Quartz crystal microbalance sensor based on nanostructured IrO2

Quartz crystal microbalance sensor based on nanostructured IrO2

Sensors and Actuators B 122 (2007) 95–100 Quartz crystal microbalance sensor based on nanostructured IrO2 T.W. Chao a , C.J. Liu a,∗ , A.H. Hsieh a ,...

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Sensors and Actuators B 122 (2007) 95–100

Quartz crystal microbalance sensor based on nanostructured IrO2 T.W. Chao a , C.J. Liu a,∗ , A.H. Hsieh a , H.M. Chang b , Y.S. Huang b , D.S. Tsai a a b

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan Received 25 February 2006; received in revised form 10 May 2006; accepted 11 May 2006 Available online 27 June 2006

Abstract Nanostructured IrO2 crystals are grown on a gold-coated quartz substrate by metal organic chemical vapor deposition (MOCVD), and their gas sensing properties are studied by quartz crystal microbalance (QCM) technique. Several morphologies, such as nanoblade, layered-column, incomplete-nanotube and square-nanorod, are observed at various combinations of substrate temperature and precursor reservoir temperature. Propionic acid is found to be adsorbed and desorbed reversibly on the IrO2 surface at room temperature, and the adsorption property depends on the nanostructure of the IrO2 . IrO2 crystals with nanoblade and layered-column morphologies show higher sensitivities to propionic acid than those with incomplete-nanotube and square-nanorod morphologies. An IrO2 QCM sensor sensitive to ppm-level propionic acid vapor at room temperature is demonstrated. © 2006 Elsevier B.V. All rights reserved. Keywords: MOCVD; IrO2 ; QCM; Gas sensor

1. Introduction Metal oxides, such as SnO2 , ZnO, TiO2 and In2 O3 , have been used extensively for gas sensors [1–4]. Most of these oxides are semiconductors, of which the electrical conductivity varies with the composition of surrounding gas atmosphere. Major drawbacks of these conductivity-based sensors are poor gas selectivity and high operating temperature. The high operating temperature is required for charge carriers of the semiconductor to overcome the activation energy barrier [5,6]. Modifications using noble metal catalysts, such as Pt, or foreign oxides, such as ZnO or In2 O3 , have been developed to achieve higher gas sensitivity and lower sensing temperature [7–9]. On the other hand, mass-based gas sensors measure the adsorption process directly, and do not require semiconductor materials as the sensing layer. Thus, the quartz crystal microbalance (QCM) sensor can usually be operated at room temperature. Among metal oxides, IrO2 possesses certain unique properties. Besides high thermal and chemical stability, it exhibits metallic behavior in electrical and optical properties [10–12], and has been used for a wide range of applications, including



Corresponding author. E-mail address: [email protected] (C.J. Liu).

0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.05.009

sensing layers for pH sensors [13,14], switching layers for electrochromic devices [15,16], and durable electrodes for chlorine and oxygen evolution [17,18]. However, there are only a few reports on the gas sensing properties of IrO2 in literature [19,20]. Nanostructured IrO2 can be synthesized by a number of methods such as reactive magnetron sputtering [11,12,21], pulsed laser ablation [22,23], solution growth [17,24], and metal-organic chemical vapor deposition (MOCVD). Various nanoscaled morphologies have been observed for IrO2 grown on single crystal sapphire and LiNbO3 substrates [25,26] by MOCVD, and IrO2 nanorods with high aspect ratios and high surface areas have been utilized as the emitter material for field emission devices [27,28]. In the present paper, the morphology evolution of IrO2 grown on gold-coated quartz crystal by MOCVD is studied, and the initial results of a QCM propionic acid vapor sensor based on the nanostructured IrO2 are reported.

2. Experimental Iridium dioxide thin layers were grown on gold-coated piezoelectric quartz crystals via MOCVD using a precursor (methylcyclopentadienyl) (1,5-cyclooctadiene) iridium, (MeCp)Ir(COD), supplied by Strem Chemicals. The substrate temperature, Ts , was controlled at 350–500 ◦ C, the total cham-

