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Langmuir 1991, 7, 2-4
Le t t ers A Surface Enhanced Raman Spectroscopy Study of the Corrosion-Inhibiting Properties of Benzimidazole and Benzotriazole on Copper Keith T. Carron,' Gi Xue, and Mary L. Lewis Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 -3838 Received May 29, 1990. In Final Form: August 9, 1990 Surface enhanced Raman spectra of benzimidazole and benzotriazole on copper have been obtained. A characteristic imidazole ring stretching mode located near 780 cm-l shows a splitting in the benzotriazolecopper surface complex. This splitting is not observed in benzimidazole. The spectrum of benzotriazole is interpreted as two distinct benzotriazole species on the surface. On the basis of this information a new structure for benzimidazole and benzotriazole on copper surfaces is proposed.
Introduction Heterocyclic nitrogen compounds have found widespread use as anticorrosion agents for copper and its alloys. The accepted procedure for the formation of compact anticorrosion films with imidazole on copper involved the formation of thick oxide coatings. The reaction between the oxidized copper and the imidazole (IMH) is1
+
Cu20 + BIMH = CU"IM + Cu H20 Recently, it has been reported by Xue et al. that imidazole reacts readily with freshly etched copper surfaces.2 It was proposed that the reaction occurred by adsorption through the pyridine nitrogen of the imidazole followed by abstraction of the pyrrole hydrogen. Adsorption through the pyridine nitrogen is believed to inductively lower the pK, of the pyrrole hydrogen. In this study we show that the thin imidazole films formed on freshly etched copper possess very good anticorrosion properties. In this work we will compare oxidation-blocking properties of benzimidazole (BIMH) and benzotriazole (BTAH). The structures of copper complexes of BIMH and BTAH have alluded chemists since they are insoluble polymers and cannot be crystallized. We have found that surface enhanced Raman spectroscopy (SERS) is an excellent technique for the study of thin films since it is very sensitive to the first couple of monolayers and the oxide formation of copper under the CuIM overlayer is clearly visible. SERS has been used to electrochemically characterize imidazoles on copper and silver electrode surface^.^ This is the first paper reporting studies on surfaces exposed under natural atmospheric conditions. We have been able to demonstrate that the order of protection against oxidation is BTAH > BIMH. Furthermore, the Raman spectra indicate that there are two BTAH species present in the surface copper complex while only one form of BIMH is present. On the basis of these observations, we have been able to propose a structure for the three-dimensional complexes.
* To whom correspondence should be addressed.
(1) Fox, P.; Lewis, G.; Boden, P. Corros. Sci. 1979, 19,457. (2) Xue, G.; Zhang, J.; Shi, G.; Wu, Y.; Shuen, B. J.Chem. SOC., Perkin Trans. 2 1989, 33. (3) Kester, J.; Furtak, T.; Bevolo, A. J. Electrochem. SOC. 1982, 129, 1716. Fleischmann, M.; Hill, I.; Mengoli, G.; Musiani, M.; Akhavan, J. Electrochim. Acta 1985,30,879. Youda, R.; Nishihara, H.; Aramaki, K. Electrochim. Acta 1990, 35, 1011.
