Langmuir 1993,9, 186-191
186
Surface Structure Determination of Thin Films of Benzimidazole on Copper Using Surface Enhanced Raman Spectroscopy Mary L. Lewis, Lars Ledung,+and Keith T. Carron' Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071 -3838 Received June 8, 1992. In Final Form: October 9, 1992 A comparison of three copper-benzimidazolecom lexes and the surface-benzimidazole specieson copper foil iepreaented. The complexes are Cur(BIM-),CuaT)BIM-)t,and Cun~(BIMH)1~(BIM-)2(SOl)c6(C2HaOH); BIMH = benzimidazole. It was found that the surface speciescorrespondeprimarily to the Cu(1)complex. The amount of Cu(1I) complex WEIS found to be solventdependent. Oxidation of the surface complex with time was not observed. The complexes are characterized with chemical analpis, Fourier transform IR, Raman spectroscopy, W-vie-near-IR, and magnetic susceptibility. The surface species wera analyzed with surfaceenhancedRaman spectroscopy. Severaloxidationstate and coordinationsensitivevibrational bands are reported.
Introduction Azole compounds have been suggested as corrosion inhibitorsfor copper and ita alloys. Azoles containmultiple sites for coordination with copper and can form multidimensional polymeric materials. Many of these systems have been extensively studied and it has been found that X-ray the chemistry at the copper surface is photoelectron spectroscopy (XPS),FT-IR, and Raman have been most extensively used in the study of these coatings. XPS has been found to be valuable in analyzing them coatings. However, it is not molecule specific and, therefore, cannot differentiate between the formation of a copper oxide underlayer and the copper-azole complex. Weha~foundthatsurfaceenhanoedRamanspectroecopy (SERS)can provide new information due to ita molecular specificity. SERS is very sensitive to the first couple of
* To whom corrmpondence should be addressed. 'Current e d k Royal Institute of Technology, S-100-44 Stockholm,Sweden.
(1) Cotton, J.; Scholea, I. Br. Corroe. J. 1967,2,1. (2)Roberta, R J. Electron Spectroec. Relot. Phenom. 1974,4,273. (3)Ogle, I.; Pol@, G.Con. Metollurg. Q. 1976,14,37. (4)Notoya, T.; Poling,G.Corr.-NACE 1976,32,316. (6) Chadwick, D.;Haehemi, T.Corros. Sci. 1978,18,39, (6) S i d e , A.;Velapoldi, R.; E r i c b n , N.Appl. Surf.Sci. 1979,3,229. (7)Fox. P.:Lewis. G.:Boden. P.Corros. SCL.1979.19.467. (8) Keeibr,J.; Furt;lk, T.;Bewlo,A. J. Electrochem. SOL; Electrochem. Sci. Technol. 1982,1716. (9)TomH.; Shanna, S. Surf. Interface Anol. 1982,4,261. (10)Y d d a , S.;Lhida, H. J. Chem. Phye. 1983, 78,6960. (11)Tompkinn, H.; h a , D.;Pasteur, G.Surf.Interface Anol. 1983, c .r..
0,lUl.
(12)Y d d a , 8.; Iahida, H. J. Moter. Sci. 1984, 19,2323. (13)Fleinchmann, M.; Hill, I.; Mengoli,G.; Mueiani, M.;Akhavan, J. Electrochim. Acto 1986,30,879. (14)G e r , D.;Gorvin, A.; Gutteridge, C.; Jackeon, A.; Raper, E. corror. Scr. 1986,!a, 1019. (16)Thierry,D.; Leygraf, C. J. Electrochem. SOC.:Electrochem. Sci. Technol. 1986,IsS,2236. (16)Muaiani, M.;Mengoli,G.; Flehhmann, M.; Lowry,R. J. Electroonol. Chem. 1987,217,187. (17)Drolet, D.;Manuta,D.; Leee,A.; Katnani, A.; Coyle, G. Inorg. Chim. Acto 1988,146,173. (18)Haehemi, T.;Hogarth, C. Electrochim. Acta 1988,33,1123. (19)Xue,G.;Ding, J.; Wu,P.;Ji, G.J. Electroonol. Chem. 1989,270, 163. (20) Xue,G.;wlang,J.; Shi, G.; Wu,Y.J. Chem. Soc., Perkin Tram. 2, 1989,33. (21)Youdn,R.;Niahihara, H.; Aramaki, K. Electrochim. Acta 1990, 36,1011. (22)Xue,G.;Ding. J. Appl. Surf. Sci. ISSO,40,327. (23)Carron, K.;Xue,G.; Lewis, M. Langmuir 1991,7, 2. (U)Xue,G.;Zhang, J. Appl. Spectroec. 1991,46,760. (26) Xue,G.;Dm, J.; Lu,P.; Dong, J. J. Phye. Chem. lSSl,96,7380. (26) Clerc, C.;Alkire, R. J. Electrochem. Soc. 1991,138,25.
