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doi:10.1016/j.ph

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Physica B 403 (2008) 50–56

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Low-temperature growth and ethanol-sensing characteristics ofquasi-one-dimensional ZnO nanostructures

Nguyen Van Hieua,b,�, Nguyen Duc Chienc

aInternational Training Institute for Materials Science (ITIMS), VietnambHanoi Advanced School of Science and Technology (HAST), Vietnam

cInstitute of Engineering Physics (IEP), Hanoi University of Technology, No. 1 Dai Co Viet, Hanoi, Vietnam

Received 8 November 2006; received in revised form 23 July 2007; accepted 3 August 2007

Abstract

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures

in the range 550–600 1C under flow of Ar or Ar/O2 as carrier gas. Uniform ZnO nanowires with diameter 15–25 nm and tetrapod

nanowires with diameter 30–50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that

the concentration of O2 in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated

from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up

to about 5.3 at the operating temperature 300 1C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the

ZnO nanostructures is also discussed and their potential application is indicated accordingly.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Nanowires; Nanorods; Nanobelts; Gas sensor

1. Introduction

In the past decades zinc oxide (ZnO) has attractedintensive research effort for its unique properties andversatile applications in transparent electronics, ultravioletlight emitters, piezoelectric devices, chemical sensors andspin electronics. Recently, quasi-one-dimensional (Q1D)ZnO nanostructures, such as nanowires, nanobelts, andnanoneedles, have been attracting tremendous researchinterest and they have been emerging as candidates for theabove-mentioned applications with much better perfor-mance and new generation of nanoscale devices [1,2] arebuilt with them.

The Q1D ZnO nanostructures can be synthesized byvarious methods, e.g. arc discharge, laser ablation,pyrolysis, electrodeposition, and chemical or physicalvapor deposition [1]. However, the most common methodto synthesize ZnO nanostructures utilizes a vapor transport

front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.08.006

ng author. Tel.: +844 8680787; fax: +84 4 8692963.

sses: hieunv-itims@mail.hut.edu.vn,

@yahoo.com (N. Van Hieu).

process based on a so-called vapor–liquid–solid (VLS)mechanism of anisotropic crystal growth. A large numberof studies have been made on the synthesis of Q1D ZnOnanostructures by direct decomposition of ZnO powder ata high temperature of 1400 1C [3,4] or carbon thermalreduction of ZnO powder at temperatures in the range880–1100 1C [5–7]. Recently, the synthesis of ZnO nanos-tructures in low-temperature range (lower than 600 1C) hasreceived much attention because it is more compatible withmicroelectronics processing. Thus, various methods havebeen developed to synthesize ZnO nanowires at tempera-tures as low as 400–600 1C [8,9]. It can be recognized thatthe diameter of Q1D ZnO nanostructures is small andcomparable to the Debye length, and chemisorption-induced surface states effectively affect the electronicstructure of the entire wires. Thus, Q1D ZnO nanostruc-tures are expected to be superior to its thin film counterpartwith regard to gas-sensing properties [2]. However,experimental data and applications of the low-tempera-ture-growth methods for chemical sensors have not beenfully explored. In this paper, we present the successfulsynthesis of different Q1D ZnO nanostructures on Si

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substrate using a simple physical vapor-deposition methodat low-temperature range of 550–600 1C. The growthmechanism of the Q1D ZnO nanostructures is discussed.The ethanol-sensing properties of these nanostructureshave been studied. A good performance of Q1D ZnOnanostructures-based sensors has been obtained. Thisresult has important implication for the development ofQ1D semiconductor metal oxide nanostructures-based gassensors.

