Journal of Physics and Chemistry of Solids 74 (2013) 979–984

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Journal of Physics and Chemistry of Solids

0022-36

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Enhanced ethanol sensing properties of multiple networked Au-dopedIn2O3 nanotube sensors

Soyeon 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 Korea

a 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 microscopy

97/$ - see front matter & 2013 Elsevier Ltd. A

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

esponding author. Tel.: þ82 32 860 7536; fax

ail 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 In2O3

nanotube sensors showed responses of 187–1219% to 50–250 ppm C2H5OH at 300 1C. These responses

are 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 In2O3

nanotube 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 3.55–3.75 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 [1–7]. 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 TeO2

nanowires as a template, particularly the enhanced ethanol gas

ll rights reserved.

: þ82 32 862 5546.

sensing properties of multiple-networked Au-doped In2O3

nanotubes.

2. Experimental

In2O3 nanotubes were synthesized on Si substrates using TeO2

nanowires 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, TeO2

nanowires were used as templates for the synthesis of In2O3

nanotubes 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. TeO2

nanowires 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 TeO2

nanowires was performed in air at 500 1C for 1 h. Sub-sequently, the resulting TeO2 nanowires were transferred to a

S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979–984980

radio 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 1.0�10�6 Torr, 2.0�10�2 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 were1.0�10�6 Torr, 2.0�10�2 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 1.0�10�3 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-doped

In2O3 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 X’pert 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 In2O3

nanotube 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 50–250 ppm, and the electrical current in the nanotubeswas monitored. The response of the In2O3 or Au-doped In2O3

nanotube sensors is defined as Ra/Rg for C2H5OH, where Ra and Rg

are 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 the

ires 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) 979–984 981

precipitation 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 ofa¼¼1.011 nm (JCPDS no. 89–4595). 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 17.83 m2/g,

Fig. 3. (a) EDX spectrum of the Au-doped In2O3 nanotubes synthesized using TeO2 na

drawn 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 In2O3

nanotubes were polycrystalline. Au was detected in the EDX

nowires 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 ppm

ethanol (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 Au–In2O3 In2O3 Au–In2O3 In2O3 Au–In2O3

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) 979–984982

spectrum (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 In2O3

nanotube (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 the

diameter 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.

Au–In2O3

nanotubes

250 250 1219 90 14 Present

work

TiO2

nanotubes

300 250 68,000 110 19 24

TiO2

nanotubes

5000 200 16 – – 25

SnO2

nanorods

300 300 3140 1 1 26

Pt–SnO2

nanopowders

100 150–

350

4000 12 360 27

ZnO

nanowires

1500 300 61 – – 28

TiO2

nanobelts

500 250 3366 1–2 1–2 29

Ag–TiO2

nanobelt

500 200 4171 1–2 1–2 29

CoFe2O4

nanopowders

50 150 7190 50 60 30

Co–ZnO

nanorods

50 350 987 – – 31

Table 3Responses, response times, and recovery times measured at different NO2

concentrations 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) 979–984 983

150, 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) In2O3

nanowire 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) [24–31]. 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-doped In2O3 nanotubesensor ranged from 351% to 1076% to 1–5 ppm NO2. This is a far

higher response value than that (�60%) of In2O3 nanowire sensorto 10 ppm NO2 [32].

In addition, the responses of both the undoped and Au-dopedIn2O3 nanotube sensors tended to increase rapidly with increas-ing ethanol gas concentration, but the response of the Au-dopedIn2O3 nanotube sensor tended to increase more with increasingethanol gas concentration than that of the undoped In2O3 nano-tube sensor. Fig. 4(e) shows the responses extracted fromFig. 4(a) and (b) as a function of the ethanol concentration.A linear relationship was observed between the response andethanol gas concentration in the range of 50–250 ppm. Theresponse of an oxide semiconductor can normally be expressedas R¼¼A [C]n

þB, where A and B are constants, and n and [C] arethe exponent and target gas concentration, respectively. Datafitting gave R¼¼4.577 [C]–201.7 and R¼¼0.124 [C]þ103.7 forthe Au-doped and undoped In2O3 nanotube sensors, respectively.The recovery time of the Au-doped In2O3 nanotubes was similarto that of the undoped In2O3 nanotubes at the same ethanolconcentration. On the other hand, the response time of the formerwas slightly longer than that of the latter at the same ethanolconcentration, but the response times were still less than 1 min.This suggests that Au doping does not affect the sensing speed ofIn2O3 nanotube sensors significantly.

