Sol–gel synthesis and characterization of the delafossite CuAlO₂

Nanocrystalline CuAlO₂ is synthesized by sol–gel method using ethylene glycol as solvent. The stages of formation are followed by thermal analysis. The X-ray diffraction pattern of the powder heat-treated at 1100 °C shows a single phase, indexed in a rhombohedral symmetry (R \(\overline{3}\) m). The apparent crystallite size (57 ± 8 nm) is determined from the Williamson–Hall plot. The direct optical transition (=3.69 eV), evaluated from the diffuse reflectance spectrum, is attributed to the charge transfer (O²⁻: 2p → Cu⁺: 4s). The oxide is p-type semiconductor, and the conduction occurs predominantly by small polaron hopping between mixed valences Cu^2+/+, due to oxygen insertion in the layered crystal. The photoelectrochemical characterization gives a flat band of 0.20 V_SCE and a hole density of 1.13 × 10¹⁸ cm⁻³. The semicircle centered on the real axis, in the electrochemical impedance spectroscopy (EIS), is due to the absence of constant phase element with a pure capacitive behavior. The straight line at 35° at low frequencies is attributed to the diffusion in the layered structure.

Temperature dependence of the electrical conductivity for CuAlO₂ synthesized by sol–gel method. Inset: the variable range hopping at low temperatures. The sol–gel method has been successfully employed for the preparation of CuAlO₂. The thermal variation of the conductivity indicates a semiconducting behavior which is frequency independent. At low temperature, a variable range hopping involves electrons jump over large distances according to the Mott law \(\sigma = \sigma_{0} \exp \left( {\frac{{T_{0} }}{T}} \right)^{0.25}\).