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Surface chemistry and electronic effects of H2(D2) on pure SnO2 and Cr-doped SnO2
367
Sensors nnd Aefua~orsB, 1.5-16 (1993) 367-371
Surface chemistry and electronic effects of Hz (D2) on pure SnO, and Cr-doped SnO, Giovanna Ghiott?, Dipnrtimento
Anna Chiorino and Wei Xiong Pan**
di Chimica Inorgonica,
Chimica Fiska e Chimica dei Maieriali,
via P.
Giuria 7, 10125 Turin (ita&)
The interaction of pure SnOz and Cr-doped SnO, with H2 (D2) at r.t. has been studied by FT-IR spectroscopy, to provide information both on surface reactions and changes in the electronic properties. On pure SnO,, H, (Dz) interaction (p = 1.33 x 10m3 Pa) causes the fast and complete loss of IR transmission, only partially restored by subsequent admission of O,, No increase in OH vibrations appears; this is justified as follows: H, is dissociatively adsorbed in the interstitial symmetric sites between oxidic ions, both on the surface and in the SnO, sublayers, as proton. On Q-doped GO,, pressures five orders of magnitude higher are necessary to see a sensible transmission loss. Simultaneously, vibrational peaks characteristic of Cr-H (Cr-D) and O-H (O-D) species appear, formed by H, (Dz) dissociation on Cr-0 pairs. The broad absorption responsible for the t~nsmission loss is actually due to the superposition of two bands of different shape. One shows an asymmetricalshape, a sharp increase in the range IOOO-2OOOcm-’(0.12-0.25 eV) and a slow drop in the high energy range. The shape and energy of the absorption edge are those expected for an electron transition from the second level of oxygen vacancies to the conduction band (CB). The other one shows a symmetrical shape, with a half width -3000~1~ and a maximum at 4750cm-’ ( zz0.6 eV). This band reveals the presenceof donor levelsat -0.6 eV from the bottom of the conduction baud. The intensity ratio of the two bands is deeply affected by the presence of Cr, that preferentially destroys the first electronic transition, showing that its major effect is to decrease the number of oxygen vacancies.
Introduction
Most commercial gas sensing devices make use of SnO*. The gas response of SnOz originates in the control exercised by chemisorbed molecules on the conductance of the near-surface region and depends both on the electronic surface states and on the rates of surface reactions involving the gases to be detected. It has been demonstrated that the use of surface additives deeply affects the gas response of SnO, [I]. We focused our interest on the different behaviour of pure and Cr-doped SnO, as revealed by interaction with H, (DJ studied by absorption FT-IR spectroscopy.
Pure SnOz was prepared following the procedure generally used for gas sensor material [2] by hydrolysis with ammonia of SnCI, water solution, followed by washing, drying at r.t. and calcining at 473 K: sample named SN-C(473). Self-supporting dii of SN-C(473)
*Author to whom the correspondence should be addmsed. **On leave from Departmentof Chemistry, Tsinghua University of Sijing, China.
were placed in an IR cell, designed to treat the samples in situ, and submitted to 3 evacuation (1 h at 1.33 x lop3 Pa)-oxidation f 1 h in pure 02, p = 6.6 x IO3Pa) cycles at 873 K, cooling down to 298 K in O2 and evacuation for 30 s at t.33 x 10e3 Pa. This material will be named hereafter SN-Of873). CrO,/SnO, samples were prepared by impregnation of the SN-C(473) powder with an aqueous solution of CrOs, with a concentration to yield a Cr content of 0.5 wt.%, and calcined at 473 K: sample SNCROS~(473~. Selfsupporting discs of SNCRO~C(473~ were submitted to the same cycles described for SN-C(473). The material will be named hereafter SNCROS-O(873). The FT-IR spectra were recorded at room temperature (r.t.). in vacuum or in controlled atmospheres, with a 1760-X Perkin-Ehner FT-IR s~trophotomer, equipped with a cryodeteztor, at a resolution of 2 cn-‘. Hi~-~~ H,, Dz and 4, from Ma&son C.P. were used.
