This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Article Cite This: J. Phys. Chem. C 2019, 123, 8463−8468 pubs.acs.org/JPCC Structure of a Superhydrophilic Surface: Wet Chemically Prepared Rutile-TiO2(110)(1 × 1) J. P. W. Treacy,† H. Hussain,†,‡ X. Torrelles,§ G. Cabailh,∥ O. Bikondoa,⊥ C. Nicklin,# G. Thornton,▽ and R. Lindsay*,†,‡ † Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, Manchester M13 9PL, United Kingdom ‡ Photon Science Institute, The University of Manchester, Manchester M13 9PL, United Kingdom § Institut de Ciència de Materials de Barcelona (CSIC), Campus UAB, 08193 Bellaterra, Spain ∥ Sorbonne Université, UMR CNRS 7588, Institut des NanoSciences de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France ⊥ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom # Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom ▽ London Centre for Nanotechnology and Chemistry Department, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom * S Supporting Information ABSTRACT: Surface X-ray diffraction has been employed to quantitatively determine the geometric structure of an X-ray-induced superhydrophilic rutile-TiO2(110)(1 × 1) surface. A scatterer, assumed to be oxygen, is found at a distance of 1.90 ± 0.02 Å above the five-fold- coordinated surface Ti atom, indicating surface hydroxylation. Two more oxygen atoms, situated further from the substrate, are also included to achieve the optimal agreement between experimental and simulated diffraction data. It is concluded that these latter scatterers are from water molecules, surface-localized through hydrogen bonding. Comparing this interfacial structure with previous studies suggests that the super- hydophilicity of titania is most likely to be a result of the depletion of surface carbon contamination coupled to extensive surface hydroxylation. ■ INTRODUCTION Ever since Wang et al.’s discovery that UV irradiation of titania surface, rutile-TiO2(110), that exhibits superhydrophilicity induced through X-ray exposure. Previously, Shirasawa et al. (SEL) have undertaken SXRD results in a superhydrophilic surface,1 there has been a great measurements from rutile-TiO2(110) to identify changes in deal of effort to both exploit and understand this novel surface structure associated with the UV-induced hydrophobic- phenomenon. Significant success has been achieved in the to-hydrophilic transition.13 They report that the application of former of these two goals, with applications including self- a wet chemical preparation (WCP) recipe to the substrate cleaning windows and antifogging mirrors.2−5 In contrast, resulted in a hydrophobic (1 × 1) surface termination, which uncertainty still remains as to the origin of the super- became hydrophilic upon UV irradiation. It is suggested that hydrophilicity. Currently, there are a number of potential this transition is associated with the presence of surface explanations to be found in the literature,1,3,6−10 but none are hydroxyls (OH), as surface five-fold-coordinated titanium supported by compelling experimental evidence. For example, atoms (Ti5c) and bridging oxygens (Ob) become hydroxylated it is proposed that the superhydrophilicity is simply a result of following the exposure to UV light. Figure 1 illustrates the the removal of surface carbon contamination.6 Other changes in interface geometry concluded in ref 13. researchers suggest that modification of the surface struc- In this Article, we revisit the structure of the super- ture/chemistry of the titania substrate (e.g., surface hydrox- hydrophilic rutile-TiO2(110)(1 × 1) surface. A WCP method ylation) underpins this macroscopic property.9 Longer range structural changes are also purported to be important, Special Issue: Hans-Joachim Freund and Joachim Sauer Festschrift including the formation of nanoscale hydrophobic and hydrophilic domains.1,11,12 Here we directly address this Received: January 9, 2019 topic, employing surface X-ray diffraction (SXRD) to Revised: February 6, 2019 quantitatively determine the structure of a model titania Published: February 6, 2019 © 2019 American Chemical Society 8463 DOI: 10.1021/acs.jpcc.9b00245 J. Phys. Chem. C 2019, 123, 8463−8468 The Journal of Physical Chemistry C Article was not possible to monitor either oxygen or water vapor concentration within the cell. SXRD data were collected at an incidence angle of 1° with the substrate at room temperature using a photon energy of hv = 17.7 keV and a 2D Pilatus photon detector. A systematic series of X-ray reflections was acquired from the sample; that is, for a given (h,k) integer, data were measured as a function of l to facilitate the generation of so-called crystal truncation rods (CTRs). h, k, and l are the reciprocal lattice