Fabrication and Structural Characterization of an Ultrathin Film of a

Communication pubs.acs.org/IC

Fabrication and Structural Characterization of an Ultrathin Film of a Two-Dimensional-Layered Metal−Organic Framework, {Fe(py)2[Ni(CN)4]} (py = pyridine) Shun Sakaida,† Tomoyuki Haraguchi,†,‡ Kazuya Otsubo,*,† Osami Sakata,§ Akihiko Fujiwara,⊥ and Hiroshi Kitagawa*,†,∥,¶ †

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Sayo-gun, Hyogo 679-5148, Japan ⊥ Department of Nanotechnology for Sustainable Energy, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ∥ Institute for Integrated Cell-Material Science, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ¶ INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-3095, Japan §

S Supporting Information *

dimensional (2-D) sheets that consist of the Fe2+ ion coordinated by the square-planar tetracyanometallate ion, and the 2-D sheets are stacked by a N-terminated ligand coordinating to Fe2+ axial positions. Structural dimensionality originates from pillar ligands: 2-D sheets are connected by π−π interactions between adjacent py ligands with a distance of 3.7 Å (see Figure 1a), while pz bridges the sheets to provide a three-dimensional (3-D) structure. To date, the use of a MOF as a thin film has been intensively investigated to utilize its various properties for device applications, such as sensing, catalysis, gas membrane, and optoelectronic devices.20−23 Several methods have been proposed for the fabrication of a MOF thin film, such as spin coating,24 electrochemical techniques,25,26 an interfacial growth approach,27 chemical vapor deposition,28 a modular assembly method,29 and metal hydroxide surface conversion.30 One obstacle to overcome during the deposition is controlling the crystal growth direction to align the channel direction in MOFs. In the bottom-up approach, deposition of a MOF on a surface is achieved by alternating soaking in solutions of building blocks (the layer-by-layer method,31−37 LbL). We recently reported the LbL fabrication of a Hofmann-type 3-D MOF,38,39 where the crystal orientation was successfully controlled, as confirmed by crystallographic techniques. On the other hand, for a 2-D Hofmann-type MOF, a few examples that have been reported to date mention a preferentially oriented thin film.40,41 Here we report the first example of the thin-film fabrication and characterization of a 2-D tetracyanonickelate-based MOF, {Fe(py)2Ni(CN)4} (film-Nipy). The deposition of film-Nipy on a metal surface was carried out using the LbL method, as shown in Figure 1b. A gold/ chromium/silicon substrate annealed in a H2 burner was soaked overnight in an ethanol (EtOH) solution of 4-mercaptopyridine to construct the self-assembled monolayer (SAM). This surfacemodified substrate was alternately soaked in two EtOH solutions, for 1 min each, at room temperature (rt; comprising 1 cycle) for

ABSTRACT: We report the fabrication and characterization of the first example of a tetracyanonickelate-based two-dimensional-layered metal−organic framework, {Fe(py)2Ni(CN)4} (py = pyridine), thin film. To fabricate a nanometer-sized thin film, we utilized the layer-by-layer method, whereby a substrate was alternately soaked in solutions of the structural components. Surface X-ray studies revealed that the fabricated film was crystalline with well-controlled growth directions both parallel and perpendicular to the substrate. In addition, lattice parameter analysis indicated that the crystal system is found to be close to higher symmetry by being downsized to a thin film.

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ver the past decade, porous coordination polymers or metal−organic frameworks (MOFs)1−4 have been intensively exploited for their potential use in switching devices, induced by external stimuli including light,5 pressure,6 temperature,7 and guest uptake.8,9 With their tremendous variations in metal ions and organic linkers, crystalline MOFs possess tunable porosity with high surface area, which is promising for versatile functional materials. Recently, cyanide-bridged Hofmann-type MOFs have attracted considerable attention because of the spin-crossover phenomenon derived from the reversible change in the Fe2+ spin states between the 1A1 (t2g6) and 5T2 (t2g4eg2) configurations.10−14 Combined with the adsorption properties of MOFs with tailored porosity, a chemoresponsive spin-state switching has been investigated based on host−guest interactions between the framework and absorbent molecules.15−17 In the exploration of the structural diversity of building blocks, much effort has been made to modify the polymeric structure and magnetic properties. Among the Hofmann-type MOFs, {Fe(py)2MII(CN)4} and {Fe(pz)MII(CN)4} (bulk-Mpy and bulk-Mpz; py = pyridine; pz = pyrazine; M = Ni, Pd, Pt) are well-known as target compounds in discussions on the framework dimensionality and cooperative spin-crossover behavior.18,19 Both compounds include two© 2017 American Chemical Society

