Fe-doped sodium aluminosilicate thin films: conductivity, microstructural organization and sensor properties

SOUD STATE EI~E~/IER

Sofia State Iomcs 74 (1994) 165-178

lUgS

Fe-doped sodium aluminosilicate thin films: conductivity, microstructural organization and sensor properties E. Bychkov, M. Bruns, U. Geckle, W. Hoffmann,

R. Schlesinger, H.J. Ache Kernforschungszentrum Karlsruhe GmbH, Institut fiir Radiochemie, D- 76021 Karlsruhe, Germany Received 5 May 1994; accepted for publication 8 September 1994

Almttact

In order to get an interracial layer providing reversible and fast ionic and electronic exchange between metallic comact and Na + ion-conducting sensor membrane, thin fdms of Fe-doped sodium aluminosilicate glass prepared by RF co-sputtering of the host glass and metallic iron have been investigated. It was found that non-reactive (At + ) and reactive (At + /02+ ) sputtered layers exhibit drastically different transport and sensor properties in accordance with STFeconversion electron M~ssbauer spectroscopic study of the local environment of iron in the films obtained. The main part of iron in the non-reactive sputtered material forms small Fe particles or clusters of 2 to 4 nm in diameter. These particles dispersed in the insulating glassy matrix cause an enormous increase of the conductivity by 9 to 10 orders of magnitude with increasing Fe content. On the other hand, room-temperature conductivity of reactive sputtered films is by a factor of 105 to 107 less than that of non-reactive sputtered samples. Both as-prepared and annealed non-reactive sputtered layers with an iron content from 3 to 12 at.% exhibit fast and reproducible redox response comparable with that of a Pt electrode. At smaller Fe concentration, redox response is hindered by low electronic conductivity. At higher iron content, oxidative and corrosion-induced phenomena affecting redox response were observed. As-prepared films reveal no Na + sensitivity even after conditioning in NaC1 solutions for at least two weeks. Annealed non-reactive sputtered layers with 3-4 at.% Fe exhibit fast and reproducible sodium ion response but only in concentrated NaCI solutions and with strongly reduced slope (20-30 mV/pNa). Small concentrations of iron do not disturb sodium ion-exchange between solution and thin film. 22Na tracer measurements of sodium uptake and loss for the obtained samples are in accordance with the sensor properties observed. It can be concluded that properly prepared and annealed films with comparable ionic and electronic conductivity, and ionic and electronic exchange current density at the interfaces are promising materials for application as an intermediate layer of an all-solid-statepotentiometric sodium sensor.

Keyword~. Conductivity; Micmstructure; Sensor; Thin films

1. Introduction Charge transfer between ion conducting sensitive m e m b r a n e s a n d electron conducting substrates or contacts o f ( m i c r o ) s e n s o r devices is often impossible because o f the missing electron conductivity o f (mostly insulating) m e m b r a n e materials. This problem can be o v e r c o m e e.g. b y means o f thin m i x e d interlayers showing both ionic a n d electronic conduc-

tivity c o m p a r a b l e to those o f the respective insulator a n d c o n d u c t o r layers o f the sensor built up. Ionic a n d electronic transport a n d exchange properties o f the mixed layer should be high enough to provide fast and reproducible response in solutions containing the ions to be exchanged a n d a p p r o p r i a t e redox couples, respectively. In this case they are suitable test systems for both reversible solid-state interfaces ( m e t a l / m i x e d layer a n d m e m b r a n e / m i x e d layer) o f an "all-

0167-2738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0167-2738(94)00187-1

166

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

solid-state" ion sensitive device. In addition, such mixed layers often exhibit distinct redox sensitivity making them suitable for redox electrodes. Commercially available sodium sensitive electrodes based on Na20-A1203-SiO2 (NAS) glasses belong to the classical ion sensors with internal reference solution and reference electrode. NAS glasses exhibit low ionic conductivity at room temperature (10-~4-10 -s S/cm, depending on sodium oxide concentration and [AI]/[Na] ratio) and are electronic insulators [ 1,2]. This means that potentiometric chemical microsensors with NAS thin film membranes should contain mixed interlayers between sensitive thin f'dm membrane and metallic contact. In order to obtain such mixed layers with comparable ionic and electronic conductivity and ionic and electronic exchange current density at both the metal/mixed layer and the undoped NAS glass/ mixed layer interfaces, co-sputtering techniques for NAS glass and metallic iron were applied. Composition, chemical states and stoichiometry as well as electric and sensor properties of the produced layers were investigated and controlled by means of various experimental methods.

2. Experimental 2. I. Film deposition

The film depositions were performed in a RF sputtering setup using 3" US II Low Profile Planar Magnetron (AP&T, Niirtingen, Germany) mounted on a standard DN 150 CF double cross recipient equipped with pre-sputtering shutter; sample positioner with ztravel allowing various distances ( 135-200 mm) between target and substrate; gas inlet system with thermovalve control providing constant operating pressure of 1 × 10-3 mbar of Ar 6.0 for non-reactive sputtering and of Ar 6.0/02 4.8 (80/20 vol.%) for reactive sputtering, respectively; deposition control oscillator (XTC/2, Leybold, Hanau, Germany) and viewport door allowing quick substrate exchanges. A base pressure of 1 × 10 -7 mbar was achieved using turbomolecular pumping unit TSU 180 H (Balzers, Liechtenstein). In order to prevent thermal damage of the target only 100 W of RF sputter power (power supply and matchwork: ENI Germany, Gerlingen)

was used yielding deposition rates of about 3-5 nm/ min leading to layer thicknesses between 30-200 nm. Sputter targets 75 mm in diameter and 5 mm thick were prepared by hot pressing NAS powder (Na20 11 tool.%, A1203 18 mol.%, SiP2 71 mol.%) produced by sol-gel techniques. A special target geometry for co-sputtering of NAS glass and metallic iron was developed in order to get glass layers with variable Fe doping. Substrates for RF co-sputtering of sodium aluminosilicate glass and metallic iron (SilSiO2(30 nm) ISi3N4(70 nm): 25×10X0.5 mm) were thoroughly cleaned with acetone and deionized water in an ultrasonic bath to remove organic impurities, etched in 1% HF, and then rinsed with deionized water and centrifuged at 3500 rpm. 2.2. Conductivity measurements

