Langmuir 1988,4,121-127
121
A Second Harmonic Generation Study of the Oxidation/Reduction Behavior of Iron in Alkaline Solutionst B. M. Biwer,* M. J. Pellin, M. W. Schauer, and D. M. Gruen Materials SciencelChemistry Divisions, Argonne Natiohal Laboratory, Argonne, Illinois 60439 Received April 21,1987. In Final Form: July 20,1987 Second harmonic generation (SHG) in conjunction with cyclic voltammetry and constant-current oxidation/reduction has revealed new information not previously obtainable by conventional electrochemical methods alone on the initial corrosion processes of an iron electrode in 0.1 M NaOH. We have shown that large amounts of dissolved oxygen do not play a significant role in the oxidation of iron in alkaline solutions. The final reduction step in going from the passive film to iron metal is shown to be a much slower process than indicated by the electrochemical data. SHG results for iron in less basic borate sdutions (pH -8.5) are very similar to those in 0.1 M NaOH, suggesting the same oxidation/reduction mechanism in both cases. There exist at least two intermediate phases on the electrode surface between the passive film at oxidative potentials and the reduced metal at hydrogen evolution potentials. The angular dependence of second harmonic intensity as a function of laser beam incidence angle for a polycrystalline surface was found to deviate from the expected results for the Fe metal at cathodic potentials. Such behavior could be due to a preferential ordering of crysfallites at the surface or the added contribution of SHG signal from buildup of charge at the surface.
Introduction Second harmonic generation (SHG) is now developing into an important surface analysis tool.' It is sensitive to submonolayer coverages2P and is able to monitor changes in surface composition4as well as ~rientation."'~ In addition, SHG is an excellent in situ technique for research in such areas as catalysis and corrosion. The effect and the basic underlying theory have been known for 25 yea.r~,1~l' but little has been done in this area until recently. Thermal surface damage has been the major stumbling block because of the high laser intensities required to obtain reasonable signal levels. The advent of picosecond lasers with the requisite peak power helps to alleviate this p r ~ b l e m . ~ There are two key factors to consider: (1)a threshold in the deposited energy appears to be required to initiate thermal damage; (2) the SHG signal is proportional to the square of the laser intensity. Assuming that the energy deposited by the laser pulse is below the surface damage threshold, a shorter pulse because of its higher intensity will generate more second harmonic signal than a longer pulse when both have the same energy. Since the SHG intensity follows the square of the laser intensity, a picosecond laser with the sgme average power as a nanosecond laser should in principle be more effective for the study of SHG from surfaces. We have previously demonstrated the capability of SHG, employing a picosecond laser, to follow the compositional changes as a function of potential of an iron electrode surface in aerated solutions of various P H . ~That work demonstrated the suitability of the SHG technique for studying processes relevant to the aqueous corrosion of metals by giving results which complement those of standard electrochemicalmethods. Moreover, the quality of the SHG spectra from Fe were found to compare well with those obtained with nanosecond NdYAG lasers using Ag electrodes,'"% even though SHG from Ag surfaces is more intense than from most other metals2sarid more than an order of magnitude greater than from Fe. Since the corrosion of iron is of major technological importance, extensive electrochemicalstudies of the metal Work supported by the U.S.Department of Energy, BES-Materials Science, under Contract W-31-109-ENG-38.
in mildly basic borate buffers and highly alkaline solutions have been conducted in the past.26 Results are not conclusive due to the many possible oxidation products and the lack of definitive in situ techniques for their characterization. The passive film is generally considered to be Fe in the 3+ state (e.g., Fe2O3, FeOOH, or Fe(OH),), which (1) For a recent review, see: Shen, Y. R. Annu. Rev. Mater. Sci. 1986, 16, 69. (2) Tom, H. W. K.; Mate, C. M.; Zhu, X. D.; Crowell, J. E.; Heinz, T. F.; Somorjai, G. A.; Shen, Y. R. Phys. Rev. Lett. 1984,52, 348. (3) Tom, H. W. K.; Mate, C. M.; Zhu, X. D.;Crowell, J. E.; Shen, Y. R.; Somorjai, G. A. Surf. Sci. 1986,172,466. (4) Biwer, B. M.; Pellin, M. J.; Schauer, M. W.; Gruen, D. M. Surf. Sci. 1986,176, 377. (5) Driscoll, T. A.; Guidotti, D. Phys. Rev. B: Condens. Matter 1983, 28, 1171. (6) Shank, C. V.; Yen, R.; Hulimann, C. Phys. Rev. Lett. 1983,51,900. (7) Heinz, T. F.: Tom, H. W. K.; Shen, Y. R. Phys. Rev. A. 1983.28, 1883. (8) Tom, H. W. K.; Heinz, T. F.; Shen, Y. R. Phys. Rev. Lett. 1983, 51. 1983. ~ - - (9) Litwin, J. A,; Sipe, J. E.; van Driel, H. M. Phys. Rev. E Condens. Matter 1985,31,5543. (10) Heinz, T. F.: LOY,M. M. T.: ThomDson, . W. A. J. Vac. Sci. Technol., E 1985, 3,.146?. (11) Tom, H. W. K.; Aumiller, G. D. Phys. Rev. B: Condens. Matter - - 7
--.
