Browsing by Author "Bouakaz, H."

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    Physical properties of the delafossite CuCoO2 synthesized by co-precipitation /hydrothermal route
    (Elsevier, 2021) Bouakaz, H.; Abbas, Moussa; Brahimi, R.; Trari, M.
    CuCoO2 crystallizing in the delafossite structure was prepared by co-precipitation/hydrothermal method at low temperature (~ 150 °C). It is stable in air up to ~400 °C beyond which it turns into the spinel. The oxide has a direct optical transition attributed to the crystal field splitting of Cu+, linearly coordinated. It exhibits p-type behavior due to O2− insertion in the layered lattice. Such property is corroborated by a cathodic photocurrent in the intensity-potential (J – E) plot and a negative slope of the capacitance potential (C−2– E) plot. The latter traced in NaOH (0.01 M) electrolyte, exhibits a straight line from which a flat band potential of +0.411 V and a hole density of 6.9 × 1023 m−3 were extracted. An exchange current density of 7.36 μA cm−2and a polarizarion resistance (6.69 kΩ cm2), derived from the semi-logarithmic curve (log(J) – E), indicate a good electrochemical stability with a medium hysteresis loop and a low H2-over potential. The anisotropy of the 2 D dimensional crystal structure allows reversible oxygen insertion in the (0 0 n) planes evidenced from the (J – E) profiles. The Electrochemical Impedance Spectroscopy (EIS) measurements give two well-distinguished depressive semicircles in the dark. The diameter of the semicircle at high frequencies (1.49 kΩ cm2) decreases down to (1.28 kΩ cm2) under irradiation, a behavior typical of a non-degenerate semi conductivity of CuCoO2
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    Semiconducting and electrochemical properties of the spinel FeCo2O4 synthetized by co-precipitation. Application to H2 production under visible light
    (Elsevier, 2023) Bouakaz, H.; Abbas, Moussa; Benallal, S.; Brahimi, R.; Trari, M.
    FeCo2O4 elaborated by co-precipitation was characterized photo-electrochemically for the first time in order to assess its performance for the hydrogen production. The X-ray diffraction revealed a single phase crystallizing in the spinel structure with a lattice constant of 8.1075 Å and a crystallite size of 35 nm. The UV–Visible diffuse reflectance of the black product exhibits an absorption above 650 nm and a direct optical transition at 1.53 eV was determined, assigned to the crystal field splitting of Co3+: 3d orbital hexa-coordinated. The transport properties indicated semi-conducting properties of FeCo2O4, the positive thermo-power (S300k = 666 μV K−1) demonstrated that holes are majority charge carriers. The Electrochemical Impedance Spectroscopy (EIS) realized in Na2SO4 (0.1 M) electrolyte showed two semicircles at high and intermediate frequencies, characteristic of the charge transfer and grain boundaries respectively. The first diameter (1249 Ω cm2) decreases under visible light down to 742 Ω cm2, thus supporting the semiconducting behavior and no inhibiting effect is observed due to the appearance of the photo-effect. The inverse of the square of the capacitance as a function of the potential (C−2 – E) exhibits a line, negatively slopped, characteristic of p-type behavior with a hole concentration of 21.9 × 1020 cm−3. The conduction band (−0.76 VSCE), made up of Co3+: 3d orbital is more cathodic than the H2-level leading to a spontaneous H2 evolution under visible irradiation (29 mW cm−2) with a liberation rate of 0.58 μmol H2/h−1/g. FeCo2O4 an showed excellent chemical stability after its reuse for the H2 production, as evidenced by X-ray diffraction