highly reproducible synthesis of very large-scale tin oxide nanowires

Contents lists available at ScienceDirect Sensors and Actuators B: Chemical Bì" 24 2⁄21 LSEVIER journal homepage: www.elsevier.com/locate/snb Highly reproducible synthesis of v

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Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

Bì" 24 2⁄21

LSEVIER

journal homepage: www.elsevier.com/locate/snb

Highly reproducible synthesis of very large-scale tin oxide nanowires used for screen-printed gas sensor

Nguyen Van Hieu*

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

Article history:

Available online xxx

A truly simple procedure was presented for highly reproducible synthesis of very large-scale SnO, nanowires (NWs) on silicon and alumina substrates The growth involves thermally evaporating SnO powder in a tube furnace with temperature, pressure, and Oz gas-flow controlled to 960°C, 0.5-5 Torr,

Keywords: and 0.4-0.6 sccm, respectively The scanning- and transmission-electron-microscopic studies show that Gas sensor the diameter and length of the nanowires vary from 50 to 150 nm and 1 to 10 jm, respectively

we sensor As-synthesized SnO2 NWs on alumina substrates were used to fabricate gas sensor by screen-printing

In oxide

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 C2) H;0H, CH3COCH3, CzHạ, CO, and H2 for studying gas-sensing properties The results reveal that as-fabricated sensors exhibit relatively reproducible and good response to ethanol gas Typically, the response to 100 ppm ethanol in air is around 11.8, and response and recovery times are around 4 and 30s, respectively

1 Introduction

In recent years, there have been extensive efforts in the syn-

thesis, characterization, and application of a new generation

of semiconductor metal oxides (SMOs) nanostructures such as

nanowires, nanorods, nanobelts, and nanotubes [1,2] These struc-

tures with a high aspect ratio (i.e., size confinement in two

coordinates) offer better crystallinity, higher integration density,

and lower power consumption [1] In addition, they demonstrate

superior sensitivity to surface chemical processes due to the large

surface-to-volume ratio and small diameter comparable to the

Debye length (a measure of the field penetration into the bulk)

[2,3] Although many different quasi-one-dimension (Q1D) nanos-

tructures of SMO such as SnQz, ZnO, In203, and TiO02 have been

investigated for gas-sensing applications, researchers have paid

greater attention to SnO» nanowires (NWs)-based sensors because

its counterparts such as a thick film, porous pellets, and thin films

are versatile in being able to sense a variety of gases [4] and are

commercially available

Presently, various synthesis methods have been reported for

producing SnOz NWs such as hydrothermal methods [5,6], thermal

decomposition of precursor powders Sn, SnO, and SnO> followed by

vapor-solid (VS) [7,8] or vapor-liquid-solid (VLS) growth [9-11]

Although 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

doi:10.1016/j.snb.2009.02.043

of SnOz NWs by the thermal decomposition using SnO as a source material, we found that it is rather difficult to synthesize the SnO2 NWs based on the previously reported procedures [1-—3,11-13] We also found experimentally that the nature of evaporation apparatus plays a very important role in the selection of the gown condi- tions such as temperature, pressure, flow-rate of carrier gas, and flow-rate of oxygen gas to successfully synthesize the SnO2 NWs Hence, the development of a simple and reproducible procedure

to synthesize SnO, NWSs is significantly meaning for gas-sensing application The fabrication of the SnO2 NWs-based gas sensors has been demonstrated by using various methods such as dielectrophoretic assembly to align on metal electrodes [12], making electrical contacts formed field effect transistor (FET) [13,14], dispersal of the NWs on prefabricated electrodes [5,15,16], deposited metal electrode on the top of the NWs [17,18], and directly grown the NWs on the electrodes [19] In summary, these techniques are used either expensive equipments such as electron-beam lithography, focus ion beam, sputtering system to fabricate the electrical contacts or a series of uncontrollable processes such as sonifica- tion and dispersal of NWs on prefabricated electrodes Due to the difficulties in synthesis and fabrication of the SnO2 NWs-based gas sensor, the practical application of the NWs sensor is still in question In this work, the thermal evaporation method was introduced to synthesize the SnO2 NWs A truly facile procedure cable

of highly producing a very large-scale of SnO2 NWs is presented As-obtained SnO» NWs on alumina substrates are used to fabricate gas sensor by screen-printing method, which is much simpler compared with previously reported methods Electrical properties and

