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Electronic structure of anodic oxide films formed on cobalt by cyclic voltammetry
Corrosion Science 48 (2006) 2971–2986 www.elsevier.com/locate/corsci
Electronic structure of anodic oxide films formed on cobalt by cyclic voltammetry M. Pontinha
a,d
, S. Faty b, M.G. Walls c, M.G.S. Ferreira M. Da Cunha Belo a,c
a,e,* ,
a
d
Instituto Superior Te´cnico, Departamento de Engenharia Quı´mica, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Faculte´ des Sciences et Techniques (VCAD), De´partement de Chimie, Dakar, Senegal c Centre d’Etudes de Chimie Me´tallurgique, CNRS, Vitry-sur-Seine, France Instituto Superior de Engenharia de Lisboa, ISEL, Departamento de Engenharia Quı´mica, Lisboa, Portugal e Universidade de Aveiro, Departamento de Engenharia Ceraˆmica e do Vidro, Aveiro, Portugal Received 20 October 2004; accepted 18 October 2005 Available online 22 December 2005
Abstract The chemical composition and degrees of oxidation of thin oxides films formed on cobalt by cyclic voltammetry have been investigated by Auger electron spectroscopy and XPS analysis. In addition the electronic properties of the films have been examined by capacitance measurements using the Mott–Schottky method and photoelectrochemistry. The analytical results show that the thickness of the cobalt oxide films increases with the number of cycles and varies from a few tens to a few hundreds of angstro¨ms. When observed by transmission electron microscopy and diffraction, the films appear compact and well crystallised (spinel structure). Capacitance measurements show that both very thin and relatively thick films exhibit p-type semiconductivity. The band structure model proposed and the interpretation of the oxidation processes in terms of lattice ionic defects, can explain the film growth mechanism. The study shows how the electric fields created by the development of space charges influence both ionic transport and electronic transfer of charges. 2005 Elsevier Ltd. All rights reserved. Keywords: Cobalt oxide films; Cyclic voltammetry; Electronic structure
* Corresponding author. Address: Instituto Superior Te´cnico, Departamento de Engenharia Quı´mica, Av. Rovisco Pais, 1049-001 Lisboa, Lisboa, Portugal. Tel.: +351 21 841 7234; fax: +351 21 840 4589. E-mail address: [email protected] (M.G.S. Ferreira).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.10.007
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1. Introduction Despite the considerable work that has been carried out on the electronic structure of passive films, there is still a lack of information concerning the exact relationships existing between the electronic and ionic processes. For the common case of a semiconducting oxide growing on a metal, the development of a single surface charge layer in the metal is accompanied by a space charge distribution in the oxide. The principal characteristic of a space charge region is that it produces a potential barrier which may have a pronounced influence on the motion of charged species. In fact, the predominant transport processes are supposed to take place in this part of the oxide, especially at low temperatures. Passive films are very thin (a few nanometers) and highly doped. The contribution of space charges to film growth processes and film properties in the case of very thin films needs further research. The present investigation concerns oxide films formed anodically on cobalt by cyclic voltammetry. This method when applied to metals like iridium [1] and ruthenium [2] yields films whose thickness increases with the number of cycles. The chemical characterisation of the oxide films is performed using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis and the structural observations conducted by transmission electron microscopy (TEM). The electronic structure is studied through capacitance measurements and photo-electrochemistry. Some theoretical models describing ionic transport under the influence of space charges are discussed. Then, on the basis of the results obtained, a model for film growth is proposed, taking into account the electronic band structure of the metal–film and film–electrolyte interfaces. 2. Experimental The cobalt used was high purity 99.995% (wt%) metal. The surface of electrodes was abraded with wet SiC paper, grit size 1200. The anodic oxide films were formed on cobalt by cyclic voltammetry in the buffer solution composed of H3BO3 (0.05 M) + Na2B4O7, 10H2O (0.75 M) and of pH 9.2 at 22 C. After cathodic treatment (1.5 V/SCE, 300 s), voltammograms were obtained by performing a potential scan between 1.1 and 0.8 V at 100 mV/s. The reference electrode used in this research was the saturated calomel electrode (SCE). The analytical experiments were performed using a 310 F Microlab (VG Scientific) equipped with a field emission electron gun, a concentric hemispherical analyser and differentially pumped ion gun. Auger spectra were taken using a 10 keV, 50 nA primary electron beam. The angle between primary beam and the surface normal was 30. Spectra were acquired in constant retard rate mode (CRR = 10 eV), the energy resolution being about 0.2%. The analyser was calibrated using the following peak energies: Cu LMM at 918.62 eV; Ag MNN at 357.80 eV and Au NVV at 70.1 eV. Ion etching was performed at a pressure of 1 · 107 mbar using high purity argon. The etching current was approximately 0.75 lA/mm2. The etched area was a crater with a diameter of 0.5 mm, whereas the electron beam had a spatial resolution of 100 nm. XPS experiments were performed on the samples surface, before and after ion sputtering. The sputtering was made using a low energy ion beam (1.5 keV) in order to minimise the effects of reduction in the oxidised metallic species. A non-monochromated Mg anode (Ka = 1253.6 eV) was used. Spectra were obtained in constant analyser energy mode (CAE = 30 eV). For this value, the energy resolution is 1 eV.
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Electron microscopic examination was performed with a JEOL FX2000 microscope on flakes of film scratched off the metallic substrate. Electrochemical impedance measurements were made using a PAR 273A potentiostat and a Brookdeal 5208 double phase synchronous detector. The Mott–Schottky plots were obtained for each film by performing a potential scan in the cathodic direction. Simultaneously, impedance measurements were performed at 3 KHz with a signal amplitude of 10 mV (rms). Capacitance values were calculated assuming a series RC network as equivalent circuit. In the photo-electrochemical study the working electrode was illuminated through a quartz window with monochromatic light; a lens was used to focus a sharp spot on the working electrode, beam area 0.36 cm2. The optical instrumentation consisted of a 150 W Xe lamp, a 250 mm F/4 monochromator, a stepper motor to control the wavelength and mechanical chopper. The grating is usable in the range 250–700 nm with 280 nm blaze wavelength and a blocking filter was used above 450 nm to eliminate any second harmonic effects. All photocurrent measurements were normalized with respect to the flux recorded by a silicon photodiode. The photocurrent was recorded by connecting the current output of the potentiostat to the lock-in amplifier and recording the voltage output at 19 Hz (the chopping frequency). The reference phase was adjusted to ensure that the lock-in displayed a zero value from the maximum signal. The recorded values were then manipulated to calculate the photocurrent and quantum efficiency values using a spreadsheet software package. 3. Results 3.1. Electrochemical behaviour of the cobalt oxides Fig. 1 shows the voltammograms obtained when the cobalt electrode undergoes an increasing number of cycles. During the first cycle, the current is fairly independent of the potential. However, as the number of cycles increases, the voltammetric plots reveal the existence of two distinct potential regions; a region of small currents situated at low potentials and a region of relatively high currents situated at more positive potentials. The most important feature of the voltammetric diagrams is the fact that the current decreases as the number of cycles increases. Such behaviour differs from that observed in the case of oxide formed by cyclic voltammetry on an iridium electrode and can be explained by assuming that the film formed on cobalt is compact while that formed on iridium exhibits high porosity [1]. 3.2. TEM observation Fig. 2 shows a TEM micrograph of the cobalt oxide film formed after 400 cycles. The film appears relatively compact. Furthermore, the electron diffraction pattern reveals a spinel structure. 3.3. Auger and XPS study Fig. 3 shows the Auger depth profiles obtained from films formed after different number of cycles. For a film formed after five cycles the sputtering time necessary to reach the
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Fig. 1. Cyclic voltammetry I = f(U) obtained on cobalt. The number of cycles is indicated in the figure.
Fig. 2. (a) TEM micrograph and (b) diffraction pattern obtained from a cobalt oxide film formed after 400 cycles.
metallic substrate is much smaller than when compared to the film formed after 400 cycles. This means that the film thickness increases greatly with the number of cycles.
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The Co2p X-ray photoelectron spectra from the cobalt films, which had been prepared by cyclic voltammetry using 400 cycles is shown in Fig. 4a. Also represented is the evolution from the surface of the film (level 1) to the bottom film–metal interface (level 5) obtained for different sputtering times. Fig. 4b presents the anhydrous and hydrated molecular fractions in each of these successive levels. The chemical states were identified by comparing the photoelectron binding energies with those obtained from the literature for cobalt. The position of shake-up satellites of the main electron peak (Co2p3/2) was also used to aid chemical identification. Cobalt compounds of valence two are paramagnetic and have a strong satellite around 6 eV above the Co2p3/2 line; Co(III) does not have any satellite. The oxide of mixed valence (Co3O4) has a weak satellite that characterizes the minor component Co(II). At deep levels in the oxide (slightly in level 3 and clearly in levels 4 and 5) the Co2p3/2 spectra exhibit a peak around 778.7 eV characteristic of cobalt metal. At all levels there is
Fig. 3. (a) Auger depth profiles obtained from the films formed after different number of cycles. (b) Co and O Auger depth profiles after 400 cycles.
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Fig. 4. (a) XPS spectra from cobalt films formed after 400 cycles. (b) Anhydrous and hydrated molecular fractions for each of the successive levels indicated in the XPS spectra.
a peak around 781.7 eV with a full width at half maximum (FWHM) of 4.5 eV, which suggests the presence of more than one component. In order to characterize further the individual components, the Co2p3/2 and O 1s spectra were deconvolved. It should be remarked that the deconvolution of the spectra was not easy because, although one knows that the spectrum of Co3O4 is composed of highspin and low-spin components, the 2p3/2 binding energies for Co3O4 (around 779.6– 779.9 eV) and high-spin cobalt in CoO are close (780–780.5 eV). At the outer surface of the film the cobalt ion is preferentially in valence III as expected because the surface is more oxidized; the satellite is thus very weak. At level 2 the satellite is stronger indicating the presence of Co in valence II, and the presence of Co(OH)2 is detected through a peak at around 781.7 eV. At deeper levels the satellite peaks obtained from the deconvolution are in accordance with the satellites of Co(II) in CoO and Co3O4. As shown in Fig. 4b close to the metal–film interface the oxides predominate, while in the region near the film–electrolyte interface the film is essentially composed of hydroxides. It can also be observed that the CoO can only be detected in the innermost part of the film, while that of Co2O3 can only be detected in the outermost part.
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3.4. Capacitance measurements (Mott–Schottky study) The properties of a semiconductor–electrolyte interface can be described by Mott– Schottky theory [3,4], according to which the space charge capacitance of a p-type semiconductor is given by 1 2 kT U FB U ¼ ð1aÞ e C 2sc qN A e0 e where e is the relative permittivity of the oxide, e0 is the vacuum permittivity, q is the elementary charge (+e for electrons and e for holes), NA is the acceptor concentration, U is the applied potential and UFB is the flat band potential. The technique is based on the fact that at a Schottky junction, the space charge layer gives rise to a space charge capacitance, CSC, that has an effective complex impedance Rł;+ (ixCSC)1. Both the ohmic part R and the capacitance CSC can be determined by superimposing an a.c. voltage of frequency x on a d.c. bias applied across the junction. Lock-in detection at frequency x allows the separation of CSC and R. Fig. 5 shows the Mott–Schottky plot (C2 versus the applied potential U) obtained with oxide films formed after different number of cycles. The measured capacitance is high in the upper part of the potential region and becomes significantly smaller in the lower potential region of the voltammograms. Two straight lines with negative slopes can be observed in the Mott–Schottky representation, which means that the film must be composed of two layers of different doping density. In fact, when a depletion region is developed we have to consider different situations. The first one corresponds to a thickness of the space charge not higher than that of the outermost layer and can be described by Eq. (1a). When the thickness of the depletion region increases and reaches the inner layer, the following modified Mott–Schottky equation holds [5]: 2 1 2 kT N a1 N a2 W 1 U FB U ¼ e N a2 ee0 C 2sc qN A e0 e
ð1bÞ
in which Na1 and Na2 are the doping densities (acceptor species) of the inner and outer layers respectively and W1 is the thickness of the outer layer. Table 1 gives the doping densities calculated using the straight lines 1 and 2 of the C2 = f(U) representation (Fig. 5). As can be observed, the doping levels calculated for the very thin films forming during the first cycles are very high (in the range 1020 cm3). However when the number of cycles increases, the doping density is situated in the range 1019 cm3. 3.5. Photoelectrochemical measurements The fundamental optical transitions (band gap) were determined from using equations established by Ga¨rtner [6] and for a semiconductor/electrode junction by Butler [7]. The photo-response near the band gap can be described by g¼
I ph ðhm Eg Þn ¼ qAW hm U
ð2Þ
where g is the quantum efficiency, Iph is the photocurrent, U is the flux of photons (energy hm), q is the electronic charge, A is a constant, W is the thickness of the space charge layer
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Fig. 5. Influence of the number of cycles on the Mott–Schottky plots. (a) 2 cycles; (b) 50 cycles; (c) 400 cycles.
and Eg the band gap energy, where n depends on the type of the transition between the valence band and the conduction band. Generally, the analysis of the photocurrent spectra for passive films shows that n = 2 was the most appropriate value to represent the photoelectrochemical behaviour observed. This value corresponds to indirect transitions in the crystalline band model or non-direct transitions in amorphous films.
