Photoelectrochemical studies of passive films

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REVIEW

STUDIES

OF PASSIVE

FILMS

SnhtbtINca

ULRICH

Laboratory,

Ltd.

ARTICLE

PHOTOELECTROCHEMICAL

Electrochemistry

Ross

Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, NY 10027, U.S.A. (Received 25 September 1985)

Abstract-The application of photoelectrochemistry as a means to study passive films is reviewed. A description of the theoretical background of photoelectrochemical behavior of passive metal electrodes is given that includes photoprocesses in crystalline and amorphous passive films, photoemission from the underlying metal, some characteristics of thin films and a summary of the photoindnced electrochemical reactions. Recent literature is reviewed distinguishing noble metals, transitions metals and valve metals. Examples that represent a characteristic type of behavior are discussed in more detail.

1. INTRODUCI’ION Photoeffects in electrochemical systems have been recognized for a long time, and the phenomenon that an additional current can be stimulated by irradiation with light, the Bequerel effect, is named after its discoverer[l]. The electrodes which Bequerel used were, in fact, metal electrodes that were covered with an oxide film. Since then, there have been continuous efforts to utilize this photoeffect in order to study properties of passive metal electrodes. Because of a limited understanding of the involved processes and also limited experimental means, results and interpretations were only qualitative and remained controversial. During the last 10-15 years, however, photoelectrochemistry with semiconductor electrodes has undergone an enormous advancement which has been largely due to the search for materials for liquid junction solar cells[2-51. Besides affording a much better understanding of a large number of systems and the processes involved in photoelectrochemical reactions, photoelectrochemistry emphasizes that solid state properties are of great importance to elcctrochemical behavior. The application of photoelectrochemistry to the study of passive films is based on the recognition that even very thin films constitute their own phase with their specific properties. Consequently, photoelectrochemistry can be used as an in situ technique to characterize passive films with respect to their optical and electronic properties. In this paper an attempt will be made to give an overview of the possibilities photoelectrochemistry offers for the investigation of passive films. Relevant theoretical background will be described, including aspects of photoelectrochemistry of amorphous materials since passive films are often highly disordered. The literature will be reviewed with the object being to distinguish noble metals, on which usually very thin films are formed (which may not be catagorized as passive films in a stricter sense), transition metals on which passive films of intermediate thickness are formed and which are of importance for corrosion 415 !?.A 31:4-A

studies, and valve metals, on which very thick films can be formed due to the possibility of high film formation potentials because of high breakdown potentials.

2. THEORETICAL

BACKGROUND

A photoelectrochemical reaction on a passive metal electrode can be described in the following simplified way. If light of a suitable energy hv is absorbed by the passive film, electrons can be excited from occupied electronic states into unoccupied ones, hv--re-

+h+.

(1)

As a consequence, the charge distribution in the film is changed, which can result in a ‘current at constant potential or a potential change at constant current which includes the open circuit condition where i = 0. The current results from an electrochemical reaction at the electrode/electrolyte interface. The changed charge distribution can also be measured as a change of the electrode capacity. Depending on the properties of the passive film with respect to its conduction type, different energetic conditions can be distinguished for an n-type or p-type semiconducting or an insulating film. Figure 1 gives a schematic representation for crystalline films without, as yet considering any effects caused by an absorption that involves localized states. Under the conditions of a depletion layer built up at thefilm+Gctrolyte interface in a semiconducting film, the electron hole pair is separated in the electric field of the space charge layer. The condition of a depletion layer is the one where a photoeffect would be observed. In the case of a p-type semiconducting film (Fig. la), the electron migrates to the surface from where it can react with an unoccupied (acceptor) state in the electrolyte which is represented by an oxidized chemical species. The hole flows to the backside contact; the resulting current is cathodic. For an n-type film (Fig. 1b) the situation is reversed. The hole migrates to the surface, reacting with an occupied (donor) state while the electron moves to the backside

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%IlMMING

b)

METAL

p-TYPE

METAL

n-TYPE

METAL

FILM

FILM

INSULATING FILM

Fig. 1. Schematic representation of an illuminated p-type (a) and n-type (b) passive film under depletion conditions and of an insulating film under flat band conditions with a simultaneous excitation in the film (hv,) and in the underlying metal (hv*) (c).

contact: the photocurrent is anodic. In an insulating film (Fig lc) the potential gradient is constant within the film and depending on the sign of the field the photocurrent is anodic or cathodic. In general, a change in the sign of the field changes the sign of the photocurrent. The potential at which this sign change occurs is called the flatband potential, Ufb. It is consequently accessible by photoelectrochemlcal techniques and can be compared with values obtained from capacity measurements. In addition to an excitation in the passive film, an excitation can happen in the underlying metal as well. For thin passive films and insulating films this has been observed and will be described later. Excitation occurs most probably from or to the Fermi level in the metal with a consecutive emission of a hot electron or hole. Internal photoemission into the passive film might also occur; it is illustrated in Fig. lc for the emission of a hot electron. It should be emphasized that photoemission is not limited to insulating films but can also happen with ntype and p-type passive films. The following description of the photoresponse will be restricted to the photocurrent,although the measurement of photopotentials is equivalent. Since, however, most measurements nowadays are performed under potentiostatic control it has become more common to measure photocurrents. Disregarding any recombination, the photocurrent of a crystalline semiconductor can be expressed by[6,7] iph = eqIo(l

-exp[

-ad,(e/kT)1’2(U-_L111)1’2]/

(1 +aL)}. (2) e denotes the electronic charge, q the quantum efficiency, I, the incident photon flux disregarding any reflections at the electrode/electrolyte interface, b: the absorption coefficient, U the electrode potential and U,, the flatband potential. The Debye length, d, is determined by important semiconductor quantities such as the dielectric constant, E, and the doping concentration, N, which is the donor concentration, Nd, for an n-type semiconductor and the acceptor concentration, N,, for a p-type material. It is given by d,

= (eeokT/eZ

N)‘/‘.

(3)

The diffusion expressed by

length

of the minority L = (Dt)“2

carriers,

L, is (4)

where D is the diffusion coefficient of the minority carriers and 7 is their lifetime. Equation (2) is derived under the assumption that the charge transfer at the electrode/electrolyte interface is not rate determining. Simplifications in Equation (2) are possible under certain conditions which will be discussed later. The photoelectrochemical behavior of passive films is largely influenced by their electronic properties. These properties as obtained from other techniques, such as capacity measurements and electron transfer reactions, will be reviewed briefly. Then, characteristics of the absorption behavior of thin films will be discussed, followed by a more detailed description of the wavelength and potential dependence of the photocurrent for crystalline and amorphous films.

