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Atsuro Fujisawa, Xuhui Xu, [Yuta Ishii](https://orcid.org/0000-0002-8957-5833), Hidekazu Shimotani, Yuta Inoue, Yuto Miyahara, Kohei Miyazaki, Yusuke Wakabayashi

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[Surface Structure Modulation of La0.6Sr0.4CoO3 Films on SrTiO3 (001) Substrate under Electrochemical Conditions](https://mdr.nims.go.jp/datasets/8a56b138-12e1-4f9b-9420-9e3deedec130)

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Surface Structure Modulation of La0.6Sr0.4CoO3 Films on SrTiO3 (001) Substrate under Electrochemical ConditionsSurface Structure Modulation of La0.6Sr0.4CoO3 Films on SrTiO3 (001)Substrate under Electrochemical ConditionsAtsuro Fujisawa, Xuhui Xu, Yuta Ishii, Hidekazu Shimotani, Yuta Inoue, Yuto Miyahara, Kohei Miyazaki,and Yusuke Wakabayashi*Cite This: https://doi.org/10.1021/acsami.5c11807 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: The surface structure of the La0.6Sr0.4CoO3 film, a typical model water-splitting catalyst, is examined under vacuum and electrochemical conditions using surfaceX-ray diffraction. The pristine sample has a two-unit-cell-thick strongly polarized SrCoO3layer at the surface, and the surface termination is predominantly a CoO2 layer with apicaloxygen atoms. After electrochemical treatment, the surface was covered with anadditional edge-shared CoO6 double layer. The polarization in the SrCoO3 region wasgreatly reduced. These structural changes were accompanied by an increase in theworking electrode current, suggesting a strong relationship between surface structuremodulation and catalytic activity. Reversible structural modulation induced by the film’selectric potential was observed and was qualitatively understood as atomic displacements caused by the local electric field andchange in the Co ionic radii.KEYWORDS: transition metal oxide surfaces, thin films, surface structure, water electrolysis, X-ray diffraction, in situ measurements1. INTRODUCTIONSpontaneous mass transport sometimes plays a significant rolein the properties of heterogeneous systems such as at surfacesand grain boundaries. Phase separation and proton conductionare typical examples of mass transport inside a material. In ironpassivation, a classical example of chemical reactions at aninterface, oxygen vacancies in the iron oxide layer flow towardthe surface to make the passive layer thicker.1−4 Mass transportin the environment often controls the rate of various chemicalreactions at solid−liquid interfaces by limiting the amount ofreactant at the interfaces.In water electrolysis in an alkaline environment, O2 isformed from abundant OH−, and therefore, the mass transportin the water side is completed immediately. In contrast, masstransport in the electrode side causes the widely observedactivation and degradation of electrocatalysts.5−8 The surfacestructure is one of the important factors affecting the oxygenevolution reaction (OER) activity.9−13 The importance of thestructural information is enhanced for oxides because the oxidestructure reflects the electronic states, such as the bondingnature, valency, and orbital and spin states of cations. Oxidecatalysts have been extensively studied because of their highenvironmental stability, including at high temperature, andhigh degree of freedom to control the chemical and electronicstructures.9,11,13−17 Spontaneous mass transport causes unin-tended structural modulation. In addition, the OER at theoxide surface is assumed to involve lattice oxygen in the so-called lattice oxygen-mediated mechanism (LOM)18 in whichspontaneous mass flow plays an important role. Therefore, insitu structural measurements of oxide catalysts are required.High-resolution surface structure analysis in controlledenvironments can be achieved using the crystal truncationrod (CTR) scattering method, a surface X-ray diffractiontechnique.19,20 Using this technique, the catalytic reactionshave been examined at gas−noble metal21−23 and liquid−noblemetal interfaces.24−26 Regarding oxide catalysts, rutile-typeoxides have been studied in detail12,27−29 because part of theCTR signal from the rutile structure is solely from oxygen,27,30which makes detailed structural analysis of the oxygensublattice easier. However, perovskite (001) surfaces do notprovide such a signal