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Table 1 Acceptor doping concentrations (Na/cm3) assuming e = 10 Number of cycles
Na1
Na2
2 5 50 100 400 2275
3.34 · 1020 5.95 · 1020 6.13 · 1020 5.61 · 1020 4.36 · 1020 2.46 · 1020
1.16 · 1020 2.30 · 1020 2.56 · 1019 2.46 · 1019 1.80 · 1019 1.34 · 1019
The energy spectrum obtained from the oxide film formed after 400 cycles reveals three important values of photon energy (Fig. 6). The first is at 1.1 eV, in the infrared. The second appears at 2.13 eV in the visible, while the third appears at 3.55 eV, in the UV region of the light spectrum. These energy values obviously correspond to transitions in cobalt oxides or hydroxides. Similar energy spectra are obtained from films formed after a smaller or larger number of cycles. From the XPS results one has to consider the presence of CoO, Co3O4 and Co2O3 and of the hydroxides. The crystalline structural units of the divalent and/or trivalent oxides are the octahedron and the tetrahedron, with the Co ions being in high-spin and low-spin configurations respectively [8]. The structure of Co3O4 is that of a spinel with a cation dis3þ 2 tribution between tetrahedral and octahedral sites written as Co2þ tet 2½Cooct ]O4 , in which 2+ 7 3+ 6 bivalent Co (d ) ions of high spin and Co (d ) ions of low spin exist. Then, if one considers the crystal field theory the following analysis can be made: in the ground state, the spectroscopic terms corresponding to the high-spin Co2+ ion (d7) are 4F and 4P. Under the influence of an octahedral ligand field the 4F term splits into three other terms which are 4 T1g(F), 4T2g(F), 4A2g(F) in order of increasing energy. The 4P term is transformed into a 4 T1g(P) term of the same energy. In the case of a tetrahedral crystal field, the split energy levels are inverted.
Fig. 6. Photo-electrochemical response obtained at 0.2 V in borate buffer solution (pH 9) with cobalt oxide films previously formed after 400 cycles.
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On the whole, if one considers only high-spin d6 and d7 structures, at least eight spinallowed transitions can be expected for this system. This very high number of transitions leads to difficulties when trying to assign them to experimentally determined energy values, particularly when the determination is indirect. Fortunately, the literature provides spectroscopic data of some cobalt compounds. A careful examination of these results, with the help of the well known Fajans Tsuhida spectroscopic series can simplify the task [9–11]. Taking into account that the 5D(d6) energy level corresponds to the high-spin Co3+ ions under the influence of an octahedral field, a possible transition may be 5
T2g ! 5 Eg
whose energy is 2.26 eV. On the other hand, if the transitions, which correspond to the configuration d7, can be related to Co2+ in octahedral crystal field, then, as a consequence of the split of the 4F energy level, one has to consider the following transitions: 4
1
T1g ðFÞ ! 4 T2g ðFÞ;
4
2
T1g ðFÞ ! 4 A2g ðFÞ;
4
3
T1g ðFÞ ! 4 T1g ðPÞ
Transitions 1 and 2 correspond to the energies of 0.92 and 2.1 eV, respectively [9]. Transition 3 has an energy of 2.5 eV, which is the difference between the ground state and the P level. In the tetrahedral crystal field the possible transitions are: 4
1
A2g ðFÞ ! 4 T2g ðFÞ;
4
2
A2g ðFÞ ! 4 T1g ðFÞ;
4
3
A2g ðFÞ ! 4 T1g ðPÞ
Transitions 1, 2 and 3 have energies of 0.5, 0.92 and 2.4 eV respectively [9]. The experimental energy values obtained by photo-electrochemistry are 1.1, 2.13 and 3.55 eV. The first may be compared to the value of the transition 4T1g(F) ! 4T2g(F) or to that of the transition 4A2g ! 4T1g. The calculated values of 2.6 and 2.1 are not very different from the experimental value of 2.13 eV. This energy is located in the visible and can be 4 related to the 4T1g(P) T1g(F) transition in the octahedral Co2+ oxide or to the 4 4 T1g(P) A2g(F) transition of the tetrahedral Co2+ ion. Finally the 3.2 eV peak is too high to be due to d–d transitions. This value situated in the UV part of the light spectrum, should arise from a charge transfer between the 2p band of oxygen and the Co ions d levels [9]. This means that one has in this case a transition from the valence band to the conduction band. 4. Discussion Mott–Schottky plots reveal that both very thin and relatively thick passive films formed on cobalt by cyclic voltammetry behave as p-type semiconductors. This means that in all cases valence band pinning occurs and a space charge region develops at the film-electrolyte interface. The existence of space charges in the case of very thin passive films has been discussed in the literature [12–17]. The growth kinetics of the anodic oxide films formed on cobalt depends in a complex manner on the situation created in the different potential regions by the ‘‘scanning’’ in the voltammetric cycles. These situations can always be related to the varying nature of the space charges developed at the metal–oxide and film–electrolyte interfaces, whatever the thickness of the film. Since the Fermi level, i.e., the electrochemical potential, is main-
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tained constant throughout the film thickness, what changes is the chemical potential l or/ and the electrostatic potential u, according to: U = EF = l + qu. This means that the chemical potentials for ionic defects and holes may be different in function of the location, while the electrochemical potential remains constant. Since the development of space charges starts from a situation of zero charge at the flatband potential, the electric field created by the applied potential at the metal–film and film–electrolyte interfaces must oppose each other. When a depleted space charge region is formed at the film–electrolyte interface, another one is also formed at the metal–film interface. This generates back-to-back electric fields in opposite directions. Morrison [18] proposes a similar configuration for electron transfer in the metal–film–electrolyte system. Fig. 7 shows that the flatband potential obtained by capacitance measurements separates a potential region of low currents (region I) from one of relatively high currents (region II). Such behaviour can be explained if one discusses the mechanisms of mass and charge transport through the film and the reactions at both metal–film interface (interface 1) and film–electrolyte interface (interface 2). One must also considers the situation created at these interfaces when the film is submitted to cyclic voltammetry. If one considers that in the oxide film the predominant defects are holes and vacancies, the latter in two ionised forms, the following equations apply at the metal–film interface [19]: 2þ Co þ V2 þ 2e Co ¼ Co
ð5aÞ
2þ Co þ V2 þ h þ 2e Co =h ¼ Co
ð5bÞ
or 2 where the symbols V2 Co and VCo =h denote unpaired and paired cation vacancy sites in the 2 oxide, e the electrons and h the holes. V2 Co and VCo =h correspond in fact to negative and neutral electrical charges respectively. Note that when Co2+ migrates to a vacancy site it creates a neutral situation.
