Influence of Single-Walled Carbon Nanotubes Enriched in

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Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene) Mihaela Aneta Baibarac, Ioan Baltog, Mirela Ilie, Bernard Humbert, Serge Lefrant, and Catalin Negrila J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11198 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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The Journal of Physical Chemistry

Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene) M. Baibarac1*, I. Baltog1, M.Ilie1,2, B. Humbert3, S. Lefrant3 and C. Negrila1

1

National Institute of Materials Physics, Lab. Optical Processes in Nanostructured Materials,

P.O. Box MG-7, Bucharest, R077125, Romania 2

University of Bucharest, Faculty of Physics, P.O. Box MG-1, 077125 Bucharest, Romania

3

Institut des Matériaux “Jean Rouxel” (UMR CNRS-Université de Nantes n°6502),

2 rue de la Houssinière, B.P. 32229, F-44322, Nantes, France

ACS Paragon Plus Environment

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ABSTRACT

A new synthesis method based on the electrochemical reduction of α, α, α’, α’-tetrabromo-pxylene (TBPX) in the presence of single-walled carbon nanotubes (SWNTs) is used to obtain composites of carbon nanotubes (CNTs) functionalized with poly(para-phenylenevinylene) (PPV). In order to separate the effects of metallic and semiconducting CNTs on their interaction with PPV, the experiments are carried out with SWNTs enriched in semiconducting (S, 99%) and metallic (M, 98%) tubes. Significant changes in the relative intensity and shift of the radial breathing vibrational mode are reported in the Raman spectra of the as-prepared SWNTs samples, i.e., as mixtures of metallic (33%) and semiconducting (66%) entities and SWNTs enriched in M tubes (98%) that result from the electrofunctionalization with PPV. This different behavior originates in the non-covalent and covalent functionalization of SWNTs enriched in M and S SWNTs tubes with PPV, respectively. A gradual decrease in the absorption of the IR bands of PPV situated in the spectral range of 750 to 1000 cm-1 as a result of the increase of the polymer weight on the blank Au support is reported. The electrofunctionalization of the asprepared SWNTs induces position changes of the PPV IR absorption bands. In the presence of S enriched SWNTs, a significant increase in the absorbance of the two IR bands peaked at 1452 and 1471 cm-1, which are assigned to the vibrational modes of the phenyl ring stretching and the quinoid structure of the PPV, respectively, is reported. It results from the covalent bonding of the PPV macromolecular chains onto the surface of the SWNTs enriched in S tubes. Our results also demonstrate that the PL quenching effect reported for the PPV/SWNTs composites is due to the metallic nanotubes.


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1. Introduction The interest in poly(para-phenylenevinylene)/carbon nanotubes (PPV/CNT) composites began in 19991 motivated by their great potential for applications in the field of photovoltaic devices. The synthesis method that is currently used for composites based on PPV and CNTs (either multi-walled carbon nanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs)) is the thermal conversion of the PPV precursor solution (hereafter labeled PPV PS) containing different weight percentages of CNTs1,2. For a better understanding of the physical properties of this composite material, various optical studies have been reported: Stokes and anti-Stokes photoluminescence (PL),3,4 electroluminescence,5 UV-VIS-NIR absorption,2 photoconductivity,6 resonant/non-resonant Stokes/anti-Stokes Raman scattering7,8 and absorption middle IR spectroscopy8. The main highlights of these studies are the following: i) SWNTs induce a percolation regime in the PPV matrix, beginning at a weight percentage of SWNTs of ≈ 2%6; ii) the changes of the UV-VIS-NIR and the PL spectra reveal significant electronic interactions between the two constituents; iii) the PL spectra reveal that SWNTs play a role in inhibiting the complete thermal conversion of the PPV precursor6 due to the functionalization of SWNTs with PPV macromolecular chains (MCs) of different length; iv) SWNTs play the role of a PPV PL quenching agent1-5; v) non-resonant/resonant Raman spectroscopy7,8 demonstrate that a noncovalent functionalization of SWNTs with PPV occurs during the thermal conversion of the PPV PS containing different weights of SWNTs. and vi) a modification of the angle of the IR active transition dipole moment of the phenyl ring C-H in the out-of-plane vibrational mode, situated at ~835 cm-1, was reported in the case of the samples obtained by the thermal conversion of the PPV PS containing different weights of SWNTs8. Note that in all of the above optical studies, the samples used were composites prepared from PPV PS containing as-prepared SWNTs, i.e., a


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mixture of metallic (33%) and semiconducting (66%) entities, with the weight percentage of CNTs ranging between 1% and 64%2-8. In comparison with previous papers2-8, the present work reports a new synthesis method for the PPV/SWNT composites, based on the electrochemical reduction of α, α, α’, α’-tetrabromo-p-xylene (TBPX) onto the CNT film. Until now, the electrochemical reduction of TBPX was only conducted on a quartz crystal coated with a gold film9 and a Pt electrode10. The method proposed in this work, involving the functionalization of SWNTs with PPV, opens new opportunities to study the different influences of SWNTs that are enriched in semiconducting (S) and metallic (M) tubes on the vibrational, PL and photoconductive properties of PPV. Therefore, we will focus the reader's attention on the optical properties of the composites based on SWNTs enriched in S and M tubes, which are electrochemically functionalized with PPV, as is shown by Raman scattering, IR spectroscopy and PL. To our knowledge, no paper in the literature has focused on studies of IR spectroscopy concerning SWNTs enriched in S and M tubes and which were functionalized with conjugated polymers (CPs). Few papers were focused on the Raman scattering and PL studies on SWNTs enriched in S and M tubes functionalized with CPs. These papers concern macromolecular compounds polydiphenylamine (PDPA)11, poly[(2,5-bisoctyloxy)-1,4-phenylene-vinylene] (BOPPV)12 and poly(2,2′-bithiophene-co-pyrene) (PBTh-Py)13. In the three cases: i) a covalent functionalization of SWNTs with macromolecular compounds was demonstrated by Raman spectroscopy and ii) a PL quenching process of composites based on PBTh-Py, PDPA and BOPPV and SWNTs enriched in M or S tubes, which manifests differently for the two types of nanotubes. The separation of SWNTs in S and M components was done through various methods, such as density gradient centrifugation14, the interaction of CNTs with the electron donor and acceptor molecules15, dielectrophoresis16 and so on. Currently, it is established that the


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thermal conversion of PPV PS containing different weight percentages of CNTs results in a noncovalent functionalization of CNTs with PPV in un-doped state.1-8 Taking into account that the electrochemical polymerization of different monomers onto CNT films can lead to different types of composite17 with various optical properties, the following questions require an answer in this paper: i) what type of composite is obtained by the electrochemical reduction of TBPX onto a CNT film?; ii) can the electrochemical synthesis of PPV onto the CNT surface lead to a shielding effect and/or an isolation of individual tubes from bundles? iii) what is the role of SWNTs enriched in S and M tubes on the PPV PL quenching process and what are the implications of S-SWNTs and M-SWNTs on the PL properties of PPV molecular compounds (MC) of different lengths? We try to answer these questions, by proposing a reaction mechanism for the electrochemical functionalization of SWNTs with PPV. 2. Experimental SWNTs enriched in metallic (98%) and semiconducting (99%) entities (labeled as MSWNTs and S-SWNTs) as well as as-prepared SWNTs, i.e. a mixture of metallic (33%) and semiconducting (66%) tubes (labeled as (M+S)-SWNTs) were purchased from NanoIntegris. The strategy used for the synthesis of the PPV/SWNTs composite involves: i) the preparation of three suspensions of M-SWNTs, S-SWNTs and (M+S)-SWNTs that have ~ 2 mg of CNTs in 2 ml of toluene under ultrasonication for 10 min; ii) the deposition of the M-SWNTs, S-SWNTs and (M+S)-SWNTs films onto rough Au supports with an area of 1 cm2 (the CNT film thickness is estimated using the protocol described in Reference 18 and by knowing that the density of the M-SWNTs, S-SWNTs and (M+S)-SWNTs

was equal to 1.1319, 2.2319 and 1.33720 g/cm3,

respectively); iii) the preparation of a solution of 0.02 M TBPX and 0.1 M tetrabuthylammonium bromide (TBAB) in dimethyl formamide (DMF). PPV and its composites were synthesized using