Please cite this article in press as: N Van Hieu, Highly reproducible synthesis of very large-scale tin oxide nanowires used for screen- printed gas sensor, Sens Actuators B: Chem (2009), doi:10.1016/j.snb.2009.02.043

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O-ring

O O O

Up-stream Substrate

Load

im ey ln-let AE

Furnace

Quartz tube

Down-strem Substrate Out-let

san

Carrier

Rotary pump

Fig 1 Thermal evaporation apparatus

gas-sensing properties are characterized with randomly selected

devices

2 Experimental

2.1 Material synthesis and characterizations

The SnOz NWs were grown in a quartz tube located in a horizon-

tal furnace with a sharp temperature gradient (Lingdberg/Blue M,

Model: TF55030A, USA) Both ends of the quartz tube were sealed

with rubber O-rings The ultimate vacuum for this configuration

was ~5 x 1073 Torr The carrier gas-line (Ar) and O2 gas-line were

connected to the left-end of a quartz tube and their flow-rate was

modulated by a digital mass-flow-control system (Aalborg, Model:

GFC17S-VALD2-A0200, USA) The right end of the quartz tube was

connected to arotary pump through a needle valve in order to main-

tain a desired pressure in the tube High purity SnO powder (Merck,

99.9%) was placed in an alumina boat as an evaporation source

Substrates with a previously deposited Au catalyst layer (thickness:

~10nm) was placed approximately 2—3 cm from the source on both

sides from the source (up-stream and down-stream) as indicated in

Fig 1 The growth process was divided into two steps Initially, the

quartz tube was evacuated to 10-2 Torr and purged several times

with Ar gas (99.99%) Subsequently, the quartz tube was evacuated

to 10-2 Torr again and the furnace temperature was increased to

960°C for 25 min It should be noted that the Ar gas-flow did not

introduced during this step This is completely different from many

previous reports on synthesizing SnO2 NWs by thermal evapora-

tion After 2-4 min, the furnace temperature reached 960 °C, oxygen

gas was added to the quartz tube at a flow rate of 0.4-0.6 sccm, and

the process was maintained for 30 min during the growth of the

SnO› NWs During the Oz addition step, the pressure inside the

tube with controlling is in the range of 0.5-5 Torr by the needle

valve The as-synthesized SnO2z NWs were characterized by scan-

ning electron microscopy (FE-SEM, Hitachi $4800), transmission

electron microscopy (TEM, JEM-100CX), energy dispersive X-ray

analysis (EDX, HORIBA EX-420 attached to the FE-SEM), and X-ray

diffraction (XRD, Philips Xpert Pro) with Cu Ka radiation generated

at a voltage of 40 kV as a source Additionally, Nikon microscope

L200 attached with Olympus digital camera was used to observe

the large-scale of SnO2 NWs on the substrates

2.2 Gas-sensor fabrication and testing

Fig 2 shows a schematic diagram of gas sensor fabrication

A patterned Au catalyst layer was deposited on the Alz,O3 sub-

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

of 100 wm) Then this substrate was used to grown SnO2 NWs by previously indicated procedures Comb-shape Au electrodes were screen-printed on the top of the SnO» NWs grown on the alumina substrate with size of 5mm x 5mm, followed heat-treatment at 600°C for 5h We fabricated a quite large numbers of gas sensors by this technique However, randomly selected sensors were tested For gas sensor characterization, the flow-through technique was employed The sensor characteristics were measured

at a temperature of 400°C using horizontal tube furnace and at various ethanol gas concentrations (10, 50, and 100ppm) Oth- erwise, the sensors were also tested with other gases such as

