devices and chemical sensing applications of metal oxide ...€¦ · devices and chemical sensing...

APPLICATION www.rsc.org/materials | Journal of Materials Chemistry

Devices and chemical sensing applications of metal oxide nanowires†

Guozhen Shen,* Po-Chiang Chen, Koungmin Ryu and Chongwu Zhou*

Received 22nd September 2008, Accepted 21st October 2008

First published as an Advance Article on the web 20th November 2008

DOI: 10.1039/b816543b

Metal oxide nanowires, with special physical properties, are ideal building blocks for a wide range of

nanoscale electronics, optoelectronics, and chemical sensing devices. This article will describe the state-

of-the-art research activities in metal oxide nanowire applications. This paper consists of three main

sections categorized by metal oxide nanowire synthesis, electronic and optoelectronic devices

applications, and chemical sensing applications. Finally, we will conclude this review with some

perspectives and outlook on the future developments in the metal oxide nanowire research area.

1. Introduction

Due to their special shapes, compositions, chemical and physical

properties, one-dimensional (1-D) metal oxide nanostructures

are the focus of current research efforts in nanotechnology since

they are the commonest minerals in the earth. 1-D metal oxide

nanostructures have now been widely used in many areas, such as

ceramics, catalysis, sensors, transparent conductive films, elec-

tro-optical and electro-chromic devices.1–5 Intensive studies have

been carried out on the synthesis of metal oxide nanowires as well

as the exploration of their novel properties. For example, 1-D

ZnO nanostructures with many different shapes, such as nano-

wires, nanobelts, nanotubes, nanorings, and nanosprings, have

been prepared using many synthesis methods. High-performance

chemical sensors have been fabricated on SnO2, ZnO, and In2O3

nanowires due to their large surface area to volume ratio.

Dr Guozhen Shen

Dr Guozhen Shen received his

Ph.D. degree in Chemistry from

University of Science and Tech-

nology of China in 2003. He

conducted his postdoctoral

research at Hanyang University,

Korea in 2004 and then joined

National Institute for Materials

Science, Japan as a visiting

researcher. Currently, he is

a research scientist in University

of Southern California. He is the

author or co-author of more

than 100 research articles and 5

book chapters. His most recent

research interests include the synthesis and characterization of one-

dimensional nanostructures and their device applications in elec-

tronics and optoelectronics.

Department of Electrical Engineering, University of Southern California,Los Angeles, CA 90089, USA. E-mail: [email protected]; [email protected]; [email protected]; Fax: +1 213 821 4208; Tel: +1 213 821 4208

† This paper is part of a Journal of Materials Chemistry theme issue onNanotubes and Nanowires. Guest editor: Z. L. Wang.

828 | J. Mater. Chem., 2009, 19, 828–839

This article will provide a comprehensive review of the state-

of-the-art research activities focused on devices and chemical

sensing applications of metal oxide nanowires, and can be

divided into three main sections. The first section briefly intro-

duces two synthesis strategies, which include top-down

approaches and bottom-up approaches, with the focus on

bottom-up approaches, for the synthesis of metal oxide nano-

wires. Next, some important electronic and optoelectronic

devices built on metal oxide nanowires are presented, which

include field-effect transistors (FETs), transparent electronics,

lasers and waveguide, nanogenerators, solar cells and photo-

catalysts, and field nanoemitters. In the third part, we will discuss

recent developments in the chemical sensing area of metal oxide

nanowires. The review will then conclude with some perspectives

and outlook on the future developments in the metal oxide

nanowire research area.

2. Synthesis of metal oxide nanowires

Till now, many methods have been developed to synthesize 1-D

metal oxide nanostructures. Basically, they can be described as

two different types: the ‘‘top-down’’ approaches and the

‘‘bottom-up’’ approaches. In this section, we will briefly discuss

Po-Chiang Chen

Po-Chiang Chen holds a B.S.

degree in Physics and a M.S. in

Optoelectronics. He is currently

working toward a Ph.D. degree

in Chemical Engineering and

Materials Science at the

University of Southern Cal-

ifornia. His research focus is on

the device applications based on

1-D nanomaterials, including

chemical sensors, transparent

electronics, and energy conver-

sion and storage devices.

