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Ethanol gas sensor based on CoFe2O4 nano-crystallines prepared by hydrothermal method
Sensors and Actuators B 120 (2006) 177–181
Ethanol gas sensor based on CoFe2O4 nano-crystallines prepared by hydrothermal method Chu Xiangfeng ∗ , Jiang Dongli, Guo Yu, Zheng Chenmou School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, PR China Received 14 December 2005; received in revised form 6 February 2006; accepted 6 February 2006 Available online 5 June 2006
Abstract CoFe2 O4 nano-crystallines were prepared by a hydrothermal method at 120–220 ◦ C. CoFe2 O4 sensors were fabricated from the CoFe2 O4 nanocrystallines and their gas sensing properties were investigated. Spinel CoFe2 O4 nano-crystallines could be obtained when the pH value was in the range of 8–14, and the morphology of the particles obtained under different conditions was almost spherical. The CoFe2 O4 nano-crystallines obtained at a pH of 8–10 exhibited n-type response to gases at low temperatures and p-type response to gases at high temperatures, while CoFe2 O4 nano-crystallines obtained at a pH of 11–14 exhibited p-type response to gases. It was found that the sensor based on the CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h (pH 8) exhibited good performance, characterized by high responses to dilute C2 H5 OH and very low responses to petrol, C6 H6 and C6 H5 CH3 when operating at 150 ◦ C. If there was no ethanol in the atmosphere, the sensor based on nano-CoFe2 O4 (180 ◦ C, 48 h, pH 8) could be used to detect triethylamine at 190 ◦ C. The sensor is a promising candidate for practical detector for dilute C2 H5 OH and triethylamine. © 2006 Published by Elsevier B.V. Keywords: Gas sensor; Nano-crystalline; CoFe2 O4
1. Introduction Demand for chemical sensors has been growing at a consistent pace in recent years due to the stringent environmental regulations that are coming into effect to reduce emissions and hazardous pollutants. Metal oxide semiconductor sensors are the most promising devices among the solid state chemical sensors, because they have many advantages such as small dimensions, low cost, low power consumption, on-line operation, and high compatibility with microelectronic processing. Hence, the metal oxide gas-sensing materials have been widely investigated for a long time [1–3]. The gas-sensing mechanism of metal oxide materials is based on the reaction between the adsorbed oxygen on the surface of the materials and the gas molecules to be detected. The state and the amount of oxygen on the surface of materials are strongly dependent on the microstructure of the materials, namely, specific area, particle size, as well as the film thickness of the sensing film. In order to obtain gas sensors with good performance, the recent research works [4–6] were ∗
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Xiangfeng). 0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2006.02.008
devoted to nano-materials because they have high specific area and contain more grain boundaries. CoFe2 O4 nano-crystallines have attracted great research interests due to their potential application as magnetic materials [7–9] and catalysts [10], but the gas-sensing properties have never been reported. In this paper, we prepared nano-crystalline CoFe2 O4 by a hydrothermal method and investigated their gassensing properties under different conditions. It was found that the sensor based on the CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h (pH 8) exhibited good performance, as characterized by high responses to dilute C2 H5 OH and (C2 H5 )3 N and very low responses to petrol, C6 H6 and C6 H5 CH3 when operating at low temperatures. 2. Experimental The raw materials were all analytical-grade reagents and used without further purification. In a typical procedure, a 0.20 M Fe(NO3 )3 solution was mixed with 0.10 M Co(NO3 )2 , an appropriate amount of a 6 M NaOH solution was added to the mixed solution to adjust the pH to 7–14, and de-ionized water was dropped into the mixed solution until the volume of the solution was about 40 ml. The mixture was stirred strongly for 30 min
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and then transferred into a 50 ml Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 120–220 ◦ C for 18–48 h. The pressure in the sealed autoclave was lower than the equilibrium vapor tension of water at the same temperature because the solution contained NaOH. After the reaction was completed, the resulting solid product was filtered and washed with water and absolute alcohol several times. After drying at 75 ◦ C for 4 h, the white powders were collected for characterization. Powder X-ray diffraction (XRD) was carried out with an X-ray diffractometer (Rigaku, D/max 2200). The morphology and the size of the products were observed by transmission electronic microscopy (TEM) using a FEI-Tecnai 12 microscope. Nano-CoFe2 O4 powder of 0.05 g and a 0.1 ml polyvinyl alcohol solution were mixed, and milled for a few