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Sensors and Actuators B 174 (2012) 594– 601

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

j o ur nal homep a ge: www.elsev ier .com/ locate /snb

Design of SnO2/ZnO hierarchical nanostructures for enhanced ethanolgas-sensing performance

Nguyen Duc Khoang, Do Dang Trung, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu ∗

International Training Institute for Materials Science, Hanoi University of Science and Technology, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:Received 5 May 2012Received in revised form 27 June 2012Accepted 30 July 2012Available online 8 August 2012

Keywords:SnO2/ZnO hierarchicalEthanol sensorsNanowires

a b s t r a c t

Designing nanostructured materials to enhance gas-sensing performance is of important key for the next-generation sensor platforms. In this paper, a design of hierarchical SnO2/ZnO nanostructures for scalablefabrication of high-performance ethanol sensors is developed based on a combination of two simple syn-thesis pathways. High-quality single crystalline SnO2 nanowire (NW) backbones were first synthesizedusing the thermal evaporation method, whereas ZnO nanorod (NR) branches were subsequently grownperpendicularly to the axis of SnO2 NWs via the hydrothermal approach. The successful synthesis ofSnO2/ZnO hierarchical nanostructures is confirmed by the results of scanning electron microscope, X-raydiffraction and photoluminescence spectrum. The ethanol-sensing properties of the SnO2/ZnO hierarchi-cal nanostructures sensors were systematically investigated and compared to those of the bare SnO2 NWssensor. The effect of growth manipulation of the SnO2/ZnO hierarchical nanostructures on the ethanolsensing characteristics was also studied. The results revealed that the design of the hierarchical nano-structures enhanced the ethanol gas response and selectivity for interfering gases such as NH3, CO, H2,CO2, and LPG. These enhancements are attributed to the enhancement of homogenous and heterogeneousNW–NW contacts. In addition, the results of this study may serve as a basis for designing various novelhierarchical nanostructures for other applications, including photocatalysis, battery electrode, solar cell,and nanosensors.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

One-dimensional (1D) semiconductor metal oxide (SMO) nano-structures have attracted increasing attention in the constructionof nanodevices ranging from (opt-) electronic devices to chemicalsensors. Nanostructures with high aspect ratio (i.e., size con-finement in two coordinates) offer better crystallinity, higherintegration density, and lower power consumption [1]. In addi-tion, they demonstrate superior sensitivity to surface chemicalprocesses because of their large surface-to-volume ratio and smalldiameter comparable with Debye length (a measure of fieldpenetration into the bulk) [1–3]. Designing mutual nanostruc-tures based on SMO have recently merged as a promising issuefor the improvement of their potential applications. In particu-lar, hierarchical nanostructures that originated from nanowires(NWs) or nanorods (NRs) provide not only large surface areamaterials but also multifunctional nanomaterials. Diverse appli-cations have been demonstrated using hierarchical nanostructure

∗ Corresponding author at: International Training Institute for Materials Science,Hanoi University of Science and Technology, No. 1, Dai Co Viet Road, Hanoi, VietNam. Tel.: +84 4 38680787; fax: +84 4 38692963.

E-mail address: hieu@itims.edu.vn (N. Van Hieu).

materials constructed from NWs and NRs, including high-efficiencydye-sensitized solar cell [4], high-performance photocatalysis [5],and gas sensors [6]. To date, numerous hierarchical nanostruc-tures of homo- and/or heterogeneous-nanostructures have beendeveloped, such as ZnO [4,7], WO3 [8], SnO2 [9,10], CdTe [11],Fe2O3 SnO2 [12], ZnO TiO2 [13], ZnO In2O3 [14], SnO2 WO3[15], and SnO2 ZnO [16–18]. Among the semiconducting metaloxides used for constructing hierarchical nanostructures, wideband gap SnO2 (3.6 eV) and ZnO (3.37 eV) are interest becauseof their advanced physical and chemical properties. In recentyears, their low dimensional nanostructures have been exten-sively investigated for novel gas-sensitive materials [19]. Thegas-sensing properties of hierarchical metal oxide nanostructureshave been comprehensively reviewed and reported in Ref. [6]. Thegas-sensing properties of the homo-hierarchical nanostructuresof SnO2 [9,10,20–23] and ZnO [7,24–27] are the most frequentlymentioned topics in the literature. Little attention has been givento the design and use of heterogeneous hierarchical SnO2/ZnOnanostructures for gas-sensing applications. ZnO and SnO2 mate-rials have been combined with special nanostructures, such asnanocomposite thin film [28], nanocomposite nanofiber [29], andcore–shell [30]. ZnO-doped SnO2 [31] have been confirmed to haveoutstanding ethanol gas-sensing properties. However, developingan effective route for the controllable fabrication of scalable 1D

