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

    http://d

    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 (Fig. 2(e)) suggests that the Au-doped In2O3nanotubes were polycrystalline. Au was detected in the EDXnowires as a template. (b) EDXS line scanning concentration profiles along a line

  • Fig. 4. (a) Dynamic response of the In2O3 nanotube gas sensor. (b) Dynamic response of the Au-doped In2O3 nanotube gas sensor. (c) Enlarged part of (a) at 250 ppmethanol (d) Enlarged part of (b) at 250 ppm ethanol. (e) Responses of the undoped and Au-doped In2O3 nanotube gas sensors as a function of the ethanol gas concentration.

    Table 1Responses, response times, and recovery times measured at different C2H5OH

    concentrations for the Au-doped and undoped In2O3 nanotube sensors at 300 1C.

    Ethanol conc. (ppm) Response (%) Response time (s) Recovery time (s)

    In2O3 AuIn2O3 In2O3 AuIn2O3 In2O3 AuIn2O3

    50 111.54 186.84 15 50 90 100

    100 116.09 238.62 12 45 110 120

    150 117.98 316.55 5 10 100 100

    200 130.43 462.86 6 3 110 110

    250 135.26 1218.92 4 5 120 110

    S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979984982spectrum (Fig. 3(a)). The Cu and C in the EDX spectrum wereattributed to the TEM grid. The EDXS line scanning concentrationprofiles along a line drawn across the diameter of an Au-doped In2O3nanotube (Fig. 3(b)) exhibited a clear tubular structure. All concen-tration profiles of the three elements, In, O and Au composing thenanotubes showed high concentrations at the two edge regions(walls) and lower concentrations at the central region.

    Fig. 4(a) and (b) shows the dynamic responses of the undopedand Au-doped In2O3 nanotubes, respectively, to an oxidizingethanol gas at 300 1C. Fig. 4(c) and (d) shows simply enlargedimages of Fig. 4(a) and (b) at 250 ppm ethanol, showing themoments of gas input and gas stop. The sensor responded well toethanol gas. The resistance decreased rapidly when the nanotubesensors were exposed to ethanol gas and recovered almost to theinitial value when the ethanol gas supply was stopped and air wasintroduced. The responses of the In2O3 nanotube sensors were notstable and reproducible for repeated testing cycles. The responsecurve showed an irregular shape near the minimum resistance foreach testing cycle, which might be due to nonuniformity in thediameter of the nanotubes. Table 1 lists the responses calculatedfrom Fig. 4(a) and (b). The In2O3 nanotube sensor showedresponses of approximately 112%, 116%, 118%, 130% and 135%to 50, 100, 150, 200 and 250 ppm ethanol, respectively. Incontrast, the Au-doped In2O3 nanotube sensor showed responsesof approximately 187%, 239%, 317%, 463% and 1219% to 50, 100,

  • Table 2Comparison of the responses, response times and recovery times of the Au-doped

    In2O3 nanotube sensor with those of oxide 1D nanostructure sensors.

    Nano-

    materials

    Ethanol

    conc.

    (ppm)

    Temp.

    (1C)Response

    (%)

    Response

    time (s)

    Recovery

    time (s)

    Ref.

    AuIn2O3nanotubes

    250 250 1219 90 14 Present

    work

    TiO2nanotubes

    300 250 68,000 110 19 24

    TiO2nanotubes

    5000 200 16 25

    SnO2nanorods

    300 300 3140 1 1 26

    PtSnO2nanopowders

    100 150

    350

    4000 12 360 27

    ZnO

    nanowires

    1500 300 61 28

    TiO2nanobelts

    500 250 3366 12 12 29

    AgTiO2nanobelt

    500 200 4171 12 12 29

    CoFe2O4nanopowders

    50 150 7190 50 60 30

    CoZnO

    nanorods

    50 350 987 31

    Table 3Responses, response times, and recovery times measured at different NO2concentrations for the Au-doped In2O3 nanotube sensor at 300 1C.

    NO2 conc.

    (ppm)

    Response

    (%)

    Response

    time

    (s)

    Recovery

    time

    (s)

    1 350.99 161 181

    2 489.96 162 181

    3 520.66 144 165

    4 817.13 158 168

    5 1075.75 153 89

    Fig. 5. (a) Dynamic response of the Au-doped In2O3 nanotube gas sensor.(b) Enlarged part of (a) at 5 ppm NO2.

    S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979984 983150, 200 and 250 ppm ethanol, respectively. Consequently, theresponses of the In2O3 nanotubes to 50, 100, 150, 200 and250 ppm ethanol gases were increased by approximately 1.7,2.1, 2.7, 3.6 and 9.0 fold by Au doping, respectively.

    These responses obtained using the Au-doped In2O3 nanotubesensor were also stronger than those (200% and 600% to 100and 250 ppm C2H5OH at 370 1C, respectively) obtained using pureIn2O3 nanowire sensors [20] despite the lower temperature (300 1C)used in the present study. The enhanced sensitivity of the Au-dopedIn2O3 nanotube sensor compared to that of pure (undoped) In2O3nanowire sensors might be due to the larger surface-to-volume ratioof the former than that of the latter as well as the effect of the Aucatalyst. Nanotubes have two surfaces (outer and inner wallsurfaces), whereas nanowires have only one surface.

    The response values of the Au-doped In2O3 nanotube sensortowards ethanol obtained in this study are comparable to othercompeting nanomaterials except for TiO2 nanotubes to 300 ppmethanol, but the response and recovery times of the former wereshorter than the latter (Table 2) [2431]. A direct comparison ofthe response between the In2O3 nanotube sensor and previousreports obtained using other nanomaterial sensors was difficultdue to the different sensing measurement systems and experi-mental conditions used in the experiments. Fig. 5 and Table 3show that the response of the Au-doped In2O3 nanotube sensor isgood not only towards C2H5OH gas but also towards other gaseslike NO2. The response values of the Au-dop...

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