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Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor
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Vacuum 82 (2008) 588–593 www.elsevier.com/locate/vacuum
Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor A.B. Bodade, A.M. Bende, G.N. Chaudhari Nano Technology Research Laboratory, Department of Chemistry, Shri Shivaji Science College, Amravati 444603, India Received 19 April 2007; received in revised form 24 August 2007; accepted 25 August 2007
Abstract This paper reports the preparation and gas-sensing characteristic of ZnO:TiO2-based hydrogen sulfide (H2S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO2 materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO2 shows excellent electrical resistance response toward H2S gas. The cross sensitivity was also checked for reducing gases like CH4, CO and H2 gas. The selectivity and sensitivity of ZnO:TiO2-based H2S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 1C. r 2007 Elsevier Ltd. All rights reserved. Keywords: ZnO; H2S sensor; Sensitivity; Selectivity
1. Introduction The use of sensors to monitor gas atmospheres represents a growing market resulting from strategies for intelligent process management, environmental protection and medicinal diagnostics as well as from the domestic, aerospace and automobile sectors. Hence, the development of fast responding, sensitive and especially highly selective gas sensor materials is of major interest. Hydrogen sulfide (H2S), a toxic gas, is often produced in coal, coal oil or natural gas industries [1]. Human exposures to H2S gas at levels higher than 250 ppm are likely to result in neurobehavioral toxicity and may even cause death [2]. Monitoring and controlling of H2S gas are crucial in laboratories and industrial areas. Present environmental and economic concerns mandate the removal of sulfur-containing compounds from the product stream of coal-derived fuel gas. In regard to removal of H2S, mixed metal-oxide sorbents [3] have been shown to be promising regenerable high temperature de-sulfurization Corresponding author.
E-mail address: [email protected] (G.N. Chaudhari). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.08.015
sorbents. Zinc titanate (Zn2TiO4), zinc ferrite (ZnFe2O4) and nickel ferrite (NiFe2O4) are the potential regenerable mixed metal-oxide sorbents which could be used to remove H2S from fuel gas at temperatures as high as 1035 K [4–6]. Ando et al. [7] reported H2S sensor with 0.5 wt% Au loaded WO3 at an operating temperature of 300 1C. Zinc oxide is an important semiconductor oxide for toxic and combustible gas-sensing applications. ZnO gas sensor elements have been fabricated in various forms, such as single crystals, wintered pellets, thick films, thin films and hetero junctions [8–13]. Semiconductor ZnO is sensitive to many sorts of gases, and has satisfactory stability, but it has some disadvantages, such as a high working temperature of 400–500 1C, poor gas selectivity and comparatively low gas sensitivity. To overcome these disadvantages of ZnO gas sensors, the researches on materials preparation, materials doping and sensors construct of ZnO have been done, and some achievements have been made [14–17]. It has been found that TiO2 as an additive could greatly improve the gas-sensing properties of sensors based on SnO2, WO3, ZnO, Ba2O3, etc. As much as we know that the effect of TiO2 on the electronic properties of ZnObased varistor had been broadly studied [18,19], the effect
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of adding TiO2 on the microstructure and sensitivity of ZnO have been investigated in detail in our previous work [20]. The search for an improved semiconductor-based sensor has expanded into new metal-oxide systems. Furthermore, it has also been reported that the gas sensitivity and selectivity of semiconductive complex oxides can be influenced by the dopants. Some workers discovered that complex oxides exhibit good sensitivity to reducing gases [21,22]. The system still attracts the attention of researchers because of its importance in practical applications. ZnO–TiO2 system materials were first used in the chemical industry as catalysts and color pigments [23]. Zinc titanates, particularly Zn2TiO4, are nowadays attractive as sorbents for removing sulfur from coal gasification product gases [24]. It has been demonstrated that zinc titanates are good dielectric materials for microwave devices [25]. Hence, they are nowadays widely applied as dielectric resonators and filters [26]. It is well known that physical and chemical of property materials depend on synthesis routes, governed by the synthesizing conditions. The aim of the present paper is to synthesize and characterize nanocrystalline ZnO:TiO2 and investigate the effect of CdO on gas-sensing properties of ZnO:TiO2-based H2S gas sensor. 2. Experimental details 2.1. Synthesis of ZnO:TiO2 nanomaterials Fig. 1 shows a flow chart of synthesis of CdO-doped ZnO:TiO2 nanomaterials. ZnO:TiO2 nanoparticles were prepared with analytical reagent grade (499.9%) zinc nitratehexahydrate (Zn(NO3)2 6H2O) and titanium nitrate Ti(NO3)4 compositional constituents in a ratio of 60 mol% ZnO and 40 mol% TiO2, which corresponded to the stoichiometric ratio 3:2 mixed with citric acid and stirred separately. The citrate obtained was mixed and subjected to magnetic stirring at 80 1C for 2 h. The solution was further heated in a pressure bomb at about 130 1C for 10 h. The dried powder was then calcined with different temperatures in the range of 300–700 1C for 6 h in order to improve crystallinity. The solution of Cd(NO3)2 was used as an additive in ZnO:TiO2 nanomaterials. The homogeneity of the compound was confirmed by X-ray diffractometer (XRD) (Model: SIEMEN D 5000) with copper target, Ka radiation (l ¼ 1.5406). The theoretical average particle sizes were estimated from the broadened XRD peak using Scherrer’s equation. The dried powder precursor was thermally analyzed to study decomposition and crystallization of the sample.
