Introduction

One of the current challenges in polymer chemistry is the development of new bio-based materials [1, 2] and low-cost polymers with specific physical properties [3,4,5,6,7,8,9,10,11] and/or biological ones [12,13,14,15,16,17,18,19,20,21]. Therefore, electro-polymerization is one of the most important routes for tailoring the physical properties of the conductive polymers for a broad range of applications [22,23,24,25,26,27]. Based on its chemical structure, carbazole molecule, as well as its derivatives underline a variety of biological properties: including anti-tumor, anti-bacterial, anti-fungal, anti-epileptic, anti-diabetic, anti-oxidative, anti-inflammatory, neuro-protective, and anticonvulsant effects as described by much research’s [28,29,30]. Poly (N-vinylcarbazole) poly (NVC), which is a conjugated polymer, has received special attention for about thirty years decade, because of its important electro-chromic properties as well as potential for the production of electroluminescent devices, photovoltaic cells and in optoelectronics [31,32,33]. For this, many studies have been focused on the synthesis of materials based NVC derivatives [34,35,36]. Sheng-Huei Hsiao et al. were studied the electro polymerization mechanism of 3, 6-di(carbazol-9-yl)-N-(4-nitrophenyl) carbazole and 3,6-di(carbazol-9-yl)-N-(4-aminophenyl) carbazole on electrode surface [37]. On other hand, PNVC-alumina (Al2O3) (nano) composite was electrochemically synthesized for corrosion protection of stainless [38]. In our previous work, copolymer poly (N-vinylcarbazole-co-aniline) doped with titanium dioxide (TiO2) was successfully prepared by direct electrochemical oxidation [39]. The work of M. Baibarac et al. were used the electrochemical polymerization of NVC on CNTs to produce PNVC/CNTs composites films which reveal their efficient application in the energy storage field [40]. In the same way, they studied the electrochemical polymerization of N-ethyl carbazole (EK) on carbon nanotubes (CNTs) films using cyclic voltammetry [41]. Recently, K. Priyadharshini were published important results about the application of poly (9-vinyl carbazole)/carbon nanotubes composites for electronic [42]. On the other hand, more attention is paid to eriochrome black T molecule as well as their polymers derivatives and their application as sensor or biosensors for different molecules. However, many studies were established regarding the electro-synthesis of poly (Eriochrome Black T) (PEBT) film modified Glassy Carbon Electrode (GCE) or Carbon Paste Electrode (CPE) for detection of dopamine [43,44,45,46], ascorbic acid [43,44,45], uric acid [43, 44, 46], adenine [47], DNA [48], methdilazine [49]. Electro-polymerization of EBT doped gold nanoparticles was also described in literature for biological molecules detection [50, 51]. In our previous work, we were synthesized poly (eriochrome black T) (PEBT)/ carbon nanotubes (CNTs) composites by cyclic voltammetry (CV) using indium tin oxide (ITO) as a support and examined the CNTs content effect on the electrochemical behavior as well as optical properties of the obtained PEBT [52]. As a continuation of this work, we describe in the present paper the preparation results of new copolymer based on N-vinylcarbazole (NVC) and Eriochrome black T (EBT) monomers, noted as poly(NVC-co-EBT) and its (nano)composites materials using carbon nanotubes (CNTs) at different content, as nano-filler. The choice of vinylcarbazole is motivated by the multiplicity of its physical properties related to its chemical structure and which implies a diversity of applications in the medical field, as cellular markers, biosensors and antibacterial coating, electronics and energy storage. The copolymer poly (NVC-co-EBT) and copolymers (nano)composites poly(NVC-co-EBT)/CNTs were synthesized by electro-deposition on indium tin oxide (ITO) substrate by cyclic voltammetry. For a fundamental characterization of the obtained materials, we report the effect of cycle number on the material behavior, the effect of CNTs content on electrochemical properties. The morphology, as well as optical band gap, were also established. Remarkably, the copolymer poly (NVC-co-EBT) exhibits a lower energy gap value as compared to the PNVC [39], implying a better semiconducting character. By adding CNTs nano-filler, this gap energy increases while preserving the semiconductor character of the material. At 10% wt. of CNTs, the gap energy converges to zero, leading to a conductor system and hence many applications areas.

Experimental part

Materials

The monomers N-vinylcarbazole (NVC) (Sigma-Aldrich, > 98%), Eriochrome black T (EBT) (Sigma-Aldrich, > 98%) were used without any purification. Carbon nanotubes (CNTs) (Sigma-Aldrich > 98%, diameter 110–170 nm, length 5–9 µm) as a doping semiconductor and lithium perchlorate (LiClO4) (Fluka), as supporting electrolyte were used as received. Chemical structures of monomers were illustrated in Fig. 1.

