- Email: [email protected]
XPS and AES study of mixed layers of RuO2 and IrO2
Vacuum/volume40/numbers 1/2/pages 91 to 94/1990
0042-207X/90S3.00+.00 Pergamon Press plc
Printed in Great Britain
XPS and AES study of mixed layers of RuO2 and IrO2 Lj A t a n a s o s k a , Institute of Technical Sciences, Serbian Academy of Sciences and Arts, Belgrade, Yugoslavia and R A t a n a s o s k i , Institute of Electrochemistry ICTM and Center for Multidisciplinary Study, University of Belgrade, Belgrade, Yugoslavia and S T r a s a t t i , Department of Physical Chemistry and Electrochemistry, University of Milan, Milan, Italy
The surface composition of RuO2+lr02 mixed oxide layers (1-2 #m thick) deposited on titanium by thermal degradation of the corresponding chlorides in aqueous or isopropanol solutions has been investigated. A surface enrichment with Ir, more pronounced for the layers obtained from isopropanol, has always been found. Electrochemical data reflecting the surface properties of RUOE+ lr02 layers confirm the excess of Ir02 at the surface.
Introduction
Results and discussion
Layers of mixed oxides on an inert substrate are widely used in the electrolytic industry ~. Mixtures of RuO2 and IrO2 are of particular interest for the reaction of oxygen evolution in acidic media 2. Recently, we have undertaken efforts to correlate the electrochemical properties of such mixed oxides with their bulk composition 3:. In order to comprehend the electrochemical behaviour of the mixed oxides an enrichment in iridium at the surface has to be assumed. Therefore to aid the understanding of the electrochemistry of the oxides surface analysis was required. XPS and AES results of mixed RuO2+IrO2 samples, confirming a surface enrichment in iridium, are reported.
Surface composition. Under the experimental circumstances of sample preparation, which were the same as for the electrochemical examination, the Auger analysis of both ruthenium and iridium requires some caution to be exercised. The AES of all samples showed that the surface was always heavily contaminated by carbon due to the prolonged contact with the furnace and laboratory atmospheres. In the Auger spectra, C (KLL) and Ru (MsN4.sN4,5) transitions appeared at 272 eV and 273 eV, respectively. As a consequence of the overlapping with the carbon signal, no adequately symmetrical peak of Ru at 273 eV could be obtained (Figure 1). The dominant Ir Auger signals for the NsN7N7 (158 eV), the NsN6N6 (164 eV) and the N4N6N6 (174 eV) transitions are partially hidden because of the superposition with the ruthenium peaks for the M 5N ,N 3 (156 eV) and the M 5N 3N 3 (188 eV) Auger transitions. The Ir (NsN704) Auger transition at 229 eV is overlapped by the Ru (MsN3Ns) transition at 231 eV. Besides, the Ir (NNN) and (NNO) Auger signals are an order of magnitude less
Experimental RuO 2q- I rO 2 layers were prepared on a thin plate of Ti by thermal decomposition (400°C ; air) of aqueous (later referred to as aqueous oxides) or isopropanol solutions (later referred to as nonaqueous oxides) of RuCI3 + IrC13 (Ventron) in the required mole ratio (see ref [3] for details in sample preparation). The samples were prepared at 10% tool composition intervals, from pure RuO2 to pure IrO2 included. A Riber UHV system equipped with an Auger cylindrical mirror analyser and an OPX 150 photoelectron spectrometer with a dual anode (Mg and A1) X-ray source were used. The Auger spectra were recorded with a primary electron beam accelerating voltage of 3 keV and an adsorbed beam current of 3 #A. The modulation amplitude of the phase sensitive detector was set at 5 eV peak to peak. The XPS spectra were taken by applying MgK, (hv = 1253.6 eV) irradiation as the photon source. Ion bombardment combined with the AES and XPS measurements was carried out with 3 keV Ar ions, emission current of I0 mA, and an argon pressure of 6 x 10 -3 Pa.
dN(E) x 2,5
LL)
Ir(MNN)
Figure 1. Auger spectrum of mixed layer o£ RuO=+ [rO= (60:40% mot).
