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Moisture effects on pure and Pd-doped SnO2 thick films analysed by FTIR spectroscopy and conductance measurements
B CHEMICAL
ELSEVIER
Sensors and Actuators B 24-25 (1995) 520-524
Moisture effects on pure and Pd-doped SnOz thick films analysed by FTIR spectroscopy and conductance measurements G. Ghiotti a, A. Chiorino a, G. Martinelli b, M.C. Carotta b aInorganic, Physical
and Material Chemistry Depwfmenf Torino University, 7 via P. Giwia, 10125 Turin, Italy b Physics Department, Femra Universi~, I2 via Paradise, 44100 Ferara, Italy
Abstract
Pure SnO, and Pd/SnO, (0.4 wt.%) pastes have been prepared starting from SnO* powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450 “C. By alternating wet and dry air, it has been observed that the conductance of pure SnO, samples changes in a reversible way only for temperatures over approximately 200 “C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnOZ samples treated in wet air at temperatures above 200 “C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5-0.18 and 0.45-0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200 “C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick films. Keywordst
FTIR spectroscopy; Gas sensors; Tin oxide
2. Experimental
1. Introduction Most commercial make
use of porous
gas detectors available at present SnO, sensing elements, mainly
because they offer high sensitivity at low operating temperature, but disadvantages such as lack of reproducibility and long-term stability are frequently observed. The adsorbed O- species deplete the outer layers of the grains of electrons, causing a Schottkybarrier conductance mechanism. It has been observed that the conductance behaviour following the interaction with combustible gas isvery sensitive to the pretreatment temperature; moreover, the presence of vapour-phase Hz0 strongly affects the magnitude of the change [l]. The origin of these effects has not yet been clearly established. The aim of this work is to analyse by Fourier-transform infrared (FMR) spectroscopy pure and Pd-doped SnO, thick films for different ambient conditions in order to correlate the conductance and the impedance variations with the different degree of surface hydration. 0925-4005/95/$09.50 0 1995 Else&r SSDI 0925-4005(94)01409-B
Science S.A.
All
rights reserved
Pure SnO, powder has been prepared with the procedure generally used for gas-sensor material: hydrolysis with ammonia of SnCl, water solution, followed by washing, drying at room temperature and calcining at 200 “C. The sensitive layer has been obtained by adding to the above-described powder only an organic vehicle with a maximum burn-out temperature of approximately 600 “C. A series of samples with 0.4 wt.% Pd as catalyst was prepared too. The paste was printed on 96% alumina substrates provided with an adequate heater element and gold conductor electrodes for electrical measurements; the samples were then fired for one hour at 850 “C. Layers detached from the support were used to obtain self-supporting disks for F’T’IRinvestigations. All the measurements have been carried in dry or 50% R.H. air and in 1000 ppm of methane in dry or wet air.
G. Ghiotfi et al. I Senwrs and Actuators B 24-25 (1995) 520-524
FHR spectra were recorded at different temperatures using a commercial heatable Aabspec cell, working in vacua or in controlled atmosphere, fitted on a Perkin-Elmer 2000 FTIR spectrophotometer with MCT detector. Samples were initially submitted to an alternate outgassing-oxidizing treatment at increasing temperature up to 650 “C, then cooled in dry air to the experimental temperature. When dry- and wet-air effects were studied, at each examined temperature, an IR spectrum was run in dly air, then after 5 min contact with 12-13 mbar of wet air and finally after 40 min contact with 40 mbar of dry air again. The sample was then cooled in dry air to the next temperature. The temperature range tested was 450-100 “C, in steps of 50 “C. The conductance measurements have been performed in dry air from 100 up to 450 “C at a heating rate of 4 “C min-‘; then the measurement has been repeated alternating 10 min of dry air and 10 min of wet air for 3 h. The impedance has been obtained by measuring separately the resistance and the capacitance of the samples in the frequency range 5-500 kHz and calculating the impedance on the basis of an equivalent circuit composed by a resistor in series with a second resistor and a capacitor connected in parallel [2].
