Fabrication of a Highly Tunable Graphene Oxide Composite through

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Fabrication of a Highly Tunable Graphene Oxide Composite through Layer-by-Layer Assembly of Highly Crystalline Polyaniline Nanofibers and Green Corrosion Inhibitors: Complementary Experimental and First-Principles Quantum Mechanics Modeling Approaches Bahram Ramezanzadeh, Pooneh Kardar, Ghasem Bahlakeh, Yasmin Hayatgheib, and Mohammad Mahdavian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04323 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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The Journal of Physical Chemistry

Fabrication of a Highly Tunable Graphene Oxide Composite through Layer-by-Layer Assembly of Highly Crystalline Polyaniline Nanofibers and Green Corrosion Inhibitors: Complementary Experimental and First-Principles Quantum Mechanics Modeling Approaches

B. Ramezanzadeh,a,* P. Kardar,a G. Bahlakeh,b Y. Hayatgheib,a M. Mahdaviana a. Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran b. Department of Engineering and Technology, Golestan University, Aliabad Katool, Iran

* [email protected], [email protected]

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ABSTRACT: Three-dimensional (3D) graphene oxide (GO) nanosheets were utilized as a unique versatile platform for fabrication of an effective anti-corrosion system through a Layer-by-Layer (L-b-L) assembly technique. In this way, the highly ordered crystalline polyaniline (Pani) nanofibers and green corrosion inhibitors (GI) were synthesized. Sustainable corrosion inhibitors were obtained from the extract of Urtica Dioica leaves. The GO-Pani-GI nanosheets were characterized by Fourier transform infrared spectroscopy (FTIR), high resolution-transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE-SEM), UV-visible spectroscopy and thermal gravimetric analysis (TGA). In addition, the adsorption features of Pani onto GO sheets and its binding propensity against GIs were assessed by applying first-principles quantum mechanics (QM) modeling approaches. The anti-corrosion properties of the GO-Pani-GI were then examined using electrochemical impedance spectroscopy (EIS) and polarization test. The results achieved from QM modeling studies demonstrated that the Pani strongly anchored to GO surfaces via physisorption mechanism. Computations further declared that all GIs interacted with Pani through intermolecular H-bonds. Moreover, the experimental investigations revealed the superior anti-corrosion performance of multilayered graphene nanocomposites.


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1. INTRODUCTION Corrosion protection of metals through deposition of polymer coatings filled with encapsulated healing agents has drawn considerable attentions during recent years. Among various methods, the Layer-by-Layer assembly of inhibitors on a solid core has been recognized with many advantages1-4. Incorporation of nanocontainers synthesized through Layer-by-Layer assembly technique into the coatings provides effective self-healing anticorrosion properties. Recently, the use of carbon-based nanomaterials has considerably received the researchers' attention. Graphene is an advanced nanomaterial, a one-atom-thick sp2 carbon lattice, with outstanding optical, electrical, thermal, and mechanical characteristics5-7. Graphene oxide is another form of carbon nanomaterials including many functional groups i.e., carboxyl and carbonyl at the edges and hydroxyl and epoxy on the basal plane. These functionalities provide reactive sites for deposition of inhibitors on the GO sheets through Layer-by-Layer assembly approach. Due to the high surface area and the charged functional groups the GO is known as an excellent platform for deposition of organic compounds8-10. Therefore, the high performance composites can be obtained in this manner. In a Layer-by-Layer technique the components can be adsorbed on the GO sheets through attractive forces such as electrostatic interactions, hydrogen bonding and van der Waals forces. Due to the negative charges carried by GO sheets the positively charged materials tend to adsorb on the GO sheets through electrostatic interactions

11-12

. The intrinsic

properties of GO sheets and advantages of the materials deposited through L-b-L system result in the creation of GO based composites with unique properties 13-15. It has been shown that a defect free monolayer of GO sheet is almost impermeable against water, oxygen and ions. There are many reports on the use of GO sheets in polymer composites for the enhancement of the barrier properties

16-19

. However, in the case of GO

sheets with defect and polymer composites with failure the barrier properties of the GO 3 ACS Paragon Plus Environment


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sheets would not be important anymore for corrosion protection. One strategy to obtain GO sheets with unique anti-corrosion properties is deposition of inhibitors on the GO sheets through L-b-L assembly. The surface charge of GO sheets in aqueous solutions is negative due to the presence of carboxylic groups. As a result, the positive materials like conductive polymers can be easily absorbed on the GO sheets via electrostatic forces. Among various conductive polymers the polyaniline (Pani) is the most popular due to its facile synthesis, special chemistry and environmental stability of Pani has been reported in literature

20-23

. Moreover, the corrosion inhibition effect

24-28

. Pani can be in different forms but the most

common state of this polymer is Emeraldine salt (ES). In a method proposed by Mohammadzadeh et al. it was shown that the highly crystalline and conductive Pani nanofibers can be deposited on the GO sheets without using any oxidizing agent. In fact, the epoxide and carbonyl groups existed on the GO sheets act as oxidizer, converting aniline into polyaniline. In this method the polyaniline nanofibers chemically interact with GO sheets 29. Deposition of Pani in Emeraldine salt state provides positive charges on the GO sheets and makes it suitable for deposition of organic inhibitors of negative charges. In this study, for the first time, the eco-friendly corrosion inhibitors extracted from natural products are considered for this purpose. These are cheap and effective green corrosion inhibitors which can be obtained from renewable sources

30-35

. One approach to inhibit the metals from corrosion in

corrosive electrolyte is direct incorporation of corrosion inhibitors. However, most of the organic inhibitors are not effective anticorrosive agents in neutral solutions. For the first time the synergistic corrosion inhibition of mild steel in the presence of Pani and GI on the GO surface is studied. The Pani and GI cannot be added to the solution directly due to some reasons. First, the Pani is insoluble in water and cannot be added to the aqueous chloride solution. Second, in the method used in this study the highly crystalline and conductive Pani nanofibers can be deposited on the GO sheets without using any oxidizing agent. In fact, the


