Water adsorption at zirconia: from the ZrO₂(111)/Pt₃Zr(0001) model system to powder samples

Abstract

We present a comprehensive study of water adsorption and desorption on an ultrathin trilayer zirconia film using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), as well as scanning tunneling microscopy (STM) at different temperatures. The saturation coverage is one H₂O per surface Zr atom, with about 12% dissociation. The monolayer TPD peak (180 K, desorption barrier 0.57 ± 0.04 eV) has a tail towards higher temperatures, caused by recombinative desorption from defect sites with dissociated water. STM shows that the defects with the strongest H₂O adsorption are found above subsurface dislocations. Additional defect sites are created by multiple water adsorption/desorption cycles; these water-induced changes were also probed by CO₂ TPD. Nevertheless, the defect density is much smaller than in previous studies of H₂O/ZrO₂. To validate our model system, transmission Fourier-transform infrared absorption spectroscopy (FTIR) studies at near-ambient pressures were carried out on monoclinic zirconia powder, showing comparable adsorption energies as TPD on the ultrathin film. The results are also compared with density functional theory (DFT) calculations, which suggest that sites with strong H₂O adsorption contain twofold-coordinated oxygen.

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1 Introduction

Many technological applications of zirconia (ZrO₂; pure or yttrium-doped, then known as yttria-stabilized zirconia YSZ) involve interaction with water. Examples are internal steam reforming in solid oxide fuel cells,¹ catalysis,² gas sensors,³ or applications as biocompatible material.⁴ However, little is known about the interaction of water with ZrO₂ surfaces on a fundamental level, which is mostly due to a lack of suitable samples. This is quite different for other oxides.^5–7 Water adsorbs only weakly on many defect-free oxide surfaces; in ultra-high vacuum (UHV) it then desorbs below room temperature (RT), typically at 160–250 K.^8–10 H₂O can bind more strongly to surfaces with defects, as shown for the prototypical example of rutile TiO₂(110).^11,12 In these cases, H₂O dissociates into an OH group, which fills an oxygen vacancy, and into a hydrogen atom, which binds to surface oxygen and forms a second OH group. On TiO₂, these OH groups are stable up to ≃490 K.¹³ On defect-free oxide surfaces, water can bind strongly if the surface termination includes highly reactive cations. It can then readily dissociate, e.g. on α-Cr₂O₃(001),¹⁴ α-Fe₂O₃(012),¹⁵ and on oxides of the alkaline earths,¹⁶ including CaO-terminated Ca₃Ru₂O₇(001).¹⁷ On RuO₂(110),¹⁸ PdO(101),¹⁹ and Fe₃O₄(001),²⁰ water binds to coordinatively unsaturated cations and forms partially dissociated structures stabilized by hydrogen bonds.

Such detailed studies are not yet available for water on ZrO₂; only few reports on H₂O adsorption can be found in the literature. On powder materials, very high adsorption enthalpies were reported for low H₂O coverages (≳2 eV on monoclinic and ≈1.5 eV on tetragonal ZrO₂), decreasing to liquid-water like binding (0.45 eV) at coverages around 2–4 H₂O/nm².^21,22 Moderate values (≈0.6 eV, derived from a TPD peak at 240–250 K) were reported for H₂O on oxidized polycrystalline Zr, i.e., a hydroxylated and, possibly, substoichiometric film.²³ To the best of our knowledge, there are no studies of H₂O adsorption on well-defined single-crystalline ZrO₂ surfaces so far. This is related to the fact that single crystals of pure ZrO₂ grown from the melt exhibit phase transformations upon cooling, thus only crystals of doped zirconia (e.g., YSZ) are available; these retain the high-temperature cubic phase. The surface chemistry of YSZ is much more complex than that of pure ZrO₂, however, as shown for adsorption of H₂O,²⁴ hydrogen,²⁵ or CO and CO₂.²⁶ Therefore, the investigation of pure ZrO₂, which is needed as a starting point of a well-grounded fundamental understanding, has to rely on thin films.

Well-defined, pure zirconia surfaces can be created as thin films, which have the additional advantage that the surface of the otherwise perfectly insulating zirconia can be probed by techniques that rely on electron transfer, such as scanning tunneling microscopy (STM). Such ZrO₂ films can be obtained by deposition of Zr in oxygen background gas.^27–30 Temperature programmed desorption (TPD) studies on 7 ML-thick ZrO₂ films on Pt(111) show three desorption peaks (190 K, 275 K, and 370 K with adsorption energies 0.46, 0.68, and 0.92 eV, respectively).^31,32 The authors suggested that water in the low-temperature peak binds molecularly while the rest is in dissociated form. The two desorption peaks at higher temperatures were assigned to defect sites.

Ultra-thin ZrO₂ films can also be prepared by oxidation of suitable Zr alloys, Pt₃Zr^33,34 or Pd₃Zr.³⁵ This approach has the advantages of better homogeneity of the films, especially a more uniform thickness, and is also less susceptible to impurity adsorption as these alloys are much less reactive than pure Zr. By a combination of STM, Auger and photoelectron spectroscopy, as well as DFT, it has been shown that the films grown by oxidation of Pt₃Zr or Pd₃Zr consist of one trilayer (O–Zr–O), structurally equivalent to a trilayer of cubic ZrO₂(111), but with additional distortions.^33,35,36 The oxide film is created by Zr diffusion to the surface when annealing in oxygen. In the case of Pt₃Zr, diffusion of Zr in the alloy is slow, thus a Zr-depleted Pt region remains below the oxide. The slightly smaller interatomic distance of Pt (0.2775 nm) in comparison with Pt₃Zr (0.281 nm) leads to a contraction of the Pt layer below the oxide film. As a consequence, misfit dislocations form between the Pt layer at the interface and the alloy below. These subsurface dislocations are visible as bright ridges in STM. The atoms in the ultra-thin ZrO₂ trilayer above the Pt layer are well ordered with an average in-plane distance of 350 pm and exhibit strong vertical buckling. These height differences are related to the superstructure (1.2 nm periodicity), resulting from the different lattice constants of the oxide and the underlying Pt layer. The unit cell of the superstructure includes 12 Zr atoms, which are accessible to adsorbates due to the large distance between the O atoms in the layer above, and the low interlayer distance between the O and Zr layers.³³ A model of the superstructure as calculated by density functional theory (DFT) is presented in the DFT chapter below.

In this work we present an extensive study of water on ZrO₂ ultra-thin films grown by oxidation of Pt₃Zr(0001) single crystals. We combine TPD, X-ray photoelectron spectroscopy (XPS), and STM with DFT calculations and show how adsorbed H₂O behaves on zirconia trilayers. Furthermore, we performed Fourier-transform infrared spectroscopy (FTIR) absorption measurements of H₂O on monoclinic ZrO₂ powder, to test the validity of the ultra-thin oxide as a model system for technological zirconia surfaces.

