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Orientation-Dependent Hydration Structures at Yttria-Stabilized Cubic Zirconia Surfaces Binyang Hou,*,†,○ Seunghyun Kim,‡,○ Taeho Kim,‡ Changyong Park,† Chi Bum Bahn,§ Jongjin Kim,∥ Seungbum Hong,∥,⊥ Su Yong Lee,# and Ji Hyun Kim*,‡ †

High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, United States ‡ Department of Nuclear Science and Engineering, School of Mechanical and Nuclear Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, South Korea § School of Mechanical Engineering, Pusan National University, Busan 46241, South Korea ∥ Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Department of Materials Science and Engineering, KAIST, Daejeon 34141, South Korea # Pohang Accelerator Laboratory, Pohang 37673, South Korea S Supporting Information *

ABSTRACT: Water interaction with surfaces is very important and plays key roles in many natural and technological processes. Because of the experimental challenges that arise when studying the interaction of water with specific crystalline surfaces, most studies on metal oxides have focused on powder samples, which averaged the interaction over different crystalline surfaces. As a result, studies on the crystalorientation-dependent interaction of water with metal oxides are rarely available in the literature. In this work, water adsorption at 8 mol % yttria-stabilized cubic single crystal zirconia (100) and (111) surfaces was studied in terms of interfacial hydration structures using high resolution X-ray reflectivity measurements. The interfacial electron density profiles derived from the structure factor analysis of the measured data show the existence of multiple layers of adsorbed water with additional peculiar metal adsorption near the oxide surfaces. Surface relaxation, depletion, and interaction between the adsorbed layers and bulk water are found to vary greatly between the two surfaces and are also different when compared to the previously studied (110) surface [Hou et al., Sci. Rep. 2016, 6, 27916]. The fractional ratio between chemisorbed and physisorbed water species were also quantitatively estimated, which turned out to vary dramatically from surface to surface. The result gives us a unique opportunity to reconsider the simplified 2:1 relation between chemisorption and physisorption, originally proposed by Morimoto et al. based on the adsorption isotherms of water on powder metal oxide samples [J. Phys. Chem. 1969, 78, 243].

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INTRODUCTION Water interaction with surfaces plays key roles in many natural and technological processes, such as corrosion, geochemistry, electrochemistry, bioadsorption, lubrication, and catalysis.3,4 Zirconia is an important material in numerous applications, such as gas sensors,5 solid oxide fuel cell electrolytes,6,7 and biomedical materials.8 It is also useful for protecting zirconium alloys in highly corrosive environments, such as those found in pressurized water reactors.9 However, zirconia suffers long-term degradation in humid or aqueous conditions due to its interaction with water.10,11 Therefore, the interaction of zirconia with water has attracted considerable interest in efforts to improve the life cycle of the materials.12 For instance, modern neutron scattering experiments have demonstrated that water molecules are both chemisorbed and physisorbed on zirconia surfaces.13−15 Similarly, water adsorption on yttria© XXXX American Chemical Society

stabilized zirconia (YSZ) surface was quantitatively studied by Raz et al.16 Their study was based on the simple relation of 2:1 ratio between the chemisorbed and physisorbed water on metal oxides proposed by Morimoto et al. nearly a half century ago.2 In that model, one water molecule is initially adsorbed onto every two metal atoms and eventually turns into two hydroxyl groups (chemisorbed) that bind to each metal atom. Every two neighboring hydroxyl groups then adsorb one water molecule through hydrogen bonding (physisorbed). More recently, the energetics of water adsorption on YSZs was also carefully measured by Costa et al. but orientation-specific surface Received: July 27, 2016 Revised: November 29, 2016 Published: November 30, 2016 A