Electronic

properties

of passiveJilms

In a large number of instances passive films are formed by a field-assisted growth process. The field strengths built up in the passive film can reach up to 10’ Vcm-‘[g]. A constant potential gradient is assumed for films with a constant stoichiometry in the film. If the activity of any chemical species involved in the formation of the film is not constant throughout the film, non-uniform fields can be assumed to exist in order to maintain a constant transport through the film. At potentials below the formation potential, the potential drop may be linear in the case of an insulating film or non-linear for a semiconducting film. Depending on the system, then, conditions of film formation as well as stoichiometry of the film contribute to the understanding of the electronic properties of the film. A very important source of electronic properties, which reflect the charge distribution in the film, is capacity measurement. Valve metals usually behave as insulators[9]. A similar picture was also assumed for other systems, such as passive iron, since the capacity

Photoelectrochemicalstudies of passivefilms of the electrode passivated at various potentials, U,, followed a linear l/C us d relation which is characteristic of dielectric fllms[ 10,111. On the other hand, as in the case of passive iron, a considerable potential dependence of the capacity was found for potentials U c U,. Such behavior is explainable by assuming that the film exhibits n-type semiconducting properties[12]. Similar results were described for nickel[ 131 where the passive film behaves like a p-type semiconductor. In both cases, the donor and acceptor concentrations, respectively, were in the order of 10zocn-3 which makes them highly doped semiconductors. Similar results have been found for thin passive films on titanium[14] and other metals. In general, capacity measurements are a good means to determine the charge distribution in the passive film, especially at the film/electrolyte interface, and to reveal solid state properties of the film, such as donor or acceptor concentration and the flatband potential. Another source of information for electronic properties of passive films is the investigation of electron transfer reactions. Electron transitions usually occur from an electronic state in the electrode to one in the electrolyte, or vice versa, with both of them at the same energy. Therefore, for any such process, the existence of electronic states in the electrolyte as well as in the electrode, ie in the passive film, is required. Various authors contributed results[1519] and models[20, 211 which demonstrated that electron transfer reactions represent some kind of probe for the passive film. The kinetics of electron transfer reactions on metals and semiconductors are quite different and a semiconducting film is usually identified by its rectifying properties. Even the existence of localized states in the band gap is reflected in the kinetics as shown by Schmickler[Zl] in his model for resonance tunneling at passive metal electrodes. The picture that is derived from such investigations can reflect solid state properties and a probing of electronic states which are relevant for electron transfer. Band structure models derived for passive films usually use band gap energies known from the bulk materials that are supposed to form the passive film. This assumption is, however, not necessarily correct, and necessary information on the band gap can better be obtained from photoelectrochemical data. Absorption

of thin films

The number of photons, N,, absorbed in the passive film is given by Lambert-Beer’s law. While for a bulk semiconductor electrode the sample thickness is large enough to absorb all incident radiation, this can be different for thin films. Possible absorption or refkction at the metal-passive film interface also has to be considered. With the film thickness, d, being the absorbing layer Equation (2) changes to . = eflZ,[l -exp(-ad)][l +Rexp(-ad)] (5) %h with R, the reflectivity of the metal. The limiting cases are R = 0 and R = 1, the film thickness then being counted once and twice, respectively. The latter case has also been considered by McAleer and Peter[22] for the case of thin passive films on titanium. Such effects of reflections at the metal-film interface are important only for d -cum1 and R $- 0. For R > 0, light is

417

absorbed by the metal which can give rise to photoemission processes. Such effects will be described later. For very thin films and not too high absorption coefficients the exponent in Equation (5) may be linearly expanded. Then the quantum efficiency and the photocurrent, respectively, become proportional to the absorption coefficient. The range of application was tested for passive titanium electrodes[23]. The number of absorbed photons normalized to incident radiation was calculated on the basis of Npllcc 1 -exp(

---ad)

(6)

using absorption data for TiOl of Memming et al@+] and a change of film thickness with formation potential, dd/dU, = 2.5 nm V- l. The result is shown in Fig. 2[23]. For low wavelengths, ie high photon energies, absorption is complete for a very thin film. For photon energies closer to the band gap energy, which is around 3 eV, NM becomes linear with U,, and with d, up to fairly thick films. This demonstrates that the approximation i,, aol holds for low tl and d. It is simultaneously demonstrated in the case of titanium that counting the film thickness twice, ie R = 1, is only justified in the case of low photon energies or of very thin films. Wavelength dependence of the photocurrent The absorption coefficient, OL,of a crystalline material depends on the photon energy in the following manner a = A (hv - EJ”/hv.

(7) For allowed transitions which are the most likely to be observed in our case, n = l/2 describes direct transitions, and n = 2 is characteristic of indirect transitions, which require a phonon participationC25, 261. The type of transition can also be determined from photocurrent spectra as long as i,,,, a a holds. The minimum energy required for indirect transitions, ES, can be evaluated from (G h~)l’~ us hu plots, and that for direct transitions E”,, from (i* l@ us hu plots. Thii information allows us to identify the material forming

1.0 -

h= 280 nm

310nm

0.8

x25

_:I5

x 100

0

20

LO Oxide

60 thickness

80 d,,

380 nm 400 nm

J 100

Inm

Fig. 2. Plot according to Equation (6) for various wavelengths taken from Ref. [23].

418

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the passive film. It also might be used to estimate to what extent the band structure in the passive film is developed with respect to bulk properties of the same material. (An example for the latter case will be given later for passive titanium.) Since passive films can be only a few atomic layers thick, no well-developed periodicity can be expected normal to the surface. The application of band structure models based on crystalline semiconductors seems to be inappropriate for very thin films as well as for thicker amorphous films. In electrochemical systems there are only a few cases where photoelectrochemical properties that are specifically related to amorphous materials have been discussed[30,31]. Some principles of the behavior of amorphous semiconductors in solid state systems are also applicable to electrochemistry. For very thin films normal to the surface or for amorphous films, only a short range order exists. This is due to a high number of non-equilibrium positions of the ions forming the films. In addition, the composition of the film can be non-stoichiometric. Although no long range order exists in an amorphous material, the basic picture of the band model remains applicable[27]. An extensive tailing of electronic states into the band gap is typical of amorphous semiconductors. The band gap as such loses its meaning; it is no longer a material constant but dependent on preparation and it is usually given by the mobility gap. It has been shown that Equation (7) is still applicable[27], although it is rather an empirical equation. It is generally assumed that selection rules are broken down[27] although this was recently argued against by Brodskv and DiVincenzor281. The exponent n can be 1, 2 or 3, with 2 being th> &ost common value. The latter case is discussed to represent the density of state function at the band edges rather than as a value for indirect transitions[29]. In one instance, photoelectrochemical investigations of amorphous Fe(III~Ti(IV)-oxides, n = l/2 has been observed which changed to n = 2 upon crystallization[31]. Another phenomenon that is frequently observed with amorphous materials is the so-called Urbach tail[32]. It is described by the following exponential relationship between the absorption coefficient and the photon energy: M = cueexp[y (hv - E,)/kT]

(8)

where a,, and y are constants. Such behavior is found as well in crystalline materials, although it is rare. Equation (8) allows for a determination of the band gap which is usually assumed to be at the point where ln a ceases to be linear with hv. For the abovementioned amorphous Fe(IIIkTi(IV)-oxides an excellent agreement has been found for E, values determined according to Equations (7) and (8). A satisfying physical explanation for the Urbach tail is yet to be found. If not all light hitting the metal/oxide contact is reflected, ie R < 1, photoemission processes from the metal are possible. This is shown schematically in Fig. 1c for a photoemission of holes into the electrolyte and an internal photoemission of electrons. Excitations from the Fermi level result in electron emission, while excitation to the Fermi level can lead to hole emission[33]. The latter process has no analogy in vacuum; it is restricted to the existence of a neighbouring phase.

