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Effect of the frequency on the gas sensing response of CoSb2O6 prepared by a colloidal method
Sensors and Actuators B 140 (2009) 149–154
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Effect of the frequency on the gas sensing response of CoSb2 O6 prepared by a colloidal method Carlos R. Michel ∗ , Alma H. Martínez-Preciado, Juan P. Morán-Lázaro Departamento de Física CUCEI, Universidad de Guadalajara, Boulevard Marcelino García Barragán 1421, 44410 Guadalajara, Jalisco, Mexico
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 2 April 2009 Accepted 3 April 2009 Available online 15 April 2009 Keywords: CoSb2 O6 microcolumns Colloidal synthesis Gas sensor Impedance measurements Electron microscopy
a b s t r a c t CoSb2 O6 with trirutile-type structure was prepared by a colloidal method using cobalt nitrate hydrate, antimony trichloride, ethylenediamine and ethyl alcohol. To obtain a precursor material, microwave radiation was applied during the evaporation process. The thermal decomposition of the precursor powder at 600 ◦ C produced single-phase polycrystalline CoSb2 O6 . By scanning electron microscopy a widespread formation of microcolumns, having a length in the range of 6–18 m, and an average diameter of 2.5 m were observed. The gas sensing properties of CoSb2 O6 thick films was evaluated by measuring the transient response of the magnitude of the impedance (|Z|) in air, CO2 and O2 . This characterization was made at 100 Hz, 1 kHz and 100 kHz (400 ◦ C). The results indicated that increasing the frequency, smaller |Z| values were registered. However, a lot more uniform and noise-free response during the detection of these gases was obtained. Polarization curves demonstrated that CoSb2 O6 responds quantitatively to changes in the CO2 concentration. However, in O2 this behavior was not observed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The emission of CO2 during recent decades has significantly contributed to the global warming, and also is associated with respiratory diseases of populations of polluted cities around the world. Apart from the strategies to reduce these emissions, the accurate determination of the concentration of environmental gases demands reliable gas sensor materials and measuring techniques. Regarding gas sensor materials, binary oxides such as SnO2 and ZnO have been intensively studied. However, ternary oxides are also of interest for this application because they show good response to gases, and have chemical and crystal structure stability [1]. Recently, nanostructured CoSb2 O6 synthesized by an aqueous solution-polymerization method has been studied by our group as a potential gas sensor material. The results indicated that this oxide is selective to CO2 and O2 , and exhibits a variation in resistance in the order of 103 during the detection of CO2 [2]. Regarding the synthesis of inorganic materials, Matijevic has used colloidal methods to prepare a wide variety of inorganic compounds such as sulfides, phosphates, oxides and hydrous oxides [3–5]. These methods are simple and direct approaches to produce materials with unique microstructural characteristics; some of which are suitable for gas sensor applications due to their large specific surface area.
∗ Corresponding author. Tel.: +52 33 33 45 41 47; fax: +52 33 33 45 41 47. E-mail address: [email protected] (C.R. Michel). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.04.007
One of the goals of this work was the synthesis of CoSb2 O6 by a non-aqueous colloidal method, using ethylenediamine in acid media. For the characterization of the resulting material as a gas sensor material, AC characterization was explored by measuring the variation of the magnitude of the impedance (|Z|) with time. Another goal was to investigate if this oxide displays a quantitative response to a variation in the gas concentration. With this purpose, polarization curves (DC) were performed. 2. Experimental CoSb2 O6 with trirutile-type structure was prepared by a nonaqueous colloidal method. In a typical synthesis, 2.28 g of Cl3 Sb (J.T. Baker), 1.45 g of Co(NO3 )2 ·6H2 O (J.T. Baker) and 0.5 ml of ethylenediamine (Sigma) were dissolved separately in 5 ml of ethyl alcohol (J.T. Baker). The resulting solutions were fully transparent, with a slight red color in the cobalt solution. First, the solutions of ethylenediamine and cobalt nitrate were mixed under vigorous stirring for 1 h, producing a transparent red solution. Then, the solution of antimony trichloride was slowly added, which produced immediately, a blue precipitate (colloidal dispersion) with pH 2. This precipitate was stirred for 2 h, and then, kept in repose for 2 days at room temperature. The evaporation of the solvent was made by microwave radiation, using a domestic microwave oven (Panasonic), operated at low power. During the evaporation, the temperature of the beaker was under 50 ◦ C to avoid splashing. The resulting precursor was placed inside an alumina crucible with cover, and heated in static air at 200 ◦ C for 8 h. Further annealing
150
C.R. Michel et al. / Sensors and Actuators B 140 (2009) 149–154
Fig. 1. Scheme of the device used for electrical measurements.
