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SnO2·Nb2O5 films for ethanol sensor, obtained by deposition of alcoholic suspensions
April 2000
Materials Letters 43 Ž2000. 166–169 www.elsevier.comrlocatermatlet
SnO 2 P Nb 2 O5 films for ethanol sensor, obtained by deposition of alcoholic suspensions I.T. Weber, E. Longo, E. Leite ) LIEC, Departamento de Quımica, UniÕersidade Federal de Sao ´ ˜ Carlos, P.O. Box 676, CEP 13565-905, Sao ˜ Carlos, SP, Brazil Received 8 June 1999; received in revised form 17 June 1999; accepted 21 September 1999
Abstract Films of Ž1 y x .SnO 2 P x Nb 2 O5 were prepared by the deposition of alcoholic suspensions, using Spin Coating Techniques. The powders used in the preparation of the suspensions were obtained by the polymeric precursor method. Results of X-ray Diffraction ŽXRD. indicate that the material has a single SnO 2 phase. Scanning Electron Microscopy ŽSEM. and Atomic Force Microscopy ŽAFM. showed quite homogeneous and porous films, without cracks and with large surface area. The electrical characterization of the samples indicates good sensitivity to ethanol at 2008C, response times in the order of 100 s and resistance in the order of 20 k V, at room temperature. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin coating techniques; Ethanol sensor; SnO 2 P Nb 2 O5 films
1. Introduction The use of SnO 2-based sensor materials is thoroughly investigated in Refs. w1–3x. It is used as powder, ceramic compacts and thin or thick films. Most of the commercially available devices are produced by typical ceramic fabrication processes. However, it has been shown that the use of films presents several advantages, such as easier application of electrodes, higher potential for in the miniaturization of devices and high mechanical resistance w4x. In order to detect small concentrations of reactive gases in air, when the oxygen partial pressure is almost constant, the surface reactions must be much
)
Corresponding author.
more important than the bulk changes, so the surface area has to be as large as possible. The use of thick and porous films satisfies such conditions. Screen-Printing is the most widely used method for the preparation of SnO 2 thick films. In this method, pastes are prepared from powders of oxides and metals containing inorganic additives and organic binders w4x. The pastes are usually printed over the electrode-covered ceramic substrate with the heater and sensor temperature at the backside. Unfortunately, this method does not allow for homogeneous doping, and produces too-thick films. In this work, an alternative route is proposed for the preparation of thick films of SnO 2 . This route consists of the preparation of Nb 2 O5-doped SnO 2 alcoholic suspensions, which are deposited by spin coating onto alumina substrata. In this way, high quality SnO 2 thick films are obtained by a low-cost
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 5 2 - 9
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
167
process without the need for any sophisticated equipment, like a vacuum chamber or a controlled atmosphere. Aiming at improving the quality of the films, increasing the adherence to the substrate and decreasing the formation of discontinuities along the film, ultrafine powders were used, prepared by the method of the polymeric precursor w5x. Fig. 2. Schematic representation of the testing chamber.
2. Experimental procedure First of all, tin and niobium citrates were prepared. These citrates reacted to form a polymeric resin, according to the process described in Fig. 1. The resins were calcined until the elimination of the organic fraction. Subsequently, they were deaggregated by milling, and calcined again at 6008C for 2 h. In this way, crystalline, extremely fine, and homogeneously doped powders were obtained.
Thirty percent solid suspension of Nb 2 O5-doped SnO 2 in ethanol was obtained, using polyvinylbutyral ŽPVB. as dispersing agent. Suspensions with a viscosity of 24 cP were deposited onto alumina substrata by spin coating. After drying at room temperature, they were thermally treated at 7008C for 1 h at a heating and cooling rate of 18Crmin. The characterization of the films was accomplished by X-ray Diffraction ŽXRD., Scanning Electron Microscopy ŽSEM., Atomic Force Microscopy ŽAFM. and Electrical Characterization. The electrical measurements were performed in a homemade testing chamber that measures the superficial resistance of the samples, as shown in Fig. 2 w6x. In this chamber, the films were exposed to an alcoholic atmosphere. The ratio between the resistance in clean air Ž R air . and the resistance in the presence of ethanol Ž R etOH . is taken as the sensitivity Ž S ., which is defined as: S s R airrR etOH
Ž 1.
