Langmuir 1985,1, 273-277 duction results in a sintering of these species, but the inability to completely reduce the platinum is indicative of this strong chemical interaction between the platinum and tin oxide. A model for the chemisorption of platinum on tin oxide is proposed. Surface hydroxyl groups are believed to be the active chemisorption sites for platinum species in solution. The chemisorption process is believed to occur through the replacement of the hydroxyl group proton with
273
the loss of a coordinated ligand from the platinum species.
Acknowledgment. We thank James R. Waggoner for his work in the development of the chemisorption platinization procedure. This work was supported by the National Science Foundation under Grants CPE-8210776 and CHE-8309445and the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. Pt, 7440-06-4; SnOz, 18282-10-5.
Auger and Raman Spectroscopic Studies of Visible Laser Microzones in the Electrochemical SERS System: Silver/2,2'-Bipyridine Ralph P. Cooney,* Merrick R. Mahoney, and Martin W. Howard Chemistry Department, University of Newcastle, Shortland, New South Wales 2308, Australia
John A. Spink CSIRO Division of Materials Science, University of Melbourne, Parkville, Victoria 3052, Australia Received September 11, 1984. In Final Form: November 9, 1984 Silver electrodes which exhibit intense surface-enhanced Raman scattering (SERS) for 2,2'-bipyridine reveal black carbon-rich laser microzones in the form of a "halo". The adsorbate-derived carbon zones are more clearly visible in this system than in the closely related system, Aglpyridine. Adsorbate carbonization processes have been studied by using Auger electron spectroscopy (AES). The AES results show that heavily carbonized zones of the halo are rich in nitrogen (presumably 2,2'-bipyridine). The carbon-film thickness in the halo (which must exceed the optical skin depth) appears to be 100-200 nm (or 300-600 carbon atom layers). The intensity-potential curves for this system exhibit unusual double intensity maxima (at -0.4 and -1.0 V). The secondary cathodic maxima at -1.0 V is tentatively attributed to resonance enhancement associated with radical ion formation of the type favored by 2,2'-bipyridine and related species. The conformation of 2,2'-bipyridine is approximately cis (C2J at both intensity maxima. Overall, there appear to be three laser photochemical thresholds in this type of system (silver corrosion, coordinated adsorbate carbonization, and oxidation decarbonization). The Ag/2,2'-bipyridine system is classified as SERS type I.
Introduction A system of classification of surface-enhanced Raman scattering (SERS) systems based on the chemical composition of the laser damage microzone (or sampling zone) has been recently pr0posed.l An updated list of classified SERS systems which exhibit evidence for laser damage'-" (1)Cooney, R. P.; Mernagh, T. P. In 'Electrochemistry: The Interfacing Science"; Rand, D. A. J.,Bond, A. M., Ed.; Elsevier: 1984. Cooney, R. P.: MemaPh. T. P. J.Electroanal. Chem. 1984.168.67. (2) Coonei, R. P.; Mernagh, T. P.; Mahoney, M. R.:Spink, J. A. J. Phys. Chem. 1983,87,5314. (3) Mernagh, T. P.; Cooney, R. P.; Turner, K. E. Chem. Phys. Lett. 1984,110, 536. (4) Macomber, S. H.; Furtak,T. E.; Devine, T. M. Chem. Phys. Lett. 1982,90,439. (5) Cooney, R. P.; Mahoney, M. R.; McQuillan, A. J. Adu. Infrared Raman Spectrosc. 1982, 9, 188. (6) Mernagh, T. P.; Cooney, R. P.; Spink, J. A. J. Raman Spectrosc.,
in press. (7) Mernagh, T. P.; Cooney, R. P. J. Raman Spectrosc., in press. (8) Mahoney, M. R.; Cooney, R. P. J. Pkys. Chem. 1983, 87, 4589. Faraday (9) Memagh, T. P.; Cooney, R. P.; Spink, J. A. J.Chem. SOC., Trans. 1 1984,80, 3469. (10)Owen, J. F.;Chen, T. T.; Chang, R. K.; Laube, B. L. J. Electroanal. Chem. 1983, 150, 389 (1983).
