Defect-Induced Water Bilayer Growth on Anatase TiO2(101

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Defect induced water bilayer growth on anatase TiO2(101) Andreas Schaefer, Valeria Lanzilotto, Ute B Cappel, Per Uvdal, Anne Borg, and Anders Sandell Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01925 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Defect induced water bilayer growth on anatase TiO2(101) A. Schaefer1, V. Lanzilotto2, U. B. Cappel2,a, P. Uvdal3, A. Borg4, A. Sandell2 1

Department of Chemistry and Chemical Engineering, and Competence Centre for Catalysis, Chalmers University of Technology, 41296 Gothenburg, Sweden 2

Dept. of Physics and Astronomy, Uppsala University, P. O. Box 516, SE-75120 Uppsala, Sweden

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Chemical Physics, Department of Chemistry, Lund University, P. O. Box 124, SE-221 00 Lund, Sweden 4

Dept. of Physics, NTNU – Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

Keywords: Metal oxides; Water adsorption; Monolayer; Dissociation; Hydroxyl groups; Titania; Photoelectron Spectroscopy; Oxygen vacancy Abstract Preparing an anatase TiO2(101) surface with a high density of oxygen vacancies and associated reduced Ti species in the near-surface region results in drastic changes in the water adsorption chemistry compared to adsorption on a highly stoichiometric surface. Using synchrotron radiation excited photoelectron spectroscopy, we observe a change in the water growth mode; from layer-by-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. Furthermore, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment occurs concomitant with effective water dissociation into hydroxyls with a very high thermal stability. The water bilayer on the reduced surface is thermally more stable than on the stoichiometric surface and it is more efficient in promoting further water dissociation upon heating. The results thus show how the presence of subsurface defects can alter the wetting mechanism of an oxide surface.

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Defect induced water bilayer growth on anatase TiO2(101)

Introduction The interaction of water with oxide surfaces is one of the most basic processes in nature and an inherent part in many technological applications. Titanium dioxide (TiO2) is in this respect a prominent example, forming the basis in self-cleaning coatings, dye-sensitized solar cells as well as in various designs for solar water splitting and photocatalysis.1-6 Molecular level understanding of the structure of water layers on solid surfaces can be extremely challenging, even when performing studies of single crystalline samples in ultrahigh vacuum (UHV). This is particularly valid for oxides and TiO2 is indeed no exception. The vast majority of the UHV work has been devoted to the thermodynamically stable (110) face of rutile TiO2.78

When prepared in UHV, TiO2(110) typically exhibits a significant density of oxygen

vacancies in the form of missing two-fold coordinated (bridging) oxygens at the surface. It is by now well established that such oxygen vacancy sites act as very efficient dissociation centers for water.7 Water can also dissociate on regular Ti-sites on the stoichiometric TiO2(110) surface9-10, but this process is quenched upon the introduction of more effective dissociation sites in the form of oxygen vacancies.11 Consequently, the chemistry of water on stoichiometric TiO2 surfaces can be complex and, in addition, substantially altered in the presence of defects. It is commonly argued that anatase TiO2 is more relevant for applications than rutile. The reason is that assemblies of TiO2 nanoparticles are often employed in devices (mainly to maximize the active surface area) and anatase is the preferred phase in the nanometer size domain.12-13 Anatase is also put forward as the most photoactive phase2, 14-15, even though phase mixtures have proven superior to the pure phases.16 The most stable termination of anatase TiO2 is the (101) surface plane, meaning that this is the dominant facet of nanocrystals. The (101) surface has a sawtooth-like appearance, featuring alternating rows of five-fold coordinated Ti (Ti5c) and two-fold coordinated O (O2c) along the [010] direction. Adsorption on the Ti5c sites defines the first layer while water adsorption on O2c rows defines the second layer. Early work1719

of water on anatase (101) advocate molecular adsorption only. Particularly noteworthy are

the results from scanning tunneling microscopy (STM) in conjunction with calculations using density functional theory (DFT), which revealed a repulsive water-water interaction within the first layer.19 This makes the water chemistry of anatase (101) a highly unusual oxide surface since it is more common that hydrogen bonded water assemblies form. The repulsive waterwater interaction on anatase (101) has also been nicely demonstrated in a very recent report, in which STM images reveal the formation of dense water chains at 80 K that transform into monomers upon heating to 190 K.20

