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Electrochemical growth of thin La2O3 films on oxide and metal surfaces
Materials Science and Engineering C 23 (2003) 123 – 128 www.elsevier.com/locate/msec
Electrochemical growth of thin La2O3 films on oxide and metal surfaces D. Stoychev a, I. Valov a, P. Stefanov b, G. Atanasova b, M. Stoycheva a, Ts. Marinova b,* a
b
Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Abstract This work presents a new electrochemical method (i.e. new electrolyte composition and electrolysis regime) for direct formation of La2O3 on ZrO2 and other oxide and metal substrates from nonaqueous electrolytes, data on the kinetics of the electrochemical processes and scanning electron microscopy (SEM) and XPS studies of the morphology, structure, dispersion and chemical composition of La2O3. D 2002 Elsevier Science B.V. All rights reserved. Keywords: La2O3; ZrO2 films; Electrodeposition; XPS; SEM
2. Experimental
and 0 –1.5 M/l LiCl (Merck) were dissolved to improve the electrical conductivity. The electrochemical studies proceeded in a standard threeelectrode electrochemical cell bonded by an electrolyte bridge to an Ag/AgCl reference electrode. The cathode substrates used were electrodes made of Au, stainless steel type 1.4301 or ZrO2 deposited on SS 1.4301 [5,6]. The counter electrode (anode) was a ring of Pt sheet. The cyclic voltametrical (CVA) and stationary polarization measurements (Wenking 68 TS1 potentiostat/galvanostat with a Wenking VSG 72 scan generator and a Philips PM 8041 recorder) were performed both for a naturally aerated solution and a deaerated medium (high-purity Ar) at 25 jC. The chemical composition of the surface of electrodeposited La layers was investigated by XPS in a VG Escalab II system (England) using MgKa radiation with an energy of 1253.6 eV. The binding energies (BE) were determined utilizing the C1s line (from an adventitious carbon) as the reference BE = 285.0 eV. The depth profiles were obtained using 3 keV Ar+ ions and a current density of 16.0 AA cm 2. The structure and morphology of the layers were characterized by scanning electron microscopy (SEM) with JEM-200CX electron microscope (Japan).
The working solutions used consisted of a saturated univalent alcohol in which 0.1 – 0.3 M/l LaCl3 (Merck)
3. Results and discussion
1. Introduction It is known that lanthanum and its oxides have a considerable stabilizing and promoting effect on different catalytically active systems for the reduction of harmful waste gases [1– 3]. The lanthanum oxide can also improve the mechanical properties of the systems due to formation of interface compounds with the substrate since, as known, La3 + forms amorphous solid solutions at relatively low temperatures. In some cases, e.g. with Al2O3 support, La2O3 hinders the interaction between the active phase and the substrate, thus preventing the formation of inactive zones or phases [4]. The present work was aimed at finding and studying compositions and a regime of electrochemical deposition of La2O3. Several types of substrates such as gold, stainless steel (SS) and zirconia deposited on SS 1.4301 [5,6] were investigated. The latter substrate was of special interest because zirconia is shown to be a very promising support of transition metal oxide-based catalysts for environmentally related reactions [7].
3.1. Electrochemical studies * Corresponding author. Tel.: +359-2-971-28-59; fax: +359-2-97128-59. E-mail address: [email protected] (Ts. Marinova).
Fig. 1a presents the potentiodynamic studies on a gold cathode in naturally aerated (dotted line) and deaerated
0928-4931/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 ( 0 2 ) 0 0 2 6 1 - 8
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Fig. 1. (a) Potentiodynamic (V = 300 mV/s) polarization curves obtained on an Au cathode in alcoholic solution containing 0.3 M/l LaCl3. The dotted lines are obtained in a naturally aerated electrolyte, and the solid lines, in deaerated argon; (b) The same as in (a) but containing 0.3 M/l LaCl3 + 2.3 M/l LiCl electrolyte.
