synthesis and high gas sensitivity of tin oxide nanotubes

A vailable online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 313–317 Synthesis and high gas sensitivity of tin oxide nanotubes G.X. Wang a,b,∗ , J.S. Park b , M.S. Park b , X.L. Gou a,b a School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia b Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia Received 24 August 2007; received in revised form 14 November 2007; accepted 14 November 2007 Available online 24 November 2007 Abstract Semiconductor tin oxide (SnO 2 ) nanotubes have been synthesised in bulk quantities using a sol–gel template (AAO membrane) synthetic technique. The morphology and crystal structure of SnO 2 nanotubes were characterised by a field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM). The as-prepared SnO 2 nanotubes are polycrystalline with an outer diameter of 200 nm, an inner diameter of about 150 nm and a length extending to tens of micrometers. SnO 2 nanotube sensors exhibited high sensitivity towards ethanol gas. © 2007 Elsevier B.V. All rights reserved. Keywords: Tin oxides; Nanotubes; Sol–gel; Gas-sensors; Nanocrystallites 1. Introduction One-dimensional (1D) nanostructures including nanotubes, nanowires, and nanoribbons have attracted both intensive and extensive research, which can be mainly attributed to their unique chemical and physical properties, and their intriguing technological applications [1,2]. In particular, 1D semicon- ductor nanostructures provide building-blocks for fabricating functional nanoscale electronic, optoelectronic, photonic, chem- ical and biomedical devices based on the bottom-up paradigm [3–7]. Among all the potential applications, nanoscale chemical and biological sensors are generally considered as one of the impor- tant areas for nanotechnology to enter into practical applications [8]. The high surface-to-volume ratio of 1D nanostructures induces extremely high sensitivity to adsorbed chemical or bio- logical species on the surface of nanosensors. Lieber et al. have developed silicon nanowire sensors and implemented them as the real-time sensors for detecting pH and biological species ∗ Corresponding author at: School of Mechanical, Materials and Mecha- tronic Engineering, University of Wollongong, Northfield Avenue, NSW 2522, Australia. Fax: +61 2 42215731. E-mail address: gwang@uow.edu.au (G.X. Wang). [9]. The principle of the Si nanowire sensors is based on the conductance (surface charge) change caused by protonation and deprotonation associated with the adsorbed molecular species. Single and multiple In 2 O 3 nanowire sensors have shown high sensitivity to NO 2 and NH 3 gas [10,11]. SnO 2 is a wide- bandgap (3.6 eV) semiconductor. The electronic conductivity of SnO 2 is significantly influenced by the effects on its sur- face states of molecular adsorption. It has been widely explored as an effective gas sensor, traditionally in the forms of thin or thick films with low sensitivity and long response time [12]. Recently, SnO 2 nanobelts have been tested for their sensitiv- ity to environmental pollutants such as CO and NO 2 [13]. Photochemical SnO 2 nanoribbon sensors have been fabricated for detecting low concentration of NO 2 at room temperature under UV light [14]. Polycrystalline SnO 2 nanowire sensors were also developed for sensing ethanol, CO and H 2 gas [15]. SnO 2 nanohole array sensors exhibited reversible response to H 2 [16]. Herein, we describe the synthesis of polycrystalline SnO 2 nanotubes using the sol–gel template method, and the fabrica- tion of SnO 2 nanotube sensors. Due to their one dimensional and tubular structure, SnO 2 nanotube sensors exhibited high sensitivity and quick response time for detecting ethanol and ammonia gas. 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.11.032 314 G.X. Wang et al. / Sensors and Actuators B 131 (2008) 313–317 2. Experimental Anodic aluminium oxide (AAO) membranes (Whatman, 200 nm pore, 60 ␮m in thickness, and 47 mm in diameter) were used as the template for preparing SnO 2 nanotubes. The chemicals used were tin(II) chloride dehydrate (SnCl 2 ·2H 2 O, Aldrich, A.C.S. reagent), sodium hydroxide (Aldrich, 98%) and hydrochloric acid (36%, Merck). SnO 2 nanotubes were synthe- sised via a sol–gel and sintering process following these steps: (i) 3.38 g SnCl 2 , 4.7 ml ethanol and 0.3 ml HCl were mixed together and aged for 24 h, during which time the colour of the solution changed from white to pale yellow and finally form- ing a transparent and highly viscous gel. Then, 0.3 ml deionised water was added to the as-prepared gel to form a solution; (ii) the AAO templates were impregnated by vacuum suction. The solution was forced to pass through the pores of the template and adhere on the pore walls; (iii) the impregnated template was dried at 100 ◦ C and then sintered at 500 ◦ C for 3 h to convert the tin hydroxide to tin oxide; (iv) after sintering, the AAO mem- brane was dissolved in 6 M NaOH solution. The undissolved SnO 2 nanotubes were collected and washed through a filter- ing process to remove Na + and Al 3+ . The crystal structures and morphologies of the SnO 2 nanotubes were characterised using X-ray diffraction (XRD, Philips 1730), field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and trans- mission electron microscopy (TEM, JEOL 2011). The specific surface area was measured by the Brunauer–Emmett–Teller (BET) method at 77 K using a NOVA 1000 high-speed gas sorption analyzer (Quantachrome Corporation, USA). The gas sensing properties of the as-prepared SnO 2 nanotubes and SnO 2 nanopowders (61 nm in average particle size (APS), Nanostruc- tured & Amorphous Materials Inc., USA) were measured using a WS-30A gas sensor measurement system. SnO 2 nanotubes and nanopowders were mixed with polyvinyl acetate (PVA) binder to form a slurry, and then pasted on to ceramic tubes (2 mm in diameter) between Au electrodes, which were connected with four platinum wires. The fabricated sensors were fitted into the gas-sensing measurement apparatus. Given amounts of ethanol and ammonia gas were injected into the testing chamber by a micro-syringe injector. The gas sensing response was defined as the ratio R air /R gas , where R air and R gas are the electrical resis- tance of the sensors in air and in gas, respectively. The gas sensing measurement was carried out at a working temperature of 200 ◦ C. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of SnO 2 nanopow- ders and SnO 2 nanotubes. All diffraction lines can be indexed to the tetragonal rutile phase (JCPDS #41-1445). It should be noted that SnO 2 nanotubes have much broader diffraction peaks and lower diffraction intensities than that of SnO 2 nanopowders, indicating a much small crystal size for the nanotubes. The aver- age crystal size of SnO 2 nanotubes was calculated to be about 15 nm using the Scherrer equation d = κλ/β cos θ. The general morphology of SnO 2 nanotubes was observed by FE-SEM and is shown in Fig. 2. The as-prepared SnO 2 nanotubes have lengths of Fig. 1. X-ray diffraction patterns of SnO 2 nanotubes and nanopowders. a few micrometers. The SnO 2 nanotubes were partially broken, which could have been induced during the sintering process or the subsequent filtering process. The inset in Fig. 2 is a top view of the SnO 2 nanotube bundle, from which we can clearly see the hollow and tubular structure with an outer diameter of 200 nm. We measured the BET surface areas of commercial nanosize SnO 2 powders and as-prepared SnO 2 nanotubes. SnO 2 nano- size powders have a BET surface area of 15.2 m 2 /g, while SnO 2 nanotubes have a surface area of 45.6 m 2 /g. The crystal structure of the SnO 2 nanotubes was further analysed by TEM and high resolution TEM (HRTEM). A general TEM image of a SnO 2 nanotube is shown in Fig. 3(a). The SnO 2 nanotubes are poly- crystalline, with the small nanosize crystals bonded together through the sintering process. Selected area electron diffrac- tion (SAED) was performed on the individual SnO 2 nanotubes (the inset in Fig. 3(a)). The indexed ring patterns confirmed the tetragonal crystal structure of the SnO 2 nanocrystals that form the nanotube. Fig. 3(b) shows a high resolution TEM image of a SnO 2 nanotube, in which the individual crystal sizes are in Fig. 2. FESEM image of SnO 2 nanotubes. The inset is a top view of SnO 2 nanotube bundle. G.X. Wang et al. / Sensors and Actuators B 131 (2008) 313–317 315 Fig. 3. (a) TEM image of a single SnO 2 nanotube. Inset: selected area electron- diffraction pattern. (b) HRTEM image of a portion of a SnO 2 nanotube. the range of 10–20 nm. The lattice spacing was measured to be 0.47 nm. SnO 2 nanotubes were tested as chemical sensors of ethanol and ammonia gas. As a comparison, the sensing properties of SnO 2 nanopodwers (APS: 61 nm) were also tested. The gas sen- sitivities were measured in air at 25 ◦ C under a relative humidity (RH = 40–50%). Through pre-testing, we first determined that the optimised sensor working temperature was 200 ◦ C, at which both SnO 2 nanotubes and SnO 2 nanopowders exhibited an opti- mal performance. Subsequently, all sensing measurements were conducted at this working temperature. Fig. 4(a) shows the real-time gas sensing response towards ethanol vapor for SnO 2 nanotube and nanopowder sensors. The ethanol vapor concen- trations were varied. Initially, the SnO 2 nanotube sensor showed similar response to the SnO 2 nanopowders at the very low concentration (10 ppm). However, as the ethanol vapor concen- tration increased, the SnO 2 nanotube sensor demonstrated larger response. In general, on increasing the gas concentrations, the response increase proportionally. Fig. 4(b) shows the gas sens- ing response versus the ethanol concentrations in the range of 10–1000 ppm. It should be noted that SnO 2 nanotubes have more than 1.5 times larger response than the corresponding nanopow- ders. This result is comparable to the previously reported ethanol Fig. 4. (a) Real-time sensing response to ethanol gas in air. Inset: equivalent electrical circuit for SnO 2 nanopowder sensor and SnO 2 nanotube sensor. (b) Sensing response vs. ethanol vapor concentration. gas sensing performance using nanocrystalline SnO 2 powders with an average crystallite size of 8 nm [17]. By analysing the transient response characteristics of SnO 2 nanotube and nanopowder sensors, we found that the response time to gas on and recovery time to gas off take less than 5 s. When examining the shape of the response curves in Fig. 4(a), we can see that the SnO 2 nanotube sensor required more response time to reach its maximum value at all concentrations when the gas was on; similarly, there was also a delay before recovery when the gas