The time necessary for a fall of 90% in the resistance is taken as response time Ž t R .. 3. Results and discussion Table 1 presents the particle characterization of the prepared powders. They show large specific areas and small grain sizes ŽTable 1.. The XRD results Table 1 Powder characterization Sample
Fig. 1. Synthesis route for the films.
code
Composition Žmol.
SBE T Žm2 rg.
d BET ˚. ŽA
d XDR ˚. ŽA
Nb1
99SnO 2 –1Nb 2 O5
38.9083
223
120
168
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
Fig. 3. Microstructure of the sample Nb1. Ža. scanning electronic micrography and Žb. atomic force micrography.
indicate only the presence of the Cassiterite phase, which confirms the solid solution between SnO 2 and Nb 2 O5 . The microscopy analysis of the films indicates that the samples are quite homogeneous, porous and rugose Žmedium rugosity, R a s 273.32 nm., as shown in Fig. 3. As a consequence, the samples have a high surface area that favors the adsorption of gaseous species. It was also noticed that the films present good adherence to the substrate. The film thickness was about 1.5 mm, and was measured by SEM using cross-section analysis. The electrical characterization showed the sensor properties of the material. The measurements were made in two stages. In both cases, a fixed concentration of 400 ppm of ethanol was used. Initially, a test for the determination of the temperature of higher sensitivity was performed. The temperature in the test ranged from 508C to 4508C. The experiments were made three times and the curves were based upon the average values. Fig. 4 shows the sensitivity and time–response curves. It was verified that the samples are more sensitive at 2008C ŽFig. 4a., where the sensitivity reaches close to 10. These S values are in agreement with the values reported in Refs. w7–9x. In a second measurement, the temperature was maintained at 2008C and the response time was determined ŽFig. 4b.. For these measurements, the reproducibility obtained was very good. The total saturation of the surface was noticed, once for times larger than t R , the resistance was stable. Unfortu-
nately, it was not possible to compare the results obtained with the literature as the resistance variation adopted in each case was not cited w7,10x.
Fig. 4. Sensitivity curves for sample Nb1. Ža. Determination of high sensitivity temperature and Žb. time–response determination.
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
4. Conclusions The method proposed in this work is quite efficient to obtain thick films of SnO 2 P Nb 2 O5 , with high superficial area, which is extremely advantageous for its application as a sensor of gases. The results of the accomplished analyses indicate that the films are quite homogeneous, porous and with good adherence to the substrata. Therefore, the deposition of films through alcoholic suspensions is a viable and low-cost alternative for the preparation of these films.
References w1x H. Teterycz, J. Kita, R. Bauer, L.J. Golonka, B.W. Licznerski, K. Nitsch, K. Wisniewski, Sensors and Actuators, B 47 Ž1998. 100.
169
w2x J. Watson, I. Kousuke, G.S.V. Colest, Meas. Sci. Technol. 4 Ž1993. 711. w3x W. Gopel, K.D. Schrienbaum, Sensors and Actuators, B ¨ 26–27 Ž1995. 1. w4x K.D. Schrienbaum, U. Weimar, W. Gopel, Sensors and ¨ Actuators, B 7 Ž1992. 709. w5x D. Gouvea, J.A. Varela, E. Longo, A. Smith, J.P. Bonnet, Eur. J. Solid State Inorg. Chem. 30 Ž9. Ž1993. 915. w6x R.L.T. Andrade, C.A. Lindino, L.O.S. Bulhoes, ˜ Quımica ´ Nova 21 Ž3. Ž1998. 348. w7x P.P. Tsai, I.C. Chen, M.H. Tzeng, Sensors and Actuators, B 13–14 Ž1993. 610. w8x P.K. Hesook et al., Sensors and Actuators, B 13–14 Ž1993. 511. w9x Y. Liu, W. Zhu, M.S. Tse, S.Y.J. Shen, Mater. Sci. Lett. 14 Ž1995. 118. w10x P. Lesark, M. Sriyudthsak, Sensors and Actuators, B 24–25 Ž1995. 504.
