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Electrochemical and photoelectrochemical behaviour of passivated Ti and Nb electrodes in nitric acid solutions
ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL BEHAVIOUR OF PASSIVATED Ti AND Nb ELECTRODES IN NITRIC ACID SOLUTIONS W. A. BADAWY,* A. FELSKE and W. J. PLIETH Freie Universitat Berlin, Institut fiir Physikalische Chemie, Takustr. 3, D-1000 Berlin 33, F.R.G. (Received
20 September
1988; in revised form 6 March
1989)
Abstract-Ti and Nb electrodes were pass&ted galvanostatically in nitric acid solutions. The electrochemical and photoelectrochemical behaviour of these electrodes was studied and compared with the behaviour of similar electrodes passivated in sulphuric acid solutions under the same conditions. The flatband potential and donor concentration of the passive films were obtained from the corresponding Mott-Schottky plots. The photoelectrochemical investigations were carried out using a pulsed laser which rcndcrcd measurements with photon energies smaller than the band gap. The results showed that the behaviour of passivated titanium in nitric acid was differing from its behaviour in sulphuric acid. The behaviour of the passivated niobium was not much aKected by the exchange of the two acids.
INTRODUCTION Valve metals, niobium and titanium are known to form stable oxide films in aqueous solutions[ 11. These metals have been subjected to intensive investigations concerning the kinetics of oxide film growth[2-111, the stability of the oxide films[l l-141 and the physical properties of the passive film itself[15-181. Nevertheless, the low-voltage passive behaviour of the metal and its photoelectrochemical properties-specially in the aggressive and, industrially, commonly used medium HNO,-appears to be rarely investigated. The great demand of stable materials used in chemical process units and application of valve metals to the electrochemical methods recently introduced into the purex process for waste minimization[19] makes the passivation of Ti and Nb and their behaviour in HNO, an important subject of further investigations. Pulsed lasers are a new light source to detect small photoeffects on semiconducting passive films formed on metals[20,21]. These pulse lasers even allow investigations with light wavelengths having energies smaller than the band gap of the semiconducting passive films formed on metals[20,21]. These pulse lasers even allow investigations with light wavelengths having energies smaller than the band gap of the semiconducting passive film. In this work we report on the behaviour of passive films formed on Ti and Nb electrodes in nitric acid solutions. To demonstrate the effect of HNO, on the passive film, a comparison with electrodes passivated in H,SO, at the same concentration is presented.
EXPERIMENTAL The details of the experimental trolytic cell were as previously
set-up and the elecdescribed[20]. The
* Permanent address: Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt.
working electrodes were made of massive cylindrical, spectroscopically pure titanium and niobium rods (Aldrich-Chemie). A stout copper wire was employed as electrica contact. The electrode was fitted into glass tubing of appropriate internal diameter with an epoxy resin, leaving an area of 0.35 cm* open at the front to the solution. The photocurrent measurements were carried out in a darkened room with a pulsed laser to illuminate the electrode surface. The main advantage of the pulsed laser in comparison to continuous light sources, was the large photocurrent/dark current ratio and the improved signal to noise ratio of the photocurrent. The used pulses had a half-width time of 10 ns with an intensity of 1=0.3 to 1.5 mJ per pulse. The small amounts of photocurrent were either (a) integrated or (b) averaged and integrated to eliminate the contribution of the electrolytic resistance and of the electrode capacity to the peak current. The typical peak current was of the order of 50 mA per light pulse. The pulsed dye laser was a FL 2001, Lambda Physik, filled with PBBO (1, = 390 nm) coumarin 307 (1, = 500 nm), or rhodamin 6C (A,,,= 58 1 nm) and pumped by an excimer laser with a repetition frequency of 7-20 Hz. All measurements were performed under potentiostatic control. Capacitance measurements were carried out technique at a frequency of using the “lock-in” 1330 Hz. The independence of frequency was checked between 0.65 and 50 kHz. Before each experiment the electrode surface was metihanically polished with diamond spray down to 0.25 pm, washed in ethanol and triply distilled water in an ultrasonic bath. Passivation of the electrodes was performed in the test solution at 2.5 mAcm_‘. After reaching the desired formation voltage, the current was interrupted and the measurements immediately carried out. The potentials were measured against the suitable reference electrode (either satured calomel electrode (see) or Hg/Hg,SO,/OS M H,SO,) and referred to the normal hydrogen electrode (nhe). The temperature was kept constant at 23°C throughout the experiments. 1711
1712
W.
RESULTS Film formation
AND
A.
BADAWY
et al.
