Gas Sensors Based on Multiwall Carbon Nanotubes Decorated with. Different Metal Oxides Nanoparticles.

Transcription

1 Gas Sensors Based on Multiwall Carbon Nanotubes Decorated with Different Metal Oxides Nanoparticles. A. Abbaspourrad, C. Verissimo, R.V. Gelamo, M. M. da Silva, A. R. Vaz, F. P. M. Rouxinol, O. L. Alves, S. A. Moshkalev Center for Semiconductor Components, University of Campinas UNICAMP, P.O Box 6061, , Campinas, SP, Brazil. Abstract In this work, we present experimental procedures that have been developed to decorate carbon nanotubes (CNTs) with metal oxides nanoparticles for gas sensing applications. For decoration, CNTs were first acid treated. Then, tin, copper and titanium oxide particles were synthesized on the surface of CNTs. For preparation of carbon nanotubes decorated with SnO 2 and CuO, the concentration of reagents and the time of sonication were found to be critical parameters to control the size and density of nanoparticles grown on the nanotubes surface. In the case of decoration with TiO 2 particles, the ph value of the solution was found to play a key role in the process. According to our results, an uniform coverage of nanotubes by TiO 2 nanoparticles surface was obtained for the ph value close to 2, whereas higher ph values led to fully covered CNTs. The SnO 2 decorated CNTs were used to fabricate a sensor in a chemical resistor configuration, and tests were successfully performed with H 2 S to show considerable changes of resistivity in the presence of the gas. The samples of decorated CNTs were characterized by HRTEM, SEM, and electrical measurements were made using a 2- terminals method. 60

2 Introduction Carbon nanotubes (CNTs) exhibit unique electronic properties, high aspect ratio, and excellent chemical and environmental stability, which make them ideal candidates for components of future nanodevices. There is a great interest in decorating of CNTs with metal, metal oxide or semiconductor nanoparticles with the exploration of new properties and applications [1-3]. Gas sensors made of carbon nanotubes have been extensively studied [4]. Sensors fabricated with single-walled carbon nanotubes (SWNTs) have been found to be very sensitive to ambient gases [5]. However, most of them employ field-effect transistor configuration (only semiconducting SWNTs can be used), or need UV light irradiation to desorb the detected gas molecules. Furthermore, multiwall carbon nanotubes (MWNTs) were found to be not very sensitive to the ambient gas. Recently, as one of new trends in nanotechnology, oxide semiconductor nanobelts have attracted much attention due to their high sensitivity for gas detection [6]. By decorating MWNTs with various metal oxides, a new family of gas sensing devices with high sensitivity can be introduced. SnO 2 is an n-type semiconductor oxide with a wide energy band gap and is it frequently employed as a gas sensing material to detect flammable, toxic and pollutant gases [7]. CuO is a p-type semiconductor which is extensively used as gas sensor material. TiO 2 is one of the most interesting candidates for gas detection applications in large scales, it has been used commercially as lambda sensor in exhaust pipes [8]. The fundamental mechanism of semiconductor gas sensors relies upon the change in electrical conductivity due to the interaction between semiconducting materials and gas molecules to be detected. When a sensor is exposed to air, oxygen is 61

3 adsorbed on the surface to form negative ions; the adsorbed oxygen extracts the conduction electrons from the near-surface region of the metal oxide grains leading to formation of an insulating surface layer. Upon exposure to reductive gas species such as CO, the arrested electrons are released by the reactions between the reductive gas and the negatively charged ions: CO+O CO 2 +e, reducing the steady-state surface oxygen concentration and donating electrons back to the bulk resulting in an increased conductivity [9]. In this study, a simple in situ chemical hydrolysis method was applied for the synthesis of CuO, TiO 2 and SnO 2 -coated MWNTs materials. Metal oxide particles were synthesized uniformly on the MWNT walls. A gas sensor prototype was fabricated with SnO 2 -coated MWNTs and the gas-sensing properties were measured at the presence of H 2 S and air. Experimental MWNTs prepared by chemical vapor deposition were purchased from Iljin Nanotech Co., Ltd (Seoul, Korea). Purity of the pristine MWNTs, as received, was 97%. The MWNTs were first heated for 2 h at 330 ºC to eliminate the residual amorphous carbon. Purified nanotubes were then treated with nitric acid for 12 h at 120 ºC, and after this washed with distilled water several times until the ph has reached 7 and dried at 80 ºC for 6 h. The so prepared acid-treated nanotubes were used for decorating with SnO 2, CuO and TiO 2 using ultrasonic bath and appropriate metal chloride salt. 62