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Pani cannot be synthesized in the absence of GO sheets. Third, addition of GI into the chloride solution could not provide good corrosion inhibition effect. This work would be a start of another work in the future, using GO-Pani-GI composite as an effective anticorrosive system in polymers. In fact, direct inclusion of the inhibitors into the polymer results in some unexpected negative effects on its properties. In this study, the GO nanosheets were utilized as unique versatile platform for the fabrication of a novel anti-corrosion system through a Layer-by-Layer assembly technique. To fabricate the GO-Pani-GI composite, the GO was initially synthesized by modified Hummers' method, and then the polyaniline fibers with positive charges were sequentially adsorbed on the negatively-charged GO sheets. Finally the adsorption of green corrosion inhibitors existed in the Urtica Dioica leaves extract, i.e., hystamine, serotonin and quercetin, on the GO-Pani sheets was investigated. The GO-Pani-GI composite made by L-b-L technique was characterized by FT-IR, TGA, XRD, UV-visible, HR-TEM and FE-SEM techniques. Additionally, in order to obtain a fundamental electronic-scale insight into the Pani adsorption onto charged GO surfaces and its interactions with the chosen green inhibitors, computational modeling studies, employing first-principles quantum mechanics (QM) approaches, were conducted on GIs/Pani and Pani/GO clusters. The anticorrosion properties of the GO-Pani-GI sheets were analyzed in saline solution on mild steel panels by EIS and polarization test. The surface chemistry and morphology of the steel panels dipped in the solution containing GO-Pani-GI was tested by FT-IR and FE-SEM/EDS analyses.

2. EXPERIMENTAL 2.1. Materials. Analytical grades of sulfuric acid (Merck Co.), expandable graphite powder (Kropfmuehl Graphite Co.), sodium nitrite (Merck Co.), hydrogen peroxide (Merck Co.) potassium permanganate (Merck Co.), aniline monomer, HCl (37%) (Merck Co.) and 5 ACS Paragon Plus Environment


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dimethylformamide (DMF, Merck Co.) were prepared. Steel panels (0.05% S, 0.5% Mn, 0.12% C, 0.3% Si, 0.045% P and balanced Fe) were prepared from Foolad Mobarakeh Co (Iran). Urtica Dioica, which grows on the north coast of Iran, was dried and powdered. 2.2. Synthesis of GO. High quality graphene oxide nanosheets were synthesized by modified Hummer’s method

36

using expandable graphite with expansion rate and grain size

of 350-700 cm3/g and 80% >300 µm. The procedure is mentioned in details in our previous works 16-19. In brief, 2 g graphite powder was added to 240 mL concentrated H2SO4 solution. After 2 h mixing 2 g NaNO3 and 12 g KMnO4 was gradually added to the solution. The mixture was mixed for 72 h and then diluted by 1200 mL deionized (DI) water. At the end, 10 mL H2O2 (35%) was added to the mixture for ending the oxidation reaction. In this way, the yellow graphite oxide suspension can be obtained. The graphite oxide mixture was centrifuged for 2 min at 4000 rpm, washed with a mixture of 1 M HCl solution and DI water for three times, and filtered at the end. The synthesis process of GO is schematically presented in Figure 1. Figure 1 2.3. Layer-by-Layer (L-b-L) Assembly of Graphene Oxide Platform with Polyaniline Nanofibers and Green Inhibitors. A highly crystalline polyaniline (Pani) nanofiber was deposited on the GO sheets through in situ polymerization of aniline. To this end, 1 mL aniline was added to 10 mL HCl (1 M) solution and stirred for 10 min. Then, 0.2 g GO sheets was added to the previous solution under nitrogen purging and stirred for one week at room temperature. Finally, the products were centrifuged and washed three times with deionized water. As schematically shown in Figure 2, the aniline polymerization on the GO sheets includes different steps. First the protonated aniline monomers adsorb on the negative sites of GO sheets. Then, the adsorbed monomers can be polymerized by the epoxide groups existed on the GO sheets. This is a novel approach for deposition of


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polyaniline on the GO sheets without using any oxidant 29. In fact, the GO sheets act as an oxidant. In this way, the highly crystalline polyaniline nanofibers in the form of Emeraldine salt (ES) can be deposited on the negative sites of GO sheets. In the next step, 0.2 g GO-Pani was added to the water solution including 1000 ppm Urtica Dioica extracts. The pH was fixed at 7.0 without adding any buffer and only by addition of small quantity of deionized water and then the mixture was stirred for 2 h at room temperature. In this step the negativelycharged corrosion inhibitors existed in the Urtica Dioica extract can be deposited on the positive sites of GO-Pani sheets. Finally, the mixture was filtered and washed with deionized water to remove the physically adsorbed compounds. Figure 2 2.4. Techniques 2.4.1. Characterization of GO and GO-Pani-GI Sheets. The morphology and microstructure of the neat GO and GO-Pani-GI sheets were characterized by HR-TEM, Tecnai G2 F20S-TWIN 200 kV and FE-SEM, Tescan Mira3 LMU, techniques. The surface chemistry of GO sheets, before and after deposition of layers, was studied by FT-IR spectrophotometer, Perkin-Elmer, in the range of 4000-400 cm-1 through a KBr Pellet procedure. The phase composition of nanosheets was studied by XRD, Philips X’Pert analysis, where the radiation source and X-ray wavelength were CuK” and 1.5406 Å, respectively. The thermal stability of nanosheets was examined by a TGA analysis, Mettler Toledo instrument, under nitrogen atmosphere, temperature range of 25-700 °C and by heating rate of 10 °C/min, respectively. The UV-vis absorption spectra of GO, GO-Pani-GI and GI in water were obtained by Hitachi U-3010 UV-Vis spectrophotometer. 2.4.2. Characterization of the Anticorrosion Properties of GO-Pani-GI Sheets. The steel panels were abraded by sand papers of 600, 800 and 1200 grits, followed by acetone degreasing. The cleaned steel sheets (1 cm2) were then dipped in 100 mL 3.5 wt.% NaCl



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solutions with and without GO-Pani-GI extract. The extract of GO-Pani-GI was prepared through addition of 0.1 g GO-Pani-GI nanosheets to 3.5 wt.% NaCl solution, stirring for 24 h and centrifugation for 2 min at 4000 rpm. An Iviuom Compactstat model electrochemical impedance spectroscopy was employed to investigate the anti-corrosion properties of the GOPani-GI sheets. The experiment was conducted at open circuit potential (OCP) in an electrochemical cell including Platinum as counter, Ag/AgCl as reference and steel panel as working electrodes. The frequency range and amplitude sinusoidal voltage of 10 kHz to 10 mHz (peak to zero) and 10 mV, respectively were selected for the measurements. Polarization test was carried out after 48 h immersion at scan rate and potential range of 1 mV/s and ±200 mV around corrosion potential, respectively.

