Low-Temperature Phase Transformation Accompanied with Charge

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Low-Temperature Phase Transformation Accompanied With Charge-Transfer Reaction of Polyiodide Ions Encapsulated in Single-Walled Carbon Nanotubes Yukihiro Yoshida, Yosuke Ishii, Nao Kato, Canghao Li, and Shinji Kawasaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07819 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Low-Temperature

Phase

Transformation

Accompanied with Charge-Transfer Reaction of Polyiodide Ions Encapsulated in Single-Walled Carbon Nanotubes Yukihiro Yoshida, Yosuke Ishii,* Nao Kato, Canghao Li, and Shinji Kawasaki* Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Gokisocho, Showa-ku, Nagoya 466-8555, Japan.

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ABSTRACT In order to understand the reason why polyiodide ions encapsulated in single-walled carbon nanotubes (SWCNTs) drastically improve the electric conductivity and the water dispersibility of SWCNTs at low temperature, we performed in-situ Raman measurements of polyiodide ions encapsulated in three kinds of SWCNTs having different mean tube diameters under low temperature down to −100ºC. It was found that for all the three samples, the Raman peak intensities of polyiodide ions increase and the G-band peak position of SWCNTs shifts toward higher wavenumber side with decreasing temperature. It means that the charge-transfer from SWCNTs to encapsulated iodine molecules increases with decreasing temperature and that holedoping level of SWCNTs increases at low temperature. Furthermore, the peak profiles changed with temperature drastically for polyiodide ions encapsulated in the SWCNT sample having the largest mean tube diameter of 2.5 nm. This change indicates the structural transformation of polyiodide ions in SWCNTs. These experimental results can be explained by the promotion of the chain-like polyiodide ion formation at low temperature. It was discussed with the control experiment using amylose that the promotion of the polyiodide ion formation at low temperature is characteristic for iodine molecules encapsulated in SWCNTs.

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1. Introduction Polyiodide ions having of various forms have attracted much interest not only from scientific fields but also from industrial fields, because they are used in low-dimensional organic conductors, solar cells, and other devices that benefit from their unique charge-transfer reactions.1-4 In fact, iodine atoms can be incorporated in many materials giving rise to chargetransfer complexes that show electrical conductivities much higher than those of the un-doped materials.5-8 In such a complex, polyiodide ions exist in many possible structures, from small and discrete polyiodides to large extended anionic networks of interconnected units (e.g. simple I−, linear I42−, V-shaped I5−, three-pronged structures of I7− and I9−).1, 9-10 These structures can be described with three building blocks of I−, I2, and I3− and the diversity of polyiodide structures can be explained by the tendency for iodine atoms to catenate. A long polyiodide ion consisting of catenated iodine atoms looks like a polymer. L. Guan et al. claimed by high resolution transmission electron microscope (TEM) observations that iodine atoms encapsulated in a hollow core of single-walled carbon nanotube (SWCNT) form a chain-like structure as a polymer, although later spectroscopic studies could not prove the chain-like structure.11 It is also known that addition of iodine to a number of non-conducting compounds such as amylose produces intensely colored complexes whose chromophore also consists of chains of polyiodide units. This color changing reaction is well known as what we call “iodine-starch reaction”.12 However, the detailed mechanism of the reaction has not been fully understood yet. The structural diversities of polyiodide ions and starch which is a random mixture of two polymers: amylose and amylopectin make the starch-iodine complex very complicated. However, many previous works indicate that linear polyiodide ions are encapsulated in the

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amylose helix.13-14 For example, it was reported by T. Michel with Extended X-ray adsorption fine structure (EXAFS) experiments that polyiodide ions exist mainly as forms of linear I5−.15 Recently, we inserted iodine molecules into SWCNTs, which have cylinder pores as amylose has, and reported that the iodine doping greatly improve the electric conductivity and the water dipersibility of SWCNTs.16 The improvement of the dispersibility can be explained by the electrostatic repulsion between SWCNTs due to the surface charges induced by the charge transfer from SWCNTs to the encapsulated iodine molecules. It should be noted that the dispersibility of SWCNTs increased with decreasing temperature. The increase of the dispersibility at low temperature indicates the increase of charge-transfer at low temperature. However, the mechanism why the charge-transfer increases at low temperature has not been clarified yet. We do not have much knowledge on how polyiodide ions behave at low temperature in a confined nano-space of SWCNTs. Therefore, we investigate in this paper the low temperature structure changes of polyiodide ions encapsulated in SWCNTs, by performing in-situ Raman measurements at low temperature for polyiodide ions encapsulated in three kinds of SWCNT samples having different mean tube diameters. 2. Methods We used three kinds of SWCNT samples having different mean tube diameters HiPco type (supplied by NanoIntegris Technologies Inc.), and SO type and EC type (both supplied by Meijo NanoCarbon Co.). All the three samples were annealed at high temperature under vacuum. The high temperature annealing not only decreases the surface defects of SWCNTs but also closes the tube ends by forming half fullerene caps at the ends.17 Subsequently, we performed airoxidation treatments to open the closed ends by burning the half fullerene caps at an appropriate

