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Formation of Metastable Water Chains on Anatase TiO(101) Arjun Dahal, and Zdenek Dohnalek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08122 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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The Journal of Physical Chemistry

Formation of Metastable Water Chains on Anatase TiO2(101) Authors: Arjun Dahala and Zdenek Dohnáleka,b,* Affiliations: a

Physical and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States. b

Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, USA Corresponding Author: *[email protected] (Z.D.)

Abstract: Anatase TiO2 is indispensable material for energy-harvesting applications and catalysis. In this study, we employ scanning tunneling microscopy (STM) and study water adsorption on most stable TiO2(101) surface of anatase. We demonstrate that at very low temperatures (80 K), water has the transient mobility that allows it to move on the surface and form extended chains. In contrast with many other oxides, these water chains are only metastable in nature. Adsorption at higher temperatures, where sustained diffusion is observed (190 K) leads to isolated water monomers in accord with prior literature. We speculate that the observed low-temperature mobility is a result of adsorption in a long-lived precursor state with a low diffusion barrier. INTRODUCTION Understanding the interactions of water with metal oxide surfaces is central to many diverse areas such as catalysis, electrochemistry, corrosion, atmospheric science, geology, astrophysics, and others.1-3 Anatase is particularly relevant because it is the most active polymorph and also the most stable nanoparticulate form of TiO2.4-5 The anatase TiO2(101) studied here, is the lowest energy surface and therefore dominates in the catalysts deployed in the applications.4, 6-7 Well-defined anatase TiO2(101) surface has saw-tooth-like morphology exposing alternating rows of two-fold-coordinated oxygen atoms (O2c) and five-fold-coordinated Ti atoms (Ti5c) along the [010] direction. Only few experimental8-14 and theoretical15-20 studies have been 1 ACS Paragon Plus Environment


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carried out to understand the interaction of water with this surface. This is in contrast with wellstudied rutile TiO2(110) surface, which is the most stable macroscopic TiO2 surface.2,

4, 21-23

Majority of the studies on anatase TiO2(101) conclude that water adsorbs molecularly. A prior combined temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) study has identified only molecular water on the surface,8 in accord with density functional theory (DFT) calculations.15,

24-25

More recently, scanning tunneling microscopy

(STM) imaging with detailed DFT calculations have also concluded that water adsorbs molecularly.9 Furthermore, DFT calculations have shown that molecular adsorption is energetically favored over dissociative adsorption by 0.44 at low coverage and by 0.28 eV at monolayer coverage.15 The theoretical studies predict that oxygen of the water molecule forms a dative bond to the surface Ti5c atoms, whereas hydrogen atoms form weak hydrogen bonds with two O2c atoms.9, 15, 24 Only a few experimental studies provide evidence for hydroxyl formation due to water dissociation on stoichiometric anatase TiO2(101).11, 14, 26 Despite these advances, little is known about how water molecules interact with each other on anatase TiO2(101). On most oxide surfaces, water-water interactions are attractive due to hydrogen bonding. As a result, hydrogen-bonded water clusters are energetically preferred over isolated molecules.23, 27-29 In contrast, recent STM measurements on anatase TiO2(101) have shown that following the adsorption at 190 K, water molecules reside preferentially spaced by empty Ti5c sites,9 indicating a lack of hydrogen bonded network. In this study, we show that water adsorption on anatase TiO2(101) at 80 K leads to the formation of metastable linear chains on Ti5c rows. Since the onset of diffusion is observed at 190 K, the chain formation at 80 K reveals transient mobility of water molecules during adsorption. This is likely a consequence of water molecules being initially trapped in the lowdiffusion-barrier precursor state before being chemisorbed on the Ti5c sites. In accord with prior studies,9 upon annealing to 190 K, these metastable chains fall apart to isolated monomers. Similarly, a direct adsorption at 190 K also leads to isolated monomers that slowly diffuse on the surface. These findings are in contrast with the behavior of water on many other oxide surfaces.23, 27, 30-31

