Relaxor ferroelectric and photo-electrochemical properties of lead-free Ba_(1-x)Eu_(2x/3)(Ti_(0.75)Zr_(0.25))O_3 ceramics. Application to chromate reduction

摘要

The perovskite Ba_(1-x)Eu_(2x/3)(Ti_(0.75)Zr_(0.25))O_3 (x = 0.025, 0.05) have been prepared by solid state reaction. A dielectric study on ceramics performed in the ranges of temperature (77-500 K) and frequency (10~2-2 × 10~5 Hz) reveal typical relaxor behavior and diffuse phase transition. The relaxor parameters (Tm, ΔTm, Δε'r/ε'r) decrease slightly with the increase of Eu-dopant concentration. The dielectric relaxation rate follows the Vogel-Fulcher relation with activation energy of 21 meV, an attempt frequency fo = 1.1 × 10~8 Hz and a static freezing temperature Tf = 95K for x = 0.025. An optical gap of 2.25 eV is obtained from the diffuse reflectance spectrum. The photo-electrochemistry throws light on the nature of the electronic bands. The Mott-Schottky plot is characteristic of n-type conduction from which a flat band potential of -0.60 V_(SCE) and an electrons density of 1.2 × 10~(18)cm~(-3) are determined for the composition x = 0.025. The upper valence band located at 6.4 eV below vacuum is made up of O~(2-): 2p orbital whereas the conduction band consists of empty 3d levels (4.13 eV) with a high reducing ability. The energetic diagram clearly assesses the photoactivity of the perovskite. The performance is improved when the crystallite size becomes comparable with the length of the nanodomains. As application, 95% of chromate (10~(-4) M) is reduced after 6h of exposition to sunlight (AMI).