SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 897–903 (1999)

Combined Ultrahigh Vacuum–ElectrochemistryStudy of Aniline Oxidation at OxidizedPolycrystalline Nickel

G. Seshadri, R. M. Sarid and J. A. Kelber*Chemistry Department, PO Box 305070, University of North Texas, Denton, TX 76203, USA

The oxidation of aniline at a polycrystalline Ni electrode in a phosphate buffered solution (pH = 5.8) hasbeen studied by the use of combined ultrahigh vacuum–electrochemistry methodology. XPS analysis showsthe presence of a surface bound aniline derived species similar to polyaniline. The amine to imine contentof this species is dependent on potential, even though voltammetry shows no redox peaks characteristicof what is conventionally referred to as polyaniline. XPS spectra show the formation of a Ni3Y species onNi when it is oxidized in a phosphate buffer solution. This species is not formed when Ni is oxidized inphosphate buffer solutions containing aniline. The overall content of oxidized Ni in the surface layer formedat anodic potentials is lower when Ni is emersed from aniline containing solutions in comparison to emersionfrom aniline free solutions. Studies of the growth of the surface bound film by a time dependent pulsedpotential method show that the growth of surface bound aniline derived polymeric species occurs after abrief induction period during which the growth of nickel oxide occurs. This aniline derived polymer speciesis found to be approximately five monolayers thick. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: Ni; aniline; electrochemistry; XPS; oxidation

INTRODUCTION

We report the oxidation of aniline at polycrystallinenickel electrodes, resulting in a surface-bound aniline-derived polymeric species. Amines have been investi-gated for use as corrosion inhibitors in both high andambient temperature conditions.1 The stability of inhibitormolecules at the adsorbate surface in aqueous solution isa critical factor in inhibitor performance.2,3 X-ray photo-electron spectroscopy studies of amine interactions withFe oxy/hydroxide surfaces indicate that high concentra-tions of amines along with long immersion times arerequired in order to obtain chemisorbed species.4 Theability to induce covalent bonding between an inhibitormolecule and a substrate would result in significantlyenhanced effectiveness of inhibitor molecules. The bond-ing of amines.RNH2/ to glassy carbon electrodes (GCE)by electrochemical oxidation has been demonstrated, asshown below5

* Correspondence to: J. A. Kelber, Chemistry Department, POBox 305070, University of North Texas, Denton, TX 76203, USA.

Contract/grant sponsor: Electric Power Research Institute.Contract/grant sponsor: Robert A. Welch Foundation.

Electrochemical oxidation of an aromatic amine suchas aniline typically results in the formation of polyanilineon the electrode surface.6 Polyaniline is an organic poly-mer that has been actively researched for several decades.6

Polyaniline shows varied and complex behavior.6 A sig-nificant portion of the research on polyaniline has beenon its growth by chemical and electrochemical means onnoble metal substrates such as Pt and Au.6 In comparison,there has been far less work done on the electropoly-merization of polyaniline on non-noble metal and alloysurfaces such as Fe, Ta, Tl, Al, Pb and steels.7,8 Filmsgrown in this manner are generally electroactive, withboth oxidation and reduction peaks appearing in cyclicvoltammograms.7,8 Electrosynthesis on Fe and on mildsteel has been carried out, but anodic dissolution of the Feelectrode during the anodic oxidation process is a prob-lem because the synthesis is typically performed underextremely acidic conditions (e.g. in H2SO4, pH < 1).9

More recently, polyaniline films formed by chemicaloxidation have been applied to mild steel electrodes.10

X-ray photoelectron spectra suggest a redox interactionbetween the polymer applied in this fashion and the steelsubstrate.10

Growth of polyaniline on non-noble metal surfaces likeFe, Ni and steels is of interest due to its potential useas a corrosion inhibitor. Open-circuit measurements, cur-rent–voltage studies and SEM studies of iron, copper,cold-rolled steel and stainless steel that had been pre-viously coated with chemically grown polyaniline sug-gest that polyaniline imparts stability against corrosion inacidic and acidic chloride environments.11 – 13 Open-circuitmeasurements and SEM studies on stainless steel coatedwith electrochemically grown polyaniline show that suchsurfaces are stable in certain chloride-containing envi-ronments but not others.14 The above studies did not

