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Sensors and Actuators B 150 (2010) 112–119

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

omparative study of gas sensor performance of SnO2 nanowires and theirierarchical nanostructures

e Viet Thong, Le Thi Ngoc Loan, Nguyen Van Hieu ∗

nternational Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No. 1, Dai Co Viet Road, Hanoi 844, Vietnam

r t i c l e i n f o

rticle history:eceived 21 April 2010eceived in revised form 25 June 2010ccepted 21 July 2010vailable online 29 July 2010

eywords:

a b s t r a c t

In this study, SnO2 nanowires (NWs) were synthesized by two different thermal evaporation processesand hierarchical SnO2 nanostructures were prepared through a combination of the two processes with-out reseeding of Au catalyst. Field emission scanning electron microscopy studies showed that theshape of the core NWs become distorted and attached to large quantities of quasi-one-dimensionalnanostructures, mostly NWs and nanobelts. The crystal structure of these as-prepared hierarchical SnO2

nanostructures was identified to coincide to the normal rutile structure. Photoluminescence properties

in oxideanowiresierarchicalas sensors

are similar among the samples with a strong peak emission band centered at 620 nm that is attributedto a defect-state-related emission. The gas sensing performance of SnO2 NWs and their hierarchicalnanostructures were simultaneously investigated by testing with liquefied petroleum gas and NH3 gasat different concentrations and operating temperatures. Results reveal that hierarchical SnO2 nanostruc-tures have enhanced gas sensing performance in comparison with SnO2 NWs materials. The gas sensingmechanism of SnO2 hierarchical nanostructures was also discussed. Results indicate that hierarchicallyporous SnO architectures are highly promising for gas sensor applications.

2

. Introduction

Development of methods for the controlled fabrication ofanostructures is currently the greatest challenge in the field ofanoscience and nanotechnology. Recently, much attention haseen directed to the use of low dimensional nanostructures such asanoparticles, nanowires (NWs), and nanotubes, as building blocks

n the production of hierarchical nanostructures because of variousovel applications. Lieber and colleagues developed a technique

or a large-scale hierarchical organization of NWs arrays for inte-rated nanosystems [1], whereas Wang and colleagues reportedhat novel hierarchical nanostructures based on NWs can be useds solar-cells [2] and nanogenerators [3]. Due to characteristic highorosity of hierarchical nanostructures, they also have been studieds photocatalysts [4] and gas sensors [5–13]. Gas sensing proper-ies of hierarchical nanostructures from metal oxides have beenomprehensively reviewed by Lee [14]. To date, the synthesis of

ierarchical nanostructures from metal oxides can be classified intohree categories. The first method involves high thermal evapo-ation of precursor powders accompanied by a low-temperatureondensation. One-step and multi-steps of the thermal evaporation

∗ Corresponding author. Tel.: +84 4 38680787; fax: +84 4 38692963.E-mail addresses: hieu@itims.edu.vn, hieunv-itims@mail.hut.edu.vn

N. Van Hieu).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.07.033

© 2010 Elsevier B.V. All rights reserved.

processes have been employed in the synthesis of different hier-archical nanostructures such as ZnO–ZnO [4–8], ZnO–SnO2 [15],SnO2–SnO2 [10,15], and ZnO–In2O3 [16], and WO3 [13,17–19]. Thesecond technique is a wet-chemical route, which is simple, environ-mentally friendly, operated low-temperatures, inexpensive, andproducts high-throughput [8,11,12]. The third option is a combi-nation of the first two methods [20]. It can be recognized that thehierarchization strategy consists of two-step vapor–liquid–solid(VLS) growths are frequently used because the hierarchical NWscan be precisely controlled through the thermal evaporation steps.The first step is to grow the core NWs and the second step is togrow the outer NWs by reseeding of nanoparticle catalysts on thecore NWs surface.

Previous studies have reported that hierarchical SnO2 nanos-tructures are relatively good gas sensors [5,10]. More or less, the gassensing properties of hierarchical SnO2 nanostructures preparedby a two-step thermal evaporation process can be enhanced by thedeposition of the gold nanoparticles in the core NWs before thesecond step of the thermal evaporation process. This is likely thematerials are functionalized with catalyst active materials (Au cat-alyst), resulting in the enhancement of gas sensing properties. This

study is the first to perform a comparison of SnO2 NWs and hierar-chical SnO2 nanostructures. SnO2 NWs are prepared through twodifferent thermal evaporation processes, with one process for coreNWs and another for outer secondary NWs. The two-step thermalevaporation process is used to synthesize hierarchical SnO2 nanos-

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L.V. Thong et al. / Sensors and

ructures while excluding the gold catalyst to eliminate any effect itay have on the gas sensing properties of the fabricated hierarchi-

al SnO2 nanostructures. The gas sensing performances of the SnO2Ws and hierarchical nanostructures are evaluated and compared.