Fig. 1
figure 1

N-Vinylcarbazole (NVC) and Eriochrome black T (EBT) monomers chemical formulas

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Preparation of poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTs (nano) composites

The (nano) composites poly (NVC-co-EBT)/CNTs were prepared by electro-polymerization using CV method. For this, the monomers NVC (6.10–3 M) and EBT (5.10–3 M) were firstly dissolved in CH3CN/ LiClO4 (0.1 M). The CNTs were added at different weight contents (3 wt. %, 5 wt. %, 10 wt. %), then sonicated for 15 min, to obtain homogeneous solution. The electro-polymerization was conducted by cyclic voltammetry in the range [− 0.5 V/SCE–1.7 V/SCE] with a scan rate of 10 mV/s, on indium tin oxide substrate (ITO). For sake of comparison, the poly (NVC-co-EBT) was synthesized in the same conditions.

Instrumental analysis

The information processing used to be carried out via a potentiostat/galvanostat kind Voltalab forty 5PGP 301) managed by way of the voltamasters of ware program using a conventional three-electrode cell (25 ml). The working electrode is glass substrate of indium tin oxide (ITO) SOLEMS, the reference electrode was a saturated calomel electrode with KCl (SCE), and the auxiliary electrode was platinum grid. Prior to the deposition process, the ITO substrates were ultrasonically cleaned in methanol, acetone and distillate water for 10 min and etched in H2SO4 45% for 2 min to activate the surface of the electrode, which was fixed at 1 cm2 area. The electrochemical impedance spectroscopy measurements were carried out under alternating voltage of 10 mV, in the frequency range between 100 kHz and 50 MHz The surface morphology of synthesized composites was highlighted according to a scanning electronic microscopy JEOL Sam- 700 1F apparatus. Optical properties of the composites were performed using Shimadzu UV–Visible spectrometer all measurements were conducted at room temperature.

Results and discussion

Electro-copolymerization of poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTs (nano)composites

Electro-polymerization of n-vinyl carbazole (NVC) and Eriochrome black T (EBT) was conducted using cyclic voltammetry method for 20 cycles, in the range [− 0.5 V–1.7 V]. As shown in Fig. 2, the voltammogram presents anodic responses screened around 0.12 V and 1.20 V, which can be assigned to eriochrome black T (EBT) and N-vinyl carbazole (NVC) monomers oxidations, respectively, to produce monomers ions units. Cathodic responses were recorded at approximate 1.09 V, for NVC monomer, followed by a second pic of EBT monomer around − 0.3 V. As described in literature [53], carbazole monomer is oxidized at around 1.20 V leading to an unstable cation radical able to couple with either another cation NVC+ or EBT+ to form another cation radical and hence the propagation of the polymer chain. It is important to note that is difficult to define exactly, if the protons loss occur before or after coupling but the lost protons are reduced in the cathodic cycle [50]. The proposed reactions mechanisms of NVC electro-polymerization in absence and presence of carbon nanotubes were described in literature by Reyna-González et al. [54, 55] and Baibarac et al. [25, 26, 40, 41]. Under the electrochemical conditions, EBT was oxidized to form benzoquinone di-imine [47, 49]. In our case, we suggest that under these operating conditions, a certain competition in the formation of the copolymer takes place without ruling out the propagation of the homopolymer chains. By increasing the anodic and cathodic currents, the growth chains were formed, implying the formation of the copolymer. On other word, the current of oxidation and reduction peaks increases during cycling, which confirm the copolymer electro-deposition. Interestingly, it is clear from Fig. 2 that the responses become stables after 20 scans, implying a self-adjustment of the copolymer film thickness at the ITO electrode. The obtained copolymer presents a good adhesion and homogeneity, as well as a high stability. The preparation of the copolymer composites poly (NVC-co-EBT)/CNTs on a transparent ITO electrode was also performed by cyclic voltammetry. Figure 3 depicts the first cyclic voltamogramms corresponding to the electro-copolymerization of poly (NVC-co-EBT) in presence of CNTs, at different percent weight of nano-filler, screened at a scan rate of 10 mV.s−1, in the range [− 0.3 V.SCE−1–1.5 V.SCE−1]. In presence of CNTs nano-fillers, the voltammograms display new redox couple. Therefore, when 3wt.% CNTs were added (poly (NVC-co-EBT)/CNT1), new oxidation peaks were appeared at 0.19 V/SCE and 0.90 V/SCE, accompanied by a cathodic wave screened at 0.32 V/SCE, respectively. In addition, the voltammogram relied to poly (NVC-co-EBT)/CNT2 records two cathodic waves at 0.51 V/SCE and at 1.01 V/SCE. However, in the case of poly (NVC-co-EBT)/CNT3, large anodic and cathodic waves were detected at 0.97 V/SCE and 1.07 V/SCE, respectively. Thus, these new oxidation and reduction potential peaks may be relied to the presence of the CNTs nano-fillers and a confirmation of the development of such interactions in presence of CNTs in the electro-polymerization solution. As shown in this figure, there is some variation in the voltammograms shape of the synthesized composites with the variation of the CNTs content. Well-defined peaks were recorded, suggesting the presence of new redox electroactive groups (function) units in the polymer chain. On other word, the CNTs nanoparticles play an important role in the doping of conductive organic polymers by increasing their electroactive surface and improving, thus their electrical conductivity.

