Photocatalytic hydrogen production on the hetero-junction CuO/ZnO

The co-precipitation is successfully used for the synthesis of the hetero-junction CuO/ZnO. Thermal analysis, ATR spectroscopy and diffuse reflectance were used to assess the photoactivity of the hetero-system for the hydrogen formation upon visible light. As expected, the X-ray diffraction shows mixed phases of CuO (tenorite) and ZnO (Wurtzite). The specific surface area is around ~7 m² g^?1 with a crystallite size lying between 20 and 49 nm. The diffuse reflectance indicates an indirect transition at 3.13 eV for ZnO and a direct transition at 1.60 eV for the sensitizer CuO. The capacitance-potential (C^?2 - E) characteristic of CuO plotted in Na₂SO₄ electrolyte exhibits p-type comportment with a flat band potential of ?0.315 V_RHE and a holes concentration of 8.7 × 10¹⁸cm^?3. The Electrochemical Impedance Spectroscopy exhibits a semicircle characteristic of the bulk material with an impedance of 1725 Ω cm² which decreases down to 453 Ω cm² under irradiation, supporting the semiconducting character of CuO. ZnO mediates the electrons transport thanks to its conduction band, formed from Zn²⁺: 4s orbital (?0.92 V_RHE); it is positioned cathodically with respect to the H₂O/H₂ level (~-0.74 V_RHE), producing a H₂ evolution under visible light illumination. The performance peaks at pH ~7 on 5% CuO/ZnO for a catalyst dose of 0.25 mg of catalyst/mL of solution in the presence of SO₃^2? as holes scavenger. A liberation rate of 340 μmol h^?1 (g of catalyst)^?1 is obtained with a quantum efficiency of 0.38% under a photons flux of 2.09 × 10¹⁹ s^?1.