0743-7463/85/2401-0273$01.50/0
is given in Table I. Several experimental techniques have been used for the identification of localized compositional and morphological damage by the laser beam. These techniques include the following (see examples in Table I): (i) scanning Auger microscopic and Auger electron spectroscopic (SAMIAES) identification of light element (e.g., carbon, oxygen) enrichment in the laser damage zone, (ii) scanning electron microscopic (SEM) examination of morphological damage resulting from laser-assisted corrosion of the metal surface, (iii) laser perforation of calibrated thin-film metal electrodes for indications of the (11)Kydd, R. A.; Cooney, R. P. J.Chem. Soc., Faraday Trans. 1 1983, 79, 2887. (12) Cooney, R. P.; Howard, M. W.; Mahoney, M. R.; Mernagh, T. P. Chem. Phys. Lett. 1981, 79, 469. (13) Guzonas, D. A.; Atkinson, G. F.;Irish, D. E. Adams, W. A. J. Electroanal. Chem. 1983,50, 457. (14) Wetzel, H.; Pettinger, B.; Wenning, U. Chem. Phys. Lett. 1980, 75, 173. (15) Howard, M. W.; Cooney, R. P. Chem. Phys. Lett. 1982,87,299. (16) Memagh, T. P.; Cooney, R. P. J. Electroanal. Chem. 1984,177, 139. (17) Kester, J. J.; Furtak, T. E.; Bevolo, A. J. J. Electrochem. SOC. 1982,129, 1716. ~~
0 1985 American Chemical Society
Cooney et al.
274 Langmuir, Vol. 1, No. 3, 1985
t-
m
2
Ne
a-
approximate depth of laser damage "craters", (iv) dependence of SERS intensities on the presence or absence of laser surface flux (or photoalteration) during the oxidation-reduction cycle (ORC) for indications of the existence of laser-assisted corrosion at anodic potentials, (v) "uniform" laser-flux perturbations of cyclic voltammograms or steady-state currents for identification of laserphotogalvanic and -photovoltaic processes, and (vi) laser-interruption studies for identification of laser-induced dissolution and/or decomposition processes associated with decay patterns exhibited by many SERS systems. Previous SAM/ AES investigations2of the Ag/ pyridine SERS system have confirmed earlier Raman spectroscopic evidence5that the sampling zone is essentially pure carbon, formed by the oxidation of coordinated pyridine by laser-photoreduciblesilver(1) chloride. Other Raman studies revealed a quasi-stoichiometric relationship between surface carbon and pyridine SERS which suggested that the intensely scattering phase is an intercalate [carbon-pyridine] and not pyridine on uncarboniaed surface sites.ls Very recently, laser perforation of thin silver film elect r o d e ~confirmed ~ that the &/pyridine SERS effect arises from semimacrwopic craters (ca. 0.03 mm in diameter and ca. 1250 atom layers in depth) formed by laser-assisted corrosion. For the same system, carbon film thickness within the laser damage zone was estimated from optical skin depths to be ca. 100 nm or 300 carbon atom layer^.^ The present SERS system, &/2,2'-bipyridine (bpy), was examined in detail for a variety of reasons. Of specific interest were the dark laser damage zones which were more distinct than the faintly visible zones reported for the closely related SERS system Ag/pyridine.4J1n'2 It was hoped that a clearer understanding of laser carbonization processes and confirmation of the [carbon-organic base] phase18 would emerge. Also, the intensity-potential curves exhibited an unusual double maxima, which may have been related to the labile cis (C,)-trans (C,) conformation change,lg Le.,
Finally, a recent note reporting SERS from this system20 assumed that the intense spectrum arises from 2,2'-bipyridine electrosorbed on clean silver surface sites. Laser carbonization and other damage effects were not considered.
