The adsorption of p-nitroaniline on silver and gold electrodes as studied with surface enhanced Raman spectroscopy (SERS)

Elccrrmhinka “cm, Vol 35, No Prmted m Great Bntarn

6, ,,p

1037-1044,

1990 0

0013-4686/90 S300+0@3 1990 Pcrgamon Pras plc

THE ADSORPTION OF p-NITROANILINE ON SILVER AND GOLD ELECTRODES AS STUDIED WITH SURFACE ENHANCED RAMAN SPECTROSCOPY (SERS) RUDOLF HOLZE Umversltat Oldenburg, Fachberelch Chemle, Carl-von-Ossletzky-Str

9-11, D-2900 Oldenburg, F R G

(Recemed 3 July 1989, m reursedform 22 August 1989)

Abstract-The adsorption of p-nltroanlhne PNA on polycrystalhne sliver and gold electrodes from neutral and acldlc electrolyte solutions saturated with PNA has been studled usmg Surface Enhanced Raman Spectroscopy With both electrode metals a perpendicular orientation of the adsorbed molecule with an mteractlon vlo the mtro-group ISconcluded based on SER spectra recorded at various electrode potentials No evidence of new surface species formed upon adsorptlon of PNA was found

INTRODUCTION Recently the adsorption of p-mtroamhne (PNA) on a polycrystalhne platinum electrode m contact with neutral or acidic sulphate electrolyte solution has been studied with modulated Electrode Reflectance Spectroscopy (ERS, the probe is light at wavelengths m the W-WS) by Schmidt and Pheth[l] Their results suggest a perpendicular onentatlon of the adsorbed molecule with the mtro-group pomtmg towards the positively charged electrode surface As part of our ongoing study of the electrochemical behavlour of various amlmes with respect to their adsorption behavlour and polymerization capability PNA was chosen as a promlsmg candidate for further studies This molecule provides at least two dlstmctly different molecular functional groups suitable for mteractlon with the electrode and 1s not prone to conductive polymer formatlon[2] The absence of the formation of conductive polymer films, which gives a strong single line m the ESR spectrum, has made electron spin resonance spectroscopy of some electrochemically formed reduced or oxldrzed forms of denvatlves of PNA posnble[3] The molecular structure of PNA with an electron drawing mtro-group m para-poatlon with respect to the ammo-group causes electronic properties of the molecule slgmficantly different from related ammes The dlelectnc constant of PNA IS 56 (at 16O”C, for comparison aniline 4 54 at 184 6”C)[4], the electric dipole moment of PNA 1s 6 3 D[5] (for comparison amhne 153 D[4]) In order to get further insight mto the adsorptive interaction between an electrode surface and the adsorbed PNA molecule as well as mto adsorptloninduced molecular changes of the adsorbed molecule zn situ vibrational spectroscopy should be particularly helpful In prmclple the adsorptlon of PNA on platinum can be studied with Surface Enhanced Raman Spectroscopy (SERS) with a method described m[6] This method baslcally consists of coating silver or gold electrodes suitable for SERS measurements with a few

layers of platinum atoms, these electrodes give well defined SER spectra with spectral features Influenced predommantly by the surface properties of platinum Since m our previous work on the adsorption of various organic molecules on electrodes mostly silver or gold electrodes were used the adsorptlon behavlour of PNA was studled first on these electrodes with SERS (ERS measurements at gold and silver electrodes are unfortunately hampered by their particular electrooptic properties[7] ) In this commumcatlon results of a SERS-study of the adsorption of PNA on gold and silver electrodes m the presence of PNA-saturated neutral and acldlc perchlorate electrolyte solutions are reported, addltlonal mformatlons obtained from W-VIS spectroscopy and cychc voltammetry (CV) as far as necessary m order to interpret the SER spectra are included

EXPERIMENTAL Cychc voltammograms of neutral and acldlc electrolyte solutions containing p-nitroanihne were recorded with a gold sheet and a silver wire electrode respectively and a potentlostat (Bank POS 73) m a conventional three electrode arrangement using an Hcell A saturated calomel electrode (see) and a gold sheet or a silver wire were used as reference and counter electrodes, respectively m cell compartments separated by glass frits All potentials are quoted us the saturated calomel electrode (E,,) CVs of PNA lrreverslbly adsorbed on the electrodes were recorded after the electrode was exposed to the electrolyte solutions saturated with PNA under electrode potential control for 5 mm The electrodes were emersed from the electrolyte solution, rmsed with plenty of water and transferred to the blank supportmg electrolyte solution Therem the CVs were recorded Ultra-violet visible spectra of electrolyte solutions were recorded with 10 mm cuvets on an Uvlkon 810 spectrometer (Kontron) interfaced to a MAC 80 A data processing system (Spectradata, Oldenburg) Because of the high sensltlvlty of the spectrometer and

