Preparation of Mn₃O₄ nanoparticles on Al-substrate for degradation of Rhodamine B under solar light

The semiconducting properties of Mn₃O₄ thin films were assessed for the heterogeneous photocatalytic oxidation of Rhodamine B (Rh B). The films were grown on Al-substrate using a spray coating, and their physical and photoelectrochemical characteristics were studied. X-ray diffraction analysis showed a well-crystallized structure of the spinel Mn₃O₄. The atomic force microscopy revealed dense and irregular grains with a mean size of ~ 50 nm and a thickness of ~ 1 µm. The direct optical transition at 2.19 eV, in agreement with the brown color, is assigned to d–d transitions within the incomplete Mn-3d shell, which enables efficient photocatalytic conversion of the solar radiation. The photoelectrochemical measurements demonstrated p-type comportment, evidenced by a negative slope in the [capacitance⁻²–potential: C⁻²–E] graph with a flat band potential of + 0.35 V_SCE and a hole density of 6.5 × 10¹⁸ cm⁻³. The electrochemical impedance spectroscopy (EIS) plot showed a semicircle due to a capacitive comportment of the bulk spinel (R_b = 70 kΩ cm²). The center positioned below the abscissa axis (− 23°) with a constant phase element (CPE). The conduction band (− 1.64 V_SCE) is more cathodic than the \({\text{O}}_{{2}} /{\text{O}}_{{2}}^{ \bullet - }\) ${\text{O}}_2/{\text{O}}_2^{\bullet -}$ level, thus catalyzing the photocatalytic process by \({\text{O}}_{{2}}^{ \bullet - }\) ${\text{O}}_2^{\bullet -}$ radicals. Under sunlight, 50% reduction of the Rh B concentration (10 ppm) was achieved within 1 h, the color removal, followed by spectrophotometry obeys a pseudo-first-order kinetic model with a rate constant of 9.9 × 10^–3 min⁻¹ (t_1/2 = 70 min).