Efficient Covalent Modification of Multiwalled Carbon Nanotubes with

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Efficient covalent modification of multiwalled carbon nanotubes with diazotized dyes in water at room temperature Asma Bensghaïer, Stephanie Lau-Truong, Mahamadou Seydou, Aazdine Lamouri, Eric Leroy, Matej Micusik, Klaudia Forro, Mohamed Beji, Jean Pinson, Maria Omastova, and Mohamed Mehdi Chehimi Langmuir, Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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Efficient covalent modification of multiwalled carbon nanotubes with diazotized dyes in water at room temperature Asma Bensghaïer1,2, Stéphanie Lau Truong3, Mahamadou Seydou3, Aazdine Lamouri3, Eric Leroy2, Matej Mičušik4, Klaudia Forro4, Mohamed Beji1, Jean Pinson3, Mária Omastová4,*, Mohamed M. Chehimi2,* 1 Université de Tunis El Manar, Faculté des Sciences, Laboratoire de Chimie Organique Structurale et Macromoléculaire (LR99ES14), Campus Universitaire, Manar II, Tunis 2092, Tunisia. 2 Université Paris Est, ICMPE (UMR7182), CNRS, UPEC, F-94320 Thiais, France 3 Univ Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086, CNRS, F-75013 Paris, France 4 Polymer Institute, Slovak Academy of Sciences, 845 41 Bratislava, Slovakia

Abstract Tetrafluoroborate salts of diazotized Azure A (AA-N2+), Neutral Red (NR-N2+) and Congo Red (CR-N2+) dyes were prepared and reacted with multiwalled carbon nanotubes (MWCNTs) at room temperature, in water without any reducing agent. The as-modified MWCNTs were examined by IRATR, Raman spectroscopy, XPS, TGA, TEM and cyclic voltammetry. The diazonium band located at ∼2350 cm-1 in the diazotized dye IR spectra vanished after attachment to the nanotubes whilst Raman D/G peak ratio slightly increased after dye covalent attachment at high initial diazonium/CNT mass ratio. XPS measurements show the loss of the F1s from BF4- anion together with a clear change in the high resolution C1s region from the modified nanotubes. Thermogravimetric analyses proved substantial mass loadings of the organic grafts levelling off at 40.5, 34.3 and 50.7 wt % for AA, NR and CR, respectively. High resolution TEM pictures confirmed the presence of 1.5-7 nm thick, continuous, amorphous layers on the nanotubes assigned to the aryl layers from the dyes. Cyclic voltammetry studies in acetonitrile (ACN) confirm grafting of the dyes; the latter retain their electrochemical behavior in the grafted state. The experimental results correlate remarkably well with quantum chemical calculations which indicate high binding energies between the dyes and the CNTs accounting for true covalent bonding (140-185 kJ/mol with CNT-aryl distance < 1.6 nm), though attachment by π stacking also contribute for obtaining stable hybrids. Finally, the pH responsive character of the robust hybrids was demonstrated by a higher degree of protonation of Neutral Red-grafted CNTs at pH 2 compared to neutral aqueous medium. This work demonstrates that diazotized dyes can be employed for the surface modification of MWCNTs in a very simple and efficient manner in water and at RT. The hybrids could be employed for many purposes such as optically pH responsive materials, biosensors, and optothermal composite actuators to name but a few. Keywords: diazonium salts, dyes, multiwalled carbon nanotubes, grafting. Corresponding authors: M. Omastová ([email protected]) M. M. Chehimi ([email protected])

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1. Introduction Diazonium surface modification has received much attention since two decades as it applies practically to all types of materials (metals, semiconductors, oxide, polymers). 1,2 ,3,4 .This surface chemistry has many appealing features: operation at room temperature, in aqueous or organic solvents and without necessarily the use of a reducing agent if the substrate has sufficient reducing power as carbon (from open circuit potential measurements 5 ). More importantly, it yields true covalent bonds between the substrate and the aryl groups resulting from the reduction of the diazonium salts. For example, diazonium salts react spontaneously with carbon nanotubes6 or gold nanoparticles.7 Conversely, sp² carbon (nano)materials8 and gold nanoparticles 9 react spontaneously with diazotized surfaces (termed “self-adhesive” surfaces8) therefore providing nanomaterial-decorated platforms, through interfacial C-C and carbon-metal bonds. Bond energies as high as 265 kJ/mol were computed by DFT; these theoretical studies permit to understand the success story of surface modification of sp² carbon (nano)materials.10 For example, multiwalled carbon nanotubes (MWCNTs) are used in many applications such as polymer nanocomposites, optoelectronics, opto-mechanical actuators and photovoltaic devices, 11 , 12 , 13 most often after modification of the outer wall through non covalent or covalent methods such as oxidation, cycloaddition and diazonium chemistry. 14 Such modification partly prevents the nanotubes from entanglement and favors their dispersion, provides robust interfacial interactions and new surface functionalities. Towards this end, the versatility of diazonium surface chemistry lies in the possibility of making functional materials starting from either small diazotized aromatic amines or from more complex ones such as calixarene,15 porphyrin16 and even polymers (the so-called “diazo resins”).17 Several methods of diazonium modification of CNTs were reported so far in the literature. Abiman et


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al.18 have demonstrated that in the presence of the reducing agent hypophosphorous acid, aryl group grafting was not favoured, and the process was even more effective without the acid but at 50 rather than 20 °C. Golosova et al. 19 reacted MWCNTs with diazonium salts in acetonitrile at RT for 5 days. The group of Tour20 dispersed SWCNTs in oleum for 3h then added NaNO2, 4-chloroaniline and AIBN. The mixture was left for 1hour at 80 °C. In another approach, Tour and co-workers21 modified SWCNTs with in situ generated diazonium salt in isopentylnitrite/water mixture at 80 °C for 12h. The same group functionalized SWCNTs in a mixture of sulfuric acid, NaNO2, ammonium persulfate (to help dispersing the tubes) and the radical

initiator

2,2′-azobis(isobutyronitrile)

(AIBN),

to

increase

the

degree

of

functionalization of SWNTs, for 3h at 80 °C. 22 Of relevance to this work, double-walled CNTs (DWCNTs) were grafted with carboxyphenyl groups by electroreduction of the diazonium precursor and then reacted with Direct Blue 71 dye through carbodiimide chemistry. Direct grafting of the dye was achieved by electrooxidation as the dye bears one amino group.23. Recently, a few papers reported on the diazotation of dyes and their reductive grafting to electrode surfaces. For example, diazotized Azure A was (electro)grafted to sp² carbon nanomaterials for gas sensing and electrocatalytic purposes 24 , 25 whereas Neutral Red diazonium salt was generated and attached to gold electrodes through GraftFast process for making of optode for the measurement of pH.26 Congo Red was also grafted to a screenprinted carbon electrode, however via an electrooxidation process, to provide electrochemical sensors of the aggregation of amyloid-β oligomers in relation to the onset of Alzheimer's disease.27

Herein, we wish to explore the propensity of isolated diazotized dyes to modify MWCNTs spontaneously. We concentrate our task on tuning the surface chemical composition of


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MWCNTs using diazotized dyes simply in water and at RT. In this regard, tetrafluoroborate salts of diazotized Azure A, Neutral Red and Congo Red dyes were prepared and reacted with MWCNTs in water, at RT and without any reducing agent, or any physical assistance, an option that has not been explored so far. The surface composition of the MWCNTs was tuned by varying the diazotized dye/MWCNT initial ratio. All modified MWCNT samples were thoroughly characterized by infrared and Raman spectroscopies, TGA and XPS. Additional TEM images were recorded to check the thickness and uniformity of the attached aryl layers. Covalent attachment of aryl layers was thoroughly investigated by cyclic voltammetry and response to pH was checked by XPS.