minutes in a mortar to form slurry. The slurry was coated onto an Al2 O3 tube at each end of which two gold leads had been installed. The Al2 O3 tube was about 8 mm in length and 2 mm in external diameter and 1.6 mm in internal diameter. A heater using Ni-Cr wire was inserted into the Al2 O3 tube to supply the operating temperature that could be controlled in the range of 100–500 ◦ C. The electrical resistance of a sensor was measured in air and in sample gases to be detected. The response (S) was defined as the ratio of the electrical resistance in air (Ra ) to that in sample gases (Rg ). Our sample gas was prepared by the following process: pure liquid (such as C6 H6 , C6 H5 CH3 and C2 H5 OH) or pure gas was injected by a syringe into a container that contained air (Pair < 1 atm), and then air was added into the container to make P = Pair + Pgas = l atm after pure liquid was volatilized totally. According to the volumes of the container and pure liquid (or pure gas), we can calculate the gas concentration. 3. Results and discussion Fig. 1 showed the XRD patterns of powders prepared at different pH values (180 ◦ C, 48 h). It could be seen that spinel CoFe2 O4 was obtained when pH value was in the range of 8–14.
Fig. 1. XRD patterns of powders prepared at different pH values (180 ◦ C, 48 h).
Fe2 O3 was obtained when pH 7 at which Co2+ could not precipitate because KspCo(OH)2 was about 1.09 × 10−15 . The XRD patterns of the products also revealed that the hydrothermal temperature and reaction time had less influence on the phase composition. Pure CoFe2 O4 could be prepared when hydrothermal temperature was 120–220 ◦ C (48 h, pH 11) and hydrothermal time was 18–48 h (pH 11, 160 ◦ C). Fig. 2 showed transmission electron microscopy (TEM) images of the as-prepared samples obtained under different conditions. The morphology of the particles was almost spherical. The particle sizes of the samples prepared under (a) pH 9.5, 210 ◦ C, 32 h (b) pH 11, 210 ◦ C, 32 h and (c) pH 13.5, 210 ◦ C, 32 h were all 20 nm, indicating that pH did not affect the particle size of the CoFe2 O4 nano-crystallines. The particle size of the samples obtained under (d) pH 11, 210 ◦ C, 24 h and (e) pH 11, 150 ◦ C, 24 h were both 15 nm, indicating that the particle size did not change obviously with the change in hydrothermal reaction temperature. From the particle size of the samples obtained under (b) pH 11, 210 ◦ C, 32 h and (d) pH 11, 210 ◦ C, 24 h, it was found that the particle size increased with increasing the hydrothermal reaction time. Fig. 3 showed the responses to 1000 ppm C2 H5 OH of seven sensors based on CoFe2 O4 nano-crystallines obtained under different conditions. The response was defined as the resistance ratio Ra /Rg , where Ra and Rg were the electrical resistances of sensor in air and that in the detected gas. The responses of the sensors were greatly affected by the pH value. The sensor based on CoFe2 O4 obtained under pH 8 exhibited the highest response to C2 H5 OH among the seven sensors prepared under different pH when operating at 110–230 ◦ C, and the maximum response was 71.9 at 150 ◦ C. As shown in Fig. 3, the responses of sensors based on CoFe2 O4 obtained under pH 8–10 were lower than 1.0 when operating at 270 and 310 ◦ C, the responses of sensors based on CoFe2 O4 obtained under pH 11–14 were also lower than 1.0 when operating at 150–310 ◦ C. The resistance of the sensors based on CoFe2 O4 increased in reducing atmosphere, which was the characteristic of p-type semiconductors; it is easy to understand that the sensors based on CoFe2 O4 exhibited p-type response to reducing gas because CoFe2 O4 was a p-type semiconductor [11]. But the responses of the sensor based on CoFe2 O4 prepared at a pH of 8–10 was higher than 1.0 when operating at low temperatures, meaning that the resistance decreased in reducing atmosphere; the phenomenon was the typical characteristic of n-type semiconductor. Bellad and Bhosale [12] also reported that CoFe2 O4 prepared under different conditions exhibited different conducting behavior. According to their results, the n-type behavior was attributed to the presence of Fe2+ and the conductivity was predominantly due to hopping of electrons from Fe2+ to Fe3+ . Other researchers [13] reported that the main conduction mechanism in iron excess ferrite was electron hopping from Fe2+ to Fe3+ ion, and the main conduction mechanism in Co3+ excess ferrite was hole hopping from Co2+ to Co3+ . When CoFe2 O4 nano-crystallines were prepared at a pH of 8–10, a little Co2+ could not form precipitate and remained in solution, so that iron excess CoFe2 O4 nano-crystallines were obtained and the sensors based on iron excess nano-crystallines exhibited n-type
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Fig. 2. Transmission electron microscopic (TEM) images of as-prepared samples under different conditions: (a) pH 9.5, 210 ◦ C, 32 h; (b) pH 11, 210 ◦ C, 32 h; (c) pH 13.5, 210 ◦ C, 32 h; (d) pH 11, 210 ◦ C, 24 h; (e) pH 11, 150 ◦ C, 24 h.