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ber pressure was 7.2 Torr, and high purity oxygen (99.999%) was used as the carrier gas with a flow rate set at 100 sccm. The oxygen flow carried the organometallic vapor from a precursor reservoir through a heated transport line. The precursor reservoir temperature, Tp , was maintained at 95–105 ◦ C and the temperature of the transport line was maintained to be higher by 10 ◦ C than that of the reservoir to prevent condensation. Details of the MOCVD system were described previously [29]. Morphology of the deposited IrO2 layer was examined by a JEOL-JSM6500F field-emission scanning electron microscope (FESEM). XRD patterns were recorded on a Rigaku DMAX-8 spectrometer equipped with a Cu K␣ source. The piezoelectric quartz crystal (AT-cut, 10 MHz) was purchased from Tai Then Electric, Taiwan, with both sides electroplated with circular gold electrodes (4.4 mm in diameter). The VOC (volatile organic compound) gas sensing experiments were performed with a QCM analyzer (SB01B, Smart Biotechnology). Piezoelectric quartz crystals were mounted inside a 1.5 L water-jacketed Pyrex detection chamber; temperature of the chamber was maintained at 30 ± 0.1 ◦ C. The chamber was first purged with N2 (99.99% purity) and 5–20 ␮L of VOC was introduced into it with a micro-syringe. After complete volatilization

of VOC, an on-line computer recorded the oscillating frequency of the quartz crystal in response to gas adsorption. The VOC concentration was calculated according to the ideal gas law as c=

22.4ρTVs × 103 273MV

(1)

where c is the concentration in ppm, ρ the density of the liquid sample in g/mL, T the temperature of the detection chamber in K, Vs the volume of the liquid sample in ␮L, M the molecular weight of the sample in g, and V is the chamber volume in L. After the experiment, the chamber was again purged with N2 to desorb the VOC from the surface. 3. Results and discussion IrO2 crystals were grown on one side of the gold-coated quartz substrate by MOCVD at various combinations of substrate temperature and degree of supersaturation. The degree of supersaturation, which depends on the arrival rate of the precursor flux at the substrate surface, was varied via adjusting the temperature of precursor reservoir [26]. FESEM images in

Fig. 1. Field-emission scanning electron microscope images of the IrO2 deposited on the gold-coated quartz substrates by MOCVD at various Ts and Tp combinations. (a) Ts = 350 ◦ C, Tp = 95 ◦ C; (b) Ts = 350 ◦ C, Tp = 105 ◦ C; (c) Ts = 450 ◦ C, Tp = 105 ◦ C; (d) Ts = 500 ◦ C, Tp = 95 ◦ C.

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Table 1 Types of IrO2 morphologies deposited at various substrate temperature and precursor reservoir temperature combinations Ts (◦ C)

500 450 400 350

Tp (◦ C) 95

105

115

130

NR NB + IT NB NB

NR NB + IT NB LC

NB + IT NB LC

LC

NR: nanorod; IT: incomplete-nanotube; NB: nanoblade; LC: layered-column.

Fig. 1 show the evolution of several morphologies. The IrO2 crystals grown at Ts = 350 ◦ C and Tp = 95 ◦ C consist of individual M-shaped nanoblades with widths around 180 nm, as shown in Fig. 1(a). As the degree of supersaturation increases, at Ts = 350 ◦ C and Tp = 105 ◦ C, the M-shaped nanoblades clustered together to form layered-columns, with column edges around 300 nm, as shown in Fig. 1(b). On the other hand, by increasing the substrate temperature, at Ts = 450 ◦ C and Tp = 105 ◦ C, IrO2 nanoblades mixed with incomplete and scrolled nanotubes are found as shown in Fig. 1(c). At even higher substrate temperature and lower degree of supersaturation, at Ts = 500 ◦ C and Tp = 95 ◦ C, nanorods with a square cross section with edges around 60 nm are formed, as shown in Fig. 1(d). Table 1 summarizes the different morphologies observed at various combinations of substrate temperature and degree of supersaturation. The layered-column morphology can be found in the low Ts –high Tp region, the nanoblade morphology is found in the slightly higher Ts or lower Tp region, the incompletenanotube morphology (in coexistence with the nanoblades) appears in the even higher Ts region, and the square-nanorod morphology exists in the high Ts –low Tp region. The existence of IrO2 crystals is confirmed by X-ray diffraction data. Fig. 2 shows the typical XRD pattern of IrO2 crystals grown on the gold-coated quartz substrate. The crystal plane (1 0 1) at 2θ = 34.7◦ is preferentially oriented in the nanostructured IrO2 layer. Other rutile IrO2 reflections are also identified, i.e. 2θ = 28.1◦ for IrO2 (1 1 0), 2θ = 40.1◦ for IrO2 (2 0 0), and 2θ = 73.2◦ for IrO2 (2 0 2). The mass of the IrO2 deposited on the QCM electrode, as well as the mass of the VOC gas adsorbed on the IrO2 layer, can be measured according to the Sauerbrey equation [30]   m −6 2 (2) f = −2.3 × 10 f0 AQCM where AQCM is the area of the QCM electrode in cm2 , f0 the fundamental frequency of quartz crystal in Hz, and m is the increase of the oscillating mass in g. The frequency shift due to the deposition of IrO2 is denoted fIrO2 , the frequency shift due to the adsorption of VOC molecules is denoted fv , and the minus sign is usually omitted for convenience. The linear relationship between f and m in the Sauerbrey equation holds when f < 0.02f0 . For the 10 MHz crystal used in this study, the theoretical upper limit for the linear range in Sauerbrey equation is 200 kHz [31].