CCC 0142–2421/99/100897–07 $17.50 Received 6 July 1998Copyright 1999 John Wiley & Sons, Ltd. Revised 5 April 1999; Accepted 6 April 1999

898 G. SESHADRIET AL.

probe metal/polyaniline or polyaniline/solution interac-tions at a fundamental level. Combined ultrahigh vac-uum–electrochemistry (UHV–EC) studies provide themeans to investigate the nature of such interactions.15 Bythis methodology, it is possible to study surfaces that arenot readily prepared by conventional bench-top methods.15

It is also possible to transfer a sample from the electro-chemistry cell to the analysis chamber, and vice versa,with no or minimal exposure to contaminants during thetransfer process.15 There is one report in the literatureinvolving the use of an anaerobic transfer electrochem-ical cell to investigate polyaniline growth on Pt.16 In thiswork, as will be shown below, the growth of the surface-bound aniline-derived species occurs not on Ni metal buton the metal oxide.

In this paper, the results of a UHV–EC study of theformation of an aniline-derived polymeric film on poly-crystalline Ni are presented. X-ray photoelectron spec-troscopy has been used in conjunction with voltammetricand pulsed potential methods. It is observed that eventhough no redox peaks characteristic of polyaniline areevident in the voltammogram in a phosphate buffer solu-tion (pH 5.8), the amine/imine ratio in the surface-boundspecies as verified by XPS analysis is very sensitive tothe potential applied to the electrode. Further, these studiesshow that when the electrode is subjected to anodic poten-tials in a phosphate buffer solution (pH 5.8) in the absenceof aniline in the solution, XPS analysis shows the pres-ence of on Ni3C-containing phase. If aniline is present inthe solution at the same pH and the electrode is subjectedto the same potential range, no Ni3C-containing phase isdetected by XPS. The fraction of the oxidized Ni compo-nent in the surface layer is found to be higher when Niis oxidized in aniline-free vs. aniline-containing solutions.Studies of the growth of the surface-bound aniline-derivedspecies on Ni by pulsed potential methods followed byXPS are also presented.

EXPERIMENTAL

Materials and reagents

The electrode material used in this study was a poly-crystalline nickel foil (Aesar, Puratronic, 99.4% pure).Electrical contact was established by spot-welding tan-talum wires on both edges of the foil. The sample tem-perature was monitored by means of a chromel–alumelthermocouple spot-welded to the back of the electrode. Allexperiments were carried out at room temperature. Sodiumphosphate monobasic (certified A.C.S. reagent, Fisher Sci-entific Corporation), sodium phosphate dibasic (>99%pure A.C.S. reagent, Aldrich Chemical Company) andaniline (>99.5% pure A.C.S. reagent, Aldrich ChemicalCompany) were used as received. All solutions were pre-pared with Millipore-grade water and purged with argonbefore use.

Sample preparation

Cleaning of the nickel foil in UHV was accomplished byargon ion bombardment (2.3 keV) at room temperature.The sample was then annealed to¾900 K. This procedure

was repeated until XPS showed the presence of metallicnickel devoid of impurities.

Ultrahigh vacuum–electrochemistry methodology

The system utilized in this study consisted of aUHV chamber with XPS capability and an antecham-ber equipped with an electrochemical cell. The twochambers were separated by a gate valve. Two dif-ferent turbomolecular pumps maintained the UHV andantechambers at pressures of<10�9 and <10�6 Torr,respectively. After sample preparation and characteriza-tion by XPS in the UHV chamber, the sample wastransported to the antechamber under vacuum for elec-trochemical experiments.