3. THEORETICAL METHOD Theoretical studies based on electronic-structure quantum mechanics methods were applied to examine the polyaniline interactions with inhibitor agents and graphene oxide surface. The molecular structure of chosen inhibitors including hystamine, serotonin and quercetin are shown in Figure 3. For the case of polyaniline, its emeraldine form was considered for QM calculations as this form of polyaniline has also been utilized in our experiments. Figure 3 (d) displays the chemical structure of emeraldine form of polyaniline. Within ab initio QM calculations, tetra-aniline was applied. In addition, for the GO interactions, a graphene oxide surface with formula C62H19O6(OH)6(COOH)3 (i.e., 2 epoxy (-C-O-C-), 2 hydroxyl (-OH), and 1 carboxyl (-COOH) group per twenty C atoms) was adopted as used in our recent works 37-38

and also in other studies 39-40. Figure 3 (e) illustrates the molecular structure of GO surface. Such a structure for GO

surface represents typical GO structure resulted from standard oxidation processes

41-42

. The

oxygenated epoxy and hydroxyl groups were randomly placed above basal plane, while all 8 ACS Paragon Plus Environment

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three carboxyl fragments were connected to edge carbon atoms. These active oxygen containing functionalities were positioned on only one side of GO surface, as employed in previous theoretical efforts 43-44. Based on the pH conditions in our experiments, it is believed that the polyaniline chains become positively-charged owing to protonation of amine/imine groups, and at the same time, the GO surfaces become negatively-charged probably owing to deprotonation of carboxyl groups. Accordingly, the intermolecular interactions for inhibitor-protonated emeraldine and protonated emeraldine-deprotonated GO clusters were investigated through QM approaches. For emeraldine protonation, a proton (H+) was bonded to amine or imine N atoms. In addition to nitrogen terminated neutral/protonated emeraldine, phenyl capped neutral/protonated emeraldine trimer was also considered for inhibitor interactions. Furthermore, a GO surface carrying negative charge was obtained by removing H atoms from two carboxyl groups, resulting in a GO surface with total charge of -2 e. Figure 3 To explore the binding affinity for inhibitor-polyaniline and polyaniline-GO clusters by using ab initio QM computations, the geometries of single molecules (i.e., hystamine, serotonin and quercetin inhibitors, neutral/protonated emeraldine and GO surface) as well as their complexes (i.e., inhibitor-neutral/protonated emeraldine and neutral/protonated emeraldine-GO) were firstly optimized by means of Hartree-Fock theory with 6-31G(d,p) basis set

45

. The geometries were further relaxed applying density functional theory (DFT)

techniques 46-47 with B3LYP hybrid functional and 6-31G(d,p) basis function 48-50. These QM calculations were performed with the use of Gaussian 09 program package

51

. The final

minimum energy geometries derived from DFT studies were subsequently utilized to analyze the binding energy of the investigated inhibitor-emeraldine and emeraldine-GO clusters.



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4. RESULTS AND DISCUSSION 4.1. Characterization of GO-Pani-GI. The morphology of the graphene oxide nanosheets modified by Pani nanofibers and green inhibitors (GI) was characterized by HRTEM (Figure 4-a1 and a2) and FE-SEM (Figure 4-b1 and b2). It can be seen from HR-TEM and FE-SEM micrographs that the surface of neat GO sheets is clean and smooth but Layerby-Layer deposition of polyaniline nanofibers and GI resulted in the creation of nanosheets with rough and uneven surface. The nanofibers interaction with GO sheets can be easily seen from these micrographs. These results confirm the successful process of the layers deposition on the sheets. Figure 4 Using FT-IR analysis the chemistry of GO, GI, GO-Pani and GO-Pani-GI nanosheets was investigated. The FT-IR spectra are presented in Figure 5. According to Figure 5, the FT-IR spectra of neat GO includes six characteristic peaks at 1725 cm-1, 1415 cm-1, 1260 cm, 1625 cm-1, 3435 cm-1, and 2950 cm-1, attributed to C=O stretching of COOH group, C-O

1

stretching of COOH group, C-O stretching of epoxide group, 1625 cm-1, aromatic C=C, OH stretching of carboxylic group, and C-H stretching, respectively 29, 52-54. FT-IR results confirm the presence of hydroxyl, carboxylic and epoxide groups on the edges and basal plane of GO sheets, which is in accordance with previous reports. Characteristic peaks related to both Pani nanofibers and GO sheets can be easily observed in the FT-IR spectra of GO-Pani. The Pani deposition on the GO sheets can be understood from the observation of C=C stretching of benzenoid ring at 1559 cm-1, C=N stretching of quinoid ring at 1480 cm-1 and C-N stretching at 1300 cm-1. The mechanism of aniline polymerization on the GO sheets is described in details in literature

29, 54-58

and schematically shown in Figure 2. In brief, the epoxide group

existed on the basal plane of GO sheets is responsible for the aniline oxidation into polyaniline nanofibers. According to literature, quercetin, quinic acid, caffeic acid, hystamine 10 ACS Paragon Plus Environment

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and serotonin are some important inhibitive organic compounds existed in the extract of Urtica Dioica leaf. The presence of these compounds in GI can be understood from the observation of many vibration bands at 1092 cm-1, related to -C-O-C stretching, 1406 cm-1, attributed to COO-, 1406 cm-1, related to aromatic ring, and 3428 cm-1, assigned to NH2 groups