DISCUSSION
and breakdown
The oxide film thickness is a function of the anodization potential. For Ti, a value lying between 1.4 and 2.5 nm V i was reported[lO, 221 and for Nb, a value of 2.1-2.5 nm V-r was given[15]. A value of 2.0 nm V-i was used if any calculations had to be done. In sulphuric acid solutions, Ti as well as Nb could be anodized to high voltages in several seconds; ey a 10 V TiO, layer was obtained in approx. 50 s. In nitric acid solutions, the anodization of the metal Ti to the same voltage--ie 10 V-took longer (approx. 15 min). In the literature different times requested for the films growths are reported which might be connected with different surface treatment. In sulphuric acid the formation potential of Ti could rise above 100 V without breakdown. ln nitric acid, at a potential drop of 12.7 V, a breakdown in the passive film was observed. The oxide film acquired a greyish, opaque appearance and turned to a spongy layer which then lost the pure blue colour of TiO,. Other experimental evidence for this behaviour provided by capacitance and photocurrent measurements (see below) shows that the more disordered structure of the oxide formed in nitric acid might be connected with localized electronic states approximately 0.84.2 eV below the band edge of the conduction band. In the case of Nb, no breakdown was observed either in H,SO, or in HNO, solution. Similar to Ti, the oxide film formation in HNO, needed much longer time than in HaSO,. As an example, the formation of a 9 V layer in nitric acid required 43 min, whereas in H,SO, and under the same conditions (2.5 mA cm ‘), it took only 5 min. In both solutions and up to 50 V, the oxide film on Nb always had a clear interference colour, ranging between light yellow and bluish-brown. Photocharge-potential
behauiour
Figure 1 illustrates the photocharge-potential behaviour of a Ti electrode passivated in 0.5 M H,SO, (curve 2) and another one passivated in 1 M HNO, (curve 1). Both were anodized galvanostatically at
1
1 N HN03
-,
1.0
1
I
2.0
I
I
3.0
E/V vs. NHE
Fig. I. Photocharge-potential relation of Ti electrodes passkated in: (1) 1 M HNO,: (2) 0.5 M H,SO, to 3.2 V (nhe). Light intensity I =0.3 mJ per pulse; 1 = 500 nm.
I
I
,
3
3
E/V vs. NHE
Fig. 2. Photocharge-potential behaviour of Nb electrodes passivated in : (1) 0.5 M H,SO,; (2) 1 M HNO, to 3.2 V (nhe). Light intensity I = 0.3 mJ per pulse; 1= 500 nm.
2.5 mA cm-’ to 3.2 V. The figure shows that the behaviour of the Ti electrode passivated in nitric acid differs from that of the electrode passivated in H,SO, solution. The varying behaviour reflects itself in the magnitude of the photocharge of the pulse per cm2 (Q,,,). The electrodes passivated and investigated in sulphuric acid did not show any photocurrent or photocharge up to 1.84 V. At more positive potentials, a weak linear increase of the photocharge with the potential during anodic polarization was recorded (curve 2). In nitric acid solution the behaviour is different. At high anodic potentials (approx. 2.5 V) an approximately exponential increase of the photocharge with the potential was recorded. While sweeping the potential cathodically, the photocharge showed a flat minimum in the potential range of 1.5 to 0.8 V. At a potential of 0.25 + 0.1 V, the photocharge passed through a maximum and then decreased again. This could be explained by localized electronic states 0.84.2 eV below the band edge of the conduction band. Obviously, these states are not generated in H,SO,. The behaviour of the passivated Nb electrode is presented in Fig. 2. The passivation potential was 3.2 V (nhe). The two curves shown in this figure demonstrate that the general behaviour of passivated Nb electrodes in nitric acid (curve 2) is not so dissimilar from the behaviour of the electrodes passivated in sulphuric acid (curve 1). In both electrolytes a maximum in the photocharge of the pulse per cm-’ (Qph) was observed. This maximum is shifted cathodically in 1 M HNO, (about 300 mV) with respect to the maximum measured in 0.5 M H,SO,. It is explained by additional electronic states 1.54.5 eV below the band edge of the conduction band. The photocharge or photocurrent measured in 1 M HNO, is slightly smaller than that measured in 0.5 M H,SO,. Capacity
I
I
0
measurements
Impedance measurements showed that, in a frequency range from 0.55 to approx. 40 kHz, the electrode capacitance is approximately frequency-independent. Therefore further capacitance measurements were made at a fixed frequency of 1330 Hz. Figure 3 illustrates the capacitance-potential behaviour of a Ti electrode passivated in 0.5 M H,SO, (1) and another one passivated in 1 M HNO, (2). both to 3.2 V (nhe).
