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Inert Layered Silicate Improves the Electrochemical Responses of a Metal Complex Polymer Miharu Eguchi,*,† Masako Momotake,† Fumie Inoue,† Takayoshi Oshima,‡,§ Kazuhiko Maeda,‡ and Masayoshi Higuchi† †
Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550, Japan § Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1, Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan ‡
S Supporting Information *
ABSTRACT: A chemically inert, insulating layered silicate (saponite; SP) and an iron(II)-based metallo-supramolecular complex polymer (polyFe) were combined via electrostatic attraction to improve the electrochromic properties of polyFe. Structural characterization indicated that polyFe was intercalated into the SP nanosheets. Interestingly, the redox potential of polyFe was lowered by combining it with SP, and the current was measurable despite the insulating nature of SP. Xray photoelectron spectroscopy showed that the decrease in the redox potential observed in the SP−polyFe hybrid was caused by the electrostatic neutralization of the Fe cation in polyFe by the negative charge on SP. Electrochemical analyses indicated that electron transfer occurred through electron hopping across the SP−polyFe hybrid. Control experiments using a metal complex composed of Fe and two 2,2′:6′,2′′-terpyridine ligands (terpyFe) showed that SP contributes to the effective electron hopping. This modulation of the electrochemical properties by the layered silicates could be applied to other electrochemical systems, including hybrids of the redox-active ionic species and ion-exchangeable adsorbents. KEYWORDS: layered silicates, organomineral hybrid complexes, metal complex polymers, electrochromism, electron hopping, electric power saving
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Mallouk12,13 reported that electrodes modified with a layered silicate concentrated a redox-active compound for electrochemical and photoelectrochemical measurements. Modification of electrodes with a layered silicate via Langmuir−Blodgett film preparation methods was reported by Yamagishi and colleagues14−16 for use in pyroelectricity and has also been used for magnetism.17,18 For photofunctional applications, the enhancement of photoisomerization,19−21 photodimerization,22,23 and photostabilization24 on layered silicates has been reported. In this study, we aimed to improve the electrochemical properties of an electrochromic metal complex on the surface of a layered silicate. Toward this goal, a metal complex polymer was introduced to the surface of a layered silicate. An iron(II)based metallo-supramolecular complex polymer, polyFe (Figure 1a), was used. polyFe consists of a ligand with a phenyl group bearing a terpyridine group at each end to
INTRODUCTION Layered silicates are major components of soil covering the earth’s surface and are important in everyday life. They are structurally flexible and are used in pottery and tiles. Certain layered silicates are employed in chemical engineering processes, such as iron making and petroleum refining. The functional properties of paper, pencil lead, paint, medicine, and cosmetics can be improved by adding layered silicates. Recently, layered silicates have also attracted attention as nanospaces that can immobilize functional molecules on or between their surfaces. Mallouk and colleagues established a method to encapsulate ionic dye molecules electrostatically with a layered silicate and an ionic polymer, and they found that the color could be changed by tuning the intramolecular interactions with the polymer.1,2 Takagi and colleagues realized color tuning on a layered silicate by combining dye molecules and the layered silicate.3,4 They also found that emission from dye molecules was enhanced on the surface of layered silicates.5−8 Sasai and colleagues added surfactants onto the surface of layered silicate to improve the emission of rhodamine 6G.9,10 Bard11 and © 2017 American Chemical Society
Received: September 2, 2017 Accepted: September 21, 2017 Published: September 21, 2017 35498
DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
Research Article
ACS Applied Materials & Interfaces
