A new type of hydrogen sensor presents an effective method for detecting hydrogen leaks in real-time, enhancing safety in industrial settings. Constructed with nano-patterned cupric oxide (CuO) nanowires (NWs) that contain voids, this sensor can identify hydrogen even at very low concentrations, offering improved response time, recovery speed, and accuracy compared to earlier CuO sensors. This innovation could significantly contribute to the safer and more dependable utilization of hydrogen in clean energy initiatives.
As we move toward more sustainable energy sources, hydrogen is gaining traction. It can be combusted like conventional fuels, generating only water as a byproduct, and can create electricity via fuel cells. However, with the rising production, use, and transport of hydrogen, safety issues arise. Hydrogen is extremely flammable at concentrations as little as 4% and is both odorless and colorless, which complicates leak detection.
To tackle these challenges, a team of researchers led by Professor Yutaka Majima from the Institute of Science Tokyo (Science Tokyo) has created a sensor capable of detecting hydrogen at very low levels with a rapid response time. Their findings were published in the journal Advanced Functional Materials on November 5, 2024.
The sensor consists of nano-patterned polycrystalline CuO nanowires that are particularly sensitive to hydrogen gas, situated on a silicon (SiO2/Si) platform featuring platinum/titanium electrodes. “We utilized electron-beam lithography and a two-step ex-situ oxidation process to establish a reliable and reproducible method for creating high-performance, nano-patterned CuO nanowire-nanogap hydrogen sensors with voids, which significantly differs from conventional free-standing single-crystal CuO nanowires that are directly grown from copper sources,” states Prof. Majima.
When the sensor interacts with hydrogen gas, it detects variations in the electrical resistance of the CuO nanowires. In the presence of air, oxygen molecules attach to the CuO NWs’ surface, forming oxygen ions (O2–, O–, O22-) that create a layer of positive charge carriers (holes) on the surface. The introduction of hydrogen causes a reaction with these oxygen ions, leading to the formation of water, which reduces the concentration of holes. Consequently, the nanowires become less conductive, allowing the sensor to identify the hydrogen’s presence and concentration by measuring the increase in resistance.
The researchers improved the sensor’s effectiveness by incorporating a pre-annealing stage in a hydrogen-rich environment, followed by a gradual oxidation in dry air. Initially, the manufactured copper (Cu) nanowires lack optimal crystallinity and form a layer of Cu oxide on their surface, which limits interaction with oxygen. The annealing process reshapes the Cu nanowires from a rectangular to a semicircular arch shape, enhancing their crystallinity. In the later oxidation phase, the Cu nanowires transform into copper oxide as copper atoms migrate outward to react with oxygen, resulting in voids that increase the surface area and provide more active sites for interaction between hydrogen and oxygen.
Thanks to these enhancements, the sensor can now identify hydrogen concentrations as low as 5 parts per billion (ppb), a far cry from previous CuO H2 sensors. Furthermore, it exhibits resistance to humidity, a common limitation in CuO gas sensors, and can swiftly detect hydrogen within just 7 seconds.
The researchers further optimized the sensor’s performance by minimizing the nanogap between the electrodes. A smaller gap creates a stronger electric field, which accelerates the movement of charge carriers, thereby enhancing the speed of the sensor’s response and recovery. With a gap measurement of 33 nm, the sensor was able to detect 1,000 ppm of H2 in merely 5 seconds, returning to baseline conditions within 10 seconds. “We are committed to expanding our development to create a broader range of gas sensors using this technique for the detection of other hazardous gases,” adds Prof. Majima.
By facilitating early detection of leaks or hazardous gas levels, this sensor can help reduce risks, allowing for the secure implementation of hydrogen technologies and supporting the transition toward a hydrogen-based economy.
