Principle Scientists: Erich Walter and Dr. Fred Favier
E.C. Walter, F. Favier, R.M. Penner*, Palladium Mesowire Arrays for Fast Hydrogen Sensors And Hydrogen-Actuated Switches, Analytical Chemistry 74 (2002) 1546.[PDF]
F. Favier, E. Walter, M. Zach, T. Benter, and R.M. Penner*, Hydrogen Sensors and Switches From Electrodeposited Palladium Mesowire Arrays, Science 293 (2001) 2227.[PDF]
Arrays of mesoscopic palladium wires, prepared by electrodeposition, form the basis for hydrogen sensors and hydrogen-actuated switches that can exhibit a response time as fast as 20 ms. These devices were constructed by electrodepositing palladium mesowires on a highly oriented pyrolytic graphite surface, and then transferring these mesowires to a cyanoacrylate film supported on a glass slide.
Figure 1. Left. Schematic diagram of a palladium mesowire array (PMA) hydrogen sensor or switch. Right. SEM image of a PMA-based hydrogen sensor.
The application of silver contacts to the ends of 10 to 100 mesowires, arrayed electrically in parallel, produced sensors and switches that exhibited a high conductivity state in the presence of hydrogen, and a low conductivity state in the absence of hydrogen, as shown in Figure 2.
Figure 2. Left. Current response of a mode I sensor to hydrogen/nitrogen mixtures (concentration of H2 in % as shown). Data were acquired in random order of H2 concentration. Data were acquired in random order of H2 concentration. Right Current amplitude versus H2 concentration for typical mode 1 and mode 2 sensors.
After an initial exposure to hydrogen, 15 to 50 nanoscopic gaps are formed in each mesowire. These breaks can be seen in the SEM image of a functional sensor shown in Figure 3.
Figure 3. SEM image of part of a a PMA-based H2 sensor that functioned for several weeks. Individual break junctions in one palladium nanowire are indicated with yellow arrows in this image.
These nanoscopic gaps or " break junctions " close in the presence of hydrogen gas and reopen in its absence as hydrogen is reversibly occluded by the palladium grains in each wire, and the palladium lattice expands and contracts by several percent. AFM images of a single break junciton functioning in this manner are shown in Figure 4. The change in resistance for sensors and switches was related to the hydrogen concentration over a range from 1% to 10%.
Figure 4. Atomic force microscope images of a Pd mesowire on a graphite surface. These images were acquired either in air or in a stream of H2 gas as indicated. A hydrogen-actuated break junction is highlighted (circle).
