Devices and chemical sensing applications of metal oxide nanowires

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1 APPLICATION 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: /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 stateof-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, electro-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 nanowires, nanobelts, nanotubes, nanorings, and nanosprings, have been prepared using many synthesis s. High-performance chemical sensors have been fabricated on SnO 2, ZnO, and In 2 O 3 nanowires due to their large surface area to volume ratio. Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA. gzshen@ustc.edu; guozhens@usc. edu; chongwuz@usc.edu; Fax: ; Tel: This paper is part of a Journal of Materials Chemistry theme issue on Nanotubes and Nanowires. Guest editor: Z. L. Wang. This article will provide a comprehensive review of the stateof-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 introduces 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 nanowires. 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 photocatalysts, 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 s 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 Dr Guozhen Shen received his Ph.D. degree in Chemistry from University of Science and Technology of China in 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 Dr Guozhen Shen than 100 research articles and 5 book chapters. His most recent research interests include the synthesis and characterization of onedimensional nanostructures and their device applications in electronics and optoelectronics. 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 California. His research focus is on the device applications based on 1-D nanomaterials, including chemical sensors, transparent electronics, and energy conversion and storage devices. 828 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

2 these two approaches developed to synthesize metal oxide nanowires 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 positions on substrates. 6,7 For example, Im et al. 6 synthesized ZnO nanowires using a complicated nanoscale spacer lithography, which can be used to detect H 2 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 microelectronic 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 Bottom-up synthesis The bottom-up approaches, in which functional electronic structures are assembled from chemically synthesized nanoscale building blocks, represent flexible alternatives to conventional top-down s. 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 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 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 is conceptually very simple and has been widely used to prepare a variety of metal oxide nanowires. Despite its simplicity, the template-directed is limited by the fact that the synthesized 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 solubilized by a liquid catalyst particle to form solid wire-like structures. 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 evaporation, chemical vapor deposition (CVD), metal-organic chemical vapor deposition (MOCVD), laser-ablation and many others. Fig. 1b and c show SEM and HRTEM images of In 2 O 3 nanowires synthesized from the VLS process using a laser-ablation, which exhibit very good crystallinity and are of single crystal nature. The inset is a TEM image of a typical In 2 O 3 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 understood as the VLS process, it has already proved to be a very important 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 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, SnO 2, CdO, and Ga 2 O 3. 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 crosssections. They usually have thickness of nm and width-tothickness ratios of 5 10 nm, respectively. 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 University of Southern California. His research interest covers carbon nanotube synthesis and applications such as nanotube circuits, chemical sensing, and OLED fabricated by carbon nanotube conductive films. He has published 3 journal papers related to carbon nanotube synthesis and OLED fabrication. 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 California in September His research group has been working at the forefront of nanoscience and nanotechnology, including synthesis and applications of carbon nanotubes and nanowires, biosensing, and nano- Dr Chongwu Zhou 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. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

