Ambivalent effect of Ni loading on gas sensing performance in SnO ₂ based gas sensor

The gas sensing characteristics of pure and 0.4-2.0 at% Ni-loaded SnO ₂ nanoparticles have been measured in dry and humid atmospheres. Approximately the same response to 50 ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. have been accomplished by loading 1.0 and 2.0 at% Ni to SnO₂. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics has been elucidated by diffuse-reflectance Fourier transform IR spectroscopy. The work function values determined from the transient of sensor resistance and contact potential difference revealed that Ni loading to SnO₂ determines the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N₂ atmosphere (>0.5 eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened.