First-Principles Considerations on Spontaneous Replacement of

Aug 13, 2008 - School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China, State Key Laboratory of Optoelectronic ...
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J. Phys. Chem. C 2008, 112, 13546–13553

First-Principles Considerations on Spontaneous Replacement of Copper by Tin in the Presence of Thiourea Guofeng Cui,*,† Xi Ke,‡ Hong Liu,† Jianwei Zhao,§ Shuqin Song,‡ and Pei Kang Shen*,‡ School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou, 510275, China, State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou, 510275, China, and Department of Chemistry, Nanjing UniVersity, Nanjing, 210093, China ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: June 26, 2008

The electrochemical quartz crystal microbalance (EQCM) and density functional theory (DFT) have been combined to study the special reaction between thiourea (TU) and metal cluster and the mechanism of the replacement of Cu by Sn in the presence of TU for the first time. The natural bond orbital (NBO) charge of the top copper atom obviously shifts toward positive values compared to the interaction behavior of single and double (SdC) regions with Cu4 cluster via the DFT method. This can explain the reason for the accelerating corrosion process at higher TU concentrations since the copper atoms can change to cuprous ions in this process. It is proven that the thermodynamically impossible replacement of Cu by Sn can occur in the presence of TU by reducing the OCP of the copper electrode to a more negative value than the redox potential of Sn2+/Sn. The DFT investigation on the interactions between delocalization (SdC) and (NsC) regions in TU and Cu4 or Sn4 cluster indicates that the highest molecular occupied orbital (HOMO) of the SdC region has better than adequate to the lowest unoccupied molecular orbital (LUMO) of Cu4 cluster than that of Sn4 cluster. The replacement mechanism deduced based on the first-principles analysis is universally applicable to the alloy deposition, corrosion inhibition, and surface treatment. 1. Introduction The requirement to develop lead-free printed wiring boards (PWB) and mercury-free alkaline batteries has motivated researchers to explore the replacement mechanism of copper atoms by tin atoms.1-3 Although many theoretical and experimental results have been reported in this field, the mechanism is still not fully understood.4 The main reason is that the true interaction mechanism between thiourea (TU) and copper or tin atoms was not clear. TU can adsorb on the surface of various metals including copper,5 iron,6 gold,7 and silver,8 and as a corrosion inhibitor to block active sites. The corrosion inhibition mechanism of TU has been intensively investigated by electrochemical techniques.9,10 However, the behavior of TU as a corrosion promoter in higher concentrations is rarely determined and discussed. It has been found that the redox potential of the metal would shift to a more negative value after adsorbed TU. This makes the replacement of the more noble metal by the less noble metal possible based on this observation. Theoretically, copper cannot be spontaneously replaced by tin in normal aqueous solution according to the standard potentials:

Cu2+ + e- ) Cu+ 2+

Cu

2+

Sn

-

+ 2e ) Cu -

+ 2e ) Sn

E ° ) 0.153 V E ° ) 0.337 V E ° ) -0.140 V

However, our previous work revealed that the replacement process of copper by tin occurs in the presence of TU based on * Corresponding author. E-mail: [email protected] (G.C.) and [email protected] (P.K.S.). † School of Chemistry and Chemical Engineering, Sun Yat-Sen University. ‡ State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University. § Department of Chemistry, Nanjing University.