This journal is ª The Royal Society of Chemistry 2009

http://www.rsc.org/materials

these two approaches developed to synthesize metal oxide

nanowires.

2.1. Top-down synthesis

‘‘Top-down’’ approaches usually utilize planar, lithographic

techniques to transfer a pre-designed pattern to a substrate which

can form complex high density structures in well-defined posi-

tions on substrates.6,7 For example, Im et al.6 synthesized ZnO

nanowires using a complicated nanoscale spacer lithography

method, which can be used to detect H2 and CO gases. Top-down

approaches have been widely used in the current microelectronic

industry. They can produce nanostructures with very uniform

shapes and electronic properties. However, as the microelec-

tronic industry advances towards ever smaller devices, top-down

approaches will soon reach their physical and economic limits,

which motivates global efforts to search for new strategies to

meet the expected demand for increased computational power as

well as for integrating low-cost and flexible computing in

unconventional environments in the future.

2.2. Bottom-up synthesis

The bottom-up approaches, in which functional electronic struc-

tures are assembled from chemically synthesized nanoscale

building blocks, represent flexible alternatives to conventional

top-down methods. They can go far beyond the limits of top-down

technology in terms of future physical and economic limits.8,9

Table 1 lists a host of 1-D metal oxide nanostructures grown from

bottom-up approaches using different techniques.10–103

To get 1-D metal oxide nanostructures using the bottom-up

approaches, one key concept is to break the growth symmetry of

materials. A straightforward method to break the symmetry for

1-D growth is the use of ‘‘hard’’ or ‘‘soft’’ templates, which may

include the edges of surface steps, carbon nanotubes, porous

membranes, surfactant, or microemulsions. This method is

conceptually very simple and has been widely used to prepare

a variety of metal oxide nanowires. Despite its simplicity, the

Koungmin Ryu

Koungmin Ryu holds a B.S

degree in Metallurgy and a M.S

degree in Materials Science and

Engineering. He is currently

a Ph.D. student at the Univer-

sity of Southern California. His

research interest covers carbon

nanotube synthesis and applica-

tions such as nanotube circuits,

chemical sensing, and OLED

fabricated by carbon nanotube

conductive films. He has pub-

lished 3 journal papers related to

carbon nanotube synthesis and

OLED fabrication.

Dr Chongwu Zhou

Dr Chongwu Zhou received his

Ph.D. in electrical engineering

from Yale University, and then

worked as a postdoctoral

research fellow at Stanford

University. He joined the faculty

at University of Southern Cal-

ifornia in September 2000. His

research group has been working

at the forefront of nanoscience

and nanotechnology, including

synthesis and applications of

carbon nanotubes and nano-

wires, biosensing, and nano-

therapy. He has won a number

of awards, including the NSF CAREER Award, the NASA TGiR

Award, the USC Junior Faculty Research Award, and the IEEE

Nanotechnology Early Career Award. He is currently an Associate

Editor for IEEE Transactions on Nanotechnology.


template-directed method is limited by the fact that the synthe-

sized nanowires are usually polycrystalline, which limits their

potential applications in many areas.

Another general strategy for the bottom-up synthesis is the use

of a ‘‘catalyst’’ to direct the 1-D growth. According to the phases

involved in the reaction, this approach can be defined as either

vapor-liquid-solid (VLS) growth,104 or solution-liquid-solid

growth (SLS).105 Fig. 1a illustrates the schematic of a typical VLS

process. During this process, a vapor phase reactant is solubi-

lized by a liquid catalyst particle to form solid wire-like struc-

tures. In this process, the catalyst is envisioned as a growth site

that defines the diameter of nanowires. According to the reaction

system, the VLS process can be divided into thermal evapora-

tion, chemical vapor deposition (CVD) method, metal-organic

chemical vapor deposition (MOCVD) method, laser-ablation

method and many others. Fig. 1b and c show SEM and HRTEM

images of In2O3 nanowires synthesized from the VLS process

using a laser-ablation method, which exhibit very good crystal-

linity and are of single crystal nature. The inset is a TEM image

of a typical In2O3 nanowire. The catalyst particle can be clearly

seen attached to the top of the nanowire, indicating the VLS

growth process. Compared with the VLS process, the SLS

process adopts a similar idea except that the reactant comes from

solution instead of the vapor phase.