Rcdts and discpssiun Iu previous work [3] the texture and mo~holo~ of the two materials were determined: electron micrographs of both materials show densely packed aggregates built up of compact small crystallites with rounded shape; the plane distances meawed by electron
@ 1993 -
Elsevk
Sequoia_ All rights resend
368
diffraction and X-ray diffraction are those of the rutile structure for both powders; average particle sizes and BET surface area are slightly perturbed by the presence of chromium (for SN-0( 873): S,,, = 39 m2 gg’, D = 115 A; for SNCROS-0(873): S,, = 23 m2 g-‘, n = 150 A). Cr-doped material showed little modifications in the shape and position of the VB-CB transition absorption edge, as revealed by the UV-Vis reflectance spectra [ 3,4] together with the presence of bands in the region of the Cr(II,III) d-d transitions at 18 000 and 14 000 cm-‘, the latter showing that the major amount of Cr is in low valence state. Minor amounts of Cr(V) and Cr(VI) in the form of chromate-like species on the surface of the SNCROS-0( 873) sample were revealed by IR spectroscopy by the presence of two weak bands at 1030 and 1015cm-’ [3]. H, (D,) adsorption on pure SnO, When H, at p = 1.33 x 10e4 Pa is adsorbed on SN-
0(873), a loss of sample transmission is immediately observed and the difference between spectra after and before H2 admission reveals the shape of the broad absorption responsible for this loss, illustrated by curve 1 in Fig. 1. The same result is obtained after adsorption of D, at p = 1.33 x 10m4Pa, as shown by curve 2 in Fig. 1. Shape and intensity of the broad absorptions are practically coincident; in both cases the transmission is completely restored by 0, admission at r.t. H, or D, interaction at p > 1.33 x 10m3Pa causes the immediate and complete loss of the sample transmission, only partially restored by subsequent contact with 0, (6.6 x lo3 Pa) for 30 min. Formation of surface hydroxyls H,(g) + 26-(s)
-
20H- + 2~ -
(1)
“‘“, j: 0.64
I-
0
owywn
0
tin
Scheme
I
oryqm
0
0xyq.n vacancy
sharing proton
has been hypothesized [5]. However, the absence of isotopic shift in the IR spectrum when D, instead of H, is adsorbed suggests that there is no contribution due to hydroxyls or deuteroxyls modes to the very broad absorption observed, its nature being completely electronic. We have suggested that H, dissociatively adsorbed is not bonded on top of 02-( s) ions, but diffuses as proton into interstitial symmetric sites between oxidie ions, both on the surface and in the SnO, sublayers, the electrons being trapped in ionic defects showing shallow levels in the bandgap [6]. This is illustrated in the following schemes for two possible surface terminations of crystallites with rutile structure. Scheme 1 illustrates the dissociative adsorption of an H, molecule on a nearly ideal (001) surface showing two oxygen vacancies, in which the electrons could be trapped, and two interstitial sites between oxidic ions, in which the protons could be arranged. A similar Hz adsorption model is shown in Scheme 2 for a nearly ideal (110) surface showing two oxygen vacancies of different type.
-I
0.46
0.30
0.16
o.ocl
Fig. 1. FT-IR absorbance spectra. The typical shapes of the very broad absorptions grown after admission of H, (curve 1) or D, (curve 2) at p = 1.33 x 10e4 Pa on an SN-O(873) sample.
0
Scheme 2.