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EXPERIMENTAL METHODS The experiments were performed in an ultrahigh vacuum (UHV) system equipped with Omicron low-temperature scanning tunneling microscope (LT-STM) system. The anatase TiO2(101) sample employed in these studies is a natural mineral crystal (5×5×1 mm3), that was cleaned by repeated cycles of Ne ion sputtering and annealing in vacuum at 920 K. As evidenced by XPS (Figure S1, Supporting Information (SI)) and large area STM images (Figure S2, SI), our sample contains a small amount of impurities (i.e. Cr, Sb) that segregate from the bulk during vacuum annealing. A small fraction (< 0.6%) of these impurities could not be completely removed even after repeated (> 20) sputter/anneal cleaning cycles. Water was dosed directly in the STM stage using a molecular beam.32 The coverage in monolayers (ML) is defined relative to the density of Ti5c sites (1 ML ≡ 5.17 × 1014 H2O/cm2) and determined by counting the molecules in the STM images. All STM images were recorded in constant-current mode at a positive sample bias of 0.7 V and tunneling current of 200 pA. A detailed description of the experimental methods is presented in the SI. The simple kinetic Monte Carlo (KMC) simulations were carried out to interpret the coverage-dependent chain-length distributions in the images. Only a random adsorption scenario was considered. In the simulations, molecules adsorbing on empty sites are immobilized, while the molecules adsorbing on top of already covered sites are allowed to diffuse randomly along the row till an empty site is found. RESULTS AND DISCUSSION An empty state STM image of clean anatase TiO2 (101) exposes alternating bright and dark rows (Figure S2b, SI). The bright rows along the [010] direction have been previously assigned to the five-fold coordinated Ti5c atoms.13 We note, that in contrast with rutile TiO2(110), oxygen vacancies are typically not present on the surface of anatase TiO2(101) and reside predominantly in the subsurface region.33 Figures 1a, b, and c present the sequential STM images of the identical surface after adsorption of 0.05, 0.08, and 0.12 ML of water at 80 K, respectively. At the initial coverage of 0.05 ML, only one type of feature, exhibiting a butterfly shape (green squares, Figure 1a), is present. No mobility is observed upon extended imaging. In accord with previous STM studies,9, 3 ACS Paragon Plus Environment


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we assign these protrusions to isolated water monomers adsorbed on Ti5c sites. Similar to the

shape observed here, in previous reports, water monomers have been imaged as features with bow tie or dumb bell like appearance. The slight variations in the appearance among the studies (including ours) is likely a consequence of different bias voltage used to acquire the STM images.

Figure 1. Coverage dependent sequential STM images of identical area on anatase TiO2(101) after adsorption of water at 80 K: (a) 0.05, (b) 0.08, and (c) 0.12 ML. The green and purple dashed squares mark monomers and dimers, respectively. The red and blue dashed circles mark pairs of monomer on next-nearest-neighbor Ti5c sites and nearest-neighbor trimers, respectively. The yellow dashed oval marks nearest-neighbor tetramer. (d, e) Color-coded line profiles along the [010] direction across different surface features.


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A line profile across the water monomer feature along the [010] direction (Figure 1d upper panel) reveals that the monomer is positioned on top of the Ti5c site. The full width at half maximum (FWHM) is 0.94 nm and the two lobes extend to neighboring Ti5c sites. Moreover, apparent contrast of the lobes also extends to the two adjacent Ti5c sites (Figure 1a). The two lobes have been attributed to hydrogen bonding with O2c and the overall butterfly-shaped appearance to the electronic effects.9 Additionally, we also observe (red circle, Figure 1a) two water monomers residing on next-nearest-neighbor Ti5c sites. The next-nearest-neighbor pair retains the appearance of two neighboring monomers as corroborated by the line scan along the [010] direction (Figure 1e, red line). We note that we did not observe any monomer features with asymmetric butterfly-shape. Such asymmetric features were observed previously in Ref.

14

and have been assigned to

hydroxyl species resulting from water dissociation. The origin of this difference is unclear but can result from the highly doped nature of the samples used in Ref.

14

to allow for the inelastic

electron tunneling spectroscopy (IETS) measurements. Alternatively, the dissociation inferred in Ref.