(solid line) alcoholic solution of LaCl3. The curves are characterized by a weak peak at about 0.740 V, which is ascribed to the formation of a lanthanum film. The further current increase is likely due to hydrogen evolution. If the well-studied mechanism of electrochemical reduction of complex zirconium ions bonded to an oxygencontaining ligand [8] is valid in this case, we can suppose that the registered peak characterizes the reduction of an oxygen-containing lanthanum complex. This assumption is based on the fact that according to Refs. [9,10], lanthanum ions exist only in complex form. Moreover, the presence or absence of oxygen in the electrolyte does not affect its potential but only the rate of the process. In deaerated electrolyte, a current decrease from i = 15.75 to 14.25 mA cm 2 is observed with peak I. The results obtained seem not to be in agreement with the thermodynamic calculations of the equilibrium potential of lanthanum E0 = 2.522 + 0.0197 [La3 +] for reaction
La3 + + 3e = La0 [9]. This value has been calculated for aqueous media for which lanthanum is known to form complexes with a high coordination degree (up to 8) [10] of water molecules. Formation of lanthanum oxide begins far before the theoretically calculated equilibrium potential of La (c 2.5 V). It occurs at a much more positive potential ( 0.4 V), which may be explained by the fact that the solvent (univalent alcohol) used has a solvation ability much weaker than that of water. In addition, formation of La oxocomplexes with different charges in alcoholic media is very probable [11]. They could undergo incomplete reduction on the cathode surface under potentials that are much more positive than the equilibrium potential calculated for La. The addition of an electrically conductive salt (LiCl) to the electrolyte (Fig. 1b) decreases, as expected, the limiting current of the lanthanum complex reduction peak down to i = 12 mA cm 2 without changing the peak potential. However, the second step with a maximum at E = 1.7 V appears, which is most probably due to a limiting current of the hydrogen reaction. According to the criterion of CVA techniques proposed by Bard and Falkner [12], the absence of peaks on the reverse scan shows that the formed layer in this range of potential did not react, i.e. the processes are irreversible. The weak maximum of the peaks is an evidence that the consecutive electrochemical processes overlap. The stationary polarization curves of the process obtained on a SS substrate are shown in Figs. 2– 4. Fig. 2 illustrates the drop of the limiting current of lanthanum complex reduction produced by the enhanced electrical conductance of the electrolyte after addition of different LiCl concentrations. These curves permit evaluation of the potential range, i.e. the current densities at which good quality lanthanum/lanthanum oxide films with a good adhesion to the substrate would be formed.
Fig. 2. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 0.3 M/l LaCl3, 0.5 M/l LiCl and 1 M/l LiCl.
D. Stoychev et al. / Materials Science and Engineering C 23 (2003) 123–128
It was also important to establish the effect of LaCl3 concentration on the kinetics of the process. The correctness of the experiment required performance of these studies with the same ionic strength achieved with various LiCl concentrations. The results obtained are given in Fig. 3. It is evident that the characteristic zones of the potentials where the reduction peaks of the lanthanum complex ( 0.740 V) and the hydrogen ions ( 1.7 V) are registered on the potentiodynamic curves also appear with the stationary ones. In the inset in Fig. 3, the limited current of lanthanum complex reduction is clearly visible. From the stationary polarization curves for the electrolytes containing different LaCl3 concentrations, the kinetic parameters (the constants a and b, from the Tafel’s equation, and the exchange current i0) of the electrochemical reaction can be determined (Table 1). The data show that with increasing concentration of lanthanum ions, the three parameters increase. This indicates an increase of the energy barrier which is to be overcome during the charge transfer needed for discharging the lanthanum ions (complexes) and can be associated with the oxide nature of the phase being formed on the steel substrate. This supposition is supported by the strong decrease of the transition coefficient a from first limited current (a1 = 0.4) to the next process (a2 = 0.17). The probable reduction of the lanthanum oxocomplex is confirmed by the results in Fig. 4. The plateau that is characteristic of all curves and is associated with oxygen reduction is only absent from the curve obtained in Ardeaerated electrolyte containing nothing but LiCl (curve 1). When the same electrolyte is rich in oxygen, the limited current of oxygen reduction is observed (curve 2). With deaerated electrolytes containing not only LiCl but also different concentrations of LaCl3, the plateau associated
Fig. 3. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 0.08, 0.18 and 0.3 M/l LaCl3.
