mater.scichina.com link.springer.com Published online 22 March 2019 | https://doi.org/10.1007/s40843-019-9410-2Sci China Mater 2019, 62(8): 1162–1168

Electronic structure evolutions driven by oxygenvacancy in SrCoO3−x filmsJiali Zhao1,5, Yi Luo2, Jia-Ou Wang1, Haijie Qian1, Chen Liu1, Xu He3, Qinghua Zhang4,5,Heyi Huang4,5, Bingbing Zhang1, Shunfang Li2, Erjia Guo4, Chen Ge4, Tieying Yang6,Xiaolong Li6, Meng He4, Lin Gu4,5, Kui-Juan Jin4,5, Kurash Ibrahim1* and Haizhong Guo2*

ABSTRACT Ionic defects, such as oxygen vacancies, play acrucial role in the magnetic and electronic states of transitionmetal oxides. Control of oxygen vacancy is beneficial to thetechnological applications, such as catalysis and energy con-version. Here, we investigate the electronic structure ofSrCoO3−x as a function of oxygen content (x). We found thatthe hybridization extent between Co 3d and O 2p increasedwith the reduction of oxygen vacancies. The valence bandmaximum of SrCoO2.5+δ has a typical O 2p characteristic. Withfurther increasing oxygen content, the Co ions transform froma high spin Co3+ to an intermediate spin Co4+, resulting in atransition of SrCoO3−x from insulator to metal. Our results onthe electronic structure evolution with the oxygen vacancies inSrCoO3−x not only illustrate a spin state transition of Co ions,but also indicate a perspective application in catalysis andenergy field.

Keywords: oxygen vacancies, SrCoO3−x, electronic structureevolution, resonant photoemission spectra

INTRODUCTIONThe characteristics of transition metal oxides (TMOs)endow them with various applications in the fields of thecutting-edge research, such as energy storage and gen-erating devices [1,2]. Oxygen vacancy has flexible degreeof freedom to manipulate the functionalities of TMOs [3]due to its strong influence on the number of electrons inthe 3d orbital of transition metals. In the cobaltites,oxygen vacancy could modify both valence and spin stateof Co ions, and thus effectively influence the physical

properties [4,5]. SrCoO3−x can be stabilized in two re-versible topotactic phases, i.e., SrCoO2.5 and SrCoO3, bysimply changing the oxygen content using in-situ or ex-situ exterior methods [5–10]. The physical properties ofthese two materials are in sharp contrast. SrCoO2.5 is anantiferromagnetic insulator with alternating CoO4 tetra-hedral and CoO6 octahedral sub-layers, whereas SrCoO3is a ferromagnetic metal composed of CoO6 octahedral.Due to a very small difference of Gibbs free energy be-tween SrCoO2.5 and SrCoO3, they can be easily trans-formed from one phase to the other at relatively lowtemperature [11–14]. Previous work had demonstratedthat the reversible topotactic phase transformation can beachieved by either physical or chemical methods. Inphysical methods, the oxygen vacancies in SrCoO3−x canbe tuned by annealing in high oxygen atmosphere orvacuum environment. The chemical approaches includeelectrolysis and direct treatment with oxidizing or redu-cing agents. Jeen et al. [7] reported phase transformationfrom SrCoO3−δ to SrCoO2.5 by annealing the sample at~210°C in vacuum. The phase transformation is re-versible as SrCoO2.5 can transform into SrCoO3−δ by an-nealing at ~350°C in pure oxygen. Recently, Lu et al. [13]illustrated the tri-state transition among the SrCoO3−δ,SrCoO2.5, and HSrCoO2.5 using electric-field driven ionictransport. Besides, SrCoO2.5 is successfully oxidized toSrCoO3 by putting the as-deposited SrCoO2.5 thin films indilute NaClO solutions [11].

