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Sensors and Actuators B 144 (2010) 425–431

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

ighly reproducible synthesis of very large-scale tin oxide nanowires used forcreen-printed gas sensor

guyen Van Hieu ∗

nternational Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Viet Nam

r t i c l e i n f o

rticle history:vailable online 5 March 2009

eywords:as sensor

a b s t r a c t

A truly simple procedure was presented for highly reproducible synthesis of very large-scale SnO2

nanowires (NWs) on silicon and alumina substrates. The growth involves thermally evaporating SnOpowder in a tube furnace with temperature, pressure, and O2 gas-flow controlled to 960 ◦C, 0.5–5 Torr,and 0.4–0.6 sccm, respectively. The scanning- and transmission-electron-microscopic studies show that

anowires sensorin oxide

the diameter and length of the nanowires vary from 50 to 150 nm and 1 to 10 �m, respectively.As-synthesized SnO2 NWs on alumina substrates were used to fabricate gas sensor by screen-printing

method. A good ohmic contact of the screen-printed NWs sensor was obtained. Randomly selected gas-sensor devices were tested with various gases such as C2H5OH, CH3COCH3, C3H8, CO, and H2 for studyinggas-sensing properties. The results reveal that as-fabricated sensors exhibit relatively reproducible andgood response to ethanol gas. Typically, the response to 100 ppm ethanol in air is around 11.8, and response

ound

and recovery times are ar

. Introduction

In recent years, there have been extensive efforts in the syn-hesis, characterization, and application of a new generationf semiconductor metal oxides (SMOs) nanostructures such asanowires, nanorods, nanobelts, and nanotubes [1,2]. These struc-ures with a high aspect ratio (i.e., size confinement in twooordinates) offer better crystallinity, higher integration density,nd lower power consumption [1]. In addition, they demonstrateuperior sensitivity to surface chemical processes due to the largeurface-to-volume ratio and small diameter comparable to theebye length (a measure of the field penetration into the bulk)

2,3]. Although many different quasi-one-dimension (Q1D) nanos-ructures of SMO such as SnO2, ZnO, In2O3, and TiO2 have beennvestigated for gas-sensing applications, researchers have paidreater attention to SnO2 nanowires (NWs)-based sensors becausets counterparts such as a thick film, porous pellets, and thin filmsre versatile in being able to sense a variety of gases [4] and areommercially available.

Presently, various synthesis methods have been reported for

roducing SnO2 NWs such as hydrothermal methods [5,6], thermalecomposition of precursor powders Sn, SnO, and SnO2 followed byapor–solid (VS) [7,8] or vapor–liquid–solid (VLS) growth [9–11].lthough there were a large number of reports on the synthesis

∗ Tel.: +84 4 8680787; fax: +84 4 8692963.E-mail addresses: hieu@itims.edu.vn, hieunv-itims@mail.hut.edu.vn.

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.02.043

4 and 30 s, respectively.© 2009 Elsevier B.V. All rights reserved.

of SnO2 NWs by the thermal decomposition using SnO as a sourcematerial, we found that it is rather difficult to synthesize the SnO2NWs based on the previously reported procedures [1–3,11–13]. Wealso found experimentally that the nature of evaporation apparatusplays a very important role in the selection of the gown condi-tions such as temperature, pressure, flow-rate of carrier gas, andflow-rate of oxygen gas to successfully synthesize the SnO2 NWs.Hence, the development of a simple and reproducible procedureto synthesize SnO2 NWs is significantly meaning for gas-sensingapplication. The fabrication of the SnO2 NWs-based gas sensorshas been demonstrated by using various methods such as dielec-trophoretic assembly to align on metal electrodes [12], makingelectrical contacts formed field effect transistor (FET) [13,14], dis-persal of the NWs on prefabricated electrodes [5,15,16], depositedmetal electrode on the top of the NWs [17,18], and directly grownthe NWs on the electrodes [19]. In summary, these techniques areused either expensive equipments such as electron-beam lithogra-phy, focus ion beam, sputtering system to fabricate the electricalcontacts or a series of uncontrollable processes such as sonifica-tion and dispersal of NWs on prefabricated electrodes. Due to thedifficulties in synthesis and fabrication of the SnO2 NWs-basedgas sensor, the practical application of the NWs sensor is still inquestion. In this work, the thermal evaporation method was intro-

duced to synthesize the SnO2 NWs. A truly facile procedure cableof highly producing a very large-scale of SnO2 NWs is presented.As-obtained SnO2 NWs on alumina substrates are used to fabricategas sensor by screen-printing method, which is much simpler com-pared with previously reported methods. Electrical properties and

426 N. Van Hieu / Sensors and Actuators B 144 (2010) 425–431

vapor

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erwise, the sensors were also tested with other gases such as100 ppm CH3COCH3, 100 ppm C3H8, 100 ppm CO, and 100 ppm H2.The gas response (S = Ra/Rg) was measured at 400 ◦C by compar-ing the resistance of the sensor in high-purity air (Ra) with thatin the target gases (Rg). Electrical characteristics (I–V curve) were

Fig. 1. Thermal e

as-sensing properties are characterized with randomly selectedevices.