Prior to carrying out the electrochemical experiments,the antechamber was filled with ultrapure nitrogen gasand maintained at a pressure of 2.5 psi above atmosphericpressure by means of a pressure-relief valve. The elec-trochemical cell utilized in these experiments is similarto those reported previously in the literature.15 Only thefront face of the sample was exposed to the electrolytemeniscus, so as to avoid shorting out the thermocouple.All potentials reported in this study are with respect toan Ag/AgCl (0.1M NaCl) reference electrode, which wasmeasured to be 0.040 V vs. SCE (saturated calomel elec-trode). Electrochemical measurements were carried out bymeans of an EG&G Princeton Applied Research Model263A potentiostat/galvanostat.

Control of the potentiostat and data acquisitionwere carried out by a computer equipped withEG&G Princeton Applied Research Model 270/250Research Electrochemistry software. At the end ofeach electrochemical experiment, the sample was alwaysremoved from the electrolyte under potential control.This process is referred to as emersion of the electrode.After the electrochemical experiments were completed,the sample was rinsed with argon-purged Millipore-gradewater to remove excess physisorbed electrolytes. Theantechamber was then pumped down to a pressure of¾10�6 Torr. Following this, the sample was transferredback to the UHV chamber for XPS analysis.

In this laboratory, it has been observed that minimalcontamination of the sample with carbon occurs (C/Ni<0.01) if it is transported from the UHV chamber to theantechamber and back to the UHV chamber without expo-sure to any electrolyte. This demonstrates that the transferprocess itself does not lead to any significant contamina-tion of the sample. However, exposure of the sample tothe electrolyte can lead to the surface being contaminatedwith carbon based impurities from the electrolyte. Thiswill be discussed in more detail later.

X-ray photoelectron spectroscopy analysis

X-ray photoelectron spectra were acquired using unmono-chromatized Al K̨ radiation operated at 15 kV and 300 Win conjunction with a hemispherical electron energy ana-lyzer in constant pass energy mode at 100 eV.The x-raysource was at 54.7° relative to the analyzer axis. All spec-tra were acquired with the surface normal aligned alongthe analyzer axis. Peak fitting of XPS spectra was accom-plished by using commercially available software: ESCAtools in the MATLAB environment.17 Shirley background

Surf. Interface Anal. 27, 897–903 (1999) Copyright 1999 John Wiley & Sons, Ltd.

ANILINE OXIDATION AT POLYCRYSTALLINE Ni 899

subtraction was utilized in this study because it has beenshown to be the most effective method for fitting the shortenergy range found in typical core level XPS spectra.18

Spectra presented in this study were calibrated by refer-encing the observed Ni 2p3/2 binding energy for a clean Nisurface to 852.3 eV.19 The XPS peak-fitting routine hasbeen described in detail elsewhere.20 Atomic sensitivityfactor values published in Ref. 19 were used in this study.

RESULTS AND DISCUSSION

Figure 1(a) shows the cyclic voltammogram of a nickelelectrode previously cleaned in UHV in 0.1M phosphatebuffer solution. Peaks A and B are associated with theoxidation of nickel.21 The Ni electrode is observed to bepassive between potentials of 0.1 and 0.9 V vs. Ag/AgCl.At potential of>0.9 V vs. Ag/AgCl there is a markedincrease in current, associated with the oxidation of water.On the return scan, no reduction peaks are observed; infact, no current associated with reduction processes isobserved until about�0.8 V vs. Ag/AgCl, where hydro-gen evolution sets in. Figure 1(b) shows the cyclic voltam-mogram of a nickel electrode (previously cleaned in UHV)in 0.1 M phosphate buffer solution containing aniline. Inaddition to peaks A and B, another oxidation peak C isobserved. This peak must be due to aniline oxidation.By the XPS evidence shown below, it will be demon-strated that aniline oxidation leads to the formation of asurface-bound aniline species similar to polyaniline. Onceagain, no reduction current is observed on the reversesweep, except at very negative potentials, where hydrogenevolution is observed. Most previous studies report thatpolyaniline does not show conductivity or redox activity

Figure 1. Cyclic voltammogram of an Ni electrode, previouslycleaned in UHV, in: (a) 0.1 M phosphate buffer solution (pH 5.8);(b) 0.015 M aniline, 0.1 M phosphate buffer solution (pH 5.8) Scanrate 0.005 V s�1 .1 µA D 10�6 A/.

at pH > 3 or 4 andattributethe lossof activity to depro-tonationof the salt form of the polymer.22–27 As will beshownbelowby XPS,films grownin this studyin a phos-phatemediumdo containsmallamountsof theprotonatedspecies.

