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Gas sensing characteristics of multi-wall carbon nanotubes
Sensors and Actuators B 81 (2001) 32±41
Gas sensing characteristics of multi-wall carbon nanotubes O.K. Varghesea, P.D. Kichambreb, D. Gongc, K.G. Onga, E.C. Dickeyc, C.A. Grimesa,* a
Department of Electrical Engineering & Materials Research Institute, 204 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b Department of Electrical & Computer Engineering, The University of Kentucky, Lexington, KY 40506, USA c Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 11 May 2001; received in revised form 16 August 2001; accepted 18 August 2001
Abstract Impedance spectroscopy was used to study the gas sensing behavior of both capacitance and resistance based sensors employing multiwall carbon nanotubes (MWNTs) as the active sensing element. Studies revealed the chemisorption of reducing gases upon the surface of the MWNTs. Increasing sensor impedance was observed with increasing humidity or partial pressures of ammonia, carbon monoxide, and carbon dioxide. The impedance changes are attributed to p-type conductivity in semiconducting MWNTs, and the formation of Schottky barriers between the metallic and semiconducting nanotubes. Reversible behavior is demonstrated for the MWNT sensors in response to humidity, carbon monoxide and carbon dioxide. The MWNT sensors strongly respond to ammonia behaving as dosimeters. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Gas sensor; Impedance
1. Introduction Carbon nanotubes [1] have received considerable attention due to their interesting physical properties [2±21]. A variety of applications have been considered including gas sensing [22,23], nano electronics [24±28], gas storage [29,30] and ®eld emission devices [31±33]. Gas sensing at room temperature is of great interest; most currently available sensors, except a few types of polymer based gas sensors, operate at elevated temperatures [34,35]. In this context, carbon nanotubes are promising due to their high surface areas, provided by their central hollow cores and the outside walls, for gas adsorption as well as their tendency to change electrical properties at room temperature in the presence of different gases [22,23,36±39]. To date reported gas sensing studies have been based either on isolated single wall carbon nanotubes (SWNTs) or on SWNT mats [22,23,37,38]. Here we report the gas sensing properties of carbon multi-wall nanotubes (MWNTs) in two sensor designs. One is a planar interdigital capacitor upon which a MWNT-SiO2 composite ®lm is placed, see Fig. 1. The second sensor design is a MWNT serpentine resistor, fabricated by photolithographically de®ning a serpentine
* Corresponding author. E-mail address: [email protected] (C.A. Grimes).
SiO2 path upon silicon, and then growing MWNTs upon the SiO2 structure, see Fig. 2. To better understand the physical sensing mechanisms of MWNTs we used impedance spectroscopy, rather than the conventional dc measurements, for our studies. Impedance spectroscopy [40] enables the representation of different gas±solid phenomena in terms of electrical equivalent circuits, allowing the results to be interpreted in a meaningful way. 2. Experimental 2.1. Nanotube growth The general method of fabricating the MWNTs used in this work has been reported earlier [41] and is brie¯y summarized here. The MWNTs were grown by pyrolysis of ferrocene and xylene under Ar/H2 atmosphere in a twostage reactor. Ferrocene has been shown to be an excellent precursor for producing Fe catalyst particles, and xylene was used as the hydrocarbon source. The liquid feed was passed through a capillary tube and preheated to 175 8C prior to entry into the reaction chamber kept at 750 8C. At this temperature, the liquid exiting the capillary was immediately volatilized and swept into the reaction zone of the furnace by a ¯ow of argon with 10% hydrogen. Prior to deposition, the substrates were cleaned in acetone, ethanol
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 2 3 - 6
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Fig. 1. Schematic diagram of the capacitive sensor. (a) The planar interdigital capacitor electrode and (b) cross-section of the sensor showing the gas sensing MWNT-SiO2 composite placed over a thin electrically insulating SiO2 layer and interdigital-electrodes.
and deionized water and subsequently dried by ¯ushing with dry argon and then mounted in the reactor. The deposition of MWNTs over quartz substrates was achieved by continuously feeding 6.5 mol% of ferrocene in xylene for 120 min, while the selective deposition of MWNTs upon the SiO2 structure was carried out by feeding approximately 0.325 mol% of ferrocene in xylene for 60 min (ferrocene concentrations above 0.325 mol% were found to grow carbon nanotubes over both patterned and un-patterned areas). After the reaction, the pre-heater and the furnace were allowed to cool to room temperature in ¯owing argon. 2.2. Sensor fabrication Planar, interdigital-electrode capacitors are commonly used in gas sensors [34]. This geometry keeps the impedance of the sensor low, avoiding problems arising with high impedance measurements [40], and provides maximum surface area for the sensor to interact with a gas. This geometry was selected for one of our sensor designs. Our preliminary studies on MWNTs grown over quartz using a parallel electrode geometry demonstrated high conductivity, which made an interdigital-electrode geometry unsuitable for the sensor with as-grown MWNTs (or MWNT mats) as the sensing element. Hence, we used a MWNT and SiO2 composite as the gas sensing element, placed in a thin layer upon the interdigital-electrodes.
Fig. 2. (a) Digital image of the resistive sensor showing the confinement of the MWNTs to the patterned serpentine silicon dioxide pattern. The silicon substrate is the bright portion in the background. (b) Schematic diagram of the serpentine resistor pattern, and (c) a cross-sectional view.
