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ABSTRACT: We found the utilization of porphyrin-based metal–organic network composed of tetra(catechol- substituted) porphyrin (cPor) and Fe ions fo...
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Robust nano-wrapping of reduced graphene oxide by metal-organic network film between Fe ions and tetra(catechol-substituted) por-phyrin Hiroaki Ozawa, Shunsuke Kusaba, Mariko Matsunaga, and Masa-aki Haga Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03828 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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Langmuir

Robust nano-wrapping of reduced graphene oxide by metal-organic network film between Fe ions and tetra(catechol-substituted) porphyrin Hiroaki Ozawa,*,† Shunsuke Kusaba,† Mariko Matsunaga,‡ and Masa-aki Haga*,† †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.

‡

Department of Electrical, Electronic, and Communication Engineering, Faculty of Science and Engineering, Chuo University 1-13-27 Kasuga, Bunkyo-ku, Tokyo, 112-8551, Japan.

KEYWORDS Porphyrin, Catechol, Reduced graphene oxide (rGO), Coordination, Soft nano-wrapping ABSTRACT: We found the utilization of porphyrin-based metal–organic network composed of tetra(catecholsubstituted) porphyrin (cPor) and Fe ions for robust wrapping materials of graphene oxide (GO), which can keep the dispersion state under the chemical reduction of GO to reduced graphene oxide (rGO) in water. The tetra(catecholsubstituted)porphyrin (cPor) was designed for soft-wrapping methods since the aromatic porphyrin moieties can be strongly adsorbed onto the surface of GO or reduced graphene oxide (rGO) via both π−π interactions and the catechol-Fe coordination network formation. The GO sheets covered with the cPor–Fe films were reduced chemically in water under retention of the wrapped nanostructure of the cPor–Fe/GO sheets. The obtained rGO composites after chemical reduction are characterized by using UV-Vis absorption, Raman, XPS spectra as well as TGA and EDX. XPS and EDX spectra showed the presence of Fe species, which originates from the coordinated Fe-catechol nodes in the wrapped cPor-Fe films. The wrapped rGO sheets could be easily handled in water due to their high solubility therein and exhibits electric conductivity. In dynamic light scattering (DLS) analysis, the average diameter of rGO composites before and after reduction changed slightly from 419 ± 309 to 663 ± 697 nm, indicating no significantly influence the chemical reduction on the size of the rGO composite or the solubility. It is expected that the soft wrapping cPor–Fe/rGO should employ the applications to prepare functional materials such as modified electrodes, catalysts, energy-storage materials, and electronic devices.

INTRODUCTION Graphene and reduced graphene oxide (rGO), i.e., twodimensional (2D) carbon nanomaterials that consist of sp2-hybridized carbon atoms, have received significant attention in the context of various applications on account of their unique physical and electronic properties.1-3 However, applications involving these carbon materials still suffer from a variety of problems that include e.g. the absence of high-yielding synthesis routes to these materials, sorting and separation issues, and low solubility. To circumvent these obstacles, covalent and noncovalent functionalization methods have been developed in order to solubilize and functionalize graphene and rGO.4-5 Particularly, the noncovalent functionalization methods have been met with great expectations, as they should operate via π–π interactions and molecular charge-transfer interactions under preservation of the intrinsic properties of graphene and rGO.6 Graphene and rGO have been solu-

bilized/functionalized using various methods and materials such as, organic solvents, aromatic molecules, polymers, surfactants, or ionic liquids.7-14 Recently, Ejima and coworkers have reported the simple and rapid assembly of films on a variety of substrate surfaces via the formation of coordination bonds between tannic acid and Fe ions, inspired by the biological surface adsorption of mussel adhesive proteins.15-16 The wrapping method based on the metal-phenolic network formation can be developed to modify the various materials surface and fabricate the metal-phenolic network films on particles, hydrogel, and capsules for applications in biomaterial and biomedical materials.17-19 Filippidi et al. have reported that the toughening elastomers containing ironcatechol complex as sacrificial, reversible cross linkers exhibited the mechanical properties having both of extensibility and stiffness.20 As for the surface modification of GO and rGO, our group has reported the soft nanowrapping of GO by using metal-organic network films