solely originating from oxygen, whichhampers the detailed examination of surface oxygen positionsin perovskite oxides under electrochemical conditions.Recent advances in analytical methods have led to manyreports of detailed structures of perovskite surfaces undervacuum conditions.31−37 Here, we report the surface structureof La1−xSrxCoO3, an active water-splitting catalyst, before andduring electrochemical treatment. For this compound, trans-mission electron microscopy at a vapor pressure of H2O of afew Pa, ex situ measurements of lattice parameters8 and surfacestructure analysis based on CTR measurements under vacuumwithout any electrochemical treatment37 have been reported.However, an in situ surface structure analysis is yet to beReceived: June 16, 2025Revised: August 19, 2025Accepted: September 22, 2025Research Articlewww.acsami.org© XXXX The Authors. Published byAmerican Chemical SocietyAhttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXThis article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on October 8, 2025 at 09:01:05 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Atsuro+Fujisawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Xuhui+Xu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuta+Ishii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hidekazu+Shimotani"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuta+Inoue"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuto+Miyahara"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kohei+Miyazaki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Wakabayashi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Wakabayashi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsami.5c11807&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=agr1&ref=pdfwww.acsami.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsami.org?ref=pdfhttps://www.acsami.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/reported. We prepared an atomically flat thin film grown on anSrTiO3 (001) surface. Under vacuum, the surface waspredominantly terminated by CoO2 planes with apical oxygenatoms on top of the Co sites. Time evolution of the surfacestructure was reported in the early stage of the electrochemicalprocess, and once saturated, nearly half of the surface wascovered with an additional CoO2 layer, i.e., CoO2 double-layertermination. The observed CoO2 double-layer structure hasedge-shared CoO6 octahedra, and this favors oxy-hydroxideformation.38 Additionally, the electric potential reversiblycontrols the surface structure, and this process involves surfacepolarization from oxygen displacement.2. METHODSLa1−xSrxCoO3 epitaxial films were grown on Nb-doped SrTiO3 (001)substrates (10 × 10 × 0.5 mm) using pulsed laser deposition with anNd:YAG laser. The wavelength, laser power, and repetition rate were266 nm, 20 mJ, and 2 Hz, respectively. During deposition, thesubstrate temperature was kept at 600 °C.The sample was mounted in a vacuum chamber or a sealedelectrochemical cell (a schematic view of the cell is shown inFigure S6) filled with 0.1 mol/L KOH aqueous solution. Prior to thecell preparation, the solution was purged with N2 gas bubbled for 15min. The electric potential V of the sample was controlled by apotentiostat relative to the Ag/AgCl reference electrode with 3 mol/LKCl aqueous solution. Throughout this paper, all potentials arequoted relative to the Ag/AgCl electrode.CTR scattering measurements were performed at BL-4C at thePhoton Factory, KEK, Japan. A synchrotron X-ray beam wasmonochromatized by a Si (111) double-crystal monochromator andfocused on the sample by a bent cylindrical mirror. A standard four-circle diffractometer was installed on the beamline, and a small two-dimensional pixel array detector (XPAD-S70, imXpad, France) wasattached on the 2θ-arm with double-slit optics. The measurementswere carefully performed to minimize radiation damage under theelectrochemical conditions. All measurements were performed atroom temperature.Quantitative analysis of the CTR intensity profiles was performedusing the Bayesian analysis software CTR-structure.39,40 In this study,we took the topmost CoO2 plane as the origin of the phasing andassumed a Gaussian distribution of the surface height so that thedetailed surface structure could be discriminated from the surfaceroughness (see Section 