4 In a typical synthesis, CuO particles was prepared by dissolving 130 mg CuCl 2 (98%) in 20 ml of distilled water and 5 mg of acid treated MWNTs were dispersed in this solution using ultrasonic bath for 10 min. The mixture was stirred for 12 h and then filtered, dried at 80 ºC for 2 h and calcined at 350 ºC for 24 h. To prepare SnO 2 -decorated MWNTs, 1 g of SnCl 2 was dissolved in 40 ml of distilled H 2 O and then 1 ml of HCl (38%) was added. Subsequently, 10 mg of the astreated MWNTs were dispersed in the above solution. This mixture was sonicated for 5 min and then stirred for 90 min at room temperature. The precipitate was then separated from the mother liquor by filtration and was washed with distilled H 2 O for several times, and then dried at 80 ºC for 6 h. In order to decorate MWNTs with TiO 2 nanoparticles, 300 µl HCl and 4 µl TiCl 4 (98%) were added to 100 ml distilled water and the mixture stirred for 30 min. Then 3 mg of acid treated MWNTs dissolved in the above solution. The mixture sonicated for 1 min and stirred for another 30 min by increasing ph up to 2.5 using NH 4 OH (1%). The precipitate was filtered and washed several times with distilled water and dried at 80 ºC for 24 h. For sensor fabrication, firstly a dispersed solution with decorated MWNTs was prepared by ultrasonic vibrating, and then the dispersed solution was dropped between Au and Pd microelectrodes (fabricated on a SiO 2 wafer). An ac electric field of 3V at 100 khz was applied then for for 2 minutes to deposit nanotubes between electrodes in a dielectrophoresis process. The electrical measurements were carried out using a 2- point probe and a HP 4145B Semiconductor Parameter Analyzer. In order to measure the sensitivity to gases, samples were placed in a chamber that allows for the measurements in vacuum (minimum pressure = 1x10-2 Torr) or under gas flow (H 2 S and synthetic air). The gases were introduced in the chamber through a needle valve coupled 63

5 in a flow meter. The gas flows used were 370 sccm for air and 30 sccm for H 2 S. The I(t) curves were obtained using a constant voltage (3V) applied to the metal electrodes, using 2-terminal probe. Results and discussion The morphology, size (aspect ratio) and electrical properties of CNTs make them an ideal material to use in sensing devices. Decoration of CNTs is of great interest to modify the properties of pristine CNTs. CNTs decorated with metal oxide nanoparticles is a novel nanotube/nanoparticle hybrid smart material, which can find promissing applications as nano sensors. As`a result of MWNTs decoration, both the specific surface area and the sensing activity can be increased significantly. For better nanotubes dispersion in a solution and deposition of nanoparticles, functionalization of nanotubes was first performed, by attaching carboxylic acid and hydroxide functional groups to their surfaces. XPS analysis of functionalized CNTs indicated that up to 20% of carbon atoms on the nanotube surface could be oxidized [10]. This functionalization step results in much better CNT dispersion because of the electrostatic repulsion between the functional groups. Furthermore, these functional groups might serve as anchor seeds for the subsequent deposition of metal oxide nanoparticles [10]. Figures 1(a) and (b) show that tin oxide nanoparticles bond on the CNT surface. The dimensions of crystalline SnO 2 nanoparticles fall in the range of 1 10 nm with the mean value of 4 nm (Fig. 1(c) and (d)). It is believed that functionalized MWNTs play dual roles during nanoparticle deposition. They not only form numerous anchor seeds for nanoparticle deposition on their surface but also form a network with a high surface area effectively preventing possible agglomeration of nanoparticles. 64

