Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 2, 2018 | https://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch014
14 Surface-Enhanced Raman Spectroscopy as a Method for Determining Surface Structures One- and Two-Dimensional Polymeric Copper Azoles Mary L . Lewis and Keith T . Carron* Department of Chemistry, University of Wyoming, Laramie, WY 82071-3838
We report the results and interpretation of a surface-enhanced Raman spectroscopic (SERS) study of benzimidazole, benzotriazole, and benzimidazole-2-thione on copper surfaces in aggressive media. We have found that SERS is an excellent tool for the study of corrosion inhibition on copper. SERS has enabled us to carry out in situ chemical analysis, isotopic substitution, and orientation determination for the inhibitors on copper. The results indicate an important role of the bridgehead heteroatom in imidazole compounds. We found that the progression of corrosion inhibition is benzimidazole < benzotriazole < benzimidazole-2-thione. Our orientation study of benzimidazole indicates that it is predominantly flat on copper surfaces.
MANY
METHODS EXIST FOR DETERMINING T H E BULK STRUCTURE OF poly
mers. O n e approach that has y i e l d e d very g o o d descriptions o f p o l y m e r i c systems is vibrational spectroscopy (1). O f t e n vibrational techniques are the only choice f o r structure determination because the amorphous nature o f m a n y polymers
rules out the use o f X - r a y diffraction techniques.
infrared a n d Raman
spectroscopy provide a spectrum
Both
o f the vibrational
modes i n molecular a n d p o l y m e r i c systems. T h e compatibility o f R a m a n * Corresponding author. 0065-2393/93/0236-0377$06.00/0 © 1993 American Chemical Society
Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on June 2, 2018 | https://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch014
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STRUCTURE-PROPERTY RELATIONS IN POLYMERS
spectroscopy w i t h all phases o f matter a n d complex environmental conditions makes it an excellent t o o l for i n situ studies o f polymers. As the p o l y m e r films b e c o m e thinner, problems b e g i n to arise w i t h sensitivity. T h e advent o f F o u r i e r transform i n f r a r e d spectroscopy ( F T I R ) has overcome this p r o b l e m f o r i n f r a r e d spectroscopy ( 2 ) . Recently d e v e l o p e d optical m u l t i c h a n n e l systems based o n charge-coupled devices ( C C D ) ( 3 ) and F o u r i e r transform R a m a n spectroscopy ( F T - R a m a n ) (4) are alleviating the sensitivity p r o b l e m for R a m a n spectroscopy. C u r r e n t l y , vibrational spectra can b e obtained v i a either technique o n monolayer films, b u t the i n s t r u m e n tation is very expensive a n d often the resulting spectra are o f l o w quality due to the n e e d f o r l o n g integration times a n d sample degradation.
Surface-Enhanced Raman Spectroscopy O n e technique that yields high-quality spectra o f monolayer films is surfaceenhanced R a m a n spectroscopy ( S E R S ) . T h e S E R S effect arises f r o m t w o sources: c h e m i c a l enhancement ( 5 ) a n d electromagnetic enhancements.
Chemical Enhancement. C h e m i c a l enhancement occurs w h e n the monolayer is c o m p o s e d o f molecules that contain groups o r atoms that c a n coordinate w i t h the metal surface. T h e resulting surface complex c a n f o r m charge transfer states w i t h energy levels i n the metal. T h i s c o n d i t i o n leads to an optical absorption a n d a surface-localized resonance R a m a n enhancement. M a n y o f the molecules studied b y S E R S b e l o n g to this class o f materials. F o r example, pyridine has b e e n exhaustively studied as a p r o b e for the S E R S effect. P y r i d i n e is a g o o d lone-pair donating ligand f o r transition metals. C a r e f u l ultrahigh v a c u u m ( U H V ) studies have shown that it is possible to observe S E R S f r o m pyridine o n substrates that, because o f their m o r p h o l o g y (smoothness), only show the c h e m i c a l effect ( 6 ) . O n the other h a n d , some substrates show b o t h c h e m i c a l enhancement f r o m c h e m i s o r b e d pyridine a n d also multilayers o f physisorbed pyridine. T h e long-range (multilayer) effect precludes the possibility o f a c h e m i c a l effect, a n d therefore, a through-space electromagnetic enhancement has also b e e n p r o p o s e d ( 7 ) . M o s t substrates w i l l show b o t h forms o f enhancement. Electromagnetic Enhancement. I n the experiments presented i n this chapter w e w i l l b e most c o n c e r n e d w i t h the electromagnetic enhance m e n t ( 8 ) . E l e c t r o m a g n e t i c enhancement occurs w h e n the surface contains roughness features smaller than the wavelength o f light. T h e physics neces sary to predict the enhancement was f o r m u l a t e d at the e n d o f the last century by L o r e n t z ( 9 ) . L o r e n t z d e r i v e d the solution f o r the response o f a dielectric sphere i n an electric field. I f the radius o f the sphere is m u c h smaller than
Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.
14.
LEWIS AND CARRON
Surface-Enhanced Raman Spectroscopy
379
the wavelength o f light, the solution to the L a p l a c e equation leads to (10)
E
*n =
,
f
E
,
(1)
0
€(ω) +
2
w h e r e E is the electric field inside the particle, E is the electric field o f the fight incident o n the sphere, a n d ε(ω) is the dielectric f u n c t i o n o f the particle material. T h e purpose f o r L o r e n t z ' s derivation was the understanding o f clouds a n d colloidal dispersions. I n most o f these systems ε(ω) is real a n d positive. H o w e v e r , i n free electron metals e(w) c a n b e negative a n d the imaginary part o f e(
