a selective nh3 gas sensor based on fe2o3–zno nanocomposites at room temperature

A selective NH 3 gas sensor based on Fe 2 O 3 –ZnO nanocompositesat room temperature State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 16

A selective NH 3 gas sensor based on Fe 2 O 3 –ZnO nanocomposites

at room temperature

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China

Received 16 April 2005; received in revised form 25 August 2005; accepted 26 August 2005

Available online 19 October 2005

Abstract

Gas sensors based on the Fe2O3–ZnO nanocomposites with different compositions of Fe:Zn was prepared by a sol–gel and spin-coating method Morphology of the Fe2O3–ZnO nanocomposites was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD, D/max-rA) and energy dispersive X-ray analysis (EDX) The results of electrical and sensing measurement indicated that the sensor with Fe:Zn = 2% exhibited fairly excellent sensitivity and selectivity to NH3at room temperature The response and recovery time of the sensor were both less than

20 s Finally, the mechanism for the improvement in the gas sensing properties was discussed

© 2005 Elsevier B.V All rights reserved

Keywords: ZnO nanoparticles; Fe2 O 3 ; Gas sensor; NH 3

1 Introduction

Since many decades, the world awareness about

environmen-tal problems and human safety is increasing with the

techno-logical development Therefore, sensors are required for many

applications Recently, the need to detect low ammonia

concen-trations has greatly increased in many fields of technological

importance, such as food technology, chemical engineering,

medical diagnosis, environmental protection, monitoring of car

interiors and industrial processes

Seiyama et al proposed the gas sensors based on ZnO thin

films for the first time[1] ZnO is sensitive to many gases of

inter-est, such as trimethylamine (TMA)[2–4], H2[5], oxygen[6–8],

H2O[9,10], ethanol[11]and NH3[12], etc It also has a rapid

response with a possibility of miniaturization However, it has

some drawbacks, such as high working temperature, normally

between 400 and 500◦C, poor gas selectivity and relatively low

gas sensitivity[13]

To overcome these disadvantages, considerable research and

development are underway There are various techniques to

modify the sensing properties of the gas sensors One critical

approach is to modify the metal oxide surface by using noble

∗Corresponding author Tel.: +86 571 8795 1667; fax: +86 571 8795 2322.

E-mail address: mseyang@zju.edu.cn (D Yang).

metals (Au, Pt or Pd)[14,15]or rare earth metals (La, Y and Ce)[16,17] ZnO(n)/CuO(p) heterocontact configuration also showed some possibility of improving the selectivity[18] Nanto

et al have reported that a sensor based on a ZnO thin film doped with Al, In or Ga could detect the ammonia gas whose concentra-tion was as low as 1 ppm[12] But the working temperature was

as high as 350◦C Recently, Ivanovskaya et al suggested that a

sensor based on␣-Fe2O3/In2O3nanocomposites exhibited high sensitivity to NO2[19]

The present work was undertaken to investigate the gas sens-ing behavior of ZnO nanoparticle thin films doped with␣-Fe2O3

nanoparticles prepared by a sol–gel and spin-coating method Morphological, structural and sensing properties at room tem-perature were studied The ultimate objective of this study is

to improve the gas selectivity and sensitivity of the nano-sized ZnO-based sensors at room temperature

2 Experimental

ZnO nanoparticles doped with␣-Fe2O3nanoparticles were prepared in a similar manner to the literature procedure[20] The␣-Fe2O3nanoparticles were synthesized by a

ultrasonically dispersed into methanol (200 ml) at about 60◦C.

solution Then, a 0.03 M solution of KOH (65 ml) in methanol

0925-4005/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.snb.2005.08.010

was added dropwise The reaction mixture was stirred for 2 h.

