Observation of an Orientation Change in Highly Oriented Layer-by

Sep 11, 2015 - Graduate School of Science and Engineering, Chuo University, Tokyo 112-8551, Japan. ‡ Department of Material and Biological Chemistry...
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Observation of an Orientation Change in Highly Oriented Layer-byLayer Films of a Ruthenium Complex upon Oxidation Reaction Katsuhiko Kanaizuka,*,†,‡,§ Sono Sasaki,*,§,∥ Takuya Nakabayashi,† Hiroyasu Masunaga,§,⊥ Hiroki Ogawa,§,# Takaaki Hikima,§ Masaki Takata,§,∇ and Masa-aki Haga*,† †

Graduate School of Science and Engineering, Chuo University, Tokyo 112-8551, Japan Department of Material and Biological Chemistry, Yamagata University, Yamagata 990-8560, Japan § RIKEN SPring-8 Center, Hyogo 679-5148, Japan ∥ Faculty of Fiber Science and Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan ⊥ Japan Synchrotron Radiation Research Institute/SPring-8, Hyogo 679-5198, Japan # Institute for Chemical Research, Kyoto University, Kyoto 611-0011, Japan ∇ Graduate School of Frontier Sciences, The University of Tokyo, Tokyo 153-8902, Japan ‡

S Supporting Information *

ABSTRACT: Layer-by-layer films composed of redox-active ruthenium dimer and Zr(IV) ions were fabricated on an indium tin oxide electrode. The fabricating behavior was monitored by cyclic voltammetry and UV−vis absorption spectral measurements. The orientation of the film was also monitored by grazing-incidence small-angle and wide-angle X-ray scattering (GISAXS) measurements, and it has been clarified that this film has a crystalline structure. The peaks obtained by GISAXS were changed upon oxidation reaction, which indicates that a change in the orientation of the ruthenium dimer occurred in the film.

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INTRODUCTION The construction of metal−organic frameworks (MOFs), or coordination polymers (CPs), has recently been studied extensively.1−3 These metal complexes bearing nanoscale space exhibit attractive performance in catalytic reactions,4 gas adsorption,5 proton-conducting properties,6 and so on. Therefore, control of the pore size or dipoles of such architectures has also been well investigated.1−3 One of the advantages in the construction of these nanoarchitectures on an electrode is the application of a potential voltage as an external stimulus to these architectures. Therefore, we have focused on the construction of crystalline redox-active complex films, such as ruthenium complexes, on an electrode. Metal complexes show many unique electronic functions; therefore, redox-active nanoarchitectures might be useful in electronic devices.7 From the viewpoint of the construction of redox-active crystalline films, we and other groups have reported on the preparation of MOFs and crystalline CPs on substrates (so-called SURMOFs) using stepwise molecular fabrication techniques in a solution; to date, various construction methods, structural analyses, and physical properties of films have been reported.8−11 While the redox-responsive SURMOFs have potential applications for energy materials and electronic devices, the studies are sparse because of the requirement for special surface-sensitive characterization techniques. One of the important subjects for redox-active crystalline films on an electrode is the possibility of an orientation change in the metal complex upon redox reaction.12 For example, the © 2015 American Chemical Society

migration of counteranions occurs both inside and outside the films when oxidation of the metal center occurs. We speculate that the motion of the anion must influence the orientation and/or structure of the films. We have employed a rigid and huge ruthenium dimer complex13 in this study.14,15 The merit of using this complex is understanding the molecular orientation. To the best of our knowledge, this paper has for the first time reported on the observation of a peak shift in grazing-incidence small-angle (GISAXS) in crystalline redoxactive complex films upon redox reaction.

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EXPERIMENTAL SECTION

Precleaned indium tin oxide (ITO) was immersed in a 0.05 mM aqueous solution of the ruthenium complex for 3 h at room temperature (Figure 1).13,16,17 After the electrode was rinsed with water, it was dried with N2 gas. In the next step, complex-immobilized ITO was dipped in a 25 mM aqueous solution of ZrOCl2 for 30 min. These two procedures were repeated for the preparation of a layer-bylayer (LbL) film. To prepare a second complex layer, this Zrterminated film was immersed in the complex solution for 3 h. Thus, a bilayer film of the Ru complex was fabricated.18−21 We established an in-situ simultaneous measurement system of GISAXS using synchrotron radiation and an electrochemical reaction using a potentiostat at BL45XU in SPring-8.22 Received: July 21, 2015 Revised: September 10, 2015 Published: September 11, 2015 10327

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Figure 3. Cyclic voltammograms of a mono- (blue), tri- (green), and pentalayer (red) at 0.1 V s−1 in a 0.1 M TBAPF6 CH3CN solution.

(EQCM) technique. Figure 4 shows the EQCM frequency response of the hexalayer vs potential together with CV. When Figure 1. Chemical structure of the ruthenium complex and a schematic image of layer-by-layer complex fabrication with Zr(IV) ions on a substrate.

