Langmuir 1993,9,1862-1867
1862
Surface Stability of Bismuth-Ruthenium Oxide Electrodes GiilsCln Giikagaq and Brendan J. Kennedy' Department of Inorganic Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Received December 15,1992. I n Final Form: March 23,1993 The effect of anodic polarization on the surface composition of Bi-Ru oxide, BizRu207+,, electrodes in acid solutionand acid-alcohol solutionshaa been studied by scanning electronmicroscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. In acidic solution the surface is unstable under either oxidation or reductive conditions, with Bi being lost from the structure. During electrolysis the surface becomes covered with particles of a Bi oxide species.
Introduction
A number of bimetallic transition-metal oxides, which exhibit metallic conductivity at room temperature, have been studied for possible use as electrocatalysts.1y2 In particular it has been demonstrated that the leadruthenium and bismuth-ruthenium pyrochloresshowhigh initial electrocatalytic activity for both oxygen reduction and evolution.36 These materials are also reported to catalyze the oxidation or isomerization of a number of simple organic compounds.6 A major problem with metal oxide electrocatalysta is that the catalytic response slowly deteriorates with time. This deterioration can be related to the instability of the surface such that corrosion and/or erosionof the electrode occurs. During potential cycling successive dissolution/ deposition cycles can occur resulting in recrystallization of the metal oxide surface. Studies by Arvia71a and others"' have shown that for precious metals such as Pt or Au, surface recrystallization can alter the electrochemical response. In general terms the effect of electrolysis on the surface rearrangement of metal oxide electrodes is considerably less well characterized than on metal electrodes, one notable exception being the PbO2 electrode.12 We have recently demonstrated that for the Pb-Ru pyrochlorethe surfacerearrangement involvespreferential leaching of Pb from the bulk and deposition onto the surface of the e1e~trode.l~ As part of a study of electroactivemetal oxide electrodes, we have examinedthe stability and activity of mixed metal Bi-Ru electrodes for oxidation of methanol. T w o polymorphs of Bi-Ru oxides are known to have metallic (1) Electrodes of Conductive Metallic Oxides; Traeatti, S., Ed.; Elsevier: Amsterdam, 1980. (2) Manoharan, R.; Goodenough,J. B. J.Electrochem. SOC.1990,137, 910.
(3)Edgell, R. G.; Goodenough, J. B.; Hamnett, A.; Naish, C. C. J. Chem. SOC.,Faraday Trans. 1 , 1983,26, 155. (4) Manoharn, R.; Paranthaman, M.; Goodenough,J. B. Eur. J. Solid State Znorg. Chem. 1989,26, 155. (5) Kannan, A. M.; Shukla, A. K.; Sathyanarayana, S. J.Electroanal. Chem.. 1990.281.339. (6) Horoktz, H. S.; Longo, J. M.; Horowitz, H.H.; Lewandowski, J. T. ACS Symp. Ser. 1985, No. 127, 143. (7) Triaca, W. E.; Kessler, T.; Canullo,J. C.; Arvia, A. J. J.Electrochem. SOC.1987,134, 1165. (8)Perdrial, C. L.; Custidiano, E.; Arvia, A. J. J. Electroanal. Chem. 1988,246,165. (9) Dillon, C. T.; Kennedy, B. J. A u t . J. Chem., in press. (10) Sumino, M.; Shibata, S. J. Electroanal. Chem. 1992, 336, 329. (11) Holland-Moritz, E.; Gordon, J.; Kanazawa, K.; Sonnerfield, R. Longmuir 1991, 7, 1981. (12) Corino, G. L.; Hill, R. J.;Jewel, A. M.; Rand, D. A. J.; Wunderlich, J. A. J. Power Sources 1985,16, 141. (13) Gokagac, G.; Kennedy, B. J. J.Electroanal. Chem., in press.
~onductivity.~~ T h e first adopta a pyrochlore type, A2B207_,,structure while the second adopta a cubicKSbOs structure.lS It has recently been suggested that the instability of lead-ruthenium pyrochlores is facilitated by the presence of Pb-rich channels within the pyrochlore s t r u ~ t u r e .Such ~ ~ channels are absent in the KSbOs structure which coaeista of three interpenetrating networks. It therefore seemed possible that the stability of BizRuz07+, with the KSbOs structure may be higher than that of the pyrochlore analogue. Studies of the surface stability of this material are now presented.