During the topotactic phase transformation, oxygenvacancy content is the only variable to trigger Co ions

1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China2 School of Physical Engineering, Zhengzhou University, Zhengzhou 450001, China3 Physique Théorique des Matériaux, Q-MAT, CESAM, Université de Liège, B-4000 Liège, Belgium4 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China5 University of Chinese Academy of Sciences, Beijing 100049, China6 Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China* Corresponding authors (emails: kurash@ihep.ac.cn (Ibrahim K); hguo@zzu.edu.cn (Guo H))

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changing from high-spin state Co3+ ions in SrCoO2.5 tointermediate-spin state Co4+ ions in SrCoO3. The physicalproperties will change with the spin state and valence ofCo ions accordingly [15]. In order to find out the un-derlying mechanism controlling the physical properties ofSrCoO2.5 and SrCoO3, one needs to understand theirelectronic structures. However, an unambiguous experi-mental observation of electronic structure changes in-duced by oxygen vacancies in SrCoO3−x is still lacking.

The amount of the oxygen vacancies of SrCoO3−x(0≤x≤0.5) was tuned by strain and chemical oxidation.Afterwards, the electronic structure evolution of SrCoO3−xdriven by variation of the oxygen vacancies was illu-strated by a combination of X-ray absorption spectra andresonant photoemission spectra. From SrCoO2.5 toSrCo2.5+δ, the variation of oxygen content wouldstrengthen the hybridization between Co 3d and O 2pwith the valence band maximum mainly having O 2pcharacteristic, resulting in the transformation from highspin Co3+ in SrCo2.5+δ to intermediate spin Co

4+ inSrCoO3−δ, and thus the transformation from insulator tometal.

EXPERIMENTAL SECTIONSrCoO2.5 thin films (24 nm-thick and 187 nm-thick) wereepitaxially grown on (001)-oriented LaAlO3 (LAO) sub-strates by pulsed laser deposition (PLD). All films weredeposited at 700°C using an excimer XeCl laser(1.5 J cm−2, 308 nm, 2 Hz) at oxygen partial pressure of10 Pa. After deposition, the films were in situ annealed for30 min and then cooled down to room temperature. Thegrowth details can be found in our previous work [16].The 24 nm-thick SrCoO2.5 film was coherently strained tothe substrate, whereas the 187 nm-thick film was partiallystrain relaxed. For comparison, the 24 nm-thick SrCoO2.5thin film was cut into two parts. One was immersed intoNaClO solution at room temperature for 20 min, denotedas oxidized-SrCoO3−x, whereas the other was kept in theas-grown state.

The crystal structure of the as-prepared brownmillerite-SrCoO2.5 (BM-SCO2.5) thin films was characterized usingan ARM-200CF (JEOL, Tokyo, Japan), state-of-the-artspherical aberration-corrected scanning transmissionelectron microscopy (STEM) techniques of the high-angleannular dark-field (HAADF) and annular bright-field(ABF)-operated being equipped with double sphericalaberration (Cs) correctors. X-ray diffraction (XRD) wascollected with Rigaku SmartLab (9 kW) High-resolution(Ge 220×2) X-Ray Diffractometer with 1.54 Å X-rays. X-ray reciprocal space mapping (RSM) of SrCoO3−x thin

films were performed with high-resolution synchrotronX-ray diffractometry at the BL14B1 beam line of ShanghaiSynchrotron Radiation Facility (SSRF) using a 1.24 Å X-rays with a Huber 5021 six-axis diffractometry. Tem-perature dependences of resistivity were performed usinga Physical Properties Measurement System (PPMS,Quantum Design Inc.). X-ray absorption spectra (XAS)and resonant photoemission spectra (RPES) were per-formed at the 4B9B beam line at Beijing SynchrotronRadiation Facility of Institute of High Energy Physics,Chinese Academy of Sciences (CAS). The ultrahigh va-cuum chamber background pressure is about2×10−10 Torr. The O-K edge and Co-L edge XAS wererecorded in the total-electron-yield (TEY) mode.

RESULTS AND DISCUSSIONFig. 1a, b show the HAADF-STEM images of BM-SCO2.5thin films with thicknesses of 24 and 187 nm, respec-tively. Both interfaces in these two films are atomicallysharp without any evidence of misfit dislocations orchemical reaction in the interface region. All films arecoherently strained by the substrate. The STEM resultdemonstrates the BM-SCO2.5 thin films are highly epi-taxial grown.

ABF-STEM can detect light atoms, such as oxygen [17].Fig. 1c, d show the ABF-STEM images of BM-SCO2.5 thinfilms with thicknesses of 24 and 187 nm, respectively.Line profiles along the vertical direction indicated by redand blue lines in ABF-STEM images of Fig. 1c, d areshown in Fig. 1e, f, respectively. The alternate stacking isobserved in Fig. 1d, originating from the fully oxygenatedoctahedral and oxygen-deficient tetrahedral sub-layers.This result is consistent with the previous report [7],confirming that the 187 nm-thick BM-SCO2.5 is stoi-chiometric.

