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15 Surface-Enhanced Raman Spectroscopy as a Method for Determining Surface Structures Thiophenol at Group Ib Metal Surfaces Keith T . Carron and Gayle Hurley Department of Chemistry, University of Wyoming, Laramie, WY 82071-3838

Raman spectra of noble metal phenyl thiolates and the corresponding surface-enhanced Raman spectroscopy (SERS) spectra of the surface species on copper, silver, and gold are reported. The SERS spectra were used to obtain orientations of the thiophenol species at the noble metal surfaces. Orientations were determined through the electric field enhancement of vibrations normal to the metal surface. The model developed in this chapter allows both the azimuthal and axial angles of C molecules at surfaces to be determined by performing the SERS measurement in media of differing indices of refraction. For silver and gold we found the axial angle θ = 85 and 76°, respectively. The azimuthal angle Фwas found to vary from 32 to 0° from silver to gold. The SERS spectra of thiophenol adsorbed onto copper were too weak for accurate angle determinations with a surrounding media other than air. However, the SERS spectrum observed on copper in air does indicate a near perpendicular orientation. 2v

PROPERTIES O F P O L Y M E R A D H E S I O N T O SURFACES are greatly affected b y the

structure o f p o l y m e r i c materials at interfaces. I n this chapter w e w i l l present a m e t h o d f o r the determination o f orientation o f a two-dimensional p o l y m e r c o m p o s e d o f noble metal p h e n y l thiolates. Organic t h i o l c o m p o u n d s f o r m self-assembled monolayers o n noble metal surfaces (1). Self-assembled monolayers c a n b e f o r m e d f r o m a large 0065-2393/93/0236~393$06.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

394

STRUCTURE-PROPERTY RELATIONS IN POLYMERS

variety o f organic thiols. A n example o f the importance o f self-assembled monolayers is seen i n the n-alkane thiols that f o r m surfaces that have the lowest surface-free energies k n o w n (2). W h e n the n-alkane is replaced b y earboxylie or hydroxyl groups, it is possible to p r o d u c e h i g h free-energy wettable surfaces. T h e macroscopic properties o f the films are d e t e r m i n e d i n part b y the nature of the organic functionality a n d i n part b y the self-assem b l e d structures that f o r m at the surface. T h e goal o f this research is to develop a spectroscopic m e t h o d b y w h i c h the surface structure o f organic thiols o n noble metal surfaces can be d e t e r m i n e d . W e chose thiophenol, p h e n y l sulfide, a n d p h e n y l disulfide as probes to develop a n d test our m e t h o d o f structure determination. Self-assembly is b e l i e v e d to occur o n noble metal surfaces due to A u - S R b o n d strengths that are weak enough to allow lateral surface diffusion, yet strong enough to prevent desorption u n d e r ambient conditions. Because m u c h o f the w o r k has b e e n o n noble metal surfaces a n d the symmetric disulfide stretches i n organic disulfides are not IR-active, many o f the spectroscopic studies have u s e d surface-enhanced R a m a n spectroscopy ( S E R S ) as the spectroscopic tool ( 3 - 6 ) . T h e first reported S E R S study o f thiols a n d disulfides o n a surface was b y Sandroff a n d H e r s c h b a c h (3), w h o f o u n d that the surface products o f t h i o p h e n o l a n d p h e n y l disulfide o n silver were identical. M o r e recently, Sobocinski et al. (4) studied alkyl thiols at electrode surfaces using S E R S a n d made comparisons between the surface interactions o f alcohols a n d thiols. A n o t h e r study o f interest is that b y Joo et al. ( 5 ) that indicates a p h o t o c h e m i c a l cleavage o f one of the C - S bonds i n organosulfides o n silver colloids. I n our study w e f o u n d that the formation o f metal thiolates o n roughened noble metal surfaces i n air is not p h o t o c h e m i cal. H o w e v e r , o u r studies w i t h colloids do c o n f i r m the results o f p h o t o c h e m istry o n colloids a n d r o u g h e n e d surfaces i n aqueous salt solutions. S E R S spectra o f thiophenol, p h e n y l sulfide, a n d p h e n y l disulfide w e r e obtained o n copper, silver, a n d g o l d surfaces. I n all cases the surface products were similar regardless o f the initial material. T h i s result indicates that the metals are capable o f b r e a k i n g the thiol, sulfide, a n d disulfide bonds. T h e surface spectra also closely resemble the b u l k R a m a n spectra o f the metal p h e n y l thiolates. I n C h a p t e r 14 w e o u t l i n e d the origin o f the surface enhancement. W e f o u n d that the general f o r m u l a for the perpendicular a n d parallel electromag netic contributions, Ε a n d E,,, to S E R S is ( 7 ) ±

