Photoelectrochemical hydrogen-evolution over p-type chalcopyrite CuInSe2

Photocatalytic H2-production has been realized over active CuInSe2, synthesized by the fusion technique. The material crystallizes in the chalcopyrite structure and exhibits p-type conductivity ascribed to copper deficiency. An optical gap of 0.95 eV was determined from the reflectance diffuse spectrum. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polaron hopping. The slope and the intercept of the Mott-Schottky plot gave a holes density NA of 5.9 x 1018 cm-3 and a flat band potential of -0.36VSCE, in perfect agreement with the photo-onset potential Von (-0.35VSCE). Hence, the conduction band, located at -1.29VSCE, allows a spontaneous H2 liberation upon visible light. In aqueous solutions, the material is stabilized by hole consumption involving X2- species (=S2- and SO32-). H2 formation would become thermodynamically easy in alkaline media and the best photoactivity was obtained in thiosulfate electrolyte (10-2 M S2O32-, 0.5 M KOH) with an evolution rate of 0.009 ml mg-1 h-1. The light induced electron transfer through the interface involves two steps mechanism where S2O32- is oxidized to SO32- and SO42- by successive reactions. Such results are corroborated by the semi logarithmic plots and photocurrent-photopotential characteristics. The influence of pH was studied with S2O32- The tendency towards saturation for prolonged irradiation is attributed to competitive reductions of the end products Sn2- and S2O62- with water and to the yellow color of polysulfide Sn2-.