1498
Langmuir 1999, 15, 1498-1502
Investigations on the Formation of RuO2/ZrO2-Based Electrocatalytic Thin Films by Surface Analysis Techniques J. Kristo´f,*,† S. Daolio,‡ A. De Battisti,§ C. Piccirillo,‡ J. Miha´ly,† and E. Horva´th| Department of Analytical Chemistry, University of Veszpre´ m, P.O. Box 158, H-8201 Veszpre´ m, Hungary, Istituto di Polarografia ed Elettrochimica Preparativa, Consiglio Nazionale delle Ricerche, Corso Stati Uniti, 4, I-35127 Padova, Italy, Dipartimento di Chimica dell’Universita´ , via L. Borsari, 46, I-44100 Ferrara, Italy, and Research Group for Analytical Chemistry, Hungarian Academy of Sciences, P.O. Box 158, H-8201 Veszpre´ m, Hungary Received May 12, 1998 Secondary ion mass spectrometry (SIMS) was used to follow the evolution of RuO2/ZrO2 film electrodes. The coating mixtures with compositions 20% Ru + 80% Zr and 50% Ru + 50% Zr prepared on titanium supports from isopropanolic solutions of RuCl3 × 3H2O and ZrOCl2 × 8H2O precursors were heated to 200, 300, and 500 °C and analyzed by SIMS. Cl--concentration depth profiles as ion intensity versus sputtering time curves showed the hydrolytic conversion of the ZrOCl2 precursor in the outer part of the film at low temperature and noble metal content and a rather uniform distribution at elevated temperatures. Zr+/Ru+ ion intensity ratios showed the relative enrichment of ruthenium in the near surface region at 500 °C, while slight accumulation of zirconia at the surface was evidenced for both compositions in harmony with the results of emission FTIR measurements. No reaction between the oxide components or between coatings and support was identified in the systems investigated.
Introduction RuO2-based mixed-oxide electrodes have a quite consolidated application in industrial chlorine production.1-3 The noble-metal oxide affords the electrode material good electric conductivity and catalytic activity. Other components, such as TiO2 and Ta2O5, essentially stabilize the performance of the catalytically active agent. Among the different possible stabilizing components, the effect of additions of TiO2 has been studied in several papers since the middle of the 1970’s1-8 due to the accepted importance of this component in industrial dimensionally stable electrodes. Release of gas products due to precursor pyrolysis, incorporation of chemical impurities, and segregation of components to the outermost part of the electrode film seem to enhance the porosity of the mixed-oxide films and therefore, their apparent catalytic activity.9-11 The formation of solid solution between the two oxide * Corresponding author. Phone/fax: +36 88 421 869. E-mail:
[email protected]. † Department of Analytical Chemistry. ‡ Istituto di Polarografia ed Elettrochimica Preparativa. § Dipartimento di Chimica dell’Universita ´. | Research Group for Analytical Chemistry. (1) Trasatti, S.; Lodi, G. In Electrodes of Conductive Metallic Oxides; Trasatti, S., Ed.; Elsevier Science Publishing Company: Amsterdam, 1980; p 301. (2) Trasatti, S.; Lodi, G. In Electrodes of Conductive Metallic Oxides; Trasatti, S., Ed.; Elsevier Science Publishing Company: Amsterdam, 1981; p 521. (3) Novak, D. M.; Tilak, B. V.; Conway, B. E. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1982; Vol. 14, p 195. (4) Hine, F.; Yasuda, M.; Yoshida, T. J. Electrochem. Soc. 1977, 124, 500. (5) Augustynski, J.; Balsenc, L.; Hinden, J. J. Electrochem. Soc. 1978, 125, 1093. (6) Gerrard, W. A.; Steele, B. C. H. J. Appl. Electrochem. 1978, 8, 417. (7) Burke, L. D.; Murphy, O. J. J. Electroanal. Chem. 1980, 112, 39. (8) Roginskaya, Yu. E.; Galyamov, B. Sh.; Belova, I. D.; Shiffrina, R. R.; Kozhevnikov, V. B.; Bystrov, V. I. Elektrokhimiya 1982, 18, 1327.
