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Hydrothermal synthesis and ethanol sensing properties of CeVO4 and CeVO4–CeO2 powders
Materials Letters 60 (2006) 1859 – 1862 www.elsevier.com/locate/matlet
Hydrothermal synthesis and ethanol sensing properties of CeVO4 and CeVO4–CeO2 powders Limiao Chen ⁎ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China Received 9 September 2005; accepted 9 December 2005 Available online 4 January 2006
Abstract CeVO4 powders with different morphologies were synthesized in basic media by surfactant-free hydrothermal process. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). From the experimental results, it has been revealed that there is an optimal pH range (pH ≤ 12.0) for the formation of pure CeVO4 and the pH plays a key role in the formation of the products of different morphologies and sizes. Increasing pH from 9 to 10 causes the morphology transformation: irregular nanoparticles → nanorods. High pH (12.0 b pH b 14.0) resulted in the formation of CeVO4–CeO2 compounds. Gas sensing behavior of CeVO4 and CeVO4–CeO2 based materials to ethanol were firstly studied. The sensors based on these materials show different response to ethanol, wherein the response is the best when using CeVO4–CeO2 compounds as material. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Crystal growth; Sensors
1. Introduction CeVO4-based materials have received considerable attention in view of their potential applications in various fields, such as counter electrodes in electrochromic device, oxidation catalysts, and components of solid oxide fuel cell (SOFC) anodes [1–4]. Synthesis of CeVO4 powders has been previously accomplished by solid-state reaction between CeO2 and V2O5 [5] and sol–gel [6] method, which needs harsh conditions. To our knowledge, there are few reports on the fabrication of CeVO4 with controllable morphology as well as the gas sensing properties. Hydrothermal method is one of the most promising solution chemical methods. The particles' size and their distribution, phase homogeneity, and morphology can be well controlled [7– 10]. The surfactant-free hydrothermal method has been used to successfully synthesize nanosized CeVO4 powder [11], but no efficient control over their size and morphology has been achieved yet. Recently, pure CeVO4 microcrystals [12] have been synthesized by an ethylenediaminetetraacetic acid-mediated hydrothermal method. ⁎ Tel./fax: +86 731 8836964. E-mail address: [email protected]. 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.037
In this paper, a surfactant-free hydrothermal process was developed to synthesize CeVO4 powders with different morphology. CeVO4–CeO2 compounds were also obtained after hydrothermal heat-treatment. Thin film type sensors are also fabricated based on the CeVO4 and CeVO4–CeO2 powders and results show that the sensor based on CeVO4–CeO2 exhibits high response to ethanol vapor. 2. Experiment 2.1. Samples' preparation The samples were prepared by hydrothermal method in the absence of any organic additives. All reagents were analytical grade pure and were purchased from Shanghai Chemical Co. Ltd. In a typical procedure, 0.001 mol NH4VO3 powder was added to 40 ml aqueous solution containing 0.001 mol Ce (NO3)3·6H2O under stirring. Sodium hydroxide solution (2M) was added dropwise into the above solution to adjust the pH to the designed basicity ranging from 9 to 14. The resulting suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and sealed tightly. Hydrothermal synthesis was carried out at 150 °C for 8 h in an electric oven without shaking
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or stirring. After cooling to room temperature naturally, the precipitates were collected, washed with distilled water and absolute ethanol several times, and then dried in air at 60 °C for 4 h. 2.2. Samples' characterization All the samples were characterized by powder X-ray diffraction (XRD) on a D8 Advance Bruker X-ray diffractometer with monochromatized Cu Kα (λ = 1.5418 Å) incident radiation. XRD patterns were recorded from 10 to 70° (2θ) with a scanning step of 0.01°. The size distribution and morphology of the samples were analyzed by transmission electron microscopy (TEM) observation on a H-800 transmission electron microscope operated at 200 kV. 2.3. Sensor element fabrication and gas-sensing procedure In a typical procedure, the as-synthesized CeVO4 and CeVO4–CeO2 powders were mixed with 2 wt.