59-61

. From the results it is obvious that the absorption peaks related to GI and Pani

appeared in the FT-IR spectrum of GO-Pani-GI but the characteristic peaks related to GO were strongly weakened. This implies the fact that the Pani nanofibers and GI could cover the whole surface of GO sheets. These results indicate that the GO-Pani-GI sheets were successfully synthesized. Figure 5 X-ray diffraction patterns of neat GO and L-b-L assembled GO-Pani-GI were provided to characterize their crystal structures. According to Figure 6-a, for the neat GO an intensive peak (001) can be seen at 2Ө=9.8°. The amount of interlayer distance for neat GO is d = 10.45 Å, which is attributed to the presence of oxygen functionalities, i.e., epoxide and carboxylic groups, on the GO sheets 29. However, for the GO-Pani-GI sheets the diffraction peak related to GO sheets (001) significantly weakened and almost disappeared, indicating that the Pani nanofibers and GI compounds could thoroughly cover the sheets. Many sharp peaks can be seen in the XRD pattern of this sample, indicating the presence of crystalline and highly ordered Pani chains on the GO sheets. The thermal stability of the neat GO and GO-Pani-GI sheets was examined through TGA analysis and the results are compared in Figure 6-b. From these results three main weight loss steps can be seen in the thermogram of neat GO at 25 to 160 °C (step 1), 160 to 300 °C (step 2) and 200 to 600 °C (step 3), attributed to loss of physically adsorbed water and pyrolysis of the hydroxyl groups, removal of stable oxygen functionalities i.e., epoxide and carboxylic, and decomposition of the main carbon skeleton of GO, respectively. The



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remained weight loss at 600 °C is about 16.26%. On the other hand, it can be clearly noted that deposition of Pani nanofibers and GI could remarkably enhance the thermal stability of GO sheets. Up to 300 °C the weight loss is negligible, reflecting that the deposited compounds reduced the pyrolysis rate of oxygen containing groups. However, the main weight loss for this sample is seen at 300 to 600 °C, which is attributed to the decomposition of Pani nanofibers and the corrosion inhibitors adsorbed. The remained weight of 58.8% was obtained at 600 °C, suggesting much higher thermal stability of L-b-L assembled GO sheets than the neat one. Figure 6 The extracts of GO, GI and GO-Pani-GI in water were investigated by UV-visible spectroscopy. It can be seen from Figure 7a that the UV-visible spectrum of GI extract includes two peaks, a sharp peak at 207 nm related to the π-π* band absorption of the aromatic rings of the inhibitors i.e., quercetin, quinic acid, caffeic acid, hystamine and serotonin

62-64

, and a less intensive and broad peak at 250 to 350 nm, assigned to the n-π*

band absorption of the functional groups i.e., C=O 61. For the case of neat GO extract, a sharp peak at 232 nm and a broad peak at 300-350 nm are observed which are attributed to the π-π* and n-π* absorption bands of aromatic rings and functional groups (i.e., carbonyl) of GO sheets, respectively 61. The absorption peaks of both GO and GI can be seen in the UV-visible spectrum of GO-Pani-GI sheets. Two distinct and sharp peaks at 210 and 262 nm are attributed to π-π* absorption bands of the aromatic rings of GI and Pani, and GO sheets, respectively. The obtained results clearly show bathochromic shift of the absorption peak of GO from 232→262 nm, indicating the interaction of Pani nanofibers and GI compounds with the surface of GO sheets. Figure 7 4.2. Results of Theoretical Studies.


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Inhibitor-polyaniline interactions: Theoretical studies applying electronic-scale quantum mechanics techniques were carried out over experiments to get fundamental understanding regarding the interactions of inhibitor agents and GO sheets with polyaniline chains. Figure 8 presents the lowest energy structures for hystamine interacting with an emeraldine, which have been extracted from DFT computations at B3LYP/6-31G(d,p) theory level. To better analyze the inhibitor-polyaniline interactions, the interactions of both neutral and protonated emeraldine were assessed. From this figure it is evident that the hystamine inhibitor interacted with both neutral and protonated emeraldine through forming hydrogen bonds (Hbonds). In equilibrated hystamine-neutral emeraldine cluster, these H-bonds appeared between amine group in emeraldine and heterocyclic imine group in hystamine, and between imine fragment of emeraldine and heterocyclic amine or ethylamine side group of hystamine. It is also apparent that both hystamine and emeraldine can act as both hydrogen donor and hydrogen acceptor in their H-bonding interactions. In protonated emeraldine, the protonated amine and imine moieties participated in H-bonding interactions by donating their H atom to heterocyclic imine N atom of hystamine. In addition, it is noted that the length of H-bonds of protonated emeraldine is shorter than that of neutral one, implying stronger hystamine interactions with protonated emeraldine as compared with neutral emeraldine. The hystamine interactions with neutral/protonated emeraldine were quantitatively examined using binding energy (∆Ebinding) term defined as: ∆Ebinding = Ehystamine/emeraldine – (Ehystamine

+

Eemeraldine). In this equation, the Ehystamine/emeraldine is the electronic energy of

optimized hystamine-emeraldine cluster, and the Ehystamine and Eemeraldine respectively denote the electronic energy of isolated hystamine and isolated emeraldine. The computed ∆Ebinding values are presented in Figure 8 for each optimized cluster. It is visible that all predicted binding energies are negative, further implying the hystamine tendency to interact with polyaniline. Based on the obtained data, it is observed that the ∆Ebinding of hystamine



