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Current Applied Physics 13 (2013) 919e924

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Current Applied Physics

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Enhanced ethanol sensing properties of TeO2/In2O3 coreeshellnanorod sensors

Hyunsung Ko, Sunghoon Park, Soyeon An, Chongmu Lee*

Department of Materials Science and Engineering, 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 6 November 2012Received in revised form9 January 2013Accepted 19 January 2013Available online 1 February 2013

Keywords:TeO2 nanorodsIn2O3 shellsGas sensorsResponseEthanol

* Corresponding author. Tel.: þ82 32 860 7536; faxE-mail address: cmlee@inha.ac.kr (C. Lee).

1567-1739/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.cap.2013.01.020

a b s t r a c t

TeO2/In2O3 coreeshell nanorods were fabricated using thermal evaporation and sputtering methods. Themultiple networked TeO2/In2O3 coreeshell nanorod sensor showed responses of 227e632%, responsetimes of 50e160 s, and recovery times of 190e220 s at ethanol (C2H5OH) concentrations of 50e250 ppm at 300 �C. The response values are 1.6e2.9 times higher and the response and recoverytimes are also considerably shorter than those of the pristine TeO2 nanorod sensor over the same C2H5OHconcentration range. The origin of the enhanced ethanol sensing properties of the coreeshell nanorodsensor is discussed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, one-dimensional (1D) nanostructure-basedsensors have become a subject of intensive research owing to theadvantages of higher sensitivity, superior spatial resolution, andrapid response associated with individual nanowire due to the highsurface-to-volume ratios compared to thin film gas sensors [1e5].Nevertheless, enhancing their sensing performance and detectionlimit is still a challenge. A range of techniques such as doping [4,5],surface functionalization [6e8], and fabrication of heterostructures[9e11] have been developed to improve the sensitivity, stability,response speed of the 1D nanostructure-based sensors.

Paratellurite (a-TeO2) crystal belongs to the tetragonal spacegroup D4

4 (P41212) having a distorted rutile structure with asym-metric covalent TeeO bonds [12]. TeO2 has a range of technologicalapplications, such as deflectors [13], modulators [14], dosimeters[15,16] optical storage material [17], laser devices [18] and gassensors [19,20], because of its useful physical properties includinga high refractive index (n ¼ 2e2.25) and high optical nonlinearity(c ¼ 10�13 to 4 � 10�12 esu) suitable for those applications [21]. Onthe other hand, there are a very limited number of reports on thesynthesis of TeO2 1D nanostructures and very few on TeO2 1D

: þ82 32 862 5546.

All rights reserved.

nanostructure sensors. In 2007, Liu et al. [20] reported the gassensing of TeO2 nanowires synthesized by the thermal evaporationof Te metal in air for the first time. They showed that TeO2 nano-wires were sensitive to toxic gases, such as NO2, NH3 and H2S. Theirresults demonstrated the possibility of making low power con-sumption gas sensors using TeO2 nanowires. This sensing perfor-mance could be enhanced further by incorporating a surfacefunctionalization technique into their simple nanowire sensors.This paper reports the enhanced sensing properties of TeO2/In2O3coreeshell nanorods in detecting ethanol (C2H5OH) gas. TeO2 be-haves as an n-type semiconductor at temperatures above 400 �C orlow oxygen partial pressures [22]. On the other hand, most TeO2,such as TeO2 single crystals, thin films and polycrystalline TeO2,were reported to show p-type semiconductor behavior undernormal conditions. The origin of the enhanced sensing properties ofthe p-type TeO2/In2O3 coreeshell nanorods is also discussed.

2. Experimental

The TeO2/In2O3 coreeshell nanorods were synthesized usingthermal evaporation and sputtering methods. First, TeO2 nanorodswere synthesized on a p-type Si (100) substrate in a quartz tubefurnace by thermal evaporation of Te powders at 400 �C in theair without using any metal catalyst and supplying any other gas.The thermal evaporation process was conducted for 1 h and thenthe furnace was cooled down to room temperature. Subsequently,

H. Ko et al. / Current Applied Physics 13 (2013) 919e924920

the TeO2 nanorods were coated with a thin In2O3 layer by sput-tering in a conventional rf magnetron sputtering system with5.0 * 10�6 Torr base vacuum. During the coating process theworking pressure was maintained at 2.0 * 10�2 Torr and the N2 gasflow rate was 20 sccm. The radio frequency (rf) sputtering powerwas 100 W and sputtering time was 20 min. The substrate tem-perature was kept at room temperature.

