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The preparation and gas-sensing properties of NiFe2O4 nanocubes and nanorods
Sensors and Actuators B 123 (2007) 793–797
The preparation and gas-sensing properties of NiFe2O4 nanocubes and nanorods Chu Xiangfeng ∗ , Jiang Dongli, Zheng Chenmou School of Chemistry and Chemical Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, PR China Received 21 April 2006; received in revised form 10 October 2006; accepted 11 October 2006 Available online 13 November 2006
Abstract NiFe2 O4 nanorods and nanocubes were prepared by a hydrothermal method without any surfactant for the first time. The length and diameter of the nanorods were about 1 m and 30 nm, respectively; the side length of the nanocubes was about 60–100 nm. It was found that the sensor based on NiFe2 O4 nanorods was relatively sensitive and selective to triethylamine. Especially, the sensitivity to 1 ppm triethylamine attained 7 when operating at 175 ◦ C. On the other hand, the sensor based on NiFe2 O4 nanocubes exhibited the opposite behavior; namely, the conductivity of the sensor increased in a reducing gas atmosphere. © 2006 Published by Elsevier B.V. Keywords: NiFe2 O4 ; Nanorod; Gas sensor
1. Introduction Now the presence of volatile organic compounds (VOCs) has become a serious concern due to strict environmental regulations on VOCs in many countries. The toxic or carcinogenic nature of VOCs is dangerous to human beings. Semiconductor metal oxides as gas-sensing materials have attracted great attention for a long time due to their advantageous features, such as higher sensitivity to ambient conditions, lower cost and simplicity in fabrication [1–3]. Apart from nano-particle and bulk sensor materials, the gas-sensing properties of one-dimensional (1D) nano-structured materials have been reported since various kinds of 1D nano-structured materials have been prepared [4–8]. The sensitivities of the sensors based on 1D nano-materials were very high because of their special surface state and morphology. Gopal Reddy et al. reported that NiFe2 O4 prepared by wetchemical precipitation exhibited sensitivity to chlorine and H2 S [9]. Some researchers investigated the sensitivity of NiFe2 O4 to a few kinds of gases [10,11]. NiFe2 O4 is reported to be a good catalyst [12]. NiFe2 O4 is a well-known magnetic material with an inverse spinel structure [13], and most efforts have been devoted to the preparation of nano-particles [14,15], nanorods
∗
Corresponding author. E-mail address: [email protected] (C. Xiangfeng).
0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2006.10.020
[16], nanowire array [17] and nanofibers [18] of NiFe2 O4 and their magnetic properties. The nanorods, nanowire arrays and nanofibers were prepared by a polyethylene glycol (PEG)assisted route, a porous anodic aluminum oxide (AAO) template method and an electrospinning method [16–18], respectively. These methods were complicated. Herein, we describe the synthesis of NiFe2 O4 nanospheres, nanocubes and nanorods by a hydrothermal method without any surfactant and investigation on their gas-sensing properties. It was found that the sensor based on NiFe2 O4 nanorods was sensitive and selective to triethylamine and the sensor based on NiFe2 O4 nanocubes exhibited the opposite behavior, i.e., the conductivity of the sensor based on NiFe2 O4 nanocubes increased in a reducing gas atmosphere.
