gated towards different gases, such as carbon monoxide (CO), NO~2~, ethanol, and H~2~ \[[@B73-sensors-20-00579]\]. High sensor responses were obtained from Zn‒In~2~O~3~ for CO, H~2,~ and ethanol respectively at a relatively lower operating temperature of 300 °C. In contrast, In~2~O~3~ nanowires showed lower sensor responses for the same gases. The superior performance of Zn‒In~2~O~3~ nanowires was attributed to the nanowire network sensors, where the large numbers of inter-nanowire junctions present along the conducting path. At each junction, a potential barrier exists which plays a dominant role in defining the resistance of the path. When reducing gases are brought into contact, the oxygen adsorbates are consumed which return the electrons back to the nanowire, leading to lower the overall channel resistance. In addition, the Zn incorporation into the In~2~O~3~ lattice creates more oxygen vacancies and increases the surface defects, which enhance the receptor function for reducing gases.

The doping of Co~3~O~4~ nanomaterials with fluorine (F) resulted in higher responses towards volatile organic compounds (VOCs) and lowered the working temperature (200 °C) compared to the fluorine-free systems \[[@B74-sensors-20-00579]\]. This behavior was attributed to the high fluorine electronegativity. The F centers attract the electron density from Co ones, enhancing, in turn, the Lewis acidity and further promoting the corresponding catalytic activity \[[@B76-sensors-20-00579]\]. The response towards the analyte gases at a lower temperature was also enhanced due to the fact that the F introduction prevents undesired free carrier annihilation in p-type Co~3~O~4~ increasing the concentration of h+ species. The introduction of niobium (Nb) into the structure of TiO~2~ nanotubes improved their response towards VOCs and CO at 200 °C \[[@B19-sensors-20-00579],[@B77-sensors-20-00579]\]. In this case, Ti atoms can be replaced by Nb in the lattice due to the similar ionic radii of Nb^5+^ and Ti^4+^ \[[@B6-sensors-20-00579