Physical and electrochemical properties of the spinel ZnBi2O4 prepared by citrate route: application towards photo degradation of methyl violet

The spinel ZnBi2O4 , prepared by citrate route, was characterized physically and photo-electrochemically. The X-ray diffraction revealed a single-phase crystallizing in a tetragonal symmetry with lattice constants: a = 7.728 (3) angstrom and c = 5.640 (6) angstrom and a crystallite size of 72 nm. A forbidden band of 2.87 eV was determined from the diffuse reflectance data, assigned to O2-: 2p-Bi3+ : 6p transition. The electronic conduction occurs by low lattice polaron hopping with n-type comportment, evidenced by the negative thermo-power (S300 K-= - 45 mu V K-1). The current density-potential J(E) characteristic, plotted in Na2SO4 solution shows an irreversible electrochemical system. The capacitance (C-2) measurement against potential (E) indicates a conduction band (4.18 eV/0.57 V-SCE), made up of Bi3+ : 6p orbital, more cathodic than the O-2/O-2(-) level (- 0.3 V-SCE) at pH similar to 7 while the valence band deriving form O2-: 2p character (+ 2.57 V-SCE) is more anodic than the OH/H2O couple. Therefore, the dissolved oxygen is spontaneously reduced into superoxide O(2)(- )with concomitant formation of OH by the photoholes. Both radicals are responsible of the oxidation of Methyl Violet on the spinel upon visible irradiation. An abatement of 83% was reached within 270 min (3.42 x 10(18 )photons s(-1)). The discoloration kinetic follows a first-order model with a half-life of 145 min. The mineralization through the Total Organic Carbon (TOC) reached 3.5%. After consecutive tests, the results show a reusability of as-prepared material and a medium stability.