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composed of tannic acid (TA) and Fe ions, leading to substrate-bound modified GO sheets.21 However, for the chemical reduction, this TA-Fe/rGO composite was chemically so weak that the TA-Fe films were easily peeled off from rGO surface by sonication treatment in water because the central sugar moiety in TA does not interact with GO or rGO surface strongly. As a result, the reduction of GO in film has been done with the solid-gas reaction between the TA-Fe/GO film and hydrazine vapor. Therefore, the handling and application of the TA-Fe film wrapped rGO were limited due to the difficulty of the homogenous dispersion of rGO composite in water without aggregation and peeling from the wrapping films. To overcome this problem related to robustness and solubility of the modified rGO, we have explored a molecular design of self-assembler to interact with rGO strongly and to form a robust organic–metal network films covered on rGO for noncovalent surface functionalization. In this paper, we report the usage of tetra(catecholsubstituted) porphyrin (cPor) as a self-assembler to form metal-organic network films with Fe ions for a softwrapping of GO sheets (Figure 1). While catechol and derivatives are known to form stable metal-tris(catechol) complex using various ions, 16,22,23 we selected Fe(III) ion referring to the our and other previous reports.15,21 The complexation between tetra(catechol-substituted) porphyrin and Fe ions provides a uniform metal-organic network film on GO sheets, since the aromatic porphyrin moieties can be strongly adsorbed onto the surface of GO. Even after the reduction of the GO sheets in these composites with hydrazine hydrate, the wrapped rGO sheets remain soluble in water, and aggregation was not observed. The obtained GO and rGO composites were characterized by optical and physical measurements, as well as by microscopy (AFM, SEM, and TEM). The soft wrapping by the metal-organic network films may occur as shown in Scheme 1. The GO was prepared by an improved version of Hummer’s method.24 The thus obtained GO contains various types of defects such as hydroxyl, epoxy, and carboxylic acid groups, and therefore, the GO surface can be modified with various materials via these defect sites.25-26 For the formation of cPor-Fe films on GO, several interactions between GO and cPor–Fe films will be included such as the multiple-point hydrogen bonding, the coordination bond between Fe and OH, COOH groups in GO, and weak π–π interaction between porphyrin and GO.21 Inspired by the simple coating methodology using

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coordination complexes of natural polyphenols and Fe(III) ions reported by Ejima et al.15, a robust films covered with cPor-Fe(III) metal-organic network having coordination nodes between cPor and FeIII ions (Scheme 1) were prepared. Significantly, the cPor-Fe network films can be strongly adsorbed to rGO surface via the multipoint interactions after the chemical reduction. Therefore, the robust cPor-Fe/rGO can be dispersed in water stably without peeling off the film from rGO even after a sonication treatment. The soft-wrapped cPor-Fe/rGO composites will enable us as a starting material to develop functional devices for a variety of applications due to the high dispensability, stability, and excellent handle ability.

EXPERIMENTAL SECTION Instruments and Methods. All starting materials and reagents were purchased from common commercial suppliers and used without further purification. Graphite flakes (natural, -325 mesh, 99.8%) were purchased from Alfa Aesar. Ultraviolet–visible spectroscopy (UV-Vis) absorption and Raman spectra were recorded on an Agilent 8453 UV-Vis spectrophotometer and a Lambda Vision MicroRAM-200 spectrometer, respectively. FT-IR spectra were recorded on a JASCO FT/IR-4200 FT-IR spectrometer. Thermogravimetric analyses were carried out on a RIGAKU Thermo plus EVO2 with a heating rate of 5°C/min under an N2 atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were carried out using a Kra-

Figure 1. Chemical structure of tetra(catechol-substituted) porphyrin (cPor).

Scheme 1. Schematic illustration of the preparation and structural details of the cPor-Fe/GO sheets.

