Characterization of PPy/Pap and AgNPs-PPy/Pap films
The FT-IR spectra of uncoated paper substrate and PPy coated Pap is shown in Fig. 1 The spectrum (a) Pap shows the typical absorption bands characteristic of cellulose. The band at 3340 cm− 1is attributed to the stretching vibration O-H bonds of the alcohol functions of cellulose (Sain et al 2006). Absorption bands located around 2924 cm− 1 and 1160 cm− 1 are respectively associated to the stretching vibration C–H in cellulose (Kaushik et al 2011) and the stretching vibrations of the C–O–C bond (He et al 2007). The band at 1030 is attributed to the stretching vibration C-O bond (Hu et al 2011).The presence of the C-H group is supported by the vibration band around 1438 cm− 1 (Ul-Islama et al 2012) and the bands near 1315 cm− 1 and 1248 cm− 1 are attributed to O–H deformation bonds.
The PPy/Pap composite displays all of the characteristic bands of the PPy phase (spectrum (b)), indicating the effective formation of PPy by in-situ oxidative polymerization. The peak located at 1540 cm− 1 is related to C-N/C = C stretching mode of the Pyrrole ring (Yuana et 2016; Feng et al 2007). The peaks at 1300 and 1160 cm− 1 are assigned to C-H in-plane vibration (Wei et al 2010), and the band at 1450 cm− 1 are due to C-N stretching of PPy (Patil et al 2013). Moreover, the peaks near 960 and 890 cm− 1 are caused by the C-H wagging vibration of doped PPy (Das et 2013) and the N-H2 wagging vibration in the PPy is initiated at 790 cm− 1 (Wang et al 2004). The characteristic band of the hydroxyl groups of the cellulose (3340 cm− 1) disappeared in the PPy/Pap composite and this disappearance is due to the physicochemical interactions of the OH groups of the cellulosic paper with the polypyrrole matrix to form the conductive composite. Moreover, it is also observed that the absorption bands at 1160 and 1030 cm− 1 are still present in the composite but with a low intensity and a low chemical shift due to the interactions between the cellulose chains and the polypyrrole cycles (Hu et al 2011). The AgNPs-PPy composite exhibits an FT-IR spectrum similar to PPy with the bands for C–N stretching and C–H plane vibration and N–H stretching but with a low intensity and a low chemical shift. These shifts reflect the interaction of the incorporated silver with the polymer backbone (Zhang et al 2014).
X-ray Photoelectron Spectroscopy
XPS was used to determine the surface elemental and chemical composition of the pristine and modified paper samples. Figure 2 displays typical survey spectra of paper, PPy/Pap and AgNPs-PPy/Pap samples. The main labeled peaks are C1s, O1s, N1s and Ag3d, centered at 285, 533, 400 and 386.3 eV, respectively. For the paper sample, additional minor peaks are Al2p (74 eV), Si2p (102 eV), Ca2p (347 eV). These elements come from the industrial formulation of the paper samples purchased at a local store. PPy exhibits other features: S2p (168 eV) from the dopant as well as Fe2p doublet (~ 712–725 eV) arising from the oxidizing agent.
N1s is relatively more intense after in situ deposition of PPy (Fig. 2, PPy/Pap) whereas a prominent Ag3d doublet is noticed after the in-situ deposition of zero valent silver nanoparticles on top of the PPy/Pap substrate. The high resolution Ag3d doublet is shown in inset of Fig. 2 indicates indeed a binding energy position of 368.3 eV for the Ag3d5/2 peak, in line with zero valent metal nanoparticles.
The C/N ratio was found to decrease in the order paper (82.4) > PPy/Pap (17.9) ~ AgNPs-PPy/Pap (18.2). This trend parallels the observation of black polypyrrole with the naked eye. It also indicates that the PPy/Pap substrate retains its C/N ratio after in situ silver deposition. Note, however, that the C/N exceeds the theoretical value of 4 for bulk polypyrrole and ~ 5 for the surface composition of polypyrrole (Beamson et al 1993). This indicates that the paper substrate is detected through the continuous PPy coating.
Figure 3 compares the narrow C1s regions from paper, PPy/Pap (1h) and AgNPs-PPy/Pap samples. Interestingly, the C1s region of the pristine paper sample is characteristic of a cellulosic substrate in that it has a prominent C-O component at ~ 286.7 eV and a second component at 288 eV due to acetal carbon type. The intense C-C/C-H C1s component is frequently observed on “real” cellulosic materials, industrially processed. The very last component at ~ 289 eV is due to carboxylic type carbon atoms, also noted by Beamson and Briggs (Omastová et al 2005).
For AgNPs-PPy/Pap sample, the fine structure of the C1s region is drastically changed: the peak is broad, and tailing. It also exhibits a subtle increase in the baseline at high binding energy side, which markedly differs from the rather flat background of the paper C1s peak in the case of the paper substrate. This step below the peak maximum is actually invariably noticed in the case of polypyrrole surfaces (Boukerma et al 2007).
Interestingly, the C1s narrow region from PPy(1h)/Pap sample has a shape that resembles that of the underlying paper, with a narrow C-C/C-H component, together with an intense C-O C1s component at ~ 286.5 eV. Although slight peak broadening can be noticed, obviously the 1h deposition time is not sufficient to have a polypyrrole coating with a C1s feature that is shaped like that of pure polypyrrole. This is one reason why in this work, we have spent time and effort tuning the deposition conditions for polypyrrole and Ag. Interestingly, despite a high C/N atomic ratio, the C1s narrow region heavily reflects the C1s shape for pure polypyrrole and that the shape observed for the cellulosic paper starts to be screened by that of polypyrrole.
