Molten Salt Flux Synthesis, Crystal Facet Design, Characterization

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Surfaces, Interfaces, and Applications

Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure and Catalytic Property of Perovskite Cobaltite Xiyang Wang, Keke Huang, Long Yuan, Shuang Li, Wei Ma, Zhongyuan Liu, and Shouhua Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08621 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Applied Materials & Interfaces

Molten Salt Flux Synthesis, Crystal Facet Design, Characterization, Electronic Structure and Catalytic Property of Perovskite Cobaltite Xiyang, Wang, Keke Huang, Long Yuan, Shuang Li, Wei Ma, Zhongyuan Liu and Shouhua Feng* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China

KEYWORDS Perovskite cobaltite; High-energy surface; Electronic structure; Molten salt; CO oxidation

ABSTRACT We present a simple and cost-effective molten salt synthetic route towards phase-pure perovskite cobaltite microcrystallines and successfully regulate different crystal facets for perovskite LaCoO3 by the strong interaction between Cl- anions and Sr2+ cations in molten salt system and polar plane. Then we take LaCoO3 (100 and 110), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) as comparison models, and characterize their crystal structure, morphology, composition, electronic state and catalytic property. XPS shows that the prepared samples with high-energy crystal facet (111) contain more surface oxygen species and active Co ions than La enrichment perovskite LaCoO3 (110 and 100) on the surface. Furthermore, combining with ambient-pressure XAS, valence band spectroscopy and density functional calculations, we find that exposed high-energy crystal facet (111) and doping Sr ions can enhance the ACS Paragon Plus Environment

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hybridization between Co cations and O anions and their O p-band center is closer to Fermi level compared with LaCoO3 (100 and 110). As expected, the samples with high-energy crystal facet (111) show better CO oxidation activity than LaCoO3 (100 and 110), and La0.7Sr0.3CoO3 (111) exhibits the highest catalytic activity. Our findings provide a new avenue to prepare high-energy facet perovskite catalysts and we also clearly reveal the relationship between surface electronic structure and intrinsic CO oxidation activity of perovskite cobaltite.

INTRODUCTION Engineering surface atom structure plays a key role in tuning physical and chemical properties of solid materials because many reaction processes occur at surface.1,2 Crystal facet engineering can not only improve the performance of functional solid materials, but also fundamentally study the relationship between surface electronic structure and the property.3-5 Perovskite oxides as an alternative cost-effective green catalyst have attracted intense attention in the field of energy and environment arising from excellent tolerance, structural stability and controllable electronic state.6-9 However, the catalytic activity of perovskite oxides is severely limited by the low active-site exposure and their further improvement lacks of the corresponding regulation strategy.10 Previous studies have anticipated that the CO oxidation activity of perovskite oxides depends on the electronic structure of B-site cation and anionic redox chemistry.11-13 Although some workers increase specific surface area of perovskites and create more surface active sites by using template method and electrospinning method, perovskite oxides synthesized by these methods are generally polycrystalline mixtures and the naive surface is still dominated by useless A-site cations, which blocks electron transfer and anion oxygen activating pathways and is not also conducive to understanding the clear relationship between surface electronic structure and intrinsic CO catalytic activity for active perovskite catalysts.14-16 Therefore, it is urgent and significant for perovskite catalysts that developing appropriate method to prepare nano and micro crystallite with high-index crystal facet.

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Synthesis of nano and micro crystallite with high-energy facets has been an important and challenging research topic because the growth rate of high-energy crystal facet is faster and is easily lost in the end.1,17 In the past decade, crystal facet tailoring of precious metals (e.g. Au, Ag, Pd and Pt) and simple metal oxides (e.g. Co3O4, Cu2O, CeO2, ZnO and TiO2) has made great progress.18-21 However, crystal facet tailoring of perovskite oxides is more challenging than that of metal and simple oxide due to compact crystal structure (ABO3) and the presence of ionic bonds (A-O) and covalent bonds (B-O).22 In our previous works, perovskite single crystal such as chromate, manganate and ferrate has been successfully prepared via dehydroxylation process in strong alkaline solution under hydrothermal condition.23-25 High-energy crystal facet of these perovskites can be modulated using cationic surfactant NH4+ because the unique tetrahedral configuration can form the linkage N–H…O–B compared with natural spherical K+ occupied in A-site.25,26 However, as most studied and high active perovskite catalyst, high-energy crystal facet tailoring of perovskite cobaltite micro crystallines is not still reported and is also more challenging than chromate, manganate and ferrate because cobalt ion has too strong coordination with hydroxyl to trigger dehydroxylation reaction under hydrothermal condition. For comparison, molten salt synthesis is high-temperature solution chemistry and crystal phase formation occurs by the reaction mechanism of solution-diffusion or solution-precipitation.27-29 This can not only avoid the problem of dehydroxylation reaction, but also regulate exposed crystal facet of solid materials by controlling the strong interaction between ions in molten salt and the polar planes.27,28,30,31 On the basis of the principle, ZnO, MgO, Co3O4, TiO2, and ZnFe2O4 with high surface energy have been successfully synthesized in molten salt fluxes.32,33 Therefore, molten salt flux synthesis method can offer an important opportunity for regulating crystal facet of perovskite cobaltite and is also simple, economical, environmentally friendly and suitable for scale production. Here, we report a molten salt flux synthetic strategy for the preparation of perovskite cobaltite with high-energy crystal facet and infer that the strong electrostatic interactions between Cl- anions and Sr2+ cations in molten salt and polar planes result in the formation of different crystal facet. Then LaCoO3 (100 and 110), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) as comparison samples are successfully ACS Paragon Plus Environment

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synthesized by molten salt method. XRD, SEM and TEM demonstrate their pure perovskite phase and exposed crystal facet. Co 2p (XPS), Co K-edge and L-edge (XAS) show that exposed crystal facet has not effect on valence state of cobalt atom and LaCoO3 doping Sr ions obviously exists some Co4+ ions. XPS and EDS indicate that the samples with high-energy crystal facet are enriched by more surface Co element compared with La enrichment LaCoO3 (100 and 110), which increases more surface active sites. Combining with X-ray absorption spectroscopy at cobalt L-, K- and oxygen K-edges, valence band spectrum, density functional calculations and CO oxidation measurements, we found that narrow electronic state between Co cations and O anions and O p-band center closer to Fermi level is vital to improving CO oxidation activity and La0.7Sr0.3CoO3 (111) exhibits super CO catalytic activity. This work not only facilitates the rational design of functional perovskite catalysts with high-energy crystal facet, but also highlights the relevance of surface electronic structure and catalytic activity and the importance of anionic redox chemistry in heterogeneous catalysis. RESULTS AND DISCUSSION

Scheme 1. Molten salt flux synthetic route of perovskite cobaltite with different crystal facet and surface morphology measured in SEM for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Molten salt flux synthesis and crystal facet design of La1-xSrxCoO3 In this work, perovskite La1-xSrxCoO3 (x=0~0.3) are successfully prepared by molten salt technique. As shown in Scheme 1, NaCl and KCl are used as molten salt to accelerate diffusion of ions. Then we ACS Paragon Plus Environment

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optimize synthetic conditions of perovskite LaCoO3 such as reaction time, reaction temperature, heating rate, the amount and proportion of NaCl and KCl and these preparative conditions are shown in Table S1. XRD data and SEM data are shown in Figure S1 and Figure S2. These experimental data show that when reaction temperature is below 700 °C, the target sample LaCoO3 contains large LaOCl mixed phases. While reaction temperature is higher than 850 °C, molten salt KCl-NaCl can volatile and splash on the wall of crucible. These results also indicate that metal ions such Na+ and K+, amount of molten salt, reaction time and heating rate have no effect on the preparation of LaCoO3 (110 and 100). Moreover, we also use NO3- ions, OH- ions and CO32- ions as molten salt to prepare LaCoO3. These results show that LaCoO3 synthesized by NO3- ions is nano particles and that prepared by CO32- ions and OH- ions is random morphology due to the formation of La(OH)3, Co(OH)2 and CoCO3 in the grinding process of reaction raw material. Therefore we can infer that Cl- ions play a key role for the synthesis of perovskite LaCoO3 with crystal facet (110) and (100) (Figure 1). Firstly, we can observe the formation of LaOCl intermediate in the process of crystal growth from XRD data of LaCoO3 prepared at 650 °C. This demonstrates that Cl- ions can coordinate with La-related species such as OLa-Cl (100) and CoOLa-Cl (110) on the surface in Figure S5, which further reduces surface free energy of crystal facets (110) and (100). Secondly, crystal facets (110) and (100) in perovskite LaCoO3 have similar surface free energy (Table S2).34,35 Thus perovskite LaCoO3 prepared in molten salt KCl-NaCl exhibits chamfer box shape (crystal facets 110 and 100). To obtain high index crystal facet (111), we need to restrict rapid growth of crystal facet (111), namely the growth of Co arrangement direction (Co LaOOO) in Figure S4. Introducing Co4+ ions can slow the growth of crystal facet (111) because the formation of Co4+ ions needs more energy in individual Co arrangement direction. However, atom arrangement in (110) and (100) direction all contains oxygen atoms such as LaO-CoO in crystal facet (100) and OOLaOCo in crystal facet (110) (Figure S4). Consequently, the growth of crystal facets (110) and (100) is mainly dominated by less formation energy of oxygen vacancy. Therefore, we try to control crystal facet (111) of perovskite LaCoO3 by introducing a small amount of Sr ions. SEM data shown in Scheme 1 demonstrate that crystal facet (111) can be successfully tailored by adding some Sr2+ ions in molten salt. ACS Paragon Plus Environment