6 (a) (b) (c) (d) Figure 1. SEM (a, b) and TEM (c, d) images of SnO 2 decorated MWNTs. SEM micrograph (Fig. 2(a)) shows good overall uniformity and dispersion of TiO 2 decorated functionalized carbon nanotubes. Large titanium oxide agglomerations were rarely observed. TEM characterization (Fig. 2(b)) showed the formation a crystalline phase for TiO 2. 65

7 (a) (b) Figure 2. SEM (a) and TEM (b) images of TiO 2 decorated MWNTs. For CuO, the syntheses led to the formation of very small nanoparticles with diameters in the range of 1-3 nm. HRTEM image (Fig. 3) also showed the absence of crystallinity for this material. Figure 3. TEM image of CuO decorated MWNT. Gas sensing device in a chemical resistor configuration (gap between Pd electrodes ~1 μm) prepared using SnO 2 decorated MWNTs connecting the electrodes, was submitted to the H 2 S atmosphere with a constant voltage applied between Pd electrodes. During the test, synthetic air (370 sccm) was kept inside the chamber and 66

8 H 2 S flow (30 sccm) was introduced for 30 seconds and turned off for 2 minutes, at a room temperature. When H 2 S was introduced to the chamber, a fast increase in the current could be seen, and the current dropped back to the original value immeadiately after the H 2 S flow was stopped (Fig. 4). This demonstrate high reversibility of the sensor. The mechanism of gas sensing is subject of future studies, probably is it based on a charge transfer occurring during interaction of H 2 S molecules with SnO 2 nanoparticles. 3,0 Air + H 2 S Air + H 2 S Air + H 2 S 2,5 Current (μa) 2,0 1,5 1,0 Air Air Air 0,5 0, Time (s) Figure 4. I ( t) curve obtained under air and H 2 S atmospheres at room temperature (T = 30 o C) using a prototype sensor with SnO 2 decorated CNTs. Conclusions Development of gas sensors based on carbon nanotubes decorated by metal oxide nanoparticles is a new research area attracting recently much interest. By selection of appropriate oxides, high sensitivity and selectivity, at low power consumption may be achieved in gas sensing using CNT based devices, with possible applications in different technological processes. In the present work, preliminary 67

9 results obtained have shown high potential of using oxide decorated CNTs for gas sensing at a room temperature. This is a great advantage over conventional gas sensors based on metal oxide thin films like SnO 2 which are known to be efficient only at temperatures as high as o C. References [1] Kong J, Franklin N, Zhou C, Chapline M, Peng S, Cho K and Dai H 2000 Science [2] Zhao L P and Gao L 2004 J. Mater. Chem [3] Sainbury T and Fitzmaurice D 2004 Chem. Mater [4] Y. M. Wong, W. P. Kang, J. L. Davidson, A. Wisitsora-at, and K. L. Soh, Sens. Actuators B 93, 327 (2003). [5] P. F. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. J. Dai, S. Peng, K. J. Cho, Nano Lett. 3, 347 (2003). [6] E. Comini, G. Faglia, G. Sberveglieri, Z. W. Pan, and Z. L. Wang, Appl. Phys. Lett. 81, 1069 (2002). [7] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, P. Taylor, Mater. Chem. Phys. 17 (1987) 577. [8] Sberveglieri G, Comini E, Faglia G, Atashbar MZ, Wlodarski W (2000) Sensors Actuators B66:139. [9] A. Kolmakov, Y. X. Zhang, G. S. Cheng, and M. Moskovits, Adv. Mater. Weinheim, Ger._ 15, 997 _2003. [10] Xie J, Zhang N and Varadan V J 2006 Smart Mater. Struct. 15 S