1 of the Supporting Information for moredetails on the diffraction theory). The sample structure wascharacterized under vacuum with a standard procedure reportedelsewhere.35 Only a subtle modulation of the surface structure wasexpected to be induced by the applied potential. Therefore,quantitative analysis was performed on the ratio of the intensitymodulation caused by the potential; the idea is similar to that for theobservation of the electric double layer based on the X-ray reflectivitymeasurement.41The surface structure model that we constructed is presented inFigure 1. The (001) surface of the perovskite structure can have BO2termination or AO termination. The occupancy parameters of theatoms up to the surface BO2 layer (the atoms presented in Figure 1a)are fixed to unity. The topmost Co site among the fully occupied sitesis labeled Btop as shown in Figure 1. Oxygen sites in the AO plane andBO2 plane are called the OI and the OII sites, respectively. The surfacestructure models are presented in (b)−(d). On top of the BtopO2layer, we assumed there was an AO plane (O(1) and As in (c)), a BO2double-layer structure (O(3) and BDL in (d)), and additional oxygenwith reduced occupancy parameters. The surface model structureshown in (d), the BO2 double-layer model was constructed based onthe reported surface structure of SrTiO3;42 only half of BDL and O(2),which is the apical oxygen to the BDL site, are occupied to maintainthe stoichiometry in the ideal BO2 double-layer termination structure(see Section 2 and Figure S1 of the Supporting Information). Thedistances of O(1)−O(4) and O(1)−O(5) are ∼1.2 Å and ∼2.7 Å,which are the covalent bond length of O−O and the hydrogen bondlength of OH−O, respectively. The other transparent spheres inFigure 1c,d show the assumed oxygen atom positions, and theiroccupancy was less than 10% under any conditions in this study. TheO(3) position overlaps As, thus, quantitative discussion on the O(3)site is impossible.The measurements were carried out in the order (i) under-vacuumCTR (Figure 2a), (ii) cyclic voltammetry (CV) first run (Figure 3a),(iii) time evolution of CTR (Figure 3b), (iv) potential dependencemeasurements of CTR at several selected scattering vectors (Figure3c−f), (v) CV second run (Figure 3a), (vi) in situ CTR for surfacestructure analysis (Figure 2c), and (vii) CV third run (Figure 3a). Thetime dependence of the potential and working electrode current ispresented in Figure S5.3. RESULTS AND ANALYSIS3.1. Under-Vacuum Structure. The CTR intensityprofiles measured on hkζ-lines under vacuum are presentedin Figure 2a together with the results of Bayesian analysis. TheFigure 1. Surface structure model used in this study. Yellow and blue octahedra show TiO6 and CoO6 octahedra. Green, red, and white spheres in(b)−(d) show the A-site, B-site, and oxygen atoms, respectively. The occupancy parameters of the atoms up to the BtopO5 pyramid are unity. (a)Overall structure of the film. The structural parameters for the atoms within the pillar shown by the black frame are refined: (b) BO2 termination,(c) AO termination, and (d) BO2 double-layer termination models. Transparent spheres show the assumed oxygen atom positions, and theiroccupancy was less than 10%.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXBhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig1&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asobtained structural parameters are presented in Figure 2b. Thehorizontal axis shows the depth, and the vertical axes show theoccupancy, atomic displacement with respect to the substratelattice dz, and isotropic atomic displacement parameter Biso.The thickness of the film is nearly 7 unit cells, and the Srconcentration for the A-site is 0.4 in the middle of the film, andnearly 1 at the surface. The Sr segregation at the surface wasalso confirmed by the analysis using the Sr K-absorption edge(see Section 3 of the Supporting Information). In the Sr-concentrated region (z ≥ 5), the cations are displaced outwardfrom the oxygen atoms as shown in the dz profiles in Figure 2b,meaning that there is electric polarization (or an electric field)at the surface. In the middle of the film, in the La0.6Sr0.4CoO3region, the relative displacement of the cations and anionsdisappears, indicating that the inside of the film is metallic. Theparameter