Tailoring The Photoelectrochemical Property In BiFeO₃ By Controlled Ferroelectric Polarization

Photoelectrochemical（PEC） or photocatalytic water splitting for hydrogen and oxygen generation has been studied for several decades. To date, the most commonly used photocatalysts in PEC systems are binary transition metal oxides, such as TiO₂ and Zn O. However, these materials can only be activated under UV light due to their large band gap. BiFeO₃（BFO） is a well-known multiferroic material, which exhibits robust ferroelectricity and relatively small band gap. Although several groups have grown various nanostructures of BFO and studied their PEC properties, it is difficult to control the ferroelectric polarization in powder and nanostructures and to study how the ferroelectric polarization affects PEC properties. Moreover, appropriate control of the polarization may significantly affect the PEC or photochemical performance of ferroelectric-based photoelectrode.Polycrystalline BFO films, due to their low cost and simple fabrication process, seem more suitable for practical applications. One key issue associated with BFO as a photoelectrode material is the rapid recombination of photo-generated electrons and holes, which has to be addressed to achieve better PEC performance. In this thesis, we carried out two parts research work:（1） Enhanced ferroelectric photoelectrochemical properties of polycrystalline BFO film by decorating with Ag nanoparticlesPolycrystalline BFO films are fabricated on Pt/Ti/SiO₂/Si（100） substrate as photoelectrode using sol-gel method. The microstructure, optical and PEC properties of the films are characterized and optimized by controlling the precursor element concentration, the annealed temperature, and the film thickness. Moreover, the PEC properties of the BFO films are dependent on ferroelectric polarization, which is mainly ascribed to the modulation of band structure at the BFO/electrolyte interface by the polarization. Further enhancement of PEC properties is obtained by decorating the samples with appropriate amounts of Ag nanoparticles. The PEC properties of BFO thin films can be further enhanced by decorating Ag NPs. The 15s-Ag/BFO photoelectrode has the maximum photocurrent density, which is almost twice that of the pure BFO photoelectrode. Such enhancement can be attributed the reduced electron-hole recombination, and localized surface plasmon resonance effect of Ag nanoparticles. This work was published in Applied Physics Letters, 2016, 108, 022902.（2） Fabrication of reduced graphene oxide/BFO polycrystalline film and its enhanced photoelectrochemical properties.BFO polycrystalline film is incorporated with reduced graphene oxide（RGO） using a facile single-step photocatalytic reaction to improve its photoresponse in visible light. Remarkable 5-fold enhancement in photocurrent density is observed on RGO/BFO composite compared with pure BFO under light illumination. This improvement is attributed to the photoresponse enhancement, and the longer electron lifetime of excited BFO as the electrons are injected to RGO instantly at the site of generation, leading to a minimized charge recombination.