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RESULTS AND DISCUSSION The chemical structure of the ruthenium complex and a schematic image of LbL complex fabrication with Zr(IV) ions on a substrate are shown in Figure 1. The layer growth was monitored using UV−vis absorption and cyclic voltammetry measurements. Figure 2A shows the UV−vis absorption Figure 4. Cyclic voltammograms of the hexalayer at 0.05 V s−1 in a 0.1 M NaClO4 aqueous solution (blue) and the EQCM (red).

the potential was scanned in the positive direction, a decrease in frequency was observed, accompanied by electrochemical oxidation from the Ru(II) to Ru(III) state (Figure S2). By GISAXS measurements, the orientation and periodic distance of a structural unit (Ru complex) in crystalline films was evaluated.22,23 When we can control the potential of the electrode modified with the complex during GISAXS measurements, the orientation of the structural unit in the complex can be known before and after the redox reaction (Figure 5A). We have monitored the intensity change in GISAXS profiles measured in the original Ru(II) state in air and in solution containing an electrolyte (Figure S3) and then in the oxidized state in solution. First, GISAXS measurement of the pentalayer film was carried out in air (Figures 5B and S4; Figure 5E shows the outof-plane intensity profiles of GISAXS patterns in Figure 5B− D). A broad peak was observed in the in-plane direction (along the qy axis), and another peak was observed in the out-of-plane direction (Figure 5Ea). This indicated that the thickness of the film (orientation of the complex) was precisely controlled. On the other hand, neighboring complexes within the film have weak interactions, such as the van der Waals interaction, among them. The periodic distance between layers (in the direction perpendicular to ITO) in the film calculated from the peak at qy = 0.25 nm−1 and qz = 2.53 nm−1 in Figure 5E(a) was ca. 2.54 nm, which indicates that the long axis of the complex may lean toward the ITO from its normal direction. The distance between neighboring complexes within the film is 2.0 nm, which is nicely fitted to the size of the ligand. This in-plane peak was not observed in the monolayer (Figure S5), which

Figure 2. (A) UV−vis absorption spectra of mono-, di-, tri-, tetra-, and pentalayers and (B) plots of peak tops vs number of layers at 322 nm (a) and 508 nm (b).

spectral change with the increase in the number of layers. The shapes of these spectra did not change during fabrication, which indicates that no electronic interaction occurs among the complexes. Figure 2B shows plots of absorbance at 322 nm based on the π−π* transition of a ligand and at 508 nm based on a metal-to-ligand charge transfer vs the number of layers. These plots were linearly increased with the increase in the number of deposition cycles, indicating sequential fabrication. Figure 3 shows cyclic voltammograms (CVs) of mono-, tri-, and pentalayers; the Ru(III/II) couple was observed at 1.0 V. The linear dependence of the plot of the anodic peak current vs scan rates indicated that the complex was immobilized on the electrode. The current increased with the number of deposition cycles. These results also indicate the successful sequential fabrication of multilayer films. XPS results also support multilayer formation (SI). After heating the film at 100 °C for several hours, no change was observed in CV, indicating the high stability of the film. Furthermore, we have studied the mass change on the multilayer films during the redox process by use of an electrochemical quartz crystal microbalance 10328

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combines X-ray diffraction and electrochemical measurements. A drastic orientation change of the complexes was observed upon redox. This change is probably triggered by the movement of counteranions. This new methodology will be useful for understanding the construction of various electroresponsive devices such as organic LEDs, solar cells, and ionconducting films.

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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.langmuir.5b02679. Electrochemical, GISAXS, and XPS data of films (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Ryo Tamura, who measured the XPS of the films. This study was financially supported by the Japanese Ministry of Education, Science, Sports, and Culture via a Grantin-Aid for priority area “Coordination Programming” (no. 21108003) and by the Institute of Science and Engineering of Chuo University. This work was partially supported by a Grantin-Aid for Scientific Research on Innovative Areas (no. 2107) of Japan Society for the Promotion of Science (JSPS). The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of the RIKEN SPring-8 Center (proposal nos. 20110077 and 20120071).

Figure 5. (A) Photograph of an in-situ simultaneous measurement system of GISAXS and the electrochemical reaction. The insert photograph shows a sample cell. A schematic image shows the GISAXS geometry for a sample installed in the cell. GISAXS profiles of pentalayers in air (B) and in KCl aqueous solution at −0.3 V (vs Ag/ AgCl) (C) and at 1.3 V (D), as well as their out-of-plane intensity profiles along qZ (E: in air (a), in KCl aqueous solution (b), and at 1.3 V (c)).

indicates that the periodic structure did not form in a single layer. Second, this film was immersed in a KCl aqueous solution, and then the potential was kept at the open circuit voltage (OCV, −0.3 V) for a few minutes. After that, the GISAXS measurement was carried out (the image and profile are shown in Figure 5C,E(b), respectively). Only one broad peak was obtained, and the periodic distance was ca. 2.72 nm, which indicates that a change in the orientation of the complex occurs. Third, the potential of the film was stepped from OCV to 1.3 V. This potential is enough to oxidize two ruthenium centers. The GISAXS profile at this potential is shown in Figure 5D. The peak shift was not observed upon applied bias from 1.3 to 0 V. We consider that counteranions can reversibly move in the film (EQCM results); on the other hand, an irreversible orientation change occurs. This change can be reproduced. We have not clearly explained this stage; however, we speculate that the orientation change based on the electric repulsion among complex occurs upon first oxidation (the repulsion force is stronger in Ru(III)−Ru(III) than in Ru(II)−Ru(II)). Also, counteranions move into the film upon oxidation. This anion movement also has a relationship with the charge storage memory and rechargeable electronic devices.17,24

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REFERENCES

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OUTLOOK To understand the orientation of a complex before and after redox, we have constructed a new measurement system that 10329

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