The discovery opens the door to the development of a new generation of gas sensors for use by sabotage units, law enforcement agencies, security organizations, as well as in a variety of industrial systems
A new study from the Rensselaer Polytechnic Institute demonstrates how graphene foam can outperform leading commercial gas sensors on the market today in detecting dangerous chemicals or bombs. The discovery opens the door to the development of a new generation of gas sensors for use by sabotage units, law enforcement agencies, security organizations, as well as in a variety of industrial systems.
The new sensor is able to repeatedly and successfully measure levels of ammonia and nitrogen dioxide in extremely low concentrations on a scale of 20 parts per million (ppm). Being composed of continuous graphene nanosheets that grow into a foam-like structure the size of a postage stamp and the thickness of fabric, the innovative sensor is flexible, lumpy and finally overcomes the shortcomings that prevented gas sensors based on nanostructures from reaching the market. The research findings were published in the scientific journal Scientific Reports.
"We are very excited about this discovery, and believe it could lead to the development of new commercial gas sensors," said engineering professor Nikhil Koratkar, one of the principal investigators. "So far, the sensors have demonstrated higher sensitivity in detecting ammonia and nitrogen dioxide under room temperature conditions than gas sensors currently on the market."
During the last decade, many researchers have shown that individual nanostructures are particularly sensitive to various chemicals and gases. However, the development and operation of a gas detection device consisting of individual nanostructures proved to be too complex and expensive and unreliable for commercial use, the researcher notes. Such an effort involves building and adjusting the position of each individual nanostructure, locating it with a microscope, using lithography to incorporate gold contacts, and then applying other expensive and slow steps. When embedded in a mobile device, such a single nanostructure may be easily damaged and rendered inactive. In addition, the removal of the trapped gas from the nanostructure could have been challenging. The newly developed structure includes all the positive properties of a single nanostructure, but is easier to work with thanks to its macroscopic size.
The researchers started by growing graphene through a structure of nickel foam. After removing the nickel foam, what remains is a hollow network of foam-like graphene. The walls of the sensor based on the foam-like graphene consist of continuous graphene sheets without any breaks or physical interruptions between them. The team of researchers decided to use this graphene foam structure as a gas detector. As a result of exposing this graphene foam to air polluted by ammonia or nitrogen dioxide, the researchers found that the gas particles stick, or are absorbed, into the foam structure. The change in the surface chemistry has a significant effect on the electrical resistance of the graphene. Measuring this change in resistance is the mechanism on which the sensor's ability to detect different gases is based. In addition, the gas sensor based on graphene is easy to clean - by applying a current of 100 milliamps through the graphene structure, the researchers were able to heat the graphene foam enough to drive away, or remove all the absorbed gas particles. This cleaning mechanism has no effect on the ability of the graphene foam to detect gases, that is - the detection process is completely reversible and the device based on this new technology will consume low energy - there will be no need for external heating devices to clean the foam - and it will be reusable thanks to this.
The researchers chose ammonia as the test gas in order to demonstrate the proof of concept for this detector. The substance ammonium nitrate is present in many explosives and is known to decompose releasing traces of ammonia. As a result, ammonia detectors are commonly used to detect the presence of explosives. Ammonia, which is also a poisonous gas, is also used in a variety of industrial and medical processes, where ammonia detectors are required to detect leaks.
The research findings showed that the new graphene foam structure detects ammonia at a concentration of 1000 parts per million within 10-5 minutes at room temperature and atmospheric pressure. The change accompanying the electrical resistance in graphene was at the level of 30 percent. This is compared to commercially available sensors composed of conductive polymers, which undergo a 30 percent resistance change within 10-5 minutes when exposed to a level of 10000 parts per million of ammonia - that is, in the same duration and resistance change, the graphene detector is ten times more sensitive. The sensitivity of the graphene detector is effective up to a level of 20 parts per million - a value much lower than those existing in contemporary devices. In addition, many of the commercially available devices consume a lot of energy since they exhibit appropriate sensitivity only at high temperatures, while the graphene detector operates at room temperature.
The team of researchers used the nitrogen dioxide compound as a second test gas - various explosives, including nitrocellulose, slowly decompose while emitting the nitrogen dioxide gas as a by-product. As a result, this compound is also used as a marker for detecting explosives. In addition, this gas is a profit pollutant emitted from the combustion chambers of many vehicles and many different environmental monitoring systems locate the concentrations of this gas in real time. The new graphene sensor detects nitrogen dioxide at a level of 100 parts per million with a 10 percent change in resistance within 10-5 minutes at room temperature and atmospheric pressure. This sensor was 10 times more sensitive than existing sensors based on conductive polymer, which detect nitrogen dioxide concentrations at the level of 1000 parts per million under the same conditions. The graphene sensor is sensitive down to 20 parts per million at room temperature.
"We regard this device as the first practical gas sensor based on nanostructures suitable for commercialization," notes the lead researcher. The new sensor can be adjusted so that it can also detect other gases besides ammonia and nitrogen dioxide.
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