Investigations of Triphenyl Phosphate and Bis-(2-ethylhexyl

Feb 1, 2007 - ethylhexyl) phosphate (BEP), against the corrosion of iron in 0.5 M H2SO4 were ... surface remains constant, while θ of BEP on the iron...
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J. Phys. Chem. C 2007, 111, 3109-3115

3109

Investigations of Triphenyl Phosphate and Bis-(2-ethylhexyl) Phosphate Self-Assembled Films on Iron Surface Using Electrochemical Methods, Fourier Transform Infrared Spectroscopy, and Molecular Simulations Wenjuan Guo,† Shenhao Chen,*,†,‡ Yuanyuan Feng,† and Chunjie Yang† School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan, Shandong 250100, P.R. China, and State Key Laboratory for Corrosion and Protection of Metal, Shenyang 110015, P.R. China ReceiVed: NoVember 1, 2006; In Final Form: December 22, 2006

The protection behaviors of two new phosphate self-assembled films, triphenyl phosphate (TPP) and bis-(2ethylhexyl) phosphate (BEP), against the corrosion of iron in 0.5 M H2SO4 were investigated using the electrochemical impedance spectroscopy (EIS), polarization curves, Fourier transform infrared (FT-IR) spectroscopy, and molecular simulations. The polarization curves showed a decrease in corrosion current density for the electrode covered with phosphates. Impedance spectra demonstrated that the charge-transfer resistance (Rct) of the electrode covered with phosphate was greater than that of the naked iron electrode. Results show that TPP molecules can adsorb on the iron surface quickly in a short time and that the coverage (θ) of the film will get to the maximum in 4 h. Subsequently, the amount of TPP molecules on the iron surface remains constant, while θ of BEP on the iron surface increases with the immersion time in 24 h. FT-IR spectroscopy was applied for the surface analysis and confirmed the adsorption of phosphates on the iron surface by monitoring the functional group peaks of the compounds. Molecular simulations were performed to illustrate the adsorption process at a molecule level and to provide some theoretical information for our experiment.

1. Introduction Molecules which contain the active group can adsorb on the metal surface and form orderly, dense, and stable structure. Among some methods in application, self-assembled films are formed by simply immersing a substrate into a solution containing surface-active material.1 It has become a common form of metal surface modification. The application field concerns corrosion protection,2-5 antimicrobial applications,6 and adhesion.7 The adsorption behavior of organic molecules on metal surface is often associated with the molecular structure. Generally, the organic molecules contain heteroatoms, such as S, O, N, or P.8-13 The most studied and probably most understood self-assembled films are that of alkanethiolates on Au surface.14-17 Nowadays some research works have been done to find some new molecules with lower toxicity as corrosion inhibitors. Lin et al. investigated the self-assembly of alkanoic acids on gold surfaces modified by underpotential deposition.18 The previous studies presented that Schiff bases were the effective inhibitors for copper corrosion.19-22 Wang et al. compared the different inhibitive effect of two new types of self-assembled films, carbazole and N-vinylcarbazole.23 The inhibition efficiencies of them for copper were 91.1% and 93.4%, respectively. Li et al. prepared Au and Ag nanoparticles and then used them to self-assemble on the copper surface.24 These films can inhibit the corrosion of copper efficiently. With the development of industry, iron is widely used in manufacturing bridges, airplanes, and buildings. It is a subject of great industrial as well as academic significance to inhibit * To whom correspondence should be addressed. Tel.: +86-53188563641. Fax: +86-531-88565167. E-mail: [email protected]. † Shandong University. ‡ State Key Laboratory for Corrosion and Protection of Metal.

the corrosion of iron. While iron tends to be corroded easily exposed to air or water, there were only a few studies about self-assembled films on the relatively reactive iron.25-27 Recently, some work has been done in the group of Shimura and Aramaki. For example, Shimura and Aramaki investigated selfassembled monolayers (SAMs) of p-toluene and p-hydroxymethylbenzene moieties on iron.28,29 The inhibition efficiencies of these two SAMs for iron in 0.5 M NaCl were not high, around 30%, a little higher than that of the toluenethiol SAMs. For iron, the overall corrosion reaction is represented by30

Fe(s) + 2H+(aq) ) Fe+2(aq) + H2(g)

(1)

The anodic partial reaction is

Fe(s) f Fe+2(aq) +2e

(2)

and the cathodic partial reaction is

2H+(aq) +2e + H2(g)

(3)

Among the previous studies, compounds with the phosphoric functional group were considered to be one of the most effective chemicals for inhibiting the corrosion of iron.31-33 Yang et al. found that inositol hexaphosphate molecule (IP6), which was an environmentally friendly inhibitor, self-assembled on a bare iron surface from a low concentration solution.34 The adsorption configuration of IP6 SAMs was deduced by using surfaceenhanced Raman scattering spectra and quantum chemistry calculation. At the different pH values, the IP6 molecules were located at the surface via four coplanar or one phosphate adsorbed on the iron surface by O atoms.