2. Experimental To prepare nano-NiFe2 O4 materials, Ni(NO3 )2 ·6H2 O (AR) and Fe(NO3 )3 ·9H2 O (AR) were dissolved in de-ionized water, letting [Ni2+ ] = 0.10 mol/L and [Fe3+ ] = 0.20 mol/L in the mixed solution. A solution of 6.0 mol/L NaOH was added drop-wise under stirring into a 20.0 mL mixed solution until the final pH value attained a designated value to form an admixture. The admixture was transferred into a Teflon autoclave (50 mL) with a stainless steel shell, and a little de-ionized water was added into the Teflon autoclave up to 80% of the total volume. The autoclave
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was kept at 120–200 ◦ C for 24–96 h and then allowed to cool to room temperature naturally. The final product was washed with de-ionized water and pure alcohol for several times to remove possible residues and then dried at 70 ◦ C for 24 h. The phase compositions of obtained samples were characterized by means of a D/max 2200 X-ray powder diffractometer (Rigaku) (XRD) with Cu K␣ radiation (λ = 0.154056 nm). The size and morphology of the sample were determined by an FEI-Tecnai 12 transmission electron microscope (TEM) and a JEM-2010HR high-resolution transmission electron microscope (HRTEM) operating at 200 kV. The paste formed from a mixture of nano-NiFe2 O4 with a polyvinyl alcohol (PVA) solution was coated onto an Al2 O3 tube on which two gold leads had been installed at each end. 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 operating temperature that could be controlled in the range of 100–500 ◦ C. Electrical resistance of a sensor was measured in air and in sample gases. The relative humidity of air was 65% RH. The sample gas was prepared by the following method: a chamber was vacuumized to about 50–80 kPa, the gas (or liquid) to be detected was injected to the chamber, and then air was put into the chamber to let the pressure in the chamber to be 101.3 kPa. The gas concentration in the chamber could be calculated according to the gas volume injected and the chamber volume. The response (S) was defined as the ratio of the electrical resistance in a sample gas (Rg ) to that in air (Ra ), but two different ratios of Rgs /Rg and Rg /Ra were used as the response data, here Rgs was the steady-state value of Rg . 3. Results and discussion Fig. 1 shows XRD patterns of NiFe2 O4 powders prepared at different pH values (160 ◦ C, 48 h). It could be seen in Fig. 1 that spinel NiFe2 O4 was obtained when pH value was in the range of 7–14. It was found that the hydrothermal reaction temperature also influenced the phase composition, there was Fe2 O3 in the product when the temperature was 120 ◦ C, and pure NiFe2 O4 was prepared when the temperature was 140–200 ◦ C (48 h, pH 13). The XRD patterns of the products also revealed that spinel NiFe2 O4 could be formed when the
Fig. 1. XRD patterns of NiFe2 O4 powders prepared at different pH values (160 ◦ C, 48 h).
hydrothermal time was 24–96 h (pH 14, 150 ◦ C). The solubility product of Fe(OH)3 and Ni(OH)2 , KspFe(OH)3 and KspNi(OH)2 , are 2.64 × 10−39 and 5.4764 × 10−16 at 298 K, respectively, but the Ksp data would change under hydrothermal conditions. Pure NiFe2 O4 was obtained because the molar ratio of Fe(OH)3 to Ni(OH)2 in the Teflon autoclave was about 2/1. Fe2 O3 may be formed in the product, when the precipitate molar ratio of Fe(OH)3 to Ni(OH)2 is higher than 2/1 and the [Fe3+ ]/[Ni2+ ] ratio in solution is lower than 2/1. Fig. 2 shows the TEM images of the as-prepared samples at different pH values (150 ◦ C, 24 h). The pH value had a significant influence on the morphology of NiFe2 O4 . When pH was 12, as shown in Fig. 2a, the shape of most particles was nearly spheres and there were a few nanocubes in the NiFe2 O4 sample, the particle size being about 10–70 nm. Nanorods were obtained when pH was 13, as depicted in Fig. 2b, the length was about 1 m and the diameter was about 30 nm. As illustrated in Fig. 2c, the shape of particles was nanocube