Novel Synthesis, Characterization, and Corrosion Inhibition Properties

Oct 15, 2010 - Colombia, Barranquilla, Colombia. ReceiVed: July 9, 2010; ReVised Manuscript ReceiVed: September 10, 2010. Functional and composite ...
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J. Phys. Chem. C 2010, 114, 18797–18804

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Novel Synthesis, Characterization, and Corrosion Inhibition Properties of Nanodiamond-Polyaniline Films Humberto Gomez,†,‡ Manoj K. Ram,*,†,§ Farah Alvi,§ Elias Stefanakos,| and Ashok Kumar†,§ Department of Mechanical Engineering, 4202 East Fowler AVenue, ENB 118, Nanotechnology Research and Education Center, and Clean Energy Research Center, UniVersity of South Florida, Tampa, Florida 33620, United States, and UniVersidad del Norte, Departamento de Ingenieria Mecanica, KM 5 Via Pto. Colombia, Barranquilla, Colombia ReceiVed: July 9, 2010; ReVised Manuscript ReceiVed: September 10, 2010

Functional and composite nanodiamonds (NDs) are rapidly emerging as promising materials for the next generation for quantum information processing, electronic material, magnetotometry, novel imaging, and IR fluorescence applications, etc. Nanocomposites of ND particles with conductive polymers [i.e., polyaniline (PANI)] displayed novel properties resulting from the molecular level interaction of diamond with PANI molecules. We have synthesized for the first time the ND-PANI nanocomposites by oxidative polymerization of aniline using ammonium peroxydisulfate [(NH4)2S2O8)] under controlled conditions. The ND-PANI nanocomposite films were characterized by UV-vis, FTIR, electrochemistry, impedance, scanning electron microscope, transmission electron microscope, and electrical conductivity techniques. Current-voltage characteristics of ND-PANI nanocomposite show the ohmic junction. The electrochemical investigation on ND-PANI revealed the wider potential values with independent redox characteristics of PANI and ND. There is an interaction of the free electron pairs of the nitrogen atoms of the PANI with a charged molecule on the surface of ND with PANI. The concept of using an active electronic barrier ND-PANI barrier built-in electric field is created at the metal surface. The NDPANI takes advantage of the excellent corrosion inhibitor characteristics of steel and aluminum due to its chain conformation and electronic properties, as demonstrated in this work. Introduction Nanodiamond (NDs) particle have gained worldwide attention due to their inexpensive large scale synthesis with narrow size (4-5 nm) distribution, facile surface functionalization, biocompatibility, quantum information processing, magnetotometry, novel imaging, and IR fluorescence applications.1-3 It is anticipated that attractive properties of NDs could be exploited for the development of new biocompatible material (for intracellular imaging and drug delivery) and devices (nonphotobleaching fluorescence).4-7 More recently, undoped ND powders consisting of diamond nanoparticles of 5 nmhavebeenincorporatedintoelectrodesforbiosensorapplications.8,9 The nanocomposite materials have also received much attention as a new form of support for electrocatalysts and biosensor, sensor, and thermal applications because of their high accessible surface area, low resistance, and high stability. Recently, Ram et al. have developed metal oxide and conducting polymer nanocomposites of titanium oxide (TiO2)-polyaniline (PANI), TiO2-polypyrrole, SnO2-polyhexylthiophene, TiO2-poly(thiophene-aniline) nanocomposites, Mn-ferrite-PANI, multiwalled carbon nanotube-poly(oanisidine), etc. using wet chemistry and have been extensively used in gas sensor and molecular electronic applications.10-17 It is known that PANI is one of the most studied materials due to its unique electrochemical, chemical, and physical properties as well as high electrical conductivity and good environmental stability in doped and pristine (undoped) states.18,19 The electronic, optical, photoelectrochemical, photoconductivity, photovoltaic, thermal, sensing * To whom correspondence should be addressed. Tel: 813-507-1571. Fax: 813-974-2353. E-mail: [email protected]. † Department of Mechanical Engineering, University of South Florida. ‡ Universidad del Norte. § Nanotechnology Research and Education Center, University of South Florida. | Clean Energy Research Center, University of South Florida.

