Design of SnO2 ZnO hierarchical nanostructures for enhanced ethanol

Sensors and Actuators B 174 (2012) 594–601

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B:

Chemical

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s n

b

Design of SnO 2/ZnO hierarchical nanostructures for enhanced ethanol gas-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 5May 2012

Received in revised form 27June 2012Accepted 30July 2012

Available online 8 August 2012

Keywords:

SnO 2/ZnO hierarchical Ethanol sensors Nanowires

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 SnO 2/ZnO nanostructures for scalable fabrication of high-performance ethanol sensors is developed based on a combination of two simple syn-thesis pathways.High-quality single crystalline SnO 2nanowire (NW)backbones were ?rst synthesized using the thermal evaporation method,whereas ZnO nanorod (NR)branches were subsequently grown perpendicularly to the axis of SnO 2NWs via the hydrothermal approach.The successful synthesis of SnO 2/ZnO hierarchical nanostructures is con?rmed by the results of scanning electron microscope,X-ray diffraction and photoluminescence spectrum.The ethanol-sensing properties of the SnO 2/ZnO hierarchi-cal nanostructures sensors were systematically investigated and compared to those of the bare SnO 2NWs sensor.The effect of growth manipulation of the SnO 2/ZnO hierarchical nanostructures on the ethanol sensing 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 NH 3,CO,H 2,CO 2,and LPG.These enhancements are attributed to the enhancement of homogenous and heterogeneous NW–NW contacts.In addition,the results of this study may serve as a basis for designing various novel hierarchical 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 construction of nanodevices ranging from (opt-)electronic devices to chemical sensors.Nanostructures with high aspect ratio (i.e.,size con-?nement in two coordinates)offer better crystallinity,higher integration density,and lower power consumption [1].In addi-tion,they demonstrate superior sensitivity to surface chemical processes because of their large surface-to-volume ratio and small diameter comparable with Debye length (a measure of ?eld penetration into the bulk)[1–3].Designing mutual nanostruc-tures based on SMO have recently merged as a promising issue for the improvement of their potential applications.In particu-lar,hierarchical nanostructures that originated from nanowires (NWs)or nanorods (NRs)provide not only large surface area materials 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,Viet Nam.Tel.:+84438680787;fax:+84438692963.

E-mail address:hieu@https://www.360docs.net/doc/c512341737.html,.vn (N.Van Hieu).

materials constructed from NWs and NRs,including high-ef?ciency dye-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 been developed,such as ZnO [4,7],WO 3[8],SnO 2[9,10],CdTe [11],Fe 2O 3SnO 2[12],ZnO TiO 2[13],ZnO In 2O 3[14],SnO 2WO 3[15],and SnO 2ZnO [16–18].Among the semiconducting metal oxides used for constructing hierarchical nanostructures,wide band gap SnO 2(3.6eV)and ZnO (3.37eV)are interest because of their advanced physical and chemical properties.In recent years,their low dimensional nanostructures have been exten-sively investigated for novel gas-sensitive materials [19].The gas-sensing properties of hierarchical metal oxide nanostructures have been comprehensively reviewed and reported in Ref.[6].The gas-sensing properties of the homo-hierarchical nanostructures of SnO 2[9,10,20–23]and ZnO [7,24–27]are the most frequently mentioned topics in the literature.Little attention has been given to the design and use of heterogeneous hierarchical SnO 2/ZnO nanostructures for gas-sensing applications.ZnO and SnO 2mate-rials have been combined with special nanostructures,such as nanocomposite thin ?lm [28],nanocomposite nano?ber [29],and core–shell [30].ZnO-doped SnO 2[31]have been con?rmed to have outstanding ethanol gas-sensing properties.However,developing an effective route for the controllable fabrication of scalable 1D

0925-4005/$–see front matter ? 2012 Elsevier B.V. All rights reserved.https://www.360docs.net/doc/c512341737.html,/10.1016/j.snb.2012.07.118

N.D.Khoang et al./Sensors and Actuators B174 (2012) 594–601595

hierarchical SnO2/ZnO nanostructures remains a great challenge. In addition,the combination of1D ZnO and SnO2nanostructures to form a heterojunction may enhance the surface-depletion effect more easily and improve the gas-sensing performance accordingly. Thus,designing SnO2/ZnO hierarchical nanostructures from NWs or NRs is expected to result in an excellent ethanol gas-sensing performance.

