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A study of the conductance and capacitance of pure and Pd-dopped SnO2 thick films
720
Semws and Actuufors B, I&i9
(1994) 7’2%723
A study of the conductance and capacitance of pure and Pd-doped SnO, thick films G. Martinelli and MS. Carotta Physics Department, Femm
University, 12 Vii Pamdiso, 44100 Fewora (Italy)
AbStract
Tin oxide based gas sensors are, at present, used in a variety of different applications and seem the most promising due to their inherent ability to undergo gas-induced conductivity changes even in the presence of low gas concentration. The gas response of thick-film tin dioxide depends crucially on electronic surface states involving absorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. Despite their high sensitivity and short response time the devices generally exhibit iow selectivity and limited long-term stability and are inguenced by inte~e~ng gases, especially water vapour. The ~nduct~i~ of ~gh-~r~i~ thick films, where the diameter of the grains is large compared with the Debye length, is described by the Schottky barrier model. The height of the energy barrier is extremely sensitive to the presence of additives, impurities, catalysts and water vapor, and the difference behveen the minimum and maximum of the energy barrier as a function of temperature has been proved suitable to be related to the sensitivity properties. SigniScant changes in the capacitance are also expected due to the large variation of charges in the surface states and in the space-charge layers during interaction with reducing gases. The aim of the present work is to study the behaviour of the space-charge and surface-state capacitance together with the coadu~n~ in the presence of reducing gas on screen-printed pure and Pd-doped samples. The ~~es~nding depletion layers, that is, the Dehye lengths of the electrons, are also discussed.
In an attempt to clarify some aspects of thick-film gas sensor behaviour, in this paper we report the conductance and capacitance responses to various stimulations of pure and Pd-doped tin dioxide printed on ahnniua substrates. The man~ac~ring process of our thick-film sensors based on SnO, paste, together with sample sizes and experimental conditions, has been descriied previously [l]. The screen-printing technology is adequate to satisfy the requirement of high specific area which is indispensable when the surface reactions are much more relevant then the bulk changes; this technol~ has also proved to be suitable for low-cost and large-scale production. The devices present a high porosity and the grain size ranges from 10 to 100 nm. The grains are interconnected by necks (closed or open) and by Schottky barriers of different heights which have the most relevant effect since their electric field acts pe~endiculariy to the conduction channel. The conductivity is generally written as
G = G, exp( - eV,lkT)
(1)
0925-4005/94/$7.00 Q 1994 Elsevier Sequoia. All rights reserved SSDI 09254005(93)01221-O
where Go 12, 31 is ahnost independent of tem~ra~re and eV, is the average activation energy. For the case of planar geometry, eV, is linked to the surface density of the absorbed oxygen ions N, by the permittivity E of the material: eV, = e2N~~2~~
(2)
where Nd is the donor concentration which is assumed constant throughout the space-charge region and equal to the bulk. The depletion layer width NJN,, can be considered to be approximately equal to the Debye length of electrons, LD = (&TieZn,)‘n
(3)
in which nb is the bulk carrier concentration. The value of the depletion layer in our samples, deduced by Hall effect measurement, is approximately 5 nm and more than 70% of the donor levels are ionized at a temperature of 350 “C 137. Disadvantages such as poor reproducibility and limited long-term stability are frequently observed in this type of sensor; moreover, its sensitivity to the ambient moisture strongly interferes with the conductive mechanism.
A better understanding of the moisture and catalyst effects may contribute to improvement of the performance of these devices.
