phys stat sol (a) 204, No 6, 1820 – 1824 (2007) / DOI 10.1002/pssa.200675318 Nanocomposite of SWNTs and SnO2 fabricated by soldering process for ammonia gas sensor application Nguyen Duc Hoa, Nguyen Van Quy, You Suk Cho, and Dojin Kim* School of Nano Science and Technology, Chungnam National University, Daejeon, Korea Received October 2006, accepted January 2007 Published online June 2007 PACS 07.07.Df, 73.63.Bd, 81.07.De We introduce a simple method to fabricate an ammonia gas sensor with a nanocomposite of single-walled carbon nanotubes (SWNTs) and SnO2 The nanocomposite showed a semiconducting property With exposure to ammonia gas, the electrical resistance of the nanocomposite sensor increases rapidly It was found that the sensor can detect the concentration of NH3 down to the 10 ppm level at room temperature The sensor exhibited a fast response time of less than 100 s and good sensing response and recovery © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Introduction The study of gas sensors has been of long time interest due to their diverse applications Many of the traditional gas sensor devices are based on the change of electrical conductance in various semiconducting oxide materials such as TiO2, SnO2, ZnO, etc [1–3] The metal-oxide-based gas sensors need to be operated at relatively high temperatures rendering their practical application difficult In the meantime, among all the surface-reacting materials, carbon nanotubes (CNTs) having unique geometry and amazing structural features appear as a potential candidate for gas sensors CNT-based gas sensors for detection of hydrogen, nitrogen dioxide, carbon dioxide, ammonia, etc, have been reported [4, 5] Although trials to enhance the sensing properties of CNTs have been made by coating or filling with novel metals [6, 7] and oxides [8, 9], the CNT-based gas sensors might suffer from several drawbacks in terms of varying structural quality depending on the synthesis method and the detailed procedure, mechanical stability related with poor adhesion to the substrate, and recovery time, etc The present work describes a simple method to fabricate a nanocomposite of single-walled carbon nanotubes (SWNTs) and SnO2 by a soldering method for application to NH3 gas detection The sensor showed a high stability by soldering CNTs to the substrate via Sn oxide The high-quality SWNTs forming a composite with the Sn oxide also showed good ammonia sensing characteristics Experimental The schematic diagram of the nanocomposite sensor structure is shown in Fig Firstly, comb-type interdigitated electrodes were patterned on sputter-deposited Ti/Pt metal films on thermally grown SiO2 The finger width is 100 µm and the gap between the fingers is 70 µm A Sn thin film of ~100 nm thickness was then deposited on the patterned substrate by sputtering The substrate was rotated during the deposition for the film thickness uniformity Sn was selected due to its low melting temperature (235 °C) Next, SWNTs were * Corresponding author: e-mail: dojin@cnu.ac.kr, Phone: +82 42 821 6639, Fax: +82 42 823 4224 © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Original Paper phys stat sol (a) 204, No (2007) 1821 Fig Schematic diagram of the interdigitated composite sensor device structure directly coated on the Sn layer by an arc-discharge process as described in detail in Ref [10] The sample was heat treated at 500 °C in N2 for h for agglomeration of Sn while grasping the CNTs over it Finally, it was heat treated at 450 °C in air for h to convert the Sn particles into SnO2 particles Sn oxide is known to be an n-type semiconductor having band gap energy 3.5 eV and often used as a sensor material [11] Thus, the fabricated sensor of SWNTs and SnO2 composite is expected to show high sensitivity at low operating temperatures [12] The heat treatment in air is necessary not only to oxidize Sn to SnO2, but also to burn out the amorphous carbon in the SWNT product and/or defective tubes This will improve the sensitivity and the recovery time of the sensor The sensor property was compared with that of pure SnO2 The morphology and quality of the SWNTs and the composite were examined by field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and transmission electron microscopy (TEM) The electrical and gas sensing properties were measured in a vacuum chamber of the volume 1500 cm3 Nitrogen was used as the dilution and carrier gases Controlled NH3 and N2 gases were injected into the chamber to establish the desired volume concentrations The total flow rate of the gases was kept constant at 500 sccm The response and recovery of the sensors were measured from the sensors’ electrical resistance changes upon the NH3 gas injection and N2 purging The sensitivity S is defined by S (%) = 100 × (Rg − Ro)/Ro, where Rg is the sensor resistance with NH3 and Ro is that with N2 All the measurements were conducted at room temperature Results and discussion A FE-SEM image of the as-synthesized SWNTs on a Sn film is shown in Fig 2a The synthesized SWNTs are homogeneously dispersed on the substrate Note that the SWNTs were deposited in-situ directly on the Fig Morphology of (a) the as-coated SWNTs on Sn film and (b) the SWNTs – SnO2 composite after the heat treatment of (a) www.pss-a.com © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 1822 Nguyen Duc Hoa et al.: Nanocomposite of SWNTs and SnO fabricated by soldering process Fig Temperature-dependent resistivity of the composite sensor revealing the semiconducting behavior of the CNTs substrate in the arc-discharge chamber, and thus they are physically attached on the substrate A separate examination of the SWNTs by Raman spectroscopy showed a very strong peak at ~1600 cm−1 with a negligible peak at ~1300 cm−1 indicating the very high quality of the synthesized tubes The diameter of the SWNTs was estimated to be ~1.4 nm from the radial breath mode peaks The TEM image showed bundles of SWNTs with some metallic nanoparticles The metallic nanoparticles were surrounded by amorphous carbon, and this amorphous carbon could be burnt out through the coming oxidation process These will be discussed in detail elsewhere The Sn films melt through the heat treatment at 500 °C in N2, and grasp the CNTs to form the composite The SWNT–Sn composite