Plasma-enhanced chemical vapor deposition carbon nanotubes for ethanol gas sensors Chia-Te Hu a , Chun-Kuo Liu a , Meng-Wen Huang b , Sen-Hong Syue a , Jyh-Ming Wu c , Yee-shyi Chang a , Jien-W. Yeh a , Han-C. Shih a,d, ⁎ a Department of Materials Science and Engineering, National Tsing Hua University Hsinchu, 30013, Taiwan, ROC b Department of Materials Science and Engineering, National Chung Hsing University Taichung, 40227, Taiwan, ROC c Materials Science and Engineering, Feng Chia University Taichung, 40724, Taiwan, ROC d Institute of Materials Science and Nanotechnology, Chinese Culture University Taipei, 11114, Taiwan, ROC abstractarticle info Available online 12 November 2008 Keywords: Carbon nanotubes Ethanol Conductance Gas sensor Adsorption Surface modification Carbon nanotubes (CNTs) have been fabricated by microwave plasma-enhanced chemical vapor deposition for detecting the presence of ethanol vapor. The conductance of the CNTs decreases when the sensors are successively exposed to ethanol vapor at room temperature. The surface of the CNTs was modified in oxygen plasma to elevate the detection sensitivity for ethanol. Successful utilization of CNTs in gas sensors may open a new window for the development of novel nanostructure gas devices. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The remarkable structural, electrical, mechanical, and chemical properties of carbon nanotubes (CNTs) have generated great interest in various fields [1–5]. Of special interest is the gas adsorption property that allows CNTs to be made as new gas materials, depending on their large surface area [6–8] and hollow geometry. Several models that account for the gas sensor applications of CNTs have been reported recently [9–16]. Hyeok et al. [17] fabricated a gas sensor from a nanocomposite by polymerizing pyrrole monomers with single wall carbon nanotubes (SWCNTs). Polypyrrole (Ppy) was prepared by a simple and straightforward in-situ chemical polymerization of pyrrole mixed with SWCNTs, and the sensor electrodes were formed by spin- casting SWCNT/Ppy onto pre-patterned electrodes. They found that the sensitivity of the nanocomposite was about ten times higher than that of Ppy. The SWCNT bundles could be nanodispersed, which may increase the specific surface area of the coated Ppy and thereby further increase the sensitivity. One of the most difficult aspects of CNT gas sensing is that most techniques are based on SWCNT field-effect transistors or require UV light irradiation to desorb the detected gas molecules. Furthermore, multiwall carbon nanotubes (MWCNTs) are not very sensitive to ambient gases [12–15]. Our objective in this investigation is to modify the surface of MWCNTs for elevating the detection sensitivity of an ethanol gas sensor. It is therefore necessary to upgrade the surface structure of MWCNTs precisely in order to realize the applications of the devices. However, only a few papers have investigated the use of MWCNTs for ethanol detection. Liang et al. [16] reported a resistance sensor fabricated from MWCNTs coated with a thin tin-oxide layer and found that the barrier height between the tin- oxide grains on the MWCNTs varies for different gases so that the sensor resistance changes markedly, which makes the sensitivity of the sensors great enough for real applications. In addition, the Shottky barrier between tin-oxide grains and MWCNTs is very low, such that electrons conduct in the MWCNTs with low resistance. So far, only noise studies on gas sensors at elevated temperatures have been carried out. Wan et al. [18] fabricated a gas sensor from a ZnO oxide nanowire; its sensitivity at an ethanol concentration of 100 ppm increased sharply as the temperature increased from 200 to 300 °C, mainly due to the enhanced reaction between the ethanol and the absorbed oxygen at an elevated temperature. This model, however, cannot be applied under room temperature conditions. Therefore, a more generalized model is required. In this paper, we present the results of ethanol gas detection by MWCNTs at room temperature and use a distinct