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Selective Wrapping of Few-walled Carbon Nanotubes by a Serpent-like Heterobimetallic Coordination Polymer Ingrid Fernandes Silva, Walace Doti do Pim, Ivo F. Teixeira, Wdeson P Barros, Ana Paula C. Teixeira, Gustavo Morari Do Nascimento, Cynthia L.M. Pereira, and Humberto Osório Stumpf J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07887 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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The Journal of Physical Chemistry

Selective Wrapping of Few-Walled Carbon Nanotubes by a Serpent-Like Heterobimetallic Coordination Polymer Ingrid F. Silvaa, Walace D. do Pima,b, Ivo F. Teixeiraa,c, Wdeson P. Barrosd, Ana Paula C. Teixeiraa, Gustavo M. do Nascimentoe, Cynthia L. M. Pereiraa and Humberto O. Stumpfa*

a

Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Av. Antônio

Carlos 6627, Belo Horizonte-MG, 31270-901, Brazil. b

Centro Federal de Educação Tecnológica de Minas Gerais, Departamento de Química, Av.

Amazonas, 5253, Belo Horizonte-MG, 30421-169, Brazil. c

d

Department of Chemistry, University of Oxford, OX1 3QR, UK. Instituto de Química, Universidade Estadual de Campinas, 13083-970, Campinas-SP,

Brazil. e

Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC.

ACS Paragon Plus Environment


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Abstract In this work, selective interactions between the constituents of the composite CNT@MnCu (2) prepared using carbon nonotubes (CNTs) (1) and the heterobimetallic chain [MnCu(opba)]n (MnCu), opba = ortho-phenylenebis(oxamate), were studied mainly by Resonance Raman spectroscopy and High-resolution Transmission Electron Microscopy (HRTEM). An apparent interaction between CNTs and MnCu complex with the wrapping of the former by the heterobimetallic complex was observed in the microscopy images. The Resonance Raman data suggest that the interations between MnCu complex and the CNTs are selective, occurring mainly with metallic CNTs independently of the diameter and excitation energy. However, for semiconducting CNTs, this interactions solely occurs with tubes having diameter higher than ca. 1.47 nm.

Introduction Since the discovery that information can be carried by spin orientation of conducting electrons, the field of spintronics has made considerable progress in the development of electronic devices both at fundamental and applied levels.1-5 Such development brought new perspectives that can lead to a revolution on the electronics industry in coming decades, making possible a new generation of devices that combines standard microelectronics with spin dependent effects, opening up a field for construction of devices with faster memories and processing speeds and lower power consumption at low electron density.3,4,6 Although several kinds of magnetic materials, from metallic to semiconducting, and organic materials can be used in the development of spintronic devices,7-9 low dimension structures are more desirable for nanoscale electronics. Thus, a promising strategy for constructing molecular spintronic devices, in the molecular scale-limit, is that of combining conducting


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materials with small magnetic molecules. A propitious class of compounds that can provide the required magnetism and dimensionality includes the composites prepared from carbon nanotubes (CNTs) and molecule-based magnets.10,11 Besides, it is possible to tune the physical properties of such composites since devices in which the conductance of the nanotubes strongly depends on the coupling with magnetic molecules1 can be build, apart from making room for multifunctional applications.10 One of the biggest challenges on the preparation of such composites is related to the nature of interaction and/or connection between its constituents (carbon nanotubes/complex). Molecular systems capable of performing π-π stacking interactions11,12 or covalent bonding1 are commonly used. Among the various complexes used to produce these composites, a system of great potential due to its magnetic properties and topological features is the MnII/CuII/opba system, where opba = ortho-phenylenebis(oxamate).13,14 In effect, this magnetic system can adopt several dimensionalities15-17 that can lead to a number of interaction modes with the CNTs. Another appealing way to promote variations of the interactions in CNTs composites is the use of ionic liquids due to their unique physical and chemical properties. The functionalization and dispersion of CNTs in ionic liquids has been widely studied,18,19 since they can act as surfactants, making easier the dispersion of CNTs due to their amphiphilic character. In another context, our group is trying to verify if selective interactions of molecule-based magnets with CNTs can be useful for purification of the later. It is known that CNTs separation with respect to different chiralities can be achieved by the use of surfactants followed by chromatography separation and other similar methods.20,21 However, heterobimetallic ferrimagnetic chains22 could selectively separate CNTs aside with high