. Experimental

The SnO2 NWs used in this study were synthesized throughwo different thermal evaporation processes. In the first process,nO2 NWs were grown at 980 ◦C on Si substrates that have previ-usly been deposited with Au catalyst, using SnO powder as theource. The second process is similar to the first, except that itas carried out at 800 ◦C with Sn powder as the source. These

wo processes, when done in series, produced hierarchical SnO2anostructures, initially synthesizing core NWs and then outeranostructures. Samples grown by the first process were termeds sample A, by the second process as sample B, and those byhe two combined processes as sample C. Both processes haveeen described in detail in our previous studies [21–23]. In brief,he SnO2 NWs were grown in a quartz tube located in a hori-ontal furnace (Lingdberg/Blue M, Model: TF55030A, USA). PurenO powder (for growing core NWs) or Sn powder (for growinguter NWs) was placed in an alumina boat as the evaporationource. Substrates with a layer of previously deposited Au cata-yst by ion sputtering were placed approximately 1.5 cm from theource. Both methods were divided into two steps. Initially, theuartz tube was evacuated to 10−2 Torr and purged several timesith high purity Ar gas (99.999%). Subsequently, the quartz tubeas evacuated to 10−2 Torr again, and the furnace temperature was

ncreased from room-temperature to 980 ◦C or 800 ◦C. When the

rowth temperature was reached, oxygen gas (99.99%) was addedo the quartz tube at a flow rate of 0.3–0.5 sccm, while maintaining

pressure of ∼2–5 Torr inside the tube. The growth temper-ture was maintained for 30 min. The morphology and crystaltructure of SnO2 NW and hierarchical SnO2 nanostructures were

Fig. 1. Gas sensor testing systems with high speed switching from the air to

ators B 150 (2010) 112–119 113

investigated through field emission scanning electron microscopy(FE-SEM), X-ray diffraction (XRD), and photoluminescence spec-troscopy (PL). The XRD (Philips Xpert Pro) was operated with aCuK� radiation generated at 40 kV. The PL spectra were acquiredat room-temperature from 360 to 910 nm using a 325 nm He–Cdlaser.

To characterize the gas sensing properties, synthesized SnO2NWs and hierarchical SnO2 nanostructures were dispersed inethanol and deposited on a pre-fabricated interdigitated electrodesubstrate using a micropipette. Numerous drops were placed onthe electrode to generate a thick film of each of the samples, whichcan eliminate the effect of SnO2 NWs film thickness and NWs den-sity on gas sensing performance [23,24]. The sensor fabricated fromthe SnO2 NWs grown at 980 ◦C was denoted as sensor A, that fromSnO2 NWs grown at 800 ◦C was denoted as sensor B, and that fromthe hierarchical SnO2 nanostructures was denoted as sensor C.

The gas sensing properties were studied by measuring all thesensors with liquid petroleum gas (LPG, 500–2000 ppm) and NH3(300–1000 ppm) at different temperatures (50–450 ◦C) using ahomemade set up (Fig. 1) with high speed switching gas flow(from/to air to/from balance gas). Balance gases (0.1% in air) werepurchased from Air Liquid Group, Singapore. The system employeda flow-through with a constant rate of 200 sccm.

3. Results and discussion

3.1. Morphology and crystal structure characterization

FE-SEM images of SnO2 NWs, samples A and B, are shown inFig. 2(a) and (b), respectively. The morphologies of NWs synthe-

sized through different techniques do not differ from each other.Structural images of hierarchical SnO2 nanostructures, sample C,are shown in Fig. 2(c) and (d). The morphology of hierarchicalSnO2 nanostructures is distinct compared with SnO2 NWs. CoreNWs appear distorted, and other NWs and nanobelts (NBs) seem

the tested gas: (a) design layout and (b) image of gas testing chamber.

114 L.V. Thong et al. / Sensors and Actuators B 150 (2010) 112–119

F ) at 80S

tvwplAace(bvblSatgggl

ig. 2. FE-SEM images of SnO2 NWs synthesized at (a: sample A) 980 ◦C, (b: sample BnO2 nanostructures reseeded with Au catalyst.