oxygen plasma treatment to enhance the sensitivity. It was found experimentally that the sensitivity of MWCNT gas sensors varies significantly with various gas concentra- tions at room temperature after oxygen plasma modification. In order to perform our study, the structure was systematically analyzed by field emission scanning electron microscopy (FE-SEM), and further quantitative analysis was conducted by high-resolution transmission electron microscopy (HRTEM). In the sensor structure, MWCNT growth occurred on an aluminum oxide substrate that was strongly held by two alligator clamps acting as electrodes for the conducting current. This assembly is highly sensitive to the presence of the Diamond & Related Materials 18 (2009) 472–477 ⁎ Corresponding author. Department of Materials Science and Engineering, National Tsing Hua University Hsinchu, 30013, Taiwan, ROC. Tel.: +886 3 5715131x33845; fax: +886 3 5710290. E-mail address: hcshih@mx.nthu.edu.tw (H C. Shih). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.057 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond ethanol molecules. The conductance of the sensors adjusts itself when they are exposed to ethanol gas. 2. Experimental In this work, microwave plasma enhanced chemical vapor deposition (MPECVD) was used to synthesize the carbon nanotubes. An iron-containing compound was used as the catalyst, which was first coated onto a non-conductive aluminum oxide substrate using a sol-gel method to promote the growth of carbon nanotubes. The substrate was placed in the MPECVD chamber, where a mixture gas of methane and hydrogen (1:10) was introduced and simultaneously decomposed by the microwaves to synthesize carbon-related materi- als. During this period, the pressure was kept at 20 Torr with a microwave power of 1.5 kW at 650 °C as measured by a thermocouple (CA). After the growth of the CNTs, the microwave chamber was cooled down to room temperature, and surface modification treat- ment was subsequently started. The oxygen plasma treatments for the MWCNTs were conducted under the following conditions: oxygen flow rate of 20 sccm, operating pressure of 0.5 Torr, microwave power of 600 W, process duration of 5–35 s, and average sample temperature of about 30 °C. It was demonstrated that the treatment result was Fig. 1. FE-SEM images of CNTs treated in oxygen plasma at a microwave power of 600 W for (a) 0 s; showing the amorphous carbon clamped on the surface of CNTs, (b) 5 s, (c) 20 s, (d) 35 s, (e) 60 s, and (f) 90 s. Fig. 2. HRTEM images of CNTs treated in oxygen plasma for (a) 0 s; denoting the amorphous carbon clamped on the surface of CNTs, (b) 20 s; showing the removal of the amorphous carbon, and (c) 35 s; CNTs becoming loosely attached, accounting for the ion sweeping over the CNTs. 473C T. Hu et al. / Diamond & Related Materials 18 (2009) 472–477 directly related to the excited species density, which was controlled by the treatment duration. The morphology of the specimens was examined by field emission scanning electron microscopy (FE-SEM, JSM-6500F). High-resolution electron microscopy (HRTEM, JEOL JEM- 2010) was performed at 200 kV with a point resolution of 0.19 nm and a lattice resolution of 0.1 nm. Adsorption isotherm experiments were performed at − 261.17 °C with a Micromeritics ASAP 2000 accelerated surface and porosimetry analysis system using N 2 gases on the MWCNT samples with masses of ∼ 0.2 g. Before each adsorption isotherm test, the nanotube bundles were exposed to a high vacuum for at least 3 h at 120 °C and b 10 − 6 Torr to ensure complete desorption of adsorbates. All measurements were conducted at − 261.17 °C; the saturation pressure p 0 for N 2 at this temperature is 760 Torr. Gas-sensing experiments were carried out using a volt-amperometric technique. During the experiment, the MWCNT-based gas sensor was placed in a sealed chamber with an electrical feedthrough. Pure ethanol gas flowed through the sealed chamber while the e lectrical properties of the MWCNTs were monitored. All such measurements were taken at 25 °C. 3. Results and