188 Sensors and Actuators, Bl (1990) 188-194

Investigations of Electrode for Oxygen Sensors Based on Lanthanum Trifluoride as Solid Electrolyte

S. HARKE, H.-D. WIEMHt)FER and W. Gc)PEL

Institute of Physical and Theoretical Chemistry of the University of Tcbingen, D-7400 Ttiingen (F.R.G.)

Abstract

Electrodes of low-temperature oxygen sensors based on LaF, as solid electrolyte are studied in relation to contact properties and electrode reac- tion mechanisms. The experimental techniques used are electrochemical impedance spectroscopy, measurements of electromotive forces (e.m.f.) and XPS and UPS photoelectron spectroscopies.

Criteria for good reference electrodes are derived from results of impedance measure- ments on solid and liquid reference contacts. Solid reference electrodes with Ag/AgF show low ohmic resistance, but are unstable at elevated temperatures, whereas Sn/SnF, reference elec- trodes show good thermal stability, but very high impedance.

From XPS, UPS, work function and imped- ance measurements we derive a model of the thermodynamics and the microscopic reaction mechanisms of gaseous oxygen and water molecules at porous Pt electrodes on LaF,. Partly reduced oxygen is formed at the LaF, surface by cathode reduction or treatment with oxygen and water at elevated temperatures and is stabilized by coadsorbed water as hydrogen peroxide.

1. Introductioo

Electrochemical oxygen sensors are of great interest in many applications. In particular, their low working temperatures are often desirable. This requirement is fulfilled by amperometric sen- sors like the Clark cell or the Mackereth cell, which both use liquid electrolytes. There are, how- ever, fields of application where solid-state sensors have major advantages over devices with liquids. This has stimulated investigations on galvanic cells with solid electrolytes that show a low- temperature Nernstian response upon variations in the oxygen partial pressure between 1 bar and lop3 bar [ 1- 121. Investigated materials are LaF, [l, 5, 10-121, B-PbF* [2,9], PbSnF, [3,4] and AgI [7, 81. Most of these electrolytes show a fast re-

sponse. In particular the presence of water at the surface seems to accelerate the response. From the slope of the open-circuit voltage versus logarithm of oxygen partial pressure, most authors deduce a one- or two-electron process for the electrode reaction. The existence of superoxide ions (O,-) or peroxide ions (HO,-) may thus be postulated at the solid electrolyte surfaces. Further experi- mental evidence, however, is not yet available about atomistic details of the elementary reaction steps.

Systematic investigations were published in 1973 for solid-state cells with the F- anion con- ductor LaF, [ 11. It was demonstrated that open- circuit voltage and current-voltage relations of cells like BilLaF,IAu and equivalent cells with electrolytes like CeF,, NdF3 and PrF, respond in a reproducible way to 02, CO*, NO*, SOz and NO. This was attributed to reversible reduction of the adsorbed gas molecules at the cathode-elec- trolyte interface. Oxygen, as an example, was assumed to react according to

02+e-+0,-

Recent results on galvanic cells with PbSnF, [ 3,4] and PbF2 [9] were interpreted in terms of the formation of peroxide according to the low- temperature electrode reaction

0,+2e-+Oz2-

The peroxide ions were assumed to occupy lattice positions in the fluorides. At temperatures above 600 K, the authors observed the ‘normal’ four- electron reduction to oxide ions, as in oxygen sensors based on O2 - anion conducting zirconia with

02+&- +202-

For LaF3 fast response was observed at room temperature after stepwise changes of oxygen par- tial pressure [5, 10-121. The proposed electrode reaction was peroxide formation [S, 10-121. Pre- treatment with water vapour at elevated tempera- tures was found to improve the response time. A reaction mechanism was suggested whereby

0 Elsevier Sequoia/Printed in The Netherlands 0925-4005/90/$3.50

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hydroxide ions formed by the water treatment participate in the electrode reaction [ 121.

We now report on systematic investigations to characterize the electrodes and the mechanism of the oxygen interaction with galvanic cells of the type: reference (doped LaF,]Pt, O,@,. Particular emphasis is put on developing criteria to charac- terize the quality of the electrode, on identifying the chemical species at the platinum electrodes and on identifying the ‘potential forming process’. This is a prerequisite to optimizing these devices with respect to long-term stability, low drifts and reproducible response.

2. Experimental

Most surface spectroscopic and electrical mea- surements were done with LaF, single crystals, which were either undoped or doped with 5 mol% SrF, (received from A. V. Chadwick [ 151). In addition, evaporated LaF, films (from Balzers, ‘coating material’) were also investigated.