Binding energies in the X-ray photoelectron spectra were corrected assuming that the peak positions of the Mg 2s peaks and C 1s peaks from the carbon surface contamination were the same for SP and SP− polyFe because there is little electronic interaction between Mg2+ cations and polyFe based on the location of Mg2+ in SP. Electrochemical Measurements. The redox properties of SP− polyFe were examined by cyclic voltammetry (CV) curves using a potentiostat (HSV-110; Hokuto Denko). A glassy carbon working electrode (diameter: 3 mm), a Ag|AgCl reference electrode, and a Pt needle counterelectrode were used. A 0.1 M tetra-n-butylammonium perchlorate (≥98%; Nacalai Tesque, Inc.) solution in acetonitrile was used as the electrolyte. Electrode samples were prepared by dropping SP−polyFe or polyFe dispersions onto the working electrode and drying. The amount of polyFe on the electrode was 0.9 nmol. Based on the molecular size estimated by Molecular Mechanics Program 3 (MM3), there were 104 polyFe layers. SP−polyFe on the electrode contained the same amounts of polyFe (0.9 nmol) and SP (2.7 μg), and there were 143 layers. Dissolved oxygen in the sample was removed by Ar bubbling. Preparation of Electrochromic Devices. Electrochromic devices were prepared based on a literature method.25 SP−polyFe aqueous dispersion (400 μL), containing polyFe (0.23 μmol) and SP (0.69 mg), and electrolyte gel in acetonitrile (400 μL, 99.5%; Wako Pure Chemical Industries, Ltd.), containing anhydrous lithium perchlorate (13 mg; Wako Pure Chemical Industries, Ltd.), poly(methyl methacrylate) (31 mg, Mw ∼ 350 000; Sigma-Aldrich), and propylene carbonate (89 μL, 98%; Wako Pure Chemical Industries, Ltd.), were applied to the conductive surfaces of indium tin oxide (ITO) glass (25 × 25 mm2; Sigma-Aldrich) in a 20 × 20 mm2 area. The substrates were left to stand overnight to dry, and the conductive surfaces were stuck together. The voltage was applied by positive and negative electrodes of a 3.0 V alkaline battery connected to the ITO substrates bearing SP−polyFe and electrolyte gel, respectively.
Figure 1. Structures of polyFe (a) and terpyFe (b).
coordinate an iron cation, forming an octahedral complex. Oneelectron-oxidized polyFe containing trivalent Fe is gradually reduced to polyFe with a divalent Fe, but the inverse reaction does not occur spontaneously because divalent Fe is thermodynamically more stable than the trivalent Fe owing to the 18-electron rule. polyFe has a metal-to-ligand charge transfer (MLCT) (d → π*) absorption at 586 nm. The color disappears when polyFe is in the trivalent state because the increase in the d−d splitting width of the trivalent state makes the MLCT too large to occur in the visible region. polyFe has been used in display devices because of this color change,25,26 which is referred to as electrochromism.27−30 Redox reactions of polyFe at the electrode surface proceed by continuous electron hopping via redox sites.31,32 Thus, the redox properties are expected to be observed even when polyFe is sandwiched between insulating silicate layers of nanometer thickness. The electrochemical properties of polyFe are expected to be modulated because polyFe interacts electrostatically with the negative charge on the layered silicate. There are no previous reports of this approach to improving the electrochromic properties by introducing a layered silicate.
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RESULTS AND DISCUSSION Structural Features. Mixing aqueous suspensions of SP and polyFe resulted in aggregation in the reaction vessel, indicating that hybridization occurred instantly via electrostatic attraction. In the following experiments, the charge ratio ([anionic charges on layered silicate]/[cationic charges of polyFe]) was adjusted to 3:2. Under these conditions, the quantitative adsorption of polyFe on SP was assumed, as the adsorption of polyFe on SP was estimated by centrifugation to be almost 100% saturated compared with the cation-exchange capacity. The hybrid structure was examined by SPM, UV−vis absorption spectroscopy, and X-ray diffraction. The SPM image of SP revealed that most of the particles were single layers with heights of 1−2 nm including the surface-adsorbed water (Figure 2a,c). The increased heights of the particles in the SPM image of the SP−polyFe hybrid confirmed hybridization of the two components (Figure 2b,d). The thinnest layers were 2−3 nm in height, where polyFe may adsorb to the SP layer on one or both sides (according to MM3 calculations, the theoretical thickness of polyFe is 0.9 nm). Some of the highest islands in the image reached around 10 nm in height. The UV−vis absorption spectra of the hybrid in dispersion (Figure 3, blue line) had an MLCT photoexcitation peak at 590 nm, which was shifted by 4 nm to a longer wavelength compared with that of polyFe aqueous solution (Figure 3, black line). Generally, shifts in the absorption peaks of adsorbates on the surfaces are related to changes in the adsorbate structure. For example, a red shift of more than 30 nm has been observed for some porphyrins on silicate surfaces due to mesosubstituent rotation.33,34 The slight shift we observed (+4 nm) indicates there was no drastic structural change in polyFe
EXPERIMENTAL SECTION
Synthesis of Materials. polyFe was prepared according to a previously reported method25,26 by mixing 4′,4⁗-(1,4-phenylene)bis(2,2′:6′,2″-terpyridine) (958 mg, 96%; Sigma-Aldrich) and iron(II) acetate (308 mg, 95%; Sigma-Aldrich) in acetic acid (1 L; Nacalai Tesque, Inc.). The mixture was refluxed at 140 °C for 24 h with stirring, filtered with a filter paper, and dried to obtain a shiny, dense, blue solid. The UV−vis absorption spectrum of an aqueous polyFe solution showed that the molar extinction coefficient was 2.9 × 104 L mol−1 cm−1 at 586 nm, considering a polymer unit as a molecule. The degree of polymerization was determined to be 590 (Viscotek 270 dual detector; Malvern Instruments Ltd.). Artificially synthesized saponite (SP; a smectite system of layered silicate) was provided by Kunimine Industries Co., Ltd. SP has a crystalline lamellar structure with atomically flat surface of 1.0 nm thickness and 20−50 nm diameter. The surfaces are negatively charged owing to the partial substitution of Si4+ with Al3+ in the crystal structure. The charge density of SP was 100 mequiv per 100 g. A 50 mg sample of SP was dispersed in 10 mL of water to afford a 5 mequiv L−1 SP dispersion where the layers were fully exfoliated to single layers. A hybrid material composed of SP and polyFe was prepared by mixing the aqueous SP dispersion and 5 × 10−3 M aqueous polyFe at room temperature. The total volume of the hybrid material was adjusted from 0.72 to 200 μL depending on the measurement equipment. Characterization. X-ray diffraction patterns were recorded on a diffractometer (Ultima III; Rigaku) using a monochromatic Cu Kα radiation at 20 mA and 40 kV. UV−vis spectroscopy (UV-2600; Shimadzu), scanning probe microscopy (SPM) in dynamic force mode using a Si-DF20 cantilever (SPA400; Seiko Instruments Inc.), and Xray photoelectron spectroscopy (XPS) using Mg Kα (ESCA-3400; Shimadzu) were used for additional characterization. 35499
DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
Research Article
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at 2θ = 6.7 was assigned to the basal spacing of SP intercalating water molecules. Electrochemical Properties. CV curves of polyFe and SP−polyFe are shown in Figure 5. The redox potential was
Figure 5. CV curves of polyFe (a) and SP−polyFe hybrid (b). Scan rate: 50 mV s−1. Figure 2. SPM images of SP (a), SP−polyFe (b), and their crosssectional profiles (c, d). The contrast scale for the height is shown below the SPM images.
shifted to a lower value, and the current was decreased by adding SP. The half-wave potential (E1/2) of the redox reaction of SP−polyFe was 0.63 V, which was 30 mV lower than that of polyFe (0.66 V). Figure 6 shows the X-ray photoelectron spectra of polyFe and SP−polyFe. The Fe 2p peak for SP−polyFe appeared at
Figure 3. UV−vis absorption spectra of aqueous polyFe (black line) and an aqueous SP−polyFe dispersion (blue line). SP does not show absorptions in the visible light region (400 < λ < 800 nm).
on or between the surfaces because polyFe has a robust sixcoordinate structure. The slight shift was likely caused by a solvent effect. The robustness of polyFe was also supported by the X-ray diffraction patterns of SP−polyFe loaded on a glass substrate (Figure 4). The basal spacing was calculated to be 1.9 nm, which was larger than the theoretical thickness of simple SP (1.0 nm).16 The theoretical thickness of polyFe was calculated by MM3 as 0.9 nm; thus, polyFe was sandwiched between surfaces without noticeable structural changes. A shoulder peak
Figure 6. X-ray photoelectron spectroscopy patterns of Fe 2p for polyFe and SP−polyFe.