In the present study,we report a controllable and scalable route for preparing SnO2/ZnO hierarchical nanostructures with SnO2NW backbones and ZnO NR branches by combining ther-mal evaporation(for SnO2NWs)and hydrothermal methods(for ZnO NRs).The comparative gas-sensing properties of the bare SnO2NWs and SnO2/ZnO hierarchical nanostructures are inves-tigated to demonstrate the potential application of hierarchical nanostructures for gas-sensing applications,in which the density and length of ZnO NRs branches are adjusted for the best ethanol-sensing performance.In addition,the gas-sensing mechanism of heterogeneous-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 through thermal evaporation and hydrothermal processes(Fig.1).In the ?rst step,SnO2NWs were synthesized according to our previous works[10].In brief,SnO2NWs were synthesized on Au-coated Si substrates through a simple thermal evaporation of Sn powders (99.9%).The source material was loaded in an alumina boat placed at the center of a quartz tube located in a horizontal-type furnace, which was heated to800?C and kept for30min during the syn-thesis of the NWs.The pressure in the quartz tube was adjusted from Torr to10Torr using O2gas with a?ow rate of0.4–0.5sccm. The as-synthesized SnO2NWs were coated with ZnO nanoparticles by spray-coating of0.01M Zn(CH3COO)2solution and subsequent heat-treatment at300?C.In the second step,the SnO2NW substrate coated with ZnO nanoparticles was immersed in an aqueous solu-tion of Zn(NO3)2(0.01M)and C6H12N4(0.01M)to allow the growth of the ZnO NR branches.The hydrothermal process was conducted at90?C for different periods(i.e.,1,2,and4h)to control the length of the NRs.After the reactions,the substrates were removed from the solution,rinsed with deionized water,and then blow dried with Ar.

The as-obtained SnO2NWs and SnO2/ZnO hierarchical nano-structures were analyzed via?eld emission scanning electron microscopy(FE-SEM,4800,Hitachi,Japan)and X-ray diffraction (XRD,Philips Xpert Pro)with Cu K?radiation generated at a volt-age of40kV as source.The photoluminescence(PL)spectrum at room temperature was acquired from360nm to910nm using a 325nm He Cd laser.

2.2.Gas sensor fabrication and characterization

For gas-sensing characterization,the as-obtained SnO2NWs and SnO2/ZnO hierarchical nanostructures were detached from the Si substrate through dispersion in isopropanol under ultrasonic treat-ment and then dried at70?C for24h.The as-obtained materials were mixed with organic binders and pasted on Pt-interdigitated electrodes with an area of800?m×1600?m.A Pt-interdigitated electrode was fabricated using a conventional photolithographic method with a?nger width of20?m and a gap size of20?m.The interdigitated electrodes were fabricated by sputtering10nm Cr and200nm Pt on a layer of silicon dioxide(SiO2)with a thickness of approximately300nm thermally grown on top of the silicon wafer.The gas-sensing characteristics of the SnO2NWs and SnO2/ZnO sen-sors were measured under identical experimental conditions.The gas concentration was controlled by changing the mixing ratio of dry parent gases and dry synthetic air.A?ow-through technique with a constant?ow rate of200sccm was used,employing a pre-viously described homemade system[10].

