Electrochemical, optical and morphological properties of poly (N-vinylcarbazole/TiO₂) and (N-vinylcarbazole/aniline)/TiO₂ copolymer prepared by electrochemical polymerization

2.1 Materials

Aniline and NVK with 99.99% purity as monomer (Aldrich product, USA), TiO₂ 99.9% (Aldrich, USA), powder at different concentrations of: 0, 10⁻²m; 3·10⁻²m; 5·10⁻²m as a doping semiconductor. The supporting electrolyte used in electropolymerization is lithium perchlorate (LiClO₄ 0.1 m) (Fluka product, Switzerland) dissolved in acetonitrile (CH₃CN).

2.2 Preparation of composite films

The composite films deposited on ITO were carried out in a glass cell obtained by CV, containing the [solution+NVK (6·10⁻³m)], or solution+[An (0.1 m)+NVK (6·10⁻³m)] or the same composition+TiO₂ at v=10 mV/s, between −0.8 and 1.8 V/SCE in which three electrodes are immerged. The working electrode is an ITO (SOLEMS) 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 reference electrode is a saturated calomel electrode with KCl (SCE) and the auxiliary electrode is platinum.

2.3 Instrumental analysis

Electrochemical impedance spectroscopy (EIS) measurements were performed using an alternative current voltage of 10 mV, at open circuit potential (Eocp), in the frequency range 50 KHz and 100 MHz.

The surface morphology of PVK, PVK/TiO₂, (PVK+PANI), (PVK+PANI)/TiO₂ films was characterized by atomic force spectroscopy (AFM) (USA) and SEM (USA). PL (Perkin Elmer LS 50B) spectroscopy and a Shimadzu UV-Visible spectrometer (Japan) were used to investigate the optical properties of the composites, PL measurements proceeded on thin films by exciting the samples with 325 nm (3.81 eV) line of a 150 W Xenon lamp. All measurements were performed at room temperature.

3.1 Electropolymerization of NVK in CH₃CN medium

Figure 1A and B shows the voltammograms recorded in the presence of LiClO₄ (0.1 m) and NVK (3·10⁻⁶m), between −0.8 and 1.8 V/SCE. The oxidation of NVK starts at 0.8 V/SCE and, on the backsweep, gives rise to a reduction wave at 0.95 V (cycle 1). These waves can be ascribed to the electrochemical behavior of carbazoles, or to the formation and reduction of the carbazolylium cation radical (8), (9), (10). The increase of redox current densities by successive CV scans showed in Figure 1A indicates the formation of the electrochemically active polymeric film on the electrode surface.

Figure 1:

Cyclic voltammograms corresponding to a solution of (NVK) 6·10⁻³ M dissolved in (LiClO₄ 0.1 m/CH₃CN) obtained with v=10 mV/s, between −0.8 and 1.8 V/SCE, (A) 30 cycling, (B) one cycle, (C) cyclic voltammograms corresponding to poly(N-vinylcarbazole) in (LiClO₄ 0.1 m/acetonitrile) solution obtained with v=10 mV/s, between −0.8 and 1.8 V/SCE, two cycles.

3.2. Cyclic voltamperometry of PVK on ITO electrode

Figure 1C presents the first two CV scans of PVK deposit on ITO. The cyclic voltamperogram revealed the same peaks showing that the PVK polymer formed has the same electrochemical behavior than that observed for the monomer.

We observed during the positive scan potential one peak at Epa₁=0.83 V/SCE and during the cathodic scan one peak at Epc₁=0.74 V/SCE. However, contrary to what was observed for the monomer, the current intensity of the oxidation and reduction peaks of the polymer film decreases during the cycling.

3.3. Effect of TiO₂concentration TiO₂

Figure 2 presents the 21st CV of (NVK) 6·10⁻³m dissolved in LiClO₄ 0.1 m/CH₃CN, obtained for different contents of TiO₂, at v=10 mV/s, between −0.8 and 1.8 V/SCE, on an ITO electrode. The recording of cyclic voltammograms shows a redox couple as those not observed in the absence of TiO₂. Thus, there is an irregular appearance and disappearance of oxidation and reduction peak potential.