Passivated
Ti and Nb electrodes
Both electrodes gave the same flatband potential (the intercept of the straight portion of the Mott-Schottky plot with the potential axis) of approx. 0 V (nhe). The donor concentration was calculated from the slope of the plot according to: N,
2 = ~ D,De
1 5X-
=/6E’
where N, is the donor concentration in cm ? e the electronic charge (1.60 x lo-is’ A s); D, the electric field constant (vacuum dielectric constant 8.85 x lo-l4 A s V-r cm-‘); 6C-‘/6E the slope of the Mott-Schottky plot and D the dielectric constant of the passive film. D was taken as 9[10]. For the passive layer of Ti formed and investigated in 1M nitric acid under the same conditions, a value of 28.5 x lOi cm-’ was obtained this being more than 6 times greater than the first one. This means that in nitric acid solutions, the number of donor terms increases. In the case of Nb electrodes, it was observed that the passivation and investigation of the passive films in nitric acid is accompanied by a decrease in the the concentration. shows donor Figure 4 Mott-Schottky plots of a Nb electrode passivated and
1 0
200
400
600
ElmV vs. NHE Fig. 3. Mott-Schottky plots of passive titanium films in; (1) 0.5 M H,SO,; (2) 1 M HNO,. Both electrodes were passivated galvanostatically at 2.5 mA cm-* to:3.2 V (nhe).
1713
investigated in 0.5 M H,S04 (1) and another in 1 M HNO, (2). Both electrodes gave roughly the same flatband potential of approx. - 175 mV (nhe). The donor concentration calculated for the electrode investigated in nitric acid is about half compared to the one investigated in sulphuric acid. The values are 3 6 x lOl9 cmW3 for the 1 M HNO, passivated eleccrnm3 for the 0.5 M H,SO, trode and 7.2 x lOi passivated one. The dielectric constant for Nb,O, was taken as 11[23]. From these results it is clear that Ti is more affected by the presence of HNO, than Nb. Effect
of the
oxide film thickness
To study the effect of the oxide film thickness on the general behaviour of the passive film in nitric acid, the electrodes were passivated to different formation potentials as described before and then investigated in the same solution. Figure 5 illustrates the variation of the photocharge with potential for titanium electrodes passivated in 1 M HNO, to 3.2, 5.2, 7.2, 10.2 and 12.2V (nhe) respectively, using a laser pulse of 0.3 mJ per pulse and a wavelength of 581 nm. The photocharge maximum (approximately 0.24.1 V below the band edge of the conduction band of the passive film) increases as the film thickness increases. At the large wavelength of 581 nm very small photocurrents independent of the film thickness were recorded in the anodic region more positive than 1 V. The dependence of the photocharge maximum on the thickness of the film may again be explained by additional electronic states in the oxide. Surface states could not explain the thickness dependence. It must be noted that the observed photocharge maximum in the case of Ti electrodes is dependent on the direction of the voltage scan. The maximum in the Q,,/potential behaviour was recorded only on going from anodic potentials into hydrogen evolution (Fig. 5, l-5). On reversing the scan direction after hydrogen evolution (which takes place near the flatband potential (0.0 V), the photocharge maximum has disappeared (Fig. 5, r). This phenomenon may be explained by the reduction of the electronic states responsible for the photocharge maximum.
J
Oh EimV vs. NHE Fig. 4. MottG%hottky plots of passive niobium films in: (1) 0.5 M H,SO,; (2) 1 M HNO,. _Bpth electrodes were passiva_ _ _. ted at 2.5 mA cm-’ to LI.L V (nhe).
2:o l:o EN vs. NHE
3.0
Fig. 5. Photochargeepotential behaviour of passivated titanium electrodes in 1 M HN03. The electrodes were passivatcd galvanostatically at 2.5 mA cm- * to:3.2 V (1); 5.2 V (2); 7.2 V (3); 10.2 V (4); 12.2 V (5). (r) The reverse scan of(5). Light intensity 1~0.3 mJ per pulse; i = 581 am.
W. A. BADAWY
1714
So far, the following model can be discussed for the photocharge maximum. In Fig. 6 the band structure is shown with localized states 0.8425 eV below the band edge of the conduction band. At more anodic potentials the states are discharged. With the potential scanned cathodically the Fermi level is raised above the localized states which then become occupied and serve as source for the photocurrent. The current increases until all states are occupied. Then the decreasing band bending leads to a decrease of the photocurrent. The chemical nature of the localized states is not yet known. Two suggestions can be discussed: (i) nitrogen oxides incorporated in the TiO,, or (ii) deep defects of TiO, resulting from the reaction with the HNO,. The states can be reduced by cathodic polarization into the hydrogen evolution region. Direct reduction or the intercalation of hydrogen into the TiO, matrix[24,25] can cause this reduction. The different behaviour after anodic and cathodic prepolarization is a characteristic property of Ti electrodes which could not be observed with the passivated Nb electrodes. Although it was observed that the behaviour of the passivated Nb electrodes was affected by hydrogen evolution, the photocharge maximum is still present afier hydrogen evolution (Fig. 2). The maximum in Fig. 2 was approximately independent of the film thickness. The effect of the film thickness on the behaviour of passivated Nb efectrodes either in 0.5 M H,SO, or in 1 M HNO, was further investigated by capacitance measurements on three Nb electrodes passivated in
p E
eI ul.
