Enhanced ethanol sensing properties of multiple networked Au-doped In2O3 nanotube sensors

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  • Journal of Physics and Chemistry of Solids 74 (2013) 979984Contents lists available at SciVerse ScienceDirectJournal of Physics and Chemistry of Solids0022-36

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    n Corr

    E-mjournal homepage: www.elsevier.com/locate/jpcsEnhanced ethanol sensing properties of multiple networked Au-dopedIn2O3 nanotube sensorsSoyeon An a, Sunghoon Park a, Hyunsung Ko a, Changhyun Jin a, Wan In Lee b, Chongmu Lee a,n

    a Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Koreab Department of Chemistry, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751, Republic of Koreaa r t i c l e i n f o

    Article history:

    Received 11 June 2012

    Received in revised form

    10 October 2012

    Accepted 17 February 2013Available online 1 March 2013

    Keywords:

    A. Oxides

    A. Nanostructures

    A. Semiconductors

    B. Vapor deposition

    C. Electron microscopy97/$ - see front matter & 2013 Elsevier Ltd. A

    x.doi.org/10.1016/j.jpcs.2013.02.016

    esponding author. Tel.: 82 32 860 7536; faxail address: cmlee@inha.ac.kr (C. Lee).a b s t r a c t

    In2O3 nanotubes were synthesized as gas sensors using TeO2 nanowires as a template. Scanning and

    transmission electron microscopy revealed the tubes to have diameters of a few hundred nanometers,

    wall thickness of 25 nm and lengths up to a few millimeters. Multiple networked Au-doped In2O3nanotube sensors showed responses of 1871219% to 50250 ppm C2H5OH at 300 1C. These responsesare far superior to those obtained by undoped In2O3 nanotubes and stronger than those obtained by

    pure In2O3 nanowires at 370 1C. In addition, the ethanol sensing mechanism of the Au-doped In2O3nanotube sensors is discussed.

    & 2013 Elsevier Ltd. All rights reserved.1. Introduction

    Indium oxide (In2O3) is as a typical n-type semiconductor witha wide direct bandgap of 3.553.75 eV that has been studiedwidely for its potential applications in solar cells, optical andelectrical devices, and gas sensors owing to its high opticaltransparency and electrical conductivity [17]. Thus far, a rangeof In2O3 nanostructures including nanocrystals [8,9], nanowires[10], nanorods [11,12] and nanobelts [13] have been synthesizedusing a range of methods and their gas-sensing properties havebeen explored. Many researchers reported that In2O3 is a promis-ing material for the detection of low concentrations of oxidizinggases, such as O3 [14,15], NO2 [16,17] and reducing gases, such asCO and H2 [18,19], but the response of pure In2O3 sensors toC2H5OH (ethanol) was not high enough. The response to 100 ppmC2H5OH was less than 3 based on pure In2O3 nanowire sensors[20]. Liu et al. prepared porous hierarchical In2O3 micro-/nano-structures, and reported that the materials obtained showed goodsensitivity and selectivity [21]. Li et al. prepared hierarchicalrod-like In2O3 microbundles and found that the microbundlesexhibited superior sensing performance to 2-chloroethanol vapor[22]. This paper reports the enhanced ethanol gas sensing proper-ties of the Au-doped In2O3 nanotubes synthesized using TeO2nanowires as a template, particularly the enhanced ethanol gasll rights reserved.

    : 82 32 862 5546.sensing properties of multiple-networked Au-doped In2O3nanotubes.2. Experimental

    In2O3 nanotubes were synthesized on Si substrates using TeO2nanowires as a template. The synthesis process consisted of threesteps: (1st step) synthesis of TeO2 nanowires by the thermalevaporation of Te powders; (2nd step) sputter-deposition of In2O3(or Au-doped In2O3) on the TeO2 nanowires; and (3rd step)removal of the TeO2 nanowires by annealing. In this study, TeO2nanowires were used as templates for the synthesis of In2O3nanotubes because TeO2 is volatile at a lower temperaturecompared to other material nanowires. For example, if we useZnO nanowires as templates, it is necessary to heat the core-shellnanowire samples above 1000 1C to remove the ZnO nanowirecores. In contrast, in the case of using TeO2 nanowires astemplates, heating to 500 1C is enough to remove them. TeO2nanowires are ideal as templates that can be removed withoutgiving the In2O3 shells a significant thermal damage. Au thinfilms, 4 nm in thickness were deposited on Si (100) by directcurrent (DC) magnetron sputtering. An alumina boat loadedwith Te powder and a Au-coated Si substrate was placed in aquartz tube, which was mounted inside a horizontal tube furnace.The thermal evaporation process for the synthesis of TeO2nanowires was performed in air at 500 1C for 1 h. Sub-sequently, the resulting TeO2 nanowires were transferred to a

    www.elsevier.com/locate/jpcswww.elsevier.com/locate/jpcshttp://dx.doi.org/10.1016/j.jpcs.2013.02.016http://dx.doi.org/10.1016/j.jpcs.2013.02.016http://dx.doi.org/10.1016/j.jpcs.2013.02.016mailto:cmlee@inha.ac.krhttp://dx.doi.org/10.1016/j.jpcs.2013.02.016