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purity level at low temperatures, when these materials reach their magnetic ordering temperature. This strategy is still challenging, but very promising, since it could allow magnetic CNT composite separation without the use of organic solvents and chromatographic columns. In this work we present the study of the interactions between the constituents of a new composite CNT@MnCu (2) prepared from carbon nanotubes (CNTs, 1) and heterobimetallic coordination

polymers.

The

later

consists

of

a

heterobimetallic

complex

{(bmim)xMny[Cu(opba)]z}, bmim = 1-butyl-3-methylimidazolium, with x = y = 2 and z = 3 for a 2D structure,23 while x = 0 and y = z = 1 for a linear22 or zig-zag23 chain (MnCu). The bulky organic cation (bmim+) is able to perform weak van der Waals interactions with CNTs. Moreover, the oxamato-based ligand (opba) was chosen due to its potential to perform π-π stacking interactions with CNTs. These interactions were studied by High-resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM) and Resonance Raman spectroscopy. Experimental The CNTs used in this work were obtained from commercial source and the manufacturer specifications are shown in supplementary information (Table S1). A suspension was prepared using 1 mg of CNTs and 10 mL of dmso. The suspension was left in a tip ultrasonic bath at 0 °C for 45 minutes (Sonic Ruptor 400/ titanium tip 5/32”) at a power of 20%, in order to increase the dispersion of the CNTs. Then the mixture was centrifuged at 10,000 rpm for 5 minutes. The solid was separated (1), washed with dmso and dried in a desiccator under vacuum for one week.


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Sample 2 was prepared by mixing 10 mL of bmimCl (0.61 mmol) solution in dmso and CNTs (1 mg) under similar conditions to sample 1. A solution of 0.06 mmol of (bmim)2[Cu(opba)]·3H2O24 in 20 mL of dmso was added slowly and under constant stirring into the suspension of CNTs, followed by the addition of MnCl2·4H2O solution (0.02 mmol in 10 mL of dmso). Then the sample 2 was centrifuged and dried at the same conditions used to sample 1. The Raman spectra were collected at excitation laser wavelengths of 785.0 nm (ELaser = 1.58 eV), 632.8 nm (ELaser = 1.96 eV) and 532.0 nm (ELaser = 2.33 eV) on a Bruker Senterra Raman spectrometer equipped with a CCD detector. All Raman spectra were collected at room temperature and using a backscattering geometry. The laser line was focused on the sample by using a 50X objective, and the power incident on the sample was kept lower than 2mW to avoid heating effects. Different acquisition times between 5 and 30 s were used for each sample in an attempt to optimize the signal-to-noise ratio of the Raman spectra. The infrared spectra of the materials were obtained using a Fourier transform infrared spectrophotometer (FTIR; Shimadzu, model IR Prestige 21) by the attenuated total reflectance (ATR) technique. High-resolution Transmission Electron Microscopy (HRTEM) images were obtained using a Tecnai G2 200 kV – SEI microscope. The samples were suspended in acetone and deposited onto a holey carbon film on 200 mesh copper grids. The Atomic Force Microscopy (AFM) images were collected using a MFP-3D-SA Asylum Research microscope and the sample was prepared by suspension of 2 in chloroform and deposited onto a piece of mica. The samples were also characterized by Thermal analyses (TG/DTG/DTA). The measurements