2. Experimental

2.1. Synthesis and characterizations

Fig. 1 shows a schematic diagram of the experimentalapparatus used in this study. It consisted of a horizontalalumina tube furnace (25 cm long and 5 cm in diameter), aquartz tube (3 cm in diameter and 80 cm long) for reaction,a rotary pump, and a gas supply and mass-flow-controlsystem. Both ends of the reaction tube were sealed withrubber O-ring. The carrier gas line (Ar) was connected onthe left end of the quartz tube, O2 gas line was connected toboth ends of the quartz tube. The rotary pump wasconnected with the right end of the quartz tube where awater bath was also connected to maintain the system atthe atmospheric pressure. The ultimate vacuum of this set-up is about 10�2 Torr. With indicated configuration,different synthesis processes of ZnO nanostructures canbe carried out such as process using the two-step control ofinflowing gas and process using a mixture of gas carrierand O2 as previously reported elsewhere [10–12]. Other-wise, a newly possible synthesis process, in which an O2

flow is injected into the reaction tube in the oppositedirection of the carrier gas flow, is being studied in ourlaboratory.

In this work, the synthesis was carried out as follows.The source material was put on a quartz board andpositioned at the center of the furnace tube. The substratesof 1 cm� 1 cm were prepared from p-type Si(1 0 0) waferand cleaned ultrasonically in ethanol and subsequently inDI water. They were placed downstream of the carrier gas

Fig. 1. Schematic diagram of experimental appa

flow. As a catalyst material, Au thin film with the thicknessof 8 nm was deposited on Si substrate using RF magnetronsputtering. The pure zinc (BDH, England, 99.9%), Ar(99.999%) and O2 (99.999%) were used. Three synthesisprocesses were conducted at temperatures in the range550–600 1C, using a temperature ramping rate 5 1C/min:(1) two-step evaporation process, (2) atmospheric evapora-tion process, and (3) O2-assisted evaporation process. Themorphology and size distribution were characterized usinga field emission scanning electron microscope (FE-SEM;4800 Hitachi, Japan). The crystalline structure of thesamples was analyzed by X-ray diffraction (XRD) usingBruker-AXS D5005.

2.2. Gas sensor fabrication and testing

The interdigitated electrode was fabricated using con-ventional photolithography method (see Fig. 2a) with afinger width of 100 mm and a gap size of 70 mm. Theinterdigitated electrode fingers were made by sputtering10 nm Ti and 200 nm Pt on a layer of silicon dioxide (SiO2)with thickness of about 100 nm thermally grown on top ofthe silicon wafer. A shadow mask was used to deposit athin (about 8 nm) Au catalytic layer on the interdigitatedelectrode area (see Fig. 2b). This Au layer forms micro-droplets after heat treatment at 500 1C as indicated inFig. 2c. Therefore, the Au layer does not influence on theelectrical resistance of the gas sensor.To perform a gas-sensing test, the gas sensor was placed

on a hot plate and held by two tungsten needles forelectrical connection, then loaded in a glass chamber asshown in Fig. 3. The electrical resistance response duringtesting was monitored by a precision semiconductorparameter analyzer HP4156A. The resistance responses ofthe sensor in air ambient and under ethanol exposurepulses were monitored. The gas-sensing characteristics ofthe fabricated sensors were measured at operating tem-perature 300 1C. The sensor response (S ¼ Ra/Rg) isdefined as the ratio of the sensor resistance in air (Ra)and in ethanol gas (Rg). Normally, the ratio DR/DC is usedto evaluate the sensitivity of the gas sensors, where DR is

ratus used to grow 1D ZnO nanostructures.

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Fig. 2. (a) Pt interdigitated electrode, (b) Pt interdigitated electrode with Au layer deposition on the top, (c) Au catalyst layer after heat treatment at

500 1C.

Fig. 3. Gas sensor testing system.

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Fig. 5. FE-SEM image of nanotetrapods synthesized by the atmospheric

evaporation process and the inset shows the high magnification of

nanotetrapods.

Fig. 4. FE-SEM image of ZnO nanostructures mixture synthesized by the

two-steps evaporation process.

N. Van Hieu, N. Duc Chien / Physica B 403 (2008) 50–56 53

the change in the relative resistance, Ra/Rg, and DC is thechange in the concentration of the detected gas. Thedefinition of sensitivity as DR/DC indicates that a higherDR at a given increase in gas concentration, DC, yields ahigher sensitivity. It can be noted that the sensor response(Ra/Rg) determines the value of DR . Therefore, in thisstudy, the sensitivity of the gas sensors was compared usingthe sensor response.