Figure 2 shows N 1s spectraof the surface-boundaniline-derivedspeciesthat form after emersinga UHV-cleanedNi electrodefrom a solution of 0.015 M anilinein a phosphatebuffer at various potentials.The widthof the N 1s spectrumindicatesthat nitrogen is presentin multiple chemicalenvironments.PreviousUHV stud-ies of aniline adsorptionof Ni(100) show the presenceof on N 1s peak at 399.3 eV at 220 K attributed tothe presenceof a monolayerof molecularaniline.28 At360 K two distinct peaks are reported at 400.0 and397.2eV, attributedto chemisorbedC6H5NH2 andC6H5N,respectively.28 According to anotherstudy in UHV byKishi etal. on Ni at 290K, two distinctpeaksoccurin theN 1sspectrumat 400.2and398.1eV dueto chemisorbedC6H5NH2 and C6H5NH, respectively.29 They report thatexposureof previously oxidized Ni to aniline at 290 Kin UHV results in a dominant featureat 400.1 eV dueto chemisorbedC6H5NH2, with a shoulderat 398.0 eVdueto chemisorbedC6H5NH.29 X-ray photoelectronspec-troscopystudiesof polyanilinereport that the N 1s spec-trum consistsof componentsat 399.4,398.3and400.9eVattributedto theamine,imine andprotonatedformsof thepolymer,respectively.10,30

Figure2(a)showstheN 1sspectrumof a UHV-cleanedNi electrodeafter a linear sweepof the potential from�1.0 to 1.2 V in a solution of 0.015 M aniline in a

Figure 2. Nitrogen 1s XPS spectra of a UHV clean Ni electrode:(a) scanned from�1.0 to 1.2 V in 0.015 M aniline, 0.1 M phosphatebuffer solution (pH 5.8), then emersed at 1.2 V; (b) scanned from�1.0 to 0.8 V in 0.015 M aniline, 0.1 M phosphate buffer solution(pH 5.8), then emersed at 0.8 V; (c) scanned from �1.0 to 0.8 V in0.015 M aniline, 0.1 M phosphate buffer solution (pH 5.8), pulsedto �1.0 V and held at �1.0 V for 10 min.

Copyright 1999JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 27, 897–903 (1999)

900 G. SESHADRIET AL.

phosphate buffer, followed by emersion at 1.2 V. As seenin the voltammogram in Fig. 1(b), aniline oxidation occurswell before 1.2 V. Also, the concentration of aniline inthe solution in this study is three times higher than thethreshold concentration of 5 mM that is required to observepolymerization of aniline.31 Therefore, the conditions usedin this study are conducive to the formation of polyaniline.Accordingly, fitting of the spectra shown in Fig. 2 hasbeen accomplished with components at binding energiesof 399.4š 0.1, 398.3š 0.1 and 400.9š 0.1 eV attributedto amine, imine and protonated nitrogen, respectively. Theamine/imine ratio for the spectrum in Fig. 2(a) is foundto be 1.99š 0.004.