The copper electrodes of the interdigital capacitive sensor, see Fig. 1, were formed on a copper-clad printed circuit board using photolithography. Prior to application of the gas sensing MWNT-SiO2 composite, to prevent shorting of the electrodes by the MWNTs the sensor was ®rst coated with a thin layer of SiO2. The gas sensing MWNT-SiO2 composite was made in the following manner: carbon nanotubes were removed from fused silica substrates and dispersed in toluene using an ultrasonic bath. The individual nanotubes thus obtained were rinsed with isopropanol and allowed to dry. The nanotubes were then dispersed in a SiO2 containing solution (20% SiO2 nanoparticles dispersed in water [42]) so as to obtain a dry-weight balance of 2:3. This solution was spin-coated onto the thin SiO2 layer deposited over the
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Fig. 3. (a) High resolution SEM image of the MWNTs grown over silicon dioxide, (b) a further magnified view showing the entanglement of the carbon nanotubes.
electrodes. The schematic diagrams of the interdigital-electrode pattern are shown in Fig. 1. In addition to the capacitive sensor, we used a serpentine resistor geometry upon which MWNTs were grown, providing a low conductivity path without compromising gas sensing surface area. The resistive sensor, shown in Fig. 2, was made in the following manner: a thick layer of silicon dioxide was grown on a silicon substrate using thermal oxidation. A SiO2 serpentine pattern (total path length 45 cm, arm width 350 mm, spacing gap between the arms 290 mm, overall dimensions 1:7 cm 1:7 cm) was de®ned from this layer by photolithography. MWNTs were then grown upon this patterned silicon dioxide layer as described in the previous section. It was observed that the nanotubes were formed exactly on the patterned SiO2 layer avoiding silicon completely. Fig. 3 shows SEM images of MWNTs grown upon the SiO2 layer. The nanotubes were found to have a diameter of 20±35 nm with lengths of a few microns; the nanotubes were not aligned normal to the substrate as they are when grown on quartz substrates [41]. 2.3. Experimental set-up The sensors were placed in a sealed Plexiglas chamber of 60 cm3 volume. Argon was used as the carrier gas throughout the work. For humidity sensing, argon was bubbled through a bottle containing water and then mixed with dry argon. The total ¯ow rate was constant at 1000 sccm, and the partial pressures of the test gases controlled with a mass ¯ow controller. Electrical contacts were made to the sensors by connecting two short copper wires to the electrodes using silver paste. For the serpentine resistor sensor, silver paste contacts were made directly to the top of the nanotubes grown over silicon dioxide. A Hewlett Packard 4192A impedance analyzer ®tted with a test ®xture (Agilent 16034E) was
employed for impedance measurements. Data was collected using a signal amplitude of 90 mV over the frequency range 5±13 MHz. All experiments were conducted at room temperature (21 8C), with the same sensor used for all measurements. Prior to each measurement the sensors were heated in vacuum at 100 8C for 1 h to remove the chemisorbed molecules. 3. Experimental results The chamber was initially ¯ushed with argon and the impedance of the two sensors measured. The total impedance (Z) was resolved into real (Z0 ) and imaginary (Z00 ) parts to construct Cole±Cole impedance plots. Figs. 4 and 5 represent the Cole±Cole plots of the capacitive and resistive sensors, respectively, in the argon environment. The experimental data was ®tted using the complex non-linear least square ®tting program, LEVM, provided by Prof. Macdonald and Solartron Analytical [43]. For all impedance plots, the dots represent experimental data and the solid-line the equivalent circuit ®t. Referring to the circuit models, R0 denotes a frequency independent Ohmic resistance, and R1 and R2 two frequency independent resistances in parallel with two frequency dependent capacitors, Cn1 (o) and Cn2 (o), respectively. According to the concept of a `universal dielectric response' introduced by Jonscher [44,45], a dispersive frequency dependent capacitance Cn(o) in parallel with a frequency independent resistance represents a semicircle in the Cole±Cole plot with its center depressed below the real axis by an angle (n 1)p/2, where n lies between 0 and 1. This non-Debye capacitance can be written as Cn o Bn (io)n 1 where Bn is a constant. Bn takes the form of an ideal frequency independent capacitance as n approaches 1. With reference to Fig. 5, the parallel combinations of
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Fig. 4. Cole±Cole plot of the capacitive sensor kept in argon environment. The equivalent circuit used for fitting the experimental data is shown adjacent to the plot.
Bn1 R1 and Bn2 R2 represent two different phenomena occurring in the material, with two different relaxation times and hence two semicircles. If the respective relaxation times t1and t2 given by t1 R1 Bn1 1=n1 and t2 R2 Bn2 1=n2 are close, as is the case for the resistive sensor, then the two semicircles overlap and it is dif®cult to distinguish between them [40], see Fig. 5. Fitting of experimental data points resulted in a n1 value of 0.96 for the capacitive sensor (Fig. 4), and for the resistive sensor (Fig. 5) values of n1 0:99 and n2 0:81; all semicircles have their centers depressed below the real axis. The n values lead to calculation of relaxation times
t1 0:11 ms for the capacitive sensor, and t1 4:3 ms and t2 1:22 ms for the resistive sensor. The relaxation times of the resistive sensor are close enough that the semicircles overlap. Since the relaxation times of each sensor are small, the sensing mechanism is inferred to be charge transfer rather than mass transport [40,46]. Therefore, R1 and R2 represent a resistance to charge transfer while Bn1 and Bn2 behave as a double layer capacitance. The capacitive sensor has a Bn1 value of 3:33 10 11 , while the resistive sensor has Bn1 5:67 10 10 and Bn2 2:87 10 9 . Cole±Cole plots of the capacitive sensor at different humidity levels are shown in Fig. 6. The impedance of
Fig. 5. Cole±Cole plot of the resistive sensor kept in argon environment. The equivalent circuit used for fitting experimental data is shown adjacent to the plot.
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Fig. 6. Cole±Cole plots of the capacitive sensor at different humidity levels.
the capacitive sensor is highly sensitive to humidity, increasing with water content, as evidenced by the large change in the arc radius. It can be seen from the equivalent circuit (Fig. 4) that at quasi-static frequencies R0 and R1 limit the current and hence the sum of these resistances equal the resistance obtained from a dc measurement. The sensitivity S can be de®ned as:
Fig. 8. Cole±Cole plots of the resistive sensor kept in presence of dry argon (0% water vapor content) and argon bubbled through water (100% water vapor content).