Figure 1. Effect of the TU concentration on the open circuit potential (OCP) variation of the copper electrode.

the open circuit potential (OCP)11 and electrochemical impedance spectroscopic (EIS) measurements.12 It showed that the metal with higher standard potential replaced by the metal with lower standard potential is possible in the presence of TU. However, we were not able to deduce the mechanism of the replacement process by solely using electrochemical methods. Here, we report the results of the replacement of Cu by Sn in the presence of TU and the origin of a noble metal replaced by a less noble metal based on the electrochemical quartz crystal microbalance (EQCM) and density functional theory (DFT) studies. The DFT method has been widely employed to study the molecular structural relationship between organic inhibitor and metal cluster.13,14 Commonly, the adsorption efficiency of corrosion inhibitor over the metal is directly linked with the energy difference between EHOMO and ELUMO. The inhibitor efficiency is better if the EHOMO of the corrosion inhibitor approaches ELUMO of the metal. However, it is necessary to investigate the interaction behavior of different delocalized

10.1021/jp8018099 CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

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Figure 2. Optimized structure and electronic property of TU obtained on the B3LYP/6-311G(d,p) level under polarizable continuum model (PCM) and solvent effect model in water solution: (a) optimized geometries (Å and degree) and NBO and (b) electrostatic potential.

Figure 3. Optimized geometry, total energy, and molecular orbital analysis of TU, Cu4 cluster, and TU adsorption compound in different coordination atoms (TU-S-Cu4 and TU-N-Cu4).

of the EQCM is its capability of in situ monitoring mass changing at the nanogram level. In this study, the EQCM and DFT methods are combined to investigate the nature of the interaction behavior of organic compound and metal.

Figure 4. Comparison of IR spectra of (a) TU and (b) copper-TU compound, in which ν denotes stretching vibration and δ denotes bending deformation.

regions in the same molecule and the metal. Additionally, EQCM has became a valuable method to analyze various electrochemical processes, such as electrodeposition,15,16 adsorption/desorption behavior of organic compounds,17-19 and corrosion behavior of metal.20-22 One of the most exciting features

2. Experimental Section 2.1. Electrochemical Measurements. The EQCM and OCP measurements were performed in a three-electrode one-compartment cell on a potentiostate/galvanostate (EG&G-PARC, 2273A, USA). A 9-MHz AT-cut gold-coated quartz crystal with 0.2 cm2 electrode area, a 1.0 × 1.0 cm2 platinum foi,l and a saturated calomel electrode (SCE) were used as working electrode, counter electrode, and reference electrode, respectively. The working electrodes, gold-coated quartz crystals, were first degreased with 10 wt % NaOH and acetone. Subsequently, copper thin film was deposited on the gold electrode in a 0.1 M cupric sulfate electrolyte. The electrode was rinsed in deionized water and dipped into 0.5 M sulfuric acid solution containing different TU concentrations ranging from 0.0 to 0.5 M,. The OCP and mass variation of the electrode were monitored simultaneously during the measurement. The replacement was performed by adding 0.1 M SnCl2 solution. The OCP

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Figure 5. The mass variation of copper electrode in various concentrations of TU from (a) 0.0 to 100 mM and (b) 100 to 500 mM monitored by EQCM in 3000 s.

Figure 6. Optimized geometries, total energy, and molecular orbital of TU, Cu4 cluster, and TU adsorption compound in different ligand number (TU-S-Cu4 and TU2-2S-Cu4).

and mass variation were recorded simultaneously once the tincontaining solution was added. All reagents were analytical grade and used as received. Solutions were prepared by using Mill-Q deionized water at a resistance of 18 MΩ or higher.

2.2. IR Spectroscopic Measurement. The colorless crystals formed on the surface of the copper electrode after the copper was immersed in 1.0 M TU and 0.5 M H2SO4 solution for 4 h. The crystals were collected and dried for infrared spectra (IR) measurement. A colorless single crystal with dimensions of 0.36

Figure 7. Optimized geometries and NBO charge distribution of TU and Cu4 cluster coordination compound: TU-S-Cu4(a) and TU2-2S-Cu4 (b).