3 Table 1 Summary of 1-D metal oxide nanostructures synthesized using different s Materials Morphology Growth Ref. ZnO Nanowires Vapor-solid 1 Vapor-liquid-solid 10 AAO template-assisted 11 Microemulsion 12 Template-free solution 13 Nanobelts Vapor-solid 2 Vapor-liquid-solid 14 Hydrothermal 15 Nanorods Template-free aqueous 16 Vapor-liquid-solid 17 Vapor-solid 18 Pulsed-laser ablation without catalyst 19 Nanotubes Vapor-solid 20 Vapor-liquid-solid 21 Solution 22 SnO 2 Nanorods Microemulsion 23 Hydrothermal 24 Solution 25 Vapor-liquid-solid 26 Nnanowires Catalyst-assisted laser ablation 27 Vapor-solid 28 Vapor-liquid-solid 29 Solution 3 Nanobelts Thermal oxidation 30 Vapor-solid 31 Laser-ablation 32 Nanotubes Templates hydrothermal 33 Aqueous solution 34 Microemulsion 35 In 2 O 3 Nanowires Catalyst-assisted laser-ablation 4 Vapor-solid 36 Vapor-liquid-solid 37 AAO-templated solution 38 Nanobelts Vapor-solid 39 Nanotubes Thermal evaporation 40 Solvothermal 41 Ga 2 O 3 Nanowires Thermal evaporation 42 Catalyst-assisted arc discharge 43 Laser-ablation 44 Catalyst-assisted vapor 45 Nanobelts Vapor-solid 46 Vapor-liquid-solid 47 Nanotubes Vapor-solid 48 WO 3 Nanowires SBA-15 templated solution 49 Thermal evaporation 50 Hydrothermal 51 Nanobelts Vapor-solid 52 V 2 O 5 Nanowires Hydrothermal 53 Nanobelts Vapor-solid 54 MgO Nanowires Vapor-solid 55 Vapor-liquid-solid 56 Catalyst-assisted laser-ablation 57 nanotubes Vapor-liquid-solid 58 TiO 2 Nanotubes AAO-templated solution 59 Solution 60 Nanowires Hydrothermal 61 Vapor-solid 62 ZrO 2 Nanowires AAO-templated solution 63 Nanorods Precursor thermal decomposition 64 Nanotubes AAO-templated solution 65 Nb 2 O 5 Nanobelts Precursor thermal decomposition 66 nanowires Vapor-liquid-solid 67 Nanotubes Precursor thermal decomposition 5 Ta 2 O 5 Nanotubes Precursor thermal decomposition 5 Table 1 (Contd.) Materials Morphology Growth Ref. MoO 3 Nanotubes Hydrothermal 68 Carbon nanotube templated 69 Vapor-solid 70 Nanowires Thermal evaporation 71 Solution 72 MnO 2 Nanowires Hydrothermal 73 SBA-15 templated synthesis 74 Nanotubes Hydrothermal 75 Fe 2 O 3 Nanowires Thermal oxidation 76 Hydrothermal 77 Nanobelts Thermal oxidation 78 Fe 3 O 4 Nanotubes MgO-templated pulsed-laser 79 deposition Nanowires Magnetic-field-induced hydrothermal 80 Co 3 O 4 Nanowires Thermal oxidation 81 Nanowires Hydrothermal 82 Nanotubes Carbon nanotube templated 83 Nanotubes Solution 84 IrO 2 Nanotubes Metal-organic CVD 85 Nanowires Metal-organic CVD 86 NiO Nanowires Wet chemical route 87 AAO-templated sol-gel 88 Nanotubes AAO-assisted solution 89 Cu 2 O Nanowires Solid-state reduction 90 Surfactant-assisted solution 91 Hydrothermal 92 CuO Nanowires Thermal oxidation 93 AAO-templated deposition 94 Solution 95 Nanobelts Solution 95 CdO Nanowires AAO-assisted electrochemical 96 deposition Chemical bath deposition 97 Nanoneedles Vapor-liquid-solid 98 Al 2 O 3 Nanotubes Pulse anodization 99 Thermal evaporation 100 Surfactant-assisted solution 101 Carbon nanotube-assisted growth 102 Nanowires, nanobelts Vapor-solid 103 Fig. 1 (a) Schematic illustrating the growth process of a VLS process. (b) SEM image and (c) TEM image of In 2 O 3 nanowires grown from VLS process. 830 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

4 Fig. 2 TEM images of ZnO nanobelts grown from the VS process. Reproduced from ref. 2: Science, 2001, 291, 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 microscopy (TEM). Detailed description and analysis of these characterization techniques can be found in some recent review papers and will not be discussed here. 106, Electronic and optoelectronic devices built on metal 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 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 (V ds ) and current (I) vs. gate voltage (V g ) are recorded to characterize the nanowire FET. Metal oxides are n-type semiconducting materials. So for the typical I V ds curves recorded from metal oxide nanowire FETs, an increase in conductance for V g > 0 and a decrease in conductance for V g < 0 are obtained. n-type FETs have been Fig. 3 Schematic of an oxide nanowire FET. fabricated on various oxide nanowires, including ZnO, In 2 O 3, SnO 2,Cu 2 O, TiO 2, CdO, etc 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 nanowires as the building blocks 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 In 2 O 3 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 transparent. 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/lightweight 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 In 2 O 3 nanowire 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 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 This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

