Facilely controlled synthesis of porous ZnO nanotubes with rich oxygen vacancies for highly sensitive and selective detection of NO2 at low temperature

In this work, biomorphic ZnO materials were prepared by simple and controllable zinc salt immersion plus air calcination method using waste willow catkins as biomass template, which present high sensing ability to trace NO2 at low energy consumption. The ZnO hollow nanotube calcined at 500 ℃ was assembled by cross-linkage of small-size nanoparticles with uniform mesoporous distribution and rich oxygen vacancies. The synergistic effect of these microstructure characteristics can not only dramatically promote the rapid gas diffusion on sensing layer, but also greatly increase active sites for surface adsorption and chemical reaction, thus enhancing gas-sensing performance. At low operating temperature of 92 °C, its fabricated sensor exhibits high response of 100.4–10 ppm NO2, which is separately 2.3, 5.6 and 12.1 times larger than those of ZnO hollow nanotube calcined at 600 ℃, thin nanorods (ZnO-TRs) and columnar nanorods (ZnO-CRs). And it is also higher than that of most reported 1-D nanostructured ZnO-based sensors. Meanwhile, this sensor also exhibits high selectivity and satisfactory response-recovery characteristics, as well as excellent humidity resistance and long-term stability. In addition, the enhanced sensing mechanism was also explored in great detail.