3.Results and discussion

3.1.Material characterization

Fig.2shows the FE-SEM images of the pristine SnO2NWs and hierarchical SnO2/ZnO nanostructures.The as-synthesized SnO2 NWs had smooth surface of single crystal with an average diameter of approximately100nm and lengths of several tens microme-ters(Fig.2a).The growth mechanism of the SnO2NWs in the present 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 after1,2,and4h growth of the ZnO NR branches,respectively.After the hydrothermal growth of ZnO NRs, the ZnO NRs branched out from the smooth SnO2NWs,forming hierarchical structures with SnO2NWs as a backbone and ZnO NRs as branches(Fig.2b).The average diameter of the ZnO NRs was approximately50nm,which was controlled by the size of the ZnO seeds.The length of ZnO NRs increased with increasing hydrothermal growth time.The average length of the ZnO NRs was approximately150,300,and600nm after1,2,and4h of growth, respectively.The growth mechanism of the ZnO NRs on the surface of SnO2NWs was as follows.When a layer of Zn(CH3COO)2solu-tion was sprayed on the surface of the SnO2NWs and heat-treated at300?C,Zn(CH3COO)2was oxidized and crystallized to form ZnO nanoparticles.The ZnO nanoparticles coated on the surface of the SnO2NW backbone played as the seeds for the growth of the ZnO NR branches during the hydrothermal process.In the early state of the hydrothermal process,the ZnO nucleated and grew out on the ZnO seeds,in which the solid ZnO nuclei were formed through the dehydration of Zn(OH)42?(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-hydroxyl complexes[35].

XRD analysis was performed to investigate the crystal struc-tures of the SnO2NWs and hierarchical SnO2/ZnO.The results are depicted in Fig.3.The XRD pattern(Fig.3a)of bare SnO2NWs exhib-ited very sharp diffraction peaks because of their high crystallinity. The typical diffraction peaks at2?of27.02?,34.44?,38.52?,and 52.10?were indexed as the(110),(101),(200),and(211)planes of tetragonal rutile SnO2,respectively.All typical diffraction peaks measured in the2?range correspond to the tetragonal structure of SnO2with lattice constants a=4.73?A and c=3.18?A.These peaks are in good agreement with those on the standard card(JCPDS,card no. 41-1445).Fig.3b and c illustrates the XRD patterns of the SnO2/ZnO hierarchical nanostructures at different hydrothermal growth peri-ods(1,2,and4h).The diffraction peak of the ZnO phase was hardly found in the XRD patterns of SnO2/ZnO samples grown for1and2h because of the relatively low amount of ZnO compared with SnO2. However,the diffraction peaks of the SnO2and ZnO phases were found to coexist in the XRD pattern of the SnO2/ZnO hierarchical nanostructures grown for4h.The typical peaks at2?of31.73?and 36.59?were well indexed as the(002)and(101)planes of ZnO, respectively.All these peaks measured in the2?range correspond to the tetragonal structure of ZnO with lattice constants of a=3.25?A and c=5.21?A.These peaks are in good agreement with those on the standard card(JCPDS card no.36-1451).No signi?cant shift was observed in the diffraction peaks.This result indicates that no

596N.D.Khoang et al./Sensors and Actuators B 174 (2012) 594–

601

Fig.1.Experimental steps to prepare the SnO 2/ZnO hierarchical nanostructures:(a)the deposition of Au catalytic layer;(b)the growth of the bare SnO 2nanowires;(c)the decoration of ZnO nanoparticles on the SnO 2nanowires surface;(d)the hydrothermal growth of ZnO nanorods;the actual SEM images of the SnO 2nanowires (e),the nanoparticles-decorated SnO 2nanowires (f),and the SnO 2/ZnO hierarchical nanostructures (h).

interface reaction exists between ZnO and SnO 2for the formation of the Zn 2SnO 4phase.

The optical characteristics of the SnO 2NWs and SnO 2/ZnO hier-archical nanostructures were also studied through PL at room temperature (Fig.4).The PL spectrum of the bare SnO 2NWs (curve 1)exhibited a broad emission peak at a visible region of 620nm

(2.0eV),which was smaller than the band gap width of the SnO 2NWs (3.6eV).Hence,the visible emission peaks cannot be ascribed to the direct recombination of a conduction electron in the Sn4d band and a hole in the O2p valence band.The semiconducting behavior of SnO 2is 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 SnO 2nanowires (a),the SnO 2/ZnO hierarchical nanostructures grown at 1h (b),2h (c),and 4h (d).