discussion Fig. 1 shows the surface morphology of the CNTS with various modification durations from the SEM analysis. Clearly visible in Fig.1(a) are the amorphous carbon layers deposited on the surface, connecting with each other, which probably result from incomplete carbon atomic piling. When CNTs were treated for 5 and 20 s, the amorphous carbon domains are eliminated, and the tubes become smoother and cleaner as shown in Fig.1(b) and (c). After 60 and 90 s, as seen in Fig.1 (e) an d (f), the carbon nanotubes become ambiguous and one can not distinguish a single carbon nanotube from the rest. From the results, it can be seen that the oxygen plasma treatment can remove the fragile parts like amorphous carbon, but too much treatment would eventually destroy the CNTs. High-resolution transmission electron microscopy (HRTEM) played a vital role, as it is the only technique that allows for real space imaging of atomic distribution in nanoparticles, particularly when the particle size is small. With consideration of the particle shape symmetry, HRTEM can be used to determine the 3D shape of small particles although the image is a 2D projection of a 3D object. The grown carbon nanotubes for TEM analysis were separated from the substrate and then ultrasonicated in ethanol. After ultrasonic treatment, a drop of liquid was then sprayed onto a carbon-coated copper grid. The CNTs were analyzed by TEM to confirm that they were really CNTs and not carbon fibers. Fig. 2 reveals that the CNTs have an inner diameter of ∼ 8 nm, an outer diameter of ∼ 23 nm, and the distance between wall layers is ∼ 0.33 nm. Comparison of the images in Fig. 2(a) and (b) indicates that the oxygen plasma modification exerted to the surface of the CNTs can remove impurities as well as amorphous carbons. This is due to the fact that the amorphous carbons are easily oxidized under the oxygen plasma species. Unfortunately, we were not able to obtain the microstructure when we continuously applied the oxygen plasma treatment for a duration longer than 20 s, as shown in Fig. 2(c). This is because ion bombardments may cause the creation of vacancies and i nterstitials in the MWCN Ts. The surface s tructure, including defects and amorphous carbons on the carbon nan owires, is removed due to the ion sweeping, which leads to an incre ase in diameter. The results from TEM in combinatio n with SEM figures reveal that the surface of the CNTs can be indeed modified by oxygen plasma treatment, but p rolonged treatments are harmful and can destroy the outside walls. Using the Br unauer–Emmett–Teller ( BET) method, t he adsorp tion isotherms of N 2 ontheMWCNTsweremeasuredforallthesamples Fig. 3. Adsorption and desorption isotherms of N 2 on modified MWCNTs at − 261.17 °C; the saturation pressure p 0 for N 2 is 760 Torr for (a) 0 s, (b) 5 s, (c) 20 s, and (d) 35 s. 474 C T. Hu et al. / Diamond & Related Materials 18 (2009) 472–477 at − 261.17 °C. Results sho wed that all of the adsorption iso therms (at − 261.17°C)fortheMWCNTsareclosetoTypeIV[1 9–20], i.e ., having h yster esis loops. The relative pressure p/p o is moder atel y high Fig. 5. Three cycles of response-recovery characteristics of the MWCNTs exposed to various ethanol concentrations for distinct modification for (a) 0 s, (b) 5 s, (c) 20 s, and (d) 35 s. Table 1 Adsorption properties of N 2 on MWCNTs, obtained by the BET method Oxygen plasma treating time (s) Specific surface area: A s (m 2 /g) Monolayer adsorption capacity: V m (cc/g STP) Single point surface area at p/p o 0.1984 (m 2 /g) Micropore area (m 2 /g) 0 91.3 21.0 90.0 14.5 5 97.0 22.2 95.0 15.2 20 102.5 24.0 100.7 16.4 35 90.1 20.0 88.3 14.3 Fig. 4. Adsorption isotherm curves for various modified CNTs at (a) low relative pressure, (b) medium relative pressure, and (c) high relative pressure. 