DOI: 10.1021/acs.jpcc.6b07550 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

z = 1/2d200 plane, and two O atoms at the z = d200 plane. The stacking of this reduced unit cell results in alternating planes of four O and two Zr atoms, respectively (Figure 1a). On the

energies were not determined as powder samples were studied.17 The structure and chemistry of anhydrous surfaces in different crystallographic orientations of zirconia, both pure ZrO2 and YSZ, in vacuum are known to be intrinsically different from each other according to theoretical studies.18−20 It has also been confirmed experimentally that their stability, configuration, and termination could vary dramatically.21 The study of water adsorption with a specific truncation plane of the zirconia surface was rarely reported and is only found in theoretical work.22 The interfacial hydration structures with atomic scale resolution are, however, yet to be studied for revealing the molecular-scale details of water reaction with the oxides. The hydration structures at specific truncation planes of the zirconia are vitally important for understanding the degradation processes, as they constitute the reaction front. With high-resolution X-ray reflectivity (HRXR), we have recently measured the hydration structure of the YSZ (110) surface with sub-Ångströ m resolution, which revealed a complex but highly ordered structure.1 In the present work, we study the orientation-dependent water interaction with YSZ surfaces in terms of near-surface metal depletion, lattice relaxation, and interfacial hydration structure. The structure factor analysis based on the nonlinear least-squares fitting to the measured X-ray reflectivity data is used to derive the electron density profiles of YSZ (100)− and (111)−water interfaces. Distinct structural differences were found among all three crystalline surfaces. The different interfacial hydration structures are described in detail, and the implication of the oxide degradation mechanism along different truncation planes is discussed. In addition, the chemisorbed and physisorbed water species on different crystalline surfaces are quantitatively assessed, showing varying relations from surface to surface. This result also gives us an opportunity to reconsider the simple 2:1 chemisorption−physisorption relation that was generalized by Morimoto et al.2 and recently adopted for zirconia surfaces.16

Figure 1. Crystal structure of yttria-stabilized cubic ZrO2: (a) side view of the (100) surface and (b) side view of the (111) surface. Teal spheres represent metal atoms, and red spheres represent oxygen atoms. With 8 mol % Y2O3 doping, the metal atom sites consist of 14.8% Y and 85.2% Zr atoms, and the oxygen sites have 96.3% occupancy.

other hand, along the surface normal direction of [111], the new unit cell exhibits hexagonal packing and the lattice constants reduce to a = a0/√2, and c = a0/√3, resulting in d111 = 2.968 Å. The unit cell consists of one and one-half O atoms at the z = 0 plane, three Zr atoms at the z = 1/4-d111 plane, three O atoms at the z = 3/4d111 plane, and one and one-half O atoms at the z = d111 plane. Along the surface normal direction, the stacking of unit cells results in periodic translations of three O, three Zr, and three O layers (Figure 1b). The d-spacing along the [110] direction is d110 = 3.634 Å, and the unit cell structures are described in detail elsewhere.1 Theoretical studies showed that, on YSZ surfaces, the singlelayer oxygen-terminated surface is energetically favorable over Zr or double oxygen layer (O−O) terminated surface.18,25 These configurations are also assumed in many studies.19,20,26−28 Therefore, in the present study we follow the single-layer oxygen-termination convention for both (100) and (111) surfaces. As a result, YSZ (100) and (111) have periodic plane arrangements of Zr−O−Zr-O···−Zr−O and O−Zr−O− O−Zr−O−···−O−Zr−O, respectively, with the outermost plane solely occupied by O atoms bridging between two metal sites (Figure 1). Note that the surface of YSZ (100) is reported to be neutral by removing a half of the oxygen atoms;19 therefore, the topmost surface layer is modeled to be occupied by two O atoms per unit cell area, leaving intrinsic vacancies on the surface. Sample Preparation. The single crystal samples were cleaned in sequential acetone, methanol, and deionized water baths with sonication for more than 15 min at each step. The procedure was repeated several times to completely remove the adventitious carbon from the surface, which was detected by Xray photoelectron spectroscopy in a previous study.1 One sample (YSZ (111) substrate) was baked in a high-temperature furnace in air at 500 °C for 24 h, and its HRXR result was compared to that without baking. The HRXRs turned out to be identical from both baked and unbaked samples, which confirmed that the room temperature cleaning procedure asdescribed was sufficient. The sample was kept in deionized water immediately after cleaning, until it was mounted to the thin film cell (Figure 2) for HRXR measurements. The exposure time to air was restricted to less than 1 min during sample mounting to minimize the effects of adsorption of adventitious carbon species on the surfaces. Before the measurements, deionized water was kept flowing through the