As solid reference electrodes we used either evaporated Ag/AgF thin layers or mixtures of Sn and SnF,, which were melted directly on the LaF, surface under dry nitrogen. Liquid reference elec- trodes consisted of Ag wires with electrolytically deposited AgCl, which were immersed in a liquid electrolyte with lop3 mol 1-i F- and Cl-. Pt working electrodes were prepared in four different ways on the surface of LaF,: (a) by sintering Pt paste; (b) by decomposing a methanolic solution of H,PtCl,; (c) by evaporating an ethanolic sus- pension of Pt black; (d) by rf sputtering Pt tilms.

Impedance measurements were carried out with a Solartron 1255FRA/1286ECI system. Surface spectroscopic investigations (XPS, UPS, ELS) were done in a combined UHV system described elsewhere [ 161.

3. Results and Discussion

3.1. Sensor Characteristics We studied the oxygen-sensing properties of

the following three galvanic cells with different reference electrodes at room temperature:

Ag(AgF]LaF, [Pt, O2 (la)

Sn, SnF,]LaF, IPt, O2 (lb)

AglAgClIF-, Cl-(LaF,(Pt, O2 (lc)

(both 10P3 M in H,O)

Cells with Sn, SnF, reference electrodes have been investigated extensively before by Miura, Kuwata, Yamazoe, Seiyama and coworkers

[ 5, lo- 141. These authors have developed prepar- ative procedures that lead to very stable oxygen sensors with short response times. Although we have not optimized our sensors in the same way, our results will be shown to be comparable.

The three types of cells showed a near-ideal Nemstian response to changing oxygen partial pressures. Figure 1 shows typical results with a liquid reference (cell lc) and a platinum electrode prepared by evaporating an ethanolic suspension of Pt black. In the simplest case, the results can be interpreted by the redox equilibrium of absorbed oxygen with reduced oxygen species O,‘- accord- ing to:

O,+ze-*O,‘- (2)

Taking into account the constant electrode poten- tial of the reference, one obtains for the open- circuit voltage or electromotive force E:

(3)

The activity of the reduced species at the LaF,/Pt interface is denoted by a(O,'- ). Figure 1 corre- sponds to a measurement in a dry gas mixture at 1 bar with different &02)/p(N2) ratios. The sen- sor was pretreated in air saturated with H,O at 100 “C for one hour. The slope of E versus log ~(0,) gives the charge number z, i.e., the number of electrons in the corresponding elec- trode reaction, if u(O,‘-) remains constant. In fact, one observes a small drift of the e.m.f. during the experiment (see Fig. 1). This means that the surface activity a(O,‘- ) changes slowly and also explains why we do not obtain integer values for z from the slope. Subtracting this drift from the absolute values of the e.m.f., we obtain

T= 300 K

atom= 1 bar

dry Na/Oa mixture

0.1

n

Fig. 1. Sensor test: e.m.f. E as a function of O2 partial pressure in a dry N,/O, mixture at a total pressure of 1 bar and a temperature of 300 K. Cell: AglAgCIICl-, F-ILaF,IPt, 0, (in N2) [Cl-, F- both 10e3 mol l-l].

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z = 1.0 & 0.1. For humid gas and after pretreat- ment with water-saturated air at 100 “C, the slope changes to z = 1.8 + 0.1. Thus, in the presence of water the absorbed oxygen seems to equilibrate with absorbed peroxidic oxygen species, resulting in an electrode reaction with two electrons per oxygen molecule. Without coadsorbed water, i.e., in dry gas, oxygen equilibrates only with superox- ide species 02-. Two reaction steps seem to be involved in the complete equilibrium of adsorbed oxygen and peroxidic species; each step leads to the transfer of one electron. The second step seems to be hindered if adsorbed water molecules are not available. The following proposed mechanism can explain these results:

(a) 02+e-*0,- (3a)

(b) 02- + H,O e HO*’ + OH- (3b)

(c) HO*’ + e- e HOz-

with the total reaction:

(3c)

0,+HzO+2e =HOz- +OH- (3)

If equilibrium (3) describes the reaction at the working electrode, the Nemst equation for the e.m.f. of the cell (1) has to be written:

E = E0 + g In p(O&o,O) a(HO,-)a(OH-) >

(4)

Here, a(HO,-) and a(OH-) denote the surface activities of the two adsorbed species HOz- and OH-. To achieve a stable e.m.f. E, well-defined and constant surface activities of these species are required. In our model, the pretreatment and con- ditioning of the LaFJPt interface determines the initial surface activities of HO*- and OH-. The situation is different from that of high-temperature oxygen sensors with stabilized zirconia, which are characterized by the electrode equilibrium

I 2+2e ‘0 - =02-

Oxide ions O2 - in stabilized cubic zirconia have a constant activity because of the high oxygen va- cancy concentration in this compound. Therefore the resulting electrode potential only changes if the oxygen partial pressure changes.