where Rargon is the resistance in the presence of argon, and Rgas R0 gas R1 gas is the resistance in the presence of a test gas. Fig. 7 shows the variation in both percentage sensitivity S and R1 as a function of water vapor concentration. The non-linear increase in R1 with humidity indicates that charge transfer is more difficult at higher humidity levels. R0 was found to be virtually independent of humidity with an average value of 87 O. Fig. 8 shows the Cole±Cole plot of the resistive sensor kept at different humidity levels. The percentage sensitivity
S for the resistive sensor is calculated using Eq. (1) with Rgas R0 gas R1 gas R2 gas . The variations in sensitivity S as well as R1 and R2 of this sensor in a humid atmosphere are shown in Fig. 9. Both R1 and R2 increase with rising humidity, with R2 increasing at a greater rate than R1 and hence playing a larger role in determining sensitivity. MWNTs demonstrate a considerable af®nity for water vapor; the response time of both sensors to increasing humidity was approximately 2±3 min, however the recovery time was found to be on the order of a few hours. In contrast with the results obtained by other research groups [23,37,38] neither sensors showed a measurable sensitivity to oxygen. However, these earlier oxygen studies used single wall carbon nanotubes (SWNT) and perhaps more importantly evacuated the chamber before and after exposure of the sensor to oxygen. In this work the sensors were exposed to ambient atmosphere, then ¯ushed with argon prior to measurements in a particular gaseous environment. The Cole±Cole plots of the capacitive sensor in response to CO2 are shown in Fig. 10, with the associated variations in
Fig. 7. The sensitivity S, and resistance R1 of the capacitive sensor in response to water vapor content.
Fig. 9. The sensitivity S, R1 and R2 of the resistive sensor to water vapor content.
S
Rgas Rargon 100% Rargon
(1)
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Fig. 10. Cole±Cole plots of the capacitive sensor in dry argon (0% CO2) and a pure CO2 atmosphere.
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Fig. 13. Cole±Cole plots of the capacitive sensor in response to varying ammonia concentrations.
Fig. 14. The sensitivity S and R1 of the capacitive sensor to ammonia content in dry argon. Fig. 11. The sensitivity S and R1 of the capacitive sensor at different CO2 concentrations (carrier gas is dry argon).
sensitivity S and R1 plotted in Fig. 11; the sensor did not show a measurable response to CO. This is opposite to the response of the resistive sensor, which did not show a measurable response to CO2, but did to CO as shown in Fig. 12. Fig. 13 shows the Cole±Cole plot of the capacitive sensor in response to varying ammonia concentrations. In contrast to the responses seen for water and carbon dioxide, as seen in Fig. 14 there is an almost linear increase in both sensitivity and R1 due to the presence of ammonia. The behavior of both sensors to ammonia is that of a dosimeter; the response time of the sensor to increasing ammonia concentration was approximately 2±3 min, however it took the sensor several days kept in vacuum at 100 8C to recover the original response. 4. Discussion
Fig. 12. Cole±Cole plot of the resistive sensor in response to CO.
Multi-wall carbon nanotubes grown over fused silica substrates [41] are conductive, appear as a frequency independent
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resistor in the Cole±Cole plot, and do not show sensitivity to any of the gases used in the present study. This is in contrast with the results obtained when these nanotubes were mixed with SiO2, as well as when they were grown upon a patterned silicon dioxide substrate. In these cases, arcs appeared in the Cole±Cole plot (Figs. 4 and 5) which clearly indicate the presence of a capacitive element due to the presence of some semiconducting or non-conducting elements in the two sensors. According to Langer et al. [47] MWNTs behave like rolled up graphene sheets with an electrical conductivity similar to crystalline graphite at room temperature or higher, with an energy band overlap similar to bulk graphite. As in graphite [18,48], MWNTs contain both holes and electrons but at room temperature generally show a metallic behavior with electrons as majority carriers due to the overlapping conduction and valence bands which varies with nanotube diameter and helicity [18,47]. MWNTs can show conductivity values in the semiconducting range [49], with the energy band overlap varying according to the nanotube chirality and hence the interaction between the different walls of the MWNT. Langer et al. [47] observed a band overlap of 3.7 meV, almost a factor of ten less than graphite, due to this interlayer interaction. The presence of defects may also reduce the conductivity of MWNTs [18,50]. Therefore, with reference to the arc in the Cole±Cole plots, it appears that semiconducting MWNTs are present among the metallic MWNTs within the sensors giving rise to a capacitive component, with the dielectric constant of the semiconducting MWNTs changing in response to gas concentrations. In the case of thick MWNT mats grown over fused silica [41] the large abundance of metallic MWNTs completely screens the effect of any semiconducting MWNTs. For the capacitive sensor, it is the dispersion of MWNTs in SiO2 which isolates the semiconducting MWNTs. For the resistive sensor, the growth density of the MWNTs (see Fig. 3) is considerably less than that of the MWNT mats reported in [41] due to a smaller amount of catalyst and shorter reaction time, which makes it possible for semiconducting MWNTs to be present without being electrically shorted. For understanding the mechanism of gas sensing, the variation of Bn, n and t for the two sensors in the presence of different gases are analyzed. Values of Bn1 and n1 for the capacitive sensor in presence of water vapor and ammonia are plotted in Figs. 15 and 16, respectively, with the corresponding relaxation times shown in Fig. 17. Fig. 18 is a plot of Bn1 , Bn2 , t1 and t2 of the resistive sensor versus humidity. It can be seen from Figs. 15 and 18 that Bn1 (and Bn2 ) for both sensors increase with increasing humidity or ammonia levels, while n1 decreases in both cases. Furthermore the variation in relaxation times t1 (and t2), Figs. 17 and 18, parallels the changes in R1 (and R2). The results indicate that there is charge transfer between the test gases and sensing element, and hence that chemisorption [34,35] of gases on the MWNTs is the dominant sensing mechanism.
Fig. 15. Variation of Bn1 and n1 of the capacitive sensor as a function of water vapor content.
Fig. 16. Variation of Bn1 and n1 of the capacitive sensor as a function of ammonia concentration.
There are two possible mechanisms in which gases can reversibly interact with the nanotubes, physisorption which does not involve charge transfer [34], and/or chemisorption which does [35]. The operational principle behind many ceramic humidity sensors is physisorption of water molecules on an initially chemisorbed hydroxyl layer [51,52]. The physisorbed water layer enables charge transport through the surface reducing the overall resistance of the sensor [53]. The process of physisorption cannot explain the
Fig. 17. Relaxation times of the capacitive sensor as a function of water vapor and ammonia concentration.
