3.3 Physicochemical characterization
The morphology of electrocatalysts formed via electrochemical methods was analyzed by SEM and is presented in Fig. 2. For the electropolymerized polypyrrole (Fig. 2 (c)) over the Toray carbon paper (Fig. 2(a)), the formation of polypyrrole in films was observed on the carbon fibers of Toray carbon paper and additionally interconnecting said fibers. Furthermore, the surface of the polymer’s films exhibited the formation of polypyrrole with a spherical morphology. For the gold deposition on the T-PPy electrodes (Fig. 2 (e)), the growth of metallic clusters on the polymer with a good distribution over the support was observed through the EDS elemental mapping (Figure h).
The elemental analysis for the T-PPy electrodes showed an atomic composition of 85.77% carbon and 14.23% oxygen; the latter can be related to an overoxidation of the polypyrrole due to the oxidation of the polymer. For the T-PPy-Au, the metallic mass composition was 64.01%, respectively. Moreover, the atomic percentage of oxygen after the deposition of the noble metal was approximately the same (13.39%), which suggests that the polymer was not further oxidized during the deposition process.
FTIR-AR analysis was carried out to study the chemical structure of Toray carbon paper, T-PPy, and T-PPy-Au (Fig. 3). At approximately 777, 891, and 917 cm− 1, three bands can be observed associated with the –CH out-of-plane vibration in heteroaromatic rings, the = C–H out-of-plane vibration, and the –CH out-of-plane bending vibration [11], [12]. The characteristic signals for polypyrrole are shown in 1032 cm− 1 attributed to the –NH bending vibration of the heterocycles, in 1277 cm− 1 the asymmetric stretching of the C–N bond, and in 1535 cm− 1 for the pyrrole 2,5 substitution [13]. In addition to the correct formation of the polymer, the obtained FTIR-AR spectra allowed us to analyze the elemental composition of the PPy after the deposition of the noble metals. After the electrodeposition of the noble metal, it can be inferred that the polymer suffered partial deprotonation due to the increase of the intensity of the signal around 1707 cm− 1, which corresponds to the combination of three double bond vibrations (C = C, C = O, and C = N)[11] This increment can be solely attributed to the C = N stretching vibration characteristic of the deprotonated form of PPy as in the EDS analysis, the atomic percentage of oxygen was not significantly modified after the noble metals electrodeposition process, which indicates that the polymer was not further oxidized. Therefore, the contribution of the C = O stretching vibration to this signal is approximately constant for the T-PPy-Au spectrum. Furthermore, this can be correlated with the decrease in the –NH bending signal at 1030 cm− 1 due to the deprotonation of the amine functional group of polypyrrole.
Table 1
ATR-FT-IR frequencies assigned for the electrodes.
|
Wavenumber (cm− 1)
|
Signal/material
|
Toray
|
T-PPy
|
T-PPy-Au
|
T-PPy-Pd
|
Out-of-plane aromatic ring deformations vibration
|
n/a
|
780
|
797
|
800
|
=C-H out-of-plane vibration
|
917
|
891
|
917
|
946
|
–CH out-of-plane bending
|
988
|
962
|
966
|
967
|
–NH bending deformation
|
n/a
|
1030
|
1046
|
1070
|
–CH bending vibration
|
1210
|
1163
|
1208
|
1209
|
Polypyrrole ring vibrations
|
n/a
|
1474
|
n/a
|
1462
|
C = C (stretching of pyrrole ring)/2,5-substituted pyrrole
|
n/a
|
1538
|
1554
|
1550
|
C = C stretching vibrations
|
n/a
|
1690
|
1706
|
1638
|
C = O stretching vibration
|
n/a
|
1690
|
1706
|
1638
|
C = N stretching vibrations
|
n/a
|
1690
|
1706
|
1638
|
Triple bonds \(\mathbf{C}\equiv \mathbf{C}\) and\(\mathbf{C}\equiv \mathbf{N}\)
|
2114
|
2115
|
2114
|
2112
|
Combination of the ester group
|
2324
|
2887
|
2288
|
2324
|
–OH (Moisture)
|
n/a
|
n/a
|
3248
|
3451
|
Figure 3 (b) shows the Raman spectra of the fabricated electrodes. For the Toray carbon paper, the characteristic bands at a Raman shift of 1320, 1573, 1608, and 2653 cm− 1 were observed, corresponding to the D band, the G band, the D’ band, and the 2D band (double resonance Raman process), respectively [14]. Additionally, the spectrum of Toray exhibited an overtone at 2463 cm− 1 that corresponds to the mode of the LO phonon (i.e., 2 LO) [15].
The spectra of T-PPy, and T-PPy-Au, exhibited a band at 682 cm− 1 assigned to the dipole moment perpendicular to the plane of the aromatic ring of the PPy chains [16]. The signal observed at 919 cm− 1 corresponds to the ring deformation band of the non-protonated PPy. The in-plane deformation associated with the polaron states of PPy is shown at 966 cm− 1 [16] The peak at 1037 cm− 1 is attributed to the in-plane C–H bending vibration [8]. The signal present at approximately 1251 cm− 1 is assigned to the antisymmetric C–H in-plane bending vibrations. The peaks that appeared at 1320 and 1376 cm− 1 correspond to the inter-ring stretching C–C vibration mode of PPy [16]. The band observed at higher frequencies of the 1376 cm− 1 signal is attributed to the antisymmetric inter-ring stretching C–N vibration of oxidized PPy [16]. The peak appearing at higher frequencies of the 1320 cm− 1 band corresponds to the neutral states of PPy [16]. The latter increases in intensity in T-PPy-Au electrode due to the deprotonation of PPy, which is in good agreement with the results obtained via ATR-FTIR. The band observed at 1495 cm− 1 is attributed to the skeletal vibrations of PPy chains derived from the C–C and C–N vibrations [8]. The signal that appears at 1592 cm-1 results from the overlap of three bands: the neutral, the polarized (polaronic), and the fully oxidized bands [16]. This band also corroborates the deprotonation of PPy after the Au electrodeposition, as it shifts to higher frequencies, which is characteristic of the deprotonation process of PPy [16].
The X-ray diffraction patterns for the synthesized electrocatalysts are shown in Fig. 3(c). The characteristic diffraction signals for Toray carbon paper appeared at a 2θ of 26.53° and 54.56°, assigned to planes (002) and (004), respectively [17]. The presence of polypyrrole was observed for the three electrodes through a diffraction signal at approximately 23.96° [18]. Based on this signal, the average distance between polymeric chains was computed, resulting in 4.622 Å for T-PPy, and 4.637 Å for T-PPy-Au, [18].
For T-PPy-Au, the Braggs diffractions for Au were indexed according to JCPDS #04-0784 [19], which corresponds to an FCC crystalline structure. Five diffractions were indexed as follows: 38.16° to (111), 44.52° to (200), 64.68° to (220), 77.6° to (311), and 12.35°, assigned to the (220) plane. The Debye-Scherrer method was used to calculate the average crystallite size for the deposited Au, which was found to be 23.45 nm.