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Effect of humidity on the gas sensing property of the tetrapod-shaped ZnO nanopowder sensor
Materials Science and Engineering B 149 (2008) 12–17
Effect of humidity on the gas sensing property of the tetrapod-shaped ZnO nanopowder sensor Zikui Bai a , Changsheng Xie a,b,∗ , Mulin Hu b , Shunping Zhang a , Dawen Zeng 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 11 July 2007; received in revised form 9 October 2007; accepted 17 November 2007
Abstract The testing chamber humidity and the storage circumstance humidity effects on the tetrapod-shaped ZnO nanopowder (named as T-ZnO) thick film sensors were investigated by measuring the resistance and sensitivity. The resistance increases gradually with increasing relative humidity (RH) in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. The sensitivity to ethanol 100 ppm in testing chambers with different humidity is in the order of 50% RH > of 75% RH > of 32% RH > of 96% RH. The sensitivity change in the storage circumstance with different RH is similar to the change in testing chamber with different RH. The stability of T-ZnO sensor is influenced evidently by the storage circumstance humidity. The water vapour in the tetrapod-shaped ZnO nanopowders was investigated by thermogravimetric analysis (TGA). The testing gas and the reactant adsorbed in the sensitive film were characterized by infrared spectrum (IR). The explanation of the observed effects was given and the mechanism of interaction of the ZnO sensing layer with H2 O was proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity; Sensitivity; Stability; Tetrapod-shaped ZnO nanopowders
1. Introduction Zinc oxide (ZnO) is a wide-band gap semiconductor that is desirable for many applications, such as piezoelectric transducers, varistors, gas sensors and transparent conducting thin films [1–3]. By reducing the size of ZnO crystals to nanoscale dimensions or controlling the morphology of ZnO crystals to tetrapod-shaped nanowhiskers [4], nanowires and nanorods [5], researchers can tailor the properties via quantum confinement and surface effect. Tetrapod-shaped ZnO nanowhiskers (named as T-ZnO) can be prepared by the method of vapourphase oxidation from metallic zinc as raw materials. The T-ZnO-based sensors have been received great attention due to their excellent sensing property to volatile organic compounds (VOCs)—benzene, toluene, xylene, ethanol and acetone [6,7]. ∗
Corresponding author at: Nanomaterial and Smart Sensor Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (C. Xie). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.11.020
The high sensitivity is attributed to the high surface-to-volume ratio and finite charge carrier concentration. The sensing mechanism of metal-oxide-based gas sensor relies on the change in electrical conductivity caused by the adsorption and reaction of gas molecules on surface. It has been well accepted that the electrical property change is the consequence of charge transfer between the sensing layer and the chemisorbed species [8,9]. The gas sensing property of gas sensors is relative to the surface absorption and contact to atmosphere. The water vapour in atmosphere is one of the chemisorbed species. For SnO2 gas sensors, many investigations have been done, and the results show that the conductivity and sensitivity of sensors depend on the RH in atmosphere. Considerable influences of the RH in atmosphere on the characteristic of gas sensors have been observed [10–12]. While for tetrapod-shaped ZnO nanopowders (T-ZnO)-based gas sensors, dedicated to this question, it can be concluded that detailed investigations directed to the study of RH influence on the characteristic of gas sensors have not been carried out yet. There is significant lack of understanding with the humidity influence on the gas sensing property of T-ZnO gas sensors.
Z. Bai et al. / Materials Science and Engineering B 149 (2008) 12–17
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In the present paper, the experiments are aimed to investigating the testing circumstance RH influence on the electrical conductivity, sensitivity of the T-ZnO sensors and the storage circumstance RH influence on the sensitivity and stability of the T-ZnO sensors. It is very significant for the T-ZnO gas sensors to be used extensively. 2. Experimental procedures 2.1. Preparation of gas sensing devices Tetrapod-shaped ZnO nanopowders (T-ZnO) were prepared by the method of vapour-phase oxidation from metallic zinc as a raw material, which was described in detail in the literature [6]. A field-emission scanning electron microscopy (FE-SEM, Sirion 200) was employed to observe morphology of powders and to determine their size. T-ZnO thick films were prepared by a screen-printing technique onto alumina substrate with pre-printed Au electrodes and the heater (RuO2 ) on the inverse of the alumina substrate by the same technique. It is a simpler and less expensive way to prepare the sensors as compared to other ways such as chemical vapour deposition, thermal evaporation, etc. The paste was prepared by mixing the T-ZnO with a given proportion to organic vehicle to be milled for 6 h. Such thick films were supposed to be heated with the temperature of 250 ◦ C for 1 h to eliminate the organic vehicle, and then sintered at 650 ◦ C for 2.5 h. The microstructure of obtained thick film was observed by field-emission scanning electron microscope (FE-SEM). The substrate with the gas sensing film, the electrodes and the heater were then soldered onto a TO-8-003 support (Yixing City Jitai Electronics) to form gas sensing devices by using gold wires and a welding machine. The devices were aged at 400 ◦ C for 6 days in the open air to relax the strains generated during the firing process. 2.2. Measurement of gas response characteristics in different humidity atmosphere Schematic diagram of the measurement system is shown in Fig. 1. The test set-up consists of a glass test chamber to host four TO-8-003 sockets for sensors, a pipeline system with a pump for the transfer of the given RH air from the bottle of salt solution to test chamber and an integrated data collection multiplexer to acquire the electrical signals from the sensors. Labview software drives all the measurement operations via a PC and a GPIB interface. The water bath temperature is fixed at 30 ◦ C. The measurement operation was not carried out until the pump had worked for 12 h to obtain the given RH in the test chamber. The required level of humidity was created by using different salt solutions which have the property to keep a constant ambient humidity [13]. By setting the saturated solutions of MgCl2 , Mg(NO3 )2 , NaCl, K2 SO4 in the bottle, the RH can be controlled to 32% RH, 50% RH, 75% RH, 96% RH, respectively. The measurement was processed in a static system with a certain RH after the pump was stopped. The ethanol vapour was injected into the test chamber by using a syringe through a rubber septum. The volume of the injected ethanol vapour was
Fig. 1. Schematic diagram of the measurement system in different relative humidity.
calculated so that the ethanol concentration in the chamber can be 100 ppm. The RH in the measurement chamber was changed from 32% RH to 96% RH. Meanwhile the ambient temperature was kept at 30 ◦ C. The effect of RH was analyzed by carrying out alcohol sensing tests on a set of T-ZnO sensors. 2.3. Stability measurement of the T-ZnO sensors in different circumstance RH The effect of environment RH on the stability of T-ZnO sensor was investigated. The sensing measurement was carried out in lab condition (25 ◦ C, 80% RH) every 3 days for eight times, which were conserved in 32% RH, 50% RH, 75% RH and 96% RH in room temperature 30 ◦ C, respectively. The measurement method was described in detail in the literature [6]. The infrared spectrum (IR) (VERTEX 70, Bruker) with the reflection absorption spectroscopy (RAS system) technology was employed to analyze the effect of storage environment humidity on the TZnO thick films, which were conserved in different environment humidity after every testing. In addition, the T-ZnO nanopowders, which were conserved in 96% RH for 7 days after calcined at 650 ◦ C for 2.5 h in air, were performed thermogravimetric analysis (TGA) (with a STA 449C instrument) at a heating rate of 10 K min−1 with air as the buffer gas. The gas sensitivity, S, is given by S = Ra /Rg , where Ra and Rg express the resistance of the sensor in air and in detecting gas, respectively. The detected vapour is alcohol, and the concentration is 100 ppm. 3. Results and discussions Fig. 2 shows the resistance and the sensitivity as a function of the RH of thick film based on T-ZnO in the test chamber with 100 ppm alcohol vapour. Such a result was gained in the maximal sensitivity temperature (370 ◦ C). It was found that the resistance in air increases with an increase of the RH in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. And the sensi-
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Z. Bai et al. / Materials Science and Engineering B 149 (2008) 12–17
Fig. 2. The change of the resistance and the sensitivity of T-ZnO sensor in different test chamber relative humidity.
tivity increases with an increase in the range of 32% RH to 50% RH and decreases when the RH increases from 50% RH to 96% RH. The effect of RH on the resistance of metal oxidation is generally accepted as that the resistance decreases with the increase of RH and such a process is reversible. For tin dioxide, there are three types of mechanisms to explain the experimentally proven increase of surface conductivity in the presence of water vapour. Two direct mechanisms, are proposed by Heiland and Kohl (for details see [14]) and the third, indirect, is suggested by Morrison and by Henrich and Cox [15,16]. As for ZnO of the same sensing mechanism, the effect of water vapour on the resistance should be homologous. Therefore, we can deduce that there should be the homologous mechanisms to explain increase of surface conductivity in the presence of water vapour. The similar two direct mechanisms are as follows: The first mechanism attributes the role of electron donor to the ‘rooted’ OH group, the one including lattice oxygen. The equation proposed is + − − H2 Ogas + ZnZn + OO = (Zn+ Zn − OH ) + (OH)O + e
(1)
− in which (Zn+ Zn − OH ) is denominated as an isolated hydroxyl or OH group and (OH)+ O is the rooted one. In the upper equation, the latter is already ionized. The reaction implies the homolytic dissociation of water and the reaction of the neutral H atom with the lattice oxygen. The latter is normally fixing two electrons and then consequently being in the (2−) state. The built-up rooted OH group, having a lower electron affinity, can become ionized and a donor (with the injection of an electron into the conduction band). The second mechanism takes into account the possibility of the reaction between the hydrogen atom and the lattice oxygen and the binding of the resulting hydroxyl group to the Zn atom. The resulting oxygen vacancy will produce, by ionisation, the additional electrons. The equation proposed by Heiland and Kohl [14] is ++ − − H2 Ogas + 2ZnZn + OO = 2(Zn+ Zn − OH ) + VO + 2e
(2)
The indirect effect could be the interaction between both the hydroxyl group and the hydrogen atom originating from the
Fig. 3. SEM morphology of tetrapod-shaped ZnO nanopowders prepared by vapour-phase oxidation.