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When the content of Sr ions is between 0.17% and 0.50% in KCl-NaCl molten salt and reaction temperature is between 750 °C and 800 °C, it is difficult to be doped in lattice of perovskite LaCoO3. Increasing the concentration of Sr ions and reaction temperature can provide more reaction energy and effective collision for doping of Sr ions. When the content of Sr is between 0.50% and 1.16% and the temperature is between 750 °C and 850 °C, a few Sr ions can be doped in lattice of perovskite LaCoO3 (doping range: 0~0.3) and it still keeps the original crystal structure (R 3c). When the doping amount of Sr is higher than 0.7, perovskite La1-xSrxCoO3 has poor crystallinity and also contains some other mixed phases. Further, LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) were successfully prepared according to above reaction mechanism.

Figure 1. (a) XRD patterns, (b) enlarge graph of peak (104) and peak (113) for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Crystal facet and structure characterization X-ray diffraction (XRD) patterns of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) are shown in Figure 1a and these perovskites all exhibit a rhombohedral structure of R 3c. The tiny peak marked by circle corresponds to Co3O4 secondary phase because crystal facet (111) contains more cobalt cation segregation phase, which agrees with the above discussed growth mechanism. Further, main peaks of these samples (104 and 113) are enlarged in Figure 1b and their peak intensity obviously decreases, indicating that introduction of Sr can decrease their crystallinity. The intensity ratio of peak (104) to peak (113) for LaCoO3 (111) and La0.7Sr0.3CoO3 (111) reduces compared with LaCoO3 (110 and 100), which can be attributed that they possess different crystallographic orientation on the surface. ACS Paragon Plus Environment

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Moreover, we also observe that the peak position shifts to high angle from LaCoO3 (110 and 100), La0.7Sr0.3CoO3 (111) to LaCoO3 (111), suggesting that exposed crystal facet (111) results in lattice shrinking and doping Sr ions of larger radius (Sr2+:0.118> La3+:0.103) leads to lattice expansion.

Figure 2. Bright-field TEM images of (a) LaCoO3 (110 and 100), (b) LaCoO3 (111) and (c) La0.7Sr0.3CoO3 (111). The corresponding FFT patterns of (d) LaCoO3 (110 and 100), (e) LaCoO3 (111) and (f) La0.7Sr0.3CoO3 (111). High-resolution TEM images for (g) LaCoO3 (110 and 100), (h) LaCoO3 (111) and (i) La0.7Sr0.3CoO3 (111). To further study their crystal structure, lattice and morphology, transmission electron microscopy is measured and TEM images of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) are shown in Figure 2 (a-c). Perovskite LaCoO3 (110 and 100) exhibits chamfer box shape and exposures (110) and (100) planes, while LaCoO3 (111) and La0.7Sr0.3CoO3 (111) show octahedral morphology and exposure (111) planes. Thickness and size of these perovskite samples make electron beam difficultly penetrated, so we embed these experimental samples into polymer resin and cut into 100 nm resin flake with the target sample using customized microtome. As seen in high-resolution HRTEM images (Figure ACS Paragon Plus Environment

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2 d-f), these samples also have a high single crystal quality. To verify their crystal structure, we do the fast Fourier transforms (FFTs) for their high resolution TEM images (Figure 2 d-f) and measure their interplanar spacings in Figure 2 (g-i). The fast Fourier transforms (FFTs) results show that LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) take a rhombohedral structure with R 3c space group, which is consistent with the corresponding XRD data. Furthermore, the interplanar spacing (d=0.271 nm) in LaCoO3 (110 and 100) is consider as (110) plane, while the interplanar spacing (d=0.254 nm) in LaCoO3 (111) can be assigned to (104) plane and the interplanar spacing (d=0.399 nm) in La0.7Sr0.3CoO3 (111) can be regarded as (012) plane. This result and main peak shift in XRD confirm that exposure of crystal facet (111) can decrease lattice spacing and introduction of Sr ions causes the distortion of octahedron and lattice expansion. Table 1 EDS, ICP and XPS results of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). EDS

ICP

XPS

La %

Sr %

Co %

La %

Sr %

Co %

La %

Sr %

Co %

Surface O Lattice O

Lattice Sr Surface Sr

LaCoO3 (110 and 100)

48.5

0

51.5

54.2

0

45.8

63.4

0

37.6

2.43

-

LaCoO3 (111)

36.2

0

63.8

39.8

0.11

59.1

51.6

4.8

43.6

3.71

0.016

La0.7Sr0.3CoO3 (111)

22.5

9.4

68.1

21.2

11.7

65.1

33.3

14.9

51.8

6.37

0.273

Surface morphology, composition and structure Surface morphology of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) is measured by scanning electron microscope (SEM). From Figure S6a We can intuitively observe that perovskite LaCoO3 synthesized in KCl-NaCl without Sr molten salt system exhibits chamfer box shape and exposes crystal facets (110) and (100). While LaCoO3 (111) in Figure S6b and La0.7Sr0.3CoO3 (111) in Figure S6c display octahedral shape and only expose crystal facet (111), which is consistent with TEM characterization and design model in Figure S6d and S6e. We also find that surface of crystal facets (110) and (100) is very smooth, while that of crystal facet (111) is very rough and contains more spiral dislocations and growth steps, indicating that the growth of LaCoO3 (110 and 100) belongs to two dimensional nucleation mechanism, and introduction of Sr ions successfully inhibits the growth of crystal facet (111) and its growth way can be attributed to screw dislocation mechanism. These results ACS Paragon Plus Environment

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verify our flux synthetic design and growth mechanism. Furthermore, to demonstrate their thermal stability, these three samples are sintered at 1000 ℃ at air for 2h and their morphology is performed using SEM. As shown in Figure S3, we can observe that these three samples do not obviously agglomerate compared to synthesized pristine samples. The morphology of LaCoO3 (110 and 100) changed from smooth chamfer box to smooth sphere, while that of LaCoO3 (111) and La0.7Sr0.3CoO3 (111) still keep original octahedron shape. Moreover, the surface of LaCoO3 (111) and La0.7Sr0.3CoO3 (111) becomes more rough due to the calcinations. These results indicate that these perovskite samples have higher thermal stability. Then we use Energy Dispersive Spectrometer (EDS) to measure their element composition and distribution. As shown in Figure S6, both LaCoO3 (110 and 100) and LaCoO3 (111) consist of La, Co and O and these elements are evenly distributed in the crystal, and only La0.7Sr0.3CoO3 (111) extra increases Sr elements. This confirms that doping of Sr ions needs higher reaction temperature and more Sr concentrations. So we can regulate crystal facet and doping amount of Sr of perovskite LaCoO3 by controlling reaction temperature and Sr concentration in molten salt. Moreover, we analyze compositions and the amount of these samples such as La, Sr and Co by EDS, ICP and XPS. These results are shown in Table 1. EDS data show that LaCoO3 (110 and 100) and LaCoO3 (111) only contain La (48.5%, 36.2%)and Co cationic elements and La0.7Sr0.3CoO3 (111) has 22.5% La, 9.4% Sr and 68.1% Co, indicating that K+ and Na+ are not doped into lattice and Sr is also not doped into LaCoO3 (111). ICP and XPS results also show that LaCoO3 (111) has a small amount of strontium and these Sr ions are surface secondary phases such as SrO, Sr(OH)2 and SrCO3 that can be further verified by fit peaks of Sr 3d in Figure S9. The samples with crystal facet (111) have more B cations and less A cations from EDS, ICP and XPS (Table 1). XPS is used to analyze surface compositions and structure and the XPS data in Table 1 show that LaCoO3 (110 and 100) has more La element (63.4%), while LaCoO3 (111) and La0.7Sr0.3CoO3 (111) contain more Co element (43.6%, 51.8%) on the surface. These results confirm the above discussed growth mechanism for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Furthermore, we also find that these synthetic samples do not contain K+, Na+ and Cl- ions by experimental measurement such as EDS (Figure S7), ACS Paragon Plus Environment

