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An alternative gas sensor material: Synthesis and electrical characterization of SmCoO3
Materials Research Bulletin 42 (2007) 84–93 www.elsevier.com/locate/matresbu
An alternative gas sensor material: Synthesis and electrical characterization of SmCoO3 Carlos Rafael Michel *, Emilio Delgado, Gloria Santilla´n, Alma H. Martı´nez, Arturo Cha´vez-Cha´vez Departamento de Fı´sica CUCEI, Universidad de Guadalajara, Blvd. M. Garcı´a Barraga´n 1421, 44410 Guadalajara, Jalisco, Mexico Received 21 December 2005; received in revised form 24 March 2006; accepted 15 May 2006 Available online 8 June 2006
Abstract Single-phase perovskite SmCoO3 was prepared by a wet-chemical synthesis technique using metal-nitrates and citric acid; after its characterization by thermal analyses and X-ray diffraction, sintering at 900 8C in air, gave single phase and well crystallized powders. The powders were mixed with an organic solvent to prepare a slurry, which was deposited on alumina substrates as thick films, using the screen-printing technique. Electrical and gas sensing properties of sintered SmCoO3 films were investigated in air, O2 and CO2, the results show that sensitivity reached a maximum value at 420 8C, for both gases. Dynamic tests revealed a better behavior of SmCoO3 in CO2 than O2, due to a fast response and a larger electrical resistance change to this gas. X-ray diffraction made on powders after electrical characterization in gases, showed that perovskite-type structure was preserved. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. Electron microscopy; D. Electrical properties
1. Introduction Considerable scientific and technological research in solid state gas sensors, has been made due to the need of environmental monitoring and measurement, in rural and urban locations. In this field only a limited number of compositions such as: SnO2, ZnO, TiO2, and few more have been studied broadly; although, some oxides with perovskite-type structure, such as SmFeO3 and La1xSrxFeO3 exhibit notable gas sensitivity and have been tested as sensors with good results [1–8]. In the case of sensors based in perovskites possessing cobalt, Ishihara et al. have found that a combination of La0.6Sr0.4CoO3 with Pt show large sensitivity, as well as selectivity to C3H6; according to these authors, the use of this material is for detection of hydrocarbons in the exhaust gas of internal combustion engines [9]. Moreover, Tunney et al. have found that thin-films of SrFeyCo1yO3 show notable oxygen sensitivity, which depends on the extent of iron substitution by cobalt and the thickness of films [10]. About the synthesis of perovskites, the ceramic method has been one of the most commonly used, however, soft chemistry routes of preparation are convenient alternative ways to obtain inorganic materials. The resulting materials by these methods exhibit a considerable smaller particle size and high porosity, compared with those synthesized by
* Corresponding author. Tel.: +52 33 33 45 41 47; fax: +52 33 33 45 41 47. E-mail address: [email protected] (C.R. Michel). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.05.008
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ceramic method. The specific surface area-to-volume ratio, results in unique properties which are greatly different from bulk materials and are ideal to surface-environment applications such as gas sensing and catalysis. Soft chemistry methods also provide a better control on stoichiometry and reduce the temperature of synthesis of materials [11,12]. In this work, single-phase SmCoO3 was prepared by a wet-chemical method, using samarium and cobalt nitrates in aqueous media containing citric acid. The calcination at 900 8C of a dried precursor, resulted in single-phase SmCoO3; which was tested as environmental gas sensor, and the results obtained were compared with those reported for similar materials. 2. Experimental The starting reagents used for the preparation of SmCoO3 were Sm(NO3)36H2O (Alfa Aesar) and Co(NO3)26H2O (J.T. Baker), which in stoichiometric amounts were dissolved in deionized water containing 0.1 mol of citric acid (Chemical Products Monterrey). The mixture was heat-dried at 75 8C for 3 h, after the complete water evaporation an exothermic reaction was observed, which produced a precursor powder. This material was analyzed by thermogravimetric and differential thermal analyses (TGA–DTA), using a SDT 2960 TA Instruments balance with a heating rate 5 8C/min, in flowing dry air. Calcination of precursor was made in air from 600 to 900 8C in a muffle-type furnace; the sintered powders were analyzed by X-ray powder diffraction (XRD), using a Rigaku Miniflex apparatus (Cu Ka radiation). Thick-films were prepared by the screen-printing technique using single-phase powders, which were mixed in isopropyl alcohol (Chemical Products Monterrey) using an ultrasonic bath, and further deposited on alumina discs. The films were heated in an oven overnight at 50 8C, and later fired at 700 8C for 2 h. Microstructure was observed by scanning electron microscopy (SEM), using a Jeol JSM 5400LV microscope. dc electrical characterization was performed using high-purity silver wires (diameter 0.2 mm), fixed with silver paste (Chemtronics) to the films. The measurements were carried out using the two-point method, from room temperature to 700 8C, using a tube-type furnace with temperature programmable control. Dynamic response of resistance experiments were performed at a constant temperature, using an Agilent 34970A data acquisition system; the films were alternatively exposed to flowing dry air, O2 (Infra, 99.99%) and CO2 (Infra, 99.8%), maintaining the gas flow rate at 150 cm3/min. 3. Results and discussion Several synthesis routes have been tested for the preparation of perovskites containing cobalt, in the case of TCoO3 (T = Y, Pr, Nd, Sm, Er, Yb, Gd, Eu, Tm