Temperature 400
(“C)
200
Conductance behavlour In Fiis. 1 and 2 the conductance in different atmospheres and for different heating rates is shown in the case of pure and Pd-doped Sn02 samples, respectively. The conductance curves are generally studied in the range 50-500 “C where several phenomena occur: 0 the desorption of 4, O,- and O- takes place at 80 “C, 150 “C and 500 “c, respectively. l the physisorbed water is entirely desorbed (41 at 150 “C. l the hydroxyl groups due to the chemisorbed water are slowly desorbed from 250 “C, but are still present at 500 “C, l the ionized catalyst particles cause a temperaturedependent modification of the surface states of the oxide. The change in the slope of the curves in Figs. 1 and 2 from 200 “C up to 400 “C is mainly due to interference between the desorbed hydroxyl groups and the change from O,- to O- species; this interference is enhanced by increasing the heating rate, giving rise to the socalled sigmoid curves (dash-dash-dot curves in Figs. 1 and 2). It can be seen that the slope of the curves Temperature 400
Fig. 2. Conductance doped sample.
vs. temperature;
heating cycle for a Pd-
starts to change at a lower temperature in the presence of a catalyst, confirming the similar shift to a lower temperature of the sensitivity to reducing agents [5].
(“C)
200
Capacitance measurements
103/T(K-‘) Fig. 1. Conductance SnOz sample.
vs. temperature;
beating cycle for a pure
The intrinsic difficulty in reproducing high-porosity material with the same morphology, the activation of several adsorption and desorption processes roughly at the same temperature and the presence of impurities and additives strongly limit the reproducibility of the responses, even if the samples have been prepared as far as possible under the same conditions. In order to separate the effects of different variables we performed capacitance measurements on the pure (without binders, additives or catalysts) and Pd-doped samples. The capacitance is expected to undergo a large variation as the surface states change [6]. The permittivity of pure SnOz is shown in Fig. 3. The apparently high permittivity means that surface states are also present at low temperatures; this high value, already observed by other authors [5, 71, can probably be justified by taking into account the presence of charged layers that can be assumed, at a very first approximation, to be series-connected capacitors, each representing a very high capacitance.
122
“”
- dry 350: _
2
.
“et
_
dry
air
in dry air
. .
*.
f
,-:. 1 t I ! 1
. :
: :
in
:‘.
0, 40 I 0 : z30 h .? 5 20 t: E blO-. n
aaqla sample
.
. 8’ - .9-
,I’
J/t
‘,
./ ,*’
i
‘. \
k. _M+ O-“““‘~“‘~~~“,,,‘~“~‘~‘~~‘~““‘,,,’~~~”~~~~ 0 200 200 500 Temperature (“C)
0
Temperature (“C) Fig. 5. Permittivity of a pure SnOz sample for different atmospheric conditions; heating cycle.
Fig. 3. Pennittivity of a pure SnOl sample; heating and cooling cycles.
Ae0.12
AE=O.OE
Temperature
(“C)
Fig. 4. Comparison behveen permittivities of pure and Pd-doped samples; heating cycle.
The dotted curve represents the behaviour of a wet sample in a stream of dry air and its first maximum is attributed to the desorption of physisorbed water. A dry sample in dry air behaves as indicated by the dash-dash-dot curve; in this case the permittivity has a maximum corresponding to the transformation from O,- to O- species. The capacitance then decreases due to the increase in the depletion layer width. In Fig. 4 the permittivity of a pure SnO, sample is compared with that of a Pd-doped sample in which the surface states induced by the catalyst interfere with 02- to O- transformation. Figures 5 and 6 relate the energy barriers to the permittivity for diBerent atmospheric conditions. There is a clear correlation between the minimum of the energy barrier and the maximum of the permittivity for the different environmental conditions. This carrelation can probably be explained by considering an initial activation of an internal electrostatic field followed by a change in the depletion layer once the surface barrier increases. Similar curves in the presence
0.4L-d----
100
400 200 300 500 Temperature (“C)
I
0
Fig. 6. Energy barriers in a pure SnOl sample for the same atmospheric conditions as described in Fig. 5.
Temperature
(“C)
Fig. 7. Permittivity of a Pd-doped sample for different atmospheric conditions; heating cycle.
of a catalyst (Fig. 7) show, in particular, the desorption of physisorbed water in wet air (dotted curve) in agreement with the previous observed [5] reversible response.
