Abstract: A good humidity-sensitive Li⁺-modified and Ca²⁺-doped PbTiO₃ nano-film (Li⁺-Ca_xPb_1-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. H₂O 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.

Key words: Li⁺-modified and Ca²⁺-doped PbTiO₃(Li-CPT), humidity sensitive nano-film, AFM, microstructure