o branch from them. Results are completely different from a pre-ious report [15]. This can be attributed to the modified procedure,here Au nanoparticles are not reseeded during the second growthrocess. This is verified by the similarity of the results reported in

iterature [15] and the results obtained (Fig. 2(e) and (f)), in whichu nanoparticles were deposited on the core NWs as reseeding cat-lysts for the outer NWs growth. The growth mechanism of SnO2ore NWs with Au nanoparticles as catalysts had initially beenlucidated by Wagner and Ellis based on the vapor–liquid–solidVLS) process, followed by many researchers [21,22,25–28]. Weelieve that the branched NWs/NBs were grown by a catalyst-freeapor–solid (VS) process because the secondary outer NWs/NBs cane directly formed from the vapor phase in the absence of the cata-

yst [29]. The following growth mechanism is proposed: some smalln droplets from Sn vapor attach to the surface of the core NWsnd form new growth sites. Once the Sn droplets completely attach

o the surface of core NWs and become nucleation sites, epitaxialrowth of branched NWs/NBs takes place. The branching patternenerated is not uniform because the Sn droplets were not homo-eneous. Core NWs become distorted because of the oxidation ofiquid Sn droplets coated the core NWs to SnO2.

0 ◦C, (c and d: sample C) hierarchical SnO2 nanostructures, and (e and f) hierarchical

Fig. 3 shows the XRD patterns of the samples. All the diffrac-tion peaks for the products match the tetragonal rutile SnO2 profile(a = 4.738 and c = 3.187) obtained from the JCPDS 77–0450 card. Assuch, nanostructures consist predominantly of pure SnO2. We haveobserved that the NWs density of the sample A was lower than thatof sample B. This is an explanation why the peaks from sample Ahave lower intensity than that from sample B.

There are several papers in the literature that report on the lumi-nescence spectra of SnO2 with different morphologies [30–33]. Itis recognized that oxygen vacancies play important roles in theseluminescence bands as well as gas sensing properties. For thesereasons, the room-temperature PL spectra of the SnO2 NWs fromsamples A, B, and C were characterized. Fig. 4 shows the roomtemperature PL spectra acquired from the samples. All spectrashow a strong visible PL band at 620 nm, which has been con-sistently attributed to oxygen vacancies that are essential defects

of metal oxides [31,33]. Despite the morphological differencesbetween SnO2 NWs and their hierarchical nanostructures, the samePL results are obtained from these samples. The reason for thisis unclear, and further studies are necessary to obtain a plausi-ble explanation. However, the hypothesis explanation is that the

L.V. Thong et al. / Sensors and Actuators B 150 (2010) 112–119 115

Fig. 3. XRD pattern of samples A, B, and C.

ot

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Nd5wribrfbu7tt

Fig. 4. PL spectra of samples A, B, and C.

xygen vacancies and sizes of nanowires and hierarchical nanos-ructures may not be much different.

.2. Gas sensing properties

To investigate and compare the gas sensing properties of SnO2Ws and hierarchical SnO2 nanostructures, sensors were tested atifferent temperatures ranging from 50 to 450 ◦C with an interval of0 ◦C to optimize working temperature. Responses were measuredith LPG and NH3 gas at concentrations of 2000 and 1000 ppm,

espectively. As shown in Fig. 5, the temperature has an obviousnfluence on the response of the sensors to LPG and NH3 gas. Theell-shaped distribution of response against operating temperatureeveals that optimum performance is achieved at 350 and 200 ◦Cor LPG and NH3 gas, respectively. Previous work has also reported

ell-shaped sensor responses [23]. The maximum response val-es to 2000 ppm LPG and 1000 ppm NH3 gas were 6, 6.2, 20.4 and.1, 7.9, 21.7 for sensors A, B, and C, respectively. For both gases,he hierarchical SnO2 nanostructure exhibits a response that ishree times that of the SnO2 NWs sensors at the optimal operat-

Fig. 5. Sensor response against operating temperature for (a) LPG and (b) NH3 gas.

ing temperatures. Responses of the two SnO2 NW sensors are verysimilar, mainly because their morphologies and wire diameters arealike (Fig. 2). Comparing the response of sensor C to the two gases(LPG, NH3), temperature can be used to selectively detect NH3. Forinstance, at 200 ◦C, the response of sensor C to 1000 ppm NH3 isabout 21.7, whereas its response to 2000 ppm LPG is about 2.5.

After the optimized temperature was determined, response pat-terns to different gas concentrations were investigated. The threesensors were tested at 350 ◦C with LPG concentrations of 500, 1000,and 2000 ppm. Fig. 6(a)–(c) shows the dynamic response-recoverycurves of sensors A, B, and C to LPG, respectively. The responseamplitudes of all the sensors increase with increasing gas concen-tration [Fig. 6(d)]. As expected, compared with SnO2 NW sensors,the sensor based on hierarchical SnO2 nanostructures (sensor C)exhibits enhanced responses for each concentration. Fig. 7 presentsthe results of testing the sensors at 200 ◦C with NH3 concentrationsof 300, 400, 500, 750 and 1000 ppm. It can be seen that hierarchicalnanostructures sensor has a higher response to NH3 gas. This can beimplied that the enhanced gas sensing response of the hierarchicalnanostructures is relevant to other gases.

