The delafossite CuAlO2 was prepared by sol–gel method and X-ray diffraction of the oxide fired at 1100 °C is characteristic of a single phase, crystallizing in a rhombohedral structure (R \(\bar{{\mathbf{3}}}\) m) with a crystallite size of 58 (± 2 nm). A grain size of 0.55 μm and a zeta potential (− 8 mV) were determined by zetammetry. A direct optical gap of 1.83 eV, obtained from the diffuse reflectance, is due to a lifting of the degeneracy of Cu+: d-d in a linear crystal field. An additional transition at 3.69 eV is due to charge transfer (O2−: 2p → Cu+: 4s). CuAlO2 exhibits p-type conduction behavior with a lattice polaron hopping between Cu2+/+ states due to the intercalation of O2− between the reticular planes (0 0 n). The electrical conductivity varies linearly with the reciprocal temperature, indicating semiconductor character which obeys to an exponential law: σ = σo exp(− 0.26 eV/kT) over the range (300–500 K). The photoelectrochemical study was undertaken in Na2SO4 electrolyte (0.1 mol dm−3); a flat band of 0.20 VSCE and a hole concentration of 1.13 × 1018 cm−3 were determined from the capacitance-potential graph. Electrochemical Impedance Spectroscopy (EIS) shows a semicircle centered on the real axis, indicating pure capacitive behavior. The inclined line at low frequencies (35°) indicates ionic diffusion in the crystal lattice. As application, the decolorization of methylene blue (MB, 92%) within 70 min under optimal conditions by hydroxyl (HO⋅) and superoxide (O2⋅−). “Photocatalysis/ultrasound (USW)” is superior to both photocatalytic oxidation (CuAlO2/Sunlight) and CuAlO2/USW. A high chemical oxygen demand (COD) is obtained at low frequency ultrasonic waves (60 kHz) corresponds to high mineralization with {USW/sunlight/CuAlO2}. The kinetics of these simultaneous processes follows a pseudo-prime model with a rate constant of 1.96 × 10−2 min−1 (t1/2 = 35 min). The sono-photo-catalytic activity, reusability and stability of CuAlO2 were found to be excellent.
