We review the synthesis, characterization, and applications of one-dimensional palladium-based nanostructures and provide perspectives on future directions in this field.
We present a unique three-dimensional palladium (Pd)-decorated crumpled reduced graphene oxide ball (Pd-CGB) nanocomposite for hydrogen (H 2 ) detection in air at room temperature. Pd-CGB nanocomposites were synthesized using a rapid continuous flame aerosol technique. Graphene oxide reduction and metal decoration occurred simultaneously in a high-temperature reducing jet (HTRJ) process to produce Pd nanoparticles that were below 5 nm in average size and uniformly dispersed in the crumpled graphene structure. The sensors made from these nanocomposites were sensitive over a wide range of H 2 concentrations (0.0025−2%) with response value, response time, and recovery time of 14.8%, 73 s, and 126 s, respectively, at 2% H 2 . Moreover, they were sensitive to H 2 in both dry and humid conditions. The sensors were stable and recoverable after 20 cycles at 2% H 2 with no degradation associated with volume expansion of Pd. Unlike two-step methods for fabricating Pd-decorated graphene sensors, the HTRJ process enables single-step formation of Pd-and other metal-decorated graphene nanocomposites with great potential for creating various gas sensors by simple drop-casting onto low-cost electrodes.
Palladium has long been explored for use in gas sensors because of its excellent catalytic properties and its unique property of forming hydrides in the presence of H 2 . However, pure Pd-based sensors usually suffer from low response and a relatively high limit of detection. Palladium nanosheets (PdNS) are of particular interest for gas sensing applications due to their high surface area and excellent electrical conductivity. Here, we demonstrate the design and fabrication of low-cost PdNS-based dual gas sensors for room-temperature detection of H 2 and CO over a wide concentration range. We fabricated sensors using multiwalled carbon nanotube@PdNS (MWCNT@PdNS) composites and compared their performance against pure PdNS devices for hydrogen sensing based on electrical resistive response. Devices using PdNS alone had a response and response time of 0.4% and 50 s, respectively, to 1% H 2 in air. MWCNT@PdNS (1:5 mass ratio) showed enhanced performance at a lower hydrogen concentration with a limit of detection (LOD H 2 ) of 5 ppm. Nearly an order of magnitude increase in response was observed on increasing the amount of MWCNT to 50 mass % in the nanocomposite, but the response fell off at low H 2 concentration. Overall, these PdNS-based sensors were found to show good repeatability, stability, and performance under humid conditions. Their response was selective for H 2 versus CH 4 , CO 2 , and NH 3 ; the response to CO was comparable in magnitude but opposite in sign to the response to H 2 . Upon simultaneous exposure to equal concentrations (10 ppm each) of H 2 and CO, the response to CO was dominant. The PdNS showed high sensitivity to CO, detecting as little as 1 ppm CO in air at room temperature. The sensitivity to CO could be used either in a stand-alone room-temperature CO detector, where H 2 is known not to be present, or in combination with CO and combustible gas detectors to distinguish H 2 from other combustible gases.
Rapid and sensitive H2 detection is important because
of its low threshold for the formation of explosive mixtures in air
(∼4%). Palladium’s unique interactions with H2 make it particularly useful in room-temperature H2 sensing,
but the formation of water by reaction with O2 at the Pd
surface can interfere with sensor response. Here, we report H2 sensors using networks of high aspect ratio and ultrathin
reduced graphene oxide (rGO)-coated palladium nanowires (Pd NWs@rGO)
with a coating of zeolite imidazole framework (ZIF-8) that serves
as a nanofiltration layer. We first produced Pd NWs in high yield
by a hydrothermal method, then sonicated them with GO and added a
reducing agent to produce Pd NWs@rGO. Thin Pd NWs promote rapid response
and high sensitivity, while rGO prevents the formation of additional
conductive channels due to expansion of Pd NWs upon H2 exposure,
ensuring monotonic sensor response. The coating of ZIF-8 reduces the
transport of molecules like oxygen and nitrogen to the Pd NWs while
allowing H2 to reach them and H2O to diffuse
away from them. The optimized sensors showed a response (relative
change in resistance) to 1% H2 in air of up to 2%, with
a response time of 5 s and a lower limit of detection of 20 ppm. Relative
to previously reported Pd-based H2 sensors with fast response,
the Pd NWs@rGO@ZIF-8 nanocomposite-based device provides a low-cost,
high-performance sensor solution for fuel-cell vehicles and similar
applications that require both rapid and sensitive H2 detection.
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