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hierarchical SnO2/ZnO nanostructures remains a great challenge.In addition, the combination of 1D ZnO and SnO2 nanostructuresto form a heterojunction may enhance the surface-depletion effectmore easily and improve the gas-sensing performance accordingly.Thus, designing SnO2/ZnO hierarchical nanostructures from NWsor NRs is expected to result in an excellent ethanol gas-sensingperformance.

In the present study, we report a controllable and scalableroute for preparing SnO2/ZnO hierarchical nanostructures withSnO2 NW backbones and ZnO NR branches by combining ther-mal evaporation (for SnO2 NWs) and hydrothermal methods (forZnO NRs). The comparative gas-sensing properties of the bareSnO2 NWs and SnO2/ZnO hierarchical nanostructures are inves-tigated to demonstrate the potential application of hierarchicalnanostructures for gas-sensing applications, in which the densityand length of ZnO NRs branches are adjusted for the best ethanol-sensing performance. In addition, the gas-sensing mechanism ofheterogeneous-hierarchical SnO2/ZnO nanostructures is also dis-cussed in the light of NW–NW contact enhancement.

2. Experimental

2.1. Material synthesis

SnO2/ZnO hierarchical nanostructures were prepared throughthermal evaporation and hydrothermal processes (Fig. 1). In thefirst step, SnO2 NWs were synthesized according to our previousworks [10]. In brief, SnO2 NWs were synthesized on Au-coated Sisubstrates through a simple thermal evaporation of Sn powders(99.9%). The source material was loaded in an alumina boat placedat the center of a quartz tube located in a horizontal-type furnace,which was heated to 800 ◦C and kept for 30 min during the syn-thesis of the NWs. The pressure in the quartz tube was adjustedfrom Torr to 10 Torr using O2 gas with a flow rate of 0.4–0.5 sccm.The as-synthesized SnO2 NWs were coated with ZnO nanoparticlesby spray-coating of 0.01 M Zn(CH3COO)2 solution and subsequentheat-treatment at 300 ◦C. In the second step, the SnO2 NW substratecoated with ZnO nanoparticles was immersed in an aqueous solu-tion of Zn(NO3)2 (0.01 M) and C6H12N4 (0.01 M) to allow the growthof the ZnO NR branches. The hydrothermal process was conductedat 90 ◦C for different periods (i.e., 1, 2, and 4 h) to control the lengthof the NRs. After the reactions, the substrates were removed fromthe solution, rinsed with deionized water, and then blow dried withAr.

The as-obtained SnO2 NWs and SnO2/ZnO hierarchical nano-structures were analyzed via field emission scanning electronmicroscopy (FE-SEM, 4800, Hitachi, Japan) and X-ray diffraction(XRD, Philips Xpert Pro) with Cu K� radiation generated at a volt-age of 40 kV as source. The photoluminescence (PL) spectrum atroom temperature was acquired from 360 nm to 910 nm using a325 nm He Cd laser.

2.2. Gas sensor fabrication and characterization

For gas-sensing characterization, the as-obtained SnO2 NWs andSnO2/ZnO hierarchical nanostructures were detached from the Sisubstrate through dispersion in isopropanol under ultrasonic treat-ment and then dried at 70 ◦C for 24 h. The as-obtained materialswere mixed with organic binders and pasted on Pt-interdigitatedelectrodes with an area of 800 �m × 1600 �m. A Pt-interdigitatedelectrode was fabricated using a conventional photolithographicmethod with a finger width of 20 �m and a gap size of 20 �m. Theinterdigitated electrodes were fabricated by sputtering 10 nm Crand 200 nm Pt on a layer of silicon dioxide (SiO2) with a thickness ofapproximately 300 nm thermally grown on top of the silicon wafer.