Scanning electron microscopy (SEM, Hitachi S-4200), trans-mission electron microscopy (TEM, Philips CM-200), energy-dis-persive X-ray spectroscopy (EDXS), and X-ray diffraction (XRD,Philips X’pert MRD diffractometer) analyses were performed on thecollected nanorod samples. The crystallographic structure wasdetermined by glancing angle X-ray diffractionwith Cu Ka radiation(0.15406 nm), a scan rate of 4�/min, and by using a sample that isgeometrically arranged at a 0.5� glancing angle with a rotatingdetector.

The gas sensing properties of plain and TeO2/In2O3 coreeshellnanorods were measured using a home-built computer-con-trolled characterization system consisting of a test chamber, a sen-sor holder, a Keithley sourcemeter-2612, mass flow controllers, anda data acquisition system. A given amount of ethanol (>99.99%) gaswas injected into the testing tube through a microsyringe, and theoutput resistance across the sensor was monitored. The resistanceof the sensor in dry air or in test gas was measured from thisvoltage.

The response of the TeO2 nanorod sensors is defined as Ra/Rg forC2H5OH, where Ra and Rg are the electrical currents in the sensorsin air and target gas, respectively. The response time is defined asthe time needed for the variation in electrical current to reach 90%of the equilibrium value after injecting the gas, and the recoverytime is defined as the time needed for the sensor to return to 90% ofthe original current in air after removing the gas.

3. Results and discussion

X-ray diffraction (XRD) measurements were conducted todetermine the crystal structures of the nanorods. The XRD patternsof the pristine TeO2 nanorods and the TeO2/In2O3 coreeshellnanorods indicate that the TeO2 cores are crystalline, whereas theIn2O3 shells are polycrystalline (Fig. 1). Most of the reflection peaksin the XRD pattern of the TeO2/In2O3 coreeshell nanorods wereassigned to the reflections of primitive tetragonal-structured rutile-type TeO2 with lattice constants of a ¼ 0.4810 nm andc¼ 0.7613 nm (JCPDS No. 78-1714) and only three small peaks fit tothe (134), (622) and (820) reflections of base-centered cubic (bcc)-

Fig. 1. XRD patterns of TeO2/In2O3 coreeshell nanorods.

structured In2O3 with lattice constants of a ¼ 1.011 nm (JCPDS No.89-4595).

Fig. 2 shows the SEM image of the TeO2/In2O3 coreeshellnanorods prepared by thermal evaporation followed by sputter-ing. Each 1D nanostructure has a rod-like morphology as can beseen in the enlarged SEM image of a typical 1D nanostructure (insetin Fig. 2). Scanning electron microscopy also shows that the syn-thesis scheme adopted in this study can grow TeO2 nanorods withdiameters of 50e150 nm and lengths up to a few hundreds ofmicrometers. The low-magnification TEM image of a typicalcoreeshell nanorods exhibits the TeO2 core at the central regionand the In2O3 shells at the two edge regions of the nanorods(Fig. 3(a)). The TEM image indicates the widths of the core and shellare w82 nm and w12 nm, respectively. The enlarged high resolu-tion TEM (HRTEM) image shows a fringe pattern in the lower darkerregion, suggesting that it is single crystals. The resolved spacingsbetween the two parallel neighboring fringes were 0.30 and0.34 nm, corresponding to the interplanar distances of the {102}and {110} lattice planes in tetragonal TeO2, respectively (Fig. 3(b)).The resolved spacing between the two parallel neighboring fringesin the lower less dark region was 0.2 nm, corresponding to theinterplanar distances of the {134} lattice plane in boy-centeredcubic In2O3. The spotty pattern in the corresponding selected areaelectron diffraction (SAED) pattern fit to the primitive tetragonal-structured-TeO2 with lattice constants of a ¼ 0.4810 nm andc ¼ 0.7613 (JCPDS No. 78-1713) (Fig. 3(c)). On the other hand, theconcentric ring pattern fit to the bcc In2O3 with lattice constants ofa ¼ 1.011 nm (JCPDS No. 89-4595).