% tetraethylorthosilicate (TEOS) and the resulting paste was cast as thin film on an alumina tube substrate (3 mm diameter, 10 mm length) provided with two platinum (Pt) electrodes of 8 mm apart and a platinum heater on the back side of the substrate in order to control the working temperature of the sensor element. After calcination in air at 400 °C for 2 h, the sensor element was installed into a HW-C30A (Hanwei Group, Henan) gas sensing intelligent apparatus, which both served as voltage sources and current reader. Each pristine sensor (in air) was found to exhibit the typical behavior of a varistor, showing an exponential decay in resistance as the voltage was ramped up. For consistency, a potential of 10 V was applied across the two Pt electrodes for all measurements, and the air coming from an ordinary fume hood was used as the background gas. For the purpose of comparison, the thickness of the thin film on the alumina tube substrate was kept identical with the same casting times (10 s) and all the gas sensing experiments were carried out in a closed plastic container (2 l in volume). C2H5OH was introduced by the use of syringe at an injection port. Before recording the data as shown in Fig. 3, the sensor elements were aged for at least 1 week. The sensitivity S was defined as the change in resistance normalized to the initial resistance of the sensor in air, S = Rgas / Rair, where Rair is the resistance of the sensor in air and Rgas is the resistance of sensor in the presence of the test gas. 3. Results and discussion The influence of solution pH on the crystalline phase of samples is shown through the XRD patterns presented in Fig. 1. For the samples prepared from the solution with pH = 9, three predominant peaks at 2θ = 23.9°, 32.3° and 47.7° due to the (200), (112) and (312) reflections of CeVO4 present, other peaks are not apparent. After increasing the pH (pH = 10, 11, 12), other diffraction peaks belonging to the crystalline CeVO4 were observed clearly (JCPDS card 12-0757), indicating the relative high crystallinity. When pH increased to 12.5, the main solid product was CeVO4, together with a small amount of Ce (OH)3 detected as parent peaks at around 2θ = 28.5°. Upon increasing the pH to 13, the intensity of the peaks centered at 28.5° increased,
Fig. 1. XRD patterns of the samples obtained in the solution with different pH: (a) pH = 9, (b) pH = 10, (c) pH = 11, (d) pH = 12, (e) pH = 12.5, (f) pH = 14.
while the intensity of the peaks indexed as CeVO4 decreased. Further increasing the pH (pH = 14) resulted in the pure CeO2 phase. From the experimental results, it seems that there exists an optimal pH range (pH ≤ 12.0) for the formation of pure CeVO4. This phenomenon can be explained by the complex interaction and balance between the chemical potential and the rate of ionic motion. The dependence of pure phase on pH has been investigated in microwave synthesis of CeVO4 [13]. In the basic media (pH = 9∼12), cerium species firstly precipitate as Ce(OH)3 particles while vanadium from NH4VO3 exists as the vanadate anion. On the microscopic level, the cerium and vanadium complex reacts in the solution to precipitate CeVO4. However, in the strong basic media (pH = 12.5∼14), Ce(OH)3 is more stable than CeVO4. Ce(OH)3 would then dehydrolyze and be oxidated as CeO2. Therefore, strong basic condition would favor the formation of CeO2. The influence of pH on the morphology of the final products was investigated. The transmission electron microscope (TEM) images showed various morphologies for the samples prepared from the aqueous solutions with different pH. The samples obtained after hydrothermal heat treatment from the solution with pH = 9 are shown in Fig. 2a. It is clearly shown that as-synthesized powders consist of small irregular nanoparticles with an average size of 10 nm. When pH is fixed at 10, rod-like particles with inhomogenous size distribution (as shown in Fig. 2b) were formed, accompanied by the appearance of few spherical particles. Further increasing the pH (pH = 11) resulted in the disappearance of the small particles (as shown in Fig. 2c). Moreover, it was observed that the surface of the rod-like particles became smooth as the particles were condensed. Based on the TEM images, we proposed that the rod-like particles might mainly be fabricated by an Ostwald ripening mechanism [14,15], i.e. the dissolving of small particles and the depositing of components on large particles. When the pH increased to 12, a large number of irregular spherical particles, together with few rod-like particles (as shown in Fig. 2d), were formed. When the pH increased to 12.5, the products formed mainly consisted of spherical and plate-like particles (as shown in Fig. 2e). In addition, small particles with diameter ranging from 10 to 30 nm were found on the surface of the spherical and plate-like particles (as shown in Fig. 2f and g, respectively). It is interesting that few individual small nanoparticles in a dispersed way were found in the solution under the present conditions even though the samples were ultrasonically treated for preparation of TEM grids. With increasing pH (pH = 13.5),
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Fig. 2. TEM images of the samples obtained in the solution with different pH value: (a) pH = 9, (b) pH = 10, (c) pH = 11, (d) pH = 12, (e, f, g) pH = 12.5, (h) pH = 14.