Biso is a measure of the positional fluctuation of eachatom from the in-plane lattice-averaged position. In defect-freecrystalline samples, Biso represents the amplitude of thermalvibrations and its typical value at room temperature is 0.5 Å2.In thin-film specimens, it often primarily reflects disorderarising from lattice defects and atomic intermixing and canreach relatively large values, on the order of 5 Å2.35,36 The Bisoparameters greatly increase around z = 5, which coincides withthe interface between La0.6Sr0.4CoO3 and SrCoO3 formed bythe Sr segregation.Other structural parameters for the atoms close to thesurface are given in Table 1. The As and BDL occupancies were22(2)% and 3(1)%, respectively, which means that 72(4)% ofthe as-grown surface is BO2 termination (Figure 1b). TheO(1) occupancy is 99(1)%, meaning that the Co at the surfaceis octahedrally coordinated.3.2. Potential Dependence of the Surface Structure.The CV results are listed in Figure 3a. The potential range ofthe CV measurement was carefully selected to avoid bubbleformation caused by the large currents. If bubbles form on theelectrode surface due to water electrolysis, they remain on thesurface because the cell is sealed. This alters the amount ofwater in the X-ray path, leading to changes in absorption and asignificant increase in signal intensity. These variations wouldnot only prevent reliable data analysis but also make it difficultFigure 2. (a) CTR intensity profiles measured under vacuum together with the result of fitting. (b) Depth profile of the obtained structuralparameters in the pillar (Figure 1a). z = 7 shows the parameters for the As and O(1) sites. (c) CTR intensity profiles in the KOH aqueous solutionat 0 V. (d) Obtained structural parameters in the pillar for 0 V data. Structural parameters at the surface (Figure 1b−d) are listed in Table 1.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXChttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig2&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asto maintain liquid contact with the sample surface. Note thatthe vertical axis in the voltammogram spans a very narrowrange. For the pristine sample, the voltammogram shows fewfeatures except for an increase in the current at +0.7 V, on theOER side, and −0.7 V, on the oxygen reduction reaction(ORR) side. Based on this profile, we examined the surfacestructure at 0 V and ± 0.6 V. After 1.5 days of X-raymeasurements, CV was recorded again, and the current wasgreatly increased. A similar change in CV caused byelectrochemical operation was also reported in ref. 8; theyattributed the increase in current as a formation of an activeelectrode surface under OER conditions. Quantitatively, it wasreported that the working electrode current is suppressed inultrathin films.43 The overall feature of the CV remainsunchanged until the end of the experiment (2.3 days from thefirst CV measurement, third run). The time evolution of theworking electrode current during the experiment is presentedin Figure S5. There was no detectable change in the workingelectrode current caused by X-ray irradiation in thesemeasurements, indicating that the photochemical reaction isnegligibleFigure 3b shows the time evolution of the CTR intensitymeasured at (0, 0, 2.04), (0, 0, 3.04), and (0, 0, 4.12). Themeasurement was done in the order of −0.6, + 0.6, −0.6, and+0.6 V. The intensity depends on both the time and potential,and the dependency varies as a function of the scatteringvector. The potential dependence is further examined bymeasuring the intensity as a function of potential at a fixedscattering vector; we call such measurements as XCVmeasurements. XCV profiles measured at (0, 0, 2.04), (0, 0,3.04), (0, 1, 3.16), and (0, 2, 2.16) are presented in Figure 3c−f. Reproducible potential dependence of the intensity and,therefore, the surface structure was observed. Only the firstXCV cycle at (0, 0, 2.04) differs from the second and thirdcycles, showing that the time evolution of the surface structurestops during this XCV measurement.We performed surface structure analysis on the 0 V datameasured after the XCV measurements. The second run of theCV measurement shown in Figure 3a was conducted justbefore the CTR measurement at 0 V. The results are presentedin Figure 2c,d and Table 1. The overall feature of the filmstructure