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Table 1 CLaCl3 (g/l) a b i0 (mA/cm2)
32 0.85 0.22 1.37 10
4
64 0.936 0.28 4.54 10
4
96 0.98 0.32 8.6 10
4
with oxygen reduction is again visible (curves 3 –5). In addition, on the basis of the current value, this plateau is by 20% higher and the curves practically coincide irrespective of the concentration of LaCl3. These results indicate reduction of a lanthanum oxocomplex on attaining definite potential values. 3.2. SEM studies Fig. 5a illustrates morphology of the La layer on the steel substrate. Obviously, the lanthanum phase is deposited as microagglomerates with sizes of 0.1 – 1 Am distributed uniformly on the whole surface and islands of macroagglomerates (f 20 –30 Am), which are relatively rare. The specific structure and morphology of the macro- and microagglomerates are given in Fig. 5b and c, respectively. The macroagglomerates consist of chaotically interwoven crystallite fibers with a diameter of the order of 0.2 – 0.5 Am and a length of hundreds of microns, whereas the microaggregates are built of well-shaped little crystals with a symmetry close to the cubic and sizes of 0.1 –0.3 Am. These small crystals have grown separately or coalesced to chains with different lengths. Since no distinct picture of the steel structure has been obtained in the SEM studies (Fig. 5c) a`nd XPS did not detect any Fe signal, it may be assumed that the steel surface is at first covered uniformly with a lanthanum oxide overlayer, after which micro- and macroaggregates begin to grow. Similar results were obtained for
Fig. 4. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 3 M/l LaCl3, 0.5 M/l LiCl and 1 M/l LiCl.
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Fig. 5. SEM micrographs of a La2O3-covered steel (1.4301) substrate: (a) a general view of the surface (1000 ); (b) typical structure and dispersion of the macroaggregates of La2O3 (10000 ); (c) typical morphology and structure of the microaggregates of La2O3 (40000 ).
the electrodeposition of La upon zirconia (characterized with spherical crystallites) deposited on SS [13]. The difference in this case consists in the predominance of macroaggregates (Fig. 6a,b). 3.3. XPS studies Fig. 7a shows the photoelectron spectra of lanthanum oxide formed electrochemically on stainless steel. The
analysis of the photoelectron spectra of the substrate surface presupposes formation of a thin hydroxide film upon the asdeposited layers. The splitting (f 3.5 eV) of the two components of the La3d5/2 peak is typical of La(OH)3 [14]. The O1s peak also shows BE = 532.0 V [14]), which is characteristic of this hydroxide. After ion bombardment of the sample for about 10 min, the splitting of the two components of the La3d5/2 peak increases to 4.2 –4.3 eV, which is typical of La2O3
Fig. 6. SEM micrographs of the surface of zirconia on which La2O3 layers are deposited: (a) distribution of the La2O3 macroaggregates (2000 ); (b) typical morphology and structure of the microaggregates of La2O3 (10000 ).
D. Stoychev et al. / Materials Science and Engineering C 23 (2003) 123–128
Fig. 7. (a) O1s and La3d5/2 spectra of La2O3 deposited electrochemically on stainless steel taken after different times of 3 keV Ar + bombardment; (b) O1s, La3d5/2 and C1s spectra of La2O3 deposited electrochemically on zirconia taken after different times of 3 keV Ar + bombardment.