. Experimental

.1. Material synthesis and characterizations

The SnO2 NWs were grown in a quartz tube located in a horizon-al furnace with a sharp temperature gradient (Lingdberg/Blue M,

odel: TF55030A, USA). Both ends of the quartz tube were sealedith rubber O-rings. The ultimate vacuum for this configurationas ∼5 × 10−3 Torr. The carrier gas-line (Ar) and O2 gas-line were

onnected to the left-end of a quartz tube and their flow-rate wasodulated by a digital mass-flow-control system (Aalborg, Model:FC17S-VALD2-A0200, USA). The right end of the quartz tube wasonnected to a rotary pump through a needle valve in order to main-ain a desired pressure in the tube. High purity SnO powder (Merck,9.9%) was placed in an alumina boat as an evaporation source.ubstrates with a previously deposited Au catalyst layer (thickness:10 nm) was placed approximately 2–3 cm from the source on both

ides from the source (up-stream and down-stream) as indicated inig. 1. The growth process was divided into two steps. Initially, theuartz tube was evacuated to 10−2 Torr and purged several timesith Ar gas (99.99%). Subsequently, the quartz tube was evacuated

o 10−2 Torr again and the furnace temperature was increased to60 ◦C for 25 min. It should be noted that the Ar gas-flow did not

ntroduced during this step. This is completely different from manyrevious reports on synthesizing SnO2 NWs by thermal evapora-ion. After 2–4 min, the furnace temperature reached 960 ◦C, oxygenas was added to the quartz tube at a flow rate of 0.4–0.6 sccm, andhe process was maintained for 30 min during the growth of thenO2 NWs. During the O2 addition step, the pressure inside theube with controlling is in the range of 0.5–5 Torr by the needlealve. The as-synthesized SnO2 NWs were characterized by scan-ing electron microscopy (FE-SEM, Hitachi S4800), transmissionlectron microscopy (TEM, JEM-100CX), energy dispersive X-raynalysis (EDX, HORIBA EX-420 attached to the FE-SEM), and X-rayiffraction (XRD, Philips Xpert Pro) with Cu K� radiation generatedt a voltage of 40 kV as a source. Additionally, Nikon microscope200 attached with Olympus digital camera was used to observehe large-scale of SnO2 NWs on the substrates.

.2. Gas-sensor fabrication and testing

Fig. 2 shows a schematic diagram of gas sensor fabrication.patterned Au catalyst layer was deposited on the Al2O3 sub-

trate by ion sputtering through a shadow mask (with mesh size

ation apparatus.

of 100 �m). Then this substrate was used to grown SnO2 NWs bypreviously indicated procedures. Comb-shape Au electrodes werescreen-printed on the top of the SnO2 NWs grown on the aluminasubstrate with size of 5 mm × 5 mm, followed heat-treatment at600 ◦C for 5 h. We fabricated a quite large numbers of gas sen-sors by this technique. However, randomly selected sensors weretested. For gas sensor characterization, the flow-through tech-nique was employed. The sensor characteristics were measuredat a temperature of 400 ◦C using horizontal tube furnace and atvarious ethanol gas concentrations (10, 50, and 100 ppm). Oth-

Fig. 2. Gas-sensor fabrication process steps.

N. Van Hieu / Sensors and Actuators B 144 (2010) 425–431 427

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ig. 3. Optical microscopes image of SnO2 NWs on the Si and Al2O3 substrates fromt different magnification on the samples from the up-stream (b and c) and the downd the down-stream (h); FE-SEM image of SnO2 nanowires with Au catalyst cap onatalyst cap of the up-stream sample (k) and the down-stream sample (m).

easured by using a Precision Semiconductor Parameter AnalyzerHP4156A).

