Characterization of As-Synthesized PANI/CeO₂ Nanocomposite Films for Enhanced Anti-corrosion Applications

2. Methods

2.1. Materials

The samples used in this study were prepared from the following materials: Polyaniline (PANI, emeraldine base, 50,000 g/mol), camphor sulfonic acid (CSA, 232.30 g/mol), N-Methyl-2-Pyrrolidone (NMP, 99.133 g/mol), Ferric oxide nanoparticles (Fe₃O₄NPs, 50-100 nm particle size), cerium dioxide nanoparticles (CeO₂NPs of size less than 50 nm). All the materials were purchased from Sigma Aldrich (formally Millipore Sigma).

2.2. Synthesis technique

To prepare PANI composite solution, 0.5 g Polyaniline and 0.12 g CSA were dissolved in 100 mL NMP. The mixture was subjected to a vigorous magnetic stirring overnight at 55 °C. To obtain a homogenous solution,

PANI composite solution was sonicated at 55 °C for three hours. Moreover, solution mixed method was utilized to synthesize PANI/CeO₂ NPs nanocomposite solutions. Under magnetic stirring for five hours, five wt.% of CeO₂ was added separately to PANI composite solution at room temperature.

The resulting PANI/CeO₂ nanocomposite solutions were sonicated for 3 hours at room temperature. To obtain the desired investigated structures, we employed casting technique to deposit PANI and PANI/CeO₂ nanocomposite films on ITO and steel substrates. To prevent any modifications on the surface morphology and to ensure complete drying of all residuals, the deposited films were dried at 40 °C at ambient condition overnight

2.2. Characterization methods

The chemical, structural, and morphological characterizations were conducted using FTIR microscope (HYPERION 3000 Bruker), XRD (Malvern Panalytical Ltd.), and SEM micrographs (Quanta FEG 450), respectively. Thermogravimetric analysis (TGA, NETZSCH) was utilized to investigate thermal stability of the as prepared PANI and PANI/CeO₂ thin films. A UV–Vis spectrophotometer (Hitachi U-3900H) with a total internal sphere was used to obtain and interpret the optical properties. A four-point probe (Microworld Inc.) hocked up to a high-resolution Keithley 2450 Sourcemeter was employed to measure and interpret the electrical conductivity. The corrosion protection performance was investigated using the polarization method.

3. Results and Discussion

The chemical, structural, and morphological characteristics of PANI and PANI/CeO₂ nanocomposite films were investigated and interpreted using the FTIR absorbance spectra (Figure 1), XRD patterns (Figure 2), SEM images and water contact angle measurements (Figure 3). Figure 1 displays the FTIR spectra of PANI and PANI/CeO₂ nanocomposite films in 500-4000 cm^-1 spectral range. For protonated PANI film, the C = N iminoquinone vibrational band appears at 660 cm^-1 [12]. Additionally, the absorption band at 825 cm^-1 is assigned to the aromatic rings. This is a strong evidence of the formation of the polymer [13]. The vibrational band that appears at 950 cm⁻¹ refers to $-{\mathrm{S}\mathrm{O}}_3\mathrm{H}$ group, ratifying the PANI protonation with CSA. Furthermore, the in-plane C - H bending vibrations within the quinoid unit (N = Q = N) appear at 1125 cm⁻¹. Also, the aromatic C − N stretching vibrations appear in the 1300-1500 cm^-1 spectral range [14]. The C - N stretching vibrations inside benzenoid (N - B - N) and quinoid (N = Q = N) rings are located at 1541 and 1651 cm^-1, respectively. The absorption bands beyond 3000 cm^-1 indicate the N - H stretching vibrations [14]. Incorporation of CeO₂ NPs into the protonated PANI films strongly moved the vibrational bands to the higher region of the spectrum. This strong shift is mainly attributed to the difference in the electronegativity between CeO₂ NPs and the PANI molecules.