3e (1 + A ( e - 1 ) )

E„ =

3 (1+A(e-1))


(1)

(2)

15.

CARRON AND HURLEY

Surface-Enhanced Raman Spectroscopy

395

e is the dielectric constant o f the metal a n d A is a geometric

factor

equivalent to ( 8 )

A =

1 - e

2

1 1+e -1 + —In 2e 1 - e

i - l a w h e r e a a n d b are t h e major a n d m i n o r axes o f the ellipsoid, respectively. T h e resonance c o n d i t i o n shifts t o w a r d the r e d as e becomes m o r e negative, w h i c h c a n have drastic effects o n the orientation analysis. F o r copper, silver, a n d gold at 647 n m , € is about —12.2, —18.4, a n d —11.5, respectively ( 9 ) , w h i c h means that t h e ratio o f modes w i t h a single p e r p e n d i c u l a r c o m p o n e n t w i l l b e enhanced 338:1 over p u r e l y parallel modes i f the molecule is o r i e n t e d p e r p e n d i c u l a r to a silver surface. W e have extended the orientation analysis to ascertain n o t only the tilt (or axial) angle o f the ζ axis, b u t also to i n c l u d e the determination o f the azimuthal rotation o f the molecule about the ζ axis. F i g u r e 1 is a d e p i c t i o n o f o u r angle definitions. T h e analysis a n d experiments h e r e i n are f o r molecules w i t h a local C symmetry. I n general, a modes are very weak i n aromatic ring systems because o f the relatively small polarization changes that occur w h e n the ring bends out o f its plane. W e are able to observe a moderately intense b m o d e . I n C h a p t e r 14, the determination o f orientation w i t h b a n d b modes was demonstrated. T h e b modes contain the a polarizability vector a n d the b modes are c o m p o s e d o f the a vector. Unfortunately, b modes are very weak i n the b u l k a n d S E R S spectra o f p h e n y l thiolates (see F i g u r e 3), w h i c h precludes accurate determinations o f surface orienta tions. T h e a modes are very strong i n the R a m a n spectra o f aromatic compounds, a n d c a n b e difficult to use i n orientation analyses because they 2v

2

l

l

2

x

2

x z

y z

2

Y

Figure 1. A definition of the axial and azimuthal angles. The axial angle θ is the angle between the C axis and the sur face. The azimuthal angle φ is the angle of rotation about the C axis of the phenyl ring. In the text θ is equal to 0 when the molecule is oriented perpendic ular to the surface; φ is 0 when the plane of the ring is parallel to the surface. 2

2


396


256

R

1.33

0

90

β

0

Figure 2. Changes in the value of R as a function of θ for various values of φ. When φ < 45°, R goes to 1/e as θ goes to 90°, and when φ > 45°, R goes to e as θ goes to 90°. Our method of angle determinations fails when φ = 45°. This indeterminacy could be alleviated by using an a mode, which contains an a component. For this figure we have assumed that the dielectric constant of the metal is — 16. 2

