An Ambient Pressure X‑ray Photoelectron Spectroscopy Study

Article pubs.acs.org/JPCC

Water Reactivity on the LaCoO3 (001) Surface: An Ambient Pressure X‑ray Photoelectron Spectroscopy Study Kelsey A. Stoerzinger,*,†,# Wesley T. Hong,†,# Ethan J. Crumlin,§ Hendrik Bluhm,∥ Michael D. Biegalski,⊥ and Yang Shao-Horn*,†,‡ †

Department of Materials Science & Engineering and ‡Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Advanced Light Source and ∥Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: The reactivity of water with the (001)pc surface of epitaxial LaCoO3 (LCO) thin films was investigated as a function of relative humidity (RH) by ambient pressure X-ray photoelectron spectroscopy. Specifically, water isobars (pH2O = 100 mTorr) were performed cooling from 300 to 25 °C, reaching a final RH of ∼0.3%. Significant changes were found in the O 1s and C 1s core-level spectra at different RHs, which were deconvoluted to yield new insights into the hydroxylation and hydration of the LCO surface. Surface hydroxyl groups were found dominant, which were accompanied by minor components including (bi)carbonates, adsorbed water, and undercoordinated/surface-dipole-influenced oxygen sites on the perovskite surface. A multilayer model was used to quantify the coverage of each species, from which the LCO (001)pc surface was found to exhibit three different regimes upon increasing RH. The water reactivity with the LCO surface proceeded by surface hydroxylatation to reach saturation (up to ∼0.5 ML), after which carbonates were found to displace hydroxyl groups, and then adsorption of water molecules.

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Fe2O3 (0001),20 react with water to form hydroxyl groups and molecular water on the surface. However, the reactivity of catalytically active oxide surfaces for oxygen electrocatalysis, such as perovskites (ABO3, with rare earth metal ions on the A site and late transition metal ions on the B site), has not been studied. This is partly due to the complexity of the XPS O 1s spectrum present on pristine ternary oxide surfaces, which has made identification of the surface oxygen species difficult. For example, the apolar TiO2 (001) termination of SrTiO3 has a single, well-defined peak associated with bulk oxygen ions in the O 1s spectrum,21 similar to those of binary oxides such as MgO and Fe2O3. For polar surfaces and/or ternary oxides with multiple terminations present, however, multiple oxygencontaining species can be observed, which creates ambiguities in the analysis of the O 1s spectrum.22−24 In particular, polar LaBO3 perovskite films with (001)pc (pseudocubic) orientation typically show one surface and one bulk feature.24−26 These species may arise from polar compensation of the surface dipole (by electronic, geometric, or chemically adsorptive means)27,28 or differences in the chemical potential among terminal planes. The physical origin of the surface feature is not well understood

INTRODUCTION Perovskite oxides are a functional class of materials with broad applications ranging from catalysis1−4 to magnetics5 and electronics.6 The reactivity of the surface with the atmosphere, specifically water vapor, plays a key role in determining the functionality of these complex oxides. Environmental changes can greatly influence oxide surface chemistry, including the coverage of surface hydroxyl groups and molecular water at ambient conditions,7,8 surface segregation of oxygen vacancies and cations,9,10 and surface reconstruction.11 Changes in the surface chemistry in turn impact the surface dipole and work function,12 surface electronic structure,13 and the mechanisms and kinetics of surface chemical reactions.14 For example, density functional theory studies have shown that the binding and coverage of OH species for Pt-alloys,15 and oxides16 correlates with the activities for the oxygen reduction and evolution reactions. However, the reactivity of oxide surfaces with the environment and how oxide surface chemistry influences their functionality remains largely unexplored under in situ conditions. Recent development of ambient pressure X-ray photoelectron spectroscopy (AP-XPS)17,18 can provide novel insight into how oxide surfaces interact with water as a function of relative humidity (RH). Several studies have shown that relatively inert oxide surfaces, such as MgO (100)19 and α© 2014 American Chemical Society