vectors. They are defined with reference to the real-space (1 × 1) unit cell of the (110) surface, described by lattice vectors (a1, a2, a3) which are parallel, to the [11̅0], [001], and [110] directions, respectively; a1 = a3 = a√2, and a2 = c, where a = 4.593 Å and c = 2.958 Å are the lattice constants of the tetragonal rutile crystal structure. A surface-sensitive reflection (i.e., one that is located well away from any bulk Bragg peak), namely, (−1, 0, 0.9), was recorded at regular intervals to monitor the surface integrity. To facilitate fully quantitative structure determination, the raw 2D diffraction images were integrated, including back- ground removal, and corrected16 to enable plots of structure Figure 1. Ball-and-stick models showing the UV-induced (hydro- factor versus perpendicular momentum transfer for each CTR phobic to hydrophilic) changes in interface geometry for rutile- TiO2(110), as concluded by SEL from SXRD data.13 Red spheres are to be compiled. This procedure generated a total of 1068 Ti atoms and darker (lighter) blue spheres are substrate (adsorbate) nonequivalent reflections from eight distinct CTRs. The ROD oxygen atoms. H atoms (pink spheres) are depicted, although they software17 was employed to simulate these data, with structural were not explicitly included in SEL’s structure determination. Possible (and nonstructural) parameters being refined to achieve the hydrogen bonds are indicated by dashed lines. overall best fit between experiment and theory. Reduced χ2 was used to evaluate the goodness of fit, which is defined as follows18 ij |F exp(hkl)| − |Fith(hkl)| yz ∑ jjjj i zz is again employed for sample preparation, but with X-rays zz N 2 1 i=1 k { being used to induce superhydrophilicity. Similar to ref 13, the 2 χ = surface is found to be extensively hydroxylated, including OH exp N−P σi ( hkl ) bound to Ti5c. The diffraction data acquired in this study, however, resemble much more closely those acquired from the N is the number of measured structure factors, P is the number hydrophobic termination in ref 13. As argued in detail below, of parameters optimized during fitting, and Fexp i (hkl) and this somewhat unexpected finding suggests that the analysis Fth i (hkl) are the experimental and theoretically calculated and interpretation of SEL13 require revision. structure factors, respectively. σexp i (hkl) is the uncertainty ■ i (hkl). χ behaves such that a value of 1 associated with Fexp 2 EXPERIMENTAL METHODS indicates that experiment and theory are essentially coincident, Concerning sample preparation, a previously published WCP with agreement decreasing with increasing χ2. The quoted recipe14 was applied to the rutile-TiO2(110) substrate (10 × precision of each fitted parameter is determined by systemati- 10 × 1 mm sample from PI-KEM), which is known to produce cally varying the parameter about its optimal value and for each a well-ordered (1 × 1) surface termination. In brief, this step optimizing all other parameters, until χ2 has increased by 1/(N − P) from its minimum value.18 ■ procedure involves sequential sonication in acetone, ethanol, and deionized water and then annealing in air at ∼973 K for ∼90 min. Subsequently, the sample is immersed in aqua regia RESULTS AND DISCUSSION solution (3:1 by volume ratio of concentrated HCl and The application of our WCP recipe to the rutile-TiO2(110) HNO3) at room temperature for ∼45 min. The final UV-ozone sample resulted in a deionized water contact angle of ∼80°. treatment step described in ref 14 was not undertaken in this This value is consistent with that reported in ref 14 for a (1 × study. Please note that the TiO2 sample remained transparent 1) surface termination subsequent to immersion in aqua regia following the application of this WCP recipe, indicating that but not exposed to UV-ozone treatment. Following exposure there was no bulk reduction.15 to I07’s photon beam, the contact angle was found to fall to Upon the completion of surface preparation, surface essentially 0°; that is, a superhydrophilic transition was hydrophilicity was evaluated by delivering a small droplet of induced by X-ray exposure. All diffraction measurements deionized water to the TiO2(110) surface using a syringe/ were undertaken with the rutile-TiO2(110)(1 × 1) surface in hypodermic needle. Once the contact angle of the deposited this state; a contact-angle measurement at the end of data droplet had been determined by visual inspection, the sample collection indicated that a value of 0° was maintained was blown dry with high-purity nitrogen. The sample was then throughout this period. Data from the (−1, 0, 0.9) reference transferred to the diffractometer located in EH1 of beamline reflection also revealed no