Experimental Section Bi&uzO,+, (y = 0.3) was prepared in the polycrystalline form by solid-statereaction of stoichiometric quantitiesof an intimate mixture of Biz03and RuOz, p r d into 13 mm diameterpelleta, as follows: 580 O C for 15 h, 650 "C for 15 h, and 750 O C for 50 h with regrinding after each heating step. It is essentialthat the sample is not heated above 750 O C to avoid formation of the pyrochlore phase. The fiial product was characterized by X-ray powder diffraction at room temperature on a Siemana DMXX) diffractometer. No lines indicative of the presence of any phaaea, other than the desired BizRuz07+,material,were observed in the diffraction pattem. The lattice parameter was a = 9.292A. Electrodes were prepared by attaching a Cu rod to the back of the pressed pellet with silver epoxy. The sample was then mounted in a Teflon holder and a thermosettingepoxy was uaed to isolate the silver epoxy and Cu rod from the solution. The mounted electrode was polished on microcloth with AlzOs polish. Electrochemicalmeasurements were obtained with an 0.Sye Micros potentioatatusing a conventionalthreeelectrodecellwith Pt gauze counter and Hg/HgzSOd (0.5 M HBOI) reference electrodes. Measurementswere preformed in deoxygenated 2.6 M HfiOdsolutions. Scanningelectron micrograph (SEM)images were collected with a Philips SEM 505 equipped with an EDAX PV9900 energy dispersion analyzer. The accelerating voltage was 20 keV. X-ray photoelectron spectia were recorded on a Kratos XSAM 800 spectrometerusing Mg Ka radiation (1253.6 eV) at 15 kV, 10 mA with a constant pass energy of 20 eV. All binding energies are referenced to the C 1s signal at 284.6 eV. The operatingpreseure was below 108Torr. Except where noted, samples were transferred through air and were not pretreated (ion etched) before measurements. Results and Discussion The cyclic voltammogram of a BizRuzO,+, electrode recorded with a scan rate of 25 mV/s between -0.6 and +0.875 V vs MMSE in 2.5 M HzS04 at 25 OC is shown in Figure la The voltammogramshowsthree anodicfeatures at -0.39, -0.28,and +0.1 V as well as a rapid increase in ~~~
(14) Abraham, F.; Nowogrocki, G.; Thomas,D. C. R. Acad. Sci. 1974, 421, 278. (15) Sleight, A. W.; Bouchard, R. J. Znorg. Chem. 1973, 12, 2314.
Q 1993 American
Chemical Society
Bi-Ru Oxide Electrodes
0.6
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Langmuir, Vol. 9, No. 7, 1993 1863
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Sweep rate 25 mV/s.
current as the potential is raised above 0.8 V, which is presumably a consequence of 0 2 evolution. While the anodic peaks at -0.39 and -0.28 V appear to be associated with the reduction process, which occurs at -0.56 V, the feature a t 0.1 V cannot be readily ascribed to a single reduction wave. Further the broadness of the oxidation feature at +0.1 V suggests this is not a simple surface redox reaction but rather correspondsto a more concerted oxidation procegs. By varying the negative potential limit, it was demonstrated that the anodic feature at +0.1 V corresponds to a reduction reaction occurring below ca. -0.45 V. The deeper the reduction, the greater the corresponding return oxidation wave. This suggests that below -0.45 V, either insbrtion of hydrogen into the lattice occura, or more likely, as is reported to occur for the related pyrochlore phase in basic solutions, bulk reduction of Bi(III)to Bi(0) omm.8 In the related compound,PbzRuz07-y theae voltammetricfeaturea are absent, further supporting the idea that they are due to the reduction of Bi(1II). The broad reduction wave tlear-0.5 V and the oxidation wave near 0.1 V, correspondingto the bulk reduction and reoxidation of Bi(III),16 dominate the cyclic voltammograms; however there are also a number of weaker features, the identity of which is somewhat more problematic. On the basis of thermodynamic data the Ru(2+/3+) couple is expected14 to occur near -0.3 V and it is possible that the features at -0.56 and -0.28 V correspond to this couple. The anodic feature at -0.39 V is then ascribed to either the reoxidation of Bi(O), which occupies a different site to the bulk of the material, or the expulsion of hydrogen from the lattice. Since the response of the oxidation feature at -0.39 V is sensitive to the lower potential limit, the latter explanation appears more likely. Limiting the lower potential limit to -0.2 V initially produces an essentially featureless cyclic voltammogram which is not reproduced here. Continued cycling resulta (16)Espinoan, A. M.; San Jose, M.T.;Tascon, M.L.;Vazquez, M. D.; Batanem, P. 5.Electrochim. Acta 1991,36, 1661. (17) Bard,A. J.;Pmns,R.; Jorden, J.Sta~dPotentMLPinAquews Solutione; Marcel Defier: New York, 1986.