Fig. 7. Schematic representation of C2(U) and I(U), which demonstrates that the flat band potential UFB separates the region of low currents (region I) from that of high currents (region II).
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At the film–solution interface (interface 2), possible reactions are: 2 Co2þ þ 2O 2 ! CoO þ VCo
ð5cÞ
2 Co2þ þ O 2 þ h ¼ CoO þ VCo =h þ 2e
ð5dÞ
or
The reactions of water decomposition and oxide and hydroxide formation involving hole consumption from the valence band could be: O2ads () O ð5eÞ 2 þh H2 O þ O ( ) HO þ OH þ h ð5fÞ 2 2 It appears that the various combinations of the exchange currents controlled by these reactions determine whether the film transports electricity as an ionic, electronic or mixed conductor. Indeed, Eq. (5a) provides a sink at interface 1 for vacancies generated at interface 2 through Eq. (5c). Thus the transport is achieved through vacancy migration and the film behaves as an ionic conductor. Furthermore, reaction (5b) at interface 1 acts as a generator of holes and as a sink for vacancy–hole pairs. Conversely at interface 2 reaction (5d) generates holes and acts as a sink for vacancy–hole pairs. Using the combination of reactions (5b) and (5d), all the electricity is transported by holes and the mass by vacancy– holes pairs of neutral charge. Thus, the film behaves as an electronic conductor. Finally, if one considers the combination (5b) and (5c) the transport is partially made by vacancies and partially by holes. In this case the film behaves as a mixed conductor. Taking into account the type of semiconductivity the capacitance response can be related to the development of a depletion region of negative charge created by cation vacancies at potentials below the flatband potential. Above the flatband potential an accumulation region of positive charges (holes) develops (Fig. 8). Thus, in region I the electric field created by the downward curvature of the valence band energy is favourable to cation migration into the film and reaction (5a) will occur at the metal–film interface. When the film is very thin, as in the case of the film formed during the first cycle, film formation can take place at the film–electrolyte interface according to reaction (5c). However according to the band structure model for thicker films the cobalt cations cannot reach interface 2 (film–electrolyte) where the opposite electric field exists. Conversely in region II of Fig. 7, above the flatband potential, the band bends downwards. Thus, there is a change in the direction of the electric field enabling the displacement of cations from an interior oxide site to a surface site, creating an interior vacancy–hole pair in the oxide. Once the cobalt cation occupies a surface site the reaction with the oxygen of the electrolyte can occur and oxide film formation ensues. It must be emphasized that the ionic partial current through the oxide film is not completely blocked at the film–electrolyte interface. Firstly the transfer of negatively charged ionic species (O2, OH) is possible. Secondly, the film is polycrystalline and the transfer of Co2+ may be possible via grain boundaries paths. Because the oxide films formed on cobalt exhibit p-type semiconductivity, the electronic exchanges involving oxygen consumption must take place in the valence band. Thus, when the space charge zone is in a depletion condition (region I in Fig. 7) the anodic processes such as injection of holes into the valence band will be favoured. Eq. (5), particularly Eqs. (5e) and (5f) represent a substantial oversimplification of the exact reaction sequence. In the case of cobalt oxide the surface is hydroxylated and the action of the holes is to oxidise a complex ionic surface layer rather than a simple ion.
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Fig. 8. Influence of the applied electrochemical potential on the band structure and the electrostatic potential distribution at the film–solution interface.
The preferred reaction will depend on the valence band energy, the energy levels associated with possible reactions and the solubility of the cation and the anion of the oxide. In the discussions concerning hole reactions one must distinguish two cases: the first one in which the ions from the semiconductor are soluble when oxidised by the holes to a higher valence state, and thus the semiconductor can dissolve. The second one is when the ions of the semiconductor are insoluble and a new phase can form. This is probably the situation of an oxide in aqueous solution with formation of hydroxides. It is interesting to note that according to Table 2, as the number of cycles increases, the flatband potential UFB1 is displaced in the cathodic direction. In contrast, the flatband potential UFB2 obtained by extrapolation of straight line 2, is displaced in the anodic direction. Simultaneously, the doping density of each part of the film decreases (Fig. 9). However, during film growth, the Na1/Na2 ratio is maintained almost constant (Table 1). Thus it can be assumed that the electronic processes taking place in each part of the film are interrelated and the electronic structure of the film may be described as a kind of p–p junction formed by a more anhydrous inner layer, and a more hydrated outer layer. Obviously it is not easy to represent the electronic structure of the oxy-hydroxides forming the outermost part of the film. Recent work [20] gives some information on the crystallographic structure of Co(OH)2, which is reported as hexagonal. It is also assumed that in the oxidation of Co(OH)2 only proton transfer occurs, without formation of a new phase. Thus the structure of the mixed Co(OH)2 and CoOOH chemical species can be considered as that of a single phase. Comparison with the semiconductor interface system helps to describe the shape of the electronic structure of the transition region as one moves
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Table 2 Influence of the number of cycles on the flat band potentials obtained by extrapolation of the straight lines 1 and 2 in Fig. 5 Number of cycles
UFB1
UFB2
2 5 50 100 400 2275
0.312 0.267 0.218 0.259 0.168 0.112
0.489 0.466 0.374 0.320 0.245 0.186
Fig. 9. Schematic representation of the capacitance results (Mott–Schottky representation) obtained with cobalt oxide films formed by a very low and a very high number of cycles. Also indicated the evolution of the flatband potential with the increasing number of cycles.
from the more anhydrous to the more hydrated part of the film. Fig. 10a shows the band structure model of the passive film. The existence of a transition region inside the film, which can be compared to a kind of p–p junction, has probably a strong influence on the reactions related to film growth (Fig. 10b). For instance, the process involving the oxidation of Co2+ to Co3+ can be expressed by the following reaction: CoðOHÞ2 ! CoOOH þ e þ Hþ
ð6Þ
In this reaction the Co2+ leaves at the surface of the oxide an electron, which fills a hole in the valence band (Fig. 10c).
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Fig. 10. Schematic representation of the anhydrous-hydrated interface: (a) electronic structure; (b) ionic and electronic transport and electronic transfer of charges; (c) reactions connected with the oxidation processes.