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XPS (Figure S8) and ICP, which can be attributed that the introduction of K+ and Na+ ions may produce high valence state Co ions or more oxygen vacancies and it need more energy for the formation crystal phase. To better study surface structure of these materials, the O 1s spectrum (Figure 3 a-c) and Sr 3d spectrum (Figure S9) are divided into surface components and lattice components. The component located at ~528.6 eV belongs to bulk O2- in perovskite (Olattice) and the other three are surface oxygen species (Osurface) that the one at ~529.6 eV is considered as perovskite lattice termination layer, the one at ~531.3 eV is surface secondary phases and the one at ~532 eV is attributed to surface adsorbed oxygen species.36,37 Surface secondary phases can be considered as oxides, hydroxides and carbonates of A-site metal such as SrCO3, Sr(OH)2, SrO, La2O3, La(OH)3 and La2(CO3)3. While perovskite termination is the structure and composition of the outermost surface of perovskite oxides and the abrupt termination differs from bulk lattice structure. Outermost surface will exhibit higher BEs than atoms situated within the bulk, and thus perovskite termination can be regarded as B-O bond (B–O assignments such as Co-O).37,38 Moreover, perovskite termination plays the key role in improving activity and stability of perovskite oxides because heterogeneous catalysis mainly happens on the surface of solid catalysts and surface segregation (A-O) in perovskite oxides does not catalytic activity. These fitting peaks are shown in Figure 3 and we calculate the content of surface O and lattice O by above multi peak fitting area. XPS data in Table 1 show that the content of surface O obviously increases from LaCoO3 (110 and 100) (2.43), LaCoO3 (111) (3.71) to La0.7Sr0.3CoO3 (111) (6.37), indicating that exposed crystal facet (111) increases more surface oxygen species and improves oxygen ion mobility, and doping Sr can increase more oxygen vacancies. Moreover, Sr 3d spectrum in Figure S9 demonstrate that Sr content in LaCoO3 (111) is mainly surface Sr such as SrCO3, Sr(OH)2 and SrO and Sr ion is not doped into perovskite lattice, while perovskite La0.7Sr0.3CoO3 (111) contains more lattice Sr (Table 1).


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Figure 3. XPS data of (a) LaCoO3 (110 and 100), (b) LaCoO3 (111) and (c) La0.7Sr0.3CoO3 (111) in O 1s regions (purple solid circles: measurement; black line: sum of fits; orange line: adsorbed oxygen; red line: surface secondary phases; magenta line: perovskite termination; blue line: lattice oxygen). (d) FTIR spectra of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Infrared spectrum of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) are performed to study their structure features (Figure 3d). A vibration band around 427 cm-1 can be assigned to the Co3+-O bond stretching vibration band (νCo-O) in the BO6 octahedron of the three perovskite samples. The vibration band around 543 cm-1 and 604 cm-1 can be considered as O–Co3+–O deformation vibration band (νO-Co-O) in the BO6 octahedron of perovskite LaCoO3 (110 and 100).39,40 The vibration band of perovskite LaCoO3 (111) shifts to a lower wavelength (532 cm-1 and 592 cm-1) compared to LaCoO3 (110 and 100) due to the difference of surface electronic structure such as surface atomic arrangement and the covalence of Co-O. While the bending vibration band around 584 cm-1 refers to O– Co–O deformation vibration band (νO-Co-O) of perovskite La0.7Sr0.3CoO3 (111), suggesting that octahedral distortion (Jahn-Teller effect) caused by doping Sr ions makes two bending vibration bands in perovskite LaCoO3 merged into one band. Moreover, a vibration band around 667 cm-1 represents the ACS Paragon Plus Environment

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Co-O bond with high covalence stretching vibration in the BO6 octahedron and the additional appearance of this vibration band could be attributed to the interaction between Co ions and O ions in perovskite LaCoO3 (111) and La0.7Sr0.3CoO3 (111), proofing that exposed high-energy crystal facet (111) can enhance the hybridization of Co-O bond.

Figure 4. Room temperature ESR spectra of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). To further study the microstructure of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111), electron paramagnetic resonance (EPR) is carried out at room temperature and their EPR spectra are shown in Figure 4. These EPR signals are mainly used to perform single electron such as oxygen species (defects) and the interaction between Co cations and O anions or Co cations (magnetic characterization) for Co-based perovskites. LaCoO3 (110 and 100) has a narrow less-intensive signal with g-value of 2.15 and ∆Hpp of 38 mT, while LaCoO3 (111) displays a narrower high-intensive EPR signal with g=2.04 and ∆Hpp=26 mT and La0.7Sr0.3CoO3 (111) exhibits a broader high-intensive signal (g=2.41 and ∆Hpp=109 mT). These EPR signals are consistent with previous reports for Co-based perovskites.41,42 Because the surface of La0.7Sr0.3CoO3 (111) and LaCoO3 (111) is B-site enrichment and LaCoO3 (111) is A-site enrichment, La0.7Sr0.3CoO3 (111) and LaCoO3 (111) is very easy to form more defect. Furthermore, from SEM images of these samples (Scheme 1 and Figure S6), we can observe that the surface of La0.7Sr0.3CoO3 (111) and LaCoO3 (111) is very rough, while that of LaCoO3 (100 and 110) is very smooth. SEM results confirm that La0.7Sr0.3CoO3 (111) and LaCoO3 (111) contain more defects on the surface. Therefore, Y-axis of LaCoO3 (100 and 110) is lower than that of La0.7Sr0.3CoO3 ACS Paragon Plus Environment

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(111) and LaCoO3 (111). Because oxygen defects concentration of La0.7Sr0.3CoO3 (111) is similar to LaCoO3 (111), their ESR signal intensity is very closer. However, La0.7Sr0.3CoO3 (111) exhibits broader signal than LaCoO3 (111). Their difference is the doping of Sr for La0.7Sr0.3CoO3 (111). When some Sr ions are doped into the lattice of LaCoO3 (111), it can produce more Co4+ ions. Compared with LaCoO3, Co4+ ions in La0.7Sr0.3CoO3 (111) can be coupled to Co3+ ions by double-exchange interactions, forming the ferromagnetic Co3+–Co4+ clusters. The competition between alien ferromagnetic Co3+–Co4+ couples and allied antiferromagnetic Co3+–Co3+ and Co4+–Co4+ couples causes a magnetic frustration, resulting in broader ESR signal.43 Furthermore, ESR signal of isolated Co4+ ions has been not detected in current reports. Electronic Structure and oxidation state XAS (X-ray absorption spectrum) as an important detection tool is widely applied to study electronic structure of functional materials. It can reflect orbital hybridization, spin state, the covalence of chemical bond and valence state by measuring the electronic state of unoccupied orbits. The O K-edge XANES spectra normalized from 520 eV to 555 eV represents the metal-O 2p hybridization of three samples. As shown in Figure 5a, peaks a and b can be ascribed as the overlapping band between Co 3d and O 2p; peak c is considered as superoxide species O2- on the surface of perovskite oxides that is profitable for the activation of oxygen molecular; peak d represents the hybridization between La 5d/Sr 4d and O 2p; peaks e and f can be considered as the Co4sp-O2p hybrid.39,44 To study the electronic states of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) in more detail, we take a closer look for peaks a and b at the superimposed O K-edge spectra (Figure 5b). LaCoO3 and La0.7Sr0.3CoO3 with crystal face (111) show a narrowed hybrid orbit between Co 3d and O 2p and their absorption peak a (O p-band center) obviously shifts to higher energy position compared with LaCoO3 (110 and 100). The sequences for energy position and orbital overlap are La0.7Sr0.3CoO3 (111) > LaCoO3 (111) > LaCoO3 (110 and 100).45 These experimental results confirm that exposed high-energy crystal face (111) improves the covalence of Co-O bond and makes O p-band center closer to Fermi energy level, and doping Sr can further enhance the hybridization between Co 3d and O2p. Furthermore, ACS Paragon Plus Environment