2. Experimental 2.1. Materials Azure A, Neutral Red and Congo Red (all Alfa Aesar products) were used as received. MWCNTs (diameter 1–10 nm, length 60-100 nm, purity >90 %) were Nanocyl 7000 products and used as received without any further purification process to remove impurities as detailed elsewhere.28 This has the advantage to keep the CNT modification steps to a minimum and thus increases the applied aspect of this work. Ether and acetone (Aldrich, spectrophotometric grade) were used as received. Deonized water was used for various cleaning and dilution processes.

2.2. Synthesis of diazotized dyes In a typical experiment, 1 g of dye is placed in a round necked bottomed flask and solubilized in 20 mL of acetone under stirring at 0 °C .Then successively 0.5 mL of HBF4 and 0.5 mL of ter-butylnitrite was cautiously added dropwise. The reaction was left to proceed at 0 °C for 15 min. The resulting solution was precipitated by addition of cold ether and filtered on a frit.


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Beautiful powders were obtained the colors of which are identical to those of the starting dyes. The diazotized dyes AA-N2+, NR-N2+ and CR-N2+ were characterized by IR (see section 3.2.1) and NMR spectroscopy. NR-N2+: 1H NMR (DMSO) δ (ppm): 7.89 (s, CH-C (N2+, BF4-)), 7.81 (q, CH-CH3), 7.49 (s, CH-N-(CH3)2), 7.40 (d, CH-N-(CH3)2), 7.01 (d, CH- CH-N-(CH3)2), 3.06 (m, N-(CH3)2). AA-N2+ : 1H-NMR (DMSO) δ (ppm): 6.44 (s, CH-CH-N-(CH3)2)), 5.2 (d, CH-CH-N-(CH3)2), 6.44 (s, CH-N-(CH3)2), 7.76 (s, CH-C(N2+, BF4-)), 7.16 (d, CH-CH-C(N2+, BF4-)), 6.61 (d, CH-CH-C(N2+, BF4-)), 3.02 (m, N-(CH3)2). CR-N2+: 1H NMR (DMSO) δ (ppm): 7.59 (d, (SO3-, Na+)C-C-CH-CH), 7.72 (s, (N=N)C-CHC(SO3-, Na+), 8.52 (t, CH-CH-CH), 8.12 (d, CH-CH-C-C(N2+, BF4-)), 8.40 (d, CH-CHC(N=N)), 8.10 (d, (N=N)C-CH-CH), 8.10 (d, CH-CH-C), 7.73 (d, CH-CH-C) The images of the resulting diazotized dyes are displayed in Figure 1.

N2+, BF4-

N

O

H 3C

N

BF4-, N2+

N

Na O S

CH3 BF4-, N2+

S Cl

N

CH3

O N

N N

N

S

CH3

Azure A diazonium

N O O Na O

BF4-, N2+

Congo Red diazonium

CH3

Neutral Red diazonium

Figure 1. Upper level: ethanol solutions of as received and diazotized dyes. Lower panel: Structure of the diazotized dyes.

2.3. Modification of carbon nanotubes MWCNTs were first sonicated for 20 min in 100 ml of distilled water (using Bioblock model S-Line, effective US power 75 W, 37 kHz). For each diazotized dye, five samples were prepared by reacting 40 mg (3.33 mmol) of ultrasonically dispersed MWCNTs with either 10,


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20, 40, 100 or 200 mg of diazonium (see footnote for the number of mmoles).‡ The mixtures were left to react under stirring for 6 h. It is to note that for the same initial masses, the number of mmoles is quasi the same for AA and NR diazonium salts but lower for CR diazonium compounds. Working with similar initial number of mmole of CR diazonium would imply much higher masses which are not convenient from the experimental viewpoint. For this reason, and for the applied aspect of the work, we compared grafting extents for the same initial masses. The resulting suspension was diluted in acetone and then centrifuged at 8000 rpm for 30 min. The precipitate was thoroughly washed with ethanol and the cycle was repeated until the washings were clear and colorless to remove any physisorbed dyes. The final step consisted in ultrasonicating the CNT-Dye hybrids in deionized water to ensure that only grafted aryl groups remain at the surface. The grafted CNT-Dye samples were dried at 60 °C overnight.

2.4. Characterization. Thermogravimetric analyses (TGA) were conducted using a Setaram instrument (Setsys Evolution 16 model). The samples were heated up from 20 to 800 °C at a linear heating rate of 10 °C/min in air. Infrared transmittance spectra were recorded using a Bruker Tensor 27 DTGS apparatus operated in the ATR mode in the 400 and 4000 cm−1 range with a resolution of 4 cm−1. Raman spectroscopy was carried out using (Horiba Labram HR evolution), excitation was at 633 nm from He–Ne Laser at room temperature. XPS spectra were recorded using a K Alpha (Thermo) fitted with a monochromatic Al Kα X-ray source (spot size: 400 µm). The pass energy was set to 200 and 50 eV for the ‡

For AA-N2BF4, NR-N2BF4 and CR-N2BF4 the MW is 390.6, 387.6 and 894.3 g/mol, respectively. The initial number of mmoles of dyes are (0.026, 0.051, 0.10, 0.256 and 0.512 mmole) for AA-N2BF4, (0.026, 0.052, 0.103, 0.258, 0.516 mmole) for NR-N2BF4 , and (0.011, 0.022, 0.044, 0.11, 0.22 mmol) for CR-N2BF4.