components can also play an important role. Its occurrence has been demonstrated for the case of ruthenium oxidetitanium oxide system in thorough microstructural investigations. On the other hand, assessment of the extent at which the different factors influence surface texture and microstructure of the oxide film is of practical importance, allowing optimal choice of precursors and of the electrode film composition. In this context an extension of the study to different synthesis procedures, making use of precursors of different nature, can be of interest. ZrOCl2‚8H2O precursor reactivity was already studied.12,13 In addition, properties of the IrO2-ZrO2 system were also considered.14 Due to the interesting results and to compare noble metal behavior, the same kind of study of the RuO2/ZrO2 system is reported here. The characterization of formation and surface properties of the mixed-oxide film catalysts has been carried out by thermal analysis and emission Fourier-transform infrared (FTIR) spectroscopy. Complementary indication on the distribution of components across the films has been obtained by secondary ion mass spectrometry (SIMS). Experimental Section Coatings of the composition 20% Ru-80% Zr and 50% Ru-50% Zr were prepared on titanium strips (size 4 × 10 (9) Daolio, S.; Facchin, B.; Pagura, C.; De Battisti, A.; Barbieri, A.; Kristo´f, J. J. Mater. Chem. 1994, 4, 1255. (10) Kristo´f, J.; Liszi, J.; De Battisti, A.; Barbieri, A.; Szabo´, P. Mater. Chem. Phys. 1994, 37, 23. (11) De Battisti, A.; Battaglin, G.; Benedetti, A.; Kristo´f, J.; Liszi, J. Chimia 1995, 49, 17. (12) Miha´ly, J.; Kristo´f, J.; Mink, J.; Nanni, L.; Patracchini, D.; De Battisti, A. Mikrochim. Acta 1997, 14, 617. (13) Daolio, S.; Kristo´f, J.; Piccirillo, C.; Gelosi, S.; Facchin, B.; Pagura, C. Rapid Comm. Mass Spectrom. 1996, 10, 1769. (14) Daolio, S.; Kristo´f, J.; Mink, J.; De Battisti, A.; Miha´ly, J.; Piccirillo, C. Rapid Comm. Mass Spectrom. 1996, 10, 1881. (15) Pagura, C.; Daolio, S.; Facchin, B. In Secondary Ion Mass Spectrometry SIMS VIII; Benninghoven, A., Jansen, K. T. F., Tumpner, J., Werner, H. W., Eds.; John Wiley: Chichester, 1992; p 239.
10.1021/la980561s CCC: $18.00 © 1999 American Chemical Society Published on Web 01/26/1999
RuO2-Based Electrocatalytic Thin Films
mm2, thickness 0.1 mm) from 0.05 M isopropanolic stock solutions of RuCl3‚3H2O and ZrOCl2‚8H2O (Fluka, Buchs, Switzerland) precursors. To avoid hydrolysis, the solutions were stabilized by the addition of concentrated hydrochloric acid (5% v/v). Prior to deposition, the strips were etched in boiling oxalic acid (10%) for 15 min, washed with distilled water, rinsed with acetone, and dried at room temperature. The coatings were prepared by applying the precursor solution mixture (after a 10-fold dilution with isopropanol) drop-by-drop onto the support and removing the solvent by hot air (60 °C). This procedure was continued until a uniform, relatively thick (400-800 nm) layer was formed. Thermoanalytical investigations and the heat treatment of the gel-like coatings to specified temperatures were carried out in a derivatograph PCtype thermoanalytical instrument (Hungarian Optical Works, Budapest, Hungary) at a heating rate of 5 °C/min in a dry oxygen atmosphere. Emission FTIR measurements were carried out by means of a Bomem MB-102 type instrument with CsI beamsplitter using a room temperature DTGS detector with a CsI window. The sample was placed on a polished steel sample stage in the horizontal position and heated to a temperature of 180 ( 0.5 °C. Emitted radiation from the heated sample was collected and collimated by an off-axis paraboloid mirror and sent to the spectrometer. Two thousand scans were made at a resolution of 4 cm-1 in all cases. SIMS investigations were performed in a custom-built instrument described elsewhere.15 A monochromatic (110 KeV) O2+ ion beam (collimated to 50-1000 µm) was generated in a mass-filtered duoplasmatron ion gun (model DP50B, VG Fisons, Loughborough, U.K.). The secondaryion optics were of the three-lens design, with a central stop, interfaced with a Balzers (Balzers, Liechtenstein) model QMA 400 quadrupole mass analyzer (mass range extended up to m/z 2048). A secondary electron multiplier (90° off-axis) was used for negative- and positive-ion detection in the counting mode. Lens potentials, quadrupole electronic units, and the detection system were controlled via a Balzers QMS 421 unit. The control of the instrument and the collection of data were carried out by two personal computers connected to a local area network. Sputtering of the coating material was carried out at 2 KeV O2+ primary ion energy and at an ion current of 400800 nA. Results and Discussion The thermogravimetric (mass loss, TG) and derivative thermogravimetric (rate of mass loss, DTG) curves of the coating system corresponding to the composition 20% Ru80% Zr are shown in Figure 1. The thermoanalytical study of the precursors alone (one-component systems) can contribute substantially to the understanding of the behavior of the mixed oxide coatings. As to the decomposition of the ZrOCl2‚8H2O film, it was found that an intramolecular hydrolysis takes place along with the liberation of crystallization water.12 It means that part of the chloride content is released as HCl (as a hydrolysis product) thereby contributing to the formation of an “oxidic” phase at very low temperatures. Hydrated RuCl3 as a film on Ti support is decomposed to RuO2 in an exothermic reaction at around 350 °C.10 In the light of preliminary thermoanalytical studies, the thermal decomposition pattern of the two-component system can be interpreted as follows. Up to about 250 °C residual solvent, crystallized water and HCl are released from the film making up about 27% of the total mass loss observed. In
Langmuir, Vol. 15, No. 4, 1999 1499
Figure 1. Thermoanalytical curves of the 20% Ru-80% Zr system on titanium support (TG, mass loss, DTG, rate of mass loss).
the 250-350 °C range, the predominating process is the formation of ZrO2. The sharp mass loss stage at 353 °C belongs to the conversion of RuCl3 to RuO2. Since most of the chloride content of the Zr precursor is lost as HCl at a relatively low temperature range, this hydrolytic decomposition can be responsible for the early development of the zirconia phase. Hydrated IrCl3 is also subject to hydrolysis, but to a limited extent, only.16 The thermoanalytical curves of the 50% Ru-50% Zr system show similar behavior with the exception that the conversion temperature of RuCl3 is reduced to 341 °C due to the excess heat released in this exothermic reaction. Based on the thermal decomposition pattern, coatings were previously heated to 200, 300, and 500 °C in the thermobalance and analyzed by SIMS. Table 1 reports the most important positive- and negative-ion species detected in surface mass spectra of both films at different temperatures. Ru-, Zr-, and Ti-containing species can be identified in the positive-ion spectra taken at all the three temperatures. At 200 °C, however, OH+ and H2O+ peaks (indicating the presence of water in the system) are observed as well. Analogously to the IrO2-ZrO2 thin films, the negative-ion spectra give more information. Ions, such as O-, Cl-, ClO-, Cl2-, TiO3-, ZrO-, ZrO2-, ZrO3-, ZrO2Cl-, Zr2O2-, Zr2O3-, RuO2-, and RuO3- give the most intense peaks. Traces of organics can be found even at higher temperatures (as indicated by fragments, e.g., CHn-, C2Hn-) entrapped in the films. For both compositions, zirconium-containing species dominate over rutheniumcontaining ones. This is a general observation valid at all temperatures, not experienced in the IrO2-ZrO2 system. It is interesting to see that higher ionic yields were obtained for ZrO+ than for Zr+ ions. Generally, this behavior is anomalous for a metal but it is characteristic of zirconium. In fact, it was observed in ZrO2 and IrO2ZrO2 thin films, too. The higher stability of ZrO+ ions can be due to the high Zr-O bond energy.17 Comparing the spectra taken for ZrO2 alone and for the ZrO2-RuO2 system, it can be seen that zirconium ionic yield is only slightly affected by ruthenia: in fact, the only change evidenced for Zr-related signals is the presence of more chloride-containing Zr species. On the contrary, (16) Kristo´f, J.; Miha´ly, J.; Mink, J.; Daolio, S.; De Battisti, A.; Nanni, L.; Piccirillo, C. J. Electroanal. Chem. 1997, 431, 99. (17) Pedley, J. B.; Marshall, E. M. J. Phys. Chem. Ref. Data 1984, 12, 967.