5 Fig. 4 (a) Cross-section view, (b) photograph, (c) optical transmission spectrum and (d) I ds -V gs characteristics of fully transparent and flexible In 2 O 3 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, Copyright ª 2001, AAA of Science. applications in optical computing, information storage, and microanalysis (Fig. 5c). Later, lasers were also observed for ZnO nanostructures with other shapes Because of its near cylindrical geometry and large refractive index (2.0), ZnO is also a natural candidate for optical waveguides. Fig. 6 shows the results of Yang et al., where optically pumped light emission guided by a ZnO nanowire and coupled into an SnO 2 nanoribbon can be clearly seen Piezoelectric nanogenerators Energy harvesting from the ambient environment has been an active research field of nanotechnology in recent years. Wang Fig. 6 (a) An optical microscope image of a ZnO nanowire guiding light into a SnO 2 nanoribbon. (b) SEM image displaying the nanowirenanoribbon junction. Reproduced from ref. 122: Science, 2004, 305, Copyright ª 2004, AAA of Science. et al. made great contributions to this field and ZnO nanowire array based piezoelectric nanogenerators have been demonstrated by them to convert mechanical energy to electricity by utilizing the coupling effect of the semiconducting and piezoelectric properties of ZnO 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 832 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

6 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 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. 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, 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 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. with different conversion efficiencies were demonstrated and the used metal oxide nanowires include TiO 2 nanorods, CuO nanorods, core/shell ZnO/Al 2 O 3, ZnO/TiO 2, ZnO/ZnSe nanowires, etc Developing semiconductor photocatalysts for water splitting and degradation of organic pollutants provides another way to solve the urgent energy and environmental issues Metal oxide nanowires gained much attention in this direction to be used as high performance photocatalysts due to their extremely enhanced surface areas D TiO 2 nanostructures, nanorods, nanowires and nanotubes, are the mostly investigated oxide nanostructures that can be used as high performance photocatalysts exhibiting better water splitting and organic degradation properties than bulk materials. 138,139 Several other oxide nanowire photocatalysts include ZnO nanowires, SnO 2 nanowires, VO 2 nanowire arrays, and many ternary oxide nanowires, such as SrSnO 4 nanowires, 142 BiVO 4 nanotubes, and AgIn(WO 4 ) 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 Field nanoemitters Field emission, also known as Fowler Nordheim tunneling, is a form of quantum tunneling in which electrons pass from an This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

7 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 / CO 2 +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. NO 2 ), the following oxidizing reaction may take place: 195 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 They studied the field emission properties of verticallyaligned ZnO nanowires grown via a VS process and found a turnon 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 nanoemitters, field emission properties of many other kinds of metal oxide nanowires were also studied, including WO 3, IrO 2, RuO 2, CuO, TiO 2, SnO 2, In 2 O 3 nanowires 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 In 2 O 3 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 configurations, working temperatures, detection limits and response times of different targeted chemicals As one can see, chemical sensors built on SnO 2, ZnO, and In 2 O 3 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 NO 2 +e / NO 2 NO 2 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 Electrical based chemical sensors A resistor based sensor is one of the easiest s 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 transistor based chemical sensor. Nanowires are dispersed on a SiO 2 / 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 represents 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 detection limits of this chemical sensor can be enhanced down to 1 ppm for ethanol as shown in Fig. 10d. 834 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