0743-7463/91/2407-0002$02.50/0
Experimental Section The chemical reagents were purchased from Aldrich and purified by crystallization from ethanol. The copper substrates were 99.999% copper foil of thickness 0.025 mm. The substrates were roughened by etchingin 12% HNOs for 4 min under vigorous stirring. This procedure is similar to that developed by Miller et aL4 The metallic copper samples were prepared by etching the copper with nitric acid, washing with distilled water, immediate immersion in a 2-3 % ethanolicsolutionof the molecule of interest, and a final washing of ethanol to remove any adsorbed reagent. SERS isotherms of the reaction showed that a limiting imidazole thickness is reached in about 40 s. Our data indicate that the azoles are reacting with the first couple of monolayers of CuzO that form at a freshly etched copper surface.6 Samples studying reactions with oxidized copper involved time and heat treatment to facilitate oxidation prior to immersion in the ethanolic reagent solution. The Raman spectra were obtained with a Jobin Yvon Mole 1000 double monochromator, an RCA 31034 PMT, and Ortec photon countingelectronics. Laser excitation was provided with a Spectra Physics 2016 Kr+ion laser. A backscattering geometry was used for all samples and a cylindrical lens was used to focus the laser in order to decrease the power density at the sample. The SERS samples were spun at 1800rpm to avoid any possible damage due to laser-induced heating. All spectra were acquired with 647-nm radiation and laser powers of 50-100 mW. A 647nm interference filter was used to remove most of the plasma lines. A plasma line at 235 cm-l was used to maintain frequency calibration between the spectra. Data collectionand storagewere interfaced through a 386-20 microcomputer. Discussion The oxidation of etched copper can be followed with SERS. This is illustrated in the spectra shown in Figure 1. The large peaks center around 585 and 520 cm-I are due to the symmetric and asymmetric stretches of Cu20. Isotherms were obtained by monitoring the ratio of the 585-cm-l peak to the background at 800 cm-l. The limiting oxide thickness was reached in about 1 h. This is in agreement with the isotherms found by Rhodin using a vacuum microbalance a t a temperature of 323 K.5 The oxide thickness after 1h is approximately six monolayers. This temperature is above room temperature, at which the spectra were obtained, but is reasonable considering the laser heating due to the relativelv high laser Dower (4) Miller, S.; Baiker, A.; Meier, M.; Wokaun, A. J.Chem. Soc., Faraday Trans. I 1984,80, 1305. ( 5 ) Rhodin, T. N. J. Am. Chem. SOC.1950, 70,5102.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 1, 1991 3
Letters
lA
A O
U
lS00 900
-1
200
cm
Figure 1. Oxidation of copper in air: (A) 11-h exposure to air; (B) 2-h exposure to air; (C)1-h exposure to air; (D) 30-min exposure to air; (E) smooth copper exposed to air for 1 h at 80 "C. A-D represent SERS spectra on roughened copper. E is a control spectrum to show that the peaks observed are surface enhanced.
I
IB I
I
A,/-
I
780
600 900
1800
cm
1
oxide
after 12 hours
200
Figure 2. SERS spectra of copper treated with BIMH: (A) roughened copper treated with BIM and exposed to air for 12 h; (B) same sample as part A prior to exposure to air.
used (200 mW) and the unusual morphology produced by etching the copper surface. Figure 2 shows the SERS spectrum of BIMH on copper before (B) and after (A) a 12-h exposure to air. Figure 2A shows that the BIMH has reacted with the thin film of CuzO which forms on metallic copper surface^.^ It can be clearly seen that the BIMH is not efficiently protecting the copper from air oxidation. The BIM peaks are identical with those observed for Cur1(BIM-)2prepared from CuSO4 and BIMH, thus indicating that the solution complex is similar to the complex present on the surface.6 The intensity of the bands scale those of BIMH. Since the spectrum is so weak, it is difficult to apply the perpen(6) Drolet,D.;Manuta,D.;Lees,A.;Katnani,A.;Coyle,G.Inorg. Chim. Acta 1988, 146, 173.
cm
1
,
200
Figure 3. SERS spectra of copper treated with BTAH: (A) roughened copper treated with BTA and exposed to air for 12 h; (B) same sample as part A prior to exposure to air.
dicular propensity rule to determine the orientation of the BIM on the surface. Tompkins et al. were able to show by use of Fourier transform infrared spectroscopy that the molecule orients itself with a preferentially horizontal orientation on the surface of ~ o p p e r .However, ~ they also report that a vertical component is present. Figure 3 illustrates the same experiment as above only using BTAH. Reaction between the BTAH and the thin film of CuzO which forms immediately on freshly etched copper is indicated by the absence of oxide peaks in Figure 3A. The CuBTA- complex was blue, and therefore, resonance enhancement increased the Raman signal. In this case, the complex was formed by reacting copper powder with a 2% ethanolic solution of BTAH. The product of CuSO4 and BTAH was fluorescent and a Raman spectrum could not be obtained. The spectrum of the bulk CuBTA- complex was similar to the SERS spectrum; however, large intensity differences were found. Of significance a band a t 780 cm-1 was found and it contained a shoulder with about the intensity of the main peak. Symmetry lowering will not increase the number of bands in BTA-. Therefore this shoulder indicates the presence of two forms of BTA-. Assuming that the two forms have similar polarizabilities, they should exist a t a ratio of 1:3. This ratio changes to nearly 1:l in the SERS spectrum (Figure 3A). This can be explained by the preferential surface enhancement of bands perpendicular to the surface. These observations are consistent with the model proposed for the surface CuBTAcomplex (see Figure 4). BTAH is known to be a better anticorrosion agent. This is illustrated from the much lower intensity of the oxide bands after 12 h of exposure to air. There is also a clear splitting of the 788-cm-I triazole in-plane ring bending mode.7y8 This band provides a good indicator of the chemical nature of the imidazole ring. In BIMH it appears a t 778 cm-1 and shifts 10 cm-' in BTAH to 788 cm-'. The band does not split in the case of -[Cu(BIM-)z]-. This can be interpreted as two forms of BTA- existing in the surface polymer as opposed to just one form with BIM-. This observation, along with the 1:3 ratio observed in the (7) Tompkins, H.; Allara,D.; Pasteur, G.Surf. Interface Anal. 1983, 5, 101. (8)Cordes, M.; Walter, J. Spectrochim. Acta 1968, 24A, 1421.