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survey and the narrow regions, respectively. Electron and argon flood guns were used to compensate for the static charge buildup of the modified nanotube samples; pristine nanotubes were analyzed without any charge compensation. The composition was determined using the manufacturer sensitivity factors. Transmission Electron Microscope (TEM) images were recorded using a FEI Tecnai F20 microscope operating at 200 kV. The samples were prepared by dispersing a small amount of nano-composite samples in ethanol and an ultrasonication of 1 min. Then a drop of the suspension was spread on a 400 mesh cupper grid covered with a holey carbon film. Then ethanol was evaporated and the sample observed in the TEM. Cyclic voltammograms were recorded on a glassy carbon (GC) (d = 2 mm) electrode sealed in glass, it was polished on polishing paper of different grades and finally with an alumina slurry of 0.04 µm and then ultrasonicated in water and alcohol for 180 s each. The counter electrode was carbon paper and the reference a saturated calomel electrode (SCE). The potentiostat was a Versastat 4 potentiostat/galvanostat/EIS from Princeton Applied Research analyzer with VersaStudio software. The voltammograms were recorded in acetonitrile (ACN) + 0.1 M NBu4BF4.

3. Results and Discussion

3.1. Strategy of grafting dyes to CNTs Figure 2 displays the general pathway to MWCNT modification by diazotized dyes; example is given for the attachment of Azure A to MWCNTs. The process is simple as it is conducted in water at RT, under stirring after ultrasonication of CNTs to achieve their dispersion. Surface modification was tuned by changing the initial mass of the dye diazonium salts (from 10 to 200 mg) while keeping the nanotube mass constant (40 mg). Very good dispersions of


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CNT-Dyes were obtained in ethanol (Figure 2, lower panel) compared to pristine CNT suspension in the same solvent. N N H2N

S Cl

CH3

N

Spontaneous reductive grafting

CH3

S Cl

t-BuNO2 HBF4

N

CH3

CH3

N

N

BF4, N2

S Cl

N

S Cl

CH3

N

CH3

CH3

CH3

Figure 2. Upper panel: general pathway to spontaneous aryl grafting of MWCNTs. Example is given for Azure A diazonium tetrafluoroborate. Lower panel: digital photograph showing good dispersion of CNT-AA in ethanol compared to pristine MWCNTs (dispersion of nanotube agglomerates). It is to note that on CNTs (in water) a chain mechanism has been proposed where aryl radical Ar• is the chain carrier produced by homolytic dediazonation of the diazonium salt in neutral medium.29

Hereafter, the samples are abbreviated CNT-AAx, CNT-NRx and CNT-CRx where x stands for the initial mass of diazotized dye (10, 20, 40, 100 or 200 mg).

3.2. Vibrational spectroscopy 3.2.1. Infrared spectroscopy IR spectroscopy was used to characterize the newly synthesized nanocomposites and the corresponding pristine and diazotized dyes (Figure 3). The IR spectrum of untreated MWCNTs is displayed in Figure S1 to help distinguishing the modification achieved by dye grafting. Figure 3 and Figure S1 show that the spectra of CNT-Dye hybrids have many similarities with that of the untreated CNTs. This is due to the aromatic rings in the base 8 ACS Paragon Plus Environment

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structures. Indeed, all spectra (Figures 3 and S1) exhibit main bands centered at 883, 1045 and 1090 cm-1 assigned to out of plane deformation of C-H bonds in aromatic compounds, C-H in plane deformation30 and C-C stretching,31 respectively. It is worth to note that the general structure of the spectra and the characteristic bands of each dye are clearly displayed in the spectra of the CNT-Dye hybrids which support the modification of the CNTs by the dyes. All spectra exhibit multiband spectral region in the 1240-1520 cm-1 range assigned to C=C and/or C=N stretching vibrations.30 Particularly, Congo Red has S=O stretching vibrations of sulfonic acid at 1061 cm-1 , C–N stretching vibrations at 1225 cm-1, C–N bending vibrations at 1352 cm-1 and N=N stretching vibrations relative to azo groups at 1584 cm-1.32 Neutral Red has a phenazine base structure the breathing vibration of which is noted at 1623-1653 cm-1.33

(a) Transmittance (a. u.)

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Congo Red

CR-N2

+

+ N2 CNT-CR200 C-H out

C=C Streching

of plane

vibration C-C Streching

C-H in plane

500

1000

vibration

1500

2000

2500

-1

Wavenumbers (cm )


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Transmittance (a. u.)

(b)

Azure A

+ AA-N 2 + N2 CNT-AA 200 C-H out

C=C and C=N

of plane

Streching vibration C-C Streching

C-H

vibration

in plane

500

1000

1500

2000

2500

Wavenumbers (cm-1) C=C Streching

(c) Transmittance (a. u.)

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

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Neutral Red

NR-N2

+

N 2+ CNT-NR200 Phenazine breathing vibration C=C and C=N C-H out

Streching vibration

of plane C-H

C-C Streching

in plane

500

1000

vibration

1500

2000

2500 -1

Wavenumbers (cm ) Figure 3. IRATR spectra of (a) CR, CR-N2+, and CNT-CR200; (b) AA, AA-N2+ and CNTAA200; and (c) NR, NR-N2+ and CNT-NR200. The effective reaction of the dye diazonium salts with CNTs leading to dediazonization and production of dye-grafted CNTs was checked by inspecting the diazonium stretching spectral region. The diazotized dyes exhibit a characteristic N≡N stretching vibration peak at 2349 cm-1 for Azure A (Figure 3b) and 2353 cm-1 for Congo Red (Figure 3a) and Neutral Red (Figure 3c) which parallels the results of Rakoczi and Oxenius.34 CNT-AA200 (Figure 3b) and CNT-NR200 (Figure 3c) samples do not display such a characteristic band therefore indicating 10 ACS Paragon Plus Environment

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effective grafting of the diazotized AA and NR dyes. IR spectroscopy was employed on many occasions to track the loss of the stretching vibration of the triple bond N≡N: Mangeney et al.35 noted the loss of the diazonium group stretching band in their study of nanodiamond modification with isolated nitrobenzenediazonium, whereas Orefuwa et al. 36 reported the absence of the N2 stretching frequencies at 2305 cm−1 as a result of the reduction of diazonium gold tetrachloride into gold NPs grafted with aryl groups. Elsewhere, Goldnanoparticle-cored dendrimers were prepared by chemical reduction of diazotized dendrimers in the presence of Au(III) with a result of total loss of the characteristic N≡N stretching band at 2245 cm-1 due to the decomposition of the diazonium compound.37 Figure 3a contrasts with Figures 3b-c in the sense that the diazonium stretching band is persisting after the reaction of CR-N2+ with CNTs. Actually, Congo Red has two aromatic amines which can be diazotized, thus the question that arises is: (i) can the two diazonium groups react simultaneously with CNTs, or (ii) via one single diazonium, or (iii) simply be adsorbed via π−π stacking given the highly conjugated nature of CR. Using the spectral data shown in Figure 3a, the intensity ratio of –N+≡N stretching band to C=C stretching vibration band (centred at 1381 cm-1) for CNT-CR200 is about 0.27 relative to the same ratio determined for the diazotized Congo red dye (ca. (IN+≡N/IC=C)CRN2+/(IN+≡N/IC=C)CNT-CR200 = 0.27). The persistence of the N+≡N stretching band is in line with reaction of CR-N2 via one diazonium group only (see DFT section). Note, however, that for the persisting diazonium group at the surface of CNT-CR can undergo partial decomposition in alcoholic medium but this was reported to occur at 50 °C38 whereas all processes of this work were conducted at RT which minimizes such dediazonization.