0743-7463f93/24oS-0186W.oO/0
monolayereon the copper surface,and the oxide formation under the copper-azole overlayer is clearly visible. The accepted procedure for the formation of compact anticorroeion films with imidazoleson copper iavolvesthe reaction between the copper oxide and the imidazole? Recently,Xue et al. have promoted a mechanisminvolving a reac$ion with copper metal rather than the oxide.28 They presented a SERS spectrum of an uncoated freshlyetched copper surface. Initially, no oxide peaks were visible. However, copper oxide is a very weak Raman scatterer and monolayer amounta of oxide may not have been detectable. They also noted a more intense S E W spectrum of an azole-treated copper surface when the surface had been freshly etched in comparison ta one that had an oxide coating from 30 min of exposure to air prior tocoating. The lower intensity from the thickoxide surface is more likely due to a lose of the S E W effect, not from a decrease in reactivity of the oxide over the metallic copper. The substrate with a thick oxide coating will be less enhanced since the SERS effect drops off ae 1/rlo from the center of curvature of the metal roughness features.27 h u l t a from severalXPS studies have quite conclusively demonstrated that the oxidation state of the copper in the surface complex of films of both benzimidazole (BIMH) and benzotriazole (BTAH) is initially compoeed of Even though X P S cannot differentiate between the formation of a copper oxide underlayer and the copper-azole complex at short exposure times, there is not an oxide underlayer present and therefore the azole copper coating is probed. Some previous studies have reported the preaence of Cu(I1)after short exposuretimes in a S or long exposures to solutions of excess ligand.2 We will provide evidence for a coating composed of a stable Cu(I)-azole complex with a small amount of a Cu(II) complex with growth of Cu(1) and Cu(I1) oxide around the surface coating. The growth of a Cu(I1)oxide along with the more predominant Cu(1) oxide is consistent with previous studies of the oxidation of untreated copper at ambient temperatures.% Determination of the surface structure requires the synthesis of bulk compounds which mimic the surface (27)Murray, C.In Surfoce Enhomed Roman Scattering; Chang, R., FurtaL, T.,W.; Plenum Press: New York, 1982;p 203. (28)Machefert, J.-M.;Lenglet, M.;Blavette,D.;Me&, A,; D'Huywr, A. In Structure ond Reoctiuity of Surfocee; Morterra, C., Zecchi~,A., Costa, G.,Ede.; Elsevier Science Publishers B.V.: Amhrdam, 1989,p 625.
Q 1993 American
Chemical Society
Langmuir, Vol. 9, No. 1, 1993 187
Thin Film of Benzimidazole on Copper
species. In the case of copper chemistry this creates difficulties due to synthetic problems associated with the insolubility of Cu(1) salta. Consequently, most of the previous studies have only examined the Cu(II)-azole complexes. Thismayhave resultedin a false interpretation of the surface species. In this paper we report a comparison of the copper-benzimidazole surface spectrum obtained using SERS to the Raman spectra of bulk Cu(1)- and Cu(ID-benzimidazole complexes.
