- Email: [email protected]
quartz crystal microbalance gas sensor for nitric oxide
Sensors and Actuators B 93 (2003) 175–180
A room temperature indium tin oxide/quartz crystal microbalance gas sensor for nitric oxide Jianqiao Hua, Furong Zhua, Jian Zhanga, Hao Gongb,* a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Department of Materials Science, National University of Singapore, Lower Kent Ridge, Singapore 119260, Singapore
Abstract Nitric oxide (NO) gas sensor using a combination of indium tin oxide (ITO) and quartz crystal microbalance is fabricated. The sensor is made from the polished AT-cut quartz crystal on which the ITO thin film is deposited using radio frequency magnetron sputtering. The sensor is sensitive to NO gas at room temperature and avoids the problems related to elevated-temperature sensors. It is shown that the sensor has a distinct negative frequency shift of 110 Hz within 600 s when it is exposed to 60 ppm of NO. The frequency-changing rate is also found to increase with NO concentration. The chemical status of elements on the ITO surface before and after NO adsorption is investigated by X-ray photoelectron spectroscopy (XPS). Possible gas sensing mechanisms are discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Indium tin oxide; Quartz crystal; Gas sensing mechanisms
1. Introduction Sensors for toxic gases have attracted much attention due to the growing concern of environmental protection and safety. A number of semiconductor oxides such as ZnO, SnO2, In2O3 and indium tin oxide (ITO) are used for different gas sensors [1–4]. Most of these sensors are based on the resistance variation when the semiconductor oxide films are exposed to target gases. Normally, the conventional resistance-type sensors are operated at an elevated temperature, usually within the range of 250–300 8C [4–8]. The requirement for user-friendly sensors drives the effort to develop more portable, sensitive and cost-effective devices. In addition, it is desirable to develop sensors working at room temperature. QCM has been used widely to monitor the change in mass loadings by measuring the shift of its resonant frequency [9]. A QCM-based sensor consists of a quartz crystal and the sensing material. A good sensing material can selectively adsorb the target species/gases. QCM devices made with metal electrodes of Pt, Au, Ag, etc. for sensor applications have been reported [10–16]. The resonant frequency of a QCM decreases when gas is adsorbed on the electrode * Corresponding author. Tel.: þ65-8746598; fax: þ65-7763604. E-mail address: [email protected] (H. Gong).
surface [17]. The corresponding frequency shift can be described by the following empirical equation [10]: Df ¼ Cf02 Dm=A;
(1)
where f0 is the resonant frequency of the unperturbed quartz, Df the QCM frequency shift due to the added mass, C a constant, Dm the added mass and A the area of the sensing material. For example, the sensitivity of QCM having a resonant frequency of 10 MHz, made with gold electrode of diameter 8 mm, is approximately 0.226 cm2 Hz/ng. In this work, we developed gas sensors with a combination of ITO and QCM and used them for nitric oxide (NO) gas sensing in atmospheric environment. Such sensors have simple layer configuration and can be operated at room temperature. The sensitivity and repeatability of the sensors are investigated. The possible sensing mechanisms are also discussed.
2. Experimental ITO films were prepared at room temperature by radio frequency magnetron sputtering using an oxidized target with In2O3 and SnO2 in a weight proportion of 9:1. The base pressure in the system was approximately 5:0 108 Torr. The total pressure of the sputtering gas mixture of argon and
0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00186-2
176
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
Gas sensing experiments were performed in a system as shown in Fig. 1 [19]. In this study, all measurements were performed at room temperature in atmospheric environment. The sensor was loaded into the main chamber and connected to a Saunders & Associate network analyzer. The frequency resolution of the analyzer is 50 Hz. The target nitric oxide gas was introduced into the test chamber using a calibrated syringe of minimum volume of 0.1 ml. In atmosphere, the concentration of the test gas, c, in parts per million (ppm), can be estimated as: c ðppmÞ ¼
Fig. 1. Schematic diagram of a gas testing system for NO detection.
hydrogen was adjusted to 3:0 103 Torr during the film deposition. In a previous work, we have shown that the presence of hydrogen during the film deposition can alter the electrical properties of ITO films [18]. In order to study the correlation between the ITO conductivity and the sensing sensitivity, we have made a set of ITO-QCM sensors with ITO electrodes prepared at the different hydrogen flow rates of 0, 1.0, 1.5 sccm in this study. AT-cut rectangular quartz blanks with dimension of 10 mm 10 mm 0:1 mm are used. The corresponding resonant frequency of the quartz substrate is about 17 MHz. An ITO-QCM sensor was fabricated using the lift-off photolithography process. The photoresist moulds were generated by lithography before ITO deposition. The thickness of ITO film obtained was 110 nm. A similar process was repeated to form the ITO electrode on the other side of the quartz crystal.