[14] The oxygen peak (Fig. 7a) forms a double structure comprising a lattice oxygen peak at about 529.8 eV and a second peak at 531.8 eV denoted as OHBE. The latter is associated with both the presence of OH groups and the effect of ion bombardment leading to a defect structure in the affected layers, accompanied by formation of O ions. The O1s and La3d5/2 spectra of La2O3 deposited electrochemically on zirconia show no substantial difference with respect to the steel surface (Fig. 7b). The only exception is the certain asymmetry on the side of lower binding energy. With both O1s and La3d5/2 lines, there is a shoulder of about 2 eV below the basic peak, which is attributed to photoemission of another separate La2O3 phase present on the surface. This assumption is also supported by the shape of the difference La3d5/2 spectrum (A B) in Fig. 7b. The spectrum B is obtained after 70-min ion bombardment when the asymmetry of La3d5/2 disappears. Such prolonged bombardment removes the lanthanum oxide overlayer of microagglomerates coating the ZrO2 surface, which leads to
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the appearance of the zirconia signal. The difference spectrum shows two peaks whose splitting (4.3 eV) is the same as that of the double La3d5/2 structure of La2O3. These findings can be attributed to the effect of electrostatic charging during the photoemission from both lanthanum phases observed in SEM micrographs (Figs. 5 and 6). In the case of a zirconia substrate, macroaggregate fibers are prevailing on the surface, and the main contribution of the O1s and La3d5/2 peaks belongs to them. These fibers have no good contact with the substrate, which leads to a higher charging during the photoemission. The low-energy shoulder of the peaks (associated with a lower charging) is due to a signal from the adjacent to zirconia La2O3 layers which are screened by the macroaggregate fibers. Fig. 7b shows that the OHBE shoulder of the O1s peak is preserved in the spectra obtained after different times of bombardment. In addition, the spectra exhibit the C1s peak with BE f 290.0 eV produced by carbon in CO32 groups. Hence, this shoulder is attributed to the presence of both OH and carbonate groups in the subsurface layers of La2O3. The composition of the deposited La2O3 films was characterized by depth profiling with 3 keV Ar+ ions. Fig. 8 illustrates the La/O ratio depending on the sputtering time of lanthanum deposited on stainless steel and zirconia. It is interesting that the La/O ratio is higher upon deposition on stainless steel than in the case of deposition on zirconia where its value is below the sesquioxide stoichiometry. This is obviously due to a difference in composition of the microaggregates forming the lanthanum film on stainless steel and the macroaggregates predominating on zirconia. It is known that rare earth oxides of the type A-M2O3 posses the hexagonal structure which can be presented as an infinite polymer complex consisting of OM4 tetrahedra with shared edges and a three- or two-dimensional packing of the layers held together by oxygen anions between
Fig. 8. The change of La/O intensity ratio for La2O3 deposited electrochemically on stainless steel and zirconia as a function of the sputtering time.
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the layers [15]. These O2 anions can easily be replaced by other anions of the type OH , CO32 or NO3 . A similar process probably occurs with electrochemically deposited layers of lanthanum oxide in the time after the electrolysis. According to Ref. [3], CO32 anions are also localized in the subsurface layers of La2O3. This could explain their presence in the spectra after ion bombardment (Fig. 7b). On the basis of the foregoing, conclusions may be drawn concerning the composition of micro- and macroaggregates forming the lanthanum films. The microaggregates are close in composition to La2O3. The higher value of the La/O ratio can be ascribed to formation of oxygen vacancies in the surface layer due to the ion bombardment. The macroaggregate fibers are probably polymer complexes [15] to which OH and CO32 groups are bonded. This explains the oxygen excess in the layers deposited on zirconia films.
4. Conclusion The present investigation shows that the electrochemical formation of La2O3 on different substrates is possible and very promising as a preparation method. The electrochemical results obtained permit the assumption that La2O3 is produced as a result of reduction of a lanthanum oxocomplex in the nonaqueous electrolyte. The films fabricated consist of micro- and macroaggregates. The latter are probably formed by polymer complexes enriched with OH and CO32 groups.
Acknowledgements This work is financially supported by European Commission through the Environment and Climate Project ENVA-CT97-0633.