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Fig. 18. Variations of Bn1 and Bn2 , and relaxation times t1 and t2, with water vapor for the resistive sensor.
present results since R1 (R1 and R2 for the resistive sensor) increases with humidity. There exists a ®nite probability of ®nding chemisorbed gases at the surface of semiconductor materials, even at room temperature, due to presence of highly active surface sites or surface defects [34]. Since the presence of ammonia, carbon monoxide and carbon dioxide lower the conductance of the sensors, it is evident that chemisorption of these gases or water molecules takes place on the MWNT surface. However, for n-type MWNTs the chemisorption of these reducing gases would have increased the electrical conductivity due to the increase in conduction band electrons. Hence, it appears that p-type semiconductor behavior is present in the MWNTs. For p-type MWNTs the adsorbed water molecules, or the reducing gas molecules, donate electrons to the valence band thereby increasing the electrical resistance. Kong et al. [22] also observed an increase in resistance in their p-type SWNT based transistors in presence of ammonia. For both sensors, the fact that R1 (and R2) as well as Bn1 (and Bn2 ) increase in the presence of the test gases shows that the capacitance arises independently of the resistance, and the rapid relaxation time (1 ms) indicates that these terms are driven by charge transfer phenomenon. The interaction mechanism of the MWNTs with the test gases can be explained as follows. It appears that semiconducting nanotubes are sandwiched between metallic ones, which gives rise to Bn1 and R1. In presence of water vapor, ammonia, carbon dioxide or carbon monoxide chemisorption takes place at the outer surface of the semiconducting MWNT. The chemisorbed reducing gases donate electrons to the valence band decreasing the number of holes, thereby increasing the separation between the Fermi level and valence band [22,39,54,55]. This forms a space charge region, i.e. Schottky barrier, at the surface of the semiconducting MWNT, increasing Bn1 and R1. It is noted that the nanotubes contain catalyst particles embedded inside them, mostly at the tip. Hence this capacitance can be attributed to a Schotttky barrier either at the contact region of a metallic MWNT and a semiconducting MWNT [57], or where the
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catalyst particles are present [28] within the MWNTs. The absorbed gases modulate the existing space charge region of the junction, the same principle used in gas sensing FETs [58]. SiO2 is known to be a good adsorber of water molecules, and hence can play a role in determining gas sensitivity in both sensors. However, physisorption of water on SiO2 leads to protonic conduction on the surface [51,56], increasing the electrical conductivity. In the present case a decreasing electrical conductance with humidity shows that SiO2 has no direct effect on the behavior. However, the adsorption of water molecules onto SiO2 may help the nanotubes to interact more easily with them [22]. While no oxygen sensitivity was observed in our sensors, it cannot be authentically stated that oxygen chemisorption is not occurring. Though we annealed the sensors in vacuum at 100 8C, this is normally not enough to remove the adsorbed oxygen species [34]. Moreover, oxygen adsorption generally takes place immediately upon exposure to atmosphere, and cannot be removed by argon ¯ushing. Thus, surface coverage might have attained a maximum value while loading the sensor inside the test chamber, hence a further increase in the oxygen concentration from the ambient cannot enhance the measured response. Finally, it should also be noted that NH3 may interact with MWNTs by replacing pre-adsorbed oxygen [22,59]. Oxygen, being an electron acceptor, increases the conductivity of p-type MWNTs as it increases the hole concentration; hence the replacement of oxygen by ammonia should reduce the conductivity. Since for both sensors Bn1 (and Bn2 ) increases with increasing ammonia concentration the interaction of ammonia with chemisorbed oxygen species can be ruled out. Consequently water vapor, ammonia, CO and CO2 appear to directly interact with the carbon nanotubes. 5. Conclusions We have explored the possibility of using MWNTs as room temperature gas/vapor sensors. Two sensor geometries were investigated, one capacitive with a MWNT-SiO2 composite placed over a planar interdigital capacitor, the other resistive with MWNTs grown upon a serpentine SiO2 pattern. Impedance spectroscopy was used to reveal the mechanism by which gases/vapors interact with MWNTs. The sensors were found to have high sensitivity to water vapor as well as ammonia, with the response to water vapor being readily reversible and the response to ammonia not, acting instead as a dosimeter. The experimental data of both sensors were simulated using appropriate equivalent circuit models, which enabled determination of the gas/ vapor sensing mechanism. Our results indicate the presence of p-type MWNTs dispersed among the predominant metallic MWNTs. Our results also indicate that chemisorption of gases on the surface of the semiconducting MWNTs is responsible for the sensing action. The presence of a Shottky
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barrier was found which may be due to the formation of such junctions between metallic and semiconducting MWNTs, or due to catalyst-MWNT junctions. Of the sensors tested the interdigital capacitor proved the more sensitive. Acknowledgements Support of this work by the National Science Foundation under contract DMR-9809686 and ECS-9988598 is gratefully acknowledged. References [1] S. Ijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56. [2] D. TomaÂnek, R.J. Enbody (Eds.), Science and Applications of Nanotubes, Kluwer Academic Publishers, New York, 1999. [3] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [4] M. Dresselhaus, G. Dresselhaus, P. Eklund, R. Saito, Carbon nanotubes, Phys. World 11 (1998) 33. [5] M.S. Dresselhaus, Down the straight and narrow, Nature 358 (1992) 195. [6] R.F. Service, Superstrong nanotubes show they are smart too, Science 281 (1998) 940. [7] P.M. Ajayan, O. Stephan, C. Colliex, D. Trauth, Aligned carbon nanotube arrays formed by cutting a polymer resin-nanotube composite, Science 265 (1994) 1212. [8] H.D. Wagner, O. Laurie, Y. Feldman, R. Tenne, Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix, Appl. Phys. Lett. 72 (1998) 188. [9] E.W. Wong, P.E. Sheehan, C.M. Lieber, Non-beam mechanics: elasticity, strength and toughness of nanorods and nanotubes, Science 277 (1997) 1971. [10] D. Qian, E.C. Dickey, R. Andrews, T. Rantell, Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites, Appl. Phys. Lett. 76 (2000) 2868. [11] Y.Y. Wei, X. Fan, G. Eres, Directed assembly of carbon nanotube circuits by selective area chemical vapor deposition on prepatterned catalyst electrode structures, J. Vacuum Sci. Technol. B 18 (2000) 3586. [12] J.N. Coleman, S. Curran, A.B. Dalton, A.P. Davey, B. McCarthy, W. Blau, R.C. Barklie, Percolation-dominated conductivity in a conjugated-polymer-carbon-nanotube composite, Phys. Rev. B 58 (1998) 7492. [13] T.W. Ebbeseb, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (1996) 54. [14] M. Bockrath, W. Liang, D. Bozovic, J.H. Hafner, C.M. Lieber, M. Tinkham, H. Park, Resonant electron scattering by defects in singlewalled carbon nanotubes, Science 291 (2001) 283. [15] S. Paulson, A. Helser, M.B. Nardelli, R.M. Taylor II, M. Falvo, R. Superfine, S. Washburn, Tunable resistance of a carbon nanotube± graphite interface, Science 290 (2000) 1742. [16] H. Dai, E.W. Wong, C.M. Lieber, Probing electrical transport in nanomaterials: condutivity of individual carbon nanotubes, Science 272 (1996) 523. [17] M. Bockrath, D.H. Cobden, P.L. McEuen, N.G. Chopra, A. Zettl, A. Thess, R.E. Smalley, Single-electron transport in ropes of carbon nanotubes, Science 275 (1997) 1922. [18] J.-C. Charlier, J.-P. Issi, Electronic structure and quantum transport in carbon nanotubes, Appl. Phys.: A Mater. Sci. Eng. 67 (1998) 79.