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The possibility of loosely bound diazonium compound via physisorption can also be ruled out given the drastic cleaning procedure of the modified CNTs which was repeatedly undertaken until the washings were clear and colorless. To account for the presence of the characteristic diazonium stretching band after spontaneous reaction with CNTs we shall consider the XPS results, particularly those comparing the spectra from the pure diazotized dye CR-N2+ and those of the corresponding CNT-CR200. In addition to the experimental XPS results we shall demonstrate, using quantum chemical calculations, that the diazotized Congo Red cannot undergo grafting through the two diazonium groups (See Section 3.7).

3.2.2. Raman spectroscopy Raman spectroscopy is widely employed to study the ordered/disordered structure of carbon nanotubes and other Csp² carbon materials by examining the D/G peak intensity ratio.39,40 The spectra of the nanocomposites are displayed in Figure 4 for the three types of CNT-Dye prepared with various initial concentrations of diazotized dyes. One can note two peaks centred at 1593 and 1308 cm-1. The former is the G-band and is assigned to the E2g phonon vibration of sp2 bonded carbon atoms in highly ordered sidewalls of carbon nanotubes, whereas the latter the so-called D-band (centered at 1308 cm-1) is attributed to disorder in the structure of sidewalls.41


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(a) ID/IG= 1.37

CNT-CR 200

ID/IG= 0.96

CNT-CR 100

ID/IG= 1.20

CNT-CR

ID/IG= 0.96

CNT-CR

ID/IG= 1.00

CNT-CR

1200

1400

20

10 MWCNTs

G

D

ID/IG= 1.30

40

1600

1800

2000

-1

Raman shift (cm )

(b) ID/IG= 1.40

CNT-AA

ID/IG= 0.87

CNT-AA 40

200

CNT-AA 100

ID/IG= 1.18

ID/IG= 1.29

D

ID/IG= 1.25 ID/IG= 1.30

1200

1300

CNT-AA

20 CNT-AA 10 MWCNTs

G

1400

1500

1600

1700

-1

Raman Shift (cm )

(c) CNT-NR 200 CNT-NR 100

ID/IG= 1.45 ID/IG= 1.18 ID/IG= 0.97

CNT-NR 40 CNT-NR 20 CNT-NR

ID/IG= 0.89

D

ID/IG= 1.07 ID/IG= 1.30

10

G MWCNTs

1200

1400

1600 -1

Raman Shift (cm )

Figure 4. Raman spectra of CNT before and after reductive grafting of dye diazonium salts at indicated initial concentrations: (a) CNT-CR, (b) CNT-AA and (c) CNT-NR.

Similar D and G bands are also observed for CNT-Dye nanocomposites, regardless the dye nature and its initial concentration. This indicates that the structure of MWCNT is maintained in the three CNT-Dye nanocomposites. The spectra, shows a high intensity of the disorder 13 ACS Paragon Plus Environment

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peak (D) mode with a large number of sp3 C-C bonded carbons as a result of the functionalization of the MWCNTs by grafting the diazotized dyes onto MWCNTs surface. The slight increase of the D/G intensity ratio in the case of CNT-CR200, CNT-AA200 and CNT-NR200 compared to the non-functionalized MWCNT indicates that the aryl top layer is covalently attached to the MWCNTs, 42 however at lesser extent than for high quality SWCNTs. 43 For lower initial mass of diazotized dyes, there is a fluctuation of the D/G intensity ratio most probably due to the defects of the underlying MWCNTs which shadow any subtle changes in the Raman spectra brought by the attachment of the dyes. Elsewhere, a decrease in the intensity of the disorder peak (D band) was even observed after modification of single walled CNTs and attributed to the sp2 C=C tangential mode because of the reduction in the number of sp3 C-C bonded caused by the defects in the nanotubes structures.44

3.3. TGA (b)

0

0

Weight loss %

-20

28% -40 -60 CR

-80

(c)

29%

-20

CNT-AA 200

-40

AA

-60 -80

CNT-CR200

-100

MWCNTs

100

200

300

400

Temperature °C

500

600

Weight loss %

(a) Weight loss %

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

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0 -20

35%

-40

NR

-60 -80 CNT-NR 200 MWCNTs

MWCNTs

-100

-100 100

200

300

400

500

600

Temprature (°C)

100

200

300

400

500

Temperature (°C)

Figure 5. Thermogravimetric analysis (TGA) plots of CNT before and after reductive grafting of dye diazonium salts: (a) CNT-CR, (b) CNT-AA and (c) CNT-NR. Thermograms of the pure dyes are also displayed the respective figures.

Thermogravimetric analysis was used to determine the mass loading of organic coatings (Fig 5). Unmodified carbon nanotubes exhibit a thermal stability up to 600 °C. Upon heating, CNT-CR exhibits three main weight loss regions (Fig 5, a); the small one of 4% at (~20-70 °C) can be attributed to physisorbed water in the grafted CNTs. The second weight loss of 28% at (~200-400 °C) can be assigned to the decomposition (Fig 5, b and c) of the aryl top layer about 29% in the case of CNT-AA and 35% for the CNT-NR at (~200-350 °C), a thermal behavior that is comparable to those of the dyes (AA and NR).


600

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The second weight loss regions in the case of CNT-AA and CNT-NR are assigned to the decomposition of MWCNTs (T> 400 °C). It is worth to note that AA is more stable after grafting: AA starts decomposing ~ 150 °C while this occurs at T > 250 °C for CNT-AA. Although NR and AA have similar structures, there is a difference between the thermal stability decomposition of phenazine (in Neutral red base structure) and phenothaizine (in Azure A base structure).45 This single heteroatom difference between AA and NR affects the thermal decomposition of the corresponding nanocomposites.

3.4. TEM

Figure 6. High resolution TEM images of (a) pristine CNTs, (b) CNT-AA200, (c) CNT-CR200, and (d) CNT-NR200. The morphology of the samples was investigated using TEM imaging. Figure 6 displays clear changes for the diazonium-treated samples compared to pristine CNTs. Interestingly, the amorphous coatings on the outer walls of the CNTs are continuous with an average thickness 15 ACS Paragon Plus Environment

Langmuir

that depends on the nature of the dye. Using TEM, it was possible to determine the thickness average values from 15 points. The thickness of the aryl layers was found to be 1.9 ± 0.30, 2.7

± 0.7 and 6.3 ± 0.7 nm for CNT-AA200, CNT-NR200 and CNT-CR200, respectively. It is to note that whilst TGA and Raman measurements suggest more grafting for AA and NR, Figure 6c shows thicker CR coating. This apparent discrepancy could be explained in terms of steric effects; indeed CR molecule is bulky and attaches to the CNTs in a way that it occupies larger volume. Probably, in this case the grafted layer seems thicker (Figure 6c) but not necessarily dense (see DFT section 3.7).

