Abstracts

Article

Electrochemical Characterization of Thin Passive Films on Nb Electrodes in H₃PO₄ Solutions

Sonia R. Biaggioª, Nerilso Bocchiª, Romeu C. Rocha-Filhoª ^* , and Francisco E. Varela^b

^aDepartamento de Química, Universidade Federal de São Carlos, C.P. 676, 13560-970 São Carlos - SP, Brazil

^bInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, (1900) La Plata, Argentina

Received: April 7, 1997; July 30, 1997

Introduction

Being a valve metal, niobium passivates with a high corrosion resistance in most common aqueous media^1-4. The passive films, constituted of amorphous niobium pentoxide³, can be easily thickened by anodic polarisation^3-7; the oxide growth occurs by high-field ion migration in the film^8,9. Amorphous niobium pentoxide has a density¹⁰ of 4.74 g cm^-3 and a rather high concentration of oxygen vacancies (10¹⁹ cm^-3) that, injected at the metal/oxide interface, act as donor states and make the oxide an n-type semiconductor^11,12; Di Quarto et al.⁴, studying films of various thickness (21 nm £ d £ 210 nm) formed in 0.5 mol/L H₂SO₄, determined their relative permittivity as being equal to 42. Niobium pentoxide crystals have a band gap of 3.4 eV¹³.

Although anodic films of niobium have been quite extensively studied (most of the data in the literature are for high-voltage anodised niobium), there is no report in the literature of a systematic study of the impedance behaviour of thin films (< 20 nm). Recently we have investigated the electrical and semiconducting properties of thin anodic films of tungsten^14,15, zirconium¹⁶ and tin-indium alloys^17,18. Herein we are extending these systematic investigations to niobium; thus, the main purpose of this work was to characterise the electrochemical behaviour and electrical properties of thin anodic passive films potentiostatically formed on niobium in aqueous 0.5 mol/L H₃PO₄ solutions, using electrochemical impedance spectroscopy. The influence of film thickness on the semiconducting properties of the thin films was investigated.

Experimental

Polycrystalline niobium rods (99.98% purity, 0.70 cm diameter) included in epoxy resin were used as working electrodes. Prior to each experiment, the electrodes were mechanically polished down to 600 grit silicon carbide paper. The counter electrode was a cylindrical platinum grid placed around the working electrode. A saturated calomel electrode (sce), coupled to a Luggin probe, was used as reference for all potential measurements. The experiments were carried out at room temperature (25 °C) in 0.5 mol/L H₃PO₄ aqueous solutions (pH 1.3), prepared from AR grade chemicals and purged with N₂ prior and during each experiment.

In order to characterise different redox processes occurring on the niobium electrode, triangular potential scans were performed between preset cathodic (E_s,c) and anodic (E_s,a) switching potentials at different scan rates v, using an Eco Chemie Autolab/GPES potentiostat/galvanostat system.

The impedance behavior study was done on thin Nb₂O₅ films potentiostatically grown. This gowth was carried out at a given formation potential (E_f) until steady-state currents were reached, usually after ca. 1 h; E_f was set between 1 V and 5 V (vs. sce). For each film, electrochemical impedance data were acquired in the 5 mHz £ f £ 10 kHz frequency range, at E_f; an rms AC signal modulation of 30 mV was used. In order to obtain data to analyse the semiconducting properties of the films, high-frequency impedance data (100 Hz £ f £ 10 kHz) were acquired at several potentials increasingly more negative than E_f. An Eco Chemie frequency response analyser module, coupled to the Autolab/GPES potentiostat/galvanostat system, was used to perform all the impedance measurements.

Results and Discussion

Voltammetric data

A voltammogram of Nb in 0.5 mol/L H₃PO₄ obtained between E_s,c = -0.8 V and E_s,a = 5 V (vs. sce) at v = 20 mV s^-1 is shown in Fig. 1. The positive-going potential scan exhibits a well-defined anodic current peak (A1) at about -0.20 V, followed by a wide passive potential region that is characteristic of the growth of anodic films on valve metals. The voltammogram also exhibits a step-wise increase of the anodic current when the electrode potential is ca. 1.8 V; this increase was previously reported by Di Quarto et al.¹² for voltammograms obtained on niobium in sulphuric acid solutionsand was tentatively assigned to the onset of oxygen evolution. No significant oxygen evolution was noticed, however; further studies in several acid solutions are now in progress in order to better characterise this oxidation process¹⁹. As the voltammetric scan is reversed, the current decreases sharply and no cathodic current contributions corresponding to the electroreduction of surface oxides are observed; at about -0.6 V, hydrogen evolution begins.