water molecule with an acid or basic group, which are also acceptor surface states. Their electronic affinity could change after the interaction. It could also be the influence of the co-adsorption of water on the adsorption of another adsorbate, which could be an electron acceptor. Henrich and Cox suggested that the pre-adsorbed oxygen could be displaced by water adsorption. In any of these mechanisms, the particular state of the surface has a major role, due to the fact that steps and surface defects will increase the dissociative adsorption. The nanowhiskers in tetrapod morphology with four needle-like feet extend from the center in [0 0 0 1] growth direction [17,18]. The diameter of the feet gradually decreased from about 40 nm at the root to a pinpoint tip and their mean length was about 450 nm, as shown in Fig. 3. Therefore, the nonpolar ZnO (1 0 1¯ 0) surfaces have amount of steps and surface defects, which can enhance the interaction of pre-adsorbed oxygen with water adsorption. Egashira et al. [19] showed by temperature-programmed desorption (TPD) and isotopic tracer studies combined with TPD that the oxygen adsorbates are rearranged in the presence of adsorbed water. It is not easy to quantify the effect of water adsorption on the charge carrier concentration. For the first mechanism of water interaction proposed by Heiland and Kohl (“rooted”, Eq. (1)), one could conclude the effect of water by considering the effect of an increased background of free charge carriers on the adsorption of oxygen. For the second mechanism proposed by Heiland and Kohl (“isolated”, Eq. (2)) one can examine the influence of water adsorption (see [20]) as an electron injection combined with the appearance of new sites for oxygen chemisorptions. Yan et al. [21] confirmed that oxygen vacancies in ZnO (1 0 1¯ 0) surfaces effectively promote the dissociative adsorption. It can be observed that according to the two direct interaction mechanisms, water vapour offered the necessary conditions for oxygen adsorption, electrons and oxygen vacancies. Therefore, a certain extent, the water adsorption should accelerate the oxygen
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adsorption. In this case one has to introduce the change in the total concentration of adsorption sites [St ] [St ] = [St0 ] + k0 pH2 O
(3)
pH2 O (partial pressure of water vapour) is proportional to the size of RH. When RH increased, the total concentration of adsorption sites [St ] increased, and the rate of coverage of hydroxyl groups and oxygen species changed. Thus, depending on the relative surface distribution and coverage of hydroxyl groups and oxygen species, an enhancing or decreasing effect of relative humidity on the resistance in air and the response to alcohol can occur. In 32% RH, there are less hydroxyl groups and oxygen species on surface of sensing films, which make the resistance and sensitivity of the T-ZnO sensor low. When relative humidity increases from 32% RH to 75% RH, the change of the coverage of oxygen species is larger than the change of the coverage of hydroxyl groups on surface of sensing films. Therefore, the resistance in air increased with the RH increasing. When humidity increases unceasingly to 96% RH, on one hand, the level of RH is too high, so that there is a large coverage with hydroxyl groups OH− , and the adsorption of oxygen can be limited. On the other hand, when the probability of OH− radicals present in two neighbor nodes of the ZnO matrix becomes sufficiently large, the catalytic synthesis of H2 O2 is expected to proceed [22]: OH− + OH− → H2 O2 + 2e
(4)
These are the facts that explain the resistance in air decrease at T = 370 ◦ C when the air relative humidity increases in the range from 75% RH to 96% RH, as shown in Fig. 2. The best response to ethanol is in 50% RH. It is contributed to the small coverage of hydroxyl groups, which cannot inhibit the ethanol adsorption. And the coverage of the oxygen species will increase [23]. Over 50% RH, the total concentration of adsorption sites [St ] increases gradually with the relative humidity increasing, and the ratio of the coverage of hydroxyl groups and oxygen species increases. Meanwhile, the interaction between the hydroxyl groups and oxygen species enhances gradually. Accordingly, the hydroxyl groups and oxygen species are rearranged on the surface of the sensitive layer [15,16,24]. In 75% RH and 96% RH, the high ratio of the coverage of hydroxyl groups can depress the high surface activity of the T-ZnO sensor and block the adsorption or reaction sites for the ethanol molecules [23], which leads to a smaller consumption and sensing signal. Therefore the sensitivity decreases with RH increasing. The relative humidity of the storage circumstance affects the sensitivity and the stability of the T-ZnO sensors too, as shown in Fig. 4. The sensitivity of T-ZnO sensors in storage circumstance with the different relative humidity is in the order of 50% RH > 75% RH > 32% RH > 96% RH to ethanol 100 ppm. The sensitivity in 32% RH and 50% RH holds the wave line 7 and 70, respectively. In 75% RH, in forepart 10 days the sensitivity retains about 30, the later declines all along to about 5. In 96% RH, at first, the sensitivity is about 5 and declines gradually after 7 days. The ZnO nanowhiskers in tetrapod morphology with four
Fig. 4. The stability of sensitivity of T-ZnO sensor in different storage circumstance relative humidity.
needle-like feet are to extend from the center in [0 0 0 1] growth direction [17,18], which is much more active than that of the nanoparticles because they possess a large ratio of surface-tovolume. The T-ZnO thick film is similar to the compact cell due to the intertexture of nanowhiskers of T-ZnO, in which there is a large number of nano-pinholes, which is helpful to the oxygen and water vapour adsorption, as shown in Fig. 5. The water vapour adsorbed into the sensitive film increases with the RH increasing. We can explain the decrease of resistance by using a revised short-circuiting pathway model [24]. The diffusion of water vapour through the nano-pinholes is involved in this model. The revised model postulates that the water vapour adsorbed on the surface can diffuse into the film bulk through the nano-pinholes, which results in a decrease of the resistance. The removal of water vapour from the nano-pinholes is more difficult than removing water vapour from the surface. This is one of the reasons why a longer pre-heating time is necessary to stabilize the sensors before using them. Fig. 6 shows the TGA results of T-ZnO nanopowders stored in 96% RH circumstance. It can be seen that, the water vapour absorbed in T-ZnO nanopowders decreases slowly within the temperature
Fig. 5. Surface SEM morphology of the thick film based on T-ZnO nanopowders.
