Fighting at the Interface: Structural Evolution during Heteroepitaxial

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Fighting at the Interface: Structural Evolution during Heteroepitaxial Growth of Cyanometallate Coordination Polymers Fengqiong Li,†,⊥ Wei Zhang,†,⊥ Arnau Carne-́ Sań chez,‡ Yoshihiro Tsujimoto,§ Susumu Kitagawa,‡ Shuhei Furukawa,*,‡ and Ming Hu*,† †

State Key Laboratory of Precision Spectroscopy (ECNU), East China Normal University, Shanghai 200241, China Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan § National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan Downloaded via UNIV OF READING on July 20, 2018 at 08:47:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

through the redox reaction of metal nodes, which provides the opportunity to investigate the guest transportation in the hybrid.34,35 In this study, two types of PBAs are selected: Na1.2FeIII0.93[FeII(CN)6] (1) as the core crystal and Na1.76Ni[Fe(CN)6]0.94 (2) as the shell crystal. Framework 1 has a facecentered cubic structure (Fm3̅m, a = 10.238 Å, JCPDS card 77-1161) (Figure S1), whereas framework 2 has a rhombohedral structure (R3̅, a = b = 7.350 Å, c = 17.300 Å) (Figure S2).36 To consider the possibility for epitaxy, we compare the lattice structure of different facets of frameworks 1 and 2 (Figure 1a). By clockwise rotating the crystal of 2 around the [11̅2] axis by 45.751° (Figure 1a), the lattice structures of the exposed facets of framework 2 are close to the facets of framework 1. According to Figure 1b, the lattice misfits between the (11̅2), (012), (1̅02) facets of 2 and the (200), (002), (020) facets of 1 are all close to −0.22%, which are significantly smaller than the misfit in other epitaxy systems,24 indicating that pseudomorphical epitaxy of 2 on 1 can be possible by clockwise rotation of 2. We synthesized cubic nanoparticles of 1 with the statistic sizes of ∼50 ± 8 nm confirmed by scanning electron microscopy (SEM) image (Figure S3a). By using these particles as seeds, the crystals of 2 are deposited on 1 via a kinetics-controlled crystallization method.37 The SEM image revealed that the obtained particles became 61 ± 10 nm while maintaining the cubic morphology (Figure S3b), indicating successful deposition of 2 on all six facets of 1. The transmission electron microscopy (TEM) image (Figure 2a1) shows different contrast between the outer and inner parts, clearly confirming the generation of core−shell structure with a thickness of the shell of ∼11 ± 2 nm. We note that the size of the core crystals is slightly reduced (48 ± 8 nm), mainly because of a slight etching of the core crystal in the solution of the reactants. The core−shell structure was also indicated by the analysis of the energy-dispersive X-ray (EDX) spectroscopy. Dominance of nickel atoms at the outermost layer (Figure 2a-3) matches with the TEM image, confirming that framework 2 is the shell. The valence information on the core−shell crystals determined by Mössbauer spectroscopy suggests the increase of the fraction of FeII, indirectly confirming the deposition of 2 on 1 (Figure S4). According

ABSTRACT: Hybridization of coordination polymers allows for combining two or more distinct structures into one material. Here, we explore the core/shell-type materials of Prussian blue analogues (PBAs) by heteroepitaxial growth and demonstrate how one phase structurally dominates the other. The volumetric ratio between the shell and core crystals determine the final structure of the hybrids. The outermost dominated the Na+ ion insertion/extraction, illustrating how the hybridization can adjust the function of PBAs.

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ybridization of crystalline solids is of great significance in developing materials with synergistic functions, which is not accessible in individual solids.1,2 Heteroepitaxial growth has been acknowledged for growing a single-crystalline solid layer on top of another crystalline solid layer.3,4 During the heteroepitaxial growth, the lattice misfit between the substrate and deposited layers triggers distortion of the interfacial structure, which strongly influences the physical property of hybrids.5,6 In most inorganic materials systems, distorted structures are confined in a minimal volumetric fraction of the surface layer at the interface due to the structural rigidity of the metal oxides or semiconductors.7 Here, we focused on coordination polymers with threedimensional framework structures, which are recognized as an emerging class of porous crystalline solids with great structural and functional feasibility.8−10 Many of the coordination polymers can deform their structures in response to external stimuli.11−16 When the flexible coordination polymers grow on substrates, the growing layers are strongly affected by the substrates and exhibit novel properties and function.17−24 In this work, we report an unprecedented structural evolution during the heteroepitaxial growth. The interfacial structural distortion can result in the change of the substrate, inducing a single-crystal to single-crystal transition. Prussian blue analogues (PBAs), a series of cyanometallate coordination polymers, are selected for this study because of the following reasons: First, most PBAs have close lattice parameters that allow for heteroepitaxial growth on top of each other.25−30 Second, the frameworks are flexible upon external stimuli.31−33 Third, their three-dimensional structures demonstrate a reversible insertion/extraction of alkaline metal ions © XXXX American Chemical Society

