Optical Gas Sensing with Solution-Processed Nanoscale Plasmonic Super Absorbers

Although gas sensors based on metal oxides are well established, they traditionally require high operative temperatures and electrical contacts, limiting in situ analyses. Optical gas sensors on the other hand can allow noncontact measurements and can also rely on light absorption to provide additional driving force to promote the gas-sensing reaction at lower temperatures. Because conventional metal oxides used in sensing applications are mostly UV absorbing, for this to work, their absorption profile should be extended into the visible range. Coupling metal oxides with optically active plasmonic nanoparticles has been a very successful way to achieve this target. Here, we demonstrate the fabrication of highly absorbing nanostructures (peak absorption of 95%) by using a combination of Au and TiO2 layers through the precise control of their thickness and lateral distribution. We started from presynthesized colloidal nanocrystals for both materials and showed the deposition of smooth, high-quality TiO2 films and the fabrication of submonolayers of Au nanocrystals. We optimized these two materials to enable light coupling and enhanced absorption when combined with a metal mirror, obtaining outstanding broadband absorption across the visible and near-infrared range. We then tested their optical gas sensing performance for ammonia detection, achieving great sensitivity and transient times, and a limit of detection lower than 10 ppm. These tests were performed at 100 °C, showcasing the advantages of these nanoarchitectures for the low-temperature detection of hazardous gases. Our work provides the demonstration and clear guidelines for the fabrication of efficient gas sensors based on superabsorbing structures.