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Fig. 6. TGA analysis of T-ZnO nanopowders in 96% RH circumstance.
range from 100 ◦ C to 615 ◦ C. The water vapour in the nanopinholes generates OH− groups through a reaction with O− and O2− in the films at above 150 ◦ C. There are two direct mechanisms to explain the decrease of resistance with the presence of water vapour as the above mentioned. The quantity of water vapour adsorbed in the T-ZnO sensors is proportional to the RH of storage circumstance. At testing temperature, the size of the remnant OH− groups in sensing layer changes with different amounts of water vapour adsorbed and different pre-heating time. The resistances of T-ZnO sensors correspond to change. This shows the instability of T-ZnO sensors. In addition, the RH of the storage circumstance affects the response-recovery process of T-ZnO sensors too. It is because the absorption water in T-ZnO thick films can affect the acid–base properties of the thick films. It is well known that there are two oxidation states of ethanol. C2 H5 OH + 21 O2 → CH3 CHO + H2 O
(5)
C2 H5 OH → C2 H4 + H2 O
(6)
The compositions of the T-ZnO thick films surface in storage environment of 96% RH and 11% RH were characterized by the infrared spectrum and shown in Fig. 7. The absorption band around 3397 cm−1 and 1127 cm−1 is assigned to the stretching vibration of O H and C O, respectively, which
are the typical characteristic band of C2 H5 OH. Wavenumber at 1435 cm−1 is considered as the stretching vibration of C C in CH3 CHO. Wavenumber at 783 cm−1 is the bending vibration of C H in C2 H4 , Wavenumber at 630 cm−1 and 620 cm−1 is the bending vibration of O H. It indicates that the compositions of the T-ZnO thick film surface are C2 H5 OH, CH3 CHO and OH groups in storage environment of 96% RH, and C2 H4 and OH groups in storage environment of 32% RH. The results are contributed to the quantity of the absorption water vapour and oxygen on T-ZnO thick film surface. On T-ZnO thick film surface in storage environment of 96% RH, there is much absorption water and little absorption oxygen. The character of the T-ZnO thick film surface is basic. Therefore, the reaction is initiated by the dehydrogenation to CH3 CHO intermediately, and C2 H5 OH is not oxidized entirely. On the contrary, on TZnO thick film surface in storage environment of 32% RH, there is little absorption water vapour and much absorption oxygen. The character of the T-ZnO thick film surface is acid. The reaction is initiated by the dehydration to C2 H4 intermediately, and C2 H5 OH is oxidized entirely. The gas sensing reactions process is different on T-ZnO thick film surface in different RH of storage environment. Recovery process of T-ZnO sensors is related to the oxygen absorption ability of sensors. The better oxygen absorption ability of the sensor is, the better recovery of the sensor and its sensitivity, as shown in Fig. 4. Neither the extremely low nor high RH in storage environment is beneficial to the response-recovery of T-ZnO sensors. The storage environment of 50% RH is the most optimization choice.
4. Conclusions The testing chamber humidity and the storage circumstance humidity effects on the T-ZnO thick film sensors were investigated by measuring the resistance and sensitivity. The resistance increases gradually with increasing relative humidity (RH) in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. The sensitivity in different testing chamber RH is in the order of 50% RH > of 75% RH > of 32% RH > of 96% RH. The sensitivity change in different storage circumstance RH is similar to the change in different testing chamber RH. The stability of TZnO sensors is influenced evidently by the storage circumstance humidity. The sensor stored in 96% RH declined fleetly, in 75% RH slower. When the testing chamber humidity and the storage circumstance humidity retained 50% RH, the T-ZnO sensors present optimal sensitivity and stability.
Acknowledgements
Fig. 7. Infrared spectrum of the T-ZnO thick film in different relative humidity storage circumstance.
The authors gratefully acknowledge the financial support by 863 Project (No. 2006AA03Z338), Nature Science Foundation of China (No. 50772040) and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology).
