Thickness-induced proton-conductivity transition in amorphous zirconium phosphate thin films

Amorphous zirconium phosphate thin films, a-ZrP_2.5O_x, revealed unique proton conductivity transition induced by reducing thickness due to the formation of highly conductive, hydrated nanolayer. The dense films made of a metaphosphate glass phase were uniformly formed over the electrode substrate by multiple spin-coating with a mixed precursor sol, as checked by TEM and RBS. When thickness d was larger than 60 nm, the proton conductivity σ across film and the activation energy E_a were not variable with d. σ abruptly increased 200 times and E_a decreases from 0.9 to 0.7 eV when d decreased from 60 to 40 nm, and it became thickness-independent again in d < 40 nm. σ of 100 nm-thick film is increased to the similar value as that of the 40 nm thick by annealing at 400 °C in H₂O/air. It was concluded that the conductivity transition could be associated with the hydration of metaphosphate nanolayer. The hydrated, high-conductive phase was very stable only when the thickness was less than 100 nm. Therefore, the films of more than hundreds nm thickness cannot change to the high-conducting hydrated phase throughout the film thickness. These unprecedented behaviors could not be explicable with a simple model based on the core space charge or continuum structural relaxation at hetrointerface.