Effect of Polar Structure on Photocatalytic Properties of Oxide Powders and Films

Abstract

Semiconductor photocatalysis has gained increasing research interests in the application of solar fuel production and wastewater treatment. A key challenge for these applications is to reduce the photogenerated charge carrier recombination losses. Materials with polar structure such as the ferroelectrics are promising to address this challenge owing to the efficient separation of charge carriers by the internal electric filed present in ferroelectric materials. In this thesis, the composition and ferroelectric effect on the photocatalytic properties of BiFeO3 and BaxSr1-xTiO3 have been investigated. BiFeO3 thin films were synthesized and employed as photoelectrodes for photocatalytic studies. Fundamental studies have been carried out to understand the complex nature of BiFeO3 and the corresponding photocatalytic performance. A bias-dependent switchable photocurrent direction in BiFeO3 thin films was observed. The BiFeO3 photoelectrode exhibits either cathodic or anodic photocurrents depending on the applied bias, presenting both the characteristics of p and n type semiconductor photoelectrodes. This phenomenon was attributed to the specific bandgap structure of the as-prepared BiFeO3, in which the depletion region can arise following both upward and downward band bending at the electrode/electrolyte solution interface. Determination of the bandgap structure of BiFeO3 photoelectrodes from its photoelectrochemical (PEC) performance suggests strategies for probing semiconductor electronic structure by PEC method and provides fundamental insight on understanding of the switchable photocurrent direction in semiconductor photoelectrodes with Fermi level position not close to the conduction/valence band edges. Meanwhile, Bi2Fe4O9, one of the parasitic phases of BiFeO3, has been found to be a promising photoelectrode material with high visible light photocatalytic activity and good stability for the first time when conducting study of the compositional effects on bismuth ferrite. Phase pure Bi2Fe4O9 (Indirect Eg ~ 2.05 eV) thin films have been prepared by the chemical solution deposition (CSD) method. It shows a photocurrent of 0.1 mA/cm2 at 1.23 V vs NHE under simulated AM1.5G illumination and 0.05 mA/cm2 under visible light (λ > 420 nm). The addition of H2O2 as a hole scavenger increased the photocurrent to 0.25 mA/cm2 under full AM1.5G illumination, indicating hole injection is one limiting factor to the performance. Single-domain BaxSr1-xTiO3 powders were prepared to further demonstrate the effect of ferroelectricity free from the domain wall contribution. Two compositions of BaxSr1-xTiO3 powders, Ba0.2Sr0.8TiO3 and Ba0.8Sr0.2TiO3 were obtained by molten salt method and their composition, structure and photocatalytic activity have been investigated. Photodegradation of organic dye molecules has been carried out to evaluate the photocatalytic activity of the BaxSr1-xTiO3 powders. The results showed that Ba0.8Sr0.2TiO3 with polar structure has a degradation rate of almost twice of that of Ba0.2Sr0.8TiO3 with non-polar structure, demonstrating that the internal electric field resulting from the ferroelectric polarization can speed the photoexcited charge carrier separation and improve the photocatalytic efficiency. This research reveals the characteristic photocatalytic behaviors of the Bismuth- and Barium-related perovskite structure materials, which will benefit the future work on ferroelectric photocatalysts.