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Langmuir tos-Shimazu AXIS-HSi spectrometer. Atomic force microscopy (AFM) images were recorded using an Agilent Technologies 5500 AFM microscope. SEM images and energy-dispersive X-ray spectroscopy elemental mapping was carried out using a Schottky Field Emission Scanning Electron microscope (SEM; JSM-7800F, JEOL) operated at 15 kV. Transmission electron microscopy (TEM) images were obtained using a Hitachi HT 7700 microscope operated at 100 kV. Dynamic light scattering was performed on an ELSZ-2000ZS (Otsuka Electronics Co., Ltd.) using a He–Ne laser (λ= 632.8 nm). The data evaluation of the DLS measurements was performed with the cumulants analysis. Materials. All reagents and solvents used for synthetic purposes were purchased from commercial suppliers and used without further purification. Preparation of graphene oxide (GO). GO was prepared according to a modification of Hummer’s method.27 Synthesis of 3-(3,4-dimethoxy-phenyl)propionaldehyde 3-(3,4-Dimethoxy-phenyl)propionaldehyde was synthesized according to a previously reported procedure.28 5,10,15,20-tetrakis(3,4-dimethoxy-3-phenylpropyl) porphyrin. Pyrrole (0.64 g, 9.54 mmol) and 3-(3,4dimethoxy-phenyl)-propionaldehyde (1.63 g, 8.39 mmol), and trifluoroacetic acid (0.5 mL) were dissolved in CHCl3 (1200 mL) and the mixture was stirred at 70 °C for 17 hours. After adding DDQ (0.592 g, 2.60 mmol), the mixture was stirred at 70 °C for 24 hours. The solvent was evaporated under reduced pressure, and the residue was purified by column chromatography on silica gel (eluent: AcOEt). The obtained solid was dissolved in CH2Cl2 and the organic layer was washed with NH3aq to remove any excess DDQ. The solvent was evaporated under reduced pressure and the product was washed with MeOH to give a purple solid (101 mg, 5%). 1H NMR (400 MHz, CDCl3): δ = 9.46 (s, 8H, β-position of pyrrole), 6.99 (dd, 4H, J = 8 Hz, 2 Hz, Ph), 6.91 (d, 4H, J = 2Hz, Ph), 6.88 (d, 4H, J =8 Hz, Ph), 5.25 (t, 8H, J = 8 Hz, CH2), 3.92 (s, 12H, OCH3), 3.78 (m, 20H, OCH3 and CH2). MALDI-TOF-MS [M + H+]/z calcd. 967.16, found 967.63. 5,10,15,20-tetrakis(3,4-dihydroxy-3-phenylpropyl) porphyrin (cPor). The synthetic route to 5,10,15,20tetrakis(3,4-dihydroxy-3-phenylpropyl) porphyrin (cPor) is shown in Scheme S1. 5,10,15,20-Tetrakis(3,4-dihydroxy3-phenylpropyl) porphyrin (53.4 mg, 0.055 mmol) was dissolved in CH2Cl2 (10 mL), before a 1 M solution of borane tribromide (BBr3) in CH2Cl2 (4.5 mL) was added dropwise. The resulting mixture was stirred for 5 hours at room temperature. The reaction mixture was washed with aqueous NaHCO3 and water to give a solid that was isolated by filtration. The obtained solid was dissolved in methanol and the solution was filtered. After evaporation of the solvent, the product was obtained as a purple solid (36 mg, 77%). 1H NMR (400 MHz, CDCl3): δ = 9.27 (s, 8H, β-position of pyrrole), 6.73-6.66 (m, 12H, Ph), 4.94 (t, 8H, J = 8 Hz, CH2), 3.52 (t, 8H, J = 8Hz, CH2). MALDI-TOF-MS [M+]/z calcd. 854.94, found 855.25.

