Palladium Nanowires that Resist Fracture
Principle Scientist: Fan Yang
Fan Yang, David K. Taggart, and Reginald M. Penner*, "Fast, Sensitive Hydrogen Gas Detection Using Palladium Nanowires that Resist Fracture", Nano Letters 9 (2009) 2177. pdf.
We started to study hydrogen sensing with palladium nanowires way back in 2001. Those nanowires, prepared using the electrochemical step edge decoration method, were about 200 nm in diameter. The first thing we discovered about these nanowires is that they spontaneously fracture upon exposure to hydrogen above about 1%. That was interesting, but very bad from a device performance perspective because it dictated a limit-of-detection of 2%. Since the lower explosion limit for hydrogen in air if 4%, 2% is much too high!
With the development of LPNE has come the ability to produce much small Pd nanowires, down to 11 nm in height and 50 nm in width. We have just started to evaluate the performance of such nanowires for the detection of hydrogen. Our first conclusion was the same as last time - even these much smaller Pd nanowires supported now on glass instead of epoxy showed the fracturing behavior of the larger ESED nanowires - and the same 2% limit-of-detection. But when we altered our method for electrodepositing these nanowires, we were quickly able to increase the grain size from 5 nm to 15 nm - a subtle difference in microstructure that produces an enormous change in the behavior of these nanowires: These "NC-15" nanowires did not fracture upon exposure to hydrogen at any concentration.
Figure 1. Pd nanowires prepared by LPNE: a) Atomic force microscope images of a typical Pd nanowire. b,d) Transmission electron microscope images of Pd nanowires prepared in the absence of EDTA (b) and presence of EDTA (c) showing the grain structure. Grain diameter histograms for wires of each type are also shown.
Here are some typical sensing data for these nanowires (Fig 2):
Figure 2. Response of two nc15-Pd nanowire sensors (inset) over a wide [H2] range. a) 0.02% to 0.1% pulses with a duration of 9 hour, b) 0.2% to 1.0% H2 pulses with a duration of 3 hour, c) 1.2% to 2.0% H2 pulses with a duration of 3.5 hour, and d) 4% to 10% H2 with a duration of 3.5 hour.
There are many surprises in the data we have collected so far and we are busy following up on these. Among them, we find that the response and recovery time depends sensitivily not only on the wire height, but also on its width (Fig 3). But why, when this smallest wire dimensions should control rate-limiting diffusion? We also find that whereas larger Pd nanowires slow appreciably in the concentration range from 1-2%, this is not true for the smallest nanowires we have tested, with heights in the 10 nm range. What is the source of this effect?
Figure 3. Comparison of temporal response characteristics for three nc15-Pd nanowires. a) Response to H2 = 10%, b) Recovery from exposure to H2 = 10%, c) Response to H2 = 0.02%, d) Recovery from exposure to H2 = 0.02%, e) Summary of response times as a function of [H2] for three nc15-Pd nanowires, and f) Summary of recovery times as a function of [H2] for the same three nanowires.
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