One-Dimensional Oxide Nanostructu...
Sensors 2010, 10, 4083-4099 doi:10.3390/s100404083 sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Review One-Dimensional Oxide Nanostructures as Gas-Sensing Materials: Review and Issues Kyoung Jin Choi 1,* and Ho Won Jang 2 1 Nano-Materials Center, Korea Institute of Science and Technology, Seoul, 130-650, Korea 2 Electronic Materials Center, Korea Institute of Science and Technology, Seoul, 130-650, Korea E-Mail: email@example.com * Author to whom correspondence should be addressed E-Mail: firstname.lastname@example.org Tel.: +82-2-958-5502 Fax: +82-2-958-5509. Received: 3 March 2010 in revised form: 15 April 2010 / Accepted: 16 April 2010 / Published: 22 April 2010 Abstract: In this article, we review gas sensor application of one-dimensional (1D) metal- oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. One of the most important steps in fabricating 1D-nanostructure devices is manipulation and making electrical contacts of the nanostructures. Gas sensors based on individual 1D nanostructure, which were usually fabricated using electron-beam lithography, have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools. In the second part of the paper, some critical issues are pointed out including long-term stability, gas selectivity, and room- temperature operation of 1D-nanostructure-based metal-oxide gas sensors. Keywords: 1-dimensional nanostructures gas sensors long-term stability gas selectivity electronic-nose room-temperature operation 1. Introduction In 1962, Seiyama et al. discovered that the electrical conductivity of ZnO could be dramatically changed by the presence of reactive gases in the air . Since then, there have been tremendous reports on the applications of semiconducting metal oxides as gas sensors due to their small dimensions, low cost, and high compatibility with microelectronic processing. Recently, one-dimensional (1D) OPEN ACCESS
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Sensors 2010, 10 4085 2. Types of Gas-Sensor Structure Based Upon 1D Oxide Nanostructures 2.1. Single 1D Nanostructure Gas Sensors Law et al.  have found that individual single-crystalline SnO2 nanoribbons have strong photoconducting response and thus detect ppm-level NO2 at room temperature by illuminating the nanoribbons with UV light of energy near the SnO2 bandgap (Eg = 3.6 eV at 300 K). Photogenerated holes recombine with trapped electrons at the surface, desorbing NO2 and other electron-trapping species: h+ + NO2-(ads) ��� NO2(gas). The space charge layer thins, and the nanoribbon conductivity rises. Ambient NO2 levels are tracked by monitoring changes in conductance in the illuminated state. The larger and faster response of individual nanoribbon sensors with 365 nm illumination than that with 254 nm illumination suggested that the presence of surface states plays a role in the photochemical adsorption- desorption behavior at room temperature. Wang and co-workers demonstrated the gas sensing ability of field-effect transistors (FETs) based on a single SnO2 nanobelt . SnO2 nanobelts were doped with surface oxygen vacancies by annealing in an oxygen-deficient atmosphere. Then the source-drain current of SnO2 nanobelt FETs could respond and recover with exposure and removal of oxygen in ambient nitrogen at 200 ��C. Later, they improved the device performance of the SnO2 nanobelt FETs . Low-resistance RuO2/Au Ohmic contacts on the SnO2 nanobelts led to high-quality n-channel depletion mode FETs with well-defined linear and saturation regimes, large on current, and on/off ratio as high as 107. The FET characteristics show a significant modification upon exposure to 0.2% H2. The channel conductance in the linear regime increases by around 17% at all gate voltages. The hydrogen reacts with and removes the oxygen adsorbed on the metal oxide surface and thus increases the electron concentration and the conductance of the nanobelt channel . Qian et al.  reported a CO sensor based on an individual Au-decorated SnO2 nanobelt. Wang and co-workers presented a high sensitivity humidity sensor based on a single SnO2 nanowire . The SnO2 nanowire based sensor had a fast and sensitive response to relative humidity in air from a wide range of environments at room temperature. In addition, it had relatively good reproducibility, and its linear response to 30���90% RH makes it easy to calibrate. The sensitivity of the single SnO2 nanowire based sensors to CO, CH4 and H2S gases at 250 ��C was improved by 50-100% through surface functionalization with ZnO or NiO nanoparticles . The heterojunction between the surface coating layers and SnO2 (i.e., n-n junction for ZnO-SnO2 and p-n junction for NiO-SnO2) and the corresponding coupling effect of the two sensing materials played a critical role in controlling device sensitivity. Besides heterojunctions, many other factors such as the size and crystalline state of surface additives and the concentration change of structure defects in the nanowires might bring a pronounced influence on the gas sensing performance of the SnO2 nanowire based device. Thus, it was difficult to use a uniform model to completely elucidate the nature of the surface additives. Despite this, it was clear that surface functionalization is a good strategy to improve the sensitivity and selectivity of the SnO2-based nanosensor. Kumar et al.  reported highly sensitive H2S sensors based on homogeneously Cu-doped SnO2 single nanowires. By Cu doping, the sensitivity of SnO2 single nanowire sensors could be increased by up to 105.