Received: April 11, 2018

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DOI: 10.1021/acs.inorgchem.8b00959 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

deposited shell crystal (2) became Fm3̅m structure despite the fact that the directly synthesized crystal (2) is R3̅. The exposed faces of the core−shell crystals are the {200} faces, revealing that the shell crystal (2) grows on the {200} faces of the core crystal (1). The thicknesses of the shells of the obtained crystals were increased to 18 ± 2 nm and 25 ± 2 nm, by changing the concentrations of the reactants for the deposition of 2 (please see Supporting Information for the experiment details, Figure S3c,d). We named the two samples as 1@218 and 1@225 to facilitate subsequent discussion. The 1@218 shows similar structure as that of 1@211 (Figure S5), whereas 1@225 exhibits a significant difference (Figure 2b). The SAED taken from a particle of 1@225 presents a diffraction pattern that can be indexed to be R3̅, suggesting that the structures of the core− shell crystals are converted to R3̅ despite the core crystal (1) being Fm3̅m structure before deposition. Because of the structure change, the exposed faces became (11̅2), (1̅12̅), (012), (01̅2̅), (1̅02), (102̅), which are also the interfaces between the core and shell. The phase structures of the 1@2x core−shell particles were further analyzed by powder X-ray diffraction (PXRD). Each phase in the physical mixture of 1 and 2 can be distinguished according to the peaks at 2θ near 24−25° (Figures S6a−c and S7). The peak at 24.80° belongs to the 220 Bragg position of 1, whereas the other two peaks at 24.67° and 24.91° are from the 110 and 014̅ positions of 2 (Figures S6c and S7). Unlike the physical mixture of 1 and 2, both the 1@211 and 1@218 particles have similar diffraction patterns corresponding to Fm3̅m structure (Figures S6d and e, S8, and S9). This result suggests that the 1@211 and 1@218 particles are of a singlephase corresponding to pure 1, which is consistent with the analysis of SAED. Remarkably, the diffraction peaks of the 1@ 225 particles match with that of pure crystal 2 and thus are fitted to R3̅ structure (Figures S6f and S10). This result indicates that the structures of the core−shell particles change to the structure of pure 2 when the shell has a thickness of 25 nm. Such single-phase structure transition observed for the 1@ 2x particles does not seem compatible with the structural prediction that a clockwise rotation of 2 on 1 expects. In contrast to our experimental results, the previously reported core−shell PBA particles were found to have double lattice groups reflecting the structures of core and shell crystals.34 The difference between the previous reports and our experiment may be caused by different crystallization pathways. In most cases, the crystallization of PBAs is not carried out via an ionby-ion or atom-by-atom pathway38−40 but rather by an aggregation-fusion route38−40 (Figure S11). This crystallization habit leads to discontinuous connection between the core and shell, leaving double lattice structures (Figure S12).41 In this work, the added sodium citrate can chelate with metal ions, slow the crystallization speed of PBAs, and then force the shell crystal to deposit via an ion-by-ion pathway,37 leading to defect-free/less interfaces. In this case, the shell/core can be directed by each other, resulting in single-phase hybrids. According to the first-principal calculation (please see the calculation details in the Supporting Information), the framework of 2 with an Fm3̅m structure is less stable than the R3̅ phase (Figure S13). However, because the core crystal 1 is of an Fm3̅m structure, shell crystal 2 is guided into a metastable Fm3̅m structure (Figure S13a). The phase change tendency of shell crystal 2 triggers a competition between the

Figure 1. (a) Scheme of the crystal structure of crystals 1 and 2 as indicated by green and orange color, respectively. By rotation of crystal 2, the two structures are comparable. (b) Projection view of the structure of various facets of crystals 1 and 2 after rotation of crystal 2. For all of the structures, Na, H, and O atoms are omitted.

