Preparation and gas-sensing properties of Ce-doped ...

Materials Science and Engineering B 137 (2007) 53–58

Preparation and gas-sensing properties of Ce-doped ZnO thin-film sensors by dip-coating Chunqiao Ge a , Changsheng Xie a,b,∗ , Shuizhou Cai b a

State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China b Nanomaterial and Smart Sensor Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 5 July 2006; received in revised form 10 October 2006; accepted 12 October 2006

Abstract CeO2 -doped ZnO thin-film gas sensors with different Ce/Zn ratios have been fabricated by dip-coating method, starting from zinc acetate dihydrate, cerium nitrate hexahydrate (Ce(NO3 )3 ·6H2 O) and anhydrous ethanol. Each layer was fired at 180 ◦ C in a conventional oven for 30 min and the final coatings were sintered at 500 ◦ C in a muffle furnace for 60 min. The microstructure and morphology of the films were characterized by XRD and FESEM, respectively. The resistance and sensitivities to volatile organic compounds were investigated on the static testing chamber. The X-ray diffraction (XRD) analysis of the films reveals the appearance of CeO2 , tetravalent cerium dioxide whose valency is different from cerium nitrate hexahydrate. The results also show that as-prepared thin films with thickness of about 5 ␮m are polycrystalline with the structure of hexagonal wurtzite type. They consist of almost spherical particles with size ranging from 40 to 65 nm. Pure ZnO and Ce-doped ZnO thin-film sensors were prepared and tested for specific sensitivity to alcohol, acetone and benzene. It is observed that 1 at.% Ce–ZnO and 5 at.% Ce–ZnO are more sensitive to volatile organic compounds (VOCs), compared with other films with the different dopant concentration. The sensitivity of 5 at.% Ce–ZnO thin-film sensors to 100 ppm alcohol reaches 80 or so at 320 ◦ C. 5 at.% Ce–ZnO thin-film sensors show good selectivity to alcohol, and thus can serve as alcohol-sensing sensors. A new physical model of the CeO2 dopant influence on the gas-sensing properties of ZnO thin films is proposed. The addition of Ce to ZnO modified the particles size distribution, electrical conductivity, the catalytic activity and thus affected gas-sensing property to some extent. © 2006 Elsevier B.V. All rights reserved. Keywords: CeO2 –ZnO; Volatile organic compounds (VOCs); Gas sensors; Sol–gel; Thin film

1. Introduction The increasing demand of fast and low cost air quality analysis techniques for domestic and industrial environmental monitoring and automotive applications is tailoring the research toward new materials and techniques to solve the problems related to the commercial sensors. Compared with the commercial sensors, metal-oxide semiconductor gas sensors, such as ZnO, SnO2 , Fe2 O3 , etc., have attracted great attention due to their advantageous features, such as high sensitivity under ambient conditions, low power consumption and simplicity in fabrication. Among them, ZnO has been shown to be useful materials for monitoring various pollutant gases like H2 S, benzene, NOx ,

∗

Corresponding author. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (C. Xie).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.10.006

NH3 , etc. [1,2], and explosive gases like CH4 , H2 , CO, etc. [3,4]. However, the sensitivities of ZnO-based materials, especially to some chemically stable gases, such as benzene, CH4 , H2 , etc., are still comparatively low. Therefore, there is a great need for making every effort to improve their gas-sensing property, from the redesign of the materials system and the mend of technology. In addition, the gas-sensing properties are related to some critical factors, such as the surface state, morphology, surfaceto-volume ratio and active center of the material. Therefore, it is necessary that morphology, surface-to-volume ratio and active center of the material should be further optimized to improve gas-sensing properties. At present, two feasible methods have been adopted in order to satisfy the above needs, namely, doping and modifying the preparation way of the material. Doping is an important and effective way to improve the properties of semiconductors. There is no exception to metal-oxide semiconductor gas sensors. In the previous studies, noble metal