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were carried out in a DTG 60H Shimadzu, with an air flow of 50 mL min‒1 and heating rate of 10 °C min‒1. Results and Discussion The HRTEM images for sample 1 show mainly Few-walled Carbon Nanotubes bundles (Figure 1.a). It was observed that the tubes have diameters in a range from 1 to 2 nm. This dispersion is compatible with the commercial specification (from 1 to 2 nm). The HRTEM images for sample 2 show a similar morphology, however, it was also observed structures with distinct morphology that can be related to the MnCu complex (Figure 1.b). Atomic Force Microscopy (AFM) images (Figure S4) show a discontinuity on the surface of the tubes, probably owing to the presence of MnCu complex on their surface. Profiles analyzed in different regions of the same tube or nanotubes bundle provide further evidence of this interaction since they show an increasing of the local CNT radius from 7 nm in the region without complex to 18 nm in the region that interaction with MnCu complex probably occurs. The HRTEM image depicted in Figure 1.c shows an isolated SWCNT helically wrapped by MnCu chains, as highlighted by arrows. Similar wrapping was observed to Few-walled Carbon Nanotubes, as shown in Figure 1.d for a four-walled CNT. Most of the images shows an apparently helical wrapping similar to those reported by Deria et al25(more HRTEM images to sample 2 are presented in supplementary information see Figure S3).


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Figure 1. HRTEM images: (a) few-walled carbon nanotubes bundles (1); (b), (c) and (d) composite CNT@MnCu (2). Figure 2 exhibits the Raman spectra obtained for the samples 1 and 2 at different laser lines. It can be seen in Figure 2 that samples 1 and 2 are dominated by the characteristic features (RBM, D, G and G' signatures) from the few-walled carbon nanotubes owing to the strong resonance Raman effect with the van Hove singularities of the nanotubes.26,27 The sharp bands from 150 to 350 cm‒1 are assigned to the radial breathing mode (RBM) of the carbon nanotubes.28 The strong bands in the frequency range from 1500 to 1650 cm‒1 are associated with the (C‒C) stretching modes (tangential G band). The mode at approximately 1300–1350 cm‒1 is related to the disorder-induced D-band feature.29



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The second order spectrum of CNTs is dominated by the G' band (not shown in the Figure), which is the highly dispersive harmonic of the D-band frequency, and it is observed here from approximately 2500–2700 cm‒1.

Figure 2. Resonance Raman spectra excited using three laser lines (785.0, 632.8, and 532.0 nm) of CNTs (1) and composite CNT@MnCu (2). In order to facilitate the visualization of the RBM bands, this entire spectral region (from 130 to 400 cm‒1) was inset with large background scale. In order to see the influence of the MnCu complex on the electronic properties of the metallic and semiconducting tubes, the detailed analysis on the RBM bands observed at Elaser = 1.58 eV is presented in Figure 3. The dt values were estimated from the empirical equation30 ωRBM(cm‒1) = 233/dt (nm) + 14 (see values in the table inside Figure 3). Calculated energy separations between the van Hove singularities [Eii(dt)] for CNTs were obtained from tightbinding calculations (Kataura plot),31-33 for the laser line at 785.0 nm (Elaser = 1.58 eV).


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According to these results, the band values below 204 cm‒1 are related to the metallic tubes from the family and the peaks from 204 to near 267 cm‒1 are related to the semiconducting tubes from the family . An increase in the relative intensities of the bands

from 204 to 267 cm‒1 can be observed from sample 1 to 2. In addition, is observed a shift of the peaks from 144 to 154 cm‒1 (∆ = +10 cm‒1), 159 to 165 cm‒1 (∆ = +6 cm‒1) and 171 to 174 cm‒1 (∆ = +3 cm‒1). It is worth to highlight that the effect was only observed for bands associated to tubes from , corresponding to nanotubes with diameter from 1.48 to 1.79 nm. However, the same effect was not observed for the bands associated to tubes from family,30 corresponding to

tubes with diameter lower than 1.23 nm.34 This may be an evidence of selective interactions between the nanotubes and the MnCu complex.