2

1

xx

are c o m p o s e d o f a linear c o m b i n a t i o n o f ot , &y , a n d a . T o solve orientations w e use the totally symmetric a stretch mode o f the p h e n y l ring. S i m p l e geometric arguments show that i f this molecule is oriented w i t h the ring p e r p e n d i c u l a r to the surface, then the intensity ratio o f a /b must b e (1.155) , where, i n this case, a indicates only the ring breathing stretch. E q u a t i o n s 1 a n d 2 indicate that the intensity ratio s h o u l d b e 1 / e for parallel orientation. A m o r e complete analysis shows that xx

y

z z

x

1

2

1

x

2

R =

sin(0)cos(c|>) + €sin(0)sin() + 1 . 1 5 5 c o s ( O ) ]

2

sin(6)sin(c|>) + esin(9)cos() + c o s ( 6 )

w h e r e R is the ratio o f relative intensities o f the S E R S bands i n relation to the b u l k o r S E R S bands w i t h different s u r r o u n d i n g dielectric constants. Ι and I correspond to the intensity o f the a a n d b bands relative to the b u l k metal thiolate o r S E R S spectrum w i t h a different s u r r o u n d i n g dielectric constant. F i g u r e 2 shows a plot o f R versus θ f o r different values o f φ . T h e p r o b l e m is that f o r a given value o f R, several values o f θ a n d φ c a n exist. A αχ

b

x

l


15.

CARRON AND HURLEY

397


nontrivial solution can be f o u n d b y either changing the excitation wavelength, thus e, or b y changing the dielectric constant o f the s u r r o u n d i n g m e d i a . W e chose the second approach because resonance R a m a n effects are p r e d i c t e d due to p h o t o c h e m i c a l degradation that was observed i n some cases. T h e rigorous expression for the

e

u s e d i n the previous equations

is

e

metal

/

e oundmgs- T h e n e w dielectric materials w e have chosen are m i x e d hexanes surr

and cyclohexane, w h i c h are not expected to adsorb to the surface a n d p e r t u r b the orientation o f the p h e n y l thiolate. T h e m i x e d hexanes a n d cyclohexane used i n o u r experiments have a dielectric constant o f 1.90 a n d 2.03,

respec

tively.

Experimental Details T h e thiols, sulfides, a n d disulfides were purchased f r o m A l d r i c h a n d used without further purification. T h e silver substrates were p r e p a r e d b y vapor deposition o f silver onto c a l c i u m - f l u o r i d e - r o u g h e n e d slides. T h e silver sub strate has p r o d u c e d enhancements o f 1.7 X 1 0 for p y r i d i n e (10). M i c r o scope slides were cleaned w i t h a concentrated a m m o n i a solution a n d cleaned w i t h a 3 0 - W radio-frequency plasma cleaner ( H a r r i c k ) for 10 m i n . A v a c u u m coating unit ( E d w a r d s ) operating at 10 ~ m b a r pressure was used for the v a c u u m depositions. T h e r o u g h e n e d substrate was p r e p a r e d b y first deposit i n g a 600-nm-thick layer o f c a l c i u m fluoride f o l l o w e d b y 50 n m o f silver. T h e depositions were carried out f r o m a resistively heated m o l y b d e n u m boat. T h e silver ( A l d r i c h ) was 9 9 . 9 % p u r e a n d the c a l c i u m fluoride (Aesar) was optical grade. T h e film thickness a n d deposition rate were m o n i t o r e d w i t h a quartz crystal microbalance. T h e silver was deposited at the rate o f 0.2 n m / s . G o l d ( G . F . G o l d s m i t h ) substrates were p r e p a r e d b y using the p r e c e d i n g m e t h o d f r o m 9 9 . 9 9 9 % p u r e gold. 5