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XPS21 and recent STM-IETS22 results have furthermore shown that a mixture of molecular and dissociated species forms on stoichiometric terraces of the (101) surface upon heating of an ice layer. Very recently, we were also able to follow up on our early XPS study by showing that partial dissociation occurs during growth of the first layer on a highly stoichiometric TiO2(101) surface. However, the relative population of the dissociated state was found to be much smaller during growth compared to that observed upon heating of a thick ice film.23 Evidently, the dissociation results from water-water interaction, which lowers the energy cost for partial dissociation on the stoichiometric surface. This supports the notion of facilitated dissociation in the presence of an extended hydrogen bonding network predicted in theoretical studies.24-25 Similar to rutile (110), cleaning of anatase (101) in UHV by sputtering and annealing brings about oxygen vacancies. Interestingly, in contrast with rutile (110), oxygen vacancies on the anatase (101) surface are unstable and tend to move subsurface.26 Low-temperature STM images show that surface oxygen vacancies start to migrate to subsurface sites at temperatures ≥ 200 K.27 In photoinduced processes, oxygen vacancies might be expected to act as charge trapping and/or recombination centers. Although the direct impact of such defects on the photoreactivity of TiO2 is not fully understood it stands clear that the influence from them will be very different for rutile TiO2(110) and anatase TiO2(101). For rutile TiO2(110), the oxygen vacancies are located on the surface and hence easily quenched by adsorption, whereas if the vacancies instead reside subsurface, as for anatase TiO2(101), their function as charge traps is not as easily quenched by adsorption. It cannot be excluded that the preference for oxygen vacancies to reside subsurface contributes to the superior photocatalytic activity of anatase over rutile.26, 28 STM images suggest that water adsorbs more strongly on the reduced surface with subsurface defects with molecules observed at 400 K.29 This was supported by densityfunctional theory (DFT) calculations that show enhanced adsorption strength on the surface with subsurface defects vs. the stoichiometric surface. Molecular adsorption was still energetically favored but a low-barrier pathway (0.26 eV) for dissociation was found 29. That the introduction of oxygen vacancies alters the water chemistry was also demonstrated in CarParrinello MD simulations.30 Simulations of disperse molecules and a monolayer indicated that, firstly, dissociation is possible on the defective surface and, secondly, that some molecules shift coordination from Ti to bridging oxygen sites while in the monolayer (ML) regime. This is in stark contrast to the perfect (101) surface, on which only Ti sites are populated up to ML coverage. The subsequent STM results in ref. 20 can be viewed as a confirmation of this result.

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Additionally, it has been proposed that the stability of surface and subsurface oxygen vacancies can be reversed upon molecular adsorption.31 In an STM study supported by DFT calculations it was discovered that if the vacancies were not too far into the bulk, they move to the surface upon O2 adsorption with the result of O2 populating an anion site. In a recent theoretical study using first-principles calculations the results suggest that a reversal of the stability of the surface and subsurface oxygen vacancies can also occur upon water adsorption.32 This interplay between water and defects allows for facile water dissociation by two alternative pathways: Either the adsorbed water molecules induce migration of the vacancy to the surface which allows for facile dissociation or water dissociates in the presence of a subsurface vacancy which leads to subsequent diffusion of the vacancy to the surface. Experimentally, the adsorption of water on stoichiometric and reduced anatase (101) has been compared in Near-Ambient-Pressure XPS (NAP-XPS) experiments.33 Water adsorption was found to take place at pressures of 0.6 mbar and above when the sample was at room temperature. Mixed molecular and dissociated adsorption was observed on both the highly reduced surface prepared by sputtering and the stoichiometric surface attained after sputtering and subsequent annealing. The similar behavior is in line with the finding of oxygen vacancies moving subsurface already at room temperature, but the cause of the dissociation was not conclusively determined. It has also been found that oxygen vacancies on anatase (101) created by e-beam irradiation are promptly hydroxylated by the dissociative adsorption of water from the residual vacuum and, in this way, stabilized at the surface.34 In this paper, we present a synchrotron radiation-based photoelectron spectroscopy study of the water/anatase (101) system. On the basis of uptake and heating series while measuring in a highly surface sensitive mode we have been able to reveal very different wetting mechanisms for highly stoichiometric and reduced anatase TiO2(101) surfaces. Specifically, we disclose the presence of different water growth modes and dissociation channels. While dissociation on the stoichiometric surface occurs by water-water interaction23, prompt dissociation occurs on the reduced surface due to the presence and migration of oxygen vacancies and associated surplus electrons. Experimental Section The experiments were conducted at the former bending magnet soft X-ray beamline D1011 at the MAX-IV Laboratory in Lund, Sweden.35-36 The end station at beamline D1011 was equipped with a SES200 electron energy analyzer (SCIENTA). The analysis chamber was

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connected via a gate valve to a chamber housing the usual equipment for cleaning and preparation of samples in UHV, that is, a sputter gun, a low-energy electron diffraction (LEED) instrument and a residual gas analyzer (RGA). The photon energies used to record the Ti 2p and O 1s spectra were 590 eV and 610 eV, respectively. The valence spectra were recorded using a photon energy of 130 eV. The spectra presented were recorded in grazing emission to enhance the surface sensitivity, i.e. with a take-off angle of 60° with respect to the surface normal. The binding energy (BE) scale was referenced to the Fermi level of a Pt foil mounted on the sample holder. The anatase TiO2(101) single crystal (SurfaceNet) was prepared by several cycles of Ar ion sputtering at 1-2 keV and annealing to 870 K. The cleaning cycles were repeated until no contaminants (most notably C, K, Si) were observed in surface sensitive photoemission spectra. The oxidized surface (denoted o-TiO2) was prepared by annealing in O2 (1.0´10-6 mbar) in order to minimize the amount of oxygen vacancies. Thus, the clean o-TiO2(101) surface was prepared following basically the same method as in the STM-IETS study.22 A moderate annealing temperature was used to reduce the risk of contaminant surface segregation (≤2 % Fe was seen after oxygen treatment). The reduced surface (denoted r-TiO2) was prepared by sputtering the oxidized surface at 1 keV for 10 minutes followed by a brief annealing to 770 K. The uptake series were measured with a water partial pressure of 1.0´10-10 mbar added, that is, the total pressure was kept at 2.0´10-10 mbar. We used heavy water as a precaution against effects caused by synchrotron light.37 However, just as in previous studies at this beamline38, we observed no significant effects of synchrotron light on the adsorbed water (see supplementary information for details). The measurement of a single O 1s spectrum in the uptake series took 122 seconds. One scan thus entailed a water dose of about 0.03 L [1 Langmuir (L) = 1.33´10-6 mbar·sec], which is an upper limit based on having a background dominated by water. The water coverage (in ML) was obtained by normalization of the integrated O 1s intensity to that of a monolayer on rutile TiO2(110), measured under exactly the same conditions. For both rutile (110) and anatase (101), 1 ML corresponds to a density of 5.2´1014 cm-2. The heating rate was held at 0.10±0.05 K/s. The mounting of the sample on a flag-style plate gives an uncertainty in the temperature reading during the heating series. To improve the accuracy we chose to adjust the temperature scale of differentiated O 1s intensities from D2O to fit with TPD data from ref. 17, giving a positive offset by 85±10 K.23