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cyclic voltammetry in a conventional three-electrodes, one-compartment cell. The working electrode was a blank Au electrode or a rough Au support covered with a film of M-SWNTs, SSWNTs, or (M+S)-SWNTs. The area of the Au electrode was 1 cm2. The counter electrode was a spiral Pt wire, while the reference electrode was a commercial Ag/AgCl electrode (3 M KCl). PPV and its composites were electrochemical synthesized over the potential range of (-2000; +2000) mV vs. a Ag/AgCl electrode at a sweep rate of 100 mV s-1 applied both on a blank Au electrode and Au electrodes coated with films of M-SWNTs, S-SWNTs, or (M+S)-SWNTs. All cyclic voltammograms were stopped at -2V vs. Ag/AgCl. The cyclic voltammograms (CVs) were recorded using a potentiostat/galvanostat from Radiometer Analytical, VOLTALAB 80 model. Raman spectra were recorded with a spectral resolution of 2 cm-1 in a backscattering geometry under excitation wavelengths of 1064 nm and 676.4 nm using a RFS100S FT Raman spectrophotometer from Bruker and a T64000 Raman spectrophotometer from Horiba Jobin Yvon, respectively. The IR spectra were recorded with a resolution of 4 cm-1 in the attenuated total reflectance (ATR) geometry, using a Fourier Transform device of Bruker, Vertex 70 FTIR spectrometer. The UV-VIS-NIR spectra were recorded on films deposited onto ITO supports with a resolution of 1 nm using a dispersive Perkin Elmer spectrometer, model Lambda 950. The PL spectra were recorded in a right-angle geometry at room temperature (RT) using a Horiba Jobin Yvon Flurolog-3 spectrometer, model FL 3-22.


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The X-ray photoelectron spectroscopy (XPS) spectra were recorded with a SPECS XPS spectrometer endowed with a Phoibos 150 electron energy analyzer with an ultimate resolution of 0.44eV (defined as the lowest obtainable FWHM of Ag3d5/2 spectrum). For excitation, a monochromated X-Ray Al Kα (1486.6eV) was used. The spectra were recorded with a Pass Energy of 3eV and were fitted using Voigt functions. 3. Results and discussions Figure 1 shows the first 30 CVs recorded on the blank Au electrode and on the rough Au supports covered with films of M-SWNTs, S-SWNTs and (M+S)-SWNTs. The electrosynthesis of PPV onto a quartz crystal covered with a gold film9 involves a reduction reaction of the monomer TBPX during the cathodic scan. During the scanning of the potential from 0 to -2 V vs. Ag/AgCl, the formation of a TBPX diradical was invoked for the first time in 2005.21 The free electrons of the diradical can there are either on one atom or on different atoms. According with Ref. [21], the absorption band peaked at 338 nm was assigned to TBPX diradical with free electrons reside on one atom. Figure 1S shows the UV-VIS absorption spectra of TBPX in the initial state and during its electrochemical reduction at -2V as well as PPV. According with Figure 1S, an absorption band peaked at 338 nm was not observed during the electrochemical reduction of TBPX in the, potential range from 0 to -2 V vs. Ag/AgCl. The absorption band of TBPX is peaked at 270 nm (4.6 eV) in good agreement with the Ref. [22]. The UV−VIS spectrum of the PPV ﬁlm electrochemical synthetized onto ITO support shows an absorption edged at 3.23 eV, which is assigned to the π−π* electronic transition.17 A new absorption band with maximum at 3.95 eV (312 nm) is observed during the electrochemical reduction of TBPX at -2V. We assign the absorption band at 312 nm to diradicals with free electrons reside on one


7


atom. In fact, scanning the potential from 0 to – 2 V vs. Ag/AgCl takes place the formation of the intermediate reaction product of the carbene type (R1R2C:, where R1 = H and R2 = Br2HC-C6H4), which corresponds to a diradical with free electrons reside on one carbon atom. Such an intermediate product is not stable.

Its short lifetime and high chemical reactivity towards

themselves result in a spontaneous dimerization reaction that ends with the formation of a dimer with the chemical formula: Br2HC-C6H4-CH=CH-C6H4-CHBr2. Successive reduction reactions of the dimer enables the growth of MC, so that on the Au electrodes surface, a PPV film will be obtained. 0,04 0,03

b) PPV-SWNT (M+S) / Au

a) PPV/Au

30 20 10 5

i (mA cm-2)

0,02

30 20 10 5

0,02

0,01

-0,01 -2

0,00

5 10 20 30

0,00

-1

0

1

2

0,02

30 20 10 5

0,00

0,02

-1

0

1

2

30 20 10 5

0,00

30 20 10 5

-0,02 -2

-0,02 -2

30 20 10 5

0,04 d) PPV-SWNT-M / Au

0,04 c) PPV-SWNT-S / Au

i (mA cm-2)

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30 20 10 5

-0,02 0 E (V) vs. Ag/AgCl

2

-2

0 E (V) vs. Ag/AgCl

2

Figure 1. Cyclic voltammograms recorded onto the blank Au electrode (a) and on the rough Au supports covered with films of (M+S)SWNTs (b), S-SWNTs (c) and M-SWNTs (d), when working electrodes were immersed into a solution of 0.02 M TBPX and 0.1 M TBAB in DMF.


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Black, red, green and blue curves in Figures a, b, c and d correspond to the 5th, 10th, 20th and 30th cycle. These reduction reactions are evidenced for the fifth CV (Figure 1) as four cathodic peaks at 0.16, -0.5, -1.05 and -1.78 V vs. Ag/AgCl. In the case of rough Au supports covered with films of M-SWNTs, S-SWNTs and (M+S)-SWNTs for cathodic scans of the fifth CV, a pronounced maximum at -0.18, -0.2 and -0.3 V vs. Ag/AgCl, respectively, can be observed. As the number of cycles increases from 5 to 30, an increase in the current density is noted for the four working electrodes. In the case of the blank Au electrode (electrode A, Figure 1a) and the rough Au supports covered with films of M-SWNTs (electrode B, Figure 1d), S-SWNTs (electrode C, Figure 1c), and (M+S)-SWNTs (electrode D, Figure 1b), a down-shift of the first cathodic peak from -0.16 to -0.18, -0.18 to -0.33, -0.2 to -0.35 and -0.3 V to -0.63 V vs. Ag/AgCl, respectively, is reported. These variations indicate a change in the reaction mechanism of the electrochemical synthesized PPV onto a Au support covered with a CNT film. The difference between the mechanisms that occur during the electrosynthesis of PPV onto a blank Au electrode and that on rough Au supports covered with films of CNTs involves the interaction of SWNT cation radicals with diradicals that result during growing the PPV MCs. Schema 1 details the reactions that occur both on the blank Au electrode and on the rough Au supports covered with films of CNTs. a)