0.5 M H,SO, to 3.2, 6.2 and 9.2 V @he), respectively (Fig. 7). In order to be sure that the passive film was not contaminated with hydrogen, the anodic scan was chosen, ie the capacity was measured from +3.2 to -0.4 V (nhe). The capacitance decreased with film thickness according to the relation C- l/d. The MottGSchoottky plots are shown in Fig. 7. A part of curve 1 of Fig. 7 was shown in Fig. 4. In Fig. 7 the MottGSchottky behaviour was observed in a relatively small potential region between -0.05 and + 0.1 V. In Fig. 4 the linear region was larger: between f0.05 and 0.4 V. Such differences were connected with different pretreatment, eg different time of prepolarization. The donor concentration calculated from the linear portion decreased as the film thickness increased (eg N,= 6.8 x 10’9cm-’ and 0.62 x 10” cm ’ for a 3.2 and a 9.2 (nhe) layer, respectively). This explains why the photocharge maxima in Fig. 2 were independent of the film thickness. The electronic states responsible for the maxima in Fig. 2 did not increase with the thickness. CONCLUSIONS The behaviour of Ti electrodes during passivation or after passivation in nitric acid is different from that in sulphuric acid solutions. The passive films undergo a breakdown at a potential of 12.7 V (nhe) and the structure of the oxide film is changed. On the other hand, Nb electrodes fail to show any form of drastic change in behaviour during polarization in nitric acid solutions. This raises the question on the stability of Ti in HNO,. Surprisingly, recent corrosion studies in HNO, carried out by the radiotracer method, did not show any dramatic increase in the role of corrosion of Ti[19]. It seems that this point needs further investigation. Acknowledgement-We from the Deutsche from the Alexander
6. Band
model of TiO, layers with localized below the conduction band.
states
1. L.Young, Anodic Oxide Films, Academic Press, London (196f). Ser. 112, 2. M. J. Joncich and L. S. Stewart, .I. electrochem. 717 (1965). 3. D. Piron and K. Nobe, Corrosion 25, 67 (1969). Electrochim. Acla 4. S. Iseki. K. Ohashi and S. Naeaura. _ 17, 2239 (1972). 5. R. Badar. G. Bouyssoux and M. Remand, Mater. Rss. Bull. 11, 525 (1976). 6. M. P. Straumanis and P. C. Chen, Corrosion 7, 229 (1951). 7. M. Stern and H. Wissenberg, J. &crrochem. Sot. 106, 755 (1959). 8. N. T. Tomas and K. Nobe, J. ulectrochem. Sot. 116,1748 (1969); 117, 622 (1970). 9. D. Laser, M. Yeniv and S. Gottesfeld, J. electrochem. Sue. 12X, 358 (1978). 10. J. W. Schultze, U. Slimming and J. Weise, Ber. Bunsenges. phys. Chem. 86, 276 (1982). 11. K. Leitner, J. W. Schultze and U. Stimming, J. electrothem. Sot. 113, 1561 (1986). 12. A. Ya. Shatalov, T. P. Bondareva and L. E. Tsygankous, J. appl. Chem. U.S.S.R. 36, 561 (1963). I
-0.3
support a grant
REFERENCES
E
Fig.
appreciate the financial Forschungsgcmcinschaft and van Humboldt-Stiftung.
-0.3
-0.I
W
0.1
0.3
0.5
vs. NHE
Fig. 7. Mott&Schottky plots of passivated niobium electrodes in 0.5 M H,SO, as function of thickness. The electrodes were passivated at 2.5 mA cm -* to:3.2 V (1); 6.2 V (2); 9.2 V (3).
Passivated
Ti a.nd Nb electrodes
13. W. A. Badawy, A. G. Gad-Allah and H. H. Rehan, J. appl. electrochem. 17, 559 (1987). 14. J. F. McAleer and L. M. Peter, Disc. Faraday Sot. 70, 67 (1980); J. electrorhem. Sot. 129, 1252 (1982). 15. F. DiQuarto, C. Sunseri and S. Piazza, Ber. Bunsenges. phys. Chem. 90, 549 (1986); 91 437 (1987). 16. S. R. Morrison, Electrochemisty o/Semiconductors and Oxidized Metal Eleclrodrs, Plenum Press, New York (1980). 17. J. C. &ant and P. Vennereau, J. electroanal. Chem. 106, 103 (1980). 18. T. Hurl& and W. Wilhelmsen, Electrochim. Acta 31, 1139 (1986). 19. G. Marx, A. Bestanpouri, R. Droste, W. Erben, W.
20. 21.
22. 23. 24. 25.