91
LjAtanasoska et al:
l•
XPS and AES study of mixed layers of Ru02 and IrO2 1.5
3~1t-/
o AES o XPS
IS
3"4p3pz Ru3d5/2
LU
n X
<06 LL 03
o
8
3r4f?/2
1.0
3r415/2 3r~3f2
t
o
0.4
o
0.2
Q
0
05
rn
8
Rut,S L. Ru4p
764
Binding energy/eV
. ~
- -
Figure 2. XPS survey spectrum of mixed layer of RuO: + IrO2 (60:40% mol).
intense than the superimposed Ru (MNN) peaks. The weaker Ir transitions, MsNTN7 at 1908 eV and M 4 N T N 7 transition at 1981 eV, do not overlap with other signals in the spectrum. However, these transitions have a very low Auger sensitivity factor (S~, = 0.0275). Compared to the Ru (MsN4.sN,.s) transition with SRu = 0.5 this can obviously introduce a considerable error in the atomic concentration evaluation. Therefore, the Auger spectra were suitable only for qualitative composition estimation. The surface quantitative analysis was carried out by XPS. A typical example of the XPS survey spectrum for the 60% tool IrO2 admixture is presented in Figure 2. The Ru 3d and Ir 4f core level spectra were analysed by an iterative curve fitting spectral routine. In the fits the Gaussian-Lorentzian band type was selected with inelastic backscattering correction. It was assumed that the peak widths as full width at half maximum (FWHW) were 1.5+0.2 eV. The area under the deconvoluted peaks provided information about the relative concentration of each type of valence state. The relative concentrations of the various species were calculated from the corresponding photoelectron peak area after correction with appropriate sensitivity factors. The XPS atomic sensitivity factors for Ir 4f7/2 and Ru 3d5/2 photoelectron lines are of identical value (1.55). In the XPS spectra, the binding energy (BE) of the C ls (285.0 eV) and Ru 3d3/2 (284 eV) photoelectron lines coincide. Thus, the Ru 3dsn, which only partially interferes with carbon peak was used for quantitative analysis. The area of Ru 3d 5/2 was determined by deconvolution of Ru 3d region. For the XPS spectra the ratio of the Ru 3d5/2 to the Ru 3d~/2 core level peaks was accepted as a reliable criterion for the amount of carbon present in the surface region. The branching ratio of the Ru 3d spin-orbit doublet in absence of carbon is 3 : 2. The symmetry factor of the 273 eV Ru Auger peak (the ratio of the maximum positive excursion to the maximum negative excursion in the dN(E)/dE) and the 3d5/= to 3d3/2 XPS ratio for Ru depend on the coating composition. As Figure 3 shows, both parameters approach the literature value 5 as the IrO2 content vanishes. A coating composition dependent carbon contamination has been observed for both aqueous and nonaqueous mixed oxides. The carbon impurity, as found by AES depth profiling, is limited to 6 nm of the surface topmost layer for coating compositions of more than 50% tool RuO2. With increasing IrO2 content, the depth of contaminated oxides becomes more than 15 nm. A plot of the surface composition of aqueous and non-aqueous 92
0.5
D
I
%tool Ru
Figure 3. Symmetry factor of Ru 273 eV Auger peak and XPS ratio ot Ru 3d~.,2 to 3d3/: peaks as a function of the mixed RuO2+IrO2 layer nominal bulk composition.
mixed binary oxides as determined by XPS vs the nominal bulk composition is shown in Figure 4. An enrichment of the surface with Ir can be observed in both cases although the enrichment is definitely higher for the non-aqueous mixed oxides. This finding agrees with the results of Hutchings et al 6 despite the different preparation procedures. The results are similar also in the details since the deviations are more appreciable for low RuO2 content. The prevalence of Ir in the external surface of the grains may be related to the different affinities of the two metals for oxygen. Thermodynamically, the affinity of Ru for oxygen is higher than that of Ir 7. This, for instance, may result in a less positive potential for the incipient oxidation of Ru with respect to Ir 3. Consequently, the presumably lower rate of Ir salt oxidation by oxygen may result in a retarded deposition of IrO2 which eventually yields in a surface enrichment with lr.
0 nono,queous , / / " " " , , , , ~
5O
I
50
% toolRu (butk
Figure 4. Surface XPS vs nominal bulk composition for aqueous and non-aqueous mixed RuO2 + IrO2 layer.
