Microstructure and Humidity-sensing Mechanism of Both Ca~(2+)-doped and Li~+-modified PbTiO_3 Nano-film

A good humidity-sensitive Li+-modified and Ca2+-doped PbTiO3 nano-film (Li+-CaxPb1-xTiO3, x=0.35, Li/Ti=1/100 mol/mol, sintered at 850℃/1h; abbreviated as Li-CPT) was prepared by the sol-spin-coating process using Ca(OAc)2, Pb(OAc)2, Li2CO3 and Ti(O-nBu)4 as starting materials. Li-CPT microstructure was studied by means of AFM. AFM observation of both morphology and linear surface roughness indicates marked reticulate grain boundaries, the linear surface roughness is within (+0.5μm)- (-1.2μm), and hence large surface area: The film surface shows the presence of polycrystalline grains with an average area of 5μm× 10μm. Such a microstructure is favorable for having a good humidity sensitivity. The crystal geometry and electron structure will change greatly after the equate-valence substitution of Ca2+ with smaller Pauling ion radius for Pb2+. The substitution decreases the lattice distortion (c/a), the non-symmetry and film cracks. There is a probable mechanism for electrical resistance-humidity sensitivity. H2 O molecules polarized and adsorbed on the Li-CPT film surface, due to the doping of Li+ ions with smaller radius, dissociate into H+ and OH-, H+ ions combine with lattice oxygen, OH- ions fill up the oxygen hole (defect site) and release free electrons which participate in electric conduction in the external electric field; hence electrical resistance will decrease with the increase of relative humidity.