Vg Vg 106 106 ; Vg þ V V
(2)
where Vg is the volume of introduced gas, and V the volume of the chamber. We have varied the gas concentration from 60 to 2000 ppm in this work. During the testing process, in order to ensure the cleanness of the sensor surface and the accurate gas concentration, the system was pumped down to 7:5 102 Torr and then purged at least three times before each new cycle of NO measurement. The time-dependent sensor responses were recorded at room temperature. The chemical status of elements on the ITO surface before and after NO adsorption was investigated by ex situ X-ray photoelectron spectroscopy (XPS). The XPS experiments were performed in a VG ESCALAB 220i-XL instrument with an Al Ka X-ray source (1486.6 eV) operated at a power of 150 W. The electron energy analyzer was set at a constant analyzer mode at pass energy of 20 eV for high-resolution spectra acquisition. The electrical properties of as-deposited ITO films were obtained by Hall effect measurement.
3. Results and discussion Three types of gas sensors, labeled as #1, #2, #3, were used for the NO detection. The ITO electrodes of the sensors #1, #2, and #3 were deposited at the hydrogen flow rates of 0,
Fig. 2. Typical time-dependent frequency shift of an ITO-QCM gas sensor (#2) as a function of NO concentration.
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
177
Fig. 3. Frequency sensitivity, Sf, as a function of NO concentration.
1.0, 1.5 sccm, respectively. There is no observed change in the resonant frequency of the sensors in air. When NO was introduced into the test chamber, the frequency started to shift. Fig. 2 shows the time-dependent frequency shift of sensor #2 exposed in different NO concentrations. It can be seen that the negative frequency shift increases with increasing NO gas concentration. The frequency shift shows an approximate linear relationship with the exposure time. The sensor has a negative frequency shift of 110 Hz within 600 s when it is exposed to a concentration of 60 ppm NO gas. We define the frequency-changing rate, Sf ¼ jDf =Dtj, or slopes of Df–Dt curves, as the parameter to reflect the sensor sensitivity [19]. The larger Sf value means the bigger frequency shift per unit time and thus the higher sensitivity. Fig. 3 shows the corresponding relationship between Sf and the NO gas concentration. It is obvious that the frequency-changing rate increases with increasing NO concentration.
The sensors are sensitive to NO gas with good repeatability of Df although the starting frequency varies in different measurements. Fig. 4 shows the repeatability experimental results for sensor #2 testing at an identical NO concentration of 590 ppm. A clear negative frequency shift was always observed when NO was introduced to the system. Similar frequency-changing rate (jDf =Dtj) of about 0.6 Hz/s was found in three replicate tests within the first 250 s, which indicates a promising reproducibility of the device. We have found that the frequency-changing rate of 0.6 Hz/s is slightly lower than 0.7 Hz/s of the fresh sample tested. This is probably due to the dissipation of some active sites on ITO surface after the first exposure to NO gas. From Fig. 4, diverse frequency-changing rates from 0.6 to 0.8 Hz/s are observed after the first 300 s. It should be noticed that we pumped out the gases in the chamber and flushed with air prior to each test. Although this procedure could remove the residual NO gas in the chamber, it might not remove all
Fig. 4. The repeatability of ITO-QCM (gas sensor #2) frequency shift exposed to the NO.
