Synthesis, characterization, and applications of a novel poly(4-[pyrrol-1-yl methyl]benzoic acid)–silver composite

The electrochemical behavior of silver was studied by cyclic voltammetry on an FTO/glass electrode in an aqueous solution containing 0.1 M NaNO₃ and 5 × 10⁻²M silver nitrate (AgNO₃). The first cycle is shown in Figure 1a. The obtained curve is characterized by the presence of a reduction peak in the vicinity of a 0.28 V/SCE corresponding to the reduction of silver cations (Ag⁺) to metallic silver (Ag). In the reverse scan, an intense oxidation peak is observed in the vicinity of a 0.53 V/SCE corresponding to silver oxidation of the deposited Ag⁺.

Figure 1

Voltammograms of silver recorded from an FTO/glass electrode immersed in an aqueous solution containing 0.1 M NaNO₃ and 5 × 10⁻²M AgNO₃ at υ = 100 mV/s with pH 5. (a) First run and (b) successive scans.

Successive scans in the same domain (Figure 1b) show a very slight increase in redox peaks. This result is due to the modification of the surface state of the FTO/glass electrode during the scans.

Investigations were carried out in order to optimize the parameters of silver deposition on the FTO/glass substrate. They include the concentration of the silver solution, the scanning speed, the charge passed by imposing a potential on the electrodes, and the pH of the electrolyte solution. The latter was adjusted by adding a few drops of HNO₃. It was noticed that a significant displacement of the silver oxidation peak reduced during the reduction toward the less positive potentials by increasing the acidity of the medium and a clear increase in the intensity of the current peaks of reduction and oxidation. Indeed, silver reduces better at more acidic pH.

The cyclic voltammetry method was used to investigate the electrochemical behavior of the monomer 4-[pyrrol-1-yl methyl]benzoic acid on an FTO/glass electrode. The electrolyte solution contained 4 × 10⁻³M monomer, 10⁻¹M acetonitrile (CH₃CN), and 10⁻¹M lithium perchlorate (LiClO₄). The cyclic voltammetry curve presented in Figure 2a is characterized by the presence of an irreversible oxidation peak around 1.3 V/SCE corresponding to monomer oxidation and consequently to the formation of a polymer film PPy-b deposited on the electrode surface (FTO/glass).

Figure 2

Cyclic voltammetry curves of the monomer on an FTO/glass electrode recorded from a solution containing 4 × 10⁻³M monomer, 10⁻¹M acetonitrile, and 10⁻¹M LiClO₄, at the scanning speed of υ = 100 mV/s. (a) First run and (b) successive scans.

Successive scans between 0 and 1.4 V/SCE led to the appearance of a redox system (E _Ox = 0.9 V/SCE and E _Red = 0.5 V/SCE), which corresponds to the reversible oxidation of the polymer, Figure 2b. The progressive increase in the oxidation and reduction peaks is explained by the growth of a polypyrrole film deposited on the surface of the electrode.

In order to insert the silver in metallic microparticles form into the polymer film, the electrode polymer/FTO/glass, which was modified by the film of PPy-b, has been dipped in a solution containing 10⁻¹ M silver nitrate (AgNO₃) during 30 min to complex the silver cations (Ag⁺) with the polymer, then rinsed with distilled water to remove the excessive Ag⁺ not retained by the polymer film. The modified electrode was then reduced to 0.35 V/SCE in an aqueous solution of 10⁻¹ M sodium nitrate (NaNO₃) in the absence of silver to precipitate the complexed silver in the form of metal microparticles.

To ascertain that silver has been inserted into the polymer film, an oxidation of the modified electrode was carried out by scanning in the silver oxidation zone. The first run of the cyclic voltammetry recorded curve, Figure 3a, is characterized by an oxidation peak in the vicinity of 0.46 V/SCE attributed to the dissolution of the silver inserted into the polymer film. In the reverse scan, a reduction peak around 0.3 V/SCE was observed, which is attributed to the reduction of silver ions formed during the dissolution of the silver.