2 Experimental and computational methods

2.1 Ultra-high vacuum setup

UHV experiments were conducted in three different systems. Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) measurements were performed in a UHV chamber described thoroughly elsewhere.³⁷ This system features a LHe flow cryostat, which can cool the sample to 20 K. The base pressure in the chamber was below 10⁻¹⁰ mbar. A Pt₃Zr(0001) single crystal (6 mm diameter) was wrapped with Ta ribbons at the circumference; these were spot-welded to thicker Ta bars leading directly to the cryostat to ensure good thermal contact. A K-type thermocouple was spot-welded directly to the backside of the crystal for accurate temperature measurements. No sample plate was used. The chamber includes a molecular-beam setup for precise gas dosing (such as D₂O) with a sharp top-hat profile in a circular area with a diameter of 3.2 mm.³⁷ This allows us to perform TPD and XPS measurements with water sticking exclusively to the center of the well-prepared crystal surface; there is no influence from water on the Ta ribbons or from the edges of the crystal. Furthermore, the molecular beam provides accurate gas doses. Since the sticking coefficient equals unity, the coverage can be given in monolayers or molecules per trilayer Zr atom. We define one monolayer (ML) as one water molecule per surface Zr atom (9.5 nm⁻², corresponding to 12 molecules per ZrO₂ trilayer unit cell). For XPS, a monochromatised Al Kα (1486.7 eV) X-ray source was used. All XPS measurements were done at 100 K and in normal emission. To reduce the influence of the residual gas on the TPD data, experiments in this chamber (TPD, XPS) were done with D₂O, while H₂O was used everywhere else.

Scanning tunneling microscopy (STM) measurements were performed in two different chambers. Low-temperature STM was performed using an Omicron LT-STM in a two-chamber UHV system with a base pressure below 1.5 × 10⁻¹¹ mbar in the STM chamber, and 4 × 10⁻¹¹ mbar in the preparation chamber. During the measurement the sample was cooled to 78 K. Room-temperature STM measurements were preformed using an Omicron μ-STM in a chamber described in ref. 33. This two-chamber system is divided into a measurement chamber (p_base < 7 × 10⁻¹¹ mbar) and a preparation chamber (p_base < 10⁻¹⁰ mbar). STM data were acquired in constant-current mode with electrochemically etched W tips. We report sample voltages for the STM images, thus positive voltages refer to tunneling into the unoccupied states of the surface. All STM images were corrected for creep of the piezo scanner as described in ref. 35.

2.2 In situ FTIR spectroscopy setup

Fourier-transform infrared absorption spectroscopy (FTIR) measurements of ZrO₂ powder were recorded in transmission mode on an Agilent Cary 660 spectrometer with a mid-infrared source and a deuterated triglycine sulfate (DTGS) detector. The powder samples were pressed into thin pellets using a pressure equivalent to 1.5 t on a 0.8 cm² area (sample diameter 10 mm, mass about 20 mg) and subsequently placed inside a homemade in situ/operando reactor cell.³⁸ This cell provides an inert all-quartz surrounding of the sample in the heated area. In situ measurements can be performed up to 1273 K under flowing and static conditions. Also, vacuum with a minimum pressure of 10⁻⁷ mbar is obtainable. The window material, BaF₂, provides access to wave numbers above 800 cm⁻¹. Experiments in flowing mode were performed using He as carrier gas. In static mode, water vapor was admitted stepwise to the evacuated cell. The water reservoir was degassed beforehand to remove dissolved atmospheric gases, in particular CO₂. All reported spectra are corrected by the spectrum of the dry pre-oxidized oxide pellet at room temperature and under vacuum, prior to exposure to water.

2.3 Sample preparation

The Pt₃Zr(0001) single crystals used for UHV studies were grown, cut and polished by MaTecK (Germany). The cleaning procedure was based on the recipe from ref. 33, with slight changes: cycles of sputtering (2 keV Ne⁺ ions in the XPS/TPD chamber, or Ar⁺ in the STM chambers; 20 min, current density ≈ 4 μA cm⁻²) and annealing (1175 K, 10 min). The last sputter cycle was applied during a linear temperature ramp from 680 K to 380 K in 20 min to reduce the density of steps and eliminate half-steps.³³ The cleanliness of the sample was checked with XPS. An ultra-thin ZrO₂ trilayer (O–Zr–O) was prepared by first annealing in oxygen (p = 1 × 10⁻⁷ mbar, 680 K, 10 min) and then in UHV (1205 K, 30 min). The first annealing step causes Zr diffusion to the surface and oxidation, but results in poorly ordered structures. In the second step the oxide forms a well-ordered ultra-thin O–Zr–O trilayer; three-dimensional (3D) ZrO₂ clusters disappear by spreading out and/or dissolving into the bulk. In the present study, the final annealing temperature was set higher than in ref. 33 and 36 to reduce the amount of ZrO₂ clusters. Although both, the annealing temperature and the annealing time, were increased compared to the recipe from Antlanger et al.,³³ the film did not break up, as verified by CO TPD, which did not show any indications of a CO signal from adsorption on the substrate.³⁶ The resulting sample is a trilayer of ZrO₂(111) on a Zr-depleted Pt₃Zr(0001) surface. In the following this is shortened to “ZrO₂/Pt₃Zr”. For the LT-STM measurements, the sample could be heated to only 1160 K; nevertheless, STM showed large areas free of ZrO₂ clusters (Fig. S1 of the ESI†). The preparation procedures are summarized in the ESI (Table S1†).

Both D₂O (TPD and XPS measurements) and H₂O (STM measurements) were purified via several freeze–pump–thaw cycles. All gases were checked for cleanliness with mass spectrometers. For TPD and XPS measurements, water was dosed using the molecular-beam doser (see above), for STM measurements water was dosed by back-filling of the chamber, making the given doses less accurate.

For the FTIR experiments, commercial powder of monoclinic ZrO₂ (zirconium (IV) oxide, 99.978%; Alfa Aesar) was used. To guarantee a well-defined, carbon- and water-depleted and sufficiently (for the subsequent experimental temperatures) sintered material, the sample was heated inside the in situ setup in pure, dry oxygen up to 1273 K and held for one hour prior to the water adsorption experiments. The sample was routinely checked by XRD for structural changes after annealing and after the water adsorption experiments. After the pretreatment, the surface area was determined as 2 m² g⁻¹ (ZrO₂) by nitrogen adsorption at 77 K according to the Brunauer–Emmett–Teller (BET) method. For BET measurements, a Quantachrome Nova 2000 Surface Area and Pore Size Analyzer was used. Gases were supplied by Messer (O₂ 5.0, He 5.0). The cooling trap for removing residual moisture from O₂ was set at ∼153 K.