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Results and Discussion Figure 1 compares Ti 2p3/2 and O 1s spectra of the o-TiO2 and r-TiO2 surfaces. The Ti 2p3/2 spectrum for the o-TiO2 surface has the Ti4+ and Ti3+ peaks at 458.8 eV and 457.2 eV. This reproduces our previous work very well, where the BE of the Ti 2p3/2 peak of the Ti4+ state was reported as 458.9 eV.21 The relative intensities are 98% and 2% although an acceptable fit can also be achieved without the inclusion of a Ti3+ component. Thus, the surface is to a very good approximation stoichiometric where 2% Ti3+ should be considered as an upper limit. The Ti 2p3/2 spectrum for r-TiO2 displays a main peak at 459.2 eV BE associated with Ti4+ and a shoulder at 457.6 eV associated with Ti3+. The relative intensities are 92% and 8%. These values can be compared to the values of 83% and 17% observed directly after sputtering, which means that the very brief annealing was sufficient to induce considerable in-diffusion of oxygen vacancies, c.f. Supplementary Information. We note that the BE of 459.2 eV for the Ti4+ peak of r-TiO2 is in excellent agreement with the BE reported by Jackman et al. for reduced anatase (101).33 We attribute the different Ti 2p BE for the two surfaces to differences in the band bending due to different abundance and distribution of the reduced Ti species. The O 1s peak of the clean surfaces has an asymmetry towards the high BE side; the origin of this is not clear. It can be due to spurious OH or be an inherent part of the substrate. The O 1s line profile shows a slightly more intense high BE shoulder for the reduced surface. One possible explanation for the asymmetry of the line profile has been provided by theory, showing that a structural relaxation of the first few atomic layers can lead to extra signals at slightly higher binding energy.24 A hard X-ray based photoelectron study by Jackman et al., however, suggests that also O 1s spectra from deeper layers exhibit the same or a similar line shape, pointing to the origin of the effect being an extra component or an intrinsic asymmetry.33 Figure 2 shows two-dimensional plots of O 1s spectra measured in real time during water growth series on the o-TiO2 and r-TiO2 surfaces at a sample temperature of 102 K. Clear differences are observed in spite of the very similar water exposures. To disclose these in detail, each spectrum was subsequently analyzed with respect to D2O and OD coverage and BE shifts by way of curve fitting. For details on the analysis of the O 1s spectra we refer to the Supplementary Information and ref.23. To give a specific example, Figure 2a compares O 1s spectra measured after exposing the two surfaces to the equivalent of 1 ML water. It stands clear that the ML formed on the r-TiO2 surface yields a higher relative intensity of the OD component and a lower intensity of the D2O component as compared to the ML formed the o-

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TiO2 surface. In addition, a significant difference in the BE shift vs. the TiO2-related peak is found. As will be shown, these observations signal very different wetting behaviors. Figure 3 summarizes the integrated intensities of the OD and D2O components, which have been directly translated into coverages (in ML). The formation of hydroxyls (OD) is monitored in Figure 3a. Here it should be noted that the given OD coverages are based on the intensity of the OD related O 1s peak obtained after subtraction of the O 1s high-BE shoulder found for the clean surface. It is readily observed that the formation of OD is different for the two surfaces. Growth at 102 K on the r-TiO2 surface results in a quick increase of the OD related O 1s signal and after only a few scans a level corresponding to 0.16±0.02 ML is reached. Further water exposure does not lead to an increase in the OD amount. In contrast, very little OD is formed on the o-TiO2 surface in the early stages, in agreement with the previously proposed low propensity towards dissociation on the perfect (101) surface. However, after about 25 scans, the OD growth rate on o-TiO2 increases significantly, reaching a maximum coverage of 0.14 ML after about 50 scans. The uptake curves for molecular water also differ for the two surfaces, Figure 3b. On o-TiO2 a clear change in uptake rate occurs at a D2O coverage of 0.90±0.05 ML. This kink signals the onset for second layer growth, from which follows that water grows in a layer-bylayer fashion on o-TiO2 at 102 K. We also note that uptakes ≥0.8 ML entails a changeover to a regime where the OD population increases faster than in the beginning. The change to a higher OD formation rate thus takes place close to the point at which adsorption of second layer water begins, supporting the notion that second layer water promotes dissociation within the first layer.23 The D2O uptake on r-TiO2 shows no change in the rate within the studied exposure regime. Based on the assumption of layer-by-layer growth on o-TiO2, it can be shown that growth on r-TiO2 is consistent with direct growth of a bilayer. Figure 4 shows the total uptake, derived as Q(D2O) + 0.5 Q(OD), as a function of exposure in ML. It is seen that the intensity increases from 1.0 to 1.3 for the o-TiO2 surface between 1 ML and 2 ML. Thus, the contribution from the first layer when covered by the second layer amounts to 0.3. It is then straightforward to calculate the intensity in the case of bilayer growth by multiplying the exposure in ML with a factor of 0.5(1+0.3) = 0.65. The agreement with the intensity increase found for growth on the r-TiO2 surface is excellent (red line in Figure 4). The O 1s BE of D2O is furthermore known to be sensitive to the water bonding configuration such that hydrogen-bonding network formation leads to a BE downshift.24, 39-41 In Figure 5, the O 1s BE of D2O relative that of TiO2 has been plotted for the growth series.