1986. 33. 8818. - - - - I
(12) Heskett, D.; Song, K. J.; Burns, A.; Plummer, E. W.; Dai, H. L. J. Chem. Phys. 1986,85, 7490. (13) Rasing. Th.: Shen. Y. R.: Kim, Mahn Won: Valint. P.. Jr.: Bock, J. Phys. Rev.-A 1985,31,537. (14) Kemnitz, Z.; Bhattacharyya, K.; Hicks, J. M.; Pinto, G. R.; Eisenthal, K. B.; Heinz, T. F. Chem. Phys. Lett. 1986,131, 285. (15) Franken, P. A,; Hill, A. E.; Peters, C. W.; Weinreich, G. Phys. Rev. Lett. 1961, 7, 118. (16) Armstrong, J. A.; Bloembergen, N.; Ducuing, J.; Pershan, P. S. Phys. Reu. 1962,127,1918. (17) Bloembergen, N.; Pershan, P. S. Phys. Rev. 1962, 128, 606. (18) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. Rev. Lett. 1981,46,1010. (19) Heinz, T. F.; Chen, C. K.; Ricard, D.; Shen, Y. R. Chem. Phys. Lett. 1981., 83. -180. -(20)M&phy, D. V.; von Raben, K. U.; Chen, T. T.; Owen, J. F.; Chang, R. K.; Laube, B. L. Surf. Sci. 1983,124, 529. (21) Chen, T. T.; von Raben, K. U.; Murphy, D. V.; Chang, R. K.; Laube, B. L. Surf. Sci. 1984, 143, 369. (22) Richmond, G. L. Chem. Phys. Lett. 1985, 113, 359. (23) Richmond, G. L. Langmuir 1986,2, 132. (24) Marshall, C. D.; Korenowski, G. M. J. Chem. Phys. 1986,85,4172. (25) Boyd, G. T.; Rasing, Th.; Leite, J. R. R.; Shen, Y. R. Phy. Rev. B Condens. Matter 1984,30,519. (26) See ref 4 and references therein.
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0743-7463/88/2404-0121$01.50/00 1988 American Chemical Society
Biwer et al.
122 Langmuir, Vol. 4, No. 1, 1988 we will refer to as an Fe(II1) species. Reduction of the passive film in borate buffers is thought by many to occur via a mixed valence oxide Fe(II-111) layer, possibly Fe304, and then directly to the metal. In alkaline solutions of higher pH, reduction of the passive film is thought to occur via an Fe(I1) species, Fe(OHI2,prior to complete reduction to the metal. There has been speculation that reduction in the latter case occm via three steps: f i t to Fe304,then to Fe(OH)z,and finally to the metal. Lending support to this idea is our previous work in aerated borate buffer and alkaline solutions where similar SHG signal responses vs potential in both kinds of solution were found. The interpretation was in terms of at least two intermediate surface compitions between the passive f i i and reduced metal regions! On the basis of this work it was concluded that the passive film, an Fe(II1) phase, is first reduced to and/or uncovers a mixed valence Fe(I1-111) species which is further reduced to an Fe(I1) compound before final reduction to the metal. The present work extends our earlier results by investigating the oxidation and reduction of an iron electrode in a deoxygenated 0.1 M NaOH solution. Since much of the previous work in this area has been with deoxygenated solutions, the primary interest is to study the SHG response of the electrode during cyclic voltammetry experiments in a solution free of oxygen so that our results could better be compared with the data of others. In addition, constant-current oxidation/reduction techniques were applied in an effort to provide additional evidence for the two intermediate steps discussed above. A further motivation for the present work was to eliminate the possibility of laser-induced reactions occurring on the iron surface due to the high laser inten~ities.~ To accomplish this, the experimental apparatus was modified over the earlier work as described in the next section.
