3.1. Electropolymerization and deposition studies
The PANI electropolymerization is based on the redox equations at the working electrode interface. The Basic reactions mechanism can be as follows:
Fig. 1(a) shows the cyclic voltammograms of ITO substrate in an acidic solution containing 0.5 M of aniline. The voltammograms were recorded under scanning potential ranges from -0.1 V to +1.2 V vs. SCE. The growth of PANI on ITO substrates is shown on the typical with the redox peak attributed located during the potentiodynamic electropolymerization of PANI at around + 0.2 V and + 0.5 V for the first cycle. And the following scans in the next cycles showed increasing the peaks corresponding to the oxidations and reductions of the polyaniline films (oxidative peaks (a1 and a2) coupled with reductive peaks (b1 and b2). However, the cyclic voltammograms well agreement with those already reported in the literature on the electro-polymerization of PANI [33]. The pair peaks are observed at 0.3 and 0.9 V vs. SCE in the third cycle clearly are associated with both polyaniline redox couples from its semiconducting state (leucoemeraldine) and to its conductive state (emeraldine), and the emeraldine to its full oxidation form (pernigraniline) [34].
Fig. 1(b) shows the voltammogram of ITO/PANI-rGO thin films recorded between −0.56 and 1.2 V vs. SCE to favor the nanocomposites growth. During the electropolymerization process of PANI film, rGO diffused slowly to the PANI sites simultaneously where redox reactions take place. The good solubility and stability of the rGO suspension in the electrolyte allows a better insertion of rGO in the PANI and to obtain more homogeneous thin films.
The rGO is formed in situ with the PANI on the ITO substrate after ion exchange subjected to ten 10 scans in the potential range, shows two pairs of redox peaks increase continuously with each cycle due to the transition between quinone/hydroquinone groups in rGO, which is typical for carbon [35]. By these functional groups of rGO, aniline monomers can be attached to rGO nanosheets (Insert Fig. 1(b) has been showing the formation groups of rGO in the 3D PANI network by FESEM characterization). Moreover, the rGO nanosheets have good conductivity, which helped dramatically to increase the current of the voltammogram cycles as shown in Fig. 1(b). The Cycles voltammograms profile Fig. 1(c) shows the oxidation peak of PANI-rGO located at 0.3 V and 0.5 V vs. SCE and the reduction peak at 0.05 V and 0.3vs. SCE for the first and last cycles. The large transition between the first and the last cycle demonstrates the increased electroactivity of the growth of the PANI-rGO film on the surface of ITO compared to the voltammogram of Fig. 1(a) thus confirming the better conductivity of the PANI/rGO film, and allowed improve the electrical performance of the film. Although the high conductivity of PANI, the rGO sheets improves a high contact interface between the PANI and the electrolyte [36] and thus explained the difference in the voltammogram cycles to the formation of PANI-rGO film with high specific surface area.
3.2. Surface morphological analysis
FESEM images was used to characterize the morphologies of the elaborated thin films, as indicated in Fig.2. The Fig. 2(a,b) shows the appearance of the rGO surface and confirms the deposition of rGO layers on ITO. As can be observed in Fig. 2(b), wrinkle-like structure, crumpled, thin and agglomerated sheets closely associated with each other. The wrinkled structure observed of rGO sheets is due to the removal of intercalated functional groups in graphitic oxide during exfoliation. An average thickness of several nanometers significantly increased the thin film effective surface area. Thus this result confirms that hydrazine hydrates have chemically reduced GO to form rGO [37].
While Fig. 2 (c,d) shows the surface morphology of the ITO/PANI thin film, which constitutes an interconnected network of PANI nanorods (NRs) represent a well-arranged and orderly uniform polymer chain structure. which makes this network useful for electron transport at the electrolyte–thin film interface [38–40]. Fig. 2(d) shows an enlarged view of ITO/PANI nanorods, which clearly shows the diameter of the PANI (around 110 nm) and the length of uniform size about 1 mm. The acquisition of a more regular and uniform microstructure offers more active sites for the fixing of pollutants.
Figure 2(e) and Fig. 2(f) show the surface morphology of the ITO/PANI-rGO and enlarged view of that thin film, respectively. It is noticeable in Fig. 2(e) that the layers of rGO that have been covered by PANI NRs are formed well-arranged with evenly dispersed. In Fig. 2(f), with an enlarged view, we could observe the nanorods of PANI that largely cover the layers of rGO and are embedded with them. We can say that it has formed bonds interaction between rGO and PANI NRs, this is what facilitates the electron transfer process. The point of the merging process is to form a fertile platform for the transfer of electrons and accelerate the electrical response.