8 Table 2 Summary of some typical chemical sensors built on metal oxide nanowires Material Device Diameter (nm) Analytes Detect limit Response time (s) Working temperature Synthesis Ref. In 2 O 3 FET 10 NO 2 NH 3,O 2 CO, H 2 NO 2 5 ppb 5 10 RT a Laser ablation 166 FET 10 NO 2,NH 3 NO ppm RT Laser ablation 167 FET 10 NH % NH3 20 RT Chemical vapor 168 deposition SnO2 Resistor and photoluminescence sensor O2, CO, NO2, ethanol CO 10 ppm; NH3 50 ppm; NO2 1 ppm C Vapor phase deposition 169 FET 60 O 2, CO K Electro-deposition 170 FET 60 O 2, CO K Electro-deposition 171 Resistor 60 O2, CO K Electro-deposition 172 Resistor (AC) 20 CO CO 5 ppm; stability 4% C 173 In-SnO2 Resistor Ethanol 10 ppm C Thermal evaporation 174 Sb-SnO 2 Resistor 40 Ethanol 10 ppm C Thermal evaporation 175 Ru-SnO 2 Resistor NO 2, liquid 50 ppm C Thermal evaporation 176 petroleum gas ZnO FET NH3, NO2 NO2 200 ppb, NH3 0.5% FET O2 Thermal evaporation 178 Resistor 30 Ethanol 50 ppm 60 RT-300 C Thermal evaporation 179 FET 60 O2 10 ppm RT Vapor-liquid-solid 180 Pd, Pt, Au, Ni,Ag,Ti doped ZnO Quartz crystal micro-balance 20 NH ppm 5 RT Thermal evaporation 181 Resistor H 2,O 3 3% O C for H 2 ;RT for O3 MBE 182 Resistor H2 10 ppm 600 RT Thermal evaporation 183 Ga-ZnO Resistor CO 320 C Thermal evaporation 184 Pd-ZnO Resistor H 2 10 ppm 20 RT 185 Pt-ZnO Resistor H ppm 600 RT MBE 186 Ga2O3 Resistor NH3, NO2 NH3 200 ppm, NO2 20 ppm Ga oxidized 187 Resistor Ethanol 1500 ppm 100 C Ga oxidized 188 Several min 250 C Thermal evaporation 189 WO3 Resistor 100 NO2, H2S NO2 50 ppb, H2S 10 ppm TeO 2 Resistor NO 2,NH 3,H 2 S NO 2 10 ppm, NH ppm, H 2 S 50 ppm 600 RT Thermal evaporation 190 V2O5 Resistor He Electro-phoresis sol-gel Resistor 10 1-Butylamine, toluene, 1-propanol 1-Butylamine 30 ppb 500 RT Aqueous solution ZnSnO 3 Resistor Ethanol 1ppm C Thermal evaporation 193 Resistor 50 O 2 Thermal evaporation 194 a RT ¼ Room temperature. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19,

9 based on metal oxide nanowire PL chemical sensors, such as SnO 2, 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 Fig. 10 (a) Schematic diagram of a single nanowire transistor structure. (b) SEM image of the chemical sensor chip integrated of a single In 2 O 3 nanowire and micromachined hotplates, where the dashed box indicates the SiN membrane. (c) SEM image of a sensing device with an In 2 O 3 nanowire bridging two electrodes. (d) Sensing response of an In 2 O 3 nanowire sensor operated at 275 C to four different ethanol concentrations (1, 10, 50, and 100 ppm). Reproduced from ref. 196: Appl. Phys. Lett., 2008, 92, Copyright ª 2007, AIP. Fig. 11 Response of a ZnO nanowire FET exposed to 10 ppm NO 2 gas. Reproduced from ref. 180: Appl. Phys. Lett., 2004, 85, Copyright ª 2004, AIP 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 s 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 SnO 2 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: In 2 O 3 nanowires, SnO 2 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. In FET based chemical sensors, Fan et al. studied oxygen and NO 2 adsorption on the ZnO nanowire surface by using individual 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 NO 2. 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 Optical and QCM based chemical sensors With the novel characterization of contactless devices, recently several research groups executed chemical sensing experiments Fig. 12 Principal component analysis (PCA) scores and loading plots of a chemical sensor array composed of four different nanostructure materials. 836 J. Mater. Chem., 2009, 19, This journal is ª The Royal Society of Chemistry 2009

10 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-ofthe-art research activities focused on devices and chemical sensing applications of metal oxide nanowires. The fascinating achievements, 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, selectivity 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. 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