and Lu), Demazeau et al. found that the use of simultaneous application of a high-oxygen pressure (40–90 kb) and a moderate temperature (500–600 8C), is required for the formation of these oxides; when the corresponding oxides T2O3 and Co3O4 were used as precursor materials (ceramic method). In this method the thermal decomposition of KClO3 is the source of oxygen [13]. However, Yoshii et al. obtained Pr1xBaxCoO3, using the ceramic method, at 1200 8C and ambient pressure, indicating that high-oxygen pressure is not indispensable in the preparation of cobalt perovskites [14]. On the other hand, the solution method has also been explored for the synthesis of these oxides; this route involves the dissolution of the corresponding nitrates, chlorides or acetates, in an appropriate liquid [15–24]. By this procedure, the reported synthesis temperatures are usually from 800 to 1100 8C, depending on the chemical composition; with the advantage of high-oxygen pressure sintering is avoided. Moreover, the effect of the molecular level homogeneity developed in solution produce, after calcination, materials with high surface-to-volume ratios [25–27], suggesting a possible application of these materials as gas sensors. In this paper, samarium and cobalt nitrates were chosen as reagents, due to their high solubility in water; citric acid was added to the solution to promote, at the end of water evaporation, its combustion with the nitrates; this exothermic reaction produced an amorphous powder (precursor), with small particle size. In order to determine the temperature of formation of SmCoO3 by this synthesis route, thermal analyses were performed on precursor material; Fig. 1 shows typical TGA–DTA profiles recorded from room temperature to 1150 8C. The TGA curve exhibits a continuous mass loss from room temperature to 374 8C, which is related to water evaporation and combustion of organic matter; this step involved a mass loss of 13.05%. From 374 to 700 8C, a reduction in the rate of gas evolution is observed; in this stage, the formation of samarium and cobalt oxides
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Fig. 1. TGA–DTA profiles of a SmCoO3 precursor sample.
from the corresponding nitrates occurred; this process counted in 6.71% of the mass loss. At 700 8C, a mass decrease can be noticed, which is associated with formation of the perovskite from Co3O4 and Sm2O3, according to the reaction: 2Co3 O4 þ 3Sm2 O3 ! 6SmCoO3d þ xO2 "
(1)
The mass loss registered at this temperature (0.73%) may be explained as follows: before the reaction between Co3O4 and Sm2O3 takes place, Co3O4 posses two Co3+ ions and one Co2+ ion; however, during the formation of SmCoO3, some Co3+ ions tend to have a more stable oxidation state: Co2+, at 1 atm oxygen partial pressure. This produces an oxygen-deficient SmCoO3d perovskite, with a mixed-valence state for cobalt: Co2+, Co3+; where Co3+ ions may exceed the amount of Co2+. Then, the mass loss observed at 700 8C is related to oxygen evolution, with the simultaneous defect formation, as oxygen vacancies in the perovskite, produced by the oxygen non-stoichiometry. At 930 8C, another mass loss is also identified, which suggests loss of oxygen in SmCoO3. On the other hand, the DTA graph, reveals peaks related to the same processes observed in the TGA profile; at 374 8C a wide endothermic peak indicates the transition to a slower gas evolution, involving at the same time, the crystallization of Co3O4 and Sm2O3. The endothermic peak placed at 700 8C indicates the temperature at which the oxygen-deficient SmCoO3d perovskite starts its formation, according with the explanation presented in the TGA results. Finally at 930 8C, the small peak suggests the evolution of oxygen from the perovskite. To study the process of crystallization on bulk samples, XRD was performed on powders calcined from 600 to 900 8C; Fig. 2 shows typical XRD patterns. At 600 8C a mixture of Co3O4 and Sm2O3 is clearly observed, whereas the pattern corresponding to 700 8C exhibits the most intense peaks of SmCoO3; however peaks of Co3O4 and Sm2O3 are also present. At 800 8C, the pattern reveals a small amount of un-reacted Co3O4, which was completely removed at 900 8C. Phase identification was made by using the JCPDF data files nos. 42-1467 for Co3O4, 43-1029 for Sm2O3 and 25-1071 for SmCoO3, the XRD pattern of SmCoO3 agrees well with its JCPDF file, however this corresponds to a sample annealed at 500 8C, under a pressure of 2000 atm oxygen. The XRD peaks for SmCoO3 were indexed using the Treor program, resulting in an orthorhombic structure with cell parameters: a = 0.52875 nm, b = 0.53495 nm, c = 0.74980 nm, and a cell volume = 0.21209 nm3 (standard deviation in a, b and c = 0.002 nm), which agree well with the values reported previously by Demazeau et al., for the same phase [13]. According with the above results, DTA–TGA profiles indicate the beginning of formation of SmCoO3 at 700 8C; however, by XRD, a pure phase was obtained at 900 8C. Fig. 3 shows three views of typical morphology of SmCoO3 thick-films; Fig. 3A exhibits a side view of a film, which was removed from the alumina substrate for its observation; an uniform film thickness of 400 mm was measured. A top view of a film (Fig. 3B), shows a surface with granular microstructure, without cracks; a micrograph obtained at a higher magnification revealed an average particle size of 366 nm (Fig. 3C). High connectivity between particles and an extensive porosity, may be also be observed from these photos. The variation in electrical conductivity with temperature in air, CO2 and O2 is shown in Fig. 4; a similar trend in the three gases is observed; however, lower values were registered in CO2. These measurements were performed initially,
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Fig. 2. XRD patters of precursor powders of SmCoO3 calcined from 600 to 900 8C, in air.