Br2HC-C6H4- CHBr2 + 2e-→ Br2HC-C6H4- CH: + 2 Br-

(1)

2 Br2HC-C6H4- CH: → Br2HC-C6H4-CH=CH-C6H4-CHBr2

(2)

Br2HC-C6H4-CH=CH-C6H4-CHBr2 + 2e- →

(3)

Br2HC-C6H4-CH=CH-C6H4-CH=CH-C6H4-CH=CH-C6H4CHBr2 + 2Br-


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…….. (-C6H4-CH=CH-)n PPV C

b)

C

- 1e

-

C

(4)

C

SWNT

Br 2

C

C

+

Br2HC

CH

+ 2Br-

C

C

(5) HC

CHBr2

C

C Br

Br C 2

C

HC C

C CHBr2

+ 4e-

4Br- +

C

C

CH

C

Br

H

Br

HC

C

C

Br

C

Br

Br

Br

C

C

C

CH

CH

CH

C

Br

CH

C

(6)

C

C C

H

n C

C

C

Br

Br

C

PPV oligomers covalently functionalized SWNTs Schema 1. The chemical mechanism of electrochemical synthesis of PPV onto the blank Au electrode (a) and on the rough Au supports covered with films of CNTs (b). The covalent functionalization of SWNTs with PPV MCs should induce further changes in the


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Raman and FTIR spectra of the two constituents in comparison with those reported for SWNTs that are non-covalently functionalized with PPV. To prove this hypothesis, Raman and FTIR spectra of the PPV oligomers electrochemically synthesized onto the Au electrodes covered with films of M-SWNTs, S-SWNTs and (M+S)-SWNTs are obtained. Figures 2 and 3 show the Raman spectra of (M+S)-SWNTs, S-SWNTs and PPV films as well as of the composites formed after the achievement of 10, 20 and 30 CVs when the two working electrodes, i.e., Au supports covered with films of (M+S)-SWNTs and S-SWNTs, were immersed in a solution of 0.02 M TBPX and 0.1 M TBAB in DMF.

1200

1400

λexc = 1064 nm

1424

1327

1174

1629

1591

PPV

e

1600

1550

100 150 200

1591

1327

1174

(M+S)-SWNT + PPV (30 CV)

d

1591

1300

164

1174


c

1278

(M+S)-SWNT + PPV (10 CV) 1174

164

1594

b

(M+S)-SWNT

a 100 150 200

1200

1273

1595

164

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Raman Intensity (a.u.)

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1400

1600

-1

Wavenumber (cm )

Figure 2. Raman spectra, recorded at λexc = 1064 nm, of the films of (M+S)-SWNTs (a) and its


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composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a (M+S)-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film. According to Figure 2e, the main Raman lines of the PPV electrochemical synthesized onto the blank Au support are situated at ~ 1174, 1327, 1424, 1550, 1591 and 1629 cm-1; these lines are assigned to the following vibrational modes: C-C stretching + C-H bending of the phenyl ring, C=C stretching + C-H bending of the vinyl group, symmetrical phenyl ring bending, C=C stretching of the phenyl ring, C-C stretching of the phenyl ring and C=C stretching of the vinyl group, respectively.17 Concerning the Raman spectra of the (M+S)-SWNTs and S-SWNTs films recorded at an excitation wavelength of 1064 nm (Figures 2a and 3a), these are identical over the 1100-1650 cm-1 spectral range and are characterized by two Raman bands peaked at 1273 and 1595 cm-1 which are assigned to the disorder state or the defects induced in the CNT architecture (labeled as the D band) and the tangential mode (TM) vibration, respectively.18 In the low frequency spectral range, Raman spectra of (M+S)-SWNTs and S-SWNTs (Figures 2a and 3a) highlight a Raman band with a maximum at 164 and 168 cm-1, respectively, each of which is attributed to the radial breathing mode (RBM) of the CNTs18. This difference in the position of the RBM band is assigned to SWNTs of different diameters.23 The position of the RBM Raman line is related to the tube diameter24, 25; the Raman bands at 164 and 168 cm-1 can be attributed to SWNTs that have a diameter of ~ 1.36 and 1.34 nm, respectively. With an increasing number of CV scans performed on Au supports covered with films of (M+S)-SWNTs and S-SWNTs, one observes a gradual decrease in the relative intensity of the RBM band along with the appearance of the Raman line at 1174 cm-1 and an up-shift of the D band without an


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increase in its intensity (Figures 2 and 3). These variations indicate a shielding effect induced by the full coverage of S-SWNT with PPV MCs.

100 150 200

1200

1400

1600

λexc = 1064 nm

1424

e

1629

1174

1550

1591

1327

PPV

1591

S-SWNT PPV (30 CV)

1591 1327

1174

d

1593

1282

1174

c

168

S--SWNT PPV (20 CV)

1594

1174

b

1282

S-SWNT PPV (10 CV) 168


a

1273

1595

168

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S-SWNT

100 150 200

1200

1400

1600

-1

Wavenumber (cm )

Figure 3. Raman spectra, recorded at λexc = 1064 nm, of the films of S-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a S-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film. An intriguing result, shown in Figures 2 and 3, is related of the value of the ratio between


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the relative intensities of the Raman lines peaked at 1595 and 1174 cm-1, which is equal to 2 and ~ 5 for the electrosynthesis of PPV onto Au supports covered with (M+S)-SWNT and S-SWNT films, respectively, as obtained after performing 30 CV scans on the working electrodes. At this stage of our study, we are tempted to explain the higher intensity of the Raman line at 1174 cm-1 observed in Figure 2 for the PPV electrosynthesized onto a Au support covered with a film of (M+S)-SWNTs as being the result of an enhancement effect induced by the generation of SPs that does not exist when PPV was deposited electrochemically onto a Au electrode covered with a S-SWNT film. The generation of SPs at the interface of the metallic tubes and PPV was demonstrated to take place for the PPV/SWNTs composites prepared by annealing conversion of PS containing different weight percentages of CNTs8. Examining Figures 4 and 5, the same behavior is not found for the Raman line at 1174 cm-1 in the case of PPV electrosynthesized onto Au supports covered with films of (M+S)SWNTs and M-SWNTs, that is, the occurrence of an enhancement of plasmonic origin in both cases. According to Figures 4 and 5, the excitation wavelength of 676.4 nm gives rise to the Raman bands labeled RBM, D and TM, which have maxima at ~ 173 (shoulder at 150 cm-1), 1307 and 1540-1578 cm-1, respectively, in the case of (M+S)-SWNTs and at ~ 176, 1315 and 1547-1582 cm-1, respectively, in the case of M-SWNTs. Recall that the asymmetric Raman band situated at ~ 1540-1547 cm-1 is assigned to the electron-phonon interaction26 and that the Raman bands at 173 and 176 cm-1 indicate a diameter of SWNTs of ~1.31 and 1.29 nm24,25, respectively. With increasing numbers of CVs, in the spectral range of 1100 to 1700 cm-1, as shown Figures 4 and 5, one observe the following: i) a gradual decrease in the relative intensity of the Raman band peaked at ~ 1540-1547 cm-1 simultaneously with an increase in the relative intensity of the D band, the variation of which in the case of PPV electrosynthesized onto a Au support covered


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with a film of (M+S)-SWNTs/M-SWNTs is accompanied by an up-shift from 1307/1315 to 1324/1318 cm-1; ii) an up-shift of the RBM band to 181 cm-1 when PPV is electrosynthesized onto a Au support covered with a film of (M+S)-SWNTs or M-SWNTs; and iii) the value of the ratio between the relative intensities of the Raman lines that peaked at 1595 and 1174 cm-1 is equal to 1.4 for the electrosynthesis of PPV onto Au supports covered with (M+S)-SWNTs or M-SWNTs films after 30 CV scans. The decrease in the intensity of the Raman band at ~ 15401547 cm-1 and the presence of the RBM Raman line indicate an isolation of the individual tubes from bundles as a result of the functionalization process.