response to gases. Cobalt excess CoFe2 O4 nano-crystallines maybe formed at a pH of 11–14 because a little Fe(OH)3 could dissolve in strong basic solution; this may be the reason why the sensors based on CoFe2 O4 nano-crystallines obtained at a pH of 11–14 exhibited p-type response to gases. It could be seen that the sensors based on CoFe2 O4 prepared at a pH of 8–10 exhibited p-type response to gases at high temperatures. Co3+ was stable and Fe2+ was not stable at high temperatures, so that the concentration of Co3+ increased and the concentration of Fe2+ in CoFe2 O4 decreased with increasing the operating temperature, which made the hole hopping from Co2+ to Co3+ to be the predominant conduction mechanism at high operating temperatures. The gas-sensing mechanism was based on the changes in conductance of CoFe2 O4 . The oxygen adsorbed on the surface of the material influenced the conductance of the CoFe2 O4 -based sensor. The amount of oxygen on the surface of the material depended on the particle size, specific area, and operating temperature of the sensor. With an increase in temperature in air, the state of oxygen adsorbed on the surface of CoFe2 O4 underwent the following reactions: O2(gas) ⇒ O2(ads) ⇒ O2 − (ads) ⇒ 2O− (ads) ⇒ 2O2− (ads) . The oxygen species captured electrons from the material, leading to the changes in hole or electron concentration in CoFe2 O4 materials. Reducing gases reacted with the oxygen adsorbed on the surface of CoFe2 O4 and the electrons captured by the oxy-
gen returned to the CoFe2 O4 material, resulting in the changes in conductance of the CoFe2 O4 . It also could be seen that the responses also influenced by the pH value even though the sensors exhibited the same response type (n-type or p-type) to reducing gases. The pH value maybe affected the amount of
Fig. 3. Responses to 1000 ppm C2 H5 OH of seven sensors based on the CoFe2 O4 nano-crystallines obtained under different conditions.
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versus triethylamine gas concentration was approximately linear. The responses to 10 ppm triethylamine and 10 ppm ethanol of the sensor were 2 and 4 when operating at 190 ◦ C and 150 ◦ C, respectively. The response to 10 ppm C2 H5 OH was higher than that to 500 ppm petrol, meaning that the sensor had good selectivity to C2 H5 OH. The response times to 50 ppm C2 H5 OH and triethylamine were about 50 and 100 s; and the recovery time were 60 s and 120 s. 4. Conclusions
Fig. 4. Responses to a few kinds of reducing gases of the sensor based on CoFe2 O4 nano-crystallines (180 ◦ C, 48 h, pH 8).
active sites on the surface of the CoFe2 O4 material that could adsorb oxygen and reducing gases. The responses to a few kinds of reducing gases of the sensor based on CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h when pH was 8 were shown in Fig. 4. The responses to 500 ppm benzene, 500 ppm toluene and 500 ppm petrol were very low, but the responses to 500 ppm triethylamine and 1000 ppm ethanol attained the maximum values at 190 ◦ C and 150 ◦ C, respectively. The sensor had good selectivity to ethanol at 150 ◦ C. If there was no ethanol in the atmosphere, the sensor could be used to detect triethylamine at 190 ◦ C. Fig. 5 depicted the correlation between the triethylamine and ethanol concentrations and responses of sensor based on CoFe2 O4 (180 ◦ C, 48 h, pH 8). The response decreased with decreasing the gas concentration. The curve shape of response
Fig. 5. Correlation between the concentrations of triethylamine and ethanol and the responses of the sensor based on CoFe2 O4 nano-crystallines (180 ◦ C, 48 h, pH 8).