Figure 2(b) shows the XPS spectrum of a UHV-cleanedNi electrode after a linear sweep of the potential from�1.0 to 0.8 V in the same solution as in Fig. 2(a), followedby emersion at 0.8 V. The XPS spectrum shows a markedincrease in the amine/imine ratio compared to that shownin Fig. 2(a). The amine/imine ratio is observed to be3.25š0.49. The XPS spectrum in Fig. 2(c) is that obtainedafter the UHV-cleaned Ni electrode was subjected to alinear sweep of the potential from�1.0 to 0.8 V, followedby pulsing the potential back to�1.0 V and holding thepotential at�1.0 V for 10 min. The amine/imine ratioobtained in this case is 5.25š 0.79. The dashed line inFig. 2 has been used to mark a binding energy value of399.4 eV. If one compares Fig. 2(c) and Fig. 2(a), it isevident that the center of the broad N 1s peak shifts from399.4 eV in Fig. 2(c) to a slightly lower binding energyin Fig. 2(a). This is in accordance with the decrease inthe amine/imine ratio on emersion at 1.2 V [Fig. 2(a)]when compared to emersion at�1.0 V [Fig. 2(c)]. Nodefinitive correlation between emersion potential and therelative amount of protonated nitrogen was observed inthese studies.

X-ray photoelectron spectroscopy measurements showno nitrogen-containing species after immersion of a nickelelectrode into the electrolyte solution containing ani-line under open-circuit conditions, followed by rinsing toremove physisorbed species. It is clear,vide supra, thatthe observed nitrogen species are due to electrochemi-cally induced oxidation and adsorption at the electrodesurface. A concern in combined UHV–EC studies is thatthe surface could undergo changes in the time that elapsesbetween completion of the electrochemical experimentand transfer of the sample back to the UHV chamber foranalysis. Such a concern is all the more important whenspecies such as amines, which are easily oxidized, areinvolved. The data presented in Fig. 2 serve to demon-strate that no significant obscuring of results occurs andthat the determination of changes in surface compositionas a function of emersion potential by UHV–EC method-ology is possible even when highly reactive species areinvolved.

The C 1s peak obtained after emersing the Ni electrodeat 0.8 V from phosphate buffer solution that is devoid ofaniline is displayed in Fig. 3(a). The C/Ni XPS atomicratio in this case is found to be 2.00š 0.54. As has beenpointed out in the experimental section, transfer betweenthe UHV chamber and antechamber does not lead tocontamination of the sample with carbon. This means thatthe carbon signal seen in Fig. 3(a) is due to the samplebeing immersed in the electrolyte and adsorbing carbon-based impurities from the electrolyte. Figure 3(b) showsthe C 1s peak after emersion at 0.8 V of the Ni electrode

Figure 3. Carbon 1s spectra of an electrode scanned from �1.0to 0.8 V and emersed at 0.8 V from: (a) 0.1 M phosphate buffer(pH 5.8); (b) 0.015 M aniline, 0.1 M phosphate (pH 5.8).

from phosphatebuffer solution containing aniline. TheC/Ni XPSatomicratio in this caseis foundto be8.2š2.2.This is a 400%increasewhencomparedto Fig. 3(b). TheC 1s peakis centeredat 284.6š 0.1 eV in both cases.

It is impossible to determinefrom the existing datawhether‘adventitious’ carbonadsorbedfrom aniline-freesolutions is also presentat the interface in the spectraof the aniline derivedfilms, or displacedduring the pro-cessof aniline oxidation. It will be shown below thatthe aniline-derivedfilm on the surface is of a multi-layerednature.Hence,even assumingthat adventitiouscarbonimpuritiesarepresent,themultilayer natureof theaniline-derivedsurfacefilm would greatly attenuatethecontributionto the XPS intensity due to theseimpuritiesin the interfaciallayer.In otherwords,thecontributionoftheseadventitiouscarbonimpuritiesto thespectrumof theaniline-derivedfilm is expectedto benegligible.For elec-trodesemersedfrom phosphatebuffer solutionscontaininganiline, the N/C atomic intensity ratio obtainedfrom thetotal areasof the N 1s andC 1s spectrais found to varybetween0.15š 0.04, with no observabledependenceonthe emersionpotential.The theoreticallycalculatedvaluefor any aniline-derivedspeciesis 0.166.