Fig. 2
figure 2

Cyclic voltammograms obtained during electro synthesis of poly (NVC-co-EBT) for 20 cycles, in the range [− 0.3–1.5 V/SCE] (NVC: 6.10−3 M, EBT: 5.10.−3 M, v = 10 mV/s, ITO electrode, in LiClO4/CH3CN)

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Fig. 3
figure 3

First cycle voltammograms relating to poly (NVC-co-EBT)/CNTs, in the range [− 0.3–1.5 V/SCE] (NVC: 6.10−3 M, EBT: 5.10.−3 M, v = 10 mV/s, ITO electrode, in LiClO4/CH3CN)

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Effect of cycle number on the copolymer behavior

Figure 4 highlights the electro-activity of the synthesized copolymers (nano)composites at 5, 10 and 20 cycle’s number. Throughout cycling, some changes in the voltamogramms shape were observed. Even thin films were formed after the first cycle and their thickness were increased by increasing the cycle’s numbers. An increase of the current intensity of the (nano)composites as compared to copolymer, whatever the cycle’s numbers, was screened. This might be relied to the presence of CNTs nanoparticles. Therefore, when the copolymer is doped with 3 wt. % of CNTs, the current intensity is remarkably increased as compared to virgin copolymer. On other word, the charge transfer becomes more important and the copolymer film more conductor. By increasing the CNTs content, poly (NVC-co-EBT)/CNT2 and poly (NVC-co-EBT)/CNT3 record a current intensity value lesser than that of poly (NVC-co-EBT)/CNT1 but they remain more important to that of the virgin copolymer. The CNTs may be act as dopant on the surface and nano-filler in bulk of the copolymer matrix.

Fig. 4
figure 4

Cyclic voltammograms relating to poly (NVC-co-EBT)/CNTs, in the range [− 0.3–1.5 V/SCE] (NVC: 6.10−3 M, EBT: 5.10.−3 M, v = 10 mV/s, ITO electrode, in LiClO4/CH3CN), for 5 cycles (a), 10 cycles (b), 20 cycles (c)

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Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) is one of the most effective techniques for analyzing the conducting polymer/electrodes interface properties [56]. It can reveal information regarding processes occurring during doping of a conducting polymer matrix. The Nyquist diagrams of poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTs composites were recorded under the same conditions as cyclic voltammetry: solvent/electrolyte system (CH3CN/LiClO4: 0.1 M), frequency range between 100 kHz and 10 MHz and at open circuit potential. The plots representing the imaginary part as a function of the real part of complex number of the impedance were reordered in Fig. 5a. In the range of low and intermediate frequencies, a linear evolution making an angle equal to or less than 45° with the reels axis was observed. This response is assigned to the diffusion process in the electrode material that is related to Warburg impedance behavior [38, 56, 57]. At high frequencies, the plots show the presence of more or less well-resolved capacitive semicircle, which confirm the charge transfer phenomenon. It is important to note that the addition of CNTs implies some decrease of the semicircle diameter and hence increase of composite conductivity. Using Z-view software, the experimental data is well fitted using the equivalent circuit combining the high-frequency internal resistance (Ri), including both the resistance of the electrolyte and that of the electrode material, in serial with three capacitive loops (Fig. 5b). The first loop associates the outer ionic charge transfer resistance (Rct1) in parallel with the double-layer capacitance (Qdl); the second one involves the inner capacitive loop (Rct2Qin) in the low-frequency domain and associates both the accumulation and the transfer of ionic charges in the inner surface (sites). The impedance modulus involves the diffusion resistance (Rd) associated with intra- and inter-grain conduction in parallel with the bulk capacitance (Qbulk) [47, 50, 51, 58, 59]. Based on the obtained results, poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTs present a semi-conductor character implying very interesting electro-chemical property, favoring its use in subsequent photo-electrochemical applications. The electrochemical parameters obtained from EIS measurements at open circuit potential are summarized in Table 1.