LjAtanasoska et al: XPS and AES study of mixed layers of Ru02 and Ir02
The inhomogeneity of the surface composition may provide evidence for the inhomogeneity of the admixture itself. No enrichment has been reported for reactively sputtered samples 2. It thus appears that the surface enrichment is probably related to the mixture homogeneity ; the better the latter, the lower surface segregation of one of the components. The greater enrichment with Ir in the case of non-aqueous mixed oxides may thus indicate a lower degree of homogeneity in non-aqueous than in aqueous mixed oxides. Iridium valency state. The valance state analysis of as-received samples in the Ru 3d and O ls regions was difficult due to carbon contamination. On the other hand, a preferential removal of oxygen took place on sputtering, with consequent shift of Ru 3d and Ir 4f peaks towards BE values typical of the metallic state. Therefore, due to the lack of carbon and iridium peaks interference the analysis of the valence state of the elements in the oxides layer in as-received samples has been concentrated on the Ir 4f region. Figure 5 shows the XPS spectra of IrO2+RuO2 samples of different compositions. A sample treated at a higher temperature (500°C) is also included. The deconvoluted spectrum exhibits two major and three minor peaks located at about 61.2 (A1), 62.6 (BI), 64.2 (A2), 65.7 (B2) and 67.5 eV (C), respectively. The BE of the Ir 4f7:2 core level of metallic Ir is located at 60.9 eV and the splitting of the spin-orbit doublet is 2.95 eV 5. The branching ratio of the 4f lines is 4 : 3. In the case of IrO:, Ir 4f core level exhibits a very specific line shape. The Ir 4f5/2 peak is more intense than the 4f7/2 signal and the extraordinarily high intensity of the saddle is always observed. This is an indication of a case of two superimposed spectra. Thus far, an interpretation of the Ir 4f region based on a curve fitting analysis has been accomplished by Hara e t a l 7 and Wertheim and Guggenheim. s Peuckert 9 and Kotz e t a l 2 have
,
,
,
,
,
pointed out the considerable broadness of the Ir 4f peaks, especially the higher binding energy tail, with no attempt for the line shape deconvolution. There is considerable disagreement in the literature about the nature of the additional Ir 4f core overlapping peaks. Hara e t a l 7 have attributed the main peaks A to Ir(III) and the satellite peaks B to Ir(IV). They have claimed that in IrO2 powder the Ir203 content is significantly high (70%). This is in contrast especially with the finding by Rutherford backscattering analysis of IrO2 on a thin plate of Ti in which a composition IrO2+x(x > 0) was proposed 4. Peukert 9 has discussed the additional peaks B in terms of Ir(VI), most probably based on Kim e t al.'s l o assumption of a Ir(VI) state. Kotz e t a l 2 have pointed out the absence of any definite correlation between BE and Ir oxidation state. They speculate the presence of Ir(III) state in oxide films based on the location of BE for IrC13 reported by Folkesson 11. However in their results the shift of 1.5 eV for Ir 4f region in respect to metallic Ir is different than the value of 2.4 eV found by Folkesson. In a recent investigation of the interaction of oxygen with a clean Ir surface Marinova and Kostov ~2 do not consider the existence of iridium oxidation state lower than four. Wertheim and Guggenheim s have claimed, on the basis of Bell e t a l ' s work ~3, that IrO 2 is the only stable form of iridium oxide at room temperature and have ruled out the existence of valence states other than + 4 . They have attributed the peculiarities of the Ir line shape to the final state screening response and anomalous peak height of the 4f5/2 component to the long tail of the 4f7/2 line. By applying a perturbation theory treatment of the delectron manybody screening response, they have obtained a good agreement with the unusually asymmetrical core electron line shape. Table 1 summarizes the positions of peaks A and B for several samples of different compositions. By constraining only the F W H M and letting the peak position vary, amazingly reproducible results concerning the BE position and the relative percentage of the Ir species have been obtained. The spin-orbit splitting was not constrained and yet it was always found to be 3 eV, which is in a very good agreement with theory. Table 1 shows also that sputtering reduces Ir to the metallic state. The shift of peaks A in respect to Ir metal is 0.9 eV, which is in agreement with the reported literature value 5. The deconvolution procedure yielded a shift of peaks B in respect to A of 1.3 eV as compared to 1.7 eV found by Hara e t al 7. The branching ratio of the spin-orbit components for both the major and satellite peaks is close to the theoretical 4 : 3.