by both the two-probe and the four-probe methods. Preliminary tests showed no appreciable difference between the results of the two methods, eventually, only the two-probe method was used; mainly due to a fast and precise data acquisition during dynamic tests. Moreover, the effect of the electrode (silver wires) was evaluated in absence of a film, in the same temperature range, and subtracted from conductivity plots. Two inflection points T1 and T2 marked in Fig. 4 indicate nominal limits of transition from low temperature semiconducting branch (T < T1), to low-activation energy branch (T > T2) with metallic conductivity. This behavior has been explained before for LaCoO3 by number of authors, Rao and Ganguly proposed that the change of electrical properties with temperature in this oxide, are intimately connected to the spin-state population of the Co3+ ions; which was confirmed by Mo¨ssbauer studies performed at different temperatures [28]. These authors also mentioned that electrical and magnetic properties are sensitive to the preparation conditions, and probably to the degree of oxidation; however, some basic features such as metallic to non-metallic transitions are present in LaMeO3 (Me = Mn, Fe, Co and Ni) perovskites. The criterion used in this paper to select the inflection points T1 and T2, shown in Fig. 4, was the same employed by several authors, and consisted that dr/dT did not show abrupt changes into a temperature range [28]. Even though, more than one slope may be present within a temperature branch, involving a slight variation in the activation energy for conduction (EA), the thermal evolution of cobalt states associated to that branch has a similar behavior. In this work, the calculated activation energies (EA) for each temperature branch and gas type were extracted from the Arrhenius graphs of Fig. 4; Table 1 shows these values. The activation energies for SmCoO3 below T1, correspond to nearly linear Arrhenius plots. Since a metallic to semiconductor transition takes place at T1, EA values below this temperature are small. For the temperature range: T1 < T < T2, EA depends on temperature, which may be observed in Fig. 4; to explain this behavior, small polaron hopping between Co2+ ions and Co3+ ions is the most accepted model, since the conductivity depends on the population of Co3+ ions, as it was mentioned before. A detailed description of the basic postulates of this model was reported elsewhere [29–31]; which explain the non-linear behavior of the Arrhenius plots in specific temperature ranges for many transition-metal oxides. Finally, above T2, EA become small showing the metal-like behavior of SmCoO3 at high temperature.
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Fig. 3. SEM photographs of: (A) side view, (B) and (C) low and high magnification top views, respectively, of SmCoO3 thick-films.
The electrical conductivity results shown in Fig. 4 were compared with those reported by Thornton et al. for the perovskites: RCoO3 (R = La, Nd, Gd, Ho, Lu and Y) [16]. Although, the conductivity graphs for these oxides have semiconductor-to-metal phase transitions, these occur at higher temperatures than SmCoO3, this is attributable to some factors such as the presence of binary cobalt oxides in the RCoO3 perovskites, and a different sample geometry and microstructure. On the other hand, the sensitivity of a material to a specific gas is defined as Rgas/Rair, where Rgas is the resistance registered in the gas, and Rair is the resistance in air. To evaluate SmCoO3 thick-films as gas sensors, sensitivity to CO2 Table 1 Calculated activation energies of conduction (EA) for SmCoO3 Gas
EA (eV) (T < T1)
EA (eV) (T1 < T < T2)
EA (eV) (T > T2)
Air CO2 O2
0.109 0.022 0.041
0.538 0.525 0.534
0.145 0.279 0.231
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Fig. 4. Arrhenius plots for SmCoO3 thick-film in air, O2 and CO2.