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La0.7Sr0.3CoO3 (111) significantly increases a new O2- species absorption peak at 532.7 eV due to synergistic effect of high-energy crystal face and Sr doping, further confirming that La0.7Sr0.3CoO3 (111) possesses more surface oxygen vacancies and outstanding oxygen mobility ability than LaCoO3 (110 and 100, 111).44,46 In addition, the Co L-edge XANES spectra of the three samples also show a clear change and their absorption spectra are shown in Figure 5c. The spectra correspond to the electron transition from Co 2p (1/2 and 3/2) to Co 3d (the unoccupied states) because of the spin-orbit interaction of the Co 2p core level and their data are normalized from 765 eV to 805 eV. For Co L3-edge and L2edge XANES of the three samples, their position for peak B at 776.6 eV and peak D at 790.9 eV remain unchanged, indicating that their valence state for Co3+ ions is the same and exposed crystal face has no significant effect on their oxidation state. While La0.7Sr0.3CoO3 (111) extra appears two absorption peak in higher energy position (peak C at 780.7 eV and peak E at 795.3 eV), which can be ascribed as the content of Co4+ ions due to doping of Sr. Furthermore, the shoulder A at lower energy region of Co L3edge peak is related to spin state of cobalt in perovskites and it has a clear increase from LaCoO3 (110 and 100) to LaCoO3 (111), La0.7Sr0.3CoO3 (111). This suggests that exposed high-energy crystal face (111) can tune spin state of surface Co ions in perovskite structure, namely increases more intermediate spin state.47,48 To confirm our opinion, we also measure Co 2p XPS data of these samples. As shown in Figure 5d, peak position of Co 2p 1/2 and 3/2 for La0.7Sr0.3CoO3 (111) shifts to higher energy compare with other two samples and that of LaCoO3 (110 and 100) and LaCoO3 (111) is the same, indicating that doping Sr can increase more Co4+ ions and exposed crystal facet does not change valence state of Co ions, but changes the covalence of C o-O bond. To further prove the number of Co4+ and Co3+, we calculate their content using chemical titration and their experimental procedure is shown in following section. Titration results show that the ration Co4+ to Co3+ is about 2.42. Co L-edge spectra directly demonstrate the existence of Co4+ ions and we also do the corresponding area integral for absorption peak of Co3+ and Co4+ ions. Experimental results also show that the number of Co4+ and Co3+ is about 2.13. These results show that their proportion is closer to 7/3 (2.33), thus we may write the formula for La0.7Sr0.3CoO3. ACS Paragon Plus Environment

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Figure 5. (a) The O K-edge XANES, (b) enlarge peak A and peak B in O K-edge XANES, (c) the Co L2 and L3-edges XANES, (d) Co 2p spectra in XPS data, (e) Raman spectra, (f) the Co K-edge XANES, (g) enlarge Co K-edge XAS spectra in the transition of Co 1s→3d, (h) enlarge X-ray absorbed edges for Co K-edge XANES, (i) enlarge Co K-edge XAS spectra in the transition of Co 1s→4p, (j) The R-space Fourier-transformed FT (K3χ(k)) of Co K-edge EXAFS spectra of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Raman spectroscopy as a powerful tool is generally used to analyze chemical bond information in solid materials via various vibrational modes and Raman data of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) are shown in Figure 5e. The lowest-energy Raman band is assigned as the Eg bending vibration of CoO6 (1 region) and the low-energy Raman band is associated with Eg quadrupole vibration (JT) of (2 region).49 The highest-energy Raman band can be attributed to the A2g breathing mode of the oxygen ion cage in CoO6 octahedron (3 region), which origins from the symmetry of the ACS Paragon Plus Environment

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local crystal structure and strong electron−phonon interactions.50,51 Eg bending vibration peak of La0.7Sr0.3CoO3 (111) (477 cm-1) shifts to higher wavenumbers compared with LaCoO3 (467 cm-1) due to doping of Sr and the presence of Co4+ ions. LaCoO3 (111) and La0.7Sr0.3CoO3 (111) extra appears a Raman peak of Eg quadrupole vibrations at 515 cm-1 due to the difference of exposed crystal face. Moreover, LaCoO3 with (110) and (100) has multiple Raman peaks in 3 region due to the mixing of spin states (three spin states: low, intermediate and high) that are A2g breathing vibration at 648.4 cm-1 (LS), 671.8 cm-1 (IS) and 689.6 cm-1 (HS), respectively, while La0.7Sr0.3CoO3 (111) and LaCoO3 (111) have an indistinctively splitting A2g breathing vibration band at 681.5 cm-1.49 These results indicate that surface atomic arrangement has important effect on spin state of cobalt and the covalence of Co-O bond. Enhancement of Raman signal can be attributed to the increase of surface roughness in crystal facet (111). To further clarify effect of exposed crystal facet and Sr doping on valence and electronic structure of Co atom, we performed XANES (X-ray near-edge spectroscopy) measurements of Co K-edge for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). From g region in Figure 5g (transitions from Co1s to Co3d), we can observe that LaCoO3 (111) contains less eg electrons compared to LaCoO3 (110 and 100) and 3d orbits of La0.7Sr0.3CoO3 (111) has no obvious split. Thus, the results confirm that exposed high-energy crystal face (111) promotes transition of low-spin state of Co atom in LaCoO3 and introduction of Sr changes band structure due to octahedral distortion. Absorbed energy position in h region and i region of Figure 5g is used to study oxidation state of Co atom. Absorbed peak (7726.7 eV, 1s→4p) in Figure 5i and absorbed edge (7723.5 eV) of Sr-substituted LaCoO3 in Figure 5h apparently shift toward a higher energy range compared with LaCoO3, and LaCoO3 (111) has a little shift in absorbed energy position and absorbed edge compared with LaCoO3 (110 and 100). These results suggest that substitute of Sr increases Co4+ contents in perovskite and exposed crystal face (111) improves the covalence of Co-O bond.52 To further study coordination structure of cobalt atom, Co Kedge EXAFS of these three samples is transformed into R-space Fourier-transformed FT (k3χ (k)). As shown in Figure 5j, the first shell presents Co-O bond and the second shell is attributed to Co-La/Sr and ACS Paragon Plus Environment

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corner-shared Co-O-Co bond in perovskite structure.52 LaCoO3 (111) increases the bond distance of CoO in first shell compared with LaCoO3 (110 and 100) due to the difference of surface atom arrangement. However, the bond distance of Co-O (0.184nm) of perovskite La0.7Sr0.3CoO3 (111) is between LaCoO3 (111) and LaCoO3 (110 and 100), which can be attributed that exposed crystal face (111) increases CoO bond length and doping Sr reduces its length due to the distortion of octahedron. Moreover, the second shell can directly reflect local structure around cobalt atom. The Co-La/Sr/Co bond length of La0.7Sr0.3CoO3 (111) shifts to high distance compared with LaCoO3 because doping Sr increases octahedral distortion and more surface defects. LaCoO3 (111) located in 0.349 nm possesses wider peak and longer bond distance in second shell than LaCoO3 (110 and 100), indicating that LaCoO3 (111) contains some intersective structural features for other two samples such as more surface defect and less octahedral distortion.39,53 Therefore, exposed crystal face has important effect on surface defect and the hybridization of Co-O bond, and doping Sr can improve oxidation state of Co atom, octahedral distortion and the covalence of Co-O bond. The correlation between electronic structure and CO oxidation activity H2-TPR data of these three samples is performed to study their redox properties. Previous studies have shown that the reduction Co-based perovskite oxides undergoes two-step process: 1) Co4+/3+→Co2+ (~665K), 2) Co2+→Co0 (~873K). In Figure 6a, we can observe that the reducing temperature of onset peak for La0.7Sr0.3CoO3 (111) is the lowest compared with other two samples, suggesting that it contains more surface active oxygen species. The reducing peak positions for La0.7Sr0.3CoO3 (111) at first step are 357 ºC, 419 ºC, while that of LaCoO3 (111) are 357 ºC, 413 ºC and that of LaCoO3 (110 and 100) are 338 ºC, 410 ºC. But chemical adsorption capacity of LaCoO3 (111) is higher than that of LaCoO3 (110 and 100) at first step. These H2-TPR results demonstrate that their redox property increases in the sequence: LaCoO3 (110 and 100) < LaCoO3 (111) < La0.7Sr0.3CoO3 (111). Furthermore, we also measured the O2-TPD data to study the amount and activity of related oxygen species. Released oxygen below 400 °C can be attributed to absorbed oxygen species and that above 400 °C can be considered as lattice oxygen. The O2-TPD results of these samples are shown in Figure 6b, we can observe that ACS Paragon Plus Environment

