Revealing the role of ferroelectric size effect on photocatalyst activity

The relationship between the photocatalytic activity of ferroelectric materials and their grain size is not clear because of the ferroelectric size effect. Here, we choose layered ferroelectric oxides Bi3TiNbO9 (BTNO) with different grain sizes prepared by hydrothermal (BTNO-H) and molten salt (BTNO-M) methods as the study models, and tried to reveal the influence process of grain size variation on the catalytic activity. The optimal hydrogen and oxygen evolution rates of BNTO-M with larger grain size are 47.1 and 3.7 times higher than those of BTNO-H2, respectively, although the latter has a larger specific surface area. Further characterization results show that the two samples are basically the same in terms of exposed facets, light absorption and surface defect states, while there are significant differences in the internal polarization fields (IPF). This result suggests that the polarization field within BTNO overtakes the specific surface area as the dominant factor in determining the photocatalytic activity in the range of ferroelectric size effect. More interestingly, the effect of the IPF on the catalytic activity is characterized by a divergence, with a severe reduction of the IPF in the c-direction leading to an ultra-low hydrogen evolution activity on the {0 0 1} exposed facet of BTNO-H nonosheets.