the spherical and plate-like particles disappeared greatly (not shown here). After adjusting pH to strongly basic condition (pH = 14), the obtained samples are irregular nanoparticles with diameter ranging from 10 to 50 nm (Fig. 2h). From Fig. 1e we can see that the powders obtained from the solution with pH = 12.5 were mixed phase. To
analyze the composition of the big particles, energy dispersive spectroscopy (EDS) analysis was used. Results show that the actual percentage of cerium (64.87%) in big particles is higher than the theoretical value (58.62%), which was caused by the formation of CeO2.
Fig. 3. Sensor response to 50 ppm ethanol vs. operating temperature for the sensor based on CeVO4–CeO2 powders.
Fig. 4. Response curve of the sensors made of: (a) CeVO4, (b) CeO2, (c) CeVO4–CeO2 particles to ethanol at 400 °C.
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4. Conclusion In summary, CeVO4 powders with irregular, spherical and rod-like images were synthesized by a surfactant-free hydrothermal method. Results showed that there is optimal pH (pH ≤ 12.0) for the formation of pure CeVO4 and the pH plays a key role in the formation of the products with different morphologies and sizes. When the pH is 14, pure CeO2 particles were obtained. Between the two pH values, CeVO4– CeO2 compounds were formed. The sensing behaviors of CeVO4 and CeVO4–CeO2 compounds to ethanol were firstly studied. Results show that the sensors based on CeVO4, CeO2 and CeVO4–CeO2 particles show different response to ethanol, wherein the response is the best when using CeVO4–CeO2 particles as material. Fig. 5. Correlation between ethanol concentration and sensitivities of sensors based on CeVO4–CeO2 compounds.
Thin film type sensors were fabricated based on the as synthesized CeVO4, CeO2 and CeVO4–CeO2 particles. The sensors made of CeVO4, CeO2 and CeVO4–CeO2 particles were denoted as S1, S2, and S3, respectively. Fig. 3 shows the gas sensitivity vs. operating temperature at 50 ppm ethanol in air for the sensor based on CeVO4–CeO2 compounds. In all cases, the intensity increased with increasing operating temperature showing a maximum at 400 °C. Fig. 4 shows the variations of response when the sensor element was exposed to 50∼1000 ppm C2H5OH, at the optimized operating temperature of 400 °C. It is clearly seen that S1 nearly has no response to the ethanol even if the sensor was exposed to ethanol with high concentration (500 ppm). S2 has an inapparent response to ethanol at the concentration of 50 ppm, but the response increased with increasing the gas concentrations (as shown in Fig. 4b). Fig. 4c shows the typical isothermal response curve when S3 was exposed to ethanol at the same concentrations. The comparison of the response curve of S3 with those of S1 and S2 shows that the sensor based on CeVO4–CeO2 has better response to ethanol. It was reported that the sensor performance (sensitivity or selectivity) of metal oxide semiconductors (SnO2, TiO2, ZnSnO4) can be modified by adding them with additives, such as CeO2, CuO, ZnO and so on [16–19]. Lee et al. [20] have demonstrated the use of the binary compound CeO2–SnO2 for H2S gas sensing at room temperature. Khodadadi et al. [21] also studied the sensing properties of the binary compound CeO2–SnO2 and found that adding a small amount of CeO2 into SnO2 significantly enhances their response to reducing gases. Although much work remains to be done to fully understand the mechanism of the response enhancement observed on CeVO4–CeO2 sensor, we speculated that this enhancement might be attributed to the heterocontacts between CeVO4 and CeO2 [22–24]. Fig. 5 shows the relationship between sensitivity and ethanol gas concentration for S3 operated at 400 °C. It can be seen from the profile that the sensitivity of the sensor increases exponentially with the ethanol concentration at first and then reaches its saturation above 1000 ppm.