Fig. 1. Flow chart for synthesis of nanocrystalline ZnO:TiO2 by polymerized complex method.
two platinum wire electrodes had been installed at each end for electrical contacts. The sensor was sintered at 600 1C for 2 h to make it rigid and to impart ceramic properties. The PVA decomposes and the strength of the final element markedly increases. The Al2O3 tube was of about 10 mm in length, 2 mm in external diameter and 1.6 mm in internal diameter. A small Ni–Cr alloy coil was placed through the tube as a heater, which provided operating temperatures at 100–350 1C and a chromel–alumel thermocouple for temperature monitoring. Adjusting the heating power controlled the temperature. Different test gases were injected into the specimen chamber through an inlet port. The output voltage across the sensor element at a 10-V input S is defined as the ratio of the change in the electrical resistance in air and test gas, DR ¼ Ra-Rg, to its resistance in dry air Ra: S ¼ ðRa Rg Þ=Ra ¼ DR=Ra .
(1)
2.2. Sensor fabrication The ZnO:TiO2:CdO powders were mixed with PVA solution to form a paste, and then the paste was applied to about 2–3 mm thickness onto an Al2O3 tube on which
The homogeneity of mixture zinc oxide and titanium oxide, doped with CdO was confirmed by XRD measurement (XRD Model: Siemen d 5000) with a copper target (Ka radiation, l ¼ 0.15406 nm).
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3. Result and discussion 3.1. Characterization of ZnO–TiO2 Fig. 2(a) shows XRD spectra of ZnO–TiO2 powder calcined at 600 1C for 3 h. It has been seen that the peak observed corresponds to ZnO and trace of the new phase Zn2TiO4. Fig. 2(b) shows the XRD spectra of CdO-doped ZnO:TiO2 calcined at 600 1C. Broadening of diffraction peaks of the sample indicates a decrease in grain size which results in an increase in porosity, but no peak related to CdO was observed, as shown in Fig. 2(b). It is therefore apparent that CdO plays an important role in the formation of a thick film and promotes the formation of pore structure. Thus, ZnO nanoparticles can react with TiO2 according to the following reaction: 2ZnO þ TiO2 ! Zn2 TiO4 .
(2) 4+
The ionic radius matching between Ti ions and Zn2+ ions is considered, which possesses the ionic radius of 0.068 and 0.074 nm, respectively. TiO2 not only reacts with ZnO along grain boundaries and from the Zn2TiO4 phase, but also diffuses into the grain bulk, where it substitutes for Zn2+ ions, thereby increasing the activity of ZnO in virtue of distortion of ZnO lattice.
Fig. 3. Sensitivity vs. operating temperature for undoped ZnO for different gases.
3.2. Gas-sensing characteristics In order to calculate the sensitivity, electrical resistance of the element was measured in the presence and absence of H2S and interfering gases, like CO and CH4 taken in a concentration of 10,000 ppm in dry air. Fig. 3 shows the response as a function of operating temperatures for undoped ZnO nanopowder calcined at 600 1C for 6 h for different reducing gases. The sensor
Fig. 4. Sensitivity vs. operating temperature for ZnO with different wt% of TiO2 for H2S calcined at 600 1C.