10.1021/jp067216t CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007

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TABLE 1: Electrochemical Impedance Spectra and Polarization Curve Parameters for Iron in 0.5 M H2SO4 without and with TPP and BEP Films CPEdl concentration (M)

immersion time (h)

naked iron

Ecorr (V/SCE)

Rct (Ω cm2)

Rs (Ω cm2)

Cdl (µF c m-2)

n

-0.529

47.4

0.304

49.5

0.88

θ (%)

icorr (µA cm-2)

IE%

538.6

CTPP ) 1 × 10-5

0.5 1 2 4 24

-0.523 -0.522 -0.517 -0.515 -0.512

181.1 195.1 215.9 216.6 216.5

0.382 0.253 0.383 0.317 0.330

31.2 30.0 27.9 26.9 27.1

0.89 0.89 0.90 0.91 0.90

73.8 75.7 78.0 78.1 78.1

143.6 133.3 120.4 120.2 120.0

73.3 75.3 77.6 77.7 77.7

CTPP ) 1 × 10-3

0.5 1 2 4 24

-0.519 -0.521 -0.519 -0.522 -0.516

193.6 211.2 231.9 229.1 228.2

0.281 0.259 0.292 0.278 0.246

30.0 28.0 26.4 26.5 26.5

0.89 0.89 0.93 0.90 0.90

75.5 77.6 79.6 79.3 79.2

134.3 123.1 112.1 113.5 114.0

75.1 77.1 79.2 78.9 78.8

CBEP ) 1 × 10-5

0.5 1 2 4 24

-0.527 -0.526 -0.526 -0.522 -0.518

68.3 117.1 135.2 180.6 204.3

0.301 0.178 0.361 0.318 0.354

29.3 28.8 30.9 38.1 31.8

0.914 0.905 0.872 0.854 0.911

30.6 59.5 64.9 73.8 76.8

380.7 222.0 192.3 144.0 127.3

29.3 58.8 64.3 73.3 76.4

CBEP ) 1 × 10-3

0.5 1 2 4 24

-0.527 -0.524 -0.525 -0.514 -0.509

71.8 129.7 157.1 213.8 258.9

0.393 0.327 0.265 0.349 0.292

33.1 27.3 30.5 42.4 40.2

0.930 0.915 0.908 0.907 0.876

34.0 63.5 69.8 77.8 81.7

362.1 200.5 165.5 121.6 100.4

32.8 62.8 69.3 77.4 81.4

The objective of this paper is to find new inhibitors for iron corrosion in sulfuric acid and to compare the effect of the molecules with different structure on the inhibitive efficiency. To inhibit the corrosion of iron, two kinds of phosphates, triphenyl phosphate (TPP) and bis-(2-ethylhexyl) phosphate (BEP), were first applied to adsorb on the surface of iron. The inhibition effects were measured using electrochemical impedance spectroscopy (EIS) and polarization curves. Results show that the two kinds of phosphates adsorb on the surface of iron and both of them have better inhibitive efficiency (IE). IE increases with the immersion time and the concentration. Fourier transform infrared (FT-IR) spectroscopy measurement confirmed the adsorption of TPP and BEP on the iron surface. Molecular simulations performed here offered insights into the adsorption process from the microscopic field. 2. Experimental Section 2.1. Chemicals. The reagents used in this experiment were all analytical grade chemicals. TPP and BEP (SCRC, Shanghai, China) were dissolved in the anhydrous ethanol to prepare 1 × 10-5 M and 1 × 10-3 M ethanol solution, respectively. Testing electrolyte of 0.5 M H2SO4 aqueous solution was prepared by diluting sulfuric acid with deionized water. 2.2. Electrodes. The iron electrode was made from a 0.2cm-diameter iron rod (Aldrich, 99.9%) imbedded in epoxy resin in a glass tube, leaving its cross section only to contact the solution. So, the geometric surface area of the iron electrode is 0.0314 cm2. 2.3. Formation of Self-Assembled Films. The iron electrode was polished on wet SiC abrasive papers (800, 2000, and 4000 # in turn) until its surface became smooth and mirrorlike bright. Then, it was rinsed with deionized water, degreased in ultrasonic ethanol, and finally dried in nitrogen stream. The pretreatment mentioned above was operated as soon as possible. Then, the electrode was immediately immersed in the TPP-containing or BEP-containing solution for self-assembly. When the films were formed, the electrode was rinsed with enough anhydrous ethanol