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interacted with protonated emeraldine is significantly higher than that of neutral emeraldine, which quantitatively affirms the stronger hystamine affinity to interact with protonated emeraldine. Such an observation is assigned to the stronger H-bonding (i.e., shorter H-bond length) interactions of protonated emeraldine. Besides the nitrogen terminated emeraldine, interactions of hystamine with phenyl terminated neutral/protonated emeraldine were also explored. Figure S1 in Supporting Information presents the final equilibrium geometry of hystamine interacted with phenyl capped neutral/protonated emeraldine trimer. It is obvious from this figure that the optimized geometries of different hystamine-phenyl capped emeraldine clusters and corresponding binding energies and H-bond lengths are similar to nitrogen terminated emeraldine tetramer. The theoretical results for intermolecular interactions of serotonin corrosion inhibitor with emeraldine are demonstrated in Figure 9. From these graphical results, it is noted that the functional groups in serotonin corrosion inhibiting agent (e.g., hydroxyl -OH and heterocyclic amine -NH fragments) participated in intermolecular H-bonds with amine group of neutral emeraldine. Similarly, serotonin molecule localized in vicinity of protonated emeraldine by establishing a hydrogen bond emerged between its hydroxyl group and protonated amine/imine in protonated emeraldine. The computed ∆Ebinding for all geometryoptimized serotonin-neutral/protonated emeraldine complexes are also given in Figure 9. It can be seen that ∆Ebinding values of serotonin are lower than those of hystamine, reflecting the stronger hystamine binding towards the emeraldine base polyaniline. Moreover, according to obtained H-bond length and ∆Ebinding, the strongest serotonin binding to neutral emeraldine occurred when it has approached the emeraldine imine moiety by its -OH group (Figure 9 (d)). The calculated binding energies also point to the fact that the serotonin inhibitor more strongly interacts with positively-charged emeraldine (Figure 9 (e) and (f)) than neutral emeraldine, an observation resulted from stronger H-bonding interactions with protonated


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functionalities of emeraldine. The final minimum energy configurations elucidated from electronic-structure QM calculations of different quercetin-neutral/protonated emeraldine complexes are provided in Figure 10. It is clear from this figure that similar to hystamine and serotonin inhibitors the quercetin bound to neutral emeraldine by involving in intermolecular H-bonds, which formed between carbonyl and different hydroxyl groups of quercetin and amine as well as imine sites of neutral emeraldine. These results evidence the fact that hydrogen bonding interactions play a crucial role in inhibitors adsorption onto polyaniline coated graphene oxide surface. Based on the energy-minimized clusters depicted in panels (f) and (g) in Figure 10, the affinity of quercetin compound to interact with protonated emeraldine is related to H-bond of its active carbonyl O atom with protonated N atom. Similar to hystamine and serotonin compounds, the computed ∆Ebinding values of quercetin hydrogen bonded with neutral as well as protonated emeraldine indicate its strengthened interactions with protonated emeraldine. These observations propose the beneficial role of polyaniline protonation in intensifying its interactions with green corrosion inhibitors. Polyaniline-graphene oxide interactions. Besides corrosion inhibitors, the interactions of neutral/protonated emeraldine with negatively-charged GO sheets were also assessed using QM methods. Figure 11 gives the lowest energy structures of both neutral and protonated emeraldine bound to charged GO surface. According to Figure 11 (a) it is noted that the neutral emeraldine approached the surface of negatively-charged graphene oxide by taking part in interfacial H-bond interactions of its amine moieties with surface-bound hydroxyl and epoxy oxygen atoms. The binding energy of -19.3 kcal/mol quantitatively affirms the attachment of neutral emeraldine onto GO surface containing negative charges. The minimum energy structures of protonated emeraldine adhered to GO surface are depicted in panels (b) and (c) in Figure 11. It is obvious that similar to neutral emeraldine the positively-



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charged one also adsorbed to GO surface by involving in intermolecular H-bonds with hydroxyl, epoxy and deprotonated carboxyl functionalities. Moreover, as shown in Figure 11 (b), one of the aromatic rings adopted a roughly parallel alignment relative to surface of GO due to π-π stacking interactions. Based on the computed ∆Ebinding values, it is found that the interactions of protonated emeraldine with GO surface are significantly stronger than those of neutral one, implying more intensified surface binding of protonated emeraldine. Such an observation is attributed to stronger hydrogen bonding and electrostatic interactions of positively-charged emeraldine as compared with neutral one. Figure 11 4.3. Anticorrosion Properties of L-b-L Assembled GO-Pani-GI Sheets. 4.3.1. Solution Phase Studies. The steel sheets were dipped in 3.5 wt.% NaCl solutions without and with GO-Pani-GI extract. The visual performance of the samples is depicted in Figure 12 after 4 and 24 h. In addition, the surface morphology and composition were studied by FE-SEM/EDS analyses (Figure 13). From Figure 12 it is apparent that as the immersion time elapsed the red corrosion rust covered the steel surface. However, the color of the GO-Pani-GI extract changed into green by increasing the immersion time, indicating the interaction of iron cations with inhibitive compounds as well as polyaniline nanofibers on the steel surface. To further analyze the results, FE-SEM micrographs were provided from the steel surface during different immersion times. It can be seen from the FE-SEM micrographs that the surface of the sample immersed in neat NaCl solution includes etched and degraded areas but a film covered the surface of the sample dipped in the solution containing GO-Pani-GI extract. At low magnification the morphology of the film is not clear but at high magnifications the flake and fiber like morphologies can be seen. The morphology of the fibers created on the steel surface is similar to the structure of Pani nanofibers. In fact, the released aniline oligomers in the extract deposited in the form of polyaniline salt. The


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aniline oligomers conversion into polyaniline fibers can be also found from the extract color changes into green 29. Combination of polyaniline nanofibers and green corrosion inhibitors can be deposited on the steel surface. From Table 2 it can be seen that the film deposited on the steel surface showed high amount of carbon and nitrogen. These are the elements existed in the green corrosion inhibitors and polyaniline structure. To confirm the film deposited on the steel surface in the extract of GO-Pani-GI the FT-IR analysis was carried out and compared with FT-IR results of GO-Pani and GI powders. According to Figure 14, the FTIR spectrum of the film deposited on the steel surface is similar to the FT-IR spectra of both GO-Pani and GI. The absorption peak related to C-H stretching can be seen at 2825-3050 cm1

in the FT-IR spectra of GO-Pani and the film deposited on the steel surface. The C=C

stretching of benzenoid ring at 1652 cm-1, C=N stretching of quinoid ring at 1475 cm-1 and CN stretching at 1291 cm-1 can be seen in the FT-IR spectrum of the film deposited on the steel surface