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C. Ge et al. / Materials Science and Engineering B 137 (2007) 53–58

additives (Pt, Pd) [5], transition-metal oxides and main-group metal oxides such as TiO2 [6], CuO [7], In2 O3 , Bi2 O3 [8], etc., were also incorporated to enhance the sensitivity and selectivity of the sensors. These metal oxides used as dopants could enhance gas-sensing properties by changing energy-band structure, mending the morphology and surface-to-volume ratio, and creating more active center at the grain boundaries. Compared with the above transitional metal oxides and main-group metal oxides, the addition of rare earth oxides (REO) to ZnO was rarely researched in the field of gas sensors except in the literature [9]. Rare earth oxides are very important in many advanced technologies especially in catalyst field. REO used as catalysts play an important role in taking active effect on cracking (or dehydrogenation) and ring-opening of hydrocarbon [10–12]. Therefore, the gas-sensing properties, especially to alkane and aromatic compound, of ZnO-based gas sensors could be improved to some extent if REO can be incorporated into ZnO nanoparticles. In addition, in the previous studies, metal-oxide semiconductor gas sensors were fabricated by two steps [13,14]: the first step, metal-oxide powders were produced and then sintered; the second step, the sintered powders were coated on ceramic tubes with two platinum electrode wires by paintbrush and then sintered again. The two-fold calcinations could easily result in some faults including agglomeration and growth of nanoparticles, and then worsen gas-sensing properties. Moreover, the coating method by paintbrush could affect the uniformity of films’ thickness. The dip-coating method was widely adopted to fabricate the uniform thin films, for example, transparent conductive oxide thin films [15]. If adopted the dip-coating method, ZnO-based precursors were coated directly on ceramic tubes and sintered once, which could effectively avoid the above faults to a great extent and improve gas-sensing properties. In the present study we have investigated, for the first time, the effect of cerium oxide as an additive in ZnO thin-film gas sensors by dip-coating method. 2. Experimental details In this experiment, zinc acetate dihydrate (Zn(CH3 COO)2 · 2H2 O) and cerium nitrate hexahydrate (Ce(NO3 )3 ·6H2 O) are used as a starting material and dopant sources, respectively, and anhydrous ethanol and monoethanolamine (MEA) are used as a solvent and stabilizer, respectively. All the reagents were analytical grade and used without further purification in our experiments. In a typical procedure of preparing Ce-doped ZnO thin films, zinc acetate dihydrate is first dissolved in a mixture which is composed of anhydrous ethanol and MEA at 60 ◦ C. The molar ratio of MEA to zinc acetate dihydrate is maintained at 1.0 and the concentration of zinc acetate is 0.025 mol/L. The mixed solution is stirred at 60 ◦ C for 1 h and continued to be stirred for 1 h after the addition of ethanolic solution of cerium nitrate hexahydrate with different Ce/Zn ratios. After the stirring, the solutions becomes clear and homogeneous which is used as the coating solution after cooling to room temperature and is allowed to age for 24 h before the deposition. The films are obtained by a dip-coating procedure. Before the deposition, the alumina ceramic tubes on which the two platinum

electrode wires had been installed at each end were ultrasonically cleaned, first in acetone and then in alcohol, and dried at room temperature in a drying ware. The deposition is carried out at room temperature in air with a controlled withdrawal speed of 1 mm/s. The immersion time of the ceramic tubes in the solution is 30 s. After the coating, the gel films are dried at 180 ◦ C immediately for 30 min in an oven. The final coatings are obtained by means of four-time repetitive dip-coatings. After the coatings on the ceramic tubes reach the thickness required, the thin films were dried in air for 24 h and then calcined at 500 ◦ C for 1 h in a muffle furnace. A small Ni–Cr alloy coil was fixed through the tube as a microheater. When sensors tested, a given amount of volatile liquid were injected into the tested chamber and mixed by a fan. The resistance of the load resistor was alternative and the heating voltage can be adjusted within a wide range. The sensitivity for gases is defined as Ra /Rg , where Rg and Ra are the resistance of the sensors in the tested gases and in the air, respectively. The structure and crystal state of the thin films were characterized on Philips X’pert X-ray diffractormeter with Cu k␣1 radiation in the 2θ range from 10 to 90 ◦ C. The morphology and size of the as-prepared thin films were analyzed by a Sirion II type field emission scanning electron microscope (FESEM). The resistance and sensitivities of the films to VOCs were investigated on the static testing chamber in our laboratory. 3. Results and discussion 3.1. Characterization of ZnO thin films Fig. 1 shows the typical XRD patterns of pure ZnO and Cedoped ZnO thin films with different Ce/Zn ratios. It is evident that all the diffraction peaks of undoped ZnO and Ce-doped ZnO, without characteristic peaks for the other impurities, could be indexed to the hexagonal wurtzite structure ZnO according to the standard JCPDS (No. 79-2205) card, and the cubic structure CeO2 from the standard JCPDS (No. 75-0390) card. At the same time, it is observed that no composite metal oxide is detected in the films. In the view of the valency of Ce3+ in the cerium nitrate hexahydrate, there is a shift of valency in the course of the calcinations of the films, which is in agreement with the literature

Fig. 1. XRD patterns of the ZnO-based powders with 0 at.% Ce (a), 1 at.% Ce (b), 5 at.% Ce (c) and 10 at.% Ce (d).