Table I. Magnetic Suraeptibility derivative l@x* (w) purple (2) white (3) red (1) expt theor
Experimental Section Preparation of Surface Films. The azole compoundswere
%C
purchased from Aldrich and purified by crystallizationfrom an ethanol/water/chloroform mixture. The copper substrates used were 99.999% copper foil (Aldrich, 0.026 mm). The copper substrateswere abrasively roughenedwith a f i e grade sandpaper followed by a chemicaletch in 12% HNOs solutionfor 4 min with vigorous stirring. The SERS enhancement was found not to depend upon the grade of sandpaper used. The foil was then washed with distilledwater and immediatelyimmersed in a 2-3 % ethanolic solution of BIMH. After removal from the BIMH solution the treated copper substrate was washed with ethanol to remove any unreacted BIMH and allowed to dry completely. Since copper is known to react instantly with oxygen, we conclude that azoles are reacting with the f i t couple of monolayers of CuzO that form at a freshly etched copper aurface.29 Preparation of Bulk Compounds. A red copperbenzimidazole complex, 1, was obtained using a procedure similar to that reported by Tompkine et al.” We mixed aqueous solutions of BIMH (inexcess) and cupricchloride. The mixture was heated, stirred, fitered, and washed with warm water to remove any unreacted material. A similar procedure, except with ethanol as the solvent, produced a purple complex, complex 2. Complex 2 was formed from an ethanolicsolutionof BIMH (in excese)and copper sulfate. The mixture was not heated, but stirred, filtered, and waehed with ethanol. A white complex, 3, was produced by a procedure similar to that reported by Xue et d.20 We mixed copper powder (in excess) and an ethanolic solution of BIMH. The copper powder was first etch$ in dilute nitric acid, thoroughly washed with water, and then waehed in ethanol. Awarm ethanolicsolution of BIMH was added to the copper and the container was capped to limit exposure to oxygen. It nee& to be emphasized that there waa not strict e l i i t i o n of oxygen. No reaction occurred when oxygenwas rigorously eliminated. The mixture was continuously stirred for eeveral days. Complex 3 was separated from the residual copper with a centrifuge. We found that 3 ie air stable when it was thoroughly washed. Bulk Raman and SERS spectra were obtained with a Spectra Physics 2026Kr+laser operatingat 647nm. The detectionsystem was a Photometrics CCD9OOO spectroscopic system. The CCD temperature was held at -102 OC. We found that optimal signal to noise was obtained with a binning of 30 pixela/group. The detector was mounted on a ISA HR-320 spectrograph with a 1200grooves/mm ion etched grated blazed for 650 m.The slit width was eet to 20 fim. The laser power was 100 mW and a line focus produced by a SO-mm cylindrical lens (Mellea Griot) was used for sample illumination. The Raman ecattered light was collected with a F1.8 Minolta camera lens with approximately a 4 1 magnificationfor f-numbermatching with the spectrograph. The FT-IRspectra of the bulk samples as KBr pelleta were obtained using a Perkin-Elmer ls00 series FT-IRin the region between 4400 and 460cm-1 with 2 cm-l resolution. The reflection W-vie-near-IR spectra of the samples were obtained using a Perkin-Elmer Lambda 9 with a diffuse reflectance attachment and a Kubelka-Monk algorithm for correction to absorbance. Magnetic susceptibilty measurementa were made on a Johnson Matthey magnetic susceptibility balance which uses the Evans method.
%S
(29) Rhodim, T.J. Am. Chem. SOC.ISM),70,6102.
4.35 2.06 0.1
red (1)
%H %N 9% Cu %@ % c1
formula
56.15 58.46 3.12 3.38 18.45 18.81 21.38 21.34
1.81 1.70 -0.0
Table I1 purple (2) expt theor 52.19 4.36 16.20 10.61 4.47 12.27
lrrtr (Ird
52.72 4.46 16.20 10.81 4.36 11.43
White
expt
(a) theor
46.74 46.53 2.77 2.79 16.85 16.50 35.00 36.17
0.00
Cu(BIM)**
C&(BIMH)la(BIM)r Cu(B1M) (S04)4.6C2HsOHC 0 Calculated by difference. * BIM = benzimidazolate anion = CTHSNZ-.BIMH = benzimidazole = C&”.