Figure 2:

Cyclic voltammograms relating to a solution of (NVK) 6·10⁻³m dissolved in LiClO₄ 0.1 m/CH₃CN, obtained for (A) 0 m TiO₂, (B) 1·10⁻²m TiO₂, (C) 3·10⁻²m TiO₂, (D) 5·10⁻²m TiO₂ shown in the figure, recorded at v=10 mV/s, between −0.8 and 1.8 V/SCE, on an ITO electrode.

Figure 2A–D shows the recording of cyclic voltammograms relating to a solution of (NVK) 6·10⁻³m dissolved in LiClO₄ 0.1 m/CH₃CN, obtained for (A) 0 m TiO₂, (B) 1·10⁻²m TiO₂, (C) 3·10⁻²m TiO₂, (D) 5·10⁻²m TiO₂, recorded at v=10 mV/s, between −0.8 and 1.8 V/SCE, on an ITO electrode.

When comparing the voltammogram of PVK before and after doping with the semiconductor of TiO₂, a large difference is observed in the form of recorded cyclic voltammograms on the ITO electrode of a PVK film doped with TiO₂.

In Figure 2B PVK doped with 10⁻²m TiO₂, an oxidation reaction developed in the potential range (0.75 V, 1.8 V) (1.37 V wave) during the anode scan. The appearance of two cathode waves located at 0.54 V and 1.03 V confirmed that there is a reaction which develops between the NVK, TiO₂ and LiClO₄. Figure 2C represents the voltammogram of PVK/3·10⁻²m TiO₂. Again we observe a change in the cyclic voltammogram pattern by a replacement of the two cathode waves by a large wave localized at 0.6 V. In Figure 2D when PVK doped with 5.10⁻²m TiO_2, an anode wave was found at 1.02 V and cathode wave was found at 0.54 V were observed. Our result is also very comparable and almost the same phenomenon that happens with that of (10), (25).

Figure 3 shows the cyclic voltammograms relating to a solution of (NVK) 6·10⁻³m dissolved in LiClO₄ 0.1 m/CH₃CN, obtained for different contents of TiO₂ only recorded at v=10 mV/s, between −0.8 and 1.8 V/SCE, on a ITO electrode. The current intensities of the oxidation and reduction peaks increase with the semiconductor content (TiO₂), and the curves show that the amount of incorporated TiO₂ increases the electro activity of the PVK/ITO electrode. This is likely to result in good contact of the TiO₂ with the PVK, which in turn contributes to a good conductivity of the film.

Figure 3:

Cyclic voltammograms relating to a solution of (NVK) 6·10⁻³m dissolved in LiClO₄ 0.1 m/CH₃CN, obtained for different contents of TiO₂ shown in the figure, recorded at v=10 mV/s, between −0.8 and 1.8 V/SCE, on an ITO electrode.

3.4 Electropolymerization of copolymer in CH₃CN medium

3.4.1 The effect of the aniline addition

The synthesis of the copolymer films was performed electrochemically. Figure 4 shows the CV plot during the electro deposition of the [NVK (6·10⁻³m)/aniline (10⁻¹) m] dissolved in CH₃CN/LiClO₄ 0.1 m solution, recorded in a potential range between −0.8 and 1.8 V/SCE, with scan rate v=10 mV/s on an ITO surface. It is noticed that the shape (intensity, potential and number of redox couple) of the cyclic voltamperogram varies with the addition of aniline. The voltamperograms (Figure 4A) show during the positive scan a potential one anodic peak at Epa₁=0.46 V/SCE, respectively, and during the cathodic scan two peaks at Epc₁=0.22 V/SCE and Epc₂=0.41 V/SCE. These peaks correspond to different oxidation and reduction states of copolymers. In Figure 4B we note that the displacement of the potential peaks is accompanied by an increase in the current intensity of the copolymer compared with PVK and PANI only, which gives us a new copolymer with a better property than that of PVK alone.