Z
Table 1. Iridium 4fv2 core level deconvolution
5 ~
0-70
e
-68
Nominal oxides composition (% mol IrOz)
)
-66
-~
BINDING
-62
fiNERY/,
-60
eV
5. Iridium 4f spectra as a function of the composition of the mixed RuO2 + IrO2 layers : (a) 20% tool IrO2 ; (b) 80% mol IrO2 ; (c) 70% mol IrO2 (500°C). Figure
20 40 60 80 100 100 (after 15 nm removed) 70 (500°C)
Major peak
Satellite peak
BE (eV)
Contribution (%)
BE (Ev)
Shift from major peak (eV)
61.2 61.3 61.3 61.1 61.2 60.3
76.7 73.0 73.9 69.0 71.0 88.0
62.7 62.7 62.7 62.3 62.5 61.3
1.4 1.4 1.3 1.2 1.3 1.0
61.3
61.3
62.5
1.2 93
LjAtanasoska et al: XPS and AES study of mixed layers of RuO2 and IrO2
The deconvolution procedure yielded an additional peak, C, which is due to the tailing on the high binding energy side and apparently without chemical back up. The appearance of peak C can be explained as a result o f the final state screening 8, i.e. conduction band interaction during the photoemission process 9. It is intriguing that the Ir peak positions do not shift with composition and temperature of preparation. This is taken to mean that Ir does not feel the presence of Ru which implies a heterogeneous structure o f the mixed oxide as assumed above. In our results, the magnitude of the satellite Ir peaks (B) obtained after deconvolution indicates a real presence of another oxidation state besides the + 4. The a m o u n t of the satellite feature B was found to be considerable, from 30% to 40% (Table 1). The shift of B peaks indicates that the other oxidation state has to be higher than 4. Due to the fact that both IrO2 and RuO2 crystallize in the same (rutile) crystal structure, an analogy can be drawn with our study of well defined single crystal surfaces of RuO2 t4. We have shown there TM a presence of Ru +6 regardless of the surface orientation and reconstruction state. These aspects call for further study during which stable Ir compounds (III, IV, VI) have to be examined. In addition, the final state state screening is certainly contributing to the shape of the Ir 4f core lines which results in the appearance of the second satellite peak (C).
Concluding remarks The surface compositional analysis of mixed oxides layers of RuO2 and IrO2 has shown an enrichment with iridium which is more pronounced for the layers obtained from isopropanol solutions. The Auger analysis o f the surfaces is complicated
94
because of the overlapping of the ruthenium and iridium peaks and the presence of carbon. XPS analysis o f the valency state of iridium has shown a presence of a state of iridium higher than + 4 .
References I A Nidola, in Electrodes of Comhwtive Metallic Oxides (Edited by S Trasatti), Part B, Elsevier (1981). 2 (a) R Kotz, H Neff and S Stucki, J Electrochem Soc, 131, 72 (1984) ; (b) R Kotz and S Stucki, Electrochim Acta, 31, 1311 (1986). 3C Angelinetta, S Trasatti, Lj Atanasoska and R Atanasoski, J Electroanal Chem, 214, 536 (1986). 4C Angelinetta, S Trasatti, Lj Atanasoska, R Atanasoski and Z Minevski, Mater Chern Phys, 22, 231 (1989). 5L E Davis, N C MacDonald, P E Palmberg, G E Riach and R E Weber, Handbook of Auyer Electron Spectroscopy, PHI, Eden Prairie, MN (1976); C D Wagner, W M Riggs, L E Davis and J F Moulder, in Handbook of X-ray Photoelectron Spectroscopy (Edited by G E Muilenberg), Perkin Elmer, Physical Electronics Division, Eden Prairie, MN (1978). 6R Hutchings, K Muller, R Kotz and S Stucki, J Mater Sci, 19, 3987 (1984). 7M Hara, K Assami, K Hashimoto and T Masumoto, Electrochim Acta, 28, 1073 (1983). 8G Wertheim and H Guggenheim, Phys Rev B, 22, 4680 (1980). 9M Peuckert, Surface Sci, 144, 451 (1984). 0K Kim, C Sell and N Winograd, in Electrochem Soc Proc Series (Edited by M Breiter), p 242, Princeton, NJ (1974). i i B Folkenson, Acta Chim Scand, 27, 287 (1973). l:Ts Marinova and K Kostov, Surface Sci, 185, 203 (1987). ~3W Bell, M Tagami and R Inyard, J Phys Chem, 70, 2048 (1966). ~4Lj Atanasoska, W O'Grady, R Atanasoski and F Pollak, Surface Sci, 202, 192 (1988).
