Barium titanate: photophysics, photocatalysis & the influence of the ferroelectric effect

Abstract

Photocatalytic and photoelectrochemical water splitting processes remain hindered by fast recombination of photogenerated electrons and holes. Ferroelectric materials are increasingly being considered to address this issue; their internal electric fields have been shown to spatially separate electrons and holes, and thus should greatly reduce recombination rates. A kinetic understanding of the extent to which electron–hole recombination can be slowed in ferroelectric materials is essential to ascertain if they can play a significant role in achieving higher solar-driven water splitting efficiencies. The focus of this thesis is an experimental investigation of charge carrier dynamics in barium titanate (BaTiO3) to observe the effect of internal electric fields on recombination rates. Time-resolved spectroscopic techniques were used in conjunction with photocatalysis studies to determine whether ferroelectricity can significantly reduce recombination rates and lead to enhanced performance. It is found that, although the transient absorption spectrum of ferroelectric BaTiO3 is similar to previously reported metal oxides, the carrier lifetimes are significantly longer, indicating the potential for ferroelectrics to be used in devices limited by fast electron–hole recombination. In the first results chapter, the transient absorption spectrum of single crystal BaTiO3 is characterised under inert atmosphere over two timescales: femtosecond–nanosecond and microsecond–second. Absorption signals due to photogenerated holes and electrons are identified using electron and hole scavengers, respectively. Comparisons are drawn between BaTiO3 and other single crystal, but non-ferroelectric, metal oxides. It is found that, on timescales relevant for water oxidation, lifetimes in BaTiO3 are at least an order of magnitude longer. In the second results chapter, the origin of long carrier lifetimes in ferroelectric BaTiO3 is explored. When the polarisation is switched off by both temperature and nanostructuring, carrier lifetimes decrease by four orders of magnitude. Recombination rates in BaTiO3 exhibit a much stronger temperature dependence than other metal oxides, which is rationalised by considering the temperature dependence of the spontaneous polarisation. The third results chapter investigates the photocatalytic performance of BaTiO3 nanopowders. It is found that, in the presence of an electron scavenger, BaTiO3 photogenerated holes are reactive and can oxidise water to produce oxygen. Transient and photoinduced absorption spectroscopies indicated that hole accumulation in a BaTiO3 sample with a higher tetragonal (ferroelectric) content, which translates to higher rates of oxygen evolution. The final results chapter probes the influence of a ferroelectric BaTiO3 substrate on α-Fe2O3 thin films. Preliminary data suggests the internal field can penetrate through the film and slow electron–hole recombination rates in α-Fe2O3.