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

597

SnO 2 JCPDS :46-1088I n t e n i s i t y (a .u .

)

70

656055504540353025202θ(Degree)

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

I n t e n s i t y (a .u .)

I n t e n s i t y (a .u .)

Wavelength (nm)

Fig.4.The PL spectrum of the bare SnO 2nanowires,hierarchical nanostructures

grown at 1

h,2h (c),and 4h (a)and its magni?cation at emission peak at 385nm (b).

a i r /R g a s )

R e s p o n s e (R 253

2

2

Fig.5.The gas response of the bare SnO 2nanowires sensor and the SnO 2/ZnO (grown for 2h)sensors to C 2H 5OH,NH 3,CO,H 2,CO 2,and LPG gases.

Therefore,the emission peak at approximately 620nm is believed

to originate from the luminescence centers formed by tin inter-stitials or dangling bonds in the SnO 2NWs.The oxygen vacancies with high density interact with interfacial tin and from a consid-erable amount of trapped states within the band gap,giving rise to a high PL intensity at room temperature [6].The PL spectra of the SnO 2/ZnO hierarchical nanostructures are also presented in Fig.4,in which curves (2),(3),and (4)correspond to the spec-tra of the ZnO NRs after 1,2,and 4h of growth,respectively.Some differences were found in the PL spectrum of the bare SnO 2NWs.Aside from the emission peak at 620nm,the PL spectra of the SnO 2/ZnO hierarchical nanostructures showed a weak emis-sion peak at 385nm (approximately 3.2eV).This peak could be attributed to the attached ZnO NRs because the PL spectrum of pure ZnO NRs shows emission peaks at approximately 380and 520nm.These peaks correspond to the near band-edge emission and deep-level/trap-state emission,respectively [37].In addition,the intensity of these emission peaks at 385and 620nm increased with increasing length of the ZnO NR branches.

3.2.Gas-sensing properties

The effects of heterogeneous hierarchical structure on the gas-sensing performance of the materials were determined by measuring the bare SnO 2NWs and SnO 2ZnO (2h)sensors with different test gases (C 2H 5OH,NH 3,CO,H 2,CO 2,and LPG)at a ?xed concentration of 100ppm and an operating temperature of 400?C.As shown in Fig.5,the responses (i.e.,R a /R g ,where R a is the resis-tance in air,and R g is the resistance in ethanol gas)of the bare SnO 2NW sensors to C 2H 5OH,NH 3,CO,H 2,CO 2,and LPG were not much different.They were proximately in the range of 1.2–2.2.Meanwhile,the responses of the SnO 2ZnO hierarchical sensors to those gases were larger (i.e.,1.5–6.2).The highest enhancement in response was observed for ethanol gas.This result indicates the potential application of the sensors for screening inebriated drivers.

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

598N.D.Khoang et al./Sensors and Actuators B 174 (2012) 594–

601

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

hierarchical sensor prepared from the branched ZnO NRs grown for 1,2,and 4h were in the range of 2.3–13.1,3.0–16.2,and 1.7–8.1,respectively.This result suggests that 2h of ZnO branch growth is optimal in designing SnO 2/ZnO hierarchical nanostructures for the best gas-sensing applications.The highest response to 25ppm ethanol of the SnO 2/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 ethanol sensors to screen intoxicated drivers,the sensor should be able to detect an ethanol concentration of approximately 200ppm,which corresponds to approximately 0.5g of C 2H 5OH per liter of blood [38].Therefore,these results suggest that SnO 2/ZnO hierarchical nanostructure sensors are effective for the enhanced detection of low ethanol gas limits.

The sensor response plotted as a function of ethanol gas con-centration is shown in Fig.7.As shown in the ?gure,the response increased with increasing ethanol gas concentration.In addition,the SnO 2/ZnO hierarchical nanostructures with ZnO NRs grown for 2h clearly exhibited the best response to ethanol gas.Its response to 25–500ppm ethanol was approximately threefold and ?vefold higher than that of bare SnO 2NWs sensors,respectively.This result suggests that the enhancement of the response becomes evident

100

10

a /R g )

1R e s p o n s e (R C 2H 5OH conc. (pp m)

Fig.7.Ethanol response of the bare SnO 2nanowires and the SnO 2/ZnO hierarchical

nanostructures sensors as a function of ethanol gas concentration.

