Effect of boundary layer thickness on ammonia gas sensing of Cr₂O₃-decorated ZnO multipods

New insights into controlling boundary layer thickness are investigated in growth of Cr₂O₃-decorated ZnO multipods on a tilted silicon substrate. In the first step, ZnO multipods were grown on a tilted silicon surface using a vapor transport method in a quartz tube. Next step, low concentration of Cr₂O₃ nanoparticles was decorated on the ZnO multipods using a PVD machine. Both arm diameter and density of the Cr₂O₃-decorated ZnO multipods were found to be strongly dependent on the boundary layer thickness. The sensing properties of the samples measured by half bridge method indicated that decoration of the Cr₂O₃ nanoparticles on the ZnO multipods significantly enhanced the response to 100 ppm NH₃ up to 85, CO₂ up to 29 and NO₂ up to 27. The response/recovery times were 25 s/48 s at 300 °C. Blue defect emission appeared in room temperature photoluminescence spectra of the sample grown in a thin boundary layer thickness.