3. Results and discussion

3.1. ZnO nanostructures synthesized from different

processes

3.1.1. The two-step evaporation process

In the two-step evaporation process, the synthesis wascarried as follows. First, the quartz tube was evacuated to10�2 Torr. Then it was purged with argon (Ar) gas forseveral times and subsequently maintained at the atmo-spheric pressure with Ar ambient. Second, the furnace tubewas heated up to 600 1C under an Ar-flow of 100 sccm.Third, the set-point temperature (600 1C) was maintainedfor 1 h. Fourth, an O2-flow of 50 sccm was then introducedthrough the quartz tube for 30min. At the end of theprocess, one could observe that the Si substrate surfacebecomes white. The morphology of ZnO nanostructureswas studied by FE-SEM as shown in Fig. 4. It can be seenthat a mixture of different morphologies such as nano-needles, nanobelts, nanorods and nanowires was grown onthe Si substrate. Thus, by using this process, we cannotobtain a uniform ZnO nanostructure. The mechanismformed the mixture of ZnO nanostructures might be verycomplicated that we cannot understand completely. How-ever, we can speculate as follows. At the beginning, underthe Ar-flow, Zn vapor transports to the substrate and itcannot form any ZnO nanostructures if there is noinjection of O2 source [12]. When O2 flow is injected tothe reaction tube, oxidation reaction that occurs during gastransport may result in different morphologies. This can becategorized into the catalyst-free vapor–solid (VS) process.It has been reported that VS process is usually capable ofproducing a variety of ZnO nanostructures [2].

3.1.2. The atmospheric evaporation process

In this process, the quartz tube was heated up to 600 1Cunder Ar-flow of 100 sccm without previously evacuatingthe reaction tube. During this step, the right end of the tubewas opened for air diffusion into the tube. The set-pointtemperature was maintained for 1 h to continuouslygrow ZnO nanostructures. In this case, the Zn sourceis physically vaporized in atmospheric pressure under theAr flow. After the reaction, the surface of the Sisubstrate appears white. The FE-SEM images of as-deposited products are shown in Fig. 5. It can be seenthat the nanotetrapods were obtained in high yield on theAu-coated Si substrate. As shown in the inset in Fig. 5, notonly separated tetrapods but also tip-connected tetrapods

were obtained. The diameter of the legs and the length ofthe nanotetrapods are 20–60 and 200–300 nm, respectively.It can be speculated that the most possible growth steps for

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25 30 35 40 45 50 55 60

0

20

40

60

80

100

120

140

(101)

Zn

Au

(10

2)

ZnO

(002)

Ln

(Cp

s)

2θ

(100)

Fig. 7. XRD pattern of ZnO nanowires grown by the O2-assisted

evaporation process at temperature 550 1C.

N. Van Hieu, N. Duc Chien / Physica B 403 (2008) 50–5654

the nanotetrapods formed by this process are as follows.Zn vapor and oxygen are transported to the downstream ofthe source reactants by controlling carrier gas. A part ofthe Zn previously deposited on the substrate reacts withoxygen diffused from the air to form small ZnO particles.These small particles can play the role of seeds forfurther growth of the nanotetrapods. Because the ZnOnucleus crystal is of trigonal pyramid shape with thewurtzite structure, the four legs of nanotetrapods canthus grow further out from the seed via a vapor solidmechanism [13,14].

3.1.3. The O2-assisted evaporation process

This evaporation process was carried out quite similar tothe two-step evaporation process. The furnace tube washeated up to 550 1C (reaction temperature) under anAr-flow of 100 sccm. When the temperature reached500 1C, an O2-flow of 0.5 sccm was introduced to thereaction tube during heating up to the set-point tempera-ture (550 1C). Afterwards, the set-point temperature(550 1C) was maintained for 1 h. The FE-SEM verificationof as-deposited product is shown in Fig. 6. Very highdensity and homogenous ZnO nanowires were obtained onthe substrate. However, there was only a relatively smallarea where the nanowires were formed, and it was smallerthan the area of the gas-sensor electrode. So this may notbe good enough for the gas sensor fabrication. Thediameter of the nanowires was in a range 15–25 nm andtheir lengths were estimated about 0.5–1.0 mm (see the insetin Fig. 6). The observed nanowires can be regarded ashaving been synthesized using a traditional VLS methodthat many researchers have reported [1]. At the furnacetemperatures in the range 420–o500 1C, Zn vapor carriedby Ar-flow cannot deposit on the substrate. When thefurnace is heated up to 500 1C, Au-nanoparticle dropletsare formed and Zn vapor is dissolved in the droplets andforms eutectic alloy droplets. Under the O2-flow, the

