Semiconductor nanowires grown molecule by molecule at the University of Toronto may represent the best hope for faster, more sensitive and more robust CO2 sensors. Such devices would be crucial in future attempts to monitor CO2 capture and storage operations.
Since the mid-1990s, researchers in the lab of Harry Ruda at U of T’s Department of Materials Science and Engineering have been refining techniques to make semiconducting wires only 20 to 30 nanometres wide and tens of micrometres long. One tool in their arsenal is molecular beam epitaxy, in which a thin stream of precursor molecules containing semiconducting materials — often indium arsenide or gallium arsenide — is directed at a metal catalyst which controls crystal growth. “Because of the size of these wires, you have quantization of electron states which you wouldn’t have in a traditional electron device,” says Ruda.
Traditional ceramic-based sensors only register a signal after the amount of CO2 that is bound is enough to change a bulk property such as resistance. By contrast, the unique quantum electrical properties of the nanowires mean it should be feasible to detect even a single molecule of CO2. That molecule can then be desorbed by applying a reverse potential. “Adsorption and desorption can be happening very quickly, because not much electrical work is required to do it,” says Ruda. This means the sensors will work faster and won’t be overcome by high CO2 concentrations.
Ruda says the only potential problem is that particulates like soot could short out the circuit; like all sensors, these will require front-end filters. He’s working with David Risk of St. Francis Xavier University, an expert in sensor automation, to design a suitable filtration system to minimize this problem. The team has received a $350,000 grant from Carbon Management Canada to create a prototype, which Ruda hopes will be ready within two years.