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EXPERIMENTAL SECTION Materials. The single crystal YSZ substrates (nominally 8 mol % Y2O3) were purchased from MTI Corporation in the dimensions of 10 × 10 × 1.0 mm3 for (100) and (111) surfaces, and 10 × 10 × 0.5 mm3 for the (110) surface.1 The lattice constant, a0, was determined to be 5.140 ± 0.008 Å from the single crystal X-ray diffraction experiment. The chemical compositions of the YSZs were analyzed by Vegard’s law for cubic zirconia,23 d = as X + b, where d is the lattice parameter, as is a constant that depends on the dopant species, X is the dopant contents, and b is a constant independent of the dopant species. The chemical compositions of the purchased YSZs turned out to match that of 9.3 mol % YSZ that has a fractional stoichiometry of 0.17 Y, 0.83 Zr, and 1.915 O atoms. However, our structure factor calculation showed that the different stoichiometries of 8 mol % and 9.3 mol % YSZs resulted in identical best-fit results in reproducing the measured HRXRs. The crystal structure of cubic YSZ is of the CaF2 type. Along the surface normal direction of [100], the unit cell exhibits its original cubic structure with the lattice constant, a0, and dspacing, d100 = a0 = 5.140 Å. Since the reflection of facecentered cubic (FCC) (100) is forbidden,24 we consider here a reduced unit cell corresponding to (200) spacing, d200 = 2.570 Å, for the surface normal direction. Thus, the reduced unit cell consists of two O atoms at the z = 0 plane, two Zr atoms at the B


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Kirkpatrick-Baez mirrors with a sample to detector distance of ∼268 mm. The single crystal (100) surface was carefully aligned parallel to the beam at the rotation center of the sample stage. The diffraction pattern was collected by an area detector (MAR345 Image Plate) with the sample rotating along the vertical axis. The 2D diffraction pattern was integrated with Dioptas33 to determine the lattice constant a0 of the cubic YSZ. The integration of the diffraction pattern is shown in Figure 3. Figure 2. Schematic illustration of the thin film cell and the experimental geometry of the X-ray reflectivity measurement. The inset is a picture of an actual sample cell with the sample length indicated. (Figure reprinted from ref 1, Nature Publishing Group 2016).

cell using a syringe to remove any unexpected contaminants. After that the water was drained out by gravitational force to form and maintain a thin film of water ca. 10 μm thickness between sample surface and the Kapton film (see Figure 2 for the geometric configuration). The inlet and outlet of the cell were closed prior to measurement. The cell was maintained as closed during the measurement. High Resolution X-ray Reflectivity Measurements. The HRXR measurements were carried out at beamline 9C of the Pohang Light Source (PLS) at the Pohang Accelerator Laboratory (PAL) in South Korea. We scanned the (00L) crystal truncation rod (CTR) intensities along the surface normal directions in θ−2θ scan geometry.29,30 This technique is essentially a one-dimensional crystallographic tool for determining the average electron density profile, and most importantly, the interfacial atomic structure where water molecules and possibly adsorbed metal species interact with the substrate surface. A detailed description of the method can be found in the review article.29 The incident X-ray energy was 15.000 keV, and the beam was focused to 0.1 mm (v) × 0.7 mm (h) by a toroidal focusing mirror with an incident flux of ∼1.1 × 1011 photons/s. A Pilatus 200K area detector was used to collect the reflected intensities, which was mounted on the 2θ arm ∼930 mm away from the sample rotation center. The scattering area on the detector was defined by a set of slits in front of the detector with a size of 8 mm (v) × 8 mm (h) for most of the data points taken. The detector slits were reduced to 4 mm (v) × 4 mm (h) to cut the strong interfering signals for the data points taken near the Bragg peaks. The background subtraction and data reduction procedure follows the description by Fenter et al.31 A set of guard slits with a size of 2 mm (v) × 2 mm (h) at a distance of ∼300 mm from the sample were used for reducing the background scattering. The measured intensities were normalized by the direct beam intensity. The errors were propagated from the counting statistics. To check the reproducibility, HRXR measurements from substrates of YSZ (100) and (111) in contact with water from different batches were carried out at beamlines 33-BM-C and 16-ID-D of the Advanced Photon Source (APS) with X-ray energies of 21.000 and 20.000 keV, respectively. The reflectivity curves are identical to those measured at beamline 9C of the PLS for both YSZ (100)− and (111)−water interfaces, and are therefore not reported here to avoid redundancy. Single Crystal X-ray Diffraction Measurement. The single crystal X-ray diffraction measurement was carried out at 16-BM-D32 of the APS with X-ray energy of 29.200 keV. The X-ray beam was focused to ∼4 μm (v) × 3 μm (h) by a set of