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Consistent with the different growth modes inferred from Figure 4, very different behaviors in the water O 1s BE are observed. In the case of water on the o-TiO2 surface, the BE relative to the O 1s peak of TiO2 is constant at 3.90±0.03 eV up to 0.90±0.05 ML, after which a dramatic decrease is noted. The coverage at which the O 1s BE starts to decrease coincides with the onset for second layer growth derived from Figure 4, indicative of hydrogen bonding network formation between the first and second layer. The total BE downshift of the O 1s peak amounts to 0.64 eV after an exposure of 1.9 ML. In the case of water on the r-TiO2 surface, we find a progressive decrease in the water O 1s BE and no plateau in the beginning. After an exposure of 1.9 ML the O 1s downshift is 0.67 eV. Thus, the exposure dependence of the BE is very different but the total BE shift of the water O 1s peak reached after about 2 ML on the two surfaces is very similar. This suggests very similar hydrogen bonding networks at 2 ML but very different pathways to get there, which, in turn, correlates very well with the intensity behavior. That is, the intensity behavior and the BE shift of the water O 1s peak consistently points to a layer-by-layer growth on o-TiO2 and bilayer growth on r-TiO2. This result confirms the earlier calculations in ref.

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, which predicted facile dissociation at a defective (reduced)

surface as well as the adsorption of molecules on O2c already in the submonolayer regime. The calculations also revealed that while the ML structures are very different on the two surfaces, the bilayer structures become quite similar in terms of water-water interaction. The number of water-water hydrogen bonds per molecule for the ML on the perfect surface was calculated to 0.03, whereas a number of 0.85 was obtained for the ML on the defective surface. For the bilayer, numbers of 2.11 (perfect surface) and 2.13 (defective surface) were obtained. This is fully consistent with the behavior of the O 1s peak of D2O presented in Figure 5, showing the absence of hydrogen bond formation ≤1 ML on the o-TiO2 surface but gives a clear indication for hydrogen bond formation ≤1 ML on the r-TiO2 surface. Likewise, the total O 1s shift reached at bilayer coverage is the same, which would then indicate a similar degree of waterwater hydrogen bonds. Water adsorption on the reduced surface also brings about changes in the substrate. In Figure 6a, it can be seen that water adsorption on the r-TiO2 surface at 102 K results in a significantly increased contribution from reduced Ti species (Ti3+) in the Ti 2p3/2 spectrum; the relative intensity of the Ti3+ component increases from 8% to 18%. The next spectrum is measured after desorption of nearly all of the molecular water by heating to 300 K. No change is observed in the relative intensities of the Ti3+ component. The increased Ti3+ contribution must therefore be associated with the remaining adsorbate, namely OD species. The oxygen vacancies are associated with surplus electrons, located at Ti sites and giving rise to Ti3+