Figure 3

Cyclic voltammograms of silver dissolution on the FTO/Glass electrode modified with a polymer film in an aqueous solution containing 0.1 M NaNO₃ and recorded at the speed of υ = 100 mV/s. (a) First run and (b) successive scans.

This result unambiguously confirms the insertion of metallic silver particles in the PPy-b film. The successive scanning, depicted in Figure 3b, showed a stability of the intensity of the oxidation and reduction peaks of silver. It seems that the silver particles are retained by the polymer film during their dissolution in Ag⁺.

EIS of the elaborated thin films prior to and after insertion of silver particles into the polymer was carried out at an open circuit potential in the frequency range of 100 kHz to 10 mHz. Nyquist diagrams obtained are shown in Figure 4. A zoom-in view in the region of low impedance is shown in Figure 4b. The shape of the diagram corresponding to the FTO electrode (curve A) is almost a straight line, characteristic of the diffusion process of a semiconductor-type material. This was recorded at an open-circuit voltage of −0.0325 V. Meanwhile, the diagram plot recorded at an open-circuit voltage of 0.025 V of the deposited polymer film PPy-b exhibited a semicircular arc in the region of high frequency (curve B) due to charge transfer processes. After complexation of the Ag⁺ cations with the polymer film and the reduction of the complex of polymer and silver in metallic microparticles (curve C), a clear reduction in the charge transfer resistance is observed, confirming the modification of the electrode surface and the insertion of the metallic silver microparticles. Herein, the diagram was recorded at an open-circuit voltage of 0.104 V. It exhibits a semicircular arc in the region of high frequencies attributed to charge transfer processes, followed by a straight line due to diffusion processes.

Figure 4

(a) Nyquist diagrams of the FTO electrode prior to and after insertion of silver microparticles into the polymer film PPy-b relative to: (A) FTO electrode, (B) FTO electrode modified by the polymer film, and (C) after insertion and reduction of silver microparticles into PPy-b. (b) A zoom-in view of the region of low impedance.

The electrical equivalent circuits that fit the Nyquist diagrams of curves B and C are depicted in Figure 4(b). Herein, R _s is the solution resistance, R _ct is the charge transfer resistance, C _p is the parallel capacitor, and Z _W is the Warburg impedance. Table 1 presents the fitted values of the electrical equivalent circuits of the electrochemical impedance measurements of curves B and C. It is worth noting the improvement in the electrical properties following the introduction of silver into the polymer film. For instance, the charge transfer resistance is reduced by 17 times once silver has been inserted and reduced.

It is worth noting that different species produces different behavior in EIS data. For instance, the insertion of silver shows a different behavior in comparison to that of cobalt [20].

Resistivity is the function of film thickness, which was determined using the mechanical profilometer. Table 2 presents the average measured resistivity values (ten spots at different positions on the surface of each sample) of different specimens. Here again, the decrease in the specimen resistivities confirms the insertion of silver into the backbone of the polymer, and thus improves the conductivity.

Figure 5 shows the SEM images of a film of the polymer PPy-b of 385 nm thickness (1.5 × 10⁻⁷ moles) deposited on an FTO substrate (Figure 5a) and that of the composite material (polymer–silver) (Figure 5b). Examination of the surface of the polymer shows that it has a more or less uniform globular morphology. After insertion of silver (1.2 × 10⁻⁷ moles ∼ equivalent to 10⁻⁵g) into the polymer film by immersion of an FTO electrode modified by the polymer film, a more concentrated globular morphology with varying grain sizes is observed on the entire surface. The bright spots observed indicate the presence of silver microparticles. This was confirmed by XRF analysis shown in Figure 6. The spectrum looks similar to that recorded using energy-dispersive spectroscopy (EDS) [17].