Received: March 25, 2014 Revised: June 29, 2014 Published: August 5, 2014 19733

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Epitaxy was confirmed by thin film X-ray diffraction (XRD), performed using a four-circle diffractometer (Bruker D8, Germany) in normal and off-normal configurations (Figure 1, and Figure S1, Supporting Information). The film thicknesses (∼22 nm) were estimated using the thickness fringes in the high-resolution 2θ−ω scans of the (002)pc diffraction peaks (“Epitaxy” software, PANalytical). Pseudocubic (pc) lattice parameters were measured to be apc = 3.859 Å and cpc = 3.790 Å from the (002) and (202) peaks. Assuming a Poisson ratio ν = 0.25 typical for perovskites,35 the relaxed lattice parameter was calculated to be âpc = 3.817 Å, in good agreement with values reported previously for bulk LCO.36 Ambient Pressure X-ray Photoelectron Spectroscopy. AP-XPS was collected at Beamline 11.0.2 at Lawrence Berkeley National Laboratory’s (LBNL) Advanced Light Source (ALS).17 All data in the main text were collected from a single beamtime under multibunch operation to minimize systematic errors resulting from beam flux and chamber cleanliness. However, we note similar trends were observed during a different beamtime under a different mode of operation (twobunch), as shown in the Supporting Information. LCO films (5 × 5 mm) were placed directly onto a ceramic heater and held in place by spring-loaded Iconel tips.37 A thermocouple was mounted directly onto the sample surface for temperature measurements and isolated from the sample holder clip with an Al2O3 spacer. The O 1s, C 1s, La 4d, and Co 3p core-level spectra were collected under UHV (∼10−7 Torr) and referenced to the C 1s binding energy of adventitious carbon (284.8 eV).38 The samples were then cleaned by heating to 300 °C in 100 mTorr O2 (measured by a calibrated membrane pressure gauge) until all carbon was removed, as verified by AP-XPS of the C 1s core level. All subsequent spectra were aligned relative to the UHVdetermined bulk oxide O 1s binding energy (528.7 eV). XPS measurements (C 1s, O 1s, La 4d, Co 3p, valence band) were collected for the clean surface in oxygen, including differential energy measurements (accessing kinetic energies from 200 to 465 eV for the O 1s) to obtain depth-resolved information on spectral contributions. A series of water vapor isobaric experiments were subsequently performed to examine the changes in surface chemistry as a function of RH, which was calculated from the actual water partial pressure (p) divided by the saturation water vapor pressure (p0 at the given sample temperature p0(T)): RH(p,T) = p/p0(T). RH was varied from 1 × 10−4% to 0.3%. Before the water isobar experiments, the chamber was evacuated to a pressure lower than 1.5 × 10−7 Torr. Subsequently, 100 mTorr H2O was introduced into the chamber, keeping the cleaned sample at 300 °C. The sample was allowed to equilibrate for ∼10 min prior to characterization. The water source was prepared from deionized water (Millipore, >18.2 MΩ·cm) and degassed by several freeze− pump−thaw cycles. The sample was then cooled in increments of 25 °C down to a final temperature of 25 °C, keeping the chamber pressure constant at ∼100 mTorr H2O. All isobar spectra were taken at an incident energy of 735 eV; the inelastic mean free path is smallest for the O 1s (∼6 Å) and about twice as large for the La 4d and Co 3p. The O 1s and C 1s core-level spectra were collected every 25 °C while the La 4d and Co 3p core-level spectra were collected every 100 °C. The combined collection time for all of the spectra at each 25 °C and 100 °C temperature interval was no longer than 5 and 15 min, respectively, to minimize the deposition of carbonaceous