Similar situations can be created on other low index faces (i.e. with high exposition probability), not reported here for sake of brevity. The partial or total destruction of the broad absorption by O2 admission can be interpreted as a consequence of the decreased concentration of electrons, now trapped by oxygen adsorbed on the surface +eWg) -O,(s) + O*-(s) (2) Oxygen vacancies are the predominant ionic intrinsic defects in SnOz crystals and powders in the range of temperatures in which our sample has been prepared [7]. Actually, shape and position of the observed electronic absorption should give information about the nature of the trapping levels. In the present case the absorption, when measurable, always shows the following spectral features: a sharp absorption edge at lOOO2000 cm-’ (0.12-0.25 eV), a slow increase reaching a maximum at 3600 cm-’ (0.45 eV), and a slow drop at higher energies. The position of the absorption edge (compatible with the ionization energy of the second level of an isolated oxygen vacancy (0.15 eV) measured in single SnO, crystals [8]) suggests it corresponds to electron transitions from this kind of level to the conduction band. However, the shape of the band is not exactly that expected for these transitions, which generally show only a continuous drop of intensities in the high energy side of the absorption [8,9], but not a second maximum as, actually, our experiment shows. The presence of more components could be the reason for the unexpected shape, indicating that not only isolated, or weakly interacting, oxygen vacancies are the intrinsic defects able to trap electrons during the reductive interaction. l& (D,) ahorption on Cr-doped SnO,
On the SNCR05-O(873) sample the admission of H, at p = 1.33 x 10m3Pa does not cause any IR response. Pressures five orders of magnitude higher are necessary to see a sensible loss in the sample transmission. The maximum loss is obtained at H, pressure of 1.33 Pa. However, the sample still shows a good transmission at this H, pressure. No further IR changes are observed for higher H, pressures. The loss in IR transparency due to the growth of the very broad electronic absorption is accompanied by the appearance of vibrational peaks (see Fig. 2, curve 1): stretching modes of free hydroxyls (the sharp bands at 3620,3595,3560 cm-‘) and hydrogen bonded hydroxyls (the broad one at 3350 cn-‘), stretching modes of hydrides both terminal and bridged (bands at 1837, 1788, 1740, 1600 cn- ‘) and bending modes of hydrogen bonded hydroxyls (1000-900 cm-‘) and free hydroxyls (the sharp band at 844 cm-‘); simultaneously the bands due to ‘chromate-like’ species at 1035 and 1015 cm-’
are eroded. The hydride and hydroxyl species, stable by evacuation at r.t., never obtained by contact of H, with pure SnO,, can be regarded as due to the H, dissociation on Cr(III,II)-O pairs and by reduction of the Cr=O(VI,V) groups. The electronic absorption is completely destroyed by the immission of 0, at r.t. (see curve 2 of Fig. 2) and more hydroxyls are formed, as shown by the difference between spectra before and after contact with oxygen (curve 3 of Fig. 2). Interaction with D, shows similar results (see Fig. 3, curve 1): the increase of the broad electronic absorption and the appearance of vibrational peaks easily assigned to deuteroxyls free (at 2670,2651,2633 cm-‘) and hydrogen bonded (at 2450 cm’) stretching modes and to deuteride species (at 1318, 1278, 1254cm-‘), stable under evacuation at r.t. Again the contact with oxygen at r.t. shows the destruction of the electronic absorption and the formation of more deuteroxyls (see curves 2 and 3 of Fig. 3). Curve 3 of both Figs. 2 and 3 shows the shape of the electronic absorption destroyed by the oxygen admission, apart for a vibrational contribution due to hydroxyls and deuteroxyls that can be easily subtracted. It is evident that the absorption is the superposition of two different components: the more intense one with a maximum at 4750 cm-‘, with a symmetrical shape and with a FWHM of about 3OOOcm-‘; the other component, much less intense, is indeed not completely separated and its shape can actually only be hypothesized, but it shows a definite sharp absorption edge at loOO2OOOcm-’ (0.12-0.25 eV), i.e. near to the ionization energy of the second level of an oxygen vacancy (0.15 eV). Our proposal is that this last component is that due to the presence of isolated, or weakly interacting, oxygen vacancies, while the first one is due to the presence of trapping levels deeper in the gap, at about 0.6eV, whose nature is difficult to understand on the basis of the experiments here reported. An explanation could be the presence of adjacent vacancies (divacancy or trivacancy). However, this is only a speculation. Comparison between SN-O(873) and SNCRO5-0(873) samples
By comparison of the results obtained on pure SN0( 873) and SNCR05-0(873), it is evident that the electronic response to Hz (D2) is enormously decreased by the presence of chromium. It is in fact necessary to increase the pressure of H, (DJ to 1.33 x lo3 Pa to observe for SNCROS-0( 873) the appearance of an electronic absorption with an intensity of the same order of magnitude as that obtained in the case of SN-0( 873) at a pressure of 1.33 x 10m4Pa. The comparison, in the case of D,, is illustrated in Fig. 4. This indicates that the number of shallow trapping levels of both types is somewhat lower with SNCROS-O(873) than with
0.6
iwo
1360
1700
8.1-l--
850
0.0 7000
1
~ 6000
5000
4000 t. -1
3000
2600
600
1050
WO
600
ce -1
Fig. 2. FT-IR absorbance spectra. (a) Typical abortions observed after admission of H, at p = 1.33x IO3Pa (curve I), and sub~q~nt evacuation and admission of Oz at p = I .33 x 103Pa (curve 2) on an SNCR05-O(873)sample. Curve 3 is the spectral difference between curves 1 and 2. (h) Expanded portions of curve I.