Mechanisms like (3a-c) are well known from the electrochemistry of oxygen electrodes in aqueous electrolytes and there have been many investigations of the intermediate steps of oxygen reduction and oxygen evolution at metal and metal oxide electrodes [ 17J. The peroxidic species HO*- (or H202) is a metastable intermediate of redox reactions with 02, whereas the superoxide radical is very reactive, particularly in the presence of water, and is therefore expected to have a short lifetime. One expects that its surface concentration is very low. The question arises, what stabilizes

adsorbed 02-, whose existence at the LaF,/Pt interface has been postulated from the results of e.m.f. measurements in dry gases.

There arc a few examples known in chemistry where 02- is stabilized in bulk phases, such as alkali superoxides, oxygen in certain semiconduct- ing metal phthalocyanines or Oz- as a point defect in certain alkali halides such as KCl. 02- as surface species was observed on metal and semi- conductor surfaces, for instance on ZnO and TiOz [ 181 as a result of chemisorption of oxygen.

Another result that supports the proposed reac- tion mechanism is that the e.m.f. of the galvanic cells also depends on the partial pressure of water. Figure 2 shows results for the e.m.f. E of a cell with a liquid reference at 300 K. At constant oxygen partial pressure, one obtains two electrons per molecule of Hz0 from the slope of the e.m.f. versus log p(H,O) curve, as expected in the proposed electrode reaction.

According to Miura et al. [ 121, oxygen sensors of the type Sn, SnF,ILaF, IPt, O2 with sputtered LaF, fIhns give a Nemstian behaviour of the e.m.f. as a function of oxygen concentration, if oxygen dissolved in aqueous solution is measured. They obtain one electron per oxygen molecule from the slope of e.m.f. versus logp(Oz) and deduce the following electrode reaction [ 121:

02+HzO+e- = HOz’ + OH- (5)

This reaction corresponds to steps (a) and (b) of the mechanism proposed in eqn. (3). The second electron transfer is not found, although water molecules are available. Peroxide is therefore not adsorbed at the LaF,/Pt interface in aqueous solutions.

Fig. 2. Sensor test: e.m.f. E as a function of I-I,0 partial pressure in air at 300 K. Cell: AghgCl(CI-, F-(LaF& Hz0 (in air) [Cl-, F- both 10e3 mol l-l].

3.2. Results of Surface Spectroscopic Measurements

Further evidence for the above-derived model can be deduced from XPS and UPS results. For this purpose we studied the interaction of water and oxygen with the LaF, surface. Figure 3 shows changes of the 0 1s core-level binding energies in the XPS spectra of a LaF, surface after different pretreatments with 02/H20 mixtures. The 0 1s emission leads to a superposition of several peaks with different chemical shifts. Chemical shifts as found earlier in the literature are attributed to OH-, 02, HzO, 02- and HO*-. An attempt at a lineshape analysis is shown in Fig. 3. The two different spectra in Fig. 3, normalized to the same maximum intensity, characterize samples pre- treated with 02/H20 at 300 K and at 500 K. OH- is clearly detectable in both cases. The signal intensity from OH- and HO*- relative to 02 - and HZ0 is increased by a factor of three for the 500 K treatment as compared to the lower 300 K treatment. The absolute signal intensity is signifi- cantly enhanced, too. We conclude that already at room temperature there is a slow hydrolysis of the LaF, surface, indicated by the presence of H20, OH- and 02- and a side reaction between O2 and H20 leading to the formation of H02-. The surface concentration of OH- and HO,- is, how- ever distinctly higher after the 500 K treatment.

XPS 01s

LaF3- surface

(b) 500 K

(a) 300 K

HOj OH- HO,

I _^. _,.^ _^^ 534 JJ” SSD

Binding Energy (eV) -

Fig. 3. XPS spectra for the 0 1s core level emission of an evaporated LaF3 layer as a function of pretreatment tempera- ture for the same gas composition (air, p(H,O) = 20 mbar). Curve (a): 300 K for 1 h. Curve (b): 500 K for 1 h.

191

As one cannot distinguish between H,O and HO2 (or 0, - ) because of the same binding energy, the formation of superoxide is not unequivocally de- tectable by XPS.

