- Email: [email protected]
Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples
Sensors and Actuators B 44 (1997) 474 – 482
Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples A. Chiorino a, G. Ghiotti a,*, F. Prinetto a, M.C. Carotta b, G. Martinelli b, M. Merli b a
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Uni6ersita` di Torino, Via Pietro Giuria 7, I-10125 Turin, Italy b INFM-Dipartimento di Fisica, Uni6ersita` di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy Received 3 June 1997; accepted 6 June 1997
Abstract The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C. © 1997 Elsevier Science S.A. Keywords: Gas sensors; SnO2 films; Methane sensing
1. Introduction It is well known that the addition of small amounts of noble metals to tin oxide can promote the gas sensitivity and the response rate of the sensing elements to inflammable gases. The aim of this work is a comparison of the microstructural properties, surface reactivity and sensitivity of a series of SnO2-based thick films. We have examined one laboratory-prepared and one commercial SnO2 powder, both pure and with addition of palladium. Concerning Pd, it has been proposed [1] that the promotion mechanism is electronic, i.e. Pd increases the work function of the SnO2
under oxidizing conditions, forming a stable oxide capable of producing a strongly electron-depleted spacecharge layer inside the SnO2, while the work function shift completely disappears when samples are exposed to reducing atmospheres. The fast oxidation of Pd(0), supported on tin oxide single crystal, to PdO in O2 (pE5 mbar) has been demonstrated by Geiger et al. [2] starting from 400°C, while at lower temperatures the oxidation reaction is too slow, even if the PdO phase should be the stable thermodynamic phase [3]. We have used a preparation method warranting the presence of Pd(II) on the starting materials. 2. Experimental
* Corresponding author. [email protected].
Fax:
+39
11
6707855;
e-mail:
0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 3 8 - 4
A commercial SnO2 powder from CERAC (named
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
475
CER) and a SnO2 powder prepared in the laboratory (named LAB) have been used. The latter has been prepared with the procedure generally used for gas-sensor materials: hydrolysis with NH3 of SnCl4 solution, followed by washing, drying at room temperature (RT) and calcination at 200°C. Pd(II)/SnO2 systems were prepared by impregnation with a water solution of Pd(NO3)2, hydrated to obtain a Pd content of 0.36 wt.% and dried at 200°C (named Pd-CER(200) and Pd-LAB(200), respectively). For spectroscopic measurements, samples in powder or pressed in self-supporting discs were pretreated in air at 850°C for 1 h (named CER(850), Pd-CER(850) and LAB(850), Pd-LAB(850), respectively). For UV–Vis spectra, samples in powder were placed in a UV–Vis cell working in vacuo or in controlled atmospheres, fitted on a Varian Cary 5 spectrophotometer equipped with a diffuse reflectance sphere. Powders were submitted to an alternate outgassing – oxidizing (in dry oxygen) treatment up to 650°C, then cooled in dry air to RT. All UV–Vis spectra were run at RT. For FT-IR measurements, samples in self-supporting discs were placed into a commercial heatable Aabspec cell working in vacuo or in controlled atmospheres, fitted on a Perkin–Elmer 2000 FT-IR spectrophotometer with MCT detector. Samples were initially submitted to an alternate outgassing – oxidizing treatment at increasing temperatures up to 650°C, then cooled to the experimental temperature (350 or 450°C) and alternatively contacted with wet air (50% RH) and 1000 ppm CH4 in wet air. When wet air and CH4/wet air effects were studied, at each examined temperature an IR spectrum was run in 5–8 mbar of wet air, then after 5 min contact with 5–8 mbar of CH4/wet air. For electrical measurements, pastes were prepared from the above powders using only organic vehicle; the pastes were printed on 96% alumina substrates provided with an adequate heater element and gold conductor electrodes. The samples were then fired for 1 h at 850°C in air. The sizes of the deposited layers were 8 mm long, 5 mm wide and 20 mm thick. The impedance measurements were performed by using a Solartron SI1260 impedance analyzer in the frequency range 10 Hz – 10 MHz. Transmission electron microscope (TEM) images were obtained with a Hitachi H-800 at 200 kV operating voltage. Specific surface area measurements were performed with a traditional BET volumetric apparatus.
cific surface area (SBET) or particle size (D) of the commercial powder. On the contrary, calcination temperature had a significant influence on SBET and particle size of laboratory samples; furthermore, for these samples, addition of Pd(II) slightly lowers the sintering process of SnO2 particles. This is reassuring, as it has been demonstrated that crystallite sizes that are low in comparison with the space-charge depth strongly affect the sensitivity of pure and doped tin oxide elements [4]. In Fig. 1(a) and (b), the micrographs of the CER(850) and LAB(850) samples are reported, respectively, to show their different microstructure. As can be seen, the CER sample shows micrographs consisting of a threedimensional network of particles with sharp borders and inhomogeneous in size, while the LAB sample exhibits particles with indented borders, very homogeneous in size, forming densely packed aggregates: the CER particles range from 20 to 200 nm, while the average diameter of the LAB(850) particles is 30 nm. As far as the electron diffraction pattern is concerned, three main reflexes were found for both CER and LAB materials. The corresponding plane distances are only a few per cent larger than those for the three main reflexes of SnO2 with rutile structure reported in the literature (JCPDS 5–0467), i.e. for 110, 101 and 211 planes. No PdO or Pd segregated particles could be found for the Pd-CER(850) sample examined by TEM. This is consistent with a very high dispersion of palladium on the surface.
3. Results and discussion
a
3.2. UV-Vis and FT-IR spectra of materials in dry air or wet air First of all, it is interesting to compare the UV–Vis reflectance and the FT-IR absorbance spectra of the four materials, pure and supporting palladium. Table 1 Texture and morphology of materials Samples
SBET a (m2 g−1)
No. of Pd atomsb (nm−2)
Dc (nm)
CERd CER(800) Pd-CER(200) Pd-CER(850) LAB(200) LAB(800) Pd-LAB (200) Pd-LAB (800)
9 9 9 9 120 10 146 18
— — 2.52 2.52 — — 0.155 1.25
20–200 20–200 20–200 20–200 * 30 * *
BET specific surface area. Assuming that all the Pd atoms are uniformly dispersed on the SnO2 surface. c Particle size for CER sample is a size range, for LAB sample is an average size. d As purchased. * Measurements not performed. b
3.1. Texture and morphology of materials Table 1 reports the pertinent data. The temperature and impregnation had no influence on either the spe-
476
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
Fig. 1. TEM images of: (a) CER(850) sample (80 000 ×); (b) LAB(850) sample (150 000 ×).