Z. Bai et al. / Materials Science and Engineering B 149 (2008) 12–17
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[13] V.I. Perelman, Short Handbook on Chemistry, Gostehizdat, Moscow, 1963. [14] G. Heiland, D. Kohl, in: T. Seiyama (Ed.), Chemical Sensor Technology, vol. 1, Kodansha, Tokyo, 1988, pp. 15–38 (Chapter 2). [15] S.R. Morrison, The Chemical Physics of Surfaces, 2nd ed., Plenum, New York, 1990. [16] V.A. Henrich, P.A. Cox, The Surface Science of Metal Oxides, Cambridge University Press, Cambridge, 1994, p. 312. [17] M. Kitano, T. Hamabe, S. Maedas, T. Okabe, J. Cryst. Growth 102 (1990) 965. [18] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Xu, Q. Zhao, B. Xiang, R.M. Wang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 144. [19] M. Egashira, M. Nakashima, S. Kawasumi, J. Chem. Soc. Chem. Commun. (1981) 1047. [20] N. Bˆarsan, R. Ionescu, Sens. Actuator B 12 (1993) 71. [21] Y.F. Yan, M.M. Al-Jassim, S.H. Wei, Phys. Rev. B 72 (2005) 161307. [22] T.T. Wol’kenstain, The Theory of Electronic Catalysis on Semiconductors, Pergamon, Oxford, 1963. [23] W. Schmid, N. Bˆarsan, U. Weimar, Sens. Actuator B 89 (2003) 232. [24] E. Sonder, L.M. Levinson, W. Katz, J. Appl. Phys. 58 (11) (1985) 420.
Effect of humidity on the gas sensing property of the tetrapod-shaped ZnO nanopowder sensor Zikui Bai a , Changsheng Xie a,b,∗ , Mulin Hu b , Shunping Zhang a , Dawen Zeng 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 11 July 2007; received in revised form 9 October 2007; accepted 17 November 2007
Abstract The testing chamber humidity and the storage circumstance humidity effects on the tetrapod-shaped ZnO nanopowder (named as T-ZnO) thick film sensors were investigated by measuring the resistance and sensitivity. The resistance increases gradually with increasing relative humidity (RH) in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. The sensitivity to ethanol 100 ppm in testing chambers with different humidity is in the order of 50% RH > of 75% RH > of 32% RH > of 96% RH. The sensitivity change in the storage circumstance with different RH is similar to the change in testing chamber with different RH. The stability of T-ZnO sensor is influenced evidently by the storage circumstance humidity. The water vapour in the tetrapod-shaped ZnO nanopowders was investigated by thermogravimetric analysis (TGA). The testing gas and the reactant adsorbed in the sensitive film were characterized by infrared spectrum (IR). The explanation of the observed effects was given and the mechanism of interaction of the ZnO sensing layer with H2 O was proposed. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity; Sensitivity; Stability; Tetrapod-shaped ZnO nanopowders
1. Introduction Zinc oxide (ZnO) is a wide-band gap semiconductor that is desirable for many applications, such as piezoelectric transducers, varistors, gas sensors and transparent conducting thin films [1–3]. By reducing the size of ZnO crystals to nanoscale dimensions or controlling the morphology of ZnO crystals to tetrapod-shaped nanowhiskers [4], nanowires and nanorods [5], researchers can tailor the properties via quantum confinement and surface effect. Tetrapod-shaped ZnO nanowhiskers (named as T-ZnO) can be prepared by the method of vapourphase oxidation from metallic zinc as raw materials. The T-ZnO-based sensors have been received great attention due to their excellent sensing property to volatile organic compounds (VOCs)—benzene, toluene, xylene, ethanol and acetone [6,7]. ∗
Corresponding author at: Nanomaterial and Smart Sensor Research Laboratory, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (C. Xie). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.11.020
The high sensitivity is attributed to the high surface-to-volume ratio and finite charge carrier concentration. The sensing mechanism of metal-oxide-based gas sensor relies on the change in electrical conductivity caused by the adsorption and reaction of gas molecules on surface. It has been well accepted that the electrical property change is the consequence of charge transfer between the sensing layer and the chemisorbed species [8,9]. The gas sensing property of gas sensors is relative to the surface absorption and contact to atmosphere. The water vapour in atmosphere is one of the chemisorbed species. For SnO2 gas sensors, many investigations have been done, and the results show that the conductivity and sensitivity of sensors depend on the RH in atmosphere. Considerable influences of the RH in atmosphere on the characteristic of gas sensors have been observed [10–12]. While for tetrapod-shaped ZnO nanopowders (T-ZnO)-based gas sensors, dedicated to this question, it can be concluded that detailed investigations directed to the study of RH influence on the characteristic of gas sensors have not been carried out yet. There is significant lack of understanding with the humidity influence on the gas sensing property of T-ZnO gas sensors.