Fig. 6. FE-SEM image of nanowires grown at 550 1C under 100 sccm Ar

gas with 0.5% O2 and the inset image shows the high magnification of the

nanowires.

growth of the solid ZnO nanowires occurs due to thesupersaturation of the liquid droplets [2].

3.2. X-ray diffraction of ZnO nanostructures grown at

different temperatures

The crystallinity of as-grown ZnO nanowires grown at550 1C and mixture of ZnO nanostructures grown at 600 1Cwere characterized using XRD and the results are shown inFigs. 7 and 8, respectively. The diffraction pattern shown inFig. 7 indicated that the ZnO nanowires are crystalline.The diffraction patterns are indexed to a typical wurtzitestructure with unit cell constants of a ¼ 0.3248 nm andc ¼ 0.2206 nm, which are in good agreement with thereported data (JCPDS Card no. 36-1451: a ¼ 0.3248 nmand c ¼ 0.2206 nm). Peaks of Au and Zn were also

25 30 35 40 45 50 55 60

0

50

100

150

200

250

(101)

(102)

(100)

(002)

Ln (

Cps)

2θ

ZnO

Fig. 8. XRD pattern of the mixed ZnO nanostructures grown by the two-

step evaporation process at 660 1C.

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0 500 1000 1500 2000 2500 3000 35000

10k

20k

30k

40k

50k

60k

R [

Ω]

Time (s)

Operating Temp.: 300 °CEthanol : 500 ppm

0 200 400 600 800 1000 1200 14000.0

2.0M

4.0M

6.0M

8.0M

10.0M

12.0MOperating Temp.: 300°CEthanol: 500 ppm

R[Ω

]

Time (s)

Fig. 9. The response curve of sensor S05 (a) and sensor S1 (b) to 500 ppm

ethanol at 300 1C.

0 1000 2000 3000 4000

500.0k

1.0M

1.5M

2.0M

2.5M

3.0M

3.5M

4.0M

Operating Temp.: 300°CEthanol: 12.5 -500 ppm

500 ppm

75 ppm25 ppm

12.5 ppm

R[Ω

]

Time (s)

Fig. 10. Response curve of ZnO tetrapod-nanowires-based sensor

operating at 300 1C.

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detected. Au peak was due to the use of Au as catalyst. TheZn peak confirms that Zn nanowires were also formed dueto the deficit of oxygen in the synthesis process. However,as shown in Fig. 8, we did not observe peaks which wecould attribute to Zn phase. This implies that at thetemperature of 600 1C, there are no Zn nanostructuresgrown from the as-synthesized products. Therefore, thisprocess could be used for the gas sensor fabrication.

3.3. Gas-sensing characterizations

Pt interdigitated electrodes with 8 nm thick Au layer onthe top were used for the gas sensor fabrication. The two-step evaporation process was first used to fabricate thesensor. The two electrodes were positioned at distances 0.5and 1 cm from Zn source during the process and thefabricated gas sensors were named S05 and S1. The gassensor testing was performed at the operating temperatureof 300 1C and 500 ppm of ethanol for the two sensors.The response curves of sensors S05 and S1 are plotted inFig. 9a and b, respectively. It can be seen that the responseof the both sensors, S (S ¼ Rair/Rgas) are not muchdifferent. However, their absolute resistances are quitedifferent. This can be attributed to the difference inthe composition of nanowires, nanobelts and nanorodsbetween the two growth positions.