Fig. 2. X-ray diffraction patterns of (a) ZnO 60% and TiO2 40% calcined at 600 1C and (b) ZnO 60%:TiO2 40%:CdO 10% calcined at 600 1C.
element shows the response 0.42 for an operating temperature 300 1C for H2S gas. Other reducing CH4, H2 and CO gases show comparatively low sensitivity. The further study aims to improve the sensitivity and impart selectivity by the addition of TiO2. Fig. 4 shows the gas-sensing characteristics of ZnOdoped 20–50 mol% TiO2 for H2S gas. This sensor element shows an improvement in H2S sensitivity from 0.42 to 0.62 with 20–40 mol%, respectively, at an operating temperature of 250 1C. A further increase in the concentration of TiO2 suppresses the sensitivity. The sensor response of 60 mol% ZnO:40 mol% TiO2 for CO, H2S, H2 and CH4 in synthetic dry air is based on
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calcination temperature and working temperature. In the range of working temperatures studied, from 100 to 350 1C, sensor response increased when temperature increased up to 250 1C for all gases. Fig. 5 shows cross sensitivity of 60% ZnO:40% TiO2. The 60% ZnO:40% TiO2 shows pronounced sensitivity 0.62 for H2S as compared to other gases, H2, CH4 and CO. Ferroni et al. [27] indicate that the increase in conduction due to substitution of Nb+5 on Ti+4 lattice shows good response for a CO sensor. It is well known that the sensitivity of the metal-oxide semiconductor sensor is mainly determined by the interaction between the target gas and the surface of the sensors. So it is certain that the greater the surface area of the materials, the stronger the interaction between adsorbed gases and sensor surface, i.e. higher the gas-sensing sensitivity. This can be seen clearly from the XRD spectra shown in Fig. 2(b). For the test gases, a major effect of the doping of different mol% of CdO on sensor elements was observed. Fig. 6 shows that an enhancement in sensitivity with an increase in the composition of CdO up to 10 mol% with further increase, reduces the sensitivity. The typical response of sensor toward the H2S is 0.81 at a low operating temperature of 225 1C with 10 mol% CdO. The resistance of ZnO:TiO2 decreases because electrons are donated to the conduction band of the sensitive films. Since the sensor is operating at higher operating temperatures, initially H2S gas molecule adsorbed at the surface of the sensitive films but at the same time desorption of gas molecules also takes place. Hence, resistance of the sensitive films increases during the desorption process. This may be the reason we observed a peak, and once equilibrium reached the response of the device become constant. As the operating temperature of the sensitive
Fig. 6. Sensitivity vs. operating temperature of 60 mol% ZnO:40 mol% TiO2 with different mol% of CdO calcined at 600 1C for H2S.
films increases, the adsorption of the gas is increased as compared to the desorption rate of the sensing gas molecule. At an optimum operating temperature (Tmax) the adsorption and desorption rates of the sensing gas molecule become equal, i.e. equilibrium is attained at this condition. With a further increase in the operating temperature of the device, the desorption rate of the gas molecules increases as compared to the adsorption rate, and diffusion of the gas decreases. Hence, the response of the device decreases at higher operating temperatures. Fig. 7 shows the cross sensitivity of ZnO:TiO2:10 mol% CdO for different reducing gases, H2, CO, LPG and H2S, as a function of the operating temperature. The sensor shows an enhancement in sensitivity and selectivity toward H2S as compared to other gases. Fig. 8 shows response characteristics of 60 mol% ZnO:40 mol% TiO2 and 60 mol% ZnO:40 mol% TiO2 doped with 10 mol% of CdO at an operating temperature of 250 1C. It has been observed that maximum sensitivity is achieved only within 1 min. The addition of CdO enhances not only the sensitivity but also rate of response. From room temperature to 250 1C, the conductance of the sensor increases with increasing temperature, that is, the intrinsic characteristic of semiconductor. However, the oxygen chemically adsorbed on the surface of ZnO:TiO2: CdO-based sensor undergoes the following reactions with rising temperature: 2 O2ðadÞ 2O 2ðadÞ 22OðadÞ 22OðadÞ .
Fig. 5. Sensitivity vs. operating temperature for 60 mol% ZnO:40 mol% TiO2 for different gases calcined at 600 1C.
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(3)
When temperature increases, the equilibrium shifts to the right, leading to an increase in conductance. When the temperature is higher than 250 1C, the effect of equilibrium (1) on conductivity is significant, and the conductivity of
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more easily than those of ZnO:TiO2. The sensitivity of the sensor is decided by the electron concentration resulting from the reaction because the sensitivity increases due to increase in the concentration of R or O2 ðadÞ . The results manifest that ZnO:TiO2:CdO can adsorb more R or O2 ðadÞ . Hence, the sensitivity increase of materials may result from the surface property change of the sensor. 3.3. Gas-sensing mechanism of ZnO:TiO2:10 mol% CdO The reducing gas R acting on the ZnO:TiO2:CdO surface can reacts as
Fig. 7. Sensitivity as a function of operating temperature for the 10 mol% CdO-doped ZnO:TiO2 calcined at 600 1C for different gases.