is formed with needle-like grains of composite The elongated grain shape originates from the reaction with the CNTs, since the heat treatment of the Sn film only revealed rounded nanoparticles of Sn During the oxidization process thereafter in air, the metallic Sn converts to SnO2 without any change in the grain shapes The final morphology of the SWNTs and SnO2 composite is shown in Fig 2b The formed SnO2 films showed an insulating property, but the composite with the CNTs showed a good conductivity with semiconducting resistance vs temperature behavior as shown in Fig The enhanced conductivity of the composite is of course due to the interconnected CNTs that form the main conducting channel An experiment to compare the sensitivity of SWNTs and of SnO2 was carried out Two sen- Fig Responses to ammonia gas of the composite and SnO2 layer © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.pss-a.com Original Paper phys stat sol (a) 204, No (2007) 1823 Fig (a) Response and recovery behaviors and (b) the sensitivity of the sensor at different ammonia gas concentrations sors of pure SnO2 and the composite of SWNTs and SnO2 were fabricated for comparison The NH3 sensing properties of the two sensors are compared in Fig When the sensor was exposed to 60 ppm of NH3, the SnO2 sensor did not show any noticeable resistance change, but the composite sensor did It clearly shows that SnO2 itself does not respond to ammonia gas, indicating that the sensing of the composite comes solely from the CNTs The resistance of the SnO2 sensor is 5.5 × 109 Ω, which is about four orders of magnitude larger than that of the composite The immunity of SnO2 to the gas and its high resistivity with respect to the sensing material or the CNTs will enhance the sensitivity of the sensor Figure 5a shows the dynamic sensing and recovery of the sensor at different concentrations of NH3 The initial test condition was set up with a flow of 500 sccm N2 for 20 For the sensing, NH3 gas was injected into the chamber for to obtain the desired concentration and then the recovery behavior was investigated by purging the chamber with N2 The resistance of the sensor increased rapidly with the NH3 injection into the chamber The SWNTs are known as a p-type semiconductor [13] and NH3 is known as an active electron donor to SWNTs It is predicted from a theoretical calculation that an NH3 molecule adsorbed on the surface of SWNTs donates 0.04 electrons to nanotubes [14] Thus, when SWNTs accept electrons from ammonia molecules, those electrons will compensate free holes to increase the resistances of the SWNTs and the SWNT-based sensor The absorption amount of NH3 on CNTs depends on the concentration of NH3 and the availability of gas-binding sites on the CNTs The adsorption sites of the SWNTs did not saturate until the investigated ammonia concentration of 0.6%, as shown by the sensitivity investigation in Fig 5b The sensor could detect the concentration of NH3 down to 10 ppm even at room temperature The sensor showed a fast response time of less than 100 s The response time here is defined by the duration for which the resistance increases to 90% of the saturation value The sensitivity of the sensor increased with the increase of the NH3 concentration Upon purging the chamber the resistance decreased to the initial value, indicating that NH3 almost completely desorbs out of the SWNTs This result suggests a uniform surface condition of the SWNTs or adsorption sites The oxidation process might burn out the defective CNTs to leave only the CNTs with perfect graphene surfaces The recovery time of the sensor estimated through fitting with an exponentially decaying function was about 3.2 The fast recovery time of our sensor in comparison with others [15, 16] is considered to be due to the non-functionalized highquality SWNTs used The recovery time depends on the bonding force of an ammonia molecule to the SWNT surface with respect to that of the nitrogen flowing out Thus, it can vary with the nitrogen flow rate The sensitivity of the sensor linearly increased with the concentration of NH3 in the range of tenths of a volume percent as shown in Fig 5b www.pss-a.com © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 1824 Nguyen Duc Hoa et al.: Nanocomposite of SWNTs and SnO fabricated by soldering process Conclusions We introduced a new simple method to fabricate an ammonia gas sensor with a composite of SWNTs and SnO2 High-quality SWNTs were directly coated on a Sn/SiO2/Si substrate by the arc-discharge method The SWNTs formed in the matrix of SnO2 provide the main conduction channel that effectively varies in its conductance upon adsorption of ammonia The sensor shows a fast response time of ~100 s and quick recovery behavior The composite sensor can detect the concentration of NH3 down to the 10 ppm level at room temperature at atmospheric pressure Acknowledgment This research was performed with the financial support of the Center for Nanostructure Materials Technology under the 21st Century Frontier R&D Program of the Ministry of Science and Technology, a Korea Research Foundation Grant (No R01- 2004-10104-0), and the BK21 program, Korea References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] S K Hazra, S Roy, and S Basu, Mater Sci Eng B 110, 195 (2004) M Kugishima, G Sakai, K Shimanoe, and N Yamazoe, Sens Actuators B 108, 130 (2005) G S Devi, V B Subrahmanyam, S C Gadkari, and S K Gupta, Anal Chim Acta 568, 41 (2006) J Andzelm, N Govind, and A Maiti, Chem Phys Lett 421, 58 (2006) L Valentini, I Armentano, J M Kenny, C Cantalini, L Lozzi, and S Santucci, Appl Phys Lett 82, 961 (2003) P 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Hoa et al.: Nanocomposite of SWNTs and SnO fabricated by soldering process Conclusions We introduced a new simple method to fabricate an ammonia gas sensor with a composite of SWNTs and SnO2 High-quality... and (b) the sensitivity of the sensor at different ammonia gas concentrations sors of pure SnO2 and the composite of SWNTs and SnO2 were fabricated for comparison The NH3 sensing properties of. .. product and/ or defective tubes This will improve the sensitivity and the recovery time of the sensor The sensor property was compared with that of pure SnO2 The morphology and quality of the SWNTs and