when pH was 14, the side length being about 60–100 nm. Fig. 2d shows the HRTEM image of a NiFe2 O4 nanorod; the interplanar spacing of 0.25 nm well agreed with the spacing between {3 1 1} planes of the NiFe2 O4 crystal (0.25 nm). The corresponding selectedarea electron diffraction (SAED) pattern (Fig. 2d) revealed that the nanorods were single crystalline. Hydrothermal reaction time also affected the morphology of NiFe2 O4 . Fig. 3 illustrates the TEM images of the samples obtained with different reaction time (pH 13, 150 ◦ C). A mixture of nanorods and nanospheres was obtained when the reaction time was 48 h, and the product was all particles when the reaction time was 96 h. The effect of reaction temperature on the morphology of the products was also investigated. The results revealed that nanorods were obtained only when the temperature was 150 ◦ C and nanospheres were formed at higher or lower temperatures. Fig. 4a shows the response values to 500 ppm triethylamine of three sensors based on the nanospheres, nanorods and nanocubes operating at different temperatures. The response of the sensor based on the nanospheres was very low when the operating temperature was 150–350 ◦ C, the maximum being only 2.7. The sensor based on the nanorods exhibited a high response when operating at 150–225 ◦ C, the maximum response was 143 at 175 ◦ C, while the response was very low when the operating temperature was higher than 250 ◦ C. The resistance of the sensors based on the nanospheres and nanorods increased in a reducing atmosphere, which is characteristic of a p-type semiconductor. The similar characteristic was also reported in literature [10]. But, the response of the sensor based on the nanocubes was lower than 1, meaning that the resistance decreased in the reducing atmosphere, the phenomenon was the typical characteristic of an n-type semiconductor. Baruwati et al. [19] also reported that the NiFe2 O4 prepared under different conditions exhibited different conducting behavior. According to their results, the ntype behavior was attributed to the presence of Fe2+ and the conductivity was predominantly due to the hopping of electrons from Fe2+ to Fe3+ , while the p-type behavior was attributed to the presence of Ni3+ and the conductivity was predominantly due to the transfer of holes from Ni2+ to Ni3+ . The sensor based
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Fig. 2. TEM images of the as-prepared samples at different pH values (150 ◦ C, 24 h): (a) pH 12; (b) pH 13; (c) pH 14; (d) SAED and HRTEM image of a single crystalline NiFe2 O4 nanorod.
Fig. 3. TEM images of samples obtained with different reaction time (pH 13, 150 ◦ C): (a) 48 h; (b) 96 h.
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Fig. 4. (a) Responses to 500 ppm triethylamine of three sensors based on nanospheres, nanorods and nanocubes operating at different temperatures. (b) Responses of the sensor based on nanorods to a few kinds of reducing gases.
Fig. 5. (a) Correlation between the triethylamine concentration and the response of the sensor based on NiFe2 O4 nanorods. (b) Response transients to 25 and 500 ppm triethylamine gas.
on the nanocubes exhibited n-type response, i.e., the resistance of the sensor in the 500 ppm triethylamine atmosphere was only 0.033 times as high as that in air when operating at 200 ◦ C. It is very difficult to explain why the NiFe2 O4 nanocubes contain the Ni3+ ions, we are now investigating the real reason. The gas-sensing mechanism is likely based on the changes in conductance of NiFe2 O4 . The oxygen adsorbed on the surface of the material influences the conductance of the NiFe2 O4 based sensor. The amount of oxygen on the surface of the material depends on the particle size, morphology, and specific area of the material, and also on the operating temperature of the sensor. With increasing the temperature in air, the state of oxygen adsorbed on the surface of NiFe2 O4 underwent the following reactions: O2(gas) ⇒ O2(ads) ⇒ O2 − (ads) ⇒ 2O− (ads) ⇒ 2O2− (ads) . The oxygen species capture electrons from the material, leading to the changes in hole or electron concentration in the NiFe2 O4 materials. Reducing gases react with the oxygen adsorbed and the electrons captured by the oxygen adsorbed, resulting in an increase in the conductance of the n-type NiFe2 O4 and a decrease in conductance of the p-type NiFe2 O4 . The responses of the sensor based on the nanorods to a few kinds of reducing gases are shown in Fig. 4b. The responses all attained the maximum values at 175 ◦ C; the responses to 1000 ppm benzene, 1000 ppm toluene, 500 ppm triethylamine
and 500 ppm methane were 28, 48, 143 and 21, respectively. The results revealed that the sensor based on the NiFe2 O4 nanorods had some selectivity to triethylamine. Fig. 5a depicts the correlation between the triethylamine concentration and the response of the sensor based on the NiFe2 O4 nanorods. The response decreased with decreasing the triethylamine gas concentration. The shape correlation of response versus gas concentration was approximately linear. The responses to low concentrations of triethylamine were high, especially, the response to 1 ppm triethylamine attained 7 when operating at 175 ◦ C. In general, the response time and recovery time are defined as the times for a sensor to reach the final signal. The response transients of the sensor of NiFe2 O4 nanorods to 25 and 500 ppm triethylamine gas are shown in Fig. 5b. The response times to 25 and 500 ppm triethylamine at 175 ◦ C were 22 and 12 s, respectively, but the recovery times were longer. 4. Conclusions In summary, we prepared NiFe2 O4 nanorods and nanocubes by a hydrothermal method without any surfactant, and investigated the gas-sensing properties. The results demonstrated that the phase composition and shape of the particles were depen-
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dent on the preparation conditions. The gas response was greatly affected by the shape of the NiFe2 O4 crystals. The sensor based on the NiFe2 O4 nanorods was sensitive and selective to low concentrations of triethylamine at 175 ◦ C; especially, the sensor could detect 1 ppm triethylamine. Thus, the sensor based on the NiFe2 O4 nanorods is promising for a practical device for detecting low concentrations of triethylamine. The sensor based on the NiFe2 O4 nanocubes exhibited the opposite behavior of the unusual conductivity increase in a reducing gas atmosphere, the response to 500 ppm triethylamine being 0.033. Acknowledgement The project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in sputtering gas, and working temperature, Sens. Actuat. B 100 (2004) 320–324. [2] U. Hoefer, J. Frank, M. Fleischer, High temperature Ga2 O3 -gas sensors and SnO2 -gas sensors: a comparison, Sens. Actuat. B 78 (2001) 6–11. [3] D. Morris, R.G. Egdell, Application of V-doped TiO2 as a sensor for detection SO2 , J. Mater. Chem. 11 (2001) 3207–3210. [4] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [5] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Adv. Mater. 15 (2003) 997–1000. [6] C. Li, D. Zhang, X. Liu, S. Han, T. Tang, J. Han, C. Zhou, In2 O3 nanowires as chemical sensors, Appl. Phys. Lett. 82 (2003) 1613–1615.
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[7] J. Chen, L. Xu, W. Li, X. Gou, ␣-Fe2 O3 nanotubes in gas sensor and lithium-ion battery applications, Adv. Mater. 17 (2005) 582–586. [8] J. Liu, X. Wang, Q. Peng, Y. Li, Vanadium pentoxide nanobelts: highly selective and stable ethanol sensor materials, Adv. Mater. 