Orientation-Dependent Hydration Structures at Yttria-Stabilized Cubic Zirconia Surfaces

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 crystal-orientation-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 Hou Sci. Rep.2016627916]. 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.196978243.

Zinc Adsorption and Hydration Structures at Yttria-Stabilized Zirconia Surfaces

“…There are plenty of vacancy sites within the surface layer, which can adapt a whole water molecule (physisorbed) or adsorb as dissociated form (chemisorbed), near the termination plane as discussed in our previous studies. 9,10 The penetrating zinc species we observed here likely replaces the vacancy-filling water at the defective sites caused by the metal depletion. The spacing between the zinc layers on the (111) surface is also larger than that on the (110) surface (indicated in Figure 3 and implied by the parameters in Zinc Partial Profiles).…”

“…Electron density profiles derived from the best-fits to the HRXRs for YSZ in contact with water (black) , and with 10 mM zinc acetate solution (red dash) at (a) the (100), (b) the (110), and (c) the (111) surfaces, and zinc partial density profiles derived from the model-independent (M.I.) (blue dashed lines) and model-dependent (M.D.)…”

“…From the subprofiles of these adsorbed zinc layers, the (110) surface shows inner-sphere (IS), outer-sphere (OS), and “extended” outer-sphere (EOS) profiles, borrowing the terminologies from Lee et al, which all stay above the terminal plane ( z = 0). These layers seem to simply replace the original hydration layers by a small fraction of the total atomic occupancy, i.e., Zn 2+ replaces some amount of metal species in the first ad-layer , as well as replacing the adsorbed water (as IS adsorption) and some water in the adsorbed water layer (as OS adsorption) and first bulk water layer (as EOS adsorption). On the other hand, the (111) surface shows quite distinct behavior in the zinc layer distribution, which includes not only the inner-sphere and outer-sphere adsorbed layers above the terminal plane but also a penetrating layer underneath it into the oxide substrate.…”

“…The single crystal yttria-stabilized zirconia or YSZ substrates (8 mol % Y 2 O 3 -stabilized ZrO 2 ) were purchased from MTI Corporation in the dimension of 10 × 10 × 1.0 mm 3 for the (100) and (111) orientations, and 10 × 10 × 0.5 mm 3 for the (110) orientation. Chemical compositions of the substrates and their d -spacings and unit cell structures along each orientation are described in detail elsewhere. , Zinc acetate dihydrate (Zn­(CH 3 COO) 2 ·2H 2 O, 99.999%, trace metal basis) was purchased from Sigma-Aldrich and used as received. The salt was dissolved in deionized water (DIW) to make the 10 mM zinc acetate solution with measured pH = 6.95 ± 0.01 at room temperature.…”

“…With synchrotron-based high-resolution X-ray reflectivity (HRXR), we have previously studied the detailed interfacial hydration structures of the 8 mol % yttria-stabilized cubic zirconia (YSZ) in (100), (110), and (111) orientations with sub-angstrom resolution and revealed the orientation-dependent water interaction with the oxide surfaces. , In this work, we study the effect of zinc adsorption on the interfacial hydration structures at room temperature and around neutral pH. With element-specific resonant anomalous X-ray reflectivity (RAXR), zinc’s partial electron density profiles are obtained from both model-dependent and model-independent analysis of the RAXR spectra along with the total hydration structures experimentally determined from HRXRs.…”