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La0.7Sr0.3CoO3 (111) has more surface absorbed oxygen species and lower onset peak than LaCoO3 (111) and LaCoO3 (110 and 100), which is very beneficial to catalytic reaction at low temperature. Their peak position is 81 °C for LaCoO3 (110 and 100), 76 °C for LaCoO3 (111) and 86 °C for La0.7Sr0.3CoO3 (111), respectively. LaCoO3 (111) has lower desorption temperature and more surface absorbed oxygen amount than LaCoO3 (110 and 100). Furthermore, La0.7Sr0.3CoO3 (111) also show better lattice oxygen mobility than LaCoO3 (110 and 100) and LaCoO3 (111) at > 400 °C. These results indicate that the content of surface active oxygen species increase in the sequence of LaCoO3 (110 and 100) < LaCoO3 (111) < La0.7Sr0.3CoO3 (111), which are consistent with XPS results.

Figure 6. (a) H2-TPR profiles, (b) O2-TPD profiles for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). To study the catalytic property of these samples, we measure CO oxidation activity of LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111). Their CO catalytic activity is shown in Figure 7a and the sequence for catalytic performance is La0.7Sr0.3CoO3 (111)> LaCoO3 (111)> LaCoO3 (110 and 100). Furthermore, the value of T10, T50 and T90 for these three samples is plotted into histogram of ACS Paragon Plus Environment

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conversion and temperature. As shown in Figure 7b, the value of T10, T50 and T90 in LaCoO3 (110 and 100) is 222 °C, 252 °C and 281 °C, respectively. Compared with LaCoO3 (110 and 100), the value of T10, T50 and T90 in LaCoO3 (111) shows a decrease of 40 °C, 34 °C and 47 °C, while that of La0.7Sr0.3CoO3 (111) shows a decrease of 72 °C, 89 °C and 87 °C, respectively. Moreover, we also measure the specific surface area (BET) and their BET values are similar for these samples, which are 0.123 m2g-1 for LaCoO3 (110 and 100), 0.135 m2g-1 for LaCoO3 (111) and 0.145 m2g-1 for La0.7Sr0.3CoO3 (111), respectively. To better study the intrinsic catalytic activity, reaction rates normalized by specific surface area are shown in Figure S14. We can observe that La0.7Sr0.3CoO3 (111) has higher reaction rate (mol s-1 m-2) than LaCoO3 (110 and 100) and LaCoO3 (111) at different temperature. To study the correlation between exposed crystal facet, electronic structure and catalytic activity, we further perform density functional theory (DFT) calculations and valence band spectrum. We build different crystal facet model and also introduce Sr ions in LaCoO3 (111). Then we use GGA+U to calculate their DOS in spin up and spin down.52,54,55 Figure 7d shows their projected density of states for the Co 3d and O 2p states obtained by DFT calculations. In the spin up, the electron density of Co 3d for different crystal facets near the Fermi level has small difference, while the electron density of O 2p valence band for crystal facet (111) near the Fermi energy is higher than that of crystal facets (110) and (100) and La0.7Sr0.3CoO3 (111) exhibit the highest electron density of O 2p valence band, increasing absorbed oxygen species and improving oxygen ion mobility. Moreover, PDOS also show that O p-center of crystal facet (111) is closer to Fermi energy than crystal facet (110) and (100) and crystal facets (110) and (100) have weaker covalence of Co-O bond, which agrees with the experimental results of O K-edge spectra. To further verify our theory calculation and experimental results, valence band spectrum of these perovskite samples are also measured to perform electronic structure near the Fermi energy (Figure 7c). The first peak near the Fermi energy can be attributed to the hybridization of Co-O bond. Compared with LaCoO3 (110 and 100), the overlap of Co-O bond for the samples with crystal facet (111) distinctly crosses the Fermi level, indicating that exposed crystal facet (111) enhances the hybridization between Co 3d and O 2p and makes the overlap of Co-O bond closer to ACS Paragon Plus Environment

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Fermi level. Combined with O K-edge absorption spectra, valence band spectra, density functional theory calculations and CO oxidation activity, we find that introducing Co4+ ions and tuning exposed facet can enhance the covalence of Co-O bond, and thus it is closer to the Fermi level for the position of O p-band center. Moreover, its CO catalytic oxidation process follow these steps: (1) O2 (g)→O2(ad)→2O- (ad); (2) CO (g)→ CO (ad); (3) CO (ad) + 2O- (ad)→CO32- (ad); (4) CO32- (ad) →CO2 (g) + O2- (ad). Activation of oxygen is the key step for CO oxidation and O p-band center closer to the Fermi level is beneficial to the activation of oxygen molecular (electron transfer), resulting in higher catalytic activity. We conclude that narrow electronic state between Co 3d and O 2p and moving the computed O p-center closer to Fermi level can increase CO oxidation activity. Therefore, perovskite La0.7Sr0.3CoO3 (111) exhibits the highest CO oxidation activity.

Figure 7. (a) CO oxidation activity, (b) 2D histogram of the 10%, 50% and 90% CO conversion vs. reaction temperature, (c) valence band spectra for LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111), (d) projected density of states for the Co 3d and O 2p obtained in the DFT calculations of LaCoO3 with different crystal facet (100, 110 and 111) and La0.7Sr0.3CoO3 (111). In the calculations, a hexa-rhombohedral unit was fixed for these samples and PDOS sets spin polarization (spin-up and spin-down). Conclusion ACS Paragon Plus Environment

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In conclusion, we demonstrate a molten salt route to the synthesis of perovskite cobaltite crystalline with high-energy crystal facet. Our experiments show that Cl- and Sr2+ ions in molten salt have a strong interaction with exposed crystal facet of perovskite cobaltite and changing Sr concentration in NaClKCl molten salt system can tune surface energy and the growth rate of crystal facet, resulting in the formation of different crystal facet. Based on this mechanism, we successfully prepared LaCoO3 (100 and 110), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) and further study their surface morphology, crystal facet, composition, electronic structure and CO catalytic activity. The best CO oxidation performance was achieved by La0.7Sr0.3CoO3 with high-energy crystal facet (111), giving rise to a decrease of 89 °C than LaCoO3 (100 and 110) at the value of T50. X-ray absorption spectroscopy, density functional theory calculations and the valence band spectrum demonstrate that exposed crystal facet has very important effect on the covalence of Co-O bond and O p-band center relative to Fermi level and these results revealed that the enhanced hybridization of Co-O band and O p-band center closer to Fermi level can improve the CO oxidation activity. We emphasize that high-energy crystal facet regulation will open a new path for the design of high-performance perovskite-type catalysts. In addition, our works also build the bridge between surface electronic structure and the intrinsic catalytic activity. Experiment Section Material synthesis La(NO3)3·6H2O (99.9%), Co(NO3)2·6H2O (99.9%), Sr(NO3)2 (99.9%), NaCl (99.9%) and KCl (99.9%) were obtained from Sinopharm Chemical Reagent Co. Ltd. All of the above reagents were used as received without any further purification. La(NO3)3·6H2O, Co(NO3)2·6H2O taken in a stoichiometric ratio were mixed in agate mortar and ground into uniform power. Then the mixture obtained above was transferred into covered corundum crucible and calcined in air. In this synthetic process, we study synthetic mechanism and optimize experimental parameters of perovskite LaCoO3 by regulating some reaction conditions such as reaction temperature, reaction time, the amount and proportion of KCl-NaCl, heating rate. After being slowly cooled to room temperature under air atmosphere, hot deionized water ACS Paragon Plus Environment