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J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
Fig. 5. Time-dependent frequency shifts of QCM using ITO films deposited under different hydrogen flow rates (sensors #1 and #3).
those NO molecules trapped on the chamber wall. The trapped NO molecules would escape slowly from the chamber wall leading to a gradual increase of NO concentration in the chamber. The frequency-changing rate was fluctuated for different measurements, which may be related to the amount and the release process of trapped NO gas. The possible corrosion of the ITO electrode may not be the cause of the diverse frequency-shift rate, as it could not explain that only the second run showed a different behavior in this case. Fig. 5 shows the frequency responses of sensors #1 and #3 working at a NO gas concentration of 1770 ppm. The frequency shift for sensor #3 was 3990 Hz, however, it was 2514 Hz for sensor #1. This implies that sensor #3, made with a higher conductive ITO of 3000 S, exhibits higher sensitivity to NO than sensor #1 with ITO of 2000 S. Fig. 6 shows the frequency shift of sensors #1 and #3 working under different gas concentrations within the same time interval of 600 s. The results show that
sample #3 made with ITO, prepared at a hydrogen flow rate of 1.5 sccm, showed a large frequency response. In a previous work, we have shown that the presence of hydrogen during the film deposition increases film conductivity and reduces the ITO surface roughness [18,20]. As such, sensor #3 had a smoother surface than sensor #1. We know that the higher sensitivity (sensor #3) can be related to either the electrical or the surface morphologic properties of the ITO films. If the surface roughness plays a crucial role in the sensor response, in principle, sensor #1 should have bigger frequency shifts than sensor #3. However, this was not found in our experiment. Therefore, the different sensor sensitivities of these two samples may be mainly due to the difference in electric properties of the ITO electrodes. We know that the use of hydrogen in the film deposition creates addition number of oxygen vacancies leading to an enhancement in the ITO conductivity. In the NO detection, these oxygen vacancies could be involved in the processes of physical or/and chemical adsorptions of NO. An increase in
Fig. 6. Frequency shift of QCM gas sensors (#1 and #3) as a function of the NO concentration.
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
179
suggesting that the gas adsorption/reaction occurred on the ITO surface. The existence of new NO3 species on the NO-exposed ITO surface indicates the reaction between NO and ITO, which causes the mass increase. The formation of new species may also change the mechanical and electronic properties of the ITO electrode. The possible ITO/quartz interfacial stress changes may also affect in the resonant frequency of the device. However, the precise verification of this interfacial effect is difficult to determine directly using XPS measurement. Behavior of ITO-QCM sensor is quite complicated. Apart from NO, some others, like water vapor and oxygen, may also influence the sensor response. For example, when the sensor is exposed to oxygen, negatively charged oxygen species could accumulate on ITO surface by grabbing electrons from the conduction band [24]. This causes the formation of an electron-depleted region near the ITO surface region and makes the ITO surface easier to absorb the target gases [24]. In order to avoid the interference, the use of dual sensors or operation of the sensors at two different temperatures [25] could be an alternative approach. The corresponding experiments are currently in progress.
4. Conclusions
Fig. 7. XPS spectra of (a) O 1s peak of ITO surface before and after NO adsorption and (b) N 1s peaks before and after the Arþ sputtering of NOexposed ITO surface.
oxygen vacancies may provide more adsorption/reaction sites, resulting in higher NO mass loading [19], or higher frequency shift. XPS has been used to examine the chemical binding energies of O 1s and N 1s for ITO films before and after NO exposure. Fig. 7(a) shows the O 1s peaks of ITO-film surface before and after NO exposure, and Fig. 7(b) shows the N 1s spectra of NO-exposed ITO surface, before and after the surface removal with argon ion sputtering. In Fig. 7(a), an extra shoulder was found in the O 1s profile from a NO-exposed ITO surface. This peak can be deconvoluted into two components with the binding energies located at 530.8 and 532.3 eV, respectively. The observed O 1s peak at 530.8–531.2 eV can be assigned to the lattice oxygen in In2O3 [21]. The peak at a binding energy of 532.3 eV can be attributed to a different oxidation state [22]. Fig. 7(b) suggest that the ITO surface may contain two different nitrogen species corresponding to the N 1s peaks at the binding energies of 407 and 400.5 eV, respectively. The weak peak at 400.5 eV may be attributed to the adsorbed NO. The strong peak at 407 eV may come from the NO3 species [23]. This suggests that new chemical species were formed on the ITO surface. After removing the surface layer, both of the two nitrogen peaks disappeared,
Gas sensors based on the ITO-coated quartz crystal microbalance have been fabricated. Such sensors are sensitive to NO gas with good repeatability at room temperature. The resonant frequency-changing rates of 0.2, 0.4, 0.7 and 2.0 Hz/s has been found when the sensor is exposed to NO gas at different concentrations of 60, 295, 590 and 1180 ppm in air, respectively. XPS analyses reveal that NO also reacts with ITO leading to the formation of NO3 species on its surface.