521
alternate pulses of dry and wet air is shown; it can be seen that the catalyst allows the sample to recover the conductance in dry air after the exposure to wet air also in the low-temperature range [3]. Similar experiments were done by testing the changes in the IR medium-region spectrum (600-7200 cm-‘) of both samples alternately contacted with dry and wet air, as the temperature was decreased from 450 to 100 “C. The behaviour shown by pure and Pd-doped samples is reported in Figs. 2 and 3, respectively, for only two temperatures (450 “C in (a) and 150 “C in (b)), selected to show the behaviour differences of the two samples. Curves 1 refer to the transmittance spectra run under dry air, curves 2 after exposure to wet air and curves 3 after readmission of dry air. Focusing our attention on the results obtained for pure SnO, at 450 “C (see Fig. 2(a)), the effect on the IR spectrum of the wet air is a transmission loss only partially restored by dry-air readmission. The difference (in absorbance) between curves 2 and 1 (see curve (a)
h-----l
a
3. Results and discussion In Fig. 1 the conductance dependence on temperature for pure and Pd-doped SnO, samples subjected to
400 C”““““““““““1
Temperature (K) 600 500
1
700
so
I
300 Temperature 200
400 (“C)
Fig. 1. Temperature dependence of the moisture response for pure and Pd-doped SnOz samples.
1
I
Ib
Fig, 2. FlIR spectra of pure St14 sample submitted to alternate dry and wet air atmospheres: (a) cycle at 450 “C; (b) cycle at 150 “C. Curves 1, transmission spectra in dry air; curves 2, after subsequent admission of 12 mbar of wet air (5 min contact); curves 3, after readmission of dry air (40 min contact). Insets, absorbance spectra differences: (a) curves 2-1; (b) curves 3- 1.
G. Ghiotti et al. f Semors and Actiators B 24-25 (1995) 520-524
1 300
Fig. 3. FI’IR spectra of Pd-doped SnOz sample submitted to alternate dry and wet air atmospheres: (a) cycle at 450 “C; (b) cycle at 150 “C. Curves 1, transmission spectra in dxy air: curves 2, after subsequent admission of 12 mbar of wet air (5 min contact); curves 3, after readmission of dly air (40 min contact). Insets, absorbance spectra differences: (a) curves 2-l; (b) curves 3- 1.
in the inset) shows the intensity of the light absorbed or scattered in the examined range, responsible for the transmission loss in wet air. The difference (in absorbance) between curves 3 and 1 (see curve (b) in the inset) is a measure of the spectral change reversibility by reexposing the sample to dry air. Curve (b) should be equal to zero in the overall range if all the spectral changes were reversible. However, even if the conductance is completely reversible, we do not expect that the same thing happens to the sample spectrum since its changes derive from different contributions that are not all necessarily reversible. The change in the conductance passing from dry to wet air should bc found in the IR spectrum as a transmission loss increasing with decreasing wavenumbers (fi), since the free electrons are known to scatter the radiation, the intensity of the scattered radiation being proportional to Y-” where n is a number depending on temperature and material. This is not the shape of curve (a), which shows a very broad absorption with superimposed very
weak bands due to vibrational modes of hydroxyls. The broad absorption is the overlap of three contributions: (i) one band with a maximum at about 1600 cm-l (0.18 eV); its position near to the ionization energy of the second level of the oxygen vacancy suggests it corresponds to electron transitions from similar levels to the conduction band (c.b.) [4]; (ii) a second band with a maximum at 3600 cm-’ (0.45 eV) tentatively assigned [4] to an electronic transition similar to that previously described, but involving oxygen divacancies; (iii) a third contribution due to the radiation scattering by the free electrons, being masked by the other two. The broad absorption is almost completely destroyed by a subsequent dry-air pulse (see curve (b)), while the intensity of hydroxyl vibrations (now the main features in curve (b)) increases. Focusing our attention on the results obtained for pure SnO, at 150 “C (see Fig. Z(b)), the effect on the IR spectrum of the wet air is still a transmission loss in the overall range, not restored by dry-air readmission. Curve (a) in this case shows a lower intensity than at 450 “C and a different shape, that is, the contribution due to the scattered radiation is now relatively higher. Obviously, curve (b) almost coincides with curve (a) owing to the irreversibility of all the spectral