1715
Schdnemann and D. Wegen, J. Iem-common. Metals 121, 507 (1986). A. Felske and W. J. Plieth, J. opt. Sot. Am. 83, 815 (1986). W. J. Plieth and K.-H. Fiirster, in Surface Studies with Lasers (Edited by Aussenegg, Leitner and Lippitsch), Springer Series in Chemical Physics, Vol. 33, p. 216, Springer Verlag, Berlin (1983). K. D. Allard, M. Ahrens and K. E. Heusler, Werkstos Korros. 26, 694 (1979). M. T. Duffy, C. C. Wang, A. Waxman and K. H. Zainiger, J. electrochem. Sot. 116, 234 (1969). M. Enyo, Electrochim. Acta 18, 163 (1973). T. Okada, Electrochim. Acta 28, 1113 (1983).
20 September
1988; in revised form 6 March
1989)
Abstract-Ti and Nb electrodes were pass&ted galvanostatically in nitric acid solutions. The electrochemical and photoelectrochemical behaviour of these electrodes was studied and compared with the behaviour of similar electrodes passivated in sulphuric acid solutions under the same conditions. The flatband potential and donor concentration of the passive films were obtained from the corresponding Mott-Schottky plots. The photoelectrochemical investigations were carried out using a pulsed laser which rcndcrcd measurements with photon energies smaller than the band gap. The results showed that the behaviour of passivated titanium in nitric acid was differing from its behaviour in sulphuric acid. The behaviour of the passivated niobium was not much aKected by the exchange of the two acids.
INTRODUCTION Valve metals, niobium and titanium are known to form stable oxide films in aqueous solutions[ 11. These metals have been subjected to intensive investigations concerning the kinetics of oxide film growth[2-111, the stability of the oxide films[l l-141 and the physical properties of the passive film itself[15-181. Nevertheless, the low-voltage passive behaviour of the metal and its photoelectrochemical properties-specially in the aggressive and, industrially, commonly used medium HNO,-appears to be rarely investigated. The great demand of stable materials used in chemical process units and application of valve metals to the electrochemical methods recently introduced into the purex process for waste minimization[19] makes the passivation of Ti and Nb and their behaviour in HNO, an important subject of further investigations. Pulsed lasers are a new light source to detect small photoeffects on semiconducting passive films formed on metals[20,21]. These pulse lasers even allow investigations with light wavelengths having energies smaller than the band gap of the semiconducting passive films formed on metals[20,21]. These pulse lasers even allow investigations with light wavelengths having energies smaller than the band gap of the semiconducting passive film. In this work we report on the behaviour of passive films formed on Ti and Nb electrodes in nitric acid solutions. To demonstrate the effect of HNO, on the passive film, a comparison with electrodes passivated in H,SO, at the same concentration is presented.
EXPERIMENTAL The details of the experimental trolytic cell were as previously
set-up and the elecdescribed[20]. The
* Permanent address: Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt.
working electrodes were made of massive cylindrical, spectroscopically pure titanium and niobium rods (Aldrich-Chemie). A stout copper wire was employed as electrica contact. The electrode was fitted into glass tubing of appropriate internal diameter with an epoxy resin, leaving an area of 0.35 cm* open at the front to the solution. The photocurrent measurements were carried out in a darkened room with a pulsed laser to illuminate the electrode surface. The main advantage of the pulsed laser in comparison to continuous light sources, was the large photocurrent/dark current ratio and the improved signal to noise ratio of the photocurrent. The used pulses had a half-width time of 10 ns with an intensity of 1=0.3 to 1.5 mJ per pulse. The small amounts of photocurrent were either (a) integrated or (b) averaged and integrated to eliminate the contribution of the electrolytic resistance and of the electrode capacity to the peak current. The typical peak current was of the order of 50 mA per light pulse. The pulsed dye laser was a FL 2001, Lambda Physik, filled with PBBO (1, = 390 nm) coumarin 307 (1, = 500 nm), or rhodamin 6C (A,,,= 58 1 nm) and pumped by an excimer laser with a repetition frequency of 7-20 Hz. All measurements were performed under potentiostatic control. Capacitance measurements were carried out technique at a frequency of using the “lock-in” 1330 Hz. The independence of frequency was checked between 0.65 and 50 kHz. Before each experiment the electrode surface was metihanically polished with diamond spray down to 0.25 pm, washed in ethanol and triply distilled water in an ultrasonic bath. Passivation of the electrodes was performed in the test solution at 2.5 mAcm_‘. After reaching the desired formation voltage, the current was interrupted and the measurements immediately carried out. The potentials were measured against the suitable reference electrode (either satured calomel electrode (see) or Hg/Hg,SO,/OS M H,SO,) and referred to the normal hydrogen electrode (nhe). The temperature was kept constant at 23°C throughout the experiments. 1711
1712
W.
RESULTS Film formation
AND
A.
BADAWY
et al.