Z. Bai et al. / Materials Science and Engineering B 149 (2008) 12–17
13
In the present paper, the experiments are aimed to investigating the testing circumstance RH influence on the electrical conductivity, sensitivity of the T-ZnO sensors and the storage circumstance RH influence on the sensitivity and stability of the T-ZnO sensors. It is very significant for the T-ZnO gas sensors to be used extensively. 2. Experimental procedures 2.1. Preparation of gas sensing devices Tetrapod-shaped ZnO nanopowders (T-ZnO) were prepared by the method of vapour-phase oxidation from metallic zinc as a raw material, which was described in detail in the literature [6]. A field-emission scanning electron microscopy (FE-SEM, Sirion 200) was employed to observe morphology of powders and to determine their size. T-ZnO thick films were prepared by a screen-printing technique onto alumina substrate with pre-printed Au electrodes and the heater (RuO2 ) on the inverse of the alumina substrate by the same technique. It is a simpler and less expensive way to prepare the sensors as compared to other ways such as chemical vapour deposition, thermal evaporation, etc. The paste was prepared by mixing the T-ZnO with a given proportion to organic vehicle to be milled for 6 h. Such thick films were supposed to be heated with the temperature of 250 ◦ C for 1 h to eliminate the organic vehicle, and then sintered at 650 ◦ C for 2.5 h. The microstructure of obtained thick film was observed by field-emission scanning electron microscope (FE-SEM). The substrate with the gas sensing film, the electrodes and the heater were then soldered onto a TO-8-003 support (Yixing City Jitai Electronics) to form gas sensing devices by using gold wires and a welding machine. The devices were aged at 400 ◦ C for 6 days in the open air to relax the strains generated during the firing process. 2.2. Measurement of gas response characteristics in different humidity atmosphere Schematic diagram of the measurement system is shown in Fig. 1. The test set-up consists of a glass test chamber to host four TO-8-003 sockets for sensors, a pipeline system with a pump for the transfer of the given RH air from the bottle of salt solution to test chamber and an integrated data collection multiplexer to acquire the electrical signals from the sensors. Labview software drives all the measurement operations via a PC and a GPIB interface. The water bath temperature is fixed at 30 ◦ C. The measurement operation was not carried out until the pump had worked for 12 h to obtain the given RH in the test chamber. The required level of humidity was created by using different salt solutions which have the property to keep a constant ambient humidity [13]. By setting the saturated solutions of MgCl2 , Mg(NO3 )2 , NaCl, K2 SO4 in the bottle, the RH can be controlled to 32% RH, 50% RH, 75% RH, 96% RH, respectively. The measurement was processed in a static system with a certain RH after the pump was stopped. The ethanol vapour was injected into the test chamber by using a syringe through a rubber septum. The volume of the injected ethanol vapour was
Fig. 1. Schematic diagram of the measurement system in different relative humidity.
calculated so that the ethanol concentration in the chamber can be 100 ppm. The RH in the measurement chamber was changed from 32% RH to 96% RH. Meanwhile the ambient temperature was kept at 30 ◦ C. The effect of RH was analyzed by carrying out alcohol sensing tests on a set of T-ZnO sensors. 2.3. Stability measurement of the T-ZnO sensors in different circumstance RH The effect of environment RH on the stability of T-ZnO sensor was investigated. The sensing measurement was carried out in lab condition (25 ◦ C, 80% RH) every 3 days for eight times, which were conserved in 32% RH, 50% RH, 75% RH and 96% RH in room temperature 30 ◦ C, respectively. The measurement method was described in detail in the literature [6]. The infrared spectrum (IR) (VERTEX 70, Bruker) with the reflection absorption spectroscopy (RAS system) technology was employed to analyze the effect of storage environment humidity on the TZnO thick films, which were conserved in different environment humidity after every testing. In addition, the T-ZnO nanopowders, which were conserved in 96% RH for 7 days after calcined at 650 ◦ C for 2.5 h in air, were performed thermogravimetric analysis (TGA) (with a STA 449C instrument) at a heating rate of 10 K min−1 with air as the buffer gas. The gas sensitivity, S, is given by S = Ra /Rg , where Ra and Rg express the resistance of the sensor in air and in detecting gas, respectively. The detected vapour is alcohol, and the concentration is 100 ppm. 3. Results and discussions Fig. 2 shows the resistance and the sensitivity as a function of the RH of thick film based on T-ZnO in the test chamber with 100 ppm alcohol vapour. Such a result was gained in the maximal sensitivity temperature (370 ◦ C). It was found that the resistance in air increases with an increase of the RH in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. And the sensi-
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Fig. 2. The change of the resistance and the sensitivity of T-ZnO sensor in different test chamber relative humidity.
tivity increases with an increase in the range of 32% RH to 50% RH and decreases when the RH increases from 50% RH to 96% RH. The effect of RH on the resistance of metal oxidation is generally accepted as that the resistance decreases with the increase of RH and such a process is reversible. For tin dioxide, there are three types of mechanisms to explain the experimentally proven increase of surface conductivity in the presence of water vapour. Two direct mechanisms, are proposed by Heiland and Kohl (for details see [14]) and the third, indirect, is suggested by Morrison and by Henrich and Cox [15,16]. As for ZnO of the same sensing mechanism, the effect of water vapour on the resistance should be homologous. Therefore, we can deduce that there should be the homologous mechanisms to explain increase of surface conductivity in the presence of water vapour. The similar two direct mechanisms are as follows: The first mechanism attributes the role of electron donor to the ‘rooted’ OH group, the one including lattice oxygen. The equation proposed is + − − H2 Ogas + ZnZn + OO = (Zn+ Zn − OH ) + (OH)O + e
(1)
− in which (Zn+ Zn − OH ) is denominated as an isolated hydroxyl or OH group and (OH)+ O is the rooted one. In the upper equation, the latter is already ionized. The reaction implies the homolytic dissociation of water and the reaction of the neutral H atom with the lattice oxygen. The latter is normally fixing two electrons and then consequently being in the (2−) state. The built-up rooted OH group, having a lower electron affinity, can become ionized and a donor (with the injection of an electron into the conduction band). The second mechanism takes into account the possibility of the reaction between the hydrogen atom and the lattice oxygen and the binding of the resulting hydroxyl group to the Zn atom. The resulting oxygen vacancy will produce, by ionisation, the additional electrons. The equation proposed by Heiland and Kohl [14] is ++ − − H2 Ogas + 2ZnZn + OO = 2(Zn+ Zn − OH ) + VO + 2e
(2)
The indirect effect could be the interaction between both the hydroxyl group and the hydrogen atom originating from the
Fig. 3. SEM morphology of tetrapod-shaped ZnO nanopowders prepared by vapour-phase oxidation.
