Characterization of SrMoO₄ synthetized by soft chemistry: application to the degradation of Rhodamine B under UV light

The scheelite SrMoO_4, prepared by soft chemistry at room temperature, was studied physically and electrochemically to assess its photocatalytic performance for the oxidation of Rhodamine B (Rh B), a recalcitrant dye. The single-phase, identified by X-ray diffraction crystallizes in a tetragonal structure. A direct band gap of (4.26 eV) is determined from the diffuse reflectance, assigned to the charge transfer O²⁻: 2p → Mo⁶⁺: 4d; such large gap of the scheelite implies the existence of intermediate states in the gap region, involved in the photo-electrochemical process. The semi-logarithmic log (J–E) plot indicates a good electrochemical stability in Na₂SO₄ electrolyte with an exchange current density of 0.29 mA cm⁻² and a polarization resistance of 0.26 kΩ cm². The linear graph capacitance⁻²—potential (C⁻²–E) was positively slopped with n-type nature, characteristic of a non-degenerate semiconductor. A flat band potential of − 0.198 V_SCE and an electronic concentration (N_D) of 5.8 × 10¹⁶ cm⁻³ were extracted. The electrochemical impedance spectroscopy (EIS), measured over a wide range \({(10}^{-3}- {10}^{5 }\mathrm{Hz}),\) showed a predominance of the bulk contribution followed by a Warburg-type diffusion at low frequencies. A specific surface area of determined by the methylene blue method. The valence band deriving from \({\mathrm{O}}^{2-}\): 2p states is more anodic than the ^•OH/H₂O couple and the Rh B was successfully oxidized under UV light. The photocatalysis was performed in a closed system with an inner-irradiation UV lamp emitting mainly at 365 nm. The degradation has reached 90% after only 20 min at pH 4.32 and ambient temperature (25 °C) with a catalyst dose of 1 g/L. The degradation of Rh B (10 mg/L), followed by UV–visible spectrophotometer, obeys a first-order kinetic with a half photocatalytic life of 5 min. The Total Organic Carbon (TOC) reached 34% after 2 h. According to the inhibitors study, the holes (h⁺) are the first active species in the Rh B photo-oxidation, followed by ^•OH radicals.