723
Conclusions The effect of moisture and a catalyst on the conductance and capacitance of screen-printed tin dioxide has been analysed. The reproducibility and long-term stability can be improved by a better understanding of the influence of each variable on the response of the devices. In this work we have pointed out the interference between hydroxyl groups with the O- species. The presence of surface states due to the catalyst has been explored and the effect of the catalyst on the reversibility of the response in a moist atmosphere has been continned. A tentative correlation between energy barrier heights and permittivity is proposed together with a justification of the very high apparent permittivity.
Acknowledgements Discussions with Professor L. Passari are gratefully acknowledged. This work was supported by Eniricerche S.p.A. and Snam S.p.A., Milan, Italy.
References
1 G. Martinelli and M.C. Carotta, Intluence of additives on the sensing properties of screen-printed SnOs gas sensors, Sensors and Actuators B, 15-16 (1993) 363-366. 2 V. Lantto, P. Romppainen and S. Leppiwuori, A study of the temperature dependence of the barrier energy in porous tin oxide, Sensors and Achuatom, 14 (1988) 149163. 3 M.C. Carotta, C. Dallara, G. Martinelli, L. Passari and A. Camanzi, CH, thick-him sensors: characterization method and theoretical explanation, Sensors and Actuators B, 3 (1991) 191-196. 4 N. Yamazoe et al, Tin oxide surface and Os, Hz0 and H, Surface Sci, 86 (1979) 334-344. J.F. M&leer, P.T. Moseley, J.O.W. Norris, D.E. Williams and B.C. Tofield, Tin dioxide gas sensors, Parts 1 and 2, I. Cheat. Sot, Fara&y Tmm., I, 83 (1987) 1323-1346 and 84 (1988) 441457. RD. Schierbaum, U. Weimar, W. Gopal and R. Kowalkowski, Conductance, work function, and catalytic activity of SnOsbased gas sensors, Sensors andActuators B, 3 (1993) 205-214. V. Lantto, Semiconductor gas sensors based on SnOz thick films, in G. Sbetvegheri (ed.), Gas Sensors, Khwer, Dordrecht, 1993.
Semws and Actuufors B, I&i9
(1994) 7’2%723
A study of the conductance and capacitance of pure and Pd-doped SnO, thick films G. Martinelli and MS. Carotta Physics Department, Femm
University, 12 Vii Pamdiso, 44100 Fewora (Italy)
AbStract
Tin oxide based gas sensors are, at present, used in a variety of different applications and seem the most promising due to their inherent ability to undergo gas-induced conductivity changes even in the presence of low gas concentration. The gas response of thick-film tin dioxide depends crucially on electronic surface states involving absorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. Despite their high sensitivity and short response time the devices generally exhibit iow selectivity and limited long-term stability and are inguenced by inte~e~ng gases, especially water vapour. The ~nduct~i~ of ~gh-~r~i~ thick films, where the diameter of the grains is large compared with the Debye length, is described by the Schottky barrier model. The height of the energy barrier is extremely sensitive to the presence of additives, impurities, catalysts and water vapor, and the difference behveen the minimum and maximum of the energy barrier as a function of temperature has been proved suitable to be related to the sensitivity properties. SigniScant changes in the capacitance are also expected due to the large variation of charges in the surface states and in the space-charge layers during interaction with reducing gases. The aim of the present work is to study the behaviour of the space-charge and surface-state capacitance together with the coadu~n~ in the presence of reducing gas on screen-printed pure and Pd-doped samples. The ~~es~nding depletion layers, that is, the Dehye lengths of the electrons, are also discussed.