0.8 A
0.6 0.4
0.2
0.0 0.6
t
27B0
2600
2600
Fig. 3. FT-IR absorbance spectra. (a) Typical absorptions observed after admission of Dz atp = 1.33x 10’Pa (curve I), and subsequent evacuation and admission of Oz at p = 1.33x 10’Pa (curve 2) on an SNCR05-O(873)sample. Curve 3 is the spectral difference between curves I and 2. (b) Expanded portions of curve I.
371 increase at the low energy side and a slower one at the high energy side. The maximum at about 3600cm-’ (0.45 eV) is therefore only an apparent maximum due to the superposition of two different absorptions.
Acknowledgements The authors are grateful to the Italian CNR P.F. ‘M.A.D.E.S.S.’ for financial support. They also gratefully acknowledge the work of Professor Pier0 Ugliengo, who helped with the surface model drawing.
O.i2
0.00 7000
6000
5000
4000 C. -i
?mo
2000
800
Fig. 4. FT-IR absorbance spectra. Comparison between the shapes of the electronic absorptions caused by admission of D1 at p = I.33 x 10m4Pa on an SN-O(873) sample (curve 1) and at p = I.33 x 10s Pa on an SNCR05-O(873) sample (curve 2). Curve 3 shows how the difference between curves 1and 2 should appear, ignoring the deuteroxyl vibrational contributions.
SN-O(873). Furthermore the comparison shows not only a change in the absolute value of the concentration of the two defects, but also the change of their relative concentrations, the absorbance assigned to the presence of oxygen vacancies being relatively less intense on Cr-doped than on pure materials. Indeed this last difference allowed us to understand the shape of the electronic absorption on pure material and to reveal the presence of two families of intrinsic defects: one assigned to oxygen vacancies and the other of a nature still unknown with energy levels at about 0.6 eV below the bottom of the conduction band. The difference between curve 1 and curve 2 of Fig. 4 coarsely shows (apart from the vibrational contribution of deuteroxyl absorptions, easily subtracted) the shape of the electronic absorption due to the oxygen vacancies), i.e. asymmetrical, as expected, with a sharp absorbance
References J. F. McAleer, P. T. Moseley, J. 0. W. Norris, D. E. Williams and B. C. Tofield, Tin dioxide gas sensors. Part 2, J. Chem. Sot., Faraday Trans. I, 84 (1988) 44-457. P. Harrison and A. Guest, Tin oxide surfaces. Part 17, J. Chem. Sot., Faraday Trans. 1, 83 (1987) 3383-3397. G. Ghiotti, A. Chiorino, Wei Xiong Pan and L. Marchese, A comparative study of SnO, and CrO,/SnOz powders: their microstructures and their different response to CO, NO, H, and CHI atmospheres, Sensors and Actuators E, 7 ( 1992) 691-699. A. Chiorino, F. Boccuzzi and G. Ghiotti, Surface chemistry and electronic effects on 0,, NO and NO/O, on SnO,, Sensors and Actuators B, 5 (1991) 189-192. J. F. McAleer, P. T. Moseley, J. 0. W. Norris and D. E. Williams, Tin dioxide gas sensors. Part I. J. Chem. Sot., Furaday Trans. 1, 83 (1987) 1323-1346, and refs. therein, G. Ghiotti, A. Chiorino and F. Bcccuzzi, IR study of surface chemistry and electronic effects of different atmospheres on SnO,, Sensors and Actuators, 19 (1989) 151-157. J. Maier and W. Giipel, Investigations of the bulk defect chemistry of polycrystalline tin(IV) oxide, J. Solid State Chem., 72 (1988) 293. Z. M. Jarzebski and J. P. Marton, Physical properties of SnO, materials, J. Electrochem. Sot., 123 (1976) 299C, 123 (1976) 333c. G.Ghiotti, A. Chiorino and F. Boccurzi, Surface chemistry and electronic effects of H, (DZ) on two different microcrystalline ZnO powders, Surface Sci., in press.
