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 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 Cu2+/+, due to oxygen insertion in the layered crystal. The photoelectrochemical characterization gives a flat band of 0.20 VSCE 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

Preparation and characterization of the semiconductor CuMnO2 by sol-gel route

Nano crystallites of the crednerite CuMnO2 are prepared by sol–gel method with two-step annealing process. The powder heated at 450°C under air flow shows a mixture of CuO, Mn2O3 and CuxMn3−xO4. However, when calcined at 900°C under N2 atmosphere, the crednerite CuMnO2 with a monoclinic structure (space group: C2/m) is obtained. The Raman spectrum shows a single peak at 679cm−1 assigned to A1g mode whereas the infrared analysis confirms the linearity of CuO23− units. The optical transition at 1.70eV, determined from the diffuse reflectance is attributed to the inter-band d-d transition of Cu+ ion. The oxide exhibits semiconducting properties with an activation energy of 0.21eV. The photo-electrochemical measurement shows p-type conduction due to O2− insertion in the two dimensional lattice. The flat band potential (+0.12 VSCE), indicates a cationic character of both valence and conduction bands deriving from Cu+: 3d orbital