Although a detailed analysis of the voltammogrammes is out of the scope of this paper, chemical and thermodynamic considerations add information supporting that peak A1 is related to the oxidation of the electrode surface to Nb₂O₅^1,20-22. It is important to mention, however, that the peak A1 potential, -0.20 V, differs considerably from -0.97 V corresponding to the Nb/Nb₂O₅ reversible electrode potential²⁰. The latter potential was estimated from standard Gibbs energy data of both products and reactants for the oxidation of Nb at pH 1.3. The tendency of niobium to form very stable oxygen compounds^1,20 strongly suggests that, during the surface preparation, an irreversible oxidation process occurs at the electrode surface involving niobium suboxides¹². Moreover, the detection of a surface film formed before anodisation was reported by several authors^4,12,21,22; Di Quarto et al.⁴ estimated the thickness of this pre-film as being about 4 nm. Thus, the presence of this previously formed film might determine a shift of the electrode oxidation potential to a value more positive than that for the Nb/Nb₂O₅ reversible process.

Electrochemical impedance data

The impedance profile of the passivated Nb electrodes was found to depend on both E_f and the operational potential (E < E_f) at which the impedance spectra were obtained. Typical complex-plane and Bode plots obtained at different E_f values are shown in Fig. 2. In the 10 mHz - 10 kHz frequency range, only one capacitive semicircle is present for all E_f values. The corrosion resistance, defined as R_p = lim_{w 0}Z(jw), becomes smaller as E_f is decreased from 5 V to 2 V and then becomes larger for E_f = 1 V. These variations in the value of R_p are in agreement with the voltammetric profile shown in Fig. 1, in which a step-wise variation in the anodic current was observed at about 1.8 V.

As pointed out by Bai and Conway²³, a detailed transfer function interpreting the whole impedance spectra cannot be elucidated by means of steady-state techniques alone, especially when the electrode process involves a multi-electron transfer process such as Nb(0) Nb(V), since information about intermediate reaction steps is not available. However, impedance data are diagnostically useful, as they can provide a variety of information, in particular on the main reactions contributing to the faradaic current at the reacting interface and on the electrical characteristics of the anodic films on the metal electrode^14-18.

An appropriate transfer function interpreting the complete set of experimental data, in the 1 V £ E_f £ 5 V potential range, should include the resistance of the passive film (R_ox) in series with the faradaic impedance of the Nb(0) Nb(V) oxidation process represented by the charge transfer resistance of the process (R_ct), and these in parallel with the capacitance (C) of the metal/passive film/electrolyte system. Thus, the impedance spectra will be discussed according to the following transfer function:

where w is the angular frequency (2pf), and the high-frequency limit, R_W, is the ohmic resistance of the electrolyte. Unfortunately, resistances R_ct and R_ox are in series in Eq. 1 and, consequently, cannot be determined separately. However, since R_ct < R_ox for this case, one can assume that the value of R_ox is directly accessible through the fitting procedure and that the value of R_ct is, thus, inaccessible. The experimental spectra are satisfactorily described by transfer function (1) if an appropriate choice of parameters is made; the theoretical functions obtained from the parameters fitted with a complex non-linear least-squares algorithm are depicted as continuous lines in Fig. 2. The values obtained for the different parameters at the potentials E_f are shown in Table 1. As can be seen from this table, the value of C decreases with increasing E_f; this capacitance can be seen as a series combination of the oxide (C_ox) and the Helmholtz layer capacitances (C_H), i.e.:II

The values of C_ox were obtained through Eq. 2, assuming a conservative constant value⁴ of 20 mF cm^-2 for C_H. Figure 3 shows that the relationship between C_ox^-1 and E_f is linear; consequently, C_ox can be directly correlated with the thickness d of the passive film using the formula for a parallel plane condenser:

where e_r denotes the relative permittivity of the film and e_o the permittivity of vacuum. Di Quarto et al.⁴ determined by optical methods that the thickness d of anodically grown Nb₂O₅ films increases linearly with the applied formation potential E_f, i.e.:

where E° is the potential at which the film starts to grow and a is the anodising ratio.

From eqs. 3 and 4 one obtains

If an average anodising ratio a of 2.2 nm V^-1 is assumed⁴, the relative permittivity of the thin passive films can be estimated from the slope of the C_ox^-1vs.E_f plot shown in Fig. 3 as being about 44. This value agrees quite well with the one previously reported by Di Quarto et al.⁴ (e_r = 42) for thicker films grown galvanostatically in sulphuric acid solutions. Finally, the estimation of E° from Fig. 3 (see Eq. 5) yields a value close to -0.55 V, a value more cathodic than the potential corresponding to anodic peak A1 observed in the voltammograms and consistent with the zero-current potential obtained for niobium in this electrolyte (see Fig. 1).