706.3 eV, which was lower than that of polyFe (708.5 eV). These results suggest that the Fe component in the SP−polyFe hybrid becomes less cationic, most likely because the negative charge on the surface of SP decreases the positive charge on Fe, increasing the highest occupied molecular orbital potential. In the case of the deposition of Fe pentacarbonyl on the surface of Pt(111), the Fe 2p XPS peak position shifted by 0.4 eV in the negative direction, as reported by Zaera in 1991.35 In our study, a more prominent effect was observed for polyFe as a result of the electrostatic effects of the anionic charges on SP. Although polyFe was sandwiched by insulating SP (65% v/ v), a measurable current was obtained with the SP−polyFe hybrid. Considering that electron hopping occurs in the electrochemical reactions of polyFe,31,32 this indicates that the electrochemical reactions of the SP−polyFe hybrid would proceed through similar electron hopping via redox sites even in the layer space. The lower current of the SP−polyFe hybrid compared with a simple polyFe could be explained by a lower electron hopping rate due to the lower density of polyFe on the surface of SP, which increased the distance between hopping sites. The pathways of electron hopping and electron transfer
Figure 4. X-ray diffraction pattern of SP−polyFe. 35500
DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
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Therefore, the redox reaction of polyFe was quasi-reversible, which means that although the diffusion rate (electron hopping rate) was high, it was lower than the electron transfer rate. However, the current for SP−polyFe increased proportionally to the square root of the scan rates (Figure 8b), although the peak shift depending on scan rates was small (Figure S1b). Hence, the redox reaction of polyFe between layers proceeded reversibly. This reversibility was attributed to an increase in the electron transfer rate relative to the electron hopping rate, which was decreased by the layer space. The slower electron hopping rate could also result from the less mobile counteranion (here the acetate ion) that should exist between the layers to compensate for the change in the oxidation state of polyFe. The results support our electron-hopping and electrontransfer model in the SP−polyFe hybrid (Figure 7b). The diffusion coefficient for the SP−polyFe hybrid on the electrode was determined to be 1.25 × 10−5 cm2 s−1 from the slope of the approximately straight line in Figure 8b. Figure S2 shows the chronoamperograms of polyFe and SP− polyFe at 1.5 V after subtracting the blank chronoamperogram. The total charges involved in the oxidation estimated by integrating dI × dt were 3.6 × 10−5 and 8.2 × 10−6 C for polyFe and SP−polyFe, respectively. Considering the polyFe loading, 42% of polyFe and 9.4% of the polyFe in SP−polyFe underwent oxidation. The percentage for the electroactive polyFe in SP−polyFe (9.4%) is close to the value (∼10%) previously reported by Ghosh and Bard for an electroactive ruthenium complex with a polymer on montmorillonite.11 The concentration of electroactive metal complexes in this kind of hybrid electrode may be improved by using a metal complex polymer with shorter metal−metal distances. Another effect of SP is that this hybridization strategy with SP was effective for a mononuclear Fe(II) complex coordinated by two 2,2′:6′,2″-terpyridine ligands (terpyFe; Figure 1b), which is a model complex for polyFe.36 The mononuclear complex alone did not exhibit a Fe3+/2+ redox peak, as the deposited terpyFe on the electrode dissolved in acetonitrile and diffused away from the electrode surface. However, clear redox peaks were observed when terpyFe was combined with SP (Figure 9) because the film-formation properties and anionic charge of SP acted as a “glue” to bind the electrode surface and terpyFe. The whole intention of the use of polyFe for previous electrochromic devices was to improve the film stability on electrodes. The use of SP with nonpolymer metal complexes
(from the metal sites of polyFe to the electrode surface) are depicted in Figure 7.
Figure 7. Electron hopping and electron transfer pathways of polyFe (a) and SP−polyFe (b) on the electrode.
Further electrochemical studies were conducted using polyFe and SP−polyFe electrodes. Figure S1 shows that the CV curves of polyFe and SP−polyFe depended on the scan rate. The faster scan rates of polyFe led to the larger shifts in the oxidation and reduction peaks to higher and lower voltages, respectively (Figure S1a). The current recorded for polyFe was not proportional to the square root of the scan rate (Figure 8a).