0042-207X/90S3.00+.00 Pergamon Press plc
Printed in Great Britain
XPS and AES study of mixed layers of RuO2 and IrO2 Lj A t a n a s o s k a , Institute of Technical Sciences, Serbian Academy of Sciences and Arts, Belgrade, Yugoslavia and R A t a n a s o s k i , Institute of Electrochemistry ICTM and Center for Multidisciplinary Study, University of Belgrade, Belgrade, Yugoslavia and S T r a s a t t i , Department of Physical Chemistry and Electrochemistry, University of Milan, Milan, Italy
The surface composition of RuO2+lr02 mixed oxide layers (1-2 #m thick) deposited on titanium by thermal degradation of the corresponding chlorides in aqueous or isopropanol solutions has been investigated. A surface enrichment with Ir, more pronounced for the layers obtained from isopropanol, has always been found. Electrochemical data reflecting the surface properties of RUOE+ lr02 layers confirm the excess of Ir02 at the surface.
Introduction
Results and discussion
Layers of mixed oxides on an inert substrate are widely used in the electrolytic industry ~. Mixtures of RuO2 and IrO2 are of particular interest for the reaction of oxygen evolution in acidic media 2. Recently, we have undertaken efforts to correlate the electrochemical properties of such mixed oxides with their bulk composition 3:. In order to comprehend the electrochemical behaviour of the mixed oxides an enrichment in iridium at the surface has to be assumed. Therefore to aid the understanding of the electrochemistry of the oxides surface analysis was required. XPS and AES results of mixed RuO2+IrO2 samples, confirming a surface enrichment in iridium, are reported.
Surface composition. Under the experimental circumstances of sample preparation, which were the same as for the electrochemical examination, the Auger analysis of both ruthenium and iridium requires some caution to be exercised. The AES of all samples showed that the surface was always heavily contaminated by carbon due to the prolonged contact with the furnace and laboratory atmospheres. In the Auger spectra, C (KLL) and Ru (MsN4.sN4,5) transitions appeared at 272 eV and 273 eV, respectively. As a consequence of the overlapping with the carbon signal, no adequately symmetrical peak of Ru at 273 eV could be obtained (Figure 1). The dominant Ir Auger signals for the NsN7N7 (158 eV), the NsN6N6 (164 eV) and the N4N6N6 (174 eV) transitions are partially hidden because of the superposition with the ruthenium peaks for the M 5N ,N 3 (156 eV) and the M 5N 3N 3 (188 eV) Auger transitions. The Ir (NsN704) Auger transition at 229 eV is overlapped by the Ru (MsN3Ns) transition at 231 eV. Besides, the Ir (NNN) and (NNO) Auger signals are an order of magnitude less
Experimental RuO 2q- I rO 2 layers were prepared on a thin plate of Ti by thermal decomposition (400°C ; air) of aqueous (later referred to as aqueous oxides) or isopropanol solutions (later referred to as nonaqueous oxides) of RuCI3 + IrC13 (Ventron) in the required mole ratio (see ref [3] for details in sample preparation). The samples were prepared at 10% tool composition intervals, from pure RuO2 to pure IrO2 included. A Riber UHV system equipped with an Auger cylindrical mirror analyser and an OPX 150 photoelectron spectrometer with a dual anode (Mg and A1) X-ray source were used. The Auger spectra were recorded with a primary electron beam accelerating voltage of 3 keV and an adsorbed beam current of 3 #A. The modulation amplitude of the phase sensitive detector was set at 5 eV peak to peak. The XPS spectra were taken by applying MgK, (hv = 1253.6 eV) irradiation as the photon source. Ion bombardment combined with the AES and XPS measurements was carried out with 3 keV Ar ions, emission current of I0 mA, and an argon pressure of 6 x 10 -3 Pa.
dN(E) x 2,5
LL)
Ir(MNN)
Figure 1. Auger spectrum of mixed layer o£ RuO=+ [rO= (60:40% mot).
