Room-Temperature Hydrogen-Selective Sensing Using Single Pt-Coated ZnO Nanowires at Microwatt Power Levels

There is great current interest in the development of low-weight, low-power-consumption hydrogen-selective sensors for use with proton-exchange membrane (PEM) and solid oxide fuel cells (SOFCs) for space craft and other applications. It is clear that nanowires and nanotubes are excellent candidates for this type of sensing, given their large surface-to-volume ratios and low weight. The ability to detect hydrogen selectively at room temperature is important because it avoids the use of on-chip heaters that add to the power consumption and weight. One method for increasing hydrogen detection sensitivity is to use a catalytic metal coating or to actually dope the sensor material with the transition metal.^{1, 2} This leads to catalytic dissociation of to atomic hydrogen, which produces a sensor response through binding to surface atoms and altering the surface potential. ZnO nanowires are attractive for a wide variety of sensing applications because of the ease of synthesis, ability to readily transfer them to cheap substrates and their bio-safe characteristics.^3–22

In this paper, we describe how the addition of sputter-deposited Pt clusters to the surface of single ZnO nanowires produces a significant increase in detection sensitivity for hydrogen at room temperature.The sensors are shown to detect ppm hydrogen at room temperature using of power.

The site-selective growth of ZnO nanowires was achieved by nucleating the nanorods on a sapphire substrate coated with Au islands as has been described in detail previously.^{8, 9, 13} ZnO nanowires were deposited by Molecular Beam Epitaxy (MBE) with a base pressure of using high purity (99.9999%) Zn metal and an plasma discharge as the source chemicals. The Zn pressure was , while the beam pressure of the mixture was . The growth time was at 600°C. The typical length of the resultant nanowires was , with typical diameters in the range of 30 to 150 nm. Selected area diffraction patterns showed the nanowires to be single-crystal. The nanowires were removed from the original substrate by sonication and transferred to a Si substrate, as reported in detail previously.²³ A shadow mask was used to pattern sputtered electrodes contacting both ends of single nanowires on the Si substrates. The separation of the electrodes was . In some cases, the nanowires were coated with discontinuous Pt cluster ( thick) deposited by sputtering. A scanning electron micrograph of the single nanowire sensor is shown in Fig. 1. Au wires were bonded to the contact pad for current–voltage (I-V) measurements performed at 25°C in a range of different ambients (vacuum, , or 100-500 ppm in ).

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The I-V characteristics from the uncoated single nanowires were linear with typical currents in the at an applied bias of 0.5 V. Figure 2 shows that the addition of the Pt-coatings increased the effective conductivity of the nanowires by over an order of magnitude. Because the Pt films are discontinuous (as evidenced by both field-emission scanning electron microscopy and atomic force microscopy), this suggests that the sputtering process itself changes the resistance of the nanowires, most likely through the introduction of oxygen vacancies which are donor states in ZnO.^{24, 25} There was a strong increase (approximately a factor of 5) in the response of the Pt-coated nanowires to hydrogen relative to the uncoated devices.

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Figure 3 shows the I-V characteristics of Pt-coated nanowires as a function of both the measurement ambient and the time after exposure to 500 ppm in . There are several aspects of the data. There was no response of either coated or uncoated nanowires to the presence of in the ambient at room temperature and indeed the I-V characteristics were independent of the measurement ambient for vacuum, air, or pure . By sharp contrast, the nanowires were sensitive to the presence of in the ambient, with the response being time-dependent. The nanowire resistance was still changing at least 15 min after the introduction of the hydrogen. An Arrhenius plot of the rate of resistance change for the nanowires exposed to the 500 ppm in for 10 min produced an activation energy of . This is larger than that expected for typical diffusion processes and suggests that the rate-limiting step may be chemisorption of hydrogen on the Pt surface. The reversible chemisorption of reactive gases at the surface of ZnO can produce a large reversible variation in the resistance.²⁶ In addition, atomic hydrogen introduces a shallow donor state into ZnO and this may play a role in the increased conductance of the nanorods.^{24, 27} The diffusion coefficient of the hydrogen is also much faster in ZnO than in any other wide bandgap semiconductor.^{28, 29} Note the very low power consumption of the nanowire sensors, which is in the range . This is approximately a factor of 25 lower than multiple ZnO nanowires operated under the same conditions and more than a factor of 50 lower than carbon nanotubes doped with Pd that were used for hydrogen detection.^{1, 2} The low power consumption is clearly of advantage in many types of remote sensing or long-term sensing applications.

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Figure 4 shows the time dependence of current (top) or relative resistance change (bottom) in both the uncoated and Pt-coated nanowires exposed to 500 ppm in . The relative resistance responses were and 50%, respectively, after 10 or 20 min exposure. By comparison, the uncoated devices showed relative resistance changes of and 3%, respectively, after 10 or 20 min exposure. The resistance change during the exposure to hydrogen was slower in the first few minutes, as is clear in Fig. 4. This may be due to removal by the atomic hydrogen of native oxide on the Pt. As the effective surface area of the Pt would increase as the oxide was removed, the rate of change of resistance due to hydrogen adsorption should also increase. At fixed voltage, the relative resistance change was linear as a function of hydrogen content in the measurement ambient up to a few percent and then increased more slowly at higher concentrations. This may indicate a saturation of bonding sites for hydrogen at high concentrations. We have not yet investigated the long-term reliability and reproducibility of the nanowire sensors, but this aspect will be key for practical applications. We have measured the time recovery characteristics of the single nanowires when hydrogen is removed from the ambient and find the recovery is limited by the time needed to flush the hydrogen out of the test fixture (a few seconds) and not by the nanowire response.

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In summary, Pt-coated ZnO single nanowires are shown to selectively detect hydrogen at room temperature with very low power consumption. The disadvantage of this approach relative to using a network of multiple nanowires is the additional processing that is needed to contact a single nanowire, but the power consumption is significantly (a factor of ) lower.

Acknowledgments