water molecule with an acid or basic group, which are also acceptor surface states. Their electronic affinity could change after the interaction. It could also be the influence of the co-adsorption of water on the adsorption of another adsorbate, which could be an electron acceptor. Henrich and Cox suggested that the pre-adsorbed oxygen could be displaced by water adsorption. In any of these mechanisms, the particular state of the surface has a major role, due to the fact that steps and surface defects will increase the dissociative adsorption. The nanowhiskers in tetrapod morphology with four needle-like feet extend from the center in [0 0 0 1] growth direction [17,18]. The diameter of the feet gradually decreased from about 40 nm at the root to a pinpoint tip and their mean length was about 450 nm, as shown in Fig. 3. Therefore, the nonpolar ZnO (1 0 1¯ 0) surfaces have amount of steps and surface defects, which can enhance the interaction of pre-adsorbed oxygen with water adsorption. Egashira et al. [19] showed by temperature-programmed desorption (TPD) and isotopic tracer studies combined with TPD that the oxygen adsorbates are rearranged in the presence of adsorbed water. It is not easy to quantify the effect of water adsorption on the charge carrier concentration. For the first mechanism of water interaction proposed by Heiland and Kohl (“rooted”, Eq. (1)), one could conclude the effect of water by considering the effect of an increased background of free charge carriers on the adsorption of oxygen. For the second mechanism proposed by Heiland and Kohl (“isolated”, Eq. (2)) one can examine the influence of water adsorption (see [20]) as an electron injection combined with the appearance of new sites for oxygen chemisorptions. Yan et al. [21] confirmed that oxygen vacancies in ZnO (1 0 1¯ 0) surfaces effectively promote the dissociative adsorption. It can be observed that according to the two direct interaction mechanisms, water vapour offered the necessary conditions for oxygen adsorption, electrons and oxygen vacancies. Therefore, a certain extent, the water adsorption should accelerate the oxygen
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adsorption. In this case one has to introduce the change in the total concentration of adsorption sites [St ] [St ] = [St0 ] + k0 pH2 O
(3)
pH2 O (partial pressure of water vapour) is proportional to the size of RH. When RH increased, the total concentration of adsorption sites [St ] increased, and the rate of coverage of hydroxyl groups and oxygen species changed. Thus, depending on the relative surface distribution and coverage of hydroxyl groups and oxygen species, an enhancing or decreasing effect of relative humidity on the resistance in air and the response to alcohol can occur. In 32% RH, there are less hydroxyl groups and oxygen species on surface of sensing films, which make the resistance and sensitivity of the T-ZnO sensor low. When relative humidity increases from 32% RH to 75% RH, the change of the coverage of oxygen species is larger than the change of the coverage of hydroxyl groups on surface of sensing films. Therefore, the resistance in air increased with the RH increasing. When humidity increases unceasingly to 96% RH, on one hand, the level of RH is too high, so that there is a large coverage with hydroxyl groups OH− , and the adsorption of oxygen can be limited. On the other hand, when the probability of OH− radicals present in two neighbor nodes of the ZnO matrix becomes sufficiently large, the catalytic synthesis of H2 O2 is expected to proceed [22]: OH− + OH− → H2 O2 + 2e
(4)
These are the facts that explain the resistance in air decrease at T = 370 ◦ C when the air relative humidity increases in the range from 75% RH to 96% RH, as shown in Fig. 2. The best response to ethanol is in 50% RH. It is contributed to the small coverage of hydroxyl groups, which cannot inhibit the ethanol adsorption. And the coverage of the oxygen species will increase [23]. Over 50% RH, the total concentration of adsorption sites [St ] increases gradually with the relative humidity increasing, and the ratio of the coverage of hydroxyl groups and oxygen species increases. Meanwhile, the interaction between the hydroxyl groups and oxygen species enhances gradually. Accordingly, the hydroxyl groups and oxygen species are rearranged on the surface of the sensitive layer [15,16,24]. In 75% RH and 96% RH, the high ratio of the coverage of hydroxyl groups can depress the high surface activity of the T-ZnO sensor and block the adsorption or reaction sites for the ethanol molecules [23], which leads to a smaller consumption and sensing signal. Therefore the sensitivity decreases with RH increasing. The relative humidity of the storage circumstance affects the sensitivity and the stability of the T-ZnO sensors too, as shown in Fig. 4. The sensitivity of T-ZnO sensors in storage circumstance with the different relative humidity is in the order of 50% RH > 75% RH > 32% RH > 96% RH to ethanol 100 ppm. The sensitivity in 32% RH and 50% RH holds the wave line 7 and 70, respectively. In 75% RH, in forepart 10 days the sensitivity retains about 30, the later declines all along to about 5. In 96% RH, at first, the sensitivity is about 5 and declines gradually after 7 days. The ZnO nanowhiskers in tetrapod morphology with four
Fig. 4. The stability of sensitivity of T-ZnO sensor in different storage circumstance relative humidity.