Experimental Section The iron electrode was studied in a 0.1 M NaOH solution prepared from distilled water and reagent grade NaOH. The electrode was the end face of a 2-mm-diameter iron wire (Puratronic, 99.9985%)sealed in a glass tube. Final polishing of the iron was accomplished with 6-pm and then 1-pm diamond paste. The new electrochemical cell itself was constructed of quartz. Teflon fittings permitted the insertion of the iron working electrode, the platinum counter electrode, and the saturated calomel reference electrode (SCE). Provisions were made to purge the cell with dry nitrogen; the cell was turned on at least 1 h prior to all experiments. The data presented here are for unstirred solutions at room temperature. The electrodes were under the control of a PAR 173potentiostat/galvanmtat in conjunctionwith a PAR 175 universal programmer and a PAR 179 digital coulometer. A picosecond mode-locked and q-switched Spectra-Physics3400 series Nd:YAG laser was used at a pulse rate of 1.7 kHz. Its frequency-doubled output (532 nm) was p-polarized and focused onto the electrode surface at an angle of incidence of 70" with respect to the surface normal. The SHG signal (266 nm) was collected by a colJimating lens and focused through the appropriate filters into a monochromator (0.3 m, MacPherson) for detection by a cooled photomultiplier tube (C31034A, RCA). The pulsecounted signal was stored by a minicomputer (DEC LSI 11/73) on disk along with the appropriate digitized potential and current data for future reference. As before, an optical microscope was used to check for visual laser damage to the electrode suface and also to monitor the size of the laser spot, which WBS approximately 1 mmz. The new electrochemicalcell was constructed to allow operation at an incident angle of 70" from the surface normal in an effort to increase the SHG signal level over that in our prior work.* In agreement with theory, polycrystalline samples have shown a dependence of the second harmonic intensity as a function of the incident laser beam The maximum intensity is ex-
,
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Figure 1. CV (top curve) and SHG signal (bottom curve) for the iron electrode in 0.1 M NaOH at a potential scan rate of 2 mV/s. perimentally observed near an angle of 70" for both metaln and nonmetalnSB surfaces. We would then expect a similar angular dependence of the signal for the electrode in both the reduced and oxidized potential regions. We observed a %fold increase in second harmonic intensity for most potentials by going from a 45O to a 70" angle of incidence. In addition, the background noise level was reduced to about one count per second by better shielding of the monochromator entrance slits from stray light. The increase in signal with an improved signal-to-noise ratio allowed us to operate with a much larger laser beam area to further reduce the possibility of damaging the electrode surface. Since changes in the SHG signal as a function of the electrode potential were seen to display the same behavior as observed when running the laser intensity near the damage threshold (tighter focus), it may be concluded that laser-inducedreactions are not responsible for the observed phenomena. There was also some concern in our previous work that the electrochemicaldata (current as a function of potential) were not necessarily representative of phenomena taking place within the laser interaction region since the electrode surface area was 100 times that of the laser spot. The present study employs a smaller electrode surface, only 3 times the area of the laser spot, so the that electrochemicaldata are almost certainly representative of the changes occurring within the laser area.
Results Cyclic Voltammetry. Our previous SHG study of iron in 0.1 M NaOH was done with an aerated s ~ l u t i o n . ~ However, much of the previous work on the corrosion of iron in alkaline solutions has been conducted in an oxygen-free e n ~ i r o n m e n t .The ~ ~ ~cyclic voltammetry work in deoxygenated 0.1 M NaOH presented here was undertaken to provide a comparison with ow previous results4 as well as to measure the role oxygen plays in the passivation of iron in alkaline solutions. The electrochemical cell was purged with Nz, and the Fe electrode was held at a potential of -1.2 V, in the re(27) Bloembergen, N.; Chang, R. K.; Jha, s. s.; Lee, C. H. Phys. Reo. 1968,174, 813.
(28) S i p , J. E.; So, V. C. Y.; Fukui, M.; Stegeman, G. I. Phys. Reo. E Condens. Matter 1980, 21, 4389. (29) b i d e r , G. A.; Schmidt, A. J.; Marowsky, G. Opt. Commun. 1983, 47,223. (30) h t r o n g , R. D.; Baurhoo, I. J.Electroanul. Chem. 1972,40,325. (31) Wieckowski, A.; Ghali, E. Electrochim. Acta 1986,30, 1423. (32) Geana, D.; El Miligy, A. A.; Lorenz, W. J. J.Appl. Electrochem. 1974, 4, 337. (33) Burke, L. D.; Lyons, M. E. G. J . Electroanal. Chem. 1986, 198, 347. (34) Schrebler Guzman, R. S.; Vilche, J. R.; Arvia, A. J. Electrochim. Acta 1979,24, 395. (35) MacDonald, D.D.; Roberts, B. Electrochim. Acta 1978,23,781.