3.2.1. Comparison of UV– Vis spectra of materials The CER(850) sample, in dry air at RT, shows a steep absorption edge characteristic of a direct valence band–conduction band (v.b. – c.b.) transition at 3.7 eV (27 423 cm − 1), a value in good agreement with data reported in the literature for SnO2 thin films [5], while
LAB(850) shows strong differences, both in shape and position of the v.b.–c.b. absorption edge, now less steep and at a lower frequency, already interpreted [6] as due to a seriously perturbed crystallographic order in the surface and subsurface layers of the LAB particles. The same differences are shown by Pd-CER(850) and
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
477
Pd-LAB(850) spectra (see Fig. 2). However, in the Pd-CER spectrum, a band at about 22 000 cm − 1, assigned to a Pd(II) – O charge transfer transition, is evident. On the contrary, in the Pd-LAB(850) spectrum, it is difficult to distinguish the eventual presence of this transition, owing to the absorption edge position and shape.
3.2.2. Comparison of FT-IR spectra The different spectral features shown by the two pure materials in wet air at 350°C are reported in Fig. 3(a) (curves 1 and 2, respectively). Looking at the expected shape of a metal oxide spectrum, the stoichiometric tin oxide should be transparent in the medium- and nearinfrared region; in particular, we should expect no absorption at n\900 cm − 1 where the high-frequency absorption edge of the longitudinal optical fundamental vibration falls. However, since we are dealing with particles of small size compared with the l of the radiation, we will expect a Rayleigh scattering from them. Furthermore, the tin oxide under the examined conditions is not stoichiometric, mainly by oxygen vacancies, and therefore we will expect scattering from free electrons in the c.b. and/or electronic transition, mainly from V + 0 (or from similar shallow levels in the band gap) to the c.b. The shape of the CER(850) spectrum (Fig. 3(b), curve 1) suggests the presence of three main contributions: (i) the absorption edge of the fundamental optical modes (curve 2); (ii) the scattering from free electrons increasing with the wavelength increase (curve 3, partially superimposed to the fundamental absorption edge); and (iii) the Rayleigh scattering from particles (curve 4; actually, the Rayleigh law, in the region of the spectrum where it prevails, is only approximately followed). No evident contribution of electronic transition from shallow levels in the band gap to the c.b. is evident, even if it could be present, Fig. 3. FT-IR spectra of pure materials in wet air at 350°C. Section (a): curve 1, CER(850); curve 2, LAB(850). Sections (b) and (c): a proposal for resolution of CER(850) and LAB(850) spectra, respectively. Curve 1, real spectra; curve 2, the absorption edge of the SnO2 fundamental optical modes; curve 3, the scattering from free electrons; curve 4, the Rayleigh scattering; curve 5, broad absorption due to electronic transitions from shallow levels in the gap to the c.b.
Fig. 2. UV – Vis – NIR diffuse reflectance spectra in dry air at RT of: Pd-CER(850), curve 1; and Pd-LAB(850), curve 2.
hidden by the other components. The LAB(850) spectrum (Fig. 3(c), curve 1) is mainly due to the superimposition of: (i) the absorption edge of the fundamental optical modes (curve 2), partially superimposed to (ii) a very intense and broad absorption due to electronic transitions from the shallow levels in the gap to the c.b. (curve 5; its shape is drawn on the basis of previous studies [6–8]); and (iii) the Rayleigh scattering from
478
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
particles (curve 4). A contribution due to the scattering from free electrons cannot be excluded, even if lower than for the CER sample, as suggested in curve 3. The weak and sharp bands at 1350 – 1200 cm − 1 are vibrations of surface carbonate-like species, originated by contact of powders with the atmospheric CO2 by cooling down to RT after the calcination at 850°C and only partially desorbed during the treatment in controlled atmosphere, described in Section 2. The fits of the IR spectra with the different components (curves 2 – 5) are, for the moment, drawn only in a qualitative way. The reason for the observed differences can be understood in the different morphology and texture of the particles of the CER and LAB materials: the presence of intrinsic surface defects, such as oxygen vacancies, oxygen divacancies able to trap electrons, and Sn2 + ions, is surely higher for the materials showing a larger surface area and smaller and badly defined crystallites with indented borders, i.e. the LAB materials. The spectroscopic results both IR and UV – Vis are consistent between them and with the resistivity values of the two pure oxides (see Section 3.4). The different spectral features shown by the two palladium-supporting materials in wet air at 350°C are reported in Fig. 4(a) (curves 1 and 2, respectively). The qualitative difference between Pd-CER and Pd-LAB spectra is of the same type shown by pure materials: the scattering from free electrons and the Rayleigh scattering being the predominant contributions to the PdCER spectrum, a broad electronic absorption and again the Rayleigh scattering being the predominant contributions to the Pd-LAB spectrum. However, the presence of palladium has strongly affected the spectrum of both CER and LAB materials, as evident in Fig. 4(b) where CER(850) and Pd-CER(850) spectra (curves 1 and 2, respectively) in wet air at 350°C are shown, and in Fig. 4(c) where LAB(850) and PdLAB(850) spectra (curves 1 and 2, respectively) in wet air at 350°C are reported. Looking at CER materials, the presence of palladium decreases the free electron scattering and increases the Rayleigh scattering, if the spectrum is considered essentially as the sum of the three contributions listed before for pure material. Looking at LAB materials, the presence of palladium decreases the electronic absorption, increases the Rayleigh scattering and shows a less steep high-frequency edge of the longitudinal fundamental vibration, if the spectrum is considered essentially as the sum of the three contributions listed before for pure material. A reasonable explication of these changes could be the following. On the basis of the thermodynamic data, the palladium stable phase [3], is PdO both on Pd-CER and Pd-LAB materials in wet air at 350°C. The decrease of the scattering from free electrons, passing from CER to Pd-CER, and of the broad absorption of electronic nature, passing from LAB to Pd-LAB spectrum, is
consistent with a strongly electron-depleted spacecharge layer inside the SnO2, under oxidizing conditions, as proposed by Yamazoe [1]. The increase of the Rayleigh scattering for both Pd-supporting materials is not easy to explain. Looking at the factors influencing the scattering from the particles, they are the volumes of the particles and the refraction indexes of the material and of the atmosphere; as the atmosphere (wet air) does not change, only the particle size and the refraction indexes of the material can be taken into consideration. As for the particle size, CER seems not to be affected by the impregnation with Pd, and Pd-LAB
Fig. 4. Section (a): FT-IR spectra of Pd-added materials in wet air at 350°C. Curve 1, Pd-CER(850); curve 2, Pd-LAB(850). Section (b): comparison between CER(850) and Pd-CER(850) spectra in wet air at 350°C (curves 1 and 2, respectively). Section (c): comparison between LAB(850) and Pd-LAB(850) spectra in wet air at 350°C (curves 1 and 2, respectively).