shown in Figure 2d is similar to that of the under-vacuum structure (Figure 2b). There is no observable changein the c-lattice spacing within the film region. This stands incontrast to previous ex situ lattice spacing measurements ofthick films before and after electrochemical treatment,8 whichreported clear lattice expansion. In that study, long-termoperation was carried out until the end of the electrode’slifetime, and lattice parameter changes were observed in thedeactivated sample. The slight change in lattice spacing in thepresent case suggests that the damage responsible for catalyticdeactivation is minor. The z positions of oxygen in the Sr-concentrated region are close to those of the cations at 0 V,meaning that the surface polarization observed under vacuumis reduced. This structural change is shown in Figure 2b,d andin Table 1 (Btop−OI of the Supporting Information). Theoccupancy of As and BDL was 11(3)% and 22(2)%,respectively. This result means that BO2 double-layertermination, which was effectively not found in the under-vacuum measurement, covers 44% of the surface (note that themaximum occupancy parameters of BDL and O(2) sites in theBO2 double-layer surface are 0.5, see Section 2). Theoccupancy of O(2) was comparable to that of BDL, meaningthat most BDL sites are octahedrally coordinated.The observed intensity modulation induced by the appliedpotential is typically smaller than 5% (see Figure S4) andtherefore, when we plot the intensity measured at ± 0.6 V onthe log scale, the results completely overlap with the 0 V resultshown in Figure 2c. It should be noted that the typicaluncertainty of the CTR intensity distribution is 20%,39 whichFigure 3. (a) Results of the CV measurements. Initial (1 st run),1.5 days later (2 nd run), and 2.3 days later (3 rd run). (b) Timeevolution of the intensity with switching potential. (c)−(f) XCVprofiles measured at (0, 0, 2.04), (0, 0, 3.04), (0, 1, 3.16), and (0, 2,2.16).Table 1. Selected Surface Structure Parameters for Vacuumand 0 V ConditionsaParameter vacuum 0 V −0.6 V +0.6 Vocc O(1) 0.99(1) 0.93(5) −0.02(2) +0.03(2)occ O(2) − 0.19(4) −0.04(3) +0.07(3)occ O(4) − 0.65(8) −0.10(8) +0.04(6)occ O(5) − 0.24(7) +0.08(8) −0.02(6)occ BDL 0.03(1) 0.22(2) +0.00(1) +0.00(1)occ As 0.22(2) 0.11(3) +0.00(1) +0.00(1)Btop−BDL (Å) − 1.87(4) −0.02(4) −0.13(2)Btop−O(1) (Å) 1.44(4) 1.46(4) +0.02(4) +0.08(6)Btop−OI (Å) 2.43(5) 2.02(5) −0.09(6) +0.13(4)BDL−O(2) (Å) − 1.84(7) −0.04(14) +0.02(6)BDL−OII (Å) − 2.34(6) −0.13(6) −0.02(5)O(1)−O(4) (Å) − 1.04(5) +0.09(5) −0.05(7)O(1)−O(5) (Å) − 2.80(6) +0.1(2) −0.16(18)aThose for −0.6 V and +0.6 V conditions are expressed by the relativevalue with respect to the 0 V condition. O(2), O(3), and O(5) werenot taken into account in the analysis for the under-vacuumcondition. Btop−BDL is the height of the BDL site measured from theBtop site.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXDhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig3&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asis mainly caused by optical misalignment. This uncertainty isapparent when comparing the intensity between two distant Qpoints. The uncertainty for the intensity measured at the sameQ point is much smaller, as shown in the XCV measurements.To derive the structural modulation caused by the appliedpotential, we calculated corrected intensity I Q( )V :I QI QI QI Q( )( )( )( )V Vcalc0Vexp0V exp=(1)where I Q( )Vexp denotes the measured intensity at potential V,and I Q( )calc0V denotes the calculated intensity based on theobtained 0 V structure. The prefactor I Q I Q( )/ ( )calc0Vexp0V correctsthe Q -dependent error caused by the optical misalignment;I Q( )V is free from the uncertainty caused by opticalmisalignment, which allows us to examine the effect of thepotential on the film structure. The results of the Bayesiananalysis performed for I V are nearly the same as those for the0 V structure except for the top surface region. The relativechange in the surface structure parameters from the 0 Vstructure is given in Table 1. Because the potential dependenceof the CTR intensity is very small, the observed surfacestructural modulation induced by the potential is also minute.The potential dependence of the surface structure isschematically presented in Figure 4. The oxygen displacesinward as the potential increases. In