Langmuir, Vol. 4, No. 1, 1988 123
SHG Study of Fe OxidationlReduction
% t
0.075 M Boric Acid/Sodium Borate 2 rnV/sec
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0.2
-0.2
-0.6
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Figure 2. Second harmonic intensity as a function of potential for the iron electrode in a borate buffer while cycling between the passive film and reduced metal regions. duced metal region, for more than 1 h to remove the oxide coating formed in air. The potential was then ramped to +0.2 V in the region of passive film formation prior to running a potential cycle between +0.2 and -1.2 V with a sweep rate of 2 mV/s. The upper curve in Figure 1 is the cyclic voltammogram (CV) for such a cycle, and the bottom curve is the corresponding second harmonic intensity as a function of potential. The CV is qualitatively the same as it is in the absence of laser irradiations and agrees with the results of other workers for iron in alkaline solutions.*36 Laser-induced reactions are therefore ruled out. It must be noted that the second cathodic wave at -1.15 V and the anodic wave at -0.6 V were not observable in an air-saturated solution on the first cycle. This behavior is in contrast to the SHG results. The SHG curve is virtually identical with that for an air-saturated solution4 with the exception of the sharp rise in intensity at -1.2 V. The initial SHG signal level, figure 1, point A, is representative of the passive film, an Fe(II1) species. During the cathodic sweep, the second harmonic intensity rises to a new level with completion of an Fe(11-111) layer at -0.8 V (point B). Note that little indication of this transition is given by the CV. Further reduction of Fe(I1) occurs by -1.05 V (Figure 1, point C) and to the final reduced metal below -1.15 V. Oxidation occurs via the reverse processes. The SHG level for the Fe to Fe(I1) transition on the oxidative sweep is not evident since the equilibrium potential is approached from the opposite direction and a different overpotential is in effect. We will see later in this paper that constant-current oxidation reveals the presence of the Fe(I1) intermediate. Experiments of this type conducted in 0.1 M solutions of LiOH and KOH also appeared to give the same result. Preliminary work with the deoxygenated borate buffer (pH 8.5) is very similar to that presented in the remainder of this paper for the deoxygenated NaOH solution (Figure 2). The rise in signal during reduction lower than -1.15 V (Figure 1) is larger and sharper than was seen in the aerated solution. Repeated potential cycling of the electrode results in an overall decrease in the second harmonic intensity and the "growth" of intensity in the Fe(I1) and -1.2-V regions. Figure 3 presents the fourth cycle of an iron electrode in a deaerated 0.1 M KOH solution. The larger signal in these areas (C and F, Figure 3) is not an
-0.2
-0.6
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SCE) Figure 3. Second harmonic intensity as a function of potential for the iron electrode in 0.1 M KOH while cycling between the passive film and reduced metal regions. Fourth cycle after Potentid (vs
preparation of a fresh electrode surface. increase in absolute count rate but an increase relative to the Fe(II1) (A) and Fe(I1-111) (B) regions. In studies on AgF3 Richmond observed a large increase in SHG signal while the electrode was polarized in the cathodic potential region when silver ions were added to the solution. In our case, iron ions are introduced into the solution upon potential cycling due to the dissolution mechanism.30 The increase in signal at point F in Figure 3 relative to points A and B could then be due to the adsorption and subsequent reduction of these iron complexes. To test this hypothesis, a fresh deoxygenated solution was introduced into the electrochemical cell with an iron electrode that had just been cycled many times. The next CV and SHG curves both show the response of an electrode cycled many times despite the initial absence of iron ions. The rise in second harmonic intensity at potentials less than -1.15 V is therefore not due to the adsorption and reduction of iron ion complexes. Examination of the electrode, cycled many times, after removal from the cell reveals the presence of a yellowbrown film. This film cannot be removed by prolonged periods of reduction in the solution. These observations are easily reproducible and indicate that investigation of the early stages of iron corrosion should be done only during the first or second potential cycle between the passive film and reduced metal regions after preparation of a fresh electrode surface. The generation of such an overlayer is well documented, but its composition is also subject to s p e ~ u l a t i o n . ~ ~ . ~ ' If the CV is begun after the electrode has been at -1.2 V for a few minutes instead of a t +0.2 V, an interesting development is observed (Figure 4). The largest second harmonic intensity is now at -1.2 V (point E in Figure 4), the lowest potential. The signal then decreases to -0.9 V (point D), where there is a sudden drop in intensity. The initial decrease in signal level could be the desorption of sodium and/or hydrogen cations from the surface as the current becomes anodic. The sharp drop in SHG signal after -0.9 V may then be attributed to the beginnings for the initial stage of Fe oxidation, possibly the adsorption of OH-. A similar ~
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(36) MncDonald, D. D.; Owen, D. J. Electrochem. SOC.1973,120, 317. (37) Huang, Z. Q.; Ord, J. L. J.Electrochem. SOC.1985, 132, 24. (38) Chen, J. M.;Bower, J. R.;Wang, C. S.;Lee, C. H. Opt. Commun. 1973, 9, 132.
Biwer et al.
124 Langmuir, Vol. 4, No. 1, 1988
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Figure 4. CV (top curve) and SHG signal (bottom curve) from the iron electrode in 0.1 M NaOH while cycling between the reduced metal and passive film regions.
Figure 6. Potential and second harmonic intensity as a function of time for the iron electrode in 0.1 M NaOH during constantcurrent oxidation at 1.5 PA.