Figure 5

SEM surface images of: (a) a film of PPy-b deposited on an FTO electrode and (b) a modified FTO/polymer (PPy-b) electrode after insertion and reduction of silver microparticles.

Figure 6

XRF spectrum measured after the insertion and reduction of silver into the polymer film PPY-b.

The AFM image, illustrated in Figure 7a, taken from a part of the polymer film deposited on an FTO/glass substrate is observed to adopt a pyramidal shape with a square and a triangular base with a height of 1.25 μm and a roughness of 157.028 nm. After the insertion of silver microparticles into the polymer film by immersing the electrode in a solution of silver nitrate for 30 min and electrochemical reduction at 0.35 V, a clear modification of the surface of the material is observed, Figure 7b, due to the covering of the pyramids with silver particles. This is confirmed by the increase in the height of the substrate from 1.25 to 2 μm. Its roughness is on the order of 349.462 nm.

Figure 7

AFM surface images of: (a) a film of PPy-b deposited on an FTO electrode and (b) a modified FTO/polymer (PPy-b) electrode after insertion and reduction of silver microparticles.

These were investigated using a spectrophotometer in the wavelength range 300–1,100 nm. The transmission spectra of different samples are shown in Figure 8.

Figure 8

Spectral distributions recorded from the films in the UV–visible range. Curve A: FTO substrate; curve B: polymer; curve C: inserted silver.

The spectrum recorded of the substrate (FTO/glass) (curve A), whose y-axis is on the right, shows at the start, λ = 300 nm, an almost zero transmission rate to reach 90% from λ = 330 nm and remains constant throughout the studied range. The starting spectrum behavior is due to the bandgap of FTO [21]. The transmission spectrum of the substrate modified by the polymer of thickness 385 nm is completely different (curve B), with its y-axis being always on the right. It displays a transmission rate of around 50% in the wavelength range λ = 700–950 nm. After the insertion of silver particles, the recorded spectrum (curve C), whose y-axis is on the left, displays new energy absorption bands, the first at λ = 500 nm and the second at λ = 700 nm. A clear reduction in the rate of transmission is observed (from 40 to 0.7%).

The I(V) characteristic of the device based on the composite (polymer–Ag) as deposited on the FTO substrate was measured. It should be mentioned that there was no deposition of electrical contacts and that the measurements were carried out directly of the sample, whether on the substrate or on the polymer sides. The shape of the curve obtained, shown in Figure 9, corresponds to that of a diode. The value of the threshold voltage which is determined by extrapolation of the linear part (first quadrant of the characteristic) is V _th = 0.214 V.

Figure 9

Characteristic I(V) of the composite (polymer + Ag)/FTO.

The reverse saturation current of such configuration was also determined by using the characteristic I(V) in a semilogarithmic scale (third quadrant). This is shown in Figure 10. Extrapolation of the linear part to V = 0 yields a reverse saturation current I _s = 0.22 mA. These parameters (V _th and I _s) are very appropriate for low power electronics applications [22,23,24].

Figure 10

Determination of the reverse saturation current of the composite.

Another potential application of the new composite material that has been investigated was in water reduction. The electrocatalytic effect of the composite material was tested in 0.1 M NaOH medium according to the following reaction: (1) 2 H 2 O + 2e − → H 2 + 2 OH − . 2{\text{H}}_{2}O+{\text{2e}}^{-}\to {\text{H}}_{2}+2{\text{OH}}^{-}. The voltammograms obtained, shown in Figure 11, show a displacement of the hydrogen release overvoltage after the insertion of the metallic silver particles in the polymer film (curve C) as compared to that of the substrate (curve A) and after modification by the polymer film (curve B). This result clearly shows the good electrocatalytic activity of the composite material. Indeed, a potential gain estimated at 350 mV has been obtained.

Figure 11

Recorded voltammograms of water reduction in 0.1 M NaOH of different electrodes at the scanning speed of 100 mV/s.