Figure 4:

Cyclic voltammograms corresponding to a solution of (NVK) 6.10⁻³m +(AN) 10⁻¹m dissolved in (LiClO₄ 0.1 m/CH₃CN) obtained with v=10 mV/s, between −0.8 and 1.8 V/SCE, (A) 30 cycling, (B) comparison of cyclic voltammograms corresponding to a solution of (NVK) 6·10⁻³m +(AN) 10⁻¹m.

3.4.2 Effect of concentration of TiO₂

The superposition of cyclic voltammograms (30 cycles) of the obtained copolymer in a solution of (0.1 m LiClO₄+ acetonitrile) in the absence and in the presence of different concentration of TiO₂, on an ITO electrode is shown in Figure 5. The voltammograms were recorded over a potential range of −0.8 to 1.8 V/SCE, at a scanning speed of 10 mV/s. It is noted that the intensity of the peak currents decreases when the semiconductor is added in solution with a concentration of 10⁻²m and 3·10⁻²m of TiO₂ compared to that of the copolymer not doped with TiO₂. At the concentration of 5·10⁻²m of TiO₂, the current increased to that of the virgin PVK. This confirms that the addition of TiO₂ at different concentration has no effect on the electrical and electrochemical properties of the copolymer.

Figure 5:

Cyclic voltammograms relating to a solution of (NVK) 6·10⁻³m+(AN) 10⁻¹m dissolved in LiClO₄ 0.1 m, obtained for different contents of TiO₂, recorded at v=10 mV/s, between −0.8 and 1.8 V/SCE, on an ITO electrode.

3.5 EIS analysis

Figure 6A shows the relative impedance diagrams of an electrode ITO modified by a film of PVK in which the TiO₂ is incorporated. The films are obtained for different concentrations of TiO₂ 0, 10⁻²m, 3·10⁻²m, and 5·10⁻²m. The curves were recorded over a frequency range between 50 KHz and 100 MHz, with a 10 mV perturbation. The modified electrode is analyzed in an aqueous solution containing (0.1 m LiClO₄+acetonitrile).

Figure 6:

Nyquist diagrams relative to films of (A) (PVK/ITO, PVK/TiO₂/ITO), (B) (copoly/ITO, copoly/TiO₂/ITO).

It is clear from this figure that the amplitude of Zr (kΩ·cm²) varies with concentration of the TiO₂. Nyquist diagrams give a semi-circle that is characteristic of a high-frequency charge transfer process (26).

It is also observed that the diameter of the semi-circle obtained towards the high frequencies decreases with the concentration of TiO₂. The resistance of the composite material is represented by the radius of the semi-circle, the smallest of which is obtained for the doped PVK with 5·10⁻²m. Thus, it can be seen that increasing the concentration decreases the radius and consequently increases the conductivity of the film which forms on the electrode. This shows that the electrochemical behavior is governed by the same diffusion process. The presence of TiO₂ in the polymer film affects the electrochemical impedance values and makes the loop more capacitive. The resistance of the electrolyte (R_Ω) and the double layer capacity (C_dl) of the composite material PVK/TiO₂ calculated after extrapolation of the Nyquist diagrams are given in Table 1.

Figure 6B shows the Nyquist diagrams corresponding to the copolymer, copolymer/TiO₂ films deposited on ITO in a solution of (0.1 m LiClO₄+acetonitrile) over a frequency range between 50 KHz and 100 mHz. It is observed that the resistivity of the copolymer films increases from 1701 Ω to 2614 Ω with the addition of aniline, thereby dismantling a decrease in conductivity.

The Nyquist diagrams in Figure 6B obtained in the presence of TiO₂ show high frequencies characteristic of a charge transfer process.