RðgasÞ 2RðadÞ;
(5)
O2ðgasÞ þ 2e 22O ðadÞ;
(6)
R þ O ðadÞ 2RO þ e :
(7)
In the absence of R, electrons are removed from ZnO:TiO2:CdO conduction band by the reduction of O2 resulting in the formation of O species and consequently their conductance decreases. When R is introduced, it reacts with O2 ðadÞ to form RO, and electrons enter the conduction band of the sensor leading to an increase in conductance. The response and recovery times of sensors made from the powders depend on the operating temperature, both being slow at low temperatures, and fast at higher temperatures. An operating temperature of about 225–250 1C was found to give the maximum sensitivity to H2S gas. The increase of the conductivity in the presence of H2S may be explained by the (7) reaction between H2S and oxygen ions on the surface of ZnO:TiO2:CdO grains, which results in the injection of electrons into the depletion layer. 4. Conclusions
Fig. 8. Gas response characteristics of (A) 60 mol% ZnO:40 mol% TiO2 and (B) 60 mol% ZnO:40 mol% TiO2-doped 10 mol% CdO for H2S gas.
sensor decreases. When the temperature is 250 1C, the conductance of the sensor increases because 2O2 ðadÞ desorbs: 2O2 ðadÞ 2O2ðgasÞ þ 4e .
(4)
The released electrons entered the conduction band of ZnO:TiO2:CdO to increase conductivity. Fig. 6 shows the sensitivity toward H2S which increases with an increase in CdO concentration up to 10 mol% at an operating temperature of 225 1C because O2 ðadÞ on the surface of ZnO:TiO2:CdO reacts with R (reducing gas)
In this paper, we reported the influence of 40-wt% TiO2 and 10-wt% CdO content on H2S sensitivity of films prepared by the polymerized complex method. From observations, the conclusions are as follows: (i) The homogeneity of the sensor confirmed by XRD measurement proposed the existence of zinc titanate only on calcination at 600 1C for 3 h. (ii) The sensitivity measurement shows improvement in sensitivity with 40 wt% TiO2 for H2S at a lower operating temperature of 250 1C. (iii) The sensitivity of ZnO:TiO2:10 mol% CdO thick films increases for an operating temperature up to 225 1C; a further increase in temperature decreases sensitivity. ZnO:TiO2:10 mol% CdO gives good response toward H2S at an operating temperature of 225 1C. (iv) The sensing mechanism of ZnO:TiO2:10 mol% CdO for reducing gases is also described.
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Acknowledgments The authors wish to thank Dr.V.G.Thakare, Principal, Shri Shivaji Science College Amravati, India, for his cooperation during this research work. This work was financially supported by Major Research Project [Project no. F.No. 32-239/2006(SR)] sanctioned by the University Grants Commission (UGC), New Delhi, India. References [1] Dorman DC, Brenneman KA, Struve MF, Miller KL, James RA, Marshall MW, et al. Teratology 2000;22:71. [2] Struve MF, Brisbois JN, Arden James R, Marshall MW, Dorman DC. Neurotoxicology 2001;22:375. [3] Siriwardane RV, Poston JA. Appl Surf Sci 1990;45:131. [4] Gopal Reddy CV, Manorama SV, Rao VJ. J Mater Sci Lett 2000; 19:778. [5] Liu Y-L, Wang H, Yang Y, Liu Z-M, Yang H-F, Shen G-L, et al. Sensors Actuators B 2004. [6] Jones TA, Mann B, Griffith JG. Sensors Actuators 1984;5:75. [7] Ando M, Juto S, Suzuki T, Tsuchida T, Nakayama C, Miura N, et al. Chem Lett 1991;335. [8] Saito S, Miyayama M, Koumoto K, Janagida H. J Am Ceram Soc 1985;68:40.