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the data handhng capabitles of the data system detection and analysis of even mmor absorption bands was easily achieved This turned out to be a major improvement allowing detectlon and discussion of spectral features which were not possible before (compare eg[ 11) SER spectra were recorded on a scanmng Coderg LRT 800 tnple monochromator spectrometer using 488 nm and 647 1 nm exciting laser light provided by Spectra Physics 164-00 and 2020- 11 laser systems The spectral bandpass was set to a value resultmg m a resolution of 5 (at 488 nm) or 7 (at 647 1 nm) cm-’ respectively The laser power at the electrode surface was approx 50 mW as measured with a laser power meter (Laser Instrumentation Ltd) The mcldent hght at the electrode surface was p-polarized In case of the 488 nm laser light an EM1 6256 S photomultlpher coupled to a dc-amphfier was used, with 647 1 nm laser light a cooled RCA C31034 photomultlpher tube and a photon counter were employed The spectroelectrochemlcal cell has been described previously m detad[8] Activation of the SERS silver electrode (polycrystalhne 99 99%, 5 mm dla , embedded m PTFE, polished with Al,O, of decreasmg particle size, the final size was 1 pm) was performed m a separate cell with 1 M KC1 solution by cycling the electrode potential between Es,= -400 mV to Es,= +250 mV for a few cycles The gold electrode (polycrystalline 99 99%, 5 mm dla, embedded m PTFE, polished down to 1 pm Al,OJ also was activated in a separate cell accordmg to a procedure described elsewhere m detad[9] This procedure basically consists of repeated electrode potential cycling of the electrode m a 0 1 M KC1 electrolyte solutlon[lO] * Activation m the spectroelectrochemlcal cell m the presence of PNA was avoided because It might have resulted m the trappmg of molecules showing vlbratlonal spectra different from spectra of purely adsorbed molecules[ 133 (Recently this apprehension has been venfied m a SERS-study of aniline adsorption on a silver electrode[14] The actlvatlon of the electrode was performed m situ m a hahde electrolyte solution contammg amhne The results were mterpreted assuming adsorbate-electrode mteractlons om mtercalated halide ions, no direct metal aniline mteraction was found On the contrary m an earher study the silver electrode was activated ex sttu as described above m the absence of amhne and subsequently exposed to the amhne containing solutlon[lS] In these measurements no interference from the hahde ions or inclusions of aniline molecules in surface silver halide layers was found, instead sdver-mtrogen modes were identified ) Electrolyte solutions were prepared from 18 Mohm water (Seralpur Pro 90 c), p-mtroamhne PNA (Merck, used as received), perchlorlc acid (Merck, G R ) and potassium perchlorate (Merck, G R ) The concentration of the supportmg electrolyte was 0 1 M The solutions were saturated with p-mtroamhne m all

*An alternate method first described by Ciao et al[lO] could not be apphed successfully m ths mvwtlgahon Development of a special potentutl scan generator[ll] and further mformatlons -. . provided ^ .by Gao et al [12] have enabled . . SWcessful apphcatlon 01 their procedure m currem WorK

electrochemical and SERS-experiments, the concentration was approximately 4 mM Only uv-vls spectra were recorded at various concentrations of PNA and constant concentrations of the supporting electrolyte as mchcated m the corresponding figures Solutions were purged with nitrogen (99 995%) after punficatlon with Oxysorb (Messer-Gneshelm) in order to reduce the oxygen trace lmpurlty level below 0 1 vpm All expenments were performed at room temperature (21°C) RESULTS AND DISCUSSION Cyckc vohmmetry The cyclic voltammogram recorded with a gold electrode m a solution of 0 1 M KC104 (pH =4 95) saturated with PNA shows m a negative-going potential scan started at E,, = - 350 mV a wave at E,,, = -775 mV which 1s caused by the reduction of the mtro-group (Fig 1) In the posltlve-gomg potential scan started at E,, = - 350 mV a poorly defined redox + 100 mV (approx) and EC,,,= wave mth E,,= - 50 mV corresponding presumably to the reversible oxidation of the ammo-group 1s observed In CVs covermg the whole electrode potential range displayed m Fig. 1 addluonal features caused by products of subsequent chemical reactIons m particular of the reduced form of PNA (see[ 161) are vlslble The redox wave was not observed m earlier mvestigatlons[l], possibly it was obscured by surface layer formation on the platinum electrode The presence of the cathodic part of the redox wave strongly supports the observation by Shenglong et al [2], m their work on the polymenzation of substituted amhnes PNA was the only compound incapable of polymer formation These authors further concluded, that a free radical assumed as an mtermedlate m the polymenzatlon process is extremely unstable m case of the electron wthdrawmg mtro-group m the paru-posltlon This should result m the absence of a polymer forming propagation reaction The CV reported here implies a stability of the oxidized species at least sufficient to allow the detection of the correspondmg reduction current of exactly the same magmtude Thus the absence of the polymenzatlon seems to be due to some other pecuhanty of PNA In part it 1scertainly due to the blocking of the para-posltlon of the benzene rmg,