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Electronic structure of anodic oxide films formed on cobalt by cyclic voltammetry M. Pontinha
a,d
, S. Faty b, M.G. Walls c, M.G.S. Ferreira M. Da Cunha Belo a,c
a,e,* ,
a
d
Instituto Superior Te´cnico, Departamento de Engenharia Quı´mica, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Faculte´ des Sciences et Techniques (VCAD), De´partement de Chimie, Dakar, Senegal c Centre d’Etudes de Chimie Me´tallurgique, CNRS, Vitry-sur-Seine, France Instituto Superior de Engenharia de Lisboa, ISEL, Departamento de Engenharia Quı´mica, Lisboa, Portugal e Universidade de Aveiro, Departamento de Engenharia Ceraˆmica e do Vidro, Aveiro, Portugal Received 20 October 2004; accepted 18 October 2005 Available online 22 December 2005
Abstract The chemical composition and degrees of oxidation of thin oxides films formed on cobalt by cyclic voltammetry have been investigated by Auger electron spectroscopy and XPS analysis. In addition the electronic properties of the films have been examined by capacitance measurements using the Mott–Schottky method and photoelectrochemistry. The analytical results show that the thickness of the cobalt oxide films increases with the number of cycles and varies from a few tens to a few hundreds of angstro¨ms. When observed by transmission electron microscopy and diffraction, the films appear compact and well crystallised (spinel structure). Capacitance measurements show that both very thin and relatively thick films exhibit p-type semiconductivity. The band structure model proposed and the interpretation of the oxidation processes in terms of lattice ionic defects, can explain the film growth mechanism. The study shows how the electric fields created by the development of space charges influence both ionic transport and electronic transfer of charges. 2005 Elsevier Ltd. All rights reserved. Keywords: Cobalt oxide films; Cyclic voltammetry; Electronic structure
* Corresponding author. Address: Instituto Superior Te´cnico, Departamento de Engenharia Quı´mica, Av. Rovisco Pais, 1049-001 Lisboa, Lisboa, Portugal. Tel.: +351 21 841 7234; fax: +351 21 840 4589. E-mail address: [email protected] (M.G.S. Ferreira).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.10.007
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1. Introduction Despite the considerable work that has been carried out on the electronic structure of passive films, there is still a lack of information concerning the exact relationships existing between the electronic and ionic processes. For the common case of a semiconducting oxide growing on a metal, the development of a single surface charge layer in the metal is accompanied by a space charge distribution in the oxide. The principal characteristic of a space charge region is that it produces a potential barrier which may have a pronounced influence on the motion of charged species. In fact, the predominant transport processes are supposed to take place in this part of the oxide, especially at low temperatures. Passive films are very thin (a few nanometers) and highly doped. The contribution of space charges to film growth processes and film properties in the case of very thin films needs further research. The present investigation concerns oxide films formed anodically on cobalt by cyclic voltammetry. This method when applied to metals like iridium [1] and ruthenium [2] yields films whose thickness increases with the number of cycles. The chemical characterisation of the oxide films is performed using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis and the structural observations conducted by transmission electron microscopy (TEM). The electronic structure is studied through capacitance measurements and photo-electrochemistry. Some theoretical models describing ionic transport under the influence of space charges are discussed. Then, on the basis of the results obtained, a model for film growth is proposed, taking into account the electronic band structure of the metal–film and film–electrolyte interfaces. 2. Experimental The cobalt used was high purity 99.995% (wt%) metal. The surface of electrodes was abraded with wet SiC paper, grit size 1200. The anodic oxide films were formed on cobalt by cyclic voltammetry in the buffer solution composed of H3BO3 (0.05 M) + Na2B4O7, 10H2O (0.75 M) and of pH 9.2 at 22 C. After cathodic treatment (1.5 V/SCE, 300 s), voltammograms were obtained by performing a potential scan between 1.1 and 0.8 V at 100 mV/s. The reference electrode used in this research was the saturated calomel electrode (SCE). The analytical experiments were performed using a 310 F Microlab (VG Scientific) equipped with a field emission electron gun, a concentric hemispherical analyser and differentially pumped ion gun. Auger spectra were taken using a 10 keV, 50 nA primary electron beam. The angle between primary beam and the surface normal was 30. Spectra were acquired in constant retard rate mode (CRR = 10 eV), the energy resolution being about 0.2%. The analyser was calibrated using the following peak energies: Cu LMM at 918.62 eV; Ag MNN at 357.80 eV and Au NVV at 70.1 eV. Ion etching was performed at a pressure of 1 · 107 mbar using high purity argon. The etching current was approximately 0.75 lA/mm2. The etched area was a crater with a diameter of 0.5 mm, whereas the electron beam had a spatial resolution of 100 nm. XPS experiments were performed on the samples surface, before and after ion sputtering. The sputtering was made using a low energy ion beam (1.5 keV) in order to minimise the effects of reduction in the oxidised metallic species. A non-monochromated Mg anode (Ka = 1253.6 eV) was used. Spectra were obtained in constant analyser energy mode (CAE = 30 eV). For this value, the energy resolution is 1 eV.
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Electron microscopic examination was performed with a JEOL FX2000 microscope on flakes of film scratched off the metallic substrate. Electrochemical impedance measurements were made using a PAR 273A potentiostat and a Brookdeal 5208 double phase synchronous detector. The Mott–Schottky plots were obtained for each film by performing a potential scan in the cathodic direction. Simultaneously, impedance measurements were performed at 3 KHz with a signal amplitude of 10 mV (rms). Capacitance values were calculated assuming a series RC network as equivalent circuit. In the photo-electrochemical study the working electrode was illuminated through a quartz window with monochromatic light; a lens was used to focus a sharp spot on the working electrode, beam area 0.36 cm2. The optical instrumentation consisted of a 150 W Xe lamp, a 250 mm F/4 monochromator, a stepper motor to control the wavelength and mechanical chopper. The grating is usable in the range 250–700 nm with 280 nm blaze wavelength and a blocking filter was used above 450 nm to eliminate any second harmonic effects. All photocurrent measurements were normalized with respect to the flux recorded by a silicon photodiode. The photocurrent was recorded by connecting the current output of the potentiostat to the lock-in amplifier and recording the voltage output at 19 Hz (the chopping frequency). The reference phase was adjusted to ensure that the lock-in displayed a zero value from the maximum signal. The recorded values were then manipulated to calculate the photocurrent and quantum efficiency values using a spreadsheet software package. 3. Results 3.1. Electrochemical behaviour of the cobalt oxides Fig. 1 shows the voltammograms obtained when the cobalt electrode undergoes an increasing number of cycles. During the first cycle, the current is fairly independent of the potential. However, as the number of cycles increases, the voltammetric plots reveal the existence of two distinct potential regions; a region of small currents situated at low potentials and a region of relatively high currents situated at more positive potentials. The most important feature of the voltammetric diagrams is the fact that the current decreases as the number of cycles increases. Such behaviour differs from that observed in the case of oxide formed by cyclic voltammetry on an iridium electrode and can be explained by assuming that the film formed on cobalt is compact while that formed on iridium exhibits high porosity [1]. 3.2. TEM observation Fig. 2 shows a TEM micrograph of the cobalt oxide film formed after 400 cycles. The film appears relatively compact. Furthermore, the electron diffraction pattern reveals a spinel structure. 3.3. Auger and XPS study Fig. 3 shows the Auger depth profiles obtained from films formed after different number of cycles. For a film formed after five cycles the sputtering time necessary to reach the
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Fig. 1. Cyclic voltammetry I = f(U) obtained on cobalt. The number of cycles is indicated in the figure.