(e.g., gas sensor, biosensor etc.), and corrosion inhibition properties of PANI conducting polymer could be improved by combining PANI with polystyrene latex, multiwalled carbon nanotubes, singlewalled nanotubes, montmorillonite, graphite and MCM-41, TiO2, and SnO2 microparticles and other nanoparticles.20-26 PANI composites possess a variety of unique properties such as electrical, mechanical, and structural properties because of the combined effect due to the close incorporation between PANI with inorganic or organic components at the molecular and atomic levels. In this work, we have extended the method to the preparation of ND-PANI nanocomposite films. Furthermore, nanocomposites of inorganic semiconductors with conductive polymers can display novel properties resulting from the molecular level interaction of the two dissimilar chemical components. The choice of NDs and PANI was motivated by a wide range of technological applications of both components, that is, transparent electrodes for photovoltaic, solar cell, and electroluminescent corrosion inhibition films. At large, our approach focuses on the fabrication, characterization, and application of ND-PANI nanocomposite film ionic flowsthereby blocking the metal dissolution and providing excellent corrosion protection. The NDs have shown stability in higher potential regions. It has been known that Fe3O4 is the passivating layer composition formed by PANI. So, PANI coatings on active metals like steel provide anodic protection, act as electronic, chemical, and physical barriers to inhibit anodic reaction, and maintain high resistance of the working electrode. Reagent and Materials Aniline (99.5%), ammonium persulfate (98%), sodium hydroxide (powder, 97%), and hydrochloric acid (37%) were all ACS grade and purchased from Sigma-Aldrich (United States). N,N-Dimethylformamide (99.8%) was purchased from Alfa

10.1021/jp106379e  2010 American Chemical Society Published on Web 10/15/2010

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SCHEME 1: Representation of ND-PANI Synthesis and Possible Nanostructures

TABLE 1: Composition of Aniline and ND in Synthesis of ND-PANI ND-PANI1 ND-PANI2 ND-PANI3

aniline (mL)

ND (g)

ND:aniline ratio

9.11 9.11 9.11

0.047 0.47 0.92

0.0051 0.051 0.101

Aesar (United States), and diamond nanoparticles (99%, diameter 5-10 nm) were purchased from International Technology Center. All of these chemicals and materials were employed as purchased without any further purification unless specified. Synthesis and Characterization of the PANI-NDs Nanocomposite Conducting Polymer The ND-PANI nanocomposite was chemically synthesized (Scheme 1) by oxidative polymerization of aniline using ammonium peroxydisulfate [(NH4)2S2O8)] under controlled conditions.27,28 The aniline to NDs ratios were kept at 1:1, 3:1, and 1:3 (Table 1) and were added in 1 M HCl solution, where the solution was cooled to 4 °C in an ice bath. Ammonium peroxydisulfate (0.025M) was also dissolved in 200 mL of 1 M HCl solution, and the solution was precooled to 4 °C. Later, [(NH4)2S2O8)] in 1 M HCl solution was added slowly in the aniline solution, and the reaction was continued for 12 h. The dark green precipitate of the ND-PANI nanocomposites recovered from the reaction vessel was filtered and washed using deionized water, methanol, acetone, and diethyl ether for the elimination of the low molecular weight polymer as well as oligomers. Furthermore, this precipitate was heated at 100 °C in a temperature-controlled oven. Such an emeraldine salt form of ND-PANI was subsequently treated by using aqueous ammonia for 24 h. Then, it was washed by using distilled water and acetone several times and dried for 6 h at 100 °C. The dark blue powder thus obtained was the emeraldine base form of the PANI-NDs conducting polymer. The emeraldine base was used for the fabrication of spin coat and solution cast films using the solubility of ND-PANI nanocomposite in dimethylformamide solvent. The ND-PANI films of various thicknesses were fabricated using spin coat and solution cast techniques and then dried at 100 °C. Fourier transform infrared (FTIR), Raman, and UV-vis optical studies on diamond-PANI conducting polymers is a preferred tool to derive information regarding the vibrational bands, Raman shift, and absorption bands of inter- and/or intragap states. Doped (dipping in 1 M HCl) and undoped vibrational bands of ND-PANI nanocomposite deposited on [100] n type silicon single crystals were measured by a FTIR spectrophotometer (Perkin-Elmer spectrum 1). The sample chamber of the spectrophotometer was continuously purged with