addition, BDL ions alsodisplace inward and the BDL−O(2) distance is unchanged. TheO(2) occupancy increases. The structural parameters for theatoms inside are nearly independent of the potential.4. DISCUSSIONFirst, the reliability of the analyzed structure is examined. Theuncertainty of all structural parameters reported in this paperwas estimated from the probability density distributionsobtained via Bayesian inference based on the experimentallymeasured intensity profiles. As shown in Figure 2a, the CTRintensity profiles measured in vacuum span nearly the entirerange along the rod direction. Although the signal nearn 0.5= + , where n is an integer, is generally too weak todetect, the intensity around (00ζ) with 1.5 is observedalmost continuously. Consequently, depth-sensitive structuralinformation was obtained for all wavevectors within theBrillouin zone, allowing the Bayesian analysis to yieldprobability distributions for each structural parameter. Incontrast, the CTR profiles measured at 0 V (Figure 2c) exhibitwide gaps near n 0.5= + , resulting from X-ray absorptionand increased background from the solution. These gaps leadto a loss of information associated with 2-fold periodicstructures. Although this affects the reliability of the depthprofile of the lattice spacings, the average spacing over two ormore unit cells remains robust. The surface terminationstructure model is well localized in real space, and itscorresponding information in reciprocal space is broadlydistributed across a wide ζ range. Therefore, the derivedsurface termination structure is considered reliable. Potential-dependent structural changes were analyzed based on subtleintensity variations shown in Figure S4, where the gaps arewider than those in Figure 2c. These wide gaps result in thelarge parameter uncertainties shown in Figure 4b.Next, we compare our result with the previously reportedLa1−xSrxCoO3 film structure37 (x = 0.2, under-vacuummeasurement without any electrochemical treatment). Ref.37 reported Sr segregation at the surface, the formation of aLaCoO3 layer underneath the dense Sr layer, SrCoO3 particleson the surface, and surface polarization. The Sr segregation atthe surface is observed in our work, meaning that Srsegregation is quite common in La1−xSrxCoO3 film growth inthe pulsed laser deposition process. Similar Sr segregation wasalso reported for La1−xSrxMnO3 film.44 The orientation of thesurface polarization reported in ref. 37 is opposite to what weobserved. We found short B−OI on the surface side as shownin Figure 2b, while they reported this on the bulk side.Local polarization at the oxide interface is controlled by thespatial distribution of the chemical species. For example, thelocal electric field at the interface between LaMnO3 andLaNiO3 points to LaNiO3.45 In the present case, our sampleand that reported in ref. 37 have different spatial distribution ofSr. Our sample does not have a LaCoO3 layer underneath orSrCoO3 particles on top, which cause different local electricfields. There is no apparent polarization inside our film, whichsuggests metallic behavior in the middle of the film electrode.In this study, a BO2 double-layer structure (Figure 1d) wasdeveloped after 1.5 days of electrochemical treatment. BDLO6octahedra are edge-shared with neighboring BtopO6 octahedra.Edge-shared structures have also been suggested in theelectrochemically formed amorphous layer grown on thesurface of the highly active catalyst Ba0.5Sr0.5Co1−xFe Ox 3 .5,6Thus, the catalytic activity of La1−xSrxCoO3 may also beenhanced by the formation of this structure. The formation ofFigure 4. (a) Schematic view of the potential (V vs Ag/AgClelectrode) dependence of the surface structure. Horizontal dashedlines are to guide the eye. (b) Relative atomic displacement withrespect to the 0 V structure.