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temperature for each SWCNT sample that was carefully determined by prior thermogravimetric (TG) measurement. The tube openings were confirmed by N2 adsorption measurements.18-19 By these series of treatments, we prepared high crystalline open-end SWCNT samples. We performed Raman measurements to check the crystallinity and synchrotron XRD measurements at BL-18C at the Photon Factory (PF: High Energy Accelerator Organization, Tsukuba, Japan) to determine the mean tube diameter. Laboratory scale powder XRD measurements were performed using Rigaku MiniFlex diffractometer, in order to observe structural changes caused by the encapsulation of iodine molecules. The nanostructure of the obtained samples was observed using a TEM (JEOL z-2500) operated at 200 kV. Iodine molecules were inserted into SWCNTs by electrochemical oxidation of iodide ions in electrolytic solution.16 To achieve electrochemical iodine doping, we fabricated threeelectrode configuration cells: paper form SWCNT working, Pt counter, and Ag/AgCl reference electrodes. 1 M NaI aqueous solution was used as an electrolyte, and we applied 1.8 V to SWCNT electrode for a few minutes. After the encapsulation treatments, we washed the SWCNT samples by distilled water to remove the iodine molecules deposited on the outer surfaces of SWCNTs and dried the washed samples. The amounts of the encapsulated iodine molecules were determined by TG measurements. In order to know the structure changes of polyiodide ions encapsulated in SWCNTs, low temperature Raman measurements were performed using a JASCO NRS-3300 spectrometer and a Linkam 10036L temperature control stage. I@SWCNTs sample was sealed in a glass capillary, which is originally used for XRD measurement, and the glass capillary was set on the cooling metal block. Temperature sensor was attached with the cooling block. We started Raman measurements at each temperature a few minutes after temperature reached to the target

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temperature. The low temperature Raman measurements of I@SWCNTs samples were done with two excitation lasers (λex = 532 nm and 633 nm). In order to assign the vibration mode of Raman peaks, ab initio calculations were performed. Structural optimizations and vibrational frequency calculations of polyiodide ions were performed based on the second order Møller–Plesset (MP2) perturbation theory using Gaussian09 software20. Dunning’s augmented double-zeta correlation consistent basis set with small-core relativistic pseudopotentials21 (aug-cc-pVDZ-PP) was used for the calculation. 3. Results & Discussion Figure 1 shows Raman spectra of three SWCNT samples (HiPco, SO, EC) used in the present study. D-band peaks observed at around 1330 cm−1 are so weak that it is very hard to find Dband peaks for SO and EC samples. Although D-band peak of HiPco sample is relatively high compared to the other two samples, its intensity ratio to G-band peak observed at around 1590 cm−1 is quite small. Therefore, we can judge that all the three SWCNT samples have high crystallinity. In low wavenumber region ( 3), we’ll discuss possible forms for these two peaks. One possibility is to assign these two peaks to different forms of I5− (e.g. V-shape and Linear shapes).39 However, the peak positions of I5−(V) and I5−(L) were reported to be 157.4 and 165.0 cm−1, respectively, which are not in good agreement with 160 and 175 cm−1 observed in the present study. It would be possible that the frequency modes of I5−(V) and I5−(L) were hardened in SWCNTs. This possibility is dismissed as follows. The frequency mode of I5−(L) was always observed at higher wavenumber side of I5−(V) peak. In fact, the order of the wavenumbers of I5−(V) and I5−(L) frequencies is reproduced by our ab initio calculations as shown in Fig. S3. Therefore, if the 160 and 175 cm−1 peaks are attributed to two forms of I5−, we have to assign 160 and 175 cm−1 peaks to I5−(V) and I5−(L), respectively. In this case, Raman peak of I5−(L) should be in resonance with 532 nm laser, as we discussed that 175 cm−1 peak is a resonant Raman peak. In order to check whether the Raman peak of I5−(L) is resonant process or not, we performed an experiment using amylose. Amylose has a helix structure and polyiodide ions are encapsulated in amylose.

13-15, 40-43

The

observed Raman spectra of amylose having linear polyiodide ions in its helix pores are shown in Fig. 7. In the Raman spectra, 110 cm−1 peak of I3− and 160 cm−1 peak, which is assigned to I5−(L), are observed. The relative intensity of 160 cm−1 peak to I3− peak at 110 cm−1 observed by 532 nm laser is almost the same as that observed by 633 nm. It means that the frequency mode of I5−(L) is not in a resonance process for 532 nm excitation. The resonance wavelength of I5− might be smaller than 532 nm. In the previous papers, it was reported that 110 cm−1 and 160 cm−1 peaks

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are enhanced by 458–477 nm and 488–514 nm lasers, respectively.4 Therefore, the assignments of 160 cm−1 and 175 cm−1 peaks to different forms of I5− should be wrong.