Conductivity measurements were carried out in planar configuration in the temperature range from 290 K to 370 K. A calibrated thermocouple was used to determine the cell temperature. Data for the impedance modulus Z and the phase angle 0 in the frequency range 100 Hz to 500 kHz were obtained for samples having resistance R < 3 Mf~. Measurements of the dc conductivity were performed for samples with higher resistance. We used non-normalized values of the conductivity for samples with the same thickness and the same distance (1 mm) between the evaporated metallic electrodes because relative changes in the conductivity dependent on iron content were important. Further systematic fourpoint measurements are planned to determine the correct cell constant. 2.3. Conversion electron M6ssbauer spectroscopy measurements

Conversion electron M6ssbauer spectroscopy (CEMS) measurements were performed at room temperature. The source was 100 mCi57Co in a rhodium host. A gas-flow (90% He-10% CH4) proportional counter was used to detect the emitted 7.3 keV conversion electrons. A constant acceleration triangular drive was used, and the data were folded to provide a constant background. The velocity scale and

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

all the data are referred to a metallic iron absorber at room temperature. The spectra were fitted with a computer least-squares procedure using both the Lorentzian line shape approximation of the Mrssbauer lines and a modified Hesse-Rfibartsch procedure [ 3 ] taking into account a distribution of the quadrupole splitting. Other experimental details and the fitting procedure were published earlier [4 ].

167

total concentration of the redox couple was 0.05 M, the constant ionic strength being I = 1.0 M. A platinum electrode was used for comparison in the redox measurements. The EMF values of the electrochemical cell ( I ) with a chemical microsensor were taken using a Metrohm 654 digital multimeter with an input impedance of 1012£1. 2.6. 22Na tracer measurements

2.4. X P S measurements

Surface analytical experiments were performed in an ESCALAB-5 electron spectrometer (VG-Scientific, East Grinstead, UK) with a base pressure of ca. 10-1o mbar. For XPS measurements the photoelectrons were excited by means of Mg Ka-radiation at a power of 200 W. The kinetic energies were measured by a 150* hemispherical energy analyzer operated in the constant analyzer energy mode (CAE). The binding energy scale was calibrated using a value of 285.0 eV for the C Is photopeak. Na 2s spectra were always taken at the beginning of an experiment in order to minimize X-ray induced sodium enrichment at the surface. For multiple peak analysis the VGS2000 software permits simultaneous fitting often gaussian components with variable Lorentzian line shape contribution and variable asymmetry.

To study 22Na uptake, the samples were dipped into a labelled sodium chloride solution for 30 s to 105 s, then blotted dry with filter paper. The residual activity of the sample was measured using a scintillation counter. The obtained value of the counting rate A was normalized to the sample area S: As = A / S ,

(2)

where As is the reduced counting rate of the sensor membrane. Samples with maximum reduced counting rate As(max), which were exposed to a labelled solution for a maximum time of ~ 105 s, were used for the 22Na loss study. They were dipped into nonradioactive NaCl solution for a certain time, blotted dry with filter paper, whereafter the reduced counting rate of the membrane was determined. Both the sorption and desorption experiments were carried out in decimolar sodium chloride solution at pH 7.

2.5. Electrochemical measurements

In order to provide a good electrical contact to the sensitive layer, part of the sputtered sample was covered with gold. The sample was placed in a specially designed electrochemical cell Ag,AgCI IKCI (3.0 M) Iltest solution [Fe-doped layer [Au,

(1) where the uncovered (sensitive) part of the sample was in contact with the test solution. A spring-loaded metallic contact was applied to the Au-covered part of the sensor membrane. A silver/silver chloride reference electrode was dipped in a test solution cavity of the cell. Sodium chloride test solutions were prepared by successive ten-fold dilutions of a 1 M NaC1 stock solution with a supporting electrolyte Mg(NO3)2 providing a constant ionic strength of I = 1.0 M. Redox measurements were performed in potassium hexacyanoferrate(II) / ( I I I ) solutions. The

3. Transport characteristics and local environment of iron 3.1. Conductivity

Co-sputtering of iron and NAS glass changes the electronic properties of the sodium aluminosilicate glass in a predictable way. For non-reactive sputtered samples, one can observe an enormous increase of conductivity with increasing Fe content by 9 to 10 orders of magnitude (Table 1 ) and drastic changes in color from light green to deep gray, probably due to intra- a n d / o r interionic optical transitions including the Fe 3d levels. Typical complex impedance plots for the as-prepared and annealed non-reactive sputtered layers are shown in Fig. 1. They consist of a distorted arc corresponding to the resistance of the layer and its geometric capacity. The resistances of the

168

E. Bychkov et aL / Solid State lonics 74 (1994) 165-178

Table 1 Room-temperatureconductivityof non-reactiveand reactivesputteredNAS-Fe films Iron concentration, (at. %)

Conductivityof non-reactive sputtered films (S)

Conductivityof reactive sputtered fdms (S)

0.0 2.6 3.8 11.8

< 10-n3 2.5(6)X10 -6 4.1(ll)x10 -s 3.6(1)X10 -4

<10-n3 8(2)X10 -13 ~10 -12 3.3( 1 0 ) x l 0 - l °

• Iron concentrationwas determinedfrom XPS data.

30

for the two types o f NAS-Fe films (Figs. 2 and 3) obeys the usual Arrhenius-like equation

(a)

17.4C .C

0

a=tro e x p ( - E , / k T )

20

0t,,=

~ N

10 05

!