6

T h e sample was dissolved i n acetone a n d was spun-coated onto the substrates. T h e concentration o f the thiol, sulfide, and disulfide was 0.01 M , a n d a 5 0 - μ L aliquot was used for the spin-coating. I m m e r s i o n was also tested as a viable application procedure. T h e spectral features were identical w i t h spun-coat samples. H o w e v e r , the quality o f the spectra was not as good as w i t h spin-coating. T h e copper substrates were p r e p a r e d f r o m 9 9 . 9 9 9 % p u r e , 0 . 0 2 5 - m m polycrystalline copper foil ( A l d r i c h ) . These substrates were roughened b y etching i n 1 2 % H N 0 for 4 m i n u n d e r vigorous stirring. T h i s p r o c e d u r e is similar to that developed b y M i l l e r et al. (11), a n d used recently b y C a r r o n et al. (12). T h e metallic copper samples were p r e p a r e d b y etching the c o p p e r w i t h nitric acid, washing w i t h distilled water, i m m e d i a t e i m m e r s i o n i n a 0 . 0 1 - M acetone solution o f the molecule o f interest, w h i c h h a d b e e n w a r m e d to 50 °C, a n d a final washing w i t h acetone to remove any physisorbed reagent. This procedure is similar to that d e v e l o p e d for coating copper w i t h azole compounds (12). 3


398


T h e R a m a n spectra w e r e obtained using a d o u b l e monochromator ( J o b i n - Y v o n M o l e 1000), a p h o t o m u l t i p l i e r ( R C A 31034), a n d p h o t o n count i n g electronics (Ortee). L a s e r excitation was p r o v i d e d w i t h an i o n laser (Spectra Physics 2025 K r + ). Backscattering geometry was used for a l l samples, a n d a cylindrical lens was used to focus the laser to decrease the p o w e r density at the sample. T h e silver a n d g o l d S E R S samples were s p u n at 1800 H z to avoid any possible damage due to laser-induced heating. T h e copper f o i l samples were not spun. A 647-nm filter was used to remove the plasma lines. B u l k R a m a n spectra w e r e obtained o n metal thiolates p r e p a r e d i n the laboratory. Silver p h e n y l thiolate ( A g S P h ) was p r e p a r e d b y mixing an excess o f t h i o p h e n o l i n water w i t h A g N O (5). T h e resulting precipitate was washed w i t h methanol. G o l d p h e n y l thiolate was synthesized t h r o u g h the r e d u c t i o n o f H A u ( I I I ) C l i n the presence o f t h i o p h e n o l (13). T h e resulting precipitate was w a s h e d w i t h methanol to remove the p h e n y l disulfide that results f r o m oxidation o f t h i o p h e n o l b y A u ( I I I ) . C o p p e r p h e n y l thiolate was synthesized b y the addition o f t h i o p h e n o l to an aqueous solution o f C u C l . A g a i n , the p r o d u c t was w a s h e d w i t h m e t h a n o l to remove p h e n y l disulfide. s

4

2

Results and Discussions Silver. F i g u r e 3 shows the R a m a n spectra o f t h i o p h e n o l a n d silver p h e n y l thiolate, a n d the S E R S spectrum o f the product o f t h i o p h e n o l reacted w i t h a silver substrate. C o m p a r i s o n o f t h i o p h e n o l a n d the silver p h e n y l thiolate shows that there is a large intensity increase i n the b a n d at 1075 c m . T h e l o w - w a v e n u m b e r shoulder o n the 1 6 0 0 - c m peak has increased i n intensity i n the metal thiolates. T h e r e is also a large intensity increase i n the 4 2 0 - c m vibration. B a n d assignments are tabulated i n T a b l e I a n d changes i n intensity are tabulated i n T a b l e II. - 1

- 1

- 1

T h e S E R S spectrum shows the largest enhancement for the 420 m o d e , w h i c h is to be expected because m o d e 420 is largely c o m p o s e d o f the a tensor (14). C o m p a r i s o n o f the S E R S spectra o f the surface product w i t h the b u l k metal thiolates shows several trends. T h e intensity ratio o f the 1 0 0 0 - c m peak relative to the 4 7 0 - c m " peak is about unity. T h i s is p r e d i c t e d for molecules that are p e r p e n d i c u l a r to the surface. z z