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[9] Pizzini S, Butta N, Norducci D, Palladino M. J Electrochem Soc 1989;136:1945. [10] Nanto H, Minami T, Takata S. J Appl Phys 1986;60:482. [11] Yamazaki T, Wada S, Noma T, Suzuki T. Sensors Actuators B 1993;13:594. [12] Basu S, Dutta A. Sensors Actuators B 1994;22:83. [13] Lou X. J Sens Trans Technol 1991;3:1. [14] Xu J, Zhu W, Chen Y. J Funct Mater 1993;24:30. [15] Xu J, Hu P, Qin J, Gu S. Gas sensing characteristics of Ru–ZnO. J Funct Mater 1998;29:281. [16] Xu J, Qin J, Zhang H, An C. Sens World 1997;38:7. [17] Zhang CC, Hu YX, Lu WZ, et al. J Eur Ceram Soc 2002;22:61. [18] Bernik S, Zupancic P, Drago K. J Eur Ceram Soc 1999;19:709. [19] Chaudhari GN, Bende AM, Bodade AB. Nano Trends 2006;1:43. [20] Huang FR, Howng W-Y. J Mater Res 1996;11:3077. [21] Tianshu Z, Ruifang Z, Yusheng S, Xinqin L. Sensors Actuators B 1996;32:185. [22] Li B, Yue Z, Li L, Zhou J, Gui Z. J Mater Sci 2002;13:415. [23] Yang J, Swisher HJ. Mat Charact 1996;37:153. [24] Kim TH, Kim HY, Byun DJ. J Korean Phys Soc 1998;32:159. [25] Wakino K, Nishikawa T, Ishikawa Y, Tamura H. Br Ceram Trans J 1990;89:39. [26] Xiangfeng C, Xingqin L, Guangyao M. Sensors Actuators B 2000; 65:64. [27] Ferroni M, Carotta MC, Guidi V, Martinelli G, Ronconi F, Richardb O, et al. Sensors Actuators B 2000;68:140.
Vacuum 82 (2008) 588–593 www.elsevier.com/locate/vacuum
Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor A.B. Bodade, A.M. Bende, G.N. Chaudhari Nano Technology Research Laboratory, Department of Chemistry, Shri Shivaji Science College, Amravati 444603, India Received 19 April 2007; received in revised form 24 August 2007; accepted 25 August 2007
Abstract This paper reports the preparation and gas-sensing characteristic of ZnO:TiO2-based hydrogen sulfide (H2S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO2 materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO2 shows excellent electrical resistance response toward H2S gas. The cross sensitivity was also checked for reducing gases like CH4, CO and H2 gas. The selectivity and sensitivity of ZnO:TiO2-based H2S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 1C. r 2007 Elsevier Ltd. All rights reserved. Keywords: ZnO; H2S sensor; Sensitivity; Selectivity
1. Introduction The use of sensors to monitor gas atmospheres represents a growing market resulting from strategies for intelligent process management, environmental protection and medicinal diagnostics as well as from the domestic, aerospace and automobile sectors. Hence, the development of fast responding, sensitive and especially highly selective gas sensor materials is of major interest. Hydrogen sulfide (H2S), a toxic gas, is often produced in coal, coal oil or natural gas industries [1]. Human exposures to H2S gas at levels higher than 250 ppm are likely to result in neurobehavioral toxicity and may even cause death [2]. Monitoring and controlling of H2S gas are crucial in laboratories and industrial areas. Present environmental and economic concerns mandate the removal of sulfur-containing compounds from the product stream of coal-derived fuel gas. In regard to removal of H2S, mixed metal-oxide sorbents [3] have been shown to be promising regenerable high temperature de-sulfurization Corresponding author.
E-mail address: [email protected] (G.N. Chaudhari). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.08.015
sorbents. Zinc titanate (Zn2TiO4), zinc ferrite (ZnFe2O4) and nickel ferrite (NiFe2O4) are the potential regenerable mixed metal-oxide sorbents which could be used to remove H2S from fuel gas at temperatures as high as 1035 K [4–6]. Ando et al. [7] reported H2S sensor with 0.5 wt% Au loaded WO3 at an operating temperature of 300 1C. Zinc oxide is an important semiconductor oxide for toxic and combustible gas-sensing applications. ZnO gas sensor elements have been fabricated in various forms, such as single crystals, wintered pellets, thick films, thin films and hetero junctions [8–13]. Semiconductor ZnO is sensitive to many sorts of gases, and has satisfactory stability, but it has some disadvantages, such as a high working temperature of 400–500 1C, poor gas selectivity and comparatively low gas sensitivity. To overcome these disadvantages of ZnO gas sensors, the researches on materials preparation, materials doping and sensors construct of ZnO have been done, and some achievements have been made [14–17]. It has been found that TiO2 as an additive could greatly improve the gas-sensing properties of sensors based on SnO2, WO3, ZnO, Ba2O3, etc. As much as we know that the effect of TiO2 on the electronic properties of ZnObased varistor had been broadly studied [18,19], the effect