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in the voltammogram shown in Figure 2 in which the two weak redox features at +0.43 and +0.70 V, are observed. These potentials are, essentially, identical to that foundla for the pyrochlore PbzRu207, and parent RuO2 materials and can be ascribed to the same processes, namely Ru(3+/4+) and Ru(4+/5+) couples, respectively. The fact that these couples are only apparent after the electrode has been cycled for a period of time suggests that these features are not characteristic of the material but rather are the result of a surface species that has formed during potential cycling. Cycling of the BizRu207+, electrode between -0.6 and +0.875 V at 25 "C in 2.5 M HzS04 for ca. 1.5 h resulted in a significant enhancement of the anodic feature at 0.1 V and the cathodic features at -0.43 and -0.56 V, Figure 3. In addition a small enhancementof the anodic features at -0.39 and -0.28 V was also observed. The currents observed during cycling continuously increased over this period of time indicating that the effective surface area of the electrodewas increasing as a consequence of surface rearrangement and/or corrosion. The time dependence of the voltammograms suggests that the two reduction features at -0.56 and -0.43 V are related to the anodic feature at 0.1 V, which is a consequence of the reoxidation of Bi(0). AB was previously mentioned the two sharper anodic features at -0.39 and -0.28 V are also related to the cathodic feature at -0.56 V and are presumablydue to the desorption of hydrogen and/or reoxidation of Ru(0) or Ru(I1). Altering the temperature to 60 "Cresulted in only minor changes in the appearance of the voltammogram. The broad anodic feature near 0.1 V remained the most noticeable feature. Cycling of the electrode at 60 "C between-0.60and+0.875Vat 25mV/sforca. 1.5hresulted in a number of changes in the appearance of the voltammogram,in particular the previously well-resolvedanodic
1864 Langmuir, Vol. 9, No. 7,1993
Giikagaq and Kennedy
features near 4 . 3 V are absent as is the relatively strong cathodic peak at 4.58 V. The large increase in the broad oxidation wave near 0.1 V suggeststhat at 60 "C the surface electrochemistry becomes dominated by the reduction and reoxidation of Bi. The growth of Bi-rich crystallites on the surface is illustrated in electron microscopy. An unfortunate consequence of the facile Bi oxidation is that it is not possible to observe the surface ruthenium redox features. Even if the lower potential limit is reduced to avoid bulk reduction of Bi(III), no well-resolved features due to Ru oxidation are observed. The addition of methanol to the electrolyte results in a rapid increase in the currents observed above 0.6 V as aresult of the catalytic oxidation of the alcohol, Figure lb. Other than this greater than 10-fold increase in current, the addition of methanol to the electrolyte did not appear to significantly alter the electrochemical response of the material. In the absence of methanol the dominant reaction is oxygen evolution, the formal potential of which is 1.23 V:
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H20 1/202 + 2H+ + 2e- Eo = 1.23 V vs SHE On ruthenium and ruthenium oxide electrodes the formation of Ru(V1) and dissolution of h o d 2 - occurs at potentials close to that of 0 2 evolution. The formation of RuV1is expected to occur near 0.5 V, as found in RuO2 electrodes,18 and while this will be shifted somewhat in the present mixed metal oxide, it is probable that this is the most likely oxidation level to be involved in the enhanced currents above 0.6 V. Analysis of the solution after oxidation in H2S04/CH30H solutions demonstrated both that dissolution of Ru occurs and that formaldehyde is produced. The catalytic oxidation of methanol to formaldehyde, can be written asI9 RuWO2(OH),+ CH30H