The EDX spectrum taken from the nanorods (Fig. 4(a)) indicatesthe presence of Te, In, and O elements, which agrees well with theresult of XRD analysis (Fig. 1). The Cu and C in the spectra are due toTEM grid. EDXS analyses clearly confirmed that TeO2/In2O3 coreeshell nanorods were synthesized successfully by showing thehigher TeO2 concentration in the central region and the higherIn2O3 concentration at both edge regions of the nanorod (Fig. 4(b)).Fig. 5(a) shows the transient response of pristine TeO2 nanorodsand TeO2/In2O3 coreeshell nanorods, respectively, at an operatingtemperature of 300 �C to C2H5OH. The sensors were exposed tosuccessive pulses of C2H5OH with the concentration ranging from50 to 250 ppm. The response is reversible, namely, the nanorodsensors show the same resistance value before and shortly aftereach ethanol pulse. Fig. 5(b) and (c), respectively, shows the

Fig. 2. SEM images of TeO2/In2O3 coreeshell nanorods. Inset, an enlarged SEM imageof a typical coreeshell nanorod.

Fig. 3. (a) Low-magnification TEM image, (b) high resolution TEM image, and (c)selected area electron diffraction pattern of TeO2/In2O3 coreeshell nanorods.

Fig. 4. (a) EDX spectrum of the TeO2/In2O3 coreeshell nanorods and (b) EDXS linescanning concentration profiles.

H. Ko et al. / Current Applied Physics 13 (2013) 919e924 921

enlarged part of the data in Fig. 5(a) measured at a C2H5OH con-centration of 250 ppm for TeO2/In2O3 coreeshell nanorods andpristine TeO2 nanorods to reveal the moments of the gas input andgas stop. The pristine TeO2 nanorods showed responses ofapproximately 142, 163, 180, 198, and 219% at C2H5OH concentra-tions of 50, 100, 150, 200, and 250 ppm, respectively (Table 1). Incontrast, the TeO2/In2O3 coreeshell nanorods showed responses of228, 334, 419, 546, and 632% at C2H5OH concentrations of 50, 100,150, 200, and 250 ppm, respectively (Table 1). Therefore, the coreeshell nanorod sensors showed responses w1.6, 2.1, 2.3, 2.8, and 2.9times larger than those of pristine TeO2 nanorod sensors by at

C2H5OH concentrations of 50, 100, 150, 200, and 250 ppm,respectively.

Fig. 5(d) shows the responses of pristine TeO2 nanorods andTeO2/In2O3 coreeshell nanorods as a function of the C2H5OH con-centration. The response of an oxide semiconductor is commonlyexpressed as R¼ A [C]n þ B, where A and B, n, and [C] are constants,exponent, and the target gas concentration, respectively. Data fit-ting gave R ¼ 0.378 [C] þ 123.757 and R ¼ 2.044 [C] þ 125.181 forthe plain TeO2 nanorod and TeO2-core/In2O3-shell microrod sen-sors, respectively. The response value of the coreeshell nanorodsensor tends to increase more rapidly than that of the pristinenanorod sensor as the C2H5OH gas concentration increases, sug-gesting that the response of the former would be far higher thanthat of the latter at high C2H5OH gas concentrations such as, forexample, a few thousands of ppm, even though the response of thecoreeshell nanorods were examined only in the C2H5OH concen-tration range of 50e250 ppm. On the other hand, both the responseand recovery times of the coreeshell nanorods were considerablydecreased by sheathing regardless of the C2H5OH concentration.

Table 2 lists the responses of the plain TeO2 nanorod sensorfabricated in this study toward C2H5OH gas along with those ofother reported nanomaterial sensors. Overall, the sensing prop-erties of the TeO2/In2O3 coreeshell nanorod sensor fabricated inthis study are comparable to those of other competing nano-materials (Table 2) [23e30]. It should be noted that the C2H5OHconcentration and the test temperature used in this study werelower than those in previous works. The response of the TeO2/In2O3

Fig. 5. (a) Dynamic responses of the plain TeO2 nanorod and TeO2/In2O3 coreeshell nanorod gas sensors. (b) Enlarged part of (a): the coreeshell nanorod at 250 ppm ethanol. (c)Enlarged part of (a): the pristine TeO2 nanorod at 250 ppm ethanol. (d) Responses of the pristine TeO2 nanorod and coreeshell nanorod gas sensors as a function of the ethanol gasconcentration.