and O2 was measured from room temperature to 700 8C; Fig. 5 shows the results obtained from these measurements. Under 230 8C the sensitivity to the gases was small; however, above this temperature, sensitivity increases up to a maximum value, placed for both gases at 420 8C. The sensitivity that exhibited SmCoO3 to CO2 (4.43), was larger than for O2 (2.77); whereas, increasing the temperature from 420 8C, a drop in sensitivity resulted for both gases. Even though, information about sensing behavior of SmCoO3 prepared by this route, is not available in our knowledge; we compare these results with those obtained by Tunney et al., from thick and thin-films of a similar perovskite: SrFeyCo1yOx [10]. They found that increasing cobalt substitution, the conductivity was increased, as a result of a reduction in the activation energy of conduction. They also observed high-oxygen sensitivity at 500 8C, in both SrFe0.5Co0.5Ox and SrFe0.75Co0.25Ox thick-films, with values of 0.6 and 0.8, respectively; whereas it was only present in SrFe0.5Co0.5Ox thin-film; demonstrating that film thickness influences the oxygen sensitivity of the film. In this work, noticeable higher values of sensitivity were observed for SmCoO3 thick-films, suggesting them as viable environmental gas sensors.
Fig. 5. Sensitivity vs. temperature plots for SmCoO3 in CO2 and O2.
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In order to study the dynamic response of resistance, SmCoO3 thick-films were alternatively exposed to air and CO2 (Fig. 6A); and air and O2 (Fig. 6B), respectively. These measurements were performed at 420 8C, which corresponds to the maximum value of sensitivity registered in both gases. A common characteristic of these graphs is that a repeated reversible pattern of resistance shifts with time is observed, the shape of them depends on the gas used. For CO2, the average time at which the resistance registered a stable value was considerably large: 8.9 min, however, 86% of the resistance change occurs within 40 s, involving an average full resistance decrease of 0.057 V. For the exposition of films to O2, see Fig. 6B, a shorter time to register a stable electrical resistance was observed (5 min), however the transient occurred slower than in CO2. In this gas, 200.4 s were needed to reach 86% of the full resistance change, whereas a resistance increase of 0.012 V was measured. From these results, SmCoO3 thick-films respond faster and with larger resistance change to CO2, than for O2. The most accepted explanation for the oxygen sensing mechanism in p-type semiconductor oxides, such as SmCoO3, is that O2 is adsorbed on the surface at high temperature; this promotes one of the two possible electron transfer processes: O2 þ 2e ! 2O ðadsÞ (2) O2 þ e ! O2 ðadsÞ
Fig. 6. Dynamic response of resistance plots for SmCoO3 thick-films in: (A) CO2 and (B) O2, both at 420 8C.
(3)
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Fig. 7. Gas selectivity plot for SmCoO3 in CO2, air and O2.
Fig. 8. (A) XRD pattern and (B) SEM image of SmCoO3, after its electrical characterization in gases.
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where (ads) in these reactions means the adsorbed species. Moreover, the surface of SmCoO3 posses cobalt ions in a mixed-valence state Co2+ and Co3+, where Co2+ deliver one electron during the electron transfer, which increase the number of holes in the conduction band of the perovskite, therefore increasing its electrical conductivity [32–34]. On the other hand, in a CO2 flow, the opposite phenomenon occurs; due to CO2 provides electrons to the SmCoO3 surface, this reduces in some extent the electrical charge carriers (holes), increasing its resistivity. The difference in resistivity measured in CO2 and O2, from dynamic experiments, suggests that the amount of Co2+ ions is smaller than Co3+ ions in SmCoO3; due to the increment in conductivity of the perovskite in presence of O2 is smaller than the increment in resistivity in CO2. Gas selectivity is another important factor to be considered in the performance of a sensor, Fig. 7 shows a typical plot of resistance versus time, registered during the alternated exposition of a SmCoO3 thick-film to air, CO2 and O2. In this graph, the change in resistance measured in CO2 is larger than in O2, according with the results displayed in Fig. 6. Besides, the pattern is highly reproducible, which suggest this material as an alternative gas sensor. To investigate the effect of a long exposition to numerous testing cycles in CO2 and O2, at 420 8C (for about 2 months), on the phase stability of SmCoO3; thick-films were characterized by XRD and SEM. Fig. 8A shows a typical XRD pattern of powder collected from a thick-film; it is evident that the perovskite-type structure was preserved, without presence of secondary phases; which indicates its good structural stability under the testing parameters. On the other hand, Fig. 8B shows a representative SEM image of a thick-film, an increase of 30% in the average grain size (520 nm), was estimated from several photos. 4. Conclusion Oxides with the perovskite-type structure containing cobalt, such as SmCoO3, seem to be good candidates for gas sensing applications, due to some outstanding characteristics: their preparation by wet-chemical synthesis is highly reproducible, exhibit fast electrical response to a change in gas composition, and possess high chemical and structural stability during long exposition to alternating gases. Acknowledgements This work was supported by PROPESTI and PRICOFIP Programs of the Universidad de Guadalajara. ED and GS are grateful to CONACYT for their master scholarships. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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An alternative gas sensor material: Synthesis and electrical characterization of SmCoO3 Carlos Rafael Michel *, Emilio Delgado, Gloria Santilla´n, Alma H. Martı´nez, Arturo Cha´vez-Cha´vez Departamento de Fı´sica CUCEI, Universidad de Guadalajara, Blvd. M. Garcı´a Barraga´n 1421, 44410 Guadalajara, Jalisco, Mexico Received 21 December 2005; received in revised form 24 March 2006; accepted 15 May 2006 Available online 8 June 2006