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of adding TiO2 on the microstructure and sensitivity of ZnO have been investigated in detail in our previous work [20]. The search for an improved semiconductor-based sensor has expanded into new metal-oxide systems. Furthermore, it has also been reported that the gas sensitivity and selectivity of semiconductive complex oxides can be influenced by the dopants. Some workers discovered that complex oxides exhibit good sensitivity to reducing gases [21,22]. The system still attracts the attention of researchers because of its importance in practical applications. ZnO–TiO2 system materials were first used in the chemical industry as catalysts and color pigments [23]. Zinc titanates, particularly Zn2TiO4, are nowadays attractive as sorbents for removing sulfur from coal gasification product gases [24]. It has been demonstrated that zinc titanates are good dielectric materials for microwave devices [25]. Hence, they are nowadays widely applied as dielectric resonators and filters [26]. It is well known that physical and chemical of property materials depend on synthesis routes, governed by the synthesizing conditions. The aim of the present paper is to synthesize and characterize nanocrystalline ZnO:TiO2 and investigate the effect of CdO on gas-sensing properties of ZnO:TiO2-based H2S gas sensor. 2. Experimental details 2.1. Synthesis of ZnO:TiO2 nanomaterials Fig. 1 shows a flow chart of synthesis of CdO-doped ZnO:TiO2 nanomaterials. ZnO:TiO2 nanoparticles were prepared with analytical reagent grade (499.9%) zinc nitratehexahydrate (Zn(NO3)2 6H2O) and titanium nitrate Ti(NO3)4 compositional constituents in a ratio of 60 mol% ZnO and 40 mol% TiO2, which corresponded to the stoichiometric ratio 3:2 mixed with citric acid and stirred separately. The citrate obtained was mixed and subjected to magnetic stirring at 80 1C for 2 h. The solution was further heated in a pressure bomb at about 130 1C for 10 h. The dried powder was then calcined with different temperatures in the range of 300–700 1C for 6 h in order to improve crystallinity. The solution of Cd(NO3)2 was used as an additive in ZnO:TiO2 nanomaterials. The homogeneity of the compound was confirmed by X-ray diffractometer (XRD) (Model: SIEMEN D 5000) with copper target, Ka radiation (l ¼ 1.5406). The theoretical average particle sizes were estimated from the broadened XRD peak using Scherrer’s equation. The dried powder precursor was thermally analyzed to study decomposition and crystallization of the sample.
Fig. 1. Flow chart for synthesis of nanocrystalline ZnO:TiO2 by polymerized complex method.
two platinum wire electrodes had been installed at each end for electrical contacts. The sensor was sintered at 600 1C for 2 h to make it rigid and to impart ceramic properties. The PVA decomposes and the strength of the final element markedly increases. The Al2O3 tube was of about 10 mm in length, 2 mm in external diameter and 1.6 mm in internal diameter. A small Ni–Cr alloy coil was placed through the tube as a heater, which provided operating temperatures at 100–350 1C and a chromel–alumel thermocouple for temperature monitoring. Adjusting the heating power controlled the temperature. Different test gases were injected into the specimen chamber through an inlet port. The output voltage across the sensor element at a 10-V input S is defined as the ratio of the change in the electrical resistance in air and test gas, DR ¼ Ra-Rg, to its resistance in dry air Ra: S ¼ ðRa Rg Þ=Ra ¼ DR=Ra .
(1)
2.2. Sensor fabrication The ZnO:TiO2:CdO powders were mixed with PVA solution to form a paste, and then the paste was applied to about 2–3 mm thickness onto an Al2O3 tube on which
The homogeneity of mixture zinc oxide and titanium oxide, doped with CdO was confirmed by XRD measurement (XRD Model: Siemen d 5000) with a copper target (Ka radiation, l ¼ 0.15406 nm).
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3. Result and discussion 3.1. Characterization of ZnO–TiO2 Fig. 2(a) shows XRD spectra of ZnO–TiO2 powder calcined at 600 1C for 3 h. It has been seen that the peak observed corresponds to ZnO and trace of the new phase Zn2TiO4. Fig. 2(b) shows the XRD spectra of CdO-doped ZnO:TiO2 calcined at 600 1C. Broadening of diffraction peaks of the sample indicates a decrease in grain size which results in an increase in porosity, but no peak related to CdO was observed, as shown in Fig. 2(b). It is therefore apparent that CdO plays an important role in the formation of a thick film and promotes the formation of pore structure. Thus, ZnO nanoparticles can react with TiO2 according to the following reaction: 2ZnO þ TiO2 ! Zn2 TiO4 .
(2) 4+
The ionic radius matching between Ti ions and Zn2+ ions is considered, which possesses the ionic radius of 0.068 and 0.074 nm, respectively. TiO2 not only reacts with ZnO along grain boundaries and from the Zn2TiO4 phase, but also diffuses into the grain bulk, where it substitutes for Zn2+ ions, thereby increasing the activity of ZnO in virtue of distortion of ZnO lattice.
Fig. 3. Sensitivity vs. operating temperature for undoped ZnO for different gases.