29, 52-56

. These results show that the film precipitated includes chemical structure

similar to polyaniline. This observation is in accordance with color changes of the extract to green after immersion of steel specimen in the extract of GO-Pani-GI and fiber like morphology of deposited film observed by FE-SEM analysis. It can be clearly seen that the FT-IR spectrum of the film deposited on the steel surface is very similar to the FT-IR spectrum of GI powder. This means that the inhibitors existed in GI i.e., Urtica Dioica leaf are quercetin, quinic acid, caffeic acid, hystamine and serotonin, successfully adsorbed on the steel surface. Figure 12 Figure 13 Figure 14 Electrochemical impedance spectroscopy was utilized for characterization of the corrosion inhibition of steel sheets immersed in the extract of GO-Pani-GI. The Nyquist



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diagrams of the samples dipped in the 3.5 wt.% NaCl solutions with and without GO-Pani-GI extract after various immersion times are shown in Figure 15. It should be noted that due to the good corrosion inhibition effect of steel sheets in the extract of GO-Pani-GI the test was continued up to 168 h. From Figure 15 it can be seen that the Nyquist diagrams of the sample dipped in 3.5 wt.% NaCl solution without GO-Pani-GI extract includes only one relaxation time, indicating that the corrosion of this sample is under charge transfer control. Utilizing proper electrical circuits the impedance data were analyzed and the electrochemical parameters including solution resistance, Rs, coating resistance, Rc, charge transfer resistance, Rct, non-ideal capacitance of coating, CPEc, and non-ideal capacitance of double layer, CPEdl were obtained. It can be seen from Figure 15 that the diameter of the semicircle decreases with increasing the immersion time for the sample dipped in the solution without extract but an ascending trend can be seen for the semicircle diameter of the sample dipped in the extract of GO-Pani-GI. This means that the corrosion action noticeably decreases with increasing the immersion time. Figure 15 and Table 2 show significant increase of Rct and Rc values by increasing the immersion time up to 168 h for the sample dipped in the extract of GO-PaniGI. Also, the increased of phase angle up to -20° can be seen at 4 h immersion. The Bode phase plots became broader by increasing the immersion time, declaring a protective film formation on the steel surface. These observations clearly indicate that as the immersion time elapses the protective film thickness increases, resulting in greater corrosion resistance for mild steel. The super-corrosion inhibition performance of GO-Pani-GI can be understood from these results. These observations are in accordance with the FE-SEM and FT-IR results. The inhibitors existed in the GO-Pani-GI extract could adsorb on the active sites of metal and inhibit the corrosion reactions through film formation and preventing the access of the corrosive agents to the active sites. Figure 15


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Table 2 The corrosion protection mechanism of steel samples dipped in the extract of GO-PaniGI was characterized by polarization test. The experiment was done after 48 h immersion on the steel samples dipped in the 3.5 wt.% solutions without and with GO-Pani-GI extract. The polarization plots and an example of Tafel extrapolation of polarization curve are given in Figure 16. Table 3 shows the parameters obtained from Tafel extrapolation including corrosion current density, icorr, corrosion potential, Ecorr, anodic Tafel slope, βa, and cathodic Tafel slope, βc. It can be seen from Figure 16 that in the presence of GO-Pani-GI extract both anodic and cathodic branches shifted towards lower current densities, indicating a mixed mode of corrosion inhibition of the GO-Pani-GI extract. In the presence of GO-Pani-GI extract the icorr significantly decreased and Ecorr slightly shifted to more positive values (anodic direction). Both anodic and cathodic Tafel slope values decreased in the presence of GO-Pani-GI extract. Although a mixed type of inhibition was observed for the GO-Pani-GI extract but the lower reduction in anodic current density and the shift of Ecorr to more positive values compared to the Ecorr of the sample dipped in the neat solution indicate that the corrosion inhibitors retarded the anodic reaction more than cathodic one. Figure 16 Table 3 In Figure 17, the adsorption mechanism of the inhibitors released from GO-Pani-GI on the steel surface is schematically shown. There are many kinds of inhibitive compounds in the extract of GI i.e., quercetin, quinic acid, caffeic acid, hystamine and serotonin 62-64. These are effective corrosion inhibitors as there are many heteroatoms like oxygen and nitrogen in their structures. These heteroatoms can interact with metal surface through complex formation with iron cations. N and O share the lone pair of electrons with metal ions i.e., Fe2+. In this way a complex between Fe2+-GI can be formed on the metal surface. This form



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of interaction mostly occurs on the anodic sites. In addition, the corrosion inhibitors can be physically adsorbed on the metal surface through π-π interactions. On the other hand, the Pani nanofibers can be adsorbed on the cathodic region through electrostatic interactions. The form of Pani deposited on the GO sheets is Emeraldin base (EB) which has positive sites that can interact with negative sites of cathodic regions. Deposition of Pani fibers on the metal surface could produce a dense and highly protective coating. It has been also mentioned in the previous reports that in the presence of Pani the oxide passive layer, i.e., Fe2O3, formation on the steel surface, mostly on the anodic sites, can be intensified

24-28

. This results in the

steel surface protection from corrosion. These show that the GO-Pani-GI is an effective anticorrosion system with multifunctional properties. Figure 17

5. CONCLUSIONS Fabrication of an advance anticorrosion system based on GO-Pani-GI through L-b-L assembly approach was considered and investigated through both experimental and computational studies. The main conclusions are listed below: • Experimental results from HR-TEM, TGA, FT-IR and UV-Visible analyses confirmed the sequential adsorption of charged components on the GO sheets. The highly crystalline polyaniline nanofibers were successfully synthesized on the GO sheets through an inventive method and then the GIs adsorbed on the GO-Pani sheets through interaction with Pani nanofibers. • The computational QM modeling results proved that the positively-charged Pani is able to strongly adsorb onto GO surfaces via physisorption (H-bonding and π-π stacking) mechanism. Computations also clarified that all GIs stabilized near protonated Pani through intermolecular H-bond interactions. 20 ACS Paragon Plus Environment

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• The GO-Pani-GI nanosheets demonstrated effective corrosion inhibition performance in saline solution on mild steel. The GO-Pani-GI sheets could inhibit the steel substrate from corrosion through multiple ways. Adsorption of GI molecules on the anodic and/or cathodic sites, deposition of Pani fibers on the steel sheets and the steel surface passivation in the presence of Pani are the main mechanism of corrosion inhibition of GO-Pani-GI system. Deposition of these compounds on the steel surface was demonstrated by FT-IR and FESEM/EDS analyses.