Though the vapor-solid (VS) process is not as clearly under-

stood as the VLS process, it has already proved to be a very

important method to synthesize 1-D metal oxide nanowires.18,28

During VS growth, no catalyst is used and the nanowires are

directly grown on the solid particles. This simple method has

been widely used to synthesize a host of metal oxide nanowires.

For example, by heating the metal oxides in a tube furnace at

high templerature, Wang et al. synthesized nanobelts of ZnO,

SnO2, CdO, and Ga2O3.2 Fig. 2 shows several typical TEM

images of the VS grown ZnO nanobelts, which have rectangular

cross-sections, different with the nanowires with round cross-

sections. They usually have thickness of 10–30 nm and width-to-

thickness ratios of 5–10 nm, respectively.

J. Mater. Chem., 2009, 19, 828–839 | 829

Table 1 Summary of 1-D metal oxide nanostructures synthesized usingdifferent methods

Materials Morphology Growth method Ref.

ZnO Nanowires Vapor-solid method 1Vapor-liquid-solid method 10AAO template-assisted method 11Microemulsion method 12Template-free solution method 13

Nanobelts Vapor-solid method 2Vapor-liquid-solid method 14Hydrothermal method 15

Nanorods Template-free aqueous method 16Vapor-liquid-solid method 17Vapor-solid method 18Pulsed-laser ablation without catalyst 19

Nanotubes Vapor-solid method 20Vapor-liquid-solid method 21Solution method 22

SnO2 Nanorods Microemulsion method 23Hydrothermal method 24Solution method 25Vapor-liquid-solid method 26

Nnanowires Catalyst-assisted laser ablation method 27Vapor-solid method 28Vapor-liquid-solid method 29Solution method 3

Nanobelts Thermal oxidation 30Vapor-solid method 31Laser-ablation method 32

Nanotubes Templates hydrothermal method 33Aqueous solution method 34Microemulsion method 35

In2O3 Nanowires Catalyst-assisted laser-ablationmethod

4

Vapor-solid method 36Vapor-liquid-solid method 37AAO-templated solution method 38

Nanobelts Vapor-solid method 39Nanotubes Thermal evaporation method 40

Solvothermal method 41Ga2O3 Nanowires Thermal evaporation method 42

Catalyst-assisted arc discharge method 43Laser-ablation method 44Catalyst-assisted vapor method 45

Nanobelts Vapor-solid method 46Vapor-liquid-solid method 47

Nanotubes Vapor-solid method 48WO3 Nanowires SBA-15 templated solution method 49

Thermal evaporation method 50Hydrothermal method 51

Nanobelts Vapor-solid method 52V2O5 Nanowires Hydrothermal method 53

Nanobelts Vapor-solid method 54MgO Nanowires Vapor-solid method 55

Vapor-liquid-solid method 56Catalyst-assisted laser-ablation

method57

nanotubes Vapor-liquid-solid method 58TiO2 Nanotubes AAO-templated solution method 59

Solution method 60Nanowires Hydrothermal method 61

Vapor-solid method 62ZrO2 Nanowires AAO-templated solution method 63

Nanorods Precursor thermal decompositionmethod

64

Nanotubes AAO-templated solution method 65Nb2O5 Nanobelts Precursor thermal decomposition

method66

nanowires Vapor-liquid-solid method 67Nanotubes Precursor thermal decomposition

method5

Ta2O5 Nanotubes Precursor thermal decompositionmethod

5

Table 1 (Contd. )

Materials Morphology Growth method Ref.

MoO3 Nanotubes Hydrothermal method 68Carbon nanotube templated method 69Vapor-solid method 70

Nanowires Thermal evaporation method 71Solution method 72

MnO2 Nanowires Hydrothermal method 73SBA-15 templated synthesis 74

Nanotubes Hydrothermal method 75Fe2O3 Nanowires Thermal oxidation method 76

Hydrothermal method 77Nanobelts Thermal oxidation method 78

Fe3O4 Nanotubes MgO-templated pulsed-laserdeposition

79

Nanowires Magnetic-field-induced hydrothermalmethod

80

Co3O4 Nanowires Thermal oxidation method 81Nanowires Hydrothermal method 82Nanotubes Carbon nanotube templated method 83Nanotubes Solution method 84

