiO2 interaction. There has been a long-standing debate on whether the water molecule is adsorbed on the surface associatively or dissociatively, especially D

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structure. The other change is the disappearance of the broad peaks at around (1 0 2) and (0 1 3) in the oxygen rods, indicating the break up of the long-range order of the second water layer (AW). In the structural analysis of the hydrophilic surface, we found that the dominant structural parameters are the anisotropic Debye−Waller factors of the topmost TiO layer, OT, and OB. As discussed below, the mean square displacements do not represent the thermal vibrational amplitudes but static positional fluctuations. We fitted the lateral and vertical fluctuation amplitudes of the OT, OB, first TiO layer, and second TiO2 layer, where we used the common amplitudes for the Ti1, Ti2, and O3 atoms of the first TiO layer, for Ti3, Ti4, and O4−O6 atoms of the second TiO2 layer, and for OT and OB atoms, in order to reduce the number of fitting parameters. The determined structural parameters are listed in Table 1. The structural model is depicted in Figure 4b, and its planar-average electron density profile is shown as the light blue line in Figure 4c. The CTR profiles calculated for the structural model are shown as the light blue lines in Figure 3. The value of χ2 is 7.5. In the analysis we fixed the occupancy of OT and OB at 1 (full occupation) because the occupancy parameter is closely coupled with the positional fluctuations so that we could not uniquely optimize these parameters simultaneously. The occupancy restriction is reasonable because if oxygen vacancies are created they will be immediately repaired in the ambient condition. We confirmed that the absence of OT and/or OB resulted in a worse fit (χ2 > 8.5). We note that in the case of the hydrophobic surface an anisotropic Debye−Waller factor did not improve the fit, and therefore the isotropic factor was used. On the hydrophilic surface, the vertical fluctuation amplitude uz of the first TiO layer is 0.24 ± 0.06 Å, and that of the OT and OB is 0.28 ± 0.12 Å, both of which are more than twice than those of the hydrophobic surface (see Table 1). The electron density profile clearly shows the structural feature (Figure 4c), where the peaks of the first and second TiO layers, OB, and OT are significantly broadened due to the positional fluctuation uz. The large uz is the main reason for the decrease of CTR scattering intensity around the anti-Bragg points. The lateral fluctuation amplitude uxy of OT and OB is significantly large, 0.7 ± 0.3 Å, indicating that the OT and OB are not rigidly bound to the lattice site. The fluctuation amplitude of OT and OB is schematically represented by the ellipsoids in Figure 4b. Another prominent change is the disappearance of the second-ordered water layer (see the electron density profile). In the analysis, we fixed the occupancy of AW to 0 since we could not find reasonable structural parameters for AW. Considering the superhydrophilicity, however, there should be adsorbed water molecules on the surface. The apparent disappearance of AW in the diffraction measurement would be because its lateral long-range order is not created on the highly fluctuated surface. It should be noted that the photoirradiated surface is reported to consist of hydrophilic and hydrophobic domains, both of which are several tens of nanometers in size and have almost the same population.6,7 In such a case, the values of the structural parameters obtained in the analysis are the average ones over the two domains. Taking the coexistence of the two phases into account, we tried to fit the unique structural parameters for each domain and their population ratio. However, the analysis did not converge into physically reasonable structures. Therefore, we conclude that the structural differences between the two domains fall within the fluctuation amplitudes uxy and uz, if the two phases coexisted in

Figure 4. Best-fit structural model for the (a) nonirradiated and (b) photoirradiated rutile−TiO2(110) surface. Red balls represent the O atoms, and light blue balls represent the Ti atoms. H atoms that can be expected to be bonded to the O atoms can not be detected in the experiment. In (a) the dashed lines outline the surface unit cell. In (b) the ellipsoids on the OT and OB represent the magnitudes of the positional fluctuations uxy and uz (see Table 1). (c) Planar average electron density profiles of the structural models of (a) and (b).