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Gas sensing characteristics of multi-wall carbon nanotubes O.K. Varghesea, P.D. Kichambreb, D. Gongc, K.G. Onga, E.C. Dickeyc, C.A. Grimesa,* a
Department of Electrical Engineering & Materials Research Institute, 204 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b Department of Electrical & Computer Engineering, The University of Kentucky, Lexington, KY 40506, USA c Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 11 May 2001; received in revised form 16 August 2001; accepted 18 August 2001
Abstract Impedance spectroscopy was used to study the gas sensing behavior of both capacitance and resistance based sensors employing multiwall carbon nanotubes (MWNTs) as the active sensing element. Studies revealed the chemisorption of reducing gases upon the surface of the MWNTs. Increasing sensor impedance was observed with increasing humidity or partial pressures of ammonia, carbon monoxide, and carbon dioxide. The impedance changes are attributed to p-type conductivity in semiconducting MWNTs, and the formation of Schottky barriers between the metallic and semiconducting nanotubes. Reversible behavior is demonstrated for the MWNT sensors in response to humidity, carbon monoxide and carbon dioxide. The MWNT sensors strongly respond to ammonia behaving as dosimeters. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon nanotubes; Gas sensor; Impedance
1. Introduction Carbon nanotubes [1] have received considerable attention due to their interesting physical properties [2±21]. A variety of applications have been considered including gas sensing [22,23], nano electronics [24±28], gas storage [29,30] and ®eld emission devices [31±33]. Gas sensing at room temperature is of great interest; most currently available sensors, except a few types of polymer based gas sensors, operate at elevated temperatures [34,35]. In this context, carbon nanotubes are promising due to their high surface areas, provided by their central hollow cores and the outside walls, for gas adsorption as well as their tendency to change electrical properties at room temperature in the presence of different gases [22,23,36±39]. To date reported gas sensing studies have been based either on isolated single wall carbon nanotubes (SWNTs) or on SWNT mats [22,23,37,38]. Here we report the gas sensing properties of carbon multi-wall nanotubes (MWNTs) in two sensor designs. One is a planar interdigital capacitor upon which a MWNT-SiO2 composite ®lm is placed, see Fig. 1. The second sensor design is a MWNT serpentine resistor, fabricated by photolithographically de®ning a serpentine
* Corresponding author. E-mail address: [email protected] (C.A. Grimes).
SiO2 path upon silicon, and then growing MWNTs upon the SiO2 structure, see Fig. 2. To better understand the physical sensing mechanisms of MWNTs we used impedance spectroscopy, rather than the conventional dc measurements, for our studies. Impedance spectroscopy [40] enables the representation of different gas±solid phenomena in terms of electrical equivalent circuits, allowing the results to be interpreted in a meaningful way. 2. Experimental 2.1. Nanotube growth The general method of fabricating the MWNTs used in this work has been reported earlier [41] and is brie¯y summarized here. The MWNTs were grown by pyrolysis of ferrocene and xylene under Ar/H2 atmosphere in a twostage reactor. Ferrocene has been shown to be an excellent precursor for producing Fe catalyst particles, and xylene was used as the hydrocarbon source. The liquid feed was passed through a capillary tube and preheated to 175 8C prior to entry into the reaction chamber kept at 750 8C. At this temperature, the liquid exiting the capillary was immediately volatilized and swept into the reaction zone of the furnace by a ¯ow of argon with 10% hydrogen. Prior to deposition, the substrates were cleaned in acetone, ethanol
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 2 3 - 6
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Fig. 1. Schematic diagram of the capacitive sensor. (a) The planar interdigital capacitor electrode and (b) cross-section of the sensor showing the gas sensing MWNT-SiO2 composite placed over a thin electrically insulating SiO2 layer and interdigital-electrodes.
and deionized water and subsequently dried by ¯ushing with dry argon and then mounted in the reactor. The deposition of MWNTs over quartz substrates was achieved by continuously feeding 6.5 mol% of ferrocene in xylene for 120 min, while the selective deposition of MWNTs upon the SiO2 structure was carried out by feeding approximately 0.325 mol% of ferrocene in xylene for 60 min (ferrocene concentrations above 0.325 mol% were found to grow carbon nanotubes over both patterned and un-patterned areas). After the reaction, the pre-heater and the furnace were allowed to cool to room temperature in ¯owing argon. 2.2. Sensor fabrication Planar, interdigital-electrode capacitors are commonly used in gas sensors [34]. This geometry keeps the impedance of the sensor low, avoiding problems arising with high impedance measurements [40], and provides maximum surface area for the sensor to interact with a gas. This geometry was selected for one of our sensor designs. Our preliminary studies on MWNTs grown over quartz using a parallel electrode geometry demonstrated high conductivity, which made an interdigital-electrode geometry unsuitable for the sensor with as-grown MWNTs (or MWNT mats) as the sensing element. Hence, we used a MWNT and SiO2 composite as the gas sensing element, placed in a thin layer upon the interdigital-electrodes.