Fig. 2. (a) TEM micrograph and (b) diffraction pattern obtained from a cobalt oxide film formed after 400 cycles.
metallic substrate is much smaller than when compared to the film formed after 400 cycles. This means that the film thickness increases greatly with the number of cycles.
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The Co2p X-ray photoelectron spectra from the cobalt films, which had been prepared by cyclic voltammetry using 400 cycles is shown in Fig. 4a. Also represented is the evolution from the surface of the film (level 1) to the bottom film–metal interface (level 5) obtained for different sputtering times. Fig. 4b presents the anhydrous and hydrated molecular fractions in each of these successive levels. The chemical states were identified by comparing the photoelectron binding energies with those obtained from the literature for cobalt. The position of shake-up satellites of the main electron peak (Co2p3/2) was also used to aid chemical identification. Cobalt compounds of valence two are paramagnetic and have a strong satellite around 6 eV above the Co2p3/2 line; Co(III) does not have any satellite. The oxide of mixed valence (Co3O4) has a weak satellite that characterizes the minor component Co(II). At deep levels in the oxide (slightly in level 3 and clearly in levels 4 and 5) the Co2p3/2 spectra exhibit a peak around 778.7 eV characteristic of cobalt metal. At all levels there is
Fig. 3. (a) Auger depth profiles obtained from the films formed after different number of cycles. (b) Co and O Auger depth profiles after 400 cycles.
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Fig. 4. (a) XPS spectra from cobalt films formed after 400 cycles. (b) Anhydrous and hydrated molecular fractions for each of the successive levels indicated in the XPS spectra.
a peak around 781.7 eV with a full width at half maximum (FWHM) of 4.5 eV, which suggests the presence of more than one component. In order to characterize further the individual components, the Co2p3/2 and O 1s spectra were deconvolved. It should be remarked that the deconvolution of the spectra was not easy because, although one knows that the spectrum of Co3O4 is composed of highspin and low-spin components, the 2p3/2 binding energies for Co3O4 (around 779.6– 779.9 eV) and high-spin cobalt in CoO are close (780–780.5 eV). At the outer surface of the film the cobalt ion is preferentially in valence III as expected because the surface is more oxidized; the satellite is thus very weak. At level 2 the satellite is stronger indicating the presence of Co in valence II, and the presence of Co(OH)2 is detected through a peak at around 781.7 eV. At deeper levels the satellite peaks obtained from the deconvolution are in accordance with the satellites of Co(II) in CoO and Co3O4. As shown in Fig. 4b close to the metal–film interface the oxides predominate, while in the region near the film–electrolyte interface the film is essentially composed of hydroxides. It can also be observed that the CoO can only be detected in the innermost part of the film, while that of Co2O3 can only be detected in the outermost part.
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3.4. Capacitance measurements (Mott–Schottky study) The properties of a semiconductor–electrolyte interface can be described by Mott– Schottky theory [3,4], according to which the space charge capacitance of a p-type semiconductor is given by 1 2 kT U FB U ¼ ð1aÞ e C 2sc qN A e0 e where e is the relative permittivity of the oxide, e0 is the vacuum permittivity, q is the elementary charge (+e for electrons and e for holes), NA is the acceptor concentration, U is the applied potential and UFB is the flat band potential. The technique is based on the fact that at a Schottky junction, the space charge layer gives rise to a space charge capacitance, CSC, that has an effective complex impedance Rł;+ (ixCSC)1. Both the ohmic part R and the capacitance CSC can be determined by superimposing an a.c. voltage of frequency x on a d.c. bias applied across the junction. Lock-in detection at frequency x allows the separation of CSC and R. Fig. 5 shows the Mott–Schottky plot (C2 versus the applied potential U) obtained with oxide films formed after different number of cycles. The measured capacitance is high in the upper part of the potential region and becomes significantly smaller in the lower potential region of the voltammograms. Two straight lines with negative slopes can be observed in the Mott–Schottky representation, which means that the film must be composed of two layers of different doping density. In fact, when a depletion region is developed we have to consider different situations. The first one corresponds to a thickness of the space charge not higher than that of the outermost layer and can be described by Eq. (1a). When the thickness of the depletion region increases and reaches the inner layer, the following modified Mott–Schottky equation holds [5]: 2 1 2 kT N a1 N a2 W 1 U FB U ¼ e N a2 ee0 C 2sc qN A e0 e
ð1bÞ
in which Na1 and Na2 are the doping densities (acceptor species) of the inner and outer layers respectively and W1 is the thickness of the outer layer. Table 1 gives the doping densities calculated using the straight lines 1 and 2 of the C2 = f(U) representation (Fig. 5). As can be observed, the doping levels calculated for the very thin films forming during the first cycles are very high (in the range 1020 cm3). However when the number of cycles increases, the doping density is situated in the range 1019 cm3. 3.5. Photoelectrochemical measurements The fundamental optical transitions (band gap) were determined from using equations established by Ga¨rtner [6] and for a semiconductor/electrode junction by Butler [7]. The photo-response near the band gap can be described by g¼
I ph ðhm Eg Þn ¼ qAW hm U
ð2Þ
where g is the quantum efficiency, Iph is the photocurrent, U is the flux of photons (energy hm), q is the electronic charge, A is a constant, W is the thickness of the space charge layer
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Fig. 5. Influence of the number of cycles on the Mott–Schottky plots. (a) 2 cycles; (b) 50 cycles; (c) 400 cycles.
and Eg the band gap energy, where n depends on the type of the transition between the valence band and the conduction band. Generally, the analysis of the photocurrent spectra for passive films shows that n = 2 was the most appropriate value to represent the photoelectrochemical behaviour observed. This value corresponds to indirect transitions in the crystalline band model or non-direct transitions in amorphous films.