I.

.

-800

I

1

-400

.

E SCE ’ mV

.

0

.

J

400

Fig 1 Cychc voltammogram of a gold sheet electrode m 0 1 M KClO, saturated with PNA, 50 mV s-l, room temperature, mtrogen purged, the background curve obtained with the supportmg electrolyte 1 not shown because the curve IS comnletelv flat over the entire Dotenhal ranlce shown here

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005

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600

620

640 “In

IO

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4””

0””

e

I””

Wavelength

/

3

nm

Fig 3 Ultra-violet-vls spectrum of 0 1 M HCIO, electrolyte solution with 0 2 mM PNA, Insert with 4 mM PNA. 10 mm cell Table 1 Spectral data of PNA and PNA adsorbed on gold and sdver electrodes at various electrode potentials, assignment of bands based on hterature data[5,20], molecular symmetry C,, (all values are wavenumbers cm-‘, Ip=m-plane, oop=out-ofplane)

Mode C-N-torston substlt mode na na oop-def Surf-O-mode NO,-rock na Ip-rmg-def oop-def C-NO,-stretch oop-def Ip-rmg-def na lp-C-C-def na Ip-def Ip-def sym -NO,-stret C-NH,-stret lpdef asym -NO,-stret lp-rmg-def NH,-def

background

Wilson mode #

Symmetry

PNA, sohd (Fig 4)

-

-

143 250 300 360 398

16a

a2 -

-

-

6b 1Oa 17a 18b

b

9a 3 14 19b 8a -

IS not

caused

‘a al -

1627

a2

b al

‘a ‘a -

by spectes

-

410 533 557 631 810 858 962 1107 1175 1230 1277 1312 1334 1390 1448 1504

a2 -

PNA,,, Au, 0 1 M KCIO,, E,,=OmV (Fig 5)

630 1255 1332 1375, 1395 1585

m the thin

electrolyte solution layer between the electrode and the cell window, since Its intensity depends upon the electrode potential

PNA,,, Au PNA,,, Ag, 0 1 M HCIO,, 0 1 M KCIO,, E,,=437mV E,=-4OOmV (Fig 7) (Fig 6)

412 505 631 855 1112 1175 1205 1266 1340 1375 (1475) 1584

PNA,,, Ag 0 1 M HCIO,, E,,=350mV (Fig 8)

415 635 860 1115 1144 1185

1332 1402 1452 1500 1595

423 635 857 1080, 1118 1150 1185

1330 -

(1544)

-

In the spectrum shown m Fig 5 (I&,=0 mV) severa1 bands can be identified (compare Table l), m all cases they correspond to modes of the ammo- and mtro-group and to m-plane modes of the benzene nng

AdsorptIon of p-mtroamlme on Ag and Au electrodes

z

I..

1500

I1

11..

.

1000 Raman shlf t/cm’

.

,

1041

,

,

500

_I

100

Fig 4 Normal Raman spectrum of solid PNA, capillary sample technique, 488 nm laser hght, resolution