Figure 2 shows the XRD patterns of PANI and PANI/CeO₂ nanocomposite films in the 10°-50° angular range. Clearly, the crystal structure of PANI is mainly determined by the synthesis conditions and the type of the protonic acid [15]. Protonated PANI film shows diffraction peaks at 14.97°, 20.72°, and 25.38° corresponding to (011), (001), and (110) diffraction crystallographic planes [16]. Additionally, a diffraction peak located at 27.86° associated with the CSA, confirming the protonation of PANI. The semi-crystalline protonated PANI with CSA has two phases. Namely, the phase in which the polymer chains are ordered (crystalline phase), and the phase in which the polymer chains are randomly distributed (amorphous phase) [17]. On the contrary, the protonated PANI/CeO₂ nanocomposite film exhibits an amorphous phase.

Figure 3 demonstrates the 1 μm scaled-SEM micrographs and water contact angle (WCA) measurements for PANI and PANI/CeO₂ nanocomposite films. As can be clearly seen, the protonated PANI film shows a rod-like shape with a WCA of 38°, indicating that the protonated PANI film has a hydrophilic nature (Figure 3a). Adding CeO₂ NPs into the PANI matrix leads to the decrease of the grain sizes as well as the WCA (24°) (Figure 3b).

The absorption spectra of PANI and PANI/CeO₂ nanocomposite films were investigated. The absorption coefficient can be expressed as, $\alpha =\left(1/d\right)\ln \left(\left(1-R\right)/T\right)$ [18,19]. The parameters $T$, $R$, and $d$ stands for the transmittance, reflectance, and film thickness, respectively. The parameter $\alpha$ exhibits a sudden decrease from 0.01 to 0.003 as incident wavelength increases from 300 to 350 nm. Beyond $\lambda =350nm$, it attains a constant vales as demonstrated by Figure 4. The vibration band appears between 400 and 480 nm is related to the superposition of the π-π* transition within the benzoin ring with the confined polarons (polaron-π*) transition [20]. Introducing CeO₂ NPs into the PANI composite matrix increases $\alpha$ in the visible region and shifts the absorption edge into the red region. The bandgap energy of both PANI and PANI/CeO₂ nanocomposite films was calculated using the Tauc plot method[21,22]. The bandgap energy of the PANI composite film was calculated to be 3.74 eV. Introducing CeO₂ NPs into the PANI matrix decreases the bandgap energy to 3.67 eV.

The two PANI and PANI/CeO₂ nanocomposite thin films samples are characterized electrically

By measuring electrical conductivity using a 4-point probe at 12 distinct points. The measured electrical conductivity of the PANI composite film is found to be 0.40 S.cm^-1. The considerably large conductivity could be attributed to the strong acid-doping of CSA, which provides additional charge carriers by protonating the imine nitrogen of the PANI backbone [23]. Incorporation of CeO₂NPs into the PANI composite film more than doubles the electrical conductivity to 1.07 S.cm^-1. To obtain a deeper insight into electrical conductivity values, the electrical conductivity maps (1 cm x 1 cm) of PANI and PANI/CeO₂ nanocomposite films are measured and presented in Figure 5. Careful examination of (Figure 5a) indicates significant variation of the conductivity across PANI composite film caused by substantial surface morphology alterations as well as the quality and conditions of the growth process. Introducing CeO₂NPs into the PANI composite matrix considerably changes the conductivity distribution. This could be attributed to the dispersion of each type of nanoparticle in the PANI matrix (Figure 5b).

Polarization method was used to elucidate the corrosion rate of PANI and PANI/CeO₂ nanocomposite films. The corrosion test was evaluated in 3.5 wt.% NaCl solution at a temperature of 298 K (Figure 6a). The corrosion potential ($E_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$) and corrosion current ($I_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$) were evaluated at the junction where the tangent of the anodic and cathodic polarization curves intersect. The corrosion rate of the nanocomposite films can be calculated using $CR=kM{I}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}/{\rho }_m$ [24,25], where $k$ is a parameter equals to $3268.5\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{A}$, $M$ stands for the molecular weight of carbon steel, ${\rho }_m$ is the density of carbon steel. The parameters $E_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$, $I_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}$ and CR of electrochemical measurements are tabulated in Table 1. It can be seen that the corrosion rate of PANI-coated carbon steel is lower than the corrosion rate of bare carbon. In addition, incorporating PANI with CeO₂ decreases the corrosion rate. The lowest corrosion rate is found for PANI/CeO₂ nanocomposite films (0.112 mm/year). The protection efficiency (${\eta }_{PROT}$(%)) can be calculated using ${\eta }_{PROT}\left(\%\right)=\left(i_{corr}^0-i_{corr}^{coat}\right)/i_{corr}^0\times 100\%$, where the parameter $i_{corr}^0$ represents corrosion current of bare carbon steel and $i_{corr}^{coat}$ stands for the corrosion current of the nanocomposite coated carbon steel [26]. The higher protection efficiency was obtained for PANI/CeO₂ nanocomposite films with a value of 92.25%.