changes (but for a small increase of hydroxyl intensity) by dry-air readmission. If the same analysis of the results performed at 450 “C on the Pd-doped sample is done, a behaviour very similar to that of pure SnO, is found (see Fig. 3(a)). Curve (a) in the inset still shows the presence of the broad absorption of electronic nature, the overlap of the three contributions, the band at 1600 cn-’ now being more prominent. Also in this case the broad absorption is drastically decreased by the subsequent dry-air pulse and hydroxyl vibration modes remain as the main features in curve (b). For the experiment performed at 150 “C (see Fig. 3(b)), the results are similar to those obtained at 450 “C, but for the intensity of the electronic absorption. However, in comparison with the pure SnO, behaviour, the main differences are the shape of the electronic absorption formed in wet air, its reversibility by dryair readmission and the hydroxyl vibration intensity. The increase of pure and Pd-doped SnO, conductance by replacing dry air with wet air is caused by the release in the c.b. of clcctrons produced in water reduction at the sample surface. However, as shown by IR measurements, some electrons remain trapped in the localized levels of oxygen monovacancies and divacancies. The relative amounts of free and trapped electrons are different in the pure SnO, and Pd-doped SnO, and for different testing temperatures. In wet air at 4.50“C the following reactions involving oxygen vacancies can be proposed:
523
G. Ghiotti et al. I Seawrs and Actuators B 24-25 (1995) 520-524
Hz0 + 20,‘-
+ Vo* + -
H,O + 20,*- + Vo+ -
20H- fVo+ +O,20H- +e-(c.b.)
(I) (2)
Obviously these reactions are equally responsible for hydroxyl formation, but reaction (2) destroys the V,+ by saturation with OH- groups and so could not explain the increase in the electronic transition related to the photoionization. By the way of contrast, reaction (1) increases the Vo’ concentration and can also be responsible for free-electron formation, since at 450 “C 70% of them are expected to be thermally ionized. A reaction similar to reaction (1) can be imagined involving the oxygen divacancies for which the probability of thermal ionization is lower, since their localized electronic levels are deeper in the energy gap between the valence band (v.b.) and c.b. So we favour reaction (l), but we obviously cannot exclude that reaction (2) could occur to some extent at this temperature. The concentration of Vo2+ and ionized divacancies that can trap electrons on the Pd-doped sample in dry air is expected to be higher than on a pure SnO, sample by Fermi energy control or oxygen spillover [5]: this agrees with the intensity of the localized electronic transition formed by a wet-air pulse, it being higher on the Pddoped than on the pure sample. The conductance reversibility at 4.50 “C by dry-air readmission can be explained by the reversibility at this temperature of the two surface reactions on both pure and Pd-doped samples, and should correspond to a decrease of surface hydroxyls. It is very surprising that dry-air readmission could increase the intensity of hydroxyl vibrations (see curve (b)). Actually, we think that this increase is only apparent and that the very weak intensity of the hydroxyl vibrations under wet air (see curve (a)) does not correspond to a lower surface concentration, but is the result of a coupling between the hydroxyl vibrations and the electron transitions absorbing in the same spectral range, which kills the intensity of the vibrations even if the hydroxyl species are present on the surface. When the electronic transition is destroyed by the dryair readmission, the remaining hydroxyls are measured by the intensity of their modes. However, as we do not know the intensity and, as a consequence, the amounts of hydroxyls formed by wet-air contact, we cannot know if the subsequent dry-air contact corresponds to a decrease of the surface hydroxyl concentration. If the conductance reversibility is related to the shift of the equilibria (1) and (2) to the left side, the remaining hydroxyls must be related to a different reaction, not difficult to find, because hydroxyls on the oxide surface can be formed by a simple heterolytic dissociation of the water on the coordinatively unsaturated M-O pairs, not involving release of electrons: Sn4++02-+HO s I
2
-
(Sn4’-OH-),+OH,-
(3)
pure SnO, T=450’C
I
” l
I
drYair
3 7 I’,
”
I
”
I I‘
1’
I
”
Real Z (Ml)
Fig. 4. Impedance spectra of pure SnOl sample in dry and wet air with and without methane (1000 ppm). Frequency ranges from 5 to 500 kHz.