The resulting solution was concentrated by the evaporation of

the solvent The resulting white product was centrifugalized,

washed with deionized water and ethanol to remove the ions

possibly remaining in the final product The ZnO nanoparticles

doped with␣-Fe2O3nanoparticles with different compositions

(molar ratio) of Fe:Zn being 0, 1, 2, 3 and 4% were prepared

Finally, five ZnO nanoparticle solutions were obtained, labeled

as sample 0, 1, 2, 3 and 4, respectively The obtained samples

were characterized by transmission electron microscopy (TEM),

which was performed with a JEM 200 CX microscope operated

at 160 kV For X-ray diffraction (XRD) (D/max-rA) and energy

dispersive X-ray analysis (EDX) (Phoenix) measurement,

pow-der samples were used, which were prepared by annealing the

final precipitates at 200◦C for 3 h.

Interdigitated Au electrodes were obtained by metal

deposi-tion on glass substrates The final precipitate was redispersible in

ethanol The Fe2O3–ZnO nanocomposite films were fabricated

on the top of the Au electrodes by spin-coating, followed by

annealing at 200◦C for 3 h before electrical and sensing

mea-surement Gas sensing behavior of the ZnO nanosensors was

measured at room temperature by using the Keithley 236 Source

Measure Unit The chamber was purged with N2until a steady baseline of the sensor resistance was reached Then, the test vapor was injected at a fixed concentration of 0.4 ppm in N2

In general, the dc voltage was fixed at 10 V, and the changes of current with time were recorded The sensor response is given

here as the current ratio Ig/Ia, where Igand Ia are the current across the sensor in the test gas and in air, respectively[22,23] The response-recovery time of the sensor is defined as the time needed to reach 90% of the original resistance

3 Results

3.1 Morphology of Fe 2 O 3 –ZnO nanocomposites

Fig 1shows TEM images of the Fe2O3–ZnO nanocompos-ites with different compositions of Fe:Zn Morphology of the pure␣-Fe2O3prepared by a hydrothermal method was nanopar-ticles (Fig 1a) Size of the nanoparticles was about 10 nm When 1% Fe2O3(molar ratio) nanoparticles (sample 1) were doped into the ZnO nanoparticles, the morphology of the result-ing sample was similar to that of the pure␣-Fe2O3 (Fig 1b)

It is obvious that the prepared nanoparticles were crystalline

Fig 1 TEM images of the ZnO–Fe 2 O 3 nanocomposites with different compositions (molar ratios) of Fe:Zn: (a) pure ␣-Fe 2 O 3 nanoparticles; (b) sample 1, inset is the selected area electron diffraction (SAED) pattern of the particles; (c) sample 2; (d) sample 3; (e) sample 4, inset is the SAED pattern of the nanorods.

Fig 2 X-ray diffraction patterns of the ZnO–Fe 2 O 3 nanocomposites with

differ-ent compositions of Fe:Zn All the diffraction peaks could be indexed according

to hexagonal structure ZnO.

from the inset ofFig 1b The morphology of sample 2 was still

nanoparticles and the size was about 10 nm (Fig 1c) However,

a few short nanorods and nanoparticles co-exited in sample 3

(Fig 1d) Almost only nanorods exited in sample 4 (Fig 1e)