1500 Langmuir, Vol. 15, No. 4, 1999
Kristo´ f et al.
Table 1. Most Important Ionic Species Observed (‚) by SIMS from Films Obtained by Coating a Titanium Support with Mixtures of Hydrated RuCl3 and ZrOCl2 and Heated to 200, 300, and 500 °C m/zc
temperature species found
12 13 16 17 18 24 25 26 32 35 48 51 52 64 70 80 90 96 102 106 122 125 134 138 139 141 150 157 172 192 212 215 228 244 260
C CH O OH H2O C2 C2H C2H2 O2 Cl Ti ClO ClOH TiO Cl2 TiO2 Zr TiO3 Ru ZrO ZrO2 ZrCl RuO2 ZrO3 ZrO3H ZrOCl RuO3 ZrO2Cl RuCl2 ZrO2Cl2 Zr2O2 Zr2Cl Zr2O3 Zr2O4 Zr2O5
200 °C positive
‚ ‚ ‚
‚ ‚
300 °C negative ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚
negative
‚ ‚a
‚ ‚ ‚ ‚
‚ ‚
‚ ‚ ‚
‚ ‚ ‚ ‚
‚ ‚ ‚ ‚
‚ ‚ ‚ ‚ ‚b
‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚
‚ ‚b
‚b
negative
‚
‚ ‚b ‚
‚
‚ ‚ ‚ ‚ ‚
‚
‚
positive
‚
‚
‚ ‚ ‚ ‚
500 °C
positive
‚ ‚
‚ ‚ ‚ ‚b
‚ ‚
‚a ‚ ‚
‚ ‚
‚a ‚
‚ ‚b
‚
‚
‚ ‚
‚
‚b ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚ ‚
‚b
a Not observed for the 20% Ru-80% Zr system. b Not observed for the 50% Ru-50% Zr system. c Mass numbers are calculated by considering the most abundant isotope for each element, i.e., 1H, 12C, 16O, 35Cl, 48Ti, 90Zr, and 102Ru.
more differences were seen for the Ru-containing phase: ionic species like RuO+, Ru2Ox+/- (x ) 0, 1, and 2) and Ru2C+ detected for the RuO2 one-component system18 are not present here. Since in previously studied systems with TiO2 as the valve metal oxide,19,20 no decrease in the ionic yield of Ru-containing species was observed; this anomaly is the result of a different matrix effect due to zirconia. No ruthenium- and zirconium-containing mixed clusters were sputtered out of these films on ion bombardment, indicating that no reaction between the two oxide components of the films takes place. This is in harmony with the chemical inertness of zirconia; the same behavior was observed for the IrO2-ZrO2 system as well. A further proof of such an inertness is given by the lack of zirconiumtitanium-mixed clusters in the spectra; rutheniumtitanium ionic species are not detected as well, indicating that in this matrix ruthenium shows low reactivity, too. Concentration depth profiles as ion intensity versus sputtering time curves give valuable information on the film evolution process and on the distribution of species as a function of depth at different temperatures. By changing the ion beam diameter between 50 and 1000 µm, it is possible to obtain “average” concentration values for ionic species sputtered out of the films. In this way the effect of inhomogeneity of the film on relative ion intensi(18) Kristo´f, J.; Daolio, S.; Piccirillo, C.; Facchin, B.; Mink, J. Surf. Sci. 1996, 348, 287. (19) Daolio, S.; Fachin, B.; Pagura, C.; De Battisti, A.; Barbieri, A. Rapid Comm. Mass Spectrom. 1994, 8, 659. (20) Daolio, S.; Kristo´f, J.; Piccirillo, C.; Pagura, C.; De Battisti, A. J. Mater. Chem. 1996, 6, 567.