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gradual decline of a large initial SHG signal followed by a sharp drop during oxidation for a Ni electrode in 0.1 M NaOH has sometimes been observed in the low potential region.39 The remainder of the curve (Figure 4) follows the same pattern as seen previously. The large second harmonic intensity at the reduced bare metal potentials relative to the signal levels in the oxidizing potential regions at 70' incidence is quite different from our previous study at 45O? The SHG signal level decreases for all potentials by changing the angle of incidence (e) back to 45O (Figure 5). The overall decrease in intensity is expected due to the angular dependence discussed in ~~
(39) Biwer, B. M.; Pellin, M. J.; Schauer, M. W.; Gruen, D. M., work
in preparation. (40) Ahaier, W.; Heusler, K. E. 2.Physik. Chem. Neue Folge 1975, 98, 161. (41) Allgaier, W.; Heusler, K. E. J. Appl. Electrochem. 1979,9, 155. (42) Marshall, C. D.; Korenowski, G. M. J.Phys. Chem. 1987,91,1289. (43) Sokolov, J.; Shih, H. D.; Bardi, U.; Jona, F.; Marcus, P. M. Solid State Commun. 1983, 48, 739. (44) Sokolov, J.; Shih, H. D.; Bardi, U.; Jona, F.; Marcus, P. M. J . Phys. C. 1984, 17, 371. (45) Lee, C. H.; Chang, R. K.; Bloembergen, N. Phys. Reu. Lett. 1967, 18, 167. (46) Corn, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. Chem. Phvs. Lett. 1984.106. 30. (47) Cim, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. J . Chem. Phys. 1984,81, 4127.
-1.2
w&AL (VSSCE) Figure 7. Second harmonic intensity from the iron electrode in 0.1 M NaOH as a function of potential during constant-current oxidation.
the Experimental Section. However, the intensity level decreases much more dramatically for the reduced metal at low potentials (--1.0 to -1.2 V) than the level at oxidative potentials. Constant-CurrentOxidation/Reduction. Additional information is gained if we now control the current instead of the potential. The Fe(I1) SHG level (point C in Figure 1) is not observed in the CV on the anodic sweep since the Fe Fe(I1) and Fe(II) Fe(I1-111) eqgibrium potentials are very close together and the associated overvoltages are different than for the cathodic sweep. Constant-current oxidation was used to look for the Fe(I1) stage during the oxidation process in 0.1 M NaOH. The CV also does not give much information on the final reduction of iron (Fe(I1) Fe) and subsequent adsorption of cations in solution. Constant-current reduction was employed to investigate this process. The second harmonic intensity and potential were monitored as a function of time during constant current (1.5 PA) oxidation of the reduced iron electrode (Figure 6). Figure 7 plots the SHG signal data as a function of potential for comparison with the CV (Figure 4). In both cases, we have a large SHG signal at the most reduced region (point E) which declines with increase in potential, due to possible desorption of H and/or Na cations, until there is a sharp drop in intensity (point D; adsorption of OH-, first stage of iron oxidation). At -0.6 V a shoulder
-
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Langmuir, Vol. 4 , No. 1, 1988 125
SHG Study of Fe OridationlReduction 0.1 M NaOH dudion
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on the curve in Figure 7 (point C) indicates the presence of the Fe(I1) phase before further oxidation to the Fe(I1111)species by -0.4 V (point B). The Fe(I1) species was not observed in the anodic sweep of the CV (Figure 4), presumably due to overvoltage considerations as was mentioned. The second harmonic intensity then decreases as the potential increases above -0.4 V as the passive film (point A) is formed. Constant-current reduction (1.75MA) also highlights the various oxidation stages, as indicated by the different SHG signal levels, and demonstrates the slower kinetics of the electrode’s final reduction step to the metal (Figure 8). Initially, the second harmonic intensity rises from the Fe(1II) level (Figure 8, point A) to that of the Fe(I1-111) level (point B). The Fe(I1-111) phase is then converted to the Fe(I1) species (point C) as represented by the gradual decrease in the SHG signal. The transition is complete after 800 s. The following sharp drop in intensity marks the beginning of the transformation from the Fe(II) species to the reduced metal, which is followed by the onset of cation adsorption (point D)after approximately 1300 s. If point D is representative of the “clean” metal surface in all cases, we must first explain why the SHG signal is much lower there than in the case of constant-current oxidation, a t -0.85 V (point D)in Figure 7 and at -0.9 V (point D)on the oxidative sweep of the CV in Figure 4. Geronov et al.4s studied the oxidationfreduction behavior of iron in alkaline solutions using Mossbauer spectroscopy. They reported the presence of residual Fe(OH)2 still existing on the surface even after passing cathodic current for extended periods of time. This result is consistent with the change in the SHG cyclic voltammograms after a few potential cycles, indicating that the composition of the electrode surface is no longer the same at the end as at the start of each cycle. It also suggests that the iron surface is still being reduced at point D in Figure 8 when the adsorption of H and Na cations occurs. This latter process is not seen in the cyclic voltammograms during the cathodic sweep due to the time necessary for the complete reduction of Fe(I1) to occur. Plotting the data from Figure 8 with the second harmonic intensity as a function of potential (Figure 9),it is seen that both reduction and cation adsorption are occurring at the same potential, -1.15 V. After sufficient time at reducing potentials, the iron surface once again gives a large SHG (48) Geronov, Y.; Tomov, T.;Georgiev, 5.J. Appl. Electrochem. 1975, 5 , 351.