Table 2Comparison of the responses, response times and recovery times of the TeO2/In2O3

coreeshell nanorod (NR) sensor with those of other oxide 1D nanostructure sensors.

Nanomaterials Ethanolconc.(ppm)

Temp.(�C)

Response(%)

Responsetime (s)

Recoverytime (s)

Ref.

TeO2/In2O3 NRs 50 300 228 160 200 Presentwork

TeO2/In2O3 NRs 250 300 634 50 190 Presentwork

TiO2 nanotubes 5000 200 16 e e [23]SnO2 nanorods 300 300 3140 1 1 [24]CeeSnO2

nanopowders200 250e450 18,500 e e [25]

PteSnO2 100 150e350 4000 12 360 [26]

H. Ko et al. / Current Applied Physics 13 (2013) 919e924922

coreeshell nanorods is far better that that of TiO2 nanotubesdespite the far lower C2H5OH concentration and the far lower testtemperature [23]. In comparison with CoFe2O4 nanopowders, theTeO2-core/In2O3-shell nanorods show somewhat lower response toC2H5OH gas and larger response and recovery times at the sametemperature [21]. On the other hand, a comparison of the TeO2/In2O3 coreeshell nanorods with Co-doped ZnO nanorods measuredat the similar C2H5OH concentration indicates that the responsevalue of the former is higher than that of the latter [30].

Both TeO2 and In2O3 in the coreeshell nanorods are p-type andn-type semiconductor oxides, respectively and the sensing mech-anism of the coreeshell nanorods is based on the surface reaction.In this study, TeO2/In2O3 coreeshell nanorods showed the farenhanced sensor response to ethanol gas. It is well known that thesurface of the sensor material is covered with chemisorbedoxygen ions such as O�, O2� and O2

� as a result of the followingreactions [31]:

O2ðgÞ/O2ðadsÞ; (1)

O2ðadsÞ þ e�/O�2 ðadsÞ; (2)

Table 1Responses, response times, and recovery times measured at different C2H5OHconcentrations for the TeO2/In2O3 nanorod sensor at 300 �C.

Ethanol cone. Response (%) Response time (s) Recovery time (s)

TeO2 TcO2/In2O3 TcO2 TcO2/In2O3 TcO2 TeO2/InO3

50 ppm 142.18 227.57 140 160 200 200100 ppm 162.93 333.59 160 100 220 210150 ppm 180.08 419.43 150 80 200 210200 ppm 197.56 546.00 140 60 200 220250 ppm 219.28 632.39 160 50 180 190

O�2 ðadsÞ þ e�/2O�ðadsÞ; (3)

O�ðadsÞ þ e�/O2�ðadsÞ: (4)

These adsorbed oxygen ions create a space charge region nearthe film surface by extracting electrons from the surface of thesensor. Adsorbed ethanol molecules react with these ionic oxygen

nanopowdersSnO2eZnO(0.05)

compositenanopowders

300 200e400 390,000 96e418 400e600 [27]

ZnOeSnO2(0.05)compositenanopowders

300 200e400 120,000 96e418 400e600 [27]

ZnO nanowires 1500 300 61 e e [28]TiO2 nanobelts 500 250 3366 1e2 1e2 [29]AgeTiO2 nanobelt 500 200 4171 1e2 1e2 [29]CoFe2O4

nanopowders50 150 7190 50 60 [21]

CoeZnO nanorods 50 350 987 e e [30]

H. Ko et al. / Current Applied Physics 13 (2013) 919e924 923

species and produce electrons via the following two reactions:(Reaction 5) dehydrogenation of ethanol to an aldehyde and (Re-action 6) dehydration of ethanol to an alkene on basic and acidicoxides, respectively [32].

