Surface nano-architecture of a metal2013organic framework

Surface nano-architecture of a metal2013organic framework

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1.646 nm c d

C N O Co

1.646 nm ππ

Figure 4 | In-plane synchrotron XRD patterns and the derived structural model for NAFS-1. a, Observed (filled circles) and calculated (calc. 1, red line: structural model with consideration of the metal-coordinated pyridine molecules and the interlayer stacking order, as illustrated in c–e; calc. 2, green line: structural model with no consideration of metal-coordinated pyridine molecules; calc. 3, blue line: structural model with no consideration of the interlayer stacking order) in-plane synchrotron XRD profiles (2θ-scan, 2θ-step =0.1◦, λ=1.554Å) for a NAFS-1 thin film (20 deposition cycles) on a Si(100) substrate (see also Supplementary Fig. S11). The inset shows a selected region of the observed and calculated diffraction profiles together with a schematic representation of the scattering geometry in this experiment. Higher statistics fine scans were collected in the vicinity of the seven observed diffraction peaks (Supplementary Fig. S7). b, Selected region of the diffraction profiles in the vicinity of the (200), (120) and (220) Bragg reflections showing the good agreement between the observed pattern (filled circles, background subtracted) and that simulated (calc. 1) when the coordinated pyridine molecules responsible for the well-ordered sheet stacking in NAFS-1 are included in the structural model (c–e). On the other hand, the absence of the (200) Bragg reflection in calc. 2—clearly observed in the experimental pattern—supports the existence of axially coordinated pyridine molecules in NAFS-1. In addition, a pronounced intensity for the (220) peak—not observed in the experimental pattern—is predicted when the ordered sheet stacking is absent (calc. 3). c, Basal plane projection of the proposed crystalline structure for NAFS-1. C atoms are shown in grey, N in blue, O in red, Co2+ ions in pale pink and Cu2+ in green. d,e, Schematic diagrams of the proposed crystalline structure for NAFS-1 viewed along the a and b axis, respectively. The axially coordinated monodentate pyridine molecules define isolated 2D sheets of thickness 1.235nm that stack along the c axis with an interlayer spacing of 0.938nm. The interdigitated layer stacking can be driven by attractive pi–pi interactions between adjacent Cu2+-coordinated pyridine molecules that protrude from the 2D sheets (marked by the pink dotted lines, see also Supplementary Fig. S9b).

by selective gas adsorption. Such gate-opening hybrids have been attracting much attention for potential applications in gas sensing and separation, or low-pressure gas storage4. The unique integration of the layer-by-layer growth technique coupled with the Langmuir–Blodgett method developed here is found to be a superior method to create crystalline MOF nanofilms endowed with gate-opening potential. In future work, we will investigate the gate-opening characteristics of NAFS-1 by in situ

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LETTERS NATUREMATERIALSDOI:10.1038/NMAT2769 monitoring of the structural changes on gas adsorption. Finally, our film growth strategy is sufficiently versatile to open the way to obtain ordered heterostructures by stacking different kinds of individual layers. Fabrication of heterostructures with suitable well-defined smooth junctions (interfaces) is key to obtaining integrated device systems. Integration is necessary in highly efficient devices not only where each constituent layer has different roles with respect to separation, storage and reactivity, but also when their functions couple to each other, as seen in biological cells.

Methods

Materials. CoTCPP was purchased from Porphyrin Systems. CuCl ·2H O (>9.9%) was purchased from Kanto Chemical. Pure grades of pyridine, chloroform and methanol were purchased from Waco Pure Chemical Industries or JunseiChemical.Allchemicalswereusedas-receivedwithoutfurthertreatment.

Substrate preparation. Silicon single crystals ((100) surface plane) were used as substrates in XRD and AFM measurements. Amorphous quartz was used as a substrate for ultraviolet–visible absorption spectroscopic measurements. The Si(100) crystals were cut into 11mm×11mm×2mm (thickness) sizes. The miscut angle of the (100) plane was less than 0.1 . The surface was polished to obtain a macroflatness of <0.01 and a microroughness (r.m.s.) of <5Å. Before the film fabrication, the substrates were immersed successively in ultrasonic baths of chloroform, acetone and ethanol, each for 30min.