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
479
Fig. 5. FT-IR absorption changes observed by 1000 ppm CH4 in wet-air interaction on: CER(850), section (a); Pd-CER(850), section (b); LAB(850), section (c); and Pd-LAB(850), section (d). Curve 1, interaction at 350°C; curve 2, interaction at 450°C. Each spectrum is reported as difference from the spectrum obtained in wet air only.
should eventually show smaller particles than LAB (see the surface area in Table 1). Thus, the only parameter that in our opinion could enhance the scattering should be an increase of the refraction index of the surface material caused by the presence of a well dispersed PdO phase. Eventually, the broadness of the high-frequency side of the SnO2 fundamental vibration observed on the Pd-LAB sample could be due to Pd – O – Sn and Pd–O– Pd surface vibration modes. We would expect a similar broadness also for the Pd-CER material. However, in this case the phenomenon, even if present, is not directly evident, partially hidden by the decrease of the free electron scattering that, in fact, could be much more pronounced than appears. The spectra of the four materials in wet air at 450°C are very similar and can be discussed on the basis of the previous arguments.
3.3. FT-IR spectra of materials passing from wet air to methane/wet air In Fig. 5, the changes in the IR absorption caused by methane interaction are summarized. To be clear, we notice that the curves reported in this figure are not the spectra of the samples under methane/wet air atmosphere, but the differences between such spectra and those registered in wet air atmosphere (some of which were previously reported). Focusing our attention on the results obtained for CER(850), the effect on the IR spectrum passing from wet air to methane/wet air is a transmission loss caused by an increase of the scattering
of the radiation by free electrons, almost identical at 350 and 450°C (see Fig. 5(a)). Fig. 5(b) shows the results for Pd-CER(850) in a similar experiment; also in this case, there is an increase in the radiation scattering by free electrons after contact with methane, almost identical for the two temperatures. However, in this case another spectroscopic feature (see the grey area in Fig. 5(b) and compare the different shape of the curves in this region with that of the curves in section (a)) is particularly evident, revealing a simultaneous erosion of the high-frequency side of the SnO2 fundamental vibration: this erosion is higher at 450 than at 350°C and could be due to the partial destruction of Pd–O–Sn or Pd–O–Pd surface vibration modes as a consequence of the PdO reduction. Fig. 5(c) and (d) shows the results obtained for LAB(850) and Pd-LAB(850) samples, respectively: the effect on the IR spectrum passing from wet air to methane/wet air is a transmission loss caused by an increase of the scattering of the radiation by free electrons and the growth of the broad absorption previously assigned [6–8] to electronic transitions involving oxygen vacancies. For these samples, unlike CER and Pd-CER samples, there is a sensible influence of the temperature on the intensity of the electronic absorption and of free electron scattered radiation. In particular, the LAB sample shows a higher increase in the intensity of both spectral features at 450°C, while PdLAB does so at 350°C. For the Pd-LAB sample both at 350 and 450°C, the erosion of the high-frequency side of the SnO2 fundamental vibration due to the PdO reduction is not directly observed, but should be
480
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
Fig. 6. Nyquist diagrams for the samples: CER(850), section (a); Pd-CER(850), section (b); LAB(850), section (c); and Pd-LAB(850), section (d).
present, partially hiding the increase of the free electron scattering. As an increase in the scattering and in the electronic absorption parallels an increase in free and trapped electrons, we can roughly correlate the response of the materials, from a spectroscopic point of view, with the increase of the intensity of the two spectral features. On this basis, concerning the Pd effect on the pure tin oxide, it makes the Pd-LAB at 350°C more sensitive than the LAB sample (compare curve 1 of Fig. 4(c) and (d)); the same does not happen at 450°C (compare curve 2 of Fig. 4(c) and (d)). Furthermore, it makes the Pd-LAB sample more sensitive at 350 than at 450°C; the same does not happen for Pd-CER. Concerning CER and Pd-CER, both at 350 and 450°C, they show a similar response.
3.4. Electrical measurements The impedance measurements in wet air (50% RH) or CH4/wet air were performed at 350 and 450°C; these temperatures have been chosen because the SnO2 thick films show the highest response to methane at 350 and 450°C with and without Pd as catalyst, respectively. The Nyquist diagrams were obtained considering an
equivalent circuit composed of a resistor and a capacitor connected in parallel mode. The resistance and the capacitance, not reported here, were measured together with the real and the imaginary part of the impedance; as has been recalled in a previous paper [10], the resistance and the capacitance behave as the imaginary and the real part of the impedance, respectively. According to the Kronig–Kramer theorem, the observed trend of the resistance and the capacitance variation could mean that the two quantities are correlated and caused by the same mechanism. In Fig. 6(a) and (c), we report the Nyquist diagram of the CER(850) and LAB(850) samples in wet air with and without methane (1000 ppm). It can be seen that the resistivity of the LAB sample is higher than that of the CER one, in agreement with the interpretation suggested in a previous paragraph about the shape of their FT-IR spectra: in fact, the CER spectrum is characterized by a strong contribution due to free electron scattering while the LAB spectrum exhibits a large absorption due to the photo-ionization of electrons from localized states to the c.b. Moreover, the UV-Vis measurements proved that the LAB sample shows surface and subsurface defects able to localize the electrons in higher amounts than the CER sample.