Materials Letters 43 Ž2000. 166–169 www.elsevier.comrlocatermatlet
SnO 2 P Nb 2 O5 films for ethanol sensor, obtained by deposition of alcoholic suspensions I.T. Weber, E. Longo, E. Leite ) LIEC, Departamento de Quımica, UniÕersidade Federal de Sao ´ ˜ Carlos, P.O. Box 676, CEP 13565-905, Sao ˜ Carlos, SP, Brazil Received 8 June 1999; received in revised form 17 June 1999; accepted 21 September 1999
Abstract Films of Ž1 y x .SnO 2 P x Nb 2 O5 were prepared by the deposition of alcoholic suspensions, using Spin Coating Techniques. The powders used in the preparation of the suspensions were obtained by the polymeric precursor method. Results of X-ray Diffraction ŽXRD. indicate that the material has a single SnO 2 phase. Scanning Electron Microscopy ŽSEM. and Atomic Force Microscopy ŽAFM. showed quite homogeneous and porous films, without cracks and with large surface area. The electrical characterization of the samples indicates good sensitivity to ethanol at 2008C, response times in the order of 100 s and resistance in the order of 20 k V, at room temperature. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Spin coating techniques; Ethanol sensor; SnO 2 P Nb 2 O5 films
1. Introduction The use of SnO 2-based sensor materials is thoroughly investigated in Refs. w1–3x. It is used as powder, ceramic compacts and thin or thick films. Most of the commercially available devices are produced by typical ceramic fabrication processes. However, it has been shown that the use of films presents several advantages, such as easier application of electrodes, higher potential for in the miniaturization of devices and high mechanical resistance w4x. In order to detect small concentrations of reactive gases in air, when the oxygen partial pressure is almost constant, the surface reactions must be much
)
Corresponding author.
more important than the bulk changes, so the surface area has to be as large as possible. The use of thick and porous films satisfies such conditions. Screen-Printing is the most widely used method for the preparation of SnO 2 thick films. In this method, pastes are prepared from powders of oxides and metals containing inorganic additives and organic binders w4x. The pastes are usually printed over the electrode-covered ceramic substrate with the heater and sensor temperature at the backside. Unfortunately, this method does not allow for homogeneous doping, and produces too-thick films. In this work, an alternative route is proposed for the preparation of thick films of SnO 2 . This route consists of the preparation of Nb 2 O5-doped SnO 2 alcoholic suspensions, which are deposited by spin coating onto alumina substrata. In this way, high quality SnO 2 thick films are obtained by a low-cost
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 5 2 - 9
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
167
process without the need for any sophisticated equipment, like a vacuum chamber or a controlled atmosphere. Aiming at improving the quality of the films, increasing the adherence to the substrate and decreasing the formation of discontinuities along the film, ultrafine powders were used, prepared by the method of the polymeric precursor w5x. Fig. 2. Schematic representation of the testing chamber.
2. Experimental procedure First of all, tin and niobium citrates were prepared. These citrates reacted to form a polymeric resin, according to the process described in Fig. 1. The resins were calcined until the elimination of the organic fraction. Subsequently, they were deaggregated by milling, and calcined again at 6008C for 2 h. In this way, crystalline, extremely fine, and homogeneously doped powders were obtained.
Thirty percent solid suspension of Nb 2 O5-doped SnO 2 in ethanol was obtained, using polyvinylbutyral ŽPVB. as dispersing agent. Suspensions with a viscosity of 24 cP were deposited onto alumina substrata by spin coating. After drying at room temperature, they were thermally treated at 7008C for 1 h at a heating and cooling rate of 18Crmin. The characterization of the films was accomplished by X-ray Diffraction ŽXRD., Scanning Electron Microscopy ŽSEM., Atomic Force Microscopy ŽAFM. and Electrical Characterization. The electrical measurements were performed in a homemade testing chamber that measures the superficial resistance of the samples, as shown in Fig. 2 w6x. In this chamber, the films were exposed to an alcoholic atmosphere. The ratio between the resistance in clean air Ž R air . and the resistance in the presence of ethanol Ž R etOH . is taken as the sensitivity Ž S ., which is defined as: S s R airrR etOH
Ž 1.