In summary, we prepared nano-CoFe2 O4 by a hydrothermal method, and investigated their gas-sensing properties. The sensors based on the CoFe2 O4 prepared at a pH of 8–10 exhibited ptype gas-response at high temperatures and n-type gas-response at low temperatures. The sensors based on CoFe2 O4 prepared at a pH of 11–14 exhibited p-type gas-response. The results demonstrated that the response was affected greatly by the preparation condition. The sensor based on nano-CoFe2 O4 (180 ◦ C, 48 h, pH 8) showed high response, good selectivity to low concentration of ethanol at 150 ◦ C; especially, the sensor could detect 10 ppm ethanol. If there was no ethanol in the atmosphere, the sensor based on the same material could be used to detect triethylamine at 190 ◦ C. Thus, the sensor is promising for practical devices for detecting low concentration of triethylamine and ethanol. Acknowledgements The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] E. Rossinyol, J. Arbiol, F. Peir´o, A. Cornet, J.R. Morante, B. Tian, T. Bo, D. Zhao, Nanostructured metal oxides synthesized by hard template method for gas sensing applications, Sens. Actuators B 109 (2005) 57–63. [2] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in the sputtering gas, and working temperature, Sens. Actuators B 100 (2004) 320–324. [3] U. Hoefer, J. Frank, M. Fleischer, High temperature Ga2 O3 -gas sensors and SnO2 -gas sensors: a comparison, Sens. Actuators B 78 (2001) 6–11. [4] Y. Liu, M. Liu, Growth of aligned square-shaped SnO2 tube arrays, Adv. Functional Mater. 15 (2005) 57–62. [5] L.G. Teoh, Y.M. Hon, J. Shieh, W.H. Lai, M.H. Hon, Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film, Sens. Actuators B 96 (2003) 219–225. [6] Y.-G. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, N. Yamazoe, Preparation of size and habit-controlled nano crystallites of tungsten oxide, Sens. Actuators B 93 (2003) 486–494. [7] X. Cao, L. Gu, Spindly cobalt ferrite nanocrystals: preparation, characterization and magnetic properties, Nanotechnology 16 (2005) 180–185. [8] N. Keller, C. Pham-Huu, T. Shiga, C. Estourn`es, J.-M. Gren`eche, M.J. Ledoux, Mild synthesis of CoFe2 O4 nanowires using carbon nanotube template: a high-coercivity material at room temperature, J. Magnetism Magnetic Mater. 272–276 (2004) 1642–1644. [9] G.B. Ji, S.L. Tang, S.K. Ren, F.M. Zhang, B.X. Gu, Y.W. Du, Simplified synthesis of single-crystalline magnetic CoFe2 O4 nanorods by a surfactantassisted hydrothermal process, J. Crystal Growth 270 (2004) 156–161.
C. Xiangfeng et al. / Sensors and Actuators B 120 (2006) 177–181 [10] Y. Tamaura, M. Tabata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature 346 (1990), 255–255. [11] M. Soliman Selim, G. Turky, M.A. Shouman, G.A. El-Shobaky, Effect of Li2 O doping on electrical properties of CoFe2 O4 , Solid State Ionics 120 (1999) 173–181.
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[12] S.S. Bellad, C.H. Bhosale, Substrate temperature dependent properties of sprayed CoFe2 O4 ferrite thin films, Thin Solid Films 322 (1998) 93– 97. [13] D.Y. Lee, D. Kim, K.H. Kim, J.S. Choi, Electrical conductivity of the spinel CoFe2 O4 solid solution, Bull. Korean Chem. Soc. 9 (1988) 333–420.