References [1] D.H. Yoon, G.M. Choi, Sens. Actuators B 45 (1997) 251. [2] M.C. Horrillo, A. Serventi, D. Rickerby, J. Gueierrez, Sens. Actuators B 58 (1999) 474. [3] C.A. Papadopoulos, D.S. Vlachos, J.N. Avaritsiotis, Sens. Actuators B 42 (1997) 95. [4] G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli, Sens. Actuators B 8 (1992) 79. [5] N.G. Patel, K.K. Makhija, C.J. Panchal, Sens. Actuators B 21 (1994) 193. [6] N.G. Patel, K.K. Makhija, C.J. Panchal, D.B. Dave, V.S. Vaishnav, Sens. Actuators B 23 (1995) 49. [7] T. Miyata, T. Hikosaka, T. Minami, Sens. Actuators B 69 (2000) 16. [8] R. Bene, I.V. Perczel, F. Re´ ti, F.A. Meyer, M. Fleisher, H. Meixner, Sens. Actuators B 71 (2000) 36. [9] B.P. Binks, Modern Characterization Methods of Surfactant Systems, 1999, p. 483. [10] G. Sauerbrey, Z. Phys. 155 (1959) 206. [11] S.R. Kim, S.A. Choi, J.D. Kim, K.H. Choi, S.K. Park, Y.H. Chang, Synth. Mater. 71 (1995) 2293.
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[12] M. Teresa, A.C. Duarte, J.P. Oliveira, Sens. Actuators B 26–27 (1995) 191. [13] H. Nanto, S. Tsubakino, M. Habara, K. Kondo, T. Morita, Y. Douguchi, H. Nakazumi, R. Waite, Sens. Actuators B 34 (1996) 312. [14] M. Teresa, P. Sergio, T. Nogueira, J.P. Oliveira, Sens. Actuators B 68 (2000) 218. [15] H. Nanto, N. Dougami, T. Mukai, M. Habara, E. Kusano, A. Kinbara, T. Ogawa, T. Oyabu, Sens. Actuators B 66 (2000) 16. [16] K. Henkel, A. Oprea, I. Paloumpa, G. Appel, D. Schmeißer, P. Kamieth, Sens. Actuators B 76 (2001) 124. [17] P. Chang, J.S. Shih, Anal. Chim. Acta 403 (2000) 39. [18] K. Zhang, F. Zhu, C.H.A. Huan, A.T.S. Wee, J. Appl. Phys. 86 (1999) 974.
[19] J. Zhang, J. Hu, F. Zhu, H. Gong, Sens. Actuators B 87 (2002) 159. [20] F. Zhu, K. Zhang, C.H.A. Huan, A.T.S. Wee, Thin Solid Films 376 (2000) 255. [21] A. Gurlo, N. Baˆ rsan, M. Ivanovskaya, U. Weimar, W. Go¨ pel, Sens. Actuators B 47 (1998) 92. [22] G. Sarala Devi, S.V. Manorama, V.J. Rao, Sens. Actuators B 56 (1999) 98. [23] X. Bao, U. Wild, M. Muhler, B. Pettinger, R. Schlogl, G. Ertl, Surf. Sci. 425 (1999) 224. [24] T. Becker, S. Muhlberger, B.C. Bosch-von, G. Muller, T. Ziemann, K.V. Hechtenberg, Sens. Actuators B 69 (1–2) (2000) 108. [25] U. Schramm, D. Meinhold, S. Winter, C. Heil, J. Mu¨ ller-Albrecht, L. Wa¨ chter, H. Hoff, C.E.O. Roesky, T. Rechenbach, P. Boeker, Sens. Actuators B 67 (2000) 219–226.