Fig. 2. The typical X-ray diffraction pattern of the IrO2 deposited on the goldcoated quartz substrate.

The growth of the IrO2 on the Au surface can be monitored with the QCM method. The value of ΔfIrO2 can be converted into the IrO2 mass loading according to the Sauerbrey equation, i.e. fIrO2 = 1 kHz is equivalent to 0.66 ␮g of IrO2 . IrO2 crystals with different morphologies exhibit different growth kinetics, as shown in Fig. 3. For the layered-column and the incomplete-nanotube morphologies, IrO2 crystals grow roughly linearly with time at 2.4 kHz/min for the 0.15 cm2 electrode surface, i.e. 11 ␮g/min/cm2 , which corresponds to approximately 9.4 nm/min assuming a hypothetically uniform layer and the density of 11.7 g/cm3 (data from MSDS). For the squarenanorod morphology deposited at Ts = 500 ◦ C and Tp = 95 ◦ C, the growth rate seems to slow down after 60 min, and for the nanoblade morphology deposited at Ts = 350 ◦ C and Tp = 95 ◦ C,

Fig. 3. The growth curves of IrO2 with different morphologies: () Ts = 350 ◦ C, Tp = 95 ◦ C; (䊉) Ts = 350 ◦ C, Tp = 105 ◦ C; () Ts = 450 ◦ C, Tp = 105 ◦ C; (♦) Ts = 500 ◦ C, Tp = 95 ◦ C.

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Fig. 5. Comparison of molar adsorptivities of the IrO2 sensor towards different VOCs at 1000 ppm concentration.

Fig. 4. Typical gas response curves for the IrO2 sensor. (a) Upon exposure to 1000 ppm propionic acid and (b) 1000 ppm n-hexylamine. IrO2 was deposited at Ts = 350 ◦ C, Tp = 95 ◦ C.

the growth rate seems to increase after 50 min. The mass measurement accuracy, however, decreases for fIrO2 > 200 kHz. Fig. 4 shows typical response curves of the QCM gas sensing experiment. Upon exposure to VOC, the resonant frequency of the IrO2 -coated quartz crystal decreases. For the adsorption experiment, the steady-state fv value is proportional to the mass of adsorbed VOC gas molecules according to the Sauerbrey equation, and the response time t80 is defined as the time it takes to reach 80% of the steady-state fv . For the desorption experiment, the detection chamber is flushed with N2 gas, and  is defined as the time required to recover the recovery time t80 80% of the steady state fv . For the IrO2 deposited at Ts = 350 ◦ C and Tp = 95 ◦ C, Fig. 4(a) shows that upon exposure to 1000 ppm  = 344 s. The propionic acid, fv = 103 Hz, t80 = 398 s and t80 reversibility of the sensor is indicated by the full recovery of the frequency during the desorption process. Fig. 4(b) shows that upon exposure to 1000 ppm n-hexylamine, fv = 101 Hz, t80 = 192 s. However, the irreversibility of the sensor is evident, since about 26% of the fv due to adsorption cannot be recovered. Blank experiments were carried out on naked gold-coated quartz substrates towards various VOCs at 1000 ppm concentration. Results show that (fv )0 = 9 Hz for propionic acid and (fv )0 = 25 Hz for n-hexylamine. Since the QCM substrates are coated with gold on both sides, the above (fv )0 values should be divided by 2 to account for the adsorption on the blank gold

surface while the sensing experiment was performed on the other side, which was covered with IrO2 . Thus, for propionic acid, 4.5 Hz out of 103 Hz, i.e. 4% of the signal in Fig. 4(a) is due to adsorption on the Au surface. For n-hexylamine, that is 12.5 Hz out of 101 Hz, i.e. 12% of the signal in Fig. 4(b) is due to adsorption on the Au surface. Blank experiments were also carried out for other VOCs: (fv )0 is 7 Hz for amylamine, 5 Hz for propanol, m-xylene or octane, and zero or less than the noise level of 1–2 Hz for all other VOCs tested in this study. For comparison of sensitivities of a QCM sensor towards different VOCs, the molar adsorptivity Am = fv /M is defined, where M is the molecular weight of the VOC. Fig. 5 shows the relative molar adsorptivities of the IrO2 sensor towards various VOCs. As most metal oxide sensors [8], IrO2 shows rather poor VOC selectivity. However, higher Am values are observed for carboxylic acids and amines, presumably due to the acid–base interaction with the oxide surface. On the other hand, the molar adsorptivities of IrO2 towards alkanes and benzene derivatives are much smaller. Fig. 6 shows that the amount of propionic acid adsorbed is proportional to the mass loading of IrO2 for various morphologies. IrO2 of the nanoblade morphology shows the highest sensitivity, followed by the layered-column IrO2 , and the incomplete-nanotube IrO2 and the square-nanorod IrO2 show much lower sensitivities. The difference in adsorptivity could be due to the difference in surface area or the difference in crystal faces exposed among various morphologies. Presumably the incomplete-nanotube morphology (Fig. 1(c)) has a lower surface area than the nanoblade or the layered-column morphologies (Fig. 1(a) and (b)), but direct measurements of surface areas by BET method were prevented due to the insufficient amount of IrO2 materials on the substrates. Fig. 7 shows the fv versus propionic acid concentration relationship in the 50–1000 ppm range for the nanoblade IrO2 deposited at Ts = 350 ◦ C and Tp = 95 ◦ C,