100 ppm CH3COCH3, 100 ppm C3Hg, 100 ppm CO, and 100 ppm Hp The gas response (S=Rg/Rg) was measured at 400°C by compar- ing the resistance of the sensor in high-purity air (Rg) with that

in the target gases (Rg) Electrical characteristics (I-V curve) were

Shadow Au Sputtering

= = = = = = = = = = F&F # mas e 2, © © | ve © se v v Fe ve v

ss 8s © nh 8 s8 s & @ = = ©= = = = v s © + v

ss es & &§& & © & se cv He 2

mask

Au _ —————>›nhnininn

catalyst layer

AIlzOaSubstrate

=n(}zprGz- ——— = >} ILIHI

nanowires

AlaOaSubstrate

Au See

electrodes

by screen-

printing AlzO3Substrate

Fig 2 Gas-sensor fabrication process steps

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Up-stream

Down-stream 8 10 12 14 16

keV

Fig 3 Optical microscopes image of SnO; NWs on the Si and AlzOs substrates from the up-stream sample (a) and the down-stream sample (e); FE-SEM images of SnO, NWs

at different magnification on the samples from the up-stream (b and c) and the down-stream (f and g); TEM images of SnOz NWs on the substrates from the up-stream (d) and the down-stream (h); FE-SEM image of SnOz nanowires with Au catalyst cap on the substrates from up-stream (i) and down-stream (1); EDX spectrum measured at the catalyst cap of the up-stream sample (k) and the down-stream sample (m)

measured by using a Precision Semiconductor Parameter Analyzer

(HP4156A)

3 Results and discussion

3.1 Morphology and microstructure characterizations

Fig 3 shows the morphology of the as-synthesized SnO2 NWs

on Si and Al,O3 substrates that was characterized by optical micro-

scope, FE-SEM, and TEM Uniform SnO2 NWs with homogeneous

entanglement were produced on a very large area (1 cm x 10cm)

on the substrates placed at up-stream and down-stream from the

source, respectively, shown in Fig 3a and e by optical microscope

and Fig 3b and f by FE-SEM Fig 3c and g shows FE-SEM images of

the samples placed at the up-stream and down-stream at higher

magnification, respectively It can be seen that the morphologies

of as-synthesized SnO2 NWs on the both sides are very similar

The diameter of the SnO2 NWs ranged from 50 to 150nm (see

Fig 3d and h) and the lengths ranged from 50 to 150m All the

NWSs were smooth and uniform along the fiber axis Actually, we

have intensively investigated the NWs morphology obtained from

the both sides for various synthesis runs by FE-SEM and TEM, and

the results reveal that their morphology are not much different

We have also tried to synthesize the NW with the same synthesis

process by using three different evaporation apparatuses, and very

similar results were obtained (not shown) This suggests that the

synthesized process proposed in the present work is very simple

and highly reproducible In other words, a very large scale of SnO›

NWSs can be obtained

Fig 3iand 1 obtained from the up-stream and down-stream show

a SnO› NWs with a catalyst particle on its tip These catalyst par-

ticles are not easily found in the FE-SEM image, because the NWs

are too long The growth mechanism of SnOz NWs in the present

work could be explained on the basis of the vapor—liquid-solid

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

the first time [24] and many researchers lately [1,5,6,8,10] In our

experiment, EDX (see Fig 3k and m for up-stream and down-stream

samples) reveals that the catalyst particles are composed of Au, Sn

and O, which indicates Au particles also play an important role in

the growth of SnO2 NWS Briefly, the NWs growth mechanism in

our experiment can be described as follows Sn vapors, as coming from the SnO source after the decomposition in SnO> (solid) and

Sn (liquid), are naturally spread out by thermal diffusion over the both substrates placed at the up-stream and the down-stream and condensed again on the substrates forming Sn—Au alloyed droplets

by reacting with the Au particles At the same time, these alloyed droplets can provide the energetically favored sites for adsorption of

Sn vapor Subsequently, the oxygen-flow, which is introduced in the reaction chamber, reacts with the liquid Sn in the droplets to form SnO 2 The peak of Si from the EDX is attributed to the contamination come from the Si substrate Fig 4a and b shows the XRD patterns of the commercial SnO» powders and the as-synthesized SnO NWs and their magnified patterns, respectively The XRD pattern of the SnO»z powders is indexed to the tetragonal rutile structure, which agrees well with the reported data from JCPDS card (77-0450) The representative XRD pattern of the SnO» NWs is identical to that

of the SnOz powders, indicating that these NWs are indeed a pure rutile phase SnOp In addition, a careful comparison between the magnified XRD patterns in Fig 4b reveals that three XRD peaks for the SnO» NWs are relatively broadened and shifted to the lower diffraction angle, as compared with the SnO, powders These obser- vations may attribute to the small size effect and tensile stress of SnO› NWs [5,6]

