Influence of Relative Humidity on CO <sub>2</sub> Interaction for Sn <sub>1-<i>x</i></sub>Gd <sub><i>x</i></sub>O <sub>(4-<i>x</i>)/2</sub>

Sn1-xGdxO(4-x)/2 is prepared by a wet chemical co-precipitation method. Doping concentrations range from 1% to 20% to determine the solubility limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Rietveld analysis highlights the presence of Gd2O3 as weak crystallization secondary phase even for 5 at. % Gd and analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The associated chemical maps reveal compositional inhomogeneities. The electrical measurements show the resistance behaviour when Gd doping level and CO2 concentration vary, highlighting the opposite sensing behaviour of SnO2 and Gd2O3 in the presence of moisture and their compensating effect at the heterojunction interface between SnO2 and Gd2O3 nanocomposite for 5 to 20%Gd doped SnO2 samples. The proposed CO2 detection mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements which allow decoupling the ionosorption processes from the dipolar interaction under in-field conditions, for SnO2 and Gd2O3 samples. The applicative aspects are completed by the selective sensitivity evaluation against potentially interfering gases.