IrO2 Nanotubes Metal-organic CVD method 85Nanowires Metal-organic CVD method 86

NiO Nanowires Wet chemical route 87AAO-templated sol-gel method 88

Nanotubes AAO-assisted solution method 89Cu2O Nanowires Solid-state reduction method 90

Surfactant-assisted solution method 91Hydrothermal method 92

CuO Nanowires Thermal oxidation method 93AAO-templated deposition method 94Solution method 95

Nanobelts Solution method 95CdO Nanowires AAO-assisted electrochemical

deposition96

Chemical bath deposition method 97Nanoneedles Vapor-liquid-solid method 98

Al2O3 Nanotubes Pulse anodization method 99Thermal evaporation method 100Surfactant-assisted solution method 101Carbon nanotube-assisted growth 102

Nanowires,nanobelts

Vapor-solid method 103

Fig. 1 (a) Schematic illustrating the growth process of a VLS process.

(b) SEM image and (c) TEM image of In2O3 nanowires grown from VLS

process.

830 | J. Mater. Chem., 2009, 19, 828–839 This journal is ª The Royal Society of Chemistry 2009

Fig. 2 TEM images of ZnO nanobelts grown from the VS process. Reproduced from ref. 2: Science, 2001, 291, 1947. Copyright ª 2001, AAA of

Science.

After growth, the obtained nanowires were characterized using

several techniques, such as X-ray diffraction (XRD), scanning

electron microscopy (SEM), and transmission electron micros-

copy (TEM). Detailed description and analysis of these charac-

terization techniques can be found in some recent review papers

and will not be discussed here.106,107

3. Electronic and optoelectronic devices built onmetal oxide nanowires

Driven by the thrust of fabricating smaller devices to create

integrated circuits with improved performance, 1-D metal oxide

nanostructures have been exploited as potential building blocks

for future nanoelectronics. In this section, we will review some

recent works on the electronic and optoelectronic devices built on

metal oxide nanowires.

3.1. Field effect transistors

The basic field effect transistor (FET) structure fabricated from

a single metal oxide nanowire is illustrated in Fig. 3. Basically,

the FET is supported on an oxidized p-type silicon substrate with

the underlying conducting silicon as the back gate electrode to

vary the electrostatic potential of the nanowire. Two metal

contacts, corresponding to the source and drain electrodes, are

defined by either electron beam lithography or photolithography

followed by evaporation of suitable metal contacts. Usually,

current (I) vs. source–drain voltage (Vds) and current (I) vs. gate

voltage (Vg) are recorded to characterize the nanowire FET.

Metal oxides are n-type semiconducting materials. So for the

typical I–Vds curves recorded from metal oxide nanowire FETs,

an increase in conductance for Vg > 0 and a decrease in

conductance for Vg < 0 are obtained. n-Type FETs have been

Fig. 3 Schematic of an oxide nanowire FET.


fabricated on various oxide nanowires, including ZnO, In2O3,

SnO2, Cu2O, TiO2, CdO, etc.108–112

By introducing suitable dopants, FETs with p-type behavior

are obtained for several metal oxide nanowires. For example,

Wang fabricated p-type FETs using P-doped ZnO nanowires.113

Lee et al. obtained p-type FETs by using N-doped ZnO nano-

wires as the building blocks.114

3.2. Transparent electronics

Transparent electronics acting as an emerging technology for the

next generation of optoelectronic devices have attracted

numerous research efforts due to thier great potential to make

a significant commercial impact in many areas.115 Metal oxides

are well known transparent conductive semiconductor materials.

Using metal oxide nanowires as the building blocks, Janes et al.

fabricated fully transparent high-performance In2O3 and ZnO

nanowire-based FETs on both glass and flexible plastic

substrates.116 Fig. 4a is the cross-section view of fully transparent

and flexible device structure. All the components used are trans-

parent. Optical image, transmission spectrum, and I–V curves of

the device fabricated on a plastic substrate are shown in Fig. 4b–d,

exhibiting very good transparency, flexibility, and performance.