substantive surface degradration. I07 at the Diamond Light Source synchrotron facility for Figure 2 shows four of the experimental CTRs acquired in SXRD measurements. It was mounted in an X-ray transparent the current study (black markers with error bars), together (cylindrical Kapton window) environmental cell. Once the cell with equivalent data collected by SEL13 from rutile-TiO2(110) was closed up, high-purity helium was flowed through it for the following UV exposure (blue markers with error bars). A duration of the SXRD measurements. It should be noted that it priori, as both data sets were recorded from superhydrophilic 8464 DOI: 10.1021/acs.jpcc.9b00245 J. Phys. Chem. C 2019, 123, 8463−8468 The Journal of Physical Chemistry C Article perfect, suggesting that the correct structural solution had not been found. On this basis, we explored other potential surface terminations, including those consistent with the presence of surface hydroxyls. The resulting overall best fit to the experimental CTRs is shown in Figure 3 (red line). To achieve this fit, 78 parameters were optimized, that is, 51 atomic coordinates, 21 Debye−Waller (DW) factors, a scale factor, surface roughness (β), three fractional occupancies, and surface fraction. The corresponding χ2 is 1.05; that is, there is an excellent level of agreement between the experimental and Figure 2. Comparison of the (0, 1, l), (1,0, l), (1, 1, l), and (2, 0, l) simulated data. experimental CTRs acquired from X-ray-induced superhydrophilic The optimum geometry of the first few atomic layers rutile-TiO2(110) in the current study with data from SEL.13 Current emerging from the best fit to the experimental CTR profiles is study: black markers with error bars; pre-UV exposure from SEL:13 depicted in Figure 4. Selected corresponding interatomic red markers with error bars; post-UV exposure from SEL:13 blue markers with error bars. Profiles are systematically offset for clarity. surfaces, it was expected that they would be very similar. However, there are significant differences. For example, the local maxima in our data at (0, 1, ∼3) and (1, 0, ∼2), are not replicated in those from SEL. In contrast, our CTR profiles are much more comparable to those reported by SEL for their pre- UV exposure (hydrophobic) surface. These data are also shown in Figure 2 as red markers with error bars. We note that on an adsorbate-free rutile-TiO2(110)(1 × 1) surface, prepared in ultrahigh vacuum (UHV), the aforementioned local maxima are associated with significant displacements of surface atoms away from their bulk positions;19 that is, they are not a direct signature of surface hydrophilicity. An absence of such features may be a result of either a more bulk-like termination or surface roughening. Considering the qualitative comparison outlined above, it was expected that fitting of our experimental SXRD data would result in the hydrophobic structure determined by SEL,13 Figure 4. Ball-and-stick models of the X-ray-induced superhydrophilic where molecular H2O is bound atop Ti5c (see Figure 1). Figure rutile-TiO2(110) surface structure determined from SXRD data. 3 shows the best fit (blue line) achieved using SEL’s Perspective (top) and plane (bottom) views are shown. Red spheres hydrophobic structure as a starting point and simply allowing are Ti atoms, and darker (lighter) blue spheres are substrate the displacement of both atomic positions and nonstructural (adsorbate) oxygen atoms. H atoms (pink spheres) are depicted, parameter values. As indicated by χ2 = 2.60, as well as visual although they were not explicitly included in the structure inspection, the experiment−theory agreement is far from determination. Possible hydrogen bonds are indicated by dashed lines. The numerical labeling of the atoms is employed in Table 1 and Table S1 for identification purposes. Symmetry-paired atoms are denoted by *. distances are listed in Table 1. A ball-and-stick model showing all atoms displaced during fitting is shown in Figure S1, along with a complete list of the optimized coordinates, DW factors, Table 1. Selected Interatomic Distances Derived from Atomic Coordinates (Table S1) of Optimized Superhydrophilic TiO2(110)(1 × 1) Structure atoms interatomic distance (Å) O(3′)−O(2′) 2.70 ± 0.06 O(3′)−O(1′) 2.65 ± 0.05 O(2′)−O(1) 2.68 ± 0.03 O(1′)−Ti(2) 1.90 ± 0.02 Figure 3. Comparison of experimental CTR data (black markers with O(1)−Ti(1) 1.83 ± 0.02 error bars), acquired from X-ray-induced superhydrophilic rutile- O(2)−Ti(1) 1.98 ± 0.02 TiO2(110), and theoretical best-fit simulations. Solid blue line O(2)−Ti(2) 1.95 ± 0.02 indicates the best fit achieved following relaxation of the hydrophobic structure reported by SEL.13 Solid red line indicates the overall best