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
The resistance and capacitance dependence on frequency, responsible of the deviation from a circular shape, is more evident on the CER sample than on the LAB one. Since the change of the resistance versus frequency is evidence of the non-linearity between the applied field and the energy barrier, we expect a different behaviour for samples with different electron mobility. The frequency at which the dependence of the resistance and the capacitance disappears is a characteristic of each sample and has to be related to the grain size and the electron mobility [10]. Actually, the differences between the two powders, indicated by the spectroscopic measurements, are mainly in the ratio between electrons in localized levels and in the conduction band; furthermore, the grain sizes and shape are very different in the two cases. All these differences can probably be correlated with the electrical behaviour; however, this phenomenon is not yet clearly understood. The addition of palladium (see Fig. 6(b) and (d)) results in an increase of the resistivity in both CER and LAB samples; this result is in agreement with a depleted space-charge layer higher than in the pure samples, as proposed by Yamazoe [1] and in agreement with the interpretation of the sample FT-IR spectra. However, this effect is much more evident in the case of the Pd-LAB sample, probably due to the larger specific surface area and to the ultrafine homogeneously sized particles. The same sample also exhibits a large response to reducing agents (methane in this experiment); the response is, as expected [9], greater at 350 than at 450°C (see Fig. 6(d)). The response to methane after palladium addition becomes greater also for the Pd-CER sample, but it remains lower at 350 than at 450°C (see Fig. 6(b)), probably because the inhomogeneity of the particles does not allow an adequate distribution of the catalyst. The Pd-LAB sample shows a strong resistance and capacitance dependence on frequency, perhaps as a consequence of the surface states modification introduced by a homogeneous distribution of the catalyst; this phenomenon may be justified by an interaction of the free charge with the surface states, as proposed in a previous paper [10]. In Fig. 7(a) and (b), the behaviour of the imaginary part of the impedance is reported. The variations of the peak heights versus temperature and their modulation versus atmosphere are in agreement with the electrical conductances under the corresponding conditions. Moreover, the values of the peaks are localized in correspondence of only one frequency. It can be deduced that the frequency at which the dependence of the resistance and the capacitance disappears is a characteristic of each sample and has to be related to the grain size and the electron mobility [10], while it is independent of ambient conditions.
481
4. Conclusions TEM observations showed that: (i) the LAB samples exhibit particles homogeneous in size with an average diameter of approximately 30 nm with indented borders; (ii) the CER samples exhibit particles not homogeneous in size (diameter range between 20 and 200 nm) with sharp borders. In agreement, LAB samples show SBET higher than CER samples. All the results obtained by UV–Vis and FT-IR spectra are consistent with the texture and microstructure of the four materials and demonstrate that the different samples show a different ratio between the electron population of localized states and of the conduction band. Electrical measurements in wet air show that LAB samples have higher resistances than CER samples, and that Pd-LAB and Pd-CER samples have a higher resistance than LAB and CER ones, respectively. Even if the spectral phenomena cannot directly correspond with the electrical ones, our spectroscopic analy-
Fig. 7. The imaginary part of the impedance for the samples: PdCER(850), section (a); and Pd-LAB(850), section (b).
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
482
sis is qualitatively in good agreement with the electrical measurements, since, looking at each couple of compared materials, a major population of localized levels has been found for the more resistant one. Furthermore, the electrical measurements in wet air show that the effect of Pd on the LAB sample is much higher than on the CER sample, but this behaviour could not be inferred from the spectroscopic measurements. The electrical measurements clearly demonstrated that LAB samples exhibit a much more satisfactory response to methane. Moreover, the Pd decreases the maximum response temperature of the LAB material. As for the FT-IR changes passing from wet air to methane/wet air, they revealed, at least qualitatively in agreement with the electrical ones, that Pd decreases the maximum response temperature of the LAB material. Again qualitatively in agreement with the electrical measurements, FT-IR changes revealed that Pd makes Pd-LAB more sensitive than the LAB sample at 350°C. This agreement is not found at 450°C. On the contrary, the FT-IR analysis of changes in CER and Pd-CER, when passing from wet air to methane/wet air, could not be directly compared with the electrical ones. The Nyquist diagrams show a higher dependence of the resistance and capacitance on frequency for CER and Pd-LAB samples than for Pd-CER and LAB ones, respectively. We suggest that this effect must be related to the different electron mobilities and to the sample sizes. At the same time, it has been demonstrated by the optical measurements that different samples present different electron energy states: the expected consequence is an electron mobility characteristic of each
.
kind of sample. Although this phenomenon requires deeper investigation, we suggest the electron mobility as a proper parameter to correlate the electrical and the optical absorption measurements.
References [1] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Actuators B 5 (1991) 7 – 19. [2] J.F. Geiger, K.D. Schierbaum, W. Go¨pel, Surface spectroscopic studies on Pd-doped SnO2, Vacuum 41 (1990) 1629 – 1632. [3] J.G. Duh, J.W. Jou, B.S. Chiou, Catalytic and gas sensing characteristics in Pd-doped SnO2, J. Electrochem. Soc. 136 (1989) 2740 – 2747. [4] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Relationship between gas sensitivity and microstructure of porous SnO2, Denki Kagaku (Chemical Sensors) 58 (1990) 1143 – 1148. [5] W. Spence, The UV absorption edge of tin oxide thin films, J. Appl. Phys. 38 (1967) 3767 – 3770. [6] G. Ghiotti, A. Chiorino, P.W. Xiong, L. Marchese, A comparative study of SnO2 and CrOx /SnO2 powders: their microstructures and their different response to CO, NO, H2, and CH4 atmospheres, Sensors and Actuators B 7 (1992) 691 – 699. [7] G. Ghiotti, A. Chiorino, P.W. Xiong, Surface chemistry and electronic effects of H2(D2) on pure SnO2 and Cr-doped SnO2, Sensors and Actuators B 15-16 (1993) 367 – 371. [8] G. Ghiotti, A. Chiorino, G. Martinelli, M.C. Carotta, Moisture effects on pure and Pd-doped SnO2 thick films analysed by FTIR spectroscopy and conductance measurements, Sensors and Actuators B 24-25 (1995) 520 – 524. [9] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, B.C. Tofield, Tin dioxide gas sensors. Part 2. The role of surface additives, J. Chem. Soc., Faraday Trans. 1 84 (1988) 441–457. [10] G. Martinelli, M.C. Carotta, L. Passari, L. Tracchi, A study of the moisture effects on SnO2 thick films by sensitivity and permittivity measurements, Sensors and Actuators B 26-27 (1995) 53 – 55.
Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples A. Chiorino a, G. Ghiotti a,*, F. Prinetto a, M.C. Carotta b, G. Martinelli b, M. Merli b a
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Uni6ersita` di Torino, Via Pietro Giuria 7, I-10125 Turin, Italy b INFM-Dipartimento di Fisica, Uni6ersita` di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy Received 3 June 1997; accepted 6 June 1997
Abstract The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C. © 1997 Elsevier Science S.A. Keywords: Gas sensors; SnO2 films; Methane sensing
1. Introduction It is well known that the addition of small amounts of noble metals to tin oxide can promote the gas sensitivity and the response rate of the sensing elements to inflammable gases. The aim of this work is a comparison of the microstructural properties, surface reactivity and sensitivity of a series of SnO2-based thick films. We have examined one laboratory-prepared and one commercial SnO2 powder, both pure and with addition of palladium. Concerning Pd, it has been proposed [1] that the promotion mechanism is electronic, i.e. Pd increases the work function of the SnO2
under oxidizing conditions, forming a stable oxide capable of producing a strongly electron-depleted spacecharge layer inside the SnO2, while the work function shift completely disappears when samples are exposed to reducing atmospheres. The fast oxidation of Pd(0), supported on tin oxide single crystal, to PdO in O2 (pE5 mbar) has been demonstrated by Geiger et al. [2] starting from 400°C, while at lower temperatures the oxidation reaction is too slow, even if the PdO phase should be the stable thermodynamic phase [3]. We have used a preparation method warranting the presence of Pd(II) on the starting materials. 2. Experimental
* Corresponding author. [email protected].
Fax:
+39
11
6707855;
e-mail:
0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 3 8 - 4
A commercial SnO2 powder from CERAC (named
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
475
CER) and a SnO2 powder prepared in the laboratory (named LAB) have been used. The latter has been prepared with the procedure generally used for gas-sensor materials: hydrolysis with NH3 of SnCl4 solution, followed by washing, drying at room temperature (RT) and calcination at 200°C. Pd(II)/SnO2 systems were prepared by impregnation with a water solution of Pd(NO3)2, hydrated to obtain a Pd content of 0.36 wt.% and dried at 200°C (named Pd-CER(200) and Pd-LAB(200), respectively). For spectroscopic measurements, samples in powder or pressed in self-supporting discs were pretreated in air at 850°C for 1 h (named CER(850), Pd-CER(850) and LAB(850), Pd-LAB(850), respectively). For UV–Vis spectra, samples in powder were placed in a UV–Vis cell working in vacuo or in controlled atmospheres, fitted on a Varian Cary 5 spectrophotometer equipped with a diffuse reflectance sphere. Powders were submitted to an alternate outgassing – oxidizing (in dry oxygen) treatment up to 650°C, then cooled in dry air to RT. All UV–Vis spectra were run at RT. For FT-IR measurements, samples in self-supporting discs were placed into a commercial heatable Aabspec cell working in vacuo or in controlled atmospheres, fitted on a Perkin–Elmer 2000 FT-IR spectrophotometer with MCT detector. Samples were initially submitted to an alternate outgassing – oxidizing treatment at increasing temperatures up to 650°C, then cooled to the experimental temperature (350 or 450°C) and alternatively contacted with wet air (50% RH) and 1000 ppm CH4 in wet air. When wet air and CH4/wet air effects were studied, at each examined temperature an IR spectrum was run in 5–8 mbar of wet air, then after 5 min contact with 5–8 mbar of CH4/wet air. For electrical measurements, pastes were prepared from the above powders using only organic vehicle; the pastes were printed on 96% alumina substrates provided with an adequate heater element and gold conductor electrodes. The samples were then fired for 1 h at 850°C in air. The sizes of the deposited layers were 8 mm long, 5 mm wide and 20 mm thick. The impedance measurements were performed by using a Solartron SI1260 impedance analyzer in the frequency range 10 Hz – 10 MHz. Transmission electron microscope (TEM) images were obtained with a Hitachi H-800 at 200 kV operating voltage. Specific surface area measurements were performed with a traditional BET volumetric apparatus.
cific surface area (SBET) or particle size (D) of the commercial powder. On the contrary, calcination temperature had a significant influence on SBET and particle size of laboratory samples; furthermore, for these samples, addition of Pd(II) slightly lowers the sintering process of SnO2 particles. This is reassuring, as it has been demonstrated that crystallite sizes that are low in comparison with the space-charge depth strongly affect the sensitivity of pure and doped tin oxide elements [4]. In Fig. 1(a) and (b), the micrographs of the CER(850) and LAB(850) samples are reported, respectively, to show their different microstructure. As can be seen, the CER sample shows micrographs consisting of a threedimensional network of particles with sharp borders and inhomogeneous in size, while the LAB sample exhibits particles with indented borders, very homogeneous in size, forming densely packed aggregates: the CER particles range from 20 to 200 nm, while the average diameter of the LAB(850) particles is 30 nm. As far as the electron diffraction pattern is concerned, three main reflexes were found for both CER and LAB materials. The corresponding plane distances are only a few per cent larger than those for the three main reflexes of SnO2 with rutile structure reported in the literature (JCPDS 5–0467), i.e. for 110, 101 and 211 planes. No PdO or Pd segregated particles could be found for the Pd-CER(850) sample examined by TEM. This is consistent with a very high dispersion of palladium on the surface.
3. Results and discussion
a
3.2. UV-Vis and FT-IR spectra of materials in dry air or wet air First of all, it is interesting to compare the UV–Vis reflectance and the FT-IR absorbance spectra of the four materials, pure and supporting palladium. Table 1 Texture and morphology of materials Samples
SBET a (m2 g−1)
No. of Pd atomsb (nm−2)
Dc (nm)
CERd CER(800) Pd-CER(200) Pd-CER(850) LAB(200) LAB(800) Pd-LAB (200) Pd-LAB (800)
9 9 9 9 120 10 146 18
— — 2.52 2.52 — — 0.155 1.25
20–200 20–200 20–200 20–200 * 30 * *
BET specific surface area. Assuming that all the Pd atoms are uniformly dispersed on the SnO2 surface. c Particle size for CER sample is a size range, for LAB sample is an average size. d As purchased. * Measurements not performed. b
3.1. Texture and morphology of materials Table 1 reports the pertinent data. The temperature and impregnation had no influence on either the spe-
476
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
Fig. 1. TEM images of: (a) CER(850) sample (80 000 ×); (b) LAB(850) sample (150 000 ×).