91
LjAtanasoska et al:
l•
XPS and AES study of mixed layers of Ru02 and IrO2 1.5
3~1t-/
o AES o XPS
IS
3"4p3pz Ru3d5/2
LU
n X
<06 LL 03
o
8
3r4f?/2
1.0
3r415/2 3r~3f2
t
o
0.4
o
0.2
Q
0
05
rn
8
Rut,S L. Ru4p
764
Binding energy/eV
. ~
- -
Figure 2. XPS survey spectrum of mixed layer of RuO: + IrO2 (60:40% mol).
intense than the superimposed Ru (MNN) peaks. The weaker Ir transitions, MsNTN7 at 1908 eV and M 4 N T N 7 transition at 1981 eV, do not overlap with other signals in the spectrum. However, these transitions have a very low Auger sensitivity factor (S~, = 0.0275). Compared to the Ru (MsN4.sN,.s) transition with SRu = 0.5 this can obviously introduce a considerable error in the atomic concentration evaluation. Therefore, the Auger spectra were suitable only for qualitative composition estimation. The surface quantitative analysis was carried out by XPS. A typical example of the XPS survey spectrum for the 60% tool IrO2 admixture is presented in Figure 2. The Ru 3d and Ir 4f core level spectra were analysed by an iterative curve fitting spectral routine. In the fits the Gaussian-Lorentzian band type was selected with inelastic backscattering correction. It was assumed that the peak widths as full width at half maximum (FWHW) were 1.5+0.2 eV. The area under the deconvoluted peaks provided information about the relative concentration of each type of valence state. The relative concentrations of the various species were calculated from the corresponding photoelectron peak area after correction with appropriate sensitivity factors. The XPS atomic sensitivity factors for Ir 4f7/2 and Ru 3d5/2 photoelectron lines are of identical value (1.55). In the XPS spectra, the binding energy (BE) of the C ls (285.0 eV) and Ru 3d3/2 (284 eV) photoelectron lines coincide. Thus, the Ru 3dsn, which only partially interferes with carbon peak was used for quantitative analysis. The area of Ru 3d 5/2 was determined by deconvolution of Ru 3d region. For the XPS spectra the ratio of the Ru 3d5/2 to the Ru 3d~/2 core level peaks was accepted as a reliable criterion for the amount of carbon present in the surface region. The branching ratio of the Ru 3d spin-orbit doublet in absence of carbon is 3 : 2. The symmetry factor of the 273 eV Ru Auger peak (the ratio of the maximum positive excursion to the maximum negative excursion in the dN(E)/dE) and the 3d5/= to 3d3/2 XPS ratio for Ru depend on the coating composition. As Figure 3 shows, both parameters approach the literature value 5 as the IrO2 content vanishes. A coating composition dependent carbon contamination has been observed for both aqueous and nonaqueous mixed oxides. The carbon impurity, as found by AES depth profiling, is limited to 6 nm of the surface topmost layer for coating compositions of more than 50% tool RuO2. With increasing IrO2 content, the depth of contaminated oxides becomes more than 15 nm. A plot of the surface composition of aqueous and non-aqueous 92
0.5
D
I
%tool Ru
Figure 3. Symmetry factor of Ru 273 eV Auger peak and XPS ratio ot Ru 3d~.,2 to 3d3/: peaks as a function of the mixed RuO2+IrO2 layer nominal bulk composition.
mixed binary oxides as determined by XPS vs the nominal bulk composition is shown in Figure 4. An enrichment of the surface with Ir can be observed in both cases although the enrichment is definitely higher for the non-aqueous mixed oxides. This finding agrees with the results of Hutchings et al 6 despite the different preparation procedures. The results are similar also in the details since the deviations are more appreciable for low RuO2 content. The prevalence of Ir in the external surface of the grains may be related to the different affinities of the two metals for oxygen. Thermodynamically, the affinity of Ru for oxygen is higher than that of Ir 7. This, for instance, may result in a less positive potential for the incipient oxidation of Ru with respect to Ir 3. Consequently, the presumably lower rate of Ir salt oxidation by oxygen may result in a retarded deposition of IrO2 which eventually yields in a surface enrichment with lr.
0 nono,queous , / / " " " , , , , ~
5O
I
50
% toolRu (butk
Figure 4. Surface XPS vs nominal bulk composition for aqueous and non-aqueous mixed RuO2 + IrO2 layer.