Received: May 3, 2017 Published: June 29, 2017 7606

DOI: 10.1021/acs.inorgchem.7b01113 Inorg. Chem. 2017, 56, 7606−7609

Communication

Inorganic Chemistry

Figure 2. LbL crystal growth of film-Nipy confirmed using IRRAS monitoring, focused on the stretching mode of the cyano group. Inset: Maximum intensity of the absorption band versus cycle number plot indicating a linear increase in the intensity during the repeated cycles. The red dashed line is a guide for the eyes.

reported for film-Ptpy.40 From these observed peaks, the lattice parameters of film-Nipy were calculated as follows: a = 15.16(3) Å, b = 7.22(1) Å, c = 7.14(2) Å, β = 91(1)°, and V = 783(3) Å3. According to the literature,13 the lattice parameters of bulk-Nipy at rt are as follows: a = 15.526(3) Å, b = 7.392(2) Å, c = 7.067(2) Å, β = 101.26(2)°, and V = 795.5(6) Å3. The lattice volume shrinkage (1.6%) when downsized to a thin film has also been reported in 3-D Hofmann-type MOFs 39 (3−7%). The remarkable shrinkage was explained by the crystal downsizing effect and the strain from heterointerface mismatch between the SAM and MOF 2-D layer. In addition, although bulk-Nipy has monoclinic symmetry (C2/m; see Figure 1), the lattice parameters for film-Nipy are close to those of a thin film of a 2-D Hofmann-type MOF film-Ptpy, which belongs to a higher symmetric space group40 [orthorhombic, Cmmm, a = 7.304(3) Å, b = 15.196(7) Å, and c = 7.377(4) Å]. We also estimated the average film thickness using the Scherrer equation, applied to the 200 peak at 2θ = 11.68°. The estimated value from the 200 peak with a fwhm of 0.55° is approximately 14.5 nm, and this value divided by the cycle number 20 is ∼7.3 Å. Hence, it can be concluded that LbL processing for 1 cycle corresponds to monolayer construction [7.581 Å given by half of a cos(β − 90°), i.e., the distance between the neighboring layers]. These results demonstrate that a highly oriented crystalline thin film was successfully fabricated using the LbL self-assembly. To demonstrate a specific response to the guest in the thin-film state, we collected the adsorption isotherm at rt for bulk-Nipy (Figure S4) and in situ XRD measurement on film-Nipy under EtOH vapor (Figure S5). Although the bulk-Nipy shows no EtOH uptake, a reversible interlayer distance change by EtOH vapor introduction was observed in film-Nipy. This result is similar to the previously reported behavior of the film-Ptpy thin film,40 indicating a crystal-downsizing effect in switching the guest-responsive property. In summary, we have reported the thin-film fabrication and structural characterization of a 2-D Hofmann-type framework, film-Nipy. The use of a simple LbL procedure at ambient conditions allows us to obtain a thin film with high crystallinity

Figure 1. (a) Crystal structure of bulk-Nipy redrawn from the literature:13 monoclinic, C2/m, a = 15.526(3) Å, b = 7.392(2) Å, c = 7.067(2) Å, β = 101.26(2)°, V = 795.5(6) Å3. Color code: C, gray; N, blue; Fe, red; Ni, green. Hydrogen atoms are omitted for clarity. (b) Schematic illustration of LbL film fabrication. After SAM modification, the self-assembly of building block molecules was used to form a thin film deposited on a gold/chromium/silicon substrate by a repeated immersion method.