nitrogen gas for 10-15 min before the data collection as well as during the measurements for the elimination of the water vapor absorption. For each sample, eight interferograms were recorded, averaged, and Fourier-transformed to produce a spectrum with a nominal resolution of 4 cm. FTIR spectra of ND-PANI nanocomposite films were obtained after proper subtraction from substrate silicon baseline. UV-vis spectra of PANI-NDs and NDs film deposited of glass and quartz were made using a Jasco V-530 UV-vis Spectrometer. Currentvoltage (I-V) characteristics characterization was performed using a Keithley electrometer (model 6517) as well as an operational amplifier in the inverting configuration. The film was deposited on a glass plate. Thin silver wires were connected to the ND-PANI nanocomposite film using silver paste. The distance between the two electrodes was around 2 mm. A positive temperature coefficient study of ND-PANI-polystyrene film was performed as a function of temperature. The film was heated in a controlled condition and I-V characteristics were measured. The morphology and size of the NDs and ND-PANI films were investigated using high-resolution transmission electron microscopy (HRTEM). The corrosion set up consisted of an electrochemical cell consisting of three electrodes. Experimental ND-PANI coatings were deposited on steel from an NMP, DMSO acid solution. The corrosion electrolytes were 1 M NaCl, 0.1 M HCl, and 0.1 M H2SO4. All studies were done at room temperature. The steel was doubly rinsed prior to electrochemical studies. The electrochemical cell contained steel coated over the metal surface on both sides, platinum as a counterelectrode, and Ag/Ag/Cl as a reference electrode. The over potential, Tafel, impedance, and CV on the films were studied in different electrolytes. Results and Discussion Depending on the origin, the ND surface is often rich in various functional groups.29 The FTIR spectra of ND show the IR bands at 3415 (NH groups), 2089, 1730 (CdO), 1652 (C-O-O-H), 1103, and 620 cm-1. The FTIR spectra of ND and ND-PANI composite films (Figure 1a) at various ND-aniline ratios, described in Table 1, have been studied on films deposited on silicon substrates using N,N-dimethylformamide solution. We can see the bands at 3239, 1588, 1506, 1308, 1149, 825, and 507 cm-1 for ND-PANI-1, whereas ND-PANI-2 shows bands at 3241, 1588, 1504, 1308, 1148, 825, and 507 cm-1. NDPANI-3 shows the bands at 3235, 2921, 2849, 1596, 1505, 1310, 1168, 1146, 827, 732, and 505 cm-1. The ND-PANI-3 contains the larger ratio of ND than aniline molecule, indicating that the band at 1600 cm-1 is due to carbon-quinoid-carbon (C-Q-O) stretch functionalities. This was partly attributed to the formation of hydrogen bonds between surface carboxyl groups, water, and

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Figure 1. (a) FTIR spectra of undoped (1) PANI, (2) ND-PANI-1, (3) ND-PANI-2, and (4) ND-PANI-3. (b) FTIR spectra of HCl-doped (1) PANI, (2) ND-PANI-1, (3) ND-PANI-2, (4) ND-PANI-3, and (5) ND.

carboxyl groups of neighboring ND particles. However, the shift in stretching frequency for C-Q-O groups could not be explained solely by hydrogen bond formation but must be due to other types of dipolar interactions, for example, lateral interactions between the functional groups on the surface.30 The spectra shown in Figure 1b are characteristics of the doped PANI emeraldine salt with a large double peak observed at 1306 and 1320 cm-1 commonly associated with the stretching mode of the radical action (-CN+•). The band observed at 1150 cm-1 corresponds to two peaks not clearly separated, the C-H stretch of the quinoid ring at 1163 cm-1 and the C-H stretch of the benzoid ring band observed at 1178 cm-1. The band observed at 1576 cm-1 can be ascribed to the stretching mode of the C-C bond in the quinoid ring with a small shoulder at 1603 cm-1 assigned to the stretching mode of the benzoid ring (C-C bond) observed at 1610 cm-1. There are few shifts in the wave numbers while going in the higher ND ratio than aniline in NDPANI-3 systems. In addition, for all samples, the bands are located at around 3240-3270 cm-1 due to N-H bond stretching. The broad absorption peak at a broad absorption band between 3700 and 3000 cm-1 is from the OH and NH groups due to the presence of diamond. Figure 2a,b shows the Raman spectroscopy of the ND and ND-PANI-1, ND-PANI-2, and ND-PANI-3 doped and undoped nanocomposite systems on silicon substrate. The Raman spec-