was off. This retard response behavior of SnO 2 nanotube sensor is typically related to the small crystal size and 1D structure of the nanotubes. It can be explained by using the equivalent electric circuit models shown in the inset in Fig. 4(a). SnO 2 nanopowders could be considered as a simple resistor because individual crystals are loosely agglomerated. There- fore, the SnO 2 nanopowder sensor shows straight lines in the response profiles. On the other hand, the SnO 2 nanotubes can be modeled as a capacitor connected in parallel with a resistor and then serially connected with another resistor. The capacitance behavior mainly comes from the grain boundaries between the tiny nanosize crystals that form the nanotubes [18]. This model 316 G.X. Wang et al. / Sensors and Actuators B 131 (2008) 313–317 Fig. 5. Real-time sensing response to ammonia gas in air. can satisfactorily explain the retarded response behavior of the SnO 2 nanotube sensor. The responses towards ammonia are shown in Fig. 5. When attempting to detect ammonia gas, the SnO 2 nanopowder sensor showed no response at low concentration, and a slight change in the resistance at high concentrations, but the response was unstable and had serious fluctuations. In contrast, the SnO 2 nan- otube sensor was active even at 10 ppm. Its response towards ammonia increased proportionally with the increasing gas con- centration. However, the overall sensing response performance towards ammonia gas is much lower than that to ethanol gas for both SnO 2 nanosize powders and nanotubes. The response curves in Figs. 4 and 5 clearly indicated a sensing mechanism that could be described as gas surface chemisorption and electron acceptance, resulting in a decrease in the sensor resistance. SnO 2 is an n-type wide band gap semiconductor. Its electronic conduction originates from point defects, which either are oxygen vacancies or foreign atoms that act as donors or acceptors. In the ambient environment, SnO 2 nanocrystals are expected to adsorb both oxygen and mois- ture, in which moisture may be adsorbed as hydroxyl groups. The adsorbed O 2− and OH − groups trap electrons from the conduction band of SnO 2 nanocrystals, inducing the formation of a depletion layer on the surface of the SnO 2 nanocrystals [19]. When exposed to ethanol vapour, CH 3 CH 2 OH molecules are chemisorbed at the active sites on the surface of the SnO 2 nanocrystals. These ethanol molecules will be oxidised by the adsorbed oxygen and lattice oxygen (O 2− )ofSnO 2 at the sensor working temperature. During this oxidation process, electrons will transfer to the surface of the SnO 2 nanocrystals to lower the number of trapped electrons, inducing a decrease in the resistance. A similar mechanism should be ascribed to the detec- tion of NH 3 gas because NH 3 is commonly considered to work as a reducing agent and to donate electrons [20]. Therefore, when exposed to NH 3 molecules, a SnO 2 sensor responds with the increased conductivity. SnO 2 nanotubes consist of small nanocrystals joined together into 1D tubular structure, resulting in many more active sites for gas chemisorption. In addition, both the inner and outer walls of SnO 2 nanotubes can adsorb a large number of gas molecules. Consequently, SnO 2 nanotubes show an enhanced sensitivity compared to the corresponding nanopowders. 4. Conclusions In summary, polycrystalline SnO 2 nanotubes have been pre- pared via the sol–gel template method. FE-SEM observation shows the tubular 1D nanostructure. TEM and HRTEM analy- sis confirmed the polycrystalline nature and tetragonal crystal structure of the SnO 2 nanotubes. The SnO 2 nanotubes exhibited an enhanced sensitivity to ethanol gas. 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Biographies G.X. Wang received his PhD degree in Materials Science and Engineering in 2001 from University of Wollongong, Australia. He is currently working as a senior lecturer atSchoolof Mechanical, Materials and MechatronicEngineering, University of Wollongong. His major research interests include nanostructured functional materials, materials chemistry in energy storage and conversion, and development of chemical and biological sensors. J.S. Park received his Master degree in Materials Engineering in 2005 from Andong National University, Korea. Currently, he is a PhD candidate at Insti- tute for Superconducting and Electronic Materials, University of Wollongong, Australia. M.S. Park received his Master degree in Materials Science and Engineering in 2005 from Korea Advanced Institute of Science and Technology, Korea. He is a currently PhD candidate at Institute for Superconducting and Electronic Materials, University of Wollongong, Australia. X.L. Gou received his PhD degree in Chemistry in2006fromNankai University, China. He is a research fellow at Institute for Superconducting and Electronic Materials, University of Wollongong, Australia. His research interests include chemical synthesis of functional nanosize inorganic materials and development of gas sensors. . online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 313–317 Synthesis and high gas sensitivity of tin oxide nanotubes G.X. Wang a,b,∗ , J.S. Park b , M.S sensors exhibited high sensitivity towards ethanol gas. © 2007 Elsevier B.V. All rights reserved. Keywords: Tin oxides; Nanotubes; Sol–gel; Gas- sensors; Nanocrystallites 1.