Figure4(a)showstheO 1sregionof a nickel electrodeafter a linear sweepof the potential in phosphatebufferfrom �1.0 to 0.8 V, followed by emersionat 0.8 V.The featuresat 529.2š 0.1, 530.8š 0.1, 532š 0.1 and533.5š 0.1 eV areattributedto oxide, hydroxide,latticewaterandmonobasicphosphateanion,respectively.19 Thepresenceof monobasicphosphatehasalso beenverifiedby thepresenceof a peakat 134.2eV (not shown),whichcorrespondsto the P 2p signal of monobasicphosphateanion.19 The oxide/hydroxideratio is found to be 0.58š0.05.Similarly, Fig. 4(b)showstheO 1sregionof anickelelectrodeaftera linearsweepof thepotentialin phosphatebuffer solution containing aniline from �1.0 to 0.8 V,followed by emersionat 0.8 V. Theoxide/hydroxideratiois found to be marginally higher at 0.70š 0.05. In both

Surf. InterfaceAnal. 27, 897–903 (1999) Copyright 1999JohnWiley & Sons,Ltd.

ANILINE OXIDATION AT POLYCRYSTALLINE Ni 901

Figure 4. Oxygen 1s spectra of a UHV clean Ni electrode:(a) scanned from�1.0 to 0.8 V in 0.1 M phosphate buffer solution(pH 5.8), then emersed at 0.8 V; (b) scanned from �1.0 to 0.8 Vin 0.015 M aniline, 0.1 M phosphate buffer solution (pH 5.8), thenemersed at 0.8 V.

cases,the phosphatecomponentcontributes�10% of thetotal oxygen signal intensity. In the caseof electrodesemersedat 1.2 V, the oxide/hydroxideratios for surfacesemersedfrom aniline-freeandaniline-containingsolutionsare found to be 0.62š 0.05 and 0.61š 0.03 (which aresimilar within experimentalerror).

Figure 5 shows the normalizedspectrafor Ni 2p3/2for the Ni electrodeafter linearly scanningthe potentialin phosphatebuffer from �1.0 to 0.8 V, followed byemersionat 0.8V. Figure5(a)is obtainedwhenno anilineis presentin solution, and Fig. 5(b) is obtainedwhenaniline is presentin solution. It is clear that there is adifferencein the two spectrain the region between854and 860 eV that must originate from the presenceofdifferentchemicalenvironmentsfor Ni. Discussionof thefitting of thesespectrafollows.

Figure 6(a) shows the Ni region of an Ni electrodeafter a linear sweepof the potential in phosphatebufferfrom �1.0 to 0.8 V, followed by emersionat 0.8 V,Ni0 representsmetallic Ni. In addition to the main corelevel, metallic Ni hasa shake-upsatelliteat higherbind-ing energy. Fitting of the metallic Ni componentin theoxidized Ni spectrahas been accomplishedby import-ing a Shirley background-correctedspectrumof the Ni2p3/2 region of metallic Ni (cleanedin UHV) into theoxidized Ni spectraand allowing the peak-fitting rou-tine to size the feature.The rationalefor this approachis that the ratio of the heightof the core-levelpeakto thesatellitepeakshouldremainlargelyunaffectedfor thesub-strateNi metal. We havesuccessfullydemonstratedthatthis approachleads to consistentfits for oxidized Fe.20

Spectralassignmentsfor Ni 2p3/2 in NiO in the litera-ture rangefrom 853.8to 854.3eV.19,32–34 In most cases,a satellitepeak¾2 eV higher than the main peakis alsoreported32–34 andNi 2p3/2 in Ni(OH)2 is reportedto be at

Figure 5. Normalized Ni 2p spectra of a UHV clean Ni electrode:(a) scanned from�1.0 to 0.8 V in 0.1 M phosphate buffer solution(pH 5.8), then emersed at 0.8 V; (b) scanned from �1.0 to 0.8 Vin 0.015 M aniline, 0.1 M phosphate buffer solution (pH 5.8), thenemersed at 0.8 V.