Sensors nnd Aefua~orsB, 1.5-16 (1993) 367-371
Surface chemistry and electronic effects of Hz (D2) on pure SnO, and Cr-doped SnO, Giovanna Ghiott?, Dipnrtimento
Anna Chiorino and Wei Xiong Pan**
di Chimica Inorgonica,
Chimica Fiska e Chimica dei Maieriali,
via P.
Giuria 7, 10125 Turin (ita&)
The interaction of pure SnOz and Cr-doped SnO, with H2 (D2) at r.t. has been studied by FT-IR spectroscopy, to provide information both on surface reactions and changes in the electronic properties. On pure SnO,, H, (Dz) interaction (p = 1.33 x 10m3 Pa) causes the fast and complete loss of IR transmission, only partially restored by subsequent admission of O,, No increase in OH vibrations appears; this is justified as follows: H, is dissociatively adsorbed in the interstitial symmetric sites between oxidic ions, both on the surface and in the SnO, sublayers, as proton. On Q-doped GO,, pressures five orders of magnitude higher are necessary to see a sensible transmission loss. Simultaneously, vibrational peaks characteristic of Cr-H (Cr-D) and O-H (O-D) species appear, formed by H, (Dz) dissociation on Cr-0 pairs. The broad absorption responsible for the t~nsmission loss is actually due to the superposition of two bands of different shape. One shows an asymmetricalshape, a sharp increase in the range IOOO-2OOOcm-’(0.12-0.25 eV) and a slow drop in the high energy range. The shape and energy of the absorption edge are those expected for an electron transition from the second level of oxygen vacancies to the conduction band (CB). The other one shows a symmetrical shape, with a half width -3000~1~ and a maximum at 4750cm-’ ( zz0.6 eV). This band reveals the presenceof donor levelsat -0.6 eV from the bottom of the conduction baud. The intensity ratio of the two bands is deeply affected by the presence of Cr, that preferentially destroys the first electronic transition, showing that its major effect is to decrease the number of oxygen vacancies.
Introduction
Most commercial gas sensing devices make use of SnO*. The gas response of SnOz originates in the control exercised by chemisorbed molecules on the conductance of the near-surface region and depends both on the electronic surface states and on the rates of surface reactions involving the gases to be detected. It has been demonstrated that the use of surface additives deeply affects the gas response of SnO, [I]. We focused our interest on the different behaviour of pure and Cr-doped SnO, as revealed by interaction with H, (DJ studied by absorption FT-IR spectroscopy.