Figure 6. Temperature dependent Raman spectra of (A) [email protected], (B) [email protected] and (C) [email protected] measured with 532-nm excitation laser.

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Figure 7. Raman spectra of (A) [email protected] and (B) I@amylose. Excitation wavelength (λex) of green and red lines are 532 nm and 633 nm, respectively. We’ll think about second possibility: 160 and 175 cm−1 peaks are assigned to I5− and I7−, respectively. In this case, the frequency mode of I7− should be in resonance to 532 nm. It would be reasonable that the resonance wavelength of I7− should be longer than that of I5− (488–514 nm), because larger polyiodide molecules should have smaller HOMO-LUMO gaps. Based on this assignment, we’ll discuss the following observations: (1) 175 cm−1 peak intensity increases compared to 160 cm−1 peak with decreasing temperature for SWCNT-2.5, (2) 175 cm−1 peaks are the dominant peak for SWCNT-1.0 and -1.5 already at room temperature. For the former

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observation, one can explain it by the transformation from I5− to I7− through the reaction of I5− + I2 → I7−. However, as shown in the Raman spectra observed by 633 nm, 160 cm−1 peak intensity does not change with temperature. It means that the number of I5− does not decrease with decreasing temperature. Therefore, instead of the transformation from I5− to I7−, we have to think simple increase of I7− (e.g. 7I2 + 2e− → 2I7−). This idea is consistent with the observation that the relative peak intensity of polyiodide ions to SWCNT G-band increases with decreasing temperature. Since the peak intensity of I5− in amylose does not change with temperature, the increase of polyiodide Raman peak is characteristic feature of SWCNTs. As we wrote already, iodine molecules are inserted into SWCNTs as a form of I2 molecule. Therefore, in the case of iodine molecules encapsulated in SWCNTs, certain amounts of I2 molecules exist in SWCNTs. Although no strong Raman peak of I2 molecules was observed, it does not indicate the absence of I2 molecules, but it is due to the small extinction coefficient of I2: Raman scattering activities of I2 molecule estimated by an ab initio calculation performed on the present study is about 140 times smaller than that of the linear I5− molecule. It should be due to the existence of I2 molecules in SWCNTs that the intensity of polyiodide Raman peaks of I@SWCNTs increases with decreasing temperature unlike in the case of amylose. Although the I2 molecules are probably randomly adsorbed in the hollow cores of SWCNT-2.5 at room temperature, they would be gathered on the inner surface of SWCNTs with decreasing temperature and transformed to polyiodide ions by charge-transfer from SWCNTs. On the other hand, since tube diameters of SWCNT-1.0 and -1.5 are so small, long polyiodide ions having Raman peak of 175 cm−1 are dominantly formed by capillary condensation effect even at room temperature.

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4. Conclusion In conclusion, polyiodide ion formation in SWCNTs at low temperature is summarized as follows. First, I2 molecules are encapsulated in SWCNTs and they start to aggregate by intermolecular interactions. Charge-transfer from SWCNTs to the aggregated I2 molecules adsorbed on the inner surface of SWCNTs produces polyiodide ions. At low temperature, the aggregation and polyiodide ion formation are promoted because the formation reaction is exothermic. The polyiodide ions, on the whole, would have a chain-like structure along to the hollow core of SWCNTs while the microscopic structures of the polyiodide ions should be written as small units (e.g. I3−, I5−, I7−). ASSOCIATED CONTENT Supporting Information. TEM image of SWCNTs (Fig. S1), Kataura-plot (Fig. S2), and theoretical Raman spectra of polyiodide ions (Fig. S3). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected], Tel.: +81-52-735-5259

*

E-mail: [email protected], Tel./Fax: +81-52-735-5221

ACKNOWLEDGMENT

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The authors thank to Research Foundation for the Electrotechnology of Chubu, and JSPS KAKENHI Grant Number 26410238 for their financial support to the present study. REFERENCES 1.

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Kong, L.; Lee, C.; Kim, S. H.; Ziegler, G. R., Characterization of Starch Polymorphic Structures Using Vibrational Sum Frequency Generation Spectroscopy. J. Phys. Chem. B 2014, 118, 1775-1783.

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The Journal of Physical Chemistry

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Page 27 of 33 λex = 532 nm

x5

x10 (c)

x5 (b)

x5

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Intensity / arb. units

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The Journal of Physical Chemistry Page 28 of 33 SWCNT

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I@SWCNT

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Page 31 of 33 The Journal of Physical Chemistry SWCNT−1.0 ( λex = 532 nm ) SWCNT−1.5 ( λex = 532 nm ) 1610

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SWCNT−2.5 ( λex = 633 nm )

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λex = 633 nm

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λex = 633 nm

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