Experimental Section The Raman spectrometer, lasers, and scanning electron micrcxscope/Augerelectron spectrometer employed in the study were described previously?J1 Spectral data were stored and processed using a Nicolet 1074 instrument computer interfaced with a Kennedy magnetic tape system. Spectroelectrochemicalcells that followed earlier design^^^^^^ were employed in this work. Silver electrode surfaces were in two forms: the 1-mm circular cross section of a sheathed silver wire and a 7-mm circular cross section of a sheathed silver rod. In both cases,Johnson-Matthey specpure silver was employed. The 1-mm silver-wire electrodes were employed in SAM/AES/SEM and optical microscopic studies of the (18)Mernagh, T.P.; Cooney, R. P. J. Raman Spectrosc. 1983,14,138. (19)Strukl, J . S.;Walter, J. L. Spectrochim. Acta, Part A 1971,27A, (20)Kim, M.;Itoh, K. Chem. Lett. 1984, 357. (21)Mahoney, M.R.;Howard,M. W.; Cooney, R. P. Chem. Phys. Lett. 1980, 71, 59. (22)Pettinger, B.;Wenning, U.; Kolb, D. M. Eer. Eunsenges. Phys. Chem. 1978,82, 1326. (23)Cooney, R. P.;Fleischmann, M.; Hendra, P. J. J. Chem. Soc., Chem. Commun. 1977, 235.
Langmuir, Vol. 1. No.3, 1985 275
Spectroscopic Studies of Visible Laser Microzones
Table 11. Auger Electron Spectmwpic (AES) Analysis of Surfam Composition of the Working Electrode from a An/Z.Z'-Bbvridlne SERS ExwrimentD atomic ratiose
Auger peak. eV
CI. 181
nonilluminated background 3.3
C.'272
halo
dark halo
center
2.7 100
2.1 100
.Laser mnditions: 514.6-nm Ar* (100 mW) foeusad. 'DaDStails of normalization proceesw are described in Table I1 (footnote a) of ref 2.
Figure I. Optical micrograph of the laser damage microzone on a withdrawn silver electrode from the SERS system: Ag/O.l M KCI, 0.03 M 22'-bipyridine. The C/Ag ratias are from the Auger data in Table 11. h e r conditions 514.5-nm Art (100 mW); time of laser exposure 30 min.
laaer damage mieraone, because the small electrode antas facilitated the unambiguous identification of laaer damage zones. Distilled water that waa refractionated under N2waa employed in the preparation of solutions. Potassium chloride used was May and Baker Pronalys reagent and the 2,2'-bipyridine was an Aldridge (99.5%) reagent. The electrolyte medium was 0.1 M KCI and 0.03 M 2,2'-bipyridine. Either 568.2-nm Kr+ (ea50 mw) or 514.5nm Ar+ (ca100mw) WBS used to excite Raman spectra. The SEW spectra recorded with these two exciting lines were very similar. The working electrode Surface was exposed to the f w d laser for (typically) 30 min. A spectra band-paen of ca.10 em-' was u s d y employed. The oxidation-reduction cycle (ORC)pretreatment, essential for intense SEW, involved a triangular sweep to 4 . 1 2 V W E ) at 5 mV 8.'. All potentials quoted were measured relative to a saturated calomel reference electrode (SCE). O p t i d micrographs were obtained by using a Nikon Metaphot binocular microscope with photomicrographic attachment. Results and Discussion (a) Auger Electron Spectroscopic (AES) Studies.
A 1-mm silver-wire electrode was subjected to an ORC, performed under 100 mW of focused 514.5-nm Ar+ laser illumination. Examination of the electrode surface, which previously exhibited intense SERS for 2,2'-bipyridine, revealed a visible laser damage zone. When viewed under an optical m i ~ ~ (at ~ magnification p e X125), the damage zone had the appearance of a black "halo" of diameter ca. 0.2 mm (see Figure 1). The black laser damage zone was more extensive and more visible in this case than for the electrode surface in &/pyridine electrochemicalSERS."J' In the latter system, the damage zone was faintly visible" a~ a grey-black zone for an ORC limit of +0.12 V and more clearly visible for an ORC limit of +0.20 V if generated with 80 mW of focused 514.5-nm Ar+. In the absence of an organic base (i.e., Ag/H20, Table I) laser corrosion of the electrode surface has been but the damage zone is not visible to the eye. These results confirm that the visibility (darkness) of the laser damage microzone depends on the ease of oxidation carbonization (see AES data, Table 11) of the organic base. This is consistent with the expectation that polycyclic aromatics would be carbonized (Le., oxidized) more readily than a monocyclic base such as pyridine."