Transparent devices built on metal oxide nanowires will

greatly enhance the performance of transparent and flexible

display approaches for heads-up displays and printable/light-

weight displays embedded within clothing or equipment. For

example, Ju et al. also demonstrated the first transparent active

matrix organic light emitting diode (AMOLED) displays driven

by transparent devices built on In2O3 nanowire.117

3.3. Lasers and waveguides

ZnO is a good candidate for room temperature UV lasers as its

exciton binding energy is approximately 60 meV, significantly

larger than those of widely used short-wavelength semiconductor

laser materials, ZnSe (22 meV), GaN (25 meV). Fig. 5a and

b show the SEM images of vertically aligned ZnO nanowire

arrays grown on sapphire substrates by using a catalyst-assisted

vapor phase transport process.118 Typical nanowires have

diameters of 20–150 nm and lengths of several micrometers.

According to the emission spectra taken from ZnO nanowire

arrays below and above the lasing threshold, these nanowires are

promising miniaturized laser light sources and may have myriad

J. Mater. Chem., 2009, 19, 828–839 | 831

Fig. 4 (a) Cross-section view, (b) photograph, (c) optical transmission spectrum and (d) Ids-Vgs characteristics of fully transparent and flexible In2O3

nanowire FETs. Reproduced from ref. 116: Nat. Nanotechnology, 2007, 2, 378. Copyright ª 2007, Nature Publishing Group.

Fig. 5 (a,b) SEM images of ZnO nanowire arrays grown on sapphire

substrates. (c) Emission spectra from nanowire arrays below and above

the lasing threshold. Reproduced from ref. 118: Science, 2001, 292, 1897.

Copyright ª 2001, AAA of Science.

Fig. 6 (a) An optical microscope image of a ZnO nanowire guiding light

into a SnO2 nanoribbon. (b) SEM image displaying the nanowire-

nanoribbon junction. Reproduced from ref. 122: Science, 2004, 305,

1269. Copyright ª 2004, AAA of Science.

applications in optical computing, information storage, and

microanalysis (Fig. 5c). Later, lasers were also observed for ZnO

nanostructures with other shapes.119–121

Because of its near cylindrical geometry and large refractive

index (2.0), ZnO is also a natural candidate for optical wave-

guides. Fig. 6 shows the results of Yang et al., where optically

pumped light emission guided by a ZnO nanowire and coupled

into an SnO2 nanoribbon can be clearly seen.122

3.4. Piezoelectric nanogenerators

Energy harvesting from the ambient environment has been an

active research field of nanotechnology in recent years. Wang

832 | J. Mater. Chem., 2009, 19, 828–839

et al. made great contributions to this field and ZnO nanowire

array based piezoelectric nanogenerators have been demon-

strated by them to convert mechanical energy to electricity by

utilizing the coupling effect of the semiconducting and piezo-

electric properties of ZnO.123–125 Fig. 7a is a typical SEM image of

the aligned ZnO nanowire arrays.127 The nanowire density was

controlled to ensure that the AFM tip can exclusively reach one

nanowire without touching another nanowire. When the AFM

tip scanned over the nanowires, the corresponding output

voltage images across the load were recorded simultaneously:

sharp, narrow output peaks were observed, as shown in Fig. 7b.

From the results shown in Fig. 7, we can see that the energy

output by one nanowire in one discharge event is �0.05 fJ, and


Fig. 7 (a) SEM image of aligned ZnO nanowire arrays. (b) Output

voltage image of the nanowire arrays. (c) A series of line profiles of

the voltage output signal. (d) Line profiles from the topography and

output voltage images across a nanowire. (e) Line profile of the voltage

output signal when the AFM tip scans across a vertical nanowire at

12.394 mm/s. (f) The resonance vibration of a nanowire after being

released by the AFM tip. Reproduced from ref. 123: Science, 2006, 312,

242. Copyright ª 2006, AAA of Science.

Fig. 8 (A) Schematic diagram of the ZnO nanowire array DSSC. (B)

Traces of current density against voltage for two different DSSCs. Inset is

a SEM image of aligned ZnO nanowires. Reproduced from ref. 128: Nat.