Results and Discussion
We prepared three bulk copper-benzimidazole compounds for comparison with the copper-benzimidazole surface coating. We were concerned with complexes formed in ethanol since ethanol is the solvent of choice for fabricatingthe surface film. Therefore, in addition to the common copper(II)-benzimidazole compound, l, that is typically prepared in aqueous solution, a second Cu(II) compound, complex 2, was prepared in ethanolicsolution. A third copper(1)-benzimidazole compound, complex 3, was also prepared. The bulk compounds were analyzed by chemical analysis,FT-IR,W-vis-near-IR, and magnetic susceptibility. The n o d r a m a n spectraofthebulk compounds were then compared to the SER spectrum of the surface coating. This has allowed us to identify the surfacespecies. The analysis of the bulk compounds will be presented followed by the SEW3 studies. Analysis of Bulk Compounds. Magnetic S u m p tibility. The magnetic susceptibility of the three bulk samples was measured and the resulta are given in Table I. The resulta indicate that 1and 2 are Cu(I1) compounds and 3 is a Cu(1) compound. Chemical Analysis. Table I1 shows the rwulta of chemical analysis (Desert Analytic, Tuecon, AZ) of the three bulk compounds prepared. Complex 1, prepared in aqueous solutions of BIMH and copper chloride, shows the simple stoichiometry of Cun(BIM)2. Complex 8, prepared in ethaaolic solutions of copper powder and BIMH, shows the simplex stoichiometry of CuI(BIM). Complex2, prepared from ethanolicsolutionsand BIMH and copper sulfate, ismore complex. The chemicalgives a stoichiometry of CU~~~(BIMH)~~(BIM)~(SO~)~ S(C2HsOH). Complex 2 is stable in hot ethanol but in water quicklyconvertetocomplex1. Inethanolthesulfab group is insufficiently solvated and, therefore,is expected to remain coordinatedwith the copper. However, in water the sulfate group is easily solvated along with the acidic proton of the imidazoleringto allow formation of complex 1.
FT-IRAnalysis. T w o normal coordinate8LLBly888 have been published on BIMH.N*31The asaigamenta given by each are quite different. Reference30used a CZVsymmetry based on strong intermolecular hydrogen bonding and (30)Cordes, M.; Waltar, J. Spectrochim. Acta, Part A 1968, BI, 1421. (31)M o b , S.;Sundaraganeaan,N.;Mink,J.Spcctrochim. Acta 19B1, 47A, 1111.
Lewis et al.
188 hngmuir, Vol. 9, No.1, 1993
15%
1600 1400 1200 1000
800
C” Figure 1. FT-IRspctraof (a) BIMH,(b)complex2, (c)complex 1, and (d) complex 3.
similarity with crystal structures of imidazole (a crystal structure has sincebeen obtained for BIMH and has shown that it indeed crystallizes with CzUsymmetry due to intermolecularhydrogen bonding with four molecules per unit cells2). Reference 30 included isotopic substitutions to aid in their analysis; however, they only used infrared analysis without complementary Raman data. In a moleculewith Czusymmetry,A2 modes will be IR inactive. Therefore, without these frequencies, the normal mode analysis is greatly compromised. A more recent normal coordinate calculation has been published by Mohan et al.31 They included both Raman and FT-IR spectra. However, ignoring any hydrogen bonding contribution, they assumed C, symmetry. Curiously, they present an FT-IRspectrum of solid BIMH in a KBr pellet that lacks any hydrogen bonding at all. This is highly suspect. Furthermore, they did not present any isotopic substitution results. Consequently,the assignmentsfrom these papers muet be treated as quite tentative. The assignmenta we have used are those of ref 30, but only when they are in agreement with our own isotopic, solution, and polarization studies. Future work in this laboratory includesour own normal coordinate calculationon BIMH to better assign the spectrum. Figure 1 shows the FT-IRspectra of the three bulk compounds along with BIMH. Only the region from 1700 to 800 cm-l is shown for clarity. The bands associated with the pyrrole hydrogen are labeled in the spectrum of BIMH (Figurela). T h e 1134-,1301-,and 1687-cm-lbands contain a component of the N-H in-plane bending mode. These bands will be monitored to help determine the type of coordination with the metal atom. Figure l b contains the FT-IRspectrum of complex 2. The chemicalanalysis indicated that this product contains coordinated sulfate groups with a copper to sulfate ratio of 6 to 1. The FT-IRspectrum contains strong bands in the 960-1160 cm-l region. These bands are indicative of a bridgingsulfate group. In a bridging sulfate, the u3 band splita into three bands due to a lowering of symmetryfrom (32)h d e , P.;Galigne, J. Acta Crytallogr. 1974, B30, 1647.