3.5. XPS 6k

C1s

CNT-CR200

N1s

(i) 395

N1s 600k

O1s

(c)

300k

(b) (a)

CNT-Congo Red

S2p S2s

CNT-Azure A

Cl2p

405

(h)

CNT-Neutral Red

(d)

400

CNT-AA200

+ N2 region

2k

(g)

CNT

+

AA-N2

0 0

410

4k

I (a. u.)

Intensity (a. u.)

900k

200

400

600

0 395

800

400

Binding energy (eV)

405

410

Binding energy (eV)

S2p 100k

F1s

50k

I (a. u.)

Na2s

I (cps)

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

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+

AA-N2

(e)

CNT-AA200

(j)

CNT-CR200

(j)

+ CR-N2

(f) 670

675

680

685

690

695

700

705

710

60

Binding energy (eV)

80

100

120

140

160

180

Binding energy (eV)

Figure 7. XPS survey regions of (a) pristine and (b-d) dye-modified CNTs; F1s spectra from (e) diazotized dye AA and (f) CNT-AA200; N1s spectra from (g) diazotized dye AA, (h) CNT-AA200 and (i) CNT-CR200, and Na2s-S2p regions from (j) diazotized dye CR and (k) CNT-CR200. 16 ACS Paragon Plus Environment

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Figure 7 displays survey regions for the pristine (Figure 7a) and modified CNTs (Figures 7 bd) using the highest initial concentration of diazonium salts (200 mg). The nanotubes are clean with a sharp C1s at ~285 eV and a minimal extent of oxidation evidenced by a very low intensity O1s peak centered at 532 eV (Figure 7a). CNT-AA200 exhibits two important peaks at 400 and 532 eV assigned to N1s and O1s, respectively (Figure 7b). The S+ atom is detected by its S2p peak at 165 eV and the chloride Cl2p feature centered at 198.5 eV (which indicates that the counter anion of S+ has not been exchanged during the formation of the diazonium tetrafluoroborate). Interestingly, for CNT-CR200 (Figure 7c) the S2p peak is located at ~168 eV. Note that neither Cl2p, nor S2p peaks are detected in the survey region of CNT-NR200 (Figure 7d) and the background is flat in the 30-285 eV spectral range. Figure 7e shows the existence of an F1s feature from AA-N2+ BF4- (Figure 7e) which vanishes after the reaction of this diazonium with CNTs to give CNT-AA200 (Figure 7f). It is interesting to check the peak shapes of the N1s regions. In this work, we start from the diazonium salts and end up with aryl-modified CNTs obtained by spontaneous reduction of the dye diazonium in the presence of the CNTs. Figure 7g displays the high resolution N1s region from AA-N2+ taken as an example and the corresponding one from CNT-AA200. Obviously, the pure diazonium salts exhibit tailed peaks extending from ~400 to 406 eV. It is well known from XPS literature on diazonium salts that the N1s peaks from the neutral and positively charged nitrogen atoms are found in the 403-404 and 405-406 eV, respectively.46,47 Elsewhere, some of us have demonstrated that clays interact with diazonium salts and the N2 group is not removed unless the diazonium-modified clay is heated up at 60 °C; in this case the N1s doublet from the two distinct nitrogen atoms of the diazonium group is observed in the 403-406 eV spectral region.48 In the actual case, dediazonization occurs spontaneously as evidenced by the loss of the N2+ features from the N1s regions of the modified nanotubes.


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Herein, the broad region is located at 403-407 eV in the sole case of AA-N2+; in contrast quasi no such feature was detected for CNT-AA200, taken as example of a modified CNT. As in IR studies, we demonstrate here that the diazonium salts have reacted with CNTs in water and at RT. In the case of CR, this dye bears two aromatic amino groups which can be diazotized as discussed above. However, the CR dye is large, highly conjugated and the diazonium groups are at opposite sides which makes it unlikely for a reaction with CNTs to occur simultaneously via the two diazonium groups. CR is planar and it follows that if one diazonium group reacts with a CNT whilst the second one is away from the CNT surface. Eventually, the second unreacted diazonium group can react with a second CNT which will result in crosslinked CNTs. Alternatively, another CR-N2+ molecule reacts with already grafted CR dyes providing polyaryl layer. The first reaction is rather unlikely as we have noted excellent dispersions of the dye-modified CNTs. For this reason, among the three diazotized dyes only CR-N2+ exhibits a small N≡N stretching band in IR spectrum due to the second diazonium group that remains pending. It follows that CNT-CR bears N2+ groups the positive charge of which must be counter-balanced by an anionic species such as BF4- arising from the HBF4 acid employed to generate the diazonium salts. However, unlike pure CR-N2+, and despite the existence of available N2 group at the surface of CNT-CR hybrids, the CNT-CR200 XPS survey scan does not display any F1s peak from BF4-. CR is a particular dye with two sulfonate groups the negative charge of which is counter-balanced by Na+ cations. In Figures 7j-k, we display the Na2s-S2p region from CR-N2+ (Figure 7j) and CNT-CR200 (Figure 7k). Upon grafting, there is an important decrease in the Na2s/S2p intensity ratio due to neutralization of the negative charge of SO3- groups by the positive charge of the second, pending N2+ from CNT-CR200. In this case, the unreacted N2+ from CNT-CR (see Figure 7i as inset of Figure 7g-hj) are counter-balanced by SO3- and not by BF4- ; consequently BF4- is no


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longer necessary to neutralize the positive charge of N2+ whilst Na+ is no longer needed to neutralize the charge of SO3-. One thus ends up with a “hybrid sulfonate CNT-CR diazonium compound” similar to the simplest one: sulfonatebenzenediazonium compound in water (where –SO3- is the charged and not in the protonated neutral form).49

Figure 8 displays the high resolution C1s survey regions for the pristine and modified CNTs (prepared with 200 mg diazonium salt). CNTs have a narrow C1s region at ~284.5 eV with minimal oxidized carbon atoms. A shake up satellite is straightforwardly observed at 291 eV. NR and AA have very similar structures and induce a broad component at 286 eV assigned to C-N, C=N, C-S+. Similarly, C-N and C–SO3 chemical environments induce higher atomic charge on the respective carbon atoms and consequently the appearance of peak broadening. For the C1s narrow regions, the experimental envelope has a full width at half maximum (FWHM) of 0.81, 1.15, 1.23 and 1.73 eV (±0.1 eV for all values) for CNT, CNTAA200, CNT-NR200 and CNT-CR200, respectively.