Spontaneous Replacement of Copper by Tin mm × 0.28 mm × 0.19 mm was employed for the data collection at 295 ( 2 K. The IR spectra of the Cu(I)-TU compound were recorded on a BRUKER EQUINOX 55 FT-IR spectrometer, using KBr pellet (400-4000 cm-1). 2.3. Computational Calculation. All calculations were carried out by the Gaussian 03 program,23 employing the hybrid Becke exchange and Lee, Yang, and Parr correlation (B3LYP) functional method.24-26 In the geometry optimization, singlepoint energy calculation, molecular orbital characteristic, Weinhold’s natural bond orbital (NBO), and electrostatic potential (ESP) analysis, 6-311G(d,p) basis sets were used for hydrogen, carbon, nitrogen, and sulfur atoms, and LAN2DZ basis sets for copper and tin atoms with the Hay and Wadt effective core pseudopotential.27 Since the interaction between TU and copper or tin atoms takes place in aqueous solution, the adsorption behavior can be greatly affected by the solvation effect of water. Therefore, the aqueous solvation effect was considered by a polarized continuum model (PCM)28-31 in the whole calculations. In PCM calculations, the dielectric constant was set at 78.3 (in the pure water of the bulk solvent)32 and temperature was set at 273.15 K. Structure and electronic parameters of TU molecules were calculated by molecular orbital theory. Additionally, to reveal the interaction behavior of TU over metal, the properties of TU and Cu4 and Sn4 clusters were investigated. 2.4. Crystalline Structure Measurement. After EQCM measurement, the electrolyte near the working electrode was sucked by an injector and the colorless crystals were obtained after the solvent evaporated. The crystallne structure was measured on a Rigaku Raxisrapid single-crystal diffractometer equipped with graphite monochromatized Mo KR (λ ) 0.71073 Å) radiation by using an ω scan mode. The single crystal with dimensions of 0.36 mm × 0.28 mm × 0.19 mm was employed for the data collection at 295 K. A total of 46747 reflections and 5804 unique ones were collected in the range of 3.29 e θ e 27.46° with Rint ) 0.0263, of which 5215 observed reflections with I > 2σ(I) were used in the succeeding structural calculations. Corrections for Lp factors and empirical absorption correction were applied to intensity data. The structure was solved by direct methods and difference Fourier syntheses. All calculations were carried out with the SHELX 97 program.33 3. Results and Discussion 3.1. Electronic Properties of TU for Adsorption Characteristics. The OCP variation of the copper electrode was investigated in the presence of TU with different concentrations. It can be clearly seen from Figure 1 that the OCP is seriously affected by the addition of TU. The potential gradually shifts to more negative potential from -0.135 to -0.764 V when the TU concentration increases from 0.0 to 0.5 M. The negatively shifting phenomenon of the metal surface potential is usually attributed to the special chemisorptions of negative species on the electrode surface.34 In this study, TU, as a neutral organic molecule, is the sole species to affect the Cu surface potential. Moreover, in our previous study,11,12 we found that the cuprous ion and TU compound [Cu(TU)4]+ was formed on the Cu electrode by the interactions between TU and Cu. Therefore, the structure and electronic properties of TU are directly linked with the negative shift of OCP. The TU molecule is special in terms of the electronic property, in which sulfur atoms are surrounded by a higher density of electrons. Figure 2 shows the optimized structure and NBO charge distribution of TU. In this structure, both N1-C and N2-C bonds have equal length (1.337 Å) and the same bond

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Figure 8. The polyhedron structure of the Cu(I)-TU complex in the crystalline structure.