References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
X. Zhang, A.B. Walters, M.A. Vannice, Appl. Catal. B4 (1994) 237. X. Zhang, A.B. Walters, M.A. Vannice, J. Catal. 155 (1995) 290. B. Klngenberg, M.A. Vannice, Chem. Mater. 8 (1996) 2755. P. Nortier, M. Soustelle, Catalysis and Automotive Pollution Control, Proc. 1st International Symposium, Brussels, 1986, Elsevier, Amsterdam, 1987, 275. P. Stefanov, D. Stoychev, I. Valov, A. Kakanakova-Georgieva, Ts. Marinova, Mater. Chem. Phys. 65 (2000) 222. P. Stefanov, D. Stoychev, M. Stoycheva, Ts. Marinova, Surf. Interface Anal. 30 (2000) 628. A. Galtayries, R. Sporken, J. Riga, G. Blanchard, R. Caudano, J. Elec. Spec. 88 (1998) 951. I. Valov, D. Stoychev, Ts. Marinova, Electrochim. Acta (in press). M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, Pergamon, Oxford, 1966. L. Genov, M. Maneva, Neorganichna chimija, 1990, Nauka i izkustvo, Sofia. J.M. Kolotirkin, Electrochimiya metalov v nevodnih rastvorah, 1974, Mir, Moscow. A. Bard, C. Falkner, Electrochemical Methods, Wiley, New York, 1980. P. Stefanov, D. Stoychev, M. Stoycheva, J. Ikonomov, Ts. Marinova, J. Surf. Interface Anal. 30 (2000) 628. H.C. Siegmann, L. Schlapbach, C.H. Brundle, Phys. Rev. Lett. 14 (1978) 972. P.E. Caro, J. Less-Common Met. 16 (1968) 367.
Electrochemical growth of thin La2O3 films on oxide and metal surfaces D. Stoychev a, I. Valov a, P. Stefanov b, G. Atanasova b, M. Stoycheva a, Ts. Marinova b,* a
b
Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
Abstract This work presents a new electrochemical method (i.e. new electrolyte composition and electrolysis regime) for direct formation of La2O3 on ZrO2 and other oxide and metal substrates from nonaqueous electrolytes, data on the kinetics of the electrochemical processes and scanning electron microscopy (SEM) and XPS studies of the morphology, structure, dispersion and chemical composition of La2O3. D 2002 Elsevier Science B.V. All rights reserved. Keywords: La2O3; ZrO2 films; Electrodeposition; XPS; SEM
2. Experimental
and 0 –1.5 M/l LiCl (Merck) were dissolved to improve the electrical conductivity. The electrochemical studies proceeded in a standard threeelectrode electrochemical cell bonded by an electrolyte bridge to an Ag/AgCl reference electrode. The cathode substrates used were electrodes made of Au, stainless steel type 1.4301 or ZrO2 deposited on SS 1.4301 [5,6]. The counter electrode (anode) was a ring of Pt sheet. The cyclic voltametrical (CVA) and stationary polarization measurements (Wenking 68 TS1 potentiostat/galvanostat with a Wenking VSG 72 scan generator and a Philips PM 8041 recorder) were performed both for a naturally aerated solution and a deaerated medium (high-purity Ar) at 25 jC. The chemical composition of the surface of electrodeposited La layers was investigated by XPS in a VG Escalab II system (England) using MgKa radiation with an energy of 1253.6 eV. The binding energies (BE) were determined utilizing the C1s line (from an adventitious carbon) as the reference BE = 285.0 eV. The depth profiles were obtained using 3 keV Ar+ ions and a current density of 16.0 AA cm 2. The structure and morphology of the layers were characterized by scanning electron microscopy (SEM) with JEM-200CX electron microscope (Japan).
The working solutions used consisted of a saturated univalent alcohol in which 0.1 – 0.3 M/l LaCl3 (Merck)
3. Results and discussion
1. Introduction It is known that lanthanum and its oxides have a considerable stabilizing and promoting effect on different catalytically active systems for the reduction of harmful waste gases [1– 3]. The lanthanum oxide can also improve the mechanical properties of the systems due to formation of interface compounds with the substrate since, as known, La3 + forms amorphous solid solutions at relatively low temperatures. In some cases, e.g. with Al2O3 support, La2O3 hinders the interaction between the active phase and the substrate, thus preventing the formation of inactive zones or phases [4]. The present work was aimed at finding and studying compositions and a regime of electrochemical deposition of La2O3. Several types of substrates such as gold, stainless steel (SS) and zirconia deposited on SS 1.4301 [5,6] were investigated. The latter substrate was of special interest because zirconia is shown to be a very promising support of transition metal oxide-based catalysts for environmentally related reactions [7].
3.1. Electrochemical studies * Corresponding author. Tel.: +359-2-971-28-59; fax: +359-2-97128-59. E-mail address: [email protected] (Ts. Marinova).