References [1] G. Picardi, F. Varsano, F. Decker, U. Opara-Krasovec, A. Surca, B. Orel, Electrochim. Acta 44 (1999) 3157. [2] J. Matta, D. Courcot, E. Abi-Aad, A. Aboukais, Chem. Mater. 14 (2002) 4118. [3] E.V. Tsipis, M.V. Patrakeev, V.V. Kharton, N.P. Vyshatko, J.R. Frade, J. Mater. Chem. 12 (2002) 3738. [4] K. Narayana, S. Masthan, V. Rao, P.K. Rao, J. Chem. Res. Synop. 9 (1997) 328. [5] W. Milligan, L. Vernon, J. Phys. Chem. 56 (1952) 145. [6] U.O. Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195. [7] X. Wang, Y. Li, J. Am. Chem. Soc. 124 (2002) 2880. [8] Y. Li, J. Wang, Z. Deng, J. Am. Chem. Soc. 123 (2001) 9904. [9] L. Chen, Y. Liu, Y. Li, J. Alloys Compd. 381 (2004) 266. [10] Z. Lu, Y. Tang, L. Chen, Y. Li, J. Crys. Growth 266 (2004) 539. [11] H. Wu, H. Xu, Q. Su, T. Chen, M. Wu, J. Mater. Chem. 13 (2003) 1223. [12] F. Luo, C. Jia, W. Song, L. You, C. Yan, Cryst. Growth Des. 5 (2005) 137. [13] H. Wang, Y. Meng, H. Yan, Inorg. Chem. Commun. 7 (2004) 553. [14] W. Wang, C. Xu, G. Wang, Y. Liu, C. Zheng, Adv. Mater. 14 (2002) 837. [15] P. Vorhees, Annu. Rev. Mater. Sci. 22 (1992) 197. [16] A. Trinchi, Y. Li, W. Wlodarski, S. Kaciulis, et al., Sens. Actuators, B, Chem. 95 (2003) 145. [17] T. Pagnier, M. Boulova, A. Galerie, A. Gaskov, G. Lucazeau, Sens. Actuators, B, Chem. 71 (2000) 134. [18] G. Devi, S. Manorama, V. Rao, J. Electrochem. Soc. 142 (1995) 228. [19] J. Yu, G. Choi, Sens. Actuators, B, Chem. 72 (2001) 141. [20] D. Lee, J. Lim, S. Lee, J. Huh, D. Lee, Sens. Actuators, B, Chem. 64 (2000) 31. [21] A. Khodadadi, S. Mohajerzadeh, Y. Mortazavi, A. Miri, Sens. Actuators, B, Chem. 80 (2001) 267. [22] R. Mochinaga, T. Yamasaki, T. Arakawa, Sens. Actuators, B, Chem. 52 (1998) 96. [23] R. Mochinaga, T. Yamasaki, T. Arakawa, Sens. Actuators, B, Chem. 66 (2000) 232. [24] R. Mochinaga, T. Arakawa, Sens. Actuators, B, Chem. 77 (2001) 196.