troscopy technique has been used extensively to study diamond materials, as it is highly sensitive to the different types of carbon bonding. It has approximately 50 times greater sensitivity toward sp2 than sp3 carbon bonding, and so, it is able to detect even small amounts of nondiamond carbon impurity. Raman spectroscopy shows bands of ND at 1982, 1700, 1627, 1484, 1339.3, 1190, and 1071 cm-1 and is a complementary technique to FTIR for the identification of vibrational modes at the surface of the ND. A typical Raman spectrum as-received ND powder is shown in Figure 2a,b, curve 1, which has the characteristic band at 1338 cm-1. The broad, asymmetric bands at 1600-1700 cm-1 are very characteristic of nanocrystalline diamond. The water peaks are predominant using FTIR method than Raman studies, but the Raman peak shifts 1623 cm-1, which shows the O-H bending vibrations in ND particles due to the confinement of water molecules.31 Table 2 shows the Raman shifts of ND-PANI (1, 2, and 3). The Raman spectrum shows an interesting trend in Figure 2b (curve 2-4) where the ND concentration increases. Raman spectra are progressive, which implies that at the ND-PANI in doped states is in either a bipolaronic form or a polaron lattice, isolated polarons as well as partially charged C-N sites or “CdN” sites with a bond order between 1 and 2 and 3 NDPANI films. One can therefore expect a change in the “polaron band” wavenumber according to whether the maximum or only a partial charge is localized on N, as well as a change in the

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Figure 2. (a) Raman spectra of undoped (1) ND, (2) PANI, (3) PANI-1, (4) ND-PANI-2, and (5) ND-PANI-3. (b) Raman spectra of doped (1) ND, (2) PANI, (3) PANI-1, (4) ND-PANI-2, and (5) ND-PANI-3.

TABLE 2: Raman Peaks (cm-1) of Nanocomposite ND-PANI ND-PANI1 ND-PANI2 ND-PANI3 1182

1171

1166

1228

1228 1255 1314 1409 1484.36 1563 1617

1222 1272 1330 1416 1484 1560 1612

1314 1480 1564 1618

the bands increase and decrease δ(CH)Q due to the presence of diamond ν(C-N) δ(ring) ν(C-N+) + Q increase; ν(CdN) + Q δ(N-H) ν(CdC)

imine band wavenumber in function of the charge on the nitrogen. We are therefore interested in two bands at 1330 and 1490 cm-1, which are characteristic of the -C-N+- and CdN stretchings in function of the charge localization in the doped form of PANI.32-35 The UV-vis absorption spectra of ND and the undoped forms of PANI, ND-PANI-1, ND-PANI-2, and ND-PANI-3 nanocomposite films are shown Figure 3a,b. The absorption spectrum of the ND indicates that the onset of the absorption edge is about 330 nm. In Figure 3a, the onset of the absorption at 330 nm corresponds to the electron jump from the valence band to the π-π* conduction band. Next, the UV-visible spectra of exhibited bands at 620 nm are due to the n-π* transitions. There is a marked difference in the undoped sample of ND-PANI-1 and NDPANI-2, which shows an absorption band at around 420-430 nm, indicating the polaron states. The band observed at 450 nm is due to the polaronic defect state emanating due to diamond conjugated with PANI film. ND-PANI-1 and NDPANI-2 show the bands (curves 3 and 4) in Figure 3a, whereas the larger ratio containing diamond does not show the polaronic state in an undoped ND-PANI nanocomposite film. Figure 3b (curves 2-5) shows the absorption bands at 370 and 420 nm