Abstract Single-phase perovskite SmCoO3 was prepared by a wet-chemical synthesis technique using metal-nitrates and citric acid; after its characterization by thermal analyses and X-ray diffraction, sintering at 900 8C in air, gave single phase and well crystallized powders. The powders were mixed with an organic solvent to prepare a slurry, which was deposited on alumina substrates as thick films, using the screen-printing technique. Electrical and gas sensing properties of sintered SmCoO3 films were investigated in air, O2 and CO2, the results show that sensitivity reached a maximum value at 420 8C, for both gases. Dynamic tests revealed a better behavior of SmCoO3 in CO2 than O2, due to a fast response and a larger electrical resistance change to this gas. X-ray diffraction made on powders after electrical characterization in gases, showed that perovskite-type structure was preserved. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. Electron microscopy; D. Electrical properties
1. Introduction Considerable scientific and technological research in solid state gas sensors, has been made due to the need of environmental monitoring and measurement, in rural and urban locations. In this field only a limited number of compositions such as: SnO2, ZnO, TiO2, and few more have been studied broadly; although, some oxides with perovskite-type structure, such as SmFeO3 and La1xSrxFeO3 exhibit notable gas sensitivity and have been tested as sensors with good results [1–8]. In the case of sensors based in perovskites possessing cobalt, Ishihara et al. have found that a combination of La0.6Sr0.4CoO3 with Pt show large sensitivity, as well as selectivity to C3H6; according to these authors, the use of this material is for detection of hydrocarbons in the exhaust gas of internal combustion engines [9]. Moreover, Tunney et al. have found that thin-films of SrFeyCo1yO3 show notable oxygen sensitivity, which depends on the extent of iron substitution by cobalt and the thickness of films [10]. About the synthesis of perovskites, the ceramic method has been one of the most commonly used, however, soft chemistry routes of preparation are convenient alternative ways to obtain inorganic materials. The resulting materials by these methods exhibit a considerable smaller particle size and high porosity, compared with those synthesized by
* Corresponding author. Tel.: +52 33 33 45 41 47; fax: +52 33 33 45 41 47. E-mail address: [email protected] (C.R. Michel). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.05.008
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ceramic method. The specific surface area-to-volume ratio, results in unique properties which are greatly different from bulk materials and are ideal to surface-environment applications such as gas sensing and catalysis. Soft chemistry methods also provide a better control on stoichiometry and reduce the temperature of synthesis of materials [11,12]. In this work, single-phase SmCoO3 was prepared by a wet-chemical method, using samarium and cobalt nitrates in aqueous media containing citric acid. The calcination at 900 8C of a dried precursor, resulted in single-phase SmCoO3; which was tested as environmental gas sensor, and the results obtained were compared with those reported for similar materials. 2. Experimental The starting reagents used for the preparation of SmCoO3 were Sm(NO3)36H2O (Alfa Aesar) and Co(NO3)26H2O (J.T. Baker), which in stoichiometric amounts were dissolved in deionized water containing 0.1 mol of citric acid (Chemical Products Monterrey). The mixture was heat-dried at 75 8C for 3 h, after the complete water evaporation an exothermic reaction was observed, which produced a precursor powder. This material was analyzed by thermogravimetric and differential thermal analyses (TGA–DTA), using a SDT 2960 TA Instruments balance with a heating rate 5 8C/min, in flowing dry air. Calcination of precursor was made in air from 600 to 900 8C in a muffle-type furnace; the sintered powders were analyzed by X-ray powder diffraction (XRD), using a Rigaku Miniflex apparatus (Cu Ka radiation). Thick-films were prepared by the screen-printing technique using single-phase powders, which were mixed in isopropyl alcohol (Chemical Products Monterrey) using an ultrasonic bath, and further deposited on alumina discs. The films were heated in an oven overnight at 50 8C, and later fired at 700 8C for 2 h. Microstructure was observed by scanning electron microscopy (SEM), using a Jeol JSM 5400LV microscope. dc electrical characterization was performed using high-purity silver wires (diameter 0.2 mm), fixed with silver paste (Chemtronics) to the films. The measurements were carried out using the two-point method, from room temperature to 700 8C, using a tube-type furnace with temperature programmable control. Dynamic response of resistance experiments were performed at a constant temperature, using an Agilent 34970A data acquisition system; the films were alternatively exposed to flowing dry air, O2 (Infra, 99.99%) and CO2 (Infra, 99.8%), maintaining the gas flow rate at 150 cm3/min. 3. Results and discussion Several synthesis routes have been tested for the preparation of perovskites containing cobalt, in the case of TCoO3 (T = Y, Pr, Nd, Sm, Er, Yb, Gd, Eu, Tm and Lu), Demazeau et al. found that the use of simultaneous application of a high-oxygen pressure (40–90 kb) and