□ A research group under Prof. Il-Doo Kim in the department of Materials Science and Engineering at Korea Advanced Institute of Science and Technology (KAIST) has developed an ultra-fast hydrogen gas detection system based on palladium (Pd) nanowire array coated with metal-organic framework (MOF) through the international collaboration with Prof. Reginald M. Penner in the department of Chemistry at University of California-Irvine. With this technology, 1 vol% of hydrogen gas can be rapidly detected in just 7 seconds.

□ Hydrogen has been regarded as an eco-friendly next-generation energy source, but it is a flammable gas that can cause an explosion even in small sparks. Since the lower explosion limit for hydrogen gas is 4 vol%, for safety, the sensors should detect the colorless and odorless hydrogen molecule quickly. The importance of sensors capable of rapidly detecting colorless and odorless hydrogen gas has been emphasized in recent guidelines issued by the U.S. Department of Energy that dictate that hydrogen sensors should detect 1 vol% of hydrogen in air in less than 60 seconds for response and recovery time.

□ Since the resistance change by the reaction of Pd with hydrogen gas is known in the 1960s, enormous efforts have been made to develop a Pd-based resistor-type hydrogen gas sensor. However, due to the influence of disturbing gases such as oxygen in the air, the performance of Pd based hydrogen sensors has not reached the level of commercialization.

□ A research team of Prof. Il-Doo Kim and Prof. Reginald M. Penner has developed an ultra-sensitive and ultra-fast hydrogen sensor that can detect hundreds of part per million levels of hydrogen gas within 60 seconds at room temperature.

□ To overcome the limitations of Pd based hydrogen sensors, the research team introduced a MOF layer on top of a Pd nanowire array. Lithographically patterned Pd nanowires were simply overcoated with a Zn based zeolite imidazole framework (ZIF-8) layer composed of Zn ions and organic ligands. ZIF-8 film is easily coated on Pd nanowires by simple dipping (for 2–6 hours) in methanol solution including Zn(NO₃)₂·6H₂O and 2-methylimidazole.

□ As the synthesized ZIF-8 is a highly porous material composed of a number of micropores of 0.34 nm and 1.16 nm, hydrogen gas with a kinetic diameter of 0.289 nm can easily penetrate inside the ZIF-8 membrane, while large molecules (> 0.34 nm) are effectively screened by MOF filter. Thus, the ZIF-8 filter on the Pd nanowires allows the predominant penetration of hydrogen molecules, leading to the acceleration of Pd based H₂ sensors with 20-fold faster recovery and response speed compared to pristine Pd nanowires at room temperature.

□ Prof. Il-Doo Kim expects that the developed ultra-fast hydrogen sensor can be useful for prevention of explosion accident by leakage of hydrogen gas. In addition, he expects that other harmful gas species in the air can be accurately detected through effective nanofiltration by using of a variety of MOF layers.

□ This study was carried out by Mr. Won-Tae Koo (first author), Prof. Il-Doo Kim (co-corresponding author), and Prof. Reginald M. Penner (co-corresponding author) and published in the online edition of ACS Nano that is a prestigious journal in the field of nanomaterials. In addition, this work was selected for cover-featured image for September issue of ACS Nano. End.

This cover image depicts a lithographically patterned Pd nanowires overcoated with a Zn based zeolite imidazole framework (ZIF-8) layer. This ZIF-8 layer functions as a nanofilter to eliminate access of impurity gas species to the Pd nanowire sensor elements while allowing the predominant penetration of hydrogen molecules, leading to the acceleration of Pd based H₂ sensors with 20-fold faster recovery and response speed compared to pristine Pd nanowires at room temperature.

Figure 1. Representative image for this paper published in ACS Nano, September, XX, 2017. 

Figure 2. Images of Pd nanowire array based hydrogen sensors, scanning electron microscopy image of a Pd nanowire covered by a metal-organic framework layer, and hydrogen sensing properties of the sensors.