Pure SnOz was prepared following the procedure generally used for gas sensor material [2] by hydrolysis with ammonia of SnCI, water solution, followed by washing, drying at r.t. and calcining at 473 K: sample named SN-C(473). Self-supporting dii of SN-C(473)
*Author to whom the correspondence should be addmsed. **On leave from Departmentof Chemistry, Tsinghua University of Sijing, China.
were placed in an IR cell, designed to treat the samples in situ, and submitted to 3 evacuation (1 h at 1.33 x lop3 Pa)-oxidation f 1 h in pure 02, p = 6.6 x IO3Pa) cycles at 873 K, cooling down to 298 K in O2 and evacuation for 30 s at t.33 x 10e3 Pa. This material will be named hereafter SN-Of873). CrO,/SnO, samples were prepared by impregnation of the SN-C(473) powder with an aqueous solution of CrOs, with a concentration to yield a Cr content of 0.5 wt.%, and calcined at 473 K: sample SNCROS~(473~. Selfsupporting discs of SNCRO~C(473~ were submitted to the same cycles described for SN-C(473). The material will be named hereafter SNCROS-O(873). The FT-IR spectra were recorded at room temperature (r.t.). in vacuum or in controlled atmospheres, with a 1760-X Perkin-Ehner FT-IR s~trophotomer, equipped with a cryodeteztor, at a resolution of 2 cn-‘. Hi~-~~ H,, Dz and 4, from Ma&son C.P. were used.
Rcdts and discpssiun Iu previous work [3] the texture and mo~holo~ of the two materials were determined: electron micrographs of both materials show densely packed aggregates built up of compact small crystallites with rounded shape; the plane distances meawed by electron
@ 1993 -
Elsevk
Sequoia_ All rights resend
368
diffraction and X-ray diffraction are those of the rutile structure for both powders; average particle sizes and BET surface area are slightly perturbed by the presence of chromium (for SN-0( 873): S,,, = 39 m2 gg’, D = 115 A; for SNCROS-0(873): S,, = 23 m2 g-‘, n = 150 A). Cr-doped material showed little modifications in the shape and position of the VB-CB transition absorption edge, as revealed by the UV-Vis reflectance spectra [ 3,4] together with the presence of bands in the region of the Cr(II,III) d-d transitions at 18 000 and 14 000 cm-‘, the latter showing that the major amount of Cr is in low valence state. Minor amounts of Cr(V) and Cr(VI) in the form of chromate-like species on the surface of the SNCROS-0( 873) sample were revealed by IR spectroscopy by the presence of two weak bands at 1030 and 1015cm-’ [3]. H, (D,) adsorption on pure SnO, When H, at p = 1.33 x 10e4 Pa is adsorbed on SN-
0(873), a loss of sample transmission is immediately observed and the difference between spectra after and before H2 admission reveals the shape of the broad absorption responsible for this loss, illustrated by curve 1 in Fig. 1. The same result is obtained after adsorption of D, at p = 1.33 x 10m4Pa, as shown by curve 2 in Fig. 1. Shape and intensity of the broad absorptions are practically coincident; in both cases the transmission is completely restored by 0, admission at r.t. H, or D, interaction at p > 1.33 x 10m3Pa causes the immediate and complete loss of the sample transmission, only partially restored by subsequent contact with 0, (6.6 x lo3 Pa) for 30 min. Formation of surface hydroxyls H,(g) + 26-(s)
-
20H- + 2~ -
(1)
“‘“, j: 0.64
I-
0
owywn
0
tin
Scheme
I
oryqm
0
0xyq.n vacancy
sharing proton
has been hypothesized [5]. However, the absence of isotopic shift in the IR spectrum when D, instead of H, is adsorbed suggests that there is no contribution due to hydroxyls or deuteroxyls modes to the very broad absorption observed, its nature being completely electronic. We have suggested that H, dissociatively adsorbed is not bonded on top of 02-( s) ions, but diffuses as proton into interstitial symmetric sites between oxidie ions, both on the surface and in the SnO, sublayers, the electrons being trapped in ionic defects showing shallow levels in the bandgap [6]. This is illustrated in the following schemes for two possible surface terminations of crystallites with rutile structure. Scheme 1 illustrates the dissociative adsorption of an H, molecule on a nearly ideal (001) surface showing two oxygen vacancies, in which the electrons could be trapped, and two interstitial sites between oxidic ions, in which the protons could be arranged. A similar Hz adsorption model is shown in Scheme 2 for a nearly ideal (110) surface showing two oxygen vacancies of different type.