These nanorods were still crystalline (inset of Fig 1e) The

growth mechanism of the nanorods was put forward based on the

above results It may be the fact that the added Fe2O3

nanopar-ticles promoted the growth of ZnO nanoparnanopar-ticles as the seeds

Then, those particles aggregated along one direction and formed

the ZnO nanorods

3.2 Structural and compositional characterization

Fig 2shows the X-ray diffraction patterns of the Fe2O3–ZnO

nanocomposites with different compositions of Fe:Zn All the

reflections in the XRD patterns can be indexed to the

hexag-onal structure ZnO with lattice constants of a = 0.3250 nm and

c = 0.5207 nm (JCPDS, 79–2205) No Fe2O3phase was detected

according to the XRD patterns It can be concluded that the

crys-tallization is relatively poor when the composition of Fe is 2%

Fig 3shows the image of EDX and composition of sample 4

Al and Si elements came from the substrates (glass) Fe, O and

Zn elements belonged to the sample It was calculated that the

composition results were almost consistent with the molar ratio

of ZnO and Fe2O3from the composition table Therefore, the

nanorods in sample 4 (Fig 1e) belonged to ZnO nanorods

3.3 Selectivity of gas sensors based on Fe 2 O 3 –ZnO

nanocomposites

Fig 4shows the selectivity of the gas sensor based on pure

ZnO nanoparticle The sensor was exposed to TMA, ethanol,

0.4 ppm at room temperature From the plots, it can be deduced

that the selectivity of the ZnO nanoparticles is poor For

compari-son, the current response of the gas sensors based on Fe2O3–ZnO

nanocomposites with different compositions of Fe:Zn as a

function of time was measured The same testing conditions

were applied for both pure ZnO nanoparticles and Fe2O3–ZnO

nanocomposites Selectivity of the gas sensors based on 2%

(Fig 5a) and 4% Fe O –ZnO nanocomposites (Fig 5b) at room

Fig 3 EDX and composition of sample 4.

temperature is shown inFig 5 It is clear that the sensitivity of both the sensors exposed to NH3is fairly high (about 10,000), whereas that to the other gases is much lower The sensitivity and selectivity are better than that of the gas sensor given in Ref

[12] These results indicate the fairly good NH3selectivity of the Fe2O3–ZnO nanocomposite thin films

3.4 Gas response and recovery characterization

Fig 6shows the gas response of the ZnO-based gas sen-sors including a pure nano-crystalline ZnO film and the four

Fe2O3–ZnO nanocomposite thin films with different composi-tions of Fe:Zn = 1, 2, 3 and 4%, when exposed to 0.4 ppm NH3

Fig 4 Selectivity of the gas sensor based on pure ZnO nanoparticle.

Fig 5 Selectivity of the gas sensors based on: (a) 2% and (b) 4% Fe 2 O 3 –ZnO

nanocomposites.

Table 1 summarizes the sensor signal (response magnitude),

response time and recovery time of the ZnO-based sensors with

different compositions of Fe:Zn The gas sensitivity of the

ZnO-based sensors increased dramatically from 100 to 10,000 as the

Fe2O3nanoparticle content increasesd initially from 0 to 2%

However, the gas sensitivity decreased dramatically when the

Fe2O3nanoparticle content increased to 3 and 4% The response

and recovery time of the sensor with Fe:Zn = 1% were more than

100 and 30 s, respectively But the response and recovery time of

the sensor with Fe:Zn = 2% were both less than 20 s According

to the gas sensitivity, response time and recovery time, it can be

concluded that Fe:Zn = 2% is the best composition

Fig 6 Response of gas sensors based on the Fe 2 O 3 –ZnO nanocomposites with

different compositions of Fe:Zn exposed to NH gas.

Table 1

NH 3 sensing properties of the Fe 2 O 3 –ZnO nanocomposite gas sensors with different compositions of Fe:Zn

Sample Sensor signal

(S = Ig/Ia )

Response time (s) Recovery time (s)

Fig 7 Reproducibility of the gas sensor based on 2% Fe 2 O 3 –ZnO nanoparticle.

3.5 Reproducibility of gas sensor based on 2%

Fe 2 O 3 –ZnO nanoparticles

Fig 7shows the reproducibility of the sensor based on 2%

Fe2O3–ZnO nanoparticles when exposed to 0.4 ppm NH3 for three times at room temperature It is clear that the response and recovery characteristics are almost reproducible and rather quick when exposed to NH3and also when exposed again to N2