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Table 1 Acceptor doping concentrations (Na/cm3) assuming e = 10 Number of cycles
Na1
Na2
2 5 50 100 400 2275
3.34 · 1020 5.95 · 1020 6.13 · 1020 5.61 · 1020 4.36 · 1020 2.46 · 1020
1.16 · 1020 2.30 · 1020 2.56 · 1019 2.46 · 1019 1.80 · 1019 1.34 · 1019
The energy spectrum obtained from the oxide film formed after 400 cycles reveals three important values of photon energy (Fig. 6). The first is at 1.1 eV, in the infrared. The second appears at 2.13 eV in the visible, while the third appears at 3.55 eV, in the UV region of the light spectrum. These energy values obviously correspond to transitions in cobalt oxides or hydroxides. Similar energy spectra are obtained from films formed after a smaller or larger number of cycles. From the XPS results one has to consider the presence of CoO, Co3O4 and Co2O3 and of the hydroxides. The crystalline structural units of the divalent and/or trivalent oxides are the octahedron and the tetrahedron, with the Co ions being in high-spin and low-spin configurations respectively [8]. The structure of Co3O4 is that of a spinel with a cation dis3þ 2 tribution between tetrahedral and octahedral sites written as Co2þ tet 2½Cooct ]O4 , in which 2+ 7 3+ 6 bivalent Co (d ) ions of high spin and Co (d ) ions of low spin exist. Then, if one considers the crystal field theory the following analysis can be made: in the ground state, the spectroscopic terms corresponding to the high-spin Co2+ ion (d7) are 4F and 4P. Under the influence of an octahedral ligand field the 4F term splits into three other terms which are 4 T1g(F), 4T2g(F), 4A2g(F) in order of increasing energy. The 4P term is transformed into a 4 T1g(P) term of the same energy. In the case of a tetrahedral crystal field, the split energy levels are inverted.
Fig. 6. Photo-electrochemical response obtained at 0.2 V in borate buffer solution (pH 9) with cobalt oxide films previously formed after 400 cycles.
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On the whole, if one considers only high-spin d6 and d7 structures, at least eight spinallowed transitions can be expected for this system. This very high number of transitions leads to difficulties when trying to assign them to experimentally determined energy values, particularly when the determination is indirect. Fortunately, the literature provides spectroscopic data of some cobalt compounds. A careful examination of these results, with the help of the well known Fajans Tsuhida spectroscopic series can simplify the task [9–11]. Taking into account that the 5D(d6) energy level corresponds to the high-spin Co3+ ions under the influence of an octahedral field, a possible transition may be 5
T2g ! 5 Eg
whose energy is 2.26 eV. On the other hand, if the transitions, which correspond to the configuration d7, can be related to Co2+ in octahedral crystal field, then, as a consequence of the split of the 4F energy level, one has to consider the following transitions: 4
1
T1g ðFÞ ! 4 T2g ðFÞ;
4
2
T1g ðFÞ ! 4 A2g ðFÞ;
4
3
T1g ðFÞ ! 4 T1g ðPÞ
Transitions 1 and 2 correspond to the energies of 0.92 and 2.1 eV, respectively [9]. Transition 3 has an energy of 2.5 eV, which is the difference between the ground state and the P level. In the tetrahedral crystal field the possible transitions are: 4
1
A2g ðFÞ ! 4 T2g ðFÞ;
4
2
A2g ðFÞ ! 4 T1g ðFÞ;
4
3
A2g ðFÞ ! 4 T1g ðPÞ
Transitions 1, 2 and 3 have energies of 0.5, 0.92 and 2.4 eV respectively [9]. The experimental energy values obtained by photo-electrochemistry are 1.1, 2.13 and 3.55 eV. The first may be compared to the value of the transition 4T1g(F) ! 4T2g(F) or to that of the transition 4A2g ! 4T1g. The calculated values of 2.6 and 2.1 are not very different from the experimental value of 2.13 eV. This energy is located in the visible and can be 4 related to the 4T1g(P) T1g(F) transition in the octahedral Co2+ oxide or to the 4 4 T1g(P) A2g(F) transition of the tetrahedral Co2+ ion. Finally the 3.2 eV peak is too high to be due to d–d transitions. This value situated in the UV part of the light spectrum, should arise from a charge transfer between the 2p band of oxygen and the Co ions d levels [9]. This means that one has in this case a transition from the valence band to the conduction band. 4. Discussion Mott–Schottky plots reveal that both very thin and relatively thick passive films formed on cobalt by cyclic voltammetry behave as p-type semiconductors. This means that in all cases valence band pinning occurs and a space charge region develops at the film-electrolyte interface. The existence of space charges in the case of very thin passive films has been discussed in the literature [12–17]. The growth kinetics of the anodic oxide films formed on cobalt depends in a complex manner on the situation created in the different potential regions by the ‘‘scanning’’ in the voltammetric cycles. These situations can always be related to the varying nature of the space charges developed at the metal–oxide and film–electrolyte interfaces, whatever the thickness of the film. Since the Fermi level, i.e., the electrochemical potential, is main-
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tained constant throughout the film thickness, what changes is the chemical potential l or/ and the electrostatic potential u, according to: U = EF = l + qu. This means that the chemical potentials for ionic defects and holes may be different in function of the location, while the electrochemical potential remains constant. Since the development of space charges starts from a situation of zero charge at the flatband potential, the electric field created by the applied potential at the metal–film and film–electrolyte interfaces must oppose each other. When a depleted space charge region is formed at the film–electrolyte interface, another one is also formed at the metal–film interface. This generates back-to-back electric fields in opposite directions. Morrison [18] proposes a similar configuration for electron transfer in the metal–film–electrolyte system. Fig. 7 shows that the flatband potential obtained by capacitance measurements separates a potential region of low currents (region I) from one of relatively high currents (region II). Such behaviour can be explained if one discusses the mechanisms of mass and charge transport through the film and the reactions at both metal–film interface (interface 1) and film–electrolyte interface (interface 2). One must also considers the situation created at these interfaces when the film is submitted to cyclic voltammetry. If one considers that in the oxide film the predominant defects are holes and vacancies, the latter in two ionised forms, the following equations apply at the metal–film interface [19]: 2þ Co þ V2 þ 2e Co ¼ Co
ð5aÞ
2þ Co þ V2 þ h þ 2e Co =h ¼ Co
ð5bÞ
or 2 where the symbols V2 Co and VCo =h denote unpaired and paired cation vacancy sites in the 2 oxide, e the electrons and h the holes. V2 Co and VCo =h correspond in fact to negative and neutral electrical charges respectively. Note that when Co2+ migrates to a vacancy site it creates a neutral situation.
Fig. 7. Schematic representation of C2(U) and I(U), which demonstrates that the flat band potential UFB separates the region of low currents (region I) from that of high currents (region II).