Hydrothermal synthesis and ethanol sensing properties of CeVO4 and CeVO4–CeO2 powders Limiao Chen ⁎ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China Received 9 September 2005; accepted 9 December 2005 Available online 4 January 2006
Abstract CeVO4 powders with different morphologies were synthesized in basic media by surfactant-free hydrothermal process. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). From the experimental results, it has been revealed that there is an optimal pH range (pH ≤ 12.0) for the formation of pure CeVO4 and the pH plays a key role in the formation of the products of different morphologies and sizes. Increasing pH from 9 to 10 causes the morphology transformation: irregular nanoparticles → nanorods. High pH (12.0 b pH b 14.0) resulted in the formation of CeVO4–CeO2 compounds. Gas sensing behavior of CeVO4 and CeVO4–CeO2 based materials to ethanol were firstly studied. The sensors based on these materials show different response to ethanol, wherein the response is the best when using CeVO4–CeO2 compounds as material. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Crystal growth; Sensors
1. Introduction CeVO4-based materials have received considerable attention in view of their potential applications in various fields, such as counter electrodes in electrochromic device, oxidation catalysts, and components of solid oxide fuel cell (SOFC) anodes [1–4]. Synthesis of CeVO4 powders has been previously accomplished by solid-state reaction between CeO2 and V2O5 [5] and sol–gel [6] method, which needs harsh conditions. To our knowledge, there are few reports on the fabrication of CeVO4 with controllable morphology as well as the gas sensing properties. Hydrothermal method is one of the most promising solution chemical methods. The particles' size and their distribution, phase homogeneity, and morphology can be well controlled [7– 10]. The surfactant-free hydrothermal method has been used to successfully synthesize nanosized CeVO4 powder [11], but no efficient control over their size and morphology has been achieved yet. Recently, pure CeVO4 microcrystals [12] have been synthesized by an ethylenediaminetetraacetic acid-mediated hydrothermal method. ⁎ Tel./fax: +86 731 8836964. E-mail address: [email protected]. 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.12.037
In this paper, a surfactant-free hydrothermal process was developed to synthesize CeVO4 powders with different morphology. CeVO4–CeO2 compounds were also obtained after hydrothermal heat-treatment. Thin film type sensors are also fabricated based on the CeVO4 and CeVO4–CeO2 powders and results show that the sensor based on CeVO4–CeO2 exhibits high response to ethanol vapor. 2. Experiment 2.1. Samples' preparation The samples were prepared by hydrothermal method in the absence of any organic additives. All reagents were analytical grade pure and were purchased from Shanghai Chemical Co. Ltd. In a typical procedure, 0.001 mol NH4VO3 powder was added to 40 ml aqueous solution containing 0.001 mol Ce (NO3)3·6H2O under stirring. Sodium hydroxide solution (2M) was added dropwise into the above solution to adjust the pH to the designed basicity ranging from 9 to 14. The resulting suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and sealed tightly. Hydrothermal synthesis was carried out at 150 °C for 8 h in an electric oven without shaking
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or stirring. After cooling to room temperature naturally, the precipitates were collected, washed with distilled water and absolute ethanol several times, and then dried in air at 60 °C for 4 h. 2.2. Samples' characterization All the samples were characterized by powder X-ray diffraction (XRD) on a D8 Advance Bruker X-ray diffractometer with monochromatized Cu Kα (λ = 1.5418 Å) incident radiation. XRD patterns were recorded from 10 to 70° (2θ) with a scanning step of 0.01°. The size distribution and morphology of the samples were analyzed by transmission electron microscopy (TEM) observation on a H-800 transmission electron microscope operated at 200 kV. 2.3. Sensor element fabrication and gas-sensing procedure In a typical procedure, the as-synthesized CeVO4 and CeVO4–CeO2 powders were mixed with 2 wt.