It has been demonstrated that the response (S = Ra/Rg) of oxidesemiconductors is usually depicted as [34–38]:

S∼ˇ [C]˛ (1)

where ˇ denotes a constant, and [C] means the concentration of thetarget gas. In practical, the ˛ is in range from 1/2 to 1, dependent onthe charge of the surface species, the stoichiometry of the elemen-tary reactions on the surface and measurement range of the target

gas. For the case of one-dimensional nanostructure SnO2, whichhas a good response to various gases, the ˛ value can be around 1[37–39]. In our case, we have also found that the sensor responseto LPG and NH3 seem to be shown a linear relationship with gasconcentration in the measured range (see Figs. 6(d) and 7(d)).

116 L.V. Thong et al. / Sensors and Actuators B 150 (2010) 112–119

Fig. 6. LPG sensing characteristics of samples (a) A, (b) B, (c) C and

Fig. 7. NH3 gas sensing characteristics of samples (a) A, (b) B, (c) C and

(d) response of fabricated sensors versus LPG concentration.

(d) response of fabricated sensors versus NH3 gas concentration.

L.V. Thong et al. / Sensors and Actuators B 150 (2010) 112–119 117

Table 1Brief summary of results reported on semiconductor LPG sensor.

Sensor type LPG (ppm) Operation T (◦C) Response References

SnO2 NWs 500 350 Ra/Rg ∼ 3.5 This workSnO2 hierarchical NWs 500 350 Ra/Rg ∼ 8.1 This workSnO2 thick film 10,000 350 (Ra–Rg)/Rg ∼ 0.93 [40]SnO2 thick film 200 300 (Ra–Rg)/Rg ∼ 0.7 [41]SnO2 thin film 800 400 (Ra–Rg)/Rg ∼ 1.38 [42]SnO2 thick film 1000 350 (Gg–Ga)/Ga ∼ 3.68 [43]SnO2 thin film 1000 345 (Ra–Rg)/Rg ∼ 0.1 [44]TGS 2612 FIGARO 1000 (VH = 5 V) (Ra–Rg)/Rg ∼ 2.1

Table 2Brief summary of results reported on semiconductor NH3 sensor.

Sensor type NH3 (ppm) Operation T (◦C) Response References

SnO2 NWs 500 300 Ra/Rg ∼ 5.1 This workSnO2 hierarchical NWs 500 300 Ra/Rg ∼ 10.9 This workSnO2 nanorods film 500 200 Ia/Ig ∼ 20 [45]Sb-doped SnO2 film 100 97 Ra/Rg ∼ 10 [46]SnO2 nanoparticle film 100 97 Ra/Rg ∼ 4 [47]Pt-doped SnO2 film 100 160 Ra/Rg ∼ 10 [48]SiO2-doped SnO2 film 100 160 Ra/Rg ∼ 5 [48]

160150(VH =

bssncgrstNhafo

abgcbfwfceotcoomwAmtsastt

between the NWs through the equation [53]:

R = R0 exp{

e�Vb

kbT

}(2)

Pure SnO2 film 100WO3-doped SnO2 film 500TGS 2444 FIGARO 10

We have measured the sensor response with relative large num-er of gas pulse sequence and did not find out any change in theensor response and the base-resistance of the sensor (data nothown). This can imply that the performance of the hierarchicalanostructure sensors has good repeatability and the hierarchi-al SnO2 nanostructures materials are promising candidates foras sensing applications. We have made comparison LPG and NH3esponse of our SnO2 NWs and SnO2 hierarchical nanostructuresensors as presented in Tables 1 and 2, respectively. It can be seenhat the responses of SnO2 hierarchical nanostructures to LPG andH3 are comparable with reported values [40–48]. However, SnO2ierarchical nanostructures have not yet optimized for practicalpplication. The sensor performance can be improved further byunctionalization with active catalysts nanoparticles for detectionf LPG and NH3 gas.