3.2. Gas-sensing characteristics In order to calculate the sensitivity, electrical resistance of the element was measured in the presence and absence of H2S and interfering gases, like CO and CH4 taken in a concentration of 10,000 ppm in dry air. Fig. 3 shows the response as a function of operating temperatures for undoped ZnO nanopowder calcined at 600 1C for 6 h for different reducing gases. The sensor
Fig. 4. Sensitivity vs. operating temperature for ZnO with different wt% of TiO2 for H2S calcined at 600 1C.
Fig. 2. X-ray diffraction patterns of (a) ZnO 60% and TiO2 40% calcined at 600 1C and (b) ZnO 60%:TiO2 40%:CdO 10% calcined at 600 1C.
element shows the response 0.42 for an operating temperature 300 1C for H2S gas. Other reducing CH4, H2 and CO gases show comparatively low sensitivity. The further study aims to improve the sensitivity and impart selectivity by the addition of TiO2. Fig. 4 shows the gas-sensing characteristics of ZnOdoped 20–50 mol% TiO2 for H2S gas. This sensor element shows an improvement in H2S sensitivity from 0.42 to 0.62 with 20–40 mol%, respectively, at an operating temperature of 250 1C. A further increase in the concentration of TiO2 suppresses the sensitivity. The sensor response of 60 mol% ZnO:40 mol% TiO2 for CO, H2S, H2 and CH4 in synthetic dry air is based on
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calcination temperature and working temperature. In the range of working temperatures studied, from 100 to 350 1C, sensor response increased when temperature increased up to 250 1C for all gases. Fig. 5 shows cross sensitivity of 60% ZnO:40% TiO2. The 60% ZnO:40% TiO2 shows pronounced sensitivity 0.62 for H2S as compared to other gases, H2, CH4 and CO. Ferroni et al. [27] indicate that the increase in conduction due to substitution of Nb+5 on Ti+4 lattice shows good response for a CO sensor. It is well known that the sensitivity of the metal-oxide semiconductor sensor is mainly determined by the interaction between the target gas and the surface of the sensors. So it is certain that the greater the surface area of the materials, the stronger the interaction between adsorbed gases and sensor surface, i.e. higher the gas-sensing sensitivity. This can be seen clearly from the XRD spectra shown in Fig. 2(b). For the test gases, a major effect of the doping of different mol% of CdO on sensor elements was observed. Fig. 6 shows that an enhancement in sensitivity with an increase in the composition of CdO up to 10 mol% with further increase, reduces the sensitivity. The typical response of sensor toward the H2S is 0.81 at a low operating temperature of 225 1C with 10 mol% CdO. The resistance of ZnO:TiO2 decreases because electrons are donated to the conduction band of the sensitive films. Since the sensor is operating at higher operating temperatures, initially H2S gas molecule adsorbed at the surface of the sensitive films but at the same time desorption of gas molecules also takes place. Hence, resistance of the sensitive films increases during the desorption process. This may be the reason we observed a peak, and once equilibrium reached the response of the device become constant. As the operating temperature of the sensitive
Fig. 6. Sensitivity vs. operating temperature of 60 mol% ZnO:40 mol% TiO2 with different mol% of CdO calcined at 600 1C for H2S.
films increases, the adsorption of the gas is increased as compared to the desorption rate of the sensing gas molecule. At an optimum operating temperature (Tmax) the adsorption and desorption rates of the sensing gas molecule become equal, i.e. equilibrium is attained at this condition. With a further increase in the operating temperature of the device, the desorption rate of the gas molecules increases as compared to the adsorption rate, and diffusion of the gas decreases. Hence, the response of the device decreases at higher operating temperatures. Fig. 7 shows the cross sensitivity of ZnO:TiO2:10 mol% CdO for different reducing gases, H2, CO, LPG and H2S, as a function of the operating temperature. The sensor shows an enhancement in sensitivity and selectivity toward H2S as compared to other gases. Fig. 8 shows response characteristics of 60 mol% ZnO:40 mol% TiO2 and 60 mol% ZnO:40 mol% TiO2 doped with 10 mol% of CdO at an operating temperature of 250 1C. It has been observed that maximum sensitivity is achieved only within 1 min. The addition of CdO enhances not only the sensitivity but also rate of response. From room temperature to 250 1C, the conductance of the sensor increases with increasing temperature, that is, the intrinsic characteristic of semiconductor. However, the oxygen chemically adsorbed on the surface of ZnO:TiO2: CdO-based sensor undergoes the following reactions with rising temperature: 2 O2ðadÞ 2O 2ðadÞ 22OðadÞ 22OðadÞ .
Fig. 5. Sensitivity vs. operating temperature for 60 mol% ZnO:40 mol% TiO2 for different gases calcined at 600 1C.