λexc = 676.4 nm

1200

1400

1600 1589

100 150 200

1174

1629

1550 1589

(M+S)-SWNT + PPV (30 CV) 1324

1174

181

e

1424

1327

PPV

c

1319

(M+S)-SWNT + PPV (20 CV) 1174

181

1587

d

1540

(M+S)-SWNT 1307

150

b

1578

173

1174


1317

181


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a 100 150 200

1200

1400

1600

-1

Wavenumber (cm )

Figure 4. Raman spectra, recorded at λexc = 676.4 nm, of the films of (M+S)-SWNTs (a) and its


15


composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a (M+S)-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.

100

200

1200

1400

1600

1550

1327

PPV

1424

1174

1629

1589

λexc = 676.4 nm

1174

1585

e

181

1585

1318

M-SWNT + PPV (30 CV)

1318

1585

M-SWNT

a 100

200

1200

1582

1317 1315

1547

b

176


1174

c


1174

181

d

181


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 53

1400

1600

-1

Wavenumber (cm )

Figure 5. Raman spectra, recorded at λexc = 676.4 nm, of the films of M-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a M-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.


16

Page 17 of 53

Additional information concerning the electrofunctionalization process of SWNTs with PPV are shown in the following via IR spectroscopy. Figure 6 shows the FTIR spectra of PPV electrosynthesized onto a blank Au electrode for different numbers of CV scans. All of the IR bands of the PPV in the form of a free film or a film deposited onto a metallic support8,27-31 prepared by the annealing conversion of PS are also observed in the case of the PPV films synthesized by electrochemical method. The IR bands at 833, 964, 1169, 1263, 1331-1347, 1421 and 1518 cm-1 are attributed to the vibrational modes of phenyl ring C-H out–of-plane bending, trans-vinylene C-H out-of-plane bending, C-H in plane bending, CHBr2 bending, benzene ring stretching, in-plane C=C stretching in phenyl ring and in-plane C-C stretching in phenyl ring8, 2731

, respectively. In Figure 6, the four IR bands of weak intensity at 874, 1396, 1450, and 1545

cm-1 are assigned to the PPV quinoid structure vibrational modes, which exist as a result of a partial doping of PPV MCs8,32.

750

900

1200

1545

1396 1421 1450 1331 1347

874

1263

1169

964

1518

833

5 10 20 30

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1350

1500 -1

Wavenumbers (cm )

Figure 6. FTIR spectra of the films of PPV electrochemical synthesized onto blank Au support


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 53

by carried out of 5 (black curve), 10 (red curve), 20 (green curve) and 30 (blue curve) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. The gradual increase of the number of CV scans from 5 to 30 (Figure 6) leads to a change in the value of the intensities of the IR bands at 833, 964 and 1518 cm-1, as evaluated by the decrease of the ratio values of I833/I1518 and I964/I1518 from 1.6 (black curve) to 0.13 (blue curve) and 0.9 (black curve) to 0.08 (blue curve), respectively. A similar behavior was reported on the PPV film deposited onto the Au support via annealing conversion of the PS, when the FTIR spectra were recorded in the grazing angle incident reflection geometry with p and s polarization8. This behavior was assigned to the preferential orientation of PPV molecules onto the metallic support.8 A significant fact is the higher intensity of the FTIR bands in the case of thin polymer film synthesized onto rough metallic supports by performing 5 CVs onto rough metallic support in comparison with the thick PPV film obtained by performing 30 CV scans. The PPV film’s thickness prepared by performing 5 and 30 CVs was estimated using the equation33: d= QM/zFAρ, where M is the molecular weight of the TBPX (421,75 g/mol), F is the Faraday constant (96485 C), ρ is the TBPX density (2.258 g/cm-3), A is the active electrode area (1 cm2), z is the number of the electrons involved (4) and Q is the charge associated to the TBPX reduction. Knowing the charge for the TBPX reduction on the Au support during recording 5 and 30 CVs that it is equal to 27.2 and 221 µC/cm2, respectively, the calculated PPV film’s thicknesses are 13 and 107 nm.33 Figure 7 shows the FTIR spectra of the composite PPV/(M+S)-SWNTs obtained on Au supports after recording of 5, 10, 20 and 30 CV. The main variations induced for (M+S)-SWNTs on the IR absorption bands of PPV are: i) regardless of the number of CVs performed during the


18

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electrosynthesis of PPV onto the blank Au support (Figure 6) and onto the Au electrode covered with a (M+S)-SWNTs film (Figure 7), in the spectral range of 750 to 950 cm-1, a down-shift of the two IR absorption bands of PPV (assigned to the vibration modes of phenyl ring C-H out–ofplane bending and trans-vinylene C-H out-of-plane) is observed to be accompanied by the appearance of an asymmetric profile with a maxima at ~ 771-779 and 916 cm-1 that are associated with the calculated vibration modes34 Ag and 2E25, respectively, of SWNTs situated at ~ 775 and 915 cm-1, respectively; ii) a gradual down-shift of the IR absorption band at 1518 cm1

(black and blue curves in Figure 6) to 1508 cm-1 (black curve in Figure 7) and then to 1489 cm-

1

(blue curves in Figure 7) when the CVs number performed onto Au electrode covered with a

(M+S)-SWNTs film increases from 5 to 30; iii) after the achievement of the first 5 cycles for the preparation of PPV films onto a blank Au support (black curve in Figure 6) and onto a Au electrode covered with a (M+S)-SWNTs film (black curve in Figure 7), a change of the intensity of the IR bands assigned to the vibrational modes in-plane C=C stretching in the phenyl ring (1421 cm-1) and quinoid structure of PPV (1450 cm-1) from 2.2 (Figure 6) to 1 (Figure 7) can be observed simultaneous with a down-shift of the latter IR band at 1442 cm-1; iv) an up-shift of the IR band assigned to the benzene ring stretching vibrational mode from 1347 cm-1 (Figure 6) to 1358 cm-1 (blue curve in Figure 7) and 1365 cm-1 (black curve in Figure 7) when onto the Au support covered with a (M+S)-SWNTs film is synthesized PPV after performing 30 and 5 CVs, respectively; and v) a decrease of the ratio between the absorbance of IR bands assigned to the vibrational modes of in-plane C=C stretching in phenyl ring (1421 cm-1) and benzene ring stretching, during the performance of 5 and 30 CVs, from 10 (black curve in Figure 6) to 1.86 (blue curve in Figure 6) and 2.6 (black curve in Figure 7) to 1 (blue curve in Figure 7), respectively, when PPV is electrosynthesized onto a blank Au support and an Au electrode


19


covered with a (M+S)-SWNTs film. These variations indicate a covalent functionalization of (M+S)-SWNTs with PPV. Such variations were not reported in the case of the FTIR spectra of the PPV/SWNTs composites obtained by annealing conversion of PS.8 In the latter case, depending on the weight percentage of SWNTs in the PPV/SWNTs composite mass, only the changes of the ratios between the absorbance of different IR bands of PPV were observed due to

750

900

1350

1508 1508

1420

1442 1440

1418

1489

1438

1496

1416 1440

1360

30

1408

20

1358

831 864 916

771

773 828

945

10

1362

950

1365

864

5

775

825

779

916

831

952

a non-covalent functionalization of SWNTs with PPV.