Mater., 2005, 4, 455. Copyright ª 2005, Nature Publishing Group.

the output voltage on the load is �8 mV. By choosing suitable

nanowire density, the power generated may be high enough to

drive a single nanowire based device. Further exploration of the

piezoelectric nanogenerator concept led to the development of

a wide variety of ZnO nanowire based piezotronic devices,

including piezoelectric field effect transistors, nanoforce sensors,

and gate diodes.126,127

3.5. Solar cells and photocatalysts

In recent years, solar energy conversion devices like solar cells,

which directly converse sunlight into electricity, have attracted

great research interest to solve the continuously increasing

energy problems. Metal oxide nanowires acting as the absorbing

layers can be used to build high performance solar cells. They

also can be used to replace the conventional quantum dot films in

dye-sensitized solar cells (DSSCs). Yang et al. built the first

DSSC using aligned ZnO nanowires as shown in Fig. 8. A direct

conversion efficiency of 1.5% is demonstrated, which is primarily

limited by the surface area of ZnO nanowire arrays.128 Driven by

this work, many DSSCs built on different metal oxide nanowires


with different conversion efficiencies were demonstrated and the

used metal oxide nanowires include TiO2 nanorods, CuO

nanorods, core/shell ZnO/Al2O3, ZnO/TiO2, ZnO/ZnSe nano-

wires, etc.129–133

Developing semiconductor photocatalysts for water splitting

and degradation of organic pollutants provides another way to

solve the urgent energy and environmental issues.134–137 Metal

oxide nanowires gained much attention in this direction to be

used as high performance photocatalysts due to their extremely

enhanced surface areas.138–145 1-D TiO2 nanostructures, nano-

rods, nanowires and nanotubes, are the mostly investigated oxide

nanostructures that can be used as high performance photo-

catalysts exhibiting better water splitting and organic degradation

properties than bulk materials.138,139 Several other oxide nanowire

photocatalysts include ZnO nanowires, SnO2 nanowires, VO2

nanowire arrays, and many ternary oxide nanowires, such as

SrSnO4 nanowires,142 BiVO4 nanotubes, and AgIn(WO4)2

nanotubes.145 With the development of better synthesis

approaches to 1-D metal oxide nanostructures, photocatalysts

with greatly improved performance are expected to be obtained.

3.6. Field nanoemitters

Field emission, also known as Fowler–Nordheim tunneling, is

a form of quantum tunneling in which electrons pass from an

J. Mater. Chem., 2009, 19, 828–839 | 833

Fig. 9 Field emission properties form vertically aligned ZnO nanowires

grown on a Si substrate. Reproduced from ref. 147: Chem. Phys. Lett.,

2005, 404, 69. Copyright ª 2005, Sciencedirect.

emitting material to the anode through a barrier in the presence

of a high electric field. It is one of the main features of metal

oxide nanowires, and is of great commercial interest in many

areas, such as displays and other electronic devices. Using metal

oxide nanowires as field nanoemitters was first reported by Lee in

2002.146 They studied the field emission properties of vertically-

aligned ZnO nanowires grown via a VS process and found a turn-

on voltage of 6.0 V/mm at a current density of 0.1 mAcm�2

(Fig. 9).147

Inspired by the work using ZnO nanowires as field nano-

emitters, field emission properties of many other kinds of metal

oxide nanowires were also studied, including WO3, IrO2, RuO2,

CuO, TiO2, SnO2, In2O3 nanowires.147–165 Compared with

carbon nanotube based emitteors, metal oxide emitters are more

stable in harsh environments and controllable in electrical

properties.

4. Chemical sensors built on metal oxide nanowires

With large surface-to-volume ratios and a Debye length

comparable to their dimensions, metal oxide nanowires have

showed great potential to be used as chemical sensors.106

Recently, the detection of a wide range of chemicals with

different nanowire sensor configurations has been reported. For

instance, Zhang and coworkers fabricated and tested an In2O3

nanowire mat sensor for which a detection limit of �5 ppb was

achieved.166 Table 2 summarizes a list of typical metal oxide

nanowire based chemical sensors with different device configu-

rations, working temperatures, detection limits and response

times of different targeted chemicals.166–194 As one can see,

chemical sensors built on SnO2, ZnO, and In2O3 nanowires have

been widely reported due to their easy synthesis, good sensitivity

to chemicals, and good stability compared to other metal oxide

nanomaterials. In addition, in spite of their sensitivity, selectivity

of chemical sensors remains one of the challenging issues in this

field.

