Efficient oxidation by electro-photocatalysis of Rhodamine B using Ba2SnO4 as photo-electrode

Abstract

The present work describes the color removal of Rhodamine B (Rh B), a recalcitrant cationic dye by electrocatalysis and electro-photocatalysis on Ba₂SnO₄ as anode. The double perovskite Ba₂SnO₄ synthesized by nitrate route was characterized by physical and electrochemical methods, a preamble of Rh B oxidation. The single phase, confirmed by X-ray diffraction, crystallizes in a tetragonal (Space Group: I4/mmm), with spherical crystallites (~ 50 nm). The zeta-sizer analysis gives an average grains size of 0.65 µm and zeta-potential of − 20 mV. The SEM analysis revealed the porosity of the oxide and the Ba–O and Sn–O bonds were confirmed by the FT-IR analysis. The direct optical gap (3.18 eV), determined by diffuse reflectance, is assigned to the charge transfer O²⁻: 2p → Sn⁴⁺: 5 s, and the double perovskite possesses a chemical inertness in the entire pH region. The Mott–Schottky plot indicates n-type behavior with a flat band potential (E_fb) of − 0.84 V_SCE, due to O²⁻ deficiency and an electron concentration N_D of 1.14 \(\times\) 10¹⁷ cm⁻³. The electrochemical impedance spectroscopy (EIS), plotted at the free potential (+ 0.5 V), reveals the bulk and grain boundaries contributions. The low electrons mobility is assigned to a narrow conduction band of Sn⁴⁺: 5 s parentage with activation energy (18 meV) in conformity with a conduction mechanism by small lattice polaron hopping. The intensity-potential J(E) profile in NaCl (10^–2 M) exhibits a small hysteresis similar to a chemical diode. The semi-logarithmic plot (logJ – E) indicates a chemical stability of Ba₂SnO₄ in the working solution (NaCl). Ba₂SnO₄ is of interest for the environmental protection and as application, Rh B (20 mg L⁻¹) was successfully oxidized by electrocatalysis with an abatement of 66% under a direct current of 150 mA, which has a bactericidal effect. An enhancement up to 92% has been reached by electro-photocatalysis; a quasi-complete discoloration occurred within 70 min in the “Electric Current-Sunlight-Ba₂SnO₄”. The Rh B elimination follows a pseudo-first-order kinetic with a rate constant of 1.96 ± 0.17 × 10^–2 min⁻¹ (t_1/2 = 35 min) and a reaction mechanism is suggested.