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

R g =R a (1+K [C ])?ˇ

(1)

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

S =(1+K [C ])ˇ

(2)

The ?tted parameters ˇand K are particularly useful because

they provide meaningful information for comparing sensor per-formances.The sensor’s response switches from zero order to ?rst order when [C ]=K ?1.Therefore,the inverse of K is called the sen-sitivity threshold [39].The power-law exponent ˇis related to the slope of the log–log plot of the sensor response versus ethanol con-centration.Thus,it can be understood as the ability of the sensor to distinguish similar concentrations [39].Table 1shows the ?tting parameters for the observed data presented in Fig.7.Evidently,the SnO 2/ZnO sensor can detect ethanol gas down to a concentration of <10ppm.The exponent ˇof the SnO 2/ZnO sensors was relatively larger than that of the SnO 2NW sensors.This result indicates that the SnO 2/ZnO sensors have better resolution in detecting ethanol gas.

In future studies,the sensitivity and selectivity of hierarchical sensors for sensing particular target gases can be further improved through functionalization with catalytic nanoparticles,such as Pd,Pt,Ag,Au,RuO 2,Co 3O 4,and NiO.

3.3.Gas-sensing mechanism

We proposed a sensing mechanism to explain the enhance-ment of the gas-sensing performance of SnO 2/ZnO hierarchical

Table 1

The ?tted parameters (ˇand K )in expression (2)obtained from experimental data presented in Fig.7.

Sensor

ˇ

1/K

SnO 2NWs

0.51100SnO 2/ZnO (1h)0.605SnO 2/ZnO (2h)0.606SnO 2/ZnO (4h)

0.62

19

N.D.Khoang et al./Sensors and Actuators B174 (2012) 594–601

599

Fig.8.The schematic illustration of the gas sensing mechanism of the bare SnO2nanowires(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 is performed by percolating the network of the NWs,where the con-ductance between two adjacent electrodes is determined by the availability of the conduction paths through the overlapping NWs [40].The gas-sensing performance of NWs and hierarchical NWs can be detected using two possible mechanisms[40,41].The?rst mechanism is based on the coaxial cable model,whereas the sec-ond mechanism is based on the percolating network theory.In the ?rst mechanism,the electron transport inside the NWs(or NRs) with diameters approximately100nm can be modulated by the electron depletion layer,which surrounds the NW surface and is formed by adsorbing the oxygen molecules on the NWs surface, extracting electrons from the conduction band of the NWs to form oxygen ions(O2?,O?,or O2?).The electron depletion layer is sup-posed to narrow the conducting channel of the NWs.When the sensors are exposed to ethanol gas,the ethanol molecules react to these oxygen ions,release electrons back into the conduction band, and reduce the depletion layer,thereby enhancing the conducting channel.The diameter of NWs plays an important role in gas-sensing performance.NWs with diameters in the range of Debye length(10–20nm for SnO2NWs)exhibit high gas response.For the second mechanism,the percolation network of electrons should pass through potential barriers at junctions between NWs.The potential barriers at the contact points between NWs effectively modulate the electron transport between two adjacent electrodes by adsorbing or desorbing gas molecules[40,41].Therefore,the second mechanism elicits a more effective in?uence on the gas-sensing performance of NW sensors[40].When the sensors are exposed to ethanol gas,the ethanol molecules react with oxygen ion at the junctions,thereby lowering the potential barrier height and enhancing the electron transport through the network NWs[1].Based on these two gas-sensing mechanisms,the ethanol response enhancement of SnO2/ZnO hierarchical nanostructures compared with SnO2NWs can be qualitatively attributed to the addition of SnO2/ZnO hetero-junctions and ZnO/ZnO junctions(Fig.8).These junctions can be considered as additional active sites,resulting in the enhancement of sensor response.

