Advanced Materials Research Vol. 620 (2013) pp 350-355 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.620.350
Synthesis and Gas Sensing Properties of SnO2 Nanostructures by Thermal Evaporation Wan Normiza Wan Mustapha1,a, Sheikh Abdul Rezan1,b*, Sabar Derita Hutagalung1,c, Nguyen Van Hieu2, Khairudin Mohamed3,a and Chan Kok You3,b 1
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia (USM), 14300 Nibong Tebal Penang, Malaysia. 2
ITIMS, Hanoi University of Science and Technology (HUST), Hanoi, Vietnam 3
School of Mechanical Engineering, USM, Malaysia.
a
[email protected], 1,b*
[email protected], 1,
[email protected], 2
[email protected], 3,
[email protected], 3,
[email protected]
Keywords: SnO2 nanowires, nanostructures, temperature, gas sensing, ethanol
Abstract. Tin oxide nanostructures (NS) were grown on silicon substrates by thermal evaporation method with three different parameters. These parameters were temperatures (650 °C, 750 °C and 850 °C), nickel catalyst concentrations (0, 5 and 10 milimoles) and tin powder source to substrate distances (2 cm, 4 cm and 6 cm). The parameters were found to affect the size and morphology of the synthesized nanostructures. Formation of nanospheres (NSs), nano-needles (NNs) and nanowires (NWs) of tin oxide were observed by Scanning Electron Microscope (SEM) at different synthesis conditions. Synthesis temperature was found to have most pronounced effect on the size and morphology of the nanostructures. Catalyst concentration has affected the porosity and growth of the nanostructures. The distance between source and substrate affected the nanostructures predominately on distribution and particle size. Energy dispersion X-ray (EDX) analysis confirms the presence of tin and oxygen in all nanostructures at all synthesis conditions. X-ray diffraction (XRD) proves the formation of tin oxide phase in all samples. Significant formation of tin oxide nanowires was observed at 850 °C. Gas sensing properties of SnO2 nanowires (NW) toward ethanol (C2H5OH) gas at 450°C with different volume concentration was measured. It was found SnO2 NW had good sensing properties for C2H5OH at 100 ppm compared to measurements made at 25-50 ppm. Introduction In recent years, one dimensional [1D] nanostructures in the form of tubes, belts, rods and wires received great attention due to its unique electronic, magnetic, optical and mechanical properties[1]. These nanostructures have wide range of application such as nano-electronics, chemical sensing, catalyst, bio-medicine and composite filler material due to high surface to volume ratio, quantum confinement effect, and the high fraction of chemically similar surface sides[2]. Among these various applications, semiconductor metal oxide-based gas sensors have attracted great attention over several decades due to their unique advantage, such as high sensitivity, low material and fabrication costs, ease of fabrication and compatibility with silicon micro fabrication. Gas sensors are important in environmental monitoring and home safety. Metal oxide sensors have been widely investigated because of nanoscale sensitivity and high compatibility with semiconductor processing methods. Recently SnO2 NWs have been used to develop an electronic nose[3]. It can analyze gases evolved during human breath by means of gaseous profile through changes in its electrical properties. SnO2 has a relatively large direct band gap of ~3.6 eV at room temperature. Advantages associated with a large band gaps include higher breakdown voltages, ability to sustain large electric fields, lower electronic noise, and high temperature and power All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 60.53.191.98-12/10/12,23:41:42)
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operation. As an n-type semiconductor, SnO2 has been actively explored for functions of detecting combustible gases such as CO, H2 and CH4. Combustible gases react with pre-adsorbed oxygen species like O(s) − or O(s) 2− to form carbon dioxide and water molecules. The reaction reduces the surface oxygen concentration, delivering some electrons back into the bulk, resulting in decreased electrical resistance. The absorption reaction on the surface of the NWs is given below by Eq. 1; CO(g) + O(s) − → CO2(g) + e−
(1)
In general, when comparing NWS with film type sensors based on small particles of SnO2, the gas response sensitivity are relatively low because of their large grain sizes. In the case of film type sensors, only a very thin layer of the films close to the surface can be activated during gas detection [4]. The sensitivity of the SnO2 can be modified by understanding the growth mechanism of the SnO2 nanowires, thus possibly allowing detection of gases at lower concentrations. Besides this, SnO2 nanowires show positive response to different gas species. These gases include ethanol, 2propanol and CO in the 2-100 parts per million (ppm) concentration ranges[5]. The more unique the conductivity patterns for the various gases towards SnO2 NWs, the better the gas response sensitivity magnitude. Materials and Methodology The tin powder used in this research has a purity of 99% (Sigma Aldrich). The synthesis of SnO2 NWs will be done using nickel nitrate (NiNO3) and gold (Au) as catalysts. These catalysts will control and accelerate the formation of tin oxide nanowires. A boron doped p-type silicon wafer was used, with orientation of (001). For NiNO3, it was spin coated on the surface of silicon substrate. The wafer was bought from by Siltronik Sdn Bhd and produce by czochraski method. This wafer serves as substrate for the deposition of SnO2 nanowires after coated with a catalyst. For the Au catalyst, sputtering method was chosen to coat the substrate. In this investigation, the Radio Corporation of America (RCA) cleaning method was used to clean the silicon substrates. For the growth process, 1 g of pure tin powder was placed in a quartz boat. Constant parameters that have been used in this project are flow rates of argon and O2 gasses (150 sccm and 50 sccm, respectively), growth time (2 hours), weight of source powder (1 g), and spin coating speed (7000 rpm) and time (30 s). Fig. 1a shows the furnace setup to synthesize the NWs. For manipulated parameters, three were investigated. These were growth temperature, distance between the source, substrate, and catalyst usage effects. The characterization of SnO2 nanostructures (NS) and NWs will be done by XRD (Bruker AXS diffractometer D8) and SEM/ EDX (Zeiss Gemini Supra 35VP). Fig. 1b shows the gas sensing test rig at ITIMS, Hanoi University[6]. Dilution of pure ethanol gas to 25-100 ppm was carried out by three mass flow controllers from an initial concentration of 1000 ppm. The resistance on the NW was measured by Keithley 2700 multimeter with data acquisition card, which measures V/I profile with respect to time. The measurement mechanism was done by 2point probe and shown in Fig. 1b.