The mechanism of enhanced gas sensing performance of hier-rchical nanostructures is still unclear. Q. Wan et al. reported thatranched SnO2 NW sensors have higher responses to ethanol thaneneral SnO2 NW sensors [10]. However, we believe that the Auatalyst previously deposited in the second growth step of SnO2ranches may have had a role in enhancing the gas sensing per-ormance. This is likely the case with SnO2 NWs functionalizedith nanoparticle catalysts [49–52]. Recently, the gas sensing per-

ormance of ZnO NWs and hierarchical ZnO nanostructures wereompared, and the gas response enhancement of the latter wasstablished and explained [5,8]. Since the gas sensing mechanismsf ZnO and SnO2 nanostructures are very similar, enhancement ofhe gas sensing performance of SnO2 hierarchical nanostructuresan be attributed to (i) higher porosity, (ii) more active centersbtained from enhanced oxygen vacancy defects, and (iii) junctionsf the core and outer NWs/NBs. To clearly understand the enhance-ent of gas sensing properties of the hierarchical nanostructures,e first explain the gas sensing mechanism of SnO2 NWs sensors.s depicted in Fig. 8, when SnO2 NWs is exposed in air, oxygenolecules will adsorb on the surface and capture electrons from

he conductance band of the NWs to become oxygen ions (active

urface complexes). The adsorbed oxygen ions on the SnO2 NWs acts electron acceptors that generate a surface depletion region on theurface of the NWs. The width of the depletion region is sensitive tohe ambient gases. As exposed to LPG and NH3 gas (donated elec-rons molecules), the reactions between these molecules and the

Ra/Rg ∼ 1 [48]Ra/Rg ∼ 0.21 [48]

5 V) Ra/Rg ∼ 0.2

active chemical complexes (oxygen ions) on the surface will lead tothe release of the trapped electrons, resulting in a decrease of thedepletion width. This leads to a decrease of the resistance of alongthe longitudinal SnO2 NWs. However, our SnO2 NWs sensors area network type and there are many NWs bridge between the twoPt electrodes forming many networking points of SnO2–SnO2 NWs(see Fig. 8(a)). The potential barrier around the networking pointsare blocked the electrons flow in a more efficient way comparedwith the depletion region on the surface. Because the barriers, withadjustable thickness and height, control the transport of electrons

Fig. 8. The schematic illustration of gas sensing mechanism of SnO2 NWs (a) andhierarchical nanostructures SnO2 (b) gas sensors.

118 L.V. Thong et al. / Sensors and Actu

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[nanothorn arrays on WO3 nanowhiskers, Adv. Mater. 18 (2006) 3105–3110.

Fig. 9. Response and recovery times of gas sensors A, B, and C.

In this equation, �Vb is the change of the barrier potentialefined as the potential in air minus that in target gas, R0 is a factor

ncluding the resistance in air and other parameters, e is the chargef an electron, kb is Boltzmann’s constant, and T is the absolute tem-erature. When the sensors are exposed to LPG and NH3, not onlyhe depletion width on the surface of the NWs, but also the height ofhe barrier potential at the networking point contact will decrease.rom the schematic illustration of gas sensing mechanism of SnO2Ws and their hierarchical nanostructures, as shown in Fig. 8(a) and

b), we can clearly see that the hierarchical nanostructures have aigher porosity and more active sites compared to the SnO2 WNsensor. More importantly, the hierarchical nanostructures sensoras much higher network point contacts (see Fig. 8(b)). This cane dominant factor in enhancing the gas sensing properties of theierarchical nanostructures.

For practical applications, the response and recovery times ofhe gas sensors are very important concerns. Fig. 9(a) and (b) showshe plots of the dynamic resistance responses of the three sen-ors to 1000 ppm LPG and 300 ppm NH3 gas, respectively. The 90%esponse time for gas exposure [t90%(air-to-gas)] and that for recov-ry [t90%(gas-to-air)] were calculated from the resistance-time datahown in Fig. 9. The t90%(air-to-gas) values for LPG and NH3 gas areround 4 and 17 s, respectively, whereas the t90%(gas-to-air) valuesor LPG and NH3 gas are around 9 and 30 s, respectively. For each of

he gases, response and recovery times do not differ greatly fromach other. These tests were performed at the corresponding opti-al temperatures of the sensors, which may have contributed to

he differences in values between the different gases.

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ators B 150 (2010) 112–119

4. Conclusion

This study demonstrated the use of a two-step thermal evap-oration method in synthesizing hierarchical SnO2 nanostructuresfor gas sensing applications. Gas sensors fabricated from SnO2 NWsand their hierarchical nanostructures were used to study and com-pare the gas sensing performance of the different nanostructures.Comparative gas sensing tests showed that hierarchical nanos-tructures exhibit enhanced gas sensing performance, which maybe attributed to high porosity, more active sites, and addition ofcore–outer junctions to the materials. Thus, synthesized hierar-chically porous SnO2 nanostructures have a great potential for gassensor applications, in addition to being used as photocatalysts andanode material for lithium-ion batteries. Moreover, this work pro-vides a simple and controllable approach that can be extended tothe synthesis of other hierarchical nanotructures based on metaloxide materials, such as SnO2, In2O3, ZnO, WO3, and TiO2.