a moderate temperature (500–600 8C), is required for the formation of these oxides; when the corresponding oxides T2O3 and Co3O4 were used as precursor materials (ceramic method). In this method the thermal decomposition of KClO3 is the source of oxygen [13]. However, Yoshii et al. obtained Pr1xBaxCoO3, using the ceramic method, at 1200 8C and ambient pressure, indicating that high-oxygen pressure is not indispensable in the preparation of cobalt perovskites [14]. On the other hand, the solution method has also been explored for the synthesis of these oxides; this route involves the dissolution of the corresponding nitrates, chlorides or acetates, in an appropriate liquid [15–24]. By this procedure, the reported synthesis temperatures are usually from 800 to 1100 8C, depending on the chemical composition; with the advantage of high-oxygen pressure sintering is avoided. Moreover, the effect of the molecular level homogeneity developed in solution produce, after calcination, materials with high surface-to-volume ratios [25–27], suggesting a possible application of these materials as gas sensors. In this paper, samarium and cobalt nitrates were chosen as reagents, due to their high solubility in water; citric acid was added to the solution to promote, at the end of water evaporation, its combustion with the nitrates; this exothermic reaction produced an amorphous powder (precursor), with small particle size. In order to determine the temperature of formation of SmCoO3 by this synthesis route, thermal analyses were performed on precursor material; Fig. 1 shows typical TGA–DTA profiles recorded from room temperature to 1150 8C. The TGA curve exhibits a continuous mass loss from room temperature to 374 8C, which is related to water evaporation and combustion of organic matter; this step involved a mass loss of 13.05%. From 374 to 700 8C, a reduction in the rate of gas evolution is observed; in this stage, the formation of samarium and cobalt oxides
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Fig. 1. TGA–DTA profiles of a SmCoO3 precursor sample.
from the corresponding nitrates occurred; this process counted in 6.71% of the mass loss. At 700 8C, a mass decrease can be noticed, which is associated with formation of the perovskite from Co3O4 and Sm2O3, according to the reaction: 2Co3 O4 þ 3Sm2 O3 ! 6SmCoO3d þ xO2 "
(1)
The mass loss registered at this temperature (0.73%) may be explained as follows: before the reaction between Co3O4 and Sm2O3 takes place, Co3O4 posses two Co3+ ions and one Co2+ ion; however, during the formation of SmCoO3, some Co3+ ions tend to have a more stable oxidation state: Co2+, at 1 atm oxygen partial pressure. This produces an oxygen-deficient SmCoO3d perovskite, with a mixed-valence state for cobalt: Co2+, Co3+; where Co3+ ions may exceed the amount of Co2+. Then, the mass loss observed at 700 8C is related to oxygen evolution, with the simultaneous defect formation, as oxygen vacancies in the perovskite, produced by the oxygen non-stoichiometry. At 930 8C, another mass loss is also identified, which suggests loss of oxygen in SmCoO3. On the other hand, the DTA graph, reveals peaks related to the same processes observed in the TGA profile; at 374 8C a wide endothermic peak indicates the transition to a slower gas evolution, involving at the same time, the crystallization of Co3O4 and Sm2O3. The endothermic peak placed at 700 8C indicates the temperature at which the oxygen-deficient SmCoO3d perovskite starts its formation, according with the explanation presented in the TGA results. Finally at 930 8C, the small peak suggests the evolution of oxygen from the perovskite. To study the process of crystallization on bulk samples, XRD was performed on powders calcined from 600 to 900 8C; Fig. 2 shows typical XRD patterns. At 600 8C a mixture of Co3O4 and Sm2O3 is clearly observed, whereas the pattern corresponding to 700 8C exhibits the most intense peaks of SmCoO3; however peaks of Co3O4 and Sm2O3 are also present. At 800 8C, the pattern reveals a small amount of un-reacted Co3O4, which was completely removed at 900 8C. Phase identification was made by using the JCPDF data files nos. 42-1467 for Co3O4, 43-1029 for Sm2O3 and 25-1071 for SmCoO3, the XRD pattern of SmCoO3 agrees well with its JCPDF file, however this corresponds to a sample annealed at 500 8C, under a pressure of 2000 atm oxygen. The XRD peaks for SmCoO3 were indexed using the Treor program, resulting in an orthorhombic structure with cell parameters: a = 0.52875 nm, b = 0.53495 nm, c = 0.74980 nm, and a cell volume = 0.21209 nm3 (standard deviation in a, b and c = 0.002 nm), which agree well with the values reported previously by Demazeau et al., for the same phase [13]. According with the above results, DTA–TGA profiles indicate the beginning of formation of SmCoO3 at 700 8C; however, by XRD, a pure phase was obtained at 900 8C. Fig. 3 shows three views of typical morphology of SmCoO3 thick-films; Fig. 3A exhibits a side view of a film, which was removed from the alumina substrate for its observation; an uniform film thickness of 400 mm was measured. A top view of a film (Fig. 3B), shows a surface with granular microstructure, without cracks; a micrograph obtained at a higher magnification revealed an average particle size of 366 nm (Fig. 3C). High connectivity between particles and an extensive porosity, may be also be observed from these photos. The variation in electrical conductivity with temperature in air, CO2 and O2 is shown in Fig. 4; a similar trend in the three gases is observed; however, lower values were registered in CO2. These measurements were performed initially,
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Fig. 2. XRD patters of precursor powders of SmCoO3 calcined from 600 to 900 8C, in air.