The intensity of oxygen ions in the 24 nm-thick BM-SCO2.5 film is stronger than that of the 187 nm-thick BM-SCO2.5 film (Fig. 1e, f), indicating the oxygen content inthe thinner film is more than that of the thicker film.Therefore, we defined the 24 nm-thick and 187 nm-thickSrCoO3−x film as SrCoO2.5+δ and SrCoO2.5 thereafter, re-spectively.

Fig. 2a shows the XRD θ-2θ scans of SrCoO2.5,SrCoO2.5+δ and 24 nm-thick oxidized-SrCoO3−x. Only(00l) peaks were observed in the XRD θ-2θ scans, in-dicating all films are of single phase and c-axis-oriented.RSM images are shown in Fig. 3. The SrCoO3−x thin filmsare coherently grown on the LAO substrates. Moreover,for the 24 nm-thick oxidized-SrCoO3−x thin film, thepeaks originating from the alternatively-stacked octahe-

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dral and tetrahedral sublayers almost disappear, whichindicates that the 24 nm-thick oxidized-SrCoO3−x thinfilm was oxidized in NaClO solution, and its oxygencontent is more than 2.75 and close to 3, therefore, wedefined this sample as SrCoO3−δ.

The lattice mismatch between LAO (a=3.792 Å) andSrCoO2.5 (a=3.905 Å) is about 2.98%. When SrCoO2.5 thinfilm grew on the LAO substrate, it suffers an in-planecompressive strain. Fig. 2b shows the enlarged part ofFig. 2a. The (008) peak of SrCoO2.5 thin film is located at

Figure 1 HAADF-STEM images of (a) 24 nm-thick and (b) 187 nm-thick BM-SCO2.5 thin films, respectively. ABF-STEM images of (c) 24 nm-thickand (d) 187 nm-thick BM-SCO2.5 thin films, respectively. (e, f) Line profiles of the inversed ABF contrasts indicated by the red and blue lines in ABFimages (c) and (d), respectively (Lines in (c) and (d) can be red and blue accordingly).

Figure 2 (a) XRD θ-2θ patterns of SrCoO2.5, SrCoO2.5+δ, and SrCoO3−δ. (b) Enlarged part of (a).

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higher 2θ angle region than that of SrCoO2.5+δ thin film. Itindicates that with the increase of the film thickness, the(008) peak shifts towards high 2θ angle region and the c-axis lattice constant decreases. The in-plane compressivestrain of the thin films relaxes with the increase of itsthickness. The (002), (006) and (0010) peaks of SrCoO2.5+δfilm appear, suggesting the alternation of CoO4 tetra-hedral layers and CoO6 octahedral layers. Therefore, theoxygen content of SrCoO2.5+δ film is no more than 2.75(0

spin state of Co ions is calculated as 0.75, 0.76, and 0.70,respectively. From the line shape and the branching ratioof Co-L edge XAS, we can conclude that Co ions in theBM-SCO2.5 and SrCoO2.5+δ are at high-spin state, whilethe Co ions in perovskite SrCoO3−δ are at intermediate-spin state, which is in agreement with the previous cal-culation and experiments [15,18–21].

O-K edge XAS originates from the transition from theO 1s core-level state to the unoccupied O 2p state. Thecompletely ionic oxides would not show any peaks in theO-K edge XAS due to the completely occupied O 2p state.Therefore, the O-K edge XAS provides not only the in-formation about the unoccupied O 2p state, but also theinformation about the hybridization between Co 3d andO 2p. Fig. 5b shows that the pre-peak E of SrCoO2.5 is tooweak to be detected. The pre-peak E is attributed to thehybridization of O 2p with Co 3d in the octahedral co-ordination. The peak C represents the superoxide speciesO2