The second kind of gas sensors was fabricated by usingthe atmospheric evaporation process. As seen in theprevious section, the observed nanotetrapods are in highyield and the film is quite porous which may be a goodsensitive material for the gas sensors. The as-fabricated gassensor was tested with various ethanol concentrations atthe operating temperature of 300 1C and the result is shownin Fig. 10. The sensor response to 500 ppm ethanol wasfound out to be about 5.3. The response and recovery timeswere determined as the time to reach 90% of the steady-state signal when the sensor was taken from air to a samplegas and from a sample gas to air, respectively. The responseand the recovery times were found to be less than 25 s.Fig. 11 depicts the correlation between the ethanol gasconcentrations and the response of the nanotetrapods ZnOsensor. It shows that the sensor response to ethanolconcentration is quite good.

The ZnO nanowires-based sensors synthesized by theprocess described in Section 3.1.3 would be expected tohave much higher sensitivity compared to the other casesdue to small diameter of the as-synthesized ZnO nano-wires. However, our current experimental set-up was notyet optimized to obtain homogenous nanowires onsufficiently large area for the gas sensor fabrication.Additionally, the metal phase of Zn is still present in theas-synthesized nanowires as indicated by XRD character-ization. So, further investigation for optimization of thesynthesis process described in Section 3.1.3 is required forthe gas sensor fabrication.

Like most semiconductor metal oxide nanoparticles-based gas sensors, the sensing properties of 1D ZnO

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10 100 10001

2

3

4

5

Sensitiv

ity (

Ra / R

g)

Ethanol concentration [ppm]

Ethanol

Operating Temp.: 300°C

Fig. 11. Sensitivity versus ethanol concentration for ZnO tetrapod-based

sensor.

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nanostructures are attributed to oxygen molecules ad-sorbed on the surface of the ZnO nanostructures whichform O2

2� ions by capturing electrons from the conductanceband. So ZnO nanostructures show a high-resistance statein the air ambient. When ZnO nanowires-based sensor isexposed to a reductive gas at moderate temperature, thegas reacts with the surface oxygen species of the nanowires,which decreases the surface concentration of O2

2� ions andincreases the electron concentration. This eventuallyincreases the conductivity of the Q1D ZnO nanostructures.However, in the case of ZnO thin film, the charge statemodification takes place only at the grain boundary orporous surface. In the case of Q1D ZnO nanostructures, itis expected that the electronic transport properties of theentire Q1D ZnO nanostructures will change effectively dueto the gas adsorption. In the light of this, one can suggestthat they are promising materials for the sensors to detectother gases. Various catalytic materials coated on the ZnOnanostructures can improve the selectivity of the gassensor. This work is currently studied by our group aswell as by many others.

4. Conclusions

We have synthesized different Q1D ZnO nanostructuressuch as nanowires, nanobelt, nanorods and tetrapods. Inparticular, uniform ZnO nanowires of 15–25 nm indiameter were successfully synthesized on Au-coated Sisubstrates via a VLS growth mechanism using the O2-assisted thermal evaporation of Zn source at 550 1C. Such a

low-temperature growth of ZnO nanowires may provide apossibility to fabricate nanodevices at low temperatures.The Q1D ZnO nanostructures could not be synthesized

with Ar carrier gas only and the partial pressure of oxygenis crucial for the growth of Q1D ZnO nanostructures bythe Zn evaporation process.The gas sensors based on the mixture of nanorods,

nanowires, and nanobelts and tetrapods nanowires weresuccessfully fabricated. The sensing characteristics toethanol gas were carefully studied. High sensitivity andfast response time were found at the operating temperatureof 300 1C. These results demonstrate that ZnO Q1Dnanostructures can be used as the gas-sensing materialsfor developing of high-performance gas sensors.

Acknowledgments

This work has been supported by Grant no. 405006(2006) from the Basic Research Program of the Ministry ofScience and Technology (MOST), and it has been partlysupported by VLIR-HUT project, code AP05/Prj3/Nr03.The authors also acknowledge partial financial supportfrom Third Italian-Vietnamese Executive Programme ofCo-operation in S&T for 2006–2008 under project titleSynthesis and Processing of Nanomaterials for Sensing,Optoelectronics and Photonic Applications.

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