. Results and discussion

.1. Morphology and microstructure characterizations

Fig. 3 shows the morphology of the as-synthesized SnO2 NWsn Si and Al2O3 substrates that was characterized by optical micro-cope, FE-SEM, and TEM. Uniform SnO2 NWs with homogeneousntanglement were produced on a very large area (1 cm × 10 cm)n the substrates placed at up-stream and down-stream from theource, respectively, shown in Fig. 3a and e by optical microscopend Fig. 3b and f by FE-SEM. Fig. 3c and g shows FE-SEM images ofhe samples placed at the up-stream and down-stream at higher

agnification, respectively. It can be seen that the morphologiesf as-synthesized SnO2 NWs on the both sides are very similar.he diameter of the SnO2 NWs ranged from 50 to 150 nm (seeig. 3d and h) and the lengths ranged from 50 to 150 �m. All theWs were smooth and uniform along the fiber axis. Actually, weave intensively investigated the NWs morphology obtained fromhe both sides for various synthesis runs by FE-SEM and TEM, andhe results reveal that their morphology are not much different.

e have also tried to synthesize the NW with the same synthesisrocess by using three different evaporation apparatuses, and veryimilar results were obtained (not shown). This suggests that theynthesized process proposed in the present work is very simplend highly reproducible. In other words, a very large scale of SnO2Ws can be obtained.

Fig. 3i and l obtained from the up-stream and down-stream showSnO2 NWs with a catalyst particle on its tip. These catalyst par-

icles are not easily found in the FE-SEM image, because the NWsre too long. The growth mechanism of SnO2 NWs in the presentork could be explained on the basis of the vapor–liquid–solid

VLS) mechanism that has been reported by Wagner and Ellis for

he first time [24] and many researchers lately [1,5,6,8,10]. In ourxperiment, EDX (see Fig. 3k and m for up-stream and down-streamamples) reveals that the catalyst particles are composed of Au, Snnd O, which indicates Au particles also play an important role inhe growth of SnO2 NWs. Briefly, the NWs growth mechanism in

-stream sample (a) and the down-stream sample (e); FE-SEM images of SnO2 NWseam (f and g); TEM images of SnO2 NWs on the substrates from the up-stream (d)ubstrates from up-stream (i) and down-stream (l); EDX spectrum measured at the

our experiment can be described as follows. Sn vapors, as comingfrom the SnO source after the decomposition in SnO2 (solid) andSn (liquid), are naturally spread out by thermal diffusion over theboth substrates placed at the up-stream and the down-stream andcondensed again on the substrates forming Sn–Au alloyed dropletsby reacting with the Au particles. At the same time, these alloyeddroplets can provide the energetically favored sites for adsorption ofSn vapor. Subsequently, the oxygen-flow, which is introduced in thereaction chamber, reacts with the liquid Sn in the droplets to formSnO2. The peak of Si from the EDX is attributed to the contaminationcome from the Si substrate. Fig. 4a and b shows the XRD patterns ofthe commercial SnO2 powders and the as-synthesized SnO2 NWsand their magnified patterns, respectively. The XRD pattern of theSnO2 powders is indexed to the tetragonal rutile structure, whichagrees well with the reported data from JCPDS card (77-0450). Therepresentative XRD pattern of the SnO2 NWs is identical to thatof the SnO2 powders, indicating that these NWs are indeed a purerutile phase SnO2. In addition, a careful comparison between themagnified XRD patterns in Fig. 4b reveals that three XRD peaksfor the SnO2 NWs are relatively broadened and shifted to the lowerdiffraction angle, as compared with the SnO2 powders. These obser-vations may attribute to the small size effect and tensile stress ofSnO2 NWs [5,6].

The thermal evaporation procedure, which was used to synthe-size the SnO2 NWs have shown some advantages in comparisonwith previous reports [5,8,10,13]. In general, Ar gas-flow is used totransport the Sn vapor from the source to the substrate. To obtain alarge-scale of SnO2 NWs with high reproducibility, the Ar flow rateis greatly needed to optimize ourselves that cannot be used fromthe literature data. It should be noted that the optimized Ar flowrate is strongly effected by various factors of evaporation apparatussuch as diameter of the reacted tubes, the temperature gradient ofthe furnace, the nature of the boat and substrate, the positions ofthe substrates and source, speed of rotary pump, directions of gas-lines in and out (vertical or horizontal), and source materials (Sn,

SnO2 powders or foils). Furthermore, with the system without usingautomatic reactive pressure control unit is difficult to control thepressure in the reacted tube. Consequently, the oxygen flow is alsoneeded to optimize correspond to the optimized Ar flow rate. Thesematters indicate that it is rather difficult to reproducibly synthesize