250k

200k

I (cps)

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

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150k

CNT-NR200

100k

CNT-CR200

50k

CNT-AA200 CNT

0 280

285

290

295

Binding energy (eV) Figure 8. High resolution C1s spectra from pristine CNTs and CNT-Dye200 hybrids.


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(N+S)/C atomic ratio

0.08

(a)

0.06

0.04

0.02

0.00

0

1

2

3

4

5

Azure A/CNT initial mass ratio

0.10

(N)/C atomic ratio

(b) 0.08

0.06

0.04

0.02

0.00

0

1

2

3

4

5

Neutral Red / CNT initial mass ratio

0.10

(N+S)/C atomic ratio

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

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0.08

0.06

0.04

0.02

0.00

0

1

2

3

4

5

CR Diazonium / CNT initial mass ratio

Figure 9. Sorption isotherms of diazotized dyes on carbon nanotubes: (a) Azure A, (b) Neutral Red, and (c) Congo Red. Graphs were constructed by plotting heteroatom to carbon atomic ratio versus initial diazonium/CNT mass ratio.


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To account for the attachment of aryl groups to the underlying CNTs, N/C and S/C atomic ratios are plotted versus the initial diazonium mass. Figure 9 displays the three isotherms determined for the diazonium dyes. Sharp increases of the atomic ratios occur at low mass loading then the plots tend to reach plateau values in the case of AA and NR (Figures 9a and b). Interestingly, for CR a distinctly different behavior is observed as the (N+S)/C atomic ratio monotonously increases (Figure 9c). The two sets of results (Figures 9 a-b on the one hand, and Figure 9c, on the other hand) parallel the findings of the TEM investigation reported above and showing thicker coating for CNT-CR200. This continuous increase of the layer indicates a fast electron transfer through the 6.3 nm layer of CR, the electrons can reach the organic layer-solution interface and further reduce the diazonium salt. Regardless the absolute mass loadings and thicknesses, it is clearly possible to tune the surface composition of the diazonium-modified CNTs by varying the initial dye/CNT mass ratio.

3.6. Electrochemical behaviour of diazotized dyes The dyes and their diazonium salts, the modified GC electrodes and MWCNT were examined in ACN + 0.1 M NBu4BF4. We first investigated the electrochemical behavior of AA;25,50,51 a reversible electron reduction wave WAA (scan rate = 0.1 V/s) is observed on a GC electrode; the redox potential is obtained as the midpoint of the anodic and cathodic peaks Epc = -0.21, Epa = -0.11, E° = - 0.16 V/SCE and ∆Ep = 0.10 V indicates a rather fast electron transfer (Figure 10a). We have examined the possible grafting of AA itself upon repetitive scanning, no decrease of the voltammogram is observed after 10 cycles. However, upon rinsing and transferring the electrode to a 0.1 M KCl solution, a voltammogram of AA can be observed that disappears after two 180 s ultrasonic rinsing in ethanol. Conversely, no adsorption can be observed if the electrode is left 30 min in the AA solution. Therefore, electrochemical driven


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adsorption is observed after repetitive scanning on the peak of AA, but no grafting takes place. This adsorption phenomenom is likely to be responsible for the very small height of the wave ip = 0.54 µA (ip/c√v = 1.7 10-3) by comparison with the reversible one electron wave of ferrocene under the same conditions and on the same electrode ip = 19 µA (ip/c√v = 60.0 103

).

We have investigated the electrochemistry of AA-N2+ under the same conditions as AA; it presents the same reversible wave as AA: WAA at E° = - 0.26 V/SCE (Figure S2); this wave remains of constant height upon repetitive scanning. Usually, the wave of the diazonium cation is located at less negative potentials than that of the parent molecule and disappears upon repetitive scanning, in ACN the wave of AA-N2+ is probably hidden in the rise of WAA and very small due to the grafting of the surface during the time necessary to prepare and start the experiment. In any case the voltammogram of AA-N2+ is very different from that of other diazonium salts. In order to compare with the spontaneous grafting on CNT, electrografting of a GC electrode was achieved by recording ten cyclic voltammograms from + 0.2 to - 0.5 V/SCE in a 1 mM solution of AA-N2+ in ACN + 0.1M NBu4BF4, the electrode was ultrasonicated in ethanol to give a modified electrode GC-AA. This modified electrode was rinsed under ultrasonication in alcohol and transferred to an ACN + 0.1 M NBu4BF4 solution; a reversible wave located at the same potential as WAA is observed, indicating the grafting of AA on the surface of the carbon electrode (Figure S3). The surface modification is also observed through a redox probe experiment: the GC-AA electrode is transferred in a 3 mM K3Fe(CN)6 + 0.1 M KCl solution, the voltammogram of the Fe(CN)63+/2+ is strongly attenuated compared to that is observed on a clean electrode, also indicating a blocking of the electrode (Figure S4). We also examined the cyclic voltammetry of AA modified carbon nanotubes: CNT-AA200; a suspension of CNT-AA200 (prepared as in the experimental section), was deposited on a GC


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electrode and the voltammogram recorded in an ACN + 0.1 M NBu4BF4 solution. The same voltammogram as that of Figure 10a is observed confirming the grafting of AA-N2+ on the surface of CNTs (Figure 10b). NR exhibits in ACN + 0.1 M NBu4BF4 on a GC electrode, a reversible wave WNR at Epc = 0.53, Epa = -0.44, E° = -0.49 V/SCE, ∆Ep = -0.90 V (Figure 10c). 52 The height of this wave is, as previously, very small ip = 2.2 µA compared to ferrocene under the same conditions. The reversible wave WNR is also observed on a GC electrode dipped for one hour an 10-3 M NR solution but is removed by ultrasonication in ethanol indicating that NR is adsorbed but not grafted on GC. NR-N2+presents on a GC electrode in ACN + 0.1M NBu4BF4 an irreversible voltammogram with a cathodic peak at -0.45 V/SCE (Figure S2).53 By repetitive scanning to -0.5 V/SCE the voltammogram decreases as usually observed with diazonium salts. After grafting the GC electrode by recording 20 cycles from 0.2 to -0.5 , rinsing in ethanol and transferring to an ACN + 0.1 M NBu4BF4 solution a partly reversible voltammogram is observed (Figure S3) with Epc = - 1.20 and Epa = - 0.58 ; E° = - 0.89 V/SCE ∆Ep = 0.41 V similar to WNR. Therefore this reversible wave can be assigned to GC-NR. The modification of the surface was also tested with a redox probe as with AA; a complete blocking of the electrode is in agreement with the formation of a film on the electrode (Figure S4). We also examined the cyclic voltammetry of NR-modified carbon nanotubes (CNT-NR200). For this purpose, a suspension of CNT-NR200 was deposited on a GC electrode and the voltammogram recorded in an ACN + 0.1 M NBu4BF4 solution. The same voltammogram as that of Figure 10c is observed confirming the grafting of NR on the surface of CNTs (Figure 10d). CR is very little soluble in ACN (c< 0.1 mM in ACN + 0.1 M NBu4BF4, GC electrode) its reversible wave is difficult to observe WCR Epc ~ -1, Epa ~ - 0.6, E° = -0.8 V/SCE (Figure