- 1

1

T h e orientation o f t h i o p h e n o l was d e t e r m i n e d b y the procedure o u t l i n e d earlier. W e obtained S E R S spectra i n air, m i x e d hexanes, a n d cyclohexane. T h e results are tabulated i n T a b l e III. T h e results are i n g o o d agreement w i t h X - r a y studies o f A g S P h that indicate that the p h e n y l ring is p e r p e n d i c u l a r to the layer o f silver ions (15). O n e p r o b l e m that can occur i n S E R S - b a s e d orientation studies is the assumption that the R a m a n spectrum o f the c h e m i s o r b e d species is equivalent to the b u l k spectrum o f the equivalent organometallic c o m p o u n d . T h e determination o f the S E R S spectrum i n two


15.

CARRON AND HURLEY

399


«•-1575 j^-1000 425 1075-*

A

\

1025—* 695

J

V

470

1 V

Β

C

Figure 3. A Raman study of thiophenol on silver. (A) SERS spectrum of thiophe nol on silver obtained with 30 mW of 647-nm laser irradiation, 3-cm~ band pass, 2-cm ~ step size, and 5-s integra tion. (B) A Raman spectrum of bulk AgSPh obtained with 5 mW of 647-nm laser irradiation, 3-cm~ bandpass, 5cm ~ step size, and 5-s integration time. (C) A Raman spectrum of thiophenol obtained with a depolarizer, 50 mW of 647-nm laser irradiation, 3-cm ~ band pass, 2-cm ~ step size, and 2-s integra tion time. 1

1

1

1

i 1800

1

cm

-ι

200

1

solvents o f d i f f e r i n g dielectric constant allowed us to check this assumption a n d to make a n orientation determination independent o f the b u l k material. T h e results f r o m T a b l e I I I indicate that, i n this case, the assumption o f equivalence o f the b u l k silver p h e n y l thiolate a n d the surface species is reasonably good. T h e changes i n φ m a y b e related to the relatively free rotation o f the p h e n y l ring o n the surface as opposed to t h e rigid structure f o u n d i n the b u l k .

Gold. O u r studies o n g o l d are shown i n F i g u r e 4. A weak b a n d corresponding to the A u - S stretch was f o u n d at 275 c m , a n d the A u S P h was stable u n d e r p r o l o n g e d irradiation at 647 n m . F r o m T a b l e I I it c a n b e seen that all modes are decreased i n intensity relative to the 470-cm ~ m o d e . T h i s relative decrease indicates that g o l d has an axial angle to the surface that is less than silver. B y using equation 3 w e f o u n d that the axial a n d azimuthal angles are 76.0 a n d 0 degrees, respectively, a n d excellent correlation to the p r e d i c t e d angles exists w h e n either hexanes o r cyclohexane was used. - 1

1


400


T a b l e I. B a n d Assignments Thiophenol (Neat)

a

Av

Assignment

278 414 464 616 698 734 834 914 988 1000 1024 1069 1092 1118 1156 1180 1270 1328 1380 1440 1478 1576 1584

(b ) (%, a ) (h) (b ) (a,) (b ) (a,) ( S - H band) (h) (a,) (a,) (b ) (%) (a ) (b ) (%) (b ) (b ) (a,) (b ) (a,) (a ) (a b)

a

2

Thiophenol (SERS) Av

Ag-Phenyl Thiolate Av

230 (Ag-S)

250 (Ag-S) 335 420 480 620 700 740 835

x

420 470 620 695 745

2

985 1000 1025 1075

2

2

x

2

1000 1025 1075 1115 1160 1185 1270

1110 1160 1180

2 2

2

x

ly

2

1375 1440 1475 1575 1600

1475 1575 1600

Band assignments are based on references 5 and 18.