Pd-doped
SnO, T=350°C
_,.j3
Fig. 5. Impedance spectra of Pd-doped SnO, sample in dry and wet air with and without methane (loo0 ppm). Same frequency range as in Fig. 4.
Lowering the temperature of the experiment, reaction (3) is responsible for the heavy surface hydroxylation, not related to conductance increase. The increase of pure and Pd-doped SnO, conductance by replacing dry air with wet air at 150 “C can still be caused by reaction (1) or (2). Owing to the shape of the broad electronic absorption in the case of pure SnO, (see Fig. 2(b), curve (a)), reaction (2) is prevailing, while reaction (1) is still important on Pd-doped SnO,. The irreversible response to moisture shown by pure SnO, could then be due to the fact that the activation energy for the inverse reactions in the equilibria (1) and (2) is too high to shift them to the left side at a temperature below 200 “C on pure SnO,. The reversible response to moisture still shown by Pd-doped SnO,
524
G. Ghiotti et al. / Sensors and A&atom B 24-25 (1995) 520-524
can be related to the catalytic properties of Pd, which enable the reverse reaction paths to change into new ones with a lower activation energy. Concerning the IR data not reported for the sake of brevity, the results and discussion for cycles at T> 200 “C are similar to those reported for cycles at 450 “C, while those at T<200 “C are similar to those reported for the cycle at 150 “C. A tentative attempt to interpret the behaviour of the impedance curves shown in Figs. 4 and 5, particularly the observed inversion of wet and dry cmves in the presence of methane for pure and Pd-doped samples, is now underway by means of the corresponding FTIR measurements.
References
U1 G. Martinelli and M.C. Carotta, Influence of additives on the sensing properties of screen-printed SnO, gas sensors, Sensors and Actuators B, 15-16 (1993) 363-366. M K.D. Schierbaum, R. Kowalkowski, U. Weimar and W. Giipel, Conductance, work function, and catalytic activity of SnO,based gas sensors, Sensors and Achutors B, 3 (1991) 191-196. [31 J.F. M&leer, P.T. Moseley, J.O.W. Norris, D.E. Williams and B.C. Tofield, Tin dioxide gas sensors, Part. 1 and 2,J. Chem. Sot. Faraday Trans. 1, 84 (1988) 441-457. 141 G. Ghiotti, A. Chiorino and W. Xiong Pan, Surface chemistry and electronic effects of H2(D2) on pure and Cr-doped SnOl, Sensors and Actuatom B, IS-16 (1993) 367-371. Bl S.R. Morrison, Selectivity in semiconductor gas sensors, Sensors and Actuatom, 12 (19S7) 425.
ELSEVIER
Sensors and Actuators B 24-25 (1995) 520-524
Moisture effects on pure and Pd-doped SnOz thick films analysed by FTIR spectroscopy and conductance measurements G. Ghiotti a, A. Chiorino a, G. Martinelli b, M.C. Carotta b aInorganic, Physical
and Material Chemistry Depwfmenf Torino University, 7 via P. Giwia, 10125 Turin, Italy b Physics Department, Femra Universi~, I2 via Paradise, 44100 Ferara, Italy
Abstract
Pure SnO, and Pd/SnO, (0.4 wt.%) pastes have been prepared starting from SnO* powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450 “C. By alternating wet and dry air, it has been observed that the conductance of pure SnO, samples changes in a reversible way only for temperatures over approximately 200 “C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnOZ samples treated in wet air at temperatures above 200 “C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5-0.18 and 0.45-0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200 “C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick films. Keywordst
FTIR spectroscopy; Gas sensors; Tin oxide
2. Experimental
1. Introduction Most commercial make
use of porous
gas detectors available at present SnO, sensing elements, mainly
because they offer high sensitivity at low operating temperature, but disadvantages such as lack of reproducibility and long-term stability are frequently observed. The adsorbed O- species deplete the outer layers of the grains of electrons, causing a Schottkybarrier conductance mechanism. It has been observed that the conductance behaviour following the interaction with combustible gas isvery sensitive to the pretreatment temperature; moreover, the presence of vapour-phase Hz0 strongly affects the magnitude of the change [l]. The origin of these effects has not yet been clearly established. The aim of this work is to analyse by Fourier-transform infrared (FMR) spectroscopy pure and Pd-doped SnO, thick films for different ambient conditions in order to correlate the conductance and the impedance variations with the different degree of surface hydration. 0925-4005/95/$09.50 0 1995 Else&r SSDI 0925-4005(94)01409-B
Science S.A.