Acknowledgment

This work has been financially supported by Vietnam’s NationalFoundation for Science and Technology Development (NAFOSTED)for a Basic Research Project (No. 103.02.95.09).

References

[1] D. Whang, S. Jin, Y. Wu, C.M. Lieber, Large-scale hierarchical organization ofnanowire arrays for integrated nanosystems, Nano Lett. 3 (2003) 1255–1259.

[2] B. Weintraub, Y. Wei, Z.L. Wang, Optical fiber/nanowire hybrid structures forefficient three-dimensional dye-sensitized solar cells, Angew. Chem. Int. 48(2009) 1–6.

[3] Y. Qin, X.D. Wang, Z.L. Wang, Microfibre–nanowire hybrid structure for energyscavenging, Nature 451 (2008) 809–813.

[4] B. Li, Y. Wang, Facile synthesis and enhanced photocatalytic performanceof flower-like ZnO hierarchical microstructures, J. Phys. Chem. C 114 (2010)890–896.

[5] Y. Zhang, J. Xu, Q. Xiang, H. Li, Q. Pan, P. Xu, Brush-like hierarchical ZnO nanos-tructures: synthesis, photoluminescence and gas sensor properties, J. Phys.Chem. C 113 (2009) 3430–3435.

[6] A. Umar, S.H. Kim, Y.-B. Hahn, Formation of hierarchical ZnO nanostructures“nanocombs”: Growth mechanism, structural and optical properties, Curr.Appl. Phys. 8 (2008) 793–797.

[7] S.R. Haldar, A. Nayak, T.K. Chini, S. Bhunia, Strong temperature and substrateeffect on ZnO nanorod flower structures in modified chemical vapor conden-sation growth, Curr Appl. Phys. 10 (2010) 942–946.

[8] J. Zhang, S. Wang, M. Xu, Y. Wang, B. Zhu, S. Zhang, W. Huang, S. Wu, Hierarchi-cally porous ZnO architectures for gas sensor application, Cryst. Growth Des. 9(2009) 3532–3537.

[9] S.J. Kim, H.-G. Kang, J. Choi, Surfactant-free preparation of ZnO dendritic struc-tures by electrochemical method, Curr. Appl. Phys. 10 (2010) 740–743.

10] Q. Wan, J. Huang, Z. Xie, T. Wang, E.N. Dattoli, W. Lu, Branched SnO2 nanowireson metallic backbones for ethanol sensors application, Appl. Phys. Lett. 92(2008), 102101-3.

11] J. Liu, Z. Guo, F. Meng, Y. Jia, T. Luo, M. Li, J. Liu, Novel single-crystalline hierar-chical structured ZnO nanorods fabricated via a wet-chemical route: combinedhigh gas sensing performance with enhanced optical properties, Cryst. GrowthDes. 9 (2009) 1716–1722.

12] J. Liu, T. Luo, F. Meng, K. Qian, Y. Wan, J. Liu, Porous hierarchical In2O3 micro-/nanostructures: preparation, formation mechanism, and their application ingas sensors for noxious volatile organic compound detection, J. Phys. Chem. C114 (2010) 4887–4894.

13] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L.Wang, Ultrasensitive and highly selective gas sensors using three dimensionaltungsten oxide nanowire networks, Appl. Phys. Lett. 88 (2006) 203101.

14] J-.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures:overview, Sens. Actuators B 140 (2009) 319–336.

15] S. Sun, G. Meng, G. Zhang, J.-P. Masse, L. Zhang, Controlled growth of SnO2

hierarchical nanostructures by a multistep thermal vapor deposition process,Chem. Eur. J. 13 (2007) 9087–9092.

16] J.Y. Lao, J.G. Wen, Z.F. Ren, Hierarchical ZnO nanostructures, Nano Lett. 2 (2002)1287–1291.

17] Y. Baek, Y. Song, K. Yong, A novel heteronanostructure system: hierarchical W

18] Z. Gu, T. Zhai, B. Gao, X. Sheng, Y. Wang, H. Fu, Y. Ma, J. Yao, Controllableassembly of WO3 nanorods/nanowires into hierarchical nanostructures, J. Phys.Chem. 110 (2006) 23829–23836.

19] J. Zhou, Y. Ding, S.Z. Deng, L. Gong, N.S. Xu, Z.L. Wang, Three-dimensional tung-sten oxide nanowire networks, Adv. Mater. 17 (2005) 2107–2110.

Actu

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

L.V. Thong et al. / Sensors and

20] C. Cheng, B. Liu, H. Yang, W. Zhou, L. un, R. Chen, S.F. Yu, J. Zhang, H. Gong, H.Sun, H.J. Fan, Hierarchical assembly of ZnO nanostructures on SnO2 backbonenanowires: low-temperature hydrothermal preparation and optical properties,ACS Nano 3 (2010) 3069–3076.