Other aromatic bases are very easily oxidized to carbon even in the absence of laser flux. For example, 1.2-diaminobenzene, will turn the entire surface of a silver electrode black in the c o u m of a typical SERS ORC." AES data (Figure 1 and Table 11) reveal that the microzone halo is carbon rich and silver depleted (C/Ag = 71.4) while the halo center is also carbon rich but less silver depleted (C/Ag = 13.0)compared to the nonilluminated background for which the C/Ag ratio is 6.4. These ratios compare with those previously reported2 for Ag/pyridine SERS (laser microzone C/Ag m; nonilluminated surface (C/Ag 5). The small quantities of silver detected in the dark halo (Table 11) are presumed to arise from silver metal particles trapped in the relatively thick carbon film (see below). The silver metal would form from the photochemical (redox) decomposition12of silver(1) bipyridine complex to silver(0) and carbon. Given the thickness and visibility of the carbon halo (Figure l ) , it is extremely improbable that the silver signal arises f"the underlying silver electrode surface. A significant trend is found for the N/Ag ratios (Table E), which change from 7.2 for the halo of heavily carbonized surface to 1.0 for the halo center to 0.4 for the nonilluminated background. These data appear to confirm that 2,2'-bipyridine (which is assumed to be the source of the N) surface content is greatest for the more heavily carbonized zones of the surface. This result supports an earlier finding that the form of the association between carbon and the organic base is one of quasi-stoichiometric intercalation rather than conventional extemal adsorption. The importance of a 'history" of carbon oxidation as a prerequisite for intermlation has been analyzed in previous papers?J','tz It is poasible that the slightly higher silver content in the oxidized (see below) halo center originatea in the very strong affmity of highly oxidized carbon (which behaves like a negatively charged colloids) for cations such as Ag+. Such cation-assimilation processes in oxidized carbon appear to be important in Ag/H,O SEW! The retention of the organic base in the carbon halo (discussed above) was not observed for the carbon microzones of the Ag/pyridine system. This is presumed to be a result of more facile desorption in the case of pyridine than for 2,2'-bipyridine because of stronger carbon-surface adsorption of polycyclic The AES data for surface oxygen (Table ID indicate an increasing trend from the background (6.4) to the halo region (7.0) to the halo center (7.3),which appears to reflect increasing laser-assisted oxidation and ultimately, at the
-
-
(26)H d . M. W.:Cooney. R P.; McQuiUm. A. J. J . Raman
(24)MaddL,A.L;Ri+ D.ln Chanetsnn ' tion of P d s r sluf.rrs'; P d W , G. D.,S i , K.5. W.,Eds.; Academic Prem New York 1976.
Spectmsc. 1980,9, 273.
(26)B m ,A P.;Kwd.C.:Anmn, F.C.J. E l c c t m o ~ lChem . 1978.
72,379.
Cooney et al.
276 Langmuir, Vol. 1, No. 3, 1985
Table 111. Dominant Lines" (AD,cm-l) in Raman Spectra of 2,2'-Bipyridine (bpy) k l b p y SERSb -0.4 V -1.0 V 997 ah 1005 m 1012 m 1052 w 1049 w 1162m 1155m
!
I
(C,)c cis-bpy in
[Fe(bpyI3l2+
(Czh)d
trans-bpy in
cryat 997 a
1025 m 1067 w 1173 ma
1045 m 1238 s
T
20 k
1270m 1302 a
1262m 1310s
1277 m 1321 a
1476 va 1555m
1476 va 1547 a
1490 va 1563 a
1587 m
1585m
1607 ma
1304 s 1448 a 1484 m 1574 a 1591 s
"v, very; e, strong; m, medium; w, weak; ah, shoulder. *Recorded with 568.2-nm Kr+ (50 mW). Similar spectra were obtained with 514.5-nm Ar' (100 mW). CFromref 29. dRaman data from this study; crystal structure given in ref 36.