% tetraethylorthosilicate (TEOS) and the resulting paste was cast as thin film on an alumina tube substrate (3 mm diameter, 10 mm length) provided with two platinum (Pt) electrodes of 8 mm apart and a platinum heater on the back side of the substrate in order to control the working temperature of the sensor element. After calcination in air at 400 °C for 2 h, the sensor element was installed into a HW-C30A (Hanwei Group, Henan) gas sensing intelligent apparatus, which both served as voltage sources and current reader. Each pristine sensor (in air) was found to exhibit the typical behavior of a varistor, showing an exponential decay in resistance as the voltage was ramped up. For consistency, a potential of 10 V was applied across the two Pt electrodes for all measurements, and the air coming from an ordinary fume hood was used as the background gas. For the purpose of comparison, the thickness of the thin film on the alumina tube substrate was kept identical with the same casting times (10 s) and all the gas sensing experiments were carried out in a closed plastic container (2 l in volume). C2H5OH was introduced by the use of syringe at an injection port. Before recording the data as shown in Fig. 3, the sensor elements were aged for at least 1 week. The sensitivity S was defined as the change in resistance normalized to the initial resistance of the sensor in air, S = Rgas / Rair, where Rair is the resistance of the sensor in air and Rgas is the resistance of sensor in the presence of the test gas. 3. Results and discussion The influence of solution pH on the crystalline phase of samples is shown through the XRD patterns presented in Fig. 1. For the samples prepared from the solution with pH = 9, three predominant peaks at 2θ = 23.9°, 32.3° and 47.7° due to the (200), (112) and (312) reflections of CeVO4 present, other peaks are not apparent. After increasing the pH (pH = 10, 11, 12), other diffraction peaks belonging to the crystalline CeVO4 were observed clearly (JCPDS card 12-0757), indicating the relative high crystallinity. When pH increased to 12.5, the main solid product was CeVO4, together with a small amount of Ce (OH)3 detected as parent peaks at around 2θ = 28.5°. Upon increasing the pH to 13, the intensity of the peaks centered at 28.5° increased,
Fig. 1. XRD patterns of the samples obtained in the solution with different pH: (a) pH = 9, (b) pH = 10, (c) pH = 11, (d) pH = 12, (e) pH = 12.5, (f) pH = 14.
while the intensity of the peaks indexed as CeVO4 decreased. Further increasing the pH (pH = 14) resulted in the pure CeO2 phase. From the experimental results, it seems that there exists an optimal pH range (pH ≤ 12.0) for the formation of pure CeVO4. This phenomenon can be explained by the complex interaction and balance between the chemical potential and the rate of ionic motion. The dependence of pure phase on pH has been investigated in microwave synthesis of CeVO4 [13]. In the basic media (pH = 9∼12), cerium species firstly precipitate as Ce(OH)3 particles while vanadium from NH4VO3 exists as the vanadate anion. On the microscopic level, the cerium and vanadium complex reacts in the solution to precipitate CeVO4. However, in the strong basic media (pH = 12.5∼14), Ce(OH)3 is more stable than CeVO4. Ce(OH)3 would then dehydrolyze and be oxidated as CeO2. Therefore, strong basic condition would favor the formation of CeO2. The influence of pH on the morphology of the final products was investigated. The transmission electron microscope (TEM) images showed various morphologies for the samples prepared from the aqueous solutions with different pH. The samples obtained after hydrothermal heat treatment from the solution with pH = 9 are shown in Fig. 2a. It is clearly shown that as-synthesized powders consist of small irregular nanoparticles with an average size of 10 nm. When pH is fixed at 10, rod-like particles with inhomogenous size distribution (as shown in Fig. 2b) were formed, accompanied by the appearance of few spherical particles. Further increasing the pH (pH = 11) resulted in the disappearance of the small particles (as shown in Fig. 2c). Moreover, it was observed that the surface of the rod-like particles became smooth as the particles were condensed. Based on the TEM images, we proposed that the rod-like particles might mainly be fabricated by an Ostwald ripening mechanism [14,15], i.e. the dissolving of small particles and the depositing of components on large particles. When the pH increased to 12, a large number of irregular spherical particles, together with few rod-like particles (as shown in Fig. 2d), were formed. When the pH increased to 12.5, the products formed mainly consisted of spherical and plate-like particles (as shown in Fig. 2e). In addition, small particles with diameter ranging from 10 to 30 nm were found on the surface of the spherical and plate-like particles (as shown in Fig. 2f and g, respectively). It is interesting that few individual small nanoparticles in a dispersed way were found in the solution under the present conditions even though the samples were ultrasonically treated for preparation of TEM grids. With increasing pH (pH = 13.5),
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Fig. 2. TEM images of the samples obtained in the solution with different pH value: (a) pH = 9, (b) pH = 10, (c) pH = 11, (d) pH = 12, (e, f, g) pH = 12.5, (h) pH = 14.