0

'

0

'

'

10

'

'

~

'

0.~

50

20

30 40 IZleoso (kOhm)

1.5

(b)

A

E t1.0

O

® .c_ 0.5 ¢tJ N m i

Coil:

,:"

ii tl

0.0 0.0

0.5

II

~.9

IN

"C.

1.0

o~

2°, ;,,: 1.5

2.0

2.5

IZleose (MOhm)

Fig. 1. Typicalcompleximpedanceplots of (a) as-preparedand (b) annealednon-reactivesputteredNAS-Fefilmswith 3.8 at.% Fe. Numbers near some experimental points in Fig. la correspond to frequenciesin kHz.Numbersin Fig. lb denotethe sample temperature. samples decrease with increasing temperature. No evidence was obtained for electrode polarization in the available low-frequency range. It was rather surprising that samples obtained by non-reactive and reactive sputtering differ significantly in their electrical characteristics. Room-temperature conductivity of the non-reactive sputtered layers is higher by a factor of 105 to 107 than that of the reactive sputtered films (Table 1 ). The temperature dependence of the conductivity a

,

(3)

where Ea is the activation energy, tro and kThave the usual meaning. It should be noted that the activation energy for the non-reactive sputtered films is very low (0.025 eV). This value of E, observed at rather high temperatures (290-360 K) is too low for small-polaron hopping [5 ]. Annealing of the nora-reactive sputtered layers reduces the conductivity by 1.5 orders of magnitude; the activation energy does not change markedly (Fig. 2). Differences in the conductivities between non-reactive and reactive sputtered films are reflected also in their activation energies. As-prepared reactive sputtered samples show a much higher activation energy of conductivity (E~=0.54 eV). In addition, annealed reactive sputtered layers exhibit higher conductivities than as-prepared samples. The room-temperature conductivity differs by a factor of 103, and the value of E~ is less by 0.22 eV (Fig. 3). To explain the different transport properties of nonreactive and reactive sputtered films, it is important to study the local environment and charge state of iron in the bulk of the films. 57Fe conversion electron M6ssbauer spectroscopy can be usefully applied for this purpose because it probes only the surface layer of solids ( ~ 200 nm thick), a thickness comparable to that of the films. 3.2. Conversion electron MiJssbauer spectroscopy (CEMS) A typical spectrum of a non-reactive sputtered sample with 3.8 at.% Fe is given in Fig. 4a. Three components (A, B and C) were found in the spec-

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

-4.0

169

_7 ¸ Non-reactive sputtered NAS-Fe films

Reactive sputtered NAS-Fe films

0.025(5) eV

-4.5 ---

-'Q--

-8

__

(a) A g - ~ A

-5.0

"~" -9 ._>

"o -5.5 c

~c -10

.2

13

13 0

for 30 mln

8 "6

._~ -6.0 (b)~

500 C for 30 mln

-12

-6.5

-7.0 2.6

2.8

3.0

3.2

3.4

3.6

1 0 0 0 / T (K ~ )

-13 2.5

2.7

2.9

3.1

3.3

3.5

1 0 0 0 / T (K-l)

Fig. 2. Temperaturedependenceof the conductivityof non-reactive sputtered NAS-Fe films with 3.8 at.% Fe: (a) before annealing; (b) after annealingat 5000Cfor 30 rain in air.

Fig. 3. Temperaturedependenceof the conductivityof reactive sputtered NAS-Fe films with 3.8 at.% Fe: (a) before annealing; (b) after annealingat 500°Cfor 30 min in air.

trum. The solid lines represent the results of a leastsquares fit of the experimental data points. The hyperfine interaction parameters (isomer shift S, quadrupole splitting QS, hyperfine magnetic field H, line width W) and the fractional area of the iron sites are summarized in Table 2. A broadened line A with small quadrupole splitting is the main component (72%) of the spectrum. The isomer shift of this line, S = - 0 . 0 2 ( 1 ) mm/s, is very close to that of metallic iron. Similar iron sites were found in CEMS studies of Fe-implanted SiO2 [6], MgO [7,8] and ZrO2-Y203 [9] layers, and in Ar- and Kr-irradiated Fe/SiO2 structures [ 10,11 ]. They were attributod to small iron particles or clusters. The mean size of the particles was found by TEM observations [6] to be 2 to 4 nm. The absence ofhyperfine magnetic splitting in our spectra, which is characteristic for metallic iron at room temperature (Fig. 4b) and occurs for iron particles larger than about 8 nm, leads to an estimate of the mean size of the iron precipitates in the as-prepared non-reactive sputtered NAS-Fe films to be of the same order of magnitude ( < 2-4 nm). Broad (0.6-0.7 ram/s) non-

Lorentzian lines of the main component are indicative of a distribution of the hyperfine interaction parameters, in particular the quadrupole splitting, typical for disordered materials. A distribution function of the quadrupole splitting, P ( Q S ) , obtained by the modified method of discrete step functions by Hesse and Riibartsch [ 3], is given in Fig. 5. It should be noted that the derived distribution function cannot be described by a single theoretical distribution because of rather high contributions to P ( Q S ) at low values of the quadrupole splitting (0.0-0.2 mm/s). There are different approaches to the theoretical description of P(QS). Henry et al. [ 12 ] used the form P ( Q S ) oc (A/J 2) exp( - d 2 / 2 d 2 ) ,

(4)

where A is the nuclear quadrupole splitting and J is the width of the distribution. Subsequently Czjzek [ 13] has shown that Eq. (4) is a special case of the distribution function (5) derived for a random distribution of charges P ( Q S ) oc ( d d - l / J d) exp( - A 2 / 2 j 2) ,

(5)

where d is the number of independent continuous

170

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

110 .

(a)

108 106 104 B

102 v

c o

100

~

~

zC

7

.

