Langmuir 1992,8, 283-290
283
X-ray Photoelectron Spectroscopy and Electrochemical Surface Characterization of IrO2 + RuO2 Electrodes I. M. Kodintsevt and S. Trasatti’ Department of Physical Chemistry and Electrochemistry, University of Milan, Via Venezian 21, 20133 Milan, Italy
M. Rubelt and A. Wieckowski* School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801
N. Kaufher Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 Received March 11,1991. In Final Form: July 15, 1991 Iron + RuOz layers on Ti prepared by thermal decomposition at 400 “C of the chlorides dissolved in acid aqueous solution have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy( X P S ) ,Auger electron spectroscopy (AES),energydispersion X-ray (EDX),inductively coupled plasma emission spectroscopy (ICPES),and cyclic voltammetry (VA) as a function of composition. The surface analysiscarried out by XPS showed surfaceenrichmentwith Ir while the in-depth composition profile by AES indicated homogeneous distribution of components below the surface. The AES atomic ratio was found to be in agreement with the nominal one as confirmed by ICPES. The voltammetric curves show features which can be used to quantitatively analyze the surface as sensitively as XPS. While in XPS the positions of Ir and Ru photoelectron lines do not change for different mixed oxides, redox transition potentials in voltammetric curves do not vary linearly with surface composition, thus pointing to some possible ‘synergistic” effects. Surface charges were obtained by integration of voltammetric curves. Their variation with composition and with potential scan rate provides some insight into the morphology and texture of the oxide layers. Reasons for the actual surface composition and for the bulk structure are discussed.
Introduction Transition-metaloxides have shown outstanding activity for a variety of electrochemical processes of technological interest. The electrocatalytic activity is most conveniently modulated if mixed oxides are used. Mixtures of oxides are also recommended to increase the resistance to Mixed oxides based on RuOz + IrOz have been proposed as efficient and stable electrocatalysts for 02 evolution in acid media.4,5 Under similar circumstances, RuOz is the more active component while IrOz imparts to it an improved resistance to anodic dissolution. It has been recently shown by one of us6+ that the details of the procedure of preparation are crucial in determining the final properties of electrocatalystswhose activity depends on the surface composition rather than on the bulk one. However, for the same composition the action of the components depends on the efficacy of the intimate + On leave from Moscow Institute of Chemical Technology,USSR.
8 Permanent address: Space Research Centre of the Polish Academy of Sciences, Ordona 21, 01-237Warsaw, Poland. (1)Trasatti, S., Ed. Electrodes of Conductive Metallic Oxides; Elsevier: Amsterdam, 1980 and 1981; Part A and Part B. (2) Trasatti, S.; O’Grady, W. E. In Adoances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-
Interscience: New York, 1980; Vol. 10, p 177. (3) Trasatti, S. In Electrochemical Hydrogen Technologies; Wendt, H., Ed.; Elsevier: Amsterdam, 1990; p 104. (4) KBtz, R.;Stucki, S. Electrochim. Acta 1986, 31, 1311. (5)Hutchings, R.;MOller, K.; Kotz, R.; Stucki, S. J.Mater. Sci. 1984, 19, 3987. (6) Angelinetta, C.; Trasatti, S.; Atanasoska, Lj. D.; Atanaeoski, R. T. J. Eleetroanal. Chem. 1986, 214, 535. (7) Angelinetta, C.; Trasatti, S.; Atanasoska, Lj. D.; Minevski, 2.S.; Atanasoski, R. T. Mater. Chem. Phys. 1989,22, 231. (8) Atanasoska, Lj.; Atanasoski, R.; Trasatti, S. Vacuum 1990,40,91.
0743-746319212408-0283$03.00/0
mixing.’ It is thus necessary to obtain a detailed surface and bulk characterization of the oxide mixture in order to be able to discriminate between the various factors governing the activity and stability. As preliminary to the study of Hz evolution on RuOp IrOz electrodes, we report in this paper the electrochemical and surface characterization of these mixtures over all the composition range. Although similar studies have been reported previously for the same nominal compositions,6-8 the results have shown that the procedure of preparation is a crucial variable whose impact has not yet been completely clarified. We have therefore deemed it necessary to investigate the properties which can be more directly affected by the preparation procedure and in what direction they may change. In this work, mixed IrOz RuOn oxides deposited on Ti at all compositionsby thermal decompositionhave been characterized by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and inductively coupled plasma emission spectroscopy (ICPES) and cyclic voltammetry (VA) at different sweep rates. Further studies concerning the cathodic behavior will be reported subsequently.
+
+
Experimental Section Electrodes. Mixed oxides were deposited on the support by thermal decompositionunder an 0 2 atmosphere at 400 “Cof an
aqueous solution of the hydrated chlorides (Fluka) (ca. 0.3 mol dm-7 acidified by HC1 so as to prevent hydrolysis. The solution of the precursors, spread onto the support,was fired for 10 min and the operation was repeated until the oxide loading, to keep to total number of moles constant, averaged 8 X 104 mol cm-2, corresponding to a nominal thickness of about 1.5 pm. Electrodeswere first prepared at 10mol % compositioninterval from pure RuO2 to pure IrOz, but the set was successively 0 1992 American Chemical Society
204 Langmuir, Vol. 8, No. 1, 1992
Kodintseu et al.
Figure 1. Scanning electron micrograph at different magnifications of Ru02 + IrO2 layers on Ti: (a) 70% RuO2 (XlOOO); (b) 90% R u O ~(X10 OOO).
integrated with more intermediate compositionswhere required, for a total of 14 compositions, two or more samples each (altogether 42 electrodes). An additional specimen, to be used for surface analyses, was prepared for each composition. All electrodes were prepared from the same batches of precursor solution, and all samples of the same compositionwere prepared in one and the same run. Supports. Titanium platelets of 10 X 10 X 0.2 mm were used as supports. A narrow stem 5 cm long served for the electrical connection. Supports were etched in 10% boiling oxalic acid for 1h immediatelybefore the coating operation. Both faces of the platelets were coated with the oxide mixture. Techniques. A number of complementary techniques, including electrochemical, emission as well as UHVsurface analysis methods, were applied to characterize the properties of the oxide layers. Electrochemicalmeasurements were performed using a fourcompartment electrochemical cell with two counterelectrodes facing the working electrode from opposite sides, and a Luggin capillary approaching the electrode from below in order to minimize uncompensated ohmic drop. All experiments were carried out in 1mol dm-3 solutions of HClOI (Fluka, puriss. p.a.) or NaOH (Fluka, puriss. p.a.). Solutions were deaerated (and stirred) by bubbling purified nitrogen. Voltammetric measurements were performed using AMEL (Milan) instrumentation. Potentials were measured against a RHE in acidic and a SCE in alkaline solution. The surface analysis was performed by X-ray photoelectron spectroscopy (XPS) using a PHI 5400 Perkin-Elmer photoelectron spectrometer. Photoelectrons were generated by Mg(Kal.2) X-ray radiation of energy 1253.6 eV. Calibration was based on the Au(4fTp)with the binding energyof 83.8 eV. General spectra (surveys)showingoverall compositionof the surfacewere recorded at 84.95 eV pass energy, whereas the data for the determination of binding energies and for curve fitting were collected at 17.9 eV pass energy. The curve fitting was performed with a PHI version 4.0 software. The objective of the measurements was to determine the oxidation states of all the elements in the films. In the present work the positions of following photoelectron lines were considered: Ir(4f), Ir(Sd), Ru(3d) and Ru(3p3/2), O(1s) as well as Fe(2p3p) if this element was detected in a given sample. The chemicalforms correspondingto the above peak positions could
be determined by comparison with the published data. No charging effects were noticed since Ir and Ru oxides are conductive. In the case of the quantitative analysis, Ir(4f,p), Ir(3d5/2),Ru(3p~p), and O(1s) lines were taken into account. Ru(3d) features could not be considered because of ita coincidence with the carbon (C(1s)line) from the carbonaceousresidues. The analyses were performed for surfaces "as received" as well as for freshly formed surfaces obtained by Ar+ sputtering of the outermost layer; 1min sputtering corresponded to removal of a layer 0.4 nm thick (calibrated for Si02). Additionally to the XPS measurements, the depth profiling was made with a scanning Auger microprobe (PHI Model 660), combined with ion bombardment, in order to assess the homogeneity of the in-depth distribution of the Ru and Ir oxides. The samples were etched by Ar+ sputtering with a rate 2.5 or 4.0 nm/min. Inductively coupled plasma emission spectroscopy (ICPES) served to determine the total amount of Ru and Ir in the oxide layers. Since RuO2 is known as one of the most difficult compounds to dissolve, the samples were fused with KOH in a zirconium crucible and afterward the fused material was dissolved in a 6 M HCk2 M HN03 mixture. Then, it was analyzed by ICPES using a Perkin-Elmer Plasma I1 emission spectrometer. The analysis was based on the intensities of Ru and Ir primary lines of wavelengths 235.791 and 204.419 nm, respectively.
Results and Discussion Microscopic and SpectroscopicCharacterization. SEM, EDX, XPS, AES, and ICPES studies were carried out with specimens specially prepared for that purpose. Representative SEM micrographs for surfaces under examination are shown in Figure 1. The observations reveal a cracked-mud structure of the oxide layers; the crevice width can be as large as the film thickness. A t high magnification, Figure lb, small particles of precipitate (10-100 nm) of an uncertain origin are detected. The distributions of Ir and Ru were recorded by EDX. Results indicated a homogeneous distribution of the components (Ir, Ru) in the surface and subsurface regions of the "island". In some cases, the signal of the underlying T i was detected in the grooves. The spatial resolution of
IrOz + RuOz Electrodes
Langmuir, Vol. 8,No. I, 1992 285
I
L " 4 0 0 2 0 0 0 B.E./eV
Figure 2. General XPS spectrum for received" RuIIrl-102 layers on Ti: (1) r = 0 (IrOt); (2) 0.5; (3) 1 (RuOz).
EDX is rather poor and no information is available on the elemental composition of the small particles of precipitate found on the surfaces. Figure 2 shows survey XPS spectra recorded for "as received" surfaces of pure RuO2, pure IrO2, and 50% IrO2 50% RuO2 oxide mixture. All major features of Ru, Ir, and 0 are visible in the spectra. The spectra indicate that the surfaces are clean and free from pronounced amounts of contaminants, except of carbon and small quantities of chlorine (in line with expectation') which are partly or totally removed by 1min of sputter cleaning. Fe was also detected in one specific case only: 70 % RuO2 + 30 % IrO2 (on the basis of the XPS analyses, a glass holder for the brush was specifically constructed so as to avoid further Fe contamination). It is noteworthy that no titanium was detected on the surface unlike a previous casea6 No sandblasting was carried out in this work prior to the coating with the oxide mixture. Table I contains the binding energies attributed to the most pronounced features of Ru (3d5 2,3d3/2, and 3p3/2), Ir (4f7/2, 4f5p, 3d5/2, and 3d3p) and (1s) photoelectron lines in the pure and mixed oxides determined for the "as received" and sputtered surfaces. For the "as delivered" surfaces of all the samples examined, the binding energies for a given species remain the same independently of the nominal composition. Shift of the photoelectron lines toward lower binding energies is observed for metals, as expected, due to an ion bombardment which induces the preferential removal of oxygen (reduction of oxides) from the films. Parts a and b of Figure 3 show the XPS spectra recorded for Ru and Ir, respectively, and Figure 4 shows spectra for 0, respectively. The chemical shifts and corresponding chemical states of the elements can be determined by taking into account the existing literature data for pure metals and their oxides. Ruthenium. The binding energies of 280.0 and 284.1 eV are reportedg for Ru(3d5p) and Ru(3d3p) photoelectron lines, respectively, in metallic ruthenium. Our results for the "as delivered" oxides in Figure 3a show the binding energies 281.0-281.2 eV (3d512)and 285.1-285.2 eV (3d3p).
+
d
(9)Wagner, C. D.,Riggs, W. M., Davis, L. E., Moulder, J. E., Muilenberg, G. E., Eds. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1979.