475C T. Hu et al. / Diamond & Related Materials 18 (2009) 472–477 when p/p o ≈ 0.06–0.7 3, and t he amount a dsorbed incr eases steeply after p/p o ≈ 0.73, as sho wn in Fig. 3. Moreover, the overall enlarged adsorption isotherms can be divided into three parts, A, B, and C, as shown in Fig. 4, which indic ates that the capabilities of the CNTs t o absorb gases under various oxygen plasma treatments follo w the order: 20 sN 5sN 0sN 35 s. This curve is in qualitative agreement with the SEM and TEM observations for the same result. As the duration of the modification increases, the gas adsorption property enhances. Unfortunately , the gas volume adsorption drops dr amatically for pr olonged e xposur e in oxygen plasma. This effect might be due to the defects and amorphous carbons, which decrease the gas adsorption capability. Applying the BET method to the data in Fig. 3 yields the monolayer adsorption capacity (V m )andthespecificsurface area of adsorption ( A s ), where molecular cross-sectional ar eas of 91.3, 97.0, 102.5, and 90. 1 m 2 /g wer e use d for N 2 duringtherespectivedurationof oxy gen plasma modification, i.e., 0, 5, 20, and 35 s . Table 1 summarizes the resulting V m and A s values of the MWCNT s. Fig. 5 gives the electrica l property curves of th e MWCNT-based sensors in a sealed chamber, which were evacuated to 10 − 5 Torr by a turbo pump. We plot a typical time evolution of the conductance at a temperature of 25 °C for a MWCNT-based sensor successively exposed to 10 ppm, 20 ppm, and 30 ppm etha nol gas. Th e do tted line represents the variation of ethan ol concentr ation, which cor responds t o the dr oppi ng time. It can be seen that as ethanol gas with a concentration of 10 ppm was introduced for 60 s, the conductance of the gas sensor decreases from its origina l electrical potential to the electrical potential due to adsorption. As the ethanol gas was removed, conductance of the gas sensor was lost. Similar behavior occurred at other concentrations of ethanol gas (20 ppm and 30 ppm). The sensing mechanism is surface conduction modulated by adsorbed gas molecules; the electrical conductivity depends strongl y on surface states pr oduced by molecular adsorption, which result in space-charge layer changes and band modulation. In our new sensors based on carbon nanotubes, a large fraction of theatoms are present at the surface and the surface properties become paramount. Oxygen is known to have good charge transfer to planar defected graphite, especially in the presence of catalytic metallic particles. Carbon nanotubes can become hole-doped in the presence of adsorbed oxygen after oxygen plasma treatment because of the electron affinity of oxy gen [21]. Owin g to the interestin g lay er ed structure, especially with ethanol intercalated, it reacts with C–Olayersthrough hydrogen bonds to charge the conductivity of the sensors and causes the current to decrease, as sh o wn in Fig. 6(a). Furthermore, the sen sitivity was highest for the specimen treated for 20 s with oxygen plasma. In order to realize the sensitivity of various surface modification durations, the sens iti vity (S) can be calculated by S=C air /C gas ,whereC air is the conductivity of the surrounding air, and C gas is the c onductivity when ethanol is i ntr oduced. Fig. 6(b)–(d) illus tr at es the sensitivity of various surface modification durations. As we can see, the sensitivity of the specimen with 20 s of oxygen plasma treatment is superior to that of the other specimens, which were treated for other time durations. The enhanced sensitivity may arise from the promoted surface-to-volume ratio of CNTs and eliminate the unsteady performance of non-treated CNTs (Fig. 5(a)). Moreover, once the ethanol is introduced, the conductivity signal drops dramatically after the modification. It might be that the defects and amorphous carbons decrease the gas adsorption activation and that oxygen plasma treatment will however overcome this disadvantage. But we are able to decrease the sensitivity if too much oxy g en plasma treatment i s applied. 4. Conclusions The MW CNT-bas ed gas sen sors made b y MPECVD fo r de tectin g ethanol molecules ha ve been studied at a pp m-lev el at room temperature. The conductance of the sensors decreases when the sensors are exposed to ethanol gas. 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