The values of R_ox obtained for the different formation potentials are also shown in Table 1. They increase with E_f, except for the value obtained at E_f = 1 V, which is exceptionally high. As pointed out above, for potentials greater than 1.8 V an additional contribution to the anodic current appears in the voltammograms of Nb in H₃PO₄. The significant decrease in the value of R_ox when E_f is varied from 1 V to 2 V indicates that the properties of the passivating film greatly change; as mentioned before, this matter is being further studied. For 2 V £ E_f£ 5 V, R_ox steadily increases, reflecting the increasing film thickness.

It is expected that for a passive layer grown at a certain E_f, the faradaic reaction associated with the formation of the film should decrease if the electrode operational potential is shifted to a value E < E_f. As pointed out previously¹⁵, this happens because the faradaic reaction decreases sharply for fields lower than the film formation field. Thus, from impedance spectra obtained at E < E_f , the values of C decrease with increasing E (see Fig. 4) until they become nearly constant at potentials close to E_f. This constant value, which was found to be dependent on the film thickness, i.e., E_f, coincides with the value of C obtained from the whole impedance spectra at E_f fitted with Eq. 1. The change in the value of C with E can be explained by assuming that the passive layers formed on niobium behave as non-stoichiometric semiconducting films. Taking into account the large band gap of Nb₂O₅ films (3.4 eV), large anodic polarisations induce space charge layers that encompass the whole film, leading to a constant capacitance, whose value reflects both the thickness and the relative permittivity of the film according to Eq. 3. Then, the whole thin film is in deep depleted conditions, with surface charges at both the oxide/solution and the metal/oxide interfaces^13-15, and C_ox is the capacitance available. On the other hand, at the lower operational potentials, C can be assumed as corresponding to the space charge capacitance (C_sc) of the semiconductor Nb₂O₅ film in the depleted region, after taking into account the value of C_H (20 mF cm^-2). Hence, for the films grown at different E_f, C increases as E_f becomes less positive (Fig. 4).

The Mott-Schottky plots (C_sc^-2vs. E) for the Nb₂O₅ films formed in H₃PO₄ are shown in Fig. 5. From these results, which clearly characterise an n-type semiconductor behaviour of the oxide film, it is possible to determine the number concentration of donors (N_D) in the space charge region as well as to estimate the flat band potential (E_FB) for the films grown at each anodic potential. This was done using the above calculated value of the relative permittivity of the Nb₂O₅ films formed in H₃PO₄ and the well-known Mott-Schottky relationship:

where N_A is the Avogadro constant, F the Faraday constant, T the thermodynamic temperature and R the molar gas constant.

As shown in Table 2, N_D decreases with E_f (i.e., increasing film thickness), indicating that the average number concentration of donors becomes smaller as the film becomes thicker. This behaviour, which is similar to the one previously reported^14,15 for thin WO₃ films anodically grown on tungsten in several electrolytes, can be partially due to a higher contribution from the surface states on decreasing the film thickness. However, an influence of the thickness on the N_D value as a consequence of a higher defect concentration in thin films cannot be excluded. Nevertheless, the values obtained for N_D are consistently larger than the ones reported by Di Quarto et al.⁴ for thicker films.

From Fig. 5 and using Eq. 6, E_FB can be estimated as about -0.72 V. This value is somewhat more negative than the values previously reported for thicker Nb₂O₅ films galvanostatically grown on Nb electrodes in 0.5 mol/L H₂SO₄, E_FB» 0.2 V (vs. sce)⁴, and in 0.1 mol/L oxalic acid, E_FB» -0.2 V (vs. sce)²⁴.

Although some doubts have been raised concerning the application of the Mott-Schottky theory to amorphous semiconductors^4,11,25, the non-linearity of the C_sc^-2vs. E plots at potentials far from the flat band potential is an expected fact for thin semiconducting films subjected to high electric fields. As mentioned above, for potentials far from E_FB, the thickness of the space charge layer becomes equal to the film thickness, and the capacitance of the interface behaves according to Eq. 5.

Acknowledgments

This work was made possible by grants from FAPESP - Fundação de Amparo à Pesquisa do Estado de São Paulo (Brazil), including a visiting-scientist fellowship to F. E. Varela; support from CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazil) is also gratefully acknowledged.

FAPESP helped in meeting the publication costs of this article

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