"

. w

98 E ILl

._,=

n-

-8

-4

0

4

8

400

(b) 300

200

100

-8

-4

0

4

8

Relative Velocity (mm/s) Fig. 4. Conversion electron Mtlssbauer spectra of (a) as-prepared non-reactive sputtered NAS-Fe film with 3.8 at.% Fe, and (b) metallic iron.

random variables, for example d= 5 for a homogeneous random distribution of charges. Eibschiitz et al. [ 14] fitted their spectra with an asymmetric Gaussian distribution ~p~ocexp{- [(.4-D)/aD]2}, 0 < A < D , P(QS)= [p20cexp{- [ (A-D) /bDI2}, D<,4
Eq. (6) (Fig. 6). In this case the goodness-of-fit parameter Z2 is by a factor of 2 better than that obtained with Eq. (5). The partial P(QS) function at low QS seems to correspond to larger Fe particles which are less distorted by their environment. Approximately one quarter of the iron atoms form larger particles. The second distribution at QS~ 0.5 m m / s probably characterizes smaller Fe clusters with higher distortion of the iron local symmetry. An average value of the quadrupole splitting QS~ 0.7 m m / s was found e.g. in oligomers of iron Fe,, where n = 2, 3 and 4 [151. The population of sites B (17%) and C (11%) is much smaller. The isomer shifts of both components (Table 2) are indicative of Fe 3+ and Fe 2+ high-spin tetrahedral complexes, respectively. Site B with S= 0.30 mm/s and QS= 0.90 mm/s is similar to those found in as-implanted and annealed ZrO2-Y203: Fe + [91, annealed SiO2:Fe + [61 and MgO:Fe + [71. Perez et al. [6,7] and Burggraaf et al. [9] have ascribed these sites to small precipitates of Fe203 ( < 25 nm). Site C with S= 0.83 mm/s and QS= 0.62 ram/ s was observed earlier in as-irradiated and annealed Fe/SiO2 ion-mixed samples [ 10,11 ]. This site was attributed to defect-associated Fe 2+ [ 16 ]. Annealing of the non-reactive sputtered sample in air at 500°C for 30 min leads to complete oxidation of the metallic iron particles and formation of small aggregates of ct-Fe203 (site D). A reduced hypertine magnetic field H = 4 8 4 ( 6 ) kOe (Table 2) instead of 518 kOe, characteristic for bulk iron(III) oxide [ 17 ], and broad lines ( W ~ 1.2 mm/s) of the observed Fe2Oa sextet seem to correspond to the iron (III) oxide particles of mean size more than ~ 14 nm. This estimation was carried out according to Kiindig et al. [ 18 ], who studied the superparamagnetic behaviour of small supported ¢t-Fe203 particles. The annealing also converts the defect-associated Fe 2+ complexes into the Fe 3+ form (site E). Similar oxidation phenomena were observed earlier for Fe-implanted oxides [ 6,7 ]. It was found that chemical and electronic states of iron are completely different in non-reactive and reactive sputtered films. As expected, the reactive sputtered NAS-Fe films contain iron mainly in the Fe 3+ high-spin state (site E'). The isomer shift of the broadened single line observed (S~ 0.2 m m / s ) gives evidence for such a conclusion (Table 2). Unfortu-

E. BFchkov et aL / Solid State lonics 74 (1994) 165-178

171

Table 2 Summary of isomer shifts S relative to u-Fe, quadrupole splittings QS, hyperfine magnetic fields H, linewidths Wand fractional areas of the iron sites in conversion electron M6ssbauer spectra of non-reactive and reactive sputtered NAS-Fe films. Uncertainties in the last digit(s) of the parameters are given in parentheses Site identification

S (ram/s)

QS (mm/s)

H (kOe)

W (mm/s)

Area (%)

as-prepared non-reactive sputtered NAS--Fe film A (Fe °) -0.02(1) B (Fe 3+) 0.30(3) C (Fe2* ) 0.83(3)

" 0.90(5) 0.62(6)

-

0.66(5) b 0.39(9) 0.32(10)

72(5) 17(4) 11 (3)

annealed non-reactive sputtered NAS--Fe £flm D (a-Fc203) 0.36(6) E (Fe 3+) 0.30(7)

0.12( 11 ) -

484(6) -

1.2(5) 2.0(7)

75(25) 25(10)

as-prepared reactive sputtered NAS--Fefdm E' (Fe 3+ ) 0.21 (7)

-

-

0.8(2)

100

• The distribution function of quadrupole splitting P(QS) is given in Fig. 5. b Lorentzian line shape approximation.

0.10 [

0.10~ Non-Reactive Sputtered NAS-Fe Thin Rims

.~ o.o8

0.06 m

i

0.04 0.02 0.00

0

0.3 0.6 0.9 1.2 QUADRUPOLE SPLn-FING(mnYs) Fig. 5. Distributionfunctionof quadrupolesplittingP(QS) for small iron particlesdispersedin the disorderednetworkof aspreparednon-reactivesputteredNAS-Fefilm. nately, the insulating properties of these films make it difficult to obtain spectra with good statistics due to surface charging. We arc, therefore, unable to discuss further details of the iron local structure in this case.

