Langmuir-Blodgett-assembled monolayer zinc ferrite nanoparticle film with unique photogenerated charge carrier separation efficiency and charge transfer behavior

Abstract

For figuring out the separation performance and transfer behavior of photogenerated charge carriers in porous spinel structured zinc ferrite (ZnFe2O4, ZFO) ultrathin films, novel ZFO nanoparticle films (LB-ZFO) were prepared on the fluorine-doped tin oxide conductive glass by the Langmuir-Blodgett (LB) method in this study. As illuminated by the light with the wavelength ranging from 320 nm to 440 nm, monolayer LB-ZFO film (LB-ZFO-1) showed relatively higher surface photovoltage than the 5-layered and 9-layered LB-ZFO films, which suggested the film thickness reduction facilitated the photogenerated charge carrier separation. Moreover, the LB-ZFO-1 film possessed lower flat band potential and larger space charge layer width in comparison with the 5-layered and 9-layered LB-ZFO films, further proving that stronger driving force was available for separating the photogenerated electrons and holes by reducing the film thickness. All the as-prepared LB-ZFO films exhibited the capacitor behavior because of their porous structure. The photoelectrocatalytic (PEC) activity of LB-ZFO-1 film was still low even though the electron transfer resistance at the LB-ZFO-1 film/electrolyte interface greatly decreased from 518.60 Ω to 92.56 Ω. Therefore, it suggests that the capacitor behavior existing in porous films is probably one of key factors that resulted in poor PEC activities of LB-ZFO films, which should be induced considerable attentions in future.