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 53

1500 -1

Wavenumbers (cm )

Figure 7. FTIR spectra of the films of PPV electrochemical synthesized onto Au support covered with (M+S)-SWNTs by carried out of 5 (curve black), 10 (red curve), 20 (green curve) and 30 (blue curve) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. As expected, the FTIR spectra of PPV in the presence of M-SWNTs and S-SWNTs show different shapes when composites are electrosynthesized under identical conditions regarding the same number of CV scans performed for the working electrode. Figure 8 is relevant in this sense.


20

The position of the IR absorption bands situated in the 750-1000 cm-1 spectral range as well as of the in-plane C-C stretching in the phenyl ring vibrational mode (1518 cm-1) is identical for the samples of the PPV type and the PPV/M-SWNTs composites electrosynthesized onto Au supports. In the 1350-1500 cm-1 spectral range, the PPV/M-SWNTs composites show two IR absorption bands, at 1420 and 1452 cm-1 (Figure 8), i.e., bands that are very close to the IR bands of PPV at ~ 1420 and 1450 cm-1 (Figure 6). a) PPV / M-SWNTs

1516 1452 1481

1420

1389

874

964

833

5 10 20 30

881

750

900

1400

1500

b) PPV / S-SWNTs 1471

5 10 20 30

1485

1452

1512

1379 1396 1421

1365

922

900

964

883

750

833

779

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Absorbance

Page 21 of 53

1400

1500 -1

Wavenumbers (cm )

Figure 8. FTIR spectra of the films of PPV electrochemical synthesized onto Au support covered with M-SWNTs (a) and S-SWNTs (b) films by carried out of 5 (black curves), 10 (red curves), 20 (green curves) and 30 (blue curves) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 53

These similarities indicate a non-covalent functionalization of M-SWNTs with PPV. The absorption bands of very low intensity with maxima at 1389 and 1481 cm-1 that are observed in the FTIR spectrum of the PPV/M-SWNT composites correspond to the down-shifted vibrational modes of the PPV quinoid structure (1396 cm-1)8,32 and SWNT A2 mode (1499 cm-1)34, respectively. In comparison with the samples of PPV and PPV non-covalently functionalized MSWNTs that are obtained after performing 5 CV scans onto Au supports (for which the more intense IR absorption bands are those situated in the 750-1000 cm-1 spectral range), in the case of the PPV/S-SWNT composites, the FTIR spectrum is dominated by a complex absorption band situated in the 1440-1500 cm-1 spectral range, which shows three component peaks at 1452, 1471 and 1485 cm-1. The new IR absorption band at 1471 cm-1 is assigned to the vibration mode of the deformation of the C-H bond.35 Enhancement of this IR absorption band, which induces also an increase in the intensity of the neighboring IR bands at 1452 and 1485 cm-1, is a convincing proof for the covalent functionalization of S-SWNTs with PPV according to Schema 1b, when the SWNT cation radicals interact with diradicals that are formed during the growth of PPV MCs of different lengths, and new covalent bonds of the type (CSWNT)2-CH- are formed. An interesting experimental fact revealed by Figure 8 is the higher intensity of IR bands at 779, 883 and 922 cm-1, which are assigned to the vibrational modes 1Ag, 1E1 and 2E25 of SWNTs34, respectively, when the PPV/S-SWNTs composite was prepared by the performance of ten CV scans onto the working electrode. The explanation of the higher intensity of the IR bands at 779, 883 and 922 cm-1 after the deposition of polymer during the first ten CVs onto the working electrode is to consider the steric effects induced by the covalent bonding of the PPV MC onto the surface of the S-SWNTs. The increase of the number of CV scans up to 30 induces a gradual decrease of the three IR bands situated in the 750-950 cm-1 spectral range, and in the final stage, these


22

Page 23 of 53

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absorption bands have a relative intensity that is smaller in comparison with the main PPV IR bands. This is explained by the fact that the greatest part of nanotubes has been covalently functionalized with macromolecular compounds. An additional argument for the covalent functionalization of S-SWNTs with PPV is shown in Figure 2S, which displays C1s spectra of SSWNTs, PPV and PPV covalently functionalized S-SWNTs. C1s spectrum of S-SWNTs (Figure 2Sa) shows a band with an asymmetric profile towards higher energy which can be deconvoluted in three bands with peaks situated at 284.3, 285.2 and 286.1 eV. The peaks at 284.3 and 285.2 eV are assigned to sp2 C=C bonds and sp3 C-C bonds, while the peak of very low intensity at 286.1 eV is attributed to C-O bonds, belonging to different contaminants of chemical compounds used in the separation process of M-SWNTs and S-SWNTs.36 According with Figure 2Sb, C1s spectrum of PPV can be deconvoluted in three bands with peaks centered at 284.4, 285.1 and 286 eV, these being assigned to sp2 C=C bonds, sp3 C-C bonds or C-H bonds and C-Br bonds.37,38 Comparison with C1s spectrum of S-SWNTs (Figure 2Sa) a significant increase in the relative intensity of the band with peak at 285.1 eV is observed in the case of the C1s spectrum of S-SWNTs after functionalization with PPV (Figure 2Sc). A change in the relative intensities of the bands with peaks at 285.1 and 286 eV as well as in their values of ratios between the relative intensities of the two bands is also observed in the case of PPV (Figure 2Sb) and S-SWNTs after the electrochemical functionalization with PPV (Figure 2Sc). In our opinion, these changes are valuable arguments for new C-C and C-Br covalent bonds, which appear when S-SWNTs are covalently functionalized with PPV according with the chemical mechanism proposed in Schema 1b. Summarizing these data, it can be concluded that the electrochemical reduction of TBPX in the presence of CNTs leads to a non-covalent functionalization of MSWNTs with PPV and a covalent functionalization of S-SWNTs with PPV.


23


The different influences of M-SWNTs and S-SWNTs on the PPV PL quenching process and the PL properties of PPV MCs of different lengths are shown in Figures 9 -11. Figure 9 shows that regardless of the working electrode used for the PPV electrosynthesis, namely, the blank Au support or the gold plate covered with films of (M+S)-SWNTs, M-SWNTs and S-SWNTs, with increasing numbers of CV scans, an increase in the intensity of the PL spectra occurs in each

6

8.0x10

5

4.0x10

5

a

PPV

b

509

30 CV

30 CV

5 CV

5 CV

500

600

400

500

d 5

9.0x10

4

0.0 400

4

5.00x10

4

2.50x10

4

600

5

1.8x10

7.50x10 PPV/(S+M)

0.00

0.0 400 2.7x10

539

1.2x10

509

case.