3.2.1. Comparison of UV– Vis spectra of materials The CER(850) sample, in dry air at RT, shows a steep absorption edge characteristic of a direct valence band–conduction band (v.b. – c.b.) transition at 3.7 eV (27 423 cm − 1), a value in good agreement with data reported in the literature for SnO2 thin films [5], while
LAB(850) shows strong differences, both in shape and position of the v.b.–c.b. absorption edge, now less steep and at a lower frequency, already interpreted [6] as due to a seriously perturbed crystallographic order in the surface and subsurface layers of the LAB particles. The same differences are shown by Pd-CER(850) and
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
477
Pd-LAB(850) spectra (see Fig. 2). However, in the Pd-CER spectrum, a band at about 22 000 cm − 1, assigned to a Pd(II) – O charge transfer transition, is evident. On the contrary, in the Pd-LAB(850) spectrum, it is difficult to distinguish the eventual presence of this transition, owing to the absorption edge position and shape.
3.2.2. Comparison of FT-IR spectra The different spectral features shown by the two pure materials in wet air at 350°C are reported in Fig. 3(a) (curves 1 and 2, respectively). Looking at the expected shape of a metal oxide spectrum, the stoichiometric tin oxide should be transparent in the medium- and nearinfrared region; in particular, we should expect no absorption at n\900 cm − 1 where the high-frequency absorption edge of the longitudinal optical fundamental vibration falls. However, since we are dealing with particles of small size compared with the l of the radiation, we will expect a Rayleigh scattering from them. Furthermore, the tin oxide under the examined conditions is not stoichiometric, mainly by oxygen vacancies, and therefore we will expect scattering from free electrons in the c.b. and/or electronic transition, mainly from V + 0 (or from similar shallow levels in the band gap) to the c.b. The shape of the CER(850) spectrum (Fig. 3(b), curve 1) suggests the presence of three main contributions: (i) the absorption edge of the fundamental optical modes (curve 2); (ii) the scattering from free electrons increasing with the wavelength increase (curve 3, partially superimposed to the fundamental absorption edge); and (iii) the Rayleigh scattering from particles (curve 4; actually, the Rayleigh law, in the region of the spectrum where it prevails, is only approximately followed). No evident contribution of electronic transition from shallow levels in the band gap to the c.b. is evident, even if it could be present, Fig. 3. FT-IR spectra of pure materials in wet air at 350°C. Section (a): curve 1, CER(850); curve 2, LAB(850). Sections (b) and (c): a proposal for resolution of CER(850) and LAB(850) spectra, respectively. Curve 1, real spectra; curve 2, the absorption edge of the SnO2 fundamental optical modes; curve 3, the scattering from free electrons; curve 4, the Rayleigh scattering; curve 5, broad absorption due to electronic transitions from shallow levels in the gap to the c.b.
Fig. 2. UV – Vis – NIR diffuse reflectance spectra in dry air at RT of: Pd-CER(850), curve 1; and Pd-LAB(850), curve 2.
hidden by the other components. The LAB(850) spectrum (Fig. 3(c), curve 1) is mainly due to the superimposition of: (i) the absorption edge of the fundamental optical modes (curve 2), partially superimposed to (ii) a very intense and broad absorption due to electronic transitions from the shallow levels in the gap to the c.b. (curve 5; its shape is drawn on the basis of previous studies [6–8]); and (iii) the Rayleigh scattering from
478
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
particles (curve 4). A contribution due to the scattering from free electrons cannot be excluded, even if lower than for the CER sample, as suggested in curve 3. The weak and sharp bands at 1350 – 1200 cm − 1 are vibrations of surface carbonate-like species, originated by contact of powders with the atmospheric CO2 by cooling down to RT after the calcination at 850°C and only partially desorbed during the treatment in controlled atmosphere, described in Section 2. The fits of the IR spectra with the different components (curves 2 – 5) are, for the moment, drawn only in a qualitative way. The reason for the observed differences can be understood in the different morphology and texture of the particles of the CER and LAB materials: the presence of intrinsic surface defects, such as oxygen vacancies, oxygen divacancies able to trap electrons, and Sn2 + ions, is surely higher for the materials showing a larger surface area and smaller and badly defined crystallites with indented borders, i.e. the LAB materials. The spectroscopic results both IR and UV – Vis are consistent between them and with the resistivity values of the two pure oxides (see Section 3.4). The different spectral features shown by the two palladium-supporting materials in wet air at 350°C are reported in Fig. 4(a) (curves 1 and 2, respectively). The qualitative difference between Pd-CER and Pd-LAB spectra is of the same type shown by pure materials: the scattering from free electrons and the Rayleigh scattering being the predominant contributions to the PdCER spectrum, a broad electronic absorption and again the Rayleigh scattering being the predominant contributions to the Pd-LAB spectrum. However, the presence of palladium has strongly affected the spectrum of both CER and LAB materials, as evident in Fig. 4(b) where CER(850) and Pd-CER(850) spectra (curves 1 and 2, respectively) in wet air at 350°C are shown, and in Fig. 4(c) where LAB(850) and PdLAB(850) spectra (curves 1 and 2, respectively) in wet air at 350°C are reported. Looking at CER materials, the presence of palladium decreases the free electron scattering and increases the Rayleigh scattering, if the spectrum is considered essentially as the sum of the three contributions listed before for pure material. Looking at LAB materials, the presence of palladium decreases the electronic absorption, increases the Rayleigh scattering and shows a less steep high-frequency edge of the longitudinal fundamental vibration, if the spectrum is considered essentially as the sum of the three contributions listed before for pure material. A reasonable explication of these changes could be the following. On the basis of the thermodynamic data, the palladium stable phase [3], is PdO both on Pd-CER and Pd-LAB materials in wet air at 350°C. The decrease of the scattering from free electrons, passing from CER to Pd-CER, and of the broad absorption of electronic nature, passing from LAB to Pd-LAB spectrum, is
consistent with a strongly electron-depleted spacecharge layer inside the SnO2, under oxidizing conditions, as proposed by Yamazoe [1]. The increase of the Rayleigh scattering for both Pd-supporting materials is not easy to explain. Looking at the factors influencing the scattering from the particles, they are the volumes of the particles and the refraction indexes of the material and of the atmosphere; as the atmosphere (wet air) does not change, only the particle size and the refraction indexes of the material can be taken into consideration. As for the particle size, CER seems not to be affected by the impregnation with Pd, and Pd-LAB
Fig. 4. Section (a): FT-IR spectra of Pd-added materials in wet air at 350°C. Curve 1, Pd-CER(850); curve 2, Pd-LAB(850). Section (b): comparison between CER(850) and Pd-CER(850) spectra in wet air at 350°C (curves 1 and 2, respectively). Section (c): comparison between LAB(850) and Pd-LAB(850) spectra in wet air at 350°C (curves 1 and 2, respectively).