at 600 ◦ C, for 5 h in static air was made in a muffle-type furnace (Thermolyne 48000). A heating rate of 100 ◦ C/h was used. After the calcination, a fluffy and black powder was obtained. The structural characterization was made by X-ray powder diffraction (XRD) at room temperature, using a Rigaku Miniflex apparatus (Cu K␣ radiation). The diffraction angle (2) was scanned from 10◦ to 70◦ . The microstructure of the powder calcined at 600 ◦ C was observed by scanning electron microscopy (SEM), using a Jeol JSM-5400LV microscope. The electrical and gas sensing characterization was made on thick films of CoSb2 O6 . These films were prepared as follows: 0.1 g of the powder was placed into a plastic vial containing 1 ml ethyl alcohol absolute, and then sonicated for 5 min. Circular thick films with 5 mm of diameter and uniform thickness 300 m were formed by depositing the suspension inside a ceramic ring, using a syringe. The ring was fixed on an alumina substrate by using four silver wires, see Fig. 1. The impedance measurements were carried out using a LCR meter (Agilent 4263B), which was operated by a computer using the LabView 8.6 software (National Instruments); the experimental setup used for these measurements is shown in Fig. 2. The variation of the current with voltage was recorded using a potentiostat/galvanostat Solartron 1285A. During the gas sensing characterization, the gases were supplied by means of a MKS Instruments 647C mass flow controller. 3. Results and discussion Fig. 3 shows the XRD pattern of the sample calcined at 600 ◦ C. In this pattern the main phase is CoSb2 O6 , which was identified by means of the ICDD file 18-0403. Besides, a small amount of Co2.33 Sb0.67 O4 , corresponding to the peak placed at 2 = 29.5◦ was identified using the ICDD card 15-0517. In this work we found
Fig. 3. X-ray powder diffraction pattern of CoSb2 O6 calcined at 600 ◦ C.
that using a colloidal synthesis method, CoSb2 O6 powder can be obtained at 600 ◦ C, which is a significantly lower temperature, than that reported for the formation of this oxide using other methods. For instance, Larcher et al. obtained this oxide at 800 ◦ C using the solid-state reaction method [6], whereas by a solution polymerization method was obtained at 700 ◦ C [2]. Fig. 4 shows the typical surface morphology of CoSb2 O6 calcined at 600 ◦ C; three different magnifications were used during this observation to show the microstructure in detail. At low magnification (Fig. 4A) it was possible to observe large laminas with irregular shape. In the thickness range of 20–40 m, on the surface of these laminas, numerous microcolumns were easily identified. At a larger magnification (Fig. 4B), individual microcolumns were observed. The entire set of microcolumns resembled a coral-like formation. When the sample was observed at 3500x or above (Fig. 4C), a faceted surface on the microcolumns was revealed. The mean diameter of the microcolumns was 2.5 m. The formation of inorganic solids possessing microstructures similar to those shown in Fig. 4 was explained several years ago by LaMer [7]. According to this author, the understanding of the nucleation processes is a key factor in the preparation of colloidal dispersions. The nucleation involves the generation of initial fragments (or nucleus) of a new and more stable phase, from a metastable mother phase. The shape developed during further particle growth is determined by the geometric characteristic of the nuclei. Then, since several nuclei, with
Fig. 2. Diagram of the experimental setup for gas sensing characterization.
C.R. Michel et al. / Sensors and Actuators B 140 (2009) 149–154
151
Fig. 5. Measured particle length distribution of CoSb2 O6 microcolumns.
behavior of these curves was associated to polaron hoping [8]. Other relevant characteristic of these graphs was that |Z| strongly depends on the frequency at low temperature, but it was almost independent of gas composition. The former was observed by the initial values of |Z|, which at 1 kHz were ∼32–35 M (Fig. 6A), whereas at 100 kHz a drastic reduction to 350 k was observed (Fig. 6B). However, the difference between the |Z| values decreased while increasing the
Fig. 4. SEM images showing the microstructure of CoSb2 O6 at magnifications of: (A) 1000×, (B) 1500×, and (C) 3500×.
different orientations, can be spontaneously formed in colloidal dispersions, the resulting solid may develop a faceted surface, such as that shown in Fig. 4. Fig. 5 shows the microcolumn length distribution, which is in the range from 6 to 18 m; the standard deviation was ±0.5 m. An average length of 10.8 m was obtained by measuring at least 60 microcolumns from several SEM photos. Due to the significant increase in the surface area produced by this method, compared with other routes where particle agglomeration is frequently found, the gas sensor properties of this material were investigated. In the first stage of the gas sensing characterization of CoSb2 O6 , the variation of the magnitude of the impedance (|Z|) with temperature was measured in flowing air, CO2 and O2 . In these experiments three frequencies were tested: 100 Hz, 1 kHz and 100 kHz, using an amplitude of 1 V. Fig. 6 shows the plots obtained at 1 kHz (A) and 100 kHz (B). At 100 Hz the graphs mainly displayed noise, without any apparent trend, which can be attributed to the large values of |Z| (>30 M). From Fig. 6, the decrease of |Z| with temperature is characteristic of the semiconductor materials, and the non-linear
Fig. 6. Variation of |Z| with temperature graphs of CoSb2 O6 thick films measured in air, O2 and CO2 , using a frequency of: (A) 1 kHz and (B) 100 kHz.