DISCUSSION
and breakdown
The oxide film thickness is a function of the anodization potential. For Ti, a value lying between 1.4 and 2.5 nm V i was reported[lO, 221 and for Nb, a value of 2.1-2.5 nm V-r was given[15]. A value of 2.0 nm V-i was used if any calculations had to be done. In sulphuric acid solutions, Ti as well as Nb could be anodized to high voltages in several seconds; ey a 10 V TiO, layer was obtained in approx. 50 s. In nitric acid solutions, the anodization of the metal Ti to the same voltage--ie 10 V-took longer (approx. 15 min). In the literature different times requested for the films growths are reported which might be connected with different surface treatment. In sulphuric acid the formation potential of Ti could rise above 100 V without breakdown. ln nitric acid, at a potential drop of 12.7 V, a breakdown in the passive film was observed. The oxide film acquired a greyish, opaque appearance and turned to a spongy layer which then lost the pure blue colour of TiO,. Other experimental evidence for this behaviour provided by capacitance and photocurrent measurements (see below) shows that the more disordered structure of the oxide formed in nitric acid might be connected with localized electronic states approximately 0.84.2 eV below the band edge of the conduction band. In the case of Nb, no breakdown was observed either in H,SO, or in HNO, solution. Similar to Ti, the oxide film formation in HNO, needed much longer time than in HaSO,. As an example, the formation of a 9 V layer in nitric acid required 43 min, whereas in H,SO, and under the same conditions (2.5 mA cm ‘), it took only 5 min. In both solutions and up to 50 V, the oxide film on Nb always had a clear interference colour, ranging between light yellow and bluish-brown. Photocharge-potential
behauiour
Figure 1 illustrates the photocharge-potential behaviour of a Ti electrode passivated in 0.5 M H,SO, (curve 2) and another one passivated in 1 M HNO, (curve 1). Both were anodized galvanostatically at
1
1 N HN03
-,
1.0
1
I
2.0
I
I
3.0
E/V vs. NHE
Fig. I. Photocharge-potential relation of Ti electrodes passkated in: (1) 1 M HNO,: (2) 0.5 M H,SO, to 3.2 V (nhe). Light intensity I =0.3 mJ per pulse; 1 = 500 nm.
I
I
,
3
3
E/V vs. NHE
Fig. 2. Photocharge-potential behaviour of Nb electrodes passivated in : (1) 0.5 M H,SO,; (2) 1 M HNO, to 3.2 V (nhe). Light intensity I = 0.3 mJ per pulse; 1= 500 nm.
2.5 mA cm-’ to 3.2 V. The figure shows that the behaviour of the Ti electrode passivated in nitric acid differs from that of the electrode passivated in H,SO, solution. The varying behaviour reflects itself in the magnitude of the photocharge of the pulse per cm2 (Q,,,). The electrodes passivated and investigated in sulphuric acid did not show any photocurrent or photocharge up to 1.84 V. At more positive potentials, a weak linear increase of the photocharge with the potential during anodic polarization was recorded (curve 2). In nitric acid solution the behaviour is different. At high anodic potentials (approx. 2.5 V) an approximately exponential increase of the photocharge with the potential was recorded. While sweeping the potential cathodically, the photocharge showed a flat minimum in the potential range of 1.5 to 0.8 V. At a potential of 0.25 + 0.1 V, the photocharge passed through a maximum and then decreased again. This could be explained by localized electronic states 0.84.2 eV below the band edge of the conduction band. Obviously, these states are not generated in H,SO,. The behaviour of the passivated Nb electrode is presented in Fig. 2. The passivation potential was 3.2 V (nhe). The two curves shown in this figure demonstrate that the general behaviour of passivated Nb electrodes in nitric acid (curve 2) is not so dissimilar from the behaviour of the electrodes passivated in sulphuric acid (curve 1). In both electrolytes a maximum in the photocharge of the pulse per cm-’ (Qph) was observed. This maximum is shifted cathodically in 1 M HNO, (about 300 mV) with respect to the maximum measured in 0.5 M H,SO,. It is explained by additional electronic states 1.54.5 eV below the band edge of the conduction band. The photocharge or photocurrent measured in 1 M HNO, is slightly smaller than that measured in 0.5 M H,SO,. Capacity
I
I
0
measurements
Impedance measurements showed that, in a frequency range from 0.55 to approx. 40 kHz, the electrode capacitance is approximately frequency-independent. Therefore further capacitance measurements were made at a fixed frequency of 1330 Hz. Figure 3 illustrates the capacitance-potential behaviour of a Ti electrode passivated in 0.5 M H,SO, (1) and another one passivated in 1 M HNO, (2). both to 3.2 V (nhe).