Ethanol gas sensor based on CoFe2O4 nano-crystallines prepared by hydrothermal method Chu Xiangfeng ∗ , Jiang Dongli, Guo Yu, Zheng Chenmou School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, PR China Received 14 December 2005; received in revised form 6 February 2006; accepted 6 February 2006 Available online 5 June 2006
Abstract CoFe2 O4 nano-crystallines were prepared by a hydrothermal method at 120–220 ◦ C. CoFe2 O4 sensors were fabricated from the CoFe2 O4 nanocrystallines and their gas sensing properties were investigated. Spinel CoFe2 O4 nano-crystallines could be obtained when the pH value was in the range of 8–14, and the morphology of the particles obtained under different conditions was almost spherical. The CoFe2 O4 nano-crystallines obtained at a pH of 8–10 exhibited n-type response to gases at low temperatures and p-type response to gases at high temperatures, while CoFe2 O4 nano-crystallines obtained at a pH of 11–14 exhibited p-type response to gases. It was found that the sensor based on the CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h (pH 8) exhibited good performance, characterized by high responses to dilute C2 H5 OH and very low responses to petrol, C6 H6 and C6 H5 CH3 when operating at 150 ◦ C. If there was no ethanol in the atmosphere, the sensor based on nano-CoFe2 O4 (180 ◦ C, 48 h, pH 8) could be used to detect triethylamine at 190 ◦ C. The sensor is a promising candidate for practical detector for dilute C2 H5 OH and triethylamine. © 2006 Published by Elsevier B.V. Keywords: Gas sensor; Nano-crystalline; CoFe2 O4
1. Introduction Demand for chemical sensors has been growing at a consistent pace in recent years due to the stringent environmental regulations that are coming into effect to reduce emissions and hazardous pollutants. Metal oxide semiconductor sensors are the most promising devices among the solid state chemical sensors, because they have many advantages such as small dimensions, low cost, low power consumption, on-line operation, and high compatibility with microelectronic processing. Hence, the metal oxide gas-sensing materials have been widely investigated for a long time [1–3]. The gas-sensing mechanism of metal oxide materials is based on the reaction between the adsorbed oxygen on the surface of the materials and the gas molecules to be detected. The state and the amount of oxygen on the surface of materials are strongly dependent on the microstructure of the materials, namely, specific area, particle size, as well as the film thickness of the sensing film. In order to obtain gas sensors with good performance, the recent research works [4–6] were ∗
Corresponding author. E-mail addresses: [email protected], [email protected] (C. Xiangfeng). 0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2006.02.008
devoted to nano-materials because they have high specific area and contain more grain boundaries. CoFe2 O4 nano-crystallines have attracted great research interests due to their potential application as magnetic materials [7–9] and catalysts [10], but the gas-sensing properties have never been reported. In this paper, we prepared nano-crystalline CoFe2 O4 by a hydrothermal method and investigated their gassensing properties under different conditions. It was found that the sensor based on the CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h (pH 8) exhibited good performance, as characterized by high responses to dilute C2 H5 OH and (C2 H5 )3 N and very low responses to petrol, C6 H6 and C6 H5 CH3 when operating at low temperatures. 2. Experimental The raw materials were all analytical-grade reagents and used without further purification. In a typical procedure, a 0.20 M Fe(NO3 )3 solution was mixed with 0.10 M Co(NO3 )2 , an appropriate amount of a 6 M NaOH solution was added to the mixed solution to adjust the pH to 7–14, and de-ionized water was dropped into the mixed solution until the volume of the solution was about 40 ml. The mixture was stirred strongly for 30 min
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and then transferred into a 50 ml Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 120–220 ◦ C for 18–48 h. The pressure in the sealed autoclave was lower than the equilibrium vapor tension of water at the same temperature because the solution contained NaOH. After the reaction was completed, the resulting solid product was filtered and washed with water and absolute alcohol several times. After drying at 75 ◦ C for 4 h, the white powders were collected for characterization. Powder X-ray diffraction (XRD) was carried out with an X-ray diffractometer (Rigaku, D/max 2200). The morphology and the size of the products were observed by transmission electronic microscopy (TEM) using a FEI-Tecnai 12 microscope. Nano-CoFe2 O4 powder of 0.05 g and a 0.1 ml polyvinyl alcohol solution were mixed, and milled for a few minutes in a mortar to form slurry. The slurry was coated onto an Al2 O3 tube at each end of which two gold leads had been installed. The Al2 O3 tube was about 8 mm in length and 2 mm in external diameter and 1.6 mm in internal diameter. A heater using Ni-Cr wire was inserted into the Al2 O3 tube to supply the operating temperature that could be controlled in the range of 100–500 ◦ C. The electrical resistance of a sensor was measured in air and in sample gases to be detected. The response (S) was defined as the ratio of the electrical resistance in air (Ra ) to that in sample gases (Rg ). Our sample gas was prepared by the following process: pure liquid (such as C6 H6 , C6 H5 CH3 and C2 H5 OH) or pure gas was injected by a syringe into a container that contained air (Pair < 1 atm), and then air was added into the container to make P = Pair + Pgas = l atm after pure liquid was volatilized totally. According to the volumes of the container and pure liquid (or pure gas), we can calculate the gas concentration. 3. Results and discussion Fig. 1 showed the XRD patterns of powders prepared at different pH values (180 ◦ C, 48 h). It could be seen that spinel CoFe2 O4 was obtained when pH value was in the range of 8–14.