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Fig. 7. |Z| vs. frequency graphs of CoSb2 O6 thick films measured at 400 ◦ C in CO2 , and equivalent RC parallel circuit (in the inset).
temperature, and at 400 ◦ C, |Z| was ∼230 k (1 kHz), whereas at 100 kHz was ∼90 k. Moreover, from Fig. 6 the decrease of |Z| with frequency resembled a low-pass filter [9]. In order to verify this fact, |Z| was measured in the frequency range from 100 Hz to 100 kHz, at 400 ◦ C. Fig. 7 shows the corresponding graph. Clearly, a decrease in |Z| while increasing the frequency was observed; which is consistent with the result obtained from a low-pass filter. This filter is composed of a resistance (Rp) and a capacitor (Cp) in parallel, as shown in the inset of Fig. 7. On the other hand, to evaluate the effect of the frequency on the gas sensing response, the variation of |Z| with time caused by the change in the gas composition (dynamic tests) was measured. The optimal temperature to perform these measurements was investigated by performing several tests from 200 to 500 ◦ C, finding that at 400 ◦ C the largest response was registered. The dynamic tests were performed as follows: dry synthetic air was supplied for approximately 5 min to obtain stable |Z| readings. Then, a test gas CO2 (or O2 ) was delivered for 3 min. Immediately after, air was supplied again. Figs. 8 and 9 show typical dynamic response graphs obtained in CO2 and O2 respectively. From Fig. 8, when CO2 was supplied at 100 Hz, an average increase in |Z| of ∼3 k was measured. When the frequencies 1 and 100 kHz were applied, the increase in |Z| was 3.9 k and 1.1 k respectively. Evidently from these results, a relationship between frequency and |Z| was not obtained. However, increasing the frequency, considerably less noisy and more well defined graphs were recorded. The noise observed in the graph obtained at 100 Hz can be attributed to flicker noise or pink noise, and is usually found in many electronic devices. Besides, it is well known that this noise decreases while increasing the applied frequency as 1/f; where f is frequency [10]. Once the noise was erased by applying a frequency of 100 kHz, sharper curves were acquired. This permitted the identification of a change in the slope of |Z| during the injection of CO2 (Fig. 8C). The values of the slopes shown in this figure were calculated. In a first stage a slope of 1.2 was obtained, whereas in the second stage this value was reduced to 0.6. It is worth to mention that after the introduction of air, a full recovery of the original |Z| value was observed, indicating the reversibility of the process. On the other hand, when oxygen was used as testing gas, the response of CoSb2 O6 had the opposite behavior than that observed in CO2 . Fig. 9 shows the results obtained at 1 kHz (A) and 100 kHz (B). At 100 Hz, the graph mainly displayed flicker noise, without
Fig. 8. |Z| vs. time plots of CoSb2 O6 measured at 100 Hz (A), 1 kHz (B) and 100 kHz (C), in air and CO2 (400 ◦ C).
any possible interpretation. In this gas, the variation of |Z| at 1 kHz was −1.7 k; whereas at 100 kHz was −600 . Moreover, a continuous decrease of the impedance can be observed in Fig. 9A and B. This decrement cannot be related to the formation of a mixed valence state in cobalt ions (2+, 3+), produced by the intake of oxygen, because antimony ions have their higher oxidation state 2− (Co2+ Sb5+ 2 O6 ) [11]. Therefore, surface phenomena such as oxygen adsorption are the more probable cause. Since air contains about 21% of oxygen, this gas was present in the entire test, and was gradually adsorbed in the surface of CoSb2 O6 reducing its impedance. The slopes at which the impedance was reduced applying 1 and 100 kHz were calculated from Fig. 9A and B. It was found that in the
C.R. Michel et al. / Sensors and Actuators B 140 (2009) 149–154
153
Fig. 10. Variation of |Z| with time for different air:CO2 flow ratios in CoSb2 O6 thick films.
Fig. 9. Variation of |Z| with time graphs measured in air and O2 , using (A) 1 kHz and (B) 100 kHz (400 ◦ C). Fig. 11. Polarization curves of CoSb2 O6 recorded in several air:CO2 mixtures (400 ◦ C).
former a slope of 0.12 was obtained; whereas in the latter was 0.02. Therefore, these results indicate that applying 100 kHz, the effect of the oxygen adsorbed from air has a smaller interference in the measurements. Moreover, the gas sensing mechanism in p-type semiconductors, such as CoSb2 O6 , it is well known that in the case of O2 , the adsorption of this gas produces an increase in the number of holes in the valence band, which decreases |Z|. The most accepted mechanism for O2 detection involves at least one of the following adsorption processes: O2 (gas) + 2e− → 2O− (ads)
(1)
−
(2)
O2 (gas) + 4e → 2O
2−
(ads)
where (ads) means the adsorbed species [12]. In the case of CO2 , several authors have proposed that this gas is adsorbed on the surface of oxides forming a very thin carbonation layer; this partial carbonation increases |Z| and when air is supplied, this carbonate layer is fully removed [13–15]. On the other hand, in order to test the ability of CoSb2 O6 to detect changes in the carbon dioxide concentration, the variation of |Z| with time was evaluated at 400 ◦ C (100 kHz). In a previous section these parameters produced the best results. In these experiments, gas flow mixtures of air and carbon dioxide (air:CO2 ), resulting of multiples 0.22 sccm were tested; Fig. 10 shows a typical graph. Clearly, by increasing the concentration of CO2 , an increase in |Z| was registered; the recovery of the original
|Z| value after the introduction of air can also be observed in this graph. In order to confirm the results obtained in AC mode for different CO2 concentrations, DC electrical characterization was performed. In these tests, polarization curves were recorded from −2 to 2 V, at 400 ◦ C. Fig. 11 displays the results acquired for the same air:CO2 mixtures used before. In this case, by increasing the concentration of CO2 a decrease in the current can be observed; besides, a nearly linear behavior is displayed for all the gas mixtures tested. These results are in agreement with previous results; however, a notable improvement was obtained in the present work, since individual graphs for each air:CO2 ratio were acquired [2]. The results obtained in AC and DC modes demonstrate that CoSb2 O6 can detect changes in the CO2 concentration; however, with oxygen the results shown a disordered behavior. 4. Conclusion Even though the use of colloidal methods for the synthesis of inorganic materials is not new, many oxide compositions possessing unique morphological characteristics can be obtained using these methods. Also, a better control on the stoichiometry and lower calcination temperatures can be achieved using these costeffective approaches. The repeatability of this synthesis method was tested by preparing the CoSb2 O6 powder several times, obtaining the same general morphological characteristics.
