3.1. Electrochemical synthesis of PPy nanowires
3.1.3. Chemical composition and functional groups of obtained PPy nanowires
i) FT- IR spectroscopy
The molecular structures of obtained PPy were characterized by Fourier Transform Infrared Spectroscopy (FT-IR) in the range of 500 - 3600 cm-1. One can noted that the FT-IR spectra was recorded to reveal the chemical composition of PPy nanowires through characteristic of absorption bands shown in Fig 3.11.
Figure 3.11. FI-IR spectrum of obtained PPy nanowires.
The characteristics of the IR bands between 1350 cm-1 and 1700 cm-1 indicates a ring transformation from aromatic form to a more quinoid structure [3].
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Peaks in this work (cm-1) Peak assignments
791 C-N stretching vibration
1963, 2375 ,
1217 C-H in plane deformation
1378 C-H ~ Quinoid
1589 C=C stretching~ Aromatic
1682, 2923 gelatin
Table 3.4. Absoption peaks in FT-IR spectrum.
It was exciting observation that the FT-IR spectrum of electrochemically synthesized doped PPy shows evidence of two typical structures of polypyrrole (the quinoid and aromatic), which are in a good agreement with previous studies [11].
N N
H H
Aromatic Quinoid
The frequency at 1378 cm-1, approximately 1380 cm-1 [5], refers to C-H stretching vibration in quinoid structure and 1589 cm-1 assigned to C=C stretching band in aromatic structure (1580 cm-1 in [9]) within rings and a single-like bond between rings. These two bands correspond to the fundamental vibrations of pyrrole ring.
Interestingly, the characteristics of two FT-IR absorption bands are in good agreement with the peaks also appear in SERS spectra (will be further discussed in next section).
Dongtao Ge [11] has reported the FT-IR spectrum of cauliflower-like PPy, as shown in Fig 3.11, there are two more absorption peaks at 1684 cm-1 (strong) and
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2923cm-1 (weak), which are attributed to the characteristics absorption peaks of gelatin [11], indicating that gelatin has been incorporated into PPy nanowires successfully.
Other study recorded the C-N stretching vibration at 791 cm-1, the stretching vibration of doped PPy at 930 cm-1 and 1215 cm-1, correspond with 791, 963 and 1217 cm-1 in this study, indicating that PPy was obtained and doped in agreement with previous work [43].
The absorption peaks of (963 cm-1) and P-H (2375 cm-1), (1217 cm-1) demonstrate that the final nanowires are doped with gelatin, phosphate and perchlorate [29].
We also tried to understand the formation principle of PPy during doping process. The quinoid structure has a lower ionization potential and a larger electron affinity than the aromatic ones [20]. As a result, upon doping the presence of a charge on the chain can provoke a local geometry relaxation from the aromatic structure towards the quinoid structure, leading to the formation of charged defects, contributing to increase conductivity.
ii) Surface Enhanced Raman Spectroscopy (SERS)
To study more intensively the functional groups in obtained PPy nanowires, firstly, we study the thickness of Pt electrode, which help us to explain the formation of PPy obtained by electrochemical method. After the electrode surface inspection, we considered the distribution of PPy products deposited on micro-electrode, which plays an important role in the enhancement of Raman intensity. Finally, we used SERS technique to evaluate the existence of typical groups and additives in PPy chain. Some initial discussion about conductivity of obtained PPy membrane is also given.
Thickness is typical aspect of a rough surface with thin metal islands exhibited, with good Raman activity, which demonstrates a microstructure smaller than 100 nm
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[17]. Surfaces with closer parking nanoparticles indeed provide more possibilities for molecules to deposite on the boundaries of nanoparticles [24].
Figure 3.12. Path of the stylus over the sample in the measurement of Pt thickness.
Figure 3.12 illustrates the measurement of the deflection of a stylus as it is drawn over an electrode. The profilometer measures the surface profile of the sample by recording the vertical position of the stylus vs. its horizontal position as the film.
The highlighted line (in the middle of the picture) presents the path of the stylus drawn over the surface. The dark areas between electrodes are dust, meanwhile surface was not be cleaned enough before the measurement.
As shown in figure 3.13, the surface morphology of the roughened Pt substrate, is 343.5 nm thick with Δ(error) = 2.3% (negligible), indicating an fine surface with close-parking nanoparticles. That might influence the formation of PPy deposited from electrolyte in potentiostat mode
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Figure 3.13. The thickness of Platinum film .
While cyclic voltammetry makes the surface of PPy film becomes roughened and smooth, results from the repelling action between oxidized units of PPy (oxidation and reduction). In this case, we proved that with such a good Pt electrode, together with the right potentiostat technique found in our study, we can prepare smooth and fine PPy membrane (in nanowie form) on surface (Fig 3.10. 3.14). In addition, we also study the role of PPy surface roughness in SERS measurement. As we know, the Raman intensity is influenced by the scattering of the exciting light from the surface of sample and the possibility of transition of vibration modes. PPy film with a less rougher surface is always accompanied by a lower extent of scattering of exciting light from the surface. As a result, a higher intensity will be recorded at the detector.
Therefore, the Raman intensity increases with the decrease of surface roughness of PPy [45].
It can be seen that PPy grew comparatively evenly onto surface of Pt substrate).
PPy nanowires filled all gaps between electrodes, leading PPy nanowires to distribute
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uniformly over the surface. As a result, Raman intensity might be enhanced with high SERS peaks at the detector.
Figure 3.14. Distribution of PPy nanowires over Pt surface electrode.
In this work, SERS was used to investigate the functional groups on the active surface of electrode which functionalized by PPy nanowires shown in Fig 3.15 and table 3.4.