Figure 3. Integrated X-ray diffraction pattern of 8 mol % YSZ obtained from the single crystal X-ray diffraction in limited reflection geometry with an X-ray energy of 29.200 keV.

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ANALYSIS AND RESULTS High-Resolution X-ray Reflectivity Analysis. The measured HRXR intensities from the YSZ (100)− and (111)−water interfaces as a function of momentum transfer, Qz = (4π/λ) sin θ, are presented in Figure 4a and Figure 5a, respectively, where λ is the wavelength of the incident X-ray and θ is the incident angle of the beam with respect to the substrate surface (Figure 2). Since the reflections of FCC (100) and (300) are forbidden,24 the first and second Bragg peaks in Figure 4a correspond to the Bragg peaks of (200) and (400), respectively. The first and second Bragg peaks in Figure 5a correspond to the Bragg peaks of (111) and (222), respectively. The measured reflectivity can be reproduced from a structure factor model based on the known substrate structure and the fitted model for the interfacial structure:29,30 I(Q z) ∝ |Fsub·uc(Q z)·FCTR (Q z) + Finterface(Q z) + Fwater(Q z)|2 (1)

Here we only briefly describe the key terms in the structure factor model. The full expression of the reflectivity and the details of each term are shown in the Supporting Information. Fsub·uc(Qz) × FCTR(Qz) corresponds to the semi-infinite single crystal substrate structure factor. Finterface(Qz) includes the near surface unit cells that deviate to any extent from the bulk, e.g., relaxation in the dspacing, surface depletion, and the adsorbed layers containing water and other species such as ions. Fwater(Qz) corresponds to the semi-infinite bulk water. The maximum Qz determines the spatial resolution of the corresponding real space structure. The electron density profile for each scattering atom can be modeled by a Gaussian distribution whose distribution width is characterized by a full width at half-maximum (fwhm). The maximum Qz ∼ 6 Å−1 in the experimental data gives an effective spatial resolution of π/ Qz,max ≈ 0.52 Å in terms of the fwhm. The derived electron density profile is then convoluted with the Gaussian broadening function (the corresponding root-mean-square width, σ = 0.22 Å). C


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Figure 4. Reflectivity data (circles with error bars) and the best-fit result (solid line) for YSZ (100)-water interface as a function of momentum transfer Qz: (a) measured X-ray reflectivity, R, and (b) normalized reflectivity, R[Qz 2sin(Qzdspacing/2)]2.

Figure 5. Reflectivity data (circles with error bars) and the best-fit result (solid line) for YSZ (111)-water interface as a function of momentum transfer Qz: (a) measured X-ray reflectivity, R, and (b) normalized reflectivity, R[Qz 2 sin(Qzdspacing/2)]2.

Figure 6. Electron density profiles derived from the best-fit results of the measured X-ray reflectivity for (a) YSZ (100)−water interface and (b) YSZ (111)−water interface.