14

can result from a different preparation procedure of water-related species. There,

multilayers of water are adsorbed at 120 K, sample is annealed to room temperature, and imaged at 5 K. We have attempted to dissociate water by colliding the molecules carrying high translational energy of 1.3 eV with the surface at 80 K as described in our previous study on rutile TiO2(110).32 No dissociation was observed even under such extreme conditions. New types of features emerge on the surface with additional exposure to water (Figures 1b and c, new features marked by purple squares, blue circles, and yellow oval). These are a consequence of the formation of various water clusters as the coverage is increased. In two cases, the butterfly features of the isolated monomers (green squares, Figure 1a) evolve into bright dumb bell features (purple squares, Figures 1b). An additional event is observed in Figure 1c. The dumb bell features are longer (FWHM of 1.32 nm) and brighter than the monomers (0.94 nm). The line profile over the dumb bell feature along the [010] direction is shown in Figure 1d (purple line). The Ti5c positions are aligned with the Ti5c positions of the original monomer (green line). We assign the dumb bell feature to two water monomers residing on nearestneighbor Ti5c sites. For simplicity, we further call it water dimer. However, it should be noted that there is likely no hydrogen bond between them.9 The line profile through the dimer (Figure 5 ACS Paragon Plus Environment


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1d, purple line) reveals its slightly asymmetric shape. Inspection of many dimers in a number of STM images reveals that the asymmetry is always observed and that the higher lobe can be on any side along the [010] direction. The reason for the asymmetry is likely related to the asymmetry of water dimer structure. Based on theoretical calculations, the optimized dimer structure has hydrogen of one of the two water molecules pointing upward possibly leading to such asymmetry.9 The next-nearest-neighbor monomer pair shown in Figure 1a (red circle) also changed after an additional water dose. The new feature (Figure 1b, blue circles) has a similar FWHM along the [010] direction, suggesting the formation of a trimer by adsorbing an additional water molecule on the middle, empty Ti5c site. The trimer feature is significantly brighter than the original next-nearest-neighbor monomer pair. An additional new trimer also appeared in Figure 1c. There, two water molecules had to adsorb on Ti5c sites neighboring the original isolated water monomer shown in Figure 1b. The trimer has an asymmetrical lineshape as indicated by the line profile along the [010] direction (Figure 1e, blue line). Similarly to the dimer, it is likely that the water molecules in the trimer are oriented differently, yielding asymmetric line shape. Larger water clusters are observed at higher coverages. For example, linear tetramer feature is highlighted by the yellow oval in Figure 1c and water chains are present in Figure 2. Figure 2a and b show water coverages of 0.20 and 0.92 ML following the dose at 80 K. The water coverage in Figure 2a gives the impression of much larger coverage because the contrast of water monomers, dimers, trimers, etc. extends to neighboring Ti5c sites. Monomers in Figure 2a can be unambiguously distinguished from the clusters and chains, as they have lower brightness. For the near monolayer coverage, Figure 2b, spherical protrusions covering all Ti5c site are observed. Several vacancies (dark) in the water chain structure can also be seen. The fact that all water molecules reside on the Ti5c sites and none in the second layer suggests that at 80 K water mobility on top of Ti5c-bound water molecules is sufficiently high to allow for their diffusion to empty Ti5c sites.


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Figure 2. STM images showing larger water coverages. (a) 0.20 and (b) 0.92 ML of water adsorbed at 80 K. (a) and (b) surfaces are annealed to 190 K and subsequently imaged at 80 K again. The respective images are shown in (c) and (d).

To gain insight into the stability of water chains, we annealed the water overlayers prepared at 80 K (Figures 2a, b) to 190 K (Figure 2c, d). We chose this annealing temperature, since it is known to be an onset of water diffusion.9 The lower coverage of 0.20 ML following the annealing is shown in Figure 2c. The water chains observed upon adsorption at 80 K (Figure 2a) mostly disappeared and butterfly shaped features for separated water monomers dominate the image (Figure 2c). This suggests that the low-temperature chains are not stable. This is in contrast with the behavior of water on many other oxide surfaces where the formation of hydrogen-bonded networks leads to stabilization.23,

27, 31

The appearance of separated water

monomers is consistent with the previous STM study, where adsorption at 190 K also yielded separated water monomers.9 Practically no changes can be seen for close to saturation coverage of 0.92 ML before (Figures 2b) and after (Figure 2d) annealing. Higher resolution observed in the image after 7 ACS Paragon Plus Environment


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annealing (Figure 2d) is simply a consequence of tip change. The absence of any significant changes at such high coverage is not surprising. Since almost all Ti5c sites are occupied, there is no space for a significant redistribution of the molecules on the Ti5c sites. The visual inspection of the images with higher coverages after water adsorption at 80 K, such as the one in Figure 2a, show a large fraction of water chains. This is surprising, considering the onset of diffusion of 190 K. To quantify the extent of water clustering at 80 K, we have analyzed images with 0.05, 0.08, 0.12, and 0.20 ML and plotted the cluster size distributions in Figure 3 (black bars). As the coverage is increased from 0.05 to 0.2 ML, the amount of water in monomers increases only slightly from 0.04 to 0.06 ML, while the coverage of water molecules in dimers increases dramatically from 0.01 to 0.05 ML. Already at 0.12 ML, the clustered water molecules (dimers, trimers, etc.) contribute to ~ 52% of the total water coverage. At 0.2 ML, monomers contribute to only ~ 29% of the total water coverage and the coverage of water in dimers becomes equivalent to that in monomers.

