Electrodeposition of nanowires for the detection of hydrogen gas
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Introduction Chemical sensing using nanowires almost always means semiconductor nanowires. Lieber and co-workers were the first to demonstrate in 20011 that chemically modified single crystalline silicon nanowires could function as ChemFETs2,3— transistors in which the gating of the channel conductivity is effected by the specific binding of ions to receptors (e.g., antibodies) present at the surface of the nanowire.4–6 Since a pioneering study of SnO2 nanobelts by Wang and co-workers in 2002,7 semiconducting nanowires composed of metal oxides, especially tin oxide (SnO2), have been extensively used as gas sensors.8–19 Although more than one mechanism has been proposed to account for the sensing properties of these nanowires, in one case, a redox reaction of a reactive gas such as SO2, H2, or NH3 with the SnO2 surface or dopant atoms present at the surface alters the electrical conductivity of the nanowire. Nanowires composed of noble metals are attractive candidates for chemical sensors because they are both ductile and chemically stable in air. The single application where electrodeposited metal nanowires have had the largest impact is that of hydrogen gas sensing. The development of sensitive, selective, power-efficient, rapid-responding, and inexpensive
hydrogen gas sensors continues to be an active area of research. Hydrogen gas is odorless and flammable above 4% in air, so H2 sensors must be deployed in proximity to devices that employ H2 as a fuel, such as within fuel cell-powered automobiles, mobile electronics, and stationary power sources.20 H2 safety sensors based upon palladium (Pd) resistors have been known since the pioneering work of Hughes and Schubert in 1992.21 Those workers prepared resistors using evaporated films of Pd-Ni (8–20 vol%) with thicknesses in the 50 nm range. Those devices produced response times of ≈10 s at 4% H2 and ≈20 s at 1% H2 while achieving a limit-of-detection (LODH ) below 2 0.1%.21 A Pd-Ni thin-film resistor achieves many design objectives for hydrogen safety sensors, including a low manufacturing cost, high sensitivity and accuracy, good stability, and a simple, rugged design, but its response times (10–20 s) are too slow by approximately an order of magnitude relative to targets set by the Department of Energy for this technology.22 Resistive sensors based upon Pd nanowires retain the simplicity of Hughes’ palladium film resistors while offering the possibility for accelerated sensor response and recovery and extremely low power consumption. In this review, we summarize recent efforts to employ electrodeposited nanowires, or components
Reginald M. Penner, University of California, Irvine, CA 92697, USA, [email protected]
MRS BULLETIN • VOLUME 35 • OCTOBER 2010 • www.mrs.org/bulletin
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ELECTRODEPOSITION OF NANOWIRES FOR THE DETECTION OF HYDROGEN GAS
of nanowires, as H2 sensors, and we highlight a new fabrication method for synthesizing palladium nanowires that streamlines the fabrication of hydrogen sensors based upon single, polycrystalline Pd nanowires.
Electrodeposition
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