The Best-Fit Results for HRXR. We performed a nonlinear least-squares fitting analysis to derive the atomic structures of YSZ (100)− and (111)−water interfaces, respectively. The best-fit models were found through a trial-and-error approach as the initial interfacial structures were completely unknown. The details of the modeling strategy, especially for the interfacial structure factor, Finterface(Qz), are specific to the individual system and are described in detail in the Supporting Information. The best-fit results for the measured HRXR data are plotted in Figure 4a and Figure 5a (solid lines) for (100) and (111)

surfaces, respectively, which reproduce the experimental data very well. The measured HRXR data normalized by their generic CTR intensities, RCTR = 1/[Qz 2 sin(Qzdspacing/2)]2, are shown in Figure 4b and Figure 5b, respectively, to emphasize the interference terms between the structure factor components expressed in eq 1. By suppressing the predominant single crystal substrate signals, the normalized HRXR spectra show more distinct oscillatory features caused by the coherent interferences of the structure factor components, such as the surface relaxation in normal direction, elemental depletion of surface layer, water adsorption, etc. The χ2s for the least-squares D



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fitting are 4.72 and 12.76 for YSZ (100)− and (111)−water interfaces, respectively. The apparently higher χ2 for YSZ (111)−water is likely due to the smaller error bars for the measured data. More details of the observed χ2 values are described in the Supporting Information. On the basis of the best-fit parameters, the electron density profiles along the surface normal direction were derived, and are shown in Figure 6a and Figure 6b for the (100) surface and the (111) surface, respectively. As the specular X-ray reflectivity only probes the structure along the surface normal direction, these profiles show the laterally averaged electron density profiles. The fitting parameters (described in the Supporting Information) and their values from the best-fit model are listed in Table 1. The nonfitting constant parameters are listed in Table S1 in the Supporting Information.