They yield the chemicalshift of 1.0-1.2 eV which is ascribed to the +4 oxidation state of ruthenium: RuO2. Fairly similar values 280.8-281.2 eV (3d5p) have been found by other g r o ~ p s ~ J +investigating '~ ruthenium oxides. Iridium. For metallic iridium, the binding energies for Ir (4f7p) and Ir(4f5/2)are 60.9 and 63.8 eV, respectively.'* The photoelectron lines shown in Figure 3b appear at 62.1 eV (4f,/2) and 65.0 eV (4f5/2) showing the chemical shifts of 1.2 eV. The signal is anomalously asymmetric in the higher binding energy range and the ratio of the peak heigth [Ir(4f7/2/Ir(4f5/21is 1:1,whereas the theoretical intensity ratio predicted for iridium is 4:3. Moreover, no change in the spectrum shape was observed under sputtering of the oxide layer; only the position of the Ir(4f) signal was shifted to the lower binding energies due to reduction of oxides by the ion bombardment. The anomalies in the core-electron line shapes in 11-02 have already been carefully considered by Wertheim and Guggenheimlgwho ascribed them to the screeningresponse of the 5d conduction electron of Ir. Since the spectrum in Figure 3b is fairly similar to the one considered in ref 19,we suggest that the chemicalshift of 1.2 eV is attributed to the presence of IrO2 in the layer analyzed. Oxygen. The asymmetry is also observed in the shape of the O(1s) photoelectron lines, Figure 4. However, differently from the Ir case, the line shape changes under ion bombardment of the surface layer. Then, the curve fitting (Gaussian-Lorentzian type with correction for inelastic scattering) was used in order to find out and to distinguish possible different chemical states of oxygen in the oxide layers. The oxygen signal from the outermost layer-Figure 4a-is composed of three species with binding energies of 530.0, 531.5, and 533.2 eV. The respective features can be attributed to: (a) 530.0 eV, oxide oxygen, as typically observed for oxides of transition metals6s2+22(the position of this signal is not influenced by ion bombardment); (b) 531.5 eV, h y d r o ~ i d e s ; ~(lc-)~ ~ 533.2 eV, adsorbed water or water of h y d r a t i ~ n . ~As ~-~~ shown in Figure 4b, the feature at 530.0 eV remains stable under ion bombardment, whereas the other two signals are influenced by the ion beam. This indicates that hydroxides and water are present in the outermost layer, only. The quantitative results obtained by XPS for iridium and ruthenium are collected in Table 11. The concentration ratio (Ir/Ru) in the outermost layer of the samples analyzed is approximately 3 times higher than the ratio inferred from the nominal composition of the oxide mixture. This excess is observed both for the "as received" and shortly sputtered surfaces. Although the enrichment of the oxide surface with Ir confirms previous finding^,"^ the possibility was checked (10)Lewerenz, H. J.; Stucki, S.; Kotz, R. Surf. Sci. 1983,126,463. (11)Kotz, R.; Lewerenz, H. J.; Stucki, S. J.Electrochem. SOC.1983, 130,835. (12)KBtz, R.;Stucki, S. J.Electrochem. SOC.1985,132,103. (13)Augustynky, J.; Koudelka, M.; Sanchez, J.;Conway, B. E. J . Electroanal. Chem. Interfacial Electrochem. 1984,160,233. (14)Kotz, R.;Lewerenz, H. J.; Bruesch, P.; Stucki, S.J.Electroanal. Chem. Interfacial Electrochem. 1983,150,209. (15)Wagner, N.; Kuhnemund, L. Cryst. Res. Technol. 1988,23,1017. (16)McEvoy, A. J.; Gissler, W. Phys. Status Solidi: A. 1982,69,K91. (17)Hara, M.; Asami, K.; Hashimoto, K.; Masumoto, T. Electrochim. Acta 1983,28,1073. (18)Peuckert, M. Surf. Sci. 1984,144,451. H. J. Phys. Reu. B: Condens. (19)Wertheim, G. K.; Guaaenheim, -_ Matter 1980,22, 4680. (20)Wandelt, J. Surf. Sci. Rep. 1982,2, 1. (21)Au, C. T.;Roberts, M. W. Chem. Phys. Lett. 1980,74,472. (22)Roberts, M. W. Pure Appl. Chem. 1981,53,2269. (23)Fisher, G. B.;Sexton, B. A. Phys. Reu. Lett. 1980,44,683. (24)Fisher, G. B.;Gland, J. L. Surf. Sci. 1980,94,446.
Kodintsev et al.
286 Langmuir, Vol. 8, No. 1, 1992 Table I. Binding Energies of Ru, Ir, and 0 in Pure Oxides and Their Mixtures BE/eV
0
0 1 0 1 0 1
30 50
6 70
0 1
16 31
62.1 61.7 62.15 61.55 62.05 61.25 61.20 62.20 61.5 61.5 61.3
65.05 64.7 65.15 64.55 65.05 64.30 64.15 65.20 64.5 64.5 64.25
298.0 297.15 298.0 297.1 298.0 296.9 296.7 298.0 297.3 297.0 296.5
313.0 313.9 312.9 313.7 312.7 312.4 313.5 312.8 312.3 312.1
0 1
100
281.1 280.9 281.0 280.7 280.45 281.25 281.0 280.5 280.4 280.95 280.85
285.1 284.8 285.1 284.8 284.5 285.25 285.1 284.6 284.6 285.05 285.0
462.9 462.5 462.85 462.05 461.9 463 462.5 462 461.8 462.7 462.3
530.2 530.1 530.0 529.9 529.95 529.9 529.8 530.0 530.0 529.8 530.0 530.0
t is the sputtering time. a
lo[
2932
289.6
288.0
282.4
2788
B.E./eV
70
0
a
66
eaa
B.E./eV
Figure 3. (a) XPS spectrum for Ru of a “asreceived” pure Ru02 layer on Ti. (b) XPS spectrum for Ir of a “as received” pure 1r02 layer on Ti. ’Ot a
.