Td to C Z , , . ~We ~ observe these at 1162, 1114, and 1072 cm-l. There is a peak at 1112 cm-I in BIMH; however, this peak is weak, whereas in complex 2 the peak at 1114 cm-l is very strong. The u1 sulfate band is observed at 973 cm-l. Goodgameand Haines also noted this split and the possibility of a bridging sulfate in a compound prepared in a manner similar to ours.34 It is not possible to monitor the N-H in-plane bending mode at 1136 cm-I due to the strong sulfate bands in this region. There are two peake in the 1600-cm-lregion. Theseoccur at 1696an 1626cm-l. This may be due to the two forma of BIMH present (the neutral BIMH and the benzimidazolate anion). A similar doublet is seen for a chlorideproduct produced in a similar manner substituting CuC12 for CuSOl for the starting reagent. Only a single band is seen in this region for the copper-benzimidazolate complexes 1 and 3. Spectra of complex 1and complex 3 are shown in parte c and d of Figure 1,respectively. These spectra are quite similar. It is noted that the N-H in-plane bending modes are absent. The accepted mechanismfor reaction includes the formation afthe benzimidazolate anion and then the formationof a polymeric complex by reaction with copper. The product, therefore, should not contain any N-H modes. Xue et al. observed N-H bands in the infrared spectrum of complex 3 and attributed this to a Cu(0)BIMH complex.2o Our magnetic susceptibility measurements, chemical analysis, and infrared spectra indicate that this cannot be a Cu(0) compound. The appearance of N-H bands was probably due to unreactsd BIMH from an insufficiently washed sample. These two compounds differ in the oxidation state of the copper, and therefore the geometry, coordination number, and strength of the Cu-N bond will be quite different. Despitethese differences,the environmentthat the BIMH moiety sees must be quite similar since the vibrational modesare so similar in intensity and frequency. There is a peak, however, in the region of the imidazole ring bending modes which is significantly shifted from complex 1to complex3. The band is at 906 cm-l in complex 1 and shifts to 919 cm-1 in complex 3. It is T b 1in0the to expect the shift to be due to the different strengt Cu-N bond. The region above 2600 cm-1 (not shown),which includes the C-H and N-H stretching region, is a large unresolved envelopedue to hydrogen bonding in both the bulk BIMH and complex 2. Complexes 1and 3 show very weak C-H stretches near 3100 cm-l and do not show N-H stretching bands. Raman Spectroscopy. Raman spectra of the bulk copper-benzimidazole complexes and bulk BIMH from 900 to 1400 cm-I are shown in Figure 2. We found two regions of the spectra which are indicative of the coordination and oxidation state of the copper. In the 9601050cm-l regionthec-Hbendingofthe bridgeheadcarbon on the imidazole shifts characteristically depending on the oxidation state of the copper. The 1260-1360 cm-1 region also contains bands which characterize the nature of the copper-benzimidazole complex. In Figure 2a we show the normal Raman spectrum of BIMH. The 960-an-’ band has been assigned to the inplane bending of the bridgehead carbon. In the spectra of the bulk copper-benzimidazole compounds this band appears to shift to higher frequency. In Figure 2b the band appears at 986 cm-l in complex 2. In Figure 2c,d the Raman spectra of complex 1 and complex 2 are shown. (33) Nakamotq K.Infrared and Raman Spectra of Inorganic and Coordination Compou& John Wiley and Sons: New York, 19SB;p 2M). (34)Goodgame, M.;Hainer, L.J. Chem. SOC.A 1966, 174.