-0.8
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Figure 9. Second harmonic intensity from the iron electrode in 0.1 M NaOH as a function of potential during constant-current
reduction.
response as seen in Figure 7 at -0.85 V (point D)and at
-0.9 V (point D)in Figure 4. Discussion Contributions to the Second Harmonic Signal. The generated second harmonic signal from an electrode surface due to an incoming electric field may come from two sources. The one source generally associated with SHG is the second-order polarization of a material. The other is the third-order hyperpolarizability, The second-o_rder eolarization of a medium is given as P J 2 ) ( 2 w )f g(2)E(w)E(w). ji(2) is the electric susceptibility, and E(w) is the e_lectricfield of an incident light wave with frequency w. PJ2)(2w)vanishes in an isotropic medium in the electric dipole approximation and exists only where there is no inversion symmetry present, as at the electrode/ solution interface. It can be seen from the form of “girijrjg
p (Ujg
+ similar terms
- w)(wjg - 2 ~ )
that, when w or 2w approsches the energy of a transition dipole of a material (7jg),PJ2)will increase dramatically. As a result, different compounds are therefore expected to generate different levels of signal under the same conditions since their electronic transitions are at varying degrees of resonance with w and 2w. The changes in our iron spectra of the second harmonic intensity as a function of potential seem to be primarily due to changes in resonance as the surface composition changes. Every change in a CV is mirrored by a change in second harmonic intensity, although the reverse is not necessarily true. The potentials at which changes take place in the CV are traceable to the occurrence of specific reactions based on thermodynamic consideration^.^ However, it is thought, for a specific surface on which no adsorption or chemical reactions are occurring, that the SHG signal will increase quadratically with a linear increase of charge at the surface of an e l e c t r ~ d e . ~This ~~~’ additiopl signal is iue_to th_e third-order_ hyperpolarizawhere is the static bility P1(2)(2w)= yEd$(W)E(w), This contribution will also electric field at the vary with change in surface composition since the term is wavelength dependent in a manner similar to ji(2). Near the potential of zero charge (pzc) of a reduced metal, where no reactions or adsorption occur, changes in second har-
126 Langmuir, Vol. 4, No. 1, 1988 monic intensity will then vary solely due to a change in electric field at the surface, as has been observed for silver
electrode^.^^ Reduced Metal Potential Region. Initial anodization of the iron electrode held in the reduced metal region at -1.2 V led to a decrease in second harmonic signal, going from point E to point D in Figures 4 and 7. Such an observation may be due to the desorption of sodium cations from the surface. Tom et al.3 observed a large increase in second harmonic intensity from a Rh(ll1) surface in ultrahigh vacuum when alkali atoms (Na, K, and Cs) were deposited on it. Chen et al.%observed the same type of increase when depositing Na on Ge. The adsorption of Na on a metal surface is known to result in a partial transfer of electronic charge to the metal. The resulting surface dipole condition is very similar to that created by the adsorption of Na+ ions on a cathodically polarized metal electrode surface as in our study. Tom et aL3 attributed their second harmonic intensity increase upon Na adsorption to a resonance effect. The Na 3s and 3p orbitals hybridize with the metal's electronic states to form bands. The Na 3s band was considered to be roughly 2 eV above the metal's Fermi level and the 3p band about another 2.1 eV higher in energy. The widths of the bands themselves were taken to be approximately 2 eV. Resonance enhancement would then be expected at their fundamental of 1.17 eV and second harmonic at 2.33 eV. Since our fundamental energy is 2.33 eV and its second harmonic at 4.66 eV, we would also expect to observe a strong resonance enhancement of the SHG signal with Na+ adsorption. A similar argument was put forward for the increase in second harmonic intensity with the adsorption of K on Rh(lll).3 Our initial work with Fe in 0.1 M kOH gives the same results as those presented here for the 0.1 M NaOH solution. It is possible thai the observed signal is enhanced due to the static field (Edc)at the surface as a result of the H+ and Na+ cations present. The formation of an iron hydride cannot be ruled out as well. Upon further anodization of the clean reduced iron at point D in Figures 4 and 7, there is a sharp drop in second harmonic intensity due to adsorption of OH-. Such a change during the intial stage of Fe oxidation could be attributed to a change in charge at the electrode surface and/or a modification of the electric susceptibility of the surface by the adsorbate. However, keep in mind that such a dramatic change was not observed