A room temperature indium tin oxide/quartz crystal microbalance gas sensor for nitric oxide Jianqiao Hua, Furong Zhua, Jian Zhanga, Hao Gongb,* a
Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore b Department of Materials Science, National University of Singapore, Lower Kent Ridge, Singapore 119260, Singapore
Abstract Nitric oxide (NO) gas sensor using a combination of indium tin oxide (ITO) and quartz crystal microbalance is fabricated. The sensor is made from the polished AT-cut quartz crystal on which the ITO thin film is deposited using radio frequency magnetron sputtering. The sensor is sensitive to NO gas at room temperature and avoids the problems related to elevated-temperature sensors. It is shown that the sensor has a distinct negative frequency shift of 110 Hz within 600 s when it is exposed to 60 ppm of NO. The frequency-changing rate is also found to increase with NO concentration. The chemical status of elements on the ITO surface before and after NO adsorption is investigated by X-ray photoelectron spectroscopy (XPS). Possible gas sensing mechanisms are discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Indium tin oxide; Quartz crystal; Gas sensing mechanisms
1. Introduction Sensors for toxic gases have attracted much attention due to the growing concern of environmental protection and safety. A number of semiconductor oxides such as ZnO, SnO2, In2O3 and indium tin oxide (ITO) are used for different gas sensors [1–4]. Most of these sensors are based on the resistance variation when the semiconductor oxide films are exposed to target gases. Normally, the conventional resistance-type sensors are operated at an elevated temperature, usually within the range of 250–300 8C [4–8]. The requirement for user-friendly sensors drives the effort to develop more portable, sensitive and cost-effective devices. In addition, it is desirable to develop sensors working at room temperature. QCM has been used widely to monitor the change in mass loadings by measuring the shift of its resonant frequency [9]. A QCM-based sensor consists of a quartz crystal and the sensing material. A good sensing material can selectively adsorb the target species/gases. QCM devices made with metal electrodes of Pt, Au, Ag, etc. for sensor applications have been reported [10–16]. The resonant frequency of a QCM decreases when gas is adsorbed on the electrode * Corresponding author. Tel.: þ65-8746598; fax: þ65-7763604. E-mail address: [email protected] (H. Gong).
surface [17]. The corresponding frequency shift can be described by the following empirical equation [10]: Df ¼ Cf02 Dm=A;
(1)
where f0 is the resonant frequency of the unperturbed quartz, Df the QCM frequency shift due to the added mass, C a constant, Dm the added mass and A the area of the sensing material. For example, the sensitivity of QCM having a resonant frequency of 10 MHz, made with gold electrode of diameter 8 mm, is approximately 0.226 cm2 Hz/ng. In this work, we developed gas sensors with a combination of ITO and QCM and used them for nitric oxide (NO) gas sensing in atmospheric environment. Such sensors have simple layer configuration and can be operated at room temperature. The sensitivity and repeatability of the sensors are investigated. The possible sensing mechanisms are also discussed.
2. Experimental ITO films were prepared at room temperature by radio frequency magnetron sputtering using an oxidized target with In2O3 and SnO2 in a weight proportion of 9:1. The base pressure in the system was approximately 5:0 108 Torr. The total pressure of the sputtering gas mixture of argon and
0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00186-2
176
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
Gas sensing experiments were performed in a system as shown in Fig. 1 [19]. In this study, all measurements were performed at room temperature in atmospheric environment. The sensor was loaded into the main chamber and connected to a Saunders & Associate network analyzer. The frequency resolution of the analyzer is 50 Hz. The target nitric oxide gas was introduced into the test chamber using a calibrated syringe of minimum volume of 0.1 ml. In atmosphere, the concentration of the test gas, c, in parts per million (ppm), can be estimated as: c ðppmÞ ¼
Fig. 1. Schematic diagram of a gas testing system for NO detection.
hydrogen was adjusted to 3:0 103 Torr during the film deposition. In a previous work, we have shown that the presence of hydrogen during the film deposition can alter the electrical properties of ITO films [18]. In order to study the correlation between the ITO conductivity and the sensing sensitivity, we have made a set of ITO-QCM sensors with ITO electrodes prepared at the different hydrogen flow rates of 0, 1.0, 1.5 sccm in this study. AT-cut rectangular quartz blanks with dimension of 10 mm 10 mm 0:1 mm are used. The corresponding resonant frequency of the quartz substrate is about 17 MHz. An ITO-QCM sensor was fabricated using the lift-off photolithography process. The photoresist moulds were generated by lithography before ITO deposition. The thickness of ITO film obtained was 110 nm. A similar process was repeated to form the ITO electrode on the other side of the quartz crystal.