− that commonly exist in the surface region of theoxygen-deficiency SrCoO3−x [22]. The intensity of peak Creduces, meanwhile the pre-peak A and peak B appearwith the increase of oxygen content. These observationscan be attributed to the hybridization of O 2p with Co 3dt2g and eg in octahedral coordination, respectively. As theoxygen content increases, peaks A and B shift toward thehigher photon energy region, where they are defined aspeaks A' and B', respectively. The calculated intensityratio of peak A and B decreases significantly with thereduction of oxygen vacancies, suggesting that Co ions inSrCoO3–δ are at intermediate-spin 4+ state, whereas Coions in SrCoO2.5+δ are at high-spin 3+ state. Peaks A andA' represent the unoccupied states of Co 3d t2g orbital ofhigh-spin Co3+ and intermediate-spin Co4+, respectively.Meanwhile, Peaks B and B' represent the unoccupied state

of the Co 3d eg orbital of high-spin Co3+ and inter-

mediate-spin Co4+, respectively. The unoccupied state ofthe Co 3d t2g orbital of the high-spin Co

3+ and the in-termediate-spin Co4+ is the same. However, the un-occupied state of Co 3d eg orbital of the intermediate-spinCo4+ is more than that of the high-spin Co3+. Therefore,the intensity ratio of peak A and B of SrCoO2.5+δ is muchlarger than that of SrCoO3−δ.

To investigate the evolution of the valence band elec-tronic structure with the variation of oxygen vacancies inSrCoO3−x, the Co 2p-3d RPES were measured and shownin Fig. 6. Three energy regions (labeled with cyan, ma-genta, and green arrows in Fig. 6a) in the Co-L edge XASwere selected to do the resonant photoemission andmeasure the electronic structure in the valence band,shown in Fig. 6b with colors corresponding to the colorsof the arrow in Fig. 6a. The selected energy regions coverthe off-resonance and on-resonance photon energy inorder to compare the on-resonance and off-resonanceeffects. The on-resonance photon energy enhances thetransition from Co 2p to Co 3d, and mainly shows thecontribution of Co 3d in valence band. From the off-resonance to on-resonance photon energy, the contribu-tion of Co 3d electrons to the valence band electronicstructure would be enhanced. For SrCoO2.5, the intensityof the peak around 4 eV below Fermi level is obviouslyenhanced at the on-resonance photon energy, indicatingthat the valence band is dominated by the Co 3d char-acteristic. For SrCoO2.5+δ, the intensity of the peak around3 eV below Fermi level decreases when the photon energyshifts from the off-resonance region to the on-resonanceregion. This result is attributed to the valence bandmaximum dominated by the O 2p characteristic. TheO 2p characteristic peak shifts toward the Fermi level

Figure 5 XAS of SrCoO2.5, SrCoO2.5+δ, and SrCoO3−δ films: (a) Co-L edge and (b) O-K edge.

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with the increase of oxygen content and results in thestronger hybridization between the Co 3d and O 2pstates, consistent with the results of the O-K edge XAS.For SrCoO3−δ, the significant enhancement of peak in-tensity around 1 eV below Fermi level at the on-re-sonance photon energy is observed. This peak has a Co 3dcharacteristic, and the valence band maximum is domi-nated by the Co 3d characteristic. The transformationfrom insulator (SrCoO2.5) to metal (SrCoO3−δ) is alsoobserved, as shown in Fig. 6c.

CONCLUSIONSWe have obtained three different oxygen-contentSrCoO3−x thin films by combing strain and chemicaloxidation method: SrCoO2.5, SrCoO2.5+δ, and SrCoO3−δ.XRD and STEM-ABF not only demonstrate all films areof good crystalline quality and single phase, but alsoqualitatively determine the oxygen content of SrCoO3−xthin films. The electronic structure evolution from aninsulator SrCoO2.5 to a metal SrCoO3−δ was explored withXAS and RPES. As the increase of oxygen content, thehybridization between Co 3d and O 2p is strengthenedand the valence band maximum exhibits a typical O 2pcharacteristic. With further increasing oxygen content,SrCoO3−x transform from a high spin Co

3+ to an inter-mediate spin Co4+, resulting in a transition of insulator tometal. Our description of the electronic structure transi-tion from insulator SrCoO2.5 to metal SrCoO3−δ is notonly important for fundamental research, but also forindustry applications, such as catalysts, solid oxide fuel

cells, and so on.