The gas-sensing characteristics of the SnO2 NWs and SnO2/ZnO sen-sors were measured under identical experimental conditions. Thegas concentration was controlled by changing the mixing ratio ofdry parent gases and dry synthetic air. A flow-through techniquewith a constant flow rate of 200 sccm was used, employing a pre-viously described homemade system [10].

3. Results and discussion

3.1. Material characterization

Fig. 2 shows the FE-SEM images of the pristine SnO2 NWs andhierarchical SnO2/ZnO nanostructures. The as-synthesized SnO2NWs had smooth surface of single crystal with an average diameterof approximately 100 nm and lengths of several tens microme-ters (Fig. 2a). The growth mechanism of the SnO2 NWs in thepresent work was explained based on the vapor–liquid–solid mech-anism [32]. More details are expounded in our previous works[33,34]. Fig. 2b–d shows the morphologies of the SnO2/ZnO hier-archical nanostructures after 1, 2, and 4 h growth of the ZnO NRbranches, respectively. After the hydrothermal growth of ZnO NRs,the ZnO NRs branched out from the smooth SnO2 NWs, forminghierarchical structures with SnO2 NWs as a backbone and ZnONRs as branches (Fig. 2b). The average diameter of the ZnO NRswas approximately 50 nm, which was controlled by the size ofthe ZnO seeds. The length of ZnO NRs increased with increasinghydrothermal growth time. The average length of the ZnO NRs wasapproximately 150, 300, and 600 nm after 1, 2, and 4 h of growth,respectively. The growth mechanism of the ZnO NRs on the surfaceof SnO2 NWs was as follows. When a layer of Zn(CH3COO)2 solu-tion was sprayed on the surface of the SnO2 NWs and heat-treatedat 300 ◦C, Zn(CH3COO)2 was oxidized and crystallized to form ZnOnanoparticles. The ZnO nanoparticles coated on the surface of theSnO2 NW backbone played as the seeds for the growth of the ZnONR branches during the hydrothermal process. In the early stateof the hydrothermal process, the ZnO nucleated and grew out onthe ZnO seeds, in which the solid ZnO nuclei were formed throughthe dehydration of Zn(OH)4

2−(aq) and Zn(NH3)42+(aq) [35]. The

ZnO crystal was supposed to grow continuously by the conden-sation of the surface hydroxyl groups with the zinc-hydroxylcomplexes [35].

XRD analysis was performed to investigate the crystal struc-tures of the SnO2 NWs and hierarchical SnO2/ZnO. The results aredepicted in Fig. 3. The XRD pattern (Fig. 3a) of bare SnO2 NWs exhib-ited very sharp diffraction peaks because of their high crystallinity.The typical diffraction peaks at 2� of 27.02◦, 34.44◦, 38.52◦, and52.10◦ were indexed as the (1 1 0), (1 0 1), (2 0 0), and (2 1 1) planesof tetragonal rutile SnO2, respectively. All typical diffraction peaksmeasured in the 2� range correspond to the tetragonal structure ofSnO2 with lattice constants a = 4.73 A and c = 3.18 A. These peaks arein good agreement with those on the standard card (JCPDS, card no.41-1445). Fig. 3b and c illustrates the XRD patterns of the SnO2/ZnOhierarchical nanostructures at different hydrothermal growth peri-ods (1, 2, and 4 h). The diffraction peak of the ZnO phase was hardlyfound in the XRD patterns of SnO2/ZnO samples grown for 1 and 2 hbecause of the relatively low amount of ZnO compared with SnO2.However, the diffraction peaks of the SnO2 and ZnO phases werefound to coexist in the XRD pattern of the SnO2/ZnO hierarchicalnanostructures grown for 4 h. The typical peaks at 2� of 31.73◦ and36.59◦ were well indexed as the (0 0 2) and (1 0 1) planes of ZnO,respectively. All these peaks measured in the 2� range correspondto the tetragonal structure of ZnO with lattice constants of a = 3.25 Aand c = 5.21 A. These peaks are in good agreement with those onthe standard card (JCPDS card no. 36-1451). No significant shiftwas observed in the diffraction peaks. This result indicates that no

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Fig. 1. Experimental steps to prepare the SnO2/ZnO hierarchical nanostructures: (a) the deposition of Au catalytic layer; (b) the growth of the bare SnO2 nanowires; (c)the decoration of ZnO nanoparticles on the SnO2 nanowires surface; (d) the hydrothermal growth of ZnO nanorods; the actual SEM images of the SnO2 nanowires (e), thenanoparticles-decorated SnO2 nanowires (f), and the SnO2/ZnO hierarchical nanostructures (h).

interface reaction exists between ZnO and SnO2 for the formationof the Zn2SnO4 phase.