2.4 Computational methods

The DFT calculations were performed with the Vienna Ab initio Simulation Package (VASP) using the projector augmented wave framework.³⁹ Earlier work has shown that dispersion effects can play a substantial role in the correct description of both the metal–zirconia³³ and the solid–water interface.^16,40,41 Therefore, the van-der-Waals corrected so-called optB86b^42,43 functional was used to properly treat dispersion effects, employing the formalism introduced by Dion et al.⁴⁴ Due to the large size of the model cell, a single k point at the Γ point was sufficient to properly describe the electronic structure of the model cell. The energy cutoff was set to 400 eV. The structures were relaxed until the residual forces were below 0.01 eV Å⁻¹. STM simulations were calculated using the Tersoff–Hamann⁴⁵ formalism. To supplement the XPS measurements, core level shifts of the oxygen 1s states were obtained in the final state approximation.⁴⁶ Dissociation barriers were calculated using the improved dimer method.^47,48

3 Results

3.1 General aspects of water adsorption and desorption

3.1.1 TPD. Temperature programmed desorption (TPD) measurements form the backbone of the analysis of the behavior of water on ZrO₂/Pt₃Zr. Fig. 1(a) shows three distinct regions in the TPD spectrum for D₂O: two desorption peaks with maxima at 150 and 180 K, and a tail extending towards high temperatures. We also checked for simultaneous desorption of other species (m/z = 3, 4, 18, 19, and 28) and found no peaks apart from the D₂O cracking products. The desorption temperature of 150 K is typical for multilayer water.⁹ When plotting this peak in a log(I) vs. 1/T plot, see Fig. 1(b), the ascending slope (dotted line) yields a desorption energy of 0.47 eV,^49,50 which is slightly lower than expected for multilayer D₂O ice (0.53 ± 0.02 eV; ref. 51). This may be due to the second-layer ice not having fully developed its crystalline order.

Fig. 1 TPD of D₂O on ZrO₂/Pt₃Zr (heating rate 1 K s⁻¹; no fresh sample preparation between spectra with different coverage). (a) The TPD spectra consist of three regions: the multilayer peak (150 K), the monolayer peak (180 K) and a high-temperature tail up to ≈540 K, see inset. (b) Logarithmic plot of the spectra in (a) vs. 1/T. The linear onset of the multilayer peak gives E_des = 0.47 eV. (c) Inversion analysis showing the desorption barrier E_desvs. coverage using ν = 10^15±1 s⁻¹. In the 0.5–0.95 ML range, E_des is 0.57 ± 0.04 eV. (d) Amount of desorbed D₂O (calculated from the integrated TPD signal) as a function of gas dose. The TPD intensities were calibrated assuming a sticking coefficient of one, thus 1 Langmuir (L) = 10⁻⁶ torr s corresponds to 0.48 ML. The red line is a linear fit.

The TPD peak with the maximum at 180 K exhibits a first-order desorption behavior and reaches saturation at a coverage of approximately 1 ML (one D₂O molecule per Zr atom in the oxide); it is therefore the monolayer peak. The area of the monolayer peak does not stay perfectly constant during repeated TPD measurements, see the section on water-induced changes, below. On the high-temperature side of the monolayer peak, starting at approximately 190 K, the desorption rate decreases more slowly than expected for a first-order peak. The signal forms a long tail and vanishes below the detection limit at ≈540 K, see inset of Fig. 1(a) (comparison with D₂O TPD of other surfaces shows that the tail is not an instrumental artifact). The tail may consist of a multitude of peaks that cannot be discerned from each other. The coverage in the tail corresponds to approximately 0.15 ML, while the rest of the monolayer makes up the peak at 180 K. When plotting the desorbed amount of water, taken from TPD, vs. the dosed amount, the intercept of a linear fit yields a dose of 0.01 ML (Fig. 1(d)). This shows, within the error of such an analysis, that no water was present on the surface before dosing.

The differential desorption energy E_des of water bound in the monolayer peak can be calculated using the inversion analysis method:^52,53 the Polanyi–Wigner formula (1), which describes the desorption rate −dθ/dt in dependence of coverage θ and temperature T,

is inverted under the assumptions of first-order desorption (n = 1), the prefactor ν being independent of θ, and a constant heating rate β:where T is the temperature and t is the time. Eqn (2) is fully defined by the measured TPD data except for ν. To determine ν and E_des, we follow ref. 54 in using eqn (1) to simulate TPD spectra that consist mainly of the monolayer peak. In our case of D₂O on ZrO₂/Pt₃Zr, the best overlap between experimental and simulated curves is achieved at ν = 10^15±1 s⁻¹. This is a typical value for water desorption.⁸Fig. 1(c) shows E_des as a function of θ, derived for an initial coverage of 1 ML. The desorption energy stays almost constant between 0.55 and 0.95 ML and amounts to 0.57 ± 0.04 eV, taking the error bars of ν into account.

At lower coverages (well inside the high-temperature tail), the results of the inversion analysis in Fig. 1(c) give adsorption energies of up to ≈1.5 eV. These energies should be treated with caution; here ν might vary considerably from the value determined for the monolayer peak (e.g. due to recombinative desorption or surface restructuring during desorption).

3.1.2 X-ray photoelectron spectroscopy. To determine whether (or to what extent) water adsorbs in molecular or dissociated form on the ZrO₂/Pt₃Zr surface, we have analyzed XPS O 1s data. When dosing 2 ML D₂O at 100 K, four different peaks are distinguished as originating from D₂O, OD, 3D ZrO₂ clusters, and the ZrO₂ trilayer, see Fig. 2(a). Fig. 2(b, c) shows the peak areas of these four different O 1s components after various flash-annealing steps. The peak at 530.1 eV originates from oxygen bound in the ZrO₂ trilayer and is very close to the reported value of 529.9 eV.³⁶ This peak does not change significantly with coverage or temperature, except for a slight decrease when the intensity is dampened by 2 ML D₂O. Within the accuracy of the fits, the peak attributed to 3D clusters also remains constant. (The trilayer and 3D ZrO₂ clusters have distinct O 1s and Zr 3d spectra;³⁶ the 3D cluster/trilayer ratio is consistent for O 1s and Zr 3d, which confirms the correctness of our peak fitting. An STM image of a 3D ZrO₂ cluster can be found in the ESI.†) The O 1s peak at 533.8 eV originates from non-dissociated D₂O.^6,55,56 When flash-annealing to 160 K, which is above the desorption temperature of the multilayer peak, yet below the desorption temperature of the monolayer peak, this peak decreases by roughly 50%, in agreement with only 1 ML remaining on the surface, see Fig. 2(c). When annealing to 180 K (maximum of the monolayer peak), the molecular D₂O peak decreases further and nearly vanishes at 200 K (beyond the monolayer peak). When annealing to 550 K, the spectrum decreases in the region 531–532 eV, i.e., between the two main peaks. This region contains signals from oxygen in zirconia clusters (E_B = 531.0 eV)³⁶ and from hydroxy groups (E_B = 531.8 eV); a shift of 1.7 eV between the trilayer oxide peak and the hydroxy peak lies within the expected range.^6,55,57,58 Since zirconia clusters do not change when annealed at 550 K, the decrease stems solely from hydroxy groups. The TPD high-temperature tail is therefore due to dissociated water, which recombines before desorption. When comparing the O 1s signal from OD groups and from the ultra-thin film to XPS simulations using the SESSA program,⁵⁹ the amount of water present as hydroxy groups is estimated as 0.12 ML. A small hydroxy component appears also on the as-prepared oxide [labelled “clean” in Fig. 2(b, c)], possibly due to adsorption from the residual gas.