O r i e n t a t i o n d e t e r m i n e d i n d e p e n d e n t o f the b u l k spectrum also m a t c h e d the b u l k values very w e l l , w h i c h indicates that the surface species closely resem bles the structure o f A u ( l ) S P h synthesized f r o m A u ( I I I ) . T h e smaller φ w i t h respect to silver c a n b e attributed to increased steric hindrance w i t h the surface as θ becomes smaller. T h e intensities relative to the 4 7 0 - c m peak i n the b u l k are larger t h a n those f o u n d w i t h silver. W e interpret this as a n increase i n b o n d polarizability i n the p h e n y l ring d u e to the decreased charge w i t h d r a w a l through the A u - S b o n d . F o r example, the relative intensity o f the 1 0 0 0 - c m b a n d , a n d most other modes, approaches the value for free t h i o p h e n o l as the metal is changed f r o m c o p p e r to silver to g o l d (see T a b l e II). T h i s increase is clearly seen i n the R a m a n spectra o f the b u l k m e t a l p h e n y l thiolates. T h e similarity i n the R a m a n s p e c t r u m o f b u l k A u S P h to t h i o p h e n o l i n comparison w i t h its dissimilarities to silver a n d c o p p e r analogues c a n explain the differences i n b o n d i n g that lead to self-assembly o f thiols o n gold. T h e p h e n y l thiolate species o n g o l d most resembles t h i o p h e n o l , a n d , therefore, c a n move laterally o n surfaces most easily. - 1

- 1


15.

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401


T a b l e I I . Changes i n Intensity Material

Frequency χ (cm ~ )

Cu

Bulk

ISERS/IBULK

420

0.54

3.67

470

1.00

1.00

1.00

695

3.54

1.00

0.28

9.83

0.98

1000

Ag

10.0

Thiophenol

6.83

1025

2.77

3.33

1.22

1075

2.54

2.00

0.78

1575

2.38

1.50

0.63

420

2.20

6.38

2.90

470

1.00

1.00

1.00

695

2.70

1.85

0.68

1000

Au

SERS

l

8.54

0.81

1025

5.60

6.08

1.08

1075

6.40

7.69

1.20

1575

7.65

8.08

1.05

420

4.60

4.13

0.90

10.6

470

1.00

1.00

1.00

695

4.00

1.26

0.32

1000

52.0

5.41

0.10

1025

18.4

5.20

0.28

1075

16.0

8.33

0.52

1575

14.8

7.60

0.51

414

25.5

464

1.00

698

31.8

1000

214.3

1024

47.8

1092

15.9

1584

15.9

Copper.

T h e R a m a n spectra associated w i t h o u r study o f t h i o p h e n o l o n c o p p e r are s h o w n i n F i g u r e 5. T h e C u - S stretch was f o u n d at 270 c m . T h e S E R S spectra o n copper w e r e weak, a n d spectra o f copper species i n solvents were unobtainable. T h e b a c k g r o u n d p r o d u c e d b y the solvents p r e v e n t e d measurement o f the 4 7 0 - c m " peak. T h e ratio o f ^ 1 0 0 0 / ^ 4 7 0 * larger than gold a n d silver. Therefore, w e w o u l d expect that t h i o p h e n o l o n c o p p e r is also oriented p e r p e n d i c u l a r to the surface. N o photochemistry was observed w i t h copper p h e n y l thiolates o r the surface species. - 1

1

s

Error Analysis.

T h e errors f o r angle determinations are shown i n T a b l e I I I . T h e error for silver is ± 4 ° f o r θ a n d ± 1 1 ° f o r φ. T h e error f o r gold is significantly smaller: θ is ± 1 . 5 ° a n d φ is + 2 ° . T h e s e errors are estimated f r o m the noise level associated w i t h the 4 7 0 - c m b a n d . T h i s peak is m u c h smaller than the 1000-cm ~ peak, a n d , therefore, is the major source - 1

1


402


Table III. Orientation of Thiophenol Intensity Ratio

Ag, Θ, φ

Au, θ, φ

(a* *) hwo/hio (hexanes) ^1000/^470 (cyclohexane)