In an attempt to clarify some aspects of thick-film gas sensor behaviour, in this paper we report the conductance and capacitance responses to various stimulations of pure and Pd-doped tin dioxide printed on ahnniua substrates. The man~ac~ring process of our thick-film sensors based on SnO, paste, together with sample sizes and experimental conditions, has been descriied previously [l]. The screen-printing technology is adequate to satisfy the requirement of high specific area which is indispensable when the surface reactions are much more relevant then the bulk changes; this technol~ has also proved to be suitable for low-cost and large-scale production. The devices present a high porosity and the grain size ranges from 10 to 100 nm. The grains are interconnected by necks (closed or open) and by Schottky barriers of different heights which have the most relevant effect since their electric field acts pe~endiculariy to the conduction channel. The conductivity is generally written as
G = G, exp( - eV,lkT)
(1)
0925-4005/94/$7.00 Q 1994 Elsevier Sequoia. All rights reserved SSDI 09254005(93)01221-O
where Go 12, 31 is ahnost independent of tem~ra~re and eV, is the average activation energy. For the case of planar geometry, eV, is linked to the surface density of the absorbed oxygen ions N, by the permittivity E of the material: eV, = e2N~~2~~
(2)
where Nd is the donor concentration which is assumed constant throughout the space-charge region and equal to the bulk. The depletion layer width NJN,, can be considered to be approximately equal to the Debye length of electrons, LD = (&TieZn,)‘n
(3)
in which nb is the bulk carrier concentration. The value of the depletion layer in our samples, deduced by Hall effect measurement, is approximately 5 nm and more than 70% of the donor levels are ionized at a temperature of 350 “C 137. Disadvantages such as poor reproducibility and limited long-term stability are frequently observed in this type of sensor; moreover, its sensitivity to the ambient moisture strongly interferes with the conductive mechanism.
A better understanding of the moisture and catalyst effects may contribute to improvement of the performance of these devices.
Temperature 400
(“C)
200
Conductance behavlour In Fiis. 1 and 2 the conductance in different atmospheres and for different heating rates is shown in the case of pure and Pd-doped Sn02 samples, respectively. The conductance curves are generally studied in the range 50-500 “C where several phenomena occur: 0 the desorption of 4, O,- and O- takes place at 80 “C, 150 “C and 500 “c, respectively. l the physisorbed water is entirely desorbed (41 at 150 “C. l the hydroxyl groups due to the chemisorbed water are slowly desorbed from 250 “C, but are still present at 500 “C, l the ionized catalyst particles cause a temperaturedependent modification of the surface states of the oxide. The change in the slope of the curves in Figs. 1 and 2 from 200 “C up to 400 “C is mainly due to interference between the desorbed hydroxyl groups and the change from O,- to O- species; this interference is enhanced by increasing the heating rate, giving rise to the socalled sigmoid curves (dash-dash-dot curves in Figs. 1 and 2). It can be seen that the slope of the curves Temperature 400
Fig. 2. Conductance doped sample.
vs. temperature;
heating cycle for a Pd-
starts to change at a lower temperature in the presence of a catalyst, confirming the similar shift to a lower temperature of the sensitivity to reducing agents [5].
(“C)
200
Capacitance measurements
103/T(K-‘) Fig. 1. Conductance SnOz sample.
vs. temperature;
beating cycle for a pure
The intrinsic difficulty in reproducing high-porosity material with the same morphology, the activation of several adsorption and desorption processes roughly at the same temperature and the presence of impurities and additives strongly limit the reproducibility of the responses, even if the samples have been prepared as far as possible under the same conditions. In order to separate the effects of different variables we performed capacitance measurements on the pure (without binders, additives or catalysts) and Pd-doped samples. The capacitance is expected to undergo a large variation as the surface states change [6]. The permittivity of pure SnOz is shown in Fig. 3. The apparently high permittivity means that surface states are also present at low temperatures; this high value, already observed by other authors [5, 71, can probably be justified by taking into account the presence of charged layers that can be assumed, at a very first approximation, to be series-connected capacitors, each representing a very high capacitance.
122
“”
- dry 350: _
2
.
“et
_
dry
air
in dry air
. .
*.
f
,-:. 1 t I ! 1
. :
: :
in
:‘.
0, 40 I 0 : z30 h .? 5 20 t: E blO-. n
aaqla sample
.