591
(3)
When temperature increases, the equilibrium shifts to the right, leading to an increase in conductance. When the temperature is higher than 250 1C, the effect of equilibrium (1) on conductivity is significant, and the conductivity of
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more easily than those of ZnO:TiO2. The sensitivity of the sensor is decided by the electron concentration resulting from the reaction because the sensitivity increases due to increase in the concentration of R or O2 ðadÞ . The results manifest that ZnO:TiO2:CdO can adsorb more R or O2 ðadÞ . Hence, the sensitivity increase of materials may result from the surface property change of the sensor. 3.3. Gas-sensing mechanism of ZnO:TiO2:10 mol% CdO The reducing gas R acting on the ZnO:TiO2:CdO surface can reacts as
Fig. 7. Sensitivity as a function of operating temperature for the 10 mol% CdO-doped ZnO:TiO2 calcined at 600 1C for different gases.
RðgasÞ 2RðadÞ;
(5)
O2ðgasÞ þ 2e 22O ðadÞ;
(6)
R þ O ðadÞ 2RO þ e :
(7)
In the absence of R, electrons are removed from ZnO:TiO2:CdO conduction band by the reduction of O2 resulting in the formation of O species and consequently their conductance decreases. When R is introduced, it reacts with O2 ðadÞ to form RO, and electrons enter the conduction band of the sensor leading to an increase in conductance. The response and recovery times of sensors made from the powders depend on the operating temperature, both being slow at low temperatures, and fast at higher temperatures. An operating temperature of about 225–250 1C was found to give the maximum sensitivity to H2S gas. The increase of the conductivity in the presence of H2S may be explained by the (7) reaction between H2S and oxygen ions on the surface of ZnO:TiO2:CdO grains, which results in the injection of electrons into the depletion layer. 4. Conclusions
Fig. 8. Gas response characteristics of (A) 60 mol% ZnO:40 mol% TiO2 and (B) 60 mol% ZnO:40 mol% TiO2-doped 10 mol% CdO for H2S gas.
sensor decreases. When the temperature is 250 1C, the conductance of the sensor increases because 2O2 ðadÞ desorbs: 2O2 ðadÞ 2O2ðgasÞ þ 4e .
(4)
The released electrons entered the conduction band of ZnO:TiO2:CdO to increase conductivity. Fig. 6 shows the sensitivity toward H2S which increases with an increase in CdO concentration up to 10 mol% at an operating temperature of 225 1C because O2 ðadÞ on the surface of ZnO:TiO2:CdO reacts with R (reducing gas)
In this paper, we reported the influence of 40-wt% TiO2 and 10-wt% CdO content on H2S sensitivity of films prepared by the polymerized complex method. From observations, the conclusions are as follows: (i) The homogeneity of the sensor confirmed by XRD measurement proposed the existence of zinc titanate only on calcination at 600 1C for 3 h. (ii) The sensitivity measurement shows improvement in sensitivity with 40 wt% TiO2 for H2S at a lower operating temperature of 250 1C. (iii) The sensitivity of ZnO:TiO2:10 mol% CdO thick films increases for an operating temperature up to 225 1C; a further increase in temperature decreases sensitivity. ZnO:TiO2:10 mol% CdO gives good response toward H2S at an operating temperature of 225 1C. (iv) The sensing mechanism of ZnO:TiO2:10 mol% CdO for reducing gases is also described.
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Acknowledgments The authors wish to thank Dr.V.G.Thakare, Principal, Shri Shivaji Science College Amravati, India, for his cooperation during this research work. This work was financially supported by Major Research Project [Project no. F.No. 32-239/2006(SR)] sanctioned by the University Grants Commission (UGC), New Delhi, India. References [1] Dorman DC, Brenneman KA, Struve MF, Miller KL, James RA, Marshall MW, et al. Teratology 2000;22:71. [2] Struve MF, Brisbois JN, Arden James R, Marshall MW, Dorman DC. Neurotoxicology 2001;22:375. [3] Siriwardane RV, Poston JA. Appl Surf Sci 1990;45:131. [4] Gopal Reddy CV, Manorama SV, Rao VJ. J Mater Sci Lett 2000; 19:778. [5] Liu Y-L, Wang H, Yang Y, Liu Z-M, Yang H-F, Shen G-L, et al. Sensors Actuators B 2004. [6] Jones TA, Mann B, Griffith JG. Sensors Actuators 1984;5:75. [7] Ando M, Juto S, Suzuki T, Tsuchida T, Nakayama C, Miura N, et al. Chem Lett 1991;335. [8] Saito S, Miyayama M, Koumoto K, Janagida H. J Am Ceram Soc 1985;68:40.
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