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Figure 3. Resonance Raman spectra (RBM band region) excited by laser line at 785.0 nm (1.58 eV) for powder samples 1 and 2. Inside the Figure are also shown the band values obtained by deconvolution of the spectra using Voigt curves. The red curves are the deconvoluted single bands, the blue dot curve is the sum of all red curves and finally the black curve is the experimental data. The changes in the intensities were similar to our previous work using solely the monometallic complex (Bu4N)2[Cu(opba)],35 however the increase of the RBM frequency values was only observed when the MnCu complex was used. It probably indicates that the electronic interactions between the MnCu complex and the CNT surface are stronger than the interactions with the monometallic complex. Although not being present in the final structure, the use of imidazolium cations may help the anchoring of the CuII complex precursor to the CNTs, leading to the formation of the MnCu complex on its surface.


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Recent studies have attributed this shift to intermolecular van der Waals interactions, which can occur between the CNTs and substrate in direct contact.36 According to Zhang et. al.,36 these interactions promote deformations in the tube, which consequently cause shifts in the RBM band. The presence of van der Waals interactions in this system can be related to interactions between the aliphatic chains of bulky cation (bmim+) and CNTs. Similar behavior was observed for Raman spectra obtained at 632.8 nm (Elaser = 1.96 eV), where mainly tubes from and families are in resonance. At higher Elaser (2.33 eV) or

532.0 nm, it were seen changes in the relative intensities and also shifts for RBM bands from 254 to 257 cm‒1 (∆ = +3 cm‒1), from 266 to 270 cm‒1 (∆ = +4 cm‒1), and from 276 to 281 cm‒ 1

(∆ = +5 cm‒1) for tubes assigned to family with minor diameter than observed for the

other two exciting radiations. However, RBM bands from 155 to 160 cm‒1 (∆ = +5 cm‒1), and family also have changed their intensities from 143 to 149 cm‒1 (∆ = +6 cm‒1) assigned to

and frequencies in the presence of the MnCu complex as can be observed in Figure 4. Hence, contrarily to that observed for lower Elaser energies, at Elaser = 2.33 eV also semiconducting tubes interact with the MnCu complex.



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Figure 4. Resonance Raman spectra (RBM band region) excited by laser line at 532.0 nm (2.33 eV) for powder samples 1 and 2. Inside the Figure are also shown the band values obtained by deconvolution of the spectra using Voigt curves. The red curves are the deconvoluted single bands, the blue dot curve is the sum of all red curves and finally the black curve is the experimental data. Considering all data from RBM region, it is possible to suggest that the MnCu complex interact mainly with metallic tubes independently of the diameter of the tube and excitation energy. However, for semiconducting tubes, this interaction solely occurs for tubes with diameter higher than ca. 1.47 nm (Figure 5). Theoretical37 and experimental38 studies reported that π-π stacking interactions are stronger with metallic CNTs. Since the MnCu chains studied in this work are based on the ligand opba (Figure S2), which has an aromatic ring, the chains are capable to perform π-π stacking interactions with the CNTs. A possible explanation for this selective behavior could be related to these interactions towards the tubes with different chiralities and diameters. Since the π-π stacking interactions are stronger with


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metallic CNTs, the MnCu chains may wrap them independently of the chain conformation. On the other hand, for the semiconducting tubes the π-π stacking interactions are weaker, therefore possibly the wrapping only happens when the tubes diameter is large enough to promote more stable conformations to the complex chain, such as the helical.

Figure 5. Selective interaction scheme between CNTs and MnCu chains formed from the reaction of Cu(opba)2- and Mn2+ ions. The wrapping is selective for metallic tubes and for semiconducting with diameter higher than 1.47 nm. Figure 6 shows the Raman spectra of CNTs samples in the spectral region from 1200 to 2700 cm‒1 excited by laser line at 785.0 nm (1.58 eV) and 532.0 nm (2.33 eV), where the characteristics D and G bands can be seen. It is possible to see that there are small modifications between the spectra. In this Elaser mainly semiconducting tubes are in resonance and according to our previous studies these tubes interact less with the complex than the metallic tubes.35 However, it is possible to see that the relative intensity of the D band and also its line width decrease in the presence of heterobimetallic complex.



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In addition, the G band (mainly the G+ component at ca. 1585 cm‒1) has an increase in its relative intensity. Similar behavior is observed at Elaser = 2.33 eV, however is observed that the G‒ component at ca. 1527 cm‒1 has a reduction in its relative intensity in the sample 2 compared to the pristine tube.