PL Intensity (counts)

508

c

PPV/S

543

508

PPV/M

540

7.50x10

4

30 CV

30 CV

5.00x10

4

5 CV

5 CV

2.50x10

4

0.00 500

600 400 500 Wavelength (nm)

600

6

1.2x10

PL Intensity ( counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 53

e

λexc = 400 nm PPV PPV / M+S PPV / S PPV / M

5

8.0x10

5

4.0x10

0.0 0

5

10

15

20

25

30

CV number

Figure 9. Photoluminescence (PL) spectra of the films of PPV electrochemically synthesized onto the blank Au support (a) and the Au electrodes covered with films of (M+S)-SWNTs (b),


24

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


M-SWNTs (c) and S-SWNTs (d) by carried out of 5 (black curves), 10 (red curves), 15 (green curves), 20 (blue curves), 25 (cyan curves) and 30 (magenta curves) cyclic voltammograms when working electrodes was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e shows PL intensity of the films of PPV electrochemically synthesized onto the blank Au support (black square) and the Au electrodes covered with films of (M+S)-SWNTs (black circle), M-SWNTs (black triangle) and S-SWNTs (open triangle) as function of cyclic voltammograms number recorded onto the working electrodes immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. All PL spectra were recorded under excitation wavelength of 400 nm and room temperature (RT). In addition, Figures 9a and 9b reveal a decrease in the PPV PL intensity when the polymer is synthesized in the presence of (M+S)-SWNTs (at ∼ 66.736 counts, magenta curve in Figure 9a) by the same number of CV scans as in the case of the blank Au support (when PL intensity was of ∼1.105.030 counts, magenta curve in Figure 9b). This result is in a good agreement with studies reported on PPV/SWNT composites prepared by the annealing conversion of PS 2-4. More information concerning the contribution of the three types of tubes, i.e., M-SWNTs, S-SWNTs and (M+S)-SWNTs, to the decrease in the PPV PL intensity is shown in Figures 9c and 9d. According to Figures 9c and 9d, one observes that after 30 CV scans, the PL intensity of the PPV synthesized in the presence of S-SWNTs is much higher (∼ 245.270 counts) in comparison with that reported in the case of M-SWNTs (∼ 65.950 counts). Some differences in the shape of the PL spectra of PPV when the polymer is electrosynthesized onto a blank Au support and Au electrodes covered with films of (M+S)-


25


SWNTs, M-SWNTs and S-SWNTs are observed in Figure 10. An analysis of the spectral components of the PL spectra of the films of PPV electrosynthesized onto the four working electrodes is shown in Figure 10. The deconvolution of the PL spectrum of PPV electrosynthesized onto a blank Au support (Figure 10a) reveals four PL bands peaked at 2.66, 2.44, 2.28 and 2.1 eV, which are very closely positioned to those reported in the case of PPV synthesized by the annealing conversion of PS2. The PL bands that have maxima at 2.66, 2.44, and 2.28 eV are assigned to the electronic emission transitions of MCs that have lengths of 4, 5, and 7-10 repeating units (RU), respectively2. The component at 2.1 eV corresponds to the vibronic replica of the first order of the PL band with a maximum at 2.44 eV2.

PPV

PPV/(S+M)

2.28

2.44

b

2.10

2.10

2.66

2.28

2.44

a

2.66

2,4

2,0

PPV/S

c

2.28

2,8

2.10

2.66

2,0

2,0

PPV/M

2.10

2,4

2,4

2.44

d

2.62

2,8

2,8

2.28

2,8

2.44


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 53

2,4

2,0

Energy (eV)

Figure 10. Spectral components of the PL spectra of the films of PPV electrochemical synthesized onto the blank Au support (a) and the Au electrodes covered with films of (M+S)-


26

Page 27 of 53

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SWNTs (b), M-SWNTs (c) and S-SWNTs (d) in the case of the thirtieth cyclic voltammograms recorded when working electrodes was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Regardless of whether the PPV is electrosynthesized in the absence or in the presence of (M+S)-SWNTs, M-SWNTs, or S-SWNTs, Figure 10 shows that the most intense PL band is the band that peaked at 2.44 eV. This fact indicates that the formation of the PPV MCs that have a length of 5 RU is not influenced by the presence of CNTs. This behavior does not occur in the case of PPV MCs that have a length of 4 or 7-10 RU. By reporting the intensity of the PL band located at 2.44 eV (labeled k2), the intensity of the emission bands at 2.66 eV (labeled k1) and 2.28 eV (labeled k3) is changed, depending on the working electrode used, as follows: i) for blank Au support, the values of the ratios Ik1/Ik2 and Ik3/Ik2 are equal with 0.7 and 1.1, respectively; ii) for a Au electrode covered with a (M+S)-SWNTs film, the values of the ratios Ik1/Ik2 and Ik3/Ik2 are equal to 0.74 and 0.72, respectively; iii) for a Au electrode covered with a M-SWNTs film, the values of the ratios Ik1/Ik2 and Ik3/Ik2 are equal with 0.4 and 0.8, respectively; and iv) for a Au electrode covered with a S-SWNTs film, the values of the ratios Ik1/Ik2 and Ik3/Ik2 are equal to 0.3 and 0.75, respectively. Summarizing these differences, we conclude that: i) when the electrochemical reduction of TBPX is performed in the presence of (M+S)-SWNTs, M-SWNTs, or S-SWNTs, a decrease with the same probability (∼ 36%) of the formation of PPV MCs that have lengths of 7-10 RU is induced and ii) in the presence of M-SWNTs or S-SWNTs, a decrease of nearly 2 times the probability of formation of PPV MCs that have a length of 4 RU (∼ 50%) is observed. Regardless of the number of CV scans used in the preparation of composites, Figure 9e


27


summarizes the influence of the (M+S)-SWNTs, M-SWNTs and S-SWNTs on the PPV PL spectra. The decrease in the intensity of the PPV PL spectra in the presence of the (M+S)SWNTs, M-SWNTs and S-SWNTs, we attempt to explain by considering the diagram of the electronic energy levels of PPV, (M+S)-SWNTs, M-SWNTs and S-SWNTs shown in Figure 11.

a1

a2

(14,5) S-SWNTs

(14,4) M-SWNTs -2

-2

LUMO

-3

-3 PPV 2.5 eV

-4

-4 -5

-5 Energy (eV) relative to vacuum

-6

-6

-7

-7

-8 HOMO -8

b1 (12, 8) (M+S)-SWNTs

b2

(10,9) (M+S)-SWNTs

-2

-2

-3

LUMO

-3 PPV 2.5 eV

-4

-4

-5

-5

-6

-6

-7

-7

-8

-8

c

HOMO

Metallic tubes

Semiconductor tubes

3.23 eV

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 53

PPV 2

3 4 Energy (eV)

5

Figure 11. The diagram of electronic energy levels of S-SWNTs (a1), M-SWNTs (a2) and semiconducting (b1) and metallic tubes (b2) in the (M+S)-SWNTs as well as PPV (the red line in Figures a1, a2, b1 and b2). UV-VIS spectrum of PPV is shown in Figure c.