-I
0.46
0.30
0.16
o.ocl
Fig. 1. FT-IR absorbance spectra. The typical shapes of the very broad absorptions grown after admission of H, (curve 1) or D, (curve 2) at p = 1.33 x 10e4 Pa on an SN-O(873) sample.
0
Scheme 2.
Similar situations can be created on other low index faces (i.e. with high exposition probability), not reported here for sake of brevity. The partial or total destruction of the broad absorption by O2 admission can be interpreted as a consequence of the decreased concentration of electrons, now trapped by oxygen adsorbed on the surface +eWg) -O,(s) + O*-(s) (2) Oxygen vacancies are the predominant ionic intrinsic defects in SnOz crystals and powders in the range of temperatures in which our sample has been prepared [7]. Actually, shape and position of the observed electronic absorption should give information about the nature of the trapping levels. In the present case the absorption, when measurable, always shows the following spectral features: a sharp absorption edge at lOOO2000 cm-’ (0.12-0.25 eV), a slow increase reaching a maximum at 3600 cm-’ (0.45 eV), and a slow drop at higher energies. The position of the absorption edge (compatible with the ionization energy of the second level of an isolated oxygen vacancy (0.15 eV) measured in single SnO, crystals [8]) suggests it corresponds to electron transitions from this kind of level to the conduction band. However, the shape of the band is not exactly that expected for these transitions, which generally show only a continuous drop of intensities in the high energy side of the absorption [8,9], but not a second maximum as, actually, our experiment shows. The presence of more components could be the reason for the unexpected shape, indicating that not only isolated, or weakly interacting, oxygen vacancies are the intrinsic defects able to trap electrons during the reductive interaction. l& (D,) ahorption on Cr-doped SnO,
On the SNCR05-O(873) sample the admission of H, at p = 1.33 x 10m3Pa does not cause any IR response. Pressures five orders of magnitude higher are necessary to see a sensible loss in the sample transmission. The maximum loss is obtained at H, pressure of 1.33 Pa. However, the sample still shows a good transmission at this H, pressure. No further IR changes are observed for higher H, pressures. The loss in IR transparency due to the growth of the very broad electronic absorption is accompanied by the appearance of vibrational peaks (see Fig. 2, curve 1): stretching modes of free hydroxyls (the sharp bands at 3620,3595,3560 cm-‘) and hydrogen bonded hydroxyls (the broad one at 3350 cn-‘), stretching modes of hydrides both terminal and bridged (bands at 1837, 1788, 1740, 1600 cn- ‘) and bending modes of hydrogen bonded hydroxyls (1000-900 cm-‘) and free hydroxyls (the sharp band at 844 cm-‘); simultaneously the bands due to ‘chromate-like’ species at 1035 and 1015 cm-’
are eroded. The hydride and hydroxyl species, stable by evacuation at r.t., never obtained by contact of H, with pure SnO,, can be regarded as due to the H, dissociation on Cr(III,II)-O pairs and by reduction of the Cr=O(VI,V) groups. The electronic absorption is completely destroyed by the immission of 0, at r.t. (see curve 2 of Fig. 2) and more hydroxyls are formed, as shown by the difference between spectra before and after contact with oxygen (curve 3 of Fig. 2). Interaction with D, shows similar results (see Fig. 3, curve 1): the increase of the broad electronic absorption and the appearance of vibrational peaks easily assigned to deuteroxyls free (at 2670,2651,2633 cm-‘) and hydrogen bonded (at 2450 cm’) stretching modes and to deuteride species (at 1318, 1278, 1254cm-‘), stable under evacuation at r.t. Again the contact with oxygen at r.t. shows the destruction of the electronic absorption and the formation of more deuteroxyls (see curves 2 and 3 of Fig. 3). Curve 3 of both Figs. 2 and 3 shows the shape of the electronic absorption destroyed by the oxygen admission, apart for a vibrational contribution due to hydroxyls and deuteroxyls that can be easily subtracted. It is evident that the absorption