Figure 3. Experimental (black bars) and simulated (red bars) water cluster size distributions. The experimentally determined distributions correspond to the 0.05, 0.08, 0.12, and 0.20 ML of water adsorbed at 80 K. The simulated distributions are determined from the Monte Carlo simulations assuming random adsorption.


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To compare the experimental distributions with what is expected for random adsorption of molecules in the absence of mobility, we performed simple Monte Carlo simulations. The red bars in Figure 3 show the simulated cluster size distributions for random adsorption. It is evident that the simulated distributions show a significantly higher population of monomers (by a factor of 2.9 at 0.20 ML) as compared to the experimentally observed populations. Further, the simulated populations of larger clusters decay much faster than the experimentally observed distributions. The high coverage of clustered molecules in experimental distributions demonstrate that water has to be mobile during adsorption at 80 K. However, the onset of sustained diffusion is observed at 190 K and the theoretically predicted water diffusion barrier along Ti5c row is 0.58 eV.34 Hence the diffusion at 80 K can only be transient in nature. Such scenario can occur, if the water molecules are initially trapped in a mobile precursor state before becoming chemisorbed on vacant Ti5c sites. While we do not know the exact nature of such precursor state, it has to satisfy the following conditions to be consistent with our observations: (a) The diffusion barrier in the precursor state has to be low (< 0.3 eV) to allow for the diffusion at 80 K. (b) The energy barrier to fall into the chemisorbed state on the Ti5c sites has to be higher than the diffusion barrier in the precursor state. This condition allows for a sufficient lifetime and mobility in the precursor state. (c) The diffusing water molecules are trapped upon encountering the already adsorbed water molecules. This condition leads to the formation of the extended chains. Such precursor state can be found for example on top of the Ti5c-bonded water molecules. We have already discussed that water can readily move on top of water-saturated surface at 80 K. The consequence is the formation of a complete first layer with no molecules trapped in the second layer as observed in Figure 2b. On bare areas of the surface, similar conditions can be possibly found on top of the ridge O2c sites. Since water binding energy on these sites is significantly smaller (~ 1.4 times) than on Ti5c sites,8 it is likely that the diffusion barrier will also be much smaller. We speculate, that water molecules that initially land on the O2c sites are transiently-bound and mobile before being captured on preferred Ti5c binding sites. We have further carried out water adsorption experiments at 190 K. Figure 4 shows images obtained at 80 K after the adsorption of 0.22 and 0.31 ML of water at 190 K. In agreement with previous STM measurements9 and with the 190 K annealed surface shown in Figure 2c, the majority species are isolated water monomers. The clusters (dimers and trimers) contribute to a small fraction of the coverage; e.g. ~ 7 and 20 % in Figure 4a and b, respectively. 9 ACS Paragon Plus Environment


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Figure 4. STM images of anatase TiO2(101) after adsorption of (a) 0.22 ML and (b) 0.31 ML of water at 190 K.

For comparison, we plot the cluster size distributions for various adsorption/annealing experiments (images shown in Figures 2a, 2c, and 4a) with ~0.2 ML of water in Figure 5. The black and green bars correspond to the adsorption of water at 80 K and 190 K, respectively. The blue bars represent the distribution after water adsorption at 80 K, and annealing to 190 K. All imaging was carried out at 80 K. Red bars correspond to cluster size distribution for simulated random adsorption of water. For 80 K adsorption experiment (black bars), water chains contribute to ~ 71% of the coverage indicating water molecules favor the formation of metastable chains as already discussed above (see Figure 3). For the 190 K adsorption and annealing experiments, monomers separated by empty Ti5c sites account for 93% of the coverage for the both experiments with only 6% and 5% being in dimer configurations, respectively. Comparisons with the simulated distribution of randomly adsorbed water (red bars) show that the experimentally observed populations of dimers are about a factor of 2 to 3 lower than expected. This implies that water molecules interact repulsively on the Ti5c rows, most likely due to dipoledipole interactions. This is consistent with previously reported theoretical calculations and TPD measurements. The theoretical calculations show that dimers are less stable by 30 meV than the two monomers residing on the next-nearest-neighbor Ti5c sites.9 The previously reported TPD measurements also indicate slight repulsive interactions. The TPD peak shifts from 260 to 250 K as the coverage increases from 0.33 to 0.66 ML.8 Using the previously calculated adsorption


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energy of 0.73 eV, this 4% decrease in the desorption temperature translates to about 30 meV decrease in binding energy in remarkable agreement with the calculations.