As shown in the spectra of Fig 3.15, the peak at 931.35 cm-1 is assigned to the symmetric stretching mode of [34,47]. Clearly, is still residual in the sample, which is coincides with the results of FT-IR (discussed in last section). The existence of this negatively charged and doped perchlorate is to balance the charge of the positively charged and oxidized PPy. Moreover, should be evenly distributed on the surface of Platinum because of high Raman intensity peak.
The enhanced peak at 972 cm-1 assigned to the vibration mode of the pyrrole ring in-plane deformation, matching well with range 972÷998 cm-1 [36].
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Figure 3.15. Surface Enhanced Raman Spectroscope of polypyrrole film deposited on Platinum surface.
In basic, the conductivity of PPy is directly decided by the conjugating length and the degree of oxidation level of PPy in this system. Both these two factors are easily evaluated from the characteristic peaks of PPy observed on SERS [44].
In Fig 3.15, we found a peak at 1592 cm-1 indicating C=C stretching of PPy , agree with peak located between 1570 and 1630 cm-1 represents the C=C back-bone stretching of PPy [7]. This strong peak might be equivalent to aromatic form of PPy appearing in FT-IR spectra. The peak position downshifts to lower wavenumbers when the conjugating length of polymer chains increases [39]. Meanwhile, the peak position found in this study is just slightly higher than 1570 cm-1, indicating to a comparatively long conjugating length of polymer chains. It should be noted that a longer conjugating length of polymer chains is contributive to higher conductivity of PPy.
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Additionally, the peaks at 1560 cm-1 and 1610 cm-1 are theoretically assigned to the C=C backbone stretching of reduced and oxidized PPy nanowire, respectively [45].
However, as presented in Fig 3.15 we just observe only a peak at 1592.6 cm-1, indicating an overlapped the two peaks. It can be seen that this prominent peak is closer to the oxidized PPy peak (approximately 1610 cm-1). Qualitatively, it can be supposed that oxidized state of PPy dominated the whole peak of C=C backbone stretching. It demonstrates that oxidation potential was well-controlled during the whole process in potentiostat mode, leading to the formation of oxidized units of PPy over the electrode’s surface. Furthermore, as above mentioned, the higher oxidized level of PPy was, the higher conductivity measured.
The conductivity of PPy is strongly increased with the Raman peak intensity of oxidized PPy, reveals in both C-H in-plane deformation of oxidized PPy and C=C stretching of oxidized PPy.
Firstly, as presented in Fig 3.15, the double peaks at 1044 cm-1 and 1086 cm-1 in SERS are assigned to be the C-H in- plane deformation of oxidized PPy [46]. It is interesting to observe the enhancement of the peak at 1044 cm-1 ascribed to the C-H bending vibration was directly related to the increased doping level of ions in the PPy shell [36]. The presence of this peak can be explained that anions participated as dopants during PPy polymerization, including phosphate and perchlorate group discussed in FT-TR spectra in Fig 3.11. However, phosphate group doesn’t appears in SERS spectra, it might be a result of that was not located near Platinum surface.
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Peaks (cm-1) in this work
Peaks (cm-1) shown in the literature
Peak assignments
931 932
972 972, 984, 998 Ring deformation
1044, 1086 1044 C-H in plane deformation
1244 1300 N-H in plane deformation
1377 1326,1392 Ring stretching ~ aromatic
1592 1560~1630 C=C backbone stretching~ Quinoid
Table 3.5. Comparison between SERS peaks in this work and those in literature.
Based on SERS spectra of Pt/PPy nanoparticle, we tried to understand the influence of the doping level in conducting polymer shell on the properties of sample.
After analyzing carefully, we find that the peak of C-H bending vibration was found to be enhanced with the increase in content of anion ( , ). The role of doping of PPy in charge transfer process can be explained according to the polaron-bipolaron theory. It is believed that doping should alter the local Fermi level, results in the formation of polarons and bipolarons and their energy gaps which are smaller than the gap of undoped state [36]. Moreover, the higher doping level of PPy indicating higher density and mobility of polarons and bipolarons, contributing to an enhancement of conductivity of PPy.
The peak at 1377 cm-1 is assigned to the ring stretching of pyrrole, represent the presence of aromatic form of PPy prepared in electrochemical polymerization. Also, it can be said that PPy rings are close to Platinum surface, thus the peak is strongly
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enhanced. Moreover, although the C=C stretching vibration is not typical in PPy structure, the enhanced peak is still very sharp and high. Hence, the entire C=C linkages in pyrrole chain might be arranged near Pt surface.
Above all, the most exciting peak is 1244 cm-1 is assigned to the vibration of β(CH)/δ(NH) [41]. In our study, the peak of N-H appears but the position shifts to a lower wavenumber compared with the peak at 1300 cm-1 of N-H in literature [45]. In addition, the amplitude of the peak at 1244 cm-1 is much smaller than that of peak at 1592 cm-1 indicating a weak enhanced peak, thus N-H groups are entirely orientated upward and far from the surface of the membrane which is believed to facilitate the DNA immobilization. This is the most interesting result achieved from our study which has not been presented in other studies [11].
In other respect, this may be ascribed to the change in chemical structure of PPy, indicating that gelatin is not only used as a ‘soft template’ for PPy growth, but also plays an important role in the orientation of N-H group on the surface of PPy membrane.
In summary, SERS spectrum proves the information about chemical structure and comparatively evaluates the conductivity (will be further discussed in the next part) of obtained PPy nanowires. Besides, the result from SERS spectrum contains helpful information about a potential DNA sensor application trend, which is the right target of this study.
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