associative adsorption than the dissociative one. Furthermore, our result is close to the bond length of 2.1−2.2 Å for the H2O adsorption reported in the recent DFT calculations.13,20 These comparisons lead to the conclusion that the OT site is dominated by associative adsorption. For the OB site, the derived Ti1−OB length is 1.84 ± 0.02 Å, the same as that of the clean surface of 1.85 ± 0.03 Å reported in the LEED and CTR studies.59,60 This value is significantly shorter than the bond length for the dissociated OH, 1.94 ± 0.07 Å27 and 1.97 ± 0.05 Å,30 which are derived in the PhD studies. The DFT study shows that the Ti−O length for the nonhydroxylated oxygen is 1.86 Å which is unchanged from the clean surface and that it is significantly elongated to 2.04 Å by the hydroxylation.30 These studies consistently indicate that the OB of the hydrophobic surface is a nonhydroxylated oxygen atom. Finally, for the AW, both of the distances from OT to AW and from OB to AW are 2.9 ± 0.1 Å, identical to the O−O length in bulk water within the uncertainty,31 suggesting that the AW interacts with both the OT and OB via hydrogen bonding. Based on the above considerations, we schematically illustrate the water adsorption structure on the hydrophobic surface in Figure 5a, which is equivalent to the recent theoretical model.13,14 3.3. Structural Transition in the Photoinduced Wettability Conversion. After the UV-light irradiation, the CTR profiles changed drastically. In Figure 3, the CTR profiles obtained from the photoirradiated hydrophilic surface (light blue circles) are compared to those of the nonirradiated hydrophobic surface (red circles). There are two prominent changes. One is the intensity decrease around the anti-Bragg points of the nonoxygen rods (0 0), (1 1), (2 0), and (0 2), as observed in the time-resolved measurements (Figure 1c). This change is mainly caused by the inhomogeneity of surface E

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Table 1. Optimal Values of the Displacement from the Bulk Position, Fluctuation Amplitude, And Site Occupancy of the Surface Atoms Indicated in Figure 4a, Derived from Structural Analysisa atom

sample

Δx/Å

Δz/Å

uxy/Å

uz/Å

occupancy

Ti1

nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated nonirradiated photoirradiated

−0.09(0.06) −0.08(0.08) −0.05(0.06) 0.04(0.04) 1.17(0.09) -

0.093(0.006) −0.03(0.01) −0.047(0.006) −0.04(0.02) 0.025(0.005) 0.00(0.03) 0.028(0.005) 0.00(0.01) 0.030(0.005) 0.00(0.01) −0.020(0.005) −0.01(0.01) 0.000(0.004) 0.00(0.01) 0.002(0.004) 0.00(0.01) 0.07(0.02) −0.04(0.16) −0.07(0.01) −0.02(0.23) −0.05(0.01) 0.09(0.05) −0.01(0.03) 0.05(0.07) 0.01(0.03) 0.06(0.04) −0.04(0.03) 0.00(0.02) 0.00(0.02) 0.00 0.00(0.03) 0.00 3.51(0.03) -

0.12(0.04) 0.16(0.07) 0.12(0.04) 0.16(0.07) 0.08 0.08(0.11) 0.08 0.08(0.11) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.12(0.04) 0.7(0.3) 0.12(0.04) 0.7(0.3) 0.12(0.04) 0.16(0.07) 0.08 0.08(0.11) 0.08 0.08(0.11) 0.08 0.08(0.11) 0.08 0.08 0.08 0.08 0.14(0.04) -

0.12(0.04) 0.24(0.06) 0.12(0.04) 0.24(0.06) 0.08 0.14(0.07) 0.08 0.14(0.07) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.12(0.04) 0.28(0.12) 0.12(0.04) 0.28(0.12) 0.12(0.04) 0.24(0.06) 0.08 0.14(0.07) 0.08 0.14(0.07) 0.08 0.14(0.07) 0.08 0.08 0.08 0.08 0.14(0.04) -

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.97(0.04) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.32(0.03) 0

Ti2 Ti3 Ti4 Ti5 Ti6 Ti7 Ti8 OT OB O3 O4 O5 O6 O7 O8 AW a

For the AW, its atomic position with respect to the bulk position of Ti1 is shown. Values in the subscript parentheses are the uncertainties. The values that were fixed in the analysis are not accompanied with the uncertainty.

3.4. Mechanism of the Photoinduced Wettability Conversion. We discuss the relation between the surface structural change and the wettability conversion, on the basis of the present results and previous reports. Recent X-ray photoemission spectroscopy measurements in a humid condition showed that the population of surface OH groups increases during the UV-light irradiation.32 In addition, a recent scanning tunneling microscopy (STM) study showed that the water molecule adsorbed at the OT site dissociates to OH by the photoirradiation, and the proton emitted from the OT is transferred to the adjacent OB and converts it to OH.33 Taking these reports and the present results into account, a possible process of the photoinduced wettability conversion is schematically illustrated in Figure 5. Figure 5a shows the hydrophobic surface, where the OT site is occupied by an intact water and the OB site is occupied by a nonhydroxylated oxygen; Figure 5b shows the proton transfer; and Figure 5c shows the resulting hydrophilic surface. The corresponding reaction equations can be described as