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10e). After recording the voltammogram of CR on a GC electrode as in Figure 10e, the electrode was rinsed in ethanol under ultrasonication for 180s, and transferred to an ACN + 0.1M NBu4BF4 solution. The voltammogram of CR was recorded indicating a very strong adsorption of CR on the surface of GC. CR-N2+ presents an ill-defined voltrammogram with a shoulder at -0.6V/SCE that decreases upon repetitive scanning and can be assigned to CR-N2+ (Figure S2). The modified electrode (GC-CR) transferred to an ACN + 0.1M NBu4BF4 presents a well-defined reversible wave Epc = - 1.07, Epa = 0.75, E° = -0.91 V/SCE, ∆Ep = 0.32 V (Figure S3), but the voltammogram of K3Fe(CN)6 is little affected by the modification of the electrode indicating a low coverage of the electrode. (Figure S3). This low coverage can be related to the presence of the bulky substituent in the ortho position of the diazonium group.54 In spite of this low coverage a thickness of 6.3 nm is observed for the film attached to the MWCNT; this indicates that the radical obtained after dediazonation of CR-N2+ reacts faster on the first grafted layer than on the surface of the CNT. Such a feature has been previously observed for nitrophenyl groups on graphene.55 CNT-CR200 sample was deposited as before on a GC electrode and the voltammogram recorded; a wave similar to WCR is observed indicating a successful grafting (Figure 10f).


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Cyclic voltammetry of dyes

Cyclic voltammetry of grafted CNT (b)

(a)

0.2 2

3 2

Current (µ µ A)

Current (µA)

1 0 -1 -2 -3

0.0 0 -0.2 -2 -0.4 -4 -0.6 -6

-4 -5

- 0.8 -8

-0.6

-0.4

-0.2

0.0

0.2

-0,6 -0,2 -0,1 0,0 -0.4 -0,3 -0.2 -0.6 -0,5 -0,4 0.0 0,1 0,2 0.2 0,3

0.4

Potential (V)

Potential (V)

(d)

(c)

0,0 0 -2,0µ -2

Current (µ µ A)

Current (µA)

1

-4,0µ -4

0

-6,0µ -6

-1

-8,0µ -8

-10 -10,0µ

-2

-12 -12,0µ

-3

-14,0µ -14 -1.6 -1,4 0.2 0,4 0.4 -1,6 -0,6 -0,4 -1.4 -1,2 -0.2 0,0 0.0 0,2 -1.0 -0,8 -1.2 -1,0 -0.4 -0,2 -0.8 -0.6

-4 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V)

0.4

Potential (V)

(e)

(f)

0.4

0.1

0.2

0.0 0.0

Current (µA)

Current (µA)

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

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-0.2 -0.4 -0.6 -0.8

-0.1 -0.2 -0.3 -0.4 -0.5

-1.0 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Potential (V)

Potential (V)

Figure 10. Cyclic voltammetry of dyes AA (a), NR (c), and CR (e) and of the corresponding grafted nanotubes CNT-AA (b), CNT-NR (d), CNT-CR (f). Conditions: c = 1 mM for AA, NR; c < 0.1 mM for CR in ACN + 0.1 M NBu4BF4; v = 100 mV s-1, reference SCE.

Altogether these electrochemical experiments confirm the spontaneous grafting on CNT of the diazonium salts of the three dyes: the very easy reduction of the corresponding diazoniums explains the spontaneous reduction by the MWCNT surface.


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3.7. Quantum chemical calculations In order to rationalize the experimental results, the mechanism of adsorption of the three molecules is investigated by using periodic density functional theory (DFT) method. We consider both the diazonium form (AA-N2+, CR-N2+ and NR-N2+) and aryl radical form (AA•, CR• and NR•) resulting from dediazoniation. The optimized geometries are shown in Figure S5. The (5, 5) CNT with diameter 6.8 Å56 is considered for the grafting as it is known to yield stronger aryl binding compared to zigzag CNT. 57. A super cell of (15 X 1 x 1) large enough to isolate the molecules is built from the primitive cell. The CNT is periodic along its main axis and vacuum space of 35 Å large enough to allow grafting molecules on the surface is added along y and z axis. Calculations were performed by means of the Vienna Ab-Initio Simulation Package (VASP 5.4.1). 58

Electron-ion interactions were described by the projector-

augmented wave (PAW) method.59 Convergence of the plane-wave expansion was obtained with a cut-off of 500 eV. The generalized gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) functional.59 Sampling in the Brillouin zone was performed on a grid of (3 x 1 x 1) k-points for the geometry optimizations. Dispersion effect is taking into account using the recent Grimme D3 method.60 The dyes are grafted on the CNT by placing N2 above carbon atoms or by placing the molecules parallel to the CNT in order to promote (π…π) interaction type. The aryl radical form is placed on the CNT by connecting the radical carbon to CNT carbon. In all cases, the geometries of complexes are fully optimized and the so-called grafting energy (∆Eg) is calculated as difference between the optimized energies of complex (ED-CNT) and isolated CNT (ECNT) and dye (ED). The optimized geometries of the complexes are presented on Figure S6.

∆Eg = ED-CNT - ED - ECNT


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Table 1. Grafting energies of diazoted molecules and aryl radical on (5,5) CNT. System Grafting type mechanism ∆Eg (kJ/mol) DD..CNT (Å) AA-N2+ N….π

physisorption

-12.9

2.99

physisorption

-43.3

3.09

physisorption

-27.3

2.86

π…π

physisorption

-70.1

3.22

π…π

physisorption

-115.8

3.30

N….π

physisorption

-13.7

2.96

π…π NR-N2+ N….π

CR-N2+

Aryl radical grafting AA•

C-CNT

chemisorption -137.8

1.59

NR•

C-CNT


1.56

CR•

C-CNT


1.59

The results indicate low physisorption for AA-N2+, NR-N2+, CR-N2+ when they adsorb by N2 groups on the nanotube with ∆Eg equal to -12.9, -27.3 and -13.7 kJ/mol, respectively. The measured minimum distance between the grafted dye is at least 2.80 Å in agreement with recent study of diatomic N2 adsorption on graphene.61,62 The grafting is also favorable by π-π interactions with energies of -48.3, -70.1 and -115.8 kJ/mol for AA-N2+, NR-N2+ and CR-N2+, respectively. The interaction is particularly strong for CR-N2+ due to its long conjugated chain. This correlates well with the large grafted layer observed in TEM for CR-N2+ compared to AA-N2+ and NR-N2+ (See section 3.4). The most important grafting energies are found for radical aryl dyes. ∆Eg are found equal to 137.8, -185.5 and -159.1 kJ/mol (See Table 1) for AA•, NR• and CR• resulting in the hybrid systems CNT-AA, CNT-NR and CNT-CR, respectively. These values correspond to the single C-C bond allowing attaching the dye radical; they are lower than C-C experimental bond energy equal to -362 kJ/mol according to the standard reference.63,64,65,66