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXEhttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?fig=fig4&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthe edge-shared BO6 octahedra structure affects the electronicenergy levels and the degree of steric hindrance, which altersthe catalytic activity.46 With a support of noble metals, it favorsoxy-hydroxide formation,38 which helps OER at the surface. Inthe present case, the CV results show a considerable increasein the electric current (Figure 3a) after developing the BO2double layer at the surface. A similar change in CV was alsoreported in ref. 8, in which redox activity was increased byforming an active surface under OER conditions. Theyattributed this increase in activity to the formation ofCoO(OH) primarily based on the observation of two kindsof Co and O in the X-ray photoelectron spectra. Ourobservation exhibits the coexistence of edge-shared andcorner-shared CoO6 octahedra, which involve two kinds ofCo and O sites. It should also be noted that our results do notexclude the formation of Co−OH bonds, as X-ray diffraction isinsensitive to hydrogen atoms.The potential dependence of the atomic displacementpresented in Figure 4 is moderate. Increased potential causesinward atomic displacement. This tendency is in accordancewith a simple view that a higher potential makes the electricfield point outward, which results in the inward displacementof oxygen. The volume of BtopO6 octahedra is increased by8(±6)% when the potential is increased from −0.6 to +0.6 V.This volumetric change suggests that the Co ion valency isreduced47,48 or a higher spin state is stabilized47 with appliedpotential. We expect a similar potential dependence for the BDLsite, but the large uncertainty in the obtained structuralparameter does not allow us to confirm this.Based on the CV results in Figure 3a, the BO2 double-layertermination surface has a larger double-layer capacity and anORR peak at −0.3 V. The peak was attributed to oxygenintercalation.18 Although oxygen intercalation may be observedthrough the structure, there was no detectable change instructure around −0.3 V; this is because of the tiny totalamount of charge. The increase in capacity is usually attributedto an increase in surface area. However, this change in double-layer capacity is not caused by the increase in the surface areabecause the CTR results show little change in the surfaceroughness. The increase in the capacity corresponds to thespace charge of ∼2 electrons per unit cell area around theinterface. Some of this space charge is explained by the changein the Btop site valency and the O(2) occupancy discussedabove. In addition, there is an expected change in the BDL sitevalency and proton addition/removal should contribute, butthis is not directly observed in our X-ray investigation. Thevoltammogram shows a positive slope after electrochemicaltreatments, which implies that the conductivity of the filmincreased. This change in conductivity was reflected in thestructure, as evidenced by reduced polarization in the verysurface region (5≤ z ≤ 7 in Figure 2b,d).Enhanced Biso parameters around the surface (Figure 2b,d)can result from a random electric field caused by the localarrangement of La/Sr or vacancies. However, a large Bisoparameter sometimes implies a reduced occupancy throughthe parameter coupling. If so, the enhanced Biso of OI and OIIseen in Figure 2b suggests a large number of oxygen vacanciesaround the surface, which are filled during the electrochemicaltreatment. Such a migration of oxygen vacancies at the surfaceunder electrochemical conditions suggests that the reactionprocess involves lattice oxygen at the surface, in accordancewith the LOM. The reduction of surface polarization andoxygen Biso around the surface suggests active atomic flowaround the surface in a 1 nm range induced by theelectrochemical conditions. This finding gives an idea of therange and magnitude of the mass flow around the surfaceunder electrochemical conditions.5. CONCLUSIONThe surface structure of the CoO2-terminated La1−xSrxCoO3film grown on SrTiO3 was examined under vacuum andelectrochemical conditions. Sr segregation was found at thefirst two AO planes, forming an ultrathin SrCoO3 layer at thesurface. This Sr segregation was stable under electrochemicaltreatment. The surface structure was modulated during theelectrochemical treatment to form BO2 double-layer termi-nation, which involves edge-shared CoO6 octahedra. Strongelectric polarization was observed in the pristine sample and itwas reduced under electrochemical conditions. The isotropicatomic displacement parameters Biso of the atoms in the rangeof 1 nm from the surface are considerably larger than those inthe interior. This tendency is unchanged for cations underelectrochemical treatment. For oxygen, in contrast, the increasein Biso at the surface is reduced after the formation of the BO2double-layer surface. The reduction of surface polarization andoxygen Biso around the surface suggests active atomic flow inthe range around 1 nm from the surface, induced by theelectrochemical conditions.