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(a)
(b)
Fig. 1: a) Furnace setup at USM b) Gas sensor testing systems at ITIMS, Hanoi University [6]. Results and Discussion Temperature plays a crucial role in affecting the shape and size of the nanostructures[7]. Fig. 2 shows the different morphology of the nanostructures (NS) at different temperatures with NiNO3 catalyst. At 650 °C, the nanostructure possesses spherical like shape with high porosity. As the temperature increases to 750 °C, nanostructure possesses facet structure with lower porosity compared to nanostructures formed at 650 °C. While at 850 °C, the nanostructure possesses a wire or needle-like structure with large diameter (116-40 nm) and relatively short width. The nanostructures formed at 850 °C also exhibit lower porosity relatively when compared with NS formed at lower temperature (Fig. 2). Fig. 3a & 3b shows the effect of catalyst on the growth on SnO2 NS and NW. When Ni-based catalyst was used at 850 °C, spherical NS was obtained. Switching to a gold-based catalyst, NW was observed. In comparison to NS synthesized in Fig. 2, the use of catalyst affected the morphology, size, aspect ratio and density of the nanostructures. Without catalyst, the size of the wire/needle like nanostructures were relatively bigger than samples that has been deposited with nickel catalyst. Besides that, nanostructures that have been deposited on the catalyst had higher concentration of SnO2 compared to those without catalyst use. For the samples without catalysts, the nanostructures exhibited relatively higher porosity. The nanostructures formed with catalyst usage were relatively denser in appearance. Gold-based catalyst was ideal for the synthesis of NWs and these samples were used for gas sensing experiments. Fig. 4a & 4b shows the EDX and XRD spectra for SnO2 NW synthesized at 850 °C with gold catalyst. In general, the morphology and the size of the nanostructures can be controlled by the use of catalyst. The metallic seeds (catalyst) act as nucleation sites that promote the growth of SnO 2 nanostructures though Vapor-Liquid-Solid (VLS) or Vapor–Solid–Solid (VSS) mechanism. However, without seeding, oxide particles sometimes can be as large as micron sizes (Figure 2). This oxide seed will deposit during the initial stage of the condensation process and large structures formed by means of VSS mechanism.
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Fig. 2: Formation of tin oxide nanostructures at different temperature without catalyst at 2 cm distance from source. a) 650 °C b) 750 °C and c) 850 °C.
(a)
(b)
Fig. 3: (a) SnO2 NS formation at 850°C with 10 milimole Ni catalyst. (b) SnO2 NW formation at 850°C with Au catalyst.
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Fig. 4: (a) XRD spectra of SnO2 NW with Au catalyst. (b) EDX analysis of SnO2 NW formation at 850 °C with Au catalyst. Fig. 5 show the gas sensing resistance in air and ethanol at different concentrations. Sensitivity of the NWs to the absorbed gases can be calculated by the ratio of Ra/Rg1. Table 1 summarizes the ethanol detection sensitivity by SnO2 NWs at different concentrations. It was found that concentration of ethanol as low as 25 ppm induced a resistive change in the NWs with sensitivity close to ~1.4. At 100 ppm, the resistive change across the NWs was larger which translated to higher sensitivity of ~2.4. In comparison to other NWs for ethanol detection such as In2O3 NWs, a sensitivity value of 2 at 100 ppm was measured[8]. This sensitivity can be enhanced in SnO2 NW by doping. Studies with Lanthanum doping showed that sensitivity can be increase up to ~62[9]. Therefore, doping of SnO2 with other rare earth elements are expected to increase the sensitivity to ethanol. The gas sensing properties of undoped SnO2 NWs shows promising results, which can be easily tailored by using different dopants.
Fig. 5: Gas sensing properties of SnO2 NW formed at 850 °C with Au catalyst to C2H5OH 1
Ra (kΩ) = resistance in air, Rg (kΩ) = resistance in the gas environment.
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Table 1: Sensitivity of SnO2 NWs at different ethanol concentrations. Concentration Ra (kΩ) Rg (kΩ) S
25 ppm 50 ppm 100 ppm 3.68E+05 3.68E+05 3.68E+05 2.71E+05 2.15E+05 1.55E+05 1.36 1.71 2.37
Conclusion The morphology and size tin oxide nanowires and other nanostructures that form in this investigation were affected by catalyst concentration and temperature significantly. The use of different parameters (temperature and catalyst) will lead to the formation of different tin oxide nanostructures/nanowires. The shapes of the nanostructures that have been identified by SEM are nanowires, nano-needles and nano-spheres. All the samples show random alignment with nonuniform size distribution. XRD and EDX results show the presence of SnO2. It was observed that optimum formation of SnO2 occurred at 850 °C. Substrate distance and use of Ni catalyst with different concentrations did not produce NWs but a combination of NNs and NSs. Gold catalyst produced NWs. Gas sensing properties of SnO2 NWs with ethanol showed good resistive change from 25-100 ppm. The sensitivity calculated was ~2.4 at 100 ppm, which was better than In2O3 NWs, reported in the literature [8]. References [1]
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