STIMMING

The absorption often described

edge for a photoemission by a Fowler plot[33].

process

qi’2 = A (hv -hv,), the threshold energy, electrode potential hv,

hve, being proportional =

e U

+

const.

is (9)

to the (10)

There is, however, an argument regarding the energy dependence of the quantum yield (see[2] for a discussion of the so-called 5/2 law). In any case, Equations (9) and (10) allow the identification of photoemission processes. They are distinguished from excitation in the passive film by their specific absorption edge and their potential dependence for emission into the electrolyte. Examples will be given later. Photocurrent spectra can be used to describe the absorption behavior of the passive film by plotting In (1 -q) us hv[22] or ipk vs hv, for the latter assuming i,, r~ GL.It allows the determination of band gap energies, possibly with a distinction between crystalline and amorphous materials. A comparison with data for bulk compounds of the same material that is supposed to form the passive film shows to what extent a similarity does or does not exist. Under all circumstances a characterization of the film is possible which describes the film in terms of the existence of electronic states that are electrochemically relevant. For the case of a simultaneous excitation in the passive film and the underlying metal the two processes can usually be distinguished by their different spectral responses.

Potential

dependence

of the photocurrent

At constant wavelength, where Q is constant, Equation (2) describes the potential dependence of the photocurrent for a crystalline semiconductor. Under certain conditions simplifications of Equation (2) are possible. For a diffusion length much larger than the (Cl width of the space charge layer, L B d, (e/W)“* I’* the photocurrent becomes potential in-u,,) dependen; as can be seen from Equation (2). Such a behavior is commonly found with bulk semiconductor electrodes which are either single crystals or well crystallized materials. For the reverse case where ad, (e/W)“’ (U - U,,) 4 1 the exponential function can be expanded, and Equation (2) changes to[7] i ph = const.ad,,

(e//C)”

(U - U&i’*.

(11)

Behavior in accordance with Equation (11) can be identified from ii,, us U plots. The slope depends on the photon energy since a is a function of hv and the extrapolation to iph = 0 should yield the flatband potential U,,. In several cases VJb values obtained from such extrapolations were higher by several hundred millivolts than I_J,, values obtained from capacity measurements. Some examples will be discussed later which demonstrate that Equation (11) cannot be used unambiguously. In the presence of surface states or when recombination is included in the derivation. different relations between photocurrent and electrode notential have been discussedF3&371. Even for high ;ecombination rates, the iJ@ relation is at most linear for crystalline semiconductors[36]. The above models describe the situation of a crystalline semiconducting material. For amorphous

Photoelectrochemical

419

studies of passive films

ELECTROLYTE

DISTANCE

Fig. 3. (a) Photoinduced excitation from a localized state at E to a state at E + hv; (b) coulombic well surrounding the state at E + hv with and without a superimposed field, F.

films, the potential dependence of iti may be altered due to the presence of a high number of localized states in the band gap. If the photoexcitation involves localized states or the electron hole pair thermalizes after excitation into locahzed states, the occurrence of a photocurrent requires the removal of the carriers from their respective traps. For zero field the probability of the carriers to escape is so low that they eventually recombine. For a field F > 0 the ionization energy of the electron in the direction of the field and the hole in the opposite direction is lowered by the amount j?F l’*. This field influence on the current is called the Poole-Frenkel effectr271. The situation of photoinduced currents involvinglo&zed states at thin amorphous films has been explored in more detail by Newmar k and Stimming[38,38a] since it seems relevant for photoelectrochemical studies with passive fihns. Figure 3 illustrates the energetic situation. In Fig. 3a an excitation process is indicated that takes place from and to a localized electronic state. Excitation from states that lie in the band (extended states) or to extended states are equally relevant. The photoexcited electron on an ionized donor experiences a coulombic force which results in an energy barrier surroundingthe trap, by - e2/4m (Fig 3b). If an _ given electric fieid is superimposed, the escape barrier is mod&d by /3F1i2. Consequently, the current depends exponentially on the square root of the electric field

or highly disordered

ip,, oc exp (+ f3F”*/kT).

(12)

Straight lines in In ipk us Flf2 plots indicate a Poole-Frenkel behavior. The PooleFrenkel effect is commonly observed for fields F z=- 1Cr’ V cm- i. For very high fields, the barrier becomes thin enough so that tunneling processes through the barrier are possible. This situation has originally been treated for the dark conductivity of amorphous materials by Hill[39], Vincent er aJ.[40] and Martin et aJ.[41]. For the calculations the following equation was used: E=E.-hv

ipl, = const. x

[

D(E)f(E)P’(E)D(E+hv)

I E = E. 1 --f(E + hv)] P(E + hv) dE

(13)

Fig. 4. Results of calculations based on Equation (13) for various hvs, taken from Refs [38, 38a].

where the constant contains the photon flux, the quantum efficiency and the electronic charge. The results of the calculations were performed using an exponential decay function for the density of states D(E) in the band gap. The Fermi function determines whether the states are occupied [j(E)] or unoccupied [l --f(E)]. The escape probability, P’, of the holes and P, of the electrons from their localized states include thermal excitation, direct tunneling and phonon assisted tunneling. The results of the calculations are shown in Fig. 4 for a band gap energy of 5 eV and for a rate determining step of the removal of the photoexcited carrier from a single trap. This applies especially to very thin films or the space charge region of a highly doped amorphous material. In such a case the electron or hole is directly exchanged with the underlying metal or the conduction band on the one side and with the electrolyte on the other side. For both phases a whole array of states D(E) is assumed at the energy of the transition. In Fig. 4 curves for various photon energies hv c E, are plotted in a Poole-Frenkel type of plot to show any deviation from that behavior more clearly. At lower fields, straight lines indicate the Poole-Frenkel effect. At higher fields, F > 5 x lo5 V cm- I, the current increases further due to tunneling, with a slope that is increasing for decreasing hv. Consequently, at higher fields the curves approach each other. At very high fields the differences between the curves of various photon energies are much less than under “normal” Poole-Frenkel conditions. It will be shown later that ion implanted passive films on hafnium electrodes show a sub-band gap photoresponse that might be explained with such a model. Photoinduced

eJectrochemica1

reactions

A measured photocurrent reflects the existence of an electrochemical process at the electrode/electrolyte interface. The following types of reactions can be distinguished.

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(i) Electron transfer reaction with a suitable redox couple in the electrolyte; an electron is exchanged with an acceptor or a hole is exchanged with a donor. (ii) Reaction with H+ or OHin the electrolyte yielding H2 or O2 as products. In non-aqueous electrolytes reactions might occur with the respective ions which make up the electrolyte. (iii) Reactions that involve the passive film itself, such as film reduction, film formation and film corrosion. For photoemission ma&ions caused by an excitation in the metal, the emitted hot electrons or holes react with a scavenger in the solution or with the electrolyte itselc42]. To summarize, the spectral behavior and the potential dependence of the photocurrent have been described for crystalline semiconducting films. Band gap energies and the flatband potential can be determined under certain limiting conditions. For passive films that exhibit only a short range order, the spectral behavior may be different from that ofcrystalline films. The potential dependence of the photocurrent can be especially different if localized states are involved. An exponential type of potential dependence according to PooleFrenkel can be expected. Photoprocesses are connected to electrochemical reactions at the interface which might be identified by electrochemical or analytical means.