5 cm-’ The mterpretatlon of SER-spectra has been dacussed prevlously[16,21,22] and 1s still subject to controversial dlscusslons The orientation and the mode of interaction of adsorbed molecules has been inferred from the absence/presence and relative intensity of bands m SER spectra as compared to the Raman spectra of the bulk substance (see eg[21]) and from band shifts of molecular modes (see eg[22]) Experimental evidence presented so far[Zl] seems to suggest, that modes of the adsorbed molecule mvolvmg motions perpendicular to the surface are particularly enhanced + In the case under dlscusslon here this would imply a perpendlcular orlentatlon of the adsorbed PNA molecule In the low wavenumber region an addltlonal band not seen with sohd PNA 1s found at 410 cm-’ This band cannot be caused by an internal mode of coadsorbed perchlorate anions, the lowest frequency of an internal mode of the ion 1s found at 460 cm- ’ [24] A surface-adsorbate vlbratlonal mode can be expected at low wavenumbers Basically varrous modes of interaction between adsorbed PNA and the electrode surface mvolvmg the aromatic electron system of the benzene rmg, hydrogen atoms at the rmg and the substltuent groups are conceivable The coordmatmg capability of the latter groups makes an mteraction with the electrode surface ora the oxygen atoms of the mtro-group or the free electron pair of the ammo group especially likely In the case of adsorption from an acidic electrolyte solution the adsorbate 1s present on a positively charged electrode surface predommantly m its unprotonated form because of electrostatic reasons, t Because of the remammg uncertamtles of this approach, which are closely related to the mapphcablhty of dipole selection rules to surface Raman spectroscopy[21] the mterpretatlon of SER spectra based on spectral shifts of bands has been dlscussed (see[22]) Unfortunately the lack of precise mformatlon on adsorptlon induced changes of vlbratlonal propertles makes such an mterpretatlon somewhat difficult Attempts to remedy this sltuatlon have been made, but so far they have provided only hmlted results restncted to small adsorbate molecules[23]

0 Raman shift/cm’

Fig 5 SER spectrum of PNA adsorbed on a gold electrode, E,, =0 mV, 0 1 M KClO,, saturated with PNA, 647 1 nm, resolution 7 cm - 1

I..

. . 1500

. .

..a

*..I.

1000 Raman shlf t I cm’

IO

Fig 6 SER spectrum of PNA adsorbed on a gold electrode, Exe=437 mV, 0 1 M HC104, saturated with PNA, 647 1 nm, resolution 7 cm- 1

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1000 Roman shlftlcm’

Fig 7 SER spectrum of PNA adsorbed on a sliver electrode, E,, = -400 mV,0 1 M KCIO,, saturated with PNA, 488 nm, resolution 5 cm-’

whereas at negative surface charges electrostatic adsorption of the ammium cation seems to occur on a mercury electrode m case of the adsorptlon of amhne from electrolyte solutions where the acid concentratlon was smaller than the amhne concentratlon[25] In case of aniline adsorbed on gold from a neutral electrolyte solution the correspondmg mtrogen-surface mode was found around 340 cm- 1[26] The difference to the value of 410 cm-’ reported here makes an adsorptlon uta the nitrogen atom less hkely An mteractlon oza the mtro-group has to be dlscussed Instead It IS supported by the strong mtenslty of the symmetric NO,-stretchmg mode The frequency of this mode 1s not shifted to a slgmficant extent In case of mtrobenzene adsorption au the mtro-group [22,27] (although no surface-adsorbate modes have been assigned m the latter work) has been concluded, too Even at electrode potentials negative to the potential of zero charge E,,, no band mdlcatmg an adsorption via the ammo- or the ammmm-group 1s found The mterpretatlon proposed here IS also m agreement with the conclusion of Schmidt and Pheth[l] A different way to identify the coordmatmg atom not used here 1s described m[28] t If the strong background 1s caused by fluorescence this would be at variance with the general observation, that a molecule close to a metalhc conductor does not show any fluorescence because of the quenchmg effect of metal-adsorbate mteractlonsC29, 301 Perhaps these mteractlons are too weak in the present case or the electronic transition found with PNA-solutions around 605 8 nm ts shifted closer to the wavelength of the laser light upon adsorptton on the electrode, thus fluorescence 1s more hkely In case of a platmum electrode the correspondmg band m ERS was found around 620 nm at a less than maxlmum surface coverage of the electrode with PNA In the present case the PNA concentration 1ssubstantially higher leadmg to a

*Basedon the model of the harmomc oscillator and SERS results obtamed with a gold electrode and vanous amomc as well as polyatonuc adsorbates Gao and Weaver have suggested[31], that the frequency of the adsorbate-surface mode depends upon the atomic mass of the coordmatmg atom of the adsorbate According to more recent reports tlus idea has not been mvestigated further (see. eg[26]), m the present case the slmllar atomic masses of oxygen and mtrogen would not allow a declslon based on tlus Idea anyway

higher degree of coverage and subsequently an even higher wavelength of this transltlon With an acidic solution SER spectra recorded at electrode potentials rangmg from E,, = - 200 mV to E,, = 500 mV (at the latter value the SERS-actmty of the electrode started to drop) have to be considered At E,,=437 mV a SER spectrum with a rather low background intensity was observed (Fig 6) It shows bands correspondmg to modes of the two substltuents and to m-plane modes of the aromatic rmg, this agam indicates a perpendicular onentatlon The assignment of the low frequency mode at 412 cm-’ to a surface oxygen mode has been discussed above

