Hydrogen photo-evolution over the spinel CuCr2O4

The photocatalytic H2 evolution over CuCr2O4 has been studied in connection with some physical properties. The black oxide is classified as a narrow band gap semiconductor crystallizing in the normal spinel structure. The photoactivity is dependent on the preparative conditions through the surface area of the powder and CuCr2O4 has been synthesized by nitrate route. The conductivity varies exponentially with temperature indicating an extrinsic conductivity between mixed valences states Cr3+/4+ located in octahedra sharing common corners. The oxide exhibits p-type conductivity with an activation energy of 0.20 eV and a hole mobility of 2.14 × 10−8 cm2/V s thermally activated. CuCr2O4 shows an excellent chemical stability, a corrosion potential of −0.775 VSCE and an exchange current density of 16 μA cm−2 have been determined from the semilogarithm plot. The capacitance measurement (C−2−V), performed in basic electrolyte, exhibits a linear behavior from which a flat band potential of 0.48 VSCE was obtained. CuCr2O4 is photoelectrochemically stable by hole consumption reactions involving X2− (=SO32- and S2O32-) as hole scavengers. The best catalytic performance for H2-photoproduction was obtained in (NaOH 0.5 M, Na2S2O3 0.025 M) with an average rate of 0.013 cm3 h−1 (mg catalyst)−1 and a quantum efficiency of 0.2% under polychromatic light. S2O32- is oxidized to SO32- and subsequently to SO42- as proven by the dual slopes in the Tafel plot. The system shows a tendency towards saturation whose deceleration is the result of the competitive reductions of the end products namely S2O62- and S2O42- with the water discharge.