ties can be minimized. Figure 2 shows the Cl- ion intensity curves, as a function of the sputtering time, for both film compositions at different temperatures. The gradual decrease in the Cl- ion intensities with the increase of the firing temperature is in complete harmony with the development of the oxide coating. Chloride distribution shows a rather uniform pattern in the films with the exception of the 20% Ru-80% Zr system at lower temperatures where a depletion can be observed in the outer part of the films. It can be supposed that the hydrolytic conversion of the hydrated ZrOCl2 component is more advanced in this region of the coating. The Cl- ion signal at 500 °C is more intense for the first system than for the second one, indicating a larger amount of residual chloride at high temperature. Because of the higher ZrOCl2 content in such coatings, it can be supposed that the conversion of the Zr precursor salt to the corresponding oxide is not complete or it could be slower at lower ruthenium contents. Also, the detection of Zr-Cl mixed clusters and the lack of Ru-Cl species at 300 and 500 °C (see Table 1) may be a further indication of residual ZrOCl2 as the main source of chloride ions. The Zr+/Ru+ ion intensity ratios at 500 °C in Figure 3 give valuable information on the distribution of the noble metal related to the stabilizing component. For both compositions a relative enrichment of ruthenium can be observed in the near-surface region of the films. At the surface, however, a slight accumulation of zirconia is evidenced for both compositions. It can therefore be concluded that the increase of the electroactive component
RuO2-Based Electrocatalytic Thin Films
Langmuir, Vol. 15, No. 4, 1999 1501
Figure 4. Emission FTIR spectra of the 20% Ru-80% Zr (curve A) and the 50% Ru-50% Zr (curve B) systems at 500 °C.
Figure 2. Cl- concentration depth profiles of the 20% Ru80% Zr (pattern A) and the 50% Ru-50% Zr (pattern B) systems at different temperatures.
Figure 3. Zr+/Ru+ ion intensity ratios for the 20% Ru-80% Zr (curve A) and the 50% Ru-50% Zr (curve B) systems at 500 °C.
Figure 5. Zr+/Ti+ and Ru+/Ti+ ion intensity ratios for the 20% Ru-80% Zr (pattern A) and the 50% Ru-50% Zr (pattern B) systems at 500 °C.
does not cause drastic changes in the distribution of the main components in the films. This observation is of high importance in the light of the behavior of the IrO2-ZrO2 system, where the segregation pattern was changed to the opposite in this concentration range with the increase of the noble metal content.14 The similarity in the distribution of species for both compositions can also be witnessed by emission FTIR measurements (Figure 4). The recorded infrared spectra show close similarity. The bands at 1060 (1058) and 1008 (1002) cm-1 belong to surface OH groups, while in the lower frequency range a superposition of ZrO2 bands (at 644 and 440 cm-1 14) on the characteristic band of RuO2 at 449 cm-1 can be observed. The electrochemical activity and mechanical stability of the electrodes are strictly related to the penetration characteristics of the electroactive component and the stabilizing agent into the support. This feature of the films
can be studied by considering the Zr+/Ti+ and Ru+/Ti+ ion intensity ratios as a function of the sputtering time (Figure 5). Zr penetration profiles show that the behavior of the stabilizing component practically does not change with the decrease of concentration. In fact, both ratios have comparable intensities; the decrease in deeper layers of the coatings to very low values is observed in less than 1000 seconds. On the contrary, differences can be experienced for noble metal penetration: curve of the 50% Ru-50% Zr system is about 1 order of magnitude higher in intensity than curve of the 20% Ru-80% Zr system. Furthermore, with increased ruthenia content, the ratio shows a maximum at approximately 100 s and then has a slow decrease toward the bulk. It means that ruthenium penetrates deeper in the support than in the other system, and this difference in the coating-support interface can affect electrocatalytic properties. Support material migra-
1502 Langmuir, Vol. 15, No. 4, 1999
tion takes place as well: in fact it can explain the noble metal depletion in the outer layers of the film. Conclusions Ionic species sputtered out of the films investigated show that, in harmony with the chemical inertness of zirconia, the oxide phases develop independently, i.e. without interaction (e.g., solid solution formation) with each other or the support. The formation mechanism of the individual phases can be investigated more easily, if the thermal history of the individual components is known. Concentration depth profiles can be successfully used for the study of species distribution across the films and for the
Kristo´ f et al.
identification of segregation phenomena. The joint use of emission FTIR and SIMS techniques is very promising in an attempt made to identify surface species as a function of film composition and firing temperature. Acknowledgment. This research was supported by Progetto Finalizzato ‘Materiali Speciale per Technologie Avanzate’ of CNR, Rome, Italy. Financial Support from the Hungarian Ministry of Culture and Education under Grants PFP-4004/1997 and OTKA T016707 is also gratefully acknowledged. LA980561S