  • S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979984980radio frequency (RF)-magnetron sputtering chamber. In2O3 thinfilms were sputter-deposited on the TeO2 nanowires. The basevacuum, chamber pressure, Ar gas flow rate, RF-power and timefor the sputter deposition were 1.0106 Torr, 2.0102 Torr,20 sccm, 100 W, and 10 min, respectively. On the other hand, inthe case of Au-doped In2O3 nanotubes formation, In2O3 and Auwere cosputtered on the synthesized TeO2 nanowires. The basevacuum, chamber pressure, Ar gas flow rate, RF-power (In2O3),DC-current (Au), and time for the sputter deposition were1.0106 Torr, 2.0102 Torr, 20 sccm, 100 W, 15 mA, and10 min, respectively. The TeO2 nanowires coated with In2O3 orAu-doped In2O3 thin films were then annealed in air at 500 1C for1 h to remove the TeO2 nanowires. The chamber pressure duringthe annealing process was 1.0103 Torr.

    The collected In2O3 or Au-doped In2O3 nanotube samples werecharacterized by scanning electron microscopy (SEM, HitachiS-4200) equipped with an energy dispersive X-ray spectrometer(EDS), transmission electron microscopy (TEM, Philips CM-200),Fig. 1. Schematic diagram of a multi-networked sensor fabricated with Au-dopedIn2O3 nanotubes.

    Fig. 2. (a) SEM image of the Au-doped In2O3 nanotubes synthesized using TeO2 nanow(c) Low-magnification TEM image of a typical Au-doped In2O3 tube. (d) Enlarged TEM i

    pattern.and X-ray diffraction (XRD, Philips Xpert MRD diffractometer).Glancing angle (0.51) Cu-Ka radiation was used for XRD.

    For the sensing measurements, 300 nm thick SiO2 thin filmswere grown on p-type (100) Si and Ni (50 nm in thickness).Au (100 nm) thin films were then deposited on a SiO2/Sisubstrate by DC-magnetron sputtering to form electrodes usingan interdigital electrode (IDE) mask. Multiple networked In2O3nanotube gas sensors were fabricated by pouring a few drops ofnanotubes-suspended in ethanol onto oxidized Si substratesequipped with a pair of IDEs with a gap length of 20 mm (Fig. 1).Multiple networked Au-doped In2O3 nanotube gas sensors werealso fabricated in a similar manner. The electrical and gas sensingproperties of the In2O3 or Au-doped In2O3 nanotubes were mea-sured at 300 1C using a home-made gas sensing measurementsystem. During the measurements, the nanotube gas sensors wereplaced in a sealed quartz tube with an electrical feed through.A set amount of C2H5OH (499.99%) gas was injected into thetesting tube through a microsyringe to obtain a C2H5OH concen-tration of 50250 ppm, and the electrical current in the nanotubeswas monitored. The response of the In2O3 or Au-doped In2O3nanotube sensors is defined as Ra/Rg for C2H5OH, where Ra and Rgare the electrical resistance of the sensors in air and the target gas,respectively. The response time is defined as the time required forthe electrical current to reach 90% of the equilibrium value afterinjecting the gas, and the recovery time is defined as the timeneeded for the sensor current to return to 90% of the originalcurrent in air after removing the gas.3. Results and discussion

    Fig. 2(a) shows a SEM image of the Au-doped In2O3 nanotubessynthesized using TeO2 nanowires as a template. Many Au nano-particles formed on the In2O3 nanotube surfaces through theires as a template. (b) XRD pattern of the Au-doped and undoped In2O3 nanotubes.

    mage of the particles at the surface of the nanotube in (c). (e) Corresponding SAED

  • S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979984 981precipitation of Au catalysts added to the nanotubes. The tubelengths were up to a few millimeters, as shown in Fig. 2(a). Anenlarged SEM image of a typical Au-doped In2O3 nanotube showed(inset in Fig. 2(a)) that the tube diameter was 100 nm. Theintensity peaks in the XRD pattern (Fig. 2(b)) were assigned to the(211), (222), (332), (134), (400), (611) and (622) reflections ofbody-centered cubic-structured In2O3 with a lattice constant ofa1.011 nm (JCPDS no. 894595). The high intensity of the (222)peak of the nanotubes indicated a strong (222) preferred orientation.The specific surface area of the In2O3 nanotubes measured using theBrunauer, Emmett and Teller (BET) method [23] was 17.83m2/g,Fig. 3. (a) EDX spectrum of the Au-doped In2O3 nanotubes synthesized using TeO2 nadrawn across the diameter of an Au-doped In2O3 nanotube.which is rather small than that of other material nanotubes reportedpreviously, possible because of the relatively large size of thenanotubes. The low-magnification TEM image (Fig. 2(c)) showedmany Au nanoparticles dispersed over the In2O3 nanotube outer wallsurface, which confirmed the tube wall thickness to be was 25 nm.High-resolution TEM image (Fig. 2(d)) revealed fringe patterns withresolved spacings of 0.29 and 0.41 nm, corresponding to the {222}and {211} lattice planes in In2O3. The large number of reflection spotson concentric circles in the corresponding selected area electrondiffraction (SAED) pattern (F