and deionized water to get rid of the TPP or BEP molecules of physical adsorption. At last, it was dried with a steam of nitrogen. 2.4. Electrochemical Measurements. The cell for electrochemical measurements was a traditional three-electrode cell. The working electrode was iron electrode, the counter electrodes were two platinum foils (1 cm × 1 cm), and the reference electrode was a saturated calomel electrode (SCE). Each platinum foil was placed in a room interconnected with the room for the working electrode. Two platinum foils were connected with each other by a copper wire. The working electrode was facing downward while the counter electrodes were placed vertically facing each other. The reference electrode was led to the surface of the working electrode through a Luggin capillary. The electrochemical measurements were performed with IM6 electrochemical workstation (ZAHNER, Germany). Impedance measurements were performed under the corrosion potential (Ecorr) with a sinusoidal potential perturbation of 5 mV in amplitude and frequency from 60 kHz to 20 mHz. Potentiodynamic polarization curves were obtained from -0.8 to -0.2 V versus SCE at a scan rate of 1 mV s-1. The data of impedance spectra and polarization curves were fitted using the software set in IM6 system. The electrolyte was deaerated by bubbling

Figure 1. Nyquist impedance spectroscopy of the naked iron in 0.5 M H2SO4 (inset is the equivalent circuit for fitting).

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Figure 2. Nyquist impedance spectra of TPP-covered and BEP-covered iron electrodes in 0.5 M H2SO4 with different immersion times. (a) CTPP ) 1 × 10-5 M; (b) CTPP ) 1 × 10-3 M; (c) CBEP ) 1 × 10-5 M; (b) CBEP ) 1 × 10-3 M.

nitrogen for 30 min before the measurements, and the temperature was kept at 25 ( 1 °C. Potentials reported in our paper were all referred to SCE. 2.5. FT-IR Spectroscopy. The samples for FT-IR measurements were iron sheets (10 mm × 10 mm × 1 mm). The polishing method was the same as what was mentioned above. After 4-h immersion, the iron sheet was rinsed with enough anhydrous ethanol and deionized water to get rid of the physical adsorption TPP or BEP molecules. At last, it was dried with a steam of nitrogen. Sixty-four scans of the infrared reflection spectra were collected at a resolution of 8.0 cm-1 in the spectral region between 4000 and 600 cm-1 using a Nicolet Nexus 670 FT-IR spectrometer equipped with a liquid-nitrogen-cooled mercury cadmium tellurium (MCT-A) detector and potassium bromide beam splitter. 2.6. Molecular Simulations. All molecular mechanics calculations were performed using the program Cerius2. The structures were optimized using COMPASS force field35 and the Smart Minimize method by high-convergence criteria. 3. Results and Discussion 3.1. Electrochemical Impedance Spectroscopy. The iron electrode was put into 0.5 M H2SO4 electrolyte for 30 min before measurement to obtain a steady value of Ecorr. The values of Ecorr are listed in Table 1. Measurements are performed to determine the impedance parameters of the iron/electrolyte or TPP-covered iron/electrolyte or BEP-covered iron/electrolyte. Figure 1 is the Nyquist impedance spectroscopy for the naked iron in 0.5 M H2SO4. It only displays a capacitive loop slightly deviating from an ideal semicircle, which is bought by surface roughness and is known as the “dispersing effect”.36,37 The small semicircle is attributed to the time constant (RctC), which is related to both the charge-transfer resistance (Rct) and the doublelayer capacitance (C).38 Considering that the impedance of a double layer does not behavior as an ideal capacitor in the presence of a dispersing effect, a constant phase element of

Figure 3. Dependence of the surface coverage of TPP (a) and BEP (b) on the immersion time.

double layer (CPEdl) is used as a substitute for the capacitor to fit more accurately the impedance behavior of the electric double