21] N.V. Hieu, Highly reproducible synthesis of very large-scale tin oxide nanowiresused for screen-printed gas sensor, Sens. Actuators B 144 (2010) 425–431.

22] N.V. Hieu, L.T.N. Loan, T. Trung, N.D. Chien, A facile thermal evaporation routefor large-area synthesis of tin oxide nanowires: characterizations and their usefor liquid petroleum gas sensor, Curr. Appl. Phys. 10 (2010) 636–641.

23] L.V. Thong, N.D. Hoa, D.T.T. Le, D.T. Viet, P.D. Tam, A.-T. Le, N.V. Hieu, On-chipfabrication of SnO2 nanowire gas sensor: the effect of growth time on sensorperformance, Sens. Actuators B 146 (2010) 361–367.

24] I.-S. Hwang, Y.-S. Kim, S.-J. Kim, B.-K. Ju, J.-H. Lee, A facile fabrication of semicon-ductor nanowires gas sensor using PDMS patterning and solution deposition,Sens. Actuators B 136 (2009) 224–229.

25] R.S. Wagner, W.C. Ellis, Vapor–liquid–solid mechanism of single crystal growth,Appl. Phys. Lett. 4 (1964) 89–90.

26] D. Calestani, M. Zha, G. Salviati, L. Lazzarini, L. Zanotti, E. Comini, G. Sberveg-lieri, Nucleation and growth of SnO2 nanowires, J. Cryst. Growth 275 (2005)2083–2087.

27] M.-R. Yang, S.-Y. Chu, R.-C. Chang, Synthesis and study of the SnO2 nanowiresgrowth, Sens. Actuators B 122 (2007) 269–273.

28] J.K. Jian, X.L. Chen, Wang.F W.J., L. Dai, Y.P. Xu, Growth and morphologies oflarge-scale SnO2 nanowires, nanobelts and nanodendrites, Appl. Phys. A 76(2003) 291–294.

29] S.H. Sun, G.W. Meng, Y.W. Wang, T. Gao, M.G. Zhang, Y.T. Tian, X.S. Peng,L.D. Zhang, Large-scale synthesis of SnO2 nanobelts, Appl. Phys. A 76 (2003)287–289.

30] S. Luo, P.K. Chu, W. Liu, M. Zhang, C. Lin, Origin of low-temperature photo-luminescence from SnO2 nanowires fabricated by thermal evaporation andannealed in different ambients, Appl. Lett. 88 (2006) 183112–183113.

31] S. Luo, J. Fan, W. Liu, M. Zhang, Z. Song, C. Lin, X. Wu, P. Chu, Synthesis and low-temperature photoluminescence properties of SnO2 nanowires and nanobelts,Nanotechnology 17 (2006) 1695–1699.

32] A. Kar, M.A. Stroscio, M. Dutta, J. Kumari, M. Meyyappan, Observation of ultra-violet emission and effect of surface states on the luminescence from tin oxidenanowires, Appl. Lett. 94 (2009) 101905–101913.

33] H.T. Chen, S.J. Xiong, X.L. Wu, J. Zhu, J.C. Shen, P.K. Chu, Tin oxide nanoribonswith vacancy structures in luminescence-sensitive oxygen sensing, Nano Lett.9 (2009) 1926–1931.

34] N. Yamazoe, K. Shimanoe, Theory of power laws for semiconductor gas sensors,Sens. Actuators B 128 (2008) 566–573.

35] G. Sakai, N. Matsunaga, K. Shimanoe, Z. Yamazoe, Theory of gas-difusion con-trolled sensitivity for thin film semiconductor gas sensor, Sens. Actuators B 80(2001) 125–131.

36] N. Yamazoe, K. Shimanoe, Theoretical approach to the gas response of oxidesemiconductor film devices under control of gas diffusion and reaction effects,Sens. Actuators B, in press.

37] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Synthesis and ethanol sensing char-acteristics of crystalline SnO2 nanorods, Appl. Phys. Lett. 87 (2005), 233503-3.

38] Y.J. Chen, L. Nie, X.Y. Xue, Y.G. Wang, T.H. Wang, Linear ethanol sensing of SnOnanorods with extremely high sensitivity, Appl. Phys. Lett. 88 (2006), 083105-3.

39] D.-F. Zhang, L.-D. Sun, G. Xu, C.-H. Yan, Size-controllable one-dimensinal SnO2

nanocrystals: synthesis, growth mechanism, and gas sensing property, Phys.

Chem. Chem. Phys. 8 (2006) 4874–4880.