The time necessary for a fall of 90% in the resistance is taken as response time Ž t R .. 3. Results and discussion Table 1 presents the particle characterization of the prepared powders. They show large specific areas and small grain sizes ŽTable 1.. The XRD results Table 1 Powder characterization Sample
Fig. 1. Synthesis route for the films.
code
Composition Žmol.
SBE T Žm2 rg.
d BET ˚. ŽA
d XDR ˚. ŽA
Nb1
99SnO 2 –1Nb 2 O5
38.9083
223
120
168
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
Fig. 3. Microstructure of the sample Nb1. Ža. scanning electronic micrography and Žb. atomic force micrography.
indicate only the presence of the Cassiterite phase, which confirms the solid solution between SnO 2 and Nb 2 O5 . The microscopy analysis of the films indicates that the samples are quite homogeneous, porous and rugose Žmedium rugosity, R a s 273.32 nm., as shown in Fig. 3. As a consequence, the samples have a high surface area that favors the adsorption of gaseous species. It was also noticed that the films present good adherence to the substrate. The film thickness was about 1.5 mm, and was measured by SEM using cross-section analysis. The electrical characterization showed the sensor properties of the material. The measurements were made in two stages. In both cases, a fixed concentration of 400 ppm of ethanol was used. Initially, a test for the determination of the temperature of higher sensitivity was performed. The temperature in the test ranged from 508C to 4508C. The experiments were made three times and the curves were based upon the average values. Fig. 4 shows the sensitivity and time–response curves. It was verified that the samples are more sensitive at 2008C ŽFig. 4a., where the sensitivity reaches close to 10. These S values are in agreement with the values reported in Refs. w7–9x. In a second measurement, the temperature was maintained at 2008C and the response time was determined ŽFig. 4b.. For these measurements, the reproducibility obtained was very good. The total saturation of the surface was noticed, once for times larger than t R , the resistance was stable. Unfortu-
nately, it was not possible to compare the results obtained with the literature as the resistance variation adopted in each case was not cited w7,10x.
Fig. 4. Sensitivity curves for sample Nb1. Ža. Determination of high sensitivity temperature and Žb. time–response determination.
I.T. Weber et al.r Materials Letters 43 (2000) 166–169
4. Conclusions The method proposed in this work is quite efficient to obtain thick films of SnO 2 P Nb 2 O5 , with high superficial area, which is extremely advantageous for its application as a sensor of gases. The results of the accomplished analyses indicate that the films are quite homogeneous, porous and with good adherence to the substrata. Therefore, the deposition of films through alcoholic suspensions is a viable and low-cost alternative for the preparation of these films.
References w1x H. Teterycz, J. Kita, R. Bauer, L.J. Golonka, B.W. Licznerski, K. Nitsch, K. Wisniewski, Sensors and Actuators, B 47 Ž1998. 100.
169
w2x J. Watson, I. Kousuke, G.S.V. Colest, Meas. Sci. Technol. 4 Ž1993. 711. w3x W. Gopel, K.D. Schrienbaum, Sensors and Actuators, B ¨ 26–27 Ž1995. 1. w4x K.D. Schrienbaum, U. Weimar, W. Gopel, Sensors and ¨ Actuators, B 7 Ž1992. 709. w5x D. Gouvea, J.A. Varela, E. Longo, A. Smith, J.P. Bonnet, Eur. J. Solid State Inorg. Chem. 30 Ž9. Ž1993. 915. w6x R.L.T. Andrade, C.A. Lindino, L.O.S. Bulhoes, ˜ Quımica ´ Nova 21 Ž3. Ž1998. 348. w7x P.P. Tsai, I.C. Chen, M.H. Tzeng, Sensors and Actuators, B 13–14 Ž1993. 610. w8x P.K. Hesook et al., Sensors and Actuators, B 13–14 Ž1993. 511. w9x Y. Liu, W. Zhu, M.S. Tse, S.Y.J. Shen, Mater. Sci. Lett. 14 Ž1995. 118. w10x P. Lesark, M. Sriyudthsak, Sensors and Actuators, B 24–25 Ž1995. 504.