Figure 3. (a) UV-vis spectra of undoped (1) ND, (2) PANI, (3) NDPANI-1, (4) ND-PANI-2, and (5) ND-PANI-3. (b) UV-vis spectra of doped (1) ND, (2) PANI, (3) ND-PANI-1, (4) ND-PANI-2, and (5) ND-PANI-3.

and the broad absorption band at 700-1000 nm. The 700-1000 nm bands are due to the protonation of PANI (polaron and bipolaron), indicative of the conducting state. The ND-PANI films show the shift of the dopant peak at 370 nm instead of

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Figure 4. TEM image revealing the morphology of the (a) ND, (b) ND-PANI-1, (c) ND-PANI-2, and (d) ND-PANI-3 thin films.

around 330 nm due to the presence of ND nanoparticles in PANI. Figure 3b reveals the interesting observation where PANI (curve 2) shows the polaronic peak due to the doping using one HCl, whereas the ND-PANI does not show any polaronic peak. The ND plays an important role in electronic transition in nanocomposite materials. The nanostructural characterization of the nanocrystalline films has therefore been employed here to elucidate film morphology using TEM. TEM micrographs of the ND, NDPANI-1, ND-PANI-2, and ND-PANI-3 nanocomposite films are shown in Figure 4a-d, respectively. Figure 4a shows a TEM image of a ND film, from which the measured average crystallite size was found to be between 5 and 10 nm. Figure 4b is a brightfield transmission electron micrograph of a film with a quite uniform distribution of conglomerates made up of diamond nanoparticles with PANI conducting polymer. Although lattice images reveal individual crystallites at atomic resolution, further TEM analysis of the ND-PANI-1 evidenced that the ND nanoparticles are encapsulated within the PANI polymeric structures. The ND-PANI-1 particle size 20-100 nm is larger than the ND nanoparticles precursor, indicating that polymerization occurs by aggregation of few nanoparticles of ND. Moreover, we have reported the TEM images of the ND-PANI film structure, and they are shown in Figure 4b-d. The ND containing the highest ratio in Figure 4d shows the domination

of ND in the ND-PANI-3 films, whereas ND-PANI-1 and NDPANI-2 show the more compact structure due to the larger aggregation emanating due to the presence of PANI in the nanocomposite structure. Figure 5 shows a HRTEM of NDPANI-3 nanocomposite where aggregated individual ND particles are characterized. Cyclic voltammetry of ND-PANI-2 deposited on the ITO plate was performed in 1.0 M HCl, and the corresponding voltammograms are shown in Figure 6a-c at different potential ranges. Figure 6d shows the CV measurement as a function of scan rate. A distinct peak centered at -0.5 V corresponds to the reduction of ND particle. The diamond can also exhibit interesting electrochemical behavior. Conductivity is induced by a charge transfer doping mechanism by the tunneling of diamond valence band electrons into the lowest unoccupied electronic levels of a solution redox couple. The cyclic voltammograms of ND-PANI-2 exhibited a pair of reversible redox peaks at oxidation peaks at -0.5, -0.2, 0.14, and 0.7 V and reduction peaks at 0.4, -0.3, and -0.7 V, respectively. The peak centered at 0.14 V is attributed to the oxidation of the emeraldine form of PANI, whereas the peak at 0.7 V is due to the pernigraniline form of PANI (otherwise known as protonation of PANI).36 However, the peak at -0.7 V can be attributed to the oxidation of diamond in the acidic medium. Interestingly, from Figure 6b, it is obvious that the ND-PANI nanocomposite

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Figure 7. Cyclic voltammetry of ND-PAN1 films vs Ag/AgCl in 0.1 M HCl at 50 mV/s scan rate: (1) ND-PANI-1, (2) ND-PANI-2, and (3) ND-PANI-3 nanocomposite films.