Fig. 2 XPS of D₂O on ZrO₂/Pt₃Zr. (a) O 1s region with 2 ML D₂O deposited at 100 K, then annealed to 160 K to remove the multilayer. (b) Comparison of spectra before and after D₂O adsorption, and after additional annealing steps. All XPS spectra were taken at 100 K. (c) Peak areas of the spectra in (b) for the peaks assigned to D₂O, OD, 3D ZrO₂ clusters, and the ZrO₂ trilayer. (d) TPD measurement (1.54 ML) for comparison with the XPS results in (b, c).

D₂O dissociation induced by X-ray beam damage was excluded by dosing 2 ML of D₂O at 100 K and annealing the sample directly to 200 K. This yielded exactly the same result as shown for 200 K in Fig. 2(b, c), showing that there is no dissociation while taking several X-ray photoelectron spectra.

3.1.3 Scanning tunneling microscopy. Fig. 3(a) shows 0.12 ± 0.04 ML of H₂O (dosed via back-filling of the chamber) on a freshly-prepared ZrO₂/Pt₃Zr surface at 100 K. This coverage corresponds to the high-temperature TPD tail. During measurements, the sample was held at 78 K. Both temperatures lie well below all desorption peaks of water, see above. Three different regions are present in the STM image: ordered, apparently uncovered and clustered areas. The water species in the ordered areas are stable during imaging; they typically appear as 80–100 pm high protrusions. The ordered areas mostly show the same periodicity as the ZrO₂ trilayer, i.e., with respect to the Pt layer below.³³ The clustered areas, presumably with a higher local coverage, appear fuzzy, indicating that the water molecules are moving under the influence of the tip. This suggests that one or two adsorption sites per unit cell are more stable than the others. Following the XPS measurements shown above, which show that the most stable adsorption sites are occupied by hydroxy groups, we attribute the ordered features to OH; this assignment is confirmed below. Dissociation of H₂O leads to two hydroxy groups, one (O_wH, also known as terminal OH) containing the oxygen atom of the water, and the other formed by a surface oxygen atom and the split-off proton (O_sH). These two OH groups should appear differently in STM. As we see only one type of distinct protrusion, we have to assume that STM shows the terminal OH, which is geometrically higher, while an adjacent O_sH is invisible due to its smaller apparent height. (This is in agreement with DFT, see below.) The coverage of one dissociated H₂O, i.e., two OH groups, per unit cell was used as the input for an XPS simulation using SESSA.⁵⁹ The result shows that this coverage accounts for ≈2/3 of the hydroxy groups seen in XPS.

Fig. 3 STM of small H₂O coverages on ZrO₂/Pt₃Zr. (a) 0.12 ± 0.04 ML H₂O, dosed at 100 K, measured at 78 K. The surface consists of stable regions with water species arranged in the structure (yellow dashes), regions with higher coverages and unstable imaging (fuzzy gray patches), and apparently uncovered regions of the ZrO₂ trilayer. (b) After dosing 15 L of H₂O at RT. Water-related species, namely O_wH, are found preferentially above subsurface dislocation lines (grey areas in the image). They locally show the order (yellow). (c) The surface after dosing 30 L of H₂O at RT and storage in UHV for 8 hours. The image was obtained with a special tip that shows OH as depressions (black). Apart from the few hydroxy groups, the clean oxide surface can be seen with atomic resolution. The red lines in the zoom-in (d) indicate that the OH are on top of Zr atoms (protrusions in the image).

When measuring H₂O on ZrO₂ at room temperature (well inside the high-temperature tail), only a low coverage of H₂O-related protrusions is found even after supplying large gas doses, see Fig. 3(b). This is in agreement with the TPD results. Again, the protrusions appear about 80–100 pm high. The ordering can still be made out locally. Water adsorbs preferentially above the subsurface dislocation lines. As mentioned above, these stem from the misfit between the Zr-depleted Pt layer between the ZrO₂ trilayer and the Pt₃Zr substrate.³³ Furthermore, water adsorbs on top of rotational domain boundaries³⁴ of the oxide film (these are rather sparse; there is none in Fig. 3). Fig. 3(c) and (d) show single water species (hydroxy groups) adsorbed on a ZrO₂ trilayer, imaged with a special tip that lets these species appear as dark depressions. Assuming that the bright protrusions in the STM image (c, d) correspond to Zr as suggested for “normal” tips in ref. 33, the red lines indicate that the adsorption site of the water species is above a Zr atom. This is consistent with the site expected for the terminal O_wH.

3.2 Water-induced surface changes

While repeating D₂O TPD measurements on ZrO₂/Pt₃Zr, the monolayer peak changed over the course of several cycles, see Fig. 4(a). There, 18 successive TPD measurements were carried out, each starting with 2 ML D₂O dosed at 100 K and then ramped to 550 K (with one exception for the dose as noted below). The monolayer peak area decreases by 8% from the first to the 18^th run, with the biggest decrease during the first cycles. At the same time, the amount of water desorbing above 185 K increases, such that the total amount of desorbed water remains constant (note that also the area of the multilayer peak stays constant within the experimental accuracy). Thus, the decreasing monolayer peak height is not due to an increasing amount of water remaining on the surface between TPD runs. These results rather show that, under the influence of repeated water adsorption/desorption cycles, adsorption sites with low E_des (T_des = 180 K) change to adsorption sites with higher E_des. Between the 8^th and the 18^th run, eight TPD cycles with 2 ML and one with 15 ML were interposed (not shown); dosing 15 ML affects the surface in the same way as 2 ML. The changing height of the 180 K peak is also visible in Fig. 1(a). (Note that the measurements in Fig. 1 were not done in the sequence of increasing or decreasing coverage, thus the 180 K peak heights at coverages above 1 ML seem to scatter randomly.) Such changes did not appear in multiple TPD cycles with CO₂, so they must be caused by the interaction of the surface with water, not by the heating/cooling cycles alone.

Fig. 4 Water-induced modification of the ZrO₂/Pt₃Zr trilayer. (a) Repeated D₂O TPD runs (heating rate 2 K s⁻¹) with a starting coverage of 2 ML each. The monolayer peak decreases while the high-temperature tail increases. (b) CO₂ TPD (initial dose ≈ 1 ML, heating rate 1 K s⁻¹) before (blue curve) and after (red) 18 D₂O TPD cycles. The increasing intensity in the 110–125 K range indicates an attractive interaction of CO₂ with the water-induced defect sites. Annealing to 1205 K restores the original state of the surface (orange, dashed).