These results reflect the well-known tendency of many fluoride surfaces to adsorb H,O and OH- strongly. At relatively high temperatures, typically above 500 K, hydrolysis of LaF, occurs in the presence of water. This results in the forma- tion of the oxides LaOF and La203 and of hy- droxide La(OH),. As a consequence, the LaF, surface and thus the LaFJPt interface of the sensor will be passivated after treatment at too high temperatures, because thick lanthanum oxide or hydroxide layers are insulating and lead to a very high electrode impedance and slow response.

In another series of experiments we studied changes A4 in the work function of LaFS surfaces with W photoelectron spectroscopy (UPS) after exposure to different partial pressures of O2 and H,O. Figure 4 gives an overview of typical results for Ar$ as evaluated from UPS spectra. The work function change A+ can be divided into a con- tribution from electron affinity, Ax, due to chang- ing surface dipole moments and a contribution A(E, - Ev)surr due to a shift of the Fermi level EF

(;I T-=400 K 1

-_ 10-e 10-m lo-'

P(O2) (Pa) -

Fig. 4. Change of work function A4 of an evaporated LaF, layer measured with UPS (HeI) as a function of partial pressures of 0, and H,O at 400 K. Curve (a): p(H,O) = 0 Pa. Curve (b): p(H,O) = 5 x lo-’ Pa. Curve (c): p(H,O) = I x 10e6Pa.

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relative to the valence band edge Ev at the surface according to

A$ = AX - A(Er - Ev),,,r

It is possible to determine the two contributions separately from a UPS spectrum. The second term indicates charge transfer between the surface and subsurface of the solid.

The UPS results in Fig. 4 show that the change in work function consists almost exclusively of contributions from A(EF - E,),,, for the systems LaFJO, and LaF,/H,O. With increasing partial pressure of 02, the work function of the LaF, surface increases, thus indicating negatively charged adsorbed oxygen. During Hz0 exposure the work function changes in the opposite direc- tion. From this we deduce positively charged sur- face species to be formed or the concentration of negatively charged species to be reduced after chemisorption of H20. Coadsorption of both H,O and O2 does not result in a simple additive effect of the two gases concerning work function changes. If oxygen is offered to LaF, at a constant partial pressure of H20, the work function first decreases, thus indicating a reaction between coadsorbed water and oxygen, before it increases again for the higher oxygen partial pressures.

3.3. Impedance of LaFJPt, O2 Working Electrodes

Impedance measurements have become a very useful tool in electrochemistry for the characteri- zation of electrodes and electrolytes [ 191. They were used here to study working electrodes and changes due to the pretreatment. Figure 5 shows a complex plane plot of the complex impedance of

Pt LaFs Pt I I

in dry Nz

300 K

0.1 Hz

Fig. 5. Complex impedance plot with blocking Pt electrodes on a LaF,( 5 mol% SrF,) single crystal. Cell: PtlLaF, IPt in dry N, at 300 K.

LaF, with blocking Pt electrodes. Values for the electrode impedance are larger than 10’ Ohm at 1 Hz. Figure 6 shows two comparable curves for the same electrodes in humid air after different pretreatment times. Curve (a) corresponds to a sample that was pretreated with humid air for one hour at a temperature of 100 “C. Curve (b) results from the same sample, but with an additional pretreatment for one hour under the same condi- tions. In all three cases, the extrapolation to high frequencies gives the ohmic resistance of the LaF3 electrolyte of about 2 x lo3 Ohm at room temper- ature for LaF, doped with 5 mol% SrF,. This corresponds to an ionic conductivity of 3.6 x lo-* (Ohm cm)-‘.

In comparison to the blocking electrodes of Fig. 5, the electrode impedance at 1 Hz in Fig. 6 is lowered by about three orders of magnitude. The treatment with O,/H,O at 100 “C lowers the elec- trode impedance, as can be seen by comparing curves (a) and (b). The frequency dependence of the electrode impedance is determined by a large Ohmic resistance in parallel with a capacitance and a diffusion contribution, the so-called War- burg impedance. The observed behaviour is in line with the model proposed in Section 3.1. Treat- ment with OJH,O at higher temperatures in- creases the amount of HzO, OH-, HOz- and probably of HOz’ being adsorbed at the surface of LaF,. The net reactions may be formulated as a coproportionation of HZ0 and 0, according to

HZ0 + $0, $ H202

and in parallel the formation of La(OH), accord- ing to

LaF, + 3H20 + La(OH), + 3HF

The electrode reaction proceeds within a thin

PtlLaF,lPt in HzO- sat. air

Fig. 6. Complex impedance plot with Pt electrodes on a LaF, (5 mol% SrF,) single crystal after pretreatment in air with p(H,O) =20 mbar at 370K for (a) 1 h and (b) 2 h. Cell: PtjLaF,IF? in H,O-saturated air at 300 K.