[16]. The average grain sizes of ZnO-based films with different Ce/Zn ratios are calculated by Deby–Scherrer equation. From this equation, the average size of the undoped and Ce-doped ZnO films was estimated to be about 35–60 nm. The SEM photographs of pure ZnO and Ce-doped ZnO thin films (Fig. 2) indicate that the films are composed of almost uniform spherical grains. The average particles size calculated in proportion to the photos is about 40–65 nm, which is in consistent with the results obtained from Deby–Scherrer equation. Moreover, the grain size tends to decrease with the increase of the concentration of the additives. Seen from the cross-section

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SEM images, the surface of the films, with the thickness of about 5 ␮m, is rough. 3.2. Resistance–temperature characteristic of the films An electrical characterization was also carried out in order to evaluate the potential use of these mixed oxides as sensing materials. Fig. 3 depicts the resistance–temperature (R–T) behavior of the pure ZnO and the Ce-doped ZnO films sensors in air. It is obvious that the resistance of ZnO thin films is distinctly decreased after the addition of the dopant. Taking the rele-

Fig. 2. Scanning electron micrographs of the films. Cross-section SEM image of pure ZnO film (a), and SEM micrographs of surface of pure ZnO (b), 1 at.% CeO2 –ZnO (c), 5 at.% CeO2 –ZnO (d) and 10 at.% CeO2 –ZnO (e).

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of the carriers. Hereby, the R–T curve of 10 at.% CeO2 –ZnO is higher than that of 1 at.% CeO2 –ZnO and 5 at.% CeO2 –ZnO from room temperature to 500 ◦ C. 3.3. Gas-sensing properties of the films

Fig. 3. The plot of resistance vs. temperature for ZnO thin films with different Ce/Zn ratios.

˚ Ce4+ = 0.96 A) ˚ into account, vant ionic radius (Zn2+ = 0.74 A, 4+ it seems reasonable for Ce to replace Zn2+ partially in the ZnO crystallites. Ce4+ may substitute the Zn2+ ions according to the following defect reaction: 1 ZnO CeO2 −→CeZn •• + Ox0 + O2 + 2e 2 where CeZn •• is the substituent defect of substituting Ce4+ for Zn2+ in the ZnO lattice, and Ox0 is the interstitial defect of oxygen ions. After the substitution, the increase of the carrier concentration results in the decrease of resistance of ZnO thin films. At the same time, the intrinsic conductance of the metal oxide semiconductor increases with the increase of the operating temperature, owing to more electrons entering into conductance band at the higher operating temperature. There is no exception to ZnO thin films. But there is a slight increase of the resistance for Cedoped ZnO thin films at 320–370 ◦ C, which is likely to be in relation to absorbed oxygen on the surface of the Ce-doped ZnO grains. It is absorbed oxygen turning into oxygen ions (O2− and O− ) by accepting free electrons in the conductance band that leads to the slight increase of the resistance. At lower temperature, the adsorption type of oxygen molecules belongs to physisorption and the adsorption attraction (i.e., Van der Waals attraction) is too weak. Therefore, the increase resulting from absorbed oxygen could not affect the dropping tendency of the resistance. The adsorption type is, however, turned into chemisorption (i.e., chemical-bond attraction) and the concentration of adsorbed oxygen molecules on surface rise gradually with the increase of the operating temperature. Therefore, more free electrons are extracted by absorbed oxygen, which induces the slight increase of the resistance at about 350 ◦ C. But the reaction (O2 + 2e → 2O− , O− + e → O2− ) is exothermic [17]. Once the operating temperature surpasses 350 ◦ C, free electrons are released, which results in decrease of the resistance again. In a addition, the R–T curve indicates that the resistance of pure ZnO is on the decline continuously with temperature, without a slight increase shown in doped ZnO, which may result from the lack of more active adsorbed-center produced by the dopant. Thirdly, redundant CeO2 phase getting together at each grain boundaries affect the mobility of the carriers across the ZnO grain boundaries, and is likely to be the recombination centers