PNA-adsorptzon

on szlver

In both KC104 and HC104 solutions saturated with PNA well defined SER-spectra with a low background intensity were recorded In the case of the KClO,-electrolyte solution SERspectra recorded at electrode potentials between E,,= -400 mV and E,,=O mV showed bands attnbuted to substltuent modes and m-plane modes of the benzene unit (see Fig 7 and Table 1) lmplymg a perpendicular orlentatlon of the adsorbate The C-N mode found m the NR-spectrum at 1390 cm-’ is shlfted upwards to 1400 cm-’ Accordmg to Colthup et al [20] electron withdrawing substltuents m the para-posltlon with respect to the ammo-group stiffen the C-N bond, this leads to a shift of the corresponding mode to higher wavenumbers Upon adsorption on an electrode with the mtro group m para-position Interacting with the electrode, this effect 1s further enhanced (see below) At low frequencies a mode around 420cm-’ IS found This band has been discussed above, it mdlcates adsorption of the molecule vza metal-oxygen mteractlon (of the mtro-group) Based on SERS-data of adsorbed halides on stlver surfaces obtained by Nichols and Hexter[32] and further supported by theoretical calculations a dver-oxygen mode can be expected at about 400 cm-’ Unfortunately the relationship between the atomic mass of the adsorbate and the wavenumber of the stretchmg vlbratlon as obtained with the hahdes does not show a simple hnear correlation as observed with adsorbates on gold electrodes[9,19] Adsorption via the ammo nrtrogen can be excluded, Since it 1s mdlcated m neutral solu-

AdsorptIon of pmtroamhne

,I

$1

1500

11

on Ag and Au electrodes

11

8

1043

“‘I(’

1000 Raman shift /cm’

500

0

Fig 8 SER spectrum of PNA adsorbed on a silver electrode, E,,= 350 mV, 0 1 M HCLO,, saturated with PNA, 488 nm, resolution 5 cn-’

tlons by an Ag-N vibration (mvolvmg the lone electron pair of the ammo-group) around 190 cm-‘[15] Burke et al have recently reported a SER spectrum of PNA adsorbed onto a sdver electrode from a solution of =0 1 M KC1 and 1 mM PNA[28] Three bands at 845, 1316 and 1610 cm-’ were found Although these bands correspond to the strongest bands m the spectrum of Fig 7 the absence of further bands m their spectra 1sdisturbing, It might be caused by the presence of coadsorbed chlonde ions (see also above m the experimental section) With an acidic electrolyte solution saturated with PNA SER-spectra of PNA adsorbed on a silver electrode recorded at electrode potentials ranging from E,, = - 100 to E,,, = 350 mV A spectrum obtained at E,,= 350 mV 1s shown m Fig 8 The low frequency mode attnbuted to the oxygen-silver stretching mode 1s observed at 420 cm-’ The remammg bands (see Table 1) correspond to modes of the two substltuents and to m-plane modes of the benzene ring, this agam implies a perpendicular orientation of the molecule In neutral and m acidic electrolyte solutions all experiments were conducted at electrode potentials positive to the Eprcr thus no addltlonal bands caused by electrostatics adsorption of amhmum catlons at potentials negative to the Epic have to be expected REFERENCES P H Schmidt and W J Pheth, J electroanal Chem 201, 163 (1986) W Shenglong, W Fosong and G Xlaohul, Synth Metals 16, 99 (1986) E T Seo, R F Nelson, J M Fntsch, L S Marcoux, D W Leedy and R N Adams, J Am them Sot g&3498 (1966),T J Stone and W A Waters, Proc them Sot ,253 (1962), W M Foxy and W A Waters, J them Sot, 6019 (1964) R C Weast (E&or), CRC Handbook of Chenustry and Phys~s, p ESI, CRC Press, Boca Raton (1985) B&tern Orgamsche Chenue, 12 711, 12 I 349, 12 II 383, 12 III 1580,12 IV 1613 M Flelschmann, Z Q Tian and L J Li, J electroanal