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
479
Fig. 5. FT-IR absorption changes observed by 1000 ppm CH4 in wet-air interaction on: CER(850), section (a); Pd-CER(850), section (b); LAB(850), section (c); and Pd-LAB(850), section (d). Curve 1, interaction at 350°C; curve 2, interaction at 450°C. Each spectrum is reported as difference from the spectrum obtained in wet air only.
should eventually show smaller particles than LAB (see the surface area in Table 1). Thus, the only parameter that in our opinion could enhance the scattering should be an increase of the refraction index of the surface material caused by the presence of a well dispersed PdO phase. Eventually, the broadness of the high-frequency side of the SnO2 fundamental vibration observed on the Pd-LAB sample could be due to Pd – O – Sn and Pd–O– Pd surface vibration modes. We would expect a similar broadness also for the Pd-CER material. However, in this case the phenomenon, even if present, is not directly evident, partially hidden by the decrease of the free electron scattering that, in fact, could be much more pronounced than appears. The spectra of the four materials in wet air at 450°C are very similar and can be discussed on the basis of the previous arguments.
3.3. FT-IR spectra of materials passing from wet air to methane/wet air In Fig. 5, the changes in the IR absorption caused by methane interaction are summarized. To be clear, we notice that the curves reported in this figure are not the spectra of the samples under methane/wet air atmosphere, but the differences between such spectra and those registered in wet air atmosphere (some of which were previously reported). Focusing our attention on the results obtained for CER(850), the effect on the IR spectrum passing from wet air to methane/wet air is a transmission loss caused by an increase of the scattering
of the radiation by free electrons, almost identical at 350 and 450°C (see Fig. 5(a)). Fig. 5(b) shows the results for Pd-CER(850) in a similar experiment; also in this case, there is an increase in the radiation scattering by free electrons after contact with methane, almost identical for the two temperatures. However, in this case another spectroscopic feature (see the grey area in Fig. 5(b) and compare the different shape of the curves in this region with that of the curves in section (a)) is particularly evident, revealing a simultaneous erosion of the high-frequency side of the SnO2 fundamental vibration: this erosion is higher at 450 than at 350°C and could be due to the partial destruction of Pd–O–Sn or Pd–O–Pd surface vibration modes as a consequence of the PdO reduction. Fig. 5(c) and (d) shows the results obtained for LAB(850) and Pd-LAB(850) samples, respectively: the effect on the IR spectrum passing from wet air to methane/wet air is a transmission loss caused by an increase of the scattering of the radiation by free electrons and the growth of the broad absorption previously assigned [6–8] to electronic transitions involving oxygen vacancies. For these samples, unlike CER and Pd-CER samples, there is a sensible influence of the temperature on the intensity of the electronic absorption and of free electron scattered radiation. In particular, the LAB sample shows a higher increase in the intensity of both spectral features at 450°C, while PdLAB does so at 350°C. For the Pd-LAB sample both at 350 and 450°C, the erosion of the high-frequency side of the SnO2 fundamental vibration due to the PdO reduction is not directly observed, but should be
480
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
Fig. 6. Nyquist diagrams for the samples: CER(850), section (a); Pd-CER(850), section (b); LAB(850), section (c); and Pd-LAB(850), section (d).
present, partially hiding the increase of the free electron scattering. As an increase in the scattering and in the electronic absorption parallels an increase in free and trapped electrons, we can roughly correlate the response of the materials, from a spectroscopic point of view, with the increase of the intensity of the two spectral features. On this basis, concerning the Pd effect on the pure tin oxide, it makes the Pd-LAB at 350°C more sensitive than the LAB sample (compare curve 1 of Fig. 4(c) and (d)); the same does not happen at 450°C (compare curve 2 of Fig. 4(c) and (d)). Furthermore, it makes the Pd-LAB sample more sensitive at 350 than at 450°C; the same does not happen for Pd-CER. Concerning CER and Pd-CER, both at 350 and 450°C, they show a similar response.
3.4. Electrical measurements The impedance measurements in wet air (50% RH) or CH4/wet air were performed at 350 and 450°C; these temperatures have been chosen because the SnO2 thick films show the highest response to methane at 350 and 450°C with and without Pd as catalyst, respectively. The Nyquist diagrams were obtained considering an
equivalent circuit composed of a resistor and a capacitor connected in parallel mode. The resistance and the capacitance, not reported here, were measured together with the real and the imaginary part of the impedance; as has been recalled in a previous paper [10], the resistance and the capacitance behave as the imaginary and the real part of the impedance, respectively. According to the Kronig–Kramer theorem, the observed trend of the resistance and the capacitance variation could mean that the two quantities are correlated and caused by the same mechanism. In Fig. 6(a) and (c), we report the Nyquist diagram of the CER(850) and LAB(850) samples in wet air with and without methane (1000 ppm). It can be seen that the resistivity of the LAB sample is higher than that of the CER one, in agreement with the interpretation suggested in a previous paragraph about the shape of their FT-IR spectra: in fact, the CER spectrum is characterized by a strong contribution due to free electron scattering while the LAB spectrum exhibits a large absorption due to the photo-ionization of electrons from localized states to the c.b. Moreover, the UV-Vis measurements proved that the LAB sample shows surface and subsurface defects able to localize the electrons in higher amounts than the CER sample.