17 (2005) 764–767. [9] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Preparation and characterization of ferrites as gas sensor materials, J. Mater. Sci. Lett. 19 (2000) 775–778. [10] Y. Liu, H. Wang, Y. Yang, Z. Liu, H. Yang, G. Shen, R. Yu, Hydrogen sulfide sensing properties of NiFe2 O4 nanopowder doped with noble metals, Sens. Actuat. B 102 (2004) 148–154. [11] L. Satyanarayana, K.M. Reddya, S.V. Manorama, Nanosized spinel NiFe2 O4 : a novel materials for the detection of liquefied petroleum gas in air, Mater. Chem. Phys. 82 (2003) 21–26. [12] K. Sreekumar, S. Sugunan, Ferrospinels based on Co and Ni prepared via a low temperature route as efficient catalysts for the selective synthesis of o-cresol and 2,6-xylenol from phenol and methanol, J. Mol. Catal. A: Chem. 185 (2002) 259–268. [13] A.S. Albuquerque, J.D. Ardisson, W.A.A. Macedo, J.L. L´opez, R. Paniago, A.I.C. Persiano, Structure and magnetic properties of nanostructured Niferrite, J. Magn. Magn. Mater. 226–230 (2001) 1379–1381. ˇ [14] V. Sepel´ ak, M. Menzel, I. Bergmann, M. Wiebcke, F. Krumeich, K.D. Becker, Structural and magnetic properties of nanosized mechanosynthesized nickel ferrite, J. Magn. Magn. Mater. 272–276 (2004) 1616–1618. [15] A. Kale, S. Gubbala, R.D.K. Misra, Magnetic behavior of nanocrystalline nickel ferrite synthesized by the reverse micelle technique, J. Magn. Magn. Mater. 277 (2004) 350–358. [16] D.E. Zhang , X.J. Zhang, X.M. Ni, H.G. Zheng, D.D. Yang, Synthesis and characterization of NiFe2 O4 megnetic nanorods via a PEG-assisted route, J. Magn. Magn. Mater. 292 (2005) 79–82. [17] D. Yu, Y. Du, Fabrication of NiFe2 O4 nanowire arrays and its magnetic properties, Acta Phys. Sin. 54 (2) (2005) 930–934. [18] D. Li, T. Herricks, Y. Xia, Magnetic nanofibers of nickel ferrite prepared by electrospinning, Appl. Phys. Lett. 83 (2003) 4586–4588. [19] B. Baruwati, K.M. Reddy, S.V. Manorama, R.K. Singh, O. Parkash, Tailored conductivity behavior in nanocrystalline nickel ferrite, Appl. Phys. Lett. 85 (2004) 2833–2835.
The preparation and gas-sensing properties of NiFe2O4 nanocubes and nanorods Chu Xiangfeng ∗ , Jiang Dongli, Zheng Chenmou School of Chemistry and Chemical Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, PR China Received 21 April 2006; received in revised form 10 October 2006; accepted 11 October 2006 Available online 13 November 2006
Abstract NiFe2 O4 nanorods and nanocubes were prepared by a hydrothermal method without any surfactant for the first time. The length and diameter of the nanorods were about 1 m and 30 nm, respectively; the side length of the nanocubes was about 60–100 nm. It was found that the sensor based on NiFe2 O4 nanorods was relatively sensitive and selective to triethylamine. Especially, the sensitivity to 1 ppm triethylamine attained 7 when operating at 175 ◦ C. On the other hand, the sensor based on NiFe2 O4 nanocubes exhibited the opposite behavior; namely, the conductivity of the sensor increased in a reducing gas atmosphere. © 2006 Published by Elsevier B.V. Keywords: NiFe2 O4 ; Nanorod; Gas sensor
1. Introduction Now the presence of volatile organic compounds (VOCs) has become a serious concern due to strict environmental regulations on VOCs in many countries. The toxic or carcinogenic nature of VOCs is dangerous to human beings. Semiconductor metal oxides as gas-sensing materials have attracted great attention for a long time due to their advantageous features, such as higher sensitivity to ambient conditions, lower cost and simplicity in fabrication [1–3]. Apart from nano-particle and bulk sensor materials, the gas-sensing properties of one-dimensional (1D) nano-structured materials have been reported since various kinds of 1D nano-structured materials have been prepared [4–8]. The sensitivities of the sensors based on 1D nano-materials were very high because of their special surface state and morphology. Gopal Reddy et al. reported that NiFe2 O4 prepared by wetchemical precipitation exhibited sensitivity to chlorine and H2 S [9]. Some researchers investigated the sensitivity of NiFe2 O4 to a few kinds of gases [10,11]. NiFe2 O4 is reported to be a good catalyst [12]. NiFe2 O4 is a well-known magnetic material with an inverse spinel structure [13], and most efforts have been devoted to the preparation of nano-particles [14,15], nanorods
∗
Corresponding author. E-mail address: [email protected] (C. Xiangfeng).