O(3)−Ti(1) 1.94 ± 0.01 fit, with β = 0.24 and a surface fraction of 0.96. O(4)−Ti(2) 1.94 ± 0.01 8465 DOI: 10.1021/acs.jpcc.9b00245 J. Phys. Chem. C 2019, 123, 8463−8468 The Journal of Physical Chemistry C Article Figure 5. Ball-and-stick models of the optimum interfacial structures determined from SXRD for X-ray-induced superhydrophilic rutile-TiO2(110) (current study), rutile-TiO2(110) subsequent to dipping in H2O(l),20 and rutile-TiO2(110) submerged in H2O(l).20 Side (top) and plane (bottom) views are shown. Red spheres are Ti atoms, and darker (lighter) blue spheres are substrate (adsorbate) oxygen atoms. Selected interatomic distances are annotated. Fractional occupancies of adsorbate oxygen atoms are indicated by the values inscribed on the lighter blue spheres. and fractional occupancies in Table S1. Neglecting the details 0.02 versus 2.09 ± 0.03 Å13). To this end, our experimental of atomic relaxation, the surface mimics the stoichiometry and data set (eight CTRs) was reduced to match that of SEL (six geometry of bulk-terminated rutile-TiO2(110)(1 × 1) but is CTRs), and fitting was undertaken. Under these conditions, we decorated with oxygen species. Focusing on Ti5c (labeled were able to effectively model the data with SEL’s hydrophobic Ti(2)), an adsorbed oxygen atom (labeled O(1′)) is located structure. On this basis, it is evident that fewer experimental atop at a distance of 1.90 ± 0.02 Å, which is consistent with CTRs leads to a local χ2 minimum, resulting in a significantly the presence of a bound terminal hydroxyl (OHt).20,21 The different surface structure. experimental distance from ref 20 is 1.95 ± 0.03 Å, with a One other matter emerging from the preceding discussion is moderately longer distance of 2.07 Å being obtained from the origin of the UV-induced change in CTR profiles observed molecular dynamics calculations.20 Two additional non- by SEL.13 Assuming that their pre-UV data is acquired from a substrate oxygen atoms (labeled O(2′) and O(3′)) are at superhydrophilic area of the rutile-TiO2(110) surface, then the somewhat greater distances from the topmost substrate atoms. observed changes cannot be accounted for by a hydrophobic− O(2′) is 2.68 ± 0.03 Å above the bridging oxygen atom hydrophilic transition. This deduction implies that UV- (labeled O(1)), with O(3′)’s nearest neighbor being O(1′) at a irradiation leads to additional interfacial modification; that is, distance of 2.65 ± 0.05 Å. These interatomic separations a unique surface structure is formed upon exposure to UV suggest that oxygen atoms O(2′) and O(3′) arise from water light. Currently, this suggestion is essentially conjecture, but is molecules, which are localized through hydrogen bonding.22 worthy of further investigation. For illustrative purposes, we have included H atoms in Figure 4 Having reconciled the results of this study with those of but stress that these species were not explicitly included during SEL,13 a comparison of the geometry of the current the generation of simulated SXRD data due to their negligible superhydrophilic termination with those reported for interfaces X-ray scattering. formed by the exposure of UHV-prepared TiO2(110)(1 × 1) Given the optimized structure displayed in Figure 4, it is to liquid water (H2O(l)) is worthwhile.20 Figure 5 compares interesting to compare this result with other pertinent studies. the current optimum structure (TiO2(110):Super) to that Focusing initially on SEL’s work,13 the present diffraction data are very similar to those acquired from their hydrophobic elucidated with SXRD following dipping of TiO2(110)(1 × 1) surface, as demonstrated in Figure 2. Because our surface is into H2O(l) and measuring ex situ in UHV (TiO2(110):Dip- superhydrophilic, as a result of X-ray exposure, this agreement H2O(l)) as well as that determined for TiO2(110) submerged presents a conundrum. One plausible explanation, arising from in H2O(l) (TiO2(110):Sub-H2O(l)). These three structures are discussion with SEL,13 is that the ∼1 mm2 footprint of the X- similar but not identical. For example, both TiO2(110):Super ray beam employed for their SXRD measurements induced and TiO2(110):Sub-H2O(l) exhibit oxygen atoms consistent superhydrophilicity only in this region. Hence the water with hydrogen-bonded H2O molecules, although their contact-angle measurement, where the droplet employed configuration differs; such scatterers are not evident in the covered a much larger surface area, did not reveal this local TiO2(110):Dip-H2O(l) data due to the acquisition in UHV. X-ray-induced superhydrophilicity. It should be noted that on Turning to Ti5c, for each structure displayed in Figure 5, the I07 almost the entire sample surface would have been exposed distance to