C2H5OHðgÞ/CH3CHOðgÞ þ H2ðgÞ; (5)

C2H5OHðgÞ/C2H4ðgÞ þ H2OðgÞ: (6)

The enhanced sensor response of the TeO2/In2O3 coreeshellnanorods to ethanol compared to that of pristine TeO2 nanorodsmight be attributed to the enhanced absorption, dehydrogenationof ethanol (Reaction 5) because of the basic property of In2O3[33,34]. The CH3CHO is subsequently oxidized by the chemisorbedoxygen ions as in the following equation:

CH3CHOðadÞ þ 5O�/2CO2 þ 2H2Oþ 5e�: (7)

In addition, it should be checked if the enhanced sensorresponse of the coreeshell nanorods is partly attributed to changesin resistance due to the surface depletion layer of each coreeshellnanorod [35,36]. C2H5OH is a reducing gas. Upon exposure toC2H5OH gas, the C2H5OH gas is adsorbed by the coreeshell nanorodsensor and electrons are released from the adsorbed C2H5OHmolecules and attracted to the In2O3-shell layers because reducinggas such as C2H5OH acts as an electron donor in the reaction. Thisreaction will result in a decrease in the depletion layer width andthus a decrease in the resistance of the nanorod sensor. On theother hand, trapped electrons are released by the In2O3-shell layerupon the supply of C2H5OH gas being stopped, leading to anincrease in the depletion layer width and thus an increase in theresistance. The electron exchange between surface states and theIn2O3-shell layer occurs within the surface layer excluding thedepletion layer. The width of the surface layer is the order of Debyelength lD which can be expressed by Ref. [37,38]:

lD ¼�

3kT=q2nc�1=2

; (8)

where 3is the static dielectric constant (8.9 � 8.85 � 10�12 F/m inIn2O3), k is the Boltzmann constant (1.38 � 10�23 J/K), T is the ab-solute temperature (573 K), q is the electrical charge of the carrier(1.6 � 10�19 C), and nc is the carrier concentration (7.3 � 1016/cm3:the value obtained by Hall measurement for the 250 nm In2O3 thinfilm deposited on the Si (100) substrate by sputtering). For theIn2O3 layer in the coreeshell nanobelts fabricated in this study, thelD value calculated for 300 �C was approximately 18.3 nm. Thismeans that C2H5OH molecules donate electrons to the TeO2 core aswell as to the In2O3-shell layer the width of which is approximately12 nm. Consequently, in the TeO2-core/In2O3-shell nanobelts, theheterojunction barrier existing at the interface of the core and shellshould also be considered because electron transport is modulatedby the heterojunction. Electron transport is modulated by theheterojunction with an adjustable energy barrier height. In otherwords, the heterojunction acts as a lever in electron transfer bywhich the electron transfer is facilitated or restrained, resulting inenhanced sensing properties of the coreeshell nanorod sensor.

Besides the enhanced absorption, dehydrogenation of ethanolmentioned above, the enhancement in response might be partlyattributed to the modulation of electron transport by the In2O3e

In2O3 homojunction at the interjection of nanorods with anadjustable energy barrier height. An energy barrier exists at theIn2O3eIn2O3 homojunction formed at the crossing point of twocoreeshell nanobelts. The electron transport is modulated by thehomojunction with an adjustable energy barrier height. In otherwords, the homojunction acts as a lever in electron transfer by

which the electron transfer is facilitated or restrained, resulting inan enhanced sensing property of the nanorod sensor.

4. Conclusions

TeO2/In2O3 coreeshell nanorods were synthesized using a two-step process: synthesis of TeO2 nanorods by the thermal evapo-ration of te powders followed by the sputtering of In2O3. The coresand shells of the nanorods were single crystal TeO2 and amorphousIn2O3, respectively. The responses of the TeO2 nanorods wereimproved approximately 2e3-fold at each C2H5OH concentrationby sheathing them with In2O3. The responses of the coreeshellnanorods are also far superior to those of the other materialmicrosensors to C2H5OH gas reported previously, even though theC2H5OH gas concentration range used in this study is somewhathigher. The improvement in the response of the TeO2 nanorods toC2H5OH gas by sheathing themwith In2O3 may be attributed to theenhanced absorption and dehydrogenation of ethanol. Besides, theenhanced sensor response of the coreeshell nanorods might bepartly attributed to the changes in resistance due to the potentialbarriers built in the In2O3eIn2O3 homojunction at the interjectionof nanorods.

Acknowledgment

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

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