SUPPORTING INFORMATION The equilibrium geometries of different hystamine-phenyl capped neutral/protonated emeraldine clusters and their corresponding binding energies. Acknowledgment The authors gratefully thank the use of School of Computer Science, Institute for Research in Fundamental Science (IPM) as the computations were done there.



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A Close-up of the Effect of Iron Oxide Type on the Interfacial Interaction between Epoxy and Carbon Steel: Combined Molecular Dynamics Simulations and Quantum Mechanics. J. Phys. Chem. C 2016, 120, 11014-11026.


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63.

Nasibi, M.; Mohammady, M.; Ghasemi, E.; Ashrafi, A.; Zaarei, D.; Rashed, G.

Corrosion Inhibition of Mild Steel by Nettle (Urtica Dioica L.) Extract: Polarization, Eis, Afm, Sem and Eds Studies. J. Adhes. Sci. Technol. 2013, 27, 1873-1885. 64.

Salehi, E.; Naderi, R.; Ramezanzadeh, B. Synthesis and Characterization of an

Effective Organic/Inorganic Hybrid Green Corrosion Inhibitive Complex Based on Zinc Acetate/Urtica Dioica. Appl. Surf. Sci. 2017, 396, 1499-1514.



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FIGURES O

(a1)

OH

HO

O

OH O O

OH O O

Modified Hummers

O OH

OH OH

O

Graphene oxide

Graphite

+

NH3

O

O

H3N

-

O

+

-

O

O

+

O

-

O

-

O

NH 3

O

-

O H3N

O

O

+

O

-

-

O

O

O

O

-

O

APS

O

O

O NH3O +

-

O

-H 3N

+

O

24 h, 25 °C

O OH

-

O

-

O

-

OH

3N

+

OH

HO

O

O

-

HO

GO+Aniline

Emarldin salt

HN N

HN

HN

HN

N

N

N HN

NH 2

N

(a2) NH

N

NH2

HO

O O O

O

O

-

O

-

HO

NH2

NH2

O

O

NH2

O

NH2

-

O

NH2

H2N

HO

NH2

O

HO

-

NH2

ONH2

O

NH2

H N

NH

N H

-

OH

O O

OH HO

O

OH

HO OH

HO

HO OH

N H

N H

N H

O

Inhibitors adsorption

Figure 1- Schematic illustration of fabrication of (a1) GO and (a2) GO-Pani-GI Ns.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


+

O

O

+

NH 3

O

H 3N

-

O

-

+

NH 3

O

O

-

O

-

O

O

O

-

-

O O

+

H 3N

O

O

O

-

O

-

O

O

(a1)

O

O

O

O NH3O +

-

O OH

-H3N

+

O

O

-

OH N+

-

O OH

O

-

O

-

3

HO

HO

Polymerization of aniline into polyaniline HN H N

N

NH 2

HN N

(a2)

O

O

H2N

-

O

-

N

HO

O

NH 2

O

-

NH

H N

NH2

N H OH

O

NH2

HN

N

H2N

O

O N

OH

N

HO

O O

O

OH

H N

OH

O O

-

O

-

H2N

N

O

Inhibitors adsorption

H N

NH2 N

OH

O

HN

HO

-

H2N

O N

NH2

Figure 2- Schematic illustration of fabrication of (a1) GO-Pani and (a2) GO-Pani-GI Ns



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(a)

N

H H N

CH2 H

C

CH2 C

N

HO

N H

NH

OH

HO

H

(d)

O

CH2

C N

OH

OH

CH2

H

(c)

HO

(b)

H H

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O

NH

N

N n

(e)

Figure 3- Molecular structure of hystamine (a), serotonin (b), quercetin (c), emeraldine type polyaniline (d), and the QM model considered for graphene oxide (e). Atoms in graphene oxide are shown in Stick style.


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(a1)


(a1)

(b1) (b1)

(a2)

(b2)

Figure 4- (a1 and a2) HR-TEM and (b1 and b2) FE-SEM micrographs (scale bar = 20 µm) of (a1, b1) GO-Pani-GI Ns and Neat GO (a2, b2)



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Figure 5- FT-IR spectra of Neat GO, green inhibitor, GI, graphene oxide modified with Pani, GO-Pani, and graphene oxide modified with Pani and then green inhibitor, GO-Pani-GI


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GO (001)

(a)

1080 1040 1000

Absolute intensity

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5

15

GO-Pani-GI Ns 5

15

25

35

45

55

65

2theta (◦) (b)

Figure 6- (a) XRD patterns and (b) TGA thermograms of Neat GO and graphene oxide modified with Pani and then green inhibitor, GO-Pani-GI



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Figure 7- UV-visible spectra of the extracts of (a1) GI, (a2) GO-Pani-GI Ns and (a3) neat GO Ns.


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(a)

(b)

∆Ebinding=-9.9

∆Ebinding=-9.3 2.01

2.03

(c)

(d) ∆Ebinding=-11.3


1.93

(e)

(f) ∆Ebinding=-28.4 2.28

1.56

∆Ebinding=-4.5

(g) 1.79

∆Ebinding=-22.0

Figure 8- The B3LYP/6-31G(d,p) optimized hystamine-neutral/protonated tetra-aniline clusters. The calculated binding energies are in kcal/mol, and the H-bonds are in angstrom (Å). The protonated amine/imine groups in tetra-aniline are encircled by dashed red color.



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(a)

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(b) ∆Ebinding=-4.1

∆Ebinding=-4.9 2.14 1.98

(c)

(d)


(e)

∆Ebinding=-5.2

1.87

(f) 1.83 1.73

∆Ebinding=-15.8

∆Ebinding=-18.3 Figure 9- The B3LYP/6-31G(d,p) optimized serotonin-neutral/protonated tetra-aniline clusters. The calculated binding energies are in kcal/mol, and the H-bonds are in angstrom (Å). The protonated amine/imine groups in tetra-aniline are encircled by dashed red color.