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was added into the corundum crucible until the resulted products were thoroughly immersed. The crucible was repeatedly treated with ultrasonic instrument until slugged samples were evenly dispersed in aqueous solution. These samples were filtered and repeatedly washed with a large amount of deionized water until there is no chlorine ion in the aqueous solution. Subsequently, the resulted black products were dried overnight at 60 °C in air. Finally, LaCoO3 (110 and 100) was prepared using 2 mmol La(NO3)3·6H2O and 2 mmol Co(NO3)2·6H2O in molten salt with 120 mmol KCl-NaCl (1:1) at 750 °C for 2h at a heating rate of 5°C min-1 and LaCoO3 (111) was prepared using 1.7 mmol La(NO3)3·6H2O, 0.5 mmol Sr(NO3)2 and 2 mmol Co(NO3)2·6H2O at the same reaction conditions with LaCoO3 (110 and 100), while La0.7Sr0.3CoO3 (111) was synthesized using 1.2 mmol La(NO3)3·6H2O, 1.2 mmol Sr(NO3)2 and 2mmol Co(NO3)2·6H2O in molten salt with 120 mmol KCl-NaCl (1:1) at 800 °C for 2h at a heating rate of 5°C min-1. The prepared samples LaCoO3 (111) and La0.7Sr0.3CoO3 (111) were extra washed with dilute acid to remove mix phases (SrCO3). Materials characterization Crystal structure of the samples such as LaCoO3 (110 and 100), LaCoO3 (111) and La0.7Sr0.3CoO3 (111) was characterized by X-ray powder diffraction (XRD) with Cu Karadiation (k= 0.15418 nm) at room temperature, which was collected on a Ultima IV diffractometer at 40 kV and 30 mA with a step scan speed of 3 °/min in an angle range of 20°~80°. SEM (scanning electron microscope images) and mapping of these samples was characterized using a Helios NanoLab 600i Dual Beam System (FEI Company, America) with an EDS equipment (EDAX) from Ametek Company. These single crystal samples were embedded into polymer resin. Then polymer resin was cured for 12h at 30 °C, for 12 h at 45 °C and for 48h 60 °C. Finally, the polymer resins with samples were cut into resin flake with 100 nm thickness using Leica RM2235, which is useful for TEM characterization. HRTEM (high resolution transmission electron microscope) of these samples was measured in a Tecnai G2 F20 at 200 kV accelerating voltage of electron beam. FTIR was measured on an IFS-66V/S infrared spectrum radiometer from 400 cm-1 to 2000 cm-1 at room temperature in vacuum condition. KBr was used to deduct the background signal and these perovskite samples were pressed into transparent pellet with the ACS Paragon Plus Environment

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weight ratio of sample to KBr, 1/100. Raman spectroscopy was performance in Renishaw inVia Raman Microscope in UK at 532 nm radiations. The X-ray absorption near-edge spectra (XANES) of O K-edge and Co L-edge were measured at the BL12B-a beamline of the National Synchrotron Radiation Laboratory (NSRL) in China. XAS spectra of these samples was collected in the total electron yield (TEY) mode under a vacuum better than 5×10-8 Pa. X-ray absorption spectra (XAS included EXAFS and XANES) of Co K-edge were characterized at the beamline BL14W1 of Shanghai Synchrotron Radiation Facility in China. Perovskite samples were mixed with appropriate LiF diluents according to the calculation of Athena soft, and these products were pressed into pellets for measuring XAS spectra. The measurement was carried out in transmission mode using Si (111) channel-cut monochromator. The Co K-edge spectra were collected from 7500 eV to 8500 eV at room temperature. Energy value of the samples was calibrated by a standard Co foil. The experimental data were analyzed using the software package IFEFFIT. The EPR spectra were measured at room temperature using a JES-FA200 ESR spectrometer. Appropriate solid samples were loaded into a quartz tube with inner diameter of 1 mm. XRay photoelectron spectra (XPS) were characterized by a Thermo ESCALab 250 analyzer at constant analyzer energy mode and the incident radiation was Al Kα X-rays (1486.6 eV). The binding energy of the acquired spectra (La 4d, Sr 3d, Co 2p, O 1s and valence band) was calibrated using the C 1s line at 284.6 eV. The elements such as Na, K, La, Sr and Co are measured by iCAP 7600 ICP-OES. These samples are thoroughly dissolved in concentrated hydrochloric acid at 200 °C to measure ICP. The cobalt oxidation state in perovskite La0.7Sr0.3CoO3 (111) such as the ratio of Co4+ to Co3+ can be calculated using chemical titration method. 15 mg La0.7Sr0.3CoO3 (111) samples are dissolved in 30 mL HCl (3 mol/L) solutions, and then 0.14g KI and 0.01g starch was added into mixed solutions. I- ions in solution can be easily oxidized into I2 in the acidic environment by Co3+ ions and Co4+ ions in La0.7Sr0.3CoO3 (111). Then, 0.015 mol L-1 Na2S2O3 was used to calibrate the content of oxidized I2. We can calculate the ration of Co4+ to Co3+ by electron transfer balance. Moreover, the titration experiment was repeated three times. The temperature-programmed reduction of H2 (H2-TPR) of these sample were measured on an autochem II 2920 auto multifunctional adsorption instrument by a temperatureACS Paragon Plus Environment

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programming system. 100 mg catalysts were heated in the flow of 10 vol. % H2/Ar (30 cm3/min) at 10 °C/min from 50 °C to 800 °C. The amount of H2 consumption was measured by thermal conductivity detector (TCD). Temperature programmed desorption (TPD) of O2 were measured by a Micromeritics ChemiSorb 2720 chemisorption analyzer. After pre-treated at 400 ºC in He for 1 h, 100 mg samples were purged under 5% O2 for 1 h, and then isothermally removed physical adsorbed O2 in He for 30 min. The TPD measurement was carried out in He atmosphere at 10 °C/min up to 800 °C at 50 mL/min. Materials performance measurement CO catalytic oxidation activity of the samples was measured in a fixed-bed quartz reactor (6 mm inner diameter). 50 mg catalysts and 100 mg quartz sands were fully mixed and the resulted products were used to evaluate activity of CO catalytic oxidation. The feed mixture gas contained 1% CO, 20% O2 and 79% Ar. The total gas flow rate was 50 cm3 min-1. The CO conversion rate was calculated by the formula α= (1-C1/C0)×100%, where α is conversion rate, C0 is the initial concentration of CO, C1 is the concentration of CO after catalytic reaction. The concentrations of the CO at different temperature were detected by gas chromatography (Agilent, GC6890N) with TCD using He gas as reference gas. Density functional theory calculations Our first-principles DFT calculations are performed using Cambridge Sequential Total Energy Package (CESTEP).56 The projector-augmented wave (PAW) potentials are used to treat electron-ion interactions and the generalized gradient approximation (GGA) is used for the electron exchange correction functions.57 The cut-off energy of the plane wave is set into 480 eV. The value of cut-off energy and k-points are tested. The convergence threshold for self-consistent-field teration is set at 2×10-5 eV and the atomic positions are fully relaxed. Considering the effects of inducing the exchange correlation correction, the value of U is set 6.5 eV for Co atom. Co atom in these perovskites is set as intermediate spin state according to our experimental results. The crystal structure of these perovskites belongs to R 3c space-group.58 ASSOCIATED CONTENT ACS Paragon Plus Environment

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic condition, XRD, SEM, EDS, XPS, XAS characterization, surface atom arrangement, and surface free energy. (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors and these authors contributed equally. Funding Sources This work was supported by National Natural Science Foundation of China (21427802, 21671076 and 21621001). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors thank Beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. The authors thank the NSRL beamline BL12B-a (National Synchrotron Radiation Laboratory) for providing beam time. The theory calculation results described in this paper were gained in the China scientific Computing Grid (ScGrid). REFERENCES (1) Kuang, Q.; Wang, X.; Jiang, Z.; Xie, Z.; Zheng, L. High-Energy-Surface Engineered Metal Oxide Micro- and Nanocrystallites and Their Applications. Acc. Chem. Res. 2014, 47, 308-318.


