Photovoltaic and photocatalytic behaviour of a ferroelectric semiconductor, lead strontium zirconate titanate, with a polarization axis perpendicular to the surface

Abstract

The photovoltaic and photocatalytic behaviour of a poled ferroelectric semiconductor, Pb_0.95Sr_0.05Zr_0.53Ti_0.47O₃, having a polarization axis perpendicular to the surface has been studied. The ferroelectric properties of this compound were revealed by a polarization vs. voltage hysteresis curve and by changes in polarization as a function of temperature, whereas the light-absorption characteristics were monitored by surface photoconductivity and ultraviolet reflection spectra. The direction of the photocurrent generated by illumination from a Xe lamp was opposite to the direction of the polarization axis, and the photovoltage was ca. three times as large as the band-gap energy (anomalous photovoltaic effects). In the photodecomposition of pure water, the evolution of hydrogen was considerably higher on the surface toward which the polarization axis pointed [the positive (+) polar surface], whereas it was negligibly small for the negative (–) polar surface; the difference in activity was more than two orders of magnitude. The stability of this oxide during the reaction was evidenced by an absence of significant changes in the peak intensities of X-ray photoelectron spectra of the constituent surface atoms. When the photocatalysts underwent heat treatment at various temperatures in a He atmosphere, the activity of the (+) polar surface increased, passed through a maximum and then decreased above the Curie temperature, while the activity of the (–) polar surface increased monotonically. Following treatment above the Curie temperature, the activities between the oppositely polarized surfaces became similar, in accordance with the disappearance of the anomalous photovoltaic effect. The role of the direction of polarization in photocatalysis is discussed.