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Figure 4. Polarization curves of iron in 0.5 M H2SO4 with and without phosphate films with different immersion times. (a) CTPP ) 1 × 10-5 M; (b) CTPP ) 1 × 10-3 M; (c) CBEP ) 1 × 10-5 M; (d) CBEP ) 1 × 10-3 M.

layer.39 So, the impedance spectroscopy can be analyzed with the equivalent circuit insetted in Figure 1, where Rct stands for the charge-transfer resistance, CPEdl a constant phase element, and Rs is the solution resistance. The impedance of CPEdl (represented as Q) is described in eq 4: n -1

ZCPE ) [Q(iw) ] frequency.40

(4)

where ω is the angular Figure 2 is the Nyquist impedance spectra of iron electrode covered with TPP films or covered with BEP films in 0.5 M H2SO4. The immersion time in phosphate solutions and the concentration of phosphate solutions were changed to investigate the effect of assembly condition on the inhibitive efficiency. Figure 2a shows the TPP-covered iron electrode in 0.5 M H2SO4 with different immersion time while the concentration of TPP is invariable at 1 × 10-5 M. The impedance spectra of TPP-covered electrodes are different from that of the naked iron electrodes both in size and shape. Compared with the Nyquist spectroscopy of naked iron electrode, the diameter size of the semicircle of TPP-covered iron electrode tends to be larger. Using the fitting software set in IM6 system, the fitting data of

the elements in the equivalent circuit are listed in Table 1. The coverage (θ) of the film can be calculated by the following formula:40

Rct0 (1 - θ) ) Rct

(5)

where Rct0 is the charge-transfer resistance of the naked electrode and Rct is the charge-transfer resistance of the TPP-covered electrode. From the data in Table 1, it can be seen that the value of the charge-transfer resistance of the naked electrode is the lowest. When the surface of iron electrode is covered with TPP films, the value of Rct increases. Generally speaking, the longer the immersion time is, the higher the value of Rct is. When the electrode is immersed in TPP solution for 30 min, θ increases markedly to 73.8% which indicates that at this time the film has better inhibitive effect for the iron corrosion. From 30 min to 1, 2, and 4 h, θ keeps on increasing and reaches 78.1% at last while the increasing rate slows down relatively. The phenomenon can be understood as the following: in the initial 30 min, TPP molecules in the solution adsorb on the naked iron

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surface quickly. Subsequently, the adsorbed molecules will form a denser film gradually. The longer the immersion time is, the more molecules the iron surface is covered with and the better the effect is. From 4 to 24 h, θ maintains constant at 78.1% instead of increasing with increasing the immersion time, which means at this time the amount of adsorbates is saturated. Figure 2b shows the TPP-covered iron electrode in 0.5 M H2SO4 with different immersion times while the concentration of TPP is invariable at 1 × 10-3 M. It displayed almost the same tendency as Figure 2a. In the initial 30 min, θ increases quickly and during the subsequent period, it increases slowly. The different phenomenon from Figure 2a is that when it is 4 h, θ decreases slightly and will stay constant afterward. This phenomenon might be interpreted for that when at a lower concentration, TPP molecules in the solution will congregate at the interface between the electrode and the solution quickly and will adsorb on the iron surface. The adsorbed molecules will rearrange at the surface which causes the lower increasing of θ. Afterward, θ will stay constant. However, at a higher concentration, the effect of repulsion force of the end groups of TPP molecules on the adsorption process cannot be neglected. When the amount of TPP molecules is close to being saturated, the end groups of TPP among the same molecule and the adjacent molecules will have the repulsion action with each other, which results in the phenomenon of θ decreasing slightly. Figure 2c and d shows the Nyquist impedance spectra of BEPcovered iron electrode in 0.5 M H2SO4 with different immersion times and concentrations. θ increases with the concentration and the immersion time. Compared with Nyquist impedance spectra of TPP-covered electrode, θ of BEP on the electrode showed a continual increasing tendency instead of increasing quickly in a short time and staying constant afterward. Figure 3 is the intuitionistic plots showing the effect of immersion time on the coverage. It obviously reflects the general increasing tendency of θ with the immersion time. At the same immersion time, θ increases with increasing the concentration. θ of adsorbates (1 × 10-3 M) on iron is a little larger than that of adsorbates (1 × 10-5 M) at the same immersion time. It can be understood easily: at the same immersion time, the higher the concentration is, namely, the more the adsorbate molecules are in the solution, the more the iron surface is covered by the adsorbate molecules. 3.2. Polarization Curves Measurements. The corrosion of naked iron or iron electrode covered with phosphate films in 0.5 M H2SO4 was also studied by potentiodynamic polarization curves. Figure 4 shows the typical polarization curves for the naked iron and the iron electrode covered with TPP or BEP films of different immersion times and concentrations. It is observed from the polarization curves that after adsorption, the current density values are decreased markedly and that Ecorr shifts in a positive direction. Values of some kinetic parameters such as Ecorr and corrosion current density (icorr) attained by extrapolation of Tafel lines and IE are listed in Table 1. IE % is calculated from eq 6:41