is the superposition of two different components: the more intense one with a maximum at 4750 cm-‘, with a symmetrical shape and with a FWHM of about 3OOOcm-‘; the other component, much less intense, is indeed not completely separated and its shape can actually only be hypothesized, but it shows a definite sharp absorption edge at loOO2OOOcm-’ (0.12-0.25 eV), i.e. near to the ionization energy of the second level of an oxygen vacancy (0.15 eV). Our proposal is that this last component is that due to the presence of isolated, or weakly interacting, oxygen vacancies, while the first one is due to the presence of trapping levels deeper in the gap, at about 0.6eV, whose nature is difficult to understand on the basis of the experiments here reported. An explanation could be the presence of adjacent vacancies (divacancy or trivacancy). However, this is only a speculation. Comparison between SN-O(873) and SNCRO5-0(873) samples
By comparison of the results obtained on pure SN0( 873) and SNCR05-0(873), it is evident that the electronic response to Hz (D2) is enormously decreased by the presence of chromium. It is in fact necessary to increase the pressure of H, (DJ to 1.33 x lo3 Pa to observe for SNCROS-0( 873) the appearance of an electronic absorption with an intensity of the same order of magnitude as that obtained in the case of SN-0( 873) at a pressure of 1.33 x 10m4Pa. The comparison, in the case of D,, is illustrated in Fig. 4. This indicates that the number of shallow trapping levels of both types is somewhat lower with SNCROS-O(873) than with
0.6
iwo
1360
1700
8.1-l--
850
0.0 7000
1
~ 6000
5000
4000 t. -1
3000
2600
600
1050
WO
600
ce -1
Fig. 2. FT-IR absorbance spectra. (a) Typical abortions observed after admission of H, at p = 1.33x IO3Pa (curve I), and sub~q~nt evacuation and admission of Oz at p = I .33 x 103Pa (curve 2) on an SNCR05-O(873)sample. Curve 3 is the spectral difference between curves 1 and 2. (h) Expanded portions of curve I.
0.8 A
0.6 0.4
0.2
0.0 0.6
t
27B0
2600
2600
Fig. 3. FT-IR absorbance spectra. (a) Typical absorptions observed after admission of Dz atp = 1.33x 10’Pa (curve I), and subsequent evacuation and admission of Oz at p = 1.33x 10’Pa (curve 2) on an SNCR05-O(873)sample. Curve 3 is the spectral difference between curves I and 2. (b) Expanded portions of curve I.
371 increase at the low energy side and a slower one at the high energy side. The maximum at about 3600cm-’ (0.45 eV) is therefore only an apparent maximum due to the superposition of two different absorptions.
Acknowledgements The authors are grateful to the Italian CNR P.F. ‘M.A.D.E.S.S.’ for financial support. They also gratefully acknowledge the work of Professor Pier0 Ugliengo, who helped with the surface model drawing.
O.i2
0.00 7000
6000
5000
4000 C. -i
?mo
2000
800
Fig. 4. FT-IR absorbance spectra. Comparison between the shapes of the electronic absorptions caused by admission of D1 at p = I.33 x 10m4Pa on an SN-O(873) sample (curve 1) and at p = I.33 x 10s Pa on an SNCR05-O(873) sample (curve 2). Curve 3 shows how the difference between curves 1and 2 should appear, ignoring the deuteroxyl vibrational contributions.
SN-O(873). Furthermore the comparison shows not only a change in the absolute value of the concentration of the two defects, but also the change of their relative concentrations, the absorbance assigned to the presence of oxygen vacancies being relatively less intense on Cr-doped than on pure materials. Indeed this last difference allowed us to understand the shape of the electronic absorption on pure material and to reveal the presence of two families of intrinsic defects: one assigned to oxygen vacancies and the other of a nature still unknown with energy levels at about 0.6 eV below the bottom of the conduction band. The difference between curve 1 and curve 2 of Fig. 4 coarsely shows (apart from the vibrational contribution of deuteroxyl absorptions, easily subtracted) the shape of the electronic absorption due to the oxygen vacancies), i.e. asymmetrical, as expected, with a sharp absorbance
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