800
900
io00
'too
~ZOO Ramon shill
IJOO i
~LOO i o n
MOO
~mo
I&
cm-l
Figure 2. SERS spectra from the system Ag/O.l M KCl, 0.03 M 2,2'-bipyridine at -0.4 (spectrum a) and -1.0 V (spectrum b). The dotted profile in the upper trace represents the underlying profile of the D (-1360 cm-') and G (-1590 cm-') bands of carbon. Exciting line 568-nmKr+ (-50 mW);band pass 10 cm-'.
halo center, limited oxidative decarbonization. One result of the oxidation of carbon at the halo center would be to render the halo-center carbon more porous in structure," which would facilitate desorption of the 2,2'-bipyridine. This may account for the lower N/Ag ratio for the halo center. Recent studies3of the faint-gray laser carbon microzones in Ag/ pyridine SERS provided evidence that the depth of laser damage was ca 1250 silver atom layers. In addition a combination of optical skin depths (or laser penetration depths) and comparative color tones for the damage zones indicated3 that carbon-film thickness under typical Ag/ pyridine SERS conditions (6100 mW of focused 514.5 nm) was ca. 100 nm. The blacker, more clearly visible halo (Figure 1) in the present study almost certainly representa a carbon film of greater thickness (Le., 100-200 nm). Carbon f i of thickness of much less than 100 nm exhibit interference colors (gold, green, blue, red, or orange)n while those of thickness ca. 100 nm are gray-black in (b) Conformation of Intercalated 2,2'-Bipyridine. SERS Spectrum. SERS spectra of 2,2'-bipyridine, recorded at 4 . 4 and -1.0 V after an ORC, is shown in Figure 2. Carbon bands (ca. 1350,1590 ~ m - can 9 ~be recognized underlying the intense SERS spectrum. The peak intensity of the 2,2'-bipyridine SERS spectrum is approximately 10 times greater than that of the carbon spectrum. Given the extent and depth (See Figure 1and Table I) of carbon (27) Reed,S.J. B.'Electron Microprobe Analjjis"; Cambridge University Press: New York, 1975; pp 180-182.
in the laser microzone, this result indicates the relative insensitivity of Raman spectroscopy in detecting surface carbon. A similar result is found for aromatic bases internally adsorbed in oxide molecular sieve channels which exhibit Raman spectra that are far more intense than the spectra of the adsorbent.28 The pattern and relative intensities of lines in the SERS spectrum at -0.4 V (Figure 2 and Table 111) indicate that the 2,2'-bipyridine (bpy) surface species responsible for the SERS spectrum are held by weak forces of attraction to the surface. The evidence for physisorption comes from the position of the line at 1005 cm-' (at -0.4 V) which is displaced only 8 cm-l from a similar dominant feature (997 cm-l) in the spectrum of free 2,2'-bipyridine (Table 111). In contrast, when 2,2'-bipyridine (bpy) is coordinated to metal ion electron acceptors (Le., models for Lewis-site chemisorption) the displacement is ca.30 cm-' (i.e., 28 cm-l for [ F e ( b p ~ ) ~ ]31 ~+ , for [Zn(bpy),ONO]NO,, and 35 cm-l cm-' for [Zn(bpy)C12])29*30In a similar way, the ringbreathing mode (vl) of pyridine in SERS spectra from silver electrodes is displaced 16 cm-' from the frequency for free ~ y r i d i n e . ~ This is substantially less than the displacement (ca. 33 cm-') for pyridine in a stable metal ~omplex.~ The pattern of lines in the SERS spectrum for 2,2'-bipyridine at -0.4 V (Table I11 and Figure 2) is very similar to the spectral pattern for [ F e ( b ~ y ) ~(i.e., ] ~ + (Czu)cis2,2'-bipyridine). It differs from the spectrum of free, 2,2'-bipyridine (which has the trans (C,) conformation) in several significant respects (Table 111),e.g., the nonappearance of dominant lines at ca. 1238 and 1448 cm-'. The 2,2'-bipyridine SERS spectrum at -0.4 V differs from the spectrum of [Fe(bpy)J2+in only one significant respect (Table 111), viz., the higher relative intensity of the line at 1005 cm-' in the SERS spectrum compared with its closest counterpart (1025 cm-') in the spectrum of the iron(I1) complex.29 The spectrum of [Zn(bpy),0NO]N03 (with a d10 metal configuration) also has a relatively intense line (1028 cm-') in this region." It appears therefore that the low intensity of the 1025 cm-' line in [Fe(b~y)~],+ (which has a d6metal ion configuration)is probably related to d,(Fen)-r*(bpy) interactions31expected to be involved (28) Nguyen, T. T.; Cooney, R. P.; Curthoya, G. C. J. Chem. Soc., Faraday Trans. 1 1976, 72,2598. (29) Clark, R. J. H.; Turtle, P. C.; Strommen, D. P.; Streuaand, B.; Kincaid, J.; Nakamoto, K. Znorg. Chem. 1977,16,84. (30)Bartlett, J.; Cooney, R. P., unpublished results. (31) Cotton, F. A,, Wilkinson, G. 'Advanced Inorganic Chemistry", 4th ed.; Wiley-Interscience: New York, 1980; pp 119-120.