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
The resistance and capacitance dependence on frequency, responsible of the deviation from a circular shape, is more evident on the CER sample than on the LAB one. Since the change of the resistance versus frequency is evidence of the non-linearity between the applied field and the energy barrier, we expect a different behaviour for samples with different electron mobility. The frequency at which the dependence of the resistance and the capacitance disappears is a characteristic of each sample and has to be related to the grain size and the electron mobility [10]. Actually, the differences between the two powders, indicated by the spectroscopic measurements, are mainly in the ratio between electrons in localized levels and in the conduction band; furthermore, the grain sizes and shape are very different in the two cases. All these differences can probably be correlated with the electrical behaviour; however, this phenomenon is not yet clearly understood. The addition of palladium (see Fig. 6(b) and (d)) results in an increase of the resistivity in both CER and LAB samples; this result is in agreement with a depleted space-charge layer higher than in the pure samples, as proposed by Yamazoe [1] and in agreement with the interpretation of the sample FT-IR spectra. However, this effect is much more evident in the case of the Pd-LAB sample, probably due to the larger specific surface area and to the ultrafine homogeneously sized particles. The same sample also exhibits a large response to reducing agents (methane in this experiment); the response is, as expected [9], greater at 350 than at 450°C (see Fig. 6(d)). The response to methane after palladium addition becomes greater also for the Pd-CER sample, but it remains lower at 350 than at 450°C (see Fig. 6(b)), probably because the inhomogeneity of the particles does not allow an adequate distribution of the catalyst. The Pd-LAB sample shows a strong resistance and capacitance dependence on frequency, perhaps as a consequence of the surface states modification introduced by a homogeneous distribution of the catalyst; this phenomenon may be justified by an interaction of the free charge with the surface states, as proposed in a previous paper [10]. In Fig. 7(a) and (b), the behaviour of the imaginary part of the impedance is reported. The variations of the peak heights versus temperature and their modulation versus atmosphere are in agreement with the electrical conductances under the corresponding conditions. Moreover, the values of the peaks are localized in correspondence of only one frequency. It can be deduced that the frequency at which the dependence of the resistance and the capacitance disappears is a characteristic of each sample and has to be related to the grain size and the electron mobility [10], while it is independent of ambient conditions.
481
4. Conclusions TEM observations showed that: (i) the LAB samples exhibit particles homogeneous in size with an average diameter of approximately 30 nm with indented borders; (ii) the CER samples exhibit particles not homogeneous in size (diameter range between 20 and 200 nm) with sharp borders. In agreement, LAB samples show SBET higher than CER samples. All the results obtained by UV–Vis and FT-IR spectra are consistent with the texture and microstructure of the four materials and demonstrate that the different samples show a different ratio between the electron population of localized states and of the conduction band. Electrical measurements in wet air show that LAB samples have higher resistances than CER samples, and that Pd-LAB and Pd-CER samples have a higher resistance than LAB and CER ones, respectively. Even if the spectral phenomena cannot directly correspond with the electrical ones, our spectroscopic analy-
Fig. 7. The imaginary part of the impedance for the samples: PdCER(850), section (a); and Pd-LAB(850), section (b).
A. Chiorino et al. / Sensors and Actuators B 44 (1997) 474–482
482
sis is qualitatively in good agreement with the electrical measurements, since, looking at each couple of compared materials, a major population of localized levels has been found for the more resistant one. Furthermore, the electrical measurements in wet air show that the effect of Pd on the LAB sample is much higher than on the CER sample, but this behaviour could not be inferred from the spectroscopic measurements. The electrical measurements clearly demonstrated that LAB samples exhibit a much more satisfactory response to methane. Moreover, the Pd decreases the maximum response temperature of the LAB material. As for the FT-IR changes passing from wet air to methane/wet air, they revealed, at least qualitatively in agreement with the electrical ones, that Pd decreases the maximum response temperature of the LAB material. Again qualitatively in agreement with the electrical measurements, FT-IR changes revealed that Pd makes Pd-LAB more sensitive than the LAB sample at 350°C. This agreement is not found at 450°C. On the contrary, the FT-IR analysis of changes in CER and Pd-CER, when passing from wet air to methane/wet air, could not be directly compared with the electrical ones. The Nyquist diagrams show a higher dependence of the resistance and capacitance on frequency for CER and Pd-LAB samples than for Pd-CER and LAB ones, respectively. We suggest that this effect must be related to the different electron mobilities and to the sample sizes. At the same time, it has been demonstrated by the optical measurements that different samples present different electron energy states: the expected consequence is an electron mobility characteristic of each
.
kind of sample. Although this phenomenon requires deeper investigation, we suggest the electron mobility as a proper parameter to correlate the electrical and the optical absorption measurements.
References [1] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Actuators B 5 (1991) 7 – 19. [2] J.F. Geiger, K.D. Schierbaum, W. Go¨pel, Surface spectroscopic studies on Pd-doped SnO2, Vacuum 41 (1990) 1629 – 1632. [3] J.G. Duh, J.W. Jou, B.S. Chiou, Catalytic and gas sensing characteristics in Pd-doped SnO2, J. Electrochem. Soc. 136 (1989) 2740 – 2747. [4] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Relationship between gas sensitivity and microstructure of porous SnO2, Denki Kagaku (Chemical Sensors) 58 (1990) 1143 – 1148. [5] W. Spence, The UV absorption edge of tin oxide thin films, J. Appl. Phys. 38 (1967) 3767 – 3770. [6] G. Ghiotti, A. Chiorino, P.W. Xiong, L. Marchese, A comparative study of SnO2 and CrOx /SnO2 powders: their microstructures and their different response to CO, NO, H2, and CH4 atmospheres, Sensors and Actuators B 7 (1992) 691 – 699. [7] G. Ghiotti, A. Chiorino, P.W. Xiong, Surface chemistry and electronic effects of H2(D2) on pure SnO2 and Cr-doped SnO2, Sensors and Actuators B 15-16 (1993) 367 – 371. [8] G. Ghiotti, A. Chiorino, G. Martinelli, M.C. Carotta, Moisture effects on pure and Pd-doped SnO2 thick films analysed by FTIR spectroscopy and conductance measurements, Sensors and Actuators B 24-25 (1995) 520 – 524. [9] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, B.C. Tofield, Tin dioxide gas sensors. Part 2. The role of surface additives, J. Chem. Soc., Faraday Trans. 1 84 (1988) 441–457. [10] G. Martinelli, M.C. Carotta, L. Passari, L. Tracchi, A study of the moisture effects on SnO2 thick films by sensitivity and permittivity measurements, Sensors and Actuators B 26-27 (1995) 53 – 55.