Fabrication of cPor-Fe/GO sheets. A mixture of 5,10,15,20-tetrakis(3,4-dihydroxy-3-phenylpropyl) porphyrin (cPor; 1 mg) in 20 mL of water was treated with trifluoroacetic acid (TFA) until cPor dissolved. The protonation of free base cPor with TFA led to a water-soluble cation and dication species. Graphene oxide (2 mg) was added to the solution, before the mixture was sonicated for 2 min. FeCl3⋅6H2O (2 mg) was added to the mixture, which was subsequently stirred for 1 min. 1 N NaOHaq. was added to the solution until the color changed from green to dark brown. The formation of Fe-catechol complex was promoted in base condition.15 The suspension was centrifuged (1000 G, 10 min) and the supernatant was taken off to remove any excess of cPor, Fe ions, and cPor-Fe complex. The remaining solid was dissolved in water (40 mL) and centrifuged (1000 G, 10 min). The obtained solid was dispersed in water (40 mL) by sonication and used for experiments. Reduction of cPor-Fe/GO by hydrazine hydrate. Hydrazine hydrate (1 mL, 30%) was added to the cPor-Fe/GO solution (40 mL). The mixture was stirred at 90 °C for 1 hour. After cooling, the suspension was centrifuged (1000 G, 30 min), before the remaining solid was dispersed in water (40 mL), followed by centrifugation (1000 G, 10 min). The obtained solid was dispersed in water (40 mL) by sonication and used for experiments.

RESULTS AND DISCUSSION The UV-Vis absorption spectra of metal-free cPor exhibits characteristic Soret and Q band absorption peaks at 422 nm and four peaks of 522, 558, 599, and 659 nm as Q

Figure 2. UV-Vis absorption spectra of cPor-Fe/rGO (broken line), cPor-Fe/GO (solid line), and cPor (dotted line). I

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Figure 3. (a) TEM images of cPor-Fe/GO. (b) AFM image and (c) line scan of cPor-Fe/GO on a mica substrate.

bands (Figure 2).29 After the formation of the cPor-Fe network films, the Soret band is bathochromically shifted (417 → 422 nm) and broadened. The four Q bands are also bathochromically shifted and broadened. The weak and broad peak around 550 nm corresponds to a mixture of charge transfer band (coordinated catechol units → FeIII) in the composite films (cf. inset in Figure 2).15, 21 In a control experiment, the UV-Vis absorption spectrum of cPorFe polymer without GO was measured (Figure. S1), which showed Soret and Q bands that were also red-shifted and broadened compared to those of cPor-Fe/Go, albeit to a lesser degree. These smaller changes may be due to interactions between porphyrin and GO. The FT-IR spectra of cPor-Fe/GO, cPor, and GO sheets are shown in Figure S2. For cPor-Fe/GO sheets, the two strong peaks at 3400 and 1630 cm-1 correspond to the O-H and C=C stretching vibrations, respectively. The intensity of the C=O peak at 1734 cm-1 decreased after soft-wrapping, and pronounced peaks at 1380 and 1055 cm-1 were observed after the formation of the cPor-Fe films. These results indicate the existence of tetra(catechol-substituted)porphyrins in the composites. The Raman spectra of cPor-Fe/GO exhibited two broad peaks at 1345 and 1596 cm-1 (Figure S3), which correspond to the D and G bands. Subsequently, X-ray photoelectron spectroscopy (XPS) measurements were carried out for cPor-Fe/GO, which revealed the C1s, N1s, O1s, and Fe2p peaks at 285, 400, 531, and 711 eV, respec-