Passivated
Ti and Nb electrodes
Both electrodes gave the same flatband potential (the intercept of the straight portion of the Mott-Schottky plot with the potential axis) of approx. 0 V (nhe). The donor concentration was calculated from the slope of the plot according to: N,
2 = ~ D,De
1 5X-
=/6E’
where N, is the donor concentration in cm ? e the electronic charge (1.60 x lo-is’ A s); D, the electric field constant (vacuum dielectric constant 8.85 x lo-l4 A s V-r cm-‘); 6C-‘/6E the slope of the Mott-Schottky plot and D the dielectric constant of the passive film. D was taken as 9[10]. For the passive layer of Ti formed and investigated in 1M nitric acid under the same conditions, a value of 28.5 x lOi cm-’ was obtained this being more than 6 times greater than the first one. This means that in nitric acid solutions, the number of donor terms increases. In the case of Nb electrodes, it was observed that the passivation and investigation of the passive films in nitric acid is accompanied by a decrease in the the concentration. shows donor Figure 4 Mott-Schottky plots of a Nb electrode passivated and
1 0
200
400
600
ElmV vs. NHE Fig. 3. Mott-Schottky plots of passive titanium films in; (1) 0.5 M H,SO,; (2) 1 M HNO,. Both electrodes were passivated galvanostatically at 2.5 mA cm-* to:3.2 V (nhe).
1713
investigated in 0.5 M H,S04 (1) and another in 1 M HNO, (2). Both electrodes gave roughly the same flatband potential of approx. - 175 mV (nhe). The donor concentration calculated for the electrode investigated in nitric acid is about half compared to the one investigated in sulphuric acid. The values are 3 6 x lOl9 cmW3 for the 1 M HNO, passivated eleccrnm3 for the 0.5 M H,SO, trode and 7.2 x lOi passivated one. The dielectric constant for Nb,O, was taken as 11[23]. From these results it is clear that Ti is more affected by the presence of HNO, than Nb. Effect
of the
oxide film thickness
To study the effect of the oxide film thickness on the general behaviour of the passive film in nitric acid, the electrodes were passivated to different formation potentials as described before and then investigated in the same solution. Figure 5 illustrates the variation of the photocharge with potential for titanium electrodes passivated in 1 M HNO, to 3.2, 5.2, 7.2, 10.2 and 12.2V (nhe) respectively, using a laser pulse of 0.3 mJ per pulse and a wavelength of 581 nm. The photocharge maximum (approximately 0.24.1 V below the band edge of the conduction band of the passive film) increases as the film thickness increases. At the large wavelength of 581 nm very small photocurrents independent of the film thickness were recorded in the anodic region more positive than 1 V. The dependence of the photocharge maximum on the thickness of the film may again be explained by additional electronic states in the oxide. Surface states could not explain the thickness dependence. It must be noted that the observed photocharge maximum in the case of Ti electrodes is dependent on the direction of the voltage scan. The maximum in the Q,,/potential behaviour was recorded only on going from anodic potentials into hydrogen evolution (Fig. 5, l-5). On reversing the scan direction after hydrogen evolution (which takes place near the flatband potential (0.0 V), the photocharge maximum has disappeared (Fig. 5, r). This phenomenon may be explained by the reduction of the electronic states responsible for the photocharge maximum.
J
Oh EimV vs. NHE Fig. 4. MottG%hottky plots of passive niobium films in: (1) 0.5 M H,SO,; (2) 1 M HNO,. _Bpth electrodes were passiva_ _ _. ted at 2.5 mA cm-’ to LI.L V (nhe).
2:o l:o EN vs. NHE
3.0
Fig. 5. Photochargeepotential behaviour of passivated titanium electrodes in 1 M HN03. The electrodes were passivatcd galvanostatically at 2.5 mA cm- * to:3.2 V (1); 5.2 V (2); 7.2 V (3); 10.2 V (4); 12.2 V (5). (r) The reverse scan of(5). Light intensity 1~0.3 mJ per pulse; i = 581 am.
W. A. BADAWY
1714
So far, the following model can be discussed for the photocharge maximum. In Fig. 6 the band structure is shown with localized states 0.8425 eV below the band edge of the conduction band. At more anodic potentials the states are discharged. With the potential scanned cathodically the Fermi level is raised above the localized states which then become occupied and serve as source for the photocurrent. The current increases until all states are occupied. Then the decreasing band bending leads to a decrease of the photocurrent. The chemical nature of the localized states is not yet known. Two suggestions can be discussed: (i) nitrogen oxides incorporated in the TiO,, or (ii) deep defects of TiO, resulting from the reaction with the HNO,. The states can be reduced by cathodic polarization into the hydrogen evolution region. Direct reduction or the intercalation of hydrogen into the TiO, matrix[24,25] can cause this reduction. The different behaviour after anodic and cathodic prepolarization is a characteristic property of Ti electrodes which could not be observed with the passivated Nb electrodes. Although it was observed that the behaviour of the passivated Nb electrodes was affected by hydrogen evolution, the photocharge maximum is still present afier hydrogen evolution (Fig. 2). The maximum in Fig. 2 was approximately independent of the film thickness. The effect of the film thickness on the behaviour of passivated Nb efectrodes either in 0.5 M H,SO, or in 1 M HNO, was further investigated by capacitance measurements on three Nb electrodes passivated in
p E
eI ul.