needle-like feet are to extend from the center in [0 0 0 1] growth direction [17,18], which is much more active than that of the nanoparticles because they possess a large ratio of surface-tovolume. The T-ZnO thick film is similar to the compact cell due to the intertexture of nanowhiskers of T-ZnO, in which there is a large number of nano-pinholes, which is helpful to the oxygen and water vapour adsorption, as shown in Fig. 5. The water vapour adsorbed into the sensitive film increases with the RH increasing. We can explain the decrease of resistance by using a revised short-circuiting pathway model [24]. The diffusion of water vapour through the nano-pinholes is involved in this model. The revised model postulates that the water vapour adsorbed on the surface can diffuse into the film bulk through the nano-pinholes, which results in a decrease of the resistance. The removal of water vapour from the nano-pinholes is more difficult than removing water vapour from the surface. This is one of the reasons why a longer pre-heating time is necessary to stabilize the sensors before using them. Fig. 6 shows the TGA results of T-ZnO nanopowders stored in 96% RH circumstance. It can be seen that, the water vapour absorbed in T-ZnO nanopowders decreases slowly within the temperature
Fig. 5. Surface SEM morphology of the thick film based on T-ZnO nanopowders.
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Fig. 6. TGA analysis of T-ZnO nanopowders in 96% RH circumstance.
range from 100 ◦ C to 615 ◦ C. The water vapour in the nanopinholes generates OH− groups through a reaction with O− and O2− in the films at above 150 ◦ C. There are two direct mechanisms to explain the decrease of resistance with the presence of water vapour as the above mentioned. The quantity of water vapour adsorbed in the T-ZnO sensors is proportional to the RH of storage circumstance. At testing temperature, the size of the remnant OH− groups in sensing layer changes with different amounts of water vapour adsorbed and different pre-heating time. The resistances of T-ZnO sensors correspond to change. This shows the instability of T-ZnO sensors. In addition, the RH of the storage circumstance affects the response-recovery process of T-ZnO sensors too. It is because the absorption water in T-ZnO thick films can affect the acid–base properties of the thick films. It is well known that there are two oxidation states of ethanol. C2 H5 OH + 21 O2 → CH3 CHO + H2 O
(5)
C2 H5 OH → C2 H4 + H2 O
(6)
The compositions of the T-ZnO thick films surface in storage environment of 96% RH and 11% RH were characterized by the infrared spectrum and shown in Fig. 7. The absorption band around 3397 cm−1 and 1127 cm−1 is assigned to the stretching vibration of O H and C O, respectively, which
are the typical characteristic band of C2 H5 OH. Wavenumber at 1435 cm−1 is considered as the stretching vibration of C C in CH3 CHO. Wavenumber at 783 cm−1 is the bending vibration of C H in C2 H4 , Wavenumber at 630 cm−1 and 620 cm−1 is the bending vibration of O H. It indicates that the compositions of the T-ZnO thick film surface are C2 H5 OH, CH3 CHO and OH groups in storage environment of 96% RH, and C2 H4 and OH groups in storage environment of 32% RH. The results are contributed to the quantity of the absorption water vapour and oxygen on T-ZnO thick film surface. On T-ZnO thick film surface in storage environment of 96% RH, there is much absorption water and little absorption oxygen. The character of the T-ZnO thick film surface is basic. Therefore, the reaction is initiated by the dehydrogenation to CH3 CHO intermediately, and C2 H5 OH is not oxidized entirely. On the contrary, on TZnO thick film surface in storage environment of 32% RH, there is little absorption water vapour and much absorption oxygen. The character of the T-ZnO thick film surface is acid. The reaction is initiated by the dehydration to C2 H4 intermediately, and C2 H5 OH is oxidized entirely. The gas sensing reactions process is different on T-ZnO thick film surface in different RH of storage environment. Recovery process of T-ZnO sensors is related to the oxygen absorption ability of sensors. The better oxygen absorption ability of the sensor is, the better recovery of the sensor and its sensitivity, as shown in Fig. 4. Neither the extremely low nor high RH in storage environment is beneficial to the response-recovery of T-ZnO sensors. The storage environment of 50% RH is the most optimization choice.
4. Conclusions The testing chamber humidity and the storage circumstance humidity effects on the T-ZnO thick film sensors were investigated by measuring the resistance and sensitivity. The resistance increases gradually with increasing relative humidity (RH) in a range of 32%–75% RH in testing chamber, while in a range of 75%–96% RH, decreases gradually with the increase of RH. The sensitivity in different testing chamber RH is in the order of 50% RH > of 75% RH > of 32% RH > of 96% RH. The sensitivity change in different storage circumstance RH is similar to the change in different testing chamber RH. The stability of TZnO sensors is influenced evidently by the storage circumstance humidity. The sensor stored in 96% RH declined fleetly, in 75% RH slower. When the testing chamber humidity and the storage circumstance humidity retained 50% RH, the T-ZnO sensors present optimal sensitivity and stability.
Acknowledgements
Fig. 7. Infrared spectrum of the T-ZnO thick film in different relative humidity storage circumstance.
The authors gratefully acknowledge the financial support by 863 Project (No. 2006AA03Z338), Nature Science Foundation of China (No. 50772040) and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology).
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