’\
6t
oxide layers) could be detected. The results of the analysis are shown in Table 111. Fairly satisfactory agreement (within f 5 ?6 ) is observed between the actual composition and the nominal one. The only exception is the last sample. However, that was the sample containing up to 12% Fe: the discrepancy can therefore be readily understood. The distribution of the components in the oxide layer was checked by AES depth profile analysis. Besides some composition variations in the outermost and near-surface layer of a thickness of 15-20 nm, the depth profiles for Ir and Ru were flat indicating a homogeneous in-depth distribution of the elements. The quantitative AES analysis of the outermost layer was made difficult by two factors: (i) the carbon contamination of the surface masks the most pronounced Ru feature a t 273 eV; (ii) the sensitivity factors depend on the oxygen content. Since the oxygen content varies sharply near the surface, it was difficult to rest on a reference standard. For the bulk, the pure RuO2 and IrO2 oxides were taken as a reference. Electrochemical Surface Characterization. The surface features of the mixed oxides were characterized in situ by cyclic voltammetry (VA). Curvesrecorded between 0.4 and 1.4 V (RHE) in acid are shown in Figure 5a. As previously observed,‘j these curves are rather featureless and are of little use to characterize the composition of the surface. However, the voltammetric charge derived by integration of these curves can be taken as a measure of the active surface area (see below). If the potential range is extended to less positive potentials down to hydrogen discharge, Figure 5b shows that some specific features are enhanced. In particular, the broad peak in the middle of the curve appears to change its position with composition. Quite tentatively, this peak can be assigned’ to the I11 IV transition of Ru and Ir. The more positive potential of the peak for Ir agrees with the known higher resistance of Ir to ~xidation.~ The curves indicate that the oxidation (and reduction) transitions are shifted to more positive potentials as the Ir content increases. Since 0 2 evolution on oxides is also related to metal redox t r a n ~ i t i o nthis , ~ ~phenomenon is less evident for IrO2 than for RuO2. Figure 6 shows a plot of E,,, the potential of the peak in Figure 5b, as a function of the nominal (bulk) composition (curve 1). The dependence is nonlinear and in accord with the predominant presence of Ir on the surface. However, if the nominal composition is replaced by the surface composition (taken from Table 11),the dependence remains nonlinear, though attenuated (curve 2). This
-
b
536 534 532 530 528 526 BE./&
Figure 4. Deconvoluted XPS spectra for O(ls) of pure IrOp on Ti: (a) “as received”; (b) 1 min of sputtering. (unlike previously) that the nominal composition could be altered during the thermal decomposition. To this purpose, some of the samples were analyzed by ICPES which enables the total contents of Ir and Ru, as well as the Ir-to-Ru ratio in the oxide layers, to be determined. Thus, the possible deviations from the nominal composition (the composition assumed in the preparation of the
(25) Trasatti, S. J.Electroanal Chem. Interfacia[Electrochem. 1980, 111, 125.
IrOp + RuOp Electrodes
Langmuir, Vol. 8, No. 1, 1992 287
Table 11. Atomic Concentrations and Concentration Ratios of Elements in RuO2 + IrOz Oxide Lavers 30% RuOz + 70% IrOz 50% RuOz + 50% IrOz 70% RuOz + 30% IrOz a b C a b C a b C % Ru 10.0 3.0 5.0 16.5 7.5 14.0 23.0 6.0 9.0 .. % Ir 23.0 22.0 36.0 16.5 18.0 29.0 10.0 7.5 11.0 %O 67.0 75.0 59.0 67.0 74.5 57.0 67.0 74.5 61.5 ( % Fe) (12.0) (18.5) Ir/Ru 2.3 7.3 7.2 1.0 1.2 1.2 2.4 2.1 0.4 a Nominal. b "As received". 1 min sputtering. Table 111. Ir-to-Ru Ratios in the Oxides Analyzed by ICPES
nominal composition RuOz RuOz RuOz RuOz
80% IrOz + 20% 70% IrOz + 30% 40% IrOz + 60% 30% Iron + 70%
Ir-to-Ru ratio nominal ICPES 4.00 3.77 2.33 2.34 0.67 0.63 0.43 0.34
a2
0
0.4
a8
a6
1.0
x(lrOA
Figure 6. Variation with composition of the peak potential in Figure5a: (1)nominal (bulk)composition; (2) surfacecomposition (from Table 11).