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(a)

(b) ∆Ebinding=-6.3

∆Ebinding=-6.4 1.99 2.01

1.97

(c)

(d)

∆Ebinding=-11.8

∆Ebinding=-7.8

1.77

1.78

(e)

(f)

∆Ebinding=-15.6

∆Ebinding=-7.7

1.64 1.88

(g)

1.74

∆Ebinding=-10.8

Figure 10- The B3LYP/6-31G(d,p) optimized quercetin-neutral/protonated tetra-aniline clusters. The calculated binding energies are in kcal/mol, and the H-bonds are in angstrom (Å). The protonated amine/imine groups in tetra-aniline are encircled by dashed red color. 39 ACS Paragon Plus Environment


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(a)

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(b)

2.23

2.36

∆Ebinding=-107.0

2.14

∆Ebinding=-19.3 1.81 1.78

(c)

∆Ebinding=-104.7

1.90

2.42

2.15

Figure 11- The B3LYP/6-31G(d,p) optimized (a) neutral tetra-aniline and (b) and (c) protonated tetra-aniline with negatively-charged GO surface. The calculated binding energies are in kcal/mol, and the H-bonds are in angstrom (Å). The protonated amine/imine groups in tetra-aniline are encircled by dashed red color.


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4h

24 h

(a1)

(a2)

(b1)

(b2)

Figure 12- Visual images from the surface of steel panels dipped in the 3.5 wt.% NaCl solutions (a1 and a2) without and (b1 and b2) with extract of L-b-L assembled GO-Pani-GI Ns for 72 h.



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(a1)

(b1)

(a2)

(b2)

(a3)

(b3)

(a4)

(b4)


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Figure 13- SEM micrographs from the surface of steel panels dipped in the 3.5 wt.% NaCl solutions (b1, b2, b3, b4) without and (a1, a2, a3, a4) with extract of L-b-L assembled GO-PaniGI Ns for 72 h; a1 and b1 (scale bar = 30 µm), a2 and b2 (scale bar = 20 µm), a3 and b3 (scale bar = 10 µm) and a4 and b4 (scale bar = 2 µm).

Figure 14- FT-IR spectra of (a1) GI powder, (a2) the film deposited on the mild steel dipped in the extract of GO-Pani-GI for 168 h, (a3) the powder of GO-Pani



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Figure 15- Nyquist diagrams for the steel sheets (1 cm2) immersed in 3.5 wt. % NaCl solutions without (a1) and (a2 and a3) with GO-Pani-GI Ns extract for various immersion times; Solid lines and marker points represent the ﬁtted and experimental data, respectively. 44 ACS Paragon Plus Environment

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Figure 16- (a1) polarization plots for the steel sheets (1 cm2) immersed in the 3.5 wt. % NaCl solutions without and with GO-Pani-GI Ns extract after 48 h; (a2) Typical Tafel extrapolated plot for extracting electrochemical parameters.



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Figure 17- Schematic adsorption mechanism of the inhibitors released from hybrid pigment on the mild steel surface dipped in 3.5 wt.% NaCl solution containing hybrid pigment extract.


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Table 1- EDS Results Obtained from Analyzing the Steel Surface after Immersion in 3.5 wt.% NaCl Solutions without and with GO-Pani-GI Extract Sample Fe (wt.%) O (wt.%) N (wt.%) C (wt.%) NaCl solution

93.1

5.8

0.5

0.6

GO-Pani-GI extract

74.2

16.1

4.5

5.2



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Page 48 of 50

Table 2- Electrochemical Parameters Extracted from Nyquist and Bode Plots through Modeling Impedance Data by Proper Electrical Equivalent Circuits, the Steel Panels (1 Cm2) Were Dipped in Test Solutions for Different Immersion Times, (±) Shows the Standard Deviation of Three Measurements Sample

Time (h)

GO-Pani-GI extract

NaCl solution

Rc (ohm.cm2)

2 4 24 48 72 168

10.97 ± 4.0 12.34 ± 8.0 1567 ± 124 2459 ± 78 3541 ± 234 4061 ± 325

-

Y0c (µS.sn.cm-2)

2 4 24 48 72 168

1043 ± 45 1025 ± 14 756 ± 23 102 ± 12 98 ± 5.0 74 ± 6.0

-

2

0.89 ± 0.02

-

4 24 48

0.88 ± 0.01 0.88 ± 0.03 0.84 ± 0.03

-

72 168

0.87 ± 0.01 0.86 ± 0.02

-

2 4 24 48 72 168

2.8 ± 0.15 3.7 ± 0.11 4.0 ± 0.23 7.7 ± 0.32 9.4 ± 0.45 14.0 ± 0.14

2.65±0.3 1.42±0.2 1.29±0.2 -

2 4 24 48 72 168

0.85 ± 0.02 0.80 ± 0.02 0.78 ± 0.01 0.83 ± 0.02 0.84 ± 0.03 0.86 ± 0.01

0.88 0.86 0.83

2 4 24 48 72 168

138.0 ± 13 95.8 ± 5.0 89.2 ± 6.0 75.9 ± 4.0 67.8 ± 5.0 55.3 ± 3.0

77 ± 86 710 ± 56 790 ± 41 -

nc

Rct ( kohm.cm2)

ndl

Y0ct (µS.sn.cm-2)


-

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Table 3- Electrochemical Parameters Obtained from Polarization Curves for the Steel Samples (1 Cm2) Immersed In 3.5 Wt.% NaCl Solutions without and with GO-Pani-GI Extract for 48 H; The Values Are the Mean of Three Replicates and (±) Corresponds to

icorr (µA/cm2)

βa(V/dec)

−βc (V/dec)

3.5 wt.% NaCl solution

Ecorr v.s. SCE (mV) -674.0 ± 4

11.5 ± 1.3

0.12 ± 0.02

0.43 ± 0.03

GO-Pani-GI extract

-695.0 ± 6

2.6 ± 0.2

0.08 ± 0.01

0.36 ± 0.02

Sample

the Standard Deviations



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TOC


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Fabrication of a Highly Tunable Graphene Oxide Composite through

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