IE% )

(

)

icorr0 - icorr icorr0

× 100

(6)

where icorr0 and icorr represent the corrosion current densities of naked iron and iron electrode covered with phosphates, respectively. It can be seen from Figure 4 that Ecorr of iron electrode covered with phosphate moved in a positive direction compared with the naked iron. Both the cathodic and anodic current

Figure 5. FT-IR spectra of TPP (a) and BEP (b) on iron surface.

densities of iron electrodes covered with phosphate films decreased significantly, which indicated that the iron surface had a weaker tendency to corrosion after the adsorption of the phosphates. The values of IE are almost identical with that of θ. Compared with eqs 5 and 6, it can be found that although θ and IE have different physical meanings, their numerical values are almost the same. Both θ and IE can reflect inhibitive effect of the films effectively. 3.3. FT-IR Spectroscopy Measurement. FT-IR is being employed as a powerful tool for analyzing the coating of inorganic substrates with films of organic molecules. At the simplest level, reflectance spectroscopy can confirm the presence of molecules adsorbed on a surface.42,43 Figure 5 shows the FTIR spectra of TPP (a) and BEP (b) on the iron electrode. The characterization peaks of the functional groups were detected, which reveals the adsorption of the related compounds on the iron surface. The FT-IR spectroscopy of TPP-covered iron surface shown in Figure 5a shows the typical two bands of C-H stretching modes, ranging from 3100 to 3000 cm-1 for aromatic C-H and from 1660 to 1300 cm-1 for aromatic CdC stretching vibrations. A weak peak at 1280 cm-1 is PdO stretching vibration for phosphates. A strong peak appearing at 1140 cm-1 reflects P-O-C (C belongs to aromatic compounds) stretch. Figure 5b shows the FT-IR spectroscopy for BEP-covered iron surface. It reveals a wide peak at 3340 cm-1 corresponding to typical O-H stretch and three peaks at 2950, 2920, and 2850 cm-1 corresponding to C-H stretch for alkanes. A weak peak at 1250 cm-1 is PdO stretching vibration for phosphates. A

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Figure 6. The models of TPP molecules (a) and BEP molecules (b) adsorbed on the Fe(111).

double peak at 1090 and 1040 cm-1 and one peak at 833 cm-1 are the P-O-C (C belongs to aliphatic compounds) stretch, whereas P-OH stretching mode is very weak in this FT-IR spectroscopy. Obvious functional group peaks of the corresponding compounds point to the presence of TPP and BEP on the surface of iron. However, some differences can be noticed compared with IR spectroscopy of the bulk samples (Acros spectra). This may be interpreted as follows. First, two different measurement methods are applied, FT-IR for the samples adsorbed on the iron surface and IR for the bulk samples, which will lead to slight differences arising from the reflection of the substrate. Second, after adsorption, the action of compounds with the iron substrate affects the peak intensity of the intramolecular bonds and the peak positions to some extent. As for TPP, the peaks related to the PdO and P-O-C stretch become weaker and red-shifted slightly to 1280 cm-1 and 1140 cm-1, respectively. As for BEP, PdO stretching mode becomes weaker and P-OH stretching mode almost disappears which may be caused by the effect of the iron substrate. The fact that the peaks corresponding to the functional groups of TPP and BEP were observed in the FT-IR spectra indicates that the two compounds adsorb on the iron surface after 4-h immersion. 3.4. Molecular Simulations. The molecular simulations were applied using Cerius2 (Accelry Company) software package to provide some elementary information for the adsorption process of TPP and BEP molecules on the iron surface. The structures were optimized using COMPASS force field and the Smart Minimize method. Figure 6a shows the model of TPP and Fe(111) before being optimized and the side and top views of the adsorption model of TPP on the surface of Fe(111). When the whole system is being optimized, TPP molecules are binding with the iron substrate by tilting at some angle to the iron surface. One O atom of P-O-C bond tends to lie lower closed