The optical characteristics of the SnO2 NWs and SnO2/ZnO hier-archical nanostructures were also studied through PL at roomtemperature (Fig. 4). The PL spectrum of the bare SnO2 NWs (curve1) exhibited a broad emission peak at a visible region of 620 nm

(2.0 eV), which was smaller than the band gap width of the SnO2NWs (3.6 eV). Hence, the visible emission peaks cannot be ascribedto the direct recombination of a conduction electron in the Sn4dband and a hole in the O2p valence band. The semiconductingbehavior of SnO2 is attributed to the oxygen vacancies in the crys-tal structure, which is also crucial to their optical properties [36].

Fig. 2. FE-SEM images of the bare SnO2 nanowires (a), the SnO2/ZnO hierarchical nanostructures grown at 1 h (b), 2 h (c), and 4 h (d).

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ZnO JCPDS :36-1451

0)

SnO2 JCPDS :46-1088

(a) SnO NW110)

01) (b) Sn O

2-ZnO: 1 h

(211)

(200)

(11

(101) (a) SnO

2NW

(c) SnO2-ZnO: 2 h

1)(110)

01)

(211)

(200)(1 (10 ( )

2

11)

00)

01)(1

01)

(110)

(101)

Inte

nis

ity (

a.u

.)

(d) SnO2-ZnO: 4 h

02)

(211

(200)(1

7065605550454035302520

(2(20

(10

2θ (Degree)

(00

Fig. 3. The XRD patterns of the bare SnO2 nanowires (a), SnO2/ZnO hierarchicalnanostructures grown at 1 h (b), 2 h (c), and 4 h (d).

(1) Sn O2

(2) Sn O2-ZnO (1 h)

(3) Sn O2-ZnO (2 h)

(4) Sn O2-ZnO (4 h)

(2)

(3)

(4)(a)

Inte

nsit

y (

a.u

.)

(1)

1000900800700600500400300

Wavelength (nm)

(1) Sn O2

(2) Sn O2-ZnO (1h)

(3) Sn O2-ZnO (2h)

(4) Sn O2-ZnO (4h)

(b)

Inte

nsit

y (

a.u

.)

395390385380375370

Wavelength (nm)

Fig. 4. The PL spectrum of the bare SnO2 nanowires, hierarchical nanostructuresgrown at 1 h, 2 h (c), and 4 h (a) and its magnification at emission peak at 385 nm(b).

6

8

10

air/R

gas)

SnO2 NWs Sensor

SnO2-ZnO(2h) Sen sor

10

0 p

pm

C2H

5O

H

10

0 p

pm

NH

3

10

0 p

pm

CO

10

0 p

pm

H2

10

0 p

pm

CO

2

100 p

pm

LP

G

2

4

Re

sp

on

se

(R

0C

2H

5OH NH

3CO H

2CO

2LPG

Fig. 5. The gas response of the bare SnO2 nanowires sensor and the SnO2/ZnO(grown for 2 h) sensors to C2H5OH, NH3, CO, H2, CO2, and LPG gases.

Therefore, the emission peak at approximately 620 nm is believedto originate from the luminescence centers formed by tin inter-stitials or dangling bonds in the SnO2 NWs. The oxygen vacancieswith high density interact with interfacial tin and from a consid-erable amount of trapped states within the band gap, giving riseto a high PL intensity at room temperature [6]. The PL spectra ofthe SnO2/ZnO hierarchical nanostructures are also presented inFig. 4, in which curves (2), (3), and (4) correspond to the spec-tra of the ZnO NRs after 1, 2, and 4 h of growth, respectively.Some differences were found in the PL spectrum of the bare SnO2NWs. Aside from the emission peak at 620 nm, the PL spectra ofthe SnO2/ZnO hierarchical nanostructures showed a weak emis-sion peak at 385 nm (approximately 3.2 eV). This peak could beattributed to the attached ZnO NRs because the PL spectrum ofpure ZnO NRs shows emission peaks at approximately 380 and520 nm. These peaks correspond to the near band-edge emissionand deep-level/trap-state emission, respectively [37]. In addition,the intensity of these emission peaks at 385 and 620 nm increasedwith increasing length of the ZnO NR branches.