up to 20 cycles (solution of 25 mM Fe(BF4)2·6H2O and 100 mM pyridine; solution of 25 mM [(C4H9)4N]2Ni(CN)4 and 100 mM pyridine). After each soaking, the surface was rinsed with EtOH. To monitor the step-by-step growth of the thin film over 20 cycles, infrared reflection absorption spectroscopy (IRRAS) data were recorded after every 5 cycles (Figure 2). A linear increase in the intensity of the cyano group stretching mode at 2166 cm−1 indicated successful LbL growth on the surface. The fabrication of film-Nipy was also characterized by IRRAS (Figure S1) and Raman spectroscopy (Figure S2); the results were compared with those of bulk-Nipy. To confirm the crystal structure of film-Nipy, we carried out surface X-ray diffraction (XRD) measurements both in the horizontal direction (in-plane scan; grazing-incidence mode) and in the vertical direction (out-of-plane scan; θ − 2θ mode), as shown in Figure 3. In the out-of-plane scan, the two peaks observed at 2θ = 11.68° and 23.45° were indexed as 200 and 400 (Figure 3a, black). For the in-plane scan, diffraction peaks indexed as 0kl from the periodic 2-D layer could be ideally observed (Figure 3b, green), and 001, 020, 002, 021, and 022 peaks were confirmed in the experimental data (Figure 3b, black). The peak observed at around 2θ = 18° cannot be attributed to the 0kl peaks (indexed as 11−1). To further investigate the crystal orientation in the thin film, we measured the rocking curve and azimuthal angle dependence for the 200 position (Figure S3). The rocking curve peak with a full width at half-maximum (fwhm) of 5.6° suggested that the 11−1 peak is derived from layer tilting relative to the substrate, as previously 7607

DOI: 10.1021/acs.inorgchem.7b01113 Inorg. Chem. 2017, 56, 7606−7609

Inorganic Chemistry

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Communication

AUTHOR INFORMATION

Corresponding Authors

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

Shun Sakaida: 0000-0002-4172-1598 Kazuya Otsubo: 0000-0003-4688-2822 Osami Sakata: 0000-0003-2626-0161 Hiroshi Kitagawa: 0000-0001-6955-3015 Present Address ‡

T.H.: Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 1628601, Japan. Author Contributions

K.O. and H.K. designed this work. S.S. prepared thin-film samples. S.S. and T.H. performed the experiments. O.S. and A.F. contributed to the synchrotron XRD measurements. S.S., K.O., and H.K. cowrote the manuscript. All authors discussed the results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology and ACCEL from the Japan Science and Technology Agency, Grants-in-Aid for Scientific Research (A) (Grants 23245012 and 26248019), a Grant-in-Aid for Young Scientists (B) (Grant 25810039), and a Grant-in-Aid for Young Scientists (A) (Grant 15H05479) from the Japan Society for the Promotion of Science. Synchrotron XRD measurements were supported by the Japan Synchrotron Radiation Research Institute (Proposals 2015A1489, 2016A1421, and 2016B1435).

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Figure 3. XRD profiles at rt for film-Nipy collected in (a) out-of-plane and (b) in-plane geometries, respectively. In each panel were plotted experimental data (black), fitting curves (red), the simulated pattern from the bulk-Nipy structure (green), and Bragg peaks (black +). Inset: Schematic representation of the experimental setup for each XRD measurement and periodic structures. *: assigned to diffraction peaks from the substrate.

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and well-controlled orientation, as confirmed by surface XRD experiments. Furthermore, the lattice parameter analysis revealed that the crystal system gets closer to higher symmetry when downsized to a thin film. On the basis of the results presented here, further work is currently in progress to investigate the MOF properties in the thin-film state,42 including further analysis of the guest sorption behavior derived from its regular porosity, the cooperative spin-crossover phenomenon in a networked structure, and other properties.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01113. Details of the experimental methods and IRRAS, Raman, and additional XRD data (PDF) 7608

DOI: 10.1021/acs.inorgchem.7b01113 Inorg. Chem. 2017, 56, 7606−7609

Communication

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DOI: 10.1021/acs.inorgchem.7b01113 Inorg. Chem. 2017, 56, 7606−7609