Langmuir, Vol. 9, No. 1, 1993 189
Thin Elma of Benzimidazole on Copper
c"
h
12'1 1135 125 1301
I
W l
985,
I
I
I
1000
I
1200
I
I
1400
AT(cm-') Ramanspectraof (a)BIMH,(b)complex2,(c)complex 1, and (d) complex 3. The bands labeled are discussed in the taxt.
F-2.
The band appears at 1020 and 1032 cm-l, respectively. (Deuteration studies of complex 1c o n f i i that this mode i n d d has a componentof the bridgehead carbon, although it may be small. Upon deuteration, only the bridgehead carbon is replaced. A 5-cm-1 shift to lower frequency is seen for this band.) The frequency shift is not as drastic in complex 2, the compound with mainly coordinated neutral BIMH molecules. This indicates a weaker Cu-N bond for these compounds causing less of a perturbation in the C-H mode. The largest shift in frequency is observed for complex 3. If only complexes 1 and 3 are compared (since these are similar compounds except for the oxidation state of the copper), it is probable that there is greater r bonding in the dlOCu(1)compound as compared to the d9 Cu(II) compound. Assuming that the u bonding is similar between the two, this would cause an increase in overall bond strength for complex 3, thus accounting for the greatest perturbation in the C-H bending mode. The 1250-1310 cm-1 region also contains bands which characterize the nature of the copper-benzimidazole complex. In this region, four bands can be found in the spectrum of BIMH. These are found at 1247,1254,1271, and 1301 cm-1. There is a single intense band (the 1271-cm-l band). The frequencies are somewhat shiftsd upon complexation with copper and the intensities are similar to the uncomplexed ligand with the notable exception of the highest frequency band. The 1247-cm-l band is assigned to an imidazole ring stretching mode. This peak can be seen to shift to lower frequency in all three compounds. These appear at 1239, 1232,and 1233cm-l for complexes 1,2, and 3, respectively. The peak at 1254cm-1 is a small shoulder in the BIMH spectrum and was not resolved in either of the papers preeenting a normal coordinate analysis of BIMH and therefore is unassigned. However, we believe this to be an imidazole ring related mode as it shifts upon coordinating with copper. The peak appears at approximately 1266cm-l in complex 1and 3 and at 1258cm-l in complex 2.
A strong band can be seen at 1277 cm-l in the Cu(I1) compounds, 1 and 2 and at 1286 cm-1 in the Cu(1)
10000
SO000
50000
cmFigure 3. Reflection UV-vis-near-IR spectra f (a) BIMH, (b) complex 3, (c) complex 1, and (d) complex 2.
compound, 3. This seems to correspond to the 1271-cm-l band of BIMH, which is assigned as an in-plane bending mode of the benzene ring. The 1301-cm-l band in BIMH has been assigned to a combination of C-N stretching and N-H bending. We believe this to be mainly a C-H in-plane bending mode of the bridgehead carbon, based on ita absence in the spectrum of the deuterated compound of complex 1. This band can be seen at 1297 cm-l in complex 1, at 1306 cm-l in complex 2, and at 1298 cm-1 as a shoulder in complex 3. This mode is quite strong in complexes 1 and 3 and shifted to somewhatlower frequency. However, in complex 2 it more closely resembles the uncomplexed ligand in intensity and is shifted to slightly higher frequency. Consequently, this band seems to be indicative of the benzimidazolatecopper complexes. The N-H in-plane bend at 1135 cm-l is absent in all three bulk complexea. Complex 2, however, ia believed to contain neutral BIMH molecules in coordinationwith the copper. It is assumed that this peak has shifted to slightly higher frequency upon coordination. This would follow the trend for coordinating amine compounds. Due to electron withdrawal from the N-H bond upon coordination, N-H stretching modes shift to lower frequencies, while N-H deformation and rocking modes shiftto higher frequencie~.~~ New bands are seen in the 1150-1200 cm-l region. The bands are broad here and there is interference with the sulfate bands, both of which preclude assigning a specific band to this mode. UV-Vis-Near-IR.Figure 3 shows the reflection UVvis-near-IR spectra of the three bulk compounds along with BIMH. Figure 3a shows the spectrum of BIMH. It contains large r-r* bands in the 36000-50000 cm-l region. The spectrum of complex 3, Figure 3b, is very similar. This is to be expected since Cu(1) is d1O and will not have d-d transitions. The spectrum of complex 1,Figure 3c, has been discussed previously by Goodgame and Hai11es.3~ They have as(35) N b o t a , K.Znfrored and Roman Spectro of Inorganic ond Coordination Compounds; John Wiley and Sons: New York, lW,p 191. (38) Carron, K.;Hurley, G. J. Phyr. Chem. 1991,96,9979.