Film preparation. CoTCPP, 1 (8.5mg) and 20µl of pyridine, 2 were dissolved together in 50ml of mixed chloroform/methanol solvent (3:1, v/v). A polytetrafluoroethylene Langmuir trough (375×75×5mm, 0.16l) was filled with 0.1MCuCl ·2H O, 3 aqueous solution as a subphase. The surface of the subphase was carefully cleaned by mild surface-touch vacuuming. The CoTCPP/pyridine solution (96µl) was spread onto the CuCl subphase with a microsyringe. Surface-pressure/area (pi–a) isotherm measurements were carried out with a KSV minitrough system using a continuous pressing speed for two barriers of 10mmmin . The 2D array of CoTCPP-py-Cu at a surface pressure of 5mNm was deposited onto the substrate by the horizontal dipping method at room temperature (process 1). The substrate with the CoTCPP-py-Cu sheet was then rinsed with flowing distilled water, immersed in distilled water for 3 min and finally dried by blowing nitrogen (process 2). To stack further layers, CoTCPP-py-Cu monolayers on the subphase were repeatedly transferred onto the substrate. The number of layers deposited is controlled by the number of cycles of sheet deposition (process 1) and rinsing/solvent immersion/drying (process 2).

The same CoTCPP/pyridine solution (160µl) was spread onto a pure water subphase for comparison. The two barriers were compressed at the same speed of 10mmmin and the 2D array was again transferred onto the substrate by the horizontal dipping method.

Ultraviolet–visible absorption spectra. Ultraviolet–visible absorption spectra of the CoTCPP-py-Cu sheets on the quartz substrate were measured with a Hitachi U-3310 spectrophotometer or a Jasco V-570 spectrophotometer at room temperature.

X-ray photoelectron spectra. XPS spectra were collected at room temperature on a PHI 5800 ESCA system (Physical electronics), equipped with a monochromatized Al Kα X-ray source of 1,486.6eV. The binding energies were calibrated with the C 1s signal (284.5eV).

Infrared spectra. Infrared spectra were collected on a JASCO FT-IR 620 spectrometer under vacuum conditions at room temperature. A transmission method was applied for the film-state samples fabricated on Si substrate and an attenuated total reflectance method was applied for the bulk-state samples with a germanium prism.

Synchrotron XRD measurements. Synchrotron XRD data were collected at room temperature with a multiaxis diffractometer at beamline BL13XU (λ=1.554Å) in SPring-8. Helium gas was supplied through the cell during the measurement. Each data set was recorded using a scintillation counter.

Out-of-plane scans were carried out in a θ−2θ scattering geometry. Rocking curve (θ-scan) and sample direction dependence (azimuthal angle, φ, scan) measurements were carried out at the (001) peak position in the out-of-plane geometry. X-ray reflectivity data were collected at the glancing angular range by a θ−2θ-scan in the out-of-plane geometry.

In the grazing-incident XRD (GIXRD) measurements, strong scattered intensity is observed when the X-ray incident angle (α) to the sample is below a critical angle. The GIXRD measurements were carried out at α =0.1 . Diffraction from the sample surface was detected in the in-plane direction with a detection angle to the surface of 0.1 . A solar slit (0.4 ) was placed between the sample and the detector to reduce background contribution.

Structuralmodelconstruction. A tetragonal unit cell with basal plane dimensions, a=b=1.6460(3)nm was determined from the seven observed Bragg diffraction peaksintheGIXRDexperiments.ThestructuralmodelforNAFS-1wasconstructed using the Materials Studio (Accelrys) software. We first introduced as a starting model the in-plane structure of bulk MOFs built by MTCPP (M = metal ion) units and adjusted it to conform to the stoichiometry of the present material. Pyridine molecules were introduced at the axial sites of both the porphyrin-centred cobalt ions and the Cu (COO) paddle-wheel units. XRD patterns were calculated with the Materials Studio or Mercury software suites.

Received 2 November 2009; accepted 19 April 2010; published online 30 May 2010

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Acknowledgements

We thank JST (CREST), NEDO and JSPS (No. 20350030 and 20655030) and the Global COE Program ‘Science for Future Molecular Systems’ for financial support, SPring-8 for access to the synchrotron X-ray facilities under the Priority Nanotechnology Support Program administered by JASRI (2008B1801, 2009A1703) and under proposal 2008B2205 and the Center of Advanced Instrumental Analysis, Kyushu University for the use of the FT-IR spectrometer and the ESCA system.

Author contributions

This work has been carried out mainly by JST-CREST and METI-NEDO projects where H.K. is a project leader and responsible for all. R.M. designed this study, interpreted the results and carried out sample preparation, XPS, infrared and AFM characterization and structural model construction. R.M. and Y.U. carried out Langmuir–Blodgett film fabrication and absorption spectra measurements. O.S. was responsible for supervising the synchrotron XRD measurements. R.M. carried out the synchrotron XRD measurements with the help of S.M. and H.Y. All authors commented on the manuscript. R.M and H.K. were responsible for writing the manuscript.

Additional information

The authors declare no competing financial interests. Supplementary information accompanies this paper on w.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions. CorrespondenceandrequestsformaterialsshouldbeaddressedtoR.M.orH.K.

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