The corrosion rate of PANI and PANI/CeO₂ nanocomposite films were calculated in the 298-338 K temperature range (Figure 6b). As the temperature of the electrochemical reaction is increased, the corrosion rate for all nanocomposite films increases. This increase is a direct consequence of the enhanced electrochemical reaction rates as well as the increase in the kinetic energy of the molecules in the electrolyte solution. Therefore, the diffusion rate of the molecules is significantly increased. To clarify the thermal activated processes of the corrosion reactions [27], the corrosion rates of PANI and PANI/CeO₂ nanocomposite films versus the reciprocal of temperature (1000/T[K]) are illustrated in Figure 6c. As can be clearly seen, corrosion rates exhibit Arrhenius-like behavior ($CR=C{R}_0\exp \left(-E_a/K_{B}T\right)$) [28]. This means that the corrosion rate of bare carbon steel and carbon steel coating by PANI, PANI/ZrO₂, PANI/Fe₃O₄, and PANI/(ZrO₂-Fe₃O₄) nanocomposite films are thermally activated. The activation energies deduced by Arrhenius fitting are tabulated in Table 1. Obviously, the highest activation energy is obtained for PANI/CeO₂ nanocomposite film. Thus, the effect of temperature on the corrosion rate of PANI/CeO₂ nanocomposite film is more pronounced than for other investigated samples.

Figure 7 illustrates the effect of coating on the Nyquist and the Bode plots of PANI and PANI/CeO₂ nanocomposite films immersed in 3.5 wt.% NaCl at 298 K. In the Nyquist plot, the curves have single capacity arcs with large radii for PANI/CeO₂ nanocomposite films compared to bare carbon steel and PANI film (Figure 7a), indicating the resistance of these nanocomposites is very high. The coating has an exceptional physical barrier influence on the electrolyte. The Z value in the Bode diagram at 0.1 Hz increases from 652 Ω.cm^-2 for bare steel to 1900 Ω.cm^-2 for PANI/CeO₂ nanocomposite film. Thus, coating the carbon steel by PANI/CeO₂ nanocomposite film enhances the corrosion resistance (Figure 7b).

4. Conclusions

This work reports the results on a novel nanocomposite coating of PANI/CeO₂ nanocomposite film for corrosion protection applications. FTIR spectra and XRD patterns confirm the formation of the nanocomposite films. Water contact angle measurements confirm that PANI/CeO₂ nanocomposite film has a hydrophilic nature. Therefore, the films are capable of interacting with water through hydrogen bonding. Optical measurements reveal that bandgap energy of the PANI composite film is 3.74 eV. Introducing CeO₂ NPs into the PANI matrix decreases the bandgap energy to 3.67 eV. Consequently, bandgap engineering in PANI/CeO₂ nanocomposite film could be a powerful technique for the design of new materials and devices based on this novel material. In addition, band diagrams with continuous bandgap variations can be generated in heterojunctions designed from this novel material and fabricated using techniques such as molecular beam epitaxy. The electrical conductivity measurements reveal that PANI composite film exhibits an electrical conductivity of 0.40 S.cm^-1. Introducing CeO₂ NPs into the PANI composite film increases the electrical conductivity by more than two-fold to 1.07 S.cm^-1. The corrosion protection performance was investigated using electrochemical measurements. The protection efficiency of PANI/CeO₂ nanocomposite film is 92.25%. In general, the efficiency of an inhibitor increases with an increase in inhibitor concentration. Obtaining excellent inhabitation for incorporating 5 wt.% of CeO_{2 in} PANI nanocomposite films is very promising for corrosion protection applications.