Gas nanosensor design packages based on tungsten oxide: mesocages, hollow spheres, and nanowires

Achieving proper designs of nanosensors for highly sensitive and selective detection of toxic environmental gases is one of the crucial issues in the field of gas sensor technology, because such designs can lead to the enhancement of gas sensor performance and expansion of their applications. Different geometrical designs of porous tungsten oxide nanostructures, including the mesocages, hollow spheres and nanowires, are synthesized for toxic gas sensor applications. Nanosensor designs with small crystalline size, large specific surface area, and superior physical characteristics enable the highly sensitive and selective detection of low concentration (ppm levels), highly toxic NO₂ among CO, as well as volatile organic compound gases, such as acetone, benzene, and ethanol. The experimental results showed that the sensor response was not only dependent on the specific surface area, but also on the geometries and crystal size of materials. Among the designed nanosensors, the nanowires showed the highest sensitivity, followed by the mesocages and hollow spheres—despite the fact that mesocages had the largest specific surface area of 80.9 m² g ^{− 1}, followed by nanowires (69.4 m² g ^{− 1}), and hollow spheres (6.5 m² g ^{− 1}). The nanowire sensors had a moderate specific surface area (69.4 m² g ^{− 1}) but they exhibited the highest sensitivity because of their small diameter (∼5 nm), which approximates the Debye length of WO₃. This led to the depletion of the entire volume of the nanowires upon exposure to NO₂, resulting in an enormous increase in sensor resistance.