best-fit results with errors parameters

YSZ (100)−Water

YSZ (111)−Water

1.173e7 ± 2.6e5 1.098 ± 0.062

1.045e7 ± 2.73e5 1.223 ± 0.063

2.382 2.606 2.559 2.567 2.570 2.570 0.270

± ± ± ± ±

0.015 0.004 0.003 0.002 0.005

± 0.011

2.867 2.972 2.967 2.975 2.973 2.968 0.435

± ± ± ± ±

0.005 0.003 0.003 0.002 0.002

± 0.007

0.167 ± 0.008

0.080 ± 0.006

0.018 ± 0.004

0.022 ± 0.002

0.008 ± 0.002

negligible

negligible

negligible

−1.589 ± 0.348

−0.717a

0.103 ± 0.022

0.114 ± 0.035

0.679 ± 0.027

1.121 ± 0.019

0.803 ± 0.027

1.735 ± 0.055

0.401 ± 0.022

0.510 ± 0.010

2.460 ± 0.017

3.137 ± 0.046

2.074 ± 0.120

4.117 ± 0.369

0.185 ± 0.027

0.352 ± 0.036

3.415 ± 0.105

6.588 ± 0.028

0.787 ± 0.109

0.247 ± 0.036

0.533 ± 0.098

0.218 ± 0.013

DISCUSSION

Surface Relaxations and Metal Depletion. The best-fit results show significant depletion of metal species in the top unit cell layers for both surfaces. Specifically, the depletion at the top unit cell is about 27.0% of the original metal occupancy on the (100) surface and 43.5% on the (111) surface, respectively (Figure 6). The depletion of metal in the second and third unit cell layers reduces to 16.7% and 1.8%, respectively, on the (100) surface, and 8.0% and 2.2%, respectively, on the (111) surface. From the fourth layer on, the depletion becomes negligible ( 5.5 Å−1 for (100) and >4.5 Å−1 for (111), respectively). Similarly, multiple modeling strategies were tested with various different possibilities, from which we could choose the best feasible scenario for reproducing the measured HRXR data. Therefore, we take the first adsorbed layer as metal adsorption but with the possibility of forming a coordination structure with terminal oxygens, vacancy-filling water, and those in the second adsorbed water layer. Since it has been reported that desorbed metal ions can be readsorbed near the surface,42 we do not rule out a possibility that the metal species in this layer could include the readsorbed yttrium ions that were initially dissolved from the substrate due to their interaction with water. Alternatively, they could be the dangling zirconium atoms which are still in the position of more depleted crystal structures just above the termination surface as discussed in detail in our previous study.1 Immediately above this layer, there exists an additional adsorbed water layer at 2.460 ± 0.017 Å for (100) surface and 3.137 ± 0.046 Å for (111) surface, indicated by the fourth red dashed lines in Figure 6, parts a and b. The peak heights of these layers in both cases look higher than that expected for the normal bulk water due to its narrower distribution width compared to that of the normal bulk water. However, the estimated water occupancies, 2.074 ± 0.120 for (100) surface is significantly lower than the expected occupancy (i.e., the maximum allowed water molecules per unit cell area: 2.59) and 4.117 ± 0.369 for (111) surface is significantly higher (i.e., the maximum allowed water molecules per unit cell area: 3.37), respectively. Note that the expected occupancies are obtained from the known 2D packing density of water, 1 molecule per 10 Å2, and the unit cell surface areas of 25.9 Å2 for the (100) surface and 33.7 Å2 for the (111) surface, respectively. The existence of the ordered second hydration layer is qualitatively consistent with the basic assumption that the vacancy filling by water and the metal−hydration interactions at the interface should strongly constrain the next-layer hydration.29 The distribution widths for the second adsorbed water layer are 0.185 ± 0.027 Å for the (100) surface and 0.352 ± 0.036 Å for the (111) surface. These distribution widths are larger than the Debye−Waller factor of lattice oxygen atoms (∼0.1 Å), indicating that the layer is likely physisorbed water layer. The position of the first bulk water layer is found at 3.415 ± 0.105 Å for the (100) surface and 6.588 ± 0.028 Å for the (111) surface, or ∼0.955 and ∼3.451 Å away from the second adsorbed water layer, respectively, indicated by the sky blue dashed lines in Figure 6, parts a and b). Meanwhile, the average distance between the second adsorbed water layer and the first bulk water layer from the YSZ (110)−water interface1 was found at ∼1.732 Å. The distance between the adsorbed water layer and the first bulk water seems to vary dramatically among the different truncation planes. It is apparent from the electron density profiles that the (100) surface (Figure 6a) and the (110) surface1 seem to have continuous water exchange

considered much more stable but was also reported to undergo a phase transformation when it was exposed to water under hydrothermal conditions.40 Based on our observation of surface relaxation and metal depletion, we propose a possible configuration of the initial stage of the oxide structure breakdown that is caused by its interaction with water as follows: Because of its intrinsic oxygen vacancies that originated from the lattice expansion of Y2O3 doping, as well as surface reconstruction on (100) and the high density of edges on (111), oxygen-terminated (100) and (111) surfaces have enough open space for the metal species to be exposed to water when they are in contact with water. The interaction of the metal species with water results in great metal depletion, possibly due to the dissolution of Y atoms that are enriched on the top surface due to Y segregation. The significant depletion of the Y species thereafter promotes the reduction of the top surface unit cell d-spacing, which is the starting point of phase transformation on the top surface. The long-term, slow, yet continuous process of the metal dissolution and surface phase transformation would eventually lead to further breakdown of the oxide structure. Interfacial Hydration. The vacancy-filling water layers at metal-depleted sites are indicated by the first two red dashed lines in Figure 6, parts a and b, for YSZ (100) and (111) surfaces, respectively. While not exactly positioned at −1.192 Å on (100) (i.e., one-half of its top unit cell d-spacing), the vacancy-filling water position in the topmost unit cell of −1.589 ± 0.348 Å still agrees well with the metal-layer position within the estimation errors. The positions of other vacancy-filling water layers at metal-depleted sites in the second topmost unit cell on YSZ (100), and the first and second topmost unit cells on YSZ (111), agree very well with their corresponding metaldepleted sites with negligible variations. For simplicity of data analysis, they are estimated by the positions of the metaldepleted sites. This analysis showed consistency with our assumption that the water species in these layers should fill the metal vacancies. The vibrational amplitudes for these water layers were initially estimated; however, they turned out to be rather small, and were close to the Debye−Waller factor for the lattice oxygen atoms. This indicates that these water molecules are strongly constrained to the metal vacancy sites and is consistent with the existence of chemisorbed water by neutron scattering experiments.13−15 The vacancy-filling water species helps in completing the terminal structure and minimizing the surface free energy.1,41 Just above the termination surface, there exists an enhanced electron density layer on both surfaces which likely contains extra metal species rather than just pure water molecules. We assume this layer consists of complex hydrated metal species as discussed in the Supporting Information. For the YSZ (100), the best-fit results show the position of this density enhanced layer to be at 0.679 ± 0.027 Å with metal occupancy of 0.803 ± 0.027 per unit cell area and vibrational amplitude of 0.401 ± 0.022 Å. For the YSZ (111), the best-fit results show the position of this density enhanced layer to be at 1.121 ± 0.019 Å with metal occupancy of 1.735 ± 0.055 per unit cell area and vibrational amplitude of 0.510 ± 0.010 Å. These layers are indicated by the third red dashed lines in Figure 6, parts a and b). The best-fit analysis in both cases yielded with no water hydration coordination. However, we should not take these results as a solid evidence of no hydration structure formation for these adsorbed metal layers, but more realistically, we should consider that these metal layers are surrounded by F