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(2) Pengzuo, C.; Kun, X.; Zhiwei, F.; Yun, T.; Junchi, W.; Xiuli, L.; Xu, P.; Hui, D.; Changzheng, W.; Yi, X. Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. 2015, 127, 14923-14927. (3) Bai, S.; Wang, L.; Li, Z.; Xiong, Y. Facet-Engineered Surface and Interface Design of Photocatalytic Materials. Adv. Sci. 2017, 4, 1600216. (4) Li, J.; Singh, U. G.; Schladt, T. D.; Stalick, J. K.; Scott, S. L.; Seshadri, R. Hexagonal YFe1−xPdxO3−δ: Nonperovskite Host Compounds for Pd2+ and Their Catalytic Activity for CO Oxidation. Chem. Mater. 2008, 20, 6567-6576. (5) Xu, K.; Cheng, H.; Liu, L.; Lv, H.; Wu, X.; Wu, C.; Xie, Y. Promoting Active Species Generation by Electrochemical Activation in Alkaline Media for Efficient Electrocatalytic Oxygen Evolution in Neutral Media. Nano Lett. 2017, 17, 578-583. (6) Gupta, S.; Kellogg, W.; Xu, H.; Liu, X.; Cho, J.; Wu, G. Bifunctional Perovskite Oxide Catalysts for Oxygen Reduction and Evolution in Alkaline Media. Chem. Asian J. 2016, 11, 10-21. (7) Zhu, J.; Li, H.; Zhong, L.; Xiao, P.; Xu, X.; Yang, X.; Zhao, Z.; Li, J. Perovskite Oxides: Preparation, Characterizations, and Applications in Heterogeneous Catalysis. ACS Catal. 2014, 4, 29172940. (8) Royer, S.; Duprez, D.; Can, F.; Courtois, X.; Batiot-Dupeyrat, C.; Laassiri, S.; Alamdari, H. Perovskites as Substitutes of Noble Metals for Heterogeneous Catalysis: Dream or Reality. Chem. Rev. 2014, 114, 10292-10368. (9) Yuqiao, G.; Yun, T.; Pengzuo, C.; Kun, X.; Jiyin, Z.; Yue, L.; Wangsheng, C.; Zhenmeng, P.; Changzheng, W.; Yi, X. Engineering the Electronic State of a Perovskite Electrocatalyst for Synergistically Enhanced Oxygen Evolution Reaction. Adv. Mater. 2015, 27, 5989-5994. (10) Tong, Y.; Guo, Y.; Chen, P.; Liu, H.; Zhang, M.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Spin-State Regulation of Perovskite Cobaltite to Realize Enhanced Oxygen Evolution Activity. Chem 2017, 3, 812-821.


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(11) Royer, S.; Duprez, D. Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides. ChemCatChem 2011, 3, 24-65. (12) Mueller, D. N.; Machala, M. L.; Bluhm, H.; Chueh, W. C. Redox Activity of Surface Oxygen Anions in Oxygen-deficient Perovskite Oxides During Electrochemical Reactions. Nat. Commun. 2015, 6, 6097. (13) Grimaud, A.; Diaz-Morales, O.; Han, B.; Hong, W. T.; Lee, Y.-L.; Giordano, L.; Stoerzinger, K. A.; Koper, M. T.; Shao-Horn, Y. Activating Lattice Oxygen Redox Reactions in Metal Oxides to Catalyse Oxygen Evolution. Nat. Chem. 2017, 9, 457-465. (14) Huang, K.; Chu, X.; Feng, W.; Zhou, C.; Si, W.; Wu, X.; Yuan, L.; Feng, S. Catalytic Behavior of Electrospinning Synthesized La0.75Sr0.25MnO3 Nanofibers in the Oxidation of CO and CH4. Chem. Eng. J. 2014, 244, 27-32. (15) Liu, Y.; Dai, H.; Deng, J.; Li, X.; Wang, Y.; Arandiyan, H.; Xie, S.; Yang, H.; Guo, G. Au/3DOM La0.6Sr0.4MnO3: Highly Active Nanocatalysts for the Oxidation of Carbon Monoxide and Toluene. J. Catal. 2013, 305, 146-153. (16) Koo, J. Y.; Kwon, H.; Ahn, M.; Choi, M.; Son, J.-W.; Han, J. W.; Lee, W. Suppression of Cation Segregation in (La,Sr)CoO3−δ by Elastic Energy Minimization. ACS Appl. Mater. interfaces 2018, 10, 8057-8065. (17) Susman, M. D.; Pham, H. N.; Datye, A. K.; Chinta, S.; Rimer, J. D. Factors Governing MgO(111) Faceting in the Thermal Decomposition of Oxide Precursors. Chem. Mater. 2018, 30, 26412650. (18) Peng, Y.-K.; Tsang, S. C. E. Facet-dependent Photocatalysis of Nanosize Semiconductive Metal Oxides and Progress of Their Characterization. Nano Today 2018, 18, 15-34. (19) Chen, Q.; Jia, Y.; Xie, S.; Xie, Z. Well-faceted Noble-metal Nanocrystals with Nonconvex Polyhedral Shapes. Chem. Soc. Rev. 2016, 45, 3207-3220. (20) Huang, M. H.; Chiu, C.-Y. Achieving Polyhedral Nanocrystal Growth with Systematic Shape Control. J. Mater. Chem. A 2013, 1, 8081-8092. ACS Paragon Plus Environment

27


Page 28 of 33

(21) Liu, B.; Ma, L.; Ning, L.-C.; Zhang, C.-J.; Han, G.-P.; Pei, C.-J.; Zhao, H.; Liu, S.-Z.; Yang, H.Q. Charge Separation between Polar {111} Surfaces of CoO Octahedrons and Their Enhanced VisibleLight Photocatalytic Activity. ACS Appl. Mater. Interfaces 2015, 7, 6109-6117. (22) Huang, K.; Yuan, L.; Feng, S. Crystal Facet Tailoring Arts in Perovskite Oxides. Inorg. Chem. Front. 2015, 2, 965-981. (23) Yuan, L.; Huang, K.; Hou, C.; Feng, W.; Wang, S.; Zhou, C.; Feng, S. Hydrothermal Synthesis and Magnetic Properties of REFe0.5Cr0.5O3 (RE = La, Tb, Ho, Er, Yb, Lu and Y) perovskite. New J. Chem. 2014, 38, 1168-1172. (24) Hou, C.; Feng, W.; Yuan, L.; Huang, K.; Feng, S. Crystal Facet Control of LaFeO3, LaCrO3, and La0.75Sr0.25MnO3. CrystEngComm 2014, 16, 2874-2877. (25) Yuan, L.; Huang, K.; Wang, S.; Hou, C.; Wu, X.; Zou, B.; Feng, S. Crystal Shape Tailoring in Perovskite Structure Rare-Earth Ferrites REFeO3 (RE = La, Pr, Sm, Dy, Er, and Y) and ShapeDependent Magnetic Properties of YFeO3. Cryst. Growth Des. 2016, 16, 6522-6530. (26) Huang, K.; Feng, W.; Yuan, L.; Zhang, J.; Chu, X.; Hou, C.; Wu, X.; Feng, S. The Effect of NH4+ on Shape Modulation of La1−xSrxMnO3 Crystals in a Hydrothermal Environment. CrystEngComm 2014, 16, 9842-9846. (27) Mann, A. K. P.; Fu, J.; DeSantis, C. J.; Skrabalak, S. E. Spatial and Temporal Confinement of Salt Fluxes for the Shape-Controlled Synthesis of Fe2O3 Nanocrystals. Chem. Mater. 2013, 25, 15491555. (28) Mao, Y.; Park, T. J.; Zhang, F.; Zhou, H.; Wong, S. S. Environmentally Friendly Methodologies of Nanostructure Synthesis. Small 2007, 3, 1122-1139. (29) Tiano, A. L.; Santulli, A. C.; Koenigsmann, C.; Feygenson, M.; Aronson, M. C.; Harrington, R.; Parise, J. B.; Wong, S. S. Toward a Reliable Synthesis of Strontium Ruthenate: Parameter Control and Property Investigation of Submicrometer-Sized Structures. Chem. Mater. 2011, 23, 3277-3288. (30) Vradman, L.; Friedland, E.; Zana, J.; Vidruk-Nehemya, R.; Herskowitz, M. Molten Salt Synthesis of LaCoO3 Perovskite. J. Mater. Sci. 2017, 52, 11383-11390. ACS Paragon Plus Environment

28

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Wu, H.; Pan, S.; Poeppelmeier, K. R.; Li, H.; Jia, D.; Chen, Z.; Fan, X.; Yang, Y.; Rondinelli, J. M.; Luo, H. K3B6O10Cl: A New Structure Analogous to Perovskite with a Large Second Harmonic Generation Response and Deep UV Absorption Edge. J. Amer. Chem. Soc. 2011, 133, 7786-7790. (32) Xu,

T.;

Zhou,

X.;

Jiang,

Z.;

Kuang,

Q.;

Xie,

Z.;

Zheng,

L.