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where the "RuO2" moiety is an appropriate surface group. A t potentials above 0.8 V, oxygen evolution will compete with methanol oxidation and the faradaic efficiency for methanol oxidation was observed to be low, typically 3-5 ?6. Increasing the temperature results in a large increase in the magnitude of the methanol oxidation currents. The surface morphologyof Bi2R~20,+~ was also studied by electron microscopy. The as-prepared material had a relatively featureless surface with no observable inhomogeneity, Figure 4. EDAX analysis demonstrated the absence of any noticeable bulk segregation, although the average Bi:Ru ratio was low, 56:44 (67:33 expected since EDAX does not detect 0),suggesting the near surface area of the material to be Bi poor. Polarization of the samples a t +0.75 V for 5 h, both with and without a 20-min precycling treatment, which involved cycling between -0.7 and +0.8 V a t 25 mV/min did not significantly alter the average surface composition, Bi 5056 ?6. Polarization did, however, result in the growth of clusters of needlelike crystallites on the surface of the material, so that the surface now appears highly inhomogenous, Figure 5a. The formation of these surface crystallites was also observed, albeit to a much lesser extent, if the sample was simply cycled. EDAX analysis showed these to be free of Ru with an approximate Bi:S ratio of 82:18, Figure 5b, indicative of a YBi2Si3nstoichiometry. The most likely species formed is a Bi(II1)sulfate such as Bi2(S04)3since it is unlikely that any simple Bi
Figure 5. Scanning electron micrograph (a, top) and EDAX spectrum (b, bottom)of a BizRu207+, electrodeafter polarization at +0.75 V in 2.5 M H2SO4 for 5 h.
(18) K6tz, R.; Lewerenz, H. J.; Stucki, S. J. Electrochem. SOC.1983, 130, 825. (19) Kennedy, B. J.; Smith, A. W.; Wagner, F. E. Aust. J. Chem. 1990, 43,913.
sulfide would be formed under the above conditions. Similar needle type crystals were formed on the surface of the electrode when it was polarized at -0.60 V,
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Langmuir, Vol. 9, No. 7, 1993 1865
Bi-Ru Oxide Electrodes
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corresponding to the bulk reduction of Bi(II1). Under these conditions the surface was covered with a large number of Bi-rich crystallites Figure 6a. Regionsadjacent to the Bi crystallites showed high Ru contents (75-80%), Figure 6b, confirmingdepletion of Bi from the pyrochlore structure. Even limiting the negative limit to -0.2 V, so to avoid reduction of Bi(III), cycling to +0.875 V resulted in the growth of Bi crystallites on the surface. Despite dissolution of Ru the surface remained Bi poor, analysis of featureless areas demonstrating that the material contained 4550% Bi, compared to an expected value of 66%. Thus it is apparent that the material is unstable under both oxidative and reductive conditions. Polarization in an acid/methanol electrolyte resulted in a similar surface morphology, with a number of large Birich crystallites being obvious. This indicates that it is the reactivity of the material in acid solution which is responsible for the surface rearrangement, rather than the involvement of the surface oxides in methanol oxidation. The Bi 4f XP spectra of the as-prepared Bi2R~20,+~ are best described by two Gaussian doublets with Bi 4f7p binding energies (BE) of 158.2 and 159.3 eV, Figure 7. Attempts to reproduce the observed spectra by a single Gaussian-shaped doublet were unsuccessful. The use of
mixed Gaussian-Lorentzian shaped lines and/or alternate background all indicated two overlapping doublets were present. The presence of two doublets can be used as evidence for the presence of two valence levels, in this case the most likely levels are Bi(II1) and Bi(V). In the oxide, Bi is expected to be present as Bi(II1)and it appears that the major signal at 158.2 eV corresponds to such a species. The higher BE species can then be ascribed to Bi(V) and indeed Felthouse et al. have postulated the presence of Bi(V)on the surface of the second polymorph, pyrochlore type Bi2R~207,.*~The presence of Bi(V) can be accounted for in two manners, either it results from substitution of Bi into the Ru sites of the KSb03 structure or, less probably, it is a surface impurity. Initially it is expected that Bi(II1) will be the stable oxidation state;2l however comparison of the ionic radii suggests that substitution of Bi(V) will be favored since the size of Bi(V), 0.74 A, is closer to that of Ru(IV), 0.67 %r, than is Bi(III), 0.96 A. Horowitz et al. have found that despite the unfavorable ionic radii in the pyrochlore, Bi(II1) can in fact substitute (20) Felthouse,T. R.; Fraundorf, P. B.; Friedman, R. M.; Schoseer, C.
