Electrochemical Study of CuFe₂O₄ Synthetized by Sol–Gel and Electro-photo-oxidation of Rhodamine B Under Sunlight

The spinel CuFe₂O₄ elaborated by sol–gel route crystallizes in a tetragonal structure with a crystallite size of 444 ± 2 nm and a zeta potential of − 35 mV. The diffuse reflectance spectroscopy and photo-electrochemistry were undertaken for its characterization. The direct gap (1.55 eV) ideal for the solar energy conversion is assigned to the transition \(: {Fe}_{oc}^{3+}:{t}_{2g}\to {Fe}_{oc}^{4+}\) $:{\mathit{Fe}}_{\mathit{oc}}^{3+}:t_{2g}\to {\mathit{Fe}}_{\mathit{oc}}^{4+}$ : \({e}_{g}\) $e_g$ in agreement with the red color, allowing more than half of the solar spectrum to be converted into chemical energy. The narrow valence band deriving from Fe³⁺: \({t}_{2g}\) $t_{2g}$ orbital induces a low electron mobility (µ = 8.91 × 10⁻¹³ cm² V⁻¹ s⁻¹). The cyclic voltammetry in Na₂SO₄ (10⁻² M) exhibits low hysteresis that resembles a chemical diode. The electrical conductivity of CuFe₂O₄ is a characteristic of a non-degenerate semiconductor with activation energy (E_a) of 0.20 eV where the electron transfer occurs by low lattice polaron hopping between mixed valences Fe⁴⁺/Fe³⁺ octahedrally coordinated. The semi-logarithmic plot (logJ–E) indicates a chemical stability of CuFe₂O₄, while the photo-chronoamperometry corroborates the p-type behavior, a result confirmed by the capacitance measurement where an electron density (N_A) of 0.176 × 10²³ cm⁻³ and a flat band potential (E_fb) equal to − 0.56 V_SCE were extracted. As application and on the basis of the potential diagram, Rhodamine B (Rh B, 20 mg L⁻¹), a cationic dye, is electrostatically attracted by the electrode surface and successfully oxidized by electrocatalysis on CuFe₂O₄. The kinetics of oxidation of Rh B followed by chemical oxygen demand (COD) analysis, which gave an abatement of 56% under a current of 150 mA, an enhancement up to 70%, was reached by electro-photocatalysis under sunlight smaller than that analyzed by UV–visible spectrophotometry (88%). The color removal follows a pseudo-first-order model with a half-life t_1/2 of 57 min; a reaction mechanism by O₂^•− and ^•OH radicals is suggested.