The thermal evaporation procedure, which was used to synthesize the SnO2 NWs have shown some advantages in comparison with previous reports [5,8,10,13] In general, Ar gas-flow is used to transport the Sn vapor from the source to the substrate To obtain a large-scale of SnO2 NWs with high reproducibility, the Ar flow rate

is greatly needed to optimize ourselves that cannot be used from the literature data It should be noted that the optimized Ar flow rate is strongly effected by various factors of evaporation apparatus such as diameter of the reacted tubes, the temperature gradient of the furnace, the nature of the boat and substrate, the positions of the substrates and source, speed of rotary pump, directions of gas- lines in and out (vertical or horizontal), and source materials (Sn, SnO, powders or foils) Furthermore, with the system without using automatic reactive pressure control unit is difficult to control the pressure in the reacted tube Consequently, the oxygen flow is also needed to optimize correspond to the optimized Ar flow rate These matters indicate that it is rather difficult to reproducibly synthesize

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the SnO, NWs on large-scale by using Ar flow for transportation of

the Sn vapor Our synthesized method is very simple The carrier

gas was not used in the NWs growth so the transportation of Sn

vapor would take place only by flow caused by thermal diffusion

The oxygen flow rate (lower than 1 sccm) was used during grow-

ing the SnO2z NWs Hence, the pressure in the reacted tube is quite

easy to control We have found that this synthesized method can

be used to grow SnO» NWs with any thermal evaporation appara-

tus Recently, we have very successfully synthesized SnO2 NWs at

low temperature (~700°C) from Sn powder source by using this

method that will be published in another paper

3.2 Electrical and gas-sensing properties

The screen-printing method for gas sensor device fabrication

proposed in this work is very much simple and this method is more

efficient compared to that adopted by previous works Hence a large

number of sensors were obtained as shown in Fig 5a FE-SEM image

of the fabricated sensor at a higher magnification is shown in Fig 5b

The patterns of the SnO2 NWs growth are shown in Fig 5c Fig 5d

represents current-voltage (I-V) characteristics of the gas sensor in

air at different temperatures The (I-V) curve of the as-fabricated gas

sensor device shows a good ohmic behavior This points out that not

only metal-semiconductor junction between the Au contact layer

and SnOz NWs but also the semiconductor—semiconductor junc-

tion between the SnO2 NWs are ohmic The ohmic behavior is very

(a) J S Sno, nanowires

- a

SnO, powders

20 25 30 35 40 45 50 55 60

29

”n 5 oO "5 < Qa a

: 1 : 1 r

29

Fig 4 XRD patterns of SnOz powders and as-synthesized SnO2 NWs (a) and their

magnified pattern (b)

Screen-printed Au electrodes

| SnO; nanowires

300

180

120

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

important to the gas-sensing properties, because the sensitivity of the gas sensor is affected by contact resistance We have measured the I-V characteristics at temperature up to 400°C and found that there is no difference in the I-V curve Hence, it could point out that the combining of the synthesis and fabrication methods in the present works is a prospective platform for large-scale fabrication

of the gas sensor, which are relatively good reliability and capable

of working in real-world environments

The gas-sensor testing by using set-up at our laboratory, which can only measure with single device each time, is time-consuming with testing a relatively large number of the sensor Therefore, only randomly selective devices were tested Fig Ga shows the responses

of the SnOz NWS sensors under exposure to 10, 50, and 100 ppm of ethanol gas at 400°C It can be seen that the resistance of the sensors in dry air is relatively large variation This can be attributed to slightly difference in the NWs density and could be a disadvantage

of the sensor fabrication method However, the responses of the sensors are not much different as shown in Fig 6b The latter issue is much 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 ethanol gas with a small fluctuation The linear dependence of the response

to ethanol gas of Q1D SnOz nanostructures was already investigated

in previous reports [6,20] This could offer a suitable application of

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a

= 192 oe f 44 :

c CRE F > Sef H 7

@ 10° 3 33 A bil] ok WL

— 4 a

” 1"

c

141 |—4—$5 ch Cˆ C—nn

c | 8 1 ©H.OH Operating temp @ 40

0 20 40 60 80 100 520 2800M 0 280 560 840 1120 1400 1680 1960 2240 :

Ethanol Concentration (ppm)