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species. Hence, the most straightforward explanation is that sub-surface oxygen vacancies and the associated surplus electrons move to the surface to initiate water dissociation, validating the scenario proposed in ref. 32. In contrast, Figure 6b shows that no Ti3+ state emerges upon D2O adsorption on the o-TiO2 surface at 102 K or upon subsequent water desorption by annealing to 470 K. Consequently, there is no evidence for defect promoted dissociation on the o-TiO2 surface. Defect out-diffusion upon water adsorption has not been reported in previous experimental studies of reduced anatase TiO2(101). There could be different reasons for this. It has been proposed that the activation energy for vacancy migration depends on the vacancy concentration. We expect the r-TiO2 surface in this work, prepared by a short annealing, to have a higher density of sub-surface oxygen vacancies than the r-TiO2 surface attained after a more extended annealing. In fact, the previous studies typically address surfaces prepared by longer annealing. Having a higher concentration of defects can be favorable for out-diffusion of the vacancies upon water adsorption and subsequent dissociation. Clearly, there is a need for further studies of surfaces where the degree of sputtering and annealing is systematically varied. Schematic illustrations summarizing the growth processes on the two surfaces are shown in Figure 7. For the r-TiO2 surface, the presence of a sufficient amount of subsurface oxygen vacancies (i) allows for prompt water dissociation and concomitant out-diffusion of the defects (ii). The oxygen vacancy in concert with associated Ti3+ species induces a coordination change of a fraction of the molecular water from Ti sites to O2c sites.32 That is, water adsorbs in the form of a bilayer (iii-iv). This behavior contrasts that of the o-TiO2 surface, on which water adsorbs in a 2D fashion and where dissociation is minimal at low coverage. Only at higher coverage, significant amounts of OD are observed. The subsequent removal of the water layer by heating was also investigated. Figures 9a and 9b compare OD and D2O coverages during heating of the water layers, formed at 102 K, on r-TiO2 and o-TiO2, respectively. The initial D2O coverage on o-TiO2 was about 3 ML. Upon heating, the D2O amount decreased rapidly, reaching 1 ML at about 220 K and the last D2O desorbs around 300 K (Figure 8b). The OD coverage shows a slight increase, with a maximum of 0.16 ML at 220 K (Figure 8a). Above 220 K, the OD coverage decreases progressively, with most of the desorption overlapping with desorption of first layer D2O. A very different behavior is observed on the r-TiO2 surface: The true D2O coverage at the start was about 2 ML (note that the starting value of 1.3 in Figure 8b corresponds to about 2 ML total coverage due to the attenuation of the first layer by the second, as discussed above). The O 1s intensity for D2O shows very little change up to 240 K (Figure 8b). A corresponding

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plateau is not observed for D2O on o-TiO2. Consequently, the 2-ML situation is considerably more stable on the r-TiO2 surface than on the o-TiO2 surface. On the r-TiO2 surface, a much more pronounced increase in the OD coverage furthermore occurs: A maximum of 0.43±0.02 ML is observed at 270 K, from which follows that heating has generated an additional 0.27 ML OD. At temperatures above 270 K, the OD amount decreases and it levels out at around 350 K at a level corresponding to 0.20±0.02 ML. That is, the OD coverage has at this point essentially returned back to the level associated with the initial dissociation at oxygen vacancies. The heating-induced dissociation on r-TiO2 deserves further scrutiny. The OD coverage starts to increase before the onset for bilayer desorption. This raises the question whether the additional dissociation is finalized while the bilayer is intact or if it also occurs upon bilayer desorption. Based on an analysis of the O 1s intensity, we find that the first of these scenarios is plausible. At 240 K, where the bilayer starts to desorb, the OD related signal has reached about 1/3 of the maximum additional intensity seen at 270 K. Provided that dissociation exclusively occurs within the first layer, the OD signal will at 240 K be attenuated by the full second layer. At 270 K, the signal from OD cannot be significantly attenuated by second layer D2O since the D2O coverage is only 0.30±0.02 ML. Consequently, the condition for the dissociation to be completed in the presence of the full bilayer is that the signal from the first layer is attenuated by a factor of three by the full second layer. An attenuation factor of about three was indeed obtained for the layer-by-layer growth on the o-TiO2 surface. We can therefore not exclude that all the additional OD is formed while having the full bilayer. This complies with the importance of second layer water for dissociation of molecules in the first layer. That is, we propose that the additional dissociation is induced by water-water interactions. The possibility to form a water structure with a higher degree of dissociation in the presence of very stable OD groups formed at defects can account for the higher stability of the bilayer on the defective surface compared to the stoichiometric surface. Two additional annealing steps were made to explore the thermal stability of the OD species formed at vacancies and it is found that the signal decreases in the temperature regime 700 – 800 K. The thermal stability of the OD groups formed at vacancies is thus much higher than hydroxyls formed at bridging oxygen vacancies on rutile (110), which recombine and desorb at 500 K. Figure 9 compares the valence spectrum for the clean r-TiO2 surface to that measured after water adsorption followed by heating to 680 K. It is clear that the features at 10.0 eV and 7.5 eV BE in the difference spectrum are due to OD, corresponding to the 3s and 1p states, respectively. This is in good agreement with valence PES spectra of the rutile (110)

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surface with hydroxylated surface oxygen vacancies, which exhibit OH 3s and 1p peaks at 10.2–10.8 and 7.6–8.0 eV BE, respectively.43-44 The valence spectra also reveal a substantial increase in the intensity of the band gap state located at 1 eV BE. The band gap state is associated with reduced Ti in the form of Ti3+ and hence correlated to the out-diffusion of oxygen vacancies previously discussed in connection with the Ti 2p spectra, Figure 6. The Ti 2p data shows furthermore that heating to 300 K does not lead to a significant change in the amount of Ti3+ while heating to 800 K produces a small decrease in the relative intensity of the Ti3+ peak, down to 13% relative intensity (cf. Figure 6a). This change is correlated with a decrease in the OH 3s and 1p intensities, suggesting that oxygen vacancies formed upon the reaction OD(a) + OD(a) ® D2O(g) + OTiO2 + Ovac diffuse to the subsurface region. This result firmly establishes the connection between OD formation and enhanced spectral contribution from reduced Ti. On a final note, we have indications that varying the surface sensitivity provides further insight into the behavior of the Ti3+ species. The sampling depth of the Ti 2p spectra shown in Figure 6 is about 8.4 Å (approximately one unit cell), illustrating that the results are strongly dominated by the behavior of the outermost 1-2 layers. However, increasing the sampling depth to > 17 Å results in a much more pronounced decrease of the Ti3+ intensity upon heating (see Supplementary Information). From this follows that deeper layers first become depleted of Ti3+ while the surplus electrons at or very close to the surface remains, in comparison, unchanged. This is clearly connected to the high stability of the OD formed at the vacancies, trapping the surplus electrons at the surface.34 Summary and Conclusions We have presented a study of how the water adsorption properties of the anatase TiO2(101) surface is affected by defects in the form of oxygen vacancies. Preparing a surface with a high density of oxygen vacancies in the near-surface region results in drastic changes in the water adsorption chemistry. Using synchrotron radiation excited photoelectron spectroscopy, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment is coupled to effective water dissociation into hydroxyls with a very high thermal stability. We also report a change in the water growth mode; from layerby-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. The water bilayer on the reduced surface is thermally more stable than on the stoichiometric surface and it is more efficient in promoting further water dissociation upon heating.