In general, CEMS results indicate that the microstructural organization in the non-reactive sputtered NAS--Fc films differs significantly from that of Fccontaining alkali aluminosilicatc glasses. For example, one observes neither before nor after thermal treatment in air the characteristic feature of ther-

~,

Non-Reactive Sputtered NAS-Fe Thin Rims

0.08

0.06 ~ 0.04 0.02~/~ 0.00

0

=' 0.3 0.6 0.9 1.2 QUADRUPOLE SPLI'I-I'ING(ram/s)

Fig. 6. Theoreticalevaluationof the derived distributionfunction P(QS) usingasymmetricGaussiandistributions (Eq. (6)) [14].

mally doped glasses with Fe 2+ low-symmetric sites with QS= 1.4-2.3 mm/s, which are dominant in glasses at low iron content and when synthesis was in reducing atmosphere [ 19 ]. On the other hand, the films behave rather similar to Fe-implanted and ionmixed oxide materials in their multiple iron sites and phase transformations during annealing. It should be noted, however, that such a large fraction of the small metallic iron particles and/or oligomeric iron clusters has never been observed in the implanted and ion-mixed samples even at the highest doses of about 10 ~7 ions/cm 2. Usually, an aggregation of small iron

172

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

precipitates of the metallic phase a-Fe was observed at very high doses [ 6,9 ].

o2(2 X2-1)+0,(2 Xt-l) am=

z-2

3.3. Microstructuralorganization and percolation phenomena +

The derived models of the microstructural organization in the NAS-Fe films allow us to explain (i) different transport properties in the non-reactive and reactive sputtered layers, and (ii) the enormous steplike increase of conductivity of the non-reactive sputtered films (Table 1 ). Reactively sputtered NAS--Fe layers contain iron mainly in the Fe 3+ state. The fraction of Fe 2+ is very low, and we, therefore, cannot expect a high electron hopping rate between donor-like (Fe 2+ ) and acceptor-like (Fe 3÷ ) centers. Consequently, the respective small-polaron hopping conductivity o, which has its maximum value at comparable concentrations of transition metal ion in different charge states, is expected to be small as well. Such behavior is described as follows [ 5 ] a = a [ F e 2+ ] [Fe 3+ ] e x p { -

½We+ Wh'~ -j,

z-2

(8) where el and o2 are the conductivities of the host and conducting phases, respectively; X~ and )(2 are the volume fractions of these phases, and z is the number of connecting bonds per site. Eq. (8) can be written in a more condensed form

(z- 2 )om=K2 -KIX2 +x/(Kz-KtX2)2+Z(z-2)O1o2,

(9)

where z

K~ = ~ ( e l - o 2 )

(10)

and

K2=(2-1)Ol-O 2 .

(11)

(7)

where a is a constant including in particular the frequencies and distances of jumps; Wd denotes the energy difference between filled (Fe 2+) and vacant (Fe a+ ) lattice sites available for small-polaron hopping, and is related to the disorder in the glass network; Wh= 1/2 Wp( Wp is the polaron binding energy). The observed increase of conductivity after annealing (Fig. 3) is probably caused by an increased iron concentration at the surface. XPS measurements show that after thermal treatment the uppermost layer of reactive sputtered samples contains twice as much iron(III) oxide. Large numbers of small metallic iron precipitates in the non-reactive sputtered films should increase conductivity due to the formation of percolation clusters. Outside the critical region, where the percolation threshold occurs, a simple effective medium percolation theory was found to be successful in describing transport properties [ 20 ]. In the effective medium approximation, the conductivity of a mixed phase system am could be written as

Ast [ 21 ] reported that the effective medium percolation theory can be used to calculate the composition dependence of the de resistivity of amorphous semiconducting AsxTe 1- x films. A comparison of the theoretical predictions and the experimental results shows excellent agreement over the entire compositional range. Later on, Sayer et al. [22,23 ] reported that the above theory describes the ionic conductivity variations in two phase systems like AgI/Ag4P207 and ~/~"-alumina surprisingly well. This model was applied to the conductivity isotherm of the non-reactive sputtered NAS-Fe films. Initial attempts to fit the experimental data using Eq. (8) showed that the specific conductivity value for pure metallic iron ( 1.0 × 105 S/cm [ 24 ] ) was too high to get useful results. Preliminary four-point measurements showed that the resulting specific conductivity of a sample with maximum iron concentration is less by at least two orders of magnitude. Therefore, an iteration procedure was used to find the best values for o2 and z. The final calculated isotherm is shown in Fig. 7 together with the experimental data points. The

173

E. Bychkov et aL ~Solid State lonics 74 (1994) 165-178

Non-reimtivesputtered NAS-Fe thin films

4. Sensor properties

-2

29eK

4.1. R e d o x sensitivity I

_4 ~

>_ C3

As-prepared and annealed non-reactive sputtered NAS-Fe films with medium iron contem ( 3-12 at.%) exhibit fast and reproducible redox response in potassium hexacyanofcrrate (II) / (III) solutions

-6

R7"

-8

E = E ° o ~ + :-~-ln

Z

8 9

aox

(12)

ared '

-10

-12-

o~,= 3.8xl u S

/

z=73 -14 0

0.03

0.06

0.09

0.12

0.15

Fe F R A C T I O N Fig. 7. Room-temperature conductivity isotherm of non-reactive sputtered NAS--Fe t'flms. The solid line represents the best fit of the experimental data points using the effective medium percolation theory [20]. The derived parameters of Eq. (8) are: o2=3.SX lO-3 S and z= 73.

best value of o2-- 3.8 × 10- 3 S seems to be reasonable because one expects that the conductivity of bulk metallic iron should be much higher than that of the small iron precipit,3tes embedded in an oxide environment. The small activation energy (0.025 eV) in contrast to the negative temperature coefficient in case of metallic conductivity also reflects some kind of interaction and/or oxidation of iron at the surface of the small metallic particles, leading to a reduced and thermally activated conductivity. The rather high number of connecting bonds per site ( z = 73) indicates a fractal topology of the infinite percolation cluster with probably multiple intersections of the conducting channels analogous to the self-similar texture model [25 ]. The estimated average size of the iron particles ( < 2-4 nm), which are sites in the infinite percolation cluster, is in good agreement with the derived value of z. The conductivity decrease after annealing the non-reactive sputtered f'dms (Fig. 2) is caused by oxidation of the iron particles and formation of small aggregates of (l-Fe203.

