Review
Recent progress in the ZnO nanostructure-based sensors

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Abstract

This review focuses on the sensors based on zinc oxide (ZnO) nanostructures, which have fascinating properties including large specific surface area, good biocompatibility, high electron mobility and piezoelectricity. Due to these versatile characteristics, ZnO nanostructures can be based upon to construct gas sensors, chemical sensors, biosensors, UV sensors, pH sensors and other sensors with different sensing mechanisms. The main structures of the sensors and factors influencing the sensitivity are also discussed.

Introduction

Nanomaterials have been extensively studied for application in various kinds of nanoscale functional devices used widely in the chemical industry, medical diagnostics, food technology, ultra-violet testing, national defense and our daily life [1], [2], [3], [4], [5]. Among these, the semiconductor nanomaterials such as ZnO, SnO2, TiO2, and ZnS receive most attention due to intriguing nanosize effects on their physical and chemical properties [6], [7], [8], [9].

ZnO nanostructures are good candidates upon which to construct functional devices, because of their low toxicity, good thermal stability, good oxidation resistibility, good biocompatibility, large specific surface area and high electron mobility [10]. ZnO is a transparent semiconductor with a direct band gap (Eg = 3.37 eV) and a large exciton binding energy (60 meV), exhibiting near UV emission, ZnO has good conductivity, also worth noting is its piezoelectricity [11]. The morphology-controlled synthesis of ZnO nanostructures have been extensively studied, the morphology of ZnO can vary from nanorods, nanotubes, nanoneedles, and nanocomb to nanoinjector, nanohelixes and nanodisks simply by adjusting preparation method and preparation conditions [12], [13], [14], [15], [16], [17]. The various morphologies and mature growth methods lead to easy preparation of ZnO-based devices. With the help of advanced micro-fabrication techniques, ZnO nanostructures have been used widely in field-effect transistors, light emitters, lasers, solar cells and sensing [18], [19], [20], [21].

Much attention has been put on ZnO nanostructures for sensing applications, several properties of ZnO are utilized; gas sensors are based on the fact that conductance changes with the reversible chemisorption process of reactive gases on the surface of ZnO [22]. Pressure sensors are based on the piezoelectric property of ZnO, first observed by Wang et al. [11]. Biosensor can be based on ZnO because of the biocompatibility and nontoxicity of ZnO [23]. Different devices with high sensing performances have been reported, however, high selective response still remains a great challenge.

In this review paper, we will comprehensively introduce the recent progress of the ZnO nanostructures-based sensors, such as gas sensors, chemical sensors, biosensors, UV sensors and pH sensors. The parameters, structures and mechanisms of each type of nano-ZnO based sensors along with factors influencing the sensitivity will be discussed.

Section snippets

Sensitivity

The sensitivity of a sensor is the ratio of the change amplitude of a sensor signal to the original amplitude, which is defined as:S=ΔRR×100%where ΔR is the change amplitude of the sensor signal, and R is the amplitude of the original signal. The signal could be the resistance, current, voltage or conductance, etc.

Taking the reductive gas ethanol as an example [24], R is the resistance of the ZnO nanostructures and the sensitivity is the ratio of the resistance change before and after being

Reductive gas sensor

ZnO nanostructures are considered as one of the most potential candidates for testing reductive gases due to their high sensitivity. Once adsorbed on the surface of ZnO nanostructures, the reductive gas molecules such as H2 [35], [36], CO [37], [38], CO2 [39], H2S [40], NH3 [33] and CH4 [41] will react with the adsorbed O2, O or O2− ions and release the electrons back to the ZnO nanostructures. Because of the large specific surface area and high electron mobility, the ZnO nanostructures will

The configuration of the sensing elements

The configuration of sensing element of ZnO nanostructures can be different according to various applications. The ZnO film, ZnO nanobridge, and the single nanowire/nanorod are the three most common configurations. They are chosen for the sensing study accordingly.

The influencing factors of the sensor based on ZnO nanostructures

There are two kinds of factors influencing the sensor based on ZnO nanostructures, namely the external and the interior influencing factors. The external ones mainly refer to the specific surface area, the temperature and the connecting electrodes, all of which influence the surface adsorption process which plays an important role in the gas sensor, the chemical sensor, the UV sensor and the humidity sensor. The interior one refers to the piezoelectric property of the ZnO nanostructures, that

Summary

The ZnO nanostructures have been widely used in different sensing applications because of their versatile properties such as large specific surface area, nontoxicity, biocompatibility, good conductivity and piezoelectricity. Objects including reductive and oxidative gases, chemical solvents, biomolecules, mass and pressure can be detected, which are summarized in Table 1. The high sensitivity and the low detection limit have promised a widespread application of the sensors based on ZnO

Acknowledgements

The authors wish to thank Prof. Wenyong LAI, Qunliang SONG and Prof. Wei WU for kindly advice. The financial supports by the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University (BJ209007), and Natural Science Research Project of Jiangsu Ordinary University (09KJB430008) are kindly acknowledged.

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