3.2. Gas-sensing properties

The effects of heterogeneous hierarchical structure on thegas-sensing performance of the materials were determined bymeasuring the bare SnO2 NWs and SnO2 ZnO (2 h) sensors withdifferent test gases (C2H5OH, NH3, CO, H2, CO2, and LPG) at a fixedconcentration of 100 ppm and an operating temperature of 400 ◦C.As shown in Fig. 5, the responses (i.e., Ra/Rg, where Ra is the resis-tance in air, and Rg is the resistance in ethanol gas) of the bareSnO2 NW sensors to C2H5OH, NH3, CO, H2, CO2, and LPG were notmuch different. They were proximately in the range of 1.2–2.2.Meanwhile, the responses of the SnO2 ZnO hierarchical sensorsto those gases were larger (i.e., 1.5–6.2). The highest enhancementin response was observed for ethanol gas. This result indicates thepotential application of the sensors for screening inebriated drivers.

The gas-sensing performances of the SnO2/ZnO hierarchicalstructures are dependent on the length of the ZnO NR branches.Therefore, to obtain the best ethanol-sensing performance ofSnO2/ZnO hierarchical sensors, we measured the response of thebare SnO2 NWs and SnO2/ZnO sensors. The branched ZnO NRswere grown at different times (i.e., 1, 2, and 4 h) with ethanol gas(25–500 ppm) at an operating temperature of 400 ◦C. The ethanol-sensing transient results are shown in Fig. 6, in which the graphswere plotted with the same scale for quickly comparing the sensorresponse. Apparently, the SnO2/ZnO hierarchical sensors exhibitedbetter ethanol response than bare SnO2 NWs. All the SnO2/ZnOhierarchical sensors showed very stable sensing and recovery char-acteristics. The responses to 25–500 ppm ethanol gas of SnO2/ZnO

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Fig. 6. Dynamic sensing response to ethanol gas (25–500 ppm) of the bare SnO2 nanowires (a) the SnO2/ZnO hierarchical nanostructures grown at 1 h (b), 2 h (c), and 4 h (d).

hierarchical sensor prepared from the branched ZnO NRs grown for1, 2, and 4 h were in the range of 2.3–13.1, 3.0–16.2, and 1.7–8.1,respectively. This result suggests that 2 h of ZnO branch growthis optimal in designing SnO2/ZnO hierarchical nanostructures forthe best gas-sensing applications. The highest response to 25 ppmethanol of the SnO2/ZnO hierarchical sensor was approximately 3.This sensor has the capacity to detect ethanol even at lower concen-trations down to sub-ppm level. In practical applications of ethanolsensors to screen intoxicated drivers, the sensor should be able todetect an ethanol concentration of approximately 200 ppm, whichcorresponds to approximately 0.5 g of C2H5OH per liter of blood[38]. Therefore, these results suggest that SnO2/ZnO hierarchicalnanostructure sensors are effective for the enhanced detection oflow ethanol gas limits.

The sensor response plotted as a function of ethanol gas con-centration is shown in Fig. 7. As shown in the figure, the responseincreased with increasing ethanol gas concentration. In addition,the SnO2/ZnO hierarchical nanostructures with ZnO NRs grown for2 h clearly exhibited the best response to ethanol gas. Its responseto 25–500 ppm ethanol was approximately threefold and fivefoldhigher than that of bare SnO2 NWs sensors, respectively. This resultsuggests that the enhancement of the response becomes evident

100 Sn O

2 NWs

SnO /ZnO (1h) S (R /R ) = (1+k[C])β

10

SnO2/ZnO (1h )

SnO2/ZnO (2h )

Sn O2/ZnO (4h )

a/R

g)

S (Ra/Rg) = (1+k[C])

1

Re

sp

on

se

(R

100010010

C2H

5OH conc. (pp m)

Fig. 7. Ethanol response of the bare SnO2 nanowires and the SnO2/ZnO hierarchicalnanostructures sensors as a function of ethanol gas concentration.

for low and high ethanol gas concentrations. The response of oxidesemiconductor gas sensors is usually depicted as [39]:

Rg = Ra(1 + K[C])−ˇ (1)

where Rg and Ra are the sensor’s resistance in ethanol and air,respectively, C is the gas concentration in ppm, and and K areconstants. The data of sensor response (S = Ra/Rg) versus ethanolgas concentration (C) can be expressed as follows:

S = (1 + K[C])ˇ (2)

The fitted parameters and K are particularly useful becausethey provide meaningful information for comparing sensor per-formances. The sensor’s response switches from zero order to firstorder when [C] = K−1. Therefore, the inverse of K is called the sen-sitivity threshold [39]. The power-law exponent is related to theslope of the log–log plot of the sensor response versus ethanol con-centration. Thus, it can be understood as the ability of the sensorto distinguish similar concentrations [39]. Table 1 shows the fittingparameters for the observed data presented in Fig. 7. Evidently, theSnO2/ZnO sensor can detect ethanol gas down to a concentrationof <10 ppm. The exponent of the SnO2/ZnO sensors was relativelylarger than that of the SnO2 NW sensors. This result indicates thatthe SnO2/ZnO sensors have better resolution in detecting ethanolgas.

In future studies, the sensitivity and selectivity of hierarchicalsensors for sensing particular target gases can be further improvedthrough functionalization with catalytic nanoparticles, such as Pd,Pt, Ag, Au, RuO2, Co3O4, and NiO.

3.3. Gas-sensing mechanism

We proposed a sensing mechanism to explain the enhance-ment of the gas-sensing performance of SnO2/ZnO hierarchical

Table 1The fitted parameters ( and K) in expression (2) obtained from experimental datapresented in Fig. 7.

Sensor 1/K

SnO2 NWs 0.51 100SnO2/ZnO (1 h) 0.60 5SnO2/ZnO (2 h) 0.60 6SnO2/ZnO (4 h) 0.62 19

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Fig. 8. The schematic illustration of the gas sensing mechanism of the bare SnO2 nanowires (a) and the SnO2/ZnO hierarchical nanostructures (b) gas sensors.

nanostructures compared with SnO2NWs (Fig. 8). The electri-cal current transport from our NWs and hierarchical sensors isperformed by percolating the network of the NWs, where the con-ductance between two adjacent electrodes is determined by theavailability of the conduction paths through the overlapping NWs[40]. The gas-sensing performance of NWs and hierarchical NWscan be detected using two possible mechanisms [40,41]. The firstmechanism is based on the coaxial cable model, whereas the sec-ond mechanism is based on the percolating network theory. In thefirst mechanism, the electron transport inside the NWs (or NRs)with diameters approximately 100 nm can be modulated by theelectron depletion layer, which surrounds the NW surface and isformed by adsorbing the oxygen molecules on the NWs surface,extracting electrons from the conduction band of the NWs to formoxygen ions (O2−, O−, or O2

−). The electron depletion layer is sup-posed to narrow the conducting channel of the NWs. When thesensors are exposed to ethanol gas, the ethanol molecules react tothese oxygen ions, release electrons back into the conduction band,and reduce the depletion layer, thereby enhancing the conductingchannel. The diameter of NWs plays an important role in gas-sensing performance. NWs with diameters in the range of Debyelength (10–20 nm for SnO2 NWs) exhibit high gas response. For thesecond mechanism, the percolation network of electrons shouldpass through potential barriers at junctions between NWs. Thepotential barriers at the contact points between NWs effectivelymodulate the electron transport between two adjacent electrodesby adsorbing or desorbing gas molecules [40,41]. Therefore, thesecond mechanism elicits a more effective influence on the gas-sensing performance of NW sensors [40]. When the sensors areexposed to ethanol gas, the ethanol molecules react with oxygenion at the junctions, thereby lowering the potential barrier heightand enhancing the electron transport through the network NWs [1].

Based on these two gas-sensing mechanisms, the ethanol responseenhancement of SnO2/ZnO hierarchical nanostructures comparedwith SnO2 NWs can be qualitatively attributed to the addition ofSnO2/ZnO hetero-junctions and ZnO/ZnO junctions (Fig. 8). Thesejunctions can be considered as additional active sites, resulting inthe enhancement of sensor response.

