J. MATER. CHEM., 1994,4(8), 1255-1258 1255

Characterization of Ru0,-based Film Electrodes by Secondary Ion Mass Spectrometry

Sergio Daolio," Bruno Facchin,a Cesare Pagura: Achille De Battisti,*b Andrea Barbierib and Janos Kristof a lstituto di Polarografia ed Elettrochimica Preparativa, Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4, 1-35020 Padova, Italy

Dipartimento di Chimica dell' Universita, via L. Borsari 46, 1-44 100 Ferrara, Italy Department of Analytical Chemistry of the University Egyetem 70, 8201 Veszprem, Hungary

Concentration depth profiles of Ti-supported Ru0,-TiO, films, prepared by pyrolysis of ruthenium(i1i) chloride hydrate and titanium diisopropoxide bis-pentane-2,4-dionate, have been studied by secondary ion mass spectrometry (SIMS). The ' " R u : ~ ~ T ~ ion intensity ratio vs. the nominal concentration of ruthenium oxide was followed for samples obtained from precursors dissolved in different solvents. The dependence on bombarding time of ion intensity profiles for other species, such as 0 -(m/z= 16), OH- (m/z= 17), 46TiO+ (m/z =62), '%OH+(m/z=63) was also studied. The mixed oxide films were characterized by cyclic voltammetry. The results of the combined research support the idea that microstructural and chemical impurities are concentrated in the outermost part of the film. This phenomenon seems to depend on the composition of the solvent used for the precursor salts.

The mechanism of charging of oxide electrodes has been studied in several papers, for the case of anodically prepared14 or thermally prepared7-12 films. According to the literature, in this process important roles are played both by electron and ion transport in the electrode materials. This is common to many different oxide systems. From a quantitative point of view, the transport properties themselves depend on the microstructure and surface morphology and consequently on a number of parameters of the preparation method. For the case of thermally prepared IrO, and RuO,, which are of interest for their catalytic properties, correlations between the conditions of the pyrolysis of the precursor salts, the annealing of oxide products and the anodic voltammetric charge q*, have been de~cribed.~ For 1r02, the influence of the nature of the solvent of the precursor salt on the electrochemical properties of the final material has also been considered." Correlations between different parameters of the preparation process and physicochemical properties of one-component systems have been In particular, it has been shown that enrichment with hydroxylated species occurs across the first tens of nm below the surface,16 involving segregation of microstructural defects in this region. This study has been extended to Ru0,- and Ir0,-based mixed oxide e l ec t rode~ . '~ -~~ In this case, the role of the film composi- tion and of the nature of the precursor salt has been con- sidered. Enrichment with Group VA or IVA metal oxide species (e.g. TaV,l7 Ti'v18,19 ) could be ascertained, also implying an increase in the number of defects in the near-surface region. This paper presents the results of a further investigation on this subject, carried out by SIMS on Ru0,-based mixed oxide electrodes. SIMS has already proved to be a reliable tool for depth profiling of main and secondary components in mixed oxide electrodes.21,22 An electrochemical study of the electrode coatings has been performed by cyclic voltammetry, comp- lementary to the SIMS characterization, and a measure of their catalytic activity.8

Experimental

The SIMS equipment was custom-built and has been described e l s e ~ h e r e . ~ ~ ' ~ ~ The samples of electrode materials were intro- duced by a fast insertion lock and exposed to an Ar' beam