and has been previously attributed to oxidized carbon species,29,30 surface hydroxyls,30 adsorbed water,31 and undercoordinated oxygen.32 Calculated predictions of the surface structure of LaBO3 (001)pc under vacuum33,34 exclude the reactivity with species such as H2O and CO2. To better understand water reactivity on perovskites, AP-XPS experiments can be used to explore surface chemistry changes and identify oxygen species based on the evolution of spectroscopic features as a function of RH. In this study, we consider the (001)pc surface of epitaxial LaCoO3 thin films as a model polar perovskite surface. AP-XPS experiments were performed in ultrahigh vacuum (UHV) and near-ambient pressure of oxygen (∼100 mTorr), as well as a range of RHs (accessed through water vapor isobaric experiments). By following changes in the surface core-level features under different environments, we identify the O 1s contributions associated with surface hydroxyls, carbonates, adsorbed water, and a surface species we attribute to undercoordinated/surface-dipole-influenced oxygen species. The binding energies and full-width-half-maxima associated with fitting these features are tabulated as reference for future XPS studies on perovskite surfaces, and trends in the coverage of surface species determined from the fitted peak areas are discussed. Considering the hydroxylation of and water adsorption on the LaCoO3 surface with respect to RH, we discuss the role of rare earth and transition metal cations in surface reactivity.

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EXPERIMENTAL METHODS Thin Film Fabrication and Characterization. Epitaxial thin films of LaCoO3 (LCO) were fabricated by pulsed laser deposition (PLD) on single crystal (001)-oriented 0.5 wt% Nb:SrTiO3 substrates with a dimension of 10 × 5 × 0.5 mm (Crystec, GmbH). Niobium-doped substrates were used for sufficient conductivity to minimize surface charging effects in XPS. The PLD target was synthesized using a solid-state reaction from stoichiometric mixtures of La2O3 and Co3O4 (Alfa Aesar, Ward Hill, MA) sintered at 1450 °C to 90% theoretical density. PLD was performed using a KrF excimer laser (λ = 248 nm) at a pulse frequency of 10 Hz and laser fluence of ∼1.50 J cm−2. A total of 5000 pulses of LCO were deposited at 610 °C under 50 mTorr O2. Film surface morphologies were examined by atomic force microscopy (AFM, Veeco) and showed root-mean-square (RMS) roughness ∼0.5 nm (Figure 1 inset).

Figure 1. High resolution X-ray diffraction (XRD) of a representative LCO film in normal configuration (002)pc. Inset: Atomic force microscopy (AFM) image of a representative LCO film surface with RMS ∼0.5 nm. The scale bar is 200 nm in length and the contrast scale is 0−3 nm. At right is a schematic of the structure of an ideal pseudocubic perovskite film, with alternating planes of LaO (+1 charge) and CoO2 (−1 charge). 19734