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species (i.e., no sensitivity to the converted hydroxyl ions from the surface oxygens). Nevertheless, it is unlikely that the systematic 2:1 ratio can be applicable to the observed ratios. The ratios between the chemisorbed and physisorbed water among all three crystalline planes of 8 mol % YSZ are listed in Table 2. The values estimated based on an assumption of maximum possible oxygen vacancies are also listed in the Table 2. The ratio of the two water species at the (110) surface appears close to 2:1; however, it is much suppressed at the (100) and (111) surfaces. The simple 2:1 ratio proposed by Morimoto et al.2 suggested that the amount of chemisorbed water is directly related to the metal species that is exposed to water. The difference we observed here could be due to the fact that the metal species at the (110) surface is directly exposed to water, while it is partially blocked by the oxygen layer on top of (100) and (111) surfaces. These apparently different ratios clearly show that the simple 2:1 relation between chemisorbed and physisorbed water species on metal−oxide surfaces does not hold for every specific crystalline truncation plane.

between the two layers, while the water exchange between these two layers on the (111) surface seems to be very limited as indicated by the apparent gap between the two layers (Figure 6b). It is known that the hydrophilicity (or hydrophobicity) of the oxide surfaces strongly depends on the presence of oxygen vacancies.43 The YSZ (111) surface among the three truncation planes has the densest hexagonal packing, and seems to have the least oxygen vacancies on the surface. Therefore, it is expected to be most hydrophobic among the three truncation planes. This is consistent with our observation that the gap between the adsorbed layers and the first bulk water layer is the largest on the (111) surface. Table 2. Ratios between Chemisorbed and Physisorbed Water at YSZ (100), (110), and (111) Surfaces orientations

(100)

without additional oxygen vacancy with maximum additional oxygen vacancy a

(110) sample (110) sample no. 2a no. 1a

(111)

1.22

2.06

1.52

0.35

1.75

2.53

2.30

0.98

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CONCLUSIONS The hydration structures of YSZ (100) and (111) surfaces were probed with high resolution X-ray reflectivity. The best-fit results showed that the intrinsic oxygen vacancies and metaldepleted sites near the oxide surface are presumably filled by chemisorbed water molecules. As indicated in the previous report at the YSZ (110)−water interface,1 there exist two additional adsorbed layers above the terminal layers, with the first adsorbed layer containing metal species and the second adsorbed layer likely consisting of physisorbed water. This feature seems to be common for all three studied YSZ surfaces, characterizing the YSZ−water interfaces. Our results also showed significant metal depletion for YSZ (100) and (111) surfaces, which is likely correlated to the surface relaxation that leads to a large reduction in the lattice constant only at the top unit cell layer. We showed semiquantitatively that the ratio between the chemisorbed and physisorbed water species varies greatly from surface to surface, which gives us a unique opportunity to reconsider the simple relation that was proposed nearly a half century ago by Morimoto et al. on specific crystal surfaces of the metal oxides. It is also shown that the exchange between the adsorbed layers and the bulk water layers is limited along the (111) surface, while the (100) and (110) surfaces seem to have continuous exchange between the adsorbed water and bulk water layers. Our results suggest that the surface energy reductions for these surfaces in contact with water are achieved quite differently. While water adsorption on oxide surfaces is generally characterized from the powder samples, our study clearly shows that the modification of surface energy and hydration structure is highly orientation-dependent and varies dramatically from surface to surface.