Chem 217, 397 (1987), L H Leung and M J Weaver, J Am them Sot 109, 5113 (1987), J electroanal Chem

217, 367 (1987) G C Schatz, m Surface Enhanced Raman Spectroscopy (Edited by R K Chang and T E Furtak), p 35, Plenum Press, New York (1982) 8 R Holze, J electroanal Chem 224, 253 (1987), Z phys Chem N F 160,45 (1988) R Holze, Sur/ SCI 202, L612 (1988) P Gao, M L Patterson, M A Tadayyom and M J Weaver, Langmuw 1(1985) 173 R Holze, Rev Se1 Instrum 60, 3348 (1989) P Gao, D Gosztola, L -W H Leung and M J Weaver, J electroanal Chem 233, 211 (1987) (a) R L Carrel], K D Beer and W Tanner, Pittsburgh Conference Ext Abstr # 102 (1987), (b) K D Beer, W Tanner and R L Garrell, J electroanal Chem 258, 313 (1989) 14 H Stundo and C Nlslnhara, J them Sot Faraday Trans I

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(1988)

15 R Holze, Electrochlm Acta 32, 1527 (1987) 16 M Heyrovsky and S Vavncka, J electroanal Chem u), 409 (1970), M Heyrovsky, S Vavncka and L Holleck, Co11 Czech them Commun 36,971 (1971), H Lund, In Organrc Electrochermstry (Edited by M M Balzer and H Lund), p 285, Marcel Dekker, New York (1983) 17 E M Genies and M Lapkowski, J electroanal Chem 236, 189 (1987) 18 J W Robinson (Editor), CRC Handbook ofSpectroscopy, Vol II, CRC Press, Cleveland (1974), Sadtler Standard Spectra, Ultra Violet Spectra, # 2243 Sadtler Research LaboratorIes Inc , Philadelphia 1975, U V-Atlas of Orgame Compounds, Vol II, Spectrum # D9-60 Butterworths 1968 19 E Lippert, Z Elektrochem, 61 (1957) 962 20 (a) N B Colthup, L H Daly and S E Wiberley, Introduction

to Infrared

and

Roman

Spectroscopy,

Academic Press, New York (1975), (b) F R Dolhsh, W G Fateley and F F Bentley, Characterrsttc Raman Frequencies of Orgamc Compounds, Wiley, New York (1974), (c) K W F Kohlrausch, Ramanspektren, Aka-

dermsche Verlagsgesellschaft, Leipzig 1943, (d) G Socrates, Infrared Characterrstw Group Frequenaes, Wiley, Chichester (1980), (e) J H S Green, SpectrochIm Acta 26A, 1503 (1970), (f) J H S Green and D J Harrison,, Spectrochtm Acta %A, 1925 (1970) 21 (a) M Moskovlts, J them Phys 77.4408 (1982) (b) R K Chang,

Ber

Bunsenges

phys

Chem

91, 296 (1987),

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(c) R P Cooney, M R Mahoney and A J McQudlan, Adv Injiared Raman Spec 9, 188 (1982), (d) R Holze, Habdltatlonsschnft, Oldenburg 1988 22 P Gao and M J Weaver, J phys Chem 89,504O (1985) 23 M R Phdpott, P S Bagus, C J Nehn, G Pacchlom, M Samant and H Sekl, 175th Electrochemical Society Meetmg, Los Angeles, USA, May 7-12,1989, Ext Abstr # 535, W Muller and P S Bagus, J Electron Spect Rel Phen 38, 103 (1986), K Hermann, W Muller and P S Bagus, J Electron Spect Rel Phen, 39, 107 (1986) 24 H Bebert, Anwendungen der Schwrngungsspektroskopte In der Anorgamschen Chemle, p 68, Sprmger-Verlag, Heidelberg (1966)

25 S L Dyatkma and B B Damaskm, Elektrokhlmaya 2, 1340 (1966)

26 R Holze, J electroanal Chem 250, 143 (1988) 27 M J Weaver, Ber Bunsenges phys Chem‘91,&0 (1987) 28 R L Burke. J R Lombardi. C Shl and W Zhana. 17Sh Electrochemical Society Meetmg, Los Angeles; Ext Abstr # 538 (1989) 29 H Metm, Prog Surf SCI 17, 153, 162 (1984) 30 D R Porterfield and A Campion, J Am them Sot 110, 408 (1988) 31 P Gao and M J Weaver, J phys Chem 89,504O (1985)

32 H Nichols and R M Hexter, J them Phys 74, 2059 (1981)