A great variety of MOX materials have been investigated for their sensitivity and selectivity to given VOCs, arising from adsorption site nature and adsorption energies dependences. These include n-type semiconducting MOXs, e.g., WO3 , ZnO , SnO2 , TiO2 , In2O3 [18]and also p-type: Co3O4 , CuO , Cr2O3 and NiO . To enhance their sensing performance (sensitivity, selectivity, stability, and response/recovery time), several methods have been developed, including: decoration with noble metal catalyst particles ; mixed-deposition of MOX nanoparticles ; morphology changes, including miniaturization ; high-aspectratio structures ; and use of hollow spheres .