when the laser was inccident at 45O, Figure 5. We saw that the reduced metal potential region did not have the expected angular dependence relative to the oxidative potential regions. One would expect all regions to have the same relative signal levels for a given 0 unless there is a favored geometry present or there is more than one contribution to the second harmonic light. The sensitivity of SHG to structure in metals was originally thought to be minimal due to the isotropic nature of the nearly free electrons present." Tom and Aumiller" have presented evidence to the contrary. A large anisotropic SHG response from a Cu(ll1) single crystal was seen due to changes in polarization or 6. The second harmonic intensity was shown to vary in a manner consistent with the in-plane surface symmetry. They suggested that the failure of previous work to detectmrface structure was due to samples without well-defined surface order. There is evidence to indicate that crystallites with the same single-crystal plane exposed to the solution might exist on the Fe electrode surface. This condition could be responsible for the observed deviation from the expected
Biwer et al. SHG 0 dependence for the reduced metal. It is known for Fe electrodes that the (110) and (211) surfaces seem to be the only stable crystal faces at various potentials in soluti~n."~lFormation of these planes can occur during either dissolution or precipitation. Surface reconstruction is also a possibility. Recent work has demonstrated that restructuring of a Ag electrode surface occurs at reduced metal potential^.^^ The reduced electrode surface could be composed of iron crystallites with the same exposed crystal face which lies in a plane parallel to the macroscopic surface. Such ordering affects only the 0 dependence and not the polarization dependence since ordering remains isotropic in the plane of the surface. Oxidation of the surface would destroy any surface symmetry since the oxide (hydroxide) structure is incompatible with that of the pure metal. A polycrystalline 0 dependence would then be expected at oxidative potentials and is observed. If a particular crystallite face is predominant at reduced metal potentials, the experiments to determine the second harmonic intensity as a function of 6 should give results identical with those for a given single-crystal surface. Angle-dependent measurements on polycrystalline and single-crystal Fe electrodes are currently under way in our laboratory to check on this possibility. Oxidative Potentials. The second harmonic intensity from the reduced and the different oxide surfaces is in the following order: Fe > Fe(I1-HI) > Fe(II) > Fe(II1). The passive film region, Fe(III), generates a SHG signal level comparable to that for iron oxidized in air, which is known to be iron in the 3+ state at the Fe203. The intensity of this level is thought to be largely due to resonance effects since w, 532 nm (2.33 eV), falls near the band-gap of FezO3,2.2 eV.50 The Fe(I1-111) region should also have a large second harmonic intensity since a mixed valence compound such as Fe304has a wide frequency absorption range51-54with resonance enhancement expected at both the fundamental w and the second harmonic 2w. The passive film itself is no more than 300 thick on the basis of the charge passed in either constant-current oxidation or reduction experiments. As a first approximation, it was assumed that the passive film was entirely composed of Fe203and that all charge passed went into its formation. Dissolution of Fe into solution as Fez+in the intermediate reactions was not accounted for and could significantly reduce the estimated thickness. Optical measurements of the passive film thickness on iron in 0.05 M NaOH placed the value at about 50 a value which is dependent on potential and pH; a higher potential and/or pH produces a thicker film. Since the second harmonic intensity levels off after 0.2 V during anodization while the passive film thickness is expected to increase, only contributions to the signal from the passive film/ solution interface are present in this potential region. As the passive film is reduced, it either exposes a thinner Fe(I1-111) layer or is converted into such a layer and the signal increases with signal originating at the metal oxide/solution interface. Experiments have been initiated using an incident frequency trapsparent to the oxide
a
(49) Brundle, C. R.; Chuang, T. J.; Wandelt, K. Surf. Sci. 1977,68,459. (50) Debnath, N. C.; Anderson, A. B. J. Electrochem. SOC. 1982,129, 2169. (51) Muret, P. Solid State Commun. 1974, 14, 1119. (52) Balberg, I.; Pinch, H. L. J. Magn. Magn. Mater. 1978, 7, 12. (53) Schlegel, A.; Alvarado, S. F.; Wachter, P. J.Phys. C 1979, 12, 1157. (54) Tanaka, T. Jpn. J. Appl. Phys. 1979,18, 1043. (55) Zakroczymski, T.; Fan, C.-J.; Szklarska-Smialowska, Z. J . Electrochern. SOC.1985,132, 2862.
Langmuir 1988,4, 127-132 overlayers in the hope of probing the metal/metal oxide interface.