4 Actuators B 144 (2010) 425–431

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28 N. Van Hieu / Sensors and

he SnO2 NWs on large-scale by using Ar flow for transportation ofhe Sn vapor. Our synthesized method is very simple. The carrieras was not used in the NWs growth so the transportation of Snapor would take place only by flow caused by thermal diffusion.he oxygen flow rate (lower than 1 sccm) was used during grow-

ng the SnO2 NWs. Hence, the pressure in the reacted tube is quiteasy to control. We have found that this synthesized method cane used to grow SnO2 NWs with any thermal evaporation appara-us. Recently, we have very successfully synthesized SnO2 NWs atow temperature (∼700 ◦C) from Sn powder source by using this

ethod that will be published in another paper.

.2. Electrical and gas-sensing properties

The screen-printing method for gas sensor device fabricationroposed in this work is very much simple and this method is morefficient compared to that adopted by previous works. Hence a largeumber of sensors were obtained as shown in Fig. 5a. FE-SEM imagef the fabricated sensor at a higher magnification is shown in Fig. 5b.he patterns of the SnO2 NWs growth are shown in Fig. 5c. Fig. 5depresents current–voltage (I–V) characteristics of the gas sensor inir at different temperatures. The (I–V) curve of the as-fabricated gas

ensor device shows a good ohmic behavior. This points out that notnly metal–semiconductor junction between the Au contact layernd SnO2 NWs but also the semiconductor–semiconductor junc-ion between the SnO2 NWs are ohmic. The ohmic behavior is very

ig. 4. XRD patterns of SnO2 powders and as-synthesized SnO2 NWs (a) and theiragnified pattern (b).

Fig. 5. As-fabricated gas sensors imaged by optical microscope (a); FE-SEM of thesensor at higher magnification (b and c); I–V characteristic of the sensors at differenttemperatures (d).

important to the gas-sensing properties, because the sensitivity ofthe gas sensor is affected by contact resistance. We have measuredthe I–V characteristics at temperature up to 400 ◦C and found thatthere is no difference in the I–V curve. Hence, it could point outthat the combining of the synthesis and fabrication methods in thepresent works is a prospective platform for large-scale fabricationof the gas sensor, which are relatively good reliability and capableof working in real-world environments.

The gas-sensor testing by using set-up at our laboratory, whichcan only measure with single device each time, is time-consumingwith testing a relatively large number of the sensor. Therefore, onlyrandomly selective devices were tested. Fig. 6a shows the responsesof the SnO2 NWs sensors under exposure to 10, 50, and 100 ppm ofethanol gas at 400 ◦C. It can be seen that the resistance of the sen-sors in dry air is relatively large variation. This can be attributed toslightly difference in the NWs density and could be a disadvantageof the sensor fabrication method. However, the responses of thesensors are not much different as shown in Fig. 6b. The latter issue ismuch more important for practical application than the former one.As also shown in Fig. 6b, the responses of all the measured sensors

are increased linearly with increasing of concentration of ethanolgas with a small fluctuation. The linear dependence of the responseto ethanol gas of Q1D SnO2 nanostructures was already investigatedin previous reports [6,20]. This could offer a suitable application of

N. Van Hieu / Sensors and Actuators B 144 (2010) 425–431 429

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ig. 6. Response characteristics of randomly tested sensors to various ethanol con-entrations at a temperature of 400 ◦C (a) and response as a function of ethanoloncentration (b).

he SnO2 NWs sensor for detecting ethanol gas. The sensitivity ofur sensors to ethanol gas is comparable with the SnO2 NWs-basedthanol sensors fabricated by other methods [6,18]. The sensitivitynd selectivity of our sensor can be greatly improved by function-lizing with catalytic nanoparticles as reported in our previous and

able 1he SnO2 NWs sensor response comparison between this work and previous works.

ensor description Measured gases

C2H5OH CH3COCH3

creen-printed SnO2 NWs sensor 100 ppm 100 ppm Ra

∼10.8nO2 nanorods sensor 100 ppm Ra/Rg∼10, 10.8b-doped SnO2 NWs 100 ppm Ra/Rg ∼2.2n-doped SnO2 NWs 200 ppm Ra/Rg ∼40nO2 nanobelts sensors 10 ppm Ra/Rg ∼25, 50 ppm

Ra/Rg ∼18.320 ppm Ra/

ingle SnO2 NWs sensor

d-doped SnO2 NWs sensor 50 ppm Ra/Rg ∼15etwork SnO2 NWs sensor

ercolating effect SnO2 NWs sensor 50 ppm (Ra − Rg)/Ra ∼3.2

a From this work.