Fig. 5
figure 5

Nyquist diagrams relating to poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTs films, recorded in a frequency range [100 kHz–10 MHz] (a), equivalent circuit (b)

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Table 1 Fitting table for the experimental data of the equivalent circuit model
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Optical properties

The absorption properties of the synthesized copolymer and corresponding (nano)composites were illustrated in Fig. 6a. Interestingly, the transmittance of the copolymer was remarkably increased by adding the CNTs nano-filler in the copolymer matrix. This result may be relied to the dispersion of the CNTs nano-filler in the electrodeposited copolymer leading to some modification of the polymer surface. Figure 6b highlights evolution of (αhν) 2 as a function of incident photon energy and the optical band gap Eg of the synthesized polymers, a determinant parameter to define the photo-electrochemistry character of the material [59].

Fig. 6
figure 6

UV–visible spectra (a) and diffuse reflectance of direct optical transition (b) of poly (NVC-co-EBT)/CNTs thin films electrodeposited on ITO surface, at different CNTs content

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(αhν)2/n = Const × (hν—Eg).

The gap energy Eg of the obtained materials was determined by the intersection of the plot extrapolationand the energy axis (hν), relied to the direct transition (n = 1). The band is defined as the separation between the lowest unoccupied molecular orbital (LUMO), corresponding to the valence band and the highest occupied molecular orbital (HUMO), corresponding to the conduction band (CB) π → π* transition [58]. As shown in Fig. 6b, the synthesized copolymer displays such improvement in the semi-conductor character, with a gap energy value of 1.95 eV, as compared to PNVC (Eg = 1.57 eV) [39]. Interestingly, this value was remarkably increased by a low doping of poly (NVC-co-EBT) with CNTs. Therefore, few CNTs content (3 wt. % and 5 wt. %) are sufficient to achieve energies gap values of 2.41 eV and 2.71 eV, respectively. This important result underlines the semi-conductor character of the poly (NVC-co-EBT)/CNT1 and poly (NVC-co-EBT)/CNT2 (nano)composites and hence their applications in photovoltaic. On the other hand, when 10 wt. % of CNTs has been added, the energy gap converges to zero value and the resulting poly (NVC-co-EBT)/CNT becomes a conductor material. As a conclusion, the synthesized poly (NVC-co-EBT) can display semi-conductor and conductor character according to the CNTs content in the polymer matrix, which increase the range of its industrial applications.

Morphology of poly (NVC-co-EBT) and poly (NVC-co-EBT)-CNTs (nano)composites

Figure 7 shows the SEM micrographs of poly (NVC-co-EBT) and poly (NVC-co EBT) /CNTs. It is clearly observed that all poly (NVC-co-EBT) coated ITO are electrodeposited and the electrode surface was completely covered by polymer. In the case of the (nano)composite poly (NVC-co-EBT)/CNTs doped with 3 wt. % CNTs, no aggregation/agglomeration of the filler was observed on the structure, confirming the homogenous dispersion of CNTs in the copolymer. The absence of agglomeration is also observed in the case of poly (NVC-co-EBT)/CNT2. So, by increasing the CNTs nanofillers content (poly (NVC-co-EBT)/CNT3), such trace micrometer-sized AC aggregates/agglomerates are visible, attesting the microcomposite formation and the bad dispersion of CNT. This result, correlate the optical properties.

Fig. 7
figure 7

SEM images of poly (NVC-co-EBT) and poly (NVC-co-EBT)/CNTS (nano)composites

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Conclusion

In this work, a new copolymer poly (NVC-co-EBT) and its corresponding (nano)composite poly (NVC-co-EBT)/CNTs were successfully synthetized via electro-polymerization route, using N-vinylcarbazole (NVC) and black eriochrome T (EBT) monomers in acetonitrile containing lithium perchlorate. The mainly study is focused on the effect of the CNTs content on electrochemical and photo-electrochemical properties of the obtained material. Therefore, thickness of the deposited polymers (nano)composites films is clearly dependent on the cycle’s number during electro-polymerization, accompanied with an increase of their current intensity as compared to virgin polymer. The energy gap of the (nano)composites poly (NVC-co-EBT)/CNTs is closely relied to the CNTs content. So, with 3 wt. % of CNTs, the energy gap value is 2.41 eV. This value was achieved 2.71 eV, at 5 wt. % of CNTs, which is an indication of the semi-conductor character of this new copolymer (nano)composite, at these CNTs doping contents. Interestingly, this copolymer becomes conductor by adding 10 wt.% of CNTs, with an energy gap equal to zero. The impact of the CNTs content and its dispersion in the polymer matrix was illustrated to describe the semi-conductor/conductor character of the material. This combined semi-conductor, conductor properties in this new synthesized polymer permits its use in many development areas and industrial fields, such as electrocatalysis, photocatalysis and photovoltaic. Such a complementary study is currently under investigation to highlight the structure, morphology, thermal and mechanical properties of the poly (NVC-co-EBT)/CNTs (nano)composites under different electro-polymerization conditions.