whose energy ranged between 1 and 5 keV and current density between 3 and 30mAcm-2. The rate of sputtering of the electrode materials was calculated to be 100-200 A min-'. The calibration curve of ruthenium ionic current was obtained from samples prepared by dropping solutions of precursor salts in the chosen ratios onto platinum supports and then evaporating the solvent at room temperature. The same bombardment conditions used for the analysis of the electrode films were applied. The electrode films were prepared accord- ing to a well established procedure, allowing good rcproduc- ibility of the film proper tie^.,^ Layer-by-layer growth of the mixed oxide films was carried out by depositing small .imounts of precursor salt mixture, firing at 400 "C under oxj gen and repeating the procedure a number of times sufficient to allow the attainment of the desired loading (corresponding in our case to a thickness of 300-400 nm). Mirror-finished citanium supports were used for the oxide films. In order to compare the influence of the preparative conditions on the final elec- trode properties, three groups of samples were prepared. Two of them, A and B, differ in the nature of the precursor salt of the ruthenium oxide. In the case of group A, RuCl3*3H2O was chosen and for group B, R u ( N O ) ( N O ~ ) ~ . Isopropyl alcohol was used as the solvent of the precursor mixtures in both cases. Group C samples were prepared from the same precursor salt mixture as group A, using an aqueous organic mixture acidified with hydrochloric acid. In all cases titanium diisopropoxy bis-pentane-2,4-dionate was used as the precur- sor of titanium oxide. The cyclic voltammetry experiments were carried out making use of a Solartron 1286 electrochemi- cal interface. The electrodes were tested in 1 mol dm-3 HC104, in the potential range 0-1.2OV us. SCE. Different potential scan rates in the range 0.010-0.300 V s-l were used.

Results and Discussion

Fig. 1 shows typical SIMS patterns (Ar' beam energy 3 keV) for a sample belonging to group A, containing 40% of RuO,. The positive-ion mass spectrum [Fig. 1 (a)] shows the presence of Ti', TiO', TiOH+ and Ru' ions. Other oxides and isotopic clusters are also present, with lower relative abun- dances. TiO,f, TiOOH', TizO+, Ti,Oi, Ti;, RuO', KuOH+, Ru: , Ru20 + , TiRu+, TiRuO ' and TiRuOl were identified

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J. MATER. CHEM., 1994, VOL. 4 1256

80-l I/ 100

x 10

150 200 250

m /u

Fig. 1 Secondary positive (a) and negative (6) ion mass spectra of a group A sample (40% of RuO,). The spectra were obtained by an 800 nA, 3 keV Ar' primary current bombardment

01 I

0.3 0.6 1 2 3 primary ion energylkev

Fig. 2 0- :OH- ion intensity ratio us. primary ion energy (same sample as in Fig. 1)

with the aid of an isotopic pattern simulation program.26 The contributions of different ionic species to partially superim- posed isotopic patterns were thus estimated. In the negative- ion mass spectrum [Fig. l(b)], the most interesting peaks are due to the ionic species H-, 0-, OH-, O,, C1-, Ti02-, Ti03-, T i020H- , Ti02-, Ti04-, Ru0,- and Ru03-, whereas other low-mass negative ions are probably due to the sample treatment. Electrode films with ruthenium concen- trations between 5 and 90mol% gave similar spectra and only the relative abundance of different isotopic patterns changes with the coating composition, the nature of the precursor or the solvent composition.

The study of selected ionic species showed the destructive effect of higher-energy primary beams. Fig. 2 shows a plot of the 0- : OH- ion intensity ratio us. primary ion energy. The ion intensity ratio increases with the bombarding energy. This information has to be borne in mind for the choice of the experimental conditions for depth-profile studies.

The depth-profiling of ruthenium and titanium ions in the electrode films were studied first. For this purpose, the depen- dence of the lo2Ru : 48Ti ion intensity ratio on sputtering time was determined. A calibration curve, allowing the direct

conversion of ion current ratios into Ru atom fractions (RuO, molar fractions), was obtained as described in the Experimental section. Fig. 3 shows the good correlation between the SIMS data and the nominal composition of the samples. Fig. 4(u) shows a typical concentration depth profile for a group A sample (20 mol% Ru02). An enrichment with titanium oxide species is evident, in agreement with results obtained with other methods for other RuO2-based''~'* or Ir0,-based electr~catalysts.'~

The depth profiles of group B electrodes show larger amounts of noble-metal species in the near-surface region, compared with group A electrodes. As shown in Fig. 4(b), for a group C electrode (10 mol% Ru02), surface segregation of ruthenium oxide species occurs when an acidified aqueous organic solvent is used for the preparation of the RuCl,.3H20 solutions. As previously mentioned, other authors ascertained important effects of the composition of the solvent of precursor salts on oxide film morphology and electrochemical behav- i o ~ r . ' ~ , , ~ In our case, the different acidity of the two solutions can change the mechanism of formation of gel precipitates during the stage of solvent evaporation (see Experimental).