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The AC electrical characterization demonstrates the important role of the applied frequency on the gas sensing properties of CoSb2 O6 thick films. The use of a frequency of 100 kHz helped to erase the noise and identify different steps of CO2 adsorption. Comparing the results obtained in CO2 and O2 , in the latter, the variation of |Z| was smaller and far less uniform. Acknowledgements Financial support from the Coordinación General Académica of the Universidad de Guadalajara and the National Council of Science and Technology of Mexico (CONACYT) under Grant I-52204 is greatly acknowledged. The authors are grateful to CONACYT for her doctorate (A.H.M.P.) and master (J.P.M.L.) scholarships.
[8] C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1996, pp. 297–299. [9] A. Williams, F.J. Taylor, Electronic Filter Design Handbook, McGraw-Hill, New York, 2006, pp. 89–135. [10] A. Snarskii, I. Bezsudnov, Giant 1/f noise in two-dimensional polycrystalline media, Physica B 403 (2008), pp. 3519–3512. [11] C.N.R. Rao, P. Ganguly (Eds.), The Metallic and Non-metallic States of Matter, Taylor & Francis, London, 1985, pp. 329–357. [12] K.W. Kolasinski, Surface Science, Wiley, Chichester, 2008, pp. 127–192. [13] H. Tsuji, A. Okamura-Yoshida, T. Shishido, H. Hattori, Dynamic behavior of carbonate species on metal oxide surface: oxygen scrambling between adsorbed carbon dioxide and oxide surface, Langmuir 19 (2003) 8793–8800. [14] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, O. Okada, H. Yanagida, Mixed oxide capacitor of CuO-BaTiO3 new type CO2 gas sensor, J. Am. Ceram. Soc. 75 (1992) 613–618. [15] Y. Nakamura, H. Zhuang, A. Kishimoto, Enhanced CO and CO2 gas sensitivity of the CuO/ZnO heterocontact made by quenched CuO ceramics, J. Electrochem. Soc. 145 (1998) 632–638.
Biographies References [1] C.R. Michel, E. López-Mena, A.H. Martínez, Grain-size effects on gas response in nanostructured Gd0.9 Ba0.1 CoO3 , Talanta 74 (2007) 235–240. [2] C.R. Michel, A.H. Martínez, S. Jiménez, Gas sensing response of nanostructured tri-rutile type CoSb2 O6 synthesized by solution-polymerization method, Sens. Actuators B 132 (2008) 45–51. [3] E. Matijevic, Preparation and properties of uniform size colloids, Chem. Mater. 5 (1993) 412–426. [4] E. Matijevic, Uniform inorganic colloid dispersions. Achievements and challenges, Langmuir 10 (1994) 8–16. [5] E. Matijevic, Monodispersed metal (hydrous) oxides—a fascinating field of colloid science, Acc. Chem. Res. 14 (1981) 22–29. [6] D. Larcher, A.S. Prakash, L. Laffont, M. Womes, J.C. Jumas, J. Olivier-Fourcade, M.S. Hedge, J.-M. Tarascon, Reactivity of antimony oxides and MSb2 O6 (M = Cu, Ni, Co), trirutile-type phases with metallic lithium, J. Electrochem. Soc. 153 (2006) A1778–A1787. [7] V.K. LaMer, Nucleation in phase transitions, Ind. Eng. Chem. 44 (1952) 1270–1277.
