A Titania Nanotube-Array Room-Temperature Sensor for Selective Detection of Low Hydrogen Concentrations
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A Titania Nanotube-Array Room-Temperature Sensor for Selective Detection of Low Hydrogen Concentrations
Oomman K. Varghese, Gopal K. Mor, Maggie Paulose, and Craig A. Grimes* Department of Electrical Engineering, and Department of Materials Science and Engineering 217 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 USA. *Email: [email protected]
ABSTRACT A tremendous variation in electrical resistance, from the semiconductor to metallic range, has been observed in titania nanotube arrays at room temperature, ≈ 25°C, in the presence of low ppm hydrogen gas concentrations (≤ 1000 ppm). The nanotube arrays are fabricated by anodizing titanium foil in an aqueous fluoride containing electrolyte solution. Subsequently, the arrays are annealed in an oxygen ambient, then coated with a 10 nm layer of palladium by evaporation. Electrical contacts are made by sputtering a small (e.g. 1 mm diameter) platinum disk atop the Pd coated nanotube-array. These sensors exhibit a resistance variation of the order of over 107 (1,000,000,000%) in the presence of 1000 ppm hydrogen at 23°C. To the best of our knowledge this dynamic change in electrical resistance the largest known response of any material, to any gas, at any temperature. The sensors demonstrate complete reversibility, repeatability, high selectivity, no drift and wide dynamic range. The nanoscale geometry of the nanotubes, in particular the points of tube-to-tube contact, is believed to be responsible for the outstanding hydrogen gas sensitivities.
INTRODUCTION The demand for a highly sensitive, selective and stable hydrogen sensor has increased in recent years due mainly to the continued and growing importance of hydrogen in fuel cell applications [1], as well as the chemical and petroleum industries. Various types of sensor technologies [2] such as Schottky junction [3-6], fiber optic [7-9], catalytic [10-12], electrochemical [13-16], field effect transistor (FET) [17-19], oxide semiconductor [20-22], and combinations of these have been developed. Metal oxide semiconductor technology is relatively simple and hence involves lower costs. However, the historical need for operating these materials at elevated temperatures to improve sensitivity has been a significant factor limiting their application. Only a few efforts employing metal oxide semiconductors for room temperature hydrogen sensing can be found in the literature [23-27]. Recently we reported on the high sensitivity of undoped titanium oxide nanotube arrays towards hydrogen at elevated temperatures [20,28]. For a sensor comprised of a 22 nm innerdiameter TiO2 nanotube array a resistance change on the order of 104 was seen in response to 1000 ppm H2 at 290°C [20]. It has now been observed that these nanotube arrays, having an inner-diameter of approximately 22 nm, tube length ranging from 200 nm to 6 µm, when coated with a thin layer (~ 10 nm) of palladium exhibit ultra-high sensitivity to hydrogen at room
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temperature (typically ≈ 23°C
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