the spherical and plate-like particles disappeared greatly (not shown here). After adjusting pH to strongly basic condition (pH = 14), the obtained samples are irregular nanoparticles with diameter ranging from 10 to 50 nm (Fig. 2h). From Fig. 1e we can see that the powders obtained from the solution with pH = 12.5 were mixed phase. To
analyze the composition of the big particles, energy dispersive spectroscopy (EDS) analysis was used. Results show that the actual percentage of cerium (64.87%) in big particles is higher than the theoretical value (58.62%), which was caused by the formation of CeO2.
Fig. 3. Sensor response to 50 ppm ethanol vs. operating temperature for the sensor based on CeVO4–CeO2 powders.
Fig. 4. Response curve of the sensors made of: (a) CeVO4, (b) CeO2, (c) CeVO4–CeO2 particles to ethanol at 400 °C.
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4. Conclusion In summary, CeVO4 powders with irregular, spherical and rod-like images were synthesized by a surfactant-free hydrothermal method. Results showed that there is optimal pH (pH ≤ 12.0) for the formation of pure CeVO4 and the pH plays a key role in the formation of the products with different morphologies and sizes. When the pH is 14, pure CeO2 particles were obtained. Between the two pH values, CeVO4– CeO2 compounds were formed. The sensing behaviors of CeVO4 and CeVO4–CeO2 compounds to ethanol were firstly studied. Results show that the sensors based on CeVO4, CeO2 and CeVO4–CeO2 particles show different response to ethanol, wherein the response is the best when using CeVO4–CeO2 particles as material. Fig. 5. Correlation between ethanol concentration and sensitivities of sensors based on CeVO4–CeO2 compounds.
Thin film type sensors were fabricated based on the as synthesized CeVO4, CeO2 and CeVO4–CeO2 particles. The sensors made of CeVO4, CeO2 and CeVO4–CeO2 particles were denoted as S1, S2, and S3, respectively. Fig. 3 shows the gas sensitivity vs. operating temperature at 50 ppm ethanol in air for the sensor based on CeVO4–CeO2 compounds. In all cases, the intensity increased with increasing operating temperature showing a maximum at 400 °C. Fig. 4 shows the variations of response when the sensor element was exposed to 50∼1000 ppm C2H5OH, at the optimized operating temperature of 400 °C. It is clearly seen that S1 nearly has no response to the ethanol even if the sensor was exposed to ethanol with high concentration (500 ppm). S2 has an inapparent response to ethanol at the concentration of 50 ppm, but the response increased with increasing the gas concentrations (as shown in Fig. 4b). Fig. 4c shows the typical isothermal response curve when S3 was exposed to ethanol at the same concentrations. The comparison of the response curve of S3 with those of S1 and S2 shows that the sensor based on CeVO4–CeO2 has better response to ethanol. It was reported that the sensor performance (sensitivity or selectivity) of metal oxide semiconductors (SnO2, TiO2, ZnSnO4) can be modified by adding them with additives, such as CeO2, CuO, ZnO and so on [16–19]. Lee et al. [20] have demonstrated the use of the binary compound CeO2–SnO2 for H2S gas sensing at room temperature. Khodadadi et al. [21] also studied the sensing properties of the binary compound CeO2–SnO2 and found that adding a small amount of CeO2 into SnO2 significantly enhances their response to reducing gases. Although much work remains to be done to fully understand the mechanism of the response enhancement observed on CeVO4–CeO2 sensor, we speculated that this enhancement might be attributed to the heterocontacts between CeVO4 and CeO2 [22–24]. Fig. 5 shows the relationship between sensitivity and ethanol gas concentration for S3 operated at 400 °C. It can be seen from the profile that the sensitivity of the sensor increases exponentially with the ethanol concentration at first and then reaches its saturation above 1000 ppm.
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