Figure 5. Voltammetric curves at 20 mV s-l in 1 mol dm" HClOd solution in the potential range (a) 0.4to 1.4V (RHE) and (b) -0.2 to1.4V (RHE) immediatelyafterastopof 5minat-0).2V(RHE): (1) pure RuOz; (2) 50% RuOz + 50% IrOz; (3) pure IrOz. The current density scale (mA cm-? is indicated.
differs from the findings of Kotz and Stucki4who reported no surface enrichment and a linear dependence of E,, on composition. The variation with surface composition observed in Figure 6 indicates that the features of IrOz predominate. These particular electrodes should therefore show kinetic and corrosion properties more typical of Iron. The study of H2 evolution is in progress. Scattered points are observed in Figure 6 at high IrOz concentrations. The reason for that is unclear. The presence of Ir in a higher oxidation state than IV could play a role. It has been ascertained elsewhere by RBSZ6 that IrOz contains an excess of oxygen. While in acid solution none of the broad peaks in the voltammetric curve can be unambiguously assigned to a single component, the situation is different in alkaline solution. Figure 7a shows that the sharp peak prior to 02 evolution (peak A) is absent for pure IrO2 samples; it is therefore due definitely to a redox transition at Ru active
a28
1
Io 0
a'
\ \ D\,8
02
a4
I
I
L
06
08
1.0
x(lr0A
Figure 7. (a) Voltammetric curves at 20 mV s-1 in 1 mol dm-3 NaOH solution in the potential range -0.6 to 0.4 V (SCE): (1) pure RuOz; (2) 50% RuOz + 50% IrOZ; (3)pure IrOz. The features chosen for the surface analysis (see text) as well as the current density scale (mA cm-2) are indicated. (b) Variation of the potential of peak A in curve 1 with composition for both seta of electrodes.
sites.27 The position of this peak is plotted as a function of composition in Figure 7b. It is intriguing that, unlike with XPS spectra, a shift in the peak position is quite appreciable especially at high IrOz content, which indicates intimate Ru-Ir interaction. Electrochemical Surface Analysis. Although any electrochemical response is molecularly nonspecific, some typical features of the voltammetric curve can be assigned to some specific redox transition and used to sample the surface concentration of the given species. (27)Burke,L.D.;Murphy,O.J.;O"eill,J.F.;Venkatesan,S.J.Chem.
(26)Trasatti, S.Electrochim. Acta 1984,29, 1503.
Soc., Faraday Trans. 1 1977, 73, 1659.
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288 Langmuir, Vol. 8, No. 1, 1992
. . . .
t 4 c
0.4
t Figure 8. (a) Surface vs bulk RuOz content. (b) Surface RuOz content from XPS vs surface RuOz content from cyclic voltammetry (VA). ( X ) XPS; (A)voltammetry between -0.6 and 0.4 V; (A)voltammetry between 0.05 and 0.35 V (SCE).
While the VA curves in Pigure 5a for acid solution are rather featureless, some new, better defined features appear in Figure 7a for basic media. In particular, the curves differ appreciably for RuOz and IrOz, the sharp peak prior to 0 2 evolution (peak A) being typical of Ru oxidation, while the large anodic current in the middle of the potential range is characteristic of IrOz. Thus, with reference to Figure 7a, the surface composition of RuOz can be derived with the formula
where subscriptsx and 1indicate the nominal mole fraction of Ru. The results are shown in Figure 8a where also the XPS data are reported. It is interesting to observe that the VA data also indicate surface enrichment with Ir, although the quantitative response depends slightly on the width of the potential window. In Figure 8b the surface composition in RuOz by XPS is compared with that by VA. It is seen that VA indicates a systematically higher value as a wider potential window is used. While in one previous case6the agreement was excellent, deviations in the same direction as here for the Ir-rich compositions were observed in another case.7 In fact, if the potential window of 0.3V is used, Figure 8b shows that the agreement between the two sets of data is again excellent. Surface Charge. The charge q*, obtained from the intergration of the VA curves shown in Figure 5a, is determined by surface redox processes assisted by proton exchange between surface OH groups and the solution
-
MOJOH), + he- + 6H+ MO,-,(OH),+, (2) q* can thus be taken as a measure of the number of protons exchanged, Le., of the active surface area, in turn related to the morphology of the layer.28 Figure 9 shows that q* varies with composition with a well-definedmaximum around 80 mol % IrOz. Previously, with the precursor dissolved in plain water, a parabolic dependence centered around 50 5% was observed,6whereas with the precursors dissolved in 2-propanol a monotonic increase in q* was the outcome.' An effect of the state of the precursors was also observed with pure I r o ~ . ~ ~ It is intriguing that the charge remains low, typical of RuOg up to a surface concentration of 60% IrOz, to increase then steadily. Thus, while the surface chemical and electrochemical properties are dominated by IrOz, the morphologyappears to be governed by RuOz. This means6 that the surface enrichment with Ir is probably related to (28) Ardizzone,S.; Carugati,A.; Trasatti, S. J.Electrochem. SOC.1982, 129, 1689. (29)Ardizzone, S.;Carugati, A.; Trasatti, S. J. Electroanal. Chem. Interfacial Electrochem. 1981, 126, 287.