All
rights reserved
Pure SnO, powder has been prepared with the procedure generally used for gas-sensor material: hydrolysis with ammonia of SnCl, water solution, followed by washing, drying at room temperature and calcining at 200 “C. The sensitive layer has been obtained by adding to the above-described powder only an organic vehicle with a maximum burn-out temperature of approximately 600 “C. A series of samples with 0.4 wt.% Pd as catalyst was prepared too. The paste was printed on 96% alumina substrates provided with an adequate heater element and gold conductor electrodes for electrical measurements; the samples were then fired for one hour at 850 “C. Layers detached from the support were used to obtain self-supporting disks for F’T’IRinvestigations. All the measurements have been carried in dry or 50% R.H. air and in 1000 ppm of methane in dry or wet air.
G. Ghiotfi et al. I Senwrs and Actuators B 24-25 (1995) 520-524
FHR spectra were recorded at different temperatures using a commercial heatable Aabspec cell, working in vacua or in controlled atmosphere, fitted on a Perkin-Elmer 2000 FTIR spectrophotometer with MCT detector. Samples were initially submitted to an alternate outgassing-oxidizing treatment at increasing temperature up to 650 “C, then cooled in dry air to the experimental temperature. When dry- and wet-air effects were studied, at each examined temperature, an IR spectrum was run in dly air, then after 5 min contact with 12-13 mbar of wet air and finally after 40 min contact with 40 mbar of dry air again. The sample was then cooled in dry air to the next temperature. The temperature range tested was 450-100 “C, in steps of 50 “C. The conductance measurements have been performed in dry air from 100 up to 450 “C at a heating rate of 4 “C min-‘; then the measurement has been repeated alternating 10 min of dry air and 10 min of wet air for 3 h. The impedance has been obtained by measuring separately the resistance and the capacitance of the samples in the frequency range 5-500 kHz and calculating the impedance on the basis of an equivalent circuit composed by a resistor in series with a second resistor and a capacitor connected in parallel [2].
521
alternate pulses of dry and wet air is shown; it can be seen that the catalyst allows the sample to recover the conductance in dry air after the exposure to wet air also in the low-temperature range [3]. Similar experiments were done by testing the changes in the IR medium-region spectrum (600-7200 cm-‘) of both samples alternately contacted with dry and wet air, as the temperature was decreased from 450 to 100 “C. The behaviour shown by pure and Pd-doped samples is reported in Figs. 2 and 3, respectively, for only two temperatures (450 “C in (a) and 150 “C in (b)), selected to show the behaviour differences of the two samples. Curves 1 refer to the transmittance spectra run under dry air, curves 2 after exposure to wet air and curves 3 after readmission of dry air. Focusing our attention on the results obtained for pure SnO, at 450 “C (see Fig. 2(a)), the effect on the IR spectrum of the wet air is a transmission loss only partially restored by dry-air readmission. The difference (in absorbance) between curves 2 and 1 (see curve (a)
h-----l
a
3. Results and discussion In Fig. 1 the conductance dependence on temperature for pure and Pd-doped SnO, samples subjected to
400 C”““““““““““1
Temperature (K) 600 500
1
700
so
I
300 Temperature 200
400 (“C)
Fig. 1. Temperature dependence of the moisture response for pure and Pd-doped SnOz samples.
1
I
Ib
Fig, 2. FlIR spectra of pure St14 sample submitted to alternate dry and wet air atmospheres: (a) cycle at 450 “C; (b) cycle at 150 “C. Curves 1, transmission spectra in dry air; curves 2, after subsequent admission of 12 mbar of wet air (5 min contact); curves 3, after readmission of dry air (40 min contact). Insets, absorbance spectra differences: (a) curves 2-1; (b) curves 3- 1.