Fig. 1. XRD patterns of powders prepared at different pH values (180 ◦ C, 48 h).
Fe2 O3 was obtained when pH 7 at which Co2+ could not precipitate because KspCo(OH)2 was about 1.09 × 10−15 . The XRD patterns of the products also revealed that the hydrothermal temperature and reaction time had less influence on the phase composition. Pure CoFe2 O4 could be prepared when hydrothermal temperature was 120–220 ◦ C (48 h, pH 11) and hydrothermal time was 18–48 h (pH 11, 160 ◦ C). Fig. 2 showed transmission electron microscopy (TEM) images of the as-prepared samples obtained under different conditions. The morphology of the particles was almost spherical. The particle sizes of the samples prepared under (a) pH 9.5, 210 ◦ C, 32 h (b) pH 11, 210 ◦ C, 32 h and (c) pH 13.5, 210 ◦ C, 32 h were all 20 nm, indicating that pH did not affect the particle size of the CoFe2 O4 nano-crystallines. The particle size of the samples obtained under (d) pH 11, 210 ◦ C, 24 h and (e) pH 11, 150 ◦ C, 24 h were both 15 nm, indicating that the particle size did not change obviously with the change in hydrothermal reaction temperature. From the particle size of the samples obtained under (b) pH 11, 210 ◦ C, 32 h and (d) pH 11, 210 ◦ C, 24 h, it was found that the particle size increased with increasing the hydrothermal reaction time. Fig. 3 showed the responses to 1000 ppm C2 H5 OH of seven sensors based on CoFe2 O4 nano-crystallines obtained under different conditions. The response was defined as the resistance ratio Ra /Rg , where Ra and Rg were the electrical resistances of sensor in air and that in the detected gas. The responses of the sensors were greatly affected by the pH value. The sensor based on CoFe2 O4 obtained under pH 8 exhibited the highest response to C2 H5 OH among the seven sensors prepared under different pH when operating at 110–230 ◦ C, and the maximum response was 71.9 at 150 ◦ C. As shown in Fig. 3, the responses of sensors based on CoFe2 O4 obtained under pH 8–10 were lower than 1.0 when operating at 270 and 310 ◦ C, the responses of sensors based on CoFe2 O4 obtained under pH 11–14 were also lower than 1.0 when operating at 150–310 ◦ C. The resistance of the sensors based on CoFe2 O4 increased in reducing atmosphere, which was the characteristic of p-type semiconductors; it is easy to understand that the sensors based on CoFe2 O4 exhibited p-type response to reducing gas because CoFe2 O4 was a p-type semiconductor [11]. But the responses of the sensor based on CoFe2 O4 prepared at a pH of 8–10 was higher than 1.0 when operating at low temperatures, meaning that the resistance decreased in reducing atmosphere; the phenomenon was the typical characteristic of n-type semiconductor. Bellad and Bhosale [12] also reported that CoFe2 O4 prepared under different conditions exhibited different conducting behavior. According to their results, the n-type behavior was attributed to the presence of Fe2+ and the conductivity was predominantly due to hopping of electrons from Fe2+ to Fe3+ . Other researchers [13] reported that the main conduction mechanism in iron excess ferrite was electron hopping from Fe2+ to Fe3+ ion, and the main conduction mechanism in Co3+ excess ferrite was hole hopping from Co2+ to Co3+ . When CoFe2 O4 nano-crystallines were prepared at a pH of 8–10, a little Co2+ could not form precipitate and remained in solution, so that iron excess CoFe2 O4 nano-crystallines were obtained and the sensors based on iron excess nano-crystallines exhibited n-type
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Fig. 2. Transmission electron microscopic (TEM) images of as-prepared samples under different conditions: (a) pH 9.5, 210 ◦ C, 32 h; (b) pH 11, 210 ◦ C, 32 h; (c) pH 13.5, 210 ◦ C, 32 h; (d) pH 11, 210 ◦ C, 24 h; (e) pH 11, 150 ◦ C, 24 h.
