A room-temperature TiO 2 -nanotube hydrogen sensor able to self-clean photoactively from environmental contamination
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Maria A. Carvalho Department of Chemical Engineering, 267 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Ooman K. Varghese Department of Electrical Engineering and Department of Materials Science and Engineering. 217 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Michael V. Pishko Department of Chemical Engineering, 267 Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Craig A. Grimes Department of Electrical Engineering and Department of Materials Science and Engineering, 217 Materials Research Laboratory, The Pensylvania State University, University Park, Pennsylvania 16802 (Received 14 August 2003; accepted 13 November 2003)
Described is a room-temperature hydrogen sensor comprised of a TiO2-nanotube array able to recover substantially from sensor poisoning through ultraviolet (UV) photocatalytic oxidation of the contaminating agent; in this case, various grades of motor oil. The TiO2 nanotubes comprising the sensor are a mixture of both anatase and rutile phases, having nominal dimensions of 22-nm inner diameter, 13.5-nm wall thickness, and 400-nm length, coated with a 10-nm-thick noncontinuous palladium layer. At 24 °C, in response to 1000 ppm of hydrogen, the sensors show a fully reversible change in electrical resistance of approximately 175,000%. Cyclic voltammograms using a 1 N KOH electrolyte under 170 mW/cm2 UV illumination show, for both a clean and an oil-contaminated sensor, anodic current densities of approximately 28 mA/cm2 at 2.5 V. The open circuit oxidation potential shows a shift from 0.5 V to –0.97 V upon UV illumination.
I. INTRODUCTION
A critical concern of any sensor platform is the potential for unwanted contamination, or poisoning, which introduces spurious measurements and generally ends the useful lifetime of a sensor. A sensor used in a noncontrolled environment faces potential contamination from volatile organic vapors, carbon soot, oil vapors, as well as dust and pollen to name but a few examples. An important advance in sensor technology would be a sensor able to self-clean, thereby extending its useful lifetime a)
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J. Mater. Res., Vol. 19, No. 2, Feb 2004 Downloaded: 19 Jun 2014
and minimizing the potential for spurious measurements. Our motivation is to develop chemical sensors of high performance that are low cost, mechanically robust, and able to self-clean thereby significantly extending their useful lifetime. TiO2 is well known for its utility as a gas sensor,1–5 and its ability to degrade photocatalytically a wide range of substances including organic materials,6,7 pesticides,8-12 and herbicides.13–18 In this work, we use sensors composed of TiO2 nanotube arrays (Fig. 1) made by an anodization technique.19 TiO2 is an n-type semiconductor; with ultraviolet photon absorption, an electron-hole pair is gener
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