LjAtanasoska et al: XPS and AES study of mixed layers of Ru02 and Ir02
The inhomogeneity of the surface composition may provide evidence for the inhomogeneity of the admixture itself. No enrichment has been reported for reactively sputtered samples 2. It thus appears that the surface enrichment is probably related to the mixture homogeneity ; the better the latter, the lower surface segregation of one of the components. The greater enrichment with Ir in the case of non-aqueous mixed oxides may thus indicate a lower degree of homogeneity in non-aqueous than in aqueous mixed oxides. Iridium valency state. The valance state analysis of as-received samples in the Ru 3d and O ls regions was difficult due to carbon contamination. On the other hand, a preferential removal of oxygen took place on sputtering, with consequent shift of Ru 3d and Ir 4f peaks towards BE values typical of the metallic state. Therefore, due to the lack of carbon and iridium peaks interference the analysis of the valence state of the elements in the oxides layer in as-received samples has been concentrated on the Ir 4f region. Figure 5 shows the XPS spectra of IrO2+RuO2 samples of different compositions. A sample treated at a higher temperature (500°C) is also included. The deconvoluted spectrum exhibits two major and three minor peaks located at about 61.2 (A1), 62.6 (BI), 64.2 (A2), 65.7 (B2) and 67.5 eV (C), respectively. The BE of the Ir 4f7:2 core level of metallic Ir is located at 60.9 eV and the splitting of the spin-orbit doublet is 2.95 eV 5. The branching ratio of the 4f lines is 4 : 3. In the case of IrO:, Ir 4f core level exhibits a very specific line shape. The Ir 4f5/2 peak is more intense than the 4f7/2 signal and the extraordinarily high intensity of the saddle is always observed. This is an indication of a case of two superimposed spectra. Thus far, an interpretation of the Ir 4f region based on a curve fitting analysis has been accomplished by Hara e t a l 7 and Wertheim and Guggenheim. s Peuckert 9 and Kotz e t a l 2 have
,
,
,
,
,
pointed out the considerable broadness of the Ir 4f peaks, especially the higher binding energy tail, with no attempt for the line shape deconvolution. There is considerable disagreement in the literature about the nature of the additional Ir 4f core overlapping peaks. Hara e t a l 7 have attributed the main peaks A to Ir(III) and the satellite peaks B to Ir(IV). They have claimed that in IrO2 powder the Ir203 content is significantly high (70%). This is in contrast especially with the finding by Rutherford backscattering analysis of IrO2 on a thin plate of Ti in which a composition IrO2+x(x > 0) was proposed 4. Peukert 9 has discussed the additional peaks B in terms of Ir(VI), most probably based on Kim e t al.'s l o assumption of a Ir(VI) state. Kotz e t a l 2 have pointed out the absence of any definite correlation between BE and Ir oxidation state. They speculate the presence of Ir(III) state in oxide films based on the location of BE for IrC13 reported by Folkesson 11. However in their results the shift of 1.5 eV for Ir 4f region in respect to metallic Ir is different than the value of 2.4 eV found by Folkesson. In a recent investigation of the interaction of oxygen with a clean Ir surface Marinova and Kostov ~2 do not consider the existence of iridium oxidation state lower than four. Wertheim and Guggenheim s have claimed, on the basis of Bell e t a l ' s work ~3, that IrO 2 is the only stable form of iridium oxide at room temperature and have ruled out the existence of valence states other than + 4 . They have attributed the peculiarities of the Ir line shape to the final state screening response and anomalous peak height of the 4f5/2 component to the long tail of the 4f7/2 line. By applying a perturbation theory treatment of the delectron manybody screening response, they have obtained a good agreement with the unusually asymmetrical core electron line shape. Table 1 summarizes the positions of peaks A and B for several samples of different compositions. By constraining only the F W H M and letting the peak position vary, amazingly reproducible results concerning the BE position and the relative percentage of the Ir species have been obtained. The spin-orbit splitting was not constrained and yet it was always found to be 3 eV, which is in a very good agreement with theory. Table 1 shows also that sputtering reduces Ir to the metallic state. The shift of peaks A in respect to Ir metal is 0.9 eV, which is in agreement with the reported literature value 5. The deconvolution procedure yielded a shift of peaks B in respect to A of 1.3 eV as compared to 1.7 eV found by Hara e t al 7. The branching ratio of the spin-orbit components for both the major and satellite peaks is close to the theoretical 4 : 3.
