High quality crednerite CuMnO 2 was prepared by solid state reaction at 950°C under argon flow. The oxide crystallizes in a monoclinically distorted delafossite structure associated to the static Jahn-Teller (J-T) effect of Mn 3? ion. Thermal analysis showed that it converts reversibly to spinel Cu x Mn 3-x O 4 at *420°C in air and further heating reform the crednerite above 940°C. CuMnO 2 is p-type, narrow semiconductor band gap with a direct optical gap of 1.31 eV. It exhibits a long-term chemical stability in basic medium (KOH 0.5 M), the semi logarithmic plot gave an exchange current density of 0.2 lA cm -2 and a corrosion potential of *-0.1 V SCE . The electrochemical oxygen insertion/desinsertion is evidenced from the intensity-potential characteristics. The flat band potential (V fb = -0.26 V SCE ) and the holes density (N A = 5.12 9 10 18 cm -3 ) were determined, respectively, by extrapolating the curve C -2 versus the potential to the intersection with C -2 = 0 and from the slope of the MottSchottky plot. From photoelectrochemical measurements, the valence band formed from Cu-3d wave function is positioned at 5.24 ± 0.02 eV below vacuum. The Nyquist representation shows straight line in the high frequency range with an angle of 65°ascribed to Warburg impedance originating from oxygen intercalation and compatible with a system under mass transfer control. The electrochemical junction is modeled by an equivalent electrical circuit thanks to the Randles model.

Nano crystallites of the crednerite CuMnO 2 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, Mn 2 O 3 and Cu x Mn 3 x O 4. However, when calcined at 900°C under N 2 atmosphere, the crednerite CuMnO 2 with a monoclinic structure (space group: C2/m) is obtained. The Raman spectrum shows a single peak at 679 cm 1 as signed to A 1g mode whereas the infrared analysis confirms the linearity of CuO 2 3 units. The optical transition at 1.70 eV, determined from the diffuse reflectance is attributed to the inter band d d transi tion of Cu þ ion. The oxide exhibits semiconducting properties with an activation energy of 0.21 eV. The photo electrochemical measurement shows p type conduction due to O 2 insertion in the two dimen sional lattice. The flat band potential (þ0.12 V SCE), indicates a cationic character of both valence and conduction bands deriving from Cu þ : 3d orbital.

Nanocrystalline CuAlO 2 is synthesized by solgel 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 (O 2-: 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 9 10 18 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. Graphical Abstract Temperature dependence of the electrical conductivity for CuAlO 2 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 CuAlO 2 . 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 r ¼ r 0 exp T 0 T À Á 0:25 .

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