Fig. 2. (a) Digital image of the resistive sensor showing the confinement of the MWNTs to the patterned serpentine silicon dioxide pattern. The silicon substrate is the bright portion in the background. (b) Schematic diagram of the serpentine resistor pattern, and (c) a cross-sectional view.
The copper electrodes of the interdigital capacitive sensor, see Fig. 1, were formed on a copper-clad printed circuit board using photolithography. Prior to application of the gas sensing MWNT-SiO2 composite, to prevent shorting of the electrodes by the MWNTs the sensor was ®rst coated with a thin layer of SiO2. The gas sensing MWNT-SiO2 composite was made in the following manner: carbon nanotubes were removed from fused silica substrates and dispersed in toluene using an ultrasonic bath. The individual nanotubes thus obtained were rinsed with isopropanol and allowed to dry. The nanotubes were then dispersed in a SiO2 containing solution (20% SiO2 nanoparticles dispersed in water [42]) so as to obtain a dry-weight balance of 2:3. This solution was spin-coated onto the thin SiO2 layer deposited over the
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Fig. 3. (a) High resolution SEM image of the MWNTs grown over silicon dioxide, (b) a further magnified view showing the entanglement of the carbon nanotubes.
electrodes. The schematic diagrams of the interdigital-electrode pattern are shown in Fig. 1. In addition to the capacitive sensor, we used a serpentine resistor geometry upon which MWNTs were grown, providing a low conductivity path without compromising gas sensing surface area. The resistive sensor, shown in Fig. 2, was made in the following manner: a thick layer of silicon dioxide was grown on a silicon substrate using thermal oxidation. A SiO2 serpentine pattern (total path length 45 cm, arm width 350 mm, spacing gap between the arms 290 mm, overall dimensions 1:7 cm 1:7 cm) was de®ned from this layer by photolithography. MWNTs were then grown upon this patterned silicon dioxide layer as described in the previous section. It was observed that the nanotubes were formed exactly on the patterned SiO2 layer avoiding silicon completely. Fig. 3 shows SEM images of MWNTs grown upon the SiO2 layer. The nanotubes were found to have a diameter of 20±35 nm with lengths of a few microns; the nanotubes were not aligned normal to the substrate as they are when grown on quartz substrates [41]. 2.3. Experimental set-up The sensors were placed in a sealed Plexiglas chamber of 60 cm3 volume. Argon was used as the carrier gas throughout the work. For humidity sensing, argon was bubbled through a bottle containing water and then mixed with dry argon. The total ¯ow rate was constant at 1000 sccm, and the partial pressures of the test gases controlled with a mass ¯ow controller. Electrical contacts were made to the sensors by connecting two short copper wires to the electrodes using silver paste. For the serpentine resistor sensor, silver paste contacts were made directly to the top of the nanotubes grown over silicon dioxide. A Hewlett Packard 4192A impedance analyzer ®tted with a test ®xture (Agilent 16034E) was
employed for impedance measurements. Data was collected using a signal amplitude of 90 mV over the frequency range 5±13 MHz. All experiments were conducted at room temperature (21 8C), with the same sensor used for all measurements. Prior to each measurement the sensors were heated in vacuum at 100 8C for 1 h to remove the chemisorbed molecules. 3. Experimental results The chamber was initially ¯ushed with argon and the impedance of the two sensors measured. The total impedance (Z) was resolved into real (Z0 ) and imaginary (Z00 ) parts to construct Cole±Cole impedance plots. Figs. 4 and 5 represent the Cole±Cole plots of the capacitive and resistive sensors, respectively, in the argon environment. The experimental data was ®tted using the complex non-linear least square ®tting program, LEVM, provided by Prof. Macdonald and Solartron Analytical [43]. For all impedance plots, the dots represent experimental data and the solid-line the equivalent circuit ®t. Referring to the circuit models, R0 denotes a frequency independent Ohmic resistance, and R1 and R2 two frequency independent resistances in parallel with two frequency dependent capacitors, Cn1 (o) and Cn2 (o), respectively. According to the concept of a `universal dielectric response' introduced by Jonscher [44,45], a dispersive frequency dependent capacitance Cn(o) in parallel with a frequency independent resistance represents a semicircle in the Cole±Cole plot with its center depressed below the real axis by an angle (n 1)p/2, where n lies between 0 and 1. This non-Debye capacitance can be written as Cn o Bn (io)n 1 where Bn is a constant. Bn takes the form of an ideal frequency independent capacitance as n approaches 1. With reference to Fig. 5, the parallel combinations of
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Fig. 4. Cole±Cole plot of the capacitive sensor kept in argon environment. The equivalent circuit used for fitting the experimental data is shown adjacent to the plot.
Bn1 R1 and Bn2 R2 represent two different phenomena occurring in the material, with two different relaxation times and hence two semicircles. If the respective relaxation times t1and t2 given by t1 R1 Bn1 1=n1 and t2 R2 Bn2 1=n2 are close, as is the case for the resistive sensor, then the two semicircles overlap and it is dif®cult to distinguish between them [40], see Fig. 5. Fitting of experimental data points resulted in a n1 value of 0.96 for the capacitive sensor (Fig. 4), and for the resistive sensor (Fig. 5) values of n1 0:99 and n2 0:81; all semicircles have their centers depressed below the real axis. The n values lead to calculation of relaxation times
t1 0:11 ms for the capacitive sensor, and t1 4:3 ms and t2 1:22 ms for the resistive sensor. The relaxation times of the resistive sensor are close enough that the semicircles overlap. Since the relaxation times of each sensor are small, the sensing mechanism is inferred to be charge transfer rather than mass transport [40,46]. Therefore, R1 and R2 represent a resistance to charge transfer while Bn1 and Bn2 behave as a double layer capacitance. The capacitive sensor has a Bn1 value of 3:33 10 11 , while the resistive sensor has Bn1 5:67 10 10 and Bn2 2:87 10 9 . Cole±Cole plots of the capacitive sensor at different humidity levels are shown in Fig. 6. The impedance of
Fig. 5. Cole±Cole plot of the resistive sensor kept in argon environment. The equivalent circuit used for fitting experimental data is shown adjacent to the plot.