Figure 9. CV curves of terpyFe with SP (red line) and terpyFe (gray line) obtained at scan rates of 50 mV s−1. The amount of terpyFe on the electrode considering a polymer unit as a molecule was 0.9 nmol. The charge ratio ([anionic charges on layered silicate]/[cationic charges of terpyFe]) was adjusted to 3:2.
Figure 8. Plots of anodic peak current vs square root of the scan rate for polyFe (a) and SP−polyFe (b). Scan rates were 5 mV s−1, 50 mV s−1, 500 mV s−1, and 5 V s−1. 35501
DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
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ACS Applied Materials & Interfaces was revealed to be applicable for these devices. The coequal currents of terpyFe with SP and SP−polyFe (see Figure 5) may be because of similar distances between the hopping sites (similar electron-hopping rates), according to the following considerations. MM3 calculations showed that the distance between the Fe cations is not greater than 1.5 nm on SP for polyFe, whereas the distance between the Fe cations of terpyFe on SP is 1.2−1.9 nm based on the average distance between the anionic charges on the SP surface (1.2 nm).37 Device Application Using SP−polyFe. Electrochromic devices consisting of two ITO electrodes with an electrolyte layer and a SP−polyFe layer between them were assembled (inset in Figure 10). The CV curve accompanying the redox
Figure 12. Profile of current vs cycle count upon sweeping the voltage between −3 and +3 V 200 times.
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SUMMARY AND CONCLUSIONS polyFe was electrostatically combined with a layered silicate (saponite; SP). Electrochemical properties were observed despite the insulating characteristics of the layered silicate because the redox reaction proceeds by continuous electron hopping via redox sites. The observed lowering of the redox potential of polyFe was ascribed to the electrostatic neutralization of cationic Fe by the anionic sites on the surface of the layered silicate, leading to the energy conservation of the electrochromic devices. The addition of SP also elicited the electrochemical properties of terpyFe because the SP acted as a glue to bind the electrode surface and terpyFe. Thus, the anionic charge on the layered silicate can be utilized effectively using a simple preparation technique that is capable of being extended to a wide range of materials. The use of other metal complexes and layered silicates will help to improve the electrochemical properties, such as the redox potential and the current, of these hybrid materials without requiring organic synthesis.
Figure 10. CV of polyFe (gray line) and SP−polyFe (blue line) in the electrochromic device under applied voltages of −3.0 to +3.0 V.
reaction at the applied voltages of −3.0 to +3.0 V is shown in Figure 10. The CV curve of the device without SP is shown by the gray line. Decreases in the redox potential (2.8 → 2.2 V) and the current were also observed in the devices (cf. Figure 5). The 11th CV curve is shown in Figure 10 because the CV curve profiles stabilized after about 10 cycles. The asymmetric nature of the curve was ascribed to the occurrence of another redox reaction besides Fe2+/Fe3+, which we could not identify (may be related to electrolyte gel or the ITO electrodes). The chromatic transitions of the device upon application of the same voltages are shown by the photographs and absorbance spectra in Figure 11. An estimated 94% of SP−polyFe (divalent) was oxidized. To test the durability of the device, the profile of current versus cycle count was observed upon repeatedly sweeping the voltage between −3 and +3 V (i.e., 0 → +3 → 0 → −3 → 0 V) 200 times (Figure 12). The current was rather constant.
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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/acsami.7b13311. CV curves of polyFe and SP−polyFe; chronoamperogram of polyFe and SP−polyFe; X-ray diffraction pattern of SP−terpyFe; UV/vis absorption spectra of aqueous terpyFe and SP−terpyFe (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Miharu Eguchi: 0000-0002-4007-7438 Kazuhiko Maeda: 0000-0001-7245-8318 Masayoshi Higuchi: 0000-0001-9877-1134 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by a PRESTO/JST program, Innovative Use of Light and Materials/Life, and a Grant-in-Aid for Young Scientists (B) (Grant No. JP15K20892), and CREST (Grant No. JPMJCR 1533) from the Japan Science and Technology Agency.
Figure 11. Chromatic transition under applied voltages of −3.0 V (a) and +3.0 V (b), and their absorption spectra (c). 35502
DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
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DOI: 10.1021/acsami.7b13311 ACS Appl. Mater. Interfaces 2017, 9, 35498−35503