3. EXPERIMENTAL

TECHNIQUES

The experimental techniques used in photoelectrochemical studies of passive films commonly use a combination of equipment used for passivation studies and that used for photoelectrochemical research with semiconductors. In Fig. 5 a block diagram of a typical experimental set-up is outlined. It is shown for a potentiostatic measurement with a monochromatic light source in the form of a polychromatic lamp and monochromator. In order to measure photopotentials, a galvanostat can be used or open circuit conditions can be applied. The light source can also be polychromatic (usually xenon or tungsten lamps) for preliminary measurements, ie in order to test under what conditions photoeffects can be expected. The

I

Potentiostot

%TMMING

advantage of the polychromatic source is its usual high intensity and its spectrum covering a wide range of wavelengths. Disadvantages are that possible heat effects may exist, especially with high power lamps, which have to be distinguished from photoeffects. A high power monochromatic light source is the laser which hasclear advantages. It can be applied to studies of the potential dependence of the photocurrent, especially where absorption coefficients are small, and to studies of sub-band gap or close-to-band gap responses. Measurements of spectra with a laser (dye laser) are usually tedious even if the equipment is available. Polychromatic sources, eg xenon lamps with quartz bulbs, which have a considerable output even below 250nm, together with a modern monochromator with digitally addressable scan rate, I range etc. are more convenient. If the photoresponse is large enough it can be measured directly. A convenient way is to use slowly chopped light, eg 0.1 Hz. Currents under dark and illuminated conditions are both recorded and, in addition, any transient behavior pertinent for the given frequency will be shown as well. In most cases, however, photocurrents are fairly small requiring methods with a better signal-to-noise ratio, for instance the lock-in technique. In this case the frequency control of the chopper sends a reference signal, fi, to the lock-in amplifier and the current equivalent signal, I, feeds the input of the lock-in amplifier. Since lock-in amplifiers tend to pick up noise, usually measurements with and without light are necessary in order to make a base line correction. The output of the lock-in amplifier can be used for the measurement of photocurrent spectra and photocurrent potential curves. In order to obtain intensitycorrected spectra, the intensity us wavelength of the light source has to be determined. This can be done by using a detector with known spectral response or a flat response in the desired wavelength range, eg a thermopile or a pyroelectric detector. Then the photocurrent can be transformed into quantum efficiencies with respect to incident radiation or, if the absorption is known, internal quantum efficiencies can becalculated. Because of various calibration curves and base line corrections, it is of advantage to use a microcomputer for data aquisition which also allows for an easier handling of the data.

4. EXAMPLES ?

Fig. 5. Schematic representation of an experimental set-up

for photoeleetroehemieal studies of passive films.

More recent work of photoelectrochemistry with passivated electrodes will now be reviewed. (For older work the review of Kuwana[43] with its comprehensive bibliography should be consulted.) Emphasis will be put on illustrative examples in order to demonstrate the various types of behavior of passive films. Some of the recent work that tries to point out the impact of an amorphous character of the passive film on photoelectrochemical behavior will be discussed. In the examples noble metals (including Ib group metals), transition metals and valve metals will be distinguished. In the section on valve metals, passive films modified by ion implantation will be included since they exhibit interesting structural properties which are reflected in their photoelectrochemical behavior.

421

Photoelectrochemical studies of passive films

Noble metals and Ib group metals Noble metals usually form thin oxide films some hundred millivolts below the oxygen evolution reaction. These oxide films are not passive films in the sense that they protect the underlying metal from dissolution. The formation process and their general behavior, however, is comparable to passive films so that treating them as such is justified. Copper and silver which both dissolve readily above their respective Thermodynamic dissolution potentials in acid solutions form passive films in neutral or basic electrolytes.

Platinum. Platinum electrodes show a complex photocurrent behavior upon uu light illumination after the formation of an oxide film[44]. In the region of the formation of a monolayer a steady state anodic photocurrent is observed. It changes to a transient type of current after formation of the so-called a-oxide which was interpreted by Vinnikov et aI.[44] by assuming a photoinduced disappearance of the oxide. With the formation of the B-oxide the photocurrent transient changes into a cathodic one. The thickness of the B-oxide seems to be stable under illumination conditions. It is concluded that different types of oxide films, n-type and p-type, are formed on platinum electrodes. Although platinum is one of the most investigated electrode materials and the properties of the oxide film have been widely studied, the abovedescribed investigation seems to be the only one with respect to photoelectrochemistry. Gold. Oxide-covered gold electrodes have been investigated with respect to photopotential and photocurrent measurements by Veselovskii et al.[4S, 461. The latter work measured the photocurrent response under UD light irradiation after oxide formation at U > 1.9 V. At these high potentials, the formation of a fairly thick oxide, possibly Au(O is assumed. The photoresponse was found to increase with oxide thickness. A very detailed and interesting study in acid and neutral solutions was undertaken by Watanabe and Gerischer[47,42]. Three main potential regimes can be distinguished in which a photoresponse is observed. At potentials V 2 1.O V, which is about 0.5 V before the oxide formation is seen in a potentiodynamic sweep, steady state photocurrents are measured at wavelengths 1 < 750 nm. The authors attribute this finding to a photoexcitation in an adsorbed OH species. Coverages of a few percent are obviously able to generate a photocurrent. At L5V > U & 1.8V transient type photocurrent response is observed which is attributed to a Auto3 layer of up to 1 nm thickness. The photocurrent increases with increasing thickness of the oxide. At potentials U > 1.9 V which is considered the range where a different oxide, possibly AUK, is formed, the photocurrent levels off although the oxide can grow up to 60nm thick. It is interesting to compare this photoelectrochemical behavior with very recent results by Bruckenstein and Shay[48] who measured the mass change during a potentiodynamic cycle of a gold electrode using the oscillating quartz technique. In both investigations three types of behavior, depending on the potential, can be distinguished. The potential ranges of the respective behaviors coincide largely in both investiga-

VII

&

1.5

3

$ 4 N

;

Au/Aua OS 0.5M

v

1.3 v

H2S04

9 e

I.1 v

2

4

3 PHOTON

ENERGY

5

hv/eV

Fig. 6. Fowler plot of photocurrent spectra with oxidecovered gold electrodes, data taken from Ref. [42]. tions. Bruckenstein and Shay[48] found a slight increase in mass starting at approx. 1.0 V, but the major increase in mass due to oxide formation was shifted by 0.2 V in the positive direction compared to the current potential curve. Between 1.4 and 1.7 V, no significant change in mass is observed. This shows that the potential range where the formation of Au,03 is assumed is mass neutral, in accordance with the assumption of an oxidation of attached OH and a place exchange reaction which does not change the mass considerably. At higher potentials, U > 1.7 V the mass increases continuously with potential. Besides the excitation in the surface film a photohole emission is assumed for higher photon energies. In Fig. 6, spectra in Fowler-type representation, q”.’ us hv, are shown for the potential range U = 0.9-1.5V [42]. While the spectra start all at one photon energy in the low energy range, straight lines are observed at hv > 4.2 eV with threshold energies that are proportional to the electrode potential. In the discussion of the energetics, the authors assign the onset of the photocurrent at 1.7eV to the band gap energy of the oxide film of AuzOs. This method is not very reliable as will be shown below for the cases of iron and nickel. It ignores that tailing, due to an extension of the density of state function into the band gap, can occur allowing absorption at hv c E,. The discussion of the energetics of the photohole emission leads to the assumption that the threshold energy is given by the upper valence band edge of liquid water. The band gap of liquid water was estimated to be > 8eV. This work by Watanabe and Gerischer is an excellent example for illustrating the sensitivity of the method and the ability of photoelectrochemistry to distinguish different surface species as well as to distinguish a parallel photoemission process from an excitation in the film.