Syntheses

of

Nano/Submicrostructured Metal Oxides with All Polar Surfaces Exposed via a Molten Salt Route. Cryst. Growth Des. 2009, 9, 192-196. (33) Yu, S.; Liu, B.; Wang, Q.; Gao, Y.; Shi, Y.; Feng, X.; An, X.; Liu, L.; Zhang, J. Ionic Liquid Assisted Chemical Strategy to TiO2 Hollow Nanocube Assemblies with Surface-Fluorination and Nitridation and High Energy Crystal Facet Exposure for Enhanced Photocatalysis. ACS appl. Mater. interfaces 2014, 6, 10283-10295. (34) Read, M. S. D.; Saiful Islam, M.; Watson, G. W.; King, F.; Hancock, F. E. Defect Chemistry and Surface Properties of LaCoO3. J. Mater. Chem. 2000, 10, 2298-2305. (35) Khan, S.; Oldman, R. J.; Cora, F.; Catlow, C. R. A.; French, S. A.; Axon, S. A. A Computational Modelling Study of Oxygen Vacancies at LaCoO3 Perovskite Surfaces. Phys. Chem. Chem. Phys. 2006, 8, 5207-5222. (36) Wang, X.; Huang, K.; Qian, J.; Cong, Y.; Ge, C.; Feng, S. Enhanced CO Catalytic Oxidation by Sr Reconstruction on the Surface of LaxSr1−xCoO3−δ. Sci. Bull. 2017, 62, 658-664. (37) Crumlin, E. J.; Mutoro, E.; Liu, Z.; Grass, M. E.; Biegalski, M. D.; Lee, Y.-L.; Morgan, D.; Christen, H. M.; Bluhm, H.; Shao-Horn, Y. Surface Strontium Enrichment on Highly Active Perovskites for Oxygen Electrocatalysis in Solid Oxide Fuel Cells. Energy Environ. Sci. 2012, 5, 60816088. (38) van der Heide, P. A. W. Systematic X-ray Photoelectron Spectroscopic Study of La1-xSrx-based Perovskite-type Oxides. Surf. Interface Anal. 2002, 33, 414-425. (39) Li, X.; Dai, H.; Deng, J.; Liu, Y.; Xie, S.; Zhao, Z.; Wang, Y.; Guo, G.; Arandiyan, H. Au/3DOM LaCoO3: High-performance Catalysts for the Oxidation of Carbon Monoxide and Toluene. Chem. Eng. J. 2013, 228, 965-975. ACS Paragon Plus Environment

29


Page 30 of 33

(40) Chen, C.-Q.; Li, W.; Cao, C.-Y.; Song, W.-G. Enhanced Catalytic Activity of Perovskite Oxide Nanofibers for Combustion of Methane in Coal Mine Ventilation Air. J. Mater. Chem. 2010, 20, 69686974. (41) Yamada, Y.; Yano, K.; Hong, D.; Fukuzumi, S. LaCoO3 Acting as an Efficient and Robust Catalyst for Photocatalytic Water Oxidation with Persulfate. Phys. Chem. Chem. Phys. 2012, 14, 57535760. (42) McAlpin, J. G.; Surendranath, Y.; Dincǎ, M.; Stich, T. A.; Stoian, S. A.; Casey, W. H.; Nocera, D. G.; Britt, R. D. EPR Evidence for Co(IV) Species Produced During Water Oxidation at Neutral pH. J. Am. Chem. Soc. 2010, 132, 6882-6883. (43) Ivanova, S.; Zhecheva, E.; Stoyanova, R. Microstructure of LaCoO3 Prepared by Freeze-drying of Metal–citrate Precursors Revealed by EPR. J. Phys. Chem. Solids 2007, 68, 168-174. (44) Karvonen, L.; Valkeapää, M.; Liu, R.-S.; Chen, J.-M.; Yamauchi, H.; Karppinen, M. O-K and Co-L XANES Study on Oxygen Intercalation in Perovskite SrCoO3-δ. Chem. Mater. 2010, 22, 70-76. (45) Wang, X.; Huang, K.; Yuan, L.; Xi, S.; Yan, W.; Geng, Z.; Cong, Y.; Sun, Y.; Tan, H.; Wu, X.; Li, L.; Feng, S. Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation. J. Phys. Chem. Lett. 2018, 9, 4146-4154. (46) Lu, N.; Zhang, P.; Zhang, Q.; Qiao, R.; He, Q.; Li, H. B.; Wang, Y.; Guo, J.; Zhang, D.; Duan, Z.; Li, Z.; Wang, M.; Yang, S.; Yan, M.; Arenholz, E.; Zhou, S.; Yang, W.; Gu, L.; Nan, C. W.; Wu, J.; Tokura, Y.; Yu, P. Electric-field Control of Tri-state Phase Transformation with a Selective Dual-ion Switch. Nature 2017, 546, 124-128. (47) Imamura, M.; Matsubayashi, N.; Shimada, H. Catalytically Active Oxygen Species in La1xSrxCoO3-δ

Studied by XPS and XAFS Spectroscopy. J. Phys. Chem. B 2000, 104, 7348-7353.

(48) Toulemonde, O.; N'Guyen, N.; Studer, F.; Traverse, A. Spin State Transition in LaCoO3 with Temperature or Strontium Doping as Seen by XAS. J. Solid State Chem. 2001, 158, 208-217.


30

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(49) Hong, W. T.; Gadre, M.; Lee, Y.-L.; Biegalski, M. D.; Christen, H. M.; Morgan, D.; Shao-Horn, Y. Tuning the Spin State in LaCoO3 Thin Films for Enhanced High-Temperature Oxygen Electrocatalysis. J. Phys. Chem. Lett. 2013, 4, 2493-2499. (50) May, K. J.; Carlton, C. E.; Stoerzinger, K. A.; Risch, M.; Suntivich, J.; Lee, Y.-L.; Grimaud, A.; Shao-Horn, Y. Influence of Oxygen Evolution during Water Oxidation on the Surface of Perovskite Oxide Catalysts. J. Phys. Chem. Lett. 2012, 3, 3264-3270. (51) Rousseau, S.; Loridant, S.; Delichere, P.; Boreave, A.; Deloume, J. P.; Vernoux, P. La(1−x)SrxCo1−yFeyO3 Perovskites Prepared by Sol–gel Method: Characterization and Relationships with Catalytic Properties for Total Oxidation of Toluene. Appl. Catal. B Environ. 2009, 88, 438-447. (52) Duan, Y.; Sun, S.; Xi, S.; Ren, X.; Zhou, Y.; Zhang, G.; Yang, H.; Du, Y.; Xu, Z. J. Tailoring the Co 3d-O 2p Covalency in LaCoO3 by Fe Substitution To Promote Oxygen Evolution Reaction. Chem. Mater. 2017, 29, 10534-10541. (53) Cheng, X.; Fabbri, E.; Nachtegaal, M.; Castelli, I. E.; El Kazzi, M.; Haumont, R.; Marzari, N.; Schmidt, T. J. Oxygen Evolution Reaction on La1–xSrxCoO3 Perovskites: A Combined Experimental and Theoretical Study of Their Structural, Electronic, and Electrochemical Properties. Chem. Mater. 2015, 27, 7662-7672. (54) Mefford, J. T.; Rong, X.; Abakumov, A. M.; Hardin, W. G.; Dai, S.; Kolpak, A. M.; Johnston, K. P.; Stevenson, K. J. Water Electrolysis on La1-xSrxCoO3-δ Perovskite Electrocatalysts. Nat. Commun. 2016, 7, 11053. (55) Haggerty, R. P.; Seshadri, R. Oxygen Stoichiometry, Crystal Structure, and Magnetism of La0.5Sr0.5CoO3 J. Phys. Condens. Matter 2004, 16, 6477-6484. (56) Segall, M. D.; Philip, J. D. L.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. First-principles Simulation: Ideas, Illustrations and the CASTEP Code. J. Phys. Condens. Matter 2002, 14, 2717. (57) Vanderbilt, D. Soft Self-consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892-7895. ACS Paragon Plus Environment

31


Page 32 of 33

(58) Sun, L.; Li, G.; Chen, W.; Hu, J. Adsorption of CO on the Oxygen Defective LaCoO3 (001) Surface: A First-principles Study. Comput. Mater. Sci. 2016, 115, 154-157.


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Molten Salt Flux Synthesis, Crystal Facet Design, Characterization

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