0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2006.10.020
[16], nanowire array [17] and nanofibers [18] of NiFe2 O4 and their magnetic properties. The nanorods, nanowire arrays and nanofibers were prepared by a polyethylene glycol (PEG)assisted route, a porous anodic aluminum oxide (AAO) template method and an electrospinning method [16–18], respectively. These methods were complicated. Herein, we describe the synthesis of NiFe2 O4 nanospheres, nanocubes and nanorods by a hydrothermal method without any surfactant and investigation on their gas-sensing properties. It was found that the sensor based on NiFe2 O4 nanorods was sensitive and selective to triethylamine and the sensor based on NiFe2 O4 nanocubes exhibited the opposite behavior, i.e., the conductivity of the sensor based on NiFe2 O4 nanocubes increased in a reducing gas atmosphere.
2. Experimental To prepare nano-NiFe2 O4 materials, Ni(NO3 )2 ·6H2 O (AR) and Fe(NO3 )3 ·9H2 O (AR) were dissolved in de-ionized water, letting [Ni2+ ] = 0.10 mol/L and [Fe3+ ] = 0.20 mol/L in the mixed solution. A solution of 6.0 mol/L NaOH was added drop-wise under stirring into a 20.0 mL mixed solution until the final pH value attained a designated value to form an admixture. The admixture was transferred into a Teflon autoclave (50 mL) with a stainless steel shell, and a little de-ionized water was added into the Teflon autoclave up to 80% of the total volume. The autoclave
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was kept at 120–200 ◦ C for 24–96 h and then allowed to cool to room temperature naturally. The final product was washed with de-ionized water and pure alcohol for several times to remove possible residues and then dried at 70 ◦ C for 24 h. The phase compositions of obtained samples were characterized by means of a D/max 2200 X-ray powder diffractometer (Rigaku) (XRD) with Cu K␣ radiation (λ = 0.154056 nm). The size and morphology of the sample were determined by an FEI-Tecnai 12 transmission electron microscope (TEM) and a JEM-2010HR high-resolution transmission electron microscope (HRTEM) operating at 200 kV. The paste formed from a mixture of nano-NiFe2 O4 with a polyvinyl alcohol (PVA) solution was coated onto an Al2 O3 tube on which two gold leads had been installed at each end. 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 operating temperature that could be controlled in the range of 100–500 ◦ C. Electrical resistance of a sensor was measured in air and in sample gases. The relative humidity of air was 65% RH. The sample gas was prepared by the following method: a chamber was vacuumized to about 50–80 kPa, the gas (or liquid) to be detected was injected to the chamber, and then air was put into the chamber to let the pressure in the chamber to be 101.3 kPa. The gas concentration in the chamber could be calculated according to the gas volume injected and the chamber volume. The response (S) was defined as the ratio of the electrical resistance in a sample gas (Rg ) to that in air (Ra ), but two different ratios of Rgs /Rg and Rg /Ra were used as the response data, here Rgs was the steady-state value of Rg . 3. Results and discussion Fig. 1 shows XRD patterns of NiFe2 O4 powders prepared at different pH values (160 ◦ C, 48 h). It could be seen in Fig. 1 that spinel NiFe2 O4 was obtained when pH value was in the range of 7–14. It was found that the hydrothermal reaction temperature also influenced the phase composition, there was Fe2 O3 in the product when the temperature was 120 ◦ C, and pure NiFe2 O4 was prepared when the temperature was 140–200 ◦ C (48 h, pH 13). The XRD patterns of the products also revealed that spinel NiFe2 O4 could be formed when the
Fig. 1. XRD patterns of NiFe2 O4 powders prepared at different pH values (160 ◦ C, 48 h).