Silver. Photoeffects are reported for silver electrodes in neutral and basic solutions[49-521. Memming et al.[49] investigated a photoinduced reaction at oxide-covered silver electrodes in 1 M KOH. They found that the photoprocess observed in the presence of Ag,O is due to an oxidation of AgzO to AgO. They also determined the spectral distribution of the photocurrent which has an onset at approx. 1 eV. The observation that a photoformed AgO layer needs a higher overpotential to be reduced than does an electrochemically formed one is assigned to the low

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conductivity of the underlying Ag,O layer. Perkins et a/.[501 observed photoeffects in the cyclic voltammogram of silver in 1 M KOH. During and after formation of Ag,O the photocurrent is anodic, but changes sign in the reverse sweep at approx. 1.3 V. The reduction peak is enhanced under illumination. The photoinduced process is considered to be an oxidation and Ag,O is considered to be an n-type ofAg+ toAg’+ semiconductor. The AgO itself did not show any direct photoeffects although it is an n-type semiconductor with a band gap of 2_2eV[53]. The idea of photoinduced Ag+ oxidation is also supported by Ross and RobertsrSt 1 who used 0.1 M KOH as their electrolyte. They, however, assume the Ag,O layer to be p-type conductins. This had earlier been found by Oshe and Rozenfel’d[52]. It seems that further work, especially to obtain more spectral information, eg as a function of the electrode potential, is necessary. Copper. The first detailed results for passivated copper electrodes were obtained by Paatsch[54]. He studied the influence of inhibitors on the corrosion behavior of copper using photopotential and photocurrent measurements in 0.1 M NaOH. While it was not clear that anodic photocurrents occured, cathodic photocurrents, associated with the oxide reduction, were observed. Such photocurrents are observed in neutral and basic solutions after the Cu(II)-oxide [CUO/CU(OH)~] reduction. The reduction of the Cur0 film in a cyclic voltammogram is enhanced under illuminationr551. This indicates, in accordance with bulk properties 01 Cu20, that this film is p-type conductina. The absorntion snectrum of Cu,Of56.571 shows an extensive peik structure which der&esfrom a number of different bands contributing to the absorption spectrum. This is still reflected in a photocurrent spectrum which was obtained in 1 M NaOH after reduction of the CUO/CU(OH)Z layer (Fig. 7)[58]. The various peaks obtained in the photocurrent spectrum can be related to the peaks in the absorption spectrum of Cu,O[56]. Using the absorption data of Brahms and Nikitine[56] andcombining it with the data in Fig. 7 (d = 2 nm), a quantum efficiency with respect to the absorbed number of photons can be estimated, which 20”/,. A similarly high value for oxideis approx. covered gold electrodes was estimated by Watanabe and Gerischer[47]. A recent study of copper in phosphoric acid by Pointu et al.[59], which was mainly performed in the potential regime where polishing occurs, showed anodic photocurrents. The spectra, however, do not allow for an identification of the film because of a wavelength spacing of only about 50nm which does not give a good enough resolution. In a later interpretation, a band structure model was discussed with conductivity types changing within the hlm[60].

Transition

metals

In the group of the transition metals are many of the technologically important materials such as iron, nickel and others. Consequently, most of the photoelectrochemical work with passive films has been done in this area, with the hope of advancing an understanding of passivity of these materials. Relatively early, Oshe and Rozenfel’d[52] determined the carrier types

STIMMING WAVELENGTH 600

X/ram

400

500

300

2.08

cu/cu*o IM

0’

’

1

2.0

2.5 PHOTON

I

3.0 ENERGY

NoOH

I

3.5

I

4.0

hu/eV

Fig. 7. Photocurrent spectrum of the passive film on copper after reduction of the CuO/Cu(OH), film; taken from Ref.

cw.

of sunerIicia1 oxides for a number of systems by recording photopotentials under pulsed illumination conditions. Their interpretation of the results was based on the semiconducting properties of the passive films. A fairly large number of investigations have Iron. been performed recentlyC13, 61-643. The picture obtained, however, is still not conclusive. Wilhelm and measurements in borate Hackerman[ 133 report buffer, pH 8.4. From the extrapolation of the linear part in the photocurrent spectrum, a band gap energy of 2.5eV is concluded. This procedure, however, is questionable since it has no real physical meaning. An analysis according to Equation (7) would be more appropriate. Taking the data of Wilhelm and Hackerman and replotting it according to Equation (7) for n = 2, ie a (t~hv)“’ us hv plot, yields a band gap energy of 1.9. Figure 8 shows the replotted data for iron and nickel; the latter will be described later. For n = l/2 ([qhv]’ us hv plot) no straight line could be obtained. (In the case of titanium, a similar result is connected to the amorphous character of thin films. See below.) A very similar curve to the one for iron in Fig. 8 was found by Abrantes and Peter[61]. They also found that the value of the band gap energy did not vary within the pH range 4.8-13.0. It is generally reported that the Dhotocurrent has a transient character[6163].‘Abrantes and Peter1611 propose a mechanism that involves the photoinduced formation of surface OH radicals which can recombine with electrons from the conduction band. Stimming[63], however. found that within an intermediate potential range with respect to the passive state of the electrode,;e approx. 0.7-1.1 Vat pH 8 and 14, the anodic transients t&r into a steady state cathodic photocurrent. In acid solutions (1 M HClO,J the photocurrent did not show

Photeeleetrochemic.al studies of passive films

423

in sulphide-containing solutions. Quantum efficiencies are Renerallv fairly hiRh for passive films. They found maximum values of 60 % for HgO and 10% for the films formed in sulphide solution. Photocurrent spectra reveal a band gap energy of 1.9 eV for the HgO film and 0.9 eV for the HgS film. The latter value is in considerable disagreement with the value of 2eV for cinnabar. There is, however, a discussion as to which modification of HgS constitutes the passive film on mercury. If metacinnabarite forms the film, which was discussed by Armstrong et aI.[67], this would be in conflict with the photoelectrochemical results since the latter one is considered a semi-metal. The authors[66] argue that the passive film might be a HgO contaminated and highly disordered metacinnabarite film which forms the HgS film and that possibly metacinnabarite is not a semi-metal.

Fig. 8. Plot of data of passivenickel and passiveiron in Ref. f131 - - using Equation (7) for the evaluation of band gap energies. a transient behavior. After prolonged polarization the photocurrent potential curves in neutral and basic solutions have a peak at lower potentials. It was found that the spectra at these lower potentials show a considerable blue shift when compared to the spectra obtained at higher potentials[64].Froelicher et 61.[62] compared the photocurrent spectra of the passive film with ones obtained for a-Fea02 and Fe304. Their conclusion is that the passive film contains Fe(H) species in the potential range before oxygen evolution sets in. The value for the Ratband potential, U, = 0.6 V, which was derived from the sign change of the photocurrent, is close (approx. 0.1 V higher) to values using obtained from capacity measurements Schottky-Mott analysisC12, 131. Although the picture of the passive film on iron that arises from the photoelectrochemical investigations is still not conclusive, it still demonstrates the capability of the method as an in situ technique. Nickel. Passive nickel was investigated in borate buffer at pH 8.4 by Wilhelm and Hackermanfl3]. Photocurrents are -cathodic indicating the p-type character of the nassive film. which is supposedly NiO. The flatband potential, U,, = 0.4 V, is in good-agreement with values obtained from capacity measurements, although approx. 0.1 V higher. The band gap energy of 3.1 eV, which is given by Wilhelm and Hackerman was obtained in the same way as described above for iron. Their data for nickel were again replotted using Equation (7). The plots in Fig. 8 show for n = 2 a band gap energy of E i = 2.2eV and for n = l/2, E$ = 3.4eV. These values are quite different from those given by Wilhelm and Hackerman but they are in excellent agreement with results obtained from ohotoelectrochemical studies of bulk NiO elec&des[65] and correspond to the band scheme derived by Dare-Edwards et al. Mercury. passivation