by both the two-probe and the four-probe methods. Preliminary tests showed no appreciable difference between the results of the two methods, eventually, only the two-probe method was used; mainly due to a fast and precise data acquisition during dynamic tests. Moreover, the effect of the electrode (silver wires) was evaluated in absence of a film, in the same temperature range, and subtracted from conductivity plots. Two inflection points T1 and T2 marked in Fig. 4 indicate nominal limits of transition from low temperature semiconducting branch (T < T1), to low-activation energy branch (T > T2) with metallic conductivity. This behavior has been explained before for LaCoO3 by number of authors, Rao and Ganguly proposed that the change of electrical properties with temperature in this oxide, are intimately connected to the spin-state population of the Co3+ ions; which was confirmed by Mo¨ssbauer studies performed at different temperatures [28]. These authors also mentioned that electrical and magnetic properties are sensitive to the preparation conditions, and probably to the degree of oxidation; however, some basic features such as metallic to non-metallic transitions are present in LaMeO3 (Me = Mn, Fe, Co and Ni) perovskites. The criterion used in this paper to select the inflection points T1 and T2, shown in Fig. 4, was the same employed by several authors, and consisted that dr/dT did not show abrupt changes into a temperature range [28]. Even though, more than one slope may be present within a temperature branch, involving a slight variation in the activation energy for conduction (EA), the thermal evolution of cobalt states associated to that branch has a similar behavior. In this work, the calculated activation energies (EA) for each temperature branch and gas type were extracted from the Arrhenius graphs of Fig. 4; Table 1 shows these values. The activation energies for SmCoO3 below T1, correspond to nearly linear Arrhenius plots. Since a metallic to semiconductor transition takes place at T1, EA values below this temperature are small. For the temperature range: T1 < T < T2, EA depends on temperature, which may be observed in Fig. 4; to explain this behavior, small polaron hopping between Co2+ ions and Co3+ ions is the most accepted model, since the conductivity depends on the population of Co3+ ions, as it was mentioned before. A detailed description of the basic postulates of this model was reported elsewhere [29–31]; which explain the non-linear behavior of the Arrhenius plots in specific temperature ranges for many transition-metal oxides. Finally, above T2, EA become small showing the metal-like behavior of SmCoO3 at high temperature.
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Fig. 3. SEM photographs of: (A) side view, (B) and (C) low and high magnification top views, respectively, of SmCoO3 thick-films.
The electrical conductivity results shown in Fig. 4 were compared with those reported by Thornton et al. for the perovskites: RCoO3 (R = La, Nd, Gd, Ho, Lu and Y) [16]. Although, the conductivity graphs for these oxides have semiconductor-to-metal phase transitions, these occur at higher temperatures than SmCoO3, this is attributable to some factors such as the presence of binary cobalt oxides in the RCoO3 perovskites, and a different sample geometry and microstructure. On the other hand, the sensitivity of a material to a specific gas is defined as Rgas/Rair, where Rgas is the resistance registered in the gas, and Rair is the resistance in air. To evaluate SmCoO3 thick-films as gas sensors, sensitivity to CO2 Table 1 Calculated activation energies of conduction (EA) for SmCoO3 Gas
EA (eV) (T < T1)
EA (eV) (T1 < T < T2)
EA (eV) (T > T2)
Air CO2 O2
0.109 0.022 0.041
0.538 0.525 0.534
0.145 0.279 0.231
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Fig. 4. Arrhenius plots for SmCoO3 thick-film in air, O2 and CO2.