It was found that CO₂ TPD is a good indicator for the water-induced surface change, see Fig. 4(b). 1 ML of CO₂, corresponding to 1 CO₂ per surface Zr atom, was dosed (at 50 K) and desorbed by ramping the temperature to 300 K before the first and after the 18^th D₂O TPD run. The CO₂ multilayer peak is at 78 K,³⁷ and the low-temperature (90–100 K) shoulder of the monolayer peak is attributed to compression of the CO₂ layer.^60,61 The monolayer peak (104 K) and the multilayer peak have roughly the same area, thus approximately one CO₂ molecule per two Zr atoms forms the monolayer. After the water adsorption/desorption cycles, the CO₂ monolayer peak area decreases and the high-temperature tail (105–130 K) increases by approximately 0.05 ML (≈10% of a CO₂ monolayer). When repeating the CO₂ TPD, there is no further change. The TPD behavior indicates that CO₂ binds more strongly to the newly-created sites. The moderate increase of the CO₂ adsorption energy points to electrostatic interaction between OH and the quadrupole moment of CO₂, rather than chemical bonding of CO₂, e.g. as bicarbonate (on TiO₂(110), the CO₂ TPD peak attributed to bicarbonate is at ≈213 K, ref. 62). We also note that the combined coverage of the monolayer (including the low-T shoulder) and tail slightly decreases after the D₂O cycles (by ≈5%). This reduction of the total amount of CO₂ bound in the first layer seems to be mainly related to less compression (weaker low-T shoulder). It is likely that the newly created sites with higher CO₂ and D₂O desorption energy pin the CO₂ layer, preventing the formation of a well-ordered compressed CO₂ layer.

We have also used STM to search for water-induced surface modifications. After one cycle of dosing 1.5 ± 0.3 ML water and annealing to 550 K, the surface appears as shown in Fig. 5(a) (STM at T = 78 K). The superstructure of the ZrO₂ trilayer and the dislocation lines appear unchanged. A small number of water-induced features with an apparent height of 120–150 pm survives the 550 K annealing; their number increases with more adsorption/desorption cycles, and amounts to approx. 0.01 ML per adsorption cycle, fitting the results from the decrease of the ML peak mentioned above. Similar to the OH formed at room-temperature adsorption, these features preferentially bind to sites above the subsurface dislocation lines. After 5 cycles, see Fig. 5(b), some ordering of these water-induced features can be made out: in the Fourier transform of the positions of these species (inset of Fig. 5(b)), the strong outer spots (marked by red circles) indicate equivalent positions with respect to the 0.35 nm ZrO₂ lattice. There are also weaker spots corresponding to the superstructure of the ZrO₂ trilayer on the oxide (blue circles), indicating that some sites in the superstructure are preferred over others. Based on the real-space images, there must be at least two such preferred positions. This behavior is similar to H₂O dosed at RT, see above. This observation can not be explained by H₂O or OH simply remaining on the surface bound to very stable adsorption sites without any surface change, as the density of water species (and therefore the density of sites) increases with the number of cycles. Likely, there is some modification (deformation) of the oxide film, leading to stronger bonding to OH. Nevertheless, the oxide lattice remains largely intact as demonstrated by the sharp FFT spots.

Fig. 5 Effect of H₂O adsorption/desorption cycles on the ZrO₂/Pt₃Zr trilayer seen by STM (T_STM = 78 K). (a) After one adsorption/desorption cycle. The water-induced bright species form preferentially on top of dislocation lines (gray bands) as well as at a rotational domain boundary [between the red arrows in the margins; the orientation of the unit cell on both sides of the DB is indicated by yellow lines]. (b) After five adsorption/desorption cycles, the coverage of the bright spots (O_wH) has increased by almost a factor of five. An FFT of the bright features (inset) indicates equivalent positions on the 0.35 nm ZrO₂ lattice and some influence by the superstructure.

3.3 Infrared measurements on monoclinic ZrO₂ powder samples

Water adsorption on a pre-sintered powder sample of monoclinic ZrO₂ was studied in isothermal (at room temperature) and isobaric experiments. For the isothermal measurements, water was dosed at pressures between 4 × 10⁻³ and 25 mbar (saturation vapor pressure). The infrared absorption spectra do not show any substantial discrete peaks but only a broad band of H-bonded species (Fig. 6(a)).²⁴ As this band does not change shape in the pressure range of interest, the amount of adsorbed water on the sample can be approximated by plotting the relative height/absorbance of this band versus the partial pressure. This approximation is justified by comparison with gravimetric experiments up to pressures where water condensation in capillaries appears, at about 80% relative humidity.²⁴ Since the absorption spectra show strong water gas-phase peaks at pressures above 10⁻² mbar, the absorption was determined at wavenumbers near 3160 cm⁻¹ (vertical line in Fig. 6(a)), where the signal intensity of gaseous water is zero (Q-branch of the first overtone of the δ rotational-vibrational mode⁶³). An absolute calibration of coverage vs. absorption is not possible due to light scattering in the pressed powder.⁶⁴ Based on gravimetric data,²⁴ the FTIR signal at saturation water pressure (p_H2O = 25 mbar) and room temperature corresponds to 20–30 molecules per nm², which is a factor of 2–3 above our previous monolayer definition of 1 H₂O per surface Zr.

Fig. 6 Infrared absorption of H₂O on sintered monoclinic ZrO₂ powder. (a) Spectra at 298 K for selected partial pressures of H₂O. Spectra are shifted vertically for clarity, and a few regions of dense gas-phase absorption peaks are indicated. (b) Differential adsorption energies from the isobaric and isothermal measurements. The vertical lines span the results for the two extreme cases of zero (top) and maximum (ice-like, bottom) adsorbate entropy. Coverage values are normalized with respect to the value at p_H2O = 25 mbar, which corresponds to 2–3 H₂O per surface Zr atom.

Initial adsorption also induces a slight negative peak between 3700 and 3800 cm⁻¹, which we attribute to a small coverage of isolated species, becoming part of the H-bonded network at higher coverage. This frequency range indicates OH bound to one or two Zr atoms. Comparison of the shape of the band of H-bonded species between different pressures shows also an initial negative signal of comparable size superimposed on the broad band above 3650 cm⁻¹. Absorption in this frequency range is frequently observed on monoclinic ZrO₂ and attributed to threefold-coordinated OH.^65,66

In addition, an isobaric experiment was performed. This experiment was performed in flowing mode by passing helium through a water vapor saturator at room temperature (p_H2O = 25 mbar) and monitoring the infrared absorption as a function of temperature. These data are less accurate than the isothermal ones due to various instrumental factors as well as the increasing appearance of ZrO₂ lattice vibrational overtones in the spectra; nevertheless the results are valuable for the analysis (see below).