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Fig. 6. Cole±Cole plots of the capacitive sensor at different humidity levels.
the capacitive sensor is highly sensitive to humidity, increasing with water content, as evidenced by the large change in the arc radius. It can be seen from the equivalent circuit (Fig. 4) that at quasi-static frequencies R0 and R1 limit the current and hence the sum of these resistances equal the resistance obtained from a dc measurement. The sensitivity S can be de®ned as:
Fig. 8. Cole±Cole plots of the resistive sensor kept in presence of dry argon (0% water vapor content) and argon bubbled through water (100% water vapor content).
where Rargon is the resistance in the presence of argon, and Rgas R0 gas R1 gas is the resistance in the presence of a test gas. Fig. 7 shows the variation in both percentage sensitivity S and R1 as a function of water vapor concentration. The non-linear increase in R1 with humidity indicates that charge transfer is more difficult at higher humidity levels. R0 was found to be virtually independent of humidity with an average value of 87 O. Fig. 8 shows the Cole±Cole plot of the resistive sensor kept at different humidity levels. The percentage sensitivity
S for the resistive sensor is calculated using Eq. (1) with Rgas R0 gas R1 gas R2 gas . The variations in sensitivity S as well as R1 and R2 of this sensor in a humid atmosphere are shown in Fig. 9. Both R1 and R2 increase with rising humidity, with R2 increasing at a greater rate than R1 and hence playing a larger role in determining sensitivity. MWNTs demonstrate a considerable af®nity for water vapor; the response time of both sensors to increasing humidity was approximately 2±3 min, however the recovery time was found to be on the order of a few hours. In contrast with the results obtained by other research groups [23,37,38] neither sensors showed a measurable sensitivity to oxygen. However, these earlier oxygen studies used single wall carbon nanotubes (SWNT) and perhaps more importantly evacuated the chamber before and after exposure of the sensor to oxygen. In this work the sensors were exposed to ambient atmosphere, then ¯ushed with argon prior to measurements in a particular gaseous environment. The Cole±Cole plots of the capacitive sensor in response to CO2 are shown in Fig. 10, with the associated variations in
Fig. 7. The sensitivity S, and resistance R1 of the capacitive sensor in response to water vapor content.
Fig. 9. The sensitivity S, R1 and R2 of the resistive sensor to water vapor content.
S
Rgas Rargon 100% Rargon
(1)
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Fig. 10. Cole±Cole plots of the capacitive sensor in dry argon (0% CO2) and a pure CO2 atmosphere.
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Fig. 13. Cole±Cole plots of the capacitive sensor in response to varying ammonia concentrations.
Fig. 14. The sensitivity S and R1 of the capacitive sensor to ammonia content in dry argon. Fig. 11. The sensitivity S and R1 of the capacitive sensor at different CO2 concentrations (carrier gas is dry argon).
sensitivity S and R1 plotted in Fig. 11; the sensor did not show a measurable response to CO. This is opposite to the response of the resistive sensor, which did not show a measurable response to CO2, but did to CO as shown in Fig. 12. Fig. 13 shows the Cole±Cole plot of the capacitive sensor in response to varying ammonia concentrations. In contrast to the responses seen for water and carbon dioxide, as seen in Fig. 14 there is an almost linear increase in both sensitivity and R1 due to the presence of ammonia. The behavior of both sensors to ammonia is that of a dosimeter; the response time of the sensor to increasing ammonia concentration was approximately 2±3 min, however it took the sensor several days kept in vacuum at 100 8C to recover the original response. 4. Discussion
Fig. 12. Cole±Cole plot of the resistive sensor in response to CO.
Multi-wall carbon nanotubes grown over fused silica substrates [41] are conductive, appear as a frequency independent
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resistor in the Cole±Cole plot, and do not show sensitivity to any of the gases used in the present study. This is in contrast with the results obtained when these nanotubes were mixed with SiO2, as well as when they were grown upon a patterned silicon dioxide substrate. In these cases, arcs appeared in the Cole±Cole plot (Figs. 4 and 5) which clearly indicate the presence of a capacitive element due to the presence of some semiconducting or non-conducting elements in the two sensors. According to Langer et al. [47] MWNTs behave like rolled up graphene sheets with an electrical conductivity similar to crystalline graphite at room temperature or higher, with an energy band overlap similar to bulk graphite. As in graphite [18,48], MWNTs contain both holes and electrons but at room temperature generally show a metallic behavior with electrons as majority carriers due to the overlapping conduction and valence bands which varies with nanotube diameter and helicity [18,47]. MWNTs can show conductivity values in the semiconducting range [49], with the energy band overlap varying according to the nanotube chirality and hence the interaction between the different walls of the MWNT. Langer et al. [47] observed a band overlap of 3.7 meV, almost a factor of ten less than graphite, due to this interlayer interaction. The presence of defects may also reduce the conductivity of MWNTs [18,50]. Therefore, with reference to the arc in the Cole±Cole plots, it appears that semiconducting MWNTs are present among the metallic MWNTs within the sensors giving rise to a capacitive component, with the dielectric constant of the semiconducting MWNTs changing in response to gas concentrations. In the case of thick MWNT mats grown over fused silica [41] the large abundance of metallic MWNTs completely screens the effect of any semiconducting MWNTs. For the capacitive sensor, it is the dispersion of MWNTs in SiO2 which isolates the semiconducting MWNTs. For the resistive sensor, the growth density of the MWNTs (see Fig. 3) is considerably less than that of the MWNT mats reported in [41] due to a smaller amount of catalyst and shorter reaction time, which makes it possible for semiconducting MWNTs to be present without being electrically shorted. For understanding the mechanism of gas sensing, the variation of Bn, n and t for the two sensors in the presence of different gases are analyzed. Values of Bn1 and n1 for the capacitive sensor in presence of water vapor and ammonia are plotted in Figs. 15 and 16, respectively, with the corresponding relaxation times shown in Fig. 17. Fig. 18 is a plot of Bn1 , Bn2 , t1 and t2 of the resistive sensor versus humidity. It can be seen from Figs. 15 and 18 that Bn1 (and Bn2 ) for both sensors increase with increasing humidity or ammonia levels, while n1 decreases in both cases. Furthermore the variation in relaxation times t1 (and t2), Figs. 17 and 18, parallels the changes in R1 (and R2). The results indicate that there is charge transfer between the test gases and sensing element, and hence that chemisorption [34,35] of gases on the MWNTs is the dominant sensing mechanism.