Vg Vg 106 106 ; Vg þ V V
(2)
where Vg is the volume of introduced gas, and V the volume of the chamber. We have varied the gas concentration from 60 to 2000 ppm in this work. During the testing process, in order to ensure the cleanness of the sensor surface and the accurate gas concentration, the system was pumped down to 7:5 102 Torr and then purged at least three times before each new cycle of NO measurement. The time-dependent sensor responses were recorded at room temperature. The chemical status of elements on the ITO surface before and after NO adsorption was investigated by ex situ X-ray photoelectron spectroscopy (XPS). The XPS experiments were performed in a VG ESCALAB 220i-XL instrument with an Al Ka X-ray source (1486.6 eV) operated at a power of 150 W. The electron energy analyzer was set at a constant analyzer mode at pass energy of 20 eV for high-resolution spectra acquisition. The electrical properties of as-deposited ITO films were obtained by Hall effect measurement.
3. Results and discussion Three types of gas sensors, labeled as #1, #2, #3, were used for the NO detection. The ITO electrodes of the sensors #1, #2, and #3 were deposited at the hydrogen flow rates of 0,
Fig. 2. Typical time-dependent frequency shift of an ITO-QCM gas sensor (#2) as a function of NO concentration.
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
177
Fig. 3. Frequency sensitivity, Sf, as a function of NO concentration.
1.0, 1.5 sccm, respectively. There is no observed change in the resonant frequency of the sensors in air. When NO was introduced into the test chamber, the frequency started to shift. Fig. 2 shows the time-dependent frequency shift of sensor #2 exposed in different NO concentrations. It can be seen that the negative frequency shift increases with increasing NO gas concentration. The frequency shift shows an approximate linear relationship with the exposure time. The sensor has a negative frequency shift of 110 Hz within 600 s when it is exposed to a concentration of 60 ppm NO gas. We define the frequency-changing rate, Sf ¼ jDf =Dtj, or slopes of Df–Dt curves, as the parameter to reflect the sensor sensitivity [19]. The larger Sf value means the bigger frequency shift per unit time and thus the higher sensitivity. Fig. 3 shows the corresponding relationship between Sf and the NO gas concentration. It is obvious that the frequency-changing rate increases with increasing NO concentration.
The sensors are sensitive to NO gas with good repeatability of Df although the starting frequency varies in different measurements. Fig. 4 shows the repeatability experimental results for sensor #2 testing at an identical NO concentration of 590 ppm. A clear negative frequency shift was always observed when NO was introduced to the system. Similar frequency-changing rate (jDf =Dtj) of about 0.6 Hz/s was found in three replicate tests within the first 250 s, which indicates a promising reproducibility of the device. We have found that the frequency-changing rate of 0.6 Hz/s is slightly lower than 0.7 Hz/s of the fresh sample tested. This is probably due to the dissipation of some active sites on ITO surface after the first exposure to NO gas. From Fig. 4, diverse frequency-changing rates from 0.6 to 0.8 Hz/s are observed after the first 300 s. It should be noticed that we pumped out the gases in the chamber and flushed with air prior to each test. Although this procedure could remove the residual NO gas in the chamber, it might not remove all
Fig. 4. The repeatability of ITO-QCM (gas sensor #2) frequency shift exposed to the NO.
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J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
Fig. 5. Time-dependent frequency shifts of QCM using ITO films deposited under different hydrogen flow rates (sensors #1 and #3).