The ethanol gas sensing mechanism of the In2O3 nanotubesensor can be modeled using the surface-depletion model [33].When the In2O3 nanotube sensor is exposed to air, it interactswith oxygen by transferring electrons from the conduction bandto the adsorbed oxygen atoms, forming ionic species, such as O� ,O2� and O2

� , as illustrated below

O2 (g)-O2 (ads) (1)

S. An et al. / Journal of Physics and Chemistry of Solids 74 (2013) 979–984984

O2 (ads)þe�- O2� (ads) (2)

O2� (ads)þe�- 2O� (ads) (3)

O� (ads)þe�- O2� (ads) (4)

A depletion region is created in the wall of the In2O3 nano-tubes due to the consumption of electrons in the surface region ofthe In2O3 nanotube walls [34], resulting in an increase in theelectrical resistance in the In2O3 nanotubes. The more the oxygenions on the surface, the thicker the surface depletion layer, thehigher the potential barrier, and the higher the electrical resis-tance [35]. Indeed, most of the nanotubes would be depletedbecause the wall thickness of the In2O3 nanotubes synthesized inthis study (Fig. 2) was as small as 25 nm.

When the sensor was exposed to ethanol gas, which is areducing gas, ethanol molecules will react with preexistingoxygen ions at the In2O3 nanotube surface to form CO2 and H2Oaccording to the following equation and the electrons werereleased back to the In2O3 nanotubes [36]:

CH3CH2OH (gas)-CH3CH2OH (ads) (5)

CH3CH2OH (ads)þ6O� (ads)-2CO2 (gas)þ3H2O (gas)þ6e� (6)

This leads to an increase in carrier concentration in the In2O3

nanotube walls and a decrease in the surface depletion layerwidth. In other words, the depleted electrons are returned to theconduction band, which results in a sharp decrease in electricalresistance in the In2O3 nanotube sensors. The original resistancewas recovered without hysteresis, which might be a uniquefeature of the In2O3 nanotube sensor with a very high surface-to-volume ratio.

In the case of Au-doped In2O3 nanotubes, the ethanol gas wasspilt over the In2O3 nanotube surface by the Au nanoparticlesprecipitated on the nanotube surface [37]. In addition, thechemisorption and dissociation of ethanol gas [38] on the Aunanoparticle surface is enhanced owing to its high catalytic orconductive nature. Consequently, the number of electronsreleased from the gas species increases. In short, a combinationof the spillover effect, enhancement of chemisorption and dis-sociation of gas and the formation of electrons results in astronger electrical response of the Au-doped In2O3 nanotubesensor to ethanol gas. On the other hand, it is unclear why theresponse and recovery times of the Au-doped In2O3 nanotubesmeasured in this study was slightly longer than those of theundoped In2O3 nanotubes. A further systematic study will beneeded to clarify this. Nevertheless the slightly longer responseand recovery times might be associated with the nonuniformdistribution of Au nanoparticles over the In2O3 nanowire surface.

4. Conclusions

In2O3 nanotubes were synthesized as gas sensors using TeO2

nanowires as a template. Scanning and transmission electronmicroscopy showed that the tubes had diameters of a fewhundred nanometers, the wall thickness of �25 nm and lengthsup to a few millimeters. The multiple networked Au-doped In2O3

nanotube sensors showed responses of 187–1187–1219% to50–250 ppm C2H5OH at 300 1C. These values are far superior tothe responses obtained by the undoped In2O3 nanotubes. Theenhanced response of the Au-doped In2O3 nanotube sensorcompared to those of the undoped In2O3 nanotube sensors mightbe due to the combination of the spillover effect, enhancement ofchemisorption and dissociation of gas, and the formation ofelectrons, resulting in a stronger electrical response of the Au-doped In2O3 nanotube sensor to ethanol gas. The responses

obtained by the Au-doped In2O3 nanotube sensors were alsostronger than those obtained by pure In2O3 nanowire sensors,which might be due to the larger surface-to-volume ratio of theformer than the latter as well as the effect of the Au catalyst.

Acknowledgment

This study was supported by the 2010 Core Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology.

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