Fig. 15. Variation of Bn1 and n1 of the capacitive sensor as a function of water vapor content.
Fig. 16. Variation of Bn1 and n1 of the capacitive sensor as a function of ammonia concentration.
There are two possible mechanisms in which gases can reversibly interact with the nanotubes, physisorption which does not involve charge transfer [34], and/or chemisorption which does [35]. The operational principle behind many ceramic humidity sensors is physisorption of water molecules on an initially chemisorbed hydroxyl layer [51,52]. The physisorbed water layer enables charge transport through the surface reducing the overall resistance of the sensor [53]. The process of physisorption cannot explain the
Fig. 17. Relaxation times of the capacitive sensor as a function of water vapor and ammonia concentration.
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Fig. 18. Variations of Bn1 and Bn2 , and relaxation times t1 and t2, with water vapor for the resistive sensor.
present results since R1 (R1 and R2 for the resistive sensor) increases with humidity. There exists a ®nite probability of ®nding chemisorbed gases at the surface of semiconductor materials, even at room temperature, due to presence of highly active surface sites or surface defects [34]. Since the presence of ammonia, carbon monoxide and carbon dioxide lower the conductance of the sensors, it is evident that chemisorption of these gases or water molecules takes place on the MWNT surface. However, for n-type MWNTs the chemisorption of these reducing gases would have increased the electrical conductivity due to the increase in conduction band electrons. Hence, it appears that p-type semiconductor behavior is present in the MWNTs. For p-type MWNTs the adsorbed water molecules, or the reducing gas molecules, donate electrons to the valence band thereby increasing the electrical resistance. Kong et al. [22] also observed an increase in resistance in their p-type SWNT based transistors in presence of ammonia. For both sensors, the fact that R1 (and R2) as well as Bn1 (and Bn2 ) increase in the presence of the test gases shows that the capacitance arises independently of the resistance, and the rapid relaxation time (1 ms) indicates that these terms are driven by charge transfer phenomenon. The interaction mechanism of the MWNTs with the test gases can be explained as follows. It appears that semiconducting nanotubes are sandwiched between metallic ones, which gives rise to Bn1 and R1. In presence of water vapor, ammonia, carbon dioxide or carbon monoxide chemisorption takes place at the outer surface of the semiconducting MWNT. The chemisorbed reducing gases donate electrons to the valence band decreasing the number of holes, thereby increasing the separation between the Fermi level and valence band [22,39,54,55]. This forms a space charge region, i.e. Schottky barrier, at the surface of the semiconducting MWNT, increasing Bn1 and R1. It is noted that the nanotubes contain catalyst particles embedded inside them, mostly at the tip. Hence this capacitance can be attributed to a Schotttky barrier either at the contact region of a metallic MWNT and a semiconducting MWNT [57], or where the
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catalyst particles are present [28] within the MWNTs. The absorbed gases modulate the existing space charge region of the junction, the same principle used in gas sensing FETs [58]. SiO2 is known to be a good adsorber of water molecules, and hence can play a role in determining gas sensitivity in both sensors. However, physisorption of water on SiO2 leads to protonic conduction on the surface [51,56], increasing the electrical conductivity. In the present case a decreasing electrical conductance with humidity shows that SiO2 has no direct effect on the behavior. However, the adsorption of water molecules onto SiO2 may help the nanotubes to interact more easily with them [22]. While no oxygen sensitivity was observed in our sensors, it cannot be authentically stated that oxygen chemisorption is not occurring. Though we annealed the sensors in vacuum at 100 8C, this is normally not enough to remove the adsorbed oxygen species [34]. Moreover, oxygen adsorption generally takes place immediately upon exposure to atmosphere, and cannot be removed by argon ¯ushing. Thus, surface coverage might have attained a maximum value while loading the sensor inside the test chamber, hence a further increase in the oxygen concentration from the ambient cannot enhance the measured response. Finally, it should also be noted that NH3 may interact with MWNTs by replacing pre-adsorbed oxygen [22,59]. Oxygen, being an electron acceptor, increases the conductivity of p-type MWNTs as it increases the hole concentration; hence the replacement of oxygen by ammonia should reduce the conductivity. Since for both sensors Bn1 (and Bn2 ) increases with increasing ammonia concentration the interaction of ammonia with chemisorbed oxygen species can be ruled out. Consequently water vapor, ammonia, CO and CO2 appear to directly interact with the carbon nanotubes. 5. Conclusions We have explored the possibility of using MWNTs as room temperature gas/vapor sensors. Two sensor geometries were investigated, one capacitive with a MWNT-SiO2 composite placed over a planar interdigital capacitor, the other resistive with MWNTs grown upon a serpentine SiO2 pattern. Impedance spectroscopy was used to reveal the mechanism by which gases/vapors interact with MWNTs. The sensors were found to have high sensitivity to water vapor as well as ammonia, with the response to water vapor being readily reversible and the response to ammonia not, acting instead as a dosimeter. The experimental data of both sensors were simulated using appropriate equivalent circuit models, which enabled determination of the gas/ vapor sensing mechanism. Our results indicate the presence of p-type MWNTs dispersed among the predominant metallic MWNTs. Our results also indicate that chemisorption of gases on the surface of the semiconducting MWNTs is responsible for the sensing action. The presence of a Shottky
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