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.5c11807Support_Fujisawa.pdf: Descriptions of (Sec. 1) diffrac-tion theory, (Sec. 2) surface model for surface structureanalysis, (Sec. 3) examination of Sr segregation, (Sec. 4)results of fitting for deriving potential dependence of thesurface structure, (Sec. 5) time dependence of theworking electrode current during the synchrotronexperiment, and (Sec. 6) schematic of the sample cell(PDF)■ AUTHOR INFORMATIONCorresponding AuthorYusuke Wakabayashi − Department of Physics, GraduateSchool of Science, Tohoku University, Sendai 980-8578,Japan; orcid.org/0000-0003-3107-0338;Email: wakabayashi@tohoku.ac.jpAuthorsAtsuro Fujisawa − Department of Physics, Graduate School ofScience, Tohoku University, Sendai 980-8578, JapanXuhui Xu − Department of Physics, Graduate School ofScience, Tohoku University, Sendai 980-8578, Japan;orcid.org/0000-0003-2653-7458Yuta Ishii − Center for Basic Research on Materials (CBRM),National Institute for Materials Science (NIMS), Tsukuba,Ibaraki 305-0047, JapanHidekazu Shimotani − Department of Physics, GraduateSchool of Science, Tohoku University, Sendai 980-8578,Japan; orcid.org/0000-0003-3238-9420Yuta Inoue − Graduate School of Engineering, KyotoUniversity, Kyoto 615-8510, Japan; orcid.org/0009-0004-8664-2254ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.5c11807ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXXFhttps://pubs.acs.org/doi/10.1021/acsami.5c11807?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsami.5c11807/suppl_file/am5c11807_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Wakabayashi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3107-0338mailto:wakabayashi@tohoku.ac.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Atsuro+Fujisawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Xuhui+Xu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-2653-7458https://orcid.org/0000-0003-2653-7458https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuta+Ishii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hidekazu+Shimotani"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3238-9420https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuta+Inoue"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0009-0004-8664-2254https://orcid.org/0009-0004-8664-2254https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yuto+Miyahara"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.5c11807?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asYuto Miyahara − Graduate School of Engineering, KyotoUniversity, Kyoto 615-8510, Japan; orcid.org/0000-0003-4662-0996Kohei Miyazaki − Graduate School of Engineering, KyotoUniversity, Kyoto 615-8510, Japan; Graduate School ofEngineering, Kobe University, Nada, Kobe 657-8501, Japan;orcid.org/0000-0001-5177-3570Complete contact information is available at:https://pubs.acs.org/10.1021/acsami.5c11807NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by a Grant-in-Aid for ScientificResearch (Japan Society for the Promotion of Science (JSPS)KAKENHI, Grant Nos. JP22H02024 and JP23K23292). Thesynchrotron radiation experiments at the Photon Factory wereperformed with the approval of the Photon Factory ProgramAdvisory Committee (Proposal Nos. 2022G016 and2024G005). We thank Heather Fish, MChem, from Edanz(https://jp.edanz.com/ac) for editing a draft of this manu-script.■ REFERENCES(1) Chao, C.; Lin, L.; Macdonald, D. A Point Defect Model forAnodic Passive Films I. Film Growth Kinetics. J. Electrochem. Soc.1981, 128, 1187−1194.(2) Macdonald, D. D.; Urquidi-Macdonald, M. Theory of Steady-State Passive Films. J. Electrochem. Soc. 1990, 137, 2395−2402.(3) Macdonald, D. The history of the Point Defect Model for thepassive state: A brief review of film growth aspects. Electrochim. Acta2011, 56, 1761−1772.(4) Fujii, H.; Wakabayashi, Y.; Doi, T. Early stages of iron anodicoxidation: Defective growth and density increase of oxide layer. Phys.Rev. Mater. 2020, 4, 033401.(5) May, K. J.; Carlton, C. E.; Stoerzinger, K. 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