Figure 5. High-resolution TEM of embedded ND particles present in nanocomposite ND-PANI-3.

exhibited distinct redox peaks of ND (at -0.7 V) and PANI (at 0.14 and 0.7 V) due to the incorporation of PANI into the ND. From these results, one can envisage that the ND-PANI nanocomposites exhibit their individual characteristic voltammetric signatures at their appropriate potentials, thereby retaining their individual redox characteristics. Figure 6b,c shows the wider potential window for the ND-PANI structures. One can envision the best of the CV properties that ND-PANI could be a corrosion inhibition material, which shows the stability at a higher potential region (-1 to 2.5 V). Figure 7 shows the cyclic voltammetry at 50 mV/s in 0.1 M HCl for diamond-PANI composite film, viz.: (1) ND-PANI-1, (2) ND-PANI-2, and (3) ND-PANI-3. The increase of ND shows the change in redox potential and, interestingly, the redox potential of less dominant in NDPANI film. The ND-PANI (1, 2, and 3) all show the

distinct potentials of ND and PANI. Figure 8 shows the CV of ND-PANI-2 films vs Ag/AgCl in 0.1 M HCl as a function of scan rate: (1) 100, (2) 50, (3) 20, (4) 10, and (5) 5 mV/s. The inset in Figure 8 shows the diffusion-controlled behavior in NDPANI films. The inset in Figure 8 shows the square of current vs square root of scan rate for ND-PANI-2 films in 0.1 M HCl for redox potential. The diffusion coefficient D0 in HCl medium for ND-PANI nanocomposites has been calculated using the Randles-Sevcik equation37 as

Ip ) (2.687 × 105)n3/2AD01/2Cv1/2

(1)

where n is the number of electrons transferred in the reaction, A is the electrode area, C is the concentration of diffusing species in the bulk of the electrolyte, and V is the sweep potential rate. By using values of n ) 2 and C ) 0.1 M HCl, the diffusion coefficient D0 was found to be 1.12 × 10-10 cm2 s-1. This value

Figure 6. Cyclic voltametry of ND-PANI-2 films vs Ag/AgCl in 0.1 M HCl at a scan rate of 50 mV/s at different potential windows for a-c curves: (a) -0.3 to -1.0 V, (b) -1.0 to 1.5 V, and (c) -1 to 2.5 V. (d) CV plots as a function of scan rate (mV/s): 1, 100; 2, 50; 3, 20; 4, 10; 5, 5; and 6, 2 mV/s.

Nanodiamond-Polyaniline Nanocomposite Films

Figure 8. CV of ND-PANI2 films vs Ag/AgCl in 0.1 M HCl as a function of scan rate: (1) 100, (2) 50, (3) 20, (4) 10, and (5) 5 mV/s. Inset: current vs square root of scan rate for ND-PANI-2 films in 0.1 M HCl for redox potential.

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Figure 10. Nyquist plot of ND-PANI as function of potential: 1, 0; 2, 0.4; 3, 0.8; and 4, 1.0 V.

Figure 9. Current-voltage characteristics of ND-PANI3 films.

Figure 11. Nquist resistance as a function of time.

is less than one-third of the equivalent diffusion coefficient obtained in the case of pristine PANI due to the presence of ND. An attempt has been made to understand the electrical behavior in ND-PANI films. The I-V characteristics of NDPANI nanocomposites made at different ratios were measured, but an interesting result was found in the ND-PANI-3 film, which contains the higher ratio of ND with aniline as compared to the mentioned ratios in Table 1. Ohmic behavior can be seen in the I-V characteristics of ND-PANI-3 film as shown in Figure 9. Figure 10 shows the I-V characteristics of NDPANI-3 films as a function of temperature. We could see the change in the resistance as a function of temperature. I-V characteristics until 150 °C were found to be quite reversible. We have seen the complete ohmic behavior in ND-PANI-3 film. Corrosion Study. Most metals and particularly steel and aluminum react with oxygen to form a protective oxide layer. The formation of the latter is inhibited by a process called galvanic corrosion. Figure 10 shows the Nyquist plot of NDPANI in 0.1 V as a function of potential. The semicircle is observed in the Nyquist plot, whereas the applied potential shows the formation of a polarized state of PANI. The applied potential polarizes the steel working electrode to 1.0 V vs Ag/ AgCl. The ability of an oxide layer to prevent corrosion depends on its electrical resistivity. The higher the resistance of the oxide layer is, the more effective it will be in preventing electron transfer. The oxide layer forms what is viewed as a passive electronic barrier at the metal surface. Conducting electroactive polymers have been used in corrosion control or protection of