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species on the surface throughout the experiment. Through the use of a beam shutter, the sample was not irradiated by X-rays between scans. Care was also taken to reduce the incident photon flux so issues such as carbon deposition were not observed. Because high intensity radiation has been shown to influence the dissociation of water on surfaces39 and was observed to deposit adventitious carbon, we performed measurements where the beam spot (250 × 250 μm) was rotated between three different locations after every condition to minimize beam influence and compared these to measurements collected when the beam spot location was held static. To quantify the coverage of different surface species found on the LCO surface, O 1s spectra were deconvoluted with Gaussian−Lorentzian (GL) peaks after a Shirley-type background subtraction. The O 1s spectra were fit using five peaks corresponding to the bulk oxide (“bulk”), hydroxyls, (bi)carbonates, “surface” oxygen, and gas-phase water; the choice of this fitting methodology can be seen directly from the raw spectra and is discussed in more detail later. The peak-fitting parameters are summarized in Table S1, Supporting Information. The relative ratio of Gaussian to Lorentzian line shape was chosen to best match the spectral shape observed experimentally (Figure S2, Supporting Information). The best fit of O 1s spectra was obtained when the “bulk” peak was fitted with a predominantly Gaussian (10% Lorentzian), the “surface” peak was fitted with 50−70% Lorentzian line shape, and all other components were kept as pure Gaussian. The integrated intensities and coverage of different species were not altered significantly when one line shape (Gaussian with 30% Lorentzian) was applied to all components (Figure S3, Supporting Information). Error bars of the peak-fitting parameters (taken as 95% confidence intervals) were determined from the variances of the fitted peak parameters, determined using Monte Carlo simulations of the experimental noise. Details on the error analysis can be found in the Supporting Information. The surface cleanliness during the experiments was monitored by XPS (Figures S4 and S5, Supporting Information). Under ambient pressure water vapor, accumulation of various carbon species such as graphitic carbon and/or hydrocarbon as well as (bi)carbonate was observed. Because the (bi)carbonate O 1s peak overlaps with the “surface” peak, its integrated area was calculated from the area of the C 1s (bi)carbonate peak using an O 1s:C 1s relative sensitivity factor (RSF), which was experimentally calibrated using the core levels of CO2 gas. This provided a value of 0.86 ± 0.03:1 (O 1s:C 1s) for an incident photon energy of 735 eV. This value depends on the experimental geometry, beam optics, and in some cases chamber pressure and should be measured for each experimental run. The total coverage of carbonate was 3 × 10−2%, 75 °C) and increased with RH. Also present was a small peak at high binding energy associated with gas-phase water (∼534−535 eV).19,20,42,43,50 The fitted intensities (Table S2, Supporting Information) and relative contributions (Table S3, Supporting Information) of these components with RH is consistent with visual inspection of the XPS spectra, where the “bulk” peak decreased, accompanied by increasing intensities of hydroxyls, carbonates, and adsorbed water. Analysis of subtle changes in the La 4d spectra further supports surface hydroxylation with increasing RH, where the most pronounced difference can be seen between 300 °C in 100 mTorr O2 and 25 °C in 100 mTorr H2O (Figure 6). The

(58% and 29% of total intensity, respectively).53 Although the satellite contribution of La2(CO3)3 has not been measured, one would expect an ionicity more similar to La(OH)3 rather than to La2O3. The satellite contribution was found to decrease with increasing RH (to 43% of total intensity), indicating an increase in ionic character. Depth profiling of a clean LCO film also indicates that the surface is more ionic; however, the impact of RH is greater. This supports our interpretation that hydroxylation (and the formation of carbonates) occurs with increasing RH and further suggests that LaO terminations exist on the surface. However, the coexistence of CoO2 terminations cannot be excluded despite the fact that minimal changes were observed in the Co 3p spectra, as the 3p core level is known to show weak sensitivity to surface oxidation state.54 A recent density functional theory study has reported that the LaO termination of the (001)pc orientation of LaCoO3 is most stable over a wide range of oxygen chemical potentials;33 however, it is well-known for perovskites that the preferred termination is sensitive to numerous parameters such as temperature, constituent chemical potentials, and the presence of adsorbates.55,56 The O 1s binding energies of surface adsorbates remained offset by a nearly constant energy from the “bulk” oxide peak upon cooling, while that of the gas-phase H2Ogas shifted to notably higher energies. The difference in binding energy between the gas-phase H2Ogas peak and the bulk oxide peak is plotted in Figure 7 as a function of temperature. This difference

Figure 7. Difference in binding energy of the water gas-phase (H2Ogas) and “bulk” oxide binding energies, which is inversely proportional to the change in work function of the LCO surface. Data collected keeping the beam spot in the same position (open) and rotating between locations (solid) are shown, with the darker solid points indicating analysis at the same location but at longer equilibration times. The relative increase in binding energy of the gas-phase peak is proportional to a decrease in work function under more humid conditions, shown on the right axis relative to 300 °C for the nonrotating spot. Error bars are defined as 95% confidence intervals estimated from Monte Carlo simulations of the experimental noise.