For determination of adsorption enthalpies, we make use of the fact that the chemical potentials μ_ad of the adsorbed and μ_g of the gas phase must be equal in equilibrium:

μ_g = H_g − TS_g = μ_ad = −E_ad − TS_ad (3)

where the enthalpy H_g and entropy S_g per molecule in the gas phase is calculated from tabulated data (assuming an ideal gas).⁶⁷E_ad is the adsorption energy per atom. We define E_ad as a positive number for stable adsorption sites, as above, thus the minus sign. The volume of the adsorbed H₂O can be neglected, i.e., its energy and enthalpy are assumed to be equal. For calculating the adsorption energy, E_ad, which can be directly compared to the DFT and TPD results above, we need to know S_ad, the entropy per molecule in the adsorbed state. S_ad has a configurational contribution due to the multiple adsorption sites available below saturation, with an upper limit of −k_B ln θ (we use the saturation coverage for θ = 1, but changing this to, e.g., 1 H₂O per surface Zr atom has only a very minor influence on the calculated E_ad). This upper limit is reached if all sites are equal and there is no interaction between adsorbates. The other contribution to S_ad comes from the adsorbate's vibrational and orientational degrees of freedom. For submonolayer coverages, the substrate provides a corrugated potential-energy landscape acting as a template for the adsorbate preventing a liquid layer, see the DFT results below. Therefore, we may safely assume that the adsorbed H₂O is bound and confined at least as strongly as in ice. Thus, we take the entropy of ice, extrapolated to temperatures above 0 °C as an upper limit of its entropy:

S^max_ad = −k_B ln θ + S^extrap_ice(T) (4)

Since S^extrap_ice(T) is almost linear in temperature between 150 and 273 K,⁶⁸ we use a linear fit, which yields S^extrap_ice/(eV K⁻¹) = (3.69 × 10⁻⁵ + 1.42 × 10⁻⁶T/K). (We have also tried using the entropy of liquid H₂O here. This would result in unphysical adsorption energies below the vaporization energy of H₂O, however, supporting our choice of ice for estimating the entropy.)

Fig. 6(b) shows the result of this calculation for both the isothermal (T = 298 K) and the isobaric (298–573 K) experiments. The bars span the range between the two limits for the entropy, zero and S^max_ad(4). For low coverage, these two experiments correspond to substantially different temperatures, thus a comparison between them can be used to estimate the entropy. As expected, the agreement is better for the upper limit of the entropy, i.e., the configurational entropy of the adsorbed layer cannot be neglected and the vibrational and orientational entropies are similar to ice. Nevertheless, we think that the actual entropy must be slightly less than the upper limit since the assumption of S^max_ad yields a reversal of the slope of the isobaric adsorption energies at low coverages, which we consider unrealistic. Therefore, our best guess is an adsorption energy of ≈0.65 eV at low coverages, decreasing to values close to the binding energy in ice at 1 ML. It should be noted that similar values of the adsorption energies could be also obtained from a Langmuir adsorption isotherm (≈0.62 eV over a large range of coverages, when assuming a constant prefactor of 10¹⁵ s⁻¹). This method was already used for adsorption studies on zirconia.^24,69 Compared with the present approach, the disadvantage of the Langmuir model is that the entropies of the gas and the adsorbed phase are hidden in the prefactor and effects such as temperature-dependent rotational and vibrational gas-phase entropies or coverage-dependent configurational entropies of the adsorbed phase will necessarily lead to a non-constant prefactor.

As mentioned, the adsorption energies in Fig. 6(b) are based on the broad absorption band and do not include the isolated species giving rise to the negative adsorption peak discussed above. Thus, one should expect the zero of the coverage scale of Fig. 6(b) a bit further to the left than shown, with higher adsorption energies at these very low coverages.

3.4 DFT calculations

The experiments by Antlanger et al.³³ have shown that the ultra-thin ZrO₂ film forms a commensurate supercell with respect to the Pt lattice, which forms the interface to the Pt₃Zr(0001) substrate below. Each unit cell contains 12 formula units of ZrO₂, which results in an average ZrO₂ in-plane lattice constant of 350 pm. Since accounting for the alloy as the substrate would require a very large unit cell (>500 atoms), a pure 5-layer Pt(111) slab served as the supporting structure of the ZrO₂ film in our calculations (d_Pt–Pt = 277.8 pm; bottom two Pt layers fixed). The viability of such a model has been shown by Meinel et al.²⁸ and for modeling the adsorption of metal adatoms by Choi et al.³⁴

3.4.1 The ZrO₂ film. Structural optimization using a simulated-annealing approach leads to a heavily distorted ZrO₂ film, shown in Fig. 7(a). With the occurrence of twofold coordinated O atoms at the surface, this structure is similar to the one found by Puigdollers et al.⁷⁰ using a PBE+U functional, and very similar to our previous result³⁴ (which is only a local energy minimum according to our present study). Probably due to the flat potential-energy landscape with a large number of local minima, also the present structure does not perfectly represent the experimental one in all details. E.g., Fig. 3(c) suggests a higher symmetry than the DFT model in Fig. 7a. Nevertheless, the main structural elements should be represented well enough to capture the key features for the interaction with the water molecules. The interlayer distances between the mean layer heights, d_O–Zr, d_Zr–O, and d_O–Pt, are 93, 72, and 223 pm, respectively, and all oxide layers are strongly buckled (125, 116, and 96 pm peak–peak for the upper O, Zr, and lower O layer). The group of six high-lying Zr atoms, marked with a dashed white triangle in Fig. 7(a), have contracted Zr–Zr distances of 323 to 347 pm. They are separated by low-lying Zr (“valleys”) with Zr–Zr distances up to 389 pm. In the high Zr triangles, the interaction between the ultra-thin oxide film and the Pt(111) substrate is dominated by the lower oxygen atoms of the trilayer, while in the valleys the Zr atoms bind strongly to the Pt surface atoms. This distortion breaks four Zr–O bonds at the surface, leading to a twofold coordination of some of the oxygen atoms at the borders of the high triangles.

Fig. 7 H₂O adsorption on a trilayer ZrO₂ film in DFT. (a) Top view of the fully-relaxed model cell including adsorption energies (in eV) at water adsorption sites. For selected sites, energies of dissociative adsorption are given after the slash. Values in parentheses indicate that the ZrO₂ film had moved and changed upon adsorption, resulting in a site of different character. The yellow rhombus indicates the unit cell, and the dashed triangle shows the group of six Zr atoms higher than the others. Twofold-coordinated oxygen atoms are marked by yellow asterisks. Side view of (b) molecularly bound and (c) dissociated water at the site marked “0.98/1.05”, with bond lengths in pm. These two configurations are indistinguishable in the STM simulations (d, e).