Da Silva Pereira and Peter[66] did studies of mercury in NaHCOS as well as

Tin. Studies of the photoelectrochemical behavior of passivated tin electrodes in borate buffer at pH 8.5 were performed by Kapusta and Hackerman[68]. Photopotential and photocurrents were measured as a function of the film thickness which was varied by polarization at different potentials or with different current densities. The passive film is supposed to be SnOz. The photoresponse (photocurrent for potentiostatic and photopotential for galvanostatic film formation) increased with increasing film thickness in the range 3-24nm. For thicker films, no further increase was found. The spectral response does not depend on the thickness of the film. An analysis of the absorption edge using an igx’ us hv plot which is the procedure given by Equation (7), yields a value of ca 2.5 eV. This value is considerably lower than the band gap of SnOz, which is 3.7 eV. Therefore, the authors argue that the value of 2.5 eV is an indirect transition while 3.7 is the “real” band gap. This conclusion is somewhat questionable since in the common definition the band gap is described as the minimum distance between valence and conduction band which also includes photon-assisted processes. As pointed out in Section 2, a straight line in an (i* hv)‘jz vs ho plot can also be characteristic of an amorphous material. The large extension of the density of states function into the band gap could be mistaken for a lower band gap energy of indirect transitions. For thin films, formed at formation potentials ZJ,, < 2.5V, a good agreement is found for the flatband potential obtained from the onset of the photocurrent and from capacity measurements. These values are close to 0.6 V. For thicker films considerable deviations are observed. The photocurrent potential curves show a distinct peak at low potentials with an onset potential at about 1.1 V. Schottky-Mott plots yield a value for the flatband potential which is more than 0.4V more positive. Although the spectra are reported to be the same for thin and thick films, it seems that, because of considerable changes in the potential dependence, different films with respect to stoichiometry, water content or structure as a function of thickness have to be assumed. Bismuth. There has been some recent work on passive bismuth[69-711 and the results indicate that it is an interesting system. Williams and Wright[69], using photopotential measurements, found band-to-

424

ULRICH

band transitions in the BiZOJ passive film as well as photoemission from the metal substrate. MetikosHukovicf701 assumed from the occurrence of anodic and cath&diGphotocurrents an “amphoteric”character of the passive film in the sense of being n-type and ptype semiconducting Bi203. A similar characterization had also been used-by Hardee and Bard[72] for the uhotocurrent behavior of thick Bi.0, films. A recent study by Castillo and Peter[71] br&ght some clarification to this confusing picture. Capacity measurements revealed that the passive film is mainly insulating; no potential dependence of the capacity was found indicating that a possible space charge layer has a thickness at least larger than the film itself. Using reflection data from the literature and applying equations similar to Equation (5), the authors calculated the absorption spectrum of the film from their anodic photocurrent data. A band gap energy of 2.8 eV due to indirect transitions was found. This was concluded from the extrapolation of the linear portion in a (a hv)“2 us hv plot. Since, however, the passive film is assumed to be amorphous, it is questionable if the assignment to indirecttransitions iscorrect. As pointed out above (see Section 2). Mott and Davisr271 assume that the selection rules’in amorphous mate%als are broken down and the above type of dependence a(hv) reflects rather the density of states close to the band edges. The cathodic photocurrent shows the same spectral response as the anodic one but at low photon energies an additional current is observed. The latter effect is explained by an internal photoemission process from the metal into the passive film, similar to the situation illustrated in Fig. lc. The threshold energy was determined from a Fowter plot ($” us hv) to be 1.4eV and it did not change with the applied field which shows that the potential drop at the metal-passive film contact does not depend on the electrode potential. On the basis of their results, the authors[74] derive a band scheme of the passive bismuth electrode. Photoelectrochemical properties of Tungsten. passive films on tungsten were investigated by Reichman and Bard[73] and in a series of papers by DiQuarto et a!.[39 74, 751. Passive films on tungsten are amorphous under certain formation conditions. This makes this system interesting for studying the influence of structural properties of the passive film on their photoelectrochemical behaviors. Amorphous films are formed at formation potentials below 70 V in 0.1 N HJPOd. Photocurrent spectra show a straight line in an (iphhv)li2 vs hv plot with an intersect at approx. 3.0eV. This value is independent of film thickness and largely independent of the film formation conditions. Polarization in 1 M HN03 for 10min results in the formation of triclinic W03 film whose absorption edge, due to indirect transitions, was found to be 2.6 eV - a considerable difference to the amorphous film. Annealing the amorphous film in argon at 350°C for 3 h results in a monoclinic film which has a similar band gap compared to the triclinic one. An interesting result was obtained for a film formed in 1 N HrSO, at 70°C for 1 h. In the (i,, hv)“’ vs hv plot, two regimes with straight lines were obtained: one with 2.7eV and the other with 3.1 eV onset. From surface analysis of the film (X-ray and SEM), it was concluded

STlMMING

that the film at the surface is highly porous but crystalline, with a composition of WOs.H,O. The conclusion of the authors is that a duplex layer of amorphous WOp underneath, and porous WO,.HsO on top forms the passive film, with both contributing to the photocurrent response. As described above, an amorphous film in comparison to a crystalline one can have a higher or lower absorption edge. In the case of the WOa passive films the localized states close to the band edges obviously do not contribute to the photocurrent, the apparent absorption edge then is higher than in the crystalline material (the opposite type of behavior will be discussed later for ion implanted passive films on hafnium electrodes). The potential dependence of the photocurrent of the amorphous films was discussed in terms of the Poole-Frenkel behavior[30]. Usually the potential dependence is an exoonential one for a Poole-Frenkel tvne of behavior. The authors[30], however, argue that-their result of a linear i,,,,(V) relation can be explained by a linear expansion of the exponential Poole-Frenkel equation [Equation (13)]. Such an assumption can be made for relatively low fields. Valve metals Passive titanium is probably the best Titanium. investigated passive metal system with respect to photoeleetrochemistry[l4, 22, 23, 76, 77, 791. This is largely due to the fact that quantum efficiencies are fairly high in TiOs which is recognized as forming the passive films. Therefore, it is easier to measure photocurrents in this system compared to systems where photocurrents are only in the order of nA or even pA. Results basically show TiO, behavior for thick films but it seems that breakdown phenomena of the film are not accounted for in the results. Paatsch[76] did photopotential measurements for films formed in 0.1 NHzS04 at potentials below approx. 1OV. At formation potentials below 6V (us see), the photopotential spectrum was found to be the same as the one for a TiOs single crystal. For higher formation potentials, U,, k 6V, the spectrum was shifted by 1Onm to higher wavelengths, which was accompanied by a color change of the passive film to blue. It was concluded that the passive film undergoes an irreversible change at higher formation potentials. Pesant and Vennereau[77] found for films formed at 6 and 15 V (ws nhe) in 0.1 M H2S04 band gap energies that were higher than those of TiOz. It is, however, not clear from their paper which method they used for the evaluation of the band gaps. From SIMS experiments which were done in coniunction with the photocurrent measurements, two dikerent species were found to form the passive film. M&leer and Peter[22] investigated thin and thick passive films formed in 1 MH+SO, and 1 MHaPOa. Film formation was usually performed at a- slow rate, 1 mV s-i being typical. Thin films formed above approx. 1.75 V show clearly TiOz behavior. The potential dependence shows basically straight f&J curves which indicates a strong field effect on the photocurrent. For thicker films, U,, > lOV, breakdown phenomena of the film becomes incorporated in the photoelectrochemical behavior. The photocurrent spectrum of a 30V film changes considerably with aging, for times up to 15 min