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At the film–solution interface (interface 2), possible reactions are: 2 Co2þ þ 2O 2 ! CoO þ VCo
ð5cÞ
2 Co2þ þ O 2 þ h ¼ CoO þ VCo =h þ 2e
ð5dÞ
or
The reactions of water decomposition and oxide and hydroxide formation involving hole consumption from the valence band could be: O2ads () O ð5eÞ 2 þh H2 O þ O ( ) HO þ OH þ h ð5fÞ 2 2 It appears that the various combinations of the exchange currents controlled by these reactions determine whether the film transports electricity as an ionic, electronic or mixed conductor. Indeed, Eq. (5a) provides a sink at interface 1 for vacancies generated at interface 2 through Eq. (5c). Thus the transport is achieved through vacancy migration and the film behaves as an ionic conductor. Furthermore, reaction (5b) at interface 1 acts as a generator of holes and as a sink for vacancy–hole pairs. Conversely at interface 2 reaction (5d) generates holes and acts as a sink for vacancy–hole pairs. Using the combination of reactions (5b) and (5d), all the electricity is transported by holes and the mass by vacancy– holes pairs of neutral charge. Thus, the film behaves as an electronic conductor. Finally, if one considers the combination (5b) and (5c) the transport is partially made by vacancies and partially by holes. In this case the film behaves as a mixed conductor. Taking into account the type of semiconductivity the capacitance response can be related to the development of a depletion region of negative charge created by cation vacancies at potentials below the flatband potential. Above the flatband potential an accumulation region of positive charges (holes) develops (Fig. 8). Thus, in region I the electric field created by the downward curvature of the valence band energy is favourable to cation migration into the film and reaction (5a) will occur at the metal–film interface. When the film is very thin, as in the case of the film formed during the first cycle, film formation can take place at the film–electrolyte interface according to reaction (5c). However according to the band structure model for thicker films the cobalt cations cannot reach interface 2 (film–electrolyte) where the opposite electric field exists. Conversely in region II of Fig. 7, above the flatband potential, the band bends downwards. Thus, there is a change in the direction of the electric field enabling the displacement of cations from an interior oxide site to a surface site, creating an interior vacancy–hole pair in the oxide. Once the cobalt cation occupies a surface site the reaction with the oxygen of the electrolyte can occur and oxide film formation ensues. It must be emphasized that the ionic partial current through the oxide film is not completely blocked at the film–electrolyte interface. Firstly the transfer of negatively charged ionic species (O2, OH) is possible. Secondly, the film is polycrystalline and the transfer of Co2+ may be possible via grain boundaries paths. Because the oxide films formed on cobalt exhibit p-type semiconductivity, the electronic exchanges involving oxygen consumption must take place in the valence band. Thus, when the space charge zone is in a depletion condition (region I in Fig. 7) the anodic processes such as injection of holes into the valence band will be favoured. Eq. (5), particularly Eqs. (5e) and (5f) represent a substantial oversimplification of the exact reaction sequence. In the case of cobalt oxide the surface is hydroxylated and the action of the holes is to oxidise a complex ionic surface layer rather than a simple ion.
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Fig. 8. Influence of the applied electrochemical potential on the band structure and the electrostatic potential distribution at the film–solution interface.
The preferred reaction will depend on the valence band energy, the energy levels associated with possible reactions and the solubility of the cation and the anion of the oxide. In the discussions concerning hole reactions one must distinguish two cases: the first one in which the ions from the semiconductor are soluble when oxidised by the holes to a higher valence state, and thus the semiconductor can dissolve. The second one is when the ions of the semiconductor are insoluble and a new phase can form. This is probably the situation of an oxide in aqueous solution with formation of hydroxides. It is interesting to note that according to Table 2, as the number of cycles increases, the flatband potential UFB1 is displaced in the cathodic direction. In contrast, the flatband potential UFB2 obtained by extrapolation of straight line 2, is displaced in the anodic direction. Simultaneously, the doping density of each part of the film decreases (Fig. 9). However, during film growth, the Na1/Na2 ratio is maintained almost constant (Table 1). Thus it can be assumed that the electronic processes taking place in each part of the film are interrelated and the electronic structure of the film may be described as a kind of p–p junction formed by a more anhydrous inner layer, and a more hydrated outer layer. Obviously it is not easy to represent the electronic structure of the oxy-hydroxides forming the outermost part of the film. Recent work [20] gives some information on the crystallographic structure of Co(OH)2, which is reported as hexagonal. It is also assumed that in the oxidation of Co(OH)2 only proton transfer occurs, without formation of a new phase. Thus the structure of the mixed Co(OH)2 and CoOOH chemical species can be considered as that of a single phase. Comparison with the semiconductor interface system helps to describe the shape of the electronic structure of the transition region as one moves
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Table 2 Influence of the number of cycles on the flat band potentials obtained by extrapolation of the straight lines 1 and 2 in Fig. 5 Number of cycles
UFB1
UFB2
2 5 50 100 400 2275
0.312 0.267 0.218 0.259 0.168 0.112
0.489 0.466 0.374 0.320 0.245 0.186
Fig. 9. Schematic representation of the capacitance results (Mott–Schottky representation) obtained with cobalt oxide films formed by a very low and a very high number of cycles. Also indicated the evolution of the flatband potential with the increasing number of cycles.
from the more anhydrous to the more hydrated part of the film. Fig. 10a shows the band structure model of the passive film. The existence of a transition region inside the film, which can be compared to a kind of p–p junction, has probably a strong influence on the reactions related to film growth (Fig. 10b). For instance, the process involving the oxidation of Co2+ to Co3+ can be expressed by the following reaction: CoðOHÞ2 ! CoOOH þ e þ Hþ
ð6Þ
In this reaction the Co2+ leaves at the surface of the oxide an electron, which fills a hole in the valence band (Fig. 10c).
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Fig. 10. Schematic representation of the anhydrous-hydrated interface: (a) electronic structure; (b) ionic and electronic transport and electronic transfer of charges; (c) reactions connected with the oxidation processes.
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[11] A. Earnshaw, T.J. Harrington, The Chemistry of the Transition Elements, Oxford Chemistry Series, Clarendon Press, Oxford, 1973. [12] N. Cabrera, N.F. Mott, Rep. Prog. Phys. 12 (1949) 163. [13] A.T. Fromhold Jr., in: The Anodic Behavior of Metals and Semiconductors Series, in: Jhon W. Diggle, Ashok K. Vijh (Eds.), Oxides and Oxide Films, 3, Marcel Dekker, New York, 1976, pp. 1–271. [14] J.F. Dewald, J. Electrochem. Soc. 102 (1955) 1. [15] H.H. Uhlig, Acta Metall. 12 (1967) 1135. [16] K. Hauffe, Oxidation of Metals, Plenum Press, New York, 1965. [17] J.N. Butler, J. Chem. Phys. 35 (1961) 636. [18] S. Roy Morrison, Electrochemistry of Semiconductors and Oxidised Metal Electrodes, Plenum Press, New York, 1980, p. 319. [19] G.T. Croff, T. Tuomi, J. Electrochem. Soc. 108 (1961) 915. [20] A. Foelske, H.H. Strehblow, Surf. Interface Anal. 34 (2002) 125.