response to gases. Cobalt excess CoFe2 O4 nano-crystallines maybe formed at a pH of 11–14 because a little Fe(OH)3 could dissolve in strong basic solution; this may be the reason why the sensors based on CoFe2 O4 nano-crystallines obtained at a pH of 11–14 exhibited p-type response to gases. It could be seen that the sensors based on CoFe2 O4 prepared at a pH of 8–10 exhibited p-type response to gases at high temperatures. Co3+ was stable and Fe2+ was not stable at high temperatures, so that the concentration of Co3+ increased and the concentration of Fe2+ in CoFe2 O4 decreased with increasing the operating temperature, which made the hole hopping from Co2+ to Co3+ to be the predominant conduction mechanism at high operating temperatures. The gas-sensing mechanism was based on the changes in conductance of CoFe2 O4 . The oxygen adsorbed on the surface of the material influenced the conductance of the CoFe2 O4 -based sensor. The amount of oxygen on the surface of the material depended on the particle size, specific area, and operating temperature of the sensor. With an increase in temperature in air, the state of oxygen adsorbed on the surface of CoFe2 O4 underwent the following reactions: O2(gas) ⇒ O2(ads) ⇒ O2 − (ads) ⇒ 2O− (ads) ⇒ 2O2− (ads) . The oxygen species captured electrons from the material, leading to the changes in hole or electron concentration in CoFe2 O4 materials. Reducing gases reacted with the oxygen adsorbed on the surface of CoFe2 O4 and the electrons captured by the oxy-
gen returned to the CoFe2 O4 material, resulting in the changes in conductance of the CoFe2 O4 . It also could be seen that the responses also influenced by the pH value even though the sensors exhibited the same response type (n-type or p-type) to reducing gases. The pH value maybe affected the amount of
Fig. 3. Responses to 1000 ppm C2 H5 OH of seven sensors based on the CoFe2 O4 nano-crystallines obtained under different conditions.
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versus triethylamine gas concentration was approximately linear. The responses to 10 ppm triethylamine and 10 ppm ethanol of the sensor were 2 and 4 when operating at 190 ◦ C and 150 ◦ C, respectively. The response to 10 ppm C2 H5 OH was higher than that to 500 ppm petrol, meaning that the sensor had good selectivity to C2 H5 OH. The response times to 50 ppm C2 H5 OH and triethylamine were about 50 and 100 s; and the recovery time were 60 s and 120 s. 4. Conclusions
Fig. 4. Responses to a few kinds of reducing gases of the sensor based on CoFe2 O4 nano-crystallines (180 ◦ C, 48 h, pH 8).
active sites on the surface of the CoFe2 O4 material that could adsorb oxygen and reducing gases. The responses to a few kinds of reducing gases of the sensor based on CoFe2 O4 nano-crystallines prepared at 180 ◦ C for 48 h when pH was 8 were shown in Fig. 4. The responses to 500 ppm benzene, 500 ppm toluene and 500 ppm petrol were very low, but the responses to 500 ppm triethylamine and 1000 ppm ethanol attained the maximum values at 190 ◦ C and 150 ◦ C, respectively. The sensor had good selectivity to ethanol at 150 ◦ C. If there was no ethanol in the atmosphere, the sensor could be used to detect triethylamine at 190 ◦ C. Fig. 5 depicted the correlation between the triethylamine and ethanol concentrations and responses of sensor based on CoFe2 O4 (180 ◦ C, 48 h, pH 8). The response decreased with decreasing the gas concentration. The curve shape of response
Fig. 5. Correlation between the concentrations of triethylamine and ethanol and the responses of the sensor based on CoFe2 O4 nano-crystallines (180 ◦ C, 48 h, pH 8).