tively (Figure S4). A deconvolution of the O1s peak resulted in the separation of three components with binding energies of 530.8, 532.2, and 533.1 eV, which were assigned to the signals from C=O, C-O, as well as oxygen atoms in water or chemisorbed oxygen species, respectively. The peak at 536.2 eV corresponds to the Na KLL Auger peak, which is due to the presence of NaOH during the sample preparation (for details, see ESI). Compared to the O1s peak of GO, the peak intensity corresponding to C-O decreased, while the peak from water increased, indicating the formation of cPor-Fe films and the incorporation of water into hydrophilic cPor-Fe film structures (Figure S4d). The Fe peak at 711.5 eV strongly suggests the presence of FeIII species.30 In XPS C1s spectra of GO, main four peaks at 284.6, 286.1, 287.2, and 288.1 corresponding to C=C, C-O, C=O, O-C=O were observed. Particularly, the C=O peak appeared remarkably due to the COOH and C=O groups in GO (Figure. S4c). The peak intensity of the C=C peak in cPor-Fe/GO relative to that of GO increased due to the presence of porphyrin (Figure. S4c). In their entirety, these results indicate the formation of cPor-Fe films on the GO surface. A thermogravimetric analysis (TGA) of GO and cPor-Fe/GO (Figure S5) revealed that GO is unstable, whereby the observed mass loss (14% at 100 °C) corresponds to the loss of adsorbed water, as well as the COOH and OH groups. In the case of cPor-Fe/GO, the observed mass loss corresponds to the loss of adsorbed water (16% at 100 °C) and a smaller mass loss (27% at 250 °C) was observed due to the covering with the cPor-Fe films. To investigate the structures of cPor–Fe/GO in detail, TEM measurements were carried out (Figure 3a), which showed sheet structures after the formation of the cPorFe films on the GO surfaces, indicating the retention of the GO-derived sheet structures. Moreover, the surface morphology of the composites is ragged, suggesting the existence of nano-sized composites that wrap the cPor-Fe network structures. The morphology of the cPor-Fe/GO sheets was also examined by AFM (Figure 3b), which confirmed a ragged surface morphology compared to that of the GO sheets. The average thickness of these ragged structures was 1.5-2 nm, which is slightly thicker than that of the GO sheets (ca. 1 nm).24 These results demonstrate that the GO surface is covered with thin cPor-Fe films. The reduction of the cPor-Fe/GO sheets in water was ac-

Figure 4. (a) Schematic preparation of cPor-Fe/rGO sheets by chemical reduction. (b) Photograph of cPor-Fe/GO (left) and cPorFe/rGO (right) in water.

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Langmuir complished by addition of hydrazine hydrate (Figure 4a). After the reduction, the cPor-Fe films are preserved by multiple-point π-π interactions between the rGO surface and the porphyrin rings in the cPor-Fe films, instead of by multiple-point hydrogen bonding (coordination bond) and weak π–π interactions between the porphyrin rings and GO.31-32 Figure 4b shows a photograph of cPor-Fefilm-wrapped GO and rGO in water. These wrapped sheets afforded homogeneous dispersions in water upon sonication. Surprisingly, even after a few weeks under basic conditions, the cPor-Fe/rGO composite was stable enough to keep its solubility in water upon sonication. Unlike a typical non-covalent modification via reversible interaction such as π-π interaction, the wrapping films was stable and irreversibly stay on the rGO surface through multipoint contact due to the formation of mealorganic network structures around rGO. As a control experiment, we tried to prepared cPor/rGO in water without Fe ions. However, without Fe ions, the soluble cPor/rGO were not obtained in water. These results indicate that the cPor-Fe films that wrap rGO keep their wrapping structure and improve the dispersion and solubility properties of rGO. To examine the solubility of cPor-Fe/rGO, we measured the size of the GO sheets by dynamic light scattering (DLS) before and after the reduction of GO with hydrazine. The average diameter of cPor-Fe/GO before and after reduction changed slightly from 419 ± 309 to 663 ± 697 nm. To confirm the DLS results, we observed the cPor-Fe/GO and cPor-Fe/rGO on ITO substrates by SEM observations (Figure S6). These SEM images exhibited the sheet structures with several tens of nm from to several µm. By considering the large experimental error for the average diameter of cPor-Fe/rGO, the size of cPor-Fe/GO and cPor-Fe/rGO were similar to that of GO (Figure S6). These results indicate that the reduction of the wrapped GO is not significantly influenced by the size of the nanosheets or the solubility. The UV-Vis absorption spectra of cPor-Fe/GO sheets in water before and after reduction are shown in Figure 2. After reduction of the GO in the composites, the Soret band was bathochromically shifted by a few nm under concomitant broadening. After the reduction of GO, the porphyrin rings interacted strongly with the rGO surface due to the restored π-conjugated system in rGO.33-34 The FT-IR spectra of cPor-Fe/GO before and after reduction (Figure. S7) show that after the reduction of GO, the in-

