Improved selectivity and low concentration hydrogen gas sensor application of Pd sensitized heterojunction n-ZnO/p-NiO nanostructures

Heterojunction based gas sensor was fabricated by forming two semiconductor nanostructures interface i.e. n-type ZnO nanorods and p-type NiO nanoplates. These nanostructures were synthesized via chemical routes. Palladium (Pd) nanoparticles (NPs) were synthesized and sensitized on the surface of heterojunction sensor material to enhance the gas response. Sensor materials were characterized using XRD, TEM, FESEM, EDS, Elemental mapping, and XPS techniques for their physicochemical properties. The n-ZnO/p-NiO heterojunction sensors with and without Pd NPs sensitization were investigated for low hydrogen gas concentration 2–100 ppm. Pd NPs sensitized n-ZnO/p-NiO heterojunction sensor showed higher response of 72% towards 100 ppm hydrogen (H2) gas concentration at the operating temperature 225 °C, whereas 53% response was noted by p-NiO/n-ZnO heterojunction sensor for 100 ppm concentration at 237 °C operating temperature. Both the sensors were investigated for selectivity studies using methane (CH4), carbon monoxide (CO), nitrogen dioxide (NO2), carbon dioxide (CO2) and H2 gases. Higher selectivity was observed towards H2 gas for both the sensors. The gas responses of both sensors were investigated at various operating temperatures and concentrations. Transient response, repeatability, and stability were also confirmed for both sensors. The gas sensing mechanism for heterojunction sensors was elucidated.