Fig. 1a presents the potentiodynamic studies on a gold cathode in naturally aerated (dotted line) and deaerated
0928-4931/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 ( 0 2 ) 0 0 2 6 1 - 8
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Fig. 1. (a) Potentiodynamic (V = 300 mV/s) polarization curves obtained on an Au cathode in alcoholic solution containing 0.3 M/l LaCl3. The dotted lines are obtained in a naturally aerated electrolyte, and the solid lines, in deaerated argon; (b) The same as in (a) but containing 0.3 M/l LaCl3 + 2.3 M/l LiCl electrolyte.
(solid line) alcoholic solution of LaCl3. The curves are characterized by a weak peak at about 0.740 V, which is ascribed to the formation of a lanthanum film. The further current increase is likely due to hydrogen evolution. If the well-studied mechanism of electrochemical reduction of complex zirconium ions bonded to an oxygencontaining ligand [8] is valid in this case, we can suppose that the registered peak characterizes the reduction of an oxygen-containing lanthanum complex. This assumption is based on the fact that according to Refs. [9,10], lanthanum ions exist only in complex form. Moreover, the presence or absence of oxygen in the electrolyte does not affect its potential but only the rate of the process. In deaerated electrolyte, a current decrease from i = 15.75 to 14.25 mA cm 2 is observed with peak I. The results obtained seem not to be in agreement with the thermodynamic calculations of the equilibrium potential of lanthanum E0 = 2.522 + 0.0197 [La3 +] for reaction
La3 + + 3e = La0 [9]. This value has been calculated for aqueous media for which lanthanum is known to form complexes with a high coordination degree (up to 8) [10] of water molecules. Formation of lanthanum oxide begins far before the theoretically calculated equilibrium potential of La (c 2.5 V). It occurs at a much more positive potential ( 0.4 V), which may be explained by the fact that the solvent (univalent alcohol) used has a solvation ability much weaker than that of water. In addition, formation of La oxocomplexes with different charges in alcoholic media is very probable [11]. They could undergo incomplete reduction on the cathode surface under potentials that are much more positive than the equilibrium potential calculated for La. The addition of an electrically conductive salt (LiCl) to the electrolyte (Fig. 1b) decreases, as expected, the limiting current of the lanthanum complex reduction peak down to i = 12 mA cm 2 without changing the peak potential. However, the second step with a maximum at E = 1.7 V appears, which is most probably due to a limiting current of the hydrogen reaction. According to the criterion of CVA techniques proposed by Bard and Falkner [12], the absence of peaks on the reverse scan shows that the formed layer in this range of potential did not react, i.e. the processes are irreversible. The weak maximum of the peaks is an evidence that the consecutive electrochemical processes overlap. The stationary polarization curves of the process obtained on a SS substrate are shown in Figs. 2– 4. Fig. 2 illustrates the drop of the limiting current of lanthanum complex reduction produced by the enhanced electrical conductance of the electrolyte after addition of different LiCl concentrations. These curves permit evaluation of the potential range, i.e. the current densities at which good quality lanthanum/lanthanum oxide films with a good adhesion to the substrate would be formed.
Fig. 2. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 0.3 M/l LaCl3, 0.5 M/l LiCl and 1 M/l LiCl.
D. Stoychev et al. / Materials Science and Engineering C 23 (2003) 123–128
It was also important to establish the effect of LaCl3 concentration on the kinetics of the process. The correctness of the experiment required performance of these studies with the same ionic strength achieved with various LiCl concentrations. The results obtained are given in Fig. 3. It is evident that the characteristic zones of the potentials where the reduction peaks of the lanthanum complex ( 0.740 V) and the hydrogen ions ( 1.7 V) are registered on the potentiodynamic curves also appear with the stationary ones. In the inset in Fig. 3, the limited current of lanthanum complex reduction is clearly visible. From the stationary polarization curves for the electrolytes containing different LaCl3 concentrations, the kinetic parameters (the constants a and b, from the Tafel’s equation, and the exchange current i0) of the electrochemical reaction can be determined (Table 1). The data show that with increasing concentration of lanthanum ions, the three parameters increase. This indicates an increase of the energy barrier which is to be overcome during the charge transfer needed for discharging the lanthanum ions (complexes) and can be associated with the oxide nature of the phase being formed on the steel substrate. This supposition is supported by the strong decrease of the transition coefficient a from first limited current (a1 = 0.4) to the next process (a2 = 0.17). The probable reduction of the lanthanum oxocomplex is confirmed by the results in Fig. 4. The plateau that is characteristic of all curves and is associated with oxygen reduction is only absent from the curve obtained in Ardeaerated electrolyte containing nothing but LiCl (curve 1). When the same electrolyte is rich in oxygen, the limited current of oxygen reduction is observed (curve 2). With deaerated electrolytes containing not only LiCl but also different concentrations of LaCl3, the plateau associated
Fig. 3. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 0.08, 0.18 and 0.3 M/l LaCl3.
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Table 1 CLaCl3 (g/l) a b i0 (mA/cm2)
32 0.85 0.22 1.37 10
4
64 0.936 0.28 4.54 10
4
96 0.98 0.32 8.6 10
4
with oxygen reduction is again visible (curves 3 –5). In addition, on the basis of the current value, this plateau is by 20% higher and the curves practically coincide irrespective of the concentration of LaCl3. These results indicate reduction of a lanthanum oxocomplex on attaining definite potential values. 3.2. SEM studies Fig. 5a illustrates morphology of the La layer on the steel substrate. Obviously, the lanthanum phase is deposited as microagglomerates with sizes of 0.1 – 1 Am distributed uniformly on the whole surface and islands of macroagglomerates (f 20 –30 Am), which are relatively rare. The specific structure and morphology of the macro- and microagglomerates are given in Fig. 5b and c, respectively. The macroagglomerates consist of chaotically interwoven crystallite fibers with a diameter of the order of 0.2 – 0.5 Am and a length of hundreds of microns, whereas the microaggregates are built of well-shaped little crystals with a symmetry close to the cubic and sizes of 0.1 –0.3 Am. These small crystals have grown separately or coalesced to chains with different lengths. Since no distinct picture of the steel structure has been obtained in the SEM studies (Fig. 5c) a`nd XPS did not detect any Fe signal, it may be assumed that the steel surface is at first covered uniformly with a lanthanum oxide overlayer, after which micro- and macroaggregates begin to grow. Similar results were obtained for
Fig. 4. Stationary polarization curves obtained on a steel (type 1.4301) substrate in an electrolyte containing 3 M/l LaCl3, 0.5 M/l LiCl and 1 M/l LiCl.
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Fig. 5. SEM micrographs of a La2O3-covered steel (1.4301) substrate: (a) a general view of the surface (1000 ); (b) typical structure and dispersion of the macroaggregates of La2O3 (10000 ); (c) typical morphology and structure of the microaggregates of La2O3 (40000 ).
the electrodeposition of La upon zirconia (characterized with spherical crystallites) deposited on SS [13]. The difference in this case consists in the predominance of macroaggregates (Fig. 6a,b). 3.3. XPS studies Fig. 7a shows the photoelectron spectra of lanthanum oxide formed electrochemically on stainless steel. The
analysis of the photoelectron spectra of the substrate surface presupposes formation of a thin hydroxide film upon the asdeposited layers. The splitting (f 3.5 eV) of the two components of the La3d5/2 peak is typical of La(OH)3 [14]. The O1s peak also shows BE = 532.0 V [14]), which is characteristic of this hydroxide. After ion bombardment of the sample for about 10 min, the splitting of the two components of the La3d5/2 peak increases to 4.2 –4.3 eV, which is typical of La2O3
Fig. 6. SEM micrographs of the surface of zirconia on which La2O3 layers are deposited: (a) distribution of the La2O3 macroaggregates (2000 ); (b) typical morphology and structure of the microaggregates of La2O3 (10000 ).
D. Stoychev et al. / Materials Science and Engineering C 23 (2003) 123–128
Fig. 7. (a) O1s and La3d5/2 spectra of La2O3 deposited electrochemically on stainless steel taken after different times of 3 keV Ar + bombardment; (b) O1s, La3d5/2 and C1s spectra of La2O3 deposited electrochemically on zirconia taken after different times of 3 keV Ar + bombardment.