3.3. XPS and UV-Vis analysis
Fig.3. Shows the XPS spectra of ITO-PANI and ITO/PANI-rGO electrodes. As is visible from Fig. 3(a), and through the presented materials, the peaks centered at 290 eV (C1s), 400 eV (N1s), and 540 eV (O1s) are all visible a wide range in the spectrum. The presence of N1s confirms the presence of polyaniline in all synthesized nanocomposites. Fig. 3(b, c) displays the deconvoluted C1s peak of PANI and PANI-rGO, revealing the presence of three different types of carbon functional groups: nonoxygenated carbon , nitrogenated carbon and oxygenated carbon [41,42].
Fig. 4 Represents the optical absorption spectra of ITO/rGO, ITO/PANI, and ITO/ PANI-rGO films. The peak in the spectra of rGO at 340 due to the π-π* absorption band demonstrates the elimination of oxygen functions in the outer portion of the GO and the electron conjugations of C-C in the basal levels of the recovered GO rendering it as rGO [43,44]. PANI exhibits well-defined three strong absorption peaks at approximately 371, 427, and 773 nm correspond to the formation of polyaniline. The peaks observed at 317 and 427 nm are attributed to the transition of electrons from the HOMO to the LUMO. These peaks correspond to the π-π* electronic transitions of the bipolar ion, benzenoid rings and polar ion excitations of the quinoid rings. The peak at 773 nm is credited to the n-π* electronic transitions between benzenoid and quinoid units [45–47]. In the UV-Vis spectra of the ITO/PANI-rGO electrode, two kinds of characteristic peaks were observed at 371 nm and 410 nm which are attributed to the PANI and rGO respectively.
3.4. Photoelectrochemical performance
The photoelectrochemical performance of ITO/PANI and ITO/PANI-rGO nanocomposites is estimated according to the ability to absorb light under intermittent light on and off. It is noticed in Fig.5(a), that the current density under dark conditions is straight and near to zero, whereas, under illumination, it is observed that there is a significant increase in photocurrent with a rectangular vertically response. Rapid and uniform photocurrent responses to all switch-on and switch-off cases were observed across all electrodes. The positive values of photocurrents are a characteristic of the n-type semiconductor [48,49], this is explained by the presence of charge carriers that transfer from HOMO to LUMO under light irradiation [50,51]. As the lighting continues, the generation of photo-excited electrons annihilates the internal electric field, as electrons and holes accumulate and lead to recombination of the charge [52]. It was noted that the photocurrent intensity of the PANI-rGO electrodes was significantly improved compared to the pure PANI electrodes.
Fig.5(b) shows the photocurrent variation as a function of the applied potential for the synthesized ITO/PANI-rGO. A range of different potentials has been used to study thin film photocurrent performance. It has been shown that rGO with high electron conductivity could aid electron transfer and suppressing the electron-hole combination [53], reaching a much higher photocurrent and faster photo response. According to Henni et al. [37] the rGO may have led to the broadening of the band gap of the PANI-rGO sample by increasing the carrier intensity in the valence band. However, it is observed that the photocurrent intensity of the electrodes increases reasonably with the increase in the potential range.
3.5. Photocatalytic activity
The prepared ITO/PANI and ITO/PANI-rGO thin films photocatalysts were investigated by following the degradation of CR pollutants under light irradiation with studies influence of pH and the temperature into the degradation of dyes organic (Fig. 6). First the maximum absorbance of the dye, in the absence of any catalysts at T=30°C and pH = 6.05, was measured scanned in the range of 300-800 nm and it was observed at a wavelength of 498 nm ascribed to Congo Red dye. The resulting spectrum is shown in Fig. 6(a). The change of absorbance was used to evaluate the photo-degradation efficiency. For 10 min in the dark, the adsorption process is achieved for adsorption-desorption equilibrium before exposing the CR to irradiation.