G. Ghiotti et al. f Semors and Actiators B 24-25 (1995) 520-524
1 300
Fig. 3. FI’IR spectra of Pd-doped SnOz sample submitted to alternate dry and wet air atmospheres: (a) cycle at 450 “C; (b) cycle at 150 “C. Curves 1, transmission spectra in dxy air: curves 2, after subsequent admission of 12 mbar of wet air (5 min contact); curves 3, after readmission of dly air (40 min contact). Insets, absorbance spectra differences: (a) curves 2-l; (b) curves 3- 1.
in the inset) shows the intensity of the light absorbed or scattered in the examined range, responsible for the transmission loss in wet air. The difference (in absorbance) between curves 3 and 1 (see curve (b) in the inset) is a measure of the spectral change reversibility by reexposing the sample to dry air. Curve (b) should be equal to zero in the overall range if all the spectral changes were reversible. However, even if the conductance is completely reversible, we do not expect that the same thing happens to the sample spectrum since its changes derive from different contributions that are not all necessarily reversible. The change in the conductance passing from dry to wet air should bc found in the IR spectrum as a transmission loss increasing with decreasing wavenumbers (fi), since the free electrons are known to scatter the radiation, the intensity of the scattered radiation being proportional to Y-” where n is a number depending on temperature and material. This is not the shape of curve (a), which shows a very broad absorption with superimposed very
weak bands due to vibrational modes of hydroxyls. The broad absorption is the overlap of three contributions: (i) one band with a maximum at about 1600 cm-l (0.18 eV); its position near to the ionization energy of the second level of the oxygen vacancy suggests it corresponds to electron transitions from similar levels to the conduction band (c.b.) [4]; (ii) a second band with a maximum at 3600 cm-’ (0.45 eV) tentatively assigned [4] to an electronic transition similar to that previously described, but involving oxygen divacancies; (iii) a third contribution due to the radiation scattering by the free electrons, being masked by the other two. The broad absorption is almost completely destroyed by a subsequent dry-air pulse (see curve (b)), while the intensity of hydroxyl vibrations (now the main features in curve (b)) increases. Focusing our attention on the results obtained for pure SnO, at 150 “C (see Fig. Z(b)), the effect on the IR spectrum of the wet air is still a transmission loss in the overall range, not restored by dry-air readmission. Curve (a) in this case shows a lower intensity than at 450 “C and a different shape, that is, the contribution due to the scattered radiation is now relatively higher. Obviously, curve (b) almost coincides with curve (a) owing to the irreversibility of all the spectral changes (but for a small increase of hydroxyl intensity) by dry-air readmission. If the same analysis of the results performed at 450 “C on the Pd-doped sample is done, a behaviour very similar to that of pure SnO, is found (see Fig. 3(a)). Curve (a) in the inset still shows the presence of the broad absorption of electronic nature, the overlap of the three contributions, the band at 1600 cn-’ now being more prominent. Also in this case the broad absorption is drastically decreased by the subsequent dry-air pulse and hydroxyl vibration modes remain as the main features in curve (b). For the experiment performed at 150 “C (see Fig. 3(b)), the results are similar to those obtained at 450 “C, but for the intensity of the electronic absorption. However, in comparison with the pure SnO, behaviour, the main differences are the shape of the electronic absorption formed in wet air, its reversibility by dryair readmission and the hydroxyl vibration intensity. The increase of pure and Pd-doped SnO, conductance by replacing dry air with wet air is caused by the release in the c.b. of clcctrons produced in water reduction at the sample surface. However, as shown by IR measurements, some electrons remain trapped in the localized levels of oxygen monovacancies and divacancies. The relative amounts of free and trapped electrons are different in the pure SnO, and Pd-doped SnO, and for different testing temperatures. In wet air at 4.50“C the following reactions involving oxygen vacancies can be proposed:
523
G. Ghiotti et al. I Seawrs and Actuators B 24-25 (1995) 520-524
Hz0 + 20,‘-
+ Vo* + -
H,O + 20,*- + Vo+ -
20H- fVo+ +O,20H- +e-(c.b.)