where E ° o x is the standard redox potential at the sensor; a,~ and ao~ are the activities of the [Fe(CN)~] 4- and [Fe(CN)6] 3- species, respectively; R, T and F have the usual meaning. The observed values of E ° o x are very close to the standard redox potential at a platinum electrode (Fig. 8). The slope of the redox response is 57-58 mV/decade. The response time of the sensors is less than one minute (it should be noted, however, that the real response time in a flow-injection system should be much faster due to better hydrodynamic conditions as compared to our measuring cell). The potential reproducibility for repeated measurements in an equimolar redox solution ( a ~ = a o ~ ) is 0.4-0.7 mY. At smaller Fe concentrations the redox response is hindered by low 500 K3[Fe(CN)e] / K 4 [Fe(CN)s] Tot~ Redo~ ~

/

400

O

~ E UJ

0.06 M

300

/

(3) Pt = ~

NAS-r-e

-

200

100

(2) NAS-Fe o -2

-1 0 1 log [Fe(lll)/Fe(ll)]

2

Fig. 8. Redox response of (1) non-reactiveand (2) reactive sputtered NAS--Fesensors, and (3) a Pt electrode.

E. Bychkov et al. / Solid State lonics 74 (I 994) 165-178

174

electronic conductivity. At higher iron content one can observe a rather strong oxidative and corrosioninduced behavior of the sensor. Some indications of such behavior can also be seen for sensors with medium iron content. Soaking in the most concentrated oxidative solution (aox/a~= 100) for a few hours results in an increase of the sensor potential at the most concentrated reduetive solution (aox/a~= 0.01 ) (Fig. 9). The original response can be restored after prolonged treatment in reductive media. Reactive sputtered samples do not exhibit redox sensitivity. Their potentials are 80-300 mV lower than that of a platinum electrode and almost independent of aoJa~d (Fig. 8). This insensitivity to redox species is probably caused by the electronic state of iron in the reactive sputtered films. The predominant Fe 3+ fraction strongly reduces the possibility of small-polaron hopping in the f'dm and simultaneously reduces the electronic exchange current density at the film/redox solution interface.

4.2. Na + sensitivity As-prepared Fe-doped reactive and non-reactive sputtered sodium aluminosilicate films fail to show

any Na + ion sensitivity even after conditioning in NaCI solution for at least two weeks (Fig. 10). This result is in good agreement with those for undoped NAS thin films reported by Becht et al. [26 ]. These authors found that thermal annealing at 500°C for 30 min promotes near-Nernstian response in the concentration range from 10 -3 M to 1 M NaC1. The same procedure was, therefore, applied to Fe-doped layers. Annealed non-reactive sputtered t'dms with 3.8 at.% Fe exhibit fast and reproducible sodium ion response after pre-conditioning in 1 M NaC1 for a few hours (Fig. 11 ). The Na + sensitivity is not very high; the low detection limit is ~ 10 -2 M, and the slope of the sensor response in concentrated NaC1 solutions is only 20 to 30 mV/pNa. However, the films reveal rather high long-term stability of the standard potential, + 3-5 mV/week (Fig. 12), and remain sensitive for at least one month. Properly annealed reactively sputtered NAS layers with high concentration of iron do not show reproducible Na + response even after three weeks in 1 M NaCI. (We cannot use iron-doped NAS films with [ Fe ] < 12 at.% because of resistivity limitations. ) One can observe some sodium ion sensitivity (8-19 mV/ pNa) above 0.1 M NaCI and only after one to two

200

400

K3[Fo(CN)6I / K 4 [Fe(CN) 6]

As-prepared NAS-Fe Thin Films

,

160

350

Soaldng Time In I M NaCl:

120

300

21 h

{ 80

E~' 250 v

UJ

1.1..I

40

20O ,t"2

NAS-Fo N o n ~

1

150 -2

113h

~

-1 tog

0

0

1

2

[Fe(lll)/Fe(ll)]

Fig. 9. Redox response of a non-reactive sputtered NAS--Fe sensot after soaking in oxidative solution (aoJa,~ = 100) for different time periods: ( 1) untreated, (2) one hour, (3) three hours.

i

4

3

2 pNa

,

i

1

0

Fig. 10. Typical response of as-prepared non-reactive sputtered NAS-Fe layers in NaCI/0.33 M Mg(NO3)2 solutions. The numbers near the calibration curves denote the soaking time in 1 M sodium chloride solution.

E. Bychkov et aL / Solid State lonics 74 (1994) 165-178 170 Annealed NAS-Fe Thin Films 160 O

~1

50

so~ano11me I n 1 M NaCI:

~

140

lh

3h

-f"

ILl

13O

120

4

NaCI / Mg(NOs)2

3

2

1

0

pNa

Fig. 11. Sodium ion responseof annealed non-reactive sputtered

NAS-Fe layersin NaC1/0.33 M Mg(NO3), solutions.The numbers near the calibrationcurvesdenote the soakingtime in 1 M sodium chloridesolution.

g

200

I~

100 •

@

175

tions from 2.6 to 3.8 at.% are very similar. As-prepared films reveal some increase of the specific activity with increasing exchange time t and saturation in the sodium uptake for t > 103 s (Fig. 13). Almost complete 22Na loss occurs during the contact of the labelled sample with non-radioactive NaC1 solution for a few seconds, and the reduced counting rate As at t> 30 s dropped to the background level. There is a significant difference in the exchange rate and amount of sodium between as-prepared (Fig. 13) and annealed (Fig. 14) films. First, the values of As for the sodium uptake are ~ 50% higher in the case of annealed samples. Secondly, one can observe two steps in the sodium loss dynamics for the annealed films. During the first fast step ( t < 30 s), approximately 70 to 75% of the 22Na content is removed from the annealed sample. Only this step occurs in the case of as-prepared films. The loss dynamics of the remaining part of 22Na is much slower. The values of As only decrease by a factor of 2 during the next three hours (Fig. 14). It should be noted also that the Z2Na uptake isotherms for the annealed samples do not show distinct saturation at high values of t. The results obtained for NAS and NAS-Fe thin films differ from those for bulk Na-containing oxide and chalcogenide glasses [27,28]. A diffusion-controlled exchange mechanism is responsible for the power law dependence of the reduced counting rate As versus the exchange time t in the latter cases

o. t~ -100 nD t-

@

~Na Exchange -200

250. 10 100 T r e a t m e n t T i m e (hours)

1000 S. 200

Fig. 12. Long-termstability of the standard potentials of annealed (a) non-reactiveand (b) reactivesputteredNAS-Felayers in NaCI/0.33 M Mg(NO3)2solutions.