Carlos R. Michel obtained his PhD in materials science in 1997 from the Universitat Autònoma de Barcelona, Spain. He is currently employed as professor–researcher in the Centro Universitario de Ciencias Exactas e Ingenierías (CUCEI) of the Universidad de Guadalajara, México. His main interests are focused on the preparation of inorganic oxides using several synthesis methods, and their potential application as environmental gas sensor materials. Alma H. Martínez-Preciado obtained her MSc in biotechnology from Universidad de Guadalajara in Biotechnology in 2004. She is currently preparing her PhD in chemical engineering. Her research fields of interest are the development of alternative methods of synthesis and the characterization of materials by electron microscopy. Juan P. Morán-Lázaro obtained his BSc in electronic engineering from the Instituto Tecnológico de Cd. Guzmán, Zapotlán, México, in 2004. He is currently preparing his MSc in physics in the Universidad de Guadalajara (CUCEI). He is working on the synthesis and characterization of inorganic materials and their application to gas sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Effect of the frequency on the gas sensing response of CoSb2 O6 prepared by a colloidal method Carlos R. Michel ∗ , Alma H. Martínez-Preciado, Juan P. Morán-Lázaro Departamento de Física CUCEI, Universidad de Guadalajara, Boulevard Marcelino García Barragán 1421, 44410 Guadalajara, Jalisco, Mexico
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 2 April 2009 Accepted 3 April 2009 Available online 15 April 2009 Keywords: CoSb2 O6 microcolumns Colloidal synthesis Gas sensor Impedance measurements Electron microscopy
a b s t r a c t CoSb2 O6 with trirutile-type structure was prepared by a colloidal method using cobalt nitrate hydrate, antimony trichloride, ethylenediamine and ethyl alcohol. To obtain a precursor material, microwave radiation was applied during the evaporation process. The thermal decomposition of the precursor powder at 600 ◦ C produced single-phase polycrystalline CoSb2 O6 . By scanning electron microscopy a widespread formation of microcolumns, having a length in the range of 6–18 m, and an average diameter of 2.5 m were observed. The gas sensing properties of CoSb2 O6 thick films was evaluated by measuring the transient response of the magnitude of the impedance (|Z|) in air, CO2 and O2 . This characterization was made at 100 Hz, 1 kHz and 100 kHz (400 ◦ C). The results indicated that increasing the frequency, smaller |Z| values were registered. However, a lot more uniform and noise-free response during the detection of these gases was obtained. Polarization curves demonstrated that CoSb2 O6 responds quantitatively to changes in the CO2 concentration. However, in O2 this behavior was not observed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The emission of CO2 during recent decades has significantly contributed to the global warming, and also is associated with respiratory diseases of populations of polluted cities around the world. Apart from the strategies to reduce these emissions, the accurate determination of the concentration of environmental gases demands reliable gas sensor materials and measuring techniques. Regarding gas sensor materials, binary oxides such as SnO2 and ZnO have been intensively studied. However, ternary oxides are also of interest for this application because they show good response to gases, and have chemical and crystal structure stability [1]. Recently, nanostructured CoSb2 O6 synthesized by an aqueous solution-polymerization method has been studied by our group as a potential gas sensor material. The results indicated that this oxide is selective to CO2 and O2 , and exhibits a variation in resistance in the order of 103 during the detection of CO2 [2]. Regarding the synthesis of inorganic materials, Matijevic has used colloidal methods to prepare a wide variety of inorganic compounds such as sulfides, phosphates, oxides and hydrous oxides [3–5]. These methods are simple and direct approaches to produce materials with unique microstructural characteristics; some of which are suitable for gas sensor applications due to their large specific surface area.
∗ Corresponding author. Tel.: +52 33 33 45 41 47; fax: +52 33 33 45 41 47. E-mail address: [email protected] (C.R. Michel). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.04.007
One of the goals of this work was the synthesis of CoSb2 O6 by a non-aqueous colloidal method, using ethylenediamine in acid media. For the characterization of the resulting material as a gas sensor material, AC characterization was explored by measuring the variation of the magnitude of the impedance (|Z|) with time. Another goal was to investigate if this oxide displays a quantitative response to a variation in the gas concentration. With this purpose, polarization curves (DC) were performed. 2. Experimental CoSb2 O6 with trirutile-type structure was prepared by a nonaqueous colloidal method. In a typical synthesis, 2.28 g of Cl3 Sb (J.T. Baker), 1.45 g of Co(NO3 )2 ·6H2 O (J.T. Baker) and 0.5 ml of ethylenediamine (Sigma) were dissolved separately in 5 ml of ethyl alcohol (J.T. Baker). The resulting solutions were fully transparent, with a slight red color in the cobalt solution. First, the solutions of ethylenediamine and cobalt nitrate were mixed under vigorous stirring for 1 h, producing a transparent red solution. Then, the solution of antimony trichloride was slowly added, which produced immediately, a blue precipitate (colloidal dispersion) with pH 2. This precipitate was stirred for 2 h, and then, kept in repose for 2 days at room temperature. The evaporation of the solvent was made by microwave radiation, using a domestic microwave oven (Panasonic), operated at low power. During the evaporation, the temperature of the beaker was under 50 ◦ C to avoid splashing. The resulting precursor was placed inside an alumina crucible with cover, and heated in static air at 200 ◦ C for 8 h. Further annealing
150
C.R. Michel et al. / Sensors and Actuators B 140 (2009) 149–154
Fig. 1. Scheme of the device used for electrical measurements.