4 Discussion

Based on the above results, the reason for the enhanced NH3

sensitivity and selectivity of the Fe2O3–ZnO nanocomposite thin film gas sensors was put forward Ivanovskaya et al.[19,24]have suggested that ethanol detection is a multi-step process involv-ing both reductive–oxidative and acid–base interactions with a sensor based on heterojunction oxide structures The reactivity

of oxides in acid–base reactions depends on the electronegativ-ity of the metal cation The electronegativelectronegativ-ity is the measure of the Lewis acid site activity The relative measure of the oxide activity in oxidation reactions can be the oxygen-oxide surface bonding energy In fact, the less the energy of oxygen atom iso-lation from the oxide surface is, the higher the oxide oxidizing ability is The reactivity of oxides in acid–base reactions depends

on the electronegativity of cations Mn+:

charge The Pauling electronegativities of Fe–O and Zn–O are

1.83 and 1.65 in Pauling units[25] So, the adsorption of the

detected gas molecules to be detected at Lewis sites would be

increased when the ZnO nanoparticles was doped with Fe2O3

nanoparticles

The oxides, which are characterized by the possibility of

metal ion reduction without oxide phase state modification,

have the greatest ability to promote oxidizing process Fe2O3

is inclined to facilitate the changing of the metal ion oxidizing

state: Fe(III)↔ Fe(II), while the oxide phase remains original

[24] That is, the complete oxidation of intermediates is going

effectively at the center of Fe2O3 nanoparticles Due to the

above-mentioned two reasons, the gas sensitivity of Fe2O3–ZnO

nanocomposites was improved

Meanwhile, there are two reasons for the fact that the

elec-tron donating ability of NH3is higher than those of ethanol and

methanol One is due to the higher electronegativity of the

oxy-gen atom than that of the nitrooxy-gen atom The other is that there is

a lone electron pair in NH3 What is more important is that

low-temperature ammonia oxidation was observed over iron oxide

[26,27] That is, the addition of Fe2O3promoted the interaction

between the gas sensor and NH3 Therefore, the NH3selectivity

was improved

However, according to the gas sensitivity, response time and

recovery time, it can be concluded that the composition of

Fe:Zn = 2% is the best composition This may be mainly due to

the morphology and the poor crystallization of the Fe2O3–ZnO

nanocomposites It is well known that the sensing mechanism

of semiconducting oxide gas sensors is based on the surface

ratio of nanoparticles (when the composition of Fe:Zn is 1 or

2%) is higher than that of nanorods (when the composition of

Fe:Zn is 3 or 4%) From the results of XRD, the crystallization

of the Fe2O3–ZnO nanoparticles was relatively poor,

indicat-ing that the defects were increased In general, there are many

oxygen vacancies in the nano-sized ZnO[29,30] So, more gas

molecules are easy to be adsorbed on the active centers[22],

which result in an increase of the sensitivity of Fe2O3–ZnO

nanoparticles Meanwhile, addition of a more amount of Fe2O3

nanoparticles may cover the active centers of ZnO In summary,

Fe2O3 nanoparticles can enhance the gas sensing properties

of the ZnO nanosensors, and the composition of Fe:Zn = 2%

is the best for the Fe2O3–ZnO nanocomposite thin film gas

sensors

5 Conclusion

Gas sensors based on Fe2O3–ZnO nanocomposites have been

prepared with different compositions of Fe:Zn The sensor with

Fe:Zn = 2% exhibited fairly excellent sensitivity and selectivity

to NH3at room temperature The response and recovery time of

the sensor were about 20 s The reproducibility of the ZnO gas

sensor with Fe:Zn = 2% was good So, the sensor could be used

for many times The increased sensitivity and selectivity to NH3

may largely be attributed to the addition of Fe2O3nanoparticles,

oxide surface and accelerate the oxidizing process

Acknowledgments

The authors would like to appreciate the financial supports

of the Natural Science Foundation of China (60225010) and

863 Project No (2004AA513024) We also thank Prof Youwen Wang for TEM and EDX measurement

References

[1] T Seiyama, A Kato, K Fjishi, M Nagatani, A new detector for gaseous components using semiconductive thin films, Anal Chem 34 (1962) 1502–1503.

[2] M Egashira, Y Shimizu, Y Takao, Trimethylamine sensor based on semiconductive metal oxides for detection of fish freshness, Sens Actu-ators B 1 (1990) 108–112.

[3] H Nanto, H Sokooshi, T Kawai, Aluminum-doped ZnO thin film gas sensor capable of detecting freshness of sea foods, Sens Actuators B 13–14 (1993) 715–717.