3.4.2 Adsorption of H₂O on the ultrathin film. To screen a large number of adsorption sites, 36 starting configurations with the H₂O molecule positioned in a 6 × 6 grid with respect to the surface unit cell were tested. After the relaxation, the H₂O molecule had always moved to one of the twelve Zr surface atoms. The adsorbed H₂O molecule does not dissociate spontaneously in any of these twelve sites. Due to the distortions of the ZrO₂ film, these H₂O adsorption sites have substantially different local geometries, which leads to a wide range of calculated adsorption energies, from 0.48 to 0.99 eV (see Fig. 7(a)). For isolated H₂O molecules, adsorption at the laterally compressed “high” Zr atoms is mostly on the weaker side, (0.48–0.77 eV), also reflected by comparably long Zr–O_w distances around 236 pm. Adsorption is stronger at valley sites (0.62–0.99 eV), where the ultra-thin ZrO₂ film is stretched, making the Zr atoms more reactive and accessible. The strongest adsorption is found where the H₂O molecule can form a hydrogen bond to an undercoordinated (twofold) oxygen atom at the border of a “high” Zr group (marked with energy 0.98 in Fig. 7(a); the site “(0.99)” is equivalent, after shifting of the film). A side view of this position with the O–H bond lengths is shown in Fig. 7(b). Due to the short H bond, dissociation via proton hopping is easy (67 meV barrier in DFT). At this site and one other (marked 0.71/0.77 in Fig. 7(a); not counting sites where the oxide film gets substantially shifted), dissociation is slightly more favored than molecular adsorption, see Fig. 7(c).

STM simulations show essentially identical images for molecular and dissociated water, see Fig. 7(d, e); the maxima are caused by the adsorbed water molecule and the terminal O_wH, respectively. This is not surprising as the difference between the two structures is only a small displacement (≈40 pm) of the lower of the two protons, and an even smaller displacement of the O_w (≈14 pm).

Upon adsorption of a full monolayer, i.e., one H₂O molecule per Zr atom, the mean adsorption energy decreases to a value of 0.68 eV. It should be noted that the calculated ground state is not formed when each Zr atom is covered by one H₂O molecule. Instead, the H₂O molecules cluster above the “high” Zr atoms, connected by H₂O chains across the valleys. However, our calculations show that the optB86 functional overestimates the formation energy of hexagonal bulk ice Ih with a calculated value of 0.74 eV compared to the experimental value (≈0.58 eV, ref. 71). This overestimation of the binding between H₂O molecules will artificially increase the tendency for the formation of water clusters. Nevertheless, our calculations show that the adsorption energy in the high-coverage limit is close to the binding energy of multilayer ice.

We have also calculated the core level binding energies of the O 1s states to confirm the identification of the different species by XPS. In the final-state approximation, taking the average binding energy of the O 1s states of the ZrO₂ film as a reference, the core levels shift towards higher binding energies by 1.1 eV and 3.55 eV for the dissociated and molecular adsorbates, respectively. This agrees well with the respective experimental values of 1.7 eV and 3.7 eV. Only small changes of the core level shift of about 0.1 eV are predicted for the different adsorption sites.

Finally, we have performed selected calculations for two ZrO₂ trilayers using the same lateral cell size as for one trilayer. These results should be seen with caution, however, as the film is strained (thicker films should approach the bulk lattice constant, which is ≈3% larger), and due to the large number of local energy minima. Nevertheless, as a general trend it appears that adsorption energies increase with thickness, in agreement to DFT calculations in the literature for monoclinic bulk ZrO₂(1̄11) showing adsorption energies up to ≈1.2 eV.^65,72

4 Discussion

Combining the data from literature and our results, a simple picture emerges for H₂O at all pure zirconia surfaces: at low coverages, H₂O is strongly bound at a few sites, with adsorption energies up to 1.5 eV or more. When these sites are saturated, experimental adsorption energies are around 0.6 eV. The TPD value of 0.45 eV for 7 ML films on Pt(111)³¹ is an exception, probably due to the simplistic assumption of the prefactor being 10¹³ s⁻¹: the TPD peak in ref. 31 is 20 K above the multilayer peak, comparable to ours. Strongly bound water seems to be mostly dissociated; the weakly bound H₂O is in molecular form at least at low temperatures on our ultrathin films. Both our DFT results for the ultrathin films and DFT calculations for H₂O on monoclinic ZrO₂ from the literature^65,72 predict dissociative adsorption at (some or all) twofold-oxygen sites, otherwise mostly molecular adsorption, in many cases forming H-bonded structures already at low coverage. This also nicely fits our XPS data as well as FTIR, where the absence of sharp peaks in FTIR spectroscopy (except for the negative peak from the initial coverage) can be explained only by ensembles of H-bonded species on the monoclinic powder sample. With the H₂O molecules on top of the Zr atoms, and in-plane Zr–Zr distances of ≈330–400 pm, the H₂O–H₂O spacing is too large for a two-dimensional ice-like network of H-bonded species (O–O spacings in ice are 275 pm). Thus, at low coverages, we should not expect extended areas of a 2D water film but only ensembles of very few of H-bonded species.

The DFT energies for water on the ultrathin film agree reasonably well with experiment, especially when considering the overestimation of H bonding with the current functional. For bulk monoclinic ZrO₂ (m-ZrO₂), the experimental adsorption energies at low coverage are very high (≳2 eV;^21,22 our FTIR study does not give any value for the adsorption energy of the species leading to the negative peak). These values are even above DFT-calculated values for flat surfaces,^65,72 probably related to defects. However, at high coverages, all experiments (including our FTIR) indicate values of ≈0.6 eV, while DFT predicts strong H₂O adsorption on bulk m-ZrO₂, with typical adsorption energies in the 0.8–1.2 eV range.^65,72 Our own test calculations for a few H₂O/m-ZrO₂ configurations show similar results. To some degree, the problem may be blamed on overestimation of the strength of H bonds (as mentioned previously); this mainly affects the energies at high coverages.

The density of sites with high adsorption energy (“defect sites”) depends strongly on the type of samples used. Astonishingly, it is rather high for 7 ML ZrO₂/Pt(111) films,³¹ when compared with the ≈15% TPD tail in our ultrathin films. For our powder samples, it may come as a surprise that almost no strongly-bound H₂O is detected. To some degree, this may be an experimental artifact: as the reference spectrum was acquired after high-temperature annealing; very strongly bound hydroxy groups or H₂O readsorbed while cooling in vacuum would be included in the reference spectrum and go undetected. The density of these species should be small, however, as indicated by the small negative peak occurring when the preadsorbed H₂O joins the H-bonded adsorbates. The low density of defect sites in our powder samples may be due to the extensive high-temperature annealing (1 h at 1273 K in pure O₂), which is also responsible for the low specific surface area.