. 8’ - .9-
,I’
J/t
‘,
./ ,*’
i
‘. \
k. _M+ O-“““‘~“‘~~~“,,,‘~“~‘~‘~~‘~““‘,,,’~~~”~~~~ 0 200 200 500 Temperature (“C)
0
Temperature (“C) Fig. 5. Permittivity of a pure SnOz sample for different atmospheric conditions; heating cycle.
Fig. 3. Pennittivity of a pure SnOl sample; heating and cooling cycles.
Ae0.12
AE=O.OE
Temperature
(“C)
Fig. 4. Comparison behveen permittivities of pure and Pd-doped samples; heating cycle.
The dotted curve represents the behaviour of a wet sample in a stream of dry air and its first maximum is attributed to the desorption of physisorbed water. A dry sample in dry air behaves as indicated by the dash-dash-dot curve; in this case the permittivity has a maximum corresponding to the transformation from O,- to O- species. The capacitance then decreases due to the increase in the depletion layer width. In Fig. 4 the permittivity of a pure SnO, sample is compared with that of a Pd-doped sample in which the surface states induced by the catalyst interfere with 02- to O- transformation. Figures 5 and 6 relate the energy barriers to the permittivity for diBerent atmospheric conditions. There is a clear correlation between the minimum of the energy barrier and the maximum of the permittivity for the different environmental conditions. This carrelation can probably be explained by considering an initial activation of an internal electrostatic field followed by a change in the depletion layer once the surface barrier increases. Similar curves in the presence
0.4L-d----
100
400 200 300 500 Temperature (“C)
I
0
Fig. 6. Energy barriers in a pure SnOl sample for the same atmospheric conditions as described in Fig. 5.
Temperature
(“C)
Fig. 7. Permittivity of a Pd-doped sample for different atmospheric conditions; heating cycle.
of a catalyst (Fig. 7) show, in particular, the desorption of physisorbed water in wet air (dotted curve) in agreement with the previous observed [5] reversible response.
723
Conclusions The effect of moisture and a catalyst on the conductance and capacitance of screen-printed tin dioxide has been analysed. The reproducibility and long-term stability can be improved by a better understanding of the influence of each variable on the response of the devices. In this work we have pointed out the interference between hydroxyl groups with the O- species. The presence of surface states due to the catalyst has been explored and the effect of the catalyst on the reversibility of the response in a moist atmosphere has been continned. A tentative correlation between energy barrier heights and permittivity is proposed together with a justification of the very high apparent permittivity.
Acknowledgements Discussions with Professor L. Passari are gratefully acknowledged. This work was supported by Eniricerche S.p.A. and Snam S.p.A., Milan, Italy.
References
1 G. Martinelli and M.C. Carotta, Intluence of additives on the sensing properties of screen-printed SnOs gas sensors, Sensors and Actuators B, 15-16 (1993) 363-366. 2 V. Lantto, P. Romppainen and S. Leppiwuori, A study of the temperature dependence of the barrier energy in porous tin oxide, Sensors and Achuatom, 14 (1988) 149163. 3 M.C. Carotta, C. Dallara, G. Martinelli, L. Passari and A. Camanzi, CH, thick-him sensors: characterization method and theoretical explanation, Sensors and Actuators B, 3 (1991) 191-196. 4 N. Yamazoe et al, Tin oxide surface and Os, Hz0 and H, Surface Sci, 86 (1979) 334-344. J.F. M&leer, P.T. Moseley, J.O.W. Norris, D.E. Williams and B.C. Tofield, Tin dioxide gas sensors, Parts 1 and 2, I. Cheat. Sot, Fara&y Tmm., I, 83 (1987) 1323-1346 and 84 (1988) 441457. RD. Schierbaum, U. Weimar, W. Gopal and R. Kowalkowski, Conductance, work function, and catalytic activity of SnOsbased gas sensors, Sensors andActuators B, 3 (1993) 205-214. V. Lantto, Semiconductor gas sensors based on SnOz thick films, in G. Sbetvegheri (ed.), Gas Sensors, Khwer, Dordrecht, 1993.