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4 Langmuir, Vol. 7, No. 1, 1991
distorted tetrahedral copper points out of the plane and is a site for a bridging BTA-. The bridging BTA- is sterically hindered from further binding through its N2 nitrogen. I t can act as a bridge through which the next planar layer can be formed. This would account for the observation that BTA- is predominantly flat on the ~ u r f a c e . ~ItJ ~also explains our observation of two BTAspecies consisting of 3-nitrogen bonded BTA-'s and 2-nitrogen bonded BTA-'s. Similarly, BIM- can grow a planar complex with one of the tetrahedral sites perpendicular to the surface. In this case all BIM-molecules are identical with bonding through both nitrogens. Therefore, splitting of the 780-cm-' BIM- peak is not observed.
A
8
Conclusion The proposed complexes for BTA- and BIM- provide an explanation for the better corrosion inhibition observed with BTA-. BIM- contains open sites at which oxygen can adsorb directly to copper and initiate the corrosion process. BTA- contains three sites that can bond with copper and, therefore, increase the density of electropositive coppers. This creates a boundary a t which the reaction
B
-
=copper
0
=Carbon
=Nitrogen
o
=Hydrogen
Figure 4. Proposed structures for (A) orthogonal view of CuBIM and (B)orthogonal view of Cu-BTA.
bulk spectrum of CuBTA, makes it possible to assign a structure for the -[Cu(BTA-)2]- surface polymer. BTAis capable of bonding through two or three of its nitrogens. Our proposed structures are illustrated in Figure 4. It consists of a nearly planar array of BTA- bonded to a copper through each of its nitrogens. The copper is assumed to be in a distorted tetrahedral geometry. This is a common geometry for d9 ~ o p p e r .Structures ~ of Cuimidazole)^ have been ascertained and the structure is a mixture of distorted tetrahedral and square planar.'OThe bulkier BIMH or BTAH are sterically hindered and do not allow this structure. Molecular modeling shows that the lowest energy configuration occurs with a distorted tetrahedral copper.ll The fourth coordination site of the (9) Wells, A. F.Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984; p 1118. (10) Jarvis, J.; Wells, A. Acta Crystallogr. 1960, 13, 1027. (11) Modeling was performed with Alchemy 11,1988,TriposAssociates.
surface + 0, 2Cu,O + 4[cu+]h + 4Le-1, cannot occur.13 The transport of Cu+ to the surface is produced by the negative potential caused by the Cu+ vacancies that occur when 02 adsorbs and dissociates on the copper surface. The thin films of Cu-inhibitor complexes were able to stop the rapid corrosion (exponential oxidation). This is probablynot due to the inability of oxygen to penetrate the films. Rather, it is due to the more positive potential a t the ambient interface and it is this more positive potential which inhibits the transport of Cu+ to the surface. Furthermore, the slower corrosion (square root oxidation) cannot occur since there is no concentration gradient of Cu+. The corrosion inhibition is a function of the number of copper sites which are blocked by the inhibitor and the electronegativity of the heteroatoms.
Acknowledgment. This work was supported in part by the Faculty Grant-in-aid from the University of Wyoming and by the NSF EPSCOR Program (Grant No. RII-8610680). Registry No. BIMH, 51-17-2; BTAH, 95-14-7; Cu, 7440-508. (12) Morito, N.;Suetaka, W. J. Jpn. Inst. Met. 1971,35, 1165. (13) Bardeen, J.; Brattain, W.; Schockley, W. J . Chem. Phys. 1946,14, 714.