Langmuir, Vol. 1, No.3, 1985 277
Spectroscopic Studies of Visible Laser Microzones
The conclusion above regarding the favored cis conformation for 2,2'-bipyridine on the surace must be considered in the context of known structures of metal complexes containing cis-l,%'-bipyridine. These structures usually have small but nonzero dihedral angles between the two aromatic rings of the ligand."s6 Such small deviations from coplanarity have been considered in the present study to justify reference to the point group Cpurather than Cz. The cathodic (5 -1.2 V) reduction of SERS intensities from silver electrode surfaces, which was studied in detail for the Aglpyridine system,18was qualitatively identified in the present system (see the extrapolated point at -1.2 V in Figure 3). That reduction in intensity for the Ag/ pyridine case was shown to be associated with the cathodic conversion of carbon to hydrocarbons.lsIz6
Conclusion
-02
-0L
-06 - O B -1 0 -1.2 applied potential I voltS(SCd
-12
Figure 3. Intensity-potential curves for the system: Ag 0.01 M KCI, 0.03 M 2,2'-bipyridine ((X) 1475 cm-',( 0 )1305 cm-, (A)
I
1557 cm-I). The extrapolated line at -1.2 V indicates the cathodic reduction in SERS intensities over 30 min. Laser conditions 568.2-nm Kr+ (50 mW).
in the vibronic enhancement mechanism for that complex.29 The SERS spectral pattern at -0.1 V is also consistent with a cis (C%)rather than a trans (C,) conformation (see Figure 2 and Table 111). Therefore, the appearance of an unusual second intensity maximum at -0.1 V (see Figure 3) cannot be explained in terms of cis-trans conformation changes. Because a second cathodic intensity maxima is not found for the closely related Ag/pyridine SERS system,6 it is very probable that the second maxima is associated with the unusual tendency of 2,2'-bipyridine and related molecules to form colored radicals. For example, 2,2'-bipyridine, 2,2'-bipyridinium cations, bridged diquaternary salts of 2,2'-bipyridine, and dialkyl-4,4'-bipyridinium (i.e., methylviologen) cations all form radical species under reducing or cathodic condition^.^^ Such colored radicals give rise to intense (nonsurface) resonance enhan~ement.~~ (32) Summers,L.A. "The Bipyridinium Herbicides"; Academic Prw: New York, 1980.
The present study has shown that 2,2'-bipyridine, under typical SERS conditions for silver electrodes, undergoes laser carbonization more readily than pyridine. The carbon-rich microzones, even under the high-vacuum sampling conditions of AES, appear to retain substantial proportions of 2,2'-bipyridine. This supports the formulation of the intensely scattering phase as a carbon intercalate. Also it appears that some laser-assisted oxidation of the carbon occurs in regions of higher laser flux. Also of interest is the evidence for radical formation at cathodic potentials. The evidence for three laser photochemical processes in the &/organic base SERS systems (silver corrosion, base carbonization, and oxidation of the carbon) precludes use of SERS as a surface analysis method in such systems. Because the adsorbate is the source of the carbon in this system it is classified' as SERS type I.
Acknowledgment. We are grateful to the Australian Research Grants Scheme for supporting this project. We are also grateful to D. Barker and J. Bartlett fo their helpful suggestions. Registry No. Ag, 7440-22-4;2,2'-bipyridine, 366-18-7. (33) Forster, M.; Girling, R. B.; Heater, R. E. J. Ramon Spectrosc.
----. (34) Walah, A.; Walsh, B.; Murphy, B.; Hathaway, B. J. Acta Crys-
1982. 12. 36. - - I
tallogr., Sect. B 1981, B37, 1512.
(35) Khan, M. A.; Tuck, D. G.Acta Crystallogr., Sect. C 1984, C40,
60.
(36) Merritt, L. L.;Schroeder, E. D. Acta Crystallogr. 1956, 9, 801.