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Our results exemplify the influence of imperfections in the anatase TiO2 lattice on the surface chemistry. Specifically, we disclose how the presence of subsurface defects can drastically influence the wetting mechanism at an oxide surface. Characterization of these defects in a controlled experimental environment is crucial as the results can be translated to imperfections in applied systems in, e.g., photochemistry and catalysis. To this end, further systematic investigations based on combined microscopic and spectroscopic experiments are expected to reveal how the chemical and physical properties of the (101) surface change as a function of density and distribution of defects. Associated Content The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information on the preparation of the o-TiO2 and r-TiO2 surfaces and related Ti 2p spectra; LEED images of pristine o-TiO2 and of r-TiO2 after exposure to water. Additional Ti 2p spectra measured with different sampling depths. Discussion of beam induced effects. Author Information Corresponding Author *E-mail: [email protected] Present address: a

Ute B. Cappel: Division of Applied Physical Chemistry, Department of Chemistry, KTH

Royal Institute of Technology, Stockholm, Sweden Notes The authors declare no competing financial interest. Acknowledgements Financial support was received from the Swedish Energy Agency (STEM) (Grant No. 366421), the Trygger foundation (Grant No. CTS 12:417), the Crafoord foundation (Grant No. 20060599), and NordForsk (Grant No. 40521). We thank the staff at the MAX IV Laboratory for their support.

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References (1) Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Carp, O.; Huisman, C. L.; Reller, A., Photoinduced Reactivity of Titanium Dioxide. Prog. Solid State Chem. 2004, 32 (1), 33-177. (3) Henderson, M. A., A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66 (6), 185-297. (4) Grätzel, M., Photoelectrochemical Cells. Nature 2001, 414, 338-344. (5) Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A., Photooxidative Self-Cleaning Transparent Titanium Dioxide Films on Glass. J. Mater. Res. 1995, 10 (11), 2842-2848. (6) Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A., Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons. ACS Nano 2010, 4 (3), 1259-1278. (7) Lun Pang, C.; Lindsay, R.; Thornton, G., Chemical Reactions on Rutile TiO2(110). Chem. Soc. Rev. 2008, 37 (10), 2328-2353. (8) Pang, C. L.; Lindsay, R.; Thornton, G., Structure of Clean and AdsorbateCovered Single-Crystal Rutile TiO2 Surfaces. Chem. Rev. 2013, 113 (6), 3887-3948. (9) Walle, L. E.; Borg, A.; Uvdal, P.; Sandell, A., Experimental Evidence for Mixed Dissociative and Molecular Adsorption of Water on a Rutile TiO2(110) Surface without Oxygen Vacancies. Phys. Rev. B 2009, 80 (23), 235436. (10) Amft, M.; Walle, L. E.; Ragazzon, D.; Borg, A.; Uvdal, P.; Skorodumova, N. V.; Sandell, A., A Molecular Mechanism for the Water–Hydroxyl Balance During Wetting of TiO2. J. Phys. Chem. C 2013, 117 (33), 17078-17083. (11) Walle, L. E.; Ragazzon, D.; Borg, A.; Uvdal, P.; Sandell, A., Competing Water Dissociation Channels on Rutile TiO2(110). Surf. Sci. 2014, 621 (Supplement C), 77-81. (12) Diebold, U., The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48 (5-8), 53-229. (13) Lazzeri, M.; Vittadini, A.; Selloni, A., Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63 (15), 155409. (14) Linsebigler, A. L.; Lu, G.; Yates, J. T., Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95 (3), 735-758. (15) Kavan, L.; Grätzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H. J., Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase. J. Am. Chem. Soc. 1996, 118 (28), 6716-6723. (16) Scanlon, D. O.; Dunnill, C. W.; Buckeridge, J.; Shevlin, S. A.; Logsdail, A. J.; Woodley, S. M.; Catlow, C. R. A.; Powell, M. J.; Palgrave, R. G.; Parkin, I. P.; Watson, G. W.; Keal, T. W.; Sherwood, P.; Walsh, A.; Sokol, A. A., Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798-801. (17) Herman, G. S.; Dohnálek, Z.; Ruzycki, N.; Diebold, U., Experimental Investigation of the Interaction of Water and Methanol with Anatase−TiO2(101). J. Phys. Chem. B 2003, 107 (12), 2788-2795. (18) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Grätzel, M., Structure and Energetics of Water Adsorbed at TiO2 Anatase (101) and (001) Surfaces. Phys. Rev. Lett. 1998, 81 (14), 2954-2957. (19) He, Y.; Tilocca, A.; Dulub, O.; Selloni, A.; Diebold, U., Local Ordering and Electronic Signatures of Submonolayer Water on Anatase TiO2(101). Nat. Mater. 2009, 8, 585-589.