Nitrogen is a unique elemental marker for polypyrrole, and it is worth to inspect its N1s in comparison to that detected at the paper surface. Figure 4a shows a quasi-symmetrical, noisy peak for pristine paper at 400.0 eV. The polypyrrole exhibits a complex shape fitted with four components centered at 398.0, 399.7, 401.1 and 402.5eV, assigned to C = N defects (6.7 %), N-H, and two positively charged nitrogen atoms noted NI+ and NII+ the contribution of which is 23.1 %, in line with true conductive polypyrrole specimen (Fig. 4b). The situation is similar for AgNPs-PPy/Pap, the positively charged nitrogen atoms of which contribute 23.8% (Fig. 4c). This demonstrates that polypyrrole coating serves as support for the in-situ deposition of silver without any significant change in its chemical structure. Interestingly, one can note an even smaller C = N contribution to the total N1s peak (3.4%), and an additional peak at 406.4 eV assigned to nitrate from AgNO3 precursor.
Scanning Electron Microscope coupled with energy dispersive X-ray spectrometry and X-ray diffraction spectroscopy of AgNPs-PPy composite films
Figure 5 (a) and (b) are the SEM micrographs of the PPy and AgNPs-PPy films, respectively. (c) EDX spectrum of the AgNPs-PPy film. (d) XRD patterns of the pristine PPy and AgNPs-PPy film.
The X-ray diffractograms of the PPy film before and after deposition of AgNPs confirmed the combination of AgNPs in the PPy matrix as shown in Fig. 5d. We can observe several peaks, indicating that the sample is crystallized. The broad peak centered at 2θ = 22.4° is due to the amorphous phase of PPy (ˇSkodovà et al 2013). The peaks located at 2θ values of 37.92°, 44.37°, 64.49° and 81.44° are matched well with the (111), (200), (220) and (222) crystal planes of the face-centered cubic Ag phase, respectively. The mean size of the crystallites, by using Scherrer’s formula for the face-centered cubic plane (111), was estimated to be of 3.7 nm. From the XRD patterns, existence of AgNPs in PPy matrix was established, which is consistent with the result of the SEM.
Electrochemical Activity Of Ppy/pap And Agnps-ppy/pap Materials
Electrochemical activity of PPy/Pap film obtained at different synthesizing durations and AgNPs-PPy/Pap electrode was investigated by cyclic voltammetry in the solution of 0.1 mol.l− 1 KOH, as shown in Fig. 6. Oxidation and reduction processes were not observed in the voltammograms and a resistive response was obtained. Repeated scans show no significant changes in the voltammograms characteristics; this is probably due to several reasons such as low conductivity of the films, pH of the medium and the high sweeping rate. This behavior was observed in the case of PPy deposits on polyester textiles (Molina et al 2008; Molina et al 2009). The current density is slightly enhanced on PPy film synthesized during 2h (Fig. 5b) and the voltammogram of the AgNPs-PPy film had higher slope (less resistance) than that for the sample of PPy, indicating that the conductive properties of the PPy composites increased with the loading of Ag nanoparticles (Lu et al 2012; Tian et al 2017].
Indeed, electrochemical impedance spectroscopy is a well-established and powerful tool in the study of electrochemical behavior of materials, in particular, the estimation of the charge-transfer resistance (Rt) which is in relation with the reactivity of the studied materials (PPy/Pap and AgNPs-PPy/Pap). Figure 7 shows the Nyquist plots of the synthesized electrodes at the equilibrium open circuit potential (-350 mV versus SCE) and in 0.1M of potassium hydroxide solution. The experiments were carried out at the frequency range of 100 kHz to 100 mHz.
The semicircle feature and the EIS line indicate that the electrode process is controlled by electrochemical reaction at high frequencies and by mass transfer at low frequencies (Sen et al 2013). The semicircle is associated with the charge transfer (Rt) occurring at the electrode/electrolyte interface and double layer capacitance. By using Setup ZSimDemo 320 program, the impedance diagrams can be fitted by a simple equivalent circuit (insert of Fig. 8) composed by Rs (solution resistance), Rt (charge transfer resistance), w (Warburg contribution) and CPE (constant phase element of double layer capacitance (Cdl)). The charge-transfer resistance (Rt) values used in equation were given by the Setup_ZSimDemo_320 program by fitting the Nyquist plots.
The results indicate that the value of Rt is low (115 ohm.cm− 2) when the PPy film is deposited on sheet during 2 h compared to those deposited during 1 h (169 Ω.cm− 2) and 3h (260 Ω.cm− 2). The lowest value of Rt (98 Ω) is found when the PPy is decorated by Ag nanoparticles deposited during 10 min on PPy. The lower charge-transfer resistance (Rt) is indicative of the improvement conductivity (Benchikh et al 2009). For the unmodified PPy electrodes, the impedance curves register a large Warburg region indicates great variation in the ion diffusion path length and an increase in ion movement obstruction in the process of ion diffusion into the interior of the electrodes. Evidently, Ag dopant in PPy promotes the ion diffusion rate and is beneficial for a fast-current response in electrochemical reactivity (Yuana et al 2016). This result was consistent with the improved reactivity responses shown in cyclic voltammograms (Fig. 6). So, the value of 2h as synthesis time was chosen as the optimum value which gave the best results in relation with PPy reactivity and the best reactivity and conductivity are obtained when PPy film is modified by silver nanoparticles deposited during 10 min.