Figure 5. Experimental (black, green, and blue bars) and simulated (red bars) cluster size distributions of water on anatase TiO2(101). The black and green bars show the distributions determined from the STM images after water adsorption at 80 and 190 K, respectively. The blue bars represent the distribution determined from the STM images of the sample prepared by adsorbing water at 80 K and annealing it to 190 K. All STM images were acquired at 80 K. The red bars represent the clusters size distributions determined from Monte Carlo simulations assuming random adsorption.

SUMMARY In summary, we have studied the adsorption of water on anatase TiO2(101) surfaces via STM imaging at 80 K and 190 K. By following the same area, we show that at 80 K water molecules form metastable clusters that yield extended chains at high coverages. The formation of water chains at such an unusually low temperatures is a result of water transient diffusion in a long-lived precursor state. This precursor state is expected to affect transient mobility of water molecules also at higher temperature as its accessibility allows for sampling of larger areas before their thermalization. At 190 K, which is the onset of sustained water mobility, these linear chains fall apart and isolated water molecules are observed as shown previously.9 Our study shows that on anatase TiO2(101) the interactions between water molecules are repulsive. This is in sharp contrast with the behavior of water molecules on most of the oxide surfaces,



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where water-water interactions are attractive due to hydrogen bonding and the growth of linear chains is favored. ASSOCIATED CONTENT Supporting Information Preparation and characterization of clean stoichiometric anatase TiO2(101), Water coverage as a function of dose at 80 K. This material is available free charge on the ACS Publications website at DOI: xxxxxx.

Notes The authors declare no competing financial interest.

Acknowledgments: This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences and performed in EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle

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(26) 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, 392-400. (27) Lee, J.; Sorescu, D. C.; Deng, X.; Jordan, K. D. Water Chain Formation on TiO2(110). J. Phys. Chem. Lett. 2013, 4, 53-57. (28) Matthiesen, J.; Hansen, J. Ø.; Wendt, S.; Lira, E.; Schaub, R.; Lægsgaard, E.; Besenbacher, F.; Hammer, B. Formation and Diffusion of Water Dimers on Rutile TiO2(110). Phys. Rev. Lett. 2009, 102, 226101. (29) Merte, L. R.; Bechstein, R.; Peng, G. W.; Rieboldt, F.; Farberow, C. A.; Zeuthen, H.; Knudsen, J.; Laegsgaard, E.; Wendt, S.; Mavrikakis, M.; Besenbacher, F. Water Clustering on Nanostructured Iron Oxide Films. Nat. Commun. 2014, 5, 4193. (30) Daschbach, J. L.; Dohnálek, Z.; Liu, S.-R.; Smith, R. S.; Kay, B. D. Water Adsorption, Desorption, and Clustering on FeO(111). J. Phys. Chem. B 2005, 109, 10362-10370. (31) Mu, R.; Cantu, D. C.; Glezakou, V.-A.; Lyubinetsky, I.; Rousseau, R.; Dohnálek, Z. Deprotonated Water Dimers: The Building Blocks of Segmented Water Chains on Rutile RuO2(110). J. Phys. Chem. C 2015, 119, 23552-23558. (32) Wang, Z.-T.; Wang, Y.-G.; Mu, R.; Yoon, Y.; Dahal, A.; Schenter, G. K.; Glezakou, V.-A.; Rousseau, R.; Lyubinetsky, I.; Dohnálek, Z. Probing Equilibrium of Molecular and Deprotonated Water on TiO2(110). Proc. Natl. Acad. Sci. 2017, 114, 1801-1805. (33) 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, 106105. (34) Agosta, L.; Gala, F.; Zollo, G. Water Diffusion on TiO2 Anatase Surface. AIP Conf. Proc. 2015, 1667, 020006.


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TOC Figure


Formation of Metastable Water Chains on Anatase TiO2(101) - The

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