193

layer at the LaFJPt interface. If concentrations of reactants or products of the electrode reaction (3) are increased, we therefore expect the experimen- tally observed decrease in electrode impedance.

3.4. Reference Electrodes The quality of a potentiometric sensor is deter-

mined by both the reference and the working electrodes. The reference electrode must have a stable electrode potential. This is determined by an electrochemical equilibrium of the ion species crossing the phase boundaries between electrode and electrolyte. If these ions can be exchanged easily between these phases, the electrode poten- tial is stabilized effectively against any distur- bances. Easy exchange of ions is equivalent to the statement that the corresponding charge-transfer resistance at this interface is low. Therefore a good reference electrode must have a low enough charge-transfer resistance. In addition the bulk components of the electrode should have a suffi- ciently high conductivity. We used impedance measurements to study three different types of reference electrodes for galvanic cells with LaF,.

Figure 7 shows a complex impedance plot for LaF, contacted with two Ag/AgF reference elec- trodes. AgF itself is a good Ag+ ion conductor. As a result the impedance of AgF bulk as well as the charge-transfer impedance at the Ag/AgF interface are negligible. The remaining contribu- tion to the impedance results from the charge transfer of F- ions at the AgF/LaF, interface as indicated in Fig. 7. At frequencies below 1 Hz, a Warburg impedance appears due to diffusion effects at the interfaces. At low temperatures Ag/ AgF is a good reference electrode for LaF, cells. Its properties are comparable to those of liquid reference electrodes, which are of widespread use in fluoride-ion sensitive electrodes based upon

Fig. 7. Complex impedance plot with two solid Ag(AgF refer- ence electrodes on a LaF, (5 mol% SrF,) single crystal at 300 K. Cell: AglAgF]LaF,(AgFIAg.

AgIAgCI( Cl-,F- ILaFs IF-$- IAgCIlAg

, 1 2

Z(real). 10’0hm -

Fig. 8. Complex impedance plot with two liquid reference electrodes on a LaF, (5 mol% SrF,) single crystal at 300 K. Cell: AglAgClICl-, F-ILaF,]Cl-, F-IAgClJAg [Cl-, F- both 10 mol l-l]. (Two additional Ag/AgCl electrodes were used to measure the potential ditTerence in a four-point arrangement.)

LaF3 single crystals. Figure 8 shows a complex impedance plot of a liquid reference. The values for the charge-transfer impedance coincide with results in the literature [20].

Figure 9 shows a complex impedance diagram of another solid reference electrode with Sn, SnF, mixtures. The impedance of this system is much higher than in the two previous examples. Figure 9 shows values for 430 K. At room temperature, the impedance of our electrodes was around IO8 Ohm. This is three orders of magnitude higher than for Ag/AgF or the liquid reference electrodes. Nevertheless, Sn, SnF, mixtures may be used as references although their impedance value reaches the higher limit for a practically useful reference electrode. In spite of this dis- advantage, the advantage of this system is its higher stability as compared to Ag/AgF. Both

Z(d) .10”0hm

Fig. 9. Complex impedance plot with two solid Sn, SnF, reference electrodes on a J_aF, (5 mol% SrF,) single crystal at 430 K. Cell: Sn, SnF&F, ]SnF, , Sn.

194

solid references have the common advantage that they make it possible to build simple all-solid-state arrangements of the sensors.

4. Conclusions

In line with previous measurements of other authors, our results show that oxygen sensors with solid electrolytes make it possible to utilize inter- esting new sensor principles operating at low tem- peratures. These principles are very similar to those of semiconductor gas sensors, because in both cases the surface chemistry of the sensor and reactions between absorbed species play a central role. Possibilities of improvement and further de- velopment concern the systematic modification of solid electrolyte surfaces, for instance by ion im- plantation or by the choice of new electrode mate- rials. Materials of interest in this respect are, e.g., thin films of metal phthalocyanines because they make it possible to stabilize reduced oxygen in the form O,-. Stabilization of surface activities of reduced oxygen species is the most important aim of this development.

Acknowledgement

This work was supported by the Bundesminis- ter fur Forschung und Technologie (F.R.G.) un- der BMFT 13A50013.

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