Figure 6. La 4d spectra at 735 eV incident photon energy taken at the same beam location for a clean film at 300 °C in 100 mTorr O2 (gray) and a film at 25 °C in 100 mTorr H2O (blue), background subtracted, and with the fitted envelope shown as a solid line. Deconvoluted spectra and constraints are shown in Figure S8, Supporting Information. The satellite features associated with La−O hybridization decrease with RH, characteristic of increased ionicity.

lanthanum spectra show strong satellite peaks, which can be attributed to charge transfer from the ligand valence band to the 4f0 orbital of the core-ionized lanthanum.51 The intensities of these satellites relative to the main peak (ionic unperturbed state) is indicative of the degree of hybridization with the ligand.52 For example, La2O3 has a greater satellite contribution than its more ionic counterpart La(OH)3,53 which suggests that changes in the relative number of La−O and La−OH bonds can be identified using the La 4d satellite features. To quantify this change, we employ the fitting methodology of Sunding et al.,53 using the binding energy constraints shown in the caption of Figure S8, Supporting Information. The contribution of the satellite peaks for a clean LCO surface (49% of total intensity) falls intermediate to those reported for La2O3 and La(OH)3

arises because the energies of the adsorbate O 1s core levels are tied to the LCO Fermi level while that of the gas-phase are relative to the vacuum level.50,57,58 The increase in the gasphase peak binding energy, which exceeds 1 eV, is indicative of a decrease in work function at the LCO surface, where similar work function reduction has been reported on Fe3O4 (001) with increasing RH.42 This change cannot be attributed solely to hydroxylation, as the coverage of OH (calculated in the following section) saturated by 150 °C while the work function continued to decrease, and significant adsorbed water on LCO (100)pc was not observed until 50 °C. A number of other factors, including surface reconstruction, change in surface 19737

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on the surface under different conditions, and the affinity of each element for hydroxylation, carbonate formation, and wetting. Lacking this information, our model effectively assumes an equal distribution of cation terminations by taking the average oxygen atomic densities and thicknesses for La and Co hydroxides/carbonates. Further insight may be gained from theoretical studies or comparison of different perovskite chemistries, guiding deeper understanding of the reactivity of oxide surfaces with the environment. Surface Adsorbate Coverage on LCO with Increasing RH. The coverage for surface adsorbates (θOH, θCO3, θH2O) on LCO as a function of temperature during the H2O isobar experiments is shown in Figure 9. Cooling from 300 to 150 °C

termination, and adsorbate reorientation (such as the rotation of an adsorbed OH dipole), may contribute to the observed decrease in work function, meriting further experimental and theoretical investigation. Estimation of Surface Adsorbate Coverage on LCO. The coverage of surface adsorbates was calculated from the relative O 1s peak areas using a multilayer electron attenuation model,19,59,60 which accounts for the number of oxygen atoms in a given adsorbate, the dimension perpendicular to the surface, and attenuation by overlaying species. The model is composed of a series of slabs, including a semi-infinite LCO crystal, a layer of “surface” oxygen assumed to originate from the perovskite surface, a layer of coexisiting OH and CO3 adsorbates, and a layer of adsorbed water on top, as shown in Figure 8. It is postulated that the “surface” oxygen layer

Figure 8. Schematic illustration of the multilayer electron attenuation model. The computed variables of the model are listed in parentheses. The “surface” oxygen layer was treated as a slab of complete coverage but with the thickness tsurf allowed to vary, where all potential contributions (undercoordinated/surface dipole influenced oxygen sites) were encompassed in a slab with model parameters equal to that of the perovskite. OH and CO3 were considered to coadsorb on the film surface with the coverage allowed to vary in the model, and adsorbed water was considered to cover the entire surface with the thickness tH2O allowed to vary. Details on the calculations and the parameters used are provided in the text and Table S4 in Supporting Information.