Figure 5. Structural model of the photoinduced wettability conversion process on the rutile−TiO2(110) surface. Hydrogen bonds are represented by blue lines in (a) and (c). In (b), the proton transfer from the OT to OB is indicated by the purple lines, and other hydrogen bonds are omitted for clarity.

our sample. In the discussion below, we avoid discussing the Ti2−OT and Ti1−OB bond lengths because of the relatively large uncertainty of more than 0.18 Å. We only refer to the Ti2−OT length of 1.98 ± 0.18 Å, which is close to the Ti−OH length rather than the Ti−H2O length (see the previous section).

(T ··· H 2OT + h+) + e− + (Ti−OB) → (Ti + •OH T) + e− + H+ + (Ti−OB) F

(1)

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resolved X-ray CTR scattering measurements. Quantitative structural analysis on static CTR data measured for the hydrophobic and hydrophilic surfaces suggest that the structural change is induced by the photoinduced dissociation of the water molecule adsorbed on the five-coordinated Ti, and the desorbed proton hydroxylates the neighboring bridging oxygen. The possible increased population of the surface OH groups might convert the surface to superhydrophilic.

(2)

where H2OT is the adsorbed water at the OT site. It is known that the photogenerated electron−hole pairs relax to the respective band edges, and finally they can reach the adsorbed waters (left-hand side of eq 1).63 The transfer rate of the hole is faster than that for electron by 1 or 2 orders of magnitude.64,65 In the right-hand side of eq 1, the H2OT reacts with the hole to become an •OHT radical, and a proton is emitted. In eq 2, the proton and electron go to the OB to form OHB, and the •OHT reacts with Ti and becomes OHT. Each of the OHT and OHB can become an active site for the water adsorption. As a consequence, the population of the adsorbed water molecules might increase (Figure 5c). The particularly large lateral positional fluctuation uxy of OT and OB on the hydrophilic surface might represent displacements induced by the hydrogen bonding between the OH groups in the adjacent sites. The hydrogen bonding for water molecules at the OT site was reported by the recent STM study,34 and the interaction would be stronger for the OH groups.35 The hydrogen bonding would also occur between OT and OB sites.36 These nearest-neighbor interactions might cause the large displacements of OT and OB from the lattice sites. The displacements of OH groups would induce a local lattice strain at the underlying TiO2 layers, and the increased positional fluctuations of the first TiO layer might represent the lattice relaxation. We speculate that the reported formation of the hydrophobic and hydrophilic domains6,7 might be caused by a strain-relief mechanism, in which the lattice strain generated in the hydrophilic domain is released in the hydrophobic one. The recent DFT study suggested that inplane tensile strains favor the dissociative water adsorption, while compressive strains stabilize the associative adsorption.20 Finally, we comment on the models previously suggested for the wettability conversion. In the early days, it was proposed that the OB vacancy produced by the photoirradiation becomes the reaction center of the water dissociation.44−48 However, this mechanism is inconsistent with experiments showing that the photoirradiation does not create OB vacancies.37 It was also suggested that a reconstruction of Ti−OH bonds is induced by the photoirradiation to increase the population of OH groups.9,49 This mechanism is denied by previous reports, which indicated that the population of OH groups on the hydrophobic surface is fairly small,21,22 and by the present results which indicate that the increase in the OH population is induced by the photoinduced dissociation of the water at the OT site. Another proposed model is that the wettability conversion is a consequence of the photocatalytic decomposition of organic contaminants.38−43 We partially agree with the idea because the removal of the hydrophobic adsorbates should certainly promote the hydrophilicity. However, we think it is not the only driving force because it was reported that the complete removal of the contamination alone does not lead to superhydrophilicity50 and that the photoinduced superhydrophilicity still occurs even when the contamination exists.48 We suggest that the increase in population of the surface OH groups is essential for the superhydrophilicity. We note that the surface OH groups can also assist with the removal of the surface contaminants.66

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 29-861-5371. ORCID

Tetsuroh Shirasawa: 0000-0001-5519-6977 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by JSPS KAKENHI Grant Numbers 24760542 and 26105008 and by JST, PRESTO. The synchrotron radiation experiments were performed at PF with the approval of Photon Factory Program Advisory Committee Proposal Numbers 2010G012, 2013S2-001, and 2014G152.

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REFERENCES

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