Lewis et al.
190 Longmuir, Vol. 9, No.1,1993 signed a distorted tetrahedral symmetry to the complex. Copper(I1)-imidazolate complexes have been found to contain half of the coppers in a square-planar geometry and halfin tetrahedral geometry.37 Manypreviousstudies have represented the Cu(I1)BIMH structure to be square planar.17Jg However, due to steric hindrance in the larger BIMH complex, square-planar geometry is not Aside from steric hindrance the complex should deviate from a perfect tetrahedral arrangementdue to Jahn-Teller distortion since Cu(II) is dg. The geometry is assumed to be nearly Du. T h e states arising from the 2Dfree-ion term in order of increasing energy are 3 2 , 2E,3 1 , and 2A1. The assignments of the two low energy bands in this spectrum are 2Bz 2Eat 7090 cm-l and z B 2 2A1 at 11300 cm-1. The large band at 20 500 cm-l is assigned to a ligand to metal charge transfer band. Assignmentshave not been made higher than 22 OOO cm-l. T h e third d-d transition PBz 2B1) may not be resolved in the reflectance spectrum as the 2B1 and 2A1energy levels are fairly close together and it will be weak since it is formally forbidden. The peak in the visible region around 25 OOO cm-1 may be a second ligand to metal charge transfer band arising from the second high lying a level of BIMH. T h e electronic absorption spectrum of complex 2 seen in Figure 3d contains a d-d band at 17 OOO cm-' with a smallshoulder at 13 8OOcm-l. The simplicityof this region indicatesa higher symmetry for this complex as compared to complex 1. The chemical analpis indicates that the number of ligandscoordinatedto copper is six. A proposed etructure for this compound includes an octahedral
-
-
anangementoftheligandswithtwoBIMHmoleculeearial and the thiid coordinating BIMH, the ethanol, and bridging sulfate or BIM- groups equatorial. T h e charge transfer band is blue shiftedfrom the complex 1compound to 30 900 cm-l. For ruthenium compounds which mimic complex 1 it has been found that the ligand a orbitals tend to be destabilizedin the imidazolatecompounds.This would result in the ligand a orbitals lying closer to the metal in complex 1. A higher energy transition is then expectedin correspondingcoordinatingBIMH compounds (complex 2).3s SERS Analysis. A comparison of bulk 1 and 3 with the surface product is diacuseed here (complex2, a Cu(1I) compoundwithcoordinatingsulfateligands,has~nruled out for comparison with the surface complex). Figure 4 shows the Raman spectrum of the CuLBIM compound, complex 1 (Figure 4a), the CuLBIM compound, complex 3 (Figure4b), and the SERSspectrum of the BIMH-treated surface (Figure 4c). The surface complex most closely matches the Cu(1) compound (complex 3) except for differences in relative intensities. These differences are to be expected due to preferential enhancementa which occur for vibrational modes perpendicular to surfaces.= In the 1OOO-cm-1region the surface spectrum gives peaks at 1001and 1031cm-l. Upon closeexamination, however, a small shoulder can be seen between the two peaks. The shoulder occurs at 1020 cm-1. This indicates a small amount of the Cu(I1) complex (this is discussed further later in thia section). In t h e 1300-cm-' region the surface spectrum shows peaks at 1266 and 1286 cm-l. A small shoulder is detectad at 1297 cm-1. This area clearly matches complex 3 much better than complex 1. The accepted procedurefor treating surfaceswith BIMH is to use an ethanolic solution of BIMH.9 We carried out a SERS study of the surface product when ethanol is (37)Jar&, J.; Web, A. Acta Cvstallogr. 1960, 13, 1027. (38)Lever, A Inorganic Electronic Spectrwcopy, 2nd ed.; Elwvier Science Publishing Company, Inc.: New York, 1984; p 308.