Figure 9. Coverage of surface species on LCO as a function of temperature in 100 mTorr H2O: adsorbed H2O (□, light blue), CO3 (Δ, gray), and OH (o, medium blue). The RH axis serves as a guide to the eye as it deviates slightly from a logarithmic relation with temperature. Error bars are defined as the 95% confidence intervals estimated from Monte Carlo simulations.

resulted in an increase of θOH from ∼0.3 to ∼0.85 ML, accompanied by a small amount of carbonate formation, reaching ∼0.15 ML at 150 °C. This condition marks a transition point, where the combined coverage of OH and CO3 is ∼1 ML (i.e., θOH + θCO3 ∼ 1). Upon further cooling, the coverage of hydroxyl groups was found to decrease while the coverage of carbonates continued to increase monotonically. This observation suggests that the continued deposition of carbonaceous species on the surface comes at the expense of the hydroxyl groups. The coverage of OH and CO3 appeared unaffected by the adsorption of water, notable at temperatures below 75 °C. The adsorption of molecular water reached ∼0.1 ML at the most humid condition. The temperature-dependent surface coverage on LCO thus has three regimes: (I) predominant hydroxyl formation (300 °C > T > 150 °C); (II) predominant carbonate formation at the expense of hydroxyl groups (T < 150 °C); (III) onset of surface wetting (T < 75 °C). Similar trends were observed during a different beamtime in which the carbonate coverage was notably higher due to residuals in the chamber from previous measurements (Figure S12, Supporting Information). The transition temperature to regime II under these conditions increased to ∼200 °C; however, the corresponding carbon coverage (θCO3 ∼0.2 ML) is similar to that at the transition in Figure 9. Influence of the Beam on Surface Adsorbate Coverage. The coverage of surface adsorbates obtained from the O 1s spectra collected from one fixed spot and rotating spots are compared in Figure 10A as a function of temperature. While the initial OH coverage was independent of irradiation, and the

includes perovskite CoO2 and LaO terminations and/or reconstructed perovskite surfaces and was treated as a slab of complete coverage but with variable thickness. OH and CO3 were considered to coadsorb on the film surface; although the combined coverage was not fixed, the computed values remained less than one monolayer. Finally, adsorbed water found at low temperatures and high RHs was considered to cover the entire surface with variable thickness. Although adsorbed water likely binds preferentially to OH and less so to CO3, our model describes adsorbed water covering both adsorbates, as it is assumed that any phase segregation of surface species such as OH and CO3 is smaller than the area from which XPS data were collected. Additional details on the calculations and the parameters used are provided in Figures S9−11 and Table S4, Supporting Information. It is appropriate to discuss the limitations in the multilayer model used in this study. As mentioned earlier, adsorbed water is assumed to form a layer of uniform density on the surface; however, experimental and theoretical studies have shown that the adsorbed water layers on oxides and carbonates can show three-dimensional island growth.61,62 Improvements on the model in this regard would require measurements by microscopic methods with spatial resolution. Furthermore, the assumption that surface hydroxyl and carbonate species can be represented by bulk hydroxides and carbonates may also be questioned. Perhaps of greatest interest for future study, however, is the degree of La- and Co-containing terminations 19738

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Lastly, it should be mentioned that the dependence of OH coverage on RH can be influenced greatly by transition metal ions and coordination. Specifically, our study stands in contrast to TiO2 (110), which has a constant OH coverage of ∼0.25 ML regardless of RH or adsorbed water,43 highlighting the need of future studies to systematically study the influence of transition metal ions on surface-water interactions.