(I) (2)
Obviously these reactions are equally responsible for hydroxyl formation, but reaction (2) destroys the V,+ by saturation with OH- groups and so could not explain the increase in the electronic transition related to the photoionization. By the way of contrast, reaction (1) increases the Vo’ concentration and can also be responsible for free-electron formation, since at 450 “C 70% of them are expected to be thermally ionized. A reaction similar to reaction (1) can be imagined involving the oxygen divacancies for which the probability of thermal ionization is lower, since their localized electronic levels are deeper in the energy gap between the valence band (v.b.) and c.b. So we favour reaction (l), but we obviously cannot exclude that reaction (2) could occur to some extent at this temperature. The concentration of Vo2+ and ionized divacancies that can trap electrons on the Pd-doped sample in dry air is expected to be higher than on a pure SnO, sample by Fermi energy control or oxygen spillover [5]: this agrees with the intensity of the localized electronic transition formed by a wet-air pulse, it being higher on the Pddoped than on the pure sample. The conductance reversibility at 4.50 “C by dry-air readmission can be explained by the reversibility at this temperature of the two surface reactions on both pure and Pd-doped samples, and should correspond to a decrease of surface hydroxyls. It is very surprising that dry-air readmission could increase the intensity of hydroxyl vibrations (see curve (b)). Actually, we think that this increase is only apparent and that the very weak intensity of the hydroxyl vibrations under wet air (see curve (a)) does not correspond to a lower surface concentration, but is the result of a coupling between the hydroxyl vibrations and the electron transitions absorbing in the same spectral range, which kills the intensity of the vibrations even if the hydroxyl species are present on the surface. When the electronic transition is destroyed by the dryair readmission, the remaining hydroxyls are measured by the intensity of their modes. However, as we do not know the intensity and, as a consequence, the amounts of hydroxyls formed by wet-air contact, we cannot know if the subsequent dry-air contact corresponds to a decrease of the surface hydroxyl concentration. If the conductance reversibility is related to the shift of the equilibria (1) and (2) to the left side, the remaining hydroxyls must be related to a different reaction, not difficult to find, because hydroxyls on the oxide surface can be formed by a simple heterolytic dissociation of the water on the coordinatively unsaturated M-O pairs, not involving release of electrons: Sn4++02-+HO s I
2
-
(Sn4’-OH-),+OH,-
(3)
pure SnO, T=450’C
I
” l
I
drYair
3 7 I’,
”
I
”
I I‘
1’
I
”
Real Z (Ml)
Fig. 4. Impedance spectra of pure SnOl sample in dry and wet air with and without methane (1000 ppm). Frequency ranges from 5 to 500 kHz.
Pd-doped
SnO, T=350°C
_,.j3
Fig. 5. Impedance spectra of Pd-doped SnO, sample in dry and wet air with and without methane (loo0 ppm). Same frequency range as in Fig. 4.
Lowering the temperature of the experiment, reaction (3) is responsible for the heavy surface hydroxylation, not related to conductance increase. The increase of pure and Pd-doped SnO, conductance by replacing dry air with wet air at 150 “C can still be caused by reaction (1) or (2). Owing to the shape of the broad electronic absorption in the case of pure SnO, (see Fig. 2(b), curve (a)), reaction (2) is prevailing, while reaction (1) is still important on Pd-doped SnO,. The irreversible response to moisture shown by pure SnO, could then be due to the fact that the activation energy for the inverse reactions in the equilibria (1) and (2) is too high to shift them to the left side at a temperature below 200 “C on pure SnO,. The reversible response to moisture still shown by Pd-doped SnO,
524
G. Ghiotti et al. / Sensors and A&atom B 24-25 (1995) 520-524
can be related to the catalytic properties of Pd, which enable the reverse reaction paths to change into new ones with a lower activation energy. Concerning the IR data not reported for the sake of brevity, the results and discussion for cycles at T> 200 “C are similar to those reported for cycles at 450 “C, while those at T<200 “C are similar to those reported for the cycle at 150 “C. A tentative attempt to interpret the behaviour of the impedance curves shown in Figs. 4 and 5, particularly the observed inversion of wet and dry cmves in the presence of methane for pure and Pd-doped samples, is now underway by means of the corresponding FTIR measurements.
References
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