0.1J

0.09 -

[r 0.06,

E

0.03

1 0 - 10 30 50 Ru (mol "/o)

Fig. 3 lo2Ru : 48Ti ion intensity ratio us. Ru concentration (mol%) for mixed precursor salt deposits. Arf beam energy 3 keV.

0.043

2 cn 0.011

r: 0

1 1

100 200 300 400 500 .-

100 200 300 400 500 t (arb. units)

Fig. 4 Typical concentration-depth profiles (Ru+ : Tif). (a) Samples of group A (isopropyl alcohol precursor solution). (b) Samples containing 10 and 20mol% of RuO, from acidic aqueous-organic precursor solution.

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J. MATER. CHEM., 1994, VOL. 4 1257

I

t

Fig.5 Ion intensity profiles (arbitrary scale for the ordinate) us. bombardment time of: 0- (m/z = 16), Ti' (m/z = 46), 46TiO+ (m/z = 62), "TiOH' (m/z = 67), obtained for an electrode containing 20 mol% of ruthenium dioxide, belonging to group A. Primary Ar+ beam: 4 keV and 25 mA cm-2.

During subsequent stages (ageing, pyrolysis), some of the features of the precipitate are retained. Much experimental work is still required to clarify this point.

Besides the distribution of the main components, the degree of hydration of the films was followed using the OH-, 0-, TiO', TiOH+, Tif profiles as monitoring parameters. Fig. 5 shows that the 46TiOf : "TiOH' (m/z =62 and 67) ion inten- sity ratio remains constant during the sputtering time and OH- ion current profiles (m/z = 17) exhibit an increase towards asymptomatic values. This indicates that hydroxylated metal species are not directly linked with the OH - species which probably originate from trapped water molecules. A systematic study of electrodes of different composition shows that the part of the film containing hydroxylated metal species is larger for electrodes containing 20 and 30 mol% RuO,. The influence of the solvent composi- tion on the TiOH+:TiO+ ion intensity ratio is relatively small. In any case, the shape of concentration depth profiles of metal ions and hydroxylated species in all the samples studied implies a defective microstructure in their outermost part.

Further information on this aspect has been obtained, in situ, by cyclic voltammetry. A typical voltammogram is shown in Fig. 6 which was obtained from a sample belonging to group A (30 mol% RuO,), in 1 mol dm-3 HC104. A single pair of peaks (anodic/cathodic) is observed and is due to the solid state redox couple Ru'~/Ru'''.~ Making use of the baseline shown in the figure,20 to a first approach the faradaic contri- bution to the total charge-storage capacity can be separated from the mainly capacitive one. The dependence of the anodic peak charge on the noble-metal nominal concentration in the electrode films (group B), is shown in Fig. 7. From this it is possible to draw the conclusion that the total number of electroactive sites in the oxide films is maximum at intermedi- ate concentrations of noble metal. Analogous results were obtained in a preliminary study on films prepared as those of group C in this work2' and also for IrO,/TiO, films." A tentative explanation for this occurrence has been sought in the larger degree of microstructural defectivity of mixed oxide films with lower noble-metal contents." SIMS results have shown that titanium oxide segregation effects are more pro- nounced for low to intermediate nominal concentrations of noble-metal oxide. This should further enhance the defective character of their surface region. It has been shown18'19 that the composition of mixed-oxide films in the near-surface region is not strongly affected by the nominal bulk composi-

10

5

N

' a 5

-5

-1 c 4

1

t

I I 1

5 0.0 0.5 1.0 1 E N vs.SCE

5

Fig. 6 Typical voltammogram obtained with a group A sample (30 mol% RuO,). Solution: 1 mol dmP3 HClO,; reference electrode SCE; potential scan rate 0.100 V s-'.