r
moo
AV(cm")
1 L '00
Figure 4. Raman spectra of (a) complex 1 and (b) complex 3 and (c) a SERS of BIMH-coatedcopper foil. The bands labeled are discussed in the text. The inset shows the preaence of a peak at 1297 cm-1. The deconvolutionroutine used an 80% Lorentzian and 20% Gaussian curvefit.
ld00
I
1200
14 30
Figure 5. SEW spectra of the copper-benzimidazole con prepared in (a) water and (b) ethanol. The bands at 1020 an 1091 cm-l are indicative of copper(II)-benzimidmlate and copper(I)-beyimihlate species,respectively. A deconvolution of the pealrs ISshown as inseta. The deconvolution routine ueed an 80%Lorentzian and 20% Gaussian curvefit.
3
replaced by water. F w 6ahows the reaulta of thisstudy. Figure Sa isthe SEW spectrum of BIMH on copperformed with ethanolasthe solvent and F w e 6bis SERSspectrum of azole-treated copper with water used as the solvent. It can be seen from the 1020-cm-1 C-H bending mode that
Thin Films of Benzimidazole on Copper
a larger percentage of the Cu(II) surface complex forms when water is used as the solvent. The inset in this figure &owe a deconvolutionof the 986-1045 cm-l region. Taking the area of the peaks we found that the percentage of Cu(II) is 72 95 larger when the film is formed in water than in ethanol. Some studies have reported the oxidation of the copper in the surface film with time using X P S . 2 9 5 In contrast, we found that the SER spectrum of the sudace complex did not change with time except for the growth of metaloxide peaks in the 5 o O - ~ m -region.% ~ We also found the bulkcompound,complex3,tobestableinair. SinceBIMH is known to be a poor corrosion inhibitor, it is our observation that the oxidation of the metal surface is occurring around the surface coating. However, since the spectrum of the surfacecoating isnot changing,this cannot be associated with the oxidation of the coating itself. We interpret the growth inthe Cu(II)peak in the XPS analyee-s to be due to the presence of copper(I1)oxide. Since XPS is not molecule specific, it is not able to discern between Cu(II) arising from the formation of surface CuO and/or oxidation of the surface complex. Finally, it is concluded that any source of the Cu(II)azole complex at the surface arises during the initial formation stage of the surface coating. Any water present in the ethanolic solution will oxidize the copper to the +2 oxidation state. However, any growth in a Cu(I1) species
Langmuir, Vol. 9, No. 1, 1993 191
after removal from the ligand solution must be due to cupric oxide since the surface coating has been found to be stable in air.
Conclusions We have made a comparison of bulk copper BIMH complexesand the surface product formed between BIMH and copper metal. We found that the surface species is predominantly complex 3 and that the proportion of Cu(I)/Cu(II)is controlled by the solvent used during the fikn fabrication. Ethanol produced films with the largest percentage of Cu(1)and is known to produce coatingswith the best oxidation pre~ention.~It may be that the corrosion which is observed with BIMH-treated surfaces is related to the Cu(I1) sites or at defecta in the Cu(1) polymeric coating created by the Cu(1I) sites. Acknowledgment. The authom acknowledge the generous support of the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 14-0&0001-G2097. The views and conclusions contained in this document are those of the authors and should not be interpreted necessarily as representing the official policies, either expressed or implied, of the U.S. govemment. We also acknowledge Drs. Patrick Sullivan and P. S. Zacharias for their helpful discussions.