at 600 ◦ C, for 5 h in static air was made in a muffle-type furnace (Thermolyne 48000). A heating rate of 100 ◦ C/h was used. After the calcination, a fluffy and black powder was obtained. The structural characterization was made by X-ray powder diffraction (XRD) at room temperature, using a Rigaku Miniflex apparatus (Cu K␣ radiation). The diffraction angle (2) was scanned from 10◦ to 70◦ . The microstructure of the powder calcined at 600 ◦ C was observed by scanning electron microscopy (SEM), using a Jeol JSM-5400LV microscope. The electrical and gas sensing characterization was made on thick films of CoSb2 O6 . These films were prepared as follows: 0.1 g of the powder was placed into a plastic vial containing 1 ml ethyl alcohol absolute, and then sonicated for 5 min. Circular thick films with 5 mm of diameter and uniform thickness 300 m were formed by depositing the suspension inside a ceramic ring, using a syringe. The ring was fixed on an alumina substrate by using four silver wires, see Fig. 1. The impedance measurements were carried out using a LCR meter (Agilent 4263B), which was operated by a computer using the LabView 8.6 software (National Instruments); the experimental setup used for these measurements is shown in Fig. 2. The variation of the current with voltage was recorded using a potentiostat/galvanostat Solartron 1285A. During the gas sensing characterization, the gases were supplied by means of a MKS Instruments 647C mass flow controller. 3. Results and discussion Fig. 3 shows the XRD pattern of the sample calcined at 600 ◦ C. In this pattern the main phase is CoSb2 O6 , which was identified by means of the ICDD file 18-0403. Besides, a small amount of Co2.33 Sb0.67 O4 , corresponding to the peak placed at 2 = 29.5◦ was identified using the ICDD card 15-0517. In this work we found
Fig. 3. X-ray powder diffraction pattern of CoSb2 O6 calcined at 600 ◦ C.
that using a colloidal synthesis method, CoSb2 O6 powder can be obtained at 600 ◦ C, which is a significantly lower temperature, than that reported for the formation of this oxide using other methods. For instance, Larcher et al. obtained this oxide at 800 ◦ C using the solid-state reaction method [6], whereas by a solution polymerization method was obtained at 700 ◦ C [2]. Fig. 4 shows the typical surface morphology of CoSb2 O6 calcined at 600 ◦ C; three different magnifications were used during this observation to show the microstructure in detail. At low magnification (Fig. 4A) it was possible to observe large laminas with irregular shape. In the thickness range of 20–40 m, on the surface of these laminas, numerous microcolumns were easily identified. At a larger magnification (Fig. 4B), individual microcolumns were observed. The entire set of microcolumns resembled a coral-like formation. When the sample was observed at 3500x or above (Fig. 4C), a faceted surface on the microcolumns was revealed. The mean diameter of the microcolumns was 2.5 m. The formation of inorganic solids possessing microstructures similar to those shown in Fig. 4 was explained several years ago by LaMer [7]. According to this author, the understanding of the nucleation processes is a key factor in the preparation of colloidal dispersions. The nucleation involves the generation of initial fragments (or nucleus) of a new and more stable phase, from a metastable mother phase. The shape developed during further particle growth is determined by the geometric characteristic of the nuclei. Then, since several nuclei, with
Fig. 2. Diagram of the experimental setup for gas sensing characterization.
C.R. Michel et al. / Sensors and Actuators B 140 (2009) 149–154
151
Fig. 5. Measured particle length distribution of CoSb2 O6 microcolumns.
behavior of these curves was associated to polaron hoping [8]. Other relevant characteristic of these graphs was that |Z| strongly depends on the frequency at low temperature, but it was almost independent of gas composition. The former was observed by the initial values of |Z|, which at 1 kHz were ∼32–35 M (Fig. 6A), whereas at 100 kHz a drastic reduction to 350 k was observed (Fig. 6B). However, the difference between the |Z| values decreased while increasing the
Fig. 4. SEM images showing the microstructure of CoSb2 O6 at magnifications of: (A) 1000×, (B) 1500×, and (C) 3500×.
different orientations, can be spontaneously formed in colloidal dispersions, the resulting solid may develop a faceted surface, such as that shown in Fig. 4. Fig. 5 shows the microcolumn length distribution, which is in the range from 6 to 18 m; the standard deviation was ±0.5 m. An average length of 10.8 m was obtained by measuring at least 60 microcolumns from several SEM photos. Due to the significant increase in the surface area produced by this method, compared with other routes where particle agglomeration is frequently found, the gas sensor properties of this material were investigated. In the first stage of the gas sensing characterization of CoSb2 O6 , the variation of the magnitude of the impedance (|Z|) with temperature was measured in flowing air, CO2 and O2 . In these experiments three frequencies were tested: 100 Hz, 1 kHz and 100 kHz, using an amplitude of 1 V. Fig. 6 shows the plots obtained at 1 kHz (A) and 100 kHz (B). At 100 Hz the graphs mainly displayed noise, without any apparent trend, which can be attributed to the large values of |Z| (>30 M). From Fig. 6, the decrease of |Z| with temperature is characteristic of the semiconductor materials, and the non-linear
Fig. 6. Variation of |Z| with temperature graphs of CoSb2 O6 thick films measured in air, O2 and CO2 , using a frequency of: (A) 1 kHz and (B) 100 kHz.