0.5 M H,SO, to 3.2, 6.2 and 9.2 V @he), respectively (Fig. 7). In order to be sure that the passive film was not contaminated with hydrogen, the anodic scan was chosen, ie the capacity was measured from +3.2 to -0.4 V (nhe). The capacitance decreased with film thickness according to the relation C- l/d. The MottGSchoottky plots are shown in Fig. 7. A part of curve 1 of Fig. 7 was shown in Fig. 4. In Fig. 7 the MottGSchottky behaviour was observed in a relatively small potential region between -0.05 and + 0.1 V. In Fig. 4 the linear region was larger: between f0.05 and 0.4 V. Such differences were connected with different pretreatment, eg different time of prepolarization. The donor concentration calculated from the linear portion decreased as the film thickness increased (eg N,= 6.8 x 10’9cm-’ and 0.62 x 10” cm ’ for a 3.2 and a 9.2 (nhe) layer, respectively). This explains why the photocharge maxima in Fig. 2 were independent of the film thickness. The electronic states responsible for the maxima in Fig. 2 did not increase with the thickness. CONCLUSIONS The behaviour of Ti electrodes during passivation or after passivation in nitric acid is different from that in sulphuric acid solutions. The passive films undergo a breakdown at a potential of 12.7 V (nhe) and the structure of the oxide film is changed. On the other hand, Nb electrodes fail to show any form of drastic change in behaviour during polarization in nitric acid solutions. This raises the question on the stability of Ti in HNO,. Surprisingly, recent corrosion studies in HNO, carried out by the radiotracer method, did not show any dramatic increase in the role of corrosion of Ti[19]. It seems that this point needs further investigation. Acknowledgement-We from the Deutsche from the Alexander
6. Band
model of TiO, layers with localized below the conduction band.
states
1. L.Young, Anodic Oxide Films, Academic Press, London (196f). Ser. 112, 2. M. J. Joncich and L. S. Stewart, .I. electrochem. 717 (1965). 3. D. Piron and K. Nobe, Corrosion 25, 67 (1969). Electrochim. Acla 4. S. Iseki. K. Ohashi and S. Naeaura. _ 17, 2239 (1972). 5. R. Badar. G. Bouyssoux and M. Remand, Mater. Rss. Bull. 11, 525 (1976). 6. M. P. Straumanis and P. C. Chen, Corrosion 7, 229 (1951). 7. M. Stern and H. Wissenberg, J. &crrochem. Sot. 106, 755 (1959). 8. N. T. Tomas and K. Nobe, J. ulectrochem. Sot. 116,1748 (1969); 117, 622 (1970). 9. D. Laser, M. Yeniv and S. Gottesfeld, J. electrochem. Sue. 12X, 358 (1978). 10. J. W. Schultze, U. Slimming and J. Weise, Ber. Bunsenges. phys. Chem. 86, 276 (1982). 11. K. Leitner, J. W. Schultze and U. Stimming, J. electrothem. Sot. 113, 1561 (1986). 12. A. Ya. Shatalov, T. P. Bondareva and L. E. Tsygankous, J. appl. Chem. U.S.S.R. 36, 561 (1963). I
-0.3
support a grant
REFERENCES
E
Fig.
appreciate the financial Forschungsgcmcinschaft and van Humboldt-Stiftung.
-0.3
-0.I
W
0.1
0.3
0.5
vs. NHE
Fig. 7. Mott&Schottky plots of passivated niobium electrodes in 0.5 M H,SO, as function of thickness. The electrodes were passivated at 2.5 mA cm -* to:3.2 V (1); 6.2 V (2); 9.2 V (3).
Passivated
Ti a.nd Nb electrodes
13. W. A. Badawy, A. G. Gad-Allah and H. H. Rehan, J. appl. electrochem. 17, 559 (1987). 14. J. F. McAleer and L. M. Peter, Disc. Faraday Sot. 70, 67 (1980); J. electrorhem. Sot. 129, 1252 (1982). 15. F. DiQuarto, C. Sunseri and S. Piazza, Ber. Bunsenges. phys. Chem. 90, 549 (1986); 91 437 (1987). 16. S. R. Morrison, Electrochemisty o/Semiconductors and Oxidized Metal Eleclrodrs, Plenum Press, New York (1980). 17. J. C. &ant and P. Vennereau, J. electroanal. Chem. 106, 103 (1980). 18. T. Hurl& and W. Wilhelmsen, Electrochim. Acta 31, 1139 (1986). 19. G. Marx, A. Bestanpouri, R. Droste, W. Erben, W.
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