Values deduced from ref 1.

Rethinking the Relationship between Chemisorbed and Physisorbed Water Species on Metal−Oxide Surfaces. Our observation shows that there are two modes of water adsorption at the all measured YSZ−water interfaces: one is the species adsorbed through site occupation (i.e., on metal depletion sites and oxygen vacancies) and the other is the species through hydrogen bonding but highly ordered (i.e., the second adsorbed water layer). The site-occupying water species is likely to be considered as chemisorbed, while the hydrogenbonded water species should be considered as physisorbed. As our method is not directly sensitive to water speciation, there is no means in the present study to determine the exact fraction of the surface oxygens that are converted to hydroxyl ions at the surface; therefore we set the observed chemisorbed water occupancies as a minimum possible chemisorption, excluding the possibilities of conversion of surface oxygens to hydroxyl ions as assumed in the earlier studies.2,11 In addition, the water species that could be potentially included in the adsorbed metal layer to form local hydration coordination is totally unaccounted. Regardless, the ratio between the chemisorbed and physisorbed water molecules at the YSZ−water interfaces, as observed here, is far different from the known average ratio based on the powder isotherm measurement (i.e., 2:1 between chemisorbed water species including the converted hydroxyl ions and the hydrogen bonded physisorbed water2) and varies dramatically from surface to surface. In the case of (100) surface, the occupancy of chemisorbed water is about 2.54 per unit cell area (= metal depletion sites of 0.27 × 2 + intrinsic oxygen vacancy of 2), while the physisorbed water occupancy is 2.074, which gives a ratio of ∼2.54:2.074 or 1.22 between the chemisorbed and physisorbed water. In the case of (111) surface, the occupancy of chemisorbed water is about 1.43 per unit cell (= metal depletion sites of 0.435 × 3 + intrinsic oxygen vacancy of 0.037 × 3), which gives a ratio of ∼1.43:4.117 or 0.35 between the two. While, in the previously studied case of the YSZ (110)-water interface,1 the ratio seems to be close to the 2:1 ratio, i.e. about 2.06 for sample no. 1 and 1.52 for sample no. 2, respectively. It is possible that we could have underestimated the actual population of the chemisorbed water

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07550. Detailed structure model, minimum momentum transfer, best-fit model for HRXR, detailed explanation of different χ2 values for the best fits, sensitivity of X-ray reflectivity in detecting the water adsorption, and a list of nonfitting parameters(PDF) G


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.H.). *E-mail: [email protected] (J.H.K.). ORCID

Binyang Hou: 0000-0003-0535-7706 Changyong Park: 0000-0002-3363-5788 Seungbum Hong: 0000-0002-2667-1983 Ji Hyun Kim: 0000-0002-3984-0686 Author Contributions ○

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. B.H. and S.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the International Collaborative Energy Technology R&D Program (No. 20138530030010) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted from the Ministry of Trade Industry and Energy. B.H. and C.P. acknowledge support from the High Pressure Collaborative Access Team (HPCAT), supported by the DOE-NNSA under Award No. DENA0001974 and the DOE-BES under Award No. DE-FG0299ER45775. The authors acknowledge the use of beamtime at beamlines 9C of the Pohang Light Source and 16-BM-D, 16ID-D, and 33-BM-C of the Advanced Photon Source. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors thank Drs. P. Chow and Y. Xiao, and Mr. C. Kenney-Benson at the HPCAT, Drs. E. Karapetrova and C.M. Schlepuetz at Sector 33 of the APS, and Dr. Y. Kim at beamline 9C of the PLS for their technical support.

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ABBREVIATIONS YSZ, yttria-stabilized zirconia; HRXR, high resolution X-ray reflectivity; CTR, crystal truncation rod

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