Article pubs.acs.org/JPCC
Formation of Metastable Water Chains on Anatase TiO2(101) Arjun Dahal† and Zdenek Dohnálek*,†,‡ †
Physical and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ‡ Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States S Supporting Information *
ABSTRACT: Anatase TiO2 is indispensable material for energy-harvesting applications and catalysis. In this study, we employ scanning tunneling microscopy 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.
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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 sawtooth-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 carried out to understand the interaction of water with this surface. This is in contrast with well-studied rutile TiO2(110) surface, which is the most stable macroscopic TiO2 surface.2,4,21−23 The 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 has 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 of hydroxyl formation © 2017 American Chemical Society
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. Because 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 low-diffusion-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 Received: August 14, 2017 Revised: August 25, 2017 Published: August 25, 2017 20413
DOI: 10.1021/acs.jpcc.7b08122 J. Phys. Chem. C 2017, 121, 20413−20418
Article
The Journal of Physical Chemistry C
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 dashed circles mark pairs of monomers on next-nearest-neighbor Ti5c sites. The blue dashed circles highlight nearest-neighbor trimers, and 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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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 Figure 1a−c presents 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,9,14 we assign these protrusions to isolated water monomers adsorbed on Ti5c sites. Similar to the shape observed here, in previous reports water monomers were imaged as features with bow-tieor dumbbell-like appearance. The slight variations in the appearance among the studies (including ours) are likely a consequence of different bias voltage used to acquire the STM images. 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 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
EXPERIMENTAL METHODS The experiments were performed in an ultrahigh vacuum (UHV) system equipped with Omicron low-temperature scanning tunneling microscope (LT-STM). 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 (20) sputter/anneal cleaning cycles. Water was dosed directly in the STM stage using a molecular beam.32 The coverage in monolayers (MLs) 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 until an empty site is found.
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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 20414
DOI: 10.1021/acs.jpcc.7b08122 J. Phys. Chem. C 2017, 121, 20413−20418
Article
The Journal of Physical Chemistry C 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 inelastic electron tunneling spectroscopy (IETS) measurements. Alternatively, the dissociation inferred in ref 14 can result from a different preparation procedure. There, multilayers of water are adsorbed at 120 K, and 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 (Figure 1b,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 dumbbell features (purple squares, Figure 1b). An additional event is observed in Figure 1c. The dumbbell features are longer (FWHM of 1.32 nm) and brighter than the monomers (0.94 nm). The line profile over the dumbbell 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 dumbbell feature to two water monomers residing on nearest-neighbor 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 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. On the basis of 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 asymmetric line shape, as indicated by the line profile along the [010] direction (Figure 1e, blue line). Similar 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,b shows 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, and so on 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
Figure 2. STM images showing larger water coverages. (a) 0.20 and (b) 0.92 ML of water adsorbed at 80 K. (a,b) Surfaces are annealed to 190 K and subsequently imaged at 80 K again. The respective images are shown in panels c and d.
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 reside 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. To gain insight into the stability of water chains, we annealed the water overlayers prepared at 80 (Figure 2a,b) to 190 K (Figure 2c,d). We chose this annealing temperature because 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 annealing (Figure 2d) is simply a consequence of tip change. The absence of any significant changes at such high coverage is not surprising. Because 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 20415
DOI: 10.1021/acs.jpcc.7b08122 J. Phys. Chem. C 2017, 121, 20413−20418
Article
The Journal of Physical Chemistry C
example, on top of the Ti5c-bonded water molecules. We have already discussed that water can readily move on top of watersaturated 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. Because 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
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 KMC simulations assuming random adsorption.
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. To compare the experimental distributions with what is expected for random adsorption of molecules in the absence of mobility, we performed simple KMC 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 with the experimentally observed populations. Furthermore, the simulated populations of larger clusters decay much faster than the experimentally observed distributions. The high coverage of clustered molecules in experimental distributions demonstrates 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. Whereas 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 (