[14] The oxygen peak (Fig. 7a) forms a double structure comprising a lattice oxygen peak at about 529.8 eV and a second peak at 531.8 eV denoted as OHBE. The latter is associated with both the presence of OH groups and the effect of ion bombardment leading to a defect structure in the affected layers, accompanied by formation of O ions. The O1s and La3d5/2 spectra of La2O3 deposited electrochemically on zirconia show no substantial difference with respect to the steel surface (Fig. 7b). The only exception is the certain asymmetry on the side of lower binding energy. With both O1s and La3d5/2 lines, there is a shoulder of about 2 eV below the basic peak, which is attributed to photoemission of another separate La2O3 phase present on the surface. This assumption is also supported by the shape of the difference La3d5/2 spectrum (A B) in Fig. 7b. The spectrum B is obtained after 70-min ion bombardment when the asymmetry of La3d5/2 disappears. Such prolonged bombardment removes the lanthanum oxide overlayer of microagglomerates coating the ZrO2 surface, which leads to
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the appearance of the zirconia signal. The difference spectrum shows two peaks whose splitting (4.3 eV) is the same as that of the double La3d5/2 structure of La2O3. These findings can be attributed to the effect of electrostatic charging during the photoemission from both lanthanum phases observed in SEM micrographs (Figs. 5 and 6). In the case of a zirconia substrate, macroaggregate fibers are prevailing on the surface, and the main contribution of the O1s and La3d5/2 peaks belongs to them. These fibers have no good contact with the substrate, which leads to a higher charging during the photoemission. The low-energy shoulder of the peaks (associated with a lower charging) is due to a signal from the adjacent to zirconia La2O3 layers which are screened by the macroaggregate fibers. Fig. 7b shows that the OHBE shoulder of the O1s peak is preserved in the spectra obtained after different times of bombardment. In addition, the spectra exhibit the C1s peak with BE f 290.0 eV produced by carbon in CO32 groups. Hence, this shoulder is attributed to the presence of both OH and carbonate groups in the subsurface layers of La2O3. The composition of the deposited La2O3 films was characterized by depth profiling with 3 keV Ar+ ions. Fig. 8 illustrates the La/O ratio depending on the sputtering time of lanthanum deposited on stainless steel and zirconia. It is interesting that the La/O ratio is higher upon deposition on stainless steel than in the case of deposition on zirconia where its value is below the sesquioxide stoichiometry. This is obviously due to a difference in composition of the microaggregates forming the lanthanum film on stainless steel and the macroaggregates predominating on zirconia. It is known that rare earth oxides of the type A-M2O3 posses the hexagonal structure which can be presented as an infinite polymer complex consisting of OM4 tetrahedra with shared edges and a three- or two-dimensional packing of the layers held together by oxygen anions between
Fig. 8. The change of La/O intensity ratio for La2O3 deposited electrochemically on stainless steel and zirconia as a function of the sputtering time.
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the layers [15]. These O2 anions can easily be replaced by other anions of the type OH , CO32 or NO3 . A similar process probably occurs with electrochemically deposited layers of lanthanum oxide in the time after the electrolysis. According to Ref. [3], CO32 anions are also localized in the subsurface layers of La2O3. This could explain their presence in the spectra after ion bombardment (Fig. 7b). On the basis of the foregoing, conclusions may be drawn concerning the composition of micro- and macroaggregates forming the lanthanum films. The microaggregates are close in composition to La2O3. The higher value of the La/O ratio can be ascribed to formation of oxygen vacancies in the surface layer due to the ion bombardment. The macroaggregate fibers are probably polymer complexes [15] to which OH and CO32 groups are bonded. This explains the oxygen excess in the layers deposited on zirconia films.
4. Conclusion The present investigation shows that the electrochemical formation of La2O3 on different substrates is possible and very promising as a preparation method. The electrochemical results obtained permit the assumption that La2O3 is produced as a result of reduction of a lanthanum oxocomplex in the nonaqueous electrolyte. The films fabricated consist of micro- and macroaggregates. The latter are probably formed by polymer complexes enriched with OH and CO32 groups.
Acknowledgements This work is financially supported by European Commission through the Environment and Climate Project ENVA-CT97-0633.
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