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They are in agreement with the recent paper of Umeyama et al.67 in which similar conjugated molecules are grafted on different CNTs. The calculated grafting energies range between -165 and -210 kJ/mol matching those reported by Jiang et al.57 for CNTs but are smaller than those reported in the case of attachment of radicals to the edges of the graphene sheet.68 It is to note, however, that since the attachment of an aryl radical creates an unpaired electron on the CNT a second attachment is very likely to occur to give a much stronger binding on the nanotube as demonstrated elsewhere.69 The (C-C) bond lengths are 1.58, 1.56 and 1.56 Å in accordance with grafting energies. The shift from experimental is due to the steric hindrance of C-H groups neighbors. This effect is more pronounced for CR• and NR• dye radicals as it can be seen in Figure 11.

Figure 11. Optimized structures of dye radicals grafted on (5, 5) CNT: AA• (a), NR• (b) and CR• (c). Carbon atoms are in cyan, oxygen in red, nitrogen in blue, sulfur in green, sodium in gray and hydrogen in yellow.


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3.8. pH-responsive character of dye-grafted CNTs So far, experimental and quantum chemical calculations undoubtedly support the covalent attachment of the dyes to the underlying CNTs. One of the applied aspects of the dyes is their response to pH. Particularly, Neutral Red is a pH indicator which changes from red color at acidic pH to yellow-orange color with a transition pH range between 5 and 8.26 It was diazotized and grafted to gold electrodes in view of making optodes for the measurement of pH.26 As a matter of fact, enzyme-modified CNTs were found to undergo reversible changes in conductance upon changes in pH which makes them suitable biosensors, 70 whereas SWCNT-modified microelectrode was shown to reversibly exhibit a relationship between the peak potential of the intrinsic protein oxidation signal and pH.71

H3C

H3C

N N CH3 CH3

N H

N N CH3 CH3

N

- HCl + HCl

Acidic form

9k

Basic form

(b)

CNT-NR200 (pH=2)

20k

(c)

CNT-NR200 ( neutral pH)

8k

%N+= 9.5 7k

I (cps)

18k

I (cps)

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

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%N+= 5.4

14k

12k

10k 6k

395

400

405

Binding energy (eV)

395

400

405

Binding energy (eV)

Figure 12. Upper level: acid-base equilibrium between the protonated and the deprotonated forms of CNT-NR200 hybrids. Lower panel: Peak-fitted N1s regions of CNT-NR200 specimens post-treated at pH 2 and as prepared in water. Herein, we examined by XPS the effect of pH on the extent of protonation of CNT-NR200. The latter was treated in an acidic aqueous solution of pH 2, washed, dried and examined by XPS in comparison to the same sample prepared in water at neutral pH. Figure 12a shows the pH response of CNT-NR200 to pH. We reasoned that pH will have an effect on the extent of 29 ACS Paragon Plus Environment

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protonated nitrogen atoms. The fitted N1s regions from the post-treated (pH 2) and the asprepared CNT-NR200 samples are compared in Figure 12b and c. The peaks are fitted with three components assigned to NH, N=N and quaternized N+ types of nitrogen atoms. Particularly, the contribution of the N+ component to the total N1s peak area contributes to 9.5 %, quasi two-fold higher than for the as-prepared CNT-NR200 sample. This simple test stresses the pH-responsive character of the dye-modified CNTs through the example of CNTNR200.

Conclusion Azure A (AA), Neutral Red (NR) and Congo Red (CR) were diazotized and grafted spontaneously to MWCNTs in water at RT without any reducing agent. The surface composition of the modified carbon nanotubes was tuned by simply changing the initial mass of the diazotized dye under test while keeping that of the nanotubes constant. The resulting CNT-AA, CNT-NR and CNT-CR nanocomposites were thoroughly characterized by infrared, Raman, XPS, TGA, TEM and electrochemistry. IR, XPS and Raman permitted to monitor the change in the surface composition and the results support the grafting of the dye diazonium salts. The grafted layers were found to be evenly distributed over the nanotube surfaces with a thickness in the 1.5-3 nm for Azure A and Neutral Red, and 6.3±0.7 nm for Congo Red. Cyclic voltammetry permitted to demonstrate that the dyes are indeed covalently attached to the CNTs. Actually, quantum chemical calculations indicate covalent grafting with high bond energies (up to 185 kJ/mol for Congo Red diazonium), but also support energetic π stacking (up to 115 kJ/mol for Congo Red). Finally, taking CNT-NR as an example, we demonstrate that this robust hybrid is pH responsive as it is much more protonated after treatment at pH 2. To summarize, this work demonstrates the efficiency of dye diazonium salt chemistry in the design of dye-functionalized carbon nanotubes, in water at RT, which could find applications as sensors,72 filters73 and composites for Braille microsystems.74

Acknowledgements The authors wish to thank Campus France and the Slovak Research and Development Agency for financial support through the Stefanik project DARLIN’ No 31785SL (SK-FR-20130033). The research was supported partially by projects VEGA 2/0149/14 (SK). A. Bensghaïer is indebted to the Tunisian Ministry of Higher Education for the provision of a Bourse d’Alternance scholarship. Quantum chemical calculations was performed using HPC resources from GENCI- [CCRT/CINES/IDRIS] (Grant 2017-[A0020807006]). 30 ACS Paragon Plus Environment

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Supporting Information IR spectrum of pristine carbon nanotubes; Electrochemical characterization of diazotized dyes by cyclic voltammetry; reductive electrografting of diazotized dyes on glassy carbon electrodes and blocking effects of the attached dyes; most stable geometries of the (diazotized) dyes, the CNTs and their complexes This material is available free of charge via the Internet at http://pubs.acs.org.

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ToC Graphic Efficient, covalent modification of multiwalled carbon nanotubes with diazotized dyes in water at room temperature Asma Bensghaïer1,2, Stéphanie Lau Truong 3, Mahamadou Seydou3, Aazdine Lamouri3, Eric Leroy2, Matej Mičušik4, Klaudia Forro4, Mohamed Beji1, Jean Pinson3, Mária Omastová4,*, Mohamed M. Chehimi2,*


Efficient Covalent Modification of Multiwalled Carbon Nanotubes with

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