to the iron surface which causes the benzene ring to tilt slightly. Figure 6b shows the model of BEP and Fe(111) before being optimized and the side and top views of the adsorption model of BEP on the surface of Fe(111). Compared with the model before optimization, BEP molecules are closer to Fe(111). O atom of O-H is closer to the iron surface which causes the alkyl chains to tilt. Results of the molecular simulations are consistent with the changes appearing in FT-IR spectra. O atoms belonging to PdO, P-O-C, and O-H groups are so close to the iron surface that the substrate can affect the characterization peaks. Molecular simulation calculations can provide important microscopic information for the adsorption process of TPP and BEP on the iron surface. 4. Conclusion The present work reveals that self-assembled films of TPP and BEP on iron surface can inhibit the corrosion of iron in 0.5 M H2SO4 efficiently. Results of electrochemical measurements show that IE increases with increasing the immersion time and concentration of the phosphates. At the same immersion time, IE of 1 × 10-3 M TPP (or BEP) is higher than that of 1 × 10-5 M TPP (or BEP). TPP can adsorb on the iron surface quickly in a short time and θ will get to 78.1% in 4 h. Subsequently, the amount of TPP molecules on the iron surface remains constant, while θ of BEP on the iron surface increases with the immersion time in 24 h. FT-IR spectroscopy, which is an effective technique of surface analysis, was applied. The functional group peaks of the corresponding compounds appear in FT-IR spectra, which confirm the adsorption of TPP and BEP on the iron surface. Molecular simulations provide some information at the molecule level for the adsorption process of TPP and BEP.

Phosphate Self-Assembled Films on Iron Surface Acknowledgment. This work was financially supported by National Natural Science Foundation of China (No.20573069) and 973 Project of China (2006CB605004). References and Notes (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Felhosi, I.; Telegdi, J.; Palinkas, G.; Kalman, E. Electrochim. Acta 2002, 47, 2335. (3) Nozawa, K.; Nishihara, H.; Aramaki, K. Corros. Sci. 1997, 39, 1625. (4) Nozawa, K.; Aramaki, K. Corros. Sci. 1999, 41, 57. (5) Aramaki, K.; Shimura, T. Corros. Sci. 2004, 46, 313. (6) Jon, S.; Seong, J.; Khademhosseini, A.; Tran, T.-N. T.; Laibinis, P. E.; Langer, R. Langmuir 2003, 19, 9989. (7) Whelan, C. M.; Kinsella, M.; Ho, H. M.; Maex, K. J. Electrochem. Soc. 2004, 151, B33. (8) Carbonell, L.; Whelan, C. M.; Kinsella, M.; Maex, K. Surperlattices Microstruct. 2004, 36, 149. (9) BuBar, R.; Nielinger, M.; Baltruschat, H. J. Electroanal. Chem. 2005, 578, 259. (10) Ashassi-Sorkhabi, H.; Majidi, M. R.; Seyyedi, K. Appl. Surf. Sci. 2004, 225, 176. (11) Khaled, K. F.; Hackerman, N. Electrochim. Acta 2003, 48, 2715. (12) Refaey, S. A. M.; Taha, F.; Abd El-Malak, A. M. Appl. Surf. Sci. 2004, 236, 175. (13) Samui, A. B.; Phadnis, S. M. Prog. Org. Coat. 2005, 54, 263. (14) Neuman, O.; Naaman, R. J. Phys. Chem. B 2006, 110, 5163. (15) Abe, K.; Takiguchi, H.; Tamada, K. Langmuir 2000, 16, 2394. (16) Fischer, D.; Curioni, A.; Andreoni, W. Langmuir 2003, 19, 3567. (17) Duan, L.; Garrett, S. J. J. Phys. Chem. B 2001, 105, 9812. (18) Lin, S. Y.; Chen, C. H.; Chan, Y. C.; Lin, C. M.; Chen, H. W. J. Phys. Chem. B 2001, 105, 4951. (19) Quan, Z. L.; Chen, S. H.; Hua, L.; Lei, S. B.; Cui, X. G. Chin. Sci. Bull. 2002, 47, 990. (20) Ma, H.; Chen, S. H.; Niu, L.; Zhao, S. Y.; Li, S. L.; Li, D. G. J. Appl. Electrochem. 2002, 32, 65.

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