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Fig. 7. |Z| vs. frequency graphs of CoSb2 O6 thick films measured at 400 ◦ C in CO2 , and equivalent RC parallel circuit (in the inset).
temperature, and at 400 ◦ C, |Z| was ∼230 k (1 kHz), whereas at 100 kHz was ∼90 k. Moreover, from Fig. 6 the decrease of |Z| with frequency resembled a low-pass filter [9]. In order to verify this fact, |Z| was measured in the frequency range from 100 Hz to 100 kHz, at 400 ◦ C. Fig. 7 shows the corresponding graph. Clearly, a decrease in |Z| while increasing the frequency was observed; which is consistent with the result obtained from a low-pass filter. This filter is composed of a resistance (Rp) and a capacitor (Cp) in parallel, as shown in the inset of Fig. 7. On the other hand, to evaluate the effect of the frequency on the gas sensing response, the variation of |Z| with time caused by the change in the gas composition (dynamic tests) was measured. The optimal temperature to perform these measurements was investigated by performing several tests from 200 to 500 ◦ C, finding that at 400 ◦ C the largest response was registered. The dynamic tests were performed as follows: dry synthetic air was supplied for approximately 5 min to obtain stable |Z| readings. Then, a test gas CO2 (or O2 ) was delivered for 3 min. Immediately after, air was supplied again. Figs. 8 and 9 show typical dynamic response graphs obtained in CO2 and O2 respectively. From Fig. 8, when CO2 was supplied at 100 Hz, an average increase in |Z| of ∼3 k was measured. When the frequencies 1 and 100 kHz were applied, the increase in |Z| was 3.9 k and 1.1 k respectively. Evidently from these results, a relationship between frequency and |Z| was not obtained. However, increasing the frequency, considerably less noisy and more well defined graphs were recorded. The noise observed in the graph obtained at 100 Hz can be attributed to flicker noise or pink noise, and is usually found in many electronic devices. Besides, it is well known that this noise decreases while increasing the applied frequency as 1/f; where f is frequency [10]. Once the noise was erased by applying a frequency of 100 kHz, sharper curves were acquired. This permitted the identification of a change in the slope of |Z| during the injection of CO2 (Fig. 8C). The values of the slopes shown in this figure were calculated. In a first stage a slope of 1.2 was obtained, whereas in the second stage this value was reduced to 0.6. It is worth to mention that after the introduction of air, a full recovery of the original |Z| value was observed, indicating the reversibility of the process. On the other hand, when oxygen was used as testing gas, the response of CoSb2 O6 had the opposite behavior than that observed in CO2 . Fig. 9 shows the results obtained at 1 kHz (A) and 100 kHz (B). At 100 Hz, the graph mainly displayed flicker noise, without
Fig. 8. |Z| vs. time plots of CoSb2 O6 measured at 100 Hz (A), 1 kHz (B) and 100 kHz (C), in air and CO2 (400 ◦ C).
any possible interpretation. In this gas, the variation of |Z| at 1 kHz was −1.7 k; whereas at 100 kHz was −600 . Moreover, a continuous decrease of the impedance can be observed in Fig. 9A and B. This decrement cannot be related to the formation of a mixed valence state in cobalt ions (2+, 3+), produced by the intake of oxygen, because antimony ions have their higher oxidation state 2− (Co2+ Sb5+ 2 O6 ) [11]. Therefore, surface phenomena such as oxygen adsorption are the more probable cause. Since air contains about 21% of oxygen, this gas was present in the entire test, and was gradually adsorbed in the surface of CoSb2 O6 reducing its impedance. The slopes at which the impedance was reduced applying 1 and 100 kHz were calculated from Fig. 9A and B. It was found that in the
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Fig. 10. Variation of |Z| with time for different air:CO2 flow ratios in CoSb2 O6 thick films.
Fig. 9. Variation of |Z| with time graphs measured in air and O2 , using (A) 1 kHz and (B) 100 kHz (400 ◦ C). Fig. 11. Polarization curves of CoSb2 O6 recorded in several air:CO2 mixtures (400 ◦ C).
former a slope of 0.12 was obtained; whereas in the latter was 0.02. Therefore, these results indicate that applying 100 kHz, the effect of the oxygen adsorbed from air has a smaller interference in the measurements. Moreover, the gas sensing mechanism in p-type semiconductors, such as CoSb2 O6 , it is well known that in the case of O2 , the adsorption of this gas produces an increase in the number of holes in the valence band, which decreases |Z|. The most accepted mechanism for O2 detection involves at least one of the following adsorption processes: O2 (gas) + 2e− → 2O− (ads)
(1)
−
(2)
O2 (gas) + 4e → 2O
2−
(ads)
where (ads) means the adsorbed species [12]. In the case of CO2 , several authors have proposed that this gas is adsorbed on the surface of oxides forming a very thin carbonation layer; this partial carbonation increases |Z| and when air is supplied, this carbonate layer is fully removed [13–15]. On the other hand, in order to test the ability of CoSb2 O6 to detect changes in the carbon dioxide concentration, the variation of |Z| with time was evaluated at 400 ◦ C (100 kHz). In a previous section these parameters produced the best results. In these experiments, gas flow mixtures of air and carbon dioxide (air:CO2 ), resulting of multiples 0.22 sccm were tested; Fig. 10 shows a typical graph. Clearly, by increasing the concentration of CO2 , an increase in |Z| was registered; the recovery of the original
|Z| value after the introduction of air can also be observed in this graph. In order to confirm the results obtained in AC mode for different CO2 concentrations, DC electrical characterization was performed. In these tests, polarization curves were recorded from −2 to 2 V, at 400 ◦ C. Fig. 11 displays the results acquired for the same air:CO2 mixtures used before. In this case, by increasing the concentration of CO2 a decrease in the current can be observed; besides, a nearly linear behavior is displayed for all the gas mixtures tested. These results are in agreement with previous results; however, a notable improvement was obtained in the present work, since individual graphs for each air:CO2 ratio were acquired [2]. The results obtained in AC and DC modes demonstrate that CoSb2 O6 can detect changes in the CO2 concentration; however, with oxygen the results shown a disordered behavior. 4. Conclusion Even though the use of colloidal methods for the synthesis of inorganic materials is not new, many oxide compositions possessing unique morphological characteristics can be obtained using these methods. Also, a better control on the stoichiometry and lower calcination temperatures can be achieved using these costeffective approaches. The repeatability of this synthesis method was tested by preparing the CoSb2 O6 powder several times, obtaining the same general morphological characteristics.