Figure 5. XPS C1s (a) and Fe2p (b) spectra of cPor-Fe/rGO on silicon substrates.

tensity of the peak at ~3400 cm-1, which corresponds to the OH stretch, decreases due to the removal of OH and COOH groups from the GO. On the other hand, the small peak at 783 cm-1, which is associated with the multiply substituted aromatic rings of catechol, remained.21 These results indicate that the cPor-Fe films are preserved after reduction, while the GO sheet was chemically reduced to rGO. To examine the chemical composition and the degree of GO reduction in the composites after reduction, XPS measurements were carried out on cPor-Fe/rGO (Figure. S8). The C1s, N1s, O1s, and Fe2p peaks of cPorFe/rGO were observed at 285, 400, 531, and 711 eV, respectively. It should be noted that the shape of the C1s peak changed significantly after reduction (Figure 5a). The C1s signal was deconvoluted into six peaks at 284.6, 286.1, 287.2, 288.1, 289.3, and 292.7 eV, which correspond to C=C, C-O, C=O, O-C=O, (CF3)CO2-, and CF3 moieties, respectively. The (CF3)CO2- and CF3 moieties are derived from CF3COOH, which was used as a source for the counter anions for the complexation between Fe and catechol units during the formation of the cPor-Fe films. The OC=O/C=C intensity ratio decreased, indicating that the cPor-Fe-wrapped GO was reduced. The O1s spectrum of cPor-Fe/rGO (Figure S8b) revealed a decreased intensity ratio of the C-O (530.9 eV) and C=O (532.1 eV) peaks relative to the Na KLL Auger peak (536.5 eV) in comparison to that in the cPor-Fe/GO spectrum, indicating the reduction of oxygenated functional groups in GO. Furthermore, an Fe peak arising from the Fe species at the catechol-Fe coordination nodes was observed (Figure 5b), and the main peak (711.5 eV) was assigned to FeIII species.26 These results indicate that the wrapping with cPor-Fe films was maintained, even after the chemical reduction. The Raman spectrum of the cPor-Fe/rGO sheet (Figure 6) showed two peaks at 1345 and 1596 cm-1, which correspond to the D and G bands. The D/G band intensity ratio is correlated to the crystallite size,35 and upon reduction of GO in the composites, the D/G ratio increased from 1.02 (cPor-Fe/GO) to 1.06 (cPor-Fe/rGO). The presence of cPor-Fe films in the composites was also examined by GO, rGO, cPor-Fe/GO, and cPor-Fe/rGO (Figures 7 and S5). The curve for GO exhibited two major weight-loss steps

Figure 6. Raman spectra of cPor-Fe/GO before (dotted line) and after (solid line) reduction on silicon substrates.

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(T = 30-100 °C and T = 100-250 °C), which correspond to the removal of adsorbed water and the loss of oxygencontaining functional groups such as COOH and OH, respectively. On the other hand, the TGA curves for rGO, cPor-Fe/rGO, and cPor-Fe/rGO revealed only the loss of adsorbed water (T < 100 °C). No substantial weight loss was observed for cPor-Fe/rGO at T = 100-300 °C. At T > 300 °C, the thermal weight loss occurred by decomposition of cPor, which strongly supports the notion of a successful reduction of the inner GO and the preservation of the wrapping cPor-Fe films after the chemical reduction. However, the thermal weight loss of cPor-Fe/GO at T > 300 °C is smaller than that of cPor-Fe/rGO. The difference of these weight loss is currently in progress. The TEM images of cPor-Fe/rGO (Figure 8a) clearly show a sheet structure and the absence of aggregates after the reduction. Moreover, the reduction of the inner GO did not

Figure 7. TGA of GO (dotted line), rGO (broken line), cPor-Fe/GO (dotted broken line), and cPor-Fe/rGO (solid line).