It is observed in Fig. 6(b) that the degradation rate of CR dye increased well using ITO/PANI-rGO compared to ITO/PANI electrodes. Rapid degradation has been observed for ITO/PANI-rGO electrodes. After 60 min, the degradation of the CR for reached close to 70%. As for the ITO/PANI electrodes, the degradation of CR reached 40%. Regarding the ITO/PANI-rGO curves, it is noticeable that the degradation rate of CR suspension increased well compared to the curves of ITO / PANI. The presence of rGO in the polyaniline structure has enhanced the photocatalytic activity, this is explained by the synergistic effect between rGO and PANI. The same effect was observed in our previous study when the rGO has been incorporated in ZnO thin layers [37] and El-Sharkaway E. A, et al [19] reported that PANI/rGO composites show a higher adsorption capacity of MB dye compared with the PANI adsorbents. Therefore, the improved substrate performances can be assigned to the interesting electroactive properties of PANI and the excellent electrical conductivity of rGO [54,55].
The pH factor also considered a very important factor and helps speed up and changed the photocatalytic process. The photocatalytic degradation activity of CR by ITO/PANI and ITO/PANI-rGO at different pH is shown in Fig. 6(c). It is observed that the CR showed high degradation efficiency in a short irradiation time at an acidic medium (reached about 90 % for both ITO/PANI and ITO/PANI-rGO). This is due to the PANI behavior that works well in an acidic medium compared to the basic medium. While in the medium with pH = 7, the degradation rate was close to 60% for ITO/PANI and 75% for ITO/PANI-rGO.
Fig. 6(d) shows the effect of temperature on the photocatalytic process. It observed that at T = 20°C the degradation rate was 58% and 66% for ITO/PANI and ITO/PANI-rGO respectively, and while the temperature increased to (T=40 °C), the rate of degradation increased and reached 80% for ITO/PANI-rGO while maintaining the same conditions. While it reached more than 80% for the ITO/PANI-rGO while continuing to rise in temperature (T = 60°C). The effect of temperature plays a very important role in photocatalytic processes as a catalyst and accelerator for the degradation of dyes. The photocatalytic mechanism of CR degradation is presented in Fig. 6(e).
The point of zero charge pHpzc is very important in studying the adsorption mechanism process as well as determining the effect of pH on adsorption. When the pH is higher than pHpzc, the catalyst surface is negatively charged and attracts cations, while it is positively charged and repels cations at pH lower than pHpzc [56][57]. The pHpzc of ITO/PANI and ITO/PANI-rGO was found to be 2.76 and 3.09 respectively as shown in Fig. 7(a,b). Since the adsorbent surface was positively charged according to the results, ITO/PANI and ITO/PANI-rGO preferred to remove the color dye at pH> pHpzc.
The adsorption kinetics of ITO/PANI and ITO/PANI-rGO was examined for CR by plotting the Ct/Co values on the function of irradiation time (Fig. 6(c)). Lower values for ITO/PANI-rGO electrodes of Ct/Co have been obtained which confirmed that the affinity of the dye pollutant towards the ITO/PANI-rGO thin film surface with enhanced photocatalytic degradation of CR at the surface. The plot of ln(C0/Ct) against t (Fig. 6(d)) display a straight line, indicating the good coincidence with the pseudo first-order equation and suggesting that the nanocomposite (ITO/PANI and ITO/PANI-rGO) owns high catalytic activity. On the other hand, the correlation coefficient of the pseudo-first-order model nearly R2 = 0.99 for all electrodes.
3.6 Antibacterial activity
Antibacterial activities of the ITO/PANI and ITO/PANI-rGO thin films were studied against one Gram-negative bacterium E. coli ATCC 25922 by the time-kill method. The results of the antimicrobial assay at the following contact times are presented as CFU of surviving cells ant rate of reduction compared with the negative test without simple as shown in Fig. 8(a,b). The results showed clearly that, all thin films exhibited antibacterial activity against the growth of E. coli gram-negative bacterial. In contrast, treatment with PANI thin film showed a statistically significant reduction in the growth of cells compared with untreated cells as reported by recent studies [58–61]. Furthermore, a noticeable difference was observed between the effect of each nanocomposite, Antibacterial efficiency of ITO/PANI/rGO was higher than ITO/PANI, and that seems similar to the resultants reported by Lin Shi [62]. Moreover, the ITO/PANI-rGO demonstrated the higher bactericidal activity with a 50% of reduction in numbers of E. coli after 60 h of exposure compared with ITO/PANI (33%), suggesting a synergistic effect of rGO and PANI. Can say that the results are acceptable compare with the area of the substrate.