Received 16 January 2019; accepted 28 February 2019;published online 22 March 2019

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Acknowledgements This work was supported by the National KeyR&D program of China (2016YFA0401002) and the National NaturalScience Foundation of China (11574365, 11474349 and 11375228). Theauthors would like to thank BL14B1 beam line of Shanghai SynchrotronRadiation Facility for technique support.

Author contributions Zhao J designed and carried out most of theexperiment, analyzed the results, and wrote the manuscript with supportfrom Ibrahim K and Guo H. Luo Y helped to perform the experiment,including film growth and XRD measurements. Wang JO, Qian H andLiu C helped to perform the XAS and RPES measurements. Yang T andLi X helped to perform the RSM measurements. Huang H performed thetransport properties measurements. Gu L and Zhang Q contributed toSTEM characterization. He M contributed to film growth. He X con-tributed to the theoretical analysis. Zhang B, Li S, Guo E, Ge C and JinKJ revised the manuscript. All authors contributed to the general dis-cussion.

Conflict of interest The authors declare no conflict of interest.

Jiali Zhao is a PhD candidate in the Institute ofHigh Energy Physics, CAS. Her current researchinterest focuses on the electronic structure andphysical properties of perovskite complex oxidesfilms.

Kurash Ibrahim received his PhD degree in 1992from the University of Liege, Belgium, worked as apostdoc fellow in the Institute of High EnergyPhysics, CAS (1993–1994), and has been a staffmember since then. He worked as a visiting re-searcher in Lawrence Berkeley National Laboratory,USA (2002), KEK and HiSOR, Japan (2003); De-partment of Physics, University of Montana State,USA (2004–2006). His research interest is on theapplication synchrotron radiation to electronicstructure investigation of condensed matter systems.

Haizhong Guo received his PhD degree in con-densed matter physics from the Institute of Phy-sics, CAS in 2005. From 2005 to 2009, he workedas a postdoc in the University of Alabama, Uni-versity of California at Davis, and Louisiana StateUniversity. He had been an associate professor inthe Institute of Physics, CAS (2009–2017). Hejoined Zhengzhou University as a professor in2017. His research interest focuses on the novelphysical phenomena emerging in the low-di-mension transition-metal oxide heterostructures.

SrCoO3−x薄膜中氧空位引起的电子结构演变赵佳丽1,5, 罗毅2, 王嘉鸥1, 钱海杰1, 刘晨1, 何旭3, 张庆华4,5,黄河意4,5, 张兵兵1, 李顺芳2, 郭尔佳4, 葛琛4, 杨铁莹6,李晓龙6, 何萌4, 谷林4,5, 金奎娟4,5, 奎热西•依布拉欣1*,郭海中2*

摘要 离子缺陷, 比如氧空位, 在过渡金属氧化物的磁性和电子结构中起着至关重要的作用. 对氧空位进行调控广泛应用于催化和能量转换领域. 我们研究了SrCoO3−x的电子结构与氧含量x的关系,发现随着氧空位含量的减少 , Co 3d和O 2p之间的杂化增强 .SrCoO2.5+δ的价带顶具有明显的O 2p特征. 随着氧含量的进一步增加, Co离子从高自旋的Co3+变成了中自旋的Co4+, 实现了绝缘体到金属的转变. 关于SrCoO3−x中氧空位引起电子结构演变的研究不仅展示了Co离子自旋的转变, 同时也为其在催化和能源领域的应用提供了理论支持.

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https://doi.org/10.1038/ncomms5669https://doi.org/10.1038/nature22389https://doi.org/10.1021/acs.jpcc.6b07544https://doi.org/10.1103/PhysRevB.98.085106http://arxiv.org/abs/1805.09524https://doi.org/10.1021/acsami.8b00791https://doi.org/10.1038/s41467-017-00121-6https://doi.org/10.1088/0953-8984/23/24/245601https://doi.org/10.1103/PhysRevLett.107.067601https://doi.org/10.1103/PhysRevLett.107.067601http://arxiv.org/abs/1104.2856https://doi.org/10.1103/PhysRevB.51.11501https://doi.org/10.1103/PhysRevB.57.13655https://doi.org/10.1021/cm9021563https://doi.org/10.1021/cm9021563

Electronic structure evolutions driven by oxygen vacancy in SrCoO3−x films INTRODUCTIONEXPERIMENTAL SECTIONRESULTS AND DISCUSSIONCONCLUSIONS