and O2 was measured from room temperature to 700 8C; Fig. 5 shows the results obtained from these measurements. Under 230 8C the sensitivity to the gases was small; however, above this temperature, sensitivity increases up to a maximum value, placed for both gases at 420 8C. The sensitivity that exhibited SmCoO3 to CO2 (4.43), was larger than for O2 (2.77); whereas, increasing the temperature from 420 8C, a drop in sensitivity resulted for both gases. Even though, information about sensing behavior of SmCoO3 prepared by this route, is not available in our knowledge; we compare these results with those obtained by Tunney et al., from thick and thin-films of a similar perovskite: SrFeyCo1yOx [10]. They found that increasing cobalt substitution, the conductivity was increased, as a result of a reduction in the activation energy of conduction. They also observed high-oxygen sensitivity at 500 8C, in both SrFe0.5Co0.5Ox and SrFe0.75Co0.25Ox thick-films, with values of 0.6 and 0.8, respectively; whereas it was only present in SrFe0.5Co0.5Ox thin-film; demonstrating that film thickness influences the oxygen sensitivity of the film. In this work, noticeable higher values of sensitivity were observed for SmCoO3 thick-films, suggesting them as viable environmental gas sensors.
Fig. 5. Sensitivity vs. temperature plots for SmCoO3 in CO2 and O2.
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In order to study the dynamic response of resistance, SmCoO3 thick-films were alternatively exposed to air and CO2 (Fig. 6A); and air and O2 (Fig. 6B), respectively. These measurements were performed at 420 8C, which corresponds to the maximum value of sensitivity registered in both gases. A common characteristic of these graphs is that a repeated reversible pattern of resistance shifts with time is observed, the shape of them depends on the gas used. For CO2, the average time at which the resistance registered a stable value was considerably large: 8.9 min, however, 86% of the resistance change occurs within 40 s, involving an average full resistance decrease of 0.057 V. For the exposition of films to O2, see Fig. 6B, a shorter time to register a stable electrical resistance was observed (5 min), however the transient occurred slower than in CO2. In this gas, 200.4 s were needed to reach 86% of the full resistance change, whereas a resistance increase of 0.012 V was measured. From these results, SmCoO3 thick-films respond faster and with larger resistance change to CO2, than for O2. The most accepted explanation for the oxygen sensing mechanism in p-type semiconductor oxides, such as SmCoO3, is that O2 is adsorbed on the surface at high temperature; this promotes one of the two possible electron transfer processes: O2 þ 2e ! 2O ðadsÞ (2) O2 þ e ! O2 ðadsÞ
Fig. 6. Dynamic response of resistance plots for SmCoO3 thick-films in: (A) CO2 and (B) O2, both at 420 8C.
(3)
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Fig. 7. Gas selectivity plot for SmCoO3 in CO2, air and O2.
Fig. 8. (A) XRD pattern and (B) SEM image of SmCoO3, after its electrical characterization in gases.
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where (ads) in these reactions means the adsorbed species. Moreover, the surface of SmCoO3 posses cobalt ions in a mixed-valence state Co2+ and Co3+, where Co2+ deliver one electron during the electron transfer, which increase the number of holes in the conduction band of the perovskite, therefore increasing its electrical conductivity [32–34]. On the other hand, in a CO2 flow, the opposite phenomenon occurs; due to CO2 provides electrons to the SmCoO3 surface, this reduces in some extent the electrical charge carriers (holes), increasing its resistivity. The difference in resistivity measured in CO2 and O2, from dynamic experiments, suggests that the amount of Co2+ ions is smaller than Co3+ ions in SmCoO3; due to the increment in conductivity of the perovskite in presence of O2 is smaller than the increment in resistivity in CO2. Gas selectivity is another important factor to be considered in the performance of a sensor, Fig. 7 shows a typical plot of resistance versus time, registered during the alternated exposition of a SmCoO3 thick-film to air, CO2 and O2. In this graph, the change in resistance measured in CO2 is larger than in O2, according with the results displayed in Fig. 6. Besides, the pattern is highly reproducible, which suggest this material as an alternative gas sensor. To investigate the effect of a long exposition to numerous testing cycles in CO2 and O2, at 420 8C (for about 2 months), on the phase stability of SmCoO3; thick-films were characterized by XRD and SEM. Fig. 8A shows a typical XRD pattern of powder collected from a thick-film; it is evident that the perovskite-type structure was preserved, without presence of secondary phases; which indicates its good structural stability under the testing parameters. On the other hand, Fig. 8B shows a representative SEM image of a thick-film, an increase of 30% in the average grain size (520 nm), was estimated from several photos. 4. Conclusion Oxides with the perovskite-type structure containing cobalt, such as SmCoO3, seem to be good candidates for gas sensing applications, due to some outstanding characteristics: their preparation by wet-chemical synthesis is highly reproducible, exhibit fast electrical response to a change in gas composition, and possess high chemical and structural stability during long exposition to alternating gases. Acknowledgements This work was supported by PROPESTI and PRICOFIP Programs of the Universidad de Guadalajara. ED and GS are grateful to CONACYT for their master scholarships. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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