metals, mostly due to its interesting electrical conductivity, unique electrochemical properties, and ease and simplicity of its synthesis.38 It has been known that Fe3O4 is the passivating layer composition formed by PANI. So, it can be concluded that PANI coatings on active metals like iron provide anodic protection, act as electronic, chemical, and physical barriers to inhibit anodic reaction, and maintain high resistance to ionic flow, thereby blocking the metal dissolution and providing excellent corrosion protection. Besides, NDs have shown stability in higher potential regions. As the potential was increased and the polymer was oxidized to a conductive state, the semicircle tightened up, and much lower values of Rp were obtained from 0.3 V and by 0.5. This means that PANI oxidizes the steel (due to iron) where it is reduced to the leucoemeraldine (LE) base. Further oxidation of iron ions lead to iron(III) oxide, and oxygen reoxidizes the LE into the ES form. The behavior of electrochemistry attributes to the shift in the potential of the redox catalytic properties of PANI-ND. For the stainless steel electrode, values of 0.08 and 0.11 V were obtained in deaerated 0.1 M H2SO4 and 0.1 M HCl (figure not shown), respectively. So, Figure 11 shows the Rp (resistance of corrosion calculated using Tafel equation) as a function of time. Interestingly, the resistance has been found to increase for 800 h due to passivating of ND-PANI with steel. The electrochemical impedance studies have also been estimated using the Tafel equation, and it shows similar behavior in the Rp. The studied Rp in Figure 11 reveals the decreasing characteristics as a function of time and found to be stable nearly for 400 h.

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Gomez et al. Acknowledgment. This research was partially supported by NSF-GOALI Grant #0928823. Supporting Information Available: Enlarged image of Figure 6b. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 12. CV of ND-PANI on steel in 0.1 M HCl as a function of time: (1) as made, (2) after 7 days, and (3) after 30 days.

We have performed the cyclic voltammograms of the NDPANI coated over steel for a month. Figure 12 shows the CV of ND-PANI on steel in 0.1 M HCl as a function of time: (1) as made, (2) after 7 days, and (3) after 30 days. The CV shows the wide oxidized and reduced peaks at 0.64 and 0.61 V in the first cycle, indicating that the ND-PANI is in the emeraldine state. A similar effect has been observed in Figure 12 (curve 2). The CV of the films after 1 month in 0.1 M solution shows the oxidized peaks at 0.58 and 0.36 V and reduced peaks at 0.59 and 0.38 V. The range of the redox peak shows that the ND-PANI is active in acidic media. The steel displayed an initial open circuit potential of 0.2 V, in the passive range, which is nearly stable over the course of 30 days of testing. It could be understood that ND-PANI reaches a steady state with oxygen, and the passivated metal oxide layer is supposed to be regenerating rather than being consumed. Conclusion For successful fabrication of the nanocomposites, dispersion of the nanoparticles into the related solvents and the specific surface functionalization with PANI molecules to obtain NDPANI are the key issues. We have synthesized nanocomposites using different ratios of ND and aniline monomer. ND-PANI nanocomposites were synthesized by chemical precipitation technique. The individual properties of PANI as well as ND remain intact after the encapsulation of ND with PANI. FTIR spectra of ND-PANI nanocomposites show that the presence of ND has a profound effect in the doping of emeraldine form to emeraldine salt state. The electrochemical characterization reveals that ND PANI salt redox properties can be studied at a wider potential range. There is an interaction of the free electron pairs of the nitrogen atoms of the PANI with a charged molecule on the surface of ND with PANI. The concept of using an active electronic barrier ND-PANI barrier built-in electric field (E) is created at the metal surface. The role of this field is to impede electron transfer from the metal surface. The ND-PANI has shown the corrosion inhibition properties over metal (steel and aluminum). The detailed study on corrosion inhibition properties of this excellent nanocomposite material is investigated in depth and will be reported elsewhere. Besides, we have observed positive temperature coefficients in this interesting material. As a global guide for future actions, this work opens new perspectives for the use of corrosion inhibition, positive temperature coefficient materials, and biosensors.

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