[4] H Nanto, H Solooshi, T Kawai, T Usuda, Zinc oxide thin-film trimethylamine sensor with high sensitivity and excellent selectivity, J Mater Sci Lett 11 (1992) 235–237.

[5] S Basu, A Dutta, Modified heterojunction based on zinc oxide thin film for hydrogen gas-sensor application, Sens Actuators B 22 (1994) 83–87.

[6] G Sbervelieri, P Nelli, S Groppelli, F Quaranta, A Valentini, L Vasanelli, Oxygen gas sensing characteristics at ambient pressure of undoped and lithium-doped ZnO-sputtered thin films, Mater Sci Eng.

B 7 (1990) 63–68.

[7] U Lampe, J M¨uller, Thin-film oxygen sensors made of reactively sput-tered ZnO, Sens Actuators B 18 (1989) 269–284.

[8] F Chaabouni, M Abbab, B Rezug, Metrological characteristics of ZnO oxygen sensor at room temperature, Sens Actuators B 100 (2004) 200–204.

[9] E Traversa, A Bearzptti, A novel humidity-detection mechanism for ZnO dense pellets, Sens Actuators B 23 (1995) 181–187.

[10] A.M Edwin Suresh Raj, C Maria Magdalane, K.S nagaraja, Zinc(II) oxide–yttrium(III) oxide composite humidity sensor, Phys Status Solidi (a) 191 (2002) 230–234.

[11] Q Wan, Q.H Li, Y.J Chen, T.H Wang, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl Phys Lett 84 (2004) 3654–3656.

[12] H Nanto, T Minami, S Takta, Zinc-oxide thin-film ammonia gas sen-sors with high sensitivity and excellent selectivity, J Appl Phys 60 (1986) 482–484.

[13] J Xu, Y Shun, Q Pan, J Qin, Sensing characteristics of double layer film of ZnO, Sens Actuators B 66 (2000) 161–163.

[14] F Boccizzi, A Chiorino, S Tsubota, M Haruta, An IR study of CO-sensing mechanism on Au/ZnO, Sens Actuators B 24–25 (1995) 540–543.

[15] S.J Gentry, T.A Jones, The role of catalysis in solid-state gas sensors, Sens Actuators B 10 (1986) 141–163.

[16] B Bhooloka Rao, Zinc oxide ceramic semi-conductor gas sensor for ethanol vapour, Mater Chem Phys 64 (2000) 62–65.

[17] X Niu, W Du, W Du, K Jiang, Preparation and gas sensing properties

of Y 3+ , La 3+ , Ce 4+ -doped ZnO, Chin Rare Earths 24 (6) (2003) 44–47 [18] M Miyayama, K Hikita, G Uozumi, H Yanagida, A.C impedance analysis of gas-sensing property in CuO/ZnO heterocontacts, Sens Actu-ators B 24–25 (1995) 383–387.

[19] M Ivanovskaya, D Kotsikau, G Faglia, P Nelli, S Irkaev, Gas-sensitive properties of thin film heterojunction structures based on Fe 2 O 3 –In 2 O 3

nanocomposites, Sens Actuators B 93 (2003) 422–430.

[20] C Pacholski, A Kornowski, H Weller, Self-assembly of ZnO: from nanodots to nanorods, Angew Chem Int Ed 41 (2002) 1188–1191 [21] Z Zhang, B.Q Wei, P.M Ajayan, Self-assembled patterns of iron oxide nanoparticles by hydrothermal chemical-vapor deposition, Appl Phys Lett 79 (2001) 4207–4209.

[22] A Gurlo, M Ivanovskaya, N Bˆarsan, M Schweizer-Berberich, U.

Weimar, W G¨opel, A Di´eguez, Grain size control in nanocrystalline

In 2 O 3 semiconductor gas sensors, Sens Actuators B 44 (1997) 327–333.