Figure 6. Resonance Raman spectra (D and G band region) excited by laser line at 785.0 nm (1.58 eV) and 532.0 nm (2.33 eV) for powder samples 1 and 2. The red curves are the deconvoluted single bands, the blue dot curve is the sum of all red curves and finally the black curve is the experimental data. For comparison purposes, the ratio of the integrated area between the D and G band (only the most intense at ca. 1585-90 cm‒1 was used) was added in the Figure. The G‒ band is mostly associated with metallic tubes, based on its observed line shape. It must be noticed that the strong electron–phonon coupling observed in metallic nanotubes gives rise to Kohn anomalies in the phonon dispersions.39,40 Hence, the influence of the metallic tubes can easily be distinguished from semiconducting tubes by the presence of the


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lower-frequency component of the G-band spectra (denoted by the metallic G‒ feature), which shows a Breit–Wigner–Fano (BWF) line shape.41,42 This characteristic was not observed in the spectra obtained when other laser lines were used, where the G band shows mainly the line shape of semiconducting tubes. The peak position of G' bands (not shown) shift to higher energies only for higher Elaser where the presence of metallic tubes is more evident by the presence of G‒ band component (frequencies values of samples 1 to 2: at 785.0 nm from 2585 to 2585 cm‒1, at 632.8 nm from 2621 to 2622 cm‒1, and at 532.0 nm from 2665 to 2672 cm‒1), which is probably related to some small charge transfer. Another spectral evidence of the electronic interaction between the CNTs and MnCu complex comes from the shift observed in the FTIR data (Figure S9), where the strongest band at ca. 1594 cm‒1 (assigned to νC=O by comparison to the [MnCu(opba)]n) shifts to 1633 cm‒1 in the composite 2. This shift could be associated to the bond tensions in the structure provoked by coupling of the MnCu complex around the cylindrical structure of the tubes. Summing up, the images of transmission electron microscopy show that the MnCu chains may be spiralling along the surface of the CNTs. The formation of this structure probably justify the changes observed in the Raman and FTIR spectra of the sample 2, which suggest that there is an electronic interaction between the MnCu complex and the CNTs. RomeroNieto et. al. attribute this effect to π-π stacking43 interactions between the nanotubes and the aromatic ring present in the opba ligand, as shown in Figure 7.



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Figure 7. Scheme of the π-π stacking interactions between the MnCu complex and SWCNT. Conclusions In this work were prepared a composite by the use of few-walled carbon nanotubes and the MnCu complex. The interactions between the constituents of this composite (carbon nanotubes/complex) were studied by transmission electron microscopy and resonance Raman spectroscopy. The HRTEM images revealed a wrapping of CNTs by the MnCu complex. Raman data also showed the presence of significant interactions between the tubes and the complex, such as van der Waals intermolecular interactions and π-π stacking interactions. The MnCu complex interacts mainly with metallic CNTs independently of the diameter and excitation energy, however, for semiconducting CNTs this interaction solely occurs with tubes having diameter higher than ca. 1.47 nm, possibly due to the difference on the π-π


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stacking interactions with metallic and semiconducting tubes. This feature can leads to a new method of selective separation of CNTs based on molecular magnetism. Associated Content Supporting Information CNTs Specification; Synthetic Scheme; HRTEM and AFM images; Thermal Analyses (TG/DTG/DTA); Infrared Spectra and Structure of the [MnCu(opba)]n. This information is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author *E-mail: [email protected]; Phone: 55-31-34094039; Fax: 55-31-34094647 Notes The authors declare no competing financial interest. Acknowledgements The authors thank the Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq), the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support and Center of Microscopy at the Universidade Federal de Minas Gerais for providing the equipment and technical support for experiments involving electron microscopy. The authors are also grateful to Professor R. M. Lago and his PhD student F. Mendonça, both from the Universidade Federal de Minas Gerais (Brazil), for carry out the thermal analysis (TGA/DTA).



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