~150

weeks pretreatment with 1 M sodium chloride solution. Standard potentials of these films exhibit large drifts and fluctuations ( + 2 0 - 4 0 mV) during repeated calibration measurements (Fig. 12).

100 om U. ¢) Ill 50 D. O~

4.3. 22Na exchange

22Na exchange isotherms for both undoped NAS thin films and NAS-Fe layers with iron concentra-

0.1 M NaCI pH 7

As-preflareclNASfilm Loss

°1o"' .... i; 2

"

.... io 3"

.... i0'

....

i'o

EXCHANGE TIME (Seconds) Fig. 13.2ZNauptake and lossisothermsfor as-preparedNAS thin films.

176

E. Bychkov et aL / Solid State l onics 74 (1994) 165-178

22Na Exchange

250 =- 200

exchange isotherm by subtracting the As values for as-prepared films Ash(t), from those for annealed samples ,4 ~m (t),

0.1 M NaCI pH 7

A

AeX~t~s~ J=Asum~'~-A]d(t)s~,J

~

150

N ~ lOO o ~ 50

Annealed NAS film

li. 0101 . . . . . . i'02 . . . . . . i'03 . . . . . . i'04 . . . .

i'05

EXCHANGE TIME (Seconds) Fig. 14. 22Na uptake and loss isotherms for NAS thin films annealed at 500°C for 30 min.

As=Aot",

(13)

where Ao is a constant, and the exponent n ~ 0.5 [27,28 ]. Both the sodium uptake and the sodium loss obey the power law dependence (13) with similar parameters A0 and n indicating that the 22Na exchange reflects the sodium ion-exchange at the glass/ solution interface [29]. Differences in amount and exchange rate of sodium between as-prepared and annealed thin films, on the one hand, and in 22Na uptake and loss, on the other hand, can be explained assuming two different types of sodium in the film: (a) exchanged sodium, and (b) adsorbed sodium. As-prepared films do not contain sites available for sodium ion-exchange. 22Na exchange isotherms for these samples reflect sodium adsorption and desorption at the film surface which do not influence the interfacial Donnan potential. The absence of the sodium ion-exchange at the as-prepared film/solution interface is strongly correlated with the insensitivity of these films to Na + ions. In contrast, properly annealed films sensitive to sodium ions contain sites available for the Na + ion-exchange in addition to those suitable for the sodium adsorption. It seems reasonable that both processes (ion-exchange and adsorption) take pla~ simultaneously at the annealed film/solution interface. Therefore, the resulting 22Na exchange isotherm is a superposition of both processes. One can estimate the fractional ion-

(14)

,

where t is the given exchange time. The resulting isotherm is shown in Fig. 15. The observed linear behavior on a log-log scale indicates that the fractional ion-exchange isotherm for annealed NAS and NASFe thin films obeys a power law (13). The power law exponent n=0.11-0.12 for the ion-exchanged sodium uptake is very close to that for the sodium loss. It suggests that the partial ion-exchange process takes place at the interface. The low value of n may be caused by (i) a non-linear diffusion, (ii) a non-diffusional mechanism of the sodium ion-exchange; and/or (iii) competitive processes, e.g., surface corrosion, formation of a modified surface layer, etc. Two different types of the ion-exchange dynamics were observed recently for A g I - P b S - A s 2 S 3 glasses studied using a l~°mAgtracer [ 30 ]. A large concentration of iron in the film hinders the sodium ion-exchange. Both as-prepared and annealed NAS-Fe films with 11.8 at.% of iron show the s a m e 22Na uptake and loss isotherms typical for the proposed sodium adsorption. As a result, the derived values of A ~ ' ( t ) assumed to be responsible for the partial ion-exchange process are very close to zero. It 1O0 80 0.1 M NaCI pH 7

~Na Uptake

60 _>

40

_0

LL

,,q

a. ¢/)

Partial 8odiumion-exchange

10

......................................... 101 102 103 104

105

EXCHANGE TIME (Seconds) Fig. 15. ( • ) Partial 22Na uptake, and (O) 22Na loss isotherms for a NAS--Fe thin film with 2.6 at.% Fe annealed at 500"C for 30 min. The As values for partial 22Na uptake were obtained by subtracting the As values for an as-prepared sample (adsorbed sodium) from those for an annealed fdm (adsorbed and ionexchanged sodium).

E. B),chkov et al. I Solid State lonics 74 (1994) 165-178

should be pointed out that the annealed films with large iron concentration do not show reproducible Na + ion response, if any. 4.4. Relationship between sensor characteristics and microstructural organization