Figure 2. (a) TEM image, SAED pattern, and EDX line scanning profile of 1@211. The blue curve represents iron, and the red curve represents nickel. (b) Characterization of 1@225.

to the thickness of the shell, the core−shell particles are named 1@211. For the epitaxy of the core−shell particles to be understood, selected area electron diffraction (SAED) was taken from a whole core−shell particle (Figure 2a-2). The diffraction spots can be indexed as Fm3̅m structure (a = 10.238 Å), which is very similar to the structure of 1. Surprisingly, no rhombohedral structure can be detected, suggesting that the B

DOI: 10.1021/acs.inorgchem.8b00959 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry shell and core. Shell 2 gives the tensile force (F2→1) to core 1, whereas core 1 offers a counter force (F1→2) to shell 2 (Figure S13c). The strength of the competitive forces is correlated to the number of unit cells, which is determined by the volume of the crystals. For the 1@211, 1@218, and 1@225 samples, the shell-to-core volumetric ratios are 2:1, 4:1, and 7:1, respectively. F2→1 becomes larger with increasing shell thickness. When F1→2 > F2→1, shell crystals 2 yield to the core crystals 1, keeping the Fm3̅m phase. When the shell crystals 2 grow larger until F1→2 < F2→1, the core crystals yield to the shell crystals, forming the R3̅ phase. The shell crystal governs electrochemical Na-ion intercalation of the single-crystalline core−shell structure according to the PBA/Na coin battery test. The discharge/charge curves of the crystals of 2 are different from the crystals of 1 because the Ni2+ ions in the crystals of 2 are inactive (Figure S14a,b). The 1 particles have two discharge/charge plateaus at 3.5 and 2.8 V vs Na/Na+, whereas 2 has only one plateau at 3.2 V vs Na/ Na+. When 1 and 2 are mixed physically, the galvanostatic discharge/charge curves show three plateaus at 2.8, 3.2, and 3.5 V vs Na/Na+, and no synergistic effect can be observed. In contrast, the discharge/charge curves and capacity of all the core−shell particles are similar to those of pure 2. Similar change is also found in the cyclic voltammograms (CV) of all the samples (Figure S15). The basic shape of the CV curves of the core−shell particles are more close to the curve of the 2 crystals. However, as the shell becomes thinner, an additional current peak at 3.0 V can be observed and becomes clearer. Figure S16 illustrates the possible transportation of Na+ ions at the interface between the PBA crystal and electrolytes. For the core−shell hybrids, only the shell crystal can contact with the electrolytes. Therefore, the Na+ ions must be filtered by the frameworks of the shell first (Figure S16c); as a result, the Na+ ions that can be delivered should be the same as shell crystal 2. Apparently, the current voltage should also be controlled by the redox potential of the surface layer, which means that the shape of the CV curves of the composite should be the same as the shell crystal. However, as demonstrated by the phase evolution of the core−shell crystal, a strong interfacial competition affects the shell crystal, especially when the shell is thin. Therefore, the redox potential of the shell should be interfered by the core crystals, leading to partial change of the potential of the redox sites on the shell. Therefore, an additional peak emerges at ∼3.0 V, suggesting a composite effect related with the unique core−shell competition phenomenon. In summary, we found a unique core−shell competition during heteroepitaxial growth of PBAs. The competition led to core−shell PBAs with single phases. Depending on the volume ratio of the shell to core, the phase structure of the shell or core could be directed by the other. Furthermore, the connected core and shell frameworks presented an outtermost layer governed Na+ ion intercalation behavior, demonstrating that the performance of PBAs could be tailored by hybridization.

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Mössbauer spectrum of the pure crystal 1, pure crystal 2, physical mixture of 1 and 2, 1@211, 1@218, and 1@225 samples. TEM image and SAED pattern of the 1@218 sample (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yoshihiro Tsujimoto: 0000-0003-2140-3362 Shuhei Furukawa: 0000-0003-3849-8038 Ming Hu: 0000-0002-5024-5650 Author Contributions ⊥

F.L. and W.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21473059). REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00959. Experimental section, PXRD, SEM, Galvanostatic charge/discharge curves, and CV curves of the PBAs. C

DOI: 10.1021/acs.inorgchem.8b00959 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00959 Inorg. Chem. XXXX, XXX, XXX−XXX