The sensing mechanism of metal oxide nanowires has been

discussed in recent publications. Briefly, the working principle of

metal oxide nanowire-based chemical sensors relies on changes

834 | J. Mater. Chem., 2009, 19, 828–839

of electrical conductivity due to the interaction of nanowires with

the surrounding environment. The charge transfer process induced

by the redox reactions between nanowire surface and tested

chemicals determines the conductance of nanowire-based chemical

sensors. For example, when a reducing gas (eg. CO) is introduced

to a chemical sensor, the following reaction happens:172

CO + O� / CO2 + e�

Here, CO reacts with adsorbed oxygen ions on the nanowire

surface and thus results in an overall increase of the electrical

conductance of metal oxide nanowires.

On the other hand, if a chemical sensor is exposed to an

oxidation gas (eg. NO2), the following oxidizing reaction may

take place:195

NO2 + e� / NO2�

NO2 serve as charge accepting molecules and withdraw electrons

from the nanowire, resulted in a reduction of electrical conductance.

Based on the above mentioned sensing mechanisms, metal

oxide nanowire-based chemical sensors are usually fabricated in

two configurations, resistors and FET devices with single or

multiple nanowire nanowires. In fact, most reported works in

this field are based on these two configurations due to easy

fabrication, good reliability, low cost, and easy integration with

heat transducers.

Instead of the sensors measuring the change of electrical

conductance, there are several other kinds of sensors, such as

photoluminescence (PL) sensors, and nanostructured ZnO

coated quartz crystal microbalance (QCM) sensors. However,

compared with the electrical chemical sensors, these sensors are

complicated and expensive.

Below we will discuss chemical sensors following different

sensor configurations, i.e. electrical, optical and nanostructure

coated QCM gas sensors and electronic noses, each with one or

two examples.

4.1. Electrical based chemical sensors

A resistor based sensor is one of the easiest methods to carry out

chemical sensing experiments by measuring the change of

conductance of the sensing element in different surrounding

environments. Fig. 10a showed a schematic drawing of a tran-

sistor based chemical sensor. Nanowires are dispersed on a SiO2/

Si substrate followed by patterning source and drain electrodes

above the dispersed nanowires. The Si substrate serves as a back

gate electrode while the chemical sensor works as FET based

sensors. To improve the sensitivity and detect inert gases, several

groups reported to integrate MEMS hotplates with chemical

sensors.178 For instances, Fig. 10b is a SEM image of the active

area of one chemical sensor chip, where the dashed box repre-

sents the SiN membrane and one nanowire is bridged by two

electrodes which can be used as a chemical sensor as shown in

Fig. 10c. The zigzag shaped micromachined hotplates provide

a facile way to control elevated temperatures with low power

consumption. With the aid of elevated temperatures, the detec-

tion limits of this chemical sensor can be enhanced down to 1

ppm for ethanol as shown in Fig. 10d.


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This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 828–839 | 835

Fig. 10 (a) Schematic diagram of a single nanowire transistor structure.

(b) SEM image of the chemical sensor chip integrated of a single In2O3

nanowire and micromachined hotplates, where the dashed box indicates

the SiN membrane. (c) SEM image of a sensing device with an In2O3

nanowire bridging two electrodes. (d) Sensing response of an In2O3

nanowire sensor operated at 275 �C to four different ethanol concen-

trations (1, 10, 50, and 100 ppm). Reproduced from ref. 196: Appl. Phys.

Lett., 2008, 92, 93111. Copyright ª 2007, AIP.

Fig. 11 Response of a ZnO nanowire FET exposed to 10 ppm NO2 gas.

Reproduced from ref. 180: Appl. Phys. Lett., 2004, 85, 5923. Copyright ª2004, AIP.

In FET based chemical sensors, Fan et al. studied oxygen and

NO2 adsorption on the ZnO nanowire surface by using indi-

vidual ZnO nanowire field-effect transistors.177 The results of

sensing experiments can be observed in Fig. 11. A considerable

variation of conductance was observed when the device was

exposed to oxygen or NO2. In addition, an electrical potential to

the back gate electrode was applied, which could help to adjust

the sensitivity range of the device or initialize the device

completely before exposure to chemicals. This can be attributed

to the fact that the Fermi level within the nanowire band gap was

manipulated by applying an external gate voltage. In addition,

a ZnO chemical sensor was fully refreshed by applying a high

negative gate bias of 60 V as shown in Fig. 11.

