1 % lanthanide-doped ZnO nanostructures as a versatile approach for state-of-the-art capacitive and resistive humidity sensors

Abstract

The effect of the 1 % lanthanide (Ln) doping on the structural, morphological, and electrical properties of the zinc oxide (ZnO) nanostructures obtained by the electrospinning-calcination method is discussed in the context of humidity sensors. X-ray diffraction (XRD) investigations, in conjunction with high-resolution X-ray photoelectron spectroscopy (XPS) confirmed the formation of (La, Ce, Pr, Nd, Sm)-ZnO nanocomposites. Also, XPS analysis demonstrated that lanthanide doping enhances the surface hydroxylation, by increasing the concentration of hydroxyl groups, promoting the surface propensity to attract water molecules. The dependence of relative permittivity and electrical resistivity on the absence and various values of the humidity were examined in relation to the nature of the dopant. Our results reveal that the undoped zinc oxide does not exhibit variation in relative electric permittivity with frequency. By contrast, in the case of the samples containing lanthanide additives, a typical semiconductor behavior is observed, with a decrease in relative electric permittivity as the frequency increases. In particular, by Ce-doping, sensitivity coefficients are improved in comparison with the undoped ZnO or for other dopants, reporting a remarkable resistance coefficient of ∼83 % in the low humidity range (e.g. 33 % RH). Meanwhile, a reduction of the electrical resistivity by approximately one order of magnitude is reported upon lanthanide doping, and a decrease of the response time from 98 s up to 62 s is achieved. These remarkable features recommend lanthanides-ZnO nanocomposites as suitable materials for the design of humidity sensors with improved characteristics.