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CONCLUSIONS In this article, we study water reactivity with the (100)pc surface of an LCO film grown on Nb:SrTiO3 by examining systematic changes in the core-level spectra at different RHs. Specifically, we observe that surface hydroxyl groups are the dominant adsorbate, along with minor components including (bi)carbonates, adsorbed water, and undercoordinated/surfacedipole-influenced oxygen sites on the perovskite surface. The coverage of these species can be estimated using a multilayer electron attenuation model, and we report three distinct surface-water interaction regimes with increasing RH: an initial hydroxylation, followed by a saturation regime after which carbonates begin to displace hydroxyl groups, and then adsorption of water molecules. The reactivity of LCO with water contrasts the case of binary transition metal oxides, where the high basicity of lanthanum and the polar nature of the (100)pc orientation of LCO contributes to greater degrees of surface hydroxylation and (bi)carbonate formation at low RH. Such AP-XPS studies on well-defined oxide surfaces can provide insights into the reactivity of oxides with the environment and surface chemistry of oxide surfaces, which is critical to develop functional oxides for catalysis, sensing, and energy storage applications.

Figure 10. Coverage of surface species on LCO as a function of temperature (A) and time (B), comparing when the analysis area was rotated (solid) to when it was not (open): adsorbed H2O (□, light blue), CO3 (Δ, light gray), and OH (o, medium blue). In comparing time, the sample that was rotated during the isobar was kept at fixed position. Error bars are defined as the 95% confidence intervals estimated from Monte Carlo simulations.

trends of OH, CO3, and adsorbed water coverage as a function of RH were comparable, rotating the sample led to a lower θOH but did not affect θCO3 or θH2O. The enhancement in the OH coverage by beam irradiation can be attributed to an increase in the dissociation constant of H2O,39 effectively reducing the barrier for hydroxylation and promoting OH binding on sites. This beam-enhanced hydroxylation was further supported by examining the coverage of OH collected from a fixed spot at 25 °C in the presence of water as a function of time. The coverage of adsorbed water remained nearly constant with time, and the slight increase in CO3 coverage is less than the marked increase in OH coverage with 15 min of beam exposure, as shown in Figure 10B. The linear trend in CO3 formation both with temperature and beam exposure suggests this process is not fully equilibrated and/or reversible in the present measurements, further confirmed by a reverse isobar where the temperature was increased back to 300 °C after cooling to 25 °C (Figure S12, Supporting Information). Comparison of LCO Surface Reactivity with Previously Studied Oxide Surfaces. Here we qualitatively compare the water reactivity on LCO (100)pc with that of binary oxides reported previously, as quantitative comparison among different studies might not be valid due to potential variations in beam-induced water dissociation. The onset of water adsorption on LCO occurs at comparable RH (∼0.01% RH) to previous studies. In contrast to MgO, Fe2O3 (0001), and Fe3O4 (001) surfaces,19,20,42 which exhibit low OH coverage prior to the onset of water adsorption (wetting), LCO (001)pc exhibits considerably higher hydroxylation and reaches a maximum OH coverage (∼0.5 ML) prior to the onset of water adsorption. The higher hydroxylation on LCO at low RH can be explained by the polar nature of the (100)pc orientation, where alternating planes of [LaO]+ and [CoO2]− require polar compensation, as well as the high basicity of Lasites, where the adsorption of OH favors LaO terminations over transition-metal-oxide terminations. In addition, water adsorption on LCO is not correlated with accelerated adsorption of OH species, in contrast to MgO, Fe2O3, and Fe3O4, but instead continued adsorption of CO3 species at the expense of OH species. This difference can be attributed to the presence of LaO-terminations on LCO (001)pc, as rare earth cations form carbonates more readily than transition metal cations do.63

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional figures and tables of raw and normalized intensities. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions #

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We give many thanks to Andrey Shavorskiy for assistance with AP-XPS measurements. This work was supported in part by the MRSEC Program of the National Science Foundation under award number DMR-0819762 and the Skoltech-MIT Center for Electrochemical Energy. The ALS and the MES beamline 11.0.2 are supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences and Materials Sciences Division of the US Department of Energy at the Lawrence Berkeley National Laboratory under Contract No. DE- AC0219739

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05CH11231. The PLD preparation performed was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. K.A.S. was supported by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1122374.

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