PHOTOCURRENT RESPONSE OF COMPOSITE PEROVSKITE OXIDE THIN FILMS WITH SPECIFIC SEMICONDUCTING AND FERROELECTRIC PROPERTIES

摘要

The goal of this study is to investigate photocatalytic semiconductor systems which are layered thin film composites built from perovskite oxide materials with characteristics such as small and large band gaps and/or ferroelectricity. In order to improve the efficiency of photocatalysis, semiconductor hetero-junctions within the developed composites have been designed to possess electronic band offsets favoring the separation of photo-induced electron and hole (e~-/h~+) pairs. Furthermore, the remanent polarization of the ferroelectric component within the composites has been utilized to induce favorable band bending at the material interface, lowering the potential barrier for electron transfer. The band offsets and ferroelectric polarization could be considered as built-in electric fields; how they interact with photo-induced e~-/h~+ would greatly affect the photocatalytic properties of the composites. In this study, various perovskite oxide thin film materials - large band gap strontium titanate (SrTiO_3), small band gap silver niobate (AgNbO_3) and ferroelectric lead lanthanum titanate (PLT) - were combined to form layered thin film composites. The composites were then adopted as photoanodes in a photoelectrochemical cell and detailed characterization of their photocurrent response was carried out under different light irradiation and ferroelectric polarization conditions. Electronic band offsets at the material interface (i.e., heterojunction) were determined by ultraviolet-visible spectrophotometry and ultraviolet photoelectron spectro-scopy. Electric field poling of the ferroelectric component was achieved by non-contact corona charging. Our results have shown that the band offsets at the SrTiO_3-AgNbO_3 heterojunction were about 1.0 eV in conduction band edge and 0.4 eV in valence band edge, promoting the rapid separation of photo-induced charge carriers; i.e., the flow of e~-from SrTiO_3 to AgNbO_3 and the flow of h~+ from AgNbO_3 to SrTiO_3. It was found that ferroelectric PLT could be used as a seeding layer for the low-temperature (500 °C) growth of SrTiO_3/AgNbO_3 thin film composites on ITO/glass substrates, forming a layered structure of SrTiO_3/AgNbO_3/PLT/ITO. In addition, the photocurrent density of the composites could be increased by depositing gold nanoparticles at the PLT-ITO interface. When the polarization of the PLT layer was poled toward the AgNbO_3 layer, the potential barrier associated with the flow of e~- to the ITO electrode was reduced by favorable band bending created at the AgNbO_3-PLT interface. This resulted in a significant increase in photocurrent density.