In summary, we prepared nano-CoFe2 O4 by a hydrothermal method, and investigated their gas-sensing properties. The sensors based on the CoFe2 O4 prepared at a pH of 8–10 exhibited ptype gas-response at high temperatures and n-type gas-response at low temperatures. The sensors based on CoFe2 O4 prepared at a pH of 11–14 exhibited p-type gas-response. The results demonstrated that the response was affected greatly by the preparation condition. The sensor based on nano-CoFe2 O4 (180 ◦ C, 48 h, pH 8) showed high response, good selectivity to low concentration of ethanol at 150 ◦ C; especially, the sensor could detect 10 ppm ethanol. If there was no ethanol in the atmosphere, the sensor based on the same material could be used to detect triethylamine at 190 ◦ C. Thus, the sensor is promising for practical devices for detecting low concentration of triethylamine and ethanol. Acknowledgements The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] E. Rossinyol, J. Arbiol, F. Peir´o, A. Cornet, J.R. Morante, B. Tian, T. Bo, D. Zhao, Nanostructured metal oxides synthesized by hard template method for gas sensing applications, Sens. Actuators B 109 (2005) 57–63. [2] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in the sputtering gas, and working temperature, Sens. Actuators B 100 (2004) 320–324. [3] U. Hoefer, J. Frank, M. Fleischer, High temperature Ga2 O3 -gas sensors and SnO2 -gas sensors: a comparison, Sens. Actuators B 78 (2001) 6–11. [4] Y. Liu, M. Liu, Growth of aligned square-shaped SnO2 tube arrays, Adv. Functional Mater. 15 (2005) 57–62. [5] L.G. Teoh, Y.M. Hon, J. Shieh, W.H. Lai, M.H. Hon, Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin film, Sens. Actuators B 96 (2003) 219–225. [6] Y.-G. Choi, G. Sakai, K. Shimanoe, Y. Teraoka, N. Miura, N. Yamazoe, Preparation of size and habit-controlled nano crystallites of tungsten oxide, Sens. Actuators B 93 (2003) 486–494. [7] X. Cao, L. Gu, Spindly cobalt ferrite nanocrystals: preparation, characterization and magnetic properties, Nanotechnology 16 (2005) 180–185. [8] N. Keller, C. Pham-Huu, T. Shiga, C. Estourn`es, J.-M. Gren`eche, M.J. Ledoux, Mild synthesis of CoFe2 O4 nanowires using carbon nanotube template: a high-coercivity material at room temperature, J. Magnetism Magnetic Mater. 272–276 (2004) 1642–1644. [9] G.B. Ji, S.L. Tang, S.K. Ren, F.M. Zhang, B.X. Gu, Y.W. Du, Simplified synthesis of single-crystalline magnetic CoFe2 O4 nanorods by a surfactantassisted hydrothermal process, J. Crystal Growth 270 (2004) 156–161.
C. Xiangfeng et al. / Sensors and Actuators B 120 (2006) 177–181 [10] Y. Tamaura, M. Tabata, Complete reduction of carbon dioxide to carbon using cation-excess magnetite, Nature 346 (1990), 255–255. [11] M. Soliman Selim, G. Turky, M.A. Shouman, G.A. El-Shobaky, Effect of Li2 O doping on electrical properties of CoFe2 O4 , Solid State Ionics 120 (1999) 173–181.
181
[12] S.S. Bellad, C.H. Bhosale, Substrate temperature dependent properties of sprayed CoFe2 O4 ferrite thin films, Thin Solid Films 322 (1998) 93– 97. [13] D.Y. Lee, D. Kim, K.H. Kim, J.S. Choi, Electrical conductivity of the spinel CoFe2 O4 solid solution, Bull. Korean Chem. Soc. 9 (1988) 333–420.