From the semicircle diameters, the transfer charge resistivity of the copolymer is increased to 10⁻²m TiO₂ resulting from the decrease in the capacitive effect which becomes less and less important for the films of copolymers modified by incorporation of the semiconductor. Then a decrease in the resistivity is observed with the increase of the TiO₂ concentration and the film becomes more conductive and more capacitive. The corresponding values, given in Table 2, were calculated after extrapolation of the Nyquist plots.

3.6 Surface morphology of PVK and copolymer films

3.6.1 SEM

From the image of the PVK film in Figure 7A, it is clearly observed that the electrodeposited polymer layer on the ITO substrate is homogeneous. The pure PVK has grains which are well interconnected and adherent.

Figure 7:

SEM images of (A) PVK/ITO, (B) PVK +(3·10⁻²m) TiO₂/ITO, (C) PVK+(5·10⁻²m) TiO₂/ITO.

The darker areas in Figure 7B represent regions with higher atomic numbers which are the conducting polymer in our case. Consequently, the lighter zones identify the areas of TiO₂ which contain pseudo spherical shapes with size distribution in the range of 15–20 nm.

The SEM images corresponding to the ITO plates modified by PVK+5·10⁻²m TiO₂ films are shown in the Figure 7C. It is clear that the particles of the inorganic semiconductor are incorporated into the organic polymer (PVK), thus modifying the morphology, no agglomeration and uniform distribution of the TiO₂ particles in the PVK film. Therefore, we consider that the semiconductor particles can be incorporated into the polymer during NVK electro polymerization which leads to pigmenting of the polymer film, and consequently allows a composite material (PVK/TiO₂) on the electrode. The improvement of the properties of the composite is explained by the fact that PVK and TiO₂ can act as electron donor and acceptor, respectively.

Figure 8A shows SEM micrographs of the copolymer. It is clearly observed that the electrodeposited copolymer layer on the ITO substrate is homogeneous and compact with a spherical shape.

Figure 8:

SEM images of (A) copolymer/ITO, (B) copolymer/(3·10⁻²m) TiO₂/ITO.

The SEM image in Figure 8B of the copolymer/TiO₂ composite shows that there is no agglomeration of TiO₂ particles in the matrix and that there is a uniform distribution of the TiO₂ and a modification of the copolymer morphology is observed.

3.6.2 AFM

The PVK, PVK/TiO₂ copolymer and copolymer/TiO₂ films were examined by AFM in the contact mode. The effect of TiO₂ content on the surface morphology of PVK and copolymer films electrochemically deposited were further evoked by AFM as presented in Figure 9. The result reveals a well-defined grain structure. Moreover, these images show the uniformity and the homogeneity of TiO₂ particles in the surface of each sample. This is confirmed by the SEM. When we compare Figure 9A and B–D, we find that the surface of the PVK film is globular in contrast. The difference between the surface morphologies of PVK and PVK+TiO₂ at different concentrations was clear. The surface is not smooth but contains few voids, compared to the pure PVK film. From these AFM measurements, the root mean square (RMS) surface roughness was calculated for a surface of 20 μm×20 μm. And the size of the grain (particle) is decreased in proportion to the percentage of TiO₂. This size is finer at 1·10⁻²m, which confirms a better distribution of TiO₂ in the copolymer. It is difficult to compare these results with the data in the literature because RMS parameters depend strongly on the scanning length and the thickness of the film.

Figure 9:

2D AFM images of (A) PVK, (B) PVK/(10⁻²m) TiO₂, (C) PVK/(3·10⁻²m) TiO₂, PVK/(5·10⁻²m) TiO₂. (A′) copoly, (B′) copoly/(10⁻²m) TiO₂, (C′) copoly/(3·10⁻²m) TiO₂, copoly/(5·10⁻²m) TiO₂ films electrodeposited on the ITO substrate.

It is clear from the 2D images in Figure 9A′–D′ that the addition of aniline and TiO₂ affects the films topography.