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affect the size, structure, or solubility of the composites. We also examined the morphology of cPor-Fe/rGO by AFM (Figure S9), which confirmed a sheet structure. The height profile images revealed that the average thickness of these sheet structures was about 1-2 nm, which is higher than that of typical rGO (less than 1 nm). After the reduction, the height of cPor-Fe/rGO became lower compared to that of cPor-Fe/GO, indicating the wrapping films are flattened without destruction by the GO reduction. To clarify the presence of cPor-Fe films around rGO, we carried out SEM measurements and element-dispersive Xray diffraction (EDX) analyses. An SEM image (Figure. 8b) showed the cPor-Fe/rGO structures on SiO2/Si substrates after evaporation of the water. The EDX spectra revealed the presence of Fe, which originates from the coordinated Fe-catechol nodes in the wrapped cPor-Fe films (Figure. S10). The presence of carbon, oxygen, nitrogen, and fluorine, which are derived from rGO, cPor, and CF3COOH, was also confirmed by XPS (Figures 5 and S8). An elemental mapping of the cPor-Fe/rGO films on Si substrates is shown in Figures. 9c-h. The nature of the cPor-Fe/rGO structure was revealed by element-mapping images of carbon, oxygen, nitrogen, and iron in the composite. The mapping for carbon, nitrogen, oxygen, and iron in the cPor-Fe/rGO films showed a homogeneous distribution on the sheet surface, which indicates that the cPor-Fe film uniformly covers the rGO surface. To further clarify the reduction of the GO sheets in cPorFe films, electrical conductivity measurements of the cPor-Fe/rGO sheets were carried out (Figure 9a and b). However, a significant conductivity for the cPor-Fe/GO films could not be obtained due to its high resistance. For GO and cPor-Fe complex, these currents also did not flow between Au gap electrodes (Figure S11). On the other hand, the cPor-Fe/rGO films showed a nonlinear I-V curve, resulting in the formation of a more conductive material. However, I-V curve of rGO between Au gap

Figure 9. (a) Illustration of an I-V measurement set up. (b) I-V curve for films of cPor-Fe/GO (dotted line) and cPor-Fe/rGO (solid line) cast between two Au electrodes that include a gap of 2 µm.

Figure 8. (a) TEM images of isolated cPor-Fe/rGO sheets. (b) SEM image and corresponding elemental-mapping images for (c) C, (d) O, (e) N, (f) F, (g) Fe, and (h) Si in cPor-Fe/rGO films on Si substrates.

electrodes was linear (Figure S11) and typical I-V curve of graphene have been reported to be linear,21, 36 and the observed change of I-V curve may be attributed to the higher resistance of the wrapping cPor-Fe films on rGO and/or defects in the graphene structure.

CONCLUSION

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Langmuir We have reported the utilization of porphyrin-based metal–organic network composed of tetra(catecholsubstituted) porphyrin (cPor) and Fe(III) ions for robust wrapping materials of graphene oxide (GO). The formation of soft-wrapped cPor-Fe films around GO sheets was confirmed by UV-Vis absorption, FT-IR, and XPS spectroscopy, as well as SEM, TEM, and AFM analyses. The GO core in cPor-Fe/Go could be reduced under preservation of the wrapped structures, and the reduction of GO in these composites was examined and confirmed by XPS, TGA, and EDX. The soft-wrapped rGO was easily solubilized in water and stable therein, and exhibits electric conductivity. The free-base porphyrin should moreover be able to capture metal ions to form the corresponding metalloporphyrins that should exhibit enhanced functionality such as photo-induced response, redox activity, or magnetism. Metalation of the porphyrins in the wrapping films and/or soft-wrapping using metalloporphyrins should thus enable the functionalization of such softwrapped GO composites, which might find applications as modified electrodes, catalysts, energy-storage materials, and electronic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis, UV-Vis absorption spectra, FT-IR spectra, Raman spectra, XPS spectra, EDX spectra, TGA, AFM, SEM, and I-V measurement. (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected].

ORCID Masa-aki Haga: 0000-0002-1230-3848

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI grant 16K05732 and JP17H05383(Coordination Asymmetry), Chuo University Joint Research Grant, the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan, and the Foundation of the Kondo Scholarship Association. We acknowledge Prof. Ho-Chol Chang and Prof. Hitoshi Shindo for their support regarding the TGA analyses and Raman measurements.

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