Hierarchical NiO Cube/Nitrogen-Doped Reduced Graphene Oxide Composite with Enhanced H 2 S Sensing Properties at Low Temperature.

A novel hierarchical NiO cube (hc-NiO)/nitrogen-doped reduced graphene oxide (N-rGO) composite is synthesized via a facile hydrothermal method and a postcalcination treatment without any templates and surfactants added. The NiO cubes assembled by abundant nanoparticles in situ grow on the surface of N-rGO layers. The combination of hc-NiO and N-rGO results in enhanced sensing properties with the contributions of the N-rGO providing high specific surface area and more efficient active sites for the adsorption of H2 S molecules and the hierarchically structured NiO cubes providing high sensitivity and distinctive selectivity to H2 S gas. At the optimal operating temperature of 92 °C, the hc-NiO/N-rGO composite based sensor shows not only high response to H2 S in a range of 0.1-100 ppm but also excellent selectivity for H2 S against the other seven gases. The gaseous product, produced from the contact of H2 S with the hc-NiO/N-rGO composite at 92 °C, is measured by GC-MS technique. The change of the surface composition and the chemical state of the hc-NiO/N-rGO composite before and after exposure to H2 S are investigated by XPS. The possible sensing mechanism of the hc-NiO/N-rGO composite is similar to that of semiconductor oxides. The H2 S molecules that absorbed on the sensor surface transform to SO2 by reacting with the adsorbed oxygen anions. Meanwhile, the electrons restricted by the surface-adsorbed oxygen return to the bulk and neutralize the holes, producing a change in resistance.