those NO molecules trapped on the chamber wall. The trapped NO molecules would escape slowly from the chamber wall leading to a gradual increase of NO concentration in the chamber. The frequency-changing rate was fluctuated for different measurements, which may be related to the amount and the release process of trapped NO gas. The possible corrosion of the ITO electrode may not be the cause of the diverse frequency-shift rate, as it could not explain that only the second run showed a different behavior in this case. Fig. 5 shows the frequency responses of sensors #1 and #3 working at a NO gas concentration of 1770 ppm. The frequency shift for sensor #3 was 3990 Hz, however, it was 2514 Hz for sensor #1. This implies that sensor #3, made with a higher conductive ITO of 3000 S, exhibits higher sensitivity to NO than sensor #1 with ITO of 2000 S. Fig. 6 shows the frequency shift of sensors #1 and #3 working under different gas concentrations within the same time interval of 600 s. The results show that
sample #3 made with ITO, prepared at a hydrogen flow rate of 1.5 sccm, showed a large frequency response. In a previous work, we have shown that the presence of hydrogen during the film deposition increases film conductivity and reduces the ITO surface roughness [18,20]. As such, sensor #3 had a smoother surface than sensor #1. We know that the higher sensitivity (sensor #3) can be related to either the electrical or the surface morphologic properties of the ITO films. If the surface roughness plays a crucial role in the sensor response, in principle, sensor #1 should have bigger frequency shifts than sensor #3. However, this was not found in our experiment. Therefore, the different sensor sensitivities of these two samples may be mainly due to the difference in electric properties of the ITO electrodes. We know that the use of hydrogen in the film deposition creates addition number of oxygen vacancies leading to an enhancement in the ITO conductivity. In the NO detection, these oxygen vacancies could be involved in the processes of physical or/and chemical adsorptions of NO. An increase in
Fig. 6. Frequency shift of QCM gas sensors (#1 and #3) as a function of the NO concentration.
J. Hu et al. / Sensors and Actuators B 93 (2003) 175–180
179
suggesting that the gas adsorption/reaction occurred on the ITO surface. The existence of new NO3 species on the NO-exposed ITO surface indicates the reaction between NO and ITO, which causes the mass increase. The formation of new species may also change the mechanical and electronic properties of the ITO electrode. The possible ITO/quartz interfacial stress changes may also affect in the resonant frequency of the device. However, the precise verification of this interfacial effect is difficult to determine directly using XPS measurement. Behavior of ITO-QCM sensor is quite complicated. Apart from NO, some others, like water vapor and oxygen, may also influence the sensor response. For example, when the sensor is exposed to oxygen, negatively charged oxygen species could accumulate on ITO surface by grabbing electrons from the conduction band [24]. This causes the formation of an electron-depleted region near the ITO surface region and makes the ITO surface easier to absorb the target gases [24]. In order to avoid the interference, the use of dual sensors or operation of the sensors at two different temperatures [25] could be an alternative approach. The corresponding experiments are currently in progress.
4. Conclusions
Fig. 7. XPS spectra of (a) O 1s peak of ITO surface before and after NO adsorption and (b) N 1s peaks before and after the Arþ sputtering of NOexposed ITO surface.
oxygen vacancies may provide more adsorption/reaction sites, resulting in higher NO mass loading [19], or higher frequency shift. XPS has been used to examine the chemical binding energies of O 1s and N 1s for ITO films before and after NO exposure. Fig. 7(a) shows the O 1s peaks of ITO-film surface before and after NO exposure, and Fig. 7(b) shows the N 1s spectra of NO-exposed ITO surface, before and after the surface removal with argon ion sputtering. In Fig. 7(a), an extra shoulder was found in the O 1s profile from a NO-exposed ITO surface. This peak can be deconvoluted into two components with the binding energies located at 530.8 and 532.3 eV, respectively. The observed O 1s peak at 530.8–531.2 eV can be assigned to the lattice oxygen in In2O3 [21]. The peak at a binding energy of 532.3 eV can be attributed to a different oxidation state [22]. Fig. 7(b) suggest that the ITO surface may contain two different nitrogen species corresponding to the N 1s peaks at the binding energies of 407 and 400.5 eV, respectively. The weak peak at 400.5 eV may be attributed to the adsorbed NO. The strong peak at 407 eV may come from the NO3 species [23]. This suggests that new chemical species were formed on the ITO surface. After removing the surface layer, both of the two nitrogen peaks disappeared,
Gas sensors based on the ITO-coated quartz crystal microbalance have been fabricated. Such sensors are sensitive to NO gas with good repeatability at room temperature. The resonant frequency-changing rates of 0.2, 0.4, 0.7 and 2.0 Hz/s has been found when the sensor is exposed to NO gas at different concentrations of 60, 295, 590 and 1180 ppm in air, respectively. XPS analyses reveal that NO also reacts with ITO leading to the formation of NO3 species on its surface.
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