10.6 8.09 5.18, 85.4, 32 4.91, 85.4, 32

52.0 6.82 3.77, 76.0, 0 2.71, 75.9, 0

^IOOO ^470

1

NOTE: The orientation of thiophenol on Ag using the SERS spectrum in air as the reference shows θ = 81.3, φ = 25. The orientation of thiophenol on Au using the SERS spectrum in air as the reference shows θ = 77.0, φ = 17.2. The error in the angles for thiophenol on Ag are approximately ±4° for θ and ±11° for φ. The error in the angles for thiophenol on Au are approximately ±1.5° for θ and ±2° for φ.

Figure 4. A Raman study of thiophenol on gold. (A) SERS spectrum of thiophe nol on gold obtained with 30 mW of 647-nm laser irradiation, 3-cm~~ band pass, 5-cm ~ step size, and 10-s integra tion. (B) A Raman spectrum of bulk AuSPh obtained with 50 mW of 647-nm laser irradiation, 3-cm~ bandpass, 2cm~ step size, and 2-s integration time. (C) Same as Figure 3C. 1

1

1

1

1800

cm

200

o f error. T h e smaller errors associated w i t h g o l d are due to the m u c h higher signal-to-noise ratio ( S / N ) i n the A u S P h spectrum. T h e better S / N for this spectrum is due to the lack o f photodegradation a n d the correspondingly higher laser powers that c o u l d be used. T h e observed errors are o n the same order o f those r e p o r t e d for I R reflection spectroscopy (16). T h e precision


15.

CARRON AND HURLEY

A

1000 y*

403


420

t

/

ft

y

1025 1075

1575

\

Β

C Figure 5. A Raman study of thiophenol on copper. (A) SERS spectrum of thio phenol on copper obtained with 10 mW of 647-nm laser irradiation, 5-cm ~ bandpass, 5-cm ~ step size, and 15-s integration. (B) A Raman spectrum of bulk CuSPh obtained with 20 mW of 647-nm laser irradiation, 3-cm ~ band pass, 3-cm ~ step size, and 3-s integra tion time. (C) Same as Figure 3C. 1

1

1i

Λ1

Α

1800

cm

-ι

A

^

200

1

1

f o u n d here is m u c h better than the ± 3 2 . 5 ° f o u n d b y W a l l s a n d B o h n ( 1 7 ) , who used a depolarization m e t h o d for angle determinations. I n general, F i g u r e 2 shows that the S / N level o f the spectra w i l l be critical as φ approaches 45°. W h e n φ is exactly equal to 45°, the described m e t h o d fails due to the inability to locate an a m o d e w i t h a k n o w n amount of a or an a m o d e . A s φ approaches 0, the R value is critical w h e n θ is large. O n the other hand, as φ approaches 90°, R is critical w h e n θ is small. W e anticipate that a complete analysis using potential-energy diagrams w i l l result i n more precise determination o f angles because two or m o r e very strong R a m a n bands can be used. x

x x

2

Summary W e have demonstrated a n e w procedure for the determination o f molecular orientation at metal surfaces. T h i s approach gives intuitively reasonable results for p h e n y l thiolates o n noble metal surfaces. O u r results indicate that thiophenol, p h e n y l sulfide, a n d p h e n y l disulfide all f o r m metal p h e n y l thio-