Z
Table 1. Iridium 4fv2 core level deconvolution
5 ~
0-70
e
-68
Nominal oxides composition (% mol IrOz)
)
-66
-~
BINDING
-62
fiNERY/,
-60
eV
5. Iridium 4f spectra as a function of the composition of the mixed RuO2 + IrO2 layers : (a) 20% tool IrO2 ; (b) 80% mol IrO2 ; (c) 70% mol IrO2 (500°C). Figure
20 40 60 80 100 100 (after 15 nm removed) 70 (500°C)
Major peak
Satellite peak
BE (eV)
Contribution (%)
BE (Ev)
Shift from major peak (eV)
61.2 61.3 61.3 61.1 61.2 60.3
76.7 73.0 73.9 69.0 71.0 88.0
62.7 62.7 62.7 62.3 62.5 61.3
1.4 1.4 1.3 1.2 1.3 1.0
61.3
61.3
62.5
1.2 93
LjAtanasoska et al: XPS and AES study of mixed layers of RuO2 and IrO2
The deconvolution procedure yielded an additional peak, C, which is due to the tailing on the high binding energy side and apparently without chemical back up. The appearance of peak C can be explained as a result o f the final state screening 8, i.e. conduction band interaction during the photoemission process 9. It is intriguing that the Ir peak positions do not shift with composition and temperature of preparation. This is taken to mean that Ir does not feel the presence of Ru which implies a heterogeneous structure o f the mixed oxide as assumed above. In our results, the magnitude of the satellite Ir peaks (B) obtained after deconvolution indicates a real presence of another oxidation state besides the + 4. The a m o u n t of the satellite feature B was found to be considerable, from 30% to 40% (Table 1). The shift of B peaks indicates that the other oxidation state has to be higher than 4. Due to the fact that both IrO2 and RuO2 crystallize in the same (rutile) crystal structure, an analogy can be drawn with our study of well defined single crystal surfaces of RuO2 t4. We have shown there TM a presence of Ru +6 regardless of the surface orientation and reconstruction state. These aspects call for further study during which stable Ir compounds (III, IV, VI) have to be examined. In addition, the final state state screening is certainly contributing to the shape of the Ir 4f core lines which results in the appearance of the second satellite peak (C).
Concluding remarks The surface compositional analysis of mixed oxides layers of RuO2 and IrO2 has shown an enrichment with iridium which is more pronounced for the layers obtained from isopropanol solutions. The Auger analysis o f the surfaces is complicated
94
because of the overlapping of the ruthenium and iridium peaks and the presence of carbon. XPS analysis o f the valency state of iridium has shown a presence of a state of iridium higher than + 4 .
References I A Nidola, in Electrodes of Comhwtive Metallic Oxides (Edited by S Trasatti), Part B, Elsevier (1981). 2 (a) R Kotz, H Neff and S Stucki, J Electrochem Soc, 131, 72 (1984) ; (b) R Kotz and S Stucki, Electrochim Acta, 31, 1311 (1986). 3C Angelinetta, S Trasatti, Lj Atanasoska and R Atanasoski, J Electroanal Chem, 214, 536 (1986). 4C Angelinetta, S Trasatti, Lj Atanasoska, R Atanasoski and Z Minevski, Mater Chern Phys, 22, 231 (1989). 5L E Davis, N C MacDonald, P E Palmberg, G E Riach and R E Weber, Handbook of Auyer Electron Spectroscopy, PHI, Eden Prairie, MN (1976); C D Wagner, W M Riggs, L E Davis and J F Moulder, in Handbook of X-ray Photoelectron Spectroscopy (Edited by G E Muilenberg), Perkin Elmer, Physical Electronics Division, Eden Prairie, MN (1978). 6R Hutchings, K Muller, R Kotz and S Stucki, J Mater Sci, 19, 3987 (1984). 7M Hara, K Assami, K Hashimoto and T Masumoto, Electrochim Acta, 28, 1073 (1983). 8G Wertheim and H Guggenheim, Phys Rev B, 22, 4680 (1980). 9M Peuckert, Surface Sci, 144, 451 (1984). 0K Kim, C Sell and N Winograd, in Electrochem Soc Proc Series (Edited by M Breiter), p 242, Princeton, NJ (1974). i i B Folkenson, Acta Chim Scand, 27, 287 (1973). l:Ts Marinova and K Kostov, Surface Sci, 185, 203 (1987). ~3W Bell, M Tagami and R Inyard, J Phys Chem, 70, 2048 (1966). ~4Lj Atanasoska, W O'Grady, R Atanasoski and F Pollak, Surface Sci, 202, 192 (1988).