Figure 6. Nickel 2p spectra of a UHV clean Ni electrode:(a) scanned from�1.0 to 0.8 V in 0.1 M phosphate buffer solution(pH 5.8), then emersed at 0.8 V; (b) scanned from �1.0 to 0.8 Vin 0.015 M aniline, 0.1 M phosphate buffer solution (pH 5.8), thenemersed at 0.8 V.

855.4eV.19,35 In this work, themainNi 2p3/2 featureasso-ciatedwith NiO is assignedat 853.8š 0.1 eV and withNi(OH)2 is assignedat 855.4š 0.1 eV. The satellitefea-turefor theoxideis not shown.As shownin Fig. 6(a),it is

Copyright 1999JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 27, 897–903 (1999)

902 G. SESHADRIET AL.

found necessary to include a component at 856.9š0.1 eV.These high-binding-energy peaks are frequently attributedto Ni3C species.36,37 The assignments for the Ni 2p3/2 fea-ture in bulk Ni2O3 in the literature vary and are reportedat 855.1, 855.8 eV, 856.5 and 857.3 eV.38 – 41 The assign-ment for the bulk Ni 2p3/2 feature in -NiOOH in theliterature is at 855.3 eV.42 Therefore, it is clear that thefeature at 856.9š0.1 eV does not correspond to any bulk-phase Ni3C compound and it could be a non-stoichiometricNi3C-containing oxide or mixed oxide–hydroxide phase.In addition, as shown in Fig. 6, a combined satellite for theNi 2p3/2 feature is assigned between 861.1 and 861.3 eV,depending on the composition of the oxide–hydroxidefilm.34,35

Figure 6(b) shows the Ni 2p3/2 spectrum for an Ni elec-trode after a linear sweep of the potential in phosphatebuffer solution containing aniline from�1.0 to 0.8 V, fol-lowed by emersion at 0.8 V. A noteworthy feature of thisspectrum is the absence of the feature at 856.9 eV. Also,although the fraction of the total Ni intensity contributedby oxidized Ni is found to be 0.34š 0.02 for the elec-trode emersed from aniline-free phosphate solutions, thevalue decreases to 0.22š0.05 for emersion from solutionscontaining aniline. Similarly, when the Ni electrode isemersed at 1.2 V, the Ni 2p3/2 spectrum (not shown) of theelectrode emersed from aniline-containing solution doesnot have a contribution from the feature at 856.9 eV. Thisfeature is found to be present in the spectrum obtainedwhen the electrode is emersed from aniline-free solutions.The fraction of the total Ni XPS signal intensity con-tributed by oxidized Ni is found to be 0.32š 0.02 whenthe electrode is emersed from aniline-free solutions. If theNi electrode is emersed from solutions containing aniline,the value is found to be 0.18š0.04. Hence it is clear,videsupra, that in the presence of the surface-bound anilinespecies the formation of the non-stoichiometric Ni3C oxideor mixed oxide–hydroxide is inhibited and that the overallcontent of oxidized Ni in the surface layer is decreased.

Determination of the time scale of the growth of theaniline-derived species is accomplished by pulsed poten-tial experiments. The Ni electrode is pulsed from an initialpotential of�1.0 to 0.8 V in a phosphate solution contain-ing aniline, for various pulse times, followed by emersionat 0.8 V. A potential of 0.8 V is chosen because this isthe lowest potential at which the N 1s XPS peak is clearlydistinguishable from the noise. Figure 7 shows the XPStotal N/total Ni atomic ratio as a function of pulse dura-tion. At pulse times of<10 s, no evidence of nitrogen isseen in the XPS spectra. Only oxidation of Ni is observedto occur. This indicates that oxidation of Ni occurs first,followed by the growth of an aniline-derived species atpulse times of>10 s. Owing to the fact that there is con-siderable attenuation of Ni XPS intensity when growth ofthe surface-bound aniline species occurs, there is a largeerror involved in the determination of the N/Ni intensityratio. Nevertheless, it is clear from Fig. 7 that at timesapproaching 100 s the N/Ni intensity ratio does not changewithin experimental error. These data indicate that, underthe conditions used above, film growth occurs on a timescale of tens of seconds and that a saturation thicknessis reached beyond which no further film growth is appar-ent. To estimate the thickness of the aniline-derived filmon the Ni surface, it is essential to calculate the fractionof the XPS intensity contributed by the first layer of Niatoms, because the nitrogen atoms will only interact with