Fig. 12 Principal component analysis (PCA) scores and loading plots of

a chemical sensor array composed of four different nanostructure

materials.

4.2. Optical and QCM based chemical sensors

With the novel characterization of contactless devices, recently

several research groups executed chemical sensing experiments

836 | J. Mater. Chem., 2009, 19, 828–839

based on metal oxide nanowire PL chemical sensors, such as

SnO2, and ZnO, etc.197,198 After exposure to chemicals, the

quenching of PL was observed.199 Although the microscopic

mechanisms are still not clear, the quenching is thought to be

related to the change of the oxidation state of the nanowire

surface before and after chemical exposure.197 In addition, the

sensing response time and recovery time are fast (merely a few

seconds), comparable with the response times of most electrical

based chemical sensors. For the QCM based sensor, it is thought

to be a mass-sensitive sensor, which can detect the change of

mass on a sensing layer. The mass of the sensing layer varies due

to the chemical reactions, adsorption, and deposition happening

above the surface of the sensing layer, while the sensor is exposed

to chemicals. QCM based sensors are also contactless devices.200

4.3. Electronic noses

The idea of electronic noses was inspired by the mechanisms of

human olfaction. In general, basic elements of an electronic nose

system include an ‘‘odour’’ sensor array, a data pre-processor,

and a pattern recognition (PARC) engine.201 There are several

methods to approach this goal, one is to make a chemical sensor

array with different nanostructured materials and the other is to

make a sensor chip with different material geometric properties

and temperature gradients (KAMINA technology). Kolmakov

et al. adapted this idea and fabricated a KAMINA sensor chip

composed of SnO2 nanowires with different nanowire densities,

which exhibited good selectivity for several chemicals.202 The

achievement not only successfully solved the ‘‘selectivity’’ issue

but also brought nanotechnology a step closer to practical

application.

Very recently, we developed a new template built with four

different semiconducting nanostructures: In2O3 nanowires, SnO2

nanowires, ZnO nanowires and single-wall C nanotubes (SWNT)

as electronic noses to detect different chemicals (Fig. 12 inset).203

n-Type metal oxide nanowires and p-type C nanotubes provide

one discrimination factor. The integrated micromachined hot

plate enables individual and accurate temperature control of

each sensor, which provides the second discrimination factor.


When this sensor array was exposed to different chemicals, good

selectivity was obtained to build up an interesting ‘‘smell-print’’

library of the detected chemicals (Fig. 12).

5. Summary

In summary, we provide a comprehensive review of the state-of-

the-art research activities focused on devices and chemical sensing

applications of metal oxide nanowires. The fascinating achieve-

ments, till now, towards the device applications of metal oxide

nanowires should inspire more and more research efforts to address

the remaining challenges in this interesting field. We tried to include

the most important topics in this review article. However, due to the

tremendous research effort and space limitations, this article is

unable to list all the exciting works reported in this field.

Although comprehensive efforts have been made towards the

synthesis of high quality metal oxide nanowires, there is still

plenty of room left unexploited. We believe that future work in

the nanowire synthesis direction should continue to focus on

generating high quality and large quantity metal oxide nanowires

in more controlled, predictable and simple ways. One key issue of

metal oxide nanowires is the growth of p-type metal oxide

nanowires or the formation of intra-nanowire p-n junctions,

which will significantly advance and widen the device application

of metal oxide nanowires.

One interesting area in the metal oxide nanowire based

chemical sensors area is still the development of high quality 1-D

metal oxide nanostructures to be used as chemical sensing

elements. The sensing issues of extremely high sensitivity, selec-

tivity and stability should be resolved. Though some research

groups have successfully detected important chemicals using 1-D

metal oxide nanostructures, the selectivity is still quite low.

Furthermore, other potential and interesting areas which need

further exploration may be the detection of very small amounts

of nerve agents such as sarin and soman, or of explosive chem-

icals for personal health and human security applications.

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