[23] W Schmid, N Bˆarsan, U Weimer, Sensing of hydrocarbons with tin

oxide sensors: possible reaction path as revealed by consumption

mea-surements, Sens Actuators B 89 (2003) 232–236.

[24] M Ivanovskaya, D Kotsikau, G Faglia, P Nelli, Influence of chemical

composition and structural factors of Fe 2 O 3 /In 2 O 3 sensor on their

selec-tivity and sensiselec-tivity to ethanol, Sens Actuators B 96 (2003) 498–503.

[25] L Pauling, The Nature of the Chemical Bond, third ed., Cornell

Uni-versity Press, Ithaca, NY, 1960, p 93.

[26] O.N Sil’chenkova, V.A Matyshak, V.N Korchak, Intermediates

deter-mined by in situ IR-spectroscopy in low-temperature oxidation of

ammo-nia on Fe 2 O 3 , Kinet Catal 40 (1999) 526–531.

[27] O.N Sil’chenkova, V.N Korchak, V.A Matyshak, The mechanism of

low-temperature ammonia oxidation on metal oxides according to the

data of spectrokinetic measurements, Kinet Catal 43 (2002) 363–371.

[28] J Watson, The tin oxide gas sensor and its application, Sens Actuators

5 (1984) 29–42.

[29] T Nagase, T Ooie, Y Makita, M Nakatsuka, K Shinozaki, N Mizutani,

A novel method for the preparation of green photoluminescent undopde

zinc oxide film involving excimer laser irradiation of a sol–gel-derived

precursor, Jpn J Appl Phys 39 (2000) 713–715.

[30] K Vanheusden, W.L Warren, C.H Seager, D.R Tallant, J.A Voigt, B.E.

Gnade, Mechanisms behind green photoluminescence in ZnO phosphor

powders, J Appl Phys 79 (1996) 7983–7990.

Biographies

Huixiang Tang was born in 1978 Now, she is PhD candidate from State

Key Lab of Silicon Materials at Zhejiang University, China Her research

project is gas sensors based on the ZnO nanoparticles.

Mi Yan was born in 1965 He has been a professor of materials science at

Zhejiang University since 1998 He graduated from Department of Materials

Science and Engineering, Southeast University in 1980 Professor Yan has been working in the fields of functional materials and surface treatment His current research interests are magnetic materials and related functional materials.

Hui Zhang received his PhD degree from State Key Laboratory of Silicon

Materials at Zhejiang University in 2004 Now, he is a teacher at Zhejiang University His research interest is preparation and application of nano-sized compound semiconductor materials.

Shenzhong Li received his master degree from State Key Laboratory of

Silicon Materials at Zhejiang University in 2005 His research interest is preparation and application of nano-sized compound semiconductor materials.

Xingfa Ma, Professor, executive director in chief of the Department of

Adhe-sives and Coatings, vice director of the Department of Sealants and Rubber Composites of Shandong Research Institute of Non-metallic Materials, Com-mittee Member of Chinese Standard of Adhesives and Sealants Now, he is a PhD candidate of materials physics and chemistry of Zhejiang University His research interests include polymer coating, interface, surface modifications, polymer-based composites and organic sensitive materials for sensor.

Mang Wang graduated from Department of Chemical Engineering of

Zhe-jiang University in 1961 Now, he is a professor in materials physics and chemistry and polymer materials at Zhejiang University He is currently the director of Reprographic Science and Engineering Society of China and a member of Specialist Group of Polymer Materials Division.

Deren Yang was born in Yangzhou, China in 1964 He received his

bache-lor degree from Department of Material Science and Engineering at Zhejiang University, and in 1991 PhD degree in the State Key Laboratory of Silicon Materials at Zhejiang University Now, he is a Cheung Kong Professor, deputy director of the State Key Laboratory of Silicon Materials at Zhejiang Uni-versity His current research interests are semiconductor materials, including growth, process and defect engineering of Czochralski silicon used for ultra-large scale integrated circuits (ULSI); preparation and application of silicon nano-wires, nano-tubes and other one dimensional semiconductor materials.