So what are these “defects” with high adsorption energy? We start this discussion by analyzing the DFT results for the ultrathin film. DFT indicates that this film is strongly distorted. Considering the sharp TPD peak (Fig. 1) and the large variation of the DFT adsorption energies (Fig. 7), we believe that DFT overestimates the heterogeneity of the surface (and, related, the distortions). Nevertheless, DFT tells us that the film structure is rather unstable, prone to distortions, and breaking of Zr–O bonds that lead to twofold-oxygen sites. We consider it likely that this kind of instability is responsible for the changes induced by multiple adsorption/desorption cycles, which create additional defect sites. DFT reveals a complex potential-energy landscape of the uncovered ultrathin oxide, but also the variety of bulk structures of similar energy⁷³ can be seen as the root of this instability. The STM results indicate stronger water bonding at one or two of the 12 Zr sites per unit cell, in agreement with the ≈12% defect sites. At room temperature, water is stable at such sites only above subsurface dislocations of the substrate, again showing that a minor disturbance (slight distortions of the substrate) significantly influences the ultrathin oxide. The calculated E_ads = 1.05 eV for the most stable site in the supercell indicates that dissociated water should be barely stable for extended times at room temperature, which nicely fits these STM results, so these defect sites are probably similar to Fig. 7(c).

DFT suggests that twofold oxygen is the site of H₂O dissociation and provides an anchor^65,72 for further H₂O by H bonding. Given that the most stable surfaces⁷⁴ of monoclinic ZrO₂ exhibit at least 1/4 twofold oxygen atoms in the surface, and our FTIR study of well-annealed m-ZrO₂ suggests a low defect concentration, it is unlikely that a twofold oxygen is sufficient to qualify as a defect site. Our DFT results (Fig. 7(a)) show substantially different adsorption energies adjacent to twofold-oxygen sites, suggesting that the details of the geometry at a twofold oxygen and the Zr atom where H₂O adsorbs (and dissociates) strongly influences the bonding strength. This implies that not all twofold oxygens provide strong bonding to H₂O. On the other hand, DFT predicts much stronger H₂O adsorption on m-ZrO₂ than found in many experiments (including ours), which would indicate that the twofold oxygen atoms on m-ZrO₂ are not present or somehow blocked in reality.

Of course, there might be also other defects. We can probably exclude the most common defect⁷⁵ of oxide surface science, oxygen vacancies: for the tetragonal ZrO₂(101) surface (equivalent to {111}-type surfaces of cubic or monoclinic ZrO₂), the formation energy of oxygen vacancies at the surface is higher than in the bulk.⁷⁰ Also for m-ZrO₂(1̄11), our calculations yield a high oxygen vacancy formation energy of 7.1 eV. This means that the concentration of oxygen vacancies at these surfaces will be negligible. Calculations for the ultrathin ZrO₂ trilayer suggest much lower oxygen vacancy formation energies (2.23 eV).⁷⁰ Oxygen vacancies, if present, should be easily filled by H₂O, and the desorption barrier at these sites should be high. Our TPD + XPS experiments exclude a significant amount of such species, however. Therefore, we believe that the concentration of oxygen vacancies at the surface of the trilayer ZrO₂ film is also negligible. Concerning minority sites, we should also consider that our ultrathin-film model system contains 3D ZrO₂ clusters.³⁶ The XPS signal from the clusters (Fig. 2(a)) is about 18% of that from the ultrathin film. Our STM images indicate that the average cluster height is at least four trilayers. With these values, an XPS simulation⁵⁹ indicates that clusters account for less than 7% of the surface area. This area is not enough to have a large impact on our XPS and TPD results; e.g., a picture where all strongly bound or dissociated water is adsorbed on the 3D clusters would be inconsistent with the area fraction of the clusters.

Let us now compare H₂O adsorption on ZrO₂ to structurally similar oxygen-terminated surfaces. The TPD peak temperature of H₂O on the ultra-thin ZrO₂/Pt₃Zr(0001) at 180 K is higher than on other oxygen-terminated oxides such as ultrathin alumina (164–168 K)^76,77 or a 2 ML-thick FeO film (H₂O: 166 K, D₂O: 169 K).⁷⁸ At least for the case of alumina, the top O–Al interlayer spacing (40 pm)⁷⁹ is less than in the present case, so the stronger bonding of H₂O on ZrO₂ must be explained by the large in-plane O–O spacing (≈350 pm) on ZrO₂; this makes the metal cations more accessible than on most other oxygen-terminated oxides, where O–O distances are around 300 pm. Strong adsorption requires that the metal cations are easily accessible; for O-terminated surfaces this means large O–O distances. Examples of structures with O–O distances similar to ZrO₂ are CeO₂(111)⁸⁰ with the main TPD peak at 200 K and In₂O₃(111) with H₂O adsorption energies of 1.2 eV (TPD peak well above room temperature).⁵⁷

Finally, how good a model system is the ultrathin ZrO₂ film for “real-world” ZrO₂ surfaces? The ultrathin film is based on a trilayer of cubic ZrO₂(111) with additional distortions and some twofold-coordinated O atoms at the surface. Thus, geometrically it is similar to the two energetically most favorable monoclinic ZrO₂ surfaces, (1̄11) and (111).⁷⁴ When comparing with ZrO₂ powder, nanocrystalline material will also exhibit a substantial number of edge and corner sites (more than the step sites in our model system), but for the high-temperature annealed material used for our FTIR study we do not expect enough of these sites to play an important role. As is generally true for ultrathin oxide films on metal substrates, the main restriction as a model system applies if adsorption involves charge transfer. In such a case, one should expect stronger bonding on the ultra-thin film than on thick films or bulk materials, where charge transfer is difficult or impossible.^34,81,82 For adsorption of H₂O, in molecular or dissociated form, there is no net charge transfer, so the fidelity of the film as a model system is probably limited by differences in geometry between the film and bulk structures. Unfortunately, the current DFT models do not agree well enough with experimental data for an exact comparison. Nevertheless, the good agreement of experimental H₂O adsorption energies is encouraging.

5 Conclusion

We have investigated the adsorption and desorption of water on a trilayer of ZrO₂ grown by oxidation of a Pt₃Zr(0001) single crystal. With a combined TPD, XPS, and STM approach, we could show that ≈88% of the water molecules in the first monolayer adsorb molecularly, with an adsorption energy of E_des = 0.57 ± 0.04 eV. This agrees well with DFT, which finds dissociative adsorption in only a few sites, with the proton accepted by twofold coordinated oxygen. Experiments also showed that the adsorption energies of dissociated water vary over a wide range, depending on local details of the structure (e.g. substrate dislocations below) and pretreatment (increasing number of “defect” sites after multiple adsorption-desorption cycles). FT-IR measurements on ZrO₂ powder showed similar adsorption energies as our UHV TPD study, indicating that the ultrathin (trilayer) ZrO₂ film is a valid model system for water adsorption on well-annealed real-world ZrO₂ surfaces.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to thank Günther Rupprechter and Christoph Rameshan for lending a Pt₃Zr(0001) sample to us. This work was supported by the Austrian Science Fund FWF (SFB program F45 Functional Oxide Surfaces and Interfaces “FOXSI”) and the Vienna Scientific Cluster (VSC). J. H. and G. S. P. acknowledge support by FWF Start grant Y847-N20.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta04137g

‡ Present address: Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 305-701, Republic of Korea.

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