Fig. 9.

(a) SEM micrograph of hollow ZnO fibers after calcination at 500 °C. (b) The resistance response during cyclic exposure to increasing NO₂ concentrations at 350 °C of sensors comprising a network of nonaligned or quasi-aligned hollow ZnO fibers and a reference ZnO thin film sensor. The NO₂ gas concentration profile is shown on top. The inset shows a SEM micrograph of quasi-aligned fibers on an interdigitated alumina substrate.

(Reprinted with permission from³⁷. © 2009 American Chemical Society.)