Al-doped ZnO for highly sensitive CO gas sensors

Al-doped ZnO (AZO) nanoparticles have been prepared using a modified sol–gel technique. The as-prepared AZO nanoparticles were annealed at 400◦C, and their morphologies and microstructural characteristics were investigated using transmission electron microscopy (TEM) and x-ray powder diffraction (XRD) analyses. Crystallites with an average size of approximately 60–70 nm and ZnO as a primary phase were observed in all samples. In addition, smaller nanoparticles (less than 5 nm) with an Al-rich structure covering the surface of larger ZnO crystallites were also noted on the Al-doped samples. Chemoresistive devices consisting of a thick layer of AZO nanoparticles on interdigitated alumina substrates have been fabricated, and their electrical and sensing characteristics for carbon monoxide were investigated. Al-doping provided a remarkable decrease in the electrical resistance of the sensing layer at the working temperature of the sensors (250–300°C). The sensors based on Al-doped ZnO exhibited a higher response than the pure ZnO sample, allowing the detection of CO at sub-ppm concentrations in air. The enhancement in sensing properties was discussed in terms of the characterization and electrical data.