L.J . Catal. 1991,157,421.
(21) Cotton,F. A.; Wilkinson,G. Adoanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988.
CbkaJaq and Kennedy
1866 Langmuir, Vol. 9,No.7, 1993
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Figun, 7. X-ray photoelectron spectra of the Bi 4f electrone in BiaRu&+,. (A).- prepared, (B) after Ar ion etching, and (C) after electrolysis. The peak near 169 eV is a S 2p line. The diamonde are the observed points and the solid linee repreeent the fit to one or two Bi 4f doublets. See text for discuesion.
for R U . In ~ ~the present case the substitution of Ru(IV) by Bi(II1) can be accomplished in one of three ways: (i) substitution of Ru(IV) by Bi(II1) with anion vacancy formation; (ii) substitution of two Ru(IV) sites by a pair (22) Horowitz, H. 9.; Longo, J. M.; Lewandomki, J. T. Mater. Res. Bull 1981, 16, 489.
of Bi(III) and Bi(V) cations with no anion vacancy; (iii) substitution of RNIV) by Bi(III) with subsequent oxidation of a Ru(IV) to RUN) and no anion vacancy. Of these three only (ii)provides for two discrete Bi oxidation states, and since substitution of the Ru sites by Bi is expectsd to be limited, then the amount of Bi(V) present is predicted to be small, as is in fact obaerved in Figure 7. T h e second possibilityis that the higher BE component is due to a surface defect. It is unlikely that the surface Bi crystalliteswould be Bi(V),since bulk Bi(W oxides are relatively unstable. Indeed Hegde and Ganguly= have found that in the simple oxide, BizO8, the Bi(6p) band lies below the 02(2p) band making it difficult to stabilize Bi(V) in such an oxide. It appears likely that the two signals are due to two different Bi(III) sites on the surface of the electrode which have different binding energies. Two observations suggest the presence of two Bi(II1) sites with di€ferent BE are preeent on the surface of BizRuz07+,. Firstly argon ion etching sufficient to remove 200Aofthesurfaceof asinter~diskresultainanoticsable decrease in the i n t e d y of the higher BE component, Figure 7, indicating that the species responsible for this is localized on the surface, and secondly electrochemical oxidation resulta in an increase in both the resolution and intensity of the higher BE component, Figure 7, although the BE remains wentially constant. Coupled with the observed growth of Bi-rich crystalliteson the surface this suggests the presence of two types of Bi sites. In principle electrochemicaloxidation could selectively oxidize any Bi(II1) which may have substituted into the Ru sites of the oxide lattice to Bi(V) while the bulk of the Bi remains Bi(II1). Such an explanation would account for the observed XPS results;however, since sampleswith the maximum number of surface crystallites have the maximum amount of the high binding energy component, the two observations seem to be related. Despite the obvious attractiveness of a two Bi oxidation state explanation we conclude that there are two surface Bi(II1)sites, one due to Bi(II1) in the oxide lattice and the second due to surface enrichment and in the extreme segregation of a separate Bi(II1) phase, possibly Biz(SO4)a. T h e differencesin the bindingenergybetween the two Bi(II1)species, ca. 1 eV, can be rationalized by noting that the extent of covalent bonding is greater in the bulk material, than in the surface species. X P S analyeisof Ru speciesis complicatedby two factore. Firstly, the diagnostic Ru 3d line lies at approximately the same BE as the C l e line. Therefore adventitious carbon impurities can, and often do, mask the Ru, or at least complicate the analysis of the Ru 3d photoelectron spectra Secondly,the Ru 3p linewhich lies in an otherwise uncluttered part of the spectra is typically broad and insensitive to changes in the ruthenium oxidation state. With these pinta in mind the Ru 3d/C 1s envelope of the as-prepared sample was analyzed using a similar protocol to that deecribed by Felthouse et aL20 This can be summarized as follows. Firstly the lowest BE Ru 3&/z line near 281 eV was estimated and the 3&z line constructed from thisusing standard ~onstraints.~ 6 Next a C 1s line was placed at 284.6 eV and any residual eignal near 282.5 was then reproducedby a second Ru 3d doublet. Finally the residual intensity above 285 eV was fitted by (23)Hegde, M.5.;Ganguly, P.Phys. Reo., 1988, BSB,4557. (24)Practical Surface Analysis, 2nded.;Briggs, D., %ah, M.P., a,; Wdey Chicheater, 1990. (25) Scofield, J. H.J. Electron Spectrosc., Relat. Phenom. 1976, 8, 129.