01
I
0.2
I
0.4
I
I
,
0.6
0.8 x ( I r 0,)
1.0
Figure 9. Variation with composition of the voltammetric charge obtained by integration of the curves in Figure 5a: (1)nominal composition; (2) surface composition (from Table 11).
the different kinetics of thermal decomposition. RuC13 is known to decompose at lower temperature than IrC13.1>30 Thus, RuOz is preformed and establishes the template morphology while IrOz is formed with some delay and accumulates on the surface. The invariance of the XPS lines of Ru and Ir with composition apparently indicates that no intimate mixing with atomic dispersion takes place or, alternatively, that no reciprocal influence on the electronic properties is operative. This conclusion contrasts with the evidence from Figure 6 suggesting a nonlinear dependence of the surface properties on surface composition. Also Kotz and Stucki4 did not observe any XPS chemical shift with composition, although on the basis of the valence band spectra they have suggested the formation of a common tzg band. This points out, also in relation with electrocatalytic s t u d i e ~ that , ~ electrochemical techniques and XPS give complementary information in that they may not probe the same surface situation. Inner and Outer Active Surface. As generally observed with oxide layers,lv6 the voltammetric charge depends on the potential sweep rate. Figure 10a shows a typical example for Ir + Ru mixed oxides. q* is seen to decrease steadily with sweep rate in a way resembling a diffusion-dependent process. The decrease of q* with the scan rate u indicates that some surface sites are less accessible to material exchange or migration. These can be pores or crack walls, as well as highly defective regions such as grain boundaries. I t has been that the dependence of q* on u can be linearized if l/q* is plotted against u ~ / ~A . typical linearization is shown in Figure lob. I t is to be noted that linearization is achieved over the entire u range, which indicates that a homogeneous effect is being taken into account. Linear extrapolation to u 0 gives q*o, the surface charge related to infinitely slow proton exchange. The related surface area is taken to measure the whole oxide surface wetted by the solution. The extrapolation to u is more difficult and less reliable since no unique straight line, based on the idea of hampered proton migration, can be obtained. In other words, if q* is plotted vs u-lI2, Figure lOc, a broken line is observed. At low sweep rates a linear dependence of lower slope is visible whose extrapolation to u gives a value of q* which is higher than those experimentally observed in the higher sweep rate range. The reason for
-
-
-
(30)Jang, G.-W.; Rajeshwar, K. J.Electrochem. SOC.1987,I34,1830. (31)Ardizzone,S.;Fregonara, G.; Traaatti, S. Electrochim. Acta 1990, 35,263.
IrOz
+ Ru02 Electrodes
6
41
Langmuir, Vol. 8, No. 1, 1992 289
b
C
Figure 10. (a) Phenomenological dependence of the voltammetric charge on potential sweep rate for two samples of the same composition (50% RuOz + 50% IrOz). (b) Dependence of the reciprocal of the voltammetric charge on the square root of the potential sweep rate for two samples of the same composition (20% RuOz + 80% IrOz). (c) Dependence of the voltammetric charge on the reciprocal of the square root of the potential sweep rate for the same samples as in part b.