Photoelectrochemiealstudies of passive films after film formation. Measurements with thin films formed with a potentiostatic step function in 1 M HClO.+[ 141 showed a band gap of 3.4eV from a (i,, hv)“’ vs hv plot. The potential dependence was generally linear, confirming the strong influence of the field. The photocurrent decreased with thickness at a constant electrode potential, demonstrating that the lower field in a thicker film has a stronger influence on the photocurrent than the additional absorption. At constant field strength, an increase of the photocurrent with thickness is observed for a wavelength of 300nm which levels off at approx. IO nm film thickness. It was concluded, in general agreement with McAleer and Peter[22], that the film is amorphous, although no clear treatment of the photoelectrochemical behavior of amorphous films was available at that time. Very recently, Leitner et aI.[23] performed a comprehensive study of the influence of the formation potential, 4 V < U,, Q 105V, on photocurrent spectra and the potential dependence. Film formation was done galvanostatically at 7 mA cm-’ up to the formation potential and then held at the respective U,, for 300s. The analysis of the spectra and the potential dependence showed four different types of behavior. This is shown in Fig. 9 in a plot of the band gap as a function of the formation potential. E’. characterizes the intersect for .ph = 0 in a (i,,,, hv)‘12 ‘vs hv plot and Ei stems from ‘ corresnondina (i,. hvj2 us hv plots. Tvne I films, UP to V,, =-15 V, d; n&t have any Et whilethe band gapfor “indirect” transitions decreases from 3.3 eV by about 0.1 eV with increasing thickness. These films were identified as clearly amorphous. Their behavior is connected to the fact that no direct band gap is found. For 16V ,( U, ,( 3OV, called type II, Ei values are observed. They decrease strongly from 3.7 to 3.25eV while E: is relatively constant at 3.3 eV. This range is interpreted as a transition between an amorphous and a microcrystalline structure of the film. For U,, > 30V up to 105V, E: stays remarkably constant at 3.55eV with a maximum variation of f 0.02 eV. This value is in good agreement with values for the direct band gap of

TiO, given by Waff and Park[78). E: = 3.22eV is found for type III films and 3.18 eV for type IV films. The main difference between type III and IV films lies in the potential dependence and the shape of the spectra. Type III spectra show a strong decrease at higher photon energies indicating surface recombination and have low quantum efficiencies in general, usually lower than the much thinner type I films. These films were considered microcrystalline with a high number of recombination centers which were more active for surface recombination than comparable states in the amorphous films. Type IV films show saturation currents and an increase of the quantum efficiency with photon energy up to approx. 5 &. They are obviously fully crvstallized and represent TiO, behavior, although the-band gap energy-is still higher by approx. 0.1 eY compared to single crystal TiOz electrodes[78a]. In aging experiments[23], a change of spectra and potential dependence could be observed which indicates a time dependent crystallization of the passive film. Naturally, these effects were the most pronounced for thin films. An interesting application of photoelectrochemical behavior in the &ray of the surface of passive titanium is described bv ButlerI791. A laser beam with a wavelength within the s$x&tm of TiO, was used to scan the surface of the electrode to trace the photoresponse at each point. Any deviation in composition of the film or defects were registered as a corresponding change in the photocurrent. Tantalum. Tantalum is supposed to form amorphous passive films of Ta20s[9]. Young[9] reported measurements of photocurrents upon illumination with polychromatic uv light. Depending on thickness, different curves were obtained as a function of the electrode potential (see Fig. 10.03 in[9]). The results were replotted in terms of a Poole-Frenkel plot which yielded one straight line for the thickness range d Tantalum is obviously another = 3&350nm[58]. example where the amorphous structure of the passive

3.6

3.2

Fig. 9.

425

Plot of band energies as a function of the formation potential of passive films on titanium as obtained from (P$IV)~vs hv plots (E:) and from (P$IV)“~ OShv plots (E:); taken from Ref. [23].

426

ULRICH STIMMING

h~330flrn

6

Ta/ Ta,05

;5j., “0

-Ir

4 ‘1.

:j5 _._.___~‘ 1 __.*._.-._._..-.-.-. -.-.----.~._._.~.-._._._~~ 0

0.5

1.0

15

2.0

u/v Fig. 10. Effect of hydrogen pretreatment of passive tantalum electrodes on the potential dependence of the sub-band gap response in 0.5 M H$O,; taken from Ref. [80].

film is clearly reflected in the photocurrent behavior. Recent investigations by White and Stimming[SO] in 0.5 M HaSO demonstrate how another structure-related property of the passive film can be reflected in the photocurrent. The band gap energy obtained from a (n hv)l” us hv plot was 3.9 & 0.05 eV for films formed between approx. 2 and 1OV by a potentiostatic step function for 30min. No straight lines could be obtained in a (r~hv)* us hv plot. A plot according to Equation (S), In q us hv (Urbach tail), which assumes CL Q r], yields a straight line between 3.4 and 4.1 eV. The photocurrent has its onset at approx. 0 V with a slightly steeper than linear increase with the potential. An interesting effect is observed after the film is exposed to hydrogen evolution (eg -2.0 V for 60 s). Besides an anodic charge in the dark of a few hundred &cm-‘, a large anodic photocurrent decreasing with increasing potential is observed. In Fig. 10, this effect is shown. The up scan is recorded just after the low potential pretreatment with a consecutive down sweep which shows the usual iph (U) behavior. The effect of an additional photocurrent after cathodic pretreatment can be observed down to wavelengths of 700nm (hv = 1.77eV). During the pretreatment, defects are obviously created by hydrogen penetration or a partial reduction of the film which gives rise to the sub-band gap response. According to Young[9], photocurrents stimulate further oxide growth. This was confirmed by Auger experiments[SO] performed with previously illuminated samples, which showed that the illuminated spot had a thickness of 50nm compared to the non-illuminated areas where the film was 30nm thick. Hufnium. Ahlgren and Sari[Sl] found that passivated hafnium electrodes show photocurrents upon uv illumination, indicating that the passive film is insulating. It is assumed that the film is formed of HfOz. In a recent investigation by Bartels et a/.[821 the passive film showed a band gap close to that of HfOr which is 5.5 eV. The photocurrent increases almost linearly with the electrode potential, indicating a strong effect of the electric field. Passive hafnium is an interesting system since structural changes induced by ion implantation are largely displayed in the photoelectrochemical properties, as will be shown in the following section. Modified films on titanium and hafnium. Photoelectrochemistry was used to study passive films on titanium and hafnium electrodes that were modified