The chemical mechanism should also be taken into account in elucidating the ethanol response enhancement of SnO2/ZnO hierarchical nanostructures.The chemical-sensing mechanism of ethanol gas is related to its decomposition and/or oxidation of ethanol molecules.The decomposition of ethanol molecules at ele-vated temperatures depends on the acid-base properties of sensing materials[42].Given that ZnO is a basic oxide[7,28],SnO2/ZnO hierarchical nanostructures may exhibit more basic properties than SnO2NWs.Therefore,dehydrogenation is favored,as depicted in Eq.(3):

C2H5OH(g)→CH3CHO(g)+H2(g)(basicoxide)(3) The CH3CHO intermediate is subsequently oxidized to form CO2 and H2O,as depicted in Eq.(4):

CH3CHO(ad)+5O

ˉ→2CO

2+2H2O+5e

ˉ(4)

These equations indicate that the electron-donating effect of ethanol gas is stronger than that of the other gases.This phe-nomenon explains why the response to ethanol gas is higher than that to the other gases at equivalent concentration(Fig.5).Besides, the3D hierarchical assembly of ZnO NRs on the core SnO2NWs makes the SnO2/ZnO hierarchical nanostructures more porous, which may also contribute to gas response enhancement.

600N.D.Khoang et al./Sensors and Actuators B174 (2012) 594–601

4.Conclusion

We reported a controllable route for the preparation of SnO2/ZnO hierarchical nanostructures by combining two conven-tional methods.The backbone SnO2NWs were?rst prepared using the thermal evaporation method.The ZnO NR branches were then grown on the SnO2NWs to form a3D hierarchical assem-bly.The gas-sensing characterizations showed that the ethanol gas-sensing performance of SnO2/ZnO hierarchical nanostructures was improved compared with that of the bare SnO2NWs.This improvement in ethanol gas response could be attributed to an additional formation of SnO2/ZnO hetero-junction.The present study may serve as a basis for designing other novel hierarchical nanostructures from oxide NWs,such as In2O3,WO3/ZnO,(p-type) Co3O4/ZnO,and(p-type)CuO/ZnO hierarchical nanostructures for gas-sensing applications.

Acknowledgement

This research is funded by Vietnam National Foundation for Science and Technology Development(NAFOSTED)under grant number103.02-2011.40.

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Biographies

Nguyen Duc Khoang received his MSc degree in Materials Science at International Training Institute for Material Science(ITIMS),Hanoi University of Science and Tech-nology(HUST),in Vietnam in2007.He is currently pursuing his PhD degree at the ITIMS,where he is working on the synthesis and applications of hybrid materials of metal oxides and carbon nanotubes.

Do Dang Trung received Engineering degree in Chemistry at the Faculty of Chemical Engineering,Hanoi University of Science Technology(HUST),Vietnam,in2004.He received MSc degree in Materials Science at the International Training Institute for Materials 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?ammable and explosive gas detectors based on nanomaterials.

N.D.Khoang et al./Sensors and Actuators B174 (2012) 594–601601

Nguyen Van Duy is currently working as a research lecturer at International Training Institute 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,in2011.His current research interests include nanomaterials,nanofabrications,characterizations,and applications to electronic devices,gas sensors,and biosensors.

Nguyen Duc Hoa obtained his PhD degree in Materials Science and Engineering in 2009at Chungnam National University in Korea.He awarded JSPS fellowship and conducted the research at National Institute for Materials Science(NIMS,Japan) from2009to2011.His research activity has covered a wide range of nanostructured materials from synthesis,fundamental,and applications.He is the author of several dozens of scienti?c articles published in reputed journals.Currently,he is a lecturer and 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)in2004,where he is currently associate professor.He received his PhD degree from the Fac-ulty of Electrical Engineering at University of Twente in The Netherlands in2004. He worked as a post-doctoral fellow at the Korea University from2006to2007. His current research interests include functional nanostructures,gas sensors,and biosensors.