3.7. Absorption measurements

To study the influence of addition of TiO₂ on the optical properties of the PVK thin films, transmittance measurements were conducted and the results are presented in Figure 10A. It was observed that the samples (PVK+10⁻² TiO₂) exhibited high transmittance (83%) in the visible region, so light can easily pass through it. When light passes, the charge carriers such as holes and electrons are produced proportional to the incident light the photoelectric transducers generate electric current when exposed to light. This confirms that this material can be used in photovoltaic cells. In the case of copolymer shown in Figure 10B which represents the transmittance spectra of copolymer and copolymer/TiO₂ thin films electrodeposited at different concentration of TiO₂, the same phenomenon was observed with no change being made with the addition of PANI.

Figure 10:

Transmittance spectra of (A) PVK, (B) PVK/TiO₂ thin films electrodeposited at different concentration of TiO₂.

In order to determine the optical band gap of this material and the type of optical absorption, Tauc (27), Burstein (28) showed that the absorption coefficient and photon energy are related by the following equation:

In the above equation, A is a constant, E_g is the band gap of the material, α is the absorption coefficient, h the Plank constant, ν is the frequency of the radiation and n has different values depending on the optical absorption process. The corresponded optical band gap energies were estimated from the plot of (αhν)² versus the photon energy (hν). The band gap was determined by the extrapolating plot. The straight-line portion of the plots shown in Figure 11A indicates the direct transition for all the samples (29). The band gap values of the samples were confined between 2.69 and 3.59 eV; they were in good agreement with theoretical band gap value of PVK which is in the range of (3.3–3.5 eV) confirmed in the literature (11), (30).

Figure 11:

Variation of (αhν)² versus hν of (A) PVK, (B) copolymer thin films electrodeposited at different percentage of TiO₂. The inset shows the corresponding variation of the band gap of the samples with TiO₂.

This curve, Figure 11A, indicates that the optical band gap decreased generally with increasing of concentration of TiO₂ compared with PVK only.

The gap values are reported in Table 3. For the optimal doping at 10⁻²m, the gap value is 2.69 eV. This is explained by the introduction of the donor levels in the band gap of PVK by the TiO₂, a consequence of an effective doping. In Figure 11B it is evident that the band gaps of the copolymer decrease with the addition of PANI to PVK. But there is no variation in the gap energy with the addition of TiO₂ in the copolymer.

The luminescence spectra were measured on films deposited on ITO substrates, using a Perkin Elmer spectrophotometer. The emission spectra of corresponding PVK, PANI and the copolymer are shown in Figure 12.

Figure 12:

PL spectrum of (A) comparison, (B) PVK, (C) copolymer.

The profiles of the PL are recorded for an excitation wavelength of 325 nm. Under this excitation, PVK showed four PL peaks well-marked, respectively (Figure 11A) at 390, 427 477 and 518 nm with a single maximum emission at 427 nm attributed to the transition in the carbazole group of PVK (31).

PANI shows three luminescence peaks at 392, at 429 (maximum emission), at 475, and at 517 nm (Figure 11A), this peaks are originated due to π*-π transition of the benzoid units, de-excitation from polaron band and bipolaron de-excitation, respectively (32).

The PL of the copolymer is located in the spectral region from 350 to 550 nm with four well-marked bands, respectively, at 398, 429, 476 and at 516 nm as well as bands of lower intensity at 520 and 418 nm. From Figure 11A it is clear that the PL spectrum did not change. While the intensity of the copolymer increases compared with that of the pure PVK and pure PANI. All values are summarized in Table 4.

From Figure 11B it is clear that the PL spectrum did not change when PVK was modified by TiO₂, but the intensity of the PVK/TiO₂ composites was increased by increasing the concentration of TiO₂ relative to that of pure PVK, and the maximum intensity present at 10⁻²m. The values of absorption peak positions of PVK and PVK/TiO₂ are summarized in Table 5. For the copolymer, the same phenomenon was observed, particularly with an increase in the TiO₂ content, the corresponding intensity of the PL peak increases (Figure 11C, Table 6).