Summary The cyclic voltammetry SHG data for the iron electrode in a deoxygenated 0.1 M NaOH solution with the laser beam incident at 4 5 O were shown not to differ from studies in an aerated s ~ l u t i o n .Such ~ a result suggests that dissolved oxygen does not play a major role in the oxidation/reduction of iron in alkaline solutions. The smaller electrode to laser beam area ratio in conjunctionwith these results also demonstrated that negligible laser-induced chemical reactions are occurring. A more careful investigation of the reduced potential region with SHG in conjunction with cyclic voltammetry and also constant-current oxidation/reduction demonstrated the slow response of the electrode to the final reduction to bare metal. SHG during constant-current oxidation was used to show the presence of an Fe(I1) phase during oxidation. This phase was not seen with SHG during oxidative potential sweeps due to overpotential considerations.
127
Changes in the second harmonic intensity during the oxidation/reduction of Fe in deaerated 0.1 M NaOH solution have been assigned on the basis of past work with iron in aerated alkaline and borate buffer solutions and present preliminary work with the electrode in deaerated borate buffer. The reduction of the passive film, Fe(III), occurs via two intermediates: first reduction to a mixed valence Fe(I1-111) compound and then an Fe(II) phase, before final reduction to the bare metal accompanied by the adsorption of cations. In any event, the reduction of iron in mildly basic borate buffers is the same for iron in more alkaline solutions, such as 0.1 M NaOH. It was also found that the second harmonic intensity from the reduced metal surface in 0.1 M NaOH is much more sensitive to the angle that the laser beam makes with the surface normal than was to be expected relative to signal levels from oxidized surfaces. Geometric contributions from crystallites with the same single-crystal face in the macroscopic surface plane have been postulated and are now under investigation. Registry No. Fe, 7439-89-6; NaOH, 1310-73-2.
Surface-Enhanced Raman Spectroscopy of Poly(2-vinylpyridine) Adsorbed on Silver Electrode Surfaces Joseph L. Lippert and E. Steven Brandt* Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received February 23,1987. I n Final Form: August 10, 1987 Surface-enhanced Raman scattering (SERS) of partially protonated poly(2-vinylpyridine) (P2VPy) adsorbed onto an electrochemicallyroughened silver electrode is reported as a function of applied potential. The species on the electrode surface is predominantly pyridinium ion when the electrode potential is positive of the point of zero charge (E.) and predominantly neutral pyridine near E,. Orientation effects involving the polymer chain are visible in the fingerprint region of these spectra. The species vs potential dependence is different for the polymer compared with low molecular weight probe molecules of pyridine and 2methylpyridine. The presence of the polymer substantially improves the stability of the active sites responsible for SERS activity to cathodic excursions to E, in chloride electrolyte. ?e differences observed with the polymer are explained on the basis of its lower solubility and the "anchoring" of positive charges within the double-layer region.
Introduction The initial observations of surface-enhanced Raman scattering (SERS) of pyridipe (Py) at roughened silver electrodes1" showing intensities 4-6 orders of magnitude greater than normal Raman spectra have been followed by numerous experiments examining the interactions of small molecules and ions adsorbed on such surfaces as a function of solution pH and applied potential."12 These (1) Flekhmann, M.; Hendra, P. J.; McQuillan, k J. Chem.Phys. Lett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) Albrecht, M. G.; Creighton, J. A. J.Am. Chem. SOC.1977,99,5216. (4) VanDuyne, R. P. In Chemical and Biochemical Applicatiom of Lasers, 4; Moore, C. B., Ed.; Academic: New York, 1979. (5) Surface Enhnced Raman Scattering; Chang, R. K.; Furtak, T. E., E&.; Plenum: New York, 1982.
0743-7463/88/2404-0127$01.50/0
experiments have demonstrated that SERS is a potentially powerful tool for examining the chemical and physical phenomena associated with adsorption and the electrode/solution interface. By contrast, relatively few experiments on macromolecules adsorbed to silver surfaces have been reported.13 (6) Allen, C. S.;VanDuyne, R. P. Chem. Phys. Lett. 1979, 63, 455. (7) Bunding, K. A.; Lombardi, J. R.; Birke, R. L. Chem. Phys. 1980,
49, 53.
(8) Dornhaus, R.; Chang, R. K. Solid State Commun. 1980, 34, 811.
(9) Dornhaus, R.; Long,M. B.; Brenner, R. E.; Chang, R. K. Surf. Sci. 1980, 93, 240. (10) Chang, H.; Hwang, K. C. J. Am. Chem. SOC.1984, 106, 6586. (11) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I,;Sun, S. C. Chem. Phys. Lett. 1984,104, 240. (12) Rodgers, D. J.; Luck, S. D.; Irish,D. E.; Guzonae, D. A.; Atkinson, G. F. J. Electroanal. Chem. 1984, 167, 237. (13) Kerker, M. Pure Appl. Chem. 1984,56, 1429.
0 1988 American Chemical Society