order of 1.5. The corresponding bond angles of H1-N1-H2, H4-N2-H3, H1-N1-C, and H4-N2-C are 118.4°, 118.6°, 119.3°, and 119.6°. Moreover, the dihedral angle of (S-C-N1-N2) is 0.0°. Obviously, the TU molecule is a typical planar and C2V symmetric structure, in which the (C-S) bond is a symmetry axis. According to NBO charge distribution analysis, the electron density that surrounds a sulfur atom is -0.515 and that that surrounds a nitrogen atom is -0.764 as illustrated in Figure 2a. However, the negative electronic characteristic of nitrogen is unable to appear since the two nitrogen atoms are closely surrounded by positively charged atoms (C, H1, H2, H3 and H4) with even the nitrogen atoms having excessive electronic density compared to other atoms, resulting in the local negative charge zone around the sulfur atom. The redistribution of the charge on TU is clearly described in its electrostatic potential analysis as shown in Figure 2b. Obviously, the sulfur atom is surrounded by the negative charge zone and the two nitrogen atoms are surrounded by the positive charge zone. Clearly, the sulfur atoms in the TU molecule and the complex of TU and cuprous that adsorbed on the surface of the Cu electrode led to a negative shift in OCP. 3.2. Interaction between TU and Cu Clusters. For the TU molecule, there are one (CdS) delocalization region and two (CsN) delocalization regions, as illustrated in Figure 2a. Those can provide higher π electron density to the LUMO orbital of metal. To analyze the activity of regions to metal atoms, the HOMO of TU and LUMO of Cu clusters are investigated as shown in Figure 3. For effective orbital overlapping, the energy difference between the orbital of HOMO of TU and LUMO of Cu clusters should be smaller so that the π electron in the HOMO can be smoothly given up to the LUMO of metal. Considering the (CdS) and (CsN) regions in the same molecule, the HOMO energy of the TU-Cu compound combined with different coordinating atoms is used as a comparison. In this case, the electron transfer will occur in the lower energy barrier process. The HOMO energy of TU-S-Cu relative to that of TU is 2.83 eV. This energy value is lower than that of TUN-Cu (3.16 eV), indicating that the electron transfer is easier in the metal-sulfur atom than that in the metal-nitrogen atom. Furthermore, the structural stability of the metal-ligand compound is analyzed in Figure 3. The total energy of TU-S-Cu4 is -1332.822565 hartrees, which is lower than that of TU-N-Cu4 (-1332.805372 hartrees) The lower energy value of the TUS-Cu means it is more stable than TU-N-Cu. Therefore, the sulfur atom can interact closely with Cu atoms and chemisorbs on the surface of Cu by providing a π electron to the LUMO of the Cu. The IR spectra of TU and its cuprous compound were compared in Figure 4. The charge characteristic of the Cu atom in the complex was measured by a single-crystal diffractometer in the complex, which is the cuprous cation as reported in the previous paper.12 The IR bands corresponding to the stretching vibrations of the -NH2 group in the TU molecule at 3380, 3279, and 3177 cm-1 are obviously broadened and shifted to higher

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TABLE 1: Bond lengths (Å) and Bond Angles (deg) in the Structure of the Cu(I)-TU Complex bond lengths/ Å CuI-S3 CuI-S2 CuI-S4 CuI-S1 CuII-S6 CuII-S5 CuII-S1 CuII-S2 CuI-CuII

2.2936 2.2884 2.2860 2.3694 2.3357 2.3448 2.2909 2.2446 2.8784

bond angles/deg S3-CuI-S2 S3-CuI-S4 S2-CuI-S4 S3-CuI-S1 S2-CuI-S1 S4-CuI-S1 S3-CuI-CuII S2-CuI-CuII S4-CuI-CuII S1-CuI-CuII

wavenumbers for the Cu(I)-TU complex compared to pure TU. The variation of the peak can be attributed to the double bond characteristic of CdN enhanced due to the formation of the SfCu bond in the coordination.35,36 Moreover, the peaks of the N-C-N stretching vibration in TU at 1477 and 1082 cm-1 shift to 1483 and 1111 cm-1 for its corresponding complex, indicating the double bond characteristic of the CdN bond was enhanced due to coordination reaction. Additionally, the peak at 730 cm-1 shifts to 718 cm-1 for the complex, which corresponds to the motion of CdS stretching. Obviously, the