Figure 7. Total N/total Ni XPS atomic intensity ratio as a functionof pulse duration for a UHV clean Ni electrode pulsed from �1.0to 0.8 V in 0.015 M aniline, 0.1 M phosphate buffer solution(pH 5.8).

the first atomic layer of Ni atoms.This is doneby usingthe relationship

∫ 1

0

e�x/�dx∫ 10

e�x/�dxD I1/Itotal .1/

Here, x is the distance below the surface in atomiclayers, � is the attenuationlength of an electron of agiven kinetic energy, I1 is the XPS intensity contributedby the Ni atoms in the first atomic layer and Itotal isthe total XPS intensity of Ni atoms. For the kineticenergy correspondingto the Ni 2p3/2 photoelectron,�is found to be ¾6 atomic layers.43 Evaluation of theabove integral results in a value of ¾15%. Therefore,¾15%of thetotal Ni 2p3/2 XPSsignalintensityoriginatesin the first atomic layer. Factoring this into the N/Niatomic concentrationratio at saturationcoverageseeninFig. 7 leadsto an N/Ni ratio of ¾5. This indicatesthatthe film formed under theseconditionsis approximatelyfive monolayersthick. A studyby Glandandco-workerson the adsorptionof aniline on Ni(100) in UHV hasdemonstratedthat only a monolayerof aniline persistson the Ni(100) surfaceat 290 K.28 Aniline multilayersare thus unstableat 300 K underUHV conditions.Thisdemonstratesthat the approximatelyfive monolayersofthe aniline-derivedspeciesseenin this studyarenot dueto adsorbedmolecularaniline but insteadare due to ananiline-derivedspeciesthat is polymeric in nature.Thispolymeric speciesmay not be what is conventionallyreferredto as polyaniline, but it is similar to it becauseboth amineandimine environmentsareobserved.

Surf. InterfaceAnal. 27, 897–903 (1999) Copyright 1999JohnWiley & Sons,Ltd.

ANILINE OXIDATION AT POLYCRYSTALLINE Ni 903

CONCLUSIONS

An aniline-derived polymer film results when oxida-tion of aniline is carried out at an Ni electrode in aphosphate-buffered solution. By the use of combinedUHV–EC methodology, it has been demonstrated thatalthough the voltammogram shows no redox peaks char-acteristic of polyaniline, the amine imine ratio in thesurface-bound aniline-derived species as determined byXPS is dependent on the potential at which the elec-trode is emersed from solution. X-ray photoelectron spec-tra show that a non-stoichiometric Ni3C-containing oxideor mixed oxide–hydroxide phase forms on Ni when itis oxidized in a phosphate buffer solution. This Ni3C-containing phase is not formed when Ni is subjected tothe same range of potentials in phosphate buffer solu-tions containing aniline. The content of oxidized Ni inthe surface layer is decreased when the Ni electrode

is emersed from aniline-containing solutions at anodicpotentials when compared to aniline-free solutions. Stud-ies of the growth of the aniline-derived polymeric film bya time-dependent pulsed potential method show that, fol-lowing an induction period during which oxidation of Nioccurs, a film of surface-bound aniline-derived polymericspecies approximately five monolayers thick grows on atime scale of tens of seconds. Future studies will focus onfactors such as the effect of the presence of other adsorbedspecies (e.g. sulfur), effects of variation of solution com-position (pH, concentration) on the growth process andfinal surface composition. The corrosion inhibition prop-erties of the film will be explored at a fundamental level.

Acknowledgements

Funding for this study provided by grants from the Electric PowerResearch Institute and the Robert A. Welch Foundation is gratefullyacknowledged. Mr Huanchi Xu is thanked for his technical assistance.

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