/ Figure 11. Voltammetric charge at u = m vs charge at u = 0 as unit slope; obtained by extrapolation; cf. Figures 10a,b: (-) (- - -) apparent linear correlation.
that is probably that q* decreases also because of uncompensated ohmic drops inside pores, cracks, and grain boundaries. These regions are excluded at high sweep rates, but according to a mechanism which differs from the simple “freezing” of proton diffusion. Thus, q* drops with r1f2 at a higher rate and the extrapolation can only be carried out based on the phenomenological dependence of q* on v. Figure 1Ocshows that a linear extrapolation has been empirically attempted on the basis of the points a t higher sweep rate. The charges extrapolated at u 0 and u * are related by the equation31
- -
where q*tot = q*,,=o is the charge related to the whole active surface, while q*out q*,,=.. is the charge related to the external surface which can still exchange protons at u = *. q*h, the difference between the previoustwo quantities,
is thus the charge related to the so-called “inner”surface, the portion less accessible to material exchange and diffusion. Figure 11shows a plot of q*outvs q*tot, viz. of the outer vs the total surface area. It is intriguing that the points gather around a straight line whose slope is slightly higher than 2. Thus, the whole surface is almost equally divided between outer and inner regions. The fact that a unique correlation is obtained suggests that all the samples, irrespective of the chemical composition, possess the same morphological texture, with the inner surface slightly higher than the outer one. Although Figure 9 shows that the features of RuO2 predominate up to about 60% IrO2, the texture of the layer, Le., its porosity and its kind of roughness, does not depend on its composition.
Conclusions Comparison with the two previous investigations6~7of 11-02 + RuOz on Ti indicates that the conditions of the precursors appear to play an outstanding role in governing the morphology and the structure of the oxide layer. In all three cases a surface enrichment with Ir has been observed, consistently with powders prepared by the Adams t e ~ h n i q u e ,but ~ differently from films obtained by reactive ~puttering.~ Nevertheless, the layers of this work showed a homogeneous depth distribution of the components with an atomic ratio very close to the nominal one. Since the layers were prepared in several equivalent steps, one may wonder why no oscillations are observed in the composition profile. It is thought that the surface enrichment is due to the different kinetics of thermal decomposition of the two precursors, RuC13 decomposing first. Thus, while the surface of the ith layer is enriched with Ir, the successive layer will be richer in Ru in the internal surface, which roughly compensates for the local difference in composition. While the quantitative surface analysis of these nonstoichiometric oxides is difficult because of the dependence of the sensitivity factor on stoichiometry, and therefore of the difficulty to set a reliable reference standard, the surface composition can be quantitatively obtained also by analyzing the voltammetric curves, provided different features attributable separately to the two components can be resolved. The results have shown that voltammetry can be as sensitive as surface spectroscopies in this respect, although the resolution of the individual features of the components appears to depend on the potential window used. One of the most important questions in electrocatalysis of mixed oxide systems is whether the two components are so intimately mixed as to give rise to a homogeneous solid system with different electronic properties capable of producing “synergic” effects. The answer is not definitive. Spectroscopic techniques such as XPS and AES do not give any evidence of the (probably very small) changes in electronic parameters brought about by atomic interactions. On the other hand, electrochemical techniques provide evidence that some interatomic charge transfer is taking place between the components with formation of a probable common electronic band as suggested in ref 4. However, the broad peak of the voltammetric curves, for instance in Figure 5b, can represent either a single surface redox transition or two voltammetrically unresolved redox processes. In the latter case, the apparent predominance of Ir as inferred from E, could be simply related to its peak being less broad than that of Ru. Alternatively, the apparent discrepancy between XPS and electrochemical surface data could be resolved,
290 Langmuir, Vol. 8, No. 1, 1992
as already mentioned in ref 4, if Ru-Ir interactions do not take place in the solid state but via the ionic double layer in solution. This would explain their undetectability in ultrahigh vacuum conditions. These aspects call for very fine investigations with single crystals. The dependence of the voltammetric charge on the potential scan rate appears to be a general feature of oxide electrodes and is attributable to the existence of hardly accessible surface regions located in pores, cracks, and grain boundaries. A separation between outer and inner surface is possible on a phenomenological basis, although only the total surface wetted by the solution can be obtained rigorously by linear extrapolation to u = 0 of l/q* vs v1I2 plots. The linear dependence of q*out on q*t,,t indicates that the texture and morphology of the oxide layer is independent of its composition, with the inner surface roughly equal to the outer surface. On the whole,
Kodintseu et ai. the results of this investigation confirm the modeling of supported oxide layers as having the morphology of compressed powders.'~3~ The cathodic behavior of these oxide electrodes and the electrocatalysis of Hz evolution will be reported in a forthcoming paper.
Acknowledgment. S.T. is grateful to the National Research Council (C.N.R., Rome) for financial support to this work. I.K. thanks the Italian Ministry for Foreign Affairs for a research fellowship. A.W. and M.R. acknowledge the financial support by the National Science Foundation (Grant NSF-DMR-89-20538). Registry No. IrO2, 12030-49-8; RuO2, 12036-10-1. (32) Trasatti, S. Croat. Chem. Acta 1990, 63, 313.