40] A.R. Phani, S. Manorama, V.J. Rao, Preparation, Characterization and electricalproperties of SnO2 based liquid petroleum gas sensor, Mater. Chem. Phys. 58(1999) 101–108.

41] M.V. Vaishampayan, R.G. Deshmukh, I.S. Mulla, Influence of Pd doping on mor-phology and LPG response of SnO2, Sens. Actuators 131 (2008) 665–672.

ators B 150 (2010) 112–119 119

42] M.H.M. Reddy, A.N. Chandorkar, E-beam deposited SnO2, Pt–SnO2 and Pd–SnO2

thin film LPG, Thin Solid Films 349 (1999) 260–265.43] M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Surface customization of

SnO2 thick films using RuO2 as a surfactant for the LPG response, Sens. ActuatorsB 122 (2007) 357–364.

44] B. Thomas, S. Benoy, K.K. Radha, Influence of Cs doping in spray deposited SnO2

thin films for LPG sensors, Sens. Actuators B 133 (2008) 404–413.45] A. Forleo, L. Francioso, S. Capone, F. Casino, P. Siciliano, O.K. Tan, H. Hui, Fab-

rication at wafer level of miniaturized gas sensors based on SnO2 nanorodsdeposited by PECVD and gas sensing characteristics, Sens. Actuators B, in press.

46] Y. Wang, Q. Mu, G. Wang, Z. Zhou, Sensing characterization to NH3 of nanocrys-talline Sb-doped SnO2 synthesized by a nonaqueous sol–gel route, Sens.Actuators B 145 (2010) 847–853.

47] Y.-D. Wang, X.-H. Wu, Q. Su, Y.-F. Li, Z.-L. Zhou, Ammonia-sensing charac-teristics of Pt and SiO2 doped SnO2 materials, Solid-State Electron. 45 (2001)347–350.

48] J. Kaur, S.C. Roy, M.C. Bhatnagar, Highly sensitive SnO2 thin film NO2 gas sensoroperating at low-temperature, Sens. Actuators B 123 (2007) 1090–1095.

49] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gassensing by individual SnO2 nanowires and nanobelts functionalized with Pdcatalyst particles, Nano Lett. 5 (2005) 667–673.

50] N.V. Hieu, H.-R. Kim, B.-K. Ju, J.-H. Lee, Enhanced performance of SnO2

nanowires ethanol sensor by functionalizing with La2O3, Sens. Actuators B 133(2008) 228–234.

51] Q. Kuang, C.-S. Lao, Z. Li, Y.-Z. Liu, Z.-X. Xie, L.-S. Zheng, Z. Lin Wang, Enhancingthe photon- and gas sensing properties of a single SnO2 nanowire based nan-odevice by nanoparticle surface functionalization, J. Phys. Chem. C 112 (2008)11539–11544.

52] I.-S. Hwang, J.-K. Choi, S.-J. Kim, K.-Y. Dong, J.-H. Kwon, B.-K. Ju, J.-H. Lee,Enhanced H2S sensing characteristics of SnO2 nanowires functionalized withCuO, Sens. Actuators B 142 (2009) 105–110.

53] V.V. Sysoev, J. Goschnick, T. Schneider, E. Strelcov, A. Kolmakov, A Gradientmicroarray electronic nose based on percolating SnO2 nanowire sensing ele-ments, Nano Lett. 7 (2007) 3182–3188.

Biographies

Le Viet Thong received his BSc degree in the Faculty of Engineering Physics, QuyNhon University, Vietnam, in 2005. He received his MSc degree in materials scienceat International Training Institute for Materials Science (ITIMS), Hanoi University ofScience and Technology (HUST), Vietnam, in October 2009. Currently, he is a lecturerat the Department of Physics, Qui Nhon University, Vietnam.

Le Thi Ngoc Loan received here BSc degree in the Faculty of Engineering Physics,Quy Nhon University, Vietnam, in 2002. She received here MSc degree in materi-als science at International Training Institute for Materials Science (ITIMS), HanoiUniversity of Science and Technology (HUST), Vietnam, in October 2008. Currently,she is pursuing her PhD degree at the ITIMS, HUST, where she is working on thesynthesis and applications of SnO2 nanowires.

Nguyen Van Hieu has been an associate professor at the International TrainingInstitute for Material Science (ITIMS), Hanoi University of Science and Technology(HUST), Vietnam, since 2009. He received his PhD degree from the Department of

Electrical Engineering, University of Twente, The Netherlands, in 2004. In 2007, heworked as a post-doctoral fellow at the Korea University. Currently, he is the vicedirector of ITIMS and a leading expert in the development of new nanomaterialsfor gas sensing applications. His current research interests include nanomaterials,nanofabrications, characterizations of and applications to electronic devices, gassensors and biosensors.