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ulating the substrate temperature and the precursor supply rate. Quartz crystal microbalance technique is utilized to monitor the IrO2 growth kinetics and to exploit the potential application of the nanostructured IrO2 as a VOC gas sensor. We find that propionic acid can be adsorbed and desorbed reversibly on the IrO2 surface at room temperature. The comparison of IrO2 with various morphologies demonstrates the effect of nanostructure on the gas sensing properties. IrO2 with nanoblade and layered-column morphologies show higher propionic acid sensitivities than those of IrO2 with incomplete-nanotube and square-nanorod morphologies. A room temperature propionic acid vapor sensor based on IrO2 is developed. Acknowledgments Fig. 6. IrO2 mass loading dependence of the QCM sensor response towards 1000 ppm propionic acid. IrO2 with different morphologies were deposited at: () Ts = 350 ◦ C, Tp = 95 ◦ C; (䊉) Ts = 350 ◦ C, Tp = 105 ◦ C; () Ts = 450 ◦ C, Tp = 105 ◦ C; (♦) Ts = 500 ◦ C, Tp = 95 ◦ C.

This work was supported by the National Science Council of Taiwan under project nos. NSC 93-2120-M-011-001 and NSC 94-2120-M-011-001. References

Fig. 7. Concentration dependence of the IrO2 QCM response towards propionic acid. The IrO2 was deposited at Ts = 350 ◦ C and Tp = 95 ◦ C.

and operated at room temperature. It has been shown recently that the operating temperature of metal oxide sensors can be lowered by oxide modification. For example, a SnO2 –ZnO composite sensor exhibits the maximum sensitivity at 250 ◦ C, which is much lower than 350 ◦ C for the pure SnO2 , for 50 ppm H2 S [8], and an In2 O3 -doped SnO2 sensor exhibits the maximum sensitivity at 75 ◦ C for 50 ppm H2 [9]. With QCM detection, a ZnO-nanowire-based sensor has been reported to show high sensitivity to ammonia in the range of 40–1000 ppm at room temperature [32]. Our data indicate that it is also possible to develop a room temperature QCM sensor based on the nanostructured IrO2 for the detection of ppm-level carboxylic acid vapor. Further studies are needed to elucidate the gas sensing mechanism, and to optimize the sensor design for better performance. 4. Conclusion Nanostructured IrO2 crystals were grown on gold-coated quartz substrate by MOCVD. Several morphologies are observed under various supersaturation conditions by manip-

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Biographies T.W. Chao received his M.S. degree in Chemical Engineering in 2005 from National Taiwan University of Science and Technology at Taipei, Taiwan. C.J. Liu received his Ph.D. degree in Physical Chemistry in 1978 from Washington University at St. Louis, USA. He is currently a professor with Department of Chemical Engineering, National Taiwan University of Science and Technology at Taipei, Taiwan. His research interests include chemical sensor, organic thin film deposition and analysis of the thin film microstructure. A.H. Hsieh is a graduate student in the Chemical Engineering Department at National Taiwan University of Science and Technology, Taipei, Taiwan. H.M. Chang received his M.S. degree in Electronic Engineering in 2005 from National Taiwan University of Science and Technology at Taipei, Taiwan. Y.S. Huang received his Ph.D. degree in Physics in 1981 from Boston College, USA. He is currently a professor with Department of Electronic Engineering, National Taiwan University of Science and Technology at Taipei, Taiwan. His research interests include crystal growth, electronic materials, and characterization of semiconductors. D.S. Tsai received his Ph.D. degree in Chemical Engineering in 1985 from The University of Notre Dame, USA. He is currently a professor with Department of Chemical Engineering, National Taiwan University of Science and Technology at Taipei, Taiwan. His research interests include ceramic materials, low-dimensional materials, piezoelectric materials and fuel cells.