Fig. Il. Spectra of the quantum efficiency of Pd implanted passive films on hafnium electrodes in 1 M HC104 for various implantation concentrations; taken from Ref. [84]. The passive films on the by ion implantation[83-851. titanium and the hafnium electrodes used for ion implantation had a thickness of 40nm for the TiO, film and 50nm for the HfOa film. Ion implantation of Pd, Fe or Xe was performed at various energies so that a practically even distribution of the implanted ion in the film was achieved. The implantation dose varied between 0.1 and 10%. It was found that the rate of redox reactions was increased by ion implantation, as was the dielectric constant of film. Xenon implantation had a similar effect, indicating that structural changes caused by the implantation process itself play an important role. Photocurrent spectra obtained for implanted HfOz exhibit a considerable red shift of the absorption edge. An example is shown in Fig. 11 for Pd implanted films[84]. In principle, ion implantation enhances the photocurrent, and the photocurrent onset for implanted films is shifted by more than 2 eV to lower photon energies. The additional absorption in the sub-band gap region clearly increases with the concentration of the implanted ion. Implantation of Fe and Xe have a similar effect which is even greater in the case of Xe. For ion implanted TiOz no such change in the absorption edge is visible. The potential dependence of the photocurrent, however, is changed towards a much stronger field dependence. The potential dependence of the photocurrent for the sub-band gap response follows a Poole-Frenkel type of behavior which is typical of amorphous materials. In Fig. 12, a log iphvs F”’ plot is shown for 5 0APd implanted HfOa films at various sub-band gap photon energies[84,85]. In the range hv = 2.5-5.0eV, straight lines are obtained which are practically parallel to each other. The dielectric constant calculated from the slope according to Equation (13), assuming a localized state to localized state excitation, is in good agreement with values obtained from capacity measurements. At high fields a characteristic break in the slope is observed; the lower the photon energy, the higher the slope. This behavior is similar to what is observed in the calculation of the photocurrent that results from an excitation to or from

Photoelectrochemical

filters, however, does not allow for a detailed analysis of the spectral behavior. Very recently, Newmark and Stimming[87] investigated iron-zirconium allows of a 33 % Fe-67 O/zr composition in 1 M NaClO,. The photocurrent spectra yield “band gap” energies for films formed in the potential range U., = 1.2-1.6 V of around 3.3 eV. The values were obtained from (‘I hv)“’ us hv plots. This energy is auite different from that of oassive films on iron (a$prox. 2 eV) and on zirconium ($3 eV) which was investigated in parallel. These first results for passive amorphous metals con&m that the passive films exhibit specific properties that are not simply a mixture of the oxides of the alloy constituting metals. It also shows that photoelectrochemistry is a valuable tool to investigate the passive film under in situ conditions.

Hf02CSXPd)

-4

-5

.B F -6

0

I

427

studies of passive films

2 F’/2,

3

1

IO2

Vi/2 c,,,-“2

5

6

Fig. 12. PooleFrenkel plot of Pd (5 %) implanted passive film on hafnium (d = 50nm) in 1 M HCIOd for various subband gap photon energies; taken from Refs [84, 853.

localized states (see Fig. 3). At high fields, the current from low lying traps (low photon energies) is enhanced more than the current that results from traps closer to the band edge (high photon energies), due to the larger contribution of tunneling. So the same general picture that is shown in Fig. 4 seems to be reflected in the experimental results at high fields. In conclusion, ion implantation changes the electronic structure of the passive films mainly through a disordering of the film. This is well reflected in and, therefore, identifiable by photoelectrochemical measurements.

Amorphous metals So far, there seem to be only two examples in which passive films on amorphous metals were studied by photoelectrochemical means[86, 871. Amorphous metals have become of much interest since they show a good corrosion resistance for specific types of metal attack (especially at grain boundaries which do not exist in a glassy material). Usually, amorphous metals are rather alloys than pure metals. In comparison to their crystalline counterparts, their passive films should most likely be amorphous, while films on crystalline substrates can be either way. Revesz and Kruger[88] have pointed out the importance of an amorphous passive film for the corrosion resistance of materials. For amorphous alloys, a passive film with new qualities and no mixture of oxides of the metallic components would be expected. Such structural and compositional properties of passive films on amorphous metals should be reflected in their photoelectrochemical behavior. Burleigh and Latanision[86] studied amorphous copper-zirconium alloys. The results demonstrate that photocurrents which were anodic can be measured and that the absorption edge is shifted to lower photon energies compared to zirconium. The technique of using a polychromatic light source with interference

5. CONCLUSIONS In the given description of the background of the photoelectrochemical behavior of passive films, it was shown that the stoichiometry and structural properties of the film are well reflected in photoelectrochemical data. Amorphous and crystalline films are different in their spectral behavior as well as in the potential dependence of the photocurrent due to the participation of localized states in amorphous films. The question, therefore, as to whether the passive film is amorphous or crystalline should be answerable by means of photoelectrochemistry, the specific advantages being that the radiation used in the experiment is not damaging to the film and that it is an in situ technique. Some of the more recent examples reviewed from the literature demonstrate this. To further exploit this potential, more elaborate techniques such as time resolved laser pulse analysis, as was done very recently by Plieth and coworkersC89, 901 for oxide-covered platinum and passive iron, respectively, the analysis of photocapacities or the development of surfaceimaging should be applied to passive films. Acknowledgements-Valuable discussions with my colleagues and students on photoelectrochemistry of passive films are appreciated. The assistance of MS A. R. Newmark in preparing the manuscript is greatly appreciated. Part of this work was financially supported by the National Science Foundation and Columbia University. REFERENCES I. E. Bequerel, C.r. h&d. S.&nc. Acad. Sci., Paris 9, 561 (1839). 2. Yu. Ya. Gurevich, Yu. V. Pleskov and Z. A. Rote&erg, Photoelectrochemistry. Consultants Bureau, New York 119801. 3. s. R. ‘Morrison, Electrochemistry at Semiconductor and Oxidized Metal Electrodes. Plenum Press, New York (1981). 4. A. Heller (Ed.), Semiconductor Liquid-Junction Solar Cells. The Electrochemical Society. Princeton, New Jersey (1977). 5. W. L. Wallace, A. J. NO& S. K. Deb and R. H. Wilson (Eds), Photoelectrochemistry: Fundamental Processes and Measurement Techniques. The Electrochemical Society

Princeton, New Jersey (1982). 6. W. W. Gaertner, Phys. Rev. 116,84

(1959).

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7. M. A. Butler, J. appl. Phys. 48, 1914 (1977). 8. K. J. Vetter, Electrochemical Kinetics. Academic Press, New York (1976). 9. L. Young, Anodic Oxide Films. Academic Press, London (1961). 59, 716 10. H. J. Engell and B. Ilschner, Z. Elektrochem. (1955). 11. R. V. Moshtev, Ber Bunsenges Phys. Chem. 72,452 (1968). 12. U. Stimming and J. W. Schultze, Ber. Bunsenges Phys. Chem. 80, 1297 (1976). J. electrochem. Sot. 13. S. M. Wilhelm and N. Hackerman, 128, 1668 (1981). 14. J. W. Schultze, U. Stimmina and J. Weise, Ber Bunsenges

Phys. Gem.

86, 276 (1982x

15. K. E. Heusler

and R. S. Yun,

Electrochim.

Acta 22, 977

(1977). 16. J. W. Schultze,

in Passivity of Metals (Edited by R. P. Frankenthal and J. Kruger), p. 82 and references therein. The Electrochemical Society. Princeton, New Jersey (1978). 17. U. Stimming and J. W. Schultze, Electrochim. Acfo 24, 852 (1979). 18. S. Kapusta and N. Hackerman, J. electrochem. Sot. 128, 327 (1981). 19. W. Schmickler and U. Stimming, Thin Solid Films 75,331

(1981). Phys. 19,217 (1977). 20. W. Schmickler and J. Ulstrup,Chem. 21. W. Schmickler, J. electroanal. Chem. 82,65 (1977); 83,387 (1977). 22. J. F. McAleer and L. M. Peter, Faraday Discuss. 70, 67

(1980). 23. K. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

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