The chemical mechanism should also be taken into accountin elucidating the ethanol response enhancement of SnO2/ZnOhierarchical nanostructures. The chemical-sensing mechanism ofethanol gas is related to its decomposition and/or oxidation ofethanol molecules. The decomposition of ethanol molecules at ele-vated temperatures depends on the acid-base properties of sensingmaterials [42]. Given that ZnO is a basic oxide [7,28], SnO2/ZnOhierarchical nanostructures may exhibit more basic properties thanSnO2 NWs. Therefore, dehydrogenation is favored, as depicted inEq. (3):

C2H5OH(g) → CH3CHO(g) + H2(g)(basicoxide) (3)

The CH3CHO intermediate is subsequently oxidized to form CO2and H2O, as depicted in Eq. (4):

CH3CHO(ad) + 5O¯ → 2CO2 + 2H2O + 5e¯ (4)

These equations indicate that the electron-donating effect ofethanol gas is stronger than that of the other gases. This phe-nomenon explains why the response to ethanol gas is higher thanthat to the other gases at equivalent concentration (Fig. 5). Besides,the 3D hierarchical assembly of ZnO NRs on the core SnO2 NWsmakes the SnO2/ZnO hierarchical nanostructures more porous,which may also contribute to gas response enhancement.

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4. Conclusion

We reported a controllable route for the preparation ofSnO2/ZnO hierarchical nanostructures by combining two conven-tional methods. The backbone SnO2 NWs were first preparedusing the thermal evaporation method. The ZnO NR branches werethen grown on the SnO2 NWs to form a 3D hierarchical assem-bly. The gas-sensing characterizations showed that the ethanolgas-sensing performance of SnO2/ZnO hierarchical nanostructureswas improved compared with that of the bare SnO2 NWs. Thisimprovement in ethanol gas response could be attributed to anadditional formation of SnO2/ZnO hetero-junction. The presentstudy may serve as a basis for designing other novel hierarchicalnanostructures from oxide NWs, such as In2O3, WO3/ZnO, (p-type)Co3O4/ZnO, and (p-type) CuO/ZnO hierarchical nanostructures forgas-sensing applications.

Acknowledgement

This research is funded by Vietnam National Foundation forScience and Technology Development (NAFOSTED) under grantnumber 103.02-2011.40.

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Biographies

Nguyen Duc Khoang received his MSc degree in Materials Science at InternationalTraining Institute for Material Science (ITIMS), Hanoi University of Science and Tech-nology (HUST), in Vietnam in 2007. He is currently pursuing his PhD degree at theITIMS, where he is working on the synthesis and applications of hybrid materials ofmetal oxides and carbon nanotubes.

Do Dang Trung received Engineering degree in Chemistry at the Faculty of ChemicalEngineering, Hanoi University of Science Technology (HUST), Vietnam, in 2004. Hereceived MSc degree in Materials Science at the International Training Institute forMaterials Science (ITIMS), HUST, in Vietnam, in October, 2010. He is currently pur-suing his PhD degree at ITIMS, where he is working on development of flammableand explosive gas detectors based on nanomaterials.

Author's personal copy

N.D. Khoang et al. / Sensors and Actuators B 174 (2012) 594– 601 601

Nguyen Van Duy is currently working as a research lecturer at International TrainingInstitute for Material Science (ITIMS), Hanoi University of Science and Technology(HUST). He received PhD degree from the Department of Electrical and Electron-ics Engineering at Sungkyunkwan University, South Korea, in 2011. His currentresearch interests include nanomaterials, nanofabrications, characterizations, andapplications to electronic devices, gas sensors, and biosensors.

Nguyen Duc Hoa obtained his PhD degree in Materials Science and Engineering in2009 at Chungnam National University in Korea. He awarded JSPS fellowship andconducted the research at National Institute for Materials Science (NIMS, Japan)from 2009 to 2011. His research activity has covered a wide range of nanostructured

materials from synthesis, fundamental, and applications. He is the author of severaldozens of scientific articles published in reputed journals. Currently, he is a lecturerand scientist at Hanoi University of Science and Technology, Vietnam.

Nguyen Van Hieu joined the International Training Institute for Material Science(ITIMS) at Hanoi University of Science and Technology (HUST) in 2004, wherehe is currently associate professor. He received his PhD degree from the Fac-ulty of Electrical Engineering at University of Twente in The Netherlands in 2004.He worked as a post-doctoral fellow at the Korea University from 2006 to 2007.His current research interests include functional nanostructures, gas sensors, andbiosensors.