ZnO-nanowire size effect induced ultra-high sensing response to ppb-level H2S

Presented is nano-size effect of ZnO nanowires (NWs) on sensitivity of hydrogen sulfide (H2S) detection in ambient atmosphere. The sensing mechanism of ZnO NWs for H2S detection is found coming from nano-size effect that governs both the sulfuration reaction when H2S is introduced and the following desulfuration reaction when the gas is removed from air. Based on the nano-material property of smaller dimension featuring higher chemical-activity at surface, the sensitivity of ZnO NWs to ultra-low concentration H2S is expected to be increased by shrinking the NW diameter. In our experiment, suspended micro hotplates for chemiresistive sensing are fabricated to investigate the nano-size sensing effect. Besides conventional ZnO NWs with the diameter as 50 nm, a novel tree-branched nano-structure of ZnO NWs (with the branch diameter as 20 nm) is in situ grown onto the sensing-area of the micro hotplate. In order to investigate the sensing effect, both the 20 nm NW sensor and the 50 nm NW sensor are used to experimentally detect H2S with concentrations in the range of ppb-ppm. The 20 nm ZnO-NWs sensor realizes resoluble sensing to 5 ppb H2S, while the 50 nm sensor can only response to H2S with concentration higher than 50 ppb. The well validated nano-size effect of thinner NWs featuring higher sensing response can be extended for various chemical sensing nano-materials.