Langmuir, Vol. 9, No.7, 1993 1867
Bi-Ru Oxide Electrodes
c
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additional C 1s lines. For the as-prepared sample it was necessary to use two Ru 3d doublets to reproduce the observed line shape, Figure 8. The BE of the stronger of these, Ru 3dsp BE = 280.9 eV, indicates that Ru(IV) is present in keeping with the formulation Bi23+R~24+0,. The 3&p line in RuO2 is at 280.9 eV. The second Ru 3dsp line at 282.6 eV may indicate the presence of a second Ru oxide, either Ru(V) or Ru(VI). RUOs is reported to have a 3dsp BE = 282.3 eV.20 This higher oxidation state Ru species may result from charge compensation of Bi(II1) substitution into the Ru sites, mechanism ii above, or balances the excess oxygen. The Ru 3p signal is a simple apin-orbit doublet and affords no additional information. An alternative explanation for the observed Ru 3d line shape is that the structure arises from final state effects. If the core hole resulta in significant screening of the conduction electrons, than an effective electron trap may be created. Such a trap can give rise to asymmetricpeaks The for allthe species involved, namely Ru, Bi, and 0.26*27 0 1speak is asymmetric;however,the presence of adsorbed
water and oxygen means that little importance can be placed on this observation. In this preaent m e this explanation can be discounted since ion etching yields a symmetricalBi 4f signal; however the Ru 3d signal remaina a doublet. Thus it appears that two oxidation etatee of Ru are present in the material. There was no evidence to indicate that reduction of the sample occurred during ion bombardment. The photoelectron spectra of a sample cycled between -0.6 and +0.876 V and then polarized at 0.76 V also showed the presence of two Ru 3d doublets at 281.6 and 282.6 eV, Figure 8. In this case it appears that both Ru(IV) and RUN)are present. If, however, the sample was oxidized without precycling,then only one Ru specieswas observed. There is a small increasein the RU 3&/2 BE after oxidation of the sample at 0.75 V at 60 OC for 6 h in 2.6 M H2SO4 to 282.1 eV from 281.6 eV. It is unclear if the higher binding energy indicates the formation of a higher oxidation state of Ru such as Ru(V) or Ru(VI), the Ru 3&p line in RUOs is 282.3 eV, or more probably reflects the difficulties in obtaining an accurate measure of the BE in the presence of a large amount of a carbon impurity. As discussed above potential cycling of the material results in surface recrystallization, and equally important as demonstrated by electron micro~copy,bismuth segregation occurs. While no pure ruthenium c r y ~ t d i t m were observed in the SEM/EDAX analysis, it is possible that extensive recrystallization of the samples yields areas where formation of Ru(IV) and Ru(V) is possible. If the surfaceis not precycled, then the depletionof Bi apparently decreasesthat stability of the high oxidation states of Ru. More extensive Bi depletion yields a 'Ru02" type surface which is known to form air-stable highly oxidized Ru surface species.
Acknowledgment. We thank Dr. I. J. Kaplin for assistancewith the electron microscopy. The purchase of the photoelectron spectrometer was made possible by grants from the ARC and the University of Sydney. This work was supported by the ARC. (26) Cox, P.A.; Gwdenough, J. B.;Tavener, P.J.; Tellea, D.;&de& R. G. J. Solid State Chem. 1986,62,360. (27) Heu, W.Y.;Kaeoweki, R. V.;Miller, T.; Chiang, T. Appl Phy6.
Lett. lSS8,52, 792.
