3.2.2. RAMAN spectroscopy
RAMAN spectroscopy is one of the most powerful method for studying and analyzing carbon materials. It provides valuable information about sp3 and sp2 carbon hybridization as well as clarifies the different structural arrangements [34]. In the present work, RAMAN analysis was used to offer information about the chemical structure of the elaborated PLA-CB and to give additional proof of the successful polymerization of MB on the surface of the electrode. The Raman spectrum of PLA conductive carbon black (Fig. 10) exhibits two main peaks at around 1333 cm− 1 and 1625 cm− 1, corresponding to D and G bands, respectively. A disorder band is known as the D band. It indicates sp3 hybridized carbon. As for the G band, it represents the planar vibration mode of sp2 hybridized carbon [19, 35, 36].
After the electropolymerization process of MB on PLA-CB, the main peak situated at 1333 cm− 1 was shifted to 1360 cm− 1. This noticeable shift is likely caused by a strong interaction between PMB and PLA-CB, resulting in the formation of new chemical bonds [2, 37]. Furthermore, after modification of the electrode, the G band at 1625 cm− 1 becomes broad and more intense. As shown in Fig. 10, MB has a characteristic peak situated at 1615 cm− 1 assigned to the C-C ring stretching [5], thus, this peak overlap with the G band and result in such behavior. It should be noted that the spectrum of MB monomer is similar in shape to the one of PMB/PLA-CB. This latter exhibits weak peaks in the region of 450 cm− 1 to 1200 cm− 1, which are related to the vibrational modes of MB. These results confirm the formation of PMB on PLA-CB.
3.2.3. X-ray photoelectron spectroscopy
Further spectroscopic analysis was achieved for the modified electrode to give a detailed characterization of the chemical structure of the polymer. Figure 11 shows C1s, N1s, O1s, and S2p XPS signals for both coated and uncoated PLA-CB electrodes. In addition, all parameters like binding energy, chemical states, and full width at half maximum (FWHM) are presented in Table 2.
Table 2
Binding energy, chemical states, and full width at half maximum (FWHM) obtained for each XPS signal.
Substrate | PMB/PLA-CB | | PLA-CB |
Element | chemical states | binding energy (eV) | FWHM | chemical states | binding energy (eV) | FWHM |
C(1s) | C-C | 284.7 | 2.64 | C-C | 284 | 2.73 |
C-S | 284.9 | 1.65 | C-H | 284.9 | 1.94 |
C-N | 286 | 2.59 | C-O-C | 286.2 | 1.63 |
C-O | 287.4 | 2.7 | C-N = O | 287.2 | 1.79 |
O-C = O | 289.1 | 1.95 | O-C = O | 288.9 | 2.06 |
O (1s) | C-O | 533.1 | 2.18 | C-O | 535.6 | 2.12 |
O-C = O | 531.7 | 2.25 | O-C = O | 533.2 | 2.27 |
O = S = O | 530.7 | 2.91 | O = N-C | 532 | 2.51 |
- | - | - | S-O | 530 | 2.63 |
N (1s) | C = N-C | 398.6 | 2.1 | C-N = O | 399.9 | 1.89 |
N-C | 399.7 | 1.89 | N-C | 398.7 | 1.87 |
N-H, N-H+ | 400.9 | 1.71 | - | - | - |
S (2p) | S-C(2p1/2) | 165 | 2.05 | S-O (1/2) | 169.8 | 2.35 |
S-C(2p3/2) | 163.8 | 2.05 | S-O (3/2) | 168.6 | 2.35 |
S+-C(2p1/2) | 166.9 | 2.47 | - | - | - |
S+-C(2p3/2) | 165.7 | 2.47 | - | - | - |
S = O(2p1/2) | 169.6 | 2.45 | - | - | - |
S = O(2p3/2) | 168.4 | 2.45 | - | - | - |
The carbon signal of PLA-CB can be decomposed into five functional components; C-C, C-H, C-O-C, C-N = O, and O-C = O situated approximately at 284, 284.9, 286.2, 287.2, 288.9 eV respectively [38]. Five peaks were also seen for PMB/ PLA-CB, still, the assignment this time is different. The bands were observed at about 284.7, 284.9, 286, 287.4, and 289 eV, and were attributed to C-C, C-S, C-N, C-O, and C = O respectively [39–41]. The appearance of C-N and C-S bands confirms the successful polymerization of MB, while the presence of C = O and C-O originating from the PLA-CB electrode indicates the formation of a very thin PMB layer.
The O(1s) XPS spectrum of the bare electrode shows four separated components at about 535.6, 533.2, 532, and 530 eV attributed to C-O, O-C = O, O = N-C, and S-O respectively. While, the signal recorded for PMB/PLA-CB can be decomposed into three functional components at 533.1, 531.7, and 530.7 eV, assigned to C-O, O-C = O, and O = S = O respectively. This low binding energy peak (O = S = O) arises from the saccharine anion which is present as a doping anion on the polymer skeleton.
Regarding the N(1s) XPS signal of PLA-CB, the spectra showed two different components. The main peak at approximately 399.6 eV is attributed to C-N = O whereas the one seen at 398.7 eV is assigned to C-N bond [42]. It is worth mentioning that the presence of nitrogen on the surface of PLA-CB might be caused by the treatment used in the filament manufacturing process. [38]. In the case of the coated electrode, three peaks situated at 398.6, 399.7, and 400.9 eV are due to C = N-C, N-C, and N-H groups respectively [40, 41, 43]. The appearance of the C = N-C bond for PMB/PLA-CB resulted from the heterocyclic nitrogen atoms (pyridine) in the MB molecule. The presence of the protonated amine can confirm the existence of ring-to-ring linkage. Furthermore, the main peak corresponding to C-N can be the result of the N-ring mechanism [41].
Finally, S(2p) was also analyzed and as seen in Fig. 11, a signal was detected for both the modified and unmodified electrodes. Before the electropolymerization process, sulfur was also detected on the PLA-CB surface as a result of the treatment used during the filament manufacturing process [38]. In the case of PMB/PLA-CB, the sulfur signal has fully changed and several peaks were observed in the XPS spectrum. Firstly, we should note that the S(2p) signal consists of three separated doublets. Due to the spin-orbit interaction, two components, 2p1/2 and 2p3/2, characterize the S(2p) spectrum. Generally, the peak-to-peak distance is about 1.18 eV and the intensity ratio is p3/2:p1/2 = 1:2, which is equal to the multiplicity [44]. The first doublet located at (163.8 eV, 165 eV) corresponds to S-C bonds. The second one with weak intensity (165.7 eV, 166.9 eV) is due to C-S+species, while the component at high binding energy (168.4 eV, 169.6 eV) is assigned to O = S = O groups [40, 45]. The sulfonic functional group on PMB/ PLA-CB originated from saccharin anions which confirms the doping phenomenon and the C-S+ doublet could be associated with charged sulfur species linked to the mesomeric structure of MB. Overall, the XPS analysis, besides SEM and Raman, has proved the formation the PMB doped by saccharin conter ions on the PLA-CB surface.