hydrothermal time was 24–96 h (pH 14, 150 ◦ C). The solubility product of Fe(OH)3 and Ni(OH)2 , KspFe(OH)3 and KspNi(OH)2 , are 2.64 × 10−39 and 5.4764 × 10−16 at 298 K, respectively, but the Ksp data would change under hydrothermal conditions. Pure NiFe2 O4 was obtained because the molar ratio of Fe(OH)3 to Ni(OH)2 in the Teflon autoclave was about 2/1. Fe2 O3 may be formed in the product, when the precipitate molar ratio of Fe(OH)3 to Ni(OH)2 is higher than 2/1 and the [Fe3+ ]/[Ni2+ ] ratio in solution is lower than 2/1. Fig. 2 shows the TEM images of the as-prepared samples at different pH values (150 ◦ C, 24 h). The pH value had a significant influence on the morphology of NiFe2 O4 . When pH was 12, as shown in Fig. 2a, the shape of most particles was nearly spheres and there were a few nanocubes in the NiFe2 O4 sample, the particle size being about 10–70 nm. Nanorods were obtained when pH was 13, as depicted in Fig. 2b, the length was about 1 m and the diameter was about 30 nm. As illustrated in Fig. 2c, the shape of particles was nanocube when pH was 14, the side length being about 60–100 nm. Fig. 2d shows the HRTEM image of a NiFe2 O4 nanorod; the interplanar spacing of 0.25 nm well agreed with the spacing between {3 1 1} planes of the NiFe2 O4 crystal (0.25 nm). The corresponding selectedarea electron diffraction (SAED) pattern (Fig. 2d) revealed that the nanorods were single crystalline. Hydrothermal reaction time also affected the morphology of NiFe2 O4 . Fig. 3 illustrates the TEM images of the samples obtained with different reaction time (pH 13, 150 ◦ C). A mixture of nanorods and nanospheres was obtained when the reaction time was 48 h, and the product was all particles when the reaction time was 96 h. The effect of reaction temperature on the morphology of the products was also investigated. The results revealed that nanorods were obtained only when the temperature was 150 ◦ C and nanospheres were formed at higher or lower temperatures. Fig. 4a shows the response values to 500 ppm triethylamine of three sensors based on the nanospheres, nanorods and nanocubes operating at different temperatures. The response of the sensor based on the nanospheres was very low when the operating temperature was 150–350 ◦ C, the maximum being only 2.7. The sensor based on the nanorods exhibited a high response when operating at 150–225 ◦ C, the maximum response was 143 at 175 ◦ C, while the response was very low when the operating temperature was higher than 250 ◦ C. The resistance of the sensors based on the nanospheres and nanorods increased in a reducing atmosphere, which is characteristic of a p-type semiconductor. The similar characteristic was also reported in literature [10]. But, the response of the sensor based on the nanocubes was lower than 1, meaning that the resistance decreased in the reducing atmosphere, the phenomenon was the typical characteristic of an n-type semiconductor. Baruwati et al. [19] also reported that the NiFe2 O4 prepared under different conditions exhibited different conducting behavior. According to their results, the ntype behavior was attributed to the presence of Fe2+ and the conductivity was predominantly due to the hopping of electrons from Fe2+ to Fe3+ , while the p-type behavior was attributed to the presence of Ni3+ and the conductivity was predominantly due to the transfer of holes from Ni2+ to Ni3+ . The sensor based
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Fig. 2. TEM images of the as-prepared samples at different pH values (150 ◦ C, 24 h): (a) pH 12; (b) pH 13; (c) pH 14; (d) SAED and HRTEM image of a single crystalline NiFe2 O4 nanorod.
Fig. 3. TEM images of samples obtained with different reaction time (pH 13, 150 ◦ C): (a) 48 h; (b) 96 h.
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Fig. 4. (a) Responses to 500 ppm triethylamine of three sensors based on nanospheres, nanorods and nanocubes operating at different temperatures. (b) Responses of the sensor based on nanorods to a few kinds of reducing gases.