Fig. 7. Transient response of randomly selected sensors (named as S1–S6) to variousgases (C2H5OH, CH3COOCH3, C3H8, CO, and H2) with concentration of 100 ppm.

other works [21–23].

As-fabricated sensors were also tested with different gases such

as CH3COCH3, C3H8, CO and H2. It can be seen that their responsecharacteristics are very similar, and the results are shown in Fig. 7.for the selected sensors. This is to suggest further that the sensor

Reference

CO H2

/Rg 100 ppm Ra/Rg ∼2.9 100 ppm Ra/Rg ∼3.4 a

[6,27][5][25]

Rg ∼2 [18,26]

100 ppm Ra/Rg ∼2,500 ppm, Ra/Rg ∼1.2,100 ppm Gg/Ga ∼1.9

20,000 ppm (in N2)(Gg − Ga)/Ga ∼0.6

[22,27–29]

50 ppm Ra/Rg ∼10 50 ppm Ra/Rg ∼33 [30]100 ppm (Gg − Ga)/Ga

∼0.5[31]

10 ppm (Ra − Rg)/Ra

∼4.1[32]

430 N. Van Hieu / Sensors and Actuat

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Fig. 8. The estimation of response–recovery time from transient response.

abrication method in the present work is quite reproducible. Addi-ionally, the responses to the measured gases of the sensors in theresent work were used to extensively compare with the previ-us works. The responses (Ra/Rg) to C2H5OH (100 ppm), CH3COCH3100 ppm), CO (100 ppm), and H2 (100 ppm) are round 11.8, 10.8,.9, and 3.4, respectively. These obtained values are comparableith most of the previous works (see Table 1 and Fig. 7).

It can be also seen that there are various SnO2 NWs-like sen-ors showed a relatively higher response, but the SnO2-doped orunctionalized with catalytic materials have been used for the NWsas sensor. For instance, the response to ethanol of our sensors cane increased with about 6 times with functionalizing with La2O3s reported [21]. This suggests that the synthesis and fabricationethods can be easily used to develop semiconductor oxides NWs

as-sensor and the gas-sensor array for detection of multi-gasespplication by functionalizing with different catalytic materials.

The dynamic response transients were obtained for the SnO2Ws sensors. The 90% response time for gas exposure (t90%(air-to-gas))nd that for recovery (t90%(gas-to-air)) were calculated from theesistance–time data shown in Fig. 8. The t90%(air-to-gas) values inhe sensing of 10, 50, and 100 ppm C2H5OH ranged from 4 to 6 s,hile the t90%(gas-to-air) value ranged from 20 to 40 s. These results

re quite comparable with the NWs-based sensor of the most of theiterature [6,8,15,17,18,21].

. Conclusion

We demonstrated that single-crystalline SnO2 NWs were suc-essfully prepared on silicon and alumina substrates throughimple thermal evaporation of SnO powder at 960 ◦C under con-rolling of pressure (0.5–5 Torr) and oxygen gas flow (0.4–0.6 sccm).t was used to synthesize in different evaporation apparatuses

ith very high reproducibility, and a very large-scale of the NWsas obtained. The as-synthesis NWs were used to fabricate gas

ensor by screen-printing method. The fabrication process doesot involve any tedious and time-consuming steps such as photor electron-beam lithography. As-fabricated SnO2 NWs sensorsxhibit relatively good performance to ethanol gas. However, theensitivity and selectivity can be improved further by surface cat-lytic doping or functionalizing or plasma treatment.

cknowledgments

The work has been supported by the National Foundation forcience & Technology Development (NAFOSTED) of Vietnam (forasic Research Project: 2009–2011), the National Key Research Pro-

[

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ors B 144 (2010) 425–431

gram for Materials Technology (Project No. KC 02-05/06-10), andthe research project of Vietnam Ministry of Education and Training(Code B2008-01-217).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.snb.2009.02.043.

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Biography

Nguyen Van Hieu received his MSc degree from the International Training Institute

for Material Science (ITIMS), Hanoi University of Technology (HUT) in 1997 and PhDdegree from the department of electrical engineering, University of Twente, Nether-lands in 2004. Since 2004, he has been a research lecturer at the ITIMS. In 2007,he worked as a post-doctoral fellow, Korea University. His current research inter-ests include nanomaterials, nanofabrications, characterizations and applications toelectronic devices, gas sensors and biosensors.