The microstructural organization of NAS-Fe films affects both the redox response and the Na + ion sensitivity. As-prepared and annealed non-reactive sputtered samples exhibit high electronic conductivity which is caused by formation of an infinite percolation cluster including either metallic nanoscale Fe particles or ct-Fe203 aggregates. In both cases the resulting high electronic density in the bulk of the film and at the fflm/redox solution interface causes fast interfacial electronic exchange and redox response of the films. Reactively sputtered NAS-Fe layers with predominant Fe 3+ fraction show neither high electronic conductivity nor redox sensitivity. Similar tendencies were observed earlier for bulk oxide glasses thermally doped with iron and other transition metals [31]. The ionic response of NAS-Fe films depends on iron concentration in the sample and the thermal pretreatment. Films with high iron content are probably not very stable in aqueous solutions. Some indications of this kind can be seen in the corrosion-induced behavior of the Fe-rich samples during redox measurements. Therefore, one can expect that the surface destruction of the films due to aqueous attack and the potential-generating sodium ion-exchange via available sites within the modified surface layer occur simultaneously, and that the former process in the case of Fe-rich samples is characterized by a higher dissolution rate. The processes were found to be the same for sodium aluminosilicate glasses with high transition metal oxide content which also show irreproducible Na + ion sensitivity [ 31 ]. The effect of the thermal pre-treatment on the local structure and on the ion-exchange properties of the films, respectively, is not yet clear. XPS measurements show that as-prepared f'flms after annealing have higher atomic density (intensities of the photopeaks measured under the same conditions increase by a factor of 2) and larger sodium concentration in the uppermost layer. It should also be noted that the ratio of bridging to non-bridging oxygen,

177

[Ob]/[Onb], decreases from 6.0 to 5.5 with annealing. However, the chemical shifts of the Si 2p, A1 2p, Na 2s and O Is photopeaks do not change after thermal treatment and are comparable to those characteristic for bulk sodium aluminosilicate glasses [32,33].

5. Conclusions

Thin films of Fe-doped sodium aluminosilicate glass prepared by RF co-sputtering of the host glass and metallic iron have been investigated. Non-reactive (Ar + ) and reactive (Ar+/O~ " ) sputtered layers exhibit drastically different transport and sensor properties. Most part of the iron in non-reactive sputtered materials forms small Fe particles or clusters of 2 to 4 nm in diameter. Reactive sputtered layers contain iron mostly in the Fe 3+ high-spin state. Iron particles dispersed in the insulating glassy matrix of non-reactive sputtered films cause an enormous increase of the conductivity by 9 to 10 orders of magnitude with increasing Fe content. Effective medium percolation theory was successfully applied to describe the observed transport properties. Roomtemperature conductivity of reactive sputtered films is by a factor of 105 to 107 less than that of non-reactive sputtered samples. As-prepared and annealed non-reactive sputtered layers with an iron content from 3 to 12 at.% exhibit fast and reproducible redox response comparable with that of a Pt electrode. Reactive sputtered samples do not show this redox sens'tivity. 22Na tracer measurements of sodium uptake and loss reveal the existence of two types of sodium in the f'flm: (a) exchanged sodium, and (b) adsorbed sodium. As-prepared films do not contain sites available for Na + exchange in good agreement with the sensor properties. Properly annealed non-reactive sputtered sodium aluminosilicate films with iron concentrations from 3 to 4 at.% exhibit fast and reproducible redox and Na + ion response and are suitable materials for use as an intermediate layer for an all-solid-state potentiometric sodium sensor. The development of such a sensor is now in progress.

178

E. Bychkov et al. / Solid State lonics 74 (1994) 165-178

Acknowledgements T h e a u t h o r s w o u l d like to t h a n k Dr. H. KleweN e b e n i u s for m a n y fruitful discussions, Dr. IC R a p tis for p r e p a r a t i o n o f the target, Dr. V. S e m e n o v a n d B. Seleznev for help in d o i n g M 6 s s b a u e r a n d tracer experiments, respectively.

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[ 16] C. Wilkinson, A.IC Cheetham, G.J. Long, P.D. Battle and D.A.O. Hope, Inorg. Chem. 23 (1984) 3136. [ 17 ] L.H. Bowen, M6ssbauer Effect Ref. Data J. 2 (1979) 76. [ 18 ] W. Kiindi& H. B6mmel, G. Constabaris and R.H. Lindquist, Phys. Rev. 142 (1966) 327. [ 19] J.M.D. Coey, J. Phys. (Paris) Colloq. 35 (1974) C6-89. [20] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574. [21 ] D.G. Ast, Phys. Rev. Lett. 33 (1974) 1042. [22 ] M. Sayer, S.L. Segel, J. Noad, J. Corey, T. Boyle, R.D. Heyding and A. Mansingh, J. Solid State Chem. 42 (1982) 191. [23] M.F. Bell, M. Sayer, D.S. Smith and P.S. Nicholson, Solid State Ionics 9/10 (1983) 731. [24] CRC Handbook of Chemistry and Physics, 62nd Ed. (CRC Press, Boca Raton, FL, 1983) p. E-82. [ 25 ] S. Kirkpatrick, in: J.C. Garland and D.B. Tanner, Electrical Transport and Optical Properties oflnhomogeneous Media (American Institute of Physics, New York, 1978) pp. 99117. [26] R. Becht, M. Bruns, W. Hoffmann and H.J. Ache, 44th Meeting Intern. SOc. Electrochem. (Berlin, September 1993). [27] G. Eisenman, in: G. Eisenman, ed., Glass Electrodes for Hydrogen and Other Cations. Principles and Practice (Dekker, New York, 1967) pp. 133-173. [28] Yu. G. Vlasov and E.A. Bychkov, Anal. Lett. 22 (1989) 1125. [29] M. Haissinsky, La Chimie Nucleaire (Masson, Paris, 1957) pp. 574--578. [30] Yu. G. Vlasov and E.A. Bychkov, in: Techn. Digest of the 4th Intern. Meeting on Chemical Sensors, ed. N. Yamazoe (Japan Association of Chemical Sensors, Tokyo, 1992) pp. 264-266. [31 ] A.M. Pisarevskii and A.V. Avdeenko, Fiz. Khim. Stekla 12 (1986) 257. [ 32 ] D. Briggs and M.P. Seah, eds., in: Practical Surface Analysis, Vol. 1, Auger and X-ray Photoelectron Spectroscopy, 2nd Ed. (Wiley, Chichester, 1990) p. 657. [33] C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G. Kinisky, H.A. Six, W.T. Jansen and J.A. Taylor, J. Vac. Sci. Technol. 21 (1982) 933.