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The AC electrical characterization demonstrates the important role of the applied frequency on the gas sensing properties of CoSb2 O6 thick films. The use of a frequency of 100 kHz helped to erase the noise and identify different steps of CO2 adsorption. Comparing the results obtained in CO2 and O2 , in the latter, the variation of |Z| was smaller and far less uniform. Acknowledgements Financial support from the Coordinación General Académica of the Universidad de Guadalajara and the National Council of Science and Technology of Mexico (CONACYT) under Grant I-52204 is greatly acknowledged. The authors are grateful to CONACYT for her doctorate (A.H.M.P.) and master (J.P.M.L.) scholarships.
[8] C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1996, pp. 297–299. [9] A. Williams, F.J. Taylor, Electronic Filter Design Handbook, McGraw-Hill, New York, 2006, pp. 89–135. [10] A. Snarskii, I. Bezsudnov, Giant 1/f noise in two-dimensional polycrystalline media, Physica B 403 (2008), pp. 3519–3512. [11] C.N.R. Rao, P. Ganguly (Eds.), The Metallic and Non-metallic States of Matter, Taylor & Francis, London, 1985, pp. 329–357. [12] K.W. Kolasinski, Surface Science, Wiley, Chichester, 2008, pp. 127–192. [13] H. Tsuji, A. Okamura-Yoshida, T. Shishido, H. Hattori, Dynamic behavior of carbonate species on metal oxide surface: oxygen scrambling between adsorbed carbon dioxide and oxide surface, Langmuir 19 (2003) 8793–8800. [14] T. Ishihara, K. Kometani, Y. Mizuhara, Y. Takita, O. Okada, H. Yanagida, Mixed oxide capacitor of CuO-BaTiO3 new type CO2 gas sensor, J. Am. Ceram. Soc. 75 (1992) 613–618. [15] Y. Nakamura, H. Zhuang, A. Kishimoto, Enhanced CO and CO2 gas sensitivity of the CuO/ZnO heterocontact made by quenched CuO ceramics, J. Electrochem. Soc. 145 (1998) 632–638.
Biographies References [1] C.R. Michel, E. López-Mena, A.H. Martínez, Grain-size effects on gas response in nanostructured Gd0.9 Ba0.1 CoO3 , Talanta 74 (2007) 235–240. [2] C.R. Michel, A.H. Martínez, S. Jiménez, Gas sensing response of nanostructured tri-rutile type CoSb2 O6 synthesized by solution-polymerization method, Sens. Actuators B 132 (2008) 45–51. [3] E. Matijevic, Preparation and properties of uniform size colloids, Chem. Mater. 5 (1993) 412–426. [4] E. Matijevic, Uniform inorganic colloid dispersions. Achievements and challenges, Langmuir 10 (1994) 8–16. [5] E. Matijevic, Monodispersed metal (hydrous) oxides—a fascinating field of colloid science, Acc. Chem. Res. 14 (1981) 22–29. [6] D. Larcher, A.S. Prakash, L. Laffont, M. Womes, J.C. Jumas, J. Olivier-Fourcade, M.S. Hedge, J.-M. Tarascon, Reactivity of antimony oxides and MSb2 O6 (M = Cu, Ni, Co), trirutile-type phases with metallic lithium, J. Electrochem. Soc. 153 (2006) A1778–A1787. [7] V.K. LaMer, Nucleation in phase transitions, Ind. Eng. Chem. 44 (1952) 1270–1277.
Carlos R. Michel obtained his PhD in materials science in 1997 from the Universitat Autònoma de Barcelona, Spain. He is currently employed as professor–researcher in the Centro Universitario de Ciencias Exactas e Ingenierías (CUCEI) of the Universidad de Guadalajara, México. His main interests are focused on the preparation of inorganic oxides using several synthesis methods, and their potential application as environmental gas sensor materials. Alma H. Martínez-Preciado obtained her MSc in biotechnology from Universidad de Guadalajara in Biotechnology in 2004. She is currently preparing her PhD in chemical engineering. Her research fields of interest are the development of alternative methods of synthesis and the characterization of materials by electron microscopy. Juan P. Morán-Lázaro obtained his BSc in electronic engineering from the Instituto Tecnológico de Cd. Guzmán, Zapotlán, México, in 2004. He is currently preparing his MSc in physics in the Universidad de Guadalajara (CUCEI). He is working on the synthesis and characterization of inorganic materials and their application to gas sensors.