In the case of n-type metal-oxide semiconductor gas sensors surface-controlled, it is generally thought that absorbed oxygen, which traps electrons from the conductance band, plays the most important role in gas sensitivity. For ZnO-based gas sensors, the changes of resistance are mainly induced by the adsorption and desorption of oxygen molecules from the surfaces of the grains. In an air environment, oxygen molecules are generally chemisorbed onto the surface of ZnO as the oxygen ions (O2 − , O− , and O2− ). After reducing gases (RG) are inleted, they readily react with the oxygen ions and liberate electrons to the conduction band, accompanied by an increase in conductivity of the film, when they are in contact with the surface of ZnO film. So the conductivity changes, i.e., sensitivity, of ZnO thin film depends on the concentration change of adsorbed oxygen ions on the surface. Fig. 4 shows the sensitivity changes of films with the operating temperature in the environments of alcohol, acetone and benzene vapors (the concentration is 100 ppm). It is obviously found that the sensitivities to 100-ppm vapor increase with operating temperature, reach its maximum at 370 ◦ C, and begin to reduce above the point. Compared with the results by twostep method in our lab [14], the sensitivity, especially to alcohol, is improved to some extent. The changes in sensitivity with operating temperature can be attributed to the fact that the adsorption type of oxygen molecules is chemisorption at higher temperature, not physisorption at lower temperature. The stronger adsorption attraction of the chemisorption could result in the higher concentration of adsorbed oxygen and the bigger change of the resistance. Hence, the sensitivities are enhanced with the increasing temperature. But the reduction in sensitivity above 370 ◦ C was due to the fact that the adsorption reaction is proverbially exothermic. At higher temperature, the reaction (O2 + 2e → 2O− , O− + e → O2− ) will proceed to left, which leads to the decrease of the trapped electrons from the conductance band. In addition, seen from the chart, sensitivity of the samples varies with the dopant concentration. The sensitivities of 1 at.% CeO2 –ZnO and 5 at.% CeO2 –ZnO films are higher than that of pure ZnO and 10 at.% CeO2 –ZnO, which can be ascribed to the more active adsorption center produced by the dopant. In addition, CeO2 phases congregating on the surface of the ZnO particles have dual effect on gas-sensing properties, namely, positive effect and negative effect. On one hand, they could benefit to dehydrogenation and ring-opening of hydrocarbon in the form of catalysts; on the other hand, they could reduce the available adsorption sites and worsen the gas-sensing properties. Fig. 5 depicts the model of ZnO inter-particles with undoping, appropriate doping and superabundant doping, respectively. In the Fig. 5(b), the appropriate CeO2 phases congregate on the surface of the ZnO particles and the positive effect dominates again the negative effect, whereas in the Fig. 5(c) the negative effect dominates again the positive effect. Therefore, appropriate doping


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Fig. 6. Concentration dependence of the sensitivity to alcohol at 370 ◦ C.

Fig. 4. Sensitivity vs. operating temperature to (a) 100 ppm alcohol, (b) 100 ppm acetone and (c) 100 ppm benzene.

benefits to the improvement of gas-sensing properties, but superabundant doping worsens the gas-sensing properties. Moreover, it is obvious that the sensitivity of the as-developed sensors to alcohol and acetone is higher than to benzene at the same concen-

tration and temperature. The difference of the sensitivity among the above VOCs may be relative with the difference of chemical bond. Fig. 6 shows the concentration dependence of sensitivity of the films. Obviously, the sensitivity to alcohol increases gradually with the increasing concentration, and the curves exhibit a tendency of quasi-linearity. But once the concentration surpasses 200 ppm, the sensitivity increases slowly, even inconspicuously. The result indicates that the concentration of the adsorbed oxygen is saturated on the limited adsorption sites. It implies that CeO2 -doped ZnO films lend themselves to air quality detector that requires the sensors to detect low concentration vapor with higher sensitivity. The selectivity, except the sensitivity, is another important property of gas sensors. The selectivity of a gas sensor to a parti cular gas may be quantitatively described as SELi = (Si / Si ) × 100%, where Si is the sensitivity of a sensor to gas i and Si represents the sum of sensitivities of a sensor to different target gases at the same concentrations [18]. Based on this definition, the selectivity to alcohol, acetone or benzene may then be described in this study as SELi = [Si /(Salcohol + Sacetone + Sbenzene )] × 100%, where i represents one of the gases as alcohol, acetone or benzene, respectively. Fig. 7 shows the selectivity changes of 5 at.% CeO2 –ZnO sensors with temperatures to alcohol, acetone and benzene. The results show that the highest alcohol selectivity (about 80%) is obtained at 320 ◦ C or so. Compared with the selectivity to alcohol, the selectivity to benzene is much lower (about 10%), indicating the sensors are not good gas sensors to selectively detect benzene in gases containing alcohol. It is not surprising that it is difficult to acti-

Fig. 5. Model of ZnO inter-particles with (a) undoping, (b) appropriate doping and (c) superabundant doping.