Fig 6 Response characteristics of randomly tested sensors to various ethanol con-

centrations at a temperature of 400°C (a) and response as a function of ethanol

concentration (b)

the SnO2 NWs sensor for detecting ethanol gas The sensitivity of

our sensors to ethanol gas is comparable with the SnO» NWs-based

ethanol sensors fabricated by other methods [6,18] The sensitivity

and selectivity of our sensor can be greatly improved by function-

Time (s)

Fig 7 Transient response of randomly selected sensors (named as $1-S6) to various gases (C2H50H, CH3COOCH3, C3Hs, CO, and Hz) with concentration of 100 ppm

alizing with catalytic nanoparticles as reported in our previous and other works [21-23]

As-fabricated sensors were also tested with different gases such

as CH3COCH3, C3Hg, CO and Hp It can be seen that their response characteristics are very similar, and the results are shown in Fig 7

Table 1

The SnOz NWS sensor response comparison between this work and previous works

~10.8

Ra/Rg ~18.3

500 ppm, Ra/Re ~1.2, (Gg — Gq)/Ga ~0.6

100 ppm Gg/G, ~1.9

~0.5

~41

4 From this work

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Basic Research Project: 2009-2011 ), the National Key Research Pro- gram for Materials Technology (Project No KC 02-05/06-10), and the 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, in the online version, at doi:10.1016/j.snb.2009.02.043

References

[1] J.G Lu, P Chang, Z Fan, Quasi-one-dimensional metal oxide materials— synthesis, properties and applications, Mater Sci Eng R 52 (2006) 49-91 [2] E Comini, Metal oxide nano-crystals for gas sensing, Anal Chim Acta 568

48.0M - c„ngnc`

He)

40.0M -

f

32.0M - mal fi 5

9

)

T

T ` T : T : T ` T 7 T ——

0 80 160 240 320 400 480 560

Time (s)

Fig 8 The estimation of response-recovery time from transient response

for the selected sensors This is to suggest further that the sensor

fabrication method in the present work is quite reproducible Addi-

tionally, the responses to the measured gases of the sensors in the

present work were used to extensively compare with the previ-

ous works The responses (Ra/Rg) to Cp H5OH (100 ppm), CH3COCH3

(100 ppm), CO (100 ppm), and Hz (100 ppm) are round 11.8, 10.8,

2.9, and 3.4, respectively These obtained values are comparable

with most of the previous works (see Table 1 and Fig 7)

It can be also seen that there are various SnOz NWs-like sen-

sors showed a relatively higher response, but the SnOz-doped or

functionalized with catalytic materials have been used for the NWs

gas sensor For instance, the response to ethanol of our sensors can

be increased with about 6 times with functionalizing with La2O3

as reported [21] This suggests that the synthesis and fabrication

methods can be easily used to develop semiconductor oxides NWs

gas-sensor and the gas-sensor array for detection of multi-gases

application by functionalizing with different catalytic materials

The dynamic response transients were obtained for the SnOz

NWSs sensors The 90% response time for gas exposure (fg9(air-to-gas) )

and that for recovery (to9%gas-to-air)) Were calculated from the

resistance-time data shown in Fig 8 The tggyair-to-gas) Values in

the sensing of 10, 50, and 100 ppm C2H50OH ranged from 4 to 6s,

while the togxgas-to-air) Value ranged from 20 to 40s These results

are quite comparable with the NWs-based sensor of the most of the

literature [6,8,15,17,18,21 ]

4 Conclusion

We demonstrated that single-crystalline SnO2 NWs were suc-

cessfully prepared on silicon and alumina substrates through

simple thermal evaporation of SnO powder at 960°C under con-

trolling of pressure (0.5-5 Torr) and oxygen gas flow (0.4—0.6 sccm)

It was used to synthesize in different evaporation apparatuses

with very high reproducibility, and a very large-scale of the NWs

was obtained The as-synthesis NWs were used to fabricate gas

sensor by screen-printing method The fabrication process does

not involve any tedious and time-consuming steps such as photo

or electron-beam lithography As-fabricated SnO2 NWs sensors

exhibit relatively good performance to ethanol gas However, the

sensitivity and selectivity can be improved further by surface cat-

alytic doping or functionalizing or plasma treatment

Acknowledgments

The work has been supported by the National Foundation for

Science & Technology Development (NAFOSTED) of Vietnam (for

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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 PhD degree 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 to electronic devices, gas sensors and biosensors

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