the atop oxygen atom is consistent with to the X-ray beam during alignment and measurement. hydroxylation (Ti5c−OHt). However, TiO2(110):Super dis- On the basis that SEL’s pre-UV-exposure surface is plays a slightly shorter Ti5c−OHt distance (1.90 ± 0.02 Å) superhydrophilic in the area probed by the X-ray beam, then than either TiO2(110):Dip-H2O(l) or TiO2(110):Sub-H2O(l) one further issue requires resolution. Specifically, despite the (1.95 ± 0.03 Å). This variation may be a result of the former similar experimental data, the discrepancy between our substrate being essentially fully oxidized, whereas the latter two optimized structure and SEL’s needs to be understood, for were somewhat reduced as a result of substrate preparation in example, the variation in the Ti(2)−O(1′) distance (1.90 ± UHV. 8466 DOI: 10.1021/acs.jpcc.9b00245 J. Phys. Chem. C 2019, 123, 8463−8468 The Journal of Physical Chemistry C Article Regarding the origin of the X-ray-induced superhydrophi- Atomic coordinates for optimum structure determined licity of titania, the current SXRD data rule out the coexistence in this study (CIF) of hydrophilic and hydrophobic domains, as analysis indicates Figure S1. Ball-and-stick model of optimized super- that almost the entire surface adopts the same geometry; that hydrophilic TiO2(110) surface, showing all atoms is, surface fraction is 0.96. It should be emphasized that the displaced during fitting of SXRD data. Table S1. present study cannot be used to definitively rule out the Optimized coordinates of atoms obtained from fitting existence of such domains on UV-exposed titania. Further- of SXRD data acquired from superhydrophilic rutile- more, because the diffraction data from TiO2(110):Dip-H2O(l) TiO2(110) (PDF) were acquired in UHV from a surface not irradiated with either UV or X-rays during dipping, the mere presence of OHt cannot be directly related to photoinduced superhydrophilicity. Given this result, one could suggest that the simple removal of surface ■ AUTHOR INFORMATION Corresponding Author carbon most likely underpins this property.6 It is, however, *Tel: +44 161 306 4824. Fax: +44 161 306 4865. E-mail: notable that the fractional occupancy of OHt (O(1′)) for robert.lindsay@manchester.ac.uk. TiO2(110):Super is approximately double that for either ORCID TiO2(110):Dip-H2O(l) or TiO2(110):Sub-H2O(l), that is, 1.00 compared to 0.45 and 0.50, respectively, as indicated in G. Cabailh: 0000-0002-8053-2132 Figure 5. Hence, increased surface hydroxylation may play a G. Thornton: 0000-0002-1616-5606 role in TiO2 superhydrophilicity, coupled to the loss of surface R. Lindsay: 0000-0001-5050-669X carbon. We remark that in ref 20 ab initio modeling suggests Notes that the presence of OHt is a result of (near) surface defects The authors declare no competing financial interest. driving surface H2O dissociation. Because the substrate in the current study is not expected to possess any significant concentration of defects, the hydroxyl species must arise from ■ ACKNOWLEDGMENTS We very much appreciate the highly constructive discussion elsewhere. Almost certainly, it is photon-induced (or photo- with Tetsuroh Shirasawa as well as his allowing us access to excited electron) chemistry that produces these OH t relevant data. This study was funded by an ERC Advanced adsorbates, which may be the reason that a higher coverage Grant ENERGYSURF (G.T.), EPSRC (UK) (EP/C541898/ is achieved; we note that this increase in surface hydroxylation 1), EU COST action CM1104, the Royal Society, M.E.C. is not simply related to carbon removal, as the surfaces in ref (Spain) through project MAT2015-68760-C2-2-P, Severo 20 are reported to be relatively carbon-free (≤0.1 monolayer). Ochoa SEV-2015-0496 Grant, and project EFA194/16/TNSI Finally, we would like to comment on a recent elegant study funded by Interreg POCTEFA Program. J.P.W.T. acknowl- suggesting that air- or aqueous-solution-exposed rutile- edges support from Diamond Light Source. ■ TiO2(110) is commonly decorated by carboxylate species.23 On the basis that SXRD is not a spectroscopic probe, there is REFERENCES always the potential for misidentification of adsorbates, (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, especially those exhibiting similar X-ray scattering character- E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Light-Induced istics (e.g., C and O). For the current study, however, we argue Amphiphilic Surfaces. Nature 1997, 388, 431−432. that this is not the case. Supporting evidence is two-fold. First, (2) Isaifan, R. J.; Samara, A.; Suwaileh, W.; Johnson, D.; Yiming, W.; Auger spectra acquired from a superhydrophilic rutile- Abdallah, A. A.; Aïssa, B. 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