The Electronic Nose: The Effect of Metal Oxide Modifications to Well-Aligned ZnO Nanowire Arrays for

Highly Sensitive and Selective Gas Detection

Haiqiao Su, Jiajun Chen, Kai Wang, Alaoddin Ahmad Ayyad and Weilie Zhou Advanced Materials Research Institute (AMRI), University of New Orleans, New Orleans, LA 70148

wzhou@uno.edu

Abstract: To mimic biological olfactory receptor arrays, a three dimensional (3D) “electronic nose” was fabricated by using well-aligned ZnO nanowire arrays modified with different metal oxides. Metal oxide nanowire arrays share 3D structures similar to mammalian olfactory receptor arrays, with thousands of vertically aligned nanowires providing a high reception area which can significantly enhance the sensors’ sensitivity. Meanwhile, with different material decorations, each array of nanowires can produce a distinguishable response for each separate analyte, which would provide a promising way to improve the selectivity. We have successfully fabricated gas sensors based on SnO2, In2O3, and WO3 coatings which have discriminated five gases which are NO2, H2S, H2, NH3, and CO. The sensitivity to NO2 and H2S has reached down to the ppb level.

Key words: Electronic nose, ZnO nanowire, Metal oxide, Gas sensor arrays.

Introduction Highly sensitive and selective chemical agent detectors have important applications in the areas of environmental emission control, public security, automotive applications, workplace hazard monitoring, medical diagnosis, and so on [1-2]. Among most of the detection technologies, including conductive polymer sensors, metal oxide conductometric sensors, and microcantilever sensors, metal oxide conductometric sensors comprise a significant part of the gas sensor component market. The reason is that metal oxides possess a broad range of electronic, chemical, and physical properties that are often highly sensitive to changes in their chemical environment. As nanotechnology has developed, metal oxide nanostructures have attracted much attention for applications in highly sensitive and selective gas sensors due to their high aspect ratio, large surface area, and availability of versatile structures as compared to their thin film counterparts [3-4]. For example, in gas sensors formed with individual In2O3 nanoparticles, a nanowire or nanowire networks can detect NO2 down to the ppb level at room-temperature [5-6].

For the purpose of mimicking the biological olfactory system with a diversity of receptor

arrays, vertically aligned 3D nanowire arrays have great potential for sensor applications, due to their extremely large surface area which provides a much stronger capability of capturing target molecules in the atmosphere [7-8]. To realize multiple receptor arrays, different metal oxides were coated onto well-aligned ZnO nanowire arrays, which can be readily grown by hydrothermal synthesis [8-9]. The electrical and gas-sensing properties of individual nanowire arrays were characterized. The electronic nose was fabricated with ZnO nanowire arrays decorated with the different metal oxides (SnO2, In2O3, and WO3) and the gas discrimination of this electronic nose was studied in detail.

Experimental The fabrication of vertically aligned semiconductor core-shell nanowire arrays generally starts with well-aligned ZnO nanowires grown by hydrothermal synthesis [10]. Fig.1 shows the schematic of the fabrication procedures for 3D sensor devices. The substrate was cleaned by a standard process, followed by the well aligned growth of ZnO nanowire arrays by hydrothermal synthesis [8]. Common thin film techniques, such as sputtering, PLD, or CVD, can be used to deposit the shell layers [8]. In this experiment, 50 nm SnO2, In2O3, and WO3 were sputtered in

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IMCS 2012 – The 14th International Meeting on Chemical Sensors 126

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IMCS 2012 – The 14th International Meeting on Chemical Sensors 127

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eferences G. Shen , Pand chemicananowires, Jdoi: 10.1039/

C. Wang, L.Metal oxideinfluencing 2106(2010);

A. Kolmakovand catalysinanostructureResearchdoi:10.1146/a

B. Cao, J.Cmonoclinic sensitive NO2323-2327(2

D. Zhang, Z. Lei, C. Zhou,using individdevices, Nadoi: 10.1021/

K. Yao, D. O’Connor, Wdetection atassembled In113, 14812 1

K. Albert, N.Stitzel, T. Vasensor arra(2000); doi: 1

J. Chen, K. ZnO nanorometal nanopselective gaNanotechnol10.1109/TNA

L. Vayssierenanowires ofMater. 15, 46200390108

S. Xu, Y. Wvertically alinorganic sucatalyst, J. 14959(2008)

S. Jha, R. Yfor SAW se2011 IEEE2010.570577

P. Chen , K, al sensing appJ. Mater. Chem/B816543B

. Yin, L. Zhae gas sens

factors, sdoi:10.3390/s

v, M. Moskovis by one-dies, Annual

34, annurev. mat

hen , X. TangWO3 nanow

O2 detection, 009), doi: 10.

Liu, C. Li, T. T Detection of dual and mu

ano Letters 4/nl0489283

Caruntu, ZW. Zhou, Part

room tempen2O3 nanopart4817 (2009);

. Lewis, C. Said, D. Walt, Cys, Chem. R10.1021/cr980

Wang, W. Zd arrays coaparticles for s detection, ogy 10, 9

ANO.2010.209

s, Growth of f ZnO from a64–466 (2003

Wei, Z. Wangigned ZnO bstrates at loAm. Chem.

, doi: 10.1021

Yadava, Patteensor array b, 1-4(2010), 74

Ryu, C. Zhouplications of mm., 19, 828-83

ang, D. Xiangsors: sensit

sensors, 10s100302088

vits, Chemicamensional mReview of 151-180

tsci.34.040203

g, W. Zhou, wire array f

J. Mater. C.1039/B81664

Tang, X. Liu, NO2 down to ultiple In2O3 4, 1919–192

Z. Zeng, J. ts per billion-erture basedticles, J. Physdoi: 10.1021/

Schauer, G. SCross-reactiveRev. 100, 20102w

Zhou, Verticalated with S

highly sensIEEE Transa

968-974 (2091423

arrayed nanoaqueous solut3); doi: 10.10

g, Patterned nanowire a

ow temperatuSoc., 130,

/ja806952j

ern recognitionbased electro

doi: 10.11

u, Devices metal oxide 39 (2009);

, R. Gao, ivity and

0, 2088-

al sensing metal-oxide

Materials (2004);

3.112141

Growth of for highly Chem., 19, 46C

S. Han, B. ppb levels nanowire

4 (2004);

Chen, C. -level H2S on self-

s. Chem. C /jp905189f

Sotzing, S. e chemical 2595–2626

ly aligned SnO2/noble sitive and actions on 11); doi:

orods and ions, Adv. 02/ adma.

growth of arrays on re without 14958–

n methods onic nose, 09/ICCIC.

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