116.22 119.66 104.46 106.54 94.83 101.36 129.79 44.83 110.53 50.02

bond angles/deg S6-CuII-S5 S6-CuII-S1 S5-CuII-S1 S6-CuII-S2 S5-CuII-S2 S1-CuII-S2 S6-CuII-CuI S5-CuII-CuI S1-CuII-CuI S2-CuII-CuI

105.60 114.62 102.28 111.41 115.02 107.76 129.94 124.37 56.68 52.11

double bond characteristic of the CdS bond is reduced due to the interaction between the sulfur atom and cuprous ion in the coordination process. Therefore, the theoretical prediction is in good agreement with the electrochemical results that TU is bonded to copper via its sulfur atom. 3.3. The Transference of TU as a Corrosion Inhibitor to Corrosion Promoter. In the EQCM plots (Figure 5), the mass loss of copper gradually increases in the corrosion electrolyte of 0.5 M H2SO4 and various concentrations of TU. The copper electrode can be corroded in 0.5 M H2SO4 even without any

Figure 9. Comparison of the typical potentials (1a, 2a, 3a) and mass (1b, 2b, 3b) variation curves of copper electrode at different TU concentrations monitored by EQCM in 0.5 M H2SO4 with and without stannous ion.

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Figure 10. Comparison of the potential (a) and mass variation rate (b) of copper electrode at TU concentration from 0.0 to 500 mM monitored by EQCM in 0.5 M H2SO4 with and without stannous ion.

Figure 11. SEM micrographs of quartz gold electrode electrodeposited by copper (a) and after being immersed 10 min in 0.5 M H2SO4, 0.1 M TU, and 0.1 M SnCl2 electrolyte (b). The insets are the corresponding digital camera pictures.

TU as shown in Figure 5a. The mass loss is tremendously more inhibited in 0.5 M H2SO4 solution containing 1-3 mM of TU than that in blank solution as shown in Figure 5a. The results revealed that TU is a typical corrosion inhibitor for copper at lower concentrations. However, the corrosion is seriously accelerated when the concentration of TU increass from 10 to 500 mM as shown in panels a and b of Figure 5, indicating

that the TU can act both as a corrosion inhibitor and a promoter for Cu depending on its concentration. The theoretical calculation shows that the relative energy of TU2-2S-Cu4 to the total energy of two TU molecules and one Cu4 cluster is 136.6295 kcal/mol. In addition, the relative energy of TU-S-Cu4 to the total energy of one TU molecule and one Cu4 cluster is 29.0004 kcal/mol as illustrated in Figure 6. Obviously, the TU-S-Cu4 structure is most stable. Therefore, the TU can chemical adsorb on the copper surface stably via the bridge bond of Cu-S. The unstable species, TU2-2S-Cu4, will emerge at higher TU concentrations. The effect of TU coordination on the charge distribution of the top copper atom in the Cu4 cluster was investigated by the NBO method and shown in Figure 7. The charge increased from 0.297 (in TU-S-Cu4), to 0.374 (in TU2-2S-Cu4), to 0.513 (in TU3-3S-Cu4) due to the increase in the ligand number from 1 to 3. Additionally, the NBO charge of the sulfur atom is -0.475 in TU-S-Cu4, which is lower than that of the total charge of two sulfur atoms -0.684 in TU2-2S-Cu4, even to -0.944 in TU3-3S-Cu4. This indicates that part of the charge of the copper atom can be deprived by chemically adsorbing sulfur atoms. Therefore, the copper atom will deviate from the Cu4 cluster when the TU concentration is increased to a critical value. The

Figure 12. Optimized geometries, total energy, and molecular orbital of TU and Sn4 cluster and their coordination compound in different coordination atoms (TU-S-Sn4 and TU-N-Sn4).