Fig. 5. (a) Correlation between the triethylamine concentration and the response of the sensor based on NiFe2 O4 nanorods. (b) Response transients to 25 and 500 ppm triethylamine gas.
on the nanocubes exhibited n-type response, i.e., the resistance of the sensor in the 500 ppm triethylamine atmosphere was only 0.033 times as high as that in air when operating at 200 ◦ C. It is very difficult to explain why the NiFe2 O4 nanocubes contain the Ni3+ ions, we are now investigating the real reason. The gas-sensing mechanism is likely based on the changes in conductance of NiFe2 O4 . The oxygen adsorbed on the surface of the material influences the conductance of the NiFe2 O4 based sensor. The amount of oxygen on the surface of the material depends on the particle size, morphology, and specific area of the material, and also on the operating temperature of the sensor. With increasing the temperature in air, the state of oxygen adsorbed on the surface of NiFe2 O4 underwent the following reactions: O2(gas) ⇒ O2(ads) ⇒ O2 − (ads) ⇒ 2O− (ads) ⇒ 2O2− (ads) . The oxygen species capture electrons from the material, leading to the changes in hole or electron concentration in the NiFe2 O4 materials. Reducing gases react with the oxygen adsorbed and the electrons captured by the oxygen adsorbed, resulting in an increase in the conductance of the n-type NiFe2 O4 and a decrease in conductance of the p-type NiFe2 O4 . The responses of the sensor based on the nanorods to a few kinds of reducing gases are shown in Fig. 4b. The responses all attained the maximum values at 175 ◦ C; the responses to 1000 ppm benzene, 1000 ppm toluene, 500 ppm triethylamine
and 500 ppm methane were 28, 48, 143 and 21, respectively. The results revealed that the sensor based on the NiFe2 O4 nanorods had some selectivity to triethylamine. Fig. 5a depicts the correlation between the triethylamine concentration and the response of the sensor based on the NiFe2 O4 nanorods. The response decreased with decreasing the triethylamine gas concentration. The shape correlation of response versus gas concentration was approximately linear. The responses to low concentrations of triethylamine were high, especially, the response to 1 ppm triethylamine attained 7 when operating at 175 ◦ C. In general, the response time and recovery time are defined as the times for a sensor to reach the final signal. The response transients of the sensor of NiFe2 O4 nanorods to 25 and 500 ppm triethylamine gas are shown in Fig. 5b. The response times to 25 and 500 ppm triethylamine at 175 ◦ C were 22 and 12 s, respectively, but the recovery times were longer. 4. Conclusions In summary, we prepared NiFe2 O4 nanorods and nanocubes by a hydrothermal method without any surfactant, and investigated the gas-sensing properties. The results demonstrated that the phase composition and shape of the particles were depen-
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dent on the preparation conditions. The gas response was greatly affected by the shape of the NiFe2 O4 crystals. The sensor based on the NiFe2 O4 nanorods was sensitive and selective to low concentrations of triethylamine at 175 ◦ C; especially, the sensor could detect 1 ppm triethylamine. Thus, the sensor based on the NiFe2 O4 nanorods is promising for a practical device for detecting low concentrations of triethylamine. The sensor based on the NiFe2 O4 nanocubes exhibited the opposite behavior of the unusual conductivity increase in a reducing gas atmosphere, the response to 500 ppm triethylamine being 0.033. Acknowledgement The project was sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] M. Bendahan, R. Boulmani, J.L. Seguin, K. Aguir, Characterization of ozone sensors based on WO3 reactively sputtered films: influence of O2 concentration in sputtering gas, and working temperature, Sens. Actuat. B 100 (2004) 320–324. [2] U. Hoefer, J. Frank, M. Fleischer, High temperature Ga2 O3 -gas sensors and SnO2 -gas sensors: a comparison, Sens. Actuat. B 78 (2001) 6–11. [3] D. Morris, R.G. Egdell, Application of V-doped TiO2 as a sensor for detection SO2 , J. Mater. Chem. 11 (2001) 3207–3210. [4] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [5] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Adv. Mater. 15 (2003) 997–1000. [6] C. Li, D. Zhang, X. Liu, S. Han, T. Tang, J. Han, C. Zhou, In2 O3 nanowires as chemical sensors, Appl. Phys. Lett. 82 (2003) 1613–1615.
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