404


lates w i t h noble metal surfaces. I n air the surface p r o d u c t was not observed to b e photoactive u n d e r l o w laser powers a n d a time p e r i o d o f several hours. H o w e v e r , i n colloids a n d continuous substrates i n the presence o f aqueous anions, photoactivity was observed ( 6 ) . S E R S spectra obtained i n m i x e d hexanes a n d cyclohexane indicate that the p h e n y l thiolate is o r i e n t e d nearly p e r p e n d i c u l a r to the surface. T h e r e is a predisposition for t h i o p h e n o l to tilt m o r e for the different metals f r o m c o p p e r to silver a n d to gold. T h e azimuthal angle, φ , a r o u n d the C axis varies between silver a n d gold. Observation that the angle φ approaches 0 as the tilt angle increases c a n b e rationalized as increased steric hindrance a r o u n d the C axis as θ becomes smaller. T h e S E R S spectra o n c o p p e r w e r e very weak, and accurate measurements i n solvents were not obtained. H o w e v e r , rela tively large βχ/έ^ ratios indicate that this species is o r i e n t e d nearly p e r p e n dicular. T h e ability o f this technique to obtain orientations without b u l k spectra o f the surface species w i l l greatly a i d i n studies where the surface complex is weak a n d cannot b e synthesized i n large quantities. 2

2

Improvements i n this approach are b e i n g explored. O n e very p r o m i s i n g approach w i l l b e to use potential energy distributions to determine the exact amount o f x, y, a n d ζ m o t i o n i n a n o r m a l coordinate. T h i s approach w o u l d allow other a modes to b e u s e d i n orientation analysis. I n aromatic systems the a modes are very strong, w h i c h w o u l d i m p r o v e angle determination errors. T h e use o f α modes w i t h a n a component w o u l d remove the redundancy associated w i t h φ = 4 5 ° that is due to o u r current use o f a n a m o d e that does not contain a n χ component. x

x

γ

x x

Y

References 1. See, for example, Bain, C.; Evall, J.; Whitesides, G. J. Am. Chem. Soc. 1989, 111, 7155. Whitesides, G.; Laibinis, P. Langmuir 1990, 6, 87. De Long, H . C.; Buttry, D. A. Langmuir 1990, 6, 1319. 2. Bain, C. D.; Troughton, Ε. B.; Tao, Y.; Evall, J.; Whitesides, G.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. 3. Sandroff, C.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. 4. Sobocinski, R. L.; Bryant, Μ. Α.; Pemberton, J. E. J. Am. Chem. Soc. 1990, 112, 6177. 5. Joo, H . J.; Kim, M . S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57. 6. Lee, T. G.; Yeom, H . W.; Oh, S.; Kim, K.; Kim, M . S. Chem. Phys. Lett. 1989, 163, 98. Joo, T. H.; Yim, Y. H.; Kim, K.; Kim, M . S. J. Phys. Chem. 1989, 93, 1422. 7. Boettcher, C. J. F. Theory of Electric Polarization; Elsevier Scientific Publishing: London, England, 1973; Vol. 1, pp 74-82. 8. van de Hulst, H . C. Light Scattering by Small Particles; Dover Publications, Inc.: New York, 1981; p 71. 9. Johnson, P. B.; Christy, R. W. Phys. Rev. Β 1972, 6, 4370. 10. Carron, K., Ph.D. Thesis, Northwestern University, Evanston, IL, 1985. 11. Miller, S.; Baiker, Α.; Meier, M.; Wokaun, A. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1305.


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12. Carron, K. T.; Xue, G.; Lewis, M . L. Langmuir 1991, 7, 2. 13. Puddephatt, R. The Chemistry of Gold; Elsevier Scientific Publishing: Oxford, England, 1978; p 61. 14. Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley: New York, 1974; p 172. 15. Dance, I. G.; Fischer, K. J.; Herath Banda, R. M.; Scudder, M . L. Inorg. Chem. 1991, 30, 183. Dance, I. G. Polyhedron 1988, 7, 2205. 16. See, for example, Porter, M . D. Anal. Chem. 1988, 60, 1143A. 17. Walls, D.; Bohn, P. W. J. Phys. Chem. 1990, 94, 2039. 18. Scott, D. W.; McCullough, J. P.; Hubbard, W. N . ; Messerly, J. F.; Hossenlopp, I. Α.; Frow, F. R.; Waddington J. Am. Chem. Soc. 1956, 78, 5463. RECEIVED for review July 15, 1991. ACCEPTED revised manuscript July 24, 1992.


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