28

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The diagrams of the electronic energy levels of CNTs are plotted using the procedure described in our previous papers11, 13. Briefly, using the Kataura plot23 and the peak position of the RBM obtained under resonant optical excitation, we determined the diameter and chirality of the semiconducting nanotubes belonging samples of the (M+S)-SWNTs and S-SWNTs types, which are equal to 1.38 and 1.35 nm, respectively, and have a chirality (n, m) of the (12, 8) and (14, 5) type, respectively. A similar procedure was used for the metallic tubes of the (M+S)SWNTs and M-SWNTs samples, the diameters of which were equal to 1.31 and 1.29 nm, respectively, and their chirality (n, m) was of the (10, 9) and (14, 4) type, respectively. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of semiconducting tubes in the (M+S)-SWNTs, the metallic tubes in the (M+S)SWNTs, the S-SWNTs, and the M-SWNTs were situated at -4.96/-4.36 eV, -4.97/-4.36 eV, 5.54/-3.79 eV, and -4.97/-4.35 eV, respectively. The HOMO and LUMO levels of PPV were calculated using cyclic voltammetry (Figure 1a) and the UV-VIS absorption spectrum (Figure 11c). Thus, knowing the onset of the oxidation potential of the PPV Eonsetox = 0.56 V and the optical band gap of the polymer, Egopt = 2.5 eV, the HOMO and LUMO levels39-41 of PPV were situated at -4.84 and -2.49 eV, respectively. Based on these data, we built diagrams a1, a2, b1 and b2 in Figure 11. A careful analysis of the LUMO and HOMO levels of PPV and CNTs enables an explanation of the more important decrease of the PPV PL intensity in the case of (M+S)SWNTs and M-SWNTs relative to S-SWNTs. As is well known, under optical excitation an exciton on the PPV MC is produced, which dissociates into an electron and a hole. Subsequently, the electron is collected at the LUMO level of SWNTs with chiralities of (10, 9), (14, 4), (12, 8) and (14, 5) that are situated in the close neighborhood, after which, by successive passing to levels of lower energy, it finally reaches the lowest LUMO level. The superposition of the


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 53

HOMO levels of PPV and SWNTs with chiralities of (10, 9), (14, 4) and (12, 8) facilitates the electrons to be collected in the valence band of the two constituents. Such a mechanism is unlikely in the case of S-SWNT, which has a HOMO level at a much lower energy than that of PPV; this mechanism will promote electrons to only recombine with the holes from the valence band of the polymer. In our opinion, these different possible de-excitation pathways are responsible for the amplification of the PPV PL quenching due to (M+S)-SWNTs, M-SWNTs or SWNTs. 4. Conclusions This paper reported new opportunities to study the influence of M-SWNTs and S-SWNTs on the vibrational and PL properties of PPV provided by the electrochemical reduction of TBPX in the presence of CNTs. Using Raman scattering and FTIR spectroscopy, it is demonstrated that the electrochemical reduction of TBPX in the presence of as-prepared SWNTs, namely, as metallic (33%) and semiconducting (66%) entities, or as S-SWNTs (99%) and M-SWNTs (98%) is an alternative method to that based on the annealing conversion of PPV PS for the preparation of composites based on PPV and CNTs. The most important results are listed below: i) an enhancement effect induced of the SPs generate at the interface PPV/metallic CNTs is evidenced by the Raman spectra recorded under an excitation wavelength of 1064 nm, in the case sample of PPV electrosynthesized onto Au support that is covered with a film of (M+S)SWNTs, in compassion with that synthesized on a Au electrode covered with a S-SWNT film. Variations of the Raman spectra in the spectral range of the RBM and TM bands indicate that the electrochemical reduction of TBPX in the presence CNTs leads to a shielding effect of SSWNTs and an isolation of the metallic individual tubes from M-SWNTs bundles.


30

Page 31 of 53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ii) using FTIR spectroscopy, it is demonstrated that the electrochemical reduction of TBPX in the presence of M-SWNTs and S-SWNTs leads to a non-covalent functionalization of M-SWNTs and a covalent functionalization of S-SWNTs with PPV, respectively. While FTIR spectra of PPV electrosynthesized onto the Au support and in the presence of M-SWNTs are similar, the FTIR spectrum of the PPV/S-SWNTs composite, is dominated by an absorption band at 1471 cm-1 that is assigned to the vibration mode of the deformation of the C-H bond, which appears as a result of the steric hindrance effects induced by the covalent bonding of PPV onto the surface of the S-SWNTs. iii) the PL spectrum of PPV electrosynthesized onto a blank Au support shows four emission bands that peaked at 2.66, 2.44, 2.28 and 2.1 eV; the first three are assigned to the electronic emission transitions of MCs that have lengths of 4, 5 and 7-10 RU, the last of which corresponds to the vibronic replica of the first order of the PL band at 2.44 eV. The electrochemical reduction of TBPX in the presence of (M+S)-SWNTs, M-SWNTs, or S-SWNTs induces a reduction of the formation of PPV MCs with lengths of 7-10 RU of ∼ 36% without a change in the weight of the PPV MCs that have a length of 5 RU; a decrease of nearly twice the probability of forming PPV MCs that have lengths of 4 RU is also reported in the presence of MSWNTs or S-SWNTs. It is demonstrated for the first time that different de-excitation pathways are responsible for the enhancement of PPV PL quenching due to (M+S)-SWNTs, M-SWNTs, or S-SWNTs. Supporting Information Available Figures: UV-VIS absorption spectra of TBPX in initial state (a) and after electrochemical reduction at -2V (b) and PPV (c). XPS C1s spectra of S-SWNTs (a), PPV (b) and PPV


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 53

covalently functionalized S-SWNTs (c). AUTHOR INFORMATION Corresponding Author *Tel: + 40 21 3690170 Fax: + 40 21 3690177 E-mail: [email protected] Funding Sources This work was funded by the Romanian National Authority for Scientific Research, CNCSUEFISCDI, Module III Bilateral Cooperation, Humbert Curien-Brancusi project, no. 1027/ 26.06.2014. ACKNOWLEDGMENT This work was performed under the auspices of Scientific Cooperation between the Institute of Materials Jean Rouxel in Nantes and Laboratory of Optical Processes in Nanostructured Materials of the National Institute of Materials Physics, Bucharest. REFERENCES (1)

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Mulazzi, E.; Perego, R.; Aarab, H.; Mihut, L.; Lefrant, S.; Faulques, E. ; Wery, J.

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Electrosynthesis and

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Poly[(2,5-bisoctyloxy)-1,4-phenylene

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38

Page 39 of 53

Table of Contents Image

1.2x106 λexc = 400 nm PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PPV PPV / M+S PPV / M PPV / S

8.0x105

4.0x105

0.0 5

10

15

20

25

30

Cv number

Quenching of PPV luminescence by different SWNTs: metallic (M) and semiconductor (S) and mixture (M+S).


39


0.04 0.03

b) PPV-SWNT (M+S) / Au

a) PPV/Au

30 20 10 5

i (mA cm-2)

0.02

30 20 10 5

0.02

0.01 0.00

-0.01 -2

-1

0

1

2

30 20 10 5

-1

0

1

2

30 20 10 5

0.02

0.00

0.00

30 20 10 5

-0.02 -2

-0.02 -2

0.04 d) PPV-SWNT-M / Au

0.04 c) PPV-SWNT-S / Au

0.02

30 20 10 5

5 10 20 30

0.00

i (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 53

0 E (V) vs. Ag/AgCl

30 20 10 5

-0.02 2

-2

0

2

E (V) vs. Ag/AgCl

Figure 1. Cyclic voltammograms recorded onto the blank Au electrode (a) and on the rough Au supports covered with films of (M+S)SWNTs (b), S-SWNTs (c) and M-SWNTs (d), when working electrodes were immersed into a solution of 0.02 M TBPX and 0.1 M TBAB in DMF. Black, red, green and blue curves in Figures a, b, c and d correspond to the 5th, 10th, 20th and 30th cycle.


1200

1400

exc = 1064 nm

1591

PPV

1424

1327

1174

e

1600

1629

100 150 200

1591

1327

1174


d

1591

1300

164

1174


c

1594

1278

(M+S)-SWNT + PPV (10 CV) 1174

164


b

(M+S)-SWNT

a 100 150 200

1200

1273

1595

164

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1550

Page 41 of 53

1400

1600

-1

Wavenumber (cm )

Figure 2. Raman spectra, recorded at λexc = 1064 nm, of the films of (M+S)-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a (M+S)-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.