13552 J. Phys. Chem. C, Vol. 112, No. 35, 2008 copper corrosion process is accelerated due to the formation of the complex of TU and copper. The theoretical calculation results can fully explain the EQCM measurement results. The molecular crystal of the Cu(I)-TU complex is illustrated in Figure 8 and Table 1. The chemical valence of the copper atom is 1.0, which has been previously reported.12 The complex consists of two Cu(I) cations and six TU molecules. The two TU molecules are bonded to the Cu(I) ion in monodentate fashion. The bond lengths of CuI-S3 and CuI-S4 are 2.2936 and 2.3282 Å, respectively, as shown in Table 1. The adjacent Cu(I) ions are bridged by S1 and S2 from the two different TU molecules through bidentate chelating modes. The bond lengths of CuI-S1 and CuII-S1 are 2.4874 and 2.3815 Å, respectively. The four-coordination environment around each Cu(I) ion can be described as a distorted tetrahedral geometry. 3.4. Spontaneous Replacement of Copper by Tin. The OCP of the Cu electrode in 0.5 M H2SO4 is more positive than that in the electrolyte containing stannous ion at the TU concentration below 0.1 M as shown in panels 1a and 2a of Figure 9 and panel a of Figure 10. However, the OCP of the Cu electrode in 0.1 M TU solution shifts toward a more negative value than that in solution containing both stannous ion and TU as illustrated in panel 3a of Figure 9 and panel a of Figure 10. In the latter case, the stannous ion can be spontaneously deposited on the surface of the Cu electrode by replacing Cu atoms. The mass loss rate of the copper electrode is significantly reduced in the electrolyte containing stannous ion as shown in panels 1b, 2b, and 3b of Figure 9 and panel b of 10. It is found that tin cannot be deposited on the Cu surface at a TU concentration less than 0.1 M, while it can be deposited at a TU concentration of more than 0.1 M. Obviously, the stannous ion can inhibit the corrosion of Cu either by changing its surface potential or by replacing the surface atoms. It is clear by comparing the SEM micrographs and the corresponding digital camera pictures of the sample before and after being immersed in 0.5 M H2SO4, 0.1 M TU, and 0.1 M SnCl2 solution that the surface of Cu was coated by tin and the surface was silver in color after the treatment (see Figure 11). The elemental analysis that shows the existence of tin has been reported elsewhere.11 It should be noted that the mass lose of the Cu electrode continues during the replacment process instead of the increase in the weight if the replacement is a two-valence process for both elements (the atomic weight of tin is 118.7 and that of copper is 63.5). This is due to the replacement is a one-valence process for Cu to form cuprous ion which was measured in previous report.12 Figure 12 shows that the relative energies of TU-S-Sn4 and TU-N-Sn4 to the stable structure are 31.8235 and 51.7476 kcal/ mol, respectively. In addition, the relative energy of TU-S-Cu4 (29.0004 kcal/mol) is lower than that of TU-S-Sn4 as illustrated in Figures 2 and 12. It is clear that the structure of TU-S-Sn4 is more stable than that of TU-N-Sn4. Furthermore, the distance of the sulfur atom to metal cluster is significantly different in the structures of TU-S-Cu4 and TU-S-Sn4. The distance of S-Cu (2.294 Å) is shorter than that of S-Sn (2.737 Å) as shown in Figures 3 and 12. Therefore, the TU-S-Cu4 compound can be easily formed in the replacement process according to the analysis of the coordination energy. The fact that the HOMO energy of TU-S-Sn4 (-0.16095 au) is closer to the HOMO energy of TU (-0.22929 au) than that of TU-S-Cu4 (-0.12527 au) indicates that the interaction between sulfur and tin is weak. Further evidence is that the distance of SfCu (in TU-S-Cu4)