tion. It is therefore reasonable to admit that, when there is an almost constant number of electroactive sites (in our case Ru ions) in the film surface region, the degree of porosity, a consequence of the microstructural defectivity, becomes the main factor controlling the charge-storage capacity. Making use of the previously mentioned simplifying assumpticm about the contributions to the total voltammetric charges, integral capacities of the different electrodes can be calculated. Maximum values are still found for samples containing 30-50 mol% ruthenium dioxide. The integral capacit Y, calcu- lated from the anodic part of the voltammogramh, varies between a maximum of 160 F rn-' and a minimum of 58 F m-2 (geometric area). Evaluation of roughness factors can be attempted, making use of data of specific capacity available in the literature. A value of 0.6 F rn-' has been proposed for r ~ t i l e . ~ ~ ' ~ ~ More recently a value of 0.8 F rn-' was proposed by Trasatti and co-workers31 for RuO,. Using the tatter, a roughness factor of 200 can be estimated for a sample contain- ing 30 mol% of RuO,. The lower limit is then 70 (80 mol% Ru02). In Fig. 7 the results of the normalization of peak charges to the effective electrode areas are reported. Some

20

15 cu

E 0 3 10 h f

Cr" v

5

a 0 20 40 60 80 100

film composition (Ru) (mol %)

Fig. 7 Dependence of the voltammetric (anodic) peak charge on the noble-metal oxide concentration in the electrode film (group A). (qan)ap; peak charge normalized to the geometric electrode area; (qan)=+ peak charge normalized to the effective electrode area.

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1258 J. MATER. CHEM., 1994, VOL. 4

residual effect of the nominal noble-metal concentration can still be observed, although they are reduced in comparison with the results referred to the geometric area. Larger surfaces are likely to favour injection/ejection of counter-ions, in our case protons, during changes in oxidation state of the elec- troactive sites in the films. In this case a linear dependence of peak currents on sl', is expected, at least in the case of larger peak charges. This has been verified for some anodic iridium oxide coating^,^,^^ for which the involvement of the volume of the electrode in the charging process has been extensively d ~ c u r n e n t e d . ~ . ~ ~ A similar situation has been met for ther- mally prepared IrO,-TiO, mixed oxide electrodes containing < 50 mol% IrO,.,' In our case, an accurate analysis showed that the peak currents are linear in s. For Ru0,-TiO, films, therefore, no evidence for control of the charging process by proton diffusion in the films has been found.

Conclusions The depth-profiling results obtained by SIMS for the two major components, Ru and Ti, agree substantially with those obtained by XPS coupled with Ar+ etching and a non- destructive technique like RBS, for a limited number of electrode compositions. At this level of the study we can therefore exclude artifacts, like preferential sputtering, due to some specific aspect of the interaction between the ion beam and the oxide matrix. The linearity of the calibration curve in Fig. 3 also favours of this assumption. Therefore, the sensitivity of the method can be fully exploited in the study of the distribution and changes of concentration of minor components, residual from incomplete decomposition of pre- cursors. This also holds for the small amount of counter-ions exchanged between electrode and solution under polarization. A study of the type and distribution of ion clusters emitted from the sample during the ion-etching can also supply more information on chemical bond features at different depths in the films.

The shape of the concentration depth profiles supports the idea that the evolution from the precursor salt film to the stoichiometric and crystalline microstructure of the final mixed oxide product is slow. Some evidence of this has been discussed already in the case of pure IrO, films on the basis of thermoanalytical and microstructural datal3'l4 and a detailed X-ray diffractometric study. lS Although a larger average crys- tallite size is generally found for RuO,, large carbon and hydrogen contents have been detected by nuclear reaction analysis and elastic recoil detection in Ru0,-TiO, films prepared by the method described in this paper.33 These films also exhibited densities as low as 3 . 0 g ~ m - ~ , although the X-ray pattern of the Ru0,-TiO, solid solution could be observed and, from the peak shape, average crystallite sizes of 15-18 nm were estimated. These data, together with the results of the present work, suggest that crystallites of the Ru0,-TiO, solid solution coexist with large intergranular areas. In the latter, besides Ru, Ti and 0, other components are accumulated in relatively large concentrations and the microstructure becomes highly defective. The porosity of these regions must be responsible for the low average density observed in other studies.33 Any further increase in the crystallite size is more or less hindered by the amorphous

microcrystalline region. This can be thought of as a residual of the gel-like structure of the precursor precipitates on the metal support, following the solvent evaporation stage. The large roughness factors obtained from cyclic voltammograms are better explained by assuming large portions of gel-like structure in the electrode films.

References

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Paper 3/07638E; Received 3 1st December, 1993

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