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Fig. 7. Selectivity vs. operating temperature to alcohol, acetone and benzene.

ZnO-based thin films with thickness of about 5 ␮m consist of almost spherical particles with size ranging from 40 to 65 nm. The gas-sensing properties of ZnO based thin films depend on the dopant concentration. CeO2 phase affects the gas sensitivity of the films by modifying their microstructure and conductivity. Among the CeO2 -doped ZnO thin-film sensors, both 1 at.% CeO2 –ZnO and 5 at.% CeO2 –ZnO sensors are, comparatively, provided with gas-sensing property to 100 ppm acetone and alcohol. In addition, the vapor concentration also affects their gas sensitivities, but the sensitivities do not rise linearly with the increase of the vapor concentration. Besides, the vapor species affect the response–recovery time. The response time for acetone and alcohol is about 10 s, but the time for benzene is longer comparatively. Acknowledgements The project supported by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology). The authors also acknowledge the financial support by Nature Science Foundation of China (Nos. 50671039 and 50271029). References

Fig. 8. Response–recovery curve of 5 at.% CeO2 –ZnO sensor at 320 ◦ C.

vate benzene at low temperatures. Since benzene is a very stable compound, activation of benzene requires catalysts and very high temperatures to break the C H bonds. Response–recovery characteristics are also the main properties of gas sensors. The response time is defined as the time for reaching 90% of the full response change of sensor when testing gas inlets. The typical response–recovery curves of 5 at.% CeO2 –ZnO sensors for acetone and alcohol are shown in Fig. 8. Seen from the figure, the response time is 10 s or so for 100 ppm acetone and alcohol and the recovery is done within 5 s. But for benzene, however, the response time is 15 s or so, owing to its longer vaporization time, and the recovery time is almost the same with acetone and alcohol. 4. Conclusion In conclusion, ZnO-based thin-film gas sensors with different Ce/Zn ratios have been fabricated by dip-coating and their gas-sensing properties to acetone, benzene and alcohol have been tested under the different operating condition. As-prepared

[1] M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Sens. Actuators B 115 (2006) 128–133. [2] S. Sen, V. Bhandarkar, K.P. Muthe, M. Roy, S.K. Deshpande, R.C. Aiyer, S.K. Gupta, J.V. Yakhmi, V.C. Sahni, Sens. Actuators B 115 (2006) 270–275. [3] J.H. Yu, G.M. Choi, Sens. Actuators B 72 (2001) 141–148. [4] N.J. Dayan, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, Thin Solid Films 325 (1998) 254–258. [5] G.S. Trivikrama Rao, D. Tarakarama Rao, Sens. Actuators B 55 (1999) 166–169. [6] B.L. Zhu, C.S. Xie, W.Y. Wang, K.J. Huang, J.H. Hu, Mater. Lett. 58 (2004) 624–629. [7] J.H. Yu, G.M. Choi, Sens. Actuators B 75 (2001) 56–61. [8] B.L. Zhu, D.W. Zeng, J. Wu, C.S. Xie, J. Mater. Sci. Mater. Electron. 14 (2003) 521–526. [9] J. Li, S.J. Bai, T.L. Ga, J. Transducer Technol. 23 (2004) 9–14 (in China). [10] L. Ge, Q.H. Huang, Y.F. Zhang, Z.Q. Shen, Eur. Polym. J. 36 (2000) 2699–2705. [11] B.A. Watson, M.T. Klein, R.H. Harding, Appl. Catal. A Gen. 160 (1997) 13–39. [12] X.L. Zhang, A.M. Zhu, X.H. Li, W.M. Gong, Catal. Today 89 (2004) 97–102. [13] X. Niu, W. Du, W. Du, Sens. Actuators B 99 (2004) 399–404. [14] B.L. Zhu, C.S. Xie, A.H. Wang, D.W. Zeng, M.L. Hu, W.Y. Wang, Mater. Res. Bull. 39 (2004) 409–415. [15] J.-H. Lee, B.-O. Park, Thin Solid Films 426 (2003) 94–99. [16] G. Neri, A. Bonavita, G. Rizzo, S. Galvagno, S. Capone, P. Siciliano, Sens. Actuators B 111–112 (2005) 78–83. [17] J.F. Chang, H.H. Kuo, I.C. Leu, M.H. Hon, Sens. Actuators B 84 (2002) 258–264. [18] G. Li, S. Kawi, Sens. Actuators B 59 (1999) 1–8.