Cui et al. is much shorter than that of SfSn (in TU-S-Sn4). Therefore, the stannous ions in TU-S-Sn4 are easier to deposit by replacing Cu. 4. Conclusions The interaction between thiourea (TU) and the copper electrode has been investigated by electrochemical quartz crystal microbalance (EQCM) and density functional theory (DFT). The EQCM results indicate that TU can transfer from corrosion inhibitor to promoter for copper dissolution by changing the TU concentrations. It was found that the open circuit potential of the copper electrode gradually decreases with the increase in the concentration of TU due to the chemisorption of sulfur atom in TU on the surface of copper. The reason for TU transferring from corrosion inhibitor to promoter at higher TU concentrations is that TU can form a stable complex with cuprous ions to reduce the free cuprous ions in the solution. The analysis of the molecule energy and molecular orbital in the complexes of TU and cuprous ions or stannous ions revealed that TU-S-Cu4 and TU-S-Sn4 are the stable complexes. The thermodynamically impossible replacement of Cu by Sn can occur in the presence of TU by reducing the OCP of the copper electrode to a more negative value than the redox potential of Sn2+/Sn. This phenomenon was explained based on the firstprinciples calculation. The replacement mechanism deduced based on the first-principles analysis is universally applicable to the alloy deposition, corrosion inhibition, and surface treatment. Acknowledgment. This work was supported by the Scientific Research Foundation for Young Teachers of Sun Yat-Sen University (2006-31000-1131214), the Guangdong Science and Technology Key Projects (2007A010700001, 2007B090400032), and Guangzhou Science and Technology Key Project (2007Z1D0051). References and Notes (1) Turbini, L. J. J. Mater. Sci.: Mater. Electron. 2007, 18, 147. (2) Nousiainen, O.; Putaala, J.; Kangasvieri, T. J. Electron. Mater. 2007, 36, 232. (3) Almeida, M. F.; Xara, S. M.; Delgado, J. Waste Manage. 2006, 26, 466. (4) Pussi, K.; AlShamaileh, E.; McLoughlin, E. Surf. Sci. 2004, 549, 24. (5) Upadhyay, D. N.; Yegnaraman, V. Mater. Chem. Phys. 2000, 62, 247. (6) Cao, P. G.; Yao, J. L.; Ren, B.; Gu, R. A.; Tian, Z. Q. J. Phys. Chem. B 2002, 106, 10150. (7) Garcia´, G.; Macagno, V. A.; Lacconi, G. I. Electrochim. Acta 2003, 48, 1273. (8) Liu, Z. J.; Wu, G. Z. Chem. Phys. Lett. 2004, 389, 298. (9) Shen, C. B.; Wang, S. G.; Yang, H. Y. Corros. Sci. 2006, 48, 1655. (10) Hepel, M.; Cateforis, E. Electrochim. Acta 2001, 46, 3801. (11) Zhao, J.; Li, N.; Cui, G. F.; Zhao, J. W. J. Electrochem. Soc. 2006, 153, C848. (12) Zhao, J.; Li, N.; Gao, S.; Cui, G. F. Electrochem. Commun. 2007, 9, 2261. (13) Turcio-Ortega, D.; Pandiyan, T.; Czuz, J.; Garcia-Ochoa, E. J. Phys. Chem. C 2007, 111, 9853. (14) Rodriguez-Valdez, L. M.; Martinez-Villafane, A.; Glossman-Mitnik, D. J. Mol. Struct. (THEOCHEM) 2005, 713, 65. (15) Lachenwitzer, A.; Magnussen, O. M. J. Phys. Chem. B 2000, 104, 7424. (16) Nicic, I.; Liang, J.; Cammarata, V.; Alanyalioglu, M.; Demir, U.; Shannon, C. J. Phys. Chem. B 2002, 106, 12247. (17) Takada, K.; Storrier, G. D.; Goldsmith, J. I.; Abruna, H. D. J. Phys. Chem. B 2001, 105, 2404. (18) Grez, P.; Celedon, C.; Molinari, A.; Oliva, A.; Orellana, M.; Schrebler, R.; DelRio, R.; Cordova, R. J. Phys. Chem. B 2005, 109, 22920. (19) Kawaguchi, T.; Yasuda, H.; Shimazu, K.; Porter, M. D. Langmuir 2000, 16, 9830.

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