100 150 200

1200

1400

exc = 1064 nm

1600

1424

e

1629

1174

1550

1591

1327

PPV

1591

S-SWNT PPV (30 CV)

1591 1327

1174

d

1593

1282

1174

c

168

S--SWNT PPV (20 CV)

1594

1174

b

1282

S-SWNT PPV (10 CV) 168


a

1273

1595

168

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 53

S-SWNT

100 150 200

1200

1400

1600

-1

Wavenumber (cm )

Figure 3. Raman spectra, recorded at λexc = 1064 nm, of the films of S-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a S-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.


100 150 200 exc = 676.4 nm

1200

1400

1600

1174

1629

1550 1589

(M+S)-SWNT + PPV (30 CV) 1324

1174

181

e

1424

1327

PPV

c

1319

(M+S)-SWNT + PPV (20 CV) 1174

181

1587

d

1540

(M+S)-SWNT 1307

150

b

1578

173

1174


1317

181


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1589

Page 43 of 53

a 100 150 200

1200

1400

1600

-1

Wavenumber (cm )

Figure 4. Raman spectra, recorded at λexc = 676.4 nm, of the films of (M+S)-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a (M+S)-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.



100

200

1200

1400

1600

1550

1327

PPV

1424

1174

1629

1589

exc = 676.4 nm

1174

1585

e

181

1585

1318


1318

1585

M-SWNT

a 100

200

1200

1582

1317 1315

1547

b

176


1174

c


1174

181

d

181


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 53

1400

1600

-1

Wavenumber (cm )

Figure 5. Raman spectra, recorded at λexc = 676.4 nm, of the films of M-SWNTs (a) and its composites obtained by the achievement of 10 (b), 20 (c) and 30 (d) cyclic voltammograms, when the Au electrode covered with a M-SWNTs film was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e corresponds to the Raman spectrum of the PPV film.


5 10 20 30

750

900

1545

1396 1421 1450

1518 1200

1331 1347

874

1263

1169

964

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


833

Page 45 of 53

1350

1500 -1

Wavenumbers (cm )

Figure 6. FTIR spectra of the films of PPV electrochemical synthesized onto blank Au support by carried out of 5 (black curve), 10 (red curve), 20 (green curve) and 30 (blue curve) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF.


750

1508

1442

1508

1440

1420 1418

1496

1438

1350

1489

831 864 916

900

1416 1440

1360

30

1408

1362

20

945

10

1358

950

1365

864

916

831 775

825

779

5

773 828 771

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 53

952


1500 -1

Wavenumbers (cm )

Figure 7. FTIR spectra of the films of PPV electrochemical synthesized onto Au support covered with (M+S)-SWNTs by carried out of 5 (curve black), 10 (red curve), 20 (green curve) and 30 (blue curve) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF.


Page 47 of 53

a) PPV / M-SWNTs

5 10 20 30

1516 1452 1481

1420

1389

874

964

833 881

750

900

1400

1500

b) PPV / S-SWNTs 1471

5 10 20 30

750

900

1485 1512

1379 1396 1421

1365

964

922

833

779

883

1452

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1400

1500 -1

Wavenumbers (cm )

Figure 8. FTIR spectra of the films of PPV electrochemical synthesized onto Au support covered with M-SWNTs (a) and S-SWNTs (b) films by carried out of 5 (black curves), 10 (red curves), 20 (green curves) and 30 (blue curves) cyclic voltammograms when working electrode was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF.


8.0x10

5

4.0x10

5

a

PPV

b

509

Page 48 of 53

30 CV

30 CV

5 CV

5 CV

500

600

400

500

d 5

9.0x10

4

0.0 400

4

5.00x10

4

2.50x10

4

600

5

1.8x10

7.50x10 PPV/(S+M)

0.00

0.0 400 2.7x10

509

6

539

1.2x10

508

c

PPV/S

543

508

PPV/M

540

7.50x10

4

30 CV

30 CV

5.00x10

4

5 CV

5 CV

2.50x10

4

0.00 500

600 400 500 Wavelength (nm)

600

6

1.2x10

PL Intensity ( counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



exc = 400 nm

e

PPV PPV / M+S PPV / S PPV / M

5

8.0x10

5

4.0x10

0.0 0

5

10

15

20

25

30

CV number

Figure 9. Photoluminescence (PL) spectra of the films of PPV electrochemically synthesized onto the blank Au support (a) and the Au electrodes covered with films of (M+S)-SWNTs (b), M-SWNTs (c) and S-SWNTs (d) by carried out of 5 (black curves), 10 (red curves), 15 (green curves), 20 (blue curves), 25 (cyan curves) and 30 (magenta curves) cyclic voltammograms when working electrodes was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. Figure e shows PL intensity of the films of PPV electrochemically synthesized onto the blank Au support (black square) and the Au electrodes covered with films of (M+S)-SWNTs (black circle), M-SWNTs (black triangle) and SSWNTs (open triangle) as function of cyclic voltammograms number recorded onto the working electrodes immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF. All PL spectra were recorded under excitation wavelength of 400 nm and room temperature (RT). ACS Paragon Plus Environment

PPV

PPV/(S+M)

2.44

b

2.10

2.10

2.66

2.28

2.44

a

2.66

2,4

2,0

PPV/S

2.28

c

2,8

2.10

2.66

2,0

2,0

PPV/M

2.10

2,4

2,4

2.44

d

2.62

2,8

2,8

2.28

2,8

2.44


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.28

Page 49 of 53

2,4

2,0

Energy (eV)

Figure 10. Spectral components of the PL spectra of the films of PPV electrochemical synthesized onto the blank Au support (a) and the Au electrodes covered with films of (M+S)SWNTs (b), M-SWNTs (c) and S-SWNTs (d) in the case of the thirtieth cyclic voltammograms recorded when working electrodes was immersed in the solution 0.02 M TBPX and 0.1 M TBAB in DMF.



a1

a2

(14,5) S-SWNTs

(14,4) M-SWNTs -2

-2

LUMO

-3

-3 PPV 2.5 eV

-4

-4 -5

-5 Energy (eV) relative to vacuum

-6

-6

-7

-7

-8 HOMO -8

b1 (12, 8) (M+S)-SWNTs

b2

(10,9) (M+S)-SWNTs

-2

-2

-3

LUMO

-3 PPV 2.5 eV

-4

-4

-5

-5

-6

-6

-7

-7

-8

-8

c

HOMO

Metallic tubes

Semiconductor tubes

3.23 eV

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 53

PPV

2

3 4 Energy (eV)

5

Figure 11. The diagram of electronic energy levels of S-SWNTs (a1), M-SWNTs (a2) and semiconducting (b1) and metallic tubes (b2) in the (M+S)-SWNTs as well as PPV (the red line in Figures a1, a2, b1 and b2). UV-VIS spectrum of PPV is shown in Figure c.


Page 51 of 53

a 4.6

2

3

4

b

5

3.95

3.14

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

3

4

5

4

5

c 3.23

2

3 Energy (eV)

Figure S1. UV-VIS absorption spectra of TBPX in initial state (a) and after electrochemical reduction at -2V (b). and PPV (c).



282

284

282

284

282

284

286.1

285.2

284

a

286

288

290

288

290

288

290

b

286

285.1

284.3

Intensity (a.u.)

286

c

285

286.1

284.4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 53

286

Bending energy (eV)

Figure S2. XPS C1s spectra of S-SWNTs (a), PPV (b) and PPV covalently functionalized SSWNTs (c)


Page 53 of 53


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