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(20) Dahal, A.; Dohnálek, Z., Formation of Metastable Water Chains on Anatase TiO2(101). J. Phys. Chem. C 2017, 121 (37), 20413-20418. (21) Walle, L. E.; Borg, A.; Johansson, E. M. J.; Plogmaker, S.; Rensmo, H.; Uvdal, P.; Sandell, A., Mixed Dissociative and Molecular Water Adsorption on Anatase TiO2(101). J. Phys. Chem. C 2011, 115 (19), 9545-9550. (22) Dette, C.; Pérez-Osorio, M. A.; Mangel, S.; Giustino, F.; Jung, S. J.; Kern, K., Single-Molecule Vibrational Spectroscopy of H2o on Anatase TiO2(101). J. Phys. Chem. C 2017, 121 (2), 1182-1187. (23) Schaefer, A.; Lanzilotto, V.; Cappel, U.; Uvdal, P.; Borg, A.; Sandell, A., First Layer Water Phases on Anatase TiO2(101). Surf Sci. 2018, 674, 25-31. (24) Patrick, C. E.; Giustino, F., Structure of a Water Monolayer on the Anatase TiO2(101) Surface. Phys. Rev. Appl. 2014, 2 (1), 014001. (25) Raju, M.; Kim, S.-Y.; van Duin, A. C. T.; Fichthorn, K. A., Reaxff Reactive Force Field Study of the Dissociation of Water on Titania Surfaces. J. Phys. Chem. C 2013, 117 (20), 10558-10572. (26) He, Y.; Dulub, O.; Cheng, H.; Selloni, A.; Diebold, U., Evidence for the Predominance of Subsurface Defects on Reduced Anatase TiO2(101). Phys. Rev. Lett. 2009, 102 (10), 106105. (27) Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W.; Aschauer, U.; Selloni, A., (Sub)Surface Mobility of Oxygen Vacancies at the TiO2 Anatase (101) Surface. Phys. Rev. Lett. 2012, 109 (13), 136103. (28) Cheng, H.; Selloni, A., Surface and Subsurface Oxygen Vacancies in Anatase TiO2 and Differences with Rutile. Phys. Rev. B 2009, 79 (9), 092101. (29) Aschauer, U.; He, Y.; Cheng, H.; Li, S.-C.; Diebold, U.; Selloni, A., Influence of Subsurface Defects on the Surface Reactivity of TiO2: Water on Anatase (101). J. Phys. Chem. C 2010, 114 (2), 1278-1284. (30) Tilocca, A.; Selloni, A., Structure and Reactivity of Water Layers on Defect-Free and Defective Anatase TiO2(101) Surfaces. J. Phys. Chem. B 2004, 108 (15), 4743-4751. (31) Setvín, M.; Aschauer, U.; Scheiber, P.; Li, Y.-F.; Hou, W.; Schmid, M.; Selloni, A.; Diebold, U., Reaction of O2 with Subsurface Oxygen Vacancies on TiO2 Anatase (101). Science 2013, 341 (6149), 988-991. (32) Li, Y.; Gao, Y., Interplay between Water and TiO2 Anatase (101) Surface with Subsurface Oxygen Vacancy. Phys. Rev. Lett. 2014, 112 (20), 206101. (33) Jackman, M. J.; Thomas, A. G.; Muryn, C., Photoelectron Spectroscopy Study of Stoichiometric and Reduced Anatase TiO2(101) Surfaces: The Effect of Subsurface Defects on Water Adsorption at near-Ambient Pressures. J. Phys. Chem. C 2015, 119 (24), 13682-13690. (34) Payne, D. T.; Zhang, Y.; Pang, C. L.; Fielding, H. H.; Thornton, G., Creating Excess Electrons at the Anatase TiO2(101) Surface. Top. Catal. 2017, 60 (6), 392-400. (35) Nyholm, R.; Svensson, S.; Nordgren, J.; Flodström, A., A Soft X-Ray Monochromator for the Max Synchrotron Radiation Facility. Nucl. Instrum. Meth. A 1986, 246 (1), 267-271. (36) Andersen, J. N.; Björneholm, O.; Sandell, A.; Nyholm, R.; Forsell, J.; Thånell, L.; Nilsson, A.; Mårtensson, N., Photoemission Spectroscopy at Max‐Lab. Synch. Rad. News 1991, 4 (4), 15-19. (37) Andersson, K.; Nikitin, A.; Pettersson, L. G. M.; Nilsson, A.; Ogasawara, H., Water Dissociation on Ru(001): An Activated Process. Phys. Rev. Lett. 2004, 93 (19), 196101.

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(38) Walle, L. E.; Ragazzon, D.; Borg, A.; Uvdal, P.; Sandell, A., Photoemission Studies of Water Dissociation on Rutile TiO2 (110): Aspects on Experimental Procedures and the Influence of Steps. Appl. Surf. Sci. 2014, 303 (Supplement C), 245-249. (39) Patrick, C. E.; Giustino, F., O 1s Core-Level Shifts at the Anatase TiO2(101)/N3 Photovoltaic Interface: Signature of H-Bonded Supramolecular Assembly. Phys. Rev. B 2011, 84 (8), 085330. (40) Garcia-Gil, S.; Arnau, A.; Garcia-Lekue, A., Exploring Large O 1s and N 1s Core Level Shifts Due to Intermolecular Hydrogen Bond Formation in Organic Molecules. Surf Sci. 2013, 613 (Supplement C), 102-107. (41) Tu, G.; Tu, Y.; Vahtras, O.; Ågren, H., Core Electron Chemical Shifts of Hydrogen-Bonded Structures. Chem. Phys. Lett. 2009, 468 (4), 294-298. (42) Tilocca, A.; Selloni, A., Vertical and Lateral Order in Adsorbed Water Layers on Anatase TiO2(101). Langmuir 2004, 20 (19), 8379-8384. (43) Brookes, I. M.; Muryn, C. A.; Thornton, G., Imaging Water Dissociation on TiO2(110). Phys. Rev. Lett. 2001, 87 (26), 266103. (44) Kurtz, R. L.; Stock-Bauer, R.; Msdey, T. E.; Román, E.; De Segovia, J., Synchrotron Radiation Studies of H2o Adsorption on TiO2(110). Surf Sci. 1989, 218 (1), 178-200.

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Figures

Figure 1. O 1s and Ti 2p spectra of the oxidized (bottom) and reduced (top) TiO2 surfaces. Contributions from Ti3+ are seen as a shoulder on the low binding energy side in the Ti 2p spectra (colored blue). For the observed asymmetry toward higher binding energy in the O 1s spectra several origins are discussed in the text.

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Figure 2. (a) O 1s datasets recorded during D2O adsorption at 102 K for the r-TiO2 (left) and o-TiO2 surfaces (right). The intensity color scale is logarithmic to enhance the features (blue corresponds to high signal intensity, light brown to background level) (b) O 1s spectra measured after exposing the surfaces to an equivalent dose of 1 ML D2O. Clearly a higher relative intensity of the OD signal and a lower relative intensity for D2O is visible for the rTiO2 surface compared to o-TiO2.

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Figure 3. Integral intensities during uptake experiments at 102 K sample temperature of: a) hydroxyl (OD) and b) water (D2O) related O 1s signals on reduced (filled circles) and oxidized (open circles) anatase TiO2(101)

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Figure 4. Total signal of adsorbed + reacted water as obtained from the OD and D2O related signals. The intensity evolutions reveal different growth modes of the adsorbed water layers on the reduced and oxidized surfaces, respectively.

Figure 5. Position of the D2O signal relative to the TiO2 signal in O 1s spectra as a function of water exposure. A decrease in the binding energy is indicative for the formation of hydrogen bonds within the adsorbed water/OD layers. Clearly, no H-bonds form on the oxidized surface at 102 K until close to 1 ML exposure, whereas a progressive shift occurs for the reduced surface. For both surfaces the total shift is quite similar after 2 ML of exposure.

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Figure 6. Ti 2p spectra of the a) reduced and b) oxidized surfaces in the clean state and after water adsorption and subsequent heating in case of the reduced surface. A clear increase of the Ti3+ related signal (blue) is seen for r-TiO2 after D2O adsorption, increasing from 8% to 18% of the total signal, thus indicating that Ovac have migrated to the surface. Heating to 300 K and desorption of D2O leads to further increase of the Ti3+ signal. The signal persists up to 800 K. No increase of the Ti3+ signal is observed for the oxidized surface upon D2O adsorption. Also after desorbing the water and heating the o-TiO2 surface to 470 K no Ti3+ signal if discernible.

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Figure 7. Growth processes of water layers on reduced (left) and oxidized (right) anatase TiO2(101). (i) Initially both surfaces are free of Ovac. (ii) Upon adsorption of D2O on r-TiO2 Ovac migrate to the surface facilitating dissociation while molecular water adsorption dominates strongly on the o-TiO2 surface. (iii) further exposure to water results in a bilayer growth mode on r-TiO2 while a layer by layer growth prevails on the o-TiO2 surface with limited dissociation of D2O. (iv) once the second water layer starts to form the propensity to OD formation increases in the first layer also for o-TiO2.

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Figure 8. Integral intensities during heating of the adsorbed water layers of a) hydroxyl (OD) and b) water (D2O) related O 1s signals on reduced (filled symbols) and oxidized (open symbols) anatase TiO2(101)

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Figure 9. Valence band spectra recorded at 130 eV photon energy of the reduced surface (black) and after D2O adsorption